A novel strategy to tag matrix metalloproteinases-positive cells for in vivo imaging of invasive and metastatic activity of tumor cells

Abstract

Matrix metalloproteinases (MMPs) are endopeptidases responsible for degrading the extracellular matrix (ECM) and remodeling tissue in both physiological and pathological processes. MMP2 and membrane-type 1 MMP (MT1-MMP) have been associated with tumor invasion, metastasis and angiogenesis; therefore, a molecular imaging strategy assessing their activity may help to predict the malignancy of tumors. Here, we established a novel method of specifically tagging the surface of MMP2- and MT1-MMP-positive cells, and applied it to the development of an optical imaging probe. We constructed a protein-based probe composed of a glutathione-S-transferase (GST)-tag (Inhibitory [I]-domain), a polypeptide as a specific substrate for both MMP2 and MT1-MMP (Cleaved [C]-domain), a transmembrane domain of the epidermal growth factor receptor (Transmembrane [TM]-domain), and DsRed2 (Fluorescent [F]-domain). In vitro experiments clearly demonstrated that, after the probe was cleaved at the C-domain by the MMPs, the resultant TM–F-domain was inserted into the cellular membrane. Optical imaging experiments in vivo demonstrated that the probe was cleaved and specifically remained in tumor xenografts in a MMP-dependent manner. These results indicate that the release of the I–C-domain through the proteolytic cleavage of the C-domain by MMP2 and MT1-MMP triggers the tagging of cellular membranes with the TM–F-domain. The present feasibility study opens the door to the development of a novel imaging probe for tumor malignancy using positron emission tomography as well as an optical imaging device.