Retinoic acid-induced blr1 expression promotes ERK2 activation and cell differentiation in HL-60 cells

CXCR5 overexpression in HL-60 cells enhances chemotaxis toward CXCL13 without anticipated interaction partners or enhanced MAPK signaling

CXCR5 is a serpentine receptor implicated in cell migration in lymphocytes and differentiation in leukocytes. It causes MAPK pathway activation and has known membrane partners for signaling. CXCR5 mRNA is reportedly expressed in neutrophils following isolation, but its role in this cellular context is unknown. CXCR5 is also expressed in HL-60 cells, a human acute myeloid leukemia line, following treatment with all-trans retinoic acid, which induces differentiation toward a neutrophil-like state. CXCR5 is necessary for this process; differentiation was crippled in CXCR5 knockout cells and enhanced in cells ectopically expressing it. Since CXCR5 has various membrane protein partners, we investigated whether CXCR5-driven all-trans retinoic acid-induced differentiation depends on its association with such partners. Pursuing this, we generated HL-60 cells overexpressing the protein. We found that CXCR5 drove migration toward its ligand, CXCL13, and probed for interactions with several candidates using flow cytometry-based Förster resonance energy transfer. Surprisingly, we did not detect interactions with any candidates, including three reported in other cellular contexts. Additionally, we observed no significant changes in all-trans retinoic acid-induced differentiation; this may be due to the stoichiometry of CXCR5 and partner receptors or CXCL13. The anticipated membrane partnerings were surprisingly apparently unnecessary for downstream CXCR5 signaling and all-trans retinoic acid-induced differentiation.

Leukemia Cells and the Cytokine Network: Therapeutic Prospects

The network and balance of cytokines is of major importance in maintaining proper homeostasis of hematopoiesis. Abnormalities in this network may result in a variety of blood disorders; however, the role of this network is not clear in leukemia. The use of antineoplastic agents has improved the survival rate of some types of leukemia, and adjunctive therapy with cytokines may be helpful. Chemotherapeutic approaches are no longer the best choice because cytotoxicity may affect normal and leukemic cells, and leukemic cells may develop resistance to the chemotherapeutic agent. Induction of differentiation to a mature phenotype and the control of apoptotic-gene expression have provided other possible alternative therapies. Combined effects of cytokines and vitamin derivatives such as retinoic acid (RA) and 1, 25 dihydroxyvitamin D3 (VD3) were found more beneficial than any of these agents individually. These agents exhibit cooperative effects, potentiate each other's effects, or both. Therefore, understanding the hematopoietic actions of these agents, their interactions with their receptors, and their differentiation signaling pathways may result In the design of new therapies. However, the role of cytokines in apoptosis is controversial because in some cases they were found to increase tumor cell resistance to apoptosis-inducing agents. Recent studies in the molecular biology of gene regulation, transcription factors, and repressors have led to new possible approaches such as differentiation therapy for the treatment of leukemia. In addition, the development of drugs that act on the molecular level such as imatinib is just the beginning of a new era in molecular targeted therapy in which the drug acts specifically on the leukemic cell. There are many possible combinations of cytokines, retinoids, and VD3, and perhaps the best therapeutic combination is yet to be described. This minireview is an update on the role of cytokines and the therapeutic potential of combinations with agents such as RA, VD3, and other chemotherapeutic agents.

6-Formylindolo(3,2-b)Carbazole (FICZ) Modulates the Signalsome Responsible for RA-Induced Differentiation of HL-60 Myeloblastic Leukemia Cells

6-Formylindolo(3,2-b)carbazole (FICZ) is a photoproduct of tryptophan and an endogenous high affinity ligand for aryl hydrocarbon receptor (AhR). It was previously reported that, in patient-derived HL-60 myeloblastic leukemia cells, retinoic acid (RA)-induced differentiation is driven by a signalsome containing c-Cbl and AhR. FICZ enhances RA-induced differentiation, assessed by expression of the membrane differentiation markers CD38 and CD11b, cell cycle arrest and the functional differentiation marker, inducible oxidative metabolism. Moreover, FICZ augments the expression of a number of the members of the RA-induced signalsome, such as c-Cbl, Vav1, Slp76, PI3K, and the Src family kinases Fgr and Lyn. Pursuing the molecular signaling responsible for RA-induced differentiation, we characterized, using FRET and clustering analysis, associations of key molecules thought to drive differentiation. Here we report that, assayed by FRET, AhR interacts with c-Cbl upon FICZ plus RA-induced differentiation, whereas AhR constitutively interacts with Cbl-b. Moreover, correlation analysis based on the flow cytometric assessment of differentiation markers and western blot detection of signaling factors reveal that Cbl-b, p-p38α and pT390-GSK3β, are not correlated with other known RA-induced signaling components or with a phenotypic outcome. We note that FICZ plus RA elicited signaling responses that were not typical of RA alone, but may represent alternative differentiation-driving pathways. In clusters of signaling molecules seminal to cell differentiation, FICZ co-administered with RA augments type and intensity of the dynamic changes induced by RA. Our data suggest relevance for FICZ in differentiation-induction therapy. The mechanism of action includes modulation of a SFK and MAPK centered signalsome and c-Cbl-AhR association.

