ORIGINAL ARTICLE The cytoskeletal network controls c-Jun translation in a UTR-dependent manner P Polak, A Oren, I Ben-Dror, D Steinberg, S Sapoznik, A Arditi-Duvdevany and L Vardimon Department of Biochemistry, George S Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel The cytoskeleton is a dynamic network that undergoes restructuring during various cellular events, influencing cell proliferation, differentiation, and apoptosis. Here, we report that accumulation of c-Jun, a member of the AP1 family of transcription factors that play a key role in normal and aberrant cell growth, dramatically increases upon depolymerization of the cytoskeleton, and that, unexpectedly, this increase is controlled translationally. Depolymerization of the actin or microtubule network induces an increase in c-Jun accumulation with no corresponding increase in c-Jun mRNA or in the half- life of the c-Jun protein, but rather in the translatability of its transcript. This increase is mediated by the untrans- lated regions (UTRs) of c-Jun mRNA, and is not dependent on activated mitogen-activated protein kinase pathways. This novel mechanism of c-Jun regulation might be relevant to physiological conditions in which c-Jun plays a pivotal role. Oncogene (2006) 25, 665–676.doi:10.1038/sj.onc.1209114; published online 10 October 2005 Keywords: c-Jun; translation control; UTRs; cytoskele- tal network; cell density Introduction The c-Jun protein is a transcription factor that forms a variety of dimeric complexes, collectively termed AP-1. It stimulates cell-cycle progression through the induc- tion of genes coding for components of the cell-cycle clock machinery, and through the repression of tumor- suppressor genes such as p53 and Rb. Growth-promot- ing agents induce the expression of c-Jun in quiescent cells, while depletion of c-Jun results in growth arrest. The ability of c-Jun to control cell proliferation is consistent with its transforming activity. The c-jun proto-oncogene is the cellular analog of the avian sarcoma virus oncogene v-jun. Overexpression of v-Jun or c-Jun results in malignant transformation of several cell lines (for a review see Herschman, 1991; Shaulian and Karin, 2002). Expression of c-Jun is markedly increased upon exposure of cells to a remarkably diverse array of extracellular stimuli. Among these are peptide growth factors, proinflammatory cytokines, oxidative and other forms of cellular stress, and ultraviolet (UV) irradiation (Ryder and Nathans, 1988; Brenner etal., 1989; Devary et al., 1992; Kolbus et al., 2000; Shaulian et al., 2000). These external stimuli cause a rapid and dramatic increase in c-jun gene transcription, mainly through activation of the mitogen-activated protein (MAPK) family of serine/threonine kinases, and in particular by Jun N-terminal kinases (JNKs) and p38, which phos- phorylate the transcription factors c-Jun, ATF2, and MEF2C and thereby activate transcription of the c-jun gene (Devary et al., 1991; Gupta et al., 1995; Han and Prywes, 1995; Karin, 1995; van Dam et al., 1995; Han et al., 1997). While transcriptional control is the main mechanism for c-Jun regulation, phosphorylation of c- Jun by JNK also reduces the ubiquitin-dependent degradation of c-Jun (Musti et al., 1997) and UV irradiation increases transcript stability (Blattner et al., 2000). Studies have shown that c-Jun expression is also regulated by cell density and by the cytoskeletal network (Reisfeld and Vardimon, 1994; Lallemand et al., 1998; Oren et al., 1999). NIH 3T3 cells or chick retinal cells that are cultured at low cell density express a high level of c-Jun, whereas at high cell density or in intact retinal tissue the expression of c-Jun is low. Depolymerization of the cytoskeleton in the intact tissue results in a rapid and sustained increase in c-Jun protein accumulation. This increase is induced by various cytoskeleton- disrupting agents, including nocodazole (Noc), vinblastine, colchicine, cytochalasin B, latrunculin A, and latrunculin B, which differ in their biochemical properties and sites of action. The molecular mechanism(s) underlying the cell density and the cytoskeleton-dependent control of c-Jun expression are not known. The cytoskeleton is a dynamic network that under- goes restructuring during a variety of cellular events including formation of cellcell or cellextracellular matrix interactions, exposure to shear stress, infiltration and invasion, and the mitotic phase of the cell cycle. Restructuring of the cytoskeleton determines cell shape and influences gene expression. Recent studies have shown that changes in cell shape and in the underlying Received 3 February 2005; revised 2 August 2005; accepted 15 August 2005; published online 10 October 2005 Correspondence: Dr L Vardimon, Department of Biochemistry, George S Wise Faculty of Life Sciences, Tel Aviv University, 69978 Tel Aviv, Israel. E-mail: vardi@post.tau.ac.il Oncogene (2006) 25, 665–676 & 2006 Nature Publishing Group All rights reserved 0950-9232/06 $30.00 www.nature.com/onc