Phosphorylation of T107 by CamKIIδ Regulates the Detoxification Efficiency and Proteomic Integrity of Glyoxalase 1.

The glyoxalase system is a highly conserved and ubiquitously expressed enzyme system, which is responsible for the detoxification of methylglyoxal (MG), a spontaneous by-product of energy metabolism. This study is able to show that a phosphorylation of threonine-107 (T107) in the (rate-limiting) Glyoxalase 1 (Glo1) protein, mediated by Ca2+/calmodulin-dependent kinase II delta (CamKIIδ), is associated with elevated catalytic efficiency of Glo1 (lower KM; higher Vmax). Additionally, we observe proteasomal degradation of non-phosphorylated Glo1 via ubiquitination does occur more rapidly as compared with native Glo1. The absence of CamKIIδ is associated with poor detoxification capacity and decreased protein content of Glo1 in a murine CamKIIδ knockout model. Therefore, phosphorylation of T107 in the Glo1 protein by CamKIIδ is a quick and precise mechanism regulating Glo1 activity, which is experimentally linked to an altered Glo1 status in cancer, diabetes, and during aging.

Cyclic AMP represses pathological MEF2 activation by myocyte-specific hypo-phosphorylation of HDAC5.

Class IIa histone deacetylases (HDACs) critically regulate cardiac function through the repression of the activity of myocyte enhancer factor 2 (MEF2)-dependent gene programs. Protein kinase D (PKD) and Ca2+/Calmodulin-dependent kinase II (CaMKII) activate MEF2 by phosphorylating distinct HDAC isoforms and thereby creating 14-3-3 binding sites for nucleo-cytoplasmic shuttling. Recently, it has been shown that this process is counteracted by cyclic AMP (cAMP)-dependent signaling. Here, we investigated the specific mechanisms of how cAMP-dependent signaling regulates distinct HDAC isoforms and determined their relative contributions to the protection from pathological MEF2 activation. We found that cAMP is sufficient to induce nuclear retention and to blunt phosphorylation of the 14-3-3 binding sites of HDAC5 (Ser259/498) and HDAC9 (Ser218/448) but not HDAC4 (Ser246/467/632). These regulatory events could be observed only in cardiomyocytes and myocyte-like cells but not in non-myocytes, pointing to an indirect myocyte-specific mode of action. Consistent with one previous report, we found that blunted phosphorylation of HDAC5 and HDAC9 was mediated by protein kinase A (PKA)-dependent inhibition of PKD. However, we show by the use of neonatal cardiomyocytes derived from genetic HDAC mouse models that endogenous HDAC5 but not HDAC9 contributes specifically to the repression of endogenous MEF2 activity. HDAC4 contributed significantly to the repression of MEF2 activity but based on the mechanistic findings of this study combined with previous results we attribute this to PKA-dependent proteolysis of HDAC4. Consistently, cAMP-induced repression of agonist-driven cellular hypertrophy was blunted in cardiomyocytes deficient for both HDAC5 and HDAC4. In conclusion, cAMP inhibits MEF2 through both nuclear accumulation of hypo-phosphorylated HDAC5 and through a distinct HDAC4-dependent mechanism.

Inflammation leads through PGE/EP3 signaling to HDAC5/MEF2-dependent transcription in cardiac myocytes.

The myocyte enhancer factor 2 (MEF2) regulates transcription in cardiac myocytes and adverse remodeling of adult hearts. Activators of G protein-coupled receptors (GPCRs) have been reported to activate MEF2, but a comprehensive analysis of GPCR activators that regulate MEF2 has to our knowledge not been performed. Here, we tested several GPCR agonists regarding their ability to activate a MEF2 reporter in neonatal rat ventricular myocytes. The inflammatory mediator prostaglandin E2 (PGE2) strongly activated MEF2. Using pharmacological and protein-based inhibitors, we demonstrated that PGE2 regulates MEF2 via the EP3 receptor, the ßγ subunit of Gi/o protein and two concomitantly activated downstream pathways. The first consists of Tiam1, Rac1, and its effector p21-activated kinase 2, the second of protein kinase D. Both pathways converge on and inactivate histone deacetylase 5 (HDAC5) and thereby de-repress MEF2. In vivo, endotoxemia in MEF2-reporter mice induced upregulation of PGE2 and MEF2 activation. Our findings provide an unexpected new link between inflammation and cardiac remodeling by de-repression of MEF2 through HDAC5 inactivation, which has potential implications for new strategies to treat inflammatory cardiomyopathies.

A proteolytic fragment of histone deacetylase 4 protects the heart from failure by regulating the hexosamine biosynthetic pathway.

The stress-responsive epigenetic repressor histone deacetylase 4 (HDAC4) regulates cardiac gene expression. Here we show that the levels of an N-terminal proteolytically derived fragment of HDAC4, termed HDAC4-NT, are lower in failing mouse hearts than in healthy control hearts. Virus-mediated transfer of the portion of the Hdac4 gene encoding HDAC4-NT into the mouse myocardium protected the heart from remodeling and failure; this was associated with decreased expression of Nr4a1, which encodes a nuclear orphan receptor, and decreased NR4A1-dependent activation of the hexosamine biosynthetic pathway (HBP). Conversely, exercise enhanced HDAC4-NT levels, and mice with a cardiomyocyte-specific deletion of Hdac4 show reduced exercise capacity, which was characterized by cardiac fatigue and increased expression of Nr4a1. Mechanistically, we found that NR4A1 negatively regulated contractile function in a manner that depended on the HBP and the calcium sensor STIM1. Our work describes a new regulatory axis in which epigenetic regulation of a metabolic pathway affects calcium handling. Activation of this axis during intermittent physiological stress promotes cardiac function, whereas its impairment in sustained pathological cardiac stress leads to heart failure.