Serum iron and hepatic transferrin receptor expression are circadian-regulated

Serum iron has long been thought to exhibit diurnal variation and is subsequently considered an unreliable biomarker of systemic iron status. Circadian regulation (endogenous [~]24-hour periodic oscillation of a biologic function) governs many critical physiologic processes. It is unknown whether serum iron levels are regulated by circadian machinery; likewise, the circadian nature of key players of iron homeostasis is unstudied. Here we show that serum iron, hepatic transferrin receptor (TFR1) gene (Tfrc) expression and erythropoietic activity exhibit circadian rhythms. Daily oscillations of serum iron, hepatic Tfrc expression and erythropoietic activity are maintained in mice housed in constant darkness, where oscillation reflects an endogenous circadian period. Oscillations of serum iron, hepatic Tfrc and erythropoietic activity were ablated when circadian machinery was disrupted in Bmal1 knockout mice. Interestingly, we find that circadian oscillations of erythropoietic activity and hepatic Tfrc expression are maintained in opposing phase, likely allowing for optimised usage and storage of serum iron whilst maintaining adequate serum levels. This study provides the first confirmatory evidence that serum iron is circadian regulated and uncovers liver-specific circadian regulation of TFR1, a major player in cellular iron uptake.