Exploiting cancer's antioxidative weakness through p53 with nanotoxicology.

Abstract

NanomedicineVol. 9, No. 4 EditorialFree AccessExploiting cancer's antioxidative weakness through p53 with nanotoxicologyMagdiel Inggrid Setyawati‡, Chor Yong Tay‡ and David Tai LeongMagdiel Inggrid Setyawati‡Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore, Singapore‡Authors contributed equallySearch for more papers by this author, Chor Yong Tay‡Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore, Singapore‡Authors contributed equallySearch for more papers by this author and David Tai LeongDepartment of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore, SingaporeSearch for more papers by this authorPublished Online:1 May 2014https://doi.org/10.2217/nnm.14.6AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareShare onFacebookTwitterLinkedInRedditEmail Keywords: anticancernanomedicinenanotoxicityoxidative stressp53Since the call to arms in the fight against cancer, many fields have proffered their best strategies to prevent, cure and care for cancer. Nanomedicine, for example, has offered its technological innovations in the form of nanoparticles (NPs) either as drug-delivery vehicles or for theranostic applications [1,2]. These NPs are generally considered to be inert and their cellular effects are assumed to be elicited by the active ingredients they deliver. However, evidence is mounting suggesting that a subset of NPs could actively interact with cells and elicits diverse cellular responses [3–5]. This suggests that the nano-sized drug carriers by themselves are biologically more active than previously thought and opens up to the possibility of using them as bona fide pharmaceutical agents for cancer therapy. In order to fully utilize these NPs, it is important to first understand how the NPs may impinge on pivotal intracellular cascades such as the p53 pathway.p53: master regulator of cellular functionsCancer in many perspectives seemed invincible. This shroud of invincibility is largely attributed to the numerous genetic mutations and epigenetic alterations in cancer cells. One highly notable altered pathway is that of p53. In as many as 50% of all human cancers one can find that p53 is either lost or mutated [6]. This is totally understandable because p53's known major function is a tumor suppressor where it controls many cellular processes, of notable relevance to survival and death. Negating p53 function confers a growth advantage to precancerous cells, thus enabling them to breach the major cellular defenses against unregulated cell growth and cancer progression.However, as a key transcription factor, the modus operandi of p53 extends beyond the realm of cell growth regulation. The role of p53 as a master regulator could also be observed in the regulation of glycolysis, repair of genotoxic injury, management of oxidative stress, motility, invasion and cellular senescence [7]. Therefore, p53 plays a decisive role in every stage of cell life ranging from fecundity, differentiation, development, cell metabolism, aging and the final stage of mortality.Cells take their cue from the environment. This is especially true for the case of p53 regulation. Depending on the nature and magnitude of the cellular stress, p53 will activate the myriad of cellular functions under its domain [8]. On this canvas of p53's influential control of key cellular processes and interactions with the extracellular environment, we hypothesized that NPs, being foreign to the cell, would also elicit distinct cellular processes engaging p53.Nanotechnology & p53: what do we know so far?To date, a growing body of evidence shows that a lion's share of metal oxide and several types of metal NPs could engage p53 cellular regulation through the mediation of intracellular reactive oxygen species (ROS) [5,9–11]. ROS refers to a group of chemically active O2 derivatives such as superoxide anion, peroxides and hydroxyl radicals that bear physiological significance as signaling molecules to regulate fundamental life processes [12]. However, excessive accumulation of NP-induced ROS may result in oxidative damage to DNA and cause genetic instability that later on promotes tumor incidence and growth [5,13]. Under this circumstance, the genotoxic damage inflicted by the aberrant level of ROS will activate the p53 pathway to suppress tumor development through either repairing the genotoxic injury or triggering apoptosis. Irreversible damage to the genetic material by ROS induced by certain NPs may prompt p53 to commit the cells to the apoptosis pathway, in order to ensure permanent removal of the injured cells and prevent the damaged genetic information from being passed down to the daughter cells. For instance, several exotic and commercial grade NPs [10–11,13] were shown to induce cell death by increasing p53 protein levels in a ROS-dependent manner [10,13–15]. However, when the genetic injury is repairable, p53 instead halts cells cycle progression and prompts the repair of sustained genetic injuries, resulting in cell survival. Such attempts by p53 to remediate the genotoxic injury were observed on human skin fibroblasts following exposure of ZnO, TiO2 and Gd2O3 NPs [10,13]. A p53-specific DNA damage response was triggered following the exposure of these NPs, as evidenced by the specific phosphorylation of the p53 protein at serine-15. Indeed, NPs can trigger cell cycle arrest and γ-H2AX-mediated DNA repair mechanism by p53 [11,16].Basal-level p53 and ROS levels are regulated through an intricate mechanism that involves crossmodulation of ROS and p53 via a redox active regulation loop. In contrast to the triggering of p53-mediated apoptosis in response to high level of ROS, a modest increase in intracellular