CYP26A1 Links WNT and Retinoic Acid Signaling: A Target to Differentiate ALDH+ Stem Cells in APC-Mutant CRC.

APC mutation is the main driving mechanism of CRC development and leads to constitutively activated WNT signaling, overpopulation of ALDH+ stem cells (SCs), and incomplete differentiation. We previously reported that retinoic acid (RA) receptors are selectively expressed in ALDH+ SCs, which provides a way to target cancer SCs with retinoids to induce differentiation. Hypotheses: A functional link exists between the WNT and RA pathways, and APC mutation generates a WNT:RA imbalance that decreases retinoid-induced differentiation and increases ALDH+ SCs. Accordingly, to restore parity in WNT:RA signaling, we induce wt-APC expression in APC-mutant CRC cells, and we assess the ability of all-trans retinoic acid (ATRA) to induce differentiation. We found that ATRA increased expression of the WNT target gene, CYP26A1, and inducing wt-APC reduced this expression by 50%. Thus, the RA and WNT pathways crosstalk to modulate CYP26A1, which metabolizes retinoids. Moreover, inducing wt-APC augments ATRA-induced cell differentiation by: (i) decreasing cell proliferation; (ii) suppressing ALDH1A1 expression; (iii) decreasing ALDH+ SCs; and (iv) increasing neuroendocrine cell differentiation. A novel CYP26A1-based network that links WNT and RA signaling was also identified by NanoString profiling/bioinformatics analysis. Furthermore, CYP26A1 inhibitors sensitized CRC cells to the anti-proliferative effect of drugs that downregulate WNT signaling. Notably, in wt-APC-CRCs, decreased CYP26A1 improved patient survival. These findings have strong potential for clinical translation.

A Review of IsomiRs in Colorectal Cancer.

As advancements in sequencing technology rapidly continue to develop, a new classification of microRNAs has occurred with the discovery of isomiRs, which are relatively common microRNAs with sequence variations compared to their established template microRNAs. This review article seeks to compile all known information about isomiRs in colorectal cancer (CRC), which has not, to our knowledge, been gathered previously to any great extent. A brief overview is given of the history of microRNAs, their implications in colon cancer, the canonical pathway of biogenesis and isomiR classification. This is followed by a comprehensive review of the literature that is available on microRNA isoforms in CRC. The information on isomiRs presented herein shows that isomiRs hold great promise for translation into new diagnostics and therapeutics in clinical medicine.

The v8-10 variant isoform of CD44 is selectively expressed in the normal human colonic stem cell niche and frequently is overexpressed in colon carcinomas during tumor development.

CD44 protein and its variant isoforms are expressed in cancer stem cells (CSCs), and various CD44 isoforms can have different functional roles in cells. Our goal was to investigate how different CD44 isoforms contribute to the emergence of stem cell (SC) overpopulation that drives colorectal cancer (CRC) development. Specific CD44 variant isoforms are selectively expressed in normal colonic SCs and become overexpressed in CRCs during tumor development. We created a unique panel of anti-CD44 rabbit genomic antibodies to 16 specific epitopes that span the entire length of the CD44 molecule. Our panel was used to comprehensively investigate the expression of different CD44 isoforms in matched pairs (n = 10) of malignant colonic tissue and adjacent normal mucosa, using two (IHC & IF) immunostaining approaches. We found that: i) CD44v8-10 is selectively expressed in the normal human colonic SC niche; ii) CD44v8-10 is co-expressed with the SC markers ALDH1 and LGR5 in normal and malignant colon tissues; iii) colon carcinoma tissues frequently (80%) stain for CD44v8-10 while staining for CD44v6 was less frequent (40%). Given that CD44v8-10 expression is restricted to cells in the normal human colonic SC niche and CD44v8-10 expression progressively increases during CRC development, CD44v8-10 expression likely contributes to the SC overpopulation that drives the development and growth of colon cancers. Since the CD44 variant v8-10 epitope is located on CD44's extracellular region, it offers great promise for targeted anti-CSC treatment approaches.

Beyond the Genetic Code: A Tissue Code?

The genetic code determines how the precise amino acid sequence of proteins is specified by genomic information in cells. But what specifies the precise histologic organization of cells in plant and animal tissues is unclear. We now hypothesize that another code, the tissue code , exists at an even higher level of complexity which determines how tissue organization is dynamically maintained. Accordingly, we modeled spatial and temporal asymmetries of cell division and established that five simple mathematical laws ("the tissue code") convey a set of biological rules that maintain the specific organization and continuous self-renewal dynamics of cells in tissues. These laws might even help us understand wound healing, and how tissue disorganization leads to birth defects and tissue pathology like cancer.

