Arsenic trioxide selectively induces acute promyelocytic leukemia cell apoptosis via a hydrogen peroxide-dependent pathway

Low concentrations of As(2)O(3) (</=1 micromol/L) induce long-lasting remission in patients with acute promyelocytic leukemia (APL) without significant myelosuppressive side effects. Several groups, including ours, have shown that 0.5 to 1 micromol/L As(2)O(3) induces apoptosis in APL-derived NB4 cells, whereas other leukemic cells are resistant to As(2)O(3) or undergo apoptosis only in response to greater than 2 micromol/L As(2)O(3). In this report, we show that the ability of As(2)O(3) to induce apoptosis in leukemic cells is dependent on the activity of the enzymes that regulate cellular H(2)O(2) content. Thus, NB4 cells have relatively low levels of glutathione peroxidase (GPx) and catalase and have a constitutively higher H(2)O(2) content than U937 monocytic leukemia cells. Glutathione-S-transferase pi (GSTpi), which is important for cellular efflux of As(2)O(3), is also low in NB4 cells. Moreover, As(2)O(3) further inhibits GPX activity and increases cellular H(2)O(2) content in NB4 but not in U937 cells. Selenite pretreatment of NB4 cells increases the activity of GPX, lowers cellular H(2)O(2) levels, and renders NB4 cells resistant to 1 micromol/L As(2)O(3). In contrast, concentrations of As(2)O(3) that alone are not capable of inducing apoptosis in NB4 cells induce apoptosis in the presence of the GPx inhibitor mercaptosuccinic acid. Similar effects are observed by modulating the activity of catalase with its inhibitor, aminotriazol. More important from a therapeutic point of view, U937 and HL-60 cells, which require high concentrations of As(2)O(3) to undergo apoptosis, become sensitive to low, clinically acceptable concentrations of As(2)O(3) when cotreated with these GPx and catalase inhibitors. The induction of apoptosis by As(2)O(3) involves an early decrease in cellular mitochondrial membrane potential and increase in H(2)O(2) content, followed by cytochrome c release, caspase 3 activation, DNA fragmentation, and the classic morphologic changes of apoptosis.

Genomic structure of the human PLZF gene

The human PLZF (promyelocytic leukaemia zinc finger) gene encodes a Krüppel-like zinc finger protein, which was identified via the reciprocal translocation t(11;17)(q23;q21) fusing it to the retinoic acid receptor alpha (RARalpha) gene in promyelocytic leukaemia. To determine its complete genomic organisation, we constructed a cosmid-map fully containing the hPLZF gene. The gene has seven exons, including a novel 5' untranslated exon, varying in size from 87 to 1358bp and spans at least 120kb. Flanking intronic sequences were identified and all splice acceptor and donor sites conformed to the gt/ag rule. Five polymorphic markers could be fine located in its vicinity. These data will facilitate mutation analysis of hPLZF in t(11;17) leukaemia cases, as well as assist mapping and loss-of-heterozygosity analysis. Here we have tested hPLZF as a possible candidate for the PGL1 locus involved in hereditary head and neck paragangliomas. However, mutation analysis revealed no aberration in 12 paraganglioma patients from different families.

Persistent primary coronary dilation induced by transatrial delivery of nitroglycerin into the pericardial space: a novel approach for local cardiac drug delivery

OBJECTIVES

We compared the effects of intrapericardial and intracoronary nitroglycerin on coronary cross-sectional area as assessed by intravascular ultrasound and demonstrated the feasibility of local cardiac drug delivery by a newly developed method to access the normal pericardial space through the right atrial appendage.

BACKGROUND

Studies of nitric oxide (NO) donors have suggested that their antiarrhythmic and antiproliferative properties are more effective when administered by the intrapericardial rather than intravascular route. We postulated that NO donors delivered intrapericardially would also cause sustained coronary vasodilation without significant systemic hypotension.

