Human Promyelocytic Leukemia HL-60 Research Articles

Design and Synthesis of Cyclic Urea Compounds: A Pharmacological Study for Retinoidal Activity.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Casein Kinase Iε Downregulates PI3K/Akt Signaling, Followed by Genotoxic Stress-Induced Apoptosis of Hematopoietic Cells.

Casein kinase I (CKI) ε is involved in cytokine-induced hematopoietic cell differentiation by directly interacting with the suppressor of cytokine signaling 3 (SOCS3) and/or β-catenin. Here, we show that CKIε plays an important role in hematopoietic cell survival through modifying Phosphatidylinositol 3-kinase (PI3K)/Akt signaling. Introduction of wild-type (WT)-CKIε into murine interleukin-3 (IL-3)-dependent myeloid progenitor 32D cells increased the sensitivity to genotoxic stress, such as γ-irradiation, anti-cancer drug and cytokine-deprivation, whereas kinase-negative (KN)-CKIε suppressed it. While IL-3-induced Akt phosphorylation at Ser473 was sustained by KN-CKIε, it was attenuated by WT-CKIε. Similarly, in human promyelocytic leukemia HL-60 cells, the increase of phosphorylated Akt induced by all-trans retinoic acid was accompanied by the decrease of CKIε expression. A specific inhibitor for CKIε, CKI-7 also increased the phospho-Akt as well as phospho-PTEN (Ser380/Thr382/383; an inactive form of PTEN). Functional expression of CKIε activated PTEN by their physical association, following the decrease of phospho-Akt. The present studies suggest that the expression of CKIε downregulates PI3K/Akt signaling through PTEN, increasing the sensitivity to apoptotic signals. Taken together with the fact that CKIε is ubiquitously expressed in normal hematopoietic cells, CKIε might be critical for prevention against genotoxic stress-induced leukomogenesis. In other words, CKIε could be a candidate of the tumor suppressor in hematological malignancies as well as cancer.

Identification of differentiation-inducing activity produced by human bone marrow stromal cell line LP101

We have previously reported that human promyelocytic leukemia HL-60 cells can be induced to differentiate into mature granulocytes when HL-60 co-cultivated with human bone marrow stromal LP101 cells. In the present study, we investigated which factors produced by LP101 cells induce HL-60 cells to differentiate into mature granulocytes. The expression of the cell surface antigen CD11b on HL-60 cells was increased after a 72-h culture with the conditioned medium (CM) obtained from LP101 cells. LP101 cells were observed to produce various cytokines, including TNF-alpha, GM-CSF and IL-6. The neutralizing antibodies against these cytokines partially suppressed the CM-induced differentiation of HL-60 cells. Recombinant TNF-alpha induced the differentiation of HL-60 cells, and GM-CSF and IL-6 additionally enhanced the effect of TNF-alpha. When the CM was divided into a low molecular weight (LMW) fraction and a high molecular weight (HMW) fraction by ultrafiltration, the LMW fraction synergistically enhanced the differentiation inducible activity of TNF-alpha. These results demonstrate that LP101 cells induce the differentiation of HL-60 cells by producing various cytokines including TNF-alpha, IL-6, and GM-CSF, and that unknown low molecular weight factors also participate.

Molecular mechanism for transcriptional activation of ganglioside GM3 synthase and its function in differentiation of HL-60 cells

Previous studies have demonstrated that the activity of GM3 synthase and GM3 content are increased during the differentiation of human promyelocytic leukemia HL-60 cells into the monocyte/macrophage lineage after phorbol 12-myristate 13-acetate (PMA) treatment. However, the molecular mechanisms involved in transcriptional activation of GM3 synthase during differentiation of PMA-induced HL-60 cells are not well understood. As evidenced by western blot analysis, PMA induced the marked activation of protein kinase C (PKC)/extracellular regulated kinases (ERKs) signal transduction pathway during the differentiation of HL-60 cells. In addition, PKC/ERKs activation induced by PMA in HL-60 cells led to the phosphorylation of cAMP-responsive element binding protein (CREB) as a transcription factor. In PMA-stimulated HL-60 cells, the PKC/ERKs-dependent CREB activation regulated expression of GM3 synthase, inducing a synthesis of ganglioside GM3 product. On the other hand, although ganglioside GM3 was shown to be able to induce the differentiation of HL-60 cells into the monocyte/macrophage lineage, effect of ganglioside GM3 on expression of CD11b as a differentiation marker in HL-60 cells has been not reported yet. Interestingly, the increased ganglioside GM3 through PKC/ERKs/CREB-dependent pathway by PMA resulted in an increase of CD11b surface antigen expression and induction of HL-60 cells adherence. Treatment with L-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP), glucosylceramide synthase inhibitor, decreased induction of not only CD11b expression but also cellular adherence by reduction of PMA-induced ganglioside GM3. Furthermore, treatment of HL-60 cells with exogenous ganglioside GM3 induced CD11b expression. These results show that the enhanced expression of GM3 synthase through PKC/ERKs-dependent CREB activation by PMA is associated with the differentiation of HL-60 cells by inducing expression of CD11b known as a monocyte/macrophage differentiation maker.

