Acknowledgments This work was supported by the

Acknowledgments This work was supported by the Natural Science Foundation of China (Nos. 21602254, 81603194), the Natural Science Foundation of Jiangsu province, China (BK20160767) and National Found for Fostering Talents of Basic Science (NFFTBS, J1310032).
Introduction Glucocorticoids (GC) play a key role in modulating the expression of a variety of genes involved in several metabolic processes and cell functions, through the interaction of these hormones with their nuclear receptor (GR). Because of the variety of physiological and pharmacological actions of GC, which include anti-inflammatory and immunosuppressive effects, synthetic GC have been widely used in clinical practice. Nevertheless, at high doses or after prolonged exposure these steroids cause adverse effects. GC-induced toxicity can be iatrogenic or the result of endogenous GC over-production, as occurs in patients with Cushing syndrome. In addition to general noxious effects, such as insulin resistance, osteoporosis, and muscle wasting, the liver is markedly affected by exposure to high levels of GC due to the pivotal role of this organ in energy metabolism and drug biotransformation. Thus, an increased risk of suffering from cholestasis and gallstone disease has been associated to long-term treatment with GC. In agreement with this concept, profound changes in bile Norfloxacin (BA) homeostasis have been described when high doses of dexamethasone (DEX) were administered to rats [1]. It should be noted that altered BA pool can be the result of impaired biosynthesis, which is mainly controlled by the expression of the key enzyme in the cholesterol-to-BA biotransformation, cholesterol 7α-hydroxylase (CYP7A1), or altered function at hepatocytes and ileal epithelium of transporters involved in BA uptake or efflux (for review see [2]). Cholestasis-associated changes reflecting impaired BA homeostasis have also been described in mice treated with DEX [3] and prednisolone [4]. Moreover, relatively high secretion of GC by the adrenal glands has been associated to cholestatic conditions, whereas endogenous GC exacerbate the liver injury and hypercholesterolemia associated with acute cholestasis in mice [5]. Interestingly, all these experimental studies have been carried out by exposing laboratory animals to high doses of GC. This leads to the question as to whether the elevation in serum BA could be secondary to hepatocyte damage induced by the hepatotoxic treatment rather than to specific GC-induced events. This prompted us to investigate the effect of GC on BA homeostasis under non-hepatotoxic conditions, paying special attention to the control of BA biosynthesis by the intestinal-hepatic FXR/FGF19 axis. Moreover, because: i) FGF21 expression has recently been reported to be sensitive to treatment with DEX through direct activation of GR [6], which seems to be involved in the protection against potentially toxic compounds, such as acetaminophen [6, 7]; and ii) FGF19/FGF21 actions emulate those of insulin/glucagon regarding glucose, lipid, and energy metabolism through different signaling pathways [8, 9], we addressed the question on whether there is also an interaction between FGF19, FGF21 and GC regarding BA homeostasis, which may have an important impact on the control of cholesterol metabolism.
Materials and methods
Results
Discussion In previous studies carried out by our group [1] and others [3, 5], it had been shown that the treatment of laboratory animals with high doses of GC alters BA homeostasis, which was characterized by enhanced serum BA concentrations. It was unclear whether this was a consequence of hepatocyte damage induced by the hepatotoxic treatment rather than due to specific GC-induced events. The present study demonstrates that although in the absence of hepatocellular injury there is not a cholestatic-like response to GC, profound alterations in the intestine/liver regulatory crosstalk occur. Thus, at a non-hepatotoxic dose, GC induce a strong signal that would normally stimulate hepatic biotransformation of cholesterol into BA, i.e., a dramatic decrease in ileal Fgf15 expression. Surprisingly, there was no considerable change in serum BA levels. This was due to the fact that the lack of Fgf15-mediated inhibition of Cyp7a1 was counterbalanced by a mechanism that includes hepatic Fgf21 up-regulation. The exact underlying events have not been fully elucidated in the present study but our results and previous reports by others permit to propose the working hypothesis that is shown in Fig. 7 and discussed below.