Polarized membrane traffic and cell polarity development is dependent on dihydroceramide synthase-regulated sphinganine turnover

Revisiting the therapeutic potential of tocotrienol

The therapeutic potential of the tocotrienol group stems from its nutraceutical properties as a dietary supplement. It is largely considered to be safe when consumed at low doses for attenuating pathophysiology as shown by animal models, in vitro assays, and ongoing human trials. Medical researchers and the allied sciences have experimented with tocotrienols for many decades, but its therapeutic potential was limited to adjuvant or concurrent treatment regimens. Recent studies have focused on targeted drug delivery by enhancing the bioavailability through carriers, self-sustained emulsions, nanoparticles, and ethosomes. Epigenetic modulation and computer remodeling are other means that will help increase chemosensitivity. This review will focus on the systemic intracellular anti-cancer, antioxidant, and anti-inflammatory mechanisms that are stimulated and/or regulated by tocotrienols while highlighting its potent therapeutic properties in a diverse group of clinical diseases.

Hepatitis C virus core protein triggers abnormal porphyrin metabolism in human hepatocellular carcinoma cells

Porphyria cutanea tarda (PCT), the most common of the human porphyrias, arises from a deficiency of uroporphyrinogen decarboxylase. Studies have shown a high prevalence of hepatitis C virus (HCV) infection in patients with PCT. While these observations implicate HCV infection as a risk factor for PCT pathogenesis, the mechanism of interaction between the virus and porphyrin metabolism is unknown. This study aimed to assess the effect of HCV core protein on intracellular porphyrin metabolism to elucidate the link between HCV infection and PCT. The accumulation and excretion of porphyrins after treatment with 5-aminolevulinic acid, a porphyrin precursor, were compared between cells stably expressing HCV core protein and controls. Cells expressing HCV core protein had lower amounts of intracellular protoporphyrin IX and heme and had higher amounts of excreted coproporphyrin III, the oxidized form of coproporphyrinogen III, compared with controls. These observations suggest that HCV core protein affects porphyrin metabolism and facilitates the export of excess coproporphyrinogen III and/or coproporphyrin III, possibly via porphyrin transporters. Real-time PCR analysis revealed that the presence of HCV core protein increased the mRNA expression of porphyrin exporters ABCG2 and FLVCR1. Western blot analysis showed a higher expression level of FLVCR1, but not ABCG2, as well as a higher expression level of mature ALAS1, which is the rate-limiting enzyme in the heme synthesis pathway, in HCV core protein-expressing cells compared with controls. The data indicate that HCV core protein induced abnormal intracellular porphyrin metabolism, with an over-excretion of coproporphyrin III. These findings may partially account for the susceptibility of HCV-infected individuals to PCT development.

Study of Ethanol-Induced Golgi Disorganization Reveals the Potential Mechanism of Alcohol-Impaired N-Glycosylation

BACKGROUND

It is known that ethanol (EtOH) and its metabolites have a negative effect on protein glycosylation. The fragmentation of the Golgi apparatus induced by alteration of the structure of largest Golgi matrix protein, giantin, is the major consequence of damaging effects of EtOH-metabolism on the Golgi; however, the link between this and abnormal glycosylation remains unknown. Because previously we have shown that Golgi morphology dictates glycosylation, we examined the effect EtOH administration has on function of Golgi residential enzymes involved in N-glycosylation.

METHODS

HepG2 cells transfected with mouse ADH1 (VA-13 cells) were treated with 35 mM EtOH for 72 hours. Male Wistar rats were pair-fed Lieber-DeCarli diets for 5 to 8 weeks. Characterization of Golgi-associated mannosyl (α-1,3-)-glycoprotein beta-1,2-N-acetylglucosaminyltransferase (MGAT1), α-1,2-mannosidase (Man-I), and α-mannosidase II (Man-II) were performed in VA-13 cells and rat hepatocytes followed by three-dimensional structured illumination microscopy (3D SIM).

