Cross-talk between tuberin, calmodulin, and estrogen signaling pathways

New Insights into the Regulation of mTOR Signaling via Ca2+-Binding Proteins

Environmental factors are important regulators of cell growth and proliferation. Mechanistic target of rapamycin (mTOR) is a central kinase that maintains cellular homeostasis in response to a variety of extracellular and intracellular inputs. Dysregulation of mTOR signaling is associated with many diseases, including diabetes and cancer. Calcium ion (Ca²⁺) is important as a second messenger in various biological processes, and its intracellular concentration is tightly regulated. Although the involvement of Ca²⁺ mobilization in mTOR signaling has been reported, the detailed molecular mechanisms by which mTOR signaling is regulated are not fully understood. The link between Ca²⁺ homeostasis and mTOR activation in pathological hypertrophy has heightened the importance in understanding Ca²⁺-regulated mTOR signaling as a key mechanism of mTOR regulation. In this review, we introduce recent findings on the molecular mechanisms of regulation of mTOR signaling by Ca²⁺-binding proteins, particularly calmodulin (CaM).

An Insight of Scientific Developments in TSC for Better Therapeutic Strategy

Tuberous sclerosis complex (TSC) is a rare genetic disease, which is characterized by noncancerous tumors in multi-organ systems in the body. Mutations in the TSC1 or TSC2 genes are known to cause the disease. The resultant mutant proteins TSC1 (hamartin) and TSC2 (tuberin) complex evade its normal tumor suppressor function, which leads to abnormal cell growth and proliferation. Both TSC1 and TSC2 are involved in several protein-protein interactions, which play a significant role in maintaining cellular homeostasis. The recent biochemical, genetic, structural biology, clinical and drug discovery advancements on TSC give a useful insight into the disease as well as the molecular aspects of TSC1 and TSC2. The complex nature of TSC disease, a wide range of manifestations, mosaicism and several other factors limits the treatment choices. This review is a compilation of the course of TSC, starting from its discovery to the current findings that would take us a step ahead in finding a cure for TSC.

Fetal cardiac rhabdomyomas treated with maternal sirolimus

OBJECTIVE

To review the pathophysiology of rhabdomyomas and the emerging option of prenatal treatment of fetal cardiac rhabdomyomas.

METHODS

We present a case of fetal rhabdomyomas causing significant hemodynamic compromise that received in utero treatment of maternal sirolimus. Genetic amniocentesis confirmed a TSC2 mutation. A treatment program was initiated with a 10-mg loading dose titrated to a goal maternal trough of 10 to 15 ng/dL. In order to follow fetal cardiac function, a sophisticated method of speckle tracking echocardiography was used before and after treatment. Obstetric ultrasound was used to monitor fetal growth, and clinical surveillance, echocardiography, and brain MRI were used to monitor postnatal growth and development through 6 months of neonatal life.

RESULTS

Sirolimus was initiated from 28 to 36 weeks of gestation with improvement of cardiac status. During this period, intrauterine growth restriction developed. Postnatally, the infant has had stable rhabdomyomas and cardiac function without reinitiating sirolimus. Brain MRI demonstrated scattered cortical tubers and subependymal nodules, and the infant has not had seizure-like activity. At 6 months of age, the infant has achieved appropriate developmental milestones.

CONCLUSION

In counseling cases of prenatal onset large obstructing rhabdomyomas and cardiac compromise, in utero sirolimus treatment can be considered.

Overexpression of the 14-3-3γ protein in uterine leiomyoma cells results in growth retardation and increased apoptosis

Protein 14-3-3γ was significantly reduced in human uterine leiomyoma compared to the adjacent normal myometrium tissue. To investigate the possible link between the reduced 14-3-3γ expression and uterine leiomyoma growth, we have overexpressed 14-3-3γ protein in uterine leiomyomal cells and its effects on cell proliferation and apoptosis were analyzed. Over-expression of 14-3-3γ was achieved by transducing into two types of uterine leiomyoma cells (primary culture cells and immortal stem cells) with a 14-3-3γ expressing adenovirus vector. Differentially expressed proteins were screened by the proteomics tool (TMT-LCTMS), followed by PANTHER database analysis to single out specifically modified signaling pathway proteins, which were confirmed by Phospho-MAPK Antibody Array and Western blots analysis. The results showed that increase in 14-3-3γ expression in both two types of human uterine leiomyoma cells inhibited cell proliferation and induced apoptosis. Proteomic screening has found 42 proteins, among 5846, that were significantly affected. PANTHER database and GeneMANIA analysis of the differentially expressed proteins have found that proteins involved in apoptosis signaling and cytoskeletal/adhesion were among the ones affected the most. Further analysis of the key signaling pathways have found that over-expression of 14-3-3γ resulted in reductions in the phosphorylations of multiple signaling molecules, including AKT, pan, ERK1/2, GSK-3 α/β, MEK1/2, Foxo1 and Vimentin. In conclusion, the loss of 14-3-3γ may have causal effects on the growth of uterine leiomyoma, which may function through modifying multiple signaling pathways, including AKT-Foxo and/or MEK1/2-ERK1/2.