METHODS

Intrapericardial nitroglycerin (200 microg) was administered in five Yorkshire pigs. Coronary cross-sectional luminal area was measured with intravascular ultrasound at various time intervals. The effects of intracoronary nitroglycerin on coronary luminal area were used for comparison.

RESULTS

Transatrial pericardial access required 1 to 3 min in all animals. Intrapericardial nitroglycerin was associated with a mean 31.7% increase in luminal area at 5 min (p < 0.001). Vasodilation peaked between 5 and 10 min and persisted for 15 min. In contrast, intracoronary nitroglycerin was associated with a smaller mean increase in luminal area (20.3% at 5 min, p < 0.01) that disappeared by 10 min. Significant systemic hypotension was observed at 3 min with intracoronary but not with intrapericardial nitroglycerin.

CONCLUSIONS

Sustained coronary vasodilation can be achieved with intrapericardial delivery of nitroglycerin without systemic hypotension. Nitric oxide donors with longer half-lives could prove beneficial in the treatment of myocardial ischemic syndromes when administered through this route. Transatrial pericardial access offers a novel route for local cardiac drug delivery.

Distinct leukemia phenotypes in transgenic mice and different corepressor interactions generated by promyelocytic leukemia variant fusion genes PLZF-RARalpha and NPM-RARalpha

Acute promyelocytic leukemia (APL) is characterized by a specific chromosome translocation involving RARalpha and one of four fusion partners: PML, PLZF, NPM, and NuMA genes. To study the leukemogenic potential of the fusion genes in vivo, we generated transgenic mice with PLZF-RARalpha and NPM-RARalpha. PLZF-RARalpha transgenic animals developed chronic myeloid leukemia-like phenotypes at an early stage of life (within 3 months in five of six mice), whereas three NPM-RARalpha transgenic mice showed a spectrum of phenotypes from typical APL to chronic myeloid leukemia relatively late in life (from 12 to 15 months). In contrast to bone marrow cells from PLZF-RARalpha transgenic mice, those from NPM-RARalpha transgenic mice could be induced to differentiate by all-trans-retinoic acid (ATRA). We also studied RARE binding properties and interactions between nuclear corepressor SMRT and various fusion proteins in response to ATRA. Dissociation of SMRT from different receptors was observed at ATRA concentrations of 0.01 microM, 0.1 microM, and 1.0 microM for RARalpha-RXRalpha, NPM-RARalpha, and PML-RARalpha, respectively, but not observed for PLZF-RARalpha even in the presence of 10 microM ATRA. We also determined the expression of the tissue factor gene in transgenic mice, which was detected only in bone marrow cells of mice expressing the fusion genes. These data clearly establish the leukemogenic role of PLZF-RARalpha and NPM-RARalpha and the importance of fusion receptor/corepressor interactions in the pathogenesis as well as in determining different clinical phenotypes of APL.

Apoptosis and growth inhibition in malignant lymphocytes after treatment with arsenic trioxide at clinically achievable concentrations

BACKGROUND

Arsenic trioxide (As2O3) can induce clinical remission in patients with acute promyelocytic leukemia via induction of differentiation and programmed cell death (apoptosis). We investigated the effects of As2O3 on a panel of malignant lymphocytes to determine whether growth-inhibitory and apoptotic effects of As2O3 can be observed in these cells at clinically achievable concentrations.

METHODS

Eight malignant lymphocytic cell lines and primary cultures of lymphocytic leukemia and lymphoma cells were treated with As2O3, with or without dithiothreitol (DTT) or buthionine sulfoximine (BSO) (an inhibitor of glutathione synthesis). Apoptosis was assessed by cell morphology, flow cytometry, annexin V protein level, and terminal deoxynucleotidyl transferase labeling of DNA fragments. Cellular proliferation was determined by 5-bromo-2'-deoxyuridine incorporation into DNA and flow cytometry and by use of a mitotic arrest assay. Mitochondrial transmembrane potential (delta psi(m)) was measured by means of rhodamine 123 staining and flow cytometry. Protein expression was assessed by western blot analysis or immunofluorescence.

