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 mu-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 mu-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 mu-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 mu-calpain at the plasma membrane, and, after mu-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 mu-calpain causes proteolysis of merlin conceivably to impair cell adhesion and/or contact inhibition of meningioma cells.
Kaneko
T
T
Department of Neurosurgery, Kanazawa University Graduate School of Medical Sciences, Kanazawa, Japan.
Yamashima
T
T
Tohma
Y
Y
Nomura
M
M
Imajoh-Ohmi
S
S
Saido
T C
TC
Nakao
M
M
Saya
H
H
Yamamoto
H
H
Yamashita
J
J
D001932:Brain Neoplasms / Q000503:physiopathology*; D002154:Calpain / Q000378:metabolism*; D002448:Cell Adhesion; D002471:Cell Transformation, Neoplastic; D005260:Female; D006801:Humans; D006861:Hydrogen Peroxide / Q000494:pharmacology; D008297:Male; D008579:Meningioma / Q000503:physiopathology*; D008875:Middle Aged; D025581:Neurofibromin 2 / Q000378:metabolism*; D016877:Oxidants / Q000494:pharmacology; D018384:Oxidative Stress; D014407:Tumor Cells, Cultured
D025581:Neurofibromin 2; D016877:Oxidants; D006861:Hydrogen Peroxide; D002154:Calpain