NEU DIFFERENTIATION FACTOR ACTIVATION OF ERBB-3 AND ERBB-4 IS CELL-SPECIFIC AND DISPLAYS A DIFFERENTIAL REQUIREMENT FOR ERBB-2

Neu differentiation factor (NDF)-induced signaling involves the activa tion of members of the ErbB family of receptor tyrosine kinases, Altho ugh ectopic expression of recombinant ErbB receptors has yielded valua ble insight into their signaling properties, the biological function a nd in vivo interplay of these receptors are still poorly understood. W e addressed this issue by studying NDF signaling in various human cell lines expressing moderate levels of all known ErbB receptors. NDF-ind uced phosphorylation of ErbB-2 and ErbB-3 was found in the breast epit helial cell line MCF10A, the breast tumor cell lines T47D and MCF7, an d the ovarian tumor cell line OVCAR3. Despite similar expression level s, NDF-induced phosphorylation of ErbB-4 was cell specific and only de tected in T47D and OVCAR3 cells, Blocking cell surface expression of E rbB-2 by intracellular expression of a single-chain antibody revealed that in these two cell lines, ErbB-2 significantly enhanced phosphoryl ation of ErbB-4. Efficient NDF-induced phosphorylation of ErbB-3 was s trictly ErbB-2 dependent in the breast tumor cell lines T47D and MCF7, while it,vas largely ErbB-2 independent in MCF10A and OVCAR3 cells. C onsequently, NDF-stimulated intracellular signaling and induction of a biological response displayed a cell-specific requirement for ErbB-2, Thus, while ErbB-2 cooperates with NDF receptors in the breast tumor cell lines, ErbB-2 independent mechanisms seem to prevail in other cel lular contexts.