Inhibition of human cancer cell growth by inducible expression of human ribonucleotide reductase antisense cDNA

Targeting ribonucleotide reductase for cancer therapy

INTRODUCTION

Ribonucleotide reductase (RR) is a unique enzyme, because it is responsible for reducing ribonucleotides to their corresponding deoxyribonucleotides, which are the building blocks required for DNA replication and repair. Dysregulated RR activity is associated with genomic instability, malignant transformation and cancer development. The use of RR inhibitors, either as a single agent or combined with other therapies, has proven to be a promising approach for treating solid tumors and hematological malignancies.

AREAS COVERED

This review covers recent publications in the area of RR, which include: i) the structure, function and regulation of RR; ii) the roles of RR in cancer development; iii) the classification, mechanisms and clinical application of RR inhibitors for cancer therapy and iv) strategies for developing novel RR inhibitors in the future.

EXPERT OPINION

Exploring the possible nonenzymatic roles of RR subunit proteins in carcinogenesis may lead to new rationales for developing novel anticancer drugs. Updated information about the structure and holoenzyme models of RR will help in identifying potential sites in the protein that could be targets for novel RR inhibitors. Determining RR activity and subunit levels in clinical samples will provide a rational platform for developing personalized cancer therapies that use RR inhibitors.

The translational regulator eIF3a: the tricky eIF3 subunit!

Regulation of gene expression is a fundamental step in cellular physiology as abnormalities in this process may lead to de-regulated growth and cancer. Translation of mRNA is mainly regulated at the rate-limiting initiation step, where many eukaryotic initiation factors (eIFs) are involved. The largest and most complex initiation factor is eIF3 which plays a role in translational regulation, cell growth and cancer. The largest subunit of eIF3 is eIF3a, although it is not required for the general function of eIF3 in translation initiation. However, eIF3a may play a role as a regulator of a subset of mRNAs and has been demonstrated to regulate the expression of p27(kip1), tyrosinated α-tubulin and ribonucleotide reductase M2 subunit. These molecules have a pivotal role in the regulation of the cell cycle. Moreover, the eIF3a mRNA is ubiquitously expressed in all tissues at different levels and is found elevated in a number of cancer types. eIF3a can modulate the cell cycle and may be a translational regulator for proteins important for entrance into S phase. The expression of eIF3a is decreased in differentiated cells in culture and the suppression of eIF3a expression can reverse the malignant phenotype and change the sensitivity of cells to cell cycle modulators. However, the role of eIF3a in cancer is still unclear. In fact, some studies have identified eIF3a to be involved in cancer development, while other results indicate that it could provide protection against evolution into higher malignancy. Together, these findings highlight the "tricky" and interesting nature of eIF3a.

Phase I trial of GTI-2040, oxaliplatin, and capecitabine in the treatment of advanced metastatic solid tumors: a California Cancer Consortium Study

BACKGROUND

GTI-2040 is a 20-mer antisense oligonucleotide targeting the mRNA of ribonucleotide reductase M2. It was combined with oxaliplatin and capecitabine in a phase I trial in patients with advance solid tumors based on previous studies demonstrating potentiation of chemotherapy with ribonucleotide reductase inhibitors.

METHODS

Patients at least 18 years of age with advanced incurable solid tumors and normal organ function as well as a Karnofsky performance status of > or =60% were eligible. One prior chemotherapy regimen for advanced disease or relapse within 12 months of adjuvant chemotherapy was required. Patients could have received prior fluoropyrimidines, including capecitabine, but not oxaliplatin. Treatment cycles were 21 days. In each cycle, GTI-2040 was given as a continuous intravenous infusion over 14 days, oxaliplatin as a 2-h intravenous infusion on day 1, and capecitabine orally twice a day for 14 days. In cycle 1 only, oxaliplatin and capecitabine were started on day 2 to allow ribonucleotide reductase mRNA levels to be measured with and without oxaliplatin and capecitabine. Doses were escalated in cohorts of three patients using a standard 3 + 3 design until the maximum tolerated dose was established, defined as no more than one first-cycle dose-limiting toxicity among six patients treated at a given dose level.