Cytotoxic C‐Benzylated Dihydrochalcones from Uvaria acuminata.

Two new C-benzylated dihydrochalcones, isochamuvaritin (1) and acumitin (2), have been isolated from the African medicinal plant Uvaria acuminata, together with the previously reported benzylbenzoate (3), uvaretin (4), isouvaretin (5), diuvaretin (6), and uvangoletin (7). The structural elucidation of compounds 1 and 2 in spectroscopic studies is described. C-Benzylated dihydrochalcones, especially 1, 2, 4, and 6, showed considerable cytotoxicity toward human promyelocytic leukemia HL-60 cells.

Design and synthesis of cyclic urea compounds: a pharmacological study for retinoidal activity

Retinoids are natural and synthetic analogues of all- trans retinoic acid (ATRA). Cancer and other serious hyperproliferative diseases are attractive therapeutic targets for retinoids. We report here the design and synthesis of novel cyclic urea compounds with retinoidal activity. YR105 exhibited potent differentiation-inducing ability toward human promyelocytic leukemia HL-60 cells at the concentration of 10 −9 M: its potency was almost equal to that of the native ligand, all- trans retinoic acid.

Aloe-emodin induced in vitro G2/M arrest of cell cycle in human promyelocytic leukemia HL-60 cells

In this study, we have evaluated the chemopreventive role of aloe-emodin in human promyelocytic leukemia HL-60 cells in vitro by studying the regulation of proliferation, cell cycle and apoptosis. Aloe-emodin inhibited cell proliferation and induced G2/M arrest and apoptosis in HL-60 cells. Investigation of the levels of cyclins B1, E and A by immunoblot analysis showed that cyclin E level was unaffected, whereas cyclin B1 and A levels increased with aloe-emodin in HL-60 cells. Investigation of the levels of cyclin-dependent kinases, Cdk1 and 2, showed increased levels of Cdk1 but the levels of Cdk2 were not effected with aloe-emodin in HL-60 cells. The levels of p27 were increased after HL-60 cells were cotreated with various concentrations of aloe-emodin. The increase of the levels of p27 may be the major factor for aloe-emodin to cause G2/M arrest in these examined cells. Flow cytometric assays and DNA fragmentation gel electrophoresis also confirmed aloe-emodin induced apoptosis in HL-60 cells. The levels of caspase-3 were increased after HL-60 cells were cotreated with 10 μM aloe-emodin for 12, 24, 48, and 72 hours. Taken together, aloe-emodin therefore appears to exert its anticarcinogenesis properties by inhibiting proliferation and inducing cell cycle arrest and apoptosis underwent activation of caspase-3 in human leukemia HL-60 cells.

Kaurene diterpene induces apoptosis in human leukemia cells partly through a caspase-8-dependent pathway.

Defects in apoptosis signaling pathways contribute to tumorigenesis and drug resistance, and these defects are often a cause of failure of chemotherapy. Thus, a major goal in chemotherapy is to find cytotoxic agents that restore the ability of tumor cells to undergo apoptosis. We previously found that an Ent-kaurene diterpene, Ent-11alpha-hydroxy-16-kauren-15-one (KD), induced apoptosis in human promyelocytic leukemia HL-60 cells. Here, we found that caspase-8, an apoptotic factor, is involved in KD-induced apoptosis. Although treatment of HL-60 cells with KD resulted in the activation of caspase-8 and -9, a caspase-8-specific inhibitor but not a caspase-9-specific inhibitor attenuated KD-induced apoptosis. Expression of a catalytically inactive caspase-8 partly attenuated KD-induced apoptosis. Treatment with KD led to a time-dependent cleavage of Bid, a substrate of caspase-8, as well as to the proteolytic processing of procaspase-8, indicating that KD treatment induces apoptosis through a caspase-8-dependent pathway. Moreover, overexpression of the drug resistance factor Bcl-2, which is frequently overexpressed in many tumors, failed to confer resistance to KD-induced cytotoxicity. Thus, KD may be a promising experimental cytotoxic agent that possibly points to new strategies to overcome a drug resistance.

