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

Abstract

Oncostatin M (OM) activates the transcription of the human low density lipoprotein receptor (LDLR) inHepG2 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 Nterminal 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.

Expression of the α5 Integrin Subunit Gene Promoter Is Positively Regulated by the Extracellular Matrix Component Fibronectin through the Transcription Factor Sp1 in Corneal Epithelial Cells in Vitro

“…Sp1 has been recently recognized as one of the few target transcription factors phosphorylated by ERK kinases (51)(52)(53). We have shown above that its ability to interact with the ␣ 5 FRE is strongly increased upon activation of the FN/␣ 5 ␤ 1 integrin-mediated signal transduction.…”

“…Most of all, it also contains the GC-rich sequence (TCCCC) located downstream of the 3Ј repeat that proved to be required for the FN responsiveness directed by the ␣ 5 FRE. Interestingly, only Sp1 and the Sp1-related protein Sp3 have been shown to bind repeat 3 (51). However, these authors have been unable to detect any OMinduced alterations in Sp1 binding by EMSA or in the ratio of hyper-versus hypophosphorylated Sp1 by Western blot analyses (51).…”

Interleukin-6 Stimulates LDL Receptor Gene Expression via Activation of Sterol-Responsive and Sp1 Binding Elements

“…Although IL-6 and OM share the same signal-transducing protein gp130 and both cytokines stimulate LDL-R in a sterol-independent manner, OM stimulation exclusively involves repeat 3 as a downstream target of mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) activation. 20,35 A further difference between OM and IL-6 is that only IL-6 directly enhances the binding of Sp1 and Sp3 to repeat 3, whereas OM does not directly upregulate Sp1 binding activity. 20 Thus, our data stand in contrast to the speculation by Li et al 35 suggesting that IL-6 and OM upregulate LDL-R by a similar mechanism.…”

Protein Phosphatase 2A (PP2A) Regulates Low Density Lipoprotein Uptake through Regulating Sterol Response Element-binding Protein-2 (SREBP-2) DNA Binding

“…All display heterogeneity in their response to SREBP-1/SREBP-2 activity, by varying the number or sequence of various SREs or specific co-factor binding sites (50 -54). The LDLR promoter responds to various signals based on the presence of multiple binding elements (55,56), which includes the serum response, Sp1, repeat 3, Egr1, and C/EBP␤ elements (56,57). Thus, PP2A may have a specific role in regulating cholesterol homeostasis through specific regulation of LDLR expression and LDL-C uptake.…”