c-myc Repression of TSC2 contributes to control of translation initiation and Myc-induced transformation

Exploiting Translation Machinery for Cancer Therapy: Translation Factors as Promising Targets

Eukaryotic protein translation has slowly gained the scientific community's attention for its advanced and powerful therapeutic potential. However, recent technical developments in studying ribosomes and global translation have revolutionized our understanding of this complex multistep process. These developments have improved and deepened the current knowledge of mRNA translation, sparking excitement and new possibilities in this field. Translation factors are crucial for maintaining protein synthesis homeostasis. Since actively proliferating cancer cells depend on protein synthesis, dysregulated protein translation is central to tumorigenesis. Translation factors and their abnormal expressions directly affect multiple oncogenes and tumor suppressors. Recently, small molecules have been used to target translation factors, resulting in translation inhibition in a gene-specific manner, opening the door for developing translation inhibitors that can lead to novel chemotherapeutic drugs for treating multiple cancer types caused by dysregulated translation machinery. This review comprehensively summarizes the involvement of translation factors in tumor progression and oncogenesis. Also, it sheds light on the evolution of translation factors as novel drug targets for developing future therapeutic drugs for treating cancer.

Low NDRG2, regulated by the MYC/MIZ-1 complex and methylation, predicts poor outcomes in DLBCL patients

Diffuse large B-cell lymphoma (DLBCL) represents the most common tumor in non-Hodgkin's lymphoma. N-Myc downstream-regulated gene 2 (NDRG2) is a tumor suppressor highly expressed in healthy tissues but downregulated in many cancers. Although cell proliferation-related metabolism rewiring has been well characterized, less is known about the mechanism of metabolic changes with DLBCL. Herein, we investigated the expressions of NDRG2, MYC and Myc-interacting zinc finger protein 1 (MIZ-1) in seven human lymphoma (mostly DLBCLs) cell lines. NDRG2 expression was inversely correlated with the expressions of MYC and MIZ-1. Further, we explored the regulatory mechanism and biological functions underlying the lymphomagenesis involving NDRG2, MYC and MIZ-1. MYC and MIZ-1 promoted DLBCL cell proliferation, while NDRG2 induced apoptosis in LY8 cells. Moreover, NDRG2 methylation was reversed by the 5-Aza-2'-deoxycytidine (5-Aza-CDR) treatment, triggering the downregulation of MYC and inhibiting DLBCL cell survival. MYC interacts with NDRG2 to regulate energy metabolism associated with mTOR. Remarkably, supporting the biological significance, the converse correlation between NDRG2 and MYC was observed in human DLBCL tumor tissues (R = -0.557). Bioinformatics analysis further validated the association among NDRG2, MYC, MIZ-1, mTOR, and related metabolism genes. Additionally, NDRG2 (P = 0.001) and MYC (P < 0.001) were identified as promising prognostic biomarkers in DLBCL patients through survival analysis. Together, our data demonstrate that the MYC/MIZ-1 complex interplays with NDRG2 to influence the proliferation and apoptosis of DLBCL cells and show the characterizations of NDRG2, MYC and MIZ-1 for metabolism features and prediction prognosis in DLBCL.

Target c-Myc to treat pancreatic cancer

C-Myc overexpression is a common finding in pancreatic cancer and predicts the aggressive behavior of cancer cells. It binds to the promoter of different genes, thereby regulating their transcription. C-Myc is downstream of KRAS and interacts with several oncogenic and proliferative pathways in pancreatic cancer. C-Myc enhances aerobic glycolysis in cancer cells and regulates glutamate biosynthesis from glutamine. It provides enough energy for cancer cells' metabolism and sufficient substrate for the synthesis of organic molecules. C-Myc overexpression is associated with chemoresistance, intra-tumor angiogenesis, epithelial-mesenchymal transition (EMT), and metastasis in pancreatic cancer. Despite its title, c-Myc is not "undruggable" and recent studies unveiled that it can be targeted, directly or indirectly. Small molecules that accelerate c-Myc ubiquitination and degradation have been effective in preclinical studies. Small molecules that hinder c-Myc-MAX heterodimerization or c-Myc/MAX/DNA complex formation can functionally inhibit c-Myc. In addition, c-Myc can be targeted through transcriptional, post-transcriptional, and translational modifications.

Crosstalk between PI3K/AKT/mTOR and WNT/β-Catenin signaling in GBM - Could combination therapy checkmate the collusion?

Glioblastoma multiforme is one of the calamitous primary glial brain tumors with extensive heterogeneity at cellular and molecular levels. While maximal surgical resection trailed by radio and chemotherapy employing temozolomide remains the gold-standard treatment for malignant glioma patients, the overall prognosis remains dismal and there exists an unmet need for effective therapeutic strategies. In this context, we hypothesize that proper understanding of signaling pathways responsible for glioblastoma multiforme proliferation would be the first trump card while searching for novel targeted therapies. Among the pathways aberrantly activated, PI3K/AKT/mTOR is the most significant pathway, that is clinically implicated in malignancies such as high-grade glioma. Further, the WNT/β-Catenin cascade is well-implicated in several malignancies, while its role in regulating glioma pathogenesis has only emerged recently. Nevertheless, oncogenic activation of both these pathways is a frequent event in malignant glioma that facilitates tumor proliferation, stemness and chemo-resistance. Recently, it has been reported that the cross-talk of PI3K/AKT/mTOR pathway with multiple signaling pathways could promote glioma progression and reduce the sensitivity of glioma cells to the standard therapy. However, very few studies had focused on the relationship between PI3K/AKT/mTOR and WNT/β-Catenin pathways in glioblastoma multiforme. Interestingly, in homeostatic and pathologic circumstances, both these pathways depict fine modulation and are connected at multiple levels by upstream and downstream effectors. Thus, gaining deep insights on the collusion between these pathways would help in discovering unique therapeutic targets for glioblastoma multiforme management. Hence, the current review aims to address, "the importance of inter-play between PI3K/AKT/mTOR and WNT/β-Catenin pathways", and put forward, "the possibility of combinatorially targeting them", for glioblastoma multiforme treatment enhancement.

Ribosomal Protein S6: A Potential Therapeutic Target against Cancer?

Ribosomal protein S6 (RPS6) is a component of the 40S small ribosomal subunit and participates in the control of mRNA translation. Additionally, phospho (p)-RPS6 has been recognized as a surrogate marker for the activated PI3K/AKT/mTORC1 pathway, which occurs in many cancer types. However, downstream mechanisms regulated by RPS6 or p-RPS remains elusive, and the therapeutic implication of RPS6 is underappreciated despite an approximately half a century history of research on this protein. In addition, substantial evidence from RPS6 knockdown experiments suggests the potential role of RPS6 in maintaining cancer cell proliferation. This motivates us to investigate the current knowledge of RPS6 functions in cancer. In this review article, we reviewed the current information about the transcriptional regulation, upstream regulators, and extra-ribosomal roles of RPS6, with a focus on its involvement in cancer. We also discussed the therapeutic potential of RPS6 in cancer.

