The extracellular region of heregulin is sufficient to promote mammary gland proliferation and tumorigenesis but not apoptosis.

Heregulin (HRG) is a member of the neuregulin family of ligands that have been shown to interact with and activate erbB receptors. A transgenic mouse model in which full-length HRG is overexpressed has proven that this protein can induce carcinomas in the murine mammary gland. These tumors display a high level of apoptosis, which appears to be mediated by the cytoplasmic domain of HRG. Because both proliferation and apoptosis play a role in tumor formation, we wished to separately view those perturbations by removing the suspected apoptosis-inducing cytoplasmic domain of HRG. We thereby sought to determine whether overexpression of the extracellular region of HRG would be sufficient to induce mammary gland carcinomas. A HRG construct lacking the cytoplasmic domain was targeted to the mammary gland using the murine mammary tumor virus promoter. Multiple lines of transgenic mice carrying the transgene developed mammary gland tumors at approximately 15 months of age. These tumors did not display high levels of apoptosis as compared with tumors from murine mammary tumor virus/full-length HRG transgenic animals. In addition, virgin transgenic mice show a persistence of terminal end bud structures, which normally disappear at the onset of puberty in wild-type mice. To examine the signal transduction pathway activated by extracellular HRG in tumors, we investigated the phosphorylation status of the epidermal growth factor receptor family members. Western blot analysis showed activation of ErbB2 and ErbB3, suggesting a possible mode of action of extracellular HRG in mammary gland carcinomas. We conclude that the extracellular and transmembrane domains of HRG are sufficient for the induction of tumorigenesis but that induction of apoptosis requires the cytoplasmic tail.