TSC1 Sets the Rate of Ribosome Export and Protein Synthesis through Nucleophosmin Translation

Abstract

Nucleophosmin (B23) is a nucleolar phosphoprotein that has been implicated in numerous cellular processes. In particular, nucleophosmin interacts with nucleolar components of newly synthesized ribosomes to promote ribosome nuclear export. Nucleophosmin 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 nucleophosmin induction and showed that hyperproliferative signals emanating from oncogenic H-Ras V12 cause tremendous increases in nucleophosmin protein expression. Nucleophosmin protein accumulation was dependent on mammalian target of rapamycin (mTOR) activation, as rapamycin completely prevented nucleophosmin induction. Consistent with this finding, genetic ablation of Tsc1, a major upstream inhibitor of mTOR, resulted in nucleophosmin protein induction through increased translation of existing nucleophosmin mRNAs. Increases in nucleophosmin protein accumulation were suppressed by reintroduction of TSC1. Induction of nucleophosmin 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 nucleophosmin nuclear export. Nucleophosmin protein accumulation in the absence of Tsc1 promoted the nuclear export of maturing ribosome subunits, providing a mechanistic link between TSC1/mTOR signaling, nucleophosmin-mediated nuclear export of ribosome subunits, protein synthesis levels, and cell growth.

Nucleophosmin Mediates Mammalian Target of Rapamycin–Dependent Actin Cytoskeleton Dynamics and Proliferation in Neurofibromin-Deficient Astrocytes

“…Furthermore, we show that NPM expression is regulated by mTOR, as mTOR inhibition by rapamycin inhibits NPM expression both in vitro and in vivo. This supports previous studies showing that NPM is highly expressed in TSC1 À/À fibroblasts and is blocked by rapamycin treatment (27).…”

“…Expression of a NPM mutant that is unable to shuttle from the nucleus to the cytoplasm blocks ribosomal export (26). In addition, treatment with rapamycin inhibits NPM protein expression in MEFs (27). In Nf1 À/À astrocytes, we also found that mTOR inhibition by rapamycin dramatically reduced NPM expression in Nf1 À/À astrocytes with no effect on wild-type astrocytes (Fig.…”

“…5A). This is in stark contrast to NPM regulation in primary MEFs where S6K is a potent inducer of NPM protein expression (27) and highlights the potential differences in mTOR signaling mechanisms between fibroblasts and astrocytes. However, when Rac1 N17 was expressed in Nf1 À/À astrocytes, NPM expression was attenuated (Fig.…”

“…Inactivation of either TSC1 or TSC2 results in mTOR pathway hyperactivation in vitro and in vivo (34,35), and inactivation of the TSC1/TSC2 complex promotes increased cell proliferation (36)(37)(38). Moreover, loss of TSC1 results in dramatic increases in NPM protein expression and NPM-dependent increases in ribosomal biogenesis (27). The TSC1/TSC2 gene products also regulate actin cytoskeleton dynamics.…”

Deconvoluting mTOR biology

“…123 Indeed, one of the major targets of mTORC1 signaling is the nucleophosmin (NPM) proto-oncogene, a TOP-containing mRNA. Growth signals, such as those emanating from RAS, stimulate the translation of existing NPM mRNAs in a rapamycin-sensitive mechanism 124 through the binding of far upstream binding protein-1 (FUBP-1) to the 3' untranslated region (UTR) of NPM mRNA. 125 In agreement with this finding, astrocytes lacking Nf1 express elevated NPM protein levels and exhibit enhanced protein synthesis.…”