Molecular basis of antifolate resistance

Sensitivity and resistance towards isoliquiritigenin, doxorubicin and methotrexate in T cell acute lymphoblastic leukaemia cell lines by pharmacogenomics

The development of drug resistance in cancer cells necessitates the identification of novel agents with improved activity towards cancer cells. In the present investigation, we compared the cytotoxicity of the chalcone flavonoide, isoliquiritigenin (ISL), with that of doxorubicin (DOX) and methotrexate (MTX) in five T cell acute lymphoblastic leukaemia (T-ALL) cell lines (Jurkat, J-Jhan, J16, HUT78 and Karpas 45). To gain insight into the molecular mechanisms which determine the response of T-ALL cells towards ISL, DOX and MTX, we applied array-based matrix comparative genomic hybridisation and microarray-based mRNA expression profiling and compared the genomic and transcriptomic profiles of the cell lines with their 50% inhibition (IC(50)) values for these three drugs. The IC(50) values for ISL did not correlate with those for DOX or MTX, indicating that ISL was still active in DOX- or MTX-unresponsive cell lines. Likewise, the genomic imbalances of chromosomal clones and mRNA expression profile significantly correlating with IC(50) values for ISL were different from thoses correlating with IC(50) values for DOX and MTX. In conclusion, ISL represents a cytotoxic natural product with activity towards T-ALL cell lines. There was no cross-resistance between ISL and DOX or MTX, and the genomic and transcriptomic profiles pointed to different molecular modes of action of ISL as compared to DOX and MTX, indicating that ISL may be a valuable adjunct for cancer therapy to treat otherwise drug-resistant tumours.

Targeting the proton-coupled folate transporter for selective delivery of 6-substituted pyrrolo[2,3-d]pyrimidine antifolate inhibitors of de novo purine biosynthesis in the chemotherapy of solid tumors

The proton-coupled folate transporter (PCFT) is a folate-proton symporter with an acidic pH optimum, approximating the microenvironments of solid tumors. We tested 6-substituted pyrrolo[2,3-d]pyrimidine antifolates with one to six carbons in the bridge region for inhibition of proliferation in isogenic Chinese hamster ovary (CHO) and HeLa cells expressing PCFT or reduced folate carrier (RFC). Only analogs with three and four bridge carbons (N-{4-[3-2-amino-4-oxo-4,7-dihydro-3H-pyrrolo[2,3-d]-pyrimidin-6-yl)propyl]benzoyl}-L-glutamic acid (compound 2) and N-{4-[4-2-amino-4-oxo-4,7-dihydro-3H-pyrrolo[2,3-d]-pyrimidin-6-yl)butyl]benzoyl}*-L-glutamic acid (compound 3), respectively) were inhibitory, with 2 ≫ 3. Activity toward RFC-expressing cells was negligible. Compound 2 and pemetrexed (Pmx) competed with [(3)H]methotrexate for PCFT transport in PCFT-expressing CHO (R2/hPCFT4) cells from pH 5.5 to 7.2; inhibition increased with decreasing pH. In Xenopus laevis oocytes microinjected with PCFT cRNA, uptake of 2, like that of Pmx, was electrogenic. Cytotoxicity of 2 toward R2/hPCFT4 cells was abolished in the presence of adenosine or 5-amino-4-imidazolecarboxamide, suggesting that glycinamide ribonucleotide formyltransferase (GARFTase) in de novo purine biosynthesis was the primary target. Compound 2 decreased GTP and ATP pools by ∼50 and 75%, respectively. By an in situ GARFTase assay, 2 was ∼20-fold more inhibitory toward intracellular GARFTase than toward cell growth or colony formation. Compound 2 irreversibly inhibited clonogenicity, although this required at least 4 h of exposure. Our results document the potent antiproliferative activity of compound 2, attributable to its efficient cellular uptake by PCFT, resulting in inhibition of GARFTase and de novo purine biosynthesis. Furthermore, they establish the feasibility of selective chemotherapy drug delivery via PCFT over RFC, a process that takes advantage of a unique biological feature of solid tumors.

Baiting proteins with C60

About 20 proteins are known to modify their activity upon interaction with C60. Their structures are present in a database that includes more than 1200 protein structures selected as possible targets for drugs and to represent the entire Protein Data Bank. The set was examined with an algorithm that appraises quantitatively the interaction of C60 and the surface of each protein. The redundancy of the set allows to establish the predictive power of the approach that finds explicitly the most probable site where C60 docks on each protein. About 80% of the known fullerene binding proteins fall in the top 10% of scorers. The close match between the model and experiments vouches for the accuracy of the model and validates its predictions. The sites of docking are shown and discussed in view of the existing experimental data available for protein-C60 interaction. A closer exam of the 10 top scorers is discussed in detail. New proteins that can interact with C60 are identified and discussed for possible future applications as drug targets and fullerene derivatives bioconjugate materials.

Polymorphisms of the gamma-glutamyl hydrolase gene and risk of relapse to acute lymphoblastic leukemia in Mexico

This study evaluated the association of -401C/T and +452C/T polymorphisms of gamma-glutamyl hydrolase and the risk of relapse to acute lymphoblastic leukemia. Genotyping was performed in 70 children with acute lymphoblastic leukemia and 140 healthy children. An association between the -401C/T polymorphism and the risk of relapse was found (p=0.028), patients with the -401T/T genotype have 10.83 (95% CI 1.30-90.14) more chance of a relapse of leukemia. No association was found between the +452C/T polymorphism and the risk of relapse. Therefore, our investigation suggests that the -401C/T polymorphism in the gamma-glutamyl hydrolase may be a factor involved in the generation of relapse to disease in patients with ALL.

Monoketocholate can decrease transcellular permeation of methotrexate across Caco-2 cell monolayers and reduce its intestinal absorption in rat

Objectives

Bile salts have been shown to decrease the absorption of methotrexate in the rat intestine by an unknown mechanism. We aimed to examine this effect.

Methods

We assessed apical-to-basolateral (AP-BL) permeation of methotrexate (5 μM) across Caco-2 cell monolayers pretreated with various concentrations (0, 0.25, 0.5, 1, 3 and 5 mm) of sodium cholate or its semisynthetic analogue, sodium 12-monoketocholate. We also determined the effect of orally administered 12-monoketocholate on the intestinal absorption of methotrexate in rats to evaluate a possible in-vitro–in-vivo correlation.

Key findings

It was found that sodium cholate and sodium 12-monoketocholate decreased the AP-BL permeation of methotrexate at low concentrations (maximal inhibition at 0.25 and 1 mm, respectively) and increased it at higher concentrations. Determination of [14C] mannitol permeation and electrical resistance of monolayers during experiments showed that membrane integrity was not compromised at low concentrations of bile salts but was disrupted at higher concentrations. Subsequently, we examined the effect of the simultaneous oral administration of sodium 12-monoketocholate (4, 20, 40 and 80 mg/kg) on the intestinal absorption of methotrexate in rats after an oral dose (5 mg/kg). The pharmacokinetic study showed that 12-monoketocholate at 4 and 20 mg/kg did not change the methotrexate area under the serum concentration–time curve whereas sodium 12-monoketocholate at 40 and 80 mg/kg significantly reduced it.

Conclusions

Sodium 12-monoketocholate appears to decrease the intestinal absorption of methotrexate in rats by inhibition of transcellular active transport.

