Activation of cdk4 and cdk2 during rat liver regeneration is associated with intranuclear rearrangements of cyclin-cdk complexes

Beyond cell cycle regulation: The pleiotropic function of CDK4 in cancer

CDK4, along with its regulatory subunit, cyclin D, drives the transition from G1 to S phase, during which DNA replication and metabolic activation occur. In this canonical pathway, CDK4 is essentially a transcriptional regulator that acts through phosphorylation of retinoblastoma protein (RB) and subsequent activation of the transcription factor E2F, ultimately triggering the expression of genes involved in DNA synthesis and cell cycle progression to S phase. In this review, we focus on the newly reported functions of CDK4, which go beyond direct regulation of the cell cycle. In particular, we describe the extranuclear roles of CDK4, including its roles in the regulation of metabolism, cell fate, cell dynamics and the tumor microenvironment. We describe direct phosphorylation targets of CDK4 and decipher how CDK4 influences these physiological processes in the context of cancer.

MicroRNA-877-5p alleviates ARDS via enhancing PI3K/Akt path by targeting CDKN1B both in vivo and in vitro

Acute respiratory distress syndrome (ARDS) is a public health problem with high morbidity and mortality worldwide due to lacking known characteristic biomarkers and timely intervention. Pulmonary edema caused by inflammation and pulmonary microvascular endothelial cell disfunction is the main pathophysiological change of ARDS. Circulating microRNAs (miRNAs) are differentially expressed between subjects who did and did not develop ARDS. Many miRNAs have been exemplified to be involved in ARDS and could represent the novel therapeutic targets, but the role of microRNA-877-5p (miR-877-5p) in ARDS and its regulatory mechanisms are still unknown. Herein, we explore the underlying function of miR-877-5p toward anesis of ARDS and addressed that miRNA-877 can reduce the release of tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, and IL-6 thus attenuating the damage of pulmonary microvascular endothelial cells (HPMECs). Have further evaluated the protein expression, we detected that miR-877-5p contributed to the relief of ARDS by suppressing Cyclin-dependent kinase inhibitor 1B (CDKN1B), which serves as a regulator of endothelial cell polarization and migration through phosphatidylinositol-3-kinase and AKT (PI3K/Akt) signaling pathway. Besides, we noticed that CDKN1B restrains cell differentiation by inhibiting Cdk2 (cyclin-dependent kinase 2), instead of Cdk4 (cyclin-dependent kinase 4), during which the nuclear translocation of CDKN1B may participate. Together, our works testified that miR-877-5p might suppress inflammatory responses and promote HPMECs regeneration via targeting CDKN1B by modulation of Cdk2 and PI3K/Akt path. These molecules likely modulating ARDS progression may inform biomarkers and therapeutic development.

The membrane-associated form of cyclin D1 enhances cellular invasion

The essential G₁-cyclin, CCND1, is a collaborative nuclear oncogene that is frequently overexpressed in cancer. D-type cyclins bind and activate CDK4 and CDK6 thereby contributing to G₁–S cell-cycle progression. In addition to the nucleus, herein cyclin D1 was also located in the cytoplasmic membrane. In contrast with the nuclear-localized form of cyclin D1 (cyclin D1^NL), the cytoplasmic membrane-localized form of cyclin D1 (cyclin D1^MEM) induced transwell migration and the velocity of cellular migration. The cyclin D1^MEM was sufficient to induce G₁–S cell-cycle progression, cellular proliferation, and colony formation. The cyclin D1^MEM was sufficient to induce phosphorylation of the serine threonine kinase Akt (Ser473) and augmented extranuclear localized 17β-estradiol dendrimer conjugate (EDC)-mediated phosphorylation of Akt (Ser473). These studies suggest distinct subcellular compartments of cell cycle proteins may convey distinct functions.

Yiqi Huoxue Recipe Improves Liver Regeneration in Rats after Partial Hepatectomy via JNK Pathway

The liver is the only visceral organ that exhibits a remarkable capability of regenerating in response to partial hepatectomy (PH) or chemical injury. Improving liver regeneration (LR) ability is the basis for the favourable treatment outcome of patients after PH, which can serve as a potential indicator for postoperative survival. The present study aimed to investigate the protective effects of Yiqi Huoxue recipe (YQHX) on LR after PH in rats and further elucidate its underlying mechanism. A two-thirds PH rat model was used in this study. Wistar rats were randomly divided into four groups: sham-operated, PH, YQHX + PH, and Fuzheng Huayu decoction (FZHY) + PH groups. All rats were sacrificed under anesthesia at 24 and 72 h after surgery. The rates of LR were calculated, and the expression levels of cyclin D1 and c-jun were determined by immunohistochemical staining. The protein levels of p-JNK1/2, JNK1/2, p-c-jun, c-jun, Bax, and Bcl-2 were detected by Western blotting, while the mRNA levels of JNK1, JNK2, c-jun, Bax, and Bcl-2 were examined by real-time polymerase chain reaction (RT-PCR). At the corresponding time points, YQHX and FZHY administration dramatically induced the protein levels of p-JNK1/2 compared to the PH group (p < 0.05), while FZHY + PH group showed prominently increase in p-JNK1/2 protein levels compared to the YQHX + PH group (p < 0.05). A similar trend was observed for the expression levels of p-c-jun. Compared to the PH group, YQHX and FZHY markedly reduced the mRNA and protein expression levels of Bax at 24 h after PH, while those in the FZHY + PH group decreased more obviously (p < 0.05). Besides, in comparison with the PH group, YQHX and FZHY administration predominantly upregulated the mRNA and protein expression levels of Bcl-2 at 24 and 72 h after PH (p < 0.05). In conclusion, YQHX improves LR in rats after PH by inhibiting hepatocyte apoptosis via the JNK signaling pathway.

Dicer1 facilitates liver regeneration in a manner dependent on the inhibitory effect of miR-21 on Pten and Rhob expression

AIMS

Tamoxifen-induced liver-specific Dicer1 deletion (iDicer1^-/-) in mature mice may provide clues demonstrating the genuine effects of acute loss of Dicer1 and miRNAs in the liver regeneration process.

MAIN METHODS

In this study, mice with tamoxifen-induced Dicer1 deletion through the Cre/LoxP system were constructed and then underwent classic 70% partial hepatectomy or CCl4-induced liver injury. To rescue the inhibitory effect of Dicer1 ablation on liver regeneration, miR-21 agomir was injected into the tail vein of iDicer1^-/- mice.

KEY FINDINGS

Unlike constitutive embryonic deletion of Dicer1, tamoxifen-induced Dicer1 deletion did not result in severe liver injury or lesions, providing an ideal model for investigating acute loss of Dicer1 and miRNAs in liver regeneration. Dicer1 deletion led to impaired liver regeneration through the inhibitory effect of miR-21 on PTEN and Rhob expression.

SIGNIFICANCE

In our previous study, we found that embryonic loss of Dicer1 impairs hepatocyte survival and leads to chronic inflammation and progenitor cell activation, while the role of Dicer1 in liver regeneration remains largely unknown. We clearly identified the promotion effect of Dicer1 on liver regeneration by increasing miR-21 expression, which inhibits the expression of two negative cell proliferation regulators, Pten and Rhob.

Loss of p27kip1 expression is associated with poor prognosis in patients with taxane-treated breast cancer

PURPOSE

Decreased expression of p27^kip1 and p57^kip2 is considered as a prognostic indicator in patients with breast cancer receiving adjuvant chemotherapy. Previous in vitro studies have reported that reduced expression of p27^kip1 and p57^kip2 is associated with resistance to taxane, which is one of the most effective chemotherapeutic agents. In this study, we investigated the association of low p27^kip1 and p57^kip2 expression with outcomes in patients with breast cancer.

