Muscle insulin resistance: assault by lipids, cytokines and local macrophages.

PURPOSE OF REVIEW: The present review outlines possible mechanisms by which high fatty acids, associated with high-fat diet and obesity, impose insulin resistance on glucose uptake into skeletal muscle. RECENT FINDINGS: It is well established that muscle insulin resistance arises in conditions of high-fatty acid availability, and correlates with accumulation of triglycerides within skeletal muscle fibres. However, it is debated whether triglycerides or other lipid metabolites such as diacylglycerols and ceramides are directly responsible. These lipid metabolites can activate serine kinases that impair insulin signalling. Accumulation of acylcarnitines and reactive oxygen species could be additional causative agents of insulin resistance. Further, the precise defects in insulin signalling in muscle caused by high intramuscular lipid (i.e. lipotoxicity) remain unclear. In parallel, proinflammatory activation within the adipose tissue of obese and high-fat fed animals or humans causes muscle insulin resistance, and is ascribed to circulating inflammatory cytokines. Recent evidence also shows proinflammatory macrophages infiltrating muscle tissue and/or intermuscular adipose tissue, and there is growing evidence that fatty acids trigger macrophages to secrete factors that directly impair insulin actions. These factors are postulated to activate stress-signalling pathways in muscle that act on the same insulin-signalling components affected by lipotoxicity. SUMMARY: Altered intramuscular lipid metabolism, circulating cytokines, and inflammatory macrophage infiltration of muscle tissue have been recently linked to muscle insulin resistance provoked by fatty acids. Each is analysed separately in this review, but they may act simultaneously and synergistically to render skeletal muscle insulin-resistant.

Protein kinase D is a key regulator of cardiomyocyte lipoprotein lipase secretion after diabetes.

The diabetic heart switches to exclusively using fatty acid (FA) for energy supply and does so by multiple mechanisms including hydrolysis of lipoproteins by lipoprotein lipase (LPL) positioned at the vascular lumen. We determined the mechanism that leads to an increase in LPL after diabetes. Diazoxide (DZ), an agent that decreases insulin secretion and causes hyperglycemia, induced a substantial increase in LPL activity at the vascular lumen. This increase in LPL paralleled a robust phosphorylation of Hsp25, decreasing its association with PKCdelta, allowing this protein kinase to phosphorylate and activate protein kinase D (PKD), an important kinase that regulates fission of vesicles from the golgi membrane. Rottlerin, a PKCdelta inhibitor, prevented PKD phosphorylation and the subsequent increase in LPL. Incubating control myocytes with high glucose and palmitic acid (Glu+PA) also increased the phosphorylation of Hsp25, PKCdelta, and PKD in a pattern similar to that seen with diabetes, in addition to augmenting LPL activity. In myocytes in which PKD was silenced or a mutant form of PKCdelta was expressed, high Glu+PA were incapable of increasing LPL. Moreover, silencing of cardiomyocyte Hsp25 allowed phorbol 12-myristate 13-acetate to elicit a significant phosphorylation of PKCdelta, an appreciable association between PKCdelta and PKD, and a vigorous activation of PKD. As these cells also demonstrated an additional increase in LPL, our data imply that after diabetes, PKD control of LPL requires dissociation of Hsp25 from PKCdelta, association between PKCdelta and PKD, and vesicle fission. Results from this study could help in restricting cardiac LPL translocation, leading to strategies that overcome contractile dysfunction after diabetes.

Acute intralipid infusion reduces cardiac luminal lipoprotein lipase but recruits additional enzyme from cardiomyocytes.

OBJECTIVE: Lipoprotein lipase (LPL) metabolizes the triglyceride (TG) core of lipoproteins. We evaluated whether circulating lipids can regulate LPL by influencing the transfer of enzyme from the myocyte to the endothelial lumen. METHODS: Acute intralipid (IL, 10% and 20%) infusion was performed in male Wistar rats. After 3 h, insulin resistance was assessed using a euglycemic hyperinsulinemic clamp. Cardiac LPL activity was determined by retrogradely perfusing the hearts with heparin. Immunogold electron microscopy visualized LPL, and heparanase was detected by immunofluorescence. Cardiac myocytes were also isolated, and heparin-releasable LPL activity was measured. RESULTS: IL infusion increased both plasma and cardiac lipids. Circulating basal plasma LPL activity increased for the duration of the infusion. Compared to control (CON) hearts, there was a substantial decrease in heparin-releasable LPL activity at the vascular lumen following 3 h of IL infusion, an effect unrelated to changes in gene and protein expression or whole-body insulin resistance. Although constant perfusion of CON hearts with heparin stripped off most of the luminal bound LPL, hearts from IL-infused animals continued to release excessive amounts of the enzyme, suggesting buildup of LPL within endothelial cells or at the endothelial basolateral surface. Immunogold labeling confirmed this observation and demonstrated robust anti-LPL staining at these sites, only in IL hearts. Perfusing hearts from IL-rats in vitro, in the absence of TG, allowed the accumulated enzyme pool to transfer to the coronary lumen. CONCLUSION: Our data suggest that acute amplification of lipids reduces cardiac luminal LPL but facilitates additional recruitment of cardiomyocyte enzyme. Should this mechanism occur globally, it could contribute towards management of hyperlipidemia.

