Conjugated linoleic acid in humans: regulation of adiposity and insulin sensitivity

Abstract

Conjugated linoleic acid (CLA) isomers, a group of positional and geometric isomers of linoleic acid [18:2(n-6)], have been studied extensively due to their ability to modulate cancer, atherosclerosis, obesity, immune function and diabetes in a variety of experimental models. The purpose of this review was to examine CLA's isomer-specific regulation of adiposity and insulin sensitivity in humans and in cultures of human adipocytes. It has been clearly demonstrated that specific CLA isomers or a crude mixture of CLA isomers prevent the development of obesity in certain rodent and pig models. This has been attributed mainly to trans-10, cis-12 CLA, both in vivo and in vitro. However, CLA's ability to modulate human obesity remains controversial because data from clinical trials using mixed isomers are conflicting. In support of some studies in humans, our group demonstrated that trans-10, cis-12 CLA prevents triglyceride (TG) accumulation in primary cultures of differentiating human preadipocytes. In contrast, cis-9, trans-11 CLA increases TG content. Closer examination has revealed that CLA's antiadipogenic actions are due, at least in part, to regulation of glucose and fatty acid uptake and metabolism. This review presents our current understanding of potential isomer-specific mechanisms by which CLA reduces human adiposity and insulin sensitivity.

FIGURE 1

Schematic representation of the mechanism by which peroxisome proliferator-activated receptor (PPAR) γ activation promotes adipocyte differentiation, insulin sensitivity and lipid accumulation in adipocytes. Activation of PPARγ by natural ligands (NL; i.e., eicosanoids, oxidized lipids, PUFA) or synthetic ligands (SL; i.e., thiazolidinediones, tyrosine analogs) initiates heterodimerization with the retinoid X receptor (RXR), which requires ligand activation by 9-cis-retinoic acid (RA). This obligate heterodimeric complex binds to peroxisome proliferator-activated receptor response elements (PPRE) in the promoters of target genes. Once bound to the promoter, the recruitment of coactivator complexes, which contain histone acetyl-transferase activity, modifies the nucleosome structure to allow for the basal transcriptional machinery to gain access to target gene promoters. Subsequent transactivation of CAAT/enhancer binding protein-α (C/ EBPα) and signal transducers and activators of transcription (STAT) 1, 5A and 5B promotes adipocyte differentiation. Activation of PPARγ promotes insulin-stimulated glucose uptake by increasing the expression of downstream targets including the insulin-stimulated glucose transporter 4 (GLUT4), phosphatidylinositol 3-kinase (PI-3-Kinase), cbl-associated protein (CAP), insulin receptor substrate (IRS) 1 and 2, and adiponectin, and by decreasing the expression of resistin. Furthermore, PPARγ activation promotes lipid storage by increasing the expression of the adipocyte fatty acid binding protein (aP2), acyl-CoA binding protein (ACBP), lipoprotein lipase (LPL), acyl-CoA synthetase (ACS), fatty acid transport protein (FATP), fatty acid translocase (CD36) and phosphoenol pyruvate carboxykinase (PEPCK), and by decreasing the expression of leptin and tumor necrosis factor-α (TNFα).

FIGURE 2

Proposed mechanism by which trans-10, cis-12 conjugated linoleic acid (CLA) decreases triglyceride (TG) accumulation in differentiating human (pre)adipocytes. CLA enters the cell through an unidentified mechanism of transport, and is shuttled into different regulatory compartments. Mechanism #1: CLA can be esterified into TG-rich lipid droplets, where it is likely to have little regulatory role, and could potentially increase TG stores. Mechanism #2: CLA can be esterified into the membrane phospholipids bi-layer, where it could alter membrane fluidity and membrane-associated signal transduction cascades. Mechanism #3: CLA can alter the abundance or activity of a currently unidentified transcription factor (TFX), which results in transcriptional repression of the peroxisome proliferator-activated receptor (PPAR) γ and its downstream targets lipoprotein lipase (LPL), acyl-CoA binding protein (ACBP), adipocyte fatty acid binding protein (aP2), the insulin-stimulated glucose transporter 4 (GLUT4) and leptin. In addition, by altering the activity or abundance of TFX, CLA decreases the expression of stearoyl-CoA desaturase 1 (SCD-1) and acetyl-CoA carboxylase (ACC), which is likely independent of CLA’s ability to reduce PPARγ signaling. By down-regulating GLUT4, ACC, and SCD-1, CLA attenuates insulin-stimulated glucose uptake, malonyl-CoA synthesis, and oleate synthesis, respectively, collectively decreasing de novo fatty acid synthesis. Decreased malonyl-CoA would impair fatty acid (FA) elongation into long-chain, unsaturated FA, limiting their availability for eicosanoid production and cell signaling, including PPARγ activation. By down-regulating LPL, ACBP and aP2, CLA attenuates FA uptake and alters intracellular trafficking to attenuate esterification into TG. Alternatively, CLA-induced alterations of FA metabolism could decrease synthesis of endogenous ligands (i.e., eicosanoids) for TFX or PPARγ.

FIGURE 3

Schematic model of insulin-stimulated glucose uptake in adipocytes. Under normal conditions, insulin binds to the extracellular α subunits of the insulin receptor (IR), which leads to intracellular autophosphorylation of β subunits of the IR. Subsequent tyrosine phosphorylation of the insulin receptor substrate (IRS) family of proteins leads to the phosphorylation and activation of phosphatidylinositol 3-kinase (PI-3K). Phosphorylation of PI-3K leads to the activation of the phosphoinositide-dependent protein kinase (PDK), which phosphorylates and activates downstream serine/threonine kinases protein kinase C (PKC) and protein kinase B (Akt/PKB). This cascade results in the translocation of the insulin-stimulated glucose transporter 4 (GLUT4) to the plasma membrane, where it facilitates intracellular flux of glucose. In addition to this PI-3K–dependent pathway, the IR can signal independently of PI-3K to stimulate GLUT4 translocation. In this case, IR autophoshorylation can lead to the tyrosine phosphorylation of c-cbl, and association of the cbl-associated protein (CAP). Subsequent binding of the SH2/SH3-containing adapter protein CrkII and the guanyl nucleotide exchange factor (C3G) activities the Rho family TC10. Activation of the TC10 plays a crucial role in the translocation of GLUT4 to the plasma membrane. Activation of both the PI-3K dependent and independent pathways is necessary for insulin-stimulated glucose uptake in adipocytes. Thiazolidinediones (TZD) increase insulin sensitivity in part by activating peroxisome proliferator-activated receptor (PPAR) γ-dependent transactivation of GLUT-4 and CAP genes. We contend that trans-10, cis-12 CLA (t10,c12-CLA)–induced insulin resistance involves antagonism of PPARγ in human adipocytes. In support of this concept, t10, c12-CLA diminishes the expression and TZD-induced activity of PPARγ (22). By antagonizing PPARγ signaling, t10, c12-CLA may indirectly reduce GLUT4 expression and insulin-stimulated glucose uptake.