Activity of TSC2 is inhibited by AKT-mediated phosphorylation and membrane partitioning

AMP-activated protein kinase signaling results in cytoplasmic sequestration of p27

Tuberin, the Tsc2 gene product, integrates the phosphatidylinositol 3-kinase/mitogen-activated protein kinase (mitogenic) and LKB1/AMP-activated protein kinase (AMPK; energy) signaling pathways, and previous independent studies have shown that loss of tuberin is associated with elevated AMPK signaling and altered p27 function. In Tsc2-null tumors and tumor-derived cells from Eker rats, we observed elevated AMPK signaling and concordant cytoplasmic mislocalization of p27. Cytoplasmic localization of p27 in Tsc2-null cells was reversible pharmacologically using inhibitors of the LKB1/AMPK pathway, and localization of p27 to the cytoplasm could be induced directly by activating AMPK physiologically (glucose deprivation) or genetically (constitutively active AMPK) in Tsc2-proficient cells. Furthermore, AMPK phosphorylated p27 in vitro on at least three sites including T170 near the nuclear localization signal, and T170 was shown to determine p27 localization in response to AMPK signaling. p27 functions in the nucleus to suppress cyclin-dependent kinase-2 (Cdk2) activity and has been reported to mediate an antiapoptotic function when localized to the cytoplasm. We found that cells with elevated AMPK signaling and cytoplasmic p27 localization exhibited elevated Cdk2 activity, which could be suppressed by inhibiting AMPK signaling. In addition, cells with elevated AMPK signaling and cytoplasmic p27 localization were resistant to apoptosis, which could be overcome by inhibition of AMPK signaling and relocalization of p27 to the nucleus. These data show that AMPK signaling determines the subcellular localization of p27, and identifies loss of integration of pathways controlling energy balance, the cell cycle, and apoptosis due to aberrant AMPK and p27 function as a feature of cells that have lost the Tsc2 tumor suppressor gene.

The TSC1-TSC2 complex: a molecular switchboard controlling cell growth

TSC1 and TSC2 are the tumour-suppressor genes mutated in the tumour syndrome TSC (tuberous sclerosis complex). Their gene products form a complex that has become the focus of many signal transduction researchers. The TSC1-TSC2 (hamartin-tuberin) complex, through its GAP (GTPase-activating protein) activity towards the small G-protein Rheb (Ras homologue enriched in brain), is a critical negative regulator of mTORC1 (mammalian target of rapamycin complex 1). As mTORC1 activity controls anabolic processes to promote cell growth, it is exquisitely sensitive to alterations in cell growth conditions. Through numerous phosphorylation events, the TSC1-TSC2 complex has emerged as the sensor and integrator of these growth conditions, relaying signals from diverse cellular pathways to properly modulate mTORC1 activity. In the present review we focus on the molecular details of TSC1-TSC2 complex regulation and function as it relates to the control of Rheb and mTORC1.

Characterization of the Rheb-mTOR signaling pathway in mammalian cells: constitutive active mutants of Rheb and mTOR

Rheb (Ras homolog enriched in brain) is a GTPase conserved from yeast to human and belongs to a unique family within the Ras superfamily of GTPases. Rheb plays critical roles in the activation of mTOR, a serine/threonine kinase that is involved in the activation of protein synthesis and growth. mTOR forms two distinct complexes, mTORC1 and mTORC2. While mTORC1 is implicated in the regulation of cell growth, proliferation, and cell size in response to amino acids and growth factors, mTORC2 is involved in actin organization. However, the mechanism of activation is not fully understood. Therefore, studies to elucidate the Rheb-mTOR signaling pathway are of great importance. Here we describe methods to characterize this pathway and to evaluate constitutive active mutants of Rheb and mTOR that we recently identified. Constitutive activity of the mutants can be demonstrated by the phosphorylation of ribosomal protein S6 kinase 1 (S6K1) and eIF4E-binding protein 1 (4E-BP1) both in vivo and in vitro after starving cells for amino acids and growth factors. In addition, formation and activity of mTORC1 and mTORC2 can be measured by immunoprecipitating these complexes and carrying out in vitro kinase assays. We also describe a protocol for rapamycin treatment, which directly inhibits mTOR and can be used to investigate the mTOR signaling pathway in cell growth, cell size, etc.

Regulation of neurogenesis and epidermal growth factor receptor signaling by the insulin receptor/target of rapamycin pathway in Drosophila

Determining how growth and differentiation are coordinated is key to understanding normal development, as well as disease states such as cancer, where that control is lost. We have previously shown that growth and neuronal differentiation are coordinated by the insulin receptor/target of rapamycin (TOR) kinase (InR/TOR) pathway. Here we show that the control of growth and differentiation diverge downstream of TOR. TOR regulates growth by controlling the activity of S6 kinase (S6K) and eIF4E. Loss of s6k delays differentiation, and is epistatic to the loss of tsc2, indicating that S6K acts downstream or in parallel to TOR in differentiation as in growth. However, loss of eIF4E inhibits growth but does not affect the timing of differentiation. We also show, for the first time in Drosophila, that there is crosstalk between the InR/TOR pathway and epidermal growth factor receptor (EGFR) signaling. InR/TOR signaling regulates the expression of several EGFR pathway components including pointedP2 (pntP2). In addition, reduction of EGFR signaling levels phenocopies inhibition of the InR/TOR pathway in the regulation of differentiation. Together these data suggest that InR/TOR signaling regulates the timing of differentiation through modulation of EGFR target genes in developing photoreceptors.

