The effects of streptozotocin diabetes on tissue specific lipase activities in the rat

Dual effects of hyperglycemia on endothelial cells and cardiomyocytes to enhance coronary LPL activity

In the diabetic heart, there is excessive dependence on fatty acid (FA) utilization to generate ATP. Lipoprotein lipase (LPL)-mediated hydrolysis of circulating triglycerides is suggested to be the predominant source of FA for cardiac utilization during diabetes. In the heart, the majority of LPL is synthesized in cardiomyocytes and secreted onto cell surface heparan sulfate proteoglycan (HSPG), where an endothelial cell (EC)-releasable β-endoglycosidase, heparanase cleaves the side chains of HSPG to liberate LPL for its onward movement across the EC. EC glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) captures this released enzyme at its basolateral side and shuttles it across to its luminal side. We tested whether the diabetes-induced increase of transforming growth factor-β (TGF-β) can influence the myocyte and EC to help transfer LPL to the vascular lumen to generate triglyceride-FA. In response to high glucose and EC heparanase secretion, this endoglycosidase is taken up by the cardiomyocyte (Wang Y, Chiu AP, Neumaier K, Wang F, Zhang D, Hussein B, Lal N, Wan A, Liu G, Vlodavsky I, Rodrigues B. Diabetes 63: 2643–2655, 2014) to stimulate matrix metalloproteinase-9 expression and the conversion of latent to active TGF-β. In the cardiomyocyte, TGF-β activation of RhoA enhances actin cytoskeleton rearrangement to promote LPL trafficking and secretion onto cell surface HSPG. In the EC, TGF-β signaling promotes mesodermal homeobox 2 translocation to the nucleus, which increases the expression of GPIHBP1, which facilitates movement of LPL to the vascular lumen. Collectively, our data suggest that in the diabetic heart, TGF-β actions on the cardiomyocyte promotes movement of LPL, whereas its action on the EC facilitates LPL shuttling.

NEW & NOTEWORTHY Endothelial cells, as first responders to hyperglycemia, release heparanase, whose subsequent uptake by cardiomyocytes amplifies matrix metalloproteinase-9 expression and activation of transforming growth factor-β. Transforming growth factor-β increases lipoprotein lipase secretion from cardiomyocytes and promotes mesodermal homeobox 2 to enhance glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1-dependent transfer of lipoprotein lipase across endothelial cells, mechanisms that accelerate fatty acid utilization by the diabetic heart.

Cardiomyocyte-endothelial cell control of lipoprotein lipase

In people with diabetes, inadequate pharmaceutical management predisposes the patient to heart failure, which is the leading cause of diabetes related death. One instigator for this cardiac dysfunction is change in fuel utilization by the heart. Thus, following diabetes, when cardiac glucose utilization is impaired, the heart undergoes metabolic transformation wherein it switches to using fats as an exclusive source of energy. Although this switching is geared to help the heart initially, in the long term, this has detrimental effects on cardiac function. These include the generation of noxious byproducts, which damage the cardiomyocytes, and ultimately result in increased morbidity and mortality. A key perpetrator that may be responsible for organizing this metabolic disequilibrium is lipoprotein lipase (LPL), the enzyme responsible for providing fat to the hearts. Either exaggeration or reduction in its activity following diabetes could lead to heart dysfunction. Given the disturbing news that diabetes is rampant across the globe, gaining more insight into the mechanism(s) by which cardiac LPL is regulated may assist other researchers in devising new therapeutic strategies to restore metabolic equilibrium, to help prevent or delay heart disease seen during diabetes. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

Lipoprotein lipase mediated fatty acid delivery and its impact in diabetic cardiomyopathy

Although cardiovascular disease is the leading cause of diabetes-related death, its etiology is still not understood. The immediate change that occurs in the diabetic heart is altered energy metabolism where in the presence of impaired glucose uptake, glycolysis, and pyruvate oxidation, the heart switches to exclusively using fatty acids (FA) for energy supply. It does this by rapidly amplifying its lipoprotein lipase (LPL-a key enzyme, which hydrolyzes circulating lipoprotein-triglyceride to release FA) activity at the coronary lumen. An abnormally high capillary LPL could provide excess fats to the heart, leading to a number of metabolic, morphological, and mechanical changes, and eventually to cardiac disease. Unlike the initial response, chronic severe diabetes "turns off" LPL, this is also detrimental to cardiac function. In this review, we describe a number of post-translational mechanisms that influence LPL vesicle formation, actin cytoskeleton rearrangement, and transfer of LPL from cardiomyocytes to the vascular lumen to hydrolyze lipoprotein-triglyceride following diabetes. Appreciating the mechanism of how the heart regulates its LPL following diabetes should allow the identification of novel targets for therapeutic intervention, to prevent heart failure. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Streptozotocin-induced diabetes enhances cardiac heparin-releasable lipoprotein lipase activity in spontaneously hypertensive rats

