Cells engage sophisticated programs of DNA damage response (DDR) and repair to guard against genetic mutations. While there is significant knowledge concerning DDR in interphase cells, much less is known about these processes in mitosis. Direct interaction between MDC1, a master DDR organizer, and a marker of DNA damage, histone γH2AX, is required to trigger robust repair. Here we show that the DNA damage-induced interaction between MDC1 and γH2AX is attenuated in mitosis. Furthermore, inhibition in the activity of the core mitotic regulator CDK1, either by pharmacological inhibition or siRNA attenuation, enhances MDC1-γH2AX colocalization in mitosis. Our findings offer key new insights into how DDR is controlled during mitosis.
Mitosis; CDK1; MDC1; γH2AX; DDR; DSB
Sphingosine kinase 1 (SK1), one of two SK enzymes, is highly regulated and has been shown to act as a focal point for the action of many growth factors and cytokines. SK1 leads to generation of sphingosine-1-phosphate (S1P) and potentially the activation of S1P receptors to mediate biologic effects. Our previous studies implicated SK1/S1P in the regulation of inflammatory processes, specifically in inflammatory bowel disease (IBD). These studies were conducted using a total body knockout mouse for SK1 and were unable to determine the source of SK1/S1P (hematopoietic or extra-hematopoietic) involved in the inflammatory responses. Therefore, bone marrow transplants were performed with wild-type (WT) and SK1-/- mice and colitis induced with dextran sulfate sodium (DSS). Irrespective of the source of SK1/S1P, bone marrow or tissue, DSS induced colitis in all mice; however, mice lacking SK1 in both hematopoietic and extra-hematopoietic compartments exhibited decreased crypt damage. Systemic inflammation was assessed, and mice with WT bone marrow demonstrated significant neutrophilia in response to DSS. In the local inflammatory response, mice lacking SK1/S1P in either bone marrow or tissue exhibited decreased induction of cytokines and less activation of STAT3 (signal transducer and activator of transcription 3). Interestingly, we determined that extra-hematopoietic SK1 is necessary for the induction of cyclooxygenase 2 (COX2) in colon epithelium in response to DSS-induced colitis. Taken together our data suggest that hematopoietic-derived SK1/S1P regulates specific aspects of the systemic inflammatory response, while extra-hematopoietic SK1 in the colon epithelium is necessary for the autocrine induction of COX2 in DSS-induced colitis.
The transcription factor Krüppel-like factor 5 (KLF5) is primarily expressed in the proliferative zone of the mammalian intestinal epithelium where it regulates cell proliferation. Studies showed that inhibition of KLF5 expression reduces proliferation rates in human colorectal cancer cells and intestinal tumor formation in mice. To identify chemical probes that decrease levels of KLF5, we used cell-based ultrahigh-throughput screening (uHTS) to test compounds in the NIH’s public domain, the Molecular Libraries Probe Production Centers Network (MLPCN) library. The primary screen involved luciferase assays in the DLD-1/pGL4.18hKLF5p cell line, which stably expressed a luciferase reporter driven by the human KLF5 promoter. A cytotoxicity counterscreen was performed in the rat intestinal epithelial cell line, IEC-6. We identified 97 KLF5-selective compounds with EC50<10 µM for KLF5 inhibition and EC50>10 µM for IEC-6 cytotoxicity. The two most potent compounds, CIDs (PubChem Compound IDs) 439501 and 5951923, were further characterized based on computational, Western blot, and cell viability analyses. Both of these compounds and two newly-synthesized structural analogs of CID 5951923 significantly reduced endogenous KLF5 protein levels and decreased viability of several colorectal cancer cell lines without any apparent impact on IEC-6 cells. Finally, when tested in the NCI-60 panel of human cancer cell lines, compound CID 5951923 was selectively active against colon cancer cells. Our results demonstrate the feasibility of uHTS in identifying novel compounds that inhibit colorectal cancer cell proliferation by targeting KLF5.
