Retinoblastoma tumor suppressor targets dNTP metabolism to regulate DNA replication

The bidirectional relationship between metabolism and cell cycle control

The relationship between metabolism and cell cycle progression is complex and bidirectional. Cells must rewire metabolism to meet changing biosynthetic demands across cell cycle phases. In turn, metabolism can influence cell cycle progression through direct regulation of cell cycle proteins, through nutrient-sensing signaling pathways, and through its impact on cell growth, which is linked to cell division. Furthermore, metabolism is a key player in mediating quiescence-proliferation transitions in physiologically important cell types, such as stem cells. How metabolism impacts cell cycle progression, exit, and re-entry, as well as how these processes impact metabolism, is not fully understood. Recent advances uncovering mechanistic links between cell cycle regulators and metabolic processes demonstrate a complex relationship between metabolism and cell cycle control, with many questions remaining.

Cellular microRNA-127-3p suppresses oncogenic herpesvirus-induced transformation and tumorigenesis via down-regulation of SKP2

Kaposi's sarcoma-associated herpesvirus (KSHV) causes the endothelial tumor KS, a leading cause of morbidity and mortality in sub-Saharan Africa. KSHV-encoded microRNAs (miRNAs) are known to play an important role in viral oncogenesis; however, the role of host miRNAs in KS tumorigenesis remains largely unknown. Here, high-throughput small-RNA sequencing of the cellular transcriptome in a KS xenograft model revealed miR-127-3p as one of the most significantly down-regulated miRNAs, which we validated in KS patient tissues. We show that restoration of miR-127-3p suppresses KSHV-driven cellular transformation and proliferation and induces G1 cell cycle arrest by directly targeting the oncogene SKP2. This miR-127-3p-induced G1 arrest is rescued by disrupting the miR-127-3p target site in SKP2 messenger RNA (mRNA) using gene editing. Mechanistically, miR-127-3p-mediated SKP2 repression elevates cyclin-dependent kinase (CDK) inhibitor p21Cip1 and down-regulates cyclin E, cyclin A, and CDK2, leading to activation of the RB protein tumor suppressor pathway and suppression of the transcriptional activities of E2F and Myc, key oncoprotein transcription factors crucial for KSHV tumorigenesis. Consequently, metabolomics analysis during miR-127-3p-induced cell cycle arrest revealed significant depletion of dNTP pools, consistent with RB-mediated repression of key dNTP biosynthesis enzymes. Furthermore, miR-127-3p reconstitution in a KS xenograft mouse model suppresses KSHV-positive tumor growth by targeting SKP2 in vivo. These findings identify a previously unrecognized tumor suppressor function for miR-127-3p in KS and demonstrate that the miR-127-3p/SKP2 axis is a viable therapeutic strategy for KS.

The multifaceted role of cell cycle regulators in the coordination of growth and metabolism

Adapting to changes in nutrient availability and environmental conditions is a fundamental property of cells. This adaptation requires a multi‐directional coordination between metabolism, growth, and the cell cycle regulators (consisting of the family of cyclin‐dependent kinases (CDKs), their regulatory subunits known as cyclins, CDK inhibitors, the retinoblastoma family members, and the E2F transcription factors). Deciphering the mechanisms accountable for this coordination is crucial for understanding various patho‐physiological processes. While it is well established that metabolism and growth affect cell division, this review will focus on recent observations that demonstrate how cell cycle regulators coordinate metabolism, cell cycle progression, and growth. We will discuss how the cell cycle regulators directly regulate metabolic enzymes and pathways and summarize their involvement in the endolysosomal pathway and in the functions and dynamics of mitochondria.

SAMHD1 Functions and Human Diseases

Deoxynucleoside triphosphate (dNTP) molecules are essential for the replication and maintenance of genomic information in both cells and a variety of viral pathogens. While the process of dNTP biosynthesis by cellular enzymes, such as ribonucleotide reductase (RNR) and thymidine kinase (TK), has been extensively investigated, a negative regulatory mechanism of dNTP pools was recently found to involve sterile alpha motif (SAM) domain and histidine-aspartate (HD) domain-containing protein 1, SAMHD1. When active, dNTP triphosphohydrolase activity of SAMHD1 degrades dNTPs into their 2'-deoxynucleoside (dN) and triphosphate subparts, steadily depleting intercellular dNTP pools. The differential expression levels and activation states of SAMHD1 in various cell types contributes to unique dNTP pools that either aid (i.e., dividing T cells) or restrict (i.e., nondividing macrophages) viral replication that consumes cellular dNTPs. Genetic mutations in SAMHD1 induce a rare inflammatory encephalopathy called Aicardi-Goutières syndrome (AGS), which phenotypically resembles viral infection. Recent publications have identified diverse roles for SAMHD1 in double-stranded break repair, genome stability, and the replication stress response through interferon signaling. Finally, a series of SAMHD1 mutations were also reported in various cancer cell types while why SAMHD1 is mutated in these cancer cells remains to investigated. Here, we reviewed a series of studies that have begun illuminating the highly diverse roles of SAMHD1 in virology, immunology, and cancer biology.

XRCC2 Regulates Replication Fork Progression during dNTP Alterations

RAD51 paralogs are essential for maintenance of genomic integrity through protection of stalled replication forks and homology-directed repair (HDR) of double-strand breaks. Here, we find that a subset of RAD51 paralogs, XRCC2 (FANCU) and its binding partner RAD51D, restrain active DNA synthesis during dinucleotide triphosphate (dNTP) alterations in a manner independent of HDR. The absence of XRCC2 is associated with increased levels of RRM2, the regulatory subunit of ribonucleotide reductase (RNR), and concomitantly high nucleotide pools, leading to unrestrained fork progression and accumulation of DNA damage during dNTP alterations. Mechanistically, this function is independent of redox signaling and RAD51-mediated fork reversal and is regulated by ataxia-telangiectasia and Rad3-related (ATR) signaling through phosphorylation of XRCC2 (Ser247). Together, these findings identify roles of RAD51 paralogs in the control of replication fork progression and maintenance of genome stability during nucleotide pool alterations.

Effect of induced dNTP pool imbalance on HIV-1 reverse transcription in macrophages

Background

Terminally differentiated/nondividing macrophages, a key target cell type of HIV-1, harbor extremely low dNTP concentrations established by a host dNTP triphosphohydrolase, SAM domain and HD domain containing protein 1 (SAMHD1). We tested whether the induction of dNTP pool imbalance can affect HIV-1 replication in macrophages. For this test, we induced a large dNTP pool imbalance by treating human primary monocyte derived macrophages with either one or three of the four deoxynucleosides (dNs), which are phosphorylated to dNTPs in cells, to establish two different dNTP imbalance conditions in macrophages.

Results

The transduction efficiency and 2-LTR circle copy number of HIV-1 GFP vector were greatly diminished in human primary macrophages treated with the biased dN treatments, compared to the untreated macrophages. We also observed the induced dNTP bias blocked the production of infectious dual tropic HIV-1 89.6 in macrophages. Moreover, biochemical DNA synthesis by HIV-1 reverse transcriptase was significantly inhibited by the induced dNTP pool imbalance. Third, the induced dNTP bias increased the viral mutant rate by approximately 20–30% per a single cycle infection. Finally, unlike HIV-1, the single dN treatment did not significantly affect the transduction of SIV_mac239-based GFP vector encoding Vpx in macrophages. This is likely due to Vpx, which can elevate all four dNTP levels even with the single dN treatment.

