Hospicells promote upregulation of the ATP-binding cassette genes by insulin-like growth factor-I via the JAK2/STAT3 signaling pathway in an ovarian cancer cell line

Role of JAK2/STAT3 Signaling Pathway in the Tumorigenesis, Chemotherapy Resistance, and Treatment of Solid Tumors: A Systemic Review

Janus kinase 2/signal transducer and activator of transcription 3 (JAK2/STAT3) pathway is a common signaling pathway used to transduce signals from the extracellular to the intracellular (nucleus) upon the binding of cytokines and growth factors to the extracellular domain of specific cell surface receptors. This signaling pathway is tightly regulated and has a multitude of biological functions such as cell proliferation, differentiation, and apoptosis. Besides, the regulated JAK2/STAT3 signaling plays a crucial role in embryonic development, hemopoiesis, and controlling the immune system. Conversely, aberrantly activated JAK2/STAT3 is frequently detected in varieties of tumors and involved in oncogenesis, angiogenesis, and metastasis of many cancer diseases that are usually refractory to the standard chemotherapy. However, the JAK3/STAT3 pathway recently emerged interestingly as a new site for the development of novel anti-tumor agents and becomes a promising therapeutic target in the treatment of many solid malignancies. Herein, this review aimed to provide insight into the JAK2/STAT3 pathway, in the hope to gain an understanding of its potential role in the pathogenesis, progression, chemotherapy resistance, and cancer therapy of solid tumors.

Growth Hormone Upregulates Mediators of Melanoma Drug Efflux and Epithelial-to-Mesenchymal Transition In Vitro and In Vivo

Simple Summary

Growth hormone (GH) action is strongly implicated in the progression and therapy resistance in several types of solid tumors which overexpress the GH receptor (GHR). The aim of our study was to characterize the effects of GH and its downstream effector insulin-like growth factor 1 (IGF-1) on melanoma using in vitro and in vivo models. We confirmed an IGF-1-independent role of elevated circulating GH in upregulating key mechanisms of therapy resistance and malignancy with analyses conducted at the molecular and cellular level. We identified that GH upregulates key mechanisms of therapy resistance and metastases in melanoma tumors in an IGF-1 dependent and independent manner by upregulating multidrug efflux pumps and EMT transcription factors. Our study reveals that GH action renders an intrinsic drug resistance phenotype to the melanoma tumors—a clinically crucial property of GH verifiable in other human cancers with GHR expression.

Abstract

Growth hormone (GH) and the GH receptor (GHR) are expressed in a wide range of malignant tumors including melanoma. However, the effect of GH/insulin-like growth factor (IGF) on melanoma in vivo has not yet been elucidated. Here we assessed the physical and molecular effects of GH on mouse melanoma B16-F10 and human melanoma SK-MEL-30 cells in vitro. We then corroborated these observations with syngeneic B16-F10 tumors in two mouse lines with different levels of GH/IGF: bovine GH transgenic mice (bGH; high GH, high IGF-1) and GHR gene-disrupted or knockout mice (GHRKO; high GH, low IGF-1). In vitro, GH treatment enhanced mouse and human melanoma cell growth, drug retention and cell invasion. While the in vivo tumor size was unaffected in both bGH and GHRKO mouse lines, multiple drug-efflux pumps were up regulated. This intrinsic capacity of therapy resistance appears to be GH dependent. Additionally, epithelial-to-mesenchymal transition (EMT) gene transcription markers were significantly upregulated in vivo supporting our current and recent in vitro observations. These syngeneic mouse melanoma models of differential GH/IGF action can be valuable tools in screening for therapeutic options where lowering GH/IGF-1 action is important.

Loss of ZIP facilitates JAK2-STAT3 activation in tamoxifen-resistant breast cancer

Tamoxifen is beneficial in treating estrogen receptor–positive breast cancer, but resistance to this treatment eventually ensues. A method to identify mechanisms of tamoxifen resistance identified the histone deacetylase ZIP, leading to the finding that increased expression of the tyrosine kinase JAK2 is one important factor. As a result of this discovery, it may be possible to use an inhibitor of JAK2 to block the aberrant activation of STAT3 caused by ZIP deficiency to help overcome or prevent tamoxifen resistance.

