Received date: June 25, 2013; Accepted date: June 26, 2013; Published date: June 28, 2013
Citation: Mantha AK (2013) APE1: A Molecule of Focus with Neuroprotective and Anti-Cancer Properties. J Biotechnol Biomater 3:e120. doi:10.4172/2155-952X.1000e120
Copyright: © 2013 Mantha AK. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Apurinic/Apyrimidinic endonuclease (APE1) is a multi-functional, central enzyme of base excision repair (BER) pathway that takes care of oxidized base damage (AP sites and strand breaks) caused by both endogenous and exogenous oxidative DNA damaging agents. In repair function, APE1 exhibits majorly abasic (AP) endonuclease activity and stable interaction(s) with BER-pathway participant proteins. Second function of APE1 is redox activation of various transcription factors (TFs e.g., c-jun, NF-kB, p53 and HIF1α) and also named as redox effector factor 1(Ref-1). In redox function, APE1 reductively activates TFs involved in regulation of gene expression for cell survival mechanisms through stable pair-wise interaction(s). Recent studies have indicated that APE1 also possesses other distinct functions such as RNA metabolism, riboendonuclease activity and protein-protein interaction for maintaining cellular homeostasis. Altered APE1 expression has been reported in various cancers and neurodegenerative diseases. Taken together such findings advocates the necessity to delineate the underlying molecular mechanism(s) for understanding its role in various biological functions, that could be translated to its application in therapeutics against human diseases like cancer, neurodegenerative diseases and other pathologies such as cardiovascular diseases.
APE1, apurinic/apyrimidinic endonuclease; BER, base excision repair; Ref-1, redox effector factor 1; ROS, reactive oxygen species; TF, transcription factor; HIF-1α, hypoxia inducible factor 1 alpha; NLS, nuclear localization signal; AD, Alzheimer’s disease; PD, Parkinson’s disease; ALS, amyotrophic lateral sclerosis; AP sites, Apurinic/Apyrimidinic sites; AP-1: Activator Protein 1; Aβ, amyloid beta; PARP-1, poly [ADP-ribose] polymerase 1; STAT3, signal transducer activator of transcription 3; WRN, Werner syndrome helicase; YB-1, Y-box-binding protein 1; XRCC-1, X-ray repair crosscomplementing protein 1; and SSBs, single strand breaks
DNA damage/lesions caused by oxidizing/alkylating exogenous agents (ionizing radiation, pesticides, and chemotherapeutic agents) and endogenous agents (metabolites, oxidative phosphorylation, and free radicals) are implicated in several human pathologies including cancer and neurodegenerative diseases . BER-pathway is the primary repair system against these DNA lesions . Two types of BER sub-pathways exists; short-patch and long-patch BER takes place in mammalian cells depending upon the type of damage, concentration of the participant BER protein(s) and thus, finally allowing the replacement of the damaged DNA gap by single nucleotide and multinucleotides respectively . APE1 is a central enzyme of BER-pathway with primary two major independent cellular functions; repair and redox [4-6]. Both functions are displayed by two different regions of the protein. Primarily, repair function through C-terminus, which involves removal of AP sites in DNA. Second function is reductive activation of several TFs displayed via disordered N-terminus which also possesses the Nuclear Localization Signal (NLS), controlling gene expression for significant cellular pathways [6,7]. Because of redox activity, APE1 is also known as redox effector factor-1 (Ref-1). It was found to be involved in reductive activation of TFs like c-Jun via a thiol exchange reaction involving Cys65 residue . Later on, it was also demonstrated to reductively activate p53 , PAX-8 , AP-1 (fos/jun) , PAX- 5 , HIF-1α and NF-κB by same redox-based process involving reduction of key Cys residues in TF’s DNA–binding domains, thus increasing their DNA-binding affinity [4,6,13]. All these TF’s regulates the transcription of various genes responsible for cell survival pathways. Thus, APE1 regulates the pathways that are involved in normal as well as cancer cell survival.
