Apoptosis of human macrophages by Flt-4 signaling: implications for atherosclerotic plaque pathology

BACKGROUND

Neointimal inflammation and angiogenesis are important contributors of progression and destabilization of the atherosclerotic plaque. While the role of vascular endothelial growth factor (VEGF) and its receptors VEGF-R1 (Flt-1) and VEGF-R2 (Flk-1) in this process has clearly been defined, expression of the VEGF-R3 (Flt-4) has only been documented on lymphatic and tumor endothelium. This study examined Flt-4 expression in human atherosclerotic plaque and explored its implications for atherosclerotic disease.

METHODS AND RESULTS

Carotid artery thrombendartherectomy specimens from 10 patients with unstable plaque were stained for Flt-4 and its specific growth factors VEGF-C and VEGF-D. Microvascular endothelial cells (MVEC) stained positive for VEGF-C and -D, but not for Flt-4. Interestingly, macrophages within inflammatory perivascular regions coexpressed Flt-4, VEGF-C and VEGF-D. In vitro studies confirmed the expression of Flt-4, VEGF-C and VEGF-D in human monocytes and cultured macrophages. Treatment of macrophages with VEGF-D induced apoptosis as determined by annexin V staining, by immunoblotting of activated caspase 3, and by the ratio of Bcl-2/Bax as well as by DNA fragmentation. Immunohistochemical studies of advanced human carotid atherosclerotic plaque confirmed the coexpression of Flt-4 with activated caspase 3 and TUNEL staining in macrophages, indicating an ongoing apoptotic process.

CONCLUSION

Human monocytes/macrophages express VEGF-C and -D and their receptor Flt-4 in vitro and in vivo within advanced atherosclerotic lesions. Flt-4, in turn, mediates monocyte/macrophage apoptosis and may this way alter plaque stability.

Blockade of A1 adenosine receptors prevents the ischaemia-induced sensitisation of adenylyl cyclase: evidence for a protein kinase C-mediated pathway

OBJECTIVE

Acute myocardial ischaemia leads to a transient sensitisation of adenylyl cyclase which may contribute to the occurrence of malignant arrhythmias and the propagation of myocardial necrosis. It is prevented by blockade of protein kinase C (PKC) which is activated in early ischaemia as shown by its translocation from the cytosol to the plasma membranes. Translocation of PKC may also occur in ischaemic preconditioning, a process thought to be induced by activation of adenosine A1 receptors. In this study it was investigated whether A1 adenosine receptors may be involved in the sensitisation of adenylyl cyclase and the activation of PKC induced by ischaemia.

METHODS

Isolated rat hearts were perfused with the specific A1 adenosine antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 1 microM) or adenosine (1 microM) prior to ischaemia induced by stop of perfusion for 5 and 10 min. Adenylyl cyclase activity was determined in plasma membranes stimulated by forskolin or stimulated via beta-receptors by isoproterenol. Total PKC activity was measured in purified plasma membranes and in the cytosolic fraction using histone III-S as a substrate.

RESULTS

Myocardial ischaemia induced a beta-receptor-independent sensitisation of adenylyl cyclase (forskolin-stimulated activity 515+/-55 vs. 384+/-30 pmol/min/mg protein) which was completely blocked by pre-perfusion with DPCPX (385+/-23 vs. 386+/-24 pmol/min/mg protein). DPCPX alone did not alter the responsiveness of adenylyl cyclase to stimulation. The stimulated adenylyl cyclase activity was increased by 20 % after pre-perfusion with adenosine, mimicking the ischaemia-induced sensitisation. The effect of adenosine was not augmented by additional ischaemia. PKC activity was translocated from the cytosol to the plasma membranes by acute ischaemia, indicating an activation of the enzyme. This effect was completely abolished by DPCPX.

CONCLUSION

These data demonstrate that in the rat heart the sensitisation of adenylyl cyclase in acute myocardial ischaemia is dependent on activation of A1 adenosine receptors. It is suggested that the sensitisation of adenylyl cyclase by adenosine or ischaemia might be mediated by an activation of PKC.

Desensitization of the pulmonary adenylyl cyclase system: a cause of airway hyperresponsiveness in congestive heart failure?

OBJECTIVES

This study was designed to investigate whether the adrenergic signal transduction in the lung and the responsiveness of airway smooth muscle to adrenergic stimulation are modulated in congestive heart failure.

BACKGROUND

Wheezing and airway hyperresponsiveness are often present in heart failure. In the failing heart, chronic adrenergic stimulation down-regulates beta-adrenergic receptors and adenylyl cyclase. We hypothesized that airway dysfunction in heart failure could be due to a similar modulation of pulmonary adrenergic signal transduction.

METHODS

Heart failure was induced in rats by aortic banding, resulting in increases in plasma norepinephrine, lung wet weight indicating congestion and left ventricular end diastolic pressure after four weeks. Beta-receptor densities in pulmonary plasma membranes were measured by radioligand binding using [125I]iodocyanopindolol. The G protein levels were determined by Western blot. Adenylyl cyclase activities in lung membranes were quantified as [32P]cAMP (cyclic adenosine-5'-monophosphate) synthesis rate. To functionally assess airway smooth muscle relaxation, carbachol-precontracted isolated tracheal strips were used.

RESULTS

Beta-receptor density was significantly decreased in heart failure from 771 +/- 89 to 539 +/- 44 fmol/mg protein without changes in receptor affinities. The beta1-/beta2-subtype ratio, however, remained constant. The G(i and alpha) and G(s alpha) protein expression was unchanged. Adenylyl cyclase activity stimulated directly with forskolin was decreased by 28%. Relaxation of tracheal strips in response to isoproterenol and forskolin, but not to papaverin, was diminished by 30%.

