Multiple myeloma is a hematological cancer that is considered incurable despite advances in treatment strategy during the last decade. Therapies targeting single pathways are unlikely to succeed due to the heterogeneous nature of the malignancy. Proliferating cell nuclear antigen (PCNA) is a multifunctional protein essential for DNA replication and repair that is often overexpressed in cancer cells. Many proteins involved in the cellular stress response interact with PCNA through the five amino acid sequence AlkB homologue 2 PCNA-interacting motif (APIM). Thus inhibiting PCNA’s protein interactions may be a good strategy to target multiple pathways simultaneously. We initially found that overexpression of peptides containing the APIM sequence increases the sensitivity of cancer cells to contemporary therapeutics. Here we have designed a cell-penetrating APIM-containing peptide, ATX-101, that targets PCNA and show that it has anti-myeloma activity. We found that ATX-101 induced apoptosis in multiple myeloma cell lines and primary cancer cells, while bone marrow stromal cells and primary healthy lymphocytes were much less sensitive. ATX-101-induced apoptosis was caspase-dependent and cell cycle phase-independent. ATX-101 also increased multiple myeloma cells’ sensitivity against melphalan, a DNA damaging agent commonly used for treatment of multiple myeloma. In a xenograft mouse model, ATX-101 was well tolerated and increased the anti-tumor activity of melphalan. Therefore, targeting PCNA by ATX-101 may be a novel strategy in multiple myeloma treatment.

IntroductionMembers of the MYC family are important oncogenes involved in the development of malignant cells. 1 This may also be the case in multiple myeloma (MM), a malignancy of antibodyproducing plasma cells in bone marrow. The activity of c-MYC in MM increases with disease stage. 2,3 The mechanism by which c-MYC is activated in each case is unclear; however, multiple signaling pathways converge on c-MYC. Translocations involving MYC and immunoglobulin genes (IG) are relatively rare in MM and considered late progression events. 4 c-MYC regulates transcription of up to 15% of the genes in human cells by binding to its obligate partner MAX. Many cancer cells may develop a dependency on c-MYC activity; and by preventing this activity, the cells may stop dividing or even undergo apoptosis. In agreement with this, short-hairpin RNA targeting MYC was shown to be lethal to a number of human myeloma cell lines. 5 A small-molecule inhibitor, termed 10058-F4, has been identified that is proposed to specifically inhibit c-MYC-MAX heterodimerization, thereby preventing transactivation of c-MYC target genes. 6,7 The inhibitor has been shown to have growth inhibitory effects on lymphoma and acute myelogenous leukemia cells. 8,9 Methods CellsMyeloma cell lines used in this study were U266, INA-6, JJN-3, KMS-12-BM, IH-1, and KJON. Details on cell culture conditions are found in supplemental Methods (available on the Blood Web site; see the Supplemental Materials link at the top of the online article). CD138 ϩ patient cells obtained through the Norwegian Myeloma Biobank were purified using RoboSep automated cell separator and Human CD138 Positive Selection Kit (Stem Cell Technologies). Bone marrow stromal cells (BMSCs) from patients were obtained by plastic adherence and cultivated as stated in supplemental Methods. The project was approved by the Regional Ethics Committee, and patients gave informed consent in accordance with the Declaration of Helsinki. Patient characteristics are described in supplemental Table 1.Cell viability measurements and quantitative RT-PCR were performed as described previously. 10 PCR TaqMan assays used were as follows: MYC, Hs00153408_m1; MYCL1, Hs00420495_m1; and GAPDH, Hs99999905_m1 (Applied Biosystems).Description of other reagents, immunoblotting, and knockdown experiments is found in supplemental Methods. Results and discussionTo address the c-MYC dependency of myeloma cells, we decided to evaluate the effect of 10058-F4 in myeloma cell lines and primary cells by in vitro studies. First, both mRNA and protein expression of c-MYC and L-MYC was determined in 6 cell lines by quantitative RT-PCR and immunoblotting ( Figure 1A). Five of the cell lines expressed c-MYC, whereas U266 only had L-MYC, as previously reported. 11 L-MYC mRNA was also detected in KMS-12-BM and to a lesser extent in INA-6 and JJN-3, although the levels of L-MYC were negligible compared with c-MYC. The effect on cell viability was evaluated in myeloma cell lines treated with increasing concentrations of 10058-F4 for 48 hour...

