INVOLVEMENT OF THE cGMP PATHWAY IN THE INHIBITION OF THE IODIDE UPTAKE AND THE EXPRESSION OF THYROID SPECIFIC GENES IN FRTL-5
Bazzara LG, Costamagna ME, Cabanillas AM, Velez ML, Fozzatti L, Masini-Repiso AM.
Depto. de Bioq. Clínica, Fac. de Cs. Químicas, Universidad Nacional de Córdoba, Córdoba, Argentina.
Previous reports indicated that nitric oxide (NO) inhibits the iodide uptake in several species and the thyroperoxidase (TPO) mRNA expression in FRTL-5 cells. This work was aimed to analyse the role of the guanylyl cyclase/cyclic GMP (cGMP) pathway in the inhibitory effect of NO on siodide uptake and in the expression of TPO and hyroglobulin (TG) mRNA in TSHt (500 mU/ml)-stimulated increased cGMP levels s(RIA) in a concentration and time-dependent manner (Table 1). The cAMP level (RIA) decreased after 48 h of incubation with 500 mM SNP and was not modified at shorter times and lower SNP concentrations. An increase of cGMP was induced by TSH (Table 1). The inhibition exerted by SNP or 8-Br-cGMP for 24 h on the iodide uptake (131I) was reversed by KT-5823 (1-10 mM), an inhibitor of thecGMP-dependent protein kinase (PKG) (Table 2). The TPO and TG mRNA expression (Northern-blot) was significantly reduced by 8-Br-cGMP. Data (24 h) were (Absorbance ratio mRNA/18 S rRNA, mean±SEM, arbitrary units): Control (C): TPO= 2.00, TG= 2.00; 8-Br-cGMP 100 mM: TPO= 1.97±0.24, TG= 1.96±0.03; 300 mM: TPO= 1.56±0.13, TG= 1.23±0.16**; 1000 mM: TPO= 1.22±0.29*, TG= 1.01±0.26** (*P<0.05, **P<0.01 vs C; n=3, ANOVA, Student-Newman-Keuls test). In conclusion, an activation of PKG seems to be involved in the NO/cGMP-induced inhibition of iodide uptake in FRTL-5. A reduction in the cAMP production could be relevant in the action of high NO/cGMP levels. Evidence is provided for a role of the cGMP pathway in the regulation of thyroid specific genes.