Design and synthesis of a near-infrared fluorescent nanofiber precursor for detecting cell-secreted urokinase activity

Abnormal proteolysis is often observed during disease progression. Up-regulation of certain tumor-associated proteases such as urokinase plasminogen activator (uPA) can be a hallmark of malignant transformation. Here we report the design and synthesis of a near-infrared nanofiber precursor (NIR–NFP) for detecting uPA activity. NIR–NFP, which is optically silent in its native state, is composed of multiple self-assembled peptide units (PEG54-BK(NIR664)SGRSANA-kldlkldlkldl-CONH2). On uPA activation, NIR–NFP releases peptide fragments (PEG54-BK(NIR664)SGR-CONH2) that contribute to a significant fluorescence amplification at 684 nm. NIR–NFP was able to detect cell-secreted uPA from human cancer cells (SKBR-3, PANC-1, MCF-7, SKOV-3, MDA-MB-231, PC-3, and HT-1080) expressing various levels of uPA. Fluorescence changes were uPA dependent, as confirmed with both Western blot analysis and enzyme activity assay. Our data suggest that an optimized preparation may be useful for imaging uPA activity in vivo.