Glucose deprivation in tuberous sclerosis complex-related tumors

Background

Cancer cells possess unique metabolic phenotypes that are determined by their underlying oncogenic pathways. Activation of the PI3K/Akt/mTOR signaling cascade promotes glycolysis and leads to glucose-dependence in tumors. In particular, cells with constitutive mTORC1 activity secondary to the loss of TSC1/TSC2 function are prone to undergo apoptosis upon glucose withdrawal in vitro, but this concept has not been tested in vivo. This study examines the effects of restricting glucose metabolism by pharmacologic and dietary means in a tuberous sclerosis complex (TSC) tumor xenograft model.

Results

Tumor-bearing mice were randomly assigned to receive unrestricted carbohydrate-free ("Carb-free") or Western-style diet in the absence or presence of 2-deoxyglucose (2-DG) in one of four treatment groups. After 14 weeks, tumor sizes were significantly different among the four treatment groups with those receiving 2-DG having the smallest tumors. Unexpectedly, the "Carb-free" diet was associated with the largest tumors but they remained responsive to 2-DG. PET imaging showed significant treatment-related changes in tumor ¹⁸fluorodeoxyglucose-uptake but the standard uptake values did not correlate with tumor size. Alternative energy substrates such as ketone bodies and monounsaturated oleic acid supported the growth of the Tsc2-/- cells in vitro, whereas saturated palmitic acid was toxic. Correspondingly, tumors in the high-fat, "Carb-free" group showed greater necrosis and liquefaction that contributed to their larger sizes. In contrast, 2-DG treatment significantly reduced tumor cell proliferation, increased metabolic stress (i.e., ketonemia) and AMPK activity, whereas rapamycin primarily reduced cell size.

Conclusions

Our data support the concept of glycolytic inhibition as a therapeutic approach in TSC whereas dietary withdrawal of carbohydrates was not effective.

Tuberous sclerosis complex-1 deficiency attenuates diet-induced hepatic lipid accumulation

Non-alcoholic fatty liver disease (NAFLD) is causally linked to type 2 diabetes, insulin resistance and dyslipidemia. In a normal liver, insulin suppresses gluconeogenesis and promotes lipogenesis. In type 2 diabetes, the liver exhibits selective insulin resistance by failing to inhibit hepatic glucose production while maintaining triglyceride synthesis. Evidence suggests that the insulin pathway bifurcates downstream of Akt to regulate these two processes. Specifically, mTORC1 has been implicated in lipogenesis, but its role on hepatic steatosis has not been examined. Here, we generated mice with hepatocyte-specific deletion of Tsc1 to study the effects of constitutive mTORC1 activation in the liver. These mice developed normally but displayed mild hepatomegaly and insulin resistance without obesity. Unexpectedly, the Tsc1-null livers showed minimal signs of steatosis even under high-fat diet condition. This 'resistant' phenotype was reversed by rapamycin and could be overcome by the expression of Myr-Akt. Moreover, rapamycin failed to reduce hepatic triglyceride levels in models of steatosis secondary to Pten ablation in hepatocytes or high-fat diet in wild-type mice. These observations suggest that mTORC1 is neither necessary nor sufficient for steatosis. Instead, Akt and mTORC1 have opposing effects on hepatic lipid accumulation such that mTORC1 protects against diet-induced steatosis. Specifically, mTORC1 activity induces a metabolic shift towards fat utilization and glucose production in the liver. These findings provide novel insights into the role of mTORC1 in hepatic lipid metabolism.

