The related retinoblastoma (pRb) and p130 proteins cooperate to regulate homeostasis in the intestinal epithelium

Significance of RB Loss in Unlocking Phenotypic Plasticity in Advanced Cancers

Cancer cells can undergo plasticity in response to environmental stimuli or under selective therapeutic pressures that result in changes in phenotype. This complex phenomenon of phenotypic plasticity is now recognized as a hallmark of cancer. Lineage plasticity is often associated with loss of dependence on the original oncogenic driver and is facilitated, in part, by underlying genomic and epigenetic alterations. Understanding the molecular drivers of cancer plasticity is critical for the development of novel therapeutic strategies. The retinoblastoma gene RB1 (encoding RB) is the first tumor suppressor gene to be discovered and has a well-described role in cell-cycle regulation. RB is also involved in diverse cellular functions beyond cell cycle including differentiation. Here, we describe the emerging role of RB loss in unlocking cancer phenotypic plasticity and driving therapy resistance across cancer types. We highlight parallels in cancer with the noncanonical role of RB that is critical for normal development and lineage specification, and the downstream consequences of RB loss including epigenetic reprogramming and chromatin reorganization that can lead to changes in lineage program. Finally, we discuss potential therapeutic approaches geared toward RB loss cancers undergoing lineage reprogramming.

Retinoblastoma Protein Paralogs and Tumor Suppression

The retinoblastoma susceptibility gene (RB1) is the first tumor suppressor gene discovered and a prototype for understanding regulatory networks that function in opposition to oncogenic stimuli. More than 3 decades of research has firmly established a widespread and prominent role for RB1 in human cancer. Yet, this gene encodes but one of three structurally and functionally related proteins that comprise the pocket protein family. A central question in the field is whether the additional genes in this family, RBL1 and RBL2, are important tumor suppressor genes. If so, how does their tumor suppressor activity overlap or differ from RB1. Here we revisit these questions by reviewing relevant data from human cancer genome sequencing studies that have been rapidly accumulating in recent years as well as pertinent functional studies in genetically engineered mice. We conclude that RBL1 and RBL2 do have important tumor suppressor activity in some contexts, but RB1 remains the dominant tumor suppressor in the family. Given their similarities, we speculate on why RB1 tumor suppressor activity is unique.

RB depletion is required for the continuous growth of tumors initiated by loss of RB

The retinoblastoma (RB) tumor suppressor is functionally inactivated in a wide range of human tumors where this inactivation promotes tumorigenesis in part by allowing uncontrolled proliferation. RB has been extensively studied, but its mechanisms of action in normal and cancer cells remain only partly understood. Here, we describe a new mouse model to investigate the consequences of RB depletion and its re-activation in vivo. In these mice, induction of shRNA molecules targeting RB for knock-down results in the development of phenotypes similar to Rb knock-out mice, including the development of pituitary and thyroid tumors. Re-expression of RB leads to cell cycle arrest in cancer cells and repression of transcriptional programs driven by E2F activity. Thus, continuous RB loss is required for the maintenance of tumor phenotypes initiated by loss of RB, and this new mouse model will provide a new platform to investigate RB function in vivo.

Merkel Cell Carcinoma: An Immunotherapy Fairy-Tale?

Merkel cell carcinoma (MCC) is a rare, highly aggressive, neuroendocrine cutaneous tumor. The incidence of MCC is growing worldwide, and the disease-related mortality is about three-fold higher than melanoma. Since a few years ago, very little has been known about this disease, and chemotherapy has been the standard of care. Nowadays, new discoveries about the pathophysiology of this neoplasm and the introduction of immunotherapy allowed to completely rewrite the history of these patients. In this review, we provide a summary of the most important changes in the management of Merkel cell carcinoma, with a focus on immunotherapy and a landscape of future treatment strategies.

Acute alcohol consumption-induced let-7a inhibition exacerbates hepatic apoptosis by regulating Rb1 in mice

Alcohol consumption is a critical risk factor for hepatic pathogenesis, including alcoholic liver diseases (ALD), but implications of alcohol-induced dysregulation of microRNA (miRNA) in ALD pathogenesis are not completely understood. In the present study, C57BL/6J male mice were treated with saline (CON; oral gavage; n = 8) or alcohol (EtOH; 3 g/kg body weight; oral gavage; n = 8) for 7 days. A total of 599 miRNAs and 158 key mRNAs related to fatty liver and hepatotoxicity pathways were assessed in mice liver tissues. The mRNA expression datasets were then utilized to predict interactions with miRNAs that were changed by alcohol consumption. Predicted miRNA-mRNA interactions were validated using in vitro miRNA transfection experiments. The results showed that let-7a was significantly decreased in the EtOH group and Rb1 mRNA was predicted as a target gene. This was further supported by an inverse correlation of RB1 and let-7a expression in mice liver tissue. Additionally, key protein expressions involved in RB1-apoptosis axis [i.e., p73, cleaved CASP-3 (cCASP-3), and cCASP-7] showed a trend of increase in the EtOH mice; this was also confirmed by capase-3 enzyme activity and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assay in livers of mice that had consumed alcohol. In line with our in vivo observations, alcohol treatment suppressed the let-7a expression and subsequently upregulated p73, cCASP-3, and cCASP-7 protein expressions in mice hepatocytes. Additional proteins in the apoptosis regulatory pathway (i.e., MDM2-p53 axis) were significantly changed in response to let-7a suppression in the cells. Taken together, the current study provides mechanistic evidence that alcohol consumption-induced let-7a suppression results in the upregulation of RB1, thereby promoting hepatic apoptosis through induction of pro-apoptotic proteins (e.g., p73), and by, at least in part, preventing MDM2-mediated p53 degradation.

Histogenesis of Merkel Cell Carcinoma: A Comprehensive Review

Merkel cell carcinoma (MCC) is a primary neuroendocrine carcinoma of the skin. This neoplasia features aggressive behavior, resulting in a 5-year overall survival rate of 40%. In 2008, Feng et al. identified Merkel cell polyomavirus (MCPyV) integration into the host genome as the main event leading to MCC oncogenesis. However, despite identification of this crucial viral oncogenic trigger, the nature of the cell in which MCC oncogenesis occurs is actually unknown. In fact, several hypotheses have been proposed. Despite the large similarity in phenotype features between MCC tumor cells and physiological Merkel cells (MCs), a specialized subpopulation of the epidermis acting as mechanoreceptor of the skin, several points argue against the hypothesis that MCC derives directly from MCs. Alternatively, MCPyV integration could occur in another cell type and induce acquisition of an MC-like phenotype. Accordingly, an epithelial as well as a fibroblastic or B-cell origin of MCC has been proposed mainly based on phenotype similarities shared by MCC and these potential ancestries. The aim of this present review is to provide a comprehensive review of the current knowledge of the histogenesis of MCC.

The integrity of the FOG-2 LXCXE pRb-binding motif is required for small intestine homeostasis

NEW FINDINGS

What is the central question of this study? Do Fog2^{Rb- / Rb-} mice present a defect of small intestine homeostasis? What is the main finding and its importance? The importance of interactions between FOG-2 and pRb in adipose tissue physiology has previously been demonstrated. Here it is shown that this interaction is also intrinsic to small intestine homeostasis and exerts extrinsic control over mouse metabolism. Thus, this association is involved in maintaining small intestine morphology, and regulating crypt proliferation and lineage differentiation. It therefore affects mouse growth and adaptation to a high-fat diet.

