Endocrine dysfunction in p27Kip1 deficient mice and susceptibility to Wnt-1 driven breast cancer.

The cyclin-dependent kinase (Cdk) inhibitor p27(Kip1) (p27) is a marker of prognosis in many cancers, including breast cancer. Low p27 expression correlates with poor prognosis, especially in hormone receptor positive breast tumors. This association suggests a role for p27 in hormone-dependent cancer. We used the Wnt-1 transgenic mouse model to further explore the role of p27 in hormone-driven breast cancer. We found that p27 deficiency did not alter breast cancer rate in either male or female Wnt-1 mice. However, we did find p27-/- females had reduced levels of serum progesterone (P) and increased variability in estradiol (E), which could have affected their cancer susceptibility. To equalize hormone levels, an additional cohort of Wnt-1 female mice was ovariectomized and implanted with slow release pellets of E and P. Although this treatment did not alter the breast cancer rate, it did accelerate the development of pituitary and gastric tumors in p27-/- mice. This study shows that while not a significant inhibitor of Wnt-1-driven breast cancer, p27 inhibits gastric tumors, whose latency is modulated by sex steroids.