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(Circulation Research. 2000;87:412.)
© 2000 American Heart Association, Inc.

Integrative Physiology

Protein Kinase C Activation Contributes to Microvascular Barrier Dysfunction in the Heart at Early Stages of Diabetes

Sarah Y. Yuan, Elena E. Ustinova, Mack H. Wu, John H. Tinsley, Wenjuan Xu, Ferenc L. Korompai, Amy C. Taulman

From the Departments of Surgery and Medical Physiology, Cardiovascular Research Institute, Texas A&M University System Health Science Center, Temple, Tex.

Correspondence to Dr Sarah Yuan, Departments of Surgery and Medical Physiology, Texas A&M University System Health Science Center, 1901 S 1st St, Bldg 4, Temple, TX 76504. E-mail yuan{at}tamu.edu

	Abstract

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Abstract—The functional disturbance of microvasculature is recognized as an initiating mechanism that underlies the development of various diabetic complications. Although a causal relationship between microvascular leakage and tissue damage has been well documented in diabetic kidneys and eyes, there is a lack of information regarding the barrier function of coronary exchange vessels in the disease state. The aim of the present study was to evaluate the permeability property of coronary microvessels during the early development of experimental diabetes with a focus on the protein kinase C (PKC)-dependent signaling mechanism. The apparent permeability coefficient of albumin (Pa) was measured in isolated and perfused porcine coronary venules. The administration of high concentrations of D-glucose induced a dose-dependent increase in the Pa value, which was prevented by blockage of PKC with its selective inhibitors bisindolylmaleimide and Goe 6976. More importantly, an elevated basal permeability to albumin was observed in coronary venules at the early onset of streptozotocin-induced diabetes. The hyperpermeability was corrected with bisindolylmaleimide and the selective PKCß inhibitor hispidin. Concomitantly, protein kinase assay showed a high PKC activity in isolated diabetic venules. Immunoblot analysis of the diabetic heart revealed a significant subcellular translocation of PKCßII and PKC from the cytosol to the membrane, indicating that the specific activity of these isoforms was preferentially elevated. The results suggest that endothelial barrier dysfunction attributed to the activation of PKC occurs at the coronary exchange vessels in early diabetes.

Key Words: diabetes • microcirculation • permeability • protein kinases

	Introduction

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Cardiovascular complications represent the major cause of morbidity and mortality in patients with diabetes mellitus. A functional disturbance in the microvasculature independent of large-vessel atherosclerosis has been implicated in the development of end-organ damage and clinical abnormalities manifested as nephropathy, retinopathy, neuropathy, and skin ulceration.¹ Characteristic changes in the microcirculation during the early stages of diabetes include autoregulatory dysfunction and increased endothelial permeability.^{1 2} Capillary leakage has been documented in various tissues of experimentally diabetic animals, including the retina,³ aorta,⁴ skin, intestine, kidney,⁵ and heart.⁶ Endothelial barrier dysfunction, occurring before microvascular sclerosis and other structural alterations, is considered to be one of the initiating mechanisms that underlies the pathogenesis of microangiopathic complications.

The precise cause of vascular hyperpermeability in diabetes has not been established. Recent evidence suggests that hyperglycemia-induced de novo synthesis of diacylglycerol and the subsequent activation of protein kinase C (PKC) constitute an important signaling pathway that leads to endothelial dysfunction.^{7 8} The PKC activity has been found to be upregulated under the hyperglycemic or diabetic condition in microangiopathy-prone tissues, such as the retina,⁹ renal glomeruli,¹⁰ aorta, and heart.¹¹ The in vivo administration of specific PKC inhibitors normalized microvascular blood flow and permeability in diabetic rats.^{12 13} In cultured endothelial cells, glucose caused a dose-dependent increase in macromolecular permeability that was abolished with the PKC inhibitors staurosporine and Goe 6976,⁷ which supports the role of PKC in the mediation of diabetes-induced endothelial barrier dysfunction.

