Michael T. Crow

The β2-Adrenergic Receptor Delivers an Antiapoptotic Signal to Cardiac Myocytes Through Gi-Dependent Coupling to Phosphatidylinositol 3′-Kinase

Abstract—Recent studies have shown that chronic β-adrenergic receptor (β-AR) stimulation alters cardiac myocyte survival in a receptor subtype-specific manner. We examined the effect of selective β1- and β2-AR subtype stimulation on apoptosis induced by hypoxia or H2O2 in rat neonatal cardiac myocytes. Although neither β1- nor β2-AR stimulation had any significant effect on the basal level of apoptosis, selective β2-AR stimulation protected myocytes from apoptosis. β2-AR stimulation markedly increased mitogen-activated protein kinase/extracellular signal–regulated protein kinase (MAPK/ERK) activation as well as phosphatidylinositol-3′-kinase (PI-3K) activity and Akt/protein kinase B phosphorylation. β1-AR stimulation also markedly increased MAPK/ERK activation but only minimally activated PI-3K and Akt. Pretreatment with pertussis toxin blocked β2-AR–mediated protection from apoptosis as well as the β2-AR–stimulated changes in MAPK/ERK, PI-3K, and Akt/protein kinase B. The selective PI-3K inhibitor, LY 29...

Alterations in cardiac gene expression during the transition from stable hypertrophy to heart failure. Marked upregulation of genes encoding extracellular matrix components.

The failing heart is characterized by impaired cardiac muscle function and increased interstitial fibrosis. Our purpose was to determine whether the functional impairment of the failing heart is associated with changes in levels of mRNA encoding proteins that modulate parameters of contraction and relaxation and whether the increased fibrosis observed in the failing heart is related to elevated expression of genes encoding extracellular matrix components. We studied hearts of 18- to 24-month-old spontaneously hypertensive rats with signs and symptoms of heart failure (SHR-F) or without evidence of failure (SHR-NF) and of age-matched normotensive Wistar-Kyoto (WKY) rats. Compared with WKY rats, SHR-NF exhibited left ventricular (LV) hypertrophy (2.2-fold) and right ventricular (RV) hypertrophy (1.5-fold), whereas SHR-F were characterized by comparable LV hypertrophy (2.1-fold) and augmented RV hypertrophy (2.4-fold; all P < .01). Total RNA was isolated from ventricles and subjected to Northern blot analysis. In SHR-F hearts, the level of alpha-myosin heavy chain mRNA was decreased in both ventricles to 1/3 and 1/5 of the SHR-NF and WKY values, respectively (both P < .01). Levels of beta-myosin heavy chain, alpha-cardiac actin, and myosin light chain-2 mRNAs were not significantly altered in hearts of SHR-NF or SHR-F. Levels of alpha-skeletal actin were twofold greater in SHR-NF hearts compared with WKY hearts and were intermediate in SHR-F hearts. Levels of atrial natriuretic factor (ANF) mRNA were elevated threefold in the LV of SHR-NF (P < .05) but were not significantly increased in the RV of SHR-NF compared with WKY rats. During the transition to failure (SHR-F versus SHR-NF), ANF mRNA levels increased an additional 1.6-fold in the LV and were elevated 4.7-fold in the RV (both P < .05). Levels of sarcoplasmic reticulum Ca(2+)-ATPase (SRCA) mRNA were maintained in the LV of hypertensive and failing hearts at levels not significantly different from WKY values. In contrast, the level of RV SRCA mRNA was 24% less in SHR-NF compared with WKY rats, and during the transition to failure, this difference was not significantly exacerbated (29% less than the WKY value). The levels of fibronectin and pro-alpha 1(I) and pro-alpha 1(III) collagen mRNAs were not significantly elevated in either ventricle of the SHR-NF group but were fourfold to fivefold higher in both ventricles of SHR-F (all P < .05).(ABSTRACT TRUNCATED AT 400 WORDS)

p53 and the hypoxia-induced apoptosis of cultured neonatal rat cardiac myocytes.

