Expression of myc-family genes in established human multiple myeloma cell lines: L-myc but not c-myc gene expression in the U-266 myeloma cell line

Targeting both BET and CBP/EP300 proteins with the novel dual inhibitors NEO2734 and NEO1132 leads to anti-tumor activity in multiple myeloma

OBJECTIVES

Two promising epigenetic therapeutic targets have emerged for the treatment of hematologic malignancies, BET and CBP/EP300 proteins. Several studies have shown that targeting these individual classes of proteins has anti-tumor activity in multiple myeloma (MM), as well as other cancers. Here, we present the first data exploring the anti-tumor activity of two novel dual inhibitors, NEO2734 and NEO1132, of both BET and CBP/EP300 proteins in MM.

METHODS

Sixteen MM cell lines (MMCLs) were treated with the dual inhibitors NEO2734 and NEO1132, the single BET inhibitors JQ1, OTX015, IBET-762, and IBET-151, and a single CBP/EP300 inhibitor CPI-637.

RESULTS

The dual inhibitor NEO2734 showed strong anti-tumor activity and was consistently highly active against all MMCLs, being as potent as JQ1 and more so than other single inhibitors. NEO2734 and NEO11132 induced a significant G1 cell cycle arrest and decreased c-MYC and IRF4 protein levels in MMCLs compared to the other single inhibitors. Sensitivity to the dual inhibitors was not dependent on a specific MM molecular subgroup but correlated with c-MYC protein expression levels.

CONCLUSIONS

The dual inhibition of BET and CBP/EP300 has potential therapeutic benefits for patients with MM.

Amino acid depletion triggered by ʟ-asparaginase sensitizes MM cells to carfilzomib by inducing mitochondria ROS-mediated cell death

Metabolic reprogramming is emerging as a cancer vulnerability that could be therapeutically exploitable using different approaches, including amino acid depletion for those tumors that rely on exogenous amino acids for their maintenance. ʟ-Asparaginase (ASNase) has contributed to a significant improvement in acute lymphoblastic leukemia outcomes; however, toxicity and resistance limit its clinical use in other tumors. Here, we report that, in multiple myeloma (MM) cells, the DNA methylation status is significantly associated with reduced expression of ASNase-related gene signatures, thus suggesting ASNase sensitivity for this tumor. Therefore, we tested the effects of ASNase purified from Erwinia chrysanthemi (Erw-ASNase), combined with the next-generation proteasome inhibitor (PI) carfilzomib. We observed an impressive synergistic effect on MM cells, whereas normal peripheral blood mononuclear cells were not affected. Importantly, this effect was associated with increased reactive oxygen species (ROS) generation, compounded mitochondrial damage, and Nrf2 upregulation, regardless of the c-Myc oncogenic-specific program. Furthermore, the cotreatment resulted in genomic instability and DNA repair mechanism impairment via increased mitochondrial oxidative stress, which further enhanced its antitumor activity. Interestingly, carfilzomib-resistant cells were found to be highly dependent on amino acid starvation, as reflected by their higher sensitivity to Erw-ASNase treatment compared with isogenic cells. Overall, by affecting several cellular programs, Erw-ASNase makes MM cells more vulnerable to carfilzomib, providing proof of concept for clinical use of this combination as a novel strategy to enhance PI sensitivity in MM patients.

Antimyeloma activity of bromodomain inhibitors on the human myeloma cell line U266 by downregulation of MYCL

Supplemental Digital Content is available in the text.

Bromodomain and extraterminal protein (BET) inhibitors suppress the expression of c-MYC. U266, a human myeloma cell line, expresses the MYCL gene, but not the c-MYC gene. Our aim was to analyse the antimyeloma activity of BET inhibitors on U266 cells. Two BET inhibitors, I-BET151 and JQ1, were tested. U266 cell proliferation decreased to 61.5 and 54.0% of the control after incubation with 500 nmol/l I-BET151 for 72 and 96 h and to 53.5 and 56.4% of control after incubation with 500 nmol/l JQ1 for 72 and 96 h by MTS tetrazolium, respectively. BET inhibitors induced cell cycle arrest at the G1 phase in U266 cells, but did not induce apoptosis by flow cytometry. According to Gene Set Enrichment Analysis, MYC-related genes were significantly downregulated in U266 cells treated with I-BET151 similar to KMS11 cells that expressed c-MYC. The MYCL1 was expressed in U266 cells, whereas c-MYC and MYCN were not by quantitative real-time reverse-transcription-PCR. Incubation with I-BET151 induced downregulation of MYCL1 in U266 cells. BET inhibitors decreased the cell proliferation in U266 cells with overexpression of MYCL less than those without overexpression of MYCL. BET inhibitors induce G1 arrest without apoptosis and interfere with the proliferation of U266 myeloma cells, which express MYCL, but not c-MYC. BET inhibitors might be active in cancers that express MYCL, but not c-MYC.