Vascular endothelial-bound lipoprotein lipase (LPL), also known as heparin-releasable LPL, catalyzes the breakdown of the triglyceride component of lipoproteins and is rate-limiting for free fatty acid transport to tissues. We previously demonstrated that heparin-releasable LPL activity increases in diabetic Wistar rat hearts, whereas with the development of hypertension in spontaneously hypertensive rats (SHR), there is a concomitant and progressive reduction in LPL activity. The objective of the present study was to examine the regulation of cardiac LPL activity in SHR-diabetic rats. Heparin perfusion of the isolated Langendorff heart induced the release of LPL activity. SHR hearts demonstrated a reduction in peak heparin-releasable LPL activity, relative to Wistar controls. However, induction of streptozotocin-induced diabetes in SHR, as in Wistar rats, increased peak heparin-releasable LPL activity in perfused hearts. The elevated heparin-releasable LPL peak could not be accounted for by enhanced LPL synthesis in that both cellular and surface-bound LPL activities in myocytes from SHR-diabetic rats were low relative to control. Chronic (12-day) insulin treatment of SHR-diabetic rats reduced the augmented heparin-releasable LPL activity and increased cell-associated LPL activity. Moreover, acute (90-minute) treatment of SHR-diabetic rats with rapid-acting insulin also reduced the heparin-releasable LPL activity to normal, although it had no effect on the low cellular LPL activity. These results demonstrate that the diabetes-induced augmentation of cardiac LPL counteracts the reduction in enzyme activity associated with hypertension. This may serve to increase the delivery of free fatty acid to the heart, and the resultant metabolic changes may lead to the severe cardiomyopathy observed in the hypertensive-diabetic rat heart.

Lipase activities in post-heparin plasma and tissues, and susceptibilities of lipoproteins in experimental diabetic rats

Heparin administration to diabetic rats caused no change in VLDL, an increase in IDL and a decrease in LDL on electrophoretic analysis of plasma lipoproteins, while the administration to control rats markedly decreased VLDL and increased IDL and LDL. Both hepatic triglyceride lipase (HTGL) and lipoprotein lipase (LPL) activities in the postheparin plasma were lower in the diabetic rats than in the controls, and the reduction of HTGL activity was greater than that of LPL activity in the diabetic rats. The LPL activity in the adipose tissue was lower in the diabetic rats than in the controls, but the activities in the cardiac and skeletal muscles were similar in the two rats. The HTGL-catalyzed fatty acid (FA) releases from the diabetic VLDL and IDL were lower than those from the normal rat VLDL and IDL, while the LPL-catalyzed FA release in the diabetic rats was not different from those in the controls. The decreases in LPL and HTGL activities and the markedly impaired susceptibility of IDL to HTGL coincide well with the postheparin changes in plasma lipoproteins in diabetic rats, an increase in IDL and a decrease in LDL.

Alteration of antioxidant status in diabetic rats by chronic exposure to psychological stressors

Antioxidant status was measured in heart, liver, kidney, lung, and erythrocytes of 2-week streptozotocin-diabetic male Wistar rats exposed to chronic intermittent psychological stress consisting of 1 h of restraint twice daily for 14 days. Diabetes reduced erythrocyte and heart and liver susceptibility to hydrogen peroxide-induced glutathione depletion. Susceptibility to peroxide-induced thiobarbituric acid reactive substance (TBARS) formation increased in erythrocytes, liver, kidney, and lung but decreased in heart. Significant changes also occurred in glutathione levels (increased in heart and decreased in liver) and in the activities of catalase (reduced in liver and kidney), glutathione reductase (elevated in heart and liver), and glutathione peroxidase (decreased in liver and lung), but not Cu,Zn-superoxide dismutase. Stress potentiated diabetes-associated hyperglycemia and attenuated diabetes-induced hyperlipidemia. In addition, the reduction in peroxide-induced glutathione depletion in heart and liver and the increased TBARS formation in kidney and lung were reversed. Similarly, the diabetes-induced induced increase in liver glutathione reductase and decreases in liver and lung glutathione peroxidase activities were abolished by stress. Thus, the relative resistance of antioxidant systems to stress can be modified under pathologic conditions in which antioxidant alterations are present.

