Down-Regulation of Insulin Signaling Is Involved in Painful Diabetic Neuropathy in Type 2 Diabetes

Background: Previous theories considered that the main cause of painful diabetic neuropathy (PDN) was due to hyperglycemia. However, recent evidence indicated that hyperinsulinemia plays a greater role in type 2 diabetic metabolisms (T2DM). Objectives: Our aim was to explore insulin signaling to determine the molecular mechanism involved in the pathogenesis of PDN in T2DM. Study Design: A randomized, double blind, controlled animal trial. Methods: We observed the localization of insulin receptor (IR) and phosphorylated insulin receptor substrate 1 (IRS-1) in the spinal cord using in situ hybridization and immunohistochemistry. Then we investigated the alternations of IR and pIRS-1 and the activity of the JAK2/STAT3 pathway by immunohistochemistry, Western Blotting, and cell culture. Finally, we detected the influence of intrathecal JAK2/STAT3 inhibitor (AG490) on nociceptive behavior and insulin signaling in ob/ob mice using Western Blotting. Results: We found that IR and pIRS-1 are mainly located in neurons in the superficial layer of the spinal dorsal horn. The expressions of IR and pIRS-1 decreased and the JAK2/STAT3 pathway activated in the spinal dorsal horn in ob/ob mice with mechanical hyperalgesia. Next, our in vitro results indicated that hyperinsulinemia and hyperglycemia impaired insulin signaling along with the activated JAK2/STAT3 pathway in differentiated human neuronal cells (SH-SY5Y). Treatment through intrathecal injection of AG490, an inhibitor of the JAK2/STAT3 pathway, alleviated mechanical hyperalgesia in ob/ob mice and prevented impaired insulin signaling in the spinal cord. Limitations: The activation of the JAK2/STAT3 pathway could not explain the mechanism of PDN in T1DM. Conclusions: We demonstrate that insulin signaling impairment in the spinal dorsal horn is associated with the activated JAK2/STAT3 pathway, which contributes to the progressive PDN in T2DM.