Tamoxifen, a widely used modulator of the estrogen receptor (ER), targets ER-positive breast cancer preferentially. We used a powerful validation-based insertion mutagenesis method to find that expression of a dominant-negative, truncated form of the histone deacetylase ZIP led to resistance to tamoxifen. Consistently, increased expression of full-length ZIP gives the opposite phenotype, inhibiting the expression of genes whose products mediate resistance. An important example is JAK2. By binding to two specific sequences in the promoter, ZIP suppresses JAK2 expression. Increased expression and activation of JAK2 when ZIP is inhibited lead to increased STAT3 phosphorylation and increased resistance to tamoxifen, both in cell culture experiments and in a mouse xenograft model. Furthermore, data from human tumors are consistent with the conclusion that decreased expression of ZIP leads to resistance to tamoxifen in ER-positive breast cancer.

PBX1 promotes the cell proliferation via JAK2/STAT3 signaling in clear cell renal carcinoma

PBX1 was abnormally overexpressed and its intracellular localization was found to be frequently amplified in many types of cancer, including renal clear cell carcinoma. PBX1 displays oncogenic activity in several different types of cells, but little is known about how signaling pathways are altered, and the function of PBX1 in renal clear cell carcinoma has not been well investigated. In this study, we demonstrate that the expression of PBX1 was significantly upregulated in 30 pairs of human tissues compared to adjacent normal tissues and the overall survival rate of PBX1-high group was significantly worse than that of PBX1-low group. Furthermore, JAK2 expression is significantly correlated to PBX1 expression in human clinical specimen and PBX1 knockdown inhibits STAT3 phosphorylation and reduced transcription of STAT3 target genes Cyclin D1. More interestingly, PBX1 knockdown inhibits ccRCC cell viability, proliferation and cell cycle progression in vivo and in vitro. Thus, our results demonstrate that PBX1 plays an oncogenic role in ccRCC via JAK2/STAT3 pathway and indicate its potential application for the treatment of human ccRCC in future.

CCL2/CCL5 secreted by the stroma induce IL-6/PYK2 dependent chemoresistance in ovarian cancer

BACKGROUND

Minimal residual disease is the main issue of advanced ovarian cancer treatment. According to the literature and previous results, we hypothesized that Mesenchymal Stromal Cells (MSC) could support this minimal residual disease by protecting ovarian cancer cells (OCC) from chemotherapy. In vitro study confirmed that MSC could induce OCC chemoresistance without contact using transwell setting. Further experiments showed that this induced chemoresistance was dependent on IL-6 OCC stimulation.

METHODS

We combined meticulous in vitro profiling and tumor xenograft models to study the role of IL-6 in MSC/OCC intereactions.

RESULTS

We demonstrated that Tocilizumab® (anti-IL-6R therapy) in association with chemotherapy significantly reduced the peritoneal carcinosis index (PCI) than chemotherapy alone in mice xenografted with OCCs+MSCs. Further experiments showed that CCL2 and CCL5 are released by MSC in transwell co-culture and induce OCCs IL-6 secretion and chemoresistance. Finally, we found that IL-6 induced chemoresistance was dependent on PYK2 phosphorylation.

CONCLUSIONS

These findings highlight the potential key role of the stroma in protecting minimal residual disease from chemotherapy, thus favoring recurrences. Future clinical trials targeting stroma could use anti-IL-6 therapy in association with chemotherapy.

Methotrexate Combined with 4-Hydroperoxycyclophosphamide Downregulates Multidrug-Resistance P-Glycoprotein Expression Induced by Methotrexate in Rheumatoid Arthritis Fibroblast-Like Synoviocytes via the JAK2/STAT3 Pathway

Objective

Rheumatoid arthritis (RA) multidrug resistance is associated with P-glycoprotein (P-gp) overexpression. We investigated the effects of methotrexate (MTX) alone and combined with 4-hydroperoxycyclophosphamide (4-HC) on P-gp expression in fibroblast-like synoviocytes (FLSs) from patients with RA and examined the signaling pathway involved.

Methods

RA-FLSs were treated with MTX, MTX + 4-HC, AG490 + MTX, or AG490 + MTX + 4-HC for 72 h. Proliferation inhibition rates were determined by MTT assay; P-gp expression was measured by flow cytometry and real-time polymerase chain reaction (RT-PCR); JAK2 and STAT3 were measured by RT-PCR and cell-based ELISA to assess STAT3 signaling.