2-Arachidonoylglycerol, an endogenous cannabinoid receptor ligand, induces accelerated production of chemokines in HL-60 cells.

2-Arachidonoylglycerol is an endogenous ligand for the cannabinoid receptors (CB1 and CB2). Previously, we provided evidence that 2-arachidonoylglycerol, but not anandamide (N-arachidonoylethanolamine), is the true natural ligand for the cannabinoid receptors. In the present study, we examined in detail the effects of 2-arachidonoylglycerol on the production of chemokines in human promyelocytic leukemia HL-60 cells. We found that 2-arachidonoylglycerol induced a marked acceleration in the production of interleukin 8. The effect of 2-arachidonoylglycerol was blocked by treatment of the cells with SR144528, a cannabinoid CB2 receptor antagonist, indicating that the effect of 2-arachidonoylglycerol is mediated through the CB2 receptor. Augmented production of interleukin 8 was also observed with CP55940, a synthetic cannabinoid, and an ether-linked analog of 2-arachidonoylglycerol. On the other hand, neither anandamide nor the free arachidonic acid induced the enhanced production of interleukin 8. A similar effect of 2-arachidonoylglycerol was observed in the case of the production of macrophage-chemotactic protein-1. The accelerated production of interleukin 8 by 2-arachidonoylglycerol was observed not only in undifferentiated HL-60 cells, but also in HL-60 cells differentiated into macrophage-like cells. Noticeably, 2-arachidonoylglycerol and lipopolysaccharide acted synergistically to induce the dramatically augmented production of interleukin 8. These results strongly suggest that the CB2 receptor and its physiological ligand, i.e., 2-arachidonoylglycerol, play important regulatory roles such as stimulation of the production of chemokines in inflammatory cells and immune-competent cells. Detailed studies on the cannabinoid receptor system are thus essential to gain a better understanding of the precise regulatory mechanisms of inflammatory reactions and immune responses.

Saucernetin-8 isolated from Saururus chinensis induced the differentiation of human acute promyelocytic leukemia HL-60 cells.

The present work was performed to investigate the effects of saucernetin-8 on proliferation and differentiation of human leukemia HL-60 cells as well as the underlying mechanisms for these effects. Saucernetin-8 exhibited a potent antiproliferative activity against HL-60 cells. This compound was also found to be a potent inducer of differentiation in human leukemia derived HL-60 cells through the examination of differentiation markers, as assessed by nitroblue tetrazolium reduction test, esterase activity assay, phagocytic activity assay, morphology change, and expression of CD14 and CD66b surface antigens. These results suggest that saucernetin-8 induces the differentiation of human leukemia cells to granulocytes and monocytes/macrophages lineage. Moreover, DNA flow-cytometry indicated that saucernetin-8 induced a G1 phase arrest of HL-60 cells. The protein and mRNA expression levels of p21 were up-regulated during saucernetin-8-dependent HL-60 cell differentiation, whereas the level of c-myc was down-regulated. Taken together, our results suggest that saucernetin-8 may have potential as a therapeutic agent in human leukemia.

Cytotoxic C-benzylated dihydrochalcones from Uvaria acuminata.

Two new C-benzylated dihydrochalcones, isochamuvaritin (1) and acumitin (2), have been isolated from the African medicinal plant Uvaria acuminata, together with the previously reported benzylbenzoate (3), uvaretin (4), isouvaretin (5), diuvaretin (6), and uvangoletin (7). The structural elucidation of compounds 1 and 2 in spectroscopic studies is described. C-Benzylated dihydrochalcones, especially 1, 2, 4, and 6, showed considerable cytotoxicity toward human promyelocytic leukemia HL-60 cells.

