Expression of Bcl-2 and Bax in cultured normal human keratinocytes and melanocytes: relationship to differentiation and melanogenesis.

Keratinocyte differentiation and melanogenesis are two major cellular processes by which the epidermal compartment of the skin acquires its protective properties. Bcl-2; an oncoprotein involved in the regulation of apoptosis, has been shown to be expressed by keratinocytes and melanocytes. To determine whether Bcl-2 and Bax, a protein which heterodimerizes with Bcl-2, may control these epidermal functions, we investigated the expression of these two oncogenes in cultivated human keratinocytes and melanocytes from the same donors, respectively induced to differentiate and to produce melanin. As determined by cytometry, we observed that these two cell types constitutively express the two proto-oncogenes. Quantification of Bcl-2 antigen sites per cell showed that Bcl-2 expression is higher in keratinocytes than in melanocytes. An increase in transglutaminase activity, a marker of keratinocyte terminal differentiation initiating cornified envelope formation, was accompanied by a decrease in Bcl-2 levels without significant modification of Bax expression. In melanocyte cultures, stimulation of the dopa-oxidase pool, a key enzyme in melanin synthesis, paralleled Bcl-2 down-regulation and Bax up-regulation. This led us to conclude that the expression of these two oncogenes and their cellular ratio are closely involved in keratinocyte differentiation and melanogenesis.