Retinoic acid-induced CD38 expression in HL-60 myeloblastic leukemia cells regulates cell differentiation or viability depending on expression levels

Vacuolin-1 enhances RA-induced differentiation of human myeloblastic leukemia cells: evidence for involvement of a CD11b/FAK/LYN/SLP-76 axis subject to endosomal regulation that drives late differentiation steps

BACKGROUND

Retinoic acid(RA), an embryonic morphogen, regulates cell differentiation. Endocytosis regulates receptor signaling that governs such RA-directed cellular processes. Vacuolin-1 is a small molecule that disrupts endocytosis, motivating interest in its effect on RA-induced differentiation/arrest. In HL-60 myeloblastic-leukemia cells, RA causes differentiation evidenced by a progression of cell-surface and functional markers, CD38, CD11b, and finally reactive oxygen species(ROS) production and G1/0 cell cycle arrest in mature cells.

RESULTS

We found that Vacuolin-1 enhanced RA-induced CD11b, ROS and G1/0 arrest, albeit not CD38. Enhanced CD11b expression was associated with enhanced activation of Focal Adhesion Kinase(FAK). Adding vacuolin-1 enhanced RA-induced tyrosine phosphorylation of FAK, Src Family Kinases(SFKs), and the adaptor protein, SLP-76, expression of which is known to drive RA-induced differentiation. Depleting CD11b cripples late stages of progressive myeloid differentiation, namely G1/0 arrest and inducible ROS production, but not expression of CD38. Loss of NUMB, a protein that supports early endosome maturation, affected RA-induced ROS and G1/0 arrest, but not CD38 expression.

CONCLUSION

Hence there appears to be a novel CD11b/FAK/LYN/SLP-76 axis subject to endosome regulation which contributes to later stages of RA-induced differentiation. The effects of vacuolin-1 thus suggest a model where RA-induced differentiation consists of progressive stages driven by expression of sequentially-induced receptors.

Targeting CD38 for acute leukemia

Acute leukemia (AL) is a hematological malignancy, and the prognosis of most AL patients hasn’t improved significantly, particularly for relapsed or refractory (R/R) AL. Therefore, new treatments for R/R adult acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) are urgently necessary. Novel developments have been made in AL treatment, including target and immune therapies. CD38 is one of the targets due to its high expression in many hematological malignancies, including multiple myeloma, ALL and a subset of AML. Consequently, targeting CD38 therapies, including CD38 monoclonal antibodies (mAbs), bispecific antibodies, and CAR-T cell therapy, exhibit promising efficacy in treating multiple myeloma without significant toxicity and are being explored in other hematological malignancies and nonhematological diseases. Herein, this review focuses on targeting CD38 therapies in ALL and AML, which demonstrate sound antileukemic effects in acute leukemia and are expected to become effective treatment methods.

Dissecting the novel partners of nuclear c-Raf and its role in all-trans retinoic acid (ATRA)-induced myeloblastic leukemia cells differentiation

All-trans retinoic acid (ATRA) is an anti-cancer differentiation therapy agent effective for acute promyelocytic leukemia (APL) but not acute myeloid leukemia (AML) in general. Using the HL-60 human non-APL AML model where ATRA causes nuclear enrichment of c-Raf that drives differentiation and G1/G0 cell cycle arrest, we now observe that c-Raf in the nucleus showed novel interactions with several prominent regulators of the cell cycle and cell differentiation. One is cyclin-dependent kinase 2 (Cdk2). ATRA treatment caused c-Raf to dissociate from Cdk2. This was associated with enhanced binding of Cdk2 with retinoic acid receptor α (RARα). Consistent with this novel Raf/CDK2/RARα axis contributing to differentiation, CD38 expression per cell, which is transcriptionally regulated by a retinoic acid response element (RARE), is enhanced. The RB tumor suppressor, a fundamental regulator of G1 cell cycle progression or arrest, was also targeted by c-Raf in the nucleus. RB and specifically the S608 phosphorylated form (pS608RB) complexed with c-Raf. ATRA treatment induced S608RB-hypophosphorylation associated with G1/G0 cell cycle arrest and release of c-Raf from RB. We also found that nuclear c-Raf interacted with SMARCD1, a pioneering component of the SWI/SNF chromatin remodeling complex. ATRA treatment diminished the amount of this protein bound to c-Raf. The data suggest that ATRA treatment to HL-60 human cells re-directed c-Raf from its historically pro-proliferation functions in the cytoplasm to pro-differentiation functions in the nucleus.

