Calpain inhibitors reduce the cornified cell envelope formation by inhibiting proteolytic processing of transglutaminase 1

Calpain I (mu-calpain) and II (m-calpain) are well known calcium-activated neutral cysteine proteases. Many reports have shown that activation of calpain is related to cataract formation, neuronal degeneration, blood clotting, ischemic injuries, muscular dystrophy and cornified cell envelope (CE) formation. Here, we report that insoluble CE formation was reduced after treatment with calpain I inhibitor (N-acetyl-leucyl-leucyl-norleucinal) on normal human epidermal keratinocytes (NHEK), whereas serine and thiol protease inhibitors had no effect on the reduction of CE. When NHEK cells were confluent, keratinocytes were treated with Various concentrations (0.5 mu M - 0.5 mM) of calpain I inhibitor or serine and thiol protease inhibitors under calcium induced differentiation. Insoluble CE formation was reduced about 90 % in the 50 mu M calpain inhibitor I treated group by day 9 of culture, whereas insoluble CE was reduced only 10 % in the same condition. Interestingly TGase activity was blocked by 90 % in the 0.5 mM calpain inhibitor treated group within 72 h, whereas TGase activity was retained by 80 % in the 0.5 mM serine protease inhibitor treated group at 7 day treatment. Therefore it can be suggested that cysteine protease calpains might be responsible for the activation of the TGase 1 enzyme to complete insoluble CE formation during epidermal differentiation.