Matrix vesicles and media vesicles as nonclassical pathways for the secretion of m-calpain from MC3T3-E1 cells

Calpain was generally believed to exist and function only in the cytoplasm. However, in-calpain has now been detected in the extracellular spaces of s ome kinds of tissue. In this study, we demonstrated the existence of m-calp ain in the medium surrounding MC3T3-E1 cultures, and its activity by zymogr aphy. At the same time, the amount of lactate dehydrogenase in medium of MC 3T3-E1 culture was extremely low compared with other cell cultures, suggest ing that m-calpain found in the culture medium of MC3T3-E1 cells originated mainly from active secretion. Moreover, the secretion of m-calpain was not blocked by brefeldin A, implying that m-calpain may be secreted by a noncl assical pathway. Recently, MC3T3-E1 has been reported to produce matrix ves icles and media vesicles, and we demonstrated m-calpain in these vesicles p roduced by MC3T3-E1 cultures. We therefore concluded that these vesicles ar e partly responsible for the secretion of m-calpain into the culture medium of MC3T3-E1 cells. (C) 2001 Academic Press.