Conjugated linoleic acid (CLA) supplementation has been reported to induce insulin resistance in animals and humans, however, the underlying mechanisms remain unclear. The aim of this study was to examine the direct effects of CLA on leptin and adiponectin secretion, two hormones with actions known to influence insulin sensitivity. Isolated rat adipocytes were incubated with CLA (1-200microM) in the absence and presence of insulin (1.6nM). CLA inhibited both basal and insulin-stimulated leptin gene expression and secretion (-30 to -40%, P<0.05-0.01). CLA also inhibited basal adiponectin production (-20 to -40%, P<0.05-0.01), but not in the presence of insulin. CLA (50-200muM) decreased basal glucose uptake (P<0.05-0.01) and significantly increased the proportion of glucose metabolized to lactate (P<0.01). Insulin treatment partially prevented the inhibitory effects of CLA on glucose uptake and induced a significant increase (P<0.05-0.01) in the percentage of glucose metabolized to lactate. A strong inverse relationship was observed between the increase in the anaerobic utilization of glucose and the decreases of both leptin and adiponectin secretion. In addition, lipolysis and the expression of the adipogenic transcription factor PPARgamma were decreased by CLA. These results indicate that CLA inhibits leptin and adiponectin secretion and suggest that increased anaerobic metabolism of glucose may be involved in these effects. The inhibition of PPARgamma could also mediate the inhibition of adiponectin induced by CLA. Furthermore, the inhibition of leptin and adiponectin production induced by CLA may contribute to insulin resistance observed in CLA-treated animals and humans.

Objective

Leptin, a hormone produced in adipocytes, is a key signal in the regulation of food intake and energy expenditure. Beta-adrenergic agonists have been shown to inhibit leptin gene expression and leptin secretion. The mechanisms underlying the inhibitory effects of beta-adrenergic agonists have not been established. In this study, we examined the effects of Trecadrine, a novel beta3-adrenergic agonist, on basal and insulin-stimulated leptin secretion in isolated rat adipocytes. Because insulin-stimulated glucose metabolism is an important regulator of leptin expression and secretion by the adipocytes, the effects of Trecadrine on indices of adipocyte metabolism were also examined.

Measurements

Isolated adipocytes were incubated with Trecadrine (10(-8)-10(-4) M) in the absence or presence of insulin (1.6 nM). Leptin secretion, glucose utilization, lactate production, glucose incorporation into CO(2) and triglyceride, as well as lipolysis (glycerol release) were determined.

Results

Trecadrine induced a concentration-dependent inhibition of basal leptin secretion. Trecadrine also decreased insulin-stimulated leptin secretion; however, the effect was not as pronounced as in the absence of insulin. Treatment of adipocytes with Trecadrine increased basal glucose utilization and produced a further increase in insulin-stimulated glucose utilization. Basal lactate production was also increased by Trecadrine; however, the proportion (percentage) of glucose carbon released as lactate was unaffected. In the presence of insulin, absolute lactate production was unaffected by Trecadrine at 96 h. However, the percentage of glucose carbon released as lactate was significantly decreased by insulin treatment, and was further decreased by the co-treatment with Trecadrine. Trecadrine induced a dose-dependent increase of the absolute amount of glucose incorporated into triglyceride. However, the percentage of glucose utilized that was incorporated into triglyceride was unaffected by Trecadrine. Trecadrine did not modify the proportion of glucose utilized that was oxidized to CO(2). Trecadrine increased glycerol release after 96 h of treatment. Glycerol release was negatively correlated with leptin secretion.

Conclusions

These results suggest that alterations of glucose metabolism are not directly involved in the effects of beta3-adrenergic agonists to inhibit leptin expression and secretion. The inverse relationship between leptin secretion and the increase of glycerol levels, which is an index of the activation of cAMP-dependent protein kinases, suggests that activation of the cAMP signaling pathway mediates the inhibitory effects of Trecadrine on leptin gene expression and secretion.