Glioblastoma multiforme is the most treatment-resistant
brain tumor. Elongation factor-2 (EF-2)
kinase (
calmodulin kinase III) is a unique
protein kinase that is overexpressed in
glioma cell lines and in human surgical specimens. Several
mitogens activate this
kinase and inhibitors block
mitogen activation and produce cell death.
Geldanamycin (GA) is a
benzoquinone ansamycin antibiotic that disrupts Hsp90-protein interactions. Because
EF-2 kinase is chaperoned by Hsp90, we investigated the effects of GA on the viability of
glioma cells, the expression of
EF-2 kinase protein, and the interaction between Hsp90 and
EF-2 kinase. GA was a potent inhibitor of the clonogenicity of four
glioma cells lines with IC(50)s ranging from 1 to 3 nM.
17-allylamino-17-demethoxygeldanamycin (17-AAG), a less toxic and less potent derivative of GA, inhibited the clonogenicity of
glioma cells with IC(50) values of 13 nM in C6 cells and 35 nM in T98G cells. Treatment of cell lines for 24-48 h of GA or
17-AAG disrupted
EF-2-kinase/Hsp90 interactions as measured by coimmunoprecipitation, resulting in a decreased amount of recoverable
kinase in cell lysates. The ability of GA to inhibit the growth of
glioma cells was abrogated by overexpressing
EF-2 kinase. In addition,
17-AAG significantly inhibited the growth of a
glioma xenograft in nude mice. These studies demonstrate for the first time the activity of GAs against human
gliomas in vitro and in vivo and suggest that destruction of
EF-2 kinase may be an important cytotoxic mechanism of this unique class of
drug.