Malignant transformation induced by cytokine genes: a comparison of the abilities of germline and mutated interleukin 3 genes to transform hematopoietic cells by transcriptional and posttranscriptional mechanisms.

The effects of inheritance of germline (gIL3) and rearranged interleukin 3 (rIL3) genes on the factor dependency and tumorigenicity of a hematopoietic cell line were compared. The rIL3 gene arose after intracisternal type A particle transposition into the 3' untranslated region (UTR). The rlL3 gene efficiently transformed cells. In contrast, factor-independent cell lines were recovered only rarely from gIL3-transfected cells, and they fell into two categories. One class inherited more than one copy of the transgene and expressed moderate levels of lL-3, whereas the other group received one copy and expressed lower amounts of lL-3. Tumorigenicity was associated with the quantity of lL-3 expression because cells from the first class were malignant, whereas those from the latter set were not. To determine which portions of the rIL3 gene were responsible for cell transformation, chimeric constructs were made containing exchanged sections of the gIL3 and rIL3 genes. The transforming region was mapped to the 3' end of the rIL3 gene, and no point mutations in the promoter region were detected. RNA gel shift assays indicated that the transposition eliminated the ability of proteins to bind the 3'UTR. These results demonstrate that the mutation of the IL-3 3'UTR by intracisternal type A particle transposition results in: (a) the inability of proteins to bind the 3'UTR; (b) prolonged mRNA half life; (c) constitutive IL-3 expression; (d) cytokine independence; and (e) tumorigenicity. Inheritance of a gIL3 gene was not normally sufficient to abrogate cytokine dependency; however, rare tumorigenic cells were isolated, implicating the importance that deregulated cytokine expression can have upon malignant transformation.