Epigenetically regulated PCDHB15 impairs aggressiveness of metastatic melanoma cells

The protocadherin proteins are cell adhesion molecules at the crossroad of signaling pathways playing a major role in neuronal development. It is now understood that their role as signaling hubs is not only important for the normal physiology of cells but also for the regulation of hallmarks of cancerogenesis. Importantly, protocadherins form a cluster of genes that are regulated by DNA methylation. We have identified for the first time that PCDHB15 gene is DNA-hypermethylated on its unique exon in the metastatic melanoma-derived cell lines and patients' metastases compared to primary tumors. This DNA hypermethylation silences the gene, and treatment with the DNA demethylating agent 5-aza-2'-deoxycytidine reinduces its expression. We explored the role of PCDHB15 in melanoma aggressiveness and showed that overexpression impairs invasiveness and aggregation of metastatic melanoma cells in vitro and formation of lung metastasis in vivo. These findings highlight important modifications of the methylation of the PCDHβ genes in melanoma and support a functional role of PCDHB15 silencing in melanoma aggressiveness.

DNA methylome combined with chromosome cluster-oriented analysis provides an early signature for cutaneous melanoma aggressiveness

Aberrant DNA methylation is a well-known feature of tumours and has been associated with metastatic melanoma. However, since melanoma cells are highly heterogeneous, it has been challenging to use affected genes to predict tumour aggressiveness, metastatic evolution, and patients’ outcomes. We hypothesized that common aggressive hypermethylation signatures should emerge early in tumorigenesis and should be shared in aggressive cells, independent of the physiological context under which this trait arises. We compared paired melanoma cell lines with the following properties: (i) each pair comprises one aggressive counterpart and its parental cell line and (ii) the aggressive cell lines were each obtained from different host and their environment (human, rat, and mouse), though starting from the same parent cell line. Next, we developed a multi-step genomic pipeline that combines the DNA methylome profile with a chromosome cluster-oriented analysis. A total of 229 differentially hypermethylated genes was commonly found in the aggressive cell lines. Genome localization analysis revealed hypermethylation peaks and clusters, identifying eight hypermethylated gene promoters for validation in tissues from melanoma patients. Five Cytosine-phosphate-Guanine (CpGs) identified in primary melanoma tissues were transformed into a DNA methylation score that can predict survival (log-rank test, p=0.0008). This strategy is potentially universally applicable to other diseases involving DNA methylation alterations.

Demethylation by low-dose 5-aza-2'-deoxycytidine impairs 3D melanoma invasion partially through miR-199a-3p expression revealing the role of this miR in melanoma

BACKGROUND

Efficient treatments against metastatic melanoma dissemination are still lacking. Here, we report that low-cytotoxic concentrations of 5-aza-2'-deoxycytidine, a DNA demethylating agent, prevent in vitro 3D invasiveness of metastatic melanoma cells and reduce lung metastasis formation in vivo.

RESULTS

We unravelled that this beneficial effect is in part due to MIR-199A2 re-expression by promoter demethylation. Alone, this miR showed an anti-invasive and anti-metastatic effect. Throughout integration of micro-RNA target prediction databases with transcriptomic analysis after 5-aza-2'-deoxycytidine treatments, we found that miR-199a-3p downregulates set of genes significantly involved in invasion/migration processes. In addition, analysis of data from melanoma patients showed a stage- and tissue type-dependent modulation of MIR-199A2 expression by DNA methylation.

CONCLUSIONS

Thus, our data suggest that epigenetic- and/or miR-based therapeutic strategies can be relevant to limit metastatic dissemination of melanoma.

DNA methyltransferase inhibitors in cancer: From pharmacology to translational studies

DNA methylation is a mammalian epigenetic mark that participates to define where and when genes are expressed, both in normal cells and in the context of diseases. Like other epigenetic marks, it is reversible and can be modulated by chemical agents. Because it plays an important role in cancer by silencing certain genes, such as tumour suppressor genes, it is a promising therapeutic target. Two compounds are already approved to treat haematological cancers, and many efforts have been carried out to discover new molecules that inhibit DNA methyltransferases, the enzymes responsible for DNA methylation. Here, we analyse the molecular mechanisms and cellular pharmacology of these inhibitors, pointing out the necessity for new pharmacological models and paradigms. The parameters of pharmacological responses need to be redefined: the aim is cellular reprogramming rather than general cytotoxicity. Thus, "epigenetic" rather than cytotoxic dosages are defined. Another issue is the delay of the response: cellular reprogramming can take several generations to produce observable phenotypes. Is this compatible with laboratory scale experiments? Finally, it is important to consider the specificity for cancer cells compared to normal cells and the appearance of resistance. We also discuss different techniques that are used and the selection of pharmacological models.

