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Diabetic neuropathy (DN) is a major complication of diabetes mellitus. We have previously reported the efficacy of Stachybotrys microspora triprenyl phenol-44D (SMTP-44D) for DN through its potential antioxidant and anti-inflammatory activities. However, the mechanisms underlying the antioxidant and anti-inflammatory activities of SMTP-44D remain unclear. The present study aimed to explore the mechanism of these effects of SMTP-44D in regard to its inhibition of soluble epoxide hydrolase (sEH) in immortalized mouse Schwann cells (IMS32) following high glucose treatment. IMS32 cells were incubated in a high glucose medium for 48 h and then treated with SMTP-44D for 48 h. After incubation, the ratio of epoxyeicosatrienoic acids (EETs) to dihydroxyeicosatrienoic acids (DHETs), oxidative stress markers, such as NADPH oxidase-1 and malondialdehyde, inflammatory factors, such as the ratio of nuclear to cytosolic levels of NF-kB and the levels of IL-6, MCP-1, MMP-9, the receptor for the advanced glycation end product (RAGE), and apoptosis, were evaluated. SMTP-44D treatment considerably increased the ratio of EETs to DHETs and mitigated oxidative stress, inflammation, RAGE induction, and apoptosis after high glucose treatment. In conclusion, SMTP-44D can suppress the induction of apoptosis by exerting antioxidant and anti-inflammatory effects, possibly through sEH inhibition. SMTP-44D can be a potential therapeutic agent against DN.

We have previously reported the efficacy of SMTP-44D for diabetic neuropathy (DN) through its potential antioxidant and anti-inflammatory activities. However, the mechanisms underlying the antioxidant and antiinflammatory activities of SMTP-44D remain unclear. The present study aimed to reveal the mechanism of these effects of soluble epoxide hydrolase (sEH) inhibition by SMTP-44D. In the in vivo assay, SMTP-44D (30 mg/kg) was administered to 200 mg/kg streptozotocin (STZ)-induced diabetic mice from the 8 to the 28 days after the injection of STZ. In the in vitro assay, IMS32 cells were incubated in a high glucose medium for 48 h and then treated with SMTP-44D (30 μM) for 48 h. The effects of the ratio of epoxyeicosatrienoic acids (EETs) to dihydroxyeicosatrienoic acids (DHETs), oxidative stress markers, and inflammatory factors by administration of SMTP-44D were assessed by LC-MS/MS, TBARS, and ELISA assay, respectively. Furthermore, apoptosis was evaluated by TUNEL assay. SMTP -44D treatment considerably increased the ratio of EETs to DHETs and mitigated oxidative stress, inflammation, and apoptosis. These results suggested that SMTP-44D can suppress the induction of apoptosis by exerting antioxidant and anti-inflammatory effects, possibly through sEH inhibition. SMTP-44D can be a potential therapeutic agent against DN.

Objective Diabetic neuropathy (DN), a major complication of diabetic mellitus, is present in more than 50% of diabetic patients. During the early stages of pathologic progression, patients with DN may experience sensory defects such as allodynia and hyperalgesia. Patients in advanced stages may present with sensory paralysis, such as nerve injury and the degeneration of axons, which are frequently accompanied by foot ulcerations, amputations, and poor prognoses. The pathogenesis of DN primarily involves oxidative stress and inflammation; thus, oxidative stress and inflammation are considered important therapeutic targets. However, few effective therapies are approved for the treatment of painful or insensate DN. The study aimed to evaluate the antioxidant and anti‐inflammatory effects of Stachybotrys microspora triprenyl phenol‐44D (SMTP‐44D) in a mouse model of DN. Methods We induced diabetes in C57BL/6J male mice by administering a single intraperitoneal injection of streptozotocin (200 mg/kg). SMTP‐44D (0.3, 3, and 30 mg/kg) was subsequently administered daily from 1 to 4 weeks after the injection of streptozotocin. The effects of SMTP‐44D were evaluated by mechanical and thermal thresholds to assess allodynia and hyperalgesia, and conduction velocity and blood flow in sciatic nerves. In addition, the levels of oxidative stress and inflammatory cytokines in sciatic nerves were determined via a thiobarbituric acid reactive substances assay and enzyme‐linked immunosorbent assay, respectively. To evaluate for neurological degeneration, G‐ratios (ratio of axon diameter to myelinated fiber diameter) and myelin thickness in the Schwann cells of sciatic nerves were measured. Results Mechanical and thermal thresholds, conduction velocity, blood flow, and myelin thickness were significantly lower in the diabetic group than in the nondiabetic group. Moreover, the levels of oxidative stress (e.g., malondialdehyde), inflammatory cytokines (e.g., tumor necrosis factor‐α, interleukin‐1β, and interleukin‐6), and G‐ratios were significantly higher in the diabetic group than in the nondiabetic group. Such results demonstrated that the DN model was successfully established. In mice with DN, treatment with SMTP‐44D significantly and dose‐dependently improved allodynia, hyperalgesia, conduction velocity of sciatic nerves, and blood flow in sciatic nerves. Furthermore, G‐ratios and myelin thickness in the Schwann cells of sciatic nerves were also significantly improved by the suppression of oxidative stress and inflammatory cytokines. Conclusion It is believed that oxidative stress and inflammation are involved in the pathological progression of DN and that they are important therapeutic targets. Our results showed that SMTP‐44D improved DN by exerting antioxidant and anti‐inflammatory effects, which suggests that SMTP‐44D is a potential novel therapeutic agent for the treatment of DN. Support or Funding Information This study was supported in part by the Japan Society for the Promotion of Science KAKENHI (Grant Number 26460346, a...