Conclusion

Collectively, these data suggest that the elevated dNTP pool imbalance can induce kinetic block and mutation synthesis of HIV-1 in macrophages.

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.

Thymidylate synthase prompts metastatic progression through the dTMP associated EMT process in pancreatic ductal adenocarcinoma

As a fundamental metabolic enzyme, anti-Thymidylate synthase (TS) strategy has been shown to be an effective therapy for human cancers. However, the genuine effects of TS in pancreatic ductal adenocarcinoma (PDA) are still conflicting. We systemically assessed the prognostic value and whether TS associated with malignant progression in PDA. Protein and mRNA expression level of TS were evaluated in en bloc PDA samples, the prognostic effect of TS expressed in cytoplasm or cytonuclear was determined separately in the first time. The impact of TS on tumor cell behaviors was assessed in in vitro assays, and the TS associated metastatic potential was further determined in two different PDA metastatic models. The retrospective clinical analysis firstly demonstrated that tumor cytonuclear TS expression was positively correlated with lymphatic metastasis and negatively correlated with the overall survival (OS) in PDA patients. The subsequent experiments further confirmed that TS depletion can effectively abate EMT (epithelial to mesenchymal) process in in vitro and decline most of the metastatic lesions in two different PDA mice models, and the deoxythymidine monophosphate (dTMP) biosynthesis malfunction resulted imbalanced dNTP pools may be the fundamental causation. Collectively, the present study suggested the prospective strategy of combined anti-TS scheme for metastatic PDA, and we strongly suggest further clinical standardization research with a large cohort to verify the prognostic value and the therapeutic potential of TS in PDA.

Control of Nucleotide Metabolism Enables Mutant p53's Oncogenic Gain-of-Function Activity

Since its discovery as an oncoprotein in 1979, investigation into p53's many identities has completed a full circle and today it is inarguably the most extensively studied tumor suppressor (wild-type p53 form or WTp53) and oncogene (mutant p53 form or mtp53) in cancer research. After the p53 protein was declared "Molecule of the Year" by Science in 1993, the p53 field exploded and a plethora of excellent reviews is now available on every aspect of p53 genetics and functional repertoire in a cell. Nevertheless, new functions of p53 continue to emerge. Here, we discuss a novel mechanism that contributes to mtp53's Gain of Functions GOF (gain-of-function) activities and involves the upregulation of both nucleotide de novo synthesis and nucleoside salvage pathways.

To Eat and to Be Eaten: Mutual Metabolic Adaptations of Immune Cells and Intracellular Bacterial Pathogens upon Infection

Intracellular bacterial pathogens (IBPs) invade and replicate in different cell types including immune cells, in particular of the innate immune system (IIS) during infection in the acute phase. However, immune cells primarily function as essential players in the highly effective and integrated host defense systems comprising the IIS and the adaptive immune system (AIS), which cooperatively protect the host against invading microbes including IBPs. As countermeasures, the bacterial pathogens (and in particular the IBPs) have developed strategies to evade or reprogram the IIS at various steps. The intracellular replication capacity and the anti-immune defense responses of the IBP's as well as the specific antimicrobial responses of the immune cells of the innate and the AIS depend on specific metabolic programs of the IBPs and their host cells. The metabolic programs of the immune cells supporting or counteracting replication of the IBPs appear to be mutually exclusive. Indeed, recent studies show that upon interaction of naïve, metabolically quiescent immune cells with IBPs, different metabolic activation processes occur which may result in the provision of a survival and replication niche for the pathogen or its eradication. It is therefore likely that within a possible host cell population subsets exist that are metabolically programmed for pro- or anti-microbial conditions. These metabolic programs may be triggered by the interactions between different bacterial agonistic components and host cell receptors. In this review, we summarize the current status in the field and discuss metabolic adaptation processes within immune cells of the IIS and the IBPs that support or restrict the intracellular replication of the pathogens.

RB1: a prototype tumor suppressor and an enigma

In this review, Dyson summarizes some recent developments in pRB research and focuses on progress toward answers for the three fundamental questions that sit at the heart of the pRB literature: What does pRB do? How does the inactivation of RB change the cell? How can our knowledge of RB function be exploited to provide better treatment for cancer patients?

The retinoblastoma susceptibility gene (RB1) was the first tumor suppressor gene to be molecularly defined. RB1 mutations occur in almost all familial and sporadic forms of retinoblastoma, and this gene is mutated at variable frequencies in a variety of other human cancers. Because of its early discovery, the recessive nature of RB1 mutations, and its frequency of inactivation, RB1 is often described as a prototype for the class of tumor suppressor genes. Its gene product (pRB) regulates transcription and is a negative regulator of cell proliferation. Although these general features are well established, a precise description of pRB's mechanism of action has remained elusive. Indeed, in many regards, pRB remains an enigma. This review summarizes some recent developments in pRB research and focuses on progress toward answers for the three fundamental questions that sit at the heart of the pRB literature: What does pRB do? How does the inactivation of RB change the cell? How can our knowledge of RB function be exploited to provide better treatment for cancer patients?

Risks at the DNA Replication Fork: Effects upon Carcinogenesis and Tumor Heterogeneity

The ability of all organisms to copy their genetic information via DNA replication is a prerequisite for cell division and a biological imperative of life. In multicellular organisms, however, mutations arising from DNA replication errors in the germline and somatic cells are the basis of genetic diseases and cancer, respectively. Within human tumors, replication errors additionally contribute to mutator phenotypes and tumor heterogeneity, which are major confounding factors for cancer therapeutics. Successful DNA replication involves the coordination of many large-scale, complex cellular processes. In this review, we focus on the roles that defects in enzymes that normally act at the replication fork and dysregulation of enzymes that inappropriately damage single-stranded DNA at the fork play in causing mutations that contribute to carcinogenesis. We focus on tumor data and experimental evidence that error-prone variants of replicative polymerases promote carcinogenesis and on research indicating that the primary target mutated by APOBEC (apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like) cytidine deaminases is ssDNA present at the replication fork. Furthermore, we discuss evidence from model systems that indicate replication stress and other cancer-associated metabolic changes may modulate mutagenic enzymatic activities at the replication fork.

Human Cytomegalovirus Can Procure Deoxyribonucleotides for Viral DNA Replication in the Absence of Retinoblastoma Protein Phosphorylation

Viral DNA replication requires deoxyribonucleotide triphosphates (dNTPs). These molecules, which are found at low levels in noncycling cells, are generated either by salvage pathways or through de novo synthesis. Nucleotide synthesis utilizes the activity of a series of nucleotide-biosynthetic enzymes (NBEs) whose expression is repressed in noncycling cells by complexes between the E2F transcription factors and the retinoblastoma (Rb) tumor suppressor. Rb-E2F complexes are dissociated and NBE expression is activated during cell cycle transit by cyclin-dependent kinase (Cdk)-mediated Rb phosphorylation. The DNA virus human cytomegalovirus (HCMV) encodes a viral Cdk (v-Cdk) (the UL97 protein) that phosphorylates Rb, induces the expression of cellular NBEs, and is required for efficient viral DNA synthesis. A long-held hypothesis proposed that viral proteins with Rb-inactivating activities functionally similar to those of UL97 facilitated viral DNA replication in part by inducing the de novo production of dNTPs. However, we found that dNTPs were limiting even in cells infected with wild-type HCMV in which UL97 is expressed and Rb is phosphorylated. Furthermore, we revealed that both de novo and salvage pathway enzymes contribute to viral DNA replication during HCMV infection and that Rb phosphorylation by cellular Cdks does not correct the viral DNA replication defect observed in cells infected with a UL97-deficient virus. We conclude that HCMV can obtain dNTPs in the absence of Rb phosphorylation and that UL97 can contribute to the efficiency of DNA replication in an Rb phosphorylation-independent manner.