Impact of breast cancer resistance protein on cancer treatment outcomes

Breast cancer resistance protein (BCRP/ABCG2) was discovered in multidrug resistant breast cancer cells having an ATP-dependent transport-based resistance phenotype. This ABC transporter functions (at least in part) as a xenobiotic protective mechanism for the organism: in the gut and biliary tract, it prevents absorption and enhances elimination of potentially toxic substances. As a placental barrier, it protects the fetus; similarly, it serves as a component of blood-brain and blood-testis barrier; BCRP is expressed in stem cells and may protect them from potentially harmful agents. Therefore, BCRP could influence cancer outcomes by (a) endogenous BCRP affecting the absorption, distribution, metabolism, and elimination of anticancer drugs; (b) BCRP expression in cancer cells may directly cause resistance by active efflux of anticancer drugs; (c) BCRP expression in cancer cells could be a manifestation of the activity of metabolic and signaling pathways that impart multiple mechanisms of drug resistance, self-renewal (stemness), and invasiveness (aggressiveness)--i.e. impart a poor prognosis--to cancers. This chapter presents a synopsis of translational clinical studies relating BCRP expression in leukemias, lymphomas, and a variety of solid tumors with clinical outcome. Data are emerging that expression of BCRP, like P-glycoprotein/ABCB1, is associated with adverse outcomes in a variety of human cancers. Whether this adverse prognostic effect results from resistance imparted to the cancer cells as the direct result of BCRP efflux of anticancer drugs, or whether BCRP expression (and also Pgp expression - coexpression of these transporters is common among poor risk cancers) serves as indicators of the activity of signaling pathways that enhance cancer cellular proliferation, metastases, genomic instability, enhance drug resistance, and oppose programmed cell death mechanisms is yet unknown.

Significance of thymidylate synthase for resistance to pemetrexed in lung cancer

Pemetrexed (MTA) is a multitargeted antifolate with promising clinical activity in lung cancer. We exposed the small cell lung cancer cell line PC6 to stepwise-increasing pemetrexed concentrations of 0.4, 1.6, and 4.0 microm, and established three pemetrexed-resistant lung cancer cell lines: PC6/MTA-0.4, PC6/MTA-1.6, and PC6/MTA-4.0 cells. To investigate the mechanisms of acquired resistance to pemetrexed, we measured the expression levels of the thymidylate synthase (TS), reduced folate carrier (RFC), and folylpoly-gamma-glutamate synthetase (FPGS) genes. TS gene expression was significantly increased in PC6/MTA-1.6 and PC6/MTA-4.0 cells relative to parental cells in a pemetrexed dose-dependent manner. In contrast, the levels of RFC gene expression in PC6/MTA-0.4 cells and FPGS in PC6/MTA-1.6 cells were significantly decreased, whereas the levels of both genes were restored in PC6/MTA-4.0 cells. Knockdown of TS expression using siRNA enhanced pemetrexed cytotoxicity in PC6/MTA-4.0 cells. The expression level of the TS gene was significantly correlated with the concentration of pemetrexed for 50% cell survival (IC(50)) in 11 non-small cell lung cancer cell lines. These results suggest that the alteration of molecular pharmacological factors in relation with pemetrexed resistance is dose-dependent, and that up-regulation of the expression of the TS gene may have an important role in the acquired resistance to pemetrexed. In addition, TS may be a predictive marker for pemetrexed sensitivity in lung cancer.

Mechanisms of multidrug resistance in cancer

The development of multidrug resistance (MDR) to chemotherapy remains a major challenge in the treatment of cancer. Resistance exists against every effective anticancer drug and can develop by numerous mechanisms including decreased drug uptake, increased drug efflux, activation of detoxifying systems, activation of DNA repair mechanisms, evasion of drug-induced apoptosis, etc. In the first part of this chapter, we briefly summarize the current knowledge on individual cellular mechanisms responsible for MDR, with a special emphasis on ATP-binding cassette transporters, perhaps the main theme of this textbook. Although extensive work has been done to characterize MDR mechanisms in vitro, the translation of this knowledge to the clinic has not been crowned with success. Therefore, identifying genes and mechanisms critical to the development of MDR in vivo and establishing a reliable method for analyzing clinical samples could help to predict the development of resistance and lead to treatments designed to circumvent it. Our thoughts about translational research needed to achieve significant progress in the understanding of this complex phenomenon are therefore discussed in a third section. The pleotropic response of cancer cells to chemotherapy is summarized in a concluding diagram.

DNA variants in region for noncoding interfering transcript of dihydrofolate reductase gene and outcome in childhood acute lymphoblastic leukemia

PURPOSE

Dihydrofolate reductase (DHFR) is the major target of methotrexate, a key component in childhood acute lymphoblastic leukemia (ALL) treatment. We recently reported an association of DHFR promoter polymorphisms with ALL outcome. Lower event-free survival correlated with haplotype *1, defined by A(-317) and C(-1610) alleles. Haplotype *1 was also associated higher DHFR expression.

EXPERIMENTAL DESIGN

Here, we analyzed adjacent 400-bp region participating in DHFR regulation as both a major promoter and a noncoding minor transcript.

RESULTS

Six polymorphisms were identified, of which five were single nucleotide polymorphisms and one was length polymorphism composed of variable number of 9-bp elements and 9-bp insertion/deletion. Haplotype analysis including all promoter polymorphisms revealed diversification of haplotype *1 into five subtypes (*1a-*1e). DNA variations of major promoter/noncoding transcript region and haplotype *1 subtypes were subsequently analyzed for the association with ALL outcome. Lower event-free survival was associated with an A allele of G(308)A polymorphism (P = 0.02) and with *1b haplotype (P = 0.01). This association was particularly striking in high-risk patients (P = 0.001) and was subsequently confirmed in independent patient cohort (P = 0.02). Haplotype *1b was the only haplotype *1 subtype associated with higher mRNA levels.

CONCLUSIONS

The study provides a new insight into DHFR regulatory variations predisposing to an event in ALL patients.

Distinct mechanistic activity profile of pralatrexate in comparison to other antifolates in in vitro and in vivo models of human cancers

PURPOSE

This study evaluated mechanistic differences of pralatrexate, methotrexate, and pemetrexed.

METHODS

Inhibition of dihydrofolate reductase (DHFR) was quantified using recombinant human DHFR. Cellular uptake and folylpolyglutamate synthetase (FPGS) activity were determined using radiolabeled pralatrexate, methotrexate, and pemetrexed in NCI-H460 non-small cell lung cancer (NSCLC) cells. The tumor growth inhibition (TGI) was assessed using MV522 and NCI-H460 human NSCLC xenografts.

RESULTS

Apparent K ( i ) values for DHFR inhibition were 45, 26, and >200 nM for pralatrexate, methotrexate, and pemetrexed, respectively. A significantly greater percentage of radiolabeled pralatrexate entered the cells and was polyglutamylatated relative to methotrexate or pemetrexed. In vivo, pralatrexate showed superior anti-tumor activity in both NSCLC models, with more effective dose-dependent TGI in the more rapidly growing NCI-H460 xenografts.

CONCLUSIONS

Pralatrexate demonstrated a distinct mechanistic and anti-tumor activity profile relative to methotrexate and pemetrexed. Pralatrexate exhibited enhanced cellular uptake and increased polyglutamylation, which correlated with increased TGI in NSCLC xenograft models.

Effects of extracellular purines on cytotoxicity of methotrexate

PURPOSE

Nucleoside and base modulation of the cytotoxicity of nucleic acid and folate antimetabolite drugs has been widely discussed. Many investigators have observed reduced toxicity due to circumvention of drug-induced inhibition of de novo purine and pyrimidine synthesis. However, exogenous purine nucleosides and bases may also enhance the cytotoxicity of even moderate concentrations of antifolate drugs (MTX and PTX) which inhibit dihydrofolate reductase. In this study, the effects of nucleosides in the medium on the cytotoxicity and deoxyribonucleoside triphosphate pools after brief exposure of cultured cells to methotrexate have been studied in cultured L1210 murine leukaemia cells.

