Expression of cytoskeletal and matrix genes following exposure to ionizing radiation: dose-rate effects and protein synthesis requirements

Experiments were designed to examine the effects of radiation dose-rate and cycloheximide on expression of cytoskeletal and matrix elements in Syrian hamster embryo cells. Results here demonstrated little effect of dose-rate for fission-spectrum neutrons when comparing expression of alpha-tubulin or fibronectin genes. Effects of cycloheximide, however, revealed several findings: (a) Cycloheximide repressed accumulation of alpha-tubulin following exposure to high dose-rate neutrons or gamma-rays. (b) Cycloheximide did not affect accumulation of mRNA for actin genes. (c) Cycloheximide abrogated the moderate induction of fibronectin-mRNA which occurred following exposure to gamma-rays and high dose-rate neutrons. These results suggest a role for labile proteins in the maintenance of alpha-tubulin and fibronectin mRNA accumulation following exposure to ionizing radiation.

Combined effects of ionizing radiation and cycloheximide on gene expression

We performed experiments to determine the effects of ionizing radiation exposure on expression of genes such as beta-actin, c-fos, histone H4, c-myc, c-jun, Rb, and p53 after exposure of Syrian hamster embryo (SHE) cells to the protein synthesis inhibitor cycloheximide. The purpose of these experiments was to determine the role of a labile protein in the radiation-induced response. The results revealed that when ionizing radiation (either fission-spectrum neutrons or gamma rays) was administered 15 min after cycloheximide treatment of SHE cells, the radiation exposure reduced cycloheximide-mediated gene induction of c-fos, histone H4, and c-jun. In addition, dose-rate differences were found when radiation exposure most significantly inhibited the cycloheximide response. Our results suggest that ionizing radiation does not act as a general protein-synthesis inhibitor and that the presence of a labile protein is required for the maintenance of specific gene transcription and mRNA accumulation after radiation exposure, especially at high dose-rates.

Salicylic acid inhibits ultraviolet- and cis-platinum-induced human immunodeficiency virus expression

Previous studies have shown that exposure of HeLa cells stably transfected with an HIV-long terminal repeat-chloramphenicol acetyltransferase (HIV-LTR-CAT) construct to many DNA-damaging agents (such as UV light) induces expression from the HIV LTR. By culturing the cells with salicylic acid we demonstrated dose-dependent repression of this UV-or cis-platinum (cis-Pt)-induced HIV expression. While salicylic acid treatment, indomethacin treatment, UV exposure, or cis-Pt treatment alone decreased viability by up to 50%, equal numbers of viable cells were used for the CAT assays. Repression was evident if salicylic acid was administered 2 h before, at the same time as, or up to 6 h after exposure to the DNA-damaging agent. The kinetics were similar for UV- and for cis-Pt-induced HIV expression, and induction was dependent on the UV dose or cis-Pt concentration added to the culture. pH changes of the media alone in the absence of salicylic acid did not affect HIV expression. Indomethacin (100 microM) did not affect UV- or cis-Pt-induced HIV expression. These results suggest a role for the prostaglandins or the cyclo-oxygenase pathway or both in HIV induction mediated by DNA-damaging agents.

The effects of multiple UV exposures on HIV-LTR expression

Previous studies have shown that cellular stress agents such as UV radiation induce transcription from the long terminal repeat (LTR) of the human immunodeficiency virus (HIV). Using HeLa cells stably transfected with the HIV-LTR sequence, which transcriptionally drives the chloramphenicol acetyl transferase (CAT) reporter gene, we examined the effects of multiple exposures to UVC (254 nm) on HIV-LTR-CAT expression. Low doses (< or = 5 J m-2) had no effect on CAT expression, but up to 29-fold induction was observed with 10 J m-2 when cells were harvested 48 h after completion of the exposure. Little difference was noted in induction levels when cells were exposed to one 25 J m-2 dose, viable cells were harvested at 24 h, 48 h or 72 h, and cell lysates were assayed for CAT expression. Two sequential 12.5 J m-2 exposures, given 24 h apart, resulted in an additive effect on CAT expression; these two exposures produced CAT activity equivalent to that induced following a single 25 J m-2 dose. This additive effect was not evident at the lower doses (< or = 5 J m-2) or at the higher doses. Maximal induction was observed using doses from 25 to 37.5 J m-2. Multiple exposures with either the low (< or = 5 J m-2) or high doses (> 25 J m-2) did not result in an additive effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of gamma rays, ultraviolet radiation, sunlight, microwaves and electromagnetic fields on gene expression mediated by human immunodeficiency virus promoter

