CELL-CYCLE PROGRESSION, MORPHOLOGY AND CONTACT INHIBITION ARE REGULATED BY THE AMOUNT OF SV40 T-ANTIGEN IN IMMORTAL HUMAN-CELLS

Expression of Simian Virus 40 (SV40) T antigen in human dermal fibrobl asts over-rides the normal controls on cell division leading to change s in cellular proliferation and life span. These changes are accompani ed by other changes in cell morphology, expression of cell specific fu nctions, and altered cell-cell interactions. In this study, we have ex amined the effects of different amounts of T antigen on cell cycle pro gression, life span and morphology in human dermal fibroblasts and dem onstrated T antigen to be a concentration dependent regulator of the c ell cycle. Using a novel, metal inducible episomal expression vector ( p735.6) which produces low basal levels of protein but high (greater t han 100-fold) levels of induction, we have compared the effects of low and high levels of T antigen expression in a precrisis and immortalis ed human line (1BRMT1). The presence of inducing agent led to maximal levels of T antigen expression and resulted in cultures with a high ra te of proliferation, an extended in vitro life span, a loss of contact inhibition of growth and a morphology characteristic of SV40-transfor med cells. In the absence of inducing agent, read-through of the T ant igen gene resulted in low but detectable levels of protein. The reduct ion in T antigen levels was accompanied by a 50% or greater reduction in the proliferative rate and restoration of cell morphology and conta ct inhibition similar to that found in non-transfected cells. The resu lts presented here demonstrate that low amounts of T antigen are suffi cient to maintain cell viability and prevent the re-expression of the senescent phenotype seen in the absence of T antigen. Similarly, the a bility of T antigen to extend the in vitro life span is not dependent on high level expression of T antigen. In contrast, the rate of prolif eration of human cells as well as the cell morphology and contact inhi bition are dependent on the amount of T antigen present. Many of the c ellular effects can be minimised or reversed by reducing T antigen exp ression.