Lymphotoxin: stimulation and regulation of colony-stimulating factors in fibroblasts.

Colony-stimulating factors (CSFs) are pivotal for proliferation and function of hematopoietic cells. We found that lymphotoxin, a product of activated lymphocytes, stimulates accumulation of granulocyte-macrophage (GM)-CSF and macrophage (M)-CSF proteins and mRNAs in fibroblasts. An increase in GM- and M-CSF mRNA levels occurred within 2 hours after addition of 1,000 U/mL lymphotoxin and levels plateaued over the next 24 hours. Tumor necrosis factor alpha (TNF alpha) was about five times more potent than lymphotoxin at low concentrations, and was nearly 1.5 to to 2 times more potent at maximally stimulating concentrations of the cytokines. Stimulation by lymphotoxin did not require either new protein synthesis or protein kinase-C stimulation. Stability studies of GM- and M-CSF transcripts in fibroblasts showed that M-CSF mRNA was five times more stable (half-life [t 1/2], 100 minutes) than GM-CSF mRNA (t 1/2, 20 minutes). Stability of these mRNAs was unchanged after stimulation of the cells with lymphotoxin. In addition, exposure of cells to 12-O-tetradecanoylphorbol 13-acetate did not alter stability of M-CSF mRNA but markedly prolonged the stability of GM-CSF mRNA. This is consistent with data showing that the AT-rich consensus region in the 3' untranslated region of many transiently expressed cytokines including GM-CSF but not M-CSF, play a major role in their mRNA stability. Our results suggest that activated lymphocytes can affect hematopoietic cell function and growth by stimulating production of CSFs by mesenchymal cells.