Morphological and biochemical analysis of Rac1 in three-dimensional epithelial cell cultures

FGFR2-activating mutations disrupt cell polarity to potentiate migration and invasion in endometrial cancer cell models

Fibroblast growth factor receptors (FGFRs) are a family of receptor tyrosine kinases that control a diverse range of biological processes during development and in adult tissues. We recently reported that somatic FGFR2 mutations are associated with shorter survival in endometrial cancer. However, little is known about how these FGFR2 mutations contribute to endometrial cancer metastasis. Here, we report that expression of the activating mutations FGFR2^N550K and FGFR2^Y376C in an endometrial cancer cell model induce Golgi fragmentation, and loss of polarity and directional migration. In mutant FGFR2-expressing cells, this was associated with an inability to polarise intracellular pools of FGFR2 towards the front of migrating cells. Such polarization defects were exacerbated in three-dimensional culture, where FGFR2 mutant cells were unable to form well-organised acini, instead undergoing exogenous ligand-independent invasion. Our findings uncover collective cell polarity and invasion as common targets of disease-associated FGFR2 mutations that lead to poor outcome in endometrial cancer patients.

Cingulin and actin mediate midbody-dependent apical lumen formation during polarization of epithelial cells

Coordinated polarization of epithelial cells is a key step during morphogenesis that leads to the formation of an apical lumen. Rab11 and its interacting protein FIP5 are necessary for the targeting of apical endosomes to the midbody and apical membrane initiation site (AMIS) during lumenogenesis. However, the machinery that mediates AMIS establishment and FIP5-endosome targeting remains unknown. Here we identify a FIP5-interacting protein, Cingulin, which localizes to the AMIS and functions as a tether mediating FIP5-endosome targeting. We analysed the machinery mediating AMIS recruitment to the midbody and determined that both branched actin and microtubules are required for establishing the site of the nascent lumen. We demonstrate that the Rac1-WAVE/Scar complex mediates Cingulin recruitment to the AMIS by inducing branched actin formation, and that Cingulin directly binds to microtubule C-terminal tails through electrostatic interactions. We propose a new mechanism for apical endosome targeting and AMIS formation around the midbody during epithelial lumenogenesis.

Polarisation of epithelial cells causes lumen formation, which is mediated by apical membrane initiation site (AMIS) and FIP5, but how this is regulated is unclear. Here, the authors identify cingulin as a FIP-5 interacting protein, recruiting the Rac1-WAVE/Scar complex to the AMIS and branched actin formation.

KIF17 regulates RhoA-dependent actin remodeling at epithelial cell-cell adhesions

The kinesin KIF17 localizes at microtubule plus-ends where it contributes to regulation of microtubule stabilization and epithelial polarization. We now show that KIF17 localizes at cell-cell adhesions and that KIF17 depletion inhibits accumulation of actin at the apical pole of cells grown in 3D organotypic cultures and alters the distribution of actin and E-cadherin in cells cultured in 2D on solid supports. Overexpression of full-length KIF17 constructs or truncation mutants containing the N-terminal motor domain resulted in accumulation of newly incorporated GFP-actin into junctional actin foci, cleared E-cadherin from cytoplasmic vesicles and stabilized cell-cell adhesions to challenge with calcium depletion. Expression of these KIF17 constructs also increased cellular levels of active RhoA, whereas active RhoA was diminished in KIF17-depleted cells. Inhibition of RhoA or its effector ROCK, or expression of LIMK1 kinase-dead or activated cofilin(S3A) inhibited KIF17-induced junctional actin accumulation. Interestingly, KIF17 activity toward actin depends on the motor domain but is independent of microtubule binding. Together, these data show that KIF17 can modify RhoA-GTPase signaling to influence junctional actin and the stability of the apical junctional complex of epithelial cells.

Synthetic matrices reveal contributions of ECM biophysical and biochemical properties to epithelial morphogenesis

Epithelial cells cultured within collagen and laminin gels proliferate to form hollow and polarized spherical structures, recapitulating the formation of a rudimentary epithelial organ. However, the contributions of extracellular matrix (ECM) biochemical and biophysical properties to morphogenesis are poorly understood because of uncontrolled presentation of multiple adhesive ligands, limited control over mechanical properties, and lot-to-lot compositional variability in these natural ECMs. We engineered synthetic ECM-mimetic hydrogels with independent control over adhesive ligand density, mechanical properties, and proteolytic degradation to study the impact of ECM properties on epithelial morphogenesis. Normal cyst growth, polarization, and lumen formation were restricted to a narrow range of ECM elasticity, whereas abnormal morphogenesis was observed at lower and higher elastic moduli. Adhesive ligand density dramatically regulated apicobasal polarity and lumenogenesis independently of cell proliferation. Finally, a threshold level of ECM protease degradability was required for apicobasal polarity and lumen formation. This synthetic ECM technology provides new insights into how cells transduce ECM properties into complex morphogenetic behaviors.

Carcinoma cells induce lumen filling and EMT in epithelial cells through soluble E-cadherin-mediated activation of EGFR

In epithelial cancers, carcinoma cells coexist with normal cells. Although it is known that the tumor microenvironment (TME) plays a pivotal role in cancer progression, it is not completely understood how the tumor influences adjacent normal epithelial cells. In this study, a three-dimensional co-culture system comprising non-transformed epithelial cells (MDCK) and transformed carcinoma cells (MSV-MDCK) was used to demonstrate that carcinoma cells sequentially induce preneoplastic lumen filling and epithelial-mesenchymal transition (EMT) in epithelial cysts. MMP-9 secreted by carcinoma cells cleaves cellular E-cadherin (encoded by CDH1) from epithelial cells to generate soluble E-cadherin (sE-cad), a pro-oncogenic protein. We show that sE-cad induces EGFR activation, resulting in lumen filling in MDCK cysts. Long-term sE-cad treatment induced EMT. sE-cad caused lumen filling by induction of the ERK signaling pathway and triggered EMT through the sustained activation of the AKT pathway. Although it is known that sE-cad induces MMP-9 release and consequent EGFR activation in tumor cells, our results, for the first time, demonstrate that carcinoma cells can induce sE-cad shedding in adjacent epithelial cells, which leads to EGFR activation and the eventual transdifferentiation of the normal epithelial cells.