RESULTS

Therapeutic concentrations of As2O3 (1-2 microM) had dual effects on malignant lymphocytes: 1) inhibition of growth through adenosine triphosphate (ATP) depletion and prolongation of cell cycle time and 2) induction of apoptosis. As2O3-induced apoptosis was preceded by delta psi(m) collapse. DTT antagonized and BSO enhanced As2O3-induced ATP depletion, delta psi(m) collapse, and apoptosis. Caspase-3 activation, usually resulting from delta psi(m) collapse, was not always associated with As2O3-induced apoptosis. As2O3 induced PML (promyelocytic leukemia) protein degradation but did not modulate expression of cell cycle-related proteins, including c-myc, retinoblastoma protein, cyclin-dependent kinase 4, cyclin D1, and p53, or expression of differentiation-related antigens.

CONCLUSIONS

Substantial growth inhibition and apoptosis without evidence of differentiation were induced in most malignant lymphocytic cells treated with 1-2 microM As2O3. As2O3 may prove useful in the treatment of malignant lymphoproliferative disorders.

Characterization of the unstable lesion by angiography, angioscopy, and intravascular ultrasound

Acute coronary syndromes are a spectrum of clinical presentations with various pathophysiologic substrates. As such, there is not one single type of lesion responsible for stable or unstable angina, acute myocardial infarction, and sudden cardiac death. Most of the information regarding the characteristics of culprit lesions derives from histopathologic studies, whether postmortem or from atherectomy samples, and from studies using angiography, angioscopy, and intravascular ultrasound. Characterization of the unstable coronary lesion is key to understanding the pathophysiology of coronary artery disease, this knowledge will allow clinicians to individualize treatment according to specific lesion types, and more importantly, will lead to strategies to identify atherosclerotic lesions in their early stages and implement preventive therapies.

Leukemia translocation protein PLZF inhibits cell growth and expression of cyclin A

The PLZF gene was identified by its fusion with the RARalpha locus in a therapy resistant form of acute promyelocytic leukemia (APL) associated with the t(11;17)(q23;q21) translocation. Here we describe PLZF as a negative regulator of cell cycle progression ultimately leading to growth suppression. PLZF can bind and repress the cyclin A2 promoter while expression of cyclin A2 reverts the growth suppressed phenotype of myeloid cells expressing PLZF. In contrast RARalpha-PLZF, a fusion protein generated in t(11;17)(q23;q21)-APL activates cyclin A2 transcription and allows expression of cyclin A in anchorage-deprived NIH3T3 cells. Therefore, cyclin A2 is a candidate target gene for PLZF and inhibition of cyclin A expression may contribute to the growth suppressive properties of PLZF. Deregulation of cyclin A2 by RARalpha-PLZF may represent an oncogenic mechanism of this chimeric protein and contribute to the aggressive clinical phenotype of t(11;17)(q23;q21)-associated APL.

Malignant cells can be sensitized to undergo growth inhibition and apoptosis by arsenic trioxide through modulation of the glutathione redox system

Arsenic trioxide (As2O3) induces clinical remission in acute promyelocytic leukemia (APL) with minimal toxicity and apoptosis in APL-derived NB4 cells at low (1 to 2 micromol/L) concentration. We examined the basis for NB4 cell sensitivity to As2O3 to identify experimental conditions that would render other malignant cells responsive to low concentrations of As2O3. The intracellular glutathione (GSH) content had a decisive effect on As2O3-induced apoptosis. Highly sensitive NB4 cells had the lowest GSH and the sensitivity of other cell lines was inversely proportional to their GSH content. The t(14;18) B-cell lymphoma cell line had low GSH levels and sensitivity to As2O3 at levels slightly higher than in APL cells. Experimental upmodulation of GSH content decreased the sensitivity to As2O3. Ascorbic acid and buthionine sulfoxide (BSO) decreased GSH to a greater extent, and rendered malignant cells more sensitive to As2O3. As2O3-induced apoptosis was not enhanced by ascorbic acid in normal cells, suggesting that the combination of ascorbic acid and As2O3 may be selectively toxic to some malignant cells. Ascorbic acid enhanced the antilymphoma effect of As2O3 in vivo without additional toxicity. Thus, As2O3 alone or administered with ascorbic acid may provide a novel therapy for lymphoma.

