The Ras-related protein Rheb is farnesylated and antagonizes Ras signaling and transformation

Differential requirement of CAAX-mediated posttranslational processing for Rheb localization and signaling

The Rheb1 and Rheb2 small GTPases and their effector mTOR are aberrantly activated in human cancer and are attractive targets for anti-cancer drug discovery. Rheb is targeted to endomembranes via its C-terminal CAAX (C = cysteine, A = aliphatic, X = terminal amino acid) motif, a substrate for posttranslational modification by a farnesyl isoprenoid. Following farnesylation, Rheb undergoes two additional CAAX-signaled processing steps, Rce1-catalyzed cleavage of the AAX residues and Icmt-mediated carboxylmethylation of the farnesylated cysteine. However, whether these post-prenylation processing steps are required for Rheb signaling through mTOR is not known. We found that Rheb1 and Rheb2 localize primarily to the endoplasmic reticulum and Golgi apparatus. We determined that Icmt and Rce1 processing is required for Rheb localization, but is dispensable for Rheb-induced activation of the mTOR substrate p70 S6 kinase (S6K). Finally, we evaluated whether farnesylthiosalicylic acid (FTS) blocks Rheb localization and function. Surprisingly, FTS prevented S6K activation induced by a constitutively active mTOR mutant, indicating that FTS inhibits mTOR at a level downstream of Rheb. We conclude that inhibitors of Icmt and Rce1 will not block Rheb function, but FTS could be a promising treatment for Rheb- and mTOR-dependent cancers.

Glycolytic flux signals to mTOR through glyceraldehyde-3-phosphate dehydrogenase-mediated regulation of Rheb

The mammalian target of rapamycin (mTOR) interacts with raptor to form the protein complex mTORC1 (mTOR complex 1), which plays a central role in the regulation of cell growth in response to environmental cues. Given that glucose is a primary fuel source and a biosynthetic precursor, how mTORC1 signaling is coordinated with glucose metabolism has been an important question. Here, we found that the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) binds Rheb and inhibits mTORC1 signaling. Under low-glucose conditions, GAPDH prevents Rheb from binding to mTOR and thereby inhibits mTORC1 signaling. High glycolytic flux suppresses the interaction between GAPDH and Rheb and thus allows Rheb to activate mTORC1. Silencing of GAPDH or blocking of the Rheb-GAPDH interaction desensitizes mTORC1 signaling to changes in the level of glucose. The GAPDH-dependent regulation of mTORC1 in response to glucose availability occurred even in TSC1-deficient cells and AMPK-silenced cells, supporting the idea that the GAPDH-Rheb pathway functions independently of the AMPK axis. Furthermore, we show that glyceraldehyde-3-phosphate, a glycolytic intermediate that binds GAPDH, destabilizes the Rheb-GAPDH interaction even under low-glucose conditions, explaining how high-glucose flux suppresses the interaction and activates mTORC1 signaling. Taken together, our results suggest that the glycolytic flux regulates mTOR's access to Rheb by regulating the Rheb-GAPDH interaction, thereby allowing mTORC1 to coordinate cell growth with glucose availability.

Specific activation of mTORC1 by Rheb G-protein in vitro involves enhanced recruitment of its substrate protein

Rheb G-protein plays critical roles in the TSC/Rheb/mTOR signaling pathway by activating mTORC1. The activation of mTORC1 by Rheb can be faithfully reproduced in vitro by using mTORC1 immunoprecipitated by the use of anti-raptor antibody from mammalian cells starved for nutrients. The low in vitro kinase activity against 4E-BP1 of this mTORC1 preparation is dramatically increased by the addition of recombinant Rheb. On the other hand, the addition of Rheb does not activate mTORC2 immunoprecipitated from mammalian cells by the use of anti-rictor antibody. The activation of mTORC1 is specific to Rheb, because other G-proteins such as KRas, RalA/B, and Cdc42 did not activate mTORC1. Both Rheb1 and Rheb2 activate mTORC1. In addition, the activation is dependent on the presence of bound GTP. We also find that the effector domain of Rheb is required for the mTORC1 activation. FKBP38, a recently proposed mediator of Rheb action, appears not to be involved in the Rheb-dependent activation of mTORC1 in vitro, because the preparation of mTORC1 that is devoid of FKBP38 is still activated by Rheb. The addition of Rheb results in a significant increase of binding of the substrate protein 4E-BP1 to mTORC1. PRAS40, a TOR signaling (TOS) motif-containing protein that competes with the binding of 4EBP1 to mTORC1, inhibits Rheb-induced activation of mTORC1. A preparation of mTORC1 that is devoid of raptor is not activated by Rheb. Rheb does not induce autophosphorylation of mTOR. These results suggest that Rheb induces alteration in the binding of 4E-BP1 with mTORC1 to regulate mTORC1 activation.

Working together: Farnesyl transferase inhibitors and statins block protein prenylation

Farnesyl transferase inhibitors (FTIs) have so far proved to have limited value as single agents in clinical trials. This PharmSight will focus on the use of a novel group of FTIs that are most effective in vitro when used in combination with the "statin" class of anti-hypercholesterolemic agents, which also block protein prenylation. We recently showed that these novel FTIs in combination with lovastatin reduce Ras prenylation and induce an apoptotic response in malignant peripheral nerve sheath cells. The combination of statins with these new FTIs may produce profound synergistic cytostatic and cytotoxic effects against a variety of tumors and other proliferative disorders. Since statins are well tolerated in the clinic, we suggest that this combination approach should be tested in in vivo models.

Rheb and mTOR regulate neuronal polarity through Rap1B

The development of polarized hippocampal neurons with a single axon and multiple dendrites depends on the activity of phosphoinositide 3-kinase (PI3K) and the GTPase Rap1B. Here we show that PI3K regulates axon specification and elongation through the GTPase Rheb and its target mammalian target of rapamycin (mTOR). Overexpression of Rheb induces the formation of multiple axons, whereas its suppression by RNA interference blocks axon specification. mTOR is a central regulator of translation that phosphorylates eIF4E-binding proteins like 4E-BP1. Axon formation was suppressed by inhibition of mTOR and expression of mTOR-insensitive 4E-BP1 mutants. Inhibition of PI3K or mTOR reduced the level of Rap1B, which acts downstream of Rheb and mTOR. The ubiquitin E3 ligase Smurf2 mediates the restriction of Rap1B by initiating its degradation. Suppression of Smruf2 by RNA interference is able to compensate the loss of Rheb. These results indicate that the mTOR pathway is required to counteract the Smurf2-initiated degradation of Rap1B during the establishment of neuronal polarity.

