Abstract
Lenalidomide selectively induces the degradation of the Ikaros proteins IKZF1 and IKZF3.
Major finding: Lenalidomide selectively induces the degradation of the Ikaros proteins IKZF1 and IKZF3.
Mechanism: Lenalidomide promotes binding of IKZF1 and IKZF3 to CRBN, a ubiquitin ligase substrate receptor.
Impact: Therapeutic agents can downregulate specific targets by altering ubiquitin ligase substrate specificity.
Lenalidomide is a thalidomide derivative that is approved for treatment of multiple myeloma and is effective in other hematologic malignancies, but the mechanistic basis for its antineoplastic activity is unknown. Using a quantitative mass spectrometry-based approach to identify lenalidomide-interacting proteins, Krönke and colleagues found that lenalidomide bound cereblon (CRBN), the substrate-recognition subunit of a ubiquitin ligase complex. Ubiquitination profiling revealed that two related transcription factors with essential roles in B- and T-cell differentiation, Ikaros family zinc finger 1 (IKZF1) and IKZF3, were specifically ubiquitinated and subsequently degraded in lenalidomide-treated multiple myeloma cells. Lu and colleagues likewise found that lenalidomide induced the selective degradation of IKZF1 and IKZF3 by screening for changes in open reading frame–luciferase fusion stability after lenalidomide treatment. Both groups found that lenalidomide decreased IKZF1 and IKZF3 protein levels by binding to CRBN and enhancing its interaction with IKZF1 and IKZF3, which promoted IKZF1 and IKZF3 ubiquitination and degradation. The specificity of this interaction was conferred by a glutamine residue within the second zinc finger of IKZF1 and IKZF3 that was not present in the highly homologous but lenalidomide-resistant Ikaros family members IKZF2 and IKZF4. Knockdown of either IKZF1 or IKZF3 led to decreased viability of lenalidomide-sensitive multiple myeloma cell lines but did not affect lenalidomide-resistant cell lines. Moreover, expression of IKZF1 or IKZF3 mutants lacking the key glutamine residue in the CRBN degron sequence or knockdown of CRBN conferred lenalidomide resistance, providing further evidence that the antitumor activity of lenalidomide in multiple myeloma is due to CRBN-dependent degradation of IKZF1 and IKZF3. In addition to identifying a mechanism of action for lenalidomide, these findings also suggest that it may be possible to selectively target undruggable proteins such as transcription factors with agents that alter ubiquitin ligase substrate specificity.
