Principles of cancer therapy: oncogene and non-oncogene addiction

Cell line-based platforms to evaluate the therapeutic efficacy of candidate anticancer agents

Efforts to discover new cancer drugs and predict their clinical activity are limited by the fact that laboratory models to test drug efficacy do not faithfully recapitulate this complex disease. One important model system for evaluating candidate anticancer agents is human tumour-derived cell lines. Although cultured cancer cells can exhibit distinct properties compared with their naturally growing counterparts, recent technologies that facilitate the parallel analysis of large panels of such lines, together with genomic technologies that define their genetic constitution, have revitalized efforts to use cancer cell lines to assess the clinical utility of new investigational cancer drugs and to discover predictive biomarkers.

DNA polymerases as potential therapeutic targets for cancers deficient in the DNA mismatch repair proteins MSH2 or MLH1

Synthetic sickness/lethality (SSL) can be exploited to develop therapeutic strategies for cancer. Deficiencies in the tumor suppressor proteins MLH1 and MSH2 have been implicated in cancer. Here we demonstrate that deficiency in MSH2 is SSL with inhibition of the DNA polymerase POLB, whereas deficiency in MLH1 is SSL with DNA polymerase POLG inhibition. Both SSLs led to the accumulation of 8-oxoG oxidative DNA lesions. MSH2/POLB SSL caused nuclear 8-oxoG accumulation, whereas MLH1/POLG SSL led to a rise in mitochondrial 8-oxoG levels. Both SSLs were rescued by silencing the adenine glycosylase MUTYH, suggesting that lethality could be caused by the formation of lethal DNA breaks upon 8-oxoG accumulation. These data suggest targeted, mechanism-based therapeutic approaches.

Generation of iPSCs from cultured human malignant cells

Induced pluripotent stem cells (iPSCs) can be generated from various differentiated cell types by the expression of a set of defined transcription factors. So far, iPSCs have been generated from primary cells, but it is unclear whether human cancer cell lines can be reprogrammed. Here we describe the generation and characterization of iPSCs derived from human chronic myeloid leukemia cells. We show that, despite the presence of oncogenic mutations, these cells acquired pluripotency by the expression of 4 transcription factors and underwent differentiation into cell types derived of all 3 germ layers during teratoma formation. Interestingly, although the parental cell line was strictly dependent on continuous signaling of the BCR-ABL oncogene, also termed oncogene addiction, reprogrammed cells lost this dependency and became resistant to the BCR-ABL inhibitor imatinib. This finding indicates that the therapeutic agent imatinib targets cells in a specific epigenetic differentiated cell state, and this may contribute to its inability to fully eradicate disease in chronic myeloid leukemia patients.

Therapeutic targeting of p53 by small molecules

Aberrant p53 function is one of the major requirements for tumor development. Reactivation of p53 function by small molecules is a promising strategy to combat cancer due to potent tumor suppressor activities of p53. Recent developments in p53 biology reveal that manipulation of p53 function might pave way to a long cancer-free life. A number of small molecules which rescue p53 function by different mechanisms, acting upstream of p53 or targeting the p53 protein itself have been identified. Notably, these molecules trigger different biological outcomes, suggesting that it might be feasible to direct p53-mediated response in a desired way. In this review I discuss the latest developments in the search for small molecules which rescue p53 function by targeting the p53 protein.

Identification of inhibitors of HSF1 functional activity by high-content target-based screening

Cancer cells are known to experience a high level of stress and may require constant repair for survival and proliferation. Recent studies showed that inhibition of heat shock factor 1 (HSF1), the key regulator for the stress-activated transcription of heat shock protein (HSP), can reduce the tumorigenic potential of cancer cells. Such a "nononcogene addiction" phenomenon makes HSF1 an attractive cancer drug target. Here, the authors report an image-based high-content screening (HCS) assay for HSF1 functional inhibitors. A heat shock-based methodology was used to stimulate the stress response followed by quantitative measurement of HSF1/HSP70 granules for compound-induced inhibitory effects. The authors discovered a small molecule from a compound library that inhibits HSF1 granule formation substantially in heat-shocked HeLa cells with IC(50) at 80 nM. Electorphoretic mobility shift of HSF1 by this compound suggested significant inhibition of HSF1 phosphorylation, accompanied by reduced expression levels of HSP70 and HSP90 after heat induction. Importantly, HeLa cells stably transfected with HSF1 shRNA were more resistant to the compound treatment under lethal temperature than cells containing HSF1, further validating an HSF1-dependent mechanism of action. The HCS assay the authors developed was robust with a Z' factor of 0.65 in a 384-well plate format, providing a valuable method for identifying small-molecule functional inhibitors of HSF1 for potential cancer treatment.

