Stimulation of prolactin gene expression by insulin

GH3 cells are a rat pituitary-derived cell line in which the expression of the growth hormone and prolactin genes is controlled by a variety of hormones including thyroid hormone. Since these cells contain insulin receptors, we explored whether these cells can be used to analyze the mechanisms involved in the regulation of gene expression by insulin. When GH3 cells were incubated with serum-free media, insulin stimulated prolactin production rates about 3-fold after 72 h and 10-fold after 96 h of incubation. Insulin stimulated prolactin mRNA levels and gene transcription rates to the same extent as the prolactin production rates. In contrast, insulin did not stimulate the rate of growth hormone production or growth hormone mRNA levels. As previously reported, thyroid hormone stimulated growth hormone production and mRNA levels. In these cells, thyroid hormone also stimulated prolactin synthesis and prolactin mRNA levels. Prolactin production and mRNA levels in cells incubated with both insulin and thyroid hormone showed a synergistic response which was about 5- to 10-fold greater than with either hormone alone. In contrast, growth hormone production rates in cells cultured with insulin and thyroid hormone was only slightly greater than cells incubated with only thyroid hormone. Half-maximal stimulation of prolactin production in cells incubated with insulin or with both insulin and thyroid hormone occurred between 2 and 5 nM insulin, suggesting that the response is mediated by the insulin receptor. These results indicate that GH3 cells should be useful in analyzing the mechanisms involved in regulation of gene expression by insulin.