Efficacy of All-Trans-Retinoic Acid in High-Risk Acute Myeloid Leukemia with Overexpression of EVI1

Introduction

EVI1 (MECOM)-positive acute myeloid leukemia (AML) cells have shown in vitro sensitivity to all-trans-retinoic acid (ATRA) by inducing differentiation, cell death, and decreased leukemic engraftment.

Methods

In this pilot study, we investigated the response to ATRA in 13 high-risk AML patients with overexpression of EVI1.

Results

Seven of the 13 patients (53.8%) achieved complete remission (CR), and response can be combined with a decreased of the leukemia stem cell pool.

Conclusion

These primary results tend to confirm in vitro results and suggest that addition of ATRA might be of benefit in the treatment of patients with EVI1-positive AML.

Roscovitine enhances All-trans retinoic acid (ATRA)-induced leukemia cell differentiation: Novel effects on signaling molecules for a putative Cdk2 inhibitor

All-trans retinoic acid (ATRA)-based differentiation therapy has been unsuccessful in treating t(15;17) negative acute myeloid leukemia (AML) patients, motivating interest in combination therapies using ATRA plus other agents. Using the t (15, 17) negative HL-60 human myeloblastic leukemia model, we find that the cyclin-dependent kinase (CDK) inhibitor, roscovitine, augments signaling by an ATRA-induced macromolecular signalsome that propels differentiation and enhances ATRA-induced differentiation. Roscovitine co-treatment enhanced ATRA-induced expression of pS259- pS289/296/301- pS621-c-Raf, pS217/221-Mek, Src Family Kinases (SFKs) Lyn and Fgr and SFK Y416 phosphorylation, adaptor proteins c-Cbl and SLP-76, Vav, and acetylated 14-3-3 in the signalsome. Roscovitine enhanced ATRA-induced c-Raf interaction with Lyn, Vav, and c-Cbl. Consistent with signalsome hyper-activation, roscovitine co-treatment enhanced ATRA-induced G1/0 arrest and expression of differentiation markers, CD11b, ROS and p47 Phox. Because roscovitine regulated Lyn expression, activation and partnering, a stably transfected Lyn knockdown was generated from wt-parental cells to investigate its function in ATRA-induced differentiation. Lyn-knockdown enhanced ATRA-induced up-regulation of key signalsome molecules, c-Raf, pS259-c-Raf, pS289/296/301-c-Raf, Vav1, SLP-76, and Fgr, but with essentially total loss of pY416-SFK. Compared to ATRA-treated wt-parental cells, differentiation markers p47 phox, CD11b, G1/G0 arrest and ROS production were enhanced in ATRA-treated Lyn-knockdown stable transfectants, and addition of roscovitine further enhanced these ATRA-inducible markers. The Lyn-knockdown cells expressed slightly higher c-Raf, pS259-c-Raf, pS289/296/301-c-Raf, and SLP-76 than wt-parental cells, and this was associated with enhanced ATRA-induced upregulation of Fgr and cell differentiation, consistent with heightened signaling, suggesting that enhanced Fgr may have compensated for loss of Lyn to enhance differentiation in the Lyn-knockdown cells.

Retinoic acid and 6-formylindolo(3,2-b)carbazole (FICZ) combination therapy reveals putative targets for enhancing response in non-APL AML

In non-acute promyelotic leukemia (APL)- non myelocytic leukemia (AML), identification of a signaling signature would predict potentially actionable targets to enhance differentiation effects of all-trans-retinoic acid (RA) and make combination differentiation therapy realizable. Components of such a signaling machine/signalsome found to drive RA-induced differentiation discerned in a FAB M2 cell line/model (HL-60) were further characterized and then compared against AML patient expression profiles. FICZ, known to enhance RA-induced differentiation, was used to experimentally augment signaling for analysis. FRET revealed novel signalsome protein associations: CD38 with pS376SLP76 and caveolin-1 with CD38 and AhR. The signaling molecules driving differentiation in HL-60 cluster in non-APL AML de novo samples, too. Pearson correlation coefficients for this molecular ensemble are nearer 1 in the FAB M2 subtype than in non-APL AML. SLP76 correlation to RXRα and p47phox were conserved in FAB M2 model and patient subtype but not in general non-APL AML. The signalsome ergo identifies potential actionable targets in AML.

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.

