Enhanced sensitivity of PTEN-deficient tumors to inhibition of FRAP/mTOR

PTEN mutations are common in sporadic microsatellite stable colorectal cancer

The tumour suppressor gene PTEN, located at chromosome sub-band 10q23.3, encodes a dual-specificity phosphatase that negatively regulates the phosphatidylinositol 3'-kinase (PI3 K)/Akt-dependent cellular survival pathway. PTEN is frequently inactivated in many tumour types including glioblastoma, prostate and endometrial cancers. While initial studies reported that PTEN gene mutations were rare in colorectal cancer, more recent reports have shown an approximate 18% incidence of somatic PTEN mutations in colorectal tumours exhibiting microsatellite instability (MSI+). To verify the role of this gene in colorectal tumorigenesis, we analysed paired normal and tumour DNA from 41 unselected primary sporadic colorectal cancers for PTEN inactivation by mutation and/or allelic loss. We now report PTEN gene mutations in 19.5% (8/41) of tumours and allele loss, including all or part of the PTEN gene, in a further 17% (7/41) of the cases. Both PTEN alleles were affected in over half (9/15) of these cases showing PTEN genetic abnormalities. Using immunohistochemistry, we have further shown that all tumours harbouring PTEN alterations have either reduced or absent PTEN expression and this correlated strongly with later clinical stage of tumour at presentation (P=0.02). In contrast to previous reports, all but one of the tumours with PTEN gene mutations were microsatellite stable (MSI-), suggesting that PTEN is involved in a distinct pathway of colorectal tumorigenesis that is separate from the pathway of mismatch repair deficiency. This work therefore establishes the importance of PTEN in primary sporadic colorectal cancer.

Molecular therapeutics: promise and challenges

The ability to analyze the genetic and epigenetic aberrations present in a particular patient's tumor on a global basis is rapidly maturing. The emerging fields of functional genomics and functional proteomics offer the opportunity to translate these advances into a full comprehension of the pathophysiology of cancer. Linking these approaches to chemical genomics and molecular therapeutics should provide an expanding repertoire of targeted therapeutics for clinical evaluation. Novel clinical trial designs that can determine the efficacy of targeted therapeutics in patients selected for aberrations in the target are needed to evaluate the wealth of new drugs becoming available. The promise of these technologies and advances in our understanding of cancer is immense, making it our responsibility to see them to fruition. These technologies should lead to a new era of individualized molecular medicine, wherein we will treat each patient with a "prescription" based on the genetic changes in each patient's tumor and their own genetic make-up, resulting in more effective and less toxic therapy.

The androgen receptor: a potential target for therapy of prostate cancer

The androgen receptor plays a pivotal role in the prostate. Its primary function is to provide responsive gene products for differentiation and growth, but under abnormal conditions it contributes to the development of prostate cancer. The goal of this review is to elucidate the molecular functions of the androgen receptor and its role in prostate cancer. Initially the function of the androgen receptor will be described. Next, the clinical diagnosis, epidemiological impact, and treatments of androgen-dependent and -independent prostate cancer will be discussed. Finally we will examine how the mechanism of androgen action has played a role in the translation of new therapies and how this may influence future treatment modalities of prostate cancer.

Novel Therapeutic Molecular Targets for Prostate Cancer: The mTor Signaling Pathway and Epidermal Growth Factor Receptor

PURPOSE

The scientific rationale and existing evidence for the use of novel molecular targets in the chemoprevention of cancer are reviewed, with special attention to prostate cancer.

MATERIALS AND METHODS

A search for relevant literature on basic science and clinical trials was conducted using PubMed/MEDLINE.

RESULTS

The emergence of molecularly targeted therapies for advanced malignancies creates an important opportunity to examine these agents for the chemoprevention of prostate cancer. Two critical targets in the proliferation and malignant transformation of normal cells, the PI3/Akt signal transduction pathway and the epidermal growth factor receptor, are currently the focus of several novel investigational therapies that are in late stage phase II and phase III studies.

CONCLUSIONS

Research to date supports consideration of these novel molecular targets as future agents in the chemoprevention of prostate cancer.

Tuberous sclerosis complex: from Drosophila to human disease

Tuberous sclerosis complex (TSC) is a human syndrome characterized by a widespread development of benign tumors. This disease is caused by mutations in the TSC1 or TSC2 tumor suppressor genes; the molecular mechanisms underlying the activity of these have long been elusive. Recent studies of Drosophila and mammalian cells demonstrate that the TSC1-TSC2 complex functions as GTPase activating protein against Rheb - a Ras-like small GTPase, which in turn regulates TOR signaling in nutrient-stimulated cell growth. These findings provide a new paradigm for how proteins involved in nutrient sensing could function as tumor suppressors and suggest novel therapeutic targets against TSC. Here, we review these exciting developments with an emphasis on Drosophila studies and discuss how Drosophila can be a powerful model system for an understanding of the molecular mechanisms of the activity of human disease genes.

Genetic alterations and aberrant expression of genes related to the phosphatidyl-inositol-3'-kinase/protein kinase B (Akt) signal transduction pathway in glioblastomas

Glioblastomas frequently carry mutations in the PTEN tumor suppressor gene on 10q23.3. The tumor suppressor properties of Pten are closely related to its inhibitory effect on the phosphatidyl-inositol-3'-kinase (Pi3k)-dependent activation of protein kinase B (Akt) signalling. Here, we report on the analysis of 17 genes related to the Pi3k/Akt signalling pathway for genetic alteration and aberrant expression in a series of 103 glioblastomas. Mutation, homozygous deletion or loss of expression of PTEN was detected in 32% of the tumors. In contrast, we did not find any aberrations in the inositol polyphosphate phosphatase like-1 gene (INPPL1), whose gene product may also counteract Pi3k-dependent Akt activation. Analysis of genes encoding proteins that may activate the pathway upstream of Pi3k revealed variable fractions of tumors with EGFR amplification (31%), PDGFRA amplification (8%), and IRS2 amplification (2%). The protein tyrosine kinase 2 (PTK2/FAK1) gene was neither amplified nor overexpressed at the mRNA level. Investigation of three genes encoding catalytic subunits of Pi3k (PIK3CA, PIK3CD, and PIK3C2B) revealed amplification of PIK3C2B (1q32) in 6 tumors (6%). Overexpression of PIK3C2B mRNA was detected in 4 of these cases. PIK3CD (1p36.2) and PIK3CA (3q26.3) were not amplified but PIK3CD mRNA was overexpressed in 6 tumors (6%). Amplification and overexpression of AKT1 was detected in a single case of gliosarcoma. The IRS1, PIK3R1, PIK3R2, AKT2, AKT3, FRAP1, and RPS6KB1 genes were neither amplified nor overexpressed in any of the tumors. Taken together, our data indicate that different genes related to the Pi3k/Akt signalling pathway may be aberrant in glioblastomas.

