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First published online 9 January 2008
doi: 10.1242/dev.009910

Development 135, 775-784 (2008)
Published by The Company of Biologists 2008
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Role of epithelial cell fibroblast growth factor receptor substrate 2{alpha} in prostate development, regeneration and tumorigenesis

Yongyou Zhang1,*, Jue Zhang1,*, Yongshun Lin1, Yongsheng Lan1, Chunhong Lin1, Jim W. Xuan2, Michael M. Shen3, Wallace L. McKeehan1, Norman M. Greenberg4 and Fen Wang1,{dagger}

1 Center for Cancer and Stem Cell Biology, Institute of Biosciences and Technology, Texas A&M Health Science Center, 2121 W. Holcombe Blvd., Houston, TX 77030-3303, USA.
2 Department of Surgery, University of Western Ontario, London, ON, N6A 4G5, Canada.
3 Departments of Medicine, and Genetics and Development, Columbia University, College of Physicians and Surgeons, Herbert Irving Comprehensive Cancer Center, 1130 St. Nicholas Avenue, Room 217B, New York, NY 10032, USA.
4 Clinical Research Division, Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue, Seattle, WA 98109-1024, USA.

{dagger} Author for correspondence (e-mail: fwang{at}ibt.tmc.edu)

Accepted 24 November 2007


   SUMMARY
TOP
 SUMMARY
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The fibroblast growth factor (FGF) regulates a broad spectrum of biological activities by activation of transmembrane FGF receptor (FGFR) tyrosine kinases and their coupled intracellular signaling pathways. FGF receptor substrate 2{alpha} (FRS2{alpha}) is an FGFR interactive adaptor protein that links multiple signaling pathways to the activated FGFR kinase. We previously showed that FGFR2 in the prostate epithelium is important for branching morphogenesis and for the acquisition of the androgen responsiveness. Here we show in mice that FRS2{alpha} is uniformly expressed in the epithelial cells of developing prostates, whereas it is expressed only in basal cells of the mature prostate epithelium. However, expression of FRS2{alpha} was apparent in luminal epithelial cells of regenerating prostates and prostate tumors. To investigate FRS2{alpha} function in the prostate, the Frs2{alpha} alleles were ablated specifically in the prostatic epithelial precursor cells during prostate development. Similar to the ablation of Fgfr2, ablation of Frs2{alpha} disrupted MAP kinase activation, impaired prostatic ductal branching morphogenesis and compromised cell proliferation. Unlike the Fgfr2 ablation, disrupting Frs2{alpha} had no effect on the response of the prostate to androgens. More importantly, ablation of Frs2{alpha} inhibited prostatic tumorigenesis induced by oncogenic viral proteins. The results suggest that FRS2{alpha}-mediated signals in prostate epithelial cells promote branching morphogenesis and proliferation, and that aberrant activation of FRS2-linked pathways might promote tumorigenesis. Thus, the prostate-specific Frs2{alpha}cn mice provide a useful animal model for scrutinizing the molecular mechanisms underlying prostatic development and tumorigenesis.

Key words: Adaptor proteins, Growth factors, Receptor tyrosine kinases, Prostate cancer, Mouse models


   INTRODUCTION
TOP
 SUMMARY
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Prostatic tumors are the most frequently diagnosed tumors in American males. Yet, the mechanisms underlying prostate tumor initiation, progression and escape from regulation by androgen are not well understood. Since many developmental events are recapitulated during tumorigenesis, understanding prostate development will aid comprehension of prostate tumorigenesis. Prostate development starts at embryonic day 17 (E17), when a group of urogenital epithelial cells in the hindgut endoderm grow out into the surrounding mesenchyme and form the anterior, ventral, dorsal and lateral prostate buds (Donjacour and Cunha, 1988Go; Sugimura et al., 1986Go). The prostatic buds then undergo extensive branching morphogenesis prior to puberty. At pubertal stages, the prostate rudiments grow rapidly; the ducts elongate from distal points and exhibit complex infolding of the intraductal mucosum. It has been shown that several growth factor families, including the fibroblast growth factor (FGF) family, play important roles in regulating prostate morphogenesis (Donjacour et al., 2003Go; Huang et al., 2005Go; Lin et al., 2007aGo; Thomson and Cunha, 1999Go).

The FGF family consists of 22 gene products controlling a wide spectrum of cellular processes. The FGF elicits regulatory signals by activating FGF receptor (FGFR) transmembrane tyrosine kinases encoded in isoforms of four genes (McKeehan et al., 1998Go; Powers et al., 2000Go; Wang and McKeehan, 2003Go). In the prostate, members of the FGF and FGFR families are partitioned between the epithelial and mesenchymal compartments and underlie directionally specific and reciprocal communication between the two compartments. Ablation of the FGF signaling disrupts prostate development (Donjacour et al., 2003Go; Huang et al., 2005Go; Lin et al., 2007aGo). Aberrant FGF signaling, in particular the reduction in the resident epithelial FGFR2IIIb, the ectopic appearance of FGFR1, and the overexpression of FGF9 in epithelial cells, is associated with prostate tumor progression (Giri et al., 1999Go; Jin et al., 2003bGo; Kwabi-Addo et al., 2001Go; Lu et al., 1999Go; McKeehan et al., 1998Go; Polnaszek et al., 2004Go; Ropiquet et al., 1999Go).

Recently, we reported that ablation of FGFR2 in prostatic epithelial precursor cells in early prostate development inhibits prostatic ductal branching morphogenesis and the acquisition of androgen dependency during development (Lin et al., 2007aGo). Similarly, ablation of Fgf10, a ligand for the FGFR2IIIb isoform, also disrupts prostate development (Donjacour et al., 2003Go; Huang et al., 2005Go). The detailed mechanism underlying how FGFR2 elicits intracellular regulatory signals in the prostate remains elusive. FRS2{alpha} is an adaptor protein that links FGFR kinases to multiple downstream signaling pathways. Activation of the MAP kinase and phosphatidylinositol 3 (PI3) kinase pathways by FGFR1 is primarily mediated via FRS2{alpha} (Kouhara et al., 1997Go; Lin et al., 1998Go; Ong et al., 1996Go; Rabin et al., 1993Go). Whether the FRS2{alpha}-mediated signals are important for FGFR2 in control of prostate development and adult tissue homeostasis remains to be determined.

