Stabilization of h-ferritin mrna in human THP-1 cells following differentiation toward macrophages

H-ferritin gene expression was highly induced in human monocytic THP-1 cells following TPA-induced differentiation toward macrophages. The induction was detected at 3 hr, reached maximal level at 12 hr and sustained up to 48 hr after exposure to TPA. The TPA-induced up-regulation of H-ferritin gene expression was also observed in other leukemic cell lines, HL60 and U937, but not in non-leukemic cells including human fibroblasts, endothelial cells and smooth muscle cells. The effect of TPA was completely blocked by protein kinase C inhibitor, H-7. Fulhermore, bacterial phospholipase C also resulted in marked increase of H-ferritin gene expression in THP-1 cells, suggesting the activation of protein kinase C was responsible for the accumulation of H-ferritin mRNA. Nuclear run-off experiment demonstrated that TPA did not increase the transcriptional rate of the H-ferritin gene. In contrast, the half life of the H-ferritin mRNA measured in the presence of actinomycin D was greatly increased in TPAtreated cells. The induction of H-ferritin gene expression by TPA required no protein synthesis. Conversely, prolonged treatment of THP-1 cells with cycloheximide, resulted in increased stability of H-ferritin mRNA and 4-5 fold increase in H-ferritin mRNA level. Taken together, these results suggest that the increased stability of H-ferritin mRNA in THP-1 is regulated by a PKC-mediated event through inactivating a RNA degrading protein factor.