Boswellic acid acetate induces differentiation and apoptosis in leukemia cell lines

Boswellic acid acetate (BC-4), a compound isolated from the herb Boswellia carterii Birdw., can induce differentiation and apoptosis of leukemia cells. Based on cell morphology and NBT reduction, BC-4 induced monocytic differentiation of myeloid leukemia HL-60, U937 and ML-1 cells at a dose under 12.5 microg/ml (24.2 microM). BC-4 was a potent inducer, with 90% of the cells showing morphologic changes and 80-90% of the cells showing NBT reduction. Specific and non-specific esterase were also increased by BC-4. Based on benzidine staining assay, BC-4 failed to induce erythroid leukemia DS-19 and K562 cells differentiation. In contrast to its selective differentiation effect, BC-4 strongly inhibited growth of all cell lines tested. The growth inhibition effect was dose- and time-dependent. In HL-60 cells, 20 microg/ml (38.8 microM) of BC-4 decreased viable cell number by 60% at 24 h, whereas at 3 days there was virtually no viable cells. Morphologic and DNA fragmentation analysis proved that BC-4 induced cell apoptosis. The dual apoptotic and differentiation effects of BC-4 suggest that it may be a powerful agent in the treatment of leukemia.

Detecting and differentiating white from red coronary thrombus by angiography in angina pectoris and in acute myocardial infarction

To determine the ability to detect thrombus by angiography, angioscopy was performed before angiography in patients undergoing interventional procedures and the data collected in a blinded fashion. These data demonstrated that the sensitivity of angiography to detect white thrombus was 50% and the specificity was 95%, whereas the sensitivity and specificity to detect red thrombus was 100%, respectively; the positive and negative predictive value of detecting thrombus in general was 89% and 83%, respectively.

Transatrial access to the normal pericardial space: a novel approach for diagnostic sampling, pericardiocentesis, and therapeutic interventions

BACKGROUND

A nonsurgical means to access the normal pericardial space could provide opportunities for diagnostic sampling and therapeutic interventions. Because there are currently no approved nonsurgical methods to accomplish this, we tested a new approach in large animals.

METHODS AND RESULTS

A catheter system was employed in a percutaneous approach from a femoral vein to pierce the right atrial appendage. Pericardial access was confirmed by placement of a radiopaque guidewire visible under fluoroscopy (6 dogs, 13 pigs). In 7 of the pigs, pericardial tamponade, produced by injection of saline or heparinized blood into the pericardial space through this route, was confirmed by fluoroscopy and hemodynamic evidence. The feasibility and safety of this access route were tested with multiple repetitions in all 19 animals. At the end of each of the 17 acute experiments, direct inspection after thoracotomy revealed no hemopericardium, laceration, or bleeding on catheter withdrawal. In 24-hour survival studies performed in 2 of the 6 dogs, the animals exhibited no behavioral signs of discomfort or untoward consequences on recovery from anesthesia. Histology revealed only a small (approximately 1-mm) fibrinous plug at the site of puncture.

CONCLUSIONS

The percutaneous approach via the right atrial appendage provides a rapid, safe route to access the normal pericardial space for diagnostic sampling and to alleviate high-volume and low-volume (<200 mL) pericardial effusions. The access route is potentially useful for selective administration of therapeutic agents, growth factors, gene vectors, and cardioactive and vasoactive agents to the heart.

