Integrated analysis of multiple receptor tyrosine kinases identifies Axl as a therapeutic target and mediator of resistance to sorafenib in hepatocellular carcinoma.

BACKGROUND: Aberrant activation of Axl is implicated in the progression of hepatocellular carcinoma (HCC). We explored the biologic significance and preclinical efficacy of Axl inhibition as a therapeutic strategy in sorafenib-naive and resistant HCC. METHODS: We evaluated Axl expression in sorafenib-naive and resistant (SR) clones of epithelial (HuH7) and mesenchymal origin (SKHep-1) using antibody arrays and confirmed tissue expression. We tested the effect of Axl inhibition with RNA-interference and pharmacologically with R428 on a number of phenotypic assays. RESULTS: Axl mRNA overexpression in cell lines (n = 28) and RNA-seq tissue datasets (n = 373) correlated with epithelial-to-mesenchymal transition (EMT). Axl was overexpressed in HCC compared to cirrhosis and normal liver. We confirmed sorafenib resistance to be associated with EMT and enhanced motility in both HuH7-SR and SKHep-1-SR cells documenting a 4-fold increase in Axl phosphorylation as an adaptive feature of chronic sorafenib treatment in SKHep-1-SR cells. Axl inhibition reduced motility and enhanced sensitivity to sorafenib in SKHep-1SR cells. In patients treated with sorafenib (n = 40), circulating Axl levels correlated with shorter survival. CONCLUSIONS: Suppression of Axl-dependent signalling influences the transformed phenotype in HCC cells and contributes to adaptive resistance to sorafenib, providing a pre-clinical rationale for the development of Axl inhibitors as a measure to overcome sorafenib resistance.

Broad targeting of resistance to apoptosis in cancer.

Apoptosis or programmed cell death is natural way of removing aged cells from the body. Most of the anti-cancer therapies trigger apoptosis induction and related cell death networks to eliminate malignant cells. However, in cancer, de-regulated apoptotic signaling, particularly the activation of an anti-apoptotic systems, allows cancer cells to escape this program leading to uncontrolled proliferation resulting in tumor survival, therapeutic resistance and recurrence of cancer. This resistance is a complicated phenomenon that emanates from the interactions of various molecules and signaling pathways. In this comprehensive review we discuss the various factors contributing to apoptosis resistance in cancers. The key resistance targets that are discussed include (1) Bcl-2 and Mcl-1 proteins; (2) autophagy processes; (3) necrosis and necroptosis; (4) heat shock protein signaling; (5) the proteasome pathway; (6) epigenetic mechanisms; and (7) aberrant nuclear export signaling. The shortcomings of current therapeutic modalities are highlighted and a broad spectrum strategy using approaches including (a) gossypol; (b) epigallocatechin-3-gallate; (c) UMI-77 (d) triptolide and (e) selinexor that can be used to overcome cell death resistance is presented. This review provides a roadmap for the design of successful anti-cancer strategies that overcome resistance to apoptosis for better therapeutic outcome in patients with cancer.

Designing a broad-spectrum integrative approach for cancer prevention and treatment.

