A small temperature rise may contribute towards the apparent induction by microwaves of heat-shock gene expression in the nematode Caenorhabditis Elegans.
We have previously reported that low intensity microwave exposure (0.75-1.0 GHz CW at 0.5 W; SAR 4-40 mW/kg) can induce an apparently non-thermal heat-shock response in Caenorhabditis elegans worms carrying hsp16-1::reporter genes. Using matched copper TEM cells for both sham and exposed groups, we can detect only modest reporter induction in the latter exposed group (15-20% after 2.5 h at 26 degrees C, rising to approximately 50% after 20 h). Traceable calibration of our copper TEM cell by the National Physical Laboratory (NPL) reveals significant power loss within the cell (8.5% at 1.0 GHz), accompanied by slight heating of exposed samples (approximately 0.3 degrees C at 1.0 W). Thus, exposed samples are in fact slightly warmer (by < or =0.2 degrees C at 0.5 W) than sham controls. Following NPL recommendations, our TEM cell design was modified with the aim of reducing both power loss and consequent heating. In the modified silver-plated cell, power loss is only 1.5% at 1.0 GHz, and sample warming is reduced to approximately 0.15 degrees C at 1.0 W (i.e., < or =0.1 degrees C at 0.5 W). Under sham:sham conditions, there is no difference in reporter expression between the modified silver-plated TEM cell and an unmodified copper cell. However, worms exposed to microwaves (1.0 GHz and 0.5 W) in the silver-plated cell also show no detectable induction of reporter expression relative to sham controls in the copper cell. Thus, the 20% "microwave induction" observed using two copper cells may be caused by a small temperature difference between sham and exposed conditions. In worms incubated for 2.5 h at 26.0, 26.2, and 27.0 degrees C with no microwave field, there is a consistent and significant increase in reporter expression between 26.0 and 26.2 degrees C (by approximately 20% in each of the six independent runs), but paradoxically expression levels at 27.0 degrees C are similar to those seen at 26.0 degrees C. This surprising result is in line with other evidence pointing towards complex regulation of hsp16-1 gene expression across the sub-heat-shock range of 25-27.5 degrees C in C. elegans. We conclude that our original interpretation of a non-thermal effect of microwaves cannot be sustained; at least part of the explanation appears to be thermal.
Dawe
Adam S
AS
Institute of Genetics, School of Biology, University of Nottingham, Nottingham, United Kingdom.
Smith
Brette
B
Thomas
David W P
DW
Greedy
Steve
S
Vasic
Nebojsa
N
Gregory
Andrew
A
Loader
Benjamin
B
de Pomerai
David I
DI
D000818:Animals; D001822:Body Burden; D001831:Body Temperature / Q000502:physiology* / Q000528:radiation effects; D017173:Caenorhabditis elegans / Q000502:physiology* / Q000528:radiation effects*; D029742:Caenorhabditis elegans Proteins / Q000378:metabolism; D004307:Dose-Response Relationship, Radiation; D005786:Gene Expression Regulation / Q000502:physiology* / Q000528:radiation effects; D006360:Heat-Shock Proteins / Q000378:metabolism*; D018869:Heat-Shock Response / Q000502:physiology* / Q000528:radiation effects; D006358:Hot Temperature; D008872:Microwaves; D011829:Radiation Dosage; D012062:Relative Biological Effectiveness; D014916:Whole-Body Irradiation
D029742:Caenorhabditis elegans Proteins; D006360:Heat-Shock Proteins