Results

MTX alone significantly induced P-gp expression and mRNA production in RA-FLSs. P-gp expression and mRNA levels were lower in the MTX + 4-HC group than in the MTX-alone group. In contrast to MTX, MTX + 4-HC reduced the STAT3 phosphorylation and downregulated JAK2 and STAT3 mRNA production. Inhibition of constitutively active STAT3 accompanied by 4-HC suppressed P-gp levels in RA-FLSs. The MTT assays revealed no significant differences in proliferation inhibition rates among groups.

Conclusions

The increased anti-P-gp effect of MTX + 4-HC versus MTX alone in RA-FLSs was mediated via inhibition of the JAK2/STAT3 pathway and may have helped reverse MDR in refractory RA patients with high-P-gp levels.

The peritoneal "soil" for a cancerous "seed": a comprehensive review of the pathogenesis of intraperitoneal cancer metastases

Various types of tumors, particularly those originating from the ovary and gastrointestinal tract, display a strong predilection for the peritoneal cavity as the site of metastasis. The intraperitoneal spread of a malignancy is orchestrated by a reciprocal interplay between invading cancer cells and resident normal peritoneal cells. In this review, we address the current state-of-art regarding colonization of the peritoneal cavity by ovarian, colorectal, pancreatic, and gastric tumors. Particular attention is paid to the pro-tumoral role of various kinds of peritoneal cells, including mesothelial cells, fibroblasts, adipocytes, macrophages, the vascular endothelium, and hospicells. Anatomo-histological considerations on the pro-metastatic environment of the peritoneal cavity are presented in the broader context of organ-specific development of distal metastases in accordance with Paget's "seed and soil" theory of tumorigenesis. The activity of normal peritoneal cells during pivotal elements of cancer progression, i.e., adhesion, migration, invasion, proliferation, EMT, and angiogenesis, is discussed from the perspective of well-defined general knowledge on a hospitable tumor microenvironment created by the cellular elements of reactive stroma, such as cancer-associated fibroblasts and macrophages. Finally, the paper addresses the unique features of the peritoneal cavity that predispose this body compartment to be a niche for cancer metastases, presents issues that are topics of an ongoing debate, and points to areas that still require further in-depth investigations.

A fast screening model for drug permeability assessment based on native small intestinal extracellular matrix

The Caco-2 cell monolayer model is widely utilized to predict drug permeability across human intestinal epithelial cells. However, at least 21 days is required for the formation and maturation of a well-tight Caco-2 cell monolayer, thereby restricting the throughput of the screening model during drug discovery. To address this challenge, a fast (7 days), and more physiologically relevant screening model integrating both the Caco-2 cell model and a small intestinal submucosa (SIS) hydrogel was developed in this study. The 7 day model exhibited desirable phenotype and functional similarity to the conventional 21 day Caco-2 model with respect to paracellular resistance, alkaline phosphatase (ALP) activities, and the mRNA expression level of three transporters (PEPT1, OATP1A2, and P-gp) as well as their mediated influx or efflux. Besides, the increased gene expression of two excretive transporters (BCRP, MRP2) and their enhanced functionality were observed in the current fast model compared to the traditional 21 day model. More importantly, a strong correlation (r² = 0.9458) was obtained between the absorptive P_app values of 19 model compounds in the 7 day model and those in the conventional 21 day model. These results revealed the pivotal role of the native extracellular matrix (SIS) in facilitating the differentiation of Caco-2 cells, leading to the reconstruction of the accelerated 7 day model, which presents a promising tool for screening drug permeability in future drug discovery.

Application of a native decellularized small intestinal extracellular matrix for the construction of a fast screening model for drug absorption evaluation.

Function of insulin-like growth factor 1 receptor in cancer resistance to chemotherapy

Drug resistance is a primary cause of chemotherapeutic failure; however, how this resistance develops is complex. A comprehensive understanding of chemotherapeutic resistance mechanisms may aid in identifying more effective drugs and improve the survival rates of patients with cancer. Insulin-like growth factor 1 receptor (IGF1R), a member of the insulin receptor family, has been extensively assessed for biological activity, and its putative contribution to tumor cell development and progression. Furthermore, researchers have attended to drugs that target IGF1R since IGF1R functions as a membrane receptor. However, how IGF1R participates in chemotherapeutic resistance remains unclear. Therefore, the present study described the IGF1R gene and its associated signaling pathways, and offered details of IGF1R-induced tumor chemoresistance associated with promoting cell proliferation, inhibition of apoptosis, regulation of ATP-binding cassette transporter proteins and interactions with the extracellular matrix. The present study offered additional explanations for tumor chemotherapy resistance and provided a theoretical basis of IGF1R and its downstream pathways for future possible chemotherapy treatment options.

