Platelet-Derived Growth Factor Receptor Regulates Myeloid and Monocytic Differentiation of HL-60 Cells

Dissecting the multifactorial nature of demyelinating disease

Chondroitin sulfate proteoglycan-4 (CSPG4) is a surface component of two key cell types (oligodendrocyte progenitor cells (OPCs) and myeloid cells) present in lysolecithin-induced lesions in mouse spinal cord. Two types of CSPG4 manipulations have been used to study the roles of these cells in myelin damage and repair: (1) OPC and myeloid-specific ablation of CSPG4, and (2) transplantation of enhanced green fluorescent protein (EGFP)-labeled progenitors to distinguish between bone marrow-derived macrophages and resident microglia. Ablation of CSPG4 in OPCs does not affect myelin damage, but decreases myelin repair, due to reduced proliferation of CSPG4-null OPCs that diminishes generation of mature oligodendrocytes for remyelination. Ablation of CSPG4 in myeloid cells greatly decreases recruitment of macrophages to spinal cord lesions, resulting in smaller initial lesions, but also in significantly diminished myelin repair. In the absence of macrophage recruitment, OPC proliferation is greatly impaired, again leading to decreased generation of myelinating oligodendrocytes. Macrophages may promote OPC proliferation via phagocytosis of myelin debris and/or secretion of factors that stimulate OPC mitosis. Microglia are not able to substitute for macrophages in promoting OPC proliferation. An additional feature of lesions in myeloid-specific CSPG4 null mice is the persistence of poorly-differentiated platelet-derived growth factor receptor α (PDGFRα)⁺ macrophages that may prolong damage.

Probing the requirement for CD38 in retinoic acid-induced HL-60 cell differentiation with a small molecule dimerizer and genetic knockout

CD38 is an ectoenzyme and receptor with key physiological roles. It metabolizes NAD⁺ to adenosine diphosphate ribose (ADPR) and cyclic ADPR, regulating several processes including calcium signalling. CD38 is both a positive and negative prognostic indicator in leukaemia. In all-trans retinoic acid (RA)-induced differentiation of acute promyelocytic leukaemia and HL-60 cells, CD38 is one of the earliest and most prominently upregulated proteins known. CD38 overexpression enhances differentiation, while morpholino- and siRNA-induced knockdown diminishes it. CD38, via Src family kinases and adapters, interacts with a MAPK signalling axis that propels differentiation. Motivated by evidence suggesting the importance of CD38, we sought to determine whether it functions via dimerization. We created a linker based on the suicide substrate arabinosyl-2′-fluoro-2′-deoxy NAD⁺ (F-araNAD⁺), dimeric F-araNAD⁺, to induce homodimerization. CD38 homodimerization did not affect RA-induced differentiation. Probing the importance of CD38 further, we created HL-60 cell lines with CRISPR/Cas9-mediated CD38 truncations. Deletion of its enzymatic domain did not affect differentiation. Apart from increased RA-induced CD11b expression, ablation of all but the first six amino acids of CD38 affected neither RA-induced differentiation nor associated signalling. Although we cannot discount the importance of this peptide, our study indicates that CD38 is not necessary for RA-induced differentiation.

Distinct NG2 proteoglycan-dependent roles of resident microglia and bone marrow-derived macrophages during myelin damage and repair

