Growth factor independence and indefinite growth ("immortalization") appear simultaneously after crisis in murine...

In murine myelocytes doubly infected with the v-myc virus and the v-Ha-ras virus, or the v-myc virus and the v-abl virus, the emergence of growth factor-independent cells is a rare event which coincides with crisis in the cultures. All growth factor-independent cells which emerge from crisis are also "immortalized", i.e., able to grow indefinitely. We wanted to determine whether the v-abl gene plays a role in the immortalization of the growth factor-independent cells. Therefore we infected murine myelocytes with v-myc virus and a v-abl mutant virus coding for a temperature-sensitive tyrosine kinase. Growth factor-independent cell clones were isolated and their properties at the nonpermissive temperature analyzed. We observed that 4 hr after the shift to the nonpermissive temperature the expression of both the genomic and subgenomic viral v-myc transcripts decreased significantly, followed by a loss of the viability of the cells. However, several variants emerged which were able to grow at the nonpermissive temperature and showed elevated expression of v-myc. The results suggest: 1) that v-abl plays a role in the immortalization of the cell clones by maintaining a critical level of v-myc expression; 2) that rare genetic changes leading to constitutive v-myc expression independent of v-abl tyrosine kinase activity can also lead to immortalization in this model system, and 3) that immortalization and growth factor independence occur in post-crisis cells and are associated with constitutive expression of v-myc.