HIF-1α controls keratinocyte proliferation by up-regulating p21 (WAF1/Cip1)

The cyclin-dependent kinase inhibitor p21(WAF1/Cip1) plays a central role in a spatial and temporal balance of epidermal keratinocyte proliferation and growth arrest. However, what controls p21 expression in keratinocytes remains uncertain. Hypoxia-inducible factor 1 alpha (HIF-1 alpha) does not only express a variety of genes essential for hypoxic adaptation, but also up-regulates p21 so as to slow down cell cycle under hypoxic conditions. In the present study, we examined the role of HIF-1 alpha in p21-mediated growth arrest of keratinocyte. Keratinocyte proliferation was arrested in the G1 phase at a high cell density. p21 was also up-regulated in a cell density-dependent manner and was found to be highly expressed in epidermal keratinocytes of normal human skins. In addition, in the same specimens and cells, we noted robust HIF-1 alpha expression. HIF-1 alpha siRNAs inhibited p21 expression and released the G1 arrest. In vivo, moreover, the intradermal injection of HIF-1 alpha siRNA attenuated p21 expression in rat epidermis and induced skin hyperplasia. Mechanistically, we propose that the production of mitochondrial reactive oxygen species and the activation of the MEK/ERK pathway are involved in the HIF-1 alpha stabilization in keratinocytes. These results imply that HIF-1 alpha functions as an up-stream player in the p21-mediated growth arrest of keratinocytes. (C) 2007 Elsevier B.V. All rights reserved.