Biology of Reproduction, Vol 53, 1286-1292, Copyright © 1995 by Society for the Study of Reproduction

ARTICLES

Relaxin-induced deoxyribonucleic acid synthesis in porcine granulosa cells is mediated by insulin-like growth factor-I

KM Ohleth and CA Bagnell
Department of Animal Sciences, Rutgers University, New Brunswick, New Jersey 08903, USA.

Relaxin stimulates in vitro DNA synthesis and cell proliferation of porcine granulosa cells (GC) and theca cells. The objective of the study reported here was to determine whether components of the ovarian insulin-like growth factor (IGF) system mediate relaxin's growth- promoting effects on porcine GC in vitro. In small follicle GC, relaxin (1-100 ng/ml) significantly (p < 0.05) increased IGF-I secretion to 25- 34% above control. Hormonal responsiveness of GC was shown by incubation with FSH (200 ng/ml), which resulted in 125% stimulation of IGF-I secretion relative to that in cells incubated alone. When IGF-I activity in the GC cultures was neutralized with a specific IGF-I antibody, relaxin (10 and 100 ng/ml)-induced [3H]thymidine incorporation was inhibited (p < 0.05). Coincubation with IGF-I antibody also suppressed basal and IGF-I (10 ng/ml)-induced [3H]thymidine incorporation into GC DNA, but had no effect on insulin (1 microgram/ml)-induced DNA synthesis, demonstrating the specificity and lack of toxicity of the IGF-I antibody. Ligand blot analysis showed no change in secretion of GC IGF binding protein (IGFBP) in response to relaxin (1, 10, and 100 ng/ml). In contrast, IGF-I (10 ng/ml) increased secretion of IGFBP-3 and -5, whereas FSH (200 ng/ml) decreased IGFBP-3 secretion and increased IGFBP-4 secretion (p < 0.05). In IGF-I receptor competition studies, IGF-I, but not relaxin, displaced [125I]IGF-I from the GC IGF-I receptor. These studies provide direct evidence for an interaction of relaxin and the ovarian IGF system. They are the first to show 1) a stimulatory effect of relaxin on IGF-I secretion; 2) the necessity of IGF-I activity for relaxin-induced GC DNA synthesis; and 3) the absence of an effect of relaxin on GC IGFBPs or IGF-I receptor. These findings support a paracrine role for relaxin in the porcine follicle and show that relaxin acts indirectly to promote follicle growth by stimulating GC IGF-I secretion.