Lack of functional retinoblastoma protein mediates increased resistance to antimetabolites in human sarcoma cell lines

The effect of phosphorylation efficiency on the oncogenic properties of the protein E7 from high-risk HPV

The Human papillomavirus (HPV) causes tumors in part by hijacking the host cell cycle and forcing uncontrolled cellular division. While there are >200 genotypes of HPV, 15 are classified as high-risk and have been shown to transform infected cells and contribute to tumor formation. The remaining low-risk genotypes are not considered oncogenic and result in benign skin lesions. In high-risk HPV, the oncoprotein E7 contributes to the dysregulation of cell cycle regulatory mechanisms. High-risk E7 is phosphorylated in cells at two conserved serine residues by Casein Kinase 2 (CK2) and this phosphorylation event increases binding affinity for cellular proteins such as the tumor suppressor retinoblastoma (pRb). While low-risk E7 possesses similar serine residues, it is phosphorylated to a lesser degree in cells and has decreased binding capabilities. When E7 binding affinity is decreased, it is less able to facilitate complex interactions between proteins and therefore has less capability to dysregulate the cell cycle. By comparing E7 protein sequences from both low- and high-risk HPV variants and using site-directed mutagenesis combined with NMR spectroscopy and cell-based assays, we demonstrate that the presence of two key nonpolar valine residues within the CK2 recognition sequence, present in low-risk E7, reduces serine phosphorylation efficiency relative to high-risk E7. This results in significant loss of the ability of E7 to degrade the retinoblastoma tumor suppressor protein, thus also reducing the ability of E7 to increase cellular proliferation and reduce senescence. This provides additional insight into the differential E7-mediated outcomes when cells are infected with high-risk verses low-risk HPV. Understanding these oncogenic differences may be important to developing targeted treatment options for HPV-induced cancers.

Anti-cancer efficacy including Rb-deficient tumors and VHL-independent HIF1α proteasomal destabilization by dual targeting of CDK1 or CDK4/6 and HSP90

A prevalent characteristic of solid tumors is intra-tumoral hypoxia. Hypoxia-inducible factor 1α (HIF1α) predominantly mediates the adaptive response to O₂ oscillation and is linked to multiple malignant hallmarks. Here we describe a strategy to robustly target HIF1α by dual inhibition of CDK(s) and heat shock protein 90 (HSP90). We show that CDK1 may contribute to HSP90-mediated HIF1α stabilization. CDK1 knockdown enhances the decrease of HIF1α by HSP90 inhibition. Dual inhibition of CDK1 and HSP90 significantly increases apoptosis and synergistically inhibits cancer cell viability. Similarly, targeting CDK4/6 using FDA-approved inhibitors in combination with HSP90 inhibition shows a class effect on HIF1α inhibition and cancer cell viability suppression not only in colorectal but also in various other cancer types, including Rb-deficient cancer cells. Dual inhibition of CDK4/6 and HSP90 suppresses tumor growth in vivo. In summary, combined targeting of CDK(s) (CDK1 or CDK4/6) and HSP90 remarkably inhibits the expression level of HIF1α and shows promising anti-cancer efficacy with therapeutic potential.

MicroRNA-1226-3p has a tumor-promoting role in osteosarcoma

Osteosarcoma is a malignant bone tumor that commonly occurs in young individuals. It accounts for 10% of solid tumors in those who are 15–19 years old. MicroRNA (miRNA/miR) dysregulation serves a crucial role in the molecular mechanism of osteosarcoma. The present study reported a novel miRNA (miR-1226-3p) and investigated its function in osteosarcoma. miR-1226-3p mimics and miR-1226-3p antisense oligonucleotides were transfected into human osteosarcoma SaOS-2 cells to alter miR-1226-3 expression, while the hFOB 1.19 cell line was used as the control. The apoptosis rate was analyzed using a dead cell apoptosis kit. TNF receptor-associated factor 3 (TRAF3) protein expression was assayed by western blotting. The results of bioinformatics and clinical specimen analyses revealed that higher expression levels of miR-1226-3p were associated with lower survival rates. Additionally, the results of experiments on cultured cells revealed that miR-1226-3p promoted the proliferation of SaOS-2 cells, while miR-1226-3p inhibition decreased cell proliferation and increased apoptosis. Furthermore, it was revealed that miR-1226-3p targeted TRAF3 in SaOS-2 cells. In conclusion, the present study suggested that miR-1226-3p promoted the proliferation of osteosarcoma cells.

Mechanisms of Resistance to Conventional Therapies for Osteosarcoma

Osteosarcoma (OS) is the most common primary bone tumor, mainly occurring in children and adolescents. Current standard therapy includes tumor resection associated with multidrug chemotherapy. However, patient survival has not evolved for the past decades. Since the 1970s, the 5-year survival rate is around 75% for patients with localized OS but dramatically drops to 20% for bad responders to chemotherapy or patients with metastases. Resistance is one of the biological processes at the origin of therapeutic failure. Therefore, it is necessary to better understand and decipher molecular mechanisms of resistance to conventional chemotherapy in order to develop new strategies and to adapt treatments for patients, thus improving the survival rate. This review will describe most of the molecular mechanisms involved in OS chemoresistance, such as a decrease in intracellular accumulation of drugs, inactivation of drugs, improved DNA repair, modulations of signaling pathways, resistance linked to autophagy, disruption in genes expression linked to the cell cycle, or even implication of the micro-environment. We will also give an overview of potential therapeutic strategies to circumvent resistance development.

Targeting Molecular Mechanisms Underlying Treatment Efficacy and Resistance in Osteosarcoma: A Review of Current and Future Strategies

Osteosarcoma is the most common primary malignant bone tumour in children and adolescents. Due to micrometastatic spread, radical surgery alone rarely results in cure. Introduction of combination chemotherapy in the 1970s, however, dramatically increased overall survival rates from 20% to approximately 70%. Unfortunately, large clinical trials aiming to intensify treatment in the past decades have failed to achieve higher cure rates. In this review, we revisit how the heterogenous nature of osteosarcoma as well as acquired and intrinsic resistance to chemotherapy can account for stagnation in therapy improvement. We summarise current osteosarcoma treatment strategies focusing on molecular determinants of treatment susceptibility and resistance. Understanding therapy susceptibility and resistance provides a basis for rational therapy betterment for both identifying patients that might be cured with less toxic interventions and targeting resistance mechanisms to sensitise resistant osteosarcoma to conventional therapies.

Clinical Implications of Methotrexate Pharmacogenetics in Childhood Acute Lymphoblastic Leukaemia

Background:

In the past two decades, a great body of research has been published regarding the effects of genetic polymorphisms on methotrexate (MTX)-induced toxicity and efficacy. Of particular interest is the role of this compound in childhood acute lymphoblastic leukaemia (ALL), where it is a pivotal drug in the different treatment protocols, both at low and high doses. MTX acts on a variety of target enzymes in the folates cycle, as well as being transported out and into of the cell by several transmembrane proteins.

Methods:

We undertook a structured search of bibliographic databases for peer-reviewed research literature using a focused review question.

Results:

This review has intended to summarize the current knowledge concerning the clinical impact of polymorphisms in enzymes and transporters involved in MTX disposition and mechanism of action on paediatric patients with ALL.

Conclusion:

In this work, we describe why, in spite of the significant research efforts, pharmacogenetics findings in this setting have not yet found their way into routine clinical practice.

Revitalizing antifolates through understanding mechanisms that govern susceptibility and resistance

In prokaryotes and eukaryotes, folate (vitamin B9) is an essential metabolic cofactor required for all actively growing cells. Specifically, folate serves as a one-carbon carrier in the synthesis of amino acids (such as methionine, serine, and glycine), N-formylmethionyl-tRNA, coenzyme A, purines and thymidine. Many microbes are unable to acquire folates from their environment and rely on de novo folate biosynthesis. In contrast, mammals lack the de novo folate biosynthesis pathway and must obtain folate from commensal microbiota or the environment using proton-coupled folate transporters. The essentiality and dichotomy between mammalian and bacterial folate biosynthesis and utilization pathways make it an ideal drug target for the development of antimicrobial agents and cancer chemotherapeutics. In this minireview, we discuss general aspects of folate biosynthesis and the underlying mechanisms that govern susceptibility and resistance of organisms to antifolate drugs.

