The prevalence of chronic kidney disease (CKD) is rising with significant cost and burden to our health system. Renal fibrosis is the common outcome of CKD independent of the underlying aetiologies. Changes in energy metabolism are emerging as a key contributor to renal fibrosis. Expression of genes regulating fatty acid and carbohydrate metabolism is reduced in the fibrotic kidney. Metabolic reprogramming has been reported in several models of renal disease, although its exact mechanism remains obscure. We aimed to determine the role of phosphorylation events that control fatty acid oxidation (FAO) and glycolysis in renal fibrosis. The folic acid nephropathy (FAN) and unilateral ureteric obstruction (UUO) models were induced in mice with knock-in (KI) mutations of phosphorylation sites (phosphosites) in acetyl CoA carboxylase 1 and 2 (ACC 1/2 KI mice), the major regulator of FAO, and 6-phosphofructo-2kinase/fructose-2,6-biphosphatase (PFKFB2 KI mice), the major regulator of glycolysis. Metformin, which activates 5'-AMP-activated protein kinase (AMPK) to increase phosphorylation of ACC, was administered to mice with FAN. ACC Ser79 phosphorylation was reduced in folate-treated tubular epithelial cells (p<0.01) and WT mice with FAN (p<0.05). Mutation of these sites in ACC1/2 KI mice with FAN or UUO caused lipid accumulation (Oil Red O p<0.01), increased triglyceride (p<0.01), increased collagen (PicroSirius red p<0.001; Masson’s Trichrome p<0.01; qRT-PCR p<0.01) and increased A-SMA (Western blot p<0.05; qRT-PCR p<0.01). Metformin administration was associated with reduced fibrosis (PicroSirius red p<0.01) and reduced lipid accumulation (Oil Red O p<0.05) in WT mice, but not in ACC1/2KI mice. Phosphorylation of the kidney isoform, PFKFB2, on residues Ser466 and Ser483 stimulates glycolysis. PFKFB2 KI mice have inactivating mutations at these phosphosites. Primary cultures of renal tubular epithelial cells from PFKFB2 KI mice had reduced ability to stimulate glycolysis when compared to WT cells (P<0.001). Absence of phosphorylation in PFKFB2 KI mice following UUO was associated with increased renal fibrosis (Picrosirius red p<0.001), increased fibronectin (Western blot p<0.05) and increased A-SMA (Western blot p<0.05). Fibrotic changes were less pronounced in the FAN model. These data suggest that reduced phosphorylation of ACC and PFKFB2 promote fibrosis and are deleterious following renal injury. Drugs that reverse this, such as metformin, may be useful in preventing and treating renal fibrosis. Fibrotic changes were most severe in PFKFB2 KI mice following UUO, highlighting the importance of glycolysis in the distal nephron.

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Peter Mount

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