Transcriptional regulation of the transferrin receptor in differentiating HL-60 leukemic cells

Stratum-specific expression of human transferrin receptor increases iron in mouse epidermis

Epidermal desquamation accounts for 20% of the body's iron loss each day. Yet, little is known about how iron content in epidermis is regulated. To test the importance of the transferrin receptor in regulating iron content in epidermis, we created transgenic mice that have stratum-specific expression of the human transferrin receptor. The keratin 14 promoter targeted the receptor primarily to basal, proliferating keratinocytes; the involucrin (Inv) promoter targeted the receptor to suprabasal, differentiating keratinocytes. There were age- and site-dependent differences in iron content in the epidermis and hair. In both types of transgenic mice, epidermal iron content increased with age and at 8 weeks was 2-3-fold greater in transgenic mice compared to littermate controls. Iron was increased up to 2-fold in hair of keratin 14-human transferrin receptor (hTfR) transgenics and 30% in Inv-hTfR transgenics. No gross or histological changes were seen in transgenic animals with increased iron in the epidermis. Ferritin expression, which was low in normal epidermis, was greatly increased in both transgenic lines, indicating that it is the likely depot for the extra iron in these animals. These data show that control of transferrin receptor expression is sufficient to regulate iron content in proliferating or differentiating keratinocytes in the epidermis.

Apicidin, a histone deacetylase inhibitor, induces differentiation of HL-60 cells

The fungal metabolite apicidin (cyclo(N-O-methyl-L-tryptophanyl-L-isoleucinyl-D-pipecolinyl-L-2-amino-8-oxodecanoyl)) inhibited the growth of HL-60 cells in a concentration-dependent manner (100-1000 nM). At higher concentrations (>300 nM), cell death was induced. At 100 nM, it induced hyperacetylation of histone H4 time-dependently, while trichostatin A induced transient hyperacetylation. Apicidin (10-100 nM) increased the cells having nitroblue tetrazolium-reducing activity and expressing CD11b but not CD14 and CD15. The expression of CD11b by apicidin was long lasting, while that by trichostatin A was transient. In K562 cells, apicidin at 10-100 nM did not inhibit cell growth nor express CD11b, CD14 and CD15. Our findings indicate that apicidin inhibits proliferation and induces the early stage of differentiation of HL-60 cells.

Regulation of the expression of enzymes involved in the replication of DNA in chemically-induced granulocytic differentiation of HL-60 leukemia cells

The expression of seven enzymes involved in the biosynthesis of DNA was measured in HL-60 promyelocytic leukemia cells treated with dimethylsulfoxide (DMSO) or all-trans retinoic acid (RA) to gain information on their role in the termination of proliferation in cells undergoing granulocytic differentiation. The steady-state levels of the mRNAs for topoisomerase I, topoisomerase II. DNA polymerase-alpha, thymidylate synthase, thymidine kinase and hypoxanthine-guanine phosphoribosyltransferase progressively declined from day 3 to day 7 of exposure to the polar solvent or the retinoid suggesting that the expression of these enzymes is coordinately regulated. In contrast, a pronounced difference between the two inducers of differentiation occurred in the expression of the mRNA of the M2 subunit of ribonucleotide reductase, with DMSO causing virtually complete inhibition of the expression of the M2 subunit of the enzyme from day 5 through day 7, with no change in the steady-state levels of the mRNA being produced by retinoic acid. Measurement of the enzymatic activities of two of these catalysts, thymidylate synthase and thymidine kinase, in cells exposed to the two inducers of maturation corroborated the findings at the level of the mRNAs, with corresponding decreases in the activity of these enzymes. The findings collectively demonstrate that the down-regulation of the expression of a relatively wide variety of enzymes involved in DNA replication occurs as late events in the granulocytic differentiation of HL-60 cells, ensuring that cellular replication cannot occur in terminally differentiated cells.

Regulation of the expression of enzymes involved in the replication of DNA in chemically induced monocytic/macrophagic differentiation of HL-60 leukemia cells

The expression of a number of housekeeping enzymes of DNA biosynthesis was measured in HL-60 promyelocytic leukemia cells undergoing monocytic/macrophagic differentiation following treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) or 1alpha,25-dihydroxyvitamin D3 (vitamin D3). Progressive decreases in the steady-state levels of the mRNAs for thymidylate synthase, topoisomerase II, and hypoxanthine guanine phosphoribosyltransferase occurred following exposure to TPA or vitamin D3. In contrast, the steady-state levels of the mRNAs for thymidine kinase, topoisomerase I, and DNA polymerase-alpha did not decrease until days 3-5 of treatment with vitamin D3 and then progressively declined thereafter. The mRNAs for thymidine kinase and topoisomerase I decreased slightly and the mRNA for DNA polymerase-alpha by 30-40%, and then remained constant between days 1 to 3 of treatment with the phorbol ester. The M2 subunit of ribonucleotide reductase exhibited an even greater difference, with no change in the steady-state concentration of mRNA over 3 days of exposure to TPA or vitamin D3. On days 5-7 of treatment with vitamin D3, essentially complete loss of the expression of the mRNA for the M2 subunit of ribonucleotide reductase occurred. Measurement of the enzymatic activities of thymidylate synthase and thymidine kinase in cells exposed to either of the inducers of maturation corroborated the findings at the level of the mRNAs, with corresponding decreases in the activity of these enzymes. The results indicate that the down-regulation of the expression of housekeeping enzymes of DNA replication occurs as late events in HL-60 cells undergoing monocytic/macrophagic differentiation, implying that the decreases in their gene expression are the result of the termination of proliferation rather than an initiating event in the cessation of DNA biosynthesis.

