Variable regulation of sensitivity to retinoic acid-induced differentiation in wild-type and retinoic acid-resistant HL-60 cells

Pharmacogenomic analysis of acute promyelocytic leukemia cells highlights CYP26 cytochrome metabolism in differential all-trans retinoic acid sensitivity

Disease relapse sometimes occurs after acute promyelocytic leukemia (APL) therapy with all-trans retinoic acid (ATRA). Among the diagnostic parameters predicting relapse, heterogeneity in the in vitro differentiation rate of blasts is an independent factor. To identify biologic networks involved in resistance, we conducted pharmacogenomic studies in APL blasts displaying distinct ATRA sensitivities. Although the expression profiles of genes invested in differentiation were similarly modulated in low- and high-sensitive blasts, low-sensitive cells showed higher levels of transcription of ATRA-target genes, transcriptional regulators, chromatin remodelers, and transcription factors. In opposition, only high-sensitive blasts expressed the CYP26A1 gene, encoding the p450 cytochrome which is known to be involved in retinoic acid catabolism. In NB4 cells, ATRA treatment activates a novel signaling pathway, whereby interleukin-8 stimulates the expression of the homeobox transcription factor HOXA10v2, an effective enhancer of CYP26A1 transcription. These data were corroborated in primary APL cells, as maturation levels correlated with CYP26A1 expression. Treatment with a retinoic acid metabolism blocking agent (RAMBA) results in high-nucleoplasmic concentrations of retinoid and growth of NB4-resistant subclones. Hence, for APL blasts associated with poor prognosis, the low CYP26A1 expression may explain high risk of resistance installation, by increased retinoid pressure. Pharmacogenomic profiles of genes involved in retinoid acid metabolism may help to optimize anticancer therapies, including retinoids.

Retinoic acid resistance in acute promyelocytic leukemia

Primary resistance of PML-RARalpha-positive acute promyelocytic leukemia (APL) to the induction of clinical remission (CR) by all-trans retinoic acid (ATRA) is rare but markedly increases in frequency after > or =2 relapses from chemotherapy-induced CRs. Nevertheless, even in de novo cases, the primary response of ATRA-naive cases is variable by several measures, suggesting involvement of heterogeneous molecular elements. Secondary, acquired ATRA resistance occurs in most patients treated with ATRA alone and in many patients who relapse from combination ATRA chemotherapy regimens despite limited ATRA exposure. Although early studies suggested that an adaptive hypercatabolic response to pharmacological ATRA levels is the principal mechanism of ATRA resistance, recent studies suggest that molecular disturbances in APL cells have a predominant role, particularly if disease relapse occurs a few months after discontinuing ATRA therapy. This review summarizes the systemic and APL cellular elements that have been linked to clinical ATRA resistance with emphasis on identifying areas of deficient information and important topics for further investigation. Overall, the subject review strongly supports the hypothesis that, although APL is an infrequent and nearly cured disease, much can be gained by understanding the complex relationship of ATRA resistance to the progression and relapse of APL, which has important implications for other leukemias and malignancies.

Malignancy: Current Clinical Practice: Current Therapeutic Options in Myelodysplastic Syndromes

Myelodysplastic syndromes (MDS) are characterized initially by ineffective hematopoiesis and subsequently the frequent development of acute myelogenous leukemias (AML). During the last 15 years, important progress has been made in the understanding of the biology and prognosis of myelodysplastic syndromes. Risk-adapted treatment strategies were established due to the high median age (60-75 years) of MDS-patients and the individual history of the disease (number of cytopenias, cytogenetical changes, transfusion requirements). The use of allogeneic bone marrow transplantation for MDS patients currently offers the only potentially curative treatment, but this treatment modality is not available for the most of the "typical" MDS-patients aged >60 years. Based on in-vitro findings analyzing the potential of several agents to differentiate or to stimulate hematopoietic progenitor cells a number of therapeutic options were evaluated in clinical trials: hematopoietic growth factors (e.g. erythropoietin, G-CSF), differentiation inducers (e.g. retinoids), or cytoprotective substances (amifostine). The role of immunsuppressive agents (antithymocyte globulin, cyclosporine A) either alone or in combination is being actively investigated. Using intensive cytotoxic treatment in patients with advanced MDS or AML after MDS complete remission rates comparable with those known from the treatment of de novo AML were reported. The therapy related toxicity (early death rate <10%) was reduced by using G-CSF given prior ("Priming") and/or after the cytotoxic treatment.

