Effect of nonsteroidal anti-inflammatory drugs (aspirin and naproxen) on inflammation-associated proteomic profiles in mouse plasma and prostate during TMPRSS2-ERG (fusion)-driven prostate carcinogenesis.

Recent preclinical studies have shown that the intake of nonsteroidal anti-inflammatory drugs (NSAIDs) aspirin and naproxen could be an effective intervention strategy against TMPRSS2-ERG fusion-driven prostate tumorigenesis. Herein, as a follow-up mechanistic study, employing TMPRSS2-ERG (fusion) positive tumors and plasma from TMPRSS2-ERG. Ptenflox/flox mice, we profiled the stage specific proteomic changes (focused on inflammatory circulating and prostate tissue/tumor-specific cytokines, chemokines, and growth factors/growth signaling-associated molecules) that contribute to prostate cancer (PCa) growth and progression in the TMPRSS2-ERG fusion-driven mouse model of tumorigenesis. In addition, the association of the protective effects of NSAIDs (aspirin 1400 ppm and naproxen 400 ppm) with the modulation of these specific molecular pathways was determined. A sandwich Elisa based membrane array-proteome profiler identifying 111 distinct signaling molecules was employed. Overall, the plasma and prostate tissue sample analyses identified 54 significant and differentially expressed cytokines, chemokines, and growth factors/growth signaling-associated molecules between PCa afflicted mice (TMPRSS2-ERG. Ptenflox/flox, age-matched noncancerous controls, NSAIDs-supplemented and no-drug controls). Bioinformatic analysis of the array outcomes indicated that the protective effect of NSAIDs was associated with reduced expression of (a) tumor promoting inflammatory molecules (M-CSF, IL-33, CCL22, CCL12, CX3CL1, CHI3L1, and CD93), (b) growth factors- growth signaling-associated molecules (Chemerin, FGF acidic, Flt-3 ligand, IGFBP-5, and PEDF), and (c) tumor microenvironment/stromal remodeling proteins MMP2 and MMP9. Overall, our findings corroborate the pathological findings that protective effects of NSAIDs in TMPSS2-ERG fusion-driven prostate tumorigenesis are associated with antiproliferative and anti-inflammatory effects and possible modulation of the immune cell enriched microenvironment.

Differential Effect of Non-Steroidal Anti-Inflammatory Drugs Aspirin and Naproxen against TMPRSS2-ERG (Fusion)-Driven and Non-Fusion-Driven Prostate Cancer.

The consumption of the non-steroidal anti-inflammatory drug (NSAID) aspirin is associated with a significant reduction in the risk of developing TMPRSS2-ERG (fusion)-positive prostate cancer (PCa) compared to fusion-negative PCa in population-based case-control studies; however, no extensive preclinical studies have been conducted to investigate and confirm these protective benefits. Thus, the focus of this study was to determine the potential usefulness of aspirin and another NSAID, naproxen, in PCa prevention, employing preclinical models of both TMPRSS2-ERG (fusion)-driven (with conditional deletion of Pten) and non-TMPRSS2-ERG-driven (Hi-Myc+/- mice) PCa. Male mice (n = 25 mice/group) were fed aspirin- (700 and 1400 ppm) and naproxen- (200 and 400 ppm) supplemented diets from (a) 6 weeks until 32 weeks of Hi-Myc+/- mice age; and (b) 1 week until 20 weeks post-Cre induction in the fusion model. In all NSAID-fed groups, compared to no-drug controls, there was a significant decrease in higher-grade adenocarcinoma incidence in the TMPRSS2-ERG (fusion)-driven PCa model. Notably, there were no moderately differentiated (MD) adenocarcinomas in the dorsolateral prostate of naproxen groups, and its incidence also decreased by ~79-91% in the aspirin cohorts. In contrast, NSAIDs showed little protective effect against prostate tumorigenesis in Hi-Myc+/- mice, suggesting that NSAIDs exert a specific protective effect against TMPRSS2-ERG (fusion)-driven PCa.

Primary Prevention and Interception Studies in RAS-Mutated Tumor Models Employing Small Molecules or Vaccines.