FRS2{alpha} is generally expressed in fetal and adult tissues (McDougall et al., 2001Go). Disruption of Frs2{alpha} alleles in early embryogenesis is lethal (Hadari et al., 2001Go). To circumvent this limitation and enable studies on later stages of development, we employed the loxP-Cre recombination system to inactivate Frs2{alpha} alleles in the prostate epithelium by crossing mice carrying loxP-flanked Frs2{alpha} (Frs2{alpha}flox) (Lin et al., 2007bGo) and Nkx3.1Cre knock-in alleles (Lin et al., 2007aGo). Similar to Fgfr2 ablation, disruption of Frs2{alpha} in the prostate epithelium reduced epithelial cell proliferation, impaired branching morphogenesis during prostate development, and impaired post-puberty androgen-dependent prostate regeneration. Unlike Fgfr2cn prostates, Frs2{alpha}cn prostates remained strictly dependent on androgen with respect to tissue homeostasis. Ablation of Frs2{alpha} significantly inhibited the initiation and progression of autochthonous mouse prostate tumors. This indicated that FRS2{alpha}-mediated mitogenic signals are important for prostatic development and regeneration, and suggest a role of aberrant FRS2{alpha}-mediated signaling in prostate tumorigenesis.


   MATERIALS AND METHODS
TOP
 SUMMARY
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animals
Mice carrying loxP-flanked Frs2{alpha} alleles, the R26R-lacZ reporter, and the Nkx3.1Cre knock-in alleles were bred and genotyped as described (Jin et al., 2003bGo; Lin et al., 2007bGo; Soriano, 1999Go). Orchiectomy and androgen therapy were carried out as described previously (Lin et al., 2007aGo). All animals were housed in the Program for Animal Resources of the Institute of Biosciences and Technology, and were handled in accordance with the principles and procedures of the Guide for the Care and Use of Laboratory Animals. All experimental procedures were approved by the Institutional Animal Care and Use Committee.

Prostate cell and organ cultures
TRAMP-C2 cells were maintained in RD (50% RPMI, 50% DMEM) medium with 5% FBS as described (Foster et al., 1997Go). For protein stability analyses, the cells (1x106 cells in 10-cm dishes) were treated with 10 µg/ml cycloheximide for the indicated times. For FGF2 response analyses, the cells (1x105 in 6-well plates) were serum starved for 24 hours prior to being treated with 10 ng/ml FGF2 for 10 minutes.

Prostate tissues were dissected at postnatal day 0.5 as described (Lamm et al., 2001Go) and transferred to 0.4 µm Millicell-CM filters (Becton Dickinson Labware Europe, Meylan, France) inside 24-well tissue culture plates. Each well contained 0.5 ml of serum-free DMEM:Ham's F-12 (1:1) supplemented with 2% ITS (12.5 µg/ml insulin, 12.5 µg/ml transferrin, 12.5 ng/ml selenious acid), 2.5 µg/ml linoleic acid-albumin (Sigma, St Louis, MO), 25 µg/ml gentamycin, 0.25 µg/ml amphotericin B and 10 nM testosterone. PI3 kinase inhibitor (LY294002) and ERK1/2 inhibitor (Calbiochem, Darmstadt, Germany) were added to the medium at a final concentration of 10 µM where indicated. Cultures were maintained for 3 days at 37°C prior to harvest for analysis.

Histology
Prostates and prostate ducts were dissected and sectioned for histological analyses as previously described (Lin et al., 2007aGo). Main ducts and distal ductal tips were quantified from at least three animals. Data are presented as mean±s.d.

Hematoxylin and Eosin staining was performed on 5-µm sections; immunohistochemical analyses and in situ hybridization were performed on 7-µm sections mounted on Superfrost/Plus slides (Fisher Scientific, Pittsburgh, PA). The antigens were retrieved by autoclaving samples in 10 mM Tris-HCl buffer (pH 10.0) for 5-10 minutes or as suggested by manufacturers of the antibodies. The source and dilutions of primary antibodies are: mouse anti-cytokeratin 8 (1:15; Fitzgerald, Concord, MA); mouse anti-smooth muscle {alpha}-actin (1:1) and mouse anti-PCNA (1:1000) from Sigma (St Louis, MO); mouse anti-P63 (1:150), rabbit anti-FRS2{alpha} (1/500) and rabbit anti-androgen receptor (1:150) from Santa Cruz (Santa Cruz, CA); mouse anti-T antigens (1:500) from BD Pharmingen (Frankin Lakes, NJ); rabbit anti-phosphoAKT (1:500) and anti-phosphoERK1/2 (1:500) from Cell Signaling Technology (Danvers, MA); rabbit anti-probasin (1:3000) from the Greenberg laboratory (Fred Hutchinson Cancer Research Center, Seattle, WA); and rabbit anti-PSP94 (1:2000) from the Xuan laboratory (University of Western Ontario, London, Canada). The TSA Plus Fluorescein System from PerkinElmer (Shelton, CT) was used to visualize anti-FRS2{alpha}, anti-phosphoAKT and anti-phosphoERK1/2 antibodies and the ExtraAvidin Peroxidase System from Sigma to visualize all other antibodies. For whole-mount lacZ staining, the prostate tissues were lightly fixed with 0.2% glutaraldehyde for 30 minutes. lacZ staining was carried out by rocking in 1 mg/ml X-Gal at room temperature overnight as described (Liu et al., 2005Go).

For proliferation analyses in 1- and 2-week-old prostates, the percentage of PCNA-positive cells in the growing duct tips was determined, as most proliferating cells were located at the most-distal part of ducts at this stage. For 4-week-old and post-pubertal regenerating prostates, the percentage of PCNA-positive cells in the whole gland was determined, as the proliferating cells were randomly distributed in the whole gland. The data were collected from at least five slides per prostate, and three prostates per genotype, and are presented as mean±s.d.

