Post-transcriptional regulation of tyrosine hydroxylase gene expression by oxygen in PC12 cells

Reduced oxygen tension (hypoxia) leads to increased stability of mRNA for tyrosine hydroxylase (TH), the rate limiting enzyme in biosynthesis of catecholamine neurotransmitters. Hypoxia increases the half life of TH mRNA from 10 to 30 hours. The increased stability of TH mRNA during hypoxia results from fast enhanced binding of a cytoplasmic protein (hypoxia inducible protein, HIP) to a pyrimidine-rich sequence within the 3' untranslated region (3'UTR) of TH mRNA. This novel cis-element is referred to as hypoxia-inducible protein binding site (HIPBS) and is located between bases 1551 and 1578 of the 3' UTR of TH mRNA. We identified that the (U/C)(C/U)CCCU motif within the HIPBS represents the optimum protein-binding site. Mutations within this region that abolish protein binding prevent also regulation of TH mRNA stability during hypoxia. UV-crosslinking and SDS-PAGE analysis of the HIPBS-protein complexes showed the presence of a major 50 kDa complex. The formation of the complex was augmented when protein extracts were obtained from PC12 cells exposed to 5% O2. Importantly, formation of the 50 kDa complex was also increased when protein extracts were obtained from carotid bodies or superior cervical ganglia from rats exposed to 10% hypoxia for twenty-four hours.