Regulation of tyrosine hydroxylase gene expression

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in catecholamine biosynthesis. In separate studies, we explored the regulation of TH gene expression by (1) oxidative stress and (2) von Hippel-Lindau tumor suppressor protein in O2-sensitive rat pheochromocytoma (PC12) cells.;1. Cellular respiration results in the formation of reactive oxygen intermediates (ROIs), such as H2O2, leading to oxidative stress and cell death. During hypoxia, reduced oxygen tension leads to decreased ROI and increased TH mRNA levels in PC12 cells, thus we hypothesized that there is a link between O2-regulated ROI production and regulation of TH gene expression. We found that ROI levels correlated inversely with TH mRNA levels. Induction of ROI formation by treatment of cells with H2O2 or iron decreased expression of TH mRNA while reduction of ROIs by exposure of cells to hypoxia, antioxidants, or iron chelators increased TH gene expression, providing evidence for regulation of TH by oxidative stress.;2. Pheochromocytomas are tumors of the adrenal medulla accompanied by increased synthesis and release of catecholamines and are often present in the von Hippel-Lindau (VHL) cancer syndrome, a disease caused by inactivation of the VHL tumor suppressor gene. We hypothesized that VHL protein (pVHL) is involved in the regulation of TH gene expression in pheochromocytoma cells. We found that overexpression of human wild type pVHL(wt) in PC12 cells resulted in a five-fold repression in TH gene expression while the truncated mutant pVHL(1--115) induced TH levels three-fold. Inhibition resulted from a block in TH transcript elongation in the downstream part of the gene that was accompanied by increased interactions between pVHL and elongin C, possibly preventing the formation of the Elongin (SIII) transcription elongation complex. pVHL(1--115) facilitated elongation of TH mRNA and increased TH promoter activity. Hypoxia alleviated the pVHL-dependent elongation block by causing a decreased nuclear interaction of pVHL and elongin B. These data provide the first in vivo evidence that pVHL and Elongin regulate gene-specific transcript elongation.