Now it is well-known that APE1 is essential for cell survival . Studies have indicated that altered APE1 expression is associated with various cancers and neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD) and Amyotrophic Lateral Sclerosis (ALS) [15-18]. APE1 overexpression has been linked with chemoresistance in cancer cells [19,20]. In neuronal cells, it provides neuroprotection against oxidative assault [21,22]. It is known that not only oxidatively damaged DNA, but also oxidatively damaged RNA (involving mRNA, rRNA and tRNA) has been suggested to be associated with neurodegenerative diseases . Recently, APE1’s role in RNA metabolism including its function as riboendonuclease has been identified, pointing to have role in post-transcriptional regulation of gene expression [24,25]. Therefore, it can be stated that APE1 and its distinct functions in repair, redox as well as in RNA metabolism (Figure 1) can be employed for possible candidate for anti-cancer and as a neuroprotection molecule for better therapies.
Figure 1: Biological functions of APE1. APE1 is a multifunctional enzyme with distinct functions: a) repair function displayed through C-terminal segment with AP endonuclease activity; b) redox function displayed through disordered N-terminus as a reductive activator of various TFs (eg., HIF- 1α, p53, AP-1, & NF-κB); and c) recently discovered activities such as RNA metabolism, riboendonuclease activity and protein-protein interaction displayed through both C- and N-terminus of this protein.
An Anti-Cancer Agent
Main treatment options for cancer includes chemotherapy and radiotherapy which exhibits their cytotoxic effects via inducing DNA damage in cancerous cells. Likewise normal healthy cells, cancerous cells also tend to repair such DNA damage via BER-pathway exploiting DNA repair protein, APE1. Therefore, obvious approach is to inhibit APE1 thereby reducing DNA-damage repair in cancerous cells. This will help to combat chemoresistance and to potentiate the cytotoxic effects of chemotherapeutic agents . Inhibition of both DNA repair function, as well as redox function can act as a rational cancer target . Data from various studies have shown that APE1 overexpression is responsible for chemoresistance in cancer cells and blocking APE1 activities have helped to sensitize cancerous cell against chemotherapeutic agents such as Bleomycin, Temozolomide (TMZ), and Methyl Methane Sulfonate (MMS) [28-30]. Some studies involving use of specific small-molecule inhibitors have tried to understand the APE1’s individual causative function responsible for cancer cell survival and proliferation [31,32]. APE1’s endonuclease activity was also found to be significantly higher in advanced tumors as compared to lower grade tumors . Currently, it is clear that both activities of APE1 play an important role in cancer cell survival and proliferation, thus research involving identification of inhibitor molecules of APE1 repair and redox function are required for development of new cancer therapeutics.
A Neuroprotective Agent
Given the fact that brain is more vulnerable to oxidative DNA damage and neurons do not regenerate readily, it becomes important to find ways for neuroprotection [33,34]. Manipulating BER-pathway by altering APE1 expression and functions in neuronal cells can help in preventing neurotoxicity caused by environmental agents (radiation) and abnormal accumulation of metabolites-mediated oxidative DNA damage . Overexpression of APE1 and increased APE1’s activities might help in neuroprotection by switching-on the cell survival mechanisms. In a study performed on nuclear extracts of brain of AD patients, increased APE1 expression was observed . Another immuno histochemical study reported elevated nuclear APE1 expression in cerebral cortex of AD patients . Elevated APE1 expression was also observed in brain and spinal cord samples of ALS patients . These finding advocates for role of elevated APE1 expression in maintaining neuronal cell survival in response to oxidative stress. It was also found that APE1 is able to protect Dorsal Root Ganglion (DRG) cells from IR-induced neurotoxicity, through its repair activity . To address the issue of responsible APE1 individual activity behind neuroprotection, various studies have been conducted. In another in-vitro study conducted on SH-SY5Y cell line, it was demonstrated that APE1’s repair function provides neuroprotection and promotes cell survival after oxidative DNA damage . Recently Mantha et al. through 2-D proteomics using PC12 and SH-SY5Y cells have identified some APE1 interacting proteins associated with neuronal cell survival pathways suggesting role of APE1 in neuroprotection against oxidative damage caused by Aβ-toxicity that is observed in case of AD .
APE1 as a Riboendonuclease
Riboendonucleases hold significance by the fact that they control mRNA degradation and hence, controls gene expression [39,40]. To date, many riboendonucleases have been identified and still, many are remaining [41,42]. Recently APE1 has been identified as a riboendonuclease that cleaves and regulates c-myc mRNA . It was demonstrated that APE1 cleaves c-myc Coding Region Determinant (CRD) RNA at UA, UG and CA sites, and they also showed that APE1 controls c-myc mRNA levels as well as half life in cells [24,25,42]. Altogether these findings suggest the possible role of APE1 in regulating other mRNAs involved in various diseases.
