JNK Regulates DKK1 Expression in Multiple Myeloma Cells.

Multiple myeloma (MM), a tumor of terminally differentiated plasma cells, depends on the bone marrow for growth and survival. MM is characterized by extensive bone loss and osteolytic lesions located at the sites of medullary plasmacytomas, suggesting that myeloma cells alter the biology of bone remodeling. Wnt signaling in osteoblasts is essential for the maintenance of osteoblast and osteoclast homeostasis and thus coupled bone turnover. MM plasma cells, but not those from healthy donors or MGUS, secrete Wnt signaling inhibitors DKK1, sFRP-2 and sFRP-3. The production of these factors contributes to osteolytic lesions by inhibiting Wnt signaling in the bone marrow milieu. It is currently not known how DKK1 expression is activated and regulated in MM. Here we report that DKK-1 expression is significantly lower in 45 myeloma cell lines (MMCL) and 5 secondary plasma cell leukemias/ pleural effusions compared to primary myeloma cells and down-regulated in all 9 primary myeloma cells with high DKK1 expression levels (Affymetrix Signal > 1,000) following co-culture with osteoclasts. This inhibition of DKK1 is associated with the simultaneous down-regulation of JUN, FOS, and FOSB, of which JUN has been shown to regulate DKK1 (Grotewald and Ruther, 2002). Ectopic expression of c-Jun in Jun negative MMCLs did not induce DKK1 expression, indicating that DKK1 transcription in this cell type is not solely dependent on c-Jun. DKK1 transcription increased 2- to 20-fold in myeloma cells from 21 of 42 primary MM following 48 hour of in-vivo exposure to thalidomide and 19 of 24 exposed to lenolidomide. The JNK-specific inhibitor SP600125 could significantly inhibit the up-regulation of DKK1 in 7 of 9 primary MM cells following in-vitro exposure to thalidomide or lenolidomide. Finally, treatment of MMCL and primary MM with the JNK activator Anisomycin resulted in up-regulation of DKK1 2- to 5-fold, but not in the presence of SP600125. These data indicate that DKK1 expression in MM cells is, in part, dependent on JNK signaling and suggests that JNK inhibitors may be an effective means of inhibiting both DKK1 expression and MM bone disease.