Type II secretory phospholipase A2 binds to ischemic flip-flopped cardiomyocytes and subsequently induces cell death

Being Eaten Alive: How Energy-Deprived Cells Are Disposed of, Mediated by C-Reactive Protein-Including a Treatment Option

In medicine, C-reactive protein (CRP) has become established primarily as a biomarker, predicting patient prognosis in many indications. Recently, however, there has been mounting evidence that it causes inflammatory injury. As early as 1999, CRP was shown to induce cell death after acute myocardial infarction (AMI) in rats and this was found to be dependent on complement. The pathological effect of CRP was subsequently confirmed in further animal species such as rabbit, mouse and pig. A conceptual gap was recently closed when it was demonstrated that ischemia in AMI or ischemia/hypoxia in the severe course of COVID-19 causes a drastic lack of energy in involved cells, resulting in an apoptotic presentation because these cells cannot repair/flip-flop altered lipids. The deprivation of energy leads to extensive expression on the cell membranes of the CRP ligand lysophosphatidylcholine. Upon attachment of CRP to this ligand, the classical complement pathway is triggered leading to the swift elimination of viable cells with the appearance of an apoptotic cell by phagocytes. They are being eaten alive. This, consequently, results in substantial fibrotic remodeling within the involved tissue. Inhibiting this pathomechanism via CRP-targeting therapy has been shown to be beneficial in different indications.

A novel phosphocholine-mimetic inhibits a pro-inflammatory conformational change in C-reactive protein

C-reactive protein (CRP) is an early-stage acute phase protein and highly upregulated in response to inflammatory reactions. We recently identified a novel mechanism that leads to a conformational change from the native, functionally relatively inert, pentameric CRP (pCRP) structure to a pentameric CRP intermediate (pCRP*) and ultimately to the monomeric CRP (mCRP) form, both exhibiting highly pro-inflammatory effects. This transition in the inflammatory profile of CRP is mediated by binding of pCRP to activated/damaged cell membranes via exposed phosphocholine lipid head groups. We designed a tool compound as a low molecular weight CRP inhibitor using the structure of phosphocholine as a template. X-ray crystallography revealed specific binding to the phosphocholine binding pockets of pCRP. We provide in vitro and in vivo proof-of-concept data demonstrating that the low molecular weight tool compound inhibits CRP-driven exacerbation of local inflammatory responses, while potentially preserving pathogen-defense functions of CRP. The inhibition of the conformational change generating pro-inflammatory CRP isoforms via phosphocholine-mimicking compounds represents a promising, potentially broadly applicable anti-inflammatory therapy.

[CRP apheresis in acute myocardial infarction and COVID-19]

C‑reactive protein (CRP) is the best-known acute phase protein. In humans, inflammation and infection are usually accompanied by an increase in CRP levels in the blood, which is why CRP is an important biomarker in daily clinical routine. CRP can mediate the initiation of phagocytosis by labeling damaged cells. This labeling leads to activation of the classical complement pathway (up to C4) and ends in the elimination of pathogens or reversibly damaged or dead cells. This seems to make sense in case of an external wound of the body. However, in the case of "internal wounds" (e.g., myocardial infarction, stroke), CRP induces tissue damage to potentially regenerable tissue by cell labeling, which has corresponding deleterious effects on cardiac and brain tissue or function. The described labeling of ischemic but potentially regenerable cells by CRP apparently also occurs in coronavirus disease 2019 (COVID-19). Parts of the lung become ischemic due to intra-alveolar edema and hemorrhage, and this is accompanied by a dramatic increase in CRP. Use of selective immunoadsorption of CRP from blood plasma ("CRP apheresis") to rapidly and efficiently lower the fulminant CRP load in the body fills this pharmacotherapeutic gap. With CRP apheresis, it is possible for the first time to remove this pathological molecule quickly and efficiently in clinical practice.

C-Reactive Protein Triggers Cell Death in Ischemic Cells

C-reactive protein (CRP) is the best-known acute phase protein. In humans, almost every type of inflammation is accompanied by an increase of CRP concentration. Until recently, the only known physiological function of CRP was the marking of cells to initiate their phagocytosis. This triggers the classical complement pathway up to C4, which helps to eliminate pathogens and dead cells. However, vital cells with reduced energy supply are also marked, which is useful in the case of a classical external wound because an important substrate for pathogens is disposed of, but is counterproductive at internal wounds (e.g., heart attack or stroke). This mechanism negatively affects clinical outcomes since it is established that CRP levels correlate with the prognosis of these indications. Here, we summarize what we can learn from a clinical study in which CRP was adsorbed from the bloodstream by CRP-apheresis. Recently, it was shown that CRP can have a direct effect on blood pressure in rabbits. This is interesting in regard to patients with high inflammation, as they often become tachycardic and need catecholamines. These two physiological effects of CRP apparently also occur in COVID-19. Parts of the lung become ischemic due to intra-alveolar edema and hemorrhage and in parallel CRP increases dramatically, hence it is assumed that CRP is also involved in this ischemic condition. It is meanwhile considered that most of the damage in COVID-19 is caused by the immune system. The high amounts of CRP could have an additional influence on blood pressure in severe COVID-19.