The PI3K-AKT-mTOR Pathway and Prostate Cancer: At the Crossroads of AR, MAPK, and WNT Signaling

Oncogenic activation of the phosphatidylinositol-3-kinase (PI3K), protein kinase B (PKB/AKT), and mammalian target of rapamycin (mTOR) pathway is a frequent event in prostate cancer that facilitates tumor formation, disease progression and therapeutic resistance. Recent discoveries indicate that the complex crosstalk between the PI3K-AKT-mTOR pathway and multiple interacting cell signaling cascades can further promote prostate cancer progression and influence the sensitivity of prostate cancer cells to PI3K-AKT-mTOR-targeted therapies being explored in the clinic, as well as standard treatment approaches such as androgen-deprivation therapy (ADT). However, the full extent of the PI3K-AKT-mTOR signaling network during prostate tumorigenesis, invasive progression and disease recurrence remains to be determined. In this review, we outline the emerging diversity of the genetic alterations that lead to activated PI3K-AKT-mTOR signaling in prostate cancer, and discuss new mechanistic insights into the interplay between the PI3K-AKT-mTOR pathway and several key interacting oncogenic signaling cascades that can cooperate to facilitate prostate cancer growth and drug-resistance, specifically the androgen receptor (AR), mitogen-activated protein kinase (MAPK), and WNT signaling cascades. Ultimately, deepening our understanding of the broader PI3K-AKT-mTOR signaling network is crucial to aid patient stratification for PI3K-AKT-mTOR pathway-directed therapies, and to discover new therapeutic approaches for prostate cancer that improve patient outcome.

Notch Inhibition via GSI Treatment Elevates Protein Synthesis in C2C12 Myotubes

The role of Notch signaling is widely studied in skeletal muscle regeneration but little is known about its influences on muscle protein synthesis (MPS). The purpose of this study was to investigate whether Notch signaling is involved in the regulation of MPS. C2C12 cells were treated with a γ-secretase inhibitor (GSI), to determine the effect of reduced Notch signaling on MPS and anabolic signaling markers. GSI treatment increased myotube hypertrophy by increasing myonuclear accretion (nuclei/myotube: p = 0.01) and myonuclear domain (myotube area per fusing nuclei: p < 0.001) in differentiating C2C12 cells. GSI treatment also elevated myotube hypertrophy in differentiated C2C12s (area/myotube; p = 0.01). In concert, GSI treatment augmented pmTOR Ser2448 (p = 0.01) and protein synthesis (using SUnSET method) in myotubes (p < 0.001). Examining protein expression upstream of mTOR revealed reductions in PTEN (p = 0.04), with subsequent elevations in pAKT Thr308 (p < 0.001) and pAKT Ser473 (p = 0.05). These findings reveal that GSI treatment elevates myotube hypertrophy through both augmentation of fusion and MPS. This study sheds light on the potential multifaceted roles of Notch within skeletal muscle. Furthermore, we have demonstrated that Notch may modulate the PTEN/AKT/mTOR pathway.

Targeting translation initiation by synthetic rocaglates for treating MYC-driven lymphomas

MYC-driven lymphomas, especially those with concurrent MYC and BCL2 dysregulation, are currently a challenge in clinical practice due to rapid disease progression, resistance to standard chemotherapy and high risk of refractory disease. MYC plays a central role by coordinating hyperactive protein synthesis with upregulated transcription in order to support rapid proliferation of tumor cells. Translation initiation inhibitor rocaglates have been identified as the most potent drugs in MYC-driven lymphomas as they efficiently inhibit MYC expression and tumor cell viability. We found that this class of compounds can overcome eIF4A abundance by stabilizing target mRNA-eIF4A interaction that directly prevents translation. Proteome-wide quantification demonstrated selective repression of multiple critical oncoproteins in addition to MYC in B cell lymphoma including NEK2, MCL1, AURKA, PLK1, and several transcription factors that are generally considered undruggable. Finally, (−)-SDS-1–021, the most promising synthetic rocaglate, was confirmed to be highly potent as a single agent, and displayed significant synergy with the BCL2 inhibitor ABT199 in inhibiting tumor growth and survival in primary lymphoma cells in vitro and in patient-derived xenograft mouse models. Overall, our findings support the strategy of using rocaglates to target oncoprotein synthesis in MYC-driven lymphomas.

Metabolism in Immune Cell Differentiation and Function

The immune system is a central determinant of organismal health. Functional immune responses require quiescent immune cells to rapidly grow, proliferate, and acquire effector functions when they sense infectious agents or other insults. Specialized metabolic programs are critical regulators of immune responses, and alterations in immune metabolism can cause immunological disorders. There has thus been growing interest in understanding how metabolic processes control immune cell functions under normal and pathophysiological conditions. In this chapter, we summarize how metabolic programs are tuned and what the physiological consequences of metabolic reprogramming are as they relate to immune cell homeostasis, differentiation, and function.

Loss of FOXP3 and TSC1 Accelerates Prostate Cancer Progression through Synergistic Transcriptional and Posttranslational Regulation of c-MYC

Although c-MYC and mTOR are frequently activated proteins in prostate cancer, any interaction between the two is largely untested. Here, we characterize the functional cross-talk between FOXP3-c-MYC and TSC1-mTOR signaling during tumor progression. Deletion of Tsc1 in mouse embryonic fibroblasts (MEF) decreased phosphorylation of c-MYC at threonine 58 (pT58) and increased phosphorylation at serine 62 (pS62), an observation validated in prostate cancer cells. Conversely, inhibition of mTOR increased pT58 but decreased pS62. Loss of both FOXP3 and TSC1 in prostate cancer cells synergistically enhanced c-MYC expression via regulation of c-Myc transcription and protein phosphorylation. This crosstalk between FOXP3 and TSC1 appeared to be mediated by both the mTOR-4EBP1-c-MYC and FOXP3-c-MYC pathways. In mice, Tsc1 and Foxp3 double deletions in the prostate led to prostate carcinomas at an early age; this did not occur in these mice with an added c-Myc deletion. In addition, we observed synergistic antitumor effects of cotreating mice with inhibitors of mTOR and c-MYC in prostate cancer cells and in Foxp3 and Tsc1 double-mutant mice. In human prostate cancer, loss of nuclear FOXP3 is often accompanied by low expression of TSC1. Because loss of FOXP3 transcriptionally induces c-Myc expression and loss of TSC1 activates mTOR signaling, these data suggest cross-talk between FOXP3-c-MYC and TSC1-mTOR signaling that converges on c-MYC to regulate tumor progression. Coadministration of c-MYC and mTOR inhibitors may overcome the resistance to mTOR inhibition commonly observed in prostate cancer cells. SIGNIFICANCE: These results establish the principle of a synergistic action of TSC1 and FOXP3 during prostate cancer progression and provide new therapeutic targets for patients who have prostate cancer with two signaling defects.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/79/7/1413/F1.large.jpg.