ABSTRACT

GATA transcription factors and their FOG cofactors play a key role in tissue-specific development and differentiation, from worms to humans. We have shown that GATA-1 and FOG-2 contain an LXCXE pRb-binding motif. Interactions between retinoblastoma protein (pRb) and GATA-1 are crucial for erythroid proliferation and differentiation, whereas the LXCXE pRb-binding site of FOG-2 is involved in adipogenesis. Fog2-knock-in mice have defective pRb binding and are resistant to obesity, due to efficient white-into-brown fat conversion. Our aim was to investigate the pathophysiological impact of FOG-2-pRb interaction on the small intestine and mouse growth. Histological analysis of the small intestine revealed architectural changes in Fog2^{Rb- / Rb-} mice, including villus shortening, with crypt expansion and a change in muscularis propria thickness. These differences were more marked in the proximo-distal part of the small intestine and were associated with an increase in crypt cell proliferation and disruption of the goblet and Paneth cell lineage. The small intestine of the mutants was unable to adapt to a high-fat diet, and had significantly lower plasma lipid levels on such a diet. Fog2^{Rb- / Rb-} mice displayed higher levels of glucose-dependent insulinotropic peptide release, and lower levels of insulin-like growth factor I release on a regular diet. Their intestinal lipid absorption was impaired, resulting in restricted weight gain. In addition to the intrinsic effects of the mutation on adipose tissue, we show here an extrinsic relationship between the intestine and the effect of FOG-2 mutation on mouse metabolism. In conclusion, the interaction of FOG-2 with pRb coordinates the crypt-villus axis and controls small intestine homeostasis.

SMAD4 Suppresses WNT-Driven Dedifferentiation and Oncogenesis in the Differentiated Gut Epithelium

The cell of origin of colon cancer is typically thought to be the resident somatic stem cells, which are immortal and escape the continual cellular turnover characteristic of the intestinal epithelium. However, recent studies have identified certain conditions in which differentiated cells can acquire stem-like properties and give rise to tumors. Defining the origins of tumors will inform cancer prevention efforts as well as cancer therapies, as cancers with distinct origins often respond differently to treatments. We report here a new condition in which tumors arise from the differentiated intestinal epithelium. Inactivation of the differentiation-promoting transcription factor SMAD4 in the intestinal epithelium was surprisingly well tolerated in the short term. However, after several months, adenomas developed with characteristics of activated WNT signaling. Simultaneous loss of SMAD4 and activation of the WNT pathway led to dedifferentiation and rapid adenoma formation in differentiated tissue. Transcriptional profiling revealed acquisition of stem cell characteristics, and colabeling indicated that cells expressing differentiated enterocyte markers entered the cell cycle and reexpressed stem cell genes upon simultaneous loss of SMAD4 and activation of the WNT pathway. These results indicate that SMAD4 functions to maintain differentiated enterocytes in the presence of oncogenic WNT signaling, thus preventing dedifferentiation and tumor formation in the differentiated intestinal epithelium.Significance: This work identifies a mechanism through which differentiated cells prevent tumor formation by suppressing oncogenic plasticity. Cancer Res; 78(17); 4878-90. ©2018 AACR.

Misidentified Human Gene Functions with Mouse Models: The Case of the Retinoblastoma Gene Family in Senescence

Although mice models rank among the most widely used tools for understanding human genetics, biology, and diseases, differences between orthologous genes among species as close as mammals are possible, particularly in orthologous gene pairs in which one or more paralogous (i.e., duplicated) genes appear in the genomes of the species. Duplicated genes can possess overlapping functions and compensate for each other. The retinoblastoma gene family demonstrates typical composite functionality in its three member genes (i.e., RB1, RB2/P130, and P107), all of which participate in controlling the cell cycle and associated phenomena, including proliferation, quiescence, apoptosis, senescence, and cell differentiation. We analyzed the role of the retinoblastoma gene family in regulating senescence in mice and humans. Silencing experiments with each member of the gene family in mesenchymal stromal cells (MSCs) and fibroblasts from mouse and human tissues demonstrated that RB1 may be indispensable for senescence in mouse cells, but not in human ones, as an example of species specificity. Furthermore, although RB2/P130 seems to be implicated in maintaining human cell senescence, the function of RB1 within any given species might differ by cell type, as an example of cell specificity. For instance, silencing RB1 in mouse fibroblasts induced a reduced senescence not observed in mouse MSCs. Our findings could be useful as a general paradigm of cautions to take when inferring the role of human genes analyzed in animal studies and when examining the role of the retinoblastoma gene family in detail.

Prognostic relevance of high atonal homolog-1 expression in Merkel cell carcinoma

BACKGROUND

It has recently been reported that atonal homolog 1 (ATOH1) gene is down-regulated in Merkel cell carcinoma (MCC) and thus may represent a tumor suppressor gene.

OBJECTIVES

We aimed to test for ATOH1 gene mutations and expression levels in MCC tissues and cell lines.

METHODS

Genomic DNA isolation and amplification via PCR was successfully performed in 33 MCCs on formalin-fixed paraffin-embedded tissue and three MCC cell lines, followed by Sanger sequencing of the whole ATOH1 gene to detect genomic aberrations. ATOH1 mRNA levels were determined by RT-PCR. Immunohistochemistry of ATOH1 was performed to quantify protein expression in tumor samples and cell lines.

RESULTS

Neither in any of the 33 MCC tissue samples nor in the three cell lines ATOH1 mutations were present. ATOH1 was expressed in all lesions, albeit at different expression levels. Univariate analysis revealed that the total immunohistology score significantly correlated with the occurrence of tumor relapse (r = 0.57; P = 0.0008). This notion was confirmed in multivariate analysis suggesting that ATOH1 expression is a potential independent predictor for tumor relapse in MCC patients (P = 0.028). MCC-related death also correlated with ATOH1 expression (r = 0.4; P = 0.025); however, ATOH1 expression did not retain its predictive value in the regression model.

CONCLUSIONS

In contrast to anecdotal reports ATOH1 expression is not lost by genetic alterations in MCC. However, protein expression of ATOH1 is increased in advanced MCC indicating that ATOH1 is involved in MCC progression.

Proteomic analysis of pRb loss highlights a signature of decreased mitochondrial oxidative phosphorylation

Nicolay et al. ablated Rb in adult mice and conducted a quantitative analysis of RNA and proteomic changes in the colon and lungs. The proteomic changes in common between Rb^KO tissues showed a striking decrease in proteins with mitochondrial functions, highlighting the importance of pRb for mitochondrial function.

The retinoblastoma tumor suppressor (pRb) protein associates with chromatin and regulates gene expression. Numerous studies have identified Rb-dependent RNA signatures, but the proteomic effects of Rb loss are largely unexplored. We acutely ablated Rb in adult mice and conducted a quantitative analysis of RNA and proteomic changes in the colon and lungs, where Rb^KO was sufficient or insufficient to induce ectopic proliferation, respectively. As expected, Rb^KO caused similar increases in classic pRb/E2F-regulated transcripts in both tissues, but, unexpectedly, their protein products increased only in the colon, consistent with its increased proliferative index. Thus, these protein changes induced by Rb loss are coupled with proliferation but uncoupled from transcription. The proteomic changes in common between Rb^KO tissues showed a striking decrease in proteins with mitochondrial functions. Accordingly, RB1 inactivation in human cells decreased both mitochondrial mass and oxidative phosphorylation (OXPHOS) function. RB^KO cells showed decreased mitochondrial respiratory capacity and the accumulation of hypopolarized mitochondria. Additionally, RB/Rb loss altered mitochondrial pyruvate oxidation from ¹³C-glucose through the TCA cycle in mouse tissues and cultured cells. Consequently, RB^KO cells have an enhanced sensitivity to mitochondrial stress conditions. In summary, proteomic analyses provide a new perspective on Rb/RB1 mutation, highlighting the importance of pRb for mitochondrial function and suggesting vulnerabilities for treatment.

