Induction of low density lipoprotein receptor (LDLR) transcription by oncostatin M is mediated by the extracellular signal-regulated kinase signalingpathway and the repeat 3 element of the LDLR promoter

Oncostatin M (OM) activates the transcription of the human low density lipo protein receptor (LDLR) in HepG2 cells through a sterol-independent mechani sm. Our previous studies showed that mutations within the repeat 3 sequence of the LDLR promoter significantly decreased OM activity on LDLR promoter luciferase reporter constructs that contain the sterol responsive element-1 (repeat 2) and Sp1 binding sites (repeats 1 and 3), In this study, we inve stigated the signal transduction pathways that are involved in OM-induced L DLR transcription, In HepG2 cells, OM induced a rapid increase in LDLR mRNA expression, with increases detected at 30 min and maximal induction at 1 h , This OM effect was not blocked by protein synthesis inhibitors, inhibitor s of p38 kinase, phosphatidylinositol 3-kinase, or c-Jun N-terminal kinase, but OM activity was completely abolished by pretreating cells with inhibit ors of the extracellular signal-regulated kinase (ERK) kinase (mitogen/ ERK kinase (MEK)), To investigate whether the repeat 3 sequence of the LDLR pr omoter is the OM-responsive element that converts ERK activation at the pro moter level, three luciferase reporters, pLDLR-TATA containing only the TAT A-like elements of the promoter, pLDLR-R3 containing repeat 3 and the TATA- like elements, and pLDLR-234 containing repeats 1, 2, 3 and the TATA-like e lements were constructed and transiently transfected into HepG2 cells. OM h ad no effect on the basal promoter construct pLDLR-TATA; however, including a single copy of repeat 3 sequence in the TATA vector (pLDLR-R3) resulted in a full OM response. The activity of OM on pLDLR-R3 was identical to that of pLDLR-234, Importantly, the ability of OM to increase luciferase activi ties in both pLDLR-R3- and pLDLR-234-transfected cells was blocked in a dos e-dependent manner by inhibition of MEK, These results demonstrate that the mitogen-activated protein kinase MEK/ERK cascade is the essential signalin g pathway by which OM activates LDLR gene transcription and provide the fir st evidence that the repeat 3 element is a new downstream target of ERK act ivation.