RESULTS

First, we detected that EtOH administration results in the loss of sialylated N-glycans on asialoglycoprotein receptor; however, the high-mannose-type N-glycans are increased. Further analysis by 3D SIM revealed that EtOH treatment despite Golgi disorganization does not change cis-Golgi localization for Man-I, but does induce medial-to-cis relocation of MGAT1 and Man-II. Using different approaches, including electron microscopy, we revealed that EtOH treatment results in dysfunction of ADP-ribosylation factor 1 (Arf1) GTPase followed by a deficiency in COPI vesicles at the Golgi. Silencing beta-COP or expression of GDP-bound mutant Arf1(T31N) mimics the EtOH effect on retaining MGAT1 and Man-II at the cis-Golgi, suggesting that (i) EtOH specifically blocks activation of Arf1, and (ii) EtOH alters the proper localization of Golgi enzymes through impairment of COPI. Importantly, the level of MGAT1 was reduced, because likely MGAT1, contrary to Man-I and Man-II, is giantin sensitive.

CONCLUSIONS

Thus, we provide the mechanism by which EtOH-induced Golgi remodeling may significantly modify formation of N-glycans.

Par1b induces asymmetric inheritance of plasma membrane domains via LGN-dependent mitotic spindle orientation in proliferating hepatocytes

Proliferating hepatocytes in the liver show an atypical, asymmetric mode of cell division, which is coordinated by Par1b and LGN and may explain the unique tissue architecture of the liver.

The development and maintenance of polarized epithelial tissue requires a tightly controlled orientation of mitotic cell division relative to the apical polarity axis. Hepatocytes display a unique polarized architecture. We demonstrate that mitotic hepatocytes asymmetrically segregate their apical plasma membrane domain to the nascent daughter cells. The non-polarized nascent daughter cell can form a de novo apical domain with its new neighbor. This asymmetric segregation of apical domains is facilitated by a geometrically distinct “apicolateral” subdomain of the lateral surface present in hepatocytes. The polarity protein partitioning-defective 1/microtubule-affinity regulating kinase 2 (Par1b/MARK2) translates this positional landmark to cortical polarity by promoting the apicolateral accumulation of Leu-Gly-Asn repeat-enriched protein (LGN) and the capture of nuclear mitotic apparatus protein (NuMA)–positive astral microtubules to orientate the mitotic spindle. Proliferating hepatocytes thus display an asymmetric inheritance of their apical domains via a mechanism that involves Par1b and LGN, which we postulate serves the unique tissue architecture of the developing liver parenchyma.

The development and maintenance of the polarized epithelial architecture and function of organs that form tubular “lumen” structures is important for normal physiology and, when deregulated, gives rise to disease. Recent studies have highlighted the importance of a strict coordination of the orientation of mitotic divisions relative to an internal axis of asymmetry in proliferating epithelial cells during this process. Hepatocytes are the predominant epithelial cells of the liver. Hepatocytes display a unique lumen-forming architecture and cellular asymmetry, but the molecular basis for this special polarized architecture is not well understood. Our study now reveals an unexpected mode of plasma membrane domain inheritance that is coupled to a cellular axis of asymmetry in proliferating mammalian hepatocytes. We show that mitotic hepatocytes asymmetrically segregate their apical plasma membrane (the membrane facing the lumen structure) along with the lumen to their daughter cells. We demonstrate that the coordinated action of two proteins, Par1b and LGN, constitutes a fundamental part of the underlying molecular mechanism. This coupling of cell division and polarity in hepatocytes is distinct from that established in other epithelial cell types. These findings are important for understanding the unique polarized tissue architecture in the developing liver.