Pooling-Based Genome-Wide Association Study Identifies Risk Loci in the Pathogenesis of Ovarian Endometrioma in Chinese Han Women

Endometriosis, regarded as a complex disease, is influenced by multiple genetic factors. Recent genome-wide association studies (GWASs) in endometriosis have identified several susceptibility loci in Caucasian and Japanese populations. However, the overlapped susceptible loci were few. This case-control study tried to identify risk loci-related genes for ovarian endometrioma in Chinese Han women from central China using DNA pooling-based GWAS. Genome DNA samples were extracted from 3038 participants in central China. Pooling-based genome-wide scan and individual genotyping were performed using Affymetrix Genome-Wide Human SNP Array 6.0 and IPLEX Gold system, which demonstrated 10 ovarian endometrioma-related novel risk loci. There were 3 of them with P value < 5 × 10^-06, separately locating in intron of insulin-like growth factor 1 receptor, chromosome 7 open reading frame 50, and Meis homeobox 1. In conclusion, the pooling-based GWAS for ovarian endometrioma identified some novel single-nucleotide polymorphisms in Chinese Han women of central China. Further assessment in other samples will be crucial to confirm the susceptibility of these results and explore the mechanisms of the related genes in the pathogenesis of ovarian endometrioma.

Cisplatin-induced CCL5 secretion from CAFs promotes cisplatin-resistance in ovarian cancer via regulation of the STAT3 and PI3K/Akt signaling pathways

Currently, acquired resistance to cisplatin (DDP) is a substantial obstacle to reducing the morbidity and mortality due to ovarian malignant tumors. Nevertheless, cisplatin plays a vital role in killing the tumor cells while it may also be a 'primer' involved in chemotherapy resistance. We found that the cisplatin-induced chemokine (C-C motif) ligand 5 (CCL5) secretion derived from cancer-associated fibroblasts (CAFs) promoted ovarian cancer cell resistance to cisplatin. Via a cytokine chip assay, we identified a spectrum of secreted proteins that were derived from the CAFs through cisplatin-induced treatment. Among these, CCL5 significantly attenuated the cytotoxic effect of cisplatin chemotherapy in vitro and in vivo. Additionally, CCL5 expression was also detected in 62 serous ovarian cancer patient tissue specimens using IHC, and the results demonstrated that chemotherapy resistant patients displayed higher expression of CCL5 than the chemo-sensitive patients (P<0.05). Mechanistically, we found that CCL5 notably increased STAT3 and Akt phosphorylation levels in ovarian cancer cells. These results indicated that cisplatin- induced CCL5 secretion derived from the CAFs may promote cisplatin resistance, which was mediated by regulation of the STAT3 and PI3K/Akt signal pathways.

Inhibition of JAK2 Reverses Paclitaxel Resistance in Human Ovarian Cancer Cells

Objective

Resistance to chemotherapy is a major factor that limits the postsurgical survival of ovarian cancer patients. Janus-activated kinase 2 (JAK2) has been implicated in cancer cell survival and the development of drug resistance in ovarian cancers. In the present study, we sought to determine whether inhibition of JAK2 reverses drug resistance in OC3/TAX300 cells, a paclitaxel-resistant human ovarian cancer cell line previously established in our laboratory.

Methods

OC3/TAX300 cells were transduced with lentivirus expressing small interference RNA (siRNA) against JAK2 and treated with JAK2 kinase inhibitor AG490.

Results

Treatment with JAK2-siRNA markedly decreased the messenger RNA and protein of JAK2 as determined by real-time polymerase chain reaction and Western blot analysis. OC3/TAX300 cells treated with JAK2-siRNA exhibited stalled growth, increased cell cycle arrest in G2/M phase, and enhanced apoptosis in response to paclitaxel. In keeping with this, JAK2-siRNA also inhibited the expression of multidrug resistance protein 1. To determine whether JAK2 promotes paclitaxel resistance via phosphorylation of signal transducer and activator of transcription 3 (STAT3), a transcription factor known to be involved in resistance to chemotherapy, we treated OC3/TAX300 cells with JAK2 kinase inhibitor AG490. Of note, AG490 reduced the level of p-STAT3 and inhibited the expression of multidrug resistance protein 1 in a dose-dependent manner.