Tumorigenic activity and therapeutic inhibition of Rheb GTPase

The AKT-mTOR pathway harbors several known and putative oncogenes and tumor suppressors. In a phenotypic screen for lymphomagenesis, we tested candidate genes acting upstream of and downstream from mTOR in vivo. We find that Rheb, a proximal activator of mTORC1, can produce rapid development of aggressive and drug-resistant lymphomas. Rheb causes mTORC1-dependent effects on apoptosis, senescence, and treatment responses that resemble those of Akt. Moreover, Rheb activity toward mTORC1 requires farnesylation and is readily blocked by a pharmacological inhibitor of farnesyltransferase (FTI). In Pten-deficient tumor cells, inhibition of Rheb by FTI is responsible for the drug's anti-tumor effects, such that a farnesylation-independent mutant of Rheb renders these tumors resistant to FTI therapy. Notably, RHEB is highly expressed in some human lymphomas, resulting in mTORC1 activation and increased sensitivity to rapamycin and FTI. Downstream from mTOR, we examined translation initiation factors that have been implicated in transformation in vitro. Of these, only eIF4E was able to enhance lymphomagenesis in vivo. In summary, the Rheb GTPase is an oncogenic activity upstream of mTORC1 and eIF4E and a direct therapeutic target of farnesyltransferase inhibitors in cancer.

Constitutively active Rheb induces oncogenic transformation

Rheb (Ras-homolog enriched in brain) is a component of the phosphatidylinositol 3-kinase (PI3K) target of rapamycin (TOR) signaling pathway, functioning as a positive regulator of TOR. Constitutively active mutants of Rheb induce oncogenic transformation in cell culture. The transformed cells are larger and contain more protein than their normal counterparts. They show constitutive phosphorylation of the ribosomal protein S6 kinase and the eukaryotic initiation factor 4E-binding protein 1, two downstream targets of TOR. The TOR-specific inhibitor rapamycin strongly interferes with transformation induced by constitutively active Rheb, suggesting that TOR activity is essential for the oncogenic effects of mutant Rheb. Rheb-induced transformation is also dependent on a C-terminal farnesylation signal that mediates localization to a cellular membrane. An engineered N-terminal myristylation signal can substitute for the farnesylation. Immunofluorescence localizes wild-type and mutant Rheb to vesicular structures in the cytoplasm, overlapping with the endoplasmic reticulum.

Biochemical and biological characterization of tumor-associated mutations of p110alpha

Signaling by class I phosphatidylinositol 3-kinase (PI3K) controls cell growth, replication, motility, and metabolism. The PI3K pathway commonly shows gain of function in cancer. Two small GTPases, Rheb (Ras homolog enriched in brain) and Ras (rat sarcoma viral oncogene), play important roles in PI3K signaling. Rheb activates the TOR (target of rapamycin) kinase in a GTP-dependent manner; it links TOR to upstream signaling components, including the tuberous sclerosis complex (TSC) and Akt (homolog of the Akt8 murine lymphoma viral oncoprotein). Constitutively active, GTP-bound Rheb is oncogenic in cell culture, and activity that requires farnesylation. Ras activates PI3K by recruitment to the plasma membrane and possibly by inducing a conformational change in the catalytic subunit p110 of PI3K. In return, Ras signaling through the MAP kinase (MAPK) pathway is activated by PIP(3), the product of PI3K. Loss of Ras function can interfere with PI3K signaling. Various lines of evidence suggest complementary roles for PI3K and MAPK signaling in oncogenesis.

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.

Estrogen-induced activation of mammalian target of rapamycin is mediated via tuberin and the small GTPase Ras homologue enriched in brain

Inhibitors of the mammalian target of rapamycin (mTOR) are currently in clinical trials for the treatment of breast cancer. The mechanisms through which mTOR are activated in breast cancer and the relationship of mTOR activation to steroid hormones, such as estrogen, that are known to influence breast cancer pathogenesis, are not yet understood. Using MCF-7 cells as a model, we found that 17-beta estradiol (E(2)) rapidly increased the phosphorylation of downstream targets of mTOR: p70 ribosomal protein S6 kinase, ribosomal protein S6, and eukaryotic initiation factor 4E-binding protein 1. The phosphoinositide-3-kinase inhibitor, wortmannin, and the mTOR inhibitor, rapamycin, blocked E(2)-induced activation of p70 ribosomal protein S6 kinase. We hypothesized that tuberin and the small GTPase Ras homologue enriched in brain (Rheb), regulators of the mTOR pathway, mediate E(2)-induced activation of mTOR. Consistent with this hypothesis, E(2) rapidly (within 5 minutes) stimulated tuberin phosphorylation at T1462, a site at which Akt phosphorylates and inactivates tuberin. E(2) also rapidly decreased the inactive, GDP-bound form of Rheb. Finally, we found that small interfering RNA down-regulation of endogenous Rheb blocked the E(2)-stimulated proliferation of MCF-7 cells, demonstrating that Rheb is a key determinant of E(2)-dependent cell growth. Taken together, these data reveal that the TSC/Rheb/mTOR pathway plays a critical role in the regulation of E(2)-induced proliferation, and highlight Rheb as a novel molecular target for breast cancer therapy.

Bnip3 mediates the hypoxia-induced inhibition on mammalian target of rapamycin by interacting with Rheb

The mammalian target of rapamycin (mTOR) is a central controller of cell growth, and it regulates translation, cell size, cell viability, and cell morphology. mTOR integrates a wide range of extracellular and intracellular signals, including growth factors, nutrients, energy levels, and stress conditions. Rheb, a Ras-related small GTPase, is a key upstream activator of mTOR. In this study, we found that Bnip3, a hypoxia-inducible Bcl-2 homology 3 domain-containing protein, directly binds Rheb and inhibits the mTOR pathway. Bnip3 decreases Rheb GTP levels in a manner depending on the binding to Rheb and the presence of the N-terminal domain. Both knockdown and overexpression experiments show that Bnip3 plays an important role in mTOR inactivation in response to hypoxia. Moreover, Bnip3 inhibits cell growth in vivo by suppressing the mTOR pathway. These observations demonstrate that Bnip3 mediates the inhibition of the mTOR pathway in response to hypoxia.

mTOR is activated in the majority of malignant melanomas

The objective of this study was to determine whether activation of the kinase mammalian target of rapamycin (mTOR) is associated with human melanoma. We found moderate or strong hyperphosphorylation of ribosomal protein S6 in 78/107 melanomas (73%). In contrast, only 3/67 benign nevi (4%) were moderately positive, and none were strongly positive. These data indicate that mTOR activation is very strongly associated with malignant, compared to benign, melanocytic lesions. Next, we tested six melanoma-derived cell lines for evidence of mTOR dysregulation. Five of the six lines showed persistent phosphorylation of S6 after 18 hours of serum deprivation, and four had S6 phosphorylation after 30 minutes of amino-acid withdrawal, indicating inappropriate mTOR activation. The proliferation of three melanoma-derived lines was blocked by the mTOR inhibitor rapamycin, indicating that mTOR activation is a growth-promoting factor in melanoma-derived cells. mTOR is directly activated by the small guanosine triphosphatase Ras homolog enriched in brain (Rheb), in a farnesylation-dependent manner. Therefore, to investigate the mechanism of mTOR activation, we used the farnesyl transferase inhibitor FTI-277, which partially blocked the growth of three of the six melanoma cell lines. Together, these data implicate activation of mTOR in the pathogenesis of melanoma, and suggest that Rheb and mTOR may be targets for melanoma therapy.