CONCLUSIONS

In heart failure, the down-regulation of pulmonary beta-receptors and concomitant decrease in adenylyl cyclase activity result in a significant attenuation of cAMP-mediated airway relaxation. These mechanisms may play a pivotal role in the pathogenesis of"cardiac asthma."

Two distinct mechanisms mediate a differential regulation of protein kinase C isozymes in acute and prolonged myocardial ischemia

An activation of protein kinase C (PKC) in acute myocardial ischemia has been shown previously using its translocation to the plasma membrane as an indirect parameter. However, whether PKC remains activated or whether other mechanisms such as altered gene expression may mediate an isozyme-specific regulation in prolonged ischemia have not been investigated. In isolated perfused rat hearts, PKC activity and the expression of PKC cardiac isozymes were determined on the protein level using enzyme activities and Western blot analyses and on the mRNA level using reverse transcriptase-polymerase chain reaction after various periods of global ischemia (1 to 60 minutes). As early as 1 minute after the onset of ischemia, PKC activity is translocated from the cytosol to the particulate fraction without change in total cardiac enzyme activity. This translocation involves all major cardiac isozymes of PKC (ie, PKCalpha, PKCdelta, PKCepsilon, and PKCzeta). This rapid, nonselective activation of PKCs is only transient. In contrast, prolonged ischemia (>/=15 minutes) leads to an increased cardiac PKC activity (119+/-7 versus 190+/-8 pmol/min per mg protein) residing in the cytosol. This is associated with an augmented, subtype-selective isozyme expression of PKCdelta and PKCvarepsilon (163% and 199%, respectively). The specific mRNAs for PKCdelta (948+/-83 versus 1501+/-138 ag/ng total RNA, 30 minutes of ischemia) and PKCepsilon (1597+/-166 versus 2611+/-252 ag/ng total RNA) are selectively increased. PKCalpha and PKCzeta remain unaltered. In conclusion, two distinct activation and regulation processes of PKC are characterized in acute myocardial ischemia. The early, but transient, translocation involves all constitutively expressed cardiac isozymes of PKC, whereas in prolonged ischemia an increased total PKC activity is associated with an isozyme-selective induction of PKCepsilon and PKCdelta. Whether these fundamentally different activation processes interact remains to be elucidated.

The cardiac adrenergic system in ischaemia: differential role of acidosis and energy depletion

OBJECTIVE

Acute myocardial ischaemia has been shown to modulate the beta-adrenergic system and to activate protein kinase C. The aim of this study was to investigate if two important components of ischaemia, i.e. energy depletion or acidosis, may contribute to these changes.

METHODS

Isolated rat hearts were perfused either with anoxia (in the absence of oxygen) or with cyanide in the absence of glucose as models of energy depletion with a loss of high energy phosphates. Alternatively, isolated hearts were perfused with acidic modified Krebs-Henseleit solution to induce acidosis.

RESULTS

Energy depletion induced by cyanide perfusion leads to an increase of beta-adrenergic receptors (81 +/- 7 vs. 50 +/- 3 fmol/mg protein, p < or = 0.05) comparable to the changes observed in ischaemia, yet without any change of total adenylyl cyclase activity or protein kinase C activity. Similar, yet less pronounced changes were induced by anoxic perfusion. Acidic perfusion, in contrast, promotes a translocation of protein kinase C to the plasma membranes, suggesting its rapid activation. Additionally, an increased total forskolin-stimulated activity of adenylyl cyclase (515 +/- 16 vs. 428 +/- 17 pmol/min/mg, p < or = 0.05) was observed. Both were comparable to the sensitization observed in early ischaemia. In acidosis, the density of beta-adrenergic receptors remained unaltered.

CONCLUSIONS

These data suggest that the regulation of cardiac beta-adrenergic receptors is susceptible to energy depletion, but not to acidosis, whereas the intracellular enzymes both adenylyl cyclase and protein kinase C may be regulated by intracellular acidosis. This is the first differentiation of distinct components of ischaemia modulating the beta-adrenergic signal transduction pathway. Both components may be operative in concert in acute myocardial ischaemia and may contribute to the regulation of these components of signal transduction observed in acute ischaemia.

Protein kinase C activation after cellular adhesion on fibronectin: partial suppression after inhibition of protein isoprenylation

The cellular events following interaction between matrix proteins and cells are important requisites for physiological mechanisms as well as the progress of a number of diseases. Cellular adhesion to fibronectin, an important component of the extracellular matrix has been demonstrated to be associated with translocation of protein kinase C (PKC) by an integrin-dependent pathway. For this process G-proteins may play an important role as coupling proteins. Membrane association and activity of G-proteins has been shown to be regulated by isoprenylation. We therefore studied whether fibronectin mediated adhesion resulted in PKC translocation and if isoprenylation of cellular proteins may play a role for this integrin-dependent pathway of PKC activation. Chinese hamster ovary (CHO) cells were pretreated with either the Hydroxy-methylglutaryl(HMG)-CoA reductase inhibitor lovastatin or prenylation inhibitor limonene. For the stimulation by extracellular matrices, CHO cells were plated on tissue culture dishes coated with fibronectin or bovine serum albumin and PKC activity was determined. To investigate direct effects of inhibition of isoprenylation on cytoskeletal organization, phalloidin-stained stress fibers were characterized after adhesion on different matrices. CHO cells seeded on fibronectin displayed over twice the PKC translocation to the particulate fraction in comparison to that measured in cells on albumin. Pretreatment of CHO cells with lovastatin or limonene resulted in partial suppression of PKC activation after cell-seeding on the specific matrix fibronectin. Changed PKC distribution was not due to a disorganization of the actin skeleton. These data show that inhibition of isoprenylation of cellular proteins, possibly small Guanosine triphosphate(GTP)-binding proteins, alters only the integrin-mediated PKC distribution but does not greatly influence constitutive PKC distribution.

