Imatinib-mediated inactivation of Akt regulates ABCG2 function in head and neck squamous cell carcinoma.
OBJECTIVE
To investigate whether the mechanism for the reversal of ABCG2 (also known as ABCP, MXR, and BCRP)-mediated drug resistance by imatinib mesylate (Gleevec, STI571; Novartis Pharmaceuticals Corp, East Hanover, New Jersey) is caused by the downregulation of Akt kinase. The adenosine triphosphatase-binding cassette protein ABCG2 has been suggested to be involved in the resistance against various anticancer drugs. Recent studies show that imatinib reverses ABCG2-mediated drug resistance to topotecan hydrochloride and SN-38. In addition, we have previously reported that imatinib downregulates Akt kinase activity, which is elevated in head and neck squamous cell carcinoma.


METHODS
Flow cytometric analysis was used to determine the levels of drug or dye extrusion from the cells.


RESULTS
We used Akt kinase inhibitors, transfection with short interfering RNA (siRNA) Akt, and the tyrosine kinase inhibitor imatinib to show that these treatments decreased the side population by 50% to 70% in Hoechst 33342 extrusion studies. Doxorubicin hydrochloride extrusion experiments also demonstrated 20% to 26% decrease in doxorubicin efflux on cells treated with imatinib, 1L6-hydroxymethyl-chiro-inositol 2-(R)-2-O-methyl-3-O-octadecylcarbonate, and transfection with siRNA Akt. With Western blot and immunofluorescence experiments, our data suggest that ABCG2 translocation is the mechanism by which imatinib and Akt regulate drug resistance. Clonogenic survival assays performed with imatinib-treated cells resulted in a dose-dependent decrease in cell survival compared with the control population.


CONCLUSIONS
Our findings demonstrate that imatinib confers greater doxorubicin retention, presumably via inhibition of Akt, which regulates ABCG2 function.
Chu
Theresa S
TS
Division of Head and Neck Surgery, Department of Surgery, University of California, San Diego, La Jolla, CA 92093, USA.
Chen
Jocelyn S
JS
Lopez
Jay Patrick
JP
Pardo
Francisco S
FS
Aguilera
Joseph
J
Ongkeko
Weg M
WM
D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters / Q000037:antagonists & inhibitors*; D000970:Antineoplastic Agents / Q000494:pharmacology*; D001549:Benzamides; D015153:Blotting, Western; D002294:Carcinoma, Squamous Cell / Q000188:drug therapy* / Q000378:metabolism; D045744:Cell Line, Tumor; D015536:Down-Regulation; D004317:Doxorubicin / Q000494:pharmacology*; D019008:Drug Resistance, Neoplasm; D005434:Flow Cytometry; D005455:Fluorescent Antibody Technique; D006258:Head and Neck Neoplasms / Q000188:drug therapy* / Q000378:metabolism; D006801:Humans; D000068877:Imatinib Mesylate; D047468:Immunoprecipitation; D009363:Neoplasm Proteins; D010879:Piperazines / Q000494:pharmacology*; D051057:Proto-Oncogene Proteins c-akt; D011743:Pyrimidines / Q000494:pharmacology*
C116614:ABCG2 protein, human; D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters; D000970:Antineoplastic Agents; D001549:Benzamides; D009363:Neoplasm Proteins; D010879:Piperazines; D011743:Pyrimidines; D004317:Doxorubicin; D000068877:Imatinib Mesylate; D051057:Proto-Oncogene Proteins c-akt