Targeted therapies and the consequent adoption of "personalized" oncology have achieved notable successes in some cancers; however, significant problems remain with this approach. Many targeted therapies are highly toxic, costs are extremely high, and most patients experience relapse after a few disease-free months. Relapses arise from genetic heterogeneity in tumors, which harbor therapy-resistant immortalized cells that have adopted alternate and compensatory pathways (i.e., pathways that are not reliant upon the same mechanisms as those which have been targeted). To address these limitations, an international task force of 180 scientists was assembled to explore the concept of a low-toxicity "broad-spectrum" therapeutic approach that could simultaneously target many key pathways and mechanisms. Using cancer hallmark phenotypes and the tumor microenvironment to account for the various aspects of relevant cancer biology, interdisciplinary teams reviewed each hallmark area and nominated a wide range of high-priority targets (74 in total) that could be modified to improve patient outcomes. For these targets, corresponding low-toxicity therapeutic approaches were then suggested, many of which were phytochemicals. Proposed actions on each target and all of the approaches were further reviewed for known effects on other hallmark areas and the tumor microenvironment. Potential contrary or procarcinogenic effects were found for 3.9% of the relationships between targets and hallmarks, and mixed evidence of complementary and contrary relationships was found for 7.1%. Approximately 67% of the relationships revealed potentially complementary effects, and the remainder had no known relationship. Among the approaches, 1.1% had contrary, 2.8% had mixed and 62.1% had complementary relationships. These results suggest that a broad-spectrum approach should be feasible from a safety standpoint. This novel approach has potential to be relatively inexpensive, it should help us address stages and types of cancer that lack conventional treatment, and it may reduce relapse risks. A proposed agenda for future research is offered.

Protamine and chloroquine enhance gene delivery and expression mediated by RNA-wrapped single walled carbon nanotubes.

The use of non-viral vectors as delivery systems in gene therapy has been extensively studied recently owing to their advantages over viral vectors. Here, we propose a new gene delivery system based on the use of RNA-wrapped single-walled carbon nanotubes (SWCNTs) complexed with the cationic protein, protamine and the drug chloroquine. Protamine was selected as a cationic protein acting as bridge between negatively charged RNA-wrapped SWCNTs and plasmid DNA. Protamine also contains a nuclear localization signal which enhances the expression of the transfected gene. The drug chloroquine, a lysosomotropic compound which has been reported to increase the transfection efficiency, was attached to RNA-wrapped SWNTs by ionic interactions. The simultaneous delivery of the drug chloroquine with plasmid DNA clearly showed an enhanced gene delivery and expression. The levels of gene expression were quantified using the luciferase reporter gene as model. Optimal conditions for transfection and gene expression were obtained and cytoxicity of the carbon nanotube complexes measured. The optimal complexes were shown to efficiently deliver plasmid DNA for efficient gene expression and may thereby be useful as gene delivery systems for gene therapy.

The prolyl-hydroxylase EGLN3 and not EGLN1 is inactivated by methylation in plasma cell neoplasia.

EGLN1 and EGLN3 are members of the egg-laying-defective 9 (EglN) prolyl-hydroxylases which during normoxia catalyse hydroxylation of the hypoxia-inducible factor (HIF)-1alpha, thereby promoting its ubiquitination by a complex containing the von Hippel-Lindau (VHL) tumour suppressor. EGLN3 also has pro-apoptotic activity in some cell types. Analyses of a well-characterised series of cases of plasma cell dyscrasias, including multiple myeloma (MM), Waldenström's macroglobulinaemia (WM) and monoclonal gammopathy of undetermined significance (MGUS) surprisingly demonstrated that the CpG island of EGLN3, and not EGLN1, is frequently methylated in these disorders. Multiple myeloma patients with a methylated EGLN3 promoter showed trends towards an increased risk of death, bone lytic lesions, anaemia, advanced stage of disease and the presence of extramedullary disease. Those individuals with methylation in the EGLN3 CpG island also had significantly lower albumin levels. These data suggest that the prolyl-hydroxylases may be a novel class of potential tumour suppressors in plasma cell neoplasia that warrant further investigation with regard to their potential utility as biomarkers. Moreover, we observed that EGLN3 is also methylated at high frequency in B-cell lymphoma subtypes, implying that loss of EGLN3 is an important epigenetic event not only in plasma cell neoplasias but also in B-cell neoplasias.

A randomized controlled trial investigating the effects of celecoxib in patients with localized prostate cancer.

UNLABELLED: Cyclooxygenase-2 (COX-2) is associated with tumour promotion, inhibition of apoptosis, angiogenesis and metastasis. Celecoxib, a selective COX-2 inhibitor was investigated, in patients with clinically localized prostate cancer using immunohistochemistry. PATIENTS AND METHODS: Patients with cT1-2 prostate cancer (n=45) were randomized to celecoxib 400mg b.d. or no treatment for four weeks prior to radical prostatectomy. Histological sections of preoperative biopsy and matched radical prostatectomy specimens were stained for markers of cell proliferation (MIB-1/Ki-67), microvessel density (CD-31 with Weidner scoring), COX-2, apoptosis (TUNEL analysis), angiogenic factors (VEGF and KDR) and HIF-1. RESULTS: Celecoxib decreased tumour cell proliferation, microvessel density, angiogenesis and HIF-1 whilst enhancing apoptosis. These effects approached statistical significance in a multivariate model and the cell proliferation index approached statistical significance on univariate analysis. CONCLUSION: In this pilot study a 4 week regimen of celecoxib resulted in measurable biological effects in prostate cancer tissue. These findings warrant further investigation.

The Targeting of RNA Polymerase I Transcription Using CX-5461 in Combination with Radiation Enhances Tumour Cell Killing Effects in Human Solid Cancers.

An increased rate of cellular proliferation is a hallmark of cancer and may be accompanied by an increase in ribosome biogenesis and dysregulation in rRNA synthesis. In this regard, CX-5461 has been developed as a novel RNA polymerase I inhibitor and is currently in Phase I/II clinical trials for solid and hematological malignancies. In the present study, interactions between CX-5461 and single-dose X-ray exposure were assessed using isobologram analysis using MTS assay and drug-induced cell death was assessed using flow cytometric, confocal microscopy and Western blot analysis. Combination treatments involving CX-5461 and single-dose X-ray exposure highlighted increased effectiveness compared to individual treatment alone in the CaSki cervical cancer line, with marked synergistic interaction occurring within the low-drug (50 nM) and low-dose radiation range (2-6 Gy). Cell lines challenged with CX-5461 demonstrated the presence of DNA damage, induction of apoptosis, autophagy and senescence alongside high percentages of G2/M cell cycle arrest. In addition, we report preferential sensitivity of ovarian cancer cells with BRCA2 mutation to this novel agent. Taken together, CX-5461 displayed a broad spectrum of activity in a panel of solid cancer cell lines with IC50 values ranging from 35 nM to >1 µM. The work described herein identifies the synergistic effects of CX-5461 in combination with X-rays in solid cancers and may also aid in the design of clinical trials involving this novel agent.

Biophysical characterization of MDR breast cancer cell lines reveals the cytoplasm is critical in determining drug sensitivity.

Dielectrophoresis (DEP) was used to examine a panel of MCF-7 cell lines comprising parental MCF-7 cells and MDR derivatives: MCF-7TaxR (paclitaxel-resistant, P-glycoprotein (P-gp) positive), MCF-7DoxR (doxorubicin-resistant MRP2 positive) plus MCF-7MDR1 (MDR1 transfected, P-gp positive). MCF-7DoxR and MCF-7MDR1 were broadly cross-resistant to natural product anticancer agents, whereas MCF-7TaxR cells were not, contrary to P-gp expression. Whilst DEP revealed modest membrane changes in MDR sub-lines, we saw significant changes in their cytoplasmic conductivity: MCF-7TaxR

Mechanisms and strategies to overcome chemotherapy resistance in metastatic breast cancer.

Resistance to chemotherapeutic agents is a significant issue in the management of patients with breast cancer. Anthracyclines, although first used over 30 years ago, are still part of the standard chemotherapy for this disease. Subsequently, the taxanes heralded a new era in chemotherapy and have been used extensively in the treatment of metastatic breast cancer. Unfortunately, along with other constituents of combination chemotherapy for metastatic breast cancer such as cyclophosphamide, these agents become increasingly ineffective in progressive disease and tumours are then deemed to be drug resistant - frequently multidrug resistant. A number of processes have been identified that can underlie clinical drug resistance, and these largely stem from in vitro laboratory-based studies in human cancer cell lines. A large proportion of these studies have focused on multidrug resistance associated with resistance to natural product anticancer agents due to the presence of putative drug transporter proteins such as P-glycoprotein, MRP1, and BCRP. Other studies have highlighted mechanisms whereby breast cancer cells show resistance to chemotherapeutic agents by altered regulation of DNA repair processes, with many other factors influencing drug detoxification processes and altering drug targets. New developmental agents with improved specificity for tumour cells, such as trastuzumab, and those with low susceptibility to common tumour-resistance mechanisms, such as ixabepilone, have provided new hope for effective treatment of breast cancer. Ixabepilone is the first in a new class of neoplastics, the epothilones. With these developments in therapy, and the technology of gene expression profiling, the future holds more promise for the development of more effective treatment for metastatic breast cancer.

The effects of cyclooxygenase-2 expression in prostate cancer cells: modulation of response to cytotoxic agents.

Cyclooxygenase (COX)-2 has emerged as an exciting target for therapeutic intervention in the management of cancer. Immunohistochemistry studies have indicated higher expression of COX-2 in cancerous versus benign prostatic tissue. We have explored the role of COX-2 in prostate cancer in terms of attenuation of apoptosis and sensitivity to pharmacological agents, including COX-2 inhibitors. The human prostate cancer cell line LNCaP was stably transfected with COX-2 (LNCaPCOX-2) and compared with the empty vector control line (LNCaPneo). Chemosensitivity testing indicated no change in sensitivity to the cytotoxic effects of COX-2 inhibitors celecoxib or sulindac or VP16. However, LNCaPCOX-2 cells showed 3-fold resistance to carboplatin, which was partially reversed by coincubation with the phosphatidylinositol 3-kinase inhibitor wortmannin. Concomitant with reduced apoptotic response to cytotoxic agents, LNCaPCOX-2 cells expressed increased levels of survivin and Bcl-2 with enhanced activation of AKT. We also investigated the effects of celecoxib on expression levels of genes relevant to prostate cancer and drug resistance in our model system using quantitative polymerase chain reaction analysis. Celecoxib treatment resulted in highly significant increases in the mRNA expression of the smooth muscle component desmin, the detoxification enzyme glutathione S-transferase pi (GSTpi), and nonsteroidal anti-inflammatory response gene (NAG-1) in the LNCaPCOX-2 cell line compared with LNCaPneo cells. Significant decreases in survivin levels and increases in GSTpi and NAG-1 appeared to be COX-2-dependent effects because they were more pronounced in LNCaPCOX-2 cells. Our findings indicate both COX-2-dependent and -independent mechanisms attributable to celecoxib and support its utility in the management of prostate cancer.

Synthesis and characterization of platinum(II) oxadiazoline complexes and their in vitro antitumor activity in platinum-sensitive and -resistant cancer cell lines.

A series of platinum(II) complexes bearing Delta (4)-1,2,4-oxadiazoline ligands have been synthesized and characterized. Their in vitro antitumor activity has been assessed in platinum-sensitive and -resistant human ovarian cancer cell lines (PEO1, PEOCisR, PEOCarboR, and SK-OV3), as well as in colon cancer (SW948) and testicular cancer cell lines (N-TERA). All compounds tested showed potent cytotoxicity in the platinum-sensitive cell lines and retained activity in the cisplatin- and carboplatin-resistant lines, with IC 50 values similar to the parental drug sensitive counterpart. We propose, therefore, that platinum(II) oxadiazoline complexes may possess a novel mechanism of action, which render them active in tumor cells, with resistance to currently used platinum anticancer agents.

Crosstalk between ERα and Receptor Tyrosine Kinase Signalling and Implications for the Development of Anti-Endocrine Resistance.