METHODS

We investigated 226 cases of breast cancer from Kangbuk SMC between 2000 and 2005. Levels of p27^kip1 and p57^kip2 expression were evaluated using immunohistochemical staining of tumor tissue microarray specimens. The relationships between the expression levels of the markers and patients' outcomes were analyzed using the Kaplan-Meier method and Cox proportional hazard model.

RESULTS

Low p57^kip2 expression was only associated with negative progesterone receptor status (p = 0.034), whereas p27^kip1 expression was associated with poor prognosis of patients receiving adjuvant chemotherapy (p = 0.005). More detailed analysis revealed that low p27^kip1 expression affects the overall survival rate of patients receiving adjuvant chemotherapy including taxane (p = 0.026), but not that of patients receiving chemotherapy without taxane.

CONCLUSIONS

Low p27^kip1 expression may be useful to predict overall survival in patients with breast cancer who are treated with taxane. Evaluation of p27^kip1 expression may provide further prognostic information beyond traditional prognostic biomarkers and an understanding of the mechanisms that impart resistance against chemotherapy.

Transforming growth factor-β in liver cancer stem cells and regeneration

Cancer stem cells have established mechanisms that contribute to tumor heterogeneity as well as resistance to therapy. Over 40% of hepatocellular carcinomas (HCCs) are considered to be clonal and arise from a stem-like/cancer stem cell. Moreover, HCC is the second leading cause of cancer death worldwide, and an improved understanding of cancer stem cells and targeting these in this cancer are urgently needed. Multiple studies have revealed etiological patterns and multiple genes/pathways signifying initiation and progression of HCC; however, unlike the transforming growth factor β (TGF-β) pathway, loss of p53 and/or activation of β-catenin do not spontaneously drive HCC in animal models. Despite many advances in cancer genetics that include identifying the dominant role of TGF-β signaling in gastrointestinal cancers, we have not reached an integrated view of genetic mutations, copy number changes, driver pathways, and animal models that support effective targeted therapies for these common and lethal cancers. Moreover, pathways involved in stem cell transformation into gastrointestinal cancers remain largely undefined. Identifying the key mechanisms and developing models that reflect the human disease can lead to effective new treatment strategies. In this review, we dissect the evidence obtained from mouse and human liver regeneration, and mouse genetics, to provide insight into the role of TGF-β in regulating the cancer stem cell niche. (Hepatology Communications 2017;1:477-493).

Impaired liver regeneration is associated with reduced cyclin B1 in natural killer T cell-deficient mice

Background

It has been shown that the proportion of natural killer T cells is markedly elevated during liver regeneration and their activation under different conditions can modulate this process. As natural killer T cells and liver injury are central in liver regeneration, elucidating their role is important.

Methods

The aim of the current study is to explore the role of natural killer T cells in impaired liver regeneration. Concanvalin A was injected 4 days before partial hepatectomy to natural killer T cells- deficient mice or to anti CD1d1-treated mice. Ki-67 and proliferating cell nuclear antigen were used to measure hepatocytes proliferation. Expression of hepatic cyclin B1 and proliferating cell nuclear antigen were evaluated by Western Blot and liver injury was assessed by ALT and histology.

Results

Natural killer T cells- deficient or mice injected with anti CD1d antibodies exhibited reduced liver regeneration. These mice were considerably resistant to ConA-induced liver injury. In the absence of NKT cells hepatic proliferating cell nuclear antigen and cyclin B1 decreased in mice injected with Concanvalin A before partial hepatectomy. This was accompanied with reduced serum interleukin-6 levels.

Conclusions

Natural killer T cells play an important role in liver regeneration, which is associated with cyclin B1 and interleukin-6.

Glutathione transferases P1/P2 regulate the timing of signaling pathway activations and cell cycle progression during mouse liver regeneration

Glutathione transferases (GST) are phase II enzymes catalyzing the detoxification of endogenous noxious compounds and xenobiotics. They also regulate phosphorylation activities of MAPKinases in a catalytic-independent manner. Previous studies have demonstrated the regulation of JNK-dependent pathway by GSTP1/2. Considering the crucial role of JNK in the early steps of the hepatocyte cell cycle, we sought to determine whether GSTP1/2 were essential for hepatocyte proliferation following partial hepatectomy (PH). Using a conventional double knockout mouse model for the Gstp1 and Gstp2 genes, we found that the lack of GSTP1/P2 reduced the rate of DNA replication and mitotic index during the first wave of hepatocyte proliferation. The lowered proliferation was associated with the decrease in TNFalpha and IL-6 plasma concentrations, reduced hepatic HGF expression and delayed and/or altered activation of STAT3, JNK and ERK1/2 signaling pathways. In addition, the expression and/or activation of cell cycle regulators such as Cyclin D1, CDK4, E2F1 and MCM7 was postponed demonstrating that the absence of GSTP1/2 delayed the entry into and progression through the G1 phase of the cell cycle and impaired the synchrony of proliferation in hepatocytes following PH. Furthermore, while JNK and its downstream targets c-Jun and ATF2 were activated during the early steps of the liver regeneration in wild-type animals, the constitutively active JNK found in the quiescent liver of Gstp1/2 knockout mice underwent a decrease in its activity after PH. Transient induction of antioxidant enzymes and nitric oxide synthase were also delayed or repressed during the regenerative response. Altogether our results demonstrate that GSTP1/2 are a critical regulators of hepatocyte proliferation in the initial phases of liver regeneration.

Smad3 signaling in the regenerating liver: implications for the regulation of IL-6 expression

Liver regeneration is vital for graft survival and adequate organ function. Smad activation regulates hepatocyte proliferation and macrophage function. The aim of the current study was to evaluate the impact of Smad3 signaling during liver regeneration in the mouse. Male C57Bl/6 wild-type (wt) mice or mice deficient in Smad3 (Smad3(-/-) ) were subjected to a 70% partial hepatectomy (pHx) or sham surgery and sacrificed 24, 42, or 48 h later. Tissue was analyzed for TGF-β signaling, the mitogenic cytokine response [i.e., tumor necrosis factor alpha, TNF-α; interleukin (IL)-6], and liver regeneration. Partial hepatectomy stimulated a strong regenerative response measured by proliferating cell nuclear antigen-positive hepatocytes 42 and 48 h post-pHx in conjunction with an increased expression of IL-6, TNF-α, and Smad2/3 phosphorylation 24 h post-pHx in both hepatocytes and nonparenchymal cells. Surprisingly, Smad3 deficiency led to reduced hepatocyte proliferation 42 h post-pHx which recovered by 48 h, a process that correlated with and was preceded by significant reductions in IL-6 expression and signal transducer and activator of transcription 3 phosphorylation, and cyclin D1 induction 24 h post-pHx. Loss of Smad3 signaling suppresses the expression of key mitogenic cytokines and delays hepatocellular regeneration. Therapies directed at finely regulating Smad3 activation early within the regenerating liver may prove useful in promoting liver cell proliferation and restoration of liver mass.

The Complex Relationship between Liver Cancer and the Cell Cycle: A Story of Multiple Regulations

The liver acts as a hub for metabolic reactions to keep a homeostatic balance during development and growth. The process of liver cancer development, although poorly understood, is related to different etiologic factors like toxins, alcohol, or viral infection. At the molecular level, liver cancer is characterized by a disruption of cell cycle regulation through many molecular mechanisms. In this review, we focus on the mechanisms underlying the lack of regulation of the cell cycle during liver cancer, focusing mainly on hepatocellular carcinoma (HCC). We also provide a brief summary of novel therapies connected to cell cycle regulation.