Src family kinase inhibitor bosutinib enhances retinoic acid-induced differentiation of HL-60 leukemia cells

The acute promyelocytic leukemia (APL) has been treated with all-trans retinoic acid (RA) for decades. While RA has largely been ineffective in non-APL AML subtypes, co-treatments combining RA and other agents are currently in clinical trials. Using the RA-responsive non-APL AML cell line HL-60, we tested the efficacy of the Src family kinase (SFK) inhibitor bosutinib on RA-induced differentiation. HL-60 has been recently shown to bear fidelity to a subtype of AML that respond to RA. We found that co-treatment with RA and bosutinib enhanced differentiation evidenced by increased CD11b expression, G₁/G₀ cell cycle arrest, and respiratory burst. Expression of the SFK members Fgr and Lyn was enhanced, while SFK activation was inhibited. Phosphorylation of several sites of c-Raf was increased and expression of AhR and p85 PI3K was enhanced. Expression of c-Cbl and mTOR was decreased. Our study suggests that SFK inhibition enhances RA-induced differentiation and may have therapeutic value in non-APL AML.

Synergistic and additive effect of retinoic acid in circumventing resistance to p53 restoration

TP53 mutations occur in ∼50% of all human tumors, with increased frequency in aggressive cancers that are notoriously difficult to treat. Additionally, p53 missense mutations are remarkably predictive of refractoriness to chemo/radiotherapy in various malignancies. These observations have led to the development of mutant p53-targeting agents that restore p53 function. An important unknown is which p53-mutant tumors will respond to p53 reactivation-based therapies. Here, we found a heterogeneous impact on therapeutic response to p53 restoration, suggesting that it will unlikely be effective as a monotherapy. Through gene expression profiling of p53^R172H -mutant lymphomas, we identified retinoic acid receptor gamma (RARγ) as an actionable target and demonstrated that pharmacological activation of RARγ with a synthetic retinoid sensitizes resistant p53-mutant lymphomas to p53 restoration, while additively improving outcome and survival in inherently sensitive tumors.

RRD-251 enhances all-trans retinoic acid (RA)-induced differentiation of HL-60 myeloblastic leukemia cells

All-trans-retinoic acid (RA) is known to induce terminal granulocytic differentiation and cell cycle arrest of HL-60 cells. Responding to an RA-induced cytosolic signaling machine, c-Raf translocates to the nucleus, providing propulsion for RA-induced differentiation. This novel mechanism is not understood, but presumably reflects c-Raf binding with nuclear gene regulatory proteins. RRD-251 is a small molecule that prevents the interaction of c-Raf and RB, the retinoblastoma tumor suppressor protein. The involvement of c-Raf and RB in RA-induced differentiation motivates interest in the effects of combined RA and RRD-251 treatment on leukemic cell differentiation.

We demonstrate that RRD-251 enhances RA-induced differentiation. Mechanistically, we find that nuclear translocated c-Raf associates with pS608 RB. RA causes loss of pS608 RB, where cells with hypophosphorylated S608 RB are G_0/G₁ restricted. Corroborating the pS608 RB hypophosphorylation, RB sequestration of E2F increased with concomitant loss of cdc6 expression, which is known to be driven by E2F. Hypophosphorylation of S608 RB releases c-Raf from RB sequestration to bind other nuclear targets. Release of c-Raf from RB sequestration results in enhanced association with GSK-3 which is phosphorylated at its S21/9 inhibitory sites. c-Raf binding to GSK-3 is associated with dissociation of GSK-3 and RARα, thereby relieving RARα of GSK-3 inhibition. RRD-251 amplifies each of these RA-induced events. Consistent with the posited enhancement of RARα transcriptional activity by RRD-251, RRD-251 increases the RARE-driven CD38 expression per cell. The RA/c-Raf/GSK-3/RARα axis emerges as a novel differentiation regulatory mechanism susceptible to RRD-251, suggesting enhancing RA-effects with RRD-251 in therapy.

Potential for subsets of wt-NPM1 primary AML blasts to respond to retinoic acid treatment

Acute myeloid leukemia (AML) has high mortality rates, perhaps reflecting a lack of understanding of the molecular diversity in various subtypes and a lack of known actionable targets. There are currently 12 open clinical trials for AML using combination therapeutic modalities including all-trans retinoic acid (RA). Mutant nucleophosmin-1, proposed as a possible marker for RA response, is the criterion for recruiting patients in three active RA phase 3 clinical trials. We tested the ability of RA alone or in combination with either bosutinib (B) or 6-formylindolo(3,2-b) carbazole (F) to induce conversion of 12 de novo AML samples toward a more differentiated phenotype. We assessed levels of expression of cell surface markers associated with differentiation, aldehyde dehydrogenase activity, and glucose uptake activity. Colony formation capacity was reduced with the combined treatment of RA and B or F, and correlated with modulation of a c-Cbl/Lyn/c-Raf-centered signalsome. Combination treatment was in most cases more effective than RA alone. Based on their responses to the treatments, some primary leukemic samples cluster closer to HL-60 cells than to other primary samples, suggesting that they may represent a hitherto undefined AML subtype that is potentially responsive to RA in a combination differentiation therapy.

