mTOR inhibitors blunt the p53 response to nucleolar stress by regulating RPL11 and MDM2 levels

Interaction of Camptothecin Anticancer Drugs with Ribosomal Proteins L15 and L11: A Molecular Docking Study

The antitumor drug topotecan (TPT) is a potent inhibitor of topoisomerase I, triggering DNA breaks lethal for proliferating cancer cells. The mechanism is common to camptothecins SN38 (the active metabolite of irinotecan) and belotecan (BLT). Recently, TPT was shown to bind the ribosomal protein L15, inducing an antitumor immune activation independent of topoisomerase I. We have modeled the interaction of four camptothecins with RPL15 derived from the 80S human ribosome. Two potential drug-binding sites were identified at Ile135 and Phe129. SN38 can form robust RPL15 complexes at both sites, whereas BLT essentially gave stable complexes with site Ile135. The empirical energy of interaction (ΔE) for SN38 binding to RPL15 is similar to that determined for TPT binding to the topoisomerase I-DNA complex. Molecular models with the ribosomal protein L11 sensitive to topoisomerase inhibitors show that SN38 can form a robust complex at a single site (Cys25), much more stable than those with TPT and BLT. The main camptothecin structural elements implicated in the ribosomal protein interaction are the lactone moiety, the aromatic system and the 10-hydroxyl group. The study provides guidance to the design of modulators of ribosomal proteins L11 and L15, both considered anticancer targets.

Epithelial to Mesenchymal Transition Relevant Subtypes with Distinct Prognosis and Responses to Chemo- or Immunotherapies in Osteosarcoma

Objective

Currently, clinical classification of osteosarcoma cannot accurately predict the survival outcomes and responses to chemo- or immunotherapies. Our goal was to classify osteosarcoma patients into clinical/biological subtypes based on EMT molecules.

Methods

This study retrospectively curated the RNA expression profiling of osteosarcoma patients from the TARGET and GSE21257 cohorts. Consensus clustering analyses were conducted in accordance with the expression profiling of prognostic EMT genes derived from univariate analyses. Immunological features were evaluated through immune cell infiltration, immune checkpoint expression, and activity of cancer immunity cycle. Drug sensitivity was estimated with the GDSC database. WGCNA approach was adopted to determine the EMT-derived genes. Following univariate analyses, a multivariate cox regression model was developed and externally verified. Predictive independency was evaluated with uni- and multivariate analyses. GSEA was presented to uncover relevant molecular mechanisms.

Results

Prognostic EMT genes across osteosarcoma patients were stratified into distinct subtypes, namely, subtypes A and B. Patients in subtype B presented desirable prognosis, high immune activation, and enhanced sensitivity to cisplatin. Meanwhile, patients in subtype A were more sensitive to gemcitabine. In total, 86 EMT-derived hub genes were determined, and an EMT score was conducted for osteosarcoma prognosis. Following external verification, this EMT score was reliably and independently predictive of patients' survival outcomes. Additionally, it was positively linked to steroid biosynthesis.

Conclusion

Overall, our findings proposed EMT-relevant molecular subtypes and signatures for predicting prognosis and therapeutic responses, contributing to personalized treatment and clinical implication for osteosarcoma.

p53 at the crossroad of DNA replication and ribosome biogenesis stress pathways

Despite several decades of intense research focused on understanding function(s) and disease-associated malfunction of p53, there is no sign of any “mid-life crisis” in this rapidly advancing area of biomedicine. Firmly established as the hub of cellular stress responses and tumor suppressor targeted in most malignancies, p53’s many talents continue to surprise us, providing not only fresh insights into cell and organismal biology, but also new avenues to cancer treatment. Among the most fruitful lines of p53 research in recent years have been the discoveries revealing the multifaceted roles of p53-centered pathways in the fundamental processes of DNA replication and ribosome biogenesis (RiBi), along with cellular responses to replication and RiBi stresses, two intertwined areas of cell (patho)physiology that we discuss in this review. Here, we first provide concise introductory notes on the canonical roles of p53, the key interacting proteins, downstream targets and post-translational modifications involved in p53 regulation. We then highlight the emerging involvement of p53 as a key component of the DNA replication Fork Speed Regulatory Network and the mechanistic links of p53 with cellular checkpoint responses to replication stress (RS), the driving force of cancer-associated genomic instability. Next, the tantalizing, yet still rather foggy functional crosstalk between replication and RiBi (nucleolar) stresses is considered, followed by the more defined involvement of p53-mediated monitoring of the multistep process of RiBi, including the latest updates on the RPL5/RPL11/5 S rRNA-MDM2-p53-mediated Impaired Ribosome Biogenesis Checkpoint (IRBC) pathway and its involvement in tumorigenesis. The diverse defects of RiBi and IRBC that predispose and/or contribute to severe human pathologies including developmental syndromes and cancer are then outlined, along with examples of promising small-molecule-based strategies to therapeutically target the RS- and particularly RiBi- stress-tolerance mechanisms to which cancer cells are addicted due to their aberrant DNA replication, repair, and proteo-synthesis demands.

