Effective radiotherapy for cancer has relied on the promise of maximally eradicating tumor cells while minimally killing normal cells. Technological advancement has provided state-of-the-art ...instrumentation that enables delivery of radiotherapy with great precision to tumor lesions with substantial reduced injury to normal tissues. Moreover, better understanding of radiobiology, particularly the mechanisms of radiation sensitivity and resistance in tumor lesions and toxicity in normal tissues, has improved the treatment efficacy of radiotherapy. Previous mechanism-based studies have identified many cellular targets that can affect radiation sensitivity, notably reactive oxygen species, DNA-damaging response signals, and tumor microenvironments. Several radiation sensitizers and protectors have been developed and clinically evaluated; however, many of these results are inconclusive, indicating that improvement remains needed. In this era of personalized medicine in which patients' genetic variations, transcriptome and proteomics, tumor metabolism and microenvironment, and tumor immunity are available. These new developments have provided opportunity for new target discovery. Several radiotherapy sensitivity-associated "gene signatures" have been reported although clinical validations are needed. Recently, several immune modifiers have been shown to associate with improved radiotherapy in preclinical models and in early clinical trials. Combination of radiotherapy and immunocheckpoint blockade has shown promising results especially in targeting metastatic tumors through abscopal response. In this article, we succinctly review recent advancements in the areas of mechanism-driven targets and exploitation of new targets from current radio-oncogenomic and radiation-immunotherapeutic approaches that bear clinical implications for improving the treatment efficacy of radiotherapy.
The development of multidrug resistance to cancer chemotherapy is a major obstacle to the effective treatment of human malignancies. It has been established that membrane proteins, notably multidrug ...resistance (MDR), multidrug resistance protein (MRP), and breast cancer resistance protein (BCRP) of the ATP binding cassette (ABC) transporter family encoding efflux pumps, play important roles in the development of multidrug resistance. Overexpression of these transporters has been observed frequently in many types of human malignancies and correlated with poor responses to chemotherapeutic agents. Evidence has accumulated showing that redox signals are activated in response to drug treatments that affect the expression and activity of these transporters by multiple mechanisms, including (a) conformational changes in the transporters, (b) regulation of the biosynthesis cofactors required for the transporter's function, (c) regulation of the expression of transporters at transcriptional, posttranscriptional, and epigenetic levels, and (d) amplification of the copy number of genes encoding these transporters. This review describes various specific factors and their relevant signaling pathways that are involved in the regulation. Finally, the roles of redox signaling in the maintenance and evolution of cancer stem cells and their implications in the development of intrinsic and acquired multidrug resistance in cancer chemotherapy are discussed.
Melanomas and other cancers that do not express argininosuccinate synthetase (AS), the rate-limiting enzyme for arginine biosynthesis, are sensitive to arginine depletion with pegylated arginine ...deiminase (ADI-PEG20). However, ADI resistance eventually develops in tumors because of AS upregulation. Although it has been shown that AS upregulation involves c-Myc, the underlying mechanisms remain unknown. Here we show that ADI-PEG20 activates Ras signaling and the effector extracellular signal-regulated kinase (ERK) and phosphoinositide 3-kinase (PI3K)/AKT/GSK-3β kinase cascades, resulting in phosphorylation and stabilization of c-Myc by attenuation of its ubiquitin-mediated protein degradation mechanism. Inhibition of the induced cell signaling pathways using PI3K/AKT inhibitors suppressed c-Myc induction and enhanced ADI-mediated cell killing. Notably, in an animal model of AS-negative melanoma, combination therapy using a PI3K inhibitor plus ADI-PEG20 yielded additive antitumor effects as compared with either agent alone. Taken together, our findings offer mechanistic insight into arginine deprivation metabolism and ADI resistance, and they illustrate how combining inhibitors of the Ras/ERK and PI3K/AKT signaling pathways may improve ADI-PEG20 anticancer responses.
