Targeting Mitochondria with ClpP Agonists as a Novel Therapeutic Opportunity in Breast Cancer.

Breast cancer is the most frequently diagnosed malignancy worldwide and the leading cause of cancer mortality in women. Despite the recent development of new therapeutics including targeted therapies and immunotherapy, triple-negative breast cancer remains an aggressive form of breast cancer, and thus improved treatments are needed. In recent decades, it has become increasingly clear that breast cancers harbor metabolic plasticity that is controlled by mitochondria. A myriad of studies provide evidence that mitochondria are essential to breast cancer progression. Mitochondria in breast cancers are widely reprogrammed to enhance energy production and biosynthesis of macromolecules required for tumor growth. In this review, we will discuss the current understanding of mitochondrial roles in breast cancers and elucidate why mitochondria are a rational therapeutic target. We will then outline the status of the use of mitochondria-targeting drugs in breast cancers, and highlight ClpP agonists as emerging mitochondria-targeting drugs with a unique mechanism of action. We also illustrate possible drug combination strategies and challenges in the future breast cancer clinic.

Flotillin-2 regulates epidermal growth factor receptor activation, degradation by Cbl-mediated ubiquitination, and cancer growth.

Epidermal growth factor receptor (EGFR) signaling is frequently dysregulated in various cancers. The ubiquitin ligase Casitas B-lineage lymphoma proto-oncogene (Cbl) regulates degradation of activated EGFR through ubiquitination and acts as an adaptor to recruit proteins required for trafficking. Here, we used stable isotope labeling with amino acids in cell culture mass spectrometry to compare Cbl complexes with or without epidermal growth factor (EGF) stimulation. We identified over a hundred novel Cbl interactors, and a secondary siRNA screen found that knockdown of Flotillin-2 (FLOT2) led to increased phosphorylation and degradation of EGFR upon EGF stimulation in HeLa cells. In PC9 and H441 cells, FLOT2 knockdown increased EGF-stimulated EGFR phosphorylation, ubiquitination, and downstream signaling, reversible by EGFR inhibitor erlotinib. CRISPR knockout (KO) of FLOT2 in HeLa cells confirmed EGFR downregulation, increased signaling, and increased dimerization and endosomal trafficking. Furthermore, we determined that FLOT2 interacted with both Cbl and EGFR. EGFR downregulation upon FLOT2 loss was Cbl dependent, as coknockdown of Cbl and Cbl-b restored EGFR levels. In addition, FLOT2 overexpression decreased EGFR signaling and growth. Overexpression of wildtype (WT) FLOT2, but not the soluble G2A FLOT2 mutant, inhibited EGFR phosphorylation upon EGF stimulation in HEK293T cells. FLOT2 loss induced EGFR-dependent proliferation and anchorage-independent growth. Lastly, FLOT2 KO increased tumor formation and tumor volume in nude mice and NSG mice, respectively. Together, these data demonstrated that FLOT2 negatively regulated EGFR activation and dimerization, as well as its subsequent ubiquitination, endosomal trafficking, and degradation, leading to reduced proliferation in vitro and in vivo.

Photochemical Dimerization of Plakinidine B Leads to Potent Inhibition of the E3 Ubiquitin-Protein Ligase CBL-B.

A new dimeric alkaloid plakoramine A [(±)-1] was identified from a marine sponge Plakortis sp. Chiral-phase HPLC separation of (±)-1 led to the purified enantiomers (+)-1 and (-)-1 which both potently inhibited CBL-B E3 ubiquitin ligase activities. The absolute configurations of the enantiomers were determined by quantum chemical calculations. Scrutinization of the purification conditions revealed a previously undescribed, nonenzymatic route to form (±)-1 via photochemical conversion of its naturally occurring monomeric counterpart, plakinidine B (2).

Mitochondrial Matrix Protease ClpP Agonists Inhibit Cancer Stem Cell Function in Breast Cancer Cells by Disrupting Mitochondrial Homeostasis.

