The mTOR pathway: Role in Aging and Cancer
mTOR Pathway Regulates Cell Proliferation & Affects Numerous Disease States
The mammalian target of rapamycin (mTOR) protein is a key regulator of cell proliferation and growth, integrating the influence of upstream signals on cell division. We offer several mTOR antibody products as well as antibodies that recognize other members of the mTOR signaling pathway for use in immunoblotting or immunoprecipitation. Here we present:
- An overview of the mTOR pathway
- Involvement of mTOR in aging and cancer
- Analyzing mTOR pathway members
- Examples from the mTOR literature
Several signaling pathway molecules are involved in both aging and cancer; for example, the tumor suppressor p53 has been referenced as a modulator of cellular senescence and organismal aging (1). Other tumor suppressors, including p16(INK4a) (2), pTEN (3) and C/EBP (4), similarly contribute to the balance between senescence and immortalization. The mTOR pathway integrates multiple signals to affect lifespan, cellular responses, and contributes to disease processes. As such, there is keen interest in evaluating new mTOR inhibitor agents as potential drug candidates, while known mTOR inhibitors such as rapamycin and its derivatives are useful tools for understanding the role of the mTOR signaling pathway in greater detail.
How does the mTOR pathway play a role in both aging and cancer? Mammalian Target Of Rapamycin (mTOR) is a serine/threonine kinase in the insulin receptor substrate (IRS) pathway that assembles into either of two complexes: mTOR complex I (mTORC1) or mTORC2. mTORC1 is activated by growth factors and nutrient levels and is inhibited by rapamycin, an immunosuppressive drug that binds to FKBP12. FKBP12 is then in turn able to bind to the FRB domain of mTOR, thereby inhibiting mTOR activity. mTORC1 activity is also inhibited by cell stress, caloric restriction, and endogenous inhibitors such as FKBP38.
The Regulatory Associated Protein Of mTORC1 (Raptor) is a subunit of the mTORC1 complex and regulates its activity. Raptor also associates with Eukaryotic translation Initiation Factor 4E-Binding Protein 1 (EIF4BP1) and ribosomal protein S6 kinase (S6K1), promoting their phosphorylation and upregulation of protein synthesis via mTORC1.
Rapamycin-Insensitive Companion Of mTOR (Rictor) is a subunit and regulator of mTORC2. It may compete with Raptor for binding to mTOR. The mTORC2 complex is activated by growth factors but, unlike mTORC1, is insensitive to nutrient levels.
Advances in understanding how mTOR phosphorylation affects association of mTOR with mTORC1 or mTORC2 has helped unravel the mechanisms of action and regulatory roles of this kinase. Watanabe and colleagues have reported four phosphorylation states for mTOR that determine the formation of the specific complexes, as well differing sensitivities of each complex to rapamycin and wortmannin (5). An overview of the mTOR pathway is shown below.
Figure 1. The mTOR pathway. From Rafaela Andrade-Vieira, et al. (2013) PLoS ONE 10.1371/journal.pone.0056567
The mTOR pathway has been found to play an important and possibly pivotal role in aging and cancer. Wu and colleagues reported that the mTOR pathway is a critical regulator of aging. Hypomorphic mice with a 75% reduction in mTOR expression relative to wild-type mice lived up to 20% longer (6). The mutant mice were also reported to have better balance and coordination, muscle strength and spatial learning, and memory than the wild-type mice.
mTOR inhibition has also been correlated with reduction in other biomarkers of aging-associated diseases such as Alzheimer's Disease. When the natural compound arctigenin was administered to an APP/PS1 transgenic mouse model of Alzheimer's Disease, the resulting Akt/mTOR pathway inhibition was correlated with ameliorated memory impairment and an increase in beta amyloid protein clearance (7).
mTOR may play a role in cancer progression as well. mTOR inhibition has been correlated with reduced recurrence of hormone receptor positive breast cancer, possibly because mTOR regulates the PI3K/Akt signaling network regulating cell proliferation, migration, and survival (8). A similar suppressive effect was reported when the PTEN/PI3K/Akt and mTOR signaling pathways were targeted as part of a treatment strategy for hormone refractory prostate cancer (9) and during inhibition of the mTORC1/S6K1 and PI3K/Akt pathways in hepatocellular carcinoma (10).
