Depletion of c-Myc inhibits human colon cancer colo 320 cells' growth

Enhancer activation from transposable elements in extrachromosomal DNA

Extrachromosomal DNA (ecDNA) is a hallmark of aggressive cancer, contributing to both oncogene amplification and tumor heterogeneity. Here, we used Hi-C, super-resolution imaging, and long-read sequencing to explore the nuclear architecture of MYC-amplified ecDNA in colorectal cancer cells. Intriguingly, we observed frequent spatial proximity between ecDNA and 68 repetitive elements which we called ecDNA-interacting elements or EIEs. To characterize a potential regulatory role of EIEs, we focused on a fragment of the L1M4a1#LINE/L1 which we found to be co-amplified with MYC on ecDNA, gaining enhancer-associated chromatin marks in contrast to its normally silenced state. This EIE, in particular, existed as a naturally occurring structural variant upstream of MYC, gaining oncogenic potential in the transcriptionally permissive ecDNA environment. This EIE sequence is sufficient to enhance MYC expression and is required for cancer cell fitness. These findings suggest that silent repetitive genomic elements can be reactivated on ecDNA, leading to functional cooption and amplification. Repeat element activation on ecDNA represents a mechanism of accelerated evolution and tumor heterogeneity and may have diagnostic and therapeutic potential.

Dual Targeting of Bromodomain and Extraterminal Domain Proteins, and WNT or MAPK Signaling, Inhibits c-MYC Expression and Proliferation of Colorectal Cancer Cells

Inhibitors of the bromodomain and extraterminal domain (BET) protein family attenuate the proliferation of several tumor cell lines. These effects are mediated, at least in part, through repression of c-MYC. In colorectal cancer, overexpression of c-MYC due to hyperactive WNT/β-catenin/TCF signaling is a key driver of tumor progression; however, effective strategies to target this oncogene remain elusive. Here, we investigated the effect of BET inhibitors (BETi) on colorectal cancer cell proliferation and c-MYC expression. Treatment of 20 colorectal cancer cell lines with the BETi JQ1 identified a subset of highly sensitive lines. JQ1 sensitivity was higher in cell lines with microsatellite instability but was not associated with the CpG island methylator phenotype, c-MYC expression or amplification status, BET protein expression, or mutation status of TP53, KRAS/BRAF, or PIK3CA/PTEN Conversely, JQ1 sensitivity correlated significantly with the magnitude of c-MYC mRNA and protein repression. JQ1-mediated c-MYC repression was not due to generalized attenuation of β-catenin/TCF-mediated transcription, as JQ1 had minimal effects on other β-catenin/TCF target genes or β-catenin/TCF reporter activity. BETi preferentially target super-enhancer-regulated genes, and a super-enhancer in c-MYC was recently identified in HCT116 cells to which BRD4 and effector transcription factors of the WNT/β-catenin/TCF and MEK/ERK pathways are recruited. Combined targeting of c-MYC with JQ1 and inhibitors of these pathways additively repressed c-MYC and proliferation of HCT116 cells. These findings demonstrate that BETi downregulate c-MYC expression and inhibit colorectal cancer cell proliferation and identify strategies for enhancing the effects of BETi on c-MYC repression by combinatorial targeting the c-MYC super-enhancer. Mol Cancer Ther; 15(6); 1217-26. ©2016 AACR.

Molecular mechanisms associated with PTHrP-induced proliferation of colon cancer cells

Parathyroid Hormone-related Protein (PTHrP) is normally produced in many tissues and is recognized for its endocrine, paracrine, autocrine and intracrine modes of action. PTHrP is also implicated in different types of cancer and its expression correlates with the severity of colon carcinoma. Using the human colon cell line Caco-2 we recently obtained evidence that PTHrP, through a paracrine pathway, exerts a protective effect under apoptotic conditions. However, if exogenous PTHrP is able or not to induce the proliferation of these intestinal tumor cells is not known. We found that PTHrP treatment increases the number of live Caco-2 cells. The hormone induces the phosphorylation and nuclear translocation of ERK 1/2, α p38 MAPK, and Akt, without affecting JNK phosphorylation. In addition, PTHrP-dependent ERK phosphorylation is reverted when PI3K activity was inhibited. Following MAPKs nuclear translocation, the transcription factors ATF-1 and CREB were activated in a biphasic manner. In addition PTHrP induces the translocation into the nucleus of β-catenin, protein that plays key role in maintaining the growth and proliferation of colorectal cancer, and increases the amount of both positive cell cycle regulators c-Myc and Cyclin D. Studies with ERK1/2, α p38 MAPK, and PI3K specific inhibitors showed that PTHrP regulates Caco-2 cell proliferation via these signaling pathways. In conclusion, the results obtained in this work expand our knowledge on the role of exogenous PTHrP in intestinal tumor cells and identify the signaling pathways that are involved in the mitogenic effect of the hormone on Caco-2 cells.

