Enhanced expression of GTP cyclohydrolase I in V-1-overexpressing PC12D cells

Three of the catecholamine-synthesizing enzymes, i.e., tyrosine hydroxylase (TH), aromatic L-amino acid decarboxylase, and dopamine β-hydroxylase, were earlier shown to be up-regulated in cloned PC12D cells overexpressing V-1, a cdc10/SWI6 motif-containing protein. GTP cyclohydrolase I (GCH) is the rate-limiting enzyme for the biosynthesis of tetrahydrobiopterin (BH₄), known as an essential cofactor for TH; and here we found the increased expression of GCH in V-1-overexpressing clones. Both GCH activity and total biopterin content were highly increased in the V-1 clones; whereas the activity of sepiapterin reductase, enzyme in the final step of the BH₄ biosynthesis, was not altered. Biochemical analyses revealed increased levels of GCH protein, mRNA, and transcription in the V-1 clones. Promoter analysis showed increased reporter activity in the construct with 150 bp of the promoter region of the human GCH gene, suggesting the involvement of cAMP-responsive element-mediated transcriptional regulation.