Immunoblotting and analyses of secretory proteins
Dissected prostate tissues or TRAMP-C2 cells were homogenized in RIPA buffer (50 mM Tris-HCl buffer pH 7.4, 1% NP40, 150 mM NaCl, 0.25% Na-deoxycholate, 1 mM EGTA, 1 mM PMSF), and extracted soluble proteins were harvested by centrifugation. Samples containing 50 µg protein were separated by SDS-PAGE and electroblotted onto nylon membranes for western analyses with the indicated antibodies. The dilutions of the antibodies are: anti-phosphoFRS2{alpha}, 1:1000; anti-phosphoERK1/2, 1:1000; anti-phosphoAKT, 1:1000; anti-AKT, 1:1000; anti-FRS2{alpha}, 1:1000; anti-ERK1/2, 1:3000; and anti-FGFR2, 1:1000. For immunoprecipitation, soluble tissue lysates containing 500 µg protein were incubated with 2 µg of the indicated antibodies overnight, and then the immunocomplexes associated with 10 µl protein A-Sepharose beads were collected by centrifugation and extracted with SDS sample buffer for western analyses. The specific bands were visualized using the ECL-Plus chemoluminescent reagents. The films were scanned with a densitometer and the bands quantified using Image J software (NIH).

Secretory proteins were extracted as described (Lin et al., 2007aGo). The protein concentrations were determined and adjusted to a final concentration of 1 mg/ml with PBS. Samples containing 25 µg protein were separated by 5-20% gradient SDS-PAGE and visualized by Coomassie Blue staining. Samples of 50 µg protein were used for western blotting analyses with anti-probasin or anti-PSP94 as indicated.

Gene expression
For in situ hybridizations, paraffin-embedded tissue sections were rehydrated, followed by 20 µg/ml protease K digestion for 7 minutes at room temperature. After prehybridization at 65°C for 2 hours, the hybridization was carried out by overnight incubation at 65°C with 0.5 µg/ml digoxigenin-labeled RNA probes for the indicated genes. Non-specifically bound probes were removed by washing four times with 0.1xDIG washing buffer at 60°C for 30 minutes. Specifically bound probes were detected using alkaline phosphatase-conjugated anti-digoxigenin antibody (Roche, Indianapolis, IN).

For real-time RT-PCR analyses, total RNA was extracted from prostates using the RiboPure Kit (Ambion, Austin, TX). The first-strand cDNAs were reverse transcribed from the RNA template using SuperScript II reverse transcriptase (Invitrogen, Carlsbad, CA) and random primers according to manufacturer's protocols. Real-time PCR analyses were carried out using the SYBR Green JumpStart Taq Ready Mix (Sigma) as instructed by the manufacturer. Relative abundances of mRNA were calculated using the comparative threshold (CT) cycle method and were normalized to β-actin or 18S rRNA as an internal control. The mean and s.d. among at least three individual experiments are shown. RT-PCR products were analyzed on 2% agarose gels for validation of products by size.


   RESULTS
TOP
 SUMMARY
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Disruption of Frs2{alpha} alleles specifically in the prostate epithelium
Previously, we reported that FGFR2 signaling is important for prostate development and acquisition of androgen responsiveness. To determine whether Frs2{alpha}, a receptor-proximal adaptor in FGF signaling, was essential for prostatic development, the loxP-Cre recombination system was used to conditionally inactivate Frs2{alpha} alleles in prostatic epithelial cell precursors (Fig. 1A). Mice carrying homozygous Frs2{alpha}cn alleles were viable and fertile. Disruption of the Frs2{alpha} in prostates was confirmed by PCR (Fig. 1B). Real-time RT-PCR analyses revealed that expression of Frs2{alpha} in 1-week-old Frs2{alpha}cn prostates was significantly reduced compared with the control prostates (Fig. 1C).


Figure 1
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  		Fig. 1. Disruption of Frs2{alpha} alleles in prostate epithelium. (A) Schematic of the floxed Frs2{alpha} alleles for conditional disruption. The genomic DNA containing coding exons 1-5 and adjacent introns is shown. Dashed boxes indicate non-coding exon sequence. The primers for PCR genotyping are indicated by arrows. Primers f1 and f2 amplify a 319 bp fragment from the floxed Frs2{alpha} allele. Primers f1 and f3 amplify a 261 bp fragment from the Frs2{alpha}-null allele; no amplification from wild-type alleles. (B) PCR genotyping for the Frs2{alpha} conditional-null alleles. Genomic DNAs extracted from each prostatic lobe of 4-week-old mice were analyzed by PCR using the primers illustrated in A. (C) Total RNAs were extracted from prostates of different ages, and Frs2{alpha} expression was assessed by real-time RT-PCR. The data were normalized to β-actin loading controls and are expressed as mean±s.d. of at least three independent experiments. Note that Frs2{alpha} alleles were intact in the stromal compartment, which is likely to account for the basal level of Frs2{alpha} expression in Frs2{alpha}cn prostates. Representative data from dorsolateral prostate are shown. (D) In situ hybridization of Frs2{alpha} expression in prostates. Enlarged image of boxed area shown on the right. Note that the expression was diminished in the epithelium of Frs2{alpha}cn prostates and mature control prostates. Arrows indicate basal cells. AP, anterior prostate; DLP, dorsolateral prostate; VP, ventral prostate; M, myristylation site; PTB, phosphotyrosine-binding site; f, Frt element; p, loxP element; F/F, homozygous Frs2{alpha}flox mice; CN, Frs2{alpha}cn mice.

 
Frs2{alpha} expression was further characterized by in situ hybridization with Frs2{alpha}-specific probes. The results showed that although Frs2{alpha} expression was constant in the stromal compartment, it exhibited a spatial and temporal expression pattern in the epithelium (Fig. 1D). In 1-week-old prostates, Frs2{alpha} was expressed relatively uniformly at high levels in the epithelium. In 2-week-old prostates, the expression became restricted to cells located at distal tips of the ductal network, where the cells were actively engaged in proliferation. In the prostates of mice at 4 weeks or older, expression of Frs2{alpha} was limited to basal epithelial and stromal cells. No Frs2{alpha} expression was detected in luminal epithelial cells. Ablation of Frs2{alpha} with Nkx3.1Cre diminished Frs2{alpha} expression in the epithelial compartment of prostates, including basal and luminal epithelial cells (Fig. 1D), whereas Frs2{alpha} expression in the stromal cells remained intact. Thus, the basal level of Frs2{alpha} expression indicated by RT-PCR in prostates of Frs2{alpha}cn and 4-week-old control mice was a constitutive property of stromal and basal cells.

