Insulin inhibits transcription of the human gene for insulin-like growth factor-binding protein-1.
Insulin rapidly lowers serum insulin-like growth factor-binding protein-1 (IGFBP-1) levels in vivo. In studies reported here, HEP G2 cells were used as a model system to investigate how insulin achieves this effect. When HEP G2 cells were incubated with 100 nM insulin for 6, 14, or 24 h, IGFBP-1 protein levels in conditioned medium fell to approximately 50% of control values. This apparently was due to a fall in the rate of IGFBP-1 protein synthesis, since HEP G2 cells incorporated 46% less [35S]methionine into IGFBP-1 during a 4-h incubation with 100 nM insulin. IGFBP-1 mRNA levels were similarly affected by 100 nM insulin, falling to 45% of control values after 2 h, and to 9% of control values after 4 h of incubation with this hormone. The fall in IGFBP-1 mRNA level is consistent with data from nuclear transcription assays. HEP G2 nuclei isolated from cells that were incubated with 100 nM insulin for 2 h synthesized only approximately 1/3 the number of IGFBP-1 transcripts as did control nuclei. Further evidence that insulin decreases IGFBP-1 gene transcription comes from transient transfections using chimeric IGFBP-1 promoter-chloramphenicol acetyltransferase reporter gene constructs. IGFBP-1 promoter activity fell to approximately 50% of control values when HEP G2 cells transfected with a construct containing the first 1205 base pairs of the IGFBP-1 promoter were incubated with 100 nM insulin for 6, 14, or 24 h. Insulin lowered both IGFBP-1 protein levels and promoter activity in a dose-dependent manner. A half-maximal effect was found at approximately 1 nM insulin and a maximal effect was found at approximately 10 nM insulin in each instance. Transfections with constructs containing smaller IGFBP-1 promoter fragments showed that the region spanning from 103 to 529 base pairs 5' to the IGFBP-1 mRNA cap site was necessary to demonstrate the inhibitory effect of insulin. These studies indicate that insulin lowers IGFBP-1 protein levels, at least in part, by rapidly decreasing the rate of IGFBP-1 gene transcription, and suggest that this insulin-mediated fall in transcription is conferred through a specific region of the IGFBP-1 promoter.
Powell
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DR
Department of Pediatrics, Baylor College of Medicine, Houston, Texas 77054.
Suwanichkul
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Cubbage
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DePaolis
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Snuggs
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Lee
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D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism; D005786:Gene Expression Regulation; D006801:Humans; D007328:Insulin / Q000502:physiology*; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008113:Liver Neoplasms; D011233:Precipitin Tests; D011401:Promoter Regions, Genetic; D013937:Thymidine Kinase / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic; D014407:Tumor Cells, Cultured
D002352:Carrier Proteins; D007328:Insulin; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I; D013937:Thymidine Kinase