Metabolic Impact of Rapamycin (Sirolimus) and B-Estradiol Using Mouse Embryonic Fibroblasts as a Model for Lymphangioleiomyomatosis

INTRODUCTION

Lymphangioleiomyomatosis (LAM) is a rare, progressive cystic lung disease that predominantly affects women of childbearing age. Exogenous rapamycin (sirolimus) has been shown to improve clinical outcomes and was recently approved to treat LAM, whereas estrogen (E₂) is implicated in disease progression. No consistent metabolic model currently exists for LAM, therefore wild-type mouse embryonic fibroblasts (MEF +/+) and TSC2 knockout cells (MEF -/-) were used in this study as a model for LAM.

METHODS

Oxygen consumption rates (OCR) and redox potential were measured to determine metabolic state across control cells, MEF +/+ and -/- cells treated with rapamycin (Rapa), and MEF +/+ and -/- cells treated with E₂. An XF96 extracellular flux analyzer from Seahorse Bioscience^® was used to measure OCR, and a RedoxSYS™ ORP was used to measure redox potential.

RESULTS

OCR of MEF -/- cells treated with rapamycin (MEF -/- Rapa) versus MEF -/- control were significantly lower across all conditions. The static oxidation reduction potential of the MEF -/- Rapa group was also lower, approaching significance. The coupling efficiency and ratio of ATP-linked respiration to maximum respiration were statistically lower in MEF -/- Rapa compared to MEF +/+ Rapa. There were no significant metabolic findings across any of the MEF cells treated with E₂. MEF -/- control cells versus MEF +/+ control cells were not found to significantly differ.

CONCLUSION

MEF cells are thought to be a feasible metabolic model for LAM, which has implications for future pharmacologic and biologic testing.

Yolk sac tumor of the ovary in a young girl with tuberous sclerosis: A case report and review of the literature

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We reported the first tuberous sclerosis patient with an ovarian yolk sac tumor.

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Although angiomyolipoma is a common benign tumor in TS patients, abdominal malignancies must be considered.

A novel androgen-regulated isoform of the TSC2 tumour suppressor gene increases cell proliferation

TSC2 (Tuberous sclerosis complex 2) is an important tumour suppressor gene, mutations within which are linked to the development of tuberous sclerosis and implicated in multiple tumour types. TSC2 protein complexes with TSC1 and blocks the ability of the Rheb (Ras homolog enriched in brain) GTPase to activate mTOR (mammalian target of rapamycin), a crucial signal transducer which regulates protein synthesis and cell growth. Here, we report the characterisation of a novel isoform of TSC2 which is under direct control of the ligand-activated androgen receptor. TSC2 isoform A (TSC2A) is derived from an internal androgen-regulated alternative promoter and encodes a 508-amino acid cytoplasmic protein corresponding to the C-terminal region of full-length TSC2, lacking the interaction domain for TSC1 and containing an incomplete interaction domain required for Rheb inactivation. Expression of TSC2A is induced in response to androgens and full-length TSC2 is co-ordinately down-regulated, indicating an androgen-driven switch in TSC2 protein isoforms. In contrast to the well-characterised suppressive effect on cell proliferation of full-length TSC2 protein, both LNCaP and HEK293 cells over-expressing TSC2 isoform A proliferate more rapidly (measured by MTT assays) and have increased levels of cells in S-phase (measured by both Edu staining and FACS analysis). Our work indicates, for the first time, a novel role for this well-known tumour suppressor gene, which encodes an activator of cell proliferation in response to androgen stimulation.