Therapeutic targeting of RAS-mutated cancers is difficult, whereas prevention or interception (treatment before or in the presence of preinvasive lesions) preclinically has proven easier. In the A/J mouse lung model, where different carcinogens induce tumors with different KRAS mutations, glucocorticoids and retinoid X receptor (RXR) agonists are effective agents in prevention and interception studies, irrespective of specific KRAS mutations. In rat azoxymethane-induced colon tumors (45% KRAS mutations), cyclooxygenase 1/2 inhibitors and difluoromethylornithine are effective in preventing or intercepting KRAS-mutated or wild-type tumors. In two KRAS-mutant pancreatic models multiple COX 1/2 inhibitors are effective. Furthermore, combining a COX and an EGFR inhibitor prevented the development of virtually all pancreatic tumors in transgenic mice. In the N-nitroso-N-methylurea-induced estrogen receptor-positive rat breast model (50% HRAS mutations) various selective estrogen receptor modulators, aromatase inhibitors, EGFR inhibitors, and RXR agonists are profoundly effective in prevention and interception of tumors with wild-type or mutant HRAS, while the farnesyltransferase inhibitor tipifarnib preferentially inhibits HRAS-mutant breast tumors. Thus, many agents not known to specifically inhibit the RAS pathway, are effective in an organ specific manner in preventing or intercepting RAS-mutated tumors. Finally, we discuss an alternative prevention and interception approach, employing vaccines to target KRAS.

Treatment Strategies and Mechanisms Associated with the Prevention of NASH-Associated HCC by a Toll-like Receptor 4 Inhibitor.

We evaluated the cancer preventive efficacy of TAK-242, an inhibitor of Toll-like receptor 4 (TLR4), in a mouse model of hepatocellular carcinoma (HCC) occurring in the context of nonalcoholic steatohepatitis (NASH). We also assessed the cellular events associated with the preventive treatment efficacy. We tested oral administration of TAK-242, at clinically relevant but toxicity-reducing doses and scheduling, in mice with hepatocyte-specific deletion of Pten (HepPten-). The optimal dose and oral gavage formulation of TAK-242 were determined to be 30 mg/kg in 5% DMSO in 30% 2-hydroxypropyl-ß-cyclodextrin. Daily oral administration of 30 mg/kg TAK-242 over 18 weeks was well tolerated and resulted in reduced development of tumors (lesions > 7.5 mm3) in HepPten- mice. This effect was accompanied by reduced macrovesicular steatosis and serum levels of alanine aminotransferase. In addition, 30 mg/kg TAK-242 daily treatment of small preexisting adenomas (lesions < 7.5 mm3) over 18 weeks, significantly reduced their progression to HCC. RNA sequencing identified 220 hepatic genes significantly altered upon TAK-242 treatment, that significantly correlated with tumor burden. Finally, cell deconvolution analysis revealed that TAK-242 treatment resulted in reduced hepatic populations of endothelial cells and myeloid-derived immune cells (Kupffer cells, Siglec-H high dendritic cells, and neutrophilic granule protein high neutrophils), while the proportion of mt-Nd4 high hepatocytes significantly increased, suggesting a decrease in hepatic inflammation and concomitant increase in mitochondrial function and oxidative phosphorylation upon TLR4 inhibition. In conclusion, this study identified treatment strategies and novel molecular and cellular mechanisms associated with the prevention of HCC in the context of NASH that merit further investigations. PREVENTION RELEVANCE: Means to prevent development of HCC or progression of small adenomas to HCC in patients with NASH are urgently needed to reduce the growing mortality due to HCC. We characterized the chemopreventive effect of oral administration of the TLR4 inhibitor TAK-242 in a model of NASH-associated HCC.

Oral administration of TRAIL-inducing small molecule ONC201/TIC10 prevents intestinal polyposis in the Apc min/+ mouse model.