Tuberous sclerosis complex is required for tumor maintenance in MYC-driven Burkitt's lymphoma

The tuberous sclerosis complex (TSC) 1/2 is a negative regulator of the nutrient‐sensing kinase mechanistic target of rapamycin complex (mTORC1), and its function is generally associated with tumor suppression. Nevertheless, biallelic loss of function of TSC1 or TSC2 is rarely found in malignant tumors. Here, we show that TSC1/2 is highly expressed in Burkitt's lymphoma cell lines and patient samples of human Burkitt's lymphoma, a prototypical MYC‐driven cancer. Mechanistically, we show that MYC induces TSC1 expression by transcriptional activation of the TSC1 promoter and repression of miR‐15a. TSC1 knockdown results in elevated mTORC1‐dependent mitochondrial respiration enhanced ROS production and apoptosis. Moreover, TSC1 deficiency attenuates tumor growth in a xenograft mouse model. Our study reveals a novel role for TSC1 in securing homeostasis between MYC and mTORC1 that is required for cell survival and tumor maintenance in Burkitt's lymphoma. The study identifies TSC1/2 inhibition and/or mTORC1 hyperactivation as a novel therapeutic strategy for MYC‐driven cancers.

Therapeutic Targeting of mTOR in T-Cell Acute Lymphoblastic Leukemia: An Update

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive blood malignancy that arises from the clonal expansion of transformed T-cell precursors. Although T-ALL prognosis has significantly improved due to the development of intensive chemotherapeutic protocols, primary drug-resistant and relapsed patients still display a dismal outcome. In addition, lifelong irreversible late effects from conventional therapy are a growing problem for leukemia survivors. Therefore, novel targeted therapies are required to improve the prognosis of high-risk patients. The mechanistic target of rapamycin (mTOR) is the kinase subunit of two structurally and functionally distinct multiprotein complexes, which are referred to as mTOR complex 1 (mTORC1) and mTORC2. These two complexes regulate a variety of physiological cellular processes including protein, lipid, and nucleotide synthesis, as well as autophagy in response to external cues. However, mTOR activity is frequently deregulated in cancer, where it plays a key oncogenetic role driving tumor cell proliferation, survival, metabolic transformation, and metastatic potential. Promising preclinical studies using mTOR inhibitors have demonstrated efficacy in many human cancer types, including T-ALL. Here, we highlight our current knowledge of mTOR signaling and inhibitors in T-ALL, with an emphasis on emerging evidence of the superior efficacy of combinations consisting of mTOR inhibitors and either traditional or targeted therapeutics.

Therapeutic dosages of aspirin counteract the IL-6 induced pro-tumorigenic effects by slowing down the ribosome biogenesis rate

Chronic inflammation is a risk factor for the onset of cancer and the regular use of aspirin reduces the risk of cancer development. Here we showed that therapeutic dosages of aspirin counteract the pro-tumorigenic effects of the inflammatory cytokine interleukin(IL)-6 in cancer and non-cancer cell lines, and in mouse liver in vivo. We found that therapeutic dosages of aspirin prevented IL-6 from inducing the down-regulation of p53 expression and the acquisition of the epithelial mesenchymal transition (EMT) phenotypic changes in the cell lines. This was the result of a reduction in c-Myc mRNA transcription which was responsible for a down-regulation of the ribosomal protein S6 expression which, in turn, slowed down the rRNA maturation process, thus reducing the ribosome biogenesis rate. The perturbation of ribosome biogenesis hindered the Mdm2-mediated proteasomal degradation of p53, throughout the ribosomal protein-Mdm2-p53 pathway. P53 stabilization hindered the IL-6 induction of the EMT changes. The same effects were observed in livers from mice stimulated with IL-6 and treated with aspirin. It is worth noting that aspirin down-regulated ribosome biogenesis, stabilized p53 and up-regulated E-cadherin expression in unstimulated control cells also. In conclusion, these data showed that therapeutic dosages of aspirin increase the p53-mediated tumor-suppressor activity of the cells thus being in this way able to reduce the risk of cancer onset, either or not linked to chronic inflammatory processes.

De novo MYC addiction as an adaptive response of cancer cells to CDK4/6 inhibition

Cyclin‐dependent kinases (CDK) are rational cancer therapeutic targets fraught with the development of acquired resistance by tumor cells. Through metabolic and transcriptomic analyses, we show that the inhibition of CDK4/6 leads to a metabolic reprogramming associated with gene networks orchestrated by the MYC transcription factor. Upon inhibition of CDK4/6, an accumulation of MYC protein ensues which explains an increased glutamine metabolism, activation of the mTOR pathway and blunting of HIF‐1α‐mediated responses to hypoxia. These MYC‐driven adaptations to CDK4/6 inhibition render cancer cells highly sensitive to inhibitors of MYC, glutaminase or mTOR and to hypoxia, demonstrating that metabolic adaptations to antiproliferative drugs unveil new vulnerabilities that can be exploited to overcome acquired drug tolerance and resistance by cancer cells.

p53 Nongenotoxic Activation and mTORC1 Inhibition Lead to Effective Combination for Neuroblastoma Therapy

Purpose: mTORC1 inhibitors are promising agents for neuroblastoma therapy; however, they have shown limited clinical activity as monotherapy, thus rational drug combinations need to be explored to improve efficacy. Importantly, neuroblastoma maintains both an active p53 and an aberrant mTOR signaling.Experimental Design: Using an orthotopic xenograft model and modulating p53 levels, we investigated the antitumor effects of the mTORC1 inhibitor temsirolimus in neuroblastoma expressing normal, decreased, or mutant p53, both as single agent and in combination with first- and second-generation MDM2 inhibitors to reactivate p53.Results: Nongenotoxic p53 activation suppresses mTOR activity. Moreover, p53 reactivation via RG7388, a second-generation MDM2 inhibitor, strongly enhances the in vivo antitumor activity of temsirolimus. Single-agent temsirolimus does not elicit apoptosis, and tumors rapidly regrow after treatment suspension. In contrast, our combination therapy triggers a potent apoptotic response in wild-type p53 xenografts and efficiently blocks tumor regrowth after treatment completion. We also found that this combination uniquely led to p53-dependent suppression of survivin whose ectopic expression is sufficient to rescue the apoptosis induced by our combination.Conclusions: Our study supports a novel highly effective strategy that combines RG7388 and temsirolimus in wild-type p53 neuroblastoma, which warrants testing in early-phase clinical trials. Clin Cancer Res; 23(21); 6629-39. ©2017 AACR.