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.

The leukemogenic fusion gene MLL-AF9 alters microRNA expression pattern and inhibits monoblastic differentiation via miR-511 repression

Background

In this study we explored the role of microRNAs (miRNAs) as mediators of leukemogenic effects of the fusion gene MLL-AF9, which results from a frequent chromosomal translocation in infant and monoblastic acute myeloid leukemia (AML).

Methods

We performed a specific and efficient knockdown of endogenous MLL-AF9 in the human monoblastic AML cell line THP1.

Results

The knockdown associated miRNA expression profile revealed 21 MLL-AF9 dependently expressed miRNAs. Gene ontology analyses of target genes suggested an impact of these miRNAs on downstream gene regulation via targeting of transcriptional modulators as well as involvement in many functions important for leukemia maintenance as e.g. myeloid differentiation, cell cycle and stem cell maintenance. Furthermore, we identified one of the most intensely repressed miRNAs, miR-511, to raise CCL2 expression (a chemokine ligand important for immunosurveillance), directly target cyclin D1, inhibit cell cycle progression, increase cellular migration and promote monoblastic differentiation. With these effects, miR-511 may have a therapeutic potential as a pro-differentiation agent as well as in leukemia vaccination approaches.

Conclusions

Our study provides new insights into the understanding of miRNAs as functional mediators of the leukemogenic fusion gene MLL-AF9 and opens new opportunities to further investigate specific therapeutic options for AML via the miRNA level.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-016-0283-5) contains supplementary material, which is available to authorized users.

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.

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.

ATRA-induced cellular differentiation and CD38 expression inhibits acquisition of BCR-ABL mutations for CML acquired resistance

Acquired resistance through genetic mutations is a major obstacle in targeted cancer therapy, but the underlying mechanisms are poorly understood. Here we studied mechanisms of acquired resistance of chronic myeloid leukemia (CML) to tyrosine kinase inhibitors (TKIs) by examining genome-wide gene expression changes in KCL-22 CML cells versus their resistant KCL-22M cells that acquire T315I BCR-ABL mutation following TKI exposure. Although T315I BCR-ABL is sufficient to confer resistance to TKIs in CML cells, surprisingly we found that multiple drug resistance pathways were activated in KCL-22M cells along with reduced expression of a set of myeloid differentiation genes. Forced myeloid differentiation by all-trans-retinoic acid (ATRA) effectively blocked acquisition of BCR-ABL mutations and resistance to the TKIs imatinib, nilotinib or dasatinib in our previously described in vitro models of acquired TKI resistance. ATRA induced robust expression of CD38, a cell surface marker and cellular NADase. High levels of CD38 reduced intracellular nicotinamide adenine dinucleotide (NAD+) levels and blocked acquired resistance by inhibiting the activity of the NAD+-dependent SIRT1 deacetylase that we have previously shown to promote resistance in CML cells by facilitating error-prone DNA damage repair. Consequently, ATRA treatment decreased DNA damage repair and suppressed acquisition of BCR-ABL mutations. This study sheds novel insight into mechanisms underlying acquired resistance in CML, and suggests potential benefit of combining ATRA with TKIs in treating CML, particularly in advanced phases.

Revealing CD38 cellular localization using a cell permeable, mechanism-based fluorescent small-molecule probe

Nicotinamide adenine dinucleotide (NAD) is increasingly recognized as an important signaling molecule that affects numerous biological pathways. Thus, enzymes that metabolize NAD can have important biological functions. One NAD-metabolizing enzyme in mammals is CD38, a type II transmembrane protein that converts NAD primarily to adenosine diphosphate ribose (ADPR) and a small amount of cyclic adenosine diphosphate ribose (cADPR). Localization of CD38 was originally thought to be only on the plasma membrane, but later reports showed either significant or solely, intracellular CD38. With the efficient NAD-hydrolysis activity, the intracellular CD38 may lead to depletion of cellular NAD, thus producing harmful effects. Therefore, the intracellular localization of CD38 needs to be carefully validated. Here, we report the synthesis and application of a cell permeable, fluorescent small molecule (SR101–F-araNMN) that can covalently label enzymatically active CD38 with minimal perturbation of live cells. Using this fluorescent probe, we revealed that CD38 is predominately on the plasma membrane of Raji and retinoic acid (RA)-treated HL-60 cells. Additionally, the probe revealed no CD38 expression in K562 cells, which was previously reported to have solely intracellular CD38. The finding that very little intracellular CD38 exists in these cell lines suggests that the major enzymatic function of CD38 is to hydrolyze extracellular rather than intracellular NAD. The fluorescent activity-based probes that we developed allow the localization of CD38 in different cells to be determined, thus enabling a better understanding of the physiological function.