Beta-blockade reduces effects of adenosine and carbachol by transregulation of inhibitory receptors and Gi proteins

Chronic blockade of stimulatory beta-adrenergic receptors may decrease inhibitory receptors of the adrenergic signal transduction system. This transregulation process might reduce the negative inotropic response of the myocardium to inhibitory receptor stimulation. Rats were treated for 6 days with the beta-blocker atenolol (2 mg/day). beta-Adrenergic receptors in cardiac plasma membranes increased from 49 +/- 6 to 75 +/- 9 fmol/mg protein (means +/- SE; P = 0.053), whereas muscarinic M2 receptors decreased (155 +/- 15 vs. 105 +/- 10 fmol/mg protein; P < or = 0.05). Moreover, inhibitory G alpha(i) proteins were reduced by 36%. The functional responses of isolated hearts to inhibitory agonists after prestimulation with isoproterenol (3 nmol/l) were significantly blunted. The Ki value for the negative inotropic response of the maximal rise in developed left ventricular pressure (dP/dt(max)) to adenosine (0.1-100 micromol/l) increased from 5.9 +/- 1.7 to 24.0 +/- 2.5 micromol/l (P < or = 0.001). A similar rightward shift of the dose-response curve was observed for the effects of adenosine on developed left ventricular pressure (LVP) and of carbachol (0.01-10 micromol/l) on LVP and dP/dt. Thus chronic beta-blockade leads to a coordinate transregulation of inhibitory receptors and Gi proteins, reducing the effects of inhibitory receptor activation of the heart. This mechanism may contribute to the beneficial effects of beta-blocker therapy in heart failure.

Changes in cardiac signal transduction systems in chronic ethanol treatment preceding the development of alcoholic cardiomyopathy

Chronic alcohol consumption has been postulated as an important pathogenetic mechanism for the development of alcoholic cardiomyopathy. This form of chronic heart failure shares with other forms of cardiomyopathy the pronounced alterations of the adrenergic signal transduction systems. These alterations include a significant reduction of beta-adrenergic receptors and a reduced responsiveness of the adenylyl cyclase. Changes of other receptor systems such as alpha-adrenergic and muscarinic receptors have not been studied extensively so far. To address the question if changes of the adrenergic signal transduction systems may occur early in the development of alcoholic cardiomyopathy and if alpha 1-adrenergic receptors and muscarinic receptors may be subjected to an altered expression even before severe impairment of the left ventricular function becomes obvious, rats were chronically fed with an alcohol diet containing 35% of total calorie intake as ethanol. In cardiac plasma membranes beta-adrenergic receptors, alpha 1-adrenergic receptors, muscarinic receptors and adenylyl cyclase activities were determined after 4 and 8 weeks of chronic alcohol treatment. After these periods of chronic alcohol diet no signs of overt heart failure such as pleural effusion or increased lung wet weight as parameters for congestion were present. Body weight gain was comparable in the controls and under chronic alcohol treatment in these adolescent rats. Both after 4 and 8 weeks of chronic alcohol treatment the density of cardiac beta-adrenergic receptors remained unchanged and all adenylyl cyclase activities remained fully responsive. In contrast, after 8 weeks of alcohol treatment the developmental increase of cardiac muscarinic receptors in the adolescent rats was greatly impaired resulting in a significantly reduced expression of these receptors even before clinical signs of heart failure. In contrast the density of cardiac alpha 1-adrenergic receptors were significantly reduced already after 4 weeks of chronic alcohol treatment with an additional impairment of the developmental increase after 8 weeks of alcohol treatment. These data characterize for the first time early changes of cardiac receptor system in chronic alcohol treatment which precede the development of overt heart failure. These changes include alpha 1-adrenergic and muscarinic receptors, but in contrast to severe heart failure, leave the beta 1-adrenergic system and the responsiveness of the adenylyl cyclase intact. Additionally these data show the developmentally increased expression of cardiac alpha 1-adrenergic and muscarinic receptors in rat heart.

Differential regulation of mRNA specific for beta 1- and beta 2-adrenergic receptors in human failing hearts. Evaluation of the absolute cardiac mRNA levels by two independent methods

In human heart failure beta-adrenergic receptors are downregulated which contributes to the reduced responsiveness to positive inotropic beta-agonists in the diseased heart. The present study addressed the question whether the number of beta-adrenergic receptors in the failing human heart is regulated at the level of the mRNA and whether the absolute steady-state levels of subtype-specific mRNAs mirror the expression of receptor-subtype proteins in human heart. In a collaborative effort, two different and independent methods, performed in two independent laboratories, reverse transcription followed by polymerase chain reaction (RT-PCR) and RNase protection assays, were used to determine the absolute steady-state levels of beta 1- and beta 2-adrenergic receptor mRNAs in control (NF) and in failing human hearts. As determined by quantitative RT-PCR the beta 1-mRNA was significantly reduced from 0.98 +/- 0.12 (n = 10) to 0.49 +/- 0.11 pg/microgram total RNA in dilated cardiomyopathy (dCMP, n = 7) and to 0.40 +/- 0.11 pg/microgram total RNA in ischemic cardiomyopathy (iCMP, n = 8). The steady-state levels of mRNA specific for beta 2-adrenergic receptors also tended to be decreased but without reaching significance (NF: 0.16 +/- 0.05, dCMP: 0.11 +/- 0.03, iCMP: 0.13 +/- 0.04 pg/microgram total RNA). RNase protection assays revealed similar values. beta 1-mRNA was found to be significantly reduced from 1.22 +/- 0.22 in NF (n = 10) to 0.63 +/- 0.14 pg/microgram total RNA in dCMP (n = 5) and to 0.52 +/- 0.1 pg/microgram total RNA in iCMP (n = 8). The beta 2-mRNA also tended to be lower in dCMP and in iCMP as compared to NF but again without reaching significance (NF: 0.14 +/- 0.02, dCMP: 0.099 +/- 0.02, iCMP 0.107 +/- 0.02 pg/microgram total RNA). This is the first study to demonstrate in parallel by two different methods performed independently in two laboratories that the ratio of beta 1- and beta 2-adrenergic receptor densities in the left ventricle of the normal human heart of about 80/20 is closely related to the absolute steady state concentrations of their specific mRNA. In addition, the magnitude of the decrease in mRNA-levels of beta 1- and beta 2-adrenergic receptors in the failing human heart closely correlates with the decrease of the respective receptor proteins. These data suggest that the predominant regulation of beta-adrenergic receptors occurs at the mRNA level.