Lysosomal trapping of palbociclib and its functional implications

Palbociclib is a selective inhibitor of cyclin-dependent kinases 4 and 6 (CDK4/6) approved for the treatment of some cancers. The main mechanism of action of palbociclib is to induce cell cycle arrest and senescence on responsive cells. Here, we report that palbociclib concentrates in intracellular acidic vesicles, where it can be readily observed due to its intrinsic fluorescence, and it is released from these vesicles upon dilution or washing out of the extracellular medium. This reversible storage of drugs into acidic vesicles is generally known as lysosomal trapping and, based on this, we uncover novel properties of palbociclib. In particular, a short exposure of cells to palbociclib is sufficient to produce a stable cell-cycle arrest and long-term senescence. Moreover, after washing out the drug, palbociclib-treated cells release the drug to the medium and this conditioned medium is active on susceptible cells. Interestingly, cancer cells resistant to palbociclib also accumulate and release the drug producing paracrine senescence on susceptible cells. Finally, other lysosomotropic drugs, such as chloroquine, interfere with the accumulation of palbociclib into lysosomes, thereby reducing the minimal dose of palbociclib required for cell-cycle arrest and senescence. In summary, lysosomal trapping explains the prolonged temporal activity of palbociclib, the paracrine activity of exposed cells, and the cooperation with lysosomotropic drugs. These are important features that may help to improve the therapeutic dosing and efficacy of palbociclib. Finally, two other clinically approved CDK4/6 inhibitors, ribociclib and abemaciclib, present a similar behavior as palbociclib, suggesting that lysosomal trapping is a property common to all three clinically-approved CDK4/6 inhibitors.

A versatile drug delivery system targeting senescent cells

Senescent cells accumulate in multiple aging-associated diseases, and eliminating these cells has recently emerged as a promising therapeutic approach. Here, we take advantage of the high lysosomal β-galactosidase activity of senescent cells to design a drug delivery system based on the encapsulation of drugs with galacto-oligosaccharides. We show that gal-encapsulated fluorophores are preferentially released within senescent cells in mice. In a model of chemotherapy-induced senescence, gal-encapsulated cytotoxic drugs target senescent tumor cells and improve tumor xenograft regression in combination with palbociclib. Moreover, in a model of pulmonary fibrosis in mice, gal-encapsulated cytotoxics target senescent cells, reducing collagen deposition and restoring pulmonary function. Finally, gal-encapsulation reduces the toxic side effects of the cytotoxic drugs. Drug delivery into senescent cells opens new diagnostic and therapeutic applications for senescence-associated disorders.

Capecitabine Efficacy Is Correlated with TYMP and RB1 Expression in PDX Established from Triple-Negative Breast Cancers

Purpose: Triple-negative breast cancer (TNBC) patients with residual disease after neoadjuvant chemotherapy have a poor outcome. We developed patient-derived xenografts (PDX) from residual tumors to identify efficient chemotherapies and predictive biomarkers in a context of resistance to anthracyclines- and taxanes-based treatments.Experimental Design: PDX were established from residual tumors of primary breast cancer patients treated in neoadjuvant setting. TNBC PDX were treated by anthracyclines, taxanes, platins, and capecitabine. Predictive biomarkers were identified by transcriptomic and immunohistologic analysis. Downregulation of RB1 was performed by siRNA in a cell line established from a PDX.Results: Residual TNBC PDX were characterized by a high tumor take, a short latency, and a poor prognosis of the corresponding patients. With the exception of BRCA1/2-mutated models, residual PDX were resistant to anthracyclines, taxanes, and platins. Capecitabine, the oral prodrug of 5-FU, was highly efficient in 60% of PDX, with two models showing complete responses. Prior treatment of a responder PDX with 5-FU increased expression of thymidylate synthase and decreased efficacy of capecitabine. Transcriptomic and IHC analyses of 32 TNBC PDX, including both residual tumors and treatment-naïve derived tumors, identified RB1 and TYMP proteins as predictive biomarkers for capecitabine response. Finally, RB1 knockdown in a cell line established from a capecitabine-responder PDX decreased sensitivity to 5-FU treatment.Conclusions: We identified capecitabine as efficient chemotherapy in TNBC PDX models established from residual disease and resistant to anthracyclines, taxanes, and platins. RB1 positivity and high expression of TYMP were significantly associated with capecitabine response. Clin Cancer Res; 24(11); 2605-15. ©2018 AACR.

Linker Histone H1.2 Directs Genome-wide Chromatin Association of the Retinoblastoma Tumor Suppressor Protein and Facilitates Its Function

The retinoblastoma tumor suppressor protein pRb is a master regulator of cellular proliferation, principally through interaction with E2F and regulation of E2F target genes. Here, we describe the H1.2 linker histone as a major pRb interaction partner. We establish that H1.2 and pRb are found in a chromatin-bound complex on diverse E2F target genes. Interrogating the global influence of H1.2 on the genome-wide distribution of pRb indicated that the E2F target genes affected by H1.2 are functionally linked to cell-cycle control, consistent with the ability of H1.2 to hinder cell proliferation and the elevated levels of chromatin-bound H1-pRb complex, which occur in growth-arrested cells. Our results define a network of E2F target genes as susceptible to the regulatory influence of H1.2, where H1.2 augments global association of pRb with chromatin, enhances transcriptional repression by pRb, and facilitates pRb-dependent cell-cycle arrest.

Testis specific Y-like 5: gene expression, methylation and implications for drug sensitivity in prostate carcinoma

BACKGROUND

TSPYL5, a putative tumor suppressor gene, belongs to the nucleosome assembly protein family. The chromosomal location of the TSPYL5 gene is 8Q22.1, and its exact role in prostate cancer etiology remains unclear. Further TSPYL5 gene and protein expression in prostate carcinoma cells and diseased tissues including its susceptibility for epigenetic silencing is unknown. Also, not known is the variation in TSPYL5 protein expression with regards to progression of prostatic carcinoma and its possible role in drug sensitivity.

METHODS

TSPYL5, DNMT-1 and DNMT-B gene expression in DU145, LNCaP and RWPE-1 cells and prostate tumor tissues was analyzed by qRT-PCR and RT-PCR. Demethylation experiments were done by treating DU145 and LNCaP cells with 5-aza-2'-deoxycytidine in vitro. Methylation analysis of TSPYL5 gene was performed by methylation specific PCR and pyrosequencing. TSPYL5 protein expression in benign and diseased prostate tumor tissues was performed by immunohistochemistry and in the cells by Western blotting.

RESULTS

TSPYL5 was differentially expressed in non-tumorigenic prostate epithelial cells (RWPE-1), androgen independent (DU145), dependent (LNCaP) prostate carcinoma cells and tissues. Methylation-specific PCR and pyrosequencing analysis identified an inverse relationship between DNA methylation and expression leading to the silencing of TSPYL5 gene. Treatment of prostate carcinoma cells in which TSPYL5 was absent or low (DU145 and LNCaP) with the demethylating agent 5-aza-2'-deoxycytidine upregulated its expression in these cells. Immunohistochemical studies clearly identified TSPYL5 protein in benign tissue and in tumors with Gleason score (GS) of 6 and 7. TSPYL5 protein levels were very low in tumors of GS ≥ 8. TSPYL5 overexpression in LNCaP cells increased the cell sensitivity to chemotherapy drugs such as docetaxel and paclitaxel, as measured by the cellular viability. Furthermore, the cells also exhibited reduced CDKN1A expression with only marginal reduction in pAKT.

CONCLUSIONS

Decrease in TSPYL5 protein in advanced tumors might possibly function as an indicator of prostate tumor progression. Its absence due to methylation-induced silencing can lead to reduced drug sensitivity in prostate carcinoma.

Secondary Craniofacial Sarcomas Following Retinoblastoma: A Systematic Review

OBJECTIVE

We conducted the largest systematic review of individual patient data to characterize secondary craniofacial sarcomas following retinoblastoma.

