The effects of low density lipoprotein on calcium transients in isolated rabbit cardiomyocytes

Oxidized low-density lipoprotein (oxLDL) affects load-free cell shortening of cardiomyocytes in a proprotein convertase subtilisin/kexin 9 (PCSK9)-dependent way

Recent studies have documented that oxidized low-density lipoprotein cholesterol (oxLDL) levels directly impact myocardial structure and function. However, the molecular mechanisms by which oxLDL affects cardiac myocytes are not well established. We addressed the question whether oxLDL modifies load-free cell shortening, a standardized readout of cardiac cellular function, and investigated whether proprotein convertase subtilisin/kexin-9 (PCSK9) is involved on oxLDL-dependent processes. Adult rat ventricular cardiomyocytes were isolated and incubated for 24 h with oxLDL. PCSK9 was silenced by administration of siRNA. Load-free cell shortening was analyzed via a line camera at a beating frequency of 2 Hz. RT-PCR and immunoblots were used to identify molecular pathways. We observed a concentration-dependent reduction of load-free cell shortening that was independent of cell damage (apoptosis, necrosis). The effect of oxLDL was attenuated by silencing of oxLDL receptors (LOX-1), blockade of p38 MAP kinase activation, and silencing of PCSK9. oxLDL increased the expression of PCSK9 and caused oxidative modification of tropomyosin. In conclusion, we found that oxLDL significantly impaired contractile function via induction of PCSK9. This is the first report about the expression of PCSK9 in adult terminal differentiated ventricular cardiomyocytes. The data are important in the light of recent development of PCSK9 inhibitory strategies.

Physiological and therapeutic regulation of PCSK9 activity in cardiovascular disease

Ischemic heart disease is the main cause of death worldwide and is accelerated by increased levels of low-density lipoprotein cholesterol (LDL-C). Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a potent circulating regulator of LDL-C through its ability to induce degradation of the LDL receptor (LDLR) in the lysosome of hepatocytes. Only in the last few years, a number of breakthroughs in the understanding of PCSK9 biology have been reported illustrating how PCSK9 activity is tightly regulated at several levels by factors influencing its transcription, secretion, or by extracellular inactivation and clearance. Two humanized antibodies directed against the LDLR-binding site in PCSK9 received approval by the European and US authorities and additional PCSK9 directed therapeutics are climbing up the phases of clinical trials. The first outcome data of the PCSK9 inhibitor evolocumab reported a significant reduction in the composite endpoint (cardiovascular death, myocardial infarction, or stroke) and further outcome data are awaited. Meanwhile, it became evident that PCSK9 has (patho)physiological roles in several cardiovascular cells. In this review, we summarize and discuss the recent biological and clinical data on PCSK9, the regulation of PCSK9, its extra-hepatic activities focusing on cardiovascular cells, molecular concepts to target PCSK9, and finally briefly summarize the data of recent clinical studies.

OxLDL stimulates cell proliferation through a general induction of cell cycle proteins

Oxidized low-density lipoprotein (oxLDL) may be involved in atherosclerosis by stimulating proliferation of cells in the vessel wall. The purpose of this study was to identify the mechanism by which oxLDL induces proliferation. Quiescent human fibroblasts and rabbit smooth muscle cells were treated with 0, 10, or 50 microg/ml oxLDL for 24-48 h. This resulted in significant increases in total cell counts at both concentrations of oxLDL, at both time points, for both types of cells. Western blot analysis revealed that oxLDL-stimulated cell proliferation was associated with significant increases in the expression of proteins that regulate entry into and progression through the cell cycle [cell division cycle 2, cyclin-dependent kinase (cdk) 2, cdk 4, cyclin B1, cyclin D1, and PCNA]. Surprisingly, the expression of cell cycle inhibitors (p21 and p27) was stimulated by oxLDL as well, but this was to a lesser extent than the effects on cell cycle-activating proteins. OxLDL also induced nuclear localization of all cell cycle proteins examined. The similar effects of oxLDL on the translocation and expression of both cell cycle-activating and -inhibiting proteins may explain the controlled proliferative phenomenon observed in atherosclerosis as opposed to the more rapid proliferative event characteristic of cancer.

Depressed phosphatidic acid-induced contractile activity of failing cardiomyocytes

The effects of phosphatidic acid (PA), a known inotropic agent, on Ca(2+) transients and contractile activity of cardiomyocytes in congestive heart failure (CHF) due to myocardial infarction were examined. In control cells, PA induced a significant increase (25%) in active cell shortening and Ca(2+) transients. The phospholipase C (PLC) inhibitor, 2-nitro-4-carboxyphenyl N,N-diphenylcarbonate, blocked the positive inotropic action induced by PA, indicating that PA induces an increase in contractile activity and Ca(2+) transients through stimulation of PLC. Conversely, in failing cardiomyocytes there was a loss of PA-induced increase in active cell shortening and Ca(2+) transients. PA did not alter resting cell length. Both diastolic and systolic [Ca(2+)] were significantly elevated in the failing cardiomyocytes. In vitro assessment of the cardiac sarcolemmal (SL) PLC activity revealed that the impaired failing cardiomyocyte response to PA was associated with a diminished stimulation of SL PLC activity by PA. Our results identify an important defect in the PA-PLC signaling pathway in failing cardiomyocytes, which may have significant implications for the depressed contractile function during CHF.

