Insulin-like growth factor-I as an amplifier of follicle-stimulating hormone action: studies on mechanism(s) and site(s) of action in cultured rat granulosa cells.
The ovarian granulosa cell has recently been shown to be a site of insulin-like growth factor-I (IGF-I) production, reception, and action. In large measure, IGF-I action (in the rat) appears contingent upon its ability to synergize with FSH, a major promoter of granulosa cell differentiation. It is the objective of the in vitro studies reported herein to elucidate the cellular mechanism(s) whereby IGF-I amplifies FSH hormonal action, placing special emphasis on the potential role of the putative intracellular second messenger cAMP in this regard. Basal FSH binding (115.7 +/- 2.1 fmol/mg cell protein) to rat granulosa cells cultured under serum-free conditions remained unchanged after 72 h of treatment with IGF-I (50 ng/ml) by itself (107.1 +/- 1.0 fmol/mg cell protein). In contrast, treatment with FSH (20 ng/ml) resulted in a significant (P less than 0.05) decrease in FSH binding capacity (but not affinity) relative to controls in either the absence or presence of IGF-I. Whereas treatment with FSH resulted in a substantial increase in forskolin-stimulatable adenylate cyclase activity (10 +/- 1.7% conversion of [3H] ATP to [3H]cAMP), concurrent treatment with IGF-I resulted in 2.2-fold enhancement of FSH action. This IGF-I effect proved dose dependent with an apparent median effective dose of 3.6 +/- 0.8 ng/ml, a concentration in keeping with its granulosa cell receptor binding affinity. Significantly, however, IGF-I proved capable of enhancing Bt2cAMP-stimulated progesterone accumulation suggesting that IGF-I may be also acting at site(s) distal to cAMP generation. Taken together, these and previous studies indicate that nanomolar concentrations of exogenously added IGF-I may be interacting with the FSH transduction signal at multiple cellular site(s) to effect amplification of FSH action.
Adashi
E Y
EY
Department of Obstetrics/Gynecology, University of Maryland School of Medicine, Baltimore 21201.
Resnick
C E
CE
Hernandez
E R
ER
May
J V
JV
Knecht
M
M
Svoboda
M E
ME
Van Wyk
J J
JJ
D000255:Adenosine Triphosphate / Q000378:metabolism; D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D003994:Bucladesine / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D005260:Female; D005640:Follicle Stimulating Hormone / Q000378:metabolism / Q000494:pharmacology*; D006107:Granulosa Cells / Q000187:drug effects*; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D011374:Progesterone / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D013002:Somatomedins / Q000494:pharmacology*
D013002:Somatomedins; D005576:Colforsin; D011374:Progesterone; D003994:Bucladesine; D007334:Insulin-Like Growth Factor I; D000255:Adenosine Triphosphate; D005640:Follicle Stimulating Hormone; D000242:Cyclic AMP; D000262:Adenylyl Cyclases