The role of hypoxia and the circadian rhythm in sleep apnoea

Abstract

Background: Obstructive sleep apnoea (OSA) causes blockage of the airways. The resulting hypoxia stabilizes hypoxia inducible factor-1α (HIF-1α) which induces hypoxic response genes. A disrupted circadian rhythm has been implicated in OSA, although the cause of this disruption and its impact remains unclear. Therefore, we investigated the relation between hypoxia, the circadian rhythm and SA.

Methods: The circadian rhythm was synchronized by 'serum shock' and hypoxia was induced in primary normal human bronchial epithelial cells by culture in a hypoxia chamber (O₂% 1.0-3.0) or by treatment with 50μM IOX2; an inhibitor of HIF-1α degradation. The effects of HIF-1α on the circadian rhythm was determined by measuring the expression of the core 'clock' genes CLOCK, BMAL1, CRY1 and PER2, by RT-qPCR. The role of HIF-1α was also investigated by HIF-1α knockdown using siRNA and by chromatin immunoprecipitation (ChIP) to confirm HIF-1α binding to hypoxia response elements in native promoters. Statistical analysis was performed by using 1-way ANOVA.

Results: Hypoxia and treatment with IOX2 both significantly elevated levels of PER2 (235%; p=0.002) compared to untreated cells. A non-significant elevation of CLOCK (176%; p=0.281), BMAL1 (295%; p=0.598) and CRY1 (215%; p=0.176) was also seen (all n=3). The increase in 'clock' gene expression was prevented by HIF-1α knockdown. ChIP analysis confirmed a direct binding of HIF-1α to the native PER2 promoter following IOX2 treatment.

Conclusion: Hypoxia in OSA causes an increase of HIF-1α which in turn disrupts the circadian rhythm in vitro. This pathway might be a potential target in tackling sleep apnoea.