Extracellular signal-regulated protein kinase (ERK) 5 is a mitogen-activated protein kinase (MAPK) that is activated by dual phosphorylation via a unique MAPK/ERK kinase 5, MEK5. The physiological importance of this signaling cascade is underscored by the early embryonic death caused by the targeted deletion of the erk5 or the mek5 genes in mice. Here, we have found that ERK5 is required for mediating the survival of fibroblasts under basal conditions and in response to sorbitol treatment. Increased Fas ligand (FasL) expression acts as a positive feedback loop to enhance apoptosis of ERK5-or MEK5-deficient cells under conditions of osmotic stress. Compared to wild-type cells, erk5À/À and mek5À/À fibroblasts treated with sorbitol display a reduced protein kinase B (PKB) activity associated with increased Forkhead box O3a (Foxo3a) activity. Based on these results, we conclude that the ERK5 signaling pathway promotes cell survival by downregulating FasL expression via a mechanism that implicates PKB-dependent inhibition of Foxo3a downstream of phosphoinositide 3 kinase.

Our work and others’ over the past few years have led to the identification of new roles of PAK1 in cardiac physiology, such as the regulation of cardiac ion channel and actomyosin function. More recent studies have revealed that PAK1-deficient mice were vulnerable to cardiac hypertrophy and readily progress to failure under sustained pressure overload and susceptible to ischemia/reperfusion injury. Our further study indicated that the PAK1 activator FTY720 was able to prevent this pressure overload-induced hypertrophy in wild-type mice without compromising their cardiac functions. A cardiac protective effect against ischemia/reperfusion injury by FTY720 was also observed in both rat and mouse models by us and others. Thus, these studies suggest that PAK1 is more important in the heart than previously thought, in particular a therapeutic potential of PAK1 activators. In the future, in-depth investigations are required to further substantiate our hypotheses on mechanisms for PAK1 function in the heart and to explore a therapeutic potential of FTY720 and other PAK1 activators in heart disease conditions.

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Funding Acknowledgements Type of funding sources: Foundation. Main funding source(s): British Heart Foundation Background Cardiovascular issues associated with diabetes, such as diabetic cardiomyopathy (DCM), can lead to heart failure. DCM is etiologically related to myocardial inflammation and can stem from a complex interplay of different cell types. Cardiomyocyte as an active mediator of the inflammatory response is an emerging concept with limited mechanistic understanding. Purpose We aimed to address the knowledge gap of cardiomyocyte endoplasmic reticulum (ER) dysfunction-mediated macrophage response and provide functional evidence for the therapeutic feasibility of managing inflammatory paracrine signals in response to diet-induced metabolic stress. Methods In vivo mouse model of high fat high sucrose diet-induced diabetes, cardiomyocyte-specific p21-activated kinase 2 (PAK2) knockout model, echocardiography, histology, 3D imaging, qPCR, co-culture of H9c2 culturing medium and bone-marrow derived macrophages, immunoblotting, macrophage isolation from myocardium, flow cytometry and AAV9-gene therapy. Results In a time-course study, diet-induced diabetic mice demonstrated an association between cardiac ER stress and sustained myocardial inflammation, with a maladaptive shift in myocardial ER stress response over time. Furthermore, as a cardiac ER dysfunction model, mice with cardiac-specific PAK2 deletion exhibited heightened myocardial inflammatory response in diabetes. Using human and mice diabetic heart samples, we show that ER stress-induced CCAAT/enhancer-binding protein homologous protein (CHOP) is a novel transcriptional regulator of high mobility group box-1 (HMGB1). Cardiac stress-induced active release of HMGB1 facilitated M1 macrophage polarization, and aggravated myocardial inflammatory signatures. Therapeutically, sequestering the extracellular HMGB1 using Glycyrrhizin conferred cardioprotection through its anti-inflammatory action. Also, as functional evidence, we showed that un-mitigated cardiac ER response due to PAK2 loss under diabetes may account as a barrier for leveraging the anti-inflammatory potential of Vildagliptin. Conclusion Collectively, we introduce an ER stress-mediated cardiomyocyte-macrophage link, altering the macrophage response in the myocardium, thereby providing insight into therapeutic prospects for diabetes-associated cardiac dysfunction.

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