Farnesyltransferase inhibitor R115777 inhibits cell growth and induces apoptosis in mantle cell lymphoma

INTRODUCTION

The cytotoxic activity of the farnesyltranseferase inhibitor R115777 was evaluated in cell lines representative of mantle cell lymphoma (MCL).

METHODS

Cell growth, proliferation, and apoptosis were analyzed in four human MCL cell lines (Granta, NCEB, REC, and UPN1) in presence of R115777, alone or in combination with vincristin, doxorubicin, bortezomib, cisplatin and cytarabine. Inhibition of farnesylation was determined by the appearance of prelamin A. The antitumor activity of R115777, administered p.o. at 100, 250 and 500 mg/kg, was determined in vivo in nude mice xenografted with UPN1 cells.

RESULTS

R115777 inhibited the growth of MCL cell lines in vitro with inhibitory concentrations ranging between 2 and 15 nM. A fifty percent decrease of cell viability was observed at concentrations comprised between 0.08 and 17 microM. Apoptosis, evaluated by annexin V and activated caspase 3 staining, was induced in all cell lines, in 40 to 71% of the cells depending on the cell lines. In addition, R115777 significantly increased the cytotoxic effect of vincristine, doxorubicin, bortezomib, cisplatin and cytarabine (p=0.001, p=0.016, p=0.006, p=0.014 and p=0.007 respectively). Exposure of MCL cell lines to R115777 during 72 hours resulted in inhibition of protein farnesylation. R115777 administered p.o. twice daily for 8 consecutive days to mice bearing established s.c. UPN1 xenograft displayed cytostatic activity at the 500 mg/kg dosage.

CONCLUSION

We have demonstrated that inhibition of farnesyltransferase by R115777 was associated with growth inhibition and apoptosis of MCL cell lines in vitro and tumor xenograft stability in vivo.

Identification and characterization of RHEBL1, a novel member of Ras family, which activates transcriptional activities of NF-kappa B

The Ras family of small GTPases regulates a wide variety of cellular functions that include cell growth, differentiation, and transformation. In this study, we identified and characterized a novel member of Ras family named RHEBL1, belonging to the Rheb branch of small GTPase proteins. The cDNA sequence contains an open reading frame of 551 bp, encoding a putative protein of 183 amino acid residues. The expression pattern of RHEBL1 showed that it was ubiquitously expressed in 17 tissues. RHEBL1 gene encodes a 20.69 kDa protein, localized in cytoplasm when overexpressed in COS7 cells. Reporter gene assays showed that overexpression of RHEBL1 in HEK 293T cells strongly activated the transcriptional activities of NF-kappa B, while the mutant (D60K) only weakly activates NF-kappa B-mediated transcription. Our findings suggest that RHEBL1 is a positive regulator of NF-kappa B-mediated gene transcription.

HDJ-2 as a Target for Radiosensitization of Glioblastoma Multiforme Cells by the Farnesyltransferase Inhibitor R115777 and the Role of the p53/p21 Pathway

Resistance of glioblastoma multiforme to radiotherapy poses a major clinical challenge. Farnesyltransferase inhibitors (FTI), such as R115777, have potential to increase radiotherapeutic benefit in this disease, although their mechanism of action is unclear. In our study with eight glioblastoma multiforme cell lines, the most sensitive ones underwent cell cycle arrest in response to FTI treatment. Radiosensitization by FTIs, however, seemed to involve other pathways. If R115777 treatment was initiated < 6 hours before irradiation, all eight glioblastoma multiforme lines were radiosensitized. However, if the time between drug and radiation was extended to 24 hours, cells harboring wild type but not mutated p53 were able to counteract drug-induced radiosensitization. The involvement of the p53/p21 pathway in the development of resistance was confirmed by showing that U87 cells transfected with human papillomavirus E6 to block p53 or interfering RNA to inhibit p21 stayed radiosensitive for 24 hours after drug treatment. The time dependency of R115777-induced radiosensitization suggested that the initial FTI target for early radiosensitization was short-lived, and that a p21-directed pathway restored resistance. Consideration of prenylated molecules that could potentially be involved led us to consider HDJ-2, a co-chaperone of heat shock protein 70. This hypothesis was strengthened by finding that cellular radiosensitivity was increased by genetic inhibition of HDJ-2, whereas overexpression conferred radioresistance. Importantly, irradiation of cells caused HDJ-2 to migrate from the cytoplasm to the nucleus, and this migration was inhibited by prior FTI treatment. These results have clinical relevance in that they help explain the variability in responses to FTIs that occurs following radiotherapy and elucidate some of the reasons for the complexity underlying FTI-induced radiosensitization.

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

Loss of tuberin, the product of TSC2 gene, increases mammalian target of rapamycin (mTOR) signaling, promoting cell growth and tumor development. However, in cells expressing tuberin, it is not known how repression of mTOR signaling is relieved to activate this pathway in response to growth factors and how hamartin participates in this process. We show that hamartin colocalizes with hypophosphorylated tuberin at the membrane, where tuberin exerts its GTPase-activating protein (GAP) activity to repress Rheb signaling. In response to growth signals, tuberin is phosphorylated by AKT and translocates to the cytosol, relieving Rheb repression. Phosphorylation of tuberin at serines 939 and 981 does not alter its intrinsic GAP activity toward Rheb but partitions tuberin to the cytosol, where it is bound by 14-3-3 proteins. Thus, tuberin bound by 14-3-3 in response to AKT phosphorylation is sequestered away from its membrane-bound activation partner (hamartin) and its target GTPase (Rheb) to relieve the growth inhibitory effects of this tumor suppressor.

Localization of Rheb to the endomembrane is critical for its signaling function

Rheb, a small GTPase, has emerged as a key molecular switch that directly regulates the activity of the mammalian target of rapamycin (mTOR). Similar to other members of the Ras superfamily, Rheb has a C-terminal CaaX box that is subject to farnesylation. This study reports that farnesylation is a key determinant of Rheb's subcellular localization and directs its association with the endomembrane. Timed imaging of live cells expressing EGFP-Rheb reveals that following brief association with the ER, Rheb localizes to highly ordered, distinct structures within the cytoplasm that display characteristics of Golgi membranes. Failure of Rheb to localize to the endomembrane impairs its ability to interact with mTOR and activate downstream targets. Consistent with the notion that the endomembrane may serve as a platform for the assembly of a functional Rheb/mTOR complex, treatment of cells with brefeldin A interferes with transmission of Rheb signals to p70S6K.

