Endocrine Abstracts

Cushing’s disease (CD) is a rare disorder of chronic hypercortisolism due to an adrenocorticotropic hormone (ACTH)-secreting pituitary tumor. Pasireotide, a multireceptor-targeted somatostatin (SST) analog with high binding affinity for the predominant SST receptor in human corticotroph tumors, SST5, has been recently approved to treat adult patients for whom surgery failed or does not represent a suitable option. However, to date, the molecular predictors of antisecretory and antiproliferative favorable responses to the medical therapy with pasireotide, are unclear. The cytoskeletal protein FLNA plays a crucial role in GH-secreting pituitary tumor responsiveness to somatostatin analogs by regulating the physiology of SST2. Aim of the present study was to investigate a possible involvement of FLNA in the modulation of SST5 expression, signaling and pasireotide-mediated downregulation. First, by co-immunoprecipitation experiments, we demonstrated that FLNA binds to SST5 in mouse tumor corticotroph AtT-20/D16v-F2 cells. FLNA genetic silencing led to statistically significant strong decrease of receptor expression in basal condition (−45.3±33.5% and −27±12%, % of basal negative control, P<0.05, in AtT-20/D16v-F2 and primary tumoral corticotroph cells). Moreover, downregulation of SST5 was observed in negative control cells incubated with 10nM pasireotide for 72h (−46±25.7%, P<0.01, % of basal), whereas no further reduction of SST5 was detected in FLNA silenced cells. Regarding SST5 signaling, FLNA expression knock down caused an impairment of SST5 inhibitory action on ERK 1/2 phosphorylation (−48±25%, P<0.05, in negative control cells vs −22±30.3%, P= ns, in FLNA silenced cells). Furthermore, by Western blot, FLNA and SST5 protein expression levels correlates in 8 human ACTH-secreting pituitary tumors tested (r=0.762, P=0.037). Interestingly, a trend of higher FLNA protein expression levels were observed in the group of primary tumor cultures that responded to pasiretoide (with reduction of p-ERK1/2 levels) compared to the not responding group (P=0.071). In conclusion, our study demonstrated that FLNA is implied in SST5 expression modulation and SST5 signal transduction mechanisms. Additional experiments are needed to unveil a possible role of FLNA in the inhibitory effect of SST5 on ACTH secretion.