It is being increasingly recognized that
nitric oxide (NO) is associated with many physiological processes, including regulation of gene expression. NO shares certain similarities with molecular
oxygen (O2). Previous studies have shown that
hypoxia up-regulates c-fos, an immediate early gene, and
tyrosine hydroxylase (TH), a late response gene that encodes rate limiting
enzyme in
catecholamine synthesis. Given the similarities between NO and O2, we hypothesized that NO inhibits
hypoxia-induced up-regulation of c-fos and TH. Experiments were performed on rat
pheochromocytoma (PC12) cells. c-fos and TH
mRNA's were analysed by Northern blot and promoter activities by reporter gene assays, respectively.
Hypoxia (1% O2 for 6 h) up-regulated c-fos and TH
mRNA and increased c-fos promoter activity.
Hypoxia-induced c-fos
mRNA expression, and promoter activities were significantly potentiated in presence of
spermine nitric oxide (SNO), a NO donor. By contrast, SNO significantly inhibited TH
mRNA expression and TH promoter activity during
hypoxia. Electrophoretic mobility shift-assay showed increased binding of
AP-1 and HIF-1
transcription factors to the TH promoter in cells exposed to
hypoxia. SNO abolished the binding of
AP-1 and HIF-1 to the TH promoter during
hypoxia, suggesting that inhibition of
hypoxia-induced TH transcription by NO are due to reduced binding of
AP-1 and HIF-1
transcription factors. These result demonstrate that NO has both positive and negative influence on gene regulation by
hypoxia and suggest that although NO resembles O2 does not always inhibit gene expression during low
oxygen.