Inactivation of the retinoblastoma gene yields a mouse model of malignant colorectal cancer

The retinoblastoma gene (Rb) is mutated at significant frequency in various human epithelial tumors, including colorectal cancer, and is strongly associated with metastatic disease. However, sole inactivation of Rb in the mouse has so far failed to yield epithelial cancers. Here, we specifically inactivate Rb and/or p53 in the urogenital epithelium and the intestine. We find that loss of both tumor suppressors is unable to yield tumors in the transitional epithelium lining the bladder, kidneys and ureters. Instead, these mice develop highly metastatic tumors of neuroendocrine, not epithelial, origin within the urogenital tract to give prostate cancer in the males and vaginal tumors in the females. Additionally, we discovered that the sole inactivation of Rb in the intestine was sufficient to induce formation of metastatic colorectal adenocarcinomas. These tumors closely mirror the human disease in regard to age of onset, histological appearance, invasiveness and metastatic potential. Like most human colorectal carcinomas, our murine Rb-deficient tumors demonstrate genomic instability and they show activation of β-catenin. Deregulation of the Wnt/β-catenin pathway is specific to the intestinal tumors, as genomic instability but not activation of β-catenin was observed in the neuroendocrine tumors. To date, attempts to generate genetically engineered mouse models of colorectal cancer tumors have yielded mostly cancer of the small intestine, which rarely occurs in humans. Our system provides the opportunity to accurately model and study colorectal cancer in the mouse via a single gene mutation.

Tumor suppression by miR-26 overrides potential oncogenic activity in intestinal tumorigenesis

To investigate the contexts in which the tumor suppressor versus oncogenic activity of miR-26 predominates in vivo, Zeitels et al. generated miR-26a transgenic mice. Despite measurable repression of Pten, elevated miR-26a levels were not associated with malignancy in transgenic animals. miR-26a expression potently suppressed intestinal adenoma formation in Apc^min/+ mice. This study reveals a tumor suppressor role for miR-26 in intestinal cancer that overrides putative oncogenic activity.

Down-regulation of miR-26 family members has been implicated in the pathogenesis of multiple malignancies. In some settings, including glioma, however, miR-26-mediated repression of PTEN promotes tumorigenesis. To investigate the contexts in which the tumor suppressor versus oncogenic activity of miR-26 predominates in vivo, we generated miR-26a transgenic mice. Despite measureable repression of Pten, elevated miR-26a levels were not associated with malignancy in transgenic animals. We documented reduced miR-26 expression in human colorectal cancer and, accordingly, showed that miR-26a expression potently suppressed intestinal adenoma formation in Apc^min/+ mice, a model known to be sensitive to Pten dosage. These studies reveal a tumor suppressor role for miR-26 in intestinal cancer that overrides putative oncogenic activity, highlighting the therapeutic potential of miR-26 delivery to this tumor type.

IGF-2 is necessary for retinoblastoma-mediated enhanced adaptation after small-bowel resection

Previously, we have demonstrated that genetically disrupting retinoblastoma protein (Rb) expression in enterocytes results in taller villi, mimicking resection-induced adaption responses. Rb deficiency also results in elevated insulin-like growth factor-2 (IGF-2) expression in villus enterocytes. We propose that postoperative disruption of Rb results in enhanced adaptation which is driven by IGF-2. Inducible, intestine-specific Rb-null mice (iRbIKO) and wild-type (WT) littermates underwent a 50% proximal small-bowel resection (SBR) at 7-9 weeks of age. They were then given tamoxifen on postoperative days (PODs) 4-6 and harvested on POD 28. The experiment was then repeated on double knockouts of both IGF-2 and Rb (IGF-2 null/iRbIKO). iRbIKO mice demonstrated enhanced resection-induced adaptive villus growth after SBR and increased IGF-2 messenger RNA (mRNA) in ileal villus enterocytes compared to their WT littermates. In the IGF-2 null/iRbIKO double-knockout mice, there was no additional villus growth beyond what was expected of normal resection-induced adaptation. Adult mice in which Rb is inducibly deleted from the intestinal epithelium following SBR have augmented adaptive growth. IGF-2 expression is necessary for enhanced adaptation associated with acute intestinal Rb deficiency.

Viral oncogene expression in the stem/progenitor cell compartment of the mouse intestine induces adenomatous polyps

Genetic and epigenetic events that alter gene expression and/or protein function or localization are thought to be the primary mechanism that drives tumorigenesis and governs the clinical behavior of cancers. Yet, a number of studies have shown that the effects of oncogene expression or tumor suppressor ablation are highly dependent on cell type. The molecular basis for this cell-type specificity and how it contributes to tumorigenesis are unknown. Here, expression of a truncated SV40 large T antigen in murine intestinal crypts promoted the formation of numerous adenomatous polyps in the colon and small intestine. In contrast, when the same T-antigen construct is expressed in villous enterocytes, the consequences are limited to hyperplasia and dysplasia. The T-antigen-induced polyps show high levels of the proto-oncogene c-Myc protein even though there is no transport of β-catenin to the nucleus. Targeting the expression of viral oncogenes to intestinal crypts or villi provides a murine model system for studying cell-type specific effects in tumorigenesis, and is particularly relevant to the study of APC/β-catenin-independent pathways contributing to the generation of intestinal polyps.

IMPLICATIONS

This mouse model system describes the formation of colon polyps in the absence of Wnt/β-catenin signaling.

Insulin-like growth factor 2 and its enterocyte receptor are not required for adaptation in response to massive small bowel resection

PURPOSE

Enhanced structural features of resection-induced intestinal adaptation have been demonstrated following the administration of multiple different growth factors and peptides. Among these, the insulin-like growth factor (IGF) system has been considered to be significant. In this study, we employ mutant mouse strains to directly test the contribution of IGF2 and its enterocyte receptor (IGF1R) toward the adaptation response to massive small bowel resection (SBR).

METHODS

IGF2-knockout (IGF2-KO) (n=8) and intestine specific IGF1R-knockout mice (IGF1R-IKO) (n=9) and their wild type (WT) littermates (n=5, n=7, respectively) underwent 50% proximal SBR. At post-operative day 7, structural adaptation was measured as crypt depth and villus height. Rates of enterocyte proliferation and apoptosis were also recorded.

RESULTS

The successful deletion of IGF2 and IGF1R expression in the enterocytes was confirmed by RT-PCR and Western blot, respectively. Normal adaptation occurred in both IGF2-KO and IGF1R-IKO mice after 50% SBR. Post-operative rates of proliferation and apoptosis in both IGF2-KO and IGF1R-IKO mice were no different than their respective controls.

CONCLUSION

IGF2 and functional IGF1R signaling in enterocytes are both dispensable for resection-induced adaptation responses. The mechanism for IGF-stimulation of intestinal adaptation may involve other ligands or cellular compartments within the intestine.

Disruption of retinoblastoma protein expression in the intestinal epithelium impairs lipid absorption

We previously demonstrated increased villus height following genetic deletion, or knockout, of retinoblastoma protein (Rb) in the intestinal epithelium (Rb-IKO). Here we determined the functional consequences of augmented mucosal growth on intestinal fat absorption and following a 50% small bowel resection (SBR). Mice with constitutively disrupted Rb expression in the intestinal epithelium (Rb-IKO) along with their floxed (wild-type, WT) littermates were placed on a high-fat diet (HFD, 42% kcal fat) for 54 wk. Mice were weighed weekly, and fat absorption, indirect calorimetry, and MRI body composition were measured. Rb-IKO mice were also subjected to a 50% SBR, followed by HFD feeding for 33 wk. In separate experiments, we examined intestinal fat absorption in mice with conditional (tamoxifen-inducible) intestinal Rb (inducible Rb-IKO) deletion. Microarray revealed that the transcriptional expression of lipid absorption/transport genes was significantly reduced in constitutive Rb-IKO mice. These mice demonstrated greater mucosal surface area yet manifested paradoxically impaired intestinal long-chain triglyceride absorption and decreased cholesterol absorption. Despite attenuated lipid absorption, there were no differences in metabolic rate, body composition, and weight gain in Rb-IKO and WT mice at baseline and following SBR. We also confirmed fat malabsorption in inducible Rb-IKO mice. We concluded that, despite an expanded mucosal surface area, Rb-IKO mice demonstrate impaired lipid absorption without compensatory alterations in energy homeostasis or body composition. These findings underscore the importance of delineating structural/functional relationships in the gut and suggest a previously unknown role for Rb in the regulation of intestinal lipid absorption.