RESULTS

The maximum tolerated dose was estimated to be the combination of GTI-2040 3 mg/kg per day for 14 days, capecitabine 600 mg/m(2) twice daily for 14 days, and oxaliplatin 100 mg/m(2) every 21 days. Dose-limiting toxicities were hematologic. GTI-2040 pharmacokinetics, obtained at steady-state on days 7 and 14, showed the high inter-patient variability previously reported. Two of six patients had stable disease at the maximum tolerated dose and one patient, with heavily pre-treated non-small cell lung cancer, had a partial response at a higher dose level. In samples from a limited number of patients, there was no clear decrease in ribonucleotide reductase expression in peripheral blood mononuclear cells during treatment.

CONCLUSION

A combination of GTI-2040, capecitabine and oxaliplatin is feasible in patients with advanced solid tumors.

Potent siRNA inhibitors of ribonucleotide reductase subunit RRM2 reduce cell proliferation in vitro and in vivo

PURPOSE

Ribonucleotide reductase (RR) is a therapeutic target for DNA replication-dependent diseases such as cancer. Here, a potent small interfering RNA (siRNA) duplex against the M2 subunit of RR (RRM2) is developed and shown to reduce the growth potential of cancer cells both in vitro and in vivo.

EXPERIMENTAL DESIGN

Three anti-RRM2 siRNAs were identified via computational methods, and the potency of these and additional "tiling" duplexes was analyzed in cultured cells via cotransfections using a RRM2-luciferase fusion construct. Knockdown of RRM2 by the best duplex candidates was confirmed directly by Western blotting. The effect of potent duplexes on cell growth was investigated by a real-time cell electronic sensing assay. Finally, duplex performance was tested in vivo in luciferase-expressing cells via whole animal bioluminescence imaging.

RESULTS

Moderate anti-RRM2 effects are observed from the three duplexes identified by computational methods. However, the tiling experiments yielded an extremely potent duplex (siR2B+5). This duplex achieves significant knockdown of RRM2 protein in cultured cells and has pronounced antiproliferative activity. S.c. tumors of cells that had been transfected with siR2B+5 preinjection grew slower than those of control cells.

CONCLUSIONS

An anti-RRM2 siRNA duplex is identified that exhibits significant antiproliferative activity in cancer cells of varying human type and species (mouse, rat, monkey); these findings suggest that this duplex is a promising candidate for therapeutic development.

Triapine and cytarabine is an active combination in patients with acute leukemia or myelodysplastic syndrome

Triapine, an iron chelator and a potent inhibitor of ribonucleotide reductase, has significant anti-leukemia activity. A phase I study of Triapine in combination with ara-C was conducted in 32 patients with refractory acute leukemia and high-risk MDS. Triapine (105 mg/m2/day 6-h infusion) was followed immediately by ara-C [100 (n=4), 200 (n=6), 400 (n=7), or 800 (n=8)mg/m2/day] as an 18-h infusion for 5 consecutive days. Dose-limiting toxicities (DLTs) were observed at the 800 mg/m2 ara-C dose level (one patient each with grade 4 mucositis; grade 4 neutropenic colitis, sepsis; grade 4 neuropathy; and grade 4 hyperbilirubinemia). Therefore, the study was amended to include an ara-C dose level of 600 mg/m2/day, no DLTs occurred in seven patients treated at this dose level. Mean Triapine C(max) and AUC were 1.13 microg/mL and 251.5 minmicrog/mL. Of 31 evaluable patients, 4 (13%) (3 AML, 1 Ph+ALL) achieved a CR (1 at a dose of 800 mg/m2; 2 at 600 mg/m2; 1 at 200mg/m2). The recommended phase II regimen is Triapine 105 mg/m2/day followed by ara-C 600 mg/m2/day for 5 consecutive days every 3-6 weeks.

Regulation of ribonucleotide reductase M2 expression by the upstream AUGs

Ribonucleotide reductase catalyzes a rate-limiting reaction in DNA synthesis by converting ribonucleotides to deoxyribonucleotides. It consists of two subunits and the small one, M2 (or R2), plays an essential role in regulating the enzyme activity and its expression is finely controlled. Changes in the M2 level influence the dNTP pool and, thus, DNA synthesis and cell proliferation. M2 gene has two promoters which produce two major mRNAs with 5′-untranslated regions (5′-UTRs) of different lengths. In this study, we found that the M2 mRNAs with the short (63 nt) 5′-UTR can be translated with high efficiency whereas the mRNAs with the long (222 nt) one cannot. Examination of the long 5′-UTR revealed four upstream AUGs, which are in the same reading frame as the unique physiological translation initiation codon. Further analysis demonstrated that these upstream AUGs act as negative cis elements for initiation at the downstream translation initiation codon and their inhibitory effect on M2 translation is eIF4G dependent. Based on the findings of this study, we conclude that the expression of M2 is likely regulated by fine tuning the translation from the mRNA with a long 5′-UTR during viral infection and during the DNA replication phase of cell proliferation.