Genetics and molecular biology of tuberous sclerosis complex

Tuberous Sclerosis Complex is a multisystem disorder exhibiting a wide range of manifestations characterized by tumour-like lesions called hamartomas in the brain, skin, eyes, heart, lungs and kidneys. Tuberous Sclerosis Complex is genetically determined with an autosomal dominant inheritance and is caused by inactivating mutations in either the TSC1 or TSC2 genes. TSC1/2 genes play a fundamental role in the regulation of phosphoinositide 3-kinase (PI3K) signalling pathway, inhibiting the mammalian target of rapamycin (mTOR) through activation of the GTPase activity of Rheb. Mutations in TSC1/2 genes impair the inhibitory function of the hamartin/tuberin complex, leading to phosphorylation of the downstream effectors of mTOR, p70 S6 kinase (S6K), ribosomal protein S6 and the elongation factor binding protein 4E-BP1, resulting in uncontrolled cell growth and tumourigenesis.

Despite recent promising genetic, diagnostic, and therapeutic advances in Tuberous Sclerosis Complex, continuing research in all aspects of this complex disease will be pivotal to decrease its associated morbidity and mortality. In this review we will discuss and analyse all the important findings in the molecular pathogenesis of Tuberous Sclerosis Complex, focusing on genetics and the molecular mechanisms that define this multisystemic disorder.

Lymphangioleiomyomatosis: what do we know and what are we looking for?

Lymphangioleiomyomatosis (LAM) is a rare disease characterised by proliferation of abnormal smooth muscle-like cells (LAM cells) leading to progressive cystic destruction of the lung, lymphatic abnormalities and abdominal tumours. It affects predominantly females and can occur sporadically or in patients with tuberous sclerosis complex. This review describes the recent progress in our understanding of the molecular pathogenesis of the disease and LAM cell biology. It also summarises current therapeutic approaches and the most promising areas of research for future therapeutic strategies.

Lymphangioleiomyomatosis

Lymphangioleiomyomatosis (LAM) is a rare disease that mainly affects women, particularly at fertile age. It is sporadic or associated with tuberous sclerosis complex. It is characterised by an abnormal proliferation of immature smooth muscle cells (SMC), which grow aberrantly in the airway, parenchyma, lymphatics and pulmonary blood vessels and which can gradually lead to respiratory failure. It affects several systems, affecting the lymphatic ganglia and causing abdominal tumours. Given its very low prevalence, a difficult to establish early diagnosis, absence of curative treatment and the difficulty in obtaining information, places LAM under the heading of the so-called Rare Diseases. There is a growing interest in the study of this disease which has led to the setting up of patient registers and an exponential growth in LAM research, both at a clinical level and cellular level.

Polymorphic variants in TSC1 and TSC2 and their association with breast cancer phenotypes

TSC1 acts coordinately with TSC2 in a complex to inhibit mTOR, an emerging therapeutic target and known promoter of cell growth and cell cycle progression. Perturbation of the mTOR pathway, through abnormal expression or function of pathway genes, could lead to tumorigenesis. TSC1 and TSC2 expression is reduced in invasive breast cancer as compared with normal mammary epithelium. Because single nucleotide polymorphisms (SNPs) in regulatory genes have been implicated in risk and age at diagnosis of breast cancers, systematic SNP association studies were performed on TSC1 and TSC2 SNPs for their associations with clinical features of breast cancer. TSC1 and TSC2 haplotypes were constructed from genotyping of multiple loci in both genes in healthy volunteers. SNPs were selected for further study using a bioinformatics approach based on SNP associations with drug response in NCI-60 cell lines and evidence of selection bias based on haplotype frequencies. Genotyping for five TSC1 and one TSC2 loci were performed on genomic DNA from 1,137 women with breast cancer. This study found that for TSC1 rs7874234, TT variant carriers had a 9-year later age at diagnosis of estrogen receptor positive (ER+), but not ER-, ductal carcinomas (P = 0.0049). No other SNP locus showed an association with age at diagnosis, nor any other breast cancer phenotype. TSC1 rs7874234 is hypothesized to be functional in ER+ breast cancer because the T allele, but not the C allele, may create an estrogen receptor element (ERE) site, resulting in increased TSC1 transcription and subsequent inhibition of mTOR.

mTOR activation, lymphangiogenesis, and estrogen-mediated cell survival: the "perfect storm" of pro-metastatic factors in LAM pathogenesis