METHODS

We conducted a systemic search of the PubMed databases and compiled a comprehensive literature review. Student t tests were used to evaluate differences between variables. Kaplan-Meier analysis was used to estimate survival. Statistical significance was assessed using a log-rank test.

RESULTS

We analyzed 220 cases of secondary craniofacial sarcomas, including 112 osteosarcomas. The average age (±SD) of onset for retinoblastoma was 1.20 ± 2.77 years. External-beam radiotherapy was delivered in 207 patients (94.0%) and chemotherapy was delivered in 53 patients (24.0%) patients. The latency period between retinoblastoma diagnosis and the onset of secondary sarcomas was 12 years. Cranial extension was found in 66 patients (30.0%). The median overall survival was worse with cranial extension (P = 0.0073). In cranial extended patients, the median survival in patients who received chemotherapy was 41 months, whereas patients who did not receive chemotherapy had a median survival of 12 months (P = 0.0020).

CONCLUSIONS

The risk of incidence of secondary sarcomas in retinoblastoma patients warrants longer follow-up periods. Moreover, chemotherapy should be considered as a potential treatment option for secondary cranial sarcomas following retinoblastoma.

CCND1 G870A polymorphism is associated with toxicity of methotrexate in childhood acute lymphoblastic leukemia

CCND1 plays a key role in cell cycle progression and may cause methotrexate (MTX) resistance, as well as its cytotoxicity. CCND1 870A variant allele is associated with altered transcripts of this gene. We hypothesized that this polymorphism may contribute to the elimination rate and hepatotoxicity of MTX in childhood acute lymphoblastic leukemia (ALL). We genotyped the CCND1 G870A polymorphism in 125 childhood ALL patients treated with HDMTX. We found no notable associations between G870A polymorphism and the risk of delayed MTX elimination. However, this polymorphism was significantly associated with an increased risk of MTX hepatotoxicity [adjusted odds ratio (OR) = 4.44, 95% confidence interval (CI) = 1.35-14.63 for AG versus GG and adjusted OR = 6.39, 95% CI = 1.82-22.43 for AA versus GG]. Our results indicated that the CCND1 G870A polymorphism may be involved in the hepatotoxicity of MTX and act as a biological marker.

Pharmacogenetics of methotrexate in acute lymphoblastic leukaemia: why still at the bench level?

PURPOSE

The antifolate drug methotrexate (MTX) was introduced into clinical practice about 60 years ago and remains an important component of different acute lymphoblastic leukemia (ALL) treatment protocols. It acts by inhibiting several enzymes in the folate pathway, thereby resulting in the disruption of folate homeostasis. To date, treatment regimens have not been personalized despite there being experimental evidence that gene polymorphisms of folate metabolizing enzymes affect MTX response. The aim of this review was to evaluate the influence of genetic polymorphisms of the enzymes involved in the MTX pathway on ALL treatment outcomes and identify factors underlining the failure to personalize MTX therapy.

METHODS

We conducted a literature search in PUBMED and Google Scholar using the following key words: methotrexate, polymorphism, acute lymphoblastic leukemia, pharmacogenetics, pharmacogenomics and personalized medicine.

RESULTS

The reasons for the failure to personalize MTX therapy may be due to (1) most studies involving single-center, small-sized cohorts, (2) differences in MTX dose across different protocols, (3) failure to consider minimal residual disease as a risk factor for post-induction treatment, (4) differences in outcome criteria between studies and (5) failure to consider the folate levels of a patient before initiation of MTX therapy. Although high-throughput techniques allow the mapping of thousands of genetic polymorphisms in a single run, it remains a major challenge to dissect out folate-metabolizing enzymes which have a high impact on the efficacy and toxicity of MTX and which, therefore, could be the targets for intervention.

CONCLUSIONS

Prospective pharmacogenetic studies which consider all of the above-mentioned factors should be undertaken to facilitate the design of personalized MTX treatment for ALL patients.

Tumour suppressor genes in chemotherapeutic drug response

Since cancer is one of the leading causes of death worldwide, there is an urgent need to find better treatments. Currently, the use of chemotherapeutics remains the predominant option for cancer therapy. However, one of the major obstacles for successful cancer therapy using these chemotherapeutics is that patients often do not respond or eventually develop resistance after initial treatment. Therefore identification of genes involved in chemotherapeutic response is critical for predicting tumour response and treating drug-resistant cancer patients. A group of genes commonly lost or inactivated are tumour suppressor genes, which can promote the initiation and progression of cancer through regulation of various biological processes such as cell proliferation, cell death and cell migration/invasion. Recently, mounting evidence suggests that these tumour suppressor genes also play a very important role in the response of cancers to a variety of chemotherapeutic drugs. In the present review, we will provide a comprehensive overview on how major tumour suppressor genes [Rb (retinoblastoma), p53 family, cyclin-dependent kinase inhibitors, BRCA1 (breast-cancer susceptibility gene 1), PTEN (phosphatase and tensin homologue deleted on chromosome 10), Hippo pathway, etc.] are involved in chemotherapeutic drug response and discuss their applications in predicting the clinical outcome of chemotherapy for cancer patients. We also propose that tumour suppressor genes are critical chemotherapeutic targets for the successful treatment of drug-resistant cancer patients in future applications.

C-terminal domain of p16(INK4a) is adequate in inducing cell cycle arrest, growth inhibition and CDK4/6 interaction similar to the full length protein in HT-1080 fibrosarcoma cells

The tumor suppressor p16(INK4a) has earned widespread attention in cancer studies since its discovery as an inhibitor of cyclin-dependent kinases (CDKs) 4/6. Structurally, it consists of four complete ankyrin repeats, believed to be involved in CDK4 interaction. According to the previous disparities concerning the importance of domains and inactivating mutations in p16, we aimed to search for the domain possessing the functional properties of the full length protein. Upon our in silico screening analyses followed by experimental assessments, we have identified the novel minimum functional domain of p16 to be the C-terminal half including ankyrin repeats III, IV and the C-terminal flanking region accompanied by loops 2 and 3. Transfection of this truncated form into HT-1080 human fibrosarcoma cells, lacking endogenous p16, revealed that it is able to inhibit cell growth and proliferation equivalent to p16(INK4a). The functional analysis showed that this fragment like p16 can interact with CDK4/6, block the entry into S phase of the cell cycle and suppress growth as indicated by colony formation assay. Identification of p16 minimum functional domain can be of benefit to the future peptidomimetic drug design as well as gene transfer for cancer therapy.

Cyclin-dependent kinase inhibitor SU9516 enhances sensitivity to methotrexate in human T-cell leukemia Jurkat cells

Methotrexate (MTX) has been used to treat various hematological malignancies. Since MTX prevents tumor cells from proliferating by inhibiting dihydrofolate reductase (DHFR), DHFR expression is a key determinant of resistance to MTX in malignant hematological tumor cells. The antiproliferative effect of MTX was significantly enhanced by the knockdown of DHFR expression by siRNA in Jurkat cells. Therefore, a novel strategy down-regulating DHFR expression seems promising for enhancing sensitivity to MTX. We found that SU9516, a cyclin-dependent kinase inhibitor, reduced the expression of both DHFR mRNA and protein. Moreover, we found that DHFR promoter activity was attenuated by SU9516 dependent on the E2F site. Finally, pretreatment with SU9516 significantly enhanced sensitivity to MTX in a colony formation assay. We conclude that a combination of cyclin-dependent kinase inhibitors and MTX may be useful for overcoming resistance to MTX.

Relationship of polymorphism of the tandem repeat sequence in the thymidylate synthase gene and the survival of stage III colorectal cancer patients receiving adjuvant 5-flurouracil-based chemotherapy

BACKGROUND

The aim of this study was to determine whether the different polymorphisms in the thymidylate synthase (TS) gene, novel G>C single nucleotide polymorphism (SNP) and variable number of tandem repeat (VNTR), may be related with disease-free survival (DFS) in patients with stage III colorectal cancer receiving adjuvant chemotherapy.

