The role of collagen structure in mitogen stimulation of ERK, cyclin D1 expression, and G1-S progression in rat hepatocytes

Stimulation of transforming growth factor-beta-1 and contact with type I collagen cooperatively facilitate irreversible transdifferentiation in proximal tubular cells

Background

By transdifferentiation, proximal tubular cells (PTC) have been considered as a source of interstitial myofibroblasts. We examined the combined effect of transforming growth factor-beta-1 (TGF-β1) stimulation and contact with type I collagen on PTC transdifferentiation.

Methods

Human kidney-2 cells were grown on type I substratum with the concurrent stimulation of TGF-β1.

Results

Following addition of TGF-β1, cells acquired an elongated fibroblastic appearance and an increase in α-smooth muscle actin (α-SMA) expression, a myofibroblastic marker. Upon addition of TGF-β1, E-cadherin expression, an epithelial marker, was reduced, while cytokeratin expression, another epithelial marker, remained unaltered. Following removal of TGF-β1, PTC regained an epithelial appearance and E-cadherin expression reverted to the unstimulated level, suggesting incomplete and reversible transdifferentiation. Addition of TGF-β1 to cells grown on type I collagen demonstrated a cooperatively increased α-SMA expression and decreased E-cadherin and cytokeratin expressions, suggesting more complete transdifferentiation. Co-stimulation of TGF-β1 and contact with type I collagen led to a stable cell phenotype and persistently decreased E-cadherin, which was not reversed upon removal of TGF-β1, indicating irreversible transdifferentiation. Addition of TGF-β1 or type I collagen caused a 4-fold increase in migratory cell number as compared to the control, whereas addition of both TGF-β1 and type I collagen led to an 11-fold increase.

Conclusions

TGF-β1 alone results in a reversible and incomplete transdifferentiation. The combination of TGF-β1 and exposure to type I collagen leads to an irreversible and complete PTC transdifferentiation.

Bone marrow stem cell therapy for liver disease

Liver disease is a rising cause of mortality and morbidity, and treatment options remain limited. Liver transplantation is curative but limited by donor organ availability, operative risk and long-term complications. The contribution of bone marrow (BM)-derived stem cells to tissue regeneration has been recognised and there is considerable interest in the potential benefits of BM stem cells in patients with liver disease. In chronic liver disease, deposition of fibrous scar tissue inhibits hepatocyte proliferation and leads to portal hypertension. Although initial reports had suggested transdifferentiation of stem cells into hepatocytes, the beneficial effects of BM stem cells are more likely derived from the ability to breakdown scar tissue and stimulate hepatocyte proliferation. Studies in animal models have yielded promising results, although the exact mechanisms and cell type responsible have yet to be determined. Small-scale clinical studies have quickly followed and, although primarily designed to examine safety and feasibility of this approach, have reported improvements in liver function in treated patients. Well-designed, controlled studies are required to fully determine the benefits of BM stem cell therapy.

Inhibitory role of Smad7 in hepatocarcinogenesis in mice and in vitro

Smad7 is a principal inhibitor of the TGFβ-Smad signalling pathway. We have investigated the functional significance of Smad7 in hepatocellular carcinoma (HCC). Smad7 knockout (KO) and wild-type (WT) mice were injected with diethylnitrosamine (DEN) to induce HCC. The effects of Smad7 on cellular features were examined in HCC cells, using a Smad7 over-expression or deletion approach. Signalling pathway components modulated by Smad7 in HCC were evaluated using luciferase reporter assay and co-immunoprecipitation. Smad7 was down-regulated in human HCCs compared with the adjacent normal tissues (p < 0.001). Smad7 KO mice were more susceptible to DEN-induced HCC than WT mice (78% versus 22%, p < 0.05). HCCs from KO mice displayed a greater proliferation activity (p < 0.05) and a reduced apoptotic index compared with WT littermates (p < 0.05). Deletion of Smad7 promoted cell proliferation in primary cultured HCC cells. In addition, over-expression of Smad7 in HCC cell lines markedly suppressed cell growth (p < 0.0001) and colony formation (p < 0.01). Cell cycle analysis revealed an increase in the G1 phase and a reduction in the S-phase populations, accompanied by up-regulation of p27(Kip1) and down-regulation of cyclin D1. Smad7 increased cell apoptosis (p < 0.01) by mediating an intrinsic [caspase-9, caspase-3 and poly(ADP-ribose) polymerase] apoptotic pathway. Moreover, Smad7 inhibited NF-κB signalling by interacting with TAB2, an upstream activator of NF-κB, and inhibited TGFβ signalling by suppressing phosphorylation of Smad3. In conclusion, loss of Smad7 enhances susceptibility to HCC. Smad7 suppresses HCC cell growth by inhibiting proliferation and G1 -S phase transition and inducing apoptosis through attenuation of NF-κB and TGFβ signalling. Smad7 acts as a potential tumour suppressor in liver.

Impact of alginate composition: from bead mechanical properties to encapsulated HepG2/C3A cell activities for in vivo implantation

Recently, interest has focused on hepatocytes' implantation to provide end stage liver failure patients with a temporary support until spontaneous recovery or a suitable donor becomes available. To avoid cell damage and use of an immunosuppressive treatment, hepatic cells could be implanted after encapsulation in a porous biomaterial of bead or capsule shape. The aim of this study was to compare the production and the physical properties of the beads, together with some hepatic cell functions, resulting from the use of different material combinations for cell microencapsulation: alginate alone or combined with type I collagen with or without poly-L-lysine and alginate coatings. Collagen and poly-L-lysine increased the bead mechanical resistance but lowered the mass transfer kinetics of vitamin B12. Proliferation of encapsulated HepG2/C3A cells was shown to be improved in alginate-collagen beads. Finally, when the beads were subcutaneously implanted in mice, the inflammatory response was reduced in the case of alginate mixed with collagen. This in vitro and in vivo study clearly outlines, based on a systematic comparison, the necessity of compromising between material physical properties (mechanical stability and porosity) and cell behavior (viability, proliferation, functionalities) to define optima hepatic cell microencapsulation conditions before implantation.

FAK regulates Cdk2 in EGF-stimulated primary cultures of hepatocytes

In this study, we report a novel role of FAK as a regulator of Cdk2 in anchorage-dependent primary cultured hepatocytes. In response to EGF, we found that S-phase entry was reduced upon FAK inhibition. This correlated with decreased protein expression and nuclear accumulation of the G1/S-phase regulator Cdk2. Further, nuclear accumulation of the Cdk2 partner cyclinE, was reduced, but not its protein level. Also, protein levels of Cdk2 were inversely linked with increased expression of the Cdk2 inhibitor p27, known to be degraded in a Cdk2-dependent manner. Also, cyclinD1 was regulated by FAK, but to a lesser extent than Cdk2. To assess the mechanism in which FAK mediates Cdk2-regulation, FAK mutants were used: FAKY397F, mutated at its integrin-regulated site, and two others mutated at docking sites for Grb2-ERK-activation (FAKY925F) and for p130Cas-Rac1-activation (FAKY861F). All three sites were central for EGF-induced ERK-activity and Cdk2 expression. In addition, FAK was important for HGF-mediated proliferation, suggesting a general mechanism for anchorage-dependent growth. Moreover, growth factor-induced cell spreading, but not survival, required FAK. Hence, integrins and growth factors cooperate in anchorage-dependent signaling events leading to proliferation and motility. In conclusion, our data suggest that FAK acts as a central coordinator of integrin and growth factor-mediated S-phase entry by its ability to regulate Cdk2.

Benzo[a]pyrene-induced transcriptomic responses in primary hepatocytes and in vivo liver: toxicokinetics is essential for in vivo-in vitro comparisons

The traditional 2-year cancer bioassay needs replacement by more cost-effective and predictive tests. The use of toxicogenomics in an in vitro system may provide a more high-throughput method to investigate early alterations induced by carcinogens. Recently, the differential gene expression response in wild-type and cancer-prone Xpa (-/-) p53 (+/-) primary mouse hepatocytes after exposure to benzo[a]pyrene (B[a]P) revealed downregulation of cancer-related pathways in Xpa (-/-) p53 (+/-) hepatocytes only. Here, we investigated pathway regulation upon in vivo B[a]P exposure of wild-type and Xpa (-/-) p53 (+/-) mice. In vivo transcriptomics analysis revealed a limited gene expression response in mouse livers, but with a significant induction of DNA replication and apoptotic/anti-apoptotic cellular responses in Xpa (-/-) p53 (+/-) livers only. In order to be able to make a meaningful in vivo-in vitro comparison we estimated internal in vivo B[a]P concentrations using DNA adduct levels and physiologically based kinetic modeling. Based on these results, the in vitro concentration that corresponded best with the internal in vivo dose was chosen. Comparison of in vivo and in vitro data demonstrated similarities in transcriptomics response: xenobiotic metabolism, lipid metabolism and oxidative stress. However, we were unable to detect cancer-related pathways in either wild-type or Xpa (-/-) p53 (+/-) exposed livers, which were previously found to be induced by B[a]P in Xpa (-/-) p53 (+/-) primary hepatocytes. In conclusion, we showed parallels in gene expression responses between livers and primary hepatocytes upon exposure to equivalent concentrations of B[a]P. Furthermore, we recommend considering toxicokinetics when modeling a complex in vivo endpoint with in vitro models.

Proliferating primary hepatocytes from the pUR288 lacZ plasmid mouse are valuable tools for genotoxicity assessment in vitro

Safety assessments of substances with regard to genotoxicity are generally based on a combination of in vitro and in vivo tests. These tests are performed according to a (tiered) test strategy whereby a positive result in vitro usually triggers further testing in vivo. A low specificity and high frequency of irrelevant positive results associated with most in vitro mammalian cell genotoxicity assays necessitates the design and validation of suitable alternatives. As such, we examined the feasibility of culturing primary hepatocytes from the pUR288 lacZ reporter mouse, and moreover, using established cultures to reliably assess genotoxic activity in vitro. Initial studies characterizing the metabolic capacity of proliferating lacZ primary hepatocytes indicated that these cells retained at least some activities important for xenobiotic metabolism: cytochrome P450 1A1 enzyme activities were markedly increased in the hepatocytes after exposure to benzo[a]pyrene, and also UDP-glucuronosyl transferase and glutathione-S-transferase activities, both Phase II enzymes, were detected. Increasing levels of phosphorylated p53 at residue serine 389 after ultraviolet treatment indicated a properly functioning p53, one of the criteria for an effective new test system. Four genotoxic substances with different mechanisms of genotoxicity, i.e., benzo[a]pyrene, bleomycin, etoposide, and cyclophosphamide, were tested in the lacZ rescue assay. For etoposide and cyclophosphamide, the induction of mutant colonies was rather low. Exposure to benzo[a]pyrene and bleomycin, however, yielded a clear concentration-dependent induction of the lacZ mutant frequency. Based on our preliminary observations, proliferating lacZ primary hepatocytes are a promising new tool for the assessment of genotoxic hazard.

Biochemical role of the collagen-rich tumour microenvironment in pancreatic cancer progression

PDAC (pancreatic ductal adenocarcinoma) is among the most deadly of human malignances. A hallmark of the disease is a pronounced collagen-rich fibrotic extracellular matrix known as the desmoplastic reaction. Intriguingly, it is precisely these areas of fibrosis in which human PDAC tumours demonstrate increased expression of a key collagenase, MT1-MMP [membrane-type 1 MMP (matrix metalloproteinase); also known as MMP-14]. Furthermore, a cytokine known to mediate fibrosis in vivo, TGF-β1 (transforming growth factor-β1), is up-regulated in human PDAC tumours and can promote MT1-MMP expression. In the present review, we examine the regulation of PDAC progression through the interplay between type I collagen (the most common extracellular matrix present in human PDAC tumours), MT1-MMP and TGF-β1. Specifically, we examine the way in which signalling events through these pathways mediates invasion, regulates microRNAs and contributes to chemoresistance.

The MAPK MEK1/2-ERK1/2 Pathway and Its Implication in Hepatocyte Cell Cycle Control

Primary cultures of hepatocytes are powerful models in studying the sequence of events that are necessary for cell progression from a G0-like state to S phase. The models mimic the physiological process of hepatic regeneration after liver injury or partial hepatectomy. Many reports suggest that the mitogen-activated protein kinase (MAPK) ERK1/2 can support hepatocyte proliferation in vitro and in vivo and the MEK/ERK cascade acts as an essential element in hepatocyte responses induced by the EGF. Moreover, its disregulation has been associated with the promotion of tumor cell growth of a variety of tumors, including hepatocellular carcinoma. Whereas the strict specificity of action of ERK1 and ERK2 is still debated, the MAPKs may have specific biological functions under certain contexts and according to the differentiation status of the cells, notably hepatocytes. In this paper, we will focus on MEK1/2-ERK1/2 activations and roles in normal rodent hepatocytes in vitro and in vivo after partial hepatectomy and in human hepatocarcinoma cells. The possible specificity of ERK1 and ERK2 in normal and transformed hepatocyte will be discussed in regard to other differentiated and undifferentiated cellular models.

The complexity of ERK1 and ERK2 MAPKs in multiple hepatocyte fate responses

Recent reports suggest that extracellular signal-regulated kinase (ERK1) and ERK2 mitogen-activated protein kinases (MAPK) may direct specific biological functions under certain contexts. In this study, we investigated the role of early and sustained epidermal growth factor (EGF) stimulation on long-term hepatocyte differentiation and the possible role of ERK1 and ERK2 in this process. We demonstrate a long-term survival and an elevated level of differentiation up to 3 weeks. The differentiation state of hepatocytes is supported by sustained expression of aldolase B, albumin, and the detoxifying enzymes CYP1A2, 2B2, and 3A23. Similarly to freshly isolated cells, cultured hepatocytes also retain the ability to respond to 3-methylcholanthrene (3MC) and phenobarbital (PB), two known CYP inducers. In addition, we show evidence that continuous MAPK/ERK kinase (MEK) inhibition enhances the level of differentiation. Using RNA interference approaches against ERK1 and ERK2, we demonstrate that this effect requires both ERK1 and ERK2 activity, whereas the specific ERK1 knockdown promotes cell survival and the specific ERK2 knockdown regulates cell proliferation. In conclusion, we demonstrate that early and sustained EGF stimulation greatly extends long-term hepatocyte survival and differentiation, and that inhibition of the ERK1/2 MAPK pathway potentiates these pro-survival/pro-differentiation phenotypes. We clearly attest that specific ERK1 and ERK2 MAPKs determine hepatocyte survival and proliferation, respectively, whereas dual inhibition is required to stabilize a highly differentiated state.

Deregulation of cancer-related pathways in primary hepatocytes derived from DNA repair-deficient Xpa-/-p53+/- mice upon exposure to benzo[a]pyrene

The current method to predict carcinogenicity of chemicals or drugs is the chronic 2-year rodent bioassay, which has disadvantages in duration, animal use, and specificity. An attractive alternative is the DNA repair-deficient Xpa(-/-)p53(+/-) mouse model that is sensitive to both genotoxic and nongenotoxic carcinogens. A next step in alternative carcinogenicity testing is the development of reliable in vitro systems. We investigated the use of primary hepatocytes, isolated from wild-type (WT) and Xpa(-/-)p53(+/-) mice, in combination with transcriptome analyses for their usefulness to predict carcinogenic features of compounds. As a proof of principle, we studied the response of hepatocytes to the genotoxic carcinogen benzo[a]pyrene (B[a]P). Upon treatment, both WT and Xpa(-/-)p53(+/-) hepatocytes appeared to be metabolically active. However, Xpa(-/-)p53(+/-) hepatocytes were more sensitive than WT hepatocytes to B[a]P treatment in terms of cell survival. In B[a]P-treated WT hepatocytes, DNA repair and cell cycle control genes were transcriptionally activated. Xpa(-/-)p53(+/-) hepatocytes were more responsive to B[a]P exposure, resulting in the downregulation of cancer-related pathways. Deregulation of mitogen-activated protein kinase signaling seems to play an essential role in this and might be the underlying reason for the increased susceptibility of Xpa(-/-)p53(+/-) mice toward carcinogens. Our conclusion is that primary hepatocytes combined with transcriptomics are promising to identify the carcinogenic features of chemicals. Furthermore, these cells seem suitable to gain further insight into the molecular mechanisms of the increased sensitivity of Xpa(-/-)p53(+/-) mice toward both genotoxic and nongenotoxic carcinogens.

Syndecan-1 and -4 differentially regulate oncogenic K-ras dependent cell invasion into collagen through α2β1 integrin and MT1-MMP

Syndecans function as co-receptors for integrins on different matrixes. Recently, syndecan-1 has been shown to be important for α2β1 integrin-mediated adhesion to collagen in tumor cells by regulating cell adhesion and migration on two-dimensional collagen. However, the function of syndecans in supporting α2β1 integrin interactions with three-dimensional (3D) collagen is less well studied. Using loss-of-function and overexpression experiments we show that in 3D collagen syndecan-4 supports α2β1-mediated collagen matrix contraction. Cell invasion through type I collagen containing 3D extracellular matrix (ECM) is driven by α2β1 integrin and membrane type-1 matrix metalloproteinase (MT1-MMP). Here we show that mutational activation of K-ras correlates with increased expression of α2β1 integrin, MT1-MMP, syndecan-1, and syndecan-4. While K-ras-induced α2β1 integrin and MT1-MMP are positive regulators of invasion, silencing and overexpression of syndecans demonstrate that these proteins inhibit cell invasion into collagen. Taken together, these data demonstrate the existence of a complex interplay between integrin α2β1, MT1-MMP, and syndecans in the invasion of K-ras mutant cells in 3D collagen that may represent a mechanism by which tumor cells become more invasive and metastatic.

The elements of human cyclin D1 promoter and regulation involved

Cyclin D1 is a cell cycle machine, a sensor of extracellular signals and plays an important role in G1-S phase progression. The human cyclin D1 promoter contains multiple transcription factor binding sites such as AP-1, NF-қB, E2F, Oct-1, and so on. The extracellular signals functions through the signal transduction pathways converging at the binding sites to active or inhibit the promoter activity and regulate the cell cycle progression. Different signal transduction pathways regulate the promoter at different time to get the correct cell cycle switch. Disorder regulation or special extracellular stimuli can result in cell cycle out of control through the promoter activity regulation. Epigenetic modifications such as DNA methylation and histone acetylation may involved in cyclin D1 transcriptional regulation.

The significance of CD14+ monocytes in peripheral blood stem cells for the treatment of rat liver cirrhosis

BACKGROUND AIMS

Circulating monocytes have been exploited as an important progenitor cell resource for hepatocytes in vitro and are instrumental in the removal of fibrosis. We investigated the significance of monocytes in peripheral blood stem cells (PBSC) for the treatment of liver cirrhosis.

METHODS

Rat CD14+ monocytes in PBSC were mobilized with granulocyte-colony-stimulating factor (G-CSF) and harvested by magnetic cell sorting (MACS). Female rats with carbon tetrachloride (CCl₄-induced liver cirrhosis were injected CM-DiI-labeled monocytes, CD14⁻ cells (1 x 10⁷ cells/rat) or saline via the portal vein.

RESULTS

Rat CD14+ and CD11b+ monocytes in PBSC were partly positive for CD34, CD45, CD44, Oct3/4 and Sox2, suggesting monocytes with progenitor capacity. Compared with CD14⁻ cell-infused and saline-injected rats, rats undergoing monocyte transplantation showed a gradually increased serum albumin level and decreased portal vein pressure, resulting in a significantly improved survival rate. Meanwhile, monocyte transplantation apparently attenuated liver fibrosis by analysis for fibronectin, α2-(1)-procollagen, α-smooth muscle aorta (SMA) and transforming growth factor (TGF)-β. Transplanted monocytes mainly clustered in periportal areas of liver, in which 1.8% cells expressed hepatocyte marker albumin and CK18. The expression level of hepatocyte growth factor (HGF), TGF-α, extracellular matrix (EGF) and vascular endothelial growth factor (VEGF) increased, while monocyte transplantation enhanced hepatocyte proliferation. On the other hand, the activities and expression of matrix metalloproteinases (MMP) increased while tissue inhibitor of metalloproteinase (TIMP)-1 expression significantly reduced in monocyte-transplanted livers. Some transplanted monocytes expressed MMP-9 and -13.

CONCLUSIONS

The data suggest that CD14+ monocytes in PBSC contribute to hepatocyte regeneration and extracellular matrix (ECM) remodeling in rat liver cirrhosis much more than CD14⁻ cells, and might offer a therapeutic alternative for patients with liver cirrhosis.

Osteogenic differentiation of marrow stromal cells on random and aligned electrospun poly(L-lactide) nanofibers

The fibrillar structure and sub-micron diameter of electrospun nanofibers can be used to reproduce the morphology and structure of the natural extracellular matrix (ECM). The objective of this work was to investigate the effect of fiber alignment on osteogenic differentiation of bone marrow stromal (BMS) cells. Random and aligned poly(L-lactide) (PLLA) nanofibers were produced by collecting the spun fibers on a stationary plate and a rotating wheel, respectively, as the ground electrode. Morphology and alignment of the BMS cells seeded on the fibers were characterized by SEM. The effect of fiber orientation on osteogenic differentiation of BMS cells was determined by measuring alkaline phosphatase (ALPase) activity, calcium content, and mRNA expression levels of osteogenic markers. There was a strong correlation between the fiber and cell distributions for the random (p=0.16) and aligned (p=0.81) fibers. Percent deviation from ideal randomness (PDIR) values indicated that cells seeded on the random fibers (PDIR=6.5%) were likely to be distributed randomly in all directions while cells seeded on the aligned fibers (PDIR=86%) were highly likely to be aligned with the direction of fibers. BMS cell seeded on random and aligned fibers had similar cell count and ALPase activity with incubation time, but the calcium content on aligned fibers was significantly higher after 21 days compared to that of random fibers (p=0.003). Osteopontin (OP) and osteocalcin (OC) expression levels of BMS cells on fibers increased with incubation time. However, there was no difference between the expression levels of OP and OC on aligned vs. random fibers. The results indicate that BMS cells aligned in the direction of PLLA fibers to form long cell extensions, and fiber orientation affected the extent of mineralization, but it had no effect on cell proliferation or mRNA expression of osteogenic markers.

High throughput assembly of spatially controlled 3D cell clusters on a micro/nanoplatform

Guided assembly of microscale tissue subunits (i.e. 3D cell clusters/aggregates) has found applications in cell therapy/tissue engineering, cell and developmental biology, and drug discovery. As cluster size and geometry are known to influence cellular responses, the ability to spatially control cluster formation in a high throughput manner could be advantageous for many biomedical applications. In this work, a micro- and nanofabricated platform was developed for this purpose, consisting of a soft-lithographically fabricated array of through-thickness microwells structurally bonded to a sheet of electrospun fibers. The microwells and fibers were manufactured from several polymers of biomedical interest. Human hepatocytes were used as model cells to demonstrate the ability of the platform to allow controlled cluster formation. In addition, the ability of the device to support studies on semi-controlled heterotypic interactions was demonstrated by co-culturing hepatocytes and fibroblasts. Preliminary experiments with other cells of interest (pancreatic cells, embryonic stem cells, and cardiomyocytes) were also conducted. Our platform possesses several advantages over previously developed microwell arrays: a more in vivo-like topographical stimulation of cells; better nutrient/waste exchange through the underlying nanofiber mat; and easy integration into standard two-chamber cell culture well systems.

Extracellular matrix modulates sensitivity of hepatocytes to fibroblastoid dedifferentiation and transforming growth factor beta-induced apoptosis

Hepatocytes in culture are a valuable tool to investigate mechanisms involved in the response of the liver to cytokines. However, it is well established that hepatocytes cultured on monolayers of dried stiff collagen dedifferentiate, losing specialized liver functions. In this study, we show that hepatocyte dedifferentiation is a reversible consequence of a specific signaling network constellation triggered by the extracellular matrix. A dried stiff collagen activates focal adhesion kinase (FAK) via Src, leading to activation of the Akt and extracellular signal-regulated kinase (ERK) 1/2 pathways. Akt causes resistance to transforming growth factor beta (TGF-beta)-induced apoptosis by antagonizing p38, whereas ERK1/2 signaling opens the route to epithelial-mesenchymal transition (EMT). Apoptosis resistance is reversible by inhibiting Akt or Src, and EMT can be abrogated by blocking the ERK1/2 pathway. In contrast to stiff collagen, a softer collagen gel does not activate FAK, keeping the hepatocytes in a state where they remain sensitive to TGF-beta-induced apoptosis and do not undergo EMT. In this culture system, inhibition of p38 as well as overexpression of constitutively active Akt causes apoptosis resistance, whereas constitutively active Ras induces EMT. Finally, we show that matrix-induced EMT is reversible by replating cells from dried stiff to soft gel collagen. Our results demonstrate that hepatocyte dedifferentiation in vitro is an active process driven by FAK-mediated Akt and ERK1/2 signaling. This leads to similar functional and morphological alterations as observed for regenerating hepatocytes in vivo and is reversible when Akt and/or ERK1/2 signaling pathways are antagonized.

CONCLUSION

Hepatocytes can exist in a differentiated and a dedifferentiated state that are reversible and can be switched by manipulating the responsible key factors of the signaling network.

Critical review of clinical trials of bone marrow stem cells in liver disease

Morbidity and mortality from cirrhosis is increasing rapidly in the Western world. Currently the only effective treatment is liver transplantation, an increasingly limited and expensive resource. Consequently, there has been great hope that stem cells may offer new therapeutic approaches in the management of liver disease. In this review we critically appraise the 11 published clinical studies of bone marrow stem cells in liver disease, and focus on the unresolved issues regarding their role. We outline the different mechanisms by which stem cells may impact on liver disease, as well as highlight the importance of the type of stem cell chosen. There are multiple different stem cell populations that have, in rodent studies, been shown to have differing effects on liver regeneration and fibrogenesis/degradation. Thus, choice of cell should reflect the desired or expected mechanism of action. The importance, and methods, of studying the fate of stem cells infused in clinical studies is emphasized as we seek to translate observations in rodents into the clinical setting. Finally, we discuss which cohorts of patients with liver disease would benefit from stem cell therapy, as well as establish minimum criteria for future clinical trials of stem cells.

Syndecan-1 supports integrin alpha2beta1-mediated adhesion to collagen

Several different receptor molecules act in concert to regulate cell adhesion. Among these are cell-surface proteoglycans and integrins, which collaborate extensively in mediating binding of cells to extracellular matrix molecules fibronectin and vitronectin. However, very little is known about possible functional synergism between proteoglycans and integrins during adhesion of cells to collagen, although collagen is the most abundant protein in the human body. Here we show that cell-surface heparan sulphate proteoglycans (HSPGs) support integrin alpha2beta1-mediated adhesion to collagen. Cells made devoid of HSPGs either by genetic means or by enzymatic digestions were unable to adhere to collagen via alpha2beta1 integrin. HSPG-deficient cells also displayed impaired spreading and actin organization on collagen. Among different HSPG molecules syndecan-1 was found to play an important role in supporting alpha2beta1 integrin-mediated adhesion. Using overexpression and knock-down experiments we demonstrated that syndecan-1, but not syndecan-2 or -4, enhanced binding of alpha2beta1 to collagen. Moreover, syndecan-1 co-localized with alpha2beta1 integrin and contributed to proper organization of cortical actin. Finally, crosstalk between syndecan-1 and alpha2beta1 integrin was found to enhance the transcription of matrix metalloproteinase-1 in response to collagen binding. Our findings thus suggest that a previously unknown link between integrin alpha2beta1 and syndecan-1 is important in regulating cell adhesion to collagen and in triggering integrin downstream signalling.

ERK2 but not ERK1 plays a key role in hepatocyte replication: an RNAi-mediated ERK2 knockdown approach in wild-type and ERK1 null hepatocytes

The mitogen-activated protein kinases (MAPKs) ERK1 and ERK2 have been implicated in various physiological events, and specific targeting of these MAPKs could affect cell proliferation in many cell types. First, to evaluate the potential specific roles of these two MAPKs, we analyzed the mitogenic response in regenerating liver after partial hepatectomy (PH) and in primary culture of hepatocytes isolated from ERK1-deficient mice. We show that ERK1 knockout and wild-type (wt) cells replicate with the same kinetics after PH in liver, in vivo, and in primary cultures of hepatocytes, in vitro. Indeed, Cyclin D1 and Cdk1 appear to be expressed concomitantly in knockout and wt cells, highlighting that hepatocytes progress in the cell cycle independently of the presence of ERK1. Second, we specifically abolished ERK2 expression by RNA interference in mouse and rat hepatocytes. We investigated whether small interfering RNA (siRNA) targeting ERK2 could specifically inhibit its expression and interfere with the process of replication. In ERK1-deficient hepatocytes, silencing ERK2 expression by RNA interference and ERK2 activation by U0126 clearly demonstrate that DNA replication is regulated by an ERK2-dependent mechanism. Furthermore, in rat wt hepatocytes, whereas ERK2 targeting inhibits late G(1) and S phase progression, ERK1 silencing is devoid of any effect on cell proliferation, indicating that ERK1 cannot rescue ERK2 deficiency.

CONCLUSION

Our results emphasize the importance of the MAPK cascade in hepatocyte replication and allow us to conclude that ERK2 is the key form involved in this regulation, in vivo and in vitro.

ERK activity and G1 phase progression: identifying dispensable versus essential activities and primary versus secondary targets

The ERK subfamily of MAP kinases is a critical regulator of S phase entry. ERK activity regulates the induction of cyclin D1, and a sustained ERK signal is thought to be required for this effect, at least in fibroblasts. We now show that early G1 phase ERK activity is dispensable for the induction of cyclin D1 and that the critical ERK signaling period is restricted to 3-6 h after mitogenic stimulation of quiescent fibroblasts. Similarly, early G1 phase ERK activity is dispensable for entry into S phase. Moreover, if cyclin D1 is expressed ectopically, ERK activity becomes dispensable throughout the G1 phase. In addition to its effect on cyclin D1, ERK activity is thought to contribute to the down-regulation of p27kip1. We found that this effect is restricted to late G1/S phase. Mechanistic analysis showed that the ERK effect on p27kip1 is mediated by Skp2 and is secondary to its effect on cyclin D1. Our results emphasize the importance of mid-G1 phase ERK activity and resolve primary versus secondary ERK targets within the G1 phase cyclin-dependent kinases.

The effect of surface chemistry on the formation of thin films of native fibrillar collagen

In this study, we used well-defined, homogeneous, gradient and patterned substrates to explore the effects of surface chemistry on the supramolecular structure of adsorbed type I collagen. Type I collagen (320microg/mL) was allowed to adsorb onto self-assembled CH(3)-, COOH-, NH(2)- and OH-terminated alkylthiolate monolayers at 37 degrees C. Atomic force microscopy, ellipsometry and phase microscopy indicated that large supramolecular collagen fibril structures (approximately 200nm in diameter, several microns long) assembled only at the hydrophobic CH(3)-terminated surfaces. By varying the surface energy using a mixture of OH- and CH(3)-terminated thiols during monolayer formation, we found that large fibril assembly occurred at surfaces with a water contact angle above 83 degrees , but not on surfaces with a water contact angle below 63 degrees . Examining a surface with a linear hydrophobicity gradient revealed that the assembly of large collagen fibrils requires a hydrophobic surface with a water contact angle of at least 78 degrees . Collagen fibril density increased over a narrow range of surface energy and reached near-maximum density on surfaces with a water contact angle of 87 degrees . These studies confirm that the supramolecular structure of adsorbed collagen is highly dependent on the underlying substrate surface chemistry. We can take advantage of this dependency to pattern areas of fibrillar and non-fibrillar collagen on a single surface. Morphology studies with vascular smooth muscle cells indicated that only collagen films formed on hydrophobic substrates mimicked the biological properties of fibrillar collagen gels.

Induction of intrahepatic cholangiocellular carcinoma by liver-specific disruption of Smad4 and Pten in mice

Cholangiocellular carcinoma (CC), the second most common primary liver cancer, is associated with a poor prognosis. It has been shown that CCs harbor alterations of a number of tumor-suppressor genes and oncogenes, yet key regulators for tumorigenesis remain unknown. Here we have generated a mouse model that develops CC with high penetrance using liver-specific targeted disruption of tumor suppressors SMAD4 and PTEN. In the absence of SMAD4 and PTEN, hyperplastic foci emerge exclusively from bile ducts of mutant mice at 2 months of age and continue to grow, leading to tumor formation in all animals at 4-7 months of age. We show that CC formation follows a multistep progression of histopathological changes that are associated with significant alterations, including increased levels of phosphorylated AKT, FOXO1, GSK-3beta, mTOR, and ERK and increased nuclear levels of cyclin D1. We further demonstrate that SMAD4 and PTEN regulate each other through a novel feedback mechanism to maintain an expression balance and synergistically repress CC formation. Finally, our analysis of human CC detected PTEN inactivation in a majority of p-AKT-positive CCs, while about half also lost SMAD4 expression. These findings elucidate the relationship between SMAD4 and PTEN and extend our understanding of CC formation.

Regulation of hepatocyte cell cycle progression and differentiation by type I collagen structure

Cell behavior is strongly influenced by the extracellular matrix (ECM) to which cells adhere. Both chemical determinants within ECM molecules and mechanical properties of the ECM network regulate cellular response, including proliferation, differentiation, and apoptosis. Type I collagen is the most abundant ECM protein in the body with a complex structure that can be altered in vivo by proteolysis, cross-linking, and other processes. Because of collagen's complex and dynamic nature, it is important to define the changes in cell response to different collagen structures and its underlying mechanisms. This chapter reviews current knowledge of potential mechanisms by which type I collagen affects cell behavior, and it presents data that elucidate specific intracellular signaling pathways by which changes in type I collagen structure differentially regulate hepatocyte cell cycle progression and differentiation. A network of polymerized fibrillar type I collagen (collagen gel) induces a highly differentiated but growth-arrested phenotype in primary hepatocytes, whereas a film of monomeric collagen adsorbed to a rigid dish promotes cell cycle progression and dedifferentiation. Studies presented here demonstrate that protein kinase A (PKA) activity is significantly elevated in hepatocytes on type I collagen gel relative to collagen film, and inhibition of this elevated PKA activity can promote hepatocyte cell cycle progression on collagen gel. Additional studies are presented that examine changes in hepatocyte cell cycle progression and differentiation in response to increased rigidity of polymerized collagen gel by fiber cross-linking. Potential mechanisms underlying these cellular responses and their implications are discussed.

3-phosphoinositide-dependent protein kinase-1 (PDK1) promotes invasion and activation of matrix metalloproteinases

Background

Metastasis is a major cause of morbidity and mortality in breast cancer with tumor cell invasion playing a crucial role in the metastatic process. PDK1 is a key molecule that couples PI3K to cell proliferation and survival signals in response to growth factor receptor activation, and is oncogenic when expressed in mouse mammary epithelial cells. We now present evidence showing that PDK1-expressing cells exhibit enhanced anchorage-dependent and -independent cell growth and are highly invasive when grown on Matrigel. These properties correlate with induction of MMP-2 activity, increased MT1-MMP expression and a unique gene expression profile.

Methods

Invasion assays in Matrigel, MMP-2 zymogram analysis, gene microarray analysis and mammary isografts were used to characterize the invasive and proliferative function of cells expressing PDK1. Tissue microarray analysis of human breast cancers was used to measure PDK1 expression in invasive tumors by IHC.

Results

Enhanced invasion on Matrigel in PDK1-expressing cells was accompanied by increased MMP-2 activity resulting from stabilization against proteasomal degradation. Increased MMP-2 activity was accompanied by elevated levels of MT1-MMP, which is involved in generating active MMP-2. Gene microarray analysis identified increased expression of the ECM-associated genes decorin and type I procollagen, whose gene products are substrates of MT1-MMP. Mammary fat pad isografts of PDK1-expressing cells produced invasive adenocarcinomas. Tissue microarray analysis of human invasive breast cancer indicated that PDK1pSer241 was strongly expressed in 90% of samples.

Conclusion

These results indicate that PDK1 serves as an important effector of mammary epithelial cell growth and invasion in the transformed phenotype. PDK1 mediates its effect in part by MT1-MMP induction, which in turn activates MMP-2 and modulates the ECM proteins decorin and collagen. The presence of increased PDK1 expression in the majority of invasive breast cancers suggests its importance in the metastatic process.

