Chloroquine-induced proinsulin misfolding in the endoplasmic reticulum underlies the attenuation of mature insulin synthesis.

Chloroquine (CQ), initially introduced for the clinical treatment of malaria, has subsequently been found to exhibit beneficial effects in combating diabetes mellitus. The anti-hyperglycemic properties of chloroquine may be attributed to its anti-inflammatory response and its ability to activate the insulin signaling pathway. However, both animal and clinical studies have yielded mixed results. Moreover, the impact of chloroquine on pancreatic β-cells, the key player of glycemic control, was not known. To fill this knowledge gap, we investigated the effects of chloroquine on pancreatic β-cell functions. Our findings revealed that while chloroquine did not alter proinsulin expression, it interfered with the conversion of proinsulin to insulin, resulting in reduced insulin levels. Using multiple independent approaches, we further showed that chloroquine disrupted proinsulin oxidative folding in the endoplasmic reticulum (ER) and impaired proinsulin trafficking from ER to Golgi, leading to ER stress and decreased insulin production. Notably, the elevated ER stress observed in chloroquine-treated β-cells was reversed upon knockout of insulin genes, indicating that chloroquine-induced β-cell ER stress primarily through the accumulation of misfolded proinsulin, rather than directly affecting ER homeostasis. Further investigation into the mechanisms underlying chloroquine-induced proinsulin misfolding revealed that the accumulation of misfolded proinsulin was not caused by autophagy inhibition or the alkaline pH of chloroquine. Instead, it was primarily due to the disruption of the interaction between proinsulin and protein disulfide isomerase (PDI). Our findings unveiled new mechanisms of chloroquine treatment and raised important safety considerations regarding the use of chloroquine in diabetes treatment.