title: Potentiation of apoptosis by histone deacetylase inhibitors and doxorubicin combination: cytoplasmic cathepsin B as a mediator of apoptosis in multiple myeloma.
abstract: BACKGROUND
  Although inhibitors of histone deacetylase inhibitors (HDACis) in combination with genotoxins potentiate apoptosis, the role of proteases other than caspases in this process remained elusive. Therefore, we examined the potentiation of apoptosis and related mechanisms of HDACis and doxorubicin combination in a panel of myeloma cell lines and in 25 primary myelomas.
  
  
  RESULTS
  At IC(50) concentrations, sodium butyrate (an HDACi) or doxorubicin alone caused little apoptosis. However, their combination potentiated apoptosis and synergistically reduced the viability of myeloma cells independent of p53 and caspase 3-7 activation. Potentiated apoptosis correlated with nuclear translocation of apoptosis-inducing factor, suggesting the induction of caspase 3- and 7-independent pathways. Consistent with this, butyrate and doxorubicin combination significantly increased the activity of cytoplasmic cathepsin B. Inhibition of cathepsin B either with a small-molecule inhibitor or downregulation with a siRNA reversed butyrate- and doxorubicin-potentiated apoptosis. Finally, ex vivo, clinically relevant concentrations of butyrate or SAHA (suberoylanilide hydroxamic acid, vorinostat, an HDACi in clinical testing) in combination with doxorubicin significantly (P<0.0001) reduced the survival of primary myeloma cells.
  
  
  CONCLUSIONS
  Cathepsin B has a prominent function in mediating apoptosis potentiated by HDACi and doxorubicin combinations in myeloma. Our results support a molecular model of lysosomal-mitochondrial crosstalk in HDACi- and doxorubicin-potentiated apoptosis through the activation of cathepsin B.
authors.affiliation: Translational Hematology and Oncology Research, Taussig Cancer Institute, Cleveland Clinic, Cleveland, OH 44195, USA. cheriyv@ccf.org
authors.forename: V
authors.initials: V
authors.lastname: Cheriyath
authors.forename: M A
authors.initials: MA
authors.lastname: Kuhns
authors.forename: M E
authors.initials: ME
authors.lastname: Kalaycio
authors.forename: E C
authors.initials: EC
authors.lastname: Borden
mesh_terms: D000971:Antineoplastic Combined Chemotherapy Protocols / Q000627:therapeutic use*; D017209:Apoptosis / Q000187:drug effects*; D020169:Caspases / Q000378:metabolism / Q000502:physiology; D002401:Cathepsin B / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000502:physiology*; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D003593:Cytoplasm / Q000378:metabolism; D004317:Doxorubicin / Q000008:administration & dosage* / Q000494:pharmacology; D004353:Drug Evaluation, Preclinical; D004357:Drug Synergism; D056572:Histone Deacetylase Inhibitors / Q000008:administration & dosage* / Q000494:pharmacology; D006801:Humans; D006877:Hydroxamic Acids / Q000008:administration & dosage / Q000494:pharmacology; D009101:Multiple Myeloma / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D034741:RNA, Small Interfering / Q000494:pharmacology; D000077337:Vorinostat
chemical_list: D056572:Histone Deacetylase Inhibitors; D006877:Hydroxamic Acids; D034741:RNA, Small Interfering; D000077337:Vorinostat; D004317:Doxorubicin; D020169:Caspases; C533912:CTSB protein, human; D002401:Cathepsin B