Oestrogen receptor subtype-specific repression of calpain expression and calpain enzymatic activity in neuronal cells -: implications for neuroprotection against Ca2+-mediated excitotoxicity

Calpains represent a superfamily of Ca2+-activated cysteine-proteases, which are important mediators of apoptosis and necrosis. In the brain, m-calpain and mu-calpain, the two ubiquitous calpain-isoforms, are strongly activated in neurones after an excitotoxic Ca2+ influx occurring, for example, during cerebral ischemia. Because oestrogen and its receptors (ER alpha/ER beta) can exert neuroprotective activity, we investigated their influence on expression of calpains and their endogenous inhibitor, calpastatin. We found that ectopic expression of ER alpha in human neuroblastoma SK-N-MC cells led to a ligand-independent constitutive down-regulation of m-calpain accompanied by an up-regulation of mu-calpain expression. Up-regulation of mu-calpain was reversed in the presence of oestrogen, which, in turn, could be blocked by co-treatment with the oestrogen-receptor antagonist ICI 182 780. Expression of calpastatin was not altered, either in the absence or in the presence of oestrogen. Additional studies revealed that ER alpha-expressing cells exhibited decreased calpain enzymatic activity and increased survival when cells were exposed to the Ca2+ ionophore, ionomycin. Since all investigated effects could be observed exclusively in the presence of ER alpha, but not ER beta, and since the effects are reduced when ER alpha and ER beta are co-expressed, our data suggest a novel ER subtype-specific neuroprotective action by repressing calpain expression and calpain activity under conditions of a massive Ca2+ influx.