The Role of CDK6 and Cyclin D3 in Keratinocyte Proliferation and Mouse Skin Carcinogenesis

CDKs (cyclin-dependent kinases) are a family of serine/threonine protein kinases, which play a critical role in regulating cell cycle. After binding and being activated by their regulatory subunit protein, D-type cyclins, CDK4/6 phosphorylate and inactivate their substrate, pRb, to permit the transcription of genes needed for cell cycle transit from G1 phase to S phase. Biochemical analysis of several human and experimental tumors shows deregulated expression and activating mutations in CDK4 and CDK6 activity. It has been widely assumed that CDK6 and CDK4 plays redundant roles since they share 71% amino acid identity, and they are regulated by the same regulatory subunit (D-type cyclins) during the G1 phase of the cell cycle. Although, recently data support the hypothesis that CDK4 and CDK6 also play non-redundant roles. Therefore, the general purpose of our studies has been to determine the role of CDK6, as well as its main regulatory partner, cyclin D3, in normal and neoplastic proliferation.;In order to investigate the role of CDK6 in tumorigenesis, we generated a transgenic mouse model of elevated CDK6 kinase activity (K5CDK6 transgenic mice). Similar to K5CDK4, overexpressed CDK6 triggered epidermal proliferation. Unexpectedly, CDK6 overexpression results in decreased skin tumor development compared with wild-type siblings. The inhibition in skin tumorigenesis was comparable to that previously reported in K5-cyclin D3 mice. These studies provide in vivo evidence that CDK4 and CDK6 play a related role as a mediator of keratinocyte proliferation but differ in apoptosis activation and skin tumor development.;We also provided insights into the compensatory mechanism between D-type cyclins. Our data show that simultaneous ablation of cyclin D1 and down-regulation of cyclin D2 resulted in robust reduction of ras-mediated skin tumorigenesis.;Interestingly, increased formation of CDK6/cyclin D3 complexes were noticed in transgenic mice overexpressing CDK6 or cyclin D3, associated with reduced tumor development. Using a K5CDK6/cyclin D3-/- compound mouse model with elevated CDK6 protein level in absence of cyclin D3, we found that the tumor inhibition effect of CDK6 is independent of cyclin D3 expression. On the other hand, cyclin D3 is necessary for CDK6 mediated tumor growth.