Blockade of MGMT expression by O6 benzyl guanine leads to inhibition of pancreatic cancer growth and induction of apoptosis.
OBJECTIVE
We sought to determine whether administration of a MGMT blocker, O(6)-benzyl guanine (O(6)BG), at an optimal biological dose alone or in combination with gemcitabine inhibits human pancreatic cancer cell growth.


METHODS
Human pancreatic cancer L3.6pl and PANC1 cells were treated with O(6)BG, either alone or in combination with gemcitabine, and the therapeutic efficacy and biological activity of these drug combinations were investigated.


RESULTS
O(6)BG sensitized pancreatic cancer cells to gemcitabine. Protein and mRNA expression of MGMT, cyclin B1, cyclin B2, cyclin A, and ki-67 were significantly decreased in the presence of O(6)BG. In sharp contrast, protein expression and mRNA message of p21(cip1) were significantly increased. Interestingly, O(6)BG increases p53-mediated p21(cip1) transcriptional activity and suppresses cyclin B1. In addition, our results indicate that p53 is recruited to p21 promoter. Furthermore, an increase in p21(cip1) and a decrease in cyclin transcription are p53 dependent. The volume of pancreatic tumors was reduced by 27% in mice treated with gemcitabine alone, by 47% in those treated with O(6)BG alone, and by 65% in those mice given combination. Immunohistochemical analysis showed that O(6)BG inhibited expression of MGMT and cyclins, and increased expression of p21(cip1). Furthermore, there was a significant decrease in tumor cell proliferation and an increase in tumor cell apoptosis.


CONCLUSIONS
Collectively, our results show that decreased MGMT expression is correlated with p53 activation, and significantly reduced primary pancreatic tumor growth. These findings suggest that O(6)BG either alone or in combination with gemcitabine may provide a novel and effective approach for the treatment of human pancreatic cancer.
Konduri
Santhi D
SD
Cancer Research Institute, M D Anderson Cancer Center Orlando, Orlando, Florida 32806, USA. santhi.konduri@orlandohealth.com
Ticku
Jonathan
J
Bobustuc
George C
GC
Sutphin
Robert M
RM
Colon
Jimmie
J
Isley
Beth
B
Bhakat
Kishor K
KK
Srivenugopal
Kalkunte S
KS
Kalkunte
Srivenugopal S
SS
Baker
Cheryl H
CH
D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology; D017209:Apoptosis / Q000187:drug effects*; D002277:Carcinoma / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D049109:Cell Proliferation / Q000187:drug effects*; D015254:DNA Modification Methylases / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D045643:DNA Repair Enzymes / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D015536:Down-Regulation / Q000187:drug effects; D004353:Drug Evaluation, Preclinical; D005786:Gene Expression Regulation / Q000187:drug effects; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects*; D016158:Genes, p53 / Q000187:drug effects; D006147:Guanine / Q000031:analogs & derivatives* / Q000494:pharmacology; D006801:Humans; D020128:Inhibitory Concentration 50; D008297:Male; D051379:Mice; D008819:Mice, Nude; D010190:Pancreatic Neoplasms / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D025521:Tumor Suppressor Proteins / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism / Q000502:physiology
D000970:Antineoplastic Agents; D025521:Tumor Suppressor Proteins; C064976:O(6)-benzylguanine; D006147:Guanine; D015254:DNA Modification Methylases; C515491:MGMT protein, human; D045643:DNA Repair Enzymes