Tuberous sclerosis gene products in proliferation control

De Novo Transcriptome Assembly and Analysis of Longevity Genes Using Subterranean Termite (Reticulitermes chinensis) Castes

The longevity phenomenon is entirely controlled by the insulin signaling pathway (IIS-pathway). Both vertebrates and invertebrates have IIS-pathways that are comparable to one another, though no one has previously described de novo transcriptome assembly of IIS-pathway-associated genes in termites. In this research, we analyzed the transcriptomes of both reproductive (primary kings “PK” and queens “PQ”, secondary worker reproductive kings “SWRK” and queens “SWRQ”) and non-reproductive (male “WM” and female “WF” workers) castes of the subterranean termite Reticulitermes chinensis. The goal was to identify the genes responsible for longevity in the reproductive and non-reproductive castes. Through transcriptome analysis, we annotated 103,589,264 sequence reads and 184,436 (7G) unigenes were assembled, GC performance was measured at 43.02%, and 64,046 sequences were reported as CDs sequences. Of which 35 IIS-pathway-associated genes were identified, among 35 genes, we focused on the phosphoinositide-dependent kinase-1 (Pdk1), protein kinase B2 (akt2-a), tuberous sclerosis-2 (Tsc2), mammalian target of rapamycin (mTOR), eukaryotic translation initiation factor 4E (EIF4E) and ribosomal protein S6 (RPS6) genes. Previously these genes (Pdk1, akt2-a, mTOR, EIF4E, and RPS6) were investigated in various organisms, that regulate physiological effects, growth factors, protein translation, cell survival, proliferation, protein synthesis, cell metabolism and survival, autophagy, fecundity rate, egg size, and follicle number, although the critical reason for longevity is still unclear in the termite castes. However, based on transcriptome profiling, the IIS-pathway-associated genes could prolong the reproductive caste lifespan and health span. Therefore, the transcriptomic shreds of evidence related to IIS-pathway genes provide new insights into the maintenance and relationships between biomolecular homeostasis and remarkable longevity. Finally, we propose a strategy for future research to decrypt the hidden costs associated with termite aging in reproductive and non-reproductive castes.

Exosome Release by Glucose Deprivation Is Important for the Viability of TSC-Null Cells

The control of exosome release is associated with numerous physiological and pathological activities, and that release is often indicative of health, disease, and environmental nutrient stress. Tuberous sclerosis complex (TSC) regulates the cell viability via the negative regulation of the mammalian target of rapamycin complex (mTORC1) during glucose deprivation. However, the mechanism by which viability of TSC-null cells is regulated by mTORC1 inhibition under glucose deprivation remains unclear. Here, we demonstrated that exosome release regulates cell death induced by glucose deprivation in TSC-null cells. The mTORC1 inhibition by rapamycin significantly increased the exosome biogenesis, exosome secretion, and cell viability in TSC-null cells. In addition, the increase in cell viability by mTORC1 inhibition was attenuated by two different types of inhibitors of exosome release under glucose deprivation. Taken together, we suggest that exosome release inhibition might be a novel way for regression of cell growth in TSC-null cells showing lack of cell death by mTORC1 inhibition.

Leucyl-tRNA synthetase 1 is required for proliferation of TSC-null cells

Uncontrolled cell proliferation associated with cancer depends on the functional abrogation of at least one of tumor suppressor. In response to nutrient cue, tuberous sclerosis complex (TSC) works as a tumor suppressor which inhibits cell growth via negative regulation of the mammalian target of rapamycin complex (mTORC1). However, the regulation mechanism of nutrient-dependent cell proliferation in TSC-null cells remains unclear. Here, we demonstrate that leucine is required for cell proliferation through the activation of leucyl-tRNA synthetase (LARS1)-mTORC1 pathway in TSC-null cells. Cell proliferation and survival were attenuated by LARS1 knock-down or inhibitors in TSC-null cells. In addition, either rapamycin or LARS1 inhibitors significantly decreased colony formation ability while their combined treatment drastically attenuated it. Taken together, we suggest that LARS1 inhibitors might considered as novel tools for the regression of tumor growth and proliferation in TSC-null tumor cells which regrow upon discontinuation of the mTORC1 inhibition.

Brain Symptoms of Tuberous Sclerosis Complex: Pathogenesis and Treatment

The mammalian target of the rapamycin (mTOR) system plays multiple, important roles in the brain, regulating both morphology, such as cellular size, shape, and position, and function, such as learning, memory, and social interaction. Tuberous sclerosis complex (TSC) is a congenital disorder caused by a defective suppressor of the mTOR system, the TSC1/TSC2 complex. Almost all brain symptoms of TSC are manifestations of an excessive activity of the mTOR system. Many children with TSC are afflicted by intractable epilepsy, intellectual disability, and/or autism. In the brains of infants with TSC, a vicious cycle of epileptic encephalopathy is formed by mTOR hyperactivity, abnormal synaptic structure/function, and excessive epileptic discharges, further worsening epilepsy and intellectual/behavioral disorders. Molecular target therapy with mTOR inhibitors has recently been proved to be efficacious for epilepsy in human TSC patients, and for autism in TSC model mice, indicating the possibility for pharmacological treatment of developmental synaptic disorders.

