Mammalian target of rapamycin: a central node of complex signaling cascades

Abstract

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that regulates cell growth and metabolism in response to diverse external stimuli. In the presence of mitogenic stimuli, mTOR transduces signals that activate the translational machinery and promote cell growth. mTOR functions as a central node in a complex net of signaling pathways that are involved both in normal physiological, as well as pathogenic events. mTOR signaling occurs in concert with upstream Akt and tuberous sclerosis complex (TSC) and several downstream effectors. During the past few decades, the mTOR-mediated pathway has been shown to promote tumorigenesis through the coordinated phosphorylation of proteins that directly regulate cell-cycle progression and metabolism, as well as transcription factors that regulate the expression of genes involved in the oncogenic processes. The importance of mTOR signaling in oncology is now widely accepted, and agents that selectively target mTOR have been developed as anti-cancer drugs. In this review, we highlight the past research on mTOR, including clinical and pathological analyses, and describe its molecular mechanisms of signaling, and its roles in the physiology and pathology of human diseases, particularly, lung carcinomas. We also discuss strategies that might lead to more effective clinical treatments of several diseases by targeting mTOR.

Keywords: lung cancer; mTOR; molecular targeting therapy; rapamycin.

Figure 1

Schematic representation of the mTOR-signalling pathway from growth factor receptor through mTOR. Arrows represent activation, whereas bars represent inhibition. Abbreviations: PI3K, phosphatidylinositol 3-kinase; PTEN, phosphatase and tensin homolog deleted on chromosome 10; AMPK, AMP-activated kinase; TSC, tuberous sclerosis complex; Rheb, Ras homologue enriched in brain; mTOR, mammalian target of rapamycin; FKBP12, immunophilin FK506-binding protein 12; FRB, FKBP12-rapamycin binding domain; 4E-BP1, eukaryotic initiation factor 4E-binding protein 1; S6K, ribosomal p70 S6 kinase.

Figure 2

Model of two mTOR signaling networks in mammalian cells: mTOR Complex 1 (mTORC1) and mTOR Complex 2 (mTORC2) and their interacting proteins. mTORC1 mediates the rapamycin-sensitive signals determining the cell size. mTORC2 signaling is rapamycin insensitive and controls the actin cytoskeleton determining the cell shape. Upstream regulators of TORC2 are not clarified. Arrows represent activation, whereas bars represent inhibition. Abbreviations: PTEN, phosphatase and tensin homolog deleted on chromosome 10; TSC, tuberous sclerosis complex; Rheb, Ras homologue enriched in brain; mTOR, mammalian target of rapamycin; raptor, regulatory-associated protein of mTOR; rictor, rapamycin-insensitive component of mTOR; 4E-BP1, eukaryotic initiation factor 4E-binding protein 1; eIF4E, eukaryotic initiation factor 4E; S6K, ribosomal p70 S6 kinase; TOS, TOR signaling motif; rS6, 40S ribosomal protein S6.

Figure 3

Immunohistochemical staining for activated mTOR (p-mTOR). Focally positive staining in a nest of squamous cell carcinoma (SCC). Intense p-mTOR staining in the acinar structure in adenocarcinoma (AC), and faint staining in large cell carcinoma (LCC). Rare positivity in small cell carcinoma (SCLC). Original magnification, ×100. Reproduced from “Critical and Diverse Involvement of Akt/Mammalian Target of Rapamycin Signaling in Human Lung Carcinomas” by Y. Dobashi, et al., Cancer. 115:107-118, 2009. Copyright John Wiley & Sons Limited. Reproduced and modified with permission.

Figure 4

Immunohistochemical staining for proteins of the mTOR pathway. A case of adenocarcinoma (harboring mutation of L858R in EGFR) showed positive stainingfor activated mTOR (p-mTOR), p-S6K and p-rS6, but not for p-4E -BP1. All activated proteins were observed in the cytoplasm, but p-S6K positivity was also found in the nuclei, and p-4E-BP1 was observed in apoptotic cells (arrow). Original magnification, ×200. mTOR, mammalian target of rapamycin; 4E-BP1, eukaryotic initiation factor 4E-binding protein 1; S6K, p70S6 kinase; rS6, 40S ribosomal protein S6. Reproduced from “Paradigm of kinase-driven pathway downstream of epidermal growth factor receptor/Akt in human lung carcinomas” by Y. Dobashi, etal., Human Pathology 42, 214-226, 2011. Copyright Elsevier Limited. Reproduced and modified with permission.

Figure 5

Protein levels analyzed by immunoblotting analysis in representative cases. The intensity of the signals was expressed as ratios relative to β-actin, which was arbitrarily assigned a value of 10 (not shown in this Figure). In the case of p-4E-BP1, the faster migrating bands were subjected to quantification. Abbreviations: N, non-neoplastic tissue; T, tumor tissue; AC, adenocarcinoma; SCC, squamous cell carcinoma; SmCC, small cell carcinoma, mTOR, mammalian target of rapamycin; S6K, p70S6 kinase; rS6, 40S ribosomal protein S6; 4E-BP1, eukaryotic initiation factor 4E-binding protein 1. Reproduced from “Paradigm of kinase-driven pathway downstream of epidermal growth factor receptor/Akt in human lung carcinomas” by Y. Dobashi, et al., Human Pathology 42, 214-226, 2011. Copyright Elsevier Limited. Reproduced and modified with permission.

Figure 6

(A). Prevalence of activation of the intermediate effectors in Akt/mTOR-mediated pathway evaluated by immunohistochemistry. The values indicate positive ratios in total cases of all non-small cell lung carcinoma. (B). Activation of effectors in mTOR/S6K/rS6 pathway in adenocarcinoma and squamous cell carcinoma evaluated by immunohistochemistry. The values indicate positive ratios in total cases. Abbreviations: mTOR, mammalian target of rapamycin; S6K, p70S6 kinase; rS6, 40S ribosomal protein S6; 4E-BP1, eukaryotic initiation factor 4E-binding protein 1.

Figure 7

Prevalence of activation in EGFR and representative intermediate effectors in Akt/mTOR-mediated pathway evaluated by immunohistochemistry. The values indicate positive ratios in total cases of non-small cell carcinoma (A) and in the cases harboring mutated EGFR (B). Abbreviations: EGFR, epidermal growth factor; mTOR, mammalian target of rapamycin; rS6, 40S ribosomal protein S6.