Cables 1 Mediates Progesterone-Induced Inhibition of Endometrial Epithelial Cell Proliferation.

Cables 1 is a cell cycle regulatory protein that has been implicated as a tumor suppressor. It is postulated that Cables 1 facilitates cell differentiation, which is supported by the finding that Cables 1 null mice exposed to chronic estrogen have evidence of endometrial hyperplasia and carcinoma in situ. Based on our previous findings, we hypothesized that loss of Cables 1 would result in delayed progesterone (P4)-induced endometrial differentiation resulting in unchecked endometrial epithelial cell proliferation. A simulated early pregnancy model was used to test this hypothesis in which Cables 1 null mice and their WT sisters (n=9 each) were ovariectomized. Two weeks post surgery, the mice were injected SC with estradiol (100 ng/day) for 2 days followed by 2 days of no treatment and then treated with either progesterone (1 mg/day) or vehicle for 3 or 4 days (n=3 each). Following treatment, the uteri from WT and mutant mice were collected, paraformaldehyde fixed, paraffin embedded and sectioned. Uterine sections were stained by immunohistochemistry for the mitotic marker phosphohistone-H3 (PH3), and were then evaluated for positive PH3 cells in the glandular and luminal epithelium. In the absence of P4, PH3 positive glandular cells demonstrated up to a three-fold increase (p<0.01) by day 4 after estradiol therapy in both the WT mice and Cables 1 null mice. Under the influence of progesterone however, differential effects were observed between WT and Cables 1 null mice in both the glandular and luminal epithelia. While both WT and mutant mice experienced a marked reduction in mitotic epithelial cells, 100 fold more luminal (p<0.001) and 10 fold more glandular (p< 0.001) PH3 positive cells were detected in the mutant female mice compared with WT females after 4 days of progesterone exposure. A similar response effect was observed in the mice exposed to 3 days of progesterone. In this model, the absence of Cables 1 blunts the inhibitory effects of P4 suggesting Cables 1 is a mediator of P4-induced differentiation and maturation of endometrial epithelial cells. In conclusion, our results indicate that Cables 1 coordinates an as yet unidentified aspect of P4-induced epithelial cell differentiation. This research was funded in part by NIH RO1 098333 (BRR) and the AMRF (BRR).