Endothelin B Receptor (Etbr) expression by human malignant plasma cells: preliminary evidence for a role of the endothelin axis in multiple myeloma.

The endothelin (ET) axis, which consists of three small peptides (ET- 1, ET-2, ET-3) and two ET receptors (ETAR and ETBR), plays an impor- tant role both in normal and tumor conditions. In solid tumors, ET-1 has been shown to act as an important paracrine or autocrine growth factor for neoplastic cells, thus participating to tumor development and progres- sion. The initial dependence of malignant plasma cells on signals fromthe bone marrow microenvironment together with our preliminary analyses showing the selective expression of the ETBR transcript by multiple myeloma (MM) cell lines, prompted us to test a possible role of the endothelin axis in supporting MM cells. Aim of our study was therefore: i) to confirm the expression of ETBR by malignant plasma cells from MM cell lines and patients’ samples at both mRNA and protein lev- el; ii) to analyze the expression of ETBR ligands by bone-marrow stro- mal cells; iii) to establish the effects of ET-1 on MM cell lines prolifera- tion and/or inhibition of apoptosis under starvation or exposure to cyto- toxic agents; and iv) to evaluate the capability of ETBR antagonists to reverse such effects. Our results demonstrated that MM cell lines (U-266, OPM-2, LP-1) expressed ETBR transcripts (RT-PCR) and proteins (West- ern blot and flow-cytometry). ETBR expression was also detected on pri- mary malignant plasma cells in 50% of patients’ bone marrow samples analyzed by flow-cytometry (Figure 1A) and immunohistochemistry. Interestingly, bone marrow-derived mesenchymal stromal cells (MSC) either undifferentiated or differentiated to adipocytes, chondrocytes and osteocytes as well as bone marrow fibroblasts and endothelial cells, revealed the expression of the ET transcripts at various levels. Moreover, in vitro experiments revealed that the addition of ET-1 rescued MM cell lines from starvation-induced apoptosis (Figure 1B). Taken together our results indicate that in MM bone marrow stromal cells may promote the survival of malignant plasma cells through the ET-1/ETBR axis. Ongo- ing analysis will confirm the capability of ET-1 to rescue MM cells from drug-induced apoptosis and the reversibility of such effects by ETBR antagonists, thus indicating their possible therapeutic application. A combined treatment in which the efficacy of established chemothera- peutic agents would be enhanced by targeting specific components of the ET axis could be a promising approach to be explored in clinical settings.