The ubiquitin-proteasome system in cardiac physiology and pathology

Upregulation of myocardial 11S-activated proteasome in experimental hyperglycemia

This study examined the hypothesis that the ubiquitin proteasome system (UPS) degrades proteins damaged by exposure to hyperglycemia. Experimental hyperglycemia was induced in male rats by treatment with streptozotocin. After 30 days, echocardiography confirmed the presence of cardiomyopathy as ejection fraction, fractional shortening, and diastolic function (E/A ratio) were decreased, and chamber diameter was increased in hyperglycemic animals. Proteasome non-ATP-dependent chymotryptic activity was increased over 2-fold in hyperglycemic hearts, but the ATP-dependent activity was decreased and levels of ubiquitinated proteins were increased. Protein levels of the PA28alpha of the 11S-activator ring were increased by 128% and the PA28beta subunit increased by 58% in the hyperglycemic hearts. The alpha3 subunit of the 20S-proteasome was increased by 82% while the catalytic beta5 subunit was increased by 68% in hyperglycemic hearts. Protein oxidation as indicated by protein carbonyls was significantly higher in hyperglycemic hearts. These studies support the conclusion that the UPS becomes dysfunctional during long term hyperglycemia. However, 11S-activated proteasome was increased suggesting a response to oxidative protein damage and a potential role for this form of the proteasome in a cardiac pathophysiology.

Molecular mechanisms and physiological significance of autophagy during myocardial ischemia and reperfusion

Autophagy is an intracellular bulk degradation process whereby cytoplasmic proteins and organelles are degraded and recycled through lysosomes. In the heart, autophagy plays a homeostatic role at basal levels, and the absence of autophagy causes cardiac dysfunction and the development of cardiomyopathy. Autophagy is induced during myocardial ischemia and further enhanced by reperfusion. Although induction of autophagy during the ischemic phase is protective, further enhancement of autophagy during the reperfusion phase may induce cell death and appears to be detrimental. In this review we discuss the functional significance of autophagy and the underlying signaling mechanism in the heart during ischemia/reperfusion.

Myosin accumulation and striated muscle myopathy result from the loss of muscle RING finger 1 and 3

Maintenance of skeletal and cardiac muscle structure and function requires precise control of the synthesis, assembly, and turnover of contractile proteins of the sarcomere. Abnormalities in accumulation of sarcomere proteins are responsible for a variety of myopathies. However, the mechanisms that mediate turnover of these long-lived proteins remain poorly defined. We show that muscle RING finger 1 (MuRF1) and MuRF3 act as E3 ubiquitin ligases that cooperate with the E2 ubiquitin-conjugating enzymes UbcH5a, -b, and -c to mediate the degradation of beta/slow myosin heavy chain (beta/slow MHC) and MHCIIa via the ubiquitin proteasome system (UPS) in vivo and in vitro. Accordingly, mice deficient for MuRF1 and MuRF3 develop a skeletal muscle myopathy and hypertrophic cardiomyopathy characterized by subsarcolemmal MHC accumulation, myofiber fragmentation, and diminished muscle performance. These findings identify MuRF1 and MuRF3 as key E3 ubiquitin ligases for the UPS-dependent turnover of sarcomeric proteins and reveal a potential basis for myosin storage myopathies.

Oxidative injury induces selective rather than global inhibition of proteasomal activity

Oxidative injury has been found to be associated with proteasomal inactivity. In this study, the extent of oxidative damage and its effects on proteasomal function has been critically assessed. Left anterior descending coronary artery was occluded (ischemia) and reperfused with or without preconditioning in male Sprague-Dawley rats. For further validation, H9c2 cardiac myoblasts cultures were used. We demonstrate that ischemia-reperfusion causes extensive endoplasmic reticulum stress as evident from the degradation of GRP78 transcript followed by its rapid induction. Western blot analysis and immunohistochemistry showed that increasing duration of ischemia and reperfusion causes accumulation of phosphorylated IkappaB (p-IkappaB), thereby suggesting proteasomal inactivity. However, similar analysis for Nrf2, a key mediator of antioxidant defense, showed sustained activation, suggesting intact proteasomal function. Preconditioning of the myocardium preserves the degradation of p-IkappaB, suggesting effective functioning of proteasome after preconditioning. Further analysis with specific proteosomal inhibitors like epoxomicin (100 nM, inhibits chymotrypsin-like activities of proteasomes) and lactacystin (2 microM, inhibits chymotrypsin as well as some trypsin-like activities of proteasomes) suggests that degradation of p-IkappaB and Keap-1 in the proteasome occurs by independent mechanisms. This study gives further insight into interrelationship between oxidative injury and catalytic function of the proteasome in heart, where oxidative injury causes selective rather than global inhibition of proteasome.

