Novel Insights into the Regulation and Function of SGK3
Derailment of PI3K-Akt signalling is frequently implicated in cancer. While Akt is a highly studied kinase, comparatively little is known about serum and glucocorticoid activated kinase 3 (SGK3), which has similar substrate specificity to Akt and is also involved in growth and survival pathways. Although Akt is considered to be the major mediator of PI3K signalling, evidence has accumulated that SGK3 can play an Akt independent role in cancer. The major aims of my PhD were to define the regulatory mechanisms that control SGK3 activity and to delineate its unique physiological function(s) through identification of SGK3 selective substrate(s). While SGK3 shares 55% sequence identity with Akt in its catalytic domain, SGK3 is unique in that it possesses a PX domain unlike any other kinase known. The PX domain localises SGK3 to PI(3)P containing endosomal membranes. This raised the possibility that SGK3 is regulated differently to Akt. In this thesis, I present data, which provided some of the first evidence that SGK3 is regulated by both PI3K Class I and Class III (Vps34). I next demonstrate that the Vps34 - SGK3 signalling axis is responsive to growth factor stimulation in an analogous manner to PI3K Class I-Akt signalling. This represents a previously unrecognised mode of Vps34 regulation, whereby IGF1 enhances PI(3)P levels through a pathway involving the UV-RAG complex of Vps34 that leads to enhanced recruitment and activation of SGK3. Moreover, I delineate how PI3K Class I exerts control over SGK3 through two different routes, one involving mTORC2 regulation and another involving the generation of a second pool of PI(3)P via sequential dephosphorylation of PI(3,4,5)P3 by SHIP2 and INPP4A/B phosphatases. These highly versatile mechanisms of SGK3 regulation explain how the Vps34-SGK3 axis is able to substitute for Akt following inhibition of PI3K Class I/Akt pathways. They also illustrate robustness of SGK3 activity that can remain active and counteract physiological conditions or stresses where either PI3K Class I or III pathways are inhibited. Finally I present data leading to the discovery of novel in vivo substrates of SGK3 (and Akt) such as PanK4. Most importantly I identify the first physiological substrate unique to SGK3, the endosomal SNARE protein, Syntaxin 12 that is not phosphorylated by Akt. This finding represents the first, non-Akt related function of SGK3 at the endosomes and further explains why SGK3 requires regulation by two distinct classes of PI3K.
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