Protein Misfolding and Cellular Stress: An Overview - Protein Misfolding and Cellular Stress in Disease and Aging - page 8

produces the transcription factor ATF4, which is also involved
in the activation of cytoprotective genes.
In addition to these protective responses, which are set into
action by acute “mild” challenges, elimination signals may also be
initiated, most probably after severe misfolding and oxidative
stress. The proapoptotic transcription activator C/EBP homolo-
gous protein (CHOP) can be activated by ATF4, and caspases
may by cleaved and activated by IRA1, thus shifting the balance
from protection to execution. If, at the same time, generation of
ROS exceeds the antioxidant capacity, apoptosis may also be
initiated.
ROS is usually created by leakage of superoxide from the
mitochondrial respiratory chain due to perturbation of mitochon-
drial structure and/or function. Indeed, this seems to be the case
during ER stress, since Ca ++ redistribution from ER to mitochon-
dria and activation of mitochondrial nitrogen oxide synthetase
(mtNOS) creating RNS is promoted by severe ER stress ( 22, 23 ).
These effects will be discussed below in connection with mito-
chondrial stress. Suffice here to say that ROS and RNS from
mitochondria may aggravate Ca ++ leakage from ER and increase
the production of ROS/RNS, thus creating a vicious cycle. If the
production of ROS/SNS is elevated beyond a certain level, the
mitochondrial permeability transition pores eventually open and
give rise to liberation of cytochrome c and execution of the apop-
tosis programme ( 29, 37, 38 ).
ROS may in addition be created inside ER by hyper-activity
of the so-called oxidative protein folding. Since ER constitutes an
oxidative milieu and many ER processed proteins contain disul-
fide bonds, this compartment, in contrast to the cytosol and
mitochondria, comprise a number of oxidoreductases by which
disulfide bonds can be created as well as redistributed and broken
if wrongly synthesized. PDI is the most important and abundant.
Although glutathione is able to reduce misplaced disulfide bonds
in polypeptides, PDI in conjunction with ER oxidoreductin 1
(Ero1p) is considered a major pathway since Ero1p is induced by
ER stress ( 39 ). Interestingly, Ero1p is a flavoenzyme which may
interact directly with molecular oxygen, producing ROS in the
form of hydrogen peroxide and thereby contribute to the burden
of oxidative stress. Further, it has been implicated, at least in neu-
ronal cells containing a nitrogen oxide synthetase, that NO may
inactivate PDI by nitrosylation, thereby inhibiting reduction
and/or redistribution of protein disulfides and aggravating the
ER stress ( 19, 40 ). Since mitochondria contain an NO synthetase
(mtNOS) ( 23 ), which is stimulated by the above discussed redis-
tribution of Ca ++ from ER, the mitochondria may generate NO.
NO is free diffusible and may migrate to the ER for nitrosylation
of proteins, such as PDI.