Biologic mechanisms for the regulation of normal human keratinocyte proliferation and differentiation

Normal human keratinocytes can be grown in serum-free medium, and the integrated control of their proliferation and differentiation can be modulated experimentally. The growth of cultured human keratinocytes can also be specifically arrested at either reversible or irreversible growth arrest states. Reversible growth arrest is induced by culture in medium containing TGF-beta or ethionine or in medium deficient of isoleucine. Irreversible growth arrest is induced by culture in razoxane-containing medium or by routine passage of keratinocytes until senescence results. The current studies were performed to determine from which growth arrest states keratinocyte differentiation occurs. Cells were therefore growth-arrested at each state, and they were then incubated in several different differentiation-promoting culture conditions. The results show that differentiation, as determined by morphologic, cytochemical, and immunofluorescent assays, can be induced from multiple reversible and irreversible growth arrest states by a series of complex biologic mechanisms. More specifically, at least three distinct stages appear to be involved in the process of keratinocyte differentiation. First, cells arrest their growth at a reversible predifferentiation state. Second, cells irreversibly lose their proliferative potential. Finally, cells express the terminally differentiated keratinocyte phenotype.