Transcriptomic Response in the Spleen after Whole-Body Low-Dose X-Ray Irradiation

As the use of medical radiation procedures continues to rise, it is imperative to further our understanding of the effects of this exposure. The spleen is not known as a particularly radiosensitive organ, although its tolerance to radiation is not well understood. Low-dose radiation exposure has been implicated in beneficial responses, particularly in cell death and DNA damage repair. In this study, adult male rats received 2, 20, 200 mGy or 4 Gy whole-body X-ray irradiation and the transcriptional response in the spleen was analyzed at 0.5, 4 and 24 h postirradiation. We analyzed expression of genes involved in apoptosis, cell cycle progression and DNA damage repair. As expected, 4 Gy irradiated animals demonstrated elevated expression of genes related to apoptosis at 0.5, 4 and 24 h postirradiation in the spleen. These animals also showed upregulation of DNA damage repair genes at 24 h postirradiation. Interestingly, the spleens of 20 mGy irradiated animals showed reduced apoptosis and cell cycle arrest compared to the spleens of sham-irradiated animals. These results further reveal that the cellular response in the spleen to whole-body irradiation differs between low- and high-dose irradiation.

The PKZ1 recombination mutation assay: a sensitive assay for low dose studies

The majority of mutation studies are performed at high doses of DNA damaging agents due to the insensitivity of most mutation assays. Extrapolation using a linear no-threshold (LNT) dose response model is then used to estimate the extent of possible DNA damage at lower doses. There is increasing evidence to suggest that the LNT model may not be correct at low doses of at least some DNA damaging agents. The pKZ1 in vivo and in vitro recombination assays have proven to be very sensitive for detection of changes in chromosomal inversion in lymphoid tissue in response to low doses of DNA damaging agents. Non-linear dose response curves for chromosomal inversion as an end-point have been identified at low doses of DNA damaging agents using this assay. Here, we review the inversion results obtained to date with the pKZ1 assays and discuss their suitability for low dose studies.

Effect of exposure to 900 MHz radiofrequency radiation on intrachromosomal recombination in pKZ1 mice

Radiofrequency (RF) radiation emitted from mobile phones is not considered to be directly genotoxic, but it may have downstream effects on cellular DNA. We studied the effect of 4 W/kg pulsed 900 MHz RF radiation on somatic intrachromosomal recombination in the spleen in the pKZ1 recombination mutagenesis model. Somatic intrachromosomal recombination inversion events were detected in spleen tissue of pKZ1 mice by histochemical staining for E. coli beta-galactosidase protein in cells in which the lacZ transgene has undergone an inversion event. pKZ1 mice were exposed daily for 30 min to plane-wave fields of 900 MHz with a pulse repetition frequency of 217 Hz and a pulse width of 0.6 ms for 1, 5 or 25 days. Three days after the last exposure, spleen sections were screened for DNA inversion events. There was no significant difference between the control and treated groups in the 1- and 5-day exposure groups, but there was a significant reduction in inversions below the spontaneous frequency in the 25-day exposure group. This observation suggests that exposure to RF radiation can lead to a perturbation in recombination frequency which may have implications for recombination repair of DNA. The biological significance of a reduction below the spontaneous frequency is not known. The number of mice in each treatment group in this study was small (n = 10 or n = 20). Therefore, repetition of this study with a larger number of animals is required to confirm these observations.

Inversion due to intrachromosomal recombination produced by carcinogens in a transgenic mouse model

Somatic intrachromosomal recombination (SICR) can result in inversions and deletions in the DNA. pKZ1 mice possess an Escherichia coli (E. coli) lacZ transgene which is only expressed after a DNA inversion involving the transgene occurs. The E. coli beta-galactosidase protein can then be detected in frozen tissue sections using a chromogenic substrate. Therefore, pKZ1 mice can be used to detect SICR inversion events in vivo in different tissues. We have tested the pKZ1 mouse for its potential as a general mutagenesis model for detecting SICR in spleen in response to carcinogens which have widely different mechanisms of genotoxicity. Animals were given a single exposure of carcinogen and spleen cells were examined 3 days later for inversion events by histochemical staining of tissue sections. Mitomycin C, X-irradiation, etoposide and methylene chloride caused significant induction of inversion events in spleen tissue, ranging from 1.6- to 4.2-fold induction with the doses used here. This is the first time that inversion events induced by these carcinogens have been specifically studied in vivo in a mouse model and the findings expand the repertoire of mutation events known to be caused by these agents. We suggest that the pKZ1 mouse can be used as a general mutagenesis model for detection of SICR events and is likely to be a useful model for studying the mechanism of SICR in response to DNA damaging agents.

Induction of somatic intrachromosomal recombination inversion events by cyclophosphamide in a transgenic mouse model

Somatic intrachromosomal recombination (SICR) can result in chromosomal inversion and deletion, mechanisms which are important in carcinogenesis. We have utilised a transgenic mouse model to study SICR inversion events in spleen cells. The transgenic construct is designed so that expression of an Escherichia coli lacZ transgene only occurs in a cell when an SICR inversion event occurs in the region of the transgene. The inversion events can then be detected by histochemical staining of frozen spleen sections for transgene expression and by polymerase chain reaction across the inversion breakpoints. The spontaneous inversion frequency in spleen rose 2-fold from 1.54 +/- 0.24 x 10(-4) (mean +/- SE) in 4-month-old transgenic mice to 3.12 +/- 0.67 x 10(-4) in 22-month-old mice. Four- or 8-month-old mice were treated with a single intraperitoneal injection of cyclophosphamide, with doses ranging from 0.01 to 100 mg/kg. The animals were killed 3 days after treatment. A significant induction of SICR inversions was detected at all doses with a 3.2-fold maximum induction of inversions detected at 10 mg/kg. These results suggest that the transgenic mouse model used here may be a sensitive model for studying the role of SICR in mutation and in studying risk assessment of environmental DNA-damaging agents.

