Determination of deoxyribonucleoside triphosphate pool sizes in ribonucleotide reductase cDNA transfected human KB cells

DHS (trans-4,4'-dihydroxystilbene) suppresses DNA replication and tumor growth by inhibiting RRM2 (ribonucleotide reductase regulatory subunit M2)

DNA replication machinery is responsible for accurate and efficient duplication of the chromosome. Since inhibition of DNA replication can lead to replication fork stalling, resulting in DNA damage and apoptotic death, inhibitors of DNA replication are commonly used in cancer chemotherapy. Ribonucleotide reductase (RNR) is the rate-limiting enzyme in the biosynthesis of deoxyribonucleoside triphosphates (dNTPs) that are essential for DNA replication and DNA damage repair. Gemcitabine, a nucleotide analog that inhibits RNR, has been used to treat various cancers. However, patients often develop resistance to this drug during treatment. Thus, new drugs that inhibit RNR are needed to be developed. In this study, we identified a synthetic analog of resveratrol (3,5,4’-trihydroxy-trans-stilbene), termed DHS (trans-4,4’-dihydroxystilbene), that acts as a potent inhibitor of DNA replication. Molecular docking analysis identified the RRM2 (ribonucleotide reductase regulatory subunit M2) of RNR as a direct target of DHS. At the molecular level, DHS induced cyclin F-mediated down-regulation of RRM2 by the proteasome. Thus, treatment of cells with DHS reduced RNR activity and consequently decreased synthesis of dNTPs with concomitant inhibition of DNA replication, arrest of cells at S-phase, DNA damage, and finally apoptosis. In mouse models of tumor xenografts, DHS was efficacious against pancreatic, ovarian, and colorectal cancer cells. Moreover, DHS overcame both gemcitabine resistance in pancreatic cancer and cisplatin resistance in ovarian cancer. Thus, DHS is a novel anti-cancer agent that targets RRM2 with therapeutic potential either alone or in combination with other agents to arrest cancer development.

Human CD4+ T cells require exogenous cystine for glutathione and DNA synthesis

Adaptive immune responses require activation and expansion of antigen-specific T cells. Whereas early T cell activation is independent of exogenous cystine (Cys2), T cell proliferation is dependent of Cys2. However, the exact roles of Cys2 in T cell proliferation still need to be determined. The aim of this study was to elucidate why activated human T cells require exogenous Cys2 in order to proliferate. We activated purified naïve human CD4+ T cells and found that glutathione (GSH) levels and DNA synthesis were dependent on Cys2 and increased in parallel with increasing concentrations of Cys2. Vice-versa, the GSH synthesis inhibitor L-buthionine-sulfoximine (BSO) and inhibition of Cys2 uptake with glutamate inhibited GSH and DNA synthesis in parallel. We further found that thioredoxin (Trx) can partly substitute for GSH during DNA synthesis. Finally, we show that GSH or Trx is required for the activity of ribonucleotide reductase (RNR), the enzyme responsible for generation of the deoxyribonucleotide DNA building blocks. In conclusion, we show that activated human T cells require exogenous Cys2 to proliferate and that this is partly explained by the fact that Cys2 is required for production of GSH, which in turn is required for optimal RNR-mediated deoxyribonucleotide synthesis and DNA replication.

Inhibition of hepatitis B virus replication by targeting ribonucleotide reductase M2 protein

Chronic hepatitis B virus (HBV) infection is a key factor for hepatocellular carcinoma worldwide. Ribonucleotide reductase (RR) regulates the deoxyribonucleoside triphosphates biosynthesis and serves as a target for anti-cancer therapy. Here, we demonstrate that RR is essential for HBV replication and the viral covalently-closed-circular DNA (cccDNA) synthesis in host liver cells. By performing computer-assisted virtual screening against the crystal structure of RR small subunit M2 (RRM2), osalmid, was identified as a potential RRM2-targeting compound. Osalmid was shown to be 10-fold more active in inhibiting RR activity than hydroxyurea, and significantly inhibited HBV DNA and cccDNA synthesis in HepG2.2.15 cells. In contrast, hydroxyurea and the RR large subunit (RRM1)-inhibitory drug gemcitabine showed little selective activity against HBV replication. In addition, osalmid also was shown to possess potent activity against a 3TC-resistant HBV strain, suggesting utility in treating drug-resistant HBV infections. Interestingly, osalmid showed synergistic effects with lamivudine (3TC) in vitro and in vivo without significant toxicity, and was shown to inhibit RR activity in vivo, thus verifying its in vivo function. Furthermore, 4-cyclopropyl-2-fluoro-N-(4-hydroxyphenyl) benzamide (YZ51), a novel derivative of osalmid, showed higher efficacy than osalmid with more potent RR inhibitory activity. These results suggest that RRM2 might be targeted for HBV inhibition, and the RRM2-targeting compound osalmid and its derivative YZ51 could be a novel class of anti-HBV candidates with potential use for hepatitis B and HBV-related HCC treatment.

