Insulin-like growth factor-I potentiates thyrotropin stimulation of adenylyl cyclase in FRTL-5 cells.
We reported that TSH and insulin-like growth factor-I (IGF-I), which were known to synergistically stimulate DNA synthesis, synergize to elevate the 1,2-diacylglycerol content of FRTL-5 thyroid cells. We presented evidence that cAMP is the proximal mediator of these actions of TSH. To further define the mechanism of this interaction, we investigated the effects of IGF-I on TSH stimulation of adenylyl cyclase. Long and short term effects of IGF-I or high doses of insulin were studied in FRTL-5 cells that were maintained in serum-, hormone-, and growth factor-free medium for 4-7 days (basal cells). When cells were incubated with high doses of insulin for 7 days and acutely stimulated, a 10-fold increase in sensitivity and a 2-fold increase in maximal responsiveness of cAMP accumulation to TSH were observed. To study shorter term effects, cells were preincubated with insulin for 3 h and then exposed to TSH, cholera toxin, or forskolin. Incubation with high doses of insulin for 3 h caused 30-300% increases in cAMP accumulation at high doses of TSH (greater than or equal to 1 mU/ml), cholera toxin (greater than 0.1 microM), and forskolin, but did not affect the EC50 for TSH. Dose-response studies were consistent with insulin acting via receptors for IGF-I, and IGF-I caused a similar effect. There was a 45% increase in adenylyl cyclase activity stimulated by TSH in membranes isolated from cells incubated with high doses of insulin for 3 h. Pretreatment of FRTL-5 cells with pertussis toxin, which ADP-ribosylates the inhibitory G-protein Gi, or adenosine, which we show inhibits cAMP accumulation by interacting with Gi, did not affect insulin/IGF-I enhancement of cAMP accumulation. We suggest that synergism of actions of TSH and IGF-I may in part be due to IGF-I enhancement of TSH stimulation of adenylyl cyclase.
Brenner-Gati
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L
Department of Medicine, Cornell University Medical College, New York, New York 10021.
Berg
K A
KA
Gershengorn
M C
MC
D037361:Adenylate Cyclase Toxin; D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D002460:Cell Line; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D007328:Insulin / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D007700:Kinetics; D037342:Pertussis Toxin; D013002:Somatomedins / Q000494:pharmacology*; D013961:Thyroid Gland / Q000187:drug effects / Q000201:enzymology*; D013972:Thyrotropin / Q000494:pharmacology*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D037361:Adenylate Cyclase Toxin; D007328:Insulin; D013002:Somatomedins; D010566:Virulence Factors, Bordetella; D005576:Colforsin; D007334:Insulin-Like Growth Factor I; D013972:Thyrotropin; D000242:Cyclic AMP; D037342:Pertussis Toxin; D000262:Adenylyl Cyclases