Research ArticleInsufficient bile acid signaling impairs liver repair in CYP27−/− mice
Introduction
It is well known that abnormally high bile acid (BA) levels result in cholestasis and liver toxicity [1], [2]. However, the potential impact of low BA levels on liver injury is unknown. We previously showed that BAs promoted liver regeneration by activating the farnesoid X receptor (FXR) [3], and loss of FXR leads to deficient liver regeneration and increases the susceptibility to toxin-induced liver injury [3], [4]. Nevertheless, it is unclear whether a reduced BA flow will impair liver regeneration and increase the risk of liver injury.
The low BA flow is observed in patients with cerebrotendinous xanthomatosis (CTX), an autosomal recessive disease [5]. CTX patients exhibit tendon xanthomas, juvenile cataracts, progressive neurological dysfunction, and elevated plasma cholestanol levels. These pathological hallmarks were attributed to the heterogeneous mutations in the gene encoding the sterol 27-hydroxylase (CYP27) [6], [7]. BAs are converted from cholesterol through two pathways in the liver. The classical pathway is initiated by cholesterol-7α-hydroxylase (CYP7A1) [8], [9], while the alternative pathway is initiated by CYP27 [10], [11]. The complete synthesis of BAs requires the alternative pathway mediated by CYP27, which catalyzes degradation of the steroid side chain. In CTX patients, dysfunctional CYP27 leads to cholestanol accumulation, and thus causes the neurodegenerative pathogenesis. CTX has been identified in a number of populations, particularly in Japanese, Sephardic Jewish, and Italian populations [12]. In the United States, the incidence was regarded to be rare. However, recent studies suggested that the prevalence of this disease might be greater than previously recognized [13]. Only a few studies were conducted to demonstrate that CTX patients were more susceptible to hepatitis in infancy [14], [15], but the potential impact of low BA flow on liver injury is not known.
An animal model with deletion of CYP27 has been generated [11]. CYP27−/− mice displayed low BA levels but normal cholesterol homeostasis and vitamin D metabolism. Due to compensatory activation of CYP3A4, CYP27−/− mice do not have neurological dysfunction or tendon xanthomas [16]. CYP27−/− mice thus provided a unique animal model to determine the impacts of reduced BA levels on liver injury in CTX patients. Additionally, the animal model is useful to evaluate whether the intake of BA sequestrants that are used to lower cholesterol levels would increase the risk of liver injury.
In this study, we performed CCl4 treatment and 70% PH in CYP27−/− mice to determine the impact of deficient BA signaling after liver injury. Our results demonstrated an essential role of physiological BA levels in liver regeneration, and thus provide insights into the risk of liver injury for CTX patients and those who take BA sequestrants.
Section snippets
Animals
CYP27+/+, CYP27+/−, and CYP27−/− mice with C57BL/6 background were maintained in a pathogen-free animal facility under a standard 12:12 h light:dark cycle. Mice were fed with the standard rodent chow and water ad libitum. 70% PH was performed as previously described [3], [17]. Px20350 (20 mg/kg) or vehicle (10% HPBCD (Sigma) in 500 mM phosphate pH 7.0) alone was treated to mice by oral garage twice a day for 2 days. Liver remnants were weighed after removal of necrotic stumps and sutures. All
CYP27−/− mice displayed enhanced CCl4-induced liver injury
Though CYP27−/− mice were reported to have reduced BA pools, hepatic and serum BA levels of these mice were not evaluated in the previous studies [11]. We measured hepatic BA levels in CYP27−/− males and found that it was 50% of those in CYP27+/+ and CYP27+/− males (Fig. 1A). However, serum BA levels were not different among the 3 genotypes (Fig. 1A), indicating that the reduction of BAs was mainly in gastrointestinal tract.
Rodents treated with i.p. injection of CCl4 are widely used to study
Discussion
In this study, we demonstrated that CYP27−/− mice are more susceptible to toxin (CCl4)-induced liver injury and display defective liver regeneration after 70% PH. Liver regrowth in CYP27−/− mice terminates before the original liver weight is restored, which is rarely seen in other animal models of deficient liver regeneration, in which liver growth is only delayed [22]. It is believed that the compensatory liver regrowth responds to constant metabolic stimuli, such as BAs, after liver injury [3]
Author contributions
Z.M. designed and performed research, wrote draft of the manuscript. N.L., X.F., and X.W. performed some experiments and provided technical support for animal studies. L.Z. assisted in acquisition of data. Y.W. and W.C. provided intellectual support in experimental design. B.F. provided animals for study and technical support. W.H. obtained the funding, designed and coordinated the project, and wrote the manuscript.
Conflict of interest
The Authors who have taken part in this study do not have a relationship with the manufacturers of the drugs involved either in the past or present and did not receive funding from the manufacturers to carry out their research. The authors received support from STOP CANCER and an Ibrahim Training Grant.
Acknowledgments
We would like to thank Kurt Jenkins for manuscript proofreading. We also thank Min Lin and Xiaosong Chen for some technical assistance. W.H. is supported by the STOP CANCER and Ibrahim Training Grant.
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Cited by (0)
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These authors contributed equally to this work.
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Present address: The Second Hospital of Jilin University, Cancer Biotherapy Center, No. 218 ZiQiang Street, Changchun, Jilin Province 130041, PR China.