| Membrane type 1 matrix metalloproteinase (MT1-MMP) is an integral membrane protein that is important in tumor growth, migration, and invasion. It has the ability to degrade ECM, non-matrix proteins such as CD44 and integrin, and activate MMP2. Semaphorin 4D (Sema4D), a membrane-bound semaphorin, is highly expressed in malignancies such as head and neck squamous cell carcinoma (HNSCC) and is known to be pro-angiogenic, promoting the growth of blood vessels into a developing tumor by acting as a chemoattractant when bound to its receptor, Plexin-B1 (PB1), on endothelial cells. Our central hypothesis is that tumor hypoxia causes an increase in Sema4D, which acts in an autocrine and paracrine manner on tumor cells to induce the overexpression of MT1-MMP, which, in turn, cleaves Sema4D and increases availability to the tumor microenvironment to promote tumor-induced angiogenesis and invasion. Using immunoblots and flow cytometry, we demonstrate that MT1-MMP increases in HNSCC cells in a Sema4D and Plexin-B1-dependent manner in hypoxia. Also, we show that RhoA and NF-kappaB (downstream effectors of Plexin-B1) are important in the regulation of cell surface MT1-MMP expression under hypoxic conditions. Consequently, tumor-induced invasion and angiogenesis are enhanced. Soluble Sema4D diffuses out from the tumor and acts as a chemoattractant for endothelial cells, which also upregulate MT1-MMP on their surface to facilitate migration through the extracellular matrix. We conclude that Sema4D controls its own availability and, therefore, its own pro-angiogenic potential through autocrine/paracrine regulation of MT1-MMP. |
