Prenatal radiation-induced limb defects mediated by Trp53-dependent apoptosis in mice

We reported previously that in utero radiation-induced apoptosis in the predigital regions of embryonic limb buds was responsible for digital defects in mice. To investigate the possible involvement of the Trp53 gene, the present study was conducted using embryonic C57BL/6J mice with different Trp53 status. Susceptibility to radiation-induced apoptosis in the predigital regions and digital defects depended on both Trp53 status and the radiation dose; i.e., Trp53 wild-type (Trp53(+/+)) mice appeared to be the most sensitive, Trp53 heterozygous (Trp53(+/-)) mice were intermediate, and Trp53 knockout (Trp53(-/-)) mice were the most resistant. These results indicate that induction of apoptosis and digital defects by prenatal irradiation in the later period of organogenesis are mediated by the Trp53 gene. These findings suggest that the wild-type Trp53 gene may be an intrinsic genetic susceptibility factor that is responsible for certain congenital defects induced by prenatal irradiation.     

Influence of prior exposure to low-dose adapting radiation on radiation-induced teratogenic effects in fetal mice with varying Trp53 function

Teratogenesis in tails and limb digits of fetal mice with varying Trp53 status was examined after exposure of pregnant females to 4 Gy gamma radiation with and without a prior 30-cGy exposure. Prior low-dose exposure modified the teratogenic effects of radiation in a manner dependent upon Trp53 status and gestation time. A 4-Gy exposure on gestation day 11 resulted in tail shortening and digit abnormalities. A 30-cGy exposure 24 h prior to a 4-Gy radiation exposure on day 11 reduced the extent of both digit abnormalities and the tail-shortening effects in Trp53(+/+) fetuses and also reduced tail shortening in Trp53(+/-) fetuses, but to a lesser extent. However, the pre-exposure enhanced the tail-shortening effects of 4 Gy in Trp53(-/-) fetuses. In contrast, a 30-cGy exposure given 24 h prior to a 4-Gy exposure on gestation day 12 had no effect on the reduced tail length resulting from the 4-Gy exposure of Trp53(+/+) or Trp53(+/-) fetuses, but it partly protected Trp53(-/-) fetuses against reduced tail length. A 4-Gy exposure alone on day 12 did not result in any increase in the frequency of digit abnormalities in Trp53(-/-) fetuses so any protective effect of the preirradiation could not be detected. However, the preirradiation did result in protection against in digit abnormalities in Trp53(+/-) fetuses. We conclude that radiation-induced teratogenesis reflects both Trp53-dependent and independent processes that lead to apoptosis, and these respond differently to prior adapting doses.     

TP53 and TP53-related genes associated with protection from apoptosis in the radioadaptive response

We investigated the effect of administering priming low-dose radiation prior to high-dose radiation on the level of apoptosis and on the expression of TP53 and TP53-related genes in mouse splenocytes. The percentage of apoptotic cells was significantly lower in TP53(+/+) mice receiving priming radiation 2 to 168 h before the high-dose irradiation, compared to TP53(+/+) mice exposed to 2 Gy alone. In contrast, TP53(+/-) mice exhibited a reduced level of apoptosis only when priming was performed for 2 or 4 h prior to the high-dose irradiation. In TP53(+/+) mice, primed mice had higher TP53 expression than mice exposed to 2 Gy. Phospho-TP53 (ser15/18) expression was the highest in mice exposed to 2 Gy and intermediate in primed mice. Expression of p21 (CDKN1A) was higher in primed mice compared with mice exposed to 2 Gy. MDM2 expression remained at a high level in all mice receiving 2 Gy. Elevated phospho-ATM expression was observed only in mice exposed to 2 Gy. We conclude that TP53 plays a critical role in the radioadaptive response and that TP53 and TP53-related genes might protect cells from apoptosis through activation of the intracellular repair system.     

Radioadaptive response for protection against radiation-induced teratogenesis

To clarify the characteristics of the radioadaptive response in mice, we compared the incidence of radiation-induced malformations in ICR mice. Pregnant ICR mice were exposed to a priming dose of 2 cGy (667 muGy/min) on day 9.5 of gestation and to a challenging dose of 2 Gy (1.04 Gy/min) 4 h later and were killed on day 18.5 of gestation. The incidence of malformations and prenatal death and fetal body weights were studied. The incidence of external malformations was significantly lower (by approximately 10%) in the primed (2 cGy + 2 Gy) mice compared to the unprimed (2 Gy alone) mice. However, there were no differences in the incidence of prenatal death or the skeletal malformations or the body weights between primed and unprimed mice. These results suggest that primary conditioning with low doses of radiation suppresses radiation-induced teratogenesis.     

