Sialic acid content of tissue-specific gp96 and its potential role in modulating gp96-macrophage interactions

Cancer-derived heat shock protein gp96 induces a tumor-specific protective immune response primarily mediated by cytotoxic T lymphocytes (CTL) directed toward cancer-associated peptides associated with gp96. Both innate and adaptive immune responses have been demonstrated using a cell culture-based signaling mechanism. When used as an extraneous vaccine, one critical interaction which must occur for an immune response to be generated is the interaction between gp96 and the antigen presenting cell (APC) surface receptors (CD91, SR-A, TLR-2, and TLR-4). Our previous study concluded that gp96 purified from various rat and human prostate cancers is differentially glycosylated based on the amino and neutral monosaccharide content, and it was postulated that the monosaccharides may play a role in its biological activity. In this report, we report differences in the cancer-specific sialic acid content of gp96 purified from normal rat prostate compared to two rat prostate cancers, MAT-LyLu and Dunning G, as well as between two human prostate cancer cells, LnCaP and DU145. We also examined the modulatory effect of sialic acid residues on the binding of gp96 to APCs and its subsequent activation. Our results supported the contention that significant differences in the sialic acid content exist between Dunning G, MAT-LyLu, and normal rat prostate gp96, which affected its binding and biochemical activity to APCs. We therefore postulate that varied glycans of HPS96, a hitherto neglected structural component, may play a pivotal role in its anticancer activity. We suggest that construction of the glycan tree is a key to identification of the necessary and sufficient elements in the structure-function activity of HSP96.