Role of the Alternative INK4A Proteins in Human Keratinocyte Senescence: Evidence for the Specific Inactivation of p16 upon Immortalization

The INK4A locus on human chromosome 9p21 encodes two genes that have been implicated in replicative senescence and tumor suppression, p16 and p14. In contrast to p16, which is up-regulated to high levels, we were unable to detect p14 protein in senescent human keratinocytes. Also, p53, an established target of p14, did not increase, suggesting that p14 is not instrumental in human keratinocyte senescence. In neoplastic keratinocyte cultures, p16 inactivation was invariably associated with the immortal phenotype, and there was evidence for the inactivation of p16, independent of p14, in 6 of 10 lines that lacked large homozygous deletions. In contrast, we failed to detect exon 1b mutations or p16-independent deletions. These results emphasize the previously proposed role for p16 in human keratinocyte senescence but do not rule out a supporting role for p14 inactivation.