Germline mutations in oncogene-induced senescence pathways are associated with multiple sessile serrated adenomas

BACKGROUND & AIMS

Little is known about the genetic factors that contribute to the development of sessile serrated adenomas (SSAs). SSAs contain somatic mutations in BRAF or KRAS early in development. However, evidence from humans and mouse models indicates that these mutations result in oncogene-induced senescence (OIS) of intestinal crypt cells. Progression to serrated neoplasia requires cells to escape OIS via inactivation of tumor suppressor pathways. We investigated whether subjects with multiple SSAs carry germline loss-of function mutations (nonsense and splice site) in genes that regulate OIS: the p16-Rb and ATM-ATR DNA damage response pathways.

METHODS

Through a bioinformatic analysis of the literature, we identified a set of genes that function at the main nodes of the p16-Rb and ATM-ATR DNA damage response pathways. We performed whole-exome sequencing of 20 unrelated subjects with multiple SSAs; most had features of serrated polyposis. We compared sequences with those from 4300 subjects matched for ethnicity (controls). We also used an integrative genomics approach to identify additional genes involved in senescence mechanisms.

RESULTS

We identified mutations in genes that regulate senescence (ATM, PIF1, TELO2,XAF1, and RBL1) in 5 of 20 subjects with multiple SSAs (odds ratio, 3.0; 95% confidence interval, 0.9–8.9; P =.04). In 2 subjects,we found nonsense mutations in RNF43, indicating that it is also associated with multiple serrated polyps (odds ratio, 460; 95% confidence interval, 23.1–16,384; P = 6.8 x 10(-5)). In knockdown experiments with pancreatic duct cells exposed to UV light, RNF43 appeared to function as a regulator of ATMATRDNA damage response.

CONCLUSIONS

We associated germline loss-of-function variants in genes that regulate senescence pathways with the development of multiple SSAs.We identified RNF43 as a regulator of the DNA damage response and associated nonsense variants in this gene with a high risk of developing SSAs.

Targeting nuclear kinases in cancer: development of cell cycle kinase inhibitors

Cellular proliferation is a tightly controlled set of events that is regulated by numerous nuclear protein kinases. The proteins involved include checkpoint kinases (CHK), cyclin-dependent kinases (CDK), which regulate the cell cycle and aurora kinases (AURK) and polo-like kinases (PLK), which regulate mitosis. In cancer, these nuclear kinases are often dysregulated and cause uncontrolled cell proliferation and growth. Much work has gone into developing novel therapeutics that target each of these protein kinases in cancer but none have been approved in patients. In this review we provide an overview of the current compounds being developed clinically to target these nuclear kinases involved in regulating the cell cycle and mitosis.

Tumor suppressor gene Rb is required for self-renewal of spermatogonial stem cells in mice

The retinoblastoma tumor suppressor gene Rb is essential for maintaining the quiescence and for regulating the differentiation of somatic stem cells. Inactivation of Rb in somatic stem cells typically leads to their overexpansion, often followed by increased apoptosis, defective terminal differentiation, and tumor formation. However, Rb's roles in germ-line stem cells have not been explored. We conditionally disrupted the Rb gene in mouse germ cells in vivo and discovered unanticipated consequences for GFRa1-protein-expressing A(single) (GFRa1(+) A(s)) spermatogonia, the major source of male germ-line stem cells. Rb-deficient GFRa1(+) A(s) spermatogonia were present at normal density in testes 5 d after birth, but they lacked the capacity for self-renewal, resulting in germ cell depletion by 2 mo of age. Rb deficiency did not affect the proliferative activity of GFRa1(+) A(s) spermatogonia, but their progeny were exclusively transit-amplifying progenitor spermatogonia and did not include GFRa1(+) A(s) spermatogonia. In addition, Rb deficiency caused prolonged proliferation of progenitor spermatogonia, transiently enlarging this population. Despite these defects, Rb deficiency did not block terminal differentiation into functional sperm; offspring were readily obtained from young males whose germ cell pool was not yet depleted. We conclude that Rb is required for self-renewal of germ-line stem cells, but contrary to its critical roles in somatic stem cells, it is dispensable for their proliferative activity and terminal differentiation. Thus, this study identifies an unexpected function for Rb in maintaining the stem cell pool in the male germ line.

ATP-noncompetitive CDK inhibitors for cancer therapy: an overview

INTRODUCTION

Cyclin-dependent kinases (CDKs) are the key drivers of cell cycle progression and are often deregulated in cancer, therefore, targeting CDKs has long been pursued as a therapeutic strategy to tackle cancer. Unfortunately, however, none of the first-generation CDK inhibitors has yielded the expected efficacy to be successfully translated to the clinic mostly because, by targeting the very conserved kinase ATP-binding site resulted to be poorly specific and quite toxic.

AREAS COVERED

Here, the authors review recent approaches aimed at developing more specific CDK inhibitors mostly through the aid of computational drug design studies and report various small molecules and peptides, which resulted in promising CDK ATP-noncompetitive inhibitors.

EXPERT OPINION

Despite few successes, these new approaches still need additional considerations to generate effective antitumoral agents. The authors discuss some of the hurdles to overcome for a successful clinical translation.

IGF-2 mediates intestinal mucosal hyperplasia in retinoblastoma protein (Rb)-deficient mice

PURPOSE

We have previously demonstrated a hyperplastic phenotype when Rb expression was disrupted within the intestinal epithelium. These findings mimic resection-induced adaptation suggesting a possible mechanistic role for Rb during adaptation. The purpose of the present study was to elucidate a mechanism for how Rb deficiency induces intestinal hyperplasia.

METHODS

Enterocytes isolated from intestine-specific Rb knockout mice (Rb-IKO) underwent a microarray to elucidate their gene expression profile. IGF2 expression was significantly elevated, which was subsequently confirmed by RT-PCR and in situ mRNA hybridization. Mice with deficient expression of IGF2 or its receptor IGF1R were therefore crossed with Rb-IKO mice to determine the significance of IGF2 in mediating the Rb-IKO intestinal phenotype.

RESULTS

Expression of IGF2 was significantly elevated in villus enterocytes of Rb-IKO mice. The mucosal hyperplasia in Rb-IKO mice was reversed when either IGF2 or IGF1R expression was genetically disrupted in Rb-IKO mice.

CONCLUSION

IGF-2 expression is significantly elevated in villus enterocytes and is required for the hyperplastic intestinal mucosal phenotype of Rb-IKO mice. The trophic effects of IGF2 require intact IGF1R signaling within the intestinal epithelium. These findings reveal novel regulatory roles for Rb in expanding intestinal mucosal surface area.

Silencing of RB1 but not of RB2/P130 induces cellular senescence and impairs the differentiation potential of human mesenchymal stem cells

Stem cell senescence is considered deleterious because it may impair tissue renewal and function. On the other hand, senescence may arrest the uncontrolled growth of transformed stem cells and protect organisms from cancer. This double function of senescence is strictly linked to the activity of genes that the control cell cycle such as the retinoblastoma proteins RB1, RB2/P130, and P107. We took advantage of the RNA interference technique to analyze the role of these proteins in the biology of mesenchymal stem cells (MSC). Cells lacking RB1 were prone to DNA damage. They showed elevated levels of p53 and p21(cip1) and increased regulation of RB2/P130 and P107 expression. These cells gradually adopted a senescent phenotype with impairment of self-renewal properties. No significant modification of cell growth was observed as it occurs in other cell types or systems. In cells with silenced RB2/P130, we detected a reduction of DNA damage along with a higher proliferation rate, an increase in clonogenic ability, and the diminution of apoptosis and senescence. Cells with silenced RB2/P130 were cultivated for extended periods of time without adopting a transformed phenotype. Of note, acute lowering of P107 did not induce relevant changes in the in vitro behavior of MSC. We also analyzed cell commitment and the osteo-chondro-adipogenic differentiation process of clones derived by MSC cultures. In all clones obtained from cells with silenced retinoblastoma genes, we observed a reduction in the ability to differentiate compared with the control clones. In summary, our data show evidence that the silencing of the expression of RB1 or RB2/P130 is not compensated by other gene family members, and this profoundly affects MSC functions.