ALDH1B1 Is Crucial for Colon Tumorigenesis by Modulating Wnt/β-Catenin, Notch and PI3K/Akt Signaling Pathways

In the normal human colon, aldehyde dehydrogenase 1B1 (ALDH1B1) is expressed only at the crypt base, along with stem cells. It is also highly expressed in the human colonic adenocarcinomas. This pattern of expression corresponds closely to that observed for Wnt/β-catenin signaling activity. The present study examines the role of ALDH1B1 in colon tumorigenesis and signalling pathways mediating its effects. In a 3-dimensional spheroid growth model and a nude mouse xenograft tumor model, shRNA-induced suppression of ALDH1B1 expression decreased the number and size of spheroids formed in vitro and the size of xenograft tumors formed in vivo by SW 480 cells. Six binding elements for Wnt/β-catenin signalling transcription factor binding elements (T-cell factor/lymphoid enhancing factor) were identified in the human ALDH1B1 gene promoter (3 kb) but shown by dual luciferase reporter assay to not be necessary for ALDH1B1 mRNA expression in colon adenocarcinoma cell lines. We examined Wnt-reporter activity and protein/mRNA expression for Wnt, Notch and PI3K/Akt signaling pathways. Wnt/β-catenin, Notch and PI3K/Akt-signaling pathways were down-regulated in SW 480 cells in which ALDH1B1 expression had been suppressed. In summary, our data demonstrate that ALDH1B1 may promote colon cancer tumorigenesis by modulating the Wnt/β-catenin, Notch and PI3K/Akt signaling pathways. Selective targeting of ALDH1B1 may represent a novel means to prevent or treat colon cancer.

Upregulated Myc expression in N-methyl nitrosourea (MNU)- induced rat mammary tumours

BACKGROUND

The most common incident cancer and cause of cancer-related deaths in women is breast cancer. The Myc gene is upregulated in many cancer types including breast cancer, and it is considered as a potential anti-cancer drug target. The present study was conducted to evaluate the Myc (gene and protein) expression pattern in an experimental mammary tumour model in rats.

MATERIALS AND METHODS

Thirty six Sprague Dawley rats were divided into: Experimental group (26 animals), which received the chemical carcinogen N-methyl nitrosourea (MNU) and a control group (10 animals), which received vehicle only. c-Myc oncoprotein and its mRNA expression pattern were evaluated using immunohistochemistry (IHC) and semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR), respectively, in normal rat mammary tissue and mammary tumours. The rat glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene was used as internal control for semi-quantitative RT-PCR.

RESULTS

Histopathological examination of mammary tissues and tumours from MNU treated animals revealed the presence of premalignant lesions, benign tumours, in situ carcinomas and invasive carcinomas. Immunohistochemical evaluation of tumour tissues showed upregulation and heterogeneous cellular localization of c-Myc oncoprotein. The expression levels of c-Myc oncoprotein were significantly elevated (75- 91%) in all the tumours. Semi-quantitative RT-PCR revealed increased expression of c-Myc mRNA in mammary tumours compared to normal mammary tissues.

CONCLUSIONS

Further large-scale investigation study is needed to adopt this experimental rat mammary tumour model as an in vivo model to study anti-cancer strategies directed against Myc or its downstream partners at the transcriptional or post-transcriptional level.

Signal transduction pathways associated with ATP-induced proliferation of colon adenocarcinoma cells

BACKGROUND

In previous work, we have demonstrated that extracellular adenosine 5'-triphosphate (ATP) acts on intestinal Caco-2 cell P2Y receptors promoting a rapid increase in the phosphorylation of ERK1/2, p46 JNK and p38 MAP kinases (MAPKs).

