Inhibition of in vivo rat liver regeneration by 2-acetylaminofluorene affects the regulation of cell cycle-related proteins

Abstract

The effects of dietary 2-acetylaminofluorene (2-AAF) on cell cycle-related proteins was studied in regenerating livers from male Wistar rats. The levels of cyclins, cyclin dependent kinases (cdks), and related proteins were studied at different times during the first cell cycle after partial hepatectomy (PH). The frequency of proliferation cell nuclear antigen (PCNA)-positive nuclei, a marker of S phase progression, was almost zero during the first 27 hours after PH in the mitoinhibited 2-AAF-treated rats, while about 50% of the nuclei were labeled 24 hours after PH in control animals. Accordingly, Western blot tests showed markedly elevated PCNA protein levels from 18 hours to the end of S phase in untreated animals but no upregulation in response to 2-AAF. Compared with control animals, animals treated with 2-AAF showed increased levels of cdk 4 and cyclin D 3 from 12 and 15 hours after PH, respectively, and altered cyclicity in cyclin D 3 expression. No effects on cyclin E were observed, while the increase in cdk 2 levels in control animals during late G 1 /S (15-27 hours) was abolished by 2-AAF. p53 was induced by 2-AAF treatment during the same period, with a peak at 24 hours. The protein detected with p21 antibodies was highly expressed in unstimulated hepatocytes in control animals, and further increased by 2-AAF. The expression was sustained until 15 hours after PH in control rats while 2-AAF-treated animals lacked detectable protein during this period; however, a transient increase was observed at 21 hours. Thus, 2-AAF affects several parameters of cell cycle regulation of possible relevance for its inhibitory effects on hepatocyte proliferation in vivo. (HEPATOLOGY 1998;27:691-696.)Many liver tumor promoters such as 2-acetylaminofluorene (2-AAF), phenobarbital, orotic acid, clofibrate and ethinylestradiol have inhibitory effects on proliferation of hepatocytes in vivo during long-term treatment, 1-5 which is believed to be an important aspect of the promotion mechanism. In the classical resistant hepatocyte model (RH-model) for rat liver carcinogenesis, where promotion is performed by dietary 2-AAF treatment combined with a partial hepatectomy (PH), the selective growth advantage of preneoplastic foci during treatment has been suggested to be caused by the inhibited proliferation of hepatocytes surrounding the lesions, combined with focal resistance to 2-AAF-induced inhibition. 1 This makes the putatively preneoplastic foci, in contrast to the surrounding tissue, able to respond to the proliferative stimulus provided by the PH. The mitoinhibitory effects of 2-AAF are caused by the formation of sulfated metabolites of N-hydroxyacetylaminofluorene, 6 which is low in the lesions compared with the surrounding hepatocytes. 7 These sulfated metabolites give rise to two different Nacetylated adducts causing major distortion of the DNA structure, and lead to an efficient DNA replication block both in vitro and in vivo. 6,[8][9] The molecular mediators of the inhibitory signals generated by 2-AAF are not known....

Liver regeneration

“…On long-term administration, AAF is a carcinogenic substance. Given for a short period of time, it blocks hepatocyte proliferation, probably by forming AAF-DNA adducts which trigger proliferative arrest via p53 and p21 (Ohlson et al, 1998). When PHx is performed in animals given AAF, hepatocytes cannot proliferate.…”

Effects of ethanol on hepatic cellular replication and cell cycle progression

“…The replacement of damaged or injured hepatocytes lost from the liver parenchyma normally occurs by the replication of mature hepatocytes [39] . The normal replication of mature hepatocytes can be inhibited by cell cycle delay or arrest [11,12,40,41] . Under conditions where normal mature hepatocyte replication is inhibited, the replication of the bipotential progenitor cells known as oval cells is increased to compensate for the inhibition of normal hepatocyte replication [40,42,43] .…”

“…The normal replication of mature hepatocytes can be inhibited by cell cycle delay or arrest [11,12,40,41] . Under conditions where normal mature hepatocyte replication is inhibited, the replication of the bipotential progenitor cells known as oval cells is increased to compensate for the inhibition of normal hepatocyte replication [40,42,43] . Because oval cells are bipotential and can differentiate into either hepatocytes or bile ductular epithelium, it has been proposed that impairment of the primary pathway of hepatocyte replacement not only leads to regeneration of hepatocytes but also to increased proliferation of ductals [44] .…”

Oxidative Stress and Oval Cell Accumulation in Mice and Humans with Alcoholic and Nonalcoholic Fatty Liver Disease

“…11,12 Oxidative stress is thought to play a major role in the pathogenesis of both alcoholic and nonalcoholic fatty liver disease (NAFLD). 13,14 The replicative activity of mature hepatocytes is also known to be inhibited in patients with alcoholic hepatitis, 15 rodents with alcohol-induced fatty livers, 16 and in some animal models of NAFLD.…”