[Effect of TSC2 gene expression downregulation by lentivirus induced RNA interference on U937 cell line and its mechanism]

目的

研究下调TSC2基因表达对白血病U937细胞系的生物学作用及其对mTOR通路活性的影响。

方法

选择TSC2高表达的U937细胞系，通过慢病毒介导的RNA干扰技术下调TSC2基因表达；采用CCK-8比色法、细胞集落形成实验和流式细胞术检测其对细胞增殖、分化和凋亡的影响；采用Western blot法和实时荧光定量PCR（RQ-PCR）法检测TSC2表达下调对mTOR通路蛋白表达及活性的影响。

结果

TSC2基因表达降低能够促进U937细胞的增殖和集落形成（P<0.05）；能够使U937细胞G₁期[（52.53±3.75）%对（75.10±4.33）%，t=6.829，P=0.002]比例明显降低，G₂/M期[（22.43±1.00）%对（15.47±1.20）%，t=−5.581，P=0.019]、S期[（25.03±4.34）%对（14.33±0.91）%，t=−5.413，P=0.013]比例升高；对细胞分化和细胞凋亡没有明显影响（P>0.05）。TSC2基因表达下调后，mTOR活性升高，磷酸化的4EBP1和S6K1蛋白活性升高，而AKT蛋白活性没有明显变化；与细胞增殖相关的基因cyclin D1、c-myc表达升高，PTEN基因表达升高，P27KIP基因和凋亡相关基因BCL-XL的表达没有明显的改变。

结论

TSC2基因表达下调可以通过调节mTOR通路活性促进白血病细胞的增殖。

Clinical significance of T cell metabolic reprogramming in cancer

Conversion of normal cells to cancer is accompanied with changes in their metabolism. During this conversion, cell metabolism undergoes a shift from oxidative phosphorylation to aerobic glycolysis, also known as Warburg effect, which is a hallmark for cancer cell metabolism. In cancer cells, glycolysis functions in parallel with the TCA cycle and other metabolic pathways to enhance biosynthetic processes and thus support proliferation and growth. Similar metabolic features are observed in T cells during activation but, in contrast to cancer, metabolic transitions in T cells are part of a physiological process. Currently, there is intense interest in understanding the cause and effect relationship between metabolic reprogramming and T cell differentiation. After the recent success of cancer immunotherapy, the crosstalk between immune system and cancer has come to the forefront of clinical and basic research. One of the key goals is to delineate how metabolic alterations of cancer influence metabolism-regulated function and differentiation of tumor resident T cells and how such effects might be altered by immunotherapy. Here, we review the unique metabolic features of cancer, the implications of cancer metabolism on T cell metabolic reprogramming during antigen encounters, and the translational prospective of harnessing metabolism in cancer and T cells for cancer therapy.

MUC16-mediated activation of mTOR and c-Myc reprograms pancreatic cancer metabolism

MUC16, a transmembrane mucin, facilitates pancreatic adenocarcinoma progression and metastasis. In the current studies, we observed that MUC16 knockdown pancreatic cancer cells exhibit reduced glucose uptake and lactate secretion along with reduced migration and invasion potential, which can be restored by supplementing the culture media with lactate, an end product of aerobic glycolysis. MUC16 knockdown leads to inhibition of mTOR activity and reduced expression of its downstream target c-MYC, a key player in cellular growth, proliferation and metabolism. Ectopic expression of c-MYC in MUC16 knockdown pancreatic cancer cells restores the altered cellular physiology. Our LC-MS/MS based metabolomics studies indicate global metabolic alterations in MUC16 knockdown pancreatic cancer cells, as compared to the controls. Specifically, glycolytic and nucleotide metabolite pools were significantly decreased. We observed similar metabolic alterations that correlated with MUC16 expression in primary tumor tissue specimens from human pancreatic adenocarcinoma cancer patients. Overall, our results demonstrate that MUC16 plays an important role in metabolic reprogramming of pancreatic cancer cells by increasing glycolysis and enhancing motility and invasiveness.

Pyruvate Kinase M2 Activates mTORC1 by Phosphorylating AKT1S1

In cancer cells, the mammalian target of rapamycin complex 1 (mTORC1) that requires hormonal and nutrient signals for its activation, is constitutively activated. We found that overexpression of pyruvate kinase M2 (PKM2) activates mTORC1 signaling through phosphorylating mTORC1 inhibitor AKT1 substrate 1 (AKT1S1). An unbiased quantitative phosphoproteomic survey identified 974 PKM2 substrates, including serine202 and serine203 (S202/203) of AKT1S1, in the proteome of renal cell carcinoma (RCC). Phosphorylation of S202/203 of AKT1S1 by PKM2 released AKT1S1 from raptor and facilitated its binding to 14-3-3, resulted in hormonal- and nutrient-signals independent activation of mTORC1 signaling and led accelerated oncogenic growth and autophagy inhibition in cancer cells. Decreasing S202/203 phosphorylation by TEPP-46 treatment reversed these effects. In RCCs and breast cancers, PKM2 overexpression was correlated with elevated S202/203 phosphorylation, activated mTORC1 and inhibited autophagy. Our results provided the first phosphorylome of PKM2 and revealed a constitutive mTORC1 activating mechanism in cancer cells.

mTORC signaling in hematopoiesis

mTOR is a serine/threonine (Ser/Thr) protein kinase that responds to multiple signals, including growth factors, amino acids, energy status, stress, and oxygen, regulates cell survival, cell growth, the cell cycle, and cell metabolism, and maintains homeostasis [1]. Increased or decreased mTORC1 activity can alter HSC function and cause hematological disorders [2, 3]. Therefore, a comprehensive knowledge of mTOR is critical to understanding how HSCs function and maintain homeostasis in the hematopoietic system. In this review, we summarize recent advances in the understanding of the mTOR signaling pathway and its roles in hematopoiesis and leukemia. We also discuss pharmacological approaches to manipulate mTOR activity.

MYC and metabolism on the path to cancer

The MYC proto-oncogene is frequently deregulated in human cancers, activating genetic programs that orchestrate biological processes to promote growth and proliferation. Altered metabolism characterized by heightened nutrients uptake, enhanced glycolysis and glutaminolysis and elevated fatty acid and nucleotide synthesis is the hallmark of MYC-driven cancer. Recent evidence strongly suggests that Myc-dependent metabolic reprogramming is critical for tumorigenesis, which could be attenuated by targeting specific metabolic pathways using small drug-like molecules. Understanding the complexity of MYC-mediated metabolic re-wiring in cancers as well as how MYC cooperates with other metabolic drivers such as mammalian target of rapamycin (mTOR) will provide translational opportunities for cancer therapy.