Abstract LB-295: Identification of the first class of proteasome inhibitors that impair 26S proteasome assembly in vitro

The Ubiquitin-Proteasome System (UPS) is the main non-lysosomial proteolytic machinery in the eukaryotic cells. Prior to their degradation proteins are adressed to the catalytic 26S proteasome complex composed of a 20S “core” that contains proteolytic activities, assembled with one or two 19S regulatory particles that bind proteins to be degraded. Due to its central role of in regulating cell cycle progression and signal transduction the UPS has been proposed as a suitable target for antineoplastic strategies. To date only catalytic inhibitors of proteasome, targeting proteolytic activities of the 20S core particle, have been characterized and/or have emerged as potent anticancer drugs (1).

However, the capacity of the UPS to degrade proteins not only depends of the catalytic activities of the 20S core, but also of the 26S proteasome assembly, a dynamic and tightly controlled cellular process. Thus, compounds impairing 26S proteasome assembly could represent a novel class of proteasome inhibitors.

With the aim of identifying such molecules we first set up a robust biochemical screening assay allowing us to follow 26S proteasome assembly in vitro. First, 26S proteasome was purified from Hela cells using affinity chromatography followed by gel filtration. Then, experimental conditions were determined for in vitro 26S disassembly into its constitutive sub-complexes (19S+20S) ands its re-assembly as an active and functional 26S entity. The differential CTL-activity between 26S and 20S proteasomes (2) was then used as the readout to follow the reassembly of 26S in vitro, prior to be confirmed using in gel overlay experiments and western-blotting. Using this biochemical assay:

1) we screened naturals compounds endowed with antiproliferative properties and described as inhibitors of the ubiquitine-proteasome pathway (3).

2) we identified structurally-related natural molecules, epidithiodioxopiperazine and diketodithiopiperazine derivatives, as the first class of molecules inhibiting proteasome assembly without significantly affecting neither 26S nor 20S catalytic activity in vitro.

Our research efforts now concentrate in showing that: 1) these molecules inhibit proteasome assembly in cells, and that 2) this inhibition can be correlated with their antiproliferative effects.

References

Sterz J. et al. Extert Opin. Invest. Drugs, 2008, 17

Rivett AJ et al., Experimental Gerontology, 2002, 37

Vandenberghe I. et al., Biochem. Pharmacol., 2008, 76

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-295.

The centrosome protein NEDD1 as a potential pharmacological target to induce cell cycle arrest

BACKGROUND

NEDD1 is a protein that binds to the gamma-tubulin ring complex, a multiprotein complex at the centrosome and at the mitotic spindle that mediates the nucleation of microtubules.

RESULTS

We show that NEDD1 is expressed at comparable levels in a variety of tumor-derived cell lines and untransformed cells. We demonstrate that silencing of NEDD1 expression by treatment with siRNA has differential effects on cells, depending on their status of p53 expression: p53-positive cells arrest in G1, whereas p53-negative cells arrest in mitosis with predominantly aberrant monopolar spindles. However, both p53-positive and -negative cells arrest in mitosis if treated with low doses of siRNA against NEDD1 combined with low doses of the inhibitor BI2536 against the mitotic kinase Plk1. Simultaneous reduction of NEDD1 levels and inhibition of Plk1 act in a synergistic manner, by potentiating the anti-mitotic activity of each treatment.

CONCLUSION

We propose that NEDD1 may be a promising target for controlling cell proliferation, in particular if targeted in combination with Plk1 inhibitors.

DNA damage induce gamma-tubulin-RAD51 nuclear complexes in mammalian cells

Rad51 protein plays an essential role in recombination repair of DNA double-strand breaks and DNA crosslinking adducts. It is part of complexes which can vary with the stage of the cell cycle and the nature of the DNA lesions. During a search for Rad51-associated proteins in CHO nuclear extracts of S-phase cells by mass spectrometry of proteins immunoprecipitated with Rad51 antibodies, we identified a centrosomal protein, gamma-tubulin. This association was confirmed by the reverse immunoprecipitation with gamma-tubulin antibodies. Both proteins copurified from HeLa cells nuclear extracts following a tandem affinity purification of double-tagged Rad51. Immunofluorescence analysis showed colocalization of both Rad51 and gamma-tubulin in discrete foci in mammalian cell nuclei. The number of colocalized foci and their overlapping area increased in the presence of DNA damage produced by genotoxic treatments either during S phase or in exponentially growing cells. These variations did not result from an overall stress because microtubule cytoskeleton poisons devoid of direct interactions with DNA, such as taxol or colcemid, did not lead to an increase of this association. The recruitment of Rad51 and gamma-tubulin in the same nuclear complex suggests a link between DNA recombination repair and the centrosome function during the cell cycle.