Ascorbic acid inhibits TPA-induced HL-60 cell differentiation by decreasing cellular H₂O₂ and ERK phosphorylation

Retinoic acid (RA), vitamin D and 12-O‑tetradecanoyl phorbol-13-acetate (TPA) can induce HL-60 cells to differentiate into granulocytes, monocytes and macrophages, respectively. Similar to RA and vitamin D, ascorbic acid also belongs to the vitamin family. High‑dose ascorbic acid (>100 µM) induces HL‑60 cell apoptosis and induces a small fraction of HL‑60 cells to express the granulocyte marker, CD66b. In addition, ascorbic acid exerts an anti‑oxidative stress function. Oxidative stress is required for HL‑60 cell differentiation following treatment with TPA, however, the effect of ascorbic acid on HL‑60 cell differentiation in combination with TPA treatment remains to be fully elucidated. The aim of the present study was to investigate the cellular effects of ascorbic acid treatment on TPA-differentiated HL-60 cells. TPA-differentiated HL-60 cells were used for this investigation, this study and the levels of cellular hydrogen peroxide (H2O2), caspase activity and ERK phosphorylation were determined following combined treatment with TPA and ascorbic acid. The results demonstrated that low‑dose ascorbic acid (5 µM) reduced the cellular levels of H2O2 and inhibited the differentiation of HL‑60 cells into macrophages following treatment with TPA. In addition, the results of the present study further demonstrated that low‑dose ascorbic acid inactivates the ERK phosphorylation pathway, which inhibited HL‑60 cell differentiation following treatment with TPA.

Retinoic acid therapy resistance progresses from unilineage to bilineage in HL-60 leukemic blasts

Emergent resistance can be progressive and driven by global signaling aberrations. All-trans retinoic acid (RA) is the standard therapeutic agent for acute promyelocytic leukemia, but 10-20% of patients are not responsive, and initially responsive patients relapse and develop retinoic acid resistance. The patient-derived, lineage-bipotent acute myeloblastic leukemia (FAB M2) HL-60 cell line is a potent tool for characterizing differentiation-induction therapy responsiveness and resistance in t(15;17)-negative cells. Wild-type (WT) HL-60 cells undergo RA-induced granulocytic differentiation, or monocytic differentiation in response to 1,25-dihydroxyvitamin D3 (D3). Two sequentially emergent RA-resistant HL-60 cell lines, R38+ and R38-, distinguishable by RA-inducible CD38 expression, do not arrest in G1/G0 and fail to upregulate CD11b and the myeloid-associated signaling factors Vav1, c-Cbl, Lyn, Fgr, and c-Raf after RA treatment. Here, we show that the R38+ and R38- HL-60 cell lines display a progressive reduced response to D3-induced differentiation therapy. Exploiting the biphasic dynamic of induced HL-60 differentiation, we examined if resistance-related defects occurred during the first 24 h (the early or "precommitment" phase) or subsequently (the late or "lineage-commitment" phase). HL-60 were treated with RA or D3 for 24 h, washed and retreated with either the same, different, or no differentiation agent. Using flow cytometry, D3 was able to induce CD38, CD11b and CD14 expression, and G1/G0 arrest when present during the lineage-commitment stage in R38+ cells, and to a lesser degree in R38- cells. Clustering analysis of cytometry and quantified Western blot data indicated that WT, R38+ and R38- HL-60 cells exhibited decreasing correlation between phenotypic markers and signaling factor expression. Thus differentiation induction therapy resistance can develop in stages, with initial partial RA resistance and moderate vitamin D3 responsiveness (unilineage maturation block), followed by bilineage maturation block and progressive signaling defects, notably the reduced expression of Vav1, Fgr, and c-Raf.

Nuclear Raf-1 kinase regulates the CXCR5 promoter by associating with NFATc3 to drive retinoic acid-induced leukemic cell differentiation

Novel functions of signaling molecules have been revealed in studies of cancer stem cells. Retinoic acid (RA) is an embryonic morphogen and stem cell regulator that controls the differentiation of a patient-derived leukemic cell line, HL-60, which is composed of progenitor cells with bipotent myelo-monocytic differentiation capability. RA treatment of HL-60 cells causes unusually long-lasting mitogen-activated protein kinase signaling, with the cells exhibiting the beginning of G0 cell cycle arrest and functional differentiation by 48 h after treatment with RA. This event coincides with the nuclear translocation of Raf-1, phosphorylated at serine 621. The present study shows how the novel localization of Raf-1 to the nucleus results in transcriptional changes that contribute to the differentiation of HL-60 cells induced by RA. We find that nuclear pS621 Raf-1 associates with NFATc3 near its cognate binding site in the promoter of CXCR5, a gene that must be up-regulated to drive RA-induced differentiation. NFATc3 becomes immunoprecipitable with anti-phosphoserine serum, and CXCR5 is transcriptionally up-regulated upon RA-induced differentiation. Inhibiting the pS621 Raf-1/NFATc3 association with PD98059 inhibits these processes and cripples RA-induced differentiation. In this novel paradigm for Raf-1 and RA function, Raf-1 has a role in driving the nuclear signaling of RA-induced differentiation of leukemic progenitor cells.