Ribosomal protein S15A augments human osteosarcoma cell proliferation in vitro

As a highly conserved housekeeping gene, the biological implications of ribosomal protein S15A (RPS15A) during various processes, including carcinogenesis, remain elusive. Herein, the authors reported that knockdown of RPS15A expression significantly inhibited human osteosarcoma U2OS cell proliferation and colony formation in vitro by using a lentivirus-mediated RNA interference (RNAi) system. Moreover, an excess accumulation of cells in the G0/G1 phase was observed in U2OS cells transduced with lentivirus targeting RPS15A, suggesting that the growth inhibition mediated by RPS15A knockdown in osteosarcoma cells was probably due to the induction of cell cycle arrest. Taken together, this study highlights the crucial role of RPS15A in promoting osteosarcoma cell proliferation, and provides a foundation for further study into the clinical potential of inhibition of RPS15A for the treatment of osteosarcoma.

Molecular rationale for the use of PI3K/AKT/mTOR pathway inhibitors in combination with crizotinib in ALK-mutated neuroblastoma

Mutations in the ALK tyrosine kinase receptor gene represent important therapeutic targets in neuroblastoma, yet their clinical translation has been challenging. The ALK^F1174L mutation is sensitive to the ALK inhibitor crizotinib only at high doses and mediates acquired resistance to crizotinib in ALK-translocated cancers. We have shown that the combination of crizotinib and an inhibitor of downstream signaling induces a favorable response in transgenic mice bearing ALK^F1174L/MYCN-positive neuroblastoma. Here, we investigated the molecular basis of this effect and assessed whether a similar strategy would be effective in ALK-mutated tumors lacking MYCN overexpression. We show that in ALK-mutated, MYCN-amplified neuroblastoma cells, crizotinib alone does not affect mTORC1 activity as indicated by persistent RPS6 phosphorylation. Combined treatment with crizotinib and an ATP-competitive mTOR inhibitor abrogated RPS6 phosphorylation, leading to reduced tumor growth and prolonged survival in ALK^F1174L/MYCN-positive models compared to single agent treatment. By contrast, this combination, while inducing mTORC1 downregulation, caused reciprocal upregulation of PI3K activity in ALK-mutated cells expressing wild-type MYCN. Here, an inhibitor with potency against both mTOR and PI3K was more effective in promoting cytotoxicity when combined with crizotinib. Our findings should enable a more precise selection of molecularly targeted agents for patients with ALK-mutated tumors.

MYC-xing it up with PIK3CA mutation and resistance to PI3K inhibitors: summit of two giants in breast cancers

Approximately 35% of breast cancers exhibit PIK3CA activating mutation. Since PIK3CA hotspot mutation is the most frequently mutated gene in human breast cancers and primarily overlaps in HER2+ as well as ER+ breast cancers, the subset of patients bearing PIK3CA activating mutation does not get fullest benefit from either anti-HER2 or anti-hormonal agents. Literature also suggests that these patients may have chemotherapy resistance. Indeed, multiple clinical trials are currently evaluating the efficacy of over 30 drugs targeting different nodal points in the PI3K-AKT-mTOR pathway in breast and other cancers. However, to date, responses of solid tumors to PI3K pathway inhibitor monotherapy remains modest with an accompanied rapid emergences of drug resistance. MYC elevation represents one of the potential modes of actions by which breast tumors develop resistance to the PI3K pathway-specific targeted therapies. As products of oncogenes, both MYC and PIK3CA are well-established onco-proteins which contribute to breast oncogenesis. However, their similarities out number their dissimilarities in the context of their specific oncogenic cellular signals. In this review we will describe the specific cellular signals initiated following alteration in the MYC gene and PIK3CA gene in breast cancers. We will interrogate how MYC gene alterations influence the action of PI3K pathway targeted drugs in the context of PIK3CA mutation towards the development PI3K inhibitor induced drug-resistance in breast cancers.

Combined inhibition of PI3K-related DNA damage response kinases and mTORC1 induces apoptosis in MYC-driven B-cell lymphomas

Pharmacological strategies capable of directly targeting MYC are elusive. Previous studies have shown that MYC-driven lymphomagenesis is associated with mammalian target of rapamycin (mTOR) activation and a MYC-evoked DNA damage response (DDR) transduced by phosphatidylinositol-3-kinase (PI3K)-related kinases (DNA-PK, ATM, and ATR). Here we report that BEZ235, a multitargeted pan-PI3K/dual-mTOR inhibitor, potently killed primary Myc-driven B-cell lymphomas and human cell lines bearing IG-cMYC translocations. Using pharmacologic and genetic dissection of PI3K/mTOR signaling, dual DDR/mTORC1 inhibition was identified as a key mediator of apoptosis. Moreover, apoptosis was initiated at drug concentrations insufficient to antagonize PI3K/mTORC2-regulated AKT phosphorylation. p53-independent induction of the proapoptotic BH3-only protein BMF was identified as a mechanism by which dual DDR/mTORC1 inhibition caused lymphoma cell death. BEZ235 treatment induced apoptotic tumor regressions in vivo that correlated with suppression of mTORC1-regulated substrates and reduced H2AX phosphorylation and also with feedback phosphorylation of AKT. These mechanistic studies hold important implications for the use of multitargeted PI3K inhibitors in the treatment of hematologic malignancies. In particular, the newly elucidated role of PI3K-related DDR kinases in response to PI3K inhibitors offers a novel therapeutic opportunity for the treatment of hematologic malignancies with an MYC-driven DDR.

The mTORC1 inhibitor everolimus prevents and treats Eμ-Myc lymphoma by restoring oncogene-induced senescence

MYC deregulation is common in human cancer. IG-MYC translocations that are modeled in Eμ-Myc mice occur in almost all cases of Burkitt lymphoma as well as in other B-cell lymphoproliferative disorders. Deregulated expression of MYC results in increased mTOR complex 1 (mTORC1) signaling. As tumors with mTORC1 activation are sensitive to mTORC1 inhibition, we used everolimus, a potent and specific mTORC1 inhibitor, to test the requirement for mTORC1 in the initiation and maintenance of Eμ-Myc lymphoma. Everolimus selectively cleared premalignant B cells from the bone marrow and spleen, restored a normal pattern of B-cell differentiation, and strongly protected against lymphoma development. Established Eμ-Myc lymphoma also regressed after everolimus therapy. Therapeutic response correlated with a cellular senescence phenotype and induction of p53 activity. Therefore, mTORC1-dependent evasion of senescence is critical for cellular transformation and tumor maintenance by MYC in B lymphocytes.

