Altered Hsp90 function in cancer: a unique therapeutic opportunity

Molecular chaperones or so-called heat shock proteins serve as central integrators of protein homeostasis within cells. In performing this function, they guide the folding, intracellular disposition, and proteolytic turnover of many key regulators of cell growth, differentiation, and survival. Recent data show essential roles for the chaperones in facilitating malignant transformation at the molecular level and support the concept that their altered utilization during oncogenesis is critical to the development of human cancers. The field is evolving rapidly, but it has become apparent that chaperones can serve as biochemical buffers at the phenotypic level for the genetic instability that is characteristic of many human cancers. Chaperone proteins thus allow tumor cells to tolerate the mutation of multiple critical signaling molecules that would otherwise be lethal. Much of the recent progress in understanding the complex role of heat shock proteins in tumorigenesis has been made possible by the discovery of several natural product antitumor antibiotics that selectively inhibit the function of the chaperone Hsp90. These agents have been used as probes to define the biological functions of Hsp90 at the molecular level and to validate it as a novel target for anticancer drug action. One of these agents, 17-allylamino,17-demethoxygeldanamycin (NSC 330507) has begun phase II clinical trials, and several second-generation compounds are now in late preclinical development. The best way to use Hsp90 inhibitors as anticancer agents remains to be defined. Trials accomplished to date, however, serve as proof of principle that Hsp90 function can be modulated pharmacologically without undue toxicity in humans. Given the redundancy and complexity of the signaling pathway abnormalities present in most cancers, the ability of Hsp90 inhibitors to alter the activity of multiple aberrant signaling molecules instead of just one or two (as most current-generation molecular therapeutics have been designed to do) may prove of unique therapeutic benefit.