Metal-Ion Transporter SLC39A14 Is Required for Cardiac Iron Loading in the Hjv Mouse Model of Iron Overload

Although iron overload-related cardiomyopathy is a leading cause of morbidity and mortality in iron-overload disorders (e.g., thalassemia major and hemochromatosis), the molecular mechanisms that mediate cardiac iron uptake and accumulation are incompletely understood. Previous studies using Slc39a14 knockout mice have revealed that SLC39A14 is required for the uptake of non-transferrin-bound iron (NTBI) by the liver and pancreas and is essential for iron loading of hepatocytes and pancreatic acinar cells. To investigate the requirement for SLC39A14 in cardiac iron accumulation, we generated cardiomyocyte-specific Slc39a14 knockout (Slc39a14 ^hrt/hrt) mice and crossed them with iron-loading hemojuvelin (Hjv) knockout mice to generate Hjv ^-/-;Slc39a14 ^hrt/hrt animals. At 12 and 24 weeks of age, cardiac nonheme iron levels were ~340% higher in Hjv ^-/- mice than in controls. By contrast, cardiac nonheme iron levels in Hjv ^-/-;Slc39a14 ^hrt/hrt mice at these ages were only ~60% higher than those than in controls, and ~65% less than those in Hjv ^-/- mice. Moreover, cardiac nonheme iron levels in Hjv ^-/-;Slc39a14 ^+/hrt (heterozygous conditional Slc39a14 knockout) mice were between those of Hjv ^-/- and Hjv ^-/-;Slc39a14 ^hrt/hrt mice, suggesting a gene-dosage effect of Slc39a14 on cardiac iron accumulation. A role for voltage-dependent calcium channels in mediating the uptake of NTBI into cardiomyocytes has been proposed based on observations of the effects of L-type calcium-channel blockers on iron uptake and accumulation in vitro and in vivo. We considered the possibility that these observations could be explained if SLC39A14 were reactive with calcium-channel blockers. To test this hypothesis, we examined the effects of blockers on the activity of SLC39A14 by using radiotracer assays in RNA-injected Xenopus oocytes expressing mouse SLC39A14. We found that 100 µM amlodipine (Amld), nifedipine, and nicardipine each afforded modest inhibition of SLC39A14-mediated ⁵⁵Fe ²⁺. Inhibition of iron transport by Amld was dose-dependent, EC ₅₀ = 167 µM ± (SEM) 30 µM. Our findings implicate SLC39A14 in mediating cardiomyocyte NTBI uptake in the mouse and raise doubts about the relative importance of calcium channels as a mechanism by which NTBI gains entry to the heart.