First, the role of p21 was analyzed in p21+/+ and p21−/− mice. Multiple Ki67-positive cells were clearly visible in p21+/+ and p21−/− mice 38 hours after PH, and there was no significant difference

between both groups (Fig. 4B). Liver mass recovery monitored by body/liver weight ratio was slightly accelerated in p21−/− mice 1 week after PH (Fig. 4C). At this time point, almost no Ki67-positive cells were detectable in either group. Overall, there were only minor differences between knockout and wild-type hepatocytes, suggesting that p21 does not play a major role for the initiation and termination of hepatocyte proliferation in healthy mice. Next, partial hepatectomies Mitomycin C were performed with Fah−/− and Fah/p21−/− mice with preexisting liver injury. We have shown that Fah−/− mice on 0% NTBC do not survive PH due to the complete p21-mediated block of hepatocyte proliferation.[2] Here, Fah-deficient mice on 2.5% NTBC for 3 months with moderate liver injury were used. Surprisingly, hepatocyte proliferation following PH was markedly inhibited in Fah−/− mice in which basal liver regeneration before PH was not impaired (Fig. 4E). Importantly, the profound cell cycle arrest was associated with a strong

induction of p21 (Fig. 4F). In contrast to Fah−/− mice, multiple Ki67-positive cells were clearly visible in Fah/p21−/− mice on 2.5% NTBC 38 hours after PH (Fig. 4E). Together, these data indicate that p21 has no lasting effect on liver regeneration 3-MA solubility dmso in healthy mice after PH. In contrast, PH in mice with preexisting liver injury leads to a strong induction of p21, which subsequently impairs liver regeneration. Several molecular pathways, in particular mitogen-activated protein kinase and mammalian target of rapamycin (mTOR), have been implicated in hepatocarcinogenesis in previous clinical and experimental studies.[3, 17,

18] Interestingly, most of these pathways are also important for liver regeneration, suggesting that they are likely candidates contributing to the cell cycle gene expression profile in tumor-prone Fah-deficient mice. To determine the role of these pathways in Fah-deficient mice, activation of JNK/c-jun, extracellular signal-regulated MCE公司 kinase (ERK), p38, and mTOR was analyzed 14 days after NTBC withdrawal and after 3 months on 2.5% NTBC. Activation of the JNK/c-jun, ERK, and p38 stress kinases did not correlate with the phenotype of Fah-deficient mice (Fig. 5A). A strong activation of the mTOR pathway, as monitored by immunoblot analysis of phosphorylated S6, was evident in Fah−/− and Fah/p21−/− mice on 0% NTBC. Similarly, a moderate phosphorylation/activation of S6 was seen in Fah−/− mice with moderate liver injury (2.5% NTBC). Interestingly, however, S6 phosphorylation was significantly reduced by 50% in Fah/p21−/− mice on 2.5% NTBC, in which hepatocyte proliferation was reduced (n = 6) (P < 0.05) (Fig. 5A,B).