Potential therapeutic effects of the simultaneous targeting of the Nrf2 and NF-κB pathways in diabetic neuropathy

Abstract

The Nuclear factor-2 erythroid related factor-2 (Nrf2) is a redox regulated transcription factor involved in the regulation of antioxidant defence systems. It drives the production of endogenous antioxidant defences and detoxifying enzymes. Nuclear factor-kappa light chain enhancer of B cells (NF-κB) is a transcription factor, involved in proinflammatory cytokine production, in addition to its immunological function. Both Nrf2 and NF-κB regulation are co-ordinated in order to maintain redox homeostasis in healthy cells. However, during pathological conditions this regulation is perturbed offering an opportunity for therapeutic intervention. Diabetic neuropathy is a condition, in which change in expression pattern of Nrf2 and NF-κB has been reported. This review aims to focus on the role of the Nrf2 and NF-κB in diabetic neuropathy and summarizes the therapeutic outcomes of various pharmacological modulators targeted at the Nrf2-NF-κB axis in diabetic neuropathy.

Keywords: Diabetic neuropathy; NF-κB; Nrf2.

Fig. 1

Possible factors in the pathophysiology of diabetic neuropathy. Hyperglycaemia induces excess formation of sorbitol through polyol pathway, advanced glycation end products (AGE), mitochondrial dysfunction and causes mitogen activated protein kinases (MAPK), poly ADP ribosyl polymerase (PARP) and protein kinase C (PKC) hyper-activation . All these pathways can contribute to nitrosative/oxidative stress in neuronal cells and endothelial cells of the vasa nervorum through depletion of endogenous antioxidant defences and excess generation of reactive oxygen species (ROS). The resulting oxidative stress leads to the activation of redox regulated transcription factors such as nuclear factor erythroid 2 related factor-2 (Nrf2), nuclear factor kappa light chain enhancer of B cells (NF-κB). Although Nrf2 is transiently activated by oxidative stress, the hyperglycaemic stress induced extracellular related kinase (ERK) activation restrain continued Nrf2 activation . A decline in Nrf2 activity and a persistent increase in NF-κB activity can lead to neuroinflammation and increased nitrosative–oxidative stress. These further lead to cumulative damage to peripheral nerve fibres, impaired blood supply to neuronal tissue , release of algogens like bradykinins and prostaglandins, which cause hypersensitivity to pain and hence, result in the development of neuropathic pain . Oxidative and nitrosative stress can also lead to massive DNA damage, which is a strong stimulator of PARP, hence causes neuronal apoptosis. All these events will culminate in the development of diabetic neuropathy (DN).

Fig. 2

Role of NF-κB and Nrf2 in diabetic neuropathy. Hyperglycaemia induced imbalance in Nrf2–NF-κB regulation can contribute to the pathogenesis of diabetic neuropathy. Enhanced NF-κB activity during the hyperglycaemic state is associated with excess production of proinflammatory cytokines such as IL-6, TNF-α, COX-2 and iNOS. These proteins and enzymes are prerequisite mediators for the initiation and amplification of inflammatory processes in neuronal cells . Reduced Nrf2 activity results in impaired antioxidant defence and is characterized by decline in superoxide dismutase (SOD), catalase and glutathione (GSH) levels. Additionally, it decreases the production of detoxifying enzymes such as, haem-oxygenase-1 (HO-I) and NADPH quinone oxidoreductase (NQO1), leading to nitrosative and oxidative stress . Neuroinflammation due to elevated NF-κB can also activate microglia and astrocytes, which further augments the release of proinflammatory mediators, thus develops a vicious cycle of inflammation. Release of algogenic mediators, such as prostaglandins, bradykinins and chemokines due to neuroinflammation, can sensitise the nerve fibres to painful stimulus and end up in sensorimotor alterations . NF-κB mediated neuroinflammation can also result in endoneurial hypoxia due to decreased blood supply to the nervous tissue and ganglion . This neuronal hypoxic condition leads to dysfunction of mitochondrial electron transport chain (etc), decreased efficiency of mitochondria, and results in increased ROS production. Nitrosative and oxidative stress generated in the nerve cells can also lead to increased AGE formation and protein kinase C (PKC) activation and increased peroxynitrite mediated PARP over-activation and apoptosis. The manifestations of neuroinflammation and oxidative stress can cumulatively cause the structural damage which can lead to the functional, sensorimotor and biochemical deficits which are characteristic of diabetic neuropathy.