Pulmonary lymphangioleiomyomatosis (LAM): progress and current challenges

Lymphangioleiomyomatosis (LAM), a rare lung disease, is characterized by the progressive proliferation, migration, and differentiation of smooth muscle (SM)-like LAM cells, which lead to the cystic destruction of the lung parenchyma, obstruction of airways and lymphatics, and loss of pulmonary function. LAM is a disease predominantly affecting women and is exacerbated by pregnancy; only a lung transplant can save the life of a patient. It has been discovered that in LAM, somatic or genetic mutations of tumor suppressor genes tuberous sclerosis complex 1 (TSC1) or TSC2 occur and the TSC1/TSC2 protein complex functions as a negative regulator of the mTOR/S6K1 signaling pathway. These two pivotal observations paved the way for the first rapamycin clinical trial for LAM. The recent discoveries that TSC1/TSC2 complex functions as an integrator of signaling networks regulated by growth factors, insulin, nutrients, and energy heightened the interest regarding this rare disease because the elucidation of disease-relevant mechanisms of LAM will promote a better understanding of other metabolic diseases such as diabetes, cancer, and cardiovascular diseases. In this review, we will summarize the progress made in our understanding of TSC1/TSC2 cellular signaling and the molecular mechanisms of LAM; we will also highlight some of the lesser explored directions and challenges in LAM research.

Loss of tuberous sclerosis complex-2 function and activation of mammalian target of rapamycin signaling in endometrial carcinoma

PURPOSE

The involvement of phosphatase and tensin homologue deleted on chromosome ten (PTEN) in endometrial carcinoma has implicated phosphatidylinositol 3-kinase signaling and mammalian target of rapamycin (mTOR) activation in this disease. Understanding the extent of mTOR involvement and the mechanism responsible for activation is important, as mTOR inhibitors are currently being evaluated in clinical trials for endometrial carcinoma. Although tuberous sclerosis complex 2 (TSC2) is the "gatekeeper" for mTOR activation, little is known about defects in the TSC2 tumor suppressor or signaling pathways that regulate TSC2, such as LKB1/AMP-activated protein kinase, in the development of endometrial carcinoma.

EXPERIMENTAL DESIGN

We determined the frequency of mTOR activation in endometrial carcinoma (primary tumors and cell lines) and investigated PTEN, LKB1, and TSC2 defects as underlying cause(s) of mTOR activation, and determined the ability of rapamycin to reverse these signaling defects in endometrial carcinoma cells.

RESULTS

Activation of mTOR was a consistent feature in endometrial carcinomas and cell lines. In addition to PTEN, loss of TSC2 and LKB1 expression occurred in a significant fraction of primary tumors (13% and 21%, respectively). In tumors that retained TSC2 expression, phosphorylation of tuberin at S939 was observed with a high frequency, indicating that mTOR repression by TSC2 had been relieved via AKT phosphorylation of this tumor suppressor. In PTEN-null and LKB1-null endometrial carcinoma cell lines with functional inactivation of TSC2, phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY294002 were able to inhibit AKT and mTOR signaling and reverse TSC2 phosphorylation. In contrast, although rapamycin inhibited mTOR signaling, it did not relieve phosphorylation of TSC2 at S939.

CONCLUSIONS

Inactivation of TSC2 via loss of expression or phosphorylation occurred frequently in endometrial carcinoma to activate mTOR signaling. High-frequency mTOR activation supports mTOR as a rational therapeutic target for endometrial carcinoma. However, whereas rapamycin and its analogues may be efficacious at inhibiting mTOR activity, these drugs do not reverse the functional inactivation of TSC2 that occurs in these tumors.

The binding of PRAS40 to 14-3-3 proteins is not required for activation of mTORC1 signalling by phorbol esters/ERK

PRAS40 binds to the mTORC1 (mammalian target of rapamycin complex 1) and is released in response to insulin. It has been suggested that this effect is due to 14-3-3 binding and leads to activation of mTORC1 signalling. In a similar manner to insulin, phorbol esters also activate mTORC1 signalling, in this case via PKC (protein kinase C) and ERK (extracellular-signal-regulated kinase). However, phorbol esters do not induce phosphorylation of PRAS40 at Thr(246), binding of 14-3-3 proteins to PRAS40 or its release from mTORC1. Mutation of Thr(246) to a serine residue permits phorbol esters to induce phosphorylation and binding to 14-3-3 proteins. Such phosphorylation is apparently mediated by RSKs (ribosomal S6 kinases), which lie downstream of ERK. However, although the PRAS40(T246S) mutant binds to 14-3-3 better than wild-type PRAS40, each inhibits mTORC1 signalling to a similar extent. Our results show that activation of mTORC1 signalling by phorbol esters does not require PRAS40 to be phosphorylated at Thr(246), bind to 14-3-3 or be released from mTORC1. It is conceivable that phorbol esters activate mTORC1 by a distinct mechanism not involving PRAS40. Indeed, our results suggest that PRAS40 may not actually be involved in controlling mTORC1, but rather be a downstream target of mTORC1 that is regulated in response only to specific stimuli, such as insulin.

Analysis of the regulatory motifs in eukaryotic initiation factor 4E-binding protein 1

Mammalian target of rapamycin complex 1 (mTORC1) phosphorylates proteins such as eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) and the S6 kinases. These substrates contain short sequences, termed TOR signalling (TOS) motifs, which interact with the mTORC1 component raptor. Phosphorylation of 4E-BP1 requires an additional feature, termed the RAIP motif (Arg-Ala-Ile-Pro). We have analysed the interaction of 4E-BP1 with raptor and the amino acid residues required for functional RAIP and TOS motifs, as assessed by raptor binding and the phosphorylation of 4E-BP1 in human cells. Binding of 4E-BP1 to raptor strongly depends on an intact TOS motif, but the RAIP motif and additional C-terminal features of 4E-BP1 also contribute to this interaction. Mutational analysis of 4E-BP1 reveals that isoleucine is a key feature of the RAIP motif, that proline is also very important and that there is greater tolerance for substitution of the first two residues. Within the TOS motif, the first position (phenylalanine in the known motifs) is most critical, whereas a wider range of residues function in other positions (although an uncharged aliphatic residue is preferred at position three). These data provide important information on the structural requirements for efficient signalling downstream of mTORC1.