Colorectal cancer (CRC) incidence is rising globally. Hence, preventing this disease is a high priority. With this aim, we determined the CRC prevention potential of the TRAIL-inducing small molecule ONC201/TIC10 using a preclinical model representing high-risk familial adenomatous polyposis (FAP) patients, Apc min/+ mice. Prior to the efficacy study, optimal and non-toxic doses of ONC201 were determined by testing five different doses of ONC201 (0-100 mg/kg body weight (BW); twice weekly by oral gavage) in C57BL/6J mice (n=6/group) for 6 weeks. BW gain, organ weights and histopathology, blood profiling, and the plasma liver enzyme profile suggested no toxicities of ONC201 at doses up to 100 mg/kg BW. For efficacy determination, beginning at six weeks of age, groups of Apc min/+ male and female mice (n≥20) treated with colon carcinogen azoxymethane (AOM) (AOM-Apc min/+) were administered ONC201 (0, 25, and 50 mg/kg BW) as above up to 20 weeks of age. At termination, efficacy was determined by comparing the incidence and multiplicity of intestinal tumors between vehicle- and drug-treated groups. ONC201 showed a strong suppressive effect against the development of both large and small intestinal tumors in male and female mice. Apc min/+ mice treated with ONC201 (50 mg/kg BW) showed >50% less colonic tumor incidence (P<0.0002) than controls. Colonic tumor multiplicity was also significantly reduced by 68% in male mice (0.44 ± 0.11 in treated vs. 1.4 ± 0.14 in controls; P<0.0001) and by 75% in female mice (0.30 ± 0.10 in treated vs. 1.19 ± 0.19 in controls; P<0.0003) with ONC201 treatment (50 mg/kg BW). Small intestinal polyps were reduced by 68% in male mice (11.40 ± 1.19 in treated vs. 36.08 ± 2.62 in controls; P<0.0001) and female mice (9.65 ± 1.15 in treated vs. 29.24 ± 2.51 in controls; P<0.0001). Molecular analysis of the tumors suggested an increase in TRAIL, DR5, cleaved caspases 3/7/8, Fas-associated death domain protein (FADD), and p21 (WAF1) in response to drug treatment. Serum analysis indicated a decrease in pro-inflammatory serum biomarkers, such as IL1ß, IL6, TNFα, G-CSF, and GM-CSF, in the ONC201-treated mice compared with controls. Our data demonstrated excellent chemopreventive potential of orally administered ONC201 against intestinal tumorigenesis in the AOM-Apc min/+ mouse model.

Characterization of stage-specific tumor progression in TMPRSS2-ERG (fusion)-driven and non-fusion-driven prostate cancer in GEM models.

In the present study, we performed a comparative stage-specific pathological and molecular marker evaluation of TMPRSS2-ERG fusion and PTEN loss-driven (TMPRSS2-ERG. Ptenflox/flox ) versus non-fusion-driven prostate tumorigenesis (Hi-Myc) in mice. Anterior, ventral, and dorsolateral prostates were collected from mice at different ages (or time points post-Cre induction). Results indicated that growth and progression of prostatic intraepithelial lesions to adenocarcinoma stages occurred in both mice models albeit at different rates. In the TMPRSS2-ERG. Ptenflox/flox mice, the initiation of tumorigenesis was slow, but subsequent progression through different stages became increasingly faster. Adenocarcinoma stage was reached early on; however, no high-grade undifferentiated tumors were observed. Conversely, in the Hi-Myc+/- mice, tumorigenesis initiation was rapid; however, progression through different stages was relatively slower and it took a while to reach the more aggressive phenotype stage. Nevertheless, at the advanced stages in the Hi-Myc+/- mice, high-grade undifferentiated tumors were observed compared to the later stage tumors observed in the fusion-driven TMPRSS2-ERG. Ptenflox/flox mice. These results were corroborated by the stage specific-pattern in the molecular expression of proliferation markers (PCNA and c-Myc); androgen receptor (AR); fusion-resultant overexpression of ERG; Prostein (SLC45-A3); and angiogenesis marker (CD-31). Importantly, there was a significant increase in immune cell infiltrations, which increased with the stage of tumorigenesis, in the TMPRSS2-ERG fusion-positive tumors relative to fusion negative tumors. Together, these findings are both novel and highly significant in establishing a working preclinical model for evaluating the efficacy of interventions during different stages of tumorigenesis in TMPRSS2-ERG fusion-driven PCa.

The HIV protease inhibitor, ritonavir, corrects diverse brain phenotypes across development in mouse model of DYT-TOR1A dystonia.

