IL-6 Shapes Circadian and Metabolic Rhythms Differently in Males and Females
IL-6 deficiency disrupts biological clocks and metabolism in sex- and diet-specific ways, revealing a novel circadian-immune-metabolic axis.
Summary
Researchers studying IL-6 knockout mice found that loss of this cytokine disrupts circadian locomotor activity, energy expenditure, and metabolic rhythms in ways that differ strikingly between sexes and dietary conditions. Males show weakened light-driven rhythms and metabolic misalignment linked to reduced VIP neuropeptide in the SCN. Females maintain circadian stability under normal light cycles but become more vulnerable under intrinsic challenges or high-fat diet. IL-6 also emerged as a novel regulator of the food-entrainable oscillator, influencing food anticipatory behavior in a sex-specific way. These findings position IL-6 as a critical bridge between immune signaling, circadian biology, and metabolic homeostasis.
Detailed Summary
Circadian disruption and metabolic disease are deeply intertwined, yet the molecular messengers linking immune signaling to biological timekeeping remain poorly defined. IL-6, a pleiotropic cytokine known for roles in inflammation and energy metabolism, displays diurnal oscillations and can modulate core clock genes like the Period family. This study asked whether IL-6 serves as a sex- and diet-sensitive integrator of circadian and metabolic rhythms.
Using IL-6 knockout (IL6KO) mice of both sexes aged 6–7 months, researchers performed comprehensive circadian phenotyping across light–dark, constant darkness, and re-entrainment protocols, alongside metabolic profiling via indirect calorimetry. Additional experiments examined fasting responses, time-restricted feeding (TRF), and high-fat diet (HFD) effects. Molecular analyses in muscle, liver, and hypothalamus assessed clock protein expression and IL-6 signaling components. Sex steroid hormones were also quantified to probe endocrine contributions.
Under standard diet and normal light conditions, IL6KO males exhibited weakened circadian rhythm strength associated with significantly reduced vasoactive intestinal peptide (VIP) in the suprachiasmatic nucleus (SCN), the brain's master clock. This correlated with energy expenditure misalignment—a lengthened period and advanced acrophase—suggesting temporal decoupling of behavioral and metabolic rhythms. Females, by contrast, maintained circadian-metabolic alignment under standard conditions, partly through coordinated reductions in both energy expenditure and food intake, and showed elevated circulating progesterone that may provide a protective hormonal buffer.
Under high-fat diet, the vulnerabilities reversed: IL6KO males showed pronounced metabolic dysfunction while IL6KO females now exhibited weakened circadian rhythms, indicating diet-induced circadian susceptibility. Fasting experiments revealed rhythmic IL-6 induction in muscle and liver, suggesting that IL-6 couples peripheral metabolic state to clock gene programs. TRF experiments uncovered sex-specific adaptations in food anticipatory activity (FAA) and substrate utilization, establishing IL-6 as a novel modulator of the food-entrainable oscillator (FEO)—a circadian system that operates independently of the SCN.
Taken together, these findings establish IL-6 as a critical mediator of circadian-metabolic plasticity, with distinct sex-dependent trade-offs between circadian stability and metabolic homeostasis. The study highlights timed modulation of IL-6 signaling as a potential therapeutic strategy for metabolic disorders rooted in circadian misalignment, and underscores the importance of including both sexes in circadian and metabolic research.
Key Findings
- IL6KO males show reduced SCN VIP levels and misaligned energy expenditure rhythms under standard light-dark conditions.
- IL6KO females maintain circadian stability under normal conditions but become vulnerable to circadian disruption on high-fat diet.
- Fasting induces rhythmic IL-6 expression in muscle and liver, coupling peripheral metabolic state to clock gene programs.
- IL-6 modulates the food-entrainable oscillator, with sex-specific effects on food anticipatory activity under both standard and high-fat diet.
- IL6KO females show elevated circulating progesterone under standard diet, potentially buffering circadian-metabolic alignment.
Methodology
IL-6 knockout and wild-type mice of both sexes (6–7 months) were phenotyped for circadian locomotor activity across light-dark, constant darkness, and re-entrainment protocols, with metabolic variables assessed by indirect calorimetry. Molecular analyses of clock proteins and IL-6 signaling were conducted in muscle, liver, and hypothalamus under ad libitum, fasting, TRF, and HFD conditions; sex steroid hormones were quantified by plasma immunoassay.
Study Limitations
All experiments were conducted in mice, and translation to human circadian-metabolic biology requires validation. Mechanistic fasting studies were performed only in males, limiting conclusions about sex differences in tissue-level IL-6 clock interactions. The study does not establish direct causality between VIP reduction and metabolic misalignment in IL6KO males.
Enjoyed this summary?
Get the latest longevity research delivered to your inbox every week.