Blocking the ZZ domain of sequestosome1/p62 suppresses myeloma growth and osteoclast formation in vitro and induces dramatic bone formation in myeloma-bearing bones in vivo.

We reported that p62 (sequestosome 1) serves as a signaling hub in bone marrow stromal cells (BMSCs) for the formation of signaling complexes, including NFκB, p38MAPK and JNK, that are involved in the increased osteoclastogenesis and multiple myeloma (MM) cell growth induced by BMSCs that are key contributors to multiple myeloma bone disease (MMBD), and demonstrated that the ZZ domain of p62 (p62-ZZ) is required for BMSC enhancement of MMBD. We recently identified a novel p62-ZZ inhibitor, XRK3F2, which inhibits MM cell growth and BMSC growth enhancement of human MM cells. In the current study, we evaluate the relative specificity of XRK3F2 for p62-ZZ, characterize XRK3F2's capacity to inhibit growth of primary MM cells and human MM cell lines, and test the in vivo effects of XRK3F2 in the immunocompetent 5TGM1 MM model. We found that XRK3F2 induces dramatic cortical bone formation that is restricted to MM containing bones and blocked the effects and upregulation of tumor necrosis factor alpha (TNFα), an osteoblast (OB) differentiation inhibitor that is increased in the MM bone marrow microenvironment and utilizes signaling complexes formed on p62-ZZ, in BMSC. Interestingly, XRK3F2 had no effect on non-MM bearing bone. These results demonstrate that targeting p62 in MM models has profound effects on MMBD.