Mutant KRAS promotes hyperplasia and alters differentiation in the colon epithelium but does not expand the presumptive stem cell pool

K-ras Mutations as the Earliest Driving Force in a Subset of Colorectal Carcinomas

K-ras oncogene is a key factor in colorectal cancer. Based on published and our data we propose that K-ras could be the oncogene responsible for the inactivation of the tumor-suppressor gene APC, currently considered as the initial step in colorectal tumorigenesis. K-ras fulfills the criteria of the oncogene-induced DNA damage model, as it can provoke well-established causes for inactivating tumor-suppressors, i.e. DNA double-strand breaks (causing allele deletion) and ROS production (responsible for point mutation). The model we propose is a variation of the currently existing model and hypothesizes that, in a subgroup of colorectal carcinomas, K-ras mutation may precede APC inactivation, representing the earliest driving force and, probably, an early biomarker of colorectal carcinogenesis. This observation is clinically useful, since it may modify the preventive colorectal cancer strategy, restricting numerically patients undergoing colonoscopies to those bearing K-ras mutation in their colorectum, either in benign polyps or the normal accompanying mucosa.

Molecular-Subtype-Specific Biomarkers Improve Prediction of Prognosis in Colorectal Cancer

Colorectal cancer (CRC) is characterized by major inter-tumor diversity that complicates the prediction of disease and treatment outcomes. Recent efforts help resolve this by sub-classification of CRC into natural molecular subtypes; however, this strategy is not yet able to provide clinicians with improved tools for decision making. We here present an extended framework for CRC stratification that specifically aims to improve patient prognostication. Using transcriptional profiles from 1,100 CRCs, including >300 previously unpublished samples, we identify cancer cell and tumor archetypes and suggest the tumor microenvironment as a major prognostic determinant that can be influenced by the microbiome. Notably, our subtyping strategy allowed identification of archetype-specific prognostic biomarkers that provided information beyond and independent of UICC-TNM staging, MSI status, and consensus molecular subtyping. The results illustrate that our extended subtyping framework, combining subtyping and subtype-specific biomarkers, could contribute to improved patient prognostication and may form a strong basis for future studies.

Interaction between Wnt/β-catenin and RAS-ERK pathways and an anti-cancer strategy via degradations of β-catenin and RAS by targeting the Wnt/β-catenin pathway

Aberrant activation of the Wnt/β-catenin and RAS-extracellular signal-regulated kinase (ERK) pathways play important roles in the tumorigenesis of many different types of cancer, most notably colorectal cancer (CRC). Genes for these two pathways, such as adenomatous polyposis coli (APC) and KRAS are frequently mutated in human CRC, and involved in the initiation and progression of the tumorigenesis, respectively. Moreover, recent studies revealed interaction of APC and KRAS mutations in the various stages of colorectal tumorigenesis and even in metastasis accompanying activation of the cancer stem cells (CSCs). A key event in the synergistic cooperation between Wnt/β-catenin and RAS-ERK pathways is a stabilization of both β-catenin and RAS especially mutant KRAS by APC loss, and pathological significance of this was indicated by correlation of increased β-catenin and RAS levels in human CRC where APC mutations occur as high as 90% of CRC patients. Together with the notion of the protein activity reduction by lowering its level, inhibition of both β-catenin and RAS especially by degradation could be a new ideal strategy for development of anti-cancer drugs for CRC. In this review, we will discuss interaction between the Wnt/β-catenin and RAS-ERK pathways in the colorectal tumorigenesis by providing the mechanism of RAS stabilization by aberrant activation of Wnt/β-catenin. We will also discuss our small molecular anti-cancer approach controlling CRC by induction of specific degradations of both β-catenin and RAS via targeting Wnt/β-catenin pathway especially for the KYA1797K, a small molecule specifically binding at the regulator of G-protein signaling (RGS)-domain of Axin.

c-Cbl mediates the degradation of tumorigenic nuclear β-catenin contributing to the heterogeneity in Wnt activity in colorectal tumors

Despite the loss of Adenomatous Polyposis Coli (APC) in a majority of colorectal cancers (CRC), not all CRCs bear hallmarks of Wnt activation, such as nuclear β-catenin. This underscores the presence of other Wnt regulators that are important to define, given the pathogenic and prognostic roles of nuclear β-catenin in human CRC. Herein, we investigated the effect of Casitas B-lineage lymphoma (c-Cbl) on nuclear β-catenin, which is an oncoprotein upregulated in CRC due to loss-of-function APC or gain-of-function CTNNB1 mutations. Despite mechanistic rationale and recent discoveries of c-Cbl's mutations in solid tumors, little is known about its functional importance in CRC. Our study in a cohort of human CRC patients demonstrated an inverse correlation between nuclear β-catenin and c-Cbl. Further investigation showed that the loss of c-Cbl activity significantly enhanced nuclear β-catenin and CRC tumor growth in cell culture and a mouse xenograft model. c-Cbl interacted with and downregulated β-catenin in a manner that was independent of CTNNB1 or APC mutation status. This study demonstrates a previously unrecognized function of c-Cbl as a negative regulator of CRC.

Apc inactivation, but not obesity, synergizes with Pten deficiency to drive intestinal stem cell-derived tumorigenesis

Obesity is a major risk factor for colorectal cancer and can accelerate Lgr5+ intestinal stem cell (ISC)-derived tumorigenesis after the inactivation of Apc However, whether non-canonical pathways involving PI3K-Akt signaling in ISCs can lead to tumor formation, and if this can be further exacerbated by obesity is unknown. Despite the synergy between Pten and Apc inactivation in epithelial cells on intestinal tumor formation, their combined role in Lgr5+-ISCs, which are the most rapidly dividing ISC population in the intestine, is unknown. Lgr5+-GFP mice were provided low-fat diet (LFD) or high-fat diet (HFD) for 8 months, and the transcriptome was evaluated in Lgr5+-ISCs. For tumor studies, Lgr5+-GFP and Lgr5+-GFP-Pten^flox/flox mice were tamoxifen treated to inactivate Pten in ISCs and provided LFD or HFD until 14-15 months of age. Finally, various combinations of Lgr5+-ISC-specific, Apc- and Pten-deleted mice were generated and evaluated for histopathology and survival. HFD did not overtly alter Akt signaling in ISCs, but did increase other metabolic pathways. Pten deficiency, but not HFD, increased BrdU-positive cells in the small intestine (P < 0.05). However, combining Pten and Apc deficiency synergistically increased proliferative markers, tumor pathology and mortality, in a dose-dependent fashion (P < 0.05). In summary, we show that HFD alone fails to drive Akt signaling in ISCs and that Pten deficiency is dispensable as a tumor suppressor in Lgr5+-ISCs. However, combining Pten and Apc deficiency in ISCs synergistically increases proliferation, tumor formation and mortality. Thus, aberrant Wnt/β-catenin, rather than PI3K-Akt signaling, is requisite for obesity to drive Lgr5+ ISC-derived tumorigenesis.

