Small hepatocytes in culture develop polarized transporter expression and differentiation

"Small Hepatocytes" in the Liver

Mature hepatocytes (MHs) in an adult rodent liver are categorized into the following three subpopulations based on their proliferative capability: type I cells (MH-I), which are committed progenitor cells that possess a high growth capability and basal hepatocytic functions; type II cells (MH-II), which possess a limited proliferative capability; and type III cells (MH-III), which lose the ability to divide (replicative senescence) and reach the final differentiated state. These subpopulations may explain the liver's development and growth after birth. Generally, small-sized hepatocytes emerge in mammal livers. The cells are characterized by being morphologically identical to hepatocytes except for their size, which is substantially smaller than that of ordinary MHs. We initially discovered small hepatocytes (SHs) in the primary culture of rat hepatocytes. We believe that SHs are derived from MH-I and play a role as hepatocytic progenitors to supply MHs. The population of MH-I (SHs) is distributed in the whole lobules, a part of which possesses a self-renewal capability, and decreases with age. Conversely, injured livers of experimental models and clinical cases showed the emergence of SHs. Studies demonstrate the involvement of SHs in liver regeneration. SHs that appeared in the injured livers are not a pure population but a mixture of two distinct origins, MH-derived and hepatic-stem-cell-derived cells. The predominant cell-derived SHs depend on the proliferative capability of the remaining MHs after the injury. This review will focus on the SHs that appeared in the liver and discuss the significance of SHs in liver regeneration.

Hepatocytic parental progenitor cells of rat small hepatocytes maintain self-renewal capability after long-term culture

The liver has a variety of functions for maintaining homeostasis, and hepatocytes play a major role. In contrast with the high regenerative capacity of mature hepatocytes (MHs) in vivo, they have not been successfully expanded ex vivo. Here we demonstrate that CD44-positive cells sorted from small hepatocyte (SH) colonies derived from a healthy adult rat liver can proliferate on a Matrigel-coated dish in serum-free chemically defined medium; in addition, a subpopulation of the cells can divide more than 50 times in a period of 17 weeks every 4-week-passage. The passage cells retained the capability to recover highly differentiated functions, such as glycogen storage, CYP activity and bile secretion. When Matrigel-treated cells from the third passage were transplanted into retrorsine/partial hepatectomy-treated rat livers, the cells engrafted to differentiate into MHs and cholangiocytes. These results suggest that long-term cultured CD44+ SHs retain hepatocytic characteristics in vitro and the capability to differentiate into hepatocytes and cholangiocytes in vivo. Thus, a newly identified subpopulation of MHs possessing the attributes of hepatocytic stem/progenitor cells can be passaged several times without losing hepatocytic characteristics.

Bile Acids Act as Soluble Host Restriction Factors Limiting Cytomegalovirus Replication in Hepatocytes

The liver constitutes a prime site of cytomegalovirus (CMV) replication and latency. Hepatocytes produce, secrete, and recycle a chemically diverse set of bile acids, with the result that interactions between bile acids and cytomegalovirus inevitably occur. Here we determined the impact of naturally occurring bile acids on mouse CMV (MCMV) replication. In primary mouse hepatocytes, physiological concentrations of taurochenodeoxycholic acid (TCDC), glycochenodeoxycholic acid, and to a lesser extent taurocholic acid significantly reduced MCMV-induced gene expression and diminished the generation of virus progeny, while several other bile acids did not exert antiviral effects. The anticytomegalovirus activity required active import of bile acids via the sodium-taurocholate-cotransporting polypeptide (NTCP) and was consistently observed in hepatocytes but not in fibroblasts. Under conditions in which alpha interferon (IFN-α) lacks antiviral activity, physiological TCDC concentrations were similarly effective as IFN-γ. A detailed investigation of distinct steps of the viral life cycle revealed that TCDC deregulates viral transcription and diminishes global translation in infected cells.

IMPORTANCE

Cytomegaloviruses are members of the Betaherpesvirinae subfamily. Primary infection leads to latency, from which cytomegaloviruses can reactivate under immunocompromised conditions and cause severe disease manifestations, including hepatitis. The present study describes an unanticipated antiviral activity of conjugated bile acids on MCMV replication in hepatocytes. Bile acids negatively influence viral transcription and exhibit a global effect on translation. Our data identify bile acids as site-specific soluble host restriction factors against MCMV, which may allow rational design of anticytomegalovirus drugs using bile acids as lead compounds.

Aggregate culture: A more accurate predictor of microcystin toxicity for risk assessment

Aggregate or spheroid culture has emerged as a more biologically relevant method for screening pharmaceutical compounds and understanding exact mechanism of action. Here in, the aggregate approach applied to the freshwater toxins, microcystins, further unearths exact mechanism(s) of toxicity and provides a markedly improved in vitro predictor of toxicity. Microcystins result in acute intoxication by binding covalently to protein phosphatase 1/2A, resulting in hepatocellular necrosis, hemorrhaging and death. Hepatocellular uptake by organic anion transporting polypeptides (OATPs), in addition to other intracellular sequelae, is considered essential for toxicity. In aggregate HepG2, expression of OAT1B1 and OATP1B3 significantly increased relative to monolayer culture. Uptake of two fluorescently labeled substrates significantly increased in aggregates compared with monolayer, confirmed by inhibition of uptake with known competitive substrates. Increased reaction oxygen species (ROS) production occurred following a three-hour exposure of microcystin LR at concentrations from 100 nM to 100 μM, with reversal by ROS scavengers, in contrast with no response in monolayers. These results suggest monolayer culture inadequately predict intracellular effects of microcystins and support evidence that aggregate culture more closely approximates in vivo form and function. The approach results in more reliable prediction of microcystin toxicity in vitro.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Liver regenerative medicine: advances and challenges

Liver transplantation is the standard care for many end-stage liver diseases. However, donor organs are scarce and some people succumb to liver failure before a donor is found. Liver regenerative medicine is a special interdisciplinary field of medicine focused on the development of new therapies incorporating stem cells, gene therapy and engineered tissues in order to repair or replace the damaged organ. In this review we consider the emerging progress achieved in the hepatic regenerative medicine within the last decade. The review starts with the characterization of liver organogenesis, fetal and adult stem/progenitor cells. Then, applications of primary hepatocytes, embryonic and adult (mesenchymal, hematopoietic and induced pluripotent) stem cells in cell therapy of liver diseases are considered. Current advances and challenges in producing mature hepatocytes from stem/progenitor cells are discussed. A section about hepatic tissue engineering includes consideration of synthetic and natural biomaterials in engineering scaffolds, strategies and achievements in the development of 3D bioactive matrices and 3D hepatocyte cultures, liver microengineering, generating bioartificial liver and prospects for fabrication of the bioengineered liver.

The new murine hepatic 3A cell line responds to stress stimuli by activating an efficient Unfolded Protein Response (UPR)

In the present study we have investigated the properties of a novel cell line (3A cells) obtained from the liver of 14.5 days post coitum (dpc) wild-type mouse embryo. 3A cells morphology was characterized by fluorescent localization of F-actin and β-catenin. The expression of specific genes and proteins essential to liver function in these cells was comparable or even more efficient then in the differentiated hepatocytic cell line MMH-D6. 3A cells also showed the capability to excrete molecules in extracellular spaces resembling functional bile canaliculi, glycogen storage activity and the ability to control retinol-binding protein 4 secretion in response to retinol deprivation. Their response to the exogenous stress stimulus induced by tunicamycin was analysed by PCR Pathway Array containing 84 genes involved in the Unfolded Protein Response (UPR). 3A cells were shown to activate the UPR following a typical stressful event, indicating that this cellular model could be further exploited to investigate hepatic proteins secretion and specific reaction to different injuries.

The role of the sodium-taurocholate cotransporting polypeptide (NTCP) and of the bile salt export pump (BSEP) in physiology and pathophysiology of bile formation

Bile formation is an important function of the liver. Bile salts are a major constituent of bile and are secreted by hepatocytes into bile and delivered into the small intestine, where they assist in fat digestion. In the small intestine, bile salts are almost quantitatively reclaimed and transported back via the portal circulation to the liver. In the liver, hepatocytes take up bile salts and secrete them again into bile for ongoing enterohepatic circulation. Uptake of bile salts into hepatocytes occurs largely in a sodium-dependent manner by the sodium taurocholate cotransporting polypeptide NTCP. The transport properties of NTCP have been extensively characterized. It is an electrogenic member of the solute carrier family of transporters (SLC10A1) and transports predominantly bile salts and sulfated compounds, but is also able to mediate transport of additional substrates, such as thyroid hormones, drugs and toxins. It is highly regulated under physiologic and pathophysiologic conditions. Regulation of NTCP copes with changes of bile salt load to hepatocytes and prevents entry of cytotoxic bile salts during liver disease. Canalicular export of bile salts is mediated by the ATP-binding cassette transporter bile salt export pump BSEP (ABCB11). BSEP constitutes the rate limiting step of hepatocellular bile salt transport and drives enterohepatic circulation of bile salts. It is extensively regulated to keep intracellular bile salt levels low under normal and pathophysiologic situations. Mutations in the BSEP gene lead to severe progressive familial intrahepatic cholestasis. The substrates of BSEP are practically restricted to bile salts and their metabolites. It is, however, subject to inhibition by endogenous metabolites or by drugs. A sustained inhibition will lead to acquired cholestasis, which can end in liver injury.

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

The subfamily of ABCC transporters consists of 13 members in mammals, including the multidrug resistance-associated proteins (MRPs), sulfonylurea receptors (SURs), and the cystic fibrosis transmembrane conductance regulator (CFTR). These proteins play roles in chemical detoxification, disposition, and normal cell physiology. ABCC transporters are expressed differentially in the liver and are regulated at the transcription and translation level. Their expression and function are also controlled by post-translational modification and membrane-trafficking events. These processes are tightly regulated. Information about alterations in the expression of hepatobiliary ABCC transporters could provide important insights into the pathogenesis of diseases and disposition of xenobiotics. In this review, we describe the regulation of hepatic ABCC transporters in humans and rodents by a variety of xenobiotics, under disease states and in genetically modified animal models deficient in transcription factors, transporters, and cell-signaling molecules.

ABCC2/Abcc2: a multispecific transporter with dominant excretory functions

ABCC2/Abcc2 (MRP2/Mrp2) is expressed at major physiological barriers, such as the canalicular membrane of liver cells, kidney proximal tubule epithelial cells, enterocytes of the small and large intestine, and syncytiotrophoblast of the placenta. ABCC2/Abcc2 always localizes in the apical membranes. Although ABCC2/Abcc2 transports a variety of amphiphilic anions that belong to different classes of molecules, such as endogenous compounds (e.g., bilirubin-glucuronides), drugs, toxic chemicals, nutraceuticals, and their conjugates, it displays a preference for phase II conjugates. Phenotypically, the most obvious consequence of mutations in ABCC2 that lead to Dubin-Johnson syndrome is conjugate hyperbilirubinemia. ABCC2/Abcc2 harbors multiple binding sites and displays complex transport kinetics.

Transplantation of embryonic small hepatocytes induces regeneration of injured liver in adult rat

Small hepatocytes as hepatic stem cells or progenitors may be transplanted to treat several end-stage liver diseases. To identify the characteristics of epithelial cells enriched from fetal liver, we used immunocytochemistry and electron micrography. All cells in the colonies were immunocytochemically positive for alpha fetoprotein and cytokeratins (CK) 7, CK8, and CK18, which are markers of hepatic progenitor. Under transmission electron microscopy, we observed the cultured cells to show naive characteristics of stem cells and to be significantly distinct from mature hepatocytes. To identity whether these small hepatocytes were able to proliferate and differentiate into mature hepatocytes, we cultured them in vitro, and, through the portal vein, and transplanted elements whose membrane were stained with red fluorescence using PKH26 linker dye, into the livers of CCl(4)-treated rats that had been subjected to two-thirds partial hepatectomy. Significant liver regeneration was observed 30 days later in rats that did or did not receive the cells. The livers of hepatocytes recipients showed sharper edges and smoother surfaces than the control group. Diffused cells labeled with red fluorescence were observed in the portal area, with branch-like red fluorescence in regions near portal areas of some lobules, suggesting that these elements were involved in the repair of liver lobules and differentiation into mature hepatocytes. Our results revealed that small hepatocytes not only have characteristics of hepatic stem cells, but also may be a source of cellular transplantation to treat liver diseases.

Cell-based therapies for metabolic liver disease

Liver transplantation is an important therapeutic option for many individuals with metabolic liver disease. Nevertheless, the invasive nature of surgery and limitations of donor organ availability have led to the search for alternatives to whole-organ transplantation. Cell-based therapies have been a particularly active area of investigation in recent years. Hepatocyte transplantations have been performed for a variety of indications, including acute liver failure, end-stage liver disease, and inborn errors of metabolism. Individuals with inborn errors of metabolism who have undergone hepatocyte transplantation have shown clinical improvement and partial correction of the underlying metabolic defect. In most cases, sustained benefits have not been observed. This may be related to inadequate cell dose, variations in the quality of hepatocyte preparations, rejection of the transplanted cells, or senescence of transplanted hepatocytes. Though initial proof of concept with hepatocyte transplantation has been demonstrated by a number of investigators, wide application of this technology has been hindered by the inability to secure a reliable and well-characterized cell source(s) for transplantation and by the challenges of sustained engraftment and expansion of transplanted cells in vivo. Cell-based therapies, including those based on stem cells or more differentiated progenitor cells, may represent the future of cell transplantation for treatment of metabolic liver disease.

Stem cells for liver tissue repair: Current knowledge and perspectives

Stem cells from extra- or intrahepatic sources have been recently characterized and their usefulness for the generation of hepatocyte-like lineages has been demonstrated. Therefore, they are being increasingly considered for future applications in liver cell therapy. In that field, liver cell transplantation is currently regarded as a possible alternative to whole organ transplantation, while stem cells possess theoretical advantages on hepatocytes as they display higher in vitro culture performances and could be used in autologous transplant procedures. However, the current research on the hepatic fate of stem cells is still facing difficulties to demonstrate the acquisition of a full mature hepatocyte phenotype, both in vitro and in vivo. Furthermore, the lack of obvious demonstration of in vivo hepatocyte-like cell functionality remains associated to low repopulation rates obtained after current transplantation procedures. The present review focuses on the current knowledge of the stem cell potential for liver therapy. We discuss the characteristics of the principal cell candidates and the methods to demonstrate their hepatic potential in vitro and in vivo. We finally address the question of the future clinical applications of stem cells for liver tissue repair and the technical aspects that remain to be investigated.

Expression of the multidrug resistance proteins MRP2 and MRP3 in human cholangiocellular carcinomas

BACKGROUND

Cholangiocellular carcinomas and gallbladder carcinomas are highly aggressive tumours with a poor prognosis and are generally regarded as chemoresistant tumours. Overexpression of ATP-binding cassette transporters of the multidrug resistance protein (MDR) and multidrug resistance-related protein (MRP) family in cancer cells is a major cause for the multidrug resistance phenotype in vitro and in vivo. To further define the role of MRP family members in biliary tract cancer, we studied the expression and localization of MRP2 and MRP3 in cholangiocellular carcinomas and gallbladder carcinomas.

MATERIALS AND METHODS

The expression and cellular localization of the multidrug resistance proteins MRP2 and MRP3 in human cholangiocellular carcinomas and gallbladder carcinomas were analysed by immunohistochemistry using isoform-specific antibodies. Expression of MRP isoforms was studied in vitro in Mz-ChA-1 cells derived from gallbladder adenocarcinoma by reverse transcription-polymerase chain reaction (RT-PCR), immunoblotting and immunofluorescence microscopy.

RESULTS

Mz-ChA-1 cells constitutively expressed MDR P-glycoproteins, MRP1, MRP2 and MRP3 by RT-PCR, immunoblotting and immunofluorescence microscopy. MRP2 and MRP3 are expressed in the respective apical and basolateral membrane domains. MRP3 was the predominant MRP isoform in gallbladder carcinomas (93%) and cholangiocellular carcinomas (57%), whereas MRP2 expression was detected in only 29% of gallbladder carcinomas and was undetectable in cholangiocellular carcinomas.

CONCLUSIONS

Our findings suggest that the intrinsic multidrug resistance of cholangiocellular and gallbladder carcinomas seems to be independent of MRP2 expression while the expression of MRP3 may contribute to the MDR phenotype.

Selective proliferation of rat hepatocyte progenitor cells in serum-free culture

This protocol details a method of obtaining selectively proliferated hepatocyte progenitor cells using hyaluronic acid (HA)-coated dishes and serum-free medium. A small hepatocyte (SH) is a hepatocyte progenitor cell of adult livers and has many hepatic functions. When the rat SH begins to proliferate, CD44 is specifically expressed. To define the purification of SH, CD44 and cytokeratin 8 are used as marker proteins. The growth of SHs is faster on HA-coated dishes than on other extracellular matrix-coated ones. The use of both DMEM/F12 medium and HA-coated dishes allows the selective proliferation of SHs in culture. The purification of SHs is approximately 85% at day 10.