Colorectal cancer; KLF5; Ultrahigh-throughput screen; Luciferase; Cell viability; Small-molecule compounds
Background & Aims
Krüppel-like factor 5 (KLF5) is transcription factor that is expressed by dividing epithelial cells of the intestinal epithelium. KLF5 promotes proliferation in vitro and in vivo and is induced by mitogens and various stress stimuli. To study the role of KLF5 in intestinal epithelial homeostasis, we examined the phenotype of mice with conditional deletion of Klf5 in the gut.
Mice were generated with intestinal-specific deletion of Klf5 (Vil-Cre;Klf5fl/fl).
Morphological changes in the small intestine and colon were examined by immunohistochemistry, immunoblotting, and real-time PCR.
Klf5 mutant mice were born at a normal Mendelian ratio but had high mortality compared to controls. Complete deletion of Klf5 from the intestinal mucosa resulted in neonatal lethality that corresponded with an absence of epithelial proliferation. Variegated intestinal-specific deletion of Klf5 in adult mice resulted in morphological changes that included a regenerative phenotype, impaired barrier function, and inflammation. Adult mutant mice exhibited defects in epithelial differentiation and migration. These changes were associated with reduced expression of Cdx 1, Cdx2, and Eph and ephrin signaling proteins. Concomitantly, Wnt signaling to β-catenin was reduced. Proliferation in regenerative crypts was associated with increased expression of the progenitor cell marker Sox9.
Deletion of Klf5 in the gut epithelium of mice demonstrated that KLF5 maintains epithelial proliferation, differentiation, and cell positioning along the crypt radial axis. Morphological changes that occur with deletion of Klf5 are associated with disruption of canonical Wnt signaling and increased expression of Sox9.
intestinal homeostasis; gastrointestinal development; genetics; GI tract
Recent advancement in understanding the role of both the genetics and molecular pathways in the formation and progression of colorectal cancer allowed the identification of factors that may be targeted for drug discovery. For the past decade various approaches have been developed to target specific steps or components of these pathways in order to prevent the development or progression of colorectal cancer. The innovation and optimization of high-throughput screening methods as well as the recent emphasis from the National Institutes of Health on translational sciences have enabled rapid progress in drug discovery in many fields, including colorectal cancer. Here we present a summary of the recent efforts of targeted high-throughput drug discovery towards pathways affected in colorectal cancer.
High-throughput screen; Colorectal cancer; Molecular pathways; Chemical probes
The zinc finger transcription factor, Krüppel-like factor 4 (KLF4), is expressed in the post-mitotic, differentiated epithelial cells lining the intestinal tract and exhibits a tumor suppressive effect on intestinal tumorigenesis. Here we report a role for KLF4 in maintaining homeostasis of intestinal epithelial cells. Mice with conditional ablation of the Klf4 gene from the intestinal epithelium were viable. However, both the rates of proliferation and migration of epithelial cells were increased in the small intestine of mutant mice. In addition, the brush-border alkaline phosphatase was reduced as was expression of ephrine-B1 in the small intestine, resulting in mispositioning of Paneth cells. In the colon of mutant mice, there was a reduction of the differentiation marker, carbonic anhydrase-1, and failure of differentiation of goblet cells. Mechanistically, deletion of Klf4 from the intestine resulted in a general activation of genes in the Wnt pathway and a global reduction in expression of genes encoding regulators of differentiation. Taken together, these data provide new insights into the function of KLF4 in regulating postnatal proliferation, migration, differentiation, and positioning of intestinal epithelial cells and demonstrate an essential role for KLF4 in maintaining normal intestinal epithelial homeostasis in vivo.
KLF4; Wnt; Proliferation; Differentiation; Migration; Paneth Cells; Goblet Cells
The zinc finger transcription factor Krüppel-like factor 4 (KLF4) regulates numerous physiologic processes including proliferation, differentiation, and development. Studies also showed that KLF4 is involved in tumorigenesis and somatic cell reprogramming. Here we aimed to assess whether KLF4 is a prognostic indicator for colon cancer.