Large T antigens of polyomaviruses: amazing molecular machines

The large tumor antigen (T antigen) encoded by simian virus 40 is an amazing molecular machine because it orchestrates viral infection by modulating multiple fundamental viral and cellular processes. T antigen is required for viral DNA replication, transcription, and virion assembly. In addition, T antigen targets multiple cellular pathways, including those that regulate cell proliferation, cell death, and the inflammatory response. Ectopic T antigen expression results in the immortalization and transformation of many cell types in culture and T antigen induces neoplasia when expressed in rodents. The analysis of the mechanisms by which T antigen carries out its many functions has proved to be a powerful way of gaining insights into cell biology. The accelerating pace at which new polyomaviruses are being discovered provides a collection of novel T antigens that, like simian virus 40, can be used to discover and study key cellular regulatory systems.

Competition between NBS1 and ATMIN controls ATM signaling pathway choice

Ataxia telangiectasia mutated (ATM) protein kinase activation by DNA double-strand breaks (DSBs) requires the Mre11-Rad50-NBS1 (MRN) complex, whereas ATM interactor (ATMIN) protein is required for ATM signaling induced by changes in chromatin structure. We show here that NBS1 and ATMIN proteins compete for ATM binding and that this mechanism controls ATM function. DSB-induced ATM substrate phosphorylation was increased in atmin mutant cells. Conversely, NBS1 deficiency resulted in increased ATMIN-dependent ATM signaling. Thus, the absence of one cofactor increased flux through the alternative pathway. Notably, ATMIN deficiency rescued the cellular lethality of NBS1-deficient cells, and NBS1/ATMIN double deficiency resulted in complete abrogation of ATM signaling and profound radiosensitivity. Hence, ATMIN and NBS1 mediate all ATM signaling by DSBs, and increased ATMIN-dependent ATM signaling explains the different phenotypes of nbs1- and atm-mutant cells. Thus, the antagonism and redundancy of ATMIN and NBS1 constitute a crucial regulatory mechanism for ATM signaling and function.

The retinoblastoma tumor suppressor and stem cell biology

Stem cells play a critical role during embryonic development and in the maintenance of homeostasis in adult individuals. A better understanding of stem cell biology, including embryonic and adult stem cells, will allow the scientific community to better comprehend a number of pathologies and possibly design novel approaches to treat patients with a variety of diseases. The retinoblastoma tumor suppressor RB controls the proliferation, differentiation, and survival of cells, and accumulating evidence points to a central role for RB activity in the biology of stem and progenitor cells. In some contexts, loss of RB function in stem or progenitor cells is a key event in the initiation of cancer and determines the subtype of cancer arising from these pluripotent cells by altering their fate. In other cases, RB inactivation is often not sufficient to initiate cancer but may still lead to some stem cell expansion, raising the possibility that strategies aimed at transiently inactivating RB might provide a novel way to expand functional stem cell populations. Future experiments dedicated to better understanding how RB and the RB pathway control a stem cell's decisions to divide, self-renew, or give rise to differentiated progeny may eventually increase our capacity to control these decisions to enhance regeneration or help prevent cancer development.

Regulation of retinoblastoma protein (Rb) by p21 is critical for adaptation to massive small bowel resection

BACKGROUND

Adaptation following massive intestinal loss is characterized by increased villus height and crypt depth. Previously, we demonstrated that p21-null mice do not adapt after small bowel resection (SBR). As retinoblastoma protein (Rb) levels are elevated in p21-null crypt cells, we first sought to determine whether Rb is required for normal adaptation. Next, we tested whether Rb expression is responsible for blocked adaptation in p21-nulls.

METHODS

Genetically manipulated mice and wild-type (WT) littermates underwent either 50% SBR or sham operation. The intestine was harvested at 3, 7, or 28 days later and intestinal adaptation was evaluated. Enterocytes were isolated and protein levels evaluated by Western blot and quantified by optical density.

RESULTS

Rb-null mice demonstrated increased villus height, crypt depth, and proliferative rate at baseline, but there was no further increase following SBR. Deletion of one Rb allele lowered Rb expression and restored resection-induced adaptation responses in p21-null mice.

CONCLUSION

Rb is specifically required for resection-induced adaptation. Restoration of adaptation in p21-null mice by lowering Rb expression suggests a crucial mechanistic role for Rb in the regulation of intestinal adaptation by p21.

What's so special about RB?

RB, p107, and p130 are highly related proteins, each capable of regulating cellular proliferation. However, only RB is frequently mutated in cancer. In this issue of Cancer Cell, Chicas et al. shed new light on this conundrum, defining a "special," nonredundant role for RB in promoting cellular senescence.

Modelling Barrett's oesophagus

Barrett's oesophagus is the replacement of normal squamous oesophageal epithelium with an intestinalized columnar epithelium. Although some insight has been gained as to what Barrett's oesophagus is, how this columnar epithelium emerges from within a stratified squamous epithelium remains an unanswered question. We have sought to determine whether oesophageal keratinocytes can be trans-differentiated into Barrett's oesophagus cells. Using an Affymetrix microarray, we found unexpectedly that gene-expression patterns in the Barrett's oesophagus were only slightly more similar to the normal small intestine than they were to the normal oesophagus. Thus gene-expression patterns suggest significant molecular similarities remain between Barrett's oesophagus cells and normal squamous oesophageal epithelium, despite their histological resemblance with intestine. We next determined whether directed expression of intestine-specific transcription factors could induce intestinalization of keratinocytes. Retroviral-mediated Cdx2 (Caudal-type homeobox 2) expression in immortalized human oesophageal keratinocytes engineered with human telomerase reverse transcriptase (EPC2-hTERT cells) could be established transiently, but not maintained, and was associated with a reduction in cell proliferation. Co-expression of cyclin D1 rescued proliferation in the Cdx2-expressing cells, but co-expression of dominant-negative p53 did not. Cdx2 expression in the EPC2-hTERT.D1 cells did not induce intestinalization. However, when combined with treatments that induce chromatin remodelling, there was a significant induction of Barrett's oesophagus-associated genes. Studies are ongoing to determine whether other intestinal transcription factors, either alone or in combination, can provoke greater intestinalization of oesophageal keratinocytes. We conclude that, on the basis of gene-expression patterns, Barrett's oesophagus epithelial cells may represent an intermediate between oesophageal keratinocytes and intestinal epithelial cells. Moreover, our findings suggest that it may be possible to induce Barrett's oesophagus epithelial cells from oesophageal keratinocytes by altering the expression of certain critical genes.

E2f1-3 switch from activators in progenitor cells to repressors in differentiating cells

In the classic paradigm of mammalian cell cycle control, Rb functions to restrict cells from entering S phase by sequestering E2F activators (E2f1, E2f2 and E2f3), which are invariably portrayed as the ultimate effectors of a transcriptional program that commit cells to enter and progress through S phase1, 2. Using a panel of tissue-specific cre-transgenic mice and conditional E2f alleles we examine the effects of E2f1, E2f2 and E2f3 triple deficiency in murine ES cells, embryos and small intestines. We show that in normal dividing progenitor cells E2F1-3 function as transcriptional activators, but contrary to current dogma, are dispensable for cell division and instead are necessary for cell survival. In differentiating cells they function in complex with Rb as repressors to silence E2F targets and facilitate exit from the cell cycle. The inactivation of Rb in differentiating cells resulted in a switch of E2F1-3 from repressors to activators, leading to the superactivation of E2F responsive targets and ectopic cell divisions, and loss of E2f1-3 completely suppressed these phenotypes. This work contextualizes the activator versus repressor functions of E2F1-3 in vivo, revealing distinct roles in dividing versus differentiating cells and in normal versus cancer-like cell cycles in vivo.

The retinoblastoma protein and its homolog p130 regulate the G1/S transition in pancreatic beta-cells

OBJECTIVE

The retinoblastoma protein family (pRb, p130, p107) plays a central role in the regulation of cell cycle progression. Surprisingly, loss of pRb in the beta-cell has no discernible effect on cell cycle control. Therefore, we explored the effects of individual loss of either p130 or p107 in addition to the simultaneous loss of both pRb/p130 on the beta-cell.

