The IRE1/XBP1 arm of the unfolded protein response suppresses the expression of NKG2D activating ligand MICA in melanoma

The UPR is a signaling pathway activated in response to endoplasmic reticulum (ER) stress. Conditions of ER stress develop following the accumulation of unfolded proteins in the ER. UPR aims to attenuate the stress by multiple mechanisms, such as promotion protein folding and enhancement the degradation of unfolded proteins. While most studies analyze the activities that the UPR induces, a few are looking at processes that the UPR represses or prevents from happening.

The developments in gene editing technologies, in particular the use of CRISPR/Cas9, allow the generation of cell lines deficient in certain genes. Using CRISPR/Cas9 mutagenesis we generated melanoma cells that lack important elements in the UPR pathway, such as IRE1, PERK, CHOP and XBP1. These cells were used for analysis the surface expression of immunomodulatory molecules. This screen identified a strong induction in the expression of the NKG2D ligand major histocompatibility class I (MHC) chain-related molecule A (MICA) in IRE1 KO and XBP1 KO cells and in the presence of an IRE1 nuclease inhibitor, indicating that the IRE1/XBP1 pathway actively suppresses MICA expression. No effect on MICA expression was observed in PERK KO or CHOP KO cells. The mechanism of regulation involves increase MICA mRNA levels in IRE1/XBP1 deficient cells, primarily by enhanced promoter activity.

Analysis of factors that regulate MICA transcription identified E2F1 as the factor that links XBP1 to MICA transcription. E2F1 expression is reduced by ER stress in an XBP1-dependent manner and abolishment of its binding to the MICA promoter strongly attenuates the promoter activity and abolishes the sensitivity to ER stress conditions. Analysis of 470 melanoma tumors establishes a reciprocal expression of XBP1s and MICA or E2F1. This study identifies a novel signaling pathway that may affect the sensitivity of melanoma to NK-dependent immunotherapy.