The role of labile iron in macrophage proinflammatory activation.

Iron is found within the cell as Fe3+, ferritin-bound iron, and Fe2+, intracellular labile iron, which can be readily used for a variety of cellular functions, especially in the mitochondria. Macrophages, a major site for iron storage and recycling, undergo significant metabolic changes upon activation. Here, we demonstrate that sequestration of the labile iron pool to the mitochondria is characteristic of the macrophage proinflammatory response. Using LPS to stimulate proinflammatory activation in bone marrow derived macrophage (BMDMs) and embryonic Kupffer cells (emKCs), we measured labile iron content and mitochondrial iron with specific probes detected by flow cytometry. The expression of key iron metabolism genes and proinflammatory cytokines were measured to assess the iron metabolism and proinflammatory responses of macrophages. Treatment with LPS in vitro and in vivo demonstrated increased the mitochondrial Fe2+ content, while the intracellular labile iron pool was decreased. The emKC population was decreased after LPS stimulation, but this effect was attenuated by depletion of ferritin. Other hepatic macrophage populations, Kupffer cell-like (KC-like) cells and recruited hepatic macrophages (RHMs), increased in response to LPS. Nrf2, a master regulator of redox homeostasis, which was repressed in emKCs but upregulated in KC-like cells and RHMs, may be key to understanding the differences in responses to LPS exhibited by these macrophage populations. Our results expand upon the interplay between iron homeostasis and the metabolic changes required for macrophage activation.