Cooperation between the H-ras oncogene and a truncated derivative of the v-myb oncogene in transformation of hamster embryo fibroblasts.

The ras oncogenes alone fully transform established (immortalized) rodent fibroblasts in a few days, but generally transform early-passage fibroblasts only partially, unless their action is complemented by that of a nuclear, immortalizing, oncogene. Here we show that transfection of second-passage Syrian hamster embryo fibroblasts (HEFs) by the EJ-H-ras oncogene coupled to the neo gene, followed by selection with G418, gives rise to apparently normal, or only slightly transformed, clonal colonies, only a few of which become established. The study of two established clonal lines showed that they acquired only after some weeks, and stepwise, the main characteristics of full neoplastic transformation, i.e. anchorage independence, reduced requirement for serum growth factors and tumorigenicity. Later both clonal lines became increasingly tumorigenic and completely independent of exogenous growth and attachment factors, without increase in the expression of the H-ras oncogene. Transfection of one of the clones, early after its isolation, with a truncated derivative of the nuclear v-myb oncogene devoid of its transcriptional negative regulatory domain and able to partially transform chicken embryo fibroblasts [(myb(KXANM)] gave rise to more transformed cells, expressing both EJ-H-ras and myb(KXANM), which became tumorigenic earlier than the controls and remained more tumorigenic later on. With more efficient transfection techniques, numerous foci of fully transformed cells were subsequently obtained, in a few days, in cultures transfected sequentially with EJ-H-ras(neo) and myb(KXANM) and in cultures co-transfected with the two oncogenes. Highly tumorigenic, serum-independent and immortalized clones expressing both oncogenes were obtained from these cultures. Hence, the truncated myb(KXANM) oncogene accelerate the stepwise transformation of unestablished HEFs by the EJ-HH-ras oncogene and, together with this oncogene, fully transforms these same cells in a single step. The two oncogenes acting in cooperation also induce cell immortalization, but myb(KXANM), by itself, is not an immortalizing oncogene. No cooperation was observed between EJ-H-ras(neo) and the unaltered v-myb oncogene.