In the coronary system, increased microvascular permeability may in large part contribute to myocardium insufficiency and ventricular dysfunction, as frequently seen in diabetic heart disease. Although the casual relationship between microvascular leakage and tissue damage has been well documented in diabetic kidney and retina, there is a paucity of information about the pathophysiological consequence of diabetes that extends particularly to the coronary exchange vessels. Therefore, the aim of the present study was to systematically examine the effect of diabetic hyperglycemia on coronary microvascular permeability, with a focus on the PKC-dependent signaling mechanism. An in situ model of intact perfused coronary venules was used to provide a direct assessment of the barrier property of venular endothelium. The results showed that albumin permeability was markedly elevated in coronary venules isolated from streptozotocin (STZ)-induced diabetic pig hearts and in venules treated with high concentrations of glucose. Upregulation of PKC activity was found in these vessels as well as in cultured coronary endothelial cells subjected to high concentrations of glucose. Correspondingly, the inhibition of PKC with pharmacological agents greatly attenuated the diabetes-induced venular hyperpermeability. We therefore suggest that the activation of PKC has an important role in the development of microvascular endothelial dysfunction during the early stages of diabetes.

	Materials and Methods

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The PKC inhibitors bisindolylmaleimide (BIM), Goe 6976, hispidin, and 2,2',3,3',4,4'-hexahydroxy-1,1'-biphenyl-6,6'-dimethanol dimethyl ether (HBDDE) were obtained from Calbiochem. STZ was ordered from Upjohn. Antibodies to PKC, PKCßI, and PKCßII were obtained from Santa Cruz, and anti-PKC was obtained from Transduction Laboratories.

Diabetes was induced in Yorkshire pigs that weighed 9 to 12 kg through the intravenous injection of STZ (150 mg/kg). Pigs were fed with a commercial diet that contains ammonium chloride (20 g/kg hog chow) the day before STZ injection. Only those that developed sustained hyperglycemia with a blood glucose level of >300 mg/dL were included in this study.

The technique of isolation and cannulation of microvessels has been described in detail in our previous publications.¹⁴ Briefly, a coronary venule of 30 to 50 µm in diameter was dissected and cannulated with 4 micropipettes (Figure 1), of which each was connected to a reservoir to allow independent control of intraluminal pressure and flow. The vessel was interchangeably perfused with a physiological salt solution and the same solution containing fluorescein isothiocyanate-albumin. The changes in the fluorescence intensity in the venule and its adjacent area were measured with a video photometer that had an optical window positioned over the vessel. The apparent solute permeability coefficient of albumin (Pa) was calculated with the equation Pa=(1/I_f)(dI_f/dt)_o(r/2), where I_f is the initial step increase in fluorescent intensity, (dI_f/dt)_o is the initial rate of gradual increase in intensity as solutes diffuse out of the vessel into the extravascular space, and r is the venular radius.^{14 15}

View larger version (31K): [in this window] [in a new window]   Figure 1. Schematic diagram showing the measurement of Pa in an isolated and cannulated coronary venule. The microvessel was cannulated with 4 micropipettes (pipette in pipette), with each pipette connected to a reservoir to allow independent control of intraluminal pressure and flow. The fluorescence intensity was measured with a video photometer with an optical window positioned over the venule and adjacent space. In each measurement, the isolated venule was first perfused with a physiological salt solution through the outer inflow pipette to establish the baseline intensity. The venular lumen was then rapidly filled with fluorochromes by switching the perfusion to the inner inflow pipette that contained fluorescein isothiocyanate (FITC)-albumin. This produced an initial step increase, followed by a gradual increase, in the intensity of fluorescence. There was a step decrease in intensity when the fluorescent-labeled molecules were washed out of the vessel lumen by switching the perfusion back to the outer inflow pipette. The Pa value was calculated with the equation Pa=(1/If)(dIf/dt)o(r/2), where If is the initial step increase in fluorescence intensity, (dIf/dt)o is the initial rate of gradual increase in intensity as solutes diffuse out of the vessel into the extravascular space, and r is the venular radius.

PKC activity was measured in both freshly isolated coronary venules and cultured coronary venular endothelial cells (CVECs) with a MESACUP protein kinase assay kit (Medical and Biological Laboratories). To obtain vessel samples, 20 to 30 coronary venules were dissected from the heart of diabetic or control pigs and quickly homogenized, and the soluble fraction of the cells was then collected through centrifugation. For cell studies, CVECs were isolated from postcapillary venules and routinely maintained as previously described.^{16 17} The cells were incubated for 24 hours with the culture media containing 50 mmol/L D-glucose and then subjected to enzymatic analysis of PKC activity.

