A ROLE FOR THE SMALL GTPASE RAC IN POLYOMAVIRUS MIDDLE-T ANTIGEN-MEDIATED ACTIVATION OF THE SERUM RESPONSE ELEMENT AND IN CELL-TRANSFORMATION

The oncogenic proteins encoded by papovaviruses, the tumor antigens, h ave been extensively used as model systems to study mitogenic signalin g and cell transformation, These proteins stimulate cell growth in cul tured cells and induce tumors in virus infected or transgenic animals, One of these proteins, polyomavirus middle-T, acts like a constitutiv ely activated tyrosine growth factor receptor, Middle-T recruits sever al cellular enzymes into a multifunctional complex located at cellular membranes, This results in the activation of cellular enzymes involve d in the regulation of cell signaling, like tyrosine kinases of the Sr c family, a phosphatidylinositol 3-kinase and a GDP/GTP exchange facto r for Ras, These activities are all required for stimulation of cell g rowth by middle-T and activate members of the MAP kinase family, Here we investigate the role of T antigen-activated pathways in the stimula tion of transcription of immediate early genes, These genes are essent ial for progression of resting cells into S phase, Our data show that Rho family GTPases play an essential role in cell transformation by mi ddle-T. Furthermore, we demonstrate that the c-fos promoter is activat ed by two Pas-initiated signaling cascades, One is Raf-dependent and r equires binding of SHC and PI 3-kinase to the middle-T complex, This p athway signals via ternary complex factor (TCF) to the serum response element (SRE) of the c-fos promoter, Signaling to TCF by Raf also depe nds on functional Pac, but not CDC42, as demonstrated in luciferase re porter assays with an ETS domain-containing promoter, The second pathw ay is Raf-independent, does not require SHC but functional PI 3-kinase , and transduces signals via Pac to serum response factor (SRF), Micro injection of dominant negative Rad blocks nuclear translocation of ERK 1 in middle-T-expressing cells, This lends support to the idea that th e two signaling cascades initiated by Pas show crosstalk at the level of MAP kinase-mediated signaling to nuclear transcription factors.