Dieckol exerts anticancer activity in human osteosarcoma (MG-63) cells through the inhibition of PI3K/AKT/mTOR signaling pathway

Background

Osteosarcoma (OS) is the most common malignant bone cancer with more metastasis and increased occurrence in children and teen-agers and being responsible for more number of morbidity and mortality worldwide.

Objective

The current exploration was planned study the in vitro anticancer actions of dieckol against human OS MG-63 cells via PI3K/AKT/mTOR signaling inhibition.

Methodology

The cytotoxicity of dieckol was scrutinized by MTT assay. Effects of dieckol on the ROS accumulation, apoptotic cell death, and MMP level in the MG-63 cells were studied by respective fluorescence staining assays. The levels of proliferative, inflammatory, and apoptotic markers in the dieckol treated MG-63 cells were scrutinized by marker specific kits. The expressions of PI3K, AKT, and mTOR was assayed by RT-PCR.

Results

The MTT assay revealed that the dieckol dose dependently prevented MG-63 cells viability and the IC50 was found at 15 µM. Dieckol treatment effectively reduced the MMP level and improved the ROS generation and apoptosis in MG-63 cells. Dieckol also regulated the proliferative (cyclin D1), inflammatory (COX-2, IL-6, TNF-α, and NF-κB), and apoptotic (caspase-3, Bax, Bcl-2) markers in the MG-63 cells. The PI3K/AKT/mTOR signaling in the MG-63 cells were effectively inhibited by the dieckol treatment.

Conclusion

In conclusion, our findings from this study recommends that the dieckol could be a talented anticancer candidate for the OS management in the future.

EBP2, a novel NPM-ALK-interacting protein in the nucleolus, contributes to the proliferation of ALCL cells by regulating tumor suppressor p53

The oncogenic fusion protein nucleophosmin-anaplastic lymphoma kinase (NPM-ALK), found in anaplastic large-cell lymphoma (ALCL), localizes to the cytosol, nucleoplasm, and nucleolus. However, the relationship between its localization and transforming activity remains unclear. We herein demonstrated that NPM-ALK localized to the nucleolus by binding to nucleophosmin 1 (NPM1), a nucleolar protein that exhibits shuttling activity between the nucleolus and cytoplasm, in a manner that was dependent on its kinase activity. In the nucleolus, NPM-ALK interacted with Epstein-Barr virus nuclear antigen 1-binding protein 2 (EBP2), which is involved in rRNA biosynthesis. Moreover, enforced expression of NPM-ALK induced tyrosine phosphorylation of EBP2. Knockdown of EBP2 promoted the activation of the tumor suppressor p53, leading to G0 /G1 -phase cell cycle arrest in Ba/F3 cells transformed by NPM-ALK and ALCL patient-derived Ki-JK cells, but not ALCL patient-derived SUDH-L1 cells harboring p53 gene mutation. In Ba/F3 cells transformed by NPM-ALK and Ki-JK cells, p53 activation induced by knockdown of EBP2 was significantly inhibited by Akt inhibitor GDC-0068, mTORC1 inhibitor rapamycin, and knockdown of Raptor, an essential component of mTORC1. These results suggest that the knockdown of EBP2 triggered p53 activation through the Akt-mTORC1 pathway in NPM-ALK-positive cells. Collectively, the present results revealed the critical repressive mechanism of p53 activity by EBP2 and provide a novel therapeutic strategy for the treatment of ALCL.

