Acute myeloblastic leukaemia cells produce soluble interleukin 6 receptor by a mechanism of alternative splicing

The aim of the present work was to investigate whether acute myeloblastic leukaemia (AML) blast cells express a soluble (s) form of interleukin 6 (IL-6) receptor (R), and if they do, what is the mechanism of production. Eight AML patient cell lines and 25 primary AML blast cell samples were investigated. The cell lines secreted high quantities of sIL-6R into their culture medium when examined by enzyme-linked immunosorbent assay (ELISA). To determine whether sIL-6R is synthesized by a mechanism of alternative splicing, RNA was analysed from all the AML blast cell samples by using reverse transcription polymerase chain reaction. In this method, primer sites flanking the transmembrane domain were utilized and the alternatively spliced IL-6R mRNA was distinguished from the non-spliced transcript form by size. All the cell lines and 64% of the primary blast cell samples expressed the alternatively spliced IL-6R mRNA. To confirm the phenomenon of alternative splicing at protein level, cytoplasmic protein fractions of the cell lines were investigated by using a sensitive adaptation of the Western blot method. All the cell lines expressed two IL-6R proteins sized 80 and 50 kDa and corresponding to the membraneous and soluble forms of IL-6R, respectively. In conclusion, the results obtained at both mRNA and protein levels strongly support alternative splicing as a mechanism of sIL-6R production in AML. Because sIL-6R modulates the effects of IL-6 on target cells, differences in sIL-6R expression levels may partially explain the previously observed diversity in IL-6-induced growth responses in AML.