METHODS

The study included 201 patients with pathologic TNM stage III colon cancer who received adjuvant 5-fluorouracil (5-FU)-based chemotherapy after surgery. DNA was extracted from fresh tumor tissue and sequenced. Patients with TS genotypes of 2R3G, 3C3G, or 3G3G were assigned to a high expression group, and those with 2R2R, 2R3C, or 3C3C, to a low expression group.

RESULTS

Frequencies of the TS tandem repeat polymorphisms among the tumor genotypes were 6.0% in 2R2R, 25.4% in 2R3R, and 68.7% in 3R3R. The low expression group included 52 patients (25.9%), and the high expression group included 149 patients (74.1%). Groups classified according to possession of VNTR, SNP, and low- or high-expression genotypes did not differ significantly in DFS. In multivariate analysis, only tumor stage showed significant prognostic value (hazard ratio (HR) 2.05, 95% CI = 1.24-3.37, P = 0.005).

CONCLUSIONS

TS polymorphisms do not predict clinical outcome of colorectal cancer patients treated with adjuvant 5-FU-based chemotherapy.

Resveratrol inhibits proliferation and promotes apoptosis of osteosarcoma cells

The phytoalexin resveratrol has been described to have chemopreventive and chemotherapeutic effects in several tumor models while its effects on osteosarcoma have not been extensively studied. Additionally, resveratrol is a potent activator of the Sirt1/Sir2 (silent information regulator 2) family of NAD-dependent deacetylases which plays a role in calorie restriction-mediated tumor suppression. In the present study, we evaluated the effect of resveratrol on growth and apoptosis in four osteosarcoma cell lines (HOS, Saos-2, U-2 OS and MG-63) and a normal human osteoblast cell line (NHOst). We found that Sirt1 protein was relatively higher expressed in the tumor cells than normal osteoblasts. Consistently, resveratrol induced apoptosis in a dose-dependent fashion in the osteosarcoma cells but had minor effect on normal osteoblasts. Also, a similar effect could be elicited by another Sirt1 activator, isonicotinamide. In addition, the pro-apoptotic effect of resveratrol could be enhanced by nutrition restriction elicited by l-asparaginase. We postulate that these effects by resveratrol are mediated via Sirt1 but further studies are needed to confirm or refute this theory.

Sister telomeres rendered dysfunctional by persistent cohesion are fused by NHEJ

Telomeres protect chromosome ends from being viewed as double-strand breaks and from eliciting a DNA damage response. Deprotection of chromosome ends occurs when telomeres become critically short because of replicative attrition or inhibition of TRF2. In this study, we report a novel form of deprotection that occurs exclusively after DNA replication in S/G2 phase of the cell cycle. In cells deficient in the telomeric poly(adenosine diphosphate ribose) polymerase tankyrase 1, sister telomere resolution is blocked. Unexpectedly, cohered sister telomeres become deprotected and are inappropriately fused. In contrast to telomeres rendered dysfunctional by TRF2, which engage in chromatid fusions predominantly between chromatids from different chromosomes (Bailey, S.M., M.N. Cornforth, A. Kurimasa, D.J. Chen, and E.H. Goodwin. 2001. Science. 293:2462–2465; Smogorzewska, A., J. Karlseder, H. Holtgreve-Grez, A. Jauch, and T. de Lange. 2002. Curr. Biol. 12:1635–1644), telomeres rendered dysfunctional by tankyrase 1 engage in chromatid fusions almost exclusively between sister chromatids. We show that cohered sister telomeres are fused by DNA ligase IV–mediated nonhomologous end joining. These results demonstrate that the timely removal of sister telomere cohesion is essential for the formation of a protective structure at chromosome ends after DNA replication in S/G2 phase of the cell cycle.

Mechanisms of antifolate resistance and methotrexate efficacy in leukemia cells

Antifolates are the first class of antimetabolites introduced to clinic about 6 decades ago. Now, after several years of administration of antifolates against malignancies and particularly leukemia, we are still trying to achieve a full understanding of the mechanisms of action and resistance to these agents. The present article covers different factors able to influence efficacy of antifolates on leukemic cells, the known mechanisms of resistance to methotrexate (MTX) and strategies to overcome these mechanisms. The dominant factors that are contributed to tolerance to cytocidal effects of MTX including pharmacokinetic factors, impaired transmembrane uptake as the most frequent rote of provoking resistance to MTX, augmented drug efflux, impaired intracellular polyglutamation as a determining process of drug efficacy, alterations in expression or activity of target enzymes and increased intracellular folate pools; and finally role of 7-hydroxymethotrexate on response or resistance to MTX will be discussed in more detail. Finally, strategies to overcome resistance to antifolates are discussed.

Pemetrexed, a multitargeted antifolate drug, demonstrates lower efficacy in comparison to methotrexate against osteosarcoma cell lines

The folate inhibitor methotrexate (MTX) is an important component of osteosarcoma (OS) treatment regimens. New generation multitargeted antifolates, such as pemetrexed (PMX), have shown promise in the treatment of various solid tumors. In this study, the in vitro efficacy of MTX and PMX was compared in OS cell lines. MTX demonstrated a superior cytotoxic effect in comparison to PMX in all tested cell lines. Apoptosis assays revealed that both MTX and PMX induce apoptosis but MTX demonstrated superior efficacy. These in vitro results suggest that PMX as a single agent may not demonstrate improved efficacy compared to MTX in OS patients.

Molecular mechanisms underlying the synergistic interaction of erlotinib, an epidermal growth factor receptor tyrosine kinase inhibitor, with the multitargeted antifolate pemetrexed in non-small-cell lung cancer cells

Because the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor erlotinib and the multitargeted antifolate pemetrexed are registered in the treatment of second-line non-small-cell lung cancer (NSCLC), empirical combinations of these drugs are being tested. This study investigated molecular mechanisms underlying their combination in six NSCLC cell lines. Cells were characterized by heterogeneous expression of pemetrexed determinants, including thymidylate synthase (TS) and dihydrofolate reductase (DHFR), and mutations potentially affecting chemosensitivity. Pharmacological interaction was studied using the combination index (CI) method, whereas cell cycle, apoptosis induction, and EGFR, extracellular signal-regulated kinases 1 and 2, and Akt phosphorylation were studied by flow cytometry, fluorescence microscopy, and enzyme-linked immunosorbent assays. Reverse-transcriptase polymerase chain reaction (RT-PCR), Western blot, and activity assays were performed to assess whether erlotinib influenced TS. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium assays demonstrated that EGFR and k-Ras mutations were related to erlotinib sensitivity, whereas TS and DHFR expression were related to pemetrexed sensitivity. Synergistic cytotoxicity was found in all cells, most pronounced with pemetrexed + erlotinib (24 h) --> erlotinib (48 h) sequence (CI, 0.09-0.40), which was associated with a significant induction of apoptosis. Pemetrexed increased EGFR phosphorylation and reduced Akt phosphorylation, which was additionally reduced by drug combination (-70.6% in H1650). Erlotinib significantly reduced TS expression and activity, possibly via E2F-1 reduction, as detected by RT-PCR and Western blot, and the combination decreased TS in situ activity in all cells. Erlotinib and pemetrexed showed a strong synergism in NSCLC cells, regardless of their genetic characteristics. Induction of apoptosis, modulation of EGFR and Akt phosphorylation, and changes in the expression of critical genes involved in pemetrexed activity contribute to this synergistic interaction and support the clinical investigation of these markers.

