Protease nexin I expression is up-regulated in human skeletal muscle by injury-related factors

Protease nexin I is a 43-50 kDa glycoprotein capable of inhibiting a number of serine proteases. In cultured differentiated human skeletal muscle (myo tubes), we previously found that protease nexin I was localized in patches at their surface where it was active and able to inhibit thrombin. To under stand the role of skeletal muscle protease nexin I after injury or in infla mmatory conditions where thrombin might be extravasated by blood vessels, w e examined the role of inflammatory factors on protease nexin I synthesis a nd secretion by myotubes in culture. By enzyme-linked immunosorbent assay ( ELISA) and Western blotting, we found that this serine protease inhibitor i s secreted by cultured human myotubes. Protease nexin I secretion is stimul ated by tumor necrosis factor-alpha, transforming growth factor-beta and in terleukin-1. Complex formation experiments with labeled thrombin reveal act ive protease nexin I bound to the surface of the treated cells. Secreted pr otease nexin 1-thrombin complex was enhanced in the presence of transformin g growth factor-beta and tumor necrosis factor-ct. Protease nexin I mRNA wa s detected by reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot analysis. Whatever the conditions, no significantly different levels were observed, indicating that the changes in cell and media protea se nexin I concentration are elicited at the translational/posttranslationa l levels. Immunocytochemical studies on human skeletal muscle biopsies of p atients suffering from inflammatory myopathies showed an overexpression of protease nexin I together with the above inflammatory factors. These findin gs suggest that skeletal muscle protease nexin I might play a role after in jury or inflammatory pathologies. (C) 1999 Wiley-Liss, Inc.