Vesicular sorting controls the polarity of expanding membranes in the C. elegans intestine

Biological tubes consist of polarized epithelial cells with apical membranes building the central lumen and basolateral membranes contacting adjacent cells or the extracellular matrix. Cellular polarity requires distinct inputs from outside the cell, e.g., the matrix, inside the cell, e.g., vesicular trafficking and the plasma membrane and its junctions.¹ Many highly conserved polarity cues have been identified, but their integration during the complex process of polarized tissue and organ morphogenesis is not well understood. It is assumed that plasma-membrane-associated polarity determinants, such as the partitioning-defective (PAR) complex, define plasma membrane domain identities, whereas vesicular trafficking delivers membrane components to these domains, but lacks the ability to define them. In vitro studies on lumenal membrane biogenesis in mammalian cell lines now indicate that trafficking could contribute to defining membrane domains by targeting the polarity determinants, e.g., the PARs, themselves.² This possibility suggests a mechanism for PARs’ asymmetric distribution on membranes and places vesicle-associated polarity cues upstream of membrane-associated polarity determinants. In such an upstream position, trafficking might even direct multiple membrane components, not only polarity determinants, an original concept of polarized plasma membrane biogenesis^3,4that was largely abandoned due to the failure to identify a molecularly defined intrinsic vesicular sorting mechanism. Our two recent studies on C. elegans intestinal tubulogenesis reveal that glycosphingolipids (GSLs) and the well-recognized vesicle components clathrin and its AP-1 adaptor are required for targeting multiple apical molecules, including polarity regulators, to the expanding apical/lumenal membrane.^5,6 These findings support GSLs’ long-proposed role in in vivo polarized epithelial membrane biogenesis and development and identify a novel function in apical polarity for classical post-Golgi vesicle components. They are also compatible with a vesicle-intrinsic sorting mechanism during membrane biogenesis and suggest a model for how vesicles could acquire apical directionality during the assembly of the functionally critical polarized lumenal surfaces of epithelial tubes.

MYO5B mutations in patients with microvillus inclusion disease presenting with transient renal Fanconi syndrome

BACKGROUND AND OBJECTIVE

: Microvillus inclusion disease (MVID) is a rare congenital enteropathy associated with brush border atrophy and reduced expression of enzymes at the enterocytes' apical surface. MVID is associated with mutations in the MYO5B gene, which is expressed in all epithelial tissues. Whether organs other than the intestine are affected in MVID is unclear. We report 2 patients with MVID that developed renal Fanconi syndrome while receiving total parenteral nutrition. Renal Fanconi syndrome has been correlated to apical plasma membrane defects in kidney proximal tubular epithelial cells. The aim of the present study was to determine whether MYO5B mutations in these patients correlate with similar apical plasma membrane defects in renal tubular epithelial cells as observed in the intestine.

METHODS

: Biopsies from kidney, duodenum, ileum, jejunum, and colon of 2 patients with MVID carrying MYO5B mutations and of age-matched controls were fixed in paraffin and analyzed with immunohistochemistry and transmission electron microscopy.

RESULTS

: Structural defects of the brush border and apical recycling endosome organization are observed in enterocytes of all of the segments of the small intestine and colon. MYO5B mutations in patients with MVID with renal Fanconi syndrome do not correlate with aberrant apical plasma membrane morphology or altered apical recycling endosome organization in renal tubular epithelial cells.

CONCLUSIONS

: MYO5B mutations have divergent effects on the apical membrane system in kidney and intestinal epithelial cells. Epithelial defects presented in MVID are therefore likely triggered by intestine-specific factors, the identification of which may provide new targets and open avenues for the development of alternative therapeutic strategies to combat this devastating disease.