Apoptotic cues from the extracellular matrix: regulators of angiogenesis

A variety of factors cooperate to regulate neovessel formation and persistence. Proangiogenic growth factors have remained an area of intense interest due to their capacity to promote endothelial cell (EC) proliferation and to initiate the angiogenic program. These growth factors are associated with increased cell survival, yet paradoxically, angiogenic ECs are more susceptible to apoptosis than quiescent ECs. Survival is regulated by cooperation between growth factor receptors and integrins, which are in turn governed by the composition of the local extracellular matrix (ECM). Integrin-mediated signaling is altered or disrupted by the presence of soluble, rather than matrix-bound ligands, thus providing a means by which ECM remodeling can influence both integrin- and growth factor-mediated events. Ultimately, the collaboration of these factors determines whether ECs survive or die, thereby regulating neovascularization.

PI3K/Akt and apoptosis: size matters

Recent research has examined Akt and Akt-related serine-threonine kinases in signaling cascades that regulate cell survival and are important in the pathogenesis of degenerative diseases and in cancer. We seek to recapitulate the research that has helped to define the current understanding of the role of the Akt pathway under normal and pathologic conditions, also in view of genetic models of Akt function. In particular, we will evaluate the mechanisms of Akt regulation and the role of Akt substrates in Akt-dependent biologic responses in the decisions of cell death and cell survival. Here, we hope to establish the mechanisms of apoptosis suppression by Akt kinase as a framework for a more general understanding of growth factor-dependent regulation of cell survival.

Organ donors with malignant gliomas: an update

The escalating shortage of organs motivates frequent reconsideration of concepts that guide the decision to accept or decline organs from donors with central nervous system (CNS) malignancy. Currently, a minority of patients who die annually of CNS malignancies are organ donors. Specifically, the organs of less than 0.5% of the 13 000 patients dying from glioma are procured and transplanted every year in the United States. This review seeks to clarify the risk of cancer transmission from transplantation of organs from donors with glioma. After considering historical precedence, we will systematically outline the clinical features of a potential organ donor with glioma that might reflect upon the risk of cancer transmission. We will then present recent knowledge regarding basic glioma biology that speaks to their metastatic potential and suggest rational strategies for the post-transplant management of recipients of organs from donors with glioma.

mTOR: A Mediator of Intracellular Homeostasis

Earlier studies have shown that mTOR plays a key role in ribosome biogenesis. In bacteria, amino acids and ATP levels independently control ribosome biogenesis. Here, we describe recent findings demonstrating that homeostatic levels of amino acids, most notably branched-chain amino acids, and ATP, independently regulate the activity of mTOR. Unlike the effects of amino acids, the effects of ATP appear to be direct. Based on these findings we propose a model by which tumor cells existing in the anaerobic environment may have an advantage in growth by exploiting the rapid, although less efficient, production of ATP to drive growth via the mTOR signaling pathway.

mTOR-mediated regulation of translation factors by amino acids

The mammalian-target-of-rapamycin (mTOR) is a multidomain protein that is important in regulating several components of the translational machinery. mTOR signalling is stimulated by hormones (e.g., insulin) and by amino acids. Our recent data suggest that TOR signalling responds to intracellular amino acids rather than to external amino acid levels. The translational repressor eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) is regulated through mTOR and undergoes phosphorylation at multiple sites, which affects its function. It contains two regulatory motifs: the C-terminal TOS motif interacts with the mTOR-binding partner, raptor, and mediates phosphorylation of specific sites in 4E-BP1. However, the N-terminal RAIP motif affects a larger range of mTOR-regulated sites. Since this motif does not bind raptor, mTOR must signal to 4E-BP1 via additional mechanisms that are independent of raptor. The kinase that phosphorylates and inhibits elongation factor 2 (eEF2 kinase) is inactivated by insulin via mTOR. Insulin decreases the ability of eEF2 kinase to bind calmodulin, its essential activator, and this effect requires mTOR signalling and a novel phosphorylation site in eEF2 kinase, Ser78. Ser78 is not phosphorylated by known components of the mTOR pathway implying the existence of novel mTOR-regulated kinases that control eEF2 kinase.

Retroviral oncogenes and TOR

Retroviruses have recruited the catalytic subunit of PI 3-kinase and its downstream target, Akt, as oncogenes. These viruses cause tumors in animals and induce oncogenic transformation in cell culture. The oncogenicity of these viruses is specifically inhibited by rapamycin; retroviruses carrying other oncogenes are insensitive to this macrolide antibiotic. Rapamycin is an inhibitor of the TOR (target of rapamycin) kinase whose downstream targets include p70 S6 kinase and the negative regulator of translation initiation 4E-BP. Emerging evidence suggests that the TOR signals transmitted to the translational machinery are essential for oncogenic transformation by the PI 3-kinase pathway.

mTOR Signaling to Translation

Over the past few years, the target of rapamycin (TOR) pathway has been implicated in the control of translation, both in yeast and in higher eukaryotes. In this review, we provide an overview of translation in eukaryotes, and discuss the mechanisms and advantages of the regulation of translation. We then describe how the TOR pathway can modulate translation in yeast and in mammals, through the modulation of the phosphorylation of key translation components, and the regulation of the abundance of ribosomes and translation factors.

mTOR as a Target for Cancer Therapy

The target of rapamycin, mTOR, acts as a sensor for mitogenic stimuli, such as insulin-like growth factors and cellular nutritional status, regulating cellular growth and division. As many tumors are driven by autocrine or paracrine growth through the type-I insulin-like growth factor receptor, mTOR is potentially an attractive target for molecular-targeted treatment. Further, a rationale for anticipating tumor-selective activity based on transforming events frequently identified in malignant disease is becoming established.

mTOR as a positive regulator of tumor cell responses to hypoxia

Rapamycin is a clinically approved immunosuppressive agent that has recently shown promising antitumor activities in human patients. In contrast to many conventional chemotherapeutic agents, rapamycin displays a remarkably high level of selectivity for certain types of tumors. The pharmacological activities of rapamycin are attributable to the functional inhibition of a single target protein, termed the mammalian target of rapamycin (mTOR). Because mTOR is widely expressed in both normal and transformed cells, variations in mTOR expression levels are likely not a primary determinant of tumor sensitivity to rapamycin. However, recent studies highlighted an intriguing link between cancer cell sensitivity to rapamycin and deregulated signaling through the phosphoinositide (PI) 3-kinase pathway. These findings have prompted a search for cancer-related responses that are jointly regulated by the PI 3-kinase signaling cascade and mTOR. The oxygen-regulated transcription factor, hypoxia-induced factor (HIF)-1, has emerged as a candidate target for both of these two highly interactive signaling proteins. Here we review evidence that mTOR functions as a positive regulator of HIF-1-dependent responses to hypoxic stress in human cancer cells.

