Galanin receptor 2 utilizes distinct signaling pathways to suppress cell proliferation and induce apoptosis in HNSCC

Galanin and its receptors, GALR1 and GALR2, are tumor suppressors and represent therapeutic targets in head and neck squamous cell carcinoma (HNSCC). In the present study, it was demonstrated that the re-expression of GALR1 in GALR1 and GALR2-negative HNSCC cells suppresses tumor cell proliferation. This is mediated via extracellular-regulated protein kinase-1/2 (ERK1/2)-dependent effects on the cyclin-dependent kinase inhibitors (CKI) and cyclin D1. In combination with galanin, GALR2 also suppressed proliferation by increasing CKI and decreasing cyclin D1 levels. In contrast to GALR1, overexpression of GALR2 also induced caspase-3-dependent apoptosis. It was identified that in GALR2-transfected cells, galanin induced activation of ERK1/2 and suppressed cell proliferation. Galanin stimulation also decreased the expression of cyclin D1 and induced apoptotic DNA ladder formation in GALR2-transfected cells. Pretreatment with the ERK1/2-specific inhibitor U0126 and pertussis toxin prevented the suppression of cyclin D1 expression, however did not affect DNA ladder formation. In conclusion, GALR2 expression in the presence of galanin exerts antitumor effects via cell cycle arrest and apoptotic pathways, and reactivation of these pathways may have therapeutic benefits in HNSCC..