Dystonias are a group of chronic movement-disabling disorders for which highly effective oral medications or disease-modifying therapies are lacking. The most effective treatments require invasive procedures such as deep brain stimulation. In this study, we used a high-throughput assay based on a monogenic form of dystonia, DYT1 (DYT-TOR1A), to screen a library of compounds approved for use in humans, the NCATS Pharmaceutical Collection (NPC; 2816 compounds), and identify drugs able to correct mislocalization of the disease-causing protein variant, ∆E302/3 hTorsinA. The HIV protease inhibitor, ritonavir, was among 18 compounds found to normalize hTorsinA mislocalization. Using a DYT1 knock-in mouse model to test efficacy on brain pathologies, we found that ritonavir restored multiple brain abnormalities across development. Ritonavir acutely corrected striatal cholinergic interneuron physiology in the mature brain and yielded sustained correction of diffusion tensor magnetic resonance imaging signals when delivered during a discrete early developmental window. Mechanistically, we found that, across the family of HIV protease inhibitors, efficacy correlated with integrated stress response activation. These preclinical results identify ritonavir as a drug candidate for dystonia with disease-modifying potential.

Effects of High-Fructose Diet vs. Teklad Diet in the MNU-Induced Rat Mammary Cancer Model: Altered Tumorigenesis, Metabolomics and Tumor RNA Expression.

Epidemiology, clinical and experimental animal studies suggest high fructose diets are detrimental to metabolic status and may contribute to tumor development. This due to increased obesity and metabolic syndrome, known risk factors for many types of cancer. We compared tumor development in N-methyl-N-nitrosourea (MNU)-treated rats fed either a high (60%)-fructose diet (HFD) or a standard diet (SD). Female Sprague-Dawley rats at 43 days of age (DOA) were fed a SD or HFD followed by administration of MNU at 50 DOA. Rats were palpated weekly and sacrificed at 190 DOA. MNU-treated rats on HFD exhibited decreased tumor latency and roughly a two-fold increase in tumor multiplicity. RNA-Seq on frozen tumors (SD vs. HFD rats) showed altered expression of approximately 10% of genes (P < 0.05). When examined by Ingenuity Pathway Analysis, multiple highly significant pathways were identified including A) mechanisms of cancer, B) Wnt pathway, C) immune response (e.g., "Th1 and Th2 activation" and "antigen presentation") and D) LXR/RXR nuclear receptor. These generalized pathways were indirectly confirmed by alterations of various interrelated disease pathways (epithelial cancers, T cell numbers and apoptosis). In a second study, serum was collected from rats on the HFD or SD pre-MNU and at the time of sacrifice. Metabolomics revealed that the HFD yielded: A) increased levels of fructose, B) increases of various monoglycerols, C) reduced levels of various diacylglycerols and oxygenated inflammatory lipids (9 and 13 HODE and 12,13 DHOME) and D) increased levels of secondary bile acids (hyodeoxycholate and 6-oxolithocholate), which may reflect microbiome changes. These metabolomic changes, which are distinct from those on a high-fat diet, may prove relevant when examining individuals who consume higher levels of fructose.

Use of Biomarker Modulation in Normal Mammary Epithelium as a Correlate for Efficacy of Chemopreventive Agents Against Chemically Induced Cancers.

In both estrogen receptor/progesterone receptor-positive (ER+/PR+) human breast cancer and in ER+/PR+ cancers in the methylnitrosourea (MNU)-induced rat model, short-term modulation of proliferation in early cancers predicts preventive/therapeutic efficacy. We determined the effects of known effective/ineffective chemopreventive agents on proliferative index (PI) in both rat mammary epithelium and small cancers. Female Sprague-Dawley rats were treated with MNU at 50 days of age. Five days later, the rats were treated with the individual compounds for a period of 14 days. At that time, normal mammary tissue from the inguinal gland area was surgically removed. After removal, the rats remained on the agents for an additional 5 months. This cancer prevention study confirmed our prior results of striking efficacy with tamoxifen, vorozole, Targretin, and gefitinib, and no efficacy with metformin, naproxen, and Lipitor. Employing a separate group of rats, the effects of short-term (7 days) drug exposure on small palpable cancers were examined. The PI in both small mammary cancers and in normal epithelium from control rats was >12%. In agreement with the cancer multiplicity data, tamoxifen, vorozole, gefitinib, and Targretin all strongly inhibited proliferation (>65%; P < 0.025) in the normal mammary epithelium. The ineffective agents metformin, naproxen, and Lipitor minimally affected PI. In the small cancers, tamoxifen, vorozole, and Targretin all reduced the PI, while metformin and Lipitor failed to do so. Thus, short-term changes in the PI in either normal mammary epithelium or small cancers correlated with long-term preventive efficacy in the MNU-induced rat model.

