pRB, a tumor suppressor with a stabilizing presence

Expanding Roles of the E2F-RB-p53 Pathway in Tumor Suppression

The transcription factor E2F links the RB pathway to the p53 pathway upon loss of function of pRB, thereby playing a pivotal role in the suppression of tumorigenesis. E2F fulfills a major role in cell proliferation by controlling a variety of growth-associated genes. The activity of E2F is controlled by the tumor suppressor pRB, which binds to E2F and actively suppresses target gene expression, thereby restraining cell proliferation. Signaling pathways originating from growth stimulative and growth suppressive signals converge on pRB (the RB pathway) to regulate E2F activity. In most cancers, the function of pRB is compromised by oncogenic mutations, and E2F activity is enhanced, thereby facilitating cell proliferation to promote tumorigenesis. Upon such events, E2F activates the Arf tumor suppressor gene, leading to activation of the tumor suppressor p53 to protect cells from tumorigenesis. ARF inactivates MDM2, which facilitates degradation of p53 through proteasome by ubiquitination (the p53 pathway). P53 suppresses tumorigenesis by inducing cellular senescence or apoptosis. Hence, in almost all cancers, the p53 pathway is also disabled. Here we will introduce the canonical functions of the RB-E2F-p53 pathway first and then the non-classical functions of each component, which may be relevant to cancer biology.

RB loss sensitizes cells to replication-associated DNA damage after PARP inhibition by trapping

The retinoblastoma tumor suppressor protein (RB) interacts physically and functionally with a number of epigenetic modifying enzymes to control transcriptional regulation, respond to replication stress, promote DNA damage response and repair, and regulate genome stability. To better understand how disruption of RB function impacts epigenetic regulation of genome stability and determine whether such changes represent exploitable weaknesses of RB-deficient cancer cells, we performed an imaging-based screen to identify epigenetic inhibitors that promote DNA damage and compromise the viability of RB-deficient cells. We found that loss of RB alone leads to high levels of replication-dependent poly-ADP ribosylation (PARylation) and that preventing PARylation by trapping PARP enzymes on chromatin enables RB-deficient cells to progress to mitosis with unresolved replication stress. These defects contribute to high levels of DNA damage and compromised cell viability. We demonstrate this sensitivity is conserved across a panel of drugs that target both PARP1 and PARP2 and can be suppressed by reexpression of the RB protein. Together, these data indicate that drugs that target PARP1 and PARP2 may be clinically relevant for RB-deficient cancers.

Osteosarcoma in Pediatric and Adult Populations: Are Adults Just Big Kids?

Malignant bone tumors are commonly classified as pediatric or adolescent malignancies, and clinical trials for these diseases have generally focused on these populations. Of primary bone cancers, osteosarcoma is among the most common. Osteosarcoma has a bimodal age distribution, with the first peak occurring in patients from 10 to 14 years old, and the second peak occurring in patients older than 65, with about 25% of cases occurring in adults between 20 and 59 years old. Notably, adult osteosarcoma patients have worse outcomes than their pediatric counterparts. It remains unclear whether age itself is a poor prognostic factor, or if inherent differences in tumor biology exist between age groups. Despite these unknowns, current treatment strategies for adults are largely extrapolated from pediatric studies since the majority of clinical trials for osteosarcoma treatments are based on younger patient populations. In light of the different prognoses observed in pediatric and adult osteosarcoma, we summarize the current understanding of the molecular etiology of osteosarcoma and how it may differ between age groups, hypothesizing why adult patients have worse outcomes compared to children.

RB loss sensitizes cells to replication-associated DNA damage by PARP inhibition

The retinoblastoma tumor suppressor protein (RB) interacts physically and functionally with a number of epigenetic modifying enzymes to control transcriptional regulation, respond to replication stress, promote DNA damage response and repair pathways, and regulate genome stability. To better understand how disruption of RB function impacts epigenetic regulation of genome stability and determine whether such changes may represent exploitable weaknesses of RB-deficient cancer cells, we performed an imaging-based screen to identify epigenetic inhibitors that promote DNA damage and compromise viability of RB-deficient cells. We found that loss of RB alone leads to high levels of replication-dependent poly-ADP ribosylation (PARylation) and that preventing PARylation through inhibition of PARP enzymes enables RB-deficient cells to progress to mitosis with unresolved replication stress and under-replicated DNA. These defects contribute to high levels of DNA damage, decreased proliferation, and compromised cell viability. We demonstrate this sensitivity is conserved across a panel of inhibitors that target both PARP1 and PARP2 and can be suppressed by re-expression of the RB protein. Together, these data indicate that inhibitors of PARP1 and PARP2 may be clinically relevant for RB-deficient cancers.

The spindle assembly checkpoint is a therapeutic vulnerability of CDK4/6 inhibitor-resistant ER+ breast cancer with mitotic aberrations

Inhibitors of cyclin-dependent kinases 4 and 6 (CDK4/6i) are standard first-line treatments for metastatic ER⁺ breast cancer. However, acquired resistance to CDK4/6i invariably develops, and the molecular phenotypes and exploitable vulnerabilities associated with resistance are not yet fully characterized. We developed a panel of CDK4/6i-resistant breast cancer cell lines and patient-derived organoids and demonstrate that a subset of resistant models accumulates mitotic segregation errors and micronuclei, displaying increased sensitivity to inhibitors of mitotic checkpoint regulators TTK and Aurora kinase A/B. RB1 loss, a well-recognized mechanism of CDK4/6i resistance, causes such mitotic defects and confers enhanced sensitivity to TTK inhibition. In these models, inhibition of TTK with CFI-402257 induces premature chromosome segregation, leading to excessive mitotic segregation errors, DNA damage, and cell death. These findings nominate the TTK inhibitor CFI-402257 as a therapeutic strategy for a defined subset of ER⁺ breast cancer patients who develop resistance to CDK4/6i.

Microarray analysis of genes with differential expression of m6A methylation in lung cancer

Purpose: N6-methyladenosine (m6A) is among the most abundant mRNA modifications in eukaryote. The aim of the present study was to investigate function of m6A mRNA methylation in lung cancer and the underlying mechanism.

Methods: Microarray analysis was performed to detect the differences in RNA expression between cancerous and adjacent non-cancerous tissue samples. The target mRNAs were subjected to Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses. Hierarchical clustering of RNAs was conducted to identify distinct m6A methylation or expression patterns between the samples.

Results: In the present study, some differentially expressed genes (DEGs) of mRNAs were identified, including up-regulated secret phosphoprotein 1 (SPP1) and down-regulated pRB. Functional enrichment analysis revealed that while differential hypermethylation was related to cell cycle, intracellular part and protein binding, the main pathway involved herpes simplex virus 1 infection related to down-regulated AKT, Araf1 and BCL2A1. In the meantime, sexual reproduction, cohesin complex and protein C-terminus binding was functionally linked to differential hypomethylation, while fluid shear stress and atherosclerosis were identified as the main pathways related to up-regulated GST and CNP.

