Novel mechanism of transcriptional activation of hepatic LDL receptor by oncostatin M

Abstract

In this paper we describe a sterol-independent regulation of low density, lipoprotein receptor (LDLR) transcription by the cytokine oncostatin M (OM) in HepG2 cells. We show that OM-induced expression is independent of cholesterol regulation and occurs at the transcriptional level. To elucidate regulatory mechanism(s), we constructed a luciferase reporter system comprising either the native LDLR promoter including repeats 1, 2, and 3, or a synthetic promoter vector containing repeats 2+3 only, allowing us to directly examine OM effects on individual elements. Specific mutants in repeats 1, 2, and 3 were made to facilitate the mapping of the OM effect on the promoter. Wildtype and mutant constructs were assayed for cholesterol and OM regulation. The results show that mutation within the core SRE-1 element of repeat 2 totally abolished cholesterol regulation but had no effect on OM inducibility. More interesting, a mutation within repeat 1 reduced basal transcription activity 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 suggest a novel regulatory role for the repeat 3 element in LDLR transcription.