Targeting the integrated networks of aggresome formation, proteasome, and autophagy potentiates ER stress-mediated cell death in multiple myeloma cells
Abstract Introduction. The viral oncolytic agent, Reolysin (RV), is a promising novel therapeutic that selectively proliferates in myeloma cells. Our group conducted a phase 1 clinical trial of single agent RV in patients with relapsed and refractory multiple myeloma (MM), and reported that treatment was well tolerated and associated with prolonged disease stability in 25% of patients. Objective responses were not evident, likely because the viral RNA present in the myeloma cells was not producing infectious viral particles. Proteasome inhibitors can lead to myeloma cell death due to increased endoplasmic reticulum (ER) stress and induction of ER-stress related apoptosis (Kelly, Oncogene, 2012). We confirmed this effect preclinically with Carfilzomib (CFZ), and hypothesized that the addition of CFZ to RV would increase viral proliferation and MM cell death sufficiently to obtain objective response in patients with relapsed MM. Methods. For this pilot trial, patients were required to have relapsed myeloma with IMWG-defined measurable disease, ANC ≥ 1,000/uL, platelet count ≥ 50,000/uL, with no creatinine requirements. Cohorts of 6 patients each were planned. Cohort 1 included patients who were CFZ na•ve or had not progressed on a CFZ containing regimen. Intravenous CFZ (20 mg/m2 days 1 and 2 of cycle 1 and 27 mg/m2 thereafter), Reolysin (3 x 1010 TCID50/day), and dexamethasone (20 mg) were administered on days 1, 2, 8, 9, 15, and 16 of a 28-day cycle (Table 1). In situ based methodologies were used to examine the distribution of CD138, CD8, NK cells (CD117 and IL-22), CD 68, PD L1, reoviral capsid protein, and reoviral RNA in bone marrow biopsies performed prior to treatment on days 1 and 9 of cycle 1. Results. Seven patients have been enrolled, four are male, and all are Caucasian. Patients have a median age of 64, and have received on average 2.4 prior lines of therapy and 4.4 prior treatments. All patients were previously exposed to Revlimid and Velcade, and 4 patients were Velcade refractory. One patient was previously treated with CFZ but was deemed to be CFZ sensitive, one patient has dialysis-dependent CKD, and all but one patient had evidence of high-risk cytogenetics on CD138-selected FISH at the time of enrollment. 6/7 patients suffered myalgias and fever after the first two doses of Reolysin, but these symptoms did not recur in any subsequent doses. Treatment has been well tolerated in 5 patients, but 2 patients were removed from study after 2 doses of combination therapy, one for congestive heart failure, and the other for gastrointestinal bleed in the setting of grade 4 thrombocytopenia and an arteriovenous malformation. Due to these 2 DLTs, patient 7 was enrolled at dose level -1 (Carfilzomib 20 mg/m2 and Reolysin 3 x 109 TCID50/day on days 1, 2, 8, 9, 15, and 16 of a 28 day cycle). Within the first 14 days following the initiation of treatment, the mean decrease in platelets for the 7 evaluable patients was 79 (50 - 139), and this included grade 4 (N = 1), and asymptomatic grade 2 (N = 3), and grade 1 (N = 3) events. All patients have had a reduction of the monoclonal protein, 5 patients remain on study, and the longest duration of response is currently 8 cycles. Responses are VGPR (N = 2), PR (N = 3), MR (N = 1), and SD (N = 1) (Figure 1). Intracellular viral replication will be reported at the meeting. Conclusion. This 3-drug regimen is relatively well tolerated in heavily treated patients with relapsed MM. Most patients experience low grade fever and myalgias after the first two doses, and patients have evidence of thrombocytopenia in cycle 1. Combination treatment is associated with reduction of the monoclonal protein in all patients, and 86% (6/7) CFZ-sensitive patients have evidence of objective response. Table 1. Combination treatment dose levels Dose level Dexamethasone (IVP) Carfilzomib (IVPB) Reolysin (IVPB) -1 20 mg/day 20 mg/m2 /day 3 x 109 TCID50/day 1 (starting dose) 20 mg/day C1 Day 1 & 2 - 20 mg/m2 /dayC1 Day 8 & onward - 27 mg/m2 /day 3 x 1010 TCID50/day Figure 1. Waterfall plot representing response of 7 patients with relapsed MM Figure 1. Waterfall plot representing response of 7 patients with relapsed MM Disclosures Off Label Use: Reolysin - oncolytic viral, anti-cancer agent.