Previous work by our group and others has shown the modulation of human immunodeficiency virus (HIV) promoter or long terminal repeat (LTR) after exposure to neutrons and ultraviolet radiations. Using HeLa cells stably transfected with a construct containing the chloramphenicol acetyl transferase (CAT) gene, the transcription of which is mediated by the HIV-LTR, we designed experiments to examine the effects of exposure to different types of radiation (such as gamma rays, ultraviolet and sunlight irradiations, electromagnetic fields and microwaves) on HIV-LTR-driven expression of CAT. These results demonstrated ultraviolet-light-induced transcription from the HIV promoter, as has been shown by others. Exposure to other DNA-damaging agents such as gamma rays and sunlight (with limited exposures) had no significant effect on transcription mediated by HIV-LTR, suggesting that induction of HIV is not mediated by just any type of DNA damage but rather may require specific types of DNA damage. Microwaves did not cause cell killing when cells in culture were exposed in high volumes of medium, and the same cells showed no changes in expression. When microwave exposure was carried out in low volumes of medium (so that excessive heat was generated) induction of HIV-LTR transcription (as assayed by CAT activity) was evident. Electromagnetic field exposures had no effect on expression of HIV-LTR. These results demonstrate that not all types of radiation and not all DNA-damaging agents are capable of inducing HIV. We hypothesize that induction of HIV transcription may be mediated by several different signals after exposure to radiation.

Cytokine and T-cell subset abnormalities in immunodeficient wasted mice

Wasted mice bear an autosomal recessive mutation (wst/wst) that manifests itself in neurologic abnormalities, immunologic deficiency, and faulty DNA repair evident by 21 days of age. The immunodeficiency is characterized by a reduction in the thymus-to-body weight ratio, low levels of IgA plasma cells at secretory sites, and increased sensitivity of T-cells to the killing effects of ionizing radiation. Experiments were designed to examine measures of T-cell activity in wasted mice. The initial experiments established that wst/wst mice have percentages of thymic and splenic Thy1+ cells equivalent to those of control littermates. Further studies of T-cell subpopulations with thymocytes revealed normal percentages of CD4+ and CD8+ cells in wst/wst mice; however, double-labeling experiments showed that CD8+ cells were predominantly CD4- in wst/wst mice, whereas in controls most CD8+ cells also expressed CD4+. Mesenteric lymph node T-cell subpopulations were similar in wasted and control mice. Because cytokines play a significant role in the regulation of the immune response and also interact with a variety of cellular systems, we examined the expression of different cytokine and related genes (IL1, IL2, IL2R, TNF, IL5, gamma-interferon, beta-TGF) in lymphoid tissues from wasted mice as well as from littermate and parental controls. Studies of RNA from lymphoid tissues of wasted mice using dot blot and Northern blot hybridizations revealed a deficiency of IL5 mRNA in thymus and spleen, decreased expression of IL2R in thymus (but not spleen), increased expression of IL1 in spleen (but not thymus), and increased expression of IL2, gamma-interferon, and beta-TGF in both spleen and thymus, relative to controls. Expression of TNF mRNA in lymphoid tissues was unaffected by the wasted mutation. These results suggest a role for cytokine imbalance in the pathogenesis of the immunodeficiency and other abnormalities of wasted mice.

