Cardiac cell loss is a prominent pathological feature associated with a number of cardiovascular disorders, including myocardial infarction, ischemia/reperfusion, chronic ischemia, and heart failure. ARC (Apoptosis Repressor with CARD/Caspase Recruitment Domain) is a recently identified muscle-specific repressor of cell death. It was originally identified as a protein that interacts with and regulates the activity of initiator caspase 8 which is associated with death receptor (TNFa, Fas) signaling. Our published and preliminary data indicate that there are additional targets for cardioprotection by ARC involving both caspase-dependent and -independent pathways. These targets are the intrinsic death signaling pathways of two major intracellular organelles, the mitochondria and the endoplasmic reticulum/Golgi complex. ARC is present in both subcellular fractions containing these organelles and staining reveals an association of ARC with the ER/developing sarcoplasmic reticulum (SR) of neonatal cardiomyocytes and a striated transverse distribution in adult cardiac muscle fibers that overlaps extensively with the SR. We hypothesize that ARCs structural association with the ER/SR and its close functional association with mitochondria are what is responsible for its ability to suppress a variety of intrinsic death stimuli, often intervening at the earliest stages in death signaling. In the mitochondria, we hypothesize that ARC interferes with BAX translocation/activation, while in the ER/SR it acts as an integral component of the protective arm of the endogenous ER stress response program. To test these hypotheses, we propose the following specific aims: Specific Aim #1 will define the molecular mechanism by which ARC inhibits BAX-induced and BAX associatedcell death as well as the functional significance of its association with a critical regulator of the mitochondrial fusion/fission apparatus. Specific Aim #2 will establish whether ARC is part of the ER stress response known as the unfolded protein response (UPR), what role it plays in ER preconditioning, and whether it suppresses the ER-specific initiator caspase 12. Specific Aim #3 will define ARC's role in regulating the apoptotic crosstalk between the ER/SR and mitochondrial death pathways through its effects on calcium dynamics and initiator caspase activation in response to hypoxia. The studies proposed will define the critical role of ARC in the management of the mitochondrial and ER/Golgi stress responses to cellular injury in the heart. [unreadable] [unreadable]