Antimicrobial activity and cytotoxicity of the essential oil of Curcuma zedoaria.

The chemical compositions of the essential oil of Curcuma zedoaria (Berg.) Rosc. were analyzed by gas chromatography-mass spectrometry (GC-MS) and showed a high content of epicurzerenone and curdione representing 46.6% and 13.7% of the total oil, respectively. The essential oil was evaluated for potential antimicrobial activity against Staphylococcus aureus, Escherichia coli, Pseudomonasa aeruginosa, Vibrio parahaemolyticus, Salmonella typhimurium and Bacillus cereus. V. parahaemolyticus was sensitive to the presence of the essential oil, while the most resistant strain appeared to be E. coli. Based on 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyl-tetrazolium bromide (MTT) assay, nitroblue tetrazolium (NBT) reduction and cell morphology, the essential oil of C. zedoaria could inhibit the proliferation of human promyelocytic leukemia HL-60 cells. These results suggest that the essential oil has the antimicrobial activity against some of Gram- positive and negative pathogenic microorganisms and the components of the extract lead to the apoptosis of human cancer cell line.

Synergistic action of piroxicam on the eicosapentaenoic acid-induced apoptosis is associated with enhanced down-regulation of anti-apoptotic Bcl-2 expression but not promoted activation of pro-apoptotic Bid protein

Eicosapentaenoic acid (EPA) is a dietary polyunsaturated fatty acid (PUFA) that is found abundantly in fish oils and induces apoptosis in human promyelocytic leukemia HL-60 cells. Cyclooxygenase (COX) converts EPA intracellularly into various inflammatory mediators that may affect the bioavailability of this fatty acid for inducing apoptosis in the cancer cells. In this study, effect of piroxicam (PRX), a COX inhibitor, on the EPA-induced apoptosis in HL-60 cells was investigated. EPA arrested cell cycle of the leukemic cells at G0/G1 phase after 8-h incubation and induced apoptosis after 24-h incubation. PRX induced neither cell-cycle arrest nor apoptosis significantly in HL-60 cells even after 48-h incubation. However, 24-h incubation with PRX followed by 24-h incubation with EPA significantly elevated both early-phase and late-phase apoptotic cells by 35% and 166% respectively, when compared to those induced by the fatty acid only. This synergistic action of PRX on the EPA-induced apoptosis was associated with enhanced down-regulation of anti-apoptotic Bcl-2 protein expression, but not promoted activation of pro-apoptotic Bid protein.

Modification of Promyelocytic Leukemia Zinc Finger Protein (PLZF) by SUMO-1 Conjugation Regulates Its Transcriptional Repressor Activity

Promyelocytic leukemia zinc finger (PLZF) protein is a sequence-specific DNA-binding protein that represses the transcriptional activity of target genes such as those for cyclin A and the interleukin-3 receptor alpha chain. The PLZF gene becomes fused to the retinoic acid receptor alpha gene as a result of the t(11, 17)(q23;q21) chromosomal translocation that is associated with acute promyelocytic leukemia. We now show that endogenous PLZF in human promyelocytic leukemia HL-60 cells is modified by conjugation with SUMO-1 (small ubiquitin-related modifier-1) and that PLZF colocalizes with SUMO-1 in the nucleus of transfected human embryonic kidney 293T cells. Site-directed mutagenesis identified lysine 242 in the RD2 domain of human PLZF as the sumoylation site. A luciferase reporter gene assay suggested that SUMO-1 modification of this residue is required for transcriptional repression by PLZF, and an electrophoretic mobility shift assay showed that this modification increases the DNA binding activity of PLZF. PLZF-mediated regulation of the cell cycle and transcriptional repression of the cyclin A2 gene were also dependent on sumoylation of PLZF on lysine 242. These results demonstrate that PLZF is modified by SUMO-1 conjugation and that this modification regulates the biological functions of PLZF.

