Toxicology of ZnO and TiO2 nanoparticles on hepatocytes: impact on metabolism and bioenergetics.

BACKGROUND AND AIM: Zinc oxide (ZnO) and titanium dioxide (TiO2) nanomaterials (NMs) are used in many consumer products, including foodstuffs. Ingested and inhaled NM can reach the liver. Whilst their effects on inflammation, cytotoxicity, genotoxicity and mitochondrial function have been explored, no work has been reported on their impact on liver intermediary metabolism. Our aim was to assess the effects of sub-lethal doses of these materials on hepatocyte intermediary metabolism. MATERIAL AND METHODS: After characterisation, ZnO and TiO2 NM were used to treat C3A cells for 4 hours at concentrations ranging between 0 and 10 µg/cm(2), well below their EC50, before the assessment of (i) glucose production and glycolysis from endogenous glycogen and (ii) gluconeogenesis and glycolysis from lactate and pyruvate (LP). Mitochondrial membrane potential was assessed using JC-10 after 0-40 µg/cm(2) ZnO. qRT-PCR was used to assess phosphoenolpyruvate carboxykinase (PEPCK) mRNA expression. Dihydroethidium (DHE) staining and FACS were used to assess intracellular reactive oxygen species (ROS) concentration. RESULTS: Treatment of cells with ZnO, but not TiO2, depressed mitochondrial membrane potential, leading to a dose-dependent increase in glycogen breakdown by up to 430%, with an increase of both glycolysis and glucose release. Interestingly, gluconeogenesis from LP was also increased, up to 10-fold and correlated with a 420% increase in the PEPCK mRNA expression, the enzyme controlling gluconeogenesis from LP. An intracellular increase of ROS production after ZnO treatment could explain these effects. CONCLUSION: At sub-lethal concentrations, ZnO nanoparticles dramatically increased both gluconeogenesis and glycogenolysis, which warrants further in vivo studies.

Oxidative stress rather than triglyceride accumulation is a determinant of mitochondrial dysfunction in in vitro models of hepatic cellular steatosis.

BACKGROUND/AIMS: There is still debate about the relationship between fat accumulation and mitochondrial function in nonalcoholic fatty liver disease. It is a critical question as only a small proportion of individuals with steatosis progress to steatohepatitis. In this study, we focused on defining (i) the effects of triglyceride accumulation and reactive oxygen species (ROS) on mitochondrial function (ii) the contributions of triglyceride, ROS and subsequent mitochondrial impairment on the metabolism of energy substrates. METHODS: Human hepatoblastoma C3A cells, were treated with various combinations of oleate, octanoate, lactate (L), pyruvate (P) and ammonia (N) acutely or for 72 h, before measurements of triglyceride concentration, cell respiration, ROS production, mitochondrial membrane potential, ketogenesis and gluconeogenesis, TCA cycle metabolite analysis and electron microscopy. RESULTS: Acutely, LPON treatment enhanced mitochondrial respiration and ROS formation. After 72 h, despite the similarities in triglyceride accumulation, LPON treatment, but not oleate, dramatically affected mitochondrial function as evidenced by decreased respiration, increased mitochondrial membrane potential and ROS formation with concomitant enhanced ketogenesis. By comparison, respiration and ROS formation remained unperturbed with oleate. Importantly, this was accompanied by an increased gluconeogenesis and ketogenesis. The addition of the antioxidant N-acetyl-L-cysteine prevented mitochondrial dysfunction and reversed metabolic changes seen with LPON, strongly suggesting ROS involvement in mediating mitochondrial impairment. CONCLUSIONS: Our data indicate that ROS formation, rather than cellular steatosis per se, impairs mitochondrial function. Thus, reduction in cellular steatosis may not always be the desired outcome without concomitant improvement in mitochondrial function and/or reducing of ROS formation.

Persistence of functional hepatocyte-like cells in immune-compromised mice.

BACKGROUND: Human embryonic stem cells (hESCs) can be efficiently differentiated to hepatocyte-like cells (HLCs) in vitro and demonstrate many of the functions and gene expression found in the adult liver. AIMS: In this study, we assess the therapeutic value of HLCs in long-term cell-based therapies in vivo. METHODS: hESC-derived HLCs were injected into the spleen of acutely injured NODscid(IL-2Rγ) null mice and analysed at various time points post-transplantation up to 3 months. RESULTS: Large clusters of human cells engrafted in the spleen after 3 days and had expanded considerably by 31 days. At these time points, we identified human cells expressing parenchymal hepatocyte markers, exhibiting biliary duct-like structures and expressing myofibroblast markers. Three months after transplantation, we could detect human HLCs that were positive for albumin and CK18 by immunostaining and human DNA by fluorescent in situ hybridisation. Moreover, we could detect secretion of human serum albumin by enzyme-linked immunoabsorbant assay. CONCLUSIONS: We observed the persistence, engraftment and function of HLCs in vivo up to 3 months post-translation; however, all murine recipients developed large splenic and liver tumours that contained endodermal and mesodermal cell types. Although our studies demonstrate that hESC-derived HLCs have the potential to play an important role in cell-based therapies, current methodologies and transplantation strategies require substantial refinement before they can be deployed safely.

