Interleukin-6 gene expression in human endothelial cells: RNA start sites, multiple IL-6 proteins and inhibition of proliferation

Interleukin-6 (IL-6) is a cytokine which is not only produced by a wide variety of different cells but one which also affects the function of diverse tissues. We have studied the expression of the IL-6 gene in freshly explanted human umbilical vein endothelial cells (HUVEC) and have also evaluated the effect of IL-6 on HUVEC proliferation. Cytokines like interleukin-1α (IL-1α) and tumor necrosis factor (TNF) as well as bacterial products such as the lipopolysaccharide (LPS) rapidly enhance production of biologically active IL-6 by HUVEC (IL-6 bioassay: increase in α 1-antichymotrypsin secretion by Hep3B2 cells and its neutralization by antiserum to E. coli -derived human IL-6). The two inducible RNA start sites in the IL-6 gene that are used in cytokine-induced fibroblasts (at +1 and −21) are also used in the same relative proportion (+1 > −21) in cytokine or LPS-induced HUVEC as determined by S 1-nuclease protection assays for IL-6 transcripts. Immunoaffinity chromatography followed by Western blotting shows that IL-6 species secreted by IL-1α-induced HUVEC are of molecular mass 23–25, 27–30 and 45 kDa as judged by SDS-PAGE under reducing conditions. Finally, rIL-6 inhibits [ 3H]-thymidine incorporation by HUVEC in a dose-dependent manner. Thus IL-6 is not only produced by HUVEC but may also affect its proliferation. The ability of the vascular endothelium to rapidly secrete IL-6 in response to inflammation-associated cytokines is of strategic value since it generates a circulatory signal which helps mobilize the acute phase plasma protein response and enlists the immune system in host defence.