Insulin-like growth factor-I inhibits transcriptional responses of transforming growth factor-beta by phosphatidylinositol 3-kinase/Akt-dependent suppression of the activation of Smad3 but not Smad2.
Insulin-like growth factor-I (IGF-I) and transforming growth factor-beta (TGF-beta) have been shown to be oncogenic and tumor suppressive, respectively, on prostate epithelial cells. Here we show that IGF-I inhibits the ability of TGF-beta to regulate expression of several genes in the non-tumorigenic rat prostatic epithelial line, NRP-152. In these cells, IGF-I also inhibits TGF-beta-induced transcriptional responses, as shown by several promoter reporter constructs, suggesting that IGF-I intercepts an early step in TGF-beta signaling. We show that IGF-I does not down-regulate TGF-beta receptor levels, as determined by both receptor cross-linking and Western blot analyses. However, Western blot analysis reveals that IGF-I selectively inhibits the TGF-beta-triggered activation Smad3 but not Smad2, while not altering expression of total Smads 2, 3, or 4. The phosphatidylinositol 3-kinase (PI3K) inhibitor, LY29004 reverses the ability of IGF-I to inhibit TGF-beta-induced transcriptional responses and the activation of Smad3, suggesting that the suppression of TGF-beta signaling by IGF-I is mediated through activation of PI3K. Moreover, we show that enforced expression of dominant-negative PI3K (DN-p85alpha) or phosphatidylinositol 3-phosphate-phosphatase, PTEN, also reverse the suppressive effect of IGF-I on TGF-beta-induced 3TP-luciferase reporter activity, whereas constitutively active PI3K (p110alphaCAAX) completely blocks TGF-beta-induced 3TP-luciferase reporter activity. Further transfection experiments including expression of constitutively active and dominant-negative Akt and rapamycin treatment suggest that suppression of TGF-beta signaling/Smad3 activation by IGF-I occurs downstream of Akt and through mammalian target of rapamycin activation. In summary, our data suggest that IGF-I inhibits TGF-beta transcriptional responses through selective suppression of Smad3 activation via a PI3K/Akt-dependent pathway.
Song
Kyung
K
Ireland Cancer Center Research Laboratories and the Department of Pharmacology, Case Western Reserve University/University Hospital of Cleveland, Cleveland, Ohio 44106, USA.
Cornelius
Susan C
SC
Reiss
Michael
M
Danielpour
David
D
D000199:Actins / Q000378:metabolism; D000818:Animals; D000903:Antibiotics, Antineoplastic / Q000494:pharmacology; D017209:Apoptosis; D015152:Blotting, Northern; D015153:Blotting, Western; D002460:Cell Line; D003432:Cross-Linking Reagents / Q000494:pharmacology; D053938:DNA Fragmentation; D004268:DNA-Binding Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004847:Epithelial Cells / Q000378:metabolism; D017930:Genes, Reporter; D007334:Insulin-Like Growth Factor I / Q000378:metabolism* / Q000502:physiology*; D008024:Ligands; D008156:Luciferases / Q000378:metabolism; D008297:Male; D051059:PTEN Phosphohydrolase; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism; D011401:Promoter Regions, Genetic; D011467:Prostate / Q000378:metabolism; D011485:Protein Binding; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D015398:Signal Transduction; D020123:Sirolimus / Q000494:pharmacology; D051899:Smad2 Protein; D051900:Smad3 Protein; D058570:TOR Serine-Threonine Kinases; D015534:Trans-Activators / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014158:Transcription, Genetic; D014162:Transfection; D016212:Transforming Growth Factor beta / Q000378:metabolism*; D053773:Transforming Growth Factor beta1; D025521:Tumor Suppressor Proteins / Q000378:metabolism
D000199:Actins; D000903:Antibiotics, Antineoplastic; D003432:Cross-Linking Reagents; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D007334:Insulin-Like Growth Factor I; D008024:Ligands; D008156:Luciferases; D051059:PTEN Phosphohydrolase; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010744:Phosphoric Monoester Hydrolases; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D020123:Sirolimus; D051899:Smad2 Protein; D051900:Smad3 Protein; D058570:TOR Serine-Threonine Kinases; D015534:Trans-Activators; D016212:Transforming Growth Factor beta; D053773:Transforming Growth Factor beta1; D025521:Tumor Suppressor Proteins; C497784:Smad2 protein, rat; C497787:Smad3 protein, rat; C508962:Tgfb1 protein, rat; C546845:mTOR protein, rat; C494920:Akt1 protein, rat