Alessandro Valli

The mTOR pathway and its role in human genetic diseases

The signalling components upstream and downstream of the protein kinase mammalian target of rapamycin (mTOR) are frequently altered in a wide variety of human diseases. Upstream of mTOR key signalling molecules are the small GTPase Ras, the lipid kinase PI3K, the Akt kinase, and the GTPase Rheb, which are known to be deregulated in many human cancers. Mutations in the mTOR pathway component genes TSC1, TSC2, LKB1, PTEN, VHL, NF1 and PKD1 trigger the development of the syndromes tuberous sclerosis, Peutz-Jeghers syndrome, Cowden syndrome, Bannayan-Riley-Ruvalcaba syndrome, Lhermitte-Duclos disease, Proteus syndrome, von Hippel-Lindau disease, Neurofibromatosis type 1, and Polycystic kidney disease, respectively. In addition, the tuberous sclerosis proteins have been implicated in the development of several sporadic tumors and in the control of the cyclin-dependent kinase inhibitor p27, known to be of relevance for several cancers. Recently, it has been recognized that mTOR is regulated by TNF-alpha and Wnt, both of which have been shown to play critical roles in the development of many human neoplasias. In addition to all these human diseases, the role of mTOR in Alzheimers disease, cardiac hypertrophy, obesity and type 2 diabetes is discussed.

The tuberous sclerosis gene products hamartin and tuberin are multifunctional proteins with a wide spectrum of interacting partners.

Mutations in the tumor suppressor genes TSC1 and TSC2, encoding hamartin and tuberin, respectively, cause the tumor syndrome tuberous sclerosis with similar phenotypes. Until now, over 50 proteins have been demonstrated to interact with hamartin and/or tuberin. Besides tuberin, the proteins DOCK7, ezrin/radixin/moesin, FIP200, IKKbeta, Melted, Merlin, NADE(p75NTR), NF-L, Plk1 and TBC7 have been found to interact with hamartin. Whereas Plk1 and TBC7 have been demonstrated not to bind to tuberin, for all the other hamartin-interacting proteins the question, whether they can also bind to tuberin, has not been studied. Tuberin interacts with 14-3-3 beta,epsilon,gamma,eta,sigma,tau,zeta, Akt, AMPK, CaM, CRB3/PATJ, cyclin A, cyclins D1, D2, D3, Dsh, ERalpha, Erk, FoxO1, HERC1, HPV16 E6, HSCP-70, HSP70-1, MK2, NEK1, p27KIP1, Pam, PC1, PP2Ac, Rabaptin-5, Rheb, RxRalpha/VDR and SMAD2/3. 14-3-3 beta,epsilon,gamma,eta,sigma,tau,zeta, Akt, Dsh, FoxO1, HERC1, p27KIP1 and PP2Ac are known not to bind to hamartin. For the other tuberin-interacting proteins this question remains elusive. The proteins axin, Cdk1, cyclin B1, GADD34, GSK3, mTOR and RSK1 have been found to co-immunoprecipitate with both, hamartin and tuberin. The kinases Cdk1 and IKKbeta phosphorylate hamartin, Erk, Akt, MK2, AMPK and RSK1 phosphorylate tuberin, and GSK3 phosphorylates both, hamartin and tuberin. This detailed summary of protein interactions allows new insights into their relevance for the wide variety of different functions of hamartin and tuberin.

Embryoid body formation of human amniotic fluid stem cells depends on mTOR

Human amniotic fluid stem cells (hAFSCs) harbor high proliferative capacity and high differentiation potential and do not raise the ethical concerns associated with human embryonic stem cells. The formation of three-dimensional aggregates known as embryoid bodies (EBs) is the principal step in the differentiation of pluripotent embryonic stem cells. Using c-Kit-positive hAFSC lines, we show here that these stem cells harbor the potential to form EBs. As part of the two kinase complexes, mTORC1 and mTORC2, mammalian target of rapamycin (mTOR) is the key component of an important signaling pathway, which is involved in the regulation of cell proliferation, growth, tumor development and differentiation. Blocking intracellular mTOR activity through the inhibitor rapamycin or through specific small interfering RNA approaches revealed hAFSC EB formation to depend on mTORC1 and mTORC2. These findings demonstrate hAFSCs to be a new and powerful biological system to recapitulate the three-dimensional and tissue level contexts of in vivo development and identify the mTOR pathway to be essential for this process.

