G protein subtype preference dictates paroxetine-enhanced serotonin receptor response in non-malignant breast...

Serotonin (5-HT) plays a key role in shaping mammary tissue morphology and regulating growth, lactation and involution. Serotonin re-uptake inhibitors (SSRIs) retain 5-HT outside the cell membrane, but their impact on breast cancer risk through altered cell proliferation is unclear. Using transcriptomic and signaling profiles and a systematic screen of receptor-selective ligands we assessed the impact of 5-HT receptor activation on non-malignant breast cell growth. Paroxetine reduced 5-HT levels in immortalized primary breast epithelial (HME-hTert) cells, mitigated free oxygen radical formation and decreased cell migration and proliferation. Thus, pathways related to cell cycle and DNA damage repair were underrepresented in the transcriptomic profile of paroxetine-treated cells. However, enriched transcipts overrepresented genes affecting neural transmission and GPCR signaling, suggesting an increase in 5-HT receptor activation. As 5-HT induced the levels of both cAMP and inositol triphosphate (IP3), the contributions of individual receptors were deciphered using receptor-selective agonists and antagonists. 5-HT receptors coupled to every G_α protein subtype were expressed and functional. The activation of the G_S-coupled receptor 5-HT₇ and the antagonists of G_i- and G_q-coupled 5-HT_1D and 5-HT_2B receptors generally suppressed proliferation. Paroxetine-dependent growth suppression was reversed by inhibitors of G_S-coupled 5-HT₇ receptor, protein kinase A, adenylyl cyclase, and agonists of G_i and G_q-coupled receptors. This matrix of interactions suggest that the anti-proliferative responses to 5-HT in non-malignant breast cells align with the G protein preference of the receptors. The potential benefits of repurposing receptor subtype-selective agents and SSRIs, and their ideal combinations, represent a novel opportunity for cancer risk reduction.