398-P: Renal Proximal Tubular Metallothionein 3 Contributes to Diabetic Kidney Lesion

Metallothionein (MT) is a cysteine-rich protein with an antioxidant. MT3, one isoform of the four MTs, is upregulated in the renal proximal tubular cells in type 2 diabetic subjects. To investigate the role of human MT3, we used the mouse sglt2 promoter for targeting human MT3 gene to renal proximal tubular cells in transgenic mice (MT3TG). MT3TG showed nodular glomerulosclerosis without hyperglycemia. Electron microscopy presented thickened glomerular basement membranes, numerous endoplasmic reticulum in proximal tubules and cytoplasmic swelling of the endothelium of peritubular capillaries accompanied with abnormal capillary formation around the glomerulus, suggesting proximal tubular MT3 induces retrogradely glomerular hypertension. Affymetrix Human Gene 1.0 ST arrays identified 43 upregulated and 81 downregulated genes between control and MT3-knockdown human renal proximal tubular epithelial cells (HRPTECs, Lonza) under hypoxia, an early event in diabetic kidney. Among them, ceruloplasmin (CP) and cytochrome b reductase 1 (CYBRD), the crucial players in the cooper and iron metabolism, were downregulated. In fact, MT3TG presented cytoplasmic co-expression of MT3, CP and CYBRD1 in renal proximal tubules. Hypoxia induced a remarkable increase in MT3 in HRPTECs, and the HIF-1alpha siRNAs abolished that. Despite of no antioxidant response element in the human MT3 promoter, Nrf2 siRNA significantly decreased hypoxia-induced MT3 expression. Moreover, MT3 and Nrf2 siRNAs inhibited hypoxia-induced CP and CYBRD1, but not HIF-1 alpha siRNA. Together with the inhibitory effects of mitochondrial inhibitors, NOX4 inhibitors and antioxidant NAC on hypoxia-induced MT3 expression, hypoxia-induced oxidative stress might be involved in the inductions of CP and CYBRD1. Of note, empagliflozin decreased hypoxia-induced MT3, CP and CYBRD1. Our study suggests that MT3 might contributes to diabetic kidney lesion via CP and CYBRD1against hypoxia-induced oxidative stress.