Vinca alkaloid-induced tubulin spiral formation correlates with cytotoxicity in the leukemic L1210 cell line

The ability of a class of C-20' modified vinca alkaloid congeners to induce tubulin spiral formation was investigated relative to their ability to inhibit microtubule assembly, their cytotoxicity against a leukemic cell line, L1210, and their measured and calculated partition coefficients. These studies were prompted by the observation that the energetics of vinca alkaloid-induced tubulin spiral polymers, or spiraling potential, is inversely related to their clinical dosage and are aimed at the long-term goal of developing the ability to predict the cytotoxic and antineoplastic properties of antimitotic drugs. We demonstrate here that vinca-induced tubulin-spiraling potential is significantly correlated with cytotoxicity against L1210 cells. This is consistent with the size of spirals formed being proportional to the relaxation time for polymer redistribution, the lifetime of cell retention, and effects on microtubule ends and dynamics. Spiraling potential also correlates with calculated but not measured partition coefficients. Surprisingly, spiraling potential does not correlate with the ability to inhibit microtubule formation with purified tubulin or microtubule protein. For the set of C-20' modified compounds studied, the largest inhibitory effects on spiraling potential and cytotoxicity are caused by multiple sites of halogen (-F, -Cl) substitution with the introduction of increased rigidity in the ring. This suggests the C-20' position interacts with a hydrogen bond acceptor or an electrophilic region on the protein that electrostatically disfavors halogen substitutions. These studies are discussed in terms of the cellular mode of action of antimitotic drugs, particularly the importance of microtubule dynamics during mitosis and the factors that regulate those dynamics.

Vinflunine (20',20'-difluoro-3',4'-dihydrovinorelbine), a novel Vinca alkaloid, which participates in P-glycoprotein (Pgp)-mediated multidrug resistance in vivo and in vitro

Vinflunine (VFL) is a novel derivative of vinorelbine (NVB, Navelbine), which has shown markedly superior antitumor activity to NVB, in various experimental animal models. To establish whether this new Vinca alkaloid participates in P-glycoprotein (Pgp)-mediated multidrug resistance (MDR), VFL-resistant murine P388 cells (P388/VFL) were established in vivo and used in conjunction with the well established MDR P388/ADR subline, to define the in vivo resistance profile for VFL. P388/VFL cells proved cross-resistant to drugs implicated in MDR (other Vinca alkaloids, doxorubicin, etoposide), but not to campothecin or cisplatin and showed an increased expression of Pgp, without any detectable alterations in topoisomerase II or in glutathione metabolism. The P388/ADR cells proved cross-resistant to VFL both in vivo and in vitro, and this VFL resistance was efficiently modulated by verapamil in vitro. Cellular transport experiments with tritiated-VFL revealed differential uptake by P388 sensitive and P388/ADR resistant cells, comparable with data obtained using tritiated-NVB. In various in vitro models of human MDR tumor cells, whilst full sensitivity was retained in cells expressing alternative non-Pgp-mediated MDR mechanisms, cross resistance was identified in Pgp-overexpressing cells. Differences were, however, noted in terms of the drug resistance profiles relative to the other Vinca, with tumor cell lines proving generally least cross-resistant to VFL. Overall, these results suggest that VFL, like other Vinca alkaloids, participates in Pgp-mediated MDR, with tumor cells selected for resistance to VFL overexpressing Pgp, yet MDR tumor cell lines proved generally less cross resistant to VFL relative to the other Vinca alkaloids.

Requirements for P-glycoprotein recognition based on structure-activity relationships in the podophyllotoxin series

Podophyllotoxin and epipodophyllotoxin react with tubulin at the same binding site as colchicine, but in contrast to colchicine, do not appear to exert their cytotoxicities by mechanisms dependent on P-glycoprotein (Pgp) expression. To investigate structural requirements for Pgp recognition a series of podophyllotoxin and epipodophyllotoxin derivatives have been synthesized. Their interactions with the multidrug resistance-related protein Pgp have been studied by evaluating their relative cytotoxicities versus P388-sensitive murine leukemic cells and a classic multidrug-resistant (MDR) Pgp-overexpressing subline (P388/ADR), and their relative tubulin polymerization inhibitory activities against microtubular proteins have been determined. Based on tridimensional structure-activity relationships within this series of compounds, structural requirements for Pgp recognition have been identified. Moreover, proposals are made for extending these criteria to other chemical classes of anticancer drugs.

