This article is available online at http://www.jlr.orgCompelling epidemiological and clinical data indicate that diabetes mellitus increases the risk for cardiac dysfunction and heart failure independently of other risk factors, such as coronary disease and hypertension. Lipotoxicity is an important contributor to cardiac dysfunction in both type 1 ( 1, 2 ) and type 2 ( 3-6 ) diabetes, which are characterized by excessive accumulation of triacylglycerols, long-chain acyl-CoAs, diacylglycerols, and ceramides in myocardium. Correlation of the increased ceramide level with development of diabetic cardiomyopathy has attracted special attention in recent years because of the well-documented ability of this sphingolipid to regulate a number of cell processes, including cell proliferation, growth arrest, differentiation and apoptosis, and the mediation of responses to stress stimuli .Ceramide is a hub of sphingolipid metabolism ( 7,8 ). The balance between ceramide-producing pathways and pathways that consume it dictates ceramide level in the cell. The de novo ceramide biosynthesis pathway, one of the major ceramide producing pathways, localizes in the endoplasmic reticulum (ER) and starts with the condensation of serine and palmitoyl-CoA producing 3-ketosphingonine, which, in turn, is rapidly converted to dihydrosphingosine. Subsequent acylation of dihydrosphingosine by a set of (dihydro)ceramide synthases (CerSs) gives rise to dihydroceramide. Finally, the removal of two hydrogens from the fatty acid chain of dihydroceramide by desaturase leads to the formation of ceramide. Other possible contributors to elevated ceramide level are: hydrolysis of SM catalyzed by SMases, Abstract Sphingolipids have been implicated as key mediators of cell-stress responses and effectors of mitochondrial function. To investigate potential mechanisms underlying mitochondrial dysfunction, an important contributor to diabetic cardiomyopathy, we examined alterations of cardiac sphingolipid metabolism in a mouse with streptozotocininduced type 1 diabetes. Diabetes increased expression of desaturase 1, (dihydro)ceramide synthase (CerS)2, serine palmitoyl transferase 1, and the rate of ceramide formation by mitochondria-resident CerSs, indicating an activation of ceramide biosynthesis. However, the lack of an increase in mitochondrial ceramide suggests concomitant upregulation of ceramide-metabolizing pathways. Elevated levels of lactosylceramide, one of the initial products in the formation of glycosphingolipids were accompanied with decreased respiration and calcium retention capacity (CRC) in mitochondria from diabetic heart tissue. In baseline mitochondria, lactosylceramide potently suppressed state 3 respiration and decreased CRC, suggesting lactosylceramide as the primary sphingolipid responsible for mitochondrial defects in diabetic hearts. Moreover, knocking down the neutral ceramidase (NCDase) resulted in an increase in lactosylceramide level, suggesting a crosstalk between glucosylceramide synthase-and NCDase-mediated ceramide utiliz...

Experimental evidence supports the role of mitochondrial ceramide accumulation as a cause of mitochondrial dysfunction and brain injury after stroke. Herein, we report that SIRT3 regulates mitochondrial ceramide biosynthesis via deacetylation of ceramide synthase (CerS) 1, 2, and 6. Reciprocal immunoprecipitation experiments revealed that CerS1, CerS2, and CerS6, but not CerS4, are associated with SIRT3 in cerebral mitochondria. Furthermore, CerS1, -2, and -6 are hyperacetylated in the mitochondria of SIRT3-null mice, and SIRT3 directly deacetylates the ceramide synthases in a NAD ؉ -dependent manner that increases enzyme activity. Investigation of the SIRT3 role in mitochondrial response to brain ischemia/reperfusion (IR) showed that SIRT3-mediated deacetylation of ceramide synthases increased enzyme activity and ceramide accumulation after IR. Functional studies demonstrated that absence of SIRT3 rescued the IR-induced blockade of the electron transport chain at the level of complex III, attenuated mitochondrial outer membrane permeabilization, and decreased reactive oxygen species generation and protein carbonyls in mitochondria. Importantly, Sirt3 gene ablation reduced the brain injury after IR. These data support the hypothesis that IR triggers SIRT3-dependent deacetylation of ceramide synthases and the elevation of ceramide, which could inhibit complex III, leading to increased reactive oxygen species generation and brain injury. The results of these studies highlight a novel mechanism of SIRT3 involvement in modulating mitochondrial ceramide biosynthesis and suggest an important role of SIRT3 in mitochondrial dysfunction and brain injury after experimental stroke.