New Therapeutic strategies for soft tissue sarcomas

The treatment of patients with metastatic soft tissue sarcomas (STS) is complex. There are limited agents available and many are associated with significant toxicity. When evaluating a patient with metastatic disease, physicians should ask themselves whether there is a role for surgery to render the patient free of disease. Combination chemotherapy in patients who have not received chemotherapy in the adjuvant setting is one option, particularly in a young patient with a good performance status. Sequential single-agent therapy for patients who are more elderly or debilitated by their disease may be more appropriate. Gemcitabine appears to be an agent with activity, particularly in patients with leiomyosarcomas. The data regarding prolonged gemcitabine infusions suggest improved activity that was predicted based on prolonged intracellular gemcitabine levels. Because of these data, the prolonged infusion schedule should be used. In addition, because of the paucity of effective agents, consideration of clinical trial participation for patients with newly diagnosed metastatic disease is appropriate, particularly in chemotherapy-insensitive histologies. The role of the newer agents (eg, ecteinascidin-743, epothilones, and mammalian target of rapamycin) is undefined. Ecteinascidin-743 has been the most extensively tested agent, and its ability to slow growth kinetics of a tumor and stabilize it clinically is intriguing. Data regarding the response to BMS-247550 will be published shortly and will help define the further role of epothilones in this disease. There is a preclinical rationale that makes the mammalian target of rapamycin inhibitors attractive for the treatment of muscle-derived neoplasms. In addition, there are cell-line data suggesting activity in rhabdomyosarcoma. These agents are being tested in adult STS and will likely be tested in pediatric histologies when there are more safety data available in that population. SU11248 will continue to be tested in patients refractory to imatinib mesylate and may well prove to be another active agent for patients with gastrointestinal stromal tumors. As depicted by the analysis of gemcitabine efficacy, agents with activity in a subgroup of STS may be overlooked by the "come one come all" approach to clinical trials in STS. Identifying key targets in specific STS will be helpful in the testing of newer molecularly targeted agents. Biologic differences will support histology-specific trials to better understand the activity of an agent in a specific disease site or specifically target a biologic pathway with relevance to the malignant potential of the disease. For future clinical trials in STS to achieve the goal of histology-specific trials, cooperative group and multi-institutional trials will be required to obtain the appropriate patients with these rare histologies. It will also be increasingly important to be committed to obtaining tumor tissue in these patients to validate hypotheses regarding tumor biology and the effectiveness of therapeutic agents.

Integration of growth factor and nutrient signaling: implications for cancer biology

Signaling networks that promote cell growth are frequently dysregulated in cancer. One regulatory network, which converges on effectors such as 4EBP1 and S6K1, leads to growth by promoting protein synthesis. Here, we discuss how this network is regulated by both extracellular signals, such as growth factors, and intracellular signals, such as nutrients. We discuss how mutations amplifying either type of signal can lead to tumor formation. In particular, we focus on the recent discovery that a tumor suppressor complex whose function is lost in tuberous sclerosis patients regulates the nutrient signal carried by the critical signaling protein TOR to the effectors 4EBP1 and S6K1. Finally, we describe how the small molecule rapamycin, which inhibits TOR and thereby the activation of these effectors, could be useful to treat tumors that have become dependent upon this pathway for growth.

HIF-1 as a target for drug development

Sensing and responding to fluxes in oxygen tension is perhaps the single most important variable in physiology, and animal tissues have developed a number of essential mechanisms to cope with the stress of low physiological oxygen levels, or hypoxia. Among these coping mechanisms is the response mediated by the hypoxia-inducible transcription factor, or HIF-1. HIF-1 is an essential component in changing the transcriptional repertoire of tissues as oxygen levels drop, and could prove to be a very important target for drug development, as treatments evolve for diseases, such as cancer, heart disease and stroke, in which hypoxia is a central aspect.

AKT activity determines sensitivity to mammalian target of rapamycin (mTOR) inhibitors by regulating cyclin D1 and c-myc expression

Prior work demonstrates that AKT activity regulates sensitivity of cells to G(1) arrest induced by mammalian target of rapamycin (mTOR) inhibitors such as rapamycin and CCI-779. To investigate this, a novel high-throughput microarray polysome analysis was performed to identify genes whose mRNA translational efficiency was differentially affected following mTOR inhibition. The analysis also allowed the assessment of steady-state transcript levels. We identified two transcripts, cyclin D1 and c-myc, which exhibited differential expression in an AKT-dependent manner: High levels of activated AKT resulted in rapamycin-induced down-regulation of expression, whereas low levels resulted in up-regulation of expression. To ectopically express these proteins we exploited the finding that the p27(kip1) mRNA was efficiently translated in the face of mTOR inhibition irrespective of AKT activity. Thus, the p27(kip1) 5'-untranslated region was fused to the cyclin D1 and c-myc coding regions and these constructs were expressed in cells. In transfected cells, expression of cyclin D1 or c-myc was not decreased by rapamycin. Most importantly, this completely converted sensitive cells to a phenotype resistant to G(1) arrest. Furthermore, the AKT-dependent differential expression patterns of these two genes was also observed in a mouse xenograft model following in vivo treatment with CCI-779. These results identify two critical downstream molecular targets whose expression is regulated by AKT activity and whose down-regulation is required for rapamycin/CCI-779 sensitivity.

Akt activation disrupts mammary acinar architecture and enhances proliferation in an mTOR-dependent manner

Activation of the serine/threonine kinase Akt/PKB positively impacts on three cellular processes relevant to tumor progression: proliferation, survival, and cell size/growth. Using a three-dimensional culture model of MCF-10A mammary cells, we have examined how Akt influences the morphogenesis of polarized epithelial structures. Activation of a conditionally active variant of Akt elicits large, misshapen structures, which primarily arise from the combined effects of Akt on proliferation and cell size. Importantly, Akt activation amplifies proliferation during the early stages of morphogenesis, but cannot overcome signals suppressing proliferation in late-stage cultures. Akt also cooperates with oncoproteins such as cyclin D1 or HPV E7 to promote proliferation and morphogenesis in the absence of growth factors. Pharmacological inhibition of the Akt effector, mammalian target of rapamycin (mTOR), with rapamycin prevents the morphological disruption elicited by Akt activation, including its effect on cell size and number, and the cooperative effect of Akt on oncogene-driven proliferation, indicating that mTOR function is required for the multiple biological effects of Akt activation during morphogenesis.