METHODS

Cell viability was determined by trypan blue exclusion assay. Colony formation was assessed by microtitration cloning assay. The deoxyribonucleotides were measured by a modification of the DNA polymerase assay. Purines were extracted with trioctylamine and 1,1,2-trichlorotrifluoroethane buffer and concentrations of purine bases were determined by HPLC.

RESULTS

Subculture of drug-treated cells in fresh medium containing 10% FCS led to greater toxicity than sub culture in 'conditioned' medium, i.e. fresh medium in which logarithmically growing cells had been cultured for 24 h before separation. Cells resuspended in fresh medium had increased dATP and sustained inhibition of dTTP levels, while cells subcultured in 'conditioned' medium had no elevation of dATP. Hypoxanthine concentration determined by HPLC in 'conditioned' medium was 0.9 microM compared to 6.7 microM in fresh medium. Resuspension of drug-treated cells in conditioned medium supplemented with 10 or 100 microM HX enhanced cytotoxicity and increased the dATP levels.

CONCLUSION

These results add further evidence that purines present in normal culture conditions are important determinants of methotrexate cytotoxicity. Elevation of dATP levels after methotrexate treatment is an important modulator of cytotoxicity.

Design, synthesis, and X-ray crystal structure of classical and nonclassical 2-amino-4-oxo-5-substituted-6-ethylthieno[2,3-d]pyrimidines as dual thymidylate synthase and dihydrofolate reductase inhibitors and as potential antitumor agents

N-{4-[(2-Amino-6-ethyl-4-oxo-3,4-dihydrothieno[2,3-d]pyrimidin-5-yl)thio]benzoyl}-L-glutamic acid 2 and 13 nonclassical analogues 2a-2m were synthesized as potential dual thymidylate synthase (TS) and dihydrofolate reductase (DHFR) inhibitors and as antitumor agents. The key intermediate in the synthesis was 2-amino-6-ethyl-5-iodothieno[2,3-d]pyrimidin-4(3H)-one, 7, to which various arylthiols were attached at the 5-position. Coupling 8 with L-glutamic acid diethyl ester and saponification afforded 2. X-ray crystal structures of 2 and 1 (the 6-methyl analogue of 2), DHFR, and NADPH showed for the first time that the thieno[2,3-d]pyrimidine ring binds in a "folate" mode. Compound 2 was an excellent dual inhibitor of human TS (IC50 = 54 nM) and human DHFR (IC50 = 19 nM) and afforded nanomolar GI50 values against tumor cells in culture. The 6-ethyl substitution in 2 increases both the potency (by 2-3 orders of magnitude) as well as the spectrum of tumor inhibition in vitro compared to the 6-methyl analogue 1. Some of the nonclassical analogues were potent and selective inhibitors of DHFR from Toxoplasma gondii.

Identification and characterization of a novel CXC chemokine in xenograft tumor induced by mas-overexpressing cells

Overexpressions of G protein-coupled receptor (GPCR) with elevated downstream signaling events have been reported in various tumors. However, the cellular mechanism that GPCR overexpression leads to tumor formation is largely unknown. The orphan GPCR mas was originally isolated from a human epidermoid carcinoma. In vivo studies of mas-overexpressing cells suggested that xenograft tumor formation was positively correlated with the levels of mas expression. Histochemical analysis indicated that xenograft tumor consisted of mas-transfected and stromal cells. Biochemical analyses revealed that cells overexpressing mas exhibited significantly increased anchorage-independent growth, whereas there was no significant difference in cell proliferation in comparison with empty vector-transfected control cells. Expression profiling using mRNA differential display and Northern analysis indicated an elevated expression of GRO and a novel CXC chemokines, tumor-induced factor (TIF), in mas-transfected cells and xenograft tumor. Bacterially expressed recombinant TIF was found to act as a neutrophil chemoattractant in a chemotactic assay. These results suggest that mas overexpression enables anchorage-independent growth of transformed cells, and interplays of secreted chemokines with stromal cells modulate xenograft tumor formation. Importantly, a novel CXC chemokine, TIF, was identified in the xenograft tumor tissues.

The relationship between response to previous systemic treatment and the efficacy of subsequent pemetrexed therapy in advanced non-small cell lung cancer

BACKGROUND

We sought to identify the relationship between response to previous systemic treatment and the efficacy of subsequent pemetrexed therapy in advanced non-small cell lung cancer (NSCLC).

PATIENTS AND METHODS

Two hundred and fifty clinical stage IIIB or IV NSCLC patients treated with pemetrexed as a second-line or further-line treatment between April 2007 and June 2008 were analyzed retrospectively. Prior therapies were divided into four types (gemcitabine-based [G], paclitaxel-based [P], docetaxel-based [D], and EGFR tyrosine kinase inhibitor [I]). Objective response rates (ORR) and progression-free survivals (PFS) for pemetrexed therapy were analyzed according to the response outcome with each previous treatment.

RESULTS

The ORR of pemetrexed therapy was higher for patients who had achieved partial response with previous [G] therapy than others (15.0% vs. 4.3%, p=0.02). In addition, median PFS for pemetrexed therapy was greater for responders to [G] than for nonresponders (3.0 months vs. 1.7 months, p=0.004). The longer PFS for responders to [G] was also shown in the analysis among patients with squamous cell carcinoma (3.2 months vs. 1.7 months, p=0.056). By univariate analyses, the variables of the responder to [G] therapy, female, adenocarcinoma, never smoking status, and ECOG performance status of 0-1 were good predictive factors for pemetrexed therapy in terms of PFS. Multivariate analysis revealed that only response to [G] had statistical significance (hazard ratio=0.62, p=0.006).

CONCLUSION

Response outcome to prior [G] therapy might predict the efficacy of subsequent pemetrexed therapy in advanced NSCLC.

Melanoma activation of 3-o-(3,4,5-trimethoxybenzoyl)-(-)-epicatechin to a potent irreversible inhibitor of dihydrofolate reductase

Human melanoma is a significant clinical problem because it is resistant to treatment by most chemotherapeutic agents, including antifolates. It is therefore a desirable goal to develop a second generation of low-toxicity antifolate drugs to overcome acquired resistance to the prevention and treatment of this skin pathology. In our efforts to improve the stability and bioavailability of green tea polyphenols for cancer therapy, we synthesized a trimethoxy derivative of epicatechin-3-gallate, which showed high antiproliferative and proapoptotic activity against melanoma. This derivative, 3-O-(3,4,5-trimethoxybenzoyl)-(-)-epicatechin (TMECG), is a prodrug that is selectively activated by the specific melanocyte enzyme tyrosinase. Upon activation, TMECG generates a stable quinone methide product that strongly inhibits dihydrofolate reductase in an irreversible manner. The treatment of melanoma cells with TMECG also affected cellular folate transport and the gene expression of DHFR, which supported the antifolate nature of this compound. In addition, its pharmacological efficacy has been confirmed in a mouse melanoma model, in which tumor growth and metastasis were inhibited, significantly enhancing the mean survival of the treated groups. TMECG, therefore, shows a potential for clinical use in melanoma therapy.

Expression and alternative splicing of folate pathway genes in HapMap lymphoblastoid cell lines

AIM

Folate is vital for cell growth and development through its important role in one-carbon metabolism - an essential process in the synthesis of amino acids and nucleic acids. Folate pathway genes have been considered as therapeutic targets of drugs for the treatment of cancer and other diseases. Racial and ethnic disparities of folate metabolism and outcome of antifolate therapies have been reported. In this study, we evaluate the genetic regulation for expression and alternative splicing of folate related genes in HapMap lymphoblastoid cell lines (LCLs) of individuals of European and African descent.

