Mitochondrially targeted anti-cancer agents

Cancer is an ever-increasing problem that is yet to be harnessed. Frequent mutations make this pathology very variable and, consequently, a considerable challenge. Intriguingly, mitochondria have recently emerged as novel targets for cancer therapy. A group of agents with anti-cancer activity that induce apoptosis by way of mitochondrial destabilisation, termed mitocans, have been a recent focus of research. Of these compounds, many are hydrophobic agents that associate with various sub-cellular organelles. Clearly, modification of such structures with mitochondria-targeting moieties, for example tagging them with lipophilic cations, would be expected to enhance their activity. This may be accomplished by the addition of triphenylphosphonium groups that direct such compounds to mitochondria, enhancing their activity. In this paper, we will review agents that possess anti-cancer activity by way of destabilising mitochondria and their possible targets. We propose that mitochondrial targeting, in particular where the agent associates directly with the target, results in more specific and efficient anti-cancer drugs of potential high clinical relevance.     

Molecular mechanism of 'mitocan'-induced apoptosis in cancer cells epitomizes the multiple roles of reactive oxygen species and Bcl-2 family proteins

Mitochondria have emerged recently as effective targets for novel anti-cancer drugs referred to as 'mitocans'. We propose that the molecular mechanism of induction of apoptosis by mitocans, as exemplified by the drug alpha-tocopheryl succinate, involves generation of reactive oxygen species (ROS). ROS then mediate the formation of disufide bridges between cytosolic Bax monomers, resulting in the formation of mitochondrial outer membrane channels. ROS also cause oxidation of cardiolipin, triggering the release of cytochrome c and its translocation via the activated Bax channels. This model may provide a general mechanism for the action of inducers of apoptosis and anticancer drugs, mitocans, targeting mitochondria via ROS production.     

Classification of mitocans,anti-cancer drugs acting on mitochondria

Mitochondria have emerged as an intriguing target for anti-cancer drugs, inherent to vast majority if not all types of tumours. Drugs that target mitochondria and exert anti-cancer activity have become a focus of recent research due to their great clinical potential (which has not been harnessed thus far). The exceptional potential of mitochondria as a target for anti-cancer agents has been reinforced by the discouraging finding that even tumours of the same type from individual patients differ in a number of mutations. This is consistent with the idea of personalised therapy, an elusive goal at this stage, in line with the notion that tumours are unlikely to be treated by agents that target only a single gene or a single pathway. This endows mitochondria, an invariant target present in all tumours, with an exceptional momentum. This train of thoughts inspired us to define a class of anti-cancer drugs acting by way of mitochondrial ‘destabilisation’, termed ‘mitocans’. In this communication, we define mitocans (many of which have been known for a long time) and classify them into several classes based on their molecular mode of action. We chose the targets that are of major importance from the point of view of their role in mitochondrial destabilisation by small compounds, some of which are now trialled as anti-cancer agents. The classification starts with targets at the surface of mitochondria and ending up with those in the mitochondrial matrix. The purpose of this review is to present in a concise manner the classification of compounds that hold a considerable promise as potential anti-cancer drugs.     

Mitocans as anti-cancer agents targeting mitochondria: lessons from studies with vitamin E analogues,inhibitors of complex II

Recently mitochondria in cancer cells have emerged as the Achilles heel for tumour destruction. Anti-cancer agents specifically targeting cancer cell mitochondria are referred to as ‘mitocans’. These compounds act by destabilising these organelles, unleashing their apoptogenic potential, resulting in the efficient death of malignant cells and suppression of tumour growth. Importantly, at least some mitocans are selective for cancer cells, and these are represented by the group of redox-silent vitamin E analogues, epitomised by α-tocopheryl succinate (α-TOS). This compound has proven itself in pre-clinical models to be an efficient anti-cancer agent, targeting complex II of the respiratory chain to displace ubiquinone binding. We propose that disrupting the electron flow of mitochondrial complex II results in generation of superoxide, triggering mitochondrial destabilisation and initiation of apoptotic pathways. Moreover, α-TOS is selective for cancer cells with their reduced anti-oxidant defenses and lower esterase activity than the normal (non-malignant) counterparts. In this mini-review we discuss the emerging significance of mitocans, as exemplified by α-TOS. Lubomir Prochazka is a visiting student of the Apoptosis Research Group, School of Medical Science, Griffith University, Southport, Qld, Australia.     

