Background and purpose: The anti-cancer agent [Arg(6), D-Trp(7,9), N(me)Phe(8)]-substance P (6-11) (SP-G) modulates gastrin releasing peptide (GRP) and arginine vasopressin signalling in small cell lung cancer cells leading to growth arrest and apoptosis. We have shown that SP-G acts as a biased agonist at GRP receptors. This work examines the hypothesis that SP-G acts as a biased agonist at the V-1A vasopressin receptor.
Experimental approach: The human V-1A receptor was expressed in CHO-K1 cells. Extracellular regulated kinase (ERK) activation and intracellular Ca2+ were measured using activation state-specific antibodies and Fura-2-AM respectively. The effect of SP-G on tumourigenicity was assessed by colony assay.
Key results: In V-1A receptor expressing cells, SP-G caused a sustained activation of ERK via a stimulation of V-1A receptor coupling to G(i). Inhibition of Gi with Pertussis toxin attenuated the inhibition by SP-G of the growth of CHO-K1 cells stably expressing the V-1A receptor. Chimeric V-1A receptors containing the second or third intracellular loop of the V-2 receptor were capable of binding vasopressin and SP-G but had altered ability to activate phospholipase C ( PLC) and ERK. The second intracellular loop of the V-1A receptor was essential for vasopressin-stimulated PLC and ERK activation but not for SP-G-induced ERK activation.
Conclusions and implications: This work provides mechanistic insight, for biased agonists at V-1A receptors and highlights a potential role for such agents as anti-cancer agents.