The chaperone GRP94 interacts with the proprotein convertase furin and regulates TGF-beta maturation in human primary M2 macrophages.

Transforming growth factor-beta (TGFβ) is a major immunosuppressive cytokine produced by various cell types, including regulatory T cells (Tregs) and M2 macrophages. In Tregs, GRP94 is known as a key protein regulating TGFβ expression by chaperoning both the TGFβ docking receptor, GARP, and the integrin αvβ8. We previously reported that GRP94 inhibition in a triple-negative breast cancer (TNBC) murine model induced a decrease in intra-tumoral CD206 + M2-like macrophages that correlated with a decrease in collagen content, an increase in CD8+ cells in tumors and a reduced tumor volume. Here, we investigated the impact of GRP94 inhibition on TGFβ expression focusing on a possible interaction with furin, a proprotein convertase responsible for the first step in TGFβ maturation. We demonstrated in human primary M2 macrophages that GRP94 interacted with furin and that GRP94 inhibition by its selective inhibitor PU-WS13 led to a decrease in furin enzymatic activity, which was associated with a decrease in TGFβ secretion. Similar results were obtained in the human TNBC cell line MDA-MB-231 using PU-WS13 and GRP94 inhibitor- 1, suggesting that our findings are not cell type-specific. Finally, we showed that GRP94 associated with LRRC33, a GARP paralog in macrophages. Together, these findings support the hypothesis that GRP94 plays a key role in regulating the TGFβ maturation pathway, not only in Tregs as previously reported, but also in M2 macrophages and tumor cells.