Vitamin D signalling in adipose tissue

Ding, Cherlyn Vitamin D signalling in adipose tissue. PhD thesis, University of Liverpool.

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Abstract

Adipose tissue inflammation is characterised by increased infiltration of macrophages and is associated with a marked increase in the synthesis and release of proinflammatory factors such as TNFα, IL-6 and MCP-1. Studies suggest that these chemokines and cytokines contribute to local tissue inflammation and levels of inflammatory mediators in the systemic circulation. The potential role of the vitamin D receptor in modulating inflammation in obesity has received increasing attention, with evidence suggesting that 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) has potent immunoregulatory effects. The data presented in this thesis showed adipose tissue expresses the vitamin D receptor, along with vitamin D metabolising enzymes CYP27B1 and CYP24A1. Treatment of adipocytes with 1,25(OH)2D3 reduced the secretion of key cytokines and chemokines involved in the inflammatory response induced by macrophage-secreted factors. MCP-1, IL-6, IL-8 and RANTES production by human adipocytes was significantly downregulated, and western blotting data supports that this was due to the inhibitory effects of 1,25(OH)2D3 on the NF-κB and MAPK signalling pathways. Treatment with 1,25(OH)2D3 also abolished macrophage-induced activation of NFκB, increasing IκBα expression and reducing NFκB p65 phosphorylation. Mitogen-activated protein kinase (MAPK) signalling activated by MC medium was also inhibited by 1,25(OH)2D3 , reducing phosphorylated p38 MAPK and phosphorylated Erk1/2. Investigations into its downstream effects showed 1,25(OH)2D3 decreased macrophage-induced inflammatory cytokine expression, reducing IL-8, MCP-1, RANTES and IL-6. To confirm the involvement of the vitamin D receptor in modulating the inflammatory response in adipocytes, two 1,25(OH)2D3 analogues, ZK159222 and ZK191784, were investigated for its effects on adipocyte-macrophage crosstalk. The anti-inflammatory effects of ZK159222 and ZK191784 are demonstrated for the first time in human adipocytes. ZK159222 treatment increased IκBα in adipocytes stimulated with macrophage-conditioned medium. In contrast, ZK159222 markedly reduced protein expression of phos-NF-κB p65. In studies of MAPK activation, ZK159222 dose-dependently decreased expression of phosphorylated p38 MAPK. ZK191784 treatment was observed to increase IκBα while reduce phosphorylated NF-κB p65 levels compared to vehicle controls. ZK191784 also showed an inhibitory effect on protein expression of phosphorylated p38 MAPK. In addition, both 1,25(OH)2D3 analogues reduced protein secretion of MCP-1 and IL-6 by human adipocytes. Both compounds also significantly reduced IL-8 expression compared to vehicle controls as well as 1,25(OH)2D3 treatment as well as MC-induced RANTES expression. Overall this research demonstrates that vitamin D3 has anti-inflammatory properties in human adipocytes, probably mediated by inhibition of the NFκB and MAPK signalling pathways. Reviewing the current evidence, the data suggests that targeting the vitamin D signalling system in adipose tissue presents a novel approach for modulating adipose tissue function, and in particular, ameliorating inflammation perpetuated by macrophage-adipocyte crosstalk in obesity.

Item Type:	Thesis (PhD)
Additional Information:	Date: 2015-03-07 (completed)
Depositing User:	Symplectic Admin
Date Deposited:	10 Dec 2015 10:46
Last Modified:	17 Dec 2022 01:33
DOI:	10.17638/02016659
URI:	https://livrepository.liverpool.ac.uk/id/eprint/2016659