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 For example, the spontaneous transformants of BALB/C–3T3 cells isolated by W. Eckhart (unpublished results) can grow in soft agar at 32 °C (the permissive temperature) but not at 39 °C (the non-permissive temperature).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 initiated1,6. 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.