Novel independent and synergistic regulation of gonadotropin-alpha subunit gene by luteinizing hormone human choriogonadotropin and gonadotropin releasing hormone in the alpha T3-1 gonadotrope cells

The alpha T3-1 cells are immortalized anterior pituitary gonadotropes which express gonadotropin-alpha subunit gene. These cells contain receptors for gonadotropin releasing hormone (GnRH) as well as for luteinizing hormone (LH) which can also bind human choriogonadotropin (hCG). Like GnRH, LH and hCG can upregulate the expression of gonadotropin-c? subunit gene. While 0.1-1.0 ng/ml hCG can upregulate, higher concentrations have no effect. However, these higher hCG concentrations can act in a synergistic manner with GnRH to increase the steady state mRNA and protein levels of gonadotropin-alpha subunit. The synergism between hCG and GnRH was mimicked by LII but not by other hormones in the glycoprotein hormone family or a or beta subunits of hCG, suggesting that the synergism is hormone specific and requires the conformation of native hormone. The hCG induced increase in gonadotropin-ce subunit mRNA levels was due to a significant increase in the half-life of gonadotropin-a subunit transcripts from 7.8 +/- 1.0 h in the controls to 16.5 +/- 3.8 h after treatment with hCG. The GnRH induced increase in gonadotropin-alpha subunit mRNA levels was due to both a significant increase in the half-life to 26.2 +/- 3.0 h as well as a significant increase in the transcription rate of the gene (159.0 +/- 7.7% of the control). A greater increase in gonadotropin-ce subunit mRNA levels following a combined treatment with GnRH and hCG was due to a further increase in half-life to 37.6 +/- 3.1 h as well as a greater increase in the transcription rate of the gene (295.1 +/- 24.2% of the control) as compared to the treatment with GnRH alone. In summary, we conclude that LH and hCG can independently and synergistically act with GnRH to increase the expression of gonadotropin-alpha subunit gene by transcriptional as well as by post-transcriptional mechanisms in alpha T3-1 cells. These effects may be important for the increase of LH levels during the preovulatory surge. (C) 1997 Elsevier Science Ireland Ltd.