Selective Apheresis of C-Reactive Protein for Treatment of Indications with Elevated CRP Concentrations

Almost every kind of inflammation in the human body is accompanied by rising C-reactive protein (CRP) concentrations. This can include bacterial and viral infection, chronic inflammation and so-called sterile inflammation triggered by (internal) acute tissue injury. CRP is part of the ancient humoral immune response and secreted into the circulation by the liver upon respective stimuli. Its main immunological functions are the opsonization of biological particles (bacteria and dead or dying cells) for their clearance by macrophages and the activation of the classical complement pathway. This not only helps to eliminate pathogens and dead cells, which is very useful in any case, but unfortunately also to remove only slightly damaged or inactive human cells that may potentially regenerate with more CRP-free time. CRP action severely aggravates the extent of tissue damage during the acute phase response after an acute injury and therefore negatively affects clinical outcome. CRP is therefore a promising therapeutic target to rescue energy-deprived tissue either caused by ischemic injury (e.g., myocardial infarction and stroke) or by an overcompensating immune reaction occurring in acute inflammation (e.g., pancreatitis) or systemic inflammatory response syndrome (SIRS; e.g., after transplantation or surgery). Selective CRP apheresis can remove circulating CRP safely and efficiently. We explain the pathophysiological reasoning behind therapeutic CRP apheresis and summarize the broad span of indications in which its application could be beneficial with a focus on ischemic stroke as well as the results of this therapeutic approach after myocardial infarction.

Secreted Phospholipase A2-IIA Modulates Transdifferentiation of Cardiac Fibroblast through EGFR Transactivation: An Inflammation-Fibrosis Link

Secreted phospholipase A₂-IIA (sPLA₂-IIA) is a pro-inflammatory protein associated with cardiovascular disorders, whose functions and underlying mechanisms in cardiac remodelling are still under investigation. We herein study the role of sPLA₂-IIA in cardiac fibroblast (CFs)-to-myofibroblast differentiation and fibrosis, two major features involved in cardiac remodelling, and also explore potential mechanisms involved. In a mice model of dilated cardiomyopathy (DCM) after autoimmune myocarditis, serum and cardiac sPLA₂-IIA protein expression were found to be increased, together with elevated cardiac levels of the cross-linking enzyme lysyl oxidase (LOX) and reactive oxygen species (ROS) accumulation. Exogenous sPLA₂-IIA treatment induced proliferation and differentiation of adult rat CFs. Molecular studies demonstrated that sPLA₂-IIA promoted Src phosphorylation, shedding of the membrane-anchored heparin-binding EGF-like growth factor (HB-EGF) ectodomain and EGFR phosphorylation, which triggered phosphorylation of ERK, P70S6K and rS6. This was also accompanied by an up-regulated expression of the bone morphogenic protein (BMP)-1, LOX and collagen I. ROS accumulation were also found to be increased in sPLA₂-IIA-treated CFs. The presence of inhibitors of the Src/ADAMs-dependent HB-EGF shedding/EGFR pathway abolished the CF phenotype induced by sPLA₂-IIA. In conclusion, sPLA₂-IIA may promote myofibroblast differentiation through its ability to modulate EGFR transactivation and signalling as key mechanisms that underlie its biological and pro-fibrotic effects.

Antioxidant tempol suppresses heart cytosolic phospholipase A2α stimulated by chronic intermittent hypoxia

Adaptation to chronic intermittent hypoxia (CIH) is associated with reactive oxygen species (ROS) generation implicated in the improved cardiac tolerance against acute ischemia–reperfusion injury. Phospholipases A2(PLA2s) play an important role in cardiomyocyte phospholipid metabolism influencing membrane homeostasis. Here we aimed to determine the effect of CIH (7000 m, 8 h/day, 5 weeks) on the expression of cytosolic PLA2(cPLA2α), its phosphorylated form (p-cPLA2α), calcium-independent (iPLA2), and secretory (sPLA2IIA) at protein and mRNA levels, as well as fatty acids (FA) profile in left ventricular myocardium of adult male Wistar rats. Chronic administration of antioxidant tempol was used to verify the ROS involvement in CIH effect on PLA2s expression and phospholipid FA remodeling. While CIH did not affect PLA2s mRNA levels, it increased the total cPLA2α protein in cytosol and membranes (by 191% and 38%, respectively) and p-cPLA2α (by 23%) in membranes. On the contrary, both iPLA2and sPLA2IIA were downregulated by CIH. CIH further decreased phospholipid n-6 polyunsaturated FA (PUFA) and increased n-3 PUFA proportion. Tempol treatment prevented only CIH-induced cPLA2α up-regulation and its phosphorylation on Ser505. Our results show that CIH diversely affect myocardial PLA2s and suggest that ROS are responsible for the activation of cPLA2α under these conditions.

PX-18 Protects Human Saphenous Vein Endothelial Cells under Arterial Blood Pressure

BACKGROUND

Arterial blood pressure-induced shear stress causes endothelial cell apoptosis and inflammation in vein grafts after coronary artery bypass grafting. As the inflammatory protein type IIA secretory phospholipase A₂ (sPLA₂-IIA) has been shown to progress atherosclerosis, we hypothesized a role for sPLA₂-IIA herein.

METHODS

The effects of PX-18, an inhibitor of both sPLA₂-IIA and apoptosis, on residual endothelium and the presence of sPLA₂-IIA were studied in human saphenous vein segments (n = 6) perfused at arterial blood pressure with autologous blood for 6 hrs.

