MULTIPLE INTRACELLULAR SIGNALLINGS ARE INVOLVED IN THYROTROPIN-RELEASING-HORMONE (TRH)-INDUCED C-FOS AND JUN-B MESSENGER-RNA LEVELS IN CLONAL PROLACTIN CELLS

In mammosomatotropes GH3B6 cells, one of the primary responses to thyrotropin-releasing hormone (TRH) is the parallel induction of two proto-oncogenes, c-fos and jun B, which code for constituents of AP1 transcription factor. To better understand the mode of action of TRH and to look for possible functions of c-fos and jun B in these cells, we have investigated the role of different intracellular signals in the induction of each proto-oncogene on the one hand, and on prolactin (PRL) release and PRL gene expression on the other hand. Northern and dot-blot analyses revealed that the activation of protein kinase C (PKC)-, Ca2+- or adenylyl cyclase-dependent pathways acutely increased both c-fos and jun B transcripts. However, a gene specific responsiveness was revealed using phorbol 12-myristate 13-acetate (TPA) and several combined treatments. The simultaneous activation of PKC and Ca2+-dependent pathways resulted in synergistic stimulations of c-fos mRNA levels only. Consistently, ionomycin plus low doses of TPA solely reproduced the potent effect of TRH on c-fos transcripts. Data collected from TRH and TPA down-regulated cells indicated that TRH probably recruits TPA-dependent PKC isoforms for stimulating c-fos but not jun B transcripts. On the contrary, the TRH-induced stimulation of either proto-oncogene likely involves Ca2+-dependent mechanisms because calcium agonists and the peptide exert non-additive effects. Finally, the synergistic stimulations observed in response to TRH combined with forskolin, indicate that adenylyl cyclase-dependent mechanisms are interconnected with TRH-induced proto-oncogene expression. The overall study also reveals that among the agonists tested, the dihydropyridine Bay K 8644 and forskolin only were capable to induce a long-lasting stimulation of c-fos and jun B mRNA levels, concomitant to increased levels of PRL transcripts, as does TRH. Considering that AP1 is assumed to be involved in signal transmission from the cell surface to the nucleus, it might be thus proposed that a common stimulation of c-fos and jun B gene expression is possibly involved in the activation of the PRL gene. On the other hand, the systematic coincidence between acute PRL release and proto-oncogenes expression suggest a role for c-fos and jun B in the control of genes involved in the secretory process.