Pulmonary Lymphangioleiomyomatosis originates in the pleural mesothelial cell population

Lymphangioleiomyomatosis (LAM) is a cystic lung disease mainly affecting women, in which degradation of the lung parenchyma is associated with a cell of unknown provenance, known as a LAM cell. LAM cells carry TSC2 mutations and can be identified in the lung parenchyma by their expression of both smooth muscle actin and antigens characteristic of melanocytes and melanocytic tumors. The nature of the cell-of-origin of LAM is controversial, and despite continued research effort remains elusive. Further, it has not been possible to culture pulmonary LAM cells in vitro, and current research relies on cells and animal models which may not recapitulate all features of the disease. We noted aberrant expression of melanoma antigens in pleural mesothelial cells in lung tissue from LAM patients, indicating that these cells could be the precursors of parenchymal LAM cells. We hypothesise that loss of tuberin function following TSC2 mutation in the mesothelial cell lineage gives rise to the cell-of-origin of pulmonary LAM (P-LAM), and of other associated conditions commonly noted in LAM patients. The unique properties of mesothelial cells provide a straightforward explanation of the diverse presentation of LAM.

TSC loss distorts DNA replication programme and sensitises cells to genotoxic stress

Tuberous Sclerosis (TSC) is characterized by exorbitant mTORC1 signalling and manifests as non-malignant, apoptosis-prone neoplasia. Previous reports have shown that TSC^-/- cells are highly susceptible to mild, innocuous doses of genotoxic stress, which drive TSC^-/- cells into apoptotic death. It has been argued that this hypersensitivity to stress derives from a metabolic/energetic shortfall in TSC^-/- cells, but how metabolic dysregulation affects the DNA damage response and cell cycle alterations in TSC^-/- cells exposed to genotoxic stress is not understood. We report here the occurrence of futile checkpoint responses and an unusual type of replicative stress (RS) in TSC1^-/- fibroblasts exposed to low-dose genotoxins. This RS is characterized by elevated nucleotide incorporation rates despite only modest origin over-firing. Strikingly, an increased propensity for asymmetric fork progression and profuse chromosomal aberrations upon mild DNA damage confirmed that TSC loss indeed proved detrimental to stress adaptation. We conclude that low stress tolerance of TSC-/- cells manifests at the level of DNA replication control, imposing strong negative selection on genomic instability that could in turn detain TSC-mutant tumours benign.

Minute amounts of hamartin wildtype rescue the emergence of tuber-like lesions in conditional Tsc1 ablated mice

Tuberous sclerosis (TSC) is a phacomatosis associated with highly differentiated malformations including tubers in the brain. Those are composed of large dysplastic neurons and 'giant cells'. Cortical tubers are frequent causes of chronic seizures and resemble neuropathologically focal cortical dysplasias (FCD) type IIb. Patients with FCDIIb, however, lack additional stigmata of TSC. Mutations and allelic variants of the TSC1 gene have been observed in patients with tubers as well as FCDIIb. Those include hamartin(R692X) and hamartin(R786X), stop mutants frequent in TSC patients and hamartin(H732Y) frequent in FCDIIb. Expression of these variants in cell culture led to aberrant distribution of corresponding proteins. We here scrutinized morphological and structural effects of these TSC1 variants by intraventricular in utero electroporation (IUE), genetically mimicking the discrete focal character and a somatic postzygotic mosaicism of the lesion, focusing on the gene dosage required for tuber-like lesions to emerge in Tsc1(flox/flox) mice. Expression of only hamartin(R692X) as well as hamartin(R786X) led to a 2-fold enlargement of neurons with high pS6 immunoreactivity, stressing their in vivo pathogenic potential. Co-electroporation of the different aberrant alleles and varying amounts of wildtype TSC1 surprisingly revealed already minimal amounts of functional hamartin to be sufficient for phenotype rescue. This result strongly calls for further studies to unravel new mechanisms for substantial silencing of the second allele in cortical tubers, as proposed by Knudson's '2-hit hypothesis'. The rescuing effects may provide a promising basis for gene therapies aiming at reconstituting hamartin expression in tubers.

Tuberous-sclerosis complex-related cell signaling in the pathogenesis of lung cancer

Background

Hamartin (TSC1) and tuberin (TSC2), encoded by the tuberous sclerosis complex (TSC) genes, form a tumor-suppressor heterodimer which is implicated in PI3K-Akt signaling and acts as a functional inhibitor of the mammalian target of rapamycin (mTOR). Dysregulation of mTOR has been assigned to carcinogenesis and thus may be involved in cancer development. We have addressed the role of hamartin, phospho-tuberin (p-TSC2) and phospho-mTOR (p-mTOR) in a series of non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC) samples.

Methods

We collected 166 NSCLC and SCLC samples for immunohistochemical studies and performed western blot analyses in NSCLC and SCLC cell lines as well as comparative analyses with EGFR phosphorylation and downstream effectors.

Results

In cell lines we found an inverse correlation between hamartin and p-mTOR expression. In surgical specimens cytoplasmic hamartin expression was observed in more than 50% of adenocarcinoma (AC) and squamous cell carcinoma (SCC) compared to 14% of SCLC. P-mTOR and p-TSC2 staining was found in a minority of cases.

There was a significant correlation between p-EGFR Tyr-1068, p-EGFR Tyr-992 and hamartin, and also between p-mTOR and p-EGFR Tyr-1173 in AC. In SCC an inverse correlation between hamartin and p-EGFR Tyr-992 was detected. Phosphorylation of TSC2 was associated with expression of MAP-Kinase. Hamartin, p-TSC2 and p-mTOR expression was not dependant of the EGFR mutation status. Hamartin expression is associated with poorer survival in SCC and SCLC.

Conclusions

Our findings confirm the inhibitory role of the tuberous sclerosis complex for mTOR activation in lung cancer cell lines. These results reveal hamartin expression in a substantial subset of NSCLC and SCLC specimens, which may be due to EGFR signaling but is not dependant on EGFR mutations. Our data provide evidence for a functional role of the tuberous sclerosis complex in lung cancer.