Regulation of intestinal myofibroblasts by KRas-mutated colorectal cancer cells through heparin-binding epidermal growth factor-like growth factor

In colorectal cancer, gain-of-function mutations in KRas play a critical role in malignant transformation. Tumor growth in colorectal cancer is known to be promoted by the intestinal myofibroblasts (IMFs) that localize adjacent to the cancer cells, but the mechanisms of interaction between KRas-mutated cancer cells and the myofibroblasts remain unclear. Here, we investigated the effects of KRas-mutated cells on the behavior of myofibroblasts by using mouse primary IMFs and cells of an IMF cell line (LmcMF) and a mouse colon epithelial cell line (aMoC1). Conditioned medium (CM) was collected from aMoC1 cells overexpressing a control vector or KRasV12 vector (KRasV12-CM), and the effects of KRasV12-CM on IMFs were analyzed by performing proliferation assays, wound-healing assays, Boyden chamber assays, and western blotting. Whereas KRasV12-CM exerted little effect on the differentiation and proliferation of primary IMFs, the CM promoted migration of both primary IMFs and LmcMF cells. In KRasV12-overexpressing aMoC1 cells, mRNA expression of heparin-binding epidermal growth factor-like growth factor (HB-EGF) was higher than in mock-transfected aMoC1 cells, and HB-EGF promoted the migration of primary IMFs and LmcMF cells. Moreover, KRasV12-CM-induced IMF migration was suppressed by dacomitinib, an inhibitor of HB-EGF receptors. Notably, in LmcMF cells, both KRasV12-CM and HB-EGF activated extracellular signal-regulated kinase (ERK) and c-jun N-terminal kinase (JNK), whereas KRasV12-CM-induced migration of IMFs was suppressed following treatment with either an ERK inhibitor (FR180204) or a JNK inhibitor (SP600125). These results suggest that HB-EGF secreted from KRas-mutated colorectal cancer cells promotes IMF migration through ERK and JNK activation, which, in turn, could support cancer progression.

KRAS mutation associated with CD44/CD166 immunoexpression as predictors of worse outcome in metastatic colon cancer

INTRODUCTION

Multiple stages of carcinogenesis in colon cancer encompass subpopulations of cancer stem cells (CSC), responsible for tumor cell transformation, growth and proliferation. CD44 and CD166 proteins are CSC markers associated with cell signaling, adhesion, migration, metastasis and lymphocytic response. The expression of CSC may be modulated by some factors, such as the KRAS gene mutation.

OBJECTIVE

Correlate the expression of CD44 and CD166 markers in metastatic colon adenocarcinoma and KRAS mutation status (wild-type/mutated) with clinical pathological features and patients' outcome.

MATERIAL AND METHODS

Fifty-eight samples of tumor tissue samples of metastatic colon adenocarcinoma were collected from patients treated with CapeOx at the HCFMRP-USP Clinical Oncology Service. Clinical and survival data were collected from medical records. KRAS status was determined by the polymerase chain reaction (PCR) technique, and analysis of immunohistochemical expression of CD44 and CD166 proteins was performed by tissue microarray.

RESULTS

The expression of CD44 and CD166 were positive in 41% and 43% of patients, respectively, and mutated KRAS was detected in 48% of patients. A significant association was found between CD166 and CD44 expression (p= 0.016), mainly in the wild-type KRAS group (p= 0.042) and patients over 65 years (p= 0.001). CD44-positive patients had 3.7-fold and 5.3-fold greater risk of liver metastasis and lung metastasis, respectively (p< 0.01), compared with CD44-negative patients. CD166-negative patients had 2.7 greater risk of lymph node involvement (0.03), compared with CD166-positive patients. KRAS mutation increased the risk of liver metastasis by 8 times (p< 0.01), and the risk of lung metastasis by 5 times (p= 0.04) in CD44-positive patients. KRAS mutation increased the risk of lymph node involvement by 8 times in CD166-negative patients (p= 0.0007).

CONCLUSION

An association between CD44 and CD166 expression was demonstrated in this study. Analysis of KRAS mutation combined with immunohistochemical expression of CD44 and CD166 identified subgroups of patients with colon adenocarcinoma at higher risk of lymph node involvement by the tumor and development of liver and lung metastasis.

BRAFV600E cooperates with CDX2 inactivation to promote serrated colorectal tumorigenesis

While 20–30% of colorectal cancers (CRCs) may arise from precursors with serrated glands, only 8–10% of CRCs manifest serrated morphology at diagnosis. Markers for distinguishing CRCs arising from ‘serrated’ versus ‘conventional adenoma’ precursors are lacking. We studied 36 human serrated CRCs and found CDX2 loss or BRAF mutations in ~60% of cases and often together (p=0.04). CDX2^Null/BRAF^V600E expression in adult mouse intestinal epithelium led to serrated morphology tumors (including carcinomas) and BRAF^V600E potently interacted with CDX2 silencing to alter gene expression. Like human serrated lesions, CDX2^Null/BRAF^V600E-mutant epithelium expressed gastric markers. Organoids from CDX2^Null/BRAF^V600E–mutant colon epithelium showed serrated features, and partially recapitulated the gene expression pattern in mouse colon tissues. We present a novel mouse tumor model based on signature defects seen in many human serrated CRCs – CDX2 loss and BRAF^V600E. The mouse intestinal tumors show significant phenotypic similarities to human serrated CRCs and inform about serrated CRC pathogenesis.

DOI:http://dx.doi.org/10.7554/eLife.20331.001

Impaired coordination between signaling pathways is revealed in human colorectal cancer using single-cell mass cytometry of archival tissue blocks

Cellular heterogeneity poses a substantial challenge to understanding tissue-level phenotypes and confounds conventional bulk analyses. To analyze signaling at the single-cell level in human tissues, we applied mass cytometry using cytometry time of flight to formalin-fixed, paraffin-embedded (FFPE) normal and diseased intestinal specimens. This technique, called FFPE-DISSECT (disaggregation for intracellular signaling in single epithelial cells from tissue), is a single-cell approach to characterizing signaling states in embedded tissue samples. We applied FFPE-DISSECT coupled to mass cytometry and found differential signaling by tumor necrosis factor-α in intestinal enterocytes, goblet cells, and enteroendocrine cells, implicating the downstream RAS-RAF-MEK pathway in determining goblet cell identity. Application of this technique and computational analyses to human colon specimens confirmed the reduced differentiation in colorectal cancer (CRC) compared to normal colon and revealed increased intratissue and intertissue heterogeneity in CRC with quantitative changes in the regulation of signaling pathways. Specifically, coregulation of the kinases p38 and ERK, the translation regulator 4EBP1, and the transcription factor CREB in proliferating normal colon cells was lost in CRC. Our data suggest that this single-cell approach, applied in conjunction with genomic annotation, enables the rapid and detailed characterization of cellular heterogeneity from clinical repositories of embedded human tissues. This technique can be used to derive cellular landscapes from archived patient samples (beyond CRC) and as a high-resolution tool for disease characterization and subtyping.

Traditional serrated adenomas and serrated carcinomas in carcinogen-treated rats

AIMS

A recent review of archived sections from early experiments in rats showed neoplasias exhibiting serrated configurations. The aim was to assess the frequency of serrated neoplasias in the colon and small intestine of carcinogen-treated rats.

METHODS

While reviewing archival sections from early experiments in Sprague-Dawley (SD) and Fisher-344 (F-344) rats, we recently detected colonic and intestinal traditional serrated adenomas (displaying serrated or microtubular patterns) and serrated carcinomas. SD rats were injected 1,2-dimethylhydrazine (DMH) for 27 weeks whereas F-344 rats were fed with a pyrolysate (GLU-1) for 24 months. Filed sections from 358 colonic and small intestinal neoplasias were re-evaluated.

