Calpain-dependent proteolysis of merlin occurs by oxidative stress in meningiomas: A novel hypothesis of tumorigenesis

BACKGROUND. The purpose of this study is to indicate that oxidative stress may contribute to occurrence of meningiomas. Recently, it was reported that aside from the neurofibromatosis type 2 (NF2) gene mutations, the calpain-dependent proteolysis of the NF2 gene product, merlin might be closely related to the development of certain NF2-related tumors. Although meningiomas are well known to occur more frequently in aged persons, it still remains unknown why calpain activation occurs predominantly in them. Because the production of free radicals with aging might be one of the causes of calpain activation especially in leptomeningeal cells being devoid of blood supply, the authors examined the relations between μ-calpain activation and merlin proteolysis induced by the oxidative stress. METHODS. The authors examined 12 patient-derived sporadic meningiomas and their primary cultured cells. Malignant glioma cell line (U-251MG), which had no relation to NF2, was used as a control. They were exposed to hydrogen peroxide (H₂O₂) for 1 hour. After oxidative stress, they were examined by Western blot and immunofluorescence microscopic analyses. RESULTS. Despite the consistent expressions of activated μ-calpain in 11 of 12 meningioma tissues, this calpain activation completely disappeared after culture; instead the full-length merlin appeared again in 8 of 11 cases. The treatment of cultured cells with hydrogen peroxide induced both μ-calpain-dependent cleavage of merlin and reduction of an intrinsic calpain inhibitor calpastatin. Such proteolysis was significantly blocked by a specific calpain inhibitor, Z-LLal. The full-length merlin was immunocytochemically colocalized with activated μ-calpain at the plasma membrane, and, after μ-calpain activation, the fragment of merlin translocated to the perinuclear cytoplasm or into the nucleus. CONCLUSIONS. These findings suggest that oxidative stress-induced activation of μ-calpain causes proteolysis of merlin conceivably to impair cell adhesion and/or contact inhibition of meningioma cells.