Chronic nicotine induces hypoxia inducible factor-2α in perinatal rat adrenal chromaffin cells: role in transcriptional upregulation of KATPchannel subunit Kir6.2
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Fetal nicotine exposure causes impaired adrenal catecholamine secretion and increased neonatal mortality during acute hypoxic challenges. Both effects are attributable to upregulation of ATP-sensitive K(+) channels (K(ATP) channels) and can be rescued by pretreatment with the blocker, glibenclamide. Although use of in vitro models of primary and immortalized, fetal-derived rat adrenomedullary chromaffin cells (i.e., MAH cells) demonstrated the involvement of α7 nicotinic ACh receptor (nAChR) stimulation and the transcription factor, HIF-2α, the latter's role was unclear. Using Western blots, we show that chronic nicotine causes a progressive, time-dependent induction of HIF-2α in MAH cells that parallels the upregulation of K(ATP) channel subunit, Kir6.2. Moreover, a common HIF target, VEGF mRNA, was also upregulated after chronic nicotine. All the above effects were prevented during co-incubation with α-bungarotoxin (100 nM), a specific α7 nAChR blocker, and were absent in HIF-2α-deficient MAH cells. Chromatin immunoprecipitation (ChIP) assays demonstrated binding of HIF-2α to a putative hypoxia response element in Kir6.2 gene promoter. Specificity of this signaling pathway was validated in adrenal glands from pups born to dams exposed to nicotine throughout gestation; the upregulation of both HIF-2α and Kir6.2 was confined to medullary, but not cortical, tissue. This study has uncovered a signaling pathway whereby a nonhypoxic stimulus (nicotine) promotes HIF-2α-mediated transcriptional upregulation of a novel target, Kir6.2 subunit. The data suggest that the HIF pathway may be involved in K(ATP) channel-mediated neuroprotection during brain ischemia, and in the effects of chronic nicotine on ubiquitous brain α7 nAChR.
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