Down-regulation of intrinsic P-glycoprotein expression in multicellular prostate tumor spheroids by reactive oxygen species

Intrinsic expression of the multidrug resistance (MDR) transporter P-glycop rotein (Pgp) may be regulated by reactive oxygen species (ROS), A transient expression of Pgp was observed during the growth of multicellular tumor sp heroids. Maximum Pgp expression occurred in tumor spheroids with a high per centage of quiescent, Ki-67-negative cells, elevated glutathione levels, in creased expression of the cyclin-dependent kinase inhibitors p27(Kip1) and p21(WAF-1) as well as reduced ROS levels and minor activity of the mitogen- activated kinase (MAPK) members c-Jun amino-terminal kinase (JNK), extracel lular signal-regulated kinase ERK1,2, and p38 MAPK, Raising intracellular R OS by depletion of glutathione with buthionine sulfoximine (BSO) or glutami ne starvation resulted in down-regulation of Pgp and p27(Kip1), whereas ERK 1,S and JNK were activated. Down-regulation of Pgp was furthermore observed with low concentrations of hydrogen peroxide and epidermal growth factor, indicating that ROS may regulate Pgp expression. The down-regulation of Pgp following BSO treatment was abolished by agents interfering with receptor tyrosine kinase signaling pathways, i,e, the protein kinase C inhibitors bi sindolylmaleimide I (BIM-1) and Ro-31-8220, the p21(ras) farnesyl protein t ransferase inhibitor III, the c-Raf inhibitor ZM 336372 and PD98059, which inhibits ERK1,2 activation. ROS involved as second messengers in receptor t yrosine kinase signaling pathways may act as negative regulators of Pgp exp ression.