Subnormal albumin gene expression is associated with weight loss in immunodeficient/DNA-repair-impaired wasted mice

OBJECTIVE

Mice bearing the autosomal recessive mutation wst express a disease syndrome of immunodeficiency, neurologic dysfunction, increased sensitivity to the killing effects of ionizing radiation, and dramatic weight loss that begins at 21 days of age and progresses until death at 28-32 days of age. Because of the reported association between abnormal liver status and weight loss, we designed experiments to examine expression of a variety of liver-specific genes in wst/wst mice relative to littermates (wst/.) and parental strain (BCF1) controls.

METHOD

Animals were individually weighed from ages 21-28 days to determine relative weight comparisons between wst/wst mice and controls. Dot blot hybridizations were set up to quantitate the accumulation of transcripts specific for alpha-fetoprotein, albumin and other liver-specific gene products.

RESULTS

These results showed a 67% reduction in albumin mRNA expression in livers derived from wst/wst mice relative to both controls. Expression of alpha-fetoprotein, as well as a variety of other liver-specific genes [secretory component (SC), metallothionein (MT-2), cytochrome P1-450 (Cyt P1-450), transferrin receptor (Tf Rec), tumor necrosis factor (TNF), and immune-associated antigen (Ia)], was unaffected.

CONCLUSIONS

These results suggest a relationship between low albumin expression and wasting syndromes in mice. In addition, our data suggest that the wasted mouse may serve as a unique model for subnormal albumin expression.

Low doses of neutrons induce changes in gene expression

Studies were designed to identify genes induced in fibroblasts after exposure to low-dose neutron radiation but not after gamma rays. Our past work had shown similar modulation of transcripts for alpha-tubulin, beta- and gamma-actins, ornithine decarboxylase and interleukin 1 after exposure to either neutrons or gamma rays. However, differences in the expression of beta-protein kinase C and c-fos genes were observed, with both being induced after exposure to gamma rays but not neutrons. Recently we have identified two genes that are induced after exposure to neutrons but not gamma rays: Rp-8 (a gene associated with apoptosis) and the long terminal repeat (LTR) of the human immunodeficiency virus (HIV). Induction of Rp-8 mRNA was demonstrated in Syrian hamster embryo (SHE) fibroblasts and was found to be induced in cells exposed to neutrons administered at low (0.005 Gy/min) and high dose rate (0.12 Gy/min). No induction of other genes associated with apoptosis such as Rp-2, bcl-2 and Tcl-30 was observed. The induction of transcription from the LTR of HIV was demonstrated in HeLa cells bearing a transfected construct of the chloramphenicol acetyl transferase (CAT) gene driven by the HIV-LTR promoter. Measurements of CAT activity and CAT transcripts after irradiation demonstrated an unresponsiveness to gamma rays over a broad range of doses (0.1-3 Gy). Twofold induction of the HIV-LTR was detected after exposure to neutrons (0.48 Gy) administered at low (0.05 Gy/min) but not high (0.12 Gy/min) dose rates. Ultraviolet-mediated HIV-LTR induction, however, was inhibited by exposure to low-dose-rate neutron irradiation. These results are interesting in light of reports that Rp-8 is induced during apoptosis and that HIV causes apoptosis.

Detection of retinoblastoma gene deletions in spontaneous and radiation-induced mouse lung adenocarcinomas by polymerase chain reaction

A polymerase chain reaction (PCR) technique has been developed to detect deletions in the mouse retinoblastoma gene using histological sections from radiation-induced and spontaneous tumors as the DNA source. Six mouse Rb gene exon fragments were amplified in a 40-cycle, 3-temperature PCR protocol. The absence of any of these fragments relative to control PCR products on a Southern blot indicated a deletion of that portion of the mouse Rb gene. Tumors chosen for analysis were lung adenocarcinomas that were judged to be the cause of death. Spontaneous tumors as well as those from irradiated mice (5.69 Gy 60Co gamma rays or 0.6 Gy JANUS neutrons, which have been found to have approximately equal radiobiological effectiveness) were analyzed for mouse Rb deletions. Tumors in 6 neutron-irradiated mice had no mouse Rb deletions. However, 1 of 6 tumors from gamma-irradiated mice (17%) and 6 of 18 spontaneous tumors from unirradiated mice (33%) showed a deletion in one or both mouse Rb alleles. All deletions detected were in the 5' region of the mouse Rb gene.