SIGNIFICANCE

This work provides novel insights into the requirements for MYC-induced oncogenesis by showing that mTORC1 activity is necessary to bypass senescence during transformation of B lymphocytes. Furthermore, tumor eradication through senescence elicited by targeted inhibition of mTORC1 identifies a previously uncharacterized mechanism responsible for significant anticancer activity of rapamycin analogues and serves as proof-of-concept that senescence can be harnessed for therapeutic benefit

Oncogenes in cell survival and cell death

The transforming effects of proto-oncogenes such as MYC that mediate unrestrained cell proliferation are countered by "intrinsic tumor suppressor mechanisms" that most often trigger apoptosis. Therefore, cooperating genetic or epigenetic effects to suppress apoptosis (e.g., overexpression of BCL2) are required to enable the dual transforming processes of unbridled cell proliferation and robust suppression of apoptosis. Certain oncogenes such as BCR-ABL are capable of concomitantly mediating the inhibition of apoptosis and driving cell proliferation and therefore are less reliant on cooperating lesions for transformation. Accordingly, direct targeting of BCR-ABL through agents such as imatinib have profound antitumor effects. Other oncoproteins such as MYC rely on the anti-apoptotic effects of cooperating oncoproteins such as BCL2 to facilitate tumorigenesis. In these circumstances, where the primary oncogenic driver (e.g., MYC) cannot yet be therapeutically targeted, inhibition of the activity of the cooperating antiapoptotic protein (e.g., BCL2) can be exploited for therapeutic benefit.

Tetraspanins stimulate protein synthesis in myeloma cell lines

Intensive protein synthesis is a unique and differential trait of multiple myeloma (MM) cells. Previously we showed that tetraspanin (CD81, CD82) overexpression in MM cell lines attenuated Akt/mTOR cascades, activated UPR, and caused autophagic death, suggesting breach of protein homeostasis. Here, we explored the role of protein synthesis in the tetraspanin-induced MM cell death. Contrary to attenuation of the major metabolic regulator, mTOR we determined elevated steady-state levels of protein in CD81N1/CD82N1 transfected MM lines (RPMI-8226, CAG). Elevated levels of immunoglobulins supported increased protein production in RPMI-8226. Changes in cell morphology consistent with elevated protein synthesis were also determined (cell, nuclei, and nucleoli sizes and ratios). Increased levels of phospho-rpS6 and decreased levels of phospho-AMPK were consistent with increased translation but independent of mTOR. Involvement of p38 and its role in tetraspanin induced translation and cell death were demonstrated. Microarray analyses of tetraspanin transfected MM cell lines revealed activation of protein synthesis signaling cascades and signals implicated in ribosome biogenesis (snoRNAs). Finally, we showed tetraspanins elevated protein synthesis was instrumental to MM cells' death. This work explores and demonstrates that excessive protein translation can be detrimental to MM cell lines and therefore may present a therapeutic target. Proteostasis is particularly important in MM because it integrates the high levels of protein production unique to myeloma cells with critically important microenvironmental cues. We suggest that increasing translation may be the path of least resistance in MM and thus may afford a novel platform for strategically designed therapy.

Dynamic network of transcription and pathway crosstalk to reveal molecular mechanism of MGd-treated human lung cancer cells

Recent research has revealed various molecular markers in lung cancer. However, the organizational principles underlying their genetic regulatory networks still await investigation. Here we performed Network Component Analysis (NCA) and Pathway Crosstalk Analysis (PCA) to construct a regulatory network in human lung cancer (A549) cells which were treated with 50 uM motexafin gadolinium (MGd), a metal cation-containing chemotherapeutic drug for 4, 12, and 24 hours. We identified a set of key TFs, known target genes for these TFs, and signaling pathways involved in regulatory networks. Our work showed that putative interactions between these TFs (such as ESR1/Sp1, E2F1/Sp1, c-MYC-ESR, Smad3/c-Myc, and NFKB1/RELA), between TFs and their target genes (such as BMP41/Est1, TSC2/Myc, APE1/Sp1/p53, RARA/HOXA1, and SP1/USF2), and between signaling pathways (such as PPAR signaling pathway and Adipocytokines signaling pathway). These results will provide insights into the regulatory mechanism of MGd-treated human lung cancer cells.

Organ size control by Hippo and TOR pathways

The determination of final organ size is a highly coordinated and complex process that relies on the precise regulation of cell number and/or cell size. Perturbation of organ size control contributes to many human diseases, including hypertrophy, degenerative diseases, and cancer. Hippo and TOR are among the key signaling pathways involved in the regulation of organ size through their respective functions in the regulation of cell number and cell size. Here, we review the general mechanisms that regulate organ growth, describe how Hippo and TOR control key aspects of growth, and discuss recent findings that highlight a possible coordination between Hippo and TOR in organ size regulation.

Systemic elevation of PTEN induces a tumor-suppressive metabolic state

Decremental loss of PTEN results in cancer susceptibility and tumor progression. PTEN elevation might therefore be an attractive option for cancer prevention and therapy. We have generated several transgenic mouse lines with PTEN expression elevated to varying levels by taking advantage of bacterial artificial chromosome (BAC)-mediated transgenesis. The "Super-PTEN" mutants are viable and show reduced body size due to decreased cell number, with no effect on cell size. Unexpectedly, PTEN elevation at the organism level results in healthy metabolism characterized by increased energy expenditure and reduced body fat accumulation. Cells derived from these mice show reduced glucose and glutamine uptake and increased mitochondrial oxidative phosphorylation and are resistant to oncogenic transformation. Mechanistically we find that PTEN elevation orchestrates this metabolic switch by regulating PI3K-dependent and -independent pathways and negatively impacting two of the most pronounced metabolic features of tumor cells: glutaminolysis and the Warburg effect.

Phospholipase D2 (PLD2) shortens the time required for myeloid leukemic cell differentiation: mechanism of action

Cell differentiation is compromised in acute leukemias. We report that mammalian target of rapamycin (mTOR) and S6 kinase (S6K) are highly expressed in the undifferentiated promyelomonocytic leukemic HL-60 cell line, whereas PLD2 expression is minimal. The expression ratio of PLD2 to mTOR (or to S6K) is gradually inverted upon in vitro induction of differentiation toward the neutrophilic phenotype. We present three ways that profoundly affect the kinetics of differentiation as follows: (i) simultaneous overexpression of mTOR (or S6K), (ii) silencing of mTOR via dsRNA-mediated interference or inhibition with rapamycin, and (iii) PLD2 overexpression. The last two methods shortened the time required for differentiation. By determining how PLD2 participates in cell differentiation, we found that PLD2 interacts with and activates the oncogene Fes/Fps, a protein-tyrosine kinase known to be involved in myeloid cell development. Fes activity is elevated with PLD2 overexpression, phosphatidic acid or phosphatidylinositol bisphosphate. Co-immunoprecipitation indicates a close PLD2-Fes physical interaction that is negated by a Fes-R483K mutant that incapacitates its Src homology 2 domain. All these suggest for the first time the following mechanism: mTOR/S6K down-regulation→PLD2 overexpression→PLD2/Fes association→phosphatidic acid-led activation of Fes kinase→granulocytic differentiation. Differentiation shortening could have a clinical impact on reducing the time of return to normalcy of the white cell counts after chemotherapy in patients with acute promyelocytic leukemia.