Virtual slides

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/9274845161175223.

mTORC1 signaling and regulation of pancreatic β-cell mass

The capacity of β cells to expand in response to insulin resistance is a critical factor in the development of type 2 diabetes. Proliferation of β cells is a major component for these adaptive responses in animal models. The extracellular signals responsible for β-cell expansion include growth factors, such as insulin, and nutrients, such as glucose and amino acids. AKT activation is one of the important components linking growth signals to the regulation of β-cell expansion. Downstream of AKT, tuberous sclerosis complex 1 and 2 (TSC1/2) and mechanistic target of rapamycin complex 1 (mTORC1) signaling have emerged as prime candidates in this process, because they integrate signals from growth factors and nutrients. Recent studies demonstrate the importance of mTORC1 signaling in β cells. This review will discuss recent advances in the understanding of how this pathway regulates β-cell mass and present data on the role of TSC1 in modulation of β-cell mass. Herein, we also demonstrate that deletion of Tsc1 in pancreatic β cells results in improved glucose tolerance, hyperinsulinemia and expansion of β-cell mass that persists with aging.

Tandem affinity purification and identification of the human TSC1 protein complex

Mutations in the TSC1 and TSC2 genes lead to tuberous sclerosis complex (TSC), which is characterized clinically by mental retardation, epilepsy, and benign tumors affecting multiple tissues. Numerous components of the TSC protein complex remain uncharacterized. Here we report the purification of the TSC1 complex under physiological conditions using a proteomic strategy. We purified the TSC1 protein complex using a tandem affinity purification method and identified a protein complex containing 139 components. Two known binding proteins of TSC1 (TSC2 and DOCK7) were identified along with other new potential partners, which cover reported and novel TSC1 functional categories. Bioinformatics and biochemical methods were used to evaluate the observed protein-protein interactions. A comparative analysis with a published expression proteomics/genomics study of TSC1 revealed more than 20 common candidates that might be functionally relevant. The data set provides new directions in which to expand our knowledge of the functions of TSC1 and the mechanisms of TSC. The results are highly reliable, which is reflected by the identification of a few reported partners of TSC1 and many TSC1/2-regulated proteins. Interestingly, many new functional categories were identified, such as DNA repair, which provide novel hints to the function of TSC1. Moreover, a few neuronal disease-related proteins that might regulate the normal functions of neurons were identified. Thus, the results suggest that many of the new interactions should be biologically significance. It will be interesting to further investigate the regulatory mechanisms of these components.

The methylation of the TSC2 promoter underlies the abnormal growth of TSC2 angiomyolipoma-derived smooth muscle cells

Tuberous sclerosis complex (TSC) is an autosomal-dominant disease that is caused by mutations in either the TSC1 or TSC2 gene. Smooth muscle-like cells (ASMs) were isolated from an angiomyolipoma of a patient with TSC. These cells lacked tuberin, were labeled by both HMB45 and CD44v6 antibodies, and had constitutive S6 phosphorylation. The cells bear a germline TSC2 intron 8-exon 9 junction mutation, but DNA analysis and polymerase chain reaction amplification failed to demonstrate loss of heterozygosity. Testing for an epigenetic alteration, we detected methylation of the TSC2 promoter. Its biological relevance was confirmed by tuberin expression and a reduction in HMB45 labeling and S6 constitutive phosphorylation after exposure to the chromatin-remodeling agents, trichostatin A and 5-azacytidine. These cells were named TSC2(-/meth) ASMs. Their proliferation required epidermal growth factor in the medium as previously described for TSC2(-/-) ASMs. Blockade of epidermal growth factor with monoclonal antibodies caused the death of TSC2(-/meth) ASMs. In addition, rapamycin effectively blocked the proliferation of these cells. Our data show for the first time that methylation of the TSC2 promoter might cause a complete loss of tuberin in TSC2 cells, and that the pathogenesis of angiomyolipomas might also originate from epigenetic defects in smooth muscle cells. Additionally, the effect of chromatin-remodeling agents in these cells suggests a further avenue for the treatment of TSC as well as lymphangioleiomyomatosis.

Mechanism of oxidative DNA damage in diabetes: tuberin inactivation and downregulation of DNA repair enzyme 8-oxo-7,8-dihydro-2'-deoxyguanosine-DNA glycosylase

OBJECTIVE

To investigate potential mechanisms of oxidative DNA damage in a rat model of type 1 diabetes and in murine proximal tubular epithelial cells and primary culture of rat proximal tubular epithelial cells.

RESEARCH DESIGN AND METHODS

Phosphorylation of Akt and tuberin, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) levels, and 8-oxoG-DNA glycosylase (OGG1) expression were measured in kidney cortical tissue of control and type 1 diabetic animals and in proximal tubular cells incubated with normal or high glucose.

RESULTS

In the renal cortex of diabetic rats, the increase in Akt phosphorylation is associated with enhanced phosphorylation of tuberin, decreased OGG1 protein expression, and 8-oxodG accumulation. Exposure of proximal tubular epithelial cells to high glucose causes a rapid increase in reactive oxygen species (ROS) generation that correlates with the increase in Akt and tuberin phosphorylation. High glucose also resulted in downregulation of OGG1 protein expression, paralleling its effect on Akt and tuberin. Inhibition of phosphatidylinositol 3-kinase/Akt significantly reduced high glucose-induced tuberin phosphorylation and restored OGG1 expression. Hydrogen peroxide stimulates Akt and tuberin phosphorylation and decreases OGG1 protein expression. The antioxidant N-acetylcysteine significantly inhibited ROS generation, Akt/protein kinase B, and tuberin phosphorylation and resulted in deceased 8-oxodG accumulation and upregulation of OGG1 protein expression.