Age-associated decrease in proteasome content and activities in human dermal fibroblasts: restoration of normal level of proteasome subunits reduces aging markers in fibroblasts from elderly persons

We measured proteasome activities and the levels of proteasome subunits in dermal fibroblasts from individuals aged 20-82 years. Proteasome activities changed with age in a biphasic manner, decreasing significantly up to 50 years of age and showing no significant change between 50 and 78 years of age. Similarly, proteasome activities in replicatively senescent dermal fibroblasts showed a passage-dependent biphasic change. We confirmed that the decreases in proteasome activities were accompanied by the accumulation of oxidized and ubiquitinated proteins. The decline in proteasome activities in aging fibroblasts was associated with a decrease in the expression of proteasome subunits. We found that the restoration of the normal level of proteasome catalytic subunits, using a lentivirus gene-delivery system, decreased the severity of the aging markers in dermal fibroblasts from elderly donors. These findings suggest that proteasome malfunction may contribute to the aging process in human skin and that the maintenance of normal proteasome activities could delay skin aging.

Ubiquitin and Ubiquitin-Like Proteins in Protein Regulation

The discovery of the ubiquitin system was awarded with the Nobel Prize in Chemistry in 2004. Labeling of intracellular proteins for degradation by a multienzymatic complex, called the proteasome, was identified as the main function of this system. Subsequently, it was discovered that the attachment of ubiquitin to proteins can modify their function without degradation. Finally, a number of other molecules were recognized to be conjugated to proteins in a manner similar to ubiquitin and were henceforth called ubiquitin-like proteins. This review provides an overview of this class of molecules and its implication for function, subcellular location, and half-life of proteins.

Loss of muscle-specific RING-finger 3 predisposes the heart to cardiac rupture after myocardial infarction

RING-finger proteins commonly function as ubiquitin ligases that mediate protein degradation by the ubiquitin-proteasome pathway. Muscle-specific RING-finger (MuRF) proteins are striated muscle-restricted components of the sarcomere that are thought to possess ubiquitin ligase activity. We show that mice lacking MuRF3 display normal cardiac function but are prone to cardiac rupture after acute myocardial infarction. Cardiac rupture is preceded by left ventricular dilation and a severe decrease in cardiac contractility accompanied by myocyte degeneration. Yeast two-hybrid assays revealed four-and-a-half LIM domain (FHL2) and gamma-filamin proteins as MuRF3 interaction partners, and biochemical analyses showed these proteins to be targets for degradation by MuRF3. Accordingly, FHL2 and gamma-filamin accumulated to abnormal levels in the hearts of mice lacking MuRF3. These findings reveal an important role of MuRF3 in maintaining cardiac integrity and function after acute myocardial infarction and suggest that turnover of FHL2 and gamma-filamin contributes to this cardioprotective function of MuRF3.

Proteomic expression analysis of cardiomyocytes subjected to proteasome inhibition

We hypothesized that impaired proteasomal function affects gene expression in cardiomyocytes. To identify those genes, a proteomics-based analysis of neonatal rat cardiac myocytes treated with the proteasome inhibitor MG132 in comparison to vehicle treated control cells was performed. MG132 treatment induced reproducible changes in the protein expression profile, which was analyzed by two-dimensional difference gel electrophoresis followed by tryptic peptide mass fingerprinting for spot identification by MALDI-TOF mass spectrometry. The identified protein alterations could be grouped into three major categories: (1) induction of small heat shock proteins (HSPs) with chaperonic function, such as HSP27, alphaB-crystallin, and cardiovascular HSP, (2) altered expression of actin associated proteins, such as cofilin-1 and transgelin, and (3) induction of antioxidant proteins, such as peroxiredoxin-1, superoxide dismutase-1, and hemeoxygenase-1. Northern blotting revealed that expression was regulated at the mRNA level. Given that proteasomal activity is decreased in cardiovascular diseases, alterations in proteasome-dependent control of mRNA expression could provide a novel mechanism by which disease progression is modulated.