Integrin-dependent signal transduction regulating cyclin D1 expression and G1 phase cell cycle progression

Integrins and growth factor receptors coordinately regulate proliferation in nontransformed cells. Coordinate signaling from these receptors controls the activation of the G1 phase cyclin-dependent kinases, largely by regulating levels of cyclin D1 and p27(kip1). Induction of cyclin D1 is one of the best understood examples of an integrin/growth factor receptor-regulated G1 phase target. This review focuses on the integrin-dependent signal transduction events that regulate the expression of cyclin D1 during G1 phase.

Type I collagen structure regulates cell morphology and EGF signaling in primary rat hepatocytes through cAMP-dependent protein kinase A

Adhesion to type 1 collagen elicits different responses dependent on whether the collagen is in fibrillar (gel) or monomeric form (film). Hepatocytes adherent to collagen film spread and proliferate, whereas those adherent to collagen gel remain rounded and growth arrested. To explore the role of potential intracellular inhibitory signals responsible for collagen gel-mediated growth arrest, cAMP-dependent protein kinase A (PKA) was examined in hepatocytes adherent to collagen film or gel. PKA activity was higher in hepatocytes on collagen gel than on film during G1 of the hepatocyte cell cycle. Inhibition of PKA using H89 increased cell spreading on collagen gel in an EGF-dependent manner, whereas activation of PKA using 8-Br-cAMP decreased cell spreading on collagen film. PKA inhibition also restored ERK activation, cyclin D1 expression and G1-S progression on collagen gel, but had no effect on cells adherent to collagen film. Analysis of EGF receptor phosphorylation revealed that adhesion to collagen gel alters tyrosine phosphorylation of the EGF receptor, leading to reduced phosphorylation of tyrosine residue 845, which was increased by inhibition of PKA. These results demonstrate that fibrillar type 1 collagen can actively disrupt cell cycle progression by inhibiting specific signals from the EGF receptor through a PKA-dependent pathway.

Regulation of growth factor signaling and cell cycle progression by cell adhesion and adhesion-dependent changes in cellular tension

The proliferation of most non-transformed cell types requires cell adhesion and cellular tension as well as exposure to mitogenic growth factors. Integrins and cadherins provide the adhesion signals, which ultimately allow for the cytoskeletal changes that control cellular tension. This review discusses the roles of integrins, cadherins, and the actin cytoskeleton as mediators of the mechanical tension critical for growth factor-dependent signaling and cell cycle progression.

Ovarian carcinoma spheroids disaggregate on type I collagen and invade live human mesothelial cell monolayers

Ovarian carcinoma patients frequently develop malignant ascites containing single and aggregated tumor cells, or spheroids. Spheroids have been shown to be resistant to many therapies, but their contribution to ovarian cancer dissemination remains undetermined. We have previously shown that ascites spheroids adhere to extracellular matrix (ECM) proteins and live human mesothelial cells via beta1 integrin subunits. Here, we assessed the ability of spheroids that were generated from the human ovarian carcinoma cell line NIH:OVCAR5 to disseminate and invade in vitro. Spheroids were seeded on ECM proteins for 24 h. While laminin and type IV collagen stimulated some cell migration, spheroids completely disaggregated on type I collagen substrates. A monoclonal antibody against the beta1 integrin subunit significantly inhibited disaggregation on all proteins tested. To test their invasive ability, spheroids were added to monolayers of live human LP9 mesothelial cells. Within 24 h, the spheroids adhered and disaggregated on top of the monolayers, and within a week had established foci of invasion encompassing a 200-fold larger surface area. Addition of a monoclonal antibody against the beta1 integrin subunit drastically reduced spheroid invasion into the mesothelial cell monolayers. GM 6001, a broad-scale matrix metalloproteinase inhibitor, also significantly blocked spheroid invasion into the mesothelial cell monolayers. Epsilon-amino-N-caproic acid, a serine protease inhibitor, partially inhibited spheroid invasion. Based on their ability to attach to, disaggregate on, and invade into live human mesothelial cell monolayers, spheroids should thus be regarded as potential contributors to the dissemination of ovarian cancer.

Differential effects of interferon alpha-2b and beta on the signaling pathways in human liver cancer cells

BACKGROUND

Interferon (IFN) has been reported to reduce the incidence of hepatocellular carcinoma (HCC) in patients with chronic hepatitis C and the recurrence of HCC after effective treatment. We examined the effect of IFNs on the proliferation and the signaling pathways of human HCC cells.

METHODS

Cellular proliferation was examined by a modified 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) assay. Activities of signaling molecules were evaluated by Western blot analysis.

RESULTS

Cellular growth was not significantly modulated by IFNalpha-2b or by IFN-beta, even though the HCC cells expressed the IFN receptors. However, extracellular signal-regulated kinase (ERK)1/2 was activated by treatment with IFNalpha-2b, and both ERK1/2 and AKT were activated by treatment with IFN-beta, implying a possible role in resistance to IFNs. Contrary to our expectations, inhibition of mitogen-activated ERK-regulating kinase (MEK) or phosphatidylinositol-3-OH kinase (PI3K) did not modulate the proliferation of HCC cells. Also, abrogation of the ERK1/2 and AKT signaling pathways did not affect cell-cycle arrest at the G1/S phase caused by IFNalpha-2b.

CONCLUSIONS

IFNalpha-2b and IFN-beta activated ERK1/2 and/or AKT independently of modulating the proliferation of HCC cells and the cell-cycle machinery. A signal transduction-based approach for HCC treatment needs to focus on other possible signaling molecules besides ERK1/2 and AKT when challenged with IFNs.

Optimization of 3-D hepatocyte culture by controlling the physical and chemical properties of the extra-cellular matrices

Hepatocytes are anchorage-dependent cells sensitive to microenvironment; the control of the physicochemical properties of the extra-cellular matrices may be useful to the maintenance of hepatocyte functions in vitro for various applications. In a microcapsule-based 3-D hepatocyte culture microenvironment, we could control the physical properties of the collagen nano-fibres by fine-tuning the complex-coacervation reaction between methylated collagen and terpolymer of hydroxylethyl methacrylate-methyl methacrylate-methylacrylic acid. The physical properties of the nano-fibres were quantitatively characterized using back-scattering confocal microscopy to help optimize the physical support for hepatocyte functions. We further enhanced the chemical properties of the collagen nano-fibres by incorporating galactose onto collagen, which can specifically interact with the asialoglycoprotein receptor on hepatocytes. By correlating a range of collagen nano-fibres of different physicochemical properties with hepatocyte functions, we have identified a specific combination of methylated and galactosylated collagen nano-fibres optimal for maintaining hepatocyte functions in vitro. A model of how the physical and chemical supports interplay to maintain hepatocyte functions is discussed.

