Induction of specific phosphodiesterase isoforms by constitutive activation of the cAMP pathway in autonomous thyroid adenomas

Thyrocytes largely depend on cAMP signaling for replication and differentia tion. This pathway may be constitutively activated by mutations of the TSH receptor (TSHR) and G(s)alpha in autonomous thyroid adenomas (ATAs). Becaus e steady state cAMP results from production by: adenylyl cyclase and degrad ation by phosphodiesterases (PDEs), we evaluated PDE activity and expressio n in ATAs with wild-type and mutant TSHR and G(s)alpha. Activating mutation s of TSHR and G(s)alpha were identified in 7 and 1 of 18 ATAs, respectively . No difference was observed in the cAMP content in ATAs with or without ac tivating mutants. In the surrounding normal thyroid tissue (NTs), PDE activ ity was 80% isobutylmethylxantkine sensitive, with the major contribution b y PDE1 and a minor contribution by PDE4. No differences were observed in PD E activities between NTs and ATAs with wild-type TSHR and G(s)alpha. In con trast, in the presence of mutant TSHRs or G(s)alpha, total PDE activity was higher. This increase was primarily due to PDE4 induction (917 +/-. 116% o ver NTs), associated with a minor PDE1 increase only in ATAs with mutant TS I-IR. By RT-PCR, increments of PDE4D and 4C messenger ribonucleic acids wer e found in the ATAs with mutant TSHR or G(s)alpha, whereas messenger ribonu cleic acids encoding other cAMP-specific PDEs were not significantly increa sed. This study provides a characterization of the PDEs expressed in human thyroid and demonstrates a dramatic PDE4 induction in the ATAs bearing muta nt TSHR or G(s)alpha genes. The increase in cAMP-degrading activity may rep resent a marker of constitutive adenylyl cyclase activation and constitutes an intracellular feedback mechanism with significant impact on the phenoty pic expression of the activating mutations.