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p97/VCP is an endoplasmic reticulum (ER)‐associated protein that belongs to the AAA (ATPases associated with diverse cellular activities) ATPase family. It has a variety of cellular functions including ER‐associated protein degradation, autophagy, and aggresome formation. Recent studies have shown emerging roles of p97/VCP and its potential as a therapeutic target in several cancer subtypes including multiple myeloma (MM). We conducted a cell‐based compound screen to exploit novel small compounds that have cytotoxic activity in myeloma cells. Among approximately 2000 compounds, OSSL_325096 showed relatively strong antiproliferative activity in MM cell lines (IC 50, 100‐500 nmol/L). OSSL_325096 induced apoptosis in myeloma cell lines, including a bortezomib‐resistant cell line and primary myeloma cells purified from patients. Accumulation of poly‐ubiquitinated proteins, PERK, CHOP, and IREα, was observed in MM cell lines treated with OSSL_325096, suggesting that it induces ER stress in MM cells. OSSL_325096 has a similar chemical structure to DBeQ, a known p97/VCP inhibitor. Knockdown of the gene encoding p97/VCP induced apoptosis in myeloma cells, accompanied by accumulation of poly‐ubiquitinated protein. IC 50 of OSSL_325096 to myeloma cell lines were found to be lower (0.1‐0.8 μmol/L) than those of DBeQ (2‐5 μmol/L). In silico protein–drug‐binding simulation suggested possible binding of OSSL_325096 to the ATP binding site in the D2 domain of p97/VCP. In cell‐free ATPase assays, OSSL_325096 showed dose‐dependent inhibition of p97/VCP ATPase activity. Finally, OSSL_325096 inhibited the growth of subcutaneous myeloma cell tumors in vivo. The present data suggest that OSSL_325096 exerts anti‐myeloma activity, at least in part through p97/VCP inhibition.

The sialic glycoprotein, MUC1, is known to be involved in the pathogenesis of various types of cancers. KL-6 is one of the surface antigens of MUC1 and also a marker of interstitial pneumonitis. A fraction of patients with myeloma (3.9%) have elevated serum KL-6 levels without any evidence of interstitial pneumonitis and their myeloma cells have high MUC1 expression. We established a myeloma cell line designated EMM1 from a patient with multiple myeloma accompanied with elevated serum KL-6. EMM1 cells expressed high levels of MUC1 compared with other myeloma cell lines. Knockdown of MUC1 in EMM1 cells induced cell cycle arrest during S phase and apoptosis, suggesting that the MUC1 expression is involved in accelerated growth of EMM1 cells. RNA-seq analysis suggests that MUC1 expression activates k-ras and TNFα-induced NFκB pathways in EMM1 cells. We injected EMM1 cells subcutaneously into Rag2-/-Jak3-/- Balb/c mice to establish a mouse xenograft model. These mice had aggressive tumor growth that was accompanied by high serum KL-6 levels. In addition, MUC1 knockdown in EMM1 cells led to inhibited tumor growth. These findings demonstrate that MUC1 serves as a potential target for developing drugs for treatment of patients with KL-6+ myeloma, and EMM1 cells and EMM1-engrafted mice are useful tools for the development of such novel agents.

Immunomodulatory drugs (IMiDs) such as thalidomide, lenalidomide, and pomalidomide are efficacious in the treatment of multiple myeloma and significantly prolong their survival. However, the mechanisms of such effects of IMiDs have not been fully elucidated. Recently, cereblon has been identified as a target binding protein of thalidomide. Lenalidomide-resistant myeloma cell lines often lose the expression of cereblon, suggesting that IMiDs act as an anti-myeloma agent through interacting with cereblon. Cereblon binds to damaged DNA-binding protein and functions as a ubiquitin ligase, inducing degradation of IKZF1 and IKZF3 that are essential transcription factors for B and T cell development. Degradation of both IKZF1 and IKZF3 reportedly suppresses myeloma cell growth. Here, we found that IMiDs act as inhibitors of DNA methyltransferases (DMNTs). We previously reported that PU.1, which is an ETS family transcription factor and essential for myeloid and lymphoid development, functions as a tumor suppressor in myeloma cells. PU.1 induces growth arrest and apoptosis of myeloma cell lines. In this study, we found that low-dose lenalidomide and pomalidomide up-regulate PU.1 expression through inducing demethylation of the PU.1 promoter. In addition, IMiDs inhibited DNMT1, DNMT3a, and DNMT3b activities in vitro. Furthermore, lenalidomide and pomalidomide decreased the methylation status of the whole genome in myeloma cells. Collectively, IMiDs exert demethylation activity through inhibiting DNMT1, 3a, and 3b, and up-regulating PU.1 expression, which may be one of the mechanisms of the anti-myeloma activity of IMiDs.

