The p21(Cip1) protein, a cyclin inhibitor, regulates the levels and the intracellular localization of CDC25A in mice regenerating livers

Cellular Senescence in Liver Disease and Regeneration

Cellular senescence is an irreversible cell cycle arrest implemented by the cell as a result of stressful insults. Characterized by phenotypic alterations, including secretome changes and genomic instability, senescence is capable of exerting both detrimental and beneficial processes. Accumulating evidence has shown that cellular senescence plays a relevant role in the occurrence and development of liver disease, as a mechanism to contain damage and promote regeneration, but also characterizing the onset and correlating with the extent of damage. The evidence of senescent mechanisms acting on the cell populations of the liver will be described including the role of markers to detect cellular senescence. Overall, this review intends to summarize the role of senescence in liver homeostasis, injury, disease, and regeneration.

Proliferation‑inhibiting pathways in liver regeneration (Review)

Liver regeneration, an orchestrated process, is the primary compensatory mechanism following liver injury caused by various factors. The process of liver regeneration consists of three stages: Initiation, proliferation and termination. Proliferation‑promoting factors, which stimulate the recovery of mitosis in quiescent hepatocytes, are essential in the initiation and proliferation steps of liver regeneration. Proliferation‑promoting factors act as the 'motor' of liver regeneration, whereas proliferation inhibitors arrest cell proliferation when the remnant liver reaches a suitable size. Certain proliferation inhibitors are also expressed and activated in the first two steps of liver regeneration. Anti‑proliferation factors, acting as a 'brake', control the speed of proliferation and determine the terminal point of liver regeneration. Furthermore, anti‑proliferation factors function as a 'steering‑wheel', ensuring that the regeneration process proceeds in the right direction by preventing proliferation in the wrong direction, as occurs in oncogenesis. Therefore, proliferation inhibitors to ensure safe and stable liver regeneration are as important as proliferation‑promoting factors. Cytokines, including transforming growth factor‑β and interleukin‑1, and tumor suppressor genes, including p53 and p21, are important members of the proliferation inhibitor family in liver regeneration. Certain anti‑proliferation factors are involved in the process of gene expression and protein modification. The suppression of liver regeneration led by metabolism, hormone activity and pathological performance have been reviewed previously. However, less is known regarding the proliferation inhibitors of liver regeneration and further investigations are required. Detailed information regarding the majority of known anti‑proliferation signaling pathways also remains fragmented. The present review aimed to understand the signalling pathways that inhbit proliferation in the process of liver regeneration.

p27Kip1 represses the Pitx2-mediated expression of p21Cip1 and regulates DNA replication during cell cycle progression

The tumor suppressor p21 regulates cell cycle progression and peaks at mid/late G1. However, the mechanisms regulating its expression during cell cycle are poorly understood. We found that embryonic fibroblasts from p27 null mice at early passages progress slowly through the cell cycle. These cells present an elevated basal expression of p21 suggesting that p27 participates to its repression. Mechanistically, we found that p27 represses the expression of Pitx2 (an activator of p21 expression) by associating with the ASE-regulatory region of this gene together with an E2F4 repressive complex. Furthermore, we found that Pitx2 binds to the p21 promoter and induces its transcription. Finally, silencing Pitx2 or p21 in proliferating cells accelerates DNA replication and cell cycle progression. Collectively, these results demonstrate an unprecedented connection between p27, Pitx2 and p21 relevant for the regulation of cell cycle progression and cancer and for understanding human pathologies associated with p27 germline mutations.

MiR-17~92 ablation impairs liver regeneration in an estrogen-dependent manner

As one of the most important post-transcriptional regulators, microRNAs (miRNAs) participate in diverse biological processes, including the regulation of cell proliferation. MiR-17~92 has been found to act as an oncogene, and it is closely associated with cell proliferation. However, its role in liver regeneration is still unclear. We generated a hepatocyte-specific miR-17~92-deficient mouse and used a mouse model with 70% partial hepatectomy (PH) or intraperitoneal injection of carbon tetrachloride to demonstrate the role of MiR-17~92 in liver regeneration. In quiescent livers, the expression of the miR-17~92 cluster showed a gender disparity, with much higher expression in female mice. The expression of four members of this cluster was found to be markedly reduced after 70% PH. The ablation of miR-17~92 led to obvious regeneration impairment during the early-stage regeneration in the female mice. Ovariectomy greatly reduced miR-17~92 expression but significantly promoted liver regeneration in wild-type mice. In addition, early regeneration impairment in miR-17~92-deficient livers could be largely restored following ovariectomy. The proliferation suppressors p21 and Pten were found to be the target effectors of miR-17~92. MiR-17~92 disruption resulted in elevated protein levels of p21 and Pten in regenerating livers. MiR-17~92 functions as a proliferation stimulator and acts in an oestrogen-dependent manner. The loss of this miRNA results in increases in p21 and Pten expression and therefore impairs liver regeneration in female mice.