Conclusions: We showed that lung cancer development involved differential expression of SPP1 and pRB mRNA, as well as m6A mRNA methylation in AKT, APAF1, BCL2A1, GST and CNP genes.

Ionizing Radiation Combined with PARP1 Inhibitor Reduces Radioresistance in Prostate Cancer with RB1/TP53 Loss

Tumor suppressor genes RB1 and TP53 are altered frequently in prostate cancer (PC), whether RB1 and TP53 inactivation promotes radioresistance remains unclear. Herein, we demonstrated that RB1 loss enhanced ionizing radiation (IR)-induced DNA damage to inhibit cell proliferation and promote cellular senescence through a TP53-dependent pathway in LNCaP cells. Furthermore, the stabilization of TP53 was regulated by ATM-mediated phosphorylation of MDM2 at Ser395. However, inactivation of RB1/TP53 reversed DNA damage-induced cellular senescence and promoted radiation survival. Importantly, combined with PARP1 inhibitor restored radiosensitivity. This finding provides a potential approach for the therapy of PC with RB1/TP53 inactivation.

MiR-486-5p Act as a Biomarker in Endometrial Carcinoma: Promotes Cell Proliferation, Migration, Invasion by Targeting MARK1

Background

miRNA expression acts as a potential biomarker in many diseases including endometrial carcinoma (EC). miR-486-5p dysregulation is observed in several tumor types, but the roles of miR-486-5p in EC are hardly ever studied.

Objective

This study aimed to analyze the expression profile of miR-486-5p in tumor tissues and serum samples of patients with EC and explore the target prediction, function analysis and validation in immortal cell lines.

Patients and Methods

A total of 42 freshly paired EC tissues, the corresponding adjacent non-neoplastic tissues and serum samples were also collected from patients with EC, and 42 matched normal serum samples were included as control group. The level of miR-486-5p expression was tested by quantitative real-time polymerase chain reaction (qRT-PCR). Cell proliferation was determined by colony formation assay and CCK-8 assay. Furthermore, functional evaluation of miR-486-5p on migration was performed by wound-healing assay and invasion was estimated by transwell invasion assay. qRT-PCR, luciferase reporter assay and Western blotting (WB) were performed to verify the targeting of MARK1 by miR-486-5p.

Results

miR-486-5p was significantly up-regulated in EC tissues and serum samples, promoting the proliferation, migration and invasive activities of EC cells by targeting MARK1.

Conclusion

These data indicated miR-486-5p as a novel molecular biomarker for diagnosing and treating EC, and MARK1 might act as a critical and functional target of miR-486-5p with the implications on cell proliferation, migration and invasiveness of EC tumor cells.

CHK2 overexpression and mislocalisation within mitotic structures enhances chromosomal instability and hepatocellular carcinoma progression

OBJECTIVE

Chromosomal instability (CIN) is the most common form of genomic instability, which promotes hepatocellular carcinoma (HCC) progression by enhancing tumour heterogeneity, drug resistance and immunity escape. CIN per se is an important factor of DNA damage, sustaining structural chromosome abnormalities but the underlying mechanisms are unknown.

DESIGN

DNA damage response protein checkpoint kinase 2 (Chk2) expression was evaluated in an animal model of diethylnitrosamine-induced HCC characterised by DNA damage and elevated mitotic errors. Chk2 was also determined in two discrete cohorts of human HCC specimens. To assess the functional role of Chk2, gain on and loss-of-function, mutagenesis, karyotyping and immunofluorescence/live imaging were performed by using HCT116, Huh7 and human hepatocytes immortalised with hTERT gene (HuS).

RESULTS

We demonstrate that mitotic errors during HCC tumorigenesis cause lagging chromosomes/DNA damage and activation of Chk2. Overexpression/phosphorylation and mislocalisation within the mitotic spindle of Chk2 contributes to induce lagging chromosomes. Lagging chromosomes and mitotic activity are reversed by knockdown of Chk2. Furthermore, upregulated Chk2 maintains mitotic activity interacting with Aurora B kinase for chromosome condensation and cytokinesis. The forkhead-associated domain of Chk2 is required for Chk2 mislocalisation to mitotic structures. In addition, retinoblastoma protein phosphorylation contributes to defective mitoses. A cohort and independent validation cohort show a strong cytoplasm to nuclear Chk2 translocation in a subset of patients with HCC.

CONCLUSIONS

The study reveals a new mechanistic insight in the coinvolvement of Chk2 in HCC progression. These findings propose Chk2 as a putative biomarker to detect CIN in HCC providing a valuable support for clinical/therapeutical management of patients.

Detection of E2F-Induced Transcriptional Activity Using a Dual Luciferase Reporter Assay

The E2F transcription factors are key targets for the retinoblastoma (RB) tumor suppressor function. The active or inactive status of RB determines the degree by which E2F-dependent gene expression will occur in a given condition. Changes in transcriptional activity in response to extracellular or intracellular stimuli are frequently measured using genetic reporter assays. In particular, dual luciferase reporter assays are most recommended for this purpose because of their improved experimental accuracy. Here we illustrate the usefulness of the dual luciferase reporter assay to detect E2F-mediated transcriptional activity upon overexpression of E2F1 in cultured cells as readout for RB status and function.

TRAP1 controls cell cycle G2-M transition through the regulation of CDK1 and MAD2 expression/ubiquitination

Regulation of tumour cell proliferation by molecular chaperones is still a complex issue. Here, the role of the HSP90 molecular chaperone TRAP1 in cell cycle regulation was investigated in a wide range of human breast, colorectal, and lung carcinoma cell lines, and tumour specimens. TRAP1 modulates the expression and/or the ubiquitination of key cell cycle regulators through a dual mechanism: (i) transcriptional regulation of CDK1, CYCLIN B1, and MAD2, as suggested by gene expression profiling of TRAP1-silenced breast carcinoma cells; and (ii) post-transcriptional quality control of CDK1 and MAD2, being the ubiquitination of these two proteins enhanced upon TRAP1 down-regulation. Mechanistically, TRAP1 quality control on CDK1 is crucial for its regulation of mitotic entry, since TRAP1 interacts with CDK1 and prevents CDK1 ubiquitination in cooperation with the proteasome regulatory particle TBP7, this representing the limiting factor in TRAP1 regulation of the G2-M transition. Indeed, TRAP1 silencing results in enhanced CDK1 ubiquitination, lack of nuclear translocation of CDK1/cyclin B1 complex, and increased MAD2 degradation, whereas CDK1 forced up-regulation partially rescues low cyclin B1 and MAD2 levels and G2-M transit in a TRAP1-poor background. Consistently, the CDK1 inhibitor RO-3306 is less active in a TRAP1-high background. Finally, a significant correlation was observed between TRAP1 and Ki67, CDK1 and/or MAD2 expression in breast, colorectal, and lung human tumour specimens. This study represents the first evidence that TRAP1 is relevant in the control of the complex machinery that governs cell cycle progression and mitotic entry and provides a strong rationale to regard TRAP1 as a biomarker to select tumours with deregulated cell cycle progression and thus likely poorly responsive to novel cell cycle inhibitors. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Unravelling the complexity of signalling networks in cancer: A review of the increasing role for computational modelling

