Silica and PM1648 modify human alveolar macrophage antigen-presenting cell activity in vitro.

Some inhaled particles are known to lead to inflammation and lung pathology, whereas others do not appear to have long-term effects. Potential mechanisms to account for these differences are only beginning to be understood. In this article we examine whether silica and PM1648 (a model urban particulate) caused selective deletion of the suppressor human alveolar macrophage (HAM) phenotype (RFD1+/7+), and whether this affected cytokine production in an antigen-presenting cell (APC) assay with autologous T lymphocytes. HAM were exposed to the bioactive particulates, silica and PM1648, for 24 hours, then isolated free of extracellular particulates and nonviable cells; HAM were then cultured with autologous lymphocytes in an 11-day APC assay. Silica exposure up-regulated a TH1 lymphocyte-derived cytokine, interferon gamma (IFN-gamma), and a TH2 lymphocyte-derived cytokine, interleukin-4 (IL-4). PM1648 exposure primarily upregulated IL-4. Neither particle exposure had a significant effect on interleukin-10 (IL-10) production. Control particulate exposures with titanium dioxide (TiO2) and wollastonite (Woll) caused no altered APC activity. Silica and PM1648 demonstrated selective toxicity to suppressor macrophages (RFD1+/7+). We propose that, because of the suppressor macrophage phenotype disabling, the activator macrophage (RFD1+/7-) operates free of the suppressor macrophage's influence, enhancing APC activity with increased lymphocyte-derived proinflammatory cytokine production.

Cell surface regulation of silica-induced apoptosis by the SR-A scavenger receptor in a murine lung macrophage cell line (MH-S).

Scavenger receptors (SR) are responsible for recognition of ligands as diverse as oxidized LDL (endogenous) to respirable particulates (exogenous). A number of recent studies have suggested that these SR ligands induce apoptosis of macrophages. However, the mechanism by which SR triggers apoptosis is not understood. This study used a murine alveolar macrophage cell line (MH-S) to investigate the role of the SR in caspase activation. The presence of SR on MH-S cells was confirmed by FACS analysis and was similar to the distribution found on murine alveolar macrophages. The activity of caspases 1, 3, and 6 was measured following a 6-h exposure to crystalline silica with and without blockers of the SR. Caspase activities were determined by hydrolysis of specific chromogenic substrates and formation of an active enzymatic form (Western for active caspase 3). Silica stimulated significant caspase activity, apoptosis, and necrosis of MH-S cells, which was attenuated by 2F8 (a blocking antibody) and polyinosinic acid (a nonspecific SR antagonist). The results indicate that the SR are necessary for caspase activation and subsequent apoptosis (as well as necrosis) caused by silica in macrophage cells.

Class A type II scavenger receptor mediates silica-induced apoptosis in Chinese hamster ovary cell line.

Macrophage scavenger receptors are known to bind endotoxins, oxidized low-density lipoproteins (Ox-LDL), and other proteins with clustered negative charges. Recent evidence indicates some particulates may also bind to the scavenger receptor and initiate apoptosis. In this study, chinese hamster ovary (CHO) cells stabily transfected with the murine class A type II scavenger receptor (SR-A II) were exposed to crystalline silica to examine the role of this receptor in apoptosis. In a 24-h culture, silica (250 microg/ml) induced significant cell injury (necrosis and apoptosis) in transfected cells (MSR II) but not in the control cells (KA-7). This effect was specific to silica, as a control particle titanium dioxide had no cytotoxic effects on the MSR II cells at equal particle mass concentrations. Furthermore, silica-induced apoptosis in the MSR II cells could be eliminated by preincubating the cells with SR-A II antagonists: polyinosinic acid or maleylated bovine serum albumin. This study further supports the hypothesis that the SR-A II is directly involved with silica toxicity and that certain scavenger receptor ligands may have an important role in regulating macrophage apoptosis.

Protection against ozone-induced pulmonary inflammation and cell death by endotoxin pretreatment in mice: role of HO-1.