Conclusions

Collectively, we conclude that the JAK2-STAT3 pathway promotes the development of paclitaxel resistance via upregulating the expression of prosurvival and antiapoptotic genes. Targeting this pathway may be effective in reversing resistance to chemotherapy in ovarian cancers.

Induction of apoptotic cell death by betulin in multidrug-resistant human renal carcinoma cells

Betulin, a triterpene from the bark of various species of birch tree, has various biological effects, including antiviral, antifungal and anticancer activities. The aim of the present study was to elucidate the mechanisms underlying the apoptotic effect of betulin in RCC4 multidrug-resistant human renal carcinoma cells. To evaluate anticancer activity, we performed cell viability and caspase activity assays, a proteome profiler array and western blot analysis in RCC4 cells. Betulin significantly decreased RCC4 cell viability in a time- and concentration-dependent manner. Betulin activated caspase family proteins, including caspase-3, -7, -8 and -9, and increased the expression of apoptosis-related proteins, including PARP and Bcl-2 family members. In an apoptosis array, betulin activated the tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) receptors TRAIL R1/DR4 and R2/DR5, and tumour necrosis factor receptor 1 (TNFR1), suggesting that betulin treatment leads to induction of apoptosis through both intrinsic and extrinsic apoptosis pathways in RCC4 cells. Notably, betulin significantly enhanced cytotoxicity and PARP cleavage in etoposide-treated RCC4 cells, and downregulated the expression of multidrug resistance protein 1 (MDR1). Taken together, our findings suggest that the anticancer effects of betulin involve induction of apoptosis and sensitisation of RCC4 cells, providing potentially useful information applicable to the use of betulin in renal cancer treatment.

Quercetin induces endoplasmic reticulum stress to enhance cDDP cytotoxicity in ovarian cancer: involvement of STAT3 signaling

There is an urgent need to make cisplatin (cDDP) more effective and less toxic in the treatment of ovarian cancer for its systemic side effects and high resistance rate. In this study, we investigated the effect of quercetin (Qu) pretreatment on the potentiation of cDDP in ovarian cancer. We found that Qu pretreatment significantly enhanced cDDP cytotoxicity in an ovarian cancer cell line and primary cancer cells. In addition, we demonstrated that Qu elicited obvious endoplasmic reticulum stress (ERS) and activated all three branches of ERS in ovarian cancer. Specific inhibitors of each ERS pathway, as well as the general ERS stabilizer tauroursodeoxycholic acid, notably diminished such enhancing effects. Furthermore, Qu notably suppressed STAT3 phosphorylation, leading to downregulation of the BCL-2 gene downstream of STAT3. Moreover, blocking ERS restored the protein levels of phosphorylated STAT3 as well as BCL-2 expression, thus abolishing the chemosensitization potency of Qu; these results revealed that Qu affected the STAT3 pathway to enhance cDDP cytotoxicity, and this effect involved ERS signaling. In a xenograft mouse model of ovarian cancer, Qu enhanced the antitumor effect of cDDP. Tumors from mice treated with cDDP in combination with Qu pretreatment had repressed STAT3 phosphorylation, lower BCL-2 and higher apoptosis levels compared with those from the other groups. Meanwhile, Qu markedly reduced the elevation of blood creatinine during cDDP intervention. These data indicate that Qu pretreatment potentiates the antitumor effects of cDDP in ovarian cancer while protecting the kidneys against damage. Therefore the strategy of Qu pretreatment may be beneficial in enhancing the therapeutic efficacy of cDDP against ovarian cancer.

Metabolic orchestration between cancer cells and tumor microenvironment as a co-evolutionary source of chemoresistance in ovarian cancer: a therapeutic implication