Abstract Introduction Although the therapeutic armamentarium against multiple myeloma has tremendously increased in recent years, it still remains an incurable disease. A highly promising novel anti-tumoral treatment strategy is to target specific non-redundant metabolic achilles heels of individual cancer entities. The semi-essential amino acid arginine can be synthesized from citrulline in most physiological tissues due to expression of the rate-limiting enzyme argininosuccinate synthetase 1 (ASS1). Various tumor entities do not express ASS1, therefore depend on the exogenous availability of arginine and pharmacological approaches to systemically deplete arginine are in phase I-III clinical development for such arginine-auxotrophic cancers. Cell death induction by arginine depletion can be dramatically enhanced by co-application of the arginine analogue canavanine. Canavanine can be used by the respective aminoacyl tRNA synthetase instead of arginine during protein translation and this leads to a highly toxic intracellular accumulation of misfolded proteins. In preliminary work we have seen that myeloma cells are largely arginine-auxotrophic and can be killed by arginine depletion and canavanine supplementation within hours, while ASS1 expressing cells are completely protected by their endogenous arginine rescue capability. Encouraging results of tumor control have already been seen in a murine myeloma model. Methods Human myeloma cell lines (NCI-H929_A2 and FD50, developed in our laboratory) were cultured and treated in RPMI-1640 medium with or without arginine. Protein levels were determinded by western blot analysis. Cell viability was measured by propidium iodide staining and flow cytometry analysis. RNA quantification was done by qRT-PCR. For autophagosome and aggresome quantification we used immunofluorescence staining (IF) and laser scanning microscopy (LSM). Results Arginine depletion and canavanine supplementation led to misfolded protein accumulation which was followed by massive apoptotic cell death. Both processes were further enhanced by co-treatment with the proteasome inhibitor bortezomib, indicated by an increase in intracellular polyubiquitinated proteins as well as higher cleaved caspase 3 levels and propidium-iodide positive cells after only 8-12 h in both tested cell lines. Unexpectedly, the endoplasmic reticulum (ER)-stress response was activated only very moderately. Expression of CHOP, a pro-apoptotic transcription factor that is highly translated under toxic ER stress, was not altered compared to control conditions. Tunicamycin-mediated induction of enhanced ER stress significantly improved the viability of arginine-starved and canavanine treated cells. This suggests that protein accumulation mainly takes place in the cytoplasm rather than the ER and tunicamycin might alleviate cell death by reduction of total protein translation. Despite severe arginine deficiency and induction of misfolded protein stress, the cells were not able to respond by an adequate upregulation of macroautophagy, as determined by an altered LC3 metabolism. The autophagic flux was significantly reduced compared to control conditions after 4-8 h of treatment. There was a strong induction of BAG3 and p62 proteins, which are both associated with chaperone-assisted autophagy as well as aggresome formation and are normally cleared via macroautophagy. Cytoplasmic aggresome formation was not detectable until onset of apoptosis. Also, no relevant modulation of phosphorylation of the autophagy inducer mTORC and the downstream kinase p70S6K1 was noted upon arginine depletion and canavanine co-treatment. Finally, ER stress induction via tunicamycin did not improve autophagic protein turnover, as determined by IF staining, LSM and western blot. Conclusions Arginine starvation in combination with canavanine supplementation induces fast and highly efficient cell death in arginine-auxotrophic myeloma cells. This novel strategy interferes with myeloma cellular metabolism by induction of misfolded protein accumulation. A relevant upregulation of potentially protective cellular strategies like ER stress responses, aggresome formation and autophagy are either not detectable or they remain insufficient. We hypothesize that our novel metabolic anti-tumor strategy is either too potent or too fast for the tumor cells to cope with its consequences. Disclosures No relevant conflicts of interest to declare.