Reciprocal regulation of autophagy and dNTP pools in human cancer cells

Ribonucleotide reductase (RNR) plays a critical role in catalyzing the biosynthesis and maintaining the intracellular concentration of 4 deoxyribonucleoside triphosphates (dNTPs). Unbalanced or deficient dNTP pools cause serious genotoxic consequences. Autophagy is the process by which cytoplasmic constituents are degraded in lysosomes to maintain cellular homeostasis and bioenergetics. However, the role of autophagy in regulating dNTP pools is not well understood. Herein, we reported that starvation- or rapamycin-induced autophagy was accompanied by a decrease in RNR activity and dNTP pools in human cancer cells. Furthermore, downregulation of the small subunit of RNR (RRM2) by siRNA or treatment with the RNR inhibitor hydroxyurea substantially induced autophagy. Conversely, cancer cells with abundant endogenous intracellular dNTPs or treated with dNTP precursors were less responsive to autophagy induction by rapamycin, suggesting that autophagy and dNTP pool levels are regulated through a negative feedback loop. Lastly, treatment with si-RRM2 caused an increase in MAP1LC3B, ATG5, BECN1, and ATG12 transcript abundance in xenografted Tu212 tumors in vivo. Together, our results revealed a previously unrecognized reciprocal regulation between dNTP pools and autophagy in cancer cells.

The conserved Lys-95 charged residue cluster is critical for the homodimerization and enzyme activity of human ribonucleotide reductase small subunit M2

Ribonucleotide reductase (RR) catalyzes the reduction of ribonucleotides to deoxyribonucleotides for DNA synthesis. Human RR small subunit M2 exists in a homodimer form. However, the importance of the dimer form to the enzyme and the related mechanism remain unclear. In this study, we tried to identify the interfacial residues that may mediate the assembly of M2 homodimer by computational alanine scanning based on the x-ray crystal structure. Co-immunoprecipitation, size exclusion chromatography, and RR activity assays showed that the K95E mutation in M2 resulted in dimer disassembly and enzyme activity inhibition. In comparison, the charge-exchanging double mutation of K95E and E98K recovered the dimerization and activity. Structural comparisons suggested that a conserved cluster of charged residues, including Lys-95, Glu-98, Glu-105, and Glu-174, at the interface may function as an ionic lock for M2 homodimer. Although the measurements of the radical and iron contents showed that the monomer (the K95E mutant) was capable of generating the diiron and tyrosyl radical cofactor, co-immunoprecipitation and competitive enzyme inhibition assays indicated that the disassembly of M2 dimer reduced its interaction with the large subunit M1. In addition, the immunofluorescent and fusion protein-fluorescent imaging analyses showed that the dissociation of M2 dimer altered its subcellular localization. Finally, the transfection of the wild-type M2 but not the K95E mutant rescued the G1/S phase cell cycle arrest and cell growth inhibition caused by the siRNA knockdown of M2. Thus, the conserved Lys-95 charged residue cluster is critical for human RR M2 homodimerization, which is indispensable to constitute an active holoenzyme and function in cells.

A small-molecule blocking ribonucleotide reductase holoenzyme formation inhibits cancer cell growth and overcomes drug resistance