Fig. 3

Crosstalk between the Nrf2 and NF-κB pathways. Oxidative stress mediated Nrf2 activation can lead to the production of antioxidant enzymes and cytoprotective enzymes such as SOD, catalase, GSH, haem-oxygenase-1 (HO-1) and NADPH quinone oxidoreductase (NQO1) and increased synthesis of GSH, NADPH and multidrug transporters. Nrf2 bind to the antioxidant response element (ARE) on DNA and maintains redox balance in body . Normally Nrf2 is bound to cytosolic repressor Kelch-like ECH-associated protein 1 (Keap1) and labelled for polyubiquitination mediated proteasomal degradation. However, during oxidative insult, the sulfhydryl groups on Keap1 are oxidised, causing the alteration in confirmation of Keap1, and hence releases Nrf2, which then binds with ARE elements of the genome along with small Maf proteins . NF-κB is another redox regulated transcription factor, involved in inflammation, immune function, cellular growth and apoptosis . p65 is a Rel protein with transactivation efficiency whereas its partner p50 does not possess transcriptional activity. Oxidative stress can cause the activation of IκB kinase (IKK). Activation of IKK causes phosphorylation of inhibitor of NF-κB, IκB and hence targets the later for polyubiquitination mediated proteasomal degradation, results in release of NF-κB, which then migrates into the nucleus, binds with the κ region of genome. With the help of other coactivators and histone acetyl transferases (HAT), NF-κB causes the transcription of proinflammatory mediators such as interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), cyclooxygenase-2 (COX-2), interleukin-1 (IL-1), intracellular adhesion molecule (ICAM) and inducible nitric oxide synthase (iNOS) . Further these two pathways are proposed to inhibit each other at their transcription level via protein–protein interactions or through secondary messenger effects. Nrf2 pathway inhibits the activation of NF-κB pathway by increasing antioxidant defences and HO-1 expression, which efficiently neutralizes ROS and detoxify toxic chemicals and hence, reduces ROS mediated NF-κB activation . Nrf2 pathway also inhibits NF-κB mediated transcription by preventing the degradation of IκB-α. Similarly, NF-κB mediated transcription reduces the Nrf2 activation by reducing the ARE gene transcription, decreases free CREB binding protein (CBP) by competing with Nrf2 for CH1-KIX domain of CBP . NF-κB also enhances the recruitment of histone deacetylase3 (HDAC3) to the ARE region by binding to Mafk and hence interferes with the transcriptional facilitation of Nrf2 .

Fig. 4

Imbalance of Nrf2–NF-κB in diabetic neuropathy and effects of simultaneous-targeting Nrf2–NF-kB pathway using pharmacological agents. Hyperglycaemia induced disturbances in Nrf2–NF-κB axis can contribute to the development of several diabetic vascular complications such as neuropathy, nephropathy and retinopathy. Enhanced NF-κB activity in diabetic neuropathy leads to the development of neuroinflammation, production of proinflammatory cytokines, nerve damage, impaired motor nerve conduction velocity (MNCV) due to demyelination, impaired neuronal blood flow (NBF) and production of algogenic mediators and hence causes pain hypersensitivity. Reduced Nrf2 activity is associated with enhanced oxidative stress in the neurons, which leads to the activation of PARP mediated neuronal apoptosis, AGE formation, PKC activation and allodynia and hyperalgesia due to the damage to the sensory fibres. However, this disturbed balance can be modulated pharmacologically to attenuate various deficits in diabetic neuropathy. The pharmacological modulation of NF-κB–Nrf2 axis by some pharmacological agents like Curcumin , Melatonin , Resveratrol and Sulforaphane produced beneficial effect by inhibiting NF-κB and activating Nrf2. The experimental outcomes of the studies of above mentioned compounds in streptozotocin (STZ) induced diabetic neuropathy model are manifested in the form of improved MNCV and NBF, decreased lipid peroxidation, IL-6, TNF-α levels, decreased expression of COX-2 and iNOS and reduced apoptosis.