Hypoxia regulates TSC1/2-mTOR signaling and tumor suppression through REDD1-mediated 14-3-3 shuttling

Hypoxia induces rapid and dramatic changes in cellular metabolism, in part through inhibition of target of rapamycin (TOR) kinase complex 1 (TORC1) activity. Genetic studies have shown the tuberous sclerosis tumor suppressors TSC1/2 and the REDD1 protein to be essential for hypoxia regulation of TORC1 activity in Drosophila and in mammalian cells. The molecular mechanism and physiologic significance of this effect of hypoxia remain unknown. Here, we demonstrate that hypoxia and REDD1 suppress mammalian TORC1 (mTORC1) activity by releasing TSC2 from its growth factor-induced association with inhibitory 14-3-3 proteins. Endogenous REDD1 is required for both dissociation of endogenous TSC2/14-3-3 and inhibition of mTORC1 in response to hypoxia. REDD1 mutants that fail to bind 14-3-3 are defective in eliciting TSC2/14-3-3 dissociation and mTORC1 inhibition, while TSC2 mutants that do not bind 14-3-3 are inactive in hypoxia signaling to mTORC1. In vitro, loss of REDD1 signaling promotes proliferation and anchorage-independent growth under hypoxia through mTORC1 dysregulation. In vivo, REDD1 loss elicits tumorigenesis in a mouse model, and down-regulation of REDD1 is observed in a subset of human cancers. Together, these findings define a molecular mechanism of signal integration by TSC1/2 that provides insight into the ability of REDD1 to function in a hypoxia-dependent tumor suppressor pathway.

The TORrid affairs of viruses: effects of mammalian DNA viruses on the PI3K-Akt-mTOR signalling pathway

The successful replication of mammalian DNA viruses requires that they gain control of key cellular signalling pathways that affect broad aspects of cellular macromolecular synthesis, metabolism, growth and survival. The phosphatidylinositol 3'-kinase-Akt-mammalian target of rapamycin (PI3K-Akt-mTOR) pathway is one such pathway. Mammalian DNA viruses have evolved various mechanisms to activate this pathway to obtain the benefits of Akt activation, including the maintenance of translation through the activation of mTOR. In addition, viruses must overcome the inhibition of this pathway that results from the activation of cellular stress responses during viral infection. This Review will discuss the range of mechanisms that mammalian DNA viruses use to activate this pathway, as well as the multiple mechanisms these viruses have evolved to circumvent inhibitory stress signalling.

Amino acids and mTOR signalling in anabolic function

Amino acids regulate signalling through the mTORC1 (mammalian target of rapamycin, complex 1) and thereby control a number of components of the translational machinery, including initiation and elongation factors. mTORC1 also positively regulates other anabolic processes, in particular ribosome biogenesis. The most effective single amino acid is leucine. A key issue is how intracellular amino acids regulate mTORC1. This does not require the TSC1/2 (tuberous sclerosis complex 1/2) complex, which is involved in the activation of mTORC1, for example, by insulin. Progress in understanding the mechanisms responsible for this will be reviewed.

Regulation of mTORC1 signaling by Src kinase activity is Akt1-independent in RSV-transformed cells

Increased activity of the Src tyrosine protein kinase that has been observed in a large number of human malignancies appears to be a promising target for drug therapy. In the present study, a critical role of the Src activity in the deregulation of mTOR signaling pathway in Rous sarcoma virus (RSV)-transformed hamster fibroblasts, H19 cells, was shown using these cells treated with the Src-specific inhibitor, SU6656, and clones of fibroblasts expressing either the active Src or the dominant-negative Src kinase-dead mutant. Disruption of the Src kinase activity results in substantial reduction of the phosphorylation and activity of the Akt/protein kinase B (PKB), phosphorylation of tuberin (TSC2), mammalian target of rapamycin (mTOR), S6K1, ribosomal protein S6, and eukaryotic initiation factor 4E-binding protein 4E-BP1. The ectopic, active Akt1 that was expressed in Src-deficient cells significantly enhanced phosphorylation of TSC2 in these cells, but it failed to activate the inhibited components of the mTOR pathway that are downstream of TSC2. The data indicate that the Src kinase activity is essential for the activity of mTOR-dependent signaling pathway and suggest that mTOR targets may be controlled by Src independently of Akt1/TSC2 cascade in cells expressing hyperactive Src protein. These observations might have an implication in drug resistance to mTOR inhibitor-based cancer therapy in certain cell types.

The tuberous sclerosis gene products hamartin and tuberin are multifunctional proteins with a wide spectrum of interacting partners