CONCLUSIONS

Hyperglycemia in type 1 diabetes and treatment of proximal tubular epithelial cells with high glucose leads to phosphorylation/inactivation of tuberin and downregulation of OGG1 via a redox-dependent activation of Akt in renal tubular epithelial cells. This signaling cascade provides a mechanism of oxidative stress-mediated DNA damage in diabetes.

Survivin expression in tuberous sclerosis complex cells

Tuberous Sclerosis Complex (TSC) is a tumor suppressor gene disorder with mutations of TSC1/TSC2 genes. This leads to the development of hamartomas that most frequently affect central nervous system, kidney, and skin. Angiomyolipomas are abdominal masses made up of muscle vessels and adipose tissues that grow mostly in proximity to kidneys and liver. Bleeding and kidney failure are the major justification for surgery. This study shows that angiomyolipoma-derived human smooth muscle TSC2-/- cells express the apoptosis inhibitor protein survivin when exposed to IGF-1. Survivin expression is also triggered whenever culture conditions perturb normal TSC2-/- cell function, such as the omission of EGF from the growth medium, the supplementation of anti-EGFR, blockade of PI3K and ERK, or inhibition of mTOR. Interestingly, single or simultaneous inhibition of PI3K by LY294002 and ERK by PD98059 does not prevent IGF-1-mediated survivin expression. Apoptogenic Smac/DIABLO, which is constitutively expressed by TSC2-/- A+ cells, is down-regulated by IGF-1 even in the presence of LY294002 and PD98059. These cells release IGF-1 by means of a negative feedback-regulated mechanism that is overrun when they are exposed to antibodies to IGF-1R, which increases the released amount by more than 400%. The autocrine release of IGF-1 may therefore be a powerful mechanism of survival of the tightly packed cells in the thick-walled vessels of TSC angiomyolipoma and in lymphangioleiomyomatosis (LAM) nodules. Future experimental therapies for TSC and LAM may result from the targeted inhibition of survivin, which may enhance sensitivity to TSC2 therapy.

Survivin expression in tuberous sclerosis complex cells

Tuberous Sclerosis Complex (TSC) is a tumor suppressor gene disorder with mutations of TSC1/TSC2 genes. This leads to the development of hamartomas that most frequently affect central nervous system, kidney, and skin. Angiomyolipomas are abdominal masses made up of muscle vessels and adipose tissues that grow mostly in proximity to kidneys and liver. Bleeding and kidney failure are the major justification for surgery. This study shows that angiomyolipoma-derived human smooth muscle TSC2-/- cells express the apoptosis inhibitor protein survivin when exposed to IGF-1. Survivin expression is also triggered whenever culture conditions perturb normal TSC2-/- cell function, such as the omission of EGF from the growth medium, the supplementation of anti-EGFR, blockade of PI3K and ERK, or inhibition of mTOR. Interestingly, single or simultaneous inhibition of PI3K by LY294002 and ERK by PD98059 does not prevent IGF-1-mediated survivin expression. Apoptogenic Smac/DIABLO, which is constitutively expressed by TSC2-/- A+ cells, is down-regulated by IGF-1 even in the presence of LY294002 and PD98059. These cells release IGF-1 by means of a negative feedback-regulated mechanism that is overrun when they are exposed to antibodies to IGF-1R, which increases the released amount by more than 400%. The autocrine release of IGF-1 may therefore be a powerful mechanism of survival of the tightly packed cells in the thick-walled vessels of TSC angiomyolipoma and in lymphangioleiomyomatosis (LAM) nodules. Future experimental therapies for TSC and LAM may result from the targeted inhibition of survivin, which may enhance sensitivity to TSC2 therapy.

Constitutively active alpha subunits of G(q/11) and G(12/13) families inhibit activation of the pro-survival Akt signaling cascade

Accumulating evidence indicates that G protein signaling plays an active role in the regulation of cell survival. Our previous study demonstrated the regulatory effects of G(i/o) proteins in nerve growth factor-induced activation of pro-survival Akt kinase. In the present study we explored the role of various members of the G(s), G(q/11) and G(12/13) subfamilies in the regulation of Akt in cultured mammalian cells. In human embryonic kidney 293 cells transiently expressing constitutively active mutants of G alpha11, G alpha14, G alpha16, G alpha12, or G alpha13 (G alpha11QL, G alpha14QL, G alpha16QL, G alpha12QL and G alpha13QL, respectively), basal phosphorylation of Akt was attenuated, as revealed by western blotting analysis using a phosphospecific anti-Akt immunoglobulin. In contrast, basal Akt phosphorylation was unaffected by the overexpression of a constitutively active G alpha(s) mutant (G alpha(s)QL). Additional experiments showed that G alpha11QL, G alpha14QL, G alpha16QL, G alpha12QL and G alpha13QL, but not G alpha(s)QL, attenuated phosphorylation of the Akt-regulated translation regulator tuberin. Moreover, they were able to inhibit the epidermal growth factor-induced Akt activation and tuberin phosphorylation. The inhibitory mechanism of Gq family members was independent of phospholipase Cbeta activation and calcium signaling because G alpha11QL, G alpha14QL and G alpha16QL remained capable of inhibiting epidermal growth factor-induced Akt activation in cells pretreated with U73122 and the intracellular calcium chelator, BAPTA/AM. Finally, overexpression of the dominant negative mutant of RhoA blocked G alpha12QL- and G alpha13QL-mediated inhibition, suggesting that activated G alpha12 and G alpha13 inhibit Akt signaling via RhoA. Collectively, this study demonstrated the inhibitory effect of activated G alpha11, G alpha14, G alpha16, G alpha12 and G alpha13 on pro-survival Akt signaling.

