mTOR signaling sets the rate of ribosome export and protein synthesis through increased nucleophosmin translation and stability

Nucleophosmin (NPM) is a nucleolar phosphoprotein that has been implicated in numerous cellular processes. In particular, NPM interacts with nucleolar components of newly synthesized ribosomes to promote ribosome nuclear export. NPM is a classic mitogen-induced protein, with changes in its expression correlating with growth factor stimulation. In this study, we examined the underlying mechanism of NPM induction and demonstrate that hyperproliferative signals emanating from oncogenic H-RasV12 cause tremendous increases in NPM protein expression. Increases of NPM protein in response to RasV12 were not the result of increased NPM mRNA induction, but were instead the result of a dramatic rise in NPM mRNA translation rates and subsequent protein stabilization. NPM protein accumulation was dependent on mTOR activation, as rapamycin completely prevented NPM induction. Consistent with this finding, genetic ablation of Tsc1, a major upstream inhibitor of mTOR, resulted in NPM protein induction through increased translation of existing NPM mRNAs. Increases in NPM protein accumulation were suppressed by re-introduction of TSC1. Induction of NPM through Tsc1 loss resulted in a greater pool of actively translating ribosomes in the cytoplasm, higher overall rates of protein synthesis and increased cell proliferation, all of which were dependent on efficient NPM nuclear export. Consistent with loss of Tsc1 causing NPM levels to increase, we also found that p70S6K1, a major downstream effector of mTOR, caused NPM protein to accumulate through increased mRNA translation and protein stability. NPM protein accumulation in response to p70 S6K1 or in the absence of Tsc1 promoted the nuclear export of maturing ribosome subunits, providing a mechanistic link between TSC1/mTOR/p70S6K1 signaling, NPM-mediated nuclear export of ribosome subunits, protein synthesis levels, and cell growth.