Selective inhibition of proliferation in colorectal carcinoma cell lines expressing mutant APC or activated B-Raf

Protective effect of miR-18a in resected liver metastases of colorectal cancer and FOLFOX treatment

BACKGROUND

Colorectal cancer ranks second in terms of cancer associated deaths worldwide, whereas miRNA play a pivotal role in the etiology of cancer and its metastases.

AIMS

Studying the expression and cellular function of miR-18a in metastatic colorectal cancer and association to progression-free survival.

METHODS AND RESULTS

Colorectal liver metastases (N = 123) and primary colorectal cancer (N = 27) where analyzed by RT-PCR and correlated with clinical follow up data. Invasion and migration assays were performed with the liver metastatic cell line LIM2099 after miR-18a knockdown. Cell viability under FOLFOX treatment and knockdown was measured. We found that the expression of miR-18a was increased 4.38-fold in liver metastases and 3.86-fold in colorectal tumor tissue compared to healthy liver tissue and colorectal mucosa, respectively (p ≤ .001). Patients with a high miR-18a expression in liver metastases had a progression-free survival (PFS) of 13.6 months versus 8.9 months in patients with low expression (N = 123; p = .024). In vitro migration of LIM2099 cells was reduced after miR-18a knockdown and cell viability was significantly increased after miR-18a knockdown and treatment with folinic acid or oxaliplatin. Subgroup analysis of PFS revealed significant benefits for patients with high miR-18a expression receiving 5-FU, folinic acid or oxaliplatin.

CONCLUSIONS

High expression of miR-18a in colorectal liver metastases might have a protective effect after resection of metastases and FOLFOX treatment regarding PFS.

Molecular Pharmacology of Multitarget Cyclin-Dependent Kinase Inhibitors in Human Colorectal Carcinoma Cells

BACKGROUND

Colorectal cancer (CRC) is a leading cause of cancer death. Certain signaling pathways are implicated in colorectal carcinogenesis. Cyclin-dependent kinases (CDKs) are commonly hyperactivated in CRC and hence multitarget CDK inhibitors serve as promising therapeutic drugs against CRC.

OBJECTIVE

Off-target effects of multitarget CDK inhibitors with differential CDK inhibitory spectrum viz. P276-00 (also known as riviciclib), roscovitine and UCN-01 on CRC cell lines of varied genetic background were delineated.

METHOD

Protein expression was analyzed for key signaling proteins by western blotting. β-catenin localization was assessed using immunofluorescence. HIF-1 transcriptional activity and target gene expression were studied by reporter gene assay and RT-PCR respectively. Anti-migratory and anti-angiogenic potential was evaluated by wound healing assay and endothelial tube formation assay.

RESULTS

CDK inhibitors modulated various signaling pathways in CRC and for certain proteins showed a highly cell line-dependent response. Riviciclib and roscovitine inhibited HIF-1 transcriptional activity and HIF-1α accumulation in hypoxic HCT116 cells. Both of these drugs also abrogated migration of HCT116 and in vitro angiogenesis in HUVECs.

CONCLUSION

Anticancer activity of multitarget CDK inhibitors can be certainly attributed to their off-target effects and should be analyzed while assessing their therapeutic utility against CRC.

Dual drug targeting to kill colon cancers

Introduction

Colorectal cancer (CRC) is driven by a small set of oncogenic and tumour suppressor mutations. However, different combinations of mutations often lead to poor tumour responses to individual anticancer drugs. We have investigated the antiproliferative and in vitro cytotoxic activity of pair‐wise combinations of inhibitors which target specific signalling pathways in colon cancer cells.

Objectives

To target specific signaling pathways pairwise with inhibitors in order to kill colon cancer cells.

Methods

The effects of different concentrations of two inhibitors on the proliferation and viability of colon cancer cell lines were measured using cell titre glow and cytotoxic assays in 2D and 3D cell micro‐cultures. One successful drug combination was used to treat a colon cancer cell line growing as a xenograft in nude mice.

Results

Colon cancer cells in non‐adherent cultures were killed more effectively by combinations of pyrvinium pamoate (a Wnt pathway inhibitor) and ABT263 (a pro‐apoptotic Bcl‐2 family inhibitor) or Ly29004 (a PI3kinase inhibitor). However, in a mouse xenograft model, the formulation and toxicity of the ABT737/PP combination prevent the use of these drugs for treatment of tumours. Fortunately, oral analogues of PP (pyrvinium phosphate, PPh) and ABT737(ABT263) have equivalent activity and can be used for treatment of mice carrying SW620 colorectal cancer xenografts. The PPh/ABT263 induced SW620 tumour cell apoptosis and reduced the rate of SW620 tumour growth.

Conclusion

By combining a Wnt signaling inhibitor (pyrvinium phosphate) and a pro‐survival inhibitor (ABT263) colon cancer cells can be killed. Combinations of Wnt signalling inhibitors with an inhibitor of the Bcl pro‐survival protein family should be considered for the treatment of patients with precancerous colon adenomas or advanced colorectal cancers with APC mutations.

The function of BTG3 in colorectal cancer cells and its possible signaling pathway

PURPOSE

B-cell translocation gene 3 (BTG3) has been identified as a candidate driver gene for various cancers, but its specific role in colorectal cancer (CRC) is poorly understood. We aimed to investigate the relationship between expression of BTG3 and clinicopathological features and prognosis, as well as to explore the effects and the role of a possible BTG3 molecular mechanism on aggressive colorectal cancer behavior.

