Myeloma cells release soluble interleukin-6R alpha in relation to disease progression by two distinct mechanisms: Alternative splicing and proteolytic cleavage

Multiple myeloma (MM) is a plasma-cell malignancy characterized by the accu mulation of malignant plasma cells within the bone marrow. Interleukin (IL) -6 is an essential survival and growth factor for myeloma cells that exerts its activity through a cell surface receptor composed of an 80-kDa ligand binding molecule (IL-6R alpha) and a 130-kDa signal-transducing molecule. O f major interest, the soluble form of the IL-6R alpha (sIL-6R alpha) is an agonistic molecule able to potentiate IL-6 activity and a strong prognostic factor in MM, In the present study, we demonstrate that purified myeloma c ells from all of the patients with MM and human myeloma cell lines release sIL-6R alpha. The level of sIL-6R alpha release correlates with disease act ivity and is clearly up-regulated during tumoral expansion in vivo and immo rtalization in vitro. Of note, this sIL-6R alpha release is strongly reduce d (50%) by a hydroxamate-based metalloproteinase inhibitor underlying the i mportance of shedding in the production of sIL-6R alpha by myeloma cells. U sing specific IL-6R alpha primers flanking the transmembrane domain, we dem onstrate by PCR the presence of two IL-6R mRNAs corresponding to the membra ne IL-6R alpha and to the sIL-6R alpha generated through alternative splici ng in myeloma cells. In conclusion, we show that: (a) native myeloma cells and human myeloma cell lines release sIL-6R alpha by two distinct mechanism s: alternative splicing and proteolytic cleavage of the membrane IL-6R alph a; and (b) the release of the sIL-6R alpha, which is an agonist of IL-6, co rrelates with disease progression, explaining in part its strong prognostic value in vivo.