Cancer induction is a highly complex process involving hundreds of different inducers but whose eventual outcome is the same. Clearly, it is essential to understand how signalling pathways and networks generated by these inducers interact to regulate cell behaviour and create the cancer phenotype. While enormous strides have been made in identifying key networking profiles, the amount of data generated far exceeds our ability to understand how it all "fits together". The number of potential interactions is astronomically large and requires novel approaches and extreme computation methods to dissect them out. However, such methodologies have high intrinsic mathematical and conceptual content which is difficult to follow. This review explains how computation modelling is progressively finding solutions and also revealing unexpected and unpredictable nano-scale molecular behaviours extremely relevant to how signalling and networking are coherently integrated. It is divided into linked sections illustrated by numerous figures from the literature describing different approaches and offering visual portrayals of networking and major conceptual advances in the field. First, the problem of signalling complexity and data collection is illustrated for only a small selection of known oncogenes. Next, new concepts from biophysics, molecular behaviours, kinetics, organisation at the nano level and predictive models are presented. These areas include: visual representations of networking, Energy Landscapes and energy transfer/dissemination (entropy); diffusion, percolation; molecular crowding; protein allostery; quinary structure and fractal distributions; energy management, metabolism and re-examination of the Warburg effect. The importance of unravelling complex network interactions is then illustrated for some widely-used drugs in cancer therapy whose interactions are very extensive. Finally, use of computational modelling to develop micro- and nano- functional models ("bottom-up" research) is highlighted. The review concludes that computational modelling is an essential part of cancer research and is vital to understanding network formation and molecular behaviours that are associated with it. Its role is increasingly essential because it is unravelling the huge complexity of cancer induction otherwise unattainable by any other approach.

MARK2 inhibits the growth of HeLa cells through AMPK and reverses epithelial-mesenchymal transition

Microtubule affinity-regulating kinases (MARKs; MARK1, MARK2, MARK3 and MARK4) act directly downstream of LKB1, the multitasking tumor-suppressor kinase, and thereby mediate its biological effects. Current understanding of the function of MARKs is greatly restricted to regulation of cell polarity. However, whether or how MARKs contribute to cellular growth control remains largely unknown. In the present study, we utilized an inducible lentiviral expression system that allows rapid MARK expression in LKB1-deficient HeLa cells, and characterized additional functions of MARKs: overexpression of MARK2 in HeLa cells resulted in a decrease in cell growth, inhibition of colony formation and arrest in G1 cell cycle phase, with AMPK as the putative downstream effector upregulating the expression of p21 and p16. MARK2 was found to play a role in F-actin reorganization and to contribute to reversal of epithelial‑mesenchymal transition (EMT) as exemplified in the case of HeLa cells that exhibited phenotypic changes, reduced cell migration and invasion. Our findings unveil the coordinated regulation of cell growth and EMT mediated by MARK2, and also provide new insights into the mechanisms underlying the anti-metastatic activity of MARK2.

Induction of p53-Independent Apoptosis and G1 Cell Cycle Arrest by Fucoidan in HCT116 Human Colorectal Carcinoma Cells

It is well known that fucoidan, a natural sulfated polysaccharide present in various brown algae, mediates anticancer effects through the induction of cell cycle arrest and apoptosis. Nevertheless, the role of tumor suppressor p53 in the mechanism action of fucoidan remains unclear. Here, we investigated the anticancer effect of fucoidan on two p53 isogenic HCT116 (p53+/+ and p53-/-) cell lines. Our results showed that inhibition of cell viability, induction of apoptosis and DNA damage by treatment with fucoidan were similar in two cell lines. Flow cytometric analysis revealed that fucoidan resulted in G1 arrest in the cell cycle progression, which correlated with the inhibition of phosphorylation of retinoblastoma protein (pRB) and concomitant association of pRB with the transcription factor E2Fs. Furthermore, treatment with fucoidan obviously upregulated the expression of cyclin-dependent kinase (CDK) inhibitors, such as p21WAF1/CIP1 and p27KIP1, which was paralleled by an enhanced binding with CDK2 and CDK4. These events also commonly occurred in both cell lines, suggesting that fucoidan triggered G1 arrest and apoptosis in HCT116 cells by a p53-independent mechanism. Thus, given that most tumors exhibit functional p53 inactivation, fucoidan could be a possible therapeutic option for cancer treatment regardless of the p53 status.

Co-option during the evolution of multicellular and developmental complexity in the volvocine green algae

Despite its major impact on the evolution of Life on Earth, the transition to multicellularity remains poorly understood, especially in terms of its genetic basis. The volvocine algae are a group of closely related species that range in morphology from unicellular individuals (Chlamydomonas) to undifferentiated multicellular forms (Gonium) and complex organisms with distinct developmental programs and one (Pleodorina) or two (Volvox) specialized cell types. Modern genetic approaches, complemented by the recent sequencing of genomes from several key species, revealed that co-option of existing genes and pathways is the primary driving force for the evolution of multicellularity in this lineage. The initial transition to undifferentiated multicellularity, as typified by the extant Gonium, was driven primarily by the co-option of cell cycle regulation. Further morphological and developmental innovations in the lineage leading to Volvox resulted from additional co-option events involving genes important for embryonic inversion, asymmetric cell division, somatic and germ cell differentiation and the structure and function of the extracellular matrix. Because of their relatively low but variable levels of morphological and developmental complexity, simple underlying genetics and recent evolutionary history, the volvocine algae are providing significant insight into our understanding of the genetics and evolution of major developmental and morphological traits.

Retinoblastoma tumor suppressor functions shared by stem cell and cancer cell strategies

Carcinogenic transformation of somatic cells resembles nuclear reprogramming toward the generation of pluripotent stem cells. These events share eternal escape from cellular senescence, continuous self-renewal in limited but certain population of cells, and refractoriness to terminal differentiation while maintaining the potential to differentiate into cells of one or multiple lineages. As represented by several oncogenes those appeared to be first keys to pluripotency, carcinogenesis and nuclear reprogramming seem to share a number of core mechanisms. The retinoblastoma tumor suppressor product retinoblastoma (RB) seems to be critically involved in both events in highly complicated manners. However, disentangling such complicated interactions has enabled us to better understand how stem cell strategies are shared by cancer cells. This review covers recent findings on RB functions related to stem cells and stem cell-like behaviors of cancer cells.

Patterns of Chromosomal Aberrations in Solid Tumors

Chromosomal abnormalities are a defining feature of solid tumors. Such cytogenetic alterations are mainly classified into structural chromosomal aberrations and copy number alterations, giving rise to aneuploid karyotypes. The increasing detection of these genetic changes allowed the description of specific tumor entities and the associated patterns of gene expression. In fact, tumor-specific landscapes of gross genomic copy number changes, including aneuploidies of entire chromosome arms and chromosomes result in a global deregulation of the transcriptome of cancer cells. Furthermore, the molecular characterization of cytogenetic abnormalities has provided insights into the mechanisms of tumorigenesis and has, in a few instances, led to the clinical implementation of effective diagnostic and prognostic tools, as well as treatment strategies that target a specific genetic abnormality.