Although anti-endocrine therapies have significantly advanced the treatment of breast cancer, they pose the problem of acquired drug resistance. The oestrogen receptor (ER)-expressing breast cancer cell lines MCF-7 and T47D alongside their in vitro derived resistant counterparts MCF-7-TR (tamoxifen-resistant) and T47D-FR (fulvestrant-resistant) showed dual resistance to fulvestrant and tamoxifen in the presence of upregulated HER1 and HER2 growth factor receptors. Our study demonstrated that tamoxifen resistance and fulvestrant resistance are associated with collateral sensitivity to the tyrosine kinase inhibitors (TKIs) lapatinib (p < 0.0001) and afatinib (p < 0.0001). Further, we found that over time, the TKIs reactivated ERα protein and/or mRNA in tamoxifen- and fulvestrant-resistant cells. Combinations of anti-endocrine agents with afatinib gave rise to significantly enhanced levels of apoptosis in both T47D-FR and MCF-7-TR in a synergistic manner versus additive effects of agents used singly. This was associated with p27kip1 induction for anti-endocrine-resistant cells versus parental cells. Our data supports the use of combination treatment utilising dual HER1/2 inhibitors in breast cancer patients showing resistance to multiple anti-endocrine agents.

Differences in the biophysical properties of membrane and cytoplasm of apoptotic cells revealed using dielectrophoresis.

We have used dielectrophoresis to determine the dielectric properties of human chronic myelogeneous leukaemic (K562) cells during apoptosis (programmed cell death). Our results indicate that K562 cells increase markedly in cytoplasmic conductivity from 0.28 S/m to 0.50 S/m within the first 4 h following treatment with staurosporine, which then lasts beyond 12 h, whilst cell shrinkage increases the capacitance of the membrane from 9.7 mF/m2 to 20 mF/m2. After 24 and 48 h of incubation with staurosporine, multiple sub-populations were detected, highlighted by the dielectric changes that the cell undergoes before death. By comparing these results with those obtained by common apoptosis monitoring techniques Annexin V and TMRE (tetramethylrhodamine ethylester), it is possible to infer the role of ion efflux in the progress of apoptosis. The use of dielectrophoresis for monitoring apoptosis offers a number of benefits as it is both rapid and non-invasive. It can also be used in parallel with other assays in high-throughput screening applications.

The role of efflux pumps in drug-resistant metastatic breast cancer: new insights and treatment strategies.

Chemotherapy plays a vital role in the treatment and management of breast cancer and is associated with significant improvements in survival. Regimens such as CMF (cyclophosphamide/methotrexate/5-fluorouracil) and, more recently, TAC (docetaxel/doxorubicin/cyclophosphamide) have been used with good response rates and complete remissions achieved in approximately 15% of cases. However, a significant proportion of women experience a recurrence of metastatic disease, with an average survival between 1-2 years. The monoclonal antibody trastuzumab is used in the treatment of HER2/neu-positive breast cancer. Although such targeted agents have heralded an exciting new era in cancer therapy, they are limited by the fact that only a subset of patients can benefit from treatment and by the emergence of resistance. Thus, the pursuit of a strategy that modulates resistance to standard chemotherapeutics remains valid. Accumulating evidence indicates that a number of mechanisms known to contribute to clinical drug resistance might be relevant to breast cancer. Tumor cell drug resistance might arise as a result of systemic pharmacologic factors, changes in the tumor microenvironment (eg, pH), cellular pharmacokinetics, drug metabolism and detoxification, drug target modifications, DNA repair, and apoptotic mechanisms. The adenotriphosphate-binding cassette membrane transporter family contributes to clinical drug resistance, especially in breast cancer. The most frequently described of this family is P-glycoprotein, followed by multidrug resistance protein-1. This review describes the factors thought to play a role in clinical breast cancer drug resistance and describes potential methods by which it might be circumvented.

Seliciclib (CYC202; r-roscovitine) in combination with cytotoxic agents in human uterine sarcoma cell lines.

