THE PLASMACYTOMA GROWTH INHIBITOR RESTRICTIN-P IS AN ANTAGONIST OF INTERLEUKIN-6 AND INTERLEUKIN-11 - IDENTIFICATION AS A STROMA-DERIVED ACTIVIN-A

A stromal protein, designated restrictin-P, that specifically kills pl asma-like cells was purified to homogeneity and shown to be identical with activin A. The specificity 60 plasma-like cells stemmed from the ability of restrictin-P/activin A to competitively antagonize the prol iferation-inducing effects of interleukin (IL) 6 and IL-11. Restrictin -P further interfered with the IL-6-induced secretion of acute phase p roteins by HepGB human hepatoma cells and with the IL-6-mediated diffe rentiation of M1 myeloblasts. A competition binding assay indicated th at restrictin-P did not interfere with the binding of IL-6 to its rece ptor on plasma-like cells, suggesting that it may act by intervening i n the signal transduction pathway of the growth factor. Indeed, concom itant addition of restrictin-P and IL-6 to cytokine-deprived B9 hybrid oma cells was followed by sustained overexpression of junB gene until cell death occurred, while IL-6 alone caused a transient increase only , This altered response to IL-6 stimulation was accompanied by a moder ate increase in STAT protein activation. Thus, in this study, we ident ified the plasmacytoma growth inhibitor, restrictin-P, as being activi n A of stromal origin. It is shown that activin A is an antagonist of IL-6 induced functions and that it modifies the IL-6 signaling pattern .