Spleen cells freshly isolated from normal mice were irradiated with 20 Gy X rays in culture. Northern blot hybridizations revealed that expression of the interleukin-1 beta (IL-1 beta) gene was induced immediately after irradiation and was increased for 2 h thereafter. Dibutyryl cyclic AMP also caused a persistent expression of the IL-1 beta gene, although it differed from X rays in that it coincidentally induced expression of the c-fos gene, which was not induced by X rays. Activation of either protein kinase C or calmodulin also induced early expression of both IL-1 beta and c-fos. Myeloid cells collected from the spleen of mice with granulocytic leukemia were X-irradiated in culture as above. The leukemia cells responded to X rays as well as to other stimuli in the same manner as the spleen cells, except that IL-1 beta mRNA was no longer detected 30 min after irradiation while c-fos was detectable for 2 h. When the leukemia cells were irradiated twice with a 3-h interval between irradiations, the second irradiation led to prolonged expression of IL-1 beta without inducing c-fos expression. These results suggest that ionizing radiation elicits early expression of the IL-1 beta gene through a mechanism that does not involve protein kinase C or A, or the transcription factor, c-fos. Whole-body irradiation of mice with 50 Gy 137Cs gamma rays also induced IL-1 beta expression in spleen but not in bone marrow or liver, although there was a delay of several hours before it was amply expressed. Furthermore, a delay as long as 24 or 72 h was observed when the radiation dose was reduced to 8.5 or 4 Gy. The results of this in vivo study suggest that the rapidity of expression of the IL-1 beta gene is dependent on the dose of radiation, and that the cells in the body cannot respond to radiation as rapidly as cells in culture.

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The activity of macrophage colony-stimulating factor (M-CSF) was found in the culture supernatant of mouse parenchymal liver cell fractions in a bone marrow colony-forming assay. The activity of an M-CSF-like substance purified by a four-step procedure was neutralized by goat anti-mouse M-CSF antiserum. M-CSF mRNA was detected in cellular RNA prepared from cultured parenchymal liver cell fractions by Northern blot analysis and also in cultured parenchymal liver cells by in situ hybridization. These results indicate that parenchymal liver cells have the capacity to produce M-CSF. We discuss the role of M-CSF in hematopoiesis, the immune response, and other biological phenomena.

In murine spleen cells, x ray irradiation induces the expression of the IL-1 beta gene at multiple phases of the peak time. We analyzed the immediate-early phase of IL-1 beta mRNA accumulation. To determine the lineage of cells that showed the immediate response to irradiation, normal spleen cells were analyzed by Northern blotting and in situ hybridization after separation by magnetic antibodies against specific cell-surface antigens. Although most of the spleen macrophages continuously expressed a low level of IL-1 beta mRNA, a portion of the macrophage population transiently accumulated large amounts of IL-1 beta message immediately after irradiation. A macrophage-like leukemia cell line that resembles these inducible macrophages was identified. A similar immediate-early and transient increase in the IL-1 beta mRNA level occurred when cultured spleen cells were irradiated with a low dose (3 Gy) of x rays. In contrast, the x ray-inducible expression of the IL-1 beta gene was immediate and continuous, not transient, in spleen cells from whole-body irradiated mice. Results of the run-on transcription assay and the determination of the decrease in the message using cultured spleen and macrophage-like leukemia cells indicated that x ray irradiation appears to activate the transcription of the IL-1 beta gene and partially stabilize the message. The results show that the x ray-induced immediate-early accumulation of IL-1 beta mRNA is regulated at both the transcriptional and post-transcriptional levels in an as yet unidentified population of spleen macrophages.

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