Cisplatin (cis diamminedichloroplatinum II, cDDP) is one of the most effective cancer chemotherapeutic agents and is used in the treatment of many types of human malignancies. However, inherent ...tumour resistance is a major barrier to effective cisplatin therapy. So far, the mechanism of cDDP resistance has not been well defined. In general, cisplatin is considered to be a cytotoxic drug, for damaging DNA and inhibiting DNA synthesis, resulting in apoptosis via the mitochondrial death pathway or plasma membrane disruption. cDDP−induced DNA damage triggers signalling pathways that will eventually decide between cell life and death. As a member of the mitogen‐activated protein kinases family, c‐Jun N‐terminal kinase (JNK) is a signalling pathway in response to extracellular stimuli, especially drug treatment, to modify the activity of numerous proteins locating in the mitochondria or the nucleus. Recent studies suggest that JNK signalling pathway plays a major role in deciding the fate of the cell and inducing resistance to cDDP‐induced apoptosis in human tumours. c‐Jun N‐terminal kinase regulates several important cellular functions including cell proliferation, differentiation, survival and apoptosis while activating and inhibiting substrates for phosphorylation transcription factors (c‐Jun, ATF2: Activating transcription factor 2, p53 and so on), which subsequently induce pro‐apoptosis and pro‐survival factors expression. Therefore, it is suggested that JNK signal pathway is a double‐edged sword in cDDP treatment, simultaneously being a significant pro‐apoptosis factor but also being associated with increased resistance to cisplatin‐based chemotherapy. This review focuses on current knowledge concerning the role of JNK in cell response to cDDP, as well as their role in cisplatin resistance.
The high-affinity copper transporter (Ctr1; SCLC31A1) plays an important role in regulating copper homeostasis because copper is an essential micronutrient and copper deficiency is detrimental to ...many important cellular functions, but excess copper is toxic. Recent research has revealed that human copper homeostasis is tightly controlled by interregulatory circuitry involving copper, Sp1, and human (hCtr1). This circuitry uses Sp1 transcription factor as a copper sensor in modulating hCtr1 expression, which in turn controls cellular copper and Sp1 levels in a 3-way mutual regulatory loop. Posttranslational regulation of hCtr1 expression by copper stresses has also been described in the literature. Because hCtr1 can also transport platinum drugs, this finding underscores the important role of hCtr1 in platinum-drug sensitivity in cancer chemotherapy. Consistent with this notion is the finding that elevated hCtr1 expression was associated with favorable treatment outcomes in cisplatin-based cancer chemotherapy. Moreover, cultured cell studies showed that elevated hCtr1 expression can be induced by depleting cellular copper levels, resulting in enhanced cisplatin uptake and its cell-killing activity. A phase I clinical trial using a combination of trientine (a copper chelator) and carboplatin has been carried out with encouraging results. This review discusses new insights into the role of hCtr1 in regulating copper homeostasis and explains how modulating cellular copper availability could influence treatment efficacy in platinum-based cancer chemotherapy through hCtr1 regulation.
Platinum (Pt)-based antitumor agents have been effective in treating many human malignancies. Drug importing, intracellular shuffling, and exporting-carried out by the high-affinity copper (Cu) ...transporter (
), Cu chaperone (Ato x1), and Cu exporters (ATP7A and ATP7B), respectively-cumulatively contribute to the chemosensitivity of Pt drugs including cisplatin and carboplatin, but not oxaliplatin. This entire system can also handle Pt drugs via interactions between Pt and the thiol-containing amino acid residues in these proteins; the interactions are strongly influenced by cellular redox regulators such as glutathione.
expression is induced by acute Cu deprivation, and the induction is regulated by the transcription factor specific protein 1 (Sp1) which by itself is also regulated by Cu concentration variations. Copper displaces zinc (Zn) coordination at the zinc finger (ZF) domains of Sp1 and inactivates its DNA binding, whereas Cu deprivation enhances Sp1-DNA interactions and increases Sp1 expression, which in turn upregulates
. Because of the shared transport system, chemosensitivity of Pt drugs can be modulated by targeting Cu transporters. A Cu-lowering agent (trientine) in combination with a Pt drug (carboplatin) has been used in clinical studies for overcoming Pt-resistance. Future research should aim at further developing effective Pt drug retention strategies for improving the treatment efficacy.
Proline, glutamine, asparagine, and arginine are conditionally non-essential amino acids that can be produced in our body. However, they are essential for the growth of highly proliferative cells ...such as cancers. Many cancers express reduced levels of these amino acids and thus require import from the environment. Meanwhile, the biosynthesis of these amino acids is inter-connected but can be intervened individually through the inhibition of key enzymes of the biosynthesis of these amino acids, resulting in amino acid starvation and cell death. Amino acid starvation strategies have been in various stages of clinical applications. Targeting asparagine using asparaginase has been approved for treating acute lymphoblastic leukemia. Targeting glutamine and arginine starvations are in various stages of clinical trials, and targeting proline starvation is in preclinical development. The most important obstacle of these therapies is drug resistance, which is mostly due to reactivation of the key enzymes involved in biosynthesis of the targeted amino acids and reprogramming of compensatory survival pathways via transcriptional, epigenetic, and post-translational mechanisms. Here, we review the interactive regulatory mechanisms that control cellular levels of these amino acids for amino acid starvation therapy and how drug resistance is evolved underlying treatment failure.