Inhibition of human mononuclear cell proliferation, interleukin synthesis, mRNA for IL-2, IL-6, and leukotriene B4 synthesis by a lipoxygenase inhibitor

Human peripheral blood mononuclear cells (PBMCs) were cultured in vitro with various stimuli and in the presence or absence of a 5-lipoxygenase (5-LO) inhibitor, A-63162, to measure its effects on PBMC proliferation, interleukin-2 receptor (IL-2R) expression, interleukin-2 (IL-2) synthesis, interleukin-6 (IL-6) synthesis, and accumulation of messenger RNA for IL-2 or IL-6. A-63162 inhibited PBMC proliferation stimulated by phytohemagglutinin (PHA) and phorbol myristate acetate (PMA) plus A23187, IL-2 receptor expression stimulated by PHA, and IL-2 or IL-6 synthesis induced by PHA plus PMA or PMA plus A23187. At the same concentration, A-63162 inhibited accumulation of mRNA for IL-2 or IL-6 and also inhibited leukotriene B4 (LTB4) synthesis. Our data indicate that the 5-LO inhibitor A-63162 has immunosuppressive activity that may be due to inhibition of LTB4 production or to direct inhibitory actions of A-63162 on IL-2 and IL-6 synthesis.

Cellular proto-oncogene expression following exposure of mice to gamma rays

Many studies have shown the importance of altered cellular proto-oncogene expression in contributing to changes in cell survival, cell transformation, and cell cycle progression. In these experiments we examined the effects of total-body exposure of BCF1 mice to gamma rays (3 Gy) in modulating expression of cellular oncogenes in both gut and liver tissues. We selected specific cellular oncogenes (c-fos, c-myc, c-src, and c-H-ras), based on their normal expression in liver and gut tissues from untreated mice. As early as 5 min following whole-body exposure of BCF1 mice to gamma rays we detected induction of mRNA specific for c-src and c-H-ras in both liver and gut tissues. Accumulation of c-fos-RNA was slightly decreased in gut but was unaffected in liver tissue from irradiated mice relative to untreated controls. Accumulation of c-myc mRNA was unaffected in all tissues examined. These experiments document that modulation of cellular proto-oncogene expression can occur as an early event in tissues following irradiation and suggest that this modulation may play a role in radiation-induced cellular changes.

Expression of cytoskeletal elements in proliferating cells following radiation exposure

Previous work has demonstrated that radiation exposure modulates the expression of a series of genes, including those that encode cytoskeletal elements. The experiments reported here were designed to examine (1) the comparative effects of neutrons administered at high versus low dose-rates, (2) the comparative effects of neutrons on cycling versus resting cells and (3) the comparative effects of neutrons versus gamma-rays on beta- and gamma-actin mRNA accumulation in Syrian hamster embryo (SHE) cells 1 and 3 h post-irradiation. JANUS fission-spectrum neutrons from Argonne National Laboratory's JANUS reactor administered at high (12 cGy/min) dose-rates had little effect on resting cells, but at very low dose-rates (0.1 cGy/min) had a repressive effect on gamma-actin mRNA accumulation. Increased accumulation of beta-actin mRNA was detected following the exposure of cells to neutrons administered at high dose-rates, but repression of beta-actin mRNA was observed when neutrons were administered at low dose-rates. Cycling cells (unexposed and neutron irradiated) in all cases expressed higher levels of all actin-specific mRNAs than resting cells; beta-actin mRNA (but not gamma-actin mRNA) was induced to a greater extent in cycling cells than in resting cells during the first hour following neutron exposure. In resting cells, however, low dose-rate neutrons were more effective than low dose-rate gamma-rays at repressing both gamma- and beta-actin mRNA accumulation. These results demonstrate the differential effects of radiation quality (neutrons versus gamma-rays) and cell-cycle state on the modulation of actin isotype-specific gene expression.

