The hVps34-SGK3 pathway alleviates sustained PI3K/Akt inhibition by stimulating mTORC1 and tumourgrowth.

Natalia Shpiro,Fiona P Bailey,Darren Cross,Claire Crafter,Pau Castel,Dario R Alessi,José Baselga,Patrick A Eyers,Ruzica Bago,Eeva Sommer

doi:10.15252/embj.201693929

Abstract

We explore mechanisms that enable cancer cells to tolerate PI3K or Akt inhibitors. Prolonged treatment of breast cancer cells with PI3K or Akt inhibitors leads to increased expression and activation of a kinase termed SGK3 that is related to Akt. Under these conditions, SGK3 is controlled by hVps34 that generates PtdIns(3)P, which binds to the PX domain of SGK3 promoting phosphorylation and activation by its upstream PDK1 activator. Furthermore, under conditions of prolonged PI3K/Akt pathway inhibition, SGK3 substitutes for Akt by phosphorylating TSC2 to activate mTORC1. We characterise 14h, a compound that inhibits both SGK3 activity and activation in vivo, and show that a combination of Akt and SGK inhibitors induced marked regression of BT‐474 breast cancer cell‐derived tumours in a xenograft model. Finally, we present the kinome‐wide analysis of mRNA expression dynamics induced by PI3K/Akt inhibition. Our findings highlight the importance of the hVps34‐SGK3 pathway and suggest it represents a mechanism to counteract inhibition of PI3K/Akt signalling. The data support the potential of targeting both Akt and SGK as a cancer therapeutic.

Highlights

The majority of human tumours harbour mutations promoting inappropriate activation of the Akt kinase, and inhibitors of Akt and its upstream activators including Class I PI3K are being evaluated in many cancer clinical trials (Liu et al, 2009; Vanhaesebroeck et al, 2012; Bauer et al, 2015)
We demonstrate that prolonged treatment of several breast cancer cell lines (ZR-75-1, CAMA-1, T47D and BT474c) harbouring mutations that activate the Akt signalling pathway with inhibitors of Class I PI3K or Akt leads to marked upregulation of SGK3 mRNA and subsequent activation of the SGK3 protein kinase
The ability to rapidly upregulate the SGK3 pathway represents an ingenious solution because SGK3 possesses similar substrate specificity as Akt and the ability to phosphorylate at least a subset of overlapping substrates and because SGK3 can be activated independently of Class I PI3K via hVps34

Summary

Introduction

The majority of human tumours harbour mutations promoting inappropriate activation of the Akt kinase, and inhibitors of Akt and its upstream activators including Class I PI3K are being evaluated in many cancer clinical trials (Liu et al, 2009; Vanhaesebroeck et al, 2012; Bauer et al, 2015). PtdIns(3,4,5) P3 and its immediate breakdown product PtdIns(3,4)P2 recruit Akt and its upstream regulator phosphoinositide-dependent kinase 1 (PDK1) to the plasma membrane via their PtdIns(3,4,5)P3/PtdIns (3,4)P2-binding PH domains (Mora et al, 2004). This induces a conformational change in Akt enabling PDK1 to phosphorylate its T-loop Thr308 residue (Alessi et al, 1997a,b; Stokoe et al, 1997; Stephens et al, 1998). A recent study has suggested that PtdIns(3,4,5)P3 stimulates Ser473 phosphorylation by binding to the PH domain of the mTORC2 complex Sin subunit (Liu et al, 2015)

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