Levels and interactions of p27, cyclin D3, and CDK4 during the formation and maintenance of the corpus luteum in mice

The cyclin-dependent kinase (CDK) inhibitor p27, the regulator of the cell cycle, is required for proper functioning of luteinizing/luteinized cells i n vivo. Since different members of the CDK family may be targeted by p27 du ring luteinization-associated cell cycle exit, this in vivo study further a nalyzed the organization of the network of cell cycle regulators that may u nderlie both the establishment and maintenance of the luteal phenotype. Mos t importantly, it shows that the luteinization process is associated with d own-regulation of CDK2 and cyclin D1, and up-regulation of p27 and cyclin D 3. Both p27 and cyclin D3 proteins not only accumulated during initial phas es of luteinization, but they remained elevated until termination of the lu teal function. Along with its accumulation, p27 lost physical contact with CDK2 and instead became associated with CDK4. In fully luteinized cells, al l cyclin D3 was incorporated into complexes with p27, some complexes being p27/cyclin D3/CDK4 trimers. Despite the significant amounts of CDK4 and CDK 6, only nonphosphorylated forms of retinoblastoma protein were detectable i n fully luteinized cells. Together, our data indicate that while inhibition of proliferation is underlaid by the progressive loss of positive regulato rs of the cell cycle, including cyclins and CDK2, maintenance of the luteal phenotype is driven by up-regulated levels of p27 and cyclin D3, at least partially owing to formation of p27/cyclin D3/CDK4 trimers.