Endothelin expression in human megakaryoblastic leukemia cell lines and normal platelet precursors

Vascular endothelial growth factor-C modulates proliferation and chemoresistance in acute myeloid leukemic cells through an endothelin-1-dependent induction of cyclooxygenase-2

High-level expression of vascular endothelial growth factor (VEGF)-C is associated with chemoresistance and adverse prognosis in acute myeloid leukemia (AML). Our previous study has found that VEGF-C induces cyclooxygenase-2 (COX-2) expression in AML cell lines and significant correlation of VEGF-C and COX-2 in bone marrow specimens. COX-2 has been reported to mediate the proliferation and drug resistance in several solid tumors. Herein, we demonstrated that the VEGF-C-induced proliferation of AML cells is effectively abolished by the depletion or inhibition of COX-2. The expression of endothelin-1 (ET-1) rapidly increased following treatment with VEGF-C. We found that ET-1 was also involved in the VEGF-C-mediated proliferation of AML cells, and that recombinant ET-1 induced COX-2 mRNA and protein expressions in AML cells. Treatment with the endothelin receptor A (ETRA) antagonist, BQ 123, or ET-1 shRNAs inhibited VEGF-C-induced COX-2 expression. Flow cytometry and immunoblotting revealed that VEGF-C induces S phase accumulation through the inhibition of p27 and the upregulation of cyclin E and cyclin-dependent kinase-2 expressions. The cell-cycle-related effects of VEGF-C were reversed by the depletion of COX-2 or ET-1. The depletion of COX-2 or ET-1 also suppressed VEGF-C-induced increases in the bcl-2/bax ratio and chemoresistance against etoposide and cytosine arabinoside in AML cells. We also demonstrated VEGF-C/ET-1/COX-2 axis-mediated chemoresistance in an AML xenograft mouse model. Our findings suggest that VEGF-C induces COX-2-mediated resistance to chemotherapy through the induction of ET-1 expression. Acting as a key regulator in the VEGF-C/COX-2 axis, ET-1 represents a potential target for ameliorating resistance to chemotherapy in AML patients.

The mitogenic activity of endothelin-1 on megakaryocytopoiesis in vitro

Endothelin (ET)-1 induces proliferation of various cells including smooth muscle cells, fibroblasts, glomerular mesangial cells, endothelial cells and osteoblasts. ET-1 also stimulates synthesis of interleukin (IL)-6 in endothelial and bone marrow stromal cells of rat. It is well known that IL-6 modulates megakaryocytopoiesis. Some studies have indicated that megakaryocytes express both ET receptors and they are targets for ET. Therefore we planned to examine the effects of ET-1 on the growth of normal megakaryocytic cells in rat bone marrow primary cell culture. Bone marrow cells were cultured at 37 degrees C, in an incubator atmosphere of 5% CO2 in air and 95% relative humidity for nine days. ET-1 at 10(-7), 10(-8 ) and 10(-11) M, and control with saline were added at the beginning of the experiment protocol. At each day, plasma clots were stained using direct-coloring thiocholin method for acetylcholinesterase activity. Although 10(-7) M ET-1 did not change the proliferation of megakaryocytic cells, this could be due to the presence of over crowded fibroblasts in the same environment. 10(-8) M ET-1 stimulated megakaryocytic cell growth to 234% over the control on the fifth day. ET-1 at a concentration of 10(-11) M also rised the megakaryocytic cell number significantly reaching up to 86% at the sixth day. Our results indicate that ET-1 may modulate the growth of megakaryocytic cells by an autocrine and/or paracrine action.

Human bone marrow platelet progenitors express and release endothelin-1

Because it has been shown that human platelet precursors from normal bone marrow express preproendothelin-1 (ET-1) mRNA, this investigation was designed to find out whether these cells could synthesize and release mature ET-1 and express ET receptors. Therefore, we examined the expression of endothelin-converting enzyme (ECE) mRNA and of mRNAs for ET receptors in cells purified from normal bone marrow and measured immunoreactive (ir)ET in their culture medium. RT-PCR applied to RNAs from platelet precursors yielded amplified fragments of the expected sizes of 567, 428, and 299 bp for ECE, ETB and ETA receptors, respectively. These cells released ir-ET into the culture medium in a time-dependent fashion. These results raise the possibility of autocrine actions of the intrinsic ET system in bone marrow platelet precursor cells.

Phosphoramidon-sensitive and -insensitive endothelin-converting enzyme in human megakaryoblastic cell lines

The human megakaryoblastic cell lines HEL, MEG-01, and DAMI express preproendothelin-1 mRNA. This investigation was designed to find out whether they could also express endothelin-converting enzyme (ECE) and release mature endothelin (ET). RT-PCR applied to RNA isolated from the cell lines amplified fragments of the expected size. The amplified cDNA of MEG-01 was submitted to restriction enzymes, which generated the expected subfragments. Membrane ECE activity was phosphoramidon-sensitive, in contrast to the cytosolic activity capable of producing ET-1 from big ET-1. The three cell lines produced ir-ET in a time-dependent manner. These results show that human megakaryoblastic cell lines express functional, phosphoramidon-sensitive and insensitive ECE activity and produce mature ET.

Change of endothelin receptor subtype in the MEG-01 human megakaryoblastic cell line

The aim of this investigation was to determine whether the endothelin receptor subtype of a megakaryoblastic cell line (MEG-01) changes during culture passages as cells undergo maturation and differentiation. On early-passage cells, binding of [125I]endothelin-1 was completely inhibited by 1 microM BQ 123 (cyclo-[D-tryptophanyl-D-aspartyl-prolyl-D-valyl-leucyl]), but not by sarafotoxin 6C. Also the endothelin-1-enhancing effect on [Ca2+]i was prevented by BQ 123, whereas sarafotoxin 6C had no effect on [Ca2+]i. In late-passage cells, endothelin ET(B) analogs, unlike endothelin ET(A) analogs, competed with binding of [125I]endothelin-1. Endothelin ET(B) receptor agonists increased [Ca2+]i while the endothelin-1-induced response was inhibited by BQ 788 ([N-[(2R,6S)-2,6-dimethyl-piperidinocarbonyl]-4-methyl-D-leucyl]-[ N(omega)-(methoxycarbonyl)-D-tryptophanyl]-D-norleucine), but not by BQ 123, although both endothelin ET(A) and ET(B) receptor mRNAs were expressed, as shown by reverse transcriptase-polymerase chain reaction. These results demonstrate that in MEG-01 cells switch from expression of endothelin ET(A) to expression of ET(B) receptors during culture. The data also suggest that late-passage MEG-01 cells look like platelets, in terms of endothelin receptor subtype.