Analysis of liver repair mechanisms in Alagille syndrome and biliary atresia reveals a role for notch signaling

Patients with Alagille syndrome (AGS), a genetic disorder of Notch signaling, suffer from severe ductopenia and cholestasis, but progression to biliary cirrhosis is rare. Instead, in biliary atresia (BA) severe cholestasis is associated with a pronounced "ductular reaction" and rapid progression to biliary cirrhosis. Given the role of Notch in biliary development, we hypothesized that defective Notch signaling would influence the reparative mechanisms in cholestatic cholangiopathies. Thus we compared phenotype and relative abundance of the epithelial components of the hepatic reparative complex in AGS (n = 10) and BA (n = 30) using immunohistochemistry and computer-assisted morphometry. BA was characterized by an increase in reactive ductular and hepatic progenitor cells, whereas in AGS, a striking increase in intermediate hepatobiliary cells contrasted with the near absence of reactive ductular cells and hepatic progenitor cells. Hepatocellular mitoinhibition index (p21(waf1)/Ki67) was similar in AGS and BA. Fibrosis was more severe in BA, where portal septa thickness positively correlated with reactive ductular cells and hepatic progenitor cells. AGS hepatobiliary cells failed to express hepatic nuclear factor (HNF) 1beta, a biliary-specific transcription factor. These data indicate that Notch signaling plays a role in liver repair mechanisms in postnatal life: its defect results in absent reactive ductular cells and accumulation of hepatobiliary cells lacking HNF1beta, thus being unable to switch to a biliary phenotype.

Characterization of two splice variants of human organic anion transporting polypeptide 3A1 isolated from human brain

In the present study we isolated two splice variants of organic anion transporting polypeptide 3A1 (OATP3A1_v1 and OATP3A1_v2) from human brain. OATP3A1_v2 lacks 18 amino acids (aa) at the COOH-terminal end (692 aa) but is otherwise similar in sequence to OATP3A1_v1 (710 aa). OATP3A1_v1 exhibits a wide tissue distribution, with expression in testis, various brain regions, heart, lung, spleen, peripheral blood leukocytes, and thyroid gland, whereas OATP3A1_v2 is predominantly expressed in testis and brain. On the cellular and subcellular levels OATP3A1_v1 could be immunolocalized in testicular germ cells, the basolateral plasma membrane of choroid plexus epithelial cells, and neuroglial cells of the gray matter of human frontal cortex. Immunolocalization of OATP3A1_v2 included Sertoli cells in testis, apical and/or subapical membranes in choroid plexus epithelial cells, and neurons (cell bodies and axons) of the gray and white matter of human frontal cortex. The rodent ortholog Oatp3a1 was also widely distributed in rat brain, and its localization included somatoneurons as well as astroglial cells. Transport studies in cRNA-injected Xenopus laevis oocytes and in stably transfected Chinese hamster ovary FlpIn cells revealed a similar broad substrate specificity for both splice variants. Transported substrates include prostaglandin (PG)E(1) and PGE(2), thyroxine, and the cyclic oligopeptides BQ-123 (endothelin receptor antagonist) and vasopressin. These studies provide further evidence for the involvement of OATPs in oligopeptide transport. They specifically suggest that OATP3A1 variants might be involved in the regulation of extracellular vasopressin concentration in human brain and thus might influence the neuromodulation of neurotransmission by cerebral neuropeptides such as vasopressin.

Functional comparison of bone marrow-derived liver stem cells: selection strategy for cell-based therapy

Several distinct subpopulations of bone marrow-derived liver progenitor cells were recently described. However, there is inadequate information comparing these subpopulations from a liver-function point of view. This study was undertaken to compare two subpopulations of liver progenitors: beta(2)-microglobulin (beta(2)m)-negative/Thy-1-positive cells, and liver progenitors obtained from the non-adherent cell fraction after a panning procedure. The cells were cultured under several conditions including high- and low-dose hepatocyte growth factor, various cellular densities, and different media. Growth characteristics, liver-specific metabolic capacity, and liver regeneration-associated gene expression were studied. Both isolation procedures yielded cells that produced albumin and metabolized ammonia into urea. The study demonstrated that the beta(2)m-negative/Thy-1-positive cell fraction metabolized ammonia into urea more efficiently and produced a superior amount of albumin compared with the panned cell fraction. The beta(2)m-negative/Thy-1-positive cell fraction could be optimal for the development of novel cell-based treatment strategies for congenital or acquired liver diseases.