Fig. 8. MYC expression promotes activation of the Stat3 and Tgfb3 involution pathways. (A) The Y705-phosphorylation status of Stat3 was determined by western blot analysis. Mammary protein lysates from MTB and MTB/TOM mice induced with doxycycline from D12.5-D15.5 of pregnancy and sacrificed at 24-hour intervals from D12.5 of pregnancy through D1PP were separated by SDS-PAGE and transferred to nitrocellulose membranes. Lysates from FVB wild-type mice harvested during virgin development, D18.5 of pregnancy and D2 of regression were included as controls. Membranes were probed with antibodies specific to Y705-phosphorylated Stat3 and total Stat3. Results are representative of experiments performed on two separate sets of animals. (B) Total RNA was extracted from mammary glands of MTB and MTB/TOM animals induced from D12.5-D15.5 of pregnancy and sacrificed at D17.5-D1PP. Expression of Tgfb3 mRNA and a direct Tgfb3 target, Serpine1, were assessed by northern hybridization analysis with radiolabeled probes.