RESULTS

DMH-treated SD rats had 215 colonic neoplasias (1.4% were serrated adenomas, 7.9% microtubular adenomas, 2.8% serrated carcinomas and 2.8% microtubular carcinomas). GLU1-treated F-344 rats had 53 colonic neoplasias (1.9% were serrated adenomas and 20.8% microtubular adenomas), and 89 small intestinal neoplasias (1.1% were serrated adenomas, 42.7% microtubular adenomas and 6.7%, microtubular carcinomas).

CONCLUSIONS

DMH/SD-rats develop serrated and microtubular adenomas and carcinomas in the colon, whereas GLU1/F-344 rats develop microtubular adenomas in the colon and microtubular adenomas and carcinomas in the small intestine. The two rat-settings emerge as suitable models to study the molecular attributes of serrated and microtubular neoplasias under the standard conditions of the laboratory. This study is the first showing that a substantial number of serrated and particularly microtubular adenomas and carcinomas develop in the colon and small intestine of experimental rats. Importantly, serrated and microtubular neoplasias in rats recreate the histology of duodenal and colonic traditional serrated neoplasias in human beings.

Gastrointestinal organoids: How they gut it out

The gastrointestinal tract is characterized by a self-renewing epithelium fueled by adult stem cells residing at the bottom of the intestinal crypt and gastric glands. Their activity and proliferation is strongly dependent on complex signaling pathways involving other crypt/gland cells as well as surrounding stromal cells. In recent years organoids are becoming increasingly popular as a new and powerful tool to study developmental or other biological processes. Organoids retain morphological and molecular patterns of the tissue they are derived from, are self-organizing, relatively simple to handle and accessible to genetic engineering. This review focuses on the developmental processes and signaling molecules involved in epithelial homeostasis and how a profound knowledge of these mechanisms allowed the establishment of a three dimensional organoid culture derived from adult gastrointestinal stem cells.

Serrated polyposis associated with a family history of colorectal cancer and/or polyps: The preferential location of polyps in the colon and rectum defines two molecular entities

Serrated polyposis (SPP) is characterized by the development of multiple serrated polyps and an increased predisposition to colorectal cancer (CRC). In the present study, we aimed to characterize, at a clinical and molecular level, a cohort of SPP patients with or without a family history of SPP and/or polyps/CRC (SPP-FHP/CRC). Sixty-two lesions from 12 patients with SPP-FHP/CRC and 6 patients with sporadic SPP were included. The patients with SPP-FHP/CRC presented with an older mean age at diagnosis (p=0.027) and a more heterogeneous histological pattern of lesions (p=0.032) than the patients with sporadic SPP. We identified two molecular forms of SPP-FHP/CRC, according to the preferential location of the lesions: proximal/whole-colon or distal colon. Mismatch repair (MMR) gene methylation [mutS homolog 6 (MSH6)/mutS homolog 3 (MSH3)] or loss of heterozygosity (LOH) of D2S123 (flanking MSH6) were detected exclusively in the former (p=3.0×10⁻⁷), in most early lesions. Proximal/whole-colon SPP-FHP/CRC presented a higher frequency of O-6-methylguanine-DNA methyltransferase (MGMT) methylation/LOH, microsatel-lite instability (MSI) and Wnt mutations (19/29 vs. 7/17; 16/23 vs. 1/14, p=2.2×10⁻⁴; 15/26 vs. 2/15, p=0.006; 14/26 vs. 4/20, p=0.02) but a lower frequency of B-raf proto-oncogene, serine/threonine kinase (BRAF) mutations (7/30 vs. 12/20, p=0.0089) than the distal form. CRC was more frequent in cases of Kirsten rat sarcoma viral oncogene homolog (KRAS)-associated proximal/whole-colon SPP-FHP/CRC than in the remaining cases (4/4 vs. 1/8, p=0.01). Thus, SPP-FHP/CRC appears to be a specific entity, presenting two forms, proximal/whole-colon and distal, which differ in the underlying tumor initiation pathways. Early MGMT and MMR gene deficiency in the former may underlie an inherited susceptibility to genotoxic stress.

Molecular pathological classification of colorectal cancer

Colorectal cancer (CRC) shows variable underlying molecular changes with two major mechanisms of genetic instability: chromosomal instability and microsatellite instability. This review aims to delineate the different pathways of colorectal carcinogenesis and provide an overview of the most recent advances in molecular pathological classification systems for colorectal cancer. Two molecular pathological classification systems for CRC have recently been proposed. Integrated molecular analysis by The Cancer Genome Atlas project is based on a wide-ranging genomic and transcriptomic characterisation study of CRC using array-based and sequencing technologies. This approach classified CRC into two major groups consistent with previous classification systems: (1) ∼16 % hypermutated cancers with either microsatellite instability (MSI) due to defective mismatch repair (∼13 %) or ultramutated cancers with DNA polymerase epsilon proofreading mutations (∼3 %); and (2) ∼84 % non-hypermutated, microsatellite stable (MSS) cancers with a high frequency of DNA somatic copy number alterations, which showed common mutations in APC, TP53, KRAS, SMAD4, and PIK3CA. The recent Consensus Molecular Subtypes (CMS) Consortium analysing CRC expression profiling data from multiple studies described four CMS groups: almost all hypermutated MSI cancers fell into the first category CMS1 (MSI-immune, 14 %) with the remaining MSS cancers subcategorised into three groups of CMS2 (canonical, 37 %), CMS3 (metabolic, 13 %) and CMS4 (mesenchymal, 23 %), with a residual unclassified group (mixed features, 13 %). Although further research is required to validate these two systems, they may be useful for clinical trial designs and future post-surgical adjuvant treatment decisions, particularly for tumours with aggressive features or predicted responsiveness to immune checkpoint blockade.

Similar but different: distinct roles for KRAS and BRAF oncogenes in colorectal cancer development and therapy resistance

Colorectal cancer (CRC) is characterized by recurrent mutations deregulating key cell signaling cascades and providing the cancer cells with novel functional traits. Among the most frequent mutations in CRC are gain-of-function missense mutations in KRAS and BRAF. Oncogenic activation of KRAS and BRAF is mutually exclusive and occurs in approximately 40% and 10% of all CRCs, respectively. Here we summarize genetic alterations currently described in the literature and databases, indicating overlapping but also specific co-occurrences with either mutated BRAF or KRAS. We describe common and potentially specific biological functions of KRAS and BRAF oncoproteins in the intestinal epithelial cells and during initiation and progression of CRC. We discuss signal transduction networks, highlighting individual functions of oncogenic KRAS and BRAF in terms of feedback loops and their impact on treatment outcome. Finally, we give an update on current strategies of targeted therapeutic intervention in oncogenic RAS-RAF signaling networks for the treatment of metastatic CRC and outline future directions.

