The role of oncogenic Ras in human skin tumorigenesis depends on the clonogenic potential of the founding keratinocytes

The role of Ras in human skin tumorigenesis induction is still ambiguous. Overexpression of oncogenic Ras causes premature senescence in cultured human cells and hyperplasia in transgenic mice. We investigate whether the oncogenic insult outcome may depend on the nature of the founding keratinocyte. We demonstrate that Ras-V12-overexpression induces senescence in primary human keratinocyte cultures, but some cells escape senescence and proliferate indefinitely. Ras-overexpression in transient-amplifying (TA)- or stem cell (SC)- enriched cultures shows that p16 levels are critical for the final result. Indeed, TA-keratinocytes expressing high levels of p16 are sensitive to Ras-V12-induced senescence whereas cells with high proliferative potential, which do not display p16, are resistant. The subpopulation, which sustains the indefinite culture growth, exhibits SC features. Bypass of senescence correlates with pRb pathway inhibition and TERT resumption. Immortalization is also sustained by activation of ERK1/2 and Akt pathways. Moreover, only transduced cultures, originating from cultures bearing SCs, induce tumors in nude mice. Our findings demonstrate that Ras-overexpression outcome depends on the clonogenic potential of the recipient keratinocyte and only the SC compartment is competent to initiate tumorigenesis.