Mitochondria are multifaceted organelles which are important for bioenergetics, biosynthesis and signaling in metazoans. Mitochondrial functions are frequently altered in cancer to promote both the energy and the necessary metabolic intermediates for biosynthesis required for tumor growth. Cancer stem cells (CSCs) contribute to chemotherapy resistance, relapse, and metastasis. Recent studies have shown that while non-stem, bulk cancer cells utilize glycolysis, breast CSCs are more dependent on oxidative phosphorylation (OxPhos) and therefore targeting mitochondria may inhibit CSC function. We previously reported that small molecule ONC201, which is an agonist for the mitochondrial caseinolytic protease (ClpP), induces mitochondrial dysfunction in breast cancer cells. In this study, we report that ClpP agonists inhibit breast cancer cell proliferation and CSC function in vitro and in vivo. Mechanistically, we found that OxPhos inhibition downregulates multiple pathways required for CSC function, such as the mevalonate pathway, YAP, Myc, and the HIF pathway. ClpP agonists showed significantly greater inhibitory effect on CSC functions compared with other mitochondria-targeting drugs. Further studies showed that ClpP agonists deplete NAD(P)+ and NAD(P)H, induce redox imbalance, dysregulate one-carbon metabolism and proline biosynthesis. Downregulation of these pathways by ClpP agonists further contribute to the inhibition of CSC function. In conclusion, ClpP agonists inhibit breast CSC functions by disrupting mitochondrial homeostasis in breast cancer cells and inhibiting multiple pathways critical to CSC function. Significance: ClpP agonists disrupt mitochondrial homeostasis by activating mitochondrial matrix protease ClpP. We report that ClpP agonists inhibit cell growth and cancer stem cell functions in breast cancer models by modulating multiple metabolic pathways essential to cancer stem cell function.

Agelasine Diterpenoids and Cbl-b Inhibitory Ageliferins from the Coralline Demosponge Astrosclera willeyana.

An extract of the coralline demosponge Astrosclera willeyana inhibited the ubiquitin ligase activity of the immunomodulatory protein Cbl-b. The bioassay-guided separation of the extract provided ten active compounds, including three new N-methyladenine-containing diterpenoids, agelasines W-Y (1-3), a new bromopyrrole alkaloid, N(1)-methylisoageliferin (4), and six known ageliferin derivatives (5-10). The structures of the new compounds were elucidated from their spectroscopic and spectrometric data, including IR, HRESIMS, and NMR, and by comparison with spectroscopic data in the literature. While all of the isolated compounds showed Cbl-b inhibitory activities, ageliferins (4-10) were the most potent metabolites, with IC50 values that ranged from 18 to 35 µM.

Sinularamides A-G, Terpenoid-Derived Spermidine and Spermine Conjugates with Casitas B-Lineage Lymphoma Proto-Oncogene B (Cbl-b) Inhibitory Activities from a Sinularia sp. Soft Coral.

An extract of a Sinularia sp. soft coral showed inhibitory activity against the E3-ubiquitin ligase casitas B-lineage lymphoma proto-oncogene B (Cbl-b). Subsequent bioassay-guided separation of the extract provided a series of terpenoid-derived spermidine and spermine amides that were named sinularamides A-G (1-7). Compounds 1-7 represent new natural products; however, sinularamide A (1) was previously reported as a synthetic end product. The structures of sinularamides A-G (1-7) were elucidated by analysis of spectroscopic and spectrometric data from NMR, IR, and HRESIMS experiments and by comparison with literature data. All of the isolated compounds showed Cbl-b inhibitory activities with IC50 values that ranged from approximately 6.5 to 33 µM.

In Vitro Ubiquitination Platform Identifies Methyl Ellipticiniums as Ubiquitin Ligase Inhibitors.

