title: Differential, tissue-specific, transcriptional regulation of apolipoprotein B secretion by transforming growth factor beta.
abstract: Apolipoprotein B (apoB) is required for the assembly and secretion of triglyceride-rich lipoproteins. ApoB synthesis is constitutive, and post-translational mechanisms modulate its secretion. Transforming growth factor beta (TGF-beta) increased apoB secretion in both differentiated and nondifferentiated Caco-2 cells and decreased secretion in HepG2 cells without affecting apolipoprotein A-I secretion. TGF-beta altered apoB secretion by changing steady-state mRNA levels and protein synthesis. Expression of SMAD3 and SMAD4 differentially regulated apoB secretion in these cells. Thus, SMADs mediate dissimilar secretion of apoB in both the cell lines by affecting gene transcription. We identified a 485-bp element, 55 kb upstream of the apob gene that contains a SMAD binding motif. This motif increased the expression of chloramphenicol acetyltransferase in Caco-2 cells treated with TGF-beta or transfected with SMADs. Hence, TGF-beta activates SMADs that bind to the 485-bp intestinal enhancer element in the apob gene and increase its transcription and secretion in Caco-2 cells. This is the first example showing differential transcriptional regulation of the apob gene by cytokines and dissimilar regulation of one gene in two different cell lines by TGF-beta. In this regulation, the presence of cytokine-responsive motif in the tissue-specific enhancer element confers cell-specific response.
authors.affiliation: Department of Anatomy and Cell Biology, SUNY Downstate Medical Center, Brooklyn, New York 11203, USA.
authors.forename: Karnail
authors.initials: K
authors.lastname: Singh
authors.forename: Olcay A
authors.initials: OA
authors.lastname: Batuman
authors.forename: Hassan O
authors.initials: HO
authors.lastname: Akman
authors.forename: Mamdouh H
authors.initials: MH
authors.lastname: Kedees
authors.forename: Varsha
authors.initials: V
authors.lastname: Vakil
authors.forename: M Mahmood
authors.initials: MM
authors.lastname: Hussain
mesh_terms: D020816:Amino Acid Motifs; D001055:Apolipoproteins B / Q000235:genetics* / Q000378:metabolism*; D015152:Blotting, Northern; D018938:Caco-2 Cells; D015500:Chloramphenicol O-Acetyltransferase / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000096:biosynthesis; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005419:Flavonoids / Q000494:pharmacology; D005786:Gene Expression Regulation; D006801:Humans; D010957:Plasmids / Q000378:metabolism; D012313:RNA / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D051900:Smad3 Protein; D051901:Smad4 Protein; D013997:Time Factors; D014018:Tissue Distribution; D015534:Trans-Activators / Q000096:biosynthesis; D014158:Transcription, Genetic; D014162:Transfection; D016212:Transforming Growth Factor beta / Q000378:metabolism*; D019076:Transgenes; D014407:Tumor Cells, Cultured
chemical_list: D001055:Apolipoproteins B; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D005419:Flavonoids; D012333:RNA, Messenger; D011994:Recombinant Proteins; C497790:SMAD3 protein, human; C497796:SMAD4 protein, human; D051900:Smad3 Protein; D051901:Smad4 Protein; D015534:Trans-Activators; D016212:Transforming Growth Factor beta; D012313:RNA; D015500:Chloramphenicol O-Acetyltransferase; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one