Hypoxia-mediated control of HIF/ARNT machinery in epidermal keratinocytes

Transcriptional activity of hypoxia-induced factor 1 (HIF1) - a heterodimer of HIF1 alpha and ARNT (HIF1 beta) - is essential for cellular adaptation to environmental stress and plays an important role in skin development, wound healing, tumorigenesis and barrier function. Using primary mouse and human epidermal keratinocytes at ambient or hypoxic (1% O-2) conditions we studied effects of hypoxia upon HIF protein expression. Significant nuclear levels of ARNT and HIF1 alpha along with high HIF1 activity in normoxic keratinocytes suggest an as yet uncharacterised oxygen-independent role for HIF pathway in the epidermis. Acute hypoxia results in an instant but transient increase of HIF1 alpha protein accompanied by a gradual decrease in its mRNA, while ARNT expression remains unchanged. In prolonged (chronic) hypoxia both HIF1 alpha and Arnt are downregulated along with decline of HIF1 activity. However, expression of classical HIF1 targets such as Selenbp1 and Vegfa remains high. Thus, keratinocytes respond to acute hypoxia with immediate block of HIF1 alpha protein degradation and concomitant increase of HIF activity, while under chronic hypoxia proangiogenic signalling is maintained through HIF1-independent pathway(s). Decline of HIF1 alpha during chronic exposure is controlled at both mRNA and protein levels, while Arnt is downregulated post-translationally. Distinct transcription levels of Hif1 alpha and Hif3 alpha splice variants under normoxia and their differential response to hypoxia suggest functional diversity of Hif-alpha isoforms and highlight the complexity of HIF machinery control in epidermal keratinocytes. (c) 2010 Elsevier B.V. All rights reserved.