Tumour-Secreted Hsp90α on External Surface of Exosomes Mediates Tumour - Stromal Cell Communication via Autocrine and Paracrine Mechanisms

Extracellular heat shock protein-90alpha (eHsp90 alpha) plays an essential role in tumour invasion and metastasis. The plasma eHsp90 alpha levels in patients with various cancers correlate with the stages of the diseases. Nonetheless, the mechanism of action by tumour-secreted eHsp90 alpha remained unclear. Here we show that eHsp90 alpha accounts for approximately 1% of the total cellular Hsp90 alpha and is associated with tumour-secreted exosomes. CRISPR-cas9 knockout of Hsp90 alpha did not affect the overall distribution and quantity of secreted exosomes, but it caused increased exosome-associated CD9 and decreased exosome-associated TSG101, Alix, and CD63. However, Hsp90 alpha-knockout tumour cells have not only lost their own constitutive motility, but also the ability to recruit stromal cells via secreted exosomes. These defects are specifically due to the lack of eHsp90 alpha on tumour cell-secreted exosomes. Anti-Hsp90 alpha antibody nullified the pro-motility activity of tumour-secreted exosomes and human recombinant Hsp90 alpha protein fully rescued the functional defects of eHsp90 alpha-free exosomes. Finally, while current exosome biogenesis models exclusively implicate the luminal location of host cytosolic proteins inside secreted exosomes, we provide evidence for eHsp90 alpha location on the external surface of tumour-secreted exosomes. Taken together, this study elucidates a new mechanism of action by exosome-associated eHsp90 alpha.