Free Fatty Acids Repress the GLUT4 Gene Expression in Cardiac Muscle via Novel Response Elements

Hyperlipidemia (HL) impairs cardiac glucose homeostasis, but the molecular mechanisms involved are yet unclear. We examined HL-regulated GLUT4 and peroxisome proliferator-activated receptor (PPAR) γ gene expression in human cardiac muscle. Compared with control patients, GLUT4 protein levels were 30% lower in human cardiac muscle biopsies from patients with HL and/or type 2 diabetes mellitus, whereas GLUT4 mRNA levels were unchanged. PPARγ mRNA levels were 30-50% lower in patients with HL and/or diabetes mellitus type 2 than in controls. Reporter studies in H9C2 cardiomyotubes showed that HL in vitro, induced by high levels of arachidonic (AA) stearic, linoleic, and oleic acids (24 h, 200 μm) repressed transcription from the GLUT4 promoter; AA also repressed transcription from the PPARγ1 and PPARγ2 promoters. Co-expression of PPARγ2 repressed GLUT4 promoter activity, and the addition of AA further enhanced this effect. 5′-Deletion analysis revealed three GLUT4 promoter regions that accounted for AA-mediated effects: two repression-mediating sequences at -443/-423 bp and -222/-197 bp, the deletion of either or both of which led to a partial derepression of promoter activity, and a third derepression-mediating sequence at -612/-587 bp that was required for sustaining this derepression effect. Electromobility shift assay further shows that AA enhanced binding to two of the three regions of cardiac nuclear protein(s), the nature of which is still unknown. We propose that HL, exhibited as a high free fatty acid level, modulates GLUT4 gene expression in cardiac muscle via a complex mechanism that includes: (a) binding of AA mediator proteins to three newly identified response elements on the GLUT4 promoter gene and (b) repression of GLUT4 and the PPARγ genes by AA.