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  • In addition to these cell autonomous effects the gut microbi


    In addition to these cell-autonomous effects, the gut microbiota may be an important determinant controlling Angptl4 in a VDR-dependent manner. It is known that certain bacterial strains and specific metabolites such as short chain fatty acids can modulate Angptl4 STA-21 [46,[49], [50], [51], [52], [53]]. Recent data also suggest a key role for VDR in the control of the gut microbiota in experimental inflammatory bowel disease [54,55]. Although beyond the scope of the current study, it is therefore tempting to speculate that VDR-dependent changes of the gut microbiota may also be relevant in the context of obesity and that these changes could be linked to differential regulation of Angptl4. Beyond Angptl4, inhibition of adipose tissue LPL in Vdr−/− mice may also be promoted by other factors. As such, jejunal expression of apolipoprotein C3 (Apoc3) was increased in Vdr−/− mice compared to Vdr+/− animals and this effect was reversed in Vdr−/− hTg (Fig. 4D, P < 0.1 each). Like Angptl4, Apoc3 is a potent inhibitor of LPL [56]. In addition to decreased lipid uptake by inhibition of LPL, we investigated whether increased adipose tissue lipolysis may contribute to the lean phenotype of Vdr−/− mice (Fig. S2). In line with earlier findings [57], Vdr−/− mice showed increased levels of parathyroid hormone – a known inducer of adipose tissue lipolysis [58,59]. However, we did neither observe an increase in fasting serum NEFAs in Vdr−/− mice nor an adipose tissue-specific up-regulation of the PTH-inducible genes Ucp1 or Dio2 [60]. These findings suggest that PTH and adipose tissue lipolysis are probably not the major physiological factors driving the phenotype of Vdr−/− mice. The independence of our findings from PTH (and calcium) are further supported by data on ten months-old Vdr+/−, Vdr−/− and Vdr−/− hTg mice fed a LFD rather than a HFD (Table S2). In this set-up, Vdr−/− animals received a low-fat rescue diet (containing high levels of calcium) resulting in completely normal serum calcium and PTH. Despite this normalization, Vdr−/− mice were much leaner and had significantly less fat mass than Vdr+/− mice. In line with our findings under HFD, this lean phenotype was partially reversed by intestinal re-expression of VDR in Vdr−/− hTg mice which had body weights that were intermediate between Vdr+/− and Vdr−/−. Previous mouse studies have demonstrated important functions of vitamin D and VDR in maintaining the intestinal barrier and protecting against intestinal inflammation. In the context of NAFLD, this has for example been suggested by a report from Su et al. showing that nutritional vitamin D deficiency increases gut permeability and plasma endotoxin levels in a HFD mouse model [61]. Mechanistically, this function of vitamin D could partially rely on modulation of certain tight junction proteins as suggested by another report demonstrating direct transcriptional regulation of claudin-2 by VDR [62]. In addition to this, intestinal VDR has been shown to protect against mucosal inflammation in experimental colitis [63,64] and contribute to systemic bile acid homeostasis by regulation of the intestinal hormone fibroblast growth factor 15 (FGF15) [65]. Whether and how these intestinal functions of VDR may affect the metabolic phenotype of our mouse model beyond the regulation of Angptl4, remains to be studied in the future. Observational clinical studies suggest an association of vitamin D deficiency with obesity and NAFLD in humans resulting in suggestions for vitamin D supplementation as a therapeutic option [[1], [2], [3], [4], [5], [6]]. Moreover, certain (although not all) previous interventional trials testing the efficacy of vitamin D supplementation in NAFLD patients described positive effects on disease measures such as liver fat content and/or improved serum markers of liver damage [[7], [8], [9], [10]]. This is in apparent contrast to the findings reported in the present study which rather suggests that a complete loss of VDR signaling protects against fat accumulation in adipose tissue and liver. In this context, it is however important to note that – besides gene regulation through VDR – vitamin D exerts non-genomic biological effects through cell surface receptors [66]. Vice versa, VDR can – in addition to vitamin D – also be activated by the secondary bile acid lithocholic acid [67] and has moreover been shown to regulate gene expression in a ligand-independent manner as well [68]. Thus, from a biochemical point of view vitamin D deficiency and inhibition of VDR activity are overlapping but partially distinct phenomena that may not necessarily result in the same physiological outputs.