Awareness of the regulation of cell signaling by post-translational ubiquitination has emerged over the past 2 decades. Like phosphorylation, post-translational modification of proteins with ubiquitin can result in the regulation of numerous cellular functions, for example, the DNA damage response, apoptosis, cell growth, and the innate immune response. In this review, we discuss recently published mechanisms by which the ubiquitin proteasome system regulates key signal transduction pathways in the heart, including MAPK JNK, calcineurin, FOXO, p53, and estrogen receptors α and β. We then explore how ubiquitin proteasome system-specific regulation of these signal transduction pathways plays a role in the pathophysiology of common cardiac diseases, such as cardiac hypertrophy, heart failure, ischemia reperfusion injury, and diabetes.

The 5=-AMP-activated protein kinase (AMPK) functions as a metabolic fuel gauge that is activated in response to environmental stressors to restore cellular energy balance. In the heart, AMPK coordinates the activation of glucose and fatty acid metabolic pathways to ensure increased production of myocardial ATP when required, such as during cardiac ischemia/reperfusion and hypertrophy, causing an increase in AMPK activity that can be viewed as both protective and maladaptive. While we understand the basic regulation of AMPK activity by kinases, recent studies have introduced the concept that AMPK is regulated by other post-translational modifications, specifically ubiquitination. These studies reported that the ubiquitin ligase cell death-inducing DFFA-like effector a ubiquitinates the ␤ subunit of AMPK to regulate its steady-state protein levels. Other investigators found that AMPK regulatory components, including the AMPK ␣ subunit and AMPK kinases NUAK1 and MARK4, can be ubiquitinated with atypical ubiquitin chains. The USP9X-deubiquitinating enzyme was identified to remove ubiquitination from both NUAK1 and MARK4. Lastly, AMPK activation increases the expression of the ubiquitin ligases MAFBx/Atrogin-1 and MuRF1. These ubiquitin ligases regulate key cardiac transcription factors to control cardiomyocyte mass and remodeling, thus suggesting another mechanism by which AMPK may function in the heart. The relevance of AMPK ubiquitination in cardiac disease has yet to be tested directly, but it likely represents an important mechanism that occurs in common cardiac diseases that may be targeted for therapy. The 5=-AMP-activated protein kinase (AMPK) functions as a metabolic fuel gauge that is activated in response to numerous environmental stressors to restore cellular and whole-body energy balance.1,2 AMPK is allosterically regulated by the competitive binding of AMP and ATP, thereby "sensing" cellular energy status and, on activation, triggers compensatory ATP-generating mechanisms while attenuating ATP-consuming processes.3 Perturbations in cardiac metabolism are closely linked to the onset and progression of cardiovascular diseases; given the central role of AMPK in regulating cellular energetics, there is considerable interest in the determination of the precise role(s) of AMPK in cardiac pathophysiological states and evaluation of the utility of modulating AMPK activity as a therapeutic intervention. 4 This review discusses AMPK function in normal and diseased hearts, with emphases on AMPK and protein degradation via the ubiquitin proteasome pathway-a potential novel approach to treating cardiovascular disease. These are significant new findings as ubiquitination is emerging as a pivotal regulatory mechanism that rivals phosphorylation in its overall significance in biology. 5 Structure and Regulation of AMPKAMPK is a highly conserved heterotrimeric enzyme consisting of three subunits, ␣, ␤, and ␥, with multiple genes encoding distinct subunit isoforms (ie, ␣1, ␣2, ␤1, ␤2, ␥1, ␥2, and ␥3). The structure and r...

Mitochondria are dynamic organelles that undergo a constant cycle of division and fusion to maintain their function. The process of mitochondrial fusion has the effect of mixing their content, allowing complementation of protein components, mtDNA repair, and distribution of metabolic intermediates. Fission, on the other hand, enables mitochondria to increase in number and capacity, and to segregate mitochondria for autophagy by the lysosome ("mitophagy"). Disruption of these protein quality control mechanisms has recently been identified in multiple cardiac diseases, including cardiac hypertrophy, heart failure, dilated cardiomyopathy, and ischemic heart disease, and is intimately tied to mitochondrial control of apoptosis. Proteins that regulate mitochondrial fusion and fission have been discovered, including Mfn1, Mfn2, and Opa1 (fusion) and Drp1 and Fis1 (fission). In this review, we discuss how these proteins are regulated by post-translational modification with ubiquitin and SUMO (small ubiquitin-like modifier). We then present what is known about the ubiquitin and SUMO ligases that regulate these post-translational modifications and regulation of mitochondrial fusion and fission, exploring their potential as therapeutic targets of cardiac disease. (Circ J 2011; 75: 2513 - 2521