We used a bone marrow transplantation approach to distinguish the activities of bone marrow-derived macrophages from the activities of central nervous system-resident microglia in phenomena associated with axon demyelination and remyelination. We transplanted wild type or germline NG2 null beta-actin-EGFP expressing bone marrow into irradiated wild type or NG2 null recipient mice, followed by analysis of lysolecithin-induced spinal cord demyelination and remyelination and quantification of Iba-1+/ F4/80+/ EGFP+ macrophages and Iba-1+/ F4/80+/ EGFP- microglia. One week after microinjection of 1% lysolecithin into the spinal cord, wild type recipients receiving NG2 null bone marrow exhibit greatly reduced infiltration of macrophages into lesions, compared to wild type recipients receiving wild type bone marrow. Wild type bone marrow recipients also exhibit larger numbers of demyelinated axons than NG2 null recipients, indicative of macrophage participation in the initial myelin damage. However, wild type bone marrow recipients also exhibit superior myelin repair at 6 weeks post-injury, compared to NG2 null bone marrow recipients, demonstrating the additional importance of macrophages in remyelination. Incompletely repaired lesions in NG2 null bone marrow recipients at 6 weeks post-injury retain elevated numbers of macrophages, in contrast to lower numbers of macrophages in more completely repaired lesions in wild type bone marrow recipients. This suggests that NG2 expression renders macrophages more effective in myelin repair and less likely to promote chronic inflammation. Effective macrophage involvement in myelin repair is due in part to effects on the proliferation and/or recruitment of oligodendrocyte progenitor cells. Reduced numbers of oligodendrocyte progenitors are seen in lesions in NG2 null bone marrow recipients, likely due to deficits in macrophage production of oligodendrocyte progenitor-relevant mitogens and in phagocytosis of inhibitory myelin debris. Microglia also appear to be important for clearance of myelin debris, as indicated by reduced phagocytosis in NG2 null recipients receiving wild type bone marrow.

Vitamin D Analogs Potentiate the Antitumor Effect of Imatinib Mesylate in a Human A549 Lung Tumor Model

In previous papers, we presented data on studies on the anticancer activity of the vitamin D₃ analogs, named PRI-2191 and PRI-2205, in different cancer models. In this study, we showed the improved antiproliferative activity of a combination of imatinib mesylate (Gleevec, GV) and cytostatic agents in in vitro studies, when used with a third compound, namely PRI-2191, in an A549 human lung cancer model. Furthermore, we analyzed the influence of both PRI-2191, as well as PRI-2205 on the anticancer activity of GV in mice bearing A549 tumors. The route of PRI-2191 analog administration showed a significant impact on the outcome of GV treatment: subcutaneous injection was more efficient and less toxic than oral gavage. Moreover, both vitamin D compounds increased the anticancer activity of GV; however, they might also potentiate some adverse effects. We also evaluated in tumor tissue the expression of VEGF, PDGF-BB, vitamin D receptor, CYP27B1, CYP24, p53 and Bcl-2, as well as PDGF receptors: α and β. We observed the upregulation of p53 expression and the downregulation of Bcl-2, as well as VEGF in A549 tumors as a result of the tested treatment. However, vitamin D analogs did not significantly influence the expression of these proteins.

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.

Optimization of potent DFG-in inhibitors of platelet derived growth factor receptorβ (PDGF-Rβ) guided by water thermodynamics

In this study we report on the hit optimization of substituted 3,5-diaryl-pyrazin-2(1H)-ones toward potent and effective platelet-derived growth factor receptor (PDGF-R) β-inhibitors. Originally, the 3,5-diaryl-pyrazin-2-one core was derived from the marine sponge alkaloid family of hamacanthins. In our first series compound 2 was discovered as a promising hit showing strong activity against PDGF-Rβ in the kinase assay (IC50 = 0.5 μM). Furthermore, 2 was shown to be selective for PDGF-Rβ in a panel of 24 therapeutically relevant protein kinases. Molecular modeling studies on a PDGF-Rβ homology model using prediction of water thermodynamics suggested an optimization strategy for the 3,5-diaryl-pyrazin-2-ones as DFG-in binders by using a phenolic OH function to replace a structural water molecule in the ATP binding site. Indeed, we identified compound 38 as a highly potent inhibitor with an IC50 value of 0.02 μM in a PDGF-Rβ enzymatic assay also showing activity against PDGF-R dependent cancer cells.

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.

Marine derived hamacanthins as lead for the development of novel PDGFRβ protein kinase inhibitors

In this study, we report on pyrazin-2(1H)-ones as lead for the development of potent adenosine triphosphate (ATP) competitive protein kinase inhibitors with implications as anti-cancer drugs. Initially, we identified the pyrazin-2(1H)-one scaffold from hamacanthins (deep sea marine sponge alkaloids) by Molecular Modeling studies as core binding motif in the ATP pocket of receptor tyrosine kinases (RTK), which are validated drug targets for the treatment of various neoplastic diseases. Structure-based design studies on a human RTK member PDGFR (platelet-derived growth factor receptor) suggested a straight forward lead optimization strategy. Accordingly, we focused on a Medicinal Chemistry project to develop pyrazin-2(1H)-ones as optimized PDGFR binders. In order to reveal Structure-Activity-Relationships (SAR), we established a flexible synthetic route via microwave mediated ring closure to asymmetric 3,5-substituted pyrazin-2(1H)-ones and produced a set of novel compounds. Herein, we identified highly potent PDGFR binders with IC₅₀ values in an enzymatic assay below µM range, and possessing significant activity against PDGFR dependent cancer cells. Thus, marine hamacanthin-derived pyrazin-2(1H)-ones showing interesting properties as lead for their further development towards potent PDGFR-inhibitors.

6-Formylindolo (3,2-b)carbazole (FICZ) enhances retinoic acid (RA)-induced differentiation of HL-60 myeloblastic leukemia cells

BACKGROUND

The aryl hydrocarbon receptor (AhR) ligand 6-Formylindolo(3,2-b)carbazole (FICZ) has received increasing attention since its identification as an endogenous AhR ligand and a photoproduct of tryptophan. FICZ and its metabolites have been detected in human fluids. We recently reported that AhR promotes retinoic acid (RA)-induced granulocytic differentiation of HL-60 myeloblastic leukemia cells by restricting the nuclear abundance of the stem cell associated transcription factor Oct4. The standard clinical management of acute promyelocytic leukemia (APL) is differentiation induction therapy using RA. But RA is not effective for other myeloid leukemias, making the mechanism of RA-induced differentiation observed in a non-APL myeloid leukemia of interest. To our knowledge, this is the first study regarding the influence of FICZ on RA-induced differentiation in any type of leukemic blasts.

METHODS

Using flow cytometry and Western blotting assays, we determined the effects of FICZ on RA-induced differentiation of HL-60 human leukemia cells. All experiments were performed in triplicate. The groups RA and FICZ + RA were compared using the Paired-Samples T-Test. Western blot figures present the typical blots.

RESULTS

We demonstrate that FICZ enhances RA-induced differentiation, assessed by the expression of the membrane differentiation marker CD11b; cell cycle arrest; and the functional differentiation marker, inducible-oxidative metabolism. FICZ causes changes in signaling events that are known to drive differentiation, and notably augments the RA-induced sustained activation of the RAF/MEK/ERK axis of the mitogen-activated protein kinase (MAPK) cascade. FICZ also augments expression of the known MAPK signaling regulatory molecules c-Cbl, VAV1, pY458 p85 PI3K, Src-family kinases (SFKs), and IRF-1, a transcription factor associated with this putative signalsome that promotes RA-induced differentiation. Moreover, FICZ in combination with RA also increases expression of AhR and even more so of both Cyp1A2 and p47phox, which are known to be transcriptionally regulated by AhR. pY1021 PDGFRβ, a marker associated with retinoic acid syndrome was also increased.

CONCLUSIONS

Our data suggest that FICZ modulates intracellular signaling pathways and enhances RA-induced differentiation.

The in-vitro antiproliferative effect of PRI-2191 and imatinib applied in combined treatment with cisplatin, idarubicin, or docetaxel on human leukemia cells