IMPORTANCE

Transforming viral oncoproteins, such as adenovirus E1A and papillomavirus E7, inactivate Rb. The standard hypothesis for how Rb inactivation facilitates infection with these viruses is that it is through an increase in the enzymes required for DNA synthesis, which include nucleotide-biosynthetic enzymes. However, HCMV UL97, which functionally mimics these viral oncoproteins through phosphorylation of Rb, fails to induce the production of nonlimiting amounts of dNTPs. This finding challenges the paradigm of the role of Rb inactivation during DNA virus infection and uncovers the existence of an alternative mechanism by which UL97 contributes to HCMV DNA synthesis. The ineffectiveness of the UL97 inhibitor maribavir in clinical trials might be better explained with a fuller understanding of the role of UL97 during infection. Furthermore, as the nucleoside analog ganciclovir is the current drug of choice for treating HCMV, knowing the provenance of the dNTPs incorporated into viral DNA may help inform antiviral therapeutic regimens.

CDK4/6 inhibitors have potent activity in combination with pathway selective therapeutic agents in models of pancreatic cancer

Pancreatic ductal adenocarcinoma (PDA) has a poor prognosis, in part, due to the therapy-recalcitrant nature of the disease. Loss of the CDK4/6 inhibitor CDKN2A is a signature genetic event in PDA. Therefore, PDA may be amenable to treatment with pharmaceutical CDK4/6 inhibitors. Surprisingly, response to CDK4/6 inhibition was highly variable in PDA models, and associated with differential suppression of gene expression. Mitotic genes were repressed and FOXM1 was uniformly attenuated; however, genes involved in DNA replication were uniquely suppressed in sensitive models. Aberrant induction of Cyclin E1 was associated with resistance, and knockdown demonstrated synergistic suppression of the cell cycle with CDK4/6 inhibition. Combination therapies are likely required for the effective treatment of disease, and drug screening demonstrated additive/antagonistic interactions with CDK4/6 inhibitors. Agents dependent on mitotic progression (taxanes/PLK1 inhibitors) were antagonized by CDK4/6 inhibition, while the response to 5-FU and gemcitabine exhibited drug specific interactions. PI3K/MTOR and MEK inhibitors potently cooperated with CDK4/6 inhibition. These agents were synergistic with CDK4/6 inhibition, blocked the aberrant upregulation of Cyclin E1, and yielded potent inhibition of tumor cell growth. Together, these data identify novel mechanisms of resistance to CDK4/6 inhibitions and provide a roadmap for combination therapies in the treatment of PDA.

Deoxyribonucleotide metabolism, mutagenesis and cancer

Cancer was recognized as a genetic disease at least four decades ago, with the realization that the spontaneous mutation rate must increase early in tumorigenesis to account for the many mutations in tumour cells compared with their progenitor pre-malignant cells. Abnormalities in the deoxyribonucleotide pool have long been recognized as determinants of DNA replication fidelity, and hence may contribute to mutagenic processes that are involved in carcinogenesis. In addition, many anticancer agents antagonize deoxyribonucleotide metabolism. Here, we consider the extent to which aspects of deoxyribonucleotide metabolism contribute to our understanding of both carcinogenesis and to the effective use of anticancer agents.

Regulation of nucleotide metabolism by mutant p53 contributes to its gain-of-function activities

Mutant p53 (mtp53) is an oncogene that drives cancer cell proliferation. Here we report that mtp53 associates with the promoters of numerous nucleotide metabolism genes (NMG). Mtp53 knockdown reduces NMG expression and substantially depletes nucleotide pools, which attenuates GTP dependent protein (GTPase) activity and cell invasion. Addition of exogenous guanosine or GTP restores the invasiveness of mtp53 knockdown cells, suggesting that mtp53 promotes invasion by increasing GTP. Additionally, mtp53 creates a dependency on the nucleoside salvage pathway enzyme deoxycytidine kinase (dCK) for the maintenance of a proper balance in dNTP pools required for proliferation. These data indicate that mtp53 harboring cells have acquired a synthetic sick or lethal phenotype relationship with the nucleoside salvage pathway. Finally, elevated expression of NMG correlates with mutant p53 status and poor prognosis in breast cancer patients. Thus, mtp53’s control of nucleotide biosynthesis has both a driving and sustaining role in cancer development.

Regulation of cancer metabolism by oncogenes and tumor suppressors

Cell proliferation requires the coordination of multiple signaling pathways as well as the provision of metabolic substrates. Nutrients are required to generate such building blocks and their form of utilization differs to significant extents between malignant tissues and their nontransformed counterparts. Thus, oncogenes and tumor suppressor genes regulate the proliferation of cancer cells also by controlling their metabolism. Here, we discuss the central anabolic functions of the signaling pathways emanating from mammalian target of rapamycin, MYC, and hypoxia-inducible factor-1. Moreover, we analyze how oncogenic proteins like phosphoinositide-3-kinase, AKT, and RAS, tumor suppressors such as phosphatase and tensin homolog, retinoblastoma, and p53, as well as other factors associated with the proliferation or survival of cancer cells, such as NF-κB, regulate cellular metabolism.

Colon cancer-associated mutator DNA polymerase δ variant causes expansion of dNTP pools increasing its own infidelity

Defects in DNA polymerases δ (Polδ) and ε (Polε) cause hereditary colorectal cancer and have been implicated in the etiology of some sporadic colorectal and endometrial tumors. We previously reported that the yeast pol3-R696W allele mimicking a human cancer-associated variant, POLD1-R689W, causes a catastrophic increase in spontaneous mutagenesis. Here, we describe the mechanism of this extraordinary mutator effect. We found that the mutation rate increased synergistically when the R696W mutation was combined with defects in Polδ proofreading or mismatch repair, indicating that pathways correcting DNA replication errors are not compromised in pol3-R696W mutants. DNA synthesis by purified Polδ-R696W was error-prone, but not to the extent that could account for the unprecedented mutator phenotype of pol3-R696W strains. In a search for cellular factors that augment the mutagenic potential of Polδ-R696W, we discovered that pol3-R696W causes S-phase checkpoint-dependent elevation of dNTP pools. Abrogating this elevation by strategic mutations in dNTP metabolism genes eliminated the mutator effect of pol3-R696W, whereas restoration of high intracellular dNTP levels restored the mutator phenotype. Further, the use of dNTP concentrations present in pol3-R696W cells for in vitro DNA synthesis greatly decreased the fidelity of Polδ-R696W and produced a mutation spectrum strikingly similar to the spectrum observed in vivo. The results support a model in which (i) faulty synthesis by Polδ-R696W leads to a checkpoint-dependent increase in dNTP levels and (ii) this increase mediates the hypermutator effect of Polδ-R696W by facilitating the extension of mismatched primer termini it creates and by promoting further errors that continue to fuel the mutagenic pathway.

