Mcl-1 and Bcl-xL cooperatively maintain integrity of hepatocytes in developing and adult murine liver

Anti-apoptotic members of the Bcl-2 family, including Bcl-2, Bcl-xL, Mcl-1, Bcl-w and Bfl-1, inhibit the mitochondrial pathway of apoptosis. Bcl-xL and Mcl-1 are constitutively expressed in the liver. Although previous research established Bcl-xL as a critical apoptosis antagonist in differentiated hepatocytes, the significance of Mcl-1 in the liver, especially in conjunction with Bcl-xL, has not been clear. To examine this question, we generated hepatocyte-specific Mcl-1-deficient mice by crossing mcl-1(flox/flox) mice and AlbCre mice and further crossed them with bcl-x(flox/flox) mice, giving Mcl-1/Bcl-xL-deficient mice. The mcl-1(flox/flox) AlbCre mice showed spontaneous apoptosis of hepatocytes after birth, as evidenced by elevated levels of serum alanine aminotransferase (ALT) and caspase-3/7 activity and an increased number of terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate nick-end labeling (TUNEL)-positive cells in the liver; these phenotypes were very close to those previously found in hepatocyte-specific Bcl-xL-deficient mice. Although mcl-1(flox/+) AlbCre mice did not display apoptosis, their susceptibility to Fas-mediated liver injury significantly increased. Further crossing of Mcl-1 mice with Bcl-xL mice showed that bcl-x(flox/+) mcl-1(flox/+) AlbCre mice also showed spontaneous hepatocyte apoptosis similar to Bcl-xL-deficient or Mcl-1-deficient mice. In contrast, bcl-x(flox/flox) mcl-1(flox/+) AlbCre, bcl-x(flox/+) mcl-1(flox/flox) AlbCre, and bcl-x(flox/flox) mcl-1(flox/flox) AlbCre mice displayed a decreased number of hepatocytes and a reduced volume of the liver on day 18.5 of embryogenesis and rapidly died within 1 day after birth, developing hepatic failure evidenced by increased levels of blood ammonia and bilirubin.

Mcl-1 is critical for blocking apoptosis in adult liver and, in the absence of Bcl-xL, is essential for normal liver development. Mcl-1 and Bcl-xL are two major anti-apoptotic Bcl-2 family proteins expressed in the liver and cooperatively control hepatic integrity during liver development and in adult liver homeostasis in a gene dose-dependent manner.