Tuberin regulates E-cadherin localization: implications in epithelial-mesenchymal transition

The tuberous sclerosis complex 2 (TSC2) gene encodes the protein tuberin, which functions as a key negative regulator of both mammalian target of rapamycin (mTOR) C1-dependent cell growth and proliferation. Loss-of-function mutations of TSC2 result in mTORC1 hyperactivity and predispose individuals to both tuberous sclerosis and lymphangioleiomyomatosis. These overlapping diseases have in common the abnormal proliferation of smooth muscle-like cells. Although the origin of these cells is unknown, accumulating evidence suggests that a metastatic mechanism may be involved, but the means by which the mTOR pathway contributes to this disease process remain poorly understood. In this study, we show that tuberin regulates the localization of E-cadherin via an Akt/mTORC1/CLIP170-dependent, rapamycin-sensitive pathway. Consequently, Tsc2(-/-) epithelial cells display a loss of plasma membrane E-cadherin that leads to reduced cell-cell adhesion. Under confluent conditions, these cells detach, grow in suspension, and undergo epithelial-mesenchymal transition (EMT) that is marked by reduced expression levels of both E-cadherin and occludin and increased expression levels of both Snail and smooth muscle actin. Functionally, the Tsc2(-/-) cells demonstrate anchorage-independent growth, cell scattering, and anoikis resistance. Human renal angiomyolipomas and lymphangioleiomyomatosis also express markers of EMT and exhibit an invasive phenotype that can be interpreted as consistent with EMT. Together, these results suggest a novel relationship between TSC2/mTORC1 and the E-cadherin pathways and implicate EMT in the pathogenesis of tuberous sclerosis complex-related diseases.

The loss of tuberin promotes cell invasion through the ß-catenin pathway

Mutations in the tumor suppressor tuberin (TSC2) are a common factor in the development of lymphangioleiomyomatosis (LAM). LAM is a cystic lung disease that is characterized by the infiltration of smooth muscle-like cells into the pulmonary parenchyma. The mechanism by which the loss of tuberin promotes the development of LAM has yet to be elucidated, although several lines of evidence suggest it is due to the metastasis of tuberin-deficient cells. Here we show that tuberin-null cells become nonadherent and invasive. These nonadherent cells express cleaved forms of β-catenin. In reporter assays, the β-catenin products are transcriptionally active and promote MMP7 expression. Invasion by the tuberin-null cells is mediated by MMP7. Examination of LAM tissues shows the expression of cleaved β-catenin products and MMP7 consistent with a model that tuberin-deficient cells acquire invasive properties through a β-catenin-dependent mechanism, which may underlie the development of LAM.

The tuberous sclerosis complex regulates trafficking of glucose transporters and glucose uptake

Human cancers often display an avidity for glucose, a feature that is exploited in clinical staging and response monitoring by using (18)F-fluoro-deoxyglucose (FDG) positron emission tomography. Determinants of FDG accumulation include tumor blood flow, glucose transport, and glycolytic rate, but the underlying molecular mechanisms are incompletely understood. The phosphoinositide-3 kinase/Akt/mammalian target of rapamycin complex (mTORC) 1 pathway has been implicated in this process via the hypoxia-inducible factor alpha-dependent expression of vascular endothelial growth factor and glycolytic enzymes. Thus, we predicted that tumors with elevated mTORC1 activity would be accompanied by high FDG uptake. We tested this hypothesis in eight renal angiomyolipomas in which the loss of tuberous sclerosis complex (TSC) 1/2 function gave rise to constitutive mTORC1 activation. Surprisingly, these tumors displayed low FDG uptake on positron emission tomography. Exploring the underlying mechanisms in vitro revealed that Tsc2 regulates the membrane localization of the glucose transporter proteins (Glut)1, Glut2, and Glut4, and, therefore, glucose uptake. Down-regulation of cytoplasmic linker protein 170, an mTOR effector, rescued Glut4 trafficking in Tsc2(-/-) cells, whereas up-regulation of Akt activity in these cells was insufficient to redistribute Glut4 to the plasma membrane. The effect of mTORC1 on glucose uptake was confirmed using a liver-specific Tsc1- deletion mouse model in which FDG uptake was reduced in the livers of mutant mice compared with wild-type controls. Together, these data show that mTORC1 activity is insufficient for increased glycolysis in tumors and that constitutive mTOR activity negatively regulates glucose transporter trafficking.