BackgroundActivins are members of the TGF-β family of ligands that have multiple biological functions in embryonic stem cells as well as in differentiated tissue. Serum levels of activin A were found to be elevated in pathological conditions such as cachexia, osteoporosis and cancer. Signaling by activin A through canonical ALK4-ACVR2 receptor complexes activates the transcription factors SMAD2 and SMAD3. Activin A has a strong affinity to type 2 receptors, a feature that they share with some of the bone morphogenetic proteins (BMPs). Activin A is also elevated in myeloma patients with advanced disease and is involved in myeloma bone disease.ResultsIn this study we investigated effects of activin A binding to receptors that are shared with BMPs using myeloma cell lines with well-characterized BMP-receptor expression and responses. Activin A antagonized BMP-6 and BMP-9, but not BMP-2 and BMP-4. Activin A was able to counteract BMPs that signal through the type 2 receptors ACVR2A and ACVR2B in combination with ALK2, but not BMPs that signal through BMPR2 in combination with ALK3 and ALK6.ConclusionsWe propose that one important way that activin A regulates cell behavior is by antagonizing BMP-ACVR2A/ACVR2B/ALK2 signaling.Electronic supplementary materialThe online version of this article (doi:10.1186/s12964-015-0104-z) contains supplementary material, which is available to authorized users.

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The rational design of inhibitors of the bHLH-ZIP oncoprotein c-Myc is hampered by a lack of structure in its monomeric state. We describe herein the design of novel, low-molecular-weight, synthetic α-helix mimetics that recognize helical c-Myc in its transcriptionally active coiled-coil structure in association with its obligate bHLH-ZIP partner Max. These compounds perturb the heterodimer’s binding to its canonical E-box DNA sequence without causing protein–protein dissociation, heralding a new mechanistic class of “direct” c-Myc inhibitors. This model was corroborated by additional biophysical methods including NMR spectroscopy and surface plasmon resonance. Several compounds demonstrated a 2-fold or greater selectivity for c-Myc–Max heterodimers over Max–Max homodimers with IC50 values as low as 5.6 µM. Finally, these compounds inhibited the proliferation of c-Myc-over-expressing cell lines in a concentration-dependent manner that correlated with the loss of expression of a c-Myc-dependent reporter plasmid despite the fact that c-Myc–Max heterodimers remained intact.

Multiple myeloma is a malignancy of plasma cells predominantly located in the bone marrow. A number of bone morphogenetic proteins (BMPs) induce apoptosis in myeloma cells in vitro, and with this study we add BMP-9 to the list. BMP-9 has been found in human serum at concentrations that inhibit cancer cell growth in vitro. We here show that the level of BMP-9 in serum was elevated in myeloma patients (median 176 pg/ml, range 8–809) compared with healthy controls (median 110 pg/ml, range 8–359). BMP-9 was also present in the bone marrow and was able to induce apoptosis in 4 out of 11 primary myeloma cell samples by signaling through ALK2. BMP-9-induced apoptosis in myeloma cells was associated with c-MYC downregulation. The effects of BMP-9 were counteracted by membrane-bound (CD105) or soluble endoglin present in the bone marrow microenvironment, suggesting a mechanism for how myeloma cells can evade the tumor suppressing activity of BMP-9 in multiple myeloma.

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Multiple myeloma (MM) is a paradigm for a malignant disease that exploits external stimuli of the microenvironment for growth and survival. A thorough understanding of the complex interactions between malignant plasma cells and their surrounding requires a detailed analysis of the transcriptional response of myeloma cells to environmental signals. We determined the changes in gene expression induced by interleukin (IL)-6, tumor necrosis factor-a, IL-21 or co-culture with bone marrow stromal cells in myeloma cell lines. Among a limited set of genes that were consistently activated in response to growth factors, a prominent transcriptional target of cytokineinduced signaling in myeloma cells was the gene encoding the serine/threonine kinase serum/glucocorticoid-regulated kinase 1 (SGK1), which is a down-stream effector of PI3-kinase. We could demonstrate a rapid, strong and sustained induction of SGK1 in the cell lines INA-6, ANBL-6, IH-1, OH-2 and MM.1S as well as in primary myeloma cells. Pharmacologic inhibition of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway abolished STAT3 phosphorylation and SGK1 induction. In addition, small hairpin RNA (shRNA)-mediated knock-down of STAT3 reduced basal and induced SGK1 levels. Furthermore, downregulation of SGK1 by shRNAs resulted in decreased proliferation of myeloma cell lines and reduced cell numbers. On the molecular level, this was reflected by the induction of cell cycle inhibitory genes, for example, CDKNA1/p21, whereas positively acting factors such as CDK6 and RBL2/p130 were downregulated. Our results indicate that SGK1 is a highly cytokine-responsive gene in myeloma cells promoting their malignant growth.