Citations

Abstract The molecular target(s) cooperating with proteasome inhibition in multiple myeloma (MM) remain unknown. We therefore measured proliferation in MM cells transfected with 13 984 small interfering RNAs in the absence or presence of increasing concentrations of bortezomib. We identified 37 genes, which when silenced, are not directly cytotoxic but do synergistically potentiate the growth inhibitory effects of bortezomib. To focus on bortezomib sensitizers, genes that also sensitized MM to melphalan were excluded. When suppressed, the strongest bortezomib sensitizers were the proteasome subunits PSMA5, PSMB2, PSMB3, and PSMB7 providing internal validation, but others included BAZ1B, CDK5, CDC42SE2, MDM4, NME7, RAB8B, TFE3, TNFAIP3, TNK1, TOP1, VAMP2, and YY1. The strongest hit CDK5 also featured prominently in pathway analysis of primary screen data. Cyclin-dependent kinase 5 (CDK5) is expressed at high levels in MM and neural tissues with relatively low expression in other organs. Viral shRNA knockdown of CDK5 consistently sensitized 5 genetically variable MM cell lines to proteasome inhibitors (bortezomib and carfilzomib). Small-molecule CDK5 inhibitors were demonstrated to synergize with bortezomib to induce cytotoxicity of primary myeloma cells and myeloma cell lines. CDK5 regulation of proteasome subunit PSMB5 was identified as a probable route to sensitization.

AbstractA paucity of validated kinase targets in human multiple myeloma has delayed clinical deployment of kinase inhibitors in treatment strategies. We therefore conducted a kinome-wide small interfering RNA (siRNA) lethality study in myeloma tumor lines bearing common t(4;14), t(14;16), and t(11;14) translocations to identify critically vulnerable kinases in myeloma tumor cells without regard to preconceived mechanistic notions. Fifteen kinases were repeatedly vulnerable in myeloma cells, including AKT1, AK3L1, AURKA, AURKB, CDC2L1, CDK5R2, FES, FLT4, GAK, GRK6, HK1, PKN1, PLK1, SMG1, and TNK2. Whereas several kinases (PLK1, HK1) were equally vulnerable in epithelial cells, others and particularly G protein–coupled receptor kinase, GRK6, appeared selectively vulnerable in myeloma. GRK6 inhibition was lethal to 6 of 7 myeloma tumor lines but was tolerated in 7 of 7 human cell lines. GRK6 exhibits lymphoid-restricted expression, and from coimmunoprecipitation studies we demonstrate that expression in myeloma cells is regulated via direct association with the heat shock protein 90 (HSP90) chaperone. GRK6 silencing causes suppression of signal transducer and activator of transcription 3 (STAT3) phosphorylation associated with reduction in MCL1 levels and phosphorylation, illustrating a potent mechanism for the cytotoxicity of GRK6 inhibition in multiple myeloma (MM) tumor cells. As mice that lack GRK6 are healthy, inhibition of GRK6 represents a uniquely targeted novel therapeutic strategy in human multiple myeloma.

Hacs1, a SH3 and SAM domain‐containing adaptor protein, is up‐regulated by IL‐4 in activated B cells and strongly expressed in dendritic cells. To elucidate the function of Hacs1 in immune regulation, we generated Hacs1 − / − mice by deletion of the SH3 and SAM domains. Hacs1 − / − mice were viable and fertile and had normal bone marrow B‐cell development and normal splenic T‐ and B‐cell populations. However, adult Hacs1 − / − mice had increased peritoneal Bla cells (IgM + CD5 + ). On immunization with T‐cell‐independent antigen TNP‐Ficoll, Hacs1 − / − mice had increased production of anti‐TNP IgM and IgG3. Purified splenic B cells from Hacs1 − / − mice showed increased cell proliferation on BCR (B‐cell receptor) stimulation. We further demonstrate that the Hacs1 − / − B cells had increased global tyrosine phosphorylation, including tyrosine kinases Lyn and Akt. Both T‐helper type 1 (T h 1) and T‐helper type 2 (T h 2) humoral responses were enhanced in Hacs1 − / − mice. In vitro bone marrow‐derived Hacs1 − / − dendritic cells showed increased IL‐12 production on stimulation with ovalbumin (OVA). This study suggests that Hacs1 is an immunoinhibitory adaptor that might be a useful target for immune suppression therapy.—Wang, D., Stewart, A. K., Zhuang, L., Zhu, Y., Wang, Y., Shi, C., Keating, A., Slutsky, A., Zhang, H., Wen, X.‐Y. Enhanced adaptive immunity in mice lacking the immunoinhibitory adaptor Hacs1. FASEBJ. 24, 947–956 (20l0). www.fasebj.org

