pp90RSK- and protein kinase C-dependent pathway regulates p42/44MAPK-induced LDL receptor transcription in HepG2 cells

• Published in: Journal of lipid research Vol. 44; no. 3; pp. 584 - 593
• Main Authors: Kapoor, Gurpreet S, Golden, Carmen, Atkins, Brett, Mehta, Kamal D
• Format: Journal Article
• Language: English
• Published: United States Elsevier 01.03.2003
• Subjects: 90 kDa ribosomal S6 kinase; CREB binding protein; Enzyme Activation; extracellular signal-regulated kinase; Gene Expression Regulation; Humans; Mitogen-Activated Protein Kinase 1 - metabolism; Mitogen-Activated Protein Kinase 3; Mitogen-Activated Protein Kinases - metabolism; Mutation; Nuclear Proteins - metabolism; Phosphorylation; Protein Kinase C - antagonists & inhibitors; Protein Kinase C - genetics; Protein Kinase C - metabolism; Protein Kinase C beta; Receptors, LDL - genetics; Response Elements - genetics; Ribosomal Protein S6 Kinases, 90-kDa - metabolism; Signal Transduction; Sp1 Transcription Factor - metabolism; Trans-Activators - metabolism; Transcription, Genetic; Tumor Cells, Cultured
• ISSN: 0022-2275
• DOI: 10.1194/jlr.M200302-JLR200

We have previously shown that different extracellular stimuli require signaling through the Raf/MEK/p42/44MAPK cascade to induce LDL receptor expression. The present studies were designed to delineate the molecular mechanisms underlying p42/44MAPK-induced LDL receptor transcription in HepG2-Delta Raf-1:ER cells, a modified HepG2 cell line in which the Raf-1/MEK/p42/44MAPK cascade can be specifically activated by anti-estradiol ICI182,780 in an agonist-specific manner. Using these cells, we show that: a) LDL receptor induction was reduced in reporter constructs containing mutation in either Sp1 or sterol-regulatory element-1 (SRE-1) sites, whereas inactivation of both sites abolished the induction; b) E1A, which inhibits CREB binding protein (CBP), a common activator of SRE-1 binding protein and Sp1, strongly repressed the induction; c) intracellular inhibition of the 90 kDa ribosomal S6 kinase (pp90RSK) cascade reduced LDL receptor induction; d) highly selective protein kinase C (PKC) inhibitors effectively abrogated the induction without affecting activation of pp90RSK; and e) overexpression of PKC beta significantly induced LDL receptor promoter activity. Taken together, these results demonstrate that pp90RSK and PKC beta are downstream effectors and Sp1, SRE-1 binding protein, and CBP are part of the transcriptional complex resulting in induction of LDL receptor expression in response to activation of the Raf/MEK/p42/44MAPK cascade. These findings identify for the first time a role for PKC beta in determining the specificity of p42/44MAPK signaling by participating with pp90RSK in regulating gene expression.