A Bax-mediated mechanism for obatoclax-induced apoptosis of cholangiocarcinoma cells

Apoptosis induction by BH3 mimetics is a therapeutic strategy for human cancer. These mimetics exert single-agent activity in cells "primed" for cell death. Primed cells are dependent upon antiapoptotic Bcl-2 proteins for survival and are characterized by the ability of the BH3 mimetic to induce cytochrome c release from their isolated mitochondria. Our aim was to examine the single-agent activity of obatoclax, a BH3 mimetic in cholangiocarcinoma cell lines. In clonogenic assays, inhibition of colony formation was observed by obatoclax treatment. Despite single-agent activity by obatoclax, the mitochondria from these cells did not release cytochrome c after incubation with this BH3 mimetic. However, immunofluorescence and cell fractionation studies identified Bax activation and translocation to mitochondria after treatment with obatoclax. shRNA targeted knockdown of Bax doubled the IC50 for obatoclax but did not abrogate its cytotoxicity, whereas knockdown of Bak did not alter the IC50. In a cell-free system, obatoclax induced an activating conformational change of Bax, which was attenuated by a site-directed mutagenesis of a previously identified protein activation site. Finally, the drug also elicited a significant in vivo response in a rodent model of this disease. In conclusion, single-agent obatoclax treatment results in Bax activation, which contributes, in part, to cell death in cholangiocarcinoma cells. These data indicate that BH3 mimetics may also function as direct activators of Bax and induce cytotoxicity in cells not otherwise primed for cell death.

Ion channels and the hallmarks of cancer

Plasma membrane (PM) ion channels contribute to virtually all basic cellular processes and are also involved in the malignant phenotype of cancer cells. Here, we review the role of ion channels in cancer in the context of their involvement in the defined hallmarks of cancer: 1) self-sufficiency in growth signals, 2) insensitivity to antigrowth signals, 3) evasion of programmed cell death (apoptosis), 4) limitless replicative potential, 5) sustained angiogenesis and 6) tissue invasion and metastasis. Recent studies have indicated that the contribution of specific ion channels to these hallmarks varies for different types of cancer. Therefore, to determine the importance of ion channels as targets for cancer diagnosis and treatment their expression, function and regulation must be assessed for each cancer.

Unfinished stories on viral quasispecies and Darwinian views of evolution

Experimental evidence that RNA virus populations consist of distributions of mutant genomes, termed quasispecies, was first published 31 years ago. This work provided the earliest experimental support for a theory to explain a system that replicated with limited fidelity and to understand the self-organization and adaptability of early life forms on Earth. High mutation rates and quasispecies dynamics of RNA viruses are intimately related to both viral disease and antiviral treatment strategies. Moreover, the quasispecies concept is being applied to other biological systems such as cancer research in which cellular mutant spectra can be also detected. This review addresses some of the unanswered questions regarding viral and theoretical quasispecies concepts as well as more practical aspects concerning resistance to antiviral treatments and pathogenesis.

Molecular classification and novel targets in hepatocellular carcinoma: recent advancements

Hepatocellular carcinoma (HCC) is one of most lethal cancers worldwide. Strategic decisions for the advancement of molecular therapies in this neoplasm require a clear understanding of its molecular classification. Studies indicate aberrant activation of signaling pathways involved in cellular proliferation (e.g., epidermal growth factor and RAS/mitogen-activated protein kinase pathways), survival (e.g., Akt/mechanistic target of rapamycin pathway), differentiation (e.g., Wnt and Hedgehog pathways), and angiogenesis (e.g., vascular endothelial growth factor and platelet-derived growth factor), which is heterogeneously presented in each tumor. Integrative analysis of accumulated genomic datasets has revealed a global scheme of molecular classification of HCC tumors observed across diverse etiologic factors and geographic locations. Such a framework will allow systematic understanding of the frequently co-occurring molecular aberrations to design treatment strategy for each specific subclass of tumors. Accompanied by a growing number of clinical trials of molecular targeted drugs, diagnostic and prognostic biomarker development will be facilitated with special attention on study design and with new assay technologies specialized for archived fixed tissues. A new class of genomic information, microRNA dysregulation and epigenetic alterations, will provide insight for more precise understanding of disease mechanism and expand the opportunity of biomarker/therapeutic target discovery. These efforts will eventually enable personalized management of HCC.