Promotion of Intestinal Epithelial Cell Turnover by Commensal Bacteria: Role of Short-Chain Fatty Acids

The life span of intestinal epithelial cells (IECs) is short (3–5 days), and its regulation is thought to be important for homeostasis of the intestinal epithelium. We have now investigated the role of commensal bacteria in regulation of IEC turnover in the small intestine. The proliferative activity of IECs in intestinal crypts as well as the migration of these cells along the crypt-villus axis were markedly attenuated both in germ-free mice and in specific pathogen–free (SPF) mice treated with a mixture of antibiotics, with antibiotics selective for Gram-positive bacteria being most effective in this regard. Oral administration of chloroform-treated feces of SPF mice to germ-free mice resulted in a marked increase in IEC turnover, suggesting that spore-forming Gram-positive bacteria contribute to this effect. Oral administration of short-chain fatty acids (SCFAs) as bacterial fermentation products also restored the turnover of IECs in antibiotic-treated SPF mice as well as promoted the development of intestinal organoids in vitro. Antibiotic treatment reduced the phosphorylation levels of ERK, ribosomal protein S6, and STAT3 in IECs of SPF mice. Our results thus suggest that Gram-positive commensal bacteria are a major determinant of IEC turnover, and that their stimulatory effect is mediated by SCFAs.

Inflammatory response in serrated precursor lesions of the colon classified according to WHO entities, clinical parameters and phenotype-genotype correlation

Studies on traditional serrated adenoma (TSA) and sessile serrated adenoma with dysplasia (SSA‐D) are rare due to the low frequency of these lesions, which are well defined by the latest WHO classification. However, introducing new morphological criteria such as intra‐epithelial lymphocytes (IELs) might facilitate colorectal polyp diagnoses. Additionally, the phenotype–genotype correlation needs to be updated as the terminology has repeatedly changed. This study analysed 516 polyps, consisting of 118 classical adenomas (CAD), 116 hyperplastic polyps (HPP), 179 SSAs, 41 SSA‐Ds, and 62 TSAs. The lesions were analysed in relation to the patients’ clinical parameters including gender, age, localisation, and size. The inflammatory background of the polyps was quantified and BRAF and KRAS mutations as well as MLH1 and CDKN2A promoter methylation were assessed. In multivariate analyses, an increase in IELs was an independent and robust new criterion for the diagnosis of SSA‐D (p < 0.001). Superficial erosions and acute neutrophil granulocytes led to reactive changes potentially resembling dysplasia. KRAS and BRAF mutations were associated with CAD/TSA and HPP/SSA, respectively. However, almost half of TSAs had a BRAF mutation and were KRAS wild type. CDKN2A seems to precede MLH1 hyper‐methylation within the serrated carcinogenesis model. The genotyping of WHO‐based entities – and especially SSA – has sharpened in comparison to previously published data. TSAs can be sub‐grouped according to their mutation status. Of note, the higher number of IELs in SSA‐D reflects their close relationship to colorectal cancers with micro‐satellite instability. Therefore, IELs might represent a new diagnostic tool for SSA‐D.

Correlation between Gene Variants, Signaling Pathways, and Efficacy of Chemotherapy Drugs against Colon Cancers

Efficacies, toxicities, and resistance mechanisms of chemotherapy drugs, such as oxaliplatin and 5-fluorouracil (5-FU), vary widely among various categories and subcategories of colon cancers. By understanding the differences in the drug efficacy and resistance at the level of protein–protein networks, we identified the correlation between the drug activity of oxaliplatin/5-FU and gene variations from the US National Cancer Institute-60 human cancer cell lines. The activity of either of these drugs is correlated with specific amino acid variant(s) of KRAS and other genes from the signaling pathways of colon cancer progression. We also discovered that the activity of a non-DNA-binding novel platinum drug, phosphaplatin, is comparable with oxaliplatin and 5-FU when it was tested against colon cancer cell lines. Our strategy that combines the knowledge from pharmacogenomics across cell lines with the molecular information from specific cancer cells is beneficial for predicting the outcome of a possible combination therapy for personalized treatment.

Molecular and histologic considerations in the assessment of serrated polyps

CONTEXT : Colorectal cancer is a heterogeneous disease resulting from different molecular pathways of carcinogenesis. Recent data evaluating the histologic features and molecular basis of the serrated polyp-carcinoma pathway have significantly contributed to more comprehensive classifications of and treatment recommendations for these tumors.

OBJECTIVE

To integrate the most recent molecular findings in the context of histologic classifications of serrated lesions and their implications in diagnostic pathology and colorectal cancer surveillance.

DATA SOURCES

Published literature focused on serrated polyps and their association with colorectal cancer.

CONCLUSIONS

Three types of serrated polyps are currently recognized: hyperplastic polyps, sessile serrated adenomas/polyps, and traditional serrated adenomas. The BRAF V600E mutation is one of the most frequent molecular abnormalities identified in hyperplastic polyps and sessile serrated adenomas. In contrast, in traditional serrated adenomas, either BRAF V600E or KRAS mutations can be frequently identified. CpG methylation has emerged as a critical molecular mechanism in the sessile serrated pathway. CpG methylation of MLH1 often leads to reduced or lost expression in dysplastic foci and carcinomas arising in sessile serrated adenomas/polyps.

Reserve stem cells: Differentiated cells reprogram to fuel repair, metaplasia, and neoplasia in the adult gastrointestinal tract

It has long been known that differentiated cells can switch fates, especially in vitro, but only recently has there been a critical mass of publications describing the mechanisms adult, postmitotic cells use in vivo to reverse their differentiation state. We propose that this sort of cellular reprogramming is a fundamental cellular process akin to apoptosis or mitosis. Because reprogramming can invoke regenerative cells from mature cells, it is critical to the long-term maintenance of tissues like the pancreas, which encounter large insults during adulthood but lack constitutively active adult stem cells to repair the damage. However, even in tissues with adult stem cells, like the stomach and intestine, reprogramming may allow mature cells to serve as reserve ("quiescent") stem cells when normal stem cells are compromised. We propose that the potential downside to reprogramming is that it increases risk for cancers that occur late in adulthood. Mature, long-lived cells may have years of exposure to mutagens. Mutations that affect the physiological function of differentiated, postmitotic cells may lead to apoptosis, but mutations in genes that govern proliferation might not be selected against. Hence, reprogramming with reentry into the cell cycle might unmask those mutations, causing an irreversible progenitor-like, proliferative state. We review recent evidence showing that reprogramming fuels irreversible metaplastic and precancerous proliferation in the stomach and pancreas. Finally, we illustrate how we think reprogrammed differentiated cells are likely candidates as cells of origin for cancers of the intestine.

Oncogenic K-Ras promotes proliferation in quiescent intestinal stem cells

K-Ras is a monomeric GTPase that controls cellular and tissue homeostasis. Prior studies demonstrated that mutationally activated K-Ras (K-Ras(G12D)) signals through MEK to promote expansion and hyperproliferation of the highly mitotically active transit-amplifying cells (TACs) in the intestinal crypt. Its effect on normally quiescent stem cells was unknown, however. Here, we have used an H2B-Egfp transgenic system to demonstrate that K-Ras(G12D) accelerates the proliferative kinetics of quiescent intestinal stem cells. As in the TAC compartment, the effect of mutant K-Ras on the quiescent stem cell is dependent upon activation of MEK. Mutant K-Ras is also able to increase self-renewal potential of intestinal stem cells following damage. These results demonstrate that mutant K-Ras can influence intestinal homeostasis on multiple levels.