Modulation of expression of virus-like elements following exposure of mice to high- and low-LET radiations

Modulation of virus expression has been reported following exposure to a variety of cellular stresses, including UV radiation and heat-shock. The experiments reported here were designed to examine expression of endogenous VL30 (virus-like 30 S) elements following exposure of whole mice to ionizing radiations. Whole mice were exposed to doses of neutrons (50 cGy) or gamma-rays (300 cGy) shown to be equally efficient in cancer production in the whole animal, and tissues were harvested at 10 and 60 min following completion of the exposure. RNA extracted from these tissues and from tissues of untreated controls was examined for VL30 RNA accumulation by dilution dot blot and Northern blot analyses. These studies revealed that neutrons repressed VL30 RNA accumulation evident within 10 min following exposure in brain, gut, thymus and spleen but not in liver, in which VL30 RNA was unaffected by radiation exposure. During this same time interval, gamma-rays induced VL30 expression in gut and brain and to a lesser extent in liver. These experiments suggest the presence of a differential molecular response following whole-body exposure to high- versus low-LET radiations. In addition, this work demonstrates that ionizing radiations may affect expression of murine endogenous viral sequences.

Enhanced expression of protein kinase C gene caused by solar radiation

Natural solar radiation (5 min of midday exposure in mid July, latitude 42 degrees N) induces protein kinase C mRNA almost two-fold in human epithelioid P3 cells in culture. This response is the same as that following tumor promotion by chemicals. The result indicates a possible role of promotion by solar UV radiation.

Differential modulation of specific gene expression following high- and low-LET radiations

Experiments were designed to examine the effects of radiation quality on specific gene expression within the first 3 h following radiation exposure in Syrian hamster embryo (SHE) cells. Preliminary work demonstrated the induction of c-fos and alpha-interferon genes following exposure to low-linear-energy-transfer (low-LET) radiations (X rays or gamma rays). More detailed experiments revealed induction of c-fos mRNA within the first 3 h following exposure to either X rays (75 cGy) or gamma rays (90 cGy). We could not detect induction of c-fos following exposure of SHE cells to fission-spectrum neutrons (high-LET) from the JANUS reactor administered at either high (12 cGy/min) or low (0.5 cGy/min) dose rates. Expression of alpha-interferon mRNA was similarly induced by low-LET radiations but only modestly by JANUS neutrons. The induction by gamma rays was dose-dependent, while induction by neutrons was specific for low doses and low dose rates. These experiments demonstrate the differential gene inductive response of cells following exposure to high- and low-LET radiations. These experiments suggest that these different qualities of ionizing radiation may have different mechanisms for inducing many of the cellular consequences of radiation exposure, such as cell survival and cell transformation.

Radiation sensitivity of T-lymphocytes from immunodeficient "wasted" mice

Mice with the autosomal recessive gene "wasted" (wst/wst) exhibit neurologic disorders, reduced mucosal immune responses, and abnormal DNA repair mechanisms. The wst/wst mouse has been proposed as a murine model for the human disorder ataxia telangiectasia. Experiments were designed to examine the sensitivity of T-cells from wasted mice to ionizing radiation. Results demonstrated that T-cell clones derived from wasted mice are more sensitive to the killing effects of gamma-rays than similar T-cell clones from control mice. Bulk thymocyte and splenic cell cultures demonstrated similar radiation sensitivity. Both thymic and splenic lymphocytes from wasted mice also expressed low proliferative responses to mitogenic stimulation with concanavalin A (Con A) that could not be attributed to an absence or reduction in T-cell number. However, following activation with Con A, cell cultures exhibited a marked decrease in the percentage of Thyl + cells in wasted mice, in contrast to cultures from control mice in which significant increases in Thyl + cells were observed. Furthermore, when cells were treated with gamma-rays in combination with Con A, Thyl + cells were decreased in control spleen and thymus, but were elevated in similarly treated wasted cultures. These changes were accompanied by an increase in cell volume in T-cells from wasted but not from control mice. These results describe the sensitivity of T-cells from wasted mice to ionizing radiation; in addition, they suggest that the wst/wst abnormality may be associated with cell cycle aberrancies.