Ikaros-1 couples cell cycle arrest of late striatal precursors with neurogenesis of enkephalinergic neurons

During central nervous system development, several transcription factors regulate the differentiation of progenitor cells to postmitotic neurons. Here we describe a novel role for Ikaros-1 in the generation of late-born striatal neurons. Our results show that Ikaros-1 is expressed in the boundary of the striatal germinal zone (GZ)/mantle zone (MZ), where it induces cell cycle arrest of neural progenitors by up-regulation of the cyclin-dependent kinase inhibitor (CDKi) p21(Cip1/Waf1). This effect is coupled with the neuronal differentiation of late precursors, which in turn is critical for the second wave of striatal neurogenesis that gives rise to matrix neurons. Consistently, Ikaros(-/-) mice had fewer striatal projecting neurons and, in particular, enkephalin (ENK)-positive neurons. In addition, overexpression of Ikaros-1 in primary striatal cultures increases the number of calbindin- and ENK-positive neurons. Our results also show that Ikaros-1 acts downstream of the Dlx family of transcription factors, insofar as its expression is lost in Dlx1/2 double knockout mice. However, we demonstrate that Ikaros-1 and Ebf-1 independently regulate the final determination of the two populations of striatal projection neurons of the matrix compartment, ENK- and substance P-positive neurons. In conclusion, our findings identify Ikaros-1 as a modulator of cell cycle exit of neural progenitors that gives rise to the neurogenesis of ENK-positive striatal neurons.

Changes in transcriptome after in vivo exposure to ionising radiation reveal a highly specialised liver response

PURPOSE

To identify transcriptional gene-networks involved in the early in vivo response of liver cells to radiation exposure and improve our understanding of the molecular processes responsible for tissue radiosensitivity.

MATERIALS AND METHODS

Transcriptome variations of liver RNA samples were measured 3 hours post-irradiation using microarray technology. The results were confirmed and extended using real-time polymerase-chain-reaction (RT-PCR).

RESULTS

We identified quantitative changes in the expression of 126 genes, most of which were observed for the first time. We show that some modifications, such as the upregulation of the cyclin-dependent kinase inhibitor 1A (Cdkn1A) gene, persisted for at least two months after the initial exposure. Other genes regulated by the transformation-related protein 53 (Trp53/p53) such as Bcl2-associated X protein (Bax) or etoposide-induced-2.4 (Ei24/PIG8) were not upregulated. Grouping differentially expressed genes into functional categories revealed that the primary response of liver cells to radiation exposure was the enhancement of oxidoreductase activity and inhibition of cell proliferation, involving cell cycle progression and apoptosis-related genes.

CONCLUSIONS

The data provides evidence of gene expression modifications associated with the hepatic response to radiation exposure. One of the main differences observed with radiation-sensitive tissues such as the spleen was cell proliferation. The comparison of our data with transcriptome modifications in different biological models enabled the identification of networks of genes that might be co-regulated. Overall, our expression data revealed genes and cellular pathways that might help to improve our understanding of the molecular basis underlying tissue radiosensitivity and to identify possible targets for novel therapeutic strategies.

Multiple division cycles and long-term survival of hepatocytes are distinctly regulated by extracellular signal-regulated kinases ERK1 and ERK2

We investigated the specific role of the mitogen-activated protein kinase (MAPK) extracellular signal-regulated kinase 1 (ERK1)/ERK2 pathway in the regulation of multiple cell cycles and long-term survival of normal hepatocytes. An early and sustained epidermal growth factor (EGF)-dependent MAPK activation greatly improved the potential of cell proliferation. In this condition, almost 100% of the hepatocytes proliferated, and targeting ERK1 or ERK2 via RNA interference revealed the specific involvement of ERK2 in this regulation. However, once their first cell cycle was performed, hepatocytes failed to undergo a second round of replication and stayed blocked in G1 phase. We demonstrated that sustained EGF-dependent activation of the MAPK/ERK kinase (MEK)/ERK pathway was involved in this blockage as specific transient inhibition of the cascade repotentiated hepatocytes to perform a new wave of replication and multiple cell cycles. We identified this mechanism by showing that this blockage was in part supported by ERK2-dependent p21 expression. Moreover, continuous MEK inhibition was associated with a lower apoptotic engagement, leading to an improvement of survival up to 3 weeks. Using RNA interference and ERK1 knockout mice, we extended these results by showing that this improved survival was due to the specific inhibition of ERK1 expression/phosphorylation and did not involve ERK2.