Myocyte cell loss is a prominent and important pathogenic feature of cardiac ischemia. We have used cultured neonatal rat cardiac myocytes exposed to prolonged hypoxia as an experimental system to identify critical factors involved in cardiomyocyte death. Exposure of myocytes to hypoxia for 48 h resulted in intranucleosomal cleavage of genomic DNA characteristic of apoptosis and was accompanied by increased p53 transactivating activity and protein accumulation. Expression of p21/WAF-1/CIP-1, a well-characterized target of p53 transactivation, also increased in response to hypoxia. Hypoxia did not cause DNA laddering or cell loss in cardiac fibroblasts. To determine whether the increase in p53 expression in myocytes was sufficient to induce apoptosis, normoxic cultures were infected with a replication-defective adenovirus expressing wild-type human p53 (AdCMV.p53). Infected cells expressed high intracellular levels of p53 protein and exhibited the morphological changes and genomic DNA fragmentation characteristic of apoptosis. In contrast, no genomic DNA fragmentation was observed in myocytes infected with the control virus lacking an insert (AdCMV.null) or in cardiac fibroblasts infected with AdCMV.p53. These results suggest that the intracellular signaling pathways activated by p53 might play a critical role in the regulation of hypoxia-induced apoptosis of cardiomyocytes.

Taxol inhibits neointimal smooth muscle cell accumulation after angioplasty in the rat.

Despite significant improvements in the primary success rate of the medical and surgical treatments for atherosclerotic disease, including angioplasty, bypass grafting, and endarterectomy, secondary failure due to late restenosis continues to occur in 30-50% of individuals. Restenosis and the later stages in atherosclerotic lesions are due to a complex series of fibroproliferative responses to vascular injury involving potent growth-regulatory molecules (such as platelet-derived growth factor and basic fibroblast growth factor) and resulting in vascular smooth muscle cell (VSMC) proliferation, migration, and neointimal accumulation. We show here, based on experiments with both taxol and deuterium oxide, that microtubules are necessary for VSMCs to undergo the multiple transformations contributing to the development of the neointimal fibroproliferative lesion. Taxol was found to interfere both with platelet-derived growth factor-stimulated VSMC migration and with VSMC migration and with VSMC proliferation, at nanomolar levels in vitro. In vivo, taxol prevented medial VSMC proliferation and the neointimal VSMC accumulation in the rat carotid artery after balloon dilatation and endothelial denudation injury. This effect occurred at plasma levels approximately two orders of magnitude lower than that used clinically to treat human malignancy (peak levels achieved in this model were approximately 50-60 nM). Taxol may therefore be of therapeutic value in preventing human restenosis with minimal toxicity.

Acute Kidney Injury Leads to Inflammation and Functional Changes in the Brain

Although neurologic sequelae of acute kidney injury (AKI) are well described, the pathogenesis of acute uremic encephalopathy is poorly understood. This study examined the short-term effect of ischemic AKI on inflammatory and functional changes of the brain in mice by inducing bilateral renal ischemia for 60 min and studying the brains 24 h later. Compared with sham mice, mice with AKI had increased neuronal pyknosis and microgliosis in the brain. AKI also led to increased levels of the proinflammatory chemokines keratinocyte-derived chemoattractant and G-CSF in the cerebral cortex and hippocampus and increased expression of glial fibrillary acidic protein in astrocytes in the cortex and corpus callosum. In addition, extravasation of Evans blue dye into the brain suggested that the blood-brain barrier was disrupted in mice with AKI. Because liver failure also leads to encephalopathy, ischemic liver injury was induced in mice with normal renal function; neuronal pyknosis and glial fibrillary acidic protein expression were not increased, suggesting differential effects on the brain depending on the organ injured. For evaluation of the effects of AKI on brain function, locomotor activity was studied using an open field test. Mice subjected to renal ischemia or bilateral nephrectomy had moderate to severe declines in locomotor activity compared with sham-operated mice. These data demonstrate that severe ischemic AKI induces inflammation and functional changes in the brain. Targeting these pathways could reduce morbidity and mortality in critically ill patients with severe AKI.

Rapamycin Inhibits α1-Adrenergic Receptor–Stimulated Cardiac Myocyte Hypertrophy but Not Activation of Hypertrophy-Associated Genes Evidence for Involvement of p70 S6 Kinase