RESEARCH DESIGN AND METHODS

Adult mice deficient in either p130 or p107 or both pRb/p130 were examined for effects on beta-cell replication, function, and survival. The Cre-Lox system was also used to inactivate pRb in wild-type and p130-deficient beta-cells in vitro.

RESULTS

In vivo loss of either p107 or p130 did not affect beta-cell replication or function. Combined pRb/p130 loss, however, resulted in dramatically accelerated proliferation as well as apoptotic cell death. Pancreas and beta-cell mass were significantly reduced in double mutants. Despite this, overall glucose tolerance was normal, except for mild postprandial hyperglycemia. Ex vivo, acute deletion of pRb in p130-deficient beta-cells also caused a striking increase in proliferation. The combined deletion of pRb/p130 upregulated islet expression of E2F2 but not E2F1.

CONCLUSIONS

These studies define an essential role for the pocket proteins in controlling the G(1)/S transition in beta-cells. When deficient in both pRb and p130, beta-cells undergo unrestrained cell cycle reentry and activation of apoptosis. These studies underscore the central role of the pRb pathway in controlling beta-cell turnover and provide new cellular targets for beta-cell regeneration.

Simian virus 40 T-antigen-mediated gene regulation in enterocytes is controlled primarily by the Rb-E2F pathway

Simian virus 40 large T antigen (TAg) contributes to cell transformation, in part, by targeting two well-characterized tumor suppressors, pRb and p53. TAg expression affects the transcriptional circuits controlled by Rb and by p53. We have performed a microarray analysis to examine the global change in gene expression induced by wild-type TAg (TAg(wt)) and TAg mutants, in an effort to link changes in gene expression to specific transforming functions. For this analysis we have used enterocytes from the mouse small intestine expressing TAg. Expression of TAg(wt) in the mouse intestine results in hyperplasia and dysplasia. Our analysis indicates that practically all gene expression regulated by TAg in enterocytes is dependent upon its binding and inactivation of the Rb family proteins. To further dissect the role of the Rb family in the induction of intestinal hyperplasia, we have screened several lines of transgenic mice expressing a truncated TAg (TAg(N136)), which is able to interfere with the Rb pathway but lacks the functions associated with the carboxy terminus of the protein. This analysis confirmed the pivotal association between the Rb pathway and the induction of intestinal hyperplasia and revealed that upregulation of p53 target genes is not associated with the tumorigenic phenotype. Furthermore, we found that TAg(N136) was sufficient to induce intestinal hyperplasia, although the appearance of dysplasia was significantly delayed.

E2F4 cooperates with pRB in the development of extra-embryonic tissues

The retinoblastoma gene, RB-1, was the first identified tumor suppressor. Rb(-/-) mice die in mid-gestation with defects in proliferation, differentiation and apoptosis. The activating E2F transcription factors, E2F1-3, contribute to these embryonic defects, indicating that they are key downstream targets of the retinoblastoma protein, pRB. E2F4 is the major pRB-associated E2F in vivo, yet its role in Rb(-/-) embryos is unknown. Here we establish that E2f4 deficiency reduced the lifespan of Rb(-/-) embryos by exacerbating the Rb mutant placental defect. We further show that this reflects the accumulation of trophectoderm-like cells in both Rb and Rb;E2f4 mutant placentas. Thus, Rb and E2f4 play cooperative roles in placental development. We used a conditional mouse model to allow Rb(-/-);E2f4(-/-) embryos to develop in the presence of Rb wild-type placentas. Under these conditions, Rb(-/-);E2f4(-/-) mutants survived to birth. These Rb(-/-);E2f4(-/-) embryos exhibited all of the defects characteristic of the Rb and E2f4 single mutants and had no novel defects. Taken together, our data show that pRB and E2F4 cooperate in placental development, but play largely non-overlapping roles in the development of many embryonic tissues.

Ets transcription factors control epithelial maturation and transit and crypt-villus morphogenesis in the mammalian intestine

Members of the Ets transcription factor family are widely expressed in both the developing and mature mammalian intestine, but their biological functions remain primarily uncharacterized. We used a dominant repressor transgene approach to probe the function of epithelial Ets factors in the homeostasis of the crypt-villus unit, the functional unit of the small intestine. We show that targeted expression in small intestinal epithelium of a fusion protein composed of the Engrailed repressor domain and the Erm DNA-binding domain (En/Erm) results in marked disruption of normal crypt-villus homeostasis, including a cell-autonomous disturbance of epithelial maturation, increased epithelial transit, severe villus dysmorphogenesis, and crypt dysmorphogenesis. The epithelial maturation disturbance is independent of the regulation of TGFbetaRII levels, in contrast to Ets-mediated epithelial differentiation during development; rather, regulation of Cdx2 expression may play a role. The villus dysmorphogenesis is independent of alterations in the crypt-villus boundary and inappropriate beta-catenin activation, and thus appears to represent a new mechanism controlling villus architectural organization. An Analysis of animals mosaic for En/Erm expression suggests that crypt nonautonomous mechanisms underlie the crypt dysmorphogenesis phenotype. Our studies thus uncover novel Ets-regulated pathways of intestinal homeostasis in vivo. Interestingly, the overall En/Erm phenotype of disturbed crypt-villus homeostasis is consistent with recently identified Ets function(s) in the restriction of intestinal epithelial tumorigenesis.

Proapoptotic function of the retinoblastoma tumor suppressor protein

The retinoblastoma protein (pRB) tumor suppressor blocks cell proliferation by repressing the E2F transcription factors. This inhibition is relieved through mitogen-induced phosphorylation of pRB, triggering E2F release and activation of cell-cycle genes. E2F1 can also activate proapoptotic genes in response to genotoxic or oncogenic stress. However, pRB's role in this context has not been established. Here we show that DNA damage and E1A-induced oncogenic stress promote formation of a pRB-E2F1 complex even in proliferating cells. Moreover, pRB is bound to proapoptotic promoters that are transcriptionally active, and pRB is required for maximal apoptotic response in vitro and in vivo. Together, these data reveal a direct role for pRB in the induction of apoptosis in response to genotoxic or oncogenic stress.

T antigen transgenic mouse models

The study of polyomavirus has benefited immensely from two scientific methodologies, cell culture and in vitro studies on one side and the use of transgenic mice as experimental models on the other. Both approaches allowed us to identify cellular products targeted by the viruses, the consequences of these interactions at the phenotypic and molecular level, and thus the potential roles of the targets within their normal cellular context. In particular, cell culture and in vitro reports suggest a model explaining partially how SV40 large T antigen contributes to oncogenic transformation. In most cases, T antigen induces cell cycle entry by inactivation of the Rb proteins (pRb, p130, and p107), thus activating E2F-dependent transcription and subsequent S-phase entry. Simultaneously, T antigen blocks p53 activity and therefore prevents the ensuing cell-cycle arrest and apoptosis. For the most part, studies of T antigen expression in transgenic mice support this model, but the use of T antigen mutants and their expression in different tissue and cell type settings have expanded our knowledge of the model system and raised important questions regarding tumorigenic mechanisms functioning in vivo.

Induction of intestinalization in human esophageal keratinocytes is a multistep process

Barrett's esophagus (BE) is the replacement of normal squamous esophageal mucosa with an intestinalized columnar epithelium. The molecular mechanisms underlying its development are not understood. Cdx2 is an intestine-specific transcription factor that is ectopically expressed in BE, but its role in this process is unclear. Herein, we describe a novel cell culture model for BE. Retroviral-mediated Cdx2 expression in immortalized human esophageal keratinocytes [EPC-human telomerase reverse transcriptase (hTERT)] could transiently be established but not maintained and was associated with a reduction in cell proliferation. Coexpression of cyclin D1, but not a dominant-negative p53, rescued proliferation in the Cdx2-expressing cells. Cdx2 expression in the EPC-hTERT.D1 cells decreased cell proliferation but did not induce intestinalization. We investigated for other treatments to enhance intestinalization and found that acidic culture conditions uniformly killed EPC-hTERT.D1.Cdx2 cells. However, treatment with 5-aza-2-deoxycytidine (5-AzaC) to demethylate epigenetically silenced genes did appear to be tolerated. Multiple Cdx2 target genes, markers of intestinal differentiation and markers of BE, were induced by this 5-AzaC treatment. More interestingly, the expression level of several of these genes was enhanced only in the EPC-hTERT.D1-Cdx2 cells treated with 5-AzaC. Two of these, SLC26a3/DRA (downregulated in adenoma) and Na+/H+ exchanger 2 (NHE2), were not previously known to be elevated in BE; however, we confirmed their elevation in BE tissue samples. 5-AzaC treatment also induced cell senescence, even at low doses. We conclude that ectopic proliferation signals, alterations in epigenetic gene regulation and the inhibition of tumor suppressor mechanisms are required for Cdx2-mediated intestinalization of human esophageal keratinocytes in BE.