Evaluation of the STAT3 inhibitor GLG­302 for the prevention of estrogen receptor­positive and ­negative mammary cancers.

Signal transducer and activator of transcription 3 (STAT3) plays a key role in the transformation of normal cells to cancerous cells. Although inhibitors of STAT3 have been shown to suppress the growth of multiple cancer types in vitro and in vivo, such agents are of particular interest for the prevention of breast cancer, which affects over 200,000 women and claims more than 40,000 lives in the United States each year. In the present study, we employed the MMTV/Neu transgenic mouse model, which develops estrogen receptor (ER)­negative, Neu­overexpressing tumors, and the Sprague­Dawley (SD) rat model, which develops ER­positive tumors upon exposure to the carcinogen 7,12­dimethylbenz[a]anthracene (DMBA), to test the efficacy of the STAT3 inhibitor GLG­302 in the prevention of mammary cancer. Orally administered GLG­302 and its trizma salt derivative reduced mammary cancer incidence, multiplicity, and tumor weights in female MMTV/Neu mice, and GLG­302 reduced tumor multiplicity and weights in female DMBA­treated rats. Consistent with the mechanism of action of STAT3 inhibitors, the reductions in mammary tumors were correlated with decreases in STAT3 phosphorylation and cell proliferation. These data suggest that GLG­302 is a novel agent with potential for prevention of mammary cancer and support the further development of STAT3 inhibitors for this cause.

Cancer Chemoprevention: Preclinical In Vivo Alternate Dosing Strategies to Reduce Drug Toxicities.

Cancer chemopreventive agents inhibit the formation of precursor lesions and/or the progression of these lesions to late stage disease. This approach to disease control has the potential to reduce the physical and financial costs of cancer in society. Several drugs that have been approved by the FDA for other diseases and have been extensively evaluated for their safety and pharmacokinetic/pharmacodynamic characteristics have the potential to be repurposed for use as cancer chemopreventive agents. These agents often mechanistically inhibit signaling molecules that play key roles in the carcinogenic process. The safety profile of agents is a primary concern when considering the administration of drugs for chemoprevention, as the drugs will be given chronically to high-risk, asymptomatic individuals. To decrease drug toxicity while retaining efficacy, several approaches are currently being explored. In this short review, we describe studies that use preclinical in vivo models to assess efficacy of alternative drug dosing strategies and routes of drug administration on chemopreventive drug efficacy. In vivo drug dosing strategies that reduce toxicity while retaining efficacy will pave the way for future cancer prevention clinical trials.

Small Molecule Inhibition of the Ubiquitin-specific Protease USP2 Accelerates cyclin D1 Degradation and Leads to Cell Cycle Arrest in Colorectal Cancer and Mantle Cell Lymphoma Models.

Deubiquitinases are important components of the protein degradation regulatory network. We report the discovery of ML364, a small molecule inhibitor of the deubiquitinase USP2 and its use to interrogate the biology of USP2 and its putative substrate cyclin D1. ML364 has an IC50 of 1.1 µm in a biochemical assay using an internally quenched fluorescent di-ubiquitin substrate. Direct binding of ML364 to USP2 was demonstrated using microscale thermophoresis. ML364 induced an increase in cellular cyclin D1 degradation and caused cell cycle arrest as shown in Western blottings and flow cytometry assays utilizing both Mino and HCT116 cancer cell lines. ML364, and not the inactive analog 2, was antiproliferative in cancer cell lines. Consistent with the role of cyclin D1 in DNA damage response, ML364 also caused a decrease in homologous recombination-mediated DNA repair. These effects by a small molecule inhibitor support a key role for USP2 as a regulator of cell cycle, DNA repair, and tumor cell growth.

