TSC1 and TSC2 tumor suppressors antagonize insulin signaling in cell growth

The TSC1-2 tumor suppressor controls insulin-PI3K signaling via regulation of IRS proteins

Insulin-like growth factors elicit many responses through activation of phosphoinositide 3-OH kinase (PI3K). The tuberous sclerosis complex (TSC1-2) suppresses cell growth by negatively regulating a protein kinase, p70S6K (S6K1), which generally requires PI3K signals for its activation. Here, we show that TSC1-2 is required for insulin signaling to PI3K. TSC1-2 maintains insulin signaling to PI3K by restraining the activity of S6K, which when activated inactivates insulin receptor substrate (IRS) function, via repression of IRS-1 gene expression and via direct phosphorylation of IRS-1. Our results argue that the low malignant potential of tumors arising from TSC1-2 dysfunction may be explained by the failure of TSC mutant cells to activate PI3K and its downstream effectors.

The mitotic-to-endocycle switch in Drosophila follicle cells is executed by Notch-dependent regulation of G1/S, G2/M and M/G1 cell-cycle transitions

The Notch signaling pathway controls the follicle cell mitotic-to-endocycle transition in Drosophila oogenesis by stopping the mitotic cycle and promoting the endocycle. To understand how the Notch pathway coordinates this process, we have identified and performed a functional analysis of genes whose transcription is responsive to the Notch pathway at this transition. These genes include the G2/M regulator Cdc25 phosphatase, String; a regulator of the APC ubiquitination complex Hec/CdhFzr and an inhibitor of the CyclinE/CDK complex, Dacapo. Notch activity leads to downregulation of String and Dacapo, and activation of Fzr. All three genes are independently responsive to Notch. In addition, CdhFzr, an essential gene for endocycles, is sufficient to stop mitotic cycle and promote precocious endocycles when expressed prematurely during mitotic stages. In contrast, overexpression of the growth controller Myc does not induce premature endocycles but accelerates the kinetics of normal endocycles. We also show that Archipelago (Ago), a SCF-regulator is dispensable for mitosis, but crucial for endocycle progression in follicle epithelium. The results support a model in which Notch activity executes the mitotic-to-endocycle switch by regulating all three major cell cycle transitions. Repression of String blocks the M-phase, activation of Fzr allows G1 progression and repression of Dacapo assures entry into the S-phase. This study provides a comprehensive picture of the logic that external signaling pathways may use to control cell cycle transitions by the coordinated regulation of the cell cycle.

mTOR inhibition reverses Akt-dependent prostate intraepithelial neoplasia through regulation of apoptotic and HIF-1-dependent pathways

Loss of PTEN function leads to activation of phosphoinositide 3-kinase (PI3K) signaling and Akt. Clinical trials are now testing whether mammalian target of rapamycin (mTOR) inhibition is useful in treating PTEN-null cancers. Here, we report that mTOR inhibition induced apoptosis of epithelial cells and the complete reversal of a neoplastic phenotype in the prostate of mice expressing human AKT1 in the ventral prostate. Induction of cell death required the mitochondrial pathway, as prostate-specific coexpression of BCL2 blocked apoptosis. Thus, there is an mTOR-dependent survival signal required downstream of Akt. Bcl2 expression, however, only partially restored intraluminal cell growth in the setting of mTOR inhibition. Expression profiling showed that Hif-1 alpha targets, including genes encoding most glycolytic enzymes, constituted the dominant transcriptional response to AKT activation and mTOR inhibition. These data suggest that the expansion of AKT-driven prostate epithelial cells requires mTOR-dependent survival signaling and activation of HIF-1 alpha, and that clinical resistance to mTOR inhibitors may emerge through BCL2 expression and/or upregulation of HIF-1 alpha activity.

Signaling control of mRNA translation in cancer pathogenesis

The regulation of translation and the control of ribosome biogenesis are essential cellular processes whose impact on cell growth and proliferation is manifested at a number of specific levels. Disruption in one or more of the steps that control protein biosynthesis has been associated with alterations in the regulation of cell growth and cell cycle progression. Consistent with this, tumor suppressors and proto-oncogenes have been found to act on these functions and may therefore regulate malignant progression by affecting the protein synthetic machinery. Although many studies have correlated deregulation of protein biosynthesis with cancer, it remains to be established whether this process is necessary and/or sufficient for neoplastic transformation and metastasis.

Target of rapamycin (TOR): an integrator of nutrient and growth factor signals and coordinator of cell growth and cell cycle progression

Cell growth (an increase in cell mass and size through macromolecular biosynthesis) and cell cycle progression are generally tightly coupled, allowing cells to proliferate continuously while maintaining their size. The target of rapamycin (TOR) is an evolutionarily conserved kinase that integrates signals from nutrients (amino acids and energy) and growth factors (in higher eukaryotes) to regulate cell growth and cell cycle progression coordinately. In mammals, TOR is best known to regulate translation through the ribosomal protein S6 kinases (S6Ks) and the eukaryotic translation initiation factor 4E-binding proteins. Consistent with the contribution of translation to growth, TOR regulates cell, organ, and organismal size. The identification of the tumor suppressor proteins tuberous sclerosis1 and 2 (TSC1 and 2) and Ras-homolog enriched in brain (Rheb) has biochemically linked the TOR and phosphatidylinositol 3-kinase (PI3K) pathways, providing a mechanism for the crosstalk that occurs between these pathways. TOR is emerging as a novel antitumor target, since the TOR inhibitor rapamycin appears to be effective against tumors resulting from aberrantly high PI3K signaling. Not only may inhibition of TOR be effective in cancer treatment, but rapamycin is an FDA-approved immunosuppressive and cardiology drug. We review here what is known (and not known) about the function of TOR in cellular and animal physiology.

Tuberin is a component of lipid rafts and mediates caveolin-1 localization: role of TSC2 in post-Golgi transport

Mutations of the TSC2 gene lead to the development of hamartomas in tuberous sclerosis complex. Their pathology exhibits features indicative of defects in cell growth, proliferation, differentiation, and migration. We have previously shown that tuberin, the TSC2 protein, resides in multiple subcellular compartments and as such may serve multiple functions. To further characterize the microsomal pool of tuberin, we found that it cofractionated with caveolin-1 in a low-density, Triton X-100-resistant fraction (i.e., lipid rafts) and regulated its localization. In cells lacking tuberin, most of the endogenous caveolin-1 was displaced from the plasma membrane to a Brefeldin-A-sensitive, post-Golgi compartment distinct from the endosome and lysosome. Correspondingly, there was a paucity of caveolae at the plasma membrane of Tsc2-/- cells. Reintroduction of TSC2, but not a disease-causing mutant, reversed the caveolin-1 localization to the membrane. Exogenously expressed caveolin-1-GFP and vesicular stomatitis virus G protein, VSVG-GFP in the Tsc2-/- cells failed to be transported to the plasma membrane and were retained in distinct post-Golgi vesicles. Our data suggest a role of tuberin in regulating post-Golgi transport without apparent effects on protein sorting. The presence of mislocalized proteins in Tsc2-/- cells may contribute to the abnormal signaling and cellular phenotype of tuberous sclerosis.