Exercise capacity and cardiac function in trained and untrained thyroid-deficient rats

This study was undertaken to ascertain the extent that impairments in a) the oxidative capacity of skeletal muscle, b) cardiac functional and biochemical correlates of contractile capacity, and c) maximal oxygen consumption (VO2 max) can be reversed in thyroid-deficient (TD) female rats subjected to an 8-wk physical training (running) program. Compared with a normal control (NC) group, thyroid deficiency caused reductions in the following: VO2 max (-32%), skeletal muscle homogenate respiratory capacity (-50%), cardiac myosin ATPase (-58%), and in situ-derived ventricular dP/dt max (-58%) (P < 0.001). The training program restored to within normal limits skeletal muscle oxidative capacity and VO2 max, but it did not improve cardiac myosin ATPase, Ca2+ regulation of myofibril ATPase, and dP/dt max relative to TD sedentary rats. However, the heart weight-to-body weight ratio was highest among the three groups in the TD trained group. These findings suggest that maximal oxygen utilization capacity of TD rats can be normalized by physical training, even though intrinsic contractile capacity of the heart could not.

Respiratory capacity and glycogen depletion in thyroid-deficient muscle

This study was undertaken to determine the effects of propylthiouracil-induced thyroid deficiency on a) the capacity of muscle homogenates to oxidize [2-14C]pyruvate and [U-14C]palmitate and b) glycogen depletion during exercise in liver and in fast-oxidative-glycogenolytic (FOG), fast-glycogenolytic (FG), and slow-oxidative (SO) muscle. Relative to the rates for normal rats, oxidation with pyruvate was reduced by 53, 68, and 58%, and palmitate by 40, 50, and 48% in FOG, FG, and SO muscle, respectively (P less than 0.05). Normal rats ran longer than thyroid-deficient rats at 26.7 m/min (87 +/- 8 vs. 37 +/- 5 min). After 40 min of running (22 m/min), the amount of glycogen consumed in normal FOG, FG, and SO muscle and in liver amounted to only 23, 12, 66, and 52%, respectively, of that for their thyroid-deficient counterparts. Also, normal rats maintained higher plasma free fatty acid levels than thyroid-deficient rats during both rest and exercise (P less than 0.05). These findings suggest that thyroid deficiency causes a reduced potential for FFA utilization in skeletal muscle that enhances its consumption of glycogen, thereby limiting endurance capacity.

Substrate oxidation specificity in different types of mammalian muscle

The maximal capacity of homogenates prepared from cardiac, fast-oxidative-glycogenolytic (FOG) vastus lateralis, fast-glycogenolytic (FG) vastus lateralis, and slow-oxidative (SO) soleus muscle to oxidize pyruvate, palmitate, alpha-glycerophosphate, and acetoacetate was assessed by measuring oxygen consumption under conditions of nonlimiting substrate and cofactors. Pyruvate oxidation varied eight-fold among the muscle types and was highest in cardiac, followed by FOG, SO, and FG muscle. Palmitate was oxidized at 97%, 85%, 77%, and 57% of the relative rate for pyruvate in cardiac, FOG, SO, and FG muscle, respectively. In contrast, alpha-glycerophosphate oxidation rates were highest in FG muscle, followed by cardiac, FOG, and SO muscle. Although cardiac muscle possessed the highest absolute rate for acetoacetate oxidation, it had the lowest capacity relative to pyruvate (19%), whereas SO muscle possessed the highest (61%). FOG and FG muscle had similar relative capacities for this substrate (30%). These results provide further evidence to suggest that mammalian muscle types are differentiated in terms of both mitochondrial mass and substrate oxidation specificity.

Effects of thyroid deficiency and sympathectomy on cardiac enzymes

The effects of thyroid deficiency (Td) and of chemical sympathectomy (Sx) were studied on marker enzymes of energy metabolism in cardiac muscle of neonatal and of adult rats. Td prevented the normal development of neonatal body weight, relative heart mass, and cardiac levels of cytochrome c (-22%), citrate synthase (-27%), phosphofructokinase (-20%) and Mg2+- and Ca2+-ATPase activity of purified myofibrils (-33%, -44%). Exogenous thyroxin replacement restored those parameters studied to normal with the exception that it persistently elevated citrate synthase activity significantly above normal control levels. Responses similar to those of Td neonates occurred when adult rats were similarly treated. Sx produced no consistent effects on respiratory and glycogenolytic marker enzymes, but caused a 20% reduction in Ca2+-ATPase activity of both neonatal and adult cardiac myofibrils. These findings suggest that cardiac muscle cells require thyroxin for normal growth and enzyme development. Also, Sx may impair cardiac functional capacity by altering Ca2+ activity of actomyosin ATPase.