Obesity is a complex disease effected by both genetic and environmental factors. The type of fat ingested seems to be an important determinant in the development or prevention of obesity. Several studies have suggested there are antiobesity properties of conjugated linoleic acid (CLA), a group of positional and geometric fatty acid isomers derived from linoleic acid (18:2 (n-6)). Leptin is a hormone secreted by the adipocytes that is implicated in the regulation of body weight via its central actions on food intake and energy expenditure. Leptin has been shown to be regulated by dietary macronutrients, and therefore the aim of the present study was to determine the direct effects of CLA on leptin secretion and expression in cultured primary adipocytes. Because glucose metabolism plays an important role in the regulation of leptin, the effects of CLA on glucose uptake and lactate production were also studied.
Epididymal fat pads were obtained from Wistar rats. All procedures were conducted according to the University Ethics Committee for the use of laboratory animals and the animals were humanely killed. Adipocytes were isolated by collagenase digestion and cultured with DMEM in the absence or presence of insulin plus several concentrations of CLA (1-200 µM). Leptin release into the media was determined by RIA, while leptin gene expression was analysed by Northern blot. Glucose and lactate were measured with a Cobas Autoanalyser.
Basal leptin secretion (3.9 ± 1.0 ng ml-1, mean ± S.E.M.) was inhibited (30-50 %) by all CLA concentrations tested. As expected, insulin caused a significant increase in leptin secretion (5.4 ± 0.9, P < 0.001). When adipocytes were co-treated with CLA in the presence of insulin, a significant dose-response decrease in the insulin-stimulated leptin production was observed (4.3 ± 1.0 (P < 0.05), 4.1 ± 1.0, 3.4 ± 0.7, 3.1 ± 0.7, 2.9 ± 0.8 (P < 0.001), at concentrations of 1, 10, 50, 100 and 200 µM, respectively). A similar inhibitory action of CLA was observed on leptin gene expression.
Basal glucose uptake (4.2 ± 0.8 µmol) was decreased (20-32%) by all CLA concentrations tested. Insulin induced a significant increase in glucose uptake (81%, P < 0.001), and CLA induced a slight but not statistically significant decrease in the insulin-stimulated glucose utilisation. Lactate production was not significantly affected by CLA, although the percentage of glucose released as lactate, which has been shown to be inversely correlated to leptin secretion, was significantly increased by CLA treatment, both in the absence (P < 0.01 for all doses tested) or presence of insulin (P < 0.01 for 50-200 µM CLA).
In summary, this study shows that the type of dietary fat can affect the regulation of genes implicated in the regulation of the body weight such as leptin. The inhibitory action of CLA on leptin secretion and expression may be mediated at least in part by the alterations induced in glucose uptake and metabolism, but further studies are needed to better understand the molecular mechanisms implicated in CLA actions on leptin.