ROS levels may instead trigger a diametrically opposing response of p53 [17]. Specifically, the p53 cascade may be engaged to extend protection against genotoxic injury by activating cellular antioxidative defense to repress intracellular ROS to its basal level [17]. Recently, low concentrations of ZnO NPs (50 μM) were reported to activate transcriptional expression of antioxidant genes such as SOD2, GPX1, SESN1, SESN2 and ALDH4A1 to restore oxidative homeostasis, improving cell viability [15]. This study suggests that the NP-triggered p53-mediated response might be more complex and less direct. Future studies should take into consideration the paradoxical stress responses mediated by p53 activation or deactivation to appreciate the full spectrum of possible NPs-p53-mediated cellular outcomes.Exploiting the combined effects of NP-induced ROS & p53 regulation for targeted cancer therapyThe indiscriminate killing of cells is a major concern for anticancer drugs and has brought about tremendous suffering to cancer patients due to the severe toxic side effects. Consequently, much effort has been devoted to developing novel targeting strategies such as the use of drug-loaded nanocapsules decorated with surface-bound targeting moieties to deliver anticancer drugs to the tumor site or take advantage of the ‘enhanced permeability retention’ effect for passive targeting [18]. Some NPs can also exploit the 'NP-induced endothelial leakiness' (NanoEL) effect that may cause a leaky phenotype to enhance access to the tumor [3]. Interestingly, several studies have also shown that metal oxide NPs such as ZnO [14], TiO2 [19], Ag [9] and Fe3O4 [20] NPs were also able to display selective killing of cancer cells without the deliberate addition of any targeting functionality, which is commonly found in many NP-based cancer treatment. A plausible explanation could be attributed to the difference in p53 status between normal and cancerous cells. Under the influence of oncogenic stimulation coupled with the loss of p53-mediated antioxidative mechanism, cancer cells are usually already in a persistent state of heightened oxidative stress, compelling the cells to display greater reliance on intracellular redox signaling [7]. The inherent oxidative stress of cancer cells is thus a unique feature that can be exploited for therapeutic purpose.This brings about an interesting notion of using NP-induced ROS as a way to overproduce intracellular ROS in order to trigger apoptosis in cancer cells while sparing the normal healthy cells with intact p53-dependent antioxidative signaling machinery. This concept was recently elegantly demonstrated by us through using a p53-knockdown BJ fibroblast cell line. While normal BJ fibroblast exhibited higher cell viability when treated with a low level of ZnO NPs, p53 knockdown sensitizes BJ fibroblasts to ZnO NP-induced apoptosis with a concomitant increase in ROS level [15]. This same result was observed in gastric cells system. When exposed to the same concentration of ZnO NPs, the lack of intact p53 in gastric cancer cells alone can determine the death response. This observation implies that the defective p53 network, as observed in most cancer cell types, provides an inherent bias towards NP-induced ROS-triggered cellular damage at low ROS levels. However, a further increase in ROS level will instead engage the p53 proapoptotic response in healthy cells, thus this strategy is not at its optimal. This warrants further studies to define the permissible operating concentration of NP-induced ROS for different p53-deficient cancer cell types, in order to achieve the cancer-targeting effect. Although not proven to date, we foresee that a similar strategy can be applied to induce preferential cell senescence as a way to restraint cancer progression even if apoptotic strategies are not available.Conclusion & future perspectiveSuccessful application of these synthetic nanosystems to medicine will require a better appreciation of how effective communication between the NPs and the complex intracellular signaling cascades can be attained in effecting certain outcomes that may be of therapeutic value. Due to its diverse biological roles, ubiquitous p53 is well poised to play the role of the central gatekeeper, directing the cells to respond to NP-inflicted damages in tandem with the cellular ROS status. To exploit the coupling effect of NP-induced ROS with the p53 signaling cascade for cancer therapy will require a better understanding of the NPs' physicochemical properties. This is to be complemented with strategies to ameliorate the inherent cytotoxic profile of NPs to normal cells. Furthermore, the nonadverse side effects of using NP-induced ROS to combat cancer should also be validated in animal models. At the same time, other important cancer discriminants, such as oncogenes, tumor suppressive miRNAs and oncomiRs, and even epigenetic regulators, could be possible players in this new paradigm of emphasizing on understanding and applying nanobiological effects to design better cancer nanomedicine.Financial & competing interests disclosureThe authors would like to acknowledge Ministry of Education, Singapore (grant no. R-397-000-136-112, R-279–000–350–112 and R-279–000–321–133 to DT Leong; and R-279–000–376–112 to CY Tay) and the NUS Department of Chemical and Biomolecular Engineering for financial support. 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R-397-000-136-112, R-279–000–350–112 and R-279–000–321–133 to DT Leong; and R-279–000–376–112 to CY Tay) and the NUS Department of Chemical and Biomolecular Engineering for financial support. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.No writing assistance was utilized in the production of this manuscript.PDF download