MATERIALS & METHODS

Gene and exon level expression and alternative splicing of folate pathway genes were compared in LCLs derived from the Centre d'Etude du Polymorphisme Humain (CEPH) from Utah (CEU) and the Yoruba from Ibadan (YRI) using a permutation-based test. A genome-wide association study was performed to search for SNPs associated with folate pathway gene expressions and alternative splicing in the combined population samples.

RESULTS

A total of 52 folate pathway genes were evaluated in the analysis of which 46 were expressed in the LCLs. There were 12 genes (26%) with differential gene-level expression and 23 genes (50%) with differential alternative splicing for exons or UTRs between the CEU and the YRI (permutation p <or= 0.05). The expression level of FPGS and the splicing indices of eight genes (ATP13A2, ASCC3L1, IFIH1, SMARCA5, SMARCA2, SETX, DDX52 and RUVBL2) were found to be associated with SNP genotypes in the combined populations (p < 3.2 x 10(-8), Bonferroni corrected p < 0.05).

CONCLUSION

Our study suggests that LCLs are an in vitro system suitable to evaluate the expression levels of folate pathway genes. The differential transcript-level expressions and the differentially alternative splicing events of exons or UTRs and associated SNP markers in 2 populations will enhance our understanding of the folate pathway and, thus, facilitate research in the areas of nutrition and folate metabolism.

Functional elements in the minimal promoter of the human proton-coupled folate transporter

The proton-coupled folate transporter (PCFT) is the dominant intestinal folate transporter, however, its promoter has yet to be revealed. Hence, we here cloned a 3.1kb fragment upstream to the first ATG of the human PCFT gene and generated sequential deletion constructs evaluated in luciferase reporter assay. This analysis mapped the minimal promoter to 157bp upstream to the first ATG. Crucial GC-box sites were identified within the minimal promoter and in its close vicinity which substantially contribute to promoter activity, as their disruption resulted in 94% loss of luciferase activity. We also identified upstream enhancer elements including YY1 and AP1 which, although distantly located, prominently transactivated the minimal promoter, as their inactivation resulted in 50% decrease in reporter activity. This is the first functional identification of the minimal PCFT promoter harboring crucial GC-box elements that markedly contribute to its transcriptional activation via putative interaction with distal YY1 and AP1 enhancer elements.

A phase II study of pemetrexed as second-line chemotherapy for the treatment of metastatic castrate-resistant prostate cancer (CRPC); Hoosier Oncology Group GU03-67

BACKGROUND

No standard therapy exists for post-docetaxel castrate-resistant prostate cancer (CRPC) patients. This trial aimed to determine the safety and efficacy of pemetrexed in post-docetaxel CRPC patients.

MATERIALS AND METHODS

CRPC patients with progression after docetaxel (Taxotere) therapy received pemetrexed (500 mg/m2) i.v. every 3 weeks. The primary end point was prostate-specific antigen (PSA) response. A pharmacogenetic analysis of the reduced folate carrier-1 gene (RFC1) G80A polymorphism was also carried out.

RESULTS

Forty-nine patients were enrolled: median age 68 years, median baseline PSA 72 ng/ml, and median Karnofsky performance status of 90. Grade 3 or 4 toxicity occurred in 20 (43%) and four patients (8%), respectively. Confirmed >50% PSA decline occurred in four patients (8%), stable PSA lasting at least 12 weeks in 10 patients (20%). A significant relationship was observed between time from prior docetaxel therapy and overall survival. Pharmacogenetic analyses of RFC1 G80A genotype frequencies showed no relationship between genotypes and clinical efficacy.

CONCLUSIONS

Pemetrexed treatment of CRPC patients after docetaxel therapy was associated with only modest clinical activity. Further investigation of pemetrexed as a single agent in a nonenriched CRPC population is unlikely to add significant clinical benefit over that seen with traditional second-line chemotherapy agents such as mitoxantrone.

Pralatrexate, a new hope for aggressive T-cell lymphomas?

Aggressive T-cell lymphomas represent a particularly poor-prognosis subgroup of lymphomas. This is especially true for patients with recurrent or refractory disease who typically have a limited response to salvage therapy and an extremely poor overall survival. There is thus a strong need to develop potentially active drugs for these malignancies. Pralatrexate is a novel antifolate designed to have high affinity for the reduced folate carrier type 1. Preclinical and clinical studies have demonstrated that pralatrexate has significant activity against T-cell lymphomas.The dose-limiting toxicity for pralatrexate is mucositis,which could be abrogated with folic acid and vitamin B12 supplementation. Pralatrexate is now being evaluated in phase II clinical trials for the treatment of peripheral T-cell lymphoma, and in a phase I/II trial in combination with gemcitabine for the treatment of non-Hodgkin's lymphoma. Because of the limited therapies available for aggressive T-cell lymphoma, pralatrexate could secure a niche for the treatment of this condition, provided on going clinical trials and future phase III trials confirm the efficacy of the drug.

Tailoring treatment of nonsmall cell lung cancer by tissue type: role of pemetrexed

Pemetrexed (ALIMTA, LY231514, MTA) is a novel multitargeted antifolate that is currently approved for the treatment of metastatic nonsmall cell lung cancer (NSCLC). Recent evidence reveals that the drug's efficacy is limited to nonsquamous lung cancer histology. As we further understand the drug's mechanisms of action, new genomic and proteomic evidence is shedding light on why some patients respond while others do not. The first goal of this review is to briefly review pemetrexed's mechanism of action, resistance patterns, toxicity profile, and pharmacokinetics. We will also review the clinical trials that led to its use in NSCLC, with special attention to data showing that pemetrexed has greater efficacy in nonsquamous histologies of NSCLC. Furthermore, we will discuss the hypotheses for the genomic and proteomic basis for this variation in efficacy. Finally, we will report the future directions for pemetrexed as a personalized agent for nonsquamous NSCLC.

Involvement of gene polymorphisms of the folate pathway enzymes in gene expression and anticancer drug sensitivity using the NCI-60 panel as a model

Folate, a vitamin of the B group involved in one-carbon group metabolism, plays an important role in DNA synthesis and methylation. Several polymorphisms in the genes involved in folate uptake and biotransformations have been shown to be associated to the risk of cancer and to anticancer drug response. We studied common polymorphisms in MTHFR (N(5,10)-methylene-tetrahydrofolate reductase), MTHFD1 (N(5,10)-methylene-tetrahydrofolate dehydrogenase), MTR (methionine synthetase) and SLC19A1 (reduced folate carrier) in the panel of 60 human tumour cell lines established by the NCI for anticancer drug screening and we tentatively associated these polymorphisms with gene expression and drug cytotoxicity as extracted from the public database of the Developmental Therapeutic Programme. We observed a consistent and highly significant association between the presence of the variant C allele of the A>C1298 polymorphism of MTHFR and the sensitivity to many anticancer drugs belonging to the classes of antifolates, antimetabolites, alkylating agents and, to a lesser extent, topoisomerase inhibitors. In contrast, the T variant allele of the C>T677 variation of MTHFR was rather associated to lower sensitivity of the cell lines towards anticancer drugs (alkylating agents, antifolates and antimetabolites) but with much lower effects than the A>C1298 variation. The polymorphisms of the other genes studied were not associated with differences in anticancer drug sensitivity, but the expression of the SLC19A1 gene was significantly correlated with the sensitivity to several drugs (antifolates, thiopurines, nitrosoureas, and DACH-platinum drugs). We concluded that the NCI-60 panel may constitute a good starting point for implementing clinical studies aimed at discovering and validating predictive genetic markers of drug efficacy and/or toxicity.

