Corticotropin-releasing hormone (CRH) is proposed to be involved in the regulation of the proliferative capacity of keratinocytes, based on its significant actions in the skin. These are mediated by CRH-R1alpha and represented by adenylate cyclase activation, Ca2+ influx, inhibition of cell proliferation and modifications in intracellular signal transduction by NF-kappaB.To define CRH action in the cell cycle we investigated its effects on the differentiation programme using the HaCaT keratinocytes model.HaCaT keratinocytes were incubated with CRH in Dulbecco's modified Eagles's medium (containing 1.8 mmol L(-1) calcium) or EpiLife (containing 0.06 mmol L(-1) calcium) medium. Cell proliferation was assessed with the MTT assay. Flow cytometry was used for the measurement of DNA content, cell size and granularity and the expression of cytokeratin 14, cytokeratin 1 and involucrin. The electrophoretic mobility shift assay was used to determine DNA binding activity by AP-1 transcription factor. Expression of cytokeratin 1 was also assessed with immunofluorescence microscopy.CRH did produce inhibition of proliferation, which was dose-dependent; the shape of the inhibition curve was determined by the media calcium concentration. CRH action was pinpointed at inhibition of the G0/1 to the S phase transition of the cell cycle. CRH also increased AP-1 binding activity, cell granularity, cytokeratin 1 and involucrin expression, and inhibited cytokeratin 14 expression.These results are consistent with CRH induction of the keratinocyte differentiation programme. Thus, the overall CRH cutaneous actions connote protective functions for the epidermis, that appear to include the triggering or acceleration of the differentiation programme.