Hiroaki Onda

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

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

Tsc2+/– mice develop tumors in multiple sites that express gelsolin and are influenced by genetic background

Tuberous sclerosis (TSC) is an autosomal dominant genetic disorder in which benign hamartomas develop in multiple organs, caused by mutations in either TSC1 or TSC2. We developed a murine model of Tsc2 disease using a gene targeting approach. Tsc2-null embryos die at embryonic days 9.5-12.5 from hepatic hypoplasia. Tsc2 heterozygotes display 100% incidence of multiple bilateral renal cystadenomas, 50% incidence of liver hemangiomas, and 32% incidence of lung adenomas by 15 months of age. Progression to renal carcinoma, fatal bleeding from the liver hemangiomas, and extremity angiosarcomas all occur at a rate of less than 10%. The renal cystadenomas develop from intercalated cells of the cortical collecting duct and uniformly express gelsolin at high levels, enabling detection of early neoplastic lesions. The tumor expression pattern of the mice is influenced by genetic background, with fewer large renal cystadenomas in the outbred Black Swiss background and more angiosarcomas in 129/SvJae chimeric mice. The slow growth of the tumors in the heterozygote mice matches the limited growth potential of the great majority of TSC hamartomas, and the influence of genetic background on phenotype correlates with the marked variability in expression of TSC seen in patients.

Mammalian target of rapamycin up-regulation of pyruvate kinase isoenzyme type M2 is critical for aerobic glycolysis and tumor growth.

Although aerobic glycolysis (the Warburg effect) is a hallmark of cancer, key questions, including when, how, and why cancer cells become highly glycolytic, remain less clear. For a largely unknown regulatory mechanism, a rate-limiting glycolytic enzyme pyruvate kinase M2 (PKM2) isoform is exclusively expressed in embryonic, proliferating, and tumor cells, and plays an essential role in tumor metabolism and growth. Because the receptor tyrosine kinase/PI3K/AKT/mammalian target of rapamycin (RTK/PI3K/AKT/mTOR) signaling cascade is a frequently altered pathway in cancer, we explored its potential role in cancer metabolism. We identified mTOR as a central activator of the Warburg effect by inducing PKM2 and other glycolytic enzymes under normoxic conditions. PKM2 level was augmented in mouse kidney tumors due to deficiency of tuberous sclerosis complex 2 and consequent mTOR activation, and was reduced in human cancer cells by mTOR suppression. mTOR up-regulation of PKM2 expression was through hypoxia-inducible factor 1α (HIF1α)-mediated transcription activation, and c-Myc–heterogeneous nuclear ribonucleoproteins (hnRNPs)-dependent regulation of PKM2 gene splicing. Disruption of PKM2 suppressed oncogenic mTOR-mediated tumorigenesis. Unlike normal cells, mTOR hyperactive cells were more sensitive to inhibition of mTOR or glycolysis. Dual suppression of mTOR and glycolysis synergistically blunted the proliferation and tumor development of mTOR hyperactive cells. Even though aerobic glycolysis is not required for breach of senescence for immortalization and transformation, the frequently deregulated mTOR signaling during multistep oncogenic processes could contribute to the development of the Warburg effect in many cancers. Components of the mTOR/HIF1α/Myc–hnRNPs/PKM2 glycolysis signaling network could be targeted for the treatment of cancer caused by an aberrant RTK/PI3K/AKT/mTOR signaling pathway.

A Mouse Model of Tuberous Sclerosis: Neuronal Loss of Tsc1 Causes Dysplastic and Ectopic Neurons, Reduced Myelination, Seizure Activity, and Limited Survival

Tuberous sclerosis (TSC) is a hamartoma syndrome caused by mutations in TSC1 or TSC2 in which cerebral cortical tubers and seizures are major clinical issues. We have engineered mice in which most cortical neurons lose Tsc1 expression during embryonic development. These Tsc1 mutant mice display several neurological abnormalities beginning at postnatal day 5 with subsequent failure to thrive and median survival of 35 d. The mice also display clinical and electrographic seizures both spontaneously and with physical stimulation, and some seizures end in a fatal tonic phase. Many cortical and hippocampal neurons are enlarged and/or dysplastic in the Tsc1 mutant mice, strongly express phospho-S6, and are ectopic in multiple sites in the cortex and hippocampus. There is a striking delay in myelination in the mutant mice, which appears to be caused by an inductive neuronal defect. This new TSC brain model replicates several features of human TSC brain lesions and implicates an important function of Tsc1/Tsc2 in neuronal development.

