Degradation of Aiolos and Ikaros Followed By Downregulation of c-Myc and then IRF4 Leads to Programmed Cell Death in Myeloma Cells Sensitive to IMiDs^® Immunomodulatory Compounds

Background: The zinc finger transcription factors, Aiolos (IKZF3) and Ikaros (IKZF1) were identified as lenalidomide (LEN) and pomalidomide (POM)-induced substrates of the cereblon (CRBN)-dependent Culin4 E3-ligase complex. While recent studies suggest that the anti-proliferative activity of LEN and POM in multiple myeloma (MM) cell lines in vitro is due in part to the targeted ubiquitination and subsequent proteasomal degradation of Aiolos and Ikaros, the downstream molecular mechanisms remain unknown. Using inducible shRNA-mediated knockdown combined with kinetic analyses, we systematically investigated the biological mechanisms associated with the degradation of Ikaros and Aiolos in MM cell lines that are sensitive to or have acquired resistance to LEN and POM.

Results: In MM1.S and U266 MM cell lines stably engineered with doxycycline (DOXY)-inducible shRNAs, knockdown of either Ikaros or Aiolos showed a reduction in cell proliferation (80%-90%) as measured by ³H-thymidine incorporation after a 4 day treatment with DOXY. We demonstrated that this anti-proliferative effect is inherently tied to and precedes the induction of apoptosis, which was maximized (60%-80% AnnV⁺/ToPro3⁺) 5 days following Aiolos or Ikaros knockdown compared with a control shRNA. shRNA-mediated knockdown of Aiolos or Ikaros was furthermore associated with decreases in both c-Myc and IRF4 protein expression levels (70%-90% and 60%-80%, respectively) that were maximized by day 4. In turn, shRNA knockdown of either c-Myc or IRF4 elicited anti-proliferative (> 80% inhibition) and pro-apoptotic (50%-80%) responses as early as 48hrs after shRNA induction. These data suggest that the reduction of c-Myc and IRF4 protein levels downstream of Aiolos and Ikaros degradation account for the apoptotic effect and marks the onset of the cytotoxic response induced by LEN and POM in MM cells.

To define the temporal order of events involving Aiolos, Ikaros, c-Myc and IRF4 in more detail, kinetic experiments following shRNA-mediated knockdown in parallel with drug treatments were performed. Data from these experiments showed that there is a distinct kinetic order of both LEN- and POM-mediated effects, initiated by immediate targeted degradation of Aiolos and Ikaros (within 90 min), followed by a decrease in c-Myc levels (24-48 hrs) with subsequent IRF4 downregulation (48-72 hrs), and finally, resulting in programmed cell death (3-5 days). Importantly, DOXY washout experiments, resulting in re-accumulation of Aiolos or Ikaros at early time points (24 hrs) partially overcame the antiproliferative effects of the shRNA-mediated knockdown of either target. Interestingly, upon the onset of c-Myc downregulation (24-48 hrs), the commitment to cell death could no longer be reversed in our experiments.

Further, we generated MM1.S and U266 cells with acquired resistance to POM (10 µM; also cross-resistant to LEN) (MM1.S/PomR and U266/PomR , respectively), in which CRBN protein expression is substantially decreased (> 90%). Consequently, in these resistant cell lines, neither Aiolos nor Ikaros are degraded in the presence of LEN or POM. However, bypass of CRBN-dependent Aiolos degradation by DOXY-induced knockdown rescued c-Myc and IRF4 downregulation and concomitant inhibition of growth (90% and 60%, respectively), suggesting that resistant MM cells with acquired CRBN loss remain dependent on Aiolos and Ikaros.

Conclusions: For the first time, our studies showed that degradation of Aiolos and Ikaros sets up a molecular sequence of events culminating in programmed cell death in MM cells. Our mechanistic studies showed that c-Myc is a key intermediate factor whose downregulation is a rate-limiting step for the transcriptional downregulation of IRF4 as well as for the commitment to cell death. Taken together, our results demonstrate a molecular sequence of events underlying the mechanism of action of cytotoxicity of LEN or POM in MM cells. Quantitative measurements of Aiolos and Ikaros degradation, and c-Myc and IRF4 downregulation in clinical samples would help validate these findings.