Continuous suppression of globin gene expression and differentiation of Friend erythroleukemia cells by phorbol 12-myristate 13-acetate (PMA) despite the loss of PMA binding sites by down regulation

Effect of electromagnetic field exposure on chemically induced differentiation of friend erythroleukemia cells

Whether exposure of humans to extremely low frequency electromagnetic fields (ELF-EMF) can cause cancer is controversial and therefore needs further research. We used a Friend erythroleukemia cell line that can be chemically induced to differentiate to determine whether ELF-EMF could alter proliferation and differentiation in these cells in a manner similar to that of a chemical tumor promoter. Exposure of this cell line to 60 Hz ELF-EMF resulted in a dose dependent inhibition of differentiation, with maximal inhibition peaking at 40% and 40 mG (4 microT). ELF-EMF at 10 mG (1.0 microT) and 25 mG (2.5 microT) inhibited differentiation at 0 and 20%, respectively. ELF-EMF at 1.0 (100) and 10.0 G (1,000 microT) stimulated cell proliferation 50% above the sham-treated cells. The activity of telomerase, a marker of undifferentiated cells, decreased 100[times] when the cells were induced to differentiate under sham conditions, but when the cells were exposed to 0.5 G (50 microT) there was only a 10[times] decrease. In summary, ELF-EMF can partially block the differentiation of Friend erythroleukemia cells, and this results in a larger population of cells remaining in the undifferentiated, proliferative state, which is similar to the published results of Friend erythroleukemia cells treated with chemical-tumor promoters.

Role of activator protein-1 and methylation function in 12-O-tetradecanoylphorbol-13-acetate--mediated inhibition of differentiation of Friend erythroleukemia cells

Friend erythroleukemia cells (FELCs) differentiate after hexamethylene-bis-acetamide treatment. This differentiation is characterized by an increase in beta-globin gene expression that is followed by appearance of the hemoglobin. Phorbol-ester tumor promoters, such as 12-O-tetradecanoylphorbol-13-acetate (TPA), inhibit differentiation of TPA-sensitive cells but not TPA-resistant cells. We have shown that the increase in beta-globin expression is inhibited by TPA in a TPA-sensitive clone but not in a TPA-resistant clone. To study the molecular mechanisms of regulation of gene expression by TPA, we examined the possible involvement of gene methylation and the TPA-responsive element (TRE). Both clones showed similar patterns of methylation around the beta-globin gene. Moreover, TPA-induced TRE binding and TRE enhancer activity were similar in both variants. These results suggest that the TPA inhibition of induced differentiation may not be explained by regulation of the methylation state. The activator protein-1 also does not play a crucial role in the sensitivity of FELCs to TPA.

A phorbol diester resistant monocytic leukemia cell line is PKC deficient

MIA C51 is a rat monocytic leukemia cell line which exhibits undifferentiated monocytic phenotype in culture. The proliferation of MIA C51 cells was not inhibited by the addition of 12-O-tetradecanoyl-phorbol-13-acetate (TPA, 1 microgram/ml) or phorbol 12, 13 dibutyrate (PDBu, 10 micrograms/ml). Comparison of MIA C51 cells to a phorbol diester-sensitive human monoblastoid U-937 cell line demonstrated that MIA C51 cells contained significantly lower number of PDBu receptors, protein kinase C (PKC) activity, and PKC protein level. Further experiments demonstrated that addition of TPA to MIA C51 cells did not induce the expression of c-fos proto-oncogene; whereas incubation of MIA C51 cells with N6, O2-dibutyryl cyclic adenosine 3',5' monophosphate (Bt2cAMP) resulted in a rapid increase of c-fos mRNA level. Thus, this cell line provides a new system for studying the signal transduction mechanisms in induced monocytic differentiation.

PolyADP ribosylation and Friend erythroleukemic-cell differentiation: action of poly(ADP-ribose) polymerase inhibitors

Previous studies have shown that benzamide and nicotinamide, two inhibitors of poly(ADP-ribose) polymerase, induce erythroid differentiation in Friend erythroleukemic cells. In contrast, we observed that two other commonly used inhibitors, i.e., 3-aminobenzamide and 3-methoxybenzamide, not merely failed to induce differentiation but actually inhibited it. Furthermore, we observed that benzamide at high concentrations induced differentiation, whereas at low concentrations, it inhibited differentiation. We propose that the induction occurring at high concentrations is due to the polar-planar properties of the molecule, while the inhibition at low concentrations might be due to the inhibition of poly(ADP-ribose) polymerase. Cells that were first exposed to an inducer (hexamethylene bisacetamide) and an inhibitor (3-aminobenzamide) and were subsequently incubated in medium lacking these substances did not differentiate. However, re-exposure to the inducer resulted in a very rapid increase in commitment to terminal differentiation. Therefore, 3-aminobenzamide appears to block differentiation prior to the commitment stage. We infer from these findings that polyADP-ribosylation is required for the terminal differentiation of Friend erythroleukemia cells.

Protein kinase C phosphorylates topoisomerase II: topoisomerase activation and its possible role in phorbol ester-induced differentiation of HL-60 cells

DNA topoisomerase II from Drosophila was phosphorylated effectively by protein kinase C. With a Km of about 100 nM, the reaction was rapid, occurring at 4 degrees C as well as at 30 degrees C and requiring as little as 0.6 ng of the protein kinase per 170 ng of topoisomerase. About 0.85 mol of phosphate could be incorporated per mol of topoisomerase II, with phosphoserine as the only phospho amino acid produced. The reaction was dependent on Ca2+ and phosphatidylserine and was stimulated by phorbol esters. Calmodulin-dependent protein kinase II, but not cyclic AMP-dependent protein kinase, was also able to phosphorylate the topoisomerase. Phosphorylation of topoisomerase II by protein kinase C resulted in appreciable activation of the topoisomerase, suggesting that it may represent a possible target for the regulation of nuclear events by protein kinase C. This possibility is supported by the finding that the phorbol ester-induced differentiation of HL-60 cells was blocked by the topoisomerase II inhibitors novobiocin and 4'-(9-acridinylamino)methanesulfon-m-anisidide(m-AMSA), but not by the inactive analog o-AMSA.