MINIREVIEW: Differentiated Mammalian Cell Lines Immortalized by Temperature Sensitive Tumor Viruses

A major goal of cell biologists is to have an in vitro cell model that simulates in vivo conditions. This cell model should be able to propagate in culture, express specialized tissue functions, and allow fundamental biological problems to be answered by a simple manipulation of the culture conditions. Unfortunately, normal differentiated cells generally do not proliferate in culture and often cease to express their specialized functions. As a result, studies have often been carried out on aberrant tumor cell lines that are capable of growth in culture and yet retain the ability to express the tissue-specific functions. However, because of their malignant state and in the absence of normal controls, conclusions from tumor cell studies may not be relevant to normal gene regulation. In the past decade, various attempts have been made to immortalize cells from normal tissues. Cell lines retaining differentiated functions have been established by transformation with chemical carcinogens (1), oncogenes (2, 3), and tumor viruses (4–6). These cell lines retain a spectrum of tissue-specific functions and have been employed in a variety of studies. However, the results obtained from studies using these transformed cell lines, which possess only a transformed phenotype, again may not simulate in vivo gene regulation. To circumvent most of these problems and, additionally, to provide a normal control, we immortalized cells with a temperature-sensitive (ts) A mutant virus of simian virus 40 (SV40). The A gene of SV40 encodes the large tumor (T) antigen which is required for the initiation and maintenance of transformation (7). The tsA mutant virus-transformed cells are conditionally transformed and only express the transformed phenotype at the permissive temperature. At the nonpermissive temperature, these cells quickly revert to a normal nontransformed phenotype. As a result, studies using SV40 tsA mutant virus-transformed cells exhibiting a differentiated phenotype provide a closer approximation of normal in vivo conditions. Most importantly, because a single cell line serves as the source for both the transformed and normal states, it is possible, for the first time, to have internally controlled experiments.