Retinoic acid-elicited RARα/RXRα signaling attenuates Aβ production by directly inhibiting γ-secretase-mediated cleavage of amyloid precursor protein

Retinoic acid (RA)-elicited signaling has been shown to play critical roles in development, organogenesis, and the immune response. RA regulates expression of Alzheimer's disease (AD)-related genes and attenuates amyloid pathology in a transgenic mouse model. In this study, we investigated whether RA can suppress the production of amyloid-β (Aβ) through direct inhibition of γ-secretase activity. We report that RA treatment of cells results in significant inhibition of γ-secretase-mediated processing of the amyloid precursor protein C-terminal fragment APP-C99, compared with DMSO-treated controls. RA-elicited signaling was found to significantly increase accumulation of APP-C99 and decrease production of secreted Aβ40. In addition, RA-induced inhibition of γ-secretase activity was found to be mediated through significant activation of extracellular signal-regulated kinases (ERK1/2). Treatment of cells with the specific ERK inhibitor PD98059 completely abolished RA-mediated inhibition of γ-secretase. Consistent with these findings, RA was observed to inhibit secretase-mediated proteolysis of full-length APP. Finally, we have established that RA inhibits γ-secretase through nuclear retinoic acid receptor-α (RARα) and retinoid X receptor-α (RXRα). Our findings provide a new mechanistic explanation for the neuroprotective role of RA in AD pathology and add to the previous data showing the importance of RA signaling as a target for AD therapy.

Differential IL-21 signaling in APCs leads to disparate Th17 differentiation in diabetes-susceptible NOD and diabetes-resistant NOD.Idd3 mice

Type 1 diabetes (T1D) is an autoimmune disease that shows familial aggregation in humans and likely has genetic determinants. Disease linkage studies have revealed many susceptibility loci for T1D in mice and humans. The mouse T1D susceptibility locus insulin-dependent diabetes susceptibility 3 (Idd3), which has a homologous genetic interval in humans, encodes cytokine genes Il2 and Il21 and regulates diabetes and other autoimmune diseases; however, the cellular and molecular mechanisms of this regulation are still being elucidated. Here we show that T cells from NOD mice produce more Il21 and less Il2 and exhibit enhanced Th17 cell generation compared with T cells from NOD.Idd3 congenic mice, which carry the protective Idd3 allele from a diabetes-resistant mouse strain. Further, APCs from NOD and NOD.Idd3 mice played a central role in this differential Th17 cell development, and IL-21 signaling in APCs was pivotal to this process. Specifically, NOD-derived APCs showed increased production of pro-Th17 mediators and dysregulation of the retinoic acid (RA) signaling pathway compared with APCs from NOD.Idd3 and NOD.Il21r-deficient mice. These data suggest that the protective effect of the Idd3 locus is due, in part, to differential RA signaling in APCs and that IL-21 likely plays a role in this process. Thus, we believe APCs provide a new candidate for therapeutic intervention in autoimmune diseases.

Modeling and analysis of retinoic acid induced differentiation of uncommitted precursor cells

Manipulation of differentiation programs has therapeutic potential in a spectrum of human cancers and neurodegenerative disorders. In this study, we integrated computational and experimental methods to unravel the response of a lineage uncommitted precursor cell-line, HL-60, to Retinoic Acid (RA). HL-60 is a human myeloblastic leukemia cell-line used extensively to study human differentiation programs. Initially, we focused on the role of the BLR1 receptor in RA-induced differentiation and G1/0-arrest in HL-60. BLR1, a putative G protein-coupled receptor expressed following RA exposure, is required for RA-induced cell-cycle arrest and differentiation and causes persistent MAPK signaling. A mathematical model of RA-induced cell-cycle arrest and differentiation was formulated and tested against BLR1 wild-type (wt) knock-out and knock-in HL-60 cell-lines with and without RA. The current model described the dynamics of 729 proteins and protein complexes interconnected by 1356 interactions. An ensemble strategy was used to compensate for uncertain model parameters. The ensemble of HL-60 models recapitulated the positive feedback between BLR1 and MAPK signaling. The ensemble of models also correctly predicted Rb and p47phox regulation and the correlation between p21-CDK4-cyclin D formation and G1/0-arrest following exposure to RA. Finally, we investigated the robustness of the HL-60 network architecture to structural perturbations and generated experimentally testable hypotheses for future study. Taken together, the model presented here was a first step toward a systematic framework for analysis of programmed differentiation. These studies also demonstrated that mechanistic network modeling can help prioritize experimental directions by generating falsifiable hypotheses despite uncertainty.

Inhibition of the redox function of APE1/Ref-1 in myeloid leukemia cell lines results in a hypersensitive response to retinoic acid-induced differentiation and apoptosis

OBJECTIVE

The standard of care for promyelocytic leukemia includes use of the differentiating agent all-trans retinoic acid (RA) and chemotherapy. RA induces cell differentiation through retinoic acid receptor (RAR) transcription factors. Because redox mechanisms influence how readily transcription factors bind to DNA response elements (RARE), the impact of small molecule (E3330) inhibition of the redox regulatory protein, apurinic-apyrimidinic endonuclease/redox effector factor (APE1/Ref-1) on RAR DNA binding and function in RA-induced myeloid leukemia cell differentiation and apoptosis was investigated.

MATERIALS AND METHODS

The redox function of APE1 was studied using the small molecule inhibitor E3330 in HL-60 and PLB acute myeloid leukemia cells. Electrophoretic mobility shift assays were employed to determine effect of inhibitor on APE1/Ref-1 redox signaling function. Trypan blue assays, Annexin-V/propidium iodide and CD11b staining, and real-time polymerase chain reaction analyses were employed to determine survival, apoptosis, and differentiation status of cells in culture.