Differential miRNA Expression Contributes to Emergence of Multiple Cancer Stem Cell Subpopulations in Human Colorectal Cancer.

One reason for lack of efficacy in cancer therapeutics is tumor heterogeneity. We hypothesize that tumor heterogeneity arises due to emergence of multiple cancer stem cell (CSC) subpopulations because miRNAs regulate expression of stem cell genes in CSCs. Our goal was to determine if: i) multiple CSC subpopulations exist in a human CRC cell population, and ii) miRNAs are differentially expressed in the different CSC subpopulations. We discovered that at least four different CSC populations (ALDH1, CD166, LGR5, LRIG1) exist in the HT29 cell line. CSC subpopulations were quantified using co-staining for multiple stem cell markers, isolated using FACS, and analyzed by NanoString miRNA profiling. The miRNA expression pattern in each CSC subpopulation was analyzed relative to miRNA expression patterns in other CSC subpopulations. Messenger RNAs predicted to be targeted by the upregulated miRNAs in each CSC subpopulation were: 1) identified using bioinformatics analyses, and 2) classified according to their predicted functions using David functional annotation analyses. We found multiple CSC subpopulations with a unique miRNA signature in each CSC subpopulation. Notably, the miRNAs expressed within one CSC subpopulation are predicted to target and downregulate the CSC genes and pathways that establish the other CSC subpopulations. Moreover, mRNAs predicted to be targeted by miRNAs in the different CSC subpopulations have different cellular functional classifications. That different CSC subpopulations express miRNAs that are predicted to target CSC genes expressed in other CSC subpopulations provides a mechanism that might explain the co-existence of multiple CSC subpopulations, tumor heterogeneity, and cancer therapy resistance.

Differential miRNA Expression Contributes to Emergence of Multiple Cancer Stem Cell Subpopulations in Human Colorectal Cancer.

One reason for lack of efficacy in cancer therapeutics is tumor heterogeneity. We hypothesize that tumor heterogeneity arises due to emergence of multiple Cancer Stem Cell (CSC) subpopulations because miRNAs regulate expression of stem cell genes in CSCs. Our goal was to determine if: i) multiple CSC subpopulations exist in a human CRC cell population, and ii) miRNAs are differentially expressed in the different CSC subpopulations. We discovered that at least four different CSC populations (ALDH1, CD166, LGR5, and LRIG1) exist in the HT29 cell line. CSC subpopulations were quantified using co-staining for multiple stem cell markers, isolated using FACS, and analyzed by NanoString miRNA profiling. The miRNA expression pattern in each CSC subpopulation was analyzed relative to miRNA expression patterns in other CSC subpopulations. Messenger RNAs predicted to be targeted by the up-regulated miRNAs in each CSC subpopulation were: 1) identified using bioinformatics analyses, and 2) classified according to their predicted functions using David functional annotation analyses. We found multiple CSC subpopulations with a unique miRNA signature in each CSC subpopulation. Notably, the miRNAs expressed within one CSC subpopulation are predicted to target and down-regulate the CSC genes and pathways that establish the other CSC subpopulations. Moreover, mRNAs predicted to be targeted by miRNAs in the different CSC subpopulations have different cellular functional classifications. That different CSC subpopulations express miRNAs that are predicted to target CSC genes expressed in other CSC subpopulations provides a mechanism that might explain the co-existence of multiple CSC subpopulations, tumor heterogeneity, and cancer therapy resistance.

HOXA9 Overexpression Contributes to Stem Cell Overpopulation That Drives Development and Growth of Colorectal Cancer.