Hsp90 inhibitors block outgrowth of EBV-infected malignant cells in vitro and in vivo through an EBNA1-dependent mechanism

EBV causes infectious mononucleosis and is associated with certain malignancies. EBV nuclear antigen 1 (EBNA1) mediates EBV genome replication, partition, and transcription, and is essential for persistence of the viral genome in host cells. Here we demonstrate that Hsp90 inhibitors decrease EBNA1 expression and translation, and that this effect requires the Gly-Ala repeat domain of EBNA1. Hsp90 inhibitors induce the death of established, EBV-transformed lymphoblastoid cell lines at doses nontoxic to normal cells, and this effect is substantially reversed when lymphoblastoid cell lines are stably infected with a retrovirus expressing a functional EBNA1 mutant lacking the Gly-Ala repeats. Hsp90 inhibitors prevent EBV transformation of primary B cells, and strongly inhibit the growth of EBV-induced lymphoproliferative disease in SCID mice. These results suggest that Hsp90 inhibitors may be particularly effective for treating EBV-induced diseases requiring the continued presence of the viral genome.

Molecular mechanisms of cardiovascular toxicity of targeted cancer therapeutics

In 2002, Hoshijima and Chien drew largely theoretical parallels between the dysregulation of the signaling pathways driving cancer and those driving cardiac hypertrophy (Hoshijima M, Chien KR. J Clin Invest. 2002;109:849-855). On the surface, this statement appeared to stretch the limits of reason, given the fact that cancer cells are known for their proliferative capacity, and adult cardiomyocytes are, except under unusual circumstances, terminally differentiated and incapable of re-entering the cell cycle. However, on closer examination, there are numerous parallels between signaling pathways that drive tumorigenesis and signaling pathways that regulate hypertrophic responses and survival in cardiomyocytes. Indeed, this issue appears to be at the core of the cardiotoxicity (often manifest as a dilated cardiomyopathy) that can result from treatment with agents typically referred to as "targeted therapeutics," which target specific protein kinases that are dysregulated in cancer. Herein, we examine the cardiotoxicity of targeted therapeutics, focusing on the underlying molecular mechanisms, thereby allowing an understanding of the problem but also allowing the identification of novel, and sometimes surprising, roles played by protein kinases in the heart.

Genetic disparity between primary tumours, disseminated tumour cells, and manifest metastasis

Recent genetic analyses of paired samples from primary tumours and disseminated tumour cells have uncovered a bewildering genetic disparity. It was therefore proposed that ectopically residing tumour cells disseminate early and develop independently into metastases parallel to the primary tumour. Alternatively, these cells may represent an irrelevant cell population unable to spawn metastases whereas only cells that disseminated late in primary tumour development (which therefore are similar to the primary tumour) will form manifest metastasis. Here, we review comparative analyses of paired samples from primary tumours and disseminated tumour cells or primary tumours and metastases. The data demonstrate a striking disparity, questioning the use of primary tumours as surrogate for the genetics of systemic cancer. In the era of molecular therapies that build upon genetic defects of tumour cells, these data call for a direct diagnostic pathology of systemic cancer.

Exploiting synthetic lethal interactions for targeted cancer therapy

Emerging data suggests that synthetic lethal interactions between mutated oncogenes/tumor suppressor genes and molecules involved in DNA damage signaling and repair can be therapeutically exploited to preferentially kill tumor cells. In this review, we discuss the concept of synthetic lethality, and describe several recent examples in which this concept was successfully implemented to target tumor cells in culture, in mouse models, and in human cancer patients.