Phosphorylation of c-Cbl and p85 PI3K driven by all-trans retinoic acid and CD38 depends on Lyn kinase activity

The leukocyte antigen CD38 is expressed after all-trans retinoic acid (ATRA) treatment in HL-60 myelogenous leukemia cells and promotes induced myeloid differentiation when overexpressed. We found that Vav1 and SLP-76 associate with CD38 in two cell lines, and that these proteins complex with Lyn, a Src family kinase (SFK) upregulated by ATRA. SFK inhibitors PP2 and dasatinib, which enhance ATRA-induced differentiation, were used to evaluate the involvement of Lyn kinase activity in CD38-driven signaling. Cells treated with ATRA for 48h followed by one hour of PP2 incubation show SFK/Lyn kinase inhibition. We observed that Lyn inhibition blocked c-Cbl and p85/p55 PI3K phosphorylation driven by the anti-CD38 agonistic mAb IB4 in ATRA-treated HL-60 cells and untreated CD38+ transfectants. In contrast, cells cultured for 48h following concurrent ATRA and PP2 treatment did not show Lyn inhibition, suggesting ATRA regulates the effects on Lyn. 48h of co-treatment preserved CD38-stimulated c-Cbl and p85/p55 PI3K phosphorylation indicating Lyn kinase activity is necessary for these events. In contrast another SFK inhibitor (dasatinib) which blocks Lyn activity with ATRA co-treatment prevented ATRA-induced c-Cbl phosphorylation and crippled p85 PI3K phosphorylation, indicating Lyn kinase activity is important for ATRA-propelled events potentially regulated by CD38. We found that loss of Lyn activity coincided with a decrease in Vav1/Lyn/CD38 and SLP-76/Lyn/CD38 interaction, suggesting these molecules form a complex that regulates CD38 signaling. Lyn inhibition also reduced Lyn and CD38 binding to p85 PI3K, indicating CD38 facilitates a complex responsible for PI3K phosphorylation. Therefore, Lyn kinase activity is important for CD38-associated signaling that may drive ATRA-induced differentiation.

The Src-family kinase inhibitor PP2 rescues inducible differentiation events in emergent retinoic acid-resistant myeloblastic leukemia cells

Retinoic acid is an embryonic morphogen and dietary factor that demonstrates chemotherapeutic efficacy in inducing maturation in leukemia cells. Using HL60 model human myeloid leukemia cells, where all-trans retinoic acid (RA) induces granulocytic differentiation, we developed two emergent RA-resistant HL60 cell lines which are characterized by loss of RA-inducible G1/G0 arrest, CD11b expression, inducible oxidative metabolism and p47^phox expression. However, RA-treated RA-resistant HL60 continue to exhibit sustained MEK/ERK activation, and one of the two sequentially emergent resistant lines retains RA-inducible CD38 expression. Other signaling events that define the wild-type (WT) response are compromised, including c-Raf phosphorylation and increased expression of c-Cbl, Vav1, and the Src-family kinases (SFKs) Lyn and Fgr. As shown previously in WT HL60 cells, we found that the SFK inhibitor PP2 significantly increases G1/G0 cell cycle arrest, CD38 and CD11b expression, c-Raf phosphorylation and expression of the aforementioned regulators in RA-resistant HL60. The resistant cells were potentially incapable of developing inducible oxidative metabolism. These results motivate the concept that RA resistance can occur in steps, wherein growth arrest and other differentiation events may be recovered in both emergent lines. Investigating the mechanistic anomalies in resistant cell lines is of therapeutic significance and helps to mechanistically understand the response to retinoic acid’s biological effects in WT HL60 cells.