RESULTS

RARα binds to its RARE in a redox-dependent manner mediated by APE1/Ref-1 redox regulation. Redox-dependent RAR-RARE binding is blocked by E3330, a small molecule redox inhibitor of APE1/Ref-1. Combination treatment of RA + E3330 results in a profound hypersensitivity of myeloid leukemia cells to RA-induced differentiation and apoptosis. Additionally, redox inhibition by E3330 results in enhanced RAR target gene, BLR-1, expression in myeloid leukemia cells.

CONCLUSIONS

The redox function of APE1/Ref-1 regulates RAR binding to its DNA RAREs influencing the response of myeloid leukemia cells to RA-induced differentiation. Targeting of APE1/Ref-1 redox function may allow manipulation of the retinoid response with therapeutic implications.

Retinoic acid induces nuclear accumulation of Raf1 during differentiation of HL-60 cells

All trans-retinoic acid (RA) is a standard therapeutic agent used in differentiation induction therapy treatment of acute promyelocytic leukemia (APL). RA and its metabolites use a diverse set of signal transduction pathways during the differentiation program. In addition to the direct transcriptional targets of the nuclear RAR and RXR receptors, signals derived from membrane receptors and the Raf-MEK-ERK pathway are required. Raf1 phosphorylation and the prolonged activation of Raf1 persisting during the entire differentiation process are required for RA-dependent differentiation of HL-60 cells. Here we identify a nuclear redistribution of Raf1 during the RA-induced differentiation of HL-60 cells. In addition, the nuclear accumulation of Raf1 correlates with an increase in Raf1 phosphorylated at serine 621. The serine 621 phosphorylated Raf1 is predominantly localized in the nucleus. The RA-dependent nuclear accumulation of Raf1 suggests a novel nuclear role for Raf1 during the differentiation process.

Gene expression analysis of human promyelocytic leukemia HL-60 cell differentiation and cytotoxicity induced by natural and synthetic retinoids

AIMS

This study analyzed gene expression profiles of human promyelocytic leukemia HL-60 cells treated with natural and synthetic retinoids (ATRA, RII and R9158), in an attempt to investigate the structure-function relationship of the retinoids in inducing cell differentiation and cytotoxicity.

MAIN METHODS

Flow cytometry was used to determine cell cycle changes in HL-60 cells following treatment (1.0 muM) with natural and synthetic retinoids (ATRA, RII and R9158), and cDNA microarrays were used to monitor the gene expression profiles of HL-60 cells treated with the various retinoids.

KEY FINDINGS

Consistent with retinoid-induced cell differentiation, treatment with these three retinoids correlated with an increase in the percentage of cells arrested in the G1/G0 phase of the cell cycle. Microarray analysis showed upregulation of known differentiation genes, adhesion molecules, and the oxidase activation pathway following retinoid treatment. Differential expression of several genes was observed in HL-60 cells treated with the three retinoids. For example, tissue remodeling protein genes, ubiquitin genes, and signal transduction genes were highly expressed in ATRA- and R9158-treated HL-60 cells, but remained unchanged in HL-60 cells treated with RII.

SIGNIFICANCE

The above findings suggest that the differentiation of HL-60 cells induced by the three retinoids occurs through similar pathways, and that there exists a structure-function relationship regarding retinoids and the induction of cell differentiation and cytotoxicity.

Human myeloblastic leukemia cells (HL-60) express a membrane receptor for estrogen that signals and modulates retinoic acid-induced cell differentiation

Estrogen receptors are historically perceived as nuclear ligand activated transcription factors. An estrogen receptor has now been found localized to the plasma membrane of human myeloblastic leukemia cells (HL-60). Its expression occurs throughout the cell cycle, progressively increasing as cells mature from G(1) to S to G(2)/M. To ascertain that the receptor functioned, the effect of ligands, including a non-internalizable estradiol-BSA conjugate and tamoxifen, an antagonist of nuclear estrogen receptor function, were tested. The ligands caused activation of the ERK MAPK pathway. They also modulated the effect of retinoic acid, an inducer of MAPK dependent terminal differentiation along the myeloid lineage in these cells. In particular the ligands inhibited retinoic acid-induced inducible oxidative metabolism, a functional marker of terminal myeloid cell differentiation. To a lesser degree they also diminished retinoic acid-induced earlier markers of cell differentiation, namely CD38 and CD11b. However, they did not regulate retinoic acid-induced G(0) cell cycle arrest. There is thus a membrane localized estrogen receptor in HL-60 myeloblastic leukemia cells that can cause ERK activation and modulates the response of these cells to retinoic acid, indicating crosstalk between the membrane estrogen and retinoic acid evoked pathways relevant to propulsion of cell differentiation.

A MAPK-positive feedback mechanism for BLR1 signaling propels retinoic acid-triggered differentiation and cell cycle arrest

MAPK signaling is required for retinoic acid (RA)-triggered G(0) cell cycle arrest and cell differentiation, but the mechanism is not well defined. In this study, RA is found to cause MAPK activation with sustained association of RAF to MEK or ERK, leading to a MAPK-dependent accumulation of p21(Waf1/Cip1) and binding to CDK2 blocking G(1)/S transition. BLR1, a chemokine receptor, was found to function as a critical component of RA-triggered MAPK signaling. Unlike wild-type parental cells, RA-treated BLR1 knock-out cells failed to show RAF and consequential MEK and ERK phosphorylation, failed to accumulate CDK inhibitors that control G(1)/S transition, and failed to differentiate and arrest in response to RA, whereas ectopically overexpressing BLR1 enhanced MAPK signaling and caused accelerated RA-induced differentiation and arrest. Ectopic overexpression of RAF enhanced BLR1 expression in response to RA, whereas inhibition of RAF or MEK by inhibitors or knockdown of RAF by short interfering RNA diminished RA-induced BLR1 expression and attenuated differentiation and growth arrest. Ectopic expression of the RAF CR3, the catalytically active domain, in the BLR1 knock-out restored RA-induced MAPK activation and the ability to differentiate and arrest, indicating that RAF effects MAPK signaling by BLR1 to propel differentiation/arrest. Taken together, RA induces cell differentiation and growth arrest through activation of a novel MAPK pathway with BLR1 as a critical component in a positive feedback mechanism that may contribute to the prolonged MAPK signaling propelling RA-induced cell cycle arrest and differentiation.