Unlike Fgfr2cn prostates that only have dorsal and lateral lobes, Frs2{alpha}cn prostates were relatively normal and consisted of anterior, dorsal, lateral and ventral prostate (AP, DP, LP and VP, respectively) lobes, although these were smaller and more transparent than wild-type prostates (see Fig. S1A in the supplementary material). The prostates of genotype Frs2{alpha}flox/flox, Frs2{alpha}flox/WT, Frs2{alpha}flox/cn, Frs2{alpha}WT/WT or Nkx3.1Cre, had no noticeable differences in organ morphology and histological structures, and were all considered as control prostates.

Ablation of Frs2{alpha} alleles in the prostate epithelium inhibits prostatic branching morphogenesis and growth
To improve direct visualization of branching morphogenesis, the R26R-lacZ reporter allele was introduced into the Frs2{alpha}cn mice by crossing with mice bearing a lacZ reporter silenced by a loxP-flanked sequence (Soriano, 1999Go). Activation of the R26R-lacZ reporter by the Cre recombinase occurred concurrently with disruption of the Frs2{alpha}flox alleles in the same cells. Whole-mount staining with X-Gal showed that similar to Fgfr2cn embryos, prostatic buds formed at E17.5 in Frs2{alpha}cn embryos with no apparent difference to controls. Although all mutant buds formed AP, DP, LP and VP lobes, fewer ductal branches were observed in the Frs2{alpha}cn prostates than in controls (Fig. 2A). We then dissected each prostatic lobe from 1-week-old mice to examine the Frs2{alpha}cn prostates in more detail. The X-Gal stain confirmed that that the complexity of the ductal network in the Frs2{alpha}cn prostates was reduced. Quantitative analyses of ducts microdissected out of 4-week-old prostates revealed a reduction in the number of ductal tips in every lobe of the Frs2{alpha}cn prostates (Fig. 2B). This indicated that the extent of branching morphogenesis, but not budding, was compromised in Frs2{alpha}cn prostates.


Figure 2
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  		Fig. 2. Disruption of Frs2{alpha} alleles inhibits ductal branching morphogenesis in prostates. (A) The urogenital tract was dissected at the indicated times, lightly fixed and then stained with X-Gal. Stained tissues representing each prostatic lobe are shown. Note the lack of significant differences between Frs2{alpha}cn (below) and control (above) prostate rudiments at E17.5 or later. (B) Prostatic lobes were dissected from mice at the indicated ages. The tissues were lightly fixed and stained with X-Gal (a,b). Note that only epithelial cells in the prostate were stained. The ductal network in each prostatic lobe from 4-week-old mice was microdissected (c-e) and the average number of tips was quantified from three prostates and is shown as mean±s.d. (f). AP, anterior prostate; DLP, dorsolateral prostate; VP, ventral prostate; B, bladder; S, seminal vesicles; U, urethra; F/F, homozygous Frs2{alpha}flox mice; CN, Frs2{alpha}cn mice; WT, wild-type Frs2{alpha}.

 
To determine whether FRS2{alpha} protein was carried over into mutant prostatic buds after ablation of expression by the Nkx3.1Cre-mediated recombination, the prostate rudiments at different stages were assessed with an anti-FRS2{alpha} antibody. FRS2{alpha} was strongly expressed in epithelial cells of control prostate rudiments and the expression gradually declined with development of the prostate. Although Nkx3.1Cre is expressed prior to E17.5, FRS2{alpha} protein remained evident in mutant epithelial cells at E18.5 and then diminished at the neonatal stage (Fig. 3A). The number of cells positive for P63 (also known as Trp63 - Mouse Genome Informatics) remained relatively constant over the same period with a slight decrease in Frs2{alpha}cn prostates at later stages (Fig. 3B). Most epithelial cells in normal prostate rudiments expressed P63 (Pu et al., 2007Go). This indicated a persistence of FRS2{alpha} protein beyond ablation of expression. Separate experiments in vitro indicated that the half-life of FRS2{alpha} was longer than that of FGFR2 in prostate epithelial tumor cells (Fig. 3C,D). These results indicated that the carryover of FRS2{alpha} after ablation by recombination might be sufficient to participate in signals that support prostatic bud formation in early prostate morphogenesis.

Presence of the proliferating cell nuclear antigen (PCNA) showed that the number of proliferating cells in Frs2{alpha}cn distal tips was reduced compared with controls at 1-2 weeks of age (Fig. 4A,B). At 3-4 weeks, the prostates were undergoing rapid growth associated with puberty. The proliferating cells were widely distributed across the prostates. Proliferation in whole dorsolateral prostate (DLP) and AP lobes in Frs2{alpha}cn prostates was also lower than in the controls (Fig. 4A,B). Post-puberty cell proliferation at 6 weeks in both Frs2{alpha}cn and control prostates was dramatically reduced. No difference was observed between Frs2{alpha}cn and control prostates at this stage (data not shown). Together, the results demonstrate that FRS2{alpha}-mediated signals are important for prostate cell proliferation during development. Since these results mirror the Fgfr2 ablation, they suggest a role of FRS2{alpha} in mediating FGFR2 mitogenic signals in the epithelial cells during prostate development.

The expression of 22 key regulatory genes was compared between control and Frs2{alpha}cn 1-week-old prostates by real-time RT-PCR. The Frs2{alpha}cn prostates exhibited reduced expression of BMP7, HOXb13, HOXd13 and NKX3.1 and increased expression of FGFR4. No significant difference was observed between Frs2{alpha}cn and control prostates in the expression of the other genes tested (Fig. 4C). HOXb13, HOXd13 and NKX3.1 are highly expressed in luminal epithelial cells and play a role in differentiation of luminal epithelial cells (Economides and Capecchi, 2003Go). The results suggest that in addition to its effects on proliferation and branching morphogenesis, ablation of FRS2{alpha} might also compromise luminal epithelial cell differentiation.


