ErbB4 is a member of the ErbB family of receptor tyrosine kinases. The ErbB family also includes EGFR and ErbB2, two of the most widely targeted receptors in cancer. However, the roles that ErbB4 plays in cancer are not as well defined as the roles played by EGFR and ErbB2. The goal of this dissertation is to explore the roles that ErbB4 plays in human malignancies.
Clinical studies of ErbB4 expression in tumor samples and some analyses of ErbB4 function in cultured tumor cell lines indicate that ErbB4 functions as a tumor suppressor. Here we demonstrate that ErbB4 expression is lost in human pancreatic tumor cell lines, suggesting that ErbB4 functions as a tumor suppressor. Moreover, the constitutively-homodimerized and –active Q646C ErbB4 mutant inhibits clonogenic proliferation in four human pancreatic tumor cell lines, further supporting the model that ErbB4 possesses tumor suppressor activities. However, ectopic expression of wild-type ErbB4 in the MIA PaCa-2 human pancreatic tumor cell line (which lacks endogenous ErbB4 expression) fails to inhibit clonogenic proliferation, even in the presence of the ErbB4 ligand Neuregulin 2beta (NRG2β). These data suggest that tumor suppressor activities of ErbB4 may be context dependent.
Clinical data and functional analyses of ErbB4 and ErbB4 ligands indicate that ErbB4 can couple to tumor cell proliferation, invasiveness, and motility, leading us to postulate that ErbB4 may function as an oncogene in some contexts. Indeed, here we demonstrate that NRG2â stimulates motility in the MCF7 human breast tumor cell line and anchorage-independent proliferation in the T47D human breast tumor cell lines, both of which endogenously express ErbB4. Gene silencing studies indicate that these effects are dependent on ErbB4 and ErbB2. These data suggest that the oncogenic activities of ErbB4 occur through ErbB4 crosstalk with another ErbB receptor.
We have explored ErbB4 crosstalk using BaF3 mouse myeloid cell lines engineered to express ErbB4 or ErbB4 mutants with various EGFR or ErbB2 constructs. ErbB4 crosstalk with either EGFR or ErbB2 is required for NRG2â to stimulate IL3 independence in BaF3 cells. ErbB2 tyrosine kinase activity, but not ErbB4 tyrosine kinase activity, is required for NRG2β to stimulate IL3 independence in BaF3/ErbB2+ErbB4 cells. Similarly, EGFR tyrosine kinase activity, but not ErbB4 kinase activity, is required for NRG2β to stimulate IL3 independence in BaF3/EGFR+ErbB4 cells. Sites of ErbB4 tyrosine phosphorylation are required for NRG2â to stimulate IL3 independence in both BaF3/EGFR+ErbB4 and BaF3/ErbB2+ErbB4 cell lines. Thus, phosphorylation of ErbB4 by EGFR or ErbB2 enables NRG2β to stimulate IL3 independence.
In summary, the data presented here indicate that homotypic ErbB4 signaling, as exemplified by the ErbB4 Q646C mutant, is coupled to tumor suppressor activities. In contrast, heterotypic ErbB4 signaling, as exemplified by ligand-induced crosstalk between ErbB4 and either EGFR or ErbB2, is coupled to oncogenic activities. Thus, the availability of other ErbB receptors for crosstalk with ErbB4 appears to play a significant role in defining the biological activities of ErbB4 and the roles that ErbB4 plays in human malignancy. Moreover, because the coupling of heterotypic ErbB4 signaling to oncogenic activities appears to be dependent on the kinase activity of EGFR and/or ErbB2, agents that target EGFR or ErbB2 may have value in the treatment of ErbB4-dependent tumors, regardless of whether these tumors overexpress the targeted receptor.