Development of mice expressing an inducible hepcidin transgene. (A) Schematic outline of the Tet ON regulatory system. The rTA protein acts as a strong transcriptional activator in the presence of doxycycline by binding to its cognate operator sequence (tetO). (B) Hepcidin mRNA levels in the livers of control mice and inducible mice (ie, double transgenic mice harboring both tetO-Hepc1 and rTA^LAP-1 transgenes), 3, 6, 9, or 18 hours after a single intraperitoneal injection of doxycycline. Total liver RNA (20 μg) was fractionated by electrophoresis, blotted, and hybridized with hepcidin and 18S-labeled probes. A typical experiment is shown. (C) Serum iron levels (μM) in animals injected with doxycycline 3, 6, 9, or 18 hours after treatment. Results are expressed relative to control serum iron (arbitrarily 100 ± standard deviation). Control animals, □; inducible mice, ▪. Statistical analysis was performed using Student t test (unpaired, 2 tailed): ^*P < .05, at least 3 animals. Thepc indicates transgenic hepcidin mRNA; endo hepc, endogenous hepcidin mRNA.