Metformin triggers ATF4-mediated UPR^mt and COX-2 inflammation in adipocytes

Metformin is the most common drug for type 2 diabetes due to its action to improve insulin sensitivity and enhance glucose uptake in tissues, including adipose tissue. As a mitochondrial complex I inhibitor, treatment with metformin may cause deleterious effects. Here, we demonstrated that treatment of adipocytes 3 T3-L1 cells with a high concentration of metformin (10 mM) led to increased reactive oxygen species (ROS) accumulation and triggered the mitochondrial unfolded protein response (UPR^mt), as revealed by increased mRNA expression of UPR^mt markers (mtHSP70, Lonp1, and FGF21). High-concentration metformin also induced COX-2 inflammation, as indicated by increased NF-κB phosphorylation and cyclooxygenase-2 (COX-2) expression. By contrast, a lower concentration (1.25 mM) showed no effects. We found that ATF4 was selectively upregulated and was required for UPR^mt and COX-2 inflammation induced by metformin. Interestingly, we showed that the integrated stress response inhibitor (ISRIB) effectively inhibited ATF4, mitigated metformin-induced UPR^mt, and reduced NF-κB/COX-2 expression. Taken together, our findings point to the undesirable effect of the high-concentration metformin in adipocytes.