Abstract 402: Selective Protein Secretion Upon Endoplasmic Reticulum Stress Protects Cardiac Myocytes

Abstract

Protein secretion is important for proper cardiac myocyte function. Many secreted proteins are synthesized and folded in the sarco- endo-plasmic reticulum (SR/ER). A number of diseases, including heart disease, alter the ER in ways that impair ER protein folding, causing ER stress, which can result in cardiac myocyte dysfunction and decreased viability. In studies aimed at assessing the effects of ER stress on cardiac myocyte viability, heart disease-related ER stress was mimicked by treating neonatal rat ventricular myocytes (NRVM) with either tunicamycin (TM) or thapsigargin (TG), which inhibit SR/ER protein glycosylation or decrease SR/ER calcium, respectively. When treated in high culture media volumes, both TM and TG caused cardiac myocyte death; however, in low culture media volumes, while TM still caused death, remarkably, TG was protective, suggesting that potentially protective factors were secreted in response to TG but not TM. To characterize these factors, the identities of proteins in control-, TM-, and TG-conditioned medium from NRVM were determined by proteomic approaches using high performance liquid chromatography and mass spectrometry. Twenty-four different proteins, known to be synthesized in the ER, were identified in control-conditioned medium. The levels of eighteen of these proteins, including extracellular matrix proteins, hormones, and growth factors were decreased in TM- and TG-conditioned medium. However, the levels of three SR/ER-resident, calcium-binding chaperones, glucose regulated protein 78 (GRP78), glucose regulated protein 94 (GRP94), and calreticulin were increased in TG-conditioned medium but not in TM-conditioned medium. Furthermore, we found that ischemia/reperfusion, which decreases SR/ER calcium, upregulated secretion of the proteins selectively secreted in response to TG. Thus, while ER stress mediated by TM or TG decreases the movement of most proteins through the secretory pathway, TG, which mimics the effects of heart disease on SR/ER calcium in cardiac myocytes, selectively enhances the secretion of a subset of proteins, which confer protection. Therefore, proteins once thought to be permanent residents of the SR/ER may have novel, extracellular, protective roles in the diseased heart.