The transfer of the small protein ubiquitin to a target protein is an intricately orchestrated process called ubiquitination that results in modulation of protein function or stability. Proper regulation of ubiquitination is essential, and dysregulation of this process is implicated in several human diseases. An example of a ubiquitination cascade that is a central signaling node in important disease-associated pathways is that of CBLB [a human homolog of a viral oncogene Casitas B-lineage lymphoma (CBL) from the Cas NS-1 murine retrovirus], a RING finger ubiquitin ligase (E3) whose substrates include a number of important cell-signaling kinases. These include kinases important in immune function that act in the T cell receptor and costimulatory pathways, the Tyro/Axl/MerTK (TAM) receptor family in natural killer (NK) cells, as well as growth factor receptor kinases like epidermal growth factor receptor (EGFR). Loss of CBLB has been shown to increase innate and adaptive antitumor immunity. This suggests that small-molecule modulation of CBLB E3 activity could enhance antitumor immunity in patients. To explore the hypothesis that enzymatic inhibition of E3s may result in modulation of disease-related signaling pathways, we established a high-throughput screen of >70,000 chemical entities for inhibition of CBLB activity. Although CBLB was chosen as a proof-of-principle target for inhibitor discovery, we demonstrate that our assay is generalizable to monitoring the activity of other ubiquitin ligases. We further extended our observed in vitro inhibition with additional cell-based models of CBLB activity. From these studies, we demonstrate that a class of natural product-based alkaloids, known as methyl ellipticiniums (MEs), is capable of inhibiting ubiquitin ligases intracellularly.

Suberitamides A-C, Aryl Alkaloids from a Pseudosuberites sp. Marine Sponge that Inhibit Cbl-b Ubiquitin Ligase Activity.

Three new aryl alkaloids named suberitamides A-C (1-3), were isolated from an extract of the marine sponge Pseudosuberites sp. collected along the coast of North Carolina. Their planar structures were established by extensive nuclear magnetic resonance (NMR) and mass spectrometry (MS) analysis. To assign the challenging relative configuration of the saturated five-membered ring in suberitamide A (1), a simple and efficient NMR protocol was applied that is based on the analysis of 2- and 3-bond 1H-13C spin-spin coupling constants using a PIP (pure in-phase) HSQMBC (heteronuclear single quantum multiple bond correlation) IPAP (in-phase and anti-phase) experiment. Suberitamides A (1) and B (2) inhibited Cbl-b, an E3 ubiquitin ligase that is an important modulator of immune cell function, with IC50 values of approximately 11 µM.

Cbl interacts with multiple E2s in vitro and in cells.

Many receptor tyrosine kinases (RTKs, such as EGFR, MET) are negatively regulated by ubiquitination and degradation mediated by Cbl proteins, a family of RING finger (RF) ubiquitin ligases (E3s). Loss of Cbl protein function is associated with malignant transformation driven by increased RTK activity. RF E3s, such as the Cbl proteins, interact with a ubiquitin-conjugating enzyme (E2) to confer specificity to the ubiquitination process and direct the transfer of ubiquitin from the E2 to one or more lysines on the target proteins. Using in vitro E3 assays and yeast two-hybrid screens, we found that Ube2d, Ube2e families, Ube2n/2v1, and Ube2w catalyze autoubiquitination of the Cbl protein and Ube2d2, Ube2e1, and Ube 2n/2v1 catalyze Cbl-mediated substrate ubiquitination of the EGFR and SYK. Phosphorylation of the Cbl protein by by Src resulted in increased E3 activity compared to unphosphorylated cbl or Cbl containing a phosphomimetic Y371E mutation. Ubiquitin chain formation depended on the E2 tested with Cbl with Ube2d2 forming both K48 and K63 linked chains, Ube2n/2v1 forming only K63 linked chains, and Ube2w inducing monoubiquitination. In cells, the Ube2d family, Ube2e family, and Ube2n/2v1 contributed to EGFR ubiquitination. Our data suggest that multiple E2s can interact with Cbl and modulate its E3 activity in vitro and in cells.

