Cell growth regulation is one of the most fundamental mechanisms of life. To guarantee the life-long maintenance of a complex organism the combination of multiple growth, mitogenic and survival signals with cell specific responses is necessary. The balance between intrinsic developmental programs and extracellular growth signals therefore determines the size of an adult organism. The best characterized key regulatory pathway that controls cell growth, and thereby cell size, is the IGF/PI3K/Akt/mTORC1 axis. mTORC1 acts as a signaling node for diverse environmental signals regulating cell growth and protein synthesis. Activation of this pathway results in the phosphorylation and activation of its two major downstream targets, the 4E-BP1 and the S6K1.
A positive function for cell size regulation has been ascribed to S6K1, however there exist contradictory findings. Recently, eIF3 was identified to provide a major link between the mTORC1/S6K1 axis by acting as a docking platform for mTOR to activate S6K1. This highlights eIF3 as a potential player in mTORC1 mediated cell size regulation.
Here, we investigated the role of S6K1 as a mediator of mTORC1 induced cell size control and whether eIF3 and S6K1 have a joint role in cell size regulation. For this, we depleted binding partners of S6K1, namely two subunits of the eIF3 complex - eIF3b and eIF3c, to study their role in cell size regulation. We show that depletion of these subunits results in a strong decrease of protein synthesis, a reduction in cell proliferation and cell size that is accompanied with a surprising increase of S6K1 activity. This hyperactive S6K1 signaling was rapamycin sensitive, identifying mTORC1 as the responsible upstream kinase. Next, a specific S6K1 inhibitor, PF-4708671, was used to simultaneously block S6K1 activity in an eIF3b and/or eIF3c deficient system. We show that S6K1 inhibition was unable to modulate cell size of eIF3b and/or eIF3c depleted cells any further, suggesting that eIF3 may regulate cell size independently of S6K1 activity. This is further proven by the restoration of eIF3b or eIF3c expression in the depleted system showing a recovery of cell size defects without affecting S6K1 activity. To address the role of eIF3 in regulating cancer cell size we finally identified two cancer associated mutations in eIF3b and eIF3c with the ability to recover from the reduced size phenotype. In summary, our results present a novel role of eIF3 in the maintenance of cell size, independently of S6K1 activity.