HOX proteins are transcription factors that regulate stem cell (SC) function, but their role in the SC origin of cancer is under-studied. Aberrant expression of HOX genes occurs in many cancer types. Our goal is to ascertain how retinoic acid (RA) signaling and the regulation of HOXA9 expression might play a role in the SC origin of human colorectal cancer (CRC). Previously, we reported that aldehyde dehydrogenase (ALDH) and other RA pathway components are co-expressed in colonic cancer SCs (CSCs) and that overpopulation of ALDH-positive CSCs occurs during colon tumorigenesis. Our hypothesis is RA signaling regulates HOXA9 expression, and dysregulated RA signaling results in HOXA9 overexpression, which contributes to CSC overpopulation in CRC. Immunostaining showed that HOXA9 was selectively expressed in ALDH-positive SCs, and HOXA9 expression was increased in CRCs compared to normal epithelium. Modulating RA signaling in CRC cells (HT29 and SW480) with ATRA and DEAB decreased cell proliferation and reduced HOXA9 expression. Bioinformatics analyses identified a network of proteins that functionally interact with HOXA9, and the genes that encode these proteins, as well as HOXA9, contain RA receptor binding sites. These findings indicate that the expression of HOXA9 and its functional network is regulated by RA signaling in normal colonic SCs, and, when dysregulated, HOXA9 may contribute to CSC overpopulation that drives CRC development and growth. Our study provides a regulatory mechanism that might be useful in developing treatments against CSC overpopulation in CRC.

Retinoids as Chemo-Preventive and Molecular-Targeted Anti-Cancer Therapies.

Retinoic acid (RA) agents possess anti-tumor activity through their ability to induce cellular differentiation. However, retinoids have not yet been translated into effective systemic treatments for most solid tumors. RA signaling is mediated by the following two nuclear retinoic receptor subtypes: the retinoic acid receptor (RAR) and the retinoic X receptor (RXR), and their isoforms. The identification of mutations in retinoid receptors and other RA signaling pathway genes in human cancers offers opportunities for target discovery, drug design, and personalized medicine for distinct molecular retinoid subtypes. For example, chromosomal translocation involving RARA occurs in acute promyelocytic leukemia (APL), and all-trans retinoic acid (ATRA) is a highly effective and even curative therapeutic for APL patients. Thus, retinoid-based target discovery presents an important line of attack toward designing new, more effective strategies for treating other cancer types. Here, we review retinoid signaling, provide an update on retinoid agents and the current clinical research on retinoids in cancer, and discuss how the retinoid pathway genotype affects the ability of retinoid agents to inhibit the growth of colorectal cancer (CRC) cells. We also deliberate on why retinoid agents have not shown clinical efficacy against solid tumors and discuss alternative strategies that could overcome the lack of efficacy.

The Role of miRNAs, miRNA Clusters, and isomiRs in Development of Cancer Stem Cell Populations in Colorectal Cancer.

MicroRNAs (miRNAs or miRs) have a critical role in regulating stem cells (SCs) during development and altered expression can cause developmental defects and/or disease. Indeed, aberrant miRNA expression leads to wide-spread transcriptional dysregulation which has been linked to many cancers. Mounting evidence also indicates a role for miRNAs in the development of the cancer SC (CSC) phenotype. Our goal herein is to provide a review of: (i) current research on miRNAs and their targets in colorectal cancer (CRC), and (ii) miRNAs that are differentially expressed in colon CSCs. MicroRNAs can work in clusters or alone when targeting different SC genes to influence CSC phenotype. Accordingly, we discuss the specific miRNA cluster classifications and isomiRs that are predicted to target the ALDH1, CD166, BMI1, LRIG1, and LGR5 SC genes. miR-23b and miR-92A are of particular interest because our previously reported studies on miRNA expression in isolated normal versus malignant human colonic SCs showed that miR-23b and miR-92a are regulators of the LGR5 and LRIG1 SC genes, respectively. We also identify additional miRNAs whose expression inversely correlated with mRNA levels of their target genes and associated with CRC patient survival. Altogether, our deliberation on miRNAs, their clusters, and isomiRs in regulation of SC genes could provide insight into how dysregulation of miRNAs leads to the emergence of different CSC populations and SC overpopulation in CRC.

APC mutations in human colon lead to decreased neuroendocrine maturation of ALDH+ stem cells that alters GLP-2 and SST feedback signaling: Clue to a link between WNT and retinoic acid signalling in colon cancer development.

