Emerging Evidences for a Protumorigenic Role of the Endothelin Axis Uncover New Therapeutic Targets in Multiple Myeloma

Multiple Myeloma (MM) is a monoclonal tumor of bone marrow (BM) plasma cell (PC), usually associated with a number of disease manifestations, including skeletal damage, anemia and immunosuppression. Despite recent therapeutic advances and an increased patient life expectancy, MM still remains an incurable malignancy. MM PC is characterized by a strong BM niche dependence, acquiring an autonomous capacity of growth only in the latter stages of disease. The endothelin (ET) axis is mainly composed of Endothelin-1 (ET-1), and 2 G-protein coupled receptors, the ET receptor A (ETAR) and ET receptor B (ETBR). In the last decades it has been demonstrated that the ET axis is able to promote the development and progression of tumor cells in an autocrine and/or paracrine fashion. Accordingly, recent experimental and clinical data obtained in solid tumors have evidenced the possibility of interfering with ET receptors (ETRs) - via specific antagonists - for therapeutic purposes. On the basis of these premises, primary aim of this study was to assess whether the ET axis may have a protumorigenic role in MM, therefore becoming a therapeutic target. The experimental evidences we obtained demonstrated that MM cell lines, primary MM PCs and BM microenvironment cells express ET-1 transcript and release the corresponding protein. Interestingly, while ETAR was constitutively expressed by PCs from healthy donors, primary malignant PCs and MM cell lines, ETBR was detected only on malignant PCs of 54 of the 100 patients enrolled in the study and in 3/5 MM cell lines. Interestingly, B lymphocytes isolated from healthy donors BM or peripheral blood (PB) or from MM patients PB, did express ETAR but not ETBR. Based on the evidence that the expression of ETBR was strictly connected to the neoplastic transformation, we next demonstrated an altered methylation status of the ETBR promoter gene in malignant PCs from ETBR-expressing MM patients and MM cell lines. The possibility of interfering with ETRs in MM was investigated in vitro using RPMI-8226 and U266 MM cell lines through the use of ETAR and ETBR selective antagonists. Both ETAR and ETBR antagonists, used alone and/or in combination, decreased RPMI-8226 and U266 cell lines viability. Based on these data we next evaluated the potential therapeutic significance in MM of Bosentan, a dual ETRs antagonist already used in clinical practice for pulmonary arterial hypertension. In agreement with our in vitro experiments, Bosentan appeared to be effective in inhibiting the viability of RPMI-8226 and U266 cell lines by reducing p-p44/42 MAPK. Furthermore, this action appeared to be synergic to that of bortezomib, a proteasome inhibitor drug already used in first-line treatment of MM. Overall our data demonstrate that the ET axis is a potential therapeutic target in MM. Further data are awaited in order to establish the prognostic significance of the aberrant expression of ETBR in MM.