Tuberous sclerosis complex 2 loss-of-function mutation regulates reactive oxygen species production through Rac1 activation

The products of the TSC1 (hamartin) and TCS2 (tuberin) tumor suppressor genes negatively regulate cell growth by inhibiting mTOR signaling. Recent research has led to the postulation that tuberin and/or hamartin are involved in tumor migration, presumably through Rho activation. Here we show that LEF-8 cells, which contain a Y1571 missense mutation in tuberin, express higher Rac1 activity than tuberin negative and positive cells. We also provide evidence of obvious lamellipodia formation in LEF-8 cells. Since the production of TSC2(Y1571H) cannot form a hetero-complex with hamartin, we further analyzed another mutant, TSC2(R611Q), which also lacks the ability to form a complex with hamartin. Introducing both forms of mutated TSC2 into COS-1 cells increased Rac1 activity as well as cell motility. We also found these two mutants interacted with Rac1. We further demonstrated that the introduction of mutated TSC2 into COS-1 cells can generate higher reactive oxygen species (ROS). These results indicate that loss-of-function mutated tuberin can activate Rac1 and thereby increase ROS production.

Activation of the mTOR pathway in sporadic angiomyolipomas and other perivascular epithelioid cell neoplasms

Angiomyolipoma (AML) belong to a family of tumors known as perivascular epithelioid cell tumors (PEComas) that share a common immunophenotypic profile of muscle and melanocytic differentiation. These tumors are clonal in nature and have a strong association with tuberous sclerosis. Genetic analyses have reported allelic imbalance at the TSC2 locus on 16p13. In the context of non-tuberous sclerosis complex (TSC), non-lymphangioleiomyomatosis-associated AMLs, and non-renal PEComas, the functional status of the TSC2 signaling pathway has not been reported. Studies over the last several years have uncovered a critical role of the TSC1/2 genes in negatively regulating the Rheb/mTOR/p70S6K cascade. Here, we examined the activity of this pathway in sporadic AMLs and PEComas using immunohistochemical and biochemical analyses. We found increased levels of phospho-p70S6K, a marker of mTOR activity, in 15 of 15 non-TSC AMLs. This was accompanied by reduced phospho-AKT expression, a pattern that is consistent with the disruption of TSC1/2 function. Western blot analysis confirmed mTOR activation concurrent with the loss of TSC2 and not TSC1 in sporadic AMLs. Similarly, elevated phospho-p70S6K and reduced phospho-AKT expression was detected in 14 of 15 cases of extrarenal PEComas. These observations provide the first functional evidence that mTOR activation is common to sporadic, non-TSC-related AMLs and PEComas. This suggests the possibility that mTOR inhibitors such as rapamycin may be therapeutic for this class of disease.

Latent Kaposi's sarcoma-associated herpesvirus infection of endothelial cells activates hypoxia-induced factors

Kaposi's sarcoma-associated herpesvirus (KSHV or HHV-8) is the etiological agent of Kaposi's sarcoma, a highly vascularized, endothelial-derived tumor. A direct role for KSHV-mediated induction of angiogenesis has been proposed based upon the nature of the neoplasia and various KSHV gene overexpression and infection model systems. We have found that KSHV infection of endothelial cells induces mRNA of hypoxia-induced factor 1alpha (HIF1alpha) and HIF2alpha, two homologous alpha subunits of the heterodimeric transcription factor HIF. HIF is a master regulator of both developmental and pathological angiogenesis, composed of an oxygen-sensitive alpha subunit and a constitutively expressed beta subunit. HIF is classically activated posttranscriptionally with hypoxia, leading to increased protein stability of HIF1alpha and/or HIF2alpha. However, we demonstrate that both alpha subunits are up-regulated at the transcript level by KSHV infection. The transcriptional activation of HIF leads to a functional increase in HIF activity under normoxic conditions, as demonstrated by both luciferase reporter assay and the increased expression of vascular endothelial growth factor receptor 1 (VEGFR1), an HIF-responsive gene. KSHV infection synergizes with hypoxia mimics and induces higher expression levels of HIF1alpha and HIF2alpha protein, and HIF1alpha is increased in a significant proportion of the latently infected endothelial cells. Src family kinases are required for the activation of HIF and the downstream gene VEGFR1 by KSHV. We also show that KS lesions, in vivo, express elevated levels of HIF1alpha and HIF2alpha proteins. Thus, KSHV stimulates the HIF pathway via transcriptional up-regulation of both HIF alphas, and this activation may play a role in KS formation, localization, and progression.