RasGRP1 opposes proliferative EGFR-SOS1-Ras signals and restricts intestinal epithelial cell growth

The character of EGFR signals can influence cell fate but mechanistic insights into intestinal EGFR-Ras signalling are limited. Here we show that two distinct Ras nucleotide exchange factors, RasGRP1 and SOS1, lie downstream of EGFR but act in functional opposition. RasGRP1 is expressed in intestinal crypts where it restricts epithelial growth. High RasGRP1 expression in colorectal cancer (CRC) patient samples correlates with a better clinical outcome. Biochemically, we find that RasGRP1 creates a negative feedback loop that limits proliferative EGFR-SOS1-Ras signals in CRC cells. Genetic Rasgrp1 depletion from mice with either an activating mutation in KRas or with aberrant Wnt signalling due to a mutation in Apc resulted in both cases in exacerbated Ras-ERK signalling and cell proliferation. The unexpected opposing cell biological effects of EGFR-RasGRP1 and EGFR-SOS1 signals in the same cell shed light on the intricacy of EGFR-Ras signalling in normal epithelium and carcinoma.

Colorectal carcinogenesis-update and perspectives

Colorectal cancer (CRC) is a very common malignancy in the Western World and despite advances in surgery, chemotherapy and screening, it is still the second leading cause of cancer deaths in this part of the world. Numerous factors are found important in the development of CRC including colonocyte metbolism, high risk luminal environment, inflammation, as well as lifestyle factors such as diet, tobacco, and alchohol consumption. In recent years focus has turned towards the genetics and molecular biology of CRC and several interesting and promising correlations and pathways have been discovered. The major genetic pathways of CRC are the Chromosome Instability Pathway representing the pathway of sporadic CRC through the K-ras, APC, and P53 mutations, and the Microsatellite Instability Pathway representing the pathway of hereditary non-polyposis colon cancer through mutations in mismatch repair genes. To identify early cancers, screening programs have been initiated, and the leading strategy has been the use of faecal occult blood testing followed by colonoscopy in positive cases. Regarding the treatment of colorectal cancer, significant advances have been made in the recent decade. The molecular targets of CRC include at least two important cell surface receptors: the epidermal growth factor receptor and the vascular endothelial growth factor receptor. The genetic and molecular knowledge of CRC has widen the scientific and clinical perspectives of diagnosing and treatment. However, despite significant advances in the understanding and treatment of CRC, results from targeted therapy are still not convincing. Future studies will determine the role for this new treatment modality.

Interleukin-17 receptor a signaling in transformed enterocytes promotes early colorectal tumorigenesis

Interleukin-17A (IL-17A) is a pro-inflammatory cytokine linked to rapid malignant progression of colorectal cancer (CRC) and therapy resistance. IL-17A exerts its pro-tumorigenic activity through its type A receptor (IL-17RA). However, IL-17RA is expressed in many cell types, including hematopoietic, fibroblastoid, and epithelial cells, in the tumor microenvironment, and how IL-17RA engagement promotes colonic tumorigenesis is unknown. Here we show that IL-17RA signals directly within transformed colonic epithelial cells (enterocytes) to promote early tumor development. IL-17RA engagement activates ERK, p38 MAPK, and NF-κB signaling and promotes the proliferation of tumorigenic enterocytes that just lost expression of the APC tumor suppressor. Although IL-17RA signaling also controls the production of IL-6, this mechanism makes only a partial contribution to colonic tumorigenesis. Combined treatment with chemotherapy, which induces IL-17A expression, and an IL-17A neutralizing antibody enhanced the therapeutic responsiveness of established colon tumors. These findings establish IL-17A and IL-17RA as therapeutic targets in colorectal cancer.

Adenoma and carcinoma components in colonic tumors show discordance for KRAS mutation

Activating mutations in KRAS are common events in the pathogenesis of colorectal carcinoma and predict response to treatment with anti-EGFR antibodies. Molecular pathology testing for KRAS mutations has become the standard of practice for patients with metastatic colorectal carcinoma. Despite the known histologic and molecular differences between adenomas and carcinomas, the concordance of KRAS mutation between adenomas and carcinomas has not been established leaving some open questions regarding the appropriate choice of tissue for KRAS mutation analysis and correct interpretation of the test results. To address these questions, we analyzed the concordance of KRAS mutation in 70 tumors that contained both adenoma and carcinoma components (2 cases of intramucosal carcinoma, 66 cases with invasion of the submucosa, and 2 invading the muscularis propria). For each case, DNA was separately isolated from the adenoma and the carcinoma component and analyzed for KRAS mutation using direct sequencing. Overall, 30 (43%) of the adenoma cases and 36 (51%) of the carcinoma cases were positive for KRAS mutation. Of the 70 cases, 16 (23%) showed discordant results. Interestingly, the fraction of discordant cases went down as the depth of carcinoma invasion increased. In summary, we identified significant KRAS mutation discordance between the adenoma and carcinoma component of the lesion. Our results suggest that effort should be made to analyze only the invasive component of the lesion and that caution should be taken when interpreting a result based on DNA extracted from noninvasive elements.

Role of the protein tyrosine phosphatase Shp2 in homeostasis of the intestinal epithelium

Protein tyrosine phosphorylation is thought to be important for regulation of the proliferation, differentiation, and rapid turnover of intestinal epithelial cells (IECs). The role of protein tyrosine phosphatases in such homeostatic regulation of IECs has remained largely unknown, however. Src homology 2-containing protein tyrosine phosphatase (Shp2) is a ubiquitously expressed cytoplasmic protein tyrosine phosphatase that functions as a positive regulator of the Ras-mitogen-activated protein kinase (MAPK) signaling pathway operative downstream of the receptors for various growth factors and cytokines, and it is thereby thought to contribute to the regulation of cell proliferation and differentiation. We now show that mice lacking Shp2 specifically in IECs (Shp2 CKO mice) develop severe colitis and die as early as 3 to 4 weeks after birth. The number of goblet cells in both the small intestine and colon of Shp2 CKO mice was markedly reduced compared with that for control mice. Furthermore, Shp2 CKO mice showed marked impairment of both IEC migration along the crypt-villus axis in the small intestine and the development of intestinal organoids from isolated crypts. The colitis as well as the reduction in the number of goblet cells apparent in Shp2 CKO mice were normalized by expression of an activated form of K-Ras in IECs. Our results thus suggest that Shp2 regulates IEC homeostasis through activation of Ras and thereby protects against the development of colitis.

Shp2/MAPK signaling controls goblet/paneth cell fate decisions in the intestine

In the development of the mammalian intestine, Notch and Wnt/β-catenin signals control stem cell maintenance and their differentiation into absorptive and secretory cells. Mechanisms that regulate differentiation of progenitors into the three secretory lineages, goblet, paneth, or enteroendocrine cells, are not fully understood. Using conditional mutagenesis in mice, we observed that Shp2-mediated MAPK signaling determines the choice between paneth and goblet cell fates and also affects stem cells, which express the leucine-rich repeat-containing receptor 5 (Lgr5). Ablation of the tyrosine phosphatase Shp2 in the intestinal epithelium reduced MAPK signaling and led to a reduction of goblet cells while promoting paneth cell development. Conversely, conditional mitogen-activated protein kinase kinase 1 (Mek1) activation rescued the Shp2 phenotype, promoted goblet cell and inhibited paneth cell generation. The Shp2 mutation also expanded Lgr5+ stem cell niches, which could be restricted by activated Mek1 signaling. Changes of Lgr5+ stem cell quantities were accompanied by alterations of paneth cells, indicating that Shp2/MAPK signaling might affect stem cell niches directly or via paneth cells. Remarkably, inhibition of MAPK signaling in intestinal organoids and cultured cells changed the relative abundance of Tcf4 isoforms and by this, promoted Wnt/β-catenin activity. The data thus show that Shp2-mediated MAPK signaling controls the choice between goblet and paneth cell fates by regulating Wnt/β-catenin activity.