METHODS

BTG3 expression was assessed by immunohistochemistry (IHC) on specimens from 140 patients with CRC. The association of BTG3 expression with clinicopathological features was examined. To confirm the biological role of BTG3 in CRC, two CRC cell lines expressing BTG3 were used and BTG3 expression was knocked down by shRNA. CCK-8, cell cycle, apoptosis, migration, and invasion assays were performed. The influence of BTG3 knockdown was further investigated by genomic microarray to uncover the potential molecular mechanisms underlying BTG3-mediated CRC development and progression.

RESULTS

BTG3 was downregulated in colorectal cancer tissues and positively correlated with pathological classification (p = 0.037), depth of invasion (p = 0.016), distant metastasis (p = 0.024), TNM stage (p = 0.007), and overall survival (OS) and disease-free survival (DFS). BTG3 knockdown promoted cell proliferation, migration, invasion, relieved G2 arrest, and inhibited apoptosis in HCT116 and LoVo cells. A genomic microarray analysis showed that numerous tumor-associated signaling pathways and oncogenes were altered by BTG3 knockdown. At the mRNA level, nine genes referred to the extracellular-regulated kinase/mitogen-activated protein kinase pathway were differentially expressed. Western blotting revealed that BTG3 knockdown upregulated PAK2, RPS6KA5, YWHAB, and signal transducer and activator of transcription (STAT)3 protein levels, but downregulated RAP1A, DUSP6, and STAT1 protein expression, which was consistent with the genomic microarray data.

CONCLUSIONS

BTG3 expression might contribute to CRC carcinogenesis. BTG3 knockdown might strengthen the aggressive colorectal cancer behavior.

The Central Role of Wnt Signaling and Organoid Technology in Personalizing Anticancer Therapy

The Wnt pathway is at the heart of organoid technology, which is set to revolutionize the cancer field. We can now predetermine a patient's response to any given anticancer therapy by exposing tumor organoids established from the patient's own tumor. This cutting-edge biomedical platform translates to patients being treated with the correct drug at the correct dose from the outset, a truly personalized and precise medical approach. A high throughput drug screen on organoids also allows drugs to be tested in limitless combinations. More recently, the tumor cells that are resistant to the therapy given to a patient were selected in culture using the patient's organoids. The resistant tumor organoids were then screened empirically to identify drugs that will kill the resistant cells. This information allows diagnosis in real-time to either prevent tumor recurrence or effectively treat the recurring tumor. Furthermore, the ability to culture stem cell-derived epithelium as organoids has enabled us to begin to understand how a stem cell becomes a cancer cell or to pin-point the genetic alteration that underlies a given genetic syndrome. Here we summarize these advances and the central role of Wnt signaling, and identify the next challenges for organoid technology.

Inhibition of Wnt/β-Catenin pathway and Histone acetyltransferase activity by Rimonabant: a therapeutic target for colon cancer

In a high percentage (≥85%) of both sporadic and familial adenomatous polyposis forms of colorectal cancer (CRC), the inactivation of the APC tumor suppressor gene initiates tumor formation and modulates the Wnt/β-Catenin transduction pathways involved in the control of cell proliferation, adhesion and metastasis. Increasing evidence showed that the endocannabinoids control tumor growth and progression, both in vitro and in vivo. We evaluated the effect of Rimonabant, a Cannabinoid Receptor 1 (CB1) inverse agonist, on the Wnt/β-Catenin pathway in HCT116 and SW48 cell lines carrying the genetic profile of metastatic CRC poorly responsive to chemotherapies. In these models, Rimonabant inhibited the Wnt/β-Catenin canonical pathway and increased β-Catenin phosphorylation; in HCT116 cells, but not in SW48, the compound also triggered the Wnt/β-Catenin non canonical pathway activation through induction of Wnt5A and activation of CaMKII. The Rimonabant-induced downregulation of Wnt/β-Catenin target genes was partially ascribable to a direct inhibition of p300/KAT3B histone acetyltransferase, a coactivator of β-Catenin dependent gene regulation. Finally, in HCT116 xenografts, Rimonabant significantly reduced tumor growth and destabilized the nuclear localization of β-Catenin. Obtained data heavily supported the rationale for the use of cannabinoids in combined therapies for metastatic CRC harbouring activating mutations of β-Catenin.

ERBB3 Positively Correlates with Intestinal Stem Cell Markers but Marks a Distinct Non Proliferative Cell Population in Colorectal Cancer

Several studies have suggested ERBB3/HER3 may be a useful prognostic marker for colorectal cancer. Tumours with an intestinal stem cell signature have also been shown to be more aggressive. Here, we investigate whether ERBB3 is associated with intestinal stem cell markers in colorectal cancer and if cancer stem cells within tumours are marked by expression of ERBB3. Expression of ERBB3 and intestinal stem cell markers (LGR5, EPHB2, CD44s and CD44v6) was assessed by qRT-PCR in primary colorectal tumours (stages 0 to IV) and matched normal tissues from 53 patients. The localisation of ERBB3, EPHB2 and KI-67 within tumours was investigated using co-immunofluorescence. Expression of ERBB3 and intestinal stem cell markers were significantly elevated in adenomas and colorectal tumours compared to normal tissue. Positive correlations were found between ERBB3 and intestinal stem cell markers. However, co-immunofluorescence analysis showed that ERBB3 and EPHB2 marked specific cell populations that were mutually exclusive within tumours with distinct proliferative potentials, the majority of ERBB3+ve cells being non-proliferative. This pattern resembles cellular organisation within normal colonic epithelium where EPHB2 labelled proliferative cells reside at the crypt base and ERBB3+ve cells mark differentiated cells at the top of crypts. Our results show that ERBB3 and intestinal stem cell markers correlate in colorectal cancers. ERBB3 localises to differentiated cell populations within tumours that are non-proliferative and distinct from cancer stem cells. These data support the concept that tumours contain discrete stem, proliferative and differentiation compartments similar to that present in normal crypts.

