SV40-Mediated Immortalization

Simian virus 40 (SV40) has been extensively used as a model system for mammalian cell replication and gene expression and has served as a highly effective “probe” for cellular functions. This has also been the case for understanding carcinogenesis since viral gene expression can result in altered cell proliferation and appearance of multiple “transformed” cell phenotypes associated with tumors. Both human and rodent SV40-transformed fibroblasts show reduced growth factor (i.e. serum) requirements, multilayer growth with increased saturation density and focus formation, and anchorage independence with growth in semi-solid media such as agar or agarose. Most relevant to this chapter, SV40 also increases the frequency at which cells become immortal. It should be noted that SV40 affects a wide variety of cell types (Tooze 1981) but this discussion will focus on fibroblasts, for convenience. Its effect on immortalization is particularly striking in human cells. Mouse cells have a quite limited life span in culture (approximatelyl0–20 generations) but can spontaneously become immortal at a measurable frequency; infection with wild-type SV40 results in almost 100% occurrence (Tevethia et al. 1998). On the other hand, normal human diploid fibroblasts virtually never spontaneously become immortal even within the greater life span of 50–60 generations observed with newborn or fetal cells in culture (McCormick and Maher 1988).