APOPTOSIS IN KERATINOCYTES IS NOT DEPENDENT ON INDUCTION OF DIFFERENTIATION

During embryological development and throughout life, regulation of th e thickness of skin is likely to involve modulation of keratinocyte pr oliferation, differentiation, and cell death. One major mechanism of c ell death is apoptosis; but the precise relationship between apoptosis and differentiation has not been well-defined. In this report, we dem onstrate that when cultured undifferentiated keratinocytes have their adhesive interactions interrupted by either enzymatic treatment (ie, t rypsin) and suspension in a semi-solid methyl cellulose medium, or exp osure to antibodies against beta 1 integrins and E-cadherin, induction of differentiation occurs (expression of involucrin), as well as apop tosis (positive terminal deoxynucleotidyl transferase (Tdt)-mediated d UTP-biotin nick end labeling (TUNEL) assay and DNA fragmentation). How ever, these events are not directly interdependent processes, as deter mined by two-color immunofluorescence staining. Thus, apoptosis can oc cur without evidence of differentiation and vice versa. The process of apoptosis in keratinocytes was dissected at the molecular level and f ound to be correlated with increased expression of Bar and decreased l evels of Bcl-x(L), with no role for either Bcl-2 or Bcl-x(S). We concl ude that keratinocytes do not need to undergo differentiation before u ndergoing apoptosis.