Toxicology of Industrial Compounds

210 USE OF LONG-TERM CULTURES OF HEPATOCYTES IN TOXICITY TESTING
been presented that part of this loss of functionality can be prevented by
several factors including soluble medium factors, extracellular matrix
components and cell-cell interactions (reviews Guillouzo et al., 1990;
Rogiers, 1993). At present no ‘ideal’ long-term hepatocyte culture model
exists but valuable alternatives are being developed which take into
account some of the factors mentioned. A recent development consists of
hepatocytes cultured in a collagen gel sandwich configuration (Dunn et al.,
1991; Lee et al., 1992). This promising system is claimed to maintain longterm
differentiation probably due to the reinstatement of the cellular
polarity of the hepatocytes as a function of the extracellular matrix (Dunn
et al., 1991; Lee et al., 1992). To date, only few results concerning xenobiotic
metabolism, are available (Koebe et al., 1994). Another recently introduced
model substantially improved the maintenance of xenobiotic metabolism
by culturing hepatocytes on a mixture of crude membrane fractions with
collagen type 1, combined with the use of culture medium supplemented
with aprotinin and selenium (Saad et al., 1993).
Also co-cultures of hepatocytes with rat epithelial cells of primitive
biliary origin represent a rather new and valuable tool in xenobiotic
biotransformation research and testing (Bégué et al., 1984a). The model
has been developed in order to mimic better the microenvironment of the
liver cells in vivo. It is until now, the only long-term hepatocyte culture
system of which enough biotransformation data exist. Co-cultures of
hepatocytes retain to a great extent the morphological and biochemical
characteristics of adult hepatocytes in vivo, including phase 1 and phase 2
xenobiotic metabolism pathways (Guillouzo, 1986; Rogiers et al., 1992;
Akrawi et al., 1993a). The following text reviews the actual knowledge
concerning xenobiotic biotransformation in cocultured hepatocytes with
emphasis on the authors’ own research.
Phase 1
reactions in co-cultured hepatocytes
It has been claimed that as much as 100 per cent of the CYP content and Naminopyrine
demethylation activity can be maintained in co-cultured rat
hepatocytes with primitive biliary duct cells (Bégué et al., 1984b). Some
other phase 1 enzymatic activities also appear to be maintained since drugs
such as ketotifen (Le Bigot et al., 1987) and testosterone (Utesch, 1992)
were metabolized by several pathways. Maier (1988) however, has
reported that aldrin epoxidase activity underwent a significant decrease as
a function of culture time. Niemann et al. (1991) was able to show that, as
was also the case for mono-cultured rat hepatocytes, enzymatic activities
belonging to the 3-methyl-cholanthrene-inducible family were better
maintained than those belonging to the phenobarbital inducible family. In
our own experiments with adult rat hepatocytes co-cultured with rat liver