Figure 3
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  		Fig. 3. Depletion of FRS2{alpha} proteins in Frs2{alpha}cn prostatic rudiments. (A) Prostates were collected at the indicated times, immunostained with the indicated antibodies, and visualized by confocal microscopy. The prostate epithelial cells were identified with anti-P63 antibody. Expression of FRS2{alpha} was assessed with anti-FRS2{alpha} antibody. Arrows indicate prostate epithelial cells. Note that FRS2{alpha} staining was diminished in Frs2{alpha}cn prostates at postnatal day 0.5 (P0.5). (B) The ratio of p63-positive cells in the epithelial compartment at different ages was calculated from three samples. Representative mean±s.d. values of data from triplicate samples are shown. (C,D) Stability of FRS2{alpha} proteins as compared with FGFR2 in prostate epithelial cells. TRAMP-C2 cells (1x106) treated with cycloheximide were lysed at the indicated times. The abundance of FRS2{alpha} was analyzed by western blot directly from the cell lysates (containing 50 µg protein). FGFR2 in cell lysates (containing 500 µg protein) was pulled down with anti-FGFR2 antibody. The specifically bound fractions were subjected to western analyses (C). The specific bands were quantitated with a densitometer and the data presented as ratios of treated to non-treated cells (D). CHX, cycloheximide; F/F, homozygous Frs2{alpha}flox mice; CN, Frs2{alpha}cn mice.

 


Figure 4
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  		Fig. 4. Compromised proliferation in Frs2{alpha}cn prostates during prepubertal and pubertal growth stages. (A) Prostate tissues were collected from control and mutant mice at the indicated ages and proliferating cells assessed by immunostaining for PCNA. Note that the proliferating cells mainly reside at the distal tips of the ductal network in 1- or 2-week-old prostates, but are randomly distributed in 4-week-old prostates of both control and mutant mice. Arrows indicate proliferating tips. Representative data from the dorsolateral prostate are shown. Insets are high-magnification views from the same sections. (B) PCNA-stained cells in distal tips of 1- or 2-week-old prostates, or whole prostate (4 weeks old), were scored. The ratio of PCNA-positive to total epithelial cells was calculated from five sections per prostate. Mean±s.d. values of data from three prostates are shown. (C) Real-time RT-PCR analyses of RNAs encoding the indicated genes from 1-week-old prostates. Data were normalized to β-actin loading controls and then presented as the relative difference to the control prostates. Mean±s.d. values of data from three prostates are shown. AP, anterior prostate; DLP, dorsolateral prostate; VP, ventral prostate; F/F, homozygous Frs2{alpha}flox mice; CN, Frs2{alpha}cn mice.

 
Ablation of Frs2{alpha} reduces activation of MAP kinase in prostatic epithelial cells
As an adaptor, FRS2{alpha} has multiple binding sites for downstream substrate that are required for FGFR to activate the MAP kinase and AKT pathways (Gotoh et al., 2004Go; Ong et al., 2000Go; Xu and Goldfarb, 2001Go). To determine whether FRS2{alpha}-mediated signals activate the MAP kinase and AKT pathways, prostates were harvested from mice at 1, 2 and 4 weeks of age and subjected to immunoblot analysis of activated FRS2{alpha}, ERK1/2 (also known as MAPK3/1 - Mouse Genome Informatics) and AKT (also known as AKT1 - Mouse Genome Informatics). FRS2{alpha} was strongly expressed and phosphorylated in 1-week-old normal prostates relative to diminished levels in 4-week-old prostates (Fig. 5A). Coincident with changes in FRS2{alpha}, phosphorylation of ERK1/2 in the control prostates was strong in early development and diminished at later stages. By contrast, phosphorylation of AKT was relatively constant. Ablation of Frs2{alpha} reduced the phosphorylation of ERK, but not AKT (Fig. 5A,B). Immunohistochemical analysis of tissues using anti-phosphorylated ERK1/2 (Fig. 5C) and anti-phosphorylated AKT antibodies (Fig. 5D) confirmed the immunoblot results. In contrast to Frs2{alpha} ablation, ablation of Fgfr2 reduced both ERK1/2 and AKT phosphorylation in addition to the expected reduction in phosphorylation of FRS2{alpha} (Fig. 5A,C,D). These results suggest that FGFR2 is a major source of FRS2{alpha} activation in prostate epithelial cells during development, and that activation of the MAP kinase pathway, but not the AKT pathway, by FGFR2 is likely to be mediated by FRS2{alpha} in prostates.


Figure 5
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  		Fig. 5. Activation of the MAP kinase pathway is compromised in Frs2{alpha}cn prostates during branching morphogenesis. (A,B) Western analyses (A) of the indicated proteins from Triton-extracts of mouse prostates at the indicated ages. β-actin was used as a loading control. Note that phosphorylated FRS2{alpha} and ERK1/2 were prominent in 1- and 2-week-old prostates and were diminished in 4-week-old prostates, whereas phosphorylated AKT remained constant in the prostates. The specific bands were quantitated (B) by densitometry. (C,D) Immunostaining of phosphorylated ERK1/2 (C) and phosphorylated AKT (D) in tissue sections of 1-week-old prostates. The specific staining was visualized with confocal microscopy. Note that phosphorylated ERK1/2 was reduced in the epithelial cells, but not in the stromal cells, of Frs2{alpha}cn prostates, and that both phosphorylated ERK1/2 and phosphorylated AKT were reduced in epithelial cells of Fgfr2cn prostates. Arrows indicate epithelial cells. (E) Newborn prostate rudiments cultured with 10 µM ERK1/2 (MEK1/2) or PI3 kinase inhibitor, as indicated, for 3 days. Arrows indicate lumens of the prostatic ductal structures. pERK1/2, phosphorylated ERK1/2; pAKT, phosphorylated AKT; F/F, homozygous Frs2{alpha}flox mice; CN, Frs2{alpha}cn mice.

 
ERK1/2 inhibitors in organ culture of newborn prostates were employed to test whether inhibition of the MAP kinase pathway mimicked the FRS2{alpha} deficiency with respect to prostatic branching morphogenesis. Similar to tissue from Frs2{alpha}cn prostates, normal prostate tissue treated with the inhibitors exhibited decreased ductal complexity indicative of compromised branching morphogenesis of prostatic buds (Fig. 5E). However, inhibition of ERK1/2 did not further decrease the ductal complexity of Frs2{alpha}cn prostates, implying that the FRS2{alpha}-mediated pathway is the major activator of the MAP kinase pathway during prostate branching morphogenesis. PI3 kinase inhibitors also significantly reduced the extent of ductal differentiation, similar to the effect of MAP kinase inhibitors. This suggests that both MAP and PI3 kinase pathways are important for prostate branching morphogenesis, but that FRS2{alpha} is primarily an effector of FGFR2 kinases for activating the MAP kinase pathway without compromise of the PI3 kinase pathway in prostate epithelial cells.