Ribonucleotide reductase (RNR) is an attractive target for anticancer agents given its central function in DNA synthesis, growth, metastasis, and drug resistance of cancer cells. The current clinically established RNR inhibitors have the shortcomings of short half-life, drug resistance, and iron chelation. Here, we report the development of a novel class of effective RNR inhibitors addressing these issues. A novel ligand-binding pocket on the RNR small subunit (RRM2) near the C-terminal tail was proposed by computer modeling and verified by site-directed mutagenesis and nuclear magnetic resonance (NMR) techniques. A compound targeting this pocket was identified by virtual screening of the National Cancer Institute (NCI) diverse small-molecule database. By lead optimization, we developed the novel RNR inhibitor COH29 that acted as a potent inhibitor of both recombinant and cellular human RNR enzymes. COH29 overcame hydroxyurea and gemcitabine resistance in cancer cells. It effectively inhibited proliferation of most cell lines in the NCI 60 human cancer panel, most notably ovarian cancer and leukemia, but exerted little effect on normal fibroblasts or endothelial cells. In mouse xenograft models of human cancer, COH29 treatment reduced tumor growth compared with vehicle. Site-directed mutagenesis, NMR, and surface plasmon resonance biosensor studies confirmed COH29 binding to the proposed ligand-binding pocket and offered evidence for assembly blockade of the RRM1-RRM2 quaternary structure. Our findings offer preclinical validation of COH29 as a promising new class of RNR inhibitors with a new mechanism of inhibition, with broad potential for improved treatment of human cancer.

Systemic delivery of siRNA nanoparticles targeting RRM2 suppresses head and neck tumor growth

Systemic delivery of siRNA to solid tumors remains challenging. In this study, we investigated the systemic delivery of a siRNA nanoparticle targeting ribonucleotide reductase subunit M2 (RRM2), and evaluated its intratumoral kinetics, efficacy and mechanism of action. Knockdown of RRM2 by an RNAi mechanism strongly inhibited cell growth in head and neck squamous cell carcinoma (HNSCC) and non-small cell lung cancer (NSCLC) cell lines. In a mouse xenograft model of HNSCC, a single intravenous injection led to the accumulation of intact nanoparticles in the tumor that disassembled over a period of at least 3days, leading to target gene knockdown lasting at least 10days. A four-dose schedule of siRNA nanoparticle delivering RRM2 siRNA targeted to HNSCC tumors significantly reduced tumor progression by suppressing cell proliferation and inducing apoptosis. These results show promise for the use of RRM2 siRNA-based therapy for HNSCC and possibly NSCLC.

Ribonucleotide reductase subunit p53R2 regulates mitochondria homeostasis and function in KB and PC-3 cancer cells

Ribonucleotide reductase (RR) is a rate-limiting enzyme that catalyzes de novo conversion of ribonucleotide 5'-diphosphates to the corresponding 2'-deoxynucleotide, essential for DNA synthesis and replication. The mutations or knockout of RR small subunit, p53R2, results in the depletion of mitochondrial DNA (mtDNA) in human, implying that p53R2 might play a critical role for maintaining mitochondrial homeostasis. In this study, siRNA against p53R2 knockdown approach is utilized to examine the impact of p53R2 depletion on mitochondria and to derive underlying mechanism in KB and PC-3 cancer cells. Our results reveal that the p53R2 expression not only positively correlates with mtDNA content, but also partakes in the proper mitochondria function, such as ATP synthesis, cytochrome c oxidase activity and membrane potential maintenance. Furthermore, overexpression of p53R2 reduces intracellular ROS and protects the mitochondrial membrane potential against oxidative stress. Unexpectedly, knockdown of p53R2 has a modest, if any, effect on mitochondrial and total cellular dNTP pools. Taken together, our study provides functional evidence that mitochondria is one of p53R2-targeted organelles and suggests an unexpected function of p53R2, which is beyond known RR function on dNTP synthesis, in mitochondrial homeostatic control.

Overexpression of RRM2 decreases thrombspondin-1 and increases VEGF production in human cancer cells in vitro and in vivo: implication of RRM2 in angiogenesis

Background

In addition to its essential role in ribonucleotide reduction, ribonucleotide reductase (RNR) small subunit, RRM2, has been known to play a critical role in determining tumor malignancy. Overexpression of RRM2 significantly enhances the invasive and metastatic potential of tumor. Angiogenesis is critical to tumor malignancy; it plays an essential role in tumor growth and metastasis. It is important to investigate whether the angiogenic potential of tumor is affected by RRM2.

Results

We examined the expression of antiangiogenic thrombospondin-1 (TSP-1) and proangiogenic vascular endothelial growth factor (VEGF) in two RRM2-overexpressing KB cells: KB-M2-D and KB-HURs. We found that TSP-1 was significantly decreased in both KB-M2-D and KB-HURs cells compared to the parental KB and mock transfected KB-V. Simultaneously, RRM2-overexpressing KB cells showed increased production of VEGF mRNA and protein. In contrast, attenuating RRM2 expression via siRNA resulted in a significant increased TSP-1 expression in both KB and LNCaP cells; while the expression of VEGF by the two cells was significantly decreased under both normoxia and hypoxia. In comparison with KB-V, overexpression of RRM2 had no significant effect on proliferation in vitro, but it dramatically accelerated in vivo subcutaneous growth of KB-M2-D. KB-M2-D possessed more angiogenic potential than KB-V, as shown in vitro by its increased chemotaxis for endothelial cells and in vivo by the generation of more vascularized tumor xenografts.

