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C-type natriuretic peptide (CNP) and its cognate guanylyl cyclase receptor, the natriuretic peptide receptor B (NPR-B) together constitute a regulatory system that controls cell function via the generation of intracellular cyclic GMP. In this report we have examined the role of cAMP signaling in the regulation of CNP and NPR-B activity in the FRTL-5 rat thyroid follicular cell line. As had been observed earlier with TSH, the cAMP mimetic, dibutyryl cAMP (dbcAMP; 1 mM) induced a significant reduction in CNP-stimulated cGMP generation that was first apparent after 6 h of treatment. The inhibitory effect of dbcAMP on NPR-B was dose dependent, with an EC50 of 0.2 mM. Pretreatment of FRTL-5 cells with either of two protein kinase A (PKA) inhibitors, KT-5720 and H-89, failed to curtail the dbcAMP reduction in NPR-B activity, suggesting that the cAMP pathway leading to inhibition of NPR-B is PKA independent. Whereas either a 30-min or a 24-h treatment with the protein kinase C-activator phorbol myristate acetate failed to alter maximal levels of CNP-stimulated cGMP, a 24-h exposure to the calcium ionophore A23187 reduced CNP-stimulated cGMP to about one-third of control. Pretreatment of FRTL-5 cells with the cell-permeable calcium chelator 1,2 bis(2-aminophenoxy)ethane-N,N,N1,N1-tetraacetic acid, tetraacetoxymethyl ester completely abrogated the cAMP-induced reduction of CNP-stimulated cGMP. Real-time PCR showed no effect of dbcAMP on NPR-B transcript at 3 and 6 h, but indicated a 40% reduction in transcript by dbcAMP at 24 h. In contrast, real-time PCR indicated a 5-fold increase in CNP transcript at 3 h, reaching 15.4-fold above control at 6 h in cells treated with dbcAMP. In addition, immunofluorescence staining of FRTL-5 cells with a specific antibody for CNP-22 showed the presence of cytoplasmic CNP that was up-regulated by incubation with either TSH or dbcAMP. These results suggested that cAMP signaling regulates the natriuretic peptide system in rat thyroid cells by increasing CNP expression, and reducing NPR-B activity. This latter action of cAMP appears to be both PKA independent and calcium dependent, and provides support for a dominant role for calcium in the regulation of NPR-B in the rat thyroid.

The natriuretic peptides signal through three receptor subtypes, of which two (NPR-A and NPR-B) are membrane-bound guanylyl cyclases for which the principal ligands are respectively atrial natriuretic factor (ANF) and C-type natriuretic peptide (CNP). In the human thyroid cell, a third receptor, NPR-C, has been implicated in the regulation of thyroglobulin, but functional roles for NPR-A and NPR-B have not yet been defined. In the present study we used RT-PCR to identify transcripts of all three receptor subtypes, both in human thyroid and in HTU-5 cells, a long-term culture of thyroid-derived cells. Both ANF and CNP induced a twofold increase in intracellular cGMP content in HTU-5 cells. Morphologic changes (a significant increase in cells of the retracted phenotype) were observed in ANF- and CNP-treated cells within 3 and 5 h of treatment respectively. Significant increases in retracted cell number were induced by ANF and CNP, but not the NPR-C-specific ring-deleted ANF analog, C-ANF(4-23), during a 15-day treatment. All three natriuretic peptides, however, induced a small (15-20%) but significant (P<0 small middle dot001) increase in DNA content per well. The stable analog of cGMP, 8-bromo-cGMP (8-BrcGMP; 1 mM), also increased the number of retracted HTU-5 cells, and was equipotent with the cAMP analog, 8-BrcAMP, in this effect. The cGMP-dependent protein kinase inhibitor, KT5823, however, had no significant effect on the ANF-induced increase in numbers of retracted cells. These results suggest that the actions of NPR-A and NPR-B, functional receptors in the human thyroid cell, may in part be mediated by cGMP-induced alterations in the cytoskeleton.