Regulation of the g1/s transition in hepatocytes: involvement of the cyclin-dependent kinase cdk1 in the DNA replication

A singular feature of adult differentiated hepatocytes is their capacity to proliferate allowing liver regeneration. This review emphasizes the literature published over the last 20 years that established the most important pathways regulating the hepatocyte cell cycle. Our article also aimed at illustrating that many discoveries in this field benefited from the combined use of in vivo models of liver regeneration and in vitro models of primary cultures of human and rodent hepatocytes. Using these models, our laboratory has contributed to decipher the different steps of the progression into the G1 phase and the commitment to S phase of proliferating hepatocytes. We identified the mitogen dependent restriction point located at the two-thirds of the G1 phase and the concomitant expression and activation of both Cdk1 and Cdk2 at the G1/S transition. Furthermore, we demonstrated that these two Cdks contribute to the DNA replication. Finally, we provided strong evidences that Cdk1 expression and activation is correlated to extracellular matrix degradation upon stimulation by the pro-inflammatory cytokine TNFα leading to the identification of a new signaling pathway regulating Cdk1 expression at the G1/S transition. It also further confirms the well-orchestrated regulation of liver regeneration via multiple extracellular signals and pathways.

Loss of transforming growth factor β adaptor protein β-2 spectrin leads to delayed liver regeneration in mice

Liver regeneration, following partial hepatectomy (PHx), occurs through precisely controlled and synchronized cell proliferation, in which quiescent hepatocytes undergo one to two rounds of replication, with restoration of liver mass and function. We previously demonstrated that loss of the Smad3/4 adaptor protein β-2 spectrin (β2SP) is associated with faster entry into S phase, and hepatocellular cancer formation. These observations led us to further pursue the role of β2SP in cell cycle progression in vivo. Liver regeneration studies with PHx in β2SP(+/-) mice reveal a surprising and significant decrease in liver/body weight ratio at 48 hours after PHx in β2SP(+/-) mice in comparison to wildtype mice. At 48 hours after PHx we also observe decreased levels of cyclin E (2.4-fold, P < 0.05), Cdk1 (7.2-fold, P < 0.05), cyclin A, pRb (Ser249/Thr252), proliferative cell nuclear antigen (PCNA), cyclin D1 with elevated levels of pCdk1 (Thr14) (3.6-fold, P < 0.05). Strikingly, at 24 hours elevated levels of p53 (4-fold, P < 0.05), phospho-p53 (ser15 and ser20), and p21 (200-fold, P < 0.05) persisting to 48 hours after PHx further correlated with raised expression of the DNA damage markers pChk2 (Thr68) and γH2AX (S139). However, compromised cell cycle progression with loss of β2SP is not rescued by inhibiting p53 function, and that G(2) /M phase arrest observed is independent and upstream of p53.

CONCLUSION

β2SP deficiency results in dysfunctional hepatocyte cell cycle progression and delayed liver regeneration at 48 hours after PHx, which is p53-independent. β2SP loss may increase susceptibility to DNA damage, impair cell cycle progression, and ultimately lead to hepatocellular cancer.

G1 cell cycle arrest signaling in hepatic injury after intraperitoneal sepsis in rats

OBJECTIVE AND DESIGN

Hepatocytes emerge from a quiescent state into a proliferative state to recover from septic injury. We hypothesize that hepatocyte cell cycle regulation after sepsis potentially contributes to the recovery of liver function.

METHODS

An animal model of sepsis was induced by cecal ligation and puncture (CLP) in rats. At serial time points after CLP, hepatocyte expression of p21, P53, cyclin D1, cyclin E, CDK2, CDK4 and PCNA was determined by immunoblot analysis, and the DNA content of isolated hepatocytes was analyzed using flow cytometry.

RESULTS

Sepsis-induced liver injury of rats was associated with G1 cell cycle arrest. Recovery of liver function was related to cell cycle progression 48 h after CLP. The upregulation of p53 and p21 correlated with G1 cell arrest 48 h after CLP. The upregulation of cyclin D1/CDK4 and cyclin E/CDK2 also correlated with the G1/S transition 48 h after CLP, resulting in PCNA expression.

CONCLUSIONS

The data suggests that G1 cell cycle arrest and p53, p21, CDKs, cyclins and PCNA expression may be involved in the injury/recovery of liver function after intraperitoneal sepsis.

Induction of UGT1A1 and CYP2B6 by an antimitogenic factor in HepG2 cells is mediated through suppression of cyclin-dependent kinase 2 activity: cell cycle-dependent expression

Hepatocyte growth factor (HGF), an antimitogenic factor for HepG2 cells, increased mRNA and protein levels of UGT1A1 and CYP2B6, as well as the endogenous cyclin-dependent kinase (CDK) inhibitors p16, p21, and p27 in HepG2 cells but not in HuH6, Caco2, or MCF7 cells. Treatment with 1,4-diamino-2,3-dicyano-1,4-bis(methylthio)butadiene (U0126) (an extracellular signal-regulated kinase inhibitor) suppressed the HGF-induced expression of UGT1A1 and CYP2B6, as well as p16, p21, and p27 in HepG2 cells. The CDK inhibitor roscovitine also enhanced the expression of UGT1A1, CYP2B6, and CYP3A4. Transfection of anti-CDK2 siRNA led to elevated levels of UGT1A1, CYP2B6, and CYP3A4 in HepG2 and SW480 cells, whereas anti-CDK4 small interfering RNA (siRNA) did not significantly enhance the expression of these enzymes. In fact, CDK2 activity was decreased in HGF-treated HepG2 cells. In cells arrested in S phase by a thymidine block and then released into a synchronous cell cycle, there was a clear dissociation among the activation of CDK2 and the expression of UGT1A1, CYP2B6, and CYP3A4. Furthermore, the induction of CYP3A4 but not UGT1A1 or CYP2B6 mRNA expression by roscovitine was repressed in pregnane X receptor (PXR) siRNA-transfected HepG2 cells. Transfection with constitutive androstane receptor siRNA or PXR siRNA in HepG2 cells did not repress the HGF-stimulated expression of UGT1A1 mRNA. Taken together, our results show that the expression of UGT1A1 and CYP2B6 is negatively regulated through a CDK2 signaling pathway linked to cell cycle progression in HepG2 and SW480 cells, the mechanism of which may differ from that of CYP3A4 expression through PXR phosphorylated by CDK2.

Cyclin-dependent kinase 1 plays a critical role in DNA replication control during rat liver regeneration

Liver regeneration is a unique process to restore hepatic homeostasis through rapid and synchronous proliferation of differentiated hepatocytes. Previous studies have shown that hepatocyte proliferation is characterized by high expression levels of the "mitotic" cyclin-dependent kinase 1 (Cdk1) during S-phase compared to other mammalian cells. In the light of findings showing that Cdk1 compensates for the loss of Cdk2 and drives S-phase in Cdk2-deficient cells derived from Cdk2 knockout mice, we took advantage of the models of liver regeneration following partial hepatectomy and primary cultures of normal rat hepatocytes to further examine the involvement of Cdk1 during DNA replication in hepatocytes and to dissect specific cell cycle regulation in hepatocytes compared to control human foreskin fibroblasts. In hepatocytes, Cdk1 exhibited a biphasic activation pattern correlating S-phase and G(2)/M transition, bound to cyclin A or B1 and localized to the nucleus during DNA replication. Importantly, small interfering RNA (siRNA)-mediated silencing of Cdk1 led to a strong decrease in DNA synthesis without affecting centrosome duplication. Furthermore, in hepatocytes arrested by the iron chelator O-Trensox in early S-phase prior to DNA replication, Cdk1/cyclin complexes were active, while replication initiation components such as the minichromosome maintenance 7 (Mcm7) protein were loaded onto DNA. Moreover, Mcm7 expression and loading onto DNA were not modified by Cdk1 silencing. Conversely, in fibroblasts, Cdk1 expression and activation were low in S-phase and its silencing did not reduce DNA synthesis.

CONCLUSION

Cdk1 is essential for DNA replication downstream formation of replication initiation complexes in hepatocytes but not in fibroblasts and, as such, our data exemplify crucial differences in the cell cycle regulation between various mammalian cell types.

Hepatic regenerative response in small-sized liver isografts in the rat

BACKGROUND

To investigate hepatic regenerative response and associated mechanisms in different-size liver grafts in the rat.