Endoplasmic reticulum stress response in cancer: molecular mechanism and therapeutic potential

In eukaryotic cells, the endoplasmic reticulum (ER) is an organelle that is responsible for protein folding and assembly, lipid and sterol biosynthesis, and free calcium storage. In the past decade, intensive research effort has been focused on intracellular stress signaling pathways from the ER that lead to transcriptional and translational reprogramming of stressed cells. These signaling pathways, which are collectively termed Unfolded Protein Response (UPR), are critical for the cell to make survival or death decision under ER stress conditions. In recent years, research in the cancer field has revealed that ER stress and the UPR are highly induced in various tumors and are closely associated with cancer cell survival and resistance to anti-cancer treatments. Identifying the UPR components that are activated or suppressed in malignancy and exploring cancer therapeutic potentials by targeting the UPR are hot research spots. In this review, we summarize the recent progress in understating UPR signaling in cancer and its related therapeutic potential.

Cancer systems biology: a network modeling perspective

Cancer is now appreciated as not only a highly heterogenous pathology with respect to cell type and tissue origin but also as a disease involving dysregulation of multiple pathways governing fundamental cell processes such as death, proliferation, differentiation and migration. Thus, the activities of molecular networks that execute metabolic or cytoskeletal processes, or regulate these by signal transduction, are altered in a complex manner by diverse genetic mutations in concert with the environmental context. A major challenge therefore is how to develop actionable understanding of this multivariate dysregulation, with respect both to how it arises from diverse genetic mutations and to how it may be ameliorated by prospective treatments. While high-throughput experimental platform technologies ranging from genomic sequencing to transcriptomic, proteomic and metabolomic profiling are now commonly used for molecular-level characterization of tumor cells and surrounding tissues, the resulting data sets defy straightforward intuitive interpretation with respect to potential therapeutic targets or the effects of perturbation. In this review article, we will discuss how significant advances can be obtained by applying computational modeling approaches to elucidate the pathways most critically involved in tumor formation and progression, impact of particular mutations on pathway operation, consequences of altered cell behavior in tissue environments and effects of molecular therapeutics.

Differential impact of tumor suppressor pathways on DNA damage response and therapy-induced transformation in a mouse primary cell model

The RB and p53 tumor suppressors are mediators of DNA damage response, and compound inactivation of RB and p53 is a common occurrence in human cancers. Surprisingly, their cooperation in DNA damage signaling in relation to tumorigenesis and therapeutic response remains enigmatic. In the context of individuals with heritable retinoblastoma, there is a predilection for secondary tumor development, which has been associated with the use of radiation-therapy to treat the primary tumor. Furthermore, while germline mutations of the p53 gene are critical drivers for cancer predisposition syndromes, it is postulated that extrinsic stresses play a major role in promoting varying tumor spectrums and disease severities. In light of these studies, we examined the tumor suppressor functions of these proteins when challenged by exposure to therapeutic stress. To examine the cooperation of RB and p53 in tumorigenesis, and in response to therapy-induced DNA damage, a combination of genetic deletion and dominant negative strategies was employed. Results indicate that loss/inactivation of RB and p53 is not sufficient for cellular transformation. However, these proteins played distinct roles in response to therapy-induced DNA damage and subsequent tumorigenesis. Specifically, RB status was critical for cellular response to damage and senescence, irrespective of p53 function. Loss of RB resulted in a dramatic evolution of gene expression as a result of alterations in epigenetic programming. Critically, the observed changes in gene expression have been specifically associated with tumorigenesis, and RB-deficient, recurred cells displayed oncogenic characteristics, as well as increased resistance to subsequent challenge with discrete therapeutic agents. Taken together, these findings indicate that tumor suppressor functions of RB and p53 are particularly manifest when challenged by cellular stress. In the face of such challenge, RB is a critical suppressor of tumorigenesis beyond p53, and RB-deficiency could promote significant cellular evolution, ultimately contributing to a more aggressive disease.

Nrf2: friend or foe for chemoprevention?

