Bax-mediated apoptosis in the livers of rats after partial hepatectomy in the retrorsine model of hepatocellular injury

Retrorsine is a member of the pyrrolizidine alkaloid family of compounds wh ose toxic effects on the liver include a long-lasting inhibition of the pro liferative capacity of hepatocytes. Despite the retrorsine-induced blockade of hepatocyte proliferation, retrorsine-exposed rats are able to reconstit ute completely their liver mass after surgical partial hepatectomy (PH) via the sustained proliferation of a population of small, incompletely differe ntiated hepatocyte-like progenitor cells (SHPCs). The extensive proliferati on of SHPCs in retrorsine-injured livers is accompanied by the progressive loss of irreversibly injured megalocytes. To study the mechanism by which r etrorsine-damaged hepatocytes are removed after PH, we performed TUNEL anal ysis to establish apoptotic indices for hepatocytes in the livers of retror sine-exposed and control rats up to 14 days post-PH, Apoptotic indices are highest (approximately 6.0%) in the livers of retrorsine-exposed rats at 1 day post-PH, gradually declining thereafter, yet remaining significantly el evated (approximately 1%) over control rats (<0.1%) at 14 days post-PH (P < .05), After PH, levels of the proapoptotic protein Bax are increased in li vers from retrorsine-exposed rats relative to the levels observed in contro l livers. Similarly, levels of the antiapoptotic protein Bcl-x(L) are signi ficantly decreased (P < .05) compared with controls at t = 0 resulting in a n increased (approximately 3.4-fold) Bax/Bcl-x protein ratio that is signif icantly elevated (P < .05) compared with controls. Finally, increased level s of Bax protein are localized to the mitochondria of retrorsine-exposed ra t livers after PH during the same time that cytochrome c is released. These observations combine to suggest that retrorsine-injured hepatocytes are re moved after PH via apoptotic pathways dependent on relative levels and loca lization of Bax and Bcl-xL protein.