The influence of low intensity electromagnetic fields on circadian clocks of cells and tissues has gained increasing scientific interest, either as a therapeutic tool or as a potential environmental hazard. Nuclear Magnetic Resonance (NMR) refers to the property of certain atomic nuclei to absorb the energy of radio waves under a corresponding magnetic field. NMR forms the basis for Magnetic Resonance Imaging, Magnetic Resonance Spectroscopy and, in a low-intensity form, for NMR therapy (tNMR). Since the circadian clock is bi-directionally intertwined with hypoxic signaling in vertebrates and mammals, we hypothesized that low intensity electromagnetic fields, such as tNMR, might not only affect circadian clocks but also Hypoxia-Inducible Factor-1α (HIF-1α). As master regulator of the hypoxic signaling pathway, HIF-1α is known to dampen the circadian amplitude under reduced oxygen availability, while the hypoxic response of cells and organisms, itself, is tightly clock controlled. In a first experiment, we investigated if tNMR is able to act as Zeitgeber for the core clock mechanism of unsynchronized zebrafish and mouse fibroblast cells, using direct light irradiation and treatment with the glucocorticoid Dexamethasone as references. tNMR significantly affected the cell autonomous clocks of unsynchronized mouse fibroblast cells NIH3-T3, but did not act as a Zeitgeber. Similar to light irradiation and in contrast to treatment with Dexamethasone, tNMR did not synchronize expression profiles of murine clock genes. However, irradiation with tNMR as well as light significantly altered mRNA and protein expression levels of Cryptochrome1, Cryptochrome2 and Clock1 for more than 24 h. Changes in mRNA and protein after different treatment durations, namely 6 and 12 h, appeared to be nonlinear. A nonlinear doseresponse relationship is known as hallmark of electromagnetic field induced effects on biological systems. The most prominent alterations were detected in murine HIF-1α protein, again in a nonlinear dose-response. In contrast to murine cells, zebrafish fibroblasts did not respond to tNMR at all. Light, a potent Zeitgeber for the peripheral clocks of fish, led to the expected synchronized clock gene oscillations of high amplitude, as did Dexamethasone. Hence, we conclude, mammalian peripheral clocks are more susceptible to tNMR than the direct light entrainable fish fibroblasts. Although light and tNMR did not act as Zeitgebers for the circadian clocks of unsynchronized murine cells, the significant observed effects might indicate downstream cellphysiological ramifications, which are worth future investigation. However, beside the effects tNMR exerts on the core clock mechanism of mammalian cells, the technology might be the first non-pharmacological approach to modify HIF-1α protein in cells and tissues. HIF-1α and the associated circadian clock play key roles in diseases with underlying ischemic background, such as infarct, stroke, and cancer and, also infectious diseases, such as Covid-19. Hence, low intensity magnet...

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