POSTTRANSCRIPTIONAL 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 biosynthesi s of catecholamine neurotransmitters. Hypoxia increases the half life of TH mRNA from 10 to 30 hours. The increased stability of TH mRNA dur ing hypoxia results from fast enhanced binding of a cytoplasmic protei n (hypoxia inducible protein, HIP) to a pyrimidine-rich sequence withi n the 3' untranslated region (3'UTR) of TH mRNA. This novel cis-elemen t 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 i dentified that the (U/C)(C/U)CCCU motif within the HIPBS represents th e optimum protein-binding site. Mutations within this region that abol ish protein binding prevent also regulation of TH mRNA stability durin g 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% O-2. Importantly, formation of the 50 kDa co mplex was also increased when protein extracts were obtained from caro tid bodies or superior cervical ganglia from rats exposed to 10% hypox ia for twenty-four hours.