An essential role for protein synthesis in oncogenic cellular transformation

The induction and maintenance of oncogenic transformation requires interference with the controls that regulate translation and transcription. The PI 3-kinase pathway, which shows gain of function in numerous and diverse human cancers, generates signals that have a positive effect on the initiation of protein synthesis. Here we review the components of the PI 3-kinase signaling pathway and the mRNA-binding protein YB-1, exploring their roles in protein synthesis and oncogenic cell transformation.

Drosophila genetic variants that change cell size and rate of proliferation affect cell communication and hence patterning

We explore in this paper the role of genetic variants that affect cell size and proliferation in the determination of organ size. We use genetic mosaics of loss or gain of function in six different loci, which promotes smaller or larger than normal cells, associated to either smaller or larger than normal territories. These variants have autonomous effects on patterning and growth in mutant territories. However, there is no correlation between cell size or rate of proliferation on the size of the mutant territory. In addition, these mosaics show non-autonomous effects on surrounding wildtype cells, consisting always in a reduction in number of non-mutant cells. In all mutant conditions the final size (and shape) of the wing is different than normal. The phenotypes of the same variants include higher density of chaetae in the notum. These autonomous and non-autonomous effects suggest that the control of size in the wing is the result of local cell communication defining canonic distances between cells in a positional-values landscape.

Estrogen-induced smooth muscle cell growth is regulated by tuberin and associated with altered activation of platelet-derived growth factor receptor-beta and ERK-1/2

The mechanisms that regulate the diverse responses to estrogen (E2) are unknown. Loss of function of the tuberous sclerosis 2 gene (TSC2), a tumor suppressor gene, has been associated with a growth-promoting effect of E2. We hypothesized that tuberin, the protein product of TSC2, binds to estrogen receptors (ER) and regulates the growth effect of E2. An in vivo association between full-length tuberin and ERalpha was observed in HEK 293 cells and ELT-3 smooth muscle cells. In contrast, poor association was observed between tuberin and ERbeta. Complex formation with ERalpha and the C-terminal end of tuberin was also observed in vivo and in vitro, indicating that binding between ERalpha and tuberin occurs at the C-terminal end of the tuberin molecule. We examined the effect of tuberin expression in ELT-3 smooth muscle cells on the growth response to E2. The growth-promoting effect of E2 in tuberin-null ELT-3 smooth muscle cells was ERalpha-specific, associated with up-regulation and activation of platelet-derived growth factor receptor-beta (PDGFRbeta) and activation of the signaling intermediate, extracellular signal-regulated kinase-1/-2 (ERK-1/2). In contrast, the expression of tuberin in ELT-3 smooth muscle cells resulted in significant abrogation of E2-stimulated growth. In parallel with this observation, the expression of tuberin in ELT-3 cells also resulted in significant inhibition of PDGFRbeta and ERK-1/2 activation in response to E2. These results demonstrate that tuberin binds specifically to ERalpha and inhibits E2-induced proliferation of ELT-3 cells. Furthermore, the opposing effects of tuberin on estrogen-induced activation of PDGFRbeta and ERK-1/-2 suggest a pivotal role for tuberin in directing the signaling events that dictate the growth response to E2.

Critical role of T-loop and H-motif phosphorylation in the regulation of S6 kinase 1 by the tuberous sclerosis complex

The tuberous sclerosis gene products Tsc1 and Tsc2 behave as tumor suppressors by restricting cell growth, a function conserved among metazoans. Recent evidence has indicated that hyperactivation of S6 kinase 1 (S6K1) may represent an important biochemical change in the development of tuberous sclerosis-associated lesions. We show here that deletion of either Tsc1 or Tsc2 or expression of the Rheb (Ras homolog enriched in brain) GTPase leads to hyperphosphorylation of S6K1 at a subset of regulatory sites, particularly those of two essential residues functionally conserved among AGC superfamily serine/threonine kinases, i.e. the activation loop (T-loop; Thr-229) and the hydrophobic motif (H-motif; Thr-389). These sites are reciprocally and dose-dependently regulated when S6K1 is coexpressed with the Tsc1-Tsc2 complex. Mutations that render S6K1 mTOR (mammalian target of rapamycin)-resistant also protect S6K1 activity and phosphorylation from down-regulation by Tsc1/2. We demonstrate that two disease-associated mutations in Tsc2 fail to negatively regulate S6K1 activity concomitant with a failure to modify T-loop and H-motif phosphorylation. Finally, we identify one pathological Tsc2 mutation that retains its ability to negatively regulate S6K1, suggesting that, in some cases, tuberous sclerosis may develop independently of S6K1 hyperactivation. These results also highlight the importance of dual control of T-loop and H-motif phosphorylation of S6K1 by the Tsc1-Tsc2 complex.

Niban gene is commonly expressed in the renal tumors: a new candidate marker for renal carcinogenesis

Functional inactivation of tuberous sclerosis 2 gene (Tsc2) leads to renal carcinogenesis in the hereditary renal carcinoma Eker rat models. Recent studies revealed a role of tuberin, a TSC2 product, in suppressing the p70 S6 kinase (p70S6K) activity via inhibition of mammalian target of rapamycin (mTOR). Phosphorylated S6 protein, a substrate of p70S6K, was expressed in the early lesions in Eker rats, and this expression was suppressed by the treatment of rapamycin, an inhibitor of mTOR. We previously isolated the novel gene Niban expressed in renal carcinogenesis of Eker rats. In this study, we demonstrated that the expression of Niban was detected from early preneoplastic lesions in Eker rats. Interestingly, in contrast to the phosphorylated S6 protein, the expression of Niban was unchanged and early lesions still remained even after treatment with rapamycin. These results might suggest the existence of another pathway independent of mTOR-S6K pathway in Tsc2 mutant renal carcinogenesis. In addition, Niban was also expressed in other renal carcinoma models, including Tsc1 and Tsc2 knockout mice, and various types of human renal cell carcinomas. Thus, Niban was commonly expressed in renal carcinomas and might be a new marker for renal carcinogenesis.