Impaired function of inhibitory G proteins during acute myocardial ischemia of canine hearts and its reversal during reperfusion and a second period of ischemia. Possible implications for the protective mechanism of ischemic preconditioning

A brief antecedent period of myocardial ischemia and reperfusion can delay cellular injury during a subsequent ischemic condition. Recent observations suggest that this protective mechanism depends on the continued activation of adenosine A1 receptors and Gi proteins. During acute myocardial ischemia, sufficient amounts of adenosine for maximal activation of adenosine A1 receptors are released, independent of a preconditioning ischemia. Hence, the protective mechanism of ischemic preconditioning may not exclusively be explained by activation of adenosine A1 receptors. As a working hypothesis, an increased responsiveness of Gi proteins toward receptor-mediated activation, leading to an increased response of Gi-regulated effectors, was tested in this study. In 47 anesthetized dogs, ischemia was induced by proximal ligation of the left anterior descending coronary artery. Animals underwent either a single period of 5 minutes of ischemia (n = 9), a single period of 15 minutes of ischemia (n = 10), 5 minutes of ischemia followed by 15 minutes of reperfusion (n = 8), 15 minutes of ischemia followed by 60 minutes of reperfusion (n = 5), or 5 minutes of ischemia followed by 15 minutes of reperfusion and a second period of 5 minutes of ischemia (n = 15). Sarcolemmal membranes were prepared from the central ischemic area and from the posterior left ventricular wall, which served as the control. During ischemia, carbochol-stimulated GTPase decreased by 38% (control, 33.5 +/- 17.7; ischemia, 24.2 +/- 15 pmol.min-1.mg protein-1; n = 9; P < .001). The decrease in carbachol-stimulated GTPase activity was associated with a 45% decrease in carbachol-mediated inhibition of adenylyl cyclase (control, 28.9 +/- 2.4% maximal inhibition; ischemia, 15.1 +/- 2.6% maximal inhibition; n = 5; P < .001). Prolongation of the ischemic period to 15 minutes did not lead to a further reduction of the Gi-mediated signal transduction. The binding properties of muscarinic receptors were not affected by ischemia. Furthermore, as demonstrated by carbachol-stimulated binding of [gamma-35S]GTP to sarcolemmal membranes, high- and low-affinity binding sites for the muscarinic antagonist carbachol, the EC50 for carbachol-stimulated GTPase activity and the substrate dependency of the high-affinity GTPase, the interaction between muscarinic receptors and inhibitory G proteins, and GTP binding to G proteins were not altered (n = 14). Immunoblotting with alpha 1- and alpha 2-specific antibodies did not indicate a loss of Gi proteins during ischemia that could explain the reduced GTPase activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of beta-adrenergic receptors in acute myocardial ischemia: subtype-selective increase of mRNA specific for beta 1-adrenergic receptors

Acute myocardial ischemia leads to a rapid increase of cardiac beta-adrenergic receptors in plasma membranes despite the release of large and desensitizing amounts of endogenous catecholamines. Part of this increase has been shown to occur at the expense of intracellular receptors. To investigate whether an additional expressional regulation of beta-adrenergic receptors due to an increase of mRNA levels is involved, the mRNA levels specific for beta 1- and beta 2-adrenergic receptors were determined after various periods of global ischemia in isolated perfused rat hearts. The subtype-specific quantification of mRNA for beta 1- and beta 2-adrenergic receptors was determined using reverse-transcription followed by PCR (RT-PCR) and RNA protection assays. RT-PCR resulted in single amplification products of the expected sizes (159 bp for beta 1-adrenergic receptors and 240 bp for beta 2-adrenergic receptors). The specificity of these amplification products was confirmed by specific restriction digests. Southern blot hybridizations with internal oligonucleotides and sequencing using the dideoxy chain termination method. For quantification purposes, the mRNAs of housekeeping gene GAPDH and of cardiac alpha-actin were determined as internal standards. Additionally, cRNAs specific for beta 1- and beta 2-adrenergic receptors were used as external standards. Brief periods of global ischemia induced a rapid increase in the steady state level of mRNA for beta 1-adrenergic receptors. There was a statistically significant rise already after 15 min by 57% compared to controls. After 30 min of ischemia the mRNA levels had almost doubled. After 60 min of ischemia, the mRNA levels specific for beta 1-adrenergic receptors tended to decrease, but remained significantly above normoxic controls. In contrast, the mRNA levels specific for beta 2-adrenergic receptors remained constant up to 60 min of global myocardial ischemia. To investigate, whether agonist occupancy of the receptors may contribute to this regulation, the effect of preperfusion with the beta-blocker alprenolol was determined. Contrary to expectation, beta-blockade did not influence the ischemia-induced increase of mRNA levels specific for beta 1-adrenergic receptors. These data demonstrate for the first time, that acute myocardial ischemia induces a rapid, and subtype-selective regulation of mRNA levels for beta 1-adrenergic receptors. However, occupation or activation of beta-adrenergic receptors by an agonist is not involved in this newly characterized regulation of mRNA for beta 1-adrenergic receptors in acute myocardial ischemia.