Methotrexate-induced senescence in human adenocarcinoma cells is accompanied by induction of p21(waf1/cip1) expression and lack of polyploidy

Human colorectal adenocarcinoma C85 cells, treated with high dose methotrexate (1 microM; IC(50)=51 nM), undergo accelerated senescence, as the cells (i) are growth arrested at the G(1) and S phases of the cell cycle, (ii) are SA-beta-galactosidase-positive, (iii) show induced expression of p21(waf1/cip1) and decreased expression of p16(INK4a), and (iv) show DNA synthesis continued at the reduced level. The fraction of C85 cells with DNA content higher than 4N is maintained at the same level (14%) in cells untreated, as well as regrown after the treatment. Multinucleation is found as the main karyotypic abnormality.

Methotrexate in pediatric osteosarcoma: response and toxicity in relation to genetic polymorphisms and dihydrofolate reductase and reduced folate carrier 1 expression

OBJECTIVE

To determine the influence of the genotype and the level of expression of different enzymes involved in folate metabolism on the response to and toxicity of high-dose methotrexate treatment in pediatric osteosarcomas.

STUDY DESIGN

DHFR and Reduced folate carrier 1 (RFC1) semiquantitative expression was analyzed in 34 primary and metastatic osteosarcoma tissues by real-time polymerase chain reaction. The following polymorphisms were also analyzed in peripheral blood from 96 children with osteosarcoma and 110 control subjects: C677T, A1298C (MTHFR), G80A (RFC1), A2756G (MTR), C1420T (SHMT), the 28bp-repeat polymorphism, and 1494del6 of the TYMS gene. Treatment toxicity was scored after each cycle according to criteria from the World Health Organization.

RESULTS

DHFR and RFC1 expression was lower in initial osteosarcoma biopsy specimens than in metastases (P = .024 and P = .041, respectively). RFC1 expression was moderately decreased in samples with poor histologic response to preoperative treatment (P = .053). Patients with osteosarcoma with G3/G4 hematologic toxicity were more frequently TT than CT/CC for C677T/MTHFR (P = .023) and GG for A2756G/MTR (P = .048 and P = .057 for gastrointestinal and hematologic toxicity, respectively).

CONCLUSIONS

The role of C677T/MTHFR and A2756G/MTR on chemotherapy-induced toxicity should be further investigated in pediatric osteosarcomas receiving high-dose methotrexate. Altered expression of DHFR and RFC1 is a feasible mechanism by which osteosarcoma cells become resistant to methotrexate.

Aberrant splicing of folylpolyglutamate synthetase as a novel mechanism of antifolate resistance in leukemia

Folylpoly-γ-gluatamate synthetase (FPGS) catalyzes the polyglutamylation and thus intracellular retention of folates and antifolates (eg, methotrexate; MTX) through the addition of multiple glutamate equivalents to their γ-carboxyl residue. Since polyglutamylation of antifolates is crucial for their pharmacological activity in leukemia, loss of FPGS function results in decreased cellular levels of polyglutamylation-dependent antifolates and consequent drug resistance. Whereas resistance to pulse exposure to antifolates is frequently associated with loss of FPGS activity, the underlying molecular mechanism remains elusive. Here we explored the molecular basis of antifolate resistance in human MTX-resistant leukemia cell lines displaying marked loss of FPGS activity. We demonstrate that these MTX-resistant cells exhibit impaired splicing of FPGS mRNA based on intron retention and/or exon skipping, thereby resulting in loss of FPGS function due to premature translation termination. Furthermore, analysis of FPGS transcripts in blood or bone marrow specimens from patients with acute lymphoblastic leukemia revealed exon 12 skipping, both at diagnosis and at relapse, the latter of which occurs after high-dose MTX-containing chemotherapy. These results constitute the first demonstration of the loss of FPGS function via aberrant mRNA splicing, thereby resulting in loss of antifolate retention and drug resistance. The clinical ramifications of these novel findings are discussed.

Involvement of breast cancer resistance protein expression on rheumatoid arthritis synovial tissue macrophages in resistance to methotrexate and leflunomide

OBJECTIVE

To determine whether multidrug-resistance efflux transporters are expressed on immune effector cells in synovial tissue from patients with rheumatoid arthritis (RA) and compromise the efficacy of methotrexate (MTX) and leflunomide (LEF).

METHODS

Synovial tissue biopsy samples obtained from RA patients before treatment and 4 months after starting treatment with MTX (n = 17) or LEF (n = 13) were examined by immunohistochemical staining and digital image analysis for the expression of the drug efflux transporters P-glycoprotein, multidrug resistance-associated protein 1 (MRP-1) through MRP-5, MRP-8, MRP-9, and breast cancer resistance protein (BCRP), and the relationship to clinical efficacy of MTX and LEF was assessed.

RESULTS

BCRP expression was observed in all RA synovial biopsy samples, both pretreatment and posttreatment, but not in control noninflammatory synovial tissue samples from orthopedic patients. BCRP expression was found both in the intimal lining layer and on macrophages and endothelial cells in the synovial sublining. Total numbers of macrophages in RA patients decreased upon treatment; in biopsy samples with persistently high macrophage counts, 2-fold higher BCRP expression was observed. Furthermore, median BCRP expression was significantly increased (3-fold) in nonresponders to disease-modifying antirheumatic drugs (DMARDs) compared with responders to DMARDs (P = 0.048). Low expression of MRP-1 was found on synovial macrophages, along with moderate expression in T cell areas of synovial biopsy specimens from one-third of the RA patients.

CONCLUSION

These findings show that the drug resistance-related proteins BCRP and MRP-1 are expressed on inflammatory cells in RA synovial tissue. Since MTX is a substrate for both BCRP and MRP-1, and LEF is a high-affinity substrate for BCRP, these transporters may contribute to reduced therapeutic efficacy of these DMARDs.

Effect of folic acid on methotrexate induction of sulfotransferases in rats

Our earlier investigation showed that MTX is an inducer of rat and human sulfotransferases. Here we report that folic acid treatment inhibited MTX induction of aryl sulfotransferase (AST-IV) in female rat liver and hydroxysteroid sulfotransferase (STa) in male rat liver. This is important for understanding the clinical mechanisms of MTX.

Polymorphism in PfMRP1 (Plasmodium falciparum multidrug resistance protein 1) amino acid 1466 associated with resistance to sulfadoxine-pyrimethamine treatment

Sulfadoxine-pyrimethamine (SP) remains widely recommended for intermittent preventive treatment against Plasmodium falciparum malaria for pregnant women and infants in Africa. Resistance to SP is increasing and associated primarily with mutations in the P. falciparum dhfr (Pfdhfr) and Pfdhps genes. This study aimed to explore the hypothetical association of genetic alterations in the P. falciparum multidrug resistance protein gene (Pfmrp1) with the in vivo response to SP by detecting the selection of single nucleotide polymorphisms (SNPs) following standard single-dose treatment administered to children with acute uncomplicated P. falciparum malaria in Tanzania. We detected significant selection of parasites carrying the Pfmrp1 1466K allele in samples from children with recrudescent infections, with 12 (100%) of 12 such samples being positive for this allele, compared to 52 (67.5%) of 77 baseline samples (P = 0.017), in parallel with the selection of the Pfdhfr Pfdhps quintuple mutant haplotype in cases of recrudescence (P = 0.001). There was no association between the 1466K SNP and the Pfdhfr Pfdhps quintuple mutation, indicating independent selections. Our data point for the first time to a role for a P. falciparum multidrug resistance protein homologue in the antimalarial activity of SP. Moreover, they add to the growing evidence of the potential importance of Pfmrp1 in antimalarial drug resistance.

