Adipokines play important roles in metabolic homeostasis and disease. We have recently identified a novel adipokine Metrnl, also known as Subfatin, for its high expression in subcutaneous fat. Here, we demonstrate a prodifferentiation action of Metrnl in white adipocytes. Adipocyte-specific knockout of Metrnl exacerbates insulin resistance induced by high-fat diet (HFD), whereas adipocyte-specific transgenic overexpression of Metrnl prevents insulin resistance induced by HFD or leptin deletion. Body weight and adipose content are not changed by adipocyte Metrnl. Consistently, no correlation is found between serum Metrnl level and BMI in humans. Metrnl promotes white adipocyte differentiation, expandability, and lipid metabolism and inhibits adipose inflammation to form functional fat, which contributes to its activity against insulin resistance. The insulin sensitization of Metrnl is blocked by PPARγ inhibitors or knockdown. However, Metrnl does not drive white adipose browning. Acute intravenous injection of recombinant Metrnl has no hypoglycemic effect, and 1-week intravenous administration of Metrnl is unable to rescue insulin resistance exacerbated by adipocyte Metrnl deficiency. Our results suggest adipocyte Metrnl controls insulin sensitivity at least via its local autocrine/paracrine action through the PPARγ pathway. Adipocyte Metrnl is an inherent insulin sensitizer and may become a therapeutic target for insulin resistance.

Hepatic concentrations of NAD + , protein levels of nicotinamide phosphoribosyltransferase (NAMPT) and several other critical enzymes regulating NAD + biosynthesis, were compared in middle-aged and aged mice or patients. The influences of NAD + decline on the steatosis and steatohepatitis were evaluated in wild-type and H247A dominant-negative, enzymically-inactive NAMPT transgenic mice (DN-NAMPT) given normal or high-fat diet (HFD). KEY RESULTSHepatic NAD + level decreased in aged mice and humans. NAMPT-controlled NAD + salvage, but not de novo biosynthesis pathway, was compromised in liver of elderly mice and humans. Given normal chow, middle-age DN-NAMPT mice displayed systemic NAD + reduction and had moderate NAFLD phenotypes, including lipid accumulation, enhanced oxidative stress, triggered inflammation and impaired insulin sensitivity in liver. All these NAFLD phenotypes, especially release of pro-inflammatory factors, Kupffer cell accumulation, monocytes infiltration, NLRP3 inflammasome pathway and hepatic fibrosis (Masson's staining and α-SMA staining), deteriorated further under HFD challenge. Oral administration of nicotinamide riboside, a natural NAD + precursor, completely corrected these NAFLD phenotypes induced by NAD + deficiency alone or HFD, whereas adenovirus-mediated SIRT1 overexpression only partially rescued these phenotypes. CONCLUSIONS AND IMPLICATIONSThese results provide the first evidence that ageing-associated NAD + deficiency is a critical risk factor for NAFLD, and suggest that supplementation with NAD + substrates may be a promising therapeutic strategy to prevent and treat NAFLD.

Aim: Metrnl is a novel secreted protein, but its physiological roles remain elusive. In this study, we investigated the tissue expression pattern of Metrnl in humans and explored its possible physiological role in the tissues with most highly expressed levels. Methods: A human tissue microarray containing 19 types of tissues from 69 donors was used to examine the tissue expression pattern of Metrnl, and the expression pattern was further verified in fresh human and mouse tissues. Intestinal epithelial cell-specific Metrnl knockout mice were generated, which were used to analyze the physiological roles of Metrnl. Results: Metrnl was the most highly expressed in the human gastrointestinal tract, and was specifically expressed in the intestinal epithelium. Consistent with this, Metrnl mRNA was also most highly expressed in the mouse gastrointestinal tract among the 14 types of tissues tested. In the intestinal epithelial cell-specific Metrnl knockout mice, the Metrnl levels in the gut fluid were significantly reduced, whereas the Metrnl serum levels showed a trend towards a reduction, but this change was not statistically significant. This cell-specific deletion of Metrnl did not affect body weight, food intake, blood glucose, colon length and histology, intestinal permeability, mucus content or mucin 2 expression under physiological conditions, but statistically decreased the expression of antimicrobial peptides, such as regenerating islet-derived 3 gamma (Reg3g) and lactotransferrin. Conclusion: Metrnl is highly expressed in the intestinal epithelial cells of humans and mice, which mainly contributes to the local gut Metrnl levels and affects the serum Metrnl level to a lesser extent. Metrnl plays a role in maintaining gut antimicrobial peptides.

Metrnl is a newly identified secreted protein highly expressed in the intestinal epithelium. This study aimed to explore the role and mechanism of intestinal epithelial Metrnl in ulcerative colitis. Metrnl −/− (intestinal epithelial cell-specific Metrnl knockout) mice did not display any phenotypes of colitis under basal conditions. However, under administration of 3% dextran sodium sulfate (DSS) drinking water, colitis was more severe in Metrnl −/− mice than in WT mice, as indicated by comparisons of body weight loss, the presence of occult or gross blood per rectum, stool consistency, shrinkage in the colon, intestinal damage, and serum levels of inflammatory factors. DSS-induced colitis activated autophagy in the colon. This activation was partially inhibited by intestinal epithelial Metrnl deficiency, as indicated by a decrease in Beclin-1 and LC3-II/I and an increase in p62 in DSS-treated Metrnl −/− mice compared with WT mice. These phenomena were further confirmed by observation of autophagosomes and immunofluorescence staining for LC3 in epithelial cells. The autophagy-related AMPK-mTOR-p70S6K pathway was also activated in DSS-induced colitis, and this pathway was partially blocked by intestinal epithelial Metrnl deficiency, as indicated by a decrease in AMPK phosphorylation and an increase in mTOR and p70S6K phosphorylation in DSS-treated Metrnl −/− mice compared with WT mice. Therefore, Metrnl deficiency deteriorated ulcerative colitis at least partially through inhibition of autophagy via the AMPK-mTOR-p70S6K pathway, suggesting that Metrnl is a therapeutic target for ulcerative colitis.