The calpain-calpastatin system in human central nervous system tumours: new insight on its prognostic value

The calpain/calpastatin proteolytic system regulates several physiopathological processes and can play opposing roles in cancer. New strategies to control alterations ofthis system for therapeutic purposes require a full understanding of the functional role played by the single protein members. For this purpose we have evaluated theinvolvement of calpain­1 in the proliferation of human meningioma and glioblastoma cells obtained from brain surgery. We demonstrate that tumour samples with analtered intracellular homeostasis of Ca2+ show increased conversion of the 80 kD inactive calpain­1 to the autolysed 75 kD form, catalytically active at mM Ca2+concentrations in vitro. Intracellular calpain­1 targets, such as spectrin and the natural calpain inhibitor calpastatin, result degraded in vivo in these tumour samples.Specifically, calpastatin undergoes a limited digestion to discrete fragments that maintain the inhibitory activity. Tumour samples with mean or high amounts of 75 kDcalpain­1 show a wide cytosolic diffusion of the inhibitor, while samples characterized by low levels of active calpain­1, retain calpastatin in cytosolic aggregates. Sincehigher calpain­1 activation does not correspond to a detectable decrease in the Ki67 cell proliferation index, the activity of calpain­1 seems to have a low prognosticsignificance in the progression of the brain tumours analysed in this study. However, the perturbation of Ca2+ homeostasis promotes an increased synthesis of MatrixMetalloproteinase­9 (MMP­9), known to be involved in tumour invasiveness, suggesting that other Ca2+­dependent processes could be related to the malignancy ofthese cells. These evidences suggest that the potentially harmful activity of autolysed 75 kD calpain­1 can be efficiently controlled by the low Mr calpastatin producedby the protease itself in vivo, allowing cell proliferation.