Phosphatidylinositol 3-kinase and Ras/mitogen-activated protein kinase signaling pathways are required for the regulation of 5-aminolevulinate synthase gene expression by insulin.
Insulin regulates the expression of several hepatic genes. Although the general definition of insulin signaling has progressed dramatically, the elucidation of the complete signaling pathway from insulin receptor to transcription factors involved in the regulation of a specific gene remains to be established. In fact, recent works suggest that multiple divergent insulin signaling pathways regulate the expression of distinct genes. 5-Aminolevulinate synthase (ALAS) is a mitochondrial matrix enzyme that catalyzes the first and rate-limiting step of heme biosynthesis. It has been reported that insulin caused the rapid inhibition of housekeeping ALAS transcription, but the mechanism involved in this repression has not been explored. The present study investigates the role of phosphatidylinositol 3-kinase (PI3-kinase) and mitogen-activated protein kinase pathways in insulin signaling relevant to ALAS inhibition. To explore this, we combined the transient overexpression of regulatory proteins involved in these pathways and the use of small cell permeant inhibitors in rat hepatocytes and HepG2 cells. Wortmannin and LY294002, PI3-kinase inhibitors, as well as lovastatin and PD152440, Ras farnesylation inhibitors, and MEK inhibitor PD98059 abolished the insulin repression of ALAS transcription. The inhibitor of mTOR/p70(S6K) rapamycin had no effect whatsoever upon hormone action. The overexpression of vectors encoding constitutively active Ras, MEK, or p90(RSK) mimicked the inhibitory action of insulin. Conversely, negative mutants of PKB, Ras, or MEK impaired insulin inhibition of ALAS promoter activity. Furthermore, inhibition of one of the pathways blocks the inhibitory effect produced by the activation of the other. Our findings suggest that factors involved in two signaling pathways that are often considered to be functionally separate during insulin action, the Ras/ERK/p90(RSK) pathway and the PI3K/PKB pathway, are jointly required for insulin-mediated inhibition of ALAS gene expression in rat hepatocytes and human hepatoma cells.
Scassa
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ME
Laboratorio de Biología Molecular, Departamento de Química Biológica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellón II Piso 4 Ciudad Universitaria, Buenos Aires, 1428, Argentina.
Guberman
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AS
Varone
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CL
Cánepa
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ET
D000624:5-Aminolevulinate Synthetase / Q000235:genetics*; D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D006528:Carcinoma, Hepatocellular; D002478:Cells, Cultured; D002867:Chromones / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015971:Gene Expression Regulation, Enzymologic / Q000502:physiology*; D005822:Genetic Vectors; D022781:Hepatocytes / Q000187:drug effects / Q000201:enzymology*; D006801:Humans; D007328:Insulin / Q000378:metabolism* / Q000494:pharmacology; D008099:Liver / Q000187:drug effects / Q000201:enzymology*; D020935:MAP Kinase Signaling System / Q000235:genetics*; D008297:Male; D009025:Morpholines / Q000494:pharmacology; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D011401:Promoter Regions, Genetic / Q000502:physiology; D017368:Protein Prenylation / Q000187:drug effects / Q000502:physiology; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000235:genetics; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D019893:Ribosomal Protein S6 Kinases / Q000235:genetics; D014158:Transcription, Genetic / Q000187:drug effects / Q000502:physiology; D014404:Tubulin / Q000235:genetics; D014407:Tumor Cells, Cultured; D000077191:Wortmannin; D018631:ras Proteins / Q000378:metabolism
D000624:5-Aminolevulinate Synthetase; D000730:Androstadienes; D002867:Chromones; D004791:Enzyme Inhibitors; D007328:Insulin; D009025:Morpholines; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger; D019893:Ribosomal Protein S6 Kinases; D014404:Tubulin; D000077191:Wortmannin; D018631:ras Proteins; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one