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...