Chronic beta-blockade transregulates inhibitory A1 adenosine and muscarinic M2 receptors of the adenylyl cyclase system

Chronic beta-blockade has evolved to an important therapeutic strategy in medicine. Not all its therapeutic effects may be explained by its direct action on the beta-adrenergic system. We therefore investigated if chronic beta-blockade in vivo or in isolated cell systems may modulate also inhibitory receptors of the adenylyl cyclase system. Chronic treatment with metoprolol for 6 days (10 mg/day) induced an increase of beta-adrenergic receptors in rat cardiac plasma membranes (53 +/- 8 vs 80 +/- 12 fmol/mg protein). Simultaneously the density of cardiac muscarinic M2 receptors decreased significantly from 150 +/- 17 to 110 +/- 12 fmol/mg protein without any change of the affinity of the receptors for their agonists or antagonists. By this mechanism chronic beta-blockade leads to an unexpected impairment of the muscarinic-mediated inhibition of the adenylyl cyclase. This transregulation of inhibitory receptors by chronic beta-blockade was not restricted to the heart but also reduced the muscarinic receptors of rat lung (35 +/- 4 vs 24 +/- 3 fmol/mg protein). Additionally, other inhibitory receptors of the adenylyl cyclase system such as the A1 adenosine receptors of rat brain were reduced by chronic beta-blockade (532 +/- 32 vs 444 +/- 26 fmol/mg protein). This transregulation of A1 adenosine receptors occurred only after chronic beta-blockade with the active stereoisomer (-)-metoprolol whereas the (+)-isomer was ineffective. The ability of the remaining A1 adenosine receptors to form the agonist-promoted high affinity state was unaltered. Their reduction, however, was sufficient to abolish the phenylisopropyl-mediated inhibition of the adenylyl cyclase. To evaluate if this regulation of various inhibitory receptors in different organs may represent a general cellular regulation mechanism, we investigated whether this transregulation also occurred in isolated cells. Isolated smooth muscle cells derived from the vas deferens (DDT1 MF-2) were cultivated in the presence of the beta-blocker atenolol (10(-5) M) for 3 days. Chronic beta-blockade in these isolated cells induced an increase of beta-adrenergic receptors and concomitantly a significant decrease of A1 adenosine receptors (460 +/- 42 vs 368 +/- 18 fmol/mg protein). The affinity of the A1 adenosine receptors for their agonists and antagonists and the ability of the remaining receptors to form the agonist-promoted high affinity state remained unaltered. In contrast, the reduction of receptor density greatly impaired the adenosine-mediated inhibition of the adenylyl cyclase. These data demonstrate that chronic beta-blockade leads to a functionally significant reduction of inhibitory receptors of the adenylyl cyclase system.(ABSTRACT TRUNCATED AT 400 WORDS)

Alpha 1-receptor-independent activation of protein kinase C in acute myocardial ischemia. Mechanisms for sensitization of the adenylyl cyclase system

The activity of the adrenergic system plays an important role in the genesis of malignant arrhythmias and the spreading of the infarcted zone in acute myocardial ischemia. Acute myocardial ischemia induces an increased activity of adenylyl cyclase. This sensitization at the enzyme level as shown in the isolated perfused rat heart occurs rapidly after the onset of ischemia (5-15 minutes) and is rapidly reversible on reperfusion. With prolonged ischemia, it is only transient and is followed by a gradual loss of the adenylyl cyclase activity. The increased activity of adenylyl cyclase is even retained after partial purification, suggesting a covalent modification of the enzyme. Blockade of alpha 1-adrenergic receptors does not prevent this sensitization, demonstrating that it occurs independently of alpha 1-adrenergic receptor activation. Only blockade of protein kinase C by various inhibitors, such as polymyxin B or staurosporine, is able to completely prevent this sensitization process. Moreover, in acute myocardial ischemia an activation of protein kinase C could be identified using its translocation from the cytosol to the particulate fraction as an indicator. Blockade of alpha 1-adrenergic receptors using prazosin fails to prevent the activation of protein kinase C and consequently the sensitization of the adenylyl cyclase system, indicating that the ischemia-induced translocation of protein kinase C occurs independently of alpha 1-adrenergic receptors. These data characterize for the first time an important interaction of two effector enzymes of two distinct signal transduction pathways, i.e., the adenylyl cyclase system and the protein kinase C system in acute myocardial ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensitization of the beta-adrenergic system in acute myocardial ischaemia by a protein kinase C-dependent mechanism

In acute myocardial infarction the occurrence of malignant arrhythmias and the spreading of the infarcted zone followed by the development of heart failure determine the clinical outcome of the disease. The activity of the adrenergic system plays an important role in both. At various levels acute myocardial ischaemia induces an inadequate activation of the adrenergic system. The increased presynaptic release of endogenous catecholamines does not promote the expected desensitization at the postsynaptic level. In contrast, acute ischaemia leads to a rapid and persistent increase of functionally coupled beta-adrenergic receptors, which in the early phase of acute ischaemia, induce an increased responsiveness of the adenylyl cyclase system to beta-adrenergic stimulation. This sensitization at the receptor level is superimposed by a receptor-independent sensitization of the adenylyl cyclase and a loss of tonic inhibition due to the functional impairment of the inhibitory G protein. At the enzyme level a transient sensitization of adenylyl cyclase in acute myocardial ischaemia is due to a modification of the enzyme, which is tightly associated with the purified enzyme. Only inhibition of protein kinase C is able to block completely the ischaemia-induced sensitization of adenylyl cyclase. Based on these data, it could be demonstrated that acute myocardial ischaemia leads to a rapid activation of protein kinase C by an as yet undefined mechanism. Beyond the sensitization of adenylyl cyclase, activation of protein kinase C may directly activate ion channels or the N+/H+ echanger, and it may induce the increased expression of oncogenes and thus crucially influence the clinical outcome of an acute myocardial infarction.