Distinct roles of two ceramide synthases, CaLag1p and CaLac1p, in the morphogenesis of Candida albicans

Lag1p and Lac1p catalyse ceramide synthesis in Saccharomyces cerevisiae. This study shows that Lag1 family proteins are generally required for polarized growth in hemiascomycetous yeast. However, in contrast to S. cerevisiae where these proteins are functionally redundant, C. albicans Lag1p (CaLag1p) and Lac1p (CaLac1p) are functionally distinct. Lack of CaLag1p, but not CaLac1p, caused severe defects in the growth and hyphal morphogenesis of C. albicans. Deletion of CaLAG1 decreased expression of the hypha-specific HWP1 and ECE1 genes. Moreover, overexpression of CaLAG1 induced pseudohyphal growth in this organism under non-hypha-inducing conditions, suggesting that CaLag1p is necessary for relaying signals to induce hypha-specific gene expression. Analysis of ceramide and sphingolipid composition revealed that CaLag1p predominantly synthesizes ceramides with C24:0/C26:0 fatty acid moieties, which are involved in generating inositol-containing sphingolipids, whereas CaLac1p produces ceramides with C18:0 fatty acid moieties, which are precursors for glucosylsphingolipids. Thus, our study demonstrates that CaLag1p and CaLac1p have distinct substrate specificities and physiological roles in C. albicans.

Apicobasal domain identities of expanding tubular membranes depend on glycosphingolipid biosynthesis

Metazoan internal organs are assembled from polarized tubular epithelia that must set aside an apical membrane domain as a lumenal surface. In a global Caenorhabditis elegans tubulogenesis screen, interference with several distinct fatty-acid-biosynthetic enzymes transformed a contiguous central intestinal lumen into multiple ectopic lumens. We show that multiple-lumen formation is caused by apicobasal polarity conversion, and demonstrate that in situ modulation of lipid biosynthesis is sufficient to reversibly switch apical domain identities on growing membranes of single post-mitotic cells, shifting lumen positions. Follow-on targeted lipid-biosynthesis pathway screens and functional genetic assays were designed to identify a putative single causative lipid species. They demonstrate that fatty-acid biosynthesis affects polarity through sphingolipid synthesis, and reveal ceramide glucosyltransferases (CGTs) as end-point biosynthetic enzymes in this pathway. Our findings identify glycosphingolipids, CGT products and obligate membrane lipids, as critical determinants of in vivo polarity and indicate that they sort new components to the expanding apical membrane.

Palmitate induces insulin resistance without significant intracellular triglyceride accumulation in HepG2 cells

Previous studies showed that increased release of free fatty acids from adipocytes leads to insulin resistance and triglyceride (TG) accumulation in the liver, which may progress into hepatic steatohepatitis. We and other investigators have previously reported that palmitate induces endoplasmic reticulum stress-mediated toxicity in several tissues. This work investigated whether palmitate could induce insulin resistance and steatosis in HepG2 cells. We treated cells with either saturated fatty acid (palmitate) or unsaturated fatty acid (oleate), and observed that palmitate significantly activated c-jun N-terminal kinase and inactivated protein kinase B. Both 4-phenylbutyric acid and glycerol significantly activated protein kinase B, confirming the involvement of endoplasmic reticulum stress in palmitate-mediated insulin resistance. Oleate, but not palmitate, significantly induced intracellular TG deposition and activated sterol regulatory element binding protein-1. Instead, diacylglycerol level and protein kinase C epsilon activity were significantly increased by palmitate, suggesting the possible role of diacylglycerol in palmitate-mediated lipotoxicity. Therefore, the present study clearly showed that palmitate impairs insulin resistance, but does not induce significant TG accumulation in HepG2 cells.

Sphingolipid transport

Sphingolipids are a family of ubiquitous membrane components that exhibit multiple functional properties fundamental to cell properties. Sphingolipid transport represents a crucial aspect in the metabolism, signaling and biological role of sphingolipids. Different mechanisms of sphingolipid movements contribute to their selective localization in different membranes but also in different portions and sides of the same membrane, thus ensuring and regulating their interaction with different enzymes and target molecules. In this chapter we will describe the knowledge of the different mechanisms ofsphingolipid movements within and between membranes, focusing on the recent advances in this field and considering the role played by selective sphingolipid molecules in the regulation of these mechanisms.