Genetic evidence that both dNTP-stabilized and strand slippage mechanisms may dictate DNA polymerase errors within mononucleotide microsatellites

Mononucleotide microsatellites are tandem repeats of a single base pair, abundant within coding exons and frequent sites of mutation in the human genome. Because the repeated unit is one base pair, multiple mechanisms of insertion/deletion (indel) mutagenesis are possible, including strand-slippage, dNTP-stabilized, and misincorportion-misalignment. Here, we examine the effects of polymerase identity (mammalian Pols α, β, κ, and η), template sequence, dNTP pool size, and reaction temperature on indel errors during in vitro synthesis of mononucleotide microsatellites. We utilized the ratio of insertion to deletion errors as a genetic indicator of mechanism. Strikingly, we observed a statistically significant bias toward deletion errors within mononucleotide repeats for the majority of the 28 DNA template and polymerase combinations examined, with notable exceptions based on sequence and polymerase identity. Using mutator forms of Pol β did not substantially alter the error specificity, suggesting that mispairing-misalignment mechanism is not a primary mechanism. Based on our results for mammalian DNA polymerases representing three structurally distinct families, we suggest that dNTP-stabilized mutagenesis may be an alternative mechanism for mononucleotide microsatellite indel mutation. The change from a predominantly dNTP-stabilized mechanism to a strand-slippage mechanism with increasing microsatellite length may account for the differential rates of tandem repeat mutation that are observed genome-wide.

Preclinical characterization of the CDK4/6 inhibitor LY2835219: in-vivo cell cycle-dependent/independent anti-tumor activities alone/in combination with gemcitabine

The G1 restriction point is critical for regulating the cell cycle and is controlled by the Rb pathway (CDK4/6-cyclin D1-Rb-p16/ink4a). This pathway is important because of its inactivation in a majority of human tumors. Transition through the restriction point requires phosphorylation of retinoblastoma protein (Rb) by CDK4/6, which are highly validated cancer drug targets. We present the identification and characterization of a potent CDK4/6 inhibitor, LY2835219. LY2835219 inhibits CDK4 and CDK6 with low nanomolar potency, inhibits Rb phosphorylation resulting in a G1 arrest and inhibition of proliferation, and its activity is specific for Rb-proficient cells. In vivo target inhibition studies show LY2835219 is a potent inhibitor of Rb phosphorylation, induces a complete cell cycle arrest and suppresses expression of several Rb-E2F-regulated proteins 24 hours after a single dose. Oral administration of LY2835219 inhibits tumor growth in human tumor xenografts representing different histologies in tumor-bearing mice. LY2835219 is effective and well tolerated when administered up to 56 days in immunodeficient mice without significant loss of body weight or tumor outgrowth. In calu-6 xenografts, LY2835219 in combination with gemcitabine enhanced in vivo antitumor activity without a G1 cell cycle arrest, but was associated with a reduction of ribonucleotide reductase expression. These results suggest LY2835219 may be used alone or in combination with standard-of-care cytotoxic therapy. In summary, we have identified a potent, orally active small-molecule inhibitor of CDK4/6 that is active in xenograft tumors. LY2835219 is currently in clinical development.

SAMHD1 specifically affects the antiviral potency of thymidine analog HIV reverse transcriptase inhibitors

Sterile alpha motif and histidine-aspartic domain-containing protein 1 (SAMHD1) is a deoxynucleoside triphosphate (dNTP) triphosphohydrolase recently recognized as an antiviral factor that acts by depleting dNTP availability for viral reverse transcriptase (RT). SAMHD1 restriction is counteracted by the human immunodeficiency virus type 2 (HIV-2) accessory protein Vpx, which targets SAMHD1 for proteosomal degradation, resulting in an increased availability of dNTPs and consequently enhanced viral replication. Nucleoside reverse transcriptase inhibitors (NRTI), one of the most common agents used in antiretroviral therapy, compete with intracellular dNTPs as the substrate for viral RT. Consequently, SAMHD1 activity may be influencing NRTI efficacy in inhibiting viral replication. Here, a panel of different RT inhibitors was analyzed for their different antiviral efficacy depending on SAMHD1. Antiviral potency was measured for all the inhibitors in transformed cell lines and primary monocyte-derived macrophages and CD4(+) T cells infected with HIV-1 with or without Vpx. No changes in sensitivity to non-NRTI or the integrase inhibitor raltegravir were observed, but for NRTI, sensitivity significantly changed only in the case of the thymidine analogs (AZT and d4T). The addition of exogenous thymidine mimicked the change in viral sensitivity observed after Vpx-mediated SAMHD1 degradation, pointing toward a differential effect of SAMHD1 activity on thymidine. Accordingly, sensitivity to AZT was also reduced in CD4(+) T cells infected with HIV-2 compared to infection with the HIV-2ΔVpx strain. In conclusion, reduction of SAMHD1 levels significantly decreases HIV sensitivity to thymidine but not other nucleotide RT analog inhibitors in both macrophages and lymphocytes.

Suppression of antifolate resistance by targeting the myosin Va trafficking pathway in melanoma

Human melanoma is a significant clinical problem. As most melanoma patients relapse with lethal drug-resistant disease, understanding and preventing mechanism(s) of resistance is one of the highest priorities to improve melanoma therapy. Melanosomal sequestration and the cellular exportation of cytotoxic drugs have been proposed to be important melanoma-specific mechanisms that contribute to multidrug resistance in melanoma. Concretely, we found that treatment of melanoma with methotrexate (MTX) altered melanogenesis and accelerated the exportation of melanosomes; however, the cellular and molecular processes by which MTX is trapped into melanosomes and exported out of cells have not been elucidated. In this study, we identified myosin Va (MyoVa) as a possible mediator of these cellular processes. The results demonstrated that melanoma treatment with MTX leads to Akt2-dependent MyoVa phosphorylation, which enhances its ability to interact with melanosomes and accelerates their exportation. To understand the mechanism(s) by which MTX activates Akt2, we examined the effects of this drug on the activity of protein phosphatase 2A, an Akt inhibitor activated by the methylation of its catalytic subunit. Taken together, this study identified a novel trafficking pathway in melanoma that promotes tumor resistance through Akt2/MyoVa activation. Because of these findings, we explored several MTX combination therapies to increase the susceptibility of melanoma to this drug. By avoiding MTX exportation, we observed that the E2F1 apoptotic pathway is functional in melanoma, and its induction activates p73 and apoptosis protease-activating factor 1 following a p53-autonomous proapoptotic signaling event.

p21-mediated RNR2 repression restricts HIV-1 replication in macrophages by inhibiting dNTP biosynthesis pathway

Macrophages are a major target cell for HIV-1, and their infection contributes to HIV pathogenesis. We have previously shown that the cyclin-dependent kinase inhibitor p21 inhibits the replication of HIV-1 and other primate lentiviruses in human monocyte-derived macrophages by impairing reverse transcription of the viral genome. In the attempt to understand the p21-mediated restriction mechanisms, we found that p21 impairs HIV-1 and simian immunodeficiency virus (SIV)mac reverse transcription in macrophages by reducing the intracellular deoxyribonucleotide (dNTP) pool to levels below those required for viral cDNA synthesis by a SAM domain and HD domain-containing protein 1 (SAMHD1)-independent pathway. We found that p21 blocks dNTP biosynthesis by down-regulating the expression of the RNR2 subunit of ribonucleotide reductase, an enzyme essential for the reduction of ribonucleotides to dNTP. p21 inhibits RNR2 transcription by repressing E2F1 transcription factor, its transcriptional activator. Our findings unravel a cellular pathway that restricts HIV-1 and other primate lentiviruses by affecting dNTP synthesis, thereby pointing to new potential cellular targets for anti-HIV therapeutic strategies.

