Direct Binding with Cereblon Mediates the Antiproliferative and Immunomodulatory Action of Lenalidomide and Pomalidomide

Abstract 738

Thalidomide, lenalidomide and pomalidomide are therapeutically active in a number of hematological malignant and premalignant conditions including myelodysplastic syndromes, multiple myeloma, and lymphomas. Clinical efficacy is ascribed to a complement of overlapping activities including direct antitumor effects, immune system activation and inhibition of stromal support of tumor growth. Thalidomide has previously been shown to bind cereblon (CRBN) a protein required for the teratogenic effects of thalidomide in zebrafish and chicken embryos (Ito et al). CRBN forms an ubiquitin E3 ligase complex with DNA damage-binding protein 1 (DDB1), cullin 4 (CUL4) and protein Rbx1 and thalidomide treatment has been shown to inhibit the ubiquitin ligase activity of the complex (Ito et al).

Using two independent biophysical methods, we demonstrate that lenalidomide and pomalidomide bind to CRBN-DDB1 complex. Fluorescence-based thermal shift analysis was carried out using purified ZZ-CRBN-DDB1. Phthalimide showed no appreciable binding to the CRBN-DDB1 complex while dose-dependent interaction of thalidomide, lenalidomide and pomalidomide was observed. Thalidomide binding showed approximately ten-fold less affinity (∼ 30 μM) than lenalidomide and pomalidomide (each ∼ 3 μM IC₅₀). Following the procedure of Ito et al, we used thalidomide analog-coupled beads (Thal-beads) and demonstrated binding of CRBN in complex with DDB1 from U266B1 myeloma cell extracts. Preincubation of the extracts with lenalidomide (100 μM) prevented CRBN binding to Thal-beads. Furthermore, the binding of CRBN was dose-dependently inhibited by preincubation with either lenalidomide or pomalidomide with IC₅₀s of 2.3 and 2.1 μM, respectively.

We then investigated whether CRBN was required for lenalidomide and pomalidomide responses associated with efficacy. First, CRBN expression was reduced in activated human T cells using CRBN siRNAs. After T cell activation, incubation with lenalidomide (1 μM) or pomalidomide (1 μM) resulted in an 11 to 14- fold-increase in IL-2 and a 5 to 10-fold increase in TNF-α. This increase was reduced ∼60% in the presence of siCRBN. Since IL-2 and TNF-α are important cytokines for tumor surveillance by activated T cells, our results indicate that some of the immunomodulatory effects of lenalidomide and pomalidomide are mediated via CRBN. We next determined if CRBN was required for the antiproliferative effect of lenalidomide and pomalidomide in myeloma cells. Multiple siRNAs were used to silence the expression of CRBN in U266B1 cells resulting in the absence of CRBN protein as determined by immunoblot analysis. Propidium-iodide staining showed that depletion of CRBN affected neither cell cycle nor proliferation of U266B1 cells. However, knockdown of CRBN markedly abrogated lenalidomide- and pomalidomide-induced delay of cell cycle progression. In addition, using lentiviral vectors we produced U266B1 cell lines with either 60% or 75% less expression of CRBN and showed that relative to the parental cell line these cells were gene dose-dependently less responsive to inhibition of proliferation by lenalidomide. The U266B1 cells in which CRBN had been decreased were also less responsive to inhibition by pomalidomide, but this compound maintained greater inhibition of proliferation than lenalidomide in the context of decreasing CRBN. Moreover, gene profile changes by lenalidomide and pomalidomide were reversed in the presence of siCRBN. In particular, induction of p21^WAF1 cyclin –dependent kinase inhibitor protein was prevented in the absence of the expression of CRBN. Similar results on different myeloma cell lines and using multiple CRBN siRNAs, confirmed the same critical role of CRBN in the antiproliferative response to lenalidomide and pomalidomide of myeloma cells.

Finally, we demonstrated that CRBN expression decreases concomitantly with the acquisition of lenalidomide resistance in H929 myeloma cells. Lenalidomide-resistant H929 cells remain responsive to inhibition by pomalidomide despite relatively reduced expression of CRBN. However, in pomalidomide-resistant DF15R myeloma cells, the complete absence of CRBN confers resistance to both lenalidomide and pomalidomide.

Our data demonstrate that CRBN is a direct target of lenalidomide and pomalidomide and plays a crucial role in the antitumor efficacy of lenalidomide and pomalidomide.