Regulation of lipoprotein lipase in the diabetic rat

Diabetes mellitus is associated with a reduction of lipoprotein lipase (LPL) activity and development of hypertriglyceridemia. In the current experiments the mechanisms involved in the regulation of LPL have been examined in control rats, streptozocin-induced diabetic rats, and diabetic rats treated chronically or with a single injection of insulin. Diabetes decreased adipose tissue LPL activity partially by decreasing immunoreactive LPL protein and the steady-state levels of LPL mRNA, but primarily by reducing the catalytic activity of LPL. Both chronic and acute insulin increased adipose tissue LPL activity by correcting the defect in the catalytic activity of LPL and increasing immunoreactive LPL protein; however, only chronic insulin restored LPL mRNA levels to normal. In the heart, LPL activity tended to be elevated with diabetes in parallel to an increase in immunoreactive LPL protein even though levels of LPL mRNA declined. Both chronic and acute insulin normalized LPL activity and immunoreactive LPL protein, while only chronic insulin corrected the levels of LPL mRNA. No changes in the catalytic activity of LPL in heart were detected among the groups. Thus, diabetes and insulin treatment regulate LPL expression pretranslationally, translationally, and post-translationally, with tissue-specific differences apparent in the mechanisms involved.

Lipid analysis of the major salivary glands in streptozotocin-diabetic rats and the effects of insulin treatment

Two separate sets of experiments were performed on female Wistar rats made diabetic with streptozotocin: (1) a time-course study where groups of three animals were removed at weekly intervals, up to 4 weeks after induction of diabetes, with an age-matched group of control (normal) animals kept for 4 weeks; (2) six further animals were made diabetic and kept for 7 weeks; three of these were given insulin in the final week. At the required time the animals were anaesthetized and the salivary glands removed and preserved by fixation or freezing. The frozen tissues were later homogenized and the protein and lipid content analysed. Histologically, intracellular lipid droplets had accumulated in the majority of the diabetic salivary glands. In the time-course experiment, the visible amount of intracellular lipid reached a maximum after 2 weeks and then decreased, with a concomitant disappearance of interstitial lipid. The increased lipid content was not attributable to any one class. The fatty acid profiles of the glands showed an increase in the percentages of C18:0 (stearic acid) and C18:2w6 (linoleic acid) and a decrease in the percentages of C18:1w9 (oleic acid) and C20:4w6 (arachidonic acid). After 1 week of insulin treatment the lipid content and the fatty acid profiles returned to normal. Thus the effect of insulin on salivary gland lipid metabolism is rapid both in its occurrence and reversibility. The effects seen in the diabetic rats are considered to be due to a lack of insulin and not to the presence of streptozotocin.

Lipid peroxidation and retinopathy in streptozotocin-induced diabetes

Using the streptozotocin (STZ)-induced diabetic rat model, we have established a time-related curve for lipid hydroperoxides (LHP) in plasma and have correlated the period corresponding to maximal increase with histologic changes in the outer retina. Measurement of thiobarbituric acid reacting substances (TBARS) provides a convenient assessment of LHP concentration in plasma. Our results demonstrate a seven-fold elevation of TBARS at 10 days post-induction which increased to fifteen times above normal at 22 days and then fell dramatically to below baseline values at 39 days. Structural damage to the retina consisted of a reduction in cell number throughout the inner and outer nuclear layers, disorganization and loss of photoreceptor segments, and dilation of the basal region of the retinal pigment epithelium. The present observations establish a correlation between LHP concentration and retinal structure and function. Taken together with other reports in the literature showing alterations of protective enzymes and antioxidants, it appears that free radicals and lipid peroxidation are involved in the etiology of diabetic retinopathy in the STZ rat model. The TBARS assay is a simple, sensitive and inexpensive method to monitor changes in oxidative status and may prove useful in diagnosis and monitoring of patients with diabetes.

Morphological effects of sympathetic nerve stimulation on rat parotid glands 3-4 weeks after the induction of streptozotocin diabetes

Male Wistar rats were fasted overnight and anaesthetized 3-4 weeks after the induction of streptozotocin diabetes. The right parotid ducts were cannulated, and parotid salivary flow was induced by stimulating the sympathetic trunk in bursts (50 Hz, 1:10). Stimulated and unstimulated glands were weighed, fixed for morphometric analysis, and assayed for total protein and amylase. Gland weights did not differ between diabetic and control rats. Nevertheless, total protein (6.17 +/- 5.40 mg) and amylase (1.32 +/- 0.49 mg) output from the gland were reduced in diabetic animals compared with controls (13.73 +/- 2.81 and 3.41 +/- 0.51 mg, respectively). Morphometric analysis of unstimulated glands showed no differences in either acinar cell profile area, or in the number of secretory granule profiles/cell in unstimulated glands. Upon sympathetic stimulation, however, the depletion of granule profiles was significantly less in diabetic than in control rats. Finally, the mean diameter of secretory granules was significantly less in diabetic (0.89 +/- 0.05 microM) than in control (1.04 +/- 0.06 microM) glands.