2β-(N-Substituted Piperazino)-5α-Androstane-3α,17β-Diols: Parallel Solid-Phase Synthesis and Antiproliferative Activity on Human Leukemia HL-60 Cells

Leukemia is the most common cancer affecting children. A steroid possessing a methylpiperazine nucleus was recently reported to inhibit the proliferation of HL-60 leukemia cells. To speed up the development of this promising potential new drug, we generated libraries of analogues using parallel solid-phase organic synthesis (SPOS). A 6-step sequence of reactions, starting from dihydrotestosterone, afforded a steroidal 2,3alpha-epoxide, which was selectively opened to give, after N-Fmoc protection, a diol with suitable stereochemistry. The difference of reactivity between 3alpha-OH and 17beta-OH was then used to allow the regioselective coupling of 17beta-OH to chloro-activated butyldiethylsilane polystyrene. We next generated three libraries of 2beta-piperazinyl-5alpha-androstane-3alpha,17beta-diol N-derivatives with 1, 2, or 3 levels of molecular diversity in acceptable yields and purities for our biological screening assay. Several members of these libraries were more potent than the lead compound, especially five members with a proline as the first level of diversity and a cyclohexylcarbonyl, methylbutyryl, cyclohexylacetyl, cyclopentylpropionyl, or hexanoyl as the second level of diversity. They efficiently inhibited HL-60 cell proliferation with IC50 values of 0.58, 0.66, 1.78, 1.98, and 2.57 microM, respectively. The present work demonstrates the potential of our SPOS approach for the optimization of a new class of cytotoxic agents.

All-trans retinoic acid induces p62DOK1 and p56DOK2 expression which enhances induced differentiation and G0 arrest of HL-60 leukemia cells

p62(DOK1) (DOK1) and p56(DOK2) (DOK2) are sequence homologs that act as docking proteins downstream of receptor or nonreceptor tyrosine kinases. Originally identified in chronic myelogenous leukemia cells as a highly phosphorylated substrate for the chimeric p210(bcr-abl) protein, DOK1 was suspected to play a role in leukemogenesis. However, p62(DOK1-/-) fibroblast knockout cells were found to have enhanced MAPK signaling and proliferation due to growth factors, suggesting negative regulatory capabilities for DOK1. The role of DOK1 and DOK2 in leukemogeneis thus is enigmatic. The data in this report show that both the DOK1 and the DOK2 adaptor proteins are constitutively expressed in the myelomonoblastic leukemia cell line, HL-60, and that expression of both proteins is induced by the chemotherapeutic differentiation causing agents, all-trans retinoic acid (atRA) and 1,25-dihydroxyvitamin D3 (VD3). Ectopic expression of either protein enhances atRA- or VD3-induced growth arrest, differentiation, and G(0)/G(1) cell cycle arrest and results in increased ERK1/2 phosphorylation. DOK1 and DOK2 are similarly effective in these capabilities. The data provide evidence that DOK1 and DOK2 proteins have a similar role in regulating cell proliferation and differentiation and are positive regulators of the MAPK signaling pathway in this context.

Retinoic acid induces expression of SLP-76: expression with c-FMS enhances ERK activation and retinoic acid-induced differentiation/G0 arrest of HL-60 cells

Retinoic acid (RA) is known to cause MAPK signaling which propels G0 arrest and myeloid differentiation of HL-60 human myeloblastic leukemia cells. The present studies show that RA up-regulated expression of SLP-76 (Src-homology 2 domain-containing leukocyte-specific phospho-protein of 76 kDa), which became a prominent tyrosine-phosphorylated protein in RA-treated cells. SLP-76 is a known adaptor molecule associated with T-cell receptor and MAPK signaling. To characterize functional effects of SLP-76 expression in RA-induced differentiation and G0 arrest, HL-60 cells were stably transfected with SLP-76. Expression of SLP-76 had no discernable effect on RA-induced ERK activation, subsequent functional differentiation, or the rate of RA-induced G0 arrest. To determine the effects of SLP-76 in the presence of a RA-regulated receptor, SLP-76 was stably transfected into HL-60 cells already overexpressing the colony stimulating factor-1 (CSF-1) receptor, c-FMS, from a previous stable transfection. SLP-76 now enhanced RA-induced ERK activation, compared to parental c-FMS transfectants. It also enhanced RA-induced differentiation, evidenced by enhanced paxillin expression, inducible oxidative metabolism and superoxide production. RA-induced RB tumor suppressor protein hypophosphorylation was also enhanced, as was RA-induced G0 cell cycle arrest. A triple Y to F mutant SLP-76 known to be a dominant negative in T-cell receptor signaling failed to enhance RA-induced paxillin expression, but enhanced RA-induced ERK activation, differentiation and G0 arrest essentially as well as wild-type SLP-76. Thus, SLP-76 overexpression in the presence of c-FMS, a RA-induced receptor, had the effect of enhancing RA-induced cell differentiation. This is the first indication to our knowledge that RA induces the expression of an adapter molecule to facilitate induced differentiation via co-operation between c-FMS and SLP-76.