Target of rapamycin (TOR)-signaling and RAIP motifs play distinct roles in the mammalian TOR-dependent phosphorylation of initiation factor 4E-binding protein 1

The translational repressor protein eIF4E-binding protein 1 (4E-BP1, also termed PHAS-I) is regulated by phosphorylation through the rapamycin-sensitive mTOR (mammalian target of rapamycin) pathway. Recent studies have identified two regulatory motifs in 4E-BP1, an mTOR-signaling (TOS) motif in the C terminus of 4E-BP1 and an RAIP motif (named after its sequence) in the N terminus. Other recent work has shown that the protein raptor binds to mTOR and 4E-BP1. We show that raptor binds to full-length 4E-BP1 or a C-terminal fragment containing the TOS motif but not to an N-terminal fragment containing the RAIP motif. Mutation of several residues within the TOS motif abrogates binding to raptor, indicating that the TOS motif is required for this interaction. 4E-BP1 undergoes phosphorylation at multiple sites in intact cells. The effects of removal or mutation of the RAIP and TOS motifs differ. The RAIP motif is absolutely required for phosphorylation of sites in the N and C termini of 4E-BP1, whereas the TOS motif primarily affects phosphorylation of Ser-64/65, Thr-69/70, and also the rapamycin-insensitive site Ser-101. Phosphorylation of N-terminal sites that are dependent upon the RAIP motif is sensitive to rapamycin. The RAIP motif thus promotes the mTOR-dependent phosphorylation of multiple sites in 4E-BP1 independently of the 4E-BP1/raptor interaction.

mTor is required for hypertrophy of Pten-deficient neuronal soma in vivo

The mechanisms that regulate mammalian cell size during development and homeostatic maintenance are poorly understood. The tumor suppressor Pten is required for correct maintenance of mammalian neuronal soma size. Selective inactivation of Pten in postnatal granule neurons of the cerebellum and dentate gyrus in mouse causes cell-autonomous hypertrophy as well as more complex phenotypes, including progressive macrocephaly, seizures, and premature death. To determine the contribution of mTor signaling to Pten-mediated growth regulation in the mammalian nervous system, we treated Pten conditional knockout mice with CCI-779, a specific mTor inhibitor. mTor inhibition decreased the seizure frequency and death rate in Pten mutant mice, prevented the increase in Pten-deficient neuronal soma size in young mice, and reversed neuronal soma enlargement in adult mice. mTor inhibition did not decrease the size of wild-type adult neurons. Thus, mTor is required for neuronal hypertrophy downstream of Pten deficiency, but is not required for maintenance of normal neuronal soma size. mTOR inhibitors may be useful therapeutic agents for diseases in brain resulting from PTEN deficiency such as Lhermitte-Duclos disease or glioblastoma multiforme.

Y box-binding protein 1 induces resistance to oncogenic transformation by the phosphatidylinositol 3-kinase pathway

Y box-binding protein 1 (YB-1) is a multifunctional protein that can act as a regulator of transcription and of translation. In chicken embryo fibroblasts transformed by the oncoproteins P3k (phosphatidylinositol 3-kinase) or Akt, YB-1 is transcriptionally down-regulated. Expression of YB-1 from a retroviral vector induces a strong cellular resistance to transformation by P3k or Akt but does not affect sensitivity to transformation by other oncoproteins, such as Src, Jun, or Qin. The YB-1-expressing cells assume a tightly adherent, flat phenotype, with YB-1 localized in the cytoplasm, and show a greatly reduced saturation density. Both cap-dependent and cap-independent translation is inhibited in these cells, but the activity of Akt remains unaffected, suggesting that YB-1 functions downstream of Akt. A YB-1 protein with a loss-of-function mutation in the RNA-binding motif no longer binds to the mRNA cap structure, is localized in the cell nucleus, does not induce the flat cellular phenotype, and fails to interfere with P3k- or Akt-induced oncogenic transformation. This mutant also does not inhibit cap-dependent or cap-independent translation. These results suggest that YB-1 acts like a rapamycin mimic, inhibiting translational events that are required in phosphatidylinositol 3-kinase-driven oncogenic transformation.

Inorganic polyphosphate stimulates mammalian TOR, a kinase involved in the proliferation of mammary cancer cells

Inorganic polyphosphate (poly P), chains of hundreds of phosphate residues linked by "high-energy" bonds as in ATP, has been conserved from prebiotic times in all cells. Poly P is essential for a wide variety of functions in bacteria, including virulence in pathogens. In this study, we observe the unique and many-fold stimulation by poly P in vitro of the protein kinase mTOR (mammalian target of rapamycin). To explore the role of poly P in mammalian cells, a yeast polyphosphatase, PPX1, was inserted into the chromosomes of MCF-7 mammary cancer cells. The transfected cells are markedly deficient in their response to mitogens, such as insulin and amino acids, as seen in their failure to activate mTOR to phosphorylate one of its substrates, PHAS-I (the initiation factor 4E-binding protein). In addition, the transfected cells are severely reduced in their growth in a serum-free medium. On the basis of these findings, we suggest that poly P (and/or PPX1) serves as a regulatory factor in the activation of mTOR in the proliferative signaling pathways of animal cells.

Involvement of the Akt/mTOR pathway on EGF-induced cell transformation

Our previous study demonstrated that phosphatidylinositol 3-kinase (PI3K) is necessary for epidermal growth factor (EGF)-induced cell transformation in mouse epidermal JB6 cells. Akt and the mammalian target of rapamycin (mTOR) are regarded as PI3K downstream effectors. Therefore, in this study, we investigated the role of Akt and mTOR on EGF-induced cell transformation in JB6 cells using rapamycin, a specific mTOR inhibitor, and cells expressing dominant negative mutants of Akt1 (DNM-Akt1). We found that the treatment of cells with rapamycin inhibited EGF-induced cell transformation but only slightly inhibited JB6 cell proliferation at 72 h. Although LY294002, a PI3K inhibitor, attenuated EGF-induced activator protein 1 (AP-1) activation, treatment with rapamycin did not affect AP-1 activity. Treatment with rapamycin inhibited EGF-induced phosphorylation and activation of ribosomal p70 S6 protein kinase (p70 S6K), an mTOR downstream target, but had no effect on phosphorylation and activation of Akt. Rapamycin also had no effect on EGF-induced phosphorylation of extracellular signal-regulated protein kinases (ERKs). We showed that introduction of DNM-Akt1 into JB6 mouse epidermal Cl 41 (JB6 Cl 41) cells inhibits EGF-induced cell transformation without blocking cell proliferation. The expression of DNM-Akt1 also suppressed EGF-induced p70 S6K activation as well as Akt activation. These results indicated an involvement of the Akt/mTOR pathway in EGF-induced cell transformation in JB6 cells.