Regulation of Microtubule-Dependent Protein Transport by the TSC2/Mammalian Target of Rapamycin Pathway

Protein transport plays a critical role in the interaction of the cell with its environment. Recent studies have identified TSC1 and TSC2, two tumor suppressor genes involved in tuberous sclerosis complex, as regulators of the mammalian target of rapamycin (mTOR) pathway. Cells deficient in TSC1 or TSC2 possess high levels of Rheb-GTP resulting in constitutive mTOR activation. We have shown previously that the TSC1/TSC2 complex is involved in post-Golgi transport of VSVG and caveolin-1 in mammalian cells. Here, we show that modulation of mTOR activity affects caveolin-1 localization and that this effect is independent of p70S6K. Tsc1- and Tsc2-null cells exhibit abnormal caveolin-1 localization that is accompanied by disorganized microtubules in the subcortical region. Analyses of green fluorescent protein-EB1 and tubulin in live mutant cells suggest a failure of the plus-ends to sense cortical signals and to halt microtubule growth. Down-regulation of CLIP-170, a putative mTOR substrate with microtubule-binding properties, rescued the abnormal microtubule arrangement and caveolin-1 localization in Tsc2-/- cells. Together, these findings highlight a novel role of the TSC2/mTOR pathway in regulating microtubule-dependent protein transport.

Aberrant beta-catenin signaling in tuberous sclerosis

The pathology associated with tuberous sclerosis complex (TSC) shows diverse phenotypes that suggest abnormal signaling of multiple pathways. Besides the negative regulatory role of the TSC1/TSC2 proteins on mTOR, we have reported an effect on beta-catenin signaling at the level of the degradation complex in vitro. The TSC1/TSC2 complex associates with GSK3 and Axin and promotes beta-catenin degradation to inhibit Wnt-stimulated TCF/LEF-dependent transcription. Here, we show that beta-catenin and its effectors, cyclin D1 and connexin 43, were up-regulated in TSC-related angiomyolipomas and lymphangioleiomyomatosis. This was supported by the failure of three disease-causing TSC2 missense mutants to inhibit Wnt signaling. Further, the interaction between TSC1/TSC2 and components of the beta-catenin degradation complex was dependent on Wnt stimulation such that binding of tuberin to GSK3 and Axin was reduced in the presence of Wnt whereas the tuberin-Dishevelled interaction was increased. GSK3 activity played a role in regulating the assembly/stability of the degradation complex. Inhibition of GSK3 by lithium chloride reduced its association with TSC1 whereas disruption of GSK3-phosphorylation sites in TSC1 reduced interaction between TSC2 and TSC1. Collectively, our data provide further evidence that beta-catenin signaling plays a role in TSC pathogenesis in vivo and suggest a novel role of GSK3 in modulating the TSC1/TSC2 complex through TSC1 phosphorylation.

Lessons from the Eker rat model: from cage to bedside

Rodent models of human diseases serve a vital role in translating bench observations to bedside therapies. In vivo manipulation of these animals allows us to explore the biologic significance of the underlying molecular and biochemical pathways. The study of human cancers has been highly enriched by the observations made from numerous transgenic mouse models. Long before the techniques of genetic engineering were discovered, Dr. Reidar Eker described one of the earliest examples of an autosomal dominant model of renal tumors in a unique strain of rats. They were used in the 1980's by Alfred Knudson to validate the "two-hit" hypothesis and to study the multi-step process of carcinogenesis. Following the identification of the Tsc2 germline mutation in the Eker rat, it became the first rodent model of tuberous sclerosis and has since been exploited in many areas of tumor biology as illustrated in the content of this issue. The focus of our review is to highlight the contribution of the Eker rat towards understanding the Tsc2 signaling pathways in tumorigenesis and evaluating potential therapeutics in the pre-clinical setting.