Differential involvement of protein kinase C in human promyelocytic leukemia cell differentiation enhanced by artemisinin

Artemisinin, a sesquiterpene lactone endoperoxide that exists in several medicinal plants, is a well-known anti-malarial agent. In this report, we investigated the effect of artemisinin on cellular differentiation in the human promyelocytic leukemia HL-60 cell culture system. Artemisinin markedly increased the degree of HL-60 leukemia cell differentiation when simultaneously combined with low doses of 1α,25-dihydoxyvitamin D 3 [1,25-(OH) 2D 3] or all- trans retinoic acid (all- trans RA). Artemisinin by itself had very weak effects on the differentiation of HL-60 cells. Cytofluorometric analysis and cell morphologic studies indicated that artemisinin potentiated 1,25-(OH) 2D 3-induced cell differentiation predominantly into monocytes and all- trans RA-induced cell differentiation into granulocytes, respectively. Extracellular-regulated kinase (ERK) inhibitors markedly inhibited HL-60 cell differentiation induced by artemisinin in combination with 1,25-(OH) 2D 3 or all- trans RA, whereas phosphatidylinositol 3-kinase (PI3-K) inhibitors did not. Particularly, protein kinase C (PKC) inhibitors inhibited HL-60 cell differentiation induced by artemisinin in combination with 1,25-(OH) 2D 3 but not with all- trans RA. Artemisinin enhanced PKC activity and protein level of PKCβI isoform in only 1,25-(OH) 2D 3-treated HL-60 cells. Taken together, these results indicate that artemisinin strongly enhanced 1,25-(OH) 2D 3- and all- trans RA-induced cell differentiation in which PKC is differentially involved in arteminisin-mediated enhancement of leukemia cell differentiation.

Involvement of Akt in mitochondria-dependent apoptosis induced by a cdc25 phosphatase inhibitor naphthoquinone analog

Vitamin K-related analogs induce growth inhibition via a cell cycle arrest through cdc25A phosphatase inhibition in various cancer cell lines. We report that 2,3-dichloro-5,8-dihydroxy-1,4-naphthoquinone (DDN), a naphthoquinone analog, induces mitochondria-dependent apoptosis in human promyelocytic leukemia HL-60 cells. DDN induced cytochrome c release, Bax translocation, cleavage of Bid and Bad, and activation of caspase-3, -8, -9 upon the induction of apoptosis. Cleavage of Bid, the caspase-8 substrate, was inhibited by the broad caspase inhibitor z-Val-Ala-Asp(OMe)-fluoromethylketone (zVAD-fmk), whereas cytochrome c release was not affected, suggesting that activation of caspase-8 and subsequent Bid cleavage occur downstream of cytochrome c release. DDN inhibited the activation of Akt detected by decreasing level of phosphorylation. Overexpression of constitutively active Akt protected cells from DDN-induced apoptosis, while dominant negative Akt moderately enhanced cell death. Furthermore, Akt prevented release of cytochrome c and cleavage of Bad in DDN-treated HL-60 cells. Taken together, DDN-induced apoptosis is associated with mitochondrial signaling which involves cytochrome c release via a mechanism inhibited by Akt.

Promyelocytic HL60 cells express NADPH oxidase and are excellent targets in a rapid spectrophotometric microplate assay for extracellular superoxide.

A great number of drugs, toxicants, and growth factors induce the generation of intermediary reactive oxygen species (ROS). The human promyelocytic leukemia HL60 cell line differentiated along the macrophage or neutrophil lineage is a model system that is frequently used for the generation of ROS by various agents. As a primary source of ROS the superoxide anion produced by an enzymatic complex, NADPH oxidase, is well established. The present study shows that nondifferentiated HL60 cells contain NADPH oxidase and can be used as a model for the assessment of oxidant as well as antioxidant compounds. The expression of the multicomponent NADPH oxidase was demonstrated in nondifferentiated HL60 cells at the molecular level by detection of the mRNAs of the components gp91phox, p47phox, and p67phox as well as functionally by phorbol 12-myristate-13-acetate (PMA)-stimulated generation of superoxide, which was susceptible to inhibition by diphenyleneiodonium. The functional assay was performed using the cells in a log growth phase by adapting a standard microplate assay based on the classic superoxide dismutase-inhibitable reduction of cytochrome c. Validation of the microplate assay was carried out both with nonadherent differentiated HL60 cells and the adherent mouse monocyte-macrophage-like RAW 264.7 cell line, as well as with various compounds of oxidant (bleomycin sulfate, cis-diammineplatinum(II), camptothecin, TNF-alpha, IL-1 beta), nonoxidant (4 alpha-PMA, piracetam), and antioxidant (alpha-tocopherol, ascorbic acid) activity. In summary, we established a highly specific, reproducible and--with the aid of the nondifferentiated HL60 cell line--time-saving superoxide microplate assay as a valuable tool for the rapid screening of compounds for oxidative and antioxidative activity.