Regulatory subunit I-controlled protein kinase A activity is required for apical bile canalicular lumen development in hepatocytes

Signaling via cAMP plays an important role in apical cell surface dynamics in epithelial cells. In hepatocytes, elevated levels of cAMP as well as extracellular oncostatin M stimulate apical lumen development in a manner that depends on protein kinase A (PKA) activity. However, neither the identity of PKA isoforms involved nor the mechanisms of the cross-talk between oncostatin M and cAMP/PKA signaling pathways have been elucidated. Here we demonstrate that oncostatin M and PKA signaling converge at the level of the PKA holoenzyme downstream of oncostatin M-stimulated MAPK activation. Experiments were performed with chemically modified cAMP analogues that preferentially target regulatory subunit (R) I or RII holoenzymes, respectively, in hepatocytes. The data suggest that the dissociation of RI- but not RII-containing holoenzymes, as well as catalytic activity of PKA, is required for apical lumen development in response to elevated levels of cAMP and oncostatin M. However, oncostatin M signaling does not stimulate PKA holoenzyme dissociation in living cells. Based on pharmacological and cell biological studies, it is concluded that RI-controlled PKA activity is essential for cAMP- and oncostatin M-stimulated development of apical bile canalicular lumens.

cAMP-dependent protein kinase A and the dynamics of epithelial cell surface domains: moving membranes to keep in shape

Cyclic adenosine monophosphate (cAMP) and cAMP-dependent protein kinase A (PKA) are evolutionary conserved molecules with a well-established position in the complex network of signal transduction pathways. cAMP/PKA-mediated signaling pathways are implicated in many biological processes that cooperate in organ development including the motility, survival, proliferation and differentiation of epithelial cells. Cell surface polarity, here defined as the anisotropic organisation of cellular membranes, is a critical parameter for most of these processes. Changes in the activity of cAMP/PKA elicit a variety of effects on intracellular membrane dynamics, including membrane sorting and trafficking. One of the most intriguing aspects of cAMP/PKA signaling is its evolutionary conserved abundance on the one hand and its precise spatial-temporal actions on the other. Here, we review recent developments with regard to the role of cAMP/PKA in the regulation of intracellular membrane trafficking in relation to the dynamics of epithelial surface domains.

Expression, localization, and inducibility by bile acids of hepatobiliary transporters in the new polarized rat hepatic cell lines, Can 3−1 and Can 10

Sinusoidal and apical transporters are responsible for the uptake and biliary elimination of many compounds by hepatocytes. Few in vitro models are however available for analyzing such functions. The expression and bile-acid inducibility of 13 transporters and two nuclear receptors were investigated in the new rat polarized lines, Can 3-1 and Can 10, and in their unpolarized parent, Fao. The relative abundance of mRNA, the protein level, and their localization were examined by real-time quantitative PCR, Western blotting, immunofluorescence, and confocal microscopy. Compared with rat liver, mRNA levels of Fao cells were: negligible for Bsep/Abcb11; lower for the uptake transporters Ntcp and Oatps; similar for SHP, FXR, and Bcrp/Abcg2; and higher (four-fold to 160-fold) for the efflux pumps Mdr1b/Abcb1b, Mdr2/Abcb4, Mrp1/Abcc1, Mrp2/Abcc2, Mrp3/Abcc3, Abcg5, and Abcg8. This profile was mostly maintained (and improved for Bsep) in Can 10. Some transporters were less well expressed in Can 3-1. In both lines, sinusoidal (Ntcp, Mrp3) and canalicular transporters (Mdr-P-glycoproteins detected with C219 antibody, Mrp2) were localized at their correct poles. Bile-acid effects on polarity and mRNA levels of transporters were analyzed after a 6-day treatment with 50 microM taurocholic, chenodeoxycholic (CDCA), or ursodeoxycholic acid (UDCA). No polarization of Fao cells was induced; Can 10 and Can 3-1 polarity was maintained. CDCA and UDCA induced marked enhancement of the volume of Can 10 bile canaliculi. CDCA upregulated Bsep, Mdr2, SHP, Mdr1b, and Oatp2/1a4 in Can 10 (two- to seven-fold) and in Fao cells. Thus, Can 10 constitutes an attractive polarized model for studying vectorial hepatobiliary transport of endogenous and xenobiotic cholephilic compounds.

