Lenalidomide plays an important role in our chemotherapeutic armamentarium against multiple myeloma, in part by exerting direct anti-proliferative and pro-apoptotic effects. Unfortunately, long-term exposure leads to the development of drug resistance through unknown mechanisms, and we therefore sought to identify pathways that could be responsible for this phenotype. Chronic drug exposure produced myeloma cell lines that were tolerant of the direct effects of lenalidomide, with a degree of resistance of up to 2,500-fold. Gene expression profiling and pathway analysis identified dysregulation of the Wnt/ß-catenin pathway as a consistent change across four independent cell isolates, and a pair of primary plasma cell samples. Acute drug treatment also increased ß-catenin transcription by 3-fold or more, and both acute and chronic exposure resulted in enhanced accumulation of ß-catenin protein by up to 20-fold or more. This produced Wnt/ß-catenin pathway activation, as judged by increased activity of a lymphoid enhancer factor/T-cell factor promoter reporter, and enhanced accumulation of the downstream targets cyclin D1 and c-Myc. Components of the ß-catenin destruction complex were also impacted by lenalidomide, which suppressed casein kinase 1a expression while augmenting glycogen synthase kinase 3a/ß phosphorylation. Stimulation of Wnt/ß-catenin signaling with recombinant Wnt-3a, or by overexpression of ß-catenin, reduced the anti-proliferative activity of lenalidomide. Conversely, suppression of ß-catenin with small hairpin RNAs restored plasma cell sensitivity to lenalidomide. Together, these findings support the hypothesis that lenalidomide mediates activation of Wnt/ß-catenin signaling in plasma cells as a mechanism of inducible chemoresistance through effects at the transcriptional and post-translational levels.

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