Potential therapeutic effects of the MTOR inhibitors for preventing ageing and progeria-related disorders

Abstract

The mammalian target of rapamycin (mTOR) pathway is an highly conserved signal transduction axis involved in many cellular processes, such as cell growth, survival, transcription, translation, apoptosis, metabolism, motility and autophagy. Recently, this signalling pathway has come to the attention of the scientific community owing to the unexpected finding that inhibition of mTOR by rapamycin, an antibiotic with immunosuppressant and chemotherapeutic properties, extends lifespan in diverse animal models. Moreover, rapamycin has been reported to rescue the cellular phenotype in a progeroid syndrome [Hutchinson-Gilford Progeria syndrome (HGPS)] that recapitulates most of the traits of physiological ageing. The promising perspectives raised by these results warrant a better understanding of mTOR signalling and the potential applications of mTOR inhibitors to counteract ageing-associated diseases and increase longevity. This review is focused on these issues.

Keywords: Hutchinson-Gilford Progeria syndrome (HGPS); ageing; lamin A; mTOR inhibitors; progeria-related diseases; rapamycin.

Figure 1

Cellular signalling upstream and downstream of mammalian target of rapamycin (mTOR) complexes. (A) mTOR pathways and functional relationships. mTOR forms two multiprotein complexes – mTOR complex (mTORC) 1 and mTORC2. The classical mTORC1‐positive inputs are growth factors, chemokines, nutrients and cell energy balance. Growth factors stimulate mTORC1 through the phosphatidylinositol (PI) 3‐kinase (PI3K)/Akt signalling pathway. Akt phosphorylates tuberous sclerosis complex 2 (TSC2). TSC2 is a GTPase‐activating protein that functions in association with TSC1 to inactivate the small G protein Rheb, which, in turn, upregulates mTORC1 activity. mTORC1 activity is required for the phosphorylation and subsequent activation of ribosomal S6 kinase 1 (S6K1), which regulates the elongation step of protein translation, ribosomal protein S6 (RPS6) and eukaryotic translation initiation factor 4B (eIF4B), ultimately promoting the initiation of translation and elongation. mTORC1 also phosphorylates and inactivates the translation inhibitor eIF4E‐binding protein 1 (4E‐BP1), which inhibits cap‐dependent translation. The mTOR pathway may be also regulated by the LKB1/AMP‐activated protein kinase (AMPK) pathway. AMPK phosphorylates TSC2, activating the TSC1/TSC2 complex and thus repressing mTORC1 activity. Moreover, AMPK phosphorylates Raptor, inducing Raptor and mTORC1 disassembly/inhibition. mTORC2 regulates cell survival through serum‐ and glucocorticoid‐activated kinase 1 (SGK1) and Akt. mTORC2 phosphorylates Akt at Ser473, priming Akt for further phosphorylation by phosphoinositide‐dependent kinase 1 (PDK1) at the Thr308 residue. Loss of phosphorylation at the Ser473 site, however, affects only some Akt substrates, such as Forkhead Homeobox type O (FOXO) transcription factors, but not TSC2, in response to growth factor signalling. mTORC2 also associates with actively translating ribosomes to phosphorylate cotranslationally Akt (at Thr450), which prevents the ubiquitination and degradation of Akt. mTORC2 is involved in the spatial control of cell growth via cytoskeletal regulation. (B) mTOR and autophagy. Under nutrient‐rich conditions, mTOR associates with and inhibits the UNC51‐like kinase (ULK) complex by phosphorylating autophagy‐related (ATG) 13 and ULK1. By contrast, stress signals or AMPK activation lead to mTORC1 dissociation from ULK1 and to the activation of the ULK complex, thereby triggering the autophagy machinery. Arrows indicate activating events, whereas perpendicular lines indicate inhibitory events. FIP200, focal adhesion kinase family‐interacting protein of 200 kD; FKBP38, FK‐506‐binding protein; GPCR, G protein‐coupled receptor; IGF1‐R, insulin‐like growth factor 1 receptor; GSK3β, glycogen synthase kinase 3β; IRS1, insulin receptor substrate 1; LKB1, liver kinase B1; mLST8, mammalian lethal with sec‐13 protein 8; PI(3,4,5)‐P3, phosphatidylinositol (3,4,5)‐trisphosphate; PRAS40, proline‐rich Akt substrate of 40‐kDa; PTEN, phosphatase and tensin homologue; SIN1, stress‐activated protein kinase‐interacting protein 1; VPS, vacuolar protein sorting

Figure 2

Mammalian target of rapamycin (mTOR) inhibitors. mTORC1 inhibitors are allosteric inhibitors that lead to the dissociation of Raptor from mTOR complex (mTORC) 1 and loss of contact between mTORC1 and its substrates. Dual phosphatidylinositol 3‐kinase PI3K/mTOR inhibitors target both PI3K and mTORC1/mTORC2, acting on the catalytic sites of PI3K and mTOR. mTORC1/mTORC2‐selective inhibitors target the catalytic site of the enzyme, thus acting on both mTORC1 and mTORC2. AMP‐activated protein kinase (AMPK) activator has a negative effect on mTORC1. Arrows indicate activating events; perpendicular lines indicate inhibitory events. FKBP38, FK‐506‐binding protein; mLST8, mammalian lethal with sec‐13 protein 8; PRAS40, proline‐rich Akt substrate of 40‐kDa

Figure 3

The mammalian target of rapamycin (mTOR) complex (mTORC1) signalling pathway contributes to ageing through various cellular processes. Active mTORC1 activates stem cell turnover, cellular senescence and protein translation (arrows), while it inhibits autophagy. These conditions contribute to the ageing of an organism. See text for details. FKBP38, FK‐506‐binding protein; mLST8, mammalian lethal with sec‐13 protein 8; PRAS40, proline‐rich Akt substrate of 40‐kDa

Figure 4

Undetectable mammalian target of rapamycin (mTOR) activity in Hutchinson–Gilford Progeria syndrome (HGPS) fibroblasts. HGPS fibroblasts at passage 20 were cultured and lysed, as described by Cenni et al 116. Immunoblotting was performed with the following antibodies: progerin (Enzo Life Sciences, Inc. Farmingdale, NY, USA.), phospho‐mTOR [P‐mTOR (Ser2448)], mTOR and microtubule‐associated protein 1 light chain 3 (LC3), respectively [#2971, #2972 and #4108 (Cell Signaling Technology Inc. Danvers, MA, USA)]. Lamin A/C (#6215, Santa Cruz Biotechnology, Inc. Dallas, TX, USA), β‐tubulin and flag (both from Sigma‐Aldrich, Saint Louis, MO, USA). The immunoblotted band between lamin A and lamin C bands corresponds to progerin