Signalling pathways activated by all-trans-retinoic acid in acute promyelocytic leukemia cells

Acute promyelocytic leukemia is a form of acute myelogenous leukemia, characterized by the t(15;17) chromososmal translocation and the presence of the abnormal PML-RARalpha fusion protein. All-trans-retinoic acid is a potent agent for the treatment of this fatal subtype of AML, and is particularly effective when combined with cytotoxic chemotherapy. The important biological activities of all-trans-retinoic acid in vitro and in vivo have provoked extensive studies over the years, aimed to define the mechanisms by which it induces its antileukemic effects. It is now well established that all-trans-retinoic acid when administered at pharmacological doses can reverse the dominant-negative effects that the PML-RARalpha oncoprotein exhibits on the functions of the wild type PML and RARalpha proteins. All-trans-retinoic acid induces gene transcription via retinoic acid responsive elements (RARE) that are present in the promoters of retinoid-responsive genes that ultimately result in the production of protein products that regulate leukemic cell differentiation and induce cell-cycle arrest. There is now accumulating evidence that additional signalling pathways are activated during all-trans-retinoic acid-treatment of cells, involving Stat-proteins, tyrosine kinases and mitogen-activated protein (Map) kinases. This review summarizes the current knowledge on the signalling cascades activated by all-trans-retinoic acid in APL cells. The clinical implications and potential translational applications from the accumulating knowledge in the field are also discussed.

retinoic acid, bromodeoxyuridine, and the Delta 205 mutant polyoma virus middle T antigen regulate expression levels of a common ensemble of proteins associated with early stages of inducing HL-60 leukemic cell differentiation

Retinoic acid (RA), bromodeoxyuridine (BrdU), and the Delta 205 mutant polyoma middle T antigen affect the expression of a common ensemble of proteins in HL-60 human myeloblastic leukemia cells. Each of these agents is known to be able to prime HL-60 cells and accelerate subsequently induced myeloid or monocytic differentiation and G0 cell cycle arrest, suggesting that they have equal or identical cellular targets relevant to the early stages of inducing cell differentiation and G0 arrest. As a test of this possibility, a survey of protein expression changes induced by RA, BrdU, or Delta 205 transfection was performed. Retinoic acid induced numerous changes within h. Bromodeoxyuridine caused larger numbers of changes, whereas Delta 205 caused a more limited number. Among the hundreds of affected proteins detected, there were comparable numbers of up- or downregulated proteins. A small number changed between undetectable and detectable expression. The affected proteins were not restricted to a single functional class and included transcription factors, receptors, signaling molecules, cytoskeletal molecules, and effectors of various cellular processes such as deoxyribonucleic acid replication, transcription, and translation. The intersect of the sets of proteins affected by RA, BrdU, and Delta 205 was identified to determine if these agents regulated a common subset of proteins. This ensemble contained the commonly upregulated proteins AF6, ABP-280, ENC-1, ESE 1, MAP2B, NTF2, casein kinase, IRF1, SRPK2, Rb2, RhoGDI, P47phox, CD45, PKR, and SIIIp15. The commonly downregulated proteins were SHC, katanin, flotillin-2/ESA, EB 1, p43/EMAPIIprecursor, Jab1, FNK. The composition of the ensemble suggested three apparent themes for cellular processes that were affected early. The themes reflected the ultimate fate of the treated precursor cells as a mature myeloid cell, namely a cell whose hallmarks are (1) motility to migrate to a target and phagocytize it, (2) inducible oxidative metabolism to reduce the target with superoxide from a respiratory burst, and (3) biosynthetic slow down consistent with conversion from cell proliferation to quiescence. Interestingly, RA appears to induce aspects of an interferon-like response of potential significance as part of a biosynthetic slow down leading to cell cycle arrest. In conclusion, three biologically disparate ways to prime cells to differentiate were used to filter out a small ensemble of commonly regulated proteins that group as either microtubule associated, oxidative metabolism machinery, or effectors of cellular responses to interferon.

A novel retinoic acid-responsive element regulates retinoic acid-induced BLR1 expression

