AKT overactivation can suppress DNA repair via p70S6 kinase-dependent downregulation of MRE11

D Piscitello,M G Vizioli,D Varshney,C Reid,D A Gillespie,P D Adams,A Seluanov,V Gorbunova,S Lilla

doi:10.1038/onc.2017.340

Abstract

Deregulated AKT kinase activity due to PTEN deficiency in cancer cells contributes to oncogenesis by incompletely understood mechanisms. Here, we show that PTEN deletion in HCT116 and DLD1 colon carcinoma cells leads to suppression of CHK1 and CHK2 activation in response to irradiation, impaired G2 checkpoint proficiency and radiosensitization. These defects are associated with reduced expression of MRE11, RAD50 and NBS1, components of the apical MRE11/RAD50/NBS1 (MRN) DNA damage response complex. Consistent with reduced MRN complex function, PTEN-deficient cells fail to resect DNA double-strand breaks efficiently after irradiation and show greatly diminished proficiency for DNA repair via the error-free homologous recombination (HR) repair pathway. MRE11 is highly unstable in PTEN-deficient cells but stability can be significantly restored by inhibiting mTORC1 or p70S6 kinase (p70S6K), downstream kinases whose activities are stimulated by AKT, or by mutating a residue in MRE11 that we show is phosphorylated by p70S6K in vitro. In primary human fibroblasts, activated AKT suppresses MRN complex expression to escalate RAS-induced DNA damage and thereby reinforce oncogene-induced senescence. Taken together, our data demonstrate that deregulation of the PI3K-AKT/ mTORC1/ p70S6K pathways, an event frequently observed in cancer, exert profound effects on genome stability via MRE11 with potential implications for tumour initiation and therapy.

Highlights

Phosphatase and tensin homologue deleted on chromosome 10(PTEN) is a tumour suppressor gene found altered in multiple sporadic tumours.[1,2] Lesions in the PTEN gene occur at almost the same frequency as TP53 alterations in certain tumour types.[3]
PTEN deficiency suppresses DNA damage signalling via MRN complex hypomorphism To investigate the impact of PTEN deficiency on DNA damage signalling, we first compared irradiation-induced activation of CHK1 and CHK2 in HCT116 colon carcinoma cells and an isogenic sister cell line in which PTEN was ablated by gene targeting
We found that the basal expression level of both MRE11 and its partner protein RAD50 were significantly diminished in HCT116 PTEN− / − cells compared with the control (Figure 1a, left)

Summary

Introduction

Phosphatase and tensin homologue deleted on chromosome 10(PTEN) is a tumour suppressor gene found altered in multiple sporadic tumours.[1,2] Lesions in the PTEN gene occur at almost the same frequency as TP53 alterations in certain tumour types.[3]. Signalling, so that impaired PTEN function leads to unrestrained activation of its downstream signals and results in high levels of constitutively active AKT.[4] AKT is key node on the PI3K pathway and controls the activation of the major signalling pathways for cell growth, survival and metabolism by phosphorylating many downstream signalling targets.[5] The amplification of PIK3CA (the gene encoding for the p110α catalytic subunit of PI3K) causes growth factor-independent constitutive activation of AKT, and is often found in ovarian and cervical cancers.[6,7,8] mTOR complex 1. (mTORC1) is a well known effector of activated AKT. AKT signals through direct phosphorylation of the TSC1/TSC2 complex to indirectly activate mTORC1.9,10 A crucial effector of mTORC1 is 40S ribosomal protein S6 kinase (S6K).[11] S6K directly regulates ribosome biogenesis, cell cycle progression, protein synthesis and metabolism.[12,13]

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