Introduction

In a study in the previous issue of Critical Care, Huang and colleagues [1] observed that, in the critically ill, adipose-fatty acid-binding protein (A-FABP) concentrations were elevated and that the serum A-FABP concentrations were independently related to serum creatinine, fasting plasma glucose, total cholesterol, tumor necrosis factor-alpha (TNF-α), albumin, and the Acute Physiology and Chronic Health Evaluation II scores, suggesting that higher A-FABP levels (>40 ng/mL) were associated with an unfavorable outcome in patients with sepsis. These results not only are interesting but also suggest that perhaps A-FABP could be used a biomarker of prognosis in the critically ill. But it is not clear why A-FABP levels should be increased in the critically ill or what this increase signifies.

Adipocyte fatty acid-binding protein (A-FABP or FABP4), also known as aP2 (adipocyte protein 2), is a carrier protein for fatty acids and is expressed primarily in adipocytes and macrophages. A-FABP belongs to the fatty acid-binding protein super-family whose members have relative molecular masses of approximately 15,000. A-FABP is a predominant cytosolic protein of mature adipocytes, accounts for approximately 6% of total cellular proteins, and is an important regulator of systemic insulin sensitivity and lipid and glucose metabolism [2]. Mice deficient in A-FABP are protected from development of hyperinsulinemia, hyperglycemia, and insulin resistance [3]. Adipocytes obtained from A-FABP-null mice had markedly reduced efficiency of lipolysis in vivo and in vitro [4] and showed a two- to three-fold decrease in fatty acid release, suggesting that A-FABP regulates efflux of fatty acids under normal physiological conditions. Acute insulin secretory response to β-adrenergic stimulation was profoundly suppressed in A-FABP−/− mice compared with their wild-type littermates [4], indicating that A-FABP could regulate systemic insulin sensitivity through its actions on other distal target tissues.

Adipose-fatty acid-binding protein and inflammation

A-FABP is also present in macrophages, and its expression in macrophages can be induced by oxidized low-density lipoprotein (LDL) [5] and Toll-like receptor (TLR) agonists [6] and suppressed by statins [7]. A-FABP modulates inflammatory cytokine production and cholesterol ester accumulation [8]. Ablation of the A-FABP gene protected against atherosclerosis [9]. This evidence suggests that A-FABP, by integrating metabolic and inflammatory pathways, provides a key link between components of metabolic syndrome, implying that blocking A-FABP protein could be considered in the treatment of heart disease, diabetes mellitus, asthma, obesity, and fatty liver disease, which are all inflammatory conditions.

In this context, it is interesting to note that A-FABP−/− mice are protected from experimental autoimmune encephalomyelitis and showed reduced levels of pro-inflammatory cytokine mRNA expression in central nervous system tissue as compared with wild-type mice. In vitro, antigen recall responses of myelin oligodendrocyte glycoprotein 35-55-immunized A-FABP−/− mice showed reduced proliferation and impaired interferon-gamma production. Dendritic cells deficient in FABPs were poor producers of pro-inflammatory cytokines-interleukin-6 (IL-6) and TNF-α- and did not promote pro-inflammatory T-cell responses, suggesting that metabolic-inflammatory pathway cross-regulation by A-FABPs plays a significant role in adaptive immune responses and inflammation [10]. These results-coupled with the observations that unsaturated fatty acids, such as palmitoleic acid, oleic acid, linoleic acid, linolenic acid, and eicosapentaenoic acid, significantly repressed the basal as well as lipopolysaccharide-induced A-FABP expression in macrophages and depletion of A-FABP expression by RNA interference (RNAi) decreased cyclooxygenase 2 (COX-2) mRNA expression and potentiated the repression by linoleic acid [11] -give interesting insights into the relationship among A-FABP, cytokines, and unsaturated fatty acids and their involvement in sepsis and other critical illnesses.

Adipose-fatty acid-binding protein, Toll-like receptors, unsaturated fatty acids, and inflammation

Recent studies [12, 13] revived the controversy of whether unsaturated fatty acids are beneficial in the management of sepsis. The fact that A-FABP is increased in sepsis, induced by oxidized LDL and TLR agonists, and suppressed by statins and enhances the production of IL-6 and TNF-α and COX-2 expression whereas unsaturated fatty acids suppress A-FABP expression and IL-6 and TNF-α production [14] indicates the complexity of the involvement of TLRs, A-FABP, cytokines, unsaturated fatty acids, and their products in sepsis.

