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

Oncostatin M (OM) activates the transcription of the human low density lipoprotein receptor (LDLR) in HepG2 cells through a sterol-independent mechanism. 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 investigated the signal transduction pathways that are involved in OM-induced LDLR 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, inhibitors of p38 kinase, phosphatidylinositol 3-kinase, or c-Jun N-terminal kinase, but OM activity was completely abolished by pretreating cells with inhibitors of the extracellular signal-regulated kinase (ERK) kinase (mitogen/ERK kinase (MEK)). To investigate whether the repeat 3 sequence of the LDLR promoter is the OM-responsive element that converts ERK activation at the promoter level, three luciferase reporters, pLDLR-TATA containing only the TATA-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 elements were constructed and transiently transfected into HepG2 cells. OM had 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 activities in both pLDLR-R3- and pLDLR-234-transfected cells was blocked in a dose-dependent manner by inhibition of MEK. These results demonstrate that the mitogen-activated protein kinase MEK/ERK cascade is the essential signaling pathway by which OM activates LDLR gene transcription and provide the first evidence that the repeat 3 element is a new downstream target of ERK activation.