Changes in chromatin conformation regulate the 5-lipoxygenase gene expression during differentiation of HL60 cells

Trichostatin A and structurally related histone deacetylase inhibitors induce 5-lipoxygenase promoter activity

5-Lipoxygenase (5-LO) mRNA expression in Mono Mac 6 cells is induced by the histone deacetylase inhibitor trichostatin A (TsA). In order to study the effects of TsA and several structurally related compounds such as MD85, D237 and M232 on 5-LO promoter activity, we have analyzed the response of a 5-lipoxygenase (5-LO) promoter luciferase reporter gene construct to histone deacetylase inhibitors in transiently transfected Mono Mac 6 and HeLa cells. We show that the activity of 5-LO promoter constructs comprising the sequences -778 to and of several successive deletions of the 5-LO promoter is strongly increased upon TsA treatment. The data suggest a significant involvement of histone deacetylases in the regulation of 5-LO gene transcription. The basal activity of the 5-LO promoter strongly depends on the presence of multiple Sp1-binding sites (GC-boxes), five of which are positioned in tandem. Deletion of the five tandemized GC-boxes in the 5-LO reporter gene construct revealed that the induction of 5-LO promoter activity by TsA seems to be independent of these GC-boxes. Methylation of 5-LO reporter gene constructs by M.Hpall reduced 5-LO promoter activity but did not prevent induction of promoter activity by TsA, although the activated reporter gene activities were lower compared to the unmethylated plasmid, indicating the dominance of methylation over TsA-sensitive histone deacetylation in silencing of the 5-LO gene. The structure-activity data obtained for histone deacetylase inhibitors suggest that this assay system might serve as a cellular screening tool for the development of HDAC inhibitors.

The biology of 5-lipoxygenase: function, structure, and regulatory mechanisms

5-Lipoxygenase (5-LO) catalyzes the two-step conversion of arachidonic acid to leukotriene A4 (LTA4). The first step consists of the oxidation of arachidonic acid to the unstable intermediate 5-hydroperoxyeicosatetraenoic acid (5-HPETE), and the second step is the dehydration of 5-HPETE to form LTA4. These events are the first committed reactions leading to the synthesis of all leukotrienes and play a critical role in controlling leukotriene production. 5-LO has evolved many complex structural features and regulatory mechanisms to allow it to fulfill this highly specialized role. The biology of 5-LO is reviewed here with an emphasis on enzymatic function, protein and gene structure, essential cofactors, and the many regulatory mechanisms controlling its expression.

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.

Inhibitors of lipoxygenase metabolism exert synergistic effects with retinoic acid on differentiation of human leukemia HL-60 cells

The interaction between drugs suppressing the production of arachidonic acid metabolites and inducers of granulocytic differentiation, i.e., all-trans retinoic acid and dimethyl sulphoxide (DMSO) was investigated using the human myeloid leukemia HL-60 cell line. The experiments were designed as a complete factorial combination of treatments and used chemiluminescence as a marker of cell oxidative burst (level of differentiation). It was clearly demonstrated that two structurally different inhibitors of 5-lipoxygenase metabolism, i.e., 3-[1-(4-chlorobenzyl)-3-t-butyl-thio-5-isopropylindol-2-yl]-2,2-di methyl propanoic acid (MK-886) and esculetin, significantly potentiated the HL-60 cell differentiation induced by retinoic acid or DMSO. Detailed mathematical evaluation of the results revealed the synergistic character of the interaction. The most significant effects were achieved with a combination of 5-lipoxygenase inhibitors and low doses of retinoic acid. These results were confirmed by analysis of cell morphology and expression of cell surface antigen CD11b after treatment of the cells with selected concentrations of agents. In contrast to those on differentiation, no additional effects of MK-886 or esculetin on cell proliferation (cell number and cell cycle parameters) and apoptosis were observed. An inhibitor of cyclooxygenases, indomethacin, affected neither cell proliferation nor differentiation of cells. The results implied that either modulation of 5-lipoxygenase metabolism or a certain type of imbalance in arachidonic acid metabolism could modulate the effects of retinoic acid or DMSO on myeloid cell differentiation.