Influence of culture media on the morphological differentiation of the PC-3 and DU145 prostatic neoplastic cell lines

The concept of differentiation of prostate cancer in terms of morphonuclear characteristics and population dynamics was investigated on the PC-3 and DU145 cell lines. A software based on the concept of Voronoi paving was set up in order to characterize the structure of these cell lines growing in vitro on histological slides. The morphonuclear characteristics were assessed by means of the digital cell image analyses of Feulgen-stained nuclei. The in vitro "morphonuclear" and "pseudo-tissular" differentiations of the PC-3 and DU145 cells were described in terms of the use of various culture media, i.e., media supplemented with either 10% (F10 medium) or 1% (F1 medium) fetal calf serum and with (or without) platelet-derived growth factor and dihydrotestosterone (PA10 and PA1 media). The present data reveal that the PC-3 cell line would be more hormone-sensitive than the DU145 one. Indeed, decreasing the FCS concentration in the culture medium while adding DHT and PDGF led to marked modifications to the morphonuclear characteristics of the PC-3 cells, but not to the DU145 cells. These modifications corresponded to an increase in nuclear size occurring concomitantly with chromatin decondensation. In the same way, spectacular modifications in terms of medium-induced pseudo-tissular differentiation were observed in the PC-3 cell line, but not in the DU145 one. Such modifications corresponded to an increase in clone size related to an increase in the mean distances between neighboring cell nuclei in a given clone. Thus, according to the criteria defined in this study, the PC-3 cell line would seem to maintain a higher degree of differentiation than the DU145 cell line.

Characterization of the differentiation of human colorectal cancer cell lines by means of Voronoi diagrams

This paper describes differentiation in terms of population dynamics through the medium of Voronoi paving which enables (via digital cell image analysis) the structure of human LOVO and HCT-15 colorectal neoplastic cell colonies growing on histological slides to be characterized. Two other tests were also used, i.e., the colorimetric MTT assay that enables the cell growth level to be determined, and a test allowing the assessment of the proliferation index, i.e., the percentage of cells in the S phase of the cell cycle. The results show that these colorectal neoplastic cells exhibited a comparatively high level of organisation in terms of the topographical distribution of nuclei within the clones when the cells were cultivated in media containing even small amounts of fetal calf serum. On the other hand, certain chemically defined media completely overturned this "pseudo-tissular" architecture. Furthermore, the colorectal cells growing in media including fetal calf serum exhibited relatively large and dense clones, undergoing an increase in the density of these clones when hormones were added to the culture medium and, concomitantly, a decrease in their proliferation. In contrast, the cells growing in chemically defined media generally exhibited smaller clones whose cell proliferation was paradoxically greater than that of the cells referred to above. This seems to bring out the importance of the part played by the cell loss factor in this cell population dynamic.

Monitoring of morphonuclear characteristics of hormone-sensitive and insensitive HCT-15 and LOVO human colorectal cells by means of digital cell image analysis

We recently set up in vitro several human colorectal neoplastic cell lines that we labelled hormone-sensitive (HS) in comparison with the original cell lines which appeared to be rather "hormone-insensitive" (HI). We describe here the cell proliferation rate and the morphonuclear characteristics of the HS and HI variants of the HCT-15 and LOVO human neoplastic colorectal cell lines which were cultured either in serum-supplemented or chemically-defined media. Morphonuclear characteristics were monitored by means of the digital cell image analyses of Feulgen-stained nuclei, while the proliferation activity of the various cell types was assessed by means of the tetrazolium-based compound (MTT) assay. The results show that it is possible to culture human LOVO and HCT-15 colorectal cells in chemically defined media. This said, growth in a chemically defined medium is difficult for these cells and is markedly less sustained than when they are cultured in serum-supplemented media. The transition of the culture from a serum-supplemented medium to a chemically defined one is accompanied by a very marked drop in cell proliferation and a number of profound changes in terms of morphonuclear characteristics. These changes basically involve chromatin decondensation, which occurs as the result of the drop in the number of large chromatin clumps in the nucleus.