Mitochondria-mediated ATP depletion by anti-cancer agents of the jasmonate family

Jasmonates are plant stress hormones that induce suppression of proliferation and death in cancer cells, while being selectively inactive towards non-transformed cells. Jasmonates can overcome apoptotic blocks and exert cytotoxic effects on drug-resistant cells expressing p53 mutations. Jasmonates induce a rapid depletion of ATP in cancer cells. Indeed, this steep drop occurs when no signs of cell death are detectable yet. Experiments using modulators of ATP synthesis via glycolysis or oxidative phosphorylation suggest that the latter is the pathway suppressed by jasmonates. Consequently, the direct effects of jasmonates on mitochondria were evaluated. Jasmonates induced cytochrome c release and swelling in mitochondria isolated from cancer cells but not from normal ones. Thus, the selectivity of jasmonates against cancer cells is rooted at the mitochondrial level, and probably exploits differences between mitochondria from normal versus cancer cells. These findings position jasmonates as promising anti-cancer drugs acting via energetic depletion in neoplastic cells.     

Family feud in chemosensitvity: p73 and mutant p53

The importance of p53 in chemotherapy-induced apoptosis of cancer cells is well established. p53 plays a critical role in the cellular response to DNA damage by regulating genes involved in cell cycle progression, apoptosis, and genomic stability. As a result, p53 tumor status is a critical determinant of both responses to anti-cancer treatment and clinical prognosis. Interestingly, tumors expressing certain mutant forms of p53 ("gain of function") are particularly resistant to chemotherapy, even when compared to cells that lack any detectable p53. Until recently, the explanation for this enhanced chemoresistance was not clear. Recent studies have shown that the p53 homologues, p73 and p63, are also activated by chemotherapies, leading to tumor cell death. Now the discovery that mutant p53 interacts with p73, and that regulation of this interaction by a p53 polymorphism can modulate chemosensitvity provide a new model for how p53-family interactions can influence the response of tumors to anti-cancer therapies. Since p53 mutations are found in more than 50% of human tumors, strategies aimed at manipulating these interactions may prove useful in enhancing the chemotherapy response, and perhaps, overcoming chemoresistance.     

Family Feud in Chemosensitvity: p73 and Mutant p53

The importance of p53 in chemotherapy-induced apoptosis of cancer cells is well established. p53 plays a critical role in the cellular response to DNA damage by regulating genes involved in cell cycle progression, apoptosis, and genomic stability. As a result, p53 tumor status is a critical determinant of both responses to anti-cancer treatment and clinical prognosis. Interestingly, tumors expressing certain mutant forms of p53 (“gain of function”) are particularly resistant to chemotherapy, even when compared to cells that lack any detectable p53. Until recently, the explanation for this enhanced chemoresistance was not clear. Recent studies have shown that the p53 homologues, p73 and p63, are also activated by chemotherapies, leading to tumor cell death. Now the discovery that mutant p53 interacts with p73, and that regulation of this interaction by a p53 polymorphism can modulate chemosensitvity provide a new model for how p53-family interactions can influence the response of tumors to anti-cancer therapies. Since p53 mutations are found in more than 50% of human tumors, strategies aimed at manipulating these interactions may prove useful in enhancing the chemotherapy response, and perhaps, overcoming chemoresistance.     

Preferential killing of cancer cells with mitochondrial dysfunction by natural compounds

Mitochondria play essential roles in cellular metabolism, redox homeostasis, and regulation of cell death. Emerging evidences suggest that cancer cells exhibit various degrees of mitochondrial dysfunctions and metabolic alterations, which may serve as a basis to develop therapeutic strategies to preferentially kill the malignant cells. Mitochondria as a therapeutic target for cancer treatment is gaining much attention in the recent years, and agents that impact mitochondria with anticancer activity have been identified and tested in vitro and in vivo using various experimental systems. Anticancer agents that directly target mitochondria or indirectly affect mitochondrial functions are collectively classified as mitocans. This review article focuses on several natural compounds that preferentially kill cancer cells with mitochondrial dysfunction, and discusses the possible underlying mechanisms and their therapeutic implications in cancer treatment. Mitocans that have been comprehensively reviewed recently are not included in this article. Important issues such as therapeutic selectivity and the relevant biochemical basis are discussed in the context of future perspectives.     

Reactive oxygen species and mitochondrial sensitivity to oxidative stress determine induction of cancer cell death by p21

p21(Waf1/Cip1/Sdi1) is a cyclin-dependent kinase inhibitor that mediates cell cycle arrest. Prolonged p21 up-regulation induces a senescent phenotype in normal and cancer cells, accompanied by an increase in intracellular reactive oxygen species (ROS). However, it has been shown recently that p21 expression can also lead to cell death in certain models. The mechanisms involved in this process are not fully understood. Here, we describe an induction of apoptosis by p21 in sarcoma cell lines that is p53-independent and can be ameliorated with antioxidants. Similar levels of p21 and ROS caused senescence in the absence of significant death in other cancer cell lines, suggesting a cell-specific response. We also found that cells undergoing p21-dependent cell death had higher sensitivity to oxidants and a specific pattern of mitochondrial polarization changes. Consistent with this, apoptosis could be blocked with targeted expression of catalase in the mitochondria of these cells. We propose that the balance between cancer cell death and arrest after p21 up-regulation depends on the specific effects of p21-induced ROS on the mitochondria. This suggests that selective up-regulation of p21 in cancer cells could be a successful therapeutic intervention for sarcomas and tumors with lower resistance to mitochondrial oxidative damage, regardless of p53 status.     