Mutations in the tumor suppressor genes TSC1 and TSC2, encoding hamartin and tuberin, respectively, cause the tumor syndrome tuberous sclerosis with similar phenotypes. Until now, over 50 proteins have been demonstrated to interact with hamartin and/or tuberin. Besides tuberin, the proteins DOCK7, ezrin/radixin/moesin, FIP200, IKKbeta, Melted, Merlin, NADE(p75NTR), NF-L, Plk1 and TBC7 have been found to interact with hamartin. Whereas Plk1 and TBC7 have been demonstrated not to bind to tuberin, for all the other hamartin-interacting proteins the question, whether they can also bind to tuberin, has not been studied. Tuberin interacts with 14-3-3 beta,epsilon,gamma,eta,sigma,tau,zeta, Akt, AMPK, CaM, CRB3/PATJ, cyclin A, cyclins D1, D2, D3, Dsh, ERalpha, Erk, FoxO1, HERC1, HPV16 E6, HSCP-70, HSP70-1, MK2, NEK1, p27KIP1, Pam, PC1, PP2Ac, Rabaptin-5, Rheb, RxRalpha/VDR and SMAD2/3. 14-3-3 beta,epsilon,gamma,eta,sigma,tau,zeta, Akt, Dsh, FoxO1, HERC1, p27KIP1 and PP2Ac are known not to bind to hamartin. For the other tuberin-interacting proteins this question remains elusive. The proteins axin, Cdk1, cyclin B1, GADD34, GSK3, mTOR and RSK1 have been found to co-immunoprecipitate with both, hamartin and tuberin. The kinases Cdk1 and IKKbeta phosphorylate hamartin, Erk, Akt, MK2, AMPK and RSK1 phosphorylate tuberin, and GSK3 phosphorylates both, hamartin and tuberin. This detailed summary of protein interactions allows new insights into their relevance for the wide variety of different functions of hamartin and tuberin.

Regulation of muscle growth by pathogen-associated molecules

Skeletal muscle demonstrates great plasticity in response to environmental and hormonal factors including pathogen-associated molecules, inflammatory cytokines, and growth factors. These signals impinge on muscle by forcing individual muscle fibers to either grow or atrophy. We recently demonstrated that skeletal muscle cells express multiple Toll-like receptors (TLR) that recognize bacterial cell wall components, such as lipopolysaccharide (LPS). Exposure of myocytes to LPS and other TLR ligands initiates an inflammatory response culminating in the autocrine production of cytokines and NO by NO synthase (NOS)2. The TLR signal through protein kinases that phosphorylate and promote the degradation of an inhibitory protein that normally retains the transcription factor, nuclear factor kappaB (NFkappaB), in the cytoplasm. Phosphorylation and degradation of the inhibitor of NFkappaB allows for translocation of NFkappaB to the nucleus and activation of inflammatory genes. Overexpression of a constitutively active inhibitor of NFkappaB kinase in skeletal muscle causes severe wasting, and we found that inhibitors of either the phosphorylation of IkappaB or its proteolytic degradation prevent TLR ligand-induced expression of cytokines and NOS2. The combination of LPS and interferon gamma dramatically enhances the magnitude and duration of LPS-stimulated NOS2 expression and reduces protein translation. Lipopolysaccharide and interferon gamma also downregulates signaling from the mammalian target of rapamycin, a kinase that directs changes in cell size. Inhibitors of NOS block the fall in muscle cell protein synthesis and restore translational signaling, indicating that activation of the NOS2-NO pathway is responsible for the observed decrease in muscle protein synthesis. Our work provides a molecular explanation for reduced muscle growth during infection. Muscle is largely self-sufficient because it expresses receptors, signaling pathways, and effectors to regulate its own size. Prolonged activation of NFkappaB and NOS2 have emerged as detrimental facets of the immune response in muscle. The interplay between inflammatory components and growth factor signaling clearly places muscle at the interface between growth and immunity.

Regulation of multisite phosphorylation and 14-3-3 binding of AS160 in response to IGF-1, EGF, PMA and AICAR

AS160 (Akt substrate of 160 kDa) mediates insulin-stimulated GLUT4 (glucose transporter 4) translocation, but is widely expressed in insulin-insensitive tissues lacking GLUT4. Having isolated AS160 by 14-3-3-affinity chromatography, we found that binding of AS160 to 14-3-3 isoforms in HEK (human embryonic kidney)-293 cells was induced by IGF-1 (insulin-like growth factor-1), EGF (epidermal growth factor), PMA and, to a lesser extent, AICAR (5-aminoimidazole-4-carboxamide-1-b-D-ribofuranoside). AS160-14-3-3 interactions were stabilized by chemical cross-linking and abolished by dephosphorylation. Eight residues on AS160 (Ser318, Ser341, Thr568, Ser570, Ser588, Thr642, Ser666 and Ser751) were differentially phosphorylated in response to IGF-1, EGF, PMA and AICAR. The binding of 14-3-3 proteins to HA-AS160 (where HA is haemagglutinin) was markedly decreased by mutation of Thr642 and abolished in a Thr642Ala/Ser341Ala double mutant. The AGC (protein kinase A/protein kinase G/protein kinase C-family) kinases RSK1 (p90 ribosomal S6 kinase 1), SGK1 (serum- and glucocorticoid-induced protein kinase 1) and PKB (protein kinase B) displayed distinct signatures of AS160 phosphorylation in vitro: all three kinases phosphorylated Ser318, Ser588 and Thr642; RSK1 also phosphorylated Ser341, Ser751 and to a lesser extent Thr568; and SGK1 phosphorylated Thr568 and Ser751. AMPK (AMP-activated protein kinase) preferentially phosphorylated Ser588, with less phosphorylation of other sites. In cells, the IGF-1-stimulated phosphorylations, and certain EGF-stimulated phosphorylations, were inhibited by PI3K (phosphoinositide 3-kinase) inhibitors, whereas the RSK inhibitor BI-D1870 inhibited the PMA-induced phosphorylations. The expression of LKB1 in HeLa cells and the use of AICAR in HEK-293 cells promoted phosphorylation of Ser588, but only weak Ser341 and Thr642 phosphorylations and binding to 14-3-3s. Paradoxically however, phenformin activated AMPK without promoting AS160 phosphorylation. The IGF-1-induced phosphorylation of the novel phosphorylated Ser666-Pro site was suppressed by AICAR, and by combined mutation of a TOS (mTOR signalling)-like sequence (FEMDI) and rapamycin. Thus, although AS160 is a common target of insulin, IGF-1, EGF, PMA and AICAR, these stimuli induce distinctive patterns of phosphorylation and 14-3-3 binding, mediated by at least four protein kinases.

