Combination of ATRA with a COX2 Inhibitor Induces Synergistic Effects On RARalpha2-Expressing Multiple Myeloma (MM) Cells.

Abstract 2809 Poster Board II-785 We recently observed that ATRA treatment selectively kills RARalpha2-expressing, while sparing RARalpha2-deficient MM cells. Previous investigations in the colon cancer cells have shown that ATRA inhibits WNT signaling through down-regulation of COX-2. Therefore, we wanted to evaluate the role of WNT signaling in ATRA-induced cell death and growth inhibition of RARalpha2-expressing myeloma cells. To our surprise, we found that ATRA treatment activated but not inhibited WNT signaling in RARalpha2-expressing myeloma cells, based on increased β-catenin levels in ATRA-treated cells. ATRA exerted minimal effects on activation of WNT signaling pathway in RARalpha2-deficient MM cells, and forced expression of RARalpha2 in RARalpha2-deficient cells restored the stimulatory activities of ATRA on the WNT signaling pathway, demonstrating that RARalpha2 expression is required for the ATRA-induced stimulation of WNT signaling in MM cells. Lithium chloride (LiCl) treatment, which activates WNT signaling, partially abrogated ATRA-induced cell death and growth inhibition in RARalpha2-expressing cells, indicating that ATRA-induced activation of WNT signaling resulted in ATRA-resistance and decreased killing of MM cells, suggesting that a combination of targeting WNT signaling pathway and ATRA treatment is necessary for ATRA-based therapy of RARalpha2-expressing myeloma. COX-2 inhibition blocks WNT signaling in colon cancer. Similarly, we found that a COX-2 inhibitor CAY10404 also blocked WNT signaling in RARalpha2-expressing cells as well as in ATRA-treated cells. Interestingly, CAY10404 activated MEK/ERK signaling pathway, while ATRA abrogated CAY10404-induced activation of MEK/ERK signaling pathway. These results demonstrate that the combination of ATRA and COX-2 inhibitor exerts synergistic inhibitory effects on both WNT and MEK/ERK signaling pathways. A combination of ATRA and the COX-2 inhibitor resulted in synergistic cytotoxicity of RARalpha2-expressing MM cells in-vitro. More importantly, the combination of ATRA and CAY10404 also resulted in a synergistic growth inhibition of established MM tumors in SCID mice. Our study demonstrates the importance of targeting WNT signaling in ATRA-based therapy in RARalpha2-expressing myeloma and provides a rationale for the combinational use of ATRA and COX-2 inhibitors. Disclosures: No relevant conflicts of interest to declare.