METHODS

Rat models of different-size-graft liver transplantation (whole, 50%-size, or 30%-size) were established, with a sham operation group serving as a control. Portal pressure, graft injury, interleukin 6 (IL-6), signal transducer and activator of transcription (Stat3), mitogen-activated protein kinase (MAPK), cyclin D1, and proliferating cell nuclear antigen (PCNA) were all assessed.

RESULTS

The portal pressure was significantly higher and hepatic injury more severe in the smaller sized groups than in the whole graft group, especially in the 30%-size grafts. Hepatic IL-6 and tumor necrosis factor-alpha (TNF-alpha) levels in the two smaller sized groups were significantly higher than in the whole graft group, while IL-6 levels appeared to be negatively associated with graft sizes. Downstream markers of IL-6, Stat3 and MAPK phosphorylation, cyclin D1, and PCNA expression were also markedly increased in the small-sized grafts compared with the whole grafts, and appeared to positively correlate with early measurements of portal pressure and subsequent hepatic injury.

CONCLUSION

Vigorous hepatic regeneration in small-for-size liver grafts may be associated with highly activated IL-6/Stat3 and MAPK signaling, which may in turn correlate with graft size, portal pressure, and hepatic injury.

Involvement of Kupffer cell-dependent signaling in T3-induced hepatocyte proliferation in vivo

Thyroid hormone-induced calorigenesis triggers liver oxidative stress with concomitant TNF-alpha production by Kupffer cells and up-regulation of gene expression. Considering that cyclin-dependent kinase-2 (CDK-2) performs essential functions for cellular proliferation, our aim was to test the hypothesis that l-3,3',5-triiodothyronine (T(3)) stimulates liver cell proliferation by upstream mechanisms involving CDK-2 expression dependent on Kupffer cell signaling. T(3) administration induced a calorigenic response at 60-70 h after treatment, with increased TNF-alpha generation and hepatic oxidative stress status, as shown by enhanced protein carbonyls and decreased glutathione content compared to controls. In this time interval, liver c-jun N-terminal kinase (JNK) phosphorylation, activator protein-1 (AP-1) DNA binding, and CDK-2 expression were enhanced, with concomitantly higher levels of the proliferation markers Ki-67 and proliferating cell nuclear antigen. These changes are abolished by administration of the Kupffer cell inactivator gadolinium chloride prior to T(3) treatment. We conclude that T(3) administration triggers liver CDK-2 expression and cellular proliferation through a cascade associated with Kupffer cell-dependent TNF-alpha generation, JNK phosphorylation, and AP-1 activation. Since CDK-2 promotes phase S progression within the cell cycle, this response may constitute a major mechanism involved in T(3)-induced liver preconditioning to ischemia/reperfusion injury.

Impaired hepatic regeneration by ischemic preconditioning in a rat model of small-for-size liver transplantation

OBJECTIVE

Graft size is one of the major risk factors in adult-to-adult living donor liver transplantation and rapid regeneration is an essential post-operative requirement. Ischemic preconditioning (IPC) has been shown to be an effective strategy in the reduction of hepatic ischemia-reperfusion injury and stimulation of liver regeneration. This study was designed to evaluate the effects of IPC on liver regeneration in small-for-size liver grafts.

METHODS

We employed a rat orthotopic liver transplantation model using small-for-size (30%) grafts, in the presence or absence (control) of IPC (10 min of ischemia followed by 15 min of reperfusion). Survival rate, graft injury, hepatocellular proliferation, cell cycle progression, Stat3 activation, as well as TNF-alpha and IL-6 expression were assessed.

RESULTS

IPC significantly enhanced the extent of graft injury and hindered hepatic regeneration in small-for-size liver grafts. The 7-day survival rate was also reduced by IPC, but failed to reach statistical significance. IPC did not affect TNF-alpha levels, but significantly decreased the elevation of IL-6 after reperfusion. These findings were correlated with down-regulation of cyclin E and cyclin D1, and decreased numbers of PCNA-positive nuclei in IPC grafts. These results were inconsistent with Stat3 activation, as P-Stat3 exhibited a stronger and prolonged pattern of expression in the IPC group, compared to controls.

CONCLUSIONS

Ischemic preconditioning may impair liver regeneration in small-for-size liver grafts by decreasing IL-6 and blunting cell cycle progression, through a mechanism at least partially independent of Stat3.

Transcription of the MAT2A gene, coding for methionine adenosyltransferase, is up-regulated by E2F and Sp1 at a chromatin level during proliferation of liver cells

Methionine adenosyltransferase (MAT) is an essential enzyme because it catalyzes the formation of S-adenosylmethionine, the main methyl donor. Two MAT-encoding genes (MAT1A, MAT2A) are found in mammals. The latter is expressed in proliferating liver, dedifferentiation and cancer, whereas MAT1A is expressed in adult quiescent hepatocytes. Here, we report studies on the molecular mechanisms controlling the induction of MAT2A in regenerating rat liver and in proliferating hepatocytes. The MAT2A is up-regulated at two discrete moments during liver regeneration, as confirmed by RNApol-ChIP analysis. The first one coincides with hepatocyte priming (i.e. G0-G1 transition), while the second one takes place at the G1-S interface. Electrophoretic mobility shift assays showed that a putative E2F sequence present in MAT2A promoter binds this factor and ChIP assays confirmed that E2F1, E2F3 and E2F4, as well as the pocket protein p130, are bound to the promoter in quiescent liver. MAT2A activation is accompanied by changes in the binding of histone-modifying enzymes to the promoter. Interestingly, p130 is not displaced from MAT2A promoter during hepatocyte priming, but it is in the late expression of the gene at the G1-S transition. Finally, the transcription factor Sp1 seems to play a decisive role in MAT2A induction, as it binds the promoter when the gene is being actively transcribed. In summary, the present work shows that the molecular mechanism of MAT2A expression is different during G0-G1 or G1-S transition and this may be related to the distinct requirements of S-adenosylmethionine during liver regeneration.

Regulation of CDK4

Cyclin-dependent kinase (CDK)4 is a master integrator that couples mitogenic and antimitogenic extracellular signals with the cell cycle. It is also crucial for many oncogenic transformation processes. In this overview, we address various molecular features of CDK4 activation that are critical but remain poorly known or debated, including the regulation of its association with D-type cyclins, its subcellular location, its activating Thr172-phosphorylation and the roles of Cip/Kip CDK "inhibitors" in these processes. We have recently identified the T-loop phosphorylation of CDK4, but not of CDK6, as a determining target for cell cycle control by extracellular factors, indicating that CDK4-activating kinase(s) might have to be reconsidered.

Cytokine signals modulated via lipid rafts mimic niche signals and induce hibernation in hematopoietic stem cells

Hematopoietic stem cells (HSCs) reside in the bone marrow (BM) niche in a noncycling state and enter the cell cycle at long intervals. However, little is known about inter- and intracellular signaling mechanisms underlying this unique property of HSCs. Here, we show that lipid raft clustering is a key event in the regulation of HSC dormancy. Freshly isolated HSCs from the BM niche lack lipid raft clustering, exhibit repression of the AKT-FOXO signaling pathway, and express abundant p57(Kip2) cyclin-dependent kinase inhibitor. Lipid raft clustering induced by cytokines is essential for HSC re-entry into the cell cycle. Conversely, inhibition of lipid raft clustering caused sustained nuclear accumulation of FOXO transcription factors and induced HSC hibernation ex vivo. These data establish a critical role for lipid rafts in regulating the cell cycle, the survival, and the entry into apoptosis of HSCs and uncover a striking similarity in HSC hibernation and Caenorhabditis elegans dauer formation.

Expression patterns of cytokine, growth factor and cell cycle-related genes after partial hepatectomy in rats with thioacetamide-induced cirrhosis

AIM: To examine the differences in the responses of normal and cirrhotic livers to partial hepatectomy in relation to the factors influencing liver regeneration.

