REPRESSION OF CYCLIN D1 EXPRESSION DOES NOT CONTRIBUTE TO INITIATION OR MAINTENANCE OF CELL-TRANSFORMATION BY ADENOVIRUS TYPE-5 E1

Expression of the gene encoding the cell cycle regulator cyclin D1 is strongly repressed in adenovirus type 5 E1 (Ad5E1)-transformed cells. Since cyclin D1 is a regulator of cell proliferation, modulation of it s abundance may affect cell cycle control. Therefore, we studied the i mportance of cyclin D1 repression for fell transformation by Ad5E1. We found that forced expression of cyclin D1 does not affect the transfo rming potential of Ad5E1. Similarly, cyclin D1 overexpression did not affect the efficiency of colony formation, the proliferation rate, or the cell cycle distribution of Ad5El-transformed cell lines, whereas t he colony formation of untransformed cell lines was strongly inhibited . Thus, repression of cyclin D1 expression is not required for initiat ion or maintenance of cell transformation by Ad5E1. In addition, we sh ow that the growth-suppressive effect of cyclin D1 correlates with cyc lin D1 binding to cdk4 rather than to proliferating cell nuclear antig en PCNA.