Regulation of mitogen-stimulated human T-cell proliferation, interleukin-2 production, and interleukin-2 receptor expression by protein kinase C inhibitor, H-7

Recently published reports suggest that the activation of protein kinase C (PKC) plays an important role in the activation pathway of many cell types. In this study, we examined the role of PKC in human T-cell proliferation, IL-2 production, and IL-2R expression, when cultured with the mitogen PHA, the PKC inhibitor H-7, and H-7 control HA1004. H-7 inhibited the PHA-stimulated [3H]thymidine uptake, IL-2 production, and IL-2R expression in a dose-related manner. Further, we found H-7 inhibited T-cell proliferation, IL-2 production, IL-2 mRNA from PHA plus PMA-stimulated cultures. We also found that H-7 inhibited the early-stage activation of PHA-stimulated cells. The presence of exogenous purified human IL-2 or rIL-4 partly reversed the immunosuppression caused by H-7. In contrast, HA1004 had no effect on cell proliferation, IL-2 production, or IL-2R expression. Our results demonstrate that PKC activation is one major pathway through which T-cells become activated.

Differential effect of ionizing radiation on transcription in repair-deficient and repair-proficient mice

Experiments were designed to examine in vivo changes in total transcription and in the expression of the c-fos gene following whole-body exposure of mice to JANUS fission-spectrum neutrons. Radiation repair-deficient (wst/wst) and -proficient (wst/., C57BL/6 x C3H F1) mice were exposed to JANUS fission-spectrum neutrons calibrated to deliver a gut dose of 50 cGy. Animals were sacrificed less than 10 or at 60 min postirradiation, and gut tissues were removed for study. Our results revealed that, in repair-proficient mice, an immediate depression (relative to untreated control) in total transcription was evident that continued through 1 h postirradiation. Conversely, radiation-sensitive wst/wst mice displayed doubled transcription levels postirradiation. Expression of c-fos was consistently depressed following radiation exposure in control and wst/wst mice. However, the depression of c-fos mRNA was delayed in wst/wst mice relative to controls. These results demonstrate abnormal regulation of transcription and of c-fos mRNA accumulation in repair-deficient wasted mice following exposure to ionizing radiation. In addition, this work documents rapid total transcriptional depression in normal mice following radiation exposure.

Regulation of protein kinase C by ionizing radiation

Members of the protein kinase C (PKC) gene family have been shown to play an important role in tumor promotion and regulation of cell growth. Experiments were designed to examine the effects of different qualities of ionizing radiation administered at a variety of doses and dose rates on the expression of PKC-specific mRNA in confluent Syrian hamster embryo cells. The results of these experiments showed that low-linear energy of transfer (LET) radiations (such as X-rays and gamma-rays) can induce increased expression of PKC mRNA within 1 h after radiation exposure. Levels of expression of PKC mRNA were increased 4- to 6-fold over unirradiated controls. Dose effects were evident, with increased accumulation of PKC mRNA at higher doses (ranging from 6 to 200 cGy). Induction of PKC mRNA occurred at a time when total cellular transcription was reduced following irradiation. Similar exposure of the cells to fission spectrum JANUS neutrons, however, had little effect on PKC mRNA expression. Modest induction (2-fold compared to untreated cells) occurred when irradiations were at very low dose rates (0.5 cGy/min). These results suggest that induction of PKC mRNA may be a step in the transformation process caused by ionizing radiation. In addition, they demonstrate that different qualities of radiation may regulate PKC differently.

Modulation of gene expression in Syrian hamster embryo cells following ionizing radiation

