A Natural ErbB4 Isoform That Does Not Activate Phosphoinositide 3-Kinase Mediates Proliferation but Not Survival or Chemotaxis

Abstract

ErbB4 is a member of the epidermal growth factor receptor (ErbB) family that mediates cellular responses activated by neuregulins (NRG) and other epidermal growth factor-like growth factors. Two naturally occurring ErbB4 isoforms, ErbB4 CYT-1 and ErbB4 CYT-2, have previously been identified. Unlike ErbB4 CYT-1, ErbB4 CYT-2 lacks a phosphoinositide 3-kinase (PI3-K)-binding site and is incapable of activating PI3-K. We have now examined the consequences of the inability of this isoform to activate PI3-K on cell proliferation, survival, and chemotaxis in response to NRG-1beta: (i) NRG-1beta stimulated proliferation of cells expressing either ErbB4 CYT-1 or ErbB4 CYT-2. Consistent with the mitogenic responsiveness, analysis of downstream signaling showed that Shc and MAPK were phosphorylated after stimulating either isoform with NRG-1beta. (ii) NRG-1beta protected cells expressing ErbB4 CYT-1 but not cells expressing ErbB4 CYT-2 from starvation-induced apoptosis as measured by effects on cell number and 4', 6-diamidino-2-phenylindole staining. Furthermore, in cells expressing ErbB4 CYT-2, Akt, a protein kinase that mediates cell survival, was not phosphorylated. (iii) NRG-1beta stimulated chemotaxis and membrane ruffling in cells expressing ErbB4 CYT-1 but not in cells expressing ErbB4 CYT-2. In summary, ErbB4 CYT-2 can mediate proliferation but not chemotaxis or survival. These results suggest a novel mechanism by which cellular responses such as chemotaxis and survival may be regulated by the expression of alternative receptor-tyrosine kinase isoforms that differ in their coupling to PI3-K signaling.