Cooperation of p27(Kip1) and p18(INK4c) in progestin-mediated cell cycle arrest in T-47D breast cancer cells

The steroid hormone progesterone regulates proliferation and differentiatio n in the mammary gland and uterus by cell cycle phase-specific actions. The long-term effect of progestins on T-47D breast cancer cells is inhibition of cellular proliferation. This is accompanied by decreased G(1) cyclin-dep endent kinase (CDK) activities, redistribution of the CDK inhibitor p27(Kip 1) among these CDK complexes, and alterations in the elation profile of cyc lin E-Cdk2 upon gel filtration chromatography, such that high-molecular-wei ght complexes predominate, This study aimed to determine the relative contr ibution of CDK inhibitors to these events. Following progestin treatment, t he majority of cyclin E- and D-CDK complexes were bound to p27(Kip1) and fe w were bound to p21(Cip1). In vitro, recombinant His(6)-p27 could quantitat ively reproduce the effects on cyclin E-Cdk2 kinase activity and the shift in molecular weight observed following progestin treatment. In contrast, cy clin DCdk4 was not inhibited by His(6)-p27 in vitro or p27(Kip1) in vivo. H owever, an increase in the expression of the Cdk4/6 inhibitor p18(INK4c) it s extensive association with Cdk4 and Cdk6 were apparent following progesti n treatment, Recombinant p18(INK4e) led to the reassortment of cyclin-CDK-C DK inhibitor complexes in vitro, with consequent decrease in cyclin E-Cdk2 activity, These results suggest a concerted model of progestin action where by p27(Kip1) and p18(INK4c) cooperate to inhibit cyclin E-Cdk2 and Cdk4. Si nce similar models have been developed for growth inhibition by transformin g growth factor beta and during adipogenesis, interaction between the Cip/K ip and INK4 families of inhibitors may be a common theme in physiological g rowth arrest and differentiation.