Genetic Evidence for the Interactions of Cyclin D1 and p27^Kip1 in Mice

Abstract

The cell cycle of cultured cells appears to be regulated by opposing actions of the cyclins together with their partners, the cyclin-dependent kinases (Cdk), and their inhibitors (Cki). Consistent with this situation null mutations in the genes for cyclin D1 and Cki p27^Kip1 in mice give opposite phenotypes of dwarfism and gigantism. To test their genetic interactions, we generated mice nullizygous for both genes. Correction of cyclin D1 or p27 null to wild-type phenotypes was observed for many but not all traits. These included, for cyclin D1^−/− mice, body weight, early lethality, retinal hypoplasia, and male aggressiveness and, for p27^−/− mice, body weight, retinal hyperplasia, and embryo implantation. p27^−/− traits that were not corrected were the aberrant estrus cycles, luteal cell proliferation, and susceptibility to pituitary tumors. This mutual correction of these phenotypes is the first genetic demonstration of the interaction of these inhibitory and stimulatory cell cycle-regulatory molecules in vivo. The molecular basis for the correction was analyzed in the neonatal retina. Retinal cellularity was rescued in the cyclin D1 null mouse by loss of p27 with only a partial restoration of phosphorylation of retinoblastoma protein (Rb) and Cdk4 activity but with a dramatic elevation of Cdk2 activity. Our data provide in vivo genetic validation of cell culture experiments that indicated that p27 acts as a negative regulator of cyclin E-Cdk2 activity and that it can be titrated away by cyclin D-Cdk4 complexes. It also supports the suggestion that the cyclin E/Cdk2 pathway can largely bypass Rb in regulating the cell cycle in vivo.

ACKNOWLEDGMENTS

We thank Andy Koff and Martine Roussel for their critical comments on the manuscript and Liang Zhu for helpful discussions. We thank A. Koff and P. Sicinski for generously providing the mice for breeding, Liyin Zhu and Bo Chen for the histological preparations of the mouse pituitaries, and James Lee for genotyping the mice.

J.W.P. is a Betty and Sheldon Feinberg senior scholar in cancer research. This work was supported in part by an NCI grant to the Albert Einstein Cancer Center, P30-13330.