Abstract Background Despite of recent advances in therapeutic strategy for multiple myeloma (MM), MM still remains incurable and novel therapeutic approach is urgently needed. We have previously demonstrated that a natural small molecule, shikonin (SHK), induced both apoptosis and necroptosis (programmed necrosis) in MM cells. In this study, we attempted to elucidate biological mechanisms of SHK in inducing apoptosis and necroptosis. Methods Six MM cell lines, KMS-12-PE, RPMI 8226, U266, KMM1, KMS-11 and a bortezomib-resistant MM cell line, KMS-11/BTZ (obtained from Kyowa Hakko Kirin Co. Ltd.), were utilized. Inhibitors of pan-caspase and necroptosis, ZVAD-fmk and Nec-1 (necrostatin-1), were employed to distinguish apoptosis and necroptosis, respectively. Cell death was analyzed using the trypan blue dye exclusion method (WST-8 assay) and flow cytometry analysis using AnnexinV/PI staining. Morphological examinations of cells were performed with May Giemza staining. Caspases, RIP1, ubiquitinated proteins, and heat shock proteins were analyzed with western blot. Knockdown of RIP1, an essential molecule for necroptosis, was performed using siRNA. ER stress was assessed by detecting activated XBP-1, which was analyzed by digestion of PCR products with ApaLI. Because the ApaLI site in XBP-1 mRNA is spliced out upon activation, the activated XBP-1 shows one large band after ApaLI digestion, while inactivated XBP-1 shows two ApaI-digested bands. Thapsigargin, sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) inhibitor, was used as an ER stress inducer. Results By screening natural compounds libraries (provided by Institute of Natural Medicine, Toyama University, Japan), we found that SHK, a natural compound derived from the root of Lithospermum erythrorhizon, induced cell death in MM cells. Apoptosis was induced at a relatively low concentration (2.5∼5 µM) and was inhibited by a caspase-inhibitor, while necroptosis was promptly induced at higher concentrations (10∼ 20 µM) within 5 hours and was completely inhibited by Nec-1. Morphological analysis showed that SHK at low concentrations induced typical apoptotic changes, such as fragmented nucleus, while SHK at higher concentrations induced necrotic morphology, such as translucent cytoplasm and swelling of cell membranes. By contrast, SHK did not induce apoptosis or necrptosis in peripheral blood mononuclear cells from healthy donors at low concentrations. SHK activated caspase-8 and -3 at low concentrations but did not at higher concentrations. RIP1, an essential molecule for necroptosis, was cleaved after treatment with SHK at low concentrations, which leads to the inhibition of necroptosis, while it was not cleaved and remained active at higher concentrations, suggesting that SHK dynamically regulates the cleavage of RIP-1. At low concentrations, shikonin induced an accumulation of ubiquitinated proteins and activated XBP-1, suggesting SHK may have a property of proteasome inhibitor eventually inducing endoplasmic reticulum stress. Finally, SHK at low concentration killed bortezomib resistant cells with lower IC50 comparing to that of the parental cells (0.91 vs 1.56 µM, respectively). Conclusions We here report, for the first time, that SHK induces apoptosis and necroptosis in MM cells at low and high concentrations, respectively, by regulating proteasome function and RIP-1 cleavage. Given the fact that SHK efficiently induces cell death in bortezomib-resistant cell line, SHK may act as a novel proteasome inhibitor for bortezomib-resistant myeloma cells. Moreover, SHK at higher concentrations, which induces nectoptosis, should be an attractive future therapeutic option potentially to eradicate MM cells. Disclosures: No relevant conflicts of interest to declare.
