The circadian clock and the hypoxic signaling pathway play critical roles in physiological homeostasis as well as in tumorgenesis. Interactions between both pathways have repeatedly been reported for mammals during the last decade, the molecular basis, though, has not been identified so far. Expression levels of oxygen-regulated and circadian clock genes in zebrafish larvae (Danio rerio) and zebrafish cell lines were significantly altered under hypoxic conditions. Thus, long-term hypoxic incubation of larvae resulted in a dampening of the diurnal oscillation amplitude of the period1 gene expression starting only several hours after start of the hypoxic incubation. A significant decrease in the amplitude of the period1 circadian oscillation in response to hypoxia and in response to the hypoxic mimic CoCl2 was also observed using a zebrafish luciferase reporter cell line in constant darkness. In addition, activity measurements of zebrafish larvae using an infrared-sensitive camera demonstrated the loss of their usual circadian activity pattern under hypoxic conditions. To explore the functional basis of the observed cross-talk between both signaling pathways ChIP assays were performed. Increasing with the duration of hypoxia, a nearly 4-fold occupancy of hypoxia-inducible factor 1 (Hif-1α) at two specific E-box binding sites located in the period1 gene control region was shown, demonstrating therewith the transcriptional co-regulation of the core clock gene by the major transcription factor of the hypoxic pathway. On the other hand, circadian transgenic zebrafish cells, simulating a repressed or an overstimulated circadian clock, modified gene transcription levels of oxygen-regulated genes such as erythropoietin and vascular endothelial growth factor 165 and altered the hypoxia-induced increase in Hif-1α protein concentration. In addition, the amount of Hif-1α protein accumulated during the hypoxic response was shown to depend on the time of the day, with one maximum during the light phase and a second one during the dark phase. The direct binding of Hif-1α to the period1 gene control region provides a mechanistic explanation for the repeatedly observed interaction between hypoxia and the circadian clock. The cross-talk between both major signaling pathways was shown for the first time to be bidirectional and may provide the advantage of orchestrating a broad range of genes and metabolic pathways to cope with altered oxygen availabilities.