The MEK/ERK pathway promotes NOTCH signalling in pancreatic cancer cells

Activation of the NOTCH receptors relies on their intracellular proteolysis by the gamma-secretase complex. This cleavage liberates the NOTCH intracellular domain (NIC) thereby allowing the translocation of NIC towards the nucleus to assemble into a transcriptional platform. Little information is available regarding the regulatory steps operating on NIC following its release from the transmembrane receptor up to its association with transcriptional partners. Interfering with these regulatory steps might potentially influences the nuclear outcome of NOTCH signalling. Herein, we exploited a reliable model to study the molecular events occurring subsequent to NOTCH1 cleavage. In pancreatic cancer cells, pulse of NOTCH1 activation led to increased expression of NOTCH target genes namely HES1 and c-MYC. We uncovered that, upon its release, the NOTCH1 intracellular domain, NIC1, undergoes a series of post-translational modifications that include phosphorylation. Most interestingly, we found that activation of the MEK/ERK pathway promotes HES1 expression. Inhibition of the gamma-secretase complex prevented the MEK/ERK-induced HES1 expression suggesting a NOTCH-dependent mechanism. Finally, higher levels of NIC1 were found associated with its transcriptional partners [CBF1, Su(H) and LAG-1] (CSL) and MASTERMIND-LIKE 1 (MAML1) upon MEK/ERK activation providing a potential mechanism whereby the MEK/ERK pathway promotes expression of NOTCH target genes. For the first time, our data exposed a signalling pathway, namely the MEK/ERK pathway that positively impacts on NOTCH nuclear outcome.

Animal models of colorectal cancer

Colorectal cancer is a heterogeneous disease that afflicts a large number of people in the USA. The use of animal models has the potential to increase our understanding of carcinogenesis, tumor biology, and the impact of specific molecular events on colon biology. In addition, animal models with features of specific human colorectal cancers can be used to test strategies for cancer prevention and treatment. In this review, we provide an overview of the mechanisms driving human cancer, we discuss the approaches one can take to model colon cancer in animals, and we describe a number of specific animal models that have been developed for the study of colon cancer. We believe that there are many valuable animal models to study various aspects of human colorectal cancer. However, opportunities for improving upon these models exist.

Biased competition between Lgr5 intestinal stem cells driven by oncogenic mutation induces clonal expansion

The concept of ‘field cancerization’ describes the clonal expansion of genetically altered, but morphologically normal cells that predisposes a tissue to cancer development. Here, we demonstrate that biased stem cell competition in the mouse small intestine can initiate the expansion of such clones. We quantitatively analyze how the activation of oncogenic K-ras in individual Lgr5⁺ stem cells accelerates their cell division rate and creates a biased drift towards crypt clonality. K-ras mutant crypts then clonally expand within the epithelium through enhanced crypt fission, which distributes the existing Paneth cell niche over the two new crypts. Thus, an unequal competition between wild-type and mutant intestinal stem cells initiates a biased drift that leads to the clonal expansion of crypts carrying oncogenic mutations.

Cancer stemness in Apc- vs. Apc/KRAS-driven intestinal tumorigenesis

Constitutive activation of the Wnt pathway leads to adenoma formation, an obligatory step towards intestinal cancer. In view of the established role of Wnt in regulating stemness, we attempted the isolation of cancer stem cells (CSCs) from Apc- and Apc/KRAS-mutant intestinal tumours. Whereas CSCs are present in Apc/KRAS tumours, they appear to be very rare (<10⁻⁶) in the Apc–mutant adenomas. In contrast, the Lin⁻CD24^hiCD29⁺ subpopulation of adenocarcinoma cells appear to be enriched in CSCs with increased levels of active β-catenin. Expression profiling analysis of the CSC-enriched subpopulation confirmed their enhanced Wnt activity and revealed additional differential expression of other signalling pathways, growth factor binding proteins, and extracellular matrix components. As expected, genes characteristic of the Paneth cell lineage (e.g. defensins) are co-expressed together with stem cell genes (e.g. Lgr5) within the CSC-enriched subpopulation. This is of interest as it may indicate a cancer stem cell niche role for tumor-derived Paneth-like cells, similar to their role in supporting Lgr5⁺ stem cells in the normal intestinal crypt. Overall, our results indicate that oncogenic KRAS activation in Apc-driven tumours results in the expansion of the CSCs compartment by increasing ®-catenin intracellular stabilization.

Smad phospho-isoforms direct context-dependent TGF-β signaling

Better understanding of TGF-β signaling has deepened our appreciation of normal epithelial cell homeostasis and its dysfunction in such human disorders as cancer and fibrosis. Smad proteins, which convey signals from TGF-β receptors to the nucleus, possess intermediate linker regions connecting Mad homology domains. Membrane-bound, cytoplasmic, and nuclear protein kinases differentially phosphorylate Smad2 and Smad3 to create C-tail (C), the linker (L), or dually (L/C) phosphorylated (p, phospho-) isoforms. According to domain-specific phosphorylation, distinct transcriptional responses, and selective metabolism, Smad phospho-isoform pathways can be grouped into 4 types: cytostatic pSmad3C signaling, mitogenic pSmad3L (Ser-213) signaling, invasive/fibrogenic pSmad2L (Ser-245/250/255)/C or pSmad3L (Ser-204)/C signaling, and mitogenic/migratory pSmad2/3L (Thr-220/179)/C signaling. We outline how responses to TGF-β change through the multiple Smad phospho-isoforms as normal epithelial cells mature from stem cells through progenitors to differentiated cells, and further reflect upon how constitutive Ras-activating mutants favor the Smad phospho-isoform pathway promoting tumor progression. Finally, clinical analyses of reversible Smad phospho-isoform signaling during human carcinogenesis could assess effectiveness of interventions aimed at reducing human cancer risk. Spatiotemporally separate, functionally different Smad phospho-isoforms have been identified in specific cells and tissues, answering long-standing questions about context-dependent TGF-β signaling.

BRAF: a driver of the serrated pathway in colon cancer

In this issue of Cancer Cell, Rad and colleagues report findings that underscore the importance of oncogenic BRAF mutation coupled with microsatellite instability, p16Ink4a inactivation, and p53 mutation in the serrated pathway of colon cancer development. These findings provide translational insights into potential therapeutic intervention for BRAF mutant colon cancers.

A genetic progression model of Braf(V600E)-induced intestinal tumorigenesis reveals targets for therapeutic intervention

We show that BRAF(V600E) initiates an alternative pathway to colorectal cancer (CRC), which progresses through a hyperplasia/adenoma/carcinoma sequence. This pathway underlies significant subsets of CRCs with distinctive pathomorphologic/genetic/epidemiologic/clinical characteristics. Genetic and functional analyses in mice revealed a series of stage-specific molecular alterations driving different phases of tumor evolution and uncovered mechanisms underlying this stage specificity. We further demonstrate dose-dependent effects of oncogenic signaling, with physiologic Braf(V600E) expression being sufficient for hyperplasia induction, but later stage intensified Mapk-signaling driving both tumor progression and activation of intrinsic tumor suppression. Such phenomena explain, for example, the inability of p53 to restrain tumor initiation as well as its importance in invasiveness control, and the late stage specificity of its somatic mutation. Finally, systematic drug screening revealed sensitivity of this CRC subtype to targeted therapeutics, including Mek or combinatorial PI3K/Braf inhibition.

