We have previously documented that interleukin 6 (IL-6) supports the growth of early murine plasma cell tumors (myelomas) and have found that the subsequent loss of IL-6-dependence represents a key step in the progression to a fully malignant cancer. Our studies with this model are aimed at (1) advancing our understanding of the cellular and molecular pathways in interleukin 6 signal transduction and IL-6 mediated cancer cell growth and (2) identifying the alterations occurring in the signal transduction pathway that result in growth factor-independent, malignant cell growth. A major aspect of our studies has focused on the transcriptional regulation and function of junB, an immediate-early response gene that is upregulated by IL-6. Using luciferase reporter expression we have identified a 200 base pair region downstream of the junB coding region that possesses the ability to upregulate the junB promotor by 75-fold. Point mutation of four enhancer elements within this region and DNA-binding analysis has revealed (1) an acute phase response element (APRE) that is necessary for junB induction by IL-6, (2) inducible binding of the Stat3 transcription factor to the APRE site, (3) an NF-kB site overlapping the APRE site that is necessary for full activity of the APRE, and (4) constitutive binding of p65 to the NF-kB site. Preliminary results also implicate other nearby enhancer elements in the upregulation of the APRE by IL-6. Studies are also underway to evaluate the role of junB in plasma cell tumor proliferation. Using antisense oligonucleotides, we previously found that junB is essential for the proliferation and survival of murine myeloma cells. We have extended this observation by overexpressing junB in IL-6-dependent cells. Preliminary results indicate that moderate overexpression substantially increases cell proliferation rates. Our studies on the mechanism of progression of IL-6-dependent tumor cells to IL-6-independent growth revealed that junB transcription is constitutively active in plasmacytoma cells that have become independent of IL-6. Reporter gene studies reveal that the APRE/Stat3 site remains under IL-6 control in IL-6-dependent cells whereas it is constitutively active in IL-6-independent cell lines, thus indicating that the loss of growth factor-dependence is """"""""upstream"""""""" of the activation of the Stat3 transcription factor and the subsequent transcriptional activation of junB .
