Changes in cell cycle-associated gene expression in a model of impaired liver regeneration

Targeting Cyclin-Dependent Kinase 1 Induces Apoptosis and Cell Cycle Arrest of Activated Hepatic Stellate Cells

Liver fibrosis is the integral process of chronic liver diseases caused by multiple etiologies and characterized by excessive deposition of extracellular matrix (ECM). During liver fibrosis, hepatic stellate cells (HSCs) transform into a highly proliferative, activated state, producing various cytokines, chemokines, and ECM. However, the precise mechanisms that license HSCs into the highly proliferative state remain unclear. Cyclin-dependent kinase 1 (CDK1) is a requisite event for the transition of the G1/S and G2/M phases in eukaryotic cells. In this study, it is demonstrated that CDK1 and its activating partners, Cyclin A2 and Cyclin B1, are upregulated in both liver fibrosis/cirrhosis patient specimens and the murine hepatic fibrosis models, especially in activated HSCs. In vitro, CDK1 is upregulated in spontaneously activated HSCs, and inhibiting CDK1 with specific small-molecule inhibitors (CGP74514A, RO-3306, or Purvalanol A) orshort hairpin RNAs (shRNAs) resulted in HSC apoptosis and cell cycle arrest by regulating Survivin expression. Above all, it is illustrated that increased CDK1 expression licenses the HSCs into a highly proliferative state and can serve as a potential therapeutic target in liver fibrosis.

Cytokine-responsive gene-2/IFN-inducible protein-10 expression in multiple models of liver and bile duct injury suggests a role in tissue regeneration

IFN-inducible protein-10 (IP-10/CXCL10) is a CXC chemokine that targets both T cells and NK cells. Elevation of IP-10 expression has been demonstrated in a number of human diseases, including chronic cirrhosis and biliary atresia. Cytokine-responsive gene-2 (Crg-2), the murine ortholog of IP-10, was induced following CCl(4) treatment of the hepatocyte-like cell line AML-12. Crg-2 expression was noted in vivo in multiple models of hepatic and bile duct injury, including bile duct ligation and CCl(4), D-galactosamine, and methylene dianiline toxic liver injuries. Induction of Crg-2 was also examined following two-thirds hepatectomy, a model that minimally injures the remaining liver, but that requires a large hepatic regenerative response. Crg-2 was induced in a biphasic fashion after two-thirds hepatectomy, preceding each known peak of hepatocyte DNA synthesis. Induction of Crg-2 was also observed in the kidney, gut, thymus, and spleen within 1 h of two-thirds hepatectomy. Characteristic of an immediate early gene, pretreatment of mice with the protein synthesis inhibitor cycloheximide before either two-thirds hepatectomy or CCl(4) injection led to Crg-2 superinduction. rIP-10 was demonstrated to have hepatocyte growth factor-inducing activity in vitro, but alone had no direct mitogenic effect on hepatocytes. Our data demonstrate that induction of Crg-2 occurs in several distinct models of liver injury and regeneration, and suggest a role for CRG-2/IP-10 in these processes.

Fas engagement accelerates liver regeneration after partial hepatectomy

Fas (CD95) is a receptor involved in induction of apoptotic cell death of Fas-bearing cells, including hepatocytes and T cells. Injection of Fas-specific antibodies into mice leads to fulminant hepatic failure and death. Fas also transduces growth-promoting signals in proliferating T cells, fibroblasts and some tumor cells. Here we show that partial hepatectomy, which triggers the immediate onset of liver regeneration, protected mice against the lethal effects of Fas-specific antibodies and prevented hepatocyte apoptosis in response to Fas engagement in vivo. Furthermore, Fas engagement accelerated liver regeneration after partial hepatectomy. Liver regeneration kinetics were delayed in mutant mice with decreased cell surface Fas expression (lpr mice). In contrast, regeneration was not delayed in lpr-cg mutant mice, which have a Fas mutation that prevents Fas-induced death but not Fas-dependent proliferative stimulation. Our results indicate that Fas engagement on cells in regenerating or healing tissues may promote cell growth.

Studies on hepatic gene expression in different liver regenerative models

We have investigated the expression of several growth-related genes in the liver after partial hepatectomy in three experimental models: normal, Dexamethasone-pretreated, and hypophysectomized rats. Dexamethasone and hypophysectomy resulted in a delay in the peak of cell replication in 6 and 18 h, respectively, when compared to the normal animals. TGFalpha mRNA expression was shifted together with the DNA synthesis, but the expression of c-myc, c-fos, c-jun, HGF, TGFbeta1, IL1beta did not delay. This result suggests that liver-derived TGFalpha but not the other factors are important in the timing of the proliferative response after partial hepatectomy.