Potential functional applications of extracellular vesicles: a report by the NIH Common Fund Extracellular RNA Communication Consortium

The NIH Extracellular RNA Communication Program's initiative on clinical utility of extracellular RNAs and therapeutic agents and developing scalable technologies is reviewed here. Background information and details of the projects are presented. The work has focused on modulation of target cell fate by extracellular vesicles (EVs) and RNA. Work on plant-derived vesicles is of intense interest, and non-mammalian sources of vesicles may represent a very promising source for different therapeutic approaches. Retro-viral-like particles are intriguing. Clearly, EVs share pathways with the assembly machinery of several other viruses, including human endogenous retrovirals (HERVs), and this convergence may explain the observation of viral-like particles containing viral proteins and nucleic acid in EVs. Dramatic effect on regeneration of damaged bone marrow, renal, pulmonary and cardiovascular tissue is demonstrated and discussed. These studies show restoration of injured cell function and the importance of heterogeneity of different vesicle populations. The potential for neural regeneration is explored, and the capacity to promote and reverse neoplasia by EV exposure is described. The tremendous clinical potential of EVs underlies many of these projects, and the importance of regulatory issues and the necessity of general manufacturing production (GMP) studies for eventual clinical trials are emphasized. Clinical trials are already being pursued and should expand dramatically in the near future.

HDAC Inhibition Overcomes Acute Resistance to MEK Inhibition in BRAF-Mutant Colorectal Cancer by Downregulation of c-FLIPL

PURPOSE

Activating mutations in the BRAF oncogene are found in 8% to 15% of colorectal cancer patients and have been associated with poor survival. In contrast with BRAF-mutant (MT) melanoma, inhibition of the MAPK pathway is ineffective in the majority of BRAFMT colorectal cancer patients. Therefore, identification of novel therapies for BRAFMT colorectal cancer is urgently needed.

EXPERIMENTAL DESIGN

BRAFMT and wild-type (WT) colorectal cancer models were assessed in vitro and in vivo. Small-molecule inhibitors of MEK1/2, MET, and HDAC were used, overexpression and siRNA approaches were applied, and cell death was assessed by flow cytometry, Western blotting, cell viability, and caspase activity assays.

RESULTS

Increased c-MET-STAT3 signaling was identified as a novel adaptive resistance mechanism to MEK inhibitors (MEKi) in BRAFMT colorectal cancer models in vitro and in vivo. Moreover, MEKi treatment resulted in acute increases in transcription of the endogenous caspase-8 inhibitor c-FLIPL in BRAFMT cells, but not in BRAFWT cells, and inhibition of STAT3 activity abrogated MEKi-induced c-FLIPL expression. In addition, treatment with c-FLIP-specific siRNA or HDAC inhibitors abrogated MEKi-induced upregulation of c-FLIPL expression and resulted in significant increases in MEKi-induced cell death in BRAFMT colorectal cancer cells. Notably, combined HDAC inhibitor/MEKi treatment resulted in dramatically attenuated tumor growth in BRAFMT xenografts.

CONCLUSIONS

Our findings indicate that c-MET/STAT3-dependent upregulation of c-FLIPL expression is an important escape mechanism following MEKi treatment in BRAFMT colorectal cancer. Thus, combinations of MEKi with inhibitors of c-MET or c-FLIP (e.g., HDAC inhibitors) could be potential novel treatment strategies for BRAFMT colorectal cancer.

CDC42 inhibition suppresses progression of incipient intestinal tumors

Mutations in the APC or β-catenin genes are well-established initiators of colorectal cancer, yet modifiers that facilitate the survival and progression of nascent tumor cells are not well defined. Using genetic and pharmacologic approaches in mouse colorectal cancer and human colorectal cancer xenograft models, we show that incipient intestinal tumor cells activate CDC42, an APC-interacting small GTPase, as a crucial step in malignant progression. In the mouse, Cdc42 ablation attenuated the tumorigenicity of mutant intestinal cells carrying single APC or β-catenin mutations. Similarly, human colorectal cancer with relatively higher levels of CDC42 activity was particularly sensitive to CDC42 blockade. Mechanistic studies suggested that Cdc42 may be activated at different levels, including at the level of transcriptional activation of the stem cell-enriched Rho family exchange factor Arhgef4. Our results indicate that early-stage mutant intestinal epithelial cells must recruit the pleiotropic functions of Cdc42 for malignant progression, suggesting its relevance as a biomarker and therapeutic target for selective colorectal cancer intervention.

RAD21 cohesin overexpression is a prognostic and predictive marker exacerbating poor prognosis in KRAS mutant colorectal carcinomas

Background:

RAD21 is a component of the cohesion complex and is integral to chromosome segregation and error-free DNA repair. RAD21 is functionally important in tumour progression but its role in colorectal carcinoma (CRC) is unclear. We therefore assessed its clinicopathological and prognostic significance in CRC, as well as its effect on chemosensitivity.

Methods:

A retrospective observation study examined RAD21 expression in 652 CRCs using a tissue microarray approach. Correlation with clinicopathological factors including gender, tumour grade, mucinous subtype, TNM stage, disease-specific survival (DSS), BRAF and KRAS mutation status, tumour p53 immunostaining, tumour microsatellite instability and tumour CpG island methylator phenotype was performed. Colorectal cancer cell clones with stable RAD21 knockdown were generated and tested for cellular sensitivity to conventional chemotherapeutic drugs.