METHODS: Cirrhosis was induced in rats by admini-stration of thioacetamide. Untreated rats were used as controls. The control rats as well as the cirrhotic rats were subjected to 70% partial hepatectomy. At different time points after hepatectomy, the livers were collected and the levels of cytokines, growth factors and cell cycle proteins were analyzed.

RESULTS: After hepatectomy, the cirrhotic remnant expressed significantly lower levels of cyclin D1, its kinase partner, cdk4, and cyclin E as compared to the controls up to 72 h post hepatectomy. Significantly lower levels of cyclin A and cdk2 were also observed while the cdk inhibitor, p27 was significantly higher. In addition, the cirrhotic group had lower IL-6 levels than the control group at all time points up to 72 h following resection.

CONCLUSION: The data from our study shows that impaired liver regeneration in cirrhotic remnants is associated with low expression of cyclins and cdks. This might be the consequence of the low IL-6 levels in cirrhotic liver remnant which would in turn influence the actions of transcription factors that regulate genes involved in cell proliferation and metabolic homeostasis during the regeneration process.

Long-term 17alpha-ethinyl estradiol treatment decreases cyclin E and cdk2 expression, reduces cdk2 kinase activity and inhibits S phase entry in regenerating rat liver

BACKGROUND/AIMS

The synthetic estrogen 17alpha-ethinyl estradiol (EE), a potent tumor promoter in rat liver, stimulates growth during short-term treatment but inhibits hepatocyte proliferation upon prolonged treatment. To identify the molecular targets of the mitoinhibitory effect of EE, the expression of proteins regulating G(1)- and S-progression were analyzed during the first cell cycle in EE-treated female Wistar rats.

METHODS

Long-term (60 days) EE treatment. Immunohistochemical staining for proliferation cell nuclear antigen (PCNA) to detect cells in S phase and quantification of mitosis. Western blot to monitor protein expression. Cdk2 kinase assay to examine histone H1 phosphorylation.

RESULTS

EE reduced the number of cells in S phase and mitosis by about 70%. Cyclin D1 and D3 were unaffected, while cdk4 was moderately decreased. Cyclin E and cdk2 were markedly decreased with concomitant marked reduction of cdk2 kinase activity. EE also decreased cyclin A and increased G1 levels of p53 and p21.

CONCLUSIONS

EE causes a cell cycle block before S-phase. The reduction of the cdk2 kinase activity, essential for G1/S-transition, might be involved in the cell cycle block. Also, EE treatment results in p53 activation and upregulation of the cdk inhibitor p21 that might contribute to the G1 arrest.

D-type cyclins and G1 progression during liver development in the rat

Initiation and progression through G1 requires the activity of signaling complexes containing cyclins (D- or E-type) and cyclin-dependent kinases (CDK4/6 and CDK2, respectively). We set out to identify the G1-phase cyclins and CDKs that are operative during late gestation liver development in the rat. This is a period during which hepatocytes show a high rate of proliferation that is, at least in part, independent of the mitogenic signaling pathways that are functional in mature hepatocytes. RNase protection assay and Western immunoblotting indicated that cyclin D1 is expressed at similar levels in fetal and adult liver. When cyclin D1 was induced after partial hepatectomy, its predominant CDK-binding partner was CDK4. In contrast, cyclins D2 and D3 predominated in fetal liver and were complexed with both CDK4 and CDK6. Little CDK6 protein was expressed in quiescent or regenerating adult liver. Cyclins E1 and E2 were both transcriptionally up-regulated in fetal liver. Activity of complexes containing cyclins E1 and E2 was higher in fetal liver, as was content of the cell cycle regulator, Rb. In fetal liver, Rb was highly phosphorylated at both cyclin D- and cyclin E-dependent sites. In conclusion, liver development is associated with a switch from cyclin D2/D3-containing complexes to cyclin D1:CDK4 complexes. We speculate that the switch in D-type cyclins may be associated with the dependence on mitogenic signaling that develops as hepatocytes mature.

A novel molecular marker of pituitary tumor transforming gene involves in a rat liver regeneration

BACKGROUND

Pituitary tumor transforming gene (PTTG), homologous to a mammalian securin, plays a pivotal role in cell transformation, however, its biological function(s) in normal tissues is not fully understood. Because the liver is a regenerative organ, the relevant biological function of PTTG in the liver would be more feasible to understand PTTG. Also, PTTG may be involved in the liver regeneration.

MATERIALS AND METHODS

Expressions of rat hepatic PTTG messengerRNA (mRNA) and cellular immunoreactivities during the cell proliferative period of the liver regeneration both in vitro and in vivo were tested.

RESULTS

PTTG expression of the rat primary hepatocyte was stimulated by HGF in a dose dependent manner, and was suppressed when hepatocyte proliferation was inhibited by transforming growth factor-beta1. A positive PTTG immunoreactive co-localizing with 5-bromo-2'-deoxyuridine (BrdU) in the hepatocyte nucleus was found and there was a concurrent sister chromatin itself by the immunofluorescent labeling of PTTG with cytokeratin 18 (CK18).

DISCUSSION

Since the correlation of PTTG mRNA expression, cell proliferation and immunoreactivity were observed in primary rat cultured hepatocytes, PTTG may be a novel marker of cell proliferation both in vitro and in vivo liver regeneration.

Blunted DNA synthesis and delayed S-phase entry following inhibition of Cdk2 activity in the regenerating rat liver

Activation of the cyclin E/Cdk2 complex may play an important role in mid-G1/S-phase progression in proliferating mammalian cells. We evaluated the effect of targeted inhibition of Cdk2 activity by CYC202 (R-roscovitine) on hepatocytes proliferation in vivo after 70% partial hepatectomy (PH) in rats. In controls, Cdk2 activity and DNA synthesis peaked 24 h after PH. CYC202 abrogated Cdk2 activity, prevented BrdU incorporation and PCNA expression and increased mortality 24 h after PH. Cyclin E and Cdk2 protein expression and complex formation was not affected by CYC202 nor was cyclin D1, Cdk4 and c-ras mRNA expression. Two consecutive injections 8 and 20 h after PH were required to elicit the inhibitory effect of CYC202, which was lost when either the injection at 8 h or at 20 h was withheld. Cdk2 activity and cell progression resumed 48 h after PH in surviving animals suggesting that CYC202 induced a reversible inhibition of the cell cycle. Our results confirm an important role for Cdk2 in hepatocytes proliferation in the regenerating liver. We demonstrate that molecular events, including Cdk2 activation, occurring within the 8th and 24th hour after PH (G1/S-phase transition) are crucial in determining whether or not DNA synthesis and hepatocytes proliferation proceed normally after PH.

TNFalpha-mediated extracellular matrix remodeling is required for multiple division cycles in rat hepatocytes

During liver regeneration, hepatocytes proliferate under the control of both proinflammatory cytokines such as tumor necrosis factor alpha (TNFalpha) and growth factors, in parallel to extracellular matrix remodeling. This study investigated mechanisms by which mitogen and extracellular matrix signals are linked for inducing proliferation of differentiated hepatocytes. The authors used adult rat hepatocytes in coculture with liver biliary cells, because cells are stably differentiated for several weeks, capable of extracellular matrix deposition, and unable to divide in response to growth factor alone. This work demonstrated that hepatocytes could undergo several proliferation waves without loss of differentiation by using alternating periods of TNFalpha/growth factor stimulation and deprivation. Three days after stimulation with TNFalpha and epidermal growth factor (EGF), up to 35% of hepatocytes divided. Demonstration was also provided that EGF alone only promoted cell progression up to late G(1), whereas TNFalpha was necessary for G(1)/S transition and Cdk1 induction. TNFalpha promoted an extracellular matrix (ECM) degradation that involved the matrix metalloproteinase MMP-9 induction through activation of NF-kappaB pathway. Finally, the authors showed that ECM remodeling signal was required for initiating any new hepatocyte division wave, in presence of mitogen. In conclusion, these results highlight that hepatocyte division is dependent on ECM deposition associated with differentiation status, and that ECM degradation signal is critical in controlling G(1)/S transition and Cdk1 induction. These results provide new insights for understanding the unique hepatocyte proliferation control and improving regeneration in patients suffering from liver damage.