Abstract We have evaluated the efficacy of a novel hydroxamic acid-derived histone deacetylase (HDAC) inhibitor, ITF2357, to promote cell death in multiple myeloma (MM) cells. HDAC inhibitors, which promote histone hyperacetylation and increase gene expression, have been evaluated as candidate agents for combating malignancies because they impact the expression of genes related to proliferation, differentiation, and survival. Exposure of MM cell lines to 1 micromolar ITF2357 led to dramatically increased levels of histone acetylation at 4 hours and 8 hours by Western analysis. Sub-micromolar concentrations of ITF2357 promoted time- and concentration-dependent cell death in MM cell lines. Using 500 nM ITF2357, a concentration potentially achievable in vivo, viability of KAS-6/1 IL-6 dependent myeloma cells was reduced to 28% of control at 24 hrs and 2% of control at 48 hours (Figure 1). In contrast, viability of normal PBMCs was 100% at 24 hours and 80% at 48 hours (Figure 2). U266 and 8226 myeloma cells were found to be sensitive to ITF-2357 in a similar fashion with U266 being least sensitive. Cell death proceeded via apoptosis as measured using Annexin V/propidium iodide staining. ITF 2357 was superior to suberoylanilide hydroxamic acid (SAHA) at inhibition of stromal cell IL-6 production. IL-1beta (10 pg/ml) was used to stimulate bone marrow stromal cell IL-6 production (105 ng/ml) after 48 hours. Concentration of ITF2357:Stromal Cell IL-6 production after 48 hours were as follows - 10 nM: 78 ng/ml; 100 nM: 79 ng/ml; 1000 nM; 32 ng/ml. SAHA at similar concentrations showed no significant decrease in stromal cell IL-6 production compared with the no drug control. In summary, ITF2357 induces significant myeloma cell apoptosis and can inhibit stromal cell IL-6 production. It represents an attractive therapeutic candidate for MM clinical trials. Figure Figure Figure Figure
Abstract Despite major advances in chemotherapy, multiple myeloma remains incurable and in need of new therapies that target novel pathways. Insufficient understanding of the molecular pathways that regulate survival in myeloma is a major impediment towards designing better therapies to prolong survival in patients or even cure the disease. This necessitates the identification of new protein targets that are crucial for the growth and survival of multiple myeloma. Just like normal plasma cells, MM cells also depend on their interactions with bone marrow stromal cells (BMSC) for survival and production of essential growth factors. We have previously shown that MM cells interact with dendritic cells (DC) in the microenvironment and in vitro can stimulate DC to produce IL-6 (ASH2010#132, ASH2011 #147, ASH2012#722). Our recent publications show that when MM cells are not in direct contact with DC, the IL-6 produced by DC can protect MM cells against dexamethasone induced cell death, while neutralizing the IL-6 with antibodies can reverse that effect (Nair et al., 2011). Unfortunately, exactly how this survival response is mediated in MM is not very clear. PIM2, a serine threonine kinase, part of the proto-oncogene group of PIM kinases has been implicated in survival in several types of cancers including prostate cancer and multiple myeloma. In our lab, microarray gene expression analysis of publicly available datasets (Figure 1) show a trend towards increased expression of PIM2 in plasma cells from myeloma patients (left panel), and significantly in the poor prognosis subgroup MAF (Zhan et al., 2006) (right panel). For the first time we show that IL-6 produced by DC may be protecting myeloma cells by up regulating PIM2 and inactivating a major protein translation inhibitor 4EBP1, which also happens to be a PIM2 target. We show that silencing PIM2 with siRNA down regulates PIM2 activity and reverses the inactivation of 4EBP1, while the latter is known to cause cell death in myeloma. We also demonstrate that neutralizing IL-6 in MM cells that either don’t produce IL-6 on their own (MM.1S) or those that do (U266), abrogates extraneous DC-IL6 ability to induce PIM2 and its downstream target 4EBP1. Recombinant IL-6 also provided similar induction of PIM2 in myeloma and increased 4EBP1 phosphorylation, which was again reversed by neutralizing the antibody against IL-6. In myeloma patients, the use of dexamethasone in frontline therapies is often complicated by the ability of the bone marrow environment to produce IL-6 that not only induce increased proliferation of MM but also help resist dexamethasone mediated cell death in myeloma. Interestingly, when we used a novel PIM2 inhibitor, JP_11646 (kindly provided by Jasco Pharmaceuticals, LLC), it not only arrested IL-6 induced proliferation even at sub-lethal doses, but also prevented IL-6 mediated rescue of myeloma cells (Figure 2). This suggests that PIM2 might be a major player in IL-6 mediated drug resistance in myeloma and targeting it may help to subvert IL-6 mediated survival in myeloma. Through RT-PCR and westerns, we also show that IL-6 modulates PIM2 expression and activity resulting in increased 4EBP1 phosphorylation (Figure 3). This was abrogated when PIM2 activity was inhibited by JP_11646 (Figure 3). We also present data that suggests IL-6 via PIM2 may be regulating other anti-apoptotic molecules downstream of IL-6 receptors including MCL-1, that is vital to MM survival. Developing PIM2 targeted therapies provides an exciting opportunity to affect the myeloma tumor microenvironment where MM induced IL-6 production from BM could be inducing drug resistance. Figure 1: Microarray expression ofPIM2 in myeloma and MAF Figure 1:. Microarray expression ofPIM2 in myeloma and MAF Figure 2: PIM2 inhibition abrogates IL-6 induced MM proliferation (A) and protection (B). Figure 2:. PIM2 inhibition abrogates IL-6 induced MM proliferation (A) and protection (B). Figure 3: Inhibiting PIM2 activity prevents PIM2 induced phosphorylation of 4EBP1 by IL-6 in myeloma Figure 3:. Inhibiting PIM2 activity prevents PIM2 induced phosphorylation of 4EBP1 by IL-6 in myeloma Disclosures Caserta: Jasco Pharmaceuticals LLC: Equity Ownership. Baldino:Jasco Pharmaceuticals LLC: Equity Ownership.
