The cellular function of the intrinsic
prion protein (PrPc) remains largely unknown. In the present study PrPc expression was investigated in multicellular prostate
tumor spheroids and was correlated to the intracellular redox state as evaluated using the
fluorescent dye 2'7'-dichlorodihydrofluorescein diacetate (
H2DCFDA). In small
tumor spheroids (diameter 100 +/- 20 microm)
reactive oxygen species (ROS) levels were increased as compared with large (diameter 250 +/- 50 microm) spheroids. ROS generation was mediated by the mitochondrial respiratory chain and a
NADPH oxidaselike
enzyme, because carbonylcyanide-m-chlorophenylhydrazone (
CCCP),
rotenone, and
diphenylene iodonium chloride (DPI) significantly reduced ROS levels. The elevated ROS were correlated to an increased expression of PrPc,
Cu/Zn superoxide dismutase (SOD-1), and
catalase in small as compared with large spheroids. In large
tumor spheroids, PrPc was predominantly expressed in the peripheral cell layers and colocalized with SOD-1 and
catalase. Raising intracellular ROS in large
tumor spheroids by
hydrogen peroxide,
menadione,
buthionine sulfoximine (BSO), and incubation in
glutamine-reduced medium increased PrPc expression. In small spheroids PrPc was downregulated after incubation with the radical scavengers dehydroascorbate (DHA) and
vitamin E. Our data indicate that PrPc expression in
tumor spheroids is related to the intracellular redox state and may participate in antioxidative defense.