DNA variants in the dihydrofolate reductase gene and outcome in childhood ALL

Dihydrofolate reductase (DHFR) is the major target of methotrexate (MTX), a key component in childhood acute lymphoblastic leukemia (ALL) treatment. A total of 15 polymorphisms in DHFR promoter were analyzed, and 3 sites (C-1610G/T, C-680A, and A-317G) were identified as sufficient to define observed haplotypes (tag single nucleotide polymorphisms [tagSNPs]). These polymorphisms were investigated for association with treatment response in 277 children with ALL. Lower event-free survival (EFS) was associated with homozygosity for the allele A-317 and C-1610 (P=.03 and .02), and with the haplotype *1, defined by both C-1610 and A-317 alleles (P=.03). The haplotype *1 conferred higher transcriptional activity (P<.01 compared with haplotypes generating minimal luciferase expression). Quantitative mRNA analysis showed higher DHFR levels for particular haplotype *1 carriers (P<.01). The analysis combining haplotype *1 with thymidylate synthase (TS) and cyclin D1 (CCND1) genotypes previously shown to affect ALL outcome showed that the number of event-predisposing genotypes was associated with increasingly lower EFS (P<.001). In conclusion, DHFR promoter polymorphisms are associated with worse ALL outcome, likely due to a higher DHFR expression. Combined effects among genes of the folate cycle can further accentuate differences in the response to the treatment.

Retinoblastoma deficiency increases chemosensitivity in lung cancer

The retinoblastoma (RB) tumor suppressor is mutated or functionally inactivated in the majority of human malignancies, and p16(INK4a)-cyclin D1-cyclin-dependent kinase 4-RB pathway aberrations are present in nearly all cases of non-small cell lung cancer (NSCLC). Here, the distinct role of RB loss in tumorigenic proliferation and sensitivity to chemotherapeutics was determined in NSCLC cells. Attenuation of RB led to a proliferative advantage in vitro and aggressive tumorigenic growth in xenograft models. Clinically, such aggressive disease is treated with genotoxic and cytotoxic chemotherapeutic agents. In vitro analysis showed that RB deficiency resulted in bypass of the checkpoint response to multiple chemotherapeutic challenges concomitant with an elevated apoptotic response. Correspondingly, RB deficiency in xenograft models led to increased chemosensitivity. However, this response was transient, and a durable response was dependent on prolonged chemotherapeutic administration. Together, these findings show that although RB deficiency enhances sensitivity to chemotherapeutic challenge, efficient and sustainable response is highly dependent on the specific therapeutic regimen, in addition to the molecular environment.

RB activity alters checkpoint response and chemosensitivity in lung cancer lines

BACKGROUND

The retinoblastoma tumor suppressor (RB) is a key regulator of cell cycle progression and is functionally inactivated in the majority of human non-small cell lung cancers (NSCLC). The specific influence of RB on therapeutic response in NSCLC remains elusive.

MATERIALS AND METHODS

We investigated the consequence of reintroduction of RB on checkpoint response and chemosensitivity in NSCLC cell lines. RB introduction into RB-proficient (NCI-H1299) and -deficient (H1734, H2172) NSCLC cells was achieved by adenoviral infection. RB/E2F target gene expression was determined by immunoblot analysis. Cell cycle response and viability after chemotherapeutic exposure were assessed by flow cytometry and MTT viability assay.

RESULTS

RB reconstitution in RB-deficient lines restored regulation of topoIIalpha, thymidylate synthase, and cyclin A. Similarly, RB overexpression in RB-proficient cells caused further regulation of some RB/E2F target genes including thymidylate synthase and topoIIalpha. In addition, RB overexpression resulted in restoration of the G1 arrest mechanism. Exposure of RB-proficient cells to cisplatin, etoposide, or 5-fluorouracil elicited arrest in various phases of the cell cycle while lines deficient for RB exhibited different checkpoint responses. However, introduction of RB restored ability to arrest following chemotherapeutic exposure. Chemotherapeutic challenge resulted in varying effects on cellular viability independent of RB status, yet restoration of RB activity conferred partial chemoresistance.

CONCLUSIONS

These results demonstrate that RB reconstitution into RB-deficient NSCLC lines establishes regulation of certain RB/E2F target genes and restores G1 arrest mechanisms. Furthermore, introduction of RB enhances the G1 checkpoint response to chemotherapeutics and decreases chemosensitivity. Knowledge of RB-dependent chemosensitivity may ultimately contribute to individualized therapy based on molecular characterization of tumors.

Pharmacogenomics of acute leukemia

Pharmacogenomics provides knowledge regarding how genetic polymorphisms affect treatment responses. Such an approach is particularly needed in cancer therapy, as most chemotherapeutics drugs affect both tumor and normal cells, are ineffective in many patients and exhibit serious side effects. Leukemia exists in two different forms, myeloid and lymphoid. Acute lymphoblastic leukemia more frequently occurs in children, whereas the risk of acute myeloid leukemia is more common in adults. Despite significant progress in the treatment of these diseases, therapy is still unsuccessful in many patients. Prognosis is particularly poor in adult acute myeloid leukemia. Treatment failure in childhood acute lymphoblastic leukemia due to drug resistance remains the leading cause of cancer-related death in children. Here, we provide an overview of pharmacogenetics studies carried out in children and adults with acute lymphoblastic leukemia and acute myeloid leukemia, attempting to find the associations between treatment responses and polymorphisms in the genes whose products are needed for metabolism, and effects of drugs used in the treatment of leukemia.

The retinoblastoma protein is an essential mediator of osteogenesis that links the p204 protein to the Cbfa1 transcription factor thereby increasing its activity

Bone formation requires the coordinated activity of numerous proteins including the transcription factor core-binding factor alpha1 (Cbfa1). Deregulation of Cbfa1 results in metabolic bone diseases including osteoporosis and osteopetrosis. The retinoblastoma protein (pRb) that is required for osteogenesis binds Cbfa1. We reported earlier that the p200 family protein p204, which is known to be involved in the differentiation of skeletal muscle myotubes, cardiac myocytes, and macrophages, also serves as a cofactor of Cbfa1 and promotes osteogenesis. In this study we established that suppression of p204 expression by an adenovirus construct encoding p204 antisense RNA inhibited osteoblast-specific gene activation by Cbfa1 in an osteogenesis assay involving the pluripotent C2C12 mesenchymal cell line. Using protein-protein interaction assays we established that Cbfa1, pRb, and p204 form a ternary complex in which pRb serves as a linker connecting p204 and Cbfa1. Chromatin immunoprecipitation assays revealed the binding of such a p204-pRb-Cbfa1 transcription factor complex to the promoter of the osteocalcin gene. The pRb requirement of the stimulation of Cbfa1 activity by p204 was established in experiments involving p204 mutants lacking one or two pRb binding (LXCXE) motifs. Such mutants failed to enhance the Cbfa1-dependent transactivation of gene expression as well as osteogenesis. Furthermore, as revealed in reporter gene and in vitro osteogenesis assays p204 synergized with pRb in the stimulation of Cbfa1-dependent gene activation and osteoblast differentiation.

RB family members as predictive and prognostic factors in human cancer

The retinoblastoma family members--pRb, pRb2/p130 and p107--are tumor suppressor genes involved in controlling four major cellular processes: growth arrest, apoptosis, differentiation and angiogenesis. Molecular genetic studies have identified abnormalities of these tumor suppressor genes in a large proportion of human cancers. These genetic alterations have emerged as significant factors in the pathogenesis and progression of many types of tumors and are therefore likely to provide relevant information to assess risk in cancer patients. There is a pressing clinical need to identify prognostic and predictive factors for patients with cancer, because there is an undeniable importance in being able to determine which patients will have a favorable outcome without further therapy (prognostic factor) and which will need some additional treatment (predictive factor). This review examines the predictive and/or prognostic role of each retinoblastoma family member in human cancer.

Chemotherapy resistance in osteosarcoma: current challenges and future directions

For patients with osteosarcoma, the use of chemotherapy has improved survival from 11% with surgical resection alone in the 1960s, to 70% by the mid-1980s. However, survival has since plateaued, despite advances in anticancer therapy. Elucidation of the mechanisms of chemoresistance and implementation of strategies to overcome chemoresistance will likely be pivotal to improving survival. In this review, the focus is on the current understanding of the mechanisms of resistance to the most commonly used agents in the treatment of osteosarcoma and the methods employed to overcome chemotherapy resistance.

