A fter diabetes mellitus, the heart has an increased reliance on fatty acids (FAs) for generation of energy.1 The majority of these FAs come from breakdown of circulating triglyceride-rich lipoproteins, a process catalyzed by lipoprotein lipase (LPL) located at the luminal side of vascular endothelial cells (ECs).2-4 ECs do not express LPL. In the heart, LPL is synthesized in cardiomyocytes and secreted to ECs. 5 We have previously reported an increase of coronary LPL in animal models of type 1 diabetes mellitus, an effect that was evident in the absence of any change in myocyte LPL gene expression. 6,7 We concluded that the augmented coronary LPL was a result of increased secretion of the enzyme from myocytes toward the coronary lumen. Regarding secretion, LPL is first transported from an intracellular pool in the myocyte to the cell surface, where it binds to heparan sulfate proteoglycans (HSPGs). 8 We have shown that this intracellular transport depends on actin cytoskeleton polymerization, a process that is magnified after diabetes mellitus.9,10 The subsequent process by which myocyte surface LPL is translocated to the coronary lumen, in addition to its replenishment after this onward movement, has not been completely elucidated.Myocyte surface HSPGs serve as a temporary docking site and an auxiliary reservoir of LPL. HSPGs are proteoglycans bearing HS side chains attached to specific serine residues of a protein core.11 Core proteins can be attached to the cell surface through a glycosylphosphatidyl inositol anchor in case of glypican, or can traverse the membrane as observed with the syndecan family.12,13 The HS side chains are polymers of repeating disaccharides which interact with multiple ligands, including antithrombin, fibroblast growth factor, and LPL. Objective-After diabetes mellitus, transfer of lipoprotein lipase (LPL) from cardiomyocytes to the coronary lumen increases, and this requires liberation of LPL from the myocyte surface heparan sulfate proteoglycans with subsequent replenishment of this reservoir. At the lumen, LPL breaks down triglyceride to meet the increased demand of the heart for fatty acid. Here, we examined the contribution of coronary endothelial cells (ECs) toward regulation of cardiomyocyte LPL secretion. Approach and Results-Bovine coronary artery ECs were exposed to high glucose, and the conditioned medium was used to treat cardiomyocytes. EC-conditioned medium liberated LPL from the myocyte surface, in addition to facilitating its replenishment. This effect was attributed to the increased heparanase content in EC-conditioned medium. Of the 2 forms of heparanase secreted from EC in response to high glucose, active heparanase released LPL from the myocyte surface, whereas latent heparanase stimulated reloading of LPL from an intracellular pool via heparan sulfate proteoglycanmediated RhoA activation. Conclusions-Endothelial heparanase is a participant in facilitating LPL increase at the coronary lumen. These observations provide an insight into the cross-talk between ECs ...

Objective-During diabetes mellitus, coronary lipoprotein lipase increases to promote the predominant use of fatty acids.We have reported that high glucose stimulates active heparanase secretion from endothelial cells to cleave cardiomyocyte heparan sulfate and release bound lipoprotein lipase for transfer to the vascular lumen. In the current study, we examined whether heparanase also has a function to release cardiomyocyte vascular endothelial growth factor (VEGF), and whether this growth factor influences cardiomyocyte fatty acid delivery in an autocrine manner. Approach and Results-Acute, reversible hyperglycemia was induced in rats, and a modified Langendorff heart perfusion was used to separate the coronary perfusate from the interstitial effluent. Coronary artery endothelial cells were exposed to high glucose to generate conditioned medium, and VEGF release from isolated cardiomyocytes was tested using endothelial cell conditioned medium or purified active and latent heparanase. Autocrine signaling of myocyte-derived VEGF on cardiac metabolism was studied. High glucose promoted latent and active heparanase secretion into endothelial cell conditioned medium, an effective stimulus for releasing cardiomyocyte VEGF. Intriguingly, latent heparanase was more efficient than active heparanase in releasing VEGF from a unique cell surface pool. VEGF augmented cardiomyocyte intracellular calcium and AMP-activated protein kinase phosphorylation and increased heparin-releasable lipoprotein lipase. Conclusions-Our data suggest that the heparanase-lipoprotein lipase-VEGF axis amplifies fatty acid delivery, a rapid and adaptive mechanism that is geared to overcome the loss of glucose consumption by the diabetic heart. If prolonged, the resultant lipotoxicity could lead to cardiovascular disease in humans.

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After diabetes, the heart has a singular reliance on fatty acid (FA) for energy production, which is achieved by increased coronary lipoprotein lipase (LPL) that breaks down circulating triglycerides. Coronary LPL originates from cardiomyocytes, and to translocate to the vascular lumen, the enzyme requires liberation from myocyte surface heparan sulfate proteoglycans (HSPGs), an activity that needs to be sustained after chronic hyperglycemia. We investigated the mechanism by which endothelial cells (EC) and cardiomyocytes operate together to enable continuous translocation of LPL after diabetes. EC were cocultured with myocytes, exposed to high glucose, and uptake of endothelial heparanase into myocytes was determined. Upon uptake, the effect of nuclear entry of heparanase was also investigated. A streptozotocin model of diabetes was used to expand our in vitro observations. In high glucose, EC-derived latent heparanase was taken up by cardiomyocytes by a caveolae-dependent pathway us ing HSPGs. This latent heparanase was converted into an active form in myocyte lysosomes, entered the nucleus, and upregulated gene expression of matrix metalloproteinase-9. The net effect was increased shedding of HSPGs from the myocyte surface, releasing LPL for its onwards translocation to the coronary lumen. EC-derived heparanase regulates the ability of the cardiomyocyte to send LPL to the coronary lumen. This adaptation, although acutely beneficial, could be catastrophic chronically because excess FA causes lipotoxicity. Inhibiting heparanase function could offer a new strategy for managing cardiomyopathy observed after diabetes.In diabetes, because glucose uptake and oxidation are impaired, the heart is compelled to use fatty acid (FA) exclusively for ATP generation (1). Multiple adaptive mechanisms, either whole-body or intrinsic to the heart, operate to make this achievable, with hydrolysis of triglyceride-rich lipoproteins being the major source of FA to the diabetic heart (2). This critical reaction is catalyzed by the vascular content of lipoprotein lipase (LPL), and we were the first to report significantly higher coronary LPL activity after diabetes (3). In the heart, LPL is synthesized by cardiomyocytes, transported to heparan sulfate (HS) proteoglycan (HSPG) binding sites on the myocyte surface, and from this temporary reservoir, the enzyme is transferred across the interstitial space to reach endothelial cells (EC) (4,5). Before this transfer, liberation of HSPG-sequestered LPL is a prerequisite and is facilitated by heparanase, an EC endoglycosidase that can cleave HS side chains on HSPGs in the extracellular matrix and on the cell surface to release bound proteins (6).Heparanase is synthesized as a latent 65-kDa precursor. After its secretion and reuptake (7), heparanase enters