Slow time-dependent cellular transformation induced at restrictive temperature by ts-src mutants

Cell lines infected with a temperature-sensitive Rous sarcoma virus have been widely used to study the temporal dynamics of various transformation parameters following downshift from the non-permissive temperature to the permissive temperature as it is considered that, at the non-permissive temperature, the infected cells exhibit the morphological and growth characteristics of normal cell whereas, at the permissive temperature, they exhibit characteristics of the transformed state. We show here that the apparently normal state in which tsPA1-infected FR3T3 cells are directed at the restrictive temperature is not a stationary stable state, but rather a transient one which continuously drifts as the cells are grown and passaged at this temperature and which eventually ends up as a new transformed state (T2) with morphological and growth properties definitely different from those belonging to the transformed state (T1) at the permissive temperature. The establishment of the transformed T2 state at the restrictive temperature occurs concomitantly with a steady accumulation of an intracellular pool of pp60v-src in the vicinity of the nucleus, whose traffic towards the plasma membrane is released following downshift to the permissive temperature, leading to the reappearance of transformation parameters characteristic of the transformed T1 state. Our finding raises the possibility that the v-src protein encoded by the tsPA1 mutant of Rous sarcoma virus may induce cellular transformation via two different pathways, leading to two different transformation states, depending on at which temperature the infected cells are grown. Various possible mechanisms that could be involved in the time-dependent establishment of a transformed state by ts-src mutants at the restrictive temperature are discussed.