Biological assays for cellular transformation.

A number of standard and widely applied procedures have been used to determine whether expression of a particular gene triggers the growth alterations that are characteristic of most oncogenes. The assays have been used extensively to evaluate the transforming potential of a wide variety of genes that encode tyrosine or serine/threonine kinases, small and heterotrimeric GTP-binding signal transduction regulators, and nuclear transcription factors, among others. Therefore, the growth-promoting characteristics of a particular gene can be compared with the properties of other genes that have been characterized by the same assays. The assays described do not represent a complete evaluation of the transforming activity of a gene. Failure to detect growth-promoting activity in any of the assays does not definitively eliminate the possibility that a particular gene is an oncogene. Specialized assays that use (nonfibroblast) recipient cells more closely approximating the likely environment of the gene of interest may provide better approaches for subsequent studies. Other biological assays for transforming potential include measurements of the adhesion properties of cells on different substrata, the ability to grow on confluent monolayers of normal cells, the ability to invade into various artificial tissue matrices, and transgenic animal models. Finally, more specific assays for biochemical alterations that reflect the transformed state can also be employed. For example, as discussed in [23] in this volume, one widely used biochemical measure of transforming potential employs transcriptional activation of genes whose promoters contain so-called oncogene-responsive elements. This, as well as other biochemical assays, can be applied to complement the biological studies described in this chapter.