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.

Signaling from Akt to FRAP/TOR targets both 4E-BP and S6K in Drosophila melanogaster

The eIF4E-binding proteins (4E-BPs) interact with translation initiation factor 4E to inhibit translation. Their binding to eIF4E is reversed by phosphorylation of several key Ser/Thr residues. In Drosophila, S6 kinase (dS6K) and a single 4E-BP (d4E-BP) are phosphorylated via the insulin and target of rapamycin (TOR) signaling pathways. Although S6K phosphorylation is independent of phosphoinositide 3-OH kinase (PI3K) and serine/threonine protein kinase Akt, that of 4E-BP is dependent on PI3K and Akt. This difference prompted us to examine the regulation of d4E-BP in greater detail. Analysis of d4E-BP phosphorylation using site-directed mutagenesis and isoelectric focusing-sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the regulatory interplay between Thr37 and Thr46 of d4E-BP is conserved in flies and that phosphorylation of Thr46 is the major phosphorylation event that regulates d4E-BP activity. We used RNA interference (RNAi) to target components of the PI3K, Akt, and TOR pathways. RNAi experiments directed at components of the insulin and TOR signaling cascades show that d4E-BP is phosphorylated in a PI3K- and Akt-dependent manner. Surprisingly, RNAi of dAkt also affected insulin-stimulated phosphorylation of dS6K, indicating that dAkt may also play a role in dS6K phosphorylation.

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.

Expression of ICAM-1, TNF-alpha, NF kappa B, and MAP kinase in tubers of the tuberous sclerosis complex

Individuals affected with tuberous sclerosis complex (TSC) develop cortical tubers characterized by disorganized cytoarchitecture and morphologically abnormal cell types, such as dysplastic neurons (DNs) and giant cells (GCs). As part of ongoing cDNA array analysis to study the molecular pathogenesis of tuber formation, we detected increased expression of intercellular adhesion molecule-1 (ICAM-1) mRNA, a cell adhesion molecule (CAM) that functions in cytokine signaling, in tubers. Western and immunohistochemical analyses revealed that ICAM-1 protein was selectively expressed in tubers, but was only minimally expressed in control cortex, adjacent nontuberal cortex, or in non-TSC focal cortical dysplasia. Increased expression of ICAM-1 was found in mice in which the Tsc1 gene was conditionally inactivated in astrocytes. Expression of molecules involved in ICAM-1 activation and cytokine signaling were increased in tubers, including tumor necrosis factor alpha (TNF-alpha), mitogen activated protein kinase (MAPK), and nuclear factor kappa B (NF-kappaB). Numerous CD68-immunoreactive macrophages were observed clustered around GCs further supporting an inflammatory response in tubers. Expression of caspase 8 and Fas support cytokine activation and detection of TUNEL reactivity suggests ongoing cell death in tubers. Specific alterations in ICAM-1, TNF-alpha, NF-kappaB1, and MAPK expression coupled with the detection of numerous CD68-immunoreactive macrophages suggests activation of proinflammatory cytokine signaling pathways in tubers that may culminate in cell death.

Molecular mechanisms through which amino acids mediate signaling through the mammalian target of rapamycin

PURPOSE OF REVIEW

The purpose of this review is to provide a summary of the current state of knowledge concerning one of the intracellular signal transduction pathways through which amino acids, and in particular leucine, regulate the initiation phase of mRNA translation. The primary focus is on a protein kinase, termed the mammalian target of rapamycin (mTOR), that is a point of convergence between amino acid and growth factor signaling to mRNA translation and thereby to cell growth.

RECENT FINDINGS

Until recently the pathway through which amino acids signal to mTOR was completely undefined. Several recent reports, however, describe the identification of proteins that modulate amino acid signaling through mTOR, that is the tuberous sclerosis complex proteins 1 and 2 and the Ras homolog enriched in brain (Rheb) protein. Tuberous sclerosis complex protein 2 is a GTPase activator protein for Rheb that is inhibited by amino acids, allowing Rheb to activate mTOR through a mechanism still to be delineated. In addition, two proteins that interact with mTOR to target it to two important substrates, eukaryotic initiation factor 4E binding protein 1 and ribosomal protein S6 kinase, have been identified. Both proteins, that is the regulatory associated protein of mTOR and G protein beta-subunit-like protein, are required for optimal signaling through mTOR by amino acids.

SUMMARY

Studies reported in the past 18 months have greatly expanded our knowledge of one of the signaling pathways through which amino acids act to regulate mTOR and also the molecular interactions that mediate the interaction between mTOR and two downstream substrates, eukaryotic initiation factor 4E binding protein 1 and ribosomal protein S6 kinase.

Loss of tuberous sclerosis complex 1 (Tsc1) expression results in increased Rheb/S6K pathway signaling important for astrocyte cell size regulation

Individuals with tuberous sclerosis complex (TSC) develop central nervous system abnormalities that may reflect astrocyte dysfunction. In an effort to model astrocyte dysfunction in TSC, we generated mice lacking Tsc1 (hamartin) expression in astrocytes and demonstrated that Tsc1-null astrocytes exhibit abnormalities in contact inhibition growth arrest. In this study, we demonstrate that hamartin-deficient astrocytes are also defective in cell size regulation. We show that the increase in Tsc1-null astrocyte size is associated with increased activation of the S6-kinase pathway. In keeping with recent reports that the hamartin/tuberin complex may regulate Rheb and downstream S6K activation, we demonstrate that expression of either Rheb or S6K in primary astrocytes results in increased S6 pathway activation, and that inhibition of Rheb activity in Tsc1-deficient astrocytes using either pharmacologic or genetic strategies markedly reduces S6 activation. Collectively, these observations suggest that TSC inactivation in astrocytes results in defective cell size regulation associated with dysregulated Rheb/mTOR/S6K pathway activity.

Interplay between growth factor and nutrient signaling: lessons from Drosophila TOR

During normal development, cellular and organismal growth is coordinately regulated. Each cell and each individual organ integrates information about nutrient availability, hormonal signals, and intrinsic growth programs. Describing the signaling pathways involved in these processes and how they are integrated is important to understand how growth is controlled during development and may also permit the development of means to curb uncontrolled growth in disease. In recent years, the biochemical analysis of cellular growth in cultured cells and the genetic dissection of growth control in model organisms has identified two conserved signaling pathways dedicated to cellular growth. The target of rapamycin (TOR) pathway regulates growth in response to nutrients, and the insulin/IGF pathways are involved in coordinating cellular growth in response to endocrine signals. This review discusses recent advances in the understanding of the interaction between these pathways, with a special focus on the contribution of the genetic analysis of these pathways in Drosophila.

