The regulatory mechanism of Hsp90α secretion and its function in tumor malignancy

Heat shock protein 90-alpha (Hsp90 alpha) is an intracellular molecular chaperone. However, it can also be secreted with the underlying regulatory mechanism remaining far from clear. Here we show that the secreted Hsp90 alpha is a C-terminal truncated form and its secretion is regulated by the C-terminal EEVD motif via interacting with proteins containing tetratricopeptide repeat domains. We also demonstrate that secretion of Hsp90 alpha is determined by the phosphorylation status at residue Thr-90, regulated by protein kinase A and protein phosphatase 5. We further demonstrate that the secretion of Hsp90 alpha is a prerequisite for its proinvasiveness function and blocking the secreted Hsp90 alpha results in significant inhibition of tumor metastasis. Meanwhile, the level of plasma Hsp90 alpha is positively correlated with tumor malignancy in clinical cancer patients. In sum, our results reveal the regulatory mechanism of Hsp90 alpha secretion, and its function in tumor invasiveness, indicating it can be a promising diagnostic marker for tumor malignancy in clinical application.