In RNA Metabolism
From DNA-RNA-Protein, each step is prone to oxidative DNA damage and ultimately, can affect cell viability. Oxidatively damaged RNA can impair protein synthesis or can lead to inaccurate translation. Therefore, cell needs to cope with damaged RNA also; likewise it copes with damaged DNA. Studies have demonstrated the role of APE1 in RNA metabolism . In an experiment, APE1-depleted cells showed accumulation of oxidized rRNA after oxidative stress, which suggests APE1’s role in cleansing oxidatively damaged RNA . Recent study has indicated APE1’s ability to cleave AP site in single stranded RNA . In an another study it has been demonstrated that APE1 interacts through N-terminal domain with nucleophosmin (NPM1) protein, a ribosome processing protein, that regulates rRNA metabolism in ribosome biogenesis . APE1 also reported to interact with YB-1 and hnRNP-L, which are involved in RNA metabolism [45,46]. All together, these findings strongly suggests for possibility of APE1 to be an evolutionary protein with previously unknown functions that needs to be understood/revealed, and possibility to have role in miRNA metabolism .
APE1 displays binary interactions with participant protein(s) of BER-pathway as well as with proteins of other biologically relevant pathways involving TFs, so as to stabilize or stimulate each other’s functions [5,47]. These interactions play important role in maintenance of cell survival. In DNA repair, weakened interactions among the key downstream participant proteins of BER-pathway can lead to reduced repair efficiency [5,47]. Important enzymes of BER-pathway and TFs to which APE1 interacts are summarized in Table 1.
|Protein Pathway||Participant Proteins||Reference|
|BER-Pathway Proteins||• XRCC-1
• pol β
• DNA ligase I
|Redox Regulation of Key TFs||• NF-κB
Table 1: Important APE1 Protein-Protein Interaction.
In addition APE1 has been shown to be interacting or cross-talking with proteins such as heat shock protein 70 (hsp70) , WRN protein , STAT3 and p300 , YB-1 for activation of multidrug resistance (MDR) gene . A recent study by Mantha et al. has addressed the role of APE1 and protein-protein interaction(s) in neuronal cell survival . This study identified various APE1 interacting proteins upon Aβ(25- 35) stress in PC12 and SH-SY5Y cells, associated with stress-dependent events and neuronal cell survival pathways. Key proteins identified are: Pyruvate kinase 3 isoform 2 (PKM2), Tropomodulin 3 (Tmod3), and hetero genous nuclear ribonucleoprotein-H1 (hn-RNP-H) . Future studies are required to unveil other APE1 and protein-protein interactions which may have key biologically significant functions.
APE1 is a key regulating enzyme of BER-pathway taking care of oxidative base damage resulting due to oxidative stress. Recent findings from various molecular studies have indicated association of differential APE1 expression pattern with various cancers and neurodegenerative disorders. It can be speculated that manipulating APE1 expression and function(s) can affect the underlying process of pathogenesis. In addition, recently discovered function of APE1 in RNA metabolism also broadens the perspectives of APE1’s exploitation as a therapeutic target for various diseases. In future, new approaches are needed to delineate the molecular mechanisms underlying APE1’s role in chemo/ radioresistance in cancer cells, and neuromodulatory role in neuronal cell survival. More proteomic studies are required to elucidate the specific APE1 protein-protein interaction in other biological pathways, in response to oxidative damage caused by exogenous and endogenous agents. Investigations elucidating the nature of molecular modifications (post-transcriptional and post-translational) and molecular signaling mechanisms regulating the functions of APE1 are need to be characterized in order to translate the APE1-mediated therapeutic interventions.
A.K.M is supported by Alzheimer’s Association, USA, NIRG-11-203527 grant. Thanks are to Ms. Shweta Thakur for assistance in drafting the article and to Dr. Monisha Dhiman, Centre for Genetic Diseases and Molecular Medicine, School of Emerging Life Science Technologies,Central University of Punjab, Bathinda for her critical comments and suggestions. Because of the short editorial/limited focus of the article, many relevant and appropriate references could not be included, for which the author sincerely apologize. The CUPB institutional number allocated for this manuscript is P-50.