Staurosporine-induced cleavage of α-smooth muscle actin during myofibroblast apoptosis

To examine the possibility that staurosporine is applicable for the treatment of abnormal scar formation such as hypertrophic scar and keloid, the cellular process during staurosporine-induced apoptosis was analyzed in myofibroblasts isolated from a rat granulation tissue pouch. Staurosporine induced myofibroblast apoptosis in a time- and dose-dependent manner with typical morphologic changes. Staurosporine (1 μM) activated caspase-3 up to 3.6-fold by cleaving pro-caspase-3 (32 kDa) to active forms (17, 19, and 20 kDa). Microfilaments mainly composed of α-smooth muscle actin, a contractile protein characterizing myofibroblasts, were degraded during staurosporine-induced apoptosis. The degradation of α-smooth muscle actin bundles was detected as early as 1 h after the treatment with staurosporine. Recombinant active caspase-3 and staurosporine-stimulated caspase-3 both cleaved purified α-smooth muscle actin in vitro. These results suggested that α-smooth muscle actin is directly degraded by caspase-3 in response to apoptotic stimuli in myofibroblasts. In addition, bleomycin (100 ng per ml) and cisplatin (1 mM) also induced myofibroblast apoptosis by activating caspase-3, suggesting that these agents have a potential therapeutic value for abnormal scar formation.