Dietary regulation of nuclear receptors in obesity-related metabolic syndrome.

In Western culture, excess visceral fat accumulation or obesity has reached epidemic proportions, resulting in metabolic syndrome. However, more than 10 years of research has shown that adipocytes also function as endocrine cells that release various bioactive substances, so called "adipocytokines or adipokines", that play a major role in the regulation of food intake, insulin sensitivity, energy metabolism, and the vascular microenvironment. Adiponectin, an adipocytokine, is considered to improve insulin sensitivity. Recently, monocyte chemoattractant protein (MCP)-1 has been reported to be a novel adipocytokine involved in the development of obesity-associated insulin resistance and atherosclerosis. Nuclear receptors, especially peroxisome proliferator-activated receptor-alpha (PPAR alpha) and PPAR gamma are ligand-activated transcription factors that regulate the metabolism of glucose and lipids. PPAR gamma is strongly expressed in adipocytes and plays a significant role in the transcriptional activation of adipocytokines including adiponectin. PPAR alpha, another PPAR isoform, is involved in the control of lipid metabolism in the liver and skeletal muscle. PPAR alpha activation causes lipid clearance via beta-oxidation enhancement. We showed that various dietary terpenoids and other natural ingredients regulate the transcription of PPAR target genes, induces the expression and secretion of adiponectin, and inhibits those of MCP-1 in adipocytes and beta-oxidation in liver. These findings indicate that dietary factor acts as an agonist of PPARs and is a valuable medical and food component for the gradual improvement of metabolic syndrome.