Astrocyte-specific TSC1 conditional knockout mice exhibit abnormal neuronal organization and seizures.

Persons affected with tuberous sclerosis complex (TSC) develop a wide range of neurological abnormalities including aberrant neuronal migration and seizures. In an effort to model TSC‐associated central nervous system abnormalities in mice, we generated two independent lines of astrocyte‐specific Tsc1 conditional knockout mice by using the Cre‐LoxP system. Astrocyte‐specific Tsc1‐null mice exhibit electroencephalographically proven seizures after the first month of age and begin to die at 3 to 4 months. Tsc1‐null mice show significant increases in astrocyte numbers throughout the brain by 3 weeks of age and abnormal neuronal organization in the hippocampus between 3 and 5 weeks. Moreover, cultured Tsc1‐null astrocytes behave similar to wild‐type astrocytes during log phase growth but demonstrate increased saturation density associated with reduced p27Kip1 expression. Collectively, our results demonstrate that astrocyte‐specific disruption of Tsc1 in mice provides a context‐dependent growth advantage for astrocytes that results in abnormalities in neuronal organization and epilepsy.

Efficacy of a Rapamycin Analog (CCI-779) and IFN- in Tuberous Sclerosis Mouse Models

Tuberous sclerosis complex (TSC) is a familial tumor disorder for which there is no effective medical therapy. Disease‐causing mutations in the TSC1 or TSC2 gene lead to increased mammalian target of rapamycin (mTOR) kinase activity in the conserved mTOR signaling pathway, which regulates nutrient uptake, cell growth, and protein translation. The normal function of TSC1 and TSC2 gene products is to form a complex that reduces mTOR kinase activity. Thus, mTOR kinase inhibition may be a useful targeted therapeutic approach. Elevated interferon‐gamma (IFN‐γ) expression is associated with decreased severity of kidney tumors in TSC patients and mouse models; therefore, IFN‐γ also has therapeutic potential. We studied cohorts of Tsc2+/− mice and a novel mouse model of Tsc2‐null tumors in order to evaluate the efficacy of targeted therapy for TSC. We found that treatment with either an mTOR kinase inhibitor (CCI‐779, a rapamycin analog) or with IFN‐γ reduced the severity of TSC‐related disease without significant toxicity. These results constitute definitive preclinical data that justify proceeding with clinical trials using these agents in selected patients with TSC and related disorders.

Tuberous Sclerosis Gene 2 Product Modulates Transcription Mediated by Steroid Hormone Receptor Family Members

Tuberous sclerosis (TSC) is a genetic disorder that results in the development of hamartomatous lesions in a variety of organ systems. Both the prevalence of the disease and the often devastating consequences of these tumors pose a serious health and medical care problem. The disease has been mapped to two distinct genetic loci in humans, and although the genes (TSC1 andTSC2) for both loci have recently been cloned, their function remains an enigma. Data presented here demonstrates that TSC2 protein can bind and selectively modulate transcription mediated by members of the steroid receptor superfamily of genes. These data place TSC2 into a growing list of nuclear receptor coregulators and strengthen the expanding body of evidence that these coregulators may play critical roles in cellular differentiation.

Tsc2 null murine neuroepithelial cells are a model for human tuber giant cells, and show activation of an mTOR pathway.