RESULTS

The presence of PX-18 in the perfusion blood induced a significant 20% reduction in endothelial cell loss compared to veins perfused without PX18, coinciding with significantly reduced sPLA₂-IIA levels in the media of the vein graft wall. In addition, PX-18 significantly attenuated caspase-3 activation in human umbilical vein endothelial cells subjected to shear stress via mechanical stretch independent of sPLA₂-IIA.

CONCLUSIONS

In conclusion, PX-18 protects saphenous vein endothelial cells from arterial blood pressure-induced death, possibly also independent of sPLA₂-IIA inhibition.

Dopamine induces lipid accumulation, NADPH oxidase-related oxidative stress, and a proinflammatory status of the plasma membrane in H9c2 cells

Excess catecholamine levels are suggested to be cardiotoxic and to underlie stress-induced heart failure. The cardiotoxic effects of norepinephrine and epinephrine are well recognized. However, although cardiac and circulating dopamine levels are also increased in stress cardiomyopathy patients, knowledge regarding putative toxic effects of excess dopamine levels on cardiomyocytes is scarce. We now studied the effects of elevated dopamine levels in H9c2 cardiomyoblasts. H9c2 cells were cultured and treated with dopamine (200 μM) for 6, 24, and 48 h. Subsequently, the effects on lipid accumulation, cell viability, flippase activity, reactive oxygen species (ROS) production, subcellular NADPH oxidase (NOX) protein expression, and ATP/ADP and GTP/GDP levels were analyzed. Dopamine did not result in cytotoxic effects after 6 h. However, after 24 and 48 h dopamine treatment induced a significant increase in lipid accumulation, nitrotyrosine levels, indicative of ROS production, and cell death. In addition, dopamine significantly reduced flippase activity and ATP/GTP levels, coinciding with phosphatidylserine exposure on the outer plasma membrane. Furthermore, dopamine induced a transient increase in cytoplasmic and (peri)nucleus NOX1 and NOX4 expression after 24 h that subsided after 48 h. Moreover, while dopamine induced a similar transient increase in cytoplasmic NOX2 and p47^phox expression, in the (peri)nucleus this increased expression persisted for 48 h where it colocalized with ROS. Exposure of H9c2 cells to elevated dopamine levels induced lipid accumulation, oxidative stress, and a proinflammatory status of the plasma membrane. This can, in part, explain the inflammatory response in patients with stress-induced heart failure.

Comparison of docosahexaenoic acid uptake in murine cardiomyocyte culture and tissue: significance to physiologically relevant studies

Long-chain n-3 (or omega 3) fatty acids, namely docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3) have been attributed cardioprotective properties. In this study, we evaluated the incorporation of DHA into cardiomyocytes and the shift in the omega 3/omega 6 ratio after supplementation of primary cardiomyocyte culture. Results are compared with atrial tissue concentrations attained after prolonged feeding of rats. The major difference between in vitro vs. in vivo supplementation is the paradoxical accumulation of arachidonic acid in cultured cardiomyocyte. However, this increase does not give rise to a higher PGE2 production after cellular stimulation, as compared with controls, possibly because of the associated inhibition of sPLA2 by DHA. Notably, in vitro supplementations with DHA 10 to 25μM approximate in vivo pharmacological treatments.

Selective apheresis of C-reactive protein: a new therapeutic option in myocardial infarction?

BACKGROUND

There is substantial evidence that C-reactive protein (CRP) mediates secondary damage of the myocardium after acute myocardial infarction (AMI). The aim of this animal trial in pigs was to specifically deplete CRP from porcine plasma after AMI and to study possible beneficial effects of the reduced CRP concentration on the infarcted area.

METHODS

Ten pigs received balloon catheter-induced myocardial infarction. CRP was depleted from five animals utilizing a new specific CRP-adsorber, five animals served as controls. The area of infarction was analyzed by cardiovascular magnetic resonance imaging on day 1 and day 14 after AMI. Porcine CRP levels were determined by ELISA.

RESULTS

CRP-apheresis resulted in a mean reduction of the CRP levels up to 48.3%. The area of infarction was significantly reduced by 30 ± 6% (P = 0.003) within 14 days in the treatment group, whereas it increased by 19 ± 11% (P = 0.260) in the controls. Fourteen days after infarction, the infarcted area revealed compact, transmural scars in the controls, whereas animals receiving CRP-apheresis showed spotted scar morphology. In the interventional group, a significantly higher left ventricular ejection fraction (LVEF) was observed after 14 days as compared to the controls (57.6 ± 2.4% vs. 46.4 ± 2.7%; P = 0.007).

CONCLUSIONS

In a pig model for AMI, we observed that selective CRP-apheresis significantly reduces CRP levels and the volume of the infarction zone after AMI. Additionally, it changes the morphology of the scars and preserves cardiac output (LVEF).

