The focus of the thesis was to investigate if the known barrier-strengthening plant secondary metabolites quercetin and berberine have an impact on porcine intestinal epithelial models in vitro (IPEC J2/PS cells) and ex vivo (PP and VE tissue specimen), and if the effect differs between both tissue types. This would underline the different barrier properties between the immunologically important PP compared to surrounding VE. Quercetin is a flavonoid with a high abundance in the human diet due to its occurrence in several fruits and vegetables. Besides the beneficial effect in different diseases due to the anti-oxidative properties, barrier-strengthening effects are described in different intestinal cell models, mainly by the upregulation of claudin 4. Those effects could be also observed in VE samples used for the experiments from the first study of this thesis (Chapter 4). The TEER was increased while the [3H]-mannitol flux was not affected. In addition to the upregulation of claudin 4, claudin 2 was remarkably reduced. No effect of quercetin on barrier function could be detected in PP tissue samples or IPEC J2/PS cells under experimental conditions. In the case of PP, the missing effect compared to VE supports the theory of different susceptibility and TJ regulation between both tissues. Regarding IPEC J2/PS cells, the lack of claudin 2 in the cell line could be causative for the absent response to quercetin. The plant alkaloid berberine has been used in traditional Chinese medicine against gastrointestinal disorders for centuries and stands in the focus of pharmacological research. Despite several beneficial effects under experimental conditions regarding diseases like diabetes or cardiovascular disorders, the toxic effects of berberine are well documented and the safety has been under evaluation by the European Food Safety Authority (EFSA) since May 2024. The incubation of VE and PP tissue over 4 h in the Ussing chamber with berberine concentrations between 0.2 μM and 200 μM had no significant effect on both tissue types. This could be attributed to the limited incubation time in the Ussing chamber compared to the cell culture models, which were used in other studies regarding the effect of berberine on small intestinal barrier function. The in vitro approach with the improved porcine cell model IPEC JS/PS showed an unexpected dose-dependent decrease of the barrier function: the TEER values decreased and the [3H]-mannitol flux increased. This was attributable to the induction of apoptosis and the reduction and internalization of sealing TJ proteins claudin 1, 3, and occludin. Based on our study, it was not possible to determine if the loss of cell-cell contacts was the reason for apoptosis induction or a consequence of it. In cancerogenic cell lines, the induction of apoptosis is well described but also occurred in the non-cancerogenic murine fibroblast cell line BALB/3T3, where different concentrations either led to cell cycle arrest or induced apoptosis. We suggest a higher susceptibility of the improved IPEC J2/PS cell model compared to cancerogenic small intestinal models like Caco-2 or even IPEC J2 cell cultures with FBS instead of PS. Our results show that the therapeutic window of berberine seems to be rather small and no beneficial effect could be observed in the porcine tissue models under the experimental conditions. This thesis was part of an overall project regarding the different barrier properties between porcine PP and VE samples, which were observed recently (Radloff et al. 2017a). Besides the two barrier-strengthening substances quercetin and berberine, the two barrier-perturbators sodium caprate and TNF were used (Radloff 2019; Droessler et al. 2021; Droessler et al. 2022). For sodium caprate, TNF and quercetin different effects on VE and PP could be observed. Those results support the theory, that PP not only has a stronger paracellular barrier function due to the higher expression of claudin 4 but also differs from VE regarding susceptibility and TJ regulation.
