Vitamin D Signaling through Induction of Paneth Cell Defensins Maintains Gut Microbiota and Improves Metabolic Disorders and Hepatic Steatosis in Animal Models

Hidekazu Tsukamoto,Linbi Cai,Zhongping Duan,Zishuo Chen,Zhi‐Xiong Jim Xiao ,Qun Sun,Richard Hu,Danmei Su,Yuan‐Ping Han ,Yanhao Lai ,Li Zhang,Sujun Zheng,Ya Nie ,Stephen J Pandol,Pengfei Wu,Moulun Luo ,Aurelia Lugea,Zhenqui Liu,Guihui Wu,Airu Zhu

doi:10.3389/fphys.2016.00498

Abstract

Metabolic syndrome (MetS), characterized as obesity, insulin resistance, and non-alcoholic fatty liver diseases (NAFLD), is associated with vitamin D insufficiency/deficiency in epidemiological studies, while the underlying mechanism is poorly addressed. On the other hand, disorder of gut microbiota, namely dysbiosis, is known to cause MetS and NAFLD. It is also known that systemic inflammation blocks insulin signaling pathways, leading to insulin resistance and glucose intolerance, which are the driving force for hepatic steatosis. Vitamin D receptor (VDR) is highly expressed in the ileum of the small intestine, which prompted us to test a hypothesis that vitamin D signaling may determine the enterotype of gut microbiota through regulating the intestinal interface. Here, we demonstrate that high-fat-diet feeding (HFD) is necessary but not sufficient, while additional vitamin D deficiency (VDD) as a second hit is needed, to induce robust insulin resistance and fatty liver. Under the two hits (HFD+VDD), the Paneth cell-specific alpha-defensins including α-defensin 5 (DEFA5), MMP7 which activates the pro-defensins, as well as tight junction genes, and MUC2 are all suppressed in the ileum, resulting in mucosal collapse, increased gut permeability, dysbiosis, endotoxemia, systemic inflammation which underlie insulin resistance and hepatic steatosis. Moreover, under the vitamin D deficient high fat feeding (HFD+VDD), Helicobacter hepaticus, a known murine hepatic-pathogen, is substantially amplified in the ileum, while Akkermansia muciniphila, a beneficial symbiotic, is diminished. Likewise, the VD receptor (VDR) knockout mice exhibit similar phenotypes, showing down regulation of alpha-defensins and MMP7 in the ileum, increased Helicobacter hepaticus and suppressed Akkermansia muciniphila. Remarkably, oral administration of DEFA5 restored eubiosys, showing suppression of Helicobacter hepaticus and increase of Akkermansia muciniphila in association with resolving metabolic disorders and fatty liver in the HFD+VDD mice. An in vitro analysis showed that DEFA5 peptide could directly suppress Helicobacter hepaticus. Thus, the results of this study reveal critical roles of a vitamin D/VDR axis in optimal expression of defensins and tight junction genes in support of intestinal integrity and eubiosis to suppress NAFLD and metabolic disorders.

Highlights

Metabolic syndrome (MetS) is becoming increasingly prevalent worldwide, and in many countries about 15–20% population bears metabolic disorders (Ford, 2005)
The mice fed with high fat diet (HFD)+vitamin D deficiency or depletion (VDD) for 18 weeks were given synthetic human α-defensin-5 (DEFA5, PDF-4415, Peptides International) via oral gavage (10 μg/dose in 0.1 ml saline) for four doses spaced over 25 days, or saline as a vehicle control
After 18 weeks of feeding, the mice fed with HFD or VDD developed moderate hepatic steatosis (Figure 1A, and the nonalcoholic fatty liver disease (NAFLD) activity score, NAS)

Summary

Introduction

Metabolic syndrome (MetS) is becoming increasingly prevalent worldwide, and in many countries about 15–20% population bears metabolic disorders (Ford, 2005). Clinically characterized and diagnosed as different diseases, MetS, nonalcoholic fatty liver disease (NAFLD), and type-II diabetes (T2D) share the common causes and biogenesis events. A “two hit theory” has been proposed to explain the transition from simple steatosis to non-alcoholic steatohepatitis (NASH), characterized by persistent hepatic inflammation, fibrosis and increased risk for cirrhosis or/and liver cancer (Day and James, 1998). Insulin resistance (IR) underlies the foundation of systemic metabolic complications, such as hyperglycemia, dyslipidemia, hyperinsulinemia, and diabetes (Shoelson et al, 2006). The cellular basis of IR is mediated in part through impairment of insulin signaling pathways, caused by pro-inflammatory cytokines produced in inflamed tissues including visceral fat (Hotamisligil et al, 1993)

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Conclusion