Abstract Introduction Starvation of tumor cells from the amino acid arginine has recently gained particular interest because of the downregulation of the rate-limiting enzyme argininosuccinate synthethase 1 (ASS1) in various cancer entities. ASS1-deficient cells cannot resynthesize arginine from citrulline and are therefore considered arginine auxotrophic. The arginine depleting enzyme arginine deiminase (ADI-PEG20, Polaris Pharmaceuticals) is currently tested in phase I-III clinical trials for different arginine auxotrophic cancers. The natural arginine analogue canavanine can compete with arginine for arginyl-tRNA-binding sites and consequently be incorporated into nascent proteins instead of arginine. Canavanine could therefore potentially further disturb intracellular protein homeostasis, especially under arginine deprivation. The sensitivity of myeloma cells towards arginine depletion strategies has not been analyzed so far. Methods Human myeloma cell lines and CD138-sorted primary human myeloma cells from patient bone marrow were screened for ASS1 expression by western blotting (WB). The cells were cultured in arginine free medium and assessed for proliferation and metabolic activity (CFSE/MTT assays), apoptosis (caspase-3 cleavage) and cell death (annexinV/propidium iodide). Canavanine was supplied in both arginine-sufficient and -deficient conditions. The level of intracellular protein stress was determined by WB and/or flow cytometry analysis for ubiquitinated proteins, phosphorylated eukaryotic initiation factor 2α (peIF2α) and the spliced isoform of the X-Box binding protein 1 (Xbp1s). Repetitive ADI-PEG20 ± canavanine application i.p. were tested in vivo in an U266 myeloma xenograft model in NOD/SCID/IL2Rcg-/- (NSG) mice. Arginine and canavanine levels in plasma were determined by HPLC. Tumor growth was measured, mice were assessed for survival, weight and side effects. Tumor tissues were analyzed for caspase-3 cleavage and Ki67 expression by immunohistochemistry. Results 5 of 6 myeloma cell lines were negative for ASS1. Also, ASS1 was either not or only weakly expressed in the majority of primary CD138+ myeloma patient samples. Arginine starvation induced an arrest of cell proliferation and/or metabolic activity of primary myeloma cells and myeloma cell lines after 18-24 h. Addition of citrulline could only rescue ASS1 positive myeloma cells due to the intracellular resynthesis of arginine. Arginine starvation alone led to delayed induction of apoptosis (e.g. 35% cell death of NCI-H929 cells after 72 h of treatment). Addition of 100 mM canavanine strongly increased cell death specifically in the context of arginine deficiency (e.g. cell death in NCI-H929 cells: 87% after 24 h, 100 % after 48h) while it was non-toxic and had no effect on cell viability under physiological arginine conditions. Co-application of canavanine induced ubiquitination of cellular proteins and led to the prolongation of a fatal unfolded protein response (UPR) as measured by markedly elevated Xbp1s levels. Prolonged UPR ultimately led to the induction of apoptosis as reflected by annexin V binding and caspase-3 cleavage. In an U266 myeloma NSG xenograft model, systemic arginine depletion by ADI-PEG20 suppressed tumor growth in vivo and significantly prolonged median survival of mice when compared with the control group (22±3 vs. 15±3 days). Canavanine treatment alone had no influence on viability (13±0 days). However, the combination of ADI-PEG20 and canavanine demonstrated the longest median survival (27±7 days). Histological examination of explanted tumors showed the highest rates of caspase-3 cleavage in the ADI-PEG20/canavanine group. Conclusion Myeloma cells are mostly arginine auxotrophic and can be selectively targeted by arginine starvation. Combination of arginine depletion with the arginine analogue canavanine leads to a highly efficient and specific tumor cell eradication and should be further optimized in multiple myeloma preclinical models. Disclosures Bomalaski: Polaris Pharmaceuticals Inc.: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees.
