Effect of inhibition of the ubiquitin-proteasome system and Hsp90 on growth and survival of rhabdomyosarcoma cells in vitro.
BACKGROUND
The ubiquitin-proteasome system (UPS) and the heat shock response (HSR) are two critical regulators of cell homeostasis, as their inhibition affects growth and survival of normal cells, as well as stress response and invasiveness of cancer cells. We evaluated the effects of the proteasome inhibitor Bortezomib and of 17-DMAG, a competitive inhibitor of Hsp90, in rhabdomyosarcoma (RMS) cells, and analyzed the efficacy of single-agent exposures with combination treatments.


METHODS
To assess cytotoxicity induced by Bortezomib and 17-DMAG in RMS cells, viability was measured by MTT assay after 24, 48 and 72 hours. Western blotting and immunofluorescence analyses were carried out to elucidate the mechanisms of action. Apoptosis was measured by FACS with Annexin-V-FITC and Propidium Iodide.


RESULTS
Bortezomib and 17-DMAG, when combined at single low-toxic concentrations, enhanced growth inhibition of RMS cells, with signs of autophagy that included intensive cytoplasmic vacuolization and conversion of cytosolic LC3-I protein to its autophagosome-associated form. Treatment with lysosomal inhibitor chloroquine facilitates apoptosis, whereas stimulation of autophagy by rapamycin prevents LC3-I conversion and cell death, suggesting that autophagy is a resistance mechanism in RMS cells exposed to proteotoxic drugs. However, combination treatment also causes caspase-dependent apoptosis, PARP cleavage and Annexin V staining, as simultaneous inhibition of both UPS and HSR systems limits cytoprotective autophagy, exacerbating stress resulting from accumulation of misfolded proteins.


CONCLUSIONS
The combination of proteasome inhibitor Bortezomib with Hsp90 inhibitor 17-DMAG, appears to have important therapeutic advantages in the treatment of RMS cells compared with single-agent exposure, because compensatory survival mechanisms that occur as side effects of treatment may be prevented.
Clinica di Oncoematologia Pediatrica, Azienda Ospedaliera-Università di Padova, Via Giustiniani 3, Padova 35128, Italy.
Marica
M
Peron
Paolo
P
Bonvini
Angelo
A
Rosolen
D000970:Antineoplastic Agents / Q000494:pharmacology / Q000633:toxicity; D017209:Apoptosis / Q000187:drug effects; D001343:Autophagy / Q000187:drug effects; D016227:Benzoquinones / Q000494:pharmacology / Q000633:toxicity; D001897:Boronic Acids / Q000494:pharmacology / Q000633:toxicity; D000069286:Bortezomib; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D018841:HSP90 Heat-Shock Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006801:Humans; D047029:Lactams, Macrocyclic / Q000494:pharmacology / Q000633:toxicity; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D011719:Pyrazines / Q000494:pharmacology / Q000633:toxicity; D012208:Rhabdomyosarcoma / Q000378:metabolism*; D025801:Ubiquitin / Q000037:antagonists & inhibitors / Q000378:metabolism*
D000970:Antineoplastic Agents; D016227:Benzoquinones; D001897:Boronic Acids; D018841:HSP90 Heat-Shock Proteins; D047029:Lactams, Macrocyclic; D011719:Pyrazines; D025801:Ubiquitin; C448659:17-(dimethylaminoethylamino)-17-demethoxygeldanamycin; D000069286:Bortezomib; D046988:Proteasome Endopeptidase Complex