Fourteen Novel Genetic Variations and Haplotype Structures of the TYMS Gene Encoding Human Thymidylate Synthase (TS)

Forty genetic variations including 14 novel ones were found in the human TYMS gene, which encodes thymidylate synthase, in 263 Japanese cancer patients who received 5-fluorouracil (FU)-based chemotherapy. Three novel variations were located within the 28-bp tandem repeat sequence in the 5'-untranslated region (UTR) and were designated 5Rc, 3Rc-ins and 4Rc. Allele frequencies were 0.021 for 5Rc, 0.006 for 3Rc-ins and 0.002 for 4Rc. Other novel variations included -133G>C and -125G>C in the 5'-UTR; IVS1-278G>A, IVS2-68C>T, IVS2-23T>C, IVS4+122_+123insATTG, IVS4-141G>A, IVS5-100A>T and IVS6-111G>A in the introns; and 1244(*302)A>G and 1264(*322)G>A in the 3'-UTR. The allele frequencies were 0.34 for IVS4+122_+123insATTG, 0.042 for -133G>C, 0.011 for IVS4-141G>A, 0.006 for -125G>C, 0.004 for IVS1-278G>A, IVS2-68C>T, 1244(*302)A>G and 1264(*322)G>A, and 0.002 for IVS2-23T>C, IVS5-100A>T and IVS6-111G>A. Using the detected polymorphisms, linkage disequilibrium (LD) analysis was performed, which divided the TYMS gene into three LD blocks. The 28-bp tandem repeat sequence in the 5'-UTR was assigned as Block 2 with a total of 7 alleles. In Blocks 1 and 3, 7 and 19 haplotypes were determined/inferred, respectively. Our findings provide fundamental and useful information for genotyping TYMS in the Japanese and probably other Asian populations.

Pharmacogenetics of methotrexate

Methotrexate (MTX) has proven efficient in the treatment of a number of malignancies, as well as non-malignant disorders characterized by a rapid cellular growth. Yet some patients might develop resistance, while others could have toxic side effects. MTX achieves its cytotoxicity through the inhibition of folate-dependent enzymes, suggesting that the genes controlling their activity or the levels of folate cofactors can modulate drug efficacy and, thus, the sensitivity of a patient to MTX. Indeed, several studies, conducted mostly in leukemia and rheumatoid arthritis patients, have addressed the potential for tailoring MTX therapy based on a patient's genetics. Several genetic variants have been shown to have a predictive role, among which the most frequently studied are those of methylenetetrahydrofolate reductase and thymidylate synthase genes. The other candidates, as well as gene-gene interactions, which may be even more important for the prediction of disease outcomes than the individual gene effects, are also briefly discussed.

Low tumor cell density environment yields survival advantage of tumor cells exposed to MTX in vitro

Stable resistance to methotrexate has been well characterized after prolonged treatment of the HT-29 colon cancer cell line, but the mechanism of cell survival at the early stages of the drug resistance process still remains unclear. Here, we demonstrate that human cancer cells in vitro are sensitive to methotrexate only above a critical cell culture density, which specifically coincides with their ability to deplete the extracellular nucleosides from a fully supplemented culture medium. At lower cell densities, extracellular nucleosides remain intact and allow salvage nucleotide synthesis that renders cells insensitive to the drug. Consistently, medium conditioned by cells seeded at standard cell densities sensitizes low cell density cultures. Extracellular nucleosides are the determinants of sensitivity because the latter effect can be mimicked with the use of inhibitors of nucleoside cellular import and reversed by supplying exogenous thymidine and hypoxanthine. Interestingly, treatment at a sensitizing cell density does not preclude the survival of less than 1% of the cells--which have no intrinsic resistance--owing to the inability of the dying cell population to condition the culture medium; this population thus survives indefinitely to continuous treatment by keeping adapted to a low cell number. This cell density-dependent adaptive process accounts for the initial steps of in vitro resistance to methotrexate (MTX) and provides a novel mechanistic insight into the cell population dynamics of cell survival and cell death during drug treatment.

Discrete signaling pathways participate in RB-dependent responses to chemotherapeutic agents

The retinoblastoma (RB) tumor suppressor has been proposed to function as a key mediator of cell cycle checkpoints induced by chemotherapeutic agents. However, these prior studies have relied on embryonic fibroblasts harboring chronic loss of RB, a condition under which compensation of RB functions is known to occur. Here we utilized primary adult fibroblasts derived from mice harboring loxP sites flanking exon 3 of the Rb gene to delineate the action of RB in the chemotherapeutic response. In this system we find that targeted disruption of Rb leads to little overt change in cell cycle distribution. However, these cells exhibited deregulation of RB/E2F target genes and became aneuploid following culture in the absence of RB. When challenged with both DNA damaging and antimetabolite chemotherapeutics, RB was required for primary adult cells to undergo DNA damage checkpoint responses and loss of RB resulted in enhanced aneuploidy following challenge. In contrast, following spontaneous immortalization and the loss of functional p53 signaling, the antimetabolite 5-fluorouracil (5-FU) failed to induce arrest despite the presence of RB. In these immortal cultures RB/E2F targets were deregulated in a complex, gene-specific manner and RB was required for the checkpoint response to camptothecin (CPT). Mechanistic analyses of the checkpoint responses in primary cells indicated that loss of RB leads to increased p53 signaling and decreased viability following both CPT and 5-FU treatment. However, the mechanism through which these agents act to facilitate cell cycle inhibition through RB were distinct. These studies underscore the critical role of RB in DNA-damage checkpoint signaling and demonstrate that RB mediates chemotherapeutic-induced cell cycle inhibition in adult fibroblasts by distinct mechanisms.

Down-regulation of thymidylate synthase expression and its steady-state mRNA by oxaliplatin in colon cancer cells

Recently, evidence has accumulated that weekly 24-h infusion of high-dose 5-fluorouracil (5-FU) with leucovorin (LV, folinic acid) biochemical modulation may improve the response rates compared with the bolus 5-FU regimens in colorectal cancer (CRC). Combining the infusional 5-FU/LV (iFL) regimens with oxaliplatin or irinotecan is widely adopted to further improve treatment efficacy. Either oxaliplatin-iFL or irinotecan-iFL may achieve an overall response rate of more than 50% in the first-line treatment. Intriguingly, in the salvage treatment for metastatic CRC patients who had failed iFL, only oxaliplatin-iFL may achieve a response rate of about 13-25%. In contrast, oxaliplatin alone or irinotecan-iFL had a very low response rate of 5% or less. To test if the oxaliplatin may reverse the iFL-related 5-FU resistance in CRC, we used DLD-1 colon adenocarcinoma cells as the in vitro study model. First, we revealed that oxaliplatin and 5-FU act synergistically on DLD-1 cells by MTT cytotoxicity assay and median drug effect analysis. Second, we treated the DLD-1 cells with serial concentrations of oxaliplatin (0.1-10 microM). Oxaliplatin treatment results in down-regulation of free thymidylate synthase (TS) protein expression by Western blotting. Further, we analyzed the TS mRNA level by reverse transcription and real-time quantitative polymerase chain reaction assay. Oxaliplatin treatment results in down-regulation of the TS mRNA level up to 40% (mean +/- SD of ratio to reference control = 0.60 +/- 0.21, range 0.42-0.84). In this study, our data provide important information explaining the reason why the combination of oxaliplatin and 5-FU results in a better objective response in 5-FU-resistant patients than oxaliplatin alone does. Our data also suggest that TS down-regulation happens at the transcriptional level. TS modulation and down-regulation had, thus, shed light on the useful potential strategy to achieve objective responses in 5-FU-resistant CRC patients.

Analysis of dihydrofolate reductase and reduced folate carrier gene status in relation to methotrexate resistance in osteosarcoma cells

BACKGROUND

To evaluate the impact of dihydrofolate reductase (DHFR) and reduced folate carrier (RFC) genes on methotrexate (MTX) resistance in osteosarcoma cells in relation to retinoblastoma (RB1) gene status.

MATERIALS AND METHODS

A series of human osteosarcoma cell lines-either sensitive or resistant to MTX-and 16 osteosarcoma tumour samples were used in this study.