Effects of rapamycin in the Eker rat model of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) presents in the pediatric population with a constellation of benign tumors that affect the brain, heart, kidney, lung, and skin. No therapy has been shown to halt disease progression or to prevent its onset. The pathogenesis of TSC stems from the inactivation of one of the two TSC genes, TSC1 and TSC2. A key function of these genes is to regulate the mammalian target of rapamycin (mTOR) pathway in response to cellular energy and nutrient and growth factor availability. Consequently, TSC-related tumors exhibit uncontrolled activation of mTOR and its effectors. Previous work has shown that a specific mTOR inhibitor, rapamycin, effectively down-regulated mTOR activity in renal tumors of Eker rats that carry a germline Tsc2 mutation. Using this model, we investigated the effects of rapamycin on pituitary and renal tumors. We observed that rats with pituitary tumors had significantly shorter survival than those without pituitary pathology. Treatment with rapamycin effectively improved their clinical state and prolonged their survival. Rapamycin also resulted in a significant decrease in the size of the Tsc2-related renal tumors. In both types of pathology, tumor response was accompanied by down-regulation of ribosomal S6 kinase activity, reduction in cell size, and induction of apoptosis. Evidence for drug resistance was found in a small percentage of lesions after prolonged therapy. When rapamycin was given before onset of disease, subsequent development of macroscopic renal tumors was reduced, but no effect on the number of microscopic precursor lesions was found. We conclude that rapamycin-sensitive mTOR activity was critical to tumor progression in the Eker rat model, but rapamycin is unlikely to eradicate all disease as a result of the development of drug resistance. Our data also suggest the role of a rapamycin-insensitive pathway during tumor initiation.

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.

Morphology of cerebral lesions in the Eker rat model of tuberous sclerosis

Tuberous sclerosis (TSC) is an autosomal dominant disorder, caused by mutations of either the TSC1 or TSC2 gene. Characteristic brain pathologies (including cortical tubers and subependymal hamartomas/giant astrocytomas) are thought to cause epilepsy, as well as other neurological dysfunction. The Eker rat, which carries a spontaneous germline mutation of the TSC2 gene (TSC2+/-), provides a unique animal model in which to study the relationship between TSC cortical pathologies and epilepsy. In the present study, we have analyzed the seizure propensity and histopathological features of a modified Eker rat preparation, in which early postnatal irradiation was employed as a "second hit" stimulus in an attempt to exacerbate cortical malformations and increase seizure propensity. Irradiated Eker rats had a tendency toward lower seizure thresholds (latencies to flurothyl-induced seizures) than seen in non-irradiated Eker rats (significant difference) or irradiated wild-type rats (non-significant difference). The majority of irradiated Eker rats exhibited dysplastic cytomegalic neurons and giant astrocyte-like cells, similar to cytopathologies observed in TSC lesions of patients. The most prominent features in these brains were hamartoma-like lesions involving large eosinophilic cells, similar to giant tuber cells in human TSC. In some cells from these hamartomas, immunocytochemistry revealed features of both neuronal and glial phenotypes, suggesting an undifferentiated or immature cell population. Both normal-appearing and dysmorphic neurons, as well as cells in the hamartomas, exhibited immunopositivity for tuberin, the protein product of the TSC2 gene.

The Role of Tumor Ablation in Bridging Patients to Liver Transplantation

HYPOTHESIS

Treatment of hepatocellular carcinoma before liver transplantation can curb local tumor progression and thereby prolong patients' transplantation eligibility.

