p19ARF-independent induction of p53 and cell cycle arrest by Raf in murine keratinocytes

In tumorigenesis of the skin, activated Ras co-operates with mutations that inactivate the tumour suppressor p53, but the molecular basis for this co-operation remains unresolved. Here we show that activation of the Raf/MAP kinase pathway in primary mouse keratinocytes leads to a p53 and p21^Cip1-dependent cycle arrest and to terminal differentiation. Raf activation in keratinocytes lacking p53 or p21^Cip1 genes leads to expression of differentiation markers, but the cells do not cease to proliferate. Thus, loss of p53 or p21^Cip1 function is necessary to disable growth-inhibitory Raf/MAP kinase signalling. Activation of oncogenes, including Ras, has been reported to stabilize and activate p53 via induction of the tumour suppressor p19^ARF. However, the response to Raf in p19^ARF–/– keratinocytes was indistinguishable from wild-type controls. Thus, p19^ARF is not essential for Raf-induced p53 induction and cell cycle arrest in keratinocytes, indicating that oncogenes engage p53 activity via multiple mechanisms.