ORIGINAL ARTICLE PIM1 kinase is destabilized by ribosomal stress causing inhibition of cell cycle progression V Iadevaia 1,4 , S Caldarola 1,4 , L Biondini 1 , A Gismondi 1 , S Karlsson 2 , I Dianzani 3 and F Loreni 1 1 Department of Biology, University Tor Vergata, Roma, Italy; 2 Department of Molecular Medicine and Gene Therapy, Lund Stem Cell Center, Lund University Hospital, Lund, Sweden and 3 Department of Medical Sciences, University of Eastern Piedmont, Novara, Italy PIM1 is a constitutively active serine/threonine kinase regulated by cytokines, growth factors and hormones. It has been implicated in the control of cell cycle progression and apoptosis and its overexpression has been associated with various kinds of lymphoid and hematopoietic malignancies. The activity of PIM1 is dependent on the phosphorylation of several targets involved in transcrip- tion, cell cycle and apoptosis. We have recently observed that PIM1 interacts with ribosomal protein (RP)S19 and cosediments with ribosomes. Defects in ribosome synthesis (ribosomal stress) have been shown to activate a p53- dependent growth arrest response. To investigate if PIM1 could have a role in the response to ribosomal stress, we induced ribosome synthesis alterations in TF-1 and K562 erythroid cell lines. We found that RP deficiency, induced by RNA interference or treatment with inhibitor of nucleolar functions, causes a drastic destabilization of PIM1. The lower level of PIM1 induces an increase in the cell cycle inhibitor p27 Kip1 and blocks cell proliferation even in the absence of p53. Notably, restoring PIM1 level by transfection causes a recovery of cell growth. Our data indicate that PIM1 may act as a sensor for ribosomal stress independently of or in concert with the known p53-dependent mechanisms. Oncogene (2010) 29, 5490–5499; doi:10.1038/onc.2010.279; published online 19 July 2010 Keywords: ribosome synthesis; cell cycle arrest; erythro- leukemia cells; PIM1; RPS19; p27 Kip1 Introduction PIM1 oncogene is highly expressed not only in hematopoietic cells, but also in other tissues, such as epithelial cells, prostate and hippocampus (Bachmann and Moroy, 2005). It is regulated by a variety of growth factors and cytokines at the transcriptional, post- transcriptional, translational and post-translational levels in response to stress conditions, such as ischemias and cellular hypoxia (Wang et al., 2001; Bachmann and Moroy, 2005; Shah et al., 2008). Oncogenic properties have been observed in animal models where PIM1 enhances the development of lymphoma and leukemia (Selten et al., 1985; van Lohuizen et al., 1989). In these experimental systems, PIM1 cooperates with MYC in lymphomagenesis, possibly by suppressing MYC- induced apoptosis. More recently, it has been shown that PIM1 colocalizes with MYC at sites of active transcription; thus, contributing to the expression of part of the MYC-regulated genes (Zippo et al., 2007). Animals deficient in PIM1 are viable and show only a minor defect in hematopoiesis (Laird et al., 1993). This suggests that PIM1 contributes to hematopoietic cell growth and survival, although its role is probably secondary to other signaling pathways. According to this, combined deficiency of PIM1 and AKT1 caused a severe impairment in hematopoietic cell growth, survival and proliferation (Hammerman et al., 2005). Several regulators of cell cycle progression and apoptosis have been identified as PIM1 targets, implicating it in the control of cell cycle and cell survival (Wang et al., 2001; Bachmann and Moroy, 2005). Among them, the cyclin- dependent kinase (CDK) inhibitor p27 Kip1 , shown to be phosphorylated and downregulated by PIM1 (Morishita et al., 2008), has been proposed as a specific controller of hematopoietic cell proliferation (Steinman, 2002; Soeiro et al., 2006). Deregulation of the expression of PIM1 kinase has been associated with the development of human malignancies, including lymphomas, leukemias, prostate cancer, squamous cell carcinoma, gastric carcinoma and colorectal carcinoma (reviewed by Shah et al., 2008). PIM1 kinase has been recently identified as an interactor of ribosomal protein (RP)S19 (Chiocchetti et al., 2005). The interaction, found in a two-hybrid screen, was confirmed by co-immunoprecipitation ex- periments. Moreover, PIM1 was shown to cosediment with ribosomal particles in sucrose gradient fractiona- tion analysis. To investigate the physiological function of the PIM1–ribosome interaction, we decided to test if alteration of ribosome synthesis could affect PIM1 expression. Ribosome synthesis is a complex process taking place sequentially in the nucleolus, in the nucleoplasm and in the cytoplasm. It involves four ribosomal RNA molecules, about 80 RPs and nearly 200 non-ribosomal factors that are required for the Received 9 January 2010; revised 9 May 2010; accepted 9 June 2010; published online 19 July 2010 Correspondence: Dr F Loreni, Department of Biology, University Tor Vergata, Via Ricerca Scientifica, Roma 00133, Italy. E-mail: loreni@uniroma2.it 4 These authors contributed equally to this work. Oncogene (2010) 29, 5490–5499 & 2010 Macmillan Publishers Limited All rights reserved 0950-9232/10 www.nature.com/onc