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.

A defect in protein farnesylation suppresses a loss of Schizosaccharomyces pombe tsc2+, a homolog of the human gene predisposing to tuberous sclerosis complex

Mutations in the human Tsc1 and Tsc2 genes predispose to tuberous sclerosis complex (TSC), a disorder characterized by the wide spread of benign tumors. Tsc1 and Tsc2 proteins form a complex and serve as a GTPase-activating protein (GAP) for Rheb, a GTPase regulating a downstream kinase, mTOR. The genome of Schizosaccharomyces pombe contains tsc1(+) and tsc2(+), homologs of human Tsc1 and Tsc2, respectively. In this study we analyzed the gene expression profile on a genomewide scale and found that deletion of either tsc1(+) or tsc2(+) affects gene induction upon nitrogen starvation. Three hours after nitrogen depletion genes encoding permeases and genes required for meiosis are less induced. Under the same condition, retrotransposons, G1-cyclin (pas1(+)), and inv1(+) are more induced. We also demonstrate that a mutation (cpp1-1) in a gene encoding a beta-subunit of a farnesyltransferase can suppress most of the phenotypes associated with deletion of tsc1(+) or tsc2(+). When a mutant of rhb1(+) (homolog of human Rheb), which bypasses the requirement of protein farnesylation, was expressed, the cpp1-1 mutation could no longer suppress, indicating that deficient farnesylation of Rhb1 contributes to the suppression. On the basis of these results, we discuss TSC pathology and possible improvement in chemotherapy for TSC.

Isolation and growth of smooth muscle-like cells derived from tuberous sclerosis complex-2 human renal angiomyolipoma: epidermal growth factor is the required growth factor

Tuberous sclerosis complex (TSC) is a tumor suppressor gene disorder characterized by mutations in the TSC1 or TSC2 genes. These mutations lead to the development of benign tumors involving smooth muscle cells, causing life-threatening lymphangioleiomyomatosis. We isolated and characterized two types of cells bearing a mutation in TSC2 exon 18 from a renal angiomyolipoma of a TSC patient: one population of alpha-actin-positive smooth muscle-like cells with loss of heterozygosity for the TSC2 gene (A(+) cells) and another of nonloss of heterozygosity keratin 8/18-positive epithelial-like cells (R(+) cells). Unlike control aortic vascular smooth muscle cells, A(+) cells required epidermal growth factor (EGF) to grow and substituting EGF with insulin-like growth factor (IGF)-1 failed to increase the cell number; however, omission of EGF did not cause cell loss. The A(+) cells constantly released IGF-1 into the culture medium and constitutively showed a high degree of S6K phosphorylation even when grown in serum-free medium. Exposure to antibodies against EGF and IGF-1 receptors caused a rapid loss of A(+) cells: 50% by 5 days and 100% by 12 days. Signal transduction mediated by EGF and IGF-I receptors is therefore involved in A(+) cell survival. These results may offer a novel therapeutic perspective for the treatment of TSC complications and lymphangioleiomyomatosis.

Identification of FIP200 interaction with the TSC1-TSC2 complex and its role in regulation of cell size control

FIP200 (focal adhesion kinase [FAK] family interacting protein of 200 kD) is a newly identified protein that binds to the kinase domain of FAK and inhibits its kinase activity and associated cellular functions. Here, we identify an interaction between FIP200 and the TSC1–TSC2 complex through FIP200 binding to TSC1. We found that association of FIP200 with the TSC1–TSC2 complex correlated with its ability to increase cell size and up-regulate S6 kinase phosphorylation but was not involved in the regulation of cell cycle progression. Conversely, knockdown of endogenous FIP200 by RNA interference reduced S6 kinase phosphorylation and cell size, which required TSC1 but was independent of FAK. Furthermore, overexpression of FIP200 reduced TSC1–TSC2 complex formation, although knockdown of endogenous FIP200 by RNA interference did not affect TSC1–TSC2 complex formation. Lastly, we showed that FIP200 is important in nutrient stimulation-induced, but not energy- or serum-induced, S6 kinase activation. Together, these results suggest a cellular function of FIP200 in the regulation of cell size by interaction with the TSC1–TSC2 complex.

Isolated unilateral tuberous sclerosis-associated renal cystic disease in a neonate

We present a male infant with antenatally detected, focal, unilateral apparently isolated renal cystic disease with morphological features of renal involvement in tuberous sclerosis. Only one previous case with similar presentation has been described. Most affected children present with either diffuse bilateral renal cystic disease or extrarenal manifestations. The major genes involved in tuberous sclerosis are now well described, and early onset of severe renal cystic disease in affected children often is related to the presence of a contiguous gene deletion syndrome involving TSC2 and PKD1 on chromosome 16.

Expression of matrix metalloproteinases and their endogenous tissue inhibitors in skin lesions from patients with tuberous sclerosis

BACKGROUND

Tuberous sclerosis complex lesions of the skin may be disfiguring to patients and can only be treated by laser or cosmetic surgery. Clarification of the molecular and structural changes involved in skin hamartomas may unravel targets for pharmacotherapy.

OBJECTIVE

We investigated the expression of matrix metalloproteinase (MMP) and its tissue inhibitor (TIMP) in fibrous plaques, angiofibromas, and lesion-free skin specimens from patients with tuberous sclerosis complex.