The mechanism of action of retinoic acid (RA) is of broad relevance to cell and developmental biology, nutrition, and cancer chemotherapy. RA is known to induce expression of the Burkitt's lymphoma receptor 1 (BLR1) gene which propels RA-induced cell cycle arrest and differentiation of HL-60 human myeloblastic leukemia cells, motivating the present analysis of transcriptional regulation of blr1 expression by RA. The RA-treated HL-60 cells used here expressed all RA receptor (RAR) and retinoid X receptor (RXR) subtypes (as detected by Northern analysis) except RXRgamma. Treatment with RAR- and RXR-selective ligands showed that RARalpha synergized with RXRalpha to transcriptionally activate blr1 expression. A 5'-flanking region capable of supporting RA-induced blr1 activation in HL-60 cells was found to contain a 205-bp sequence in the distal portion that was necessary for transcriptional activation by RA. Within this sequence DNase I footprinting revealed that RA induced binding of a nuclear protein complex to an element containing two GT boxes. Electromobility shift assays (EMSAs) and supershift assays showed that this element bound recombinant RARalpha and RXRalpha. Without RA there was neither complex binding nor transcriptional activation. Both GT boxes were needed for binding the complex, and mutation of either GT box caused the loss of transcriptional activation by RA. The ability of this cis-acting RAR-RXR binding element to activate transcription in response to RA also depended on downstream sequences where an octamer transcription factor 1 (Oct1) site and a nuclear factor of activated T cells (NFATc) site between this element and the transcriptional start, as well as a cyclic AMP response element binding factor (CREB) site between the transcriptional start and first exon of the blr1 gene, were necessary. Each of these sites bound its corresponding transcription factor. A transcription factor-transcription factor binding array analysis of nuclear lysate from RA-treated cells indicated several prominent RARalpha binding partners; among these, Oct1, NFATc3, and CREB2 were identified by competition EMSA and supershift and chromatin immunoprecipitation assays as components of the complex. RA upregulated expression of these three factors. In sum the results of the present study indicate that RA-induced expression of blr1 expression depends on a novel RA response element. This cis-acting element approximately 1 kb upstream of the transcriptional start consists of two GT boxes that bind RAR and RXR in a nuclear protein complex that also contains Oct1, NFATc3, and CREB2 bound to their cognate downstream consensus binding sites.

Optimization of nb-4 and hl-60 differentiation for use in opsonophagocytosis assays

Production of effective vaccine formulations is dependent on the availability of assays for the measurement of protective immune responses. The development and standardization of in vitro human cell-based assays for functional opsonophagocytic antibodies require critical evaluation and optimization of the preparation of cells for the assay. We report evaluation of a number of protocols with two continuous cell lines (NB-4 and HL-60) for the provision of differentiated cells for use in functional assays. Flow cytometric analysis of CD11b antigen expression, as a marker of differentiation, indicated that all-trans-retinoic acid (ATRA) gave improved differentiation (>80% of cells differentiated at 96 h) when compared with dimethylformamide (DMF) (<60% of cells differentiated at 96 h). Morphological changes during differentiation toward a neutrophil-like phenotype were assessed by scanning electron microscopy. HL-60 and NB-4 cells treated with ATRA showed more spreading and flattening than cells treated with DMF, further evidence that they may have achieved a more differentiated phenotype. The number of cell divisions in culture appeared to be critical because cell lines maintained in exponential growth for >40 passages failed to express CD11b antigen or show morphological changes associated with differentiation after exposure to either differentiation-inducing reagent. Late-passage cells also demonstrated increased tolerance to DMF. Our results indicated that ATRA supplemented with vitamin D(3) and granulocyte colony-stimulating factor affords robust, rapid, and reproducible differentiation of both cell types.

Ectopic expression of CXCR5/BLR1 accelerates retinoic acid- and vitamin D(3)-induced monocytic differentiation of U937 cells

The product of the blr1 gene is a CXC chemokine receptor (CXCR5) that regulates B lymphocyte migration and has been implicated in myelomonocytic differentiation. The U937 human leukemia cell line was used to study the role of blr1 in retinoic acid-regulated monocytic leukemia cell growth and differentiation. blr1 mRNA expression was induced within 12 hr by retinoic acid in U937 cells. To determine whether the early induction of blr1 might regulate inducible monocytic cell differentiation, U937 cells were stably transfected with blr1 (U937/blr1 cells). Ectopic expression of blr1 caused no significant cell cycle or differentiation changes, but caused the U937/blr1 cells to differentiate faster when treated with either retinoic acid or 1alpha,25-dihydroxyvitamin D(3). Treated with retinoic acid, U937/blr1 cells showed a greater increase in the percentage of CD11b expressing cells than vector control cells. Retinoic acid also induced a higher percentage of functionally differentiated blr1 transfectants as assessed by nitroblue tetrazolium reduction. U937/blr1 cells underwent moderate growth inhibition on treatment with retinoic acid. Similar results occurred with 1alpha,25-dihydroxyvitamin D(3). Because blr1 was induced early during cell differentiation and because its overexpression accelerated monocytic differentiation, it may be important for signals controlling cell differentiation.

Retinoic acid causes MEK-dependent RAF phosphorylation through RARalpha plus RXR activation in HL-60 cells

Retinoic acid (RA) is known to cause the myeloid differentiation of HL-60 human myeloblastic leukemia cells in a process requiring MEK-dependent ERK2 activation. This RA-induced ERK2 activation appears after approximately 4 h and persists until the cells are differentiated and G0 arrested (Yen et al, 1998). This motivates the question of whether RA also activated RAF as part of a typical RAF/MEK/MAPK cascade. Retinoic acid is shown here to also increase the phosphorylation of RAF, but in an unusual way. Surprisingly, increased RAF phosphorylation is first detectable after 12 to 24 hours by phosphorylation-induced retardation of polyacrylamide gel electrophoretic mobility. The RA-induced increased RAF phosphorylation is still apparent after 72 hours of treatment when most cells are differentiated and G0 arrested. There is a progressive dose-response relationship with 10(-8), 10(-7), and 10(-6) M RA. The RA-induced RAF phosphorylation corresponds to increased in vitro kinase activity. Inhibition of MEK with a PD98059 dose which inhibits ERK2 phosphorylation and subsequent cell differentiation also inhibits RAF phosphorylation. RA-induced MEK-dependent RAF phosphorylation is not due to changes in the amount of cellular MEK. The induced RAF phosphorylation, as well as anteceding ERK2 activation, depends on ligand-induced activation of both an RARalpha receptor and an RXR receptor. This and the slow kinetics of activation suggest a need for prior RA-induced gene expression. In summary, RA induces a MEK-dependent prolonged RAF activation, whose slow onset occurs after ERK2 activation but still well before cell cycle arrest and cell differentiation. The RA-induced increased RAF phosphorylation thus differs from typical mitogenic growth factor signaling, features that may contribute to cell cycle arrest and differentiation instead of division as the cellular outcome.