The multiple connections between pRB and cell metabolism

The pRB tumor suppressor is traditionally seen as an important regulator of the cell cycle. pRB represses the transcriptional activation of a diverse set of genes by the E2F transcription factors and prevents inappropriate S-phase entry. Advances in our understanding of pRB have documented roles that extend beyond the cell cycle and this review summarizes recent studies that link pRB to the control of cell metabolism. pRB has been shown to regulate glucose tolerance, mitogenesis, glutathione synthesis, and the expression of genes involved in central carbon metabolism. Several studies have demonstrated that pRB directly targets a set of genes that are crucial for nucleotide metabolism, and this seems likely to represent one of the ways by which pRB influences the G1/S-phase transition and S-phase progression.

Directed phenotype switching as an effective antimelanoma strategy

Therapeutic resistance in melanoma and other cancers arises via irreversible genetic, and dynamic phenotypic, heterogeneity. Here, we use directed phenotype switching in melanoma to sensitize melanoma cells to lineage-specific therapy. We show that methotrexate (MTX) induces microphthalmia-associated transcription factor (MITF) expression to inhibit invasiveness and promote differentiation-associated expression of the melanocyte-specific Tyrosinase gene. Consequently, MTX sensitizes melanomas to a tyrosinase-processed antifolate prodrug 3-O-(3,4,5-trimethoxybenzoyl)-(-)-epicatechin (TMECG), that inhibits the essential enzyme DHFR with high affinity. The combination of MTX and TMECG leads to depletion of thymidine pools, double-strand DNA breaks, and highly efficient E2F1-mediated apoptosis in culture and in vivo. Importantly, this drug combination delivers an effective and tissue-restricted antimelanoma therapy in vitro and in vivo irrespective of BRAF, MEK, or p53 status.

Intracellular nucleotide levels and the control of retroviral infections

Retroviruses consume cellular deoxynucleoside triphosphates (dNTPs) to convert their RNA genomes into proviral DNA through reverse transcription. While all retroviruses replicate in dividing cells, lentiviruses uniquely replicate in nondividing cells such as macrophages. Importantly, dNTP levels in nondividing cells are extremely low, compared to dividing cells. Indeed, a recently discovered anti-HIV/SIV restriction factor, SAMHD1, which is a dNTP triphosphohydrolase, is responsible for the limited dNTP pool of nondividing cells. Lentiviral reverse transcriptases (RT) uniquely stay functional even at the low dNTP concentrations in nondividing cells. Interestingly, Vpx of HIV-2/SIVsm proteosomally degrades SAMHD1, which elevates cellular dNTP pools and accelerates lentiviral replication in nondividing cells. These Vpx-encoding lentiviruses rapidly replicate in nondividing cells by encoding both highly functional RTs and Vpx. Here, we discuss a series of mechanistic and virological studies that have contributed to conceptually linking cellular dNTP levels and the adaptation of lentiviral replication in nondividing cells.

Leishmania induces survival, proliferation and elevated cellular dNTP levels in human monocytes promoting acceleration of HIV co-infection

Leishmaniasis is a parasitic disease that is widely prevalent in many tropical and sub-tropical regions of the world. Infection with Leishmania has been recognized to induce a striking acceleration of Human Immunodeficiency Virus Type 1 (HIV-1) infection in coinfected individuals through as yet incompletely understood mechanisms. Cells of the monocyte/macrophage lineage are the predominant cell types coinfected by both pathogens. Monocytes and macrophages contain extremely low levels of deoxynucleoside triphosphates (dNTPs) due to their lack of cell cycling and S phase, where dNTP biosynthesis is specifically activated. Lentiviruses, such as HIV-1, are unique among retroviruses in their ability to replicate in these non-dividing cells due, at least in part, to their highly efficient reverse transcriptase (RT). Nonetheless, viral replication progresses more efficiently in the setting of higher intracellular dNTP concentrations related to enhanced enzyme kinetics of the viral RT. In the present study, in vitro infection of CD14+ peripheral blood-derived human monocytes with Leishmania major was found to induce differentiation, marked elevation of cellular p53R2 ribonucleotide reductase subunit and R2 subunit expression. The R2 subunit is restricted to the S phase of the cell cycle. Our dNTP assay demonstrated significant elevation of intracellular monocyte-derived macrophages (MDMs) dNTP concentrations in Leishmania-infected cell populations as compared to control cells. Infection of Leishmania-maturated MDMs with a pseudotyped GFP expressing HIV-1 resulted in increased numbers of GFP+ cells in the Leishmania-maturated MDMs as compared to control cells. Interestingly, a sub-population of Leishmania-maturated MDMs was found to have re-entered the cell cycle, as demonstrated by BrdU labeling. In conclusion, Leishmania infection of primary human monocytes promotes the induction of an S phase environment and elevated dNTP levels with notable elevation of HIV-1 expression in the setting of coinfection.

Leishmaniasis is a parasitic disease that infects several human host immune cells, including neutrophils, monocytes, and macrophages. Moreover, while HIV-1 infects monocytes and macrophages, only the infected macrophages productively release viral progenies. Importantly, patients coinfected with both pathogens progress more rapidly to AIDS. In this study, we examine how Leishmania major changes the cellular environment of monocytes in vitro. We found that Leishmania-infected monocytes actively mature into macrophages in the absence of GM-CSF, and that these cells up-regulate the expression of ribonucleotide reductase, an enzyme that catalyzes the formation of deoxynucleoside triphosphates (dNTPs). We confirmed the elevation of dNTP concentrations using a very sensitive dNTP assay for monocytes and monocyte-maturated macrophages. Collectively, these data support a model in which infection of monocytes with Leishmania elevates the intracellular dNTP pools, which is one of the natural anti-viral blocks to HIV-1 infection in monocytes and macrophages in patients.

Melanoma coordinates general and cell-specific mechanisms to promote methotrexate resistance

Melanoma, the most aggressive form of skin cancer, is notoriously resistant to all current modalities of cancer therapy, including to the drug methotrexate. Melanosomal sequestration and cellular exportation of methotrexate have been proposed to be important melanoma-specific mechanisms that contribute to the resistance of melanoma to methotrexate. In addition, other mechanisms of resistance that are present in most epithelial cancer cells are also operative in melanoma. This report elucidates how melanoma orchestrates these mechanisms to become extremely resistant to methotrexate, where both E2F1 and checkpoint kinase 1 (Chk1), two molecules with dual roles in survival/apoptosis, play prominent roles. The results indicated that MTX induced the depletion of dihydrofolate in melanoma cells, which stimulated the transcriptional activity of E2F1. The elevate expression of dihydrofolate reductase and thymidylate synthase, two E2F1-target genes involved in folate metabolism and required for G(1) progression, favored dTTP accumulation, which promoted DNA single strand breaks and the subsequent activation of Chk1. Under these conditions, melanoma cells are protected from apoptosis by arresting their cell cycle in S phase. Excess of dTTP could also inhibit E2F1-mediated apoptosis in melanoma cells.