Conclusion

These findings suggest a positive role of RRM2 in tumor angiogenesis and growth through regulation of the expression of TSP-1 and VEGF.

Mechanisms by which herpes simplex virus DNA polymerase limits translesion synthesis through abasic sites

Results suggest a high probability that abasic (AP) sites occur at least once per herpes simplex virus type 1 (HSV-1) genome. The parameters that control the ability of HSV-1 DNA polymerase (pol) to engage in AP translesion synthesis (TLS) were examined because AP lesions could influence the completion and fidelity of viral DNA synthesis. Pre-steady-state kinetic experiments demonstrated that wildtype (WT) and exonuclease-deficient (exo-) pol could incorporate opposite an AP lesion, but full TLS required absence of exo function. Virtually all of the WT pol was bound at the exo site to AP-containing primer-templates (P/Ts) at equilibrium, and the pre-steady-state rate of excision by WT pol was higher on AP-containing than on matched DNA. However, several factors influencing polymerization work synergistically with exo activity to prevent HSV-1 pol from engaging in TLS. Although the pre-steady-state catalytic rate constant for insertion of dATP opposite a T or AP site was similar, ground-state-binding affinity of dATP for insertion opposite an AP site was reduced 3-9-fold. Single-turnover running-start experiments demonstrated a reduced proportion of P/Ts extended to the AP site compared to the preceding site during processive synthesis by WT or exo- pol. Only the exo- pol engaged in TLS, though inefficiently and without burst kinetics, suggesting a much slower rate-limiting step for extension beyond the AP site.

An increase of cytochrome C oxidase mediated disruption of gemcitabine incorporation into DNA in a resistant KB clone

Mechanistic aberrations leading to Gemcitabine (2',2'-dFdCyd,2,2-difluorodeoxycytidine, Gem) resistance may include alteration in its transport, metabolism and incorporation into DNA. To explore the mechanism of Gem resistance, the restriction fragment differential display PCR (RFDD-PCR) was employed to compare the mRNA expression patterns of KBGem (Gem resistant), KBHURs (hydroxyurea resistant) and KBwt (parental KB cell). Nine gene fragments were overexpressed specifically in the KBGem clone. Sequencing and BLAST results showed that three fragments represent cytochrome C oxidase (CCOX, respiration complex IV) subunit III (CCOX3). The cDNA microarray confirmed that the mRNAs of CCOX and ATP synthase subunits were upregulated in KBGem as compared to KBwt and KBHURs. The increase in CCOX1 protein and activity led to the increase of free ATP concentration, which is consistent with the gene expression profile of KBGem. Furthermore, the sensitivity to Gem could be reversed by sodium azide, a CCOX inhibitor. Following the treatment of sodium azide, the cellular accumulation of [3H]-Gem increased in a dose (of azide)-dependent manner, which is associated with increase of [3H]-Gem incorporation into DNA in KBGem. In summary, an increase of CCOX activity and free ATP level may reduce the transport, metabolism and DNA incorporation of Gem, resulting in Gem resistance.

Ribonucleotide reductase subunits M2 and p53R2 are potential biomarkers for metastasis of colon cancer

BACKGROUND

Ribonucleoside diphosphate reductase plays a key role in converting ribonucleoside diphosphate to 2'-deoxyribonucleoside diphosphate, which is necessary for DNA repair and replication. To determine if human ribonucleotide reductase small subunit M2 (hRRM2) and p53-dependent human ribonucleotide reductase small subunit R2 (p53R2) play roles on invasion ability of cancer cells, the gene transferring technique was used to construct stable hRRM2 and p53R2 overexpression transfectants. Increase of hRRM2 dramatically enhanced the cell migration in KB and PC-3 cells, but p53R2 overexpression reduced cellular invasion potential to 50% and 40% in KB and PC-3 cells, respectively. Furthermore, hRRM2 enhanced cancer cells to induce the cell migration of Human Umbilical Vein Endothelial Cells, but p53R2 reduced this ability in transfectants.

