Hypoxia and adrenergic function: Molecular mechanisms related to Egr-1 and Sp1 activation

Hypoxia is shown to regulate the stress hormone epinephrine through its biosynthesis by phenylethanolamine N-methyltransferase (PNMT) via PNMT gene activation and transcription factors Egr-1 and Sp1 in adrenal medulla-derived PC12 cells. Moderate hypoxia (5% oxygen) markedly stimulates PNMT promoter-driven luciferase activity in the cells. Hypoxia increases Egr-1 and Sp1 mRNA and nuclear protein content and Egr-1 and Sp1 protein–DNA binding complex formation. Subsequent to transcription factor induction, endogenous PNMT mRNA and protein also increase. Egr-1 and Sp1 binding site inactivation or Egr-1 and Sp1 siRNA inhibit PNMT promoter stimulation by hypoxia. Hypoxia elevates protein kinase A (PKA), phospholipase C (PLC), phosphoinositide 3-kinase, protein kinase C, ERK1/2 mitogen-activated protein kinase and p38 mitogen-activated protein kinase expression while selective inhibitors of these signaling enzymes abrogate hypoxic induction of the PNMT promoter and the rise in Egr-1, Sp1 and PNMT mRNA and protein. PC12 cells lacking PKA or PLCγ-1 show significant reduction in PNMT promoter activation by hypoxia. Signaling inhibitors do not affect these responses or reduce hypoxic induction of the PNMT promoter to a lesser extent. Findings suggest that Egr-1 and Sp1 through synergistic interaction are critical transcriptional activators for hypoxic stress-regulated adrenergic function controlled via cAMP/PKA and PLC signaling. Identification of Sp1 as a mediator of hypoxia-induced transcriptional activation of PNMT has not been previously been shown. The effects of hypoxia on PNMT and thereby epinephrine may have important ramifications for the stress hormone epinephrine, its ability to regulate behavioral and physiological processes associated with stress and stress-elicited illness.

Research highlights
► Hypoxia transcriptionally activates the epinephrine-synthesizing enzyme, phenylethanolmine N-methyltransferase (PNMT).
► Hypoxic activation of the PNMT gene occurs via Egr-1 and Sp1.
► Egr-1 and Sp1 are indirectly induced following hypoxic activation of HIF1a.
► Signaling mechanisms associated with hypoxic induction of PNMT include cAMP-PKA, PLC and downstream, PKC, IP3, ERK1/2 and p38.