The E5 oncoprotein of human papillomavirus type 16 enhances endothelin-1-induced keratinocyte growth

Human keratinocytes express ETA receptors and produce endothelin-1 (ET-1), which stimulates growth response. Previously, we reported that a twofold increase in ETA receptors is present in human papillomavirus type 16 (HPV16) immortalized keratinocytes and that ET-1 induces enhanced proliferative response in these cell lines compared to normal cells. The present studies examine whether the E5 gene of HPV16 is responsible for the enhanced activity of ET-1 in HPV-transfected keratinocytes. The presence of the E5 gene in growth factor-starved keratinocytes induced the DNA synthesis and enhanced the mitogenic activity of ET-1 or epidermal growth factor. The selection of primary keratinocytes in growth factor-free medium with the addition of ET-1 as a growth factor showed that E5-transfected keratinocytes were able to grow and to form a higher number of larger colonies with respect to untransfected cells. This effect seems to be related to the interaction of E5 with the mitogenic signaling pathway of ET-1 rather than to an increase in the expression of the receptors for ET-1. In conclusion, our data demonstrate that E5 enhances ligand signaling in keratinocytes outside the EGF pathway by the amplification of the proliferative effect of ET-1/ETA receptor signaling.