Lineage tracing reveals multipotent stem cells maintain human adenomas and the pattern of clonal expansion in tumor evolution

The genetic and morphological development of colorectal cancer is a paradigm for tumorigenesis. However, the dynamics of clonal evolution underpinning carcinogenesis remain poorly understood. Here we identify multipotential stem cells within human colorectal adenomas and use methylation patterns of nonexpressed genes to characterize clonal evolution. Numerous individual crypts from six colonic adenomas and a hyperplastic polyp were microdissected and characterized for genetic lesions. Clones deficient in cytochrome c oxidase (CCO(-)) were identified by histochemical staining followed by mtDNA sequencing. Topographical maps of clone locations were constructed using a combination of these data. Multilineage differentiation within clones was demonstrated by immunofluorescence. Methylation patterns of adenomatous crypts were determined by clonal bisulphite sequencing; methylation pattern diversity was compared with a mathematical model to infer to clonal dynamics. Individual adenomatous crypts were clonal for mtDNA mutations and contained both mucin-secreting and neuroendocrine cells, demonstrating that the crypt contained a multipotent stem cell. The intracrypt methylation pattern was consistent with the crypts containing multiple competing stem cells. Adenomas were epigenetically diverse populations, suggesting that they were relatively mitotically old populations. Intratumor clones typically showed less diversity in methylation pattern than the tumor as a whole. Mathematical modeling suggested that recent clonal sweeps encompassing the whole adenoma had not occurred. Adenomatous crypts within human tumors contain actively dividing stem cells. Adenomas appeared to be relatively mitotically old populations, pocketed with occasional newly generated subclones that were the result of recent rapid clonal expansion. Relative stasis and occasional rapid subclone growth may characterize colorectal tumorigenesis.

Genetic reconstitution of tumorigenesis in primary intestinal cells

Animal models for human colorectal cancer recapitulate multistep carcinogenesis that is typically initiated by activation of the Wnt pathway. Although potential roles of both genetic and environmental modifiers have been extensively investigated in vivo, it remains elusive whether epithelial cells definitely require interaction with stromal cells or microflora for tumor development. Here we show that tumor development could be simply induced independently of intestinal microenvironment, even with WT murine primary intestinal cells alone. We developed an efficient method for lentiviral transduction of intestinal organoids in 3D culture. Despite seemingly antiproliferative effects by knockdown of adenomatous polyposis coli (APC), we managed to reproducibly induce APC-inactivated intestinal organoids. As predicted, these organoids were constitutively active in the Wnt signaling pathway and proved tumorigenic when injected into nude mice, yielding highly proliferative tubular epithelial glands accompanied by prominent stromal tissue. Consistent with cellular transformation, tumor-derived epithelial cells acquired sphere formation potential, gave rise to secondary tumors on retransplantation, and highly expressed cancer stem cell markers. Inactivation of p53 or phosphatase and tensin homolog deleted from chromosome 10, or activation of Kras, promoted tumor development only in the context of APC suppression, consistent with earlier genetic studies. These findings clearly indicated that genetic cooperation for intestinal tumorigenesis could be essentially recapitulated in intestinal organoids without generating gene-modified mice. Taken together, this in vitro model for colon cancer described herein could potentially provide unique opportunities for carcinogenesis studies by serving as a substitute or complement to the currently standard approaches.

Sox9 induction, ectopic Paneth cells, and mitotic spindle axis defects in mouse colon adenomatous epithelium arising from conditional biallelic Apc inactivation

We generated transgenic mice in which human CDX2 gene elements control expression of a tamoxifen-regulated Cre protein (CDX2P-CreER(T2)) to allow for inducible gene targeting in intestinal epithelium. After tamoxifen dosing of CDX2P-CreER(T2) mice, Cre activity was detected in the distal ileal, cecal, colonic, and rectal epithelium, with selected crypt base, transit amplifying, and surface cells all capable of activating Cre function. Four weeks after tamoxifen dosing of CDX2P-CreER(T2) mice carrying a Cre-activated fluorescent reporter, single crypts were uniformly fluorescence positive or negative, reflecting Cre activation in crypt stem cells. Biallelic inactivation of the Apc tumor suppressor gene via the CDX2P-CreER(T2) transgene in colon epithelium led to acute alterations in cell proliferation, apoptosis, and morphology, along with mitotic spindle misorientation, β-catenin nuclear localization, and induction of the intestinal stem cell markers Lgr5 and Musashi-1 and the Sox9 transcription factor. Normal mouse colon epithelium lacks Paneth cells, a key small intestine niche cell type, and Paneth cell differentiation is dependent on Sox9 function. In Apc-deficient colon epithelium, ectopic Paneth-like cells were seen outside the crypt base, such as new crypt budding sites. Our data indicate Apc inactivation via CDX2P-CreER(T2) targeting in mouse colon epithelium is sufficient to induce adenomatous changes and the generation of Paneth-like cells from neoplastic progenitors, with potentially significant roles in colon adenoma development and progression.

Pathology of rodent models of intestinal cancer: progress report and recommendations

In October 2010, a pathology review of rodent models of intestinal neoplasia was held at The Jackson Laboratory. This review complemented 2 other concurrent events: a workshop on methods of modeling colon cancer in rodents and a conference on current issues in murine and human colon cancer. We summarize the results of the pathology review and the committee's recommendations for tumor nomenclature. A virtual high-resolution image slide box of these models has been developed. This report discusses significant recent developments in rodent modeling of intestinal neoplasia, including the role of stem cells in cancer and the creation of models of metastatic intestinal cancer.

Assessment of the mode of action underlying development of rodent small intestinal tumors following oral exposure to hexavalent chromium and relevance to humans

Abstract Chronic exposure to high concentrations of hexavalent chromium (Cr(VI)) in drinking water causes intestinal adenomas and carcinomas in mice, but not in rats. Cr(VI) causes damage to intestinal villi and crypt hyperplasia in mice after only one week of exposure. After two years of exposure, intestinal damage and crypt hyperplasia are evident in mice (but not rats), as are intestinal tumors. Although Cr(VI) has genotoxic properties, these findings suggest that intestinal tumors in mice arise as a result of chronic mucosal injury. To better understand the mode of action (MOA) of Cr(VI) in the intestine, a 90-day drinking water study was conducted to collect histological, biochemical, toxicogenomic and pharmacokinetic data in intestinal tissues. Using MOA analyses and human relevance frameworks proposed by national and international regulatory agencies, the weight of evidence supports a cytotoxic MOA with the following key events: (a) absorption of Cr(VI) from the intestinal lumen, (b) toxicity to intestinal villi, (c) crypt regenerative hyperplasia and (d) clonal expansion of mutations within the crypt stem cells, resulting in late onset tumorigenesis. This article summarizes the data supporting each key event in the MOA, as well as data that argue against a mutagenic MOA for Cr(VI)-induced intestinal tumors.

