Long-term hormonal regulation of the cAMP-specific phosphodiesterases in cultured FRTL-5 thyroid cells

Thyrotropin (TSH) and pharmacological agents that elevate intracellular cAM P concentrations potentiate the mitogenic response of FRTL-5 thyroid cells to insulin-like growth factor-I (IGF-I). This study was undertaken to deter mine the role of cAMP phosphodiesterases (PDEs) in this TSH-dependent regul ation. Incubation of FRTL-5 cells with TSH, forskolin, or dibutyryl cAMP gr adually induced the PDE activity, and treatment for 24 h produced a marked increase in type 4 high affinity cAMP PDEs. Under basal conditions, transcr ipts corresponding to PDE4A, PDE4B, PDE4C, and PDE4D were present. Stimulat ion for 24 h by TSH, forskolin or dibutyryl cAMP induced an increase in mRN A levels of PDE4B, PDE4D, and PDE4C. To understand the role of this cAMP-de pendent PDE regulation in the potentiation of the mitogenic, response to IG F-I, thymidine incorporation into DNA in response to IGF-I and TSH was meas ured in the absence or presence of PDE inhibitors. Exposure of the cells to 3-isobutyl-1-methylxanthine (IBMX) or RO 20-1724 had opposing effects on t hymidine incorporation into DNA, depending on the stimulus applied. When IG F-I was used alone, both IBMX and RO 20-1724 potentiated IGF-I-stimulated t hymidine, incorporation. However, when IGF-I and TSH at high concentrations were used in combination, these PDE inhibitors blocked thymidine incorpora tion into DNA. In addition,, these inhibitors depressed the synergistic inc rease in cyclin D1 and cyclin D- or cyclin E-associated cyclin-dependent ki nase (CDK) activity that is induced by TSH and IGF-I. Increased CDK activit ies have been shown to play a crucial role in progression through the G(1)/ S phase of the cell cycle. These data demonstrate that TSH produces marked changes in the cAMP degradative pathway of FRTL-5 cells by regulating the e xpression of cAMP PDEs. The regulation of the intracellular cAMP levels by this mechanism may contribute to the TSH- and IGF-I-dependent control of th e entry into the S phase of the cell cycle through changes in the cyclin/CD K system in FRTL-5 cells. (C) 2001 Elsevier Science B.V. All rights reserve d.