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 (myotubes), we previously found that protease nexin I was localized in patches at their surface where it was active and able to inhibit thrombin. To understand the role of skeletal muscle protease nexin I after injury or in inflammatory conditions where thrombin might be extravasated by blood vessels, we examined the role of inflammatory factors on protease nexin I synthesis and secretion by myotubes in culture. By enzyme‐linked immunosorbent assay (ELISA) and Western blotting, we found that this serine protease inhibitor is secreted by cultured human myotubes. Protease nexin I secretion is stimulated by tumor necrosis factor‐α, transforming growth factor‐β and interleukin‐1. Complex formation experiments with labeled thrombin reveal active protease nexin I bound to the surface of the treated cells. Secreted protease nexin I‐thrombin complex was enhanced in the presence of transforming growth factor‐β and tumor necrosis factor‐α. Protease nexin I mRNA was 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 protease nexin I concentration are elicited at the translational/posttranslational levels. Immunocytochemical studies on human skeletal muscle biopsies of patients suffering from inflammatory myopathies showed an overexpression of protease nexin I together with the above inflammatory factors. These findings suggest that skeletal muscle protease nexin I might play a role after injury or inflammatory pathologies. J. Cell. Physiol. 179:305–314, 1999. © 1999 Wiley‐Liss, Inc.