ONC201 kills breast cancer cells in vitro by targeting mitochondria.

We report a novel mechanism of action of ONC201 as a mitochondria-targeting drug in cancer cells. ONC201 was originally identified as a small molecule that induces transcription of TNF-related apoptosis-inducing ligand (TRAIL) and subsequently kills cancer cells by activating TRAIL death receptors. In this study, we examined ONC201 toxicity on multiple human breast and endometrial cancer cell lines. ONC201 attenuated cell viability in all cancer cell lines tested. Unexpectedly, ONC201 toxicity was not dependent on either TRAIL receptors nor caspases. Time-lapse live cell imaging revealed that ONC201 induces cell membrane ballooning followed by rupture, distinct from the morphology of cells undergoing apoptosis. Further investigation found that ONC201 induces phosphorylation of AMP-dependent kinase and ATP loss. Cytotoxicity and ATP depletion were significantly enhanced in the absence of glucose, suggesting that ONC201 targets mitochondrial respiration. Further analysis indicated that ONC201 indirectly inhibits mitochondrial respiration. Confocal and electron microscopic analysis demonstrated that ONC201 triggers mitochondrial structural damage and functional impairment. Moreover, ONC201 decreased mitochondrial DNA (mtDNA). RNAseq analysis revealed that ONC201 suppresses expression of multiple mtDNA-encoded genes and nuclear-encoded mitochondrial genes involved in oxidative phosphorylation and other mitochondrial functions. Importantly, fumarate hydratase deficient cancer cells and multiple cancer cell lines with reduced amounts of mtDNA were resistant to ONC201. These results indicate that cells not dependent on mitochondrial respiration are ONC201-resistant. Our data demonstrate that ONC201 kills cancer cells by disrupting mitochondrial function and further suggests that cancer cells that are dependent on glycolysis will be resistant to ONC201.

CRKL mediates EML4-ALK signaling and is a potential therapeutic target for ALK-rearranged lung adenocarcinoma.

Anaplastic lymphoma kinase (ALK) gene rearrangements are oncogenic drivers in a small subset of patients with non-small-cell lung cancer (NSCLC). The ALK inhibitors are highly effective in NSCLC patients harboring ALK rearrangements; however, most patients acquire resistance to the therapy following an initial response. Mechanisms of acquired resistance are complex. We used LC-MS/MS-based phosphotyrosine-peptide profiling in the EML4-ALK rearranged H3122 and H2228 cells treated with ALK inhibitors, to identify downstream effectors of ALK. We then used Western blot, siRNA experiments, cell proliferation, viability and migration assays to validate our findings. We identified CRKL as a novel downstream effector of ALK signaling. We demonstrated that CRKL tyrosine phosphorylation was repressed by pharmacological inhibition or small interfering RNA (siRNA) knockdown of ALK in the ALK-rearranged cells. More importantly, CRKL knockdown attenuated their cell proliferation, viability, and migration, but it had no effect on ALK phosphorylation and expression in these cells. Furthermore, CRKL tyrosine phosphorylation was inhibited by dasatinib (an inhibitor of ABL and SRC kinases), which in combination with the ALK inhibitor crizotinib displayed a synergistic inhibitory effect in vitro. In conclusion, our study suggests that CRKL is a key downstream effector of ALK, and combined inhibition of ALK and CRKL may represent an effective strategy for treating ALK-rearranged NSCLC patients.

The TRAIL receptor agonist drozitumab targets basal B triple-negative breast cancer cells that express vimentin and Axl.