The retinoblastoma protein tumor suppressor is important for appropriate osteoblast differentiation and bone development

Mutation of the retinoblastoma (RB) tumor suppressor gene is strongly linked to osteosarcoma formation. This observation and the documented interaction between the retinoblastoma protein (pRb) and Runx2 suggests that pRb is important in bone development. To assess this hypothesis, we used a conditional knockout strategy to generate pRb-deficient embryos that survive to birth. Analysis of these embryos shows that Rb inactivation causes the abnormal development and impaired ossification of several bones, correlating with an impairment in osteoblast differentiation. We further show that Rb inactivation acts to promote osteoblast differentiation in vitro and, through conditional analysis, establish that this occurs in a cell-intrinsic manner. Although these in vivo and in vitro differentiation phenotypes seem paradoxical, we find that Rb-deficient osteoblasts have an impaired ability to exit the cell cycle both in vivo and in vitro that can explain the observed differentiation defects. Consistent with this observation, we show that the cell cycle and the bone defects in Rb-deficient embryos can be suppressed by deletion of E2f1, a known proliferation inducer that acts downstream of Rb. Thus, we conclude that pRb plays a key role in regulating osteoblast differentiation by mediating the inhibition of E2F and consequently promoting cell cycle exit.

Retinoblastoma protein (pRb), but not p107 or p130, is required for maintenance of enterocyte quiescence and differentiation in small intestine

The function of retinoblastoma protein (pRb) in the regulation of small intestine epithelial cell homeostasis has been challenged by several groups using various promoter-based Cre transgenic mouse lines. Interestingly, different pRb deletion systems yield dramatically disparate small intestinal phenotypes. These findings confound the function of pRb in this dynamic tissue. In this study, Villin-Cre transgenic mice were crossed with Rb (flox/flox) mice to conditionally delete pRb protein in small intestine enterocytes. We discovered a novel hyperplasia phenotype as well as ectopic cell cycle reentry within villus enterocytes in the small intestine. This phenotype was not seen in other pRb family member (p107 or p130) null mice. Using a newly developed crypt/villus isolation method, we uncovered that expression of pRb was undetectable, whereas proliferating cell nuclear antigen, p107, cyclin E, cyclin D3, Cdk2, and Cdc2 were dramatically increased in pRb-deficient villus cells. Cyclin A, cyclin D1, cyclin D2, and Cdk4/6 expression was not affected by absent pRb expression. pRb-deficient villus cells appeared capable of progressing to mitosis but with higher rates of apoptosis. However, the cycling villus enterocytes were not completely differentiated as gauged by significant reduction of intestinal fatty acid-binding protein expression. In summary, pRb, but not p107 or p130, is required for maintaining the postmitotic villus cell in quiescence, governing the expression of cell cycle regulatory proteins, and completing of absorptive enterocyte differentiation in the small intestine.

Loss of nucleoplasmic LAP2alpha-lamin A complexes causes erythroid and epidermal progenitor hyperproliferation

Lamina-associated polypeptide (LAP) 2alpha is a chromatin-associated protein that binds A-type lamins. Mutations in both LAP2alpha and A-type lamins are linked to human diseases called laminopathies, but the molecular mechanisms are poorly understood. The A-type lamin-LAP2alpha complex interacts with and regulates retinoblastoma protein (pRb), but the significance of this interaction in vivo is unknown. Here we address the function of the A-type lamin-LAP2alpha complex with the use of LAP2alpha-deficient mice. We show that LAP2alpha loss causes relocalization of nucleoplasmic A-type lamins to the nuclear envelope and impairs pRb function. This causes inefficient cell-cycle arrest in dense fibroblast cultures and hyperproliferation of epidermal and erythroid progenitor cells in vivo, leading to tissue hyperplasia. Our results support a disease-relevant model in which LAP2alpha defines A-type lamin localization in the nucleoplasm, which in turn affects pRb-mediated regulation of progenitor cell proliferation and differentiation in highly regenerative tissues.

Loss of Rb1 in the gastrointestinal tract of Apc1638N mice promotes tumors of the cecum and proximal colon

To examine the role of Rb1 in gastrointestinal (GI) tumors, we generated mice with an Apc(1638N) allele, Rb(tm2brn) floxed alleles, and a villin-cre transgene (RBVCA). These animals had exon 19 deleted from Rb1 throughout the GI tract. We have shown previously that Rb1 deficiency is insufficient for GI tumor initiation, with inactivation of an Apc allele capable of overcoming the insufficiency. In this study we demonstrate that RBVCA mice have reduced median survival because of an increase in tumor incidence and multiplicity in the cecum and the proximal colon. Large intestinal tumors are predominantly adenomas, whereas the tumors of the small intestine are a mixture of adenomas and adenocarcinomas. We find truncation mutations to the second Apc allele in tumors of both the large and small intestine. Expression profiles of duodenal and cecal tumors relative to each other show unique gene subsets up and down regulated. Substantial expression patterns compare to human colorectal cancer, including recapitulation of embryonic genes. Our results indicate that Rb1 has significant influence over tumor location in the GI tract, and that both cecal and duodenal tumors initiate through inactivation of Apc. Expression profile analysis indicates the two tumor types differentially regulate distinct sets of genes that are over-expressed in a majority of human colorectal carcinomas.

E2F1 represses beta-catenin transcription and is antagonized by both pRB and CDK8

The E2F1 transcription factor can promote proliferation or apoptosis when activated, and is a key downstream target of the retinoblastoma tumour suppressor protein (pRB). Here we show that E2F1 is a potent and specific inhibitor of beta-catenin/T-cell factor (TCF)-dependent transcription, and that this function contributes to E2F1-induced apoptosis. E2F1 deregulation suppresses beta-catenin activity in an adenomatous polyposis coli (APC)/glycogen synthase kinase-3 (GSK3)-independent manner, reducing the expression of key beta-catenin targets including c-MYC. This interaction explains why colorectal tumours, which depend on beta-catenin transcription for their abnormal proliferation, keep RB1 intact. Remarkably, E2F1 activity is also repressed by cyclin-dependent kinase-8 (CDK8), a colorectal oncoprotein. Elevated levels of CDK8 protect beta-catenin/TCF-dependent transcription from inhibition by E2F1. Thus, by retaining RB1 and amplifying CDK8, colorectal tumour cells select conditions that collectively suppress E2F1 and enhance the activity of beta-catenin.

Retinoblastoma function is essential for establishing lung epithelial quiescence after injury

The retinoblastoma gene product (RB) regulates cell cycle, quiescence, and survival in a cell type-dependent and environment-dependent manner. RB function is critical in the pulmonary epithelium, as evidenced by nearly universal RB inactivation in lung cancer and increased lung cancer risk in persons with germline RB gene mutations. Lung carcinomas occur in the context of epithelial remodeling induced by cytotoxic damage. Whereas the role of RB in development and normal organ homeostasis has been extensively studied, RB function in the context of cellular injury and repair has remained largely unexplored. In the current studies, the RB gene was selectively deleted in the respiratory epithelium of the mouse. Although RB was not required for establishing or maintaining quiescence during lung homeostasis, RB was essential for establishing quiescence during epithelial repair after injury. Notably, aberrant cell cycle progression was sustained for 9 months after injury in RB-deficient lungs. Prenatal and postnatal RB ablation had similar effects, providing evidence that timing of RB loss was not critical to the outcome and that the injury-induced phenotype was not secondary to compensatory alterations occurring during development. These data show that RB is essential for repair of the respiratory epithelium after cytotoxic damage and support a critical unique role for RB in the context of epithelial remodeling after injury. Because human cancers are associated with chronic cellular damage, these findings have important new implications for RB-mediated tumor suppression.