Regulation of p53 stability and apoptosis by a ROR agonist

Activation of p53 function leading to cell-cycle arrest and/or apoptosis is a promising strategy for development of anti-cancer therapeutic agents. Here, we describe a novel mechanism for stabilization of p53 protein expression via activation of the orphan nuclear receptor, RORα. We demonstrate that treatment of cancer cells with a newly described synthetic ROR agonist, SR1078, leads to p53 stabilization and induction of apoptosis. These data suggest that synthetic ROR agonists may hold utility in the treatment of cancer.     

Mitochondrial targeting drug lonidamine triggered apoptosis in doxorubicin-resistant HepG2 cells

Mitochondria play a crucial role in the induction and execution of apoptosis. Accordingly, recent suggestions have been made to use agents that directly act on mitochondria to trigger apoptosis so that drug-sensitive and-resistant tumour cells can be eliminated. To test this hypothesis, human hepatocarcinoma HepG2 and its derivative R-HepG2 with doxorubicin (Dox) resistance as a result of expression of P-glycoprotein were used to investigate the effect of lonidamine (LND), a new mitochondrial targeting drug, on the induction of apoptosis. Results from our study indicate that R-HepG2 cells were more sensitive to LND than parental cells in terms of cytotoxicity determined by alamar blue assay. Cell death induced by LND was associated with the hallmarks of apoptosis such as mitochondrial membrane depolarization, release of cytochrome c, phosphatidyl-serine externalization and DNA fragmentation. Moreover, combined treatment of cells with Dox and LND elicited more cell death. Taken together, our results suggest a potential use of LND as an anti-cancer drug to bypass drug resistance and to trigger tumour destruction through apoptosis in HepG2 and R-HepG2 cells.     

The promise and obstacle of p53 as a cancer therapeutic agent

p53 is a tumor suppressor gene that is mutated in greater than 50% of human cancers. The action of p53 as a tumor suppressor involves inhibition of cell proliferation through cell cycle arrest and/or apoptosis. Loss of p53 function therefore allows the uncontrolled proliferation associated with cancerous cells. While design of most anti-cancer agents has focused on targeting and inactivating cancer promoting targets, such as oncogenes, recent attention has been given to restoring the lost activity of tumor suppressor genes. Because the loss of p53 function is so prevalent in human cancer, this protein is an ideal candidate for such therapy. Several gene therapeutic strategies have been employed in the attempt to restore p53 function to cancerous cells. These approaches include introduction of wild-type p53 into cells with mutant p53; the use of small molecules to stabilize mutant p53 in a wild-type, active conformation; and the introduction of agents to prevent degradation of p53 by proteins that normally target it. In addition, because mutant p53 has oncogenic gain of function activity, several approaches have been investigated to selectively target and kill cells harboring mutant p53. These include the introduction of mutant viruses that cause cell death only in cells with mutant p53 and the introduction of a gene that, in the absence of functional p53, produces a toxic product. Many obstacles remain to optimize these strategies for use in humans, but, despite these, restoration of p53 function is a promising anti-cancer therapeutic approach.     

Anti-cancer effects of JKA97 are associated with its induction of cell apoptosis via a Bax-dependent and p53-independent pathway

p53, one of the most commonly mutated genes in human cancers, is thought to be associated with cancer development. Hence, screening and identifying natural or synthetic compounds with anti-cancer activity via p53-independent pathway is one of the most challenging tasks for scientists in this field. Compound JKA97 (methoxy-1-styryl-9H-pyrid-[3,4-b]-indole) is a small molecule synthetic anti-cancer agent, with unknown mechanism(s). In this study we have demonstrated that the anti-cancer activity of JKA97 is associated with apoptotic induction via p53-independent mechanisms. We found that co-incubation of human colon cancer HCT116 cells with JKA97 inhibited HCT116 cell anchorage-independent growth in vitro and tumorigenicity in nude mice and also induced a cell apoptotic response, both in the cell culture model and in a tumorigenesis nude mouse model. Further studies showed that JKA97-induced apoptosis was dramatically impaired in Bax knock-out (Bax(-/-)) HCT116 cells, whereas the knock-out of p53 or PUMA did not show any inhibitory effects. The p53-independent apoptotic induction by JKA97 was confirmed in other colon cancer and hepatocarcinoma cell lines. In addition, our results showed an induction of Bax translocation and cytochrome c release from the mitochondria to the cytosol in HCT116 cells, demonstrating that the compound induces apoptosis through a Bax-initiated mitochondria-dependent pathway. These studies provide a molecular basis for the therapeutic application of JKA97 against human cancers with p53 mutations.     