PATIENTS AND METHODS

To further determine the role of human ribonucleotide reductase subunits on cancer metastasis, a tissue array, including 59 primary and 49 metastatic colon adenocarcinoma samples, was used. Immunohistochemistry was used to evaluate the relationship between human ribonucleotide reductase subunits and metastasis.

RESULTS

Univariate and multivariate analysis revealed that p53R2 is negatively related to the metastasis of colon adenocarcinoma samples (odds ratio, 0.23; P < 0.05); hRRM2 increases the risk of metastasis in colon cancer, but did not show significantly. Thus, opposing regulation of hRRM2 and p53R2 in invasion potential might play a critical role in determining the invasion and metastasis phenotype in cancer cells.

CONCLUSION

The expression level of ribonucleotide reductase small subunits could serve as biomarkers to predict the malignancy potential of human cancers in the future.

Metastasis-suppressing potential of ribonucleotide reductase small subunit p53R2 in human cancer cells

PURPOSE

Previous gene transfection studies have shown that the accumulation of human ribonucleotide reductase small subunit M2 (hRRM2) enhances cellular transformation, tumorigenesis, and malignancy potential. The latest identified small subunit p53R2 has 80% homology to hRRM2. Here, we investigate the role of p53R2 in cancer invasion and metastasis.

EXPERIMENTAL DESIGN

The immunohistochemistry was conducted on a tissue array including 49 primary and 59 metastatic colon adenocarcinoma samples to determine the relationship between p53R2 expression and metastasis. A Matrigel invasive chamber was used to sort the highly invasive cells and to evaluate the invasion potential of p53R2.

RESULTS

Univariate and multivariate analyses revealed that p53R2 is negatively related to the metastasis of colon adenocarcinoma samples (odds ratio, 0.23; P<0.05). The decrease of p53R2 is associated with cell invasion potential, which was observed in both p53 wild-type (KB) and mutant (PC-3 and Mia PaCa-2) cell lines. An increase in p53R2 expression by gene transfection significantly reduced the cellular invasion potential to 54% and 30% in KB and PC-3 cells, respectively, whereas inhibition of p53R2 by short interfering RNA resulted in a 3-fold increase in cell migration.

CONCLUSIONS

Opposite regulation of hRRM2 and p53R2 in invasion potential might play a critical role in determining the invasion and metastasis phenotype in cancer cells. The expression level of ribonucleotide reductase small subunits may serve as a biomarker to predict the malignancy potential of human cancers in the future.

A phase I study of oxaliplatin in combination with gemcitabine: correlation of clinical outcome with gene expression

PURPOSE

Oxaliplatin has in vitro activity similar to or higher than other platinum agents. Preclinically, gemcitabine has demonstrated synergy when combined with platinum compounds. These facts formed the rationale for determining the maximum tolerated dose (MTD) of gemcitabine in combination with oxaliplatin.

METHODS

Eligible patients with advanced incurable solid tumors were given oxaliplatin 130 mg/m2 as a 2-h infusion on day 1 followed by escalating doses of gemcitabine given over 30 min on day 1 and 8 of a 21-day cycle.

RESULTS

A total of 43 patients were enrolled, including 30 patients at the MTD in an expanded cohort. At a gemcitabine dose of 800 mg/m2, 1/6 patients had a dose limiting toxicity (DLT) (grade 3 blurred vision and memory loss). At 1,000 mg/m2, 1/6 patients had a DLT (grade 3 increase in AST). At 1,200 mg/m2, 2/3 patients had a DLT (grade 4 thrombocytopenia and grade 3 confusion). The MTD of gemcitabine with 130 mg/m2 of oxaliplatin was therefore 1,000 mg/m2. The clearances of gemcitabine and ultrafilterable platinum are within the ranges previously reported for single agents. A patient with colon cancer had a partial response, and 21 patients had a best response of stable disease. In patients with tumor biopsies treated at the MTD, decreased ribonucleotide reductase M2 expression correlated with response.

CONCLUSION

Treatment with gemcitabine and oxaliplatin was well tolerated with primarily hematologic toxicity at the MTD. Study of biochemical correlates of response remain of interest thought current results remain exploratory.