The exocyst gene Sec10 regulates renal epithelial monolayer homeostasis and apoptotic sensitivity

The highly conserved exocyst protein complex regulates polarized exocytosis of subsets of secretory vesicles. A previous study reported that shRNA knockdown of an exocyst central subunit, Sec10 (Sec10-KD) in Madin-Darby canine kidney (MDCK) cells disrupted primary cilia assembly and 3D cyst formation. We used three-dimensional collagen cultures of MDCK cells to further investigate the mechanisms by which Sec10 and the exocyst regulate epithelial polarity, morphogenesis, and homeostasis. Sec10-KD cysts initially demonstrated undisturbed lumen formation although later displayed significantly fewer and shorter primary cilia than controls. Later in cystogenesis, control cells maintained normal homeostasis, while Sec10-KD cysts displayed numerous apoptotic cells extruded basally into the collagen matrix. Sec10-KD MDCK cells were also more sensitive to apoptotic triggers than controls. These phenotypes were reversed by restoring Sec10 expression with shRNA-resistant human Sec10. Apico-basal polarity appeared normal in Sec10-KD cysts, whereas mitotic spindle angles differed significantly from controls, suggesting a planar cell polarity defect. In addition, analysis of renal tubules in a newly generated kidney-specific Sec10-knockout mouse model revealed significant defects in primary cilia assembly and in the targeted renal tubules; abnormal epithelial cell extrusion was also observed, supporting our in vitro results. We hypothesize that, in Sec10-KD cells, the disrupted exocyst activity results in increased apoptotic sensitivity through defective primary cilia signaling and that, in combination with an increased basal cell extrusion rate, it affects epithelial barrier integrity and homeostasis.

Abl Kinases Regulate HGF/Met Signaling Required for Epithelial Cell Scattering, Tubulogenesis and Motility

Tight regulation of receptor tyrosine kinases (RTKs) is crucial for normal development and homeostasis. Dysregulation of RTKs signaling is associated with diverse pathological conditions including cancer. The Met RTK is the receptor for hepatocyte growth factor (HGF) and is dysregulated in numerous human tumors. Here we show that Abl family of non-receptor tyrosine kinases, comprised of Abl (ABL1) and Arg (ABL2), are activated downstream of the Met receptor, and that inhibition of Abl kinases dramatically suppresses HGF-induced cell scattering and tubulogenesis. We uncover a critical role for Abl kinases in the regulation of HGF/Met-dependent RhoA activation and RhoA-mediated actomyosin contractility and actin cytoskeleton remodeling in epithelial cells. Moreover, treatment of breast cancer cells with Abl inhibitors markedly decreases Met-driven cell migration and invasion. Notably, expression of a transforming mutant of the Met receptor in the mouse mammary epithelium results in hyper-activation of both Abl and Arg kinases. Together these data demonstrate that Abl kinases link Met activation to Rho signaling and Abl kinases are required for Met-dependent cell scattering, tubulogenesis, migration, and invasion. Thus, inhibition of Abl kinases might be exploited for the treatment of cancers driven by hyperactivation of HGF/Met signaling.

Differential sensitivity of epithelial cells to extracellular matrix in polarity establishment

Establishment of apical-basal polarity is crucial for epithelial sheets that form a compartment in the body, which function to maintain the environment in the compartment. Effects of impaired polarization are easily observed in three-dimensional (3-D) culture systems rather than in two-dimensional (2-D) culture systems. Although the mechanisms for establishing the polarity are not completely understood, signals from the extracellular matrix (ECM) are considered to be essential for determining the basal side and eventually generating polarity in the epithelial cells. To elucidate the common features and differences in polarity establishment among various epithelial cells, we analyzed the formation of epithelial apical-basal polarity using three cell lines of different origin: MDCK II cells (dog renal tubules), EpH4 cells (mouse mammary gland), and R2/7 cells (human colon) expressing wild-type α-catenin (R2/7 α-Cate cells). These cells showed clear apical-basal polarity in 2-D cultures. In 3-D cultures, however, each cell line displayed different responses to the same ECM. In MDCK II cells, spheroids with a single lumen formed in both Matrigel and collagen gel. In R2/7 α-Cate cells, spheroids showed similar apical-basal polarity as that seen in MDCK II cells, but had multiple lumens. In EpH4 cells, the spheroids displayed an apical-basal polarity that was opposite to that seen in the other two cell types in both ECM gels, at least during the culture period. On the other hand, the three cell lines showed the same apical-basal polarity both in 2-D cultures and in 3-D cultures using the hanging drop method. The three lines also had similar cellular responses to ECM secreted by the cells themselves. Therefore, appropriate culture conditions should be carefully determined in advance when using various epithelial cells to analyze cell polarity or 3-D morphogenesis.

Prediction of individual response to anticancer therapy: historical and future perspectives

Since the introduction of chemotherapy for cancer treatment in the early 20th century considerable efforts have been made to maximize drug efficiency and at the same time minimize side effects. As there is a great interpatient variability in response to chemotherapy, the development of predictive biomarkers is an ambitious aim for the rapidly growing research area of personalized molecular medicine. The individual prediction of response will improve treatment and thus increase survival and life quality of patients. In the past, cell cultures were used as in vitro models to predict in vivo response to chemotherapy. Several in vitro chemosensitivity assays served as tools to measure miscellaneous endpoints such as DNA damage, apoptosis and cytotoxicity or growth inhibition. Twenty years ago, the development of high-throughput technologies, e.g. cDNA microarrays enabled a more detailed analysis of drug responses. Thousands of genes were screened and expression levels were correlated to drug responses. In addition, mutation analysis became more and more important for the prediction of therapeutic success. Today, as research enters the area of -omics technologies, identification of signaling pathways is a tool to understand molecular mechanism underlying drug resistance. Combining new tissue models, e.g. 3D organoid cultures with modern technologies for biomarker discovery will offer new opportunities to identify new drug targets and in parallel predict individual responses to anticancer therapy. In this review, we present different currently used chemosensitivity assays including 2D and 3D cell culture models and several -omics approaches for the discovery of predictive biomarkers. Furthermore, we discuss the potential of these assays and biomarkers to predict the clinical outcome of individual patients and future perspectives.

Three-dimensional organotypic culture: experimental models of mammalian biology and disease

Mammalian organs are challenging to study as they are fairly inaccessible to experimental manipulation and optical observation. Recent advances in three-dimensional (3D) culture techniques, coupled with the ability to independently manipulate genetic and microenvironmental factors, have enabled the real-time study of mammalian tissues. These systems have been used to visualize the cellular basis of epithelial morphogenesis, to test the roles of specific genes in regulating cell behaviours within epithelial tissues and to elucidate the contribution of microenvironmental factors to normal and disease processes. Collectively, these novel models can be used to answer fundamental biological questions and generate replacement human tissues, and they enable testing of novel therapeutic approaches, often using patient-derived cells.

Detection of atomic scale changes in the free volume void size of three-dimensional colorectal cancer cell culture using positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) provides a direct measurement of the free volume void sizes in polymers and biological systems. This free volume is critical in explaining and understanding physical and mechanical properties of polymers. Moreover, PALS has been recently proposed as a potential tool in detecting cancer at early stages, probing the differences in the subnanometer scale free volume voids between cancerous/healthy skin samples of the same patient. Despite several investigations on free volume in complex cancerous tissues, no positron annihilation studies of living cancer cell cultures have been reported. We demonstrate that PALS can be applied to the study in human living 3D cell cultures. The technique is also capable to detect atomic scale changes in the size of the free volume voids due to the biological responses to TGF-β. PALS may be developed to characterize the effect of different culture conditions in the free volume voids of cells grown in vitro.

Mutant p53 cooperates with knockdown of endogenous wild-type p53 to disrupt tubulogenesis in Madin-Darby canine kidney cells

Mutation of the p53 gene is the most common genetic alteration in human malignances and associated clinically with tumor progression and metastasis. To determine the effect of mutant p53 on epithelial differentiation, we developed three-dimensional culture (3-D) of Madin-Darby canine kidney (MDCK) cells. We found that parental MDCK cells undergo a series of morphological changes and form polarized and growth-arrested cysts with hollow lumen, which resembles branching tubules in vitro. We also found that upon knockdown of endogenous wild-type p53 (p53-KD), MDCK cells still form normal cysts in 3-D culture, indicating that p53-KD alone is not sufficient to disrupt cysts formation. However, we found that ectopic expression of mutant R163H (human equivalent R175H) or R261H (human equivalent R273H) in MDCK cells leads to disruption of cyst polarity and formation of invasive aggregates, which is further compounded by knockdown of endogenous wild-type p53. Consistently, we found that expression of E-cadherin, β-catenin, and epithelial-to-mesenchymal transition (EMT) transcription factors (Snail-1, Slug and Twist) is altered by mutant p53, which is also compounded by knockdown of wild-type p53. Moreover, the expression level of c-Met, the hepatocyte growth factor receptor and a key regulator of kidney cell tubulogenesis, is enhanced by combined knockdown of endogenous wild-type p53 and ectopic expression of mutant R163H or R261H but not by each individually. Together, our data suggest that upon inactivating mutation of the p53 gene, mutant p53 acquires its gain of function by altering morphogenesis and promoting cell migration and invasion in part by upregulating EMT and c-Met.

Specific fate decisions in adult hepatic progenitor cells driven by MET and EGFR signaling

The relative contribution of hepatocyte growth factor (HGF)/MET and epidermal growth factor (EGF)/EGF receptor (EGFR), two key signal transduction systems in the normal and diseased liver, to fate decisions of adult hepatic progenitor cells (HPCs) has not been resolved. Here, we developed a robust culture system that permitted expansion and genetic manipulation of cells capable of multilineage differentiation in vitro and in vivo to examine the individual roles of HGF/MET and EGF/EGFR in HPC self-renewal and binary cell fate decision. By employing loss-of-function and rescue experiments in vitro, we showed that both receptors collaborate to increase the self-renewal of HPCs through activation of the extracellular signal-regulated kinase (ERK) pathway. MET was a strong inducer of hepatocyte differentiation by activating AKT and signal transducer and activator of transcription (STAT3). Conversely, EGFR selectively induced NOTCH1 to promote cholangiocyte specification and branching morphogenesis while concomitantly suppressing hepatocyte commitment. Furthermore, unlike the deleterious effects of MET deletion, the liver-specific conditional loss of Egfr facilitated rather than suppressed progenitor-mediated liver regeneration by switching progenitor cell differentiation toward hepatocyte lineage. These data provide new insight into the mechanisms regulating the stemness properties of adult HPCs and reveal a previously unrecognized link between EGFR and NOTCH1 in directing cholangiocyte differentiation.

P63 regulates tubular formation via epithelial-to-mesenchymal transition

P63, a p53 family member, is expressed as TA and ΔN isoforms. Interestingly, both TAp63 and ΔNp63 are transcription factors, and regulate both common and distinct sets of target genes. p63 is required for survival of some epithelial cell lineages, and lack of p63 leads to loss of epidermis and other epithelia in humans and mice. Here, we explored the role of p63 isoforms in cell proliferation, migration and tubulogenesis by using Madin-Darby Canine Kidney (MDCK) tubular epithelial cells in two- or three-dimensional (2-D or 3-D) culture. We found that like downregulation of p53, downregulation of p63 and TAp63 decreases expression of growth-suppressing genes, including p21, PUMA and MIC-1, and consequently promotes cell proliferation and migration in 2-D culture. However, in 3-D culture, downregulation of p63, especially TAp63, but not p53, decapacitates MDCK cells to form a cyst structure through enhanced epithelial-to-mesenchymal transition (EMT). In contrast, downregulation of ΔNp63 inhibits MDCK cell proliferation and migration in 2-D culture, and delays but does not block MDCK cell cyst formation and tubulogenesis in 3-D culture. Consistent with this, downregulation of ΔNp63 markedly upregulates growth-suppressing genes, including p21, PUMA and MIC-1. Taken together, these data suggest that TAp63 is the major isoform required for tubulogenesis by maintaining an appropriate level of EMT, whereas ΔNp63 fine-tunes the rate of cyst formation and tubulogenesis by maintaining an appropriate expression level of genes involved in cell cycle arrest and apoptosis.

Association of the epithelial-to-mesenchymal transition phenotype with responsiveness to the p21-activated kinase inhibitor, PF-3758309, in colon cancer models

The p21-activated kinase (PAK) family of serine/threonine kinases, which are overexpressed in several cancer types, are critical mediators of cell survival, motility, mitosis, transcription, and translation. In the study presented here, we utilized a panel of colorectal cancer (CRC) cell lines to identify potential biomarkers of sensitivity or resistance that may be used to individualize therapy to the PAK inhibitor PF-03758309. We observed a wide range of proliferative responses in the CRC cell lines exposed to PF-03758309, this response was recapitulated in other phenotypic assays such as anchorage-independent growth, three-dimensional (3D) tumor spheroid formation, and migration. Interestingly, we observed that cells most sensitive to PF-03758309 exhibited up-regulation of genes associated with a mesenchymal phenotype (CALD1, VIM, ZEB1) and cells more resistant had an up-regulation of genes associated with an epithelial phenotype (CLDN2, CDH1, CLDN3, CDH17) allowing us to derive an epithelial-to-mesenchymal transition (EMT) gene signature for this agent. We assessed the functional role of EMT-associated genes in mediating responsiveness to PF-3758309, by targeting known genes and transcriptional regulators of EMT. We observed that suppression of genes associated with the mesenchymal phenotype conferred resistance to PF-3758309, in vitro and in vivo. These results indicate that PAK inhibition is associated with a unique response phenotype in CRC and that further studies should be conducted to facilitate both patient selection and rational combination strategies with these agents.

Role of membrane traffic in the generation of epithelial cell asymmetry

Epithelial cells have an apical-basolateral axis of polarity, which is required for epithelial functions including barrier formation, vectorial ion transport and sensory perception. Here we review what is known about the sorting signals, machineries and pathways that maintain this asymmetry, and how polarity proteins interface with membrane-trafficking pathways to generate membrane domains de novo. It is becoming apparent that membrane traffic does not simply reinforce polarity, but is critical for the generation of cortical epithelial cell asymmetry.

Na,K-ATPase β-subunit cis homo-oligomerization is necessary for epithelial lumen formation in mammalian cells

Na,K-ATPase is a hetero-oligomer of an α- and a β-subunit. The α-subunit (Na,K-α) possesses the catalytic function, whereas the β-subunit (Na,K-β) has cell-cell adhesion function and is localized to the apical junctional complex in polarized epithelial cells. Earlier, we identified two distinct conserved motifs on the Na,K-β(1) transmembrane domain that mediate protein-protein interactions: a glycine zipper motif involved in the cis homo-oligomerization of Na,K-β(1) and a heptad repeat motif that is involved in the hetero-oligomeric interaction with Na,K-α(1). We now provide evidence that knockdown of Na,K-β(1) prevents lumen formation and induces activation of extracellular regulated kinases 1 and 2 (ERK1/2) mediated by phosphatidylinositol 3-kinase in MDCK cells grown in three-dimensional collagen cultures. These cells sustained cell proliferation in an ERK1/2-dependent manner and did not show contact inhibition at high cell densities, as revealed by parental MDCK cells. This phenotype could be rescued by wild-type Na,K-β(1) or heptad repeat motif mutant of Na,K-β(1), but not by the glycine zipper motif mutant that abrogates Na,K-β(1) cis homo-oligomerization. These studies suggest that Na,K-β(1) cis homo-oligomerization rather than hetero-oligomerization with Na,K-α(1) is involved in epithelial lumen formation. The relevance of these findings to pre-neoplastic lumen filling in epithelial cancer is discussed.

Establishing epithelial glandular polarity: interlinked roles for ARF6, Rac1, and the matrix microenvironment

The depletion of ARF6 in epithelial cysts causes a striking inversion of glandular orientation. This requires temporal Rac1 inactivation and is accompanied in basement membrane cultures by improperly assembled laminins. In collagen I, these inverted cysts promote integrin-linked fibril linearization reminiscent of matrix remodeling in disease.

Epithelial cysts comprise the structural units of the glandular epithelium. Although glandular inversion in epithelial tumors is thought to be a potential mechanism for the establishment of metastatic disease, little is known about the morphogenic cues and signaling pathways that govern glandular polarity and organization. Using organotypic cultures of Madin-Darby canine kidney cells in reconstituted basement membrane, we show that cellular depletion of the small GTP-binding protein ARF6 promotes the formation of inverted cysts, wherein the apical cell membrane faces the cyst exterior, and the basal domain faces the central lumen, while individual cell polarity is maintained. These cysts are also defective in interactions with laminin at the cyst–matrix interface. This inversion of glandular orientation is accompanied by Rac1 inactivation during early cystogenesis, and temporal activation of Rac1 is sufficient to recover the normal cyst phenotype. In an unnatural collagen I microenvironment, ARF6-depleted, inverted epithelial cysts exhibit some loss of cell polarity, a marked increase in Rho activation and Rac1 inactivation, and striking rearrangement of the surrounding collagen I matrix. These studies demonstrate the importance of ARF6 as a critical determinant of glandular orientation and the matrix environment in dictating structural organization of epithelial cysts.

Pak1 regulates the orientation of apical polarization and lumen formation by distinct pathways

The development of the basic architecture of branching tubules enclosing a central lumen that characterizes most epithelial organs crucially depends on the apico-basolateral polarization of epithelial cells. Signals from the extracellular matrix control the orientation of the apical surface, so that it faces the lumen interior, opposite to cell-matrix adhesion sites. This orientation of the apical surface is thought to be intrinsically linked to the formation of single lumens. We previously demonstrated in three-dimensional cyst cultures of Madin-Darby canine kidney (MDCK) cells that signaling by β1 integrins regulates the orientation of the apical surface, via a mechanism that depends on the activity of the small GTPase Rac1. Here, we investigated whether the Rac1 effector Pak1 is a downstream effector in this pathway. Expression of constitutive active Pak1 phenocopies the effect of β1 integrin inhibition in that it misorients the apical surface and induces a multilumen phenotype. The misorientation of apical surfaces depends on the interaction of active Pak1 with PIX proteins and is linked to defects in basement membrane assembly. In contrast, the multilumen phenotype was independent of PIX and the basement membrane. Therefore, Pak1 likely regulates apical polarization and lumen formation by two distinct pathways.

EB1-recruited microtubule +TIP complexes coordinate protrusion dynamics during 3D epithelial remodeling

BACKGROUND

Epithelial remodeling, in which apical-basal polarized cells switch to a migratory phenotype, plays a central role in development and disease of multicellular organisms. Although dynamic microtubules (MTs) are required for directed migration on flat surfaces, how MT dynamics are controlled or contribute to epithelial remodeling in a more physiological three-dimensional (3D) environment is not understood. We use confocal live-cell imaging to analyze MT function and dynamics during 3D epithelial morphogenesis and remodeling of polarized Madin-Darby canine kidney epithelial cells that undergo partial epithelial-to-mesenchymal transition in response to hepatocyte growth factor (HGF).

RESULTS

We find that HGF treatment increases MT growth rate before morphological changes are evident and that large numbers of MTs grow into HGF-induced cell extensions independent of centrosome reorientation. Using lentivirus-mediated small hairpin RNA, we demonstrate that EB1, an adaptor protein that mediates recruitment of numerous other +TIP proteins to growing MT plus ends, is required for this HGF-induced MT reorganization. We further show that protrusion and adhesion dynamics are disorganized and that vesicular trafficking to the tip of HGF-induced cell extensions is disrupted in EB1-depleted cells.

CONCLUSIONS

We conclude that EB1-mediated interactions with growing MTs are important to coordinate cell shape changes and directed migration into the surrounding extracellular matrix during epithelial remodeling in a physiological 3D environment. In contrast, EB1 is not required for the establishment or maintenance of apical-basal cell polarity, suggesting different functions of +TIPs and MTs in different types of cell polarity.

Live-cell imaging of migrating cells expressing fluorescently-tagged proteins in a three-dimensional matrix

Traditionally, cell migration has been studied on two-dimensional, stiff plastic surfaces. However, during important biological processes such as wound healing, tissue regeneration, and cancer metastasis, cells must navigate through complex, three-dimensional extracellular tissue. To better understand the mechanisms behind these biological processes, it is important to examine the roles of the proteins responsible for driving cell migration. Here, we outline a protocol to study the mechanisms of cell migration using the epithelial cell line (MDCK), and a three-dimensional, fibrous, self-polymerizing matrix as a model system. This optically clear extracellular matrix is easily amenable to live-cell imaging studies and better mimics the physiological, soft tissue environment. This report demonstrates a technique for directly visualizing protein localization and dynamics, and deformation of the surrounding three-dimensional matrix. Examination of protein localization and dynamics during cellular processes provides key insight into protein functions. Genetically encoded fluorescent tags provide a unique method for observing protein localization and dynamics. Using this technique, we can analyze the subcellular accumulation of key, force-generating cytoskeletal components in real-time as the cell maneuvers through the matrix. In addition, using multiple fluorescent tags with different wavelengths, we can examine the localization of multiple proteins simultaneously, thus allowing us to test, for example, whether different proteins have similar or divergent roles. Furthermore, the dynamics of fluorescently tagged proteins can be quantified using Fluorescent Recovery After Photobleaching (FRAP) analysis. This measurement assays the protein mobility and how stably bound the proteins are to the cytoskeletal network. By combining live-cell imaging with the treatment of protein function inhibitors, we can examine in real-time the changes in the distribution of proteins and morphology of migrating cells. Furthermore, we also combine live-cell imaging with the use of fluorescent tracer particles embedded within the matrix to visualize the matrix deformation during cell migration. Thus, we can visualize how a migrating cell distributes force-generating proteins, and where the traction forces are exerted to the surrounding matrix. Through these techniques, we can gain valuable insight into the roles of specific proteins and their contributions to the mechanisms of cell migration.

Molecular mechanisms controlling vascular lumen formation in three-dimensional extracellular matrices

Considerable progress has been made toward a molecular understanding of how cells form lumen and tube structures in three-dimensional (3D) extracellular matrices (ECM). This progress has occurred through work performed with endothelial and epithelial cell models using both in vitro and in vivo approaches. Despite the apparent similarities between endothelial and epithelial cell lumen and tube formation mechanisms, there are clear distinctions that directly relate to their functional differences. This review will focus on endothelial cell (EC) lumen formation mechanisms which control blood vessel formation during development and postnatal life. Of great interest is that an EC lumen signaling complex has been identified which controls human EC lumen and tube formation in 3D matrices and which coordinates integrin-ECM contacts, cell surface proteolysis, cytoskeletal rearrangements, and cell polarity. This complex consists of the collagen-binding integrin α2β1, the collagen-degrading membrane-type 1 matrix metalloproteinase (MT1-MMP), junction adhesion molecule (Jam)C, JamB, polarity proteins Par3 and Par6b, and the Rho GTPase Cdc42-GTP. These interacting proteins are necessary to stimulate 3D matrix-specific signaling events (including activation of protein kinase cascades that regulate the actin and microtubule cytoskeletons) to control the formation of EC lumens and tube networks. Also, EC lumen formation is directly coupled to the generation of vascular guidance tunnels, enzymatically generated ECM conduits that facilitate EC tube remodeling and maturation. Mural cells such as pericytes are recruited along EC tubes within these tunnel spaces to control ECM remodeling events resulting in vascular basement membrane matrix assembly, a key step in tube maturation and stabilization.

Phosphoinositides in cell architecture

Inositol phospholipids have been implicated in almost all aspects of cellular physiology including spatiotemporal regulation of cellular signaling, acquisition of cellular polarity, specification of membrane identity, cytoskeletal dynamics, and regulation of cellular adhesion, motility, and cytokinesis. In this review, we examine the critical role phosphoinositides play in these processes to execute the establishment and maintenance of cellular architecture. Epithelial tissues perform essential barrier and transport functions in almost all major organs. Key to their development and function is the establishment of epithelial cell polarity. We place a special emphasis on highlighting recent studies demonstrating phosphoinositide regulation of epithelial cell polarity and how individual cells use phosphoinositides to further organize into epithelial tissues.

Distinct roles of cadherin-6 and E-cadherin in tubulogenesis and lumen formation

Classic cadherins are important regulators of tissue morphogenesis. The predominant cadherin in epithelial cells, E-cadherin, has been extensively studied because of its critical role in normal epithelial development and carcinogenesis. Epithelial cells may also coexpress other cadherins, but their roles are less clear. The Madin Darby canine kidney (MDCK) cell line has been a popular mammalian model to investigate the role of E-cadherin in epithelial polarization and tubulogenesis. However, MDCK cells also express relatively high levels of cadherin-6, and it is unclear whether the functions of this cadherin are redundant to those of E-cadherin. We investigate the specific roles of both cadherins using a knockdown approach. Although we find that both cadherins are able to form adherens junctions at the basolateral surface, we show that they have specific and mutually exclusive roles in epithelial morphogenesis. Specifically, we find that cadherin-6 functions as an inhibitor of tubulogenesis, whereas E-cadherin is required for lumen formation. Ablation of cadherin-6 leads to the spontaneous formation of tubules, which depends on increased phosphoinositide 3-kinase (PI3K) activity. In contrast, loss of E-cadherin inhibits lumen formation by a mechanism independent of PI3K.

Molecular basis for endothelial lumen formation and tubulogenesis during vasculogenesis and angiogenic sprouting

Many studies reveal a fundamental role for extracellular matrix-mediated signaling through integrins and Rho GTPases as well as matrix metalloproteinases (MMPs) in the molecular control of vascular tube morphogenesis in three-dimensional (3D) tissue environments. Recent work has defined an endothelial cell (EC) lumen signaling complex of proteins that controls these vascular morphogenic events. These findings reveal a signaling interdependence between Cdc42 and MT1-MMP to control the 3D matrix-specific process of EC tubulogenesis. The EC tube formation process results in the creation of a network of proteolytically generated vascular guidance tunnels in 3D matrices that are utilized to remodel EC-lined tubes through EC motility and could facilitate processes such as flow-induced remodeling and arteriovenous EC sorting and differentiation. Within vascular guidance tunnels, key dynamic interactions occur between ECs and pericytes to affect vessel remodeling, diameter, and vascular basement membrane matrix assembly, a fundamental process necessary for endothelial tube maturation and stabilization. Thus, the EC lumen and tube formation mechanism coordinates the concomitant establishment of a network of vascular tubes within tunnel spaces to allow for flow responsiveness, EC-mural cell interactions, and vascular extracellular matrix assembly to control the development of the functional microcirculation.

KIF17 stabilizes microtubules and contributes to epithelial morphogenesis by acting at MT plus ends with EB1 and APC

Epithelial polarization is associated with selective stabilization and reorganization of microtubule (MT) arrays. However, upstream events and downstream consequences of MT stabilization during epithelial morphogenesis are still unclear. We show that the anterograde kinesin KIF17 localizes to MT plus ends, stabilizes MTs, and affects epithelial architecture. Targeting of KIF17 to plus ends of growing MTs requires kinesin motor activity and interaction with EB1. In turn, KIF17 participates in localizing adenomatous polyposis coli (APC) to the plus ends of a subset of MTs. We found that KIF17 affects MT dynamics, polymerization rates, and MT plus end stabilization to generate posttranslationally acetylated MTs. Depletion of KIF17 from cells growing in three-dimensional matrices results in aberrant epithelial cysts that fail to generate a single central lumen and to polarize apical markers. These findings implicate KIF17 in MT stabilization events that contribute to epithelial polarization and morphogenesis.

Pak1 regulates branching morphogenesis in 3D MDCK cell culture by a PIX and beta1-integrin-dependent mechanism

Branching morphogenesis is a fundamental process in the development of the kidney. This process gives rise to a network of ducts, which form the collecting system. Defective branching can lead to a multitude of kidney disorders including agenesis and reduced nephron number. The formation of branching tubules involves changes in cell shape, cell motility, and reorganization of the cytoskeleton. However, the exact intracellular mechanisms involved are far from understood. We have used the three-dimensional (3D) Madin-Darby canine kidney (MDCK) cell culture system to study how p21-activated kinase 1 (Pak1), which is an important regulator of the cytoskeleton, modulates branching. Our data reveal that Pak1 plays a crucial role in regulating branching morphogenesis. Expression of a dominant-negative Pak1 mutant (DN-Pak1) in MDCK cysts resulted in the spontaneous formation of extensions and branching tubules. Cellular contractility and levels of phosphorylated myosin light chain (pMLC) were increased in DN-Pak1 cells in collagen. Expression of a DN-Pak1 mutant that does not bind to PIX (DN-Pak1-DeltaPIX) failed to form extensions in collagen and did not have increased contractility. This shows that the DN-Pak1 mutant requires PIX binding to generate extensions and increased contractility in 3D culture. Furthermore, a beta1-integrin function-blocking antibody (AIIB2) inhibited the formation of branches and blocked the increased contractility in DN-Pak1 cysts. Taken together, our work shows that DN-Pak1-induced branching morphogenesis requires PIX binding and beta1-integrin signaling.

Pancreatic ductal morphogenesis and the Pdx1 homeodomain transcription factor

Embryonic development of the pancreas is marked by an early phase of dramatic morphogenesis, in which pluripotent progenitor cells of the developing pancreatic epithelium give rise to the full array of mature exocrine and endocrine cell types. The genetic determinants of acinar and islet cell lineages are somewhat well defined; however, the molecular mechanisms directing ductal formation and differentiation remain to be elucidated. The complex ductal architecture of the pancreas is established by a reiterative program of progenitor cell expansion and migration known as branching morphogenesis, or tubulogenesis, which proceeds in mouse development concomitantly with peak Pdx1 transcription factor expression. We therefore evaluated Pdx1 expression with respect to lineage-specific markers in embryonic sections of the pancreas spanning this critical period of duct formation and discovered an unexpected population of nonislet Pdx1-positive cells displaying physical traits of branching. We then established a 3D cell culture model of branching morphogenesis using primary pancreatic duct cells and identified a transient surge of Pdx1 expression exclusive to branching cells. From these observations we propose that Pdx1 might be involved temporally in a program of gene expression sufficient to facilitate the biochemical and morphological changes necessary for branching morphogenesis.

Culturing MDCK cells in three dimensions for analyzing intracellular dynamics

Epithelial cells grown in three-dimensional (3-D) cultures of extracellular matrix differentiate into a multicellular structure of polarized cells. This process shares many characteristics with the physiological development of an epithelial tissue and the formation of polarity in epithelial cells. Imaging 3-D cultures of polarized epithelial cells is therefore a powerful tool to study epithelial architecture and morphogenesis under close-to-physiological conditions. The new generation of confocal microscopes allows live-cell imaging of fluorescently tagged molecules in these cultures. This opens up new opportunities for studying how molecules behave and are distinguished asymmetrically within a 3-D setting. This unit discusses technical aspects for culturing and imaging MDCK 3-D culture for both fixed 3-D cultures and live-cell imaging.

Liver progenitor cells fold up a cell monolayer into a double-layered structure during tubular morphogenesis

Bile ducts are hepatic tubular structures that are lined by cholangiocytes, a type of liver epithelial cell. Cholangiocytes first form a single layer of cells, termed the ductal plate, surrounding the portal vein, which eventually remodels into the branching tubular network of bile ducts. The process of bile duct morphogenesis is not yet clear: a conventional model where cholangiocytes proliferate to duplicate a single layer of the ductal plate before lumen formation seems inconsistent with the observation that proliferation is dramatically reduced when hepatoblasts, liver progenitor cells, differentiate into cholangiocytes. Here, we developed a new culture system in which a liver progenitor cell line, HPPL, reorganizes from a monolayer to tubular structures in response to being overlaid with a gel containing type I collagen and Matrigel. We found that some of the HPPL in the monolayer depolarized and migrated to fold up the monolayer into a double-cell layer. These morphogenetic processes occurred without cell proliferation and required phosphatidylinositol 3-kinase and Akt activity. Later in morphogenesis, luminal space was generated between the two cell layers. This process, in particular enlargement of the apical lumen, involved transcriptional activity of HNF1beta. Thus, using this sandwich culture system, we could segregate tubulogenesis of bile ducts into distinct steps and found that the PI3K/Akt pathway and HNF1beta regulated different steps of the morphogenesis. Although the process of tubulogenesis in culture specifically resembled early bile duct formation, involvement of these two key players suggests that the sandwich culture might help us to find common principles of tubulogenesis in general.

Modelling tissues in 3D: the next future of pharmaco-toxicology and food research?

