UC Irvine

The BCR/ABL oncogene encodes an activated tyrosine kinase that causes human chronic myelogenous leukemia. The mechanism of transformation, however, is complex and not well understood. One of the important contributions of BCR to transformation is believed to be dimerization or oligomerization of ABL, thereby activating ABL tyrosine kinase activity. We reasoned that if ABL was dimerized through other mechanisms, activation of the tyrosine kinase activity should also result, and the activated kinase may also be transforming. Erythropoietin is known to activate its receptor by causing dimerization, and therefore a synthetic oncogene was created by linking the extracytoplasmic and transmembrane domains of the EPO receptor with c-ABL. This chimeric receptor was stably expressed in Ba/F3 cells and, in the absence of EPO, had no detectable biological effect on the cells. EPO, however, induced a rapid, dose-dependent activation of ABL tyrosine kinase activity and phosphorylation of several cellular proteins. The major target proteins have been identified, and are very similar to the known substrates of BCR/ABL, including Shc, CBL, CRKL, and several proteins in the cytoskeleton. EPO treatment also resulted in biological effects that were remarkably similar to those of BCR/ABL, including improved viability, altered integrin function, and a weak mitogenic signal. The biological effects were in part dose-dependent, in that low EPO concentrations enhanced viability but did not cause proliferation. At high EPO doses, kinase activation was maximal, and a mitogenic effect was also revealed. In nude mice, Ba/F3 cells expressing this chimeric receptor did not cause detectable disease without administration of pharmacologic doses of EPO. If EPO was given intraperitoneally 5 days a week, however, a dose-dependent lethal leukemia resulted. This ligand-regulatable oncogene mimics some of the biological effects of BCR/ABL, and analysis of ABL mutants in this system will be useful to dissect the signaling pathways that cause CML.