Genetic analysis of TOR signaling in Drosophila

Over a 4-day period of development, Drosophila larvae undergo a roughly 1,000-fold increase in mass. This impressive growth requires a continuous source of dietary protein; in the absence of amino acids, growth is arrested and various larval tissues display characteristic cell-cycle, metabolic, and structural changes. Mutations in the Drosophila target of rapamycin (dTOR) gene result in strikingly similar phenotypes, suggesting that dTOR acts in a signaling pathway responsive to nutrient availability. Genetic epistasis experiments indicate that dTOR is also required for cell growth in response to insulin and PI3K signaling, and that S6K activation can partially rescue dTOR loss of function. Thus dTOR has roles in both nutrient- and growth factor-mediated signaling, and may act to coordinate the activities of these pathways during development. Here we describe the use of mutations in dTOR to dissect its role in various signaling events, to gain insight into TOR protein structure, and to identify novel factors involved in TOR signaling.

TOR signaling

The mammalian target of rapamycin, mTOR, is a protein Ser-Thr kinase that functions as a central element in a signaling pathway involved in the control of cell growth and proliferation. The activity of mTOR is controlled not only by amino acids, but also by hormones and growth factors that activate the protein kinase Akt. The signaling pathway downstream of Akt leading to mTOR involves the protein products of the genes mutated in tuberous sclerosis, TSC1 and TSC2, and the small guanosine triphosphatase, Rheb. In cells, mTOR is found in a complex with two other proteins, raptor and mLST8. In this review, we describe recent progress in understanding the control of the mTOR signaling pathway and the role of mTOR-interacting proteins.

Latitudinal clines inDrosophila melanogaster: Body size, allozyme frequencies, inversion frequencies, and the insulin-signalling pathway

Many latitudinal clines exist in Drosophila melanogaster: in adult body size, in allele frequency at allozyme loci, and in frequencies of common cosmopolitan inversions. The question is raised whether these latitudinal clines are causally related. This review aims to connect data from two very different fields of study, evolutionary biology and cell biology, in explaining such natural genetic variation in D. melanogaster body size and development time. It is argued that adult body size clines, inversion frequency clines, and clines in allele frequency at loci involved in glycolysis and glycogen storage are part of the same adaptive strategy. Selection pressure is expected to differ at opposite ends of the clines. At high latitudes, selection on D. melanogaster would favour high larval growth rate at low temperatures, and resource storage in adults to survive winter. At low latitudes selection would favour lower larval critical size to survive crowding, and increased male activity leading to high male reproductive success. Studies of the insulin-signalling pathway in D. melanogaster point to the involvement of this pathway in metabolism and adult body size. The genes involved in the insulin-signalling pathway are associated with common cosmopolitan inversions that show latitudinal clines. Each chromosome region connected with a large common cosmopolitan inversion possesses a gene of the insulin transmembrane complex, a gene of the intermediate pathway and a gene of the TOR branch. The hypothesis is presented that temperate D. melanogaster populations have a higher frequency of a 'thrifty' genotype corresponding to high insulin level or high signal level, while tropical populations possess a more 'spendthrift' genotype corresponding to low insulin or low signal level.

Regulation of Cell Cycles in Drosophila Development: Intrinsic and Extrinsic Cues

An intriguing aspect of cell cycle regulation is how cell growth and division are coordinated with developmental signals to produce properly patterned organisms of the appropriate size. Using the foundation laid by a detailed understanding of the regulators that intrinsically control progression through the cell cycle, links between developmental signals and the cell cycle are being elucidated. Considerable progress has been made using Drosophila melanogaster, both in identifying new cell cycle regulators that respond to developmental cues and in defining the impact of extrinsic signals on homologs of mammalian oncogenes and tumor suppressors. In this review, we discuss each cell cycle phase, highlighting differences between archetypal and variant cell cycles employed for specific developmental strategies. We emphasize the interplay between developmental signals and cell cycle transitions. Developmental control of checkpoints, cell cycle exit, and cell growth are also addressed.

TSC2 mediates cellular energy response to control cell growth and survival

Mutations in either the TSC1 or TSC2 tumor suppressor gene are responsible for Tuberous Sclerosis Complex. The gene products of TSC1 and TSC2 form a functional complex and inhibit the phosphorylation of S6K and 4EBP1, two key regulators of translation. Here, we describe that TSC2 is regulated by cellular energy levels and plays an essential role in the cellular energy response pathway. Under energy starvation conditions, the AMP-activated protein kinase (AMPK) phosphorylates TSC2 and enhances its activity. Phosphorylation of TSC2 by AMPK is required for translation regulation and cell size control in response to energy deprivation. Furthermore, TSC2 and its phosphorylation by AMPK protect cells from energy deprivation-induced apoptosis. These observations demonstrate a model where TSC2 functions as a key player in regulation of the common mTOR pathway of protein synthesis, cell growth, and viability in response to cellular energy levels.

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.

Loss of Tsc1/Tsc2 activates mTOR and disrupts PI3K-Akt signaling through downregulation of PDGFR

Tuberous sclerosis (TSC) is a familial tumor syndrome due to mutations in TSC1 or TSC2, in which progression to malignancy is rare. Primary Tsc2(-/-) murine embryo fibroblast cultures display early senescence with overexpression of p21CIP1/WAF1 that is rescued by loss of TP53. Tsc2(-/-)TP53(-/-) cells, as well as tumors from Tsc2(+/-) mice, display an mTOR-activation signature with constitutive activation of S6K, which is reverted by treatment with rapamycin. Rapamycin also reverts a growth advantage of Tsc2(-/-)TP53(-/-) cells. Tsc1/Tsc2 does not bind directly to mTOR, however, nor does it directly influence mTOR kinase activity or cellular phosphatase activity. There is a marked reduction in Akt activation in Tsc2(-/-)TP53(-/-) and Tsc1(-/-) cells in response to serum and PDGF, along with a reduction in cell ruffling. PDGFRalpha and PDGFRbeta expression is markedly reduced in both the cell lines and Tsc mouse renal cystadenomas, and ectopic expression of PDGFRbeta in Tsc2-null cells restores Akt phosphorylation in response to serum, PDGF, EGF, and insulin. This activation of mTOR along with downregulation of PDGFR PI3K-Akt signaling in cells lacking Tsc1 or Tsc2 may explain why these genes are rarely involved in human cancer. This is in contrast to PTEN, which is a negative upstream regulator of this pathway.

