Photodynamic
therapy (PDT) is based on the photoinduced generation of highly cytotoxic
singlet oxygen, and other reactive oxygen species (ROS), which induce oxidative
cell injury that leads to necrotic or apoptotic cell death. Photodynamic
killing of pathologically changed cells provides a treatment for cancer and
some non-cancerous diseases. Cell death processes are known to be controlled by
cell signaling systems. Therefore, the modification of signaling pathways may
modulate the cellular response to PDT. The present chapter is devoted to data
on mechanisms regulating the cellular respiration.
1.
The Principal Signaling Pathways
The
main finding in modern cell biology is the discovery of a complex cell
regulation system consisting of many interacting signaling pathways leading
from the cell surface receptors into the cytoplasm, further into the nucleus,
and back to the cytoplasm, the cell surface and the extra cellular medium. This
system consists of numerous intercellular signaling molecules, surface
receptors, cytoplasmic signaling cascades, transcription factors and
executioner proteins, which altogether determine a cell's response to external
impacts. Some of these factors regulate cell survival, and others control cell
death. Numerous, if not all, signaling pathways take part in the cellular
responses to PDT.
Everybody
who starts to study cell signaling processes meets two significant problems:
(i) the huge complexity of the cell signaling system, and (ii) imperfect
scientific terminology. The number of cellular proteins is estimated to be
about 105. The most of them participate in intracellular signaling and
regulation. Only a small fraction of the signaling proteins have been
identified so far, and their molecular functions are often not determined. The
signaling proteins usually form protein pathways or cascades aimed to execute
various cellular functions. However, not all components of these pathways are
determined, and their activity is not well defined. The signaling pathways may
control different sets of effector proteins in different cell lines or even in
the same cells under different conditions. These effector proteins form the
physiological state of the cell. But their number and activity are unknown. The
interaction of different signaling pathways strongly complicates the issue.
Another
problem is the absence of any system for naming the signaling proteins. The
latter are mainly senseless abbreviations. Since nobody can keep in mind about
105 such names, researchers working in different fields hardly understand each
other. Moreover, some proteins that were discovered several times in different
organisms may have different names. But later, when they are identified as the
same protein, their names may consist of all primary names, for example,
JNK/SAPK, or p21/Cip1/WAF. In the present chapter, I consider only several tens
of known signaling proteins that are believed to play central roles in cellular
functions. The major known signaling pathways and potential cellular PDT
targets are schematically presented at Figure1. Extracellular signaling
molecules (hormones, neuromediators, cytokines, or growth factors) are
recognized by receptors coupled to G-proteins (RCGP), or by receptor tyrosine
kinases (RTK). Extracellular matrix or adjacent cells are recognized by the
cell adhesion receptors (integrins, cadherins). There are several signaling
pathways in the cell leading from these receptors into the cytoplasm and the nucleus.
The
calcium pathway is initiated by Ca2+ influx through the plasma membrane (PM),
or by Ca2+ release from Ca2+-storing organelles: endoplasmic reticulum (ER) or
mitochondria. The release of Ca2+ from ER may be initiated either by RCGP or by
RTK. Ca2+ accumulation in the cytosol activates the downstream Ca2+-dependent
signaling proteins, such as calmodulin (CaM), calmodulin-dependent kinase II
(CaMKII), protein kinase C (PKC), etc.
In
the cAMP signaling pathway, ligand binding to RCGP activates G-protein, which
stimulates adenylate cyclase (AC) to produce cyclic adenosine monophosphate
(cAMP). cAMP-dependent protein kinase A (PKA) then activates diverse cellular
reactions.
Receptor
tyrosine kinase may initiate the signaling pathways mediated by: (i)
phospholipase C and Ca2+; (ii) phosphatydilinositol 3-kinase (PI3K) and protein
kinase B/Akt; (iii) mitogen-activated protein kinases (MAPKs). There are three
well-characterized MAP kinases in the MAPK family: extracellularly regulated
kinase (ERK), c-Jun terminal kinase (JNK), and protein kinase p38. ERKs are
stimulated by cytokines or other chemical signals. They regulate cell
proliferation and survival. JNK and p38 are involved in cell reactions to
environmental stress: division, survival, or apoptosis.
Integrins,
cell adhesion receptors, may activate focal adhesion kinase that regulates
cytoskeleton remodeling, cell shape and motility. All these protein kinases
regulate numerous effector proteins and transcription factors, which determine
cell responses to external impacts. Each cell has an individual set of such
proteins that determine its physiological state and provide specific responses.
2.
Possible ROS Sensors
Oxidative
stress is associated not only with general cell injury, but also with
up-regulation of some cellular proteins. These generally include cell
protection enzymes (catalase, superoxide dismutase, hemoxygenase-1, ferritin,
glutathione peroxidase, glutathione reductase, quinone reductase, thioredoxin,
thioredoxin reductase, and cyclooxygenase-2), proteins involved in
intercellular interactions, and transcription factors (AP-1, ATF/CREB, ETS,
C/EBP, NF-κB) (Turpaev, 2002). PDT activates or up-regulates diverse proteins
involved in cell protection (chaperons, superoxide dismutase, heme
oxygenase-1), signal transduction and transcription control (cyclooxygenase-2,
p38-MAPK, JNK, FAK, PI 3-kinase, Akt/PKB, AP-1, NF-κB), and apoptosis
(caspases, Bcl-2 family proteins, nucleases). This is evidently mediated by the
cellular regulatory systems (Oleinick et al., 2002; Almeida et al., 2004;
Castano et al., 2005; Nowis et al., 2005; Uzdensky, 2008). The primary event in
this pathway is recognition of PDT-generated ROS. This suggests the presence of
specific ROS sensors in the cell. Alternatively, the cell should recognize
results of gross damage such as ionic disbalance, impairment of the energy
metabolism, and destruction of cellular constituents.
Relatively
long-lived ROS, like hydrogen peroxide (H2O2) and superoxide-anion (O2.-), are
produced as minor by-products of oxidative phosphorylation in normal cells. In
stress conditions, their production is significantly enlarged (Skulachev,
1999). Short-lived singlet oxygen 1O2, which is produced during PDT, is two
orders more active. However, unlike H2O2 and O2.-, animal cells don't meet
singlet oxygen in their life, and possibly don't need in specific 1O2 sensors.
Nevertheless, the complex cell response to oxidative stress involving diverse
signaling pathways implies the presence of ROS sensors reacting to either
changes in the cellular redox potential, or peroxide accumulation, or general
ROS level. Such sensors have not yet been found, but potential candidates for
this role are discussed in the literature. (Anatoly B. Uzdensky et.al. 2012) Russian University
