Due to the lack of a precursor

Due to the lack of a precursor for serotonin, a tryptophan-deficient diet for 3 weeks led to a reduced serum serotonin level and confirmed the successful dysregulation of the peripheral serotonergic system during normal liver function and during liver insufficiency. Concomitant reductions in the levels of the 5-HT1B and PKA proteins in response to serotonergic system dysfunction during liver insufficiency suggest that the 5-HT1B receptor is directly stimulated by serum serotonin. Specifically, one important mechanism that regulates 5-HT1B receptor function and is negatively correlated with PKA signaling is receptor desensitization and internalization following serotonin-induced activation (Barnes and Sharp, 1999). However, the serum serotonin level after serotonergic system dysfunction during liver insufficiency was only slightly but significantly increased compared to that in rats after induced serotonergic system dysfunction during normal liver function. One possible explanation for this is the release of serotonin from minor reserves of peripheral serotonin, such as lymphocytes, monocytes, macrophages, and mast cells, which can stimulate the 5-HT1B receptor. Extending this mechanism, the increased testosterone level observed in response to serotonergic system dysfunction during liver insufficiency led to decreased levels of proinflammatory cytokines. A previous study showed that both physiological and supraphysiological concentrations of testosterone reduce the Bestatin hydrochloride of IL-1β (Corcoran et al., 2010). Moreover, simultaneous increase in the levels of testosterone, serum serotonin, anti-inflammatory IL-4, and STAT6 protein in the nuclear fraction of the liver after serotonergic system dysregulation during liver insufficiency suggests the presence of a testosterone - IL-4 - STAT6 interlink under control of the serotonergic system during liver insufficiency. Consistent with this proposed interlink, one role of PKA is to inhibit the production of the anti-inflammatory cytokine IL-4 (Zhou et al., 2007). In addition, the activated JAK/STAT6 signaling pathway represents the main mediator of IL-4 signaling (Nelms et al., 1999; Nappo et al., 2017), and significant levels of activated STAT6 are observed in primary prostate tissue (Ni et al., 2002), which are controlled by the serum testosterone level (Izumi et al., 2017). Notably, liver failure reduces GnRH secretion by the hypothalamus and leads to secondary testicular failure (Mowat et al., 1976). Moreover, hypophysectomy in male rats decreases CYP2C11 expression to 25 to 30% (Waxman and Holloway, 2009). In contrast, the CYP3A2 isoform is not downregulated by hypophysectomy (Waxman and Holloway, 2009; Li et al., 2015). Steady-state mRNA levels of CYP3A isoforms were not changed during liver insufficiency. Suppression of these isoforms was observed only at the protein and activity levels, suggesting a nongenomic mechanism of CYP3A isoform downregulation during liver insufficiency. Nonetheless, transcriptional activation of CYP3A isoforms was observed after induced dysfunction of the serotonergic system during normal liver function. Moreover, the activity of these isoforms was also increased, while the protein level of CYP3A2 was not changed, suggesting that posttranslational stabilization of this isoform is also under control of the serotonergic system. Dysfunction of the serotonergic system during liver insufficiency led to suppression of CYP3A isoforms at the gene, protein and activity levels by a mechanism based on posttranscriptional and posttranslational processes. The altered immune profile suggested the key role of the glucocorticoid (GC) receptor in this mechanism. The glucocorticoid receptor crucially regulates the transcription of mediators of the immune system by both suppressing (IL-1β, TNFα and IL-6) and stimulating (IL-4 and TGF-β1, although the latter is controversial) the production of a large number of proinflammatory or anti-inflammatory cytokines (Elenkov, 2004; Franchimont, 2004). Moreover, the GR participates in the PXR-mediated induction of the expression of CYP3A isoforms through a unique integrative mechanism (Huss and Kasper, 2000; Pascussi et al., 2008; De Martin et al., 2014). In our study, the level of the PXR protein, a confirmed mediator of CYP3A isoform expression, was increased after serotonergic system dysfunction only during normal liver function. Dysfunction of serotonergic system during liver insufficiency results in a decreased GR protein level and increased STAT6 and ERK1/2 protein levels in the nuclear fraction of the liver, suggesting the creation of conditions for alterations in GR phosphorylation that are important for subcellular localization of the GR protein. The specific site of GR phosphorylation is currently unknown and will be examined in a future study. Consistent with this interpretation, previous studies have shown that GR is a phosphoprotein containing numerous phosphorylation sites, including sites for pERK, p38 MAPK, PKC, and PKA, and alterations in GR phosphorylation status affect its subcellular localization and nuclear-cytoplasmic shuttling (Jans and Hübner, 1996; Ismaili and Garabedian, 2004). Moreover, the transcriptional interference between the GR-mediated and IL-4-mediated signal transduction pathways is based on the mutual transcriptional repressive activities of GR and STAT6 in T lymphocytes, whereas increased STAT6 activity is effectively blocked by an MEK/ERK inhibitor (Biola et al., 2000; Shaul and Segar, 2007; So et al., 2007).