Our group reported a significant association between hexokinase II overexpression and chemoresistance in ovarian cancer, suggesting that aerobic glycolysis in the so-called Warburg effect might contribute to cancer progression. However, a growing body of evidence indicates contradictory findings with regard to the Warburg effect, such as high mitochondrial activity in highly invasive tumors and low ATP contribution of glycolysis in ovarian cancer. As a solution for the dilemma of the Warburg effect, the "reverse Warburg effect" was proposed in which aerobic glycolysis might occur in the stromal compartment of the tumor rather than in the cancer cells, indicating that the glycolytic tumor stroma feed the cancer cells through a type of symbiotic relationship. The reverse Warburg effect acting on the relationship between cancer cells and cancer-associated fibroblasts has evolved into dynamic interplay between cancer cells and multiple tumor stromal compartments, including cancer-associated fibroblasts, the extracellular matrix, endothelial cells, mesenchymal stem cells, adipocytes, and tumor-associated macrophages. Peritoneal cavities including ascites and the omentum also form a unique environment that is highly receptive for carcinomatosis in the advanced stages of ovarian cancer. The complicated but ingeniously orchestrated stroma-mediated cancer metabolism in ovarian cancer provides great heterogeneity in tumors with chemoresistance, which makes the disease thus far difficult to cure by single stromal-targeting agents. This review will discuss the experimental and clinical evidence of the cross-talk between cancer cells and various components of tumor stroma in terms of heterogeneous chemoresistance with focal points for therapeutic intervention in ovarian cancer.

A subset of bone marrow stromal cells regulate ATP-binding cassette gene expression via insulin-like growth factor-I in a leukemia cell line

The importance of the insulin-like growth factor, IGF, as a signaling axis in cancer development, progression and metastasis is highlighted by its effects on cancer cells, notably proliferation and acquired resistance. The role of the microenvironment within which cancer cells evolve and which mediates this effect is far from clear. Here, the involvement of IGF-I in inducing multidrug resistance in a myeloid leukemia cell line, grown in the presence of bone marrow-derived stromal cells called 'Hospicells' (BMH), is demonstrated. We found that i) drug sensitive as well as resistant leukemia cells express IGF-I and its receptor IGF-IR. However, the resistant cells were found to secrete high levels of IGF-I. ii) Presence of exogenous IGF-I promoted cell proliferation, which decreased when an inhibitor of IGF-IR (picropodophyllin, PPP) was added. iii) BMH and IGF-I are both involved in the regulation of genes of the ATP binding cassette (ABC) related to resistance development (MDR1, MRP1, MRP2, MRP3 and BCRP). iv) The levels of ABC gene expression by leukemia cells were found to increase in the presence of increasing numbers of BMH. However, these levels decreased when IGF-IR was inhibited by addition of PPP. v) Co-culture of the drug-sensitive leukemia cells with BMH induced protection against the action of daunorubicin. This chemoresistance was amplified by the presence of IGF-I whereas it decreased when IGF-IR was inhibited. Our results underline the role of microenvironment in concert with the IGF-1 pathway in conferring drug resistance to leukemia cells.

CEP-33779 antagonizes ATP-binding cassette subfamily B member 1 mediated multidrug resistance by inhibiting its transport function

The overexpression of ATP-binding cassette (ABC) transporters often leads to the development of multidrug resistance (MDR), which is the major factor contributing to the failure of chemotherapy. The objective of this study was to investigate the enhancement of CEP-33779, a small-molecule inhibitor of Janus kinase 2 (JAK2), on the efficacy of conventional chemotherapeutic agents in MDR cells with overexpression of P-glycoprotein (ABCB1), multidrug resistance-associated protein 1 (ABCC1) and breast cancer resistance protein (ABCG2). Our results showed that CEP-33779, at nontoxic concentrations, significantly sensitized ABCB1 overexpressing MDR cells to its anticancer substrates. CEP-33779 significantly increased intracellular accumulation and decreased the efflux of doxorubicin by inhibiting the ABCB1 transport function. Furthermore, CEP-33779 did not alter the expression of ABCB1 both at protein and mRNA levels but did stimulate the activity of ABCB1 ATPase. CEP-33779 was predicted to bind within the large hydrophobic cavity of homology modeled ABCB1. In addition, the down-regulation of JAK2 by shRNA altered neither the expression of ABCB1 nor the cytotoxic effect of chemotherapeutic agents in ABCB1-overexpressing cells. Significantly, CEP-33779 enhanced the efficacy of vincristine against the ABCB1-overexpressing and drug resistant KBv200 cell xenograft in nude mice. In conclusion, we conclude that CEP-33779 enhances the efficacy of substrate drugs in ABCB1-overexpressing cells by directly inhibiting ABCB1 transport function. The findings encouraged to further study on the combination therapy of CEP-33779 with conventional chemotherapeutic agents in ABCB1 mediated-MDR cancer patients.