RESULTS

In U-2OS MTX-resistant variants, and in other RB1-positive cell lines, MTX resistance was associated with increased levels of DHFR and with a slight decrease of RFC gene expression. In Saos-2 MTX-resistant variants, and in another RB1-negative cell line, development of MTX resistance was associated with a decrease in expression of RFC, without any significant involvement of DHFR. In osteosarcoma clinical samples, amplification of the DHFR gene at clinical onset appeared to be more frequent in RB1-positive compared with RB1-negative tumours.

CONCLUSIONS

Amplification of the DHFR gene may occur more frequently in the presence of RB1-mediated negative regulation of its activity and can be present at clinical onset in osteosarcoma patients. Simultaneous evaluation of RFC, DHFR and RB1 gene status at the time of diagnosis may become the basis for the identification of potentially MTX-unresponsive osteosarcoma patients, who could benefit from treatment protocols with alternative antifolate drugs.

The influence of cyclin D1 (CCND1) 870A>G polymorphism and CCND1-thymidylate synthase (TS) gene-gene interaction on the outcome of childhood acute lymphoblastic leukaemia

The 870A>G polymorphism in the cyclin D1 (CCND1) gene modulates mRNA splicing, leading to altered protein that may affect the regulation of the G1/S cell-cycle checkpoint. This polymorphism has been reported to influence susceptibility to and progression of several malignancies. Furthermore, the change of retinoblastoma protein regulation mediated by CCND1 may play a role in the development of methotrexate (MTX) resistance via an associated higher activity of enzymes that are inhibited by MTX. This study shows that children with acute lymphoblastic leukaemia (ALL) who are homozygous for the CCND1 A variant have a lower probability of event-free survival (P = 0.006) compared to carriers of the G variant. A significant result is retained in the presence of other prognostic factors. This impact is even more apparent in individuals who are also homozygous for thymidylate synthase (TS) triple repeat (P < 0.00005), which has previously been shown to influence the outcome of childhood ALL.

Targeting angiogenic processes by combination low-dose paclitaxel and radiation therapy

Tumor growth and angiogenesis are interdependent. Paclitaxel and radiation therapy are commonly used in the clinic, in a number of disease sites, requiring high dosages of both drug and radiation for cure. Paclitaxel (Taxol) is a diterpenoid with antitumor activity against a variety of human neoplasms and can amplify the cytotoxic effect of ionizing radiation in vitro, presumably by inducing arrest at metaphase, known to be a very radiosensitive phase of the cell cycle. Little is known about how angiogenesis is affected by paclitaxel when the combination of paclitaxel and radiation are used. We have evaluated the combination of paclitaxel and radiation at various concentrations, on cytokine-induced angiogenesis in vitro with the goal of determining whether reduction of radiation and paclitaxel doses is possible without sacrificing efficacy. We have found that paclitaxel inhibited endothelial cell proliferation, migration, and tube formation (differentiation) at one-tenth the concentration needed to achieve a similar effect on tumor cell lines. In combination with radiation, inhibition of endothelial cell function was additive and increased twofold. The combination of low-dose paclitaxel and radiation suggests a complementary strategy with potential clinical ramifications to target angiogenesis-dependent malignancies.

Retinoblastoma tumor suppressor targets dNTP metabolism to regulate DNA replication

The retinoblastoma tumor suppressor, RB, is a negative regulator of the cell cycle that is inactivated in the majority of human tumors. Cell cycle inhibition elicited by RB has been attributed to the attenuation of CDK2 activity. Although ectopic cyclins partially overcome RB-mediated S-phase arrest at the replication fork, DNA replication remains inhibited and cells fail to progress to G(2) phase. These data suggest that RB regulates an additional execution point in S phase. We observed that constitutively active RB attenuates the expression of specific dNTP synthetic enzymes: dihydrofolate reductase, ribonucleotide reductase (RNR) subunits R1/R2, and thymidylate synthase (TS). Activation of endogenous RB and related proteins by p16ink4a yielded similar effects on enzyme expression. Conversely, targeted disruption of RB resulted in increased metabolic protein levels (dihydrofolate reductase, TS, RNR-R2) and conferred resistance to the effect of TS or RNR inhibitors that diminish available dNTPs. Analysis of dNTP pools during RB-mediated cell cycle arrest revealed significant depletion, concurrent with the loss of TS and RNR protein. Importantly, the effect of active RB on cell cycle position and available dNTPs was comparable to that observed with specific antimetabolites. Together, these results show that RB-mediated transcriptional repression attenuates available dNTP pools to control S-phase progression. Thus, RB employs both canonical cyclin-dependent kinase/cyclin regulation and metabolic regulation as a means to limit proliferation, underscoring its potency in tumor suppression.

The molecular genetics of therapeutic resistance in malignant astrocytomas

The adverse prognosis associated with malignant astrocytomas (MA) is due in part to the development of resistance by the tumor to chemo- and radiotherapy-induced cytotoxic damage. The mechanisms of resistance are poorly understood but function at the level of the endothelial cell, the blood-brain barrier and the neoplastic cell itself. The classic examples of drug resistance proteins, such as the p-glycoprotein/multidrug resistance protein 1, have been identified within MA biopsy specimens. However, it is questionable to what degree, if at all, these proteins contribute directly to the evolution and prognosis of the MA. Surprisingly, there are specific genes, not traditionally associated with resistance, which appear increasingly relevant to both tumor progression and insensitivity to cytotoxic damage. These genes are involved in cell cycle regulation, and include the retinoblastoma susceptibility gene (Rb), the tumor suppressor gene p53, as well as those encoding the cyclins, their kinases and inhibitors. The interaction between the products of these genes and intratumoral environmental factors appears to involve a dynamic and prognostically adverse selection process. It is from this perspective that the mechanism(s) of hypoxic-ischaemic selection for resistance and its therapeutic repercussions will be analyzed.

The emergence of resistance to targeted cancer therapeutics

Drug resistance has always been a concern in cancer treatment, often blamed on the genetic complexity and instability of tumor cells. While studies of cancer cell lines have implicated an array of potential mechanisms, it has been difficult to translate these insights into clinically meaningful improvements in cancer treatment. The successful deployment of molecularly targeted therapeutics in some cancers has led to widespread optimism that this approach will become broadly applicable. Despite their early promise in the clinic, the novel therapeutics are often plagued with the age old problem of acquired drug resistance. Progress in understanding why certain patients respond and why some develop resistance can be made rapidly through studies of the drug target in tumor tissue from patient. One important lesson is that many cancers, even in the most advanced stages, continue to rely on a limited number of critical oncogenic signals for maintenance of the malignant phenotype. This article reviews the mechanisms of drug resistance to a variety of cancer therapeutics and provides an approach for how measures of drug target activity can be incorporated into clinical trial design.

Novel aspects of resistance to drugs targeted to dihydrofolate reductase and thymidylate synthase

Drug resistance is often a limiting factor in successful chemotherapy. Our laboratory has been interested in studying mechanisms of resistance to drugs that are targeted to the thymidylate biosynthesis pathway especially those that target thymidylate synthase (TS) and dihydrofolate reductase (DHFR). We have used leukemia as a model system to study resistance to methotrexate (MTX) and colorectal cancer as the model system to study 5-fluorouracil (5-FU) resistance. In leukemias, we and others have shown that transport, efflux, polyglutamylation and hydrolase activities are major determinants of MTX resistance. We have further reported that some leukemic cells have an increase in DHFR gene copy number possibly contributing to the resistant phenotype. Recently, we have begun to study in detail the molecular mechanisms that govern translational regulation of DHFR in response to MTX as an additional resistance mechanism. Studies thus far involving colorectal tumors obtained from patients have focused predominantly on the predictive value of levels of TS expression and p53 mutations in determining response to 5-FU. Although the predictive value of these two measures appears to be significant, given the variety of resistance to 5-FU observed in cell lines, it is not likely that these are the only measures predictive of response or responsible for acquired resistance to this drug. The enzyme uridine-cytidine monophosphate kinase (UMPK) is an essential and rate-limiting enzyme in 5-FU activation while dihydropyrimidine dehydrogenase (DPD) is a catabolic enzyme that inactivates 5-FU. Alterations in UMPK and DPD may therefore explain failure of 5-FU response in the absence of alterations in TS or p53. Transcription factors that regulate TS may also influence drug sensitivity. We have found that mRNA levels of the E2F family of transcription factors correlates with TS message levels and are higher in lung metastases than in liver metastases of colorectal cancers. Moreover, gene copy number of the E2F-1 gene appears to be increased in a significant number of samples obtained from metastases of colorectal cancer. We have also generated mutants of both DHFR and TS that confer resistance to MTX as well as 5-FU by random as well as site-directed mutagenesis. These mutants used alone or as fusion cDNAs of the mutants have proven to be useful in transplant studies where transfer of these mutant cDNAs to bone marrow cells have been shown to confer drug resistance to recipients. The fusion cDNAs of DHFR such as the DHFR-herpes simplex virus type 1 thymidine kinase (HSVTK) are also useful for regulation of gene expression in vivo using MTX as the small molecule regulator that can be monitored by positron emission tomography (PET) scanning or by optical imaging using a fusion construct such as DHFR-EGFP.