Supersensitivity of the adenylyl cyclase system in acute myocardial ischemia: evaluation of three independent mechanisms

Malignant arrhythmias and the spreading of the infarcted zone in acute myocardial ischemia may be influenced by the sympathetic system. It has been known for quite some time that acute ischemia leads to an increased release of endogenous catecholamines. Adaptive mechanisms at the postsynaptic level such as receptor desensitization, which are operative under normoxic conditions, are abolished in acute myocardial ischemia. On the contrary, three newly characterized, distinct mechanisms lead to a transiently increased activity of the beta-adrenergic system in the early phase of acute ischemia: 1) Functionally coupled beta-adrenergic receptors are rapidly and persistently increased at the cell surface due to the impairment of beta-agonist-promoted uncoupling and internalization. 2) Despite the reversible increase of inhibitory, muscarinic M2 receptors, the inhibitory pathway of the adenylyl cyclase systems becomes ineffective since the coupling protein, Gi, is rapidly impaired. Both the Gi-linked GTPase-activity and the binding of [gamma-35S]GTP are reduced by 25-30% without any loss of the total protein. Stimulatory effects prevail at the G-protein level since in the early period of ischemia the stimulatory G-protein, Gs, remains intact. 3) The adenylyl cyclase is transiently sensitized by about 30%. This increased activity is closely associated with the partially purified enzyme and may be due to a rapidly reversible covalent modification. Prolonged ischemia, in contrast, results in a general decrease of the cyclase activity notwithstanding any changes at the receptor or G-protein level. The individual mechanisms may play distinct and/or complimentary roles in the early sensitization of the adenylyl cyclase system in acute myocardial ischemia.

Dual sensitization of the adrenergic system in early myocardial ischemia: independent regulation of the beta-adrenergic receptors and the adenylyl cyclase

Acute myocardial ischemia provokes sensitization of the adenylyl cyclase system. This sensitization can be differentiated in a receptor-specific and an enzyme-specific sensitization. The receptor-linked sensitization is characterized by an increase of beta-adrenergic receptors in the plasma membranes after 15 mins of global ischemia (49.8 +/- 3.6 to 67 +/- 6 fmol/mg protein) followed by a further increase (89 +/- 4 fmol/mg protein) after 50 min of ischemia in isolated perfused hearts. Concomitantly functionally coupled receptors which are able to bind the beta-agonist with high affinity, increased by 32% after 15 min and by 57% after 50 min of ischemia. The affinities of the receptors for their agonists or their antagonists remain unchanged. Maximally isoproterenol-stimulated adenylyl cyclase activity rose from 66 +/- 7 to 101 +/- 10 pmol cAMP/min/mg protein after 15 min of global ischemia indicating the beta-receptor-specific sensitization of the beta-adrenergic system. This sensitization was followed by a gradual decline of the adenylyl cyclase activity after 30 and 50 min of global ischemia. Additionally, 15 min of myocardial ischemia induced an enzyme-linked sensitization of the adenylyl cyclase activity as indicated by an increase of the forskolin-stimulated activity by about 25% (300 +/- 20 vs 378 +/- 25 pmol cAMP/min/mg protein). In contrast after 50 min of ischemia the total adenylyl cyclase activity declined (232 +/- 24 pmol cAMP/min/mg protein) despite the persistent increase of beta-adrenergic receptors in the plasma membranes. These data demonstrate that the enzyme-specific sensitization is only transient. The early sensitization and late inactivation of the adenylyl cyclase activity occurred independently of receptor activation and could not be prevented by beta-blockade (10(-6) M alprenolol). Cyanide perfusion (1 mM), used to block energy metabolism, lead to energy depletion similar to acute myocardial ischemia. This resulted in an increase of functionally coupled receptors with a time course comparable to that of global ischemia. Additional perfusion with desensitizing concentrations of the beta-agonist isoproterenol did not induce uncoupling or internalization of beta-adrenergic receptors in cyanide treated hearts, suggesting that the rise in functionally coupled receptors is due to a redistribution in part caused by the abolition of continuous receptor internalization. In contrast, the enzyme-linked sensitization is independent of cellular localization of the beta-adrenergic receptors. The increased activity was carried by the enzyme even after partial purification with solubilization and wheat germ affinity chromatography. These data suggest an ischemia-induced, covalent modification of the adenylyl cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenergic receptors and sensitization of adenylyl cyclase in acute myocardial ischemia

