Tumor necrosis factor and interleukin-1 down-regulate receptors for substance P in human astrocytoma cells

This study examined the influence of cytokines on substance P (SP) receptors (NK1 subtype) in the human astrocytoma cell line UC11. Following trypsinization and passage, the density of SP receptors in these cells was rather low but gradually increased several fold over the course of a few days in culture. Frequent replacement of the growth medium enhanced the density of receptors even more, suggesting that growth factors in the culture medium may determine the levels of receptor. Exposure of the cells to sub-nanomolar concentrations of tumor necrosis factor (TNF alpha) or interleukin-1 beta (IL1 beta), but not interleukin-2 or interleukin-6, decreased the density of SP receptors. This was accompanied by a decrease in the ability of SP to stimulate inositolphosphate formation. The ability of histamine to activate inositolphosphate formation was not influenced by the cytokines. The decrease in SP receptor density was readily reversible on washout of the cytokines. The EC50 for TNF alpha was approximately 0.5 ng/ml, the EC50 for IL1 beta was approximately 0.1 ng/ml. Radioligand binding studies with [125I]TNF alpha indicated the presence of a low density of high affinity binding sites for this ligand: Kd = 2.5 +/- 0.6 ng/ml, Bmax = 14.8 +/- 2.7 fmol bound/mg protein (assuming trimeric form of ligand bound). The most likely explanation for the cytokine effect is an inhibition of the synthesis of new receptors.