A novel pathway activated by somatostatin receptor type 2 (SST2): Inhibition of pituitary tumor cell migration and invasion through cytoskeleton protein recruitment

Abstract

The pharmacological therapy of GH-secreting pituitary tumors is based on somatostatin (SS) analogs that reduce GH secretion and cell proliferation by binding mainly SS receptors type 2 (SST2). Antimigratory effects of SS have been demonstrated in different cell models, but no data on pituitary tumors are available. Aims of our study were to evaluate SST2 effects on migration and invasion of human and rat tumoral somatotrophs, and to elucidate the molecular mechanism involved focusing on the role of cofilin and filamin A (FLNA). Our data revealed that SST2 agonist BIM23120 significantly reduced GH3 cells migration (-22% ± 3.6%, p < 0.001) and invasion on collagen IV (-31.3% ± 12.2%, p < 0.01), both these effects being reproduced by octreotide and pasireotide. Similar results were obtained in primary cultured cells from human GH-secreting tumors. These inhibitory actions were accompanied by a marked increase in RhoA/ROCK-dependent cofilin phosphorylation (about 2.7-fold in GH3 and 2.1-fold in human primary cells). Accordingly, the anti-invasive effect of the SS analog was mimicked by the overexpression in GH3 cells of the S3D phosphomimetic cofilin mutant, and abolished by both phosphodeficient S3A cofilin and a specific ROCK inhibitor that prevented cofilin phosphorylation. Moreover, FLNA silencing and FLNA dominant-negative mutants FLNA19-20 and FLNA21-24 transfection demonstrated that FLNA plays a scaffold function for SST2-mediated cofilin phosphorylation. Accordingly, cofilin recruitment to agonist-activated SST2 was completely lost in FLNA silenced cells. In conclusion, we demonstrated that SST2 inhibits rat and human tumoral somatotrophs migration and invasion through a molecular mechanism that involves FLNA-dependent cofilin recruitment and phosphorylation.

Cytoskeleton Protein Filamin A Is Required for Efficient Somatostatin Receptor Type 2 Internalization and Recycling through Rab5 and Rab4 Sorting Endosomes in Tumor Somatotroph Cells

“…As far as GPCRs expressed in pituitary cells are concerned, SST 2 represents the main molecular target for the medical therapy with SSA of GH-secreting tumors in acromegalic patients. Previous data obtained by our group showed that FLNA is a key modulator of SST 2 stabilization and signaling in tumor somatotroph cells [14, 33], and that transient SST 2 –FLNA interactions occur in living CHO cells to preferentially localize receptors along actin fibers, allowing efficient ligand-promoted SST 2 recruitment to clathrin-coated pits and internalization [15].…”

Cytoskeleton actin-binding proteins in clinical behavior of pituitary tumors

“…Another mechanism by which FLNA mediates SSTR2 antiproliferative action has been demonstrated in pancreatic neuroendocrine tumors, where a competition of FLNA with PI3K regulatory subunit p85 for the binding to SSTR2 occurs (Najib et al 2012), and where FLNA is required for SSTR2 expression and signaling (Vitali et al 2016). FLNA is also required to mediate the inhibitory effects of SSTR2 on cell migration and invasion in GH-secreting pituitary tumors (Peverelli et al 2018a), by mediating the recruitment to activated SSTR2 of components of the cofilin pathway, as discussed below.…”

“…The specific SSTR2 agonist BIM23120 inhibited migration and invasion on collagen IV in both primary cultured cells from human GH-secreting tumors and GH3 cell line (Peverelli et al 2018a). It is of clinical relevance noting that these effects are reproduced by the two SSAs used in the pharmacological therapy of pituitary tumors: octreotide, with high preferential binding affinity for SSTR2, and pasireotide, with a broader spectrum of affinity for different receptor subtypes and high binding affinity to SST5.…”

“…To date, DRD2 coupling with RhoA/ROCK signaling pathway has been described only in mouse striatal neurons (Deyts et al 2009, Galan-Rodriguez et al 2017, but the molecular mechanism involved has never been investigated. As described below, FLNA may function as scaffold linking together GPCRs and components of the cofilin pathway (Peverelli et al 2018a), suggesting that DRD2 effect on P-cofilin may be mediated by FLNA. However, G proteins coupled to DRD2, or beta arrestins, involved in DRD2 internalization (Kim et al 2001) and able to activate Rho (Barnes et al 2005, Ma et al 2012, may be potential molecular players connecting DRD2 to cofilin pathway.…”

“…Recently, an anti-migratory effect of SSTR2 in GH-secreting tumor cells has been discovered, in addition to its recognized antiproliferative and pro-apoptotic actions (Peverelli et al 2018a). This previously unknown SSTR2 ability to actively participate to cytoskeleton remodeling is mediated by FLNA and cofilin.…”