Because membrane translocation has been widely used as an indication of PKC activation,¹⁸ we further performed immunoblot analysis on freshly dissected porcine heart tissues to characterize the subcellular distribution of PKC isoforms, including PKC, PKCßI, PKCßII, and PKC. After homogenization of the tissue, the cytosolic and membranous fractions were separated through ultracentrifugation, followed by solubilization in a lysis buffer containing SDS. The fractionated samples were then subjected to Western blot analyses with monoclonal antibodies directed against the individual isoenzymes.

For data analysis, n is given as the number of animals (in PKC assays) or vessels (in permeability studies). At each experimental condition, the values of Pa from different venules were averaged and reported as mean±SEM. ANOVA was performed to evaluate the significance of intergroup differences, and a value of P<0.05 was considered significant for the comparisons.

An expanded Materials and Methods section can be found in an online data supplement available at http://www.circresaha.org.

	Results

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Effect of High Glucose on Venular Permeability
Figure 2 demonstrates the effect of high concentrations of D-glucose on the permeability of isolated coronary venules. An increase in glucose concentration caused a rapid, dose-dependent increase in albumin permeability. A maximal response was observed at 60 minutes after the administration of glucose. The changes in Pa in response to various concentrations of glucose were as follows: from the basal value of 3.02±0.32x10^-6 cm/s to 5.15±0.36x10^-6 cm/s at 12.5 mmol/L (n=8, P<0.05), from 3.17±0.28x10^-6 cm/s to 6.45±0.67x10^-6 cm/s at 25 mmol/L (n=10, P<0.05), from 2.86±0.43x10^-6 cm/s to 6.79±0.75x10^-6 cm/s at 50 mmol/L (n=6, P<0.05), and from 2.52±0.29x10^-6 cm/s to 9.01±1.46x10^-6 cm/s at 100 mmol/L (n=9, P<0.05).

View larger version (45K): [in this window] [in a new window]   Figure 2. Effect of D-glucose on Pa in isolated coronary venules. Numbers in parentheses indicate the number of vessels studied. *Significant difference vs control value at 5 mmol/L glucose.

The hyperpermeability response to glucose was markedly suppressed during inhibition of PKC (Figure 3). Administration of the PKC inhibitors BIM (10^-5 mol/L) (n=7) and Goe 6976 (10^-7 mol/L) (n=6) did not significantly alter the baseline permeability, but they completely prevented the increase in permeability caused by the high concentration (50 mmol/L) of glucose.

View larger version (35K): [in this window] [in a new window]   Figure 3. Glucose-induced changes in venular permeability during inhibition of PKC with BIM (10-5 mol/L) and Goe 6976 (10-7 mol/L). Numbers in parentheses indicate the number of vessels studied. *Significant difference vs control group at 5 mmol/L glucose.

Effect of Diabetes on Venular Permeability
STZ-induced diabetes significantly impaired the barrier function of coronary venules in pigs as early as 4 weeks after administration of the drug (Figure 4A). Specifically, the coronary venular permeability was 2.85±0.19x10^-6 cm/s in nondiabetic pigs at the basal condition (n=27), 2.62±0.62x10^-6 cm/s at 2 weeks of diabetes (n=4, P>0.05 versus nondiabetic), 4.11±0.33x10^-6 cm/s at 4 weeks of diabetes (n=9, P<0.05 versus nondiabetic), and 6.69±0.55x10^-6 cm/s at 6 to 8 weeks of diabetes (n=15, P<0.05 versus nondiabetic).

View larger version (41K): [in this window] [in a new window]   Figure 4. Time course of diabetes-induced changes in permeability (A) and PKC activity (B) in isolated coronary venules. Numbers in parentheses indicate the number of vessels studied. *Significant difference vs nondiabetic group.