Low levels of human copper transporter 1 (hCtr1) mRNA are associated with a shorter progression-free survival after platinum-based therapy. Pretreatment with a copper-lowering agent such as trientine ...enhanced hCtr1-mediated platinum uptake. Therefore, we conducted a pilot study (NCT01178112) of carboplatin and trientine with the goal of resensitizing patients with advanced cancer to platinum chemotherapy. This case report reviews the outcomes of 5 patients with platinum-resistant high-grade epithelial ovarian cancer enrolled on the study to date. Overall, they tolerated treatment well. Severe adverse events that occurred in 2 patients were myelosuppression, notably anemia requiring transfusion. Dose-limiting toxicity was not observed within the first 28 days (cycle 1). After 2 cycles of therapy, partial remission was achieved in 1 patient (10+ months), stable disease in 3 patients (2, 3.5+, and 5 months, respectively), and 1 patient had progressive disease. These cases provide preliminary clinical evidence that the role of decreasing copper levels in reversing platinum resistance merits additional clinical investigation. Evaluation of this novel strategy is warranted in larger studies to assess the efficacy of this approach for treating platinum-resistant advanced epithelial ovarian cancer in patients with high copper levels.
Platinum-based antitumor agents have been effective in the treatments of many human malignancies but the ultimate success of these agents is often compromised by development of drug resistance. One ...mechanism associated with resistance to platinum drugs is reduced intracellular accumulation owing to impaired drug intake, enhanced outward transport, or both. Mechanisms for transporting platinum drugs were not known until recent demonstrations that import and export transporters involved in maintenance copper homeostasis are also involved in the transport of these drugs. Ctr1, the major copper influx transporter, has been convincingly demonstrated to transport cisplatin and its analogues, carboplatin, and oxaliplatin. Evidence also suggests that the two copper efflux transporters ATP7A and ATP7B regulate the efflux of cisplatin. These observations are intriguing, because conventional thinking of the inorganic physiologic chemistry of cisplatin and copper is quite different. Hence, understanding the underlying mechanistic aspects of these transporters is critically important. While the mechanisms by which hCtr1, ATP7A and ATP7B transport copper ions have been studied extensively, very little is known about the mechanisms by which these transporters shuffle platinum-based antitumor agents. This review discusses the identification of copper transporters as platinum drug transporters, the structural-functional and mechanistic aspects of these transporters, the mechanisms that regulate their expression, and future research directions that may eventually lead to improved efficacy of platinum-based-based drugs in cancer chemotherapy through modulation of their transporters' activities.
The human high-affinity copper transporter (hCtr1) plays an important role in the regulation of intracellular copper homeostasis.
hCtr1 is involved in the transport of platinum-based antitumor agents ...such as cisplatin (CDDP); however, the mechanisms that
regulate hCtr1-mediated transport of these agents have not been well elucidated. We compared the mechanisms of hCtr1-mediated
transport of copper and CDDP. We found that replacements of several methionine residues that are essential for hCtr1-mediated
copper transport conferred a dominant-negative effect on the endogenous hCtr1âs function, resulting in reduced rates of Cu(I)
and CDDP transport and increased resistance to the toxicities of copper and CDDP treatments. Kinetic constant analyses revealed
that although these mutations reduced maximal transport rates ( V max ) for Cu(I) and CDDP, reduction of K m only for Cu(I) but not for CDDP was observed. Mutation in Gly167, which is located in the third transmembrane domain and
is involved in helix packing of hCtr1, also conferred dominant-negative property of Cu(I) transport but not of CDDP transport.
Deleting the N-terminal 45 amino acids that contain two methionine-rich motifs resulted in cytoplasmic localization of the
hCtr1 and abolished the dominant-negative function of these mutants. Nonetheless, these mutations did not affect the capacities
of hCtr1 oligomerization induced by copper or CDDP, suggesting a distinct structural requirement between metal transport and
oligomerization. Finally, we also observed that expressing the dominant-negative hCtr1 mutants up-regulates endogenous hCtr1
mRNA expression, consistent with our previous report that intracellular copper homeostasis and homeostatic levels of hCtr1
mRNA are mutually regulated.
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