Sebastian, Sinto

Background The precise molecular mechanism and targets of lenalidomide specific anti-Multiple Myeloma (MM) activity remain largely unknown, although, cereblon (CRBN) has been identified as a central mediator of activity with IRF-4 acting as a downstream molecule. Lenalidomide resistance in MM cells which, despite CRBN depletion, are able to restore their IRF-4 levels suggests the existence of alternative pathways. Methods To measure apoptosis and gene expression analysis 1x 106cells were incubated with 10μm to 50μm of lenalidomide for 24 to 96 hours before they were examined by annexin-PI and FACS analysis. Gene and protein expression were measured by RT-PCR, western blot, and immunohistochemistry. For lenalidomide followed bortezomib serial studies, cells were incubated with 10μm of lenalidomide for 48 hours and later treated with bortezomib for 24 to 48 hrs. CRBN and immunoglobulin light chain knock-down were performed by lentiviral mediated shRNA. Results We found lenalidomide induces Endoplasmic reticulum stress (ERS) via accumulation of intracellular immunoglobulin proteins, but only in the presence of CRBN in a panel of MM cell lines. This accumulation of intracellular proteins resulted in ERS as documented by western blot and RT-PCR in OPM2 and MMIS cell lines. We next utilized OPM2 cells knocked down for CRBN and control, non-target (NT), and treated with different doses of lenalidomide. After three days of treatment an ERS response was occurring in OPM2-NT cells but not in CRBN knockdown cells. Using Western blot we found decreased XBP-1u and increased XBP-1s, ERS marker proteins, in CRBN positive (NT) cells after 72 hours of lenalidomide treatment, but not in CRBN knock-down cells. Another ERS marker GRP78/BiP was also clearly accumulated after lenalidomide treatment in CRBN positive cells. Inactivation of the tumor suppressor p53 by degradation is a mechanism utilized by cells to adapt to ERS has been reported. We also found that p53 protein down regulated after lenalidomide treatment in CRBN positive OPM2 and MMIS cells. We have analyzed other isogenic cell lines MM1.S (lenalidomide-sensitive) and isogenic MM1.Sres (lenalidomide-resistant) HMCLs and found that lenalidomide induces ERS in MM1.S but not MM1.Sres cells. CRBN is important for lenalidomide to induce immunoglobulin protein accumulation and ERS was further proved by over-expressing CRBN in OCI-My5 MM cell line and subsequent lenalidomide treatment. To test the effect of light chain accumulation as ER stressors, we knocked down immunoglobulin light chain lambda in human MM cell line OPM2. Knock-down of lambda light chain in OPM2 cells resulted in lenalidomide resistance and it induced endoplasmic reticulum stress mediated anti-myeloma activity. Our preliminary data reveals that lenalidomide mediated progressive ERS can positively enhance bortezomib induced apoptosis in an in-vitro MM model. We pre-treated myeloma cell lines with lenalidomide and subsequently treated with bortezomib. MM cells pre-treated with lenalidomide for two days and there after treated with bortezomib clearly shown increased sensitivity to bortezomib induced apoptosis than non-pre-lenalidomide treated cells. Conclusion Here we show that lenalidomide causes accumulation of immunoglobulin light chains, inducing progressive ER stress, resulting in anti-myeloma activity and that effect depends on the presence of CRBN. Proteasome inhibitors such as bortezomib also sensitize MM cells to endoplasmic reticulum stress-mediated apoptosis. Our data demonstrated the synergistic effect of lenalidomide and proteasome inhibitors, likely due to the augmented protein stress. This data support the use of serial therapy utilizing pre-lenalidomide treatment followed by bortezomib or carfilzomib treatment. Disclosures: Fonseca: Medtronic: Consultancy; Otsuka: Consultancy; Celgene: Consultancy; Genzyme: Consultancy; BMS: Consultancy; Lilly: Consultancy; Onyx: Consultancy, Research Funding; Binding Site: Consultancy; Millennium: Consultancy; AMGEN: Consultancy; Cylene: Research Funding; Prognostication of MM based on genetic categorization of the disease: Prognostication of MM based on genetic categorization of the disease, Prognostication of MM based on genetic categorization of the disease Patents & Royalties.

