Transcriptional regulation of human microsomal triglyceride transfer protein by hepatocyte nuclear factor-4

Microsomal triglyceride transfer protein (MTP) catalyzes the assembly of triglyceride (TG)-rich apolipoprotein B-containing liver (e.g., VLDL) and intestinal (e.g., chylomicron) lipoproteins. The human MTP gene promoter is reported here to associate in vivo with endogenous hepatocyte nuclear factor-4 (HNF-4 ) and to be transactivated or transsuppressed by overexpressed or by dominant negative HNF-4 , respectively. Human MTP (hMTP) transactivation by HNF-4 is accounted for by the concerted activity of distal ( 83/ 70) and proximal ( 50/ 38) direct repeat 1 elements of the hMTP promoter that bind HNF-4 . Transactivation by HNF-4 is specifically antagonized by chicken ovalbumin upstream promoter. Transcriptional activation of hMTP by HNF-4 is mediated by HNF-4 domains engaged in ligand binding and ligand-driven transactivation and is further complemented by HNF-4 /HNF-1 synergism that involves the HNF-4 activation function 1 (AF-1) domain. hMTP transactivation by HNF-4 is specifically inhibited by , -tetramethyl-hexadecanedioic acid acting as an HNF-4 antagonist ligand. hMTP transactivation by HNF-4 may account for the activation or inhibition of MTP expression and the production of TG-rich lipoproteins by agonist (e.g., saturated fatty acids) or antagonist [e.g., (n-3) PUFA, hypolipidemic fibrates, or Methyl-substituted dicarboxylic acid (Medica) compounds] HNF-4 ligands. —Sheena, V., R. Hertz, J. Nousbeck, I. Berman, J. Magenheim, and J. BarTana. Transcriptional regulation of human microsomal triglyceride transfer protein by hepatocyte nuclear factor-4 . J. Lipid Res. 2005. 46: 328–341. Supplementary key words lipoproteins • nuclear receptors • hypolipidemic drugs • Medica 16 Microsomal triglyceride transfer protein (MTP) is expressed in liver and intestine, where it plays a central role in the assembly and secretion of triglyceride (TG)-rich, apolipoprotein B (apoB)-containing lipoproteins (e.g., liver VLDL and intestinal chylomicrons). MTP catalytic subunit heterodimerizes with protein disulfide isomerase to catalyze the lipidation of the newly synthesized apoB with TG, cholesteryl esters, and phospholipids in the endoplasmic reticulum lumen (reviewed in 1, 2). In the absence of MTP, lipoprotein assembly is blocked, resulting in apoB ubiquitination and its subsequent proteolysis (3, 4). Blocking lipoprotein assembly may lead to hypo/abetalipoproteinemia, hypotriglyceridemia, and hypocholesterolemia at the expense of hepatic steatosis (5–8). On the other hand, overexpression of MTP may result in increased apoB lipidation, its incorporation into plasma apoB-containing lipoproteins, and hyperlipidemia (9). In addition to apoB lipidation and lipoprotein assembly, MTP has recently been implicated in regulating the ability of CD1d-bearing cell types (e.g., hepatocytes, intestinal epithelial cells) to effect CD1d-restricted antigen presentation (10). Thus, abrogation of MTP function may lead to protection from CD1d-mediated hepatitis and/or colitis. The 5 -flanking 200 bp of the MTP gene promoter are highly conserved in human and hamster (11). The human MTP (hMTP) promoter consists of consensus sequences for sterol regulatory element binding protein (SREBP; 124/ 116), insulin ( 122/ 111), activator protein 1 (AP-1; 109/ 104), and hepatocyte nuclear factor-1 (HNF-1 ; 103/ 98). Transfection analysis has indicated that the hMTP promoter activity was reduced by more than 95% by deleting the promoter to 87 bp or by mutating its HNF-1 or AP-1 elements (11). The hMTP promoter is further controlled by more distal promoter elements. Of particular interest is the G493T mutation, which results in higher promoter activity with a concomitant increase in the production of postprandial chylomicrons and VLDL (12, 13). Abbreviations: AF-1, activation function 1; AP-1, activator protein 1; apoB, apolipoprotein B; CHIP, chromatin immunoprecipitation; CIDICA, , -tetrachloro-tetradecanedioic acid; COUP-TF, chicken ovalbumin upstream promoter transcription factor; D-DR1, distal direct repeat 1; D-mut, mutated distal direct repeat 1; DR1, direct repeat 1; GFP, green fluorescent protein; hMTP, human microsomal triglyceride transfer protein; HNF-1 , hepatocyte nuclear factor-1 ; HNF-4 , hepatocyte nuclear factor4 ; Medica 16, , -tetramethyl-hexadecanedioic acid; MTP, microsomal triglyceride transfer protein; P-DR1, proximal direct repeat 1; P-mut, mutated proximal direct repeat 1; PPAR , peroxisome proliferator-activated receptor ; PUFA, polyunsaturated fatty acid; RXR, retinoid X receptor; SREBP, sterol regulatory element binding protein; TG, triglyceride. 1 To whom correspondence should be addressed. e-mail: [email protected] Manuscript received 28 September 2004 and in revised form 3 November 2004. Published, JLR Papers in Press, November 16, 2004. DOI 10.1194/jlr.M400371-JLR200 by gest, on N ovem er 7, 2017 w w w .j.org D ow nladed fom