Novel therapeutic strategies targeting the epidermal growth factor receptor (EGFR) family and its downstream effectors in breast cancer

From the early experience with tamoxifen to the current use of Herceptin, targeted therapy has been proven to be an important part of breast cancer (BC) treatment. In the last decade, advances in molecular biology have allowed scientists to design highly individualized, 'smart' pharmaceuticals, capable of manipulating the growth factor pathways and the genes that are involved in the development and maintenance of the malignant phenotype. The epidermal growth factor receptor (EGFR) family, as one of the best studied growth factor pathways in cancer, resembles a 'treasure island' by providing a wide range of biologically relevant targets involved in breast carcinogenesis. While a large number of new agents targeting this pathway are continuingly being tested in preclinical experiments, clinicians are witnessing the migration of some of these agents to daily practice. The aim of this review is to provide clinicians with an updated synopsis of the most advanced anti-erbB therapeutic strategies with activity against BC.

PTEN: one gene, many syndromes

PTEN, on 10q23.3, encodes a major lipid phosphatase which signals down the phosphoinositol-3-kinase/Akt pathway and effects G1 cell cycle arrest and apoptosis. Germline PTEN mutations have been found to occur in 80% of classic Cowden syndrome (CS), 60% of Bannayan-Riley-Ruvalcaba syndrome (BRRS), up to 20% of Proteus syndrome (PS), and approximately 50% of a Proteus-like syndrome (PSL). CS is a heritable multiple hamartoma syndrome with a high risk of breast, thyroid, and endometrial carcinomas. BRRS is a congenital autosomal dominant disorder characterized by megencephaly, developmental delay, lipomatosis, and speckled penis. PS and PSL had never been associated with risk of malignancy. Finding germline PTEN mutations in patients with BRRS, PS, and PSL suggests equivalent risks of developing malignancy as in CS with implications for medical management. The mutational spectra of CS and BRRS overlap, with many of the mutations occurring in exons 5, 7, and 8. Genotype-phenotype association analyses have revealed that the presence of germline PTEN mutations is associated with breast tumor development, and that mutations occurring within and 5' of the phosphatase motif were associated with multi-organ involvement. Pooled analysis of PTEN mutation series of CS and BRRS occurring in the last five years reveals that 65% of CS-associated mutations occur in the first five exons encoding the phosphatase domain and the promoter region, while 60% of BRRS-associated mutations occur in the 3' four exons encoding mainly the C2 domain. Somatic PTEN mutations occur with a wide distribution of frequencies in sporadic primary tumors, with the highest frequencies in endometrial carcinomas and glioblastoma multiform. Several mechanisms of PTEN inactivation occur in primary malignancies derived from different tissues, but a favored mechanism appears to occur in a tissue-specific manner. Inappropriate subcellular compartmentalization and increased/decreased proteosome degradation may be two novel mechanisms of PTEN inactivation. Further functional work could reveal more effective means of molecular-directed therapy and prevention.

Insulin-induced Drosophila S6 kinase activation requires phosphoinositide 3-kinase and protein kinase B

An important mechanism by which insulin regulates cell growth and protein synthesis is through activation of the p70 ribosomal S6 protein kinase (S6K). In mammalian cells, insulin-induced PI3K (phosphoinositide 3-kinase) activation, generates the lipid second messenger PtdIns(3,4,5) P (3), which is thought to play a key role in triggering the activation of S6K. Although the major components of the insulin-signalling pathway are conserved in Drosophila, recent studies suggested that S6K activation does not require PI3K in this system. To investigate further the role of dPI3K (Drosophila PI3K) in dS6K (Drosophila S6K) activation, we examined the effect of two structurally distinct PI3K inhibitors on insulin-induced dS6K activation in Kc167 and S2 Drosophila cell lines. We found that both inhibitors prevented insulin-stimulated phosphorylation and activation of dS6K. To investigate further the role of the dPI3K pathway in regulating dS6K activation, we also used dsRNAi (double-stranded RNA-mediated interference) to decrease expression of dPI3K and the PtdIns(3,4,5) P (3) phosphatase dPTEN ( Drosophila phosphatase and tensin homologue deleted on chromosome 10) in Kc167 and S2 cells. Knock-down of dPI3K prevented dS6K activation, whereas knock-down of dPTEN, which would be expected to increase PtdIns(3,4,5) P (3) levels, stimulated dS6K activity. Moreover, when the expression of the dPI3K target, dPKB (Drosophila protein kinase B), was decreased to undetectable levels, we found that insulin could no longer trigger dS6K activation. This observation provides the first direct demonstration that dPKB is required for insulin-stimulated dS6K activation. We also present evidence that the amino-acid-induced activation of dS6K in the absence of insulin, thought to be mediated by dTOR (Drosophila target of rapamycin), which is unaffected by the inhibition of dPI3K by wortmannin. The results of the present study support the view that, in Drosophila cells, dPI3K and dPKB, as well dTOR, are required for the activation of dS6K by insulin.

Beyond Herceptin and Gleevec

Identification of the key role of protein kinases as potential oncoproteins has led to the emergence of a new era of target-directed therapies. Among a variety of novel therapeutic strategies two have shown the most promise and led to a variety of therapeutic agents in clinical development. One approach utilises humanised monoclonal antibodies generated against the extracellular domain of transmembrane protein kinases. The second approach is the generation of small molecule ATP analogues targeting the kinase domain itself. The approval of agents such as Herceptin for the treatment of advanced breast cancer and Gleevec for chronic myelogenous leukemia and gastrointestinal stromal tumours are the first examples of gene-based cancer drugs and represent the first example of a novel strategy in anti-cancer therapy.

TSC2 regulates VEGF through mTOR-dependent and -independent pathways

Inactivation of the TSC2 tumor suppressor protein causes tuberous sclerosis complex (TSC), a disease characterized by highly vascular tumors. TSC2 has multiple functions including inhibition of mTOR (mammalian target of Rapamycin). We found that TSC2 regulates VEGF through mTOR-dependent and -independent pathways. TSC2 loss results in the accumulation of HIF-1alpha and increased expression of HIF-responsive genes including VEGF. Wild-type TSC2, but not a disease-associated TSC2 mutant, downregulates HIF. Rapamycin normalizes HIF levels in TSC2(-/-) cells, indicating that TSC2 regulates HIF by inhibiting mTOR. In contrast, Rapamycin only partially downregulates VEGF in this setting, implying an mTOR-independent link between TSC2 loss and VEGF. This pathway may involve chromatin remodeling since the HDAC inhibitor Trichostatin A downregulates VEGF in TSC2(-/-) cells.

