Volume 62, Issue 1, Pages 219-233 (January 2015) Acid sphingomyelinase-ceramide system in steatohepatitis: A novel target regulating multiple pathways  Carmen Garcia-Ruiz, Jose M. Mato, Dennis Vance, Neil Kaplowitz, José C. Fernández-Checa  Journal of Hepatology  Volume 62, Issue 1, Pages 219-233 (January 2015) DOI: 10.1016/j.jhep.2014.09.023 Copyright © 2014 European Association for the Study of the Liver Terms and Conditions

Fig. 1 ASMase in steatohepatitis: One target, multiple pathways. ASMase becomes activated in liver samples of patients and experimental models of ASH and NASH. Recent evidence shows that ASMase is required for endoplasmic reticulum (ER) stress, autophagy and lysosomal membrane permeabilization (LMP) in ASH/NASH, which in turn mediates insulin resistance (I/R), lipogenesis, fibrosis and regulates steatosis. LMP is a novel pathway of lipotoxicity that cross-talks with mitochondria to induce apoptosis/necrosis. Moreover, ASMase activation depletes hepatic S-adenosyl-L-methionine (SAM) and phosphatidylcholine (PC), which are key intermediates in liver physiology via methylation of multiple substrates (DNA, proteins and lipids), maintenance of antioxidant and GSH defence, and membrane integrity. Thus, ASMase contributes to liver steatosis, liver injury, inflammation and fibrosis, characteristic of ASH/NASH. ASMase targeting may be a novel target of potential relevance for ASH/NASH by disabling multiple pathogenic pathways. Journal of Hepatology 2015 62, 219-233DOI: (10.1016/j.jhep.2014.09.023) Copyright © 2014 European Association for the Study of the Liver Terms and Conditions

Fig. 2 Hepatic methionine metabolism. Diet-derived methionine is transformed by MAT1A to SAM, which is then used for methylation reactions generating SAH and homocysteine. Homocysteine is remethylated to methionine by betaine or 5MTHF in reactions catalysed by BHMT and methionine synthase (MS). Betaine is derived from diet or mitochondria by diffusion following choline oxidation. MS links the methionine cycle to the folate cycle. Homocysteine escapes the methionine cycle via its transformation to cystathionine in the transsulfuration pathway, which provides cysteine for the synthesis of glutathione (GSH). SAM is transported to mitochondria to promote the methylation of mitochondrial components and the transport of GSH from cytosol by maintenance of mitochondrial membrane-fluidity. Several alterations in the methionine metabolism are associated with steatohepatitis, indicated by the red and blue arrows denoting decreases and increases, respectively. Decreased GNMT leads to increased SAM which contributes to steatohepatitis (see text for details). Journal of Hepatology 2015 62, 219-233DOI: (10.1016/j.jhep.2014.09.023) Copyright © 2014 European Association for the Study of the Liver Terms and Conditions

Fig. 3 Pathways of hepatic PC generation and role in hepatic steatosis. Liver PC is principally generated from choline via the CDP-choline arm of the Kennedy pathway. An additional pathway of PC generation is the methylation of PE by PEMT using SAM as a methyl donor. PC regulates hepatic steatosis, as PC is essential for VLDL secretion. Moreover, as shown in certain contexts, such as in the liver of LCCTα null mice, PC prevents the unconventional proteolytic processing of SREBP in the ER by proteases SP1/SP2 that lead to the activation of lipogenesis pathways. PC is a substrate for the synthesis of SM catalysed by SMS in the Golgi. PC depletion results in a secondary increase of ceramide levels. Journal of Hepatology 2015 62, 219-233DOI: (10.1016/j.jhep.2014.09.023) Copyright © 2014 European Association for the Study of the Liver Terms and Conditions

Fig. 4 Central role of ASMase in the regulation of MAT1A, PC homeostasis and lipid metabolism. Alterations in methionine metabolism, reflected as increased levels of Hcy, decreased ratio of SAM/SAH, and ASMase activation engage in a feed forward loop via the ASMase-mediated reduction of MAT1A expression. ASMase-induced generation of ceramide and sphingosine can inhibit the CDP-choline pathway of PC de novo biosynthesis, leading to decreased levels of PC. In addition, ASMase-mediated SAM depletion could contribute to decreased phosphatidylethanolamine methylation, resulting in low PC generation. ASMase activation promotes autophagy and LMP and is required for ER stress. While LMP contributes to lipotoxicity, the role autophagy in steatosis is controversial (dashed line). ER stress not only promotes lipogenesis and fatty liver via activation of the SREBP transcription factors, but regulates expression of the mitochondrial transporting protein StARD1, which stimulates cholesterol trafficking to mitochondria, contributing to oxidative stress and hepatocellular manifestations of steatohepatitis. Journal of Hepatology 2015 62, 219-233DOI: (10.1016/j.jhep.2014.09.023) Copyright © 2014 European Association for the Study of the Liver Terms and Conditions

Journal of Hepatology 2015 62, 219-233DOI: (10. 1016/j. jhep. 2014. 09 Copyright © 2014 European Association for the Study of the Liver Terms and Conditions