Epidermal growth factor-induced hepatocellular carcinoma: gene expression profiles in precursor lesions, early stage and solitary tumours

Epidermal growth factor is an important mitogen for hepatocytes. Its overexpression promotes hepatocellular carcinogenesis. To identify the network of genes regulated through EGF, we investigated the liver transcriptome during various stages of hepatocarcinogenesis in EGF2B transgenic mice. Targeted overexpression of IgEGF induced distinct hepatocellular lesions and eventually solid tumours at the age of 6-8 months, as evidenced by histopathology. We used the murine MG U74Av2 oligonucleotide microarrays to identify transcript signatures in 12 tumours of small (n=5, pooled), medium (n=4) and large sizes (n=3), and compared the findings with three nontumorous transgenic livers and four control livers. Global gene expression analysis at successive stages of carcinogenesis revealed hallmarks linked to tumour size. A comparison of gene expression profiles of nontumorous transgenic liver versus control liver provided insight into the initial events predisposing liver cells to malignant transformation, and we found overexpression of c-fos, eps-15, TGIF, IGFBP1, Alcam, ets-2 and repression of Gas-1 as distinct events. Further, when gene expression profiles of small manifested tumours were compared with nontumorous transgenic liver, additional changes were obvious and included overexpression of junB, Id-1, minopontin, villin, claudin-7, RR M2, p34cdc2, cyclinD1 and cyclinB1 among others. These genes are therefore strongly associated with tumour formation. Our study provided new information on the tumour stage-dependent network of EGF-regulated genes, and we identified candidate genes linked to tumorigenes and progression of disease.

An increase in the expression of ribonucleotide reductase large subunit 1 is associated with gemcitabine resistance in non-small cell lung cancer cell lines

The mechanisms of resistance to the antimetabolite gemcitabine in non-small cell lung cancer have not been extensively evaluated. In this study, we report the generation of two gemcitabine-selected non-small cell lung cancer cell lines, H358-G200 and H460-G400. Expression profiling results indicated that there was evidence for changes in the expression of 134 genes in H358-G200 cells compared with its parental line, whereas H460-G400 cells exhibited 233 genes that appeared to be under- or overexpressed compared with H460 cells. However, only the increased expression of ribonucleotide reductase subunit 1 (RRM1), which appeared in both resistant cell lines, met predefined analysis criteria for genes to investigate further. Quantitative PCR analysis demonstrated H358-G200 cells had a greater than 125-fold increase in RRM1 RNA expression. Western blot analysis confirmed high levels of RRM1 protein in this line compared with the gemcitabine-sensitive parent. No significant change in the expression of RRM2 was observed in either cell line, although both gemcitabine-resistant cell lines had an approximate 3-fold increase in p53R2 protein. A partial revertant of H358-G200 cells had reduced levels of RRM1 protein (compared with G200 cells), without observed changes in RRM2 or p53R2. In vitro analyses of ribonucleotide reductase activity demonstrated that despite high levels of RRM1 protein, ribonucleotide reductase activity was not increased in H358-G200 cells when compared with parental cells. The cDNA encoding RRM1 from H358-G200 cells was cloned and sequenced but did not reveal the presence of any mutations. The results from this study indicate that the level of RRM1 may affect gemcitabine response. Furthermore, RRM1 may serve as a biomarker for gemcitabine response.

Association of differentially expressed genes with activation of mouse hepatic stellate cells by high-density cDNA mircoarray

AIM: To characterize the gene expression profiles associated with activation of mouse hepatic stellate cell (HSC) and provide novel insights into the pathogenesis of hepatic fibrosis.

METHODS: Mice HSCs were isolated from BALB/c mice by in situ perfusion of collagenase and pronase and single-step density Nycodenz gradient. Total RNA and mRNA of quiescent HSC and culture-activated HSC were extracted, quantified and reversely transcripted into cDNA. cDNAs from activated HSC were labeled with Cy5 and cDNAs from the quiescent HSC were labeled with Cy3, which were mixed with equal quantity, then hybridized with cDNA chips containing 4000 genes. Chips were washed, scanned and analyzed. Increased expression of 4 genes and decreased expression of one gene in activated HSC were confirmed by reverse transcription- polymerase chain reaction (RT-PCR).