Molecular events associated with accelerated proliferative response in rat livers when partial hepatectomy is preceded by a sham operation

BACKGROUND

When a sham operation is performed 6 h before partial hepatectomy (PH), the regenerative response is accelerated suggesting that sham operation itself contributes to cellular events leading to proliferation.

MATERIALS AND METHODS

In order to examine the mechanisms implicated in this acceleration, we compared the activation of several factors associated with the progression through the cell cycle at various times after PH and after PH preceded by sham operation (S6 h + PH). The effect of a single sham (S) and two combined sham operations (S6 h + S) was also examined. Nonoperated rats were used as controls (C).

RESULTS

The early factors NF-kappaB and Stat3 were activated after S6 h + PH and S6 h + S. C-jun expression was increased 0.5 h and 2 h after PH and 6 h after sham. There was no further increase in S6 h + PH and S6 h + S. In contrast, c-myc expression returned to baseline levels after S6 h and a new increase was observed 2 h after S6 h + PH but not after S6 h + S. P53 mRNA was significantly expressed 6 h after S6 h + PH, but at a level similar than that observed 6 and 12 h after PH alone. An earlier increase in c-Ha-ras mRNA and cyclin E protein was found in S6 h + PH, in comparison with PH alone.

CONCLUSIONS

The first divergent response between the two combined models involved c-myc expression. However, major differences related to the accelerated liver regenerative response observed after S6 h + PH were found at late time points associating an earlier expression of c-Ha-ras and nuclear cyclin E.

P38SAPK2 phosphorylates cyclin D3 at Thr-283 and targets it for proteasomal degradation

Cyclin D3 plays a critical role in maturation of precursor T cells and their levels are tightly regulated during this process. Alteration of cyclin D3 levels has been proposed to be important in the development of different human cancers, including malignancies of the lymphoid system. Thus, we have analysed the mechanisms involved in the regulation of cyclin D3 levels. Our results indicate that cyclin D3 is degraded via proteasome and that Thr-283 is essential for its degradation. Wild-type cyclin D3 but not the Thr-283A mutant accumulated ubiquitylated forms after treatment with proteasome inhibitors. We also observed that different type of stresses promote the Thr-283-dependent in vivo degradation of cyclin D3. The analysis of the kinases involved in Thr-283 phosphorylation indicates that all the members of the p38SAPK family of serine-threonine kinases are able to phosphorylate cyclin D3 at this specific site. Moreover, we found that the overexpression of p38alphaSAPK2 induce the decrease of cyclin D3 in vivo. These results indicate that p38SAPK might be involved in the regulation of cyclin D3 levels and suggest that this mechanism is involved in the maturation of precursor T-cells. Alterations of this mechanism might be important for oncogenesis.

Negative regulation of liver regeneration by innate immunity (natural killer cells/interferon-gamma)

BACKGROUND AND AIMS

Hepatic lymphocytes are composed mainly of natural killer (NK) cells and NKT cells, which play key roles in innate immune responses against pathogens and tumors in the liver. This report analyzes the effects of activation of innate immunity by viral infection or the toll-like receptor 3 (TLR3) ligand on liver regeneration.

METHODS

The partial hepatectomy (PHx) method was used as a model of liver regeneration. Murine cytomegalovirus (MCMV) infection and the TLR3 ligand polyinosinic-polycytidylic acid [poly(I:C)] were used to activate innate immunity.

RESULTS

NK cells are activated after PHx, as evidenced by producing interferon (IFN)-gamma. Infection with MCMV or injection of poly(I:C) further activates NK cells to produce IFN-gamma and attenuates liver regeneration in the PHx model. Depletion of NK cells or disruption of either the IFN-gamma gene or the IFN-gamma receptor gene enhances liver regeneration and partially abolishes the negative effects of MCMV and polyI:C on liver regeneration, whereas NKT cells may only play a minor role in suppression of liver regeneration. Adoptive transfer of IFN-gamma +/+ NK cells, but not IFN-gamma -/- NK cells, restores the ability of polyI:C to attenuate liver regeneration in NK-depleted mice. Finally, administration of polyI:C or IFN-gamma enhances expression of several antiproliferative proteins, including STAT1, IRF-1, and p21cip1/waf1 in the livers of partially hepatectomized mice.

CONCLUSIONS

Our findings suggest that viral infection and the TLR3 ligand negatively regulate liver regeneration via activation of innate immunity (NK/IFN-gamma), which may play an important role in the pathogenesis of viral hepatitis.

Dexamethasone inhibits early regenerative response of rat liver after cold preservation and transplantation

Regeneration is crucial for the recovery of hepatic mass following liver transplantation. Glucocorticoids, immunosuppressive and antiinflammatory agents commonly used in transplantation, are known to inhibit the expression of specific cytokines and growth factors. Some of these proteins, namely tumor necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6), play a critical role in the initiation of liver regeneration. Following cold preservation and reperfusion of the transplanted liver, the normal recovery process is marked by increased expression of TNF-alpha and IL-6, followed by activation of cytokine-responsive transcription factors and progression of the cell cycle resulting in hepatocyte proliferation. We hypothesized that glucocorticoids may influence the repair mechanisms initiated after extended cold preservation and transplantation. Using a rat orthotopic liver transplant model, recipient animals were treated with dexamethasone at the time of transplantation of liver grafts with prolonged cold storage (16 hours). Treatment with dexamethasone suppressed and delayed the expression of TNF-alpha and IL-6 compared with animals receiving no treatment and attenuated downstream nuclear factor kappaB (NF-kappaB), signal transduction and activator of transcription 3 (STAT3), and activation protein 1 (AP-1) activation. This suppression was accompanied by poor cell-cycle progression, delayed cyclin D1 nuclear transposition, and impaired hepatocyte proliferation by BrdU uptake. Histologically, the liver grafts in treated animals demonstrated more injury than controls, which appeared to be necrosis, rather than apoptosis. In conclusion, these data provide evidence that the administration of glucocorticoids at the time of transplantation inhibits the initiation of the regenerative process and may have a deleterious effect on the recovery of liver grafts requiring significant regeneration. This may be particularly relevant for transplantation of partial liver grafts in the living donor setting.

Changes of hepatocyte cell cycle after partial hepatectomy in cirrhotic rat liver

AIM

To investigate the effects of cyclins and oncogene proteins on liver regeneration after partial hepatectomy in cirrhotic rats.

METHODS

By employing immunohistochemistry and in situ hybridization, we observed the changes of liver cyclin A, D and oncogene proteins in the model of rat liver cirrhosis(CCL₄) after partial hepatectomy.

RESULTS

The expressions and distributions of hepatocyte cyclin A and D were similar, which were mainly in the cytoplasm and nuclei of the cells. Around the central veins, the expressions were high and early at about 6 h postoperation. During liver regeneration, the expressions of cyclin A and D mRNA were marked and locally distributed. Cyclin B and Rb proteins were distributed in the cytoplasm and nuclei of the cells. Cyclin B and Rb proteins had stronger expressions in the liver at 6-24 h after operation. P27 expressions occured at 24 h after operation, and were most marked at 1 wk after operation. RB protein also showed stronger expressions.

CONCLUSION

Liver regeneration is dependent on a muti-pathway mechanism.

Retrorsine: a kinetic study of its influence on rat liver regeneration in the portal branch ligation model

BACKGROUND

Retrorsine, a naturally occurring pyrrolizidine alkaloid, impairs liver regeneration after partial hepatectomy by mechanisms that are still unclear.

AIM

The aim of the study was to clarify the influence of retrorsine on cell cycle progression in the regenerating liver lobes of rats after portal branch ligation (PBL).

METHODS

Liver weight, protein and DNA contents, DNA synthesis (5'-bromodeoxyuridine (BrdU) incorporation) and cellular levels of Cyclin E, CDK-2, CDK-4 and proliferating cell nuclear antigen (PCNA) were assessed before and 24, 48, 72 and 168 h after PBL.

RESULTS

Before surgery, higher levels of cyclin E, CDK-2, CDK-4 and PCNA as well as BrdU incorporation were found in the liver of retrorsine-treated rats than in untreated rats. Liver weight gain, protein and DNA synthesis as well as induction of cell cycle related proteins were all strongly impaired by retrorsine in the regenerating lobes after PBL.

CONCLUSIONS

In conclusion, retrorsine impairs liver regeneration in the PBL model not only by an S or G2/M phase block, but also by a block located before the G1/S transition of the cell cycle.

Regulation and role of p21 and p27 cyclin-dependent kinase inhibitors during hepatocyte differentiation and growth

Unlike a large number of cell types that undergo terminal differentiation associated with permanent withdrawal from the cell cycle, mature quiescent hepatocytes retain high proliferative potential. We report here a specific behavior of members of the Cip/Kip family of cyclin-dependent kinase (Cdk) inhibitors during development of the rat liver and proliferation of normal hepatocytes. Expression of p21, p27, and p57 transcripts and proteins was downregulated during the differentiation process to low or undetectable levels in adult liver. In contrast to p27, p21 protein increased in a mitogen-dependent manner in isolated hepatocytes and its expression pattern correlated with that of cyclin D1. In proliferating hepatocytes, p21 was predominantly associated with cyclin D1, these proteins were colocalized in the nucleus and p21-associated retinoblastoma protein (pRb) kinase activity increased in parallel with that of cyclin D1. Overexpression of p21 in mitogen-stimulated hepatocytes reduced DNA synthesis. In contrast, inhibition of p21 expression by antisense or small interfering RNAs oligonucleotides accelerated S phase entry. Finally, expression of p21 and cyclin D1, but not p27 proteins was regulated by MAPK kinase/extracellular signal-regulated kinase and phosphatidylinositol 3-kinase-ferric-reducing ability power/mammalian target of rapamycin signal transduction pathways. In conclusion, these results demonstrate a specific and differential regulation of p21 and p27 during hepatocyte differentiation and proliferation that may contribute to the control of quiescent differentiated hepatic cell proliferating activity.

Loss of p27(Kip1) accelerates DNA replication after partial hepatectomy in mice

BACKGROUND

Liver regeneration after partial hepatectomy (PH) is accomplished by a synchronous replication of hepatocytes. Both positive and negative regulators of cyclin-dependent protein kinase (Cdk) have been implicated in hepatocyte proliferation, but their specific roles in vivo remain to be clarified. To investigate the specific role of p27(Kip1), a member of the Cip/Kip family of Cdk inhibitors, in cell-cycle regulation during liver regeneration, p27-knockout mice were studied after PH.

MATERIALS AND METHODS

Under ether anesthesia, mice were subjected to 70% PH. Animals were sacrificed at intervals after the surgery, and the remnant liver was harvested and analyzed.

RESULTS

In p27-deficient mice, the timing of DNA synthesis was significantly accelerated with a perturbation in the ordered distribution of proliferating cells in the hepatic lobule. p27 deficiency, however, did not affect the whole population of cycling cells, the number of apoptotic cells, or liver injury and mortality after PH.

CONCLUSION

These data provide in vivo evidence that p27 functions as a brake in the "start" of the hepatocyte cell cycle, thereby coordinating temporally and spatially the onset of DNA synthesis of hepatocytes within the hepatic lobules.

Differential regulation of cyclins D1 and D3 in hepatocyte proliferation

Substantial evidence suggests that cyclin D1 plays a pivotal role in the control of the hepatocyte cell cycle in response to mitogenic stimuli, whereas the closely related protein cyclin D3 has not been extensively evaluated. In the current study, we examined the regulation of cyclins D1 and D3 during hepatocyte proliferation in vivo after 70% partial hepatectomy (PH) and in culture. In contrast to cyclin D1, which was nearly undetectable in quiescent liver and substantially up-regulated after PH, cyclin D3 was constitutively expressed and induced only modestly. In the regenerating liver, the concentration of cyclin D3 was only about 10% of that of cyclin D1. Cyclin D1 formed complexes primarily with cyclin-dependent kinase 4 (cdk4), which were markedly activated in the regenerating liver and readily sequestered the cell cycle inhibitory proteins, p21 and p27. Cyclin D3 bound to both cdk4 and cdk6. Cyclin D3/cdk6 activity was readily detectable in quiescent liver and changed little after PH, and this complex appeared to play a minor role in sequestering p21 and p27. In cultured hepatocytes, epidermal growth factor or insulin had little effect, but the combination of these agents substantially induced cyclin D1 and cell cycle progression. Inhibition of Mek1 or phosphoinositide 3-kinase markedly inhibited cyclin D1 expression and replication. In contrast, cyclin D3 was expressed in the absence of mitogens and was only modestly affected by these manipulations. In addition, growth-inhibitory extracellular matrix conditions inhibited cyclin D1 but not cyclin D3 expression. In conclusion, these results support the concept that cyclin D1 is critically regulated by extracellular stimuli that control proliferation, whereas cyclin D3 is regulated through different pathways and plays a distinct role in the liver.

Steatosis is not sufficient to cause an impaired regenerative response after partial hepatectomy in rats

BACKGROUND/AIMS

Fatty liver is known to be associated with increased mortality and morbidity after liver resection. The ability of fatty liver to regenerate after two-thirds partial hepatectomy was studied in three different models of steatosis in rats: obese Zucker rats, orotic acid-fed Wistar rats and Wistar rats fed a methionine-low, choline-deficient diet.

METHODS

Liver regeneration was assessed 24 h after partial hepatectomy by bromodeoxyuridine incorporation (immunohistochemistry), proliferating cell nuclear antigen, cyclin E and cyclin-dependent kinase 2 protein expression (Western blot analysis) and cyclin-dependent kinase 2 activity (kinase assays using histone H1 as a substrate).

RESULTS

No significant difference of proliferative response was found between orotic acid or methionine-low, choline-deficient diet-fed and control Wistar rats 24 h after partial hepatectomy. In contrast, hepatocyte proliferation in obese Zucker rats after partial hepatectomy was significantly reduced when compared with their lean controls.

CONCLUSIONS

Steatosis per se does not impair liver regeneration. The reduced liver regeneration observed in obese Zucker rats may not be due to fatty infiltration itself but to other factors such as leptin receptor dysfunction.

Rb localization and phosphorylation kinetics correlate with the cellular phenotype of cultured breast adenocarcinoma cells

Retinoblastoma protein (Rb) expression has been correlated with state of differentiation, proliferation rate, and metastatic potential in breast adenocarcinomas and established cell lines. These observations, based on immunoreactivity of total Rb rather than hypophosphorylated protein, do not address the relationship between functional Rb and indicators of an aggressive transformed cellular phenotype. We hypothesized that the distribution of functional Rb and the kinetics of Rb phosphorylation would differ between cell lines representing immortalized mammary epithelium (MCF10A), differentiated nonmetastatic mammary adenocarcinoma (MCF-7), and poorly differentiated, highly metastatic mammary adenocarcinoma (MDA-MB-231) and that these differences would be informative of the cellular phenotype. Direct immunofluorescence microscopy was used to compare qualitatively the subcellular localization of total and hypophosphorylated Rb protein in synchronized and asynchronous cells. This technique was also used to quantitatively assess the amounts of hypophosphorylated Rb throughout the cell cycle in these representative cell lines. Total Rb stained more prominently than hypophosphorylated Rb in the nucleus of all asynchronous cells. Rb phosphorylation was more rapid in MCF-7 cells than in MCF10A cells, whereas Rb dephosphorylation appeared deregulated in MDA-MB-231 cells. We conclude that assessment of hypophosphorylated Rb may be more useful than assessment of total Rb for the evaluation of transformed breast adenocarcinoma phenotypes.

