Effects of geldanamycin, a heat-shock protein 90-binding agent, on T cell function and T cell nonreceptor protein tyrosine kinases

The benzoquinoid ansamycins geldanamycin (GA), herbimycin, and their deriva tives are emerging as novel therapeutic agents that act by inhibiting the 9 0-kDa heat-shock protein hsp90, We report that GA inhibits the proliferatio n of mitogen-activated T cells. GA is actively toxic to both resting and ac tivated T cells; activated T cells appear to be especially vulnerable. The mechanism by which GA acts is reflected by its effects on an essential hsp9 0-dependent protein, the T cell-specific nonreceptor tyrosine kinase lck, G A treatment depletes lck levels in cultured T cells by a kinetically slow d ose-dependent process, Pulse-chase analyses indicate that GA induces the ve ry rapid degradation of newly synthesized lck molecules. GA also induces a slower degradation of mature lck populations. These results correlate with global losses in protein tyrosine kinase activity and an inability to respo nd to TCR stimuli, but the activity of mature lck is not immediately compro mised. Although the specific proteasome inhibitor lactacystin provides marg inal protection against GA-induced lck depletion, proteasome inhibition als o induces changes in lck detergent solubility independent of GA application . There is no other evidence for the involvement of the proteosome. Lysosom e inhibition provides quantitatively superior protection against degradatio n. These results indicate that pharmacologic inhibition of hsp90 chaperone function may represent a novel immunosuppressant strategy, and elaborate on the appropriate context in which to interpret losses of lck as a reporter for the pharmacology of GA in whole organisms.