Adaptive response in embryogenesis: IV. Protective and detrimental bystander effects induced by X radiation in cultured limb bud cells of fetal mice

The radioadaptive response and the bystander effect represent important phenomena in radiobiology that have an impact on novel biological response mechanisms and risk estimates. Micromass cultures of limb bud cells provide an in vitro cellular maturation system in which the progression of cell proliferation and differentiation parallels that in vivo. This paper presents for the first time evidence for the correlation and interaction in a micromass culture system between the radioadaptive response and the bystander effect. A radioadaptive response was induced in limb bud cells of embryonic day 11 ICR mice. Conditioning irradiation of the embryonic day 11 cells with 0.3 Gy resulted in a significant protective effect against the occurrence of apoptosis, inhibition of cell proliferation, and differentiation induced by a challenging dose of 5 Gy given the next day. Both protective and detrimental bystander effects were observed; namely, irradiating 50% of the embryonic day 11 cells with 0.3 Gy led to a successful induction of the protective effect, and irradiating 70% of the embryonic day 12 cells with 5 Gy produced a detrimental effect comparable to that seen when all the cells were irradiated. Further, the bystander effect was markedly decreased by pretreatment of the cells with an inhibitor to block the gap junction-mediated intercellular communication. These results indicate that the bystander effect plays an important role in both the induction of a protective effect by the conditioning dose and the detrimental effect of the challenge irradiation. Gap junction-mediated intercellular communication was suggested to be involved in the induction of the bystander effect.     

Assessing DNA damage and health risk using biomarkers

Increased genomic instability has been found associated with cancer and aging. The p53 tumor suppressor protein is a major determinant of genomic instability as a regulator of cell cycle control and apoptosis in response to DNA damage. To investigate the rate of age-related mutation accumulation in the absence of p53, we crossed Trp53 null mice with transgenic mice harboring a lacZ mutational target gene. In the hybrid animals, lacZ mutation frequencies at early age (i.e. at about 2 months) were found to be the same as in the control lacZ animals. However, up until about 6 months, when the Trp53-knockout mice usually die from cancer, mutations were found to accumulate with age in the spleen, and to a lesser extent in the liver, at a more rapid rate than in the control Trp53(+/+) or Trp53(+/-), lacZ hybrid mice. Treatment of 2-3-month-old Trp53(-/-), lacZ hybrid mice with the powerful mutagen ethyl nitrosourea (ENU) resulted in a higher number of mutations induced in the liver but not in the spleen, as compared to the Trp53(+/+), lacZ mice. These results suggest that p53 is not an important determinant of gene mutation induction, either spontaneously during development or after treatment with a mutagen. The accelerated age-related accumulation of mutations in normal spleen and liver could be explained by the defect in apoptosis, which would prevent severely damaged cells from being eliminated.     

Teratogenic effects of mild heat stress during mouse embryogenesis: effect of Trp53

Hyperthermia can be teratogenic in fetal mice exposed during organogenesis, an effect considered to be due to heat-induced apoptosis of cells in the developing organs. We exposed pregnant mice carrying Trp53(+/+), Trp53(+/-) and Trp53(-/-) fetuses to mild whole-body hyperthermia that raised their core temperature to 40.5 degrees C for 60 min on either day 10 or 11 of gestation. On day 18 of gestation, the fetuses were removed from control and hyperthermia-treated mice and genotyped, and tail length was measured. Limb digits were examined for abnormalities. Tail length in unheated control fetuses was influenced by Trp53 status. A complete lack of functional Trp53 (Trp53(-/-)) but not partial lack of function (Trp53(+/-)) resulted in shorter tails compared to Trp53(+/+) fetuses, indicating a role for Trp53 in the regulation of tail lengthening in mouse fetuses. In all three genotypes, hyperthermia on gestation day 10 resulted in tails shorter than unheated controls, and hyperthermia on day 11 resulted in tails longer than controls. There was no effect on limb digit abnormalities. The data suggest that Trp53-dependent or independent apoptosis may not be directly involved in heat-induced teratogenesis, but that the primary teratogenic effect of heat results from the disruption of another tail length-regulating process that is independent of Trp53. However, the nature of the teratogenic outcome of that disruption depends on the gestation time. The ability of Trp53 to additionally regulate the tail lengthening process was also sensitive to the effects of heat, but that sensitivity again depended on the time of the heat stress during gestation.     

