Introduction
Understanding how IL6 regulates central and peripheral nutrient homeostasis is complicated by contradictory and multi-systemic effects under various physiological states [
1]. IL6 is best known as a pro-inflammatory cytokine that regulates innate immunity and the acute-phase response. However, IL6 also has tissue-specific effects that can differ in humans and rodents, depending on context and timing of stimulation [
2]. IL6 promotes chronic inflammation, whereas it displays anti-inflammatory effects during acute inflammatory stimuli [
3].
Obesity and its progression to diabetes are associated with chronic inflammation characterised by secretion of the proinflammatory cytokines resistin, TNFα and IL6 from adipocytes [
4]. Epidemiological data confirm that elevated circulating IL6 correlates with adiposity in humans [
5]. IL6 is generally thought to promote systemic insulin resistance, especially during obesity, because it is secreted from fat cells of insulin-resistant humans [
5]. However, in type 2 diabetes patients the plasma concentrations of IL6 and TNFα may best reflect the level of adiposity rather than insulin sensitivity during the euglycaemic–hyperinsulinaemic clamp [
6]. Yet TNFα might also be a principle cause of dysregulated insulin signalling, as it stimulates production of IL6, IL1 and C-reactive peptide [
7]. It is possible that IL6 opposes the action of TNFα upon insulin sensitivity, as physical exercise promotes secretion of IL6 from skeletal muscle, while improving insulin sensitivity and nutrient homeostasis [
1,
8]. The question of how IL6 integrates multiple signalling cascades to coordinate nutrient homeostasis in mammals remains unanswered.
Cell-based experiments and in vivo studies in rodents show that IL6 promotes insulin resistance [
9]. In vivo, 90 min after IL6 injections plasma glucose and insulin concentrations increase [
10]. Infusion of IL6 for 3 h causes hepatic and muscle insulin resistance [
11,
12]. In addition, hepatic insulin receptor signalling improves in
ob/ob mice treated with neutralising antibodies against IL6 [
13]. Recently, electrotransfer of murine
IL6 cDNA into skeletal muscle promoted liver inflammation and hyperinsulinaemia in mice [
14].
Unlike in rodent studies, infusion of recombinant human IL6 (hIL6) to sustain physiological concentrations in healthy individuals or patients with diabetes increases lipolysis in the absence of adverse effects and enhances glucose infusion rates during a euglycaemic–hyperinsulinaemic clamp [
15‐
17]. Moreover, adipose-derived hIL6 can have autocrine effects that increase leptin secretion and fat oxidation, and reduce expression and activity of lipoprotein lipase in human adipose tissues, a phenomenon that might attenuate progression of obesity and diabetes [
18]. Human IL6 also displays anti-inflammatory characteristics by inhibiting TNFα and IL1, and activating IL1 receptor antagonist and IL10 [
19‐
21]. Moreover, in rodents IL6 has central effects similar to those of leptin in promotion of nutrient homeostasis and peripheral insulin sensitivity [
1,
22]. Thus, the role of IL6 in the regulation of nutrient homeostasis is contradictory and incompletely resolved, possibly confounded by differences between human and murine cytokine action [
1].
Leptin is secreted from adipose tissue in proportion to fat stores, informing the central nervous system of the peripheral energy supply. Dysregulated leptin action (
ob/ob mice) increases food intake, while reducing energy expenditure. In addition,
ob/ob mice display severe obesity and insulin resistance that progresses to diabetes [
23]. However, ordinary obesity in mice and humans is associated with elevated leptin concentrations, suggesting leptin resistance in the central nervous system as a principle cause [
24,
25]. Interestingly, IL6 might be required for a normal leptin response, as adult
Il6
−/−
mice develop hyperphagia and obesity, which is difficult to prevent by peripheral leptin injections [
26].
To establish the long-term systemic effect of hIL6 upon nutrient homeostasis in mice, we investigated glucose tolerance, energy expenditure and insulin action in transgenic C57BL/6J mice and ob/ob mice that secrete hIL6 constitutively into the circulation. Our results show that hIL6 promotes central leptin action in mice, together with its beneficial effects upon nutrient homeostasis.
