Differential regulation of lymphotoxin and tumor necrosis factor genes in human T lymphocytes

Abstract

Lymphotoxin (LT) and tumor necrosis factor (TNF) are related cytokines that share many biological effects. The genes for LT and TNF are adjacent to each other on chromosome 6 in man, but previous data indicate that the kinetics of their production differ markedly. To explain the mechanisms for this difference, we compared the regulation of these two genes in human T lymphocytes, isolated from peripheral blood, after stimulation with the mitogens concanavalin A and phorbol myristate acetate. Differences in the kinetics of protein secretion were paralleled by differences in cognate mRNA accumulation. TNF mRNA accumulated rapidly after stimulation, peaked by 6 h, and returned to unstimulated (base-line) levels by 24 h. In contrast, LT mRNA accumulated slowly after stimulation, usually peaked at approximately 18 h, and remained increased above base-line levels at 48-72 h. By nuclear transcription run-on assays, increased transcription of TNF mRNA and LT mRNA was demonstrated after stimulation. However, TNF transcription peaked earlier and appeared to be 4-10 times greater than that of the LT gene. In contrast, the half-life of LT mRNA was 8-10-fold longer than that of TNF mRNA as demonstrated by actinomycin D pulse-chase experiments. Cycloheximide did not block LT or TNF mRNA accumulation, indicating that new protein synthesis was not required for induction of either gene. These results suggest strongly that the LT and TNF genes are regulated differently in human T lymphocytes after mitogen stimulation. TNF mRNA accumulates rapidly primarily because of increased transcription and decreases rapidly related to its brief half-life. In contrast, LT mRNA accumulates more slowly but persists much longer; the accumulation of this mRNA appears to be controlled largely by post-transcriptional mechanisms.