Mitochondrial p53 activates Bak and causes disruption of a Bak-Mcl1 complex

The tumour suppressor activity of the p53 protein has been explained by its ability to induce apoptosis in response to a variety of cellular stresses. Thus, understanding the mechanism by which p53 functions in the execution of cell death pathways is of considerable importance in cancer biology. Recent studies have indicated that p53 has a direct signalling role at mitochondria in the induction of apoptosis, although the mechanisms involved are not completely understood. Here we show that, after cell stress, p53 interacts with the pro-apoptotic mitochondrial membrane protein Bak. Interaction of p53 with Bak causes oligomerization of Bak and release of cytochrome c from mitochondria. Notably, we show that formation of the p53-Bak complex coincides with loss of an interaction between Bak and the anti-apoptotic Bcl2-family member Mcl1. These results are consistent with a model in which p53 and Mcl1 have opposing effects on mitochondrial apoptosis by interacting with, and modulating the activity of, the death effector Bak.     

Polo-like kinase 1 inhibitors,mitotic stress and the tumor suppressor p53

Polo-like kinase 1 has been established as one of the most attractive targets for molecular cancer therapy. In fact, multiple small-molecule inhibitors targeting this kinase have been developed and intensively investigated. Recently, it has been reported that the cytotoxicity induced by Plk1 inhibition is elevated in cancer cells with inactive p53, leading to the hypothesis that inactive p53 is a predictive marker for the response of Plk1 inhibition. In our previous study based on different cancer cell lines, we showed that cancer cells with wild type p53 were more sensitive to Plk1 inhibition by inducing more apoptosis, compared with cancer cells depleted of p53. In the present work, we further demonstrate that in the presence of mitotic stress induced by different agents, Plk1 inhibitors strongly induced apoptosis in HCT116 p53^+/+ cells, whereas HCT116 p53^−/− cells arrested in mitosis with less apoptosis. Depletion of p53 in HCT116 p53^+/+ or U2OS cells reduced the induction of apoptosis. Moreover, the surviving HCT116 p53^−/− cells showed DNA damage and a strong capability of colony formation. Plk1 inhibition in combination with other anti-mitotic agents inhibited proliferation of tumor cells more strongly than Plk1 inhibition alone. Taken together, the data underscore that functional p53 strengthens the efficacy of Plk1 inhibition alone or in combination by strongly activating cell death signaling pathways. Further studies are required to investigate if the long-term outcomes of losing p53, such as low differential grade of tumor cells or defective DNA damage checkpoint, are responsible for the cytotoxicity of Plk1 inhibition.     

Targeting of p53 and its homolog p73 by protoporphyrin IX

The p53 tumor suppressor is recognized as a promising target for anti-cancer therapies. We previously reported that protoporphyrin IX (PpIX) disrupts the p53/murine double minute 2 (MDM2) complex and leads to p53 accumulation and activation of apoptosis in HCT 116 cells. Here we show the direct binding of PpIX to the N-terminal domain of p53. Furthermore, we addressed the induction of apoptosis in HCT 116 p53-null cells by PpIX and revealed interactions between PpIX and p73. We propose that PpIX disrupts the p53/MDM2 or MDMX and p73/MDM2 complexes and thereby activates the p53- or p73-dependent cancer cell death.     

p53-dependent inhibition of TrxR1 contributes to the tumor-specific induction of apoptosis by RITA

Thioredoxin reductase 1 (TrxR1) is a key regulator in many redox-dependent cellular pathways, and is often overexpressed in cancer. Several studies have identified TrxR1 as a potentially important target for anticancer therapy. The low molecular weight compound RITA (NSC 652287) binds p53 and induces p53-dependent apoptosis. Here we found that RITA also targets TrxR1 by non-covalent binding, followed by inhibition of its activity in vitro and by inhibition of TrxR activity in cancer cells. Interestingly, a novel ~130 kDa form of TrxR1, presumably representing a stable covalently linked dimer, and an increased generation of reactive oxygen species (ROS) were induced by RITA in cancer cells in a p53-dependent manner. Similarly, the gold-based TrxR inhibitor auranofin induced apoptosis related to oxidative stress, but independently of p53 and without apparent induction of the ~130 kDa form of TrxR1. In contrast to the effects observed in cancer cells, RITA had no impact on TrxR or ROS formation in normal fibroblasts (NHDF). The inhibition of TrxR1 can sensitize tumor cells to agents that induce oxidative stress and may directly trigger cell death. Thus, our results suggest that a unique p53-dependent effect of RITA on TrxR1 in cancer cells might synergize with p53-dependent induction of pro-apoptotic genes and oxidative stress, thereby leading to a robust induction of cancer cell death, without affecting non-transformed cells.