Levels of KLF4 were measured by immunohistochemical analysis of a tissue microarray containing 367 independent colon cancer sections. Univariate data analysis was performed in addition to construction of multivariate models with several clinicopathologic factors to evaluate KLF4 as an independent predictor of survival and cancer recurrence (disease-free survival).
Colon cancer tissues had significantly overall lower KLF4 levels compared to non-cancer tissues (P < 0.0001). Using logistic regression, a trend was noted for decreased odds of KLF4 expression in higher stages of tumors. In both univariate and multivariate analyses, KLF4 was a significant predictor of survival and recurrence.
KLF4 expression is significantly down-regulated in colon cancer and loss of KLF4 is an independent predictor of survival and recurrence.
These findings suggest that KLF4 may serve as a prognostic biomarker for colon cancer.
KLF4; Colon Cancer; Tissue microarray; Survival; Recurrence
The Krüppel-like factor (KLF) family of transcription factors regulates diverse biological processes that include proliferation, differentiation, growth, development, survival, and responses to external stress. Seventeen mammalian KLFs have been identified, and numerous studies have been published that describe their basic biology and contribution to human diseases. KLF proteins have received much attention because of their involvement in the development and homeostasis of numerous organ systems. KLFs are critical regulators of physiological systems that include the cardiovascular, digestive, respiratory, hematological, and immune systems and are involved in disorders such as obesity, cardiovascular disease, cancer, and inflammatory conditions. Furthermore, KLFs play an important role in reprogramming somatic cells into induced pluripotent stem (iPS) cells and maintaining the pluripotent state of embryonic stem cells. As research on KLF proteins progresses, additional KLF functions and associations with disease are likely to be discovered. Here, we review the current knowledge of KLF proteins and describe common attributes of their biochemical and physiological functions and their pathophysiological roles.
Krüppel-Like Factor; Proliferation; Differentiation; Inflammation; Cardiovascular Diseases; Tumorigenesis; Fat Metabolism; Induced Pluripotent Stem Cell
Krüppel-like factor 4 (KLF4) is a zinc-finger transcription factor with diverse regulatory functions in proliferation, differentiation, and development. KLF4 also plays a role in inflammation, tumorigenesis, and reprogramming of somatic cells to induced pluripotent stem (iPS) cells. To gain insight into the mechanisms by which KLF4 regulates these processes, we conducted DNA microarray analyses to identify differentially expressed genes in mouse embryonic fibroblasts (MEFs) wild type and null for Klf4.
Expression profiles of fibroblasts isolated from mouse embryos wild type or null for the Klf4 alleles were examined by DNA microarrays. Differentially expressed genes were subjected to the Database for Annotation, Visualization and Integrated Discovery (DAVID). The microarray data were also interrogated with the Ingenuity Pathway Analysis (IPA) and Gene Set Enrichment Analysis (GSEA) for pathway identification. Results obtained from the microarray analysis were confirmed by Western blotting for select genes with biological relevance to determine the correlation between mRNA and protein levels.
One hundred and sixty three up-regulated and 88 down-regulated genes were identified that demonstrated a fold-change of at least 1.5 and a P-value < 0.05 in Klf4-null MEFs compared to wild type MEFs. Many of the up-regulated genes in Klf4-null MEFs encode proto-oncogenes, growth factors, extracellular matrix, and cell cycle activators. In contrast, genes encoding tumor suppressors and those involved in JAK-STAT signaling pathways are down-regulated in Klf4-null MEFs. IPA and GSEA also identified various pathways that are regulated by KLF4. Lastly, Western blotting of select target genes confirmed the changes revealed by microarray data.
These data are not only consistent with previous functional studies of KLF4’s role in tumor suppression and somatic cell reprogramming, but also revealed novel target genes that mediate KLF4’s functions.
