Project Summary: Heart disease is the leading cause of mortality and is often preceded by pathological cardiac hypertrophy due to chronic hypertension and sustained increases in cardiac afterload. While initially an adaptive response to maintain cardiac output and systemic blood supply, pathological cardiac hypertrophy eventually leads to a decompensated state of heart failure (HF). The initial adaptive aspect of hypertrophic growth requires increases in protein synthesis, which must be balanced by adequate protein folding, and degradation of misfolded, potentially toxic proteins. Without this balance, cardiac myocytes cannot maintain protein homeostasis, or proteostasis, which threatens functional cellular integrity and viability. The endoplasmic reticulum (ER) is a central node in proteostasis, with ~40% of proteins trafficked through this organelle, underscoring the importance of ER proteostasis in the heart. An important feature of ER proteostasis is recognition and degradation of terminally misfolded proteins through ER associated degradation (ERAD). While ERAD canonically recognizes misfolded ER proteins, we have recently identified the first and only described non-ER substrate for ERAD, serum/glucocorticoid regulated kinase 1 (SGK1), a pro-growth, cytosolic (non-ER) kinase. Intriguingly, SGK1 is not misfolded, yet is targeted to the cytosolic face of the ER for proteasome- mediated degradation via ERAD and requires the ER E3 ubiquitin ligase, HRD1. Further, we have found that SGK1 degradation by ERAD is impaired in human hypertrophic heart failure and in a mouse model of pressure overload-induced heart failure. SGK1 promotes growth in cancer cells, and our work and others’ have demonstrated relevance for SGK1 in cardiac pathology, but none have investigated whether SGK1 is required for the development of pathological cardiac hypertrophy and if this is mediated by impairing its degradation at the ER via ERAD. Our hypothesis is that the cytosolic kinase, SGK1, drives pathological cardiac hypertrophy in the pressure-overloaded heart and that inhibiting SGK1 ubiquitylation at the ER slows its degradation by proteasomes, which increases SGK1 levels and enhances its pro-hypertrophic effects in the heart. We will address this hypothesis using primary cultured cardiac myocytes and mice subjected to pressure overload in the following specific aims: (1) Determine the effects of depleting endogenous SGK1 on cardiac myocyte hypertrophy; and (2) Examine the effects on cardiac myocyte hypertrophy of ectopically expressed wild type SGK1, or a degradation resistant mutant form of SGK1 that cannot be ubiquitylated but retains ER localization. Pursuit of these aims will determine whether SGK1 is a promising target in mitigating pathological cardiac hypertrophy and whether SGK1 degradation by non-canonical ERAD is an important mechanistic aspect of cardiac pathophysiology that can be modulated therapeutically.

Project Narrative: Cardiovascular disease accounts for 1 in 3 individuals in the US, often resulting from pressure-overload induced growth of the heart and consequent heart failure, a chronic condition for which we have no cure and a great unmet clinical need. The proposed work will study newly discovered aspects of vital cellular processes that are disrupted during pathological growth and contribute to progressive heart failure. Our studies will provide insight into whether targeting these disrupted cellular processes can serve as an effective therapeutic strategy to reduce mortality from heart disease.