Granulocyte-macrophage colony-stimulating factor expression is regulated at transcriptional and posttranscriptional levels in a murine bone marrow stromal cell line

We have reported modulation by cytokines interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) and by hormonal cyclic-adenosine-monophosphate (cAMP) agonists, of hematopoietic growth factor production in the murine marrow adherent cell line +/+-1.LDA11. Previously, we reported that increased intracellular cAMP levels inhibited bioactive granulocyte-macrophage colony-stimulatory factor (GM-CSF) production stimulated by IL-1 or by the synergistic stimulus of IL-1 plus TNF-α. On the other hand, increased intracellular cAMP stimulated IL-6 synthesis in +/+-1.LDA11 cells. In addition, cAMP was additive with either IL-1 or IL-1 plus TNF-α in inducing production of soluble IL-6. In the present study, these observations were pursued mechanistically at the level of messenger RNA (mRNA) production. Northern blot analysis of steady-state mRNA for GM-CSF revealed induction by treatment of +/+-1.LDA11 cells with IL-1 or with TNF-α. The combined stimulation by IL-1 plus TNF-α resulted in supra-additive increases in GM-CSF expression by +/+-1.LDA11. Addition to stromal cells of the soluble cAMP agonist 8-bromo-cAMP (8BrcAMP) at 0.5 to 1 mM stimulated IL-6 mRNA expression acting alone, and it was additive with IL-1 or IL-1 plus TNF-α in stimulating IL-6 expression. On the other hand, 8BrcAMP inhibited GM-CSF mRNA expression stimulated by IL-1 or IL-1 plus TNF-α. Inhibition of GM-CSF mRNA by 8BrcAMP was time-dependent, starting 120 to 180 minutes posttreatment. In addition, inhibition of GM-CSF transcript expression in +/+-1.LDA11 by 8BrcAMP required the expression of a labile protein. Nuclear run-on assays revealed that GM-CSF and IL-6 genes were transcriptionally induced in +/+-1.LDA11 by incubation with IL-1 plus TNF-α. IL-6 transcription was further enhanced by 8BrcAMP co-incubation. More sensitive experiments using a luciferase reporter vector containing the GM-CSF promoter region were necessary to convincingly establish the role of TNF-α and 8BrcAMP on transcriptional induction of the GM-CSF gene in +/+1.LDA11 stromal cells. Considering these results and an effect of 8BrcAMP on decreasing GM-CSF transcript stability in actinomycin-D (act-D) decay experiments, we conclude that the inhibitory effect of 8BrcAMP on GM-CSF expression is exerted at the posttranscriptional level. These data demonstrate that the intracellular level of cAMP has an important discriminatory role on expression of the cytokines GM-CSF and IL-6 in a model stromal cell line.