Abstract Background The clinical symptoms of rheumatoid arthritis (RA), such as morning stiffness and joint pain, exhibit marked diurnal variation, indicating that its pathophysiological processes may be regulated by circadian rhythms. However, the circadian characteristics of relevant inflammatory markers, bone‑destructive factors, and core clock genes, as well as their interrelationships, remain to be fully elucidated. Methods A collagen‐induced arthritis (CIA) model was established in rats maintained under a 12‐h light/dark cycle. Serum and joint tissues were collected at six zeitgeber time (ZT) points (ZT2, 6, 10, 14, 18, 22). Serum cytokine levels were measured by enzyme‐linked immunosorbent assay. The protein expression of macrophage migration inhibitory factor (MIF), Dickkopf (DKK‐1), receptor activator of nuclear factor kappa‐B ligand (RANKL), and osteoprotegerin (OPG) in joint tissues was analyzed by Western blotting, and the mRNA expression of core clock genes ( Clock , Bmal1 , Per2 , Cry1 ) was quantified by reverse transcription‐quantitative polymerase chain reaction. Data were analyzed using GraphPad Prism and IBM SPSS Statistics. Circadian rhythmicity was assessed using the R package “CircaCompare”. Results In the CIA rats, serum levels of pro‐inflammatory cytokines (tumor necrosis factor‐α, interleukin‐6, interleukin‐17, interleukin‐1β, MIF) and bone‐destructive factors (DKK‐1, RANKL) were elevated during the dark phase, while OPG was decreased. In inflamed joint tissues, protein levels of MIF, DKK‐1, and RANKL displayed significant circadian oscillations peaking during the dark phase, in antiphase to OPG. Furthermore, analysis of core clock genes revealed that the positive regulatory elements, Clock and Bmal1 , lost their statistical rhythmicity, while the negative elements were impaired: Per2 was arrhythmic in both control and CIA groups, and Cry1 exhibited a significantly attenuated oscillation amplitude. Conclusion These findings indicate a temporal coupling among nocturnal systemic inflammation, local circadian clock disruption, and dysregulated bone metabolism in CIA rats. This coupling provides molecular insights into the diurnal symptom fluctuations characteristic of RA and suggests that chronotherapy targeting MIF or DKK‐1 warrants further investigation.
Zhang et al. (Mon,) studied this question.