Epidermal Sensing of Oxygen Regulates Systemic Hypoxic Response

Amphibians respond to changes in environmental oxygen at least in part through their skin, and frogs can use their permeable skin to derive oxygen directly from the atmosphere. Mammalian skin, however, has generally been thought of as an impermeable barrier, with no direct communication between outside environment and inner respiratory physiology. Mammals are known to sense changes in oxygen pressure by carotid bodies that regulate cardiovascular and respiratory response and by the kidneys and liver that regulate erythropoiesis by erythropoietin production. Boutin and colleagues, however, have created a series of experiments that demonstrate unanticipated regulation of erythropoiesis by novel regulation of renal erythropoietin production via epidermal O₂ sensing.

The erythropoietin (EPO) gene is one of many “hypoxia-regulated” genes whose expression is controlled by the master transcription factors, hypoxia-inducible factors-1 and -2 (HIF-1 and HIF-2), each composed of dimers of α and β subunits. Only the HIF α subunits are regulated by hypoxia, and their expression is controlled post-transcriptionally. Under normoxic conditions, the prolyl residues of HIFs are hydroxylated by the enzyme prolyl hydroxylase, which allows the von Hippel-Lindau protein (pVHL) to bind to HIF α, leading to rapid degradation by the ubiquitin-proteasome pathway. During hypoxic conditions, HIF α is stabilized (by not being targeted for proteasome degradation) and forms a transcriptional complex with HIF β that leads to increased expression of multiple target genes involved in diverse processes, including cell proliferation and survival, metabolism, angiogenesis, and erythropoiesis. HIF-1α and HIF-2α exhibit high sequence homology but have different mRNA expression patterns. HIF-1α is expressed ubiquitously, whereas HIF-2α expression is restricted to certain tissues. Both HIF-1α and HIF-2α are regulated by identical mechanisms during hypoxia and form a heterodimer with the same HIF-β subunit. HIF-1 is the principal regulator of EPO gene transcription in the kidney. In other tissues, such as brain and liver (that generates ~ 20 percent of circulating erythropoietin), EPO gene transcription is HIF-2-dependent.

Boutin and colleagues created a mouse with conditional deletion of Vhl in epidermal keratinocytes, which caused cutaneous vasodilation and increased expression of Hif-1α and Hif-2α. Although keratinocytes do not make erythropoietin, the erythropoietin level was nonetheless found to be increased, and the mouse became polycythemic. Further studies of this epidermal Vhl knockout mouse revealed that the elevated levels of hif-1 caused upregulation of inducible nitric oxide synthase, which in turn led to increased cutaneous nitric oxide (NO), a potent vasodilator. This NO-induced skin vasodilation resulted in decreased perfusion of other organs, most notably the liver, with subsequent hypoxia-induced, increased expression of hepatic Hif-2α, which in turn caused increased expression of the Epo gene. In follow-up experiments using mice with wild-type Vhl, the authors deleted the cutaneous genes for either Hif-1α or Hif-2α. Unexpectedly, under conditions of normoxia, the loss of Hif-1α and Hif-2α had no effect on erythropoietin levels. Under hypoxic conditions, however, the Hif-1α epidermal knockout mice did not display an appropriate increase in renal Epo gene transcription and were unable to mount an appropriate renal erythropoietin response. These experiments show the importance of epidermal hif in sensing environmental oxygen levels and regulating systemic hypoxic responses in mice, with physiologic regulation mediated primarily by hif-1α while the pathologic loss of Vhl is mediated primarily by hif-2α.