Exosomal and extracellular HMGB1 have opposite effects on SASH1 expression in rat astrocytes and glioma C6 cells.

Exosomes are a type of extracellular vesicles derived from cells and mediators of intercellular communication. Different cell types have their own unique exosomes for exchanging information. We previously found that SASH1, a tumor suppressor, was lowly expressed or absent in glioma tissues and glioma C6 cells, but the structure and function of the corresponding exosomes had been unclear. Hence, we aimed to investigate whether exosomes generated from normal glial cells and glioma cells form different protein patterns and whether those derived from normal glial cells affect SASH1 expression in glioma cells. We collected exosomes from astrocytes and C6 cells and identified their exosomal proteins through mass spectrometry. We also performed gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) analyses, whose results showed that both the total and unique exosomal proteins from each cell type were similar. Moreover, the KEGG analysis revealed different clusters of unique exosomal proteins in glial cells and glioma cells. In the normal glial cells, the top clusters were mainly involved in processes with RNA transcripts and proteins, whereas in glioma cells the clusters were attributed to PI3K-Akt signaling, cell adhesion, and cancer-related pathways. Western blot analysis showed that HMGB1 exists in exosomes derived from cultured astrocytes, although its expression was higher in glioma C6 cells. Furthermore, we found that exosomes extracted from astrocytes could increase SASH1 expression in C6 cells (P = 0.040), whereas those derived from HMGB1-depleted astrocytes could not (P = 0.6133). The expression levels of SASH1 decreased after the addition of extracellular recombinant HMGB1 protein, whereas that of TLR4 increased. Our study is the first to demonstrate that HMGB1 plays different roles depending on its form: as an extracellular protein, HMGB1 decreases SASH1 expression, but as an exosomal protein, HMGB1 increases SASH1 expression. Nevertheless, the mechanism, which partly depends on the TLR4 pathway, behind these opposing effects requires further study. Our novel findings on the structure-dependent roles of the cytokine HMGB1 in promoting or inhibiting cancer provide a fresh insight into the interactions of cancer cells with the microenvironment.