Selection for methotrexate resistance in mammalian cells bearing a Drosophila dihydrofolate reductase transgene: Methotrexate resistance in transgenic mammalian cells

Antifolates, such as methotrexate (MTX), are the treatment of choice for numerous cancers. MTX inhibits dihydrofolate reductase (DHFR), which is essential for cell growth and proliferation. Mammalian cells can acquire resistance to antifolate treatment through a variety of mechanisms but decreased antifolate titers due to changes in drug efflux or influx, or alternatively, the amplification of the DHFR gene are the most commonly acquired resistance mechanisms. In Drosophila, however, a resistant phenotype has only been observed to occur by mutation resulting in a MTX-resistant DHFR. It is unclear if differences in gene structure and/or genome organization between Drosophila and mammals contribute to the observed differences in acquired drug resistance. To investigate if gene structure is involved, Drosophila Dhfr cDNA was transfected into a line of CHO cells that do not express endogenous DHFR. These transgenic cells, together with wild-type CHO cells, were selected for 19 months for resistance to increasing concentrations of MTX, from 50- to 200-fold over the initial concentration. Since Drosophila Dhfr appears to have been amplified several fold in the selected transgenic mammalian cells, a difference in genome organization may contribute to the mechanism of MTX resistance.

Folate receptor beta as a potential delivery route for novel folate antagonists to macrophages in the synovial tissue of rheumatoid arthritis patients

OBJECTIVE

To determine the expression of folate receptor beta (FRbeta) in synovial biopsy tissues and peripheral blood lymphocytes from rheumatoid arthritis (RA) patients and to identify novel folate antagonists that are more selective in the targeting and internalization of FRbeta than methotrexate (MTX).

METHODS

Immunohistochemistry and computer-assisted digital imaging analyses were used for the detection of FRbeta protein expression on immunocompetent cells in synovial biopsy samples from RA patients with active disease and in noninflammatory control synovial tissues. FRbeta messenger RNA (mRNA) levels were determined by reverse transcription-polymerase chain reaction analysis. Binding affinities of FRbeta for folate antagonists were assessed by competition experiments for 3H-folic acid binding on FRbeta-transfected cells. Efficacy of FRbeta-mediated internalization of folate antagonists was evaluated by assessment of antiproliferative effects against FRbeta-transfected cells.

RESULTS

Immunohistochemical staining of RA synovial tissue showed high expression of FRbeta on macrophages in the intimal lining layer and synovial sublining, whereas no staining was observed in T cell areas or in control synovial tissue. Consistently, FRbeta mRNA levels were highest in synovial tissue extracts and RA monocyte-derived macrophages, but low in peripheral blood T cells and monocytes. Screening of 10 new-generation folate antagonists revealed 4 compounds for which FRbeta had a high binding affinity (20-77-fold higher than for MTX). One of these, the thymidylate synthase inhibitor BCG 945, displayed selective targeting against FRbeta-transfected cells.

CONCLUSION

Abundant FRbeta expression on activated macrophages in synovial tissue from RA patients deserves further exploration for selective therapeutic interventions with high-affinity-binding folate antagonists, of which BCG 945 may be a prototypical representative.

Structural determinants of imidazoacridinones facilitating antitumor activity are crucial for substrate recognition by ABCG2

Symadex is the lead acridine compound of a novel class of imidazoacridinones (IAs) currently undergoing phase II clinical trials for the treatment of various cancers. Recently, we have shown that Symadex is extruded by ABCG2-overexpressing lung cancer A549/K1.5 cells, thereby resulting in a marked resistance to certain IAs. To identify the IA residues essential for substrate recognition by ABCG2, we here explored the ability of ABCG2 to extrude and confer resistance to a series of 23 IAs differing at defined residue(s) surrounding their common 10-azaanthracene structure. Taking advantage of the inherent fluorescent properties of IAs, ABCG2-dependent efflux and drug resistance were determined in A549/K1.5 cells using flow cytometry in the presence or absence of fumitremorgin C, a specific ABCG2 transport inhibitor. We find that a hydroxyl group at one of the R1, R2, or R3 positions in the proximal IA ring was essential for ABCG2-mediated efflux and consequent IA resistance. Moreover, elongation of the common distal aliphatic side chain attenuated ABCG2-dependent efflux, thereby resulting in the retention of parental cell sensitivity. Hence, the current study offers novel molecular insight into the structural determinants that facilitate ABCG2-mediated drug efflux and consequent drug resistance using a unique platform of fluorescent IAs. Moreover, these results establish that the IA determinants mediating cytotoxicity are precisely those that facilitate ABCG2-dependent drug efflux and IA resistance. The possible clinical implications for the future design of novel acridines that overcome ABCG2-dependent multidrug resistance are discussed.

Gene expression profiling of leukemia T-cells resistant to methotrexate and 7-hydroxymethotrexate reveals alterations that preserve intracellular levels of folate and nucleotide biosynthesis

In vitro treatment of human T-cell leukemia cells with 7-hydroxymethotrexate, the major metabolite of methotrexate resulted in acquired resistance as a result of the complete loss of folypolyglutamate synthetase (FPGS) activity. This was in contradistinction to the major modality of antifolate resistance of impaired drug transport in leukemia cells exposed to methotrexate. To identify the genes associated with methotrexate and 7-hydroxymethotrexate resistance, we herein explored the patterns of genome-wide expression profiles in these antifolte-resistant leukemia sublines. mRNA levels of the reduced folate carrier, the primary influx transporter of folates and antifolates, were down-regulated more than two-fold in methotrexate-resistant cells. The dramatic loss of FPGS activity in 7-hydroxymethotrexate-resistant cells was associated with alterations in the expression of various genes aimed at preserving reduced folates and/or enhancing purine nucleotide biosynthesis, e.g. methylene tetrahydrofolate reductase, glycinamide ribonucleotide formyltransferase, adenosine deaminase, cystathionine beta synthase, as well as the ATP-dependent folate exporters BCRP/ABCG2 and MRP1/ABCC1. The observed changes in gene expression were generally not paralleled by acquired DNA copy numbers alterations, suggesting transcriptional regulatory mechanisms. Interestingly, gene expression of DNA/RNA metabolism and transport genes were more profoundly altered in methotrexate-resistant subline, whereas in 7-hydroxymethotrexate-resistant cells, the most profoundly affected groups of genes were those encoding for proteins involved in metabolism and cellular proliferation. Thus, the present investigation provides evidence that 7-hydroxymethotrexate induces gene expression alterations and an antifolate resistance modality that are distinct from its parent drug methotrexate.

Correlation Between Thymidylate Synthase Protein Expression and Gene Polymorphism with Clinicopathological Parameters in Colorectal Carcinoma

5-Fluorouracil (5-FU) represents the basis of chemotherapy for colorectal carcinoma, inhibiting thymidylate synthase (TS), an essential enzyme for DNA replication. Previous studies have associated high TS protein expression by tumor cells with poor outcome of patients with colorectal carcinoma, but others have refuted these findings. In view of the potential role of TS as predictive parameter and the lack of consensus in the literature, the present study compared 2 methods: protein expression and gene polymorphism, correlating them with clinicopathological findings. Immunohistochemical detection of TS in tumor cells and detection of gene polymorphism in the blood were performed in 32 patients with colorectal carcinoma treated with 5-FU. No correlation was found between TS protein expression and gene polymorphism. Neither method correlated with survival, tumor staging, and tumor histological grading. This result possibly reflects a complex tumor response to 5-FU therapy, where TS is just one of the involved proteins.