BACKGROUND: Inhibition of cyclin-dependent kinases (CDKs) has recently emerged as an interesting approach to treat human malignancies. This was explored in human leiomyosarcoma (LMS) lines, which represent a tumour associated with poor survival, chemo-unresponsiveness and deregulation of cell cycle components. MATERIALS AND METHODS: Using isobologram analysis with MTT chemosensitivity testing, the effects of the CDK inhibitor seliciclib (CYC202, R-roscovitine) when used alone or in combination with paclitaxel was studied in uterine cancer cell lines. Apoptotic endpoints were also examined via Annexin V assay using flow cytometry and Western blotting. RESULTS: Overall seliciclib combined with paclitaxel proved synergistic for all cell lines. This was concomitant with an enhanced apoptotic effect and downregulation of the LAP survivin. CONCLUSION: Our data support the use of seliciclib as part of combination therapy for uterine cancer.

Proteomics Analysis of Ovarian Cancer Cell Lines and Tissues Reveals Drug Resistance-associated Proteins.

BACKGROUND: Carboplatin and paclitaxel form the cornerstone of chemotherapy for epithelial ovarian cancer, however, drug resistance to these agents continues to present challenges. Despite extensive research, the mechanisms underlying this resistance remain unclear. MATERIALS AND METHODS: A 2D-gel proteomics method was used to analyze protein expression levels of three human ovarian cancer cell lines and five biopsy samples. Representative proteins identified were validated via western immunoblotting. Ingenuity pathway analysis revealed metabolomic pathway changes. RESULTS: A total of 189 proteins were identified with restricted criteria. Combined treatment targeting the proteasome-ubiquitin pathway resulted in re-sensitisation of drug-resistant cells. In addition, examination of five surgical biopsies of ovarian tissues revealed α-enolase (ENOA), elongation factor Tu, mitochondrial (EFTU), glyceraldehyde-3-phosphate dehydrogenase (G3P), stress-70 protein, mitochondrial (GRP75), apolipoprotein A-1 (APOA1), peroxiredoxin (PRDX2) and annexin A (ANXA) as candidate biomarkers of drug-resistant disease. CONCLUSION: Proteomics combined with pathway analysis provided information for an effective combined treatment approach overcoming drug resistance. Analysis of cell lines and tissues revealed potential prognostic biomarkers for ovarian cancer.

Receptor tyrosine kinase (RTK) inhibition is effective in chemosensitising EGFR-expressing drug resistant human ovarian cancer cell lines when used in combination with cytotoxic agents.

This study has focused on the use of RTK inhibitors in the treatment of ovarian cancer. We have used the human ovarian cancer cell line PEO1 alongside two in-house derived drug resistant variants: PEO1CarboR (8-fold acquired resistance to carboplatin and cisplatin) and the Pgp expressing PEO1TaxR (15-fold acquired resistance to paclitaxel). These variant cell lines were shown to have a higher expression of EGFR 1.6- and 2.0-fold increase, respectively, compared with the parental cell line. We have shown that the RTK inhibitor GW282974A (an analogue of GW2016; lapatinib) is effective in chemosensitisation of drug resistant EGFR over-expressing cells giving rise to a synergistic effect when used in combination with either cisplatin or paclitaxel in chemosensitivity assays. These effects were also seen at the level of apoptosis using the Annexin V assay and expression levels of the IAP Survivin. A reduction in the downstream signalling effector phosphorylated ERK was seen in both resistant cell lines when GW282974A was used in combination with either cisplatin or paclitaxel. This reduction was not so apparent in cells treated with the single agent GW282974A or cytotoxic agent. Interestingly, we did not show evidence for an enhanced sensitivity to the RTK inhibitor in our EGFR expressing resistant lines versus parental PEO1 cells. However, the paclitaxel resistant cell line appeared more sensitive to the chemosensitising effects of GW282974A, in line with its increased EGFR expression. Our data suggest that RTK inhibition is effective in circumvention of tumour cell drug resistance that occurs in conjunction with EGFR overexpression.