KLF4; microarray; MEF; DAVID; GSEA; IPA; SAM; FDR
Krüppel-like factor 4 (KLF4) is a zinc finger transcription factor expressed in the differentiated epithelial cells lining of the intestine. Under physiological conditions, KLF4 inhibits cell proliferation. Conversely, KLF4 mediates proinflammatory signaling in macrophages and its overexpression in the esophageal epithelium activates cytokines, leading to inflammation-mediated esophageal squamous cell cancer formation in mice. Here, we tested whether KLF4 has a proinflammatory activity in experimental colitis in mice.
Villin-Cre;Klf4fl/fl mice with intestine-specific Klf4 deletion (Klf4ΔIS) and control mice with floxed Klf4 gene (Klf4fl/fl) were treated or not with 3% dextran sodium sulfate (DSS) for 7 days to induce colitis. Additionally, WT mice were administered or not, nanoparticles loaded with scrambled or Klf4-siRNA, and concomitantly given DSS.
Compared with DSS-treated Klf4fl/fl mice, DSS-treated Klf4ΔIS mice were significantly less sensitive to DSS-induced colitis. DSS treatment of Klf4fl/fl mice induced Klf4 expression in the crypt zone of the colonic epithelium. DSS-treated Klf4ΔIS mice had increased proliferation relative to DSS-treated control mice. DSS treatment induced NF-κB signaling pathway in Klf4fl/fl mice colon but not Klf4ΔIS mice. Additionally, WT mice given DSS and nanoparticle/Klf4-siRNA were less sensitive to colitis and had reduced Klf4 expression and while maintaining the proliferative response in the colonic epithelium.
Our results indicate that Klf4 is an important mediator of DSS-induced colonic inflammation by modulating NF-κB signaling pathway and could be involved in the pathogenesis and/or propagation of inflammatory bowel disease. Thus, Klf4 may represent a novel therapeutic target in inflammatory bowel disease.
Klf4; DSS; NF-κB
Background: Krüppel-like factor 4 (KLF4) is a zinc-finger transcription factor with diverse regulatory functions in proliferation, differentiation, and development. KLF4 also plays a role in inflammation, tumorigenesis, and reprogramming of somatic cells to induced pluripotent stem (iPS) cells. To gain insight into the mechanisms by which KLF4 regulates these processes, we conducted DNA microarray analyses to identify differentially expressed genes in mouse embryonic fibroblasts (MEFs) wild type and null for Klf4. Methods: Expression profiles of fibroblasts isolated from mouse embryos wild type or null for the Klf4 alleles were examined by DNA microarrays. Differentially expressed genes were subjected to the Database for Annotation, Visualization and Integrated Discovery (DAVID). The microarray data were also interrogated with the Ingenuity Pathway Analysis (IPA) and Gene Set Enrichment Analysis (GSEA) for pathway identification. Results obtained from the microarray analysis were confirmed by Western blotting for select genes with biological relevance to determine the correlation between mRNA and protein levels. Results: One hundred and sixty three up-regulated and 88 down-regulated genes were identified that demonstrated a fold-change of at least 1.5 and a P-value < 0.05 in Klf4-null MEFs compared to wild type MEFs. Many of the up-regulated genes in Klf4-null MEFs encode proto-oncogenes, growth factors, extracellular matrix, and cell cycle activators. In contrast, genes encoding tumor suppressors and those involved in JAK-STAT signaling pathways are down-regulated in Klf4-null MEFs. IPA and GSEA also identified various pathways that are regulated by KLF4. Lastly, Western blotting of select target genes confirmed the changes revealed by microarray data. Conclusions: These data are not only consistent with previous functional studies of KLF4's role in tumor suppression and somatic cell reprogramming, but also revealed novel target genes that mediate KLF4's functions.