METHODS

Gene expression of MMPs and TIMP-1 was measured by reverse transcription polymerase chain reaction, gelatinase activity by gelatin zymography, and the content of collagenases and TIMPs by enzyme-linked immunosorbent assay.

RESULTS

Compared with lesion-free specimens, hamartomas exhibited decreased levels of TIMPs and messenger RNA expression of TIMP-1, and increased content of MMP-1 and MMP-13 and activity of MMP-9, although gelatinase gene expression was diminished. Gene expression of MMP-15 and MMP-17 was not affected but was diminished for MMP-14.

CONCLUSION

The significant variations of the above extracellular matrix molecules between lesion-free specimens and tuberous sclerosis complex hamartomas overall favors a collagenous protein-degrading microenvironment in affected skin, and argue in support of antiprotease treatment for disfiguring skin lesions.

Update on the cytogenetics and molecular genetics of chordoma

Chordoma is a rare mesenchymal tumour of complex biology for which only histologic and immunohistochemical criteria have been defined, but no biomarkers predicting the clinical outcome and response to treatment have yet been recognised. We herein review the interdisciplinary information achieved by epidemiologists, neurosurgeons and basic scientists on chordoma, usually a sporadic tumour, which also includes a small fraction of familial cases. Main focus is on the current knowledge of the genetic alterations which might pinpoint candidate genes and molecular mechanisms shared by sporadic and familiar chordomas. Due to the scarcity of the investigated tumour specimens and the multiple chromosome abnormalities found in tumours with aberrant karyotypes, conventional cytogenetics and Fluorescence In Situ Hybridization failed to detect recurrent chordoma-specific chromosomal rearrangements. Genome-wide approaches such as Comparative Genomic Hybridization (CGH) are yet at an initial stage of application and should be implemented using BAC arrays either genome-wide or targeting selected genomic regions, disclosed by Loss of Heterozygosity (LOH) studies. An LOH region was shown by a systematic study on a consistent number of chordomas to encompass 1p36, a genomic interval where a candidate gene was suggested to reside. Despite the rarity of multiplex families with chordoma impaired linkage studies, a chordoma locus could be mapped to chromosome 7q33 by positive lod score in three independent families. The role in chordomagenesis of the Tuberous Sclerosis Complex (TSC) genes has been proved, but the extent of involvement of TSC1 and TSC2 oncosuppressors in chordoma remains to be assessed. In spite of the scarce knowledge on the genetics and molecular biology of chordoma, recent initiation of clinical trials using molecular-targeted therapy, should validate new molecular targets and predict the efficacy of a given therapy. Comparative genetic and biomolecular studies should enhance the molecular taxonomy of chordoma which might have a prognostic significance and better orient the therapeutic options.

Insulin signaling pathways in cortical dysplasia and TSC-tubers: tissue microarray analysis

To evaluate the possible roles of the Akt/PKB-mTOR-p70S6K-S6 and cap-dependent translation (eIF4G) pathways in the pathogenesis of tuberous sclerosis complex (TSC)-associated cortical tubers and focal cortical dysplasia (FCD), we performed qualitative and semiquantitative immunohistochemical evaluation on surgically resected corticectomy specimens to detect phosphorylated molecules as activated downstream targets of the signaling pathways. A tissue microarray paraffin block was constructed from 63 archival specimens of surgically resected TSC tubers, FCDs with balloon cells, cortical dysplasia without balloon cells, and histologically normal-appearing neocortex obtained from cases with Rasmussen encephalitis, cystic-gliotic encephalopathy, and temporal lobe epilepsy. Abnormal neuroglial cells were positive for phospho-S6 and phospho-eIF4G with various staining intensities in FCDs and TSC tubers. Both proteins were much less abundantly expressed in normal-appearing neocortex. Phospho-mTOR expression was observed in neurons in all groups. The expression of phospho-S6 and phospho-eIF4G was associated with dysplastic lesions (p < 0.05), and the cytoplasmic phospho-p70S6K expression was most specific for and abundant in TSC tubers and much less prominent in other groups (p < 0.01). These results suggest that constitutive activation of cytoplasmic p70S6K plays a pivotal role in the pathogenesis of TSC tubers and that FCDs possess a distinct mechanism for activation of S6 and eIF4G.

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 Papillomavirus 16 E6 Oncoprotein Interferences with Insulin Signaling Pathway by Binding to Tuberin

Tuberous sclerosis complex (TSC) is a genetic disorder caused by mutations in either TSC1 or TSC2 tumor suppressor gene. TSC1 and TSC2 products, Harmatin and Tuberin, form the functional complex to serve as the negative regulator for insulin-induced phosphorylation of S6 kinase and elF4E-binding protein 1. High-risk human papillomavirus (HPV) infection is the necessary cause for cervical cancer. E6 oncoprotein encoded by HPV plays a pivotal role in carcinogenesis by interference with the host intracellular protein functions. In this study, we show that HPV16 E6 interacts with tumor suppressor gene TSC2 product, Tuberin, and results in the phosphorylation of S6 kinase and S6 even in the absence of insulin. The overexpression of Tuberin overcomes the effect of E6 on S6 kinase phosphorylation. Binding with HPV16 E6 causes the proteasome-mediated degradation of Tuberin. A DILG motif and an ELVG motif located in the carboxyl-terminal of Tuberin are required for E6 binding. In addition, the Tuberin interaction region in E6 has been mapped in the amino-terminal portion of HPV16 E6, which is different from the binding domain with p53. These results provide a possible link between E6-induced oncogenesis and the insulin-stimulated cell proliferation signaling pathway.