APC mutations drive human colorectal cancer (CRC) development. A major contributing factor is colonic stem cell (SC) overpopulation. But, the mechanism has not been fully identified. A possible mechanism is the dysregulation of neuroendocrine cell (NEC) maturation by APC mutations because SCs and NECs both reside together in the colonic crypt SC niche where SCs mature into NECs. So, we hypothesized that sequential inactivation of APC alleles in human colonic crypts leads to progressively delayed maturation of SCs into NECs and overpopulation of SCs. Accordingly, we used quantitative immunohistochemical mapping to measure indices and proportions of SCs and NECs in human colon tissues (normal, adenomatous, malignant), which have different APC-zygosity states. In normal crypts, many cells staining for the colonic SC marker ALDH1 co-stained for chromogranin-A (CGA) and other NEC markers. In contrast, in APC-mutant tissues from familial adenomatous polyposis (FAP) patients, the proportion of ALDH+ SCs progressively increased while NECs markedly decreased. To explain how these cell populations change in FAP tissues, we used mathematical modelling to identify kinetic mechanisms. Computational analyses indicated that APC mutations lead to: 1) decreased maturation of ALDH+ SCs into progenitor NECs (not progenitor NECs into mature NECs); 2) diminished feedback signaling by mature NECs. Biological experiments using human CRC cell lines to test model predictions showed that mature GLP-2R+ and SSTR1+ NECs produce, via their signaling peptides, opposing effects on rates of NEC maturation via feedback regulation of progenitor NECs. However, decrease in this feedback signaling wouldn't explain the delayed maturation because both progenitor and mature NECs are depleted in CRCs. So the mechanism for delayed maturation must explain how APC mutation causes the ALDH+ SCs to remain immature. Given that ALDH is a key component of the retinoic acid (RA) signaling pathway, that other components of the RA pathway are selectively expressed in ALDH+ SCs, and that exogenous RA ligands can induce ALDH+ cancer SCs to mature into NECs, RA signaling must be attenuated in ALDH+ SCs in CRC. Thus, attenuation of RA signaling explains why ALDH+ SCs remain immature in APC mutant tissues. Since APC mutation causes increased WNT signaling in FAP and we found that sequential inactivation of APC in FAP patient tissues leads to progressively delayed maturation of colonic ALDH+ SCs, the hypothesis is developed that human CRC evolves due to an imbalance between WNT and RA signaling.

MicroRNA Expression Profiling of Normal and Malignant Human Colonic Stem Cells Identifies miRNA92a as a Regulator of the LRIG1 Stem Cell Gene.

MicroRNAs (miRNAs) have a critical role in regulating stem cells (SCs) during development, and because aberrant expression of miRNAs occurs in various cancers, our goal was to determine if dysregulation of miRNAs is involved in the SC origin of colorectal cancer (CRC). We previously reported that aldehyde dehydrogenase (ALDH) is a marker for normal and malignant human colonic SCs and tracks SC overpopulation during colon tumorigenesis. MicroRNA expression was studied in ALDH-positive SCs from normal and malignant human colon tissues by Nanostring miRNA profiling. Our findings show that: (1) A unique miRNA signature distinguishes ALDH-positive CRC cells from ALDH-positive normal colonic epithelial cells, (2) Expression of four miRNAs (miRNA200c, miRNA92a, miRNA20a, miRNA93) are significantly altered in CRC SCs compared to normal colonic SCs, (3) miRNA92a expression is also upregulated in ALDH-positive HT29 CRC SCs as compared to ALDH-negative SCs, (4) miRNA92a targets the 3'UTR of LRIG1 SC gene, and (5) miRNA92a modulates proliferation of HT29 CRC cells. Thus, our findings indicate that overexpression of miRNA92a contributes to the SC origin of CRC. Strategies designed to modulate miRNA expression, such as miRNA92a, may provide ways to target malignant SCs and to develop more effective therapies against CRC.

Autocatalytic Tissue Polymerization Reaction Mechanism in Colorectal Cancer Development and Growth.

The goal of our study was to measure the kinetics of human colorectal cancer (CRC) development in order to identify aberrant mechanisms in tissue dynamics and processes that contribute to colon tumorigenesis. The kinetics of tumor development were investigated using age-at-tumor diagnosis (adenomas and CRCs) of familial adenomatous coli (FAP) patients and sporadic CRC patients. Plots of age-at-tumor diagnosis data as a function of age showed a distinct sigmoidal-shaped curve that is characteristic of an autocatalytic reaction. Consequently, we performed logistics function analysis and found an excellent fit (p < 0.05) of the logistic equation to the curves for age-at-tumor diagnoses. These findings indicate that the tissue mechanism that becomes altered in CRC development and growth involves an autocatalytic reaction. We conjecture that colonic epithelium normally functions as a polymer of cells which dynamically maintains itself in a steady state through an autocatalytic polymerization mechanism. Further, in FAP and sporadic CRC patients, mutation in the adenomatous polyposis coli (APC) gene increases autocatalytic tissue polymerization and induces tumor tissues to autocatalyze their own progressive growth, which drives tumor development in the colon.

