Filamin A (FLNA) Plays an Essential Role in Somatostatin Receptor 2 (SST2) Signaling and Stabilization After Agonist Stimulation in Human and Rat Somatotroph Tumor Cells

Somatostatin receptor type 2 (SST2) is the main pharmacological target of medical therapy for GH-secreting pituitary tumors, but molecular mechanisms regulating its expression and signaling are largely unknown. The aim of this study was to investigate the role of cytoskeleton protein filamin A (FLNA) in SST2 expression and signaling in somatotroph tumor cells. We found a highly variable expression of FLNA in human GH-secreting tumors, without a correlation with SST2 levels. FLNA silencing in human tumoral cells did not affect SST2 expression and localization but abolished the SST2-induced reduction of cyclin D1(-37% +/- 15% in control cells, P < .05 vs basal) and caspase-3/7 activation (+63% +/- 31% in control cells, P < .05 vs basal). Overexpression of a FLNA dominant-negative mutant that specifically prevents SST2-FLNA binding reduced SST2 expression after prolonged agonist exposure (-55% +/- 5%, P < .01 vs untreated cells) in GH3 cells. Moreover, SST2-induced apoptotic effect (77% +/- 54% increase of caspase activity, P < .05 vs basal) and SST2-mediated ERK1/2 inhibition (48% +/- 17% reduction of ERK1/2 phosphorylation, P < .01 vs basal) were abrogated in cells overexpressing another FLNA mutant that prevents FLNA interaction with partner proteins but not with SST2, suggesting a scaffold function of FLNA in somatotrophs. In conclusion, these data demonstrate that FLNA is involved in SST2 stabilization and signaling in tumoral somatotrophs, playing both a structural and functional role.