Epithelial cell cycling predicts p53 responsiveness to gamma-irradiation during post-natal mammary gland development

The tumor suppressor gene, TP53, plays a major role in surveillance and repair of radiation-induced DNA damage. In multiple cell types, including mammary epithelial cells, abrogation of p53 (encoded by Trp53) function is associated with increased tumorigenesis. We examined gamma-irradiated BALB/c-Trp53(+/+) and -Trp53(-/-) female mice at five stages of post-natal mammary gland development to determine whether radiation-induced p53 activity is developmentally regulated. Our results show that p53-mediated responses are attenuated in glands from irradiated virgin and lactating mice, as measured by induction of p21/WAF1 (encoded by Cdkn1a) and apoptosis, while irradiated early- and mid-pregnancy glands exhibit robust p53 activity. There is a strong correlation between p53-mediated apoptosis and the degree of cellular proliferation, independent of the level of differentiation. In vivo, proliferation is intimately influenced by steroid hormones. To determine whether steroid hormones directly modulate p53 activity, whole organ cultures of mammary glands were induced to proliferate using estrogen plus progesterone or epidermal growth factor plus transforming growth factor-alpha and p53 responses to gamma-irradiation were measured. Regardless of mitogens used, proliferating mammary epithelial cells show comparable p53 responses to gamma-irradiation, including expression of nuclear p53 and p21/WAF1 and increased levels of apoptosis, compared to non-proliferating irradiated control cultures. Our study suggests that differences in radiation-induced p53 activity during post-natal mammary gland development are influenced by the proliferative state of the gland, and may be mediated indirectly by the mitogenic actions of steroid hormones in vivo.     

Radiation-induced teratogenic effects in fetal mice with varying Trp53 function: influence of prior heat stress

Teratogenesis induced by radiation in fetal mice has been closely linked to Trp53-dependent apoptosis. This study examined teratogenesis in tails and limb digits of fetal mice with varying Trp53 status after a 4-Gy radiation exposure, with and without a prior 40.5 degrees C, 60-min heat stress. Irradiation earlier in gestation (day 11) produced greater effects than later (day 12) exposure, but in both cases the maximum teratogenic effect of radiation occurred in Trp53 normal fetuses, the minimum in Trp53 null fetuses, and intermediate effects in Trp53 heterozygotes, indicating dominance of Trp53-dependent apoptosis. Heat stress 24 h prior to irradiation on day 11 did not alter the teratogenic effects in Trp53 normal or heterozygous fetuses, but it reduced effects in the Trp53 null fetuses. Conversely, heat stress immediately before irradiation on day 11 amplified teratogenesis in Trp53 null fetuses, still with only a small or no effect on fetuses with full or partial Trp53 function, respectively. These results indicate little effect of mild heat on Trp53-dependent apoptosis after irradiation, but they also suggest heat-induced amplification of Trp53-independent processes that led to apoptosis when heat was delivered near the time of radiation exposure, and heat-induced protection of that process when sufficient expression time was allowed. However, Trp53-dependent apoptosis, when functional, acted as the ultimate determinant of radiation-induced teratogenic effects during early organogenesis. On gestation day 12, radiation effects were diminished, but heat stress 24 h prior to radiation exposure had a large amplifying effect in Trp53 normal or heterozygous fetuses. In the absence of functional Trp53, the sensitizing effect of the heat was diminished. The results may suggest that at later times in organ development, DNA repair is more active, allowing some cells to escape radiation-induced Trp53-dependent apoptosis. However, heat may be able to significantly inhibit this active repair and increase the teratogenic effect of radiation. A diminished effect in the absence of functional Trp53 is consistent with an influence of heat on inhibiting DNA repair, but with a diminished probability of apoptosis.     

Biology of marrow stromal cell lines derived from long-term bone marrow cultures of Trp53-deficient mice.

