Upregulation of the calcium-dependent protease, calpain, during keratinocyte differentiation

Calpain is a ubiquitous neutral calcium-activated thiol protease that is im plicated in various cellular functions including exocytosis, cell fusion, a poptosis and proliferation. The calpain system is composed of the enzymes C L-calpain and m-calpain and their endogenous inhibitor. calpastatin. We emp loyed the spontaneously immortalized human HaCaT keratinocytes, which retai n their ability to differentiate in vitro and in vivo. to study the modulat ion of the calpain system during keratinocyte differentiation. The cellular levels of keratinocyte differentiation markers and of the components of th e calpain system were monitored by immunoblotting. Three established differ entiation stimuli: increase in cell density as a function of time in cultur e, elevation of extracellular calcium concentration and exposure to 1.25-di hydroxyvitamin D-3 enhanced the expression of the three keratinocyte differ entiation markers keratin 10, involucrin and transglutaminase. The differen tiation of HaCaT cells was accompanied by elevation of the components of th e calpain system, although the pattern of increase varied according to the specific differentiation stimulus. ii higher increase in calpains as compar ed with the increase in calpastatin suggests an increase in net calpain act ivity during differentiation. Such an increase may play a part in the diffe rentiation process itself and/or in the regulation of key events in differe ntiating keratinocyte metabolism.