Short Communication Endothelin Receptor Blockade Inhibits Proliferation of Kaposi’s Sarcoma Cells

Endothelin-1 (ET-1) has been shown to be mitogenic for endothelial and several tumor cells through an autocrine mechanism. In this study we evaluated whether the tumorigenic KS IMM cell line deriving from Kaposi’s sarcoma (KS), a highly angiogenic tumor, is susceptible to ET-1 mitogenic activity. By reverse transcriptase-polymerase chain reaction, we detected ET-1 mRNA expression and both ETA receptor (ETAR) and ETBR mRNA transcripts in the KS IMM cells. High concentrations of ET-1 are released from the KS IMM cells and competition-binding studies demonstrated that these cells also express functional ETAR and ETBR with high affinity for ET-1 and ET-1/ ET-3, respectively. Expression of ET-1 and cognate receptors could be detected by immunohistochemical method in vitro , in KS IMM xenograft, and in tissue sections of a human KS lesion. Furthermore ET-1 induces a marked and dose-dependent increase in [H]thymidine incorporation comparable to that elicited by vascular endothelial growth factor. Addition of both selective ETBR antagonist (BQ 788) and ETAR antagonist (BQ 123), completely blocked ET-1-induced mitogenic response and reduced the basal growth rate of unstimulated cells, suggesting that both receptors mediated the proliferative signal. Such findings demonstrate that ET-1 participates on KS pathogenesis acting as an autocrine growth factor and that ET-1 receptor antagonists may thus be novel candidates for therapeutic intervention. (Am J Pathol 2001, 158:841–847) Kaposi’s sarcoma (KS) is a highly angiogenic tumor characterized by aberrant proliferation of vascular structures with proliferation of endothelial and spindle (tumor) cells and enhanced vascular permeability. Angiogenic molecules produced by KS dictate the progression of the lesion by autocrine and paracrine mechanisms and are likely to play a central role in the development and progression of KS. The KS-derived spindle cells induce vascular lesions when inoculated subcutaneously in the nude mouse thus suggesting that these cells secrete factors that induce angiogenesis and enhance vascular permeability. The production in vitro of autocrine growth factors, which include basic fibroblast growth factor, interleukins 1 and 6, and vascular endothelial growth factor (VEGF), by the KS cells, has been previously reported. Endothelin-1 (ET-1), is constitutively produced by endothelial cells and in elevated amounts in many tumors. ET-1 acts through two distinct subtypes of G proteincoupled receptors, namely ETA and ETB. ETAR shows selectivity for ET-1 whereas ETBR binds both ET-1 and ET-3. In tumors, such as in ovarian and cervical carcinoma, ET-1 is overexpressed and acts as an autocrine growth factor selectively through ETAR, as demonstrated by the inhibitory effects induced by specific ETAR antagonists. The observation that ET-1 is a mitogen for endothelial and tumor cells raises the possibility that ET-1 contributes to the pathogenesis of KS. In this report, we demonstrate that KS IMM, an immortalized KS-derived cell line that retains most of the features of the parental tumor and can induce KS-like sarcomas when injected subcutaneously in nude mice, expresses in vitro and in vivo ETA and ETB receptors, and secretes the potent mitogenic peptide ET-1 that acts as an autocrine growth factor. These findings, together with the inhibitory effect of ET-1 receptor antagonists on cell proliferation, suggest that ET-1 plays an important role in the KS progression