IPI-504, a novel and soluble HSP-90 inhibitor, blocks the unfolded protein response in multiple myeloma cells.
BACKGROUND
Inhibitors of heat shock protein (Hsp) 90 induce apoptosis in multiple myeloma (MM) cells, but the molecular mechanisms underlying this cytotoxic outcome are not clear. Here, we investigate the effect of IPI-504, a novel and highly soluble inhibitor of the Hsp90 ATPase activity, on the unfolded protein response (UPR) in MM cells. The UPR is a stress response pathway triggered by sensors located at the endoplasmic reticulum (ER) membrane whose function is to reduce an excessive accumulation of misfolded protein in the ER. During normal development of B-lymphocytes to antibody-producing plasma cells, a partial UPR has been described, where IREalpha and ATF-6 are stimulated, whereas the third sensor, PERK, is not induced.


METHODS
Levels of the activated forms of the three main UPR sensors ATF-6, XBP-1 and PERK/eIF-2 were monitored in two different MM cells lines and one non-MM cell lines under various experimental conditions including incubation with increasing concentration of IPI-504. Also, MM cells were incubated with IPI-504 and several apoptosis markers were monitored.


RESULTS
We show here that a partial UPR is constitutively activated in plasma cell-derived MM cells and that IPI-504 can potently inhibit this pathway. IPI-504 achieves this by inactivating the transcription factors XBP1 and ATF6. In addition, IPI-504 also blocks the tunicamycin-induced phosphorylation of eIF2 by PERK. Dose-response and time course experiments reveal that IPI-504's inhibitory effect on the UPR parallels its cytotoxic and pro-apoptotic effects on MM cells.


CONCLUSIONS
The results presented here suggest that the IPI-504-induced apoptosis might be, in part, mediated by the inhibition of the partial UPR. Other malignancies that rely on intact and efficient UPR to survive could be considered as new indications for Hsp90 inhibitors.
Patterson
Jon
J
Infinity Pharmaceuticals, Inc., 780 Memorial Drive, Cambridge, MA 02139, USA.
Palombella
Vito J
VJ
Fritz
Christian
C
Normant
Emmanuel
E
D051702:Activating Transcription Factor 6 / Q000235:genetics / Q000502:physiology; D000970:Antineoplastic Agents / Q000494:pharmacology*; D000998:Antiviral Agents / Q000494:pharmacology; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology; D016227:Benzoquinones / Q000494:pharmacology*; D015153:Blotting, Western; D016923:Cell Death / Q000187:drug effects; D045744:Cell Line, Tumor; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D015852:Eukaryotic Initiation Factor-2 / Q000235:genetics / Q000502:physiology; D017930:Genes, Reporter / Q000235:genetics; D018841:HSP90 Heat-Shock Proteins / Q000037:antagonists & inhibitors*; D006801:Humans; D047029:Lactams, Macrocyclic / Q000494:pharmacology*; D008156:Luciferases / Q000235:genetics; D009101:Multiple Myeloma / Q000188:drug therapy* / Q000378:metabolism* / Q000473:pathology; D009687:Nuclear Proteins / Q000235:genetics / Q000502:physiology; D010766:Phosphorylation; D017510:Protein Folding; D000071701:Regulatory Factor X Transcription Factors; D014157:Transcription Factors; D014162:Transfection; D014415:Tunicamycin / Q000494:pharmacology; D000071717:X-Box Binding Protein 1; D019892:eIF-2 Kinase / Q000235:genetics / Q000502:physiology
D051702:Activating Transcription Factor 6; D000970:Antineoplastic Agents; D000998:Antiviral Agents; D016227:Benzoquinones; D004268:DNA-Binding Proteins; D015852:Eukaryotic Initiation Factor-2; D018841:HSP90 Heat-Shock Proteins; D047029:Lactams, Macrocyclic; D008156:Luciferases; D009687:Nuclear Proteins; D000071701:Regulatory Factor X Transcription Factors; D014157:Transcription Factors; D014415:Tunicamycin; D000071717:X-Box Binding Protein 1; D019892:eIF-2 Kinase; C497111:ATF6 protein, human; C000605543:XBP1 protein, human; C112765:tanespimycin