Baicalein induced in vitro apoptosis undergo caspases activity in human promyelocytic leukemia HL-60 cells

In this study, we evaluated the potential apoptosis effects of baicalein on human promyelocytic leukemia HL-60 cells in vitro. Apoptosis induction, cell viability, morphology and caspase-3 activity were then performed to determine by flow cytometric assay, DNA gel electrophoresis, anti-ADP-ribose stain and determination of caspase-3 activity. There is a significant difference in cell death of HL-60 cells that was detected between baicalein-treated and untreated groups. Furthermore, there was a further significant increase in apoptosis induction when cells were treated with baicalein compared to without baicalein treated groups. Flow cytometric assays and DNA fragmentation gel electrophoresis also confirmed baicalein induced apoptosis in HL-60 cells. Baicalein also promoted caspase-3 activity then leading to cleavage of poly-ADP-ribose polymerase finally leading to DNA fragmentation occurrence. Furthermore, the baicalein-induced apoptosis was markedly blocked by the broad-spectrum caspase inhibitor, z-VAD-fmk. Taken together, these results suggest that treatment of human leukemia HL-60 cells with baicalein induced apoptosis through activation of caspase-3 activity.

Bezafibrate Induces a Mitochondrial Derangement in Human Cell Lines: A PPAR-Independent Mechanism for a Peroxisome Proliferator

Bezafibrate is a hypolipidemic drug that belongs to the group of peroxisome proliferators because it binds to peroxisome proliferator-activated receptors type alpha (PPARs). Peroxisome proliferators produce a myriad of extraperoxisomal effects, which are not necessarily dependent on their interaction with PPARs. An investigation on the peculiar activities of bezafibrate could clarify some of the molecular events and the relationship with the biochemical and pharmacological properties of this class of compounds. In this view, the human acute promyelocytic leukemia HL-60 cell line and human rabdomiosarcoma TE-671 cell line were cultured in media containing bezafibrate and a number of observations such as spectrophotometric analysis of mitochondrial respiratory chain enzymes, NMR metabolite determinations, phosphofructokinase enzymatic analysis, and differentiation assays were carried on. Bezafibrate induced a derangement of NADH cytochrome c reductase activity accompanied by metabolic alterations, mainly a shift to anaerobic glycolysis and an increase of fatty acid oxidation, as shown by NMR analysis of culture supernatants where acetate, lactate, and alanine levels increased. On the whole, the present results suggest a biochemical profile and a therapeutic role of this class of PPARs ligands more complex than those previously proposed.

Comparison of the gene expression profiles of monocytic versus granulocytic lineages of HL-60 leukemia cell differentiation by DNA microarray analysis

It is now recognized that precise patterns of differentially expressed genes ultimately direct a particular cell toward a given lineage. In this study, we compared the expression profiles of cancer-related genes by cDNA microarray analysis during the differentiation of human promyelocytic leukemia HL-60 cells into either monocytes or granulocytes. RNA was isolated at times 0, 6, 12, 24, 36, 48, and 72 h following stimulation of differentiation with all- trans retinoic acid (all- trans RA) or 1,25-dihydroxyvitamin D 3 [1,25-(OH) 2D 3], and hybridized to the microarray gene chips containing 872 genes related to cell-cycles, oncogenes and leukemias. Several genes were commonly or differentially regulated during cell differentiation into either lineage, as demonstrated by both hierarchical and self-organizing map clustering analysis. At 72 h the expression levels of 45 genes were commonly up- or down-regulated at least a twofold in both lineages. Most importantly, 32 genes including α-L-fucosidase gene and adducin γ subunit gene were up- or down-regulated only in all- trans RA-treated HL-60 cells, while 12 genes including interleukin 1β and hypoxia-inducible factor 1α were up- or down-regulated only in 1,25-(OH) 2D 3-treated HL-60 cells. The expression of selected genes was confirmed by Northern blot analysis. As expected, some genes identified have not been examined during HL-60 cell differentiation into either lineage. The identification of genes associated with a specific differentiation lineage may give important insights into functional and phenotypic differences between two lineages of HL-60 cell differentiation.