Downregulation of 14-3-3 sigma prevents clonal evolution and leads to immortalization of primary human keratinocytes

In human epidermal keratinocytes, replicative senescence, is determined by a progressive decline of clonogenic and dividing cells, Its timing is contr olled by clonal evolution, that is, by the continuous transition from stem cells to transient amplifying cells. We now report that downregulation of 1 4-3-3 sigma, which is specifically expressed in human stratified epithelia, prevents keratinocyte clonal evolution, thereby forcing keratinocytes into the stem cell compartment. This allows primary human keratinocytes to read ily escape replicative senescence. 14-3-3 sigma-dependent bypass of senesce nce is accompanied by maintenance of telomerase activity and by downregulat ion of the p16(INK4a) tumor suppressor gene, hallmarks of keratinocyte immo rtalization. Taken together, these data therefore suggest that inhibition o f a single endogenous gene product fosters immortalization of primary human epithelial cells without the need of exogenous oncogenes and/or oncoviruse s.