Identification of novel single amino acid changes that result in hyperactivation of the unique GTPase, Rheb, in fission yeast

Rheb GTPase is a key player in the control of growth, cell cycle and nutrient uptake that is conserved from yeast to humans. To further our understanding of the Rheb pathway, we sought to identify hyperactivating mutations in the Schizosaccharomyces pombe Rheb, Rhb1. Hyperactive forms of Rhb1 were found to result from single amino acid changes at valine-17, serine-21, lysine-120 or asparagine-153. Expression of these mutants confers resistance to canavanine and thialysine, phenotypes which are similar to phenotypes exhibited by cells lacking the Tsc1/Tsc2 complex that negatively regulates Rhb1. The thialysine-resistant phenotype of the hyperactive Rhb1 mutants is suppressed by a second mutation in the effector domain. Purified mutant proteins exhibit dramatically decreased binding of GDP, while their GTP binding is not drastically affected. In addition, some of the mutant proteins show significantly decreased GTPase activities. Thus the hyperactivating mutations are expected to result in an increase in the GTP-bound/GDP-bound ratio of Rhb1. By using the hyperactive mutant, Rhb1(K120R), we have been able to demonstrate that Rhb1 interacts with Tor2, one of the two S. pombe TOR (Target of Rapamycin) proteins. These fission yeast results provide the first evidence for a GTP-dependent association of Rheb with Tor.

Development of farnesyl transferase inhibitors: a review

Farnesyl transferase inhibitors are a new class of biologically active anticancer drugs. The exact mechanism of action of this class of agents is, however, currently unknown. The drugs inhibit farnesylation of a wide range of target proteins, including Ras. It is thought that these agents block Ras activation through inhibition of the enzyme farnesyl transferase, ultimately resulting in cell growth arrest. In preclinical models, the farnesyl transferase inhibitors showed great potency against tumor cells; yet in clinical studies, their activity was far less than anticipated. Reasons for this disappointing clinical outcome might be found in the drug-development process. In this paper, we outline an algorithm that is potentially useful for the development of biologically active anticancer drugs. The development of farnesyl transferase inhibitors, from discovery to clinical trials, is reviewed on the basis of this algorithm. We found that two important steps of this algorithm were underestimated. First, understanding of the molecular biology of the defective pathway has mainly been focused on H-Ras activation, whereas activation of K-Ras or other farnesylated proteins is probably more important in tumorigenesis. Inhibition of farnesylation is possibly not sufficient, because geranylgeranylation might activate K-Ras and suppress the effect of farnesyl transferase inhibitors. Furthermore, a well-defined proof of concept in preclinical and clinical studies has not been achieved. Integrating the proposed algorithm in future studies of newly developed biologically active anti-cancer drugs might increase the rate of success of these compounds in patients.

Tuberous sclerosis complex: molecular pathogenesis and animal models

Mutations in one of two genes, TSC1 and TSC2, result in a similar disease phenotype by disrupting the normal interaction of their protein products, hamartin and tuberin, which form a functional signaling complex. Disruption of these genes in the brain results in abnormal cellular differentiation, migration, and proliferation, giving rise to the characteristic brain lesions of tuberous sclerosis complex (TSC) called cortical tubers. The most devastating complications of TSC affect the central nervous system and include epilepsy, mental retardation, autism, and glial tumors. Relevant animal models, including conventional and conditional knockout mice, are valuable tools for studying the normal functions of tuberin and hamartin and the way in which disruption of their expression gives rise to the variety of clinical features that characterize TSC. In the future, these animals will be invaluable preclinical models for the development of highly specific and efficacious treatments for children affected with TSC.

Thematic review series: Lipid Posttranslational Modifications. Farnesyl transferase inhibitors

Some proteins undergo posttranslational modification by the addition of an isoprenyl lipid (farnesyl- or geranylgeranyl-isoprenoid) to a cysteine residue proximal to the C terminus. Protein isoprenylation promotes membrane association and contributes to protein-protein interactions. Farnesylated proteins include small GTPases, tyrosine phosphatases, nuclear lamina, cochaperones, and centromere-associated proteins. Prenylation is required for the transforming activity of Ras. Because of the high frequency of Ras mutations in cancer, farnesyl transferase inhibitors (FTIs) were investigated as a means to antagonize Ras function. Evaluation of FTIs led to the finding that both K- and N-Ras are alternatively modified by geranylgeranyl prenyltransferase-1 in FTI-treated cells. Geranylgeranylated forms of Ras retain the ability to associate with the plasma membrane and activate substrates. Despite this, FTIs are effective at inhibiting the growth of human tumor cells in vitro, suggesting that activity is dependent on blocking the farnesylation of other proteins. FTIs also inhibit the in vivo growth of human tumor xenografts and sensitize these models to chemotherapeutics, most notably taxanes. Several FTIs have entered clinical trials for various cancer indications. In some clinical settings, primarily hematologic malignancies, FTIs have displayed evidence of single-agent activity. Clinical studies in progress are exploring the antitumor activity of FTIs as single agents and in combination. This review will summarize the basic biology of FTIs, their antitumor activity in preclinical models, and the current status of clinical studies with these agents.

Rheb activation of mTOR and S6K1 signaling

More than 10 years ago, Rheb (Ras homolog enriched in brain) was identified as a highly conserved protein that is a member of the Ras superfamily of small GTPases, which play critical roles in cell growth and proliferation. Recently, a convergence of genetic and biochemical evidence from yeast, Drosophila, and mammalian cells has placed Rheb upstream of the mammalian target of rapamycin (mTOR) and immediately downstream of the tumor suppressors TSC1 (hamartin) and TSC2 (tuberin). Rheb plays a key role in the regulation of cell growth in response to growth factors, nutrients, and amino acids linking PI3K and TOR signaling. Rheb activation of the nutrient and energy-sensitive TOR pathway leads to the direct phosphorylation of two known downstream translational control targets by mTOR, the 40S ribosomal S6 kinase 1 (S6K1) and the eukaryotic translation initiation factor 4E (eIF4E)- binding protein 1 (4E-BP1). Appropriate regulation of this pathway is crucial for the proper control of cell growth, proliferation, survival, and differentiation. Inappropriate regulation of these signaling molecules, therefore, can lead to a variety of human diseases. In this chapter, we describe cell biological and biochemical methods commonly used to study Rheb activation and dissect its role in the mTOR-signaling pathway.

Novel triazole based inhibitors of Ras farnesyl transferase

A novel series of potent inhibitors of Ras farnesyl transferase possessing a 1,2,4-triazole pharmacophore is described. These inhibitors were discovered from a parallel synthesis effort and were subsequently optimized to in vitro IC(50) value of less than 1nM.

