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

Abstract

Calpains represent a superfamily of Ca2+-activated cysteine-proteases, which are important mediators of apoptosis and necrosis. In the brain, m-calpain and μ-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 (ERα/ERβ) can exert neuroprotective activity, we investigated their influence on expression of calpains and their endogenous inhibitor, calpastatin. We found that ectopic expression of ERα in human neuroblastoma SK-N-MC cells led to a ligand-independent constitutive down-regulation of m-calpain accompanied by an up-regulation of μ-calpain expression. Up-regulation of μ-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 ERα-expressing cells exhibited decreased calpain enzymatic activity and increased survival when cells were exposed to the Ca 2+ ionophore, ionomycin. Since all investigated effects could be observed exclusively in the presence of ERα, but not ERβ, and since the effects are reduced when ERα and ERβ 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. © 2006 The Authors.