Small-Molecule Sigma1 Modulator Induces Autophagic 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 I integral membrane glycoprotein that is cotranslationally 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 antitumor 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 pharmacologic 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 coculture 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 IFNγ-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. Visual Overview: http://mcr.aacrjournals.org/content/molcanres/16/2/243/F1.large.jpg. Mol Cancer Res; 16(2); 243–55. ©2017 AACR.