Farnesyltransferase inhibitors reverse altered growth and distribution of actin filaments in Tsc-deficient cells via inhibition of both rapamycin-sensitive and -insensitive pathways

Farnesyltransferase inhibitors (FTI) have been developed as anticancer drugs and are currently being evaluated in clinical trials. In this study, we have examined the effects of FTIs on Tsc-null cells to gain insight into their effects on farnesylated Rheb GTPase. This protein is involved in the activation of mTOR/S6K signaling and is down-regulated by the Tsc1/Tsc2 complex. Both Tsc1(-/-) and Tsc2(-/-) mouse embryonic fibroblasts exhibit constitutive activation of S6K and grow in the absence of serum. Two different FTI compounds, the clinical compound BMS-214662 and the newly described BMS-225975, inhibit the constitutive activation of mTOR/S6K signaling and block serum-free growth of the Tsc-null mouse embryonic fibroblasts. We have also found that Tsc-null mouse embryonic fibroblasts grow under anchorage-independent conditions and that both FTI compounds inhibit this soft agar growth. These FTI effects are similar to those observed with rapamycin. Another interesting phenotype of Tsc-null mouse embryonic fibroblasts is that they are round and contain actin filaments predominantly at the cell periphery. The addition of FTIs, but not rapamycin, led to the reappearance of intracellular actin filaments and reduction of peripheral actin filaments. The ability of FTI to rearrange actin filaments seems to be largely mediated by the inhibition of Rheb protein, as induction of intracellular actin filaments by FTI was much less efficient in Tsc2-null cells expressing Rheb (M184L), a geranylgeranylated mutant Rheb that can bypass farnesylation. These results reveal that FTIs inhibit Rheb, causing two different effects in Tsc-deficient cells, one on growth and the other on actin filament distribution.

Rheb binds and regulates the mTOR kinase

BACKGROUND

The target of rapamycin (TOR), in complex with the proteins raptor and LST8 (TOR complex 1), phosphorylates the p70S6K and 4E-BP1 to promote mRNA translation. Genetic evidence establishes that TOR complex activity in vivo requires the small GTPase Rheb, and overexpression of Rheb can rescue TOR from inactivation in vivo by amino-acid withdrawal. The Tuberous Sclerosis heterodimer (TSC1/TSC2) functions as a Rheb GTPase activator and inhibits TOR signaling in vivo.

RESULTS

Here, we show that Rheb binds to the TOR complex specifically, independently of its ability to bind TSC2, through separate interactions with the mTOR catalytic domain and with LST8. Rheb binding to the TOR complex in vivo and in vitro does not require Rheb guanyl nucleotide charging but is modulated by GTP and impaired by certain mutations (Ile39Lys) in the switch 1 loop. Nucleotide-deficient Rheb mutants, although capable of binding mTOR in vivo and in vitro, are inhibitory in vivo, and the mTOR polypeptides that associate with nucleotide-deficient Rheb in vivo lack kinase activity in vitro. Reciprocally, mTOR polypeptides bound to Rheb(Gln64Leu), a mutant that is nearly 90% GTP charged, exhibit substantially higher protein kinase specific activity than mTOR bound to wild-type Rheb.

CONCLUSIONS

The TOR complex 1 is a direct target of Rheb-GTP, whose binding enables activation of the TOR kinase.

Differential membrane localization of ERas and Rheb, two Ras-related proteins involved in the phosphatidylinositol 3-kinase/mTOR pathway

Two Ras-related proteins, ERas and Rheb, which are involved in the phosphatidylinositol 3-kinase pathway, display high GTP affinity and have atypical CAAX motifs. The factors governing the intracellular localization of ERas and Rheb are incompletely understood. In the current study, we show by confocal microscopy that ERas is localized to the plasma membrane, whereas Rheb is confined to the endomembranes. Membrane localization of the two proteins was abolished by mutation of the cysteine of the CAAX motif. Membrane targeting was also abolished by a farnesyltransferase inhibitor but not by a geranylgeranyltransferase inhibitor. In mouse fibroblasts deficient in either Rce1 (Ras converting enzyme 1) or Icmt (isoprenylcysteine carboxyl methyltransferase), ERas was mislocalized mainly to the Golgi apparatus, whereas Rheb showed diffuse localization. Mutation of cysteines in the hypervariable region of ERas prevented the plasma membrane localization of ERas, very strongly suggesting that palmitoylation of the cysteines is essential for membrane targeting. The hypervariable region of Rheb does not contain cysteines or polybasic residues, and when it was replaced with the hypervariable region of H-Ras, Rheb displayed plasma membrane localization. These data indicate that ERas shares the same posttranslational modifications with H-Ras and N-Ras and is localized at the plasma membrane. Rheb also shares the same membrane-targeting pathway but because of the absence of palmitoylation is located on endomembranes.

The farnesyl transferase inhibitor (FTI) SCH66336 (lonafarnib) inhibits Rheb farnesylation and mTOR signaling. Role in FTI enhancement of taxane and tamoxifen anti-tumor activity

Lonafarnib (SCH66336) is a farnesyl transferase inhibitor (FTI) that inhibits the post-translational lipid modification of H-Ras and other farnesylated proteins. K- and N-Ras are also substrates of farnesyl transferase; however, upon treatment with FTIs, they are alternatively prenylated by geranylgeranyl transferase-1. Despite the failure to abrogate prenylation of K- and N-Ras, growth of many tumors in preclinical models is inhibited by FTIs. This suggests that the anti-proliferative action of FTIs is dependent on blocking the farnesylation of other proteins. Rheb (Ras homologue enriched in brain) is a farnesylated small GTPase that positively regulates mTOR (mammalian target of rapamycin) signaling. We found that Rheb and Rheb2 mRNA were elevated in various tumor cell lines relative to normal cells. Peptides derived from the carboxyl termini of human Rheb and Rheb2 are in vitro substrates for farnesyl transferase but not geranylgeranyl transferase-1. Rheb prenylation in cell culture was completely inhibited by SCH66336, indicating a lack of alternative prenylation. SCH66336 treatment also inhibited the phosphorylation of S6 ribosomal protein, a downstream target of Rheb and mTOR signaling. SCH66336 did not inhibit S6 phosphorylation in cells expressing Rheb-CSVL, a mutant construct of Rheb designed to be geranylgeranylated. Importantly, expression of Rheb-CSVL also abrogated SCH66336 enhancement of tamoxifen- and docetaxel-induced apoptosis in MCF-7 breast cancer cells and ES-2 ovarian cancer cells, respectively. Further, inhibition of Rheb signaling by rapamycin treatment, small interfering RNA, or dominant negative Rheb enhanced tamoxifen- and docetaxel-induced apoptosis, similar to FTI treatment. These studies demonstrated that Rheb is modified by farnesylation, is not a substrate for alternative prenylation, and plays a role in SCH66336 enhancement of the anti-tumor response to other chemotherapeutics.

