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Diabetic kidney disease (DKD) is the leading cause of end-stage renal disease (ESRD) worldwide. Approximately, one-third of diabetic patients with transient exposure to hyperglycemia still develop DKD despite receiving intensified glycemic control, which propose an essential concept “metabolic memory”. The underlying mechanisms of "metabolic memory" include a range of pathogenic factors in connection with DKD, such as inflammation and epigenetic modifications. DNA methylation is a representative epigenetic modification and has been implicated in the pathogenesis of DKD. DNA methyltransferase 1 (DNMT1) is required for the maintenance of all methylation in the genome. It has recently been reported that the renal expression of DNMT1 was increased in DKD patients. The upregulation of DNMT1 can activate inflammation. However, inflammation is well known to induce insulin resistance, playing a critical role in the pathogenesis of DKD. Therefore, we hypothesized that the DNMT1-seletive inhibitor might improve DKD by alleviating inflammation and insulin resistance.
Male diabetic db/db (C57BLKS/J-LepRdb/LepRdb) mice and age-matched wild-type (BKS) mice were used in our study. At 8 weeks of age, diabetic db/db mice were given intraperitoneal injection of the DNMT1 inhibitor 5-azacytidine (1 mg/kg body weight or 2 mg/kg body weight) or saline as control every other day. After 12 weeks of treatment, the blood, urine and kidney samples were collected for measurements.
The renal expression of DNMT1, but not of DNMT3A and DNMT3B, was increased in diabetic kidneys. The upregulation of DNMT1 was significantly inhibited in 5-azacytidine–treated db/db mice. In db/db mice, the renal expression levels of NF-κB, TNF-ɑ, MIP-1α, IL-1β and IL-6 were remarkably enhanced relative to those in nondiabetic BKS mice. 5-azacytidine alleviated the renal expression of these inflammatory mediators and also reduced blood levels of IL-6 and TNF-α in db/db mice.In addition, 5-azacytidine concentration-dependently reduced serum levels of glucose and improved systemic insulin sensitivity. The urinary albumin excretion and serum creatinine in db/db mice were markedly decreased after 5-azacytidine treatment. This was accompanied by alleviation of glomerular hypertrophy, mesangial matrix expansion, glomerular basement membrane thickness and the extent of foot process effacement. Cell experiment results showed that inflammatory factor TNF-a could promote the expression of DNMT1 through the NF-κB/SP1 signaling pathway, however, inhibition of DNMT1 could reduce the activation of NF-κB pathway, suggesting the interaction between inflammation and aberrant DNA methylation.
Our findings suggest a mutual effect between inflammation and aberrant DNA methylation in DKD. Inflammatory factors resulted in the upregulation of DNMT1 in DKD, leading to aberrant DNA methylation. However, the DNMT1 inhibitor 5-azacytidine improved DKD by reducing inflammation and insulin resistance. Inhibition of DNMT1 may be a new therapeutic target for treating DKD.