Long-term hormonal regulation of the cAMP-specific phosphodiesterases in cultured FRTL-5 thyroid cells.
Thyrotropin (TSH) and pharmacological agents that elevate intracellular cAMP concentrations potentiate the mitogenic response of FRTL-5 thyroid cells to insulin-like growth factor-I (IGF-I). This study was undertaken to determine the role of cAMP phosphodiesterases (PDEs) in this TSH-dependent regulation. Incubation of FRTL-5 cells with TSH, forskolin, or dibutyryl cAMP gradually induced the PDE activity, and treatment for 24 h produced a marked increase in type 4 high affinity cAMP PDEs. Under basal conditions, transcripts corresponding to PDE4A, PDE4B, PDE4C, and PDE4D were present. Stimulation for 24 h by TSH, forskolin or dibutyryl cAMP induced an increase in mRNA levels of PDE4B, PDE4D, and PDE4C. To understand the role of this cAMP-dependent PDE regulation in the potentiation of the mitogenic response to IGF-I, thymidine incorporation into DNA in response to IGF-I and TSH was measured 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 thymidine incorporation into DNA, depending on the stimulus applied. When IGF-I was used alone, both IBMX and RO 20-1724 potentiated IGF-I-stimulated thymidine incorporation. However, when IGF-I and TSH at high concentrations were used in combination, these PDE inhibitors blocked thymidine incorporation into DNA. In addition, these inhibitors depressed the synergistic increase in cyclin D1 and cyclin D- or cyclin E-associated cyclin-dependent kinase (CDK) activity that is induced by TSH and IGF-I. Increased CDK activities 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 expression of cAMP PDEs. The regulation of the intracellular cAMP levels by this mechanism may contribute to the TSH- and IGF-I-dependent control of the entry into the S phase of the cell cycle through changes in the cyclin/CDK system in FRTL-5 cells.
Takahashi
S I
SI
Department of Pediatrics, University of North Carolina at Chapel Hill, 27599, USA. atkshin@mail.ecc.u-tokyo.ac.jp
Nedachi
T
T
Fukushima
T
T
Umesaki
K
K
Ito
Y
Y
Hakuno
F
F
Van Wyk
J J
JJ
Conti
M
M
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D015105:3',5'-Cyclic-AMP Phosphodiesterases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D015152:Blotting, Northern; D002457:Cell Extracts / Q000737:chemistry; D002460:Cell Line; D002478:Cells, Cultured; D002852:Chromatography, Ion Exchange; D000242:Cyclic AMP / Q000096:biosynthesis; D004247:DNA / Q000096:biosynthesis; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects*; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D013961:Thyroid Gland / Q000187:drug effects* / Q000201:enzymology; D013972:Thyrotropin / Q000494:pharmacology*; D013997:Time Factors
D002457:Cell Extracts; D004791:Enzyme Inhibitors; D012333:RNA, Messenger; D007334:Insulin-Like Growth Factor I; D013972:Thyrotropin; D004247:DNA; D000242:Cyclic AMP; D015105:3',5'-Cyclic-AMP Phosphodiesterases; D000262:Adenylyl Cyclases; D015056:1-Methyl-3-isobutylxanthine