CerS5 deficiency promotes liver fibrosis development in non-alcoholic fatty liver disease

BACKGROUND

Hepatocyte lipotoxicity mediated by sphingolipids was considered one of important factors in NAFLD development. Knocking out key enzymes for sphingolipids synthesis, such as DES-1, SPHK1 and CerS6, could reduce hepatocyte lipotoxicity and improve NAFLD progression. Previous studies showed that roles of CerS5 and CerS6 in sphingolipids metabolism were similar, but the role of CerS5 was controversial in NAFLD development. This study aimed to clarify the role and mechanism of CerS5 in NAFLD development.

METHODS

Hepatocyte conditional CerS5 knockout (CerS5 CKO) and wild type (WT) mice were fed with standard control diet (SC) and choline-deficient, l-amino acid-defined, high-fat diet (CDAHFD) and then divided into four groups: CerS5 CKO-SC, CerS5 CKO-CDAHFD, WT-SC and WT-CDAHFD. RT-PCR, IHC and WB were used to analyze the expression of inflammatory, fibrosis and bile acids (BA) metabolism factors. RNA-seq was used to analyze differences of transcriptional levels of liver molecules among the four groups. Metabolomics was used to measured differences of hepatic BAs among the four groups.

RESULTS

Hepatocyte specific knockout of CerS5 did not increase or reduce the severity of 8-weeks CDAHFD induced hepatic steatosis and inflammation, but significantly worsened the progression of liver fibrosis in these mice. At the molecular level, hepatocyte specific knockout of CerS5 did not increase or reduce expression of hepatic inflammatory factors: CD68, F4/80 and MCP-1, but increased expression of hepatic fibrosis factors: α-SMA, COL1α and TGF-β in mice fed with CDAHFD. Transcriptome analysis showed that hepatocyte specific knockout of CerS5 significantly decreased the expression of hepatic cyp27a1, and decreased expression of cyp27a1 was further validated by RT-PCR and WB. Considering that cyp27a1 was a key enzyme in the alternative pathway of BA synthesis, we further found that hepatic BA pools in CerS5 CKO mice were more conducive to the progression of liver fibrosis, which were characterized by elevated hydrophobic 12α-OH BAs and decreased hydrophilic non-12α-OH BAs.

CONCLUSION

CerS5 played an important role in the progression of NAFLD related fibrosis, and hepatocyte specific knockout of CerS5 accelerated the progression of NAFLD related fibrosis, which was possibly due to the inhibition of BA synthesis alternative pathway by knocking out hepatocyte CerS5.

Sphingolipid-Induced Programmed Cell Death is a Salicylic Acid and EDS1-Dependent Phenotype in Arabidopsis Fatty Acid Hydroxylase (Fah1, Fah2) and Ceramide Synthase (Loh2) Triple Mutants

Ceramides (Cers) and long-chain bases (LCBs) are plant sphingolipids involved in the induction of plant programmed cell death (PCD). The fatty acid hydroxylase mutant fah1 fah2 exhibits high Cer levels and moderately elevated LCB levels. Salicylic acid glucoside level is increased in this mutant, but no cell death can be detected by trypan blue staining. To determine the effect of Cers with different chain lengths, fah1 fah2 was crossed with ceramide synthase mutants longevity assurance gene one homologue1-3 (loh1, loh2 and loh3). Surprisingly, only triple mutants with loh2 show cell death detected by trypan blue staining under the selected conditions. Sphingolipid profiling revealed that the greatest differences between the triple mutant plants are in the LCB and LCB-phosphate (LCB-P) fraction. fah1 fah2 loh2 plants accumulate LCB d18:0, LCB t18:0 and LCB-P d18:0. Crossing fah1 fah2 loh2 with the salicylic acid (SA) synthesis mutant sid2-2 and with the SA signaling mutants enhanced disease susceptibility 1-2 (eds1-2) and phytoalexin deficient 4-1 (pad4-1) revealed that lesions are SA- and EDS1-dependent. These quadruple mutants also confirm that there may be a feedback loop between SA and sphingolipid metabolism as they accumulated less Cers and LCBs. In conclusion, PCD in fah1 fah2 loh2 is a SA- and EDS1-dependent phenotype, which is likely due to accumulation of LCBs.

Ceramide synthases: insights into the expression and prognosis of lung cancer

CerSs (ceramide synthases), a group of enzymes that catalyze the formation of ceramides from sphingoid base and acyl-CoA substrates. As far, six types of CerSs (CerS1-CerS6) have been found in mammals. Each of these enzymes have unique characteristics, but maybe more noteworthy is the ability of individual CerS isoform to produce a ceramide with a characteristic acyl chain distribution. As key regulators of sphingolipid metabolism, CerSs highlight their unique characteristics and have emerging roles in regulating programmed cell death, cancer and many other aspects of biology. However, the role of CerSs in lung cancer has not been fully elucidated. In this study, there was no significant change in the sequence or copy number of CerSs gene, which could explain the stability of malignant tumor development through COSMIC database. In addition, gene expression in lung cancer was examined using the Oncomine^TM database, and the prognostic value of each gene in non-small cell lung cancer (NSCLC) was analyzed by Kaplan-Meier analysis. The results showed that high mRNA expression levels of CerS2, CerS3, CerS4 and CerS5 in all NSCLC patients were associated with improved prognosis. Among them, CerS2 and CerS5 are also highly expressed in adenocarcinoma (Ade), but not in squamous cell carcinoma (SCC). In contrast, high or low expression of CerS1 and CerS6 no difference was observed in patients with NSCLC, Ade and SCC. Integrated the data of this study suggested that these CerSs may be a potential tumor markers or drug target of new research direction.