Induction of mitochondrial nitrative damage and cardiac dysfunction by chronic provision of dietary omega-6 polyunsaturated fatty acids.

Increased awareness of obesity has led to a dietary shift toward "heart-friendly" vegetable oils containing omega-6 polyunsaturated fatty acid (omega-6 PUFA). In addition to its beneficial effects, omega-6 PUFA also exhibits proinflammatory and prooxidative properties. We hypothesized that chronic dietary omega-6 PUFA can induce free radical generation, predisposing the cardiac mitochondria to oxidative damage. Male Wistar rats were fed a diet supplemented with 20% w/w sunflower oil, rich in omega-6 PUFA (HP) or normal laboratory chow (LP) for 4 weeks. HP feeding augmented phospholipase A(2) activity and breakdown of cardiolipin, a mitochondrial phospholipid. HP hearts also demonstrated elevated inducible nitric oxide synthase expression, loss of Mn superoxide dismutase, and increased mitochondrial nitrotyrosine levels. In these hearts, oxidative damage to mitochondrial DNA (mDNA) was demonstrated by 8-hydroxyguanosine immunopositivity, overexpression of DNA repair enzymes, and a decrease in the mRNA expression of specific respiratory subunits encoded by the mDNA. Functionally, at higher workloads, HP hearts also demonstrated a greater decline in cardiac work than LP, suggesting a compromised mitochondrial reserve. Our study, for the first time, demonstrates that consumption of a high fat diet rich in omega-6 PUFA for only 4 weeks instigates mitochondrial nitrosative damage and causes cardiac dysfunction at high afterloads.

Palmitate-activated macrophages confer insulin resistance to muscle cells by a mechanism involving protein kinase C θ and ε.

BACKGROUND: Macrophage-derived factors contribute to whole-body insulin resistance, partly by impinging on metabolically active tissues. As proof of principle for this interaction, conditioned medium from macrophages treated with palmitate (CM-PA) reduces insulin action and glucose uptake in muscle cells. However, the mechanism whereby CM-PA confers this negative response onto muscle cells remains unknown. METHODOLOGY/PRINCIPAL FINDINGS: L6-GLUT4myc myoblasts were exposed for 24 h to palmitate-free conditioned medium from RAW 264.7 macrophages pre-treated with 0.5 mM palmitate for 6 h. This palmitate-free CM-PA, containing selective cytokines and chemokines, inhibited myoblast insulin-stimulated insulin receptor substrate 1 (IRS1) tyrosine phosphorylation, AS160 phosphorylation, GLUT4 translocation and glucose uptake. These effects were accompanied by a rise in c-Jun N-terminal kinase (JNK) activation, degradation of Inhibitor of κBα (IκBα), and elevated expression of proinflammatory cytokines in myoblasts. Notably, CM-PA caused IRS1 phosphorylation on Ser1101, and phosphorylation of novel PKCθ and ε. Co-incubation of myoblasts with CM-PA and the novel and conventional PKC inhibitor Gö6983 (but not with the conventional PKC inhibitor Gö6976) prevented PKCθ and ε activation, JNK phosphorylation, restored IκBα mass and reduced proinflammatory cytokine production. Gö6983 also restored insulin signalling and glucose uptake in myoblasts. Moreover, co-silencing both novel PKC θ and ε isoforms in myoblasts by RNA interference, but not their individual silencing, prevented the inflammatory response and restored insulin sensitivity to CM-PA-treated myoblasts. CONCLUSIONS/CLINICAL SIGNIFICANCE: The results suggest that the block in muscle insulin action caused by CM-PA is mediated by novel PKCθ and PKCε. This study re-establishes the participation of macrophages as a relay in the action of fatty acids on muscle cells, and further identifies PKCθ and PKCε as key elements in the inflammatory and insulin resistance responses of muscle cells to macrophage products. Furthermore, it portrays these PKC isoforms as potential targets for the treatment of fatty acid-induced, inflammation-linked insulin resistance.

