Stefin B (
cystatin B) is an endogenous inhibitor of
cysteine proteinases localized in the nucleus and the cytosol. Loss-of-function mutations in the
stefin B gene (CSTB) gene were reported in patients with
Unverricht-Lundborg disease (EPM1). Our previous results showed that thymocytes isolated from
stefin B-deficient mice are more sensitive to apoptosis induced by the
protein kinase C (PKC) inhibitor staurosporin (STS) than the wild-type control cells. We have also shown that the increased expression of
stefin B in the nucleus of T98G
astrocytoma cells delayed cell cycle progression through the S phase. In the present study we examined if the nuclear or cytosolic functions of
stefin B are responsible for the accelerated induction of apoptosis observed in the cells from
stefin B-deficient mice. We have shown that the overexpression of
stefin B in the nucleus, but not in the cytosol of
astrocytoma T98G cells, delayed
caspase-3 and -7 activation. Pretreatment of cells with the pan-
caspase inhibitor
z-Val-Ala-Asp(OMe)-fluoromethylketone completely inhibited
caspase activation, while treatment with the inhibitor of calpains- and
papain-like
cathepsins (2S,3S)-trans-epoxysuccinyl-leucylamido-3-methyl-butane ethyl
ester did not prevent
caspase activation. We concluded that the delay of
caspase activation in T98G cells overexpressing
stefin B in the nucleus is independent of
cathepsin inhibition.