Short- and long-term effects of chromosome mis-segregation and aneuploidy

Dividing cells that experience chromosome mis-segregation generate aneuploid daughter cells, which contain an incorrect number of chromosomes. Although aneuploidy interferes with the proliferation of untransformed cells, it is also, paradoxically, a hallmark of cancer, a disease defined by increased proliferative potential. These contradictory effects are also observed in mouse models of chromosome instability (CIN). CIN can inhibit and promote tumorigenesis. Recent work has provided insights into the cellular consequences of CIN and aneuploidy. Chromosome mis-segregation per se can alter the genome in many more ways than just causing the gain or loss of chromosomes. The short- and long-term effects of aneuploidy are caused by gene-specific effects and a stereotypic aneuploidy stress response. Importantly, these recent findings provide insights into the role of aneuploidy in tumorigenesis.

Expression Analysis of Genes Involved in the RB/E2F Pathway in Astrocytic Tumors

Astrocytic gliomas, which are derived from glial cells, are considered the most common primary neoplasias of the central nervous system (CNS) and are histologically classified as low grade (I and II) or high grade (III and IV). Recent studies have shown that astrocytoma formation is the result of the deregulation of several pathways, including the RB/E2F pathway, which is commonly deregulated in various human cancers via genetic or epigenetic mechanisms. On the basis of the assumption that the study of the mechanisms controlling the INK4/ARF locus can help elucidate the molecular pathogenesis of astrocytic tumors, identify diagnostic and prognostic markers, and help select appropriate clinical treatments, the present study aimed to evaluate and compare methylation patterns using bisulfite sequencing PCR and evaluate the gene expression profile using real-time PCR in the genes CDKN2A, CDKN2B, CDC6, Bmi-1, CCND1, and RB1 in astrocytic tumors. Our results indicate that all the evaluated genes are not methylated independent of the tumor grade. However, the real-time PCR results indicate that these genes undergo progressive deregulation as a function of the tumor grade. In addition, the genes CDKN2A, CDKN2B, and RB1 were underexpressed, whereas CDC6, Bmi-1, and CCND1 were overexpressed; the increase in gene expression was significantly associated with decreased patient survival. Therefore, we propose that the evaluation of the expression levels of the genes involved in the RB/E2F pathway can be used in the monitoring of patients with astrocytomas in clinical practice and for the prognostic indication of disease progression.

The stress of coping with E2F loss

E2F transcription factors are key regulators of cellular proliferation, and altered E2F activity is a common feature of tumor cells. Thus, E2F targeting is being explored as a therapeutic strategy in cancer. Importantly, recent mouse knockout studies show that concomitant loss of E2f1/E2f2 activity is associated with increased genomic instability and oncogenic potential in normal differentiating cells, a finding that might have implications for cancer therapy.

Tumor-suppressive p53 Signaling Empowers Metastatic Inhibitor KLF17-dependent Transcription to Overcome Tumorigenesis in Non-small Cell Lung Cancer

Metastasis, which is controlled by concerted action of multiple genes, is a complex process and is an important cause of cancer death. Krüppel-like factor 17 (KLF17) is a negative regulator of metastasis and epithelial-mesenchymal transition (EMT) during cancer progression. However, the underlying molecular mechanism and biological relevance of KLF17 in cancer cells are poorly understood. Here, we show that tumor suppressor protein p53 plays an integral role to induce KLF17 expression in non-small cell lung cancer (NSCLC). p53 is recruited to the KLF17 promoter and results in the formation of p53-DNA complex. p53 enhances binding of p300 and favors histone acetylation on the KLF17 promoter. Mechanistically, p53 physically interacts with KLF17 and thereby enhances the anti-metastatic function of KLF17. p53 empowers KLF17-mediated EMT genes transcription via enhancing physical association of KLF17 with target gene promoters. Nutlin-3 recruits KLF17 to EMT target gene promoters and results in the formation of KLF17-DNA complex via a p53-dependent pathway. p53 depletion abrogates DNA binding affinity of KLF17 to EMT target gene promoters. KLF17 is critical for p53 cellular activities in NSCLC. Importantly, KLF17 enhances p53 transcription to generate a novel positive feedback loop. KLF17 depletion accelerates lung cancer cell growth in response to chemotherapy. Mechanistically, we found that KLF17 increases the expression of tumor suppressor genes p53, p21, and pRB. Functionally, KLF17 required p53 to suppress cancer cell invasion and migration in NSCLC. In conclusion, our study highlights a novel insight into the anti-EMT effect of KLF17 via a p53-dependent pathway in NSCLC, and KLF17 may be a new therapeutic target in NSCLC with p53 status.

Novel actions of next-generation taxanes benefit advanced stages of prostate cancer

PURPOSE

To improve the outcomes of patients with castration-resistant prostate cancer (CRPC), there is an urgent need for more effective therapies and approaches that individualize specific treatments for patients with CRPC. These studies compared the novel taxane cabazitaxel with the previous generation docetaxel, and aimed to determine which tumors are most likely to respond.

EXPERIMENTAL DESIGN

Cabazitaxel and docetaxel were compared via in vitro modeling to determine the molecular mechanism, biochemical and cell biologic impact, and cell proliferation, which was further assessed ex vivo in human tumor explants. Isogenic pairs of RB knockdown and control cells were interrogated in vitro and in xenograft tumors for cabazitaxel response.

RESULTS

The data herein show that (i) cabazitaxel exerts stronger cytostatic and cytotoxic response compared with docetaxel, especially in CRPC; (ii) cabazitaxel induces aberrant mitosis, leading to pyknotic and multinucleated cells; (iii) taxanes do not act through the androgen receptor (AR); (iv) gene-expression profiling reveals distinct molecular actions for cabazitaxel; and (v) tumors that have progressed to castration resistance via loss of RB show enhanced sensitivity to cabazitaxel.

CONCLUSIONS

Cabazitaxel not only induces improved cytostatic and cytotoxic effects, but also affects distinct molecular pathways, compared with docetaxel, which could underlie its efficacy after docetaxel treatment has failed in patients with CRPC. Finally, RB is identified as the first potential biomarker that could define the therapeutic response to taxanes in metastatic CRPC. This would suggest that loss of RB function induces sensitization to taxanes, which could benefit up to 50% of CRPC cases.

Genetic and Epigenetic Discoveries in Human Retinoblastoma

Retinoblastoma is a rare pediatric cancer of the retina. Nearly all retinoblastomas are initiated through the biallelic inactivation of the retinoblastoma tumor susceptibility gene (RB1). Whole-genome sequencing has made it possible to identify secondary genetic lesions following RB1 inactivation. One of the major discoveries from retinoblastoma sequencing studies is that some retinoblastoma tumors have stable genomes. Subsequent epigenetic studies showed that changes in the epigenome contribute to the rapid progression of retinoblastoma following RB1 gene inactivation. In addition, gene amplification and elevated expression of p53 antagonists, MDM2 and MDM4, may also play an important role in retinoblastoma tumorigenesis. The knowledge gained from these recent molecular, cellular, genomic, and epigenomic analyses are now being integrated to identify new therapeutic approaches that can help save lives and vision in children with retinoblastoma, with fewer long-term side effects.