We offer several antibodies1 that can be used to analyze members of the mTOR signaling pathway by Western blot or immunoprecipitation.:
|mTOR||10343A||WB, IP||Recognizes rat & human mTOR|
|mLST8||28013A||WB, IP||Antigen: Human mLST8 N-terminal synthetic peptide|
|PRAS40||28033A||WB, IP||Phospho-specific antibody (Cat.# 28035A) for WB recognizes PRAS40 Ser-183|
|Raptor||28011A, 28037A||WB, IP||Cat.# 28011A raised against a synthetic peptide corresponding to a middle region of human Rictor. Cat.# 28037A raised to an N-terminal peptide of human Rictor.|
|Rheb||28015A||WB, IP||Antigen: synthetic peptide corresponding to C-terminal portion of human Rheb|
|Rictor||28039A||WB, IP||Antigen: synthetic human Rictor p|
1Offered in partnership with IBL Co., Ltd., Japan.
References listed below cite IBL or Immuno-Biological Laboratories as
the kit manufacturer.
2WB, Western Blot; IP, Immunoprecipitation
Branched-chain amino acids prevent insulin-induced hepatic tumor cell proliferation by inducing apoptosis through mTORC1 and mTORC2-dependent mechanisms. Hagiwara, A.,. et al. (2012) J. Cellular Physiol. 227(5):2097.
Branched-chain amino acid (BCAA), often used as a treatment agent for hepatocellular carcinoma, was found to enhance the negative feedback loop of mTORC1/S6K1, resulting in a greater inhibition of the PI3K/Akt pathway. BCAA treatment was also found to suppress mTORC2 phosphorylation of Akt. Immunoprecipitation experiments were performed using the mouse monoclonal mTOR antibody (Cat.# 10343A).
Regulation of folliculin (the BHD gene product) phosphorylation by Tsc2-mTOR pathway. Piao, X., et al. (2009) Biochem. Biophys. Res. Comm. 389:16.
Birt-Hogg-Dubé syndrome (BHDS) is an autosomal dominant genetic disease that predisposes patients to fibrofolliculomas, lung cysts, and renal neoplasia. BHD is a tumor suppressor that encodes folliculin (FLCN). The mTORC1 complex was known to regulate FLCN downstream of the mTORC1 negative regulator tuberin, the product of tuberous sclerosis 2 gene (TSC2), but the precise mechanisms were unknown. The authors identified multiple sites at which FLCN is phosphorylated by an mTORC1-dependent pathway, which then affects the interaction of FLCN with AMPK-activated protein kinase (AMPK). Antibodies to Raptor and Rictor from Immuno-Biological Laboratories were used in this study.
The mechanism of insulin-stimulated 4E-BP protein binding to mammalian target of rapamycin (mTOR) complex 1 and its contribution to mTOR complex 1 signaling. Rapley, J., et al. (2011) J. Biol. Chem. 286:38043.
The interaction of mTORC1 with its substrates PRAS40, S6K1, Rheb, and 4E-BP was investigated as a function of insulin and amino acid levels. Rheb activation was found to lead directly to mTOR catalytic activity, while PRAS40 dissociation did not lead directly to mTORC1 signaling. For immunoprecipitation experiments, anti-mTOR, anti-PRAS40 (Pro238), and anti-raptor (R1) antibodies from Immuno-Biological Laboratories were used.
General References - mTOR Pathway
1. Rufini, A., et al. (2013) Oncogene 32(43):5129.
2. Coppé J.P., et al. (2011) J. Biol. Chem. 286(42):36396.
3. Kitagishi, Y. and Matsuda, S. (2013) Int. J. Mol. Med. 31(3):511.
4. Iakova P., et al. (2003) Cell 113:495.
5. Watanabe, R., et al. (2011) J. Nucl. Med. 52:497.
6. Wu, J.J., et al. (2013) Cell Reports 4(5):913.
7. Zhu, Z., et al. (2013) J. Neurosci. 33(32):13138.
8. Villarreal-Garza, C., et al. (2012) Ann. Oncol. 23(10):2526.
9. Suh, Y., et al. (2010) Carcinogenesis 31(8)1424.
10. Hagiwara, A., et al. (2012) J. Cellular Physiol. 227(5):2097.