Estrogen and estrogen receptors chauffeur the sex-biased autophagic action in liver

Autophagy, or cellular self-digestion, is an essential cellular process imperative for energy homeostasis, development, differentiation, and survival. However, the intrinsic factors that bring about the sex-biased differences in liver autophagy are still unknown. In this work, we found that autophagic genes variably expresses in the steroidogenic tissues, mostly abundant in liver, and is influenced by the individual's sexuality. Starvation-induced autophagy in a time-dependent female-dominated manner, and upon starvation, a strong gender responsive circulating steroid-HK2 relation was observed, which highlighted the importance of estrogen in autophagy regulation. This was further confirmed by the enhanced or suppressed autophagy upon estrogen addition (male) or blockage (female), respectively. In addition, we found that estrogen proved to be the common denominator between stress management, glucose metabolism, and autophagic action in female fish. To understand further, we used estrogen receptor (ER)α- and ER-β2-knockout (KO) medaka and found ER-specific differences in sex-biased autophagy. Interestingly, starvation resulted in significantly elevated mTOR transcription (compared with control) in male ERα-KO fish while HK2 and ULK activation was greatly decreased in both KO fish in a female oriented fashion. Later, ChIP analysis confirmed that, NRF2, an upstream regulator of mTOR, only binds to ERα, while both ERα and ERβ2 are effectively pulled down the HK2 and LC3. FIHC data show that, in both ER-KO fish, LC3 nuclear-cytoplasmic transport and its associated pathways involving SIRT1 and DOR were greatly affected. Cumulatively, our data suggest that, ERα-KO strongly affected the early autophagic initiation and altered the LC3 nuclear-cytoplasmic translocation, thereby influencing the sex-biased final autophagosome formation in medaka. Thus, existence of steroid responsive autophagy regulatory-switches and sex-biased steroid/steroid receptor availability influences the gender-skewed autophagy. Expectedly, this study may furnish newer appreciation for gender-specific medicine research and therapeutics.

Emerging Role of the Nucleolar Stress Response in Autophagy

Autophagy represents a conserved self-digestion program, which allows regulated degradation of cellular material. Autophagy is activated by cellular stress, serum starvation and nutrient deprivation. Several autophagic pathways have been uncovered, which either non-selectively or selectively target the cellular cargo for lysosomal degradation. Autophagy engages the coordinated action of various key regulators involved in the steps of autophagosome formation, cargo targeting and lysosomal fusion. While non-selective (macro)autophagy is required for removal of bulk material or recycling of nutrients, selective autophagy mediates specific targeting of damaged organelles or protein aggregates. By proper action of the autophagic machinery, cellular homeostasis is maintained. In contrast, failure of this fundamental process is accompanied by severe pathophysiological conditions. Hallmarks of neuropathological disorders are for instance accumulated, mis-folded protein aggregates and damaged mitochondria. The nucleolus has been recognized as central hub in the cellular stress response. It represents a sub-nuclear organelle essential for ribosome biogenesis and also functions as stress sensor by mediating cell cycle arrest or apoptosis. Thus, proper nucleolar function is mandatory for cell growth and survival. Here, I highlight the emerging role of nucleolar factors in the regulation of autophagy. Moreover, I discuss the nucleolar stress response as a novel signaling pathway in the context of autophagy, health and disease.

EBF1-Mediated Upregulation of Ribosome Assembly Factor PNO1 Contributes to Cancer Progression by Negatively Regulating the p53 Signaling Pathway

The RNA-binding protein PNO1 is critical for ribosome biogenesis, but its potential role in cancer remains unknown. In this study, online data mining, cDNA, and tissue microarrays indicated that PNO1 expression was higher in colorectal cancer tissue than in noncancerous tissue, and its overexpression was associated with worse patient survival. Gain-of-function and loss-of-function studies demonstrated that PNO1 knockdown suppressed growth of colorectal cancer cells in vitro and in vivo, while PNO1 overexpression promoted colorectal cancer cell proliferation in vitro. In colorectal cancer cells expressing wild-type p53, PNO1 knockdown enhanced expression of p53 and its downstream gene p21, and reduced cell viability; these effects were prevented by p53 knockout and attenuated by the p53 inhibitor PFT-α. Moreover, PNO1 knockdown in HCT116 cells decreased levels of 18S rRNA, of 40S and 60S ribosomal subunits, and of the 80S ribosome. It also reduced global protein synthesis, increasing nuclear stress and inhibiting MDM2-mediated ubiquitination and p53 degradation. Overexpressing EBF1 suppressed PNO1 promoter activity and decreased PNO1 mRNA and protein, inhibiting cell proliferation and inducing cell apoptosis through the p53/p21 pathway. In colorectal cancer tissues, the expression of EBF1 correlated inversely with PNO1. Data mining of online breast and lung cancer databases showed increased PNO1 expression and association with poor patient survival; PNO1 knockdown reduced cell viability of cultured breast and lung cancer cells. Taken together, these findings indicate that PNO1 is overexpressed in colorectal cancer and correlates with poor patient survival, and that PNO1 exerts oncogenic effects, at least, in part, by altering ribosome biogenesis. SIGNIFICANCE: This study identifies the ribosome assembly factor PNO1 as a potential oncogene involved in tumor growth and progression of colorectal cancer.