RESULTS: A total of 835 differentially expressed genes were identified by cDNA chip between activated and quiescent HSC, and 465 genes were highly expressed in activated HSC. The differentially expressed genes included those involved in protein synthesis, cell-cycle regulation, apoptosis, and DNA damage response.

CONCLUSION: Many genes implicated in intrahepatic inflammation, fibrosis and proliferation were up-regulated in activated HSC. cDNA microarray is an effective technique in screening for differentially expressed genes between two different situations of the HSC. Further analysis of the obtained genes will help understand the molecular mechanism of activation of HSC and hepatic fibrosis.

Phase I and pharmacokinetic study of the ribonucleotide reductase inhibitor, 3-aminopyridine-2-carboxaldehyde thiosemicarbazone, administered by 96-hour intravenous continuous infusion

PURPOSE

3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP; Triapine; Vion Pharmaceuticals Inc, New Haven, CT) is a potent inhibitor of ribonucleotide reductase, with activity in preclinical tumor model systems. A phase I trial was initiated to determine the dose-limiting toxicities, maximum-tolerated dose, and pharmacokinetics of a 96-hour intravenous (IV) continuous infusion in patients with advanced cancer.

PATIENTS AND METHODS

Initially, courses were administered every 3 weeks, using an accelerated titration design. Subsequently, courses were administered every 2 weeks, and the dose was escalated in cohorts of three to six patients.

RESULTS

Twenty-one patients were enrolled, seven on the every-3-week schedule and 14 on the every-other-week schedule. Three of six patients at 160 mg/m(2)/d developed dose-limiting toxicities including neutropenia, hyperbilirubinemia, and nausea or vomiting. Based on these initial results, the dose for 3-AP was re-escalated beginning at 80 mg/m(2)/d but administered every 2 weeks. At 120 mg/m(2)/d, three of seven patients had dose-limiting but reversible asthenia, hyperbilirubinemia, and azotemia or acidosis; however, in the case of renal and hepatic adverse events, the events were related to pre-existing borderline abnormal organ function. Therefore, the recommended phase II dose for 3-AP administered by 96-hour IV infusion is 120 mg/m(2)/d every 2 weeks. Detailed pharmacokinetic studies demonstrated linear kinetics up to 160 mg/m(2), with substantial inter-patient variability. There was no correlation between dose and clearance (R(2) = 0.0137). There were no objective responses, but there was prolonged stabilization of disease or decreases in serum tumor markers associated with stable disease in four patients.

CONCLUSION

The 96-hour infusion of 3-AP is safe and well tolerated at the recommended phase II doses. Phase II trials of Triapine are ongoing.

Role of eIF3 p170 in controlling synthesis of ribonucleotide reductase M2 and cell growth

Translation initiation in eukaryotes is a rate-limiting step in protein synthesis. It is a complicated process that involves many eukaryotic initiation factors (eIFs). Altering the expression level or the function of eIFs may influence the synthesis of some proteins and consequently cause abnormal cell growth and malignant transformation. P170, the largest putative subunit of eIF3, has been found elevated in human breast, cervical, esophageal, and lung cancers, suggesting that p170 may have a potential role in malignant transformation and/or cell growth control. Our recent studies suggested that p170 is likely a translational regulator and it may mediate the effect of mimosine on the translation of a subset mRNAs. Mimosine, a plant nonprotein amino acid, inhibits mammalian DNA synthesis, an essential event of cell growth. The rate-limiting step in DNA synthesis is the conversion of the ribonucleotides to their corresponding deoxyribonucleotides catalysed by ribonucleotide reductase of which the activity is regulated by the level of its M2 subunit. It has been reported that inhibiting the activity of M2 also inhibits cell growth. To understand the relationship between protein and DNA synthesis and between p170 and cell growth control, we investigated in this study whether p170 regulates the synthesis of M2 and, thus, cell growth. We found that altering the expression level of p170 changes the synthesis rate of both M2 and DNA. Decreasing p170 expression in human lung cancer cell line H1299 and breast cancer cell line MCF7 significantly reversed their malignant growth phenotype. However, the overall [35S]methionine incorporation following dramatic decrease in p170 expression was only approximately 25% less than the control cells. These observations, together with our previous findings, suggest that p170 may regulate the translation of a subset mRNAs and its elevated expression level may be important for cancer cell growth and for maintaining their malignant phenotype.