Human vascular smooth muscle cells. Target for and source of tumor necrosis factor.

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Abstract
TNF-alpha (also known as cachectin) may produce many of its important effects in vivo by actions on blood vessels. Endothelial cells are well known to respond to TNF-alpha. We investigated whether vascular smooth muscle cells (SMC), the most abundant cell type in most vessels, also respond to TNF-alpha and the related cytokine lymphotoxin (TNF-beta). Both human rTNF-alpha and beta (0.1 to 100 ng/ml) induced transient accumulation of IL-1 mRNA by adult human vascular SMC that peaked between 1 and 4 h. The inhibitor of RNA synthesis actinomycin D (1 microgram/ml) blocked the induction of IL-1 mRNA, whereas inhibition of protein synthesis with cycloheximide (1 microgram/ml) resulted in a marked "superinduction" of both IL-1 alpha and IL-1 beta mRNA species. TNF-alpha treatment also increased intracellular biologically active IL-1 and subsequent release of IL-1 activity from SMC. Metabolic labeling and immunoprecipitation with specific antibodies demonstrated de novo synthesis of IL-1 alpha and IL-1 beta precursors in TNF-treated or lymphotoxin-treated SMC. TNF-alpha also activated other SMC functions including the concentration-dependent release of PGE2 from SMC, and time-dependent induction of the gene for (2'-5')-oligoadenylate synthetase, an enzyme thought to mediate the anti-viral and anti-proliferative actions of IFN. We also explored whether SMC, which both produce and respond to IL-1, might also express either of the TNF genes. Bacterial LPS (10 micrograms/ml) caused slight accumulation of TNF-alpha transcripts. Incubation of SMC for 4 h with inhibitors of protein synthesis alone caused little or no elevation of TNF-alpha mRNA, but simultaneous addition of LPS ("superinduction" conditions) induced large amounts of TNF-alpha (but not TNF-beta) mRNA. Cells treated with anisomycin (1 microgram/ml) and LPS, then washed to remove this reversible inhibitor of protein synthesis, released TNF-alpha into the medium, as assessed by the L929 cytotoxicity assay and by metabolic labeling and immunoprecipitation. Thus, SMC both respond to both TNF and lymphotoxin and can produce TNF-alpha, a cytokine with numerous effects on vascular cells of potential significance in the pathophysiology of septic shock and other inflammatory conditions.