Neuroprotective effects of vitamin D on high fat diet- and palmitic acid-induced enteric neuronal loss in mice

Sara Larsson,Ulrikke Voss

doi:10.1186/s12876-018-0905-9

Abstract

BackgroundThe role of vitamin D in obesity and diabetes is debated. Obese and/or diabetic patients have elevated levels of free fatty acids, increased susceptibility to gastrointestinal symptoms and are suggested to have altered vitamin D balance. The enteric nervous system is pivotal in regulating gastrointestinal activity and high fat diet (HFD) has been shown to cause loss of enteric neurons in ileum and colon. This study investigates the effect of vitamin D on HFD- and palmitic acid-induced enteric neuronal loss in vivo and in vitro.MethodsMice were fed either a normal diet (ND) or HFD supplemented with varying levels of vitamin D (from 0x to 20x normal vitamin D level) for 19 weeks. Ileum and colon were analyzed for neuronal numbers and remodeling. Primary cultures of myenteric neurons from mouse small intestine were treated with palmitic acid (4x10-4M) and/or 1α,25-hydroxy-vitamin D3 (VD, 10-11- 10-7M) with or without modulators of lipid metabolism and VD pathways. Cultures were analyzed by immunocyto- and histochemical methods.ResultsVitamin D supplementation had no effect on enteric neuronal survival in the ND group. HFD caused substantial loss of myenteric neurons in ileum and colon. Vitamin D supplementation between 0-2x normal had no effect on HFD-induced neuronal loss. Supplementation with 20x normal, prevented the HFD-induced neuronal loss. In vitro supplementation of VD prevented the palmitic acid-induced neuronal loss. The VD receptor (VDR) was not identified in enteric neurons. Enteric glia expressed the alternative VD receptor, protein disulphide isomerase family A member 3 (PDIA3), but PDIA3 was not found to mediate the VD response in vitro. Inhibition of peroxisome proliferator-activated receptor gamma (PPARγ) and immune neutralization of isocitrate lyase prevented the VD mediated neuroprotection to palmitic acid exposure.ConclusionsResults show that VD protect enteric neurons against HFD and palmitic acid induced neuronal loss. The mechanism behind is suggested to be through activation of PPARγ leading to improved neuronal peroxisome function and metabolism of neuronal lipid intermediates.

Highlights

The role of vitamin D in obesity and diabetes is debated
Since animal studies have shown that vitamin D supplementation protects against high fat diet (HFD) induced metabolic parameters [1,2,3], several efforts have been made to elucidate the role of vitamin D in type-2 diabetes and in obesity, as well as in their complications [4,5,6]
VDR dimerizes with the retinoid-X receptor (RXR) forming a heterodimer, that in turn binds to specific vitamin D3 (VD) response elements (VDREs). [13, 14] it is suggested that initiation of VDR mediated responses cannot account for the fast responses seen upon VD stimulation in certain cell types e.g. osteoblasts and intestinal epithelial cells. [15,16,17] Protein disulphide isomerase family A member 3 (PDIA3), known as 1,25MAARS, has been suggested as an alternative VD receptor, capable of mediating the fast VD responses and acting as a modulator of cellular function independent of VDR [15, 18]

Summary

Introduction

The role of vitamin D in obesity and diabetes is debated. Obese and/or diabetic patients have elevated levels of free fatty acids, increased susceptibility to gastrointestinal symptoms and are suggested to have altered vitamin D balance. Since animal studies have shown that vitamin D supplementation protects against high fat diet (HFD) induced metabolic parameters [1,2,3], several efforts have been made to elucidate the role of vitamin D in type-2 diabetes and in obesity, as well as in their complications [4,5,6]. Whether produced in the skin through UV radiation or ingested in the diet, vitamin D (25-hydroxy vitamin D3) is in the body converted to its active form 1α,25-hydroxy-vitamin D3 (VD), by the enzyme 1α-hydroxylase known as cytochrome P450 This enzyme is found mainly in the kidneys but other cells types including immune cells and neurons have been shown to express this enzyme [11, 12]. VD is reported to decrease reactive oxygen species (ROS) by increasing levels of the key anti-oxidative enzymes superoxide dismutase (SOD), catalase and glutathione (GSH) [20,21,22]

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