Jw. Liu et al., NOVEL MECHANISM OF TRANSCRIPTIONAL ACTIVATION OF HEPATIC LDL RECEPTORBY ONCOSTATIN M, Journal of lipid research, 38(10), 1997, pp. 2035-2048
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.