Survival of acute myeloid leukemia cells requires PI3 kinase activation

The mechanisms that regulate the growth and survival of acute myeloid leukemia (AML) cells are largely unknown. We hypothesized that constitutive activation of phosphatidyl-inositide 3 kinase (PI3 kinase) could regulate survival in primary cells from patients with AML. Here we demonstrate that Akt, a critical substrate of PI3 kinase, is activated in AML blasts. In a short-term culture system, most AML patient samples showed a dose-dependent decrease in survival after incubation with the PI3 kinase inhibitor LY294002. This decrease in survival was partially due to the induction of apoptosis. Furthermore, we have shown that p70 S6 kinase and 4EBP-1, downstream mediators of Akt signaling, also are phosphorylated in AML blasts. Phosphorylation of these proteins is inhibited by the mTOR inhibitor RAD001. Incubation of AML blasts with RAD001 induces only a small decrease in survival of the cells; however, when combined with Ara-C, RAD001 enhances the toxicity of Ara-C. These results demonstrate that constitutive activation of the PI3 kinase pathway is necessary for the survival of AML blasts and that targeting of this pathway with pharmacologic inhibitors may be of clinical benefit in treatment of AML.

Overcoming resistance of cancer cells to apoptosis

Discovery of the B cell lymphoma gene 2 (Bcl-2 gene) led to the concept that development of cancers required the simultaneous acquisition, not only of deregulated cell division, but also of resistance to programmed cell death or apoptosis. Apoptosis is arguably the common pathway to cell death resulting from a range of therapeutic initiatives, so that understanding the basis for the resistance of cancer cells to apoptosis may hold the key to development of new treatment initiatives. Much has already been learnt about the apoptotic pathways in cancer cells and proteins regulating these pathways. In most cells, apoptosis is dependent on the mitochondrial dependent pathway. This pathway is regulated by pro- and anti-apoptotic members of the Bcl-2 family, and manipulation of these proteins offers scope for a number of treatment initiatives. Effector caspases activated by the mitochondrial pathway or from death receptor signaling are under the control of the inhibitor of apoptosis protein (IAP) family. Certain proteins from mitochondrial can, however, competitively inhibit their binding to effector caspases. Information about the structure of these proteins has led to initiatives to develop therapeutic agents to block the IAP family. In addition to development of selective agents based on these two (Bcl-2 and IAP) protein families, much has been learnt about signal pathways that may regulate their activity. These in turn might provide additional approaches based on selective regulators of the signal pathways.

Prostate cancer: a dynamic illness with shifting targets

Despite high response rates and palliative clinical benefits, androgen ablation does not cure advanced prostate cancer because of the inevitable emergence of resistant cells. Many new therapies under development for prostate cancer target pathways and molecules that contribute to the growth and survival of these cells. The rational and effective use of targeted therapies to eradicate resistant populations of tumour cells should be grounded on the premise that prostate cancer is a dynamic disease that evolves as it progresses, and that specific molecular determinants mediating sensitivity and resistance may be relevant only during specific states of the disease. Directed approaches must account for this changing dynamic so that clinical outcomes may be improved.

Evidence that phosphatidylinositol 3-kinase- and mitogen-activated protein kinase kinase-4/c-Jun NH2-terminal kinase-dependent Pathways cooperate to maintain lung cancer cell survival

Cancer cells in which the PTEN lipid phosphatase gene is deleted have constitutively activated phosphatidylinositol 3-kinase (PI3K)-dependent signaling and require activation of this pathway for survival. In non-small cell lung cancer (NSCLC) cells, PI3K-dependent signaling is typically activated through mechanisms other than PTEN gene loss. The role of PI3K in the survival of cancer cells that express wild-type PTEN has not been defined. Here we provide evidence that H1299 NSCLC cells, which express wild-type PTEN, underwent proliferative arrest following treatment with an inhibitor of all isoforms of class I PI3K catalytic activity (LY294002) or overexpression of the PTEN lipid phosphatase. In contrast, overexpression of a dominant-negative mutant of the p85alpha regulatory subunit of PI3K (Deltap85) induced apoptosis. Whereas PTEN and Delta85 both inhibited activation of AKT/protein kinase B, only Deltap85 inhibited c-Jun NH2-terminal kinase (JNK) activity. Cotransfection of the constitutively active mutant Rac-1 (Val12), an upstream activator of JNK, abrogated Deltap85-induced lung cancer cell death, whereas constitutively active mutant mitogen-activated protein kinase kinase (MKK)-1 (R4F) did not. Furthermore, LY294002 induced apoptosis of MKK4-null but not wild-type mouse embryo fibroblasts. Therefore, we propose that, in the setting of wild-type PTEN, PI3K- and MKK4/JNK-dependent pathways cooperate to maintain cell survival.

Prostate intraepithelial neoplasia induced by prostate restricted Akt activation: the MPAKT model

To determine whether Akt activation was sufficient for the transformation of normal prostate epithelial cells, murine prostate restricted Akt kinase activity was generated in transgenic mice (MPAKT mice). Akt expression led to p70S6K activation, prostatic intraepithelial neoplasia (PIN), and bladder obstruction. mRNA expression profiles from MPAKT ventral prostate revealed similarities to human cancer and an angiogenic signature that included three angiogenin family members, one of which was found elevated in the plasma of men with prostate cancer. Thus, the MPAKT model may be useful in studying the role of Akt in prostate epithelial cell transformation and in the discovery of molecular markers relevant to human disease.

Phospholipase D confers rapamycin resistance in human breast cancer cells

mTOR (mammalian target of rapamycin) is a protein kinase that regulates cell cycle progression and cell growth. Rapamycin is a highly specific inhibitor of mTOR in clinical trials for the treatment of breast and other cancers. mTOR signaling was reported to require phosphatidic acid (PA), the metabolic product of phospholipase D (PLD). PLD, like mTOR, has been implicated in survival signaling and the regulation of cell cycle progression. PLD activity is frequently elevated in breast cancer. We have investigated the effect of rapamycin on breast cancer cell lines with different levels of PLD activity. MCF-7 cells, with relatively low levels of PLD activity, were highly sensitive to the growth-arresting effects of rapamycin, whereas MDA-MB-231 cells, with a 10-fold higher PLD activity than MCF-7 cells, were highly resistant to rapamycin. Elevating PLD activity in MCF-7 cells led to rapamycin resistance; and inhibition of PLD activity in MDA-MB-231 cells increased rapamycin sensitivity. Elevated PLD activity in MCF-7 cells also caused rapamycin resistance for S6 kinase phosphorylation and serum-induced Myc expression. These data implicate mTOR as a critical target for survival signals generated by PLD and suggest that PLD levels in breast cancer could be a valuable indicator of the likely efficacy of rapamycin treatment.