Gene expression signatures for HOXA4, HOXA9, and HOXD10 reveal alterations in transcriptional regulatory networks in colon cancer.

We previously reported that HOXA4, HOXA9, and HOXD10 are selectively expressed in colonic stem cells (SCs) and their overexpression contributes to colorectal cancer (CRC). Our goals here were to determine how these HOX genes are transcriptionally regulated and whether transcriptional dysregulation of HOX genes occurs in CRC. Accordingly, we used correlation analysis to identify genes that are expression-correlated or anticorrelated with HOXA4, HOXA9, and HOXD10. We then used Gene Ontology (GO) analysis to functionally classify these genes. The GO results for both HOXA4 and HOXD10 correlated gene sets for normal colon and CRC show functions mostly classified as developmental, transcriptional regulation, and DNA binding. This raised the question: Are these gene sets regulated by the same transcription factors (TFs)? Consequently, we used promoter analysis and interaction network toolset (PAINT) to identify commonly shared transcription response elements. The results indicated that completely different sets of TFs coregulate HOXA4 and HOXD10 (but not HOXA9) and their expression-correlated genes. And predicted TFs are altered in CRC compared with normal colon. Taken together, analysis of gene signatures correlated with expression of HOXA4 and HOXD10 indicates how these HOX genes are: (a) transcriptionally regulated in the normal colon; (b) dysregulated in CRC. This discovery provides a mechanism for targeting CRC SCs.

The anti-cancer effect of retinoic acid signaling in CRC occurs via decreased growth of ALDH+ colon cancer stem cells and increased differentiation of stem cells.

BACKGROUND: Tumorigenesis is driven by stem cell (SC) overpopulation. Because ALDH is both a marker for SCs in many tissues and a key enzyme in retinoid acid (RA) signaling, we studied RA signaling in normal and malignant colonic SCs. HYPOTHESIS: RA signaling regulates growth and differentiation of ALDH+ colonic SCs; dysregulation of RA signaling contributes to SC overpopulation and colorectal cancer (CRC) development. METHODS: We analyzed normal and malignant colonic tissues and CRC cell lines to see if retinoid receptors (RXR & RAR) are exclusively expressed in ALDH+ SCs, and if RA signaling changes during CRC development. We determined whether RA signaling regulates cancer SC (CSC) proliferation, differentiation, sphere formation, and population size. RESULTS: RXR & RAR were expressed in ALDH+ colonic SCs, but not in MCM2+ proliferative cells. Western blotting/immunostaining of CRCs revealed that RA signaling components become overexpressed in parallel with ALDH overexpression, which coincides with the known overpopulation of ALDH+ SCs that occurs during, and drives, CRC development. Treatment of SCs with all-trans retinoic acid (ATRA) decreased proliferation, sphere formation and ALDH+ SC population size, and induced differentiation along the neuroendocrine cell (NEC) lineage. CONCLUSIONS: Retinoid signaling, by regulating ALDH+ colonic CSCs, decreases SC proliferation, sphere formation, and population size, and increases SC differentiation to NECs. Dysregulation of RA signaling in colonic SCs likely contributes to overpopulation of ALDH+ SCs and CRC growth. IMPLICATIONS: That retinoid receptors RXR and RAR are selectively expressed in ALDH+ SCs indicates RA signaling mainly occurs via ALDH+ SCs, which provides a mechanism to selectively target CSCs.

Role of HOX Genes in Stem Cell Differentiation and Cancer.

HOX genes encode an evolutionarily conserved set of transcription factors that control how the phenotype of an organism becomes organized during development based on its genetic makeup. For example, in bilaterian-type animals, HOX genes are organized in gene clusters that encode anatomic segment identity, that is, whether the embryo will form with bilateral symmetry with a head (anterior), tail (posterior), back (dorsal), and belly (ventral). Although HOX genes are known to regulate stem cell (SC) differentiation and HOX genes are dysregulated in cancer, the mechanisms by which dysregulation of HOX genes in SCs causes cancer development is not fully understood. Therefore, the purpose of this manuscript was (i) to review the role of HOX genes in SC differentiation, particularly in embryonic, adult tissue-specific, and induced pluripotent SC, and (ii) to investigate how dysregulated HOX genes in SCs are responsible for the development of colorectal cancer (CRC) and acute myeloid leukemia (AML). We analyzed HOX gene expression in CRC and AML using information from The Cancer Genome Atlas study. Finally, we reviewed the literature on HOX genes and related therapeutics that might help us understand ways to develop SC-specific therapies that target aberrant HOX gene expression that contributes to cancer development.