Enhanced growth inhibition and differentiation of fluorodeoxyuridine-treated human colon carcinoma cells by phenylbutyrate

The effect of phenylbutyrate (PB), a nontoxic differentiation inducer, in human colon carcinoma cell lines treated with 5-fluorodeoxyuridine (FUdR) was evaluated. Two HT-29 human colon carcinoma subclones (U4 well differentiated and U9 poorly differentiated) were equally growth inhibited by 16 h of FUdR (0.2 microM) treatment but recovered cell growth in 3-6 days after the removal of FUdR. PB as a single agent had minimal effect on cell growth, but after FUdR treatment, PB inhibited cell growth for 12 days. The inhibition of cell growth in FUdR-treated cells by PB was more sustained in U4 than U9 cells and was associated with an increased and sustained expression of p21waf1 protein, secretion of transforming growth factor beta1, mediators of p53-dependent or -independent G1 cell cycle arrest, and an increase in the alkaline phosphatase activity as well, considered a marker of differentiation in colon carcinoma cells. These effects of PB were seen only in FUdR-pretreated cells because PB alone had minimal effect on the expression of these genes. The sequential use of FUdR followed by PB in patients with colon carcinoma should be explored because two subclones of HT29, irrespective of their state of differentiation, respond to this clinically achievable regimen.

The promyelocytic leukemia zinc finger protein affects myeloid cell growth, differentiation, and apoptosis

The promyelocytic leukemia zinc finger (PLZF) gene, which is disrupted in therapy-resistant, t(11;17)(q23;q21)-associated acute promyelocytic leukemia (APL), is expressed in immature hematopoietic cells and is down-regulated during differentiation. To determine the role of PLZF in myeloid development, we engineered expression of PLZF in murine 32Dcl3 cells. Expression of PLZF had a dramatic growth-suppressive effect accompanied by accumulation of cells in the G0/G1 compartment of the cell cycle and an increased incidence of apoptosis. PLZF-expressing pools also secreted a growth-inhibitory factor, which could explain the severe growth suppression of PLZF-expressing pools that occurred despite the fact that only half of the cells expressed high levels of PLZF. PLZF overexpression inhibited myeloid differentiation of 32Dcl3 cells in response to granulocyte and granulocyte-macrophage colony-stimulating factors. Furthermore, cells that expressed PLZF appeared immature as demonstrated by morphology, increased expression of Sca-1, and decreased expression of Gr-1. These findings suggest that PLZF is an important regulator of cell growth, death, and differentiation. Disruption of PLZF function associated with t(11;17) may be a critical event leading to APL.

Dephosphorylation of Vav is associated with the induction of mouse erythroleukemia cell differentiation: effects of orthovanadate and levamisole

Mouse erythroleukemia (MEL) cell erythroid differentiation induced by dimethyl sulfoxide or hexamethylene bisacetamide (HMBA) is accompanied by the production of hemoglobin, terminal cell division and decreases in lactate production and fructose 2,6-bisphosphate levels. A number of studies have suggested that decreases in the cellular level of protein phosphotyrosine content may play a role in MEL cell differentiation. In particular, it was shown that the expression of several protein tyrosine phosphatase genes accompany this process and that the transfection of one of these genes into MEL cells followed by its subsequent expression induced eythroid differentiation. However, none of the physiological substrates for these protein tyrosine phosphatases have been identified. It is shown here that MEL cell differentiation is accompanied by decreases in tyrosine phosphorylation of Vav and possibly of the erythropoietin receptor (EpoR). Immunoprecipitation of the EpoR and analysis of co-precipitated proteins, indicates that the EpoR associates with Vav, STAT5 and an unidentified 60 Kd protein, . HMBA-induced erythroid differentiation abrogates these associations. The phosphatase inhibitors, Na3VO4 and levamisole, inhibit HMBA-induced differentiation as well as the association of the EpoR with Vav, STAT5 and the 60 Kd protein. This is of interest since Na3VO4, at the concentrations used here, has been shown to be a potent inhibitor of the activity of protein tyrosine phosphatases. These results suggest that levamisole, at least indirectly, acts by a molecular mechanism similar to that of Na3VO4 and that the loss of the association of the EpoR with Vav, STAT5, and and/or the reduction in the level of tyrosine phosphorylation of these proteins may play a role in MEL cell differentiation.