KLF4; microarray; MEF; DAVID; GSEA; IPA; SAM; FDR
The p53 tumor suppressor inhibits the proliferation of cells which undergo prolonged activation of the mitotic checkpoint. However, the function of this antiproliferative response is not well defined. Here we report that p53 suppresses structural chromosome instability following mitotic arrest in human cells. In both HCT116 colon cancer cells and normal human fibroblasts, DNA breaks occurred during mitotic arrest in a p53-independent manner, but p53 was required to suppress the proliferation and structural chromosome instability of the resulting polyploid cells. In contrast, cells made polyploid without mitotic arrest exhibited neither significant structural chromosome instability nor p53-dependent cell cycle arrest. We also observed that p53 suppressed both the frequency and structural chromosome instability of spontaneous polyploids in HCT116 cells. Furthermore, time-lapse videomicroscopy revealed that polyploidization of p53−/− HCT116 cells is frequently accompanied by mitotic arrest. These data suggest that a function of the p53-dependent postmitotic response is the prevention of structural chromosome instability following prolonged activation of the mitotic checkpoint. Accordingly, our study suggests a novel mechanism of tumor suppression for p53, as well as a potential role for p53 in the outcome of antimitotic chemotherapy.
p53; cell cycle arrest; chromosomal instability; DNA damage; mitotic checkpoint; polypoidization
Inactivation of the tumor suppressor adenomatous polyposis coli, with the resultant activation of β-catenin, is the initiating event in the development of a majority of colorectal cancers. Krüppel-like factor 5 (KLF5), a proproliferative transcription factor, is highly expressed in the proliferating intestinal crypt epithelial cells. To determine whether KLF5 contributes to intestinal adenoma formation, we examined tumor burdens in ApcMin/+ mice and ApcMin/+/Klf5+/− mice. Compared with ApcMin/+ mice, ApcMin/+/Klf5+/− mice had a 96% reduction in the number of intestinal adenomas. Reduced tumorigenicity in the ApcMin/+/Klf5+/− mice correlated with reduced levels and nuclear localization of β-catenin as well as reduced expression of two β-catenin targets, cyclin D1 and c-Myc. In vitro studies revealed a physical interaction between KLF5 and β-catenin that enhanced the nuclear localization and transcriptional activity of β-catenin. Thus, KLF5 is necessary for the tumor-initiating activity of β-catenin during intestinal adenoma formation in ApcMin/+ mice, and reduced expression of KLF5 offsets the tumor-initiating activity of the ApcMin mutation by reducing the nuclear localization and activity of β-catenin.
Colorectal cancer is one of the leading causes of cancer mortality and morbidity worldwide. Previous studies indicate that the zinc finger-containing transcription factor Krüppel-like factor 5 (KLF5) positively regulates proliferation of intestinal epithelial cells and colorectal cancer cells. Importantly, inhibition of KLF5 expression in intestinal epithelial cells and colorectal cancer cells by pharmacologic or genetic means reduces their rate of proliferation. To identify additional and novel small molecules that inhibit KLF5 expression and thus colorectal cancer proliferation, we developed a reporter assay using colorectal cancer cell line (DLD-1) that stably expressed a luciferase reporter gene directed by 1,959 bp of the human KLF5 promoter upstream of the ATG start codon and performed a cell-based high-throughput screen with the Library of Pharmacologically Active Compounds that contains 1,280 biologically active compounds. The screen identified 8 potential inhibitors and 6 potential activators of the KLF5 promoter. Three potential inhibitors, wortmannin, AG17, and AG879, were further evaluated by secondary analyses. All three significantly reduced both KLF5 promoter-luciferase activity and protein level in DLD-1 cells in a dose- and time-dependent manner when compared with controls. They also significantly reduced the rate of proliferation of DLD-1 and two other colorectal cancer cell lines, HCT116 and HT29. Our results show the principle of using high-throughput screening to identify small-molecule compounds that modulate KLF5 activity and consequently inhibit colorectal cancer proliferation.