Phase I Study of the Farnesyltransferase Inhibitor BMS-214662 Given Weekly in Patients with Solid Tumors

Purpose: A phase I trial of BMS-214662, a selective farnesyltransferase inhibitor with significant preclinical antitumor activity in which drug was given as a weekly 1-hour infusion for four of six weeks, was conducted to evaluate the tolerability, pharmacokinetics, and pharmacodynamic effect on farnesyltransferase activity in peripheral blood mononuclear cells.

Experimental Design: BMS-214662 was given to 27 patients with solid tumors at 10 escalating dose levels (28-220 mg/m2) allowing intrapatient dose escalation; pharmacokinetics and pharmacodynamics were done at the first seven dose levels.

Results: Grade 4 neutropenia (four patients) was the most common dose-limiting toxicity followed by aminotransferase elevation (grade 3 alanine aminotransferase and grade 4 aspartate aminotransferase) and grade 3 dehydration. Most frequent toxicities were neutropenia in 11 (14%), anemia in 15 (19%), fatigue in 9 (12%), and nausea and diarrhea in 6 (8%) of courses, respectively. One minor response lasting 18 weeks in a patient with non–small cell lung cancer, serum calcitonin level reduction accompanied by disease stabilization in two of four patients with medullary thyroid carcinoma, and stable disease in 16 of 25 evaluable patients was seen. No correlation was observed between dose and Cmax, total body clearance (mean, 26.15 ± 10.88 L per hour per m2), volume of distribution at steady state (mean, 39.51 ± 17.91 L/m2), or half-life (mean, 2.63 ± 1.81 hours); a moderate correlation existed between dose given and systemic drug exposure (AUC). Substantial inhibition of peripheral blood mononuclear cell farnesyltransferase activity but near complete recovery by 24 hours was seen.

Conclusion: BMS-214667 was well tolerated as a weekly 1-hour i.v. infusion for four of six weeks with evidence of pharmacodynamic effect. The study was terminated before maximum tolerated dose was reached. Alternative schedules of drug administration might result in improved pharmacodynamic profile.

TSC1-2 tumour suppressor and regulation of mTOR signalling: linking cell growth and proliferation?

Understanding the relationship between growth and proliferation in multicellular organisms requires identification of the key regulators of growth control, and an understanding of how they regulate growth and how growth is linked to cell proliferation. Recent progress in understanding the mechanisms of growth control indicates that the tuberous sclerosis complex tumour-suppressor TSC1-2 serves as a point of integration between growth-stimulatory and growth-suppressive signalling upstream of a small GTPase, Rheb. However, Rheb-induced growth might not explain the additional effects of TSC1-2 upon cell proliferation.

[Involvement of PI3K/Akt pathway in prostate cancer. Potential strategies for developing targeted therapies]

Because of the unavailability of effective therapies to block or reverse the progression of androgen-independent prostate cancer, it seems obvious to target growth signaling pathways for which frequently recurring mutations have been identified. Acquired mutations of the PTEN gene have been reported in several tumor types, including up to 30% - 60% of prostate cancer tumors. This results in constitutive activation of the PI3K/Akt pathway which then represents a major target to prevent dysfunctions in cell growth, survival and motility. Our experience and, therefore, our own tools allow us to design new inhibitors of growth factor receptor tyrosine kinase, PDK-1 and farnesyltransferase activities. These original compounds could selectively switch off one or several steps of the multifunctional pathway and constitute lead compounds in the design of new classes of potent drugs.

Akt-dependent cell size regulation by the adhesion molecule on glia occurs independently of phosphatidylinositol 3-kinase and Rheb signaling

The role of cell adhesion molecules in mediating interactions with neighboring cells and the extracellular matrix has long been appreciated. More recently, these molecules have been shown to modulate intracellular signal transduction cascades critical for cell growth and proliferation. Expression of adhesion molecule on glia (AMOG) is downregulated in human and mouse gliomas, suggesting that AMOG may be important for growth regulation in the brain. In this report, we examined the role of AMOG expression on cell growth and intracellular signal transduction. We show that AMOG does not negatively regulate cell growth in vitro or in vivo. Instead, expression of AMOG in AMOG-deficient cells results in a dramatic increase in cell size associated with protein kinase B/Akt hyperactivation, which occurs independent of phosphatidylinositol 3-kinase activation. AMOG-mediated Akt phosphorylation specifically activates the mTOR/p70S6 kinase pathway previously implicated in cell size regulation, but it does not depend on tuberous sclerosis complex/Ras homolog enriched in brain (Rheb) signaling. These data support a novel role for a glial adhesion molecule in cell size regulation through selective activation of the Akt/mTOR/S6K signal transduction pathway.

Signaling by target of rapamycin proteins in cell growth control

Target of rapamycin (TOR) proteins are members of the phosphatidylinositol kinase-related kinase (PIKK) family and are highly conserved from yeast to mammals. TOR proteins integrate signals from growth factors, nutrients, stress, and cellular energy levels to control cell growth. The ribosomal S6 kinase 1 (S6K) and eukaryotic initiation factor 4E binding protein 1(4EBP1) are two cellular targets of TOR kinase activity and are known to mediate TOR function in translational control in mammalian cells. However, the precise molecular mechanism of TOR regulation is not completely understood. One of the recent breakthrough studies in TOR signaling resulted in the identification of the tuberous sclerosis complex gene products, TSC1 and TSC2, as negative regulators for TOR signaling. Furthermore, the discovery that the small GTPase Rheb is a direct downstream target of TSC1-TSC2 and a positive regulator of the TOR function has significantly advanced our understanding of the molecular mechanism of TOR activation. Here we review the current understanding of the regulation of TOR signaling and discuss its function as a signaling nexus to control cell growth during normal development and tumorigenesis.

Structural basis for the unique biological function of small GTPase RHEB

The small GTPase Rheb displays unique biological and biochemical properties different from other small GTPases and functions as an important mediator between the tumor suppressor proteins TSC1 and TSC2 and the mammalian target of rapamycin to stimulate cell growth. We report here the three-dimensional structures of human Rheb in complexes with GDP, GTP, and GppNHp (5'-(beta,gamma-imide)triphosphate), which reveal novel structural features of Rheb and provide a molecular basis for its distinct properties. During GTP/GDP cycling, switch I of Rheb undergoes conformational change while switch II maintains a stable, unusually extended conformation, which is substantially different from the alpha-helical conformation seen in other small GTPases. The unique switch II conformation results in a displacement of Gln64 (equivalent to the catalytic Gln61 of Ras), making it incapable of participating in GTP hydrolysis and thus accounting for the low intrinsic GTPase activity of Rheb. This rearrangement also creates space to accommodate the side chain of Arg15, avoiding its steric hindrance with the catalytic residue and explaining its noninvolvement in GTP hydrolysis. Unlike Ras, the phosphate moiety of GTP in Rheb is shielded by the conserved Tyr35 of switch I, leading to the closure of the GTP-binding site, which appears to prohibit the insertion of a potential arginine finger from its GTPase-activating protein. Taking the genetic, biochemical, biological, and structural data together, we propose that Rheb forms a new group of the Ras/Rap subfamily and uses a novel GTP hydrolysis mechanism that utilizes Asn1643 of the tuberous sclerosis complex 2 GTPase-activating protein domain instead of Gln64 of Rheb as the catalytic residue.

