Proteasomal regulation of cardiac hypertrophy: is demolition necessary for building?

An important role for A20-binding inhibitor of nuclear factor-kB-1 (ABIN1) in inflammation-mediated endothelial dysfunction: an in vivo study in ABIN1 (D485N) mice

Introduction

The link between cardiovascular disease (CVD) and patients with chronic inflammation is not clearly understood. We examined a knock-in mouse expressing a poly-ubiquitin-binding-defective mutant of the protein ABIN1 (ABIN1(D485N)), which develops a systemic lupus erythematosus-like autoimmune disease because of the hyperactivation of IκB kinases (IκKs) and mitogen-activated protein kinases (MAPKs). These mice were used to determine the potential role of these signaling pathways in inflammation-mediated CVD development.

Methods

Laser Doppler imaging in combination with the iontophoresis of vasoactive chemicals were used to assess endothelium-dependent vasodilatation in vivo in ABIN1 (D485N)) mutant defective (n = 29) and wild-type (WT) control (n = 26) mice. Measurements were made at baseline, and animals were subdivided to receive either chow or a proatherogenic diet for 4 weeks, after which, follow-up assessments were made. Paired and unpaired t tests, and ANOVA with post hoc Bonferroni correction were used for statistical significance at P <0.05.

Results

Endothelium-dependent vasodilatation to acetylcholine was attenuated at 4 weeks in ABIN1(D485N)-chow-fed mice compared with age-matched WT-chow-fed mice (P <0.05). The magnitude of attenuation was similar to that observed in WT-cholesterol-fed animals (versus WT-chow, P <0.01). ABIN1(D485N)-cholesterol-fed mice had the poorest endothelium-dependent responses compared with other groups (P <0.001). ABIN1(D485N)-chow-fed mice had increased plasma interleukin-6 (IL-6) levels (versus WT-chow, P <0.001), and this was further elevated in ABIN1(D485N)-cholesterol-fed mice (versus ABIN1(D485N)-chow; P <0.05). IL-1α was significantly greater in all groups compared with WT-chow (P <0.01). ABIN1(D485N) mice showed significant cardiac hypertrophy (P <0.05).

Conclusions

The ABIN(D485N) mice display endothelial dysfunction and cardiac hypertrophy, which is possibly mediated through IL-6 and, to a lesser degree, IL-1α. These results suggest that the ABIN1-mediated hyperactivation of IKKs and MAPKs might mediate chronic inflammation and CVD development.

Proteasome inhibition represses unfolded protein response and Nox4, sensitizing vascular cells to endoplasmic reticulum stress-induced death

Background

Endoplasmic reticulum (ER) stress has pathophysiological relevance in vascular diseases and merges with proteasome function. Proteasome inhibition induces cell stress and may have therapeutic implications. However, whether proteasome inhibition potentiates ER stress-induced apoptosis and the possible mechanisms involved in this process are unclear.

Methodology/Principal Findings

Here we show that proteasome inhibition with MG132, per se at non-lethal levels, sensitized vascular smooth muscle cells to caspase-3 activation and cell death during ER stress induced by tunicamycin (Tn). This effect was accompanied by suppression of both proadaptive (KDEL chaperones) and proapoptotic (CHOP/GADD153) unfolded protein response markers, although, intriguingly, the splicing of XBP1 was markedly enhanced and sustained. In parallel, proteasome inhibition completely prevented ER stress-induced increase in NADPH oxidase activity, as well as increases in Nox4 isoform and protein disulfide isomerase mRNA expression. Increased Akt phosphorylation due to proteasome inhibition partially offset the proapoptotic effect of Tn or MG132. Although proteasome inhibition enhanced oxidative stress, reactive oxygen species scavenging had no net effect on sensitization to Tn or MG132-induced cell death.

Conclusion/Relevance

These data indicate unfolded protein response-independent pathways whereby proteasome inhibition sensitizes vascular smooth muscle to ER stress-mediated cell death. This may be relevant to understand the therapeutic potential of such compounds in vascular disease associated with increased neointimal hyperplasia.