Ozone is a ubiquitous air pollutant that can cause acute pulmonary inflammation and cell injury and may contribute to the exacerbation of chronic pulmonary diseases. The molecular mechanisms of ozone-induced cell injury, as well as protective mechanisms against ozone-injury, are not well understood. Since ozone is a reactive oxidant, and heme oxygenase-1 (HO-1) is an antioxidant enzyme induced by many oxidative stimuli, we hypothesized that HO-1 is one of the protective mechanisms against ozone-induced cell injury, as well as pulmonary inflammation. In the current study, C57Bl/6 mice were pretreated with a low level of endotoxin (lipopolysaccharide, LPS) (0.5 mg/kg) to induce HO-1, and 16 h later were exposed to 1 ppm ozone for 3 h. Endotoxin pretreatment caused a significant protection against ozone-induced pulmonary inflammation and cell injury in bronchoalveolar lavage (BAL) cells. The protection by endotoxin pretreatment against ozone-induced inflammation and necrosis in BAL cells was abolished by the cotreatment with a heme oxygenase inhibitor, tin protoporphyrin IX dichloride (SnPP), suggesting that HO-1 is responsible for the protection against ozone-induced pulmonary inflammation and BAL cell necrosis. Therefore, since HO-1 is induced following ozone exposure, HO-1 may contribute to the development of cellular adaptation to chronic ozone exposure.

Effect of acrolein on human alveolar macrophage NF-kappaB activity.

Acrolein is an environmental pollutant that is known to suppress respiratory host defense against infections; however, the mechanism of the decrease in host defense is not yet clear. We have previously reported that acrolein inhibited endotoxin-induced cytokine release and induced apoptosis in human alveolar macrophages, suggesting that the inhibition of cytokine release and/or cytotoxicity to alveolar macrophages may, in part, be responsible for acrolein-induced immunosuppression in the lung. Because nuclear factor-kappaB (NF-kappaB) is an important transcription factor for a number of cytokine genes and is also an important regulator of apoptosis, the effect of acrolein on NF-kappaB activity was examined by electrophoresis mobility shift assay. Acrolein caused a dose-dependent inhibition of endotoxin-induced NF-kappaB activation as well as an inhibition of basal level NF-kappaB activity. Because IkappaB is a principal regulator of NF-kappaB activity in the nucleus, changes in IkappaB were determined by Western blotting. Acrolein-inhibited IkappaB phosphorylation leads to an increase in cellular IkappaB levels preventing NF-kappaB nuclear translocation and is likely the mechanism of acrolein-induced inhibition of NF-kappaB activity. The role of basal level NF-kappaB in acrolein-induced apoptosis was also examined. An NF-kappaB inhibitor (MG-132) also induced apoptosis in human alveolar macrophages, suggesting that a certain basal level NF-kappaB activity may be required for macrophage cell survival. Taken together, our results suggest that the acrolein-inhibited endotoxin-induced NF-kappaB activation decreased the basal level NF-kappaB activity, which may be responsible for the inhibition of cytokine release and the induction of apoptosis in human alveolar macrophages.

Alveolar macrophage apoptosis and TNF-alpha, but not p53, expression correlate with murine response to bleomycin.

Apoptosis is considered to be a protective mechanism that limits lung injury. However, apoptosis might contribute to the inflammatory burden present in the injured lung. The exposure of mice to bleomycin (BLM) is a well-established model for the study of lung injury. BLM exposure induces DNA damage and enhances tumor necrosis factor (TNF)-alpha expression in the lung. To evaluate the importance of alveolar macrophage (AM) apoptosis in the pathogenesis of lung injury, we exposed BLM-sensitive (C57BL/6) and BLM-resistant (BALB/c) mice to BLM (120 mg/kg) and studied the induction of apoptosis [by light-microscopy changes (2, 8, 12, 24, 48, and 72 h) and annexin V uptake by flow cytometry (24 h)], the secretion of TNF-alpha (measured by ELISA), and the expression of p53 (by immunoblotting) in AM retrieved from these mice. BLM, but not vehicle, induced apoptosis in AM from both murine strains. The numbers of apoptotic AM were significantly greater (P < 0.001) in C57BL/6 mice (52.9%) compared with BALB/c mice (40.8%) as demonstrated by annexin V uptake. BLM induction of apoptosis in AM was preceded by an increased secretion of TNF-alpha in C57BL/6 but not in BALB/c mice. Furthermore, double TNF-alpha receptor-deficient mice, developed on a C57BL/6 background, demonstrated significantly (P < 0.001) lower numbers of apoptotic AM compared with C57BL/6 and BALB/c mice. BLM also enhanced p53 expression in AM from both murine strains. However, p53-deficient mice developed BLM-induced lung injury, exhibited similar lung cell proliferation (measured as proliferating cell nuclear antigen immunostaining), and accumulated similar amounts of lung hydroxyproline (65 +/- 6.9 microgram/lung) as did C57BL/6 (62 +/- 6.5 microgram/lung) mice. Therefore, AM apoptosis is occurring during BLM-induced lung injury in a manner that correlates with murine strain sensitivity to BLM. Furthermore, TNF-alpha secretion rather than p53 expression contributes to the difference in murine strain response to BLM.tumor necrosis factor; strain susceptibility

Expression of TNF and the necessity of TNF receptors in bleomycin-induced lung injury in mice.

