LST8 is a component of both mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2). 
Herein, to examine the role of LST8, a common component of mTOR complexes, in 
the regulation of mTORC1 and mTORC2, first, we showed over-expression of LST8 in 
HepG2 to markedly enhance basal phosphorylation levels of not only p70 S6 kinase 
but also Akt. In contrast, LST8 knockdown by siRNA in HepG2 decreased 
phosphorylation levels of both p70 S6 kinase and Akt. These results indicate the 
LST8 expression level to determine basal mTORC1 and mTORC2 activities, since 
LST8 appears to be the component present at the lowest level in both mTORC1 and 
mTORC2 complexes. Previously, we reported S6 kinase phosphorylation to be 
reduced by over-expression of the Cterminally deleted Raptor mutant (Raptor-ΔCT) 
not binding to mTOR or LST8, while phosphorylation levels of Akt were markedly 
enhanced with no alteration in IRS-1 phosphorylation or PI 3-kinase activity. 
Using Raptor-ΔCT, we investigated the competition for association with LST8 
between mTORC1 and mTORC2. Over-expression of Raptor-ΔCT abolished formation of 
the Raptor, S6 kinase, mTOR and LST8 complex, while the amount of LST8 in the 
Rictor-mTOR complex was increased. Therefore, it is likely that Raptor-mTOR and 
Rictor-mTOR complexes compete for association with LST8, and this mechanism may 
contribute to the reciprocal negative regulations of mTORC1 and mTORC2 
activities, in terms of their LST8 components.: