Oxidative stress induces actin-cytoskeletal and tight-junctional alterations in hepatocytes by a Ca2+ -dependent, PKC-mediated mechanism: protective effect of PKA.

Oxidative stress elevates Ca2+ and, presumably, activates Ca2+ -dependent PKCs. We analyzed the participation of Ca2+ -dependent PKCs in actin disorganization and tight-junctional impairment induced by the pro-oxidant tert-butylhydroperoxide (tBOOH) in isolated rat hepatocyte couplets. tBOOH (100 microM) augmented radical oxygen species (ROS), as indicated by increased lipid peroxidation (+217%, p < 0.05) and intracellular production of 2',7'-dichlorofluorescein (+36%, p < 0.05). Cytosolic Ca2+ and PKCalpha translocation to membrane, an indicator of PKCalpha activation, were also elevated by tBOOH (+100 and +79%, respectively, p < 0.05). tBOOH increased the number of couplets displaying membrane blebs (+278%, p < 0.001) and caused redistribution of F-actin. tBOOH induced tight-junctional impairment, as indicated by a reduction in the percentage of couplets retaining presecreted cholyllysylfluorescein in their canalicular vacuoles (-54%, p < 0.001). tBOOH induced redistribution of the tight-junctional-associated protein ZO-1. All these events were prevented by the panspecific PKC inhibitors H7 and staurosporine, the Ca2+ -dependent PKC inhibitor Gö6976, the intracellular Ca2+ chelator BAPTA/AM, and the PKA activator dibutyryl-cyclic AMP. Furthermore, PKC inhibition and PKA activation not only prevented but also fully reversed tBOOH-induced blebbing. Conversely, tBOOH-induced ROS formation and Ca2+ elevation remained unchanged. We conclude that ROS induce hepatocellular actin-cytoskeleton rearrangement and tight-junctional impairment by a PKC-mediated, Ca2+ -dependent mechanism, which is counteracted by PKA.

CD40 mediated human cholangiocyte apoptosis requires JAK2 dependent activation of STAT3 in addition to activation of JNK1/2 and ERK1/2.

CD40 is critically involved in Fas-mediated cholangiocyte apoptosis during liver inflammation, but the underlying signalling events are poorly understood. Our recent work implicated AP-1 in CD40-induced cholangiocyte apoptosis, but suggested involvement of other signalling pathways. Because STAT3 has been implicated in liver regeneration we investigated this signalling pathway during CD40 mediated cholangiocyte apoptosis. Western immunoblotting, electrophoretic mobility gel shift assays, In situ DNA end labelling and caspase-3 activity were used to investigate intracellular signalling and apoptosis in primary human cholangiocytes following CD40 activation. CD40-activation induced caspase-3 dependent cholangiocyte apoptosis and 3-fold increases in JNK/ERK phosphorylation (concomitant with increased AP-1 binding activity) and 4-fold increases in pSTAT3, which were sustained for up to 24 h. Protein levels of c-Jun, c-Fos and pSTAT3 confirmed the upregulation. Phosphorylation of p38 remained unchanged suggesting that this MAP kinase was not involved in CD40 mediated apoptosis. Increased JAK2 phosphorylation accompanied increased STAT3 phosphorylation after CD40 ligation. Cholangiocytes were also shown to express JAK1 and 3 which was phosphorylated following control stimulation with TNFalpha or IL2 respectively but not after CD40 ligation. JNK, ERK and JAK2 inhibitors partially abrogated apoptosis and when used in combination reduced it to basal levels. In conclusion, induction of CD40-mediated cholangiocyte apoptosis requires JAK2-mediated phosphorylation of STAT3 as well as sustained JNK1/2, ERK1/2 activation. This study demonstrates that STAT3 can function as a proapoptotic factor in primary human liver epithelial cells.

Transcriptional regulation of cyclin A2 by RASSF1A through the enhanced binding of p120E4F to the cyclin A2 promoter.

Recent advances in the study of RASSF1A, the candidate tumor suppressor gene, indicate a possible role of RASSF1A in cell cycle regulation; however, very little is known regarding molecular mechanisms underlying this control. Using small interfering RNA to knockdown endogenous RASSF1A in the breast tumor cell line HB2 and in the cervical cancer cell line HeLa, we identify that a key player in cell cycle progression, cyclin A2, is concomitantly increased at both protein and mRNA levels. In A549 clones stably expressing RASSF1A, cyclin A2 levels were diminished compared with vector control. A known transcriptional regulator of cyclin A2, p120(E4F) (a repressor of cyclin A2), has been shown previously by our group to interact with RASSF1A. We show that levels of p120(E4F) are not affected by RASSF1A small interfering RNA in HB2 and HeLa cells. However, electrophoretic mobility shift assays indicate that knockdown of endogenous RASSF1A in HB2 and HeLa cells leads to a reduction in the binding capacity of p120(E4F) to the cyclin A2 promoter, whereas in the A549 clone stably expressing RASSF1A the binding capacity is increased. These data are further corroborated in vitro by the luciferase assay and in vivo by chromatin immunoprecipitation experiments. Together, these data identify the cyclin A2 gene as a cellular target for RASSF1A through p120(E4F) and for the first time suggest a transcriptional mechanism for RASSF1A-dependent cell cycle regulation.

RASSF1A interacts with microtubule-associated proteins and modulates microtubule dynamics.

The candidate tumor suppressor gene RASSF1A is inactivated in many types of adult and childhood cancers. However, the mechanisms by which RASSF1A exerts its tumor suppressive functions have yet to be elucidated. To this end, we performed a yeast two-hybrid screen to identify novel RASSF1A-interacting proteins in a human brain cDNA library. Seventy percent of interacting clones had homology to microtubule-associated proteins, including MAP1B and VCY2IP1/C19ORF5. RASSF1A association with MAP1B and VCY2IP1/C19ORF5 was subsequently confirmed in mammalian cell lines. This suggested that RASSF1A may exert its tumor-suppressive functions through interaction with the microtubules. We demonstrate that RASSF1A associates with the microtubules, causing them to exist as hyperstabilized circular bundles. We found that two naturally occurring tumor-associated missense substitutions in the RASSF1A coding region, C65R and R257Q, perturb the association of RASSF1A with the microtubules. The C65R and R257Q in addition to VCY2IP1/C19ORF5 showed reduced ability to induce microtubule acetylation and were unable to protect the microtubules against the depolymerizing action of nocodazole. In addition, wild-type RASSF1A but not the C65R or the R257Q is able to block DNA synthesis. Our data identify a role for RASSF1A in the regulation of microtubules and cell cycle dynamics that could be part of the mechanism(s) by which RASSF1A exerts its growth inhibition on cancer cells.

Identification of the E1A-regulated transcription factor p120 E4F as an interacting partner of the RASSF1A candidate tumor suppressor gene.

Epigenetic inactivation of the candidate tumor suppressor gene RASSF1A is a frequent and critical event in the pathogenesis of many human cancers. The RASSF1A protein contains a Ras association domain, suggesting a role in Ras-like signaling pathways, and has also been implicated in cell cycle progression. However, the preliminary data suggests that the RASSF1A gene product is likely to have multiple functions. To identify novel RASSF1A functions, we have sought to identify interacting proteins by yeast two-hybrid analysis in a human brain cDNA library. We identified the E1A-regulated transcription factor p120(E4F) as a RASSF1A interacting partner in yeast and mammalian cells, and demonstrated that RASSF1A protein and p120(E4F) form a complex in vivo. The interaction between RASSF1A and p120(E4F) was confirmed by both in vitro and in vivo pull downs and coimmunoprecipitation assays. In addition, specific inactivation of RASSF1A by short interfering RNA disrupts binding of RASSF1A to p120(E4F) in coimmunoprecipitation assays. In addition, we demonstrated enhanced G(1) cell cycle arrest and S phase inhibition by propidium iodide staining of p120(E4F) in the presence of RASSF1A. As p120(E4F) has been reported previously to interact with p14ARF, retinoblastoma, and p53, these findings provide an important link between the function of RASSF1A and other major human tumor suppressor genes.

Identification of novel gene expression targets for the Ras association domain family 1 (RASSF1A) tumor suppressor gene in non-small cell lung cancer and neuroblastoma.

RASSF1A is a recently identified 3p21.3 tumor suppressor gene. The high frequency of epigenetic inactivation of this gene in a wide range of human sporadic cancers including non-small cell lung cancer (NSCLC) and neuroblastoma suggests that RASSF1A inactivation is important for tumor development. Although little is known about the function of RASSF1A, preliminary data suggests that it may have multiple functions. To gain insight into RASSF1A functions in an unbiased manner, we have characterized the expression profile of a lung cancer cell line (A549) transfected with RASSF1A. Initially we demonstrated that transient expression of RASSF1A into the NSCLC cell line A549 induced G(1) cell cycle arrest, as measured by propidium iodide staining. Furthermore, annexin-V staining showed that RASSF1A-expressing cells had an increased sensitivity to staurosporine-induced apoptosis. We then screened a cDNA microarray containing more than 6000 probes to identify genes differentially regulated by RASSF1A. Sixty-six genes showed at least a 2-fold change in expression. Among these were many genes with relevance to tumorigenesis involved in transcription, cytoskeleton, signaling, cell cycle, cell adhesion, and apoptosis. For 22 genes we confirmed the microarray results by real-time RT-PCR and/or Northern blotting. In silico, we were able to confirm the majority of these genes in other NSCLC cell lines using published data on gene expression profiles. Furthermore, we confirmed 10 genes at the RNA level in two neuroblastoma cell lines, indicating that these RASSF1A target genes have relevance in non-lung cell backgrounds. Protein analysis of six genes (ETS2, Cyclin D3, CDH2, DAPK1, TXN, and CTSL) showed that the changes induced by RASSF1A at the RNA level correlated with changes in protein expression in both non-small cell lung cancer and neuroblastoma cell lines. Finally, we have used a transient assay to demonstrate the induction of CDH2 and TGM2 by RASSF1A in NSCLC cell lines. We have identified several novel targets for RASSF1A tumor suppressor gene both at the RNA and the protein levels in two different cellular backgrounds. The identified targets are involved in diverse cellular processes; this should help toward understanding mechanisms that contribute to RASSF1A biological activity.

Differential induction of nuclear factor-kappaB and activator protein-1 activity after CD40 ligation is associated with primary human hepatocyte apoptosis or intrahepatic endothelial cell proliferation.

CD40, a tumor necrosis factor receptor superfamily member, is up-regulated on intraheptatic endothelial cells (IHEC) and epithelial cells during inflammatory liver disease, and there is evidence that the functional outcome of CD40 ligation differs between cell types. Ligation of CD40 on cholangiocytes or hepatocytes results in induction of Fas-mediated apoptosis, whereas ligation of IHEC CD40 leads to enhanced chemokine secretion and adhesion molecule expression. We now report that differential activation of two transcription factors, nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1), in primary human hepatocytes or IHEC, is associated with and may explain, in part, the different responses of these cell types to CD40 ligation. CD40 ligation induced a rise in NF-kappaB activity in hepatocytes,which peaked at 2 h and returned to baseline by 24 h; however, IHEC CD40 ligation resulted in a sustained up-regulation of NF-kappaB (>24 h). In hepatocytes, CD40 ligation led to sustained up-regulation of AP-1 activity >24 h associated with increased protein levels of RelA (p65), c-Jun, and c-Fos, whereas no induction of AP-1 activity was observed in IHECs. Analysis of mitogen-activated protein kinase phosphorylation (phospho-extracellular signal-regulated kinase 1/2 and phospho-c-Jun NH(2)-terminal kinase 1/2) and expression of inhibitor kappaBalpha were entirely consistent, and thus confirmed the profiles of NF-kappaB and AP-1 signaling and the effects of the selective inhibitors assessed using electrophoretic mobility shift assay or Western immunoblotting. CD40 ligation resulted in induction of apoptosis in hepatocytes after 24 h, but on IHECs, CD40 ligation resulted in proliferation. Inhibition of (CD40-mediated) NF-kappaB activation prevented IHEC proliferation and led to induction of apoptosis. Selective extracellular signal-regulated kinase and c-Jun NH(2)-terminal kinase inhibitors reduced levels of apoptosis in (CD40-stimulated) hepatocytes by approximately 50%. We conclude that differential activation of these two transcription factors in response to CD40 ligation is associated with differences in cell fate. Transient activation of NF-kappaB and sustained AP-1 activation is associated with apoptosis in hepatocytes, whereas prolonged NF-kappaB activation and a lack of AP-1 activation in IHECs result in proliferation.

Hepatic expression of secondary lymphoid chemokine (CCL21) promotes the development of portal-associated lymphoid tissue in chronic inflammatory liver disease.

The chronic inflammatory liver disease primary sclerosing cholangitis (PSC) is associated with portal inflammation and the development of neolymphoid tissue in the liver. More than 70% of patients with PSC have a history of inflammatory bowel disease and we have previously reported that mucosal addressin cell adhesion molecule-1 is induced on dendritic cells and portal vascular endothelium in PSC. We now show that the lymph node-associated chemokine, CCL21 or secondary lymphoid chemokine, is also strongly up-regulated on CD34(+) vascular endothelium in portal associated lymphoid tissue in PSC. In contrast, CCL21 is absent from LYVE-1(+) lymphatic vessel endothelium. Intrahepatic lymphocytes in PSC include a population of CCR7(+) T cells only half of which express CD45RA and which respond to CCL21 in migration assays. The expression of CCL21 in association with mucosal addressin cell adhesion molecule-1 in portal tracts in PSC may promote the recruitment and retention of CCR7(+) mucosal lymphocytes leading to the establishment of chronic portal inflammation and the expanded portal-associated lymphoid tissue. This study provides further evidence for the existence of portal-associated lymphoid tissue and is the first evidence that ectopic CCL21 is associated with lymphoid neogenesis in human inflammatory disease.