Protein levels of α1-tubulin, protein disulfide isomerase, tropomyosins and vimentin are regulated by the tuberous sclerosis gene products

Tuberous sclerosis (TSC) is an autosomal dominant tumour suppressor gene syndrome affecting about 1 in 6000 individuals. It is characterized by mental retardation and epilepsy. A variety of tumours characteristically occur in different organs of TSC patients. Typically, highly epileptogenic dysplastic lesions (tubers) composed of abnormal shaped neurones can be detected in the cerebral cortex. Two tumour suppressor genes have been shown to be responsible for this disease: TSC1, encoding hamartin, and TSC2, encoding tuberin. In this study we performed a proteomic approach of two-dimensional gel electrophoresis with subsequent mass spectrometrical identification of protein spots after ectopic overexpression of human TSC1 or TSC2. We found the protein levels of alpha1-tubulin, protein disulfide isomerase, tropomyosin 3 and 5 and vimentin to be regulated by the two tuberous sclerosis gene products. The here presented findings suggest that deregulation of the control of these target proteins might contribute to the development of tumours in tuberous sclerosis patients. These data provide important new insights into the molecular development of this disease especially since alpha1-tubulin, protein disulfide isomerase and certain tropomyosins have also been implicated in the regulation of neuronal differentiation.

Tuberous Sclerosis Complex 2 Gene Product Interacts with Human SMAD Proteins

Tuberin (TSC2) is a tumor suppressor gene. At the cellular level, tuberin is required as a critical regulator of cell growth, neuronal differentiation, and tumor suppression. Here we report a critical role for tuberin in late stage myeloid cell differentiation. Tuberin strongly augments transforming growth factor (TGF)-beta1 signal transduction pathways, including SMAD activation. We also demonstrate that the amino-terminal region of tuberin interacts specifically with the MH2 domain of SMAD2 and SMAD3 proteins to regulate TGF-beta1-responsive genes such as p21(CIP). Inhibition of tuberin expression by Tsc2 antisense greatly reduces the ability of TGF-beta to transcriptionally regulate p21(CIP), p27(KIP), and cyclin A leading to an abrogation of the antiproliferative effects of TGF-beta1. Also, inhibition of tuberin expression during stimulation of monocytic differentiation with vitamin D(3) and TGF-beta1 significantly impaired myeloid cell growth inhibition and differentiation. Together, the data demonstrate the presence of a novel activation process following TGF-beta1 stimulation that requires tuberin-dependent activity.

Tuberin and hamartin expression is reduced in the majority of subependymal giant cell astrocytomas in tuberous sclerosis complex consistent with a two-hit model of pathogenesis

Subependymal giant cell astrocytomas are distinctive brain tumors that are seen only in tuberous sclerosis complex. Although histologically benign, they cause both moribidity and occasional mortality owing to progressive growth in some patients. Tuberous sclerosis complex is an autosomal dominant genetic disorder with a high sporadic case rate that is due to mutations in either of two genes, TSC1 and TSC2, encoding hamartin and tuberin, respectively. The pathogenesis of subependymal giant cell astrocytomas in tuberous sclerosis complex is uncertain. In this study, we examined the expression of tuberin and hamartin in subependymal giant cell astrocytomas from nine patients with tuberous sclerosis complex by immunohistochemistry with confocal microscopy. Loss of hamartin expression was seen in all subependymal giant cell astrocytomas, including five from patients with germline TSC2 mutations and two from patients with germline TSC1 mutations. The subependymal giant cell astrocytomas of six of nine patients had no expression of tuberin as well, whereas three patients retained some tuberin expression. Tuberin expression was seen in one patient with a TSC2 germline mutation and two patients whose mutational status was not determined. Overall, these data indicate a loss of both tuberin and hamartin expression in the subependymal giant cell astrocytomas of patients with both TSC1 and TSC2 mutations and are consistent with a two-hit disease pathogenesis model for the development of subependymal giant cell astrocytomas.

Thyroid-stimulating hormone/cAMP and glycogen synthase kinase 3beta elicit opposing effects on Rap1GAP stability

Beyond regulating Rap activity, little is known regarding the regulation and function of the Rap GTPase-activating protein Rap1GAP. Tuberin and E6TP1 protein levels are tightly regulated through ubiquitin-mediated proteolysis. A role for these RapGAPs, along with SPA-1, as tumor suppressors has been demonstrated. Whether Rap1GAP performs a similar role was investigated. We now report that Rap1GAP protein levels are dynamically regulated in thyroid-stimulating hormone (TSH)-dependent thyroid cells. Upon TSH withdrawal, Rap1GAP undergoes a net increase in phosphorylation followed by proteasome-mediated degradation. Sequence analysis identified two putative destruction boxes in the Rap1GAP C-terminal domain. Glycogen synthase kinase 3beta (GSK3beta) phosphorylated Rap1GAP immunoprecipitated from thyroid cells, and GSK3beta inhibitors prevented phosphorylation and degradation of endogenous Rap1GAP. Co-expression of GSK3beta and Rap1GAP in human embryonic kidney 293 cells stimulated proteasome-dependent Rap1GAP turnover. Mutational analysis established a role for serine 525 in the regulation of Rap1GAP stability. Overexpression of Rap1GAP in thyroid cells impaired TSH/cAMP-stimulated p70S6 kinase activity and cell proliferation. These data are the first to show that Rap1GAP protein levels are tightly regulated and are the first to support a role for Rap1GAP as a tumor suppressor.