A Configurable Three-Dimensional Microenvironment in a Microfluidic Channel for Primary Hepatocyte Culture

We have developed a technique for the in situ three-dimensional (3D) immobilization of primary rat hepatocytes within a localized matrix in a microfluidic channel that provides a 3D microenvironment incorporating both a configurable 3D matrix and fluid perfusion. This is based on the laminar flow complex coacervation of a pair of oppositely charged polyelectrolytes, i.e., methylated collagen and a terpolymer of HEMA-MMA-MAA. 3D collagen matrices were formed with minimal gelation times (<8 min), were able to entrap cells under aqueous noncytotoxic conditions, and permitted culture media to be perfused in the microchannel by virtue of the spatial confinement of the 3D matrix on one side of the channel. The architecture and stability of the collagen matrix could be configured by the use of different material combinations and changes in the polyelectrolyte flow rates and retention time. Primary rat hepatocytes cultured for 24 h in the 3D matrix within the microchannel showed comparable or enhanced cytochrome P450 7-ethoxyresorufin-O-deethylation activity with static controls. The configurable 3D microenvironment in the microfluidic channel may be a potential 3D culture model of primary hepatocytes for drug testing applications.

Regulation of human hepatocytes by P2Y receptors: control of glycogen phosphorylase, Ca2+, and mitogen-activated protein kinases

In the rat both short-term liver function, such as glycogen metabolism, and long-term events such as proliferation after partial hepatectomy, are in part controlled by release of nucleotides such as ATP acting on hepatocyte P2Y(1) and P2Y(2) receptors (members of a family of P2Y receptors for extracellular nucleotides such as ATP and UTP). Here, we have studied P2Y receptor regulation of signaling pathways involved in glycogen phosphorylase activation and proliferation of primary human hepatocytes. Stimulation of cultured hepatocytes with either ATP and UTP, but not UDP or 2-methylthio ADP, led to concentration-dependent increases in cytosolic free Ca(2+) concentration ([Ca(2+)](c); EC(50) for ATP = 3.3 microM, for UTP = 2.3 microM) and [(3)H]inositol (poly)phosphates (EC(50) for ATP = 9.4 microM, for UTP = 15.4 microM). ATP and UTP also stimulated glycogen phosphorylase in human hepatocytes, each with a threshold for activation of less than 1 microM. Application of 2-methylthio ADP up to 100 microM was ineffective. Phosphorylation of both extracellular signal-related kinase and c-Jun N-terminal kinase was stimulated by ATP and UTP, but not by 2-methylthio ADP or UDP, either alone or when costimulated with epidermal growth factor. In conclusion, in human hepatocytes P2Y receptors control both glycogen metabolism and proliferation-associated responses such as increased [Ca(2+)](c) and mitogen-activated protein kinase cascades. Regulation seems to be primarily through P2Y(2) receptors. In contrast with previous studies on rat hepatocytes, there is an absence of responses mediated by P2Y(1) receptors.

Extracellular matrix-dependent myosin dynamics during G1-S phase cell cycle progression in hepatocytes

Cell spreading and proliferation are tightly coupled in anchorage-dependent cells. While adhesion-dependent proliferation signals require an intact actin cytoskeleton, and some of these signals such as ERK activation have been characterized, the role of myosin in spreading and cell cycle progression under different extracellular matrix (ECM) conditions is not known. Studies presented here examine changes in myosin activity in freshly isolated hepatocytes under ECM conditions that promote either proliferation (high fibronectin density) or growth arrest (low fibronectin density). Three different measures were obtained and related to both spreading and cell cycle progression: myosin protein levels and association with cytoskeleton, myosin light chain phosphorylation, and its ATPase activity. During the first 48 h in culture, corresponding with transit through G1 phase, there was a six-fold increase in both myosin protein levels and myosin association with actin cytoskeleton. There was also a steady increase in myosin light chain phosphorylation and ATPase activity with spreading, which did not occur in non-spread, growth-arrested cells on low density of fibronectin. Myosin-inhibiting drugs blocked ERK activation, cyclin D1 expression, and S phase entry. Overexpression of the cell cycle protein cyclin D1 overcame both ECM-dependent and actomyosin-dependent inhibition of DNA synthesis, suggesting that cyclin D1 is a key event downstream of myosin-dependent cell cycle regulation.

Evidence for ERK1/2 phosphorylation controlling contact inhibition of proliferation in Madin-Darby canine kidney epithelial cells

Increasing cell density arrests epithelial cell proliferation by a process termed contact inhibition. We investigated mechanisms of contact inhibition using a model of contact-inhibited epithelial cells. Hepatocyte growth factor (HGF) treatment of contact-inhibited Madin-Darby canine kidney (MDCK) cells stimulated cell proliferation and increased levels of phosphorylated ERK1/2 (phospho-ERK1/2) and cyclin D1. MEK inhibitors PD-98059 and U0126 inhibited these HGF-dependent changes, indicating the dependence on phosphorylation of ERK1/2 during HGF-induced loss of contact inhibition. In relation to contact-inhibited high-density cells, low-density MDCK cells proliferated and had higher levels of phospho-ERK1/2 and cyclin D1. PD-98059 and U0126 inhibited low-density MDCK cell proliferation. Trypsinization of high-density MDCK cells immediately increased phospho-ERK1/2 and was followed by a transient increase in cyclin D1 levels. Reformation of cell junctions after trypsinization led to decreases in phospho-ERK1/2 and cyclin D1 levels. High-density MDCK cells express low levels of both cyclin D1 and phospho-ERK1/2, and treatment of these cells with fresh medium containing HGF but not fresh medium alone for 6 h increased phospho-ERK1/2 and cyclin D1 levels compared with cells without medium change. These data provide evidence that HGF abrogates MDCK cell contact inhibition by increasing ERK1/2 phosphorylation and levels of cyclin D1. These results suggest that in MDCK cells, contact inhibition of cell proliferation in the presence of serum occurs by cell density-dependent regulation of ERK1/2 phosphorylation.