EphA2 receptor tyrosine kinase in mammary gland development and breast cancer induced osteolysis

Abstract

Eph receptor tyrosine kinases are membrane bound receptors often expressed by normal epithelial cells but are frequently overexpressed in many human cancers. Of the many Eph receptors, EphA2 is present at low levels in normal mammary tissue but highly expressed in breast cancer and correlative with a poor patient prognosis. The focus of this thesis is to understand the endogenous role of EphA2 in mammary gland development and how overexpression can contribute to the poor prognosis through metastasis often seen with breast cancers overexpressing EphA2. Using EphA2-deficient animals, this thesis work demonstrates for the first time that EphA2 receptor function is required for mammary epithelial growth and branching morphogenesis. Loss of EphA2 decreased penetration of mammary epithelium into fat pad, reduced epithelial proliferation, and inhibited epithelial branching. These defects appear to be intrinsic to loss of EphA2 in epithelium, as transplantation of EphA2-deficient mammary tissue into wild-type recipient stroma recapitulated these defects. In addition, HGF-induced mammary epithelial branching morphogenesis was significantly reduced in EphA2-deficient cells relative to wild-type cells, which correlated with elevated basal RhoA activity. These results suggest that EphA2 receptor acts as a positive regulator in mammary gland development, functioning downstream of HGF to regulate branching through inhibition of RhoA. Breast cancer metastasis to bone is a major cause of morbidity and mortality in patients. Analysis of human breast-to-bone metastasis samples revealed EphA2 positive staining on tumor cells in close proximity to osteoclast at the tumor-bone interface. To define the role of EphA2 in tumor cell-host bone cell interactions, mouse tibias were injected with osteolytic breast tumor cells lacking EphA2 activity. Our data showed that inhibition of EphA2 activity significantly decreased tumor-induced osteolysis compared to controls. Further in vitro analysis revealed that blocking EphA2 function resulted in defective precursor maturation into functional osteoclasts. A human antibody targeted against EphA2 decreased breast tumor induced osteolysis in vivo. Our studies indicate the selective inhibition of EphA2 at the tumor-bone interface may be a benefit for the treatment of breast-to-bone metastases