Retransformation of a thermosensitive BALB/c–3T3 transformant by murine sarcoma virus at the non-permissive temperature

Abstract

CERTAIN fibroblast cell lines exhibit some of the properties of transformed cells at one temperature while retaining normal properties at a higher temperature. For example, the spontaneous transformants of BALB/C–3T3 cells isolated by W. Eckhart (unpublished results) can grow in soft agar at 32 °C (the permissive temperature) but not at 39 °C (the non-permissive temperature). They show transformed morphology at 32 °C but normal morphology at 39 °C (ref. 1). They have certain temperature-sensitive surface properties such as ability to grow in Agarose, loss of a transformation-sensitive iodinatable glycoprotein (LETS)^2,3, increase in total cell proteolytic activity⁴ and loss of the growth promoting activity of fibroblast growth factor⁵. But they behave normally with respect to the regulation of intracellular events. Unlike transformed fibro-blasts, both at permissive and non-permissive temperatures, BALB/C–3T3 cells exhibit density-dependent growth inhibition⁵. They cease growing as a result of serum limitation and express the typical increases in protein synthesis, ribosomal RNA synthesis and DNA synthesis when growth is initiated^1,6. The evidence therefore suggests that the trans-formants have lost their potential to express a transformed phenotype at the non-permissive temperature. I now show that this is not an irreversible loss by demonstrating that murine sarcoma virus (MSV) can transform the cells at the non-permissive temperature.