Brain lipoprotein lipase is responsive to nutritional and hormonal modulation

Functional lipoprotein lipase activity was recently described in rat brain. The present study was performed to further characterize the biologic significance of brain lipoprotein lipase (heparin releasable component) and elucidate regulatory factors. Comparative studies were performed on tissue (brain, adipose, and heart) heparin releasable lipoprotein lipase in the fasted and diabetic (streptozotocin 100 mg/kg BW IP) rat. Both fasting (96 hours) and diabetes (ten days) significantly decreased brain (cortical) (P less than .05) and adipose (epididymal fat pad) (P less than .001) lipoprotein lipase activity. In contrast, heart muscle enzyme activity was significantly increased (P less than .001) in response to fasting and diabetes. Refeeding (Purina chow 96 hours) and insulin replacement (96 hours) reversed these changes in tissue lipoprotein lipase consequent to fasting and diabetes, respectively. There was a positive correlation between the changes in serum insulin concentration and adipose lipoprotein lipase, but there was no correlation between this parameter and brain or heart lipoprotein lipase. In addition, although T3 therapy normalized the low T3 state associated with both fasting and diabetes, it had no effect on the enzyme activity in the studied tissues. However, subsequent studies demonstrated that hypothyroidism (2 weeks post thyroidectomy) significantly decreased brain lipoprotein lipase activity (P less than .001) and increased both the adipose (P less than .025) and heart (P less than .025) enzyme activity. T3 replacement (0.8 micrograms/100 BW/d for 1 week) reversed the effects of hypothyroidism. However, the relationship between brain enzyme activity and serum T3 was nonlinear as hyperthyroidism tended to reduce brain LPL activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Triacylglycerol lipase activities in isolated myocardial cells from chronically diabetic rat hearts

Diabetes was induced by the administration of streptozotocin (55 mg/kg or 70 mg/kg) to rats. After 21-25 days, myocardial cells (myocytes) were isolated from control and diabetic rat hearts. Rates of endogenous lipolysis, measured as the output of glycerol, were elevated in the chronically diabetic myocytes. Lipoprotein lipase activity was reduced in homogenates of diabetic myocytes. Neutral triacylglycerol lipase activity was increased in myocytes from rats made diabetic with the lower dose of streptozotocin, but not in myocytes from diabetic rats given the higher dose. Diabetes had no effect on acid lysosomal lipase activity.

Effect of diabetes on acid and neutral triacylglycerol lipase and on lipoprotein lipase activities in isolated myocardial cells from rat heart

A neutral triacylglycerol lipase activity that is separate and distinct from lipoprotein lipase (LPL) could be measured in homogenates of myocardial cells if protamine sulphate and high concentrations of albumin were included in the assay. This neutral lipase was predominantly particulate, with the highest relative specific activity in microsomal subcellular fractions. The induction of diabetes by the administration of streptozotocin to rats resulted in a decrease in LPL activity in myocyte homogenates and in particulate subcellular fractions, but the percentage of cellular LPL activity that was released during incubation of myocytes with heparin was normal. In contrast, neutral lipase activity was increased in diabetic myocyte homogenates and microsomal fractions. Acid triacylglycerol lipase activity was not changed in diabetic myocytes. The decrease in LPL in myocytes owing to diabetes may result in the decreased functional LPL activity at the capillary endothelium of the diabetic heart.

Lipid accumulation in the major salivary glands of streptozotocin-diabetic rats

Four-five months after the induction of diabetes, salivary tissues of male Wistar rats were preserved by glutaraldehyde fixation or rapid freezing in dry-ice cooled hexane. Fixed tissues were either processed and embedded for light and electron microscopy, or frozen and, together with unfixed tissues, sectioned and stained with Oil Red 0 or by the calcium-lipase method. All diabetic glands had considerably more intracellular lipid than control ones. Lipid accumulation within parenchymal cells varied with the type of gland, and was more pronounced in animals with the highest serum-glucose levels. Serous cells of parotid and sublingual glands accumulated the greatest amount of lipid; lesser amounts were present in seromucous acinar cells of submandibular glands; little or none in mucous acinar cells of sublingual glands. There was no lipid in striated, granular or excretory ducts. Histochemical staining suggested that the intracellular lipid was mainly triglyceride which may accumulate by increased uptake for use as an energy source, or by decreased use in the synthesis of secretory granule and plasma-membrane material.