NF-κB inhibitory action of resveratrol: A probable mechanism of neuroprotection in experimental diabetic neuropathy

Abstract

Resveratrol has shown array of biological actions, and is under clinical development for various disease conditions. The etiology of diabetic neuropathy revolves around oxidative stress, AGE formation, lipid peroxidation etc. All these stimulate inflammatory processes and NF-κB cascade is considered as one of the major players of inflammatory response. Activation of NF-κB results in elevated levels of inflammatory mediators. COX-2 and TNF-α activity have also been correlated with inflammatory damage in the pathophysiology of diabetic neuropathy (DN). Therefore we investigated the effect of resveratrol on NF-κB inflammatory cascade, COX-2, TNF-α and IL-6 levels in experimental DN.

We found that resveratrol protected against various functional and behavioral deficits in diabetic neuropathy in line with our earlier published reports. In this study we found that the resveratrol treatment decreased the expression of p65 and IκB-α in treated rats. Treatment also ameliorated the elevated levels of TNF-α, IL-6 and COX-2. Resveratrol treatment produced significant decrease in nerve MDA levels in treated animals which may also be contributing to reduction in neuro-inflammation. This study confirms the NF-κB inhibitory activity and anti-inflammatory activity of resveratrol which may contribute to neuroprotection in diabetic neuropathy apart from its antioxidant effect.

Introduction

Diabetes is attaining an epidemic stature throughout the world and the scenario in developing nations is worsening day by day. One of the most debilitating and common complication of diabetes is diabetic neuropathy which affects more than 60% diabetics. The clinical manifestations of diabetic neuropathy may include numbness, burning and tingling sensation, intractable pain, etc., [1], [2]. In spite of decades of research in this field there is still no approved therapy for treating diabetic neuropathy. Only few symptomatic treatments are available which lessen the neuropathic pain which is one of cardinal features of diabetic neuropathy. The neuropathy progresses with decreasing nerve functionality and decreasing nerve blood perfusion, which ends up in malnourished nerve and lead to permanent nerve damage [3], [4].

Pathophysiological factors of diabetic neuropathy include persistent hyperglycemia, microvascular insufficiency, oxidative stress, nitrosative stress, PARP over activation, defective neurotrophism, autoimmune-mediated nerve destruction, etc., [1], [5], [6]. The factors like oxidative stress, AGE formation, lipid peroxidation which results from diabetic condition can stimulate inflammatory processes [7]. Diabetic conditions are associated with chronic low-grade inflammation, evidenced by raised surrogate markers of inflammation which include C-reactive protein (CRP), IL-6 and TNF-α. These further contribute to pathogenesis of diabetes and diabetic neuropathy [8]. Many of the major pathophysiological pathways of diabetic neuropathy may converge at a common platform, i.e., NF-κB activation which is the cause of inflammation resulting in tissue injury at various sites [9].

Inflammation is a complex process, but a central mechanism is activation of the NF-κB cascade. Changes in cytokine levels, ROS and AGEs/advanced lipoxygenation end products (ALEs) generated by hyperglycemia, as in diabetic condition leads to NF-κB activation. In transthyretin-related familial amyloidotic polyneuropathy, up-regulation of the p50 NF-κB subunit was found in peripheral nerves [10]. p65 subunit of NF-κB was found to be over expressed in sural nerve macrophages in acute and chronic inflammatory demyelinating polyneuropathies which points towards the role of NF-κB in genesis of inflammatory demyelination [11]. Demyelination is well proven contributor to the conduction deficits seen in clinical as well as in experimental diabetic neuropathy. NF-κB has also been reviewed for its role and contribution in causing various other diseases like cancer, rheumatoid arthritis, atherosclerosis, etc., [12].

Resveratrol (trans-3,4′,5-trihydroxystilbene), a natural polyphenolic, non-flavonoid antioxidant, is a phytoalexin found in many plants including grapes, nuts and berries. There are numerous studies that have documented beneficial effects of resveratrol such as cardiovascular and cancer preventive properties [13], [14]. We have previously reported the beneficial effects of resveratrol in experimental diabetic neuropathy [3], [15]. Resveratrol has shown anti-inflammatory activity and its effect has been documented on SIRT and Nrf2 pathways and effect on COX-2 in various experimental models [13]. We assessed the effect of resveratrol treatment on NF-κB inflammatory cascade in experimental diabetic neuropathy. We also tried to find out whether the NF-κB inhibitory activity contributes to the neuroprotection shown in the rat model of experimental diabetic neuropathy.

The study to evaluate NF-κB activity of resveratrol was performed at two different dose levels (10 and 20 mg/kg) in the STZ induced rat model of diabetic neuropathy. The rationale of dose selection was based on our previous study in which we found the effectiveness of resveratrol treatment at theses two doses. As resveratrol is having antioxidant effect we evaluated lipid peroxidation in sciatic nerves. We also studied nerve levels of inflammatory mediators (TNF-α and IL-6 levels). COX-2 has been implicated in causing inflammation and its resolutions, so we investigated the effect of resveratrol treatment on COX-2 levels in experimental diabetic neuropathy. For studying the NF-κB activity we performed the expression studies of NF-κB, IκB-α and p-IκB-α levels in sciatic nerve.