To investigate the effect of Trp53 (formerly known as p53) on stromal cells of the hematopoietic microenvironment, long-term bone marrow cultures were established from mice in which the Trp53 gene had been inactivated by homologous recombination (Trp53(-/-)) or their wild-type littermates (Trp53(+/+)). Long-term bone marrow cultures from Trp53(-/-) mice continued to produce nonadherent cells for 22 weeks, while Trp53(+/+) cultures ceased production after 15 weeks. There was a significant increase in the number of nonadherent cells produced in Trp53(-/-) long-term bone marrow cultures beginning at week 9 and continuing to week 22 (P < 0.02). The Trp53(-/-) cultures also showed significantly increased cobblestone island formation indicative of early hematopoietic stem cell-containing colonies beginning at week 10 (P < 0.01). Cobblestone islands persisted until weeks 15 and 22 in Trp53(+/+) and Trp53(-/-) cultures, respectively. Co-cultivation experiments in which Trp53(+/+) Sca1(+)lin- enriched hematopoietic stem cells were plated on Trp53(-/-) stromal cells showed increased cobblestone island formation compared to Trp53(-/-) Scal+lin- cells plated on Trp53(+/+) or Trp53(-/-) stromal cells. Radiation survival curves for clonal bone marrow stromal cells revealed a similar D0 for the Trp53(+/+) and Trp53(-/-) cell lines (1.62 +/- 0.16 and 1.49 +/- 0. 08 Gy, respectively; P = 0.408), and similar n (8.60 +/- 3.23 and 10.71 +/- 0.78, respectively) (P = 0.491). Cell cycle analysis demonstrated a G2/M-phase arrest that occurred 6 h after irradiation for both Trp53(+/+) and Trp53(-/-) stromal cell lines. After 10 Gy irradiation, there was no significant increase in the frequency of apoptosis detected in Trp53(+/+) compared to Trp53(-/-) marrow stromal cell lines. In the stromal cell lines, ICAM-1 was constitutively expressed on Trp53(+/+) but not Trp53(-/-) cells; however, a 24-h exposure to TNF-alpha induced detectable ICAM-1 on Trp53(-/-) cells and increased expression on Trp53(+/+) cells. To test the effect of Trp53 on the radiation biology of hematopoietic progenitor cells, the 32D cl 3 cell line was compared with a subclone in which expression of an E6 inserted transgene accelerates ubiquitin-dependent degradation of Trp53, thus preventing accumulation of Trp53 after genotoxic stress. The radiation survival curves were similar with no significant difference in the D0 or n, or in the percentage of cells undergoing apoptosis after 10 Gy irradiation between the two cell lines. Cells of the 32D-E6 cell line displayed a G2/M-phase arrest 6 h after 10 Gy, while cells of the parent line exhibited both a G2/M-phase arrest and a G1-phase arrest at 24 and 48 h. The results suggest a complex mechanism of action of Trp53 on the interactions between stromal and hematopoietic cells in long-term bone marrow cultures.     

Trp53 affects the developmental anomaly of clefts of the palate in irradiated mouse embryos but not clefts of the lip with or without the palate

Trp53-deficient mice exhibit increased incidences of developmental anomalies when irradiated, probably due to lack of Trp53-dependent apoptosis. A/J strain-derived CL/Fr mice develop clefts of the lip with or without the palate (CL/P) in approximately one-fifth of the embryos. We produced Trp53-deficient CL/Fr mice and examined the susceptibility to spontaneous development of CL/P and clefts of palate only (CPO), which differ in their developmental mechanisms, CL/P resulting from clefts of the primary palate and CPO from clefts of the secondary palate. The effect of radiation on the two phenotypes was also studied. Unexpectedly, no increase in the frequency of CL/P was observed under either condition, indicating that Trp53 deficiency does not contribute to genesis of CL/P. On the other hand, radiation enhanced the incidence of CPO in Trp53(+/+) embryos but not in Trp53(+/-) and Trp53(-/-/) embryos, suggesting that the absence or presence of only one allele of Trp53 is insufficient to hinder differentiation and proliferation of cells involved in the secondary palate formation. These results indicate that Trp53 function adversely affects the development of CPO when certain damaging agents such as radiation are given.     