CONCLUSION

Our results emphasize that transient MAPK inhibition allows multiple cell cycles in primary cultures of hepatocytes and that ERK2 has a key role in the regulation of S phase entry. Moreover, we revealed a major and distinct role of ERK1 in the regulation of hepatocyte survival. Taken together, our results represent an important advance in understanding long-term survival and cell cycle regulation of hepatocytes.

A mesoscale model of G1/S phase transition in liver regeneration

The liver regenerates and maintains its function and size after injury by counterbalancing cell death with compensatory cell division. During liver regeneration, injured sites release cytokines, which stimulate normally quiescent hepatocytes to re-enter cell division cycle. Using a mesoscale approach, we have implemented the first mathematical model that describes cytokine-induced dedifferentiation of hepatocytes and the subsequent initiation of DNA synthesis (G0/G1 and G1/S phase transitions of the cell cycle). The model accurately reproduces experimentally measured kinetics of various signaling intermediates and DNA synthesis in hepatocytes for varying degrees of liver damage, in both wild type and knockout backgrounds. Liver regeneration is known to be a robust process, as liver mass reconstitution still occurs in various knockout mice (albeit with different kinetics). We analyze the robustness of the model using methods of control analysis. Moreover, we discuss the system's bandpass filtering properties and delays, which arise from feedbacks and nested feed-forward loops.

Akt-mediated liver growth promotes induction of cyclin E through a novel translational mechanism and a p21-mediated cell cycle arrest

The control of hepatocyte growth is relevant to the processes of liver regeneration, development, metabolic homeostasis, and cancer. A key component of growth control is the protein kinase Akt, which acts downstream of mitogens and nutrients to affect protein translation and cell cycle progression. In this study, we found that transient transfection of activated Akt triggered a 3-4-fold increase in liver size within days but only minimal hepatocyte proliferation. Akt-induced liver growth was associated with marked up-regulation of cyclin E but not cyclin D1. Analysis of liver polyribosomes demonstrated that the post-transcriptional induction of cyclin E was associated with increased translational efficiency of this mRNA, suggesting that cell growth promotes expression of this protein through a translational mechanism that is distinct from the cyclin D-E2F pathway. Treatment of Akt-transfected mice with rapamycin only partially inhibited liver growth and did not prevent the induction of cyclin E protein, indicating that target of rapamycin activity is not necessary for this response. In the enlarged livers, cyclin E-Cdk2 complexes were present in high abundance but were inactive due to increased binding of p21 to these complexes. Akt transfection of p21(-/-) mice promoted liver growth, activation of Cdk2, and enhanced hepatocyte proliferation. In conclusion, growth promotes cyclin E expression through a novel translational mechanism in the liver, suggesting a new link between cell growth and the cell cycle machinery. Furthermore, p21 suppresses proliferation in the overgrown livers and may play a role in preventing cell cycle progression in response to organ size homeostatic mechanisms.

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c-Myc regulates cell size and ploidy but is not essential for postnatal proliferation in liver

The c-Myc protein is a transcription factor implicated in the regulation of multiple biological processes, including cell proliferation, cell growth, and apoptosis. In vivo overexpression of c-myc is linked to tumor development in a number of mouse models. Here, we show that perinatal inactivation of c-Myc in liver causes disorganized organ architecture, decreased hepatocyte size, and cell ploidy. Furthermore, c-Myc appears to have distinct roles in proliferation in liver. Thus, postnatal hepatocyte proliferation does not require c-Myc, whereas it is necessary for liver regeneration in adult mice. These results show novel physiological functions of c-myc in liver development and hepatocyte proliferation and growth.

p21(WAF1/Cip1) is not involved in kainic acid-induced apoptosis in murine cerebellar granule cells

Kainic acid (KA) treatment induced neuronal death and apoptosis in murine cerebellar granule cells (CGNs) cultures from both wild-type and knockout p21(-/-) mice. There was not statistically significant difference in the percentage of neuronal apoptosis among strains. KA-induced neurotoxicity was prevented in the presence of NBQX (20 microM) and GYKI 52446 (20 microM), but not by z-VAD-fmk, suggesting that caspases are not involved in the apoptotic process. Data suggest that p21(WAF/Cip) was unable to modulate KA-induced apoptosis in murine CGNs.