Multi-scale modeling of APC and [Formula: see text]-catenin regulation in the human colonic crypt.

Stem cell renewal and differentiation in the human colonic crypt are linked to the [Formula: see text]-catenin pathway. The spatial balance of Wnt factors in proliferative cells within the crypt maintain an appropriate level of cellular reproduction needed for normal crypt homeostasis. Mutational events at the gene level are responsible for deregulating the balance of Wnt factors along the crypt, causing an overpopulation of proliferative cells, a loss of structure of the crypt domain, and the initiation of colorectal carcinomas. We formulate a PDE model describing cell movement and reproduction in a static crypt domain. We consider a single cell population whose proliferative capabilities are determined by stemness, a quantity defined by intracellular levels of adenomatous polyposis coli (APC) scaffold protein and [Formula: see text]-catenin. We fit APC regulation parameters to biological data that describe normal protein gradients in the crypt. We also fit cell movement and protein flux parameters to normal crypt characteristics such as renewal time, total cell count, and proportion of proliferating cells. The model is used to investigate abnormal crypt dynamics when subjected to a diminished APC gradient, a scenario synonymous to mutations in the APC gene. We find that a 25% decrease in APC synthesis leads to a fraction of 0.88 proliferative, which is reflective of normal-appearing FAP crypts. A 50% drop in APC activity yields a fully proliferative crypt showing a doubling of the level of stemness, which characterizes the initial stages of colorectal cancer development. A sensitivity analysis of APC regulation parameters shows the perturbation of factors that is required to restore crypt dynamics to normal in the case of APC mutations.

Overexpression of HOXA4 and HOXA9 genes promotes self-renewal and contributes to colon cancer stem cell overpopulation.

Because HOX genes encode master regulatory transcription factors that regulate stem cells (SCs) during development and aberrant expression of HOX genes occurs in various cancers, our goal was to determine if dysregulation of HOX genes is involved in the SC origin of colorectal cancer (CRC). We previously reported that HOXA4 and HOXD10 are expressed in the colonic SC niche and are overexpressed in CRC. HOX gene expression was studied in SCs from human colon tissue and CRC cells (CSCs) using qPCR and immunostaining. siRNA-mediated knockdown of HOX expression was used to evaluate the role of HOX genes in modulating cancer SC (CSC) phenotype at the level of proliferation, SC marker expression, and sphere formation. All-trans-retinoic-acid (ATRA), a differentiation-inducing agent was evaluated for its effects on HOX expression and CSC growth. We found that HOXA4 and HOXA9 are up-regulated in CRC SCs. siRNA knockdown of HOXA4 and HOXA9 reduced: (i) proliferation and sphere-formation and (ii) gene expression of known SC markers (ALDH1, CD166, LGR5). These results indicate that proliferation and self-renewal ability of CRC SCs are reduced in HOXA4 and HOXA9 knockdown cells. ATRA decreased HOXA4, HOXA9, and HOXD10 expression in parallel with reduction in ALDH1 expression, self-renewal, and proliferation. Overall, our findings indicate that overexpression of HOXA4 and HOXA9 contributes to self-renewal and overpopulation of SCs in CRC. Strategies designed to modulate HOX expression may provide ways to target malignant SCs and to develop more effective therapies for CRC.

Increased Musashi-2 and Decreased NUMB Protein Levels Observed in Human Colorectal Cancer are reverted to Normal Levels by ATRA-Induced Cell Differentiation.

