GSTK1 and RETREG1/FAM134B-Mediated Reticulophagy Attenuates Tubular Injury in Diabetic Nephropathy Through Endoplasmic Reticulum Stress and Apoptosis

Background: Reticulophagy is a key process to recovery from endoplasmic reticulum (ER) stress and for maintaining ER homeostasis by selectively removing damaged ER and its components. However, its precise mechanisms in diabetic nephropathy (DN) remain unclear. Methods: We detected the expression of reticulophagy regulator 1 (RETREG1/FAM134B) in tubular cells under diabetic conditionsby using immunoblotting, real-time PCR, and and immunostaining. Diabetic mouse models in proximal tubular cell-specific RETREG1 and glutathione s-transferase kappa 1 (GSTK1) knockout backgrounds were generated to clarify the role of RETREG1 in glucotoxicity-mediated renal tubular injury. In addition, immunoprecipitation coupled with mass spectrometry (IP-MS) showed that GSTK1 could interact with RETREG1. Results: We found that the expression of RETREG1 was decreased in the tubular cells in DN patients and animal models, which was positively correlated with estimated glomerular filtration rate (eGFR) and negatively associated with tubulointerstitial damage. Proximal tubule-specific knockout of RETREG1 exacerbated reticulophagy abnormalities in diabetic mice induced by high-fat diet (HFD) combined with streptozotocin (STZ), which was accompanied by increased ER stress, apoptosis of tubular cells and tubulointerstitial fibrosis. In vitro, overexpression of RETREG1 notably restored reticulophagy, alleviated ER stress and apoptosis in HK-2 cells, a human proximal tubular cell line, treated with high glucose. Mechanistically, immunoprecipitation coupled with mass spectrometry (IP-MS) suggested that RETREG1 could interact with GSTK1. Silencing of GSTK1 further aggravated the reduction of reticulophagy and tubular injury both in vivo and in vitro. These effects in in vitro were partially blocked by overexpressing RETREG1. Conclusion: These findings suggest that GSTK1/RETREG1 exerts a protective role in tubular injury through restoring reticulophagy and mitigating ER stress of tubular cells in DN. Funding: Government Support – Non-U.S.