Hypoxic stress-induced changes in adrenergic function: role of HIF1α

Sustaining epinephrine-elicited behavioral and physiological responses during stress requires replenishment of epinephrine stores. Egr-1 and Sp1 contribute by stimulating the gene encoding the epinephrine-synthesizing enzyme, phenylethanolamine N-methyltransferase (PNMT), as shown for immobilization stress in rats in adrenal medulla and for hypoxic stress in adrenal medulla-derived PC12 cells. Hypoxia (5% O-2) also activates hypoxia inducible factor (HIF) 1 alpha, increasing mRNA, nuclear protein and nuclear protein/hypoxia response element binding complex formation. Hypoxia and HIF1 alpha over-expression also elevate PNMT promoter-driven luciferase activity in PC12 cells. Hypoxia may be limiting as HIF1 alpha over-expression increases luciferase expression to no greater extent than oxygen reduction alone. HIF1 alpha inducers CoCl2 or deferoxamine elevate luciferase as well. PC12 cells harboring a HIF1 alpha expression construct show markedly higher levels of Egr-1 and Sp1 mRNA and nuclear protein and PNMT mRNA and cytoplasmic protein. Inactivation of Egr-1 and Sp1 binding sites in the proximal -893 bp of PNMT promoter precludes HIF1 alpha stimulation while a potential hypoxia response element (-282 bp) in the promoter shows weak HIF1 alpha affinity at best. These findings are the first to suggest that hypoxia activates the proximal rat PNMT promoter primarily via HIF1 alpha induction of Egr-1 and Sp1 rather than by co-activation by Egr-1, Sp1 and HIF1 alpha. In addition, the rise in HIF1 alpha protein leading to Egr-1 and Sp1 stimulation of PNMT appears to include HIF1 alpha gene activation rather than simply prevention of HIF1 alpha proteolytic degradation.