Insulin Stimulates Long-Chain Fatty Acid Utilization by Rat Cardiac Myocytes Through Cellular Redistribution of FAT/CD36

Abstract

The existence of an intracellular pool of fatty acid translocase (FAT/CD36), an 88-kDa membrane transporter for long-chain fatty acids (FAs), and the ability of insulin to induce translocation events prompted us to investigate the direct effects of insulin on cellular uptake of FA by the heart. Insulin (0.1 nmol/l and higher) increased FA uptake by isolated rat cardiac myocytes by 1.5-fold. This insulin-induced increase in FA uptake was completely blocked by phloretin, sulfo-N-succinimidylpalmitate ( T he heart is among the most sensitive organs to insulin action. This hormone has multiple effects on cardiac intermediary metabolism, one of the best described being the stimulation of glucose transport through translocation of GLUT4 from intracellular vesicular structures to the cardiac sarcolemma (1). Compared with insulin's effects on glucose utilization by the heart, little information is available about its effects on cardiac long-chain fatty acid (FA) utilization. Insulin is known to decrease FA oxidation through inhibition of carnitine palmitoyltransferase I, the effect of which is most likely mediated through increased intracellular levels of malonyl-CoA (2,3). In skeletal muscle, the insulin-mediated drop in FA oxidation is accompanied by an increase in FA esterification into intracellular triacylglycerol stores (4). Because the stimulation of FA esterification exceeded the inhibition of FA oxidation, it can be deduced that the net effect on FA uptake was stimulatory and expected to lead to accumulation of neutral lipid in muscle (4).There is essentially no information available about the potential influence of insulin on transport of FA across the cardiac sarcolemma. A regulatory role of insulin on FA transport has not been considered because of the generally accepted notion that cellular uptake of FA is governed by passive diffusion and merely dependent on the metabolic demands of the tissue (5,6). However, the accumulating evidence that membrane-associated proteins are involved in cellular FA uptake (7,8) opens the possibility that insulin may also influence FA utilization at the level of transsarcolemmal transport. There is functional evidence for the involvement in FA transport of three distinct sarcolemmal proteins: the 43-kDa plasma membrane FA binding protein (FABPpm), the 62-kDa FA transport protein (FATP), and the 88-kDa FA translocase (FAT), the rat homologue of human CD36 (9,10). Using cardiac myocytes (7) and heart giant vesicles (11), we found that the bulk of cardiac FA uptake is mediated by the FA transporter FAT/CD36. Furthermore, the generation of transgenic mouse models for FAT/CD36 helped to reveal the in vivo importance of this transporter in cardiac and skeletal muscle FA utilization. First, muscle-specific FAT/CD36 overexpressor mice have demonstrated lowered plasma triacylglycerols and FA concentrations together with elevated glucose and insulin levels (12). Second, CD36 knockout mice exhibit elevated levels of plasma FA and

Heterogeneity in limb fatty acid kinetics in type 2 diabetes

“…The decrease of palmitate uptake with insulin has been reported previously for the leg [7,30] and the arm [31] of healthy subjects. Nevertheless, when fatty acid levels were maintained during the insulin infusion, the fatty acid uptake by skeletal muscle was actually increased in rat muscle preparations [32,33] and in cultured cells from the vastus lateralis muscle [28,29]. These observations imply that the reduction in limb fatty acid uptake with insulin is caused more by reduced limb fatty acid availability than by lessened skeletal muscle insulin sensitivity.…”

Insulin stimulates fatty acid transport by regulating expression of FAT/CD36 but not FABPpm

“…In the LDM fraction there was no mitochondrial contamination, as determined by the absence of the mitochondrial enzyme cytochrome oxidase (data not shown). We have previously reported that the PM fraction was enriched with the ouabain-sensitive p-nitrophenylphosphatase (13.5-fold enrichment) accompanied by the decrease in sarcoplasmatic EGTA-sensitive Ca 2ϩ -ATPase (3.6-fold decline) (19).…”

“…To examine the effects of 2-h insulin (10 nM) exposure on palmitate uptake by cardiac myocytes, we used the procedure of Luiken et al (19). For these purposes, 0.6 ml of a [1-14 C]palmitate-BSA complex was added to 1.8 ml of cardiomyocyte suspension (final palmitate concentration 100 M, palmitate/BSA ratio of 0.3).…”

“…Cardiac myocytes were isolated from male adult rats according to the procedure described by Fisher et al (11), as modified by Luiken et al (19). Briefly, rats were anesthetized using Somnotol (50 -60 mg/100 g ip) combined with heparin (300 IU/100 g ip).…”

“…In recent years, it has been shown that rates of LCFA transport can be regulated acutely and chronically. In heart and skeletal muscle exposed briefly to insulin (15 min) and in contracting muscle (Ͻ30 min), rates of LCFA transport are increased due to the translocation of FAT/CD36 from an intracellular depot to the plasma membrane (4,18,19). Expression of FAT/CD36 mRNA is upregulated by LCFAs in neonatal cardiac myocytes (30), although the functional metabolic consequences of these changes on LCFA transport and metabolism have not been determined.…”