Although the PKC inhibitor BIM did not alter the basal permeability of normal, nondiabetic vessels, it dose-dependently attenuated diabetes-induced hyperpermeability (Figure 5). In venules from pigs at 8 weeks of diabetes, the Pa value was 7.47±1.12x10^-6 cm/s (n=7). This value was reduced with BIM to 6.38±1.45x10^-6 cm/s at 10^-7 mol/L (n=5, P>0.05), 4.42±1.45x10^-6 cm/s at 10^-6 mol/L (n=7, P<0.05), and 3.67±0.81x10^-6 cm/s at 10^-5 mol/L (n=7, P<0.05). Furthermore, as shown in Figure 6, although the selective PKC inhibitor HBDDE (8x10^-5 mol/L) did not affect the high Pa in diabetic venules (n=4), the selective ß-isoform inhibitor hispidin (4x10^-6 mol/L) greatly attenuated diabetes-induced hyperpermeability in coronary venules (6.69±0.55x0^-6 cm/s before and 3.06±0.29x0^-6 cm/s after hispidin, n=7, P<0.05).

View larger version (65K): [in this window] [in a new window]   Figure 5. Effect of the PKC inhibitor BIM on venular hyperpermeability induced by diabetes. Number in parentheses indicate the number of vessels studied. *Significant difference vs nondiabetic group.

View larger version (49K): [in this window] [in a new window]   Figure 6. Changes in permeability of diabetic coronary venules before and after the inhibition of PKC with HBDDE and PKCß with hispidin. Numbers in parentheses indicate the number of vessels studied. *Significant difference vs before the inhibitors.

PKC Activity
The protein kinase assay showed that the overall enzymatic activity of PKC was upregulated in diabetic venules (Figure 4B), with a time course that correlated with that of diabetes-induced venular hyperpermeability. In freshly isolated coronary venules, PKC activity at 2 weeks of diabetes was 100±12.02% of the control value obtained from nondiabetic pigs (n=3, P>0.05), 163.99±72.06% of control at 4 weeks (n=3, P>0.05), and 328.33±51.74% of control at 6 to 8 weeks (n=3, P<0.05). In cultured CVECs, high glucose (50 mmol/L) treatment induced a 73.71±10.78% increase in PKC activity. In Western blot analysis (Figure 7), diabetes did not alter the subcellular distribution of PKC and PKCßI but induced a significant translocation of PKCßII and PKC from the cytosol to the membrane, indicating an upregulation in the activity of the ßII- and -isoforms.

View larger version (40K): [in this window] [in a new window]   Figure 7. Western blot analysis of cytosolic (C) and membranous (M) distribution of PKC isoenzymes in control and diabetic porcine hearts. A, Representative immunoblots of PKC, PKCßI, PKCßII, and PKC. B, Densitometric analysis of 4 experiments with separate pigs.

	Discussion

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The results of the present study demonstrated that an elevation of the glucose concentration in the perfusion media induced a rapid, dose-dependent increase in the permeability of coronary venules. The hyperpermeability response was prevented during pharmacological inhibition of PKC. More importantly, an elevated basal permeability was found in coronary venules at 4 to 8 weeks after STZ-induced diabetes, which was concomitant with an increase in PKC activity in the same tissue. Furthermore, Western blot analysis of the diabetic heart tissue revealed that the membranous distributions of PKCßII and PKC were preferentially increased, whereas PKC and PKCßI did not significantly change. Correlatively, a wide-spectrum PKC inhibitor and a PKCß-specific blockade were able to restore the barrier function of the diabetic coronary venules. The results support a role for PKC in the mediation of microvascular endothelial dysfunction at the early stages of diabetes.

Microvascular endothelial dysfunction plays an important role in the development of diabetic cardiovascular complications.¹ As an early pathophysiological alteration, microvascular leakage precedes structural abnormalities in the microvasculature.¹⁹ Increased extravasation of albumin from microvessels is seen in various tissues of diabetic subjects.^{3 4 5 6} In rats, vascular hyperpermeability is demonstrated as early as 4 weeks after the onset of STZ-induced diabetes.²⁰ However, due to the technical limitations, diabetes-induced early functional changes in the coronary microvascular system remain largely unclarified, and even less is known about the permeability property of coronary exchange vessels under hyperglycemic conditions. In this regard, the present study is the first to show that the coronary venular barrier function is damaged by short-term diabetes. The increase in the permeability of porcine coronary venules occurred 4 weeks after the onset of diabetes, and the hyperpermeability effect was comparably seen in the same types of vessels subjected to high glucose stimulation. This finding is in agreement with the hypothesis that hyperglycemia triggers microvascular endothelial dysfunction during the early period of diabetes.¹⁹ The rapid permeability response of venules to diabetes and high glucose indicates that an impairment of endothelial barrier function may be an initial pathophysiological change in the development of diabetic heart disease. In support of our data, other experiments have demonstrated a dose-dependent increase in transendothelial flux of albumin in cultured cells incubated with high concentrations of glucose.^{7 21 22} Electron microscopy observation of intact perfused rat hearts²³ revealed the formation of endothelial gaps upon hyperglucose stimulation.