The multidrug resistance protein breast cancer resistance protein (BCRP) protects adipose-derived stem cells against ischemic damage

Adipose tissue-derived stem cells (ASCs) are promising candidates for regenerative therapy, like after myocardial infarction. However, when transplanted into the infarcted heart, ASCs are jeopardized by the ischemic environment. Interestingly, it has been shown that multidrug resistance (MDR) proteins like the breast cancer resistance protein (BCRP) and P-glycoprotein (P-gp) have a protective effect in haematopoietic stem cells. In ASC, however, only expression of BCRP was shown until now. In this study, we therefore analysed the expression and functional activity of BCRP and P-gp and their putative function in ischemia in ASC. BCRP and P-gp protein expression was studied over time (passages 2-6) using western blot analysis and immunohistochemical staining. MDR activity was analysed using protein-specific substrate extrusion assays. Ischemia was induced using metabolic inhibition. All analyses demonstrated protein expression and activity of BCRP in ASCs. In contrast, only minor expression of P-gp was found, without functional activity. BCRP expression was most prominent in early passage ASCs (p2) and decreased during culture. Finally, ischemia induced expression of BCRP. In addition, when BCRP was blocked, a significant increase in dead ASCs was found already after 1 h of ischemia. In conclusion, ASCs expressed BCRP, especially in early passages. In addition, we now show for the first time that BCRP protects ASCs against ischemia-induced cell death. These data therefore indicate that for transplantation of ASCs in an ischemic environment, like myocardial infarction, the optimal stem cell protective effect of BCRP theoretically will be achieved with early culture passages ASCs.

Cytosolic phospholipase A(2)α protects against ischemia/reperfusion injury in the heart

Studies with sPLA(2) Group X, and cPLA(2) α gene-targeted mice suggest that absence of sPLA(2) Group X results in protection from ischemia/reperfusion (I/R) injury in the heart, and absence of cPLA(2) α Group IV is protective in the brain. Although latter studies might suggest a similar deleterious role for cPLA(2) α in I/R injury in the heart, the pathophysiology of stroke is intricately related to excitotoxicity and cannot necessarily be extrapolated to the heart. We report here that unlike findings in the brain, cPLA(2) α((-/-)) mice have exaggerated injury following I/R in vivo. In contrast, there is no difference in injury induced by simulated ischemia in cardiomyocytes isolated from cPLA(2) α((-/-)) versus cPLA(2) α((+/+)) mice. This suggests that cPLA(2) α does not have an important cardiomyocyte autonomous effect on ischemic injury. Prostaglandin E(2) (PGE(2) ) levels are significantly reduced in the hearts of the cPLA(2) α((-/-)) mice, and the enhanced injury is ameliorated by treatment with the PGE analog, misoprostol. We demonstrate that cPLA(2) α is cardioprotective in vivo, and this is likely via cPLA(2) α-mediated production of cardioprotective eicosanoids. These studies are the first to identify a protective role for cPLA(2) in I/R injury in any organ and raise concerns over long-term inhibition of cPLA(2).

Homocysteine induces phosphatidylserine exposure in cardiomyocytes through inhibition of Rho kinase and flippase activity

AIMS

Increased levels of homocysteine (Hcy) form an independent risk factor for cardiovascular disease. In a previous study we have shown that Hcy induced phosphatidylserine (PS) exposure to the outer leaflet of the plasma membrane in cardiomyocytes, inducing a pro-inflammatory phenotype. In the present study the mechanism(s) involved in Hcy-induced PS exposure were analyzed.

METHODS

H9c2 rat cardiomyoblasts were subjected to 2.5 mM D,L-Hcy and analyzed for RhoA translocation and activity, Rho Kinase (ROCK) activity and expression and flippase activity. In addition, the effect of ROCK inhibition with Y27632 on Hcy-induced PS exposure and flippase activity was analyzed. Furthermore, GTP and ATP levels were determined.

RESULTS

Incubation of H9c2 cells with 2.5 mM D,L-Hcy did not inhibit RhoA translocation to the plasma membrane. Neither did it inhibit activation of RhoA, even though GTP levels were significantly decreased. Hcy did significantly inhibit ROCK activation, but not its expression, and did inhibit flippase activity, in advance of a significant decrease in ATP levels. ROCK inhibition via Y27632 did not have significant added effects on this.

CONCLUSION

Hcy induced PS exposure in the outer leaflet of the plasma membrane in cardiomyocytes via inhibition of ROCK and flippase activity. As such Hcy may induce cardiomyocytes vulnerable to inflammation in vivo in hyperhomocysteinaemia patients.

Inhibition of Rho-ROCK signaling induces apoptotic and non-apoptotic PS exposure in cardiomyocytes via inhibition of flippase

Subsequent to myocardial infarction, cardiomyocytes within the infarcted areas and border zones expose phosphatidylserine (PS) in the outer plasma membrane leaflet (flip-flop). We showed earlier that in addition to apoptosis, this flip-flop can be reversible in cardiomyocytes. We now investigated a possible role for Rho and downstream effector Rho-associated kinase (ROCK) in the process of (reversible) PS exposure and apoptosis in cardiomyocytes. In rat cardiomyoblasts (H9c2 cells) and isolated adult ventricular rat cardiomyocytes Clostridium difficile Toxin B (TcdB), a Rho GTPase family inhibitor, C3 transferase (C3), a Rho(A,B,C) inhibitor and the ROCK inhibitors Y27632 and H1152 were used to inhibit Rho-ROCK signaling. PS exposure was assessed via flow cytometry and fluorescent digital imaging microscopy using annexin V. Akt expression and phosphorylation were analyzed via Western blot, and Akt activity was inhibited by wortmannin. The cellular concentration activated caspase 3 was determined as a measure of apoptosis, and flippase activity was assessed via flow cytometry using NBD-labeled PS. TcdB, C3, Y27632 and H1152 all significantly increased PS exposure. TcdB, Y27632 and H1152 all significantly inhibited phosphorylation of the anti-apoptotic protein Akt and Akt inhibition by wortmannin lead to increased PS exposure. However, only TcdB and C3, but not ROCK- or Akt inhibition led to caspase 3 activation and thus apoptosis. Notably, pancaspase inhibitor zVAD only partially inhibited TcdB-induced PS exposure indicating the existence of apoptotic and non-apoptotic PS exposure. The induced PS exposure coincided with decreased flippase activity as measured with NBD-labeled PS flip-flop. In this study, we show a regulatory role for a novel signaling route, Rho-ROCK-flippase signaling, in maintaining asymmetrical membrane phospholipid distribution in cardiomyocytes.