The phosphatase inhibitors, orthovanadate and levamisole, inhibit induction of erythroid differentiation and abrogate the associated inhibition of glycolysis

Alterations in protein tyrosine phosphate (PTP), lactate, and fructose 2,6-bisphosphate (F-2,6-P2) levels have been associated with induced MEL cell differentiation and commitment to terminal cell division (TCD). The possible relationships of perturbations in PTP metabolism and reduction in lactate formation during differentiation were investigated utilizing sodium orthovanadate, Na3VO4, primarily an inhibitor of PTP phosphatases, and levamisole, considered an alkaline phosphatase inhibitor. Both of these compounds were found to effectively inhibit the TCD-associated differentiation induced by DMSO, HMBA, and Na butyrate and to abrogate the differentiation-associated reduction in lactate accumulation due to these agents. However, they were found not to inhibit hemin-induced hemoglobin synthesis which is independent of TCD and does not alter lactate metabolism. Two brominated levamisole analogs, L-p-bromotetramisole and D-p-bromotetramisole, were also found to be inhibitors of TCD-associated differentiation and to be effective at even lower concentrations than levamisole. The changes in TCD-associated differentiation and lactate production exhibited the same concentration-dependence with respect to the inhibitors. These findings strengthened the theory that TCD-associated differentiation, decreased lactate production, and sensitivity to phosphatase inhibitors are all associated. Since the induction of MEL cell differentiation has been shown to be associated with, and is thought to be due to, the induction of PTP phosphatase activity and Na3VO4 is thought to inhibit the differentiation by inhibiting PTP phosphatase activity, the effect of levamisole on PTP levels was determined. Levamisole, like Na3VO4, was found to increase the tyrosine phosphate levels of proteins of similar molecular weights in intact cells in both the presence and absence of a differentiation inducer. Several phosphotyrosine-containing, similarly sized proteins were particularly affected by differentiation induction and by Na3VO4 and levamisole treatment. Changes in the levels of tyrosine phosphate-containing proteins of approximately 92-96, 60, and 38 kd were particularly noticeable. The induction of differentiation reduced PTP levels and inhibition of differentiation due to treatment with either Na3VO4 or levamisole increased their levels. These data suggest relationships between signal transduction pathways involved in differentiation and TCD, the regulation of lactate and F-2,6-P2 metabolism, and PTP levels.

Reduced retinoic acid-sensitivities of nuclear receptor corepressor binding to PML- and PLZF-RARalpha underlie molecular pathogenesis and treatment of acute promyelocytic leukemia

Typical acute promyelocytic leukemia (APL) is associated with expression of the PML-RARalpha fusion protein and responsiveness to treatment with all-trans retinoic acid (ATRA). A rare, but recurrent, APL has been described that does not respond to ATRA treatment and is associated with a variant chromosomal translocation and expression of the PLZF-RARalpha fusion protein. Both PML- and PLZF-RARalpha possess identical RAR sequences and inhibit ATRA-induced gene transcription as well as cell differentiation. We now show that the above-mentioned oncogenic fusion proteins interact with the nuclear receptor corepressor N-CoR and, in comparison with the wild-type RARalpha protein, their interactions display reduced sensitivities to ATRA. Although pharmacologic concentration of ATRA could still induce dissociation of N-CoR from PML-RARalpha, it had a very little effect on its association with the PLZF-RARalpha fusion protein. This ATRA-insensitive interaction between N-CoR and PLZF-RARalpha was mediated by the N-terminal PLZF moiety of the chimera. It appears that N-CoR/histone deacetylase corepressor complex interacts directly in an ATRA-insensitive manner with the BTB/POZ-domain of the wild-type PLZF protein and is required, at least in part, for its function as a transcriptional repressor. As the above-noted results predict, histone deacetylase inhibitors antagonize oncogenic activities of the PML-RARalpha fusion protein and partially relieve transcriptional repression by PLZF as well as inhibitory effect of PLZF-RARalpha on ATRA response. Taken together, our results demonstrate involvement of nuclear receptor corepressor/histone deacetylase complex in the molecular pathogenesis of APL and provide an explanation for differential sensitivities of PML- and PLZF-RARalpha-associated leukemias to ATRA.

