INSULIN-LIKE GROWTH-FACTOR-I, A COMPONENT OF SERUM THAT ENABLES PORCINE CUMULUS CELLS TO EXPAND IN RESPONSE TO FOLLICLE-STIMULATING-HORMONEIN-VITRO

The present study was aimed at determining whether insulinlike growth factor-1 (IGF-1) is a component of fetal bovine serum (FBS) that enabl es porcine cumulus cells to expand in response to FSH in vitro. Cumulu s-oocyte complexes (COG) obtained from 4- to 6-mm follicles of prepube rtal gilts were cultured at 39 degrees C for 24 h in media that contai ned human recombinant IGF-1 (50 ng/ml), FBS (15% v:v) or their combina tion, with or without FSH (1.5 mu g/ml), and cumulus expansion was sco red microscopically. Expansion was FSH dependent and was observed only when IGF-1, FBS, or both were present. The proportion of FSH-stimulat ed COC exhibiting full expansion in response to IGF-1 alone did not di ffer significantly (p > 0.05) from the proportion in those cultured wi th FBS or IGF-1+FBS (79 +/- 2.8% vs. 84 +/- 2.2% or 76 +/- 6.2%, respe ctively). In a concentration-response study, FSH-stimulated expansion was observed in a significant proportion of COC (32 +/- 2.8% vs. 0% co ntrol) at 1 ng/ml IGF-1, with the proportions increasing dose-dependen tly to maximal values between in and 75 ng/ml IGF-1, and decreasing at higher IGF-1 concentrations. Exposure of COC to an IGF-1 receptor (IG F-1R)-neutralizing antibody (Ab) for 90 min before addition of FSH and FBS dose-dependently inhibited cumulus expansion, with maximal inhibi tion at 10 mu g/ml (1 +/- 1.0% vs. 68 +/- 1.1% control). In the absenc e of FBS or IGF-1, some COC had a tendency toward slight expansion whe n cultured with FSH, and the Ab completely inhibited that effect, sugg esting that this may be due to endogenous IGF-1 production by the COG. The Ab effect was reversible and was eliminated by washing twice with fresh medium followed by culture for an additional 24 h in the presen ce of FSH and FBS. Expression of IGF-1R mRNA in the isolated oocyte an d cumulus cells was determined by reverse tramscriptase polymerase cha in reaction using sequence-specific primers. The IGF-1R message was de tected in both the oocyte and cumulus cells. Collectively, these obser vations suggest that IGF-1 is a component of serum that enables cumulu s cells to expand in response to FSH in vitro, and that the effect is receptor mediated. Since IGF-1 is present in the follicle in vive, it may have a physiological role during gonadotropin-induced cumulus expa nsion.