Mitotic abnormalities are a common feature of human cancer cells, and recent studies have provided evidence that such abnormalities may play a causative, rather than merely incidental role, in tumorigenesis. One such abnormality is prolonged activation of the mitotic checkpoint, which can be provoked by a number of the gene changes which drive tumor formation. At the same time, antimitotic chemotherapeutics exert their clinical efficacy through the large-scale induction of prolonged mitotic checkpoint activation, indicating that mitotic arrest is influential in both the formation and treatment of human cancer. However, how this influence occurs is not well-understood. In this perspective, we will discuss the current evidence in support of the potential mechanisms by which prolonged activation of the mitotic checkpoint affects both tumorigenesis and antimitotic chemotherapy.
Aneuploidy; Antimitotic chemotherapy; Apoptosis; Cell cycle arrest; Centrosomes; Chromosomal instability; Checkpoint; DNA damage; Mitosis; Polypoidy; Tumorigenesis
The zinc finger-containing transcription factor, Krüppel-like factor 4 (KLF4), inhibits cell proliferation. An in vivo tumor suppressive role for KLF4 is demonstrated by the recent finding that Klf4 haploinsufficiency in ApcMin/+ mice promotes intestinal tumorigenesis. Studies also show that KLF4 is required for the terminal differentiation of goblet cells in the mouse intestine. The Notch signaling pathway suppresses goblet cell formation and is up-regulated in intestinal tumors. Here we investigated the relationship between Notch signaling and KLF4 expression in intestinal epithelial cells. The rate of proliferation of HT29 human colon cancer cells was reduced when treated with the γ-secretase inhibitor dibenzazepine (DBZ) to inhibit Notch or siRNA directed against Notch. KLF4 levels were increased in DBZ- or Notch siRNA-treated cells. Conversely, over-expression of Notch in HT29 cells reduced KLF4 levels, suppressed KLF4 promoter activity and increased proliferation rate. Treatment of ApcMin/+ mice with DBZ resulted in a 50% reduction in the number of intestinal adenomas compared to the vehicle-treated group (p < 0.001). Both the normal-appearing intestinal mucosa and adenomas obtained from DBZ-treated ApcMin/+ mice had increased goblet cell numbers and Klf4 staining accompanied by reduced cyclin D1 and Ki67 staining when compared to those from vehicle-treated mice. Results of these studies indicate that Notch signaling suppresses KLF4 expression in intestinal tumors and colorectal cancer cells. Inhibition of Notch signaling increases KLF4 expression and goblet cell differentiation, and reduces proliferation and tumor formation. KLF4 is therefore a potential mediator for the anti-tumor effect of Notch inhibitors such as DBZ.
KLF4; goblet cells; γ-secretase inhibitor; ApcMin/+ mouse; adenomas
The potential for clinical application of pluripotent embryonic stem cells is immense but hampered by moral and ethical complications. Recent advances in the reprogramming of somatic cells by defined factors to a state that resemble embryonic stem cells have created tremendous excitement in the field. Four factors, Sox2, Oct4, Klf4 and c-Myc, when exogenously introduced into somatic cells, can lead to the formation of induced pluripotent stem (iPS) cells that have the capacity for self-renewal and differentiation into tissues of all three germ layers. In this review, we focus on the role of Krüppel-like factors (KLFs) in regulating somatic cell reprogramming. KLFs are zinc finger-containing transcription factors with diverse biological functions. We first provide an overview of the KLF family of regulatory proteins, paying special attention to the established biological and biochemical functions of KLF4 and KLF5. We then review the role of KLFs in somatic cell reprogramming and delineate the putative mechanism by which KLFs participates the establishment and self-renewal of iPS cells. Further research is likely to provide additional insight into the mechanisms of somatic cell reprogramming and refinement of the technique with which to generate clinically relevant iPS cells.