Radioadaptive response: Efficient repair of radiation-induced DNA damage in adapted cells

To verify the hypothesis that the induction of a novel, efficient repair mechanism for chromosomal DNA breaks may be involved in the radioadaptive response, the repair kinetics of DNA damage has been studied in cultured Chinese hamster V79 cells with single-cell gel electrophoresis. The cells were adapted by priming exposure with 5 cGy of γ-rays and 4-h incubation at 37°C. There were no indication of any difference in the initial yields of DNA double-strand breaks induced by challenging doses from non-adapted cells and from adapted cells. The rejoining of DNA double-strand breaks was monitored over 120 min after the adapted cells were challenged with 5 or 1.5 Gy, doses at the same level to those used in the cytogenetical adaptive response. The rate of DNA damage repair in adapted cells was higher than that in non-adapted cells, and the residual damage was less in adapted cells than in non-adapted cells. These results indicate that the radioadaptive response may result from the induction of a novel, efficient DNA repair mechanism which leads to less residual damage, but not from the induction of protective functions that reduce the initial DNA damage.     

Adaptive response in embryogenesis: V. Existence of two efficient dose-rate ranges for 0.3 Gy of priming irradiation to adapt mouse fetuses

The adaptive response is an important phenomenon in radiobiology. A study of the conditions essential for the induction of an adaptive response is of critical importance to understanding the novel biological defense mechanisms against the hazardous effects of radiation. In our previous studies, the specific dose and timing of radiation for induction of an adaptive response were studied in ICR mouse fetuses. We found that exposure of the fetuses on embryonic day 11 to a priming dose of 0.3 Gy significantly suppressed prenatal death and malformation induced by a challenging dose of radiation on embryonic day 12. Since a significant dose-rate effect has been observed in a variety of radiobiological phenomena, the effect of dose rate on the effectiveness of induction of an adaptive response by a priming dose of 0.3 Gy administered to fetuses on embryonic day 11 was investigated over the range from 0.06 to 5.0 Gy/min. The occurrence of apoptosis in limb buds, incidences of prenatal death and digital defects, and postnatal mortality induced by a challenging dose of 3.5 Gy given at 1.8 Gy/min to the fetuses on embryonic day 12 were the biological end points examined. Unexpectedly, effective induction of an adaptive response was observed within two dose-rate ranges for the same dose of priming radiation, from 0.18 to 0.98 Gy/ min and from 3.5 to 4.6 Gy/min, for reduction of the detrimental effect induced by a challenging dose of 3.5 Gy. In contrast, when the priming irradiation was delivered at a dose rate outside these two ranges, no protective effect was observed, and at some dose rates elevation of detrimental effects was observed. In general, neither a normal nor a reverse dose- rate effect was found in the dose-rate range tested. These results clearly indicated that the dose rate at which the priming irradiation was delivered played a crucial role in the induction of an adaptive response. This paper provides the first evidence for the existence of two dose-rate ranges for the same dose of priming radiation to successfully induce an adaptive response in mouse fetuses.     

Ionizing radiation down-regulates p53 protein in primary Egr-1-/- mouse embryonic fibroblast cells causing enhanced resistance to apoptosis

In this study, we sought to investigate the mechanism of the proapoptotic function of Egr-1 in relation to p53 status in normal isogenic cell backgrounds by using primary MEF cells established from homozygous (Egr-1(-/-)) and heterozygous (Egr-1(+/-)) Egr-1 knock-out mice. Ionizing radiation caused significantly enhanced apoptosis in Egr-1(+/-) cells (22.8%; p < 0.0001) when compared with Egr-1(-/-) cells (3.5%). Radiation elevated p53 protein in Egr-1(+/-) cells in 3-6 h. However, in Egr-1(-/-) cells, the p53 protein was down-regulated 1 h after radiation and was completely degraded at the later time points. Radiation elevated the p53-CAT activity in Egr-1(+/-) cells but not in Egr-1(-/-) cells. Interestingly, transient overexpression of EGR-1 in p53(-/-) MEF cells caused marginal induction of radiation-induced apoptosis when compared with p53(+/+) MEF cells. Together, these results indicate that Egr-1 may transregulate p53, and both EGR-1 and p53 functions are essential to mediate radiation-induced apoptosis. Rb, an Egr-1 target gene, forms a trimeric complex with p53 and MDM2 to prevent MDM2-mediated p53 degradation. Low levels of Rb including hypophosphorylated forms were observed in Egr-1(-/-) MEF cells before and after radiation when compared with the levels observed in Egr-1(+/-) cells. Elevated amounts of the p53-MDM2 complex and low amounts of Rb-MDM-2 complex were observed in Egr-1(-/-) cells after radiation. Because of a reduction in Rb binding to MDM2 and an increase in MDM2 binding with p53, p53 is directly degraded by MDM2, and this leads to inactivation of the p53-mediated apoptotic pathway in Egr-1(-/-) MEF cells. Thus, the proapoptotic function of Egr-1 may involve the mediation of Rb protein that is essential to overcome the antiapoptotic function of MDM2 on p53.     

