Ansamycins exert their effects by binding
heat shock protein 90 (Hsp90) and targeting important signalling molecules for degradation via the
proteasome pathway. We wanted to study the effect of
geldanamycin (GA) and its derivative
17-allylamino-17-demethoxygeldanamycin (17-AAG) on
glioblastoma cell lines. We show that these cells are growth inhibited by
ansamycins by being arrested in G(2)/M and, subsequently, cells undergo apoptosis. The
protein levels of cell division cycle 2 (cdc2)
kinase and cell division cycle 25c (cdc25c) were downregulated upon GA and
17-AAG treatment and cdc2
kinase activity was inhibited. However, other
proteins involved in the G(2)/M checkpoint were not affected. The cdc2 and cdc25c
mRNA levels did not show significant differences upon
ansamycin treatment, but the stability of cdc2
protein was reduced. The association of cdc2 and cdc25c with p50(cdc37), an Hsp90 co-chaperone, decreased, but the interaction of cdc2 and cdc25c with the Hsp70 co-chaperone increased after
ansamycin treatment.
Proteasome inhibitors were able to rescue the cdc2 downregulation, but not the cdc25c reduction. However,
calpain inhibitors were able to rescue the cdc25c downregulation, suggesting that cdc25c is proteolysed by calpains in the presence of
ansamycins, and not by the
proteasome. We conclude that
ansamycins downregulate cdc2 and cdc25c by two different mechanisms.