The secreted protease Adamts18 links mammary epithelial hormone action to extracellular matrix changes and stem cell niche function

Estrogens and progesterone control mammary gland development and breast carcinogenesis via their cognate receptors expressed in a subset of cells of the luminal layer of the mammary epithelium. The extracellular matrix (ECM) including the basement membrane (BM) is important in breast physiology and tumorigenesis but how epithelial hormone receptor signaling and ECM are linked mechanistically is unclear. We identify the secreted protease Adamts18 as critical intermediary. Luminal estrogen and progesterone receptor signaling via upregulation of Wnt4 expression and ensuing canonical Wnt signaling activation in basal cells control Adamts18 expression there. The protease has an epithelial-intrinsic role in stem cell activation. We identify multiple binding partners in the interstitial ECM and BM and show that ADAMTS18 cleaves fibronectin in vitro. Its deletion results in increased fibronectin, collagen I and IV, and laminin deposition in pubertal glands. Adamts18 interacts genetically with Col18a1, which encodes a proteoglycan that is BM-specific, in stem cell regulation. Adamts18 inactivation impairs Hippo signaling and reduces Fgfr2 expression and signaling, which are vital for stem cell function. Our findings link epithelial hormone signaling to BM remodeling by Adamts18, and define the BM as an essential stem cell niche component.