Ribosomal Proteins Control Tumor Suppressor Pathways in Response to Nucleolar Stress

Ribosome biogenesis includes the making and processing of ribosomal RNAs, the biosynthesis of ribosomal proteins from their mRNAs in the cytosol and their transport to the nucleolus to assemble pre-ribosomal particles. Several stresses including cellular senescence reduce nucleolar rRNA synthesis and maturation increasing the availability of ribosome-free ribosomal proteins. Several ribosomal proteins can activate the p53 tumor suppressor pathway but cells without p53 can still arrest their proliferation in response to an imbalance between ribosomal proteins and mature rRNA production. Recent results on senescence-associated ribogenesis defects (SARD) show that the ribosomal protein S14 (RPS14 or uS11) can act as a CDK4/6 inhibitor linking ribosome biogenesis defects to the main engine of cell cycle progression. This work offers new insights into the regulation of the cell cycle and suggests novel avenues to design anticancer drugs.

Reduced Expression of PROX1 Transitions Glioblastoma Cells into a Mesenchymal Gene Expression Subtype

The homeodomain transcription factor PROX1 has been linked to several cancer types, including gliomas, but its functions remain to be further elucidated. Here we describe a functional role and the prognostic value of PROX1 in glioblastoma. Low expression of PROX1 correlated with poor overall survival and the mesenchymal glioblastoma subtype signature. The latter finding was recapitulated in vitro, where suppression or overexpression of PROX1 in glioma cell cultures transitioned cells to a mesenchymal or to a nonmesenchymal glioblastoma gene expression signature, respectively. PROX1 modulation affected proliferation rates that coincided with changes in protein levels of CCNA1 and CCNE1 as well as the cyclin inhibitors CDKN1A, CDKN1B, and CDKN1C. Overexpression of SOX2 increased PROX1 expression, but treatment with a CDK2 inhibitor subsequently decreased PROX1 expression, which was paralleled by decreased SOX2 levels. The THRAP3 protein was a novel binding partner for PROX1, and suppression of THRAP3 increased both transcript and protein levels of PROX1. Together, these findings highlight the prognostic value of PROX1 and its role as a regulator of glioblastoma gene expression subtypes, intratumoral heterogeneity, proliferation, and cell-cycle control.Significance: These findings demonstrate the role and prognostic value of PROX1 in glioblastomas; low PROX1 levels correlate with a mesenchymal gene expression subtype and shorter survival in glioblastoma tumors. Cancer Res; 78(20); 5901-16. ©2018 AACR.

Molecular determinants of Drosophila immunophilin FKBP39 nuclear localization

FK506-binding proteins (FKBPs) belong to a distinct class of immunophilins that interact with immunosuppressants. They use their peptidyl-prolyl isomerase (PPIase) activity to catalyze the cis-trans conversion of prolyl bonds in proteins during protein-folding events. FKBPs also act as a unique group of chaperones. The Drosophila melanogaster peptidyl-prolyl cis-trans isomerase FK506-binding protein of 39 kDa (FKBP39) is thought to act as a transcriptional modulator of gene expression in 20-hydroxyecdysone and juvenile hormone signal transduction. The aim of this study was to analyze the molecular determinants responsible for the subcellular distribution of an FKBP39-yellow fluorescent protein (YFP) fusion construct (YFP-FKBP39). We found that YFP-FKBP39 was predominantly nucleolar. To identify the nuclear localization signal (NLS), a series of YFP-tagged FKBP39 deletion mutants were prepared and examined in vivo. The identified NLS signal is located in a basic domain. Detailed mutagenesis studies revealed that residues K188 and K191 are crucial for the nuclear targeting of FKBP39 and its nucleoplasmin-like (NPL) domain contains the sequence that controls the nucleolar-specific translocation of the protein. These results show that FKBP39 possesses a specific NLS in close proximity to a putative helix-turn-helix (HTH) motif and FKBP39 may bind DNA in vivo and in vitro.

mTOR: An attractive therapeutic target for osteosarcoma?