PTEN: tumour suppressor, multifunctional growth regulator and more

The tumour suppressor gene PTEN is mutated in a wide range of human cancers at a frequency roughly comparable with p53. In addition, germline PTEN mutations are associated with several dominant growth disorders. The molecular and cellular basis of these disorders has been elucidated by detailed in vivo genetic analysis in model organisms, in particular the fruit fly and mouse. Studies in the fly have shown that PTEN's growth regulatory functions are primarily mediated via its lipid phosphatase activity, which specifically reduces the cellular levels of phosphatidylinositol 3,4,5-trisphosphate. This activity antagonizes the effects of activated PI3-kinase in the nutritionally controlled insulin receptor pathway, thereby reducing protein synthesis and restraining cell and organismal growth, while also regulating other biological processes, such as fertility and ageing. Remarkably, this range of functions appears to be conserved in all higher organisms. PTEN also plays a role as a specialized cytoskeletal regulator, which, for example, is involved in directional movement of some migratory cells and may be important in metastasis. Furthermore, conditional knockouts in the mouse have recently revealed functions for PTEN in other processes, such as cell type specification and cardiac muscle contractility. Genetic approaches have therefore revealed a surprising diversity of global and cell type-specific PTEN-regulated functions that appear to be primarily controlled by modulation of a single phosphoinositide. Together with evidence from studies in cell culture that suggests links between PTEN and other growth regulatory genes such as p53, these studies provide new insights into PTEN-linked disorders and are beginning to suggest potential clinical strategies to combat these and other diseases.

Pam and its ortholog highwire interact with and may negatively regulate the TSC1.TSC2 complex

Tuberous Sclerosis Complex (TSC) is an autosomal dominant disorder associated with mutations in TSC1, which codes for hamartin, or TSC2, which codes for tuberin. The brain is one of the most severely affected organs, and CNS lesions include cortical tubers and subependymal giant cell astrocytomas, resulting in mental retardation and seizures. Tuberin and hamartin function together as a complex in mammals and Drosophila. We report here the association of Pam, a protein identified as an interactor of Myc, with the tuberin-hamartin complex in the brain. The C terminus of Pam containing the RING zinc finger motif binds to tuberin. Pam is expressed in embryonic and adult brain as well as in cultured neurons. Pam has two forms in the rat CNS, an approximately 450-kDa form expressed in early embryonic stages and an approximately 350-kDa form observed in the postnatal period. In cortical neurons, Pam co-localizes with tuberin and hamartin in neurites and growth cones. Although Pam function(s) are yet to be defined, the highly conserved Pam homologs, HIW (Drosophila) and RPM-1 (Caenorhabditis elegans), are neuron-specific proteins that regulate synaptic growth. Here we show that HIW can genetically interact with the Tsc1.Tsc2 complex in Drosophila and could negatively regulate Tsc1.Tsc2 activity. Based on genetic studies, HIW has been implicated in ubiquitination, possibly functioning as an E3 ubiquitin ligase through the RING zinc finger domain. Therefore, we hypothesize that Pam, through its interaction with tuberin, could regulate the ubiquitination and proteasomal degradation of the tuberin-hamartin complex particularly in the CNS.

Inactivation of the tuberous sclerosis complex-1 and -2 gene products occurs by phosphoinositide 3-kinase/Akt-dependent and -independent phosphorylation of tuberin

The tuberous sclerosis complex (TSC) is a genetic disorder that is caused through mutations in either one of the two tumor suppressor genes, TSC1 and TSC2, that encode hamartin and tuberin, respectively. Interaction of hamartin with tuberin forms a heterodimer that inhibits signaling by the mammalian target of rapamycin to its downstream targets: eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) and ribosomal protein S6 kinase 1 (S6K1). During mitogenic sufficiency, the phosphoinositide 3-kinase (PI3K)/Akt pathway phosphorylates tuberin on Ser-939 and Thr-1462 that inhibits the tumor suppressor function of the TSC complex. Here we show that tuberin-hamartin heterodimers block protein kinase C (PKC)/MAPK- and phosphatidic acid-mediated signaling toward mammalian target of rapamycin-dependent targets. We also show that two TSC2 mutants derived from TSC patients are defective in repressing phorbol 12-myristate 13-acetate-induced 4E-BP1 phosphorylation. PKC/MAPK signaling leads to phosphorylation of tuberin at sites that overlap with and are distinct from Akt phosphorylation sites. Phosphorylation of tuberin by phorbol 12-myristate 13-acetate was reduced by treatment of cells with either bisindolylmaleimide I or UO126, inhibitors of PKC and MAPK/MEK (MAPK/ERK kinase), respectively, but not by wortmannin (an inhibitor of PI3K). This work reveals that both PI3K-independent and -dependent mechanisms modulate tuberin phosphorylation in vivo.

A genetic screen for modifiers of the lats tumor suppressor gene identifies C-terminal Src kinase as a regulator of cell proliferation in Drosophila

Disrupting mechanisms that control cell proliferation, cell size and apoptosis can cause changes in animal and tissue size and contribute to diseases such as cancer. The LATS family of serine/threonine kinases control tissue size by regulating cell proliferation and function as tumor suppressor genes in both Drosophila and mammals. In order to understand the role of lats in size regulation, we performed a genetic modifier screen in Drosophila to identify components of the lats signaling pathway. Mutations in the Drosophila homolog of C-terminal Src kinase (dcsk) were identified as dominant modifiers of both lats gain-of-function and loss-of-function phenotypes. Homozygous dcsk mutants have enlarged tissue phenotypes similar to lats and FACS and immunohistochemistry analysis of these tissues revealed that dcsk also regulates cell proliferation during development. Animals having mutations in both dcsk and lats display cell overproliferation phenotypes more severe than either mutant alone, demonstrating these genes function together in vivo to regulate cell numbers. Furthermore, homozygous dcsk phenotypes can be partially suppressed by overexpression of lats, indicating that lats is a downstream mediator of dcsk function in vivo. Finally, we show that dCSK phosphorylates LATS in vitro at a conserved C-terminal tyrosine residue, which is critical for normal LATS function in vivo. Taken together, these results demonstrate a role for dCSK in regulating cell numbers during development by inhibiting cell proliferation and suggest that lats is one of the mediators of the dcsk phenotype.

