Even in the absence of ERK activity, stimulation of HepG2 cells with TPA induces a 2- to 2
Even in the absence of ERK activity, stimulation of HepG2 cells with TPA induces a 2- to 2.5-fold increase in receptor mRNA levels relative to untreated cells (P= 0.006). and by activating JNK through two protein kinase C-independent mechanisms. Finally, prolonged JNK activation increased steady-state levels of receptor mRNA and protein, and significantly enhanced cellular LDL-binding activity. These data suggest that JNK may play an important role in posttranscriptional control of LDL receptor expression, thus constituting a novel mechanism to enhance plasma LDL clearance by liver cells. Keywords:transmission transduction, mitogen-activated protein kinase, lipoprotein metabolism, mRNA turnover, gene expression Mammalian cells require cholesterol for the synthesis of membranes, steroid hormones, and bile salts. Cholesterol is principally synthesized in the liver, but is packaged into serum lipoproteins for transport to peripheral tissues (1). LDL is the most abundant cholesterol-carrying vehicle in human plasma, and considerable epidemiological evidence maintains that elevated plasma LDL levels are a major risk factor for atherosclerosis, myocardial infarction, Betulinic acid and related mortality (24). Cellular LDL uptake is usually Rabbit Polyclonal to Collagen I alpha2 (Cleaved-Gly1102) mediated by the LDL receptor, an integral plasma membrane glycoprotein that is expressed in all cell types but most abundantly in the liver (5), where internalized cholesterol may be excreted either directly or after metabolic conversion into bile acids (6,7). The importance of hepatic LDL receptors in systemic cholesterol excretion is usually exemplified by patients suffering from familial hypercholesterolemia, an autosomal dominant disorder whereby one or both LDL receptor alleles do not encode functional receptors (as examined in Refs.1,4,8). Introduction of functional hepatic LDL receptors to homozygous familial hypercholesterolemia patients by liver transplantation (9) or ex lover vivo gene therapy (10) significantly lowers plasma cholesterol in these individuals and enhances prognosis. Hepatic LDL receptor expression Betulinic acid is also enhanced in patients taking statin drugs, which inhibit de novo cholesterol synthesis (as examined in Ref.11). Since cholesterol synthesis is restricted principally to the liver, these compounds deplete intracellular cholesterol in this tissue, which in turn enhances transcription of the LDL receptor gene through activation of the sterol-regulatory element binding protein (SREBP) family of transcription factors (12). These examples demonstrate that therapeutic strategies that increase the expression of functional LDL receptors Betulinic acid in liver cells improve LDL clearance from your circulation, which in turn slows atherosclerotic development and reduces the risk of coronary heart disease. The regulation of hepatic LDL receptor expression has been extensively analyzed using the highly differentiated human hepatocarcinoma cell collection HepG2 (13). In this cell collection, LDL receptor levels are suppressed in response to cholesterol and lipoprotein loading (14). However, expression of LDL receptors is also regulated through several transmission transduction pathways in HepG2 cells, including the cyclic AMP, diacylglycerol-protein kinase C (PKC), and mitogen-activated protein kinase (MAPK) pathways (1517). Recent studies have probed the associations between these signaling cascades in the control of receptor expression. Treating HepG2 cells with the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) potently but transiently induces accumulation of LDL receptor mRNA, including transcriptional activation of the receptor gene (18) and stabilization of its encoded mRNA (15). Induction of receptor gene transcription by TPA appears to require the isoform of PKC and is principally mediated through the p42/44 extracellular signal-regulated kinase (ERK) signaling pathway (18). Stabilization of receptor mRNA by TPA requires sequences in the distal 3 untranslated region (3UTR), possibly including elements related to theAlurepetitive sequence that are located in this domain name of the receptor transcript (19). Recent studies also statement that LDL receptor mRNA stability is enhanced in HepG2 cells following treatment with the herbal alkaloid berberine through activation of the ERK pathway, including sequences in the proximal 3UTR of the receptor mRNA (20). The objective of this study was to characterize intracellular signaling pathways controlling the decay kinetics of LDL receptor mRNA. Using TPA-treated HepG2 cells as a model system, we show that activation of PKC stabilizes receptor mRNA concomitant with activation of the three major MAPK pathways. Unlike the inhibition of LDL receptor mRNA decay induced by berberine, stabilization of this transcript following PKC activation was not influenced by chemical inhibitors of either.