Actions of growth factors in the follicle
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In this paper we have examined the possibility that soluble factors produced by the thecal and granulosa cells may be essential local modulators of follicular development. The observations that insulin could influence both the growth and the differentiation of granulosa cells were important in establishing the concept that peptides could act as amplifiers of the actions of gonadotrophins. Insulin alone did not influence aromatase activity significantly but acted synergistically with FSH to augment aromatase activity in rat granulosa cells. Unlike aromatase activity, insulin alone was able to significantly stimulate 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) activity, the maximum level achieved approaching that obtained with high concentrations of FSH. To determine if insulin could influence other parameters of granulosa cell function in addition to steroidogenesis, we measured a component of extracellular matrix, fibronectin, previously shown to be inhibited by FSH. Treatment with insulin independently inhibited the increase in fibronectin secretion observed in control cultures. Also, insulin alone was able to stimulate quiescent bovine granulosa cells to incorporate [3H]thymidine into DNA under serum-conditions. The concentration of insulin required in these experiments was higher than physiological levels suggesting that other insulin-like growth factors may be involved. Our work and that of others has shown that IGF1 can mimic the actions of insulin and is effective at lower concentrations. A possible source of IGF1 production in the follicle was sought initially by collecting rat granulosa cell conditioned medium, and assessing biological activity and immunoreactivity. Conditioned medium augmented the actions of FSH on aromatase activity and alone stimulated 3 beta-HSD, indicating the presence of insulin-like bioactivity. A positive reaction on immunoblots using specific antiserum confirmed the presence of immunoreactive IGF1. Conditioned medium from thecal cells contained a growth-promoting activity (TcGF) that did not augment FSH-induced aromatase activity. The production of growth factors locally within the follicle may represent the self-amplifying mechanism that enables the dominant follicle to complete its developmental program and ovulate.
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