Frs2{alpha} ablation compromises androgen-induced prostatic cell proliferation without effect on androgen-dependent secretory function
Although Frs2{alpha}cn prostates were smaller overall, they exhibited a similar epithelial infolding to normal wild-type prostates (see Fig. S1 in the supplementary material). No significant difference was observed in the display of luminal cell cytokeratin 8 (also known as keratin 8 - Mouse Genome Informatics), or {alpha}-actin in stromal cells, or androgen receptor (AR) in both luminal epithelial and stromal cells, between Frs2{alpha}cn and control prostates (see Fig. S2A in the supplementary material). Frs2{alpha}cn prostates exhibited near-normal levels of secretory proteins, although some reduction in the production of probasin and PSP94 (also known as MSMB - Mouse Genome Informatics) was noted in the VP lobe (see Fig. S2B,C in the supplementary material).

Previously, we reported that ablation of the Fgfr2 alleles interfered with the strict androgen-dependent properties of prostate with respect to tissue homeostasis. To test whether ablation of Frs2{alpha} exhibited a similar effect, 8-week-old mice were deprived androgens by orchiectomy. Similar to the effect on control prostates, the androgen deprivation induced apoptosis in Frs2{alpha}cn prostates within 2 days (data not shown) and the tissues atrophied in 2 weeks (see Fig. S3 in the supplementary material). This is in marked contrast to Fgfr2cn prostates, which remained largely unresponsive to the same androgen deprivation after 2 weeks (Lin et al., 2007aGo). This suggested that resident epithelial cell FGFR2 instructed the prostate to acquire strict dependence on androgen via pathways other than those mediated by FRS2{alpha}.


Figure 6
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  		Fig. 6. Compromised proliferation activities in regenerating Frs2{alpha}cn prostates induced by androgens. (A,B) Expression of Frs2{alpha} in the epithelium of regenerating prostates. Expression of Frs2{alpha} was assessed by in situ hybridization (A) or real-time RT-PCR (B). Data were normalized to the abundance of 18S rRNA and are expressed as mean±s.d. of at least three prostates. The background of Frs2{alpha} expression in the conditional mutants is from the stroma. Red arrows, epithelial cells; black arrows, stromal cells. (C,D) Reduced androgen-induced proliferation in Frs2{alpha}cn prostates. Androgen was restored to mice 14 days after orchiectomy to induce prostate regeneration. Tissues were harvested at the indicated days after administration of androgen. Proliferating cells were revealed by immunostaining with anti-PCNA antibody (C). Red arrows, epithelial cells; black arrows, stromal cells. (D) Representative data from the dorsolateral prostate are shown. PCNA-positive cells were scored and reported as ratios of proliferating cells to total cell numbers. Mean±s.d. values of data from at least three prostates are shown. (E) The average wet tissue weights of normal prostates before castration (Cont), 2 weeks after the operation (Cas), and 14 days after administration of androgen (Reg). Mean±s.d. values of data from three samples are shown. F/F, homozygous Frs2{alpha}flox mice; CN, Frs2{alpha}cn mice.

 
During regeneration induced by restoration of androgen, expression of FRS2{alpha} was apparent in the restored prostate luminal epithelial cells (Fig. 6A). Real-time RT-PCR analyses confirmed that Frs2{alpha} expression was significantly increased in regenerating prostates as compared with the castrated remnants (Fig. 6B). This suggested that FRS2{alpha} plays a role in prostate regeneration. To test this, the rate of epithelial cell proliferation was compared among normal, FRS2{alpha} and FGFR2-deficient regenerating prostates induced by androgen. PCNA immunostaining of proliferating cells revealed that, similar to the effect of Fgfr2 ablation, disruption of Frs2{alpha} significantly reduced proliferation activity in the prostate epithelium (Fig. 6). Taken together, the results indicate that FRS2{alpha} potentially mediates mitogenic signals of FGFR2 during androgen-induced proliferation, but FGFR2 support of androgen-dependent gene expression in the prostate is mediated by FRS2{alpha}-independent pathways.

Ablation of Frs2{alpha} in the prostatic epithelium inhibits prostatic tumorigenesis
Although FRS2{alpha} was not expressed in luminal epithelial cells of mature normal prostates, it appeared prominently in luminal epithelial cells in prostate tumors in the TRAMP mouse (Fig. 7A,B), the tumors of which were induced by oncogenic T antigens targeted to prostate epithelial cells (Kaplan-Lefko et al., 2003Go). This was coincident with the ectopic appearance of FGFR1 in foci of high-grade PIN and overt tumor cells in the TRAMP prostates (Fig. 7A). These results are consistent with the previous finding that the ectopic appearance of normally stromal resident FGFR1 is often associated with prostate tumor progression (Jin et al., 2003aGo; Kwabi-Addo et al., 2001Go). The significantly increased phosphorylation of FRS2{alpha}, ERK1/2 and AKT in TRAMP-C2 epithelial cell lines derived from the TRAMP tumors induced by FGF2, indicated the presence of active ectopic FGFR1 signaling in the tumor cells (Fig. 7C). TRAMP-C2 cells are characterized by expression of ectopic FGFR1 (Foster et al., 1999Go). FGF2 is specifically recognized by ectopic FGFR1, but not by resident FGFR2IIIb, in prostate epithelial cells (McKeehan et al., 1998Go). Thus, it is likely that FRS2{alpha} plays a role in mediation of ectopic FGFR1 in prostate tumors.

To determine whether FRS2{alpha} had any impact on tumorigenesis in the TRAMP mice, the Frs2{alpha}cn and TRAMP mice were crossed. Histological analysis showed that by 10 weeks, the majority of TRAMP mice carrying the floxed Frs2{alpha} alleles developed the expected low-to-high-grade PIN lesions. PIN foci were apparent across the whole prostate with an estimated 80% of the prostate exhibiting various degrees and grades of PIN lesions (Fig. 7D,E). Although the TRAMP transgenic T antigens were highly expressed in the Frs2{alpha}cn prostates, PIN lesions across the Frs2{alpha}cn prostates were fewer in number and the majority of the lesions were lower grade than in control TRAMP mice (Fig. 7D,E). By 24 weeks, most control TRAMP mice developed advanced, poorly differentiated tumors, whereas tumors in the Frs2{alpha}cn/TRAMP hybrids were well-differentiated (Fig. 7D). Application of a Kaplan-Meier survival analysis showed that ablation of Frs2{alpha} alleles significantly increased the life span of TRAMP mice (Fig. 7F). These results strongly suggest that ablation of FRS2{alpha}-mediated signaling pathways in prostate epithelial cells inhibits prostate tumor initiation and progression in the TRAMP mice. and this mostly likely occurs through the adaptor function of FRS2{alpha} with ectopic FGFR1.