Smooth muscle-like cells in pulmonary lymphangioleiomyomatosis

Proliferation, migration, and differentiation of smooth muscle (SM)-like lymphangioleiomyomatosis (LAM) cells in the lungs are pathologic manifestations of pulmonary LAM, a rare lung disease predominantly afflicting women and exacerbated by pregnancy. LAM cells form nodules throughout the lung without any predominant localization, but can also form small cell clusters dispersed within lung parenchyma. LAM cells have the appearance of "immature" SM-like cells, irregularly distributed within the nodule in contrast to organized SM cell layers in airways and vasculature. Progressive growth of LAM cells leads to the cystic destruction of the lung parenchyma, obstruction of airways and lymphatics, and loss of pulmonary function. Pathogenetically, LAM occurs from somatic or genetic mutations of tumor suppressor genes tuberous sclerosis complex 1 (TSC1) or TSC2. The TSC1/TSC2 protein complex is an integrator of signaling networks regulated by growth factors, insulin, nutrients, and energy. The observation that the TSC1/TSC2 functions as a negative regulator of the mammalian target of rapamycin (mTOR)/p70 S6 kinase (S6K1) signaling pathway yielded the first rapamycin clinical trial for LAM. Although LAM is a rare lung disease, the elucidation of disease-relevant mechanisms of LAM will provide a better understanding of not only SM-like cell growth, migration, and differentiation in LAM but may also offer insights into other metabolic diseases such as cardiovascular diseases, diabetes, and cancer. In this article, we will summarize the progress made in our understanding of LAM, and we will focus on how dysregulation of TSC1/TSC2 signaling results in abnormal proliferation and migration of SM-like LAM cells.

Modulation of host cell stress responses by human cytomegalovirus

Human cytomegalovirus (HCMV) induces cellular stress responses during infection due to nutrient depletion, energy depletion, hypoxia and synthetic stress, e.g., endoplasmic reticulum (ER) stress. Cellular stress responses initiate processes that allow the cell to survive the stress; some of these may be beneficial to HCMV replication while others are not. Several studies show that HCMV manipulates stress response signaling in order to maintain beneficial effects while inhibiting detrimental effects. The inhibition of translation is the most common effect of stress responses that would be detrimental to HCMV infection. This chapter will focus on the mechanisms by which cap-dependent translation is maintained during HCMV infection through alterations of the phosphatidylinositol-3' kinase (PI3K)-Akt-tuberous sclerosis complex (TSC)-mammalian target of rapamycin (mTOR) signaling pathway. The emerging picture is that HCMV affects this pathway in multiple ways, thus ensuring that cap-dependent translation is maintained despite the induction of stress responses that would normally inhibit it. Such dramatic alterations of this pathway lead to questions of what other beneficial effects the virus might gain from these changes and how these changes may contribute to HCMV pathogenesis.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

Selective inhibition of growth of tuberous sclerosis complex 2 null cells by atorvastatin is associated with impaired Rheb and Rho GTPase function and reduced mTOR/S6 kinase activity

Inactivating mutations in the tuberous sclerosis complex 2 (TSC2) gene, which encodes tuberin, result in the development of TSC and lymphangioleiomyomatosis (LAM). The tumor suppressor effect of tuberin lies in its GTPase-activating protein activity toward Ras homologue enriched in brain (Rheb), a Ras GTPase superfamily member. The statins, 3-hydroxy-3-methylglutaryl CoA reductase inhibitors, have pleiotropic effects which may involve interference with the isoprenylation of Ras and Rho GTPases. We show that atorvastatin selectively inhibits the proliferation of Tsc2-/- mouse embryo fibroblasts and ELT-3 smooth muscle cells in response to serum and estrogen, and under serum-free conditions. The isoprenoids farnesylpyrophosphate (FPP) and geranylgeranylpyrophosphate (GGPP) significantly reverse atorvastatin-induced inhibition of Tsc2-/- cell growth, suggesting that atorvastatin dually targets a farnesylated protein, such as Rheb, and a geranylgeranylated protein, such as Rho, both of which have elevated activity in Tsc2-/- cells. Atorvastatin reduced Rheb isoprenylation, GTP loading, and membrane localization. Atorvastatin also inhibited the constitutive phosphorylation of mammalian target of rapamycin, S6 kinase, and S6 found in Tsc2-/- cells in an FPP-reversible manner and attenuated the high levels of phosphorylated S6 in Tsc2-heterozygous mice. Atorvastatin, but not rapamycin, attenuated the increased levels of activated RhoA in Tsc2-/- cells, and this was reversed by GGPP. These results suggest that atorvastatin may inhibit both rapamycin-sensitive and rapamycin-insensitive mechanisms of tuberin-null cell growth, likely via Rheb and Rho inhibition, respectively. Atorvastatin may have potential therapeutic benefit in TSC syndromes, including LAM.

Development and characterization of gemcitabine-resistant pancreatic tumor cells

BACKGROUND

Pancreatic cancer is an exceptionally lethal disease with an annual mortality nearly equivalent to its annual incidence. This dismal rate of survival is due to several factors including late presentation with locally advanced, unresectable tumors, early metastatic disease, and rapidly arising chemoresistance. To study the mechanisms of chemoresistance in pancreatic cancer we developed two gemcitabine-resistant pancreatic cancer cell lines.