DESIGN

Retrospective case-control pilot study. Twelve of 39 patients receiving liver transplantation for hepatocellular carcinoma had treatment before transplantation. Pretreatment included radiofrequency ablation (n = 8), percutaneous ethanol injection (n = 2), both modalities (n = 1), and tumor resection (n = 1). Twelve control subjects without pretreatment who were age-, sex-, and score-matched on the Model for End-stage Liver Disease and Child-Turcotte-Pugh classification were selected. The primary outcome measure was the waiting period for transplantation.

RESULTS

Patients with pretreatment waited on the transplant list significantly longer than their counterparts without pretreatment (median, 484 vs 253 days; P =.03).

CONCLUSIONS

Treatment before transplantation with tumor ablation or resection is associated with a longer waiting period on the transplant list. This enables patients who might otherwise be removed from the list because of tumor progression to receive transplantation.

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

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

Multiple roles of the tuberous sclerosis complex genes

Soon after proposing the "two-hit" hypothesis for tumorigenesis, Knudson pursued further experimental validation of the concept by using a rat model of dominantly inherited renal tumor. Today, the Eker rat is one of the best characterized models of tuberous sclerosis complex (TSC) and has been used extensively for study of the function of the TSC2 tumor suppressor gene. Along with TSC1, these two genes behave as expected for tumor suppressor genes with evidence for loss of heterozygosity in tumors and suppression of growth when expressed in proliferating cells. Despite much experimental work, the mechanisms of these genes have remained elusive until recently. This review summarizes some of the current concepts in our understanding of the biological and biochemical function of the TSC genes.

The tuberin-hamartin complex negatively regulates beta-catenin signaling activity

Tuberous sclerosis complex (TSC) is characterized by the formation of hamartomas in multiple organs resulting from mutations in the TSC1 or TSC2 gene. Their protein products, hamartin and tuberin, respectively, form a functional complex that affects cell growth, differentiation, and proliferation. Several lines of evidence, including renal tumors derived from TSC2+/- animals, suggest that the loss or inhibition of tuberin is associated with up-regulation of cyclin D1. As cyclin D1 can be regulated through the canonical Wnt/beta-catenin signaling pathway, we hypothesize that the cell proliferative effects of hamartin and tuberin are partly mediated through beta-catenin. In this study, total beta-catenin protein levels were found to be elevated in the TSC2-related renal tumors. Ectopic expression of hamartin and wild-type tuberin, but not mutant tuberin, reduced beta-catenin steady-state levels and its half-life. The TSC1-TSC2 complex also inhibited Wnt-1 stimulated Tcf/LEF luciferase reporter activity. This inhibition was eliminated by constitutively active beta-catenin but not by Disheveled, suggesting that hamartin and tuberin function at the level of the beta-catenin degradation complex. Indeed, hamartin and tuberin co-immunoprecipitated with glycogen synthase kinase 3 beta and Axin, components of this complex in a Wnt-1-dependent manner. Our data suggest that hamartin and tuberin negatively regulate beta-catenin stability and activity by participating in the beta-catenin degradation complex.

