Peroxisome proliferator-activated receptor (PPAR) alpha and PPARbeta/delta, but not PPARgamma, modulate the expression of genes involved in cardiac lipid metabolism.

Long-chain fatty acids (FA) coordinately induce the expression of a panel of genes involved in cellular FA metabolism in cardiac muscle cells, thereby promoting their own metabolism. These effects are likely to be mediated by peroxisome proliferator-activated receptors (PPARs). Whereas the significance of PPARalpha in FA-mediated expression has been demonstrated, the role of the PPARbeta/delta and PPARgamma isoforms in cardiac lipid metabolism is unknown. To explore the involvement of each of the PPAR isoforms, neonatal rat cardiomyocytes were exposed to FA or to ligands specific for either PPARalpha (Wy-14,643), PPARbeta/delta (L-165041, GW501516), or PPARgamma (ciglitazone and rosiglitazone). Their effect on FA oxidation rate, expression of metabolic genes, and muscle-type carnitine palmitoyltransferase-1 (MCPT-1) promoter activity was determined. Consistent with the PPAR isoform expression pattern, the FA oxidation rate increased in cardiomyocytes exposed to PPARalpha and PPARbeta/delta ligands, but not to PPARgamma ligands. Likewise, the FA-mediated expression of FA-handling proteins was mimicked by PPARalpha and PPARbeta/delta, but not by PPARgamma ligands. As expected, in embryonic rat heart-derived H9c2 cells, which only express PPARbeta/delta, the FA-induced expression of genes was mimicked by the PPARbeta/delta ligand only, indicating that FA also act as ligands for the PPARbeta/delta isoform. In cardiomyocytes, MCPT-1 promoter activity was unresponsive to PPARgamma ligands. However, addition of PPARalpha and PPARbeta/delta ligands dose-dependently induced promoter activity. Collectively, the present findings demonstrate that, next to PPARalpha, PPARbeta/delta, but not PPARgamma, plays a prominent role in the regulation of cardiac lipid metabolism, thereby warranting further research into the role of PPARbeta/delta in cardiac disease.

Peroxisome proliferator-activated receptors: lipid binding proteins controling gene expression.

The peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily. Since their discovery in the beginning of the nineties the three isoforms (PPARalpha, beta/delta and gamma, encoded by different genes) have been implicated in the regulation of almost every single aspect of lipid metabolism and, consequently, in diseases that involve disturbances in lipid metabolism (obesity, diabetes, atherosclerosis, cardiac failure). Although their prominent role in these processes has hardly been disputed, the way in which the activity of these transcription factors is regulated under physiological and pathological conditions awaits further clarification. An unresolved issue has been the nature of the natural ligand of these receptors. Biochemical studies have shown that the PPAR isoforms are rather promiscuous with respect to ligand binding, with a large variety of naturally occurring lipid-like substances acting as low-affinity ligands. More recently this concept has been confirmed by crystallographic studies on the ligand-binding pocket. In addition to ligand availability, the trans-activating capacity likely depends on phosphorylation status of the PPARs and on the recruitment of auxiliary proteins (co-activators and corepressors). Accordingly, the biological activity of these key-regulators of metabolism is controlled at multiple levels, which enables each tissue to fine tune its metabolic machinery to the demands of the body in a specific fashion.