Anchoring of protein kinase A-regulatory subunit IIalpha to subapically positioned centrosomes mediates apical bile canalicular lumen development in response to oncostatin M but not cAMP

Oncostatin M and cAMP signaling stimulate apical surface-directed membrane trafficking and apical lumen development in hepatocytes, both in a protein kinase A (PKA)-dependent manner. Here, we show that oncostatin M, but not cAMP, promotes the A-kinase anchoring protein (AKAP)-dependent anchoring of the PKA regulatory subunit (R)IIalpha to subapical centrosomes and that this requires extracellular signal-regulated kinase 2 activation. Stable expression of the RII-displacing peptide AKAP-IS, but not a scrambled peptide, inhibits the association of RIIalpha with centrosomal AKAPs and results in the repositioning of the centrosome from a subapical to a perinuclear location. Concomitantly, common endosomes, but not apical recycling endosomes, are repositioned from a subapical to a perinuclear location, without significant effects on constitutive or oncostatin M-stimulated basolateral-to-apical transcytosis. Importantly, however, the expression of the AKAP-IS peptide completely blocks oncostatin M-, but not cAMP-stimulated apical lumen development. Together, the data suggest that centrosomal anchoring of RIIalpha and the interrelated subapical positioning of these centrosomes is required for oncostatin M-, but not cAMP-mediated, bile canalicular lumen development in a manner that is uncoupled from oncostatin M-stimulated apical lumen-directed membrane trafficking. The results also imply that multiple PKA-mediated signaling pathways control apical lumen development and that subapical centrosome positioning is important in some of these pathways.

Characterization of WIF-B9/R cells as an in vitro model with hepatocyte-like polarity and enhanced expression of canalicular ABC transporters involved in phase III of hepatic detoxification

The rat hepatoma/human fibroblast hybrid cell line WIF-B9 was developed to be used in studies requiring maintained hepatocyte-like polarity. To enhance their usefulness in order to investigate hepatic phase III detoxification process, we have characterized a subline of WIF-B9 cells (WIF-B9/R) obtained by exposure to progressively increasing cisplatin concentrations (up to 10 microM) and double sub-clonal selection. As compared to WIF-B9 cells, the cytostatic effect of cisplatin and doxorubicin on WIF-B9/R cells was lower (>10-fold), whereas the ability to reduce cell loading of cisplatin, doxorubicin, rhodamine 123 and calcein was higher. As their parent cells, WIF-B9/R cells express hepatocyte-like polarity. However, they have enhanced stable expression of Mdr1, Mrp1, Mrp2, Mrp3 and BCRP, but not Bsep/BSEP, as determined by real-time quantitative RT-PCR and western blot. Differentiation to hepatocyte-like phenotype was characterized by the formation of canalicular-like structures, containing in their membranes immunocytochemically detectable Mdr1, Mrp2 and BCRP. Functionality of these ABC transporters was evaluated by using specific substrates and inhibitors. Thus, canalicular-like structures were able to concentrate calcein, rhodamine 123 and doxorubicin. Moreover, verapamil, probenecid and Hoechst-33342 inhibited doxorubicin efflux and enhanced its content in WIF-B9/R cells. Probenecid inhibited calcein efflux and increased calcein cell load, but had no effect on cell loading of rhodamine 123, which was increased by verapamil and Hoechst-33342. In conclusion, WIF-B9/R cells are a useful model of polarized cells to study, in the absence of Bsep/BSEP, hepatic phase III of the detoxification process of several compounds whose canalicular transport is mediated by ABC proteins.

