Human primary hepatocytes represent a gold standard in in vitro liver research. Due to their low availability and high costs alternative liver cell models with comparable morphological and biochemical characteristics have come into focus. The human hepatocarcinoma cell line HepG2 is often used as a liver model for toxicity studies. However, under two-dimensional (2D) cultivation conditions the expression of xenobiotic-metabolizing enzymes and typical liver markers such as albumin is very low. Cultivation for 21 days in a three-dimensional (3D) Matrigel culture system has been reported to strongly increase the metabolic competence of HepG2 cells. In our present study we further compared HepG2 cell cultivation in three different 3D systems: collagen, Matrigel and Alvetex culture. Cell morphology, albumin secretion, cytochrome P450 monooxygenase enzyme activities, as well as gene expression of xenobiotic-metabolizing and liver-specific enzymes were analyzed after 3, 7, 14, and 21 days of cultivation. Our results show that the previously reported increase of metabolic competence of HepG2 cells is not primarily the result of 3D culture but a consequence of the duration of cultivation. HepG2 cells grown for 21 days in 2D monolayer exhibit comparable biochemical characteristics, CYP activities and gene expression patterns as all 3D culture systems used in our study. However, CYP activities did not reach the level of HepaRG cells. In conclusion, the increase of metabolic competence of the hepatocarcinoma cell line HepG2 is not due to 3D cultivation but rather a result of prolonged cultivation time.

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In general, xenobiotic metabolizing enzymes (XMEs) are expressed in lower levels in the extrahepatic tissues than in the liver, making the former less relevant for the clearance of xenobiotics. Local metabolism, however, may lead to tissue-specific adverse responses, e.g. organ toxicities, allergies or cancer. This review summarizes the knowledge on the expression of phase I and phase II XMEs and transporters in extrahepatic tissues at the body's internal-external interfaces. In the lung, CYPs of families 1, 2, 3 and 4 and epoxide hydrolases are important phase I enzymes, while conjugation is less relevant. In skin, phase I-related enzymatic reactions are considered less relevant. Predominant skin XMEs are phase II enzymes, whereby glucuronosyltransferases (UGT) 1, glutathione-S-transferase (GST) and N-acetyltransferase (NAT) 1 are important for detoxification. The intestinal epithelium expresses many transporters and phase I XME with high levels of CYP3A4 and CYP3A5 and phase II metabolism is mainly related to UGT, NAT and Sulfotransferases (SULT). In the kidney, conjugation reactions and transporters play a major role for excretion processes. In the bladder, CYPs are relevant and among the phase II enzymes, NAT1 is involved in the activation of bladder carcinogens. Expression of XMEs is regulated by several mechanisms (nuclear receptors, epigenetic mechanisms, microRNAs). However, the understanding why XMEs are differently expressed in the various tissues is fragmentary. In contrast to the liver - where for most XMEs lower expression is demonstrated in early life - the XME ontogeny in the extrahepatic tissues remains to be investigated.

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PA cross the intestinal barrier structure-dependently. The passage of the noncyclic PA Hn and Em is reduced by an ABCB1-driven efflux into the gastrointestinal lumen resulting in a decreased oral bioavailability.