Eicosapentaenoic acid, but not oleic acid, stimulates beta-oxidation in adipocytes.

The beneficial roles of dietary fish oil in lowering serum TAG levels in animals and humans have been attributed in part to the high content of two n-3 polyunsaturated very long-chain FA, EPA, and DHA. Recent studies show that EPA induces mitochondrial beta-oxidation in hepatocytes, which might contribute to the systemic lipid-lowering effect. Whether EPA affects FA storage or oxidation in adipocytes is not clear. To investigate this possibility, 3T3-L1 adipocytes incubated with EPA (100 microM) for 24 h were assayed for beta-oxidation, carnitine palmitoyl transferase 1 (CPT-1) activity, protein, and mRNA expression of CPT-1. For comparison, cells treated with oleic acid, octanoic acid, and clofibrate, a synthetic ligand for peroxisome proliferator-activated receptor alpha were also analyzed. Mitochondria were isolated by differential centrifugation, and the mitochondrial membrane acyl chain composition was measured by GLC. EPA increased the oxidation of endogenous FA but did not inhibit lipogenesis. Oleic acid and clofibrate did not affect FA oxidation or lipogenesis, whereas octanoic acid suppressed the oxidation of endogenous FA and inhibited lipogenesis. Increased beta-oxidation by EPA was associated with increased CPT-1 activity but without changes in its mRNA and protein expression. EPA treatment increased the percentage of this FA in the mitochondrial membrane lipids. We suggest that EPA increased the activity of CPT-1 and beta-oxidation in adipocytes by altering the structure or dynamics of the mitochondrial membranes.

Medium-chain Fatty acids attenuate agonist-stimulated lipolysis, mimicking the effects of starvation.

OBJECTIVE: To test the hypothesis that incorporation of medium-chain fatty acids (FAs) into adipocyte triglycerides alters intracellular lipolysis. RESEARCH METHODS AND PROCEDURES: 3T3-L1 adipocytes were pretreated with octanoate for various incubation periods. After the removal of exogenous FAs, cells were incubated with different lipolytic agonists. To determine the effects on lipolysis, we measured the following: the release of glycerol and FAs, lipase activity, protein levels of hormone-sensitive lipase (HSL), and perilipin A; translocation of HSL; phosphorylation of perilipin A; and levels of cellular adenosine triphosphate, cyclic adenosine monophosphate, and H2O2. To compare the effects of starvation with those caused by octanoate pretreatment, we measured glycerol release and H2O2 generation in rat adipocytes of starved donors. RESULTS: Pretreatment of adipocytes with octanoate in vitro increased basal lipolysis but decreased the cellular response for agonists. The same effects were seen in starvation in vivo. Preincubation with octanoate for 48 hours did not affect basal lipase activity, HSL, and perilipin protein levels, but it reduced agonist-stimulated perilipin phosphorylation and HSL translocation toward fat droplets. This was associated with a reduction in basal cellular adenosine triphosphate levels and agonist-stimulated cyclic adenosine monophosphate generation. Starvation and octanoate pretreatment both increased intracellular H2O2 concentrations, which might also contribute to the inhibition on agonist-stimulated lipolysis. DISCUSSION: Pretreatment with octanoate seems to induce changes in adipocyte lipolysis in a pattern mimicking the effects of starvation. Such changes could contribute, in part, to weight loss in animals and humans associated with dietary medium-chain FAs.

Octanoate inhibits triglyceride synthesis in 3T3-L1 and human adipocytes.

To understand how medium-chain fatty acids (FA) influence lipid metabolism in adipocytes, we studied the effects of octanoate on the oxidation of glucose and endogenous palmitate, cellular O(2) consumption, mitochondrial membrane potential, lipid synthesis from long-chain FA, glucose and lactate. We found that octanoate significantly suppressed the esterification of oleate into triglycerides (TG) in both 3T3-L1 and human adipocytes. Octanoate also significantly suppressed de novo FA synthesis. These effects were associated with octanoate-mediated reductions in the activities of acyl CoA:1,2-diacylglycerol acyltransferase (DGAT) and acetyl CoA carboxylase (ACC). Cells pretreated with octanoate had reduced mRNA levels for a number of lipid metabolism genes, including of DGAT, ACC and stearoyl CoA desaturase-1. On the other hand, octanoate did not acutely perturb cellular O(2) consumption or mitochondrial membrane potential. Together, these results suggest that octanoate affected adipocyte function by reducing TG synthesis but not by enhancing oxidation.