Bone density is controlled by interactions between osteoclasts, which resorb bone, and osteoblasts, which deposit it. The semaphorins and their receptors, the plexins, originally shown to function in the immune system and to provide chemotactic cues for axon guidance, are now known to play a role in this process as well. Emerging data have identified Semaphorin 4D (Sema4D) as a product of osteoclasts acting through its receptor Plexin-B1 on osteoblasts to inhibit their function, tipping the balance of bone homeostasis in favor of resorption. Breast cancers and other epithelial malignancies overexpress Sema4D, so we theorized that tumor cells could be exploiting this pathway to establish lytic skeletal metastases. Here, we use measurements of osteoblast and osteoclast differentiation and function in vitro and a mouse model of skeletal metastasis to demonstrate that both soluble Sema4D and protein produced by the breast cancer cell line MDA-MB-231 inhibits differentiation of MC3T3 cells, an osteoblast cell line, and their ability to form mineralized tissues, while Sema4D-mediated induction of IL-8 and LIX/CXCL5, the murine homologue of IL-8, increases osteoclast numbers and activity. We also observe a decrease in the number of bone metastases in mice injected with MDA-MB-231 cells when Sema4D is silenced by RNA interference. These results are significant because treatments directed at suppression of skeletal metastases in bone-homing malignancies usually work by arresting bone remodeling, potentially leading to skeletal fragility, a significant problem in patient management. Targeting Sema4D in these cancers would not affect bone remodeling and therefore could elicit an improved therapeutic result without the debilitating side effects.

The semaphorins and plexins, originally shown to control axon growth cone guidance, are now known to influence many aspects of cell adhesion and migration. Our group and others have demonstrated that carcinomas over express Semaphorin 4D (S4D), particularly under conditions of hypoxia, for the purpose of supporting tumor-induced angiogenesis. However, S4D is a transmembrane protein that must be proteolytically cleaved to attract endothelial cells into a malignancy, a process that in head and neck squamous cell carcinoma (HNSCC) is controlled by membrane type 1-matrix metalloproteinase (MT1-MMP). We wanted to test the possibility that S4D acts through its receptor, Plexin-B1 (PB1), to enhance activity of endothelial cell MT1-MMP, facilitating extracellular matrix degradation and further promoting angiogenesis. We observed upregulated transcription, expression and cell surface localization of MT1-MMP in endothelial cells treated with S4D, occurring in a PB1, RhoA and NF-κB-dependent manner. We also demonstrated that S4D-induced endothelial cell collagen degradation, migration and capillary tube formation are suppressed when PB1 is silenced or its downstream effectors RhoA and NF-κB are inhibited. Endothelial MT1-MMP contributes to the ability of these cells to digest their way through the basement membrane and extracellular matrix, a requirement for the creation of the vascular networks that supply a developing tumor with oxygen and nutrients and allow it access to the systemic circulation for metastasis. These results demonstrate that S4D promotes angiogenesis at multiple levels, and suggest that it would be a valid target for anti-angiogenic intervention in the treatment of solid malignancies. Citation Format: Amr Bugshan, Zaid Khoury, Asma Buhamrah, John R. Basile. Expression and activity of MT1-MMP in endothelial cells is regulated by Semaphorin 4D [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1953.