PU.1 is an Ets family transcription factor, which is necessary for differentiation of both myeloid and lymphoid lineages. It was previously reported that conditional knockout of the upstream regulatory element (URE) located in 14 kb 5' of the PU.1 gene resulted in down-regulation of PU.1 expression in granulocytes and B lymphocytes by 80% compared to that of wild type and induced acute myeloid leukemia and CLL-like diseases in mice. Since the URE contains a suppressor region for PU.1 expression in T cells, such mice express PU.1 in T cells and develop T cell lymphoma. Thus, the failure of proper expression of PU.1 in certain differentiation stages in certain cell lineages appears to result in hematological malignancies. We previously reported that PU.1 is down-regulated in various myeloma cell lines. In addition, PU.1 is expressed in normal plasma cells and PU.1 is down-regulated in myeloma cells of certain myeloma patients, who appear to have poor prognosis. In those myeloma cell lines, the promoter and URE of the PU.1 gene are highly methylated. A demethylation agent, 5-aza-2'-deoxycytidine, induced PU.1 up-regulation, growth arrest, and apoptosis in myeloma cell lines, KMS12PE and KHM11. In addition, conditionally expressed PU.1 induced cell growth arrest and apoptosis in PU.1-low-negative myeloma cell lines, U266 and KMS12PE, suggesting that PU.1 is a tumor suppressor for myeloma cells. To elucidate the mechanisms of the cell growth arrest and apoptosis in myeloma cells induced by PU.1, we performed DNA microarray analysis to compare gene expression levels before and after PU.1 expression. Among cell-cycle related genes, p21WAF1/CIP1 was found up-regulated in U266 cells, while among apoptosis related genes, TRAIL was highly up-regulated in both U266 and KMS12PE cell lines. With further investigation, we concluded that PU.1 directly transactivated the TRAIL gene in myeloma cells, leading to apoptosis. Based on the DNA microarray data generated, we found that IRF4 is downregulated in U266 myeloma cells after PU.1 induction. It has been reported that knockdown of IRF4 induces apoptosis in myeloma cell lines. Therefore, we examined whether IRF4 was down-regulated in three myeloma cell lines, U266, KMS12PE, and KHM11 following PU.1 induction. Conditional expression of PU.1 by tet-off system induced IRF4 down-regulation in U266and KMS12PE cells. With lentiviral transduction method, ectopic expression of PU.1 also induced IRF4 down-regulation, cell-cycle arrest, and apoptosis in KHM11 cells. To investigate the role of IRF4 in PU.1-expressing U266 cells, we stably expressed IRF4, partially rescuing U266 cells from apoptosis. IRF4 is known to directly bind to the IRF7 promoter and down-regulate IRF7 expression in activated B cell-like (ABC) subtype of diffuse large B-cell lymphoma cells. Therefore, we examined whether IRF4 bound to the IRF7 promoter in KMS12PEand U266cells using chromatin immunoprecipitation assays. We found that IRF4 directly bound to the IRF7 promoter in both myeloma cell lines. When we overexpressed PU.1, IRF4 levels were decreased and the IRF4 binding to the IRF7 promoter was significantly reduced in those cell lines. Moreover, knockdown of IRF7 significantly rescued PU.1-expressing U266cells from apoptosis. These data strongly suggest that PU.1-induced apoptosis is associated with IRF4 down-regulation and subsequent IRF7 up-regulation in myeloma cells. Since IRF4 is essential transcription factor for myeloma cell survival, up-regulation of PU.1 by demethylation agents, including 5-aza-2'-deoxycytidine may serve as a promising therapeutic modality of multiple myeloma by inducing down-regulation of IRF4. Disclosures No relevant conflicts of interest to declare.

