The human melanocyte: a model system to study the complexity of cellular aging and transformation in non-fibroblastic cells

The melanocyte is a neural crest-derived cell that localizes in humans to s everal organs including the epidermis, eye, inner ear and leptomeninges. In the skin, melanocytes. synthesize and transfer melanin pigments to surroun ding keratinocytes, leading to skin pigmentation and protection against sol ar exposure. We have investigated the process of replicative senescence and accompanying irreversible cell cycle arrest, in melanocytes in culture. As was found in other cell types, progressive telomere shortening appears to trigger replicative senescence in normal melanocytes. In addition, senescen ce is associated with increased binding of the cyclin-dependent kinase inhi bitor (CDK-1) p16(lNK4a) to CDK4, down-regulation of cyclin E protein level s (and consequent loss of cyclin E/ CDK2 activity), underphosphorylation of the retinoblastoma protein RB and subsequent increased levels of E2F4-RB r epressive complexes. In contrast to fibroblasts, however, the CDK-1s p21(Wa f-1) and P27(Kip-1) are also down-regulated. These changes appear to be imp ortant for replicative senescence because they do not occur in melanocytes that overexpress the catalytic subunit of the enzyme telomerase (hTERT), or in melanomas, which are tumors that originate from melanocytes or melanobl asts. In contrast to unmodified melanocytes, hTERT overexpressing (telomeri zed) melanocytes displayed telomerase activity, stable telomere lengths and an extended replicative life span. However, telomerized melanocytes show c hanges in cell cycle regulatory proteins, including increased levels of cyc lin E, p21(Waf-1) and p27(Kip-1). Cyclin E, p21(Waf-1) and p27(Kip-1) are a lso elevated in many primary melanomas, whereas p16(INK4a) is mutated or de leted in many invasive and metastatic melanomas. Thus, the molecular mechan isms leading to melanocyte senescence and transformation differ significant ly from fibroblasts. This suggests that different cell types may use differ ent strategies to halt the cell cycle in response to telomere attrition and thus prevent replicative immortality. (C) 2001 Elsevier Science Inc. All r ights reserved.