Alpha₂-adrenoceptor agonists trigger prolactin signaling in breast cancer cells

Breast cancer is the most frequent malignancy among women worldwide. We have described the expression of α₂-adrenoceptors in breast cancer cell lines, associated with increased cell proliferation and tumor growth. A mitogenic autocrine/paracrine loop of prolactin (Prl) has been described in breast cancer cells. We hypothesized that the α₂-adrenergic enhancement of proliferation could be mediated, at least in part, by this Prl loop. In both T47D and MCF-7 cell lines, the incubation with the α₂-adrenergic agonist dexmedetomidine significantly increased Prl release into the culture medium (measured by the Nb2 bioassay), this effect being reversed by the α₂-adrenergic antagonist rauwolscine. No change in Prl receptors (PrlR) was observed by RT-qPCR in these cell lines. In IBH-6 cells a decrease in Prl secretion was observed at the lower dexmedetomidine concentration. The signaling pathways involved in ovine Prl (oPrl) and dexmedetomidine action were also assessed. Both compounds significantly activated STAT5 and ERK in all three cell lines. In T47D and MCF-7 cell lines also AKT was activated by both Prl and dexmedetomidine. We therefore describe the STAT5 phosphorylation by an α₂-adrenergic agonist, dexmedetomidine. In T47D cells, the α₂-adrenergic stimulation of cell proliferation is probably mediated, at least in part, by the Prl autocrine/paracrine loop, because this effect is abrogated by the specific PrlR antagonist Δ1-9-G129R-hPrl. The implication of Prl loop describes a novel mechanism of action of this GPCR.