Sigma1 Modulator Induces Degradation of PD-L1

Emerging evidence suggests that Sigma1 (SIGMAR1, also known as sigma-1 receptor) is a unique ligand-regulated integral membrane scaffolding protein that contributes to cellular protein and lipid homeostasis. Previously, we demonstrated that some small molecule modulators of Sigma1 alter endoplasmic reticulum (ER) associated protein homeostasis pathways in cancer cells, including the unfolded protein response and autophagy. Programmed death-ligand 1 (PD-L1) is a type 1 integral membrane glycoprotein that is co-translationally inserted into the ER and is processed and transported through the secretory pathway. Once at the surface of cancer cells, PD-L1 acts as a T-cell inhibitory checkpoint molecule and suppresses anti-tumor immunity. Here, we demonstrate that in Sigma1-expressing triple negative breast and androgen-independent prostate cancer cells, PD-L1 protein levels were suppressed by RNAi knockdown of Sigma1 and by small molecule inhibition of Sigma1. Sigma1-mediated action was confirmed by pharmacological competition between Sigma1 selective inhibitor and activator ligands. When administered alone, the Sigma1 inhibitor decreased cell surface PD-L1 expression and suppressed functional interaction of PD-1 and PD-L1 in a co-culture of T-cells and cancer cells. Conversely, the Sigma1 activator increased PD-L1 cell surface expression, demonstrating the ability to positively and negatively modulate Sigma1 associated PD-L1 processing. We discovered that the Sigma1 inhibitor induced degradation of PD-L1 via autophagy, by a mechanism distinct from bulk macroautophagy or general ER-stress associated autophagy. Finally, the Sigma1 inhibitor suppressed interferon-γ induced PD-L1. Our data demonstrate that small molecule Sigma1 modulators can be used to regulate PD-L1 in cancer cells and trigger its degradation by selective autophagy. Implications: Sigma1 modulators sequester and eliminate PD-L1 by autophagy, thus preventing functional PD-L1 expression at the cell surface. This posits Sigma1 modulators as novel therapeutic agents in PD-L1/PD-1 blockade strategies that regulate the tumor immune microenvironment.

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PublicationsMartin Lehr