Neuroprotective effects of a nanocrystal formulation of sPLA(2) inhibitor PX-18 in cerebral ischemia/reperfusion in gerbils

The group IIA secretory phospholipase A2 (sPLA(2)-IIA) has been studied extensively because of its involvement in inflammatory processes. Up-regulation of this enzyme has been shown in a number of neurodegenerative diseases including cerebral ischemia and Alzheimer's disease. PX-18 is a selective sPLA(2) inhibitor effective in reducing tissue damage resulting from myocardial infarction. However, its use as a neuroprotective agent has been hampered due to its low solubility. In this study, we test the possible neuroprotective effects of PX-18 formulated as a suspension of nanocrystals. Transient global cerebral ischemia was induced in gerbils by occlusion of both common carotid arteries for 5 min. Four days after ischemia/reperfusion (I/R), extensive delayed neuronal death, DNA damage, and increases in reactive astrocytes and microglial cells were observed in the hippocampal CA1 region. PX-18 nanocrystals (30 and 60 mg/kg body wt) and vehicle controls were injected i.p. immediately after I/R. PX-18 nanocrystal injection significantly reduced delayed neuronal death, DNA damage, as well as glial cell activation. These findings demonstrated the effective neuroprotection of PX-18 in the form of nanocrystal against I/R-induced neuronal damage. The results also suggest that nanocrystals hold promise as an effective strategy for the delivery of compounds with poor solubility that would otherwise be precluded from preclinical development.

AMP-activated protein kinase influences metabolic remodeling in H9c2 cells hypertrophied by arginine vasopressin

Substrate use switches from fatty acids toward glucose in pressure overload-induced cardiac hypertrophy with an acceleration of glycolysis being characteristic. The activation of AMP-activated protein kinase (AMPK) observed in hypertrophied hearts provides one potential mechanism for the acceleration of glycolysis. Here, we directly tested the hypothesis that AMPK causes the acceleration of glycolysis in hypertrophied heart muscle cells. The H9c2 cell line, derived from the embryonic rat heart, was treated with arginine vasopressin (AVP; 1 μM) to induce a cellular model of hypertrophy. Rates of glycolysis and oxidation of glucose and palmitate were measured in nonhypertrophied and hypertrophied H9c2 cells, and the effects of inhibition of AMPK were determined. AMPK activity was inhibited by 6-[4-(2-piperidin-1- yl-ethoxy)-phenyl]-3-pyridin-4-yl-pyrrazolo-[1,5-a]pyrimidine (compound C) or by adenovirus-mediated transfer of dominant negative AMPK. Compared with nonhypertrophied cells, glycolysis was accelerated and palmitate oxidation was reduced with no significant alteration in glucose oxidation in hypertrophied cells, a metabolic profile similar to that of intact hypertrophied hearts. Inhibition of AMPK resulted in the partial reduction of glycolysis in AVP-treated hypertrophied H9c2 cells. Acute exposure of H9c2 cells to AVP also activated AMPK and accelerated glycolysis. These elevated rates of glycolysis were not altered by AMPK inhibition but were blocked by agents that interfere with Ca2+ signaling, including extracellular EGTA, dantrolene, and 2-aminoethoxydiphenyl borate. We conclude that the acceleration of glycolysis in AVP-treated hypertrophied heart muscle cells is partially dependent on AMPK, whereas the acute glycolytic effects of AVP are AMPK independent and at least partially Ca2+ dependent.

Cyclin-dependent kinase 2 signaling regulates myocardial ischemia/reperfusion injury

Ischemia/reperfusion (I/R) injury to the heart is accompanied by the upregulation and posttranslational modification of a number of proteins normally involved in regulating cell cycle progression. Two such proteins, cyclin-dependent kinase-2 (Cdk2) and its downstream target, the retinoblastoma gene product (Rb), also play a critical role in the control of apoptosis. Myocardial ischemia activates Cdk2, resulting in the phosphorylation and inactivation of Rb. Blocking Cdk2 activity reduces apoptosis in cultured cardiac myocytes. Genetic or pharmacological inhibition of Cdk2 activity in vivo during I/R injury led to a 36% reduction in infarct size (IFS), when compared to control mice, associated with a reduction in apoptotic myocytes. To confirm that Rb was the critical target in Cdk2-mediated I/R injury, we determined the consequences of I/R injury in cardiac-specific Rb-deficient mice (CRb(L/L)). IFS was increased 140% in CRb(L/L) mice compared to CRb+/+ controls. TUNEL positive nuclei and caspase-3 activity were augmented by 92% and 36%, respectively, following injury in the CRb(L/L) mice demonstrating that loss of Rb in the heart significantly exacerbates I/R injury. These data suggest that Cdk2 signaling pathways are critical regulators of cardiac I/R injury in vivo and support a cardioprotective role for Rb.

