Oncogene-induced senescence does not require the p16 INK4a or p14ARF melanoma tumor suppressors

Oncogene-induced senescence is considered to act as a potent barrier to cell transformation, and has been seen in vivo during the early stages of tumor development. Human nevus cells frequently express oncogenic N-RAS or B-RAF, and are thought to be permanently growth arrested. Many studies have suggested that the p16 INK4a and, to a lesser extent, the p14ARF tumor suppressor proteins act as critical triggers of oncogene-induced senescence in nevi, and thus these proteins represent major inhibitors of progression to melanoma. There have also been reports, however, showing that p16 INK4a and/or p14ARF is not sufficient to execute the oncogene-induced senescence program. In this study, we examined the impact of melanoma-associated N-RAS Q61K on melanocyte senescence and utilized RNA-interference vectors to directly assess the individual contribution of human p14ARF and p16 INK4a genes to the N-RAS-induced senescence program. We formally show that cultured human melanocytes can initiate an effective oncogene-mediated senescence program in the absence of INK4a/ARF-encoded proteins. Our data are consistent with observations showing that senescent nevus cells do not always express p16 INK4a, and highlight the need to thoroughly explore INK4a/ARF-independent molecular pathways of senescence in human melanocytes.