Osteosarcoma (OS) is a common primary malignant bone tumor with high morbidity and mortality in children and young adults. How to improve poor prognosis of OS due to resistance to chemotherapy remains a challenge. Recently, growing findings show activation of mammalian target of rapamycin (mTOR), is associated with OS cell growth, proliferation, metastasis. Targeting mTOR may be a promising therapeutic approach for treating OS. This review summarizes the roles of mTOR pathway in OS and present research status of mTOR inhibitors in the context of OS. In addition, we have attempted to discuss how to design a better treatment project for OS by combining mTOR inhibitor with other drugs.

mTOR Cross-Talk in Cancer and Potential for Combination Therapy

The mammalian Target of Rapamycin (mTOR) pathway plays an essential role in sensing and integrating a variety of exogenous cues to regulate cellular growth and metabolism, in both physiological and pathological conditions. mTOR functions through two functionally and structurally distinct multi-component complexes, mTORC1 and mTORC2, which interact with each other and with several elements of other signaling pathways. In the past few years, many new insights into mTOR function and regulation have been gained and extensive genetic and pharmacological studies in mice have enhanced our understanding of how mTOR dysfunction contributes to several diseases, including cancer. Single-agent mTOR targeting, mostly using rapalogs, has so far met limited clinical success; however, due to the extensive cross-talk between mTOR and other pathways, combined approaches are the most promising avenues to improve clinical efficacy of available therapeutics and overcome drug resistance. This review provides a brief and up-to-date narrative on the regulation of mTOR function, the relative contributions of mTORC1 and mTORC2 complexes to cancer development and progression, and prospects for mTOR inhibition as a therapeutic strategy.

NVP-BEZ235 synergizes cisplatin sensitivity in osteosarcoma

Osteosarcoma(OS) remains a major health concern in childhood and adolescence, although cisplatin is one of the gold standard chemotherapeutic drugs in the treatment of OS, chemoresistant to cisplatin is common. Phosphoinositide 3-kinase (PI3K)-Akt-mammalian target of rapamycin inhibitor (mTOR) pathway and autophagy regulates chemosensitivity incancer cells. In this study, we hypothesized that NVP-BEZ235, a dual inhibitor of PI3K/mTOR, could synergize cisplatin sensitivity in OS. In vitro, NVP-BEZ235 plus cisplatinexerted a synergistic effect on cell proliferation inhibition and apoptosis induction. Cisplatin could activate PI3K-Akt-mTOR pathway activity in early times, whereas, NVP-BEZ235 could inhibit PI3K-Akt -mTOR pathway activity all the times alone or combined with cisplatin. What's more, NVP-BEZ235 could switch function of autophagy induced by cisplatin to synergize cisplatin sensitivity. In vivo, pronounced decrease in tumor cell proliferation and increase in apoptosisin combination-treated mouse xenograft models compared with cisplatin or NVP-BEZ235 treated models. All these results suggest NVP-BEZ235 could synergize cisplatin sensitivity in OS, combination of NVP-BEZ235 with cisplatin could represent a novel therapeutic strategy for treatment of OS.

Heterozygous loss of TSC2 alters p53 signaling and human stem cell reprogramming

Tuberous sclerosis complex (TSC) is a pediatric disorder of dysregulated growth and differentiation caused by loss of function mutations in either the TSC1 or TSC2 genes, which regulate mTOR kinase activity. To study aberrations of early development in TSC, we generated induced pluripotent stem cells using dermal fibroblasts obtained from patients with TSC. During validation, we found that stem cells generated from TSC patients had a very high rate of integration of the reprogramming plasmid containing a shRNA against TP53. We also found that loss of one allele of TSC2 in human fibroblasts is sufficient to increase p53 levels and impair stem cell reprogramming. Increased p53 was also observed in TSC2 heterozygous and homozygous mutant human stem cells, suggesting that the interactions between TSC2 and p53 are consistent across cell types and gene dosage. These results support important contributions of TSC2 heterozygous and homozygous mutant cells to the pathogenesis of TSC and the important role of p53 during reprogramming.