The 70-kD S6 kinase (p70S6K) has been implicated in the regulation of protein synthesis in many cell types and in the angiotensin II-stimulated hypertrophy of cardiac myocytes. Our purpose was to determine whether p70S6K plays a role in cardiomyocyte hypertrophy induced by the alpha 1-adrenergic receptor (alpha 1-AR) agonist phenylephrine (PE). PE stimulated the activity of p70S6K > 3-fold, and this increase was blocked by rapamycin, an immunosuppressant macrolide that selectively inhibits p70S6K. When administered for 3 days, PE stimulated a 30% increase in total protein content, a 2-fold increase in the incorporation of [14C]phenylalanine (14C-Phe) into protein, and a 50% increase in two-dimensional myocyte area. Rapamycin pretreatment (> or = 500 pg/mL) significantly inhibited each of these PE-stimulated changes. Two days of PE treatment resulted in a 1.6-fold increase in total RNA yield per dish, a 2-fold increase in incorporation of [14C]uridine into myocyte RNA, and increases in relative mRNA levels of the hypertrophy-associated atrial natriuretic factor (ANF, 2.1-fold) and skeletal alpha-actin (SK, 2.2-fold) genes. Although rapamycin abolished the PE-stimulated increases in total RNA and incorporation of [14C]uridine, it had no effect on the induction of the ANF and SK genes. LY294002, a specific inhibitor of phosphatidylinositol 3-kinase (PI3-K) activity, inhibited PE-stimulated increases in p70S6K activity and the incorporation of labeled precursors into myocyte protein and RNA. These results demonstrate that p70S6K is activated by the hypertrophic agent PE and that a PI3-K or PI3-K-like activity is required for p70S6K activation and myocyte hypertrophy. The data suggest that p70S6K activation may be required for PE-stimulated hypertrophy of cardiac myocytes. Our results demonstrate that intracellular signaling pathways responsible for transcriptional and translational responses diverge early after alpha 1-AR stimulation in cardiac myocytes.

Sustained β1-Adrenergic Stimulation Modulates Cardiac Contractility by Ca2+/Calmodulin Kinase Signaling Pathway

A tenet of &bgr;1-adrenergic receptor (&bgr;1AR) signaling is that stimulation of the receptor activates the adenylate cyclase-cAMP-protein kinase A (PKA) pathway, resulting in positive inotropic and relaxant effects in the heart. However, recent studies have suggested the involvement of Ca2+/calmodulin-dependent protein kinase II (CaMKII) in &bgr;1AR-stimulated cardiac apoptosis. In this study, we determined roles of CaMKII and PKA in sustained versus short-term &bgr;1AR modulation of excitation-contraction (E-C) coupling in cardiac myocytes. Short-term (10-minute) and sustained (24-hour) &bgr;1AR stimulation with norepinephrine similarly enhanced cell contraction and Ca2+ transients, in contrast to anticipated receptor desensitization. More importantly, the sustained responses were largely PKA-independent, and were sensitive to specific CaMKII inhibitors or adenoviral expression of a dominant-negative CaMKII mutant. Biochemical assays revealed that a progressive and persistent CaMKII activation was associated with a rapid desensitization of the cAMP/PKA signaling. Concomitantly, phosphorylation of phospholamban, an SR Ca2+ cycling regulatory protein, was shifted from its PKA site (16Ser) to CaMKII site (17Thr). Thus, &bgr;1AR stimulation activates dual signaling pathways mediated by cAMP/PKA and CaMKII, the former undergoing desensitization and the latter exhibiting sensitization. This finding may bear important etiological and therapeutical ramifications in understanding &bgr;1AR signaling in chronic heart failure.

Bax regulates primary necrosis through mitochondrial dynamics

The defining event in apoptosis is mitochondrial outer membrane permeabilization (MOMP), allowing apoptogen release. In contrast, the triggering event in primary necrosis is early opening of the inner membrane mitochondrial permeability transition pore (mPTP), precipitating mitochondrial dysfunction and cessation of ATP synthesis. Bcl-2 proteins Bax and Bak are the principal activators of MOMP and apoptosis. Unexpectedly, we find that deletion of Bax and Bak dramatically reduces necrotic injury during myocardial infarction in vivo. Triple knockout mice lacking Bax/Bak and cyclophilin D, a key regulator of necrosis, fail to show further reduction in infarct size over those deficient in Bax/Bak. Absence of Bax/Bak renders cells resistant to mPTP opening and necrosis, effects confirmed in isolated mitochondria. Reconstitution of these cells or mitochondria with wild-type Bax, or an oligomerization-deficient mutant that cannot support MOMP and apoptosis, restores mPTP opening and necrosis, implicating distinct mechanisms for Bax-regulated necrosis and apoptosis. Both forms of Bax restore mitochondrial fusion in Bax/Bak-null cells, which otherwise exhibit fragmented mitochondria. Cells lacking mitofusin 2 (Mfn2), which exhibit similar fusion defects, are protected to the same extent as Bax/Bak-null cells. Conversely, restoration of fused mitochondria through inhibition of fission potentiates mPTP opening in the absence of Bax/Bak or Mfn2, indicating that the fused state itself is critical. These data demonstrate that Bax-driven fusion lowers the threshold for mPTP opening and necrosis. Thus, Bax and Bak play wider roles in cell death than previously appreciated and may be optimal therapeutic targets for diseases that involve both forms of cell death.

