Function and regulation of the intrinsic
prion protein (PrPc) are largely unknown. In the present study the regulation of PrPc expression by
growth factors and
cytokines that increase intracellular
reactive oxygen species (ROS) levels was studied in
glioma and
neuroblastoma cells grown as multicellular
tumor spheroids. PrPc
protein was significantly increased when
glioma spheroids were treated with either
ATP,
nerve growth factor (
NGF),
epidermal growth factor (
EGF), or
tumor necrosis factor alpha (
TNF-alpha), whereas
mRNA levels as evaluated by
Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) remained unchanged.
ATP,
NGF,
EGF, and
TNF-alpha raised intracellular ROS levels as evaluated using the redox-sensitive fluorescence
dye 2'7'-dichlorodihydrofluorescein diacetate (
H2DCFDA). The observed elevation in PrPc was completely abolished in the presence of the
free radical scavengers vitamin E and
ebselen, as well as following pretreatment with the
NADPH-oxidase inhibitor diphenylen iodonium
chloride (DPI), indicating that PrPc levels are regulated by intracellular ROS. The correlation of PrPc expression to the intracellular ROS levels was investigated by the use of
neuroblastoma cells overexpressing either mutant V210I PrP, or wild-type PrPc. It was observed that the intracellular redox state was significantly reduced in PrPc as well as V210I PrP overexpressing cells as compared to non-transfected cells. Consequently, the observed elevation of ROS following treatment with
ATP was completely abolished in PrP overexpressing cells. Our data are in line with the assumption that PrPc plays a role as
free radical scavenger and/or sensor molecule for oxidative stress.