The carnitine/acylcarnitine carrier (CAC) is an integral protein of the inner mitochondrial membrane that transports acylcarnitine esters (in exchange for free carnitine) into mitochondria where the acyl groups are oxidized. It therefore plays an essential role in fatty acid β-oxidation, which is the major source of energy for heart and skeletal muscles during fasting and physical exercise. The human CAC is encoded by the SLC25A20 gene that maps to chromosome 3p21.31. Alterations in the SLC25A20 gene are responsible for CAC deficiency, which is an autosomal recessive and nonpopulation-specific disorder presenting an equal male-to-female ratio. CAC deficiency (OMIM 212138) presents two phenotypes: the first and more severe form with early onset in the neonatal period and the second milder form with onset in infancy. Patients usually display a good correlation between phenotype and reduction of CAC activity. Early recognition is crucial for CAC-deficient patients and at present the only long-term treatment consists of fasting prevention with frequent meals, a diet rich in carbohydrates, low in lipids, and supplemented with essential polyunsaturated fatty acids. We have found that CAC mRNA level in HepG2 cells is increased by both statins and fibrates in a dose-dependent manner. Fluvastatin and GW7647 also cause a CAC mRNA increase in rat hepatocytes and myocytes. Furthermore, when they are used in combination, a synergistic effect is observed. Because statins and fibrates are known to act on gene transcription by activating PPARα, we checked whether the PPRE regulatory element is present in the CAC gene promoter. Indeed, this promoter contains a PPRE sequence from -99 to -80 bp, which shares 80.9% identity with the canonical PPRE. The fact that the effects of both statins and fibrates on gene reporter activity are abolished by mutations in the PPRE site clearly indicates that the CAC transcriptional activation by these drugs is mediated by PPRE. In addition, the activation of CAC gene transcription by forskolin provides evidence that the PKA pathway regulates CAC gene expression by phosphorylation of PPARα. Finally, 9-cis-retinoic acid, the ligand of RXRα which heterodimerizes with PPARα for binding to PPRE, enhances CAC gene expression. The results of the present investigation on the regulation of CAC gene expression extend our knowledge of the molecular mechanisms by which statins, fibrates and retinoic acid exert hypolipidemic action. Furthermore, on the basis of our results we propose that patients affected by CAC deficiency, who present a mild phenotype with some residual activity and for whom pharmacological strategies are at present very limited, might benefit from treatment with statins and fibrates acting via stimulation of CAC gene expression.

STATINS, FIBRATES AND RETINOIC ACID UPREGULATE MITOCHONDRIAL ACYLCARNITINE CARRIER GENE EXPRESSION

INFANTINO, VITTORIA;
2010

Abstract

The carnitine/acylcarnitine carrier (CAC) is an integral protein of the inner mitochondrial membrane that transports acylcarnitine esters (in exchange for free carnitine) into mitochondria where the acyl groups are oxidized. It therefore plays an essential role in fatty acid β-oxidation, which is the major source of energy for heart and skeletal muscles during fasting and physical exercise. The human CAC is encoded by the SLC25A20 gene that maps to chromosome 3p21.31. Alterations in the SLC25A20 gene are responsible for CAC deficiency, which is an autosomal recessive and nonpopulation-specific disorder presenting an equal male-to-female ratio. CAC deficiency (OMIM 212138) presents two phenotypes: the first and more severe form with early onset in the neonatal period and the second milder form with onset in infancy. Patients usually display a good correlation between phenotype and reduction of CAC activity. Early recognition is crucial for CAC-deficient patients and at present the only long-term treatment consists of fasting prevention with frequent meals, a diet rich in carbohydrates, low in lipids, and supplemented with essential polyunsaturated fatty acids. We have found that CAC mRNA level in HepG2 cells is increased by both statins and fibrates in a dose-dependent manner. Fluvastatin and GW7647 also cause a CAC mRNA increase in rat hepatocytes and myocytes. Furthermore, when they are used in combination, a synergistic effect is observed. Because statins and fibrates are known to act on gene transcription by activating PPARα, we checked whether the PPRE regulatory element is present in the CAC gene promoter. Indeed, this promoter contains a PPRE sequence from -99 to -80 bp, which shares 80.9% identity with the canonical PPRE. The fact that the effects of both statins and fibrates on gene reporter activity are abolished by mutations in the PPRE site clearly indicates that the CAC transcriptional activation by these drugs is mediated by PPRE. In addition, the activation of CAC gene transcription by forskolin provides evidence that the PKA pathway regulates CAC gene expression by phosphorylation of PPARα. Finally, 9-cis-retinoic acid, the ligand of RXRα which heterodimerizes with PPARα for binding to PPRE, enhances CAC gene expression. The results of the present investigation on the regulation of CAC gene expression extend our knowledge of the molecular mechanisms by which statins, fibrates and retinoic acid exert hypolipidemic action. Furthermore, on the basis of our results we propose that patients affected by CAC deficiency, who present a mild phenotype with some residual activity and for whom pharmacological strategies are at present very limited, might benefit from treatment with statins and fibrates acting via stimulation of CAC gene expression.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11563/22307
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