Effect of modulation of iron availability on IRP activity in monocytes of controls and GH patients maturing to macrophages in vitro. Monocytes from 5 controls, 5 untreated, and 4 treated unrelated GH patients were grown for different days in unsupplemented medium (♦) or in the presence of either 50 μg/mL ferric ammonium citrate (▪) or 50 mmol/L desferrioxamine (•). Iron loading was achieved by continuous culture in the presence of the iron salt and iron deprivation by adding desferrioxamine 16 hours before cell harvest. Lysates were prepared from cells freshly isolated (day 0) or cultured for 3 and 6 days, and IRP activity was measured by RNA-bandshift assay as described in the legend to Fig 1. Values were calculated as described in Materials and Methods and are expressed as means with SD. IRP activity was repressed by iron loading and upregulated by iron chelation at all stages of differentiation in RE cells of both controls and GH patients.