Research interest in lymphangioleiomyomatosis (LAM) has grown dramatically in the past decade, particularly among cancer biologists. There are at least two reasons for this: first, the discovery in the year 2000 that LAM cells carry TSC2 gene mutations, linking LAM with cellular pathways including the PI3K/Akt/mTOR axis, and allowing the Tuberous Sclerosis Complex (TSC)-regulated pathways that are believed to underlie LAM pathogenesis to be studied in cells, yeast, Drosophila, and mice. A second reason for the rising interest in LAM is the discovery that LAM cells can travel to the lung, including repopulating a donor lung after lung transplantation, despite the fact that LAM cells are histologically benign. This "benign metastasis" underpinning suggests that elucidating LAM pathogenesis will unlock a set of fundamental mechanisms that underlie metastatic potential in the context of a cell that has not yet undergone malignant transformation. Here, we will outline the data supporting the metastatic model of LAM, consider the biochemical and cellular mechanisms that may enable LAM cells to metastasize, including both cell autonomous and non-cell autonomous factors, and highlight a mouse model in which estrogen promotes the metastasis and survival of TSC2-deficient cells in a MEK-dependent manner. We propose a multistep model of LAM cell metastasis that highlights multiple opportunities for therapeutic intervention. Taken together, the metastatic behavior of LAM cells and the involvement of tumor-related signaling pathways lead to optimism that cancer-related paradigms for diagnosis, staging, and therapy will lead to therapeutic breakthroughs for women living with LAM.

Dopamine neuron stimulating actions of a GDNF propeptide

BACKGROUND

Neurotrophic factors, such as glial cell line-derived neurotrophic factor (GDNF), have shown great promise for protection and restoration of damaged or dying dopamine neurons in animal models and in some Parkinson's disease (PD) clinical trials. However, the delivery of neurotrophic factors to the brain is difficult due to their large size and poor bio-distribution. In addition, developing more efficacious trophic factors is hampered by the difficulty of synthesis and structural modification. Small molecules with neurotrophic actions that are easy to synthesize and modify to improve bioavailability are needed.

METHODS AND FINDINGS

Here we present the neurobiological actions of dopamine neuron stimulating peptide-11 (DNSP-11), an 11-mer peptide from the proGDNF domain. In vitro, DNSP-11 supports the survival of fetal mesencephalic neurons, increasing both the number of surviving cells and neuritic outgrowth. In MN9D cells, DNSP-11 protects against dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA)-induced cell death, significantly decreasing TUNEL-positive cells and levels of caspase-3 activity. In vivo, a single injection of DNSP-11 into the normal adult rat substantia nigra is taken up rapidly into neurons and increases resting levels of dopamine and its metabolites for up to 28 days. Of particular note, DNSP-11 significantly improves apomorphine-induced rotational behavior, and increases dopamine and dopamine metabolite tissue levels in the substantia nigra in a rat model of PD. Unlike GDNF, DNSP-11 was found to block staurosporine- and gramicidin-induced cytotoxicity in nutrient-deprived dopaminergic B65 cells, and its neuroprotective effects included preventing the release of cytochrome c from mitochondria.

CONCLUSIONS

Collectively, these data support that DNSP-11 exhibits potent neurotrophic actions analogous to GDNF, making it a viable candidate for a PD therapeutic. However, it likely signals through pathways that do not directly involve the GFRalpha1 receptor.

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

PURPOSE

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

EXPERIMENTAL DESIGN

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

RESULTS

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

CONCLUSIONS

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

The tuberous sclerosis complex proteins – a GRIPP on cognition and neurodevelopment

Tuberous sclerosis complex (TSC) is a multi-system disorder associated with mutations in the TSC1 (hamartin) or TSC2 (tuberin) genes. The neurocognitive features of TSC show wide variability and have generally been attributed to structural brain abnormalities and/or seizures. We review the fundamental roles of TSC1 and TSC2 in cell signalling and propose that because the hamartin-tuberin complex (hereafter referred to as TSC1-2) acts as a global regulator and integrator of a range of physiological processes ('GRIPP') the neurocognitive manifestations of TSC result directly from cell-signalling abnormalities. Under the GRIPP hypothesis, the spectrum of neurodevelopmental abnormalities is caused by the biochemical consequences of individual TSC1 and TSC2 mutations. Recognizing the importance of signalling disruption in the brain might improve our understanding of other neurocognitive disorders.

