The hemopexin domain of MMP3 is responsible for mammary epithelial invasion and morphogenesis through extracellular interaction with HSP90β.

Matrix metalloproteinases (MMPs) are crucial mediators in sculpting tissue architecture and are required for many physiological and pathological processes. MMP3 has been shown to regulate branching morphogenesis in the mammary gland. Ectopic expression of proteolytically active MMP3 in mouse mammary epithelia triggers supernumerary lateral branching and, eventually, tumors. Using a three-dimensional collagen-I (Col-1) gel assay that simulates epithelial invasion and branching, we show that it is the hemopexin domain that directs these processes. Using three different engineered constructs containing a variation on MMP3 structural domains, we confirmed the importance of the hemopexin domain also in primary organoids of the mammary gland. A proteomic screen of MMP3-binding partners surprisingly revealed that the intracellular chaperone heat-shock protein 90 β (HSP90β) is present extracellularly, and its interaction with the hemopexin domain of MMP3 is critical for invasion. Blocking of HSP90β with inhibitory antibodies added to the medium abolished invasion and branching. These findings shift the focus from the proteolytic activity of MMP3 as the central player to its hemopexin domain and add a new dimension to HSP90β's functions by revealing a hitherto undescribed mechanism of MMP3 regulation. Our data also may shed light on the failure of strategies to use MMP inhibitors in cancer treatment and other related disorders.