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-glycoprotein (Pgp) may be regulated by reactive oxygen species (ROS). A transient expression of Pgp was observed during the growth of multicellular tumor spheroids. Maximum Pgp expression occurred in tumor spheroids with a high percentage of quiescent, Ki-67-negative cells, elevated glutathione levels, increased expression of the cyclin-dependent kinase inhibitors p27Kip1 and p21WAF-1 as well as reduced ROS levels and minor activity of the mitogen-activated kinase (MAPK) members c-Jun amino-terminal kinase (JNK), extracellular signal-regulated kinase ERK1,2, and p38 MAPK. Raising intracellular ROS by depletion of glutathione with buthionine sulfoximine (BSO) or glutamine starvation resulted in down-regulation of Pgp and p27Kip1, whereas ERK1,2 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 bisindolylmaleimide I (BIM-1) and Ro-31-8220, the p21ras farnesyl protein transferase inhibitor III, the c-Raf inhibitor ZM 336372 and PD98059, which inhibits ERK1,2 activation. ROS involved as second messengers in receptor tyrosine kinase signaling pathways may act as negative regulators of Pgp expression.