Cortical tubers are developmental brain malformations in the tuberous sclerosis complex (TSC) that cause epilepsy and autism in TSC patients whose pathogenesis is uncertain. Tsc2 null murine neuroepithelial progenitor (NEP) cells display persistent growth when growth factors are withdrawn, express GFAP at high levels, and have reduced expression of a set of early neuronal lineage markers. Tsc2 null NEP cells exhibit aberrant differentiation into giant cells that express both beta III-tubulin and GFAP and that are morphologically similar to giant cells in human tubers. Tsc2 null giant cells and tuber giant cells have similar transcriptional profiles. Tsc2 null NEP cells express high levels of phosphorylated S6kinase, S6, Stat3, and 4E-BP-1, which is reversed by treatment with rapamycin, an inhibitor of mTOR. We conclude that giant cells in human tubers likely result from a complete loss of TSC2 expression and activation of an mTOR pathway during cortical development.

Heterozygosity for the tuberous sclerosis complex (TSC) gene products results in increased astrocyte numbers and decreased p27-Kip1 expression in TSC2+/- cells.

Tuberous sclerosis complex (TSC) is an autosomal dominant tumor predisposition syndrome characterized by benign proliferations (hamartomas). In the brain, individuals with TSC develop autism, mental retardation and seizures associated with focal cortical dysplasias, subependymal nodules, and subependymal giant cell astrocytomas (SEGAs). We hypothesize that dysregulated astrocyte function due to mutations in the tumor suppressor genes, TSC1 and TSC2, may contribute to the pathogenesis of these brain abnormalities. In this report, we demonstrate that mice heterozygous for a targeted defect in either the Tsc1 or Tsc2 genes(Tsc1+/− and Tsc2+/− mice) exhibit a 1.5-fold increase in the number of astrocytes in vivo. Whereas increased astrocyte numbers in vivo were suggestive of a proliferative advantage, Tsc2+/− primary astrocyte cultures did not show a cell-autonomous growth advantage, anchorage-independent growth, increased saturation density, or increased fluid-phase endocytosis compared to wild type astrocytes. Tsc2 null mouse embryonic fibroblasts (MEFs) however, did exhibit increased saturation density compared to Tsc2 wild type controls. In both Tsc2+/− astrocytes and Tsc2 null mouse embryonic fibroblasts, p27-Kip1 expression was decreased compared to wild type cells, and was reversed by tuberin re-expression in Tsc2−/− MEFs. In contrast, no change in endocytosis was observed upon tuberin re-expression in Tsc2−/− MEFs. Collectively, these results suggest Tsc heterozygosity may provide a non-cell-autonomous growth advantage for astrocytes that may involve p27-Kip1 expression.

Functional Tyrosine Kinase Inhibitor Profiling : A Generally Applicable Method Points to a Novel Role of Platelet-Derived Growth Factor Receptor-β in Tuberous Sclerosis

Tumors often exhibit activation of specific tyrosine kinases, which may allow targeting of therapy through inhibition of tyrosine kinase signaling. This strategy has been used successfully in the development of STI571 (gleevec), an inhibitor of bcr-abl tyrosine kinase that has been used successfully in the treatment of chronic myelogenous leukemia. STI571 also shows activity against c-kit and platelet-derived growth factor receptor-beta (PDGFRbeta) tyrosine kinase signaling, thus potentially expanding the number of tumors that may respond to it. We describe a simple and rapid method to assess functional activity of tyrosine kinase signaling that is broadly applicable to tumor types. As proof of principle, we have applied it to cells that serve as models of the autosomal-dominant tumor syndrome tuberous sclerosis (TS). We found that TS model cells derived from tuberin heterozygous mice and from a human renal angiomyolipoma are highly sensitive to PDGFR antagonists and that these cells express PDGFRbeta. Given that PDGFRbeta signaling is inhibited by STI571, we found that SV7tert human angiomyolipoma cells are sensitive to STI571. Thus, we describe a novel but simple method of determining the functional tyrosine kinase profile of a neoplastic cell and our results suggest that STI571 might be useful in the treatment of neoplasms commonly seen in patients with TS.