Endocrine Abstracts

Medullary thyroid carcinoma (MTC) is a rare neuroendocrine tumor originating from parafollicular thyroid C cells that produce calcitonin, accounting for 5%-10% of thyroid cancers. In all inherited cases of MTC, and in about 40% of sporadic cases, activating mutations of the receptor tyrosine kinase proto-oncogene RET are found. Signaling pathways involved in cell proliferation, survival and motility are triggered by constitutively active RET, but the mechanisms underlying malignant transformation of C cells have been only partially elucidated. Neck surgery is the mainstay of treatment. New modalities of target treatments, including cabozantinib and vandetanib, two tyrosine kinase inhibitors, and selpercatinib and pralsetanib, two selective ret inhibitors have shown to be promising in advanced and recurrent disease. However, patients with unresectable MTC are difficult to treat and the prognosis remains unfavorable. The cytoskeletal protein filamin A (FLNA) is involved in the regulation of different signaling pathways mediated by various tyrosine kinase receptors, but there are no data on the role of FLNA in the RET-mediated pathway. Aim of this study was to test the role of FLNA in regulating RET expression, RET and ERK1/2 phosphorylation and cell proliferation, in human TT cell line, harboring the RET mutation C634W, and primary cultures cells deriving from surgically removed MTC. Our data showed that genetic silencing of FLNA in TT cell line significantly reduced RET expression (-18.54 (30.79)% vs control cells, P value<0.01) and its phosphorylation (-35.9 (37.4)% vs control cells, P value<0.01); moreover, it inhibited ERK1/2 phosphorylation (-40.73 (33.33)% vs control cells, P value<0.01) in accordance with cell proliferation reduction (-33 (21.08)% vs control cells, P value<0.01). Furthermore, we found a decrease in calcitonin secretion (-19.78 (17.57)% vs control cells, P value<0.05) in FLNA silenced cells. In primary cultured cells derived from MTC (n=4 RET wild type and n=1 C634S RET), FLNA knockdown reduced cell proliferation, regardless of RET mutational status. In conclusion, these data demonstrated that FLNA is required for RET expression and signaling and promotes cell growth in MTC cells, suggesting FLNA may represent as a novel possible target for effective treatment of MTC.