Absence of the cell cycle inhibitor p21Cip1 reduces LPS-induced NO release and activation of the transcription factor NF-?B in mixed glial cultures

We have studied possible differences in glial activation between cells from wild-type and p21Cip1-/- mice. We compared the effect of serum mitogenic stimulation on proliferation rate and on the total number of glial cells after 7 days of culture. No differences between wild-type and p21Cip1-/- glial cells were observed. We also compared the effect of lipopolysaccharide (LPS) from Escherichia coli, an agent widely used to induce glial activation. Nitric oxide (NO) and tumor necrosis factor-alpha (TNF-alpha) release, and nuclear factor kappa-B (NF-kappaB) activation were evaluated as indicators of glial activation. We observed an attenuation of NO release and NF-kappaB activation in p21Cip1-/- glial cells when compared with glial cells from wild-type mice. In contrast, TNF-alpha release was enhanced in p21Cip1-/- glia. These results suggest that the cell cycle inhibitor p21Cip1 plays a role in the inflammatory response induced by LPS.

The Cdk inhibitor p21 is required for necrosis, but it inhibits apoptosis following toxin-induced liver injury

Liver injury and repair were examined in wild type, p21Waf1/Cip1, and p27Kip1-deficient mice following carbon tetrachloride (CCl4) administration. In wild type liver, p21 expression is induced in a biphasic manner following injection of CCl4, with an early peak of p21 expression occurring in pericentral hepatocytes at 6 h, prior to evidence of injury, and a second peak succeeding regenerative proliferation. In contrast, p27 is present throughout the quiescent liver, but its expression decreases following CCl4 injection. Surprisingly, p21-deficient animals were resistant to CCl4-induced necrotic injury, indicating that rapid induction of p21 in pericentral hepatocytes following CCl4 injection contributes to subsequent necrosis. Expression of cytochrome P450 2E1, which plays an essential role in CCl4-induced necrotic injury, was not affected in p21-deficient mice. Although they had the least injury, p21-deficient mice had the highest levels of hepatic proliferation that correlated with increases in hyperphosphorylated retinoblastoma protein and Cyclin A gene expression. Increased replication in p21-deficient livers was counteracted by an increase in hepatocyte apoptosis as detected by caspase-3 activation. p21 plays distinct and opposing roles regulating hepatocyte survival during injury and subsequent repair, with early induction of p21 contributing to necrotic injury and later expression to cessation of proliferation and hepatocyte survival.

The Forkhead Box m1b transcription factor is essential for hepatocyte DNA replication and mitosis during mouse liver regeneration

The Forkhead Box (Fox) proteins are an extensive family of transcription factors that shares homology in the winged helix DNA-binding domain and whose members play essential roles in cellular proliferation, differentiation, transformation, longevity, and metabolic homeostasis. Liver regeneration studies with transgenic mice demonstrated that FoxM1B regulates the onset of hepatocyte DNA replication and mitosis by stimulating expression of cell cycle genes. Here, we demonstrate that albumin-promoter-driven Cre recombinase-mediated hepatocyte-specific deletion of the Foxm1b Floxed (fl) targeted allele resulted in significant reduction in hepatocyte DNA replication and inhibition of mitosis after partial hepatectomy. Reduced DNA replication in regenerating Foxm1b(-/-) hepatocytes was associated with sustained increase in nuclear staining of the cyclin-dependent kinase (Cdk) inhibitor p21(Cip1) (p21) protein between 24 and 40 h after partial hepatectomy. Furthermore, increased nuclear p21 levels and reduced expression of Cdc25A phosphatase coincided with decreases in Cdk2 activation and hepatocyte progression into S-phase. Moreover, the significant reduction in hepatocyte mitosis was associated with diminished mRNA levels and nuclear expression of Cdc25B phosphatase and delayed accumulation of cyclin B1 protein, which is required for Cdk1 activation and entry into mitosis. Cotransfection studies demonstrate that FoxM1B protein directly activated transcription of the Cdc25B promoter region. Our present study shows that the mammalian Foxm1b transcription factor regulates expression of cell cycle proteins essential for hepatocyte entry into DNA replication and mitosis.