Date of Graduation
8-2012
Document Type
Dissertation (PhD)
Program Affiliation
Cancer Biology
Degree Name
Doctor of Philosophy (PhD)
Advisor/Committee Chair
Dos Sarbassov
Committee Member
Xin Lin
Committee Member
Mong-Hong Lee
Committee Member
Georg Halder
Committee Member
Hui-Kuan Lin
Abstract
Growth factor signaling promotes anabolic processes via activation of the
PI3K-Akt kinase cascade. Deregulation of the growth factor-dependent PI3K-Akt
pathway was implicated in tumorigenesis. Akt is an essential serine/threonine
protein kinase that controls multiple physiological functions such as cell growth,
proliferation, and survival to maintain cellular homeostasis. Recently, the
mammalian Target of Rapamycin Complex 2 (mTORC2) was identified as the
main Akt Ser-473 kinase, and Ser-473 phosphorylation is required for Akt
hyperactivation. However, the detailed mechanism of mTORC2 regulation in
response to growth factor stimulation or cellular stresses is not well understood.
In the first project, we studied the regulation of the mTORC2-Akt signaling
under ER stress. We identified the inactivation of mTORC2 by glycogen synthase
kinase-3β (GSK-3β). Under ER stress, the essential mTORC2 component, rictor,
is phosphorylated by GSK-3β at Ser-1235. This phosphorylation event results in
the inhibition of mTORC2 kinase activity by interrupting Akt binding to mTORC2.
Blocking rictor Ser-1235 phosphorylation can attenuate the negative impacts of
GSK-3β on mTORC2/Akt signaling and tumor growth. Thus, our work
demonstrated that GSK-3β-mediated rictor Ser-1235 phosphorylation in response
to ER stress interferes with Akt signaling by inhibiting mTORC2 kinase activity.
In the second project, I investigated the regulation of the mTORC2 integrity.
We found that basal mTOR kinase activity depends on ATP level, which is tightly
regulated by cell metabolism. The ATP-sensitive mTOR kinase is required for
SIN1 protein phosphorylation and stabilization. SIN1 is an indispensable subunit of
mTORC2 and is required for the complex assembly and mTORC2 kinase activity.
Our findings reveal that mTOR-mediated phosphorylation of SIN1 is critical for
maintaining complex integrity by preventing SIN1 from lysosomal degradation.
In sum, our findings verify two distinct mTORC2 regulatory mechanisms via
its components rictor and SIN1. First, GSK-3β-mediated rictor Ser-1235
phosphorylation results in mTORC2 inactivation by interfering its substrate binding
ability. Second, mTOR-mediated Ser-260 phosphorylation of SIN1 preserves its
complex integrity. Thus, these two projects provide novel insights into the
regulation of mTORC2.
Keywords
cell signaling, mTORC2, Akt, GSK-3 beta, ER stress, complex integrity