The oxytocin receptor antagonist atosiban inhibits cell growth via a "biased agonist" mechanism

In human myometrial cells, the promiscuous coupling of the oxytocin receptors (OTRs) to G_q and G_i leads to contraction. However, the activation of OTRs coupled to different G protein pathways can also trigger opposite cellular responses, e.g. OTR coupling to G_i inhibits, whereas its coupling to G_q stimulates, cell proliferation. Drug analogues capable of promoting a selective receptor-G protein coupling may be of great pharmacological and clinical importance because they may target only one specific signal transduction pathway. Here, we report that atosiban, an oxytocin derivative that acts as a competitive antagonist on OTR/G_q coupling, displays agonistic properties on OTR/G_i coupling, as shown by specific ³⁵S-labeled guanosine 5′-3-O-(thio) trisphosphate ([³⁵S]GTPγS) binding. Moreover, atosiban, by acting on a G _i-mediated pathway, inhibits cell growth of HEK293 and Hadin-Darby canine kidney cells stably transfected with OTRs and of DU145 prostate cancer cells expressing endogenous OTRs. Notably, atosiban leads to persistent ERK1/2 activation and p21^WAF1/CIP1 induction, the same signaling events leading to oxytocin-mediated cell growth inhibition via a G_i pathway. Finally, atosiban exposure did not cause OTR internalization and led to only a modest decrease (20%) in the number of high affinity cell membrane OTRs, two observations consistent with the finding that atosiban did not lead to any desensitization of the oxytocin-induced activation of the G_q- phospholipase C pathway. Taken together, these observations indicate that atosiban acts as a "biased agonist" of the human OTRs and thus belongs to the class of compounds capable of selectively discriminating only one among the multiple possible active conformations of a single G protein-coupled receptor, thereby leading to the selective activation of a unique intracellular signal cascade.