The Rheb family of GTP-binding proteins

Rheb proteins represent a novel and unique family of the Ras superfamily GTP-binding proteins that is conserved from yeast to human. Biochemical studies establish that they bind and hydrolyze GTP. Molecular modeling studies reveal a few structural differences between Rheb and Ras, which may suggest that residues involved in biochemical activities differ between the two G-proteins. The function of Rheb has been studied in a number of organisms that point to the involvement of Rheb in cell growth and cell cycle progression. In addition, studies in fungi suggest that Rheb is involved in arginine uptake. Further studies in Drosophila and mammalian cells have shown that the effects of Rheb on growth and cell cycle progression are mediated by the effect on the insulin/TOR/S6K signaling pathway. These studies have also shown that a complex consisting of the tuberous sclerosis gene products, Tsc1/Tsc2, serves as a GTPase activating protein (GAP) for Rheb, implying Rheb's role in this genetic disorder. Finally, Rheb proteins have been shown to be farnesylated and small molecule inhibitors of protein farnesyltransferase can block the ability of Rheb to activate the TOR/S6K signaling.

The Tuberous Sclerosis Complex Genes in Tumor Development

The study of hereditary tumor syndromes has laid a solid foundation toward understanding the genetic basis of cancer. One of the latest examples comes from the study of tuberous sclerosis complex (TSC). As a member of the phakomatoses, TSC is characterized by the appearance of benign tumors, most notably in the central nervous system, kidney, heart, lung, and skin. While classically described as "hamartomas," the pathology of the lesions has features suggestive of abnormal cellular proliferation, size, differentiation, and migration. Occasionally, tumors progress to become malignant (i.e., renal cell carcinoma). The genetic basis of this disease has been attributed to mutations in one of two unlinked genes, TSC1 and TSC2. Cells undergo bi-allelic inactivation of either gene to give rise to tumors in a classic tumor suppressor "two-hit" paradigm. The functions of the TSC1 and TSC2 gene products, hamartin and tuberin, respectively, have remained ill defined until recently. Genetic, biochemical, and biologic analyses have highlighted their role as negative regulators of the mTOR signaling pathway. Tuberin, serving as a substrate of AKT and AMPK, mediates mTOR activity by coordinating inputs from growth factors and energy availability in the control of cell growth, proliferation, and survival. Emerging evidence also suggests that the TSC 1/2 complex may play a role in modulating the activity of beta-catenin and TGFbeta. These findings provide novel functional links between the TSC genes and other tumor suppressors responsible for Cowden's disease (PTEN), Peutz-Jeghers syndrome (LKB1), and familial polyposis (APC). Common sporadic cancers such as prostate, lung, colon, endometrium, and breast have ties to these genes, highlighting the potential role of the TSC proteins in human cancers. Rapamycin, a specific mTOR inhibitor, has potent antitumoral activities in preclinical models of TSC and is currently undergoing phase I/II clinical studies.

Human RAS superfamily proteins and related GTPases

The tumor oncoproteins HRAS, KRAS, and NRAS are the founding members of a larger family of at least 35 related human proteins. Using a somewhat broader definition of sequence similarity reveals a more extended superfamily of more than 170 RAS-related proteins. The RAS superfamily of GTP (guanosine triphosphate) hydrolysis-coupled signal transduction relay proteins can be subclassified into RAS, RHO, RAB, and ARF families, as well as the closely related Galpha family. The members of each family can, in turn, be arranged into evolutionarily conserved branches. These groupings reflect structural, biochemical, and functional conservation. Recent findings have provided insights into the signaling characteristics of representative members of most RAS superfamily branches. The analysis presented here may serve as a guide for predicting the function of numerous uncharacterized superfamily members. Also described are guanosine triphosphatases (GTPases) distinct from members of the RAS superfamily. These related proteins employ GTP binding and GTPase domains in diverse structural contexts, expanding the scope of their function in humans.

Mouse models of tuberous sclerosis complex

The most devastating complications of tuberous sclerosis complex affect the central nervous system and include epilepsy, mental retardation, autism, and glial tumors. Mutations in one of two genes, TSC1 and TSC2, result in a similar disease phenotype by disrupting the normal interaction of their protein products, hamartin and tuberin, which form a functional signaling complex. Disruption of these genes in the brain results in abnormal cellular differentiation, migration, and proliferation, giving rise to characteristic brain lesions called cortical tubers. Relevant animal models, including conventional and conditional knockout mice, are valuable tools for studying the normal functions of tuberin and hamartin and how disruption of their expression gives rise to the variety of clinical features that characterize tuberous sclerosis complex. In the future, these animals will be invaluable preclinical models for the development of highly specific and efficacious treatments for children affected with tuberous sclerosis complex.

Biochemical and functional characterizations of small GTPase Rheb and TSC2 GAP activity

Tuberous sclerosis complex (TSC) is a genetic disease caused by a mutation in either the tsc1 or tsc2 tumor suppressor gene. Recent studies have demonstrated that TSC2 displays GAP (GTPase-activating protein) activity specifically towards the small G protein Rheb and inhibits its ability to stimulate the mTOR signaling pathway. Rheb and TSC2 comprise a unique pair of GTPase and GAP, because Rheb has high basal GTP levels and TSC2 does not have the catalytic arginine finger found in Ras-GAP. To investigate the function of TSC2 and Rheb in mTOR signaling, we analyzed the TSC2-stimulated Rheb GTPase activity. We found that Arg15, a residue equivalent to Gly12 in Ras, is important for Rheb to function as a substrate for TSC2 GAP. In addition, we identified asparagine residues essential for TSC2 GAP activity. We demonstrated a novel catalytic mechanism of the TSC2 GAP and Rheb that TSC2 uses a catalytic "asparagine thumb" instead of the arginine finger found in Ras-GAP. Furthermore, we discovered that farnesylation and membrane localization of Rheb is not essential for Rheb to stimulate S6 kinase (S6K) phosphorylation. Analysis of TSC1 binding defective mutants of TSC2 shows that TSC1 is not required for the TSC2 GAP activity but may function as a regulatory component in the TSC1/TSC2 complex. Our data further demonstrate that GAP activity is essential for the cellular function of TSC2 to inhibit S6K phosphorylation.