Previously, we found that GST-tagged tumor necrosis factor-related apoptosis inducing ligand preferentially killed triple-negative breast cancer (TNBC) cells with a mesenchymal phenotype by activating death receptor 5 (DR5). The purpose of this study was to explore the sensitivity of breast cancer cell lines to drozitumab, a clinically tested DR5-specific agonist; identify potential biomarkers of drozitumab-sensitive breast cancer cells; and determine if those biomarkers were present in tumors from patients with TNBC. We evaluated viability, caspase activity, and sub-G1 DNA content in drozitumab-treated breast cancer cell lines and we characterized expression of potential biomarkers by immunoblot. Expression levels of vimentin and Axl were then explored in 177 TNBC samples from a publically available cDNA microarray dataset and by immunohistochemistry (IHC) in tumor tissue samples obtained from 53 African-American women with TNBC. Drozitumab-induced apoptosis in mesenchymal TNBC cell lines but not in cell lines from other breast cancer subtypes. The drozitumab-sensitive TNBC cell lines expressed the mesenchymal markers vimentin and Axl. Vimentin and Axl mRNA and protein were expressed in a subset of human TNBC tumors. By IHC, ~15 % of TNBC tumors had vimentin and Axl expression in the top quartile for both. These findings indicate that drozitumab-sensitive mesenchymal TNBC cells express vimentin and Axl, which can be identified in a subset of human TNBC tumors. Thus, vimentin and Axl may be useful to identify TNBC patients who would be most likely to benefit from a DR5 agonist.

LAMC2 enhances the metastatic potential of lung adenocarcinoma.

Lung cancer is the number one cancer killer, and metastasis is the main cause of high mortality in lung cancer patients. However, mechanisms underlying the development of lung cancer metastasis remain unknown. Using genome-wide transcriptional analysis in an experimental metastasis model, we identified laminin γ2 (LAMC2), an epithelial basement membrane protein, to be significantly upregulated in lung adenocarcinoma metastatic cells. Elevated LAMC2 increased traction force, migration, and invasion of lung adenocarcinoma cells accompanied by the induction of epithelial-mesenchymal transition (EMT). LAMC2 knockdown decreased traction force, migration, and invasion accompanied by EMT reduction in vitro, and attenuated metastasis in mice. LAMC2 promoted migration and invasion via EMT that was integrin ß1- and ZEB1-dependent. High LAMC2 was significantly correlated with the mesenchymal marker vimentin expression in lung adenocarcinomas, and with higher risk of recurrence or death in patients with lung adenocarcinoma. We suggest that LAMC2 promotes metastasis in lung adenocarcinoma via EMT and may be a potential therapeutic target.

A specific missense mutation in GTF2I occurs at high frequency in thymic epithelial tumors.

We analyzed 28 thymic epithelial tumors (TETs) using next-generation sequencing and identified a missense mutation (chromosome 7 c.74146970T>A) in GTF2I at high frequency in type A thymomas, a relatively indolent subtype. In a series of 274 TETs, we detected the GTF2I mutation in 82% of type A and 74% of type AB thymomas but rarely in the aggressive subtypes, where recurrent mutations of known cancer genes have been identified. Therefore, GTF2I mutation correlated with better survival. GTF2I ß and δ isoforms were expressed in TETs, and both mutant isoforms were able to stimulate cell proliferation in vitro. Thymic carcinomas carried a higher number of mutations than thymomas (average of 43.5 and 18.4, respectively). Notably, we identified recurrent mutations of known cancer genes, including TP53, CYLD, CDKN2A, BAP1 and PBRM1, in thymic carcinomas. These findings will complement the diagnostic assessment of these tumors and also facilitate development of a molecular classification and assessment of prognosis and treatment strategies.

Whole genome and transcriptome sequencing of a B3 thymoma.