The development and validation of reliable in vitro methods alternative to conventional in vivo studies in experimental animals is a well-recognised priority in the fields of pharmaco-toxicology and food research. Conventional studies based on two-dimensional (2-D) cell monolayers have demonstrated their significant limitations: the chemically and spatially defined three-dimensional (3-D) network of extracellular matrix components, cell-to-cell and cell-to-matrix interactions that governs differentiation, proliferation and function of cells in vivo is, in fact, lost under the simplified 2-D condition. Being able to reproduce specific tissue-like structures and to mimic functions and responses of real tissues in a way that is more physiologically relevant than what can be achieved through traditional 2-D cell monolayers, 3-D cell culture represents a potential bridge to cover the gap between animal models and human studies. This article addresses the significance and the potential of 3-D in vitro systems to improve the predictive value of cell-based assays for safety and risk assessment studies and for new drugs development and testing. The crucial role of tissue engineering and of the new microscale technologies for improving and optimising these models, as well as the necessity of developing new protocols and analytical methods for their full exploitation, will be also discussed.

The transforming Rho family GTPase Wrch-1 disrupts epithelial cell tight junctions and epithelial morphogenesis

Wrch-1, an atypical and transforming Rho GTPase, regulates cellular activities including proliferation and actin organization, but its functions and effectors remain poorly characterized. We show here that Wrch-1 distributes along the apical and basolateral membranes in MDCK cells and binds the cell polarity protein Par6 in a GTP-dependent manner. Activated Wrch-1 negatively regulates the kinetics of tight junction (TJ) assembly during epithelial cell polarization but has no detectable effect on overall cell polarity in confluent monolayers. It also causes a dramatic cytoskeletal reorganization and multilayering in cells grown in two-dimensional culture and disrupts cystogenesis of cells grown in three-dimensional (3D) culture. Similarly, short hairpin RNA-mediated knockdown of Wrch-1 perturbs cystogenesis in 3D culture, suggesting that tight regulation of Wrch-1 activity is necessary for normal epithelial morphogenesis. A weakly transforming effector domain mutant of activated Wrch-1 that inhibits Par6 binding abrogates the ability of Wrch-1 to disrupt TJ formation, actin organization, and epithelial morphogenesis. We hypothesize that Wrch-1-induced morphological and growth transformation may occur in part through Par6-mediated disruption of TJs and actin organization.

Met receptor tyrosine kinase degradation is altered in response to the leucine-rich repeat of the Listeria invasion protein internalin B

Entry of the bacterial pathogen Listeria monocytogenes into host epithelial cells is critical for infection and virulence. One major pathway for Listeria entry involves binding of the bacterial protein Internalin B to the host receptor tyrosine kinase Met (hepatocyte growth factor receptor). Activation of Met and downstream signaling cascades is critical for Listeria entry. Internalin B is composed of several structural domains including an N-terminal leucine-rich repeat that is sufficient for binding Met and stimulating downstream signal transduction. Internalin B is monomeric, whereas the leucine-rich repeat is dimeric when expressed as an isolated fragment. The different quaternary states of Internalin B and the leucine-rich repeat suggest that these two Met ligands might cause distinct biological effects. Here we demonstrate that Internalin B and the leucine-rich repeat fragment exhibit agonist properties that differentially influence Met down-regulation in lysosomes. Specifically, Met stability is increased in response to the leucine-rich repeat fragment compared with Internalin B. Interestingly, Internalin B and the leucine-rich repeat stimulate equivalent rates of clathrin-mediated Met internalization. However, the leucine-rich repeat is defective in promoting lysosomal down-regulation of Met and instead enhances receptor recycling to the cell surface. In addition, the leucine-rich repeat causes prolonged Met activation (phosphorylation) and increased cell motility compared with Internalin B. Taken together, our findings indicate that individual domains of Internalin B differentially regulate Met trafficking. The ability of the leucine-rich repeat fragment to promote Met recycling could account for the increased cell motility induced by this ligand.

Three-dimensional lithographically defined organotypic tissue arrays for quantitative analysis of morphogenesis and neoplastic progression

Here, we describe a simple micromolding method to construct three-dimensional arrays of organotypic epithelial tissue structures that approximate in vivo histology. An elastomeric stamp containing an array of posts of defined geometry and spacing is used to mold microscale cavities into the surface of type I collagen gels. Epithelial cells are seeded into the cavities and covered with a second layer of collagen. The cells reorganize into hollow tissues corresponding to the geometry of the cavities. Patterned tissue arrays can be produced in 3-4 h and will undergo morphogenesis over the following 1-3 d. The protocol can easily be adapted to study a variety of tissues and aspects of normal and neoplastic development.

Association of tetraspanin CD9 with transmembrane TGF{alpha} confers alterations in cell-surface presentation of TGF{alpha} and cytoskeletal organization

Ligand presentation is a major determinant of receptor activation. The epidermal growth factor receptor (EGFR), a tyrosine kinase receptor, is activated by growth factors of the transforming growth factor alpha (TGFalpha) family. The tetraspanin CD9 interacts with transmembrane TGFalpha and decreases its ectodomain shedding to release soluble TGFalpha. Here we report that CD9 has a role in the maturation of transmembrane TGFalpha and its stabilization at the cell surface, and in the cell-surface distribution in polarized epithelial cells. Furthermore, coexpression of CD9 and TGFalpha confers changes in cytoskeletal organization with a decrease in actin stress fibers and focal adhesions, and changes in RhoA and Rac1 GTPase activity. These alterations are reversed by blocking EGFR signaling. Finally, we demonstrate changes in cell adhesion and migration resulting from coexpression of TGFalpha with CD9. These results provide insight into the role of CD9 in the presentation of TGFalpha in epithelial and carcinoma cells, whose physiology is driven by ligand-induced EGFR activation.

Par3 functions in the biogenesis of the primary cilium in polarized epithelial cells

Par3 is a PDZ protein important for the formation of junctional complexes in epithelial cells. We have identified an additional role for Par3 in membrane biogenesis. Although Par3 was not required for maintaining polarized apical or basolateral membrane domains, at the apical surface, Par3 was absolutely essential for the growth and elongation of the primary cilium. The activity reflected its ability to interact with kinesin-2, the microtubule motor responsible for anterograde transport of intraflagellar transport particles to the tip of the growing cilium. The Par3 binding partners Par6 and atypical protein kinase C interacted with the ciliary membrane component Crumbs3 and we show that the PDZ binding motif of Crumbs3 was necessary for its targeting to the ciliary membrane. Thus, the Par complex likely serves as an adaptor that couples the vectorial movement of at least a subset of membrane proteins to microtubule-dependent transport during ciliogenesis.