Endothelial heparanase secretion after acute hypoinsulinemia is regulated by glucose and fatty acid.

Following diabetes, the heart increases its lipoprotein lipase (LPL) at the coronary lumen by transferring LPL from the cardiomyocyte to the endothelial lumen. We examined how hyperglycemia controls secretion of heparanase, the enzyme that cleaves myocyte heparan sulphate proteoglycan to initiate this movement. Diazoxide (DZ) was used to decrease serum insulin and generate hyperglycemia. A modified Langendorff technique was used to separate coronary from interstitial effluent, which were assayed for heparanase and LPL. Within 30 min of DZ, interstitial heparanase increased, an effect that closely mirrored an augmentation in interstitial LPL. Endothelial cells were incubated with palmitic acid (PA) or glucose, and heparanase secretion was determined. PA increased intracellular heparanase, with no effect on secretion of this enzyme. Unlike PA, glucose dose-dependently lowered endothelial intracellular heparanase, which was strongly associated with increased heparanase activity in the incubation medium. Preincubation with cytochalasin D or nocodazole prevented the high glucose-induced depletion of intracellular heparanase. Our data suggest that following hyperglycemia, translocation of LPL from the cardiomyocyte cell surface to the apical side of endothelial cells is dependent on the ability of the fatty acid to increase endothelial intracellular heparanase followed by rapid secretion of this enzyme by glucose, which requires an intact microtubule and actin cytoskeleton.

AMP-activated protein kinase confers protection against TNF-{alpha}-induced cardiac cell death.

AIMS: Although a substantial role for 5' adenosine monophosphate-activated protein kinase (AMPK) has been established in regulating cardiac metabolism, a less studied action of AMPK is its ability to prevent cardiac cell death. Using established AMPK activators like dexamethasone (DEX) or metformin (MET), the objective of the present study was to determine whether AMPK activation prevents tumour necrosis factor-alpha (TNF-alpha) induced apoptosis in adult rat ventricular cardiomyocytes. METHODS AND RESULTS: Cardiomyocytes were incubated with DEX, MET, or TNF-alpha for varying durations (0-12 h). TNF-alpha-induced cell damage was evaluated by measuring caspase-3 activity and Hoechst staining. Protein and gene estimation techniques were employed to determine the mechanisms mediating the effects of AMPK activators on TNF-alpha-induced cardiomyocyte apoptosis. Incubation of myocytes with TNF-alpha for 8 h has increased caspase-3 activation and apoptotic cell death, an effect that was abrogated by DEX and MET. The beneficial effect of DEX and MET was associated with stimulation of AMPK, which led to a rapid and sustained increase in Bad phosphorylation. This event reduced the interaction between Bad and Bcl-xL, limiting cytochrome c release and caspase-3 activation. Addition of Compound C to inhibit AMPK reduced Bad phosphorylation and prevented the beneficial effects of AMPK against TNF-alpha-induced cytotoxicity. CONCLUSION: Our data demonstrate that although DEX and MET are used as anti-inflammatory agents or insulin sensitizers, respectively, their common property to phosphorylate AMPK promotes cardiomyocyte cell survival through its regulation of Bad and the mitochondrial apoptotic mechanism.

Cleavage of protein kinase D after acute hypoinsulinemia prevents excessive lipoprotein lipase-mediated cardiac triglyceride accumulation.

OBJECTIVE: During hypoinsulinemia, when cardiac glucose utilization is impaired, the heart rapidly adapts to using more fatty acids. One means by which this is achieved is through lipoprotein lipase (LPL). We determined the mechanisms by which the heart regulates LPL after acute hypoinsulinemia. RESEARCH DESIGN AND METHODS: We used two different doses of streptozocin (55 [D-55] and 100 [D-100] mg/kg) to induce moderate and severe hypoinsulinemia, respectively, in rats. Isolated cardiomyocytes were also used for transfection or silencing of protein kinase D (PKD) and caspase-3. RESULTS: There was substantial increase in LPL in D-55 hearts, an effect that was absent in severely hypoinsulinemic D-100 animals. Measurement of PKD, a key element involved in increasing LPL, revealed that only D-100 hearts showed an increase in proteolysis of PKD, an effect that required activation of caspase-3 together with loss of 14-3-3zeta, a binding protein that protects enzymes against degradation. In vitro, phosphomimetic PKD colocalized with LPL in the trans-golgi. PKD, when mutated to prevent its cleavage by caspase-3 and silencing of caspase-3, was able to increase LPL activity. Using a caspase inhibitor (Z-DEVD) in D-100 animals, we effectively lowered caspase-3 activity, prevented PKD cleavage, and increased LPL vesicle formation and translocation to the vascular lumen. This increase in cardiac luminal LPL was associated with a striking accumulation of cardiac triglyceride in Z-DEVD-treated D-100 rats. CONCLUSIONS After severe hypoinsulinemia, activation of caspase-3 can restrict LPL translocation to the vascular lumen. When caspase-3 is inhibited, this compensatory response is lost, leading to lipid accumulation in the heart.