We examined the modulation of gene expression in Syrian hamster embryo (SHE) cells at various times following exposure to low doses of ionizing radiation. Early passage SHE cells were irradiated in plateau phase (greater than 95% G0-G1 cells) with 21-cGy fission-spectrum neutrons, 75-cGy X-rays, or 90-cGy gamma-rays, none of which induced more than 10% loss in cell viability. RNA harvested at various times after exposure was examined for levels of particular RNA species by dot blot and Northern blot hybridizations. Levels of beta-actin-specific RNA decreased within 15 min after exposure of the cells. The kinetics of repression of beta-actin mRNA were similar for all qualities of radiation (X-rays, gamma-rays, and neutrons) for 12 h post-irradiation. Within 1 h after neutron exposure (21 cGy), we observed a decrease in accumulation of RNA species (relative to RNA from nonirradiated cells) encoding the enzyme ornithine decarboxylase; this decrease continued for up to 12 h. Similar results were obtained with gamma- and X-rays. RNA encoding interleukin 1, however, was induced by 3 h after neutron irradiation but reduced to background levels by 7 h. Amounts of rRNA remained constant in all experiments, although total transcription on a per cell basis was reduced within 15 min following irradiation and did not return to normal until 7 h post-irradiation. No alterations, relative to untreated control cells, in overall cell viability or the rate of cell cycle progression were observed in cells either immediately or within 24 h post-irradiation. Our results demonstrate modulation of specific genes following low-dose irradiation. In addition, our findings suggest that some molecular responses to different qualities of ionizing radiation (X-rays, gamma-rays, and neutrons) may be similar.

Effects of ionizing radiation on expression of genes encoding cytoskeletal elements: kinetics and dose effects

We examined the modulation in expression of genes encoding three cytoskeletal elements (beta-actin, gamma-actin, and alpha-tubulin) in Syrian hamster embryo (SHE) cells following exposure to ionizing radiations. Early-passage SHE cells were irradiated in plateau phase with various low doses (12-200 cGy) of neutrons, gamma-rays, or x-rays. RNA samples were prepared from cells at different times postexposure and were analyzed for levels of specific transcripts by northern blots. The results revealed that alpha-tubulin was induced by both high-linear energy of transfer (LET) (neutrons) and low-LET (gamma-rays and x-rays) radiations with similar kinetics. The peak in alpha-tubulin mRNA accumulation occurred between 1 and 3 h postexposure; for gamma-actin mRNA, accumulation was similarly induced. For both gamma-actin and alpha-tubulin, the higher the dose during the first hour postexposure (up to 200 cGy gamma-rays), the greater the level of mRNA induction. In contrast, mRNA specific for beta-actin showed decreased accumulation during the first hour following radiation exposure, and remained low up to 3 h postexposure. These results document the differential modulation of genes specific for cytoskeletal elements following radiation exposure. In addition, they demonstrate a decrease in the ratio of beta-actin:gamma-actin mRNA within the first 3 h following gamma-ray exposure. These changes in mRNA accumulation are similar to those reported in some transformed cell lines and in cells treated with tumor promoters, which suggests a role for changes in actin- and tubulin-mRNA expression in radiation-mediated transformation.

Theiler's virus-induced demyelination in mice immunosuppressed with anti-IgM and in mice expressing the xid gene

Intracerebral infection with Theiler's murine encephalomyelitis virus produces chronic immune-mediated demyelination in susceptible strains of mice. We examined the role of Ig in the pathogenesis of demyelination. In susceptible SJL/J mice (H-2s), suppression of B cell responses with IgG fraction of goat anti-mu (anti-mu IgG) from birth resulted in increased numbers and severity of demyelinating lesions in the spinal cord 35 days after infection. In contrast, treatment of resistant C57BL/10 (H-2b), C57BL/6 (H-2b), or B10.D2 (H-2d) mice with anti-mu IgG had no apparent effect since these mice did not develop demyelination or inflammation in the spinal cord following infection. Similar results were obtained with certain strains of B-cell deficient mice that exhibit the xid gene mutation. Male CBA/NJ (xid) showed increased meningeal inflammation and demyelination compared to male CBA/J mice. However, B6.CBAN, C3.CBAN, or C.CBAn mice showed no or minimal evidence of demyelination despite the presence of the xid mutation. In the SJL/J mouse, the majority of the humoral immune response to virus antigen was restricted to the IgG2b and IgM isotypes. These data indirectly support the hypothesis that immunoglobulins protect partially against development of virus-induced demyelination in susceptible but not resistant animals. In addition, the data argue strongly against the hypothesis that TMEV-induced demyelination is mediated predominantly by humoral autoimmune or humoral viral immune mechanisms.