THE RETINOBLASTOMA PROTEIN: A MASTER TUMOR SUPPRESSOR ACTS AS A LINK BETWEEN CELL CYCLE AND CELL ADHESION

RB1 was the first tumor suppressor gene discovered. Over four decades of work have revealed that the Rb protein (pRb) is a master regulator of biological pathways influencing virtually every aspect of intrinsic cell fate including cell growth, cell-cycle checkpoints, differentiation, senescence, self-renewal, replication, genomic stability and apoptosis. While these many processes may account for a significant portion of RB1's potency as a tumor suppressor, a small, but growing stream of evidence suggests that RB1 also significantly influences how a cell interacts with its environment, including cell-to-cell and cell-to-extracellular matrix interactions. This review will highlight pRb's role in the control of cell adhesion and how alterations in the adhesive properties of tumor cells may drive the deadly process of metastasis.

Biochemical and cellular analysis of Ogden syndrome reveals downstream Nt-acetylation defects

The X-linked lethal Ogden syndrome was the first reported human genetic disorder associated with a mutation in an N-terminal acetyltransferase (NAT) gene. The affected males harbor an Ser37Pro (S37P) mutation in the gene encoding Naa10, the catalytic subunit of NatA, the major human NAT involved in the co-translational acetylation of proteins. Structural models and molecular dynamics simulations of the human NatA and its S37P mutant highlight differences in regions involved in catalysis and at the interface between Naa10 and the auxiliary subunit hNaa15. Biochemical data further demonstrate a reduced catalytic capacity and an impaired interaction between hNaa10 S37P and Naa15 as well as Naa50 (NatE), another interactor of the NatA complex. N-Terminal acetylome analyses revealed a decreased acetylation of a subset of NatA and NatE substrates in Ogden syndrome cells, supporting the genetic findings and our hypothesis regarding reduced Nt-acetylation of a subset of NatA/NatE-type substrates as one etiology for Ogden syndrome. Furthermore, Ogden syndrome fibroblasts display abnormal cell migration and proliferation capacity, possibly linked to a perturbed retinoblastoma pathway. N-Terminal acetylation clearly plays a role in Ogden syndrome, thus revealing the in vivo importance of N-terminal acetylation in human physiology and disease.

Regulation of mitotic cytoskeleton dynamics and cytokinesis by integrin-linked kinase in retinoblastoma cells

During cell division integrin-linked kinase (ILK) has been shown to regulate microtubule dynamics and centrosome clustering, processes involved in cell cycle progression, and malignant transformation. In this study, we examine the effects of downregulating ILK on mitotic function in human retinoblastoma cell lines. These retinal cancer cells, caused by the loss of function of two gene alleles (Rb1) that encode the retinoblastoma tumour suppressor, have elevated expression of ILK. Here we show that inhibition of ILK activity results in a concentration-dependent increase in nuclear area and multinucleated cells. Moreover, inhibition of ILK activity and expression increased the accumulation of multinucleated cells over time. In these cells, aberrant cytokinesis and karyokinesis correlate with altered mitotic spindle organization, decreased levels of cortical F-actin and centrosome de-clustering. Centrosome de-clustering, induced by ILK siRNA, was rescued in FLAG-ILK expressing Y79 cells as compared to those expressing FLAG-tag alone. Inhibition of ILK increased the proportion of cells exhibiting mitotic spindles and caused a significant G2/M arrest as early as 24 hours after exposure to QLT-0267. Live cell analysis indicate ILK downregulation causes an increase in multipolar anaphases and failed cytokinesis (bipolar and multipolar) of viable cells. These studies extend those indicating a critical function for ILK in mitotic cytoskeletal organization and describe a novel role for ILK in cytokinesis of Rb deficient cells.

Cross-species genomic and epigenomic landscape of retinoblastoma

Genetically engineered mouse models (GEMMs) of human cancer are important for advancing our understanding of tumor initiation and progression as well as for testing novel therapeutics. Retinoblastoma is a childhood cancer of the developing retina that initiates with biallelic inactivation of the RB1 gene. GEMMs faithfully recapitulate the histopathology, molecular, cellular, morphometric, neuroanatomical and neurochemical features of human retinoblastoma. In this study, we analyzed the genomic and epigenomic landscape of murine retinoblastoma and compared them to human retinoblastomas to gain insight into shared mechanisms of tumor progression across species. Similar to human retinoblastoma, mouse tumors have low rates of single nucleotide variations. However, mouse retinoblastomas have higher rates of aneuploidy and regional and focal copy number changes that vary depending on the genetic lesions that initiate tumorigenesis in the developing murine retina. Furthermore, the epigenetic landscape in mouse retinoblastoma was significantly different from human tumors and some pathways that are candidates for molecular targeted therapy for human retinoblastoma such as SYK or MCL1 are not deregulated in GEMMs. Taken together, these data suggest there are important differences between mouse and human retinoblastomas with respect to the mechanism of tumor progression and those differences can have significant implications for translational research to test the efficacy of novel therapies for this devastating childhood cancer.

Inhibition of retinoblastoma mRNA degradation through Poly (A) involved in the neuroprotective effect of berberine against cerebral ischemia

Berberine is one kind of isoquinoline alkaloid with anti-apoptotic effects on the neurons suffering ischemia. To address the explanation for these activities, the berberine-induced cell cycle arrest during neurons suffering ischemia/reperfusion had been studied in the present study. According to the in vitro neurons with oxygen-glucose deprivation and in vivo ICR mice with cerebral ischemia/reperfusion, it was found that berberine could protect the mRNA of retinoblastoma (Rb) from degradation through its function on the poly(A) tail. The prolonged half-life of retinoblastoma 1 (gene of Rb, RB1) mRNA level secures the protein level of retinoblastoma, which facilitates cell cycle arrest of neurons in the process of ischemia/reperfusion and subsequently avoids cells entering in the apoptotic process. The poly(A) tail of RB1 mRNA, as a newly identified target of berberine, could help people focus on the interaction between berberine and mRNA to further understand the biological activities and functions of berberine.