Overgrowth syndromes: is dysfunctional PI3-kinase signalling a unifying mechanism?

Studies in drosophila and animal models have shown that the phosphoinositide-3-kinase (PI3-kinase) axis plays a central role in normal development, defining the number and size of cells in tissues. Dysfunction of this pathway leads to growth anomalies and has been established to play a key role in the pathogenesis of Cowden syndrome and tuberous sclerosis. It is probable that dysfunction of this pathway is the basis of other disorders especially those typified by asymmetric overgrowth.

Drosophila Rheb GTPase is required for cell cycle progression and cell growth

Precise body and organ sizes in the adult animal are ensured by a range of signaling pathways. In a screen to identify genes affecting hindgut morphogenesis in Drosophila, we identified a P-element insertion in dRheb, a novel, highly conserved member of the Ras superfamily of G-proteins. Overexpression of dRheb in the developing fly (using the GAL4:UAS system) causes dramatic overgrowth of multiple tissues: in the wing, this is due to an increase in cell size; in cultured cells, dRheb overexpression results in accumulation of cells in S phase and an increase in cell size. Using a loss-of-function mutation we show that dRheb is required in the whole organism for viability (growth) and for the growth of individual cells. Inhibition of dRheb activity in cultured cells results in their arrest in G1 and a reduction in size. These data demonstrate that dRheb is required for both cell growth (increase in mass) and cell cycle progression; one explanation for this dual role would be that dRheb promotes cell cycle progression by affecting cell growth. Consistent with this interpretation, we find that flies with reduced dRheb activity are hypersensitive to rapamycin, an inhibitor of the growth regulator TOR. In cultured cells, the effect of overexpressing dRheb was blocked by the addition of rapamycin. These results imply that dRheb is involved in TOR signaling.

The functions of insulin signaling: size isn't everything, even in Drosophila

Mammalian insulin and insulin-like growth factors (IGFs) signal through several receptors with different ligand specificities to regulate metabolism and growth. This regulation is defective in diabetes and in a wide variety of human tumors. Recent analysis in Drosophila melanogaster has revealed that insulin-like molecules (known as DILPs in flies) also control growth and metabolism, but probably do so by signaling through a single insulin receptor (InR). The intracellular signaling molecules regulated by this receptor are highly evolutionarily conserved. Work in flies has helped to dissect the network of InR-regulated intracellular signaling pathways and identify some of the critical players in these pathways and in interacting signaling cascades. Surprisingly, these studies have shown that DILPs control tissue and body growth primarily by regulating cell growth and cell size. Changes in cell growth produced by these molecules may subsequently modulate the rate of cell proliferation in a cell type-specific fashion. At least part of this growth effect is mediated by two small groups of neurons in the Drosophila brain, which secrete DILPs into the circulatory system at levels that are modulated by nutrition. This signaling center is also involved in DILP-dependent control of the fly's rate of development, fertility, and life span. These surprisingly diverse functions of InR signaling, which appear to be conserved in all higher animals, reflect a central role for this pathway in coordinating development, physiology, and properly proportioned growth of the organism in response to its nutritional state. Studies in flies are providing important new insights into the biology of this system, and the identification of novel components in the InR-regulated signaling cascade is already beginning to inform the development of new therapeutic strategies for insulin-linked diseases in the clinic.

hippo encodes a Ste-20 family protein kinase that restricts cell proliferation and promotes apoptosis in conjunction with salvador and warts

The coordination between cell proliferation and cell death is essential to maintain homeostasis within multicellular organisms. The mechanisms underlying this regulation are yet to be completely understood. Here, we report the identification of hippo (hpo) as a gene that regulates both cell proliferation and cell death in Drosophila. hpo encodes a Ste-20 family protein kinase that binds to and phosphorylates the tumor suppressor protein Salvador (Sav), which is known to interact with the Warts (Wts) protein kinase. Loss of hpo results in elevated transcription of the cell cycle regulator cyclin E and the cell-death inhibitor diap1, leading to increased proliferation and reduced apoptosis. Further, we show that hpo, sav, and wts define a pathway that regulates diap1 at the transcriptional level. A human homolog of hpo completely rescues the overgrowth phenotype of Drosophila hpo mutants, suggesting that hpo might play a conserved role for growth control in mammals.

Tuberous sclerosis complex gene products, Tuberin and Hamartin, control mTOR signaling by acting as a GTPase-activating protein complex toward Rheb

BACKGROUND

Tuberous Sclerosis Complex (TSC) is a genetic disorder that occurs through the loss of heterozygosity of either TSC1 or TSC2, which encode Hamartin or Tuberin, respectively. Tuberin and Hamartin form a tumor suppressor heterodimer that inhibits the mammalian target of rapamycin (mTOR) nutrient signaling input, but how this occurs is unclear.

RESULTS

We show that the small G protein Rheb (Ras homolog enriched in brain) is a molecular target of TSC1/TSC2 that regulates mTOR signaling. Overexpression of Rheb activates 40S ribosomal protein S6 kinase 1 (S6K1) but not p90 ribosomal S6 kinase 1 (RSK1) or Akt. Furthermore, Rheb induces phosphorylation of eukaryotic initiation factor 4E binding protein 1 (4E-BP1) and causes 4E-BP1 to dissociate from eIF4E. This dissociation is completely sensitive to rapamycin (an mTOR inhibitor) but not wortmannin (a phosphoinositide 3-kinase [PI3K] inhibitor). Rheb also activates S6K1 during amino acid insufficiency via a rapamycin-sensitive mechanism, suggesting that Rheb participates in nutrient signaling through mTOR. Moreover, Rheb does not activate a S6K1 mutant that is unresponsive to mTOR-mediated signals, confirming that Rheb functions upstream of mTOR. Overexpression of the Tuberin-Hamartin heterodimer inhibits Rheb-mediated S6K1 activation, suggesting that Tuberin functions as a Rheb GTPase activating protein (GAP). Supporting this notion, TSC patient-derived Tuberin GAP domain mutants were unable to inactivate Rheb in vivo. Moreover, in vitro studies reveal that Tuberin, when associated with Hamartin, acts as a Rheb GTPase-activating protein. Finally, we show that membrane localization of Rheb is important for its biological activity because a farnesylation-defective mutant of Rheb stimulated S6K1 activation less efficiently.

CONCLUSIONS

We show that Rheb acts as a novel mediator of the nutrient signaling input to mTOR and is the molecular target of TSC1 and TSC2 within mammalian cells.