Expression of growth regulatory genes in a SCID mouse-human model of intestinal epithelial regeneration

Analysis of human intestinal epithelial regeneration has been limited. This study has used a novel SCID mouse-human model to test the hypothesis that distinct stages of human intestinal epithelial regeneration are accompanied by differential expression of growth regulatory genes. Disaggregated epithelium, which included crypt cell aggregates, was isolated from human fetal small intestine and transplanted subcutaneously in SCID mice. This method induced a coordinated regeneration response and enabled temporal separation of cell populations at different stages of histogenesis and cytodifferentiation. Graft epithelium was identified using a specific anti-human monoclonal antibody (MAb 5D3) against cytokeratins 8 and 18. Functional epithelial lineages were identified by appropriate markers. Growth regulatory genes relevant to proliferation and apoptosis, including Bcl-2, p53 and Ki67, were assayed at different stages of regeneration. During early regeneration, Bcl-2, p53, and Ki67 were expressed throughout the epithelial compartment. On completion of regeneration, these genes were expressed only in crypt epithelium and were absent from villi. This study has established a novel SCID mouse-human model of intestinal epithelial regeneration. During early regeneration, increased Bcl-2 and Ki67 expression may indicate suppression of apoptosis and enhanced proliferation respectively, consistent with expansion of the stem cell fraction. The p53 gene may influence pathways of differentiation during regeneration, analogous to its role during development.

Differential regulation of the retinoblastoma family of proteins during cell proliferation and differentiation

In the present study we have analysed the regulation of pocket protein expression and post-transcriptional modifications on cell proliferation and differentiation, both in vivo and in vitro. There are marked changes in pocket protein levels during these transitions, the most striking differences being observed between p130 and p107. The mechanisms responsible for regulating pocket protein levels seem to be dependent on both cell type and pocket protein, in addition to their dependence on the cell growth status. Changes in retinoblastoma protein and p107 levels are independent of their state of phosphorylation. However, whereas p130 phosphorylation to forms characteristic of quiescent/differentiated cells results in the accumulation of p130 protein, phosphorylation of p130 to one or more forms characteristic of cycling cells is accompanied by down-regulation of its protein levels. We also show here that the phosphorylation status and protein levels of p130 and p107 are regulated in vivo as in cultured cells. In vivo, changes in p130 forms are correlated with changes in E2F complexes. Moreover, the modulation of p130 and p107 status during cell differentiation in vitro is consistent with the patterns of protein expression and phosphorylation status found in mouse tissues. Thus in addition to the direct disruption of pocket protein/E2F complexes induced by cyclin/cyclin-dependent kinase, the results we report here indicate that the differential modulation of pocket protein levels constitutes a major mechanism that regulates the pool of each pocket protein that is accessible to E2F and/or other transcription factors.

Abnormalities of liver regeneration: a review

Structural and functional changes during liver regeneration have been studied extensively in experimental animals following partial hepatectomy or hepatic injury induced by noxious substances. These observations have been extended to evaluate abnormalities of liver regeneration which contribute to chronic hepatitis, cirrhosis and/or liver cancer in man. This is facilitated by the simultaneous perfusion of flash frozen percutaneous biopsies or explanted liver in an acrylic chamber with tritiated thymidine and proline to evaluate DNA and collagen synthesis, respectively. Such investigations indicate that chronic liver damage is associated with replication of mesenchymal, ductular and parenchymal cells, accompanied by increased fibrogenesis. The regenerative response of the liver after noxious injury in experimental animals and man is associated with the release of cytokines, increase of growth response genes and change in telomerase activity. The ability to monitor morphological, genetic and biochemical parameters provides new information on the kinetics of the reparative process in hepatobiliary disease. Abnormal liver regeneration and its untoward effects including tumorigenesis may be modified by altering nutrients, blocking antigens or receptors, and inhibiting metabolites which regulate cell replication and collagen deposition.

Hepatic protein synthesis in the regenerating rat liver after hepatopancreatectomy

To evaluate the effects of hepatopancreatectomy on the regenerative process of the liver, the serum protein changes, hepatic protein synthesis (HPS), and bromodeoxyuridine (BrdU) labeling index were measured in rats. Sprague-Dawley rats were divided into four groups according to the type of resection: A simple laparotomy was performed in the sham group; 68% of the liver was excised in the Hx group; 45% of the pancreas was excised in the Px group; and 45% of the pancreas and 68% of the liver were excised simultaneously in the HPx group. Serum total protein and albumin levels were significantly lower in the HPx group compared to the other three groups on postoperative day (POD) 3 (P < 0.05). HPS was markedly increased in the Hx and HPx groups. In the Hx group, it was significantly higher, peaking on POD 2, compared to the HPx group (P < 0.05), while in the HPx group, it was significantly higher compared to the Hx group (P < 0.05), peaking on POD 3. The BrdU labeling index, as a marker of DNA synthesis, was significantly suppressed in the HPx group on POD 1 compared to the Hx group (P < 0.05). Thus, compared to hepatectomy alone, hepatopancreatectomy suppresses DNA synthesis, causing a delay in the increase of protein synthesis in the regenerating liver, resulting in a more marked decrease in the serum protein level.