Results:

RAD21 expression was significantly correlated with male gender (56.7% vs 43.3%, P=0.02), well-differentiated histology (14.4% vs 4.0%, P=0.0001), higher T-stage (36.1% vs 27.0%, P=0.01), presence of metastasis (18.8% vs 12.6%, P=0.03), and shorter DSS (hazard ratio (HR) 1.4, 95% CI 1.1 to 1.9, P=0.01) in both univariate and multivariate analysis. RAD21 expression was associated with shorter DSS in patients with KRAS mutant tumours (HR:2.6, 95% CI:1.4–4.3, P=0.001) and in patients receiving adjuvant chemoradiotherapy (HR:1.9, 95% CI:1.2–3.0, P=0.008). Colorectal cancer cells with RAD21 knockdown exhibited enhanced sensitivity to 5-fluorouracil, either alone or in combination with oxaliplatin.

Conclusions:

RAD21 expression in CRC is associated with aggressive disease especially in KRAS mutant tumours and resistance to chemoradiotherapy. RAD21 may be an important novel therapeutic target.

BRAF V600E is a determinant of sensitivity to proteasome inhibitors

A critical step toward defining tailored therapy in patients with cancer is the identification of genetic interactions that may impair-or boost-the efficacy of selected therapeutic approaches. Cell models able to recapitulate combinations of genetic aberrations are important to find drug-genotype interactions poorly affected by the heterogeneous genetics of human tumors. In order to identify novel pharmacogenomic relationships, we employed an isogenic cell panel that reconstructs cancer genetic scenarios. We screened a library of 43 compounds in human hTERT-HME1 epithelial cells in which PTEN or RB1 were silenced in combination with the targeted knockin of cancer-associated mutations in EGFR, KRAS, BRAF, or PIK3CA oncogenes. Statistical analysis and clustering algorithms were applied to display similar drug response profiles and mutation-specific patterns of activity. From the screen, we discovered that proteasome inhibitors show selectivity toward BRAF V600E-mutant cells, irrespective of PTEN or RB1 expression. Preferential targeting of BRAF-mutant cells by proteasome inhibitors was corroborated in a second BRAF V600E isogenic model, as well as in a panel of colorectal cancer cell lines by the use of the proteasome inhibitor carfilzomib. Notably, carfilzomib also showed striking in vivo activity in a BRAF-mutant human colorectal cancer xenograft model. Vulnerability to proteasome inhibitors is dependent on persistent BRAF signaling, because BRAF V600E blockade by PLX4720 reversed sensitivity to carfilzomib in BRAF-mutant colorectal cancer cells. Our findings indicated that proteasome inhibition might represent a valuable targeting strategy in BRAF V600E-mutant colorectal tumors.

Colon tumour secretopeptidome: insights into endogenous proteolytic cleavage events in the colon tumour microenvironment

The secretopeptidome comprises endogenous peptides derived from proteins secreted into the tumour microenvironment through classical and non-classical secretion. This study characterised the low-Mr (<3kDa) component of the human colon tumour (LIM1215, LIM1863) secretopeptidome, as a first step towards gaining insights into extracellular proteolytic cleavage events in the tumour microenvironment. Based on two biological replicates, this secretopeptidome isolation strategy utilised differential centrifugal ultrafiltration in combination with analytical RP-HPLC and nanoLC-MS/MS. Secreted peptides were identified using a combination of Mascot and post-processing analyses including MSPro re-scoring, extended feature sets and Percolator, resulting in 474 protein identifications from 1228 peptides (≤1% q-value, ≤5% PEP) - a 36% increase in peptide identifications when compared with conventional Mascot (homology ionscore thresholding). In both colon tumour models, 122 identified peptides were derived from 41 cell surface protein ectodomains, 23 peptides (12 proteins) from regulated intramembrane proteolysis (RIP), and 12 peptides (9 proteins) generated from intracellular domain proteolysis. Further analyses using the protease/substrate database MEROPS, (http://merops.sanger.ac.uk/), revealed 335 (71%) proteins classified as originating from classical/non-classical secretion, or the cell membrane. Of these, peptides were identified from 42 substrates in MEROPS with defined protease cleavage sites, while peptides generated from a further 205 substrates were fragmented by hitherto unknown proteases. A salient finding was the identification of peptides from 88 classical/non-classical secreted substrates in MEROPS, implicated in tumour progression and angiogenesis (FGFBP1, PLXDC2), cell-cell recognition and signalling (DDR1, GPA33), and tumour invasiveness and metastasis (MACC1, SMAGP); the nature of the proteases responsible for these proteolytic events is unknown. To confirm reproducibility of peptide fragment abundance in this study, we report the identification of a specific cleaved peptide fragment in the secretopeptidome from the colon-specific GPA33 antigen in 4/14 human CRC models. This improved secretopeptidome isolation and characterisation strategy has extended our understanding of endogenous peptides generated through proteolysis of classical/non-classical secreted proteins, extracellular proteolytic processing of cell surface membrane proteins, and peptides generated through RIP. The novel peptide cleavage site information in this study provides a useful first step in detailing proteolytic cleavage associated with tumourigenesis and the extracellular environment. This article is part of a Special Issue entitled: An Updated Secretome.

Human tissue-engineered colon forms from postnatal progenitor cells: an in vivo murine model

AIM

Loss of colon reservoir function after colectomy can adversely affect patient outcomes. In previous work, human fetal intestinal cells developed epithelium without mesenchyme following implantation in mice. However, for humans, postnatal tissue would be the preferred donor source. We generated tissue-engineered colon (TEC) from postnatal human organoid units.