Re-expression of proteins involved in cytokinesis during cardiac hypertrophy

Cardiomyocytes stop dividing after birth and postnatal heart growth is only achieved by increase in cell volume. In some species, cardiomyocytes undergo an additional incomplete mitosis in the first postnatal week, where karyokinesis takes place in the absence of cytokinesis, leading to binucleation. Proteins that regulate the formation of the actomyosin ring are known to be important for cytokinesis. Here we demonstrate for the first time that small GTPases like RhoA along with their downstream effectors like ROCK I, ROCK II and Citron Kinase show a developmental stage specific expression in heart, with high levels being expressed in cardiomyocytes only at stages when cytokinesis still occurs (i.e. embryonic and perinatal). This suggests that downregulation of many regulatory and cytoskeletal components involved in the formation of the actomyosin ring may be responsible for the uncoupling of cytokinesis from karyokinesis in rodent cardiomyocytes after birth. Interestingly, when the myocardium tries to adapt to the increased workload during pathological hypertrophy a re-expression of proteins involved in DNA synthesis and cytokinesis can be detected. Nevertheless, the adult cardiomyocytes do not appear to divide despite this upregulation of the cytokinetic machinery. The inability to undergo complete division could be due to the presence of stable, highly ordered and functional sarcomeres in the adult myocardium or could be because of the inefficiency of degradation pathways, which facilitate the division of differentiated embryonic cardiomyocytes by disintegrating myofibrils.

Heart failure and protein quality control

The heart is constantly under mechanical, metabolic, and thermal stress, even at baseline physiological conditions, and cardiac stress may increase as a result of environmental or intrinsic pathological insults. Cardiomyocytes are continuously challenged to efficiently and properly fold nascent polypeptides, traffic them to their appropriate cellular locations, and keep them from denaturing in the face of normal and pathological stimuli. Because deployment of misfolded or unfolded proteins can be disastrous, cells, in general, and cardiomyocytes, in particular, have developed a multilayered protein quality control system for maintaining proper protein conformation and for reorganizing and removing misfolded or aggregated polypeptides. Here, we examine recent data pointing to the importance of protein quality control in cardiac homeostasis and disease.

Proteasome dysfunction inhibits surfactant protein gene expression in lung epithelial cells: mechanism of inhibition of SP-B gene expression

Surfactant proteins maintain lung function through their actions to reduce alveolar surface tension and control of innate immune responses in the lung. The ubiquitin proteasome pathway is responsible for the degradation of majority of intracellular proteins in eukaryotic cells, and proteasome dysfunction has been linked to the development of neurodegenerative, cardiac, and other diseases. Proteasome function is impaired in interstitial lung diseases associated with surfactant protein C (SP-C) mutation mapping to the BRICHOS domain located in the proSP-C protein. In this study we determined the effects of proteasome inhibition on surfactant protein expression in H441 and MLE-12 lung epithelial cells to understand the relationship between proteasome dysfunction and surfactant protein gene expression. Proteasome inhibitors lactacystin and MG132 reduced the levels of SP-A, SP-B, and SP-C mRNAs in a concentration-dependent manner in H441 and MLE-12 cells. In H441 cells, lactacystin and MG132 inhibition of SP-B mRNA was associated with similar decreases in SP-B protein, and the inhibition was due to inhibition of gene transcription. Proteasome inhibitors decreased thyroid transcription factor-1 (TTF-1)/Nkx2.1 DNA binding activity, and the reduced TTF-1 DNA binding activity was due to reduced expression levels of TTF-1 protein. These data indicated that the ubiquitin proteasome pathway is essential for the maintenance of surfactant protein gene expression and that disruption of this pathway inhibits surfactant protein gene expression via reduced expression of TTF-1 protein.

Optimal determination of heart tissue 26S-proteasome activity requires maximal stimulating ATP concentrations

The ubiquitin-proteasome system has been implicated in both cardiac physiology and pathophysiology. Research in this area has been hampered by the lack of a simple, reproducible method to assess 26S-proteasome peptidase activities. The current report demonstrates that one reason for lack of reproducibility is the myriad of ATP concentrations, many of them excessive, which have been used to stimulate peptidase activity. The chymotrypsin-like or caspase-like activities of 26S-proteasome in cardiac tissue isolates were determined using Suc-LLVY-AMC or Z-LLE-AMC, respectively, over a range of ATP concentrations up to 2 mmol/L. The optimal ATP concentration to assess both peptidase activities was found to be in the low micromolar range (from 6 to 100 micromol/L) depending on the cardiac tissue isolate protein (10 to 90 microg protein) contained in the reaction. Increasing ATP beyond the optimal range was inhibitory. In general, chymotrypsin-like and caspase-like activities could be stimulated 2- to 2.5-fold and 1.4- to 1.8-fold, respectively, over basal (ATP, 0 micromol/L), and could be effectively inhibited with lactacystin or Z-Pro-Nle-Asp-CHO, respectively. Based on these observations, an optimized method is presented for ex vivo determination of cardiac 26S-proteasome peptidase activities which was used to confirm inactivation of this complex by myocardial ischemia and reperfusion.