KLF; iPS cells; ES cells; Reprogramming; Somatic cells
Background & Aims
Krüppel-like factor 5 (KLF5) is a transcription factor that is highly expressed in proliferating crypt cells of the intestinal epithelium. KLF5 has a pro-proliferative effect in vitro and is induced by mitogenic and stress stimuli. To determine whether KLF5 is involved in mediating proliferative responses to intestinal stressors in vivo, we examined its function in a mouse model of transmissible murine colonic hyperplasia (TMCH), which is triggered by colonization of the mouse colon by the bacterial pathogen, Citrobacter rodentium.
Heterozygous Klf5 knockout (Klf5+/−) mice were generated from embryonic stem cells carrying an insertional disruption of the Klf5 gene. Klf5+/− mice or wild-type (WT) littermates were infected with C. rodentium by oral gavage. At various time points post-infection (p.i.), mice were sacrificed and distal colons harvested. Colonic crypt heights were determined morphometrically from sections stained with hematoxylin and eosin. Frozen tissues were stained by immunofluorescence using antibodies against Klf5 and the proliferation marker, Ki67, to determine Klf5 expression and numbers of proliferating cells per crypt.
Infection of WT mice with C. rodentium resulted in a 2-fold increase in colonic crypt heights at 14 days p.i. and was accompanied by a 1.7-fold increase in Klf5 expression. Infection of Klf5+/− mice showed an attenuated induction of Klf5 expression, and hyperproliferative responses to C. rodentium were reduced in the Klf5+/− animals as compared to WT littermates.
Our study demonstrates that Klf5 is a key mediator of crypt cell proliferation in the colon in response to pathogenic bacterial infection.
Background & Aims
Krüppel-like factor 5 (KLF5) is a zinc finger-containing transcription factor that regulates cell proliferation. Oncogenic KRAS mutations are commonly found in colorectal cancers. We aimed to determine whether KLF5 mediates KRAS functions during intestinal tumorigenesis.
The effects of KLF5 on proliferation and transformation were examined in IEC-6 intestinal epithelial cells stably transfected with an inducible KRASV12G. KLF5 expression was examined in intestinal tumors derived from transgenic mice expressing KRASV12G under a villin promoter and in human colorectal cancers with mutated KRAS.
Induction of KRASV12G in IEC-6 cells resulted in increased expression of KLF5, accompanied by an increased rate of proliferation and anchorage-independent growth. Inhibition of KLF5 expression by MEK inhibitors or KLF5-specific small interfering RNA (siRNA) reduced proliferation and anchorage-independent growth despite KRASV12G induction. Human colorectal cancer cell lines with mutated KRAS contained high levels of KLF5 and reduction of KLF5 by MEK inhibitors or KLF5 siRNA also led to reduced proliferation and transformation. In vivo, both intestinal tumors derived from mice transgenic for villin-KRASV12G and human primary colorectal cancers with mutated KRAS contained high levels of KLF5 and increased staining of the proliferative marker, Ki67.
Elevated levels of KLF5 protein are strongly correlated with activating KRAS mutations in intestinal tumors both in vitro and in vivo. Inhibition of KLF5 expression in these tumor cells resulted in significantly reduced rates of proliferation and transforming activities. We conclude that KLF5 is an important mediator of oncogenic KRAS transforming functions during intestinal tumorigenesis.
The mitotic checkpoint is a mechanism that arrests the progression to anaphase until all chromosomes have achieved proper attachment to mitotic spindles. In cancer cells, satisfaction of this checkpoint is frequently delayed or prevented by various defects, some of which have been causally implicated in tumorigenesis. At the same time, deliberate induction of mitotic arrest has proved clinically useful, as antimitotic drugs that interfere with proper chromosome-spindle interactions are effective anticancer agents. However, how mitotic arrest contributes to tumorigenesis or antimitotic drug toxicity is not well defined. Here, we report that mitotic chromosomes can acquire DNA breaks during both pharmacologic and genetic induction of mitotic arrest in human cancer cells. These breaks activate a DNA damage response, occur independently of cell death, and subsequently manifest as karyotype alterations. Such breaks can also occur spontaneously, particularly in cancer cells containing mitotic spindle abnormalities. Moreover, we observed evidence of some breakage in primary human cells. Our findings thus describe a novel source of DNA damage in human cells. They also suggest that mitotic arrest may promote tumorigenesis and antimitotic toxicity by provoking DNA damage.