Inhibition of Na(+)/H(+) exchange at the beginning of reperfusion is cardioprotective in isolated, beating adult cardiomyocytes

Stimulation of Na(+)/H(+)exchange during ischemia-reperfusion results in cardiac damage. However, it is unclear whether the Na(+)/H(+)exchanger is active during the ischemic period or during reperfusion. Adult beating cardiomyocytes were exposed to an ischemia mimetic solution for 90 min and then reperfused with a normal solution for 30 min. 5-(N,N-dimethyl)-amiloride (DMA), a blocker of the Na(+)/H(+)exchanger, was administered during ischemia and the first 3 min of reperfusion or only during the first 3 min of reperfusion. Administration of DMA only upon reperfusion resulted in increased cell survival (81+/-1%, P<0.05) compared to using the drug during ischemia and reperfusion (63+/-3%) and in the absence of drug (60+/-1%). During ischemia, pH(i)was lower when DMA was present in the ischemic solution. The inhibition of the Na(+)/H(+)exchanger retarded the recovery of pH during reperfusion. The highest recovery of active cell shortening was observed when DMA was used at the beginning of reperfusion. The use of DMA also reduced the level of passive cell shortening during reperfusion, and when used at the beginning of reperfusion significantly increased the recovery of Ca(2+)transients. Our results demonstrate that the exchanger is primarily active during reperfusion and that inhibition of the exchanger solely at this time has a strong cardioprotective effect.

Lesions in ryanodine channels in smooth muscle cells exposed to oxidized low density lipoprotein

The purpose of the present investigation was to investigate the subcellular basis responsible for the loss of vasoreactivity in atherosclerotic vessels. We have chosen to focus on the potential of oxidized low density lipoprotein (oxLDL), an important atherogenic agent, to alter sarcoplasmic reticulum (SR) structure and function. Vascular smooth muscle cells (VSMCs) were exposed for 1 to 6 days to low concentrations of minimally oxidized LDL. ATP was used to probe SR function in VSMCs. ATP can increase [Ca(2+)](i) in control VSMCs because of a release of Ca(2+) from the SR. However, after chronic exposure to oxLDL, cells lose their ability to increase [Ca(2+)](i) in response to ATP. These cells also exhibit a depressed rise in [Ca(2+)](i) after exposure to ryanodine. These effects were associated with a decreased immunoreactivity for the ryanodine-sensitive Ca(2+)-release channels in the SR of oxLDL-treated cells. Immunohistochemical analysis of aortic sections obtained from rabbits fed a cholesterol-supplemented diet revealed a significant decrease in the immunoreactivity for ryanodine channels in the plaque and in the medial layer underlying the plaque. In summary, our data identify oxLDL as a component within the atherosclerotic milieu capable of inducing a decrease in smooth muscle ryanodine channel density. This alteration is associated with a significant defect in the ability of the SR within the smooth muscle cell to regulate Ca(2+). These lesions may contribute to the altered vasoreactivity exhibited by atherosclerotic vessels.

Chronic exposure of smooth muscle cells to minimally oxidized LDL results in depressed inositol 1,4,5-trisphosphate receptor density and Ca(2+) transients

Oxidized LDL (oxLDL) (0.1 mg/mL) increased [Ca(2+)](i) in vascular smooth muscle cells (VSMCs) within 5 to 10 seconds of incubation. This increase was mediated via an inositol 1,4,5-trisphosphate (IP(3))-dependent release of Ca(2+) from the sarcoplasmic reticulum. However, atherosclerosis is a gradual process in which VSMCs are more likely exposed to low concentrations of oxLDL over extended periods rather than acute exposures. It is very possible, therefore, that lower [oxLDL] and longer exposure times may induce a very different response with regard to regulation of [Ca(2+)](i). VSMCs were incubated with 4- to 100-fold lower [oxLDL] for up to 6 days. The conditions were not cytotoxic. Basal [Ca(2+)](i) was not altered. Surprisingly, however, after chronic exposure to oxLDL, a brief addition of oxLDL (0.1 mg/mL) or norepinephrine failed to elicit the expected rise in Ca(2+)(i). Because the acute effects of oxLDL on control cells were mediated through an IP(3)-dependent pathway, we investigated the integrity of the VSMC IP(3) receptors. Immunocytochemical analysis and Western blots revealed a depression in the density of IP(3) receptors after chronic exposure of VSMCs to oxLDL. These changes in IP(3) receptors have significance under atherosclerotic conditions as well. Immunocytochemical analysis revealed a decrease in IP(3) receptor density in the medial layer under atherosclerotic plaques in situ. Our data, therefore, demonstrate a striking difference between the acute and chronic effects of oxLDL on VSMC calcium. Whereas acute exposure to oxLDL stimulates [Ca(2+)](i), chronic exposure results in depressed Ca(2+) transients, apparently through a decrease in IP(3) receptor density. These changes have functional implications for the atherosclerotic vessel in vivo, and our data implicates oxLDL in this process.

