Epidermal growth factor induction of apolipoprotein A-I is mediated by the Ras-MAP kinase cascade and Sp1.
Insulin induces apolipoprotein A-I, apoA-I gene transcription via a membrane receptor with intrinsic tyrosine kinase activity. This finding prompted us to ask whether the gene is stimulated by epidermal growth factor (EGF), EGF a peptide hormone that binds to another member of the receptor superfamily with tyrosine kinase activity. Our data showed that like insulin, EGF increased abundance of apoA-I protein and transcription of the gene in human hepatoma, Hep G2 cells. The effects of both hormones appeared direct because their induction of apoA-I gene transcription was not affected by the protein synthesis inhibitor, cycloheximide. Although both insulin and EGF stimulate apoA-I expression, each hormone binds to a distinct membrane receptor thus suggesting differential intracellular signaling. Therefore, we used a panel of inhibitors to define the pathway(s) that mediate the actions of these hormones. Whereas, the actions of EGF required only the Ras-mitogen-activated protein, MAP kinase, those of insulin were mediated by equal participation of both the Ras-MAP kinase and protein kinase C, PKC cascades. Despite differences in signaling pathways triggered by each hormone receptor, the activation of apoA-I transcription required the participation of a single transcription factor, Sp1. Furthermore, EGF induction of transcription was attenuated by mutating the MAP kinase site at amino acid, Thr(266) rendering Sp1 phosphorylation deficient. In summary, EGF stimulation of apoA-I expression is mediated solely by the Ras-MAP kinase cascade and enhanced activity of this pathway requires Sp1 with an intact phosphorylation site at Thr(266). However, insulin induction of this gene is different and requires both Ras-MAP kinase and PKC pathways but their actions are also mediated by Sp1.
Endocrine Research Group, Departments of Medicine, University of Calgary, Alberta, Canada T2N 4N1.
X L
XL
Zheng
S
S
Matsubara
C
C
Diao
M D
MD
Hollenberg
N C
NC
Wong
D000595:Amino Acid Sequence; D016632:Apolipoprotein A-I / Q000378:metabolism*; D001665:Binding Sites; D015153:Blotting, Western; D015500:Chloramphenicol O-Acetyltransferase / Q000378:metabolism; D003513:Cycloheximide / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004815:Epidermal Growth Factor / Q000378:metabolism*; D066246:ErbB Receptors / Q000378:metabolism; D005799:Genes, Dominant / Q000235:genetics; D017930:Genes, Reporter; D011905:Genes, ras / Q000235:genetics; D006801:Humans; D007328:Insulin / Q000378:metabolism / Q000494:pharmacology; D020935:MAP Kinase Signaling System; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D010766:Phosphorylation; D010957:Plasmids / Q000378:metabolism; D011485:Protein Binding; D011493:Protein Kinase C / Q000378:metabolism; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D011799:Quinazolines / Q000494:pharmacology; D012190:Retroviridae / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction; D016329:Sp1 Transcription Factor / Q000378:metabolism*; D013912:Threonine / Q000737:chemistry / Q000235:genetics; D013997:Time Factors; D014158:Transcription, Genetic; D014162:Transfection; D014407:Tumor Cells, Cultured; D015854:Up-Regulation; D018631:ras Proteins / Q000378:metabolism*
D016632:Apolipoprotein A-I; D004791:Enzyme Inhibitors; D007328:Insulin; D011500:Protein Synthesis Inhibitors; D011799:Quinazolines; D016329:Sp1 Transcription Factor; D013912:Threonine; D004815:Epidermal Growth Factor; D003513:Cycloheximide; D015500:Chloramphenicol O-Acetyltransferase; D066246:ErbB Receptors; D011493:Protein Kinase C; D018631:ras Proteins; C088860:4-((3-bromophenyl)amino)-6,7-dimethoxyquinazoline