Hydrogen peroxide and hydroxyl radicals mediate palmitate-induced cytotoxicity to hepatoma cells: relation to mitochondrial permeability transition

We studied the toxicological responses of a human hepatoblastoma cell line (HepG2/C3A) to various free fatty acids (FFA) in order to identify the relation between reactive oxygen species (ROS) production and mitochondrial permeability transition (MPT). Exposure to the saturated FFA, palmitate, led to a time-dependent ROS production and hydrogen peroxide release as well as a loss of mitochondrial potential. The cytotoxicity of palmitate was significantly reduced by treating with scavengers of hydrogen peroxide, hydroxyl radical and the spin trap alpha-(4-pyridyl-1-oxide)-N-tert-butyl nitrone (POBN). Superoxide dismutase (SOD) mimics, nitric oxide scavenger, and inhibitor of de novo ceramide synthesis had no effect on the toxicity. MPT-inhibitor, cyclosporine, prevented the loss of mitochondrial potential but did not reduce the cytotoxicity. In contrast, inhibiting mitochondrial complexes I and III reduced the early potential loss and the cytotoxicity. These results suggest that palmitate-cytotoxicity to hepatoma cells is mediated through the production of H2O2 and *OH and independent of MPT.

Rho kinase, myosin-II, and p42/44 MAPK control extracellular matrix-mediated apical bile canalicular lumen morphogenesis in HepG2 cells

The molecular mechanisms that regulate multicellular architecture and the development of extended apical bile canalicular lumens in hepatocytes are poorly understood. Here, we show that hepatic HepG2 cells cultured on glass coverslips first develop intercellular apical lumens typically formed by a pair of cells. Prolonged cell culture results in extensive organizational changes, including cell clustering, multilayering, and apical lumen morphogenesis. The latter includes the development of large acinar structures and subsequent elongated canalicular lumens that span multiple cells. These morphological changes closely resemble the early organizational pattern during development, regeneration, and neoplasia of the liver and are rapidly induced when cells are cultured on predeposited extracellular matrix (ECM). Inhibition of Rho kinase or its target myosin-II ATPase in cells cultured on glass coverslips mimics the morphogenic response to ECM. Consistently, stimulation of Rho kinase and subsequent myosin-II ATPase activity by lipoxygenase-controlled eicosatetranoic acid metabolism inhibits ECM-mediated cell multilayering and apical lumen morphogenesis but not initial apical lumen formation. Furthermore, apical lumen remodeling but not cell multilayering requires basal p42/44 MAPK activity. Together, the data suggest a role for hepatocyte-derived ECM in the spatial organization of hepatocytes and apical lumen morphogenesis and identify Rho kinase, myosin-II, and MAPK as potentially important players in different aspects of bile canalicular lumen morphogenesis.

Efficient trafficking of MDR1/P-glycoprotein to apical canalicular plasma membranes in HepG2 cells requires PKA-RIIalpha anchoring and glucosylceramide

In hepatocytes, cAMP/PKA activity stimulates the exocytic insertion of apical proteins and lipids and the biogenesis of bile canalicular plasma membranes. Here, we show that the displacement of PKA-RIIalpha from the Golgi apparatus severely delays the trafficking of the bile canalicular protein MDR1 (P-glycoprotein), but not that of MRP2 (cMOAT), DPP IV and 5'NT, to newly formed apical surfaces. In addition, the direct trafficking of de novo synthesized glycosphingolipid analogues from the Golgi apparatus to the apical surface is inhibited. Instead, newly synthesized glucosylceramide analogues are rerouted to the basolateral surface via a vesicular pathway, from where they are subsequently endocytosed and delivered to the apical surface via transcytosis. Treatment of HepG2 cells with the glucosylceramide synthase inhibitor PDMP delays the appearance of MDR1, but not MRP2, DPP IV, and 5'NT at newly formed apical surfaces, implicating glucosylceramide synthesis as an important parameter for the efficient Golgi-to-apical surface transport of MDR1. Neither PKA-RIIalpha displacement nor PDMP inhibited (cAMP-stimulated) apical plasma membrane biogenesis per se, suggesting that other cAMP effectors may play a role in canalicular development. Taken together, our data implicate the involvement of PKA-RIIalpha anchoring in the efficient direct apical targeting of distinct proteins and glycosphingolipids to newly formed apical plasma membrane domains and suggest that rerouting of Golgi-derived glycosphingolipids may underlie the delayed Golgi-to-apical surface transport of MDR1.