The phosphatidylinositol-3 kinase (PI3K)-Akt pathway suppresses neurite branch formation in NGF-treated PC12 cells

BACKGROUND

Previous studies have shown that phosphatidylinositol-3 kinase (PI3K) plays an important role in NGF (nerve growth factor)-induced neurite elongation. However, the roles of the PI3K pathway in neurite branch formation were not fully understood. Also, it was not clear where the PI3K pathway is activated during branch formation.

RESULTS

We found that the treatment of PC12 cells with the PI3K inhibitor LY294002 resulted in a marked increase in the number of neurite branch points, suggesting a suppressive role of PI3K in neurite branch formation. Expression of a constitutively active form of Akt, a downstream effector of PI3K, decreased the number of branch points, whereas that of a dominant-negative form of Akt increased it. In contrast, inhibition of neither Rac, mTOR nor GSK3, other effectors of PI3K, promoted branch formation. Importantly, the phosphorylated form of endogenous Akt was localized at the tips of growth cones, but devoid of small branches in NGF-treated PC12 cells. A GFP-fusion protein of the plekstrin-homology (PH) domain of Akt was also localized at the tips of growth cones.

CONCLUSIONS

The PI3K-Akt pathway thus plays a key role in suppression of neurite branch formation in NGF-treated PC12 cells.

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.

Regulation of PDGF production and ERK activation by estrogen is associated with TSC2 gene expression

Mechanisms that regulate the growth response to estrogen (17beta-estradiol, E2) are poorly understood. Recently, loss of function of the tuberous sclerosis complex 2 (TSC2) gene has been associated with E2-related conditions that are characterized by benign cellular proliferation. We examined the growth response to E2 in vascular smooth muscle cells (VSMCs) that possess wild-type TSC2 and compared them with ELT-3 smooth muscle cells that do not express TSC2. In TSC2-expressing VSMCs, growth inhibition in response to E2 was associated with downregulation of platelet-derived growth factor (PDGF), PDGF receptor (PDGFR), and limited activation of extracellular signal-regulated kinase (ERK). In contrast, the growth-promoting effect of E2 in TSC2-null ELT-3 cells was associated with induction of PDGF, robust phosphorylation of PDGFR, and sustained activation of ERK. Furthermore, in ELT-3 cells, cellular growth and ERK activation by E2 were inhibited by the PDGFR inhibitor tyrphostin AG 17 and by PDGF-neutralizing antibody. These results demonstrate that autocrine production of PDGF and augmentation of the ERK pathway leads to estrogen-induced cellular proliferation in TSC2-null cells, a pathway that was downregulated in cells that express TSC2. Understanding the mechanisms that regulate the diverse responses to the steroid hormone estrogen could lead to novel approaches to the treatment of estrogen-related diseases that are characterized by aberrant cell proliferation.

Mechanisms of benzene-induced hematotoxicity and leukemogenicity: cDNA microarray analyses using mouse bone marrow tissue

Although the mechanisms underlying benzene-induced toxicity and leukemogenicity are not yet fully understood, they are likely to be complicated by various pathways, including those of metabolism, growth factor regulation, oxidative stress, DNA damage, cell cycle regulation, and programmed cell death. With this as a background, we performed cDNA microarray analyses on mouse bone marrow tissue during and after a 2-week benzene exposure by inhalation. Our goal was to clarify the mechanisms underlying the hematotoxicity and leukemogenicity induced by benzene at the level of altered multigene expression. Because a few researchers have postulated that the cell cycle regulation mediated by p53 is a critical event for benzene-induced hematotoxicity, the present study was carried out using p53-knockout (KO) mice and C57BL/6 mice. On the basis of the results of large-scale gene expression studies, we conclude the following: (a) Benzene induces DNA damage in cells at any phase of the cell cycle through myeloperoxidase and in the redox cycle, resulting in p53 expression through Raf-1 and cyclin D-interacting myb-like protein 1. (b) For G1/S cell cycle arrest, the p53-mediated pathway through p21 is involved, as well as the pRb gene-mediated pathway. (c) Alteration of cyclin G1 and Wee-1 kinase genes may be related to the G2/M arrest induced by benzene exposure. (d) DNA repair genes such as Rad50 and Rad51 are markedly downregulated in p53-KO mice. (e) p53-mediated caspase 11 activation, aside from p53-mediated Bax gene induction, may be an important pathway for cellular apoptosis after benzene exposure. Our results strongly suggest that the dysfunction of the p53 gene, possibly caused by strong and repeated genetic and epigenetic effects of benzene on candidate leukemia cells, may induce fatal problems such as those of cell cycle checkpoint, apoptosis, and the DNA repair system, finally resulting in hemopoietic malignancies. Our cDNA microarray data provide valuable information for future investigations of the mechanisms underlying the toxicity and leukemogenicity of benzene.

Rheb GTPase is a direct target of TSC2 GAP activity and regulates mTOR signaling

Tuberous sclerosis complex (TSC) is a genetic disease caused by mutation in either TSC1 or TSC2. The TSC1 and TSC2 gene products form a functional complex and inhibit phosphorylation of S6K and 4EBP1. These functions of TSC1/TSC2 are likely mediated by mTOR. Here we report that TSC2 is a GTPase-activating protein (GAP) toward Rheb, a Ras family GTPase. Rheb stimulates phosphorylation of S6K and 4EBP1. This function of Rheb is blocked by rapamycin and dominant-negative mTOR. Rheb stimulates the phosphorylation of mTOR and plays an essential role in regulation of S6K and 4EBP1 in response to nutrients and cellular energy status. Our data demonstrate that Rheb acts downstream of TSC1/TSC2 and upstream of mTOR to regulate cell growth.

Tumour suppressors hamartin and tuberin: intracellular signalling

Tumour suppressors hamartin and tuberin, encoded by tuberous sclerosis complex 1(TSC1) and TSC2 genes, respectively, are critical regulators of cell growth and proliferation. Mutations in TSC1 and TSC2 genes are the cause of an autosomal dominant disorder known as tuberous sclerosis complex (TSC). Another genetic disorder, lymphangioleiomyomatosis (LAM), is also associated with mutations in the TSC2 gene. Hamartin and tuberin control cell growth by negatively regulating S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E binding protein 1 (4E-BP1), potentially through their upstream modulator mammalian target of rapamycin (mTOR). Growth factors and insulin promote Akt/PKB-dependent phosphorylation of tuberin, which in turn, releases S6K1 from negative regulation by tuberin and results in the activation of S6K1. Although much has been written regarding the molecular genetics of TSC and LAM, which is associated with either the loss of or mutation in the TSC1 and TSC2 genes, few reviews have addressed the intracellular signalling pathways regulated by hamartin and tuberin. The current review will fill the gap in our understanding of their role in cellular signalling networks, and by improving this understanding, an integrated picture regarding the normal function of tuberin and hamartin is beginning to emerge.

