Expression of Bcl-2 family during liver regeneration and identification of Bcl-x as a delayed early response gene

• Published in: The American journal of pathology Vol. 150; no. 6; pp. 1985 - 1995
• Main Authors: Tzung, SP, Fausto, N, Hockenbery, DM
• Format: Journal Article
• Language: English
• Published: Bethesda, MD ASIP 01-06-1997; American Society for Investigative Pathology
• Subjects: Animals; bcl-X Protein; Biological and medical sciences; Blotting, Western; Cell cycle, cell proliferation; Cell Division; Cell physiology; Cycloheximide - pharmacology; Fundamental and applied biological sciences. Psychology; Gene Expression Regulation - drug effects; Immunohistochemistry; Liver - metabolism; Liver Regeneration - physiology; Male; Mice; Mice, Inbred C3H; Mice, Inbred Strains; Molecular and cellular biology; Proto-Oncogene Proteins - metabolism; Proto-Oncogene Proteins c-bcl-2 - metabolism; RNA - metabolism; Time Factors; Immunohistochemistry; Rodentia; Hepatectomy; Pathology; Regeneration; Vertebrata; Mammalia; Partial; Mouse; Surgery; Animal; C-Onc gene; Modulation; Cell cycle; Immunoblotting assay; Molecular biology; Protooncogene; Apoptosis
• ISSN: 0002-9440; 1525-2191

Induction of Bcl-2 and Bcl-x has been demonstrated in mitogen-stimulated lymphocytes in vitro, suggesting that these two apoptosis modulators may also play a role during proliferation. To explore this possibility in a physiological setting, mRNA expression of various Bcl-2 family members was examined during liver regeneration induced by partial hepatectomy, a well characterized in vivo model of cell cycle progression. After a 60% partial hepatectomy in C3H/HeN mice, the steady-state levels of Bcl-x mRNA exhibited a cyclical pattern, with peaks at 4 hours (early G1) and 48 to 72 hours (G1 phase of the second hepatocyte cell cycle). A1 and Bcl-2 mRNA were not detected, and the levels of two Mcl-1 mRNA species remained low without significant changes. The three pro-apoptotic members of the family, Bak, Bad, and Bax, all showed an early decline in mRNA levels when Bcl-x transcripts increased, followed by later peaks at 12, 24, and 48 to 72 hours, respectively. Experiments were subsequently conducted in C3H/HeJ mice, an endotoxin-resistant strain with slower liver regeneration marked by a protracted G1 phase. Even though immediate-early gene responses measured by c-myc induction remained intact, the timing of Bcl-x mRNA expression was delayed in C3H/HeJ mice. When C3H/HeN mice were pretreated with cycloheximide before hepatectomy, the early peak of Bcl-x mRNA at 4 hours was essentially abrogated whereas the immediate-early gene c-myc was hyperinduced, thus implicating Bcl-x as a delayed early response gene during liver regeneration. Bcl-x was localized in hepatocytes and by both immunohistochemistry and Western blot analysis, Bcl-xL protein reached highest levels at 12 hours (mid-G1), consistent with the expression of a delayed early gene. In summary, the expression profiles of Bcl-2 family members during liver regeneration suggest a cell-cycle-dependent regulation as well as a physiological role for these apoptosis-modulating genes during growth and proliferation.