Imatinib mesylate (Gleevec, STI571) is a specific inhibitor of the Bcr/Abl fusion tyrosine kinase that exhibits potent antileukemic effects in chronic myelogenous leukemia. Bcr/Abl-positive K562 and Bcr/Abl-negative HL-60 human leukemia cells were used to investigate the effect of PRI-2191, a calcitriol analog, on the biological effects of imatinib combined with other anticancer drugs. The results show that PRI-2191 enhances the antiproliferative effect of imatinib on HL-60 cells. When these two agents together are applied with either docetaxel or cisplatin, but not with idarubicin, the antiproliferative effect could still be enhanced. Moreover, when the interaction between the chemotherapy agents was antagonistic or additive, PRI-2191 could even shift it to synergism. This effect correlated with an accumulation of HL-60 cells in the G0/G1 phase of the cell cycle and a decrease in the percentage of cells in the G2/M and S stage in the ternary combinations used. PRI-2191 did not influence apoptosis induced by imatinib alone or in ternary combinations with all the chemotherapy agents used. These results may suggest that the stronger antiproliferative effect of the combined treatment with PRI-2191 on HL-60 cells is related to cell cycle arrest rather than to the induction of apoptosis.

Activation of the aryl hydrocarbon receptor AhR Promotes retinoic acid-induced differentiation of myeloblastic leukemia cells by restricting expression of the stem cell transcription factor Oct4

Retinoic acid (RA) is used to treat leukemia and other cancers through its ability to promote cancer cell differentiation. Strategies to enhance the anticancer effects of RA could deepen and broaden its beneficial therapeutic applications. In this study, we describe a receptor cross-talk system that addresses this issue. RA effects are mediated by RAR/RXR receptors that we show are modified by interactions with the aryl hydrocarbon receptor (AhR), a protein functioning both as a transcription factor and a ligand-dependent adaptor in an ubiquitin ligase complex. RAR/RXR and AhR pathways cross-talk at the levels of ligand-receptor and also receptor-promoter interactions. Here, we assessed the role of AhR during RA-induced differentiation and a hypothesized convergence at Oct4, a transcription factor believed to maintain stem cell characteristics. RA upregulated AhR and downregulated Oct4 during differentiation of HL-60 promyelocytic leukemia cells. AhR overexpression in stable transfectants downregulated Oct4 and also decreased ALDH1 activity, another stem cell-associated factor, enhancing RA-induced differentiation as indicated by cell differentiation markers associated with early (CD38 and CD11b) and late (neutrophilic respiratory burst) responses. AhR overexpression also increased levels of activated Raf1, which is known to help propel RA-induced differentiation. RNA interference-mediated knockdown of Oct4 enhanced RA-induced differentiation and G(0) cell-cycle arrest relative to parental cells. Consistent with the hypothesized importance of Oct4 downregulation for differentiation, parental cells rendered resistant to RA by biweekly high RA exposure displayed elevated Oct4 levels that failed to be downregulated. Together, our results suggested that therapeutic effects of RA-induced leukemia differentiation depend on AhR and its ability to downregulate the stem cell factor Oct4.

ATRA-induced HL-60 myeloid leukemia cell differentiation depends on the CD38 cytosolic tail needed for membrane localization, but CD38 enzymatic activity is unnecessary

Leukocyte antigen CD38 expression is an early marker of all-trans retinoic acid (ATRA) stimulated differentiation in the leukemic cell line HL-60. It promotes induced myeloid maturation when overexpressed, whereas knocking it down is inhibitory. It is a type II membrane protein with an extracellular C-terminal enzymatic domain with NADase/NADPase and ADPR cyclase activity and a short cytoplasmic N-terminal tail. Here we determined whether CD38 enzymatic activity or the cytoplasmic tail is required for ATRA-induced differentiation. Neither a specific CD38 ectoenzyme inhibitor nor a point mutation that cripples enzymatic activity (CD38 E226Q) diminishes ATRA-induced differentiation or G1/0 arrest. In contrast a cytosolic deletion mutation (CD38 Δ11-20) prevents membrane expression and inhibits differentiation and G1/0 arrest. These results may be consistent with disrupting the function of critical molecules necessary for membrane-expressed CD38 signal transduction. One candidate molecule is the Src family kinase Fgr, which failed to undergo ATRA-induced upregulation in CD38 Δ11-20 expressing cells. Another is Vav1, which also showed only basal expression after ATRA treatment in CD38 Δ11-20 expressing cells. Therefore, the ability of CD38 to propel ATRA-induced myeloid differentiation and G1/0 arrest is unimpaired by loss of its ectoenzyme activity. However a cytosolic tail deletion mutation disrupted membrane localization and inhibited differentiation. ATRA-induced differentiation thus does not require the CD38 ectoenzyme function, but is dependent on a membrane receptor function.

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.

c-Cbl interacts with CD38 and promotes retinoic acid-induced differentiation and G0 arrest of human myeloblastic leukemia cells

Retinoic acid (RA) is known to regulate cell growth and differentiation. In HL-60 human myeloblastic leukemia cells, it causes mitogen-activated protein kinase (MAPK) signaling leading to myeloid differentiation and G(0) cell cycle arrest. This communication reports that expression of the Cbl adaptor caused enhanced extracellular signal-regulated kinase 2 activation and promoted RA-induced differentiation and G(0)-arrest. Stable transfectants ectopically expressing c-Cbl underwent myeloid differentiation faster than wild-type (wt) cells when treated with RA. In contrast, c-Cbl knockdown stable transfectants differentiated slower than wt cells when treated with RA. Cells ectopically expressing c-Cbl had enhanced CD38 expression when treated with RA, and cells ectopically expressing CD38 had enhanced c-Cbl expression, even without with RA, suggesting an interaction between c-Cbl and CD38. Fluorescence resource energy transfer and coimmunoprecipitation showed that c-Cbl and CD38 bind each other. RA causes the gradual down-regulation and eventual loss of c-Cbl expression, resulting in loss of the Cbl-CD38 interaction, suggesting that c-Cbl plays a relatively early role in promoting RA-induced differentiation. RA-induced differentiation can thus be propelled by c-Cbl and by CD38, both of which bind together, enhance the expression of each other, and cause MAPK signaling. There thus seems to be a cooperative role for c-Cbl and CD38, reflected in their direct binding, in propulsion of RA-induced differentiation.

Human dendritic cell line models for DC differentiation and clinical DC vaccination studies

Dendritic cells (DC) are increasingly applied in the immunotherapy of cancer. As the development of a standardized DC vaccine product is often hampered by the limited availability of DC precursors and inter- and intra-donor variability, and the preparation of individual vaccines is labor-intensive, it would be preferable to use DC from a readily available and unlimited source, such as cell lines can provide. It has been described that leukemia-derived cell lines are able to differentiate into functional DC, creating possibilities for the development of highly reproducible DC vaccines and providing in vitro model systems for in-depth studies about DC physiology. This review discusses the different human DC cell line differentiation models described so far. Based on the available data, characteristics that determine the ability of leukemia cells to differentiate along the different precursor stages into functional DC will be formulated. In addition, evidence will be provided that the human CD34+ acute myeloid leukemia cell line MUTZ-3 provides DC that exhibit the functional properties that are crucial for the in vivo generation of CTL-mediated immunity and thus, currently, represents the most valuable, sustainable model system for myeloid DC differentiation and clinical DC vaccination studies.

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.

Inhibiting the platelet derived growth factor receptor increases signs of retinoic acid syndrome in myeloid differentiated HL-60 cells

PDGFR inhibitors are successfully used in a number of cancer treatments. The standard treatment for acute promyelocytic leukemia (APL) involves differentiation therapy with retinoic acid (RA). However, the relapse rates are significant. In the present work we evaluated the effects of RA therapy in the presence of PDGFR inhibitor, AG1296. Adding AG1296 with RA increased secretion of TNF-alpha, IL-8, and MMP-9 expression. This treatment induced higher levels of ICAM-1 endothelial cell expression, and increased cellular mobility. Inhibiting PDGFR enhanced RA-induced expression of integrin. Integrin ligand increased differentiation markers CD11b, inducible oxidative metabolism and PDGFR-beta phosphorylation. While the neutrophil-endothelial cell interactions are strengthened by the combined treatment, the endothelium-substratum interactions are weakened, a situation common in RAS.

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.