Speaking of combing Velcade with other regimens . . . From AACR, now rescheduled for July in D.C.
>>Geldanamycin combined with PS-341 synergistically activates a cellular stress response, causes massive accumulation of ubiquitinated proteins and shows enhanced antitumor activity.
Edward G. Mimnaugh, Wanping Xu, Jen S. Isaacs, Michele Vos, Len M. Neckers, National Cancer Institute, NIH, Rockville, MD.
Geldanamycin (GA) specifically binds to the Hsp90 protein chaperone and promotes ubiquitin-dependent proteasomal degradation of many of its client proteins. ps-341 is a potent, reversible, but long-acting proteasome inhibitor. Separately, ps-341 and 17-AAG, a GA analog with reduced host toxicity, are currently being evaluated clinically as novel anticancer agents. We reasoned that combining GA with ps-341 to destabilize Hsp90 client proteins while simultaneously blocking their degradation might result in improved antitumor cytotoxicity. Indeed, GA plus ps-341 were additive in inhibiting the proliferation of MCF-7 carcinoma cells, when measured by the MTT assay. Importantly, when combined with ps-341, the usual cytostatic activity of GA became cytocidal. ps-341, but not GA, potently inhibited proteasome activity. Individually, GA and ps-341 increased Hsp90, Hsp70 and GRP78/BiP stress-response proteins, while in cells exposed to both drugs, these chaperones were superinduced and were substantially relocalized to the detergent-insoluble, pellet fraction of cell lysates. GA alone only slightly increased protein ubiquitination, but GA combined with ps-341 enhanced protein ubiquitination well beyond the level that was stabilized by ps-341. The bulk of accumulated ubiquitinated proteins were localized in the pellet fraction, suggesting they were aggregated. In cells treated with GA plus ps-341, immunofluorescence revealed strong ubiquitin localization in what appeared to be aggresomes. The dual drug treatment also caused the formation of prominent and abundant cytosolic vacuoles, which fluorescence-tagged organelle marker experiments indicated originated from endoplasmic reticulum, did not contain Hsp70 and were not lysosome derived. Cycloheximide co-treatment with GA & ps-341 prevented vacuole formation and blocked protein ubiquitination, suggesting that newly-synthesized, misfolded proteins might be sequestered within vacuoles as well as in aggresomes in proteasome-inhibited cells. By simultaneously disrupting Hsp90 function while blocking the proteasome degradation pathway, GA and ps-341 strongly stimulate the accumulation of ubiquitinated proteins, induce a robust cellular stress response and ultimately kill tumor cells.<<
17-AAG is from Kosan
Cheers, Tuck |