Preparation of HeLa Total Membranes and Assay of Lipid-inhibition of Serine Palmitoyltransferase Activity

Serine palmitoyltranferase (SPT) is a pyridoxal 5' phosphate (PLP)-dependent enzyme that catalyzes the first and rate-limiting step of de novo synthesis of sphingolipids. SPT activity is homeostatically regulated in response to increased levels of sphingolipids. This homeostatic regulation of SPT is mediated through small ER membrane proteins termed the ORMDLs. Here we describe a procedure to assay ORMDL dependent lipid inhibition of SPT activity. The assay of SPT activity using radiolabeled L-serine was developed from the procedure established by the Hornemann laboratory. The activity of SPT can also be measured using deuterated L-serine but it requires mass spectrometry, which consumes money, time and instrumentation. The ORMDL dependent lipid inhibition of SPT activity can be studied in both cells and in a cell free system. This assay procedure is applicable to any type of mammalian cell. Here we provide the detailed protocol to measure SPT activity in the presence of either short chain (C8-ceramide) or long chain ceramide (C24-ceramide). One of the greatest advantages of this protocol is the ability to test insoluble long chain ceramides. We accomplished this by generating long chain ceramide through endogenous ceramide synthase by providing exogenous sphingosine and 24:1 acyl CoA in HeLa cell membranes. This SPT assay procedure is simple and easy to perform and does not require sophisticated instruments.

Function of ceramide synthases on growth, ganoderic acid biosynthesis and sphingolipid homeostasis in Ganoderma lucidum

Ceramide synthases (CERSs) catalyse an N-acyltransferase reaction using long-chain base (LCB) and fatty acyl-coenzyme A (CoA) as substrates to synthesize ceramide (Cer), which is the backbone of all complex sphingolipids. In the present study, three CERSs (LAG1, LAG2 and LAG3) form Ganoderma lucidum were analysed. The silencing of lag1 by RNA interference reduced ganoderic acid biosynthesis and Cer and complex sphingolipids contents, which contain long-chain-fatty-acids (LCFAs, including C16 and C18). In contrast, the silencing of lag2 or lag3 did not result in obvious phenotypic and sphingolipid homeostasis changes, although the lag2/lag3 double-silenced mutants exhibited increased ganoderic acid biosynthesis as well as reduced growth, reduced Cer and complex sphingolipids contents, which contain very-long-chain fatty acids (VLCFAs, including C22, C24 and C26). The results of the present study indicate that the three assayed CERSs have distinct physiological functions and substrate specificities in G. lucidum.

Yeast ceramide synthases, Lag1 and Lac1, have distinct substrate specificity

LAG1 was the first longevity assurance gene discovered in Saccharomyces cerevisiae. The Lag1 protein is a ceramide synthase and its homolog, Lac1, has a similar enzymatic function but no role in aging. Lag1 and Lac1 lie in an enzymatic branch point of the sphingolipid pathway that is interconnected by the activity of the C4 hydroxylase, Sur2. By uncoupling the enzymatic branch point and using lipidomic mass spectrometry, metabolic labeling and in vitro assays we show that Lag1 preferentially synthesizes phyto-sphingolipids. Using photo-bleaching experiments we show that Lag1 is uniquely required for the establishment of a lateral diffusion barrier in the nuclear envelope, which depends on phytoceramide. Given the role of this diffusion barrier in the retention of aging factors in the mother cell, we suggest that the different specificities of the two ceramide synthases, and the specific effect of Lag1 on asymmetrical inheritance, may explain why Δlag1 cells have an increased lifespan while Δlac1 cells do not.

Highlighted Article: Distinct substrate specificities of Lag1 and Lac1, the two yeast ceramide synthases, are revealed, shedding light on their physiological roles.

The enigma of ceramide synthase regulation in mammalian cells

Ceramide synthases (CerS) are key enzymes in the lipid metabolism of eukaryotic cells. Their products, ceramides (Cer), are components of cellular membranes but also mediate signaling functions in physiological processes such as proliferation, skin barrier function and cerebellar development. In pathophysiological processes such as multiple sclerosis and tumor progression, ceramide levels are altered, which can be ascribed, partly, to dysregulation of CerS gene transcription. Most publications deal with the effects of altered ceramide levels on physiological and pathophysiological processes, but the regulation of the appropriate CerS is frequently not investigated. This is insufficient for the clarification of the role of ceramides, because most ceramide species are generated by at least two CerS. The mechanisms of CerS regulation are manifold and it seems that each CerS isoform is regulated individually. For this reason, we discuss the different CerS separately in this review. From transcriptional regulation to alteration of protein activity, the possibilities to influence CerS are diverse. Furthermore, CerS are influenced by a variety of molecules including hormones and lipids. Without claiming completeness, we provide a résumé of the regulatory mechanisms for each CerS in mammalian cells and how dysregulation of these mechanisms during physiological processes may lead to pathophysiological processes.