We investigated whether chronic in vivo treatment with the peroxisome proliferator-activated receptor alpha agonist Wy-14,643 attenuates cardiac contractile function by impairing mitochondrial respiration. Wy-14,643 (25 mg kg(-1) day(-1)) was administered to Wistar rats by oral gavage for 14 consecutive days, after which ex vivo heart function, myocardial mitochondrial respiratory capacity, and metabolic gene expression were determined. Body and heart weights were not significantly altered following 14 days of Wy-14,643 administration. Heart perfusion studies showed significantly reduced systolic and developed pressures, while the rate pressure product declined by 36 +/- 2.6% (P < 0.01 vs. vehicle) after 14 days of Wy-14,643 treatment. State 3 mitochondrial respiration was lower in the Wy-14,643 group (P = 0.06 vs. vehicle). State 4 respiration and oligomycin-insensitive proton leak were significantly increased compared with matched controls. The rate of ADP phosphorylation was also decreased by 44.9 +/- 1.9% (P < 0.05 vs. vehicle). Pyruvate dehydrogenase kinase 4 (PDK4) and uncoupling protein 3 (UCP3) transcript levels were upregulated, while cytochrome oxidase II (COXII) expression was decreased following Wy-14,643 treatment. This study demonstrates that chronic in vivo Wy-14,643 administration impaired cardiac contractile function in parallel with decreased mitochondrial respiratory function and increased uncoupling.

We hypothesized the coordinate induction of mitochondrial regulatory genes in the hypertrophied right ventricle to sustain mitochondrial respiratory capacity and contractile function in response to increased load. Wistar rats were exposed to hypobaric hypoxia (11% O(2)) or normoxia for 2 wk. Cardiac contractile and mitochondrial respiratory function were separately assessed for the right and left ventricles. Transcript levels of several mitochondrial regulators were measured. A robust hypertrophic response was observed in the right (but not left) ventricle in response to hypobaric hypoxia. Mitochondrial O(2) consumption was increased in the right ventricle, while proton leak was reduced vs. normoxic controls. Citrate synthase activity and mitochondrial DNA content were significantly increased in the hypertrophied right ventricle, suggesting higher mitochondrial number. Transcript levels of nuclear respiratory factor-1, peroxisome proliferator-activated receptor-gamma-coactivator-1alpha, cytochrome oxidase (COX) subunit II, and uncoupling protein-2 (UCP2) were coordinately induced in the hypertrophied right ventricle following hypoxia. UCP3 transcript levels were significantly reduced in the hypertrophied right ventricle vs. normoxic controls. Exposure to chronic hypobaric hypoxia had no significant effects on left ventricular mitochondrial respiration or contractile function. However, COXIV and UCP2 gene expression were increased in the left ventricle in response to chronic hypobaric hypoxia. In summary, we found coordinate induction of several genes regulating mitochondrial function and higher mitochondrial number in a model of physiological right ventricular hypertrophy, linking the efficiency of mitochondrial oxidative phosphorylation and respiratory function to sustained contractile function in response to the increased load.

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Chronic hypobaric hypoxia (CHH) increases load on the right ventricle (RV) resulting in RV hypertrophy. We hypothesized that CHH elicits distinct responses, i.e., the hypertrophied RV, unlike the left ventricle (LV), displaying enhanced mitochondrial respiratory and contractile function. Wistar rats were exposed to 4 weeks CHH (11% O2) versus normoxic controls. RV/body weight ratio increased (P < 0.001 vs. control) while RV systolic and developed pressures were higher. However, LV systolic and developed pressures were significantly reduced. Mitochondrial O2 consumption was sustained in the hypertrophied RV, ADP/O increased (P < 0.01 vs. control) and proton leak significantly decreased. Conversely, LV mitochondrial O2 consumption was attenuated (P < 0.05 vs. control) and proton leak significantly increased. In parallel, expression of mitochondrial regulators was upregulated in the hypertrophied RV but not the LV. Our data show that the hypertrophied RV induces expression of mitochondrial regulatory genes linking respiratory capacity and enhanced efficiency to sustained contractile function.

The ubiquitin proteasome system plays a role in regulating protein activity and is integral to the turnover of damaged and worn proteins. In this review, we discuss the recently described relationship between the ubiquitin proteasome system and the cardiac creatine kinase/phosphocreatine shuttle, an essential component of ATP generation and energy shuttling within the heart. The ubiquitin ligase muscle ring finger-1 (MuRF1) binds creatine kinase, leading to its ubiquitination and possible degradation. MuRF1 may also be integral in the regulation of creatine kinase activity in vivo. Since there is a close relationship between the cardiac creatine kinase/ phospho-creatine shuttle activity and heart failure, these findings suggest that MuRF1's role in protein quality control of creatine kinase may be vital to the regulation and maintenance of cardiac energetics to protect against heart failure.