Multiple mechanisms of resistance to methotrexate and novel antifolates in human CCRF-CEM leukemia cells and their implications for folate homeostasis

We determined the mechanisms of resistance of human CCRF-CEM leukemia cells to methotrexate (MTX) vs. those to six novel antifolates: the polyglutamatable thymidylate synthase (TS) inhibitors ZD1694, multitargeted antifolate, pemetrexed, ALIMTA (MTA) and GW1843U89, the non-polyglutamatable inhibitors of TS, ZD9331, and dihydrofolate reductase, PT523, as well as DDATHF, a polyglutamatable glycinamide ribonucleotide transformylase inhibitor. CEM cells were made resistant to these drugs by clinically relevant intermittent 24 hr exposures to 5-10 microM of MTX, ZD1694, GW1843U89, MTA and DDATHF, by intermittent 72 hr exposures to 5 microM of ZD9331 and by continuous exposure to stepwise increasing concentrations of ZD9331, GW1843U89 and PT523. Development of resistance required only 3 cycles of intermittent drug exposure to ZD1694 and MTA, but 5 cycles for MTX, DDATHF and GW1843U89 and 8 cycles for ZD9331. The predominant mechanism of resistance to ZD1694, MTA, MTX and DDATHF was impaired polyglutamylation due to approximately 10-fold decreased folylpolyglutamate synthetase activity. Resistance to intermittent exposures to GW1843U89 and ZD9331 was associated with a 2-fold decreased transport via the reduced folate carrier (RFC). The CEM cell lines resistant to intermittent exposures to MTX, ZD1694, MTA, DDATHF, GW1843U89 and ZD9331 displayed a depletion (up to 4-fold) of total intracellular reduced folate pools. Resistance to continuous exposure to ZD9331 was caused by a 14-fold increase in TS activity. CEM/GW70, selected by continuous exposure to GW1843U89 was 50-fold resistant to GW1843U89, whereas continuous exposure to PT523 generated CEM/PT523 cells that were highly resistant (1550-fold) to PT523. Both CEM/GW70 and CEM/PT523 displayed cross-resistance to several antifolates that depend on the RFC for cellular uptake, including MTX (95- and 530-fold). CEM/GW70 cells were characterized by a 12-fold decreased transport of [3H]MTX. Interestingly, however, CEM/GW70 cells displayed an enhanced transport of folic acid, consistent with the expression of a structurally altered RFC resulting in a 2.6-fold increase of intracellular folate pools. CEM/PT523 cells displayed a markedly impaired (100-fold) transport of [3H]MTX along with 12-fold decreased total folate pools. In conclusion, multifunctional mechanisms of resistance in CEM cells have a differential impact on cellular folate homeostasis: decreased polyglutamylation and transport defects lead to folate depletion, whereas a structurally altered RFC protein can provoke expanded intracellular folate pools.

Relationship of p15 and p16 gene alterations to elevated dihydrofolate reductase in childhood acute lymphoblastic leukaemia

The downstream effects of p15 and p16 gene deletions and loss of transcripts on dihydrofolate reductase (DHFR) were examined in 63 B-precursor (BP) acute lymphoblastic leukaemia (ALL) samples. p15 and/or p16 gene deletions were seen in 6% and 8%, respectively, of BP-ALL samples; however, losses of p15 and/or p16 transcripts were seen in 26 out of 63 (41%) samples. Loss of p15 transcripts (36.5%) exceeded that for p16 (17.5%). For the 26 BP-ALLs that lacked p15 and/or p16 transcripts, only six (23%) exhibited low levels of DHFR by flow cytometry assay with Pt430, a fluorescent anti-folate. Conversely, 18 out of 37 (49%) BP-ALL samples with intact p15 and/or p16 genes and transcripts showed low levels of DHFR (P = 0.04). In p15- and p16-null K562 cells transfected with a tetracycline-inducible p15 cDNA construct, induction of p15 transcripts and protein was accompanied by decreased growth rates, decreased S-phase fraction, decreased retinoblastoma protein phosphorylation, and markedly reduced levels of DHFR transcripts and protein. Collectively, our results suggest that losses of p15 and/or p16 gene expression result in elevated levels of DHFR in BP-ALL in children. However, additional downstream factors undoubtedly also contribute to elevated levels of this enzyme target.

Repression of human reduced folate carrier gene expression by wild type p53

The relationship between loss of functional p53 and human reduced folate carrier (hRFC) levels and function was examined in REH lymphoblastic leukemia cells, which express wild type p53, and in p53-null K562 cells (K562(pTet-on/p53)) engineered to express wild type p53 under control of a tetracycline-inducible promoter. Activation of p53 in REH cells by treatment with daunorubicin was accompanied by decreased ( approximately 5-fold) levels of hRFC transcripts and methotrexate transport. Treatment of K562(pTet-on/p53) cells with doxycycline resulted in a dose-dependent expression of p53 protein and transcripts, increased p21 protein, decreased dihydrofolate reductase, and G(1) arrest with decreased numbers of cells in S-phase. p53 induction was accompanied by up to 3-fold decreases in hRFC transcripts transcribed from the upstream hRFC-B promoter and similar losses of hRFC protein and methotrexate uptake capacity. Expression of p15 in an analogous inducible system in K562 cells resulted in a nearly identical decrease of S-phase cells and dihydrofolate reductase without effects on hRFC levels or activity. When the hRFC-B promoter was expressed as full-length and basal promoter-luciferase reporter constructs in K562(pTet-on/p53) cells, induction of p53 with doxycycline resulted in a 3-fold loss of promoter activity, which was reversed by cotransfection with a trans-dominant-negative p53. These studies show that wild type p53 acts as a repressor of hRFC gene expression, via a mechanism that is independent of its effects on cell cycle progression.

Enzyme-catalyzed therapeutic agent (ECTA) design: activation of the antitumor ECTA compound NB1011 by thymidylate synthase

The in vivo administration of enzyme-inhibiting drugs for cancer and infectious disease often results in overexpression of the targeted enzyme. We have developed an enzyme-catalyzed therapeutic agent (ECTA) approach in which an enzyme overexpressed within the resistant cells is recruited as an intracellular catalyst for converting a relatively non-toxic substrate to a toxic product. We have investigated the potential of the ECTA approach to circumvent the thymidylate synthase (TS) overexpression-based resistance of tumor cells to conventional fluoropyrimidine [i.e. 5-fluorouracil (5-FU)] cancer chemotherapy. (E)-5-(2-Bromovinyl)-2'-deoxy-5'-uridyl phenyl L-methoxyalaninylphosphoramidate (NB1011) is a pronucleotide analogue of (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVdU), an antiviral agent known to be a substrate for TS when in the 5'-monophosphorylated form. NB1011 was synthesized and found to be at least 10-fold more cytotoxic to 5-FU-resistant, TS-overexpressing colorectal tumor cells than to normal cells. This finding demonstrates that the ECTA approach to the design of novel chemotherapeutics results in compounds that are selectively cytotoxic to tumor cell lines that overexpress the target enzyme, TS, and therefore may be useful in the treatment of fluoropyrimidine-resistant cancer.