title: Bid agonist regulates murine hepatocyte proliferation by controlling endoplasmic reticulum calcium homeostasis.
abstract: BH3-interacting domain death agonist (Bid), a BH3-only B cell lymphoma 2 family molecule, is generally known for its importance in activating the mitochondrial apoptosis pathway after death receptor engagement, particularly in hepatocytes. However, Bid also promotes hepatocyte proliferation during liver regeneration and carcinogenesis. This study was designed to examine the hypothesis that Bid regulates endoplasmic reticulum calcium concentration ([Ca(2+)](ER)) homeostasis to affect hepatocyte proliferation. We found that serum-stimulated hepatocyte proliferation was dependent on calcium, and the depletion of calcium with thapsigargin or ethylene glycol tetraacetic acid (EGTA) inhibited the proliferation. Subcellular fractionation showed that a portion of Bid was inserted into the endoplasmic reticulum (ER)-enriched membranes, and single-cell calcium imaging indicated that Bid was important for maintaining the [Ca(2+)](ER) level. Bid-deficient hepatocytes manifested delayed and reduced serum-stimulated proliferation, which was corrected by ionomycin or reconstitution of Bid, particularly an ER-targeted Bid. Finally, B cell lymphoma 2-associated X protein (Bax) could also be 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, which suggested that Bid and Bax worked by the same regulatory mechanism in [Ca(2+)](ER) control.
  
  
  CONCLUSIONS
  Bid regulates hepatocyte proliferation by positively affecting [Ca(2+)](ER) homeostasis, and this could be important for liver regeneration and carcinogenesis.
authors.affiliation: Department of Pathology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261, USA.
authors.forename: Hong-Min
authors.initials: HM
authors.lastname: Ni
authors.forename: Catherine J
authors.initials: CJ
authors.lastname: Baty
authors.forename: Na
authors.initials: N
authors.lastname: Li
authors.forename: Wen-Xing
authors.initials: WX
authors.lastname: Ding
authors.forename: Wentao
authors.initials: W
authors.lastname: Gao
authors.forename: Min
authors.initials: M
authors.lastname: Li
authors.forename: Xiaoyun
authors.initials: X
authors.lastname: Chen
authors.forename: Jianjie
authors.initials: J
authors.lastname: Ma
authors.forename: George K
authors.initials: GK
authors.lastname: Michalopoulos
authors.forename: Xiao-Ming
authors.initials: XM
authors.lastname: Yin
mesh_terms: D000818:Animals; D051023:BH3 Interacting Domain Death Agonist Protein / Q000235:genetics / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D049109:Cell Proliferation; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D022781:Hepatocytes / Q000166:cytology / Q000502:physiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D008817:Mice, Mutant Strains; D018160:Receptors, Cytoplasmic and Nuclear
chemical_list: D051023:BH3 Interacting Domain Death Agonist Protein; C494815:Bid protein, mouse; C431936:Nr5a2 protein, mouse; D018160:Receptors, Cytoplasmic and Nuclear; D002118:Calcium