Discussion
Despite evidence of pleotropic and contradictory actions of IL6 upon glucose tolerance in rodent models and human studies, our experiments show clearly that overexpression of hIL6 in brain and lung of hIL6
tg mice reduces daily food consumption and promotes energy expenditure. Consistent with the reduced adiposity, circulating insulin decreases and glucose tolerance improves, confirming that hIL6 promotes systemic insulin sensitivity, especially in animals on HFD. Moreover, circulating leptin and daily food consumption decreases, suggesting that hIL6 improves central leptin sensitivity or action.
Previous reports have shown that central leptin signalling requires IL6-mediated signals for a normal response. Thus
Il6
−/−
mice slowly develop obesity while circulating leptin increases, and obese
Il6
−/−
mice do not respond to intracranial leptin injections [
26]. By contrast, circulating leptin decreases significantly in h
IL6
tg mice on chow or HFD. Since leptin signalling is required in the hypothalamus to suppress appetite and promote energy expenditure, hIL6 apparently augments leptin action: otherwise the h
IL6
tg mice would consume more food and accumulate adipose mass [
23]. In our study, only
ob/ob
IL6 mice responded significantly to low-dose leptin injections with greater locomotor activity accompanied by decreased body weight and food consumption. Thus our results support the hypothesis that life-long hIL6 promotes central leptin signalling, which prevents diet-induced obesity in mice.
The signalling subunit gp130 of the IL6 receptor complex is similar structurally to the intracellular tail of the signalling-isoform of the leptin receptor, isoform b (LepRb) [
37]. Consistent with the shared regulation of STAT3 phosphorylation by leptin and IL6,
Pomc and
Agrp expression in our study was nearly normal in h
IL6
tg mice on a HFD. However, the effect of hIL6 upon
Pomc and
Agrp regulation appears to occur through its effects upon leptin signalling, as
ob/ob and
ob/ob
IL6 mice were equally hyperphagic.
STAT3 to SOCS3 signalling is stimulated by leptin in the hypothalamus and throughout the body by numerous factors including IL6, IFN-γ, IL10, CNTF (ciliary neurotrophic factor) and other gp130 signalling cytokines [
2]. However, the leptin response increased while SOCS3 production also increased in the hypothalamus of lean h
IL6
tg mice, suggesting that SOCS3 does not inexorably block the leptin signal. Direct comparison of hypothalamic STAT3 phosphorylation in
ob/ob and
ob/ob
IL6 mice shows that hIL6 weakly promoted STAT3 phosphorylation in the absence of leptin. Thus, hIL6 largely promotes the leptin-stimulated STAT3 to SOCS3 cascade, which maintains the normal relation between leptin and SOCS3.
We posit that IL6 receptor α-neurons are separate from LepRb-neurons, since hIL6 failed to normalise body weight or food intake in
ob/ob
IL6 mice. However, IL6 receptor α-neurons might converge upon a common efferent circuit, ordinarily regulated by LepRb neurons, to augment leptin signalling in
ob/ob mice or wild-type mice on the HFD. A similar relation appears to exist between CNTF receptor neurons and LepRb neurons [
2]. LepRb and the CNTF receptor share structural homology and can activate similar signalling pathways in the hypothalamus. In the absence of CNTF receptor, CNTF can activate gp130 through a homodimer of IL6 receptor and leukaemia inhibitory factor receptor (LIFR) [
38]. CNTF can ameliorate obesity by circumventing diet-induced leptin resistance [
39]. It remains to be investigated whether CNTF mediates any of the central effects of IL6.
Cell-based experiments suggest that the IL6-stimulated STAT3 to SOCS3 cascade causes hepatic insulin resistance by inhibiting insulin receptor signalling and increasing IRS1 degradation [
10]. In parallel with increasing SOCS3 concentrations, IRS1 concentrations in the present study decreased in the postprandial liver of h
IL6
tg mice; however, hIL6 prevented the near complete loss of insulin-stimulated insulin receptor autophosphorylation and the downstream phosphorylation of IRS2 and Akt
ser473 in animals on HFD, a finding consistent with improved systemic glucose tolerance. Recently, the negative effect of SOCS3 on insulin action has been questioned, since liver-specific
Stat3
−/−
mice with low SOCS3 concentrations were unable to suppress hepatic glucose production [
32]. The strongest effect of mIL6 upon liver metabolism might depend upon hypothalamic STAT3 signalling, which mediates the normalising effect of leptin on hepatic insulin action in rats on a HFD [
35]. Intracerebral ventricular insulin infusion has been shown to increase levels of mIL6 in the liver, which can increase hepatic STAT3 and through that suppress expression of gluconeogenic enzymes [
32]. Thus hepatic IL6 to STAT3 signalling triggered by brain insulin action could play an important role in nutrient homeostasis. However, in animals on a chow diet hIL6 might not be sufficient, because STAT3 phosphorylation did not increase in fasted h
IL6
tg mice, while increasing equally in wild-type and h
IL6
tg mice. Whereas the HFD inhibited hepatic STAT3 phosphorylation in our study, hIL6 strongly promoted basal and postprandial STAT3 phosphorylation. Thus a postprandial signal, perhaps initiated by insulin and/or leptin in the hypothalamus, appears to be essential for hepatic STAT3 phosphorylation.