Functional interaction of mammalian target of rapamycin complexes in regulating mammalian cell size and cell cycle

Dysregulation of the mammalian target of rapamycin (mTOR) kinase pathway is centrally involved in a wide variety of cancers and human genetic diseases. In mammalian cells, mTOR is part of two different kinase complexes: mTORC1 composed of mTOR, raptor and mLST8, and mTORC2 containing mTOR, rictor, sin1 and mLST8. Whereas, mTORC1 is known to be a pivotal regulator of cell size and cell cycle control, the question whether the recently discovered mTORC2 complex is involved in these processes remains elusive. We report here that the mTORC1-mediated consequences on cell cycle and cell size are separable and do not involve effects on mTORC2 activity. However, we show that mTORC2 itself is a potent regulator of mammalian cell size and cell cycle via a mechanism involving the Akt/TSC2/Rheb cascade. Our data are of relevance for the understanding of the molecular development of the many human diseases caused by deregulation of upstream and downstream effectors of mTOR.

Tuberin, p27 and mTOR in different cells

Mutations in the genes TSC1 or TSC2 cause the autosomal dominantly inherited tumor suppressor syndrome tuberous sclerosis, which is characterized by the development of tumors, named hamartomas, in different organs. The TSC gene products, hamartin and tuberin, form a complex, of which tuberin is assumed to be the functional component. Both, hamartin and tuberin have been implicated in the control of the cell cycle by activating the cyclin-dependent kinase inhibitor p27 and in cell size regulation by inhibiting the mammalian target of rapamycin (mTOR) a regulator of the p70 ribosomal protein S6 kinase (p70S6K) and its target the ribosomal protein S6. The tuberin/hamartin complex was shown to protect p27 from protein degradation. Within the mTOR signaling pathway tuberin harbors GTPase activating (GAP) potential toward Rheb, which is a potent regulator of mTOR. In this study, we have analyzed the protein levels of tuberin, p27, cyclin D1, mTOR and phospho mTOR Ser2448 (activated mTOR), S6 and phospho S6 Ser240/244 (activated S6) and as controls α-tubulin and topoisomerase IIβ, in ten different cells, including primary normal cells, immortalized and transformed cell lines.

Skp2 inversely correlates with p27 and tuberin in transformed cells

The cyclin-dependent kinase inhibitor p27Kip1 (p27) is a major gatekeeper of the mammalian cell cycle progression known to be regulated by both, its subcellular localization and its degradation. To allow entrance into S phase and thereby mammalian cell cycle progression p27 must be degraded by a skp2-containing E3 ubiquitin ligase whose task is to target p27 for degradation by the proteasome. The tumor suppressor gene product tuberin directly binds to p27 and protects it from degradation via skp2. Whereas, p27 and tuberin are known to be localized to both, the cytoplasm and the nucleus, the localization of skp2 remained elusive. Here we demonstrate that skp2 is a cytoplasmic and nuclear protein. In addition we found an inverse correlation of the endogenous protein levels of skp2 with p27 and tuberin in different transformed cells and under different growth conditions. These data allow new important insights into this molecular network of cell cycle control.

New insights into the role of the tuberous sclerosis genes in leukemia.

The genes TSC1 and TSC2, encoding hamartin and tuberin, respectively, have been shown to be involved in the development of the autosomal dominantly inherited tumor syndrome tuberous sclerosis (TSC). However, inactivation of these genes has also been demonstrated to be associated with sporadic bladder cancer, ovarian and gall bladder carcinoma, non-small-cell carcinoma of the lung, breast cancer, pancreatic cancer, astrocytoma, xanthoastrocytoma, ependymomas, oral squamous cell carcinoma and endometrial cancer. The hamartin/tuberin protein complex plays a central role in the regulation of the mammalian target of rapamycin (mTOR) signalling network. A wide variety of components of the mTOR cascade have been demonstrated to be involved in many different human cancers. Mutations in several mTOR pathway component genes are known to cause specific monogenic human genetic diseases and this signalling cascade has been shown to be of relevance for Alzheimers disease, type 2 diabetes, obesity and hypertrophy. Consequently, e.g. clinical trials for the treatment with rapamycin, a negative regulator of mTOR, of hamartomas in TSC have already been initiated. Now the first evidence is provided for an involvement of the TSC genes in acute leukemia.

CDKs as therapeutic targets for the human genetic disease tuberous sclerosis

The tuberous sclerosis gene 2 product tuberin is an important regulator of the mammalian target of rapamycin (mTOR). In addition, tuberin is known to bind to the cyclin‐dependent kinase (CDK) inhibitor p27Kip1 (p27) and to regulate its stability and localization via mTOR‐independent mechanisms. Recently, evidence has been provided that tuberin also affects p27 localization via regulating mTOR′s potential to activate the serum‐ and glucocorticoid‐inducible kinase (SGK1) to phosphorylate p27. Taken together, these findings strengthen the argument that besides mTOR‐inhibitors, such as rapamycin analogues, p27 and CDKs could also be considered targets for hamartoma therapeutics in tuberous sclerosis.