Orexin A but not orexin B regulates lipid metabolism and leptin secretion in isolated porcine adipocytes

It is well known that orexins are involved in the metabolism and endocrine function of rodent adipocytes, but there are no data on other animal species, including pigs. Therefore, in this study, we tested the hypothesis that orexin A (OxA) and orexin B (OxB) modulate the metabolism and endocrine functions of isolated porcine adipocytes and adipose tissue explants. Moreover, we characterized the possible mechanism of OxA action in porcine adipocytes. According to the results, both orexin receptor 1 and orexin receptor 2 were expressed in the porcine adipose tissue. We found that OxA suppressed the release of glycerol from porcine adipocytes both in the absence (basal lipolysis; P < 0.05) and in the presence (stimulated lipolysis; P < 0.05) of isoproterenol. Orexin A increased basal and insulin-stimulated glucose uptake (P < 0.05), as well as it enhanced the rate of glucose incorporation into lipids with insulin (stimulated lipogenesis; P < 0.01) or without insulin (basal; P < 0.05). We have also shown that OxA stimulated the mRNA expression of glucose transporter 4 (P < 0.05) and its translocation into the plasma membrane (P < 0.01). Moreover, OxA upregulated the mRNA expression of leptin in isolated porcine adipocytes (P < 0.05) and increased the secretion of leptin (P < 0.05). We have also demonstrated one of the possible mechanisms of OxA action in adipocytes. In the presence of extracellular-signal-regulated kinase 1 and 2 (ERK1/2) inhibitor, the effect of OxA was not detectable in porcine adipocytes, which indicates that this peptide increased cell viability via ERK1/2 pathway (P < 0.05). However, OxB did not show any effect on the metabolism and endocrine functions of porcine adipocytes. In summary, we have shown for the first time that OxA has a significant impact on the intensity of lipolysis, glucose uptake, lipogenesis, as well as on the expression and secretion of leptin. Therefore, we conclude that OxA but not OxB regulates lipid metabolism in porcine adipose tissue and that this regulation is partly mediated via ERK1/2 pathway. The action of orexins should be further explored to better understand their role in the regulation of adiposity in pigs.