Expression ratio of CCND1 to CDKN2A mRNA predicts RB1 status of cultured cancer cell lines and clinical tumor samples

Background

The retinoblastoma product (RB1) is frequently deregulated in various types of tumors by mutation, deletion, or inactivation through association with viral oncoproteins. The functional loss of RB1 is recognized to be one of the hallmarks that differentiate cancer cells from normal cells. Many researchers are attempting to develop anti-tumor agents that are preferentially effective against RB1-negative tumors. However, to identify patients with RB1-negative cancers, it is imperative to develop predictive biomarkers to classify RB1-positive and -negative tumors.

Results

Expression profiling of 30 cancer cell lines composed of 16 RB1-positive and 14 RB1-negative cancers was performed to find genes that are differentially expressed between the two groups, resulting in the identification of an RB1 signature with 194 genes. Among them, critical RB1 pathway components CDKN2A and CCND1 were included. We found that microarray data of the expression ratio of CCND1 and CDKN2A clearly distinguished the RB1 status of 30 cells lines. Measurement of the CCND1/CDKN2A mRNA expression ratio in additional cell lines by RT-PCR accurately predicted RB1 status (12/12 cells lines). The expression of CCND1/CDKN2A also correlated with RB1 status in xenograft tumors in vivo. Lastly, a CCND1/CDKN2A assay with clinical samples showed that uterine cervical and small cell lung cancers known to have a high prevalence of RB1-decifiency were predicted to be 100% RB1-negative, while uterine endometrial or gastric cancers were predicted to be 5-22% negative. All clinically normal tissues were 100% RB1-positive.

Conclusions

We report here that the CCND1/CDKN2A mRNA expression ratio predicts the RB1 status of cell lines in vitro and xenograft tumors and clinical tumor samples in vivo. Given the high predictive accuracy and quantitative nature of the CCND1/CDKN2A expression assay, the assay could be utilized to stratify patients for anti-tumor agents with preferential effects on either RB1-positive or -negative tumors.

Thymidylate synthase inhibition induces p53-dependent and p53-independent apoptotic responses in human urinary bladder cancer cells

PURPOSE

In search for more effective clinical protocols, the antimetabolite drug 5-fluorouracil (5-FU) has been successfully included in new regimens of bladder cancer combination chemotherapy. In the present study, we have investigated the effects of 5-FU treatment on apoptosis induction in wild-type and mutant p53 urinary bladder cancer cells.

METHODS

We have used MTT-based assays, FACS analysis, Western blotting and semi-quantitative RT-PCR in RT4 and RT112 (grade I, wild-type p53), as well as in T24 (grade III, mutant p53) and TCCSUP (grade IV, mutant p53) human urinary bladder cancer cell lines.

RESULTS

In the urothelial bladder cancer cell lines RT4 and T24, 5-FU-induced TS inhibition proved to be associated with cell type-dependent (a) sensitivity to the drug, (b) Caspase-mediated apoptosis, (c) p53 stabilization and activation, as well as Rb phosphorylation and E2F1 expression and (d) transcriptional regulation of p53 target genes and their cognate proteins, while an E2F-dependent transcriptional network did not seem to be critically engaged in such type of responses.

CONCLUSIONS

We have shown that in the wild-type p53 context of RT4 cells, 5-FU-triggered apoptosis was prominently efficient and mainly regulated by p53-dependent mechanisms, whereas the mutant p53 environment of T24 cells was able to provide notable levels of resistance to apoptosis, basically ascribed to E2F-independent, and still unidentified, pathways. Nevertheless, the differential vulnerability of RT4 and T24 cells to 5-FU administration could also be associated with cell-type-specific transcriptional expression patterns of certain genes critically involved in 5-FU metabolism.

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.

High prevalence of retinoblastoma protein loss in triple-negative breast cancers and its association with a good prognosis in patients treated with adjuvant chemotherapy

BACKGROUND

Triple-negative breast cancer (TNBC) is an aggressive disease, nevertheless exhibiting a high response rate to chemotherapy. Since the retinoblastoma protein (pRb) loss confers a high sensitivity to chemotherapy regimens, we evaluated the prevalence of pRb loss in TNBCs and its relevance on the clinical outcome of patients treated with adjuvant chemotherapy.

PATIENTS AND METHODS

pRb status was prospectively evaluated by immunocytochemistry in 518 consecutive patients with complete receptor information. The predictive value of pRb status in TNBCs was determined according to the adjuvant therapeutic treatments.

RESULTS

Fifty-three tumors were identified as TNBCs. The prevalence of pRb loss was significantly higher in TNBCs than in the other cancer subtypes. All patients with TNBCs lacking pRb and treated with systemic chemotherapy (cyclophosphamide, methotrexate and 5-fluorouracil) were disease free at a medium follow-up time of 109 months, whereas the clinical outcome of those expressing pRb was significantly poorer (P = 0.008). Analysis of disease-free survival including the established anatomo-clinical prognostic parameters indicated pRb loss as the only significant predictive factor.

CONCLUSIONS

pRb loss is much more frequent in TNBCs than in the other breast cancer subtypes. Patients with TNBCs lacking pRb had a very favorable clinical outcome if treated with conventional adjuvant chemotherapy.

Tailoring to RB: tumour suppressor status and therapeutic response

The retinoblastoma tumour suppressor (RB) is a crucial regulator of cell-cycle progression that is invoked in response to a myriad of anti-mitogenic signals. It has been hypothesized that perturbations of the RB pathway confer a synonymous proliferative advantage to tumour cells; however, recent findings demonstrate context-specific outcomes associated with such lesions. Particularly, loss of RB function is associated with differential response to wide-ranging therapeutic agents. Thus, the status of this tumour suppressor may be particularly informative in directing treatment regimens.

Loss of retinoblastoma tumor suppressor protein makes human breast cancer cells more sensitive to antimetabolite exposure

PURPOSE

The RB tumor-suppressor activity may influence the therapeutic response in human breast cancers. The effect of adjuvant therapy on clinical outcome of breast cancer patients was analyzed, and the sensitivity to 5-fluorouracil (5-FU) and methotrexate was investigated in MCF-7 and HCT-116 human cancer cells, according to their RB status.

EXPERIMENTAL DESIGN

RB protein (pRB) expression was prospectively evaluated by immunocytochemistry in 518 consecutive patients and its predictive value was determined according to the adjuvant therapeutic treatments. MCF-7 and HCT-116 human cancer cells silenced for RB1 expression were treated with 5-FU and methotrexate, at the same concentrations and time exposures as determined in the interstitium of breast cancers of patients treated with adjuvant chemotherapy.

RESULTS

Multivariate analysis of disease-free survival, including all the established clinical and histopathologic prognostic variables, indicated that the absence of pRB expression was the only predictive factor of good clinical outcome in patients treated with standard systemic chemotherapy (cyclophosphamide, methotrexate, and 5-FU) but not in patients treated with endocrine therapy alone. 5-FU and methotrexate significantly reduced the growth rate of RB1-silenced but not of control MCF-7 and HCT-116 cells. This was likely due to the absence of a DNA damage checkpoint with accumulation of DNA double-strand breaks in RB1-silenced but not in control cells.

