Metformin triggers ATF4-mediated UPRmt 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 3T3-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 (UPRmt), as revealed by increased mRNA expression of UPRmt 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 UPRmt and COX-2 inflammation induced by metformin. Interestingly, we showed that the integrated stress response inhibitor (ISRIB) effectively inhibited ATF4, mitigated metformin-induced UPRmt, and reduced NF-κB/COX-2 expression. Taken together, our findings point to the undesirable effect of the high-concentration metformin in adipocytes.