   DISCUSSION
TOP
 SUMMARY
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Disruption of Frs2{alpha} in prostate epithelium impairs prostatic morphogenesis
Members of the FGF family play cell-specific roles in prostatic development, function, compartmental homeostasis and tumorigenesis (Lin et al., 2007aGo; McKeehan et al., 1998Go). Here we report that the expression of FRS2{alpha}, a major adaptor protein in the FGF signaling cascade, is spatiotemporally expressed and is closely associated with prostatic cell proliferation. Although diminished in the adult prostatic epithelium, expression of FRS2{alpha} was activated in epithelial cells of regenerating and tumor prostates. Tissue-specific ablation of Frs2{alpha} in prostatic epithelial precursor cells compromised prostatic branching morphogenesis and growth. Furthermore, ablation of Frs2{alpha} alleles retarded prostatic tumor formation and progression in the autochthonous TRAMP mouse model of prostate cancer. The data demonstrate that FRS2{alpha} is important for prostatic development and growth, and imply that aberrant activation of FRS2{alpha}-mediated signaling might contribute to prostatic tumorigenesis.


Figure 7
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  		Fig. 7. Ablation of FRS2{alpha} in the prostatic epithelium inhibits tumorigenesis in the TRAMP prostatic tumor model. (A,B) Expression of Frs2{alpha} and Fgfr1 at the mRNA level was assessed in TRAMP prostates by in situ hybridization (A) and RT-PCR (B). Red arrows, epithelial cells; black arrows, stromal cells. Inset, wild-type control showing no expression of Fgfr1 in the epithelial compartment. T1 and T2, different, individual TRAMP tumors; C2, the C2 cell line derived from TRAMP tumors; PWT, prostates of 6-month-old wild-type mice; PTRAMP, prostates of 3-week-old TRAMP mice in which PIN lesions are minimal. Brain and liver were used as positive controls; β-actin as a loading control. (C) Treatment of TRAMP-C2 cells with FGF2 (10 ng/ml) for 10 minutes after serum starvation for 24 hours. Cells were lysed and the lysates analyzed by western blot with the indicated antibodies. (D) Prostate tissue sections were prepared from TRAMP mice with homozygous Frs2{alpha}-null or Frs2{alpha} floxed alleles at the indicated ages and stained with Hematoxylin and Eosin (HE) or analyzed by immunohistotaining with anti-T-antigen antibody. Blue arrows, focal lesions; green arrows, normal epithelial cells. (E) PIN foci in prostates of 10-week-old TRAMP mice were identified as defined (Park et al., 2002Go), and the percentage area occupied by lesions quantitated. Mean±s.d. values of data from five prostates are shown. (F) Mortality of TRAMP mice with the indicated Frs2{alpha} alleles was determined from daily observation over 250 days. The percentage of mice that survived to the indicated ages is shown. pFRS2{alpha}, phosphorylated FRS2{alpha}; pERK1/2, phosphorylated ERK1/2; pAKT, phosphorylated AKT; F/F, homozygous TRAMP-Frs2{alpha}flox mice; CN, TRAMP-Frs2{alpha}cn mice.

 
FRS2{alpha} in mediation of the FGF10/FGFR2 signals for prostate development and growth
During prostatic organogenesis, FGF7 and FGF10 are expressed in the mesenchyme, and FGFR2IIIb, the cognate receptor for FGF7/10, is expressed in the epithelium of the distal parts of elongating and branching ducts in prostatic rudiments (Huang et al., 2005Go; Thomson and Cunha, 1999Go). Ablation of FGFR2 in prostate epithelial cell precursors inhibits cell proliferation and cellularity throughout prostatic development (Lin et al., 2007aGo). Because of its role as a general adaptor bridging activated FGFR kinases and the MAP kinase, ablation of Frs2{alpha} potentially compromises activity of the FGF7/10-FGFR2IIIB axis. Together with the data showing that ablation of Fgfr2 concurrently reduced activation of FRS2{alpha} and the MAP kinase pathway, the results suggest that FGFR2 signals for activating the MAP kinase pathway and regulating cell proliferation in prostate epithelial cells were mediated by FRS2{alpha}. By contrast, phosphorylated AKT was reduced in Fgfr2cn, but not in Frs2{alpha}cn, prostates (Fig. 5). This indicates that FGFR2 activates the AKT pathway via an FRS2{alpha}-independent mechanism. Unlike Fgfr2 ablation, which abrogates anterior and ventral prostatic bud formation, ablation of FRS2{alpha} did not disrupt development of prostatic buds. It is not uncommon that the prostate has a lobe-specific response to morphoregulatory genes (Economides and Capecchi, 2003Go; Podlasek et al., 1997Go; Pu et al., 2007Go). Because the apparent half-life of FRS2{alpha} is longer than that of FGFR2, our results could not resolve whether FGFR2 signals in prostatic bud formation were mediated by FRS2{alpha}-independent pathways or by the carryover of FRS2{alpha}. Further experiments are needed to clarify this issue.

Several signaling pathways, including those of SHH, Notch, BMP and FGF, have been implicated in prostate development. Ablation of Frs2{alpha} or Fgfr2 alleles reduced expression of HOXd13. In addition, ablation of Frs2{alpha} reduced expression of BMP7 and HOXb13 and increased expression of FGFR4, FGF7, FGF10 and BMP4. Of this set of regulators, ablation of Fgfr2 reduces expression of BMP4 (Lin et al., 2007aGo). This difference suggested that FGFR2 signaling in prostate epithelial cells is at least in part mediated by FRS2{alpha}, but that FGFR2 is not the only upstream activator of FRS2{alpha} in the prostate. Notably, HOXb13 is only expressed in ventral prostates. Concurrent inactivation mutations of both Hoxb13 and Hoxd13 cause severe hypoplasia in ventral prostate morphogenesis (Economides and Capecchi, 2003Go). Our results suggest that both Hoxb13 and Hoxd13 might be downstream targets of FRS2-mediated signaling cascades in the prostate, and that FRS2{alpha}-mediated signals are crucial for ventral epithelial cell differentiation. The expression of PSP94 and probasin, which are secretory proteins characteristic of mature luminal epithelial cells, was consistently and significantly reduced in Frs2{alpha}cn ventral prostate (see Fig. S2B in the supplementary material). These results suggest that ablation of Frs2{alpha} compromises luminal epithelial cell differentiation in ventral prostate. Further experiments will be carried out to test this possibility.

