Alterations of the glyoxylate metabolism in hepatic steatosis – a risk factor for hyperoxaluria
Creators: name: Jashari, Adelina
Publisher: name: Technische Universität Dortmund
Year: 2020
Subjects: subject: Non-alcoholic fatty liver disease
subject: Agxt
subject: Kidney stones
subject: Hyperoxaluria
subject: 540
subject: 570
subject: Medizinische Chemie
subject: Leber
subject: Nierenstenkrankheit

description: Non-alcoholic fatty liver disease (NAFLD) comprises a wide spectrum of liver diseases and is the leading cause of liver diseases worldwide. Features of the metabolic syndrome and extrahepatic diseases, like cardiovascular diseases and kidney stone disease, strongly associate with NAFLD. A previously identified steatosis-associated downregulation of the alanine-glyoxylate aminotransferase (AGXT) was suggested to be one molecular link explaining the connection of NAFLD with the development of calcium oxalate stones. AGXT is responsible for the hepatic glyoxylate detoxification and its deficiency results in primary hyperoxaluria type 1 (PH1), which is a hereditary disorder leading to the formation of calcium oxalate kidney stones. Furthermore, the AGXT downregulation in steatotic conditions was proposed to be influenced by DNA methylation since the AGXT promoter is hypermethylated in steatosis. In the course of this thesis, further alterations of the glyoxylate metabolism were revealed in the ob/ob and Western diet (WD) mouse models of NAFLD contributing to a better understanding of the steatosis-associated modifications. These alterations indicating an increased hepatic oxalate production in steatotic conditions were confirmed by elevated oxalate excretion of steatotic hepatocytes from WD-fed and ob/ob mice upon hydroxyproline exposure. Furthermore, the increased oxalate production in the fatty liver was validated in vivo by elevated oxalate levels in the plasma from the hepatic vein of WD-fed mice. Additionally, oxalate levels in the plasma and urine of ob/ob mice were increased to a higher extent by dietary hydroxyproline compared to ob/+ mice, showing an enhanced susceptibility towards hydroxyproline also in vivo. The selective sensitivity of steatotic hepatocytes to hydroxyproline contrasted with that of primary hepatocytes from Agxt knock-out (Agxt-/-) mice, which excreted more oxalate when exposed to all oxalate precursors than hepatocytes from wild type control mice. This suggested, that only mitochondrial glyoxylate detoxification is compromised in hepatic steatosis. Rescuing the Agxt expression in the hepatocytes of ob/ob mice by AAV-mediated gene transfer was able to reduce oxalate production from hydroxyproline. Moreover, inhibition of hydroxyproline catabolism normalised the oxalate excretion from ob/ob hepatocytes after consumption of hydroxyproline. These results provided clear evidence that the downregulation of Agxt is, at least partially, responsible for the steatosis-accompanied increased hepatic oxalate production in NAFLD mouse models. This supported the hypothesis of the steatosis-associated downregulation of Agxt being a molecular link explaining the strong association between NALFD and kidney stone disease. The translational relevance of these findings was studied in a cohort of overweight and obese children and adolescents with biopsy proven NAFLD and corresponding 24 h urine samples. The steatosis percentage positively correlated with the amount of urinarily excreted oxalate, showing the relevance of steatosis induced hyperoxaluria in human NAFLD. In order to further understand the deregulation of Agxt expression in steatosis and the dependency to DNA methylation, its transcriptional upregulation in response to glucagon was studied in ob/ob and 6 weeks WD-fed mice. When treated with glucagon, an increase of the Agxt mRNA expression was missing in ob/ob mice but not in 6 weeks WD-fed mice, most probably due to the higher hypermethylation of the Agxt promoter in ob/ob mice. This supported the importance of the Agxt promoter hypermethylation regarding the regulation of the Agxt gene expression. These results were confirmed in vitro in glucagon stimulated primary hepatocytes from the two different NAFLD mouse models and in an in vitro steatosis model. Furthermore, the phosphorylation of Creb was lost earlier in hepatocytes from ob/ob, 6 weeks WD-fed mice and in an in vitro steatosis model. All in all, these findings support the thesis that steatosis associated AGXT promoter hypermethylation might repress the AGXT gene expression leading to an impaired response towards glucagon.