Abstract Background: Multiple myeloma (MM) cells evade apoptosis through multiple mechanisms thus enabling it to evade therapy. The Bcl2 family of anti-apoptotic proteins is aberrantly expressed in MM cell lines and patient cells. Yet, pharmacological intervention of this family appears to have significant activity only in molecular subgroups of MM patients. This clearly suggests alternate mechanisms of overcoming apoptotic signals in MM cells in addition to the Bcl2 family, through proteins such as IAPs. We have previously shown that simultaneous inhibition of the three major IAP proteins, namely cIAP1, cIAP2 and XIAP is required to induce pronounced apoptosis in MM cells. However, IAP inhibition results in apoptosis in only some MM cell lines and patient cells. Given that levels of Bcl2 family proteins are unaffected by IAP inhibition, we hypothesized that combined inhibition of the IAP proteins using a SMAC mimetic LCL161 and the Bcl2 family proteins using a pan-Bcl2 inhibitor obatoclax (OBX) will lead to more pronounced and synergistic cell death in a broader subgroup of MM patients. Methods: LCL161 was synthesized by Novartis Inc. (Basel, Switzerland). OBX was purchased from Selleckchem (Houston, USA). Stock solutions were made in DMSO, and subsequently diluted in RPMI-1640 medium for use. MM cell lines were cultured in RPMI 1640 containing 10% fetal bovine serum (20% serum for primary patient cells) supplemented with L-Glutamine, penicillin, and streptomycin. Cytotoxicity was measured using the MTT viability assay and proliferation using thymidine uptake. Apoptosis was measured using flow cytometry upon cell staining with Annexin V-FITC and propidium iodide (PI) for cell lines and patient cells. Immunoblotting was done on cell extracts at various time points following incubation with the drugs in order to study the cell signaling pathways and a Results: LCL161/OBX combination induced synergistic cytotoxicity and anti-proliferative effects on a broad range of human MM cell lines, including drug resistant cell lines like DOX40 and MM1R. Components of the bone marrow microenvironment including bone marrow stromal cells and tumor promoting cytokines (VEGF, IGF and IL6) were unable to protect MM cells from the effects of the drug combination. We saw a time dependent increase in apoptosis, with the combination inducing significantly more apoptosis than either of the single agents alone. Examining the mechanism of action of the drug combination showed clear inhibition of the IAP proteins, activation of caspases 9, 8, 3 and Bid by LCL161 and the combination and up regulation of the pro-apoptotic proteins Bim, Bid, Puma and Noxa and accumulation of LC3-II by OBX and the combination. Using chloroquine along with the OBX, we were able to demonstrate that OBX induced protective autophagy and the addition of LCL161 was able to overcome this protective effect induced after single agent OBX treatment. Since protective autophagy can be induced by the ER stress response, we then examined the expression levels of proteins involved in this pathway. We observed clear induction of ER stress mediated UPR pathway by both the drugs. However, LCL161 and OBX induced different branches of the UPR pathway. OBX activated the ATF6 and pErk/peif2α/ATF4 branches of the UPR, both of which have been implicated in cell survival during ER stress. ATF4 under irrecoverable ER stress can lead to increase in transcription of CHOP and cause apoptosis. We therefore examined levels of CHOP and observed no induction of CHOP post treatment with either of the drugs or the combination. LCL161, however differentially modulated the IRE1 branch of the UPR by down regulating Xbp-1 splicing, which is a pro survival activity of IREI and up regulating pJNK, which indicated a pro-apoptotic activity induced by IRE1 post irrecoverable ER stress This indicated that the ER stress induced apoptosis is triggered by LCL161, which might be important to overcome the ER induced protective effects induced by OBX. Conclusion: Taken together, our studies indicate that LCL161/OBX combination induces synergistic cell death through modulation of apoptosis, authophagy and the ER stress response. Disclosures No relevant conflicts of interest to declare.