METHODS

Resistant cells were obtained by culturing L3.6pl and AsPC-1 cells in serially increasing concentrations of gemcitabine. Stable cultures were obtained that were 40- to 50-fold increased in resistance relative to parental cells. Immunofluorescent staining was performed to examine changes in beta-catenin and E-cadherin localization. Protein expression was determined by immunoblotting. Migration and invasion were determined by modified Boyden chamber assays. Fluorescence-activated cell sorting (FACS) analyses were performed to examine stem cell markers.

RESULTS

Gemcitabine-resistant cells underwent distinct morphological changes, including spindle-shaped morphology, appearance of pseudopodia, and reduced adhesion characteristic of transformed fibroblasts. Gemcitabine-resistant cells were more invasive and migratory. Gemcitabine-resistant cells were increased in vimentin and decreased in E-cadherin expression. Immunofluorescence and immunoblotting revealed increased nuclear localization of total beta-catenin. These alterations are hallmarks of epithelial-to-mesenchymal transition (EMT). Resistant cells were activated in the receptor protein tyrosine kinase, c-Met and increased in expression of the stem cell markers CD (cluster of differentiation)24, CD44, and epithelial-specific antigen (ESA).

CONCLUSIONS

Gemcitabine-resistant pancreatic tumor cells are associated with EMT, a more-aggressive and invasive phenotype in numerous solid tumors. The increased phosphorylation of c-Met may also be related to chemoresistance and EMT and presents as an attractive adjunctive chemotherapeutic target in pancreatic cancer.

IKK beta suppression of TSC1 links inflammation and tumor angiogenesis via the mTOR pathway

TNFalpha has recently emerged as a regulator linking inflammation to cancer pathogenesis, but the detailed cellular and molecular mechanisms underlying this link remain to be elucidated. The tuberous sclerosis 1 (TSC1)/TSC2 tumor suppressor complex serves as a repressor of the mTOR pathway, and disruption of TSC1/TSC2 complex function may contribute to tumorigenesis. Here we show that IKKbeta, a major downstream kinase in the TNFalpha signaling pathway, physically interacts with and phosphorylates TSC1 at Ser487 and Ser511, resulting in suppression of TSC1. The IKKbeta-mediated TSC1 suppression activates the mTOR pathway, enhances angiogenesis, and results in tumor development. We further find that expression of activated IKKbeta is associated with TSC1 Ser511 phosphorylation and VEGF production in multiple tumor types and correlates with poor clinical outcome of breast cancer patients. Our findings identify a pathway that is critical for inflammation-mediated tumor angiogenesis and may provide a target for clinical intervention in human cancer.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

Intermediate filament scaffolds fulfill mechanical, organizational, and signaling functions in the cytoplasm

Intermediate filaments (IFs) are cytoskeletal polymers whose protein constituents are encoded by a large family of differentially expressed genes. Owing in part to their properties and intracellular organization, IFs provide crucial structural support in the cytoplasm and nucleus, the perturbation of which causes cell and tissue fragility and accounts for a large number of genetic diseases in humans. A number of additional roles, nonmechanical in nature, have been recently uncovered for IF proteins. These include the regulation of key signaling pathways that control cell survival, cell growth, and vectorial processes including protein targeting in polarized cellular settings. As this discovery process continues to unfold, a rationale for the large size of this family and the context-dependent regulation of its members is finally emerging.

AKT/PKB signaling: navigating downstream

The serine/threonine kinase Akt, also known as protein kinase B (PKB), is a central node in cell signaling downstream of growth factors, cytokines, and other cellular stimuli. Aberrant loss or gain of Akt activation underlies the pathophysiological properties of a variety of complex diseases, including type-2 diabetes and cancer. Here, we review the molecular properties of Akt and the approaches used to characterize its true cellular targets. In addition, we discuss those Akt substrates that are most likely to contribute to the diverse cellular roles of Akt, which include cell survival, growth, proliferation, angiogenesis, metabolism, and migration.

Identification of TBC7 having TBC domain as a novel binding protein to TSC1-TSC2 complex

TBC7, a TBC (Tre-2/Bub2/Cdc16) 1 domain protein, was identified as a novel binding protein to the TSC1-TSC2 tumor suppressor complex by peptide mass fingerprinting analysis of the proteins immunoprecipitated with FLAG-epitope tagged TSC1 and TSC2 from the transfected mammalian cells. The in vivo and in vitro association of TBC7 and the TSC1-TSC2 complex was confirmed by the co-immunoprecipitation and pull-down analysis, respectively, and TBC7 was revealed to bind to the C-terminal half region of TSC1, which is distinct from the binding site with TSC2. The immunofluorescence microscopy and subcellular fractionation showed that TBC7 co-localizes with the tumor suppressor complex in the endomembrane. Overexpression of TBC7 enhanced ubiquitination of TSC1 and increased phosphorylation of S6 protein by S6 kinase, that is located in the mTOR-signaling pathway. These results indicate TBC7 could take a part in the negative regulation of the tumor suppressor complex through facilitating the downregulation of TSC1.

Novel functions of vimentin in cell adhesion, migration, and signaling

Vimentin is the major intermediate filament (IF) protein of mesenchymal cells. It shows dynamically altered expression patterns during different developmental stages and high sequence homology throughout all vertebrates, suggesting that the protein is physiologically important. Still, until recently, the real tasks of vimentin have been elusive, primarily because the vimentin-deficient mice were originally characterized as having a very mild phenotype. Recent studies have revealed several key functions for vimentin that were not obvious at first sight. Vimentin emerges as an organizer of a number of critical proteins involved in attachment, migration, and cell signaling. The highly dynamic and complex phosphorylation of vimentin seems to be a likely regulator mechanism for these functions. The implicated novel vimentin functions have broad ramifications into many different aspects of cell physiology, cellular interactions, and organ homeostasis.

mTOR pathway as a target in tissue hypertrophy

Recent work has shown that the mTOR (mammalian target of rapamycin) pathway is an integral cell growth regulator. The mTOR pathway involves two functional complexes, TORC1 and TORC2, which have been defined by both their association with raptor or rictor, respectively, and their sensitivity to short-term rapamycin inhibition. Loss of tumor suppressors TSC1 or TSC2 leads to aberrant activation of TORC1, which has been implicated in the control of cell size. As a result, both physiologic and pathologic tissue hypertrophy are associated with TORC1 activation. Some clinical examples include skeletal and cardiac muscle hypertrophy, vascular restenosis, and compensatory nephrotic hypertrophy. Clarification of the mTOR pathway may lead to increased understanding of both the etiology and consequences of aberrant cell size regulation. This review covers some of the biochemical regulation of the mTOR pathway that may be important to the regulation of cell size, and it will present several potential clinical applications where the control of cell size may be biologically significant.

