Galanin is a
neuropeptide that is widely distributed in the central and peripheral nervous systems. In a previous study, we showed that a
small cell lung carcinoma (SCLC) cell line, SBC-3A, released progalanin but not
galanin, and that progalanin was then converted to galanin(1-20), the active form. Because the galanin(1-20) had undergone hydrolysis at Arg and Lys residues, the
protease concerned was surmised to have a
trypsin-like activity. The present study was performed to identify the
trypsin-like
protease which had previously been found to activate progalanin in this
tumor tissue. The
protease was isolated using chromatography and electrophoresis, and identified in
tumor extracts from SBC-3A
tumor-bearing mice; the major
protease was found to be
plasmin. We next confirmed that extracellular processing of progalanin occurs in SCLC
tumor tissue (
tumors produced by the implantation of SBC-3A cells into mice), and in two types of
breast tumor tissue (obtained by implantation into mice of BT-549 and MDA-MB-436 cells). In cell culture, processed forms of progalanin were undetectable in SBC-3A, BT-549 or MDA-MB-436 cells. Conversely, gel filtration chromatography analysis of
tumor extracts from SBC-3A, BT-549 and MDA-MB-436-bearing mice, revealed that
galanin-like immunoreactivity (
galanin-LI) in these
tumor extracts was due to the presence of progalanin (14 kDa) and galanin(1-20) (2 kDa). Moreover,
trypsin-like
protease activity was elevated, and
plasmin was expressed abundantly in SBC-3A, BT-549 and MDA-MB-436
tumors in mice. In addition,
tranexamic acid, a
plasmin inhibitor, inhibited progalanin conversion to galanin(1-20). The present study revealed that
plasmin was present in
tumor tissue, and that it was responsible for processing progalanin to galanin(1-20) in the extracellular environment.