The question of whether IL6 has positive or negative effects on metabolism is the subject of continuing controversy [
8]. The hypothesis that IL6 induces insulin resistance is challenged by findings that regular physical exercise increases insulin sensitivity while promoting production and release of IL6 from contracting skeletal muscle [
40,
41]. IL6 can also increase peripheral insulin sensitivity and glucose tolerance by activating AMP-activated protein kinase (AMPK) in muscle [
17,
42]. Here, however, hIL6 had no effect on AMPK phosphorylation or activity in h
IL6
tg mice (data not shown). Further investigation regarding a potential of AMPK to mediate some of the effects of hIL6 is required.
The relation between IL6 and leptin in the central nervous system might play an important role on the effect of exercise upon nutrient homeostasis. Moderate exercise promotes peripheral insulin sensitivity and suppresses weight gain [
43]. Human IL6 secretion from skeletal muscle is dramatically increased during and after exercise [
44]. Our results are consistent with the hypothesis that the effect of exercise upon nutrient homeostasis and insulin sensitivity might be mediated through central effects of muscle-derived IL6 in promoting central leptin signalling.
Our results are consistent with the hypothesis that decreased fat mass in h
IL6
tg mice, especially those on HFD, arises through increased energy expenditure. Thus oxygen consumption, CO
2 production and physical activity were increased in the h
IL6
tg mice. These data are consistent with previous reports that a single intracranial injection of IL6 increases oxygen consumption and energy expenditure by rats [
22,
26].
Chronic cerebral expression of mIL6, using an
IL6 transgene under the control of glial fibrillary acidic protein (
GFAP) promoter, activates the hypothalamic–pituitary–adrenal axis, which increases corticosterone concentrations in stressed mice [
45]. However, in our experiments, plasma corticosterone concentrations were barely increased in unstressed h
IL6
tg mice compared with control mice and increased equally in both mice during stress (data not shown). However, as in our h
IL6
tg mice, the plasma leptin concentration was reduced in
GFAP-IL6 transgenic mice [
45]. Since circulating IL6 was not elevated in those
GFAP-IL6 mice, those results support the hypothesis that hIL6 promotes leptin action in the central nervous system.
Previous reports have shown that transgenic
IL6 causes various pathologies of the immune system that can be fatal to mice [
46,
47]. Human
IL6 in C57BL/6J mice under the control of human immunoglobulin heavy-chain enhancer develop mesangial proliferative glomerulonephritis with massive IgG1 plasmacytosis [
48].
MTI/IL6 transgenic mice expressing murine IL6 constitutively in the liver developed progressive kidney damage and died between 12 and 20 weeks of age [
49]. Hepatic inflammation occurs in transgenic mouse secreting mIL6 from muscle [
14]. Despite the above, our h
IL6
tg mice with circulating hIL6 secreted from brain and lung never displayed hepatic inflammation, acute inflammatory response or systemic inflammation. Perhaps the sites of IL6 secretion are critical for its systemic effect. In any case, h
IL6
tg mice provide a unique system to investigate the role of hIL6 in central and peripheral nutrient homeostasis.
In summary, hIL6 protects mice from insulin resistance and obesity. Since this effect was not observed in ob/ob mice, hIL6 apparently augments central leptin action without substituting for leptin. Due to its immunoreactive nature, IL6 might never be a successful therapeutic treatment strategy; however, prolonged treatment with IL6 homologues with high accessibility to the central nervous system might show therapeutic promise in anti-obesity therapy.