Modulation of intrinsic in vitro resistance to carboplatin by edatrexate in the A549 human nonsmall cell lung cancer cell line.

Edatrexate (10-ethyl-deazaaminopterin) is a methotrexate analog that has been shown to have greater antitumor activity and improved therapeutic index compared to its parent compound in preclinical systems. We have evaluated the ability of edatrexate to modulate the intrinsic resistance of the lung adenocarcinoma A549 cell line to carboplatin. Concentration effects, exposure time and schedule dependence were assessed. Modulation of resistance was observed with edatrexate treatment (0.2 microM for 1 h) prior to carboplatin. The concentrations of carboplatin to achieve IC50 at the 1-, 3-, and 24-h IC50 were decreased by a mean of 16.8 times (12.2-22.2) with edatrexate preexposure. In contrast, there was little modulation observed of carboplatin resistance when carboplatin was administered prior to edatrexate. In addition, schedule dependency experiments were performed using the method described by Chou and Talalay, in which the ratio of carboplatin to edatrexate was constant or nonconstant, and both the potency of effects and the shapes of the concentration-effect curves were taken into account in a computerized analysis. These experiments also demonstrated schedule dependency. Although both treatments resulted in a reduced IC50 vs. carboplatin alone, the reduction was much greater when edatrexate was added first (12.59 vs. 2.59 times). We conclude that the combination of edatrexate and carboplatin demonstrates schedule-dependent modulation of intrinsic carboplatin resistance in this in vitro model at clinically achievable edatrexate plasma levels (0.01 to 10 microM). The greatest modulatory synergism was observed in the setting of edatrexate treatment before carboplatin. Our findings suggest a potentially useful schedule when combining edatrexate and carboplatin for the treatment of malignant disease.

Schedule-dependent synergism of edatrexate and cisplatin in combination in the A549 lung-cancer cell line as assessed by median-effect analysis.

The methotrexate analog edatrexate has been shown to have greater antitumor activity and an improved therapeutic index as compared with its parent compound in preclinical systems. These studies suggest that edatrexate may have a broad role in the treatment of solid tumors. Information regarding edatrexate in combination with other chemotherapeutic agents is limited. We evaluated the interaction of edatrexate with cisplatin in vitro as assessed by median-effect analysis in the A549 human lung-cancer cell line. The effects of dose, exposure time, and schedule dependence were assessed. Cytotoxicity was evaluated using the tetrazolium-based colorimetric (MTT) assay. The inhibitory concentration producing 50% absorbance (IC50 for edatrexate with 1 h exposure was 1.4 microM. For all combination experiments, the edatrexate dose was fixed at 0.2 microM (IC10) whereas cisplatin (CDDP) concentrations were varied for 1-, 3-, and 24-h exposures either before or after edatrexate treatment. Drug interactions were assessed using the combination-index method as defined by median-effect analysis. A synergistic interaction was documented in experiments when edatrexate was applied prior to CDDP (combination index, < 1). The combination studies in which edatrexate was used prior to CDDP resulted in significant reduction of all three CDDP IC50 values: 1-h IC50, from 30.0 to 3.9 microM; 3-h IC50, from 21.3 to 1.4 microM; and 24-h IC50, from 1.7 to 0.03 microM. In contrast, synergism was not observed in experiments in which edatrexate treatment occurred after cisplatin exposure. Median-effect analysis is a useful method of determining drug interactions. In the present study, the combination of edatrexate and CDDP demonstrated schedule-dependent synergism, with the synergism being observed only in the setting of edatrexate treatment before CDDP exposure. Due to the potential broad spectrum of activity of edatrexate plus CDDP, further studies are warranted to determine the mechanism responsible for the synergism and to investigate this combination in a variety of tumor models.

Modulation of homeobox gene expression alters the phenotype of human hematopoietic cell lines.

We have previously reported that certain genes of the HOX2 cluster of homeobox genes on human chromosome 17 are specifically expressed in human leukemic cell lines with erythroid potential, suggesting that these genes are involved in hematopoietic differentiation. We now show that the expression of the HOX 2.2 gene decreases during erythropoietin-induced differentiation of the erythroid cell line MB02. In order to study the role of the HOX 2.2 homeobox gene in hematopoiesis, vectors producing sense or antisense transcripts were introduced into K562 and HEL cells, pluripotent lines with erythroid and myeloid features. Overexpression of HOX 2.2 is associated with loss of erythroid features in both lines and an increase in certain myelomonocytic markers in K562 cells. Expression of antisense HOX 2.2 is associated with an increase in erythroid features in HEL cells and a mild decrease in myeloid characteristics in K562 cells. Overexpression of the adjacent HOX 2.1 gene in K562 cells does not produce similar phenotype changes. These data demonstrate that modulation of a specific HOX 2 homeobox gene can change the phenotype of somatic cells and suggest that certain HOX 2 genes play a role in blood cell differentiation.