Rosiglitazone-induced heart remodelling is associated with enhanced turnover of myofibrillar protein and mTOR activation

We investigated cardiac hypertrophy elicited by rosiglitazone treatment at the level of protein synthesis/degradation, mTOR, MAPK and AMPK signalling pathways, cardiac function and aspects of carbohydrate/lipid metabolism. Hearts of rats treated or not with rosiglitazone (15 mg/kg day) for 21 days were evaluated for gene expression, protein synthesis, proteasome and calpain activities, signalling pathways, and function by echocardiography. Rosiglitazone induced eccentric heart hypertrophy associated with increased expression of ANP, BNP, collagen I and III and fibronectin, reduced heart rate and increased stroke volume. Rosiglitazone robustly increased heart glycogen content ( approximately 400%), an effect associated with increases in glycogenin and UDPG-PPL mRNA levels and glucose uptake, and a reduction in glycogen phosphorylase expression and activity. Cardiac triglyceride content, lipoprotein lipase activity and mRNA levels of enzymes involved in fatty acid oxidation were also reduced by the agonist. Rosiglitazone-induced cardiac hypertrophy was associated with an increase in myofibrillar protein content and turnover (increased synthesis and an enhancement of calpain-mediated myofibrillar degradation). In contrast, 26S beta5 chymotryptic proteasome activity and mRNA levels of 20S beta2 and beta5 and 19S RPN 2 proteasome subunits along with the ubiquitin ligases atrogin and CHIP were all reduced by rosiglitazone. These morphological and biochemical changes were associated with marked activation of the key growth-promoting mTOR signalling pathway, whose pharmacological inhibition with rapamycin completely blocked cardiac hypertrophy induced by rosiglitazone. The study demonstrates that both arms of protein balance are involved in rosiglitazone-induced cardiac hypertrophy, and establishes the mTOR pathway as a novel important mediator therein.

Muscle ring finger 1 mediates cardiac atrophy in vivo

Pathological cardiac hypertrophy, induced by various etiologies such as high blood pressure and aortic stenosis, develops in response to increased afterload and represents a common intermediary in the development of heart failure. Understandably then, the reversal of pathological cardiac hypertrophy is associated with a significant reduction in cardiovascular event risk and represents an important, yet underdeveloped, target of therapeutic research. Recently, we determined that muscle ring finger-1 (MuRF1), a muscle-specific protein, inhibits the development of experimentally induced pathological; cardiac hypertrophy. We now demonstrate that therapeutic cardiac atrophy induced in patients after left ventricular assist device placement is associated with an increase in cardiac MuRF1 expression. This prompted us to investigate the role of MuRF1 in two independent mouse models of cardiac atrophy: 1) cardiac hypertrophy regression after reversal of transaortic constriction (TAC) reversal and 2) dexamethasone-induced atrophy. Using echocardiographic, histological, and gene expression analyses, we found that upon TAC release, cardiac mass and cardiomyocyte cross-sectional areas in MuRF1(-/-) mice decreased approximately 70% less than in wild type mice in the 4 wk after release. This was in striking contrast to wild-type mice, who returned to baseline cardiac mass and cardiomyocyte size within 4 days of TAC release. Despite these differences in atrophic remodeling, the transcriptional activation of cardiac hypertrophy measured by beta-myosin heavy chain, smooth muscle actin, and brain natriuretic peptide was attenuated similarly in both MuRF1(-/-) and wild-type hearts after TAC release. In the second model, MuRF1(-/-) mice also displayed resistance to dexamethasone-induced cardiac atrophy, as determined by echocardiographic analysis. This study demonstrates, for the first time, that MuRF1 is essential for cardiac atrophy in vivo, both in the setting of therapeutic regression of cardiac hypertrophy and dexamethasone-induced atrophy.

Cardiac muscle protein catabolism in diabetes mellitus: activation of the ubiquitin-proteasome system by insulin deficiency

Protein degradation is increased by both insulin deficiency and insulin resistance in humans and animal models. In skeletal muscle this insulin-dependent increase in protein degradation involves activation of both caspase-3 and the ubiquitin-proteasome system. The influence of abnormal insulin signaling on protein metabolism in cardiac muscle is not well understood; therefore, we measured protein degradation in cardiac muscle of mice with streptozotocin-induced diabetes. Insulin deficiency increased both total muscle proteolysis (measured as tyrosine release in muscle slices or extracts) and the degradation of the myofibrillar protein actin (measured as the appearance of a 14-kDa actin fragment). Expression of ubiquitin mRNA and chymotrypsin-like activity in the proteasome were increased, indicating activation of the ubiquitin-proteasome system in diabetic mouse heart. We also evaluated possible signaling pathways that might regulate cardiac muscle proteolysis. Insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation, and Akt phosphorylation were decreased. Insulin replacement prevented the decrease in IRS-1/Akt phosphorylation, the increase in proteolysis, and attenuated the increase in ubiquitin mRNA. We conclude that insulinopenia accelerates proteolysis in cardiac muscle by reducing IRS-1/Akt signaling, which leads to activation of the ubiquitin-proteasome proteolytic pathway.