Tuberous sclerosis genes regulate cellular 14-3-3 protein levels

The genes TSC1, encoding hamartin, and TSC2, encoding tuberin are responsible for tuberous sclerosis. This autosomal dominant tumor suppressor gene syndrome affects about 1 in 6000 individuals. A variety of tumors characteristically occur in different organs of tuberous sclerosis patients and are believed to result from defects in cell cycle/cell size control. We performed a proteomics approach of two-dimensional gel electrophoresis with subsequent mass spectrometrical identification of protein spots after ectopic overexpression of human TSC1 or TSC2. We found the cellular levels of four isoforms of the 14-3-3 protein family, 14-3-3 gamma, 14-3-3, 14-3-3 sigma, and 14-3-3 zeta, to be regulated by the two tuberous sclerosis gene products. In the same experiments the protein levels of keratin 7, capZ alpha-1 subunit, ezrin, and nedasin were not affected by ectopic TSC1 or TSC2. Western blot analyses confirmed the deregulation of 14-3-3 proteins upon ectopic overexpression of TSC1 and TSC2. A TSC1 mutant not encoding the transmembrane domain and the tuberin-binding domain but harbouring most of the coiled-coil region and the ERM protein interaction domain of hamartin did not affect 14-3-3 protein levels. The here presented findings suggest that deregulation of 14-3-3 protein amounts might contribute to the development of tumors in tuberous sclerosis patients. These data provide important new insights into the molecular development of this disease especially since both, the TSC genes and the 14-3-3 proteins, are known to be involved in mammalian cell cycle control.

Regulation of PCNA and CAF-1 expression by the two tuberous sclerosis gene products

Tuberous sclerosis is an autosomal dominant tumor suppressor gene syndrome affecting about 1 in 6000 individuals. Two genes have been shown to be responsible for this disease: TSC1, encoding hamartin and TSC, encoding tuberin. A variety of tumors characteristically occur in different organs of tuberous sclerosis patients and are believed to result from defects in cell cycle/cell size control. In this study, we performed two-dimensional gel electrophoresis with subsequent mass spectrometrical identification of protein spots after overexpression of TSC1 or TSC2. We found expression of PCNA and the p48 subunit of CAF-1 to be regulated by two tuberous sclerosis gene products. CAF-1 and PCNA interact as major regulators of chromatin assembly during DNA repair. We suggest that deregulation of the control of chromatin assembly might contribute to development of tumors in tuberous sclerosis patients and provide important new insights into the molecular development, especially since deregulation of chromatin assembly and DNA repair results in genomic instability, a hallmark of tumor development.

Tumour suppressors hamartin and tuberin: intracellular signalling

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

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.

Pediatric liver tumors and hepatic ontogenesis: common and distinctive pathways

Several types of pediatric liver tumors exhibit structural features apparently reflecting processes which normally occur during hepatic ontogenesis: some hepatoblastomas mimic distinct phases of hepatogenesis, including the formation of mesenchymal structures closely associated with immature epithelia, and there are tumors almost exclusively consisting of complex mesenchymal patterns. Current classifications of hepatoblastomas refer to the identification of more or less mature (differentiated) single or mixed components seen in histologic preparations. These do not, however, attempt to integrate ontogenic pathways, in contrast for example, to nephroblastoma and associated lesions, where such a view has proved to be highly fruitful. Based on the fact that an enormous amount of knowledge has recently been accumulated regarding hepatic ontogenesis, time may have come to look at these tumors with a new eye. In what follows, we aim at trying to analyze distinct features of pediatric hepatic tumors (except vascular tumors) within the background of ontogenesis. Some key steps of hepatogenesis and the regulatory factors involved may, in the future, deliver an armamentarium to search for novel molecular mechanisms involved in tumorigenic pathways.

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.

Multicompartmental distribution of the tuberous sclerosis gene products, hamartin and tuberin

Mutations of the TSC1 and TSC2 genes give rise to the clinical disorder of tuberous sclerosis characterized by the development of hamartomas predominantly affecting the central nervous system, kidney, skin, lung, and heart. The function of the gene products, hamartin and tuberin, is not well understood but we have previously suggested a role in vesicular transport. To define the subcellular compartment(s) involved with these two proteins, biochemical characterization of hamartin and tuberin was performed in primary tissues and cell lines. Fractionation of cell lysates identified both proteins in the cytosolic, microsomal, and cytoskeletal compartments. In each of these fractions, hamartin and tuberin formed a stable complex in coimmunoprecipitation analyses. Further, they colocalized extensively in discrete, vesicular structures in the cytoplasm. Within the microsomal compartment, hamartin and tuberin behaved as peripheral membrane proteins that associate with the cytosolic leaflet of membranous domains. Immunoisolation of tuberin-bound vesicles using magnetic beads showed an enrichment of rap1, rab5, and caveolin-1, all of which have been found in specialized lipid microdomains, caveolae. Our data suggest that hamartin and tuberin are multicompartmental proteins that partially reside in caveolin-1-enriched structures and potentially affect their signaling.

Renal cell carcinoma

Renal cell carcinoma continues to be a difficult malignancy to treat because of its ability to spread asymptomatically and its inherent resistance to conventional chemotherapy. However, molecular genetic studies bring new insights into the pathogenesis of this disorder and may provide new targets against which novel chemotherapeutic agents could be developed. Nephron-sparing surgery is also gaining wider acceptance as favorable long-term, cancer-free survival data emerge from clinical trials. For metastatic disease, cytokine therapy continues to be the mainstay of treatment despite marginal efficacy and a significant side-effect profile.