The ribonucleotide reductase subunit M2B subcellular localization and functional importance for DNA replication in physiological growth of KB cells

Ribonucleoside diphosphate reductase (EC 1.17.4.1) (RR) is a potential target for antineoplastic agents due to its crucial role in DNA replication and repair. The expression and activity of RR subunits are highly regulated to maintain an optimal dNTP pool, which is required to maintain genetic fidelity. The human RR small subunit M2B (p53R2) is thought to contribute to DNA repair in response to DNA damage. However, it is not clear whether M2B is involved in providing dNTPs for DNA replication under physiological growth conditions. Serum starvation synchronized studies showed that a rapid increase of M2B was associated with cyclin E, which is responsible for regulation of G(1)/S-phase transition. A living cell sorting study that used KB cells in normal growth, further confirmed that M2B increased to maximum levels at the G(1)/S-phase transition, and decreased with DNA synthesis. Confocal studies revealed that M2B redistributed from the cytoplasm to the nucleus earlier than hRRM2 in response to DNA replication. Nuclear accumulation of M2B is associated with dynamic changes in dNTP at early periods of serum addition. By using M2B-shRNA expression vectors, inhibition of M2B may result in growth retardation in KB cells. We conclude that M2B may translocate from the cytoplasm into the nucleus and allow dNTPs to initiate DNA synthesis in KB cells under physiological conditions. Thus, our findings suggested that M2B might play an important role for initiating DNA replication of KB cells in normal growth.

Nuclear factor Y regulation and promoter transactivation of human ribonucleotide reductase subunit M2 gene in a Gemcitabine resistant KB clone

Overexpression of human ribonucleotide reductase subunit M2 (hRRM2) has been shown as a potential factor causing Gemcitabine (Gem) resistance. We hypothesized the nuclear factor Y (NF-Y) would transcriptionally regulate hRRM2 and contribute to overexpression of hRRM2 in a Gem resistant clone. A luciferase and gel shift assay, and a Southwestern blot were employed to analyze the promoter activity of hRRM2. The data exhibited the hRRM2 promoter was upregulated almost 5-fold in the Gem resistant KB clone (KBGem) via three sequential CCAAT boxes located in the proximal promoter region. Nuclear extracts from KB and KBGem could interact with the CCAAT motif of the hRRM2 proximal promoter region, and could form DNA-protein complexes with different binding patterns. The complexes could be further recognized with antibodies against NF-Y subunits A and B. Histone deacetylases (HDAC) involvement in NF-Y transcription repression in the KBGem clone was examined. A HDAC activity assay revealed a 3-fold decrease of HDAC activity in the KBGem clone compared to KB cells. Parental cells were treated with trichostatin A (TSA), a HDAC inhibitor. NF-Y transactivation was induced, resulting in an increase of hRRM2 expression. This led to an expanded dCTP pool and an abrogated [3H]Gemcitabine incorporation. In addition, microarray analysis results showed most of the proliferation-related genes were upregulated in KBGem. This finding was consistent with enhanced NF-Y transactivation in KBGem. In summary, upregulation of NF-Y transactivation increased hRRM2 transcription, which played a pivotal role in the Gem resistant KB clone.

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

Ribonucleotide reductase (RR) is a rate-limiting enzyme in DNA synthesis and repair. The enzyme consists of two dissimilar subunits, M1 and M2. It is known that the M2 subunit plays a role in tumorgenicity and metastasis. In this study, we transfected human oropharyngeal KB cancer cells with human RR M1 and M2 antisense cDNA expressed by an inducible vector system. The transfectants were double-selected with hygromycin and G418. The clones, designated KB-M1AS, KB-M2AS and KB-CAT, represented transfectant clones that contained M1 antisense cDNA, M2 antisense cDNA, and a CAT reporter gene, respectively. In a colony-forming assay, colony formation for the KB-M2AS clone decreased approximately 50% when M2 antisense mRNA expression was induced by isopropylthiogalactose (IPTG). However, the KB-M1AS clone revealed no significant inhibition under IPTG induction. RR enzyme activity, as measured by 14CDP reduction assay, revealed a 30% decrease in the IPTG-induced KB-M2AS clone relative to non-IPTG-induced samples at 144 hours. As shown by Northern blot, expression of the M2 antisense mRNA showed peaks at 48 hours and 144 hours after induction by IPTG. M2 antisense mRNA expression induced by IPTG was 33-fold greater than the uninduced control at 144 hours. Western blot analysis showed that the M2 subunit protein level decreased in the KB-M2AS clone beginning at 72 hours after induction and continued to decrease to 50% of the uninduced control at 144 hours, then showed a slight recovery at 168 hours. In conclusion, M2 antisense mRNA expression by an inducible system can effectively decrease RR M2 protein expression, reduce enzyme activity, and inhibit growth. Furthermore, this approach can be employed in future antisense investigations.