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Recent studies have revealed in normal thyroid tissue the presence of the transcript of several phosphodiesterases (PDEs), enzymes responsible for the hydrolysis of cyclic nucleotides. In this work, we analyzed the expression of PDE5 in a series of human papillary thyroid carcinomas (PTCs) presenting or not BRAF V600E mutation and classified according to ATA risk criteria. Furthermore, we tested the effects of two PDE5 inhibitors (sildenafil, tadalafil) against human thyroid cancer cells. PDE5 gene and protein expression were analyzed in two different cohorts of PTCs by real-time PCR using a TaqMan micro-fluid card system and Western blot. MTT and migration assay were used to evaluate the effects of PDE5 inhibitors on proliferation and migration of TPC-1, BCPAP, and 8505C cells. In a first series of 36 PTCs, we found higher expression levels of PDE5A in tumors versus non-tumor (normal) tissues. PTCs with BRAF mutation showed higher levels of mRNA compared with those without mutation. No significant differences were detected between subgroups with low and intermediate ATA risk. Upregulation of PDE5 was also detected in tumor tissue proteins. Similar results were obtained analyzing the second cohort of 50 PTCs. Moreover, all tumor tissues with high PDE5 levels showed reduction of Thyroglobulin, TSH receptor, Thyroperoxidase, and NIS transcripts. In thyroid cancer cells in vitro, sildenafil and tadalafil determined a reduction of proliferation and cellular migration. Our findings demonstrate for the first time an overexpression of PDE5 in PTCs, and the ability of PDE5 inhibitors to block the proliferation of thyroid cancer cells in culture, therefore, suggesting that specific inhibition of PDE5 may be proposed for the treatment of these tumors.

IntroductionA causative role for activating mutations of the TSHreceptor (TSHR) has been demonstrated in the majority ofthyroid hyperfunctioning nodules, as well as for thefamilial and sporadic congenital nonautoimmune hyper-thyroidism [1, 2] and also the rare hyperfunctioning car-cinomas [3, 4]. More than 30 different amino acidsubstitutions have been described so far, and their char-acterization, together with in vitro mutagenesis studieshave been exploited for a structure–function analysis of thereceptor, leading to a structural model in which the areasinvolved in the binding to TSH or signal transduction havebeen depicted [5]. The emerging picture suggests theexistence of a molecular constraint which limits the con-stitutive activity of the receptor, removed by the interactionof TSH with the extracellular (EX) portion, but with animportant role played also by the transmembrane regions inthe dimerization process and the functional activation ofthe receptor [5, 6]. Herein we describe the discovery of twounusual heterozygous mutations occurring in two of seventhyroid functioning nodules detected as single nodules or ina multinodular goiter.Materials and methodsSpecimen of nodular tissues were collected and immedi-ately frozen after surgical removal from seven patients withsingle or multinodular hyperfunctioning goiter. Non-nod-ular normal tissue from the contralateral lobe was alsocollected. After RNA extraction and cDNA synthesis, PCRamplification of the exons 9 and 10 of the TSHR gene wasperformed as previously described [3, 4] and the amplifiedproducts sequenced with a BigDye Terminator version 3.1Cycle Sequencing kit in an automated 3130xl analyzer(both from Applied Biosystems, Foster City, CA, USA).The study protocol was approved by the local ethicscommittee.ResultsSequencing of TSH receptor gene in a small cohort ofseven patients with thyroid hyperfunctioning nodulesrevealed the presence, in two cases, of unusual somaticheterozygous mutations. The first was a deletion of D403