MATERIALS & METHODS

Organoid units were prepared from human colon waste specimens, loaded onto biodegradable scaffolds and implanted into immunocompromised mice. After 4 weeks, human TEC was harvested. Immunofluorescence staining confirmed human origin, identified differentiated epithelial cell types and verified the presence of supporting mesenchyme.

RESULTS

Human TEC demonstrated a simple columnar epithelium. Immunofluorescence staining demonstrated human origin and the three differentiated cell types of mature colon epithelium. Key mesenchymal components (smooth muscle, intestinal subepithelial myofibroblasts and ganglion cells) were seen.

CONCLUSION

Colon can form from human progenitor cells on a scaffold in a mouse host. This proof-of-concept experiment is an important step in transitioning TEC to human therapy.

Colon cancer cells adopt an invasive phenotype without mesenchymal transition in 3-D but not 2-D culture upon combined stimulation with EGF and crypt growth factors

BACKGROUND

The intestinal crypt homeostasis is maintained by a combination of growth factors including Wnt, R-Spondin1, Noggin and the epidermal growth factor (EGF). In human colorectal cancer, the Wnt pathway is constitutively activated through genetic and epigenetic alterations in as many as 11 genes encoding components of this crypt stem-cell maintenance mechanism. Although the proliferation of colon cancer cells does not require Wnt, it is possible that colon cancer cells can still respond to the crypt growth factors in the colonic microenvironment. A number of studies have shown that epithelial cells behave differently in 3-D versus 2-D cultures. Because the 3-D conditions more closely mimic the in vivo environment, we examined the effects of Wnt and other crypt growth factors on colon cancer cell growth in 3-D culture.

METHODS

Colon cancer cells were grown in 3-D matrigel supplemented with different combinations of crypt growth factors and colonies were examined for morphology and pathways.

RESULTS

When colon cancer cells were cultured in 3-D with EGF, they grew as round spheroid colonies. However, colon cancer cells also grew as flat, disc-like colonies when cultured with EGF plus Wnt, R-Spondin1 and Noggin. Disc colonies were found to have comparable levels of E-cadherin as the spheroid colonies, but showed decreased E-cadherin at the cell-matrix contact sites. Disc colonies also elaborated F-actin rich protrusions (FRP) at the cell-matrix edge, reminiscent of an invasive phenotype but without the expression of vimentin. These E-cadherin and F-actin alterations were not induced by the four growth factors in 2-D culture. Formation of the disc colonies was inhibited by the knockdown of β-catenin and by protein kinase inhibitors such as gefitinib, imatinib and MK-2206. Furthermore, withdrawal of the crypt growth factors was able to revert the disc colonies to spheroid growth, showing that the invasive phenotype was reversible dependent on the availability of growth factors.

CONCLUSIONS

These findings show that colon cancer cells remain responsive to the growth factors in the crypt microenvironment and can be induced to undergo morphological transformation in the more physiologically relevant 3-D culture.

Concomitant BRAF and PI3K/mTOR blockade is required for effective treatment of BRAF(V600E) colorectal cancer

PURPOSE

BRAF(V600E) mutations are associated with poor clinical prognosis in colorectal cancer (CRC). Although selective BRAF inhibitors are effective for treatment of melanoma, comparable efforts in CRC have been disappointing. Here, we investigated potential mechanisms underlying this resistance to BRAF inhibitors in BRAF(V600E) CRC.

EXPERIMENTAL DESIGN

We examined phosphoinositide 3-kinase (PI3K)/mTOR signaling in BRAF(V600E) CRC cell lines after BRAF inhibition and cell viability and apoptosis after combined BRAF and PI3K/mTOR inhibition. We assessed the efficacy of in vivo combination treatment using a novel genetically engineered mouse model (GEMM) for BRAF(V600E) CRC.

RESULTS

Western blot analysis revealed sustained PI3K/mTOR signaling upon BRAF inhibition. Our BRAF(V600E) GEMM presented with sessile serrated adenomas/polyps, as seen in humans. Combination treatment in vivo resulted in induction of apoptosis and tumor regression.

CONCLUSIONS

We have established a novel GEMM to interrogate BRAF(V600E) CRC biology and identify more efficacious treatment strategies. Combination BRAF and PI3K/mTOR inhibitor treatment should be explored in clinical trials.

APRIL induces tumorigenesis and metastasis of colorectal cancer cells via activation of the PI3K/Akt pathway

A proliferation-inducing ligand (APRIL) is highly expressed in colorectal cancer (CRC) tissues and cell lines. However, the biological functions and precise signals elicited by APRIL in CRC have not been fully understood. Here, we used small interfering RNA to selectively deplete APRIL and to determine its tumorigenic effects in a CRC cell line SW480 both in vitro and in vivo. Knockdown of APRIL in SW480 cells was associated with modulation of cell proliferation as well as reduction of cell migration and invasion in vitro. Moreover APRIL-knockdown SW480 cells displayed markedly inhibited tumor growth and decreased metastasis to the liver in immunodeficient mice upon subcutaneous injection. Importantly, we observed that downregulation of APRIL in SW480 cells resulted in greatly decreased activity of phosphoinositide 3-kinase (PI3K)/Akt pathway. In addition, we observed that recombinant human APRIL mediated activation of the PI3K/Akt pathway in CRC cells resulting in induced expression of important cell cycle proteins and matrix metalloproteinases in a PI3K/Akt dependent manner. This was concurrent with marked cell growth viability as well as increased cell migration and invasion. Together, these compelling data suggest that APRIL-induced tumorigenesis and metastasis of CRC cells may be accomplished through activation of the PI3K/Akt pathway. These findings may lead to a better understanding of the biological effects of APRIL and may provide clues for identifying novel therapeutic and preventive molecular markers for CRC.