CONCLUSIONS

The absence of pRB expression renders human breast cancer cells more sensitive to 5-FU and methotrexate and predicts a good clinical outcome for patients treated with adjuvant chemotherapy. We suggest that patients with RB-negative breast cancers should be treated with systemic chemotherapy.

Folate-mediated one-carbon metabolism

Tetrahydrofolate (THF) polyglutamates are a family of cofactors that carry and chemically activate one-carbon units for biosynthesis. THF-mediated one-carbon metabolism is a metabolic network of interdependent biosynthetic pathways that is compartmentalized in the cytoplasm, mitochondria, and nucleus. One-carbon metabolism in the cytoplasm is required for the synthesis of purines and thymidylate and the remethylation of homocysteine to methionine. One-carbon metabolism in the mitochondria is required for the synthesis of formylated methionyl-tRNA; the catabolism of choline, purines, and histidine; and the interconversion of serine and glycine. Mitochondria are also the primary source of one-carbon units for cytoplasmic metabolism. Increasing evidence indicates that folate-dependent de novo thymidylate biosynthesis occurs in the nucleus of certain cell types. Disruption of folate-mediated one-carbon metabolism is associated with many pathologies and developmental anomalies, yet the biochemical mechanisms and causal metabolic pathways responsible for the initiation and/or progression of folate-associated pathologies have yet to be established. This chapter focuses on our current understanding of mammalian folate-mediated one-carbon metabolism, its cellular compartmentation, and knowledge gaps that limit our understanding of one-carbon metabolism and its regulation.

Pretreatment with 5-FU enhances cisplatin cytotoxicity in head and neck squamous cell carcinoma cells

PURPOSE

In the treatment of head and neck malignancy, cisplatin and 5-FU have been used the most as chemotherapeutic agents. The difference in efficacies of these is unclear and controversial. To investigate more effective schedule, we analyzed the cytotoxicity in different treatment sequence with two agents in vitro and the mechanism for different effectiveness.

METHODS

UM-SCC-23 and UM-SCC-81B, head and neck squamous cell carcinoma cell lines, were analyzed for cellular killing in alternative sequence treatment with cisplatin and 5-FU. The treatment schedule was designed based on the clinical regimen. To determine the mechanism for the difference of cytotoxicity with each schedule, cell cycle distributions of both cells after 5-FU treatment with various durations were analyzed by flow-cytometry and immunostaining with anti-PCNA and anti-BrdU.

RESULTS

5-FU pretreatment followed by cisplatin treatment showed higher cell killing in both types of cells than the reverse treatment schedule. In the cell cycle analysis and immunostaining after the treatment of 5-FU, the rate of PCNA-positive cells was increased from 24 to 144 h in both cells. The rate of BrdU-positive cells of UM-SCC-81B in flow-cytometry was also increased, while that of UM-SCC-23 was gradually decreased. These data suggested that the cells treated with 5-FU for more than 144 h were still in the S-phase with or without DNA synthesis.

CONCLUSIONS

In head and neck carcinoma cells, we showed 5-FU pretreatment enhanced cisplatin cytotoxicity. The result of cell cycle analysis and immunostaining showed S-phase arrest by treatment of prolonged 5-FU treatment. The very long arrest in S-phase might be a mechanism to enhance cisplatin cytotoxicity by 5-FU pretreatment. We thus suggest pretreatment with 5-FU to enhance the effectiveness of cisplatin-based chemotherapy.

Therapeutic potential of CDK inhibitor NU2058 in androgen-independent prostate cancer

Antiandrogens are initially effective in controlling prostate cancer (CaP), the second most common cancer in men, but resistance, associated with the loss of androgen-regulated cell cycle control, is a major problem. At present there is no effective treatment for androgen-independent prostate cancer (AIPC). Cellular proliferation is driven by cyclin-dependent kinases (CDKs) with kinase inhibitors (for example, p27) applying the breaks. We present the first investigation of the therapeutic potential of CDK inhibitors, using the guanine-based CDK inhibitor NU2058 (CDK2 IC(50)=17 microM, CDK1 IC(50)=26 microM), in comparison with the antiandrogen bicalutamide (Casodex) in AIPC cells. A panel of AIPC cells was found to be resistant to Casodex-induced growth inhibition, but with the exception of PC3 (GI(50)=38 microM) and CWR22Rv1 (GI(50)=46 microM) showed similar sensitivity to NU2058 (GI(50)=10-17 microM) compared to androgen-sensitive LNCaP cells (GI(50)=15 microM). In LNCaP cells and their Casodex-resistant derivative, LNCaP-cdxR, growth inhibition by NU2058 was accompanied by a concentration-dependent increase in p27 levels, reduced CDK2 activity and pRb phosphorylation, a decrease in early gene expression and G1 cell cycle phase arrest in both cell lines. In response to Casodex, there were similar observations in LNCaP cells (GI(50)=6+/-3 microM Casodex) but not in LNCaP-cdxR cells (GI(50)=24+/-5 microM Casodex).

SWI/SNF activity is required for the repression of deoxyribonucleotide triphosphate metabolic enzymes via the recruitment of mSin3B

The SWI/SNF chromatin remodeling complex plays a critical role in the coordination of gene expression with physiological stimuli. The synthetic enzymes ribonucleotide reductase, dihydrofolate reductase, and thymidylate synthase are coordinately regulated to ensure appropriate deoxyribonucleotide triphosphate levels. Particularly, these enzymes are actively repressed as cells exit the cell cycle through the action of E2F transcription factors and the retinoblastoma tumor suppressor/p107/p130 family of pocket proteins. This process is found to be highly dependent on SWI/SNF activity as cells deficient in BRG-1 and Brm subunits fail to repress these genes with activation of pocket proteins, and this deficit in repression can be complemented, via the ectopic expression of BRG-1. The failure to repress transcription does not involve a blockade in the association of E2F or pocket proteins p107 and p130 with promoter elements. Rather, the deficit in repression is due to a failure to mediate histone deacetylation of ribonucleotide reductase, dihydrofolate reductase, and thymidylate synthase promoters in the absence of SWI/SNF activity. The basis for this is found to be a failure to recruit mSin3B and histone deacetylase proteins to promoters. Thus, the coordinate repression of deoxyribonucleotide triphosphate metabolic enzymes is dependent on the action of SWI/SNF in facilitating the assembly of repressor complexes at the promoter.

Transgenic expression of human thymidylate synthase accelerates the development of hyperplasia and tumors in the endocrine pancreas

Thymidylate synthase (TS) is an essential enzyme for DNA synthesis and repair and elevated levels of TS have been identified as an important prognostic biomarker for colorectal cancer and several other common human malignancies. In addition, TS gene expression has been linked with cell-cycle regulation and cell proliferation through the ability of retinoblastoma protein to repress the transcriptional activation of E2F target genes such as TS. Therefore, overproduction of TS could participate in the progression to a neoplastic phenotype. Consistent with this model, a recent study has suggested that ectopic TS expression can induce a transformed phenotype in mammalian cells. To investigate the role of deregulated TS activity in tumor development, we generated transgenic mice that express high levels of catalytically active human TS (hTS) exclusively in the pancreas and low levels of hTS in multiple other tissues. Analyses of pancreatic tissue in TS transgenic mice revealed abnormalities within the endocrine pancreas, ranging from pancreatic islet hyperplasia to the detection of islet cell tumors. Overexpression of hTS in murine islets provides a new model to study genetic alterations associated with the progression from normal cells to hyperplasia to islet cell tumors, and suggests that this mouse model may be useful for regulating TS activity in vivo for development of cancer prevention and new therapies.