Lineage-restricted expression of homeobox-containing genes in human hematopoietic cell lines.

We investigated the role of homeobox-containing genes in human hematopoiesis because homeobox genes (i) control cell fate in the Drosophila embryo, (ii) are expressed in specific patterns in human embryos, and (iii) appear to function as transcription factors that control cell phenotype in other mammalian organs. Using four homeobox probes from the HOX2 locus and a previously undescribed homeobox cDNA (PL1), we screened mRNAs from 18 human leukemic cell lines representing erythroid, myeloid, and T- and B-cell lineages. Complex patterns of lineage-restricted expression are observed: some are restricted to a single lineage, while others are expressed in multiple lineages. No single homeobox gene is expressed in all types of hematopoietic cells, but each cell type exhibits homeobox gene expression. HOX2.2 and -2.3 homeobox-containing cDNAs were cloned from an erythroleukemia cell (HEL) cDNA library, while the homeobox cDNA PL1 was isolated from a monocytic cell (U-937) library. Differentiation of HEL and K-562 cells with various inducers results in modulation of specific homeobox transcripts. In addition, HOX2.2 is expressed in normal bone marrow cells. We have demonstrated (i) lineage-restricted expression of five homeobox genes in erythroid and monocytic cell lines; (ii) expression of additional homeobox genes in other cell lineages (HL-60 and lymphoid cells); (iii) expression of one homeobox gene in normal marrow cells; and (iv) modulation of expression during differentiation. These data suggest that these genes play a role in human hematopoietic development and lineage commitment.

Expression of retinoic acid receptor alpha mRNA in human leukemia cells.

The expression of the newly described human retinoic acid receptor alpha (RAR alpha) in six nonlymphoid and six lymphoid leukemia cell lines and nine freshly obtained samples of leukemia cells from patients with acute nonlymphoid leukemia was assessed by Northern blot analysis, using a full length cDNA clone of RAR alpha as probe. RAR alpha was expressed in all 12 cell lines and in all fresh leukemia samples as two major transcripts of 2.6 and 3.5 kb in size. Levels of RAR alpha expression and transcript sizes in retinoid-sensitive cells (such as HL60 or fresh promyelocytic leukemia cells) were not different from those in other samples. Moreover, expression of RAR alpha was not significantly modulated by exposure to cis-retinoic acid (cisRA) in either cisRA-responsive or unresponsive cells. By using a 3' fragment of the RAR alpha gene as a probe, we confirmed that the transcripts visualized did not represent the homologous RAR beta gene. RAR alpha appears to be expressed in most human leukemia cells regardless of the type of biologic response to retinoic acid.

Expression of homeobox genes in human erythroleukemia cells.

Because homeobox-containing genes play a major role in embryogenesis and tissue identity in Drosophila and because similar genes encode tissue-specific transcription factors in mammalian cells, we hypothesized that homeobox genes might plan a role in hematopoietic differentiation and lineage commitment. We therefore surveyed a number of human leukemic cell lines for expression of homeobox-containing genes by Northern gel analysis with probes from the Hox 2 cluster of homeobox genes on chromosome 17. We observed transcripts for Hox 2.1, 2.2, 2.3 and 2.6 in the erythroid line HEL and for Hox 2.3 and 2.6 in the erythroid line K562. Using homeobox-specific probes we confirmed that the transcripts visualized contained the homeodomains for each gene as well as the flanking sequences. The myeloid lines HL60, KG1 and U937 did not express specific transcripts for any of the 4 genes studied. However, all these cell lines demonstrated bands when probed at low stringency with certain Hox 2 probes, indicating the expression of other homologous but as yet unidentified homeobox genes. Expression of Hox 2.3 and 2.6 was seen in some T and B lymphoid cell lines. Induction of differentiation in HEL cells resulted in complex modulation of expression of the Hox 2 genes. We have therefore observed erythroid-restricted expression of certain Hox 2 homeobox containing genes in human erythroid cell lines and modulation of that expression with differentiation, suggesting a role for these genes in the regulation of hematopoiesis. Different homeobox genes appear to be expressed in non-erythroid leukemic cell lines.