Abstract Multiple myeloma is the second most common hematological malignancy in the U.S. with an estimated 30,700 new diagnoses in 2018. It is a clonal disease of plasma cells that, despite recent therapeutic advances, remains incurable. Myeloma cells retain numerous characteristics of normal plasma cells including reliance on survival signals in the bone marrow for long term viability. However, malignant transformation of plasma cells imparts the ability to proliferate, causing harmful bone lesions in patients, and in advanced stages independence of the bone-marrow microenvironment. Therefore, we are investigating the molecular mechanisms of myeloma cell survival that allow them to become extramedullary. We identified syntenin-1 (SDCBP) as a protein involved in myeloma cell survival and a potential therapeutic target. Syntenin-1 is an adapter protein that has been shown to regulate surface expression of several transmembrane proteins by binding with membrane phospholipids and mediating vesicular trafficking of proteins throughout the cell. Syntenin-1 regulates the surface expression of CD138, a plasma/myeloma cell marker. Syntenin-1 has been shown to regulate apoptosis in numerous cancer cell lines including breast cancer, glioma, and pancreatic cancer but its role in multiple myeloma survival has not been studied. To determine if syntenin-1 expression has an effect on myeloma cell survival, we utilized the CoMMpass dataset (IA12), a longitudinal study of myeloma patients that includes transcriptomic analysis throughout treatment. We found that patients with the highest expression of syntenin-1 mRNA (top quartile) had significantly worse overall survival, progression-free survival, and a shorter response duration than those in the bottom quartile of expression. To determine if syntenin-1 has a role in myeloma cell survival, we used short hairpin RNA to knock down syntenin-1 (shsyn) in RPMI 8226 and MM1.s myeloma cell lines. We then determined the amount of cell death using Annexin-V staining flow cytometry four days following lentiviral infection. We found increased cell death in syntenin-1-silenced cells compared to our empty vector control in both RPMI 8226 (control=42.17%, shsyn=71.53%, p=0.04) and MM1.s cell lines (control=8.57%, shsyn=29.9%, p=0.04) suggesting that syntenin-1 is important for myeloma cell survival. Syntenin-1 contains two PDZ domains that allow it to bind to receptor proteins via their corresponding PDZ-binding motifs. We therefore wanted to look at correlation of syntenin-1 expression with CD138 and CD86, two PDZ-binding domain containing proteins expressed on the surface of myeloma cells. Using the CoMMpass dataset, we found patients with high expression of syntenin-1 had a median expression of CD86 that was twice as high as the total population (P<0.0001) while syntenin-1-low patients expressed CD86 at levels that were half as much as the population (P<0.0001). In contrast, there was no clear relationship between syntenin-1 and CD138 mRNA expression. Indeed if one takes into account all patients, there is a positive correlation between CD86 and syntenin-1 expression (r=0.228, P<0.0001) while there is a negative correlation between CD138 and syntenin-1 (r=-0.1923, P<0.0001). The correlation with CD86 but not CD138 suggests a previously undescribed role for syntenin-1 in myeloma cells. Our lab has previously shown that expression of CD86 is necessary for myeloma cell survival, and signals via its cytoplasmic domain to confer drug resistance. Silencing syntenin-1 results in a decrease in CD86 surface expression. However, there is no change in CD86 transcript or total cellular CD86 protein levels in our shsyn treated cells. Moreover, knockdown of CD86 resulted in increased protein expression and transcript levels of syntenin-1. Taken together, these data suggest that syntenin-1 may regulate CD86 expression on the cell surface. Our data supports a novel role for syntenin-1 in myeloma cell viability and as a potential regulator of CD86 surface expression. The role of syntenin-1 has not previously been explored in multiple myeloma and determining its molecular function is warranted as it may be an attractive target for therapeutic treatment of the disease. Disclosures Lonial: Amgen: Research Funding. Boise:AstraZeneca: Honoraria; Abbvie: Consultancy.