Vav1 in differentiation of tumoral promyelocytes

The multidomain protein Vav1, in addition to promote the acquisition of maturation related properties by normal hematopoietic cells, is a key player in the ATRA- and PMA-induced completion of the differentiation program of tumoral myeloid precursors derived from APL. This review is focussed on the role of Vav1 in differentiating promyelocytes, as part of interconnected networks of functionally related proteins ended to regulate different aspects of myeloid maturation. The role of Vav1 in determining actin cytoskeleton reorganization alternative to the best known function as a GEF for small G proteins is discussed, as well as the binding of Vav1 with cytoplasmic and nuclear signaling molecules which provides a new perspective in the modulation of nuclear architecture and activity. In particular, new hints are provided on the ability of Vav1 to determine the nuclear amount of proteins implicated in modulating mRNA production and stability and in regulating the ATRA-dependent protein expression also by direct interaction with transcription factors known to drive the ATRA-induced maturation of myeloid cells. The reviewed findings summarize the major advances in the understanding of additional, non conventional functions connected with the vast interactive potential of Vav1.

Interferon regulatory factor-1 binds c-Cbl, enhances mitogen activated protein kinase signaling and promotes retinoic acid-induced differentiation of HL-60 human myelo-monoblastic leukemia cells

All-trans retinoic acid (RA) and interferons (IFNs) have efficacy in treating certain leukemias and lymphomas, respectively, motivating interest in their mechanism of action to improve therapy. Both RA and IFNs induce interferon regulatory factor-1 (IRF-1). We find that in HL-60 myeloblastic leukemia cells which undergo mitogen activated protien kinase (MAPK)-dependent myeloid differentiation in response to RA, IRF-1 propels differentiation. RA induces MAPK-dependent expression of IRF-1. IRF-1 binds c-Cbl, a MAPK related adaptor. Ectopic IRF-1 expression causes CD38 expression and activation of the Raf/MEK/ERK axis, and enhances RA-induced differentiation by augmenting CD38, CD11b, respiratory burst and G0 arrest. Ectopic IRF-1 expression also decreases the activity of aldehyde dehydrogenase 1, a stem cell marker, and enhances RA-induced ALDH1 down-regulation. Interestingly, expression of aryl hydrocarbon receptor (AhR), which is RA-induced and known to down-regulate Oct4 and drive RA-induced differentiation, also enhances IRF-1 expression. The data are consistent with a model whereby IRF-1 acts downstream of RA and AhR to enhance Raf/MEK/ERK activation and propel differentiation.

Induction of retinoid X receptor activity and consequent upregulation of p21WAF1/CIP1 by indenoisoquinolines in MCF7 cells

Retinoid X receptor (RXR) has been targeted for the chemoprevention and treatment of cancer. To discover potential agents acting through RXRs, we utilized an RXR response element (RXRE)-luciferase reporter gene assay. Following extensive screening, 3-amino-6-(3-aminopropyl)-5,6-dihydro-5,11-dioxo-11H-indeno[1,2-c]isoquinoline dihydrochloride (AM6-36) was found to induce RXRE-luciferase activities. AM6-36 inhibited COX-2 expression and anchorage-independent growth with 12-O-tetradecanoylphorbol 13-acetate-stimulated JB6 Cl41 cells, induced the expression of CD38 in HL-60 cells, and attenuated the growth of N-methyl-N-nitrosourea-induced mammary tumors in rats. Consistent with other reports describing the antiproliferative effects of RXR agonists in breast cancers, AM6-36 showed growth inhibition with cultured MCF7 breast cancer cells, accompanied by G(2)/M-phase arrest at lower concentrations and enhanced S-phase arrest at higher concentrations. On the basis of DNA microarray analysis, AM6-36 upregulated the expression of CDKN1A, a target gene of RXR, by 35-fold. In accord with this response, the expression of the corresponding protein, p21(WAF1/CIP1), was increased in the presence of AM6-36. Induction of p21 by AM6-36 was abrogated following transient knockdown of RXRα, demonstrating that the effect of AM6-36 on the expression of p21 is closely related to modulation of RXRα transcriptional activity. Intestinal permeability was suggested with Caco-2 cells and limited metabolism resulted when AM6-36 was incubated with human liver microsomes. Oral administration with rats resulted in 0.8 μg/mL, 4.3 μg/g, and 0.3 μg/g in serum, liver, and mammary gland, respectively. In sum, these data suggest that AM6-36 is a promising lead for the treatment or prevention of breast cancer and provide a strong rationale for testing in more advanced antitumor systems.