Induction of memory-associated immediate early genes by nerve growth factor in rat primary cortical neurons and differentiated mouse Neuro2A cells

Activation of several immediate early genes (IEGs) is crucial for long-term memory formation in vivo. In vitro methods of inducing these genes have not been investigated extensively. Here we present data demonstrating that application of the neurotrophin, nerve growth factor (NGF), to both rat primary neuronal cultures and differentiated mouse neuroblastoma 2A (N2A) cultures reliably induces expression of several IEGs, including Zif268, Nur77 and Arc, each of which have been linked to memory consolidation. These findings provide an in vitro model in which to test other agents that might modulate the induction of memory-associated genes.

Regulation of B-Raf kinase activity by tuberin and Rheb is mammalian target of rapamycin (mTOR)-independent

Tuberous sclerosis complex (TSC) is a tumor suppressor gene syndrome with manifestations that can include seizures, mental retardation, autism, and tumors in the brain, retina, kidney, heart, and skin. The products of the TSC1 and TSC2 genes, hamartin and tuberin, respectively, heterodimerize and inhibit the mammalian target of rapamycin (mTOR). We found that tuberin expression increases p42/44 MAPK phosphorylation and B-Raf kinase activity. Short interfering RNA down-regulation of tuberin decreased the p42/44 MAPK phosphorylation and B-Raf activity. Expression of Rheb, the target of the GTPase-activating domain of tuberin, inhibited wild-type B-Raf kinase but not activated forms of B-Raf. The interaction of endogenous Rheb with B-Raf was enhanced by serum and by Ras overexpression. A farnesylation-defective mutant of Rheb co-immunoprecipitated with and inhibited B-Raf but did not activate ribosomal protein S6 kinase, indicating that farnesylation is not required for B-Raf inhibition by Rheb and that B-Raf inhibition and S6 kinase activation are separable activities of Rheb. Consistent with this, inhibition of B-Raf and p42/44 MAPK by Rheb was resistant to rapamycin in contrast to Rheb activation of S6 kinase, which is rapamycin-sensitive. Taken together these data demonstrate that inhibition of B-Raf kinase via Rheb is an mTOR-independent function of tuberin.

Follicle-stimulating hormone activation of hypoxia-inducible factor-1 by the phosphatidylinositol 3-kinase/AKT/Ras homolog enriched in brain (Rheb)/mammalian target of rapamycin (mTOR) pathway is necessary for induction of select protein markers of follicular differentiation

We sought to elucidate the role of AKT in follicle-stimulating hormone (FSH)-mediated granulosa cell (GC) differentiation. Our results define a signaling pathway in GCs whereby the inactivating phosphorylation of tuberin downstream of phosphatidylinositol (PI) 3-kinase/AKT activity leads to Rheb (Ras homolog enriched in brain) and subsequent mTOR (mammalian target of rapamycin) activation. mTOR then stimulates translation by phosphorylating p70 S6 kinase and, consequently, the 40 S ribosomal protein S6. Activation of this pathway is required for FSH-mediated induction of several follicular differentiation markers, including luteinizing-hormone receptor (LHR), inhibin-alpha, microtubule-associated protein 2D, and the PKA type IIbeta regulatory subunit. FSH also promotes activation of the transcription factor hypoxia-inducible factor-1 (HIF-1). FSH-stimulated HIF-1 activity is inhibited by the PI 3-kinase inhibitor LY294002, the Rheb inhibitor FTI-277 (farnesyltransferase inhibitor-277), and the mTOR inhibitor rapamycin. Finally, we find that the FSH-mediated up-regulation of reporter activities for LHR, inhibin-alpha, and vascular endothelial growth factor is dependent upon HIF-1 activity, because a dominant negative form of HIF-1alpha interferes with the up-regulation of these genes. These results show that FSH enhances HIF-1 activity downstream of the PI 3-kinase/AKT/Rheb/mTOR pathway in GCs and that HIF-1 activity is necessary for FSH to induce multiple follicular differentiation markers.

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.

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.

High affinity for farnesyltransferase and alternative prenylation contribute individually to K-Ras4B resistance to farnesyltransferase inhibitors

Farnesyltransferase inhibitors (FTIs) block Ras farnesylation, subcellular localization and activity, and inhibit the growth of Ras-transformed cells. Although FTIs are ineffective against K-Ras4B, the Ras isoform most commonly mutated in human cancers, they can inhibit the growth of tumors containing oncogenic K-Ras4B, implicating other farnesylated proteins or suggesting distinct functions for farnesylated and for geranylgeranylated K-Ras, which is generated when farnesyltransferase is inhibited. In addition to bypassing FTI blockade through geranylgeranylation, K-Ras4B resistance to FTIs may also result from its higher affinity for farnesyltransferase. Using chimeric Ras proteins containing all combinations of Ras background, CAAX motif, and K-Ras polybasic domain, we show that either a polybasic domain or an alternatively prenylated CAAX renders Ras prenylation, Ras-induced Elk-1 activation, and anchorage-independent cell growth FTI-resistant. The polybasic domain alone increases the affinity of Ras for farnesyltransferase, implying independent roles for each K-Ras4B sequence element in FTI resistance. Using microarray analysis and colony formation assays, we confirm that K-Ras function is independent of the identity of the prenyl group and, therefore, that FTI inhibition of K-Ras transformed cells is likely to be independent of K-Ras inhibition. Our results imply that relevant FTI targets will lack both polybasic and potentially geranylgeranylated methionine-CAAX motifs.

Identification of dominant negative mutants of Rheb GTPase and their use to implicate the involvement of human Rheb in the activation of p70S6K

Rheb GTPases represent a unique family of the Ras superfamily of G-proteins. Studies on Rheb in Schizosaccharomyces pombe and Drosophila have shown that this small GTPase is essential and is involved in cell growth and cell cycle progression. The Drosophila studies also raised the possibility that Rheb is involved in the TOR/S6K signaling pathway. In this paper, we first report identification of dominant negative mutants of S. pombe Rheb (SpRheb). Screens of a randomly mutagenized SpRheb library yielded a mutant, SpRhebD60V, whose expression in S. pombe results in growth inhibition, G1 arrest, and induction of fnx1+, a gene whose expression is induced by the disruption of Rheb. Alteration of the Asp-60 residue to all possible amino acids by site-directed mutagenesis led to the identification of two particularly strong dominant negative mutants, D60I and D60K. Characterization of these dominant negative mutant proteins revealed that D60V and D60I exhibit preferential binding of GDP, while D60K lost the ability to bind both GTP and GDP. A possible use of the dominant negative mutants in the study of mammalian Rheb was explored by introducing dominant negative mutations into human Rheb. We show that transient expression of the wild type Rheb1 or Rheb2 causes activation of p70S6K, while expression of Rheb1D60K mutant results in inhibition of basal level activity of p70S6K. In addition, Rheb1D60K and Rheb1D60V mutants blocked nutrient- or serum-induced activation of p70S6K. This provides critical evidence that Rheb plays a role in the mTOR/S6K pathway in mammalian cells.