C4b-binding protein is present in affected areas of myocardial infarction during the acute inflammatory phase and covers a larger area than C3

BACKGROUND

During myocardial infarction reduced blood flow in the heart muscle results in cell death. These dying/dead cells have been reported to bind several plasma proteins such as IgM and C-reactive protein (CRP). In the present study we investigated whether fluid-phase complement inhibitor C4b-binding protein (C4BP) would also bind to the infarcted heart tissue.

METHODS AND FINDINGS

Initial studies using immunohistochemistry on tissue arrays for several cardiovascular disorders indicated that C4BP can be found in heart tissue in several cardiac diseases but that it is most abundantly found in acute myocardial infarction (AMI). This condition was studied in more detail by analyzing the time window and extent of C4BP positivity. The binding of C4BP correlates to the same locations as C3b, a marker known to correlate to the patterns of IgM and CRP staining. Based on criteria that describe the time after infarction we were able to pinpoint that C4BP binding is a relatively early marker of tissue damage in myocardial infarction with a peak of binding between 12 hours and 5 days subsequent to AMI, the phase in which infiltration of neutrophilic granulocytes in the heart is the most extensive.

CONCLUSIONS

C4BP, an important fluid-phase inhibitor of the classical and lectin pathway of complement activation binds to jeopardized cardiomyocytes early after AMI and co-localizes to other well known markers such as C3b.

Secretory type II phospholipase A2 in culprit coronary lesions is associated with myocardial infarction

BACKGROUND

Secretory type-II phospholipase A(2) (sPLA(2)-II) is a cardiovascular risk marker since higher levels of this acute phase protein imply an increased risk for coronary artery disease. Moreover, it is hypothesized that local activity of sPLA(2)-II in the atherosclerotic plaque facilitates an inflammatory response to induce plaque instability or rupture. We have studied the presence of sPLA(2)-II in culprit lesions in the coronary arteries of patients with acute myocardial infarction (AMI) or angina pectoris.

MATERIALS AND METHODS

We performed a histological examination of culprit lesions in 41 patients with stable (SAP) or unstable angina pectoris (UAP), or AMI using directed coronary atherectomy (DCA). Frozen slides were analysed immuno-histochemically for the presence of sPLA(2)-II, macrophages and smooth muscle cells. Immunopositive areas were calculated as a percentage of the total tissue area using image analysis software.

RESULTS

Intracellular sPLA(2)-II was found in atherosclerotic lesions in the macrophages of the intima as well as in vascular smooth muscle cells. Next to this, extracellular sPLA(2)-II depositions were also found. These depositions were significantly more extensive in patients with AMI, i.e. 26%(median)[6%(25th(percentile))-44%(75th(percentile))] of the intima area, than in patients with SAP 0%(median) (0%(25th)-10%(75th); P = 0.013) or UAP 0%(median) (0%(25th)-0%(75th); P = 0.04).

CONCLUSIONS

Extracellular sPLA(2)-II is more abundantly present in atherosclerotic culprit lesions that have led to myocardial infarction. This suggests a role for extracellular sPLA(2)-II in the development of complications of atherosclerotic lesions in coronary arteries.

H9c2 cardiomyoblasts produce thyroid hormone

Thyroid hormone acts on a wide range of tissues. In the cardiovascular system, thyroid hormone is an important regulator of cardiac function and cardiovascular hemodynamics. Although some early reports in the literature suggested an unknown extrathyroidal source of thyroid hormone, it is currently thought to be produced exclusively in the thyroid gland, a highly specialized organ with the sole function of generating, storing, and secreting thyroid hormone. Whereas most of the proteins necessary for thyroid hormone synthesis are thought to be expressed exclusively in the thyroid gland, we now have found evidence that all of these proteins, i.e., thyroglobulin, DUOX1, DUOX2, the sodium-iodide symporter, pendrin, thyroid peroxidase, and thyroid-stimulating hormone receptor, are also expressed in cardiomyocytes. Furthermore, we found thyroglobulin to be transiently upregulated in an in vitro model of ischemia. When performing these experiments in the presence of 125 I, we found that 125 I was integrated into thyroglobulin and that under ischemia-like conditions the radioactive signal in thyroglobulin was reduced. Concomitantly we observed an increase of intracellularly produced, 125 I-labeled thyroid hormone. In conclusion, our findings demonstrate for the first time that cardiomyocytes produce thyroid hormone in a manner adapted to the cell's environment.