The unsaturated fatty acid arachidonic acid (AA) is the precursor of two series of prostaglandins (PGs), two series of thromboxanes (TXs), and four series of leukotrienes (LTs); eicosapentaenoic acid (EPA) is the precursor of three series of PGs, three series of TXs, and five series of LTs. PGs, TXs, and LTs formed from EPA are less pro-inflammatory in nature compared with PGs, TXs, and LTs formed from AA but still are pro-inflammatory in nature [15]. Interestingly, AA, EPA, and docosahexaenoic acid (DHA) suppress the production of pro-inflammatory cytokines IL-6 and TNF-α both in vitro and in vivo. In addition, free radicals act on unsaturated fatty acids to give rise to F2-isoprostanes, a group of pro-inflammatory substances, whereas nitric oxide and unsaturated fatty acids interact to form nitrolipids that have anti-inflammatory action [15].

Furthermore, AA is the precursor of lipoxin A4 (LXA4), and EPA and DHA give rise to resolvins and DHA gives rise to protectins; resolvins and protectins have potent anti-inflammatory actions, resolve inflammation, and enhance wound healing by suppressing the production of free radicals, myeloperoxidase, IL-6, TNF-α, and HMGB1 (high-mobility box 1) and antagonize the pro-inflammatory actions of leukotrienes [15]. Resolvins and protectins block TLR-mediated activation of macrophages [15] and enhance the anti-bacterial action of antibiotics and augment the clearance of bacteria [16]. This implies that the optimal production of lipoxins, resolvins, and protectins from unsaturated fatty acids is essential for clearing the invading pathogens, resolving inflammation, and preventing tissue damage.

Balance between pro- and anti-inflammatory events and molecules determines prognosis

Under normal physiological conditions, a balance between pro- and anti-inflammatory molecules is maintained. A tilting of this balance in favor of the former would lead to initiation and progression of inflammation, insulin resistance, and consequent tissue damage [17]. Such an inflammatory process could be due not only to increased expression of A-FABP and augmented production of pro-inflammatory molecules but also to deficiency of anti-inflammatory molecules such as IL-4, IL-10, lipoxins, resolvins, and protectins (Figure 1). In this context, it is noteworthy that A-FABP suppresses cardiomyocyte contraction [18] whereas unsaturated fatty acids seem to prevent cardiac failure [19], reinforcing close interaction(s) among various pro- and anti-inflammatory molecules and their role in sepsis. Hence, a better understanding of the interaction(s) among these molecules in inflammatory conditions is essential in order to develop meaningful and novel therapeutic strategies especially for sepsis.

Figure 1
figure 1

Interaction(s) among adipose-fatty acid-binding protein (A-FABP), Toll-like receptor (TLR), cytokines, free radicals, unsaturated fatty acids, and their products and inflammation and resolution of inflammation. Infections, injuries (including surgery), and high-fat diet activate macrophages and TLRs, leading to secretion of increased amounts of pro-inflammatory cytokines that, in turn, produce an excess of free radicals. Pro-inflammatory cytokines, TLRs, and free radicals activate A-FABP and cyclooxygenase 2 (COX-2), leading to increased production of pro-inflammatory prostaglandins (PGs), leukotrienes (LTs), and thromboxanes (TXs) from unsaturated fatty acids and decreases in the synthesis and release of anti-inflammatory lipoxins (LXs), resolvins (RSVs), and protectins (PRTs). Blocking the expression of A-FABP and TLRs will suppress inflammation. Unsaturated fatty acids and LXs, RSVs, and PRTs are expected to suppress macrophage activation and expression of A-FABP and TLRs and inhibit inflammation. Increased expression of A-FABP also occurs in obesity, type 2 diabetes mellitus (DM), and coronary heart disease (CHD). Interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), macrophage activation, increased expression of A-FABP, and increased production of PGs, LTs, TXs, and free radicals enhance insulin resistance in infections and sepsis. AA, arachidonic acid; DHA, docosahexaenoic acid; EPA, eicosapentaenoic acid; HMGB1, high-mobility group box 1; LPS, lipopolysaccharide; NL, nitrolipid; NO, nitric oxide; ROS, reactive oxygen species.

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