GFP reporter mice for the retinoblastoma-related cell cycle regulator p107

The RB tumor suppressor gene is mutated in a broad range of human cancers, including pediatric retinoblastoma. Strikingly, however, Rb mutant mice develop tumors of the pituitary and thyroid glands, but not retinoblastoma. Mouse genetics experiments have demonstrated that p107, a protein related to pRB, is capable of preventing retinoblastoma, but not pituitary tumors, in Rb-deficient mice. Evidence suggests that the basis for this compensatory function of p107 is increased transcription of the p107 gene in response to Rb inactivation. To begin to address the context-dependency of this compensatory role of p107 and to follow p107 expression in vivo, we have generated transgenic mice carrying an enhanced GFP (eGFP) reporter inserted into a bacterial artificial chromosome (BAC) containing the mouse p107 gene. Expression of the eGFP transgene parallels that of p107 in these transgenic mice and identifies cells with a broad range of expression level for p107, even within particular organs or tissues. We also show that loss of Rb results in the upregulation of p107 transcription in specific cell populations in vivo, including subpopulations of hematopoietic cells. Thus, p107 BAC-eGFP transgenic mice serve as a useful tool to identify distinct cell types in which p107 is expressed and may have key functions in vivo, and to characterize changes in cellular networks accompanying Rb deficiency.

Coordinated regulation of intestinal functions in C. elegans by LIN-35/Rb and SLR-2

LIN-35 is the sole C. elegans representative of the pocket protein family, which includes the mammalian Retinoblastoma protein pRb and its paralogs p107 and p130. In addition to having a well-established and central role in cell cycle regulation, pocket proteins have been increasingly implicated in the control of critical and diverse developmental and cellular processes. To gain a greater understanding of the roles of pocket proteins during development, we have characterized a synthetic genetic interaction between lin-35 and slr-2, which we show encodes a C2H2-type Zn-finger protein. Whereas animals harboring single mutations in lin-35 or slr-2 are viable and fertile, lin-35; slr-2 double mutants arrest uniformly in early larval development without obvious morphological defects. Using a combination of approaches including transcriptome profiling, mosaic analysis, starvation assays, and expression analysis, we demonstrate that both LIN-35 and SLR-2 act in the intestine to regulate the expression of many genes required for normal nutrient utilization. These findings represent a novel role for pRb family members in the maintenance of organ function. Our studies also shed light on the mechanistic basis of genetic redundancy among transcriptional regulators and suggest that synthetic interactions may result from the synergistic misregulation of one or more common targets.

Cdx1, a dispensable homeobox gene for gut development with limited effect in intestinal cancer

The homeobox gene Cdx1 is involved in anteroposterior patterning in embryos and its expression selectively persists in the intestinal epithelium throughout life. In human colon cancers, Cdx1 is overexpressed in few cases and lost in the majority of adenocarcinomas. We used mouse models of gain and loss-of-function to investigate the role of Cdx1 in intestinal development and cancers. Transgenic mice overexpressing Cdx1 and knockout mice exhibited a morphologically normal intestine. Cell proliferation, specification into the four differentiated lineages and migration along the crypt-villus axis were unchanged compared to wild-type mice. Changing Cdx1 caused an inverse and dose-dependent modification of the expression of the paralogous gene Cdx2, indicating that Cdx1 fine-tunes Cdx2 activity. Transgenenic and knockout mice failed to spontaneously develop tumours. Overexpressing Cdx1 was without incidence on the frequency of intestinal tumours induced chemically by azoxymethane treatment or genetically in Apc(Delta14/+) mice. However, it augmented the severity of the tumours in Apc(Delta14/+) mice. Inversely, the loss-of-function of Cdx1 in knockout mice was without incidence on the growth of tumours induced by azoxymethane. We conclude that Cdx1 is dispensable for intestinal development and that its overexpression could increase malignancy in early stages of tumourigenesis.

Differential effects of oncogenic K-Ras and N-Ras on proliferation, differentiation and tumor progression in the colon

Kras is commonly mutated in colon cancers, but mutations in Nras are rare. We have used genetically engineered mice to determine whether and how these related oncogenes regulate homeostasis and tumorigenesis in the colon. Expression of K-Ras(G12D) in the colonic epithelium stimulated hyperproliferation in a Mek-dependent manner. N-Ras(G12D) did not alter the growth properties of the epithelium, but was able to confer resistance to apoptosis. In the context of an Apc-mutant colonic tumor, activation of K-Ras led to defects in terminal differentiation and expansion of putative stem cells within the tumor epithelium. This K-Ras tumor phenotype was associated with attenuated signaling through the MAPK pathway, and human colon cancer cells expressing mutant K-Ras were hypersensitive to inhibition of Raf, but not Mek. These studies demonstrate clear phenotypic differences between mutant Kras and Nras, and suggest that the oncogenic phenotype of mutant K-Ras might be mediated by noncanonical signaling through Ras effector pathways.

Casein kinase II motif-dependent phosphorylation of human papillomavirus E7 protein promotes p130 degradation and S-phase induction in differentiated human keratinocytes

The E7 proteins of human papillomaviruses (HPVs) promote S-phase reentry in differentiated keratinocytes of the squamous epithelia to support viral DNA amplification. In this study, we showed that nuclear p130 was present in the differentiated strata of several native squamous epithelia susceptible to HPV infection. In contrast, p130 was below the level of detection in HPV-infected patient specimens. In submerged and organotypic cultures of primary human keratinocytes, the E7 proteins of the high-risk mucosotrophic HPV-18, the benign cutaneous HPV-1, and, to a lesser extent, the low-risk mucosotropic HPV-11 destabilized p130. This E7 activity depends on an intact pocket protein binding domain and a casein kinase II (CKII) phosphorylation motif. Coimmunoprecipitation experiments showed that both E7 domains were important for binding to p130 in extracts of organotypic cultures. Metabolic labeling in vivo demonstrated that E7 proteins were indeed phosphorylated in a CKII motif-dependent manner. Moreover, the efficiencies of the E7 proteins of various HPV types or mutations to induce S-phase reentry in spinous cells correlated with their relative abilities to bind and to destabilize p130. Collectively, these data support the notion that p130 controls the homeostasis of the differentiated keratinocytes and is therefore targeted by E7 for degradation to establish conditions permissive for viral DNA amplification.

The homeodomain transcription factor Cdx1 does not behave as an oncogene in normal mouse intestine

The Caudal-related homeobox genes Cdx1 and Cdx2 are intestine-specific transcription factors that regulate differentiation of intestinal cell types. Previously, we have shown Cdx1 to be antiproliferative and to promote cell differentiation. However, other studies have suggested that Cdx1 may be an oncogene. To test for oncogenic behavior, we used the murine villin promoter to ectopically express Cdx1 in the small intestinal villi and colonic surface epithelium. No changes in intestinal architecture, cell differentiation, or lineage selection were observed with expression of the transgene. Classic oncogenes enhance proliferation and induce tumors when ectopically expressed. However, the Cdx1 transgene neither altered intestinal proliferation nor induced spontaneous intestinal tumors. In a murine model for colitis-associated cancer, the Cdx1 transgene decreased, rather than increased, the number of adenomas that developed. In the polyps, the expression of the endogenous and the transgenic Cdx1 proteins was largely absent, whereas endogenous Villin expression was retained. This suggests that transgene silencing was specific and not due to a general Villin inactivation. In conclusion, neither the ectopic expression of Cdx1 was associated with changes in intestinal cell proliferation or differentiation nor was there increased intestinal cancer susceptibility. Our results therefore suggest that Cdx1 is not an oncogene in normal intestinal epithelium.