Ischemia induces nuclear NOX2 expression in cardiomyocytes and subsequently activates apoptosis

In previous work we have demonstrated increased expression of NOX2 in cardiomyocytes of infarcted human hearts. In the present manuscript we investigated the functional role of NOX2 in ischemically challenged H9c2 cells, a rat cardiomyoblast cell line, and adult rat cardiomyocytes. Expression of NOX2 in H9c2 cells was confirmed by RT-PCR. In Western-blot experiments, increased NOX2 expression was detected during ischemia, which was inhibited by transcription and translation inhibitors. Surprisingly, under ischemia, in addition to an increased cytosolic expression, NOX2 was localized mainly in the nucleus of apoptotic cardiomyocytes, where it colocalized with nitrotyrosine residues and activated caspase 3. Inhibition of reactive-oxygen-species generation with the flavoenzyme inhibitor diphenylene iodonium (DPI) and the NADPH-oxidase inhibitor apocynin led to a significantly decreased induction of apoptosis as assessed by quantification of caspase-3 activity and by TUNEL analysis. These results demonstrate that NOX2 is expressed in the nucleus of cardiomyocytes during apoptosis and that it likely participates in proapoptotic signaling. To the best of our knowledge, this is the first demonstration of nuclear NOX2 expression and its involvement in cardiomyocyte apoptosis.

CRP and the disposal of dying cells: consequences for systemic lupus erythematosus and rheumatoid arthritis

C reactive protein (CRP) levels directly correlate with the disease activity of many inflammatory diseases, e.g. sepsis, infection, and various autoimmunopathies such as rheumatoid arthritis (RA). In contrast, insufficient CRP levels are implicated in the development of systemic lupus erythematosus (SLE). This article reports on the level-depended effects of CRP in various diseases. In detail we show that increased and decreased levels of CRP, as demonstrated in patients with RA and SLE, respectively can contribute to disease progression.

Secretory PLA2-IIA: a new inflammatory factor for Alzheimer's disease

Secretory phospholipase A₂-IIA (sPLA₂-IIA) is an inflammatory protein known to play a role in the pathogenesis of many inflammatory diseases. Although this enzyme has also been implicated in the pathogenesis of neurodegenerative diseases, there has not been a direct demonstration of its expression in diseased human brain. In this study, we show that sPLA₂-IIA mRNA is up-regulated in Alzheimer's disease (AD) brains as compared to non-demented elderly brains (ND). We also report a higher percentage of sPLA₂-IIA-immunoreactive astrocytes present in AD hippocampus and inferior temporal gyrus (ITG). In ITG, the majority of sPLA₂-IIA-positive astrocytes were associated with amyloid β (Aβ)-containing plaques. Studies with human astrocytes in culture demonstrated the ability of oligomeric Aβ_1–42 and interleukin-1β (IL-1β) to induce sPLA₂-IIA mRNA expression, indicating that this gene is among those induced by inflammatory cytokines. Since exogenous sPLA₂-IIA has been shown to cause neuronal injury, understanding the mechanism(s) and physiological consequences of sPLA₂-IIA upregulation in AD brain may facilitate the development of novel therapeutic strategies to inhibit the inflammatory responses and to retard the progression of the disease.

Secretory phospholipase A2 of group IIA: Is it an offensive or a defensive player during atherosclerosis and other inflammatory diseases?

Since its discovery in the serum of patients with severe inflammation and in rheumatoid arthritic fluids, the secretory phospholipase A2 of group IIA (sPLA2-IIA) has been chiefly considered as a proinflammatory enzyme, the result of which has been very intense interest in selective inhibitors of sPLA2-IIA in the hope of developing new and efficient therapies for inflammatory diseases. The recent discovery of the antibacterial properties of sPLA2-IIA, however, has raised the question of whether the upregulation of sPLA2-IIA during inflammation is to be considered uniformly negative and the hindrance of sPLA2-IIA in every instance beneficial. The aim of this review is for this reason, along with the results of various investigations which argue for the proinflammatory and proatherogenic effects of an upregulation of sPLA2-IIA, also to array data alongside which point to a protective function of sPLA2-IIA during inflammation. Thus, it could be shown that sPLA2-IIA, apart from the bactericidal effects, possesses also antithrombotic properties and indeed plays a possible role in the resolution of inflammation and the accelerated clearance of oxidatively modified lipoproteins during inflammation via the liver and adrenals. Based on these multipotent properties the knowledge of the function of sPLA2-IIA during inflammation is a fundamental prerequisite for the development and establishment of new therapeutic strategies to prevent and treat severe inflammatory diseases up to and including sepsis.

Clusterin: a protective mediator for ischemic cardiomyocytes?

We examined the relationship between clusterin and activated complement in human heart infarction and evaluated the effect of this protein on ischemic rat neonatal cardiomyoblasts (H9c2) and isolated adult ventricular rat cardiomyocytes as in vitro models of acute myocardial infarction. Clusterin protects cells by inhibiting complement and colocalizes with complement on jeopardized human cardiomyocytes after infarction. The distribution of clusterin and complement factor C3d was evaluated in the infarcted human heart. We also analyzed the protein expression of clusterin in ischemic H9c2 cells. The binding of endogenous and purified human clusterin on H9c2 cells was analyzed by flow cytometry. Furthermore, the effect of clusterin on the viability of ischemically challenged H9c2 cells and isolated adult ventricular rat cardiomyocytes was analyzed. In human myocardial infarcts, clusterin was found on scattered, morphologically viable cardiomyocytes within the infarcted area that were negative for complement. In H9c2 cells, clusterin was rapidly expressed after ischemia. Its expression was reduced after reperfusion. Clusterin bound to single annexin V-positive or annexin V and propidium iodide-positive H9c2 cells. Clusterin inhibited ischemia-induced death in H9c2 cells as well as in isolated adult ventricular rat cardiomyocytes in the absence of complement. We conclude that ischemia induces the upregulation of clusterin in ischemically challenged, but viable, cardiomyocytes. Our data suggest that clusterin protects cardiomyocytes against ischemic cell death via a complement-independent pathway.