Cofactor chemogenomics

Cofactors are organic molecules, most of them originating from vitamins, that bind to enzymes making them able to catalyze defined reactions. A cofactor-based chemogenomics approach exploits the presence of a cofactor-binding domain to develop compound scaffolds tailored to mimic the cofactor and to replace it within target enzyme classes. As a result, a loss of function is observed. An expansion of the cofactor scaffold to include structural/chemical features derived from the substrate, that usually binds at cofactor adjacent sites, increases the specificity of the enzyme fishing. This approach has been so far applied only to NAD(P)(+)-dependent enzymes. However, it is suitable for all other cofactors, with difficulties, for some of them, originated by very tight binding. In the case of cofactors covalently bound to the enzyme, the competition between the natural cofactor and the cofactor scaffold mimic can only occur during enzyme folding.

PCFT/SLC46A1 promoter methylation and restoration of gene expression in human leukemia cells

The proton-coupled folate transporter (PCFT/SLC46A1) displays optimal and prominent folate and antifolate transport activity at acidic pH in human carcinoma cells but poor activity in leukemia cells. Consistently herein, human leukemia cell lines expressed poor PCFT transcript levels, whereas various carcinoma cell lines showed substantial PCFT gene expression. We identified a CpG island with high density at nucleotides -200 through +100 and explored its role in PCFT promoter silencing. Leukemia cells with barely detectable PCFT transcripts consistently harbored 85-100% methylation of this CpG island, whereas no methylation was found in carcinoma cells. Treatment with 5-Aza-2'-deoxycytidine which induced demethylation but not with the histone deacetylase inhibitor trichostatin A, restored 50-fold PCFT expression only in leukemia cells. These findings constitute the first demonstration of the dominant epigenetic silencing of the PCFT gene in leukemia cells. The potential translational implications of the restoration of PCFT expression in chemotherapy of leukemia are discussed.

FDH: an aldehyde dehydrogenase fusion enzyme in folate metabolism

FDH (10-formyltetrahydrofolate dehydrogenase, Aldh1L1, EC 1.5.1.6) converts 10-formyltetrahydrofolate (10-formyl-THF) to tetrahydrofolate and CO(2) in a NADP(+)-dependent reaction. It is a tetramer of four identical 902 amino acid residue subunits. The protein subunit is a product of a natural fusion of three unrelated genes and consists of three distinct domains. The N-terminal domain of FDH (residues 1-310) carries the folate binding site and shares sequence homology and structural topology with other enzymes utilizing 10-formyl-THF as a substrate. In vitro it functions as 10-formyl-THF hydrolase, and evidence indicate that this activity is a part of the overall FDH mechanism. The C-terminal domain of FDH (residues 400-902) originated from an aldehyde dehydrogenase-related gene and is capable of oxidation of short-chain aldehydes to corresponding acids. Similar to classes 1 and 2 aldehyde dehydrogenases, this domain exists as a tetramer and defines the oligomeric structure of the full-length enzyme. The two catalytic domains are connected by an intermediate linker (residues 311-399), which is a structural and functional homolog of carrier proteins possessing a 4'-phosphopantetheine prosthetic group. In the FDH mechanism, the intermediate linker domain transfers a formyl, covalently attached to the sulfhydryl group of the phosphopantetheine arm, from the N-terminal domain to the C-terminal domain. The overall FDH mechanism is a coupling of two sequential reactions, a hydrolase and a formyl dehydrogenase, bridged by a substrate transfer step. In this mechanism, one domain provides the folate binding site and a hydrolase catalytic center to remove the formyl group from the folate substrate, another provides a transfer vehicle between catalytic centers and the third one contributes the dehydrogenase machinery further oxidizing formyl to CO(2).

Folate deprivation induces BCRP (ABCG2) expression and mitoxantrone resistance in Caco-2 cells

Folates can induce the expression and activity of the breast-cancer-resistance-protein (BCRP) and the multidrug-resistance-protein-1 (MRP1). Our aim was to study the time-dependent effect of folate deprivation/supplementation on (i) BCRP and MRP expression and (ii) on drug resistance mediated by these transporters. Therefore Caco-2 colon cancer cells usually grown in standard RPMI-medium containing supraphysiological folic acid (FA) concentrations (2.3 muM; high-folate, HF) were gradually adapted to more physiological folate concentrations (1 nM leucovorin (LV) or 1 nM FA; low-folate, LF), resulting in the sublines Caco-2-LF/LV and Caco-2-LF/FA. Caco-2-LF/LV and LF/FA cells exhibited a maximal increase of 5.2- and 9.6-fold for BCRP-mRNA and 3.9- and 5.7-fold for BCRP protein expression, respectively, but no major changes on MRP expression. Overexpression of BCRP in the LF-cells resulted in 3.6- to 6.3-fold resistance to mitoxantrone (MR), which was completely reverted by the BCRP inhibitor Ko143. On the other hand, LF-adapted cells were markedly more sensitive to methotrexate than the HF-counterpart, both after 4-hr (9,870- and 23,923-fold for Caco-2-LF/LV and LF/FA, respectively) and 72-hr (11- and 22-fold for Caco-2-LF/LV and LF/FA, respectively) exposure. Immunofluorescent staining observed with a confocal-laser-scan-microscope revealed that in Caco-2 cells (both HF and LF), BCRP is mainly located in the cytoplasm. In conclusion, folate deprivation induces BCRP expression associated with MR resistance in Caco-2 cells. The intracellular localization of BCRP in these cells suggests that this transporter is not primarily extruding its substrates out of the cell, but rather to an intracellular compartment where folates can be kept as storage.

The reduced folate carrier (RFC) is cytotoxic to cells under conditions of severe folate deprivation. RFC as a double edged sword in folate homeostasis

The reduced folate carrier (RFC), a bidirectional anion transporter, is the major uptake route of reduced folates essential for a spectrum of biochemical reactions and thus cellular proliferation. However, here we show that ectopic overexpression of the RFC, but not of folate receptor alpha, a high affinity unidirectional folate uptake route serving here as a negative control, resulted in an approximately 15-fold decline in cellular viability in medium lacking folates but not in folate-containing medium. Moreover to explore possible mechanisms of adaptation to folate deficiency in various cell lines that express the endogenous RFC, we first determined the gene expression status of the following genes: (a) RFC, (b) ATP-driven folate exporters (i.e. MRP1, MRP5, and breast cancer resistance protein), and (c) folylpoly-gamma-glutamate synthetase and gamma-glutamate hydrolase (GGH), enzymes catalyzing folate polyglutamylation and hydrolysis, respectively. Upon 3-7 days of folate deprivation, semiquantitative reverse transcription-PCR analysis revealed a specific approximately 2.5-fold decrease in RFC mRNA levels in both breast cancer and T-cell leukemia cell lines that was accompanied by a consistent fall in methotrexate influx, serving here as an RFC transport activity assay. Likewise a 2.4-fold decrease in GGH mRNA levels and approximately 19% decreased GGH activity was documented for folate-deprived breast cancer cells. These results along with those of a novel mathematical biomodeling devised here suggest that upon severe short term (i.e. up to 7 days) folate deprivation RFC transport activity becomes detrimental as RFC, but not ATP-driven folate exporters, efficiently extrudes folate monoglutamates out of cells. Hence down-regulation of RFC and GGH may serve as a novel adaptive response to severe folate deficiency.