Health reflects the ability of an organism to adapt to stress. Stresses--metabolic, proteotoxic, mitotic, oxidative and DNA-damage stresses--not only contribute to the etiology of cancer and other chronic degenerative diseases but are also hallmarks of the cancer phenotype. Activation of the Kelch-like ECH-associated protein 1 (KEAP1)-NF-E2-related factor 2 (NRF2)-signaling pathway is an adaptive response to environmental and endogenous stresses and serves to render animals resistant to chemical carcinogenesis and other forms of toxicity, whilst disruption of the pathway exacerbates these outcomes. This pathway can be induced by thiol-reactive small molecules that demonstrate protective efficacy in preclinical chemoprevention models and in clinical trials. However, mutations and epigenetic modifications affecting the regulation and fate of NRF2 can lead to constitutive dominant hyperactivation of signaling that preserves rather than attenuates cancer phenotypes by providing selective resistance to stresses. This review provides a synopsis of KEAP1-NRF2 signaling, compares the impact of genetic versus pharmacologic activation and considers both the attributes and concerns of targeting the pathway in chemoprevention.

Whole chromosome instability caused by Bub1 insufficiency drives tumorigenesis through tumor suppressor gene loss of heterozygosity

Genetic alterations that promote chromosome missegregation have been proposed to drive tumorigenesis through loss of whole chromosomes containing key tumor suppressor genes. To test this unproven idea, we bred Bub1 mutant mice that inaccurately segregate their chromosomes onto p53(+/-), Apc(Min/+), Rb(+/-), or Pten(+/-) backgrounds. Bub1 insufficiency predisposed p53(+/-) mice to thymic lymphomas and Apc(Min/+) mice to colonic tumors. These tumors consistently lacked the nonmutated tumor suppressor allele but had gained a copy of the mutant allele. In contrast, Bub1 insufficiency had no impact on tumorigenesis in Rb(+/-) mice and inhibited prostatic intraepithelial neoplasia formation in Pten(+/-) mice. Thus, Bub1 insufficiency can drive tumor formation through tumor suppressor gene loss of heterozygosity, but only in restricted genetic and cellular contexts.

Overexpression of RAD51 suppresses recombination defects: a possible mechanism to reverse genomic instability

RAD51, a key protein in the homologous recombinational DNA repair (HRR) pathway, is the major strand-transferase required for mitotic recombination. An important early step in HRR is the formation of single-stranded DNA (ss-DNA) coated by RPA (a ss-DNA-binding protein). Displacement of RPA by RAD51 is highly regulated and facilitated by a number of different proteins known as the 'recombination mediators'. To assist these recombination mediators, a second group of proteins also is required and we are defining these proteins here as 'recombination co-mediators'. Defects in either recombination mediators or co-mediators, including BRCA1 and BRCA2, lead to impaired HRR that can genetically be complemented for (i.e. suppressed) by overexpression of RAD51. Defects in HRR have long been known to contribute to genomic instability leading to tumor development. Since genomic instability also slows cell growth, precancerous cells presumably require genomic re-stabilization to gain a growth advantage. RAD51 is overexpressed in many tumors, and therefore, we hypothesize that the complementing ability of elevated levels of RAD51 in tumors with initial HRR defects limits genomic instability during carcinogenic progression. Of particular interest, this model may also help explain the high frequency of TP53 mutations in human cancers, since wild-type p53 represses RAD51 expression.

Medical treatments: in association or alone, their role and their future perspectives: novel molecular-targeted therapy for hepatocellular carcinoma

Accumulated understanding of the molecular networks in the state of oncogene addiction, i.e., the "Achilles' heel of cancer," has led to the development of novel targeted therapies. Using genome-wide gene expression and network analysis, we have identified "Aurora kinase B" as a unique molecule to predict the lethal recurrence of hepatocellular carcinoma (HCC) even after curative hepatectomy. Comparative genomic hybridization (CGH)-array analysis revealed the genomic instability was closely related to Aurora kinase B expression in HCC. Then, we analyzed the in vitro and in vivo effects of a selective inhibitor of Aurora kinase B on human HCC cells. Treatment with Aurora B inhibitor in vitro resulted in polyploidy and apoptotic cell death. The growth of orthotopic liver tumors was significantly suppressed by the Aurora B inhibitor. Our preclinical studies indicate that Aurora kinase B is a promising molecular target "Achilles' heel" for the treatment of aggressive HCC.

