Antagonistic effect of small-molecule inhibitors of Wnt/β-catenin in multiple myeloma

Multiple Myeloma (MM) is the second most frequent hematological cancer in the US and is characterized by the clonal proliferation of neoplastic plasma cells in association with elevated serum monoclonal protein levels. Clinical manifestations of MM include lytic bone lesions, anemia, immunodeficiency, and renal impairment. It is evident from earlier studies that maintenance of MM within the bone marrow (BM) microenvironment largely depends on a number of factors including cytokines IL-6 and the family of Wnt ligands that are secreted by the BM stromal cells (BMSCs) (1–5). Wnt ligand specifically activates the canonical Wnt pathway which is mediated through the key transcriptional effector β-catenin (β-cat). In unstimulated cells, β-cat is phosphorylated by the Axin/APC/GSK3β-mediated destruction complex (DC), and is a target for ubiquitination and subsequent proteasome-mediated degradation. Wnt induction blocks the activity of the DC, thereby resulting in the cytosolic accumulation of non-phosphorylated, active β-cat, which translocates to the nucleus and together with T-cell factor/Lymphoid Enhancer Factor (TCF/LEF) transcriptional factors, activates transcription of downstream target genes. The Wnt pathway has been shown to play an important role in the regulation of cancer cell proliferation and differentiation (2, 6–8). Previous studies have shown that malignant MM plasma cells over express β-cat, including its N-terminally non-phosphorylated form, suggesting that β-cat/TCF-mediated transcription may be active in MM cells (9). Studies using both in vitro and in vivo models have shown that Wnt-β-cat signaling mediates critical events in the development of MM and thus indicates related phenotypic changes in plasma cells(10). Although a recent study reports the therapeutic efficacy of bortezomib via Wnt-independent stabilization of β-cat (11), a role for Wnt signaling in MM remains unclear. Dickkopf-1 (DKK1), a soluble inhibitor of Wnt/β catenin signaling, functions by binding to the Wnt co-receptor LRP5 to regulate its function on the cell surface in MM cells (12). However, deregulation of CRT in cancer development makes the β-cat-TCF complex as an ideal target for therapeutic approaches (13–15). Given the dual role of Wnt in normal bone formation and in myeloma disease, our interest was to test the chemosensitivity of recently identified small molecule inhibitors of β-cat regulated transcription (iCRTs) that are designed to specifically target β-cat/TCF-regulated transcription (16). Using human MM cell lines and patient derived BMMC that express nuclear β-cat, we report that iCRTs (oxazole and thiazole) are effective in down regulating nuclear β-cat, and reducing cell proliferation. Our findings further indicated a significant decrease in the level of vasculoendothelial growth factor (VEGF), in cells treated with iCRTs. Although our attempts to test the in vivo efficacy of iCRTs in preclinical models are in progress, we provide the first in vitro data evaluation of iCRTs as potential Wnt/β-cat/VEGF pathway antagonists in MM that could effectively block or decrease the disease progression at clinically relevant doses.