A critical role for HSP90 in cancer cell invasion involves interaction with the extracellular domain of HER-2

HSP90 is a ubiquitously expressed molecular chaperone that controls the folding, assembly, intracellular disposition, and proteolytic turnover of many proteins, most of which are involved in signal transduction processes. Recently, a surface form of HSP90 has been identified and associated with cell migration events. In this paper, we explore the interaction of surface HSP90 with HER-2, a receptor-like glycoprotein and member of the ErbB family of receptor tyrosine kinases that play central roles in cellular proliferation, differentiation, and migration as well as in cancer progress. The involvement of HSP90 in the regulation of HER-2 has been attributed so far to receptor stabilization via interaction with its cytoplasmic kinase domain. Here we present evidence, using glutathione S-transferase pull-down and transfection assays, for a novel interaction between surface HSP90 and the extracellular domain of HER-2. Specific disruption of this interaction using mAb 4C5, a function-blocking monoclonal antibody against HSP90, inhibits cell invasion accompanied by altered actin dynamics in human breast cancer cells under ligand stimulation conditions with heregulin. Additionally, disruption of surface HSP90/HER-2 interaction leads to inhibition of heregulin-induced HER-2-HER-3 heterodimer formation, reduced HER-2 phosphorylation, and impaired downstream kinase signaling. Interestingly, this disruption does not affect HER-2 internalization. Our data suggest that surface HSP90 is involved in heregulin-induced HER-2 activation and signaling, leading to cytoskeletal rearrangement, essential for cell invasion.