Exogenous activation of LKB1/AMPK signaling induces G₁ arrest in cells with endogenous LKB1 expression

The tumor suppressor protein LKB1 is a serine/threonine kinase that plays a critical role in cell proliferation, and its inactivation has been linked to tumorigenesis in various cancer types. Current understanding of the LKB1 function is largely restricted to results from experiments on LKB1‑deficient cancer cells, while the regulation and activity of endogenous LKB1 has been rarely investigated. In a previous study, we showed that LKB1 knockdown in two healthy cell lines accelerates cell cycle progression through the G1/S checkpoint by inhibition of the p53 and p16 pathways. In the present study, we examined the effects of overexpression of LKB1 on two healthy and one cancer cell line. Administration of exogenous LKB1 activated LKB1/AMPK signaling and arrested the cell cycle at the G1 phase in an LKB1-dependent manner. G1 arrest induced by LKB1 was accompanied by the downregulation of cyclin D1 and cyclin D3, and the upregulation of p53, p21 and p16, while no differences were detected for CDK4, CDK6, cyclin E, p15 and p27. These results indicated that exogenous activation of LKB1/AMPK signaling inhibits the G1/S cell cycle transition, even in cells with an endogenous expression of LKB1. Findings of the present study extend earlier observations on LKB1‑inactivated neoplastic cells and provide novel insights into the growth-inhibitory effects of LKB1.

MiR-26b, upregulated in Alzheimer's disease, activates cell cycle entry, tau-phosphorylation, and apoptosis in postmitotic neurons

MicroRNA (miRNA) functions in the pathogenesis of major neurodegenerative diseases such as Alzheimer's disease (AD) are only beginning to emerge. We have observed significantly elevated levels of a specific miRNA, miR-26b, in the defined pathological areas of human postmortem brains, starting from early stages of AD (Braak III). Ectopic overexpression of miR-26b in rat primary postmitotic neurons led to the DNA replication and aberrant cell cycle entry (CCE) and, in parallel, increased tau-phosphorylation, which culminated in the apoptotic cell death of neurons. Similar tau hyperphosphorylation and CCE are typical features of neurons in pre-AD brains. Sequence-specific inhibition of miR-26b in culture is neuroprotective against oxidative stress. Retinoblastoma protein (Rb1), a major tumor suppressor, appears as the key direct miR-26b target, which mediates the observed neuronal phenotypes. The downstream signaling involves upregulation of Rb1/E2F cell cycle and pro-apoptotic transcriptional targets, including cyclin E1, and corresponding downregulation of cell cycle inhibitor p27/Kip1. It further leads to nuclear export and activation of Cdk5, a major kinase implicated in tau phosphorylation, regulation of cell cycle, and death in postmitotic neurons. Therefore, upregulation of miR-26b in neurons causes pleiotropic phenotypes that are also observed in AD. Elevated levels of miR-26b may thus contribute to the AD neuronal pathology.

How does ethanol induce apoptotic cell death of SK-N-SH neuroblastoma cells

A body of evidence suggests that ethanol can lead to damage of neuronal cells. However, the mechanism underlying the ethanol-induced damage of neuronal cells remains unclear. The role of mitogen-activated protein kinases in ethanol-induced damage was investigated in SK-N-SH neuroblastoma cells. 3-[4,5-Dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide cell viability assay, DNA fragmentation detection, and flow cytometric analysis showed that ethanol induced apoptotic cell death and cell cycle arrest, characterized by increased caspase-3 activity, DNA fragmentation, nuclear disruption, and G₁ arrest of cell cycle of the SK-N-SH neuroblastoma cells. In addition, western blot analysis indicated that ethanol induced a lasting increase in c-Jun N-terminal protein kinase activity and a transient increase in p38 kinase activity of the neuroblastoma cells. c-Jun N-terminal protein kinase or p38 kinase inhibitors significantly reduced the ethanol-induced cell death. Ethanol also increased p53 phosphorylation, followed by an increase in p21 tumor suppressor protein and a decrease in phospho-Rb (retinoblastoma) protein, leading to alterations in the expressions and activity of cyclin dependent protein kinases. Our results suggest that ethanol mediates apoptosis of SK-N-SH neuroblastoma cells by activating p53-related cell cycle arrest possibly through activation of the c-Jun N-terminal protein kinase-related cell death pathway.

Cancer metabolism: feeding a worm to starve a tumor

The tumor suppressor Rb is known to have its hand in many pots. New findings have added another pot to the mix - cell metabolism. This may lead to a better understanding of Rb mutant phenotypes and Rb's roles in oncogenesis.

Whole chromosome instability resulting from the synergistic effects of pRB and p53 inactivation

Whole chromosome instability (CIN) is a common feature of cancer cells and has been linked to increased tumor evolution and metastasis. Several studies have shown that the loss of the pRB tumor suppressor causes mitotic defects and chromosome mis-segregation. pRB is inactivated in many types of cancer and this raises the possibility that the loss of pRB may be a general cause of CIN in tumors. Paradoxically, retinoblastoma tumor cells have a relatively stable karyotype and currently the circumstances in which pRB inactivation causes CIN in human cancers are unclear. Here we utilize a fluorescence in situ hybridization-based approach to score numerical heterogeneity in chromosome copy number as a readout of CIN. Using this technique, we show that high levels of CIN correlate with the combined inactivation of pRB and p53 and that this association is evident in two independent panels of cancer cell lines. Retinoblastoma cell lines characteristically retain a wild-type TP53 gene, providing an opportunity to test the relevance of this functional relationship. We show that retinoblastoma cell lines display mitotic defects similar to those seen when pRB is depleted from non-transformed cells, but that the presence of wild-type p53 suppresses the accumulation of aneuploid cells. A similar synergy between pRB and p53 inactivation was observed in HCT116 cells. These results suggest that the loss of pRB promotes segregation errors, whereas loss of p53 allows tolerance and continued proliferation of the resulting, genomically unstable cancer cells. Hence, it is the cooperative effect of inactivation of both pRB and p53 tumor suppressor pathways that promotes CIN.

Chlamydia trachomatis inhibits telomeric DNA damage signaling via transient hTERT upregulation

Epidemiological data exist to support a positive association between Chlamydia trachomatis (Ctr) infection and gynecological cancers; however, putative cellular mechanisms for this association are lacking. Here, we identified Ctr-induced perturbations to host cell phenotypes in vitro that persisted after clearance of infection and could directly contribute to host cell transformation. In particular, human telomerase catalytic subunit (hTERT) mRNA expression and catalytic subunit activity were increased in acute infected late passage IMR90E1A cells. hTERT upregulation was accompanied by recruitment of ceramide, a known regulator of hTERT, to the chlamydial inclusion and was abrogated following doxycycline-mediated infection clearance. In cells cleared of Ctr infection, average telomere length was slightly increased and immunofluorescence staining of the DNA damage marker γH2A.X was reduced after clearance of infection compared with cells that had not been infected. Reduced p53 binding to the promoter of the cell cycle checkpoint regulator p21 was also detected in cells cleared of infection and p21 levels were reduced; moreover, this cell population exhibited increased resistance to etoposide-induced DNA damage. Thus, Ctr infection altered cell aging and survival pathways, which persisted after infection clearance. Cells that survive infection are likely to exhibit altered physiology, as evidenced by an increased resistance to DNA damage-induced apoptosis, which may support cellular transformation.