Molecular pathology of thymomas is poorly understood. Genomic aberrations are frequently identified in tumors but no extensive sequencing has been reported in thymomas. Here we present the first comprehensive view of a B3 thymoma at whole genome and transcriptome levels. A 55-year-old Caucasian female underwent complete resection of a stage IVA B3 thymoma. RNA and DNA were extracted from a snap frozen tumor sample with a fraction of cancer cells over 80%. We performed array comparative genomic hybridization using Agilent platform, transcriptome sequencing using HiSeq 2000 (Illumina) and whole genome sequencing using Complete Genomics Inc platform. Whole genome sequencing determined, in tumor and normal, the sequence of both alleles in more than 95% of the reference genome (NCBI Build 37). Copy number (CN) aberrations were comparable with those previously described for B3 thymomas, with CN gain of chromosome 1q, 5, 7 and X and CN loss of 3p, 6, 11q42.2-qter and q13. One translocation t(11;X) was identified by whole genome sequencing and confirmed by PCR and Sanger sequencing. Ten single nucleotide variations (SNVs) and 2 insertion/deletions (INDELs) were identified; these mutations resulted in non-synonymous amino acid changes or affected splicing sites. The lack of common cancer-associated mutations in this patient suggests that thymomas may evolve through mechanisms distinctive from other tumor types, and supports the rationale for additional high-throughput sequencing screens to better understand the somatic genetic architecture of thymoma.

Copy number aberrations of genes regulating normal thymus development in thymic epithelial tumors.

PURPOSES: To determine whether the deregulation of genes relevant for normal thymus development can contribute to the biology of thymic epithelial tumors (TET). EXPERIMENTAL DESIGN: Using array comparative genomic hybridization, we evaluated the copy number aberrations of genes regulating thymus development. The expression of genes most commonly involved in copy number aberrations was evaluated by immunohistochemistry and correlated with patients' outcome. Correlation between FOXC1 copy number loss and gene expression was determined in a confirmation cohort. Cell lines were used to test the role of FOXC1 in tumors. RESULTS: Among 31 thymus development-related genes, PBX1 copy number gain and FOXC1 copy number loss were presented in 43.0% and 39.5% of the tumors, respectively. Immunohistochemistry on a series of 132 TETs, including those evaluated by comparative genomic hybridization, revealed a correlation between protein expression and copy number status only for FOXC1 but not for PBX1. Patients with FOXC1-negative tumors had a shorter time to progression and a trend for a shorter disease-related survival. The correlation between FOXC1 copy number loss and mRNA expression was confirmed in a separate cohort of 27 TETs. Ectopic FOXC1 expression attenuated anchorage-independent cell growth and cell migration in vitro. CONCLUSION: Our data support a tumor suppressor role of FOXC1 in TETs.

Poly(ADP-ribose) polymerase inhibition enhances p53-dependent and -independent DNA damage responses induced by DNA damaging agent.

Targeting DNA repair with poly(ADP-ribose) polymerase (PARP) inhibitors has shown a broad range of anti-tumor activity in patients with advanced malignancies with and without BRCA deficiency. It remains unclear what role p53 plays in response to PARP inhibition in BRCA-proficient cancer cells treated with DNA damaging agents. Using gene expression microarray analysis, we find that DNA damage response (DDR) pathways elicited by veliparib (ABT-888), a PARP inhibitor, plus topotecan comprise the G1/S checkpoint, ATM, and p53 signaling pathways in p53-wildtype cancer cell lines and BRCA1, BRCA2 and ATR pathway in p53-mutant lines. In contrast, topotecan alone induces the G1/S checkpoint pathway in p53-wildtype lines and not in p53-mutant cells. These responses are coupled with G2/G1 checkpoint effectors p21(CDKN1A) upregulation, and Chk1 and Chk2 activation. The drug combination enhances G2 cell cycle arrest, apoptosis and a marked increase in cell death relative to topotecan alone in p53-wildtype and p53-mutant or -null cells. We also show that the checkpoint kinase inhibitor UCN-01 abolishes the G2 arrest induced by the veliparib and topotecan combination and further increases cell death in both p53-wildtype and -mutant cells. Collectively, PARP inhibition by veliparib enhances DDR and cell death in BRCA-proficient cancer cells in a p53-dependent and -independent fashion. Abrogating the cell-cycle arrest induced by PARP inhibition plus chemotherapeutics may be a strategy in the treatment of BRCA-proficient cancer.

MicroRNA expression and clinical outcome of small cell lung cancer.