Cell size regulation by the human TSC tumor suppressor proteins depends on PI3K and FKBP38

TSC1 and TSC2 are responsible for the tumor suppressor gene syndrome tuberous sclerosis (TSC). Mammalian TSC genes have been shown to be involved in cell cycle regulation. Recently, in Drosophila, these data have been confirmed and TSC genes have further been demonstrated to affect cell size control. Here we provide supporting data for the fact that the latter function is conserved in mammals. Human TSC1 and TSC2 trigger mammalian cell size reduction and a dominant-negative TSC2 mutant induces increased size. These effects occur in all cell cycle phases, are dependent on the activity of the phosphoinositide-3-kinase and are abolished by co-overexpression of a dominant-negative Akt mutant. Two independent naturally occurring and disease-causing mutations within the TSC2 gene eliminate tuberin's capacity to affect cell size control, emphasizing the relevance of this function for the development of the disease. The same mutations have earlier been shown not to affect tuberin's antiproliferative capacity. That the consequences of modulated TSC gene expression on cell proliferation and on cell size can be assigned to separable functions is further supported by two findings: A mutation within the TSC1 gene, earlier shown to still harbor anti-proliferative effects, was found to eliminate the cell size regulating functions. An important mammalian cell size regulator, c-Myc, was found to inhibit tuberin's antiproliferative capacity, but to have no effects on tuberin-dependent cell size control. To obtain further mechanistical insights, microarray screens for genes involved in TSC1- or TSC2-mediated cell size effects were performed. Antisense experiments revealed that the so observed regulation of the FK506-binding protein, FKBP38, plays a role in TSC gene-dependent cell size regulation. These data provide new insights into mammalian cell size regulation and allow a better understanding of the function of human TSC genes.

Expression of Drosophila FOXO regulates growth and can phenocopy starvation

BACKGROUND

Components of the insulin signaling pathway are important regulators of growth. The FOXO (forkhead box, sub-group "O") transcription factors regulate cellular processes under conditions of low levels of insulin signaling. Studies in mammalian cell culture show that activation of FOXO transcription factors causes cell death or cell cycle arrest. The Caenorhabditis elegans homologue of FOXO, Daf-16, is required for the formation of dauer larvae in response to nutritional stress. In addition, FOXO factors have been implicated in stress resistance and longevity.

RESULTS

We have identified the Drosophila melanogaster homologue of FOXO (dFOXO), which is conserved in amino acid sequence compared with the mammalian FOXO homologues and Daf-16. Expression of dFOXO during early larval development causes inhibition of larval growth and alterations in feeding behavior. Inhibition of larval growth is reversible upon discontinuation of dFOXO expression. Expression of dFOXO during the third larval instar or at low levels during development leads to the generation of adults that are reduced in size. Analysis of the wings and eyes of these small flies indicates that the reduction in size is due to decreases in cell size and cell number. Overexpression of dFOXO in the developing eye leads to a characteristic phenotype with reductions in cell size and cell number. This phenotype can be rescued by co-expression of upstream insulin signaling components, dPI3K and dAkt, however, this rescue is not seen when FOXO is mutated to a constitutively active form.

CONCLUSIONS

dFOXO is conserved in both sequence and regulatory mechanisms when compared with other FOXO homologues. The establishment of Drosophila as a model for the study of FOXO transcription factors should prove beneficial to determining the biological role of these signaling molecules. The alterations in larval development seen upon overexpression of dFOXO closely mimic the phenotypic effects of starvation, suggesting a role for dFOXO in the response to nutritional adversity. This work has implications in the understanding of cancer and insulin related disorders, such as diabetes and obesity.

Rheb is a direct target of the tuberous sclerosis tumour suppressor proteins

Mutations in the TSC1 or TSC2 genes cause tuberous sclerosis, a benign tumour syndrome in humans. Tsc2 possesses a domain that shares homology with the GTPase-activating protein (GAP) domain of Rap1-GAP, suggesting that a GTPase might be the physiological target of Tsc2. Here we show that the small GTPase Rheb (Ras homologue enriched in brain) is a direct target of Tsc2 GAP activity both in vivo and in vitro. Point mutations in the GAP domain of Tsc2 disrupted its ability to regulate Rheb without affecting the ability of Tsc2 to form a complex with Tsc1. Our studies identify Rheb as a molecular target of the TSC tumour suppressors.

Insulin activation of Rheb, a mediator of mTOR/S6K/4E-BP signaling, is inhibited by TSC1 and 2

Tumor suppressor genes evolved as negative effectors of mitogen and nutrient signaling pathways, such that mutations in these genes can lead to pathological states of growth. Tuberous sclerosis (TSC) is a potentially devastating disease associated with mutations in two tumor suppressor genes, TSC1 and 2, that function as a complex to suppress signaling in the mTOR/S6K/4E-BP pathway. However, the inhibitory target of TSC1/2 and the mechanism by which it acts are unknown. Here we provide evidence that TSC1/2 is a GAP for the small GTPase Rheb and that insulin-mediated Rheb activation is PI3K dependent. Moreover, Rheb overexpression induces S6K1 phosphorylation and inhibits PKB phosphorylation, as do loss-of-function mutations in TSC1/2, but contrary to earlier reports Rheb has no effect on MAPK phosphorylation. Finally, coexpression of a human TSC2 cDNA harboring a disease-associated point mutation in the GAP domain, failed to stimulate Rheb GTPase activity or block Rheb activation of S6K1.

Rheb is an essential regulator of S6K in controlling cell growth in Drosophila

Understanding the mechanisms through which multicellular organisms regulate cell, organ and body growth is of relevance to developmental biology and to research on growth-related diseases such as cancer. Here we describe a new effector in growth control, the small GTPase Rheb (Ras homologue enriched in brain). Mutations in the Drosophila melanogaster Rheb gene were isolated as growth-inhibitors, whereas overexpression of Rheb promoted cell growth. Our genetic and biochemical analyses suggest that Rheb functions downstream of the tumour suppressors Tsc1 (tuberous sclerosis 1)-Tsc2 in the TOR (target of rapamycin) signalling pathway to control growth, and that a major effector of Rheb function is ribosomal S6 kinase (S6K).