Tuberin nuclear localization can be regulated by phosphorylation of its carboxyl terminus

Tuberin, the tuberous sclerosis 2 (TSC2) gene product, has been identified as a tumor suppressor protein genetically implicated in the pathology of tuberous sclerosis and the female-specific lung disease lymphangioleiomyomatosis. Tuberin and its predominant cytoplasmic binding partner hamartin have been shown to complex with a variety of intracellular signaling regulators and affect the processes of protein translation, cellular proliferation, cellular migration, and cellular transcription. In previous studies, we have presented evidence for tuberin binding to the calcium-dependent intracellular signaling protein calmodulin (CaM), overlap of tuberin CaM binding domain with a binding domain for estrogen receptor alpha, and the phosphorylation-associated nuclear localization of tuberin. In the study presented here, we expand our findings on the mechanism of tuberin nuclear localization to show that the CaM-estrogen receptor-alpha binding domain of tuberin can also serve as a tuberin nuclear localization sequence. Furthermore, we identify an Akt/p90 ribosomal S6 kinase-1 phosphorylation site within the carboxyl terminus of tuberin that can regulate tuberin nuclear localization and significantly affect the ability of tuberin to modulate estrogen genomic signaling events. These findings suggest a link between tuberin nuclear localization and a variety of intracellular signaling events that have direct implications with respect to the role of tuberin in the pathology of tuberous sclerosis and lymphangioleiomyomatosis.

Molecular pathogenesis of lymphangioleiomyomatosis: lessons learned from orphans

Lymphangioleiomyomatosis (LAM) is a rare progressive cystic lung disease affecting young women. The pivotal observation that LAM occurs both spontaneously and as part of the tuberous sclerosis complex (TSC) led to the hypothesis that these disorders share common genetic and pathogenetic mechanisms. In this review we describe the evolution of our understanding of the molecular and cellular basis of LAM and TSC, beginning with the discovery of the TSC1 and TSC2 genes and the demonstration of their involvement in sporadic (non-TSC) LAM. This was followed by rapid delineation of the signaling pathways in Drosophila melanogaster with confirmation in mice and humans. This knowledge served as the foundation for novel therapeutic approaches that are currently being used in human clinical trials.

Hamartin and tuberin: working together for tumour suppression

TSC1 and TSC2 are two recently identified tumour suppressor genes encoding hamartin and tuberin, respectively, and involved in pathogenesis of tuberous sclerosis, neurological disorder connected with the development of hamartomas in numerous organ systems, including the brain, kidneys, heart and liver. Both protein products of TSC1 and TSC2 form an intracellular complex exerting GTPase-activating (GAP) activity towards a small G protein, Ras homologue enriched in brain (Rheb). Inhibition of Rheb is important for the regulation of mTOR pathway, while mutation of hamartin or tuberin results in uncontrolled cell cycle progression. Tuberin, possessing the Rheb-GAP domain, is phosphorylated by several kinases that confer the signals of growth factor stimulation or low cellular energy levels. Such a modification of tuberin influences its activity within the complex with hamartin and positively or negatively modulates mTOR-regulated protein translation and cellular proliferation. Current article describes biochemical properties of hamartin and tuberin, their known regulatory phosphorylation sites and binding partners.

Tuberous sclerosis complex: linking growth and energy signaling pathways with human disease

The most exciting advances in the tuberous sclerosis complex (TSC) field occurred in 1993 and 1997 with the cloning of the TSC2 and TSC1 genes, respectively, and in 2003 with the identification of Rheb as the target of tuberin's (TSC2) GTPase activating protein (GAP) domain. Rheb has a dual role: it activates mTOR and inactivates B-Raf. Activation of mTOR leads to increased protein synthesis through phosphorylation of p70S6K and 4E-BP1. Upon insulin or growth factor stimulation, tuberin is phosphorylated by several kinases, including AKT/PKB, thereby suppressing its GAP activity and activating mTOR. Phosphorylation of hamartin (TSC1) by CDK1 also negatively regulates the activity of the hamartin/tuberin complex. Despite these biochemical advances, exactly how mutations in TSC1 or TSC2 lead to the clinical manifestations of TSC is far from being understood. Two of the most unusual phenotypes in TSC are the apparent metastasis of benign cells carrying TSC1 and TSC2 mutations, resulting in pulmonary lymphangiomyomatosis, and the ability of cells with TSC1 or TSC2 mutations to differentiate into the separate components of renal angiomyolipomas (vessels, smooth muscle and fat). We will discuss how the TSC signaling pathways are affected by mutations in TSC1 or TSC2, focusing on how these mutations may lead to the renal and pulmonary manifestations of TSC.