BACKGROUND: Musashi stem cell (SC) proteins (MSI-1 & MSI-2) are known to become over expressed during colorectal tumorigenesis in humans and mice. MSI-1 overexpression induces tumorigenesis through Notch activation via inactivation of NUMB. Previous studies also show that MSI-2 overexpression in mice induces intestinal tumorigenesis but the mechanism is independent of NUMB. However, whether the MSI-2/NUMB pathway contributes to colorectal cancer (CRC) development in humans is still undetermined. METHODS: We evaluated expression of MSI-2 and NUMB proteins in matched normal and CRC patient samples, as well as in human CRC cell lines. We also determined whether induction of cellular differentiation by all-trans retinoic acid (ATRA) influences MSI-2 and NUMB expression. RESULTS: Analysis of matched patient tissue samples and CRC cell lines showed that MSI-2 protein expression is significantly increased and NUMB expression is decreased in CRCs compared to the normal colonic tissue. Immunostaining of normal and adenomatous colonic epithelium revealed that MSI-1+ andMSI-2+ SCs reside in the SC niche and they become overpopulated during colon tumorigenesis. Moreover, promoting cellular differentiation by ATRA reduces MSI-2 protein levels, while increasing NUMB protein levels in human CRC cell lines. CONCLUSIONS: MSI-2/NUMB protein expression is altered during colon tumorigenesis, and indicates that MSI-2/NUMB signaling in human colonic stem cells is closely linked to normal colonic epithelial homeostasis. IMPLICATIONS: The ability to normalize MSI-2/NUMB signaling by inducing differentiation of cancer SCs suggests a novel therapeutic approach for CRC treatment.

An miRNA Expression Signature for the Human Colonic Stem Cell Niche Distinguishes Malignant from Normal Epithelia.

Malignant transformation of tissue stem cells (SC) may be the root of most cancer. Accordingly, we identified miRNA expression patterns in the normal human colonic SC niche to understand how cancer stem cells (CSC) may arise. In profiling miRNA expression in SC-enriched crypt subsections isolated from fresh, normal surgical specimens, we identified 16 miRNAs that were differentially expressed in the crypt bottom, creating an SC signature for normal colonic epithelia (NCE). A parallel analysis of colorectal cancer tissues showed differential expression of 83 miRNAs relative to NCE. Within the 16 miRNA signature for the normal SC niche, we found that miR-206, miR-007-3, and miR-23b individually could distinguish colorectal cancer from NCE. Notably, miR-23b, which was increased in colorectal cancer, was predicted to target the SC-expressed G protein-coupled receptor LGR5. Cell biology investigations showed that miR-23b regulated CSC phenotypes globally at the level of proliferation, cell cycle, self-renewal, epithelial-mesenchymal transition, invasion, and resistance to the colorectal cancer chemotherapeutic agent 5-fluorouracil. In mechanistic experiments, we found that miR-23b decreased LGR5 expression and increased ALDH+ CSCs. CSC analyses confirmed that levels of LGR5 and miR-23b are inversely correlated in ALDH+ CSCs and that distinct subpopulations of LGR5+ and ALDH+ CSCs exist. Overall, our results define a critical function for miR-23b, which, by targeting LGR5, contributes to overpopulation of ALDH+ CSCs and colorectal cancer. Cancer Res; 77(14); 3778-90. ©2017 AACR.

A kinetic model to study the regulation of ß-catenin, APC, and Axin in the human colonic crypt.

The Wnt/[Formula: see text]-catenin pathway plays a crucial role in stem cell renewal and differentiation in the normal human colonic crypt. The balance between [Formula: see text]-catenin and APC along the crypt axis determines its normal functionality. The mechanism that deregulates this balance may give insight into the initiation of colorectal cancer. This is significant because the spatial dysregulation of [Formula: see text]-catenin by the mutated tumor suppressor gene/protein APC in human colonic crypts is responsible for the initiation and growth of colorectal cancer. We consider a regulatory function that promotes APC synthesis within the cell and its effect on the accumulation of the Wnt target protein, [Formula: see text]-catenin. It is evident that an APC gradient exists along the crypt axis; however, the mechanism by which APC expression is regulated within the cell is not well known. We investigate the dynamics of an APC regulatory mechanism with an increased level of Axin at the subcellular level. Model output shows an increase of APC for a diminished Wnt signal, which explains the APC gradient along the crypt. We find that the dynamic interplay between [Formula: see text]-catenin, APC, and Axin produces oscillatory behavior, which is controlled by the Wnt stimulus. In the presence of reduced functional APC, the oscillations are amplified, which suggests that the cell remains in a more proliferative state for longer periods of time. Increased Axin levels (typical of mammalian cells) reduce oscillatory behavior and minimize the levels of [Formula: see text]-catenin within the cell while raising the levels of APC.