A soluble interleukin 6 receptor isolated from conditioned medium of human breast cancer cells is encoded by a differentially spliced mRNA.

The human interleukin 6 receptor (IL-6R) is expressed on cells as a transmembrane protein of 80 kDa (gp80, ligand binding unit), or as a smaller soluble counterpart (sIL-6R, approximately 55 kDa). Recombinant or natural sIL-6Rs bind IL-6 and stimulate biological activity by association with the signal transducing subunit gp130 at the cell surface. The origin of sIL-6Rs is not clear. Haematopoietic cells express, in addition to the gp80 mRNA, an IL-6R mRNA where the transmembrane domain is spliced out, predicting a shorter protein with a modified basic sequence at the C-terminus. We show that the spliced mRNA is expressed in human T47D breast carcinoma cells and soluble IL-6R protein is indeed secreted by these cells. An antibody against the C-terminus of the spliced protein detects a 55-65 kDa glycosylated species in sIL-6R purified from T47D supernatant by classical and immunoaffinity chromatography. The spliced T47D IL-6R, glycosylated or after removal of O- and N-linked polysaccharides, has the same size as a recombinant spliced IL-6R from CHO cells. The recombinant spliced IL-6R acts on cells as an IL-6 agonist to stimulate transcription from IL-6 inducible enhancers.