A model of low-flow ischemia and reperfusion in single, beating adult cardiomyocytes

The present study was undertaken to comprehensively characterize low-flow ischemia and reperfusion in single adult cardiomyocytes and to determine whether it is important to control contractile activity. The ischemia-mimetic solution was hypoxic, acidic (pH 6.0), and deficient in glucose but contained elevated KCl. Cardiomyocytes were stimulated to contract throughout ischemia and during reperfusion with control perfusate. After the ischemia-reperfusion insult, cells exhibited poor recovery of active cell shortening, a decrease in passive cell length, increased frequency of necrosis, lower ATP content, and evidence of the generation of oxygen-derived free radicals within the cells. Intracellular lactate concentration increased, pH decreased, and Ca(2+) transients were depressed during the ischemic insult, but the latter two parameters recovered partially on reperfusion. Basal intracellular Ca(2+) concentration was elevated during ischemia and early into reperfusion. Recovery was attenuated in cells that were electrically stimulated to contract throughout ischemia. The duration of ischemia, stimulation frequency, and composition of the ischemia-mimetic solution were important variables. The inclusion of 10 mM lactate in the ischemia-mimetic solution significantly aggravated all the parameters examined above. Our data demonstrate that 1) an ischemia-mimetic solution administered to single, isolated adult cardiomyocytes can reproduce many of the responses observed in whole hearts, 2) caution should be used in adding lactate to an ischemic solution, and 3) it is important to stimulate contractile activity throughout ischemia to reproduce the effects of ischemia in whole hearts.

The importance of lipid solubility in antioxidants and free radical generating systems for determining lipoprotein proxidation

The oxidative modification of low-density lipoprotein (LDL) plays an important role in atherosclerosis. Protecting LDL from oxidation has been shown to reduce the risk of coronary heart disease. In this study, we compared the protective effects of two lipophilic antioxidants (vitamin E and lazaroid) with two hydrophilic antioxidants (trolox and vitamin C) in the presence of several different free radical generating systems. Vitamin E (IC50 = 5.9 microM) and lazaroid (IC50 = 5.0 microM) were more effective in inhibiting lipid peroxidation caused by a Fe-ADP free radical generating system than vitamin C (IC50 = 5.2 x 10(3) microM) and trolox (IC5 = 1.2 x 10(3) microM). Preincubation of lipoproteins with a lipophilic antioxidant increased the protective effect against various free radicals. Preincubation with hydrophilic antioxidants did not have an effect. We also tested the efficacy of the antioxidants when the free radicals were generated within the lipid or the aqueous environment surrounding the LDL. For this purpose, we used the peroxyl generating azo-compounds AMVN (2,2'-azobis(2,4-dimethylvaleronitrile)) and AAPH (2,2'azobis(2-amidinopropane) dihydrochloride). All of the antioxidants tested were more effective against free radicals generated in a water soluble medium than they were against free radicals generated in a lipid environment. In conclusion, our data demonstrate that lipid solubility is an important factor for both the antioxidant and the free radical generating systems in determining the extent of lipid peroxidation in LDL. Our data also demonstrate that antioxidant efficacy in one set of experimental conditions may not necessarily translate into a similar degree of protection in another set of conditions where lipophilicity is a variable.

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.

Decrease in myocardial Na(+)-K(+)-ATPase activity and ouabain binding sites in hypercholesterolemic rabbits

OBJECTIVES

The purpose of this study was to explore the effect of high dietary cholesterol on the lipid composition, Na(+)-K(+)-ATPase activity and ouabain receptor property of the myocardial sarcolemma.

METHODS

Male New Zealand white rabbits were fed with standard chow or standard chow supplemented with 0.5% (w/w) cholesterol and 10% (w/w) coconut oil to induce hypercholesterolemia. After 8 weeks, the rabbits were sacrificed; a myocardial sarcolemma fraction was then prepared from the left ventricular myocardium and analyzed for lipid composition. Assay of Na(+)-K(+)-ATPase activity and 3H-ouabain binding studies were performed in the myocardial sarcolemma from the control and cholesterol-fed rabbits.

RESULTS

The cholesterol content, but not the phospholipid content, of the sarcolemma was significantly greater in the cholesterol-fed group, thus, resulting in an increased cholesterol/phospholipid molar ratio in the cholesterol-fed group. In addition, a decrease in Na(+)-K(+)-ATPase activity was also found in this group. The decrease in Na(+)-K(+)-ATPase activity was selective, since the Mg(++)-ATPase and 5'-nucleotidase activities remained unchanged. In the 3H-ouabain binding study, a decrease in the number of maximum binding sites, but not the binding affinity, for 3H-ouabain was found in the cholesterol-fed group.

CONCLUSIONS

High dietary cholesterol induces higher levels of cholesterol not only in the plasma, but also in the myocardial sarcolemma. These changes result in decreased myocardial Na(+)-K(+)-ATPase activity mediated by a reduction in the maximum number of binding sites for ouabain but not a change in binding affinity.