Targeting HSP70 for cancer therapy

In a recent issue of Molecular Cell, Leu et al. (2009) demonstrate that a chemical inhibitor of HSP70 exerts prominent tumor-selective cytotoxic effects, thereby lending further support to the notion that non-oncogene addiction constitutes a promising target for anticancer therapy.

Caspase 2-mediated tumor suppression involves survivin gene silencing

One of the pivotal functions of endogenous tumor suppression is to oppose aberrant cell survival, but the molecular requirements of this process are not completely understood. Here, we show that caspase 2, a death effector with largely unknown functions, represses transcription of the survivin gene, a general regulator of cell division and cytoprotection in tumors. This pathway involves caspase 2 proteolytic cleavage of the NFκB activator, RIP1. In turn, loss of RIP1 abolishes transcription of NFκB target genes, including survivin, resulting in deregulated mitotic transitions, enhanced apoptosis, and suppression of tumorigenicity, in vivo. Therefore, caspase 2 functions as an endogenous inhibitor of NFκB-dependent cell survival, and this mechanism may contribute to tumor suppression in humans.

CRX is a diagnostic marker of retinal and pineal lineage tumors

Background

CRX is a homeobox transcription factor whose expression and function is critical to maintain retinal and pineal lineage cells and their progenitors. To determine the biologic and diagnostic potential of CRX in human tumors of the retina and pineal, we examined its expression in multiple settings.

Methodology/Principal Findings

Using situ hybridization and immunohistochemistry we show that Crx RNA and protein expression are exquisitely lineage restricted to retinal and pineal cells during normal mouse and human development. Gene expression profiling analysis of a wide range of human cancers and cancer cell lines also supports that CRX RNA is highly lineage restricted in cancer. Immunohistochemical analysis of 22 retinoblastomas and 13 pineal parenchymal tumors demonstrated strong expression of CRX in over 95% of these tumors. Importantly, CRX was not detected in the majority of tumors considered in the differential diagnosis of pineal region tumors (n = 78). The notable exception was medulloblastoma, 40% of which exhibited CRX expression in a heterogeneous pattern readily distinguished from that seen in retino-pineal tumors.

Conclusions/Significance

These findings describe new potential roles for CRX in human cancers and highlight the general utility of lineage restricted transcription factors in cancer biology. They also identify CRX as a sensitive and specific clinical marker and a potential lineage dependent therapeutic target in retinoblastoma and pineoblastoma.

Suppression of cancer cell growth by promoting cyclin D1 degradation

The cyclin D1 proto-oncoprotein is a crucial regulator in cell-cycle progression, and aberrant overexpression of cyclin D1 is linked to tumorigenesis of many different cancer types. By screening ubiquitinated cyclin D1 as a substrate with a deubiquitinase library, we have identified USP2 as a specific deubiquitinase for cyclin D1. USP2 directly interacts with cyclin D1 and promotes its stabilization by antagonizing ubiquitin-dependent degradation. Conversely, USP2 knockdown destabilizes cyclin D1 and induces growth arrest in the human cancer lines where cell growth is dependent on cyclin D1 expression. Of note, cyclin D1 is not universally required for cell-cycle progression. Inactivation of USP2 has either very mild effects on cell growth in normal human fibroblasts or no effect in the cancer cells that do not express cyclin D1. These findings suggest that targeting USP2 is an effective approach to induce growth suppression in the cancer cells addicted to cyclin D1 expression.

Signaling to p53: ribosomal proteins find their way

Inherently disparate cell growth and division, which are intimately coupled through a delicate network of intracellular and extracellular signaling, require ribosomal biogenesis. A number of events imparting instability to ribosomal biogenesis can cause nucleolar stress. In response to this stress, several ribosomal proteins bind to MDM2 and block MDM2-mediated p53 ubiquitination and degradation, resulting in p53-dependent cell cycle arrest. By doing so, the ribosomal proteins play a crucial role in connecting deregulated cell growth with inhibition of cell division. The ribosomal protein-MDM2-p53 signaling pathway provides a molecular switch that may constitute a surveillance network monitoring the integrity of ribosomal biogenesis.

