HYDROXYINDOLE-O-METHYLTRANSFERASE IN Y-79 CELLS - REGULATION BY SERUM

Hydroxyindole-O-methyltransferase (HIOMT) catalyzes the last step in t he synthesis of melatonin. In the present study, the regulation of HIO MT expression was examined in the human Y-79 retinoblastoma cell line. Cells were grown in suspension culture using medium supplemented with 10% fetal calf serum (FCS). HIOMT activity and mRNA were strongly red uced when FCS was substituted with 0.1% bovine serum albumin (BSA), an d were restored by addition of FCS. The effect of FCS on HIOMT express ion was relatively selective, because the abundance of mRNA encoding a ctin, G3PDH or interphotoreceptor retinoid-binding protein did not cha nge following serum deprivation. However, S-antigen (arrestin) mRNA wa s regulated by serum coordinately with HIOMT mRNA, suggesting that S-a ntigen expression is also controlled by a serum factor. The effect of serum on HIOMT expression was net duplicated by treatment with a serie s of known differentiating factors, nor was it reduced by dialysis or stripping procedures which remove steroids, growth factors and thyroid hormones.