DIFFERENTIAL EXPRESSION AND FUNCTIONALLY COOPERATIVE ROLES FOR THE RETINOBLASTOMA FAMILY OF PROTEINS IN EPIDERMAL DIFFERENTIATION

Terminal differentiation requires cell cycle withdrawal, suggesting th e involvement of negative cell cycle controllers in the process, We ha ve analysed the involvement of the retinoblastoma family of proteins ( pRb, p107 and p130) in epidermal proliferation and differentiation. Th ese proteins play key roles as inhibitors of cell cycle progression an d are involved in muscle and neuron differentiation, We found that dur ing in vitro differentiation of human HaCaT keratinocytes, pRb, p107 a nd p130 are sequentially expressed, in contrast to the co-expression o bserved during cell cycle progression in the same cells. Immunofluores cence studies on skin sections revealed the presence of pRb and p107 i n basal and suprabasal cell layers, whilst p130 is restricted to cells already committed to differentiation in the suprabasal compartments, To explore the functional significance of the differential expression of these proteins, transfection experiments mere performed in HaCaT ke ratinocytes. We observed that the forced over-expression of pRb, p107 or p130 individually did not induce differentiation of the transfected cells, However, the co-transfection of pRb and p107 induced the expre ssion of early differentiation markers (keratin k10) and triple transf ectants pRb + p107 + p130 expressed markers representative of later st ages of epidermal differentiation (involucrin). Finally, we observed t hat these three proteins repress keratinocyte proliferation, although to a different extent (p107 > pRb greater than or equal to p130). Thes e results indicate that the members of the pRb family play specific, y et coordinated roles during epidermal differentiation, and that the or dered progression along the different stages of this process results f rom the effects of different combinations of these proteins.