Effect of Sphingomyelin Synthase Gene Family, as Phospholipase Cs, on liver function

Abstract

Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are two essential phospholipids for cell formation, growth, and death. The steady state levels of PC and PE are controlled by their biosynthesis (Kennedy Pathway) and catabolic pathways through different phospholipases, including phospholipase C (PLC). Mammalian phospholipase Cs (PLC) are a group of enzymes which produce a diacylglycerol(DAG) and a phosphorylated molecule, such as phosphorylcholine and phosphorylethanolamine. Many studies have been conducted on mammalian phosphatidylcholine-PLC (PC-PLC), and many biological functions could be related with phosphatidylethanolamine-PLC (PE-PLC). However, the genes which are responsible for mammalian PC-PLC and PE-PLC have not been cloned. Given the fact that tricyclodecan-9-yl-potassium xanthate (D609) can inhibit both PC-PLC and sphingomyelin synthase (SMS) activities, and all SMS gene family members have a conservative catalytic domain (mediating the nucleophilic attack on the phosphodiester bond of PC and/or PE), we hypothesized that all SMS gene family members have PLC activity. In the current study, we showed that SMS1 and SMS2are PC-PLC, while SMSr is PE-PLC. Further, we demonstrated that SMS1/SMS2/SMSr triple deficiency significantly increases PE and PC, whereas significantly decreases DAG in the liver and plasma, compared with control. Furthermore, we found that liver-specific Sms1/Sms2 double knockout (KO) mice have severe liver inflammation, fibrosis, and tumor, and all these are associated with SMS1 but not SMS2 deficiency. Importantly, we found that SMSr deficiency can rescue, at least partially, the pathology caused by SMS1/SMS2 deficiency. We then investigated the mechanism. We found that liver SMS1 deficiency promotes an over-production of glucosylceramide which, in tune, can stimulate hepatocyte to produce more inflammatory factors, including TGFβ1. The elevated TGFβ1 then can stimulate hepatic stellate cells to over-express genes which are related with fibrosis, such as collagen 1α1 and platelet-derived growth factor receptors, resulting in liver fibrosis. Moreover, we found that PE treatment (mimic the condition of SMSr deficiency), in vitro or in vivo, can significantly reduce hepatocyte TGFβ1 production and, consequently, hepatic stellate cell collagen1α1 production. PE treatment further can reduce TGFβ1-associated β-cateninnucleus translocation, thus reducing tumorigenesis.

We concluded that 1) all SMS gene family members have PLC activity; SMS1 and SMS2 are two PC-PLCs; and SMSr is aPE-PLC; 2) SMS1 deficiency-mediated glucosylceramide accumulation causes liver inflammation, fibrosis, and tumorigenesis. All these pathological phenotypes can be corrected, at least partially, by SMSr deficiency-mediated PE accumulation or PE supplementation. This study potentially provides a new mechanism for the development of nonalcoholic steatohepatitis (NASH) and its treatment.