Arsenic trioxide cooperates with all trans retinoic acid to enhance mitogen-activated protein kinase activation and differentiation in PML-RARalpha negative human myeloblastic leukemia cells

Arsenic trioxide (ATO) synergistically promotes all trans retinoic acid (ATRA)-induced differentiation of PML-RARalpha negative HL-60 myeloblastic leukemia cells. In PML-RARalpha positive myeloid leukemia cells, ATO is known to cause degradation of PML-RARalpha with subsequent induced myeloid differentiation. We found that ATO by itself does not cause differentiation of the PML-RARalpha negative HL-60 cells, but enhances ATRA's capability to cause differentiation. ATO augmented ATRA-induced RAF/MEK/ERK axis signaling, expression of CD11b and p47(PHOX), and inducible oxidative metabolism. ATO enhanced ATRA-induced population growth retardation without evidence of apoptosis or enhanced G1/G0 growth arrest. Compared to ATRA-treated cells, the ATRA plus ATO-treated cells progressed more slowly through the cell cycle as detected by a slower rate of accumulation in G2/M following nocodazole treatment. Hoechst/PI staining showed that low-dose ATO did not induce apoptosis. In summary, our results indicate that ATO in conjunction with ATRA is of potential chemotherapeutic use in PML-RARalpha negative leukemias.

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.

c-Cbl tyrosine kinase-binding domain mutant G306E abolishes the interaction of c-Cbl with CD38 and fails to promote retinoic acid-induced cell differentiation and G0 arrest

Retinoic acid (RA) causes HL-60 human myeloblastic leukemia cell myeloid differentiation that is dependent on MAPK signaling. The process is propelled by c-Cbl, which binds the CD38 receptor as part of a signaling complex generating MAPK signaling. Here we report that the capability of c-Cbl to do this is lost in the G306E tyrosine kinase-binding domain mutant. Unlike wild-type (WT) c-Cbl, the G306E mutant c-Cbl fails to propel RA-induced differentiation, and disrupts the normal association with CD38. The G306E mutant does, like WT c-Cbl, co-immunoprecipitate with Vav, Slp-76, and p38. But unlike WT c-Cbl, does not cause MAPK signaling. In contrast, the C381A Ring finger domain mutant functions like WT c-Cbl. It binds CD38 and is part of the same apparent c-Cbl/Slp-76/Vav/p38 signaling complex. The C381A mutant causes MAPK signaling and propels RA-induced differentiation. In addition to HL-60 cells and their WT or mutant c-Cbl stable transfectants, the c-Cbl/Vav/Slp-76 complex is also found in NB4 cells where c-Cbl was previously also found to bind CD38. The data are consistent with a model in which the G306E mutant c-Cbl forms a signaling complex that includes Slp-76, Vav, and p38; but does not drive MAPK signaling because it fails to bind the CD38 receptor. Without the G306E mutation the c-Cbl unites CD38 with the signaling complex and delivers a MAPK signal that drives RA-induced differentiation. The results demonstrate the importance of the Gly306 residue in the ability of c-Cbl to propel RA-induced differentiation.

Mechanism-based small molecule probes for labeling CD38 on live cells

CD38 is a type II transmembrane glycoprotein with multiple functions. It acts as an ecto-enzyme as well as a receptor. The enzymatic activity catalyzes the formation of two potent Ca(2+) releasing agents: cyclic adenosine diphosphate ribose (cADPR) from nicotinamide adenine dinucleotide (NAD) and nicotinic acid adenine dinucleotide phosphate (NAADP) from NAD phosphate (NADP). The receptor function of CD38 leads to the phosphorylation of intracellular signaling proteins and the up-regulation of cytokine production in immune cells. These two functions of CD38 underlie its involvement in various biological processes, such as hormone secretion, immune cell differentiation, and immune responses. Clinically, CD38 is used as a negative prognosis marker for chronic lymphatic leukemia (CLL). However, a clear molecular understanding of CD38's role in physiology and pathology is still lacking. To facilitate the study of CD38 at cellular and molecular levels, here we report a mechanism-based method for fluorescently labeling CD38 on live cells. This labeling method does not interfere with the receptor function of CD38 and the downstream signaling. The labeling method is thus a useful tool to study the receptor function of CD38 in live cells. In addition, since the mechanism-based labeling also inhibits the enzymatic activity of CD38, it should be useful for dissecting the receptor function of CD38 without interference from its enzyme function in complicated biological processes.

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.

Nicotinamide cooperates with retinoic acid and 1,25-dihydroxyvitamin D(3) to regulate cell differentiation and cell cycle arrest of human myeloblastic leukemia cells

Nicotinamide, the amide derivative of vitamin B(3), cooperates with retinoic acid (RA), a form of vitamin A, and 1,25-dihydroxyvitamin D(3) (D3), to regulate cell differentiation and proliferation of human myeloblastic leukemia cells. In human myeloblastic leukemia cells, RA or D3 are known to cause MAPK signaling leading to myeloid or monocytic differentiation and G0 cell cycle arrest. In this process, RA or D3 induces the early expression of CD38, a receptor that causes ERK signaling and propels further differentiation. Our study demonstrates that nicotinamide in combination with RA or D3 affected induced expression levels of CD38, CD11b and CD14, suggesting a cooperative function of nicotinamide and RA or D3. Nicotinamide transiently retarded the initial RA- or D3-induced expression of CD38, which subsequently reached the same nearly 100% expression. Nicotinamide induced ERK activation and further enhanced the RA-induced ERK activation, but the D3-induced ERK activation was diminished by nicotinamide, although levels still exceeded those induced by RA, suggesting lineage-specific nicotinamide responses. Nicotinamide enhanced both RA- and D3-induced CD11b expression, inducible oxidative metabolism, and G0 cell cycle arrest, accelerating their induced occurrence in all instances. Consistent with this, the RA- or D3-induced downregulation of PARP was enhanced by nicotinamide. Nicotinamide thus regulated RA- or D3-induced differentiation and G0 arrest, causing a transient delay in certain early aspects of the progression to terminal differentiation but ultimately accelerating the occurrence of terminally, functionally differentiated G0 cells.

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 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.

Expression of CD38 in Human Neuroblastoma Sh-SY5Y Cells

Human CD38 antigen is a 42–45 kDa type II transmembrane glycoprotein with a short N-terminal cytoplasmic domain and a long C-terminal extracellular region. It is widely expressed in different cell types including thymocytes, activated T cells, and terminally differentiated B cells (plasma cells) and it is involved in cellular proliferation and adhesion. CD38 acts as an ectocyclase that converts NAD+ to the Ca2+-releasing second messenger cyclic ADP-ribose (cADPR). It has been also demonstrated that increased extracellular levels of NAD+ and cADPR are involved in inflammatory diseases and in cellular damage, such as ischemia. In the present study, we have characterized the expression of CD38 in human neuroblastoma SH-SY5Y cell line. All-trans-retinoic acid (ATRA) treatment was used to induce cell differentiation. Our results indicate that: a) even if SH-SY5Y cells have a negative phenotype express CD38 at nuclear level, ATRA treatment does not influence this pattern; b) CD38 localizing to the nucleus may co-localize with p80-coilin positive nuclear-coiled bodies; c) purified nuclei, by Western blot determinations using anti-CD38 antibodies, display a band with a molecular mass of −42 kDa; d) SH-SY5Y cells show nuclear ADP-ribosyl cyclase due to CD38 activity; e) the basal level of CD38 mRNA shows a time-dependent increase after treatment with ATRA. These results suggest that the presence of constitutive fully functional CD38 in the SH-SY5Y nucleus has some important implications for intracellular generation of cADP-ribose and subsequent nucleoplasmic calcium release.

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.

Induction of kinase suppressor of RAS-1(KSR-1) gene by 1, alpha25-dihydroxyvitamin D3 in human leukemia HL60 cells through a vitamin D response element in the 5'-flanking region

Differentiation therapy is being developed as an additional therapeutic option for the treatment of several forms of cancer, including myeloid leukemia. In model systems, the physiologically active form of vitamin D, 1, alpha25-dihydroxyvitamin D3 (1,25D), induces monocytic differentiation of human myeloid cells, but the mechanism is not clear. We report here, the first direct connection between the signal provided by 1,25D and the molecular circuitry known to be involved in monocytic differentiation. Specifically, we show that 1,25D selectively increases the expression of the gene encoding kinase suppressor of Ras-1 (KSR-1) in HL60 cells, while other differentiation-inducing agents such as 12-O-tetradecanoylphorbol-13-acetate, retinoic acid or dimethyl sulfoxide do not significantly increase KSR-1 expression. Further, the upregulation of KSR-1 gene by 1,25D is competed by ZK159222, an antagonist of vitamin D receptor (VDR) action, and can occur in the presence of protein synthesis inhibitor cycloheximide, showing that the effect is direct. Most importantly, we have identified a vitamin D responsive element (VDRE) in the promoter region of the human KSR-1 gene, to which VDR binds in a 1,25D-dependent manner, in vitro and in vivo. This binding is paralleled by increased association of RNA polymerase II with the transcription start site of KSR-1 gene, and the VDRE is functional in reporter assays. Our findings offer a potential mechanism for a signaling pathway that contributes to 1,25D-induced monocytic differentiation of human myeloid leukemia cells.