BCR-ABL and constitutively active erythropoietin receptor (cEpoR) activate distinct mechanisms for growth factor-independence and inhibition of apoptosis in Ba/F3 cell line

Abstract

The interleukin-3 dependent murine Ba/F3 cell line has been widely used as an experimental model of cell transformation by BCR – ABL oncogenes as assessed by induction of growth-factor-independence and inhibition of apoptosis in vitro. The signaling pathways used by BCR – ABL oncogenes to exert these effects are unknown. To gain insights into this phenomenon, we have introduced the p190- and p210-encoding BCR – ABL oncogenes as well as the constitutively activated oncogenic murine erythropoietin receptor (cEpoR) into Ba/F3 and compared the behavior of individual clones in response to apoptotic stimuli. Both p210 and p190 BCR – ABL vectors induced IL-3-independent growth and the same result was obtained with the cEpo-R vector. Individual clones of Ba/F3 cells expressing BCR – ABL exhibited significant resistance to apoptosis induced by either etoposide, serum deprivation or growth-factor withdrawal. In contrast, Ba/F3 cells expressing the constitutively active cEpoR behaved like parental Ba/F3 cells undergoing apoptosis when similarly treated with etoposide or upon serum deprivation. Bc12 and Bax levels were similar in all BCR – ABL and cEpoR-transfected clones. However, in band-shift assays, nuclear extracts from growth-factor-independent Ba/F3 clones expressing cEpoR had no detectable STAT activity as opposed to the constitutive STAT activation detected in all Ba/F3 clones expressing p210 or p190 BCR – ABL. Our results indicate that although both constitutively activated cEpoR and BCR – ABL oncogenes induce growth-factor independence in Ba/F3 cells, only BCR – ABL is able to protect cells from etoposide and serum-deprivation-induced apoptosis and induce a strong constitutive activation of STAT factors, suggesting a role for these molecules in the anti-apoptotic activity of BCR – ABL.