Adrenocortical carcinomas (ACCs) are rare endocrine tumors with poor prognosis. The insulin-like growth factor 2 (IGF2) is overexpressed in the great majority of ACC, and IGF2/IGF1R pathway acts as a proliferative autocrine loop, but to date IGF1R-targeted therapies have demonstrated a limited efficacy and the molecular mechanisms regulating this pathway are still unknown. The cytoskeleton acting-binding protein filamin A (FLNA), determinant in cancer progression and metastasis in different tumors, affects the intracellular trafficking and signalling of many receptors, including growth factor receptors such as EGFR, but a possible role of FLNA in regulating IGF1R has never been investigated. The aims of this study are: 1) to test FLNA involvement in modulating IGF1R signaling in human ACC cell lines H295R and SW13; 2) to evaluate FLNA expression in ACCs and a possible correlation with IGF1R pathway activation. By immunoprecipitation we found that IGF1R interacted with FLNA in basal condition, with an increased recruitment of FLNA after IGF2 stimulation, in both cell lines. As expected, IGF2 promoted H295R and SW13 cell proliferation and migration and accordingly increased ERK and cofilin activation. Interestingly, all these tumorigenic actions of IGF2 were potentiated in the absence of FLNA. Indeed, in cell silenced for FLNA, IGF2 induced a further increase of proliferation (+69±40% in SW13 and +11±1.3% in H295R, P<0.05), migration (+10±7% in SW13 and +17±8% in H295R, P<0.05), p-ERK/tot-ERK ratio (+1.14±0.2 fold in SW13, P<0.05) and a decrease of p-cofilin/tot-cofilin ratio (−0.20±0.02 fold in SW13, P<0.05) vs IGF2-stimulated control cells. Furthermore, FLNA silencing in SW13 cells was associated to an increase of IGF1R expression after IGF2 stimulation (+1.60±0.7 fold vs IGF2-stimulated control cells, P<0.05), suggesting that FLNA is involved in receptor downregulation. Finally, western blot analysis showed significantly lower FLNA expression in ACCs (n=5) than in adrenocortical adenomas (n=19) (FLNA/GAPDH ratio 0.98±1.4 and 4.37±2.5, respectively, P<0.01). Moreover, FLNA expression levels in ACC samples were negatively correlated with ERK phosphorylation status. In conclusion, we demonstrated that low levels of FLNA enhance IGF2/IGF1R pathway activation in adrenocortical tumor cells, suggesting FLNA as a new factor possibly influencing the response to the therapy with IGF1R-target drugs.
18 - 21 May 2019
European Society of Endocrinology