Relevant cAMP-specific phosphodiesterase isoforms in human pituitary: effect of Gs(alpha) mutations.
Both cAMP production by adenylyl cyclase and cAMP degradation by phosphodiesterases account for intracellular cAMP levels. We previously demonstrated an increased phosphodiesterase activity in GH-secreting adenomas bearing the gsp oncogene. Here we characterize both the activity and the expression of cAMP-specific phosphodiesterase genes in the human pituitary and in gsp+ and gsp- GH-secreting adenomas and analyze the impact of this intracellular feedback mechanism on the levels of cAMP-responsive element-binding protein phosphorylation. Normal pituitary and gsp- GH-secreting adenomas showed similar phosphodiesterase activities, and 7-fold higher levels were observed in gsp+ tumors. In these tumors the increased activity was mainly owing to isobutyl-methyl-xanthine-sensitive phosphodiesterase 4 and to isobutyl-methyl-xanthine-insensitive isoforms. By semiquantitative RT-PCR, all phosphodiesterase 4 transcripts were expressed in the normal and tumoral pituitary. However, the levels of phosphodiesterase 4C and 4D messenger RNAs were significantly higher in gsp+ than in gsp- GH-secreting adenomas and normal pituitary. Expression of the thyroid-specific isobutyl-methyl-xanthine-insensitive phosphodiesterase 8B was absent in the normal pituitary but detectable in almost all GH-secreting adenomas and higher in gsp+ (P < 0.02). Therefore, this study provides a characterization of phosphodiesterase expression in human pituitary and demonstrates a dramatic induction of the cAMP-specific phosphodiesterases 4C and phosphodiesterases 4D and phosphodiesterases 8B in gsp+ GH-secreting adenomas. Similar levels of cAMP-responsive element-binding protein phosphorylation were observed in gsp- and gsp+ GH-secreting adenomas; however, phosphodiesterase blockade caused an increase in cAMP-responsive element-binding protein phosphorylation that was significantly higher in gsp+ than in gsp- adenomas. Because cAMP-responsive element-binding protein represents the principal end point of the cAMP pathway, these results suggest that the enhanced phosphodiesterase activity may have a significant impact on the phenotypic expression of gsp mutations.
Persani
L
L
Institute of Endocrine Sciences, University of Milan, Istituto Auxologico Italiano IRCCS, Milan 20145, Italy. persani@auxologico.it
Borgato
S
S
Lania
A
A
Filopanti
M
M
Mantovani
G
G
Conti
M
M
Spada
A
A
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D015105:3',5'-Cyclic-AMP Phosphodiesterases / Q000235:genetics* / Q000378:metabolism; D000236:Adenoma / Q000201:enzymology / Q000235:genetics*; D017362:Cyclic AMP Response Element-Binding Protein / Q000378:metabolism; D019205:GTP-Binding Protein alpha Subunits, Gs / Q000235:genetics*; D019382:Human Growth Hormone / Q000378:metabolism; D006801:Humans; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D015513:Oncogene Proteins / Q000235:genetics; D010766:Phosphorylation; D010902:Pituitary Gland / Q000201:enzymology*; D010911:Pituitary Neoplasms / Q000201:enzymology / Q000235:genetics*; D012333:RNA, Messenger / Q000235:genetics; D012016:Reference Values; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014158:Transcription, Genetic; D014407:Tumor Cells, Cultured
D017362:Cyclic AMP Response Element-Binding Protein; D007527:Isoenzymes; D015513:Oncogene Proteins; D012333:RNA, Messenger; D019382:Human Growth Hormone; D015105:3',5'-Cyclic-AMP Phosphodiesterases; D019205:GTP-Binding Protein alpha Subunits, Gs; D015056:1-Methyl-3-isobutylxanthine