Uptake and metabolism of [3H]retinoic acid delivered to human foreskin keratinocytes either bound to serum albumin or added directly to the culture medium

Retinoic acid (RA), a potent modulator of cell proliferation and differentiation is present in plasma bound to serum albumin. The biologic significance or source of plasma RA is not clear. Although most cellular RA is believed to be made in situ via the oxidation of retinol, plasma RA could potentially provide target cells with a source of preformed RA. To investigate RA uptake, we have used a model system of human foreskin keratinocytes (HKc) cultured in serum-free media to compare the uptake and metabolism of [3H]RA added directly to the culture medium in ethanol to that delivered bound to bovine serum albumin (BSA). [3H]RA added directly to the culture medium was rapidly taken up by HKc during the first 10 min of incubation (25-35% of the applied RA), no further accumulation occurred between 10 min and 90 min, and then cell-associated radioactivity rapidly decreased to about 3-5% of the applied dose by 12 h. In contrast, when [3H]RA was delivered to HKc bound to BSA, total cell-associated radioactivity reached about 2.5% of the applied dose by 5 min, increased to 3-5% of the applied radioactivity by 1 h, and no further accumulation or loss occurred over the next 23 h. The uptake by HKc of [3H]RA delivered bound to BSA or added directly to the culture medium was not influenced by pre-treatment of the cells for 72 h with unlabeled RA or by excess unlabeled RA added at the time of uptake. Analysis of the cells and media by high-performance liquid chromatography for RA metabolites found that [3H]RA added directly to the medium is rapidly converted by HKc to polar compounds that are subsequently excreted back into the medium. Also, RA added directly to the medium was susceptible to degradation in the absence of cells. In marked contrast, [3H]RA added to the media bound to BSA was much less susceptible to degradation in the absence of cells, and few [3H]RA metabolites were found in the media even after exposure to HKc for 24 h. The binding of RA to albumin clearly protects RA from conversion to polar metabolites, and also provides for a controlled delivery of RA from the aqueous extracellular environment to the cell surface.

Treatment of refractory undifferentiated acute myelogenous leukemia with all-trans-retinoic acid

A patient is described with undifferentiated acute myeloblastic leukemia refractory to two courses of daunorubicin and cytosine arabinoside. Because some the myeloblasts developed morphologic features of promyelocytes, the patient was treated with all-trans-retinoic acid (ATRA) in an attempt to promote maturation. Cytogenetic studies and sensitive molecular analysis did not reveal any abnormality classically associated with acute promyelocytic leukemia. Serial bone marrow biopsies demonstrated myeloid maturation, and the patient uneventfully went into a sustained complete remission. A review of the literature confirms this to be an apparently hitherto undescribed response to ATRA that may have therapeutic implications in similar patients.

Retinoylation of proteins in leukemia, embryonal carcinoma, and normal kidney cell lines: differences associated with differential responses to retinoic acid

In HL60 cells a nuclear protein of Mr 55,000 is retinoylated, with the formation of a thioester bond. To gain further knowledge on the role of retinoylation we studied it in cell lines with varied responses to retinoic acid (RA). Compared to HL60 the extent of retinoylation (mol/cell) was about fivefold higher in HL60/MRI, a mutant which is more sensitive to RA than HL60. Retinoylation occurred to the same extent and at similar rates in HL60 and in HL60/RA-res, a mutant resistant to differentiation by RA. One-dimensional polyacrylamide gel electrophoresis patterns for the three HL60 cell lines were similar. However, two-dimensional polyacrylamide gel electrophoresis patterns of the three HL60 cell lines were distinct. While we saw the same major retinoylated protein of Mr 55,000 in the three cell lines, the HL60/RA-res cells also contained a high level of a protein with the same Mr and a lower pI. The extent of retinoylation was greater in the RA-sensitive embryonal carcinoma cell line, PCC4.aza1R, than in a RA-resistant cell line, PCC4.(RA)-2. One-dimensional polyacrylamide gel electrophoresis patterns of retinoylated proteins of the embryonal carcinoma cell lines were different from HL60 and from each other. The retinoylation pattern of the normal canine kidney cell line (MDCK) was different from either HL60 or the embryonal carcinoma cells. These results showed the retinoylation was widespread and that the response to RA of different cell types may depend on the retinoylation of specific proteins.

Effect of butylated hydroxytoluene on bilineage differentiation of the human HL-60 myeloblastic leukemia cell

Butylated hydroxytoluene (BHT), which has both antioxidant and membrane active properties, has been reported to affect cellular differentiation. We studied its effect on the bipotent lineage differentiation of the important HL-60 human myeloblastic leukemia cell line using reduction of nitroblue tetrazolium, cell cycle analysis, population growth rate, monoclonal antibodies, and morphology. BHT markedly accelerated retinoic acid-induced myelocytic differentiation and dihydroxyvitamin D3-induced monocytic differentiation in a concentration and time-dependent manner. Butylated hydroxyanisole (BHA) had a comparable effect. Preincubation with the compounds was not necessary to evoke the acceleration Other antioxidants and inhibitors of eicosanoid synthesis were inactive. We conclude that the important food preservatives BHT and BHA accelerate the kinetics of terminal differentiation of human leukemia and that this effect is likely due at least in part to their membrane active properties.