Acute diabetes moderates trafficking of cardiac lipoprotein lipase through p38 mitogen-activated protein kinase-dependent actin cytoskeleton organization.

OBJECTIVE: Heart disease is a leading cause of death in diabetes and could occur because of excessive use of fatty acid for energy generation. Our objective was to determine the mechanisms by which AMP-activated protein kinase (AMPK) augments cardiac lipoprotein lipase (LPL), the enzyme that provides the heart with the majority of its fatty acid. RESEARCH DESIGN AND METHODS: We used diazoxide in rats to induce hyperglycemia or used 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) and thrombin to directly stimulate AMPK and p38 mitogen-activated protein kinase (MAPK), respectively, in cardiomyocytes. RESULTS: There was a substantial increase in LPL at the coronary lumen following 4 h of diazoxide. In these diabetic animals, phosphorylation of AMPK, p38 MAPK, and heat shock protein (Hsp)25 produced actin cytoskeleton rearrangement to facilitate LPL translocation to the myocyte surface and, eventually, the vascular lumen. AICAR activated AMPK, p38 MAPK, and Hsp25 in a pattern similar to that seen with diabetes. AICAR also appreciably enhanced LPL, an effect reduced by preincubation with the p38 MAPK inhibitor SB202190 or by cytochalasin D, which inhibits actin polymerization. Thrombin activated p38 MAPK in the absence of AMPK phosphorylation. Comparable with diabetes, activation of p38 MAPK and, subsequently, Hsp25 phosphorylation and F-actin polymerization corresponded with an enhanced LPL activity. SB202190 and silencing of p38 MAPK also prevented these effects induced by thrombin and AICAR, respectively. CONCLUSIONS: We propose that AMPK recruitment of LPL to the cardiomyocyte surface (which embraces p38 MAPK activation and actin cytoskeleton polymerization) represents an immediate compensatory response by the heart to guarantee fatty acid supply when glucose utilization is compromised.

Acute dexamethasone-induced increase in cardiac lipoprotein lipase requires activation of both Akt and stress kinases.

Following dexamethasone (DEX), cardiac energy generation is mainly through utilization of fatty acids (FA), with DEX animals demonstrating an increase in coronary lipoprotein lipase (LPL), an enzyme that hydrolyzes lipoproteins to FA. We examined the mechanisms by which DEX augments cardiac LPL. DEX was injected in rats, and hearts were removed, or isolated cardiomyocytes were incubated with DEX (0-8 h), for measurement of LPL activity and Western blotting. Acute DEX induced whole body insulin resistance, likely an outcome of a decrease in insulin signaling in skeletal muscle, but not cardiac tissue. The increase in luminal LPL activity after DEX was preceded by rapid nongenomic alterations, which included phosphorylation of AMPK and p38 MAPK, that led to phosphorylation of heat shock protein (HSP)25 and actin cytoskeleton rearrangement, facilitating LPL translocation to the myocyte cell surface. Unlike its effects in vivo, although DEX activated AMPK and p38 MAPK in cardiomyocytes, there was no phosphorylation of HSP25, nor was there any evidence of F-actin polymerization or an augmentation of LPL activity up to 8 h after DEX. Combining DEX with insulin appreciably enhanced cardiomyocyte LPL activity, which closely mirrored a robust elevation in phosphorylation of HSP25 and F-actin polymerization. Silencing of p38 MAPK, inhibition of PI 3-kinase, or preincubation with cytochalasin D prevented the increases in LPL activity. Our data suggest that, following DEX, it is a novel, rapid, nongenomic phosphorylation of stress kinases that, together with insulin, facilitates LPL translocation to the myocyte cell surface.