AbstractAs multiple myeloma tumors universally dysregulate cyclin D genes we conducted high-throughput chemical library screens for compounds that induce suppression of cyclin D2 promoter transcription. The top-ranked compound was a natural triterpenoid, pristimerin. Strikingly, the early transcriptional response of cells treated with pristimerin closely resembles cellular responses elicited by proteosome inhibitors, with rapid induction of heat shock proteins, activating transcription factor 3 (ATF3), and CHOP. Enzymatic assays and immunoblotting confirm that pristimerin rapidly (< 90 minutes) and specifically inhibits chymotrypsin-like proteosome activity at low concentrations (< 100 nM) and causes accumulation of cellular ubiquitinated proteins. Notably, cytotoxic triterpenoids including pristimerin inhibit NF-κB activation via inhibition of IKKα or IKKβ, whereas proteosome inhibitors instead suppress NF-κB function by impairing degradation of ubiquitinated IκB. By inhibiting both IKK and the proteosome, pristimerin causes overt suppression of constitutive NF-κB activity in myeloma cells that may mediate its suppression of cyclin D. Multiple myeloma is exquisitely sensitive to proteosome or NF-κB pathway inhibition. Consistent with this, pristimerin is potently and selectively lethal to primary myeloma cells (IC50 < 100 nM), inhibits xenografted plasmacytoma tumors in mice, and is synergistically cytotoxic with bortezomib—providing the rationale for pharmaceutical development of triterpenoid dual-function proteosome/NF-κB inhibitors as therapeutics for human multiple myeloma and related malignancies.

AbstractD-cyclins are regulators of cell division that act in a complex with cyclin-dependent kinases to commit cells to a program of DNA replication. D-cyclins are overexpressed in many tumors, including multiple myeloma and leukemia, and contribute to disease progression and chemoresistance. To better understand the role and impact of D-cyclins in hematologic malignancies, we conducted a high throughput screen for inhibitors of the cyclin D2 promoter and identified the drug cyproheptadine. In myeloma and leukemia cells, cyproheptadine decreased expression of cyclins D1, D2, and D3 and arrested these cells in the G0/G1 phase. After D-cyclin suppression, cyproheptadine induced apoptosis in myeloma and leukemia cell lines and primary patient samples preferentially over normal hematopoietic cells. In mouse models of myeloma and leukemia, cyproheptadine inhibited tumor growth without significant toxicity. Cyproheptadine-induced apoptosis was preceded by activation of the mitochondrial pathway of caspase activation and was independent of the drug's known activity as an H1 histamine and serotonin receptor antagonist. Thus, cyproheptadine represents a lead for a novel therapeutic agent for the treatment of malignancy. Because the drug is well tolerated and already approved in multiple countries for clinical use as an antihistamine and appetite stimulant, it could be moved directly into clinical trials for cancer.

AbstractThe oncogene c-maf is frequently overexpressed in multiple myeloma cell lines and patient samples and contributes to increased cellular proliferation in part by inducing cyclin D2 expression. To identify regulators of c-maf, we developed a chemical screen in NIH3T3 cells stably overexpressing c-maf and the cyclin D2 promoter driving luciferase. From a screen of 2400 off-patent drugs and chemicals, we identified glucocorticoids as c-maf–dependent inhibitors of cyclin D2 transactivation. In multiple myeloma cell lines, glucocorticoids reduced levels of c-maf protein without influencing corresponding mRNA levels. Subsequent studies demonstrated that glucocorticoids increased ubiquitination-dependent degradation of c-maf and up-regulated ubiquitin C mRNA. Moreover, ectopic expression of ubiquitin C recapitulated the effects of glucocorticoids, demonstrating regulation of c-maf protein through the abundance of the ubiquitin substrate. Thus, using a chemical biology approach, we identified a novel mechanism of action of glucocorticoids and a novel mechanism by which levels of c-maf protein are regulated by the abundance of the ubiquitin substrate.