Targeted therapies: the rare cancer paradigm

This review analyzes the state of the art of targeted therapies for several tumors, starting from the paradigmatic example of Imatinib treatment in chronic myelogenous leukemia (CML). We discuss how rare tumors can be models for various mechanisms of receptor tyrosine kinase (RTK) activation, and provide the opportunity to develop new therapies also for more common cancer types. We discuss the activation of the downstream RTK effectors as further targets for therapies in colorectal cancer. Finally, we highlight how a novel multidimensional approach which adds an in silico dimension to the in vitro and in vivo approach, can predict clinical results.

Mechanism of lapatinib-mediated radiosensitization of breast cancer cells is primarily by inhibition of the Raf>MEK>ERK mitogen-activated protein kinase cascade and radiosensitization of lapatinib-resistant cells restored by direct inhibition of MEK

BACKGROUND AND PURPOSE

We recently showed that lapatinib, an EGFR/HER2 inhibitor, radiosensitized breast cancer cells of the basal and HER2+ subtypes. The purpose of this study was to identify the downstream signaling pathways responsible for lapatinib-mediated radiosensitization in breast cancer.

MATERIALS AND METHODS

Response of EGFR downstream signaling pathways was assessed by Western blot and clonogenic cell survival assays in breast tumor cells after irradiation (5Gy), lapatinib, CI-1040, or combined treatment.

RESULTS

In SUM102 cells, an EGFR+ basal breast cancer cell line, exposure to ionizing radiation elicited strong activation of ERK1/2 and JNK, which was blocked by lapatinib, and weak/no activation of p38, AKT or STAT3. Direct inhibition of MEK1 with CI-1040 resulted in 95% inhibition of surviving colonies when combined with radiation while inhibition of JNK with SP600125 had no effect. Lapatinib-mediated radiosensitization of SUM102 cells was completely abrogated with expression of constitutively active Raf. Treatment of lapatinib-resistant SUM185 cells with CI-1040 restored radiosensitization with 45% fewer surviving colonies when combined with radiation.

CONCLUSIONS

These data suggest that radiosensitization by lapatinib is mediated largely through inhibition of MEK/ERK and that direct inhibition of this pathway may provide an additional avenue of radiosensitization in EGFR+ or HER2+ breast cancers.

Loss-of-function genetic screens as a tool to improve the diagnosis and treatment of cancer

A major impediment to the effective treatment of cancer is the molecular heterogeneity of the disease, which is also reflected in an equally diverse pattern of clinical responses to therapy. Currently, only few drugs are available that can be used safely and effectively to treat cancer. To improve this situation, the development of novel and highly specific targets for therapy is of utmost importance. Possibly even more importantly, we need better tools to predict which patients will respond to specific therapies. Such drug response biomarkers will be instrumental to individualize the therapy of patients having seemingly similar cancers. In this study, we discuss how RNA interference-based genetic screens can be used to address these two pressing needs in the care for cancer patients.

Is resistance useless? Multidrug resistance and collateral sensitivity

When cancer cells develop resistance to chemotherapeutics, it is frequently conferred by the ATP-dependent efflux pump P-glycoprotein (MDR1, P-gp, ABCB1). P-gp can efflux a wide range of cancer drugs; its expression confers cross-resistance, termed "multidrug resistance" (MDR), to a wide range of drugs. Strategies to overcome this resistance have been actively sought for more than 30 years, yet clinical solutions do not exist. A less understood aspect of MDR is the hypersensitivity of resistant cancer cells to other drugs, a phenomenon known as "collateral sensitivity" (CS). This review highlights the extent of this effect for the first time, and discusses hypotheses (e.g. generation of reactive oxygen species) to account for the underlying generality of this phenomenon, and proposes exploitation of CS as a strategy to improve response to chemotherapy.

Irreversible pan-ErbB tyrosine kinase inhibitors and breast cancer: current status and future directions

Aberrant activation of HER2 through overexpression has been shown to play an important role in some breast cancers. Therapies against this receptor including the monoclonal antibody, trastuzumab, or the small tyrosine kinase inhibitor, lapatinib have shown to improve the prognosis of such patients. Despite overexpressing HER2, some patients do not respond to these targeted treatments or progress after a short period of time. Irreversible tyrosine kinase inhibitors have been developed to bypass several pathways that could be involved in this resistance. In vitro, these agents have been shown to be more potent and to prolong target inhibition. Clinical development of these agents is ongoing and early results are promising. This review will describe the biologic rationale that justifies the development of these agents in breast cancer focusing on the current status and future directions.