Parvin-beta inhibits breast cancer tumorigenicity and promotes CDK9-mediated peroxisome proliferator-activated receptor gamma 1 phosphorylation

Parvin-beta is a focal adhesion protein downregulated in human breast cancer cells. Loss of Parvin-beta contributes to increased integrin-linked kinase activity, cell-matrix adhesion, and invasion through the extracellular matrix in vitro. The effect of ectopic Parvin-beta expression on the transcriptional profile of MDA-MB-231 breast cancer cells, which normally do not express Parvin-beta, was evaluated. Particular emphasis was placed upon propagating MDA-MB-231 breast cancer cells in three-dimensional culture matrices. Interestingly, Parvin-beta reexpression in MDA-MB-231 cells increased the mRNA expression, serine 82 phosphorylation (mediated by CDK9), and activity of the nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPARgamma), and there was a concomitant increase in lipogenic gene expression as a downstream effector of PPARgamma. Importantly, Parvin-beta suppressed breast cancer growth in vivo, with associated decreased proliferation. These data suggest that Parvin-beta might influence breast cancer progression.

Chinese medicinal compound delisheng has satisfactory anti-tumor activity, and is associated with up-regulation of endostatin in human hepatocellular carcinoma cell line HepG2 in three-dimensional culture

AIM: To investigate the multicellular resistance of human hepatocellular carcinoma HepG2 cells in three-dimensional culture to delisheng, 5-fluorouracil and adriamycin, and the possible molecular mechanisms of delisheng.

METHODS: Human hepatocellular carcinoma HepG2 cells were cultured with a liquid overlay technique. After the formation of multicellular spheroids, morphology was analyzed by phase contrast microscopy, scanning electron microscopy and transmission electron microscopy. Sensitivity of HepG2 cells to delisheng, 5-fluorouracil and adriamycin was investigated by MTT assay in multicelluar spheroids and monolayers. Vascular endothelial growth factor (VEGF) and endostatin expression were analyzed in multicellular spheroids treated with delisheng, 5-fluorouracil, adriamycin and negative control PBS, with immunohistochemical staining.

RESULTS: Multicellular spheroids exhibited structural characteristics somewhat different to those in monolayers. The cells in three-dimensional cell culture turned out to be less sensitive to delisheng, 5-fluorouracil and adriamycin than the cells cultured in monolayer. This showed that delisheng had a satisfactory cells inhibition ratio compared to 5-fluorouracil and adriamycin. Immunohistochemical staining showed that VEGF and endostatin expression was positive during growth as multicellular spheroids, and endostatin expression in spheroids with treatment of delisheng was higher than that with 5-fluorouracil, adriamycin and PBS (139.35 ± 7.83, 159.23 ± 10.34, 162.83 ± 3.47 and 148.48 ± 11.06, P < 0.05).

CONCLUSION: Chinese medicine compound delisheng has satisfactory anti-tumor activity in HepG2 cells in three-dimensional culture, and the effects are associated with up-regulation of endostatin.

The third dimension bridges the gap between cell culture and live tissue

Moving from cell monolayers to three-dimensional (3D) cultures is motivated by the need to work with cellular models that mimic the functions of living tissues. Essential cellular functions that are present in tissues are missed by 'petri dish'-based cell cultures. This limits their potential to predict the cellular responses of real organisms. However, establishing 3D cultures as a mainstream approach requires the development of standard protocols, new cell lines and quantitative analysis methods, which include well-suited three-dimensional imaging techniques. We believe that 3D cultures will have a strong impact on drug screening and will also decrease the use of laboratory animals, for example, in the context of toxicity assays.

Effects of 3-methoxy-2(5H)-furanone-containing extracts from Narthecium ossifragum (L.) Huds. on renal tubular cells in vitro

Narthecium ossifragum, a perennial herb of the lily family, causes toxic renal tubular necrosis in several ruminant species. 3-Methoxy-2(5H)-furanone (3M2F) has been identified as a nephrotoxin present in N. ossifragum extracts. We studied effects of three different 3M2F-containing fractions isolated from N. ossifragum and synthetic 3M2F on the porcine kidney cell line LLC-PK1. In some of the experiments, we included the glioma cell lines U251 and BT4Cn to compare the effects of the toxin on LLC-PK1 cells to the effect on these cell lines. The synthetic 3M2F was shown to be only mildly toxic, and the most purified fraction from N. ossifragum showed the highest degree of toxicity in our studies. When monolayer cultures were exposed to increasing amounts of 3M2F-containing extract, a dose-dependent increase in cell death was observed. Similarly, reduced neutral red uptake and 3H-thymidine uptake (DNA synthesis) was observed. There was increased apoptotic activity in the LLC-PK1 cells with increasing concentration of 3M2F-containing extract. Multicellular three-dimensional spheroids from LLC-PK1 cells stopped fluid transport, showed degenerative changes and collapsed totally 6 h after extract exposure. Our findings indicate junctional damage, reduced cellular endocytosis and DNA-synthesis as well as induction of apoptosis as possible mechanisms for the acute tubular necrosis observed in ruminant species.

Liver progenitor cells develop cholangiocyte-type epithelial polarity in three-dimensional culture

Cholangiocytes are cellular components of the bile duct system of the liver, which originate from hepatoblasts during embryonic liver development. Although several transcription factors and signaling molecules have been implicated in bile duct development, its molecular mechanism has not been studied in detail. Here, we applied a three-dimensional (3D) culture technique to a liver progenitor cell line, HPPL, to establish an in vitro culture system in which HPPL acquire differentiated cholangiocyte characteristics. When HPPL were grown in a gel containing Matrigel, which contains extracellular matrix components of basement membrane, HPPL developed apicobasal polarity and formed cysts, which had luminal space inside. In the cysts, F-actin bundles and atypical protein kinase C were at the apical membrane, E-cadherin was localized at the lateral membrane, and beta-catenin and integrin alpha6 were located at the basolateral membrane. HPPL in cysts expressed cholangiocyte markers, including cytokeratin 19, integrin beta4, and aquaporin-1, but not a hepatocyte marker, albumin. Furthermore, HPPL transported rhodamine 123, a substrate for multidrug resistance gene products, from the basal side to the central lumen. These data indicate that HPPL develop cholangiocyte-type epithelial polarity in 3D culture. Phosphatidylinositol 3-kinase signaling was essential for proliferation and survival of HPPL in culture, whereas laminin-1 was a crucial component of Matrigel for inducing epithelial polarization of HPPL. Because HPPL cysts display structural and functional similarities with bile ducts, the 3D culture of HPPL recapitulates in vivo cholangiocyte differentiation and is useful to study the molecular mechanism of bile duct development in vitro.