Polyomavirus small t antigen prevents retinoic acid-induced retinoblastoma protein hypophosphorylation and redirects retinoic acid-induced G0 arrest and differentiation to apoptosis

Polyomavirus small t antigen (ST) impedes late features of retinoic acid (RA)-induced HL-60 myeloid differentiation as well as growth arrest, causing apoptosis instead. HL-60 cells were stably transfected with ST. ST slowed the cell cycle, retarding G2/M in particular. Treated with RA, the ST transfectants continued to proliferate and underwent apoptosis. ST also impeded the normally RA-induced hypophosphorylation of the retinoblastoma tumor suppressor protein consistent with failure of the cells to arrest growth. The RA-treated transfectants expressed CD11b, an early cell surface differentiation marker, but inducible oxidative metabolism, a later and more mature functional differentiation marker, was largely inhibited. Instead, the cells underwent apoptosis. ST affected significant known components of RA signaling that result in G0 growth arrest and differentiation in wild-type HL-60. ST increased the basal amount of activated ERK2, which normally increases when wild-type cells are treated with RA. ST caused increased RARalpha expression, which is normally down regulated in RA-treated wild-type cells. The effects of ST on RA-induced myeloid differentiation did not extend to monocytic differentiation and G0 arrest induced by 1,25-dihydroxy vitamin D3, whose receptor is also a member of the steroid-thyroid hormone superfamily. In this case, ST abolished the usually induced G0 arrest and retarded, but did not block, differentiation without inducing apoptosis, thus uncoupling growth arrest and differentiation. In sum, the data show that ST disrupted the normal RA-induced program of G0 arrest and differentiation, causing the cells to abort differentiation and undergo apoptosis.

Retinoic acid-induced blr1 expression requires RARalpha, RXR, and MAPK activation and uses ERK2 but not JNK/SAPK to accelerate cell differentiation

Upstream signaling requirements of retinoic acid (RA)-induced blr1 expression and downstream signaling consequences of blr1 over-expression in a human myeloid leukemia cell line demonstrate that mitogen-activated protein kinase (MAPK) signaling complexes are involved in both avenues. RA-induced myeloid differentiation and G1/G0 growth arrest of HL-60 cells is known to require the activation of the RARalpha and RXR retinoid receptors, as well as activation of the MAPK, ERK2. Transcriptional activation of the Burkitt's lymphoma receptor 1 (blr1) gene occurs early during RA-induced differentiation of HL-60 cells and requires these same three activating processes. The use of retinoid ligands that activate either the RARalpha or the RXR retinoid receptors revealed that blr1 mRNA induction was detectable only when both RARalpha and RXR were activated. Neither the RARalpha nor RXR selective ligands alone induced expression of blr1, but the combination of the two ligands induced the expression of blr1 to the same extent as RA. The MAPKK (MEK) inhibitor, PD98059, was used to determine whether extracellular signal-regulated kinase (ERK2) activation was necessary for induction of blr1 mRNA. PD98059 inhibited induced blr1 mRNA expression, due to RA or activated RARalpha plus RXR ligands, indicating that ERK2 activation is necessary for blr1 mRNA expression. Previous studies showed that ectopic expression of blr1 also caused increased MAPK activation, in particular ERK2, and subsequently accelerated RA-induced differentiation and G1/G0 growth arrest. Inhibition of ERK2 activation inhibited differentiation of blr1 transfectants, suggesting that the accelerated differentiation reflected blr1-enhanced ERK2 activation. The present data also demonstrate that ectopic expression of blr1 increased JNK/SAPK activity, but JNK/ SAPK activation was not needed for accelerated RA-induced differentiation and growth arrest. The results show that the signals known to be required for HL-60 differentiation, activated RARalpha, RXR, and ERK2, are necessary for blr1 mRNA expression. Downstream consequences of blr1 overexpression include enhanced MAPK signaling.

Differential display as an approach to study differentiation and differentiation therapy in AML

Different subgroups of acute myeloid leukemia (AML) can be defined by the specific non-random chromosomal translocation that is present within the abnormal cell types. In one type of AML, acute promyelocytic leukemia (APL), the block in the normal process of differentiation can be circumvented by the addition of a chemical inducer, in this case retinoic acid. This is due to the defect in APL affecting the retinoic acid receptor gene. This type of therapy has become known as differentiation therapy. However, most types of leukemia do not respond to the retinoic acid, and therefore methods of differentiation therapy need to be developed by targeting other genes involved in the leukemia process. This requires the molecular characterizations of the genes that are expressed during differentiation and in particular those genes that show a differential expression in inducer sensitive cells and those resistant to induced differentiation. Therefore, therapeutic agents could be developed to specifically target these genes. This article describes how the technique of differential display, as one of several possible methods of molecular screening, may allow the identification of genes which can be targeted to induce differentiation.