Effects of vitamin E and its derivatives on diabetic nephropathy in Rats and identification of diacylglycerol kinase subtype involved in the improvement of diabetic nephropathy

Abstract

Background: Diabetes is a significant social issue. Controlling diabetic complications such as nephropathy is very important for QOL of diabetic patients. One of the mechanisms which causes diabetic complications is the abnormal activation of protein kinase C (PKC) by increased diacylglycerol (DG) from hyperglycemia. Diacylglycerol kinase (DGK) can attenuate PKC activity by converting DG to phosphatidic acid. Thus far, d-α-tocopherol (VtE) treatment has been shown to prevent early changes of diabetic renal dysfunctions by activating DGK. However, it is still unknown whether VtE derivatives improve diabetic nephropathy similarly to VtE, and which DGK subtype is activated by VtE and/or the derivatives. Objective: The purpose of the study was to investigate effects of VtE and its derivatives on diabetic nephropathy in rats, in addition to identifying the DGK subtype involved in the improvement of nephropathy in vivo. Methods: To induce diabetes in rats, six weeks old male Sprague-Dawley rats were intraperitonealy administrated 65 mg/kg streptozocin (STZ) in 20 mM citrate buffer. For two or eight weeks, 40 mg/kg VtE, 44 mg/kg acetate VtE (aVtE) or 49.3 mg/kg succinate VtE (sVtE) was intraperitonealy administrated every other day after STZ administration. The blood glucose level, body weight, and kidney weight, in addition to urinary volume, albumin, and BUN were measured every week after STZ administration. Additionally, in order to identify the DGK subtype activated by VtE and aVtE, the DGK subtype expressed in the rat glomerulus was checked by RT-PCR and western blotting, and the activity in the glomerulus from the rats before and after the VtE and aVtE treatments were measured in the presence or absence of EGTA. Results: Averages of kidney weight and BUN of rats treated with VtE, aVtE and sVtE for 8 weeks were reduced, compared to the control. However, the intraperitoneal administration of sVtE was toxic. Additionally, the amount of urine volume and urinary albumin significantly improved by the two-weeks treatment of VtE and aVtE. mRNA of DGKα, γ, δ, ϵ, and ζ were detected in the glomerulus, but only the protein of DGKα and DGKδ were detected as calcium-dependent and independent subtype respectively. Both VtE and aVtE activated DGK in the glomerulus. However, the calcium dependent DGK subtype was mainly activated by aVtE, with not only the calciumdependent subtype but also the calcium-independent subtype being activated in the case of VtE. In other words, DGKα was activated by aVtE and DGKδ was additionally enhanced in the case of VtE. Conclusion: These results clearly indicated that aVtE as well as VtE improved diabetic nephropathy, with the activation of DGKα and/or δ being potentially involved with this improvement.