Constitutive expression of hypoxia-inducible factor-1α in keratinocytes during the repair of skin wounds in horses

As a transient hypoxic state exists within skin wounds in horses and may be important for the healing process, this study sought to identify a molecular hypoxia response occurring in horse limb and body wounds healing by second intention. Hypoxia-inducible factor 1 alpha (HIF1 alpha) protein expression was studied throughout repair by Western blotting and immunofluorescence. Paradoxically, HIF1 alpha was strongly expressed in intact skin and its expression decreased dramatically following wounding (p < 0.01), despite the expected hypoxic state within the wounded tissue. HIF1 alpha levels reincreased in parallel with the epithelialization process, and more rapidly in body wounds than in limb wounds (p < 0.01). HIF1 alpha localized predominantly to the keratinocyte layer, in which it was constitutively expressed throughout healing. The HIF1 alpha target gene cyclin-dependent kinase inhibitor 1A (CDKN1A) showed a pattern of expression similar to HIF1 alpha throughout the healing process and also localized to the keratinocyte layer, suggesting that HIF1 alpha may regulate its constitutive expression. The HIF1 alpha target genes vascular endothelial growth factor A (VEGFA) and solute carrier family 2 (facilitated glucose transporter) member 1 (SLC2A1) however did not have a pattern of expression similar to HIF1 alpha, at the mRNA level. We conclude that HIF1 alpha is expressed in a continuous and hypoxia-independent manner in equine keratinocytes in both intact and wounded skin, and may regulate the expression of CDKN1A in this cell type.