Hepatic regulation of fatty acid synthase by insulin and T3: evidence for T3 genomic and nongenomic actions.
Fatty acid synthase (FAS) is a key enzyme of hepatic lipogenesis responsible for the synthesis of long-chain saturated fatty acids. This enzyme is mainly regulated at the transcriptional level by nutrients and hormones. In particular, glucose, insulin, and T(3) increase FAS activity, whereas glucagon and saturated and polyunsaturated fatty acids decrease it. In the present study we show that, in liver, T(3) and insulin were able to activate FAS enzymatic activity, mRNA expression, and gene transcription. We localized the T(3) response element (TRE) that mediates the T(3) genomic effect, on the FAS promoter between -741 and -696 bp that mediates the T(3) genomic effect. We show that both T(3) and insulin regulate FAS transcription via this sequence. The TRE binds a TR/RXR heterodimer even in the absence of hormone, and this binding is increased in response to T(3) and/or insulin treatment. The use of H7, a serine/threonine kinase inhibitor, reveals that a phosphorylation mechanism is implicated in the transcriptional regulation of FAS in response to both hormones. Specifically, we show that T(3) is able to modulate FAS transcription via a nongenomic action targeting the TRE through the activation of a PI 3-kinase-ERK1/2-MAPK-dependent pathway. Insulin also targets the TRE sequence, probably via the activation of two parallel pathways: Ras/ERK1/2 MAPK and PI 3-kinase/Akt. Finally, our data suggest that the nongenomic actions of T(3) and insulin are probably common to several TREs, as we observed similar effects on a classical DR4 consensus sequence.
Radenne
Anne
A
Département des Sciences Biologiques, Centre de recherche BioMed, Université du Québec, CP 8888, Succursale Centreville, Montreal, Canada H36 3P8.
Akpa
Murielle
M
Martel
Caroline
C
Sawadogo
Sabine
S
Mauvoisin
Daniel
D
Mounier
Catherine
C
D000818:Animals; D015153:Blotting, Western; D002478:Cells, Cultured; D002642:Chick Embryo; D024202:Electrophoretic Mobility Shift Assay; D048049:Extracellular Signal-Regulated MAP Kinases / Q000502:physiology; D064429:Fatty Acid Synthases / Q000235:genetics / Q000378:metabolism*; D007328:Insulin / Q000494:pharmacology*; D008099:Liver / Q000187:drug effects / Q000201:enzymology / Q000502:physiology*; D020928:Mitogen-Activated Protein Kinases / Q000502:physiology; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D010957:Plasmids / Q000235:genetics; D011401:Promoter Regions, Genetic / Q000235:genetics; D014176:Protein Biosynthesis / Q000187:drug effects; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D020218:Response Elements; D015398:Signal Transduction / Q000502:physiology; D014162:Transfection; D014284:Triiodothyronine / Q000235:genetics / Q000494:pharmacology*
D007328:Insulin; D012333:RNA, Messenger; D014284:Triiodothyronine; D064429:Fatty Acid Synthases; D019869:Phosphatidylinositol 3-Kinases; D048049:Extracellular Signal-Regulated MAP Kinases; D020928:Mitogen-Activated Protein Kinases