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p19ARF-independent induction of p53 and cell cycle arrest by Raf in murine keratinocytes
E Roper1, W Weinberg, F M Watt
1Imperial Cancer Research Fund, London, UK.
Abstract:
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 p21Cip1-dependent cycle arrest and to terminal differentiation. Raf activation in keratinocytes lacking p53 or p21Cip1 genes leads to expression of differentiation markers, but the cells do not cease to proliferate. Thus, loss of p53 or p21Cip1 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 p19ARF. However, the response to Raf in p19ARFI-/- keratinocytes was indistinguishable from wild-type controls. Thus, p19ARF is not essential for Raf-induced p53 induction and cell cycle arrest in keratinocytes, indicating that oncogenes engage p53 activity via multiple mechanisms.
Insights
Activated Ras in skin cancer co-operates with p53 inactivation. Raf pathway activation causes cell cycle arrest dependent on p53 and p21Cip1, but not p19ARF, revealing multiple oncogene mechanisms.
Area of Science:
- Oncology
- Molecular Biology
- Cell Biology
Background:
- Activated Ras collaborates with p53 mutations in skin tumorigenesis.
- The molecular mechanisms underlying this cooperation are not fully understood.
Purpose of the Study:
- To investigate the role of the Raf/MAP kinase pathway in skin cell proliferation and differentiation.
- To elucidate the involvement of p53, p21Cip1, and p19ARF in Raf-mediated cellular responses.
Main Methods:
- Primary mouse keratinocyte culture.
- Analysis of cell cycle arrest and differentiation markers.
- Gene knockout studies (p53, p21Cip1, p19ARF).
Main Results:
- Raf activation in keratinocytes induced p53 and p21Cip1-dependent cell cycle arrest and differentiation.
- Loss of p53 or p21Cip1 function abrogated Raf-induced growth inhibition but not differentiation.
- p19ARF was not essential for Raf-induced p53 activation and cell cycle arrest.
Conclusions:
- p53 and p21Cip1 are crucial for inhibiting proliferation in response to Raf pathway activation.
- Loss of p53 or p21Cip1 function is necessary to bypass Raf-induced growth arrest.
- Oncogenes utilize multiple pathways to activate p53, independent of p19ARF in keratinocytes.