A Novel Chemotherapeutic Agent to Treat Tumors with DNA Mismatch Repair Deficiencies.

Impairing the division of cancer cells with genotoxic small molecules has been a primary goal to develop chemotherapeutic agents. However, DNA mismatch repair (MMR)-deficient cancer cells are resistant to most conventional chemotherapeutic agents. Here we have identified baicalein as a small molecule that selectively kills MutSα-deficient cancer cells. Baicalein binds preferentially to mismatched DNA and induces a DNA damage response in a MMR-dependent manner. In MutSα-proficient cells, baicalein binds to MutSα to dissociate CHK2 from MutSα leading to S-phase arrest and cell survival. In contrast, continued replication in the presence of baicalein in MutSα-deficient cells results in a high number of DNA double-strand breaks and ultimately leads to apoptosis. Consistently, baicalein specifically shrinks MutSα-deficient xenograft tumors and inhibits the growth of AOM-DSS-induced colon tumors in colon-specific MSH2 knockout mice. Collectively, baicalein offers the potential of an improved treatment option for patients with tumors with a DNA MMR deficiency. Cancer Res; 76(14); 4183-91. ©2016 AACR.

ATAD5 deficiency decreases B cell division and Igh recombination.

Mammalian ATPase family AAA domain-containing protein 5 (ATAD5) and its yeast homolog enhanced level of genomic instability 1 are responsible for unloading proliferating cell nuclear antigen from newly synthesized DNA. Prior work in HeLa and yeast cells showed that a decrease in ATAD5 protein levels resulted in accumulation of chromatin-bound proliferating cell nuclear antigen, slowed cell division, and increased genomic instability. In this study, B cells from heterozygous (Atad5(+/m)) mice were used to examine the effects of decreased cell proliferation on Ab diversity. ATAD5 haploinsufficiency did not change the frequency or spectrum of somatic hypermutation in Ab genes, indicating that DNA repair and error-prone DNA polymerase η usage were unaffected. However, immunized Atad5(+/m) mice had decreased serum IgG1 Abs, demonstrating a functional effect on class switch recombination. The mechanism of this altered immune response was then examined following ex vivo stimulation of splenic B cells, where Atad5(+/m) cells accumulated in the S phase of the cell cycle and had reduced proliferation compared with wild-type cells. These haploinsufficient cells underwent a significant decline in activation-induced deaminase expression, resulting in decreased switch region DNA double-strand breaks and interchromosomal translocations in the Igh locus. Class switch recombination to several isotypes was also reduced in Atad5(+/m) cells, although the types of end-joining pathways were not affected. These results describe a defect in DNA replication that affects Igh recombination via reduced cell division.

Cell-based high-throughput screens for the discovery of chemotherapeutic agents.

With modern advances in robotics and data processing, high-throughput screening (HTS) is playing an increasingly growing role in the drug discovery process. The ultimate success of HTS depends upon the development of assays that are robust and reproducible in miniaturized formats, have low false-positive rates, and can identify drugs that offer improvements over those currently on the market. One example of such an assay is the ATAD5-luciferase HTS assay, which identified three antioxidants that could kill cancer cells without inducing mutagenesis. Here we discuss the ATAD5- luciferase assay and expand upon the value of HTS in identifying other potential cancer drugs, focusing on cell-based assays that involve DNA damage or repair pathways.

High-throughput genotoxicity assay identifies antioxidants as inducers of DNA damage response and cell death.

Human ATAD5 is a biomarker for identifying genotoxic compounds because ATAD5 protein levels increase posttranscriptionally in response to DNA damage. We screened over 4,000 compounds with a cell-based quantitative high-throughput ATAD5-luciferase assay detecting genotoxic compounds. We identified 22 antioxidants, including resveratrol, genistein, and baicalein, that are currently used or investigated for the treatment of cardiovascular disease, type 2 diabetes, osteopenia, osteoporosis, and chronic hepatitis, as well as for antiaging. Treatment of dividing cells with these compounds induced DNA damage and resulted in cell death. Despite their genotoxic effects, resveratrol, genistein, and baicalein did not cause mutagenesis, which is a major side effect of conventional anticancer drugs. Furthermore, resveratrol and genistein killed multidrug-resistant cancer cells. We therefore propose that resveratrol, genistein, and baicalein are attractive candidates for improved chemotherapeutic agents.

