PURPOSE: Evasion from chemotherapy-induced apoptosis due to p53 loss strongly 
contributes to drug resistance. Identification of specific targets for the 
treatment of drug-resistant p53-null tumors would therefore increase the 
effectiveness of cancer therapy.
EXPERIMENTAL DESIGN: By using a kinase-directed short hairpin RNA library and 
HCT116p53KO drug-resistant colon carcinoma cells, glycogen synthase kinase 3 
beta (GSK3B) was identified as a target whose silencing bypasses drug resistance 
due to loss of p53. p53-null colon cancer cell lines with different sets of 
mutations were used to validate the role of GSK3B in sustaining resistance and 
to characterize cell death mechanisms triggered by chemotherapy when GSK3B is 
silenced. In vivo xenograft studies were conducted to confirm resensitization of 
drug-resistant cells to chemotherapy upon GSK3 inhibition. Colon cancer samples 
from a cohort of 50 chemotherapy-treated stage II patients were analyzed for 
active GSK3B expression.
RESULTS: Downregulation of GSK3B in various drug-resistant p53-null colon cancer 
cell lines abolished cell viability and colony growth after drug addition 
without affecting cell proliferation or cell cycle in untreated cells. Cell 
death of 5-fluorouracil (5FU)-treated p53-null GSK3B-silenced colon carcinoma 
cells occurred via PARP1-dependent and AIF-mediated but RIP1-independent 
necroptosis. In vivo studies showed that drug-resistant xenograft tumor mass was 
significantly reduced only when 5FU was given after GSK3B inhibition. Tissue 
microarray analysis of colon carcinoma samples from 5FU-treated patients 
revealed that GSK3B is significantly more activated in drug-resistant versus 
responsive patients.
CONCLUSIONS: Targeting GSK3B, in combination with chemotherapy, may represent a 
novel strategy for the treatment of chemotherapy-resistant tumors.