Insulin-like growth factor-I effects on gonadotropin-releasing hormone biosynthesis in GT1-7 cells

The immortalized GT1-7 cell line synthesizes and secretes GnRH, the key hormone of reproduction. However, GT1-7 cells lack the normal inputs from neurotransmitters, growth factors, and steroids, which are involved in the maturation and maintenance of GnRH neurons in the brain. We examined the effects of the neurotrophic factor insulin-like growth factor-I (IGF-I) on GnRH gene expression and the mechanism for these changes. Initially, effects of IGF-I on GnRH gene expression were determined by ribonuclease protection assay. In time-course experiments, IGF-I treatment caused significant increases in nuclear GnRH primary transcript levels, an index of GnRH gene transcription, 4 and 8 h after initiation of IGF-I treatment. GnRH messenger RNA (mRNA) levels in the cytoplasm were stimulated by IGF-I at 24 h of treatment. IGF-I also affected GT1-7 cell morphology, with an increase in process extension and cell-cell contacts. In contrast, GnRH peptide levels in the medium were initially stimulated and then suppressed by IGF-I, indicating an uncoupling of biosynthesis and secretion. The increase in GnRH mRNA levels induced by IGF-I is probably caused by a transcriptional mechanism, as evidenced by the increase in GnRH primary transcript levels before a change in GnRH mRNA levels, as well as our finding of a similar GnRH mRNA half-life for both control and IGF-I-treated cells. Interestingly, GT1-7 cells themselves were observed to express IGF-I immunoreactivity, suggesting the possibility of autoregulation by this neurotrophic factor. It is concluded that IGF-I is an important modulator of GnRH gene expression and release in the GT1-7 cell line. The reported stimulatory effects of IGF-I in vivo, and its hypothesized role in the development of GnRH neurons in the brain, suggest that IGF-I may make the GT1-7 cells line more like a mature GnRH neuron, as a model for future studies.