Triptolide Improves Endoplasmic Reticulum Stress-mediated Podocyte Apoptosis under High Glucose Conditions

Abstract

Abstract Background Diabetic kidney disease (DKD) is an important complication of diabetes in which endoplasmic reticulum stress (ERS) plays an important role and triptolide (TP) is effective in the treatment of DKD. To investigate the inhibition of ERS-mediated apoptosis in podocytes by TP in a high glucose environment in vivo and in vitro. Methods 1. DKD rat models were established by a high-fat, high-sugar diet combined with intraperitoneal streptozotocin (STZ) injection and randomly divided into model group (DKD group), 4-phenylbutyrate (4-PBA) group (DKD + 4-PBA) and TP group (DKD + TP); another 10 rats were routinely maintained as the normal control group (NC group). The DKD + 4-PBA and TP groups were treated with the corresponding drugs by gavage for 4 weeks, and the model and normal groups received equal amounts of saline containing DMSO by gavage daily. Changes in blood glucose, urine microalbumin (UMA), and some liver and kidney function indices were determined before and after treatment. Structural changes in the kidney were observed and GRP78 was detected by Western blot (WB). 2. The human renal podocyte hyperglycemia model and the thapsigargin (TG)-induced ERS model were established and perturbed by TP, respectively. WB, immunofluorescence, flow cytometry, and qPCR were used to monitor ERS, apoptosis, and changes in key molecules of related pathways in podocytes. Results 1. Both TP and 4-PBA reduced UMA levels in DKD rats (P < 0.01), alleviated glomerular mesangial expansion and tubular injury in DKD rats, reduced synaptic fusion and deletion, apoptotic vesicle formation and podocyte number in DKD rats, and downregulated overexpression of ERS marker protein GRP78 (P < 0.01). There was no significant effect on blood glucose, liver, or kidney function (P > 0.05). 2. In human podocytes induced by TG or high glucose, TP downregulates gene and protein overexpression of GRP78 and alleviates ER ultrastructural changes and podocyte apoptosis in the ERS state. TP downregulated the expression of marker proteins for ERS and unfolded protein response, including CHOP, IRE1α, P-IRE1α, and P-JNK, and also blocked the nuclear translocation of ATF6, with significant inhibition of the CHOP/GADD153 gene-activated transcription pathway and the c-JUN N-terminal kinase (JNK) pathway among the three pathways induced by ERS, but not observed for the caspase-12 (caspase-4) activation pathway. Conclusions Inhibition of ERS improves DKD, and the therapeutic effects of TP in DKD are achieved, at least in part, by inhibiting ERS to protect podocytes.