Hypoxia leads to abnormal epidermal differentiation via HIF-independent pathways

Atmospheric oxygen is important for the epidermis, as the skin epidermis is not greatly affected by blood circulation. Therefore, it is necessary to understand the effect of hypoxic signals on the epidermis as some environmental stimuli can induce skin hypoxia. Here, we investigated how hypoxia (1% O2) affected skin equivalents (SEs) and normal human epidermal keratinocytes. We found that hypoxia specifically decreased the protein levels of keratin 1 (K1)/keratin 10 (K10), a representative marker of the epidermal spinous layer in the epidermis. However, hypoxia-inducible factors, the major regulators of hypoxia, did not affect hypoxia-induced down-regulation of K1/K10. We also found that N-acetyl-l-cysteine (NAC), a reactive oxygen species scavenger, antagonized the hypoxia-induced reduction of K1/K10 in keratinocytes and SEs. In contrast to the findings for NAC, inhibitors that blocked reactive oxygen species generation did not cause recovery of K1/K10 protein levels under hypoxic conditions. Taken together, these results indicate that hypoxia leads to abnormal keratinocyte differentiation by down-regulating K1/K10 and that this phenomenon can be ameliorated by NAC.