Mammalian target of rapamycin: a new molecular target for breast cancer

The mammalian target of rapamycin (mTOR), a downstream effector of the phosphatidylinositol 3-kinase (PI3K)/Akt (protein kinase B) signaling pathway that mediates cell survival and proliferation, is a prime strategic target for anticancer therapeutic development. By targeting mTOR, the immunosuppressant and antiproliferative agent rapamycin inhibits signals required for cell cycle progression, cell growth, and proliferation. Both rapamycin and novel rapamycin analogues with more favorable pharmaceutical properties, such as CCI-779, RAD 001, and AP23573, are highly specific inhibitors of mTOR. In essence, these agents gain function by binding to the immunophilin FK506 binding protein 12 and the resultant complex inhibits the activity of mTOR. Because mTOR activates both the 40S ribosomal protein S6 kinase (p70s6k) and the eukaryotic initiation factor 4E-binding protein-1, rapamycin-like compounds block the actions of these downstream signaling elements, which results in cell cycle arrest in the G1 phase. Rapamycin and its analogues also prevent cyclin-dependent kinase (CDK) activation, inhibit retinoblastoma protein phosphorylation, and accelerate the turnover of cyclin D1, leading to a deficiency of active CDK4/cyclin D1 complexes, all of which potentially contribute to the prominent inhibitory effects of rapamycin at the G1/S boundary of the cell cycle. Rapamycin and rapamycin analogues have demonstrated impressive growth-inhibitory effects against a broad range of human cancers, including breast cancer, in preclinical and early clinical evaluations. In breast cancer cells, PI3K/Akt and mTOR pathways seem to be critical for the proliferative responses mediated by the epidermal growth factor receptor, the insulin growth factor receptor, and the estrogen receptor. Furthermore, these pathways may be constitutively activated in cancers with many types of aberrations, including those with loss of PTEN suppressor gene function. Therefore, the development of inhibitors of mTOR and related pathways is a rational therapeutic strategy for breast and other malignancies that possess a wide range of aberrant molecular constituents. This review will summarize the principal mechanisms of action of rapamycin and rapamycin derivatives, as well as the potential utility of these agents as anticancer therapeutic agents with an emphasis on breast cancer. The preliminary results of early clinical evaluations with rapamycin analogues and the unique developmental challenges that lie ahead will also be discussed.

Phase I and pharmacokinetic study of CCI-779, a novel cytostatic cell-cycle inhibitor, in combination with 5-fluorouracil and leucovorin in patients with advanced solid tumors

BACKGROUND

CCI-779 is a novel ester of the immunosuppressive agent sirolimus that exerts cytostatic effects by the inhibition of the translation of cell-cycle regulatory proteins. We investigated the maximum tolerated dose (MTD) and pharmacokinetics (PK) of CCI-779 in combination with leucovorin (LV) and 5-fluorouracil (5-FU) in patients with advanced solid tumors.

PATIENTS AND METHODS

Patients were treated with LV at 200 mg/m(2) as a 1-h i.v. infusion directly followed by continuous 24-h i.v. infusion of 5-FU, in the first patient at 2000 mg/m(2) and in subsequent patients at 2600 mg/m(2). CCI-779 was administered directly prior to LV as a 30-min i.v. infusion at a starting dose of 15 mg/m(2) beginning at day 8 and escalated in subsequent cohorts of patients. One cycle consisted of six weekly administrations followed by 1 week of rest. Blood samples were drawn to assess PK of CCI-779 as well as its effect on steady-state 5-FU exposures.

RESULTS

Twenty-eight patients entered the study, the majority having tumor types for which 5-FU is used as a treatment. CCI-779 doses of 15, 25, 45 and 75 mg/m(2) were investigated. Skin toxicity (rash) was prominent at all dose levels examined. Stomatitis was the dose-limiting toxicity (DLT) for 75 mg/m(2) doses of CCI-779. Subsequently the cohort at 45 mg/m(2) was expanded to a total of 15 patients, and at this dose level two treatment-related deaths occurred due to mucositis with bowel perforation. Based on the toxicities observed, it was decided to discontinue the study. Partial responses were observed in three patients with gastrointestinal tumors. No pharmacokinetic interaction between CCI-779 and 5-FU was observed.

CONCLUSIONS

The safety profiles of CCI-779 and 5-FU/LV suggest an overlap of drug-related toxicities, and the administration of these drugs at these doses and schedule resulted in unacceptable toxicity and therefore cannot be recommended. If CCI-779 is to be used in combination with 5-FU/LV, other doses or schedules of administration will need to be explored.

Mitogenic signaling cascades in glial tumors

Gliomas are primary central nervous system tumors that arise from astrocytes, oligodendrocytes, or their precursors. Gliomas can be classified into several groups according to histological features. A number of genetic alterations have been identified in human gliomas; these generally affect either signal transduction pathways activated by receptor tyrosine kinases or cell cycle growth arrest pathways. These observed genetic alterations are now being used to complement histopathological diagnosis. The aim of the present review is to give a broad overview of the receptor tyrosine kinase signaling machinery involved in gliomagenesis, with an emphasis on the cooperative interaction between receptor tyrosine kinase signaling and the cell cycle-regulatory machinery. Understanding molecular features of primary glial tumors will eventually allow for target-selective intervention in distinct glioma subsets and a more rational approach to adjuvant therapies for these refractory diseases.

Sustained activation of the JNK cascade and rapamycin-induced apoptosis are suppressed by p53/p21(Cip1)

Under serum-free conditions, rapamycin, an inhibitor of mammalian target of rapamycin (mTOR), induces apoptosis of cells lacking functional p53. Cells expressing wild-type p53 or p21(Cip1)arrest in G1 and remain viable. In cells lacking functional p53, rapamycin or amino acid deprivation induces rapid and sustained activation of apoptosis signal-regulating kinase 1 (ASK1), c-Jun N-terminal kinase, and elevation of phosphorylated c-Jun that results in apoptosis. This stress response depends on expression of eukaryotic initiation factor 4E binding protein 1 and is suppressed by p21(Cip1) independent of cell cycle arrest. Rapamycin induces p21(Cip1) binding to ASK1, suppressing kinase activity and attenuating cellular stress. These results suggest that inhibition of mTOR triggers a potentially lethal response that is prevented only in cells expressing p21(Cip1).