Bleomycin (BLM) induction of lung fibrosis in mice is an established model to study the mechanism of pulmonary fibrosis. Cytokine secretion has been implicated as a fundamental component of the lung fibrotic process observed in response to BLM. Among the cytokines implicated in lung fibrosis, Tumor necrosis factor (TNF) alpha has been considered to play a fundamental role. In the present study, we characterized the cellular sources of TNF during BLM-induced lung injury and examined the importance of TNF receptors in this process. To characterize the expression of TNF, we utilized two strains of mice, one sensitive (C57BL/6) and one resistant (BALB/c) to BLM-induced lung injury. Mice received BLM (120 mg/kg total) or saline, as control, by multiple subcutaneous injections. BLM induced the development of inflammation in subpleural areas only in the lungs of BLM-sensitive mice. These subpleural areas were characterized by infiltration of CD68-positive macrophages and increased collagen deposition. BLM enhanced the expression of TNF mRNA in BLM-sensitive, but not in BLM-resistant, mice. In situ hybridization studies localized the expression of TNF in the areas of BLM-induced inflammation in 6% and 27% of macrophages at 14 and 21 days post BLM treatment. In addition to TNF, BLM exposure resulted in the upregulated expression of transforming growth factor (TGF)-beta 1, but not interleukin (IL)-1, mRNA in the lungs of both murine strains at 14 and 21 days. This upregulated expression of TGF-beta 1 mRNA was greater in the lungs of BLM-sensitive mice. In separate experiments, double TNF receptor knockout mice were exposed to BLM. These animals demonstrated an increased expression of TNF, but not TGF-beta 1, mRNA in response to BLM and did not exhibit histologic evidence of lung injury following BLM exposure. In summary, the upregulation of TNF mRNA in macrophages correlated with the appearance of inflammation following BLM exposure and was limited to the BLM-sensitive strain. Furthermore, in addition to the release of the TNF ligand, it appears that the presence of TNF receptors is necessary for the development of BLM-induced lung injury, and signaling through these receptors may contribute to the regulation of the TGF-beta 1 mRNA expression observed in response to bleomycin. These results provide further support for a role of macrophages and TNF in the induction of lung inflammation.

Potential involvement of 4-hydroxynonenal in the response of human lung cells to ozone.

Ozone is a photochemically generated pollutant that can cause acute pulmonary inflammation and induce cellular injury and may contribute to the development or exacerbation of chronic lung diseases. Despite much research, the mechanisms of ozone- and oxidant-induced cellular injury are still uncertain. Ozone and secondary free radicals have been reported to cause the formation of aldehydes in biological fluids. One of the most toxic aldehydes formed during oxidant-induced lipid peroxidation is 4-hydroxynonenal (HNE). HNE reacts primarily with Cys, Lys, and His amino acids, altering protein function and forming protein adducts. The purpose of this study was to determine whether HNE could account for the acute effects of ozone on lung cells. Human subjects were exposed to 0.4 parts/million ozone or air for 1 h with exercise (each subject served as his/her own control). Six hours after ozone exposure, cells obtained by airway lavage were examined for apoptotic cell injury, and cells from bronchoalveolar lavage were examined for apoptosis, presence of HNE adducts, and expression of stress proteins. Significant apoptosis was evident in airway lung cells after ozone exposure. Western analysis demonstrated an increase in a 32-kDa HNE protein adduct and a number of stress proteins, viz., 72-kDa heat shock protein and ferritin, in alveolar macrophages (AM) after ozone exposure. All of these effects could be replicated by in vitro exposure of AM to HNE. Consequently, the in vitro results and demonstration of HNE protein adducts after ozone exposure are consistent with a potential role for HNE in the cellular toxic effects of ozone.

Urban particle-induced apoptosis and phenotype shifts in human alveolar macrophages.

Epidemiological studies report a small but positive association between short-term increases in airborne particulate matter and small increases in morbidity and mortality from respiratory and cardiovascular disease in urban areas. However, the lack of a mechanistic explanation to link particle exposure and human health effects makes it difficult to validate the human health effects. The present study tested the hypothesis that urban particles could cause apoptosis of human alveolar macrophages(AM) and a shift of their phenotypes to a higher immune active state, which would provide a mechanism to explain an inflammatory response. Freshly isolated human AM were incubated for 24 hr with urban particles (#1648 and #1649), Mount Saint Helen's ash (MSH), and residual oil fly ash (ROