Journal of Cellular Biochemistry 90:253–266 (2003) ARTICLES Suppression of AP-1 Constitutive Activity Interferes With Polyomavirus MT Antigen Transformation Ability Sheila Maria Brochado Winnischofer, Maria Leonor Sarno de Oliveira, and Mari Cleide Sogayar* Instituto de Quı ´mica-Universidade de Sa ˜o Paulo, CP 26077, 05513-970 Sa ˜o Paulo, Brasil Abstract Polyomavirus (Py) encodes a potent oncogene, the middle T antigen (MT), that induces cell transformation by binding to and activating several cytoplasmic proteins which take part in transduction of growth factors-induced mitogenic signal to the nucleus. We have previously reported that the AP-1 transcriptional complex is a target for MT during cell transformation although, its activation was not sufficient for establishment of the transformed phenotype. Here we show that expression of a dominant-negative cJun mutant in MT transformed cell lines inhibits its transformation ability, indicating that constitutive AP-1 activity is necessary for cell transformation mediated by MT. Evidences also suggest that proliferation of MT transformed cells in low serum concentrations and their ability to form colonies in agarose are controlled by distinct mechanisms. J. Cell. Biochem. 90: 253 – 266, 2003. ß 2003 Wiley-Liss, Inc. Key words: polyomavirus; middle T antigen; AP-1 activation; cell transformation The early region of polyomavirus (Py) encodes three tumor antigens, namely, large T (LT), middle T (MT), and small T (ST). The LT antigen causes immortalization of primary cell lines [Schlegel and Benjamin, 1978; Rassoulzadegan et al., 1982; Cherington et al., 1986] by binding to the product of the Rb tumor suppressor gene (Retinoblastoma) [Freund et al., 1992]. It has been shown that interaction between LT and RB affects p53-dependent cell cycle arrest [Doherty and Freund, 1997]. More recently, it has been suggested that coexpression of Py MT and ST antigens can inhibit both p53-induced cell cycle arrest and apoptosis [Qian and Wiman, 2000]. The ST antigen is able to induce cell prolifera- tion, in a manner dependent on its binding to protein phosphatase 2A [Mullane et al., 1998]. ST expression promotes both high saturation densities and changes in the cell cytoskeleton [Liang et al., 1984; Cherington et al., 1986]. Since MT alone can induce morphological transformation and alterations in the growth properties of established cell lines [Schlegel and Benjamin, 1978; Treisman et al., 1981; Cherington et al., 1986], it is believed that it plays a central role during Py transformation and tumorigenesis. The MT phosphoprotein associates with the cell membrane and does not appear to display any enzymatic activity of its own. Instead, it exerts its effects in the host cell by interacting with, and altering the activities of essential growth regulatory proteins that have been implicated in normal cell growth control [Pallas et al., 1988; Armelin and Oliveira, 1996; Oliveira et al., 1999; Ichaso and Dilworth, 2001; Dilworth, 2002]. Examples of cellular proteins that bind to MT are: pp60 c-src , c-yes, and c-fyn kinases [Courtneidge and Smith, 1983; Kornbluth et al., 1986; Kaplan et al., 1987; Horak et al., 1989; Su et al., 1995; Dunant et al., 1996], phosphatidylinositol 3-kinase (PI3K) [Whitman et al., 1985], protein phos- phatase 2A [Glenn and Eckhart, 1993; Camp- bell et al., 1995], Shc [Campbell et al., 1994; Dilworth et al., 1994; Blaikie et al., 1997], phospholipase Cg (PLCg) [Su et al., 1995], and 14-3-3 [Pallas et al., 1994; Cullere ´ et al., 1998]. Association of MT antigen with these cellular proteins occurs, in general, through SH2 or PTB domains present in these proteins and MT ß 2003 Wiley-Liss, Inc. Maria Leonor S. de Oliveira is on leave of absence from the Centro de Biotecnologia-Instituto Butantan, 05503-900 Sa ˜o Paulo, Brasil. Grant sponsor: FAPESP; CNPq; FINEP; and PRP-USP. *Correspondence to: Mari Cleide Sogayar, Chemistry Institute, University of Sa ˜o Paulo, CP 26077, Sa ˜o Paulo 05513-970, SP, Brazil. E-mail: mcsoga@iq.usp.br Received 7 May 2003; Accepted 23 June 2003 DOI 10.1002/jcb.10628