Background: Extracellular Hsp90 (eHsp90) is implicated in cancer cell motility and invasion. Results: Tumor eHsp90 regulates ERK signaling, EZH2 expression, and histone methylation to facilitate prostate tumor growth and invasion. Conclusion: Newly characterized epigenetic functions of eHsp90 contribute to prostate tumorigenesis. Significance: Epigenetic modulation by eHsp90 may be a key facilitator in the transition of indolent to aggressive disease, highlighting a novel molecular effector of disease progression.

Heat shock proteins (Hsps) represent a diverse group of chaperones that play a vital role in the protection of cells against numerous environmental stresses. Although our understanding of chaperone biology has deepened over the last decade, the “atypical” extracellular functions of Hsps have remained somewhat enigmatic and comparatively understudied. The heat shock protein 90 (Hsp90) chaperone is a prototypic model for an Hsp family member exhibiting a duality of intracellular and extracellular functions. Intracellular Hsp90 is best known as a master regulator of protein folding. Cancers are particularly adept at exploiting this function of Hsp90, providing the impetus for the robust clinical development of small molecule Hsp90 inhibitors. However, in addition to its maintenance of protein homeostasis, Hsp90 has also been identified as an extracellular protein. Although early reports ascribed immunoregulatory functions to extracellular Hsp90 (eHsp90), recent studies have illuminated expanded functions for eHsp90 in wound healing and cancer. While the intended physiological role of eHsp90 remains enigmatic, its evolutionarily conserved functions in wound healing are easily co-opted during malignancy, a pathology sharing many properties of wounded tissue. This review will highlight the emerging functions of eHsp90 and shed light on its seemingly dichotomous roles as a benevolent facilitator of wound healing and as a sinister effector of tumor progression.

Histone variants are known to play a central role in genome regulation and maintenance. However, many variants are inaccessible by antibody-based methods or bottom-up tandem mass spectrometry due to their highly similar sequences. For many, the only tractable approach is with intact protein top-down tandem mass spectrometry. Here, ultrahigh-resolution FT-ICR MS and MS/MS yield quantitative relative abundances of all detected HeLa H2A and H2B isobaric and isomeric variants with a label-free approach. We extend the analysis to identify and relatively quantitate 16 proteoforms from 12 sequence variants of histone H2A and 10 proteoforms of histone H2B from three other cell lines: human embryonic stem cells (WA09), U937, and a prostate cancer cell line LaZ. The top-down MS/MS approach provides a path forward for more extensive elucidation of the biological role of many previously unstudied histone variants and post-translational modifications.

Heat-shock protein 90 (Hsp90), a highly conserved molecular chaperone, is frequently upregulated in tumors, and remains an attractive anti-cancer target. Hsp90 is also found extracellularly, particularly in tumor models. Although extracellular Hsp90 (eHsp90) action is not well defined, eHsp90 targeting attenuates tumor invasion and metastasis, supporting its unique role in tumor progression. We herein investigated the potential role of eHsp90 as a modulator of cancer stem-like cells (CSCs) in prostate cancer (PCa). We report a novel function for eHsp90 as a facilitator of PCa stemness, determined by its ability to upregulate stem-like markers, promote self-renewal, and enhance prostasphere growth. Moreover, eHsp90 increased the side population typically correlated with the drug-resistant phenotype. Intriguingly, tumor cells with elevated surface eHsp90 exhibited a marked increase in stem-like markers coincident with increased expression of the epithelial to mesenchymal (EMT) effector Snail, indicating that surface eHsp90 may enrich for a unique CSC population. Our analysis of distinct effectors modulating the eHsp90-dependent CSC phenotyperevealed that eHsp90 is a likely facilitator of stem cell heterogeneity. Taken together, our findings provide unique functional insights into eHsp90 as a modulator of PCa plasticity, and provide a framework towards understanding its role as a driver of tumor progression.