Phenylalanine Hydroxylase in Cultured Hepatocytes I. HORMONAL CONTROL OF ENZYME LEVELS

Abstract The presence of phenylalanine hydroxylase as a constitutive enzyme in two clonal cell lines (H4-II-E-C3 and MH1C1) derived from rat hepatomas and cultured in vitro is described; eight other clonal cell strains originating from rat or rabbit liver did not contain detectable amounts of this enzyme. The levels of phenylalanine hydroxylase in the two hepatoma cell lines were stimulated by hydrocortisone, corticosterone, dexamethasone or N6-O2'-dibutyryl-3',5'-cyclic adenosine monophosphate. The stimulation of the levels of the enzyme by hydrocortisone in the H4-II-E-C3 line was shown to depend on de novo protein synthesis as it was inhibited by cycloheximide. It is postulated that the hepatoma cell lines are probably good models for studying the factors regulating the expression of the phenylalanine hydroxylase gene.