How to induce non-polarized cells of hepatic origin to express typical hepatocyte polarity: generation of new highly polarized cell models with developed and functional bile canaliculi

Few in vitro models expressing complex hepatocyte polarity are available. We used the unpolarized rat Fao cell line to isolate the polarized WIF-B line. These complex rat-human hybrid cells form functional simple bile canaliculi. To obtain Fao-derived polarized models with a simpler chromosome content and developed bile canaliculi, we employed two approaches. Partial success was achieved with monochromosomal hybrids. As shown by the immunolocalization of apical, basolateral, and tight-junctional proteins, monochromosomal hybrid 11-3 cells were polarized. They formed simple functional bile canaliculi and transiently expressed the typical polarity of simple epithelial cells. One subclone blocked in this polarity state was isolated. A more robust approach was provided by spheroid culture, a three-dimensional system that strengthens cell-cell contacts. Transient spheroid culture induced irreversible polarization of Fao cells. This induction occurred in most spheroids (approximately 1% of the cells). From populations enriched in stably polarized cells, we generated new polarized cell models, designated Can. Can 3-1 cells formed simple functional bile canaliculi when plated at high density. Regardless of plating density, Can 9 and Can 10 cells formed long tubular branched canaliculi competent for vectorial transport of organic anions and bile acids, and involving several dozen adjacent cells. Thus, we have generated new cell models stably expressing typical hepatocyte polarity. Among these models, Can 9 and Can 10 are the first capable of forming functional, highly developed bile canaliculi similar to those formed in vivo.

Improved visualization and quantitative analysis of fluorescent membrane sterol in polarized hepatic cells

Dehydroergosterol is a natural yeast sterol which has recently been employed for direct observation of intracellular sterol transport by UV microscopy. Here, methods are described for improved visualization and quantification of dehydroergosterol in the membranes of polarized HepG2 cells. Using a new online assay, it is shown that dehydroergosterol derived from a cyclodextrin complex inserted into the plasma membrane with a half time of t(1/2) approximately 34 s. Based on a detailed bleaching analysis of dehydroergosterol, slightly different bleaching rates for dehydroergosterol in the basolateral and canalicular membrane were found, indicating different fluorophore environments. Bleaching correction in concert with 3D imaging allows for detection of dehydroergosterol enrichment in microvilli of the canalicular membrane forming the biliary canaliculus. Evidence is provided that some dehydroergosterol accumulating in a subapical compartment or apical recycling compartment can rapidly (t(1/2) approximately 2 min) exchange in vesicles towards the biliary canaliculus while the majority of dehydroergosterol does not redistribute from this compartment. The rapidly exchanging pool resembles only a small portion of the total subapical compartment or apical recycling compartment-associated dehydroergosterol (about 15-30%). Kinetic modelling supports the theory that the subapical compartment or apical recycling compartment to biliary canaliculus transport pathway for sterol is unidirectional. This pathway might be important for rapid biliary transport of free sterol produced by hydrolysis of cholesteryl esters derived from high density lipoprotein.

Evolving endosomes: how many varieties and why?

The cell biologist's insight into endosomal diversity, in terms of both form and function, has increased dramatically in the past few years. This understanding has been promoted by the availability of powerful new techniques that allow imaging of both cargo and machinery in the endocytic process in real time, and by our ability to inhibit components of this machinery by RNA interference. The emerging picture from these studies is of a highly complex, dynamic and adaptable endosomal system that interacts at various points with the secretory system of the cell.