Promotion of interferon-gamma production by natural killer cells via suppression of murine peritoneal macrophage prostaglandin E₂ production using intravenous anesthetic propofol

Abstract

Propofol is an intravenous anesthetic, widely used for general anesthesia during surgery, which inevitably involves tissue trauma with inflammation. At sites of inflammation, prostanoids, especially prostaglandin E₂ (PGE₂), are abundant. This study addresses the effect of propofol on macrophage PGE₂ production. Using thioglycollate-elicited murine peritoneal macrophages, propofol (7.5–30 μM) suppressed lipopolysaccharide-induced PGE₂ production. The suppression was via the direct inhibition of cyclooxygenase (COX) enzyme activity and due neither to the downregulation of COX expression nor the inhibition of arachidonic acid release from plasma membranes. In macrophage:natural killer (NK) cell co-culture, propofol dramatically increased interferon-gamma (IFN-γ) production, and the actions of propofol were mimicked by a selective COX-2 inhibitor, NS-398, as well as the selective EP4 receptor antagonist L-161,982, suggesting a role of PGE₂ suppression in the upregulation of IFN-γ production. Furthermore, in purified NK cell culture, PGE₂ directly suppressed the production of IFN-γ by activated NK cells, which was reversed by selective inhibition of EP4 activity. Taken together, our results show that, in macrophage:NK cell co-culture, propofol, through the suppression of macrophage PGE₂ production, upregulates NK cell IFN-γ production by alleviating EP4 receptor-mediated suppression of IFN-γ production. Propofol may potentially exert considerable influence on inflammation and immunity by suppressing PGE₂ synthesis.

Introduction

Prostanoids, including prostaglandins and thromboxanes, are abundant at sites of inflammation [1], [2]. Among prostanoids, prostaglandin E₂ (PGE₂) is especially important in both modulating inflammation and determining the characteristics of the immune reaction that follows. For example, PGE₂ can exacerbate inflammation by facilitating plasma protein extravasations via vasodilatation and increased vascular permeability [3], [4]. On the other hand, PGE₂ may suppress interferon-gamma (IFN-γ) production from natural killer (NK) cells, as well as helper T (Th) cells, which may lead to the downregulation of innate immunity and the deviation of adaptive immunity away from the Th1 type [3], [5], [6], [7], [8]. To further complicate things, in certain situations, PGE₂ can facilitate Th1 cell differentiation and Th17 cell expansion, leading to a progression of autoimmune diseases [9], [10]. These protean effects of PGE₂ may depend on various interactions among numerous kinds of immune and non-immune cells, which involve the production of cytokines and other lipid mediators in response to PGE₂, the surface expression of PGE₂ receptors (EP) on PGE₂-responding cells, and a variety of participating cells with varying maturation-activation statuses at the local inflammatory site [1], [3]. Although many immune (e.g., macrophages, dendritic cells (DC), and neutrophils) and non-immune (e.g., fibroblast and endothelial cells) cells can produce PGE₂, macrophages are considered to be one of the most powerful cells in the modulation of inflammation and immune function through their ability to produce a large amount of PGE₂ [1].

Prostanoids are produced by cyclooxygenase (COX) and specific prostanoid synthases [11], [12]. The COX enzyme has at least two isoforms; COX-1 is constitutively expressed in most cells, whereas COX-2 expression is undetectable during the resting state and induced dramatically upon stimulation, particularly in immune cells [11], [12]. The COX substrate is arachidonic acid (AA), which is synthesized from the cleavage of the lipid membrane by the action of cytosolic phospholipase A₂ (cPLA₂) and certain forms of secretory phospholipase A₂, whose activities are also influenced by inflammatory stimulation [13], [14]. After AA is converted to PGH₂ by COX, cytosolic PGE synthase and microsomal PGE synthase convert PGH₂ to PGE₂. The major prostanoids produced in a particular cell are determined by the expression profiles of the specific prostanoid synthases, and the major prostanoids produced in macrophages are PGE₂ and thromboxane (TX) A₂ [1].

Propofol (2,6-diisopropylphenol) is a widely used intravenous anesthetic for general anesthesia and patient sedation [15], [16]. Apart from its anesthetic/hypnotic effects, propofol also possesses non-anesthetic effects, including some that are immunologically relevant [15], [16], [17], [18]. Because of propofol's basic phenol structure, it has a potent anti-oxidative effect [19], [20], [21]. Propofol is also reported to have effects on nitric oxide (NO) production, stimulating constitutive NO production and inhibiting inducible NO production [22], [23]. Propofol's effects on cytokine production have been reported in numerous studies, including tumor necrosis factor-alpha (TNF-α), interleukin (IL)-6, IL-8, and IL-10 [17], [24], [25], [26]. Although the results are somewhat conflicting due, conceivably, to widely different experimental designs, propofol appears to suppress the production of pro-inflammatory cytokines from immune cells [17], [22], [23], [24], [25], [26]. Recently, we found that propofol suppresses PGE₂ production by the human monocytic cell line THP-1 [27] and human peripheral monocytes [28], and we found that these suppressive effects are likely due to the inhibition of COX enzyme activity. In the present study, we show that propofol suppresses COX activity in murine peritoneal macrophages, leading to decreased PGE₂ production. We also show that, in macrophage:NK cell co-culture, propofol dramatically increases IFN-γ production by NK cells, probably through the suppression of PGE₂ production by macrophages. These results suggest that propofol has the potential to exert considerable influence on inflammation and immunity by suppressing PGE₂ synthesis.