The tuberous sclerosis complex: balancing proliferation and survival

Mutations in genes encoding either hamartin [TSC1 (tuberous sclerosis complex 1)] or tuberin (TSC2) result in a multisystem disorder characterized by the development of benign tumours and hamartomas in several organs. The TSC1 and TSC2 proteins form a complex that lies at the crossroad of many signalling pathways integrating the energy status of the cell with signals induced by nutrients and growth factors. The TSC1/2 complex is a critical negative regulator of mTORC1 [mTOR (mammalian target of rapamycin) complex 1], and by that controls anabolic processes to promote cell growth, proliferation and survival. In the present paper, we review recent evidence highlighting the notion that the TSC1/2 complex simultaneously controls mTOR-dependent and mTOR-independent signals critical for the balancing of cell proliferation and cell death.

The autophagy conundrum in cancer: influence of tumorigenic metabolic reprogramming

Tumorigenesis is often accompanied by metabolic changes that favor rapid energy production and increased biosynthetic capabilities. These metabolic adaptations promote the survival and proliferation of tumor cells, and in conjunction with the hypoxic and metabolically challenged tumor microenvironment, influence autophagic activity. Autophagy is a catabolic process that allows cellular macromolecules to be broken down and re-utilized as metabolic precursors. Stimulation of autophagy promotes the survival of tumor cells under stressful metabolic and environmental conditions, and counters the potentially deleterious effects of mitochondrial dysfunction and the ROS that these organelles generate. However, inhibition of autophagy has also been reported to fuel tumorigenesis. In spite of the advances in our understanding of the relationship between autophagy and tumorigenesis, it remains unclear whether the therapeutic approaches targeting autophagy should aim to increase or decrease autophagic flux in tumor tissues in human patients. Here, we review how metabolic reprogramming influences autophagic activity in tumors, and discuss how inhibition of autophagy might be exploited to target tumor cells that show altered metabolism.

Driving gradual endogenous c-myc overexpression by flow-sorting: intracellular signaling and tumor cell phenotype correlate with oncogene expression

Insulin-exposed rat mammary cancer cells were flow sorted based on a c-myc reporter plasmid encoding a destabilized green fluorescent protein. Sorted cells exhibited gradual increases in c-myc levels. Cells overexpressing c-myc by only 10% exhibited phenotypic changes attributable to c-myc overexpression, such as cell cycle disturbances, increased cell size, and overexpression of the S6 ribosomal protein. Cells overexpressing c-myc by 70% exhibited additional phenotypic changes typical of c-myc overexpression, such as increased histone H3 phosphorylation, and reduced adherence. Sorted cells also exhibited overexpression of the IGF-1R, and slightly elevated expression of the IR. Increased susceptibility to the mitogenic effect of insulin was seen in a small proportion of the sorted cells, and insulin was more effective in activating the p44/42 MAPK pathway, but not the PI3K pathway, in the sorted cells than in the nonsorted cell population. To our knowledge, this is the first in vitro system allowing functional coupling between mitogenic signaling by a well-defined growth factor and gradual overexpression of the normal, endogenous c-myc gene. Thus, our flow-sorting approach provides an alternative modeling of the receptor-mediated carcinogenic process, compared to the currently used approaches of recombinant constitutive or conditional overexpression of oncogenic transmembrane receptor tyrosine kinases or oncogenic transcription factors.

Frontier of epilepsy research - mTOR signaling pathway

Studies of epilepsy have mainly focused on the membrane proteins that control neuronal excitability. Recently, attention has been shifting to intracellular proteins and their interactions, signaling cascades and feedback regulation as they relate to epilepsy. The mTOR (mammalian target of rapamycin) signal transduction pathway, especially, has been suggested to play an important role in this regard. These pathways are involved in major physiological processes as well as in numerous pathological conditions. Here, involvement of the mTOR pathway in epilepsy will be reviewed by presenting; an overview of the pathway, a brief description of key signaling molecules, a summary of independent reports and possible implications of abnormalities of those molecules in epilepsy, a discussion of the lack of experimental data, and questions raised for the understanding its epileptogenic mechanism.

E2F2 suppresses Myc-induced proliferation and tumorigenesis

Deregulation of E2F transcriptional activity as a result of alterations in the p16-cyclin D-Rb pathway is a hallmark of cancer. However, the roles of the different E2F family members in the process of tumorigenesis are still being elucidated. Studies in mice and humans suggest that E2F2 functions as a tumor suppressor. Here we demonstrate that E2f2 inactivation cooperates with transgenic expression of Myc to enhance tumor development in the skin and oral cavity. In fact, hemizygosity at the E2f2 locus was sufficient to increase tumor incidence in this model. Loss of E2F2 enhanced proliferation in Myc transgenic tissue but did not affect Myc-induced apoptosis. E2F2 did not behave as a simple activator of transcription in epidermal keratinocytes but instead appeared to differentially regulate gene expression dependent on the individual target. E2f2 inactivation also altered the changes in gene expression in Myc transgenic cells by enhancing the increase of some genes, such as cyclin E, and reversing the repression of other genes. These findings demonstrate that E2F2 can function as a tumor suppressor in epithelial tissues, perhaps by limiting proliferation in response to Myc.

Regulation of gene expression in hepatic cells by the mammalian Target of Rapamycin (mTOR)

Background

We investigated mTOR regulation of gene expression by studying rapamycin effect in two hepatic cell lines, the non-tumorigenic WB-F344 cells and the tumorigenic WB311 cells. The latter are resistant to the growth inhibitory effects of rapamycin, thus providing us with an opportunity to study the gene expression effects of rapamycin without confounding effects on cell proliferation.

Methodology/Principal Findings

The hepatic cells were exposed to rapamycin for 24 hr. Microarray analysis on total RNA preparations identified genes that were affected by rapamycin in both cell lines and, therefore, modulated independent of growth arrest. Further studies showed that the promoter regions of these genes included E-box-containing transcription factor binding sites at higher than expected rates. Based on this, we tested the hypothesis that c-Myc is involved in regulation of gene expression by mTOR by comparing genes altered by rapamycin in the hepatic cells and by c-Myc induction in fibroblasts engineered to express c-myc in an inducible manner. Results showed enrichment for c-Myc targets among rapamycin sensitive genes in both hepatic cell lines. However, microarray analyses on wild type and c-myc null fibroblasts showed similar rapamycin effect, with the set of rapamycin-sensitive genes being enriched for c-Myc targets in both cases.

