In addition, a proteomic study comparing the insulin secretion of glucose-responsive and non-responsive insulinoma cell lines identified lower levels of UPR-related chaperone proteins such as glucose-regulated protein 78kDa (GRP78), glucose-regulated protein 90kDa (GRP94), protein disulfide isomerase (PDI), and endoplasmic reticulum protein 29 (ERP29) in non-responsive cells, suggesting that the depletion of such proteins may affect β-cell function [65]. Here, P4HB is linked to pancreatic insulinoma.