The critical role of oxalate in the liver-kidney axis and its effect on glucose metabolism
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Date
2024
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Abstract
Glucose metabolism is a highly regulated and conserved pathway orchestrated by several factors like hormones, nutritional state and enzymes. The organs, which are responsible to maintain normal blood glucose levels are the liver the kidney. Under prolonged fasting, the body makes use of the de novo production of glucose from non-carbohydrate substrates – mainly lactate, glycerol and amino acids. This process is called gluconeogenesis. In the non-alcoholic fatty liver disease (NAFLD) hepatic gluconeogenesis is frequently elevated which results in hyperglycemia and manifests the progression to type 2 diabetes mellitus. In a previous study, a steatosis-associated downregulation of the alanine-glyoxylate aminotransferase (Agxt) was observed in mouse models and patients with NAFLD, and this was accompanied by increased urinary oxalate levels. The physiological function of Agxt is to detoxify glyoxylate to prevent the formation of the harmful waste product oxalate. Therefore, decreased Agxt expression in the fatty liver could represent a mechanism that explains a higher risk of hyperoxaluria in patients with NAFLD. In addition to the well-established role of Agxt in preventing oxalate production, evidence for a role of Agxt in amino acid-driven gluconeogenesis exists, which has to date not been completely understood. At the same time, oxalate has been reported to inhibit pyruvate carboxylase, a key enzyme of the gluconeogenesis pathway. Altogether, this leads to the hypothesis, that Agxt may support gluconeogenesis by restricting oxalate generation. To investigate this hypothesis, the inhibitory effect of oxalate on gluconeogenesis was first studied in vitro. The exposure of oxalate and the oxalate precursor hydroxyproline to human and murine hepatocytes in this assay revealed an inhibitory effect of oxalate on the glucose production from pyruvate, lactate and alanine, but not from glycerol. The findings demonstrated a critical role and a direct influence of oxalate on glucose synthesis from precursors that require pyruvate carboxylase to enter the gluconeogenesis pathway.To translate the in vitro findings to an in vivo model, glucose homeostasis was studied in AgxtKO mice, which display high plasma and urinary oxalate levels. The results revealed no changes in the plasma glucose levels, but, intriguingly, a decreased body weight loss after an overnight starvation. In plasma, levels of the glucogenic amino acid glutamine were decreased after fasting in Agxt-deficient compared to wildtype mice. Glucose metabolism-associated gene expression analysis in the liver showed no difference between AgxtKO and wt mice, whereas in the kidney, gene expression changes were observed, suggesting upregulation of compensatory pathways to overcome oxalate inhibition, such as glycerol-driven gluconeogenesis. The results demonstrated that renal gluconeogenesis is more affected by oxalate in the kidney of hyperoxaluric AgxtKO mice, and when hepatic Agxt deficiency appears it affects muscle protein breakdown and glutamine release. This indicates a role of Agxt in gluconeogenesis, since other key enzymes of gluconeogenesis were also upregulated. The results suggest a small contribution of Agxt in glucose homeostasis via regulation of oxalate generation within hepatocytes. In the absence of Agxt, mice can maintain glycemia. However, the levels of oxalate generated in the liver appear to affect the kidney, as judged by the observed gene expression changes. We propose that in kidney, gluconeogenesis from glycerol and other compensatory pathways will be predominant when Agxt is downregulated and oxalate levels become high enough to inhibit gluconeogenesis via pyruvate carboxylase. Conversely, upregulation of Agxt – e.g. by glucagon or pyruvate availability - will prevent oxalate generation and allow pyruvate-driven gluconeogenesis. The hypothesis that levels of oxalate may determine substrate utilization for glucose production in glucose-producing organs like liver and kidney in health and disease should be further explored using 13C metabolic flux analysis. Finally, further experiments are also needed to understand the influence of hepatic Agxt on muscle protein breakdown.
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Gluconeogenesis, Oxalate, Pyruvate, Liver-kidney crosstalk