Kristine M. Wadosky

The story so far: post-translational regulation of peroxisome proliferator-activated receptors by ubiquitination and SUMOylation

Many studies have implicated the peroxisome proliferator-activated receptor (PPAR) family of nuclear receptor transcription factors in regulating cardiac substrate metabolism and ATP generation. Recently, evidence from a variety of cell culture and organ systems has implicated ubiquitin and small ubiquitin-like modifier (SUMO) conjugation as post-translational modifications that regulate the activity of PPAR transcription factors and their coreceptors/coactivators. Here we introduce the ubiquitin and SUMO conjugation systems and extensively review how they have been shown to regulate all three PPAR isoforms (PPARα, PPARβ/δ, and PPARγ) in addition to the retinoid X receptor and PPARγ coactivator-1α subunits of the larger PPAR transcription factor complex. We then present how the specific ubiquitin (E3) ligases have been implicated and review emerging evidence that post-translational modifications of PPARs with ubiquitin and/or SUMO may play a role in cardiac disease. Because PPAR activity is perturbed in a variety of forms of heart disease and specific proteins regulate this process (E3 ligases), this may be a fruitful area of investigation with respect to finding new therapeutic targets.

Muscle ring finger 1 and muscle ring finger 2 are necessary but functionally redundant during developmental cardiac growth and regulate E2F1-mediated gene expression in vivo

Muscle ring finger (MuRF) proteins have been implicated in the transmission of mechanical forces to nuclear cell signaling pathways through their association with the sarcomere. We recently reported that MuRF1, but not MuRF2, regulates pathologic cardiac hypertrophy in vivo. This was surprising given that MuRF1 and MuRF2 interact with each other and many of the same sarcomeric proteins experimentally.

Muscle RING finger-1 attenuates IGF-I-dependent cardiomyocyte hypertrophy by inhibiting JNK signaling

Recent studies implicate the muscle-specific ubiquitin ligase muscle RING finger-1 (MuRF1) in inhibiting pathological cardiomyocyte growth in vivo by inhibiting the transcription factor SRF. These studies led us to hypothesize that MuRF1 similarly inhibits insulin-like growth factor-I (IGF-I)-mediated physiological cardiomyocyte growth. We identified two lines of evidence to support this hypothesis: IGF-I stimulation of cardiac-derived cells with MuRF1 knockdown 1) exhibited an exaggerated hypertrophy and, 2) conversely, increased MuRF1 expression-abolished IGF-I-dependent cardiomyocyte growth. Enhanced hypertrophy with MuRF1 knockdown was accompanied by increases in Akt-regulated gene expression. Unexpectedly, MuRF1 inhibition of this gene expression profile was not a result of differences in p-Akt. Instead, we found that MuRF1 inhibits total protein levels of Akt, GSK-3β (downstream of Akt), and mTOR while limiting c-Jun protein expression, a mechanism recently shown to govern Akt, GSK-3β, and mTOR activities and expression. These findings establish that MuRF1 inhibits IGF-I signaling by restricting c-Jun activity, a novel mechanism recently identified in the context of ischemia-reperfusion injury. Since IGF-I regulates exercise-mediated physiological cardiac growth, we challenged MuRF1(-/-) and MuRF1-Tg+ mice and their wild-type sibling controls to 5 wk of voluntary wheel running. MuRF1(-/-) cardiac growth was increased significantly over wild-type control; conversely, the enhanced exercise-induced cardiac growth was lost in MuRF1-Tg+ animals. These studies demonstrate that MuRF1-dependent attenuation of IGF-I signaling via c-Jun is applicable in vivo and establish that further understanding of this novel mechanism may be crucial in the development of therapies targeting IGF-I signaling.

Regulation of the calpain and ubiquitin-proteasome systems in a canine model of muscular dystrophy

Introduction: Previous studies have tested the hypothesis that calpain and/or proteasome inhibition is beneficial in Duchenne muscular dystrophy, based largely on evidence that calpain and proteasome activities are enhanced in the mdx mouse. Methods: mRNA expression of ubiquitin‐proteasome and calpain system components were determined using real‐time polymerase chain reaction in skeletal muscle and heart in the golden retriever muscular dystrophy model. Similarly, calpain 1 and 2 and proteasome activities were determined using fluorometric activity assays. Results: We found that less than half of the muscles tested had increases in proteasome activity, and only half had increased calpain activity. In addition, transcriptional regulation of the ubiquitin‐proteasome system was most pronounced in the heart, where numerous components were significantly decreased. Conclusion: This study illustrates the diversity of expression and activities of the ubiquitin‐proteasome and calpain systems, which may lead to unexpected consequences in response to pharmacological inhibition. Muscle Nerve, 2011

MuRF1 mono-ubiquitinates TRα to inhibit T3-induced cardiac hypertrophy in vivo

Thyroid hormone (TH) is recognized for its role in cellular metabolism and growth and participates in homeostasis of the heart. T3 activates pro-survival pathways including Akt and mTOR. Treatment with T3 after myocardial infarction is cardioprotective and promotes elements of physiological hypertrophic response after cardiac injury. Although T3 is known to benefit the heart, very little about its regulation at the molecular level has been described to date. The ubiquitin proteasome system (UPS) regulates nuclear hormone receptors such as estrogen, progesterone, androgen, and glucocorticoid receptors by both degradatory and non-degradatory mechanisms. However, how the UPS regulates T3-mediated activity is not well understood. In this study, we aim to determine the role of the muscle-specific ubiquitin ligase muscle ring finger-1 (MuRF1) in regulating T3-induced cardiomyocyte growth. An increase in MuRF1 expression inhibits T3-induced physiological cardiac hypertrophy, whereas a decrease in MuRF1 expression enhances T3s activity both in vitro and in cardiomyocytes in vivo MuRF1 interacts directly with TRα to inhibit its activity by posttranslational ubiquitination in a non-canonical manner. We then demonstrated that a nuclear localization apparatus that regulates/inhibits nuclear receptors by sequestering them within a subcompartment of the nucleus was necessary for MuRF1 to inhibit T3 activity. This work implicates a novel mechanism that enhances the beneficial T3 activity specifically within the heart, thereby offering a potential target to enhance cardiac T3 activity in an organ-specific manner.