PU.1 is an essential transcription factor for hematopoiesis and important for differentiation of both myeloid and lymphoid lineages. In mice conditionally knocked-out of 3.4 kb length of the enhancer region located in14 kb 5’ upstream of the PU.1 gene (URE), PU.1 is down-regulated in myeloid cells and B cells by 20% of that of wild type, and such mice develop acute myeloid leukemia and CLL-like diseases. These data strongly suggest that PU.1 has tumor suppressor activity in hematopoietic cells. We previously reported that human PU.1 is down-regulated in the majority of myeloma cell lines through the methylation of the promoter and the 17 kb upstream enhancer region (URE) of the PU.1 gene that is homologous to that in 14 kb 5’ upstream of the murine PU.1 gene. Conditionally expressed PU.1 with tet-off system induced cell growth arrest and apoptosis in two myeloma cell lines, KMS12PE and U266, suggesting that the down-regulation of PU.1 is necessary for myeloma cell growth. We have also reported that PU.1 is expressed in normal plasma cells and in contrast, PU.1 is down-regulated in primary myeloma cells from a subset of myeloma patients, who appear to have poor prognosis. In the present study, to test whether PU.1 has tumor suppressor activity in vivo, we generated xenograft mouse models. 0.6 - 1 x 107 KMS12PE cells were subcutaneously injected in 16 immunodeficient mice (Rag2-/- Jak3-/- bulb/c). The mice were then administered doxycycline through drinking water. Half of the mice (N=8) stopped taking doxycycline when the tumor sizes reached 1 cm in diameter, whereas the other half (N=8) kept taking doxycycline. Although the tumors in the mice taking doxycycline continued to grow, the tumor growth in the mice not taking doxycycline significantly slowed down. Flow cytometry analysis of the tumors in the mice that stopped taking doxycycline revealed that the cells from the tumor had completely lost PU.1 expression. Moreover, when U266 cells conditionally expressing PU.1 were subcutaneously injected to another 10 mice and the same experiment was conducted, although the tumors in the mice taking doxycycline (N=5) kept growing, the tumors in the mice not taking doxycycline (N=5), did not grow any further. The present data suggest that PU.1 serves as a tumor a suppressor in the multiple myeloma cell lines as examined in vivo. Disclosures No relevant conflicts of interest to declare.

PU.1 is an Ets family transcription factor, which is essential for differentiation of both myeloid and lymphoid lineages. It was previously reported that conditional knockout of the upstream enhancer region (URE) located in 14 kb 5’ of the murine PU.1 gene resulted in down-regulation of PU.1 expression in granulocytes and B lymphocytes by 80% compared to that of wild type and induced acute myeloid leukemia and CLL-like diseases in mice. Therefore, down-regulation of PU.1 in myeloid and B cell lineages results in hematological malignancies. We previously reported that PU.1 is down-regulated in 5 out of 7 myeloma cell lines as well as primary myeloma cells from a subset of myeloma patients; that the promoter and the URE located in 17 kb 5’ of the human PU.1 gene that is homologous to that in 14 kb 5’ of murine PU.1 gene are highly methylated in these cell lines; and that conditionally expressed PU.1 with tet-off system induces cell growth arrest and apoptosis in myeloma cell lines, U266 and KMS12PE, suggesting that the down-regulation of PU.1 is necessary for myeloma cell growth. Here, to evaluate tumor suppressor activity of PU.1 in mature B and plasma cells in vivo, we generated Cγ1-Cre PU.1 knockout mice by crossing Cγ1-Cre and PU.1-loxP mice. We confirmed that PU.1 alleles were both conditionally deleted in the maturation stages of B cells from post germinal center B to plasma cells. By 18-24 months of age, about 77.7% (10 of 13) of the knockout mice had developed serum M proteins. To induce B cell differentiation to plasma cells, those mice were immunized with NP-CGG and 76.9% (20 of 26) of the mice developed serum M protein. ELISA of sera from those mice revealed that IgG was not elevated compared to those from the PU.1-loxP mice, which was thought because Cγ1-Cre locus fails to produce IgG1. Instead, a small number (5 of 20) of the mice showed relatively large amounts of IgM and/or IgA. When 11 such mice were sacrificed, 7 had developed splenomegaly and/or intestinal B cell lymphoma. Immunostaining revealed that B220+ cells had infiltrated into the tumors and various organs including the spleen, liver, and bone marrow. Those cells were monoclonal for κ chain and partly CD138 positive. When we transplanted those tumor cells into Rag2-/- Jak3-/- immunedeficient mice, all the mice died within 3 weeks. Thus, PU.1 apparently functions as a tumor suppressor in mature B cells and its deletion in late B cell maturation stages produces B cell lymphoma with M proteinemia. The remaining 4 mice developed high titer IgM and/or IgA levels and flow cytometry of bone marrow cells and splenocytes revealed that those cells were monoclonal for κ chain and positive for B220 and IgM and/or IgA, suggesting that those mice suffered from multiple myeloma or monoclonal gammopathy with undetermined sighnificance (MGUS). These data strongly suggest that conditional knockout of PU.1 in post germinal center B and plasma cells results in B cell lymphoma and plasma cell neoplasms related to multiple myeloma. Disclosures No relevant conflicts of interest to declare.