Cardiac glycogen accumulation after dexamethasone is regulated by AMPK.

Glycogen is an immediate source of glucose for cardiac tissue to maintain its metabolic homeostasis. However, its excess brings about cardiac structural and physiological impairments. Previously, we have demonstrated that in hearts from dexamethasone (Dex)-treated animals, glycogen accumulation was enhanced. We examined the influence of 5'-AMP-activated protein kinase (AMPK) on glucose entry and glycogen synthase as a means of regulating the accumulation of this stored polysaccharide. After Dex, cardiac tissue had a limited contribution toward the development of whole body insulin resistance. Measurement of glucose transporter 4 (GLUT4) at the plasma membrane revealed an excess presence of this transporter protein at this location. Interestingly, this was accompanied by an increase in GLUT4 in the intracellular membrane fraction, an effect that was well correlated with increased GLUT4 mRNA. Both total and phosphorylated AMPK increased after Dex. Immunoprecipitation of Akt substrate of 160 kDa (AS160) followed by Western blot analysis demonstrated no change in Akt phosphorylation at Ser(473) and Thr(308) in Dex-treated hearts. However, there was a significant increase in AMPK phosphorylation at Thr(172), which correlated well with AS160 phosphorylation. In Dex-treated hearts, there was a considerable reduction in the phosphorylation of glycogen synthase, whereas glycogen synthase kinase-3-beta phosphorylation was augmented. Our data suggest that AMPK-mediated glucose entry combined with the activation of glycogen synthase and a reduction in glucose oxidation (Qi et al., Diabetes 53: 1790-1797, 2004) act together to promote glycogen storage. Should these effects persist chronically in the heart, they may explain the increased morbidity and mortality observed with long-term excesses in endogenous or exogenous glucocorticoids.

AMPK control of myocardial fatty acid metabolism fluctuates with the intensity of insulin-deficient diabetes.

Flexibility in substrate selection is essential for the heart to maintain production of energy and contractile function, and is managed through multiple mechanisms including PPAR-alpha and AMP-activated protein kinase (AMPK). Rats injected with 55 mg/kg STZ (D55) were kept for 4 days (acute diabetes; D55-A) prior to termination. Fatty acid (FA) oxidation increased in D55-A hearts, with no significant change in gene expression of PPAR-alpha, or its downstream targets. However, both AMPK and ACC phosphorylation were significantly higher in these hearts, effects that were reversed by insulin. Unexpectedly, when the duration of diabetes in D55 rats was extended to 6 weeks (chronic diabetes; D55-C), AMPK and ACC phosphorylation were comparable in control and D55-C hearts. In D55-C rat hearts, lack of AMPK activation was closely associated to an overload of plasma and cardiac lipids. To validate the relationship between lipids and cardiac AMPK activation, we either induced more severe diabetes (100 mg/kg STZ to provoke both hyperglycemia and hyperlipidemia acutely; D100-A) or infused intralipid (IL) to enlarge circulating lipids. There was no difference in cardiac AMPK and ACC phosphorylation in D100-A rats compared to control. Measurement of AMPK and ACC phosphorylation in control and D55-A hearts revealed that their phosphorylation was inhibited by acute intralipid infusion. Our data suggest that activation of AMPK is an adaptation that would ensure adequate cardiac energy production when glucose utilization is compromised. However, in severe diabetes, with the addition of augmented plasma and heart lipids, AMPK activation is prevented, and control of FA oxidation is likely through alternate mechanisms. Given that AMPK plays an important role in preventing cardiac ischemic/reperfusion damage, it is possible that in these diabetic hearts, the accelerated damage observed during exposure to ischemia/reperfusion could be a likely outcome of a compromised activation of AMPK.

Altered cardiac fatty acid composition and utilization following dexamethasone-induced insulin resistance.