Induction of intrachromosomal homologous recombination in human cells by raltitrexed, an inhibitor of thymidylate synthase

Thymidylate deprivation brings about "thymineless death" in prokaryotes and eukaryotes. Although the precise mechanism for thymineless death has remained elusive, inhibition of the enzyme thymidylate synthase (TS), which catalyzes the de novo synthesis of TMP, has served for many years as a basis for chemotherapeutic strategies. Numerous studies have identified a variety of cellular responses to thymidylate deprivation, including disruption of DNA replication and induction of DNA breaks. Since stalled or collapsed replication forks and strand breaks are generally viewed as being recombinogenic, it is not surprising that a link has been demonstrated between recombination induction and thymidylate deprivation in bacteria and lower eukaryotes. A similar connection between recombination and TS inhibition has been suggested by studies done in mammalian cells, but the relationship between recombination and TS inhibition in mammalian cells had not been demonstrated rigorously. To gain insight into the mechanism of thymineless death in mammalian cells, in this work we undertook a direct investigation of recombination in human cells treated with raltitrexed (RTX), a folate analog that is a specific inhibitor of TS. Using a model system to study intrachromosomal homologous recombination in cultured fibroblasts, we provide definitive evidence that treatment with RTX can stimulate accurate recombination events in human cells. Gene conversions not associated with crossovers were specifically enhanced several-fold by RTX. Additional experiments demonstrated that recombination events provoked by a double-strand break (DSB) were not impacted by treatment with RTX, nor was error-prone DSB repair via nonhomologous end-joining. Our work provides evidence that thymineless death in human cells is not mediated by corruption of DSB repair processes and suggests that an increase in chromosomal recombination may be an important element of cellular responses leading to thymineless death.

Molecular basis of bortezomib resistance: proteasome subunit beta5 (PSMB5) gene mutation and overexpression of PSMB5 protein

The proteasome inhibitor bortezomib is a novel anticancer drug that has shown promise in the treatment of refractory multiple myeloma. However, its clinical efficacy has been hampered by the emergence of drug-resistance phenomena, the molecular basis of which remains elusive. Toward this end, we here developed high levels (45- to 129-fold) of acquired resistance to bortezomib in human myelomonocytic THP1 cells by exposure to stepwise increasing (2.5-200 nM) concentrations of bortezomib. Study of the molecular mechanism of bortezomib resistance in these cells revealed (1) an Ala49Thr mutation residing in a highly conserved bortezomib-binding pocket in the proteasome beta5-subunit (PSMB5) protein, (2) a dramatic overexpression (up to 60-fold) of PSMB5 protein but not of other proteasome subunits including PSMB6, PSMB7, and PSMA7, (3) high levels of cross-resistance to beta5 subunit-targeted cytotoxic peptides 4A6, MG132, MG262, and ALLN, but not to a broad spectrum of chemotherapeutic drugs, (4) no marked changes in chymotrypsin-like proteasome activity, and (5) restoration of bortezomib sensitivity in bortezomib-resistant cells by siRNA-mediated silencing of PSMB5 gene expression. Collectively, these findings establish a novel mechanism of bortezomib resistance associated with the selective overexpression of a mutant PSMB5 protein.

MDM2 regulates dihydrofolate reductase activity through monoubiquitination

MDM2 is a ubiquitin ligase that is best known for its essential function in the negative regulation of p53. In addition, MDM2 expression is associated with tumor progression in a number of common cancers, and in some cases, this has been shown to be independent of p53 status. MDM2 has been shown to promote the degradation of a number of other proteins involved in the regulation of normal cell growth and proliferation, including MDM4 and RB1. Here, we describe the identification of a novel substrate for the MDM2 ubiquitin ligase: dihydrofolate reductase (DHFR). MDM2 binds directly to DHFR and catalyses its monoubiquitination and not its polyubiquitination. In addition, MDM2 expression reduces DHFR activity in a p53-independent manner, but has no effect upon the steady-state level of expression of DHFR. We show that changes in MDM2 expression alter folate metabolism in cells as evidenced by MDM2-dependent alteration in the sensitivity of cells to the antifolate drug methotrexate. Furthermore, we show that the ability of MDM2 to inhibit DHFR activity depends upon an intact MDM2 RING finger. Our studies provide for the first time a link between MDM2, an oncogene with a critical ubiquitin ligase activity and a vital one-carbon donor pathway involved in epigenetic regulation, and DNA metabolism, which has wide ranging implications for both cell biology and tumor development.

Intravitreal methotrexate resistance in a patient with primary intraocular lymphoma

PURPOSE

To describe the clinical course of a patient with multiple recurrences of primary intraocular lymphoma (PIOL).

DESIGN

Interventional case report,

METHODS

Retrospective chart review.

RESULTS

A 57-year-old female treated with multiple intravitreal methotrexate injections became refractory to intravitreal methotrexate after a year. Lymphoma cells evaluated using immunocytochemistry and confocal microscopy showed aberrant multidrug resistance-related protein (MRP) and decreased reduced folate carrier (RFC) and folate binding protein (FBP) expression compared to PIOL cells from another patient clinically responsive to methotrexate.

CONCLUSIONS

This case suggests that alterations in the transport of methotrexate across the cell membrane might contribute to resistance following repeated intravitreal injections.

Resistance to chemotherapy in cancer: a complex and integrated cellular response

Inherent and acquired resistance pathways account for the high rate of failure in cancer chemotherapy. The mechanisms or pathways mediating resistance may be classified as pharmacokinetic (i.e. alter intratumour drug exposue) or pharmacodynamic (i.e. failure to elicit cytotoxicity). More often than not, the resistant phenotype is characterised by alterations in multiple pathways. Consequently, the pathways may act synergistically or generate a broad spectrum of resistance to anticancer drugs. There has been a great deal of systematic characterisation of drug resistance in vitro. However, translating this greater understanding into clinical efficacy has rarely been achieved. This review explores the phenomenon of drug resistance in cancer and highlights the gap between in vitro and in vivo observations. This gap presents a major obstacle in overcoming drug resistance and restoring sensitivity to chemotherapy.

Up regulation in gene expression of chromatin remodelling factors in cervical intraepithelial neoplasia

BACKGROUND

The highest rates of cervical cancer are found in developing countries. Frontline monitoring has reduced these rates in developed countries and present day screening programs primarily identify precancerous lesions termed cervical intraepithelial neoplasias (CIN). CIN lesions described as mild dysplasia (CIN I) are likely to spontaneously regress while CIN III lesions (severe dysplasia) are likely to progress if untreated. Thoughtful consideration of gene expression changes paralleling the progressive pre invasive neoplastic development will yield insight into the key casual events involved in cervical cancer development.

RESULTS

In this study, we have identified gene expression changes across 16 cervical cases (CIN I, CIN II, CIN III and normal cervical epithelium) using the unbiased long serial analysis of gene expression (L-SAGE) method. The 16 L-SAGE libraries were sequenced to the level of 2,481,387 tags, creating the largest SAGE data collection for cervical tissue worldwide. We have identified 222 genes differentially expressed between normal cervical tissue and CIN III. Many of these genes influence biological functions characteristic of cancer, such as cell death, cell growth/proliferation and cellular movement. Evaluation of these genes through network interactions identified multiple candidates that influence regulation of cellular transcription through chromatin remodelling (SMARCC1, NCOR1, MRFAP1 and MORF4L2). Further, these expression events are focused at the critical junction in disease development of moderate dysplasia (CIN II) indicating a role for chromatin remodelling as part of cervical cancer development.

CONCLUSION

We have created a valuable publically available resource for the study of gene expression in precancerous cervical lesions. Our results indicate deregulation of the chromatin remodelling complex components and its influencing factors occur in the development of CIN lesions. The increase in SWI/SNF stabilizing molecule SMARCC1 and other novel genes has not been previously illustrated as events in the early stages of dysplasia development and thus not only provides novel candidate markers for screening but a biological function for targeting treatment.