Both stat3-activation and Stat3-independent BCL2 downregulation are important for interleukin-6 induced apoptosis of 1A9-M cells

A unique subclone of a bone marrow-derived stromal cell line, BMS2.4, produ ces soluble factors that inhibit proliferation of several types of hematopo ietic cell lines. An understanding of these molecules may be informative ab out negative regulatory circuits that can potentially limit blood cell form ation. We used expression cloning to identify interleukin-g (IL-6) as one f actor that suppressed growth of a pre-B-cell variant line, 1A9-M. Moreover, IL-6 induced macrophage-differentiation and apoptosis of 1A9-M cells. Duri ng this process, IL-6 downregulated expression of BCL2 in 1A9-M cells and s timulated BCL-XL expression, but had no effect on p53, Bar, or Bak gene exp ression. Mechanisms for transduction of IL-6-induced signals were then eval uated in Il-Gi-stimulated 1A9-M cells. Whereas the signal transducer and ac tivator of transcription 3 (Stat3) was phosphorylated and activated, there was no effect on either Stat1 or Stat5. The importance of BCL2 and Stat3 on Ib-fi-induced macrophage-differentiation and apoptosis was studied with 1A 9-M cells expressing human BCL2 or a dominant-negative form of Stat3, respe ctively. IL-6-induced apoptosis, but not macrophage-differentiation, was bl ocked by continuously expressed BCL2. A dominant-negative form of Stat3 inh ibited both macrophage-differentiation and apoptosis induced by IL-6. Howev er, diminished Stat3 activity did not prevent IL-6-induced downregulation o f the BCL2 gene. Therefore, activation of Stat3 is essential for IL-6-induc ed macrophage-differentiation and programmed cell death in this model. Wher eas overexpression of BCL2 abrogates the apoptotic response, Stat3-independ ent signals appear to downregulate expression of the BCL2 gene. (C) 1999 by The American Society of Hematology.