Regulation of redox homeostasis by ATF4-MTHFD2 axis during white adipose tissue browning.

Maintaining redox balance is crucial for mitochondrial homeostasis. During browning of white adipocytes, both the quality and quantity of mitochondria undergo dramatic changes. However, the mechanisms controlling the redox balance in the mitochondria during this process remain unclear. In this study, we demonstrate that thermogenic activation occurs before mitochondrial biogenesis during cold-induced browning of inguinal white adipose tissue (iWAT) and is accompanied by increased mitochondrial stress and integrated stress response (ISR) signaling. Specifically, cold exposure enhances the expression of ATF4, an ISR effector. Adipocyte-specific deletion of ATF4 results in increased energy expenditure, but paradoxically leads to a lower core body temperature, and heightened pro-inflammation in iWAT after cold exposure, which is restored by the antioxidant, MitoQ. Mechanistically, ATF4 regulates the redox balance through MTHFD2, an enzyme involved in mitochondrial redox homeostasis by NADPH generation. Cold exposure upregulates MTHFD2 expression in an ATF4-dependent manner, and its inhibition by DS18561882 in vivo leads to impaired cold-induced mitochondrial respiration similar to the effects of ATF4 loss. These findings suggest that ATF4 is essential for redox balance via MTHFD2, thereby affecting tissue homeostasis during iWAT browning.