Acute myocardial ischemia leads to a gradual increase in beta-adrenergic receptors at the cell surface. This increase occurs rapidly after onset of global ischemia (15 minutes) and persists even after prolonged periods of global ischemia. This alteration can be observed both in vivo and in vitro in isolated perfused hearts. Several groups have previously shown that ischemia induces a local release of endogenous catecholamines. Here, we show that these endogenous catecholamines are sufficiently high to induce receptor desensitization with internalization of beta-adrenergic receptors in normal hearts. In acute myocardial ischemia, however, agonist-promoted internalization and functional uncoupling of beta-adrenergic receptors is abolished. Consequently, the balance of internalization and externalization of receptors is shifted toward an increase in functionally coupled receptors at the cell surface. Similarly, but inconsistently, the density of alpha 1-adrenergic receptors in the plasma membrane is increased in acute myocardial ischemia. In regard to function, the increase of coupled beta-adrenergic receptors leads to an augmented responsiveness of the adenylyl cyclase system to beta-adrenergic stimulation. This receptor-specific sensitization is superimposed by a transient increase of total adenylyl cyclase activity in the very early phase of global ischemia (0-20 minutes). The enhanced activity of adenylyl cyclase to direct stimulation is tightly associated with the partially purified enzyme, suggesting a covalent modification of the enzyme molecule. However, after prolonged periods (greater than 30 minutes) of global ischemia, the ischemia-induced enzyme-specific sensitization is displaced by a general reduction in enzyme activity, both in vivo and in vitro. The persistent sensitization at the receptor level then meets an unresponsive adenylyl cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Independent sensitization of beta-adrenoceptors and adenylate cyclase in acute myocardial ischaemia

1. Acute myocardial ischaemia provokes sensitization of the adenylate cyclase system. This sensitization could be differentiated in a receptor-linked and an enzyme-linked sensitization. The increase in the number of beta-adrenoceptors in the plasma membranes was observed already after 15 min of global ischaemia (50 +/- 2 to 67 +/- 6 fmol mg-1 protein) and persisted after 50 min of ischaemia. The maximally isoprenaline-stimulated adenylate cyclase activity rose from 66 +/- 7 to 100 +/- 10 pmol cAMP min-1 mg-1 protein after 15 min of global ischaemia indicating the receptor-mediated sensitization of the beta-adrenergic system. However, after 50 min of ischaemia the isoprenaline-stimulated adenylate cyclase was reduced by about 50% despite the continuous increase of beta-adrenoceptors in the plasma membranes. 2. Additionally direct stimulation of the adenylate cyclase by forskolin revealed an increased enzyme activity after 15 min of global ischaemia (300 +/- 20 vs 378 +/- 25 pmol cAMP min-1 mg-1). Prolonged periods of ischaemia, however, caused a decline of the total adenylate cyclase activity (232 +/- 24 pmol cAMP min-1 mg-1 protein). This demonstrates an enzyme-specific sensitization of the adenylate cyclase, which in contrast to the rise in beta-adrenoceptors is only transient. This enzyme-specific sensitization or the late inactivation of the enzyme occur independently of receptor activation and cannot be prevented by beta-adrenoceptor blockade (10(-6) M alprenolol) prior to the ischaemic insult.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a noncatalytic domain in AMP deaminase that influences binding to myosin

AMP deaminase (AMP-D) plays a critical role in energy metabolism in skeletal muscle. Prior studies have demonstrated AMP-D binds to myosin heavy chain in vitro, and it decorates the end of the A band in the myofibril. The present study presents evidence that proteolytic removal of 14 kilodaltons, presumably from the carboxy terminus, of the native 80K peptide does not eliminate catalytic activity but this deletion has a pronounced influence on binding of AMP-D to myosin in the presence of ATP. Comparison of the sequence of the rat skeletal muscle form of AMP-D to that of yeast AMP-D demonstrates conservation of an ATP binding site in the carboxy-terminal domain of the rat protein. These results provide a mechanism for regulating binding of AMP-D to myosin heavy chain in response to changes in ATP concentration and suggest a potential function for AMP-D/myosin complex formation in myocytes.

Binding of adenylosuccinate synthetase to contractile proteins of muscle

The muscle isozyme of adenylosuccinate synthetase (AdSS), an enzyme of the purine nucleotide cycle, has previously been shown to bind to purified F-actin in buffers of low ionic strength and pH (Ogawa et al. Eur. J. Biochem. 85: 331-338, 1978). We have extended these observations by measuring the association of both crude and purified AdSS with the contractile proteins of muscle in buffers of physiological ionic strength and pH. Under these conditions, the enzyme binds to F-actin, actin-tropomyosin complexes, reconstructed thin filaments, and myofibrils but not to myosin. The apparent dissociation constant of 1.2 microM and binding maximum of 2.6 nmol enzyme/mg myofibrils indicate that binding of AdSS to myofibrils can be physiologically significant. The results suggest that AdSS in muscle may be associated with the thin filament of myofibrils.

Cloning and sequence of rat myoadenylate deaminase cDNA. Evidence for tissue-specific and developmental regulation

Myoadenylate deaminase is the muscle-specific isoform of AMP deaminase (EC 3.5.4.6), an enzyme which plays a special role in energy metabolism in skeletal muscle. A 2.3-kilobase cDNA encoding this enzyme has been cloned from a lambda gt10 library prepared from rat skeletal muscle using oligonucleotide probes designed from AMP deaminase peptide sequences. This cDNA was sequenced, and the amino acid sequence of this isoform of AMP deaminase was deduced. Sequences homologous to this cDNA are identified in the genome of eukaryotes as diverse as yeast and man. Tissue-specific expression of a 2.5-kilobase AMP deaminase transcript is demonstrated, and the abundance of this transcript as well as the 80-kDa adult, muscle-specific peptide of AMP deaminase increase in parallel during postnatal skeletal muscle development. In the adult animal, the abundance of this transcript and AMP deaminase activity are differentially expressed in various skeletal muscle fiber types. We conclude that AMP deaminase sequences have been highly conserved during evolution, and in mammals there is developmental and tissue-specific control of expression of this gene.