A number of hypotheses have been proposed to explain the injurious effect of hyperglycemia, including the polyol pathway, nonenzymatic glycation, oxidant generation, and PKC activation.^{24 25} Of these mechanisms, the PKC signaling has been increasingly recognized as an early and common pathway that leads to vascular complications.^{18 26 27} It is well accepted that elevated blood glucose levels increase the de novo synthesis of diacylglycerol, which in turn stimulates PKC.²⁸ The vascular effects of PKC activation are characterized by endothelial dysfunction and microcirculatory disturbance.^{8 29} Recent studies have revealed a direct relationship between the activity of PKC and vascular permeability in various organs.^{26 30 31} Oral administration of the selective PKC inhibitor LY333531 can normalize the microcirculation in retina and kidney of diabetic rats.²⁶ Another PKC inhibitor, LY290181, prevents glucose-induced increases in endothelial permeability and corresponding vascular changes in vivo.¹³ Our experiments also support the central role of PKC activation in the pathophysiological regulation of microvascular permeability in diabetes.

PKC is a family of serine/threonine kinases that consists of >10 isoforms with distinct cellular function and different expression patterns.²⁶ Previous investigations^{11 32 33} have revealed augmentation of PKC, PKCß, and PKC activity in the myocardium of diabetic rats. Despite the fact that various combinations of PKC isoforms are activated in the diabetic heart, PKCßII activity seems to be predominant in all vascular tissues.¹⁸ Indeed, the ß-isoenzyme was found to be upregulated in porcine hearts from the present study, and the PKCß inhibitor displayed a significant effect in blocking the hyperpermeability response to diabetes in coronary microvessels. Therefore, it is likely that the microvascular endothelial barrier dysfunction in the heart during early diabetes is, at least in part, attributed to the activation of PKCß. In this regard, PKCß may be a potential therapeutic target for vascular complications and cardioangiopathy in diabetes. Regarding the other isoforms, although our experiment showed an elevated PKC activity in the heart, we were unable to define its role in the regulation of venular permeability due to the lack of specific inhibitors.

It should be noted that the previous information regarding the exchange process in diabetes was obtained either through the measurement of tracer clearance, the result of which is complicated by hemodynamic effects and extrinsic factors, or through in vitro assays on cultured cells derived from large vessels, which renders difficulties in the identification of mechanisms distinct to particular microvascular beds. In contrast, our intact perfused venule model enabled a direct assessment of microvascular permeability under conditions in which various influencing factors were tightly controlled, producing results that are physiologically relevant and specific to the exchange vessels of the heart. Furthermore, the correlative measurement of PKC activity in the diabetic venules directly supports the pharmacological approaches used in the evaluation of PKC-elicited coronary hyperpermeability.

In conclusion, the present study indicates that the permeability of coronary venules is increased in the early stages of experimental diabetes. We suggest that activation of PKCß in response to increased glucose levels is involved in the signaling mechanisms that underlie diabetes-induced coronary microvascular dysfunction.

	Acknowledgments

 
This work was supported by National Heart, Lung, and Blood Institute grant HL-61507 and by the Scott and White Hospital Wigley Fund. Dr Yuan is a recipient of National Institutes of Health Research Career Award KO2-HL-03606.

Received March 16, 2000; revision received July 11, 2000; accepted July 11, 2000.

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				Q. Liang, E. C. Carlson, R. V. Donthi, P. M. Kralik, X. Shen, and P. N. Epstein Overexpression of Metallothionein Reduces Diabetic Cardiomyopathy Diabetes, January 1, 2002; 51(1): 174 - 181. [Abstract] [Full Text] [PDF]

				A. I. Vinik, T. Erbas, T. S. Park, K. B. Stansberry, J. A. Scanelli, and G. L. Pittenger Dermal Neurovascular Dysfunction in Type 2 Diabetes Diabetes Care, August 1, 2001; 24(8): 1468 - 1475. [Abstract] [Full Text] [PDF]

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