Alterations in cyclin-dependent kinase 2 function during differentiation of primary human keratinocytes.

Terminal differentiation of epithelial cells is intimately linked to cell-cycle withdrawal. The tight coupling of these two processes is critical to maintenance of epidermal tissue homeostasis and is frequently disrupted in squamous cell carcinoma. To identify possible molecular targets of epithelial carcinogenesis, we investigated the regulatory pathways that couple cellular differentiation and proliferation in primary cultures of human keratinocytes and found that the cyclin-dependent kinase inhibitors (CKIs) p21cip1/waf1 and p27kip1 were induced early during differentiation of human keratinocytes, whereas p15ink4B was induced later in differentiation. The induction of p21c1/waf1 was mediated by both transcriptional and non-transcriptional mechanisms, and the activities of cyclin A/cyclin-dependent kinase (cdk) 2 and cyclin E/cdk2 complexes were specifically inhibited during keratinocyte differentiation. In contrast, p21cip1/wafl did not associate with cdk4, and the activities of cdk4 complexes remained unchanged. Hence, our results support the model that multiple CKIs participate in linking cellular proliferation and differentiation in human keratinocytes by specific modulation of cdk2 activity.