Caf1 regulates translocation of ribonucleotide reductase by releasing nucleoplasmic Spd1-Suc22 assembly

Appropriate supply of deoxyribonucleotides by the ribonucleotide reductase (RNR) complex is essential for DNA replication and repair. One recent model for the RNR activation in Schizosaccharomyces pombe is translocation of the regulatory subunit Suc22 from the nucleoplasm to the cytoplasm. The RNR inhibitory protein Spd1, which retains Suc22 in the nucleoplasm, is rapidly degraded upon DNA-replication stress, resulting in release of Suc22 to form the active RNR complex in the cytoplasm. Here, we show that Caf1, a component of the Ccr4-Not complex, is responsible for resistance of the replication stress and control of the Suc22 translocation. Caf1 is required not only for the stress-induced translocation of Suc22 from nucleoplasm to cytoplasm but also for the degradation of nucleoplasmic Spd1. DNA-replication stress appears to allow Caf1 to interact with Suc22, resulting in release of the nucleoplasmic Spd1-Suc22 assembly. Taken together, these results suggest a novel function of Caf1 as a key regulator in the stress-induced RNR activation.

Ribonucleotide reductase small subunit p53R2 facilitates p21 induction of G1 arrest under UV irradiation

p53R2, which is one of the two known ribonucleotide reductase small subunits (the other being M2), is suggested to play an important role in supplying deoxynucleotide triphosphates (dNTP) for DNA repair during the G(1) or G(2) phase of the cell cycle. The ability of p53R2 to supply dNTPs for repairing DNA damages requires the presence of a functional p53 tumor suppressor. Here, we report in vivo physical interaction and colocalization of p53R2 and p21 before DNA damage. Mammalian two-hybrid assay further indicates that the amino acids 1 to 113 of p53R2 are critical for interacting with the NH(2)-terminal region (amino acids 1-93) of p21. The binding between p21 and p53R2 decreases inside the nucleus in response to UV, the time point of which corresponds to the increased binding of p21 with cyclin-dependent kinase-2 (Cdk2), and the decreased Cdk2 activity in the nucleus at G(1). Interestingly, p53R2 dissociates from p21 but facilitates the accumulation of p21 in the nucleus in response to UV. On the other hand, the ribonucleotide reductase activity increases at the corresponding time in response to UV. These data suggest a new function of p53R2 of cooperating with p21 during DNA repair at G(1) arrest.

Retinoblastoma tumor suppressor: where cancer meets the cell cycle

The retinoblastoma tumor suppressor gene, Rb, was the first tumor suppressor identified and plays a fundamental role in regulation of progression through the cell cycle. This review details facets of RB protein function in cell cycle control and focuses on specific questions that remain intensive areas of investigation.

DNA precursor metabolism and genomic stability

Intracellular concentrations of the four deoxyribonucleoside triphosphates (dNTPs) are closely regulated, and imbalances in the four dNTP pools have genotoxic consequences. Replication errors leading to mutations can occur, for example, if one dNTP in excess drives formation of a non-Watson-Crick base pair or if it forces replicative DNA chain elongation past a mismatch before DNA polymerase can correct the error by 3' exonuclease proofreading. This review focuses on developments since 1994, when the field was last reviewed comprehensively. Emphasis is placed on the following topics: 1) novel aspects of dNTP pool regulation, 2) dNTP pool asymmetries as mutagenic determinants, 3) dNTP metabolism and hypermutagenesis of retroviral genomes, 4) dNTP metabolism and mutagenesis in the mitochondrial genome, 5) chemical modification of nucleotides as a premutagenic event, 6) relationships between dNTP metabolism, genome stability, aging, and cancer.

c-Myc, genome instability, and tumorigenesis: the devil is in the details

The c-myc oncogene acts as a pluripotent modulator of transcription during normal cell growth and proliferation. Deregulated c-myc activity in cancer can lead to excessive activation of its downstream pathways, and may also stimulate changes in gene expression and cellular signaling that are not observed under non-pathological conditions. Under certain conditions, aberrant c-myc activity is associated with the appearance of DNA damage-associated markers and karyotypic abnormalities. In this chapter, we discuss mechanisms by which c-myc may be directly or indirectly associated with the induction of genomic instability. The degree to which c-myc-induced genomic instability influences the initiation or progression of cancer is likely to depend on other factors, which are discussed herein.

Modification of human immunodeficiency virus type 1 reverse transcriptase to target cells with elevated cellular dNTP concentrations

Retroviruses and DNA viruses utilize cellular dNTPs as substrates for their DNA polymerases during viral replication in infected cells. However, because of S phase-dependent dNTP biosynthesis, the availability of cellular dNTPs significantly varies among cell types (e.g. dividing versus nondividing cells and normal versus tumor cells). Here we tested whether alterations in the dNTP utilization efficiency and dNTP binding affinity of viral DNA polymerases can switch viral infection specificity to cell types with different dNTP concentrations. We employed an HIV-1 reverse transcriptase (RT) mutant (Q151N), which is catalytically active only at high dNTP concentrations because of its reduced dNTP binding affinity. Indeed, the modified HIV-1 vector harboring the Q151N mutant RT preferentially transduced tumor cells containing higher cellular dNTP concentrations than primary cells (e.g. human lung fibroblasts (HLFs) and human keratinocytes). Although the wild type HIV-1 vector transduced both HLFs and tumor cells, the Q151N vector failed to transduce HLFs and keratinocytes but efficiently transduced tumor cells. Pretreatment of HLFs with deoxynucleosides, which increase cellular dNTP pools, enabled the mutant vector to transduce HLFs, suggesting that the transduction failure of the RT mutant vector to primary cells is because of inefficient reverse transcription in low cellular dNTP environments. We also observed that the Q151N vector expressing herpes simplex virus-thymidine kinase renders tumor cells sensitive to gancyclovir. This study validates a novel strategy in which modifications of viral DNA polymerases in various vector systems allow the delivery of target genes exclusively to tumor cells exploiting elevated cellular dNTP concentration as a tumor cell-specific host factor.

Cancer chemotherapy by deoxynucleotide depletion and E2F-1 elevation

We propose that the lethality of commonly used anticancer drugs, e.g., methotrexate and cis-platinum are due, at least in part, to an increase of the E2F-1-mediated apoptotic cascade. The drugs directly or indirectly decrease deoxynucleoside triphosphates. The E2F family acts to provide control of S phase by transcribing genes required for deoxynucleoside triphosphate and DNA synthesis. Thus, a mechanism for control of E2F-1 is essential, a signal safeguarding against aberrant or uncontrolled cell proliferation. We have proposed a feedback control by NTPs that down-regulates E2F-1. Here, we provide evidence in support of this hypothesis.