Nuclear expression of β-catenin promotes RB stability and resistance to TNF-induced apoptosis in colon cancer cells

Tumor necrosis factor (TNF)-α promotes tumor development under chronic inflammation. Because TNF also activates caspase-8, selective inhibition of TNF-induced extrinsic apoptosis would be required for inflammation-associated tumor growth. In a mouse model of inflammation-associated colon carcinogenesis, we found nuclear expression of β-catenin in tumors of wild-type, but not mutant, mice that were made resistant to TNF-induced apoptosis by a germline mutation blocking caspase cleavage of the retinoblastoma (RB) protein, despite similar frequencies of β-catenin exon-3 mutations in these two genetic backgrounds. TNF-induced apoptosis was also attenuated in human colon cancer cell lines with genetically activated β-catenin. However, we found that HCT116 cells, which contain an activated allele of β-catenin but do not express nuclear β-catenin, were sensitive to TNF-induced apoptosis. In HCT116 cells, TNF stimulated efficient RB cleavage that preceded chromatin condensation. In contrast, TNF did not induce RB cleavage in colon cancer cells expressing nuclear β-catenin and these cells could be sensitized to basal and/or TNF-induced apoptosis by the knockdown of β-catenin or RB. In the apoptosis-resistant colon cancer cells, knockdown of β-catenin led to a reduction in the RB protein without affecting RB mRNA. Furthermore, ectopic expression of the caspase-resistant, but not the wild-type, RB re-established resistance to TNF-induced caspase activation in colon cancer cells without β-catenin. Together, these results suggest that nuclear β-catenin-dependent RB stabilization suppresses TNF-induced apoptosis in caspase-8-positive colon cancer cells.

β-Catenin-dependent lysosomal targeting of internalized tumor necrosis factor-α suppresses caspase-8 activation in apoptosis-resistant colon cancer cells

Tumor necrosis factor-α (TNF)-induced apoptotic activation of caspase-8 requires internalization of its receptor. This study shows that constitutively activated β-catenin is required to facilitate the lysosomal delivery of internalized TNF, the inhibition of caspase-8 activation, and the suppression of apoptosis in colon cancer cells.

The Wnt/β-catenin pathway is constitutively activated in more than 90% of human colorectal cancer. Activated β-catenin stimulates cell proliferation and survival, however, its antiapoptotic mechanisms are not fully understood. We show here that activated β-catenin is required to suppress caspase-8 activation, but only in colon cancer cells that are resistant to tumor necrosis factor-α (TNF)-induced apoptosis. We found that lysosomal delivery of internalized TNF occurred at a faster pace in apoptosis-resistant than in apoptosis-sensitive colon cancer cells. Retardation of endosomal trafficking through vacuolar ATPase (V-ATPase) inhibition enhanced caspase-8 activation in apoptosis-resistant but not apoptosis-sensitive cells. Interestingly, knockdown of β-catenin also prolonged TNF association with the early endosome and enhanced caspase-8 activation in apoptosis-resistant but not apoptosis-sensitive colon cancer cells. In a mouse model of inflammation-associated colon tumors, we found nuclear expression of β-catenin, resistance to TNF-induced apoptosis, and reactivation of apoptosis in vivo after cotreatment of TNF with a V-ATPase inhibitor. Together these results suggest that activated β-catenin can facilitate endosomal trafficking of internalized TNF to suppress caspase-8 activation in colon cancer cells.

Identifying mutated proteins secreted by colon cancer cell lines using mass spectrometry

Secreted proteins encoded by mutated genes (mutant proteins) are a particularly rich source of biomarkers being not only components of the cancer secretome but also actually implicated in tumorigenesis. One of the challenges of proteomics-driven biomarker discovery research is that the bulk of secreted mutant proteins cannot be identified directly and quantified by mass spectrometry due to the lack of mutated peptide information in extant proteomics databases. Here we identify, using an integrated genomics and proteomics strategy (referred to iMASp - identification of Mutated And Secreted proteins), 112 putative mutated tryptic peptides (corresponding to 57 proteins) in the collective secretomes derived from a panel of 18 human colorectal cancer (CRC) cell lines. Central to this iMASp was the creation of Human Protein Mutant Database (HPMD), against which experimentally-derived secretome peptide spectra were searched. Eight of the identified mutated tryptic peptides were confirmed by RT-PCR and cDNA sequencing of RNA extracted from those CRC cells from which the mutation was identified by mass spectrometry. The iMASp technology promises to improve the link between proteomics and genomic mutation data thereby providing an effective tool for targeting tryptic peptides with mutated amino acids as potential cancer biomarker candidates. This article is part of a Special Issue entitled: Integrated omics.

Wnt proteins in mineralized tissue development and homeostasis

The past decade has seen rapid advancement in the dissection of the molecular events and players in the development and homeostasis of mineralized tissues, that is, teeth and bones. Much of this is due to research efforts toward the regeneration of these organs and also to develop treatments for pathologies of bone, especially osteoporosis. Of late, great interest has been focused on the Wnt family of proteins and their involvement in tooth and bone development and in the regulation of postnatal bone mass. The purpose of this review is to summarize these findings and to explore new areas of Wnt research such as Wnt?bone morphogenetic protein interactions and the exciting revelation of systemic serotonin being involved in bone mass regulation.

