ER stress-associated transcription factor CREB3 is essential for normal Ca <superscript>2+</superscript> , ATP, and ROS homeostasis.

In mammalian cells, mitochondrial respiration produces reactive oxygen species (ROS) such as superoxide (O 2 - ), which is then converted by the SOD1 enzyme into hydrogen peroxide (H 2 O 2 ), the predominant form of cytosolic ROS. ROS at high levels can be toxic, but below this threshold are important for physiological processes acting as a second messenger similar to Ca 2+ . Mitochondrial Ca 2+ influx from the ER increases ATP and ROS production, while ATP and ROS can regulate Ca 2+ homeostasis, leading to an intricate interplay between Ca 2+ , ROS, and ATP synthesis. The Unfolded Protein Response (UPR) proteins ATF6α and XBP1 contribute to protection from oxidative stress through upregulation of Sod1 and Catalase genes. Here, UPR-associated protein CREB3 is shown to play a role in balancing Ca 2+ , ROS, and ATP homeostasis. Creb3-deficient mouse embryonic fibroblast cells (MEF -/- ) were susceptible to H 2 O 2 -induced oxidative stress while having a functioning antioxidant gene expression response compared to MEF +/+ . MEF -/- cells also contained elevated basal cytosolic ROS levels, which was attributed to drastically increased basal mitochondrial respiration and spare respiratory capacity relative to MEF +/+ . MEF -/- cells also showed an increase in endoplasmic reticulum Ca 2+ release and mitochondrial Ca 2+ levels hinting at a potential cause for MEF -/- cell mitochondrial dysfunction. These results suggest that CREB3 is essential for maintaining proper Ca 2+ , ATP, and ROS homeostasis in mammalian cells.

Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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