Glutaminase inhibition in multiple myeloma induces apoptosis via MYC degradation

Multiple Myeloma (MM) is an incurable hematological malignancy affecting millions of people worldwide. As in all tumor cells both glucose and more recently glutamine have been identified as important for MM cellular metabolism, however there is some dispute as to the role of glutamine in MM cell survival. Here we show that the small molecule inhibitor compound 968 effectively inhibits glutaminase and that this inhibition induces apoptosis in both human multiple myeloma cell lines (HMCLs) and primary patient material. The HMCL U266 which does not express MYC was insensitive to both glutamine removal and compound 968, but ectopic expression of MYC imparted sensitivity. Finally, we show that glutamine depletion is reflected by rapid loss of MYC protein which is independent of MYC transcription and post translational modifications. However, MYC loss is dependent on proteasomal activity, and this loss was paralleled by an equally rapid induction of apoptosis. These findings are in contrast to those of glucose depletion which largely affected rates of proliferation in HMCLs, but had no effects on either MYC expression or viability. Therefore, inhibition of glutaminolysis is effective at inducing apoptosis and thus serves as a possible therapeutic target in MM.

Addiction to c-MYC in multiple myeloma

In multiple myeloma, c-MYC is activated and contributes to the malignant phenotype. Targeting MYC by short hairpin RNA induced cell death in myeloma cell lines; however, cell lines are generated from samples taken in advanced stages of the disease and may not reflect patient cells adequately. In this study, we used the selective small molecule inhibitor of MYC-MAX heterodimerization, 10058-F4, on myeloma cell lines as well as primary myeloma cells, and we show that inhibition of c-MYC activity efficiently induces myeloma cell death. Moreover, in cocultures of cell lines with bone marrow stromal cells from myeloma patients, the inhibitor still induces apoptosis. Our results provide further evidence that myeloma cells are addicted to c-MYC activity and that c-MYC is a promising therapeutic target in multiple myeloma.

Control of apoptosis in human multiple myeloma by insulin-like growth factor I (IGF-I)

Human multiple myeloma (MM) is characterized by the expansion of neoplastic plasmablasts/plasma cells with complex genetic aberrations and high dependence for survival and growth on cytokines produced in the bone marrow microenvironment. As tools in the study of MM about 80 authentic MM cell lines and a few relevant in vivo mouse models are available. The dependence on insulin-like growth factor receptor (IGF-IR) signaling in the development and maintenance of the malignant phenotype in a variety of cancers is a rationale for attempts to improve tumor treatment by selectively inhibiting the IGF-IR in malignant cells by neutralizing antibodies, dominant negative IGF-IR, and IGF-IR siRNA. Testing the hypothesis that abrogating IGF-IR-mediated signaling of survival should make MM cells more susceptible to apoptosis, our studies have so far provided proof-of-principle by the demonstration that inhibition of a signaling pathway stimulating survival renders cells susceptible to drug-induced apoptosis when the drug (dexamethasone) and inhibitor (rapamycin) converge on the same target, that is p70(S6K). The recent publication of the three-dimensional structure of the IGF-IR kinase domain has facilitated the development of IGF-IR inhibitors of the cyclolignan family, that is picropodophyllin, with capacity to distinguish also in vivo between the IGF-IR and the insulin receptor. Studies in vitro and in vivo with picropodophyllin show promising effects, that is apoptosis induction and growth arrest, and have made it possible to evaluate the biological and therapeutic effects of inhibition of the IGF-IR signaling in MM.

Multiple myeloma regression mediated by bruceantin

PURPOSE

Bruceantin has been shown to induce cell differentiation in a number of leukemia and lymphoma cell lines. It also down-regulated c-MYC, suggesting a correlation of down-regulation with induction of cell differentiation or cell death. In the present study, we focused on multiple myeloma, using the RPMI 8226 cell line as a model.