Aneuploidy in health, disease, and aging

Aneuploidy, an aberrant number of chromosomes, has been recognized as a feature of human malignancies for over a century, but compelling evidence for causality was largely lacking until mouse models for chromosome number instability were used. These in vivo studies have not only uncovered important new insights into the extremely complex aneuploidy-cancer relationship but also into the molecular mechanisms underlying proper and aberrant chromosome segregation. A series of diverse mouse models for the mitotic checkpoint protein BubR1 has provided evidence for a provocative novel link between aneuploidization and the development of age-related pathologies.

Molecular aspects of cancer cell resistance to chemotherapy

Cancer cell resistance to chemotherapy is still a heavy burden that impairs treatment of cancer patients. Both intrinsic and acquired resistance results from the numerous genetic and epigenetic changes occurring in cancer cells. Most of the hallmarks of cancer cells provide general mechanisms to sustain stresses such as the ones induced by chemotherapeutic drugs. Moreover, specific changes in the target bring resistance to specific drugs like modification in nucleotide synthesis enzymes upon anti-metabolite exposure, in microtubule composition upon spindle poison treatment, in topoisomerase activity upon topoisomerase inhibitor incubation or in intracellular signaling pathways when targeting tyrosine kinase receptors. Finally, the stemness properties of a few cancer cells as well as components of the tumor stroma, like fibroblasts and tumor-associated macrophages but also hypoxia, also help tumor to resist to anticancer agents. These processes provide an additional level of complexity to the understanding of the tumor resistance phenomenon. This review aims to describe the different general mechanisms as well as some examples of specific on target modifications inducing cancer cell resistance to chemotherapy at the molecular level. Perspectives to develop more efficient treatment, using genomic signature or more specific biomarkers to characterize putative resistance mechanisms in patients before choosing the more appropriate treatment, will also be discussed.

The Role of F-Box Proteins during Viral Infection

The F-box domain is a protein structural motif of about 50 amino acids that mediates protein–protein interactions. The F-box protein is one of the four components of the SCF (SKp1, Cullin, F-box protein) complex, which mediates ubiquitination of proteins targeted for degradation by the proteasome, playing an essential role in many cellular processes. Several discoveries have been made on the use of the ubiquitin–proteasome system by viruses of several families to complete their infection cycle. On the other hand, F-box proteins can be used in the defense response by the host. This review describes the role of F-box proteins and the use of the ubiquitin–proteasome system in virus–host interactions.

Molecular pathways: targeting Mdm2 and Mdm4 in cancer therapy

The p53 tumor suppressor is activated in response to cellular stresses to induce cell-cycle arrest, cellular senescence, and apoptosis. The p53 gene is inactivated by mutations in more than 50% of human tumors. In addition, tumor cells dampen p53 activities via overexpression of p53-negative regulators, in particular 2 structurally related proteins, Mdm2 and Mdm4. And yet, Mdm2 and Mdm4 possess p53-independent activities, which also contribute to tumor formation and progression. Given that Mdm2 and Mdm4 inhibit p53 activities to promote tumor development, small molecules and peptides were developed to abrogate the inhibition of p53 by Mdm proteins. Antitumor activities of these molecules have already been confirmed in preclinical studies and early-phase clinical trials. These research endeavors and clinical advances constitute the main focus of this review.

Genetic instability: tipping the balance

Tumor cells typically contain a genome that is highly divergent from the genome of normal, non-transformed cells. This genetic divergence is caused by a number of distinct changes that the tumor cell acquires during its transformation from a normal cell into a tumorigenic counterpart. Changes to the genome include mutations, deletions, insertions, and also gross chromosomal aberrations, such as chromosome translocations and whole chromosome gains or losses. This genetic disorder of the tumor cell has complicated the identification of crucial driver mutations that cause cancer. Moreover, the large genetic divergence between different tumors causes them to behave very differently, and makes it difficult to predict response to therapy. In addition, tumor cells are genetically unstable and frequently acquire new mutations and/or gross chromosomal aberrations as they divide. This is beneficial for the overall capacity of a tumor to adapt to changes in its environment, but newly acquired genetic alterations can also compromise the genetic dominance of the tumor cell and thus affect tumor cell viability. Here, we review the mechanisms that can cause gross chromosomal aberrations, and discuss how these affect tumor cell viability.

YB-1: oncoprotein, prognostic marker and therapeutic target?

Hanahan and Weinberg have proposed the ‘hallmarks of cancer’ to cover the biological changes required for the development and persistence of tumours [Hanahan and Weinberg (2011) Cell 144, 646–674]. We have noted that many of these cancer hallmarks are facilitated by the multifunctional protein YB-1 (Y-box-binding protein 1). In the present review we evaluate the literature and show how YB-1 modulates/regulates cellular signalling pathways within each of these hallmarks. For example, we describe how YB-1 regulates multiple proliferation pathways, overrides cell-cycle check points, promotes replicative immortality and genomic instability, may regulate angiogenesis, has a role in invasion and metastasis, and promotes inflammation. We also argue that there is strong and sufficient evidence to suggest that YB-1 is an excellent molecular marker of cancer progression that could be used in the clinic, and that YB-1 could be a useful target for cancer therapy.

Deciphering the retinoblastoma protein phosphorylation code

Multisite phosphorylation modulates the function of regulatory proteins with complex signaling properties and outputs. The retinoblastoma tumor suppressor protein (Rb) is inactivated by cyclin-dependent kinase (Cdk) phosphorylation in normal and cancer cell cycles, so understanding the molecular mechanisms and effects of Rb phosphorylation is imperative. Rb functions in diverse processes regulating proliferation, and it has been speculated that multisite phosphorylation might act as a code in which discrete phosphorylations control specific activities. The idea of an Rb phosphorylation code is evaluated here in light of recent studies of Rb structure and function. Rb inactivation is discussed with an emphasis on how multisite phosphorylation changes Rb structure and associations with protein partners.

miR-106a overexpression and pRB downregulation in sporadic colorectal cancer

Rb1 plays an important role in cell cycle progression and therefore may be involved in malignant transformation of colonic cells. The aim of our research was to define the potential role of Rb1 as a prognostic biomarker in tumorigenesis of sporadic colorectal cancer, and to examine the role of miR-106a in Rb1 regulation as it functionally binds to 3'UTR of transcribed mRNA. We examined LOH and promoter methylation status. Real-time PCR was used for Rb1 mRNA and miR-106a, and immunohistochemistry for protein expression analysis. All the results obtained from patients' samples were correlated with the clinicopathological parameters in order to determine its influence on the sporadic colorectal carcinogenesis. LOH showed no correlation with mRNA and pRb expression. 51.5% of tumor samples were scored negative for pRb staining. Despite this finding, we detected overexpression of Rb1 mRNA in tumor samples in comparison to the adjacent normal tissue (p=0.023). mRNA overexpression was consistent with Rb1 promoter methylation analysis results, which showed no methylation in the investigated samples. Expression analysis of miR-106a in the patients samples showed its overexpression in colorectal cancer (p<10(-4)). Negative pRb score was expected according to the definition of tumor suppressor genes and their proposed role in the malignant transformation of the cells. The observed discrepancy between mRNA and protein expression can be explained by a regulatory mechanism that inhibits translation, such as microRNA silencing. Our results suggest that miR-106a might have a regulatory role for Rb1 in sporadic colorectal cancer.

