HSP90 inhibition radiosensitize head and neck carcinoma via EGFR degradation

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We have recently discovered that EGFR is activated on exposure to cisplatin and gemcitabine in head and neck cancer cell lines. This activation of EGFR leads to ubiquitin-mediated degradation of the receptor itself, causing cellular toxicity and radiation sensitization. We hypothesized that the receptor degradation is a critical event in drug induced toxicity and radiation sensitization. In order to test our hypothesis, we screened several head and neck cancer cell lines that are either sensitive or resistant to cisplatin. We have found that EGFR is not degraded in cisplatin-resistant cell lines. We hypothesized that if we could promote EGFR degradation, we could then sensitize these cells to cisplatin-mediated cytotoxicity radiosensitization. For this purpose we chose a novel approach towards targeting EGFR using Hsp90 inhibition. We chose this approach because it is known that Hsp90 is a major chaperone for ErbB-2, which shares major sequence homology with EGFR at the Hsp90 binding site.
 Clonogenic survival assays were performed to determine the optimum dose of cisplatin and geldanamycin and their combination. We carried out Immunoblotting of EGFR and HSP90 signaling mediators in cisplatin sensitive and resistant head and neck cancer cell lines after treating them with cisplatin, geldanamycin and their combinations. Flow cytometry was also carried out in these experimental conditions. Our data show that (1) Hsp90 is activated by cisplatin only in cisplatin-sensitive cells, (2) EGFR and Hsp90 interact and can be co-immunoprecipitated, and (3) geldanamycin, an inhibitor of Hsp90, accelerates the degradation of EGFR in cisplatin-resistant cells, leading to both cellular toxicity and significant radiosensitization.
 These findings demonstrate that EGFR degradation after chemotherapy depends on Hsp90. Furthermore, they suggest that the new generation of geldanamycin analogues that are entering the clinic may potentiate cisplatin-mediated cytotoxicity and radiosensitization via EGFR degradation.