Deletion of ferroportin in murine myeloid cells increases iron accumulation and stimulates osteoclastogenesis in vitro and in vivo

Abstract

Osteoporosis, osteopenia, and pathological bone fractures are frequent complications of iron-overload conditions such as hereditary hemochromatosis, thalassemia, and sickle cell disease. Moreover, animal models of iron overload have revealed increased bone resorption and decreased bone formation. Although systemic iron overload affects multiple organs and tissues, leading to significant changes on bone modeling and remodeling, the cell autonomous effects of excessive iron on bone cells remain unknown. Here, to elucidate the role of cellular iron homeostasis in osteoclasts, we generated two mouse strains in which solute carrier family 40 member 1 (), a gene encoding ferroportin (FPN), the sole iron exporter in mammalian cells, was specifically deleted in myeloid osteoclast precursors or mature cells. The FPN deletion mildly increased iron levels in both precursor and mature osteoclasts, and its loss in precursors, but not in mature cells, increased osteoclastogenesis and decreased bone mass Of note, these phenotypes were more pronounced in female than in male mice. studies revealed that the elevated intracellular iron promoted macrophage proliferation and amplified expression of nuclear factor of activated T cells 1 (Nfatc1) and PPARG coactivator 1β (Pgc-1β), two transcription factors critical for osteoclast differentiation. However, the iron excess did not affect osteoclast survival. While increased iron stimulated global mitochondrial metabolism in osteoclast precursors, it had little influence on mitochondrial mass and reactive oxygen species production. These results indicate that FPN-regulated intracellular iron levels are critical for mitochondrial metabolism, osteoclastogenesis, and skeletal homeostasis in mice.

Regulation of tissue iron homeostasis: the macrophage “ferrostat”

“…In addition, we believe that tightly regulated iron donation by Mɸs is a critical piece of this repair-to-resolution process. Indeed, loss of Mɸ iron donation delays or impairs wound healing in mice (64,65,83). Thus, we propose that Mɸs continually supply iron to cells undergoing repair until cell regeneration/ repair is completed or near completion.…”

“…Resident Mɸs are indispensable for bone remodeling, as evidenced by the well-studied population of osteoclasts that drive bone resorption. Iron is implicated in the regulation of osteoclast function, such that iron release via Fpn is necessary for normal osteoclastogenesis and global skeletal homeostasis in mice (83). For instance, mice that lack Fpn activity in osteoclasts have accelerated osteoclastogenesis and skeletal resorption.…”

Estrogens decrease osteoclast number by attenuating mitochondria oxidative phosphorylation and ATP production in early osteoclast precursors

“…The amount of non-mitochondrial oxygen consumption was determined by inhibiting the respiratory chain activity with an antimycin A and rotenone cocktail (10 µM). These data were used to calculate the mitochondrial basal respiration, ATP-linked respiration, reserve respiratory capacity, and proton leak as we previously described 34,35 .…”

A trans-eQTL network regulates osteoclast multinucleation and bone mass

“…Generation of mice with conditional deletion of Slc40a1 in the macrophage-osteoclast lineage using LysMCre mice was consistent with in vitro data and resulted in a phenotype of increased bone mass and strength (Figure 5 and figure supplement 4). Thus, Slc40a1 regulates bone structure and strength via direct actions in the macrophage-osteoclast lineage, further supporting the conclusion that MMnet plays a key role in the physiological regulation of bone mass by osteoclasts.Nevertheless, Wang and colleagues(Wang et al, 2018) recently reported that deletion of Slc40a1 in the myeloid lineage using LysMCre mice causes decreased bone mass and mineralisation in female, but not male, mice. Wang et al also reported that deletion of Slc40a1 in terminally differentiated osteoclasts using CtskCre mice resulted in no skeletal abnormalities in either sex.…”