Frequency and mechanisms of factor independence in IL-3-dependent cell lines

Interleukin-3 (IL-3) -dependent cell lines were tested for spontaneous mutation frequency with respect to two markers, growth factor-independent growth and 8-azaguanine-resistant growth. The mutation frequency for the growth factor-independent growth was approximately equal to that of 8-azaguanine-resistant growth. Investigation of the growth factor-independent phenotypes indicated that at least two different mechanisms for growth factor independence existed. One mechanisms was the activation of the IL-3 gene by mutation resulting in the constitutive expression of IL-3 and autocrine regulated growth. A second mechanisms results in IL-3-independent growth in a manner in which (1) constitutive tyrosine kinase activity was not seen and (2) c-myc transcription was constitutively activated. Interestingly, in these cells both the tyrosine phosphorylation and c-myc transcriptional pathways are still activatable following IL-3 stimulation. At present, the exact nature of the mutation that results in this phenotype is not known.