Colorectal cancer (CRC) is a major cause of morbidity and mortality from cancers in the United States. Recent studies have revealed the paradigm in which sequential genetic changes (mutations) result in the progression from normal colonic tissues to frank carcinoma. In particular, the study of hereditary colorectal cancer and polyposis syndromes such as familial adenomatous polyposis and hereditary nonpolyposis colon cancer has contributed enormously to the understanding of the pathogenesis of CRC. Here we describe some of the common genetic pathways in CRC and the mechanisms of action for some of the key genes involved in the formation of CRC. The understanding of the genetic pathways and functions in CRC may lead to the development of novel therapeutic approaches for treating this deadly disease.
Mitosis is a crucial part of the cell cycle. A successful mitosis requires the proper execution of many complex cellular behaviors. Thus, there are many points at which mitosis may be disrupted. In cancer cells, chronic disruption of mitosis can lead to unequal segregation of chromosomes, a phenomenon known as chromosomal instability. A majority of colorectal tumors suffer from this instability, and recent studies have begun to reveal the specific ways in which mitotic defects promote chromosomal instability in colorectal cancer.
The zinc finger transcription factor Krüppel-like factor 4 (KLF4) is frequently down-regulated in colorectal cancer. Previous studies showed that the expression of KLF4 was activated by the colorectal cancer tumor suppressor adeno-matous polyposis coli (APC) and that KLF4 repressed the Wnt/β-catenin pathway. Here, we examined whether KLF4 plays a role in modulating intestinal tumorigenesis by comparing the tumor burdens in mice heterozygous for the ApcMin allele (ApcMin/+) and those heterozygous for both the ApcMin and Klf4 alleles (Klf4+/−/ApcMin/+). Between 10 and 20 weeks of age, Klf4+/−/ApcMin/+ mice developed, on average, 59% more intestinal adenomas than ApcMin/+ mice (P < 0.0001). Immunohistochemical staining showed that Klf4 protein levels were lower in the normal-appearing intestinal tissues of Klf4+/−/ApcMin/+ mice compared with wild-type, Klf4+/−, or ApcMin/+ mice. In contrast, the levels of β-catenin and cyclin D1 were higher in the normal-appearing intestinal tissues of Klf4+/−/ApcMin/+ mice compared with the other three genotypes. Klf4 levels were further decreased in adenomas from both ApcMin/+ and Klf4+/−/ApcMin/+ mice compared with their corresponding normal-appearing tissues. Reverse transcription-PCR showed an inverse correlation between adenoma size and Klf4 mRNA levels in both Klf4+/−/ApcMin/+ and ApcMin/+ mice. There was also a progressive loss of heterozygosity of the wild-type Apc allele in adenomas with increasing size from Klf4+/−/ApcMin/+ and ApcMin/+ mice. Results from this study show that KLF4 plays an important role in promoting the development of intestinal adenomas in the presence of ApcMin mutation.
The Krüppel-like factor (KLF) proteins are zinc finger–containing transcription factors that exert important functions in regulating diverse biologic processes such as growth, proliferation, differentiation, development, inflammation, and apoptosis. Many KLFs have also been shown to play significant roles in tumorigenesis of various organs and tissues. Three in particular—KLF4, KLF5, and KLF6—are often dysregulated in tumors of the gastrointestinal tract, including colorectal cancer. This article reviews the functions of these three KLFs in normal gastrointestinal biology and their pathobiologic roles in colorectal cancer.