Wnt signaling and colon tumorigenesis--a view from the periphery

In this brief overview we discuss the association between Wnt signaling and colon cell biology and tumorigenesis. Our current understanding of the role of Apc in the β-catenin destruction complex is compared with potential roles for Apc in cell adhesion and migration. The requirement for phosphorylation in the proteasomal-mediated degradation of β-catenin is contrasted with roles for phospho-β-catenin in the activation of transcription, cell adhesion and migration. The synergy between Myb and β-catenin regulation of transcription in crypt stem cells during Wnt signaling is discussed. Finally, potential effects of growth factor regulatory systems, Apc or truncated-Apc on crypt morphogenesis, stem cell localization and crypt fission are considered.

LGR5 is a negative regulator of tumourigenicity, antagonizes Wnt signalling and regulates cell adhesion in colorectal cancer cell lines

Background

LGR5 (Leucine-rich repeat-containing G-protein coupled receptor 5) is the most established marker for intestinal stem cells. Mouse models show that LGR5+ cells are the cells of origin of intestinal cancer, and LGR5 expression is elevated in human colorectal cancers, however very little is known about LGR5 function or its contribution to the stem cell phenotype and to colorectal cancer.

Principal Findings

We have modulated the expression of LGR5 by RNAi (inhibitory RNAs) or overexpression in colorectal cancer cell lines. Paradoxically, ablation of LGR5 induces increased invasion and anchorage-independent growth, and enhances tumourigenicity in xenografts experiments. Conversely, overexpression of LGR5 augments cell adhesion, reduces clonogenicity and attenuates tumourigenicity. Expression profiling revealed enhanced wnt signalling and upregulation of EMT genes upon knockdown of LGR5, with opposite changes in LGR5 overexpressing cells. These findings suggest that LGR5 is important in restricting stem cells to their niche, and that loss of LGR5 concomitant with activated wnt signalling may contribute to the invasive phenotype of colorectal carcinomas.

Recent advances in understanding the role of diet and obesity in the development of colorectal cancer

Colorectal cancer (CRC) is a major cause of premature death in the UK and many developed countries. However, the risk of developing CRC is well recognised to be associated not only with diet but also with obesity and lack of exercise. While epidemiological evidence shows an association with factors such as high red meat intake and low intake of vegetables, fibre and fish, the mechanisms underlying these effects are only now being elucidated. CRC develops over many years and is typically characterised by an accumulation of mutations, which may arise as a consequence of inherited polymorphisms in key genes, but more commonly as a result of spontaneously arising mutations affecting genes controlling cell proliferation, differentiation, apoptosis and DNA repair. Epigenetic changes are observed throughout the progress from normal morphology through formation of adenoma, and the subsequent development of carcinoma. The reasons why this accumulation of loss of homoeostatic controls arises are unclear but chronic inflammation has been proposed to play a central role. Obesity is associated with increased plasma levels of chemokines and adipokines characteristic of chronic systemic inflammation, and dietary factors such as fish oils and phytochemicals have been shown to have anti-inflammatory properties as well as modulating established risk factors such as apoptosis and cell proliferation. There is also some evidence that diet can modify epigenetic changes. This paper briefly reviews the current state of knowledge in relation to CRC development and considers evidence for potential mechanisms by which diet may modify risk.

Oncogenic Kras promotes chemotherapy-induced growth factor shedding via ADAM17

Oncogenic mutations in Kras occur in 40% to 45% of patients with advanced colorectal cancer (CRC). We have previously shown that chemotherapy acutely activates ADAM17, resulting in growth factor shedding, growth factor receptor activation, and drug resistance in CRC tumors. In this study, we examined the role of mutant Kras in regulating growth factor shedding and ADAM17 activity, using isogenic Kras mutant (MT) and wild-type (WT) HCT116 CRC cells. Significantly higher levels of TGF-α and VEGF were shed from KrasMT HCT116 cells, both basally and following chemotherapy treatment, and this correlated with increased pErk (phosphorylated extracellular signal regulated kinase)1/2 levels and ADAM17 activity. Inhibition of Kras, MEK (MAP/ERK kinase)1/2, or Erk1/2 inhibition abrogated chemotherapy-induced ADAM17 activity and TGF-α shedding. Moreover, we found that these effects were not drug or cell line specific. In addition, MEK1/2 inhibition in KrasMT xenografts resulted in significant decreases in ADAM17 activity and growth factor shedding in vivo, which correlated with dramatically attenuated tumor growth. Furthermore, we found that MEK1/2 inhibition significantly induced apoptosis both alone and when combined with chemotherapy in KrasMT cells. Importantly, we found that sensitivity to MEK1/2 inhibition was ADAM17 dependent in vitro and in vivo. Collectively, our findings indicate that oncogenic Kras regulates ADAM17 activity and thereby growth factor ligand shedding in a MEK1/2/Erk1/2-dependent manner and that KrasMT CRC tumors are vulnerable to MEK1/2 inhibitors, at least in part, due to their dependency on ADAM17 activity.

Nonsense mediated decay resistant mutations are a source of expressed mutant proteins in colon cancer cell lines with microsatellite instability

Background

Frameshift mutations in microsatellite instability high (MSI-High) colorectal cancers are a potential source of targetable neo-antigens. Many nonsense transcripts are subject to rapid degradation due to nonsense-mediated decay (NMD), but nonsense transcripts with a cMS in the last exon or near the last exon-exon junction have intrinsic resistance to nonsense-mediated decay (NMD). NMD-resistant transcripts are therefore a likely source of expressed mutant proteins in MSI-High tumours.

