Ca2+ and BMP-6 signaling regulate E2F during epidermal keratinocyte differentiation

The epidermis consists of a squamous epithelium continuously replenished by committed stem cells, which can either self-renew or differentiate. We dem onstrated previously that E2F genes are differentially expressed in develop ing epidermis (Dagnino, L., Fry, C. J., Bartley, S. M., Farnham, P., Gallie , B. L., and Phillips, R. A. (1997) Cell Growth Differ. 8, 553-563). Thus, we hypothesized that various E2F proteins likely play distinct growth regul atory roles in the undifferentiated stem cells and in terminally differenti ated keratinocytes. To further understand the function of E2F genes in epid ermal morphogenesis, we have examined the expression, regulation, and prote in-protein interactions of E2F factors in undifferentiated cultured murine primary keratinocytes or in cells induced to differentiate with Ca2+ or BMP -6 ((b) under bar one (m) under bar orphogenetic (p) under bar (p) under ba r rotein (6) under bar). We find similar patterns of E2F regulation with bo th differentiating agents and demonstrate a switch in expression from E2F-1 , -2, and -3 in undifferentiated, proliferating cells to E2F-5 in terminall y differentiated keratinocytes. Inhibition of keratinocyte proliferation by transforming growth factor-beta did not enhance E2F-5 protein levels, sugg esting that this response is specific to differentiation rather than revers ible cell cycle withdrawal. E2F-5 up-regulation is also accompanied by form ation of heteromeric nuclear complexes containing E2F5, p130, and histone d eacetylase (HDAC) 1. Overexpression of E2F5 specifically inhibited DNA synt hesis in undifferentiated keratinocytes in an HDAC-dependent manner, sugges ting that E2F-5(.)p130(.)HDAC1 complexes are likely involved in the permane nt withdrawal from the cell cycle of keratinocytes responding to differenti ation stimuli.