Dysregulation of RNA polymerase I transcription during disease

Transcription of the ribosomal RNA genes by the dedicated RNA polymerase I enzyme and subsequent processing of the ribosomal RNA are fundamental control steps in the synthesis of functional ribosomes. Dysregulation of Pol I transcription and ribosome biogenesis is linked to the etiology of a broad range of human diseases. Diseases caused by loss of function mutations in the molecular constituents of the ribosome, or factors intimately associated with RNA polymerase I transcription and processing are collectively termed ribosomopathies. Ribosomopathies are generally rare and treatment options are extremely limited tending to be more palliative than curative. Other more common diseases are associated with profound changes in cellular growth such as cardiac hypertrophy, atrophy or cancer. In contrast to ribosomopathies, altered RNA polymerase I transcriptional activity in these diseases largely results from dysregulated upstream oncogenic pathways or by direct modulation by oncogenes or tumor suppressors at the level of the RNA polymerase I transcription apparatus itself. Ribosomopathies associated with mutations in ribosomal proteins and ribosomal RNA processing or assembly factors have been covered by recent excellent reviews. In contrast, here we review our current knowledge of human diseases specifically associated with dysregulation of RNA polymerase I transcription and its associated regulatory apparatus, including some cases where this dysregulation is directly causative in disease. We will also provide insight into and discussion of possible therapeutic approaches to treat patients with dysregulated RNA polymerase I transcription. This article is part of a Special Issue entitled: Transcription by Odd Pols.

The Retinoblastoma pathway regulates stem cell proliferation in freshwater planarians

Freshwater planarians are flatworms of the Lophotrochozoan superphylum and are well known for their regenerative abilities, which rely on a large population of pluripotent adult stem cells. However, the mechanisms by which planarians maintain a precise population of adult stem cells while balancing proliferation and cell death, remain to be elucidated. Here we have identified, characterized, and functionally tested the core Retinoblastoma (Rb) pathway components in planarian adult stem cell biology. The Rb pathway is an ancient and conserved mechanism of proliferation control from plants to animals and is composed of three core components: an Rb protein, and a transcription factor heterodimer of E2F and DP proteins. Although the planarian genome contains all components of the Rb pathway, we found that they have undergone gene loss from the ancestral state, similar to other species in their phylum. The single Rb homolog (Smed-Rb) was highly expressed in planarian stem cells and was required for stem cell maintenance, similar to the Rb-homologs p107 and p130 in vertebrates. We show that planarians and their phylum have undergone the most severe reduction in E2F genes observed thus far, and the single remaining E2F was predicted to be a repressive-type E2F (Smed-E2F4-1). Knockdown of either Smed-E2F4-1 or its dimerization partner Dp (Smed-Dp) by RNAi resulted in temporary hyper-proliferation. Finally, we showed that known Rb-interacting genes in other systems, histone deacetylase 1 and cyclinD (Smed-HDAC1; Smed-cycD), were similar to Rb in expression and phenotypes when knocked down by RNAi, suggesting that these established interactions with Rb may also be conserved in planarians. Together, these results showed that planarians use the conserved components of the Rb tumor suppressor pathway to control proliferation and cell survival.

Retinal horizontal cells lacking Rb1 sustain persistent DNA damage and survive as polyploid giant cells

The retinoblastoma tumor susceptibility gene, Rb1, is a key regulator of the cell cycle, and mutations in this gene have been found in many human cancers. Prior studies showed that retina-specific knockout of Rb1 in the mouse results in the formation of abnormally large horizontal cells, but the development, fate, and genomic status of these cells remain unknown. In this study, we conditionally inactivate Rb1 in early retinal progenitors and show that the loss of Rb1 leads to the rapid degeneration of most retinal cells except horizontal cells, which persist as giant cells with aberrant centrosome content, DNA damage, and polyploidy/aneuploidy. We observed inappropriate cell cycle entry of Rb1-deficient horizontal cells during the first postnatal weeks, which dropped off abruptly by P30. Despite extensive DNA damage in Rb1-deficient horizontal cells, these cells can still enter mitosis. Adult Rb1-deficient horizontal cells display elevated DNA content (5N-34N) that varied continuously, suggesting the presence of aneuploidy. We also found evidence of supernumerary and disoriented centrosomes in a rare population of mitotic cells in the mutant retinas. Overall our data demonstrate that horizontal cells are a remarkably robust cell type and can survive for months despite extensive DNA damage and elevated genome content.

Enteric-delivered rapamycin enhances resistance of aged mice to pneumococcal pneumonia through reduced cellular senescence

Rapamycin, a potent immunomodulatory drug, has shown promise in the amelioration of numerous age-associated diseases including cancer, Alzheimer's disease and cardiac hypertrophy. Yet the elderly, the population most likely to receive therapeutic rapamycin, are already at increased risk for infectious disease; thus concern exists that rapamycin may exacerbate age-associated immune dysfunctions and worsen infection outcomes. Herein, we examined the impact of enteric delivered rapamycin monotherapy (eRapa) on the susceptibility of aged (22-24month) C57BL/6 mice to Streptococcus pneumoniae, the leading bacterial cause of community-acquired pneumonia. Following challenge with S. pneumoniae, administration of eRapa conferred modest protection against mortality. Reduced mortality was the result of diminished lung damage rather than reduced bacterial burden. eRapa had no effect on basal levels of Interleukin (IL)-1α, IL-6, IL-10, IL-12p70, KC, Interferon-γ, Tumor necrosis factor α and Monocyte chemotactic protein-1 in whole lung homogenates or during pneumococcal pneumonia. Previously we have demonstrated that cellular senescence enhances permissiveness for bacterial pneumonia through increased expression of the bacterial ligands Laminin receptor (LR), Platelet-activating factor receptor (PAFr) and Cytokeratin 10 (K10). These proteins are co-opted by S. pneumoniae and other respiratory tract pathogens for host cell attachment during lung infection. UM-HET3 mice on eRapa had reduced lung cellular senescence as determined by levels of the senescence markers p21 and pRB, but not mH2A.1. Mice on eRapa also had marked reductions in PAFr, LR, and K10. We conclude that eRapa protected aged mice against pneumonia through reduced lung cellular senescence, which in turn, lowered bacterial ligand expression.

Genome instability: does genetic diversity amplification drive tumorigenesis?

Recent data show that catastrophic events during one cell cycle can cause massive genome damage producing viable clones with unstable genomes. This is in contrast with the traditional view that tumorigenesis requires a long-term process in which mutations gradually accumulate over decades. These sudden events are likely to result in a large increase in genomic diversity within a relatively short time, providing the opportunity for selective advantages to be gained by a subset of cells within a population. This genetic diversity amplification, arising from a single aberrant cell cycle, may drive a population conversion from benign to malignant. However, there is likely a period of relative genome stability during the clonal expansion of tumors - this may provide an opportunity for therapeutic intervention, especially if mechanisms that limit tolerance of aneuploidy are exploited.