PROJECT SUMMARY/ABSTRACT Heart failure (HF) is a major cause of mortality worldwide, and identifying novel therapies to treat HF represents an urgent clinical need. The long-term vision of my laboratory is that apolipoproteins can be used to treat HF. We have discovered that reduced circulating levels of apolipoprotein M (ApoM) are associated with increased mortality in human HF. Each standard deviation reduction in ApoM is associated with a doubling of mortality risk in HF, an association that is independent of B-type natriuretic peptide, coronary artery disease, and other known risk factors. ApoM is made almost exclusively by the liver, secreted by hepatocytes, and binds the bioactive lipid sphingosine-1-phosphate (S1P) on HDL particles in the circulation, ultimately activating G-protein coupled S1P receptors on various cell types; however, the precise mechanism by which ApoM may increase HF survival is unknown. To understand mechanisms of cardioprotection by ApoM, we utilized a doxorubicin cardiotoxicity (DoxTox) model. Dox is utilized to treat multiple human cancers, but its use is limited by DoxTox and long-term HF. We have discovered that Dox reduces ApoM in humans and mice. In DoxTox models, increasing ApoM improves survival and prevents Dox-induced cardiac dysfunction. In a clinically relevant acute myeloid leukemia model, preliminary studies indicate ApoM does not interfere with Dox anti-cancer efficacy. Our preliminary data suggest that ApoM attenuates Dox-induced autophagic impairment in the myocardium. We find ApoM increases autophagic flux and preserves nuclear transcription factor EB (TFEB), a master regulator of autophagy and lysosomal biogenesis implicated in multiple cardiomyopathies. Our data suggest ApoM-driven autophagy and preservation of nuclear TFEB are protective mechanisms generalizable to other cardiomyopathies. This R01 proposal tests the hypothesis that ApoM, via canonical S1P signaling, enhances myocardial autophagy and preserves nuclear TFEB to attenuate DoxTox. Aim 1 tests whether hepatic S1P production is required for ApoM-mediated myocardial autophagy; Aim 2 tests whether the S1P receptor at the level of cardiomyocyte is required for autophagy, and Aim 3 tests whether cardiomyocyte TFEB is required for the cardioprotective effects of ApoM. Aim 3 also utilizes the innovative technique of CUT&RUN sequencing to determine whether ApoM directs TFEB to specific transcriptional targets, which will help elucidate or confirm other pathways downstream of TFEB directed by ApoM. Success of these aims will identify mechanisms by which ApoM can attenuate DoxTox and improve outcomes in HF.