Abstract:

The late endosomal/lysosomal adaptor MAPK and mTOR activator 1 (LAMTOR1) is an important regulator protein in the response to energy stress. Public gene expression data shows that the expression of LAMTOR1 is abnormally high in nonalcoholic steatohepatitis (NASH) and hepatocellular carcinoma (HCC); hence, LAMTOR1 may play an important role in the development of NASH and HCC. Therefore, exploring the LAMTOR1 regulatory mechanism in the progression of NASH and malignant transformation of liver inflammation may be crucial for translational medicine. First, a NASH mouse model was established by feeding a methionine choline-deficient (MCD) diet. Hematoxylin-eosin staining of liver tissues showed successful modeling of inflammatory injury in the mouse liver. Immunoblot analysis confirmed LAMTOR1- and LAMTOR1-mediated protein expression in LAMTOR1 specifically depleted mouse livers. Subsequently, metabolic profiling of liver tissues was performed using an ultra-performance liquid chromatography-time-of-flight mass spectrometry strategy. Based on the retention time, m/z value, and tandem mass spectra, 134 metabolites were identified. Among these, the levels of 45 metabolite were significantly influenced by hepatic LAMTOR1 depletion. According to the metabolomics results, uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) was significantly upregulated in LAMTOR1-depleted (LAMTOR1^LKO) hepatocyte tissues. As the final product of the hexosamine biosynthetic pathway (HBP), alteration in UDP-GlcNAc levels may regulate LAMTOR1 and metabolic regulatory genes downstream of HBP. Moreover, there was an obvious increase in the levels of several methylation-related metabolites. Thus, upregulated S-adenosylmethionine, S-adenosylhomocysteine, and N6,N6,N6-trimethyl-L-lysine indicated that LAMTOR1 may regulate the process of DNA or protein methylation. In addition, downregulation of 9-oxo-octadecadienoate, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) was also observed in LAMTOR1^LKO mice liver tissues. Alterations in polyunsaturated fatty acids, such as EPA and DHA, link LAMTOR1 to inflammatory and immune processes, which are known to play important roles in NASH pathogenesis. These metabolic disorders demonstrated that LAMTOR1 significantly contributed to the metabolic mechanism of NASH. Furthermore, gene expression correlations were analyzed to interpret the regulatory role of LAMTOR1 from the perspective of genetic networks. Owing to a paucity of liver whole-transcriptome studies in NASH, correlation analysis was performed based on HCC transcriptome data from public databases. First, a negatively regulated relationship was observed between LAMTOR1 and MAT1A (R=-0.47). MAT1A encodes methionine adenosyltransferase 1A, an essential enzyme that catalyzes the formation of S-adenosylmethionine. Based on the upregulation of UDP-GlcNAc under hepatocyte LAMTOR1 depletion, it was predicted that LAMTOR1 positively influenced MGAT1 (R=0.47), a gene encoding alpha-1,3-mannosyl-glycoprotein 2-beta-N-acetylglucosaminyltransferase. Together with changes in succinyladenosine caused by hepatocyte LAMTOR1 deletion, predicted correlation results showed that LAMTOR1 may also participate in the pathogenesis through the positive regulatory relationship with ADSL (R=0.59). The ADSL gene provides instructions for making an enzyme called adenylosuccinate lyase, which can dephosphorylate the substrate succinyladenosine. In this study, LAMTOR1 was identified to specifically regulate multiple key metabolic pathways based on both NASH mouse models and gene expression correlations. These results illustrate the important role of LAMTOR1 in the progression of NASH and malignant transformation of liver inflammation, which provides a theoretical basis for the diagnosis and treatment of NASH or possible NASH-driven HCC.

Key words: liquid chromatography-mass spectrometry (LC-MS), metabolomics, LAMTOR1, non-alcoholic steatohepatitis, malignant transformation