• Authors: Maher, C. M., Thomas, J. D., Haas, D. A., Longen, C. G., Oyer, H. M., Tong, J. Y., Kim, F. J.
  • Year: 2018
  • Journal: Mol Cancer Res 16 243-255
  • Applications: in vitro / DNA, siRNA / INTERFERin, jetPRIME
  • Cell types:
    1. Name: MDA-MB-231
      Description: Human breast adenocarcinoma cells
      Known as: MDAMB231
    2. Name: Mouse embryonic immortalized fibroblasts
      Description: Immortalized mouse embryonic fibroblasts
      Known as: iMEF
    3. Name: PC-3
      Description: Human prostate carcinoma cells
      Known as: PC3, PC 3


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 IFNgamma-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: Mol Cancer Res; 16(2); 243-55. (c)2017 AACR.