Influence of suramin alone or in combination with DHT and PDGF on the cell proliferation of benign and malignant human prostatic tissues in organ cultures

We studied the suramin-induced influence on the cell proliferation of 16 benign and 6 malignant lesions of the human prostate maintained in vitro as organ cultures. The cell proliferation was assessed by nuclear labeling with tritiated thymidine autoradiography. We also studied the dihydrotestosterone (DHT)-and platelet-derived growth factor (PDGF)-induced modulation of suramin influence on such prostate organ culture cell proliferation. Our results indicate that more than half of the benign prostatic tissues showed cell proliferation which was modulated by DHT and/or PDGF, while none of the six carcinomas responded to such hormonal stimulation. Suramin alone inhibited the cell proliferation of only 19% of the prostate organ culture under study, while in combination with DHT and/or PDGF this inhibition level reached 48%. However, we occasionally observed that S alone or in combination with DHT and/or PDGF was also able to stimulate prostate cell proliferation. We think that organ cultures of human prostatic tissues might represent a helpful pre-clinical tool to study the anti-tumoral influence of suramin, which is a new antineoplastic generative compound.

In vitro influence of gastrin, oestradiol and gonadotropin-releasing hormone on HCT-15 and LoVo human colorectal neoplastic cell proliferation

We set up in vitro several human colorectal neoplastic cell lines that we labelled "hormone-sensitive" (HS) in comparison to the original cell lines which appeared to be rather "hormone-insensitive" (HI). We used LoVo and HCT-15 human colorectal neoplastic cell lines and studied the influence of 17 beta-oestradiol (E2), gastrin and two gonadotropin-releasing hormone (GnRH) analogues, HRF and buserelin, on the proliferation of the HS and HI variants of the LoVo and HCT-15 cell lines. Cell proliferation was evaluated by a colorimetric assay, the MTT test. Our results show that E2, gastrin, HRF and buserelin did not induce a significant stimulatory influence on the HI variants of the LoVo and HCT-15 cells, i.e. the cells that were cultured in a hormone-free 10% FCS-supplemented medium. In sharp contrast, the colorectal cells cultured for 30 passages in an E2 and/or gastrin + 1% FCS-supplemented medium showed a marked tropic response to E2, gastrin, HRF and buserelin. However, the HS variants of the HCT-15 cells appeared less sensitive to the two GnRH analogues than did the HS variants of the LoVo cells.

The combination of the tetrazolium derivative reduction (MTT) and digital cell image analysis to monitor in vitro the cytotoxicity of anti-neoplastic drugs

We assessed the in vitro drug-induced cytotoxicity by means of the rapid low-cost but weakly sensitive Thiazolyl blue (MTT) test, and the less rapid, higher cost, but highly sensitive cell image analysis (CIA) test. We studied the influence of three drugs, i.e. a vinca-alkaloid (Navelbine), an alkylating investigational agent (PE1001), and an intercalating drug, i.e. adriamycin, on the proliferation (MTT test) and cell kinetics (CIA test) of the mouse MXT and the MCF-7 mammary cancer cell lines. Adriamycin and PE1001 decreased MXT and MCF-7 cell proliferation in a dose-dependent manner as assessed by both the MTT and CIA tests. Navelbine was highly cytotoxic in a dose-dependent manner. We demonstrated that Navelbine arrests cells in the M phase without altering the G2 phase. In sharp contrast, PE1001 arrest cells in the G2 phase without altering the M phase. An adriamycin-induced effect was apparent on S phase. Thus, the MTT test allows the screening of a great number of drugs analyzed at the cell proliferation level and, in the case of MTT positive drug-induced response, the CIA test enabled the drug-induced effect at cell cycle kinetic level to be investigated.

Characterization of a spontaneously transformed pulmonary embryonic rat (PER) epithelial cell line

A spontaneously transformed pulmonary embryonic rat epithelial cell line (PER) is described in terms of growth, tumorigenicity, growth factor responsiveness and biosynthetic capacity. At low-passage subcultures, PER cells grew as a monolayer and did not form colonies in soft agar. After long-term subcultivation, they lost contact inhibition, became anchorage-independent and formed tumours in nude mice. Low concentrations of foetal calf serum permit the maximum growth rate. The multiplication and metabolic activity, assessed by 2-deoxy-D-glucose uptake, was significantly stimulated by growth factors. PER cells synthesized collagen types I, III, IV and V, laminin and fibronectin, and organized a pericellular matrix made up of only basement membrane components (type IV collagen and laminin) and fibronectin. These data enabled us to define PER cells as a transformed epithelial cell line evolving towards malignancy with long-term subcultivation. These cells appeared to be a valuable tool in studies of tumour cell-matrix interactions and regulation of growth factor receptors in tumorigenesis.