EXPERIMENTAL DESIGN

The effects of bruceantin on c-MYC levels and apoptosis were examined by immunoblotting, 4',6-diamidino-2-phenylindole staining, evaluation of caspase-like activity, and 3,3'-dihexyloxacarbocyanine iodide staining. The potential of bruceantin to inhibit primary tumor growth was assessed with RPMI 8226 xenografts in SCID mice, and apoptosis in the tumors was evaluated by the terminal deoxynucleotidyl transferase-mediated nick end labeling assay.

RESULTS

c-MYC was strongly down-regulated in cultured RPMI 8226 cells by treatment with bruceantin for 24 h. With U266 and H929 cells, bruceantin did not regulate c-MYC in this manner. Apoptosis was induced in the three cell lines. In RPMI 8226 cells, apoptosis occurred through proteolytic processing of procaspases and degradation of poly(ADP-ribose) polymerase. The mitochondrial pathway was also involved. Because RPMI 8226 cells were the most sensitive, they were used in a xenograft model. Bruceantin treatment (2.5-5 mg/kg) resulted in a significant regression of tumors without overt toxicity. Apoptosis was significantly elevated in tumors derived from animals treated with bruceantin (37%) as compared with the control tumors (14%).

CONCLUSIONS

Bruceantin interferes with the growth of RPMI 8226 cells in cell culture and xenograft models. These results suggest that bruceantin should be reinvestigated for clinical efficacy against multiple myeloma and other hematological malignancies.

Diverse karyotypic abnormalities of the c-myc locus associated with c-myc dysregulation and tumor progression in multiple myeloma

Translocations involving c-myc and an Ig locus have been reported rarely in human multiple myeloma (MM). Using specific fluorescence in situ hybridization probes, we show complex karyotypic abnormalities of the c-myc or L-myc locus in 19 of 20 MM cell lines and approximately 50% of advanced primary MM tumors. These abnormalities include unusual and complex translocations and insertions that often juxtapose myc with an IgH or IgL locus. For two advanced primary MM tumors, some tumor cells contain a karyotypic abnormality of the c-myc locus, whereas other tumor cells do not, indicating that this karyotypic abnormality of c-myc occurs as a late event. All informative MM cell lines show monoallelic expression of c-myc. For Burkitt's lymphoma and mouse plasmacytoma tumors, balanced translocation that juxtaposes c-myc with one of the Ig loci is an early, invariant event that is mediated by B cell-specific DNA modification mechanisms. By contrast, for MM, dysregulation of c-myc apparently is caused principally by complex genomic rearrangements that occur during late stages of MM progression and do not involve B cell-specific DNA modification mechanisms.

Increasing multiple myeloma mortality among the elderly: a manifestation of aging and differential survival

Increasing multiple myeloma incidence and mortality among the elderly in industrialized nations has been attributed to associated environmental carcinogens. Age-specific multiple myeloma mortality rates in the United States from 1968 to 1989 were analyzed using the Strehler-Mildvan modification of the Gompertz relationship between aging and mortality. The results suggest that worsening environmental influences are not responsible for increasing multiple myeloma mortality among the elderly. Differential survival, a concept originally popularized by Charles Darwin, and its effect upon the surviving gene pool in an aging population is an alternative explanation for increasing multiple myeloma incidence and mortality in the elderly.

L-myc and N-myc in hematopoietic malignancies

The myc proto-oncogenes encode nuclear DNA-binding phosphoproteins which regulate cell proliferation and differentiation. The c-myc gene is implicated in hematopoietic malignancies on the basis of its frequent deregulation in naturally occurring leukemias and lymphomas. Recent evidence suggests that also the N-myc and L-myc genes may have a role in normal and malignant hematopoiesis. N-myc and to a certain degree L-myc can substitute for c-myc in transformation assays in vitro, and their overexpression can block the differentiation of leukemia cell lines. Immunoglobulin heavy chain enhancer (IgH) -driven overexpression of N-myc or L-myc genes cause lymphatic and myeloid tumors, respectively, in transgenic mice. Furthermore, the L-myc and N-myc genes are expressed in several human leukemias and leukemia cell lines, L-myc predominantly in myeloid and N-myc both in myeloid and in some lymphoid leukemias. All N/L-myc positive leukemias and leukemia cell lines coexpress the c-myc gene, thus exemplifying a lack of negative cross-regulation between the different myc genes in leukemia cells. Taken together, these data suggest that L-myc and N-myc may participate in the growth regulation of hematopoietic cells.