V-Ha-ras-dependent expression of interleukin-3 mRNA in premalignant PB-3c mast cells correlates with the formation of tumors without interleukin-3 gene rearrangements

Retroviral transduction of v-Ha-ras into the interleukin-3 (IL-3) dependent PB-3c cells results in the generation of IL-3 autocrine tumors without (class-I tumors) or with (class-II tumors) insertion of endogenous retroviral elements into the IL-3 locus. In this study, we examined the frequency of both tumor classes arising from the IL-3-dependent PB-3c clone 15. This cell line was previously found to be unable to suppress IL-3 expression of class-I tumor cells after cell fusion and to be highly tumorigenic following ZIP-ras-neo transduction. To identify possible preexisting determinants of tumor progression, clone 15 cells were subcloned either before or after retroviral ZIP-ras-neo infection and tested for tumorigenicity. All 20 sublines with clonal cellular background, but with heterogeneous integration sites, were tumorigenic. In contrast, only 12 of 19 sublines with clonal integration sites formed tumors. None of the 42 tumor cell lines analyzed by Southern blotting showed evidence of IL-3 gene rearrangement. Only the tumorigenic sublines showed v-Ha-ras transcripts on Northern blots and low levels of IL-3 mRNA detectable by RT-PCR, as well as recovery of IL-3-independent cells after crisis in vitro. Expression of IL-3 mRNA and tumorigenicity were found in somatic cell hybrids of clone 15 with 15 v-Ha-ras, but not in cell hybrids with the class-I tumor suppressor-positive clone 20 v-Ha-ras. These data suggest that RT-PCR signals for IL-3 are a predictive marker in premalignant cells for the formation of tumors without IL-3 gene rearrangements, which depend on v-Ha-ras expression and a cooperating cellular function of recessive nature.