Nucleophosmin protein expression level, but not threonine 198 phosphorylation, is essential in growth and proliferation

Nucleophosmin (NPM), an oligomeric phosphoprotein and nucleolar target of the ARF tumor suppressor, contributes to several critical cellular processes. Previous studies have shown that the human NPM's phosphorylation by cyclin E cyclin-dependent kinase 2 (cdk2) on threonine (Thr) 199 regulates its translocation from the centrosome during cell cycle progression. Given our previous finding that ARF directly binds NPM, impeding its transit to the cytoplasm and arresting cells before S-phase entry, we hypothesized that ARF might also inhibit NPM phosphorylation. However, ARF induction did not impair phosphorylation of the cdk2 target residue in murine NPM, Thr(198). Furthermore, phosphorylation of Thr(198) occurred throughout the cell cycle and was concomitant with increases in overall NPM expression. To investigate the cell's presumed requirement for NPM-Thr(198) phosphorylation in promoting the processes of growth and proliferation, we examined the effects of a non-phosphorylatable NPM mutant, T198A, in a clean cell system in which endogenous NPM had been removed by RNA interference. Here, we show that the T198A mutant is fully capable of executing NPM's described roles in nucleocytoplasmic shuttling, ribosome export and cell cycle progression. Moreover, the proliferative defects observed with stable NPM knockdown were restored by mutant NPM-T198A expression. Thus, we demonstrate that the reduction in NPM protein expression blocks cellular growth and proliferation, whereas phosphorylation of NPM-Thr(198) is not essential for NPM's capacity to drive cell cycle progression and proliferation. Oncogene (2009) 28, 3209-3220; doi: 10.1038/onc.2009.178; published online 29 June 2009