Abstract Abstract 1846 Background The low frequency of TP53 (p53) alterations (<10%) in MM makes this tumor an ideal candidate for p53-activation therapies. Unfortunately, DNA alkylating agents (cyclophosphamide and melphalan), when used alone, cannot fully exploit p53 mediated apoptosis. It is currently unknown the extent to which DNA alkylating agents and p53 activating small molecules are activating functional p53 signaling to induce apoptosis in primary MM cells, because, p53 mainly induces cell cycle arrest/DNA repair or apoptosis in the DNA damage response. Emerging studies demonstrate that PARP activation and p53 poly(ADP-ribosyl)ation can negatively influence apoptosis induction but the exact molecular mechanisms are unknown. Methods For apoptosis and gene expression analysis 05 × 106 cells were incubated with 2.5 μM of melphalan, 10 μM of olaparib and 2.5 μM of nutlin-3a alone and/or combinations of melphalan with olparib and/or nutlin3a with olaprib for 24 to 72 hours before they were examined for cell death by the annexin-PI and FACS analysis. Gene and protein expression were measured by RT-PCR, western blot, and immunohistochemistry. For combination studies, cells were incubated with 0.194–50 μM of melphalan and 0.625–40 μM of either olaparib or ABT-888 for 72 hours. p53 knock-down performed by lentiviral mediated shRNA. Vk*MYC mice with significant gammopathy (>20g/l on SPEP) used for in-vivo drug combination studies. Results Preliminary results support our principle hypothesis that, a PARP inhibitor (olaparib) enhanced Melphalan sensitivity in wild type p53 MM cell lines H929 and MM1S but not in p53 mutated or homozygously deleted cell lines U266 and KMS11, respectively. Moreover, we demonstrate that p53 knock-down decreased the synergistic effect of combining a PARP inhibitor with melphalan and Nutlin-3a. Again, combining a PARP inhibitor with the DNA alkylating agent melphalan induced clear PARP cleavage, a signature of apoptosis in wild type p53 MM cell lines H929 and MM1S. No significant PARP cleavage was observed after melphalan or olaparib treatment alone in H929 and MMIS. As expected, the p53 mutant cell line U266 did not show any PARP cleavage and p21 up-regulation after both drug treatments. Here we propose that the hyper-activated p53 from PARP inhibition along with DNA alkylating agents (melphalan) and/or p53 activating agents (nutlin-3a) result in better responses. Finally, in a murine xenograft model of human MM, olaparib potentiated melphalan activity in vivo, with significant reduction in M spike level. Conclusion Our studies indicate that DNA alkylating agents activate wild type p53, but leads to DNA repair and cell survival and this can be abolished by PARP inhibition in wild type p53 expressing MM cases. Moreover, our study also found that olaparib, and ABT-888 - PARP1 inhibitors currently in clinical trials - display different dose responses in MM cell lines and in the abilities of individual PARP inhibitors to sensitize MM cell lines to melphalan varied to a great extent in a cell context- and cell line specific manner. Disclosures: No relevant conflicts of interest to declare.

Abstract It is well established that 17p13(del) and p53 mutations represent poor prognosis in MM patients. We have previously shown that p53 is a haploinsufficient tumor suppressor in myeloma and p53 expression level is correlated with functional status of the pathway. In addition, p53 expression levels do not always correlate with p53 genotype. The absence of p53 expression in myeloma cells severely abolishes the integrity of its downstream signaling response, thus, driving disease aggressiveness and drug resistance. There is a pressing need for a more effective treatment strategy for high-risk patients harbouring p53 abnormalities. PRIMA-1 is a first-in-class drug that was identified to specifically kill cancer cells harboring mutant p53. It has shown substantial efficacy in various solid tumors with mutant as well as non-mutant p53. However, the discovery of the anti-myeloma activity of PRIMA-1 is still at its infancy. By utilizing human myeloma cell lines (HMCLs) of different p53 status as our study model, we aim to identify the efficacy of PRIMA-1 in MM and to dissect the mechanisms behind PRIMA-1-induced toxicity. Despite its known ability to reactivate mutant-p53 function, we did not observe any restoration of mutant p53 transcriptional activities. The expression levels of the p53-direct-targets, p21, MDM2 and PUMA, were unchanged upon treatment in the p53-mutant-HMCLs. In fact, PRIMA-1 was found to be able to decrease the cell viability of all the HMCLs, irrespective of their p53 status (Mutant, WT, Null), strongly indicating a p53-independent response. Importantly, PRIMA-1 showed the highest cytotoxicity in a p53-deficient background (IC50 of 16uM in -/-, 55uM in WT/WT and 70uM in mutant HMCLs). This finding was validated when p53 silencing in a WT HMCL significantly enhanced its sensitivity to PRIMA-1. GEP analysis on the two most sensitive cell lines (JJN3 and KMS11) demonstrated significant enrichment of genes related to unfolded (p=1.90E-06 in JJN3 and 6.12E-08 in KMS11) and misfolded protein (p=2.99E-06 in JJN3 and 8.21E-08 in KMS11) responses. Subsequent validation by Western blot and qPCR confirmed the activation of ER stress pathway in response to PRIMA-1, evidenced by the up-regulation of ER stress markers such as GRP78, GADD34 and CHOP. Further investigations then revealed that the PERK/CHOP axis was most important as CHOP knockdown saw an attenuation of MM cells sensitivity to PRIMA-1-induced apoptosis. Recently, it was reported that cells with increased ER stress are more sensitive to bortezomib. Indeed, we showed that PRIMA-1 synergizes with bortezomib, and was able to rescue bortezomib resistance when both compounds were co-incubated in a bortezomib-resistant-HMCL. In conclusion, we identified a novel pathway by which PRIMA-1 exerts its toxicity in MM through induction of ER stress. This may be a novel therapeutic strategy to induce ER stress to sensitize cells to proteasome inhibitor. Owing to the versatility of the drug in targeting myeloma cells with or without p53 abnormalities; PRIMA-1 can potentially be used alone or in combination with other drugs such as bortezomib in a broad range of MM including those with high-risk disease. Disclosures No relevant conflicts of interest to declare.

Key Points IMiDs inhibit TrxR-mediated intracellular decomposition of H2O2 and caused oxidative stress in MM cells. MM cells with lower antioxidative capacity were more vulnerable to lenalidomide-induced H2O2 accumulation and its associated cytotoxicity.