The heat shock protein HSP90 serves as a chaperone for receptor protein kinases, steroid receptors and other intracellular signaling molecules. Targeting HSP90 with ansamycin antibiotics disrupts normal protein processing and promotes degradation of clients of the HSP90 complex, usually by ubiquitination. PDGFRα is a tyrosine kinase receptor upregulated and activated in several malignancies. Endogenous PDGFRα interacts with HSP90, as demonstrated by co-immunoprecipitation of the complex from lysates of C272hTert ovarian cancer cells and in U118 glyoblastoma cells. Treatment of ovarian cancer cells (C272hTert and C848) and glioblastoma cancer cell lines (U118 and ShSy5) with the HSP90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17AAG, 1μM) promotes degradation of the PDGFRα within 3 hours. Likewise, phospho-Akt and total Akt are degraded after treatment with 17AAG. PDGFRα mediated cell proliferation is inhibited by 17AAG, as demonstrated by BRDU incorporation in C272hTert ovarian cells stimulated with PDGF-AA. In contrast, PDGFRα levels are not affected by 17AAG in normal human smooth muscle cells (HSMC) or in murine fibroblasts (NIH3T3). These results suggest that the conformation of the HSP90/PDGFR complex in normal cells may not be disrupted as efficiently by 17-AAG in non-transformed as in transformed cells. PDGFRα degradation by 17-AAG is prevented by treatment with a proteasome inhibitor (MG132). High molecular weight, ubiquitinated forms of the receptor are found in cells treated with 17AAG and MG132. Also, localization of PDGFRα at the cellular membrane is disrupted by treatment with 17AAG. In the presence of MG132, increased cytoplasmic and membrane staining are noted, while treatment with 17AAG and MG132 leads to intense cytoplasmic PDGFR staining. These results suggest that PDGFRα interacts with HSP90 in cancer cells. Targeting this complex with a geldanamycin derivative promotes degradation of the receptor and inhibits PDGFR induced cancer cell proliferation. These findings suggest that HSP90 inhibitors might have therapeutic role in malignancies dependent on the PDGFR.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]