Tuberin: a stimulus-regulated tumor suppressor protein controlled by a diverse array of receptor tyrosine kinases and G protein-coupled receptors

Tuberin, a tumor suppressor protein, is involved in various cellular functions including survival, proliferation, and growth. It has emerged as an important effector regulated by receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs). Regulation of tuberin by RTKs and GPCRs is highly complex and dependent on the type of receptors and their associated signaling molecules. Apart from Akt, the first kinase recognized to phosphorylate and inactivate tuberin upon growth factor stimulation, an increasing number of kinases upstream of tuberin have been identified. Furthermore, recruitment of different scaffolding adaptor components to the activated receptors appears to play an important role in the regulation of tuberin activity. More recently, the differential regulation of tuberin by various G protein family members have also been intensively studied, it appears that G proteins can both facilitate (e.g., G(i/o)) as well as inhibit (e.g., G(q)) tuberin phosphorylation. In the present review, we attempt to summarize our emerging understandings of the roles of RTKs, GPCRs, and their cross-talk on the regulation of tuberin.

The TSC1 gene product hamartin interacts with NADE

Hamartomatous brain lesions are a hallmark of brain pathology of tuberous sclerosis complex (TSC). To elucidate the mechanism of tumor development in the brain of TSC, we identified NADE (p75NTR-associated cell death executor) as an interactor for TSC1 gene product hamartin using a yeast two-hybrid system. In a pull-down assay, endogenous NADE was purified with the immobilized coiled-coil domain (CCD) of hamartin from the PC12h cell lysate. Immunofluorescence and immunoprecipitation confirmed the interaction of hamartin and NADE in cultured neurons and mouse brain lysate. Hamartin constitutively associated with NADE to prevent its proteasomal degradation. Suppression of hamartin with TSC1 small interfering RNA (siRNA) caused reduction of NADE and failed to lead to NGF-induced apoptosis in PC12h cells. These results indicate that hamartin binds to NADE to regulate neuronal cell function and loss of this association is likely to contribute to the brain pathology in TSC.

PDGFRs are critical for PI3K/Akt activation and negatively regulated by mTOR

The receptor tyrosine kinase/PI3K/Akt/mammalian target of rapamycin (RTK/PI3K/Akt/mTOR) pathway is frequently altered in tumors. Inactivating mutations of either the TSC1 or the TSC2 tumor-suppressor genes cause tuberous sclerosis complex (TSC), a benign tumor syndrome in which there is both hyperactivation of mTOR and inhibition of RTK/PI3K/Akt signaling, partially due to reduced PDGFR expression. We report here that activation of PI3K or Akt, or deletion of phosphatase and tensin homolog (PTEN) in mouse embryonic fibroblasts (MEFs) also suppresses PDGFR expression. This was a direct effect of mTOR activation, since rapamycin restored PDGFR expression and PDGF-sensitive Akt activation in Tsc1-/- and Tsc2-/- cells. Akt activation in response to EGF in Tsc2-/- cells was also reduced. Furthermore, Akt activation in response to each of EGF, IGF, and PMA was reduced in cells lacking both PDGFRalpha and PDGFRbeta, implying a role for PDGFR in transmission of growth signals downstream of these stimuli. Consistent with the reduction in PI3K/Akt signaling, in a nude mouse model both Tsc1-/- and Tsc2-/- cells had reduced tumorigenic potential in comparison to control cells, which was enhanced by expression of either active Akt or PDGFRbeta. In conclusion, PDGFR is a major target of negative feedback regulation in cells with activated mTOR, which limits the growth potential of TSC tumors.

Methods for studying signal-dependent regulation of translation factor activity

The translational machinery of mammalian cells is regulated through the phosphorylation of a number of its components, especially translation factor proteins. These include factors involved in the initiation and elongation stages of translation, and proteins that modify their activity. Examples include eukaryotic initiation factor (eIF) 4E, eukaryotic elongation factor (eEF) 2, and eIF4E-binding protein 1 (4E-BP1). Their phosphorylation is mediated by protein kinases that, in turn, are regulated by specific intracellular signaling pathways. These pathways include those mediated via the mammalian target of rapamycin (mTOR), the ERK and p38 MAP kinase pathways, and protein kinase B (Akt). These pathways are activated by hormones (e.g., insulin), growth factors, mitogens, and other extracellular stimuli. In some cases, amino acids also modulate the pathway (e.g., mTOR). Procedures are described for determining the states of phosphorylation and/or activity of several translation factors, and of kinases that phosphorylate them. We also outline procedures for assessing the states of activation of relevant signaling pathways. In addition, we provide guidelines on using small molecule inhibitors to assess the involvement of specific signaling pathways in controlling translation factors and protein synthesis.

