Thyrotropin regulates autophosphorylation and kinase activity in both the insulin and the insulin-like growth factor-I receptors in FRTL5 cells.
TSH regulation of insulin and insulin-like growth factor-I (IGF-I) receptor kinases has been studied in FRTL5 cultured thyroid cells. Preincubation of intact cells with TSH increased by 2-fold insulin and IGF-I receptor autophosphorylation and phosphorylation of the p175 endogenous substrate for the receptors. Enhanced phosphorylations reached a maximum within 30 min, were maintained for 30 min more, and vanished after 120 min of TSH incubation. TSH dose-responses exhibited half-maximal and maximal effects at 1 and 10 pM, respectively. In vitro, insulin as well as IGF-I receptors purified from cells treated with 10 pM TSH also exhibited 2-fold enhanced receptor autophosphorylation and kinase activity toward the exogenous substrate poly(Glu,Tyr) (4:1). At variance with TSH, cell incubation with either 8-bromo-cAMP or the protein kinase-C activator 12-O-tetradecanoylphorbol-13-acetate inhibited insulin and IGF-I receptor kinases. In intact cells, TSH stimulation of insulin and IGF-I receptor kinases was accompanied by enhanced turnover of phosphate on autophosphorylated receptors, increased receptor tyrosine phosphorylation, and decreased receptor serine/threonine phosphorylation in response to insulin. Incubation of in vivo labeled insulin and IGF-I receptors with extracts from TSH-treated cells also decreased receptor phosphoserine and phosphothreonine content. Furthermore, preincubation of insulin and IGF-I receptors with extracts from TSH-treated cells enhanced in vitro autophosphorylation. The latter effect was inhibited by the serine/threonine phosphatase inhibitors fluoride and okadaic acid, but not by the tyrosine phosphatase inhibitor vanadate. The data suggest that in FRTL5 cells, TSH induces the activity of a Ser/Thr protein phosphatase, which dephosphorylates insulin and IGF-I receptors and enhances their endogenous kinases.
Condorelli
G
G
Centro di Endocrinologia ed Oncologia Sperimentale del C.N.R., University of Naples, II Medical School, Italy.
Formisano
P
P
Miele
C
C
Beguinot
F
F
D000818:Animals; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004988:Ethers, Cyclic / Q000494:pharmacology; D005459:Fluorides / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D019319:Okadaic Acid; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors; D010766:Phosphorylation / Q000187:drug effects; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D051381:Rats; D011972:Receptor, Insulin / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000378:metabolism*; D017451:Receptors, Somatomedin; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013961:Thyroid Gland / Q000166:cytology* / Q000201:enzymology* / Q000648:ultrastructure; D013972:Thyrotropin / Q000494:pharmacology*
D004988:Ethers, Cyclic; D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D019319:Okadaic Acid; D007334:Insulin-Like Growth Factor I; D013972:Thyrotropin; D000242:Cyclic AMP; D011505:Protein-Tyrosine Kinases; D011972:Receptor, Insulin; D010749:Phosphoprotein Phosphatases; D013755:Tetradecanoylphorbol Acetate; D005459:Fluorides