Cell cycle-related protein expression in regenerating fibrotic and cirrhotic rat liver following partial hepapectomy

Abstract

Background and Aims: We previously reported that cyclin D1 accumulation facilitates assembly of enzymatically active cyclin Dl-cdk 4 complexes as liver cells progress through (31 phase in regenerative normal rat liver after partial hepatectomy (PH, BBRC 245: 70,1990). Clinically, liver cirrhosis is associated with prominent mortality, while cirrhotic liver is considered to regenerate less actively than normal liver. Although impaired regeneration creates several therapeutic problems, the mechanisms responsible for this step have not yet been elucidated. The aim of this study was to characterize the expression of cell cycle-related proteins in the regenerating fibrotic and cirrhotic rat liver. Methods: Liver fibrosis (F group) or cirrhosis (LC group) was induced by intraperitoneal injections of porcine serum (0.5 ml) or thioacetamide (200mg/kg B.W. in saline) into male F344 rats, twice a week for 12 weeks respectively. Control rats (C group) received intraperitoneal injections of 0.5 ml saline at the same time points. The rats were subjected to 70% PH (C and F groups) and 45% PH (LC groups), rsspectJvely, since more than 40% of the rats with cirrhosis died alter 70% PH. Development of hepatic fibrosis and cirrhosis was confirmed by Masson-trichrome staining. DNA synthesis was determined by the proliferating cell nuclear antigen (PCNA) labeling index. Protein expression of cyclin D1, cdk 4, and CDK inhibitors (p21 and p27) was measured throughout 48 hrs after PH by western blotting. In addition, the rate of formation of cyclin D1cdk 4 complexes was estimated. Results : PCNA labeling index increased significantly higher in C and F groups than that in LC groups after PH, although the index was higher in LC groups before hepatectomy. Interestingly, drastic induction of cyclin D1 and gradual increase of cdk 4 protein occurred after PH in C and F groups, whereas induction of cyclin D1 was not detected in LC group. In contrast, cdk 4 protein was detected at a high level in cirrhotic liver and did not change following PH. Although induction of p21 was detected in all groups, the level in LC group was lower than other groups for 48 hrs. p27 was constantly expressed at high levels in all groups. No significant change of this protein level was observed. Conclusion: The impaired regeneration of the cirrhotic liver is associated with a lowered level of cyclin D1 expression, whereas CDK inhibitors (p21 and p27) may not contribute to this process.