IgM colocalises with complement and C reactive protein in infarcted human myocardium

AIMS

Reperfusion of ischaemic myocardium after acute myocardial infarction (AMI) can induce ischaemia/reperfusion (I/R) injury, as a result of local activation of the complement system. C reactive protein (CRP) is involved in this activation. This study analysed the potential role of IgM in complement activation in the infarcted human myocardium.

METHODS

Immunochemical analysis was carried out on heart specimens from 59 patients who died from AMI. Serial slides of frozen tissue from the infarction site were stained for IgM, complement factors C3d and C5b-9 (membrane attack complex), and CRP.

RESULTS

IgM deposits were found on the plasma membrane, cross striations, and in the cytoplasm of jeopardised cardiomyocytes in infarcts of one to five days duration. IgM depositions were remarkably similar to those of CRP and both complement factors. The relative staining intensities of IgM and CRP varied greatly among patients.

CONCLUSIONS

Similar to CRP, IgM targets complement locally to jeopardised cardiomyocytes in the human heart after AMI. Localisation patterns and relative staining intensities suggest that IgM and CRP recognise similar epitopes in the ischaemic heart, but that the relative contribution of each protein to complement activation in the ischaemic myocardium differs among patients.

Serum Levels of Type II Secretory Phospholipase A2 and the Risk of Future Coronary Artery Disease in Apparently Healthy Men and Women

OBJECTIVES

To study the prospective relationship between serum levels of type II secretory phospholipase A2 (sPLA2) and the risk of future coronary artery disease (CAD) in apparently healthy men and women.

METHODS AND RESULTS

We conducted a prospective nested case-control study among apparently healthy men and women aged 45 to 79 years. Cases (n=1105) were people in whom fatal or nonfatal CAD developed during follow-up. Controls (n=2209) were matched by age, sex, and enrollment time. sPLA2 levels were significantly higher in cases than controls (9.5 ng/mL; interquartile range [IQR], 6.4 to 14.8 versus 8.3 ng/mL; IQR, 5.8 to 12.6; P<0.0001). sPLA2 plasma levels significantly correlated with age, body mass index, systolic blood pressure, high-density lipoprotein (HDL) cholesterol levels, and C-reactive protein (CRP) levels. Taking into account matching for sex and age and adjusting for body mass index, smoking, diabetes, systolic blood pressure, low-density lipoprotein cholesterol, HDL cholesterol, and CRP levels, the risk of future CAD was 1.34 (1.02 to 1.71; P=0.02) for people in the highest sPLA2 quartile, compared with those in the lowest (P for linearity=0.03).

CONCLUSIONS

Elevated levels of sPLA2 were associated with an increased risk of future CAD in apparently healthy individuals. The magnitude of the association was similar to that observed between CRP and CAD risk, and both associations were independent.

Defects in the disposal of dying cells lead to autoimmunity

The fast and efficient uptake of dying cells is of main importance to prevent contact of the immune system with intracellular autoantigens. Insufficient clearance of the latter is discussed to drive the humoral autoimmune response in systemic lupus erythematosus. Many adaptor molecules and receptors are involved in the recognition of dying cells. In this paper we focus on the involvement of phosphatidylserine, glycoproteins, and complement and DNaseI in the clearance of apoptotic and necrotic cells, respectively. Furthermore, extracellular danger signals released from necrotic cells are discussed and the uptake process of primary necrotic cells is investigated in detail. Last but not least, the character and origin of clearance defects observed in some systemic lupus erythematosus patients is presented.

Annexin A5 scintigraphy of forearm as a novel in vivo model of skeletal muscle preconditioning in humans

BACKGROUND

Nonlethal ischemia and reperfusion reduce ischemia-reperfusion-induced cell death, a phenomenon called ischemic preconditioning. In animal models, this potent endogenous protection is mimicked in vivo by administration of adenosine. In humans, exploitation of ischemic preconditioning is hindered by the lack of an appropriate in vivo model to study this phenomenon. To solve this problem, we aimed to set up an easy-to-use human in vivo model to study ischemic or pharmacological preconditioning.

METHODS AND RESULTS

Healthy male volunteers performed unilateral ischemic handgrip. At reperfusion, we intravenously injected technetium-99m-labeled Annexin A5, a presumed marker of ischemic injury, and we imaged both forearms and hands simultaneously with a gamma camera. Region of interest analysis (counts per pixel) and subsequent calculation of the percentage difference in radioactivity between experimental and control hands (thenar muscle; mean+/-SE) revealed significant uptake to the ischemically exercised tissue (26+/-3% at 4 hours after reperfusion; P<0.05). This selective localization of Annexin A5 was reduced by ischemic preconditioning (10 minutes of ischemia plus reperfusion before ischemic exercise) or by infusion of adenosine into the brachial artery to 6+/-1% and 10+/-3%, respectively (P<0.05 versus ischemic exercise alone), resembling observations in animal models with infarct size as an end point. Appropriate control experiments supported our conclusion.

CONCLUSIONS

Annexin A5 scintigraphy can be applied to test pharmacological or physiological interventions for their ability to prevent ischemia-reperfusion injury.