METHODS AND RESULTS

In this study, we investigated whether the extracellular ATP-P2Y receptor signalling pathways were required for the proliferation of Caco-2 cells. Confocal microscopy and immunobloting studies showed that ERK1/2 and JNK translocate into the nucleus of the cells stimulated by ATP, where they participate, together with p38 MAPK, in the phosphorylation of JunD, ATF-1 and ATF-2 transcription factors. In addition, ATP through the activation of MAPKs induces the expression of the immediate early genes products of the Jun family, c-Fos and MAP kinase phosphatase-1 (MKP-1). Moreover, ERK1/2 and p38 MAPK are involved in the phosphorylation of MKP-1 in Caco-2 cells. Of physiological significance, in agreement with the mitogenic role of the MAPK cascade, ATP increased Caco-2 cell proliferation, and this effect was blocked by UO126, SB203580 and SP600125, the specific inhibitors of ERK1/2, p38 MAPK and JNK1/2, respectively.

CONCLUSION

Extracellular ATP induces proliferation of Caco-2 human colonic cancer cells by activating MAPK cascades and modulation of transcription factors.

GENERAL SIGNIFICANCE

These findings and identification of the specific P2Y subtype receptors involved in the mitogenic effect of ATP on Caco-2 cells might be relevant for understanding tumor cell development, resistance to treatment regimens and the design of new therapeutic strategies.

Triptolide is an inhibitor of RNA polymerase I and II-dependent transcription leading predominantly to down-regulation of short-lived mRNA

Triptolide, a natural product extracted from the Chinese plant Tripterygium wilfordii, possesses antitumor properties. Despite numerous reports showing the proapoptotic capacity and the inhibition of NF-kappaB-mediated transcription by triptolide, the identity of its cellular target is still unknown. To clarify its mechanism of action, we further investigated the effect of triptolide on RNA synthesis in the human non-small cell lung cancer cell line A549. Triptolide inhibited both total RNA and mRNA de novo synthesis, with the primary action being on the latter pool. We used 44K human pan-genomic DNA microarrays and identified the genes primarily affected by a short treatment with triptolide. Among the modulated genes, up to 98% are down-regulated, encompassing a large array of oncogenes including transcription factors and cell cycle regulators. We next observed that triptolide induced a rapid depletion of RPB1, the RNA polymerase II main subunit that is considered a hallmark of a transcription elongation blockage. However, we also show that triptolide does not directly interact with the RNA polymerase II complex nor does it damage DNA. We thus conclude that triptolide is an original pharmacologic inhibitor of RNA polymerase activity, affecting indirectly the transcription machinery, leading to a rapid depletion of short-lived mRNA, including transcription factors, cell cycle regulators such as CDC25A, and the oncogenes MYC and Src. Overall, the data shed light on the effect of triptolide on transcription, along with its novel potential applications in cancers, including acute myeloid leukemia, which is in part driven by the aforementioned oncogenic factors.

The knockdown of c-myc expression by RNAi inhibits cell proliferation in human colon cancer HT-29 cells in vitro and in vivo

We investigated the effects of RNA interference-mediated silencing of the c-myc gene on celluar proliferation and apoptosis in human colon cancer HT-29 cells in vitro and in vivo. A small interfering RNA (siRNA) targeting c-myc was designed, the DNA template was synthesized, and the siRNA was obtained by in vitro transcription. After siRNA transfection into HT-29 and human neuroblastoma IMR-32 cells with Lipofectamine 2000™, the proliferation of the HT-29 and IMR-32 cells was assessed via 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) colorimetry, and Hoechst 33258 staining was used to observe cell apoptosis. Following gene transfer to HT-29 cells, the expression of c-myc mRNA was examined via reverse transcription polymerase chain reaction, and the level of the protein via Western blot assay. Growth curves were constructed and in vivo experiments were performed on nude mice to assess the effects of c-myc silencing on tumor growth. The c-myc expression in the tumor tissue was measured by reverse transcription polymerase chain reaction and subsequently by immunohistochemistry. Our paper demonstrates that the delivery of siRNA directed against c-myc not only efficiently down-regulated the expression of c-myc, inhibited the proliferation of HT-29 cells and induced apoptosis in vitro, but also suppressed the growth of colon cancer cells in vivo.