There is increasing evidence localizes the mitochondrial chaperone heat shock protein (HSP)60, outside the cell, where it mediates interactions between immune cells and other body tissues. However, the mechanisms by which HSP60 is secreted into the extracellular environment are not fully understood. Recent studies have shown that HSP60 is actively released by a nonconventional secretion mechanism, the lipid raft-exosome pathway. In the present study, we show for the first time that HSP60, produced by 3-methylcholantrene-induced fibrosarcoma tumour cells, is secreted through the conventional endoplasmic reticulum-Golgi secretory pathway. Confocal microscopy using anti-TGN38 and anti-HSP60 antibodies together with monensin, a Golgi transport inhibitor, demonstrated the relocation of HSP60 to the Golgi of malignant cells but not primary fibroblast cells subjected to heat shock or fibroblast cell lines. Transmission electron microscopy, flow cytometry and cell fractionation of cell treated with brefeldin A, an inhibitor of endoplasmic reticulum to Golgi protein transport, further indicated that HSP60 is present both in the endoplasmic reticulum and the Golgi complex of malignant cells. We found a single mRNA with a mitochondrial targeting sequence encoding for HSP60 in the malignant cells but two HSP60 translation products, namely the native unmodified protein and a protein post-translationally modified by N-glycosylation. The N-glycans observed were composed of high-mannose structures and bi-, tri-and tetraantennary complex type structures occupying sites of the three potential glycosylation sites present on HSP60. Accordingly, we propose that HSP60 in malignant cells is transported through the endoplasmic reticulum-Golgi secretion pathway, where it acquires N-glycans, and thus can affect the immunological properties of the proteins in the tumour microenvironment. Structured digital abstractl Grp94 and Hsp60 colocalize by cosedimentation through density gradient (View interaction) Abbreviations 3-MCA, 3-methylcholantrene; BFA, brefeldin A; EGFA, epidermal growth factor receptor; ER, endoplasmic reticulum; FACS, fluorescenceactivated cell sorting; FGF-2, fibroblast growth factor-2; FITC, fluorescein isothiocyanate; HRP, horseradish peroxidase; HSP, heat shock protein; MFI, mean fluorescence intensity; NP-HPLC, normal phase-HPLC; TEM, transmission electron microscopy; TLR, Toll-like receptor.

We evaluated the patterns of sialylation on fibrosarcoma cell lines arising following 3-methylcholanthrene treatments of wild-type and IL-1alpha-deficient mice; the former induced progressive tumors, whereas the latter cell lines induced regressing tumors or failed to develop into tumors in mice due to immune rejection. In regressing tumors, terminating alpha2-6-Neu5Ac residues were present at lower levels than in progressively growing tumors. In both tumor cells, the amount of alpha2-6-Neu5Ac residues was higher by an order of magnitude relative to the amount expressed in primary fibroblasts harvested from IL-1alpha-deficient and wild-type mice. We focused on membrane proteins, which may interact with the immune system. Interestingly, HSP65, grp75, and gp96 were found on the surfaces of malignant cells and were shown to possess sialylated N-glycans. The amount of trisialylated glycans on gp96 and HSP65 and monosialylated glycans on grp75 of regressing cells was significantly lower than in progressively growing cells, suggesting a dependency of these specific glycoforms on anti-tumor immunity.

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