Assessment of the classical MDR phenotype in epithelial ovarian carcinoma using primary cultures: a feasibility study.

This in vitro feasibility study has assessed a number of techniques and their applicability when looking at the role of multidrug resistance (MDR) in solid tumours. Fresh tumour material was obtained from 34 patients, (11 previously treated, 23 untreated) with ovarian adenocarcinoma. Doxorubicin sensitivity was measured using the MTT assay +/- the cyclosporins, Pgp expression was assessed by immunocytochemistry with the MRK-16 MoAb and flow cytometry was used to assess intracellular drug accumulation +/- PSC 833. 85% of samples showed some evidence of modest chemosensitisation by the cyclosporins (median 1.74-fold). We saw a marked variation in the number of Pgp positive cells between patients (1-87%, median 31%). 63% of samples tested showed an enhancement of DNR accumulation in the presence of PSC 833, with a median increase of 7% (sample range 0-29%). The present study highlights some of the technical difficulties encountered when working with fresh tumour material ex vivo. We conclude that screening of patients for their suitability to enter clinical trials incorporating MDR modulating agents is technically demanding, but feasible.

Polo Like Kinase 2 Tumour Suppressor and cancer biomarker: new perspectives on drug sensitivity/resistance in ovarian cancer.

The polo-like kinase PLK2 has recently been identified as a potential theranostic marker in the management of chemotherapy sensitive cancers. The methylation status of the PLK2 CpG island varies with sensitivity to paclitaxel and platinum in ovarian cancer cell lines. Importantly, extrapolation of these in vitro data to the clinical setting confirms that the methylation status of the PLK2 CpG island predicts outcomes in patients treated with carboplatin and paclitaxel chemotherapy. A second cell cycle regulator, p57Kip2, is also subject to epigenetic silencing in carboplatin resistance in vitro and in vivo, emphasising that cell cycle regulators are important determinants of sensitivity to chemotherapeutic agents and providing insights into the phenomenon of collateral drug sensitivity in oncology. Understanding the mechanistic basis and identification of robust biomarkers to predict collateral sensitivity may inform optimal use of chemotherapy in patients receiving multiple lines of treatment.

Polo-like kinase Plk2 is an epigenetic determinant of chemosensitivity and clinical outcomes in ovarian cancer.

Resistance to platinum- and taxane-based chemotherapy remains a major clinical impediment to effective management of epithelial ovarian cancer (EOC). To gain insights into resistance mechanisms, we compared gene and confirmed expression patterns of novel EOC cell lines selected for paclitaxel and carboplatin resistance. Here, we report that resistance can be conferred by downregulation of the Polo-like kinase Plk2. Mechanistic investigations revealed that downregulation occurred at the level of transcription via associated DNA methylation of the CpG island in the Plk2 gene promoter in cell lines, primary tumors, and patient sera. Inhibitory RNA (RNAi)-mediated knockdown and ectopic overexpression established a critical functional role for Plk2 in determining apoptotic sensitivity to paclitaxel and carboplatin. In drug-resistant human EOC cell lines, Plk2 promoter methylation varied with the degree of drug resistance and transcriptional silencing of the promoter. RNAi-dependent knockdown of Plk2 abrogated G(2)-M cell-cycle blockade by paclitaxel, conferring resistance to both paclitaxel and platinum. Conversely, ectopic expression of Plk2 restored sensitivity to G(2)-M cell-cycle blockade and cytotoxicity triggered by paclitaxel. In clinical cases, DNA methylation of the Plk2 CpG island in tumor tissue was associated with a higher risk of relapse in patients treated postoperatively with carboplatin and paclitaxel (P = 0.003). This trend was also reflected in the analysis of matched serum samples. Taken together, our results implicate Plk2 as a clinically important determinant of chemosensitivity, in support of the candidacy of Plk2 as a theranostic marker to inform EOC management.