IRS-I expression on the luteinized rat ovary: IGF-I and cyclic AMP effects on IRS-I tyrosine phosphorylation.
The expression of insulin receptor substrate-I (IRS-I) mRNA was demonstrated in rat luteal cells by Northern blot analysis, in situ hybridization as well as by reverse transcriptase polymerase chain reaction. Western blot with a polyclonal anti IRS-I antibody showed the presence of a 183 kDa protein which corresponds to the size of IRS-I reported in other tissues. Further studies were performed to determine whether human chorionic gonadotropin (hCG) can interact with the insulin-like growth factor-I (IGF-I) signaling pathway to increase tyrosine phosphorylation of IRS-I. While hCG alone was ineffective in stimulating the phosphorylation of IRS-I, IGF-I mediated phosphorylation of IRS-I was increased by prior exposure to hCG. These results were further confirmed by the immunoprecipitation of IRS-I from the lysate of hCG- and IGF-I-treated luteal cell cultures followed by Western blotting with anti-phosphotyrosine antibody. Similarly, pretreatment with forskolin also increased IGF-I stimulated IRS-I phosphorylation. The increased tyrosine phosphorylation of IRS-I seen in response to IGF-I stimulation following treatment with either hCG or forskolin was not due to an increase in IRS-I content. Furthermore, IGF-I receptor tyrosine kinase activity was not affected by forskolin, suggesting that the increase in IRS-I tyrosine phosphorylation was not the result of an increase in its activity. Thus, we conclude that hCG/LH and IGF-I signaling pathways 'cross-talk' to increase the levels of IRS-I tyrosine phosphorylation. The observed increase in IRS-I tyrosine phosphorylation may be the result of an increase in the stability of the phosphorylated form of IRS-I.
Talavera
F
F
Department of Obstetrics and Gynecology, University of Michigan Medical Center Endocrine Laboratory, Ann Arbor 48109-0278, USA.
Chen
Z
Z
Menon
K M
KM
D000818:Animals; D006063:Chorionic Gonadotropin / Q000494:pharmacology; D003338:Corpus Luteum / Q000378:metabolism*; D000242:Cyclic AMP / Q000494:pharmacology*; D005260:Female; D055504:Insulin Receptor Substrate Proteins; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D008184:Luteal Cells / Q000378:metabolism; D007986:Luteinizing Hormone / Q000494:pharmacology; D010750:Phosphoproteins / Q000096:biosynthesis* / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley
D006063:Chorionic Gonadotropin; C526292:IRS1 protein, human; D055504:Insulin Receptor Substrate Proteins; C526296:Irs1 protein, rat; D010750:Phosphoproteins; D007334:Insulin-Like Growth Factor I; D007986:Luteinizing Hormone; D000242:Cyclic AMP