Neural and glial-mediated effects of growth factors acting via tyrosine kinase receptors on luteinizing hormone-releasing hormone neurons

It is becoming increasingly evident that the secretory activity of LHRH neurons is regulated not only by transsynaptic inputs but also by trophic molecules of glial and neuronal origin. The present experiments were undertaken to gain insights into the potential cell-cell mechanisms by which basic fibroblast growth factor (bFGF) and transforming growth factor-α (TGFα), two growth factors produced in the hypothalamus, may affect LHRH neuronal function. Northern blot analysis showed that the LHRH-producing cell line GT1-7 contains the messenger RNA (mRNA) encoding the type 1 fibroblast growth factor receptor (FGFR-1) but not that encoding the epidermal growth factor (EGF) receptors, which mediates the biological actions of both TGFα and EGF. Ligand-induced receptor phosphorylation experiments demonstrated that GT1-7 cells possess biologically active FGFR-1s but not EGF receptors. Exposure of the cells to bFGF resulted not only in FGFR-1 tyrosine phosphorylation, but also in tyrosine phosphorylation of phospholipase Cγ, one of the initial enzymes in the intracellular signaling cascade initiated by FGFR activation. GT1-7 cells proliferated in response to this activation. Despite the presence of biologically active receptors, bFGF did not significantly stimulate release of the mature LHRH decapeptide. Instead, bFGF increased the steady-state levels of the mRNA encoding the LHRH precursor processing endoprotease PC2, with a time course comparable to that of phorbol esters, suggesting that, as shown in the companion paper, the actions of the growth factor on LHRH neurons involve facilitation of the initial step in LHRH prohormone processing. The increase in PC2 gene expression was not accompanied by changes in LHRH mRNA levels. Unlike these direct actions of bFGF on GT-1 cells, TGFα appears to act indirectly via astroglial intermediacy. Exposure of GT1-7 cells to TGFα or EGF failed to affect several parameters of cellular activity including LHRH release, LHRH and PC2 mRNA levels, and cell proliferation. In contrast, astrocyte culture medium conditioned by treatment with TGFα led to sustained stimulation of LHRH release with no changes in LHRH gene expression and a transient increase in PC2 mRNA levels. Although no definitive evidence for the presence of FGFR-1 in normal LHRH neurons could be obtained by either double immunohistochemistry or double in situ hybridization procedures, fetal LHRH neurons in primary culture responded to bFGF with neurite outgrowth. Thus, normal LHRH neurons may have an FGFR-1 content too low for detection by regular histochemical procedures, and/or detectable expression of the receptor may be confined to a much earlier developmental stage. The mitogenic effect of bFGF on GT1-7 cells supports this possibility and suggests a role for FGF in the cell proliferation events that precede acquisition of the LHRH neuronal phenotype. It appears that once this phenotype is established, bFGF may promote the differentiation of LHRH neurons. The results also suggest that the secretory capacity of LHRH neurons develops under a dual trophic influence, one on peptide processing exerted directly by bFGF on early neurons, and another on LHRH release, exerted by TGFα via the intermediacy of astroglial cells.