br Conflict of interest br Introduction Colorectal cancer CR

Conflict of interest
Introduction Colorectal cancer (CRC) is one of the major cause of tumor-related morbidity and mortality worldwide. Poor prognosis and consequences of its metastatic spread make CRC the second most common cause of cancer-related deaths in western countries [1]. Apart from other factors such as environmental and genetic, it has been shown that increased reactive oxygen species (ROS) produced by cytokines, chemokines, and growth factors contribute to colon cancer growth [2]. Normally ROS is reduced directly by glutathione (GSH) and/or through various anti-oxidative enzymes. However, excessive ROS cause lipid peroxidation and lipid aldehydes formation. Lipid aldehydes such as hydroxy-trans-2-nonenal (HNE) are reduced by aldose reductase (AR; AKR1B1) to corresponding alcohol, 1, 4-dihydroxynonene (DHN) or conjugated by GSH to form GS-lipid aldehydes such as GS-HNE which is also reduced by AR to GS-DHN [3]. We have demonstrated earlier that GS-DHN activates PKC, PI3K, and PLC which in turn activate transcription factors NF-κB and AP-1 [4], [5]. Inhibition of AR by fidarestat, a specific inhibitor of AR, prevents the bFGF- and PDGF-induced activation of NF-κB, expression of cyclooxygenase-2 (Cox-2), and production of prostaglandin E2 (PGE2) which promote colon cancer ampar growth [6], [7]. Although it is well known that deregulation of mitochondrial biogenesis plays a significant role in the progression of cancer growth [8], [9], the role of AR in the regulation of mitochondrial biogenesis is not known. Recent studies show that nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is activated under oxidative stress and binds to the antioxidant response element (ARE) in the 5′-promoter region of cytoprotective genes and induces the expression of a number of antioxidant proteins such as superoxide dismutase (SOD), glutathione reductase, NAD(P)H:quinone oxidoreductase 1 (NQO1), and heme oxygenase-1 (HO-1) which prepare the cells to withstand oxidative stress [10], [11]. Also, Nrf2 regulates mitochondrial biogenesis by interacting with various mitochondrial proteins and maintains the levels of peroxisome proliferator-activated receptor gamma coactivator (PGC-1α) and Nrf1 [12]. It has also been shown that PGC-1α co-activates Nrf1 which further activates the mitochondrial transcription factor A (TFAM) and transcribes proteins involved in mitochondrial DNA repair [13], [14]. Further, AMP-activated protein kinase (AMPK) is an energy sensor of the cells and works as a key regulator of mitochondrial biogenesis by upregulating PGC1α [13], [14]. TFAM could also modulate mitochondrial DNA and upregulate p53 (a tumor suppressor) transcriptional activity and play a crucial role in mitochondrial biogenesis.
Materials and methods
Results
Discussion We have shown earlier that AR inhibitors prevent CRC growth and metastasis in vitro and in vivo models by inhibiting the NF-κB-mediated pro-inflammatory pathways [4], [5]. In this study, we show that AR inhibitor, fidarestat increases the activity of Nrf2-mediated anti-oxidative pathways and treatment of CRC with fidarestat before the addition of EGF augmented it. Further, AR inhibitor-induced EGF-stimulated mitochondrial biogenesis and decreased mitochondrial DNA damage. These results suggest a novel bifunctional role of AR inhibition; i.e., by preventing the NF-κB-mediated pro-inflammatory pathways and increasing the Nrf2–mediated anti-oxidative/anti-inflammatory pathways, AR inhibitors could act as chemopreventive drugs. Further, Nrf2 is an important transcription factor that controls cellular redox equilibrium by regulating the expression of antioxidant and anti-inflammatory mediators such as GST, NQO1, CAT, SOD, and HO-1 by binding to respective antioxidant response elements and plays a major role in protecting the cells under oxidative stress conditions [18]. However, Nrf2 has been shown to play protective as well as harmful effects in various forms of cancers [19]. Some studies indicate that expression of Nrf2 causes chemoresistance while several other studies suggest that Nrf2 expression decreases tumor burden [20], [21]. Specifically, Nrf2 null mice showed increased tumor growth in azoxymethane- and dextran sodium sulfate-induced colon cancer models as compared to control mice [22], [23]. Similarly, Nrf2 null mice have been shown to aggressively develop tumors in benzo(a)pyrene- and N-nitrosobyutyl(4-hydroxybutyl)amine –induced gastric and bladder cancers [24], [25]. These studies indicate that Nrf2 protects against oxidative stress-induced tumor growth initiated by oxidants such as chemical carcinogens, cytokines, and growth factors. In the current study, we have shown that fidarestat increases the expression, nuclear translocation, and DNA binding activity of Nrf2 in CRC cells. Consistently, fidarestat pretreatment further activated EGF-induced Nrf2 expression and transcriptional activity. Our studies also suggest that fidarestat increases the expression of Nrf2-mediated downstream anti-oxidative proteins such as HO1, NQO1, and SOD and it augments the EGF-induced anti-oxidative proteins. These studies indicate that pre-incubation with fidarestat prepares the cells against oxidative stress by inducing Nrf2/HO1 pathway which contributes to its anti-oxidative/anti-inflammatory properties. Consistent with our data, several studies have also shown that natural chemopreventive agents such as curcumin, cinnamaldehyde, sulforaphane, and resveratrol exert their effects by increasing the expression of Nrf2 pathway-mediated defense responses such as expression of phase-II detoxification enzymes and transporters which protect cells from xenobiotic and carcinogenic stresses [23], [26], [27], [28], [29]. Similarly, some synthetic Nrf2 activators such as Oltipraz and dimethyl fumarate have also been shown to ameliorate various cancer forms including colon cancer [30], [31], [32].