HSP90 and GRP94 are homologous cellular chaperones found in cytosol and endoplasmic reticulum, respectively. Several years ago, we discovered that members of the benzoquinone ansamycin class of antibiotics, including herbimycin A and geldanamycin (GA) bound to HSP90 and GRP94 and disrupted certain multi-molecular complexes of which these proteins were a part. We have utilized pharmacologic disruption of HSP90 and GRP94 activity to study the function of these chaperones in cellular signal transduction. Multiple signal transduction proteins interact with these charperones, including the kinases src, erbB2, c-raf-1, Akt, Kit, Met, Bcr-Abl, the transcription factor HIF-1alpha, and mutated (but not wild type) p53. A general consequence of pharmacologic disruption of the chaperone/signal protein complex is the resultant marked instability and incorrect subcellular localization of the signalling protein. The instability is due to stimulation of targeted degradation of the signalling protein by the 26S proteasome proteolytic complex following chaperone dissociation. We made the novel observation that HSP90 associates with the cytosolic kinase RIP, a key component of the TNF signalling pathway which leads to NFkB activation. We have determined that disruption of RIP stability by geldanamycin prevents NFkB induction by TNF, but not TNF signalling to Jnk, thus sensitizing cells to the apoptotic properties of TNF. We have additionally observed that another kinase associated with cell survival, Akt, is sensitive to geldanamycin. Geldanamycin blocks NFkB induction by a wide variety of stimuli other than TNF, including chemotherapeutic drugs and IL-1. Its ability to do this may relate to its destabilization of Akt. Benzoquinone ansamycins (geldanamycin) had been the only agents capable of specifically interfering in HSP90/GRP94 function. Recently, we identified radicicol as representing a novel class of natural product capable of binding to HSP90. Both radicicol and the ansamycins bind to HSP90 at an amino terminal nucleotide pocket. Most recently, we have identified a third class of natural product, novobiocin, which also binds to HSP90, although at a lower affinity than either benzoquinone ansamycins or radicicol. Nonetheless, novobiocin appears to cause the same biologic effects on """"""""client proteins"""""""" as ansamycins and radicicol. Surprisingly, novobiocin appears to interact with a carboxyl terminal region on HSP90, which is in fact a previously unrecognized second nucleotide binding site. Preliminary animal testing has revealed no toxicity after twice daily administration of novobiocin for more than one month. This regimen demonstrates significant anti-tumor activity in a transgenic murine model of erbB2-driven breast cancer. We have observed that geldanamycin reverses beta-catenin tyrosine phosphorylation in melanoma cells, probably due to the rapid loss of erbB2 from these cells. In untreated cells, erbB2 and beta-catenin can be readily co-precipitated. Loss of beta-catenin tyrosine phosphorylation leads to an increased association with E-cadherin and decreased cell motility in vitro. This is the first indication that modulation of the tyrosine phosphorylation status of beta catenin in melanoma cells is associated with decreased motility. The fact that beta-catenin tyrosine phosphorylation seems to be mediated, in 3/3 melanoma cell lines examined, by erbB2 - a geldanamycin-sensitive tyrosine kinase - suggests that geldanamycin treatment may be anti-metastatic. This hypothesis is currently being tested in an in vivo metastasis model.In collaboration with Brian Blagg of the Univ. of Kansas, we have identified a series of novobiocin derivatives that demonstrate improved binding affinity and anti-Hsp90 activity compared to the parental compound, and we have demonstrated the ability of several of these derivatives to deplete Hsp90 client proteins in tumor cells.The ErbB family of receptor tyrosine kinases contains four members.
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