Rapamycin-insensitive regulation of 4e-BP1 in regenerating rat liver

In cultured cells, growth factor-induced phosphorylation of two translation modulators, p70 S6 kinase and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), is blocked by nanomolar concentrations of the immunosuppressant rapamycin. Rapamycin also attenuates liver regeneration after partial hepatectomy, but it is not known if this growth-suppressive effect is due to dephosphorylation of p70 S6 kinase and/or 4E-BP1. We found that partial hepatectomy induced a transient increase in liver p70 S6 kinase activity and 4E-BP1 phosphorylation as compared with sham-operated rats. The amount of p70 S6 kinase protein in regenerating liver did not increase, but active kinase from partially hepatectomized animals was highly phosphorylated. Phosphorylated 4E-BP1 from regenerating liver was unable to form an inhibitory complex with initiation factor 4E. Rapamycin blocked the activation of p70 S6 kinase in response to partial hepatectomy in a dose-dependent manner, but 4E-BP1 phosphorylation was not inhibited. By contrast, functional phosphorylation of 4E-BP1 induced by injection of cycloheximide or growth factors was partially reversed by the drug. The mammalian target of rapamycin (mTOR) has been proposed to directly phosphorylate 4E-BP1. Western blot analysis using phospho-specific antibodies showed that phosphorylation of Thr-36/45 and Ser-64 increased in response to partial hepatectomy in a rapamycin-resistant manner. Thus, rapamycin inhibits p70 S6 kinase activation and liver regeneration, but not functional phosphorylation of 4E-BP1, in response to partial hepatectomy. These results indicate that the effect of rapamycin on 4E-BP1 function in vivo can be significantly different from its effect in cultured cells.

Induction of hepatocyte proliferation and liver hyperplasia by the targeted expression of cyclin E and skp2

Cells in culture become competent to replicate in the absence of growth factor after progressing beyond the late G1 restriction point, suggesting that a set of genes expressed during G1 phase is sufficient to trigger completion of the cell cycle. However, this has not been demonstrated in an in vivo system. In this study, we examined whether transfection of genes associated with the G1/S transition could trigger hepatocyte replication. Co-transfection of cyclin E and skp2 synergistically promoted cell cycle progression in cultured primary hepatocytes in the absence of mitogen or in the presence of growth inhibitors. Furthermore, transfection of hepatocytes in vivo with cyclin E and skp2 promoted abundant hepatocyte replication and hyperplasia of the liver. These studies confirm that transfection with a small number of genes can trigger proliferation of quiescent hepatocytes in vivo, and suggest that therapies to enhance liver regeneration by targeting cell cycle control genes may be feasible.

Effects of retinoic acid on proliferation, phenotype and expression of cyclin-dependent kinase inhibitors in TGF-beta1-stimulated rat hepatic stellate cells

AIM:To study the molecular mechanisms of retinoic acid (RA)on prolix-feration and expression of cyclin-dependent kinase inhibitors (CKI), i.e.p16, p21 and p27 in cultured rat hepatic stellate cells (HSC) stimulated with transforming growth factor beta 1 (TGF-beta1).METHODS:HSC were isolated from healthy rat livers and cultured.After stimulated with 1mg/L TGF-beta1, subcultured HSC were treated with or without 1nmol/L RA. MTT assay, immunocytochemistry (ICC) for p16, p21, p27 and alpha-smooth muscle actin (alpha-SMA) protein, in situ hybridization (ISH) for retinoic acid receptor beta 2 (RAR-beta2) and p16, p21 and p27 mRNA and quantitative image analysis (partially) were performed.RESULTS:inhibited HSC proliferation (41.50%,P<0.05),decreased the protein level of alpha-SMA (55.09%, P<0.05), and induced HSC to express RAR-beta2 mRNA. In addition, RA increased the protein level of p16 (218.75%, P <0.05) and induced p21 protein expression; meanwhile, p27 was undetectable by ICC in both control and RA-treated HSC. However, RA had no influence on the mRNA levels of p16, p21 or p27 as determined by ISH.CONCLUSION:Up-regulation of p16 and p21 on post-transcriptional level may contribute, in part, to RA inhibition of TGF-beta 1-initiated rat HSC activation in vitro.

Hyperstimulation with interleukin 6 inhibits cell cycle progression after hepatectomy in mice

Interleukin 6 (IL-6) is an important mediator of hepatocyte proliferation after hepatectomy. However, elevated IL-6 levels are found in patients with chronic liver disease. Therefore, it is unclear if hyperstimulation with IL-6 may have an influence on liver regeneration. We investigated whether a strong activation of IL-6-dependent pathways may change the course of hepatocyte proliferation after hepatectomy. Transgenic mice overexpressing the human soluble IL-6 receptor/gp80 (hsgp80) in hepatocytes were stimulated with or without hepatectomy with human IL-6 (hIL-6). Nuclear extracts were prepared and activation of gp130-dependent pathways was studied by Western blot and gel shift experiments. Cell cycle progression of hepatocytes after hepatectomy was investigated by monitoring cell cycle-specific factors. hIL-6 strongly activates Stat3 for more than 48 hours in human soluble hsgp80 transgenic mice. In contrast, no major differences were evident in the regulation of the Ras/MAP kinase pathway compared with wild-type (wt) mice. Also when hsgp80 mice were stimulated with hIL-6 3 hours before hepatectomy Stat3 is activated for more than 72 hours, whereas in unstimulated mice this event is restricted to the early hours. Strong activation of Stat3 resulted in a delay and inhibition of hepatocyte proliferation as measured by 5-bromo-2'-deoxyuridine (BrdU) staining and Cyclin A and E expression. This observation directly correlates with the induction of the cell cycle inhibitor p21. In summary, strong IL-6-dependent activation of Stat3 before hepatectomy results in delay and inhibition of cell cycle progression after hepatectomy. Therefore our results suggest that hyperstimulation with IL-6 can inhibit liver regeneration.

Differential association of p21Cip1 and p27Kip1 with cyclin E-CDK2 during rat liver regeneration

BACKGROUND/AIMS

The cell cycle inhibitors p21Cip1 and p27Kip1 regulate liver regeneration by modulating the activity of cyclin-dependent kinases (CDKs). However, the specific role of these inhibitors in the regulation of CDK2 activity during liver regeneration remains unknown. The aim of this study was to examine the association of p21Cip1 and p27Kip1 with cyclin E-CDK2 and cyclin A-CDK2 complexes during rat liver regeneration and to correlate the association of both inhibitors with CDK2 activity.

METHODS

The association of p21Cip1 or p27Kip1 with cyclin E-CDK2 or cyclin A-CDK2 and the activities of these complexes were analyzed by immunoprecipitation of rat liver homogenates obtained at different times after a partial hepatectomy (PH), followed by Western blotting or kinase assays.

RESULTS

High amounts of p27Kip1 bound to cyclin E-CDK2 were observed during the first 13 h after PH, when CDK2 activity was very low. At 24 h, when CDK2 activity was maximal, the amount of bound-p27Kip1 decreased strongly. The amount of p21Cip1 bound to these complexes was low during the first 13 h but subsequently increased. No cyclin A-CDK2 complexes were found during the first 13 h after PH. At 24 h, complexes containing low levels of both inhibitors were detected and at 28 h, a significant increase in p21Cip1 and p27Kip1 associated with cyclin A-CDK2 was observed.

CONCLUSIONS

p27Kip1 acts as a brake on cyclin E-CDK2 activity during the first 13 h after a PH. Both p21Cip1 and p27Kip1 down-regulate cyclin A-CDK2 activity at 28 h after PH, after its maximal activation.