Proteomics in studying cancer stem cell biology

Normal multipotent tissue stem cells (SCs) are the driving force behind tissue turnover and repair. The cancer stem cell theory holds that tumors also contain stem-like cells that drive tumor growth and metastasis formation. However, very little is known about the regulation of SC maintenance pathways in cancer and how these are affected by cancer-specific genetic alterations and by treatment. Proteomics is emerging as a powerful tool to identify the signaling complexes and pathways that control multi- and pluri-potency and SC differentiation. Here, the authors review the novel insights that these studies have provided and present a comprehensive strategy for the use of proteomics in studying cancer SC biology.

The mysterious human epidermal cell cycle, or an oncogene-induced differentiation checkpoint

Fifteen years ago, we reported that proto-oncogene MYC promoted differentiation of human epidermal stem cells, a finding that was surprising to the MYC and the skin research communities. MYC was one of the first human oncogenes identified, and it had been strongly associated with proliferation. However, it was later shown that MYC could induce apoptosis under low survival conditions. Currently, the notion that MYC promotes epidermal differentiation is widely accepted, but the cell cycle mechanisms that elicit this function remain unresolved. We have recently reported that keratinocytes respond to cell cycle deregulation and DNA damage by triggering terminal differentiation. This mechanism might constitute a homeostatic protection face to cell cycle insults. Here, I discuss recent and not-so-recent evidence suggesting the existence of a largely unexplored oncogene-induced differentiation response (OID) analogous to oncogene-induced apoptosis (OIA) or senescence (OIS). In addition, I propose a model for the role of the cell cycle in skin homeostasis maintenance and for the dual role of MYC in differentiation.

A role for the epidermal growth factor receptor signaling in development of intestinal serrated polyps in mice and humans

BACKGROUND & AIMS

Epithelial cancers can be initiated by activating mutations in components of the mitogen-activated protein kinase signaling pathway such as v-raf murine sarcoma viral oncogene homolog B1 (BRAF), v-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog (KRAS), or epidermal growth factor receptor (EGFR). Human intestinal serrated polyps are a heterogeneous group of benign lesions, but some progress to colorectal cancer. Tumors that arise from these polyps frequently contain activating mutations in BRAF or KRAS, but little is known about the role of EGFR activation in their development.

METHODS

Polyp samples were obtained from adults during screening colonoscopies at Mount Sinai Hospital in New York. We measured levels of EGFR protein and phosphorylation in human serrated polyps by immunohistochemical and immunoblot analyses. We generated transgenic mice that express the ligand for EGFR, Heparin-binding EGF-like growth factor (HB-EGF), in the intestine.

RESULTS

EGFR and the extracellular-regulated kinases (ERK)1/2 were phosphorylated in serrated areas of human hyperplastic polyps (HPPs), sessile serrated adenomas, and traditional serrated adenomas. EGFR and ERK1/2 were phosphorylated in the absence of KRAS or BRAF activating mutations in a subset of HPP. Transgenic expression of the EGFR ligand HB-EGF in the intestines of mice promoted development of small cecal serrated polyps. Mice that expressed a combination of HB-EGF and US28 (a constitutively active, G-protein-coupled receptor that increases processing of HB-EGF from the membrane) rapidly developed large cecal serrated polyps. These polyps were similar to HPPs and had increased phosphorylation of EGFR and ERK1/2 within the serrated epithelium. Administration of pharmacologic inhibitors of EGFR or MAPK to these transgenic mice significantly reduced polyp development.

CONCLUSIONS

Activation of EGFR signaling in the intestine of mice promotes development of serrated polyps. EGFR signaling also is activated in human HPPs, sessile serrated adenomas, and traditional serrated adenomas.

Clinicopathologic and genetic characterization of traditional serrated adenomas of the colon

Traditional serrated adenomas (TSAs) are a type of colorectal polyp with neoplastic potential. Immunohistochemical analysis and sequencing were performed on 24 TSAs from 23 patients to characterize the molecular genetics of TSAs. Abnormal Ki-67 and p53 labeling were observed in 7 (29%) of 24 and 6 (25%) of 24 TSAs, respectively; both types were significantly associated with the presence of conventional epithelial dysplasia (P = .0005 and P = .0001, respectively). Activating KRAS mutation was identified in 11 TSAs (46%) and was mutually exclusive with activating BRAF mutations, which were seen in 7 TSAs (29%). Abnormal p53 nuclear labeling in a TSA was significantly associated with BRAF mutation status (P = .04), whereas no relationship was found for β-catenin labeling patterns. The overall morphologic features of TSA do not correlate with the genetic status of the KRAS and BRAF genes. However, conventional epithelial dysplasia and abnormal p53 labeling in a TSA are seen more often in the setting of BRAF mutation.

Expression of Abelson interactor 1 (Abi1) correlates with inflammation, KRAS mutation and adenomatous change during colonic carcinogenesis

Background

Abelson interactor 1 (Abi1) is an important regulator of actin dynamics during cytoskeletal reorganization. In this study, our aim was to investigate the expression of Abi1 in colonic mucosa with and without inflammation, colonic polyps, colorectal carcinomas (CRC) and metastases as well as in CRC cell lines with respect to BRAF/KRAS mutation status and to find out whether introduction of KRAS mutation or stimulation with TNFalpha enhances Abi1 protein expression in CRC cells.

Methodology/Principal Findings

We immunohistochemically analyzed Abi1 protein expression in 126 tissue specimens from 95 patients and in 5 colorectal carcinoma cell lines with different mutation status by western immunoblotting. We found that Abi1 expression correlated positively with KRAS, but not BRAF mutation status in the examined tissue samples. Furthermore, Abi1 is overexpressed in inflammatory mucosa, sessile serrated polyps and adenomas, tubular adenomas, invasive CRC and CRC metastasis when compared to healthy mucosa and BRAF-mutated as well as KRAS wild-type hyperplastic polyps. Abi1 expression in carcinoma was independent of microsatellite stability of the tumor. Abi1 protein expression correlated with KRAS mutation in the analyzed CRC cell lines, and upregulation of Abi1 could be induced by TNFalpha treatment as well as transfection of wild-type CRC cells with mutant KRAS. The overexpression of Abi1 could be abolished by treatment with the PI3K-inhibitor Wortmannin after KRAS transfection.

Conclusions/Significance

Our results support a role for Abi1 as a downstream target of inflammatory response and adenomatous change as well as oncogenic KRAS mutation via PI3K, but not BRAF activation. Furthermore, they highlight a possible role for Abi1 as a marker for early KRAS mutation in hyperplastic polyps. Since the protein is a key player in actin dynamics, our data encourages further studies concerning the exact role of Abi1 in actin reorganization upon enhanced KRAS/PI3K signalling during colonic tumorigenesis.

Adult mammalian stem cells: the role of Wnt, Lgr5 and R-spondins

After its discovery as oncogen and morphogen, studies on Wnt focused initially on its role in animal development. With the finding that the colorectal tumour suppressor gene APC is a negative regulator of the Wnt pathway in (colorectal) cancer, attention gradually shifted to the study of the role of Wnt signalling in the adult. The first indication that adult Wnt signalling controls stem cells came from a Tcf4 knockout experiment: mutant mice failed to build crypt stem cell compartments. This observation was followed by similar findings in multiple other tissues. Recent studies have indicated that Wnt agonists of the R-spondin family provide potent growth stimuli for crypts in vivo and in vitro. Independently, Lgr5 was found as an exquisite marker for these crypt stem cells. The story has come full circle with the finding that the stem cell marker Lgr5 constitutes the receptor for R-spondins and occurs in complex with Frizzled/Lrp.