ERBB Expression in Philadelphia Chromosome-Positive ALL as a Potential Therapeutic Target.

Philadelphia chromosome-positive ALL (Ph+ALL) strikes older children and adults, is difficult to treat and often has poor outcomes with conventional therapies. Tremendous advances have been made in other cancers through identification of essential signaling molecules that can be inhibited by small molecule medicines. ERBB proteins are such targets for several adult carcinomas, including breast and lung cancer. Expression of one member of the ERBB family, Her-2, is associated with chemotherapy resistance in one report of adult B-ALL. We examined the expression, activation status and contribution to growth and survival of ERBB receptors in Ph+ALL cell lines Z33, Z119 and Z181. Phosphorylated Her-2 was detected by western blot in all 3 lines, as was trace amounts of Her-3. Western-blot identified expression of the p80 form of Her-4 in Z119, which correlated with flow cytometry assessment of Her-4. Expression in Z33 of EGFR, detected by western blot, correlated with down-regulation of both EGFR and Her-2 from the cell surface, as assessed by flow cytometry. Internalization of EGFR homo- and hetero-dimers is associated with receptor phosphorylation and active signaling. To assess the physiologic importance of ERBB signaling in Ph+ALL, cells were cultured in the pan-ERBB inhibitor CI-1033 (Pfizer Global Research and Development), which is specific for ERBB family receptors in tissue culture up to 20 μM concentrations. All lines demonstrated reduced cell yield after CI-1033 exposure, with average IC50 of 0.783 for Z119, 1.178 for Z181 and 1.737 for Z33. The increased sensitivity of Z119 to CI-1033 correlated with an increased rate of apoptosis: 3 μM CI-1033 caused a 26% increase in the fraction of sub-diploid Z119 cells at 24 hours, compared to 13% for Z181 and 16% for Z33. Higher sensitivity to ERBB inhibition also correlated with growth factor resistance, since Z33 and Z181 are dependent upon autocrine GM-CSF for growth while Z119 grows independent of known autocrine secretion of hematopoietic growth factors. These data suggest that Ph+ALL cells may acquire GM-CSF independence by relying more on ERBB-derived signals, since these receptors are capable of activating similar second messenger pathways. Our data suggest that, surprisingly, the ERBB family of receptor tyrosine kinases are intact and functioning in Ph+ALL, and inhibition of these signals may provide benefit to Ph+ALL patients, either in combination with imatinib or in patients who develop imatinib resistance.