Synergistic Effects of Hyperglycemia and Dyslipidemia on Diabetic Neuropathy-High Glucose Aggravates Oxidized LDL-Induced Schwann Cell Death via Hyperactivation of TLR4

Diabetic neuropathy is a progressive complication of diabetes mellitus, with persistent pain and loss of sensory function. Oxidized LDL (oxLDL), an oxidized form of LDL, has been shown to play a significant role in the development of neuronal dysfunction. Circulating oxLDL is elevated in type 2 diabetes, leading to higher cardiovascular risk. However, the combined effects of hyperglycemia and oxLDL on the progression of diabetic neuropathy remain unknown. Here, we investigated the synergistic effects of high glucose on oxLDL-induced schwann cell death and its detailed mechanism. Immortalized adult mouse Schwann (IMS32) cells were treated with oxLDL (0, 150, 300 μg/mL) under normal glucose (NG, 5.5 mM) or high glucose (HG, 25 mM) conditions for 24 hours. Following treatment, we evaluated cell viability, gene, and protein expression of Toll-like receptor 4 (TLR4), a receptor for oxLDL. Under NG conditions, 300 μg/mL oxLDL, but not 150 μg/mL reduced the cell viability. Moreover, under HG conditions, both 150 μg/mL and 300 μg/mL oxLDL decreased the cell viability. Likewise, the gene and protein expression of TLR4 was synergistically upregulated by the combination of oxLDL and HG. We further evaluated the effects of TLR4 inhibition on the cell viability and apoptotic caspase-3 activity. The cells were pretreated with TAK-242, a TLR4 inhibitor, for 2 hours prior to 150 μg/mL of oxLDL treatment under NG or HG conditions. TAK-242 pretreatment attenuated the oxLDL-induced cell death in HG conditions. The caspase-3 activity was increased by oxLDL in HG conditions, but not in NG. Furthermore, the increased caspase-3 activity by oxLDL and HG was abolished by TAK-242. These results indicate that high glucose and oxLDL induce activation of TLR4, leading to exacerbated schwann cell apoptosis. The dysregulation of TLR4 signal in diabetes complicated with dyslipidemia may contribute to the pathogenesis of diabetic neuropathy. Disclosure W.Nihei: None. A.Kato: None. T.Himeno: None. M.Kondo: None. J.Nakamura: None. H.Kamiya: Research Support; Kissei Pharmaceutical Co., Ltd., Ono Pharmaceutical Co., Ltd., Eli Lilly Japan K.K., Merck & Co., Inc., Sumitomo Dainippon Pharma Co., Ltd., Takeda Pharmaceutical Co., Ltd., Mitsubishi Tanabe Pharma Corporation, Japan Tobacco Inc., Novo Nordisk, Taisho Pharmaceutical Holdings Co., Ltd., Speaker's Bureau; Sanofi, Novo Nordisk, Sumitomo Dainippon Pharma Co., Ltd., Boehringer Ingelheim Japan, Inc., Eli Lilly Japan K.K., Daiichi Sankyo, Ono Pharmaceutical Co., Ltd., Kissei Pharmaceutical Co., Ltd., Mitsubishi Tanabe Pharma Corporation, Kowa Company, Ltd., Novartis Pharma K.K., Merck & Co., Inc., Sanwa Kagaku Kenkyusho, Otsuka Pharmaceutical Co., Ltd., Taisho Pharmaceutical Holdings Co., Ltd., Astellas Pharma Inc., Terumo Corporation, Fukuda Denshi, LifeScan Diabetes Institute, EA Pharma Co., Ltd, KAKEN PHARMACEUTICAL CO., LTD, AstraZeneca. K.Sango: None. K.Kato: None.