Stress and mTORture signaling

The TOR (target of rapamycin) pathway is an evolutionarily conserved signaling module regulating cell growth (accumulation of mass) in response to a variety of environmental cues such as nutrient availability, hypoxia, DNA damage and osmotic stress. Its pivotal role in cellular and organismal homeostasis is reflected in the fact that unrestrained signaling activity in mammals is associated with the occurrence of disease states including inflammation, cancer and diabetes. The existence of TOR homologs in unicellular organisms whose growth is affected by environmental factors, such as temperature, nutrients and osmolarity, suggests an ancient role for the TOR signaling network in the surveillance of stress conditions. Here, we will summarize recent advances in the TOR signaling field with special emphasis on how stress conditions impinge on insulin/insulin-like growth factor signaling/TOR signaling.

Mind the GAP: Wnt Steps onto the mTORC1 Train

The TSC1/2 tumor-suppressor complex controls protein synthesis through the regulation of mTOR. In this issue of Cell, Inoki et al. (2006) report that the kinases GSK3 and AMPK cooperate in the activation of TSC2 to inhibit mTOR activity. Surprisingly, the phosphorylation of TSC2 by GSK3 is markedly suppressed by Wnt signaling. This suggests that components of the mTOR pathway may be therapeutic targets for diseases linked to hyperactive Wnt signaling.

"Translating" tumor hypoxia: unfolded protein response (UPR)-dependent and UPR-independent pathways

Poor oxygenation (hypoxia) is present in the majority of human tumors and is associated with poor prognosis due to the protection it affords to radiotherapy and chemotherapy. Hypoxia also elicits multiple cellular response pathways that alter gene expression and affect tumor progression, including two recently identified separate pathways that strongly suppress the rates of mRNA translation during hypoxia. The first pathway is activated extremely rapidly and is mediated by phosphorylation and inhibition of the eukaryotic initiation factor 2alpha. Phosphorylation of this factor occurs as part of a coordinated endoplasmic reticulum stress response program known as the unfolded protein response and activation of this program is required for hypoxic cell survival and tumor growth. Translation during hypoxia is also inhibited through the inactivation of a second eukaryotic initiation complex, eukaryotic initiation factor 4F. At least part of this inhibition is mediated through a Redd1 and tuberous sclerosis complex 1/2-dependent inhibition of the mammalian target of rapamycin kinase. Inhibition of mRNA translation is hypothesized to affect the cellular tolerance to hypoxia in part by promoting energy homeostasis. However, regulation of translation also results in a specific increase in the synthesis of a subset of hypoxia-induced proteins. Consequently, both arms of translational control during hypoxia influence gene expression and phenotype. These hypoxic response pathways show differential activation requirements that are dependent on the level of oxygenation and duration of hypoxia and are themselves highly dynamic. Thus, the severity and duration of hypoxia can lead to different biological and therapeutic consequences.

Effect of TRB3 on insulin and nutrient-stimulated hepatic p70 S6 kinase activity

Insulin and nutrients activate hepatic p70 S6 kinase (S6K1) to regulate protein synthesis. Paradoxically, activation of S6K1 also leads to the development of insulin resistance. In this study, we investigated the effect of TRB3, which acts as an endogenous inhibitor of Akt, on S6K1 activity in vitro and in vivo. In cultured cells, overexpression of TRB3 completely inhibited insulin-stimulated S6K1 activation by mammalian target of rapamycin, whereas knockdown of endogenous TRB3 increased both basal and insulin-stimulated activity. In C57BL/6 mice, adenoviral overexpression of TRB3 inhibited insulin-stimulated activation of hepatic S6K1. In contrast, overexpression of TRB3 did not inhibit nutrient-stimulated S6K1 activity. We also investigated the effect of starvation, feeding, or insulin treatment on TRB3 levels and S6K1 activity in the liver of C57BL/6 and db/db mice. Both insulin and feeding activate S6K1 in db/db mice, but only insulin activates in the C57BL/6 strain. TRB3 levels were 3.5-fold higher in db/db mice than C57BL/6 mice and were unresponsive to feeding or insulin, whereas both treatments reduced TRB3 in C57BL/6 mice. Akt was activated by insulin alone in the C57BL/6 strain and but not in db/db mice. Both insulin and feeding activated mammalian target of rapamycin similarly in these mice; however, feeding was unable to activate the downstream target S6K1 in C57BL/6 mice. These results suggest that the nutrient excess in the hyperphagic, hyperinsulinemic db/db mouse primes the hepatocyte to respond to nutrients resulting in elevated S6K1 activity. The combination of elevated TRB3 and constitutive S6K1 activity results in decreased insulin signaling via the IRS-1/phosphatidylinositol 3-kinase/Akt pathway.

A role for 14-3-3 in insulin-stimulated GLUT4 translocation through its interaction with the RabGAP AS160

Translocation of the insulin-regulated glucose transporter GLUT4 to the cell surface is dependent on the phosphatidylinositol 3-kinase/Akt pathway. The RabGAP (Rab GTPase-activating protein) AS160 (Akt substrate of 160 kDa) is a direct substrate of Akt and plays an essential role in the regulation of GLUT4 trafficking. We have used liquid chromatography tandem mass spectrometry to identify several 14-3-3 isoforms as AS160-interacting proteins. 14-3-3 proteins interact with AS160 in an insulin- and Akt-dependent manner via an Akt phosphorylation site, Thr-642. This correlates with the dominant negative effect of both the AS160(T642A) and the AS160(4P) mutants on insulin-stimulated GLUT4 translocation. Introduction of a constitutive 14-3-3 binding site into AS160(4P) restored 14-3-3 binding without disrupting AS160-IRAP (insulin-responsive amino peptidase) interaction and reversed the inhibitory effect of AS160(4P) on GLUT4 translocation. These data show that the insulin-dependent association of 14-3-3 with AS160 plays an important role in GLUT4 trafficking in adipocytes.