Induction of protein kinase C mRNA in cultured lymphoblastoid T cells by iron-transferrin but not by soluble iron

Iron-transferrin (FeTF) is an essential growth factor required for proliferation of lymphoid cells. FeTF activates protein kinase C (PKC) in the lymphoblastoid T-cell line, CCRF-CEM. We have treated CEM cells with human FeTF, then examined levels of PKC mRNA by hybridization analysis using cDNA probes specific for α-, β-, and γ-PKC subspecies. CEM cell mRNA hybridized with the β-subspecies probe but not with probes for α- or γ-subspecies. After exposure to FeTF an increase in PKC-β mRNA was detectable at 10 minutes, peaked at 12 hours, and was sustained for 72 hours. Nuclear transcription assays demonstrated that rates of PKC-β mRNA transcription were increased in FeTF-treated cells. By contrast, steady state levels of PKC-β mRNA did not increase after treatment of cells with apotransferrin or gallium TF. Similarly, treatment with soluble iron as ferric ammonium citrate did not increase steady state levels of PKC-β mRNA, despite producing a marked increase in cellular ferritin content. Ferritin increased from a baseline value of 63 ng/10⁶ cells to 98 and 100 ng/10⁶ cells in CEM cells treated for 1 hour with ferric ammonium citrate or FeTF, respectively. FeTF did not increase cytoplasmic-free calcium in CEM cells loaded with fura-2, indicating that binding of FeTF to transferrin receptors did not open membrane Ca²⁺ channels or release intracellular Ca²⁺. In addition, pretreatment of cells with desferrioxamine, but not ferrioxamine, blocked the FeTF-induced increase in PKC-β transcripts. Therefore, iron as FeTF (not soluble iron or nonferric TF) stimulates transcription of the CEM cell PKC-β gene. Transcriptional rate of the PKC-β gene does not correlate with cellular iron content as judged by ferritin measurements. Furthermore, the requirement for FeTF does not appear to reflect activation of a classic agonist pathway as judged by stable cellular Ca²⁺. These data suggest that delivery of iron by FeTF to one or more specific cellular compartments may stimulate PKC-β gene transcription in CEM cells. This is a US government work. There are no restrictions on its use.