Knock-in of the Cre cDNA to the Nkx3.1 locus disrupts one Nkx3.1 allele. Consistent with an earlier report that verified reduced expression of Nkx3.1 in heterozygous Nkx3.1-null prostates (Bhatia-Gaur et al., 1999Go), our results showed a reduction in expression of Nkx3.1 in Frs2{alpha}cn prostates. By contrast, expression of Nkx3.1 in Fgfr2cn prostates that also carry only one Nkx3.1 allele remained similar to that in controls (Lin et al., 2007aGo). Others reported that Nkx3.1 expression in prostate is regulated both by androgen and FGF signaling pathways (Pu et al., 2007Go). Thus, it is possible that expression of Nkx3.1 is repressed by FGFR2 via FRS2{alpha}-independent pathways and that the repression is alleviated upon ablation of FGFR2 and to a degree sufficient to result in a net expression of Nkx3.1 equal to that of wild-type prostates. It is also possible that loss of FGFR2 activates the non-canonical expression of Nkx3.1.

Ablation of Frs2{alpha} alleles attenuates androgen-induced prostatic regeneration
Ablation of Fgfr2 alleles compromises the acquisition of dependence of the prostate on androgen with respect to tissue homeostasis. By contrast, FRS2{alpha}-deficient prostates remained strictly androgen-dependent. Similar to controls, Frs2{alpha}cn prostates responded to androgen deprivation within 24 hours (see Fig. S3 in the supplementary material and data not shown). This and the fact that FRS2{alpha} was very low or absent in luminal epithelial cells of mature prostates, suggest that FRS2{alpha} is not essential to FGFR2 signaling with regard to its effects on the androgen-dependence of adult prostates.

The prostate is one of the few organs that have regenerative capacity in mice. Deprivation of androgen by orchiectomy and subsequent restoration induces rapid regression and regeneration, respectively. Although expression of FRS2{alpha} was very low or absent in the mature resting prostate epithelium, its expression was significantly increased in the regenerating epithelial cells induced by androgen. Although, overall, the androgen-induced regeneration of the prostate was not blocked by the absence of FRS2{alpha}, the rate and extent of proliferation during the process were compromised (Fig. 6). Thus, the elevated expression of FRS2{alpha} in regenerating epithelial cells contributes to the rate of androgen-induced prostate regeneration, but is not essential for it.

FRS2{alpha}-mediated signals in prostatic tumorigenesis
FRS2{alpha} has four tyrosine phosphorylation sites for GRB2 binding that link the FGFR kinase to the PI3 kinase/AKT pathway, and two for SHP2 (also known as PTPN11 - Mouse Genome Informatics) binding that link it to the MAP kinase pathway and are important for mediating mitogenic signals of FGFR (Gotoh et al., 2004Go; Kouhara et al., 1997Go; Ong et al., 1996Go; Ong et al., 1997Go; Xu and Goldfarb, 2001Go). The four GRB2-binding sites are required for mediating signals to activate the FiRE enhancer element of the mouse syndecan 1 gene (Zhang et al., 2007Go). We previously reported that FRS2{alpha} phosphorylation by FGFR kinases in rat prostate tumor cells is FGFR isotype-specific and associated with the promotion of cell proliferation by the presence of ectopic FGFR1 (Wang et al., 2002Go). Here we showed that from the early branching morphogenesis to mature stages, the expression of FRS2{alpha} in luminal epithelial cells of mouse prostates was proportional to the number of cells actively engaged in proliferation. Expression of FRS2{alpha} was transiently activated in the luminal epithelial cells during regeneration and appeared in PIN lesions and tumors where the cells were also actively proliferating. Our unpublished results using in situ hybridization have confirmed that that Fgfr1 is not expressed in prostate epithelial cells during development, or during androgen-induced regeneration, or in resting mature prostates. Moreover, prostate epithelial cell-specific ablation of Fgfr1 using Nkx3.1Cre induced no detectable abnormalities in prostate development or regeneration of mature prostates. However, FGRF1 was apparently expressed in the epithelial cells of lesion foci in the TRAMP prostate (Fig. 7A). Thus, the coincidence of the ectopic appearance of FGFR1 coupled with the constitutive expression of FRS2{alpha} might support and drive the neoplastic properties of the tumors, particularly with respect to mitogenesis. Prevention of expression of ectopic FGFR1 in tumor epithelial cells by a cross between ablation of the Fgfr1 alleles specifically in prostate epithelial cells and the TRAMP mice should shed light on this subject. Together, the results reveal that the ectopic FGFR1/FRS2{alpha} signaling axis is a feature of the TRAMP tumor epithelial cells. Disruption of the axis by ablation of the Frs2{alpha} alleles is likely to interfere with tumor initiation and progression in the TRAMP model.

In summary, we report that FRS2{alpha} is spatiotemporally expressed in the prostatic epithelium and plays a role in prostate development, androgen-driven regeneration and tumorigenesis. The major role appears to be in mediation of prostatic epithelial cell proliferation in these three processes, rather than a direct effect on androgen responsiveness and differentiated function. The prostate epithelial cell-specific Frs2{alpha}cn mice provide an additional useful model for scrutinizing not only the molecular mechanisms underlying prostate growth and development, but also how prostate cancer cells escape from strict controls on tissue homeostasis to propagate autonomously.

Supplementary material
Supplementary material for this article is available at http://dev.biologists.org/cgi/content/full/135/4/775/DC1


   ACKNOWLEDGMENTS
 
We thank Mary Cole for critical reading of the manuscript. The work was supported by AHA0655077Y from the America Heart Association, DAMD17-03-0014 (F.W.) from the US Department of Defense, and CA96824 (F.W.), CA115985 (M.M.S.), DK076602 (M.M.S.), CA59971 (W.L.M.) and CA84296 (N.M.G.) from the NIH.


   Footnotes
 
* These authors contributed equally to this work Back


   REFERENCES
TOP
 SUMMARY
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 


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