Tuberous sclerosis as an underlying basis for infantile spasm

The study of the molecular pathogenesis of epilepsy in tuberous sclerosis has taken on a new dimension with the identification of the TSC1 and TSC2 genes. While the development of seizures is ultimately related to mutations in one of the two genes, the mechanism underlying the genotype-phenotype relationship remains a puzzle. This chapter, presented arguments in favor of the hypothesis that abnormal cortical excitability originates in and around focal areas of structural malformations (i.e., cortical tubers and dysplasia) and that these "lesions" are the biologic consequences of tuberin and/or hamartin dysfunction. This model relies on the concept of a multistep process occurring early in cortical development whereby certain progenitor cells in the germinal layer of the ventricular zone destined for the cortex undergo inactivation of the TSC1 or TSC2 locus (Fig. 2). Immature neuroepithelial cells carrying "two-hit" mutations at either locus are believed to proliferate, migrate, and differentiate abnormally, resulting in the formation of "dysplastic" cells that are heterotopic in distribution. The pathology of the classic tuber suggests a clonal expansion of the bizarre-appearing giant cells that display incomplete, multilineage, and often ambiguous phenotype. Further, they infiltrate the six-layered structure of the cortex to form a poorly circumscribed area containing a mixture of cell types to create a highly disorganized region of a neuronal and glial network. Whether arising from the dysplastic "two-hit" target cells themselves or adjacent "innocent" bystander neurons as a result of aberrant cell-cell interaction, abnormal epileptic discharges originate from these structural abnormalities. The mechanism of how TSC1 and TSC2 inactivation causes tuber to develop is not known, but emerging experimental evidence suggests a disruption of the hamartin-tuberin "haloenzyme" in the regulation of cell size and number via the insulin signaling pathway and a p27/CDK-dependent mechanism. Biochemically, TSC1/TSC2 may associate with cytoskeletal components and vesicular adaptors in regulating sorting and trafficking of newly synthesized and recycling proteins in the post-Golgi compartments. As such, spatial and temporal localization of proteins may be affected in tuberin or hamartin-deficient neuronal cells where proper synaptic delivery of neurotransmitters plays an important role in normal cerebral function. We are in the earliest stages of understanding the role of TSC genes in epileptogenesis. To test the hypothesis outlined earlier, there is a need to create in vitro and in vivo models, as direct human experimentation is not feasible. To date, there are several rodent models of TSC, both spontaneous and recombinant strains. Unfortunately, none has consistently developed spontaneous cortical tubers, although one example was reported in an otherwise asymptomatic Eker rat (Mizuguchi et al., 2000). If the "two-hit" hypothesis is operational in tubers, as seen in other TSC lesions, it follows that radiation and chemical carcinogens should have a quantitative and qualitative effect on the development of these cerebral malformations. In preliminary experiments, we have found evidence of areas of cortical dysplasia in Eker rats irradiated early in life (Fig. 3). These dysplastic [figure: see text] cells stained positively with NeuN, consistent with the immunophenotype of cells in tubers. Alternatively, one can analyze the in vivo and in vitro characteristics of neuroprogenitor cells that are deficient of hamartin or tuberin. While homozygous mutants of TSC1 and TSC2 are lethal during midgestation, one of several techniques can be used to derive mutant neuroepithelial cells, including the procurement of -/- cells prior to embryonic deaths and subsequent cortical transplantation into syngeneic animals, development of conditional "knock outs," or chimeric mutants. These approaches, with their unique advantages and disadvantages, will be helpful in gaining insights into the development of cortical tubers and their electrophysiologic consequences.

Activated mammalian target of rapamycin pathway in the pathogenesis of tuberous sclerosis complex renal tumors

Disruption of the TSC1 or TSC2 gene leads to the development of tumors in multiple organs, most commonly affecting the kidney, brain, lung, and heart. Recent genetic and biochemical studies have identified a role for the tuberous sclerosis gene products in phosphoinositide 3-kinase signaling. On growth factor stimulation, tuberin, the TSC2 protein, is phosphorylated by Akt, thereby releasing its inhibitory effects on p70S6K. Here we demonstrate that primary tumors from tuberous sclerosis complex (TSC) patients and the Eker rat model of TSC expressed elevated levels of phosphorylated mammalian target of rapamycin (mTOR) and its effectors: p70S6K, S6 ribosomal protein, 4E-BP1, and eIF4G. In the Eker rat, short-term inhibition of mTOR by rapamycin was associated with a significant tumor response, including induction of apoptosis and reduction in cell proliferation. Surprisingly, these changes were not accompanied by significant alteration in cyclin D1 and p27 levels. Our data provide in vivo evidence that the mTOR pathway is aberrantly activated in TSC renal pathology and that treatment with rapamycin appears effective in the preclinical setting.

