Cyclin D- and E-dependent kinases and the p57(KIP2) inhibitor: Cooperativeinteractions in vivo

This study examines in vivo the role and functional interrelationships of c omponents regulating exit from the G(1) resting phase into the DNA syntheti c (S) phase of the cell cycle. Our approach made use of several key experim ental attributes of the developing mouse lens, namely its strong dependence on pRb in maintenance of the postmitotic state, the down-regulation of cyc lins D and E and up-regulation of the p57(KIP2) inhibitor in the postmitoti c lens fiber cell compartment, and the ability to target transgene expressi on to this compartment. These attributes provide an ideal in vivo context i n which to examine the consequences of forced cyclin expression and/or of l oss of p57(KIP2) inhibitor function in a cellular compartment that permits an accurate quantitation of cellular proliferation and apoptosis rates in s itu. Here, we demonstrate that, despite substantial overlap in cyclin trans gene expression levels, D-type and E cyclins exhibited clear functional dif ferences in promoting entry into S phase. In general, forced expression of the D-type cyclins was more efficient than cyclin E in driving lens fiber c ells into S phase. In the ease of cyclins D1 and D2, ectopic proliferation required their enhanced nuclear localization through CDK4 coexpression. Hig h nuclear levels of cyclin E and CDK2, while not sufficient to promote effi cient exit from G(1), did act synergistically with ectopic cyclin D/CDK4. T he functional differences between D-type and E cyclins was most evident in the p57(KIP2)-deficient lens wherein cyclin D overexpression induced a rate of proliferation equivalent to that of the pRb null lens, while overexpres sion of cyclin E did not increase the rate of proliferation over that induc ed by the loss of p57(KIP2) function. These in vivo analyses provide strong biological support for the prevailing view that the antecedent actions of cyclin D/CDK4 act cooperatively with cyclin E/CDK2 and antagonistically wit h p57(KIP2) to regulate the G(1)/S transition in a cell type highly depende nt upon pRb.