Simulated microgravity decreases circulating iron in rats: Role of inflammation-induced hepcidin upregulation

During spaceflight, humans exposed to microgravity exhibit an increase of iron storage and a reduction of circulating iron. Such perturbations could promote oxidative stress and anaemia in astronauts. The mechanism by which microgravity modulates iron metabolism is still unknown. Herein, we hypothesized that microgravity upregulates hepcidin, a hormone produced by the liver that is the main controller of iron homeostasis. To test this hypothesis, rats were submitted to hindlimb unloading (HU), the reference model to mimic the effects of microgravity in rodents. After 7 days, the mRNA level of hepcidin was increased in the liver of HU rats (+74%, P = 0.001). In agreement with the biological role of hepcidin, we found an increase of spleen iron content (+78%, P = 0.030) and a decrease of serum iron concentration (−35%, P = 0.002) and transferrin saturation (−25%, P = 0.011) in HU rats. These findings support a role of hepcidin in microgravity‐induced iron metabolism alteration. Furthermore, among the signalling pathways inducing hepcidin mRNA expression, we found that only the interleukin‐6/signal transducer and activator of transcription 3 (IL‐6/STAT3) axis was activated by HU, as shown by the increase of phospho‐STAT3 (+193%, P < 0.001) and of the hepatic mRNA level of haptoglobin (+167%, P < 0.001), a STAT3‐inducible gene, in HU rats. Taken together, these data support the idea that microgravity may alter iron metabolism through an inflammatory process upregulating hepcidin.