Conclusions/Significance

There is considerable overlap in the regulation of gene expression by mTOR and c-Myc. However, regulation of gene expression through mTOR is c-Myc-independent and cannot be attributed to the involvement of specific transcription factors regulated by the rapamycin-sensitive mTOR Complex 1.

Understanding and Targeting the Eukaryotic Translation Initiation Factor eIF4E in Head and Neck Cancer

The eukaryotic translation initiation factor eIF4E is elevated in about 30% of human malignancies including HNSCC where its levels correlate with poor prognosis. Here, we discuss the biochemical and molecular underpinnings of the oncogenic potential of eIF4E. Studies in human leukemia specimens, and later in a mouse model of prostate cancer, strongly suggest that cells with elevated eIF4E develop an oncogene dependency to it, making them more sensitive to targeting eIF4E than normal cells. We describe several strategies that have been suggested for eIF4E targeting in the clinic: the use of a small molecule antagonist of eIF4E (ribavirin), siRNA or antisense oligonucleotide strategies, suicide gene therapy, and the use of a tissue-targeting 4EBP fusion peptide. The first clinical trial targeting eIF4E indicates that ribavirin effectively targets eIF4E in poor prognosis leukemia patients and more importantly leads to striking clinical responses including complete and partial remissions. Finally, we discuss the relevance of these findings to HNSCC.

The regulation of protein synthesis in cancer

Translational control of cancer is a multifaceted process, involving alterations in translation factor levels and activities that are unique to the different types of cancers and the different stages of disease. Translational alterations in cancer include adaptations of the tumor itself, of the tumor microenvironment, an integral component in disease, and adaptations that occur as cancer progresses from development to local disease and ultimately to metastatic disease. Adaptations include the overexpression and increased activity of specific translation factors, the physical or functional loss of translation regulatory components, increased production of ribosomes, selective mRNA translation, and alteration of signal transduction pathways to permit unfettered activation of protein synthesis. There is intense clinical interest to capitalize on the emerging new understanding of translational control in cancer by targeting specific components of the translation apparatus that are altered in disease for the development of specific cancer therapeutics. Clinical trial data are nascent but encouraging, suggesting that translational control constitutes an important new area for drug development in human cancer.

c-Myc and eIF4F constitute a feedforward loop that regulates cell growth: implications for anticancer therapy

The Myc/Max/Mad family of transcription factors and the eukaryotic initiation factor 4F (4F) complex play fundamental roles in regulating cell growth, proliferation, differentiation, and oncogenic transformation. Recent findings indicate that the role of Myc during cell growth and proliferation is linked to an increase in eIF4F activity in a feedforward relationship, providing a possible molecular mechanism of cell transformation by Myc. Developing therapeutics to inhibit eIF4F and/or Myc could be a potential treatment for a wide range of human cancers.

cMyc increases cell number through uncoupling of cell division from cell size in CHO cells

Background

Over the past decades, the increase in maximal cell numbers for the production of mammalian derived biologics has been in a large part due to the development of optimal feeding strategies. Engineering of the cell line is one of probable approaches for increasing cell numbers in bioreactor.

Results

We have demonstrated that the over-expression of the c-myc gene in immortalised CHO cells can increase proliferation rate and maximal cell density in batch culture compared to the control. The changes were attributed to a rapid transition into S-phase from a shortened duration of G₁ phase and to the uncoupling of cell size from cell proliferation. To achieve the >70% increase in maximal cell density without additional supply of nutrients the cells underwent an overall reduction of 14% in size as well as a significant decrease in glucose and amino acid consumption rate. Consequently, the total biomass accumulation did not show a significant change from the control. The amount of hSEAP-hFc activity of the over expressing c-myc cell line was found to be within 0.7% of the control.

Conclusion

It is shown that the manipulation of cell cycle kinetics and indirectly cell metabolism gives higher cell densities in CHO batch cultures. The unaltered apoptotic rate supported the proposition that the increase in cell number was a result of enhance cell cycle kinetics and cellular metabolism rather than increasing viability. Production of hSEAP-hFc from a constitutive c-myc over-expressing cell line did not increase with the increase in cell number.

A Rab escort protein integrates the secretion system with TOR signaling and ribosome biogenesis

The coupling of environmental conditions to cell growth and division is integral to cell fitness. In Saccharomyces cerevisiae, the transcription factor Sfp1 couples nutrient status to cell growth rate by controlling the expression of ribosome biogenesis (Ribi) and ribosomal protein (RP) genes. Sfp1 is localized to the nucleus in rich nutrients, but upon nutrient limitation or target of rapamycin (TOR) pathway inhibition by rapamycin, Sfp1 rapidly exits the nucleus, leading to repression of the Ribi/RP regulons. Through systematic cell-based screens we found that many components of the secretory system influence Sfp1 localization. Notably, the essential Rab escort protein Mrs6 exhibited a nutrient-sensitive interaction with Sfp1. Overexpression of Mrs6 prevented nuclear localization of Sfp1 in rich nutrients, whereas loss of Mrs6 resulted in nuclear Sfp1 localization in poor nutrients. These effects were specific to Sfp1 and independent of the protein kinase C (PKC) pathway, suggesting that Mrs6 lies in a distinct branch of TOR and ribosome biogenesis regulation. Rapamycin-resistant alleles of MRS6 were defective in the cytoplasmic retention of Sfp1, the control of cell size, and in the repression of the Ribi/RP regulons. The Sfp1-Mrs6 interaction is a nexus for growth regulation that links the secretory system and TOR-dependent nutrient signaling to ribosome biogenesis.

Growth controls connect: interactions between c-myc and the tuberous sclerosis complex-mTOR pathway

Among other signals, cell growth is particularly controlled by the target of rapamycin (TOR) pathway that includes the tuberous sclerosis complex genes (TSC1/2), and through transcriptional effects regulated by c-myc. Overexpression of Drosophila Myc and TSC1/2 cause opposing growth and proliferation defects. Despite this relationship, direct regulatory connections between Myc and the TSC have only recently been evaluated. Other than studies of p53 regulation, little consideration has been given to transcriptional regulation of the TSC genes. Here we review evidence that transcriptional controls are potentially important regulators of TSC2 expression, and that Myc is a direct repressor of its expression. Since tuberin loss de-represses Myc protein, the connection between these two growth regulators is positioned to act as a feed-forward loop that would amplify the oncogenic effects of decreased tuberin or increased Myc. Further experiments will be needed to clarify the mechanisms underlying this important connection, and evaluate its overall contribution to cancers caused by TSC loss or Myc gain.