Elevated in vivo frequencies of mutant T cells with altered functional expression of the T-cell receptor or hypoxanthine phosphoribosyltransferase genes in p53-deficient mice

We have studied the effects of a defect in the p53 gene on spontaneous and radiation-induced somatic mutation frequencies in vivo by measuring T-cell receptor (TCR) and hypoxanthine phosphoribosyltransferase (HPRT) mutant frequencies (MFs) in p53 deficient mice both before and after exposure to X-irradiation. In the absence of irradiation, the TCR and HPRT mutant frequencies were roughly two-fold higher in p53 null (-/-) mice than in wild-type (+/+) mice. Unexpectedly, the TCR and HPRT MFs were slightly lower in heterozygote p53 (+/-) than in wild-type (+/+) mice, however. After 2 weeks 2Gy whole body irradiation the TCR and HPRT MFs were about two-fold higher in the p53 null (-/-) and p53 (+/-) mice than in the wild-type. Taken together, these findings suggest that a defect in the p53 gene may lead to TCR and HPRT mutants being recovered at higher frequencies in both irradiated and unirradiated mice, but it should be emphasized that the effects we have observed are not particularly strong, albeit that they are statistically significant. Interestingly, several of the highest TCR MF values that we observed in the course of our experiments were recorded in p53 (-/-) animals that had developed thymomas and hence appeared to be cancer prone.     

Effect of low-level radiation on the death of male germ cells

Hormetic and adaptive responses induced by low-level radiation in hematopoietic and immune systems have been observed, as shown by stimulatory effects on cell growth and resistance to subsequent radiation-induced cytogenetic damage. However, in terms of cell death by apoptosis, the effects of low-level radiation are controversial: Some studies showed decreased apoptosis in response to low-level radiation while others showed increased apoptosis. This controversy may be related to the radiation doses or dose rates and also, more importantly, to the cell types. Testes are one of the most radiosensitive organs. The loss of male germ cells after exposure to ionizing radiation has been attributed to apoptosis. In the present study, the effects of low-level radiation at doses up to 200 mGy on mouse male germ cells in terms of apoptosis and the expression of apoptosis-related proteins were examined at different times after whole-body exposure of mice to low-level radiation. In addition, the effect of pre-exposure to low-level radiation on subsequent cell death induced by high doses of radiation was examined to explore the possibility of low-level radiation-induced adaptive response. The results showed that low-level radiation in the dose range of 25-200 mGy induced significant increases in apoptosis in both spermatogonia and spermatocytes, with the maximal effect at 75 mGy. The increased apoptosis is most likely associated with Trp53 protein expression. Furthermore, 75 mGy low-level radiation given pre-irradiation led to an adaptive response of seminiferous germ cells to subsequent high-level radiation-induced apoptosis. These results suggest that low-level radiation induces increased apoptosis in male germ cells but also induces a significant adaptive response that decreases cell death after a subsequent high-dose irradiation.     

Bcl-2 mediates sex-specific differences in recovery of mice from LPS-induced signs of sickness independent of IL-6.

Chronic pulmonary diseases are more common in boys than in girls. Therefore, we investigated the differences in signs of sickness in male and female mice that were exposed to lipopolysaccharide (LPS) by intranasal instillation. Because apoptosis is important in the resolution of inflammation, we tested the hypothesis that reduced levels of Bcl-2, a regulator of apoptosis, may play a role in gender-specific differences in response to inflammation. Bcl-2 wild-type (+/+) female mice recovered from an LPS-induced drop in body temperature and loss in body weight significantly faster than male (+/+) mice. Female heterozygous (+/-) mice showed reduced Bcl-2 levels and exhibited a slower recovery than female (+/+) mice that was similar to the recovery pattern in male (+/+) and (+/-) mice. Interleukin-6 (IL-6) activity levels in the bronchoalveolar lavage fluid were higher in male than in female mice but were not different between (+/+) and (+/-) mice. We conclude that Bcl-2 plays a role in mediating the faster recovery of female (+/+) mice from LPS-induced signs of sickness independent of IL-6. These studies indicate that apoptotic mechanisms may be involved in gender-specific differences in chronic pulmonary diseases.