2 DISTINCT SIGNALING PATHWAYS ARE INVOLVED IN THE CONTROL OF THE BIPHASIC JUNB TRANSCRIPTION INDUCED BY INTERLEUKIN-6 IN THE B-CELL HYBRIDOMA-7TD1

We have measured the level of junB mRNA in the B hybridoma cell line 7 TD1, under interleukin-6 (IL-6) stimulation. IL-6 increases junB mRNA in a biphasic fashion, The first early-induced peak was transient and likely corresponds to the well documented typical junB mRNA, stimulate d in response to numerous growth factors, including IL-6, At variance, the second peak which has never been reported previously, lasted seve ral hours, As a consequence of its effect on junB mRNA, IL-6 stimulate d, in a biphasic fashion, the nuclear accumulation of the JunB protein , In this study, we demonstrated that IL-6 regulation occurred exclusi vely at the transcriptional level and that the bimodal increase of jun B mRNA and JunB protein can be accounted for by a biphasic stimulation of junB transcription. Furthermore, our data point to two major diffe rences between the mechanism of control of the early and the late IL-6 -induced junB transcription waves. First, cycloheximide strongly poten tiated the transcription of the second wave, whereas it failed to affe ct the early-induced burst, Second, tyrphostin, a tyrosine kinase inhi bitor, impaired the expression of the first but not the second junB mR NA peak, Conversely, genistein, another tyrosine kinase inhibitor, tot ally abolished the expression of the second peak of junB mRNA whereas it did not affect the expression of the first peak. Altogether these d ata indicate that, in 7TD1 cells, IL-6 controls junB transcription in a biphasic fashion by means of two separate transduction pathways.