NOVEL MECHANISM OF TRANSCRIPTIONAL ACTIVATION OF HEPATIC LDL RECEPTORBY ONCOSTATIN M

In this paper we describe a sterol-independent regulation of low densi ty lipoprotein receptor (LDLR) transcription by the cytokine oncostati n M (OM) in HepG2 cells. We show that OM-induced expression is indepen dent of cholesterol regulation and occurs at the transcriptional level . To elucidate regulatory mechanism(s), we constructed a luciferase re porter system comprising either the native LDLR promoter including rep eats 1, 2, and 3, or a synthetic promoter vector containing repeats 23 only, allowing us to directly examine OM effects on individual eleme nts. Specific mutants in repeats 1, 2, and 3 were made to facilitate t he mapping of the OM effect on the promoter. Wildtype and mutant const ructs were assayed for cholesterol and OM regulation. The results show that mutation within the core SRE-1 element of repeat 2 totally aboli shed cholesterol regulation but had no effect on OM inducibility. More interesting, a mutation within repeat 1 reduced basal transcription a ctivity to 10% of the native promoter, but OM induction was unaltered. However, the identical mutation engineered in repeat 3 significantly decreased OM induction of LDLR promoter activity. These results sugges t a novel regulatory role for the repeat 3 element in LDLR transcripti on.