Bid Regulates Murine Hepatocyte Proliferation by Controlling ER Calcium Homeostasis

Bid, a BH3-only Bcl-2 family molecule, is generally known for its importance in activating the mitochondrial apoptosis pathway following death receptor engagement, particularly in hepatocytes. However, Bid also promotes hepatocyte proliferation during liver regeneration and carcinogenesis. This study aimed to examine the hypothesis that Bid regulated endoplasmic reticulum calcium ([Ca 2+ ] ER ) homeostasis to affect hepatocyte proliferation. We found that serum-stimulated hepatocyte proliferation was dependent on calcium and depletion of calcium using thapsigargin or EGTA inhibited the proliferation. Subcellular fractionation found that a portion of Bid was inserted in the ER-enriched membranes and single-cell calcium imaging indicated that Bid was important for maintaining [Ca 2+ ] ER level. Bid-deficient hepatocytes manifested a delayed and reduced serum-stimulated proliferation, which was corrected by ionomycin or reconstitution of Bid, particularly an ER-targeted Bid. Finally, Bax could be also found in the ER-enriched membranes and Bax deficiency caused the same proliferation defect. However, Bid/Bax double deletion in hepatocytes did not further augment the defect, suggesting that Bid and Bax worked by the same regulatory mechanism in [Ca 2+ ] ER control.