Opposite functions for E2F1 and E2F4 in human epidermal keratinocyte differentiation

Proteins of the retinoblastoma family (pRb, p107, and p130) modulate cell p roliferation, a function related to their capacity to control the activity of the E2F transcription factor family. The Rb proteins also control cell d ifferentiation in different tissues. We have recently described their invol vement in human epidermal keratinocyte differentiation (Paramio, J. M., Lai n, S., Segrelles, C., Lane, E. B., and Jorcano, J. L. (1998) Oncogene 17, 9 49-957), Here we show that E2F proteins are also involved in this process. We found that E2F1 and E2F4 are expressed differentially during the in vitr o differentiation of human epidermal keratinocytes, with the former uniform ly present throughout the process, whereas the second is predominantly expr essed at the onset of differentiation. This pattern is also observed in hum an skin by confocal microscopy. Electrophoretic mobility shift assays and i mmunoprecipitation experiments demonstrated that the complexes formed by E2 F1 and E2F4 and Rb family proteins vary throughout in vitro keratinocyte di fferentiation. In agreement with this observation, several E2F-responsive g enes are differentially regulated during this process. To test the function al implications of these observations, we transfected Ha-CaT keratinocytes with plasmids coding for E2F1 and E2F4. Transfected cells display opposite in vitro differentiation properties. Although E2F1-transfected cells are un able to differentiate, E2F4-transfected cells show an increased differentia tion rate compared with Neo-transfected control cells. Our data demonstrate that the differential and coordinated expression and interaction of E2F an d Rb proteins modulate the process of epidermal differentiation and provide clear evidence that members of the E2F family of transcription factors pla y specific and opposite roles during cell differentiation.