Mechanism of activation of SGK3 by growth factors via the Class 1 and Class 3 PI3Ks.

Nazma Malik,Hannah Tovell,Annika Hornberger,Thomas Macartney,Karen E Anderson,Alan R Prescott,Dario R Alessi

doi:10.1042/bcj20170650

Abstract

Derailment of the PI3K-AGC protein kinase signalling network contributes to many human diseases including cancer. Recent work has revealed that the poorly studied AGC kinase family member, SGK3, promotes resistance to cancer therapies that target the Class 1 PI3K pathway, by substituting for loss of Akt kinase activity. SGK3 is recruited and activated at endosomes, by virtue of its phox homology domain binding to PtdIns(3)P. Here, we demonstrate that endogenous SGK3 is rapidly activated by growth factors such as IGF1, through pathways involving both Class 1 and Class 3 PI3Ks. We provide evidence that IGF1 enhances endosomal PtdIns(3)P levels via a pathway involving the UV-RAG complex of hVPS34 Class 3 PI3K. Our data point towards IGF1-induced activation of Class 1 PI3K stimulating SGK3 through enhanced production of PtdIns(3)P resulting from the dephosphorylation of PtdIns(3,4,5)P3. Our findings are also consistent with activation of Class 1 PI3K promoting mTORC2 phosphorylation of SGK3 and with oncogenic Ras-activating SGK3 solely through the Class 1 PI3K pathway. Our results highlight the versatility of upstream pathways that activate SGK3 and help explain how SGK3 substitutes for Akt following inhibition of Class 1 PI3K/Akt pathways. They also illustrate robustness of SGK3 activity that can remain active and counteract physiological conditions or stresses where either Class 1 or Class 3 PI3K pathways are inhibited.

Highlights

serum and glucocorticoid-regulated kinase-3 (SGK3) belongs to the protein kinase A (AGC) family of protein kinases that orchestrate a wide range of biological responses including controlling cell growth, proliferation, metabolism and intracellular trafficking possesses [1]
No SGK3 activity or T-loop phosphorylation was observed in SGK3 knock-out HEK293 cells analysed in parallel, validating the assay approach (Figure 1A)
We note that addition of 14 h and MK2206 to SGK3 knock-out cells reduces the low levels of N-myc downstream-regulated gene-1 (NDRG1) phosphorylation further compared with MK2206 alone

Summary

Introduction

SGK3 (serum- and glucocorticoid-regulated kinase family member 3) belongs to the AGC family of protein kinases that orchestrate a wide range of biological responses including controlling cell growth, proliferation, metabolism and intracellular trafficking possesses [1]. Like other members of the AGC kinases including closely related Akt isoforms, SGK3 is activated following phosphorylation of its kinase domain T-loop Thr320 residue by PDK1 (3-phosphoinositide-dependent kinase-1) and phosphorylation of its C-terminal Ser422 hydrophobic motif residue by mTORC2 (mTOR complex 2) [3,4]. Like other AGC kinase members, phosphorylation of the hydrophobic residue of SGK3 by mTORC2 promotes T-loop phosphorylation and activation by PDK1 [5,6,7]. The two other SGK isoforms, namely SGK1 and SGK2, do not possess any phosphoinositide-binding domain and their activity is regulated by mTORC2 phosphorylating the hydrophobic motif residue, triggering activation by PDK1 [5,8,9]. PtdIns(3)P can be generated through breakdown of PtdIns(3,4,5)P3

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Results

Conclusion