Phosphatidylinositol 3-kinase/Akt pathway regulates tuberous sclerosis tumor suppressor complex by phosphorylation of tuberin

Normal cellular functions of hamartin and tuberin, encoded by the TSC1 and TSC2 tumor suppressor genes, are closely related to their direct interactions. However, the regulation of the hamartin-tuberin complex in the context of the physiologic role as tumor suppressor genes has not been documented. Here we show that insulin or insulin growth factor (IGF) 1 stimulates phosphorylation of tuberin, which is inhibited by the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 but not by the mitogen-activated protein kinase inhibitor PD98059. Expression of constitutively active PI3K or active Akt, including Akt1 and Akt2, induces tuberin phosphorylation. We further demonstrate that Akt/PKB associates with hamartin-tuberin complexes, promoting phosphorylation of tuberin and increased degradation of hamartin-tuberin complexes. The ability to form complexes, however, is not blocked. Akt also inhibits tuberin-mediated degradation of p27(kip1), thereby promoting CDK2 activity and cellular proliferation. Our results indicate that tuberin is a direct physiological substrate of Akt and that phosphorylation of tuberin by PI3K/Akt is a major mechanism controlling hamartin-tuberin function.

Tsc tumour suppressor proteins antagonize amino-acid-TOR signalling

Target of Rapamycin (TOR) mediates a signalling pathway that couples amino acid availability to S6 kinase (S6K) activation, translational initiation and cell growth. Here, we show that tuberous sclerosis 1 (Tsc1) and Tsc2, tumour suppressors that are responsible for the tuberous sclerosis syndrome, antagonize this amino acid-TOR signalling pathway. We show that Tsc1 and Tsc2 can physically associate with TOR and function upstream of TOR genetically. In Drosophila melanogaster and mammalian cells, loss of Tsc1 and Tsc2 results in a TOR-dependent increase of S6K activity. Furthermore, although S6K is normally inactivated in animal cells in response to amino acid starvation, loss of Tsc1-Tsc2 renders cells resistant to amino acid starvation. We propose that the Tsc1-Tsc2 complex antagonizes the TOR-mediated response to amino acid availability. Our studies identify Tsc1 and Tsc2 as regulators of the amino acid-TOR pathway and provide a new paradigm for how proteins involved in nutrient sensing function as tumour suppressors.

Tuberin regulates p70 S6 kinase activation and ribosomal protein S6 phosphorylation. A role for the TSC2 tumor suppressor gene in pulmonary lymphangioleiomyomatosis (LAM)

Although the cellular functions of TSC2 and its protein product, tuberin, are not known, somatic mutations in the TSC2 tumor suppressor gene are associated with tumor development in lymphangioleiomyomatosis (LAM). We found that ribosomal protein S6 (S6), which exerts translational control of protein synthesis and is required for cell growth, is hyperphosphorylated in the smooth muscle-like cell lesions of LAM patients compared with smooth muscle cells from normal human blood vessels and trachea. Smooth muscle (SM) cells derived from these lesions (LAMD-SM) also exhibited S6 hyperphosphorylation, constitutive activation of p70 S6 kinase (p70S6K), and increased basal DNA synthesis. In parallel, TSC2-/- smooth muscle cells (ELT3) and TSC2-/- epithelial cells (ERC15) also exhibited hyperphosphorylation of S6, constitutive activation of p70S6K, and increased basal DNA synthesis. Re-introduction of wild type tuberin into LAMD-SM, ELT3, and ERC15 cells abolished phosphorylation of S6 and significantly inhibited p70S6K activity and DNA synthesis. Rapamycin, an immunosuppressant, inhibited hyperphosphorylation of S6, p70S6K activation, and DNA synthesis in LAMD-SM cells. Interestingly, the basal levels of phosphatidylinositol 3-kinase, Akt/protein kinase B, and p42/p44 MAPK activation were unchanged in LAMD-SM and ELT3 cells relative to levels in normal human tracheal and vascular SM. These data demonstrate that tuberin negatively regulates the activity of S6 and p70S6K specifically, and suggest a potential mechanism for abnormal cell growth in LAM.