ADAM17 Mediates Epidermal Growth Factor Receptor Transactivation and Vascular Smooth Muscle Cell Hypertrophy Induced by Angiotensin II

BACKGROUND

Angiotensin II (Ang II) promotes growth of vascular smooth muscle cells (VSMCs) via epidermal growth factor (EGF) receptor (EGFR) transactivation mediated through a metalloprotease-dependent shedding of heparin-binding EGF-like growth factor (HB-EGF). However, the identity of the metalloprotease responsible for this process remains unknown.

METHODS AND RESULTS

To identify the metalloprotease required for Ang II-induced EGFR transactivation, primary cultured aortic VSMCs were infected with retrovirus encoding dominant negative (dn) mutant of ADAM10 or ADAM17. EGFR transactivation induced by Ang II was inhibited in VSMCs infected with dnADAM17 retrovirus but not with dnADAM10 retrovirus. However, Ang II comparably stimulated intracellular Ca2+ elevation and JAK2 tyrosine phosphorylation in these VSMCs. In addition, dnADAM17 inhibited HB-EGF shedding induced by Ang II in A10 VSMCs expressing the AT1 receptor. Moreover, Ang II enhanced protein synthesis and cell volume in VSMCs infected with control retrovirus, but not in VSMCs infected with dnADAM17 retrovirus.

CONCLUSIONS

ADAM17 activated by the AT1 receptor is responsible for EGFR transactivation and subsequent protein synthesis in VSMCs. These findings demonstrate a previously missing molecular mechanism by which Ang II promotes vascular remodeling.

Mapping the Murine Cardiac 26S Proteasome Complexes

The importance of proteasomes in governing the intracellular protein degradation process has been increasingly recognized. Recent investigations indicate that proteasome complexes may exist in a species- and cell-type-specific fashion. To date, despite evidence linking impaired protein degradation to cardiac disease phenotypes, virtually nothing is known regarding the molecular composition, function, or regulation of cardiac proteasomes. We have taken a functional proteomic approach to characterize 26S proteasomes in the murine heart. Multidimensional chromatography was used to obtain highly purified and functionally viable cardiac 20S and 19S proteasome complexes, which were subjected to electrophoresis and tandem mass spectrometry analyses. Our data revealed complex molecular organization of cardiac 26S proteasomes, some of which are similar to what were reported in yeast, whereas others exhibit contrasting features that have not been previously identified in other species or cell types. At least 36 distinct subunits (17 of 20S and 19 of 19S) are coexpressed and assembled as 26S proteasomes in this vital cardiac organelle, whereas the expression of PA200 and 11S subunits were detected with limited participation in the 26S complexes. The 19S subunits included a new alternatively spliced isoform of Rpn10 (Rpn10b) along with its primary isoform (Rpn10a). Immunoblotting and immunocytochemistry verified the expression of key alpha and beta subunits in cardiomyocytes. The expression of 14 constitutive alpha and beta subunits in parallel with their three inducible subunits (beta1i, beta2i, and beta5i) in the normal heart was not expected; these findings represent a distinct level of structural complexity of cardiac proteasomes, significantly different from that of yeast and human erythrocytes. Furthermore, liquid chromatography/tandem mass spectroscopy characterized 3 distinct types of post-translational modifications including (1) N-terminal acetylation of 19S subunits (Rpn1, Rpn5, Rpn6, Rpt3, and Rpt6) and 20S subunits (alpha2, alpha5, alpha7, beta3, and beta4); (2) N-terminal myristoylation of a 19S subunit (Rpt2); and (3) phosphorylation of 20S subunits (eg, alpha7)). Taken together, this report presents the first comprehensive characterization of cardiac 26S proteasomes, providing critical structural and proteomic information fundamental to our future understanding of this essential protein degradation system in the normal and diseased myocardium.

Phylogenetic origin of LI-cadherin revealed by protein and gene structure analysis

The intestine specific LI-cadherin differs in its overall structure from classical and desmosomal cadherins by the presence of seven instead of five cadherin repeats and a short cytoplasmic domain. Despite the low sequence similarity, a comparative protein structure analysis revealed that LI-cadherin may have originated from a five-repeat predecessor cadherin by a duplication of the first two aminoterminal repeats. To test this hypothesis, we cloned the murine LI-cadherin gene and compared its structure to that of other cadherins. The intron-exon organization, including the intron positions and phases, is perfectly conserved between repeats 3-7 of LI-cadherin and 1-5 of classical cadherins. Moreover, the genomic structure of the repeats 1-2 and 3-4 is identical for LI-cadherin and highly similar to that of the repeats 1-2 of classical cadherins. These findings strengthen our assumption that LI-cadherin originated from an ancestral cadherin with five domains by a partial gene duplication event.