An Rb family-independent E2F3 transcription factor variant impairs STAT5 signaling and mammary gland remodeling during pregnancy in mice.

E2F transcription factors are regulated by binding to the retinoblastoma (Rb) tumor suppressor family of proteins. Previously, we reported an mutation that disrupts the binding with Rb proteins without affecting the transcriptional activity of E2F. We also showed that mouse embryonic fibroblasts with an mutation exhibit increased E2F activity and more rapid cell proliferation. In this report, we analyzed mice to further characterize the consequences of Rb family-independent E2F3 activity. We found that homozygous mice were viable and had no obvious developmental defects or tumor growth. Our results also indicated that cells largely retain normal control of cell proliferation However, female mice had partial nursing defects. Examination of the mammary glands revealed increased caveolin-1 (CAV1) expression, reduced prolactin receptor/Stat5 signaling, and impaired pregnancy-induced cell proliferation and differentiation. Of note, ChIP experiments disclosed that E2F3 binds the CAV1 promoter. Furthermore, E2F3 overexpression induced CAV1 expression, and CRISPR/CAS9-mediated E2F3 knockout reduced CAV1 levels and also increased prolactin receptor-induced Stat5 signaling in mammary epithelial cells. Our results suggest that the Rb family-independent E2F3 variant inhibits pregnancy-induced mammary gland cell proliferation and differentiation by up-regulating CAV1 expression and inhibiting Stat5 signaling.