Adenovirus-mediated gene transfer of the human tissue inhibitor of metalloproteinase-2 blocks vascular smooth muscle cell invasiveness in vitro and modulates neointimal development in vivo

BACKGROUND Endovascular injury induced by balloon withdrawal leads to the increased activation of matrix metalloproteinases (MMPs) in the vascular wall, allowing smooth muscle cells (SMCs) to digest the surrounding extracellular matrix (ECM) and migrate from the media into the intima. The objective of this study was to examine the effects of a replication-deficient adenovirus carrying the cDNA for human tissue inhibitor of metalloproteinase-2 (AdCMV.hTIMP-2) on SMC function in vitro and neointimal development in the injured rat carotid artery. METHODS AND RESULTS Infection of cultured rat aortic SMCs at a multiplicity of infection of 100 with AdCMV.hTIMP-2 resulted in high-level expression of hTIMP-2 mRNA and protein secretion into the medium. Conditioned media (CM) from AdCMV. hTIMP-2-infected but not control virus (AdCMV.null or AdCMV. betagal)-infected SMCs inhibited MMP-2 activity on gelatin zymograms as well as the chemoattractant-directed migration of SMCs across reconstituted basement membrane proteins in the Boyden chamber assay. In contrast, AdCMV.hTIMP-2 CM had no effect on chemoattractant-directed migration of SMCs occurring in the absence of an ECM barrier or on the proliferation of cultured neointimal SMCs. Delivery of AdCMV.hTIMP-2 (2.5x10(9) pfu) to the carotid artery wall at the time of balloon withdrawal injury inhibited SMC migration into the intima by 36% (P<0.05) at 4 days and neointimal area by 53% (P<0.01) at 8 days and by 12% (P=NS) at 21 days after injury. AdCMV.hTIMP-2 had no effect on medial area. CONCLUSIONS Adenovirus-mediated hTIMP-2 gene transfer inhibits SMC invasiveness in vitro and in vivo and delays neointimal development after carotid injury.

Phosphatidylinositol 3-kinase functionally compartmentalizes the concurrent Gs signaling during β2-adrenergic stimulation

Compartmentation of intracellular signaling pathways serves as an important mechanism conferring the specificity of G protein-coupled receptor (GPCR) signaling. In the heart, stimulation of &bgr;2-adrenoceptor (&bgr;2-AR), a prototypical GPCR, activates a tightly localized protein kinase A (PKA) signaling, which regulates substrates at cell surface membranes, bypassing cytosolic target proteins (eg, phospholamban). Although a concurrent activation of &bgr;2-AR-coupled Gi proteins has been implicated in the functional compartmentation of PKA signaling, the exact mechanism underlying the restriction of the &bgr;2-AR-PKA pathway remains unclear. In the present study, we demonstrate that phosphatidylinositol 3-kinase (PI3K) plays an essential role in confining the &bgr;2-AR-PKA signaling. Inhibition of PI3K with LY294002 or wortmannin enables &bgr;2-AR-PKA signaling to reach intracellular substrates, as manifested by a robust increase in phosphorylation of phospholamban, and markedly enhances the receptor-mediated positive contractile and relaxant responses in cardiac myocytes. These potentiating effects of PI3K inhibitors are not accompanied by an increase in &bgr;2-AR-induced cAMP formation. Blocking Gi or G&bgr;&ggr; signaling with pertussis toxin or &bgr;ARK-ct, a peptide inhibitor of G&bgr;&ggr;, completely prevents the potentiating effects induced by PI3K inhibition, indicating that the pathway responsible for the functional compartmentation of &bgr;2-AR-PKA signaling sequentially involves Gi, G&bgr;&ggr;, and PI3K. Thus, PI3K constitutes a key downstream event of &bgr;2-AR-Gi signaling, which confines and negates the concurrent &bgr;2-AR/Gs-mediated PKA signaling.