Interleukin-32 upregulates the expression of ABCA1 and ABCG1 resulting in reduced intracellular lipid concentrations in primary human hepatocytes

Background and aims: The role of interleukin (IL-) 32 in inflammatory conditions is well-established, however, the mechanism behind its role in atherosclerosis remains unexplained. Our group reported a promoter single nucleotide polymorphism in IL-32 associated with higher high-density lipoprotein (HDL) concentrations. We hypothesize that endogenous IL-32 in liver cells, a human monocytic cell line and carotid plaque tissue, can affect atherosclerosis by regulating (HDL) cholesterol homeostasis via expression of cholesterol transporters/mediators. Methods: Human primary liver cells were stimulated with recombinant human (rh) TNF alpha and poly I:C to study the expression of IL-32 and mediators in cholesterol pathways. Additionally, IL-32 was overexpressed in HepG2 cells and overexpressed and silenced in THP-1 cells to study the direct effect of IL-32 on cholesterol transporters expression and function. Results: Stimulation of human primary liver cells resulted in induction of IL-32 alpha, IL-32 beta and IL-32 gamma mRNA expression (p < 0.01). A strong correlation between the expression of IL-32 gamma and ABCA1, ABCG1, LXR alpha and apoA1 was observed (p < 0.01), and intracellular lipid concentrations were reduced in the presence of endogenous IL-32 (p < 0.05). Finally, IL32 gamma and ABCA1 mRNA expression was upregulated in carotid plaque tissue and when IL-32 was silenced in THP-1 cells, mRNA expression of ABCA1 was strongly reduced. Conclusions: Regulation of IL-32 in human primary liver cells, HepG2 and THP-1 cells strongly influences the mRNA expression of ABCA1, ABCG1, LXR alpha and apoA1 and affects intracellular lipid concentrations in the presence of endogenous IL-32. These data, for the first time, show an important role for IL32 in cholesterol homeostasis. (C) 2018 The Authors. Published by Elsevier B.V.