PHOSPHORYLATION OF HSP27 - THE MOLECULAR MECHANISM FOR MOBILE PHONE
RADIATION-INDUCED INCREASE IN BLOOD-BRAIN BARRIER PERMEABILITY
D. Leszczynski. Bio-NIR Research Group, Radiobiology Laboratory, STUK
- Radiation and Nuclear Safety Authority, Helsinki, Finland
BACKGROUND:
The question of whether mobile phone radiation (RF-EMF) is hazardous to
health remains unanswered. In earlier study (1) has been demonstrated
that the 1-hour non-thermal exposure of human endothelial cell line EA.hy926
to SAR of 2W/kg (900MHz GSM signal) leads, among others, to:
i) transient increase in phosphorylation of hsp27 stress response protein,
which was prevented by SB203580, a specific inhibitor of p38 mitogen-activated
protein kinase (p38MAPK), and
(ii) transient changes in protein expression levels of hsp27 and p38MAPK.
Phosphorylated hsp27 has been shown to regulate, among others, cell apoptosis
- due to inhibition of the proteolytic activation of pro-caspase-9, and
stability of stress fibers - due to increasing polymerization of actin.
The latter, when occurring in endothelial cells lining brain's capillary
blood vessels, might be of critical importance for the functioning of
blood-brain barrier because stabilization of stress fibers was shown to
cause: (i) cell shrinkage, leading to opening of spaces between cells,
(ii) increase in the permeability and pinocytosis of endothelial monolayer,
(iii) increase information of apoptosis-unrelated blebs on the surface
of endothelial cells, which may obstruct blood flow through capillary
blood vessels, (iv) stronger responsiveness of endothelial cells to estrogen
and, when stimulated by this hormone, to secrete larger than normally
amounts of basic fibroblast growth factor (bFGF) which could, in endocrine
manner, stimulate de-differentiation and proliferation of endothelial
cells leading to the associated with proliferative state - cell shrinkage
and unveiling of basal membrane.
HYPOTHESIS:
Based on the above, has been proposed (1) that: the activation (phosphorylation)
of hsp27 by mobile phone radiation might be the molecular mechanism regulating
(i) caspase-9-dependent apoptotic pathway and (ii) increase in blood-brain
barrier permeability, which has been observed in some animal experiments.
OBJECTIVE:
To determine whether physiological responses of endothelial cells, which
are associated with the hsp27 phosphorylation and might affect permeability
of blood-brain barrier (stability of stress fibers, cell size/shape),
occur in the mobile phone radiation exposed cultures of human endothelial
cells.
METHODS:
Human endothelial cell line EA.hy926 cells, grown on microscope cover
slides, were exposed for 1h to 900MHz GSM signal at an average SAR of
2W/kg. Temperature of cell cultures remained throughout irradiation period
at 37+0.3 o C thus the effects reported here are of non-thermal nature.
Cells on cover slides were fixed either immediately or 1h after the end
of irradiation. The expression of hsp27 was determined by indirect immunohistochemistry
in order to confirm that the cells respond to irradiation in the same
way as in the previous study (1). The appearance of cells (size, shape)
and stabilization of stress fibers was determined by staining of the cells
with phalloidin that was labeled with fluorescent-dye (AlexiaFluor).
RESULTS AND DISCUSSION:
As expected, 1h exposure of cells to mobile phone radiation increased
expression of hsp27. However, in order to increase hsp27 expression by
heat shock was required 3h incubation of cells at 43 o C (1h exposure
had no effect). This observation, together with the measurements showing
that temperature of medium was throughout RF-EMF exposure period at 37+0.3
o C, suggest that the observed here effects are of non-thermal nature.
The stability of stress fibers, as determined by the pattern of staining
with phalloidin-AlexiaFluor, increased after 1h irradiation and did not
decline during the 1h of post-irradiation incubation (Figure 1 A,B,C).
Induction of the stability of stress fibers caused cells to shrink. As
seen in Figure 1 A,B,C - brightly stained cells with stabilized stress
fibers rounded-up and appeared as smaller than the adjacent non-stained
cells. These rounded-up, stress fibers expressing, cells contacted in-between
only through thin pseudopods (Figure 1D,E,F). The observed hsp27-related
increase in the stability of stress fibers and caused by it changes in
cell shape and size of EA.hy926 cells support the proposed above hypothesis
that the hsp27/p38MAPK stress signaling pathway might be the molecular
mechanism regulating mobile phone radiation-induced permeability of blood-brain
barrier. Furthermore, the possible RF-EMF-induced breakage of the blood-brain
barrier, if occurring repeatedly over a long period of time might become
a health hazard because of the possible extra-capillary accumulation of
molecules that might cause brain tissue damage.
Reference: Leszczynski D. & Joenväärä S. (2002) Differentiation, in press
|
