Interleukin-6 As A Mediator Of Circadian And Sex Differences In Exercise-induced Stat3 And Autophagy Signaling

Circadian rhythms are highly conserved across tissues and play an important role in regulating how the heart responds to physiological and pathological stress. Exercise is known to be cardioprotective, yet the impact of exercise timing on molecular signaling is still being explored. Interleukin-6 (IL-6) is an exercise-induced cytokine which has been shown to be important to cardioprotection and metabolism. It is currently unknown how exercise timing modulates the IL-6 signaling pathway in the heart. PURPOSE: To investigate how time-of-day and IL-6 influence exercise-induced activation of STAT3 and its downstream signaling pathways in male and female mouse hearts. METHODS: Wild-type and IL-6 knockout male and female mice (~10 weeks of age) were housed under strict 12:12 L:D conditions and habituated to treadmill running at low speeds during the dark phase (~ZT 12-14). Mice were exposed to a 1 hour bout of treadmill exercise (15 m/min, 0% grade) at either ZT0-1 or ZT12-13. Hearts were collected immediately after exercise or from time-matched sedentary controls, snap-frozen in liquid nitrogen, and western blotting was performed to examine STAT3 activation and downstream markers of anabolic signaling (4EBP1) and autophagy (LC3, p62). RESULTS: In WT mice, pSTAT3 in males showed a time and exercise interaction, with the strongest increase at ZT0 and a smaller effect at ZT12, while females had elevated pSTAT3 with exercise at ZT0 but not ZT12. In KO mice, exercise increased pSTAT3 at both ZT0 and ZT12 in females, and selectively at ZT0 in males, with ZT0 exercise higher than ZT12. For p4EBP1, WT males showed a time and exercise interaction, with ZT0 exercise higher than ZT12, whereas females showed no significant changes. LC3 increased with exercise in WT mice, particularly in males, whereas changes in KO mice were less consistent. p62 was largely unchanged except for higher levels in male KO mice at ZT0. Overall, exercise effects were strongest for pSTAT3. CONCLUSION: Exercise-induced STAT3 activation in the heart is partially sex- and time-dependent, whereas downstream anabolic and autophagy signaling is primarily observed in WT hearts. These results highlight exercise timing as a critical determinant of cardiac molecular signaling and suggest that IL-6 is necessary for modulation of downstream pathways.