Methods

Using antibodies to the conserved N-termini of predicted mutant proteins, we analysed MSI-High colorectal cancer cell lines for examples of naturally expressed mutant proteins arising from frameshift mutations in coding microsatellites (cMS) by immunoprecipitation and Western Blot experiments. Detected mutant protein bands from NMD-resistant transcripts were further validated by gene-specific short-interfering RNA (siRNA) knockdown. A genome-wide search was performed to identify cMS-containing genes likely to generate NMD-resistant transcripts that could encode for antigenic expressed mutant proteins in MSI-High colon cancers. These genes were screened for cMS mutations in the MSI-High colon cancer cell lines.

Results

Mutant protein bands of expected molecular weight were detected in mutated MSI-High cell lines for NMD-resistant transcripts (CREBBP, EP300, TTK), but not NMD-sensitive transcripts (BAX, CASP5, MSH3). Expression of the mutant CREBBP and EP300 proteins was confirmed by siRNA knockdown. Five cMS-bearing genes identified from the genome-wide search and without existing mutation data (SFRS12IP1, MED8, ASXL1, FBXL3 and RGS12) were found to be mutated in at least 5 of 11 (45%) of the MSI-High cell lines tested.

Conclusion

NMD-resistant transcripts can give rise to expressed mutant proteins in MSI-High colon cancer cells. If commonly expressed in primary MSI-High colon cancers, MSI-derived mutant proteins could be useful as cancer specific immunological targets in a vaccine targeting MSI-High colonic tumours.

Independent interactions of phosphorylated β-catenin with E-cadherin at cell-cell contacts and APC at cell protrusions

Background

The APC tumour suppressor functions in several cellular processes including the regulation of β-catenin in Wnt signalling and in cell adhesion and migration.

Findings

In this study, we establish that in epithelial cells N-terminally phosphorylated β-catenin specifically localises to several subcellular sites including cell-cell contacts and the ends of cell protrusions. N-terminally phosphorylated β-catenin associates with E-cadherin at adherens junctions and with APC in cell protrusions. We isolated APC-rich protrusions from stimulated cells and detected β-catenin, GSK3β and CK1α, but not axin. The APC/phospho-β-catenin complex in cell protrusions appears to be distinct from the APC/axin/β-catenin destruction complex. GSK3β phosphorylates the APC-associated population of β-catenin, but not the cell junction population. β-catenin associated with APC is rapidly phosphorylated and dephosphorylated. HGF and wound-induced cell migration promote the localised accumulation of APC and phosphorylated β-catenin at the leading edge of migrating cells. APC siRNA and analysis of colon cancer cell lines show that functional APC is required for localised phospho-β-catenin accumulation in cell protrusions.

Conclusions

We conclude that N-terminal phosphorylation of β-catenin does not necessarily lead to its degradation but instead marks distinct functions, such as cell migration and/or adhesion processes. Localised regulation of APC-phospho-β-catenin complexes may contribute to the tumour suppressor activity of APC.

Analysis of cellular phosphatidylinositol (3,4,5)-trisphosphate levels and distribution using confocal fluorescent microscopy

We have developed an immunocytochemistry method for the semiquantitative detection of phosphatidylinositol (3,4,5)-trisphosphate (PI(3,4,5)P3) at the cell plasma membrane. This protocol combines the use of a glutathione S-transferase-tagged pleckstrin homology (PH) domain of the general phosphoinositides-1 receptor (GST-GRP1PH) with fluorescence confocal microscopy and image segmentation using cell mask software analysis. This methodology allows the analysis of PI(3,4,5)P3 subcellular distribution in resting and epidermal growth factor (EGF)-stimulated HEK293T cells and in LIM1215 (wild-type phosphoinositide 3-kinase (PI3K)) and LIM2550 (H1047R mutation in PI3K catalytic domain) colonic carcinoma cells. Formation of PI(3,4,5)P3 was observed 5min following EGF stimulation and resulted in an increase of the membrane/cytoplasm fluorescence ratio from 1.03 to 1.53 for HEK293T cells and from 2.2 to 3.3 for LIM1215 cells. Resting LIM2550 cells stained with GST-GRP1PH had an elevated membrane/cytoplasm fluorescence ratio of 9.8, suggesting constitutive PI3K activation. The increase in the membrane/cytoplasm fluorescent ratio was inhibited in a concentration-dependent manner by the PI3K inhibitor LY294002. This cellular confocal imaging assay can be used to directly assess the effects of PI3K mutations in cancer cell lines and to determine the potential specificity and effectiveness of PI3K inhibitors in cancer cells.

Dkk-1 inhibits intestinal epithelial cell migration by attenuating directional polarization of leading edge cells

Wnt signaling pathways regulate proliferation, motility, and survival in a variety of human cell types. Dickkopf-1 (Dkk-1) is a secreted Wnt antagonist that has been proposed to regulate tissue homeostasis in the intestine. In this report, we show that Dkk-1 is secreted by intestinal epithelial cells after wounding and that it inhibits cell migration by attenuating the directional orientation of migrating epithelial cells. Dkk-1 exposure induced mislocalized activation of Cdc42 in migrating cells, which coincided with a displacement of the polarity protein Par6 from the leading edge. Consequently, the relocation of the microtubule organizing center and the Golgi apparatus in the direction of migration was significantly and persistently inhibited in the presence of Dkk-1. Small interfering RNA-induced down-regulation of Dkk-1 confirmed that extracellular exposure to Dkk-1 was required for this effect. Together, these data demonstrate a novel role of Dkk-1 in the regulation of directional polarization of migrating intestinal epithelial cells, which contributes to the effect of Dkk-1 on wound closure in vivo.