The comparison between conditioned media and serum-free media in human embryonic stem cell culture and differentiation.

Human embryonic stem cells (hESCs) offer an inexhaustible supply of human somatic cell types through their ability to self-renew while retaining pluripotency. As such, hESC-derived cell types are important for applications ranging from in vitro modeling to therapeutic use. However, for their full potential to be realized, both the growth of the undifferentiated cells and their derivatives must be performed in defined culture conditions. Many research groups maintain hESCs using mouse embryonic fibroblasts (MEF) and MEF conditioned medium (CM). The use of murine systems to support hESCs has been imperative in developing hESC technology; however, they suffer from some major limitations including lack of definition, xenobiotic nature, batch-to-batch variation, and labor-intensive production. Therefore, hESC culture definition is essential if hESC lines, and their derivatives are to be quality assured and manufactured to GMP. We have initiated the process of standardizing hESC tissue culture and have employed two serum-free media: mTeSR (MT) and Stem Pro (SP). hESCs were maintained in a pluripotent state, for over 30 passages using MT and SP. Additionally, we present evidence that hESCs maintained in MT and SP generate equivalent levels of human hepatic endoderm as observed with CM. This data suggests that MT and SP are effective replacements for MEF-CM in hESC culture, contributing to the standardization of hESC in vitro models and ultimately their application.

Bone marrow stem cells contribute to alcohol liver fibrosis in humans.

Bone marrow-derived stem cell (BMSC) contribution to liver repair varies considerably and recent evidence suggests these cells may contribute to liver fibrosis. We investigated the mobilization and hepatic recruitment of bone marrow (BM) stem cells in patients with alcohol liver injury and their contribution to parenchymal/non-parenchymal liver cell lineages. Liver biopsies from alcoholic hepatitis (AH) patients and male patients, who received a female liver transplant and developed AH, were analyzed for BM stem cell content by fluorescence in situ hybridization and immunostaining. Y chromosome analysis was performed, along with co-staining for hepatocyte, biliary, myofibroblast, and Ki-67 markers. Blood CD34(+) levels were quantified in AH patients by flow cytometry. AH patients had increased CD34(+) cell counts in liver tissue (1.834% +/- 0.605%; P < 0.05) and in blood (0.195% +/- 0.063%; P < 0.05) as compared with matched controls (0.299% + 0.208% and 0.067% +/- 0.01%). A proportion of hepatic myofibroblasts were BM-derived (7.9%-26.8%) as deemed by the co-localization of Y chromosome/alpha-smooth muscle actin (alpha-SMA) staining. In the cross-sex liver grafts with AH, 5.025% of the myofibroblasts were co-staining for CD34, suggesting that a population of CD34(+) cells were contributing to the hepatic myofibroblast population. There was no evidence of BM contribution to hepatocyte or biliary cell differentiation, nor evidence of increased hepatocyte regeneration. Alcohol liver injury mobilizes CD34(+) stem cells into the circulation and recruits them into the liver. These BMSCs contribute to the hepatic myofibroblast population but not to parenchymal lineages and do not promote hepatocyte repair.

Highly efficient differentiation of hESCs to functional hepatic endoderm requires ActivinA and Wnt3a signaling.

Human embryonic stem cells (hESCs) are a valuable source of pluripotential primary cells. To date, however, their homogeneous cellular differentiation to specific cell types in vitro has proven difficult. Wnt signaling has been shown to play important roles in coordinating development, and we demonstrate that Wnt3a is differentially expressed at critical stages of human liver development in vivo. The essential role of Wnt3a in hepatocyte differentiation from hESCs is paralleled by our in vitro model, demonstrating the importance of a physiologic approach to cellular differentiation. Our studies provide compelling evidence that Wnt3a signaling is important for coordinated hepatocellular function in vitro and in vivo. In addition, we demonstrate that Wnt3a facilitates clonal plating of hESCs exhibiting functional hepatic differentiation. These studies represent an important step toward the use of hESC-derived hepatocytes in high-throughput metabolic analysis of human liver function.