Cancer attractors: a systems view of tumors from a gene network dynamics and developmental perspective

Cell lineage commitment and differentiation are governed by a complex gene regulatory network. Disruption of these processes by inappropriate regulatory signals and by mutational rewiring of the network can lead to tumorigenesis. Cancer cells often exhibit immature or embryonic traits and dysregulated developmental genes can act as oncogenes. However, the prevailing paradigm of somatic evolution and multi-step tumorigenesis, while useful in many instances, offers no logically coherent reason for why oncogenesis recapitulates ontogenesis. The formal concept of "cancer attractors", derived from an integrative, complex systems approach to gene regulatory network may provide a natural explanation. Here we present the theory of attractors in gene network dynamics and review the concept of cell types as attractors. We argue that cancer cells are trapped in abnormal attractors and discuss this concept in the light of recent ideas in cancer biology, including cancer genomics and cancer stem cells, as well as the implications for differentiation therapy.

Raf-1 and squamous cell carcinoma: Rok-ing the boat

Squamous cell carcinoma (SCC) is the second most common form of nonmelanoma skin cancer. In this issue of Cancer Cell, Ehrenreiter et al. unveil a critical role for the Raf-1/Rok-alpha interaction in the pathogenesis of SCCs, thus paving the way for the development of therapeutic modalities to treat this malignancy.

Anticancer immunochemotherapy using adjuvants with direct cytotoxic effects

Conventional chemotherapeutics may induce immunogenic cancer cell death or stimulate immune effectors via so-called off-target effects. The study by Besch et al. in this issue of the JCI now demonstrates that agents designed to stimulate the innate immune system by activating intracellular pattern recognition receptors can kill cancer cells in a direct, cell-autonomous fashion (see the related article beginning on page 2399). The authors show that ligation of viral RNA sensors, such as RIG-I or MDA-5, by viral RNA mimetics triggers mitochondrial apoptosis in human melanoma cells in an IFN-independent fashion. The data suggest that tumor cell killing and immunostimulation may synergize for optimal anticancer immunochemotherapy.

A genome-wide RNAi screen identifies multiple synthetic lethal interactions with the Ras oncogene

Oncogenic mutations in the small GTPase Ras are highly prevalent in cancer, but an understanding of the vulnerabilities of these cancers is lacking. We undertook a genome-wide RNAi screen to identify synthetic lethal interactions with the KRAS oncogene. We discovered a diverse set of proteins whose depletion selectively impaired the viability of Ras mutant cells. Among these we observed a strong enrichment for genes with mitotic functions. We describe a pathway involving the mitotic kinase PLK1, the anaphase-promoting complex/cyclosome, and the proteasome that, when inhibited, results in prometaphase accumulation and the subsequent death of Ras mutant cells. Gene expression analysis indicates that reduced expression of genes in this pathway correlates with increased survival of patients bearing tumors with a Ras transcriptional signature. Our results suggest a previously underappreciated role for Ras in mitotic progression and demonstrate a pharmacologically tractable pathway for the potential treatment of cancers harboring Ras mutations.

The Use and Refinement of Rodent Models in Anti-cancer Drug Discovery: A Review

This review describes the changing use of tumour models in rodents (predominantly mice) as employed over the last four decades in anti-cancer drug discovery, and the refinements in the experimental methods used. Such models are required to examine the complexities of cancer biology (e.g. tumour angiogenesis, invasion and metastasis, host immunity factors) and the impact of potential therapies (e.g. drug pharmacokinetics, pharmacodynamics and therapeutic index), and they have produced efficacious human therapeutics. Animal welfare considerations have driven refinements to animal models of cancer over time, with the most dramatic refinements being facilitated by the move away from inherently cytotoxic therapeutic approaches toward targeted inhibitors of disease-related processes. Whereas, four decades ago, the impact of disease burden was used as an endpoint in the absence of defined mechanistic parameters, acute pharmacodynamic measures are now increasingly used to minimise the adverse effects of disease and experimental procedures in a given animal. The changes in the UK guidelines on the use of rodents in preclinical cancer testing are also used as an illustration of the progressive refinement in tumour models and drug testing.