Dynamic regulation of PCNA ubiquitylation/deubiquitylation.

Proliferating Cell Nuclear Antigen (PCNA) ubiquitylation plays a crucial role in maintaining genomic stability during DNA replication. DNA damage stalling the DNA replication fork induces PCNA ubiquitylation that activates DNA damage bypass to prevent the collapse of DNA replication forks that could potentially produce double-strand breaks and chromosomal rearrangements. PCNA ubiquitylation dictates the mode of bypass depending on the level of ubiquitylation; monoubiquitylation and polyubiquitylation activate error-prone translesion synthesis and error-free template switching, respectively. Due to the error-prone nature of DNA damage bypass, PCNA ubiquitylation needs to be tightly regulated. Here, we review the molecular mechanisms to remove ubiquitin from PCNA including the emerging role of USP1 and ELG1 in this fascinating process.

Mechanism of the internal ribosome entry site-mediated translation of serine hydroxymethyltransferase 1.

The 5'-untranslated region (UTR) of serine hydroxymethyltransferase 1 (SHMT1) contains an internal ribosome entry site (IRES) that regulates SHMT1 expression, a rate-limiting enzyme in de novo thymidylate biosynthesis. In this study, we show that the SHMT1 IRES is the first example of a cellular IRES that is poly(A) tail-independent. Interactions between the 5'-UTR and 3'-UTR functionally replaced interactions between the poly(A) tail and the poly(A)-binding protein (PABP) to achieve maximal IRES-mediated translational efficiency. Depletion of the SHMT1 IRES-specific trans-acting factor (ITAF) CUG-binding protein 1 (CUGBP1) from in vitro translation extracts or deletion of the CUGBP1 binding site on the 3'-UTR of the SHMT1 transcript decreased the IRES activity of non-polyadenylylated biscistronic mRNAs relative to polyadenylylated biscistronic mRNAs and resulted in a requirement for PABP. We also identified a novel ITAF, heterogeneous nuclear ribonucleoprotein H2 (hnRNP H2), that stimulates SHMT1 IRES activity by binding to the 5'-UTR of the transcript and interacting with CUGBP1. Collectively, these data support a model for the IRES-mediated translation of SHMT1 whereby the circularization of the mRNA typically provided by the eukaryotic initiation factor (eIF) 4G/PABP/poly(A) tail interaction is achieved instead through the hnRNP H2/CUGBP1-mediated interaction of the 5'- and 3'-UTRs of the SHMT1 transcript. This circularization enhances the IRES activity of SHMT1 by facilitating the recruitment and/or recycling of ribosomal subunits, which bind to the transcript in the middle of the 5'-UTR and migrate to the initiation codon via eIF4A-mediated scanning.

A UV-responsive internal ribosome entry site enhances serine hydroxymethyltransferase 1 expression for DNA damage repair.

Thymidine nucleotides are required for faithful DNA synthesis and repair, and their de novo biosynthesis is regulated by serine hydroxymethyltransferase 1 (SHMT1). The SHMT1 transcript contains a heavy chain ferritin, heterogeneous nuclear ribonucleoprotein H2, and CUG-binding protein 1-responsive internal ribosome entry site (IRES) that regulates SHMT1 translation. In this study a non-lethal dose of UVC is shown to increase SHMT1 IRES activity and protein levels in four different cell lines. The mechanism for the UV-induced activation of the SHMT1 IRES involves an increase in heavy chain ferritin and heterogeneous nuclear ribonucleoprotein H2 expression and the translocation of CUG-binding protein 1 from the nucleus to the cytoplasm. The UV-induced increase in SHMT1 translation is accompanied by an increase in the small ubiquitin-like modifier-dependent nuclear localization of the de novo thymidylate biosynthesis pathway and a decrease in DNA strand breaks, indicating a role for SHMT1 and nuclear folate metabolism in DNA repair.

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.