PT - JOURNAL ARTICLE AU - Cara A. Rabik AU - Melissa L. Fishel AU - Julianne L. Holleran AU - Kristen Kasza AU - Mark R. Kelley AU - Merrill J. Egorin AU - M. Eileen Dolan TI - Enhancement of Cisplatin [<em>cis</em>-Diammine Dichloroplatinum (II)] Cytotoxicity by <em>O</em><sup>6</sup>-Benzylguanine Involves Endoplasmic Reticulum Stress AID - 10.1124/jpet.108.141291 DP - 2008 Nov 01 TA - Journal of Pharmacology and Experimental Therapeutics PG - 442--452 VI - 327 IP - 2 4099 - http://jpet.aspetjournals.org/content/327/2/442.short 4100 - http://jpet.aspetjournals.org/content/327/2/442.full SO - J Pharmacol Exp Ther2008 Nov 01; 327 AB - O6-Benzylguanine (BG) enhances cisplatin [cis-diammine dichloroplatinum (II)]-induced cytotoxicity and apoptosis in head and neck cancer cell lines by an unknown mechanism. We investigated the effect of cisplatin with and without BG on two targets of damage: DNA and the endoplasmic reticulum (ER). We chose three cancer cell lines to ascertain the mechanism of BG-enhanced cytotoxicity: SQ20b head and neck and SKOV-3x ovarian cancer cell lines, where BG enhanced cisplatin cytotoxicity, and A549 nonsmall cell lung cancer line, where BG did not enhance cisplatin cytotoxicity. All three lines had an increase in DNA damage when BG was added to cisplatin treatment, as evidenced by increased platination and phosphorylated histone H2AX formation. The increase in cisplatin-induced DNA damage after treatment with BG plus cisplatin is not sufficient to increase cytotoxicity or apoptosis in A549 cells. We evaluated the effect of cisplatin on the ER and observed increased caspase 12 cleavage in SQ20b and SKOV-3x cells, but not in A549 cells, after treatment with BG plus cisplatin versus cisplatin alone. Growth arrest and DNA damage inducible (GADD) 153, an ER stress-response gene, is up-regulated after treatment with BG plus cisplatin compared with cisplatin alone in SQ20b and SKOV-3x cells, but not in A549 cells. ER stress-induced apoptosis is an integral part of the mechanism by which BG enhances cisplatin. Inhibition of ER stress in the SQ20b cell line by salubrinal, an inhibitor of eIF2α dephosphorylation, or GADD153 small interfering RNA, abrogated BG-enhancement of cisplatin cytotoxicity and apoptosis through caspase 3 and 12 cleavage. These data indicate GADD153 up-regulation plays an important role in BG-enhanced cisplatin cytotoxicity and apoptosis. The American Society for Pharmacology and Experimental Therapeutics