Effect of iron and retinoic acid on the control of transferrin receptor and ferritin in the human promonocytic cell line U937

The effect of changes in iron availability and induction of differentiation on transferrin receptor expression and ferritin levels has been examined in the promonocytic cell line U937. Addition of iron (as 200 micrograms/ml saturated transferrin) or retinoic acid (1 microM) both caused approx. 70% reduction in the average number of surface transferrin receptors, while the iron chelator desferrioxamine caused an 84% increase. Comparable changes also occurred in the levels of transferrin receptor mRNA. Neither iron nor retinoic acid significantly altered the half-life of transferrin receptor mRNA in the presence of actinomycin D (approx. 75 min) but a 10-fold increase in stability occurred in the presence of desferrioxamine. Iron and retinoic acid both caused an increase in intracellular ferritin levels (approx. 4-and 3-fold, respectively), while desferrioxamine reduced ferritin levels by approx. two-thirds. The effect of iron and retinoic acid added together did not differ greatly from that of each agent alone. None of the treatments greatly affected levels of L-ferritin mRNA. Virtually no H-ferritin mRNA was detected in U937 cells. These results show that changes in ferritin and transferrin receptor caused by treatment with retinoic acid are similar to those induced by excess iron, and suggest that changes in these proteins during cell differentiation are due to redistribution of intracellular iron into the regulatory pool(s), rather than to iron-independent mechanisms.

Characterization of the defect in a variant of HL-60 promyelocytic leukemia cells with reduced transferrin receptor expression

The mechanism by which a clone of HL-60 human promyelocytic leukemia cells designated Tf-Gel-1 expresses reduced levels of the transferrin receptor (TfR) was investigated. Tf-Gel-1 was developed by continuous exposure of HL-60 cells to human iron-saturated transferrin covalently linked to the plant toxin gelonin (Tf-Gel); this variant was five- to sixfold more resistant to Tf-Gel than parental HL-60 cells. The amount of cell surface, as well as of solubilized, TfR and the cycling pools of TfR in Tf-Gel-1 cells, as measured by the binding of [125I]Tf, were all decreased to 20-30% of the levels present in parental cells. The growth of Tf-Gel-1 cells was independent of exogenous Fe3+ and was comparable to that of parental HL-60 cells. Despite the lower levels of TfRs, the Tf-Gel-1 clone retained the capacity to alter receptor expression, depending upon the phase of growth and the intracellular iron concentration, and to down-regulate TfRs in response to inducers of differentiation. Southern hybridization of cellular DNA with TfR cDNA did not reveal differences between parental and Tf-Gel-1 cells in the level and arrangement of the TfR gene. Basal and inducible (repressible) levels of TfR mRNA from Tf-Gel-1 cells, as measured by northern hybridization of cellular RNA with TfR cDNA, were comparable to those of parental cells. Metabolic labeling of cells with [35S]methionine, followed by immunoprecipitation of TfRs, demonstrated that the amount of radioactivity incorporated into TfRs in Tf-Gel-1 cells was reduced to a degree that approximated the decrease in [125I]Tf binding. Cell surface TfRs prepared from exponentially growing parental cells labeled with 125I by the solid-phase lactoperoxidase-glucose oxidase method existed as a doublet, with one form being phosphorylated and the other not phosphorylated. In contrast, Tf-Gel-1 cells not only contained diminished amounts of TfRs but also contained only the phosphorylated form of TfRs in the surface membrane. The decrease in the surface membrane concentration of the TfR in Tf-Gel-1 cells was specific for this glycoprotein, since the levels of other cell surface antigens, such as CD13, CD15 and CD45, were normal in Tf-Gel-1 cells. A reduction in the incorporation of [3H]mannose into the acid-insoluble fraction of cells and an increase in sensitivity to ricin suggested that Tf-Gel-1 cells possessed an aberration in carbohydrate metabolism.

Plasma membrane and intracellular pools of transferrin receptors decline during in vitro cultivation of U937 cells

Transferrin receptor expression in the monocyte-like cell line U937 was investigated during in vitro cultivation. U937 cells expressed a single class of high affinity surface transferrin receptors (KD approximately 4 nM), with apparent subunit Mr of 90-95,000 Da as determined by SDS-reducing PAGE. [125I]-transferrin binding studies on detergent-solubilized cells revealed that half to two-thirds of the total functional binding sites were located intracellularly. Radioligand binding, immunofluorescence and flow cytometry studies were performed on intact, detergent-solubilized, or saponin-permeabilized cells, using either transferrin or the anti-transferrin receptor monoclonal antibody OKT9 IgG. These studies demonstrated that functional and antigenic transferrin receptor levels were maximal on cells 24 h after subculture at low density and declined during the culture period. Scatchard analysis of radioligand binding data suggested that the decline in functional transferrin binding sites resulted from a decline in the number of available receptors. These results demonstrate that in U937 cells there is a density-dependent regulation of transferrin receptor expression, resulting in a loss of functional and antigenic receptors from both plasma membrane and intracellular locations.

A rapid redistribution of the transferrin receptor to the cell surface of HL-60 cells and K562 cells upon treatment with dimethyl sulfoxide due to slowing of endocytosis

Treatment of two human leukemia cell lines with 1.25% dimethyl sulfoxide at 37 degrees C results in a rapid increase in the number of transferrin receptors on the cell surface detected by fluorescein-labeled anti-transferrin receptor antibodies. Both HL-60 cells, a human myeloid cell line, and K562 cells, a human erythroid-myeloid cell line, showed a 25-65% increase in cell surface transferrin binding in parallel experiments. Scatchard plot analysis of the data indicates that the number of receptors increases while the affinity of transferrin for the receptor remains the same. This rapid increase in the number of receptors at the cell surface appears to be due to a slowing of endocytosis rather than an increase in externalization of the receptor.