Interleukin-18 null mutation increases weight and food intake and reduces energy expenditure and lipid substrate utilization in high-fat diet fed mice

Highlight

• Results from interleukin-18 knockout mice suggest that it may curb appetite and promote lipid utilization in healthy adults consuming high-fat diets.

Abstract

Objective

The proinflammatory cytokine interleukin-18 (IL-18) putatively modulates food intake and energy metabolism, but the effects of IL-18 in high-fat diet fed animals are unknown. Whether IL-18 alters basal metabolic rate or metabolic processes of living is unknown. Here, we tested the hypothesis that IL-18 modulates weight gain, energy intake, whole-body energy expenditure, and utilization of lipid as a fuel substrate in high-fat diet fed mice.

Methods

Food intake, whole-body metabolism, and motor activity of IL-18 knockout mice were compared to those of wildtype littermates; anorectic effects of intracerebroventricular IL-18 administration were compared between IL-18 receptor knockout, IL-18/IL-18R knockout and wildtype mice.

Results

Chow-reared IL-18 knockout mice were overweight at 6 months of age and then gained excess weight on both low-fat and high-fat diets, ate more high-fat diet, and showed reduced whole-body energy expenditure and increased respiratory exchange ratios. Reductions in energy expenditure of IL-18 knockout mice were seen across fasting vs. feeding conditions, low- vs. high-fat diets, high vs. low levels of physical activity and times of day, suggesting actions on basal metabolic rate. The circadian amplitude of energy expenditure, but not respiratory exchange ratio, food intake, or motor activity, also was blunted in IL-18 knockout mice. Central IL-18 administration reduced high-fat diet intake in wildtype mice, but not in mice lacking the IL-18 receptor.

Conclusion

The loss-of-function results support the hypothesis that endogenous IL-18 suppresses appetite and promote energy expenditure and lipid fuel substrate utilization not only during sickness, but also in healthy adults consuming high-fat diets.

Introduction

Better understanding the molecular controls of energy metabolism may inform the treatment of obesity. Interleukin-18 (IL-18), an 18 kDa multifunctional cytokine discovered for its proinflammatory and interferon-γ-inducing properties (Okamura et al., 1995), produces diverse effects via activation of the IL-18 receptor complex (Born et al., 2000, Torigoe et al., 1997), an IL-1/Toll-like superfamily receptor. Recent findings suggest that IL-18 may be a physiologic modulator of food intake and energy metabolism. Unlike classic proinflammtory cytokines that mediate the sickness response (e.g., IL-1β, IL-6, TNF-α), IL-18 also is constitutively expressed in non-immune cells and derived partly from adipocytes. Similar to other adipocytokines, its circulating levels relate to metabolic state, including fat mass, weight loss, hyperglycemia, and dietary fat intake (Esposito et al., 2002a, Esposito et al., 2002b, Esposito et al., 2003). Intracebrebroventricular or peripheral administration of IL-18 potently suppressed chow intake, feed efficiency and weight regain in fasted mice, without promoting sickness-like behavior (Zorrilla et al., 2007). Conversely, mice partially (Il18^+/−) or totally deficient (IL18^−/−) in IL-18 overate chow and purified low-fat diet by young adulthood (Zorrilla et al., 2007). Adult Il18^−/− mice showed increased feed efficiency; indirect calorimetry revealed reduced energy expenditure in low-fat diet-fed female Il18^−/− mice and increased respiratory exchange ratios (RER) (VCO₂/VO₂) in mutants of both sexes (Zorrilla et al., 2007). By mid-adulthood, Il18^−/− mice became obese (Netea et al., 2006, Zorrilla et al., 2007). Similar delayed-onset obesity phenotypes were observed in IL-18 receptor knockout (KO) mice and in IL-18-binding protein overexpressing mice (Netea et al., 2006).

The present studies sought to determine the effects of the IL-18 null genotype in mice fed high-fat diet. Previous calorimetry studies in IL-18 KO mice were performed using low-fat diet (Zorrilla et al., 2007). Few humans eat low-fat diets, however, and the indirect calorimetric profile of IL-18 null mice is unknown. High-fat diets can produce different rates of energy expenditure as compared with low-fat diets (Bandini et al., 1994, Ebbeling et al., 2012), in relation to the different energy and macronutrient intakes elicited by each. High-fat diets also promote greater relative utilization of lipids as a fuel substrate vs. low-fat diets (McNeill et al., 1988, Rumpler et al., 1991, Verboeket-van de Venne et al., 1994). As a result of these differences, many studies of transgenic mice have observed strikingly different metabolic phenotypes with high-fat diet exposure (Gordon et al., 2008, Klockener et al., 2011, Kusudo et al., 2012, Lee et al., 2007, Paula et al., 2010, Strader et al., 2004, Sutton et al., 2006, Wortley et al., 2004, Zigman et al., 2005). Potentially consistent with a role for IL-18 in metabolic adaptations to high-fat diet, high-fat meals increase circulating IL-18 levels. Therefore, the present study tested the hypothesis that IL-18 null mutation also reduces whole-body energy expenditure and utilization of lipid as a fuel substrate in high-fat diet fed mice.

Energy expenditure can be subdivided into components that reflect the basal metabolic rate of minimally maintaining the organism as compared to phasic components of energy expenditure related to activities of living, including physical activity, thermic effects of food intake and adaptive thermogenesis (Even and Nadkarni, 2012). In our previous study of IL-18 KO mice, whole-body energy metabolism was studied in free-feeding mice, and the genotypes exhibited differences in food intake and motor activity (Zorrilla et al., 2007). Thus, it remains unclear whether phasic components of energy expenditure are responsible for the observed differences in total daily energy expenditure or whether IL-18 KO mice may exhibit a reduced basal metabolic rate. To differentiate between the hypotheses that basal metabolic processes vs. phasic metabolic processes (e.g., activity, absorptive phase of feeding) contribute to IL-18 genotype effects on total daily energy expenditure, the present study measured whole-body energy expenditure of IL-18 KO and wildtype mice under both fasting and feeding conditions within each of the dark cycle and light cycle. Concurrent motor activity was measured.

A third goal was to determine the circadian-dependence of the IL-18 phenotype on food intake and energy expenditure. In our initial study, hyperphagia of low-fat diet was most evident during the mid-to-late dark cycle and respiratory exchange ratios were observed at some, but not other, times of day. On the other hand, genotype differences in energy expenditure of low-fat diet-fed mice and circulating IL-18 levels were consistent across a 24-h period (Zorrilla et al., 2007). Still, circadian variations in sensitivity to IL-18 might exist, as has been seen for IL-1β and IFN-γ (Opp and Toth, 1997). We therefore performed a cosinor analysis of chronobiologic differences in the food intake, energy expenditure, respiratory exchange ratios and motor activity of high-fat diet-fed IL-18 KO vs. wildtype mice.

A final pharmacological study sought to determine whether brain IL-18 systems modulate the control of high-fat diet intake and the mediating role of the IL-18R therein.