Identification of molecular subtypes of glioblastoma by gene expression profiling

Epidermal growth factor receptor (EGFR) overexpression occurs in nearly 50% of cases of glioblastoma (GBM), but its clinical and biological implications are not well understood. We have used Affymetrix high-density oligonucleotide arrays to demonstrate that EGFR-overexpressing GBMs (EGFR+) have a distinct global gene transcriptional profile. We show that the expression of 90 genes can distinguish EGFR+ from EGFR nonexpressing (EGFR-) GBMs, including a number of genes known to act as growth/survival factors for GBMs. We have also uncovered two additional novel molecular subtypes of GBMs, one of which is characterized by coordinate upregulation of contiguous genes on chromosome 12q13-15 and expression of both astrocytic and oligodendroglial genes. These results define distinct molecular subtypes of GBMs that may be important in disease stratification, and in the discovery and assessment of GBM treatment strategies.

Downstream effectors of oncogenic ras in multiple myeloma cells

Ectopic expression of mutated K-ras or N-ras in the interleukin 6 (IL-6)-dependent ANBL6 multiple myeloma cell line induces cytokine-independent growth. To investigate the signaling pathways activated by oncogenic ras that may stimulate IL-6-independent growth, we compared ANBL6 cells stably transfected with mutated K or N-ras genes with wild-type ras-expressing control cells identically transfected with an empty vector. Upon depletion of IL-6, both mutated ras-containing myeloma lines demonstrated constitutive activation of mitogen-activated extracellular kinase 2(MEK)/extracellular signal-regulated kinase (ERK), phosphatidylinositol-3 kinase (PI3-kinase)/AKT, mammalian target of rapamycin (mTOR)/p70S6-kinase, and nuclear factor kappa B (NF-kappa B) pathways. In contrast, signal transducer and activator of transcription-3 (STAT-3) was not constitutively tyrosine phosphorylated in mutant ras-expressing cells. We used several maneuvers in attempts to selectively target these constitutively active pathways. The mTOR inhibitors rapamycin and CCI-779, the PI3-kinase inhibitor LY294002, and the MEK inhibitor PD98059 all significantly curtailed growth of mutant ras-containing cells. Farnesyl transferase inhibitors, used to target ras itself, had modest effects only against mutant N-ras-containing cells. Growth of mutant N-ras-containing myeloma cells was also inhibited by acute expression of the IKB superrepressor gene, which abrogated NF-kappa B activation. These results indicate that several pathways contributing to stimulation of cytokine-independent growth are activated downstream of oncogenic ras in myeloma cells. They also suggest that therapeutic strategies that target these pathways may be particularly efficacious in patients whose myeloma clones contain ras mutations.

Issues and progress with protein kinase inhibitors for cancer treatment

Identification of the key roles of protein kinases in cancer has led to extensive efforts to develop kinase inhibitors for the treatment of a wide range of cancers, and more than 30 such agents are now in clinical trials. Here, we consider the crucial issues in the development of kinase inhibitors for cancer, and discuss strategies to address the challenges raised by these issues in the light of preclinical and clinical experiences so far.

Advances in molecular therapies in patients with brain tumors

BACKGROUND

We are witnessing the development of new treatment modalities for primary brain tumors. An area under intense investigation is the use of small molecules targeting intracellular signaling pathways that interfere with growth, invasion, and metastasis of high-grade gliomas.

METHODS

We review clinical trials of small molecules in adults with brain tumors. This search included electronic databases, specialty journals, textbooks, proceedings, and Web sites of the National Cancer Institute and other cooperative brain tumor groups in Europe and the United States.

RESULTS

Several drugs with the ability to down-regulate the growth and invasion of malignant gliomas are at various stages of testing. Most of these focus on interfering with oncogenic and tumor survival pathways. Examples include inhibitors of tyrosine kinases, farnesyltransferases, and matrix metalloproteinases. These molecules are at different stages of testing, and a conclusive picture of which drug is most effective, either alone or in combination, needs better definition. The metalloproteinase inhibitor marimastat with temozolomide has given the best results to date in phase II trials, increasing the rate of 6-month progression-free survival for recurrent glioblastoma multiforme and anaplastic gliomas.

CONCLUSIONS

As our understanding of the biology of gliomas increases and new drugs targeting specific molecular pathways enter well-designed cooperative trials, the control and prognosis of these tumors should improve.

The C terminus of initiation factor 4E-binding protein 1 contains multiple regulatory features that influence its function and phosphorylation

Eukaryotic initiation factor 4E (eIF4E) binds the mRNA cap structure and forms eIF4F complexes that recruit 40S subunits to the mRNA. Formation of eIF4F is blocked by eIF4E-binding proteins such as 4E-BP1, which interacts with eIF4E via a motif in the center of its 118-residue sequence. 4E-BP1 plays key roles in cell proliferation, growth, and survival. Binding of 4E-BP1 to eIF4E is regulated by hierarchical multisite phosphorylation. Here we demonstrate that three different features in the C terminus of 4E-BP1 play distinct roles in regulating its phosphorylation and function. Firstly, we identify a new phosphorylation site in its C terminus (S101). A serine or glutamate at this position is required for efficient phosphorylation at Ser65. A second C-terminal site, S112, directly affects binding of 4E-BP1 to eIF4E without influencing phosphorylation of other sites. Thirdly, a conserved C-terminal motif influences phosphorylation of multiple residues, including rapamycin-insensitive sites. These relatively long-range effects are surprising given the reportedly unstructured nature of 4E-BP1 and may imply that phosphorylation of 4E-BP1 and/or binding to eIF4E induces a more-ordered structure. 4E-BP2 and -3 lack phosphorylatable residues corresponding to both S101 and S112. However, in 4E-BP3, replacement of the alanine at the position corresponding to S112 by serine or glutamate did not confer the ability to be released from eIF4E in response to insulin.

Does the ribosome translate cancer?

Ribosome biogenesis and translation control are essential cellular processes that are governed at numerous levels. Several tumour suppressors and proto-oncogenes have been found either to affect the formation of the mature ribosome or to regulate the activity of proteins known as translation factors. Disruption in one or more of the steps that control protein biosynthesis has been associated with alterations in the cell cycle and regulation of cell growth. Therefore, certain tumour suppressors and proto-oncogenes might regulate malignant progression by altering the protein synthesis machinery. Although many studies have correlated deregulation of protein biosynthesis with cancer, it remains to be established whether this translates directly into an increase in cancer susceptibility, and under what circumstances.