Non‐alcoholic fatty liver disease alters expression of genes governing hepatic nitrogen conversion

Abstract

Background & Aims We recently showed that the functional capacity for ureagenesis is deficient in non‐alcoholic fatty liver disease (NAFLD) patients. The aim of this study was to assess expression of urea cycle‐related genes to elucidate a possible gene regulatory basis to the functional problem. Methods Liver mRNA expression analyses within the gene pathway governing hepatic nitrogen conversion were performed in 20 non‐diabetic, biopsy‐proven NAFLD patients (8 simple steatosis; 12 non‐alcoholic steatohepatitis [NASH]) and 12 obese and 14 lean healthy individuals. Sixteen NAFLD patients were included for gene expression validation. Relationship between gene expressions and functional capacity for ureagenesis was described. Results Gene expression of most urea cycle‐related enzymes were downregulated in NAFLD vs both control groups; markedly so for the urea cycle flux‐generating carbamoyl phosphate synthetase (CPS1) (~3.5‐fold, P < .0001). In NASH, CPS1 downregulation paralleled the deficit in ureagenesis (P = .03). Additionally, expression of several genes involved in amino acid uptake and degradation, and the glucagon receptor gene, were downregulated in NAFLD. Conversely, glutamine synthetase (GS) expression increased >1.5‐fold (P ≤ .03), inversely related to CPS1 expression (P = .004). Conclusions NAFLD downregulated the expression of urea cycle‐related genes. Downregulation of urea cycle flux‐generating CPS1 correlated with the loss of functional capacity for ureagenesis in NASH. On gene level, these changes coincided with an increase in the major ammonia scavenging enzyme GS. The effects seemed related to a fatty liver as such rather than NASH or obesity. The findings support gene regulatory mechanisms involved in the deficient ureagenesis of NAFLD, but it remains unexplained how hepatocyte fat accumulation exerts these effects.

Glucagon acutely regulates hepatic amino acid catabolism and the effect may be disturbed by steatosis

“…In support of this, hypermethylation of promoter regions of the urea cycle genes Otc and Cps1 has been reported both in animals models of NASH and in patients with NAFLD [ 30 ]. In addition to Cps1 , two amino acid transporters, Slc7a2 and Slc38a2 , were downregulated in mice on HFD + FW as well as in Gcgr −/- mice, which is in line with human data showing downregulation in patients with NAFLD [ 31 , 46 ]. SLC38A2 transports neutral amino acids (i.e., alanine), and, in support of its possible role in the liver-alpha cell axis, transcription of Slc38a2 was increased in rats by feeding a high-protein diet, by glucagon administration, and in hepatocytes incubated with forskolin (an activator of adenylyl cyclase) [ 47 ].…”

“…It was recently shown that patients with non-alcoholic fatty liver disease (NAFLD) have reduced capacity for ureagenesis [ 29 ], and that gene expression of urea cycle-related enzymes is downregulated in NAFLD patients [ 30 , 31 ]. To test whether this is also the case in mice, animals were fed a high-fat diet and 15% fructose water (HFD + FW) for 8–10 weeks to induce a physiologically relevant model of hepatic steatosis ( Figure 4 D).…”

Nonalcoholic Fatty Liver Disease Impairs the Liver–Alpha Cell Axis Independent of Hepatic Inflammation and Fibrosis

“…(3,31,32) Glucagon's impact on ureagenesis includes transcriptional changes in genes (such as carbamoylphosphate synthase 1 [CPS1], glutamic-oxaloacetic transaminase 1 [GOT1], and solute carrier family 25 member 18 [SLC25A18]) that regulate hepatic nitrogen conversion in NAFLD, as has been shown in HOMA-IR Glucagon-alanine index R 2 = 0.165 P < 0.05 several studies. (26,33,34) Eriksen et al (34) found that the genes most involved with urea cycle-related enzymes were down-regulated in patients with both NAFL and NASH compared to lean and obese controls without NAFLD. The altered expression explains the decrease observed in in vivo functional capacity for ureagenesis.…”

Functional Microbiomics Reveals Alterations of the Gut Microbiome and Host Co‐Metabolism in Patients With Alcoholic Hepatitis

“…Impaired urea cycle in fatty liver disease has been reported previously. (29)(30)(31)(32) The capacity of urea synthesis is decreased in patients with compromised liver function but increases in patients with inflammation. (33) Interestingly, both mechanisms are involved in alcoholic hepatitis.…”