Glucocorticoid therapy is often associated with impaired insulin sensitivity and cardiovascular disease. The present study was designed to evaluate cardiac fatty acid (FA) composition and metabolism following acute dexamethasone (Dex) treatment. Using the euglycemic hyperinsulinemic clamp, rats injected with Dex demonstrated a reduced glucose infusion rate. This whole body insulin resistance was also associated with a heart-specific increase in pyruvate dehydrogenase kinase 4 gene expression and a reduction in the rate of glucose oxidation. Dex treatment increased basal and postheparin plasma lipolytic activity. In the heart, palmitic and oleic acid levels were higher after 4 h of Dex and decreased to control (CON) levels within 8 h. Measurement of polyunsaturated FAs demonstrated a drop in linoleic and gamma-linolenic acid, with an increase in arachidonic acid (AA) after acute Dex injection. Tissue FA can be either oxidized or stored as triglyceride (TG). At 4 h, Dex augmented cardiac TG accumulation. However, this increase in tissue TG could not be maintained, such that at 8 h following Dex, TG declined to CON levels. AMP-activated protein kinase (AMPK) activation is known to promote FA oxidation through its control of acetyl-CoA carboxylase (ACC). Acute Dex promoted ACC phosphorylation, and increased cardiac palmitate oxidation, likely through its effects in increasing AMPK phosphorylation and total AMPK protein and gene expression. Whether these acute effects of Dex on FA oxidation, TG storage, and arachidonic acid accumulation can be translated into increased cardiovascular risk following chronic therapy has yet to be determined.

Cardiomyocyte apoptosis induced by short-term diabetes requires mitochondrial GSH depletion.

Oxidative stress due to excessive reactive oxygen species (ROS) and depleted antioxidants such as glutathione (GSH) can give rise to apoptotic cell death in acutely diabetic hearts and lead to heart disease. At present, the source of these cardiac ROS or the subcellular site of cardiac GSH loss [i.e., cytosolic (cGSH) or mitochondrial (mGSH) GSH] has not been completely elucidated. With the use of rotenone (an inhibitor of the electron transport chain) to decrease the excessive ROS in acute streptozotocin (STZ)-induced diabetic rat heart, the mitochondrial origin of ROS was established. Furthermore, mitochondrial damage, as evidenced by loss of membrane potential, increases in oxidative stress, and reduction in mGSH was associated with increased apoptosis via increases in caspase-9 and -3 activities in acutely diabetic hearts. To validate the role of mGSH in regulating cardiac apoptosis, L-buthionine-sulfoximine (BSO; 10 mmol/kg ip), which blocks GSH synthesis, or diethyl maleate (DEM; 4 mmol/kg ip), which inactivates preformed GSH, was administered in diabetic rats for 4 days after STZ administration. Although both BSO and DEM lowered cGSH, they were ineffective in reducing mGSH or augmenting cardiomyocyte apoptosis. To circumvent the lack of mGSH depletion, BSO and DEM were coadministered in diabetic rats. In this setting, mGSH was undetectable and cardiac apoptosis was further aggravated compared with the untreated diabetic group. In a separate group, GSH supplementation induced a robust amplification of mGSH in diabetic rat hearts and prevented apoptosis. Our data suggest for the first time that mGSH is crucial for modulating the cell suicide program in short-term diabetic rat hearts.

Lysophosphatidic acid-mediated augmentation of cardiomyocyte lipoprotein lipase involves actin cytoskeleton reorganization.

The lipoprotein lipase (LPL)-augmenting property of lysophosphatidylcholine requires the formation of lysophosphatidic acid (LPA) (J Mol Cell Cardiol 37: 931-938, 2004). Given that the actin cytoskeleton has been implicated in regulating cardiomyocyte LPL, we examined whether LPL secretion after LPA involves actin cytoskeleton reassembly. Incubation of myocytes with LPA (1-100 nM) increased basal and heparin-releasable LPL (HR-LPL), an effect that was independent of shifts in LPL mRNA. The influence of LPA on myocyte LPL was reflected at the coronary lumen, with substantial increases of the enzyme at this location. Incubation of myocytes with cytochalasin D not only blocked LPA-induced augmentation of HR-LPL but also abrogated filamentous actin formation. These effects of LPA were likely receptor mediated. Exposure of myocytes to LPA facilitated significant membrane translocation of RhoA and its downstream effector Rho kinase I (ROCK I), and blocking this effect with Y-27632 appreciably reduced basal and HR-LPL activity. Incubation of adipose tissue with LPA also significantly enhanced basal and HR-LPL activity, suggesting that sarcomeric actin likely has a limited role in influencing the LPL secretory function of LPA in the myocyte. Comparable to LPA, hyperglycemia also caused significant membrane translocation of RhoA and ROCK I in hearts isolated from diazoxide-treated animals, effects that were abrogated using insulin. Overall, our data suggest that comparable to hyperglycemia, LPA-induced increases in cardiac LPL occurred via posttranscriptional mechanisms and processes that likely required RhoA activation and actin polymerization. Whether this increase in LPL augments triglyceride deposition in the heart leading to eventual impairment in contractile function is currently unknown.