Evidence for sequential expression of multiple AMP deaminase isoforms during skeletal muscle development

AMP deaminase (myoadenylate deaminase; EC 3.5.4.6) is an integral part of the myofibril in skeletal muscle, and this enzyme plays an important role in energy metabolism in this tissue. We report here the identification of three AMP deaminase isoforms during skeletal muscle development in the rat. An embryonic isoform is expressed in the developing hindlimb of the rat between 7 and 14 days of gestation. This isoform is not unique to skeletal muscle or the embryo as it is also expressed in many nonmuscle tissues of the perinatal and adult rat. A perinatal isoform of AMP deaminase that is restricted to skeletal muscle is produced 4-6 days before birth and persists for 2-3 weeks of postnatal life. An adult, skeletal muscle-specific isoform of AMP deaminase appears at birth and reaches maximal levels after 3 weeks of postnatal development. We conclude from these studies there is a developmentally controlled program that leads to the sequential expression of AMP deaminase isoforms during the transition from embryonic to adult skeletal muscle.

Binding of phosphorylase a and b to skeletal muscle thin filament proteins

Phosphorylase plays an important role in energy generation during muscle contraction. We have demonstrated that purified rabbit skeletal muscle phosphorylase a and phosphorylase b bind to rabbit muscle F-actin, F-actin-tropomyosin, F-actin-tropomyosin-troponin, and myofibrils. Neither phosphorylase a nor phosphorylase b binds to myosin. Phosphorylase a and b bind to F-actin with S0.5 values of 1.5 X 10(-6) and 2.1 X 10(-6) M, respectively. At saturation, 0.035 mol of phosphorylase a and b is bound for every seven G-actin monomers in the F-actin polymer. Using the F-actin-tropomyosin-troponin complex as opposed to F-actin as a binding target, there are five- and threefold increases in the maximal binding capacity for phosphorylase a and phosphorylase b, respectively, without a significant change in the S0.5 value for either form of the enzyme. A similar stoichiometry and affinity of phosphorylase binding are observed when myofibrils are used as the binding target. Ca2+ ions and AMP increase the maximal binding capacity for phosphorylase a to myofibrils while ATP decreases the Bmax. Our study suggests that in skeletal muscle, phosphorylase a and phosphorylase b may interact with the thin filament, and that this binding to thin filament proteins may be controlled by changes in sarcoplasmic concentration of Ca2+ and ligands of phosphorylase during muscle contraction.

The use of nucleotide phosphorothioate diastereomers to define the structure of metal-nucleotide bound to GTP-AMP and ATP-AMP phosphotransferases from beef-heart mitochondria

The diastereomers of adenosine 5'-O-[1-thio]triphosphate (ATP[alpha S]) and adenosine 5'-O-[2-thio]triphosphate (ATP[beta S]) were utilized to seek unambiguous assignment of Mg2+ coordination to ATP when bound to ATP-AMP phosphotransferase from beef heart mitochondria (AK2). Similarly, the diastereomers of guanosine 5'-O-[thio]triphosphate (GTP[alpha S]) and guanosine 5'-O-[2-thio]triphosphate (GTP[beta S]) were utilized to seek unambiguous assignment of Mg2+ coordination to GTP when bound to GTP-AMP phosphotransferase from beef heart mitochondria (AK3). Furthermore the diastereomers of guanosine 5'-O-[1-thio]diphosphate (GDP-[alpha S]) have been used to assign Mg2+ coordination to GDP when bound to AK3. The ratios (V for isomer Sp)/(V for isomer Rp) obtained in the presence of Mg2+ and Cd2+ are compared to those already published for ATP-AMP phosphotransferases from pig muscle (AK1) [Kalbitzer et al. (1983) Eur. J. Biochem. 133, 221-227] and from baker's yeast (AKy) [Tomasselli and Noda (1983) Eur. J. Biochem. 132, 109-115]. In all cases, coordination of Mg2+ to the beta-phosphate via the pro-R oxygen is present, as shown by reversal of specificity for the diastereomers of ATP [beta S] or GTP [beta S] respectively on changing the metal ion. In contrast, there is no reversal of specificity for the diastereomers of ATP [alpha S] or GTP[alpha S], or for GDP[alpha S] in the case of AK3 for the reverse reaction, indicating that there is no interaction of the metal with the alpha-phosphate group. The observed stereospecificity for the alpha-thiophosphate is consistent with the assumption of an interaction of the pro-R oxygen of the alpha-phosphate group with the enzyme.

The structural isomerisation of human-muscle adenylate kinase as studied by 1H-nuclear magnetic resonance

Human muscle adenylate kinase (ATP:AMP phosphotransferase, EC 2.7.4.3.) was studied by 1H-nuclear magnetic resonance spectroscopy. The C-2 and C-4 proton resonances of the active-center histidine His-36 could be identified; the pK of His-36 was determined as 6.1. The pK of His-189 is very low (4.9) although it is located at the surface of the protein. Other resonance lines are discussed in comparison with NMR spectra of porcine adenylate kinase [McDonald et al. (1975) J. Biol. Chem. 250, 6947-6954]. A pH-dependent structural isomerization as shown by X-ray crystallography in the pig enzyme [Pai et al. (1977) J. Mol. Biol. 114, 37-45] was not observed for human adenylate kinase in solution. However, the binding of adenosine(5')pentaphospho(5')adenosine (Ap5A), a bisubstrate inhibitor, to adenylate kinase causes an overall change of the NMR spectrum indicative of a large conformational change of the enzyme. The exchange rate (koff) for Ap5A was estimated as 10 s-1 and decreases by addition of Mg2+. On the basis of these values and the known dissociation constant it is likely that the binding of Ap5A is a diffusion-controlled process kon being 10(8) M-1 s-1. In conclusion, the system Ap5A/Mg2+/human adenylate kinase, which has been studied by NMR spectroscopy and X-ray diffraction in parallel, is suitable for analyzing the induced fit postulated by Jencks for all kinase-catalyzed reactions.