Regular articleCharacterisation of some cytotoxic endpoints using rat liver and HepG2 spheroids as in vitro models and their application in hepatotoxicity studies. II. Spheroid cell spreading inhibition as a new cytotoxic marker
Introduction
The major concerns in adopting in vitro models to predict in vivo effects are data comparability and reliability. To increase the reliability and predictability of in vitro data to human and other animals, some fundamental aspects, such as similarity of cell functionality, comparability of “dosing” concentrations, and reliability of endpoints must be well defined or “framed.” The simple indicators, LD50 and LC50, play an essential role in animal experiments because they can be used as reference doses for subsequent studies and for comparing relative toxicities among different chemicals and across species. In most in vitro studies, while such a reference scheme has not been adopted, the EC50 or IC50 values indicate a specific endpoint Ponsoda et al 1995, Jinno et al 1997, Clemedson and Ekwall 1999, Ekwall 1999 and LD50 indicated by cell viability (Bironaite and Öllinger, 1997) has been used in some studies. As a result, a wide concentration range (5000-fold, highest/lowest), often with a 10-fold difference between two concentrations can be expected in some in vitro studies (Walker et al., 2000). A wide concentration range is neither a good approach for mechanistic or concentration–response studies nor an efficient practice. On the other hand, some in vitro studies tend to use very high concentrations, which may be difficult to reach in vivo. This may make the data less valuable to the in vivo situation because the data lack comparability in concentration between the two models. Therefore, a good in vitro model should adopt a “dosing” concentration reference system to make data comparable with in vivo data. To achieve this, we must find a way to link the concentrations between in vitro and in vivo models.
The similarity of liver spheroid models to intact tissues in structure and specific functions have been reported by many researchers and reviewed by Mueller-Klieser (1997). For example, under certain culture conditions, such as gyrotation, single liver cells can reorganize to form multicellular, three-dimensional spheroids, in which cells resume cell–cell contacts as seen in vivo Wu et al 1996, Yumoto et al 1996, Walker et al 2000, Tzanakakis et al 2001. The cells in spheroids survive and retain liver specific function much longer than cells cultured as monolayers Landry et al 1985, Lazar et al 1995, Eschbach et al 1997. Cells from adult rat pancreatic islets are capable of self-assembling to form aggregates that secrete insulin (Halban et al., 1987). Normal human adult breast epithelial stem cells formed three-dimensional “organoids” almost identical to those found in vivo (Chang et al., 2001). Mammary gland epithelial cells are able to reorganize into hollow, alveolar-like structures and retain the functions of secreting milk proteins (Barcellos-Hoff et al., 1989). Our recent work using magic angle spinning 1H-NMR, showed that the “finger print” (spectrum) of liver spheroids more closely resembles intact liver than isolated hepatocytes (Bollard et al., 2002). More and more evidence shows that, in some important aspects, a spheroid may resemble a viable, miniorgan that can be maintained long term. Therefore, the usefulness of liver spheroids and spheroids grown from hepatic cell lines as in vitro models is worthy of further characterisation.
This study investigated the cell-spreading behaviour of individual rat liver and HepG2 spheroids and their application to in vitro toxicology studies. Our early observations showed that liver cells in spheroids can spread out under certain culture conditions. This phenomenon may be exploited as a simple and useful endpoint to examine the in vitro “acute” cytotoxicity of a test compound to compare relative toxicity among different test agents. The utility of the cell-spreading phenomenon of rat liver and HepG2 spheroids was characterised using four model hepatoxicants, d-galatosamine, propranolol, diclofenac, and paracetamol, as tested in Part I of this study.
Section snippets
Chemicals and medium
l-Glutamine was obtained from GibcoBril. Foetal bovine serum (FBS), hepatocyte medium, Minimum Essential Medium Eagle (MEM), penicillin and streptomycin sulfate, toxicants, and other chemicals were purchased from Sigma (Sigma-Aldrich Company Ltd) unless otherwise indicated.
Liver spheroid culture
Liver cell preparation and spheroid culture were as stated in Part I.
HepG2 spheroid culture
The HepG2 cell line (Human Caucasian Hepatocyte Carcinomal cells from ECACC) was prepared as described in Part I.
Culture media
The culture medium used for cell-spreading
Spheroid cell spreading
After formation of spheroids from single liver cells under rotation-mediated culture conditions, the individual cells maintained in normal static culture conditions can spread out from the spheroids. Higher concentrations of FCS encouraged cell spreading. Cell spreading in the medium containing 10 % FCS was much better than 5 % FCS in terms of percentage, area, and speed of spreading. There was no significant difference between 10 and 15 % FCS in cell spreading (data not shown). Fig. 1 shows
Discussion
Cytotoxicity of a test compound is identified by a variety of cytotoxic endpoints. The commonly used in vitro cytotoxic endpoints include enzyme release such as LDH, γ-GT, GPT, and GOT Scaife 1985, Verhulst et al 1998, metabolic and functional parameters including neutral red uptake, dimethylthiazole-diphenyl tetrazolium bromide (MTT) metabolism (Sellers et al., 1994; Mosmann, 1983), and pyruvic acid consumption (Verhulst et al., 1998), cell viability (Jinno et al., 1997), and many other
Acknowledgements
The authors thank Dr. David Patton for his support in electron microscopy work.
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