The role of microRNAs in small-cell lung carcinoma (SCLC) is largely unknown. miR-34a is known as a p53 regulated tumor suppressor microRNA in many cancer types. However, its therapeutic implication has never been studied in SCLC, a cancer type with frequent dysfunction of p53. We investigated the expression of a panel of 7 microRNAs (miR-21, miR-29b, miR-34a/b/c, miR-155, and let-7a) in 31 SCLC tumors, 14 SCLC cell lines, and 26 NSCLC cell lines. We observed significantly lower miR-21, miR-29b, and miR-34a expression in SCLC cell lines than in NSCLC cell lines. The expression of the 7 microRNAs was unrelated to SCLC patients' clinical characteristics and was neither prognostic in term of overall survival or progression-free survival nor predictive of treatment response. Overexpression or downregulation of miR-34a did not influence SCLC cell viability. The expression of these 7 microRNAs also did not predict in vitro sensitivity to cisplatin or etoposide in SCLC cell lines. Overexpression or downregulation of miR-34a did not influence sensitivity to cisplatin or etoposide in SCLC cell lines. In contrast to downregulation of the miR-34a target genes cMET and Axl by overexpression of miR-34a in NSCLC cell lines, the intrinsic expression of cMET and Axl was low in SCLC cell lines and was not influenced by overexpression of miR-34a. Our results suggest that the expression of the 7 selected microRNAs are not prognostic in SCLC patients, and miR-34a is unrelated to the malignant behavior of SCLC cells and is unlikely to be a therapeutic target.

Enhanced growth inhibition by combined DNA methylation/HDAC inhibitors in lung tumor cells with silenced CDKN2A.

Aberrant hypermethylation at CpG sites within the CDKN2A gene is associated with silencing and has been proposed as a target for reactivation using both DNA methylation and histone deacetylation inhibitors. This study investigates the role of selecting tumor samples with a silenced as compared to deleted CDKN2A locus when assessing the efficacy of DNA methyltransferase inhibitor, zebularine, combined with the HDAC inhibitor, depsipeptide. Non-small cell lung cancer cell lines with defined CDKN2A status were analyzed by MTS assay to determine the effect of zebularine or zebularine combined with depsipeptide on tumor cell growth. We observed that zebularine treatment resulted in inhibition of cell growth in 11 out of 12 lung cancer cell lines with silenced CDKN2A, but no cell growth inhibition was detected in the 7 lung cancer cell lines tested with deleted CDKN2A (p>0.001). In addition, we found that the combination of 30 microM zebularine and 6 or 7 nM depsipeptide resulted in a synergistic inhibition of cell growth in tumor cells with silenced CDKN2A (p<0.001, CI=0.70 and 0.57, respectively) but not in tumor cells with deleted CDKN2A. In conclusion, tumor cells with methylated CDKN2A are more sensitive to zebularine than cell lines with deleted CDKN2A and the combination of zebularine/depsipeptide results in a synergistic effect on cell growth inhibition that is also linked with the presence of silenced CDKN2A. Thus, combination of DNA methyltransferase and HDAC inhibitors may be a potential treatment for lung cancer patients, but careful selection of patients will be needed to optimize the benefit of this regimen.

Elevated levels of thymidylate synthase linked to neoplastic transformation of mammalian cells.

Thymidylate synthase (TS), an enzyme that is essential for DNA synthesis and repair has been identified as an important biomarker for colorectal and other human cancers. The elevated steady-state levels of TS found in many common human malignancies have been thought to represent a secondary event in tumor formation. However, it has recently been demonstrated that the deregulated levels of ectopic TS may also have a causal effect on tumorgenesis since overexpression of human TS transforms immortalized mammalian cells to a malignant phenotype. Since the levels of TS are regulated by E2F-1 and thus are linked to the cell cycle pathway, regulating TS activity may be an important factor for the control of cell cycle progression and for the development of therapeutic strategies and cancer prevention.