AKT induces senescence in human cells via mTORC1 and p53 in the absence of DNA damage: implications for targeting mTOR during malignancy

Abstract

The phosphatidylinositol 3-kinase (PI3K)/AKT and RAS oncogenic signalling modules are frequently mutated in sporadic human cancer. Although each of these pathways has been shown to play critical roles in driving tumour growth and proliferation, their activation in normal human cells can also promote cell senescence. Although the mechanisms mediating RAS-induced senescence have been well characterised, those controlling PI3K/AKT-induced senescence are poorly understood. Here we show that PI3K/AKT pathway activation in response to phosphatase and tensin homolog (PTEN) knockdown, mutant PI3K, catalytic, α polypeptide (PIK3CA) or activated AKT expression, promotes accumulation of p53 and p21, increases cell size and induces senescence-associated β-galactosidase activity. We demonstrate that AKT-induced senescence is p53-dependent and is characterised by mTORC1-dependent regulation of p53 translation and stabilisation of p53 protein following nucleolar localisation and inactivation of MDM2. The underlying mechanisms of RAS and AKT-induced senescence appear to be distinct, demonstrating that different mediators of senescence may be deregulated during transformation by specific oncogenes. Unlike RAS, AKT promotes rapid proliferative arrest in the absence of a hyperproliferative phase or DNA damage, indicating that inactivation of the senescence response is critical at the early stages of PI3K/AKT-driven tumourigenesis. Furthermore, our data imply that chronic activation of AKT signalling provides selective pressure for the loss of p53 function, consistent with observations that PTEN or PIK3CA mutations are significantly associated with p53 mutation in a number of human tumour types. Importantly, the demonstration that mTORC1 is an essential mediator of AKT-induced senescence raises the possibility that targeting mTORC1 in tumours with activated PI3K/AKT signalling may exert unexpected detrimental effects due to inactivation of a senescence brake on potential cancer-initiating cells.

Akt1 is the principal Akt isoform regulating apoptosis in limiting cytokine concentrations

“…This is consistent with recent observations that the oncogenic effects of Akt activation are most apparent when the tumour suppressor function of p53 is lost. 28 Interestingly, in our hands, the deletion of p53 also renders cells resistant to apoptosis triggered by Akt inhibitory drugs. Although Akt activation is a frequent feature of many malignancies, including acute myeloid leukaemia, 29 our data indicate that Akt activation is not sufficient to promote leukaemiagenesis.…”

“…31 Several mechanisms may link cytokine-mediated Akt activation to p53. First, the p53-regulator MDM2 is a substrate of Akt, 4,28,32 and Akt can control the abundance of p53 by regulating this E3 ligase responsible for p53 turnover. Second, at least in the context of enforced Akt activation, Akt may maintain glucose uptake in the face of IL-3 derivation, perhaps by maintaining the activity of specific glucose transporters.…”

Early-onset Evans syndrome, immunodeficiency, and premature immunosenescence associated with tripeptidyl-peptidase II deficiency

“…36 One conceivable target is the Akt/mTOR pathway. Its activation has previously been linked to senescence associated b-galactosidase activity in human fibroblasts 37 and to CD8 T-cell senescence. 38 In fact, recent observations in patients with gain-of-function mutations in the PI3K catalytic subunit or loss of the regulatory subunit p85 suggest that activation of this pathway may be associated with immunodeficiency and autoimmunity in human patients.…”

Impaired caudal fin‐fold regeneration in zebrafish deficient for the tumor suppressor Pten

“…However, this response occurs in tumor cells, whereas the cells of developing ptena −/− ptenb −/− embryos are otherwise normal. Oncogenes expressed in normal healthy cells induce senescence (Gorgoulis & Halazonetis, 2010), and hyper‐active AKT has been shown to do the same in mouse embryonic and human fibroblasts (Astle et al., 2012; Chen et al., 2005). Moreover, T‐cell senescence occurs in some immunodeficient patients with germline gain‐of‐function mutations in PI3K (Lucas et al., 2014).…”

“…These responses are elicited in immortalized cell lines or cell lines following irradiation‐induced DNA damage, which are not normal, WT cells or conditions. Hyper‐active AKT in normal human cells initiates the release of pro‐inflammatory cytokines, a hallmark of senescence‐associated secretory phenotype (Astle et al., 2012), which is thought to induce apoptosis of senescent cells (Freund, Orjalo, Desprez, & Campisi, 2010). Thus, the elevated p‐AKT in ptena −/− ptenb −/− embryos may induce senescence and apoptosis.…”