TMBIM6 promotes diabetic tubular epithelial cell survival and albumin endocytosis by inhibiting the endoplasmic reticulum stress sensor, IRE1α

Aim: Reduced albumin reabsorption in proximal tubular epithelial cells (PTECs), resulting from decreased megalin plasma membrane (PM) localization due to prolonged endoplasmic reticulum (ER) stress, potentially contributes to albuminuria in early diabetic kidney disease (DKD). To examine this possibility, we investigated the cytoprotective effect of TMBIM6 in promoting diabetic PTEC survival and albumin endocytosis by attenuating ER stress with an IRE1α inhibitor, KIRA6.

Methods and results: Renal TMBIM6 distribution and expression were determined by immunohistochemistry, western blotting, and qPCR, whereas tubular injury was evaluated in db/db mice. High-glucose (HG)-treated HK-2 cells were either treated with KIRA6 or transduced with a lentiviral vector for TMBIM6 overexpression. ER stress was measured by western blotting and ER-Tracker Red staining, whereas Apoptosis was determined by performing TUNEL assays. Megalin expression was measured by immunofluorescence, and albumin endocytosis was evaluated after incubating cells with FITC-labeled albumin. Tubular injury and TMBIM6 downregulation occurred in db/db mouse renal cortical tissues. Both KIRA6 treatment and TMBIM6 overexpression inhibited ER stress by decreasing the levels of phosphorylated IRE1α, XBP1s, GRP78, and CHOP, and stabilizing ER expansion in HG-treated HK-2 cells. TUNEL assays performed with KIRA6-treated or TMBIM6-overexpressing cells showed a significant decrease in Apoptosis, consistent with the significant downregulation of Bax and upregulation of Bcl-2, as measured by immunoblotting. Both KIRA6 and TMBIM6 overexpression promoted megalin PM localization and restored albumin endocytosis in HG-treated HK-2 cells.

Conclusion: TMBIM6 promoted diabetic PTEC survival and albumin endocytosis by negatively regulating the IRE1α branch of ER stress.