New roles for Dicer in the nucleolus and its relevance to cancer

The nucleolus is a distinct compartment of the nucleus responsible for ribosome biogenesis. Mis-regulation of nucleolar functions and of the cellular translation machinery has been associated with disease, in particular with many types of cancer. Indeed, many tumor suppressors (p53, Rb, PTEN, PICT1, BRCA1) and proto-oncogenes (MYC, NPM) play a direct role in the nucleolus, and interact with the RNA polymerase I transcription machinery and the nucleolar stress response. We have identified Dicer and the RNA interference pathway as having an essential role in the nucleolus of quiescent Schizosaccharomyces pombe cells, distinct from pericentromeric silencing, by controlling RNA polymerase I release. We propose that this novel function is evolutionarily conserved and may contribute to the tumorigenic pre-disposition of DICER1 mutations in mammals.

The roles of RRP15 in nucleolar formation, ribosome biogenesis and checkpoint control in human cells

The nucleolus controls ribosome biogenesis and its perturbation induces nucleolar stress that inhibits cell cycle progression and activates checkpoint responses. Here, we investigate the roles of ribosomal RNA processing protein, RRP15, in nucleolar formation, ribosome biogenesis, cell cycle progression and checkpoint control in human cells. RRP15 is localized in the nucleolus and required for nucleolar formation. In contrast to the budding yeast Rrp15p that was reported as a component of pre-60S subunits, RRP15 is found in both pre-40S and pre-60S subunits and involved in regulating rRNA transcription and ribosome biogenesis. Perturbation of RRP15 induces nucleolar stress that activates RPL5/RPL11/5S rRNA (RP)-Mdm2-p53 axis checkpoint response and arrests cells at G1-G1/S in p53-proficient non-transformed RPE1 cells but not in p53-deficient HeLa and MCF7 tumor cells. Instead, p53-deficient HeLa and MCF7 cells with RRP15-dependent nucleolar stress enter S-phase with S-phase perturbation that activates ATR-Chk1- γH2AX axis DNA replication/damage checkpoint response, delaying S-G2/M progression and, ultimately, causing cell death. The selective checkpoint response, cell cycle inhibition and/or cytotoxicity induced by RRP15-dependent nucleolar stress in p53-proficient non-transformed cells and p53-deficient tumor cells suggest that RRP15 might be a potential target for cancer therapy.

Machine learning for predicting lifespan-extending chemical compounds

Increasing age is a risk factor for many diseases; therefore developing pharmacological interventions that slow down ageing and consequently postpone the onset of many age-related diseases is highly desirable. In this work we analyse data from the DrugAge database, which contains chemical compounds and their effect on the lifespan of model organisms. Predictive models were built using the machine learning method random forests to predict whether or not a chemical compound will increase Caenorhabditis elegans' lifespan, using as features Gene Ontology (GO) terms annotated for proteins targeted by the compounds and chemical descriptors calculated from each compound's chemical structure. The model with the best predictive accuracy used both biological and chemical features, achieving a prediction accuracy of 80%. The top 20 most important GO terms include those related to mitochondrial processes, to enzymatic and immunological processes, and terms related to metabolic and transport processes. We applied our best model to predict compounds which are more likely to increase C. elegans' lifespan in the DGIdb database, where the effect of the compounds on an organism's lifespan is unknown. The top hit compounds can be broadly divided into four groups: compounds affecting mitochondria, compounds for cancer treatment, anti-inflammatories, and compounds for gonadotropin-releasing hormone therapies.

mTOR signaling in osteosarcoma: Oncogenesis and therapeutic aspects (Review)

The mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase that belongs to the phosphoinositide-3-kinase (PI3K)-related kinase family. Oncogenic activation of mTOR signaling significantly contributes to the progression of different types of cancers including osteosarcoma (OS; the most common primary malignant tumor of bone). In the present study, we review the association of the mTOR signaling pathway with OS, and the possible effective treatment strategies by targeting this pathway. In the metastatic behavior of OS, one of the most common actionable aberrations was found in the PI3K/Akt/mTOR pathway. Upon phosphorylation, activated mTOR contributes to OS cellular transformation and poor cancer prognosis via downstream effectors such as S6K1, 4EBP1 and eIF4E, which are overexpressed in OS. Targeting the mTOR complex is a significant approach in cancer therapeutic research, and of course, rapamycin is the primary inhibitor of mTOR. Various other chemotherapeutic molecules have also shown potential activity against mTOR. As mTOR is a new promising oncological target and blockade of the mTOR pathway with selective inhibitors has significant potential in OS therapeutic research, the development of the optimal dose, regimen and a rationale for the use of mTOR inhibitors in combination with other anticancer agents may provide a successful treatment strategy for OS.

Nucleolus-derived mediators in oncogenic stress response and activation of p53-dependent pathways

Rapid growth and division of cells, including tumor ones, is correlated with intensive protein biosynthesis. The output of nucleoli, organelles where translational machineries are formed, depends on a rate of particular stages of ribosome production and on accessibility of elements crucial for their effective functioning, including substrates, enzymes as well as energy resources. Different factors that induce cellular stress also often lead to nucleolar dysfunction which results in ribosome biogenesis impairment. Such nucleolar disorders, called nucleolar or ribosomal stress, usually affect cellular functioning which in fact is a result of p53-dependent pathway activation, elicited as a response to stress. These pathways direct cells to new destinations such as cell cycle arrest, damage repair, differentiation, autophagy, programmed cell death or aging. In the case of impaired nucleolar functioning, nucleolar and ribosomal proteins mediate activation of the p53 pathways. They are also triggered as a response to oncogenic factor overexpression to protect tissues and organs against extensive proliferation of abnormal cells. Intentional impairment of any step of ribosome biosynthesis which would direct the cells to these destinations could be a strategy used in anticancer therapy. This review presents current knowledge on a nucleolus, mainly in relation to cancer biology, which is an important and extremely sensitive element of the mechanism participating in cellular stress reaction mediating activation of the p53 pathways in order to counteract stress effects, especially cancer development.

PRAS40 deregulates apoptosis in Ewing sarcoma family tumors by enhancing the insulin receptor/Akt and mTOR signaling pathways

EWS expression in Ewing sarcoma family tumors (ESFTs) is decreased due to the haploinsufficiency elicited by chromosomal translocation. The abnormal expression levels of EWS and its downstream factors contribute to the manifestation of ESFTs. Previously, we reported that increased Proline-rich Akt substrate of 40 kDa (PRAS40), which is encoded by an EWS mRNA target, promotes the development of ESFTs. However, the mechanism remains elusive. To clarify the role of PRAS40 in ESFTs, we silenced PRAS40 expression in ESFT cells using siRNAs and found increased levels of terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cells. Cleaved caspase 3 levels and cytochrome C release were increased simultaneously. Furthermore, with PRAS40 knockdown, the phosphorylation of Akt and mTOR downstream factors, i.e., S6K and S6, was attenuated notably. Ectopic expression of PRAS40 increased Akt and S6 phosphorylation. Activation of Akt only partially reversed the apoptosis induced by PRAS40 knockdown, and downregulation of S6 phosphorylation by PRAS40 silencing could not be sufficiently restored via Akt activation. Searching the upstream factors in this pathway, the autophosphorylation of insulin receptor (IR) was found to be inhibited significantly by PRAS40 silencing but increased by PRAS40 overexpression. Therefore, PRAS40 may enhance IR phosphorylation to facilitate Akt and mTOR signaling leading to the apoptosis deregulation in ESFTs. Moreover, in vivo results confirmed that PRAS40 deletion suppressed the growth of ESFT xenografts and downregulated IR and S6 phosphorylation. Our findings suggest a novel functioning model for PRAS40, which represents a novel therapeutic target for ESFTs.

Research progress on the multidrug resistance mechanisms of osteosarcoma chemotherapy and reversal

Osteosarcoma (OS) is the most common and aggressive primary malignant type of bone cancer in children and adolescents. Chemotherapy is one of the most important treatments for OS. Although cancer therapy has improved over the past few decades, survival outcomes for OS patients remain unsatisfactory. One of the primary reasons for the failure of current treatments is that patients with stage IV cancer often develop resistance to anticancer agents. This article will review multidrug resistance (MDR) mechanisms of OS and strategies for overcoming resistance.