Arsenic trioxide as an inducer of apoptosis and loss of PML/RAR alpha protein in acute promyelocytic leukemia cells

BACKGROUND

Retinoids, which are derivatives of vitamin A, induce differentiation of acute promyelocytic leukemia (APL) cells in vitro and in patients. However, APL cells develop resistance to retinoic acid treatment. Arsenic trioxide (As2O3) can induce clinical remission in patients with APL, including those who have relapsed after retinoic acid treatment, by inducing apoptosis (programmed cell death) of the leukemia cells. In this study, we investigated the molecular mechanisms by which As2O3 induces apoptosis in retinoic acid-sensitive NB4 APL cells, in retinoic acid-resistant derivatives of these cells, and in fresh leukemia cells from patients.

METHODS

Apoptosis was assessed by means of DNA fragmentation analyses, TUNEL assays (i.e., deoxyuridine triphosphate labeling of DNA nicks with terminal deoxynucleotidyl transferase), and flow cytometry. Expression of the PML/RAR alpha fusion protein in leukemia cells was assessed by means of western blotting, ligand binding, and immunohistochemistry. Northern blotting and ribonuclease protection assays were used to evaluate changes in gene expression in response to retinoic acid and As2O3 treatment.

RESULTS AND CONCLUSIONS

As2O3 induces apoptosis without differentiation in retinoic acid-sensitive and retinoic acid-resistant APL cells at concentrations that are achievable in patients. As2O3 induces loss of the PML/RAR alpha fusion protein in NB4 cells, in retinoic-acid resistant cells derived from them, in fresh APL cells from patients, and in non-APL cells transfected to express this protein. As2O3 and retinoic acid induce different patterns of gene regulation, and they inhibit the phenotypes induced by each other. Understanding the molecular basis of these differences in the effects of As2O3 and retinoic acid may guide the clinical use of arsenic compounds and provide insights into the management of leukemias that do not respond to retinoic acid.

Angioscopy of culprit coronary lesions in unstable angina pectoris and correlation of clinical presentation with plaque morphology

This study demonstrates that plaque disruption and thrombus are absent in a considerable number of patients with unstable angina and that culprit lesion morphologies as assessed by angioscopy may differ among the various clinical subsets of patients. Although plaque disruption and thrombus undoubtedly play an important role in the pathogenesis of unstable angina, alternative mechanisms may be responsible for ischemia in some patients.

Induction of erythroid differentiation is associated with inhibition of glycolysis and a decrease in fructose 2,6-bisphosphate levels

The fraction of glucose metabolized to lactate is dramatically reduced during erythroid differentiation of mouse erythroleukemia (MEL) cells induced by dimethyl sulfoxide (DMSO), hexamethylene bisacetamide (HMBA), or sodium butyrate treatment. In order to determine the mechanism of the reduction in lactate production, several enzymatic steps in glucose catabolism were investigated. No changes in glycolytic enzyme levels were found during differentiation that could account for the alteration in lactate production and alterations in pyruvate kinase activity are known not to occur during MEL cell differentiation. Further, utilizing D-mannoheptulose, a specific inhibitor of hepatic-/tumor-specific glucokinase, no dependence on the activity of this enzyme for growth or differentiation was observed. Therefore, the possibility was entertained that the decrease in lactate production reflected a decrease in fructose 2,6-bisphosphate (F-2,6-P-2) which is a major regulator of the lactate production due to its ability to allosterically stimulate phosphofructokinase-1 (PFK-1) activity. PFK-1 cannot function in the absence of F-2,6-P-2 when only a suboptimal concentration of one of its substrates, fructose-6-phosphate (F-6-P), is present. When assayed under limiting F-6-P concentrations, it was found that following DMSO- or HMBA-induced differentiation, PFK-1 activity was decreased 7-20-fold. This finding suggested that F-2,6-P-2 levels might be controlling lactate production in this system. In keeping with this idea, marked decreases in F-2,6-P-2 levels were found to occur during DMSO- or HMBA-induced differentiation. These data suggest that decreasing F-2,6-P-2 levels account for the decrease in lactate accumulation that occurs during MEL cell differentiation.

Leukemia-associated retinoic acid receptor alpha fusion partners, PML and PLZF, heterodimerize and colocalize to nuclear bodies

In acute promyelocytic leukemia (APL), the typical t(15;17) and the rare t(11;17) translocations express, respectively, the PML/RARalpha and PLZF/RARalpha fusion proteins (where RARalpha is retinoic acid receptor alpha). Herein, we demonstrate that the PLZF and PML proteins interact with each other and colocalize onto nuclear bodies (NBs). Furthermore, induction of PML expression by interferons leads to a recruitment of PLZF onto NBs without increase in the levels of the PLZF protein. PML/RARalpha and PLZF/RARalpha localize to the same microspeckled nuclear domains that appear to be common targets for the two fusion proteins in APL. Although PLZF/RARalpha does not affect the localization of PML, PML/RARalpha delocalizes the endogenous PLZF protein in t(15;17)-positive NB4 cells, pointing to a hierarchy in the nuclear targeting of these proteins. Thus, our results unify the molecular pathogenesis of APL with at least two different RARalpha gene translocations and stress the importance of alterations of PLZF and RARalpha nuclear localizations in this disease.

Sequence-specific DNA binding and transcriptional regulation by the promyelocytic leukemia zinc finger protein

Chromosomal translocation t(11;17)(q23;21) is associated with a retinoic acid-resistant form of acute promyelocytic leukemia. The translocation fuses the RARalpha gene to the PLZF gene, resulting in the formation of reciprocal fusion proteins, hypothesized to play prominent roles in leukemogenesis. Promyelocytic leukemia zinc finger (PLZF) encodes a transcription factor with nine Krüppel-like zinc fingers, seven of which are retained in the t(11;17) fusion protein RARalpha-PLZF. We identified a specific DNA-binding site for the PLZF protein and showed that PLZF binds to this site through its most carboxyl seven zinc fingers. In co-transfection experiments, PLZF repressed transcription through its cognate binding site. This repression function of PLZF was mapped to two regions on the protein, including the evolutionarily conserved POZ domain. In contrast, the RARalpha-PLZF protein activated transcription of a promoter containing a PLZF response element. These results suggest that RARalpha-PLZF, generated in acute promyelocytic leukemia, is an aberrant transcription factor that can deregulate the expression of PLZF target genes and contribute to leukemogenesis.

Cloning of a gene (RIG-G) associated with retinoic acid-induced differentiation of acute promyelocytic leukemia cells and representing a new member of a family of interferon-stimulated genes

In a cell line (NB4) derived from a patient with acute promyelocytic leukemia, all-trans-retinoic acid (ATRA) and interferon (IFN) induce the expression of a novel gene we call RIG-G (for retinoic acid-induced gene G). This gene codes for a 58-kDa protein containing 490 amino acids with several potential sites for post-translational modification. In untreated NB4 cells, the expression of RIG-G is undetectable. ATRA treatment induces the transcriptional expression of RIG-G relatively late (12-24 hr) in a protein synthesis-dependent manner, whereas IFN-alpha induces its expression early (30 min to 3 hr). Database search has revealed a high-level homology between RIG-G and several IFN-stimulated genes in human (ISG54K, ISG56K, and IFN-inducible and retinoic acid-inducible 58K gene) and some other species, defining a well conserved gene family. The gene is composed of two exons and has been mapped by fluorescence in situ hybridization to chromosome 10q24, where two other human IFN-stimulated gene members are localized. A synergistic induction of RIG-G expression in NB4 cells by combined treatment with ATRA and IFNs suggests that a collaboration exists between their respective signaling pathways.