UPF1 deficiency enhances mitochondrial ROS which promotes an immunosuppressive microenvironment in pancreatic ductal adenocarcinoma

Upstream frameshift 1 (UPF1) is an RNA helicase involved in a number of mRNA regulatory processes including nonsense-mediated decay. Mutations in the UPF1 locus that reduce its expression have been associated with adenosquamous carcinoma of the pancreas, a particularly aggressive form of the disease. To determine the effect of Upf1 suppression in a murine model of pancreatic adenocarcinoma, we silenced with shRNA Upf1 in cells derived from an autochthonous tumor in an LSL-KrasG12D/+; Trp53R172H/+; Pdx-1Cre/+ mouse (KPC) and orthotopically implanted these cells in the pancreas of C57BL/6 mice. Tumors derived from Upf1-deficient cells were markedly larger than those derived from control cells, a difference observed only in immunocompetent mice. The immune infiltrate of Upf1-deficient tumors was enriched in myeloid-derived suppressor cells (MDSCs) and depleted of CD8+ cells compared to control KPC tumors. Upf1-deficient KPC cells secreted inflammatory cytokines including G-CSF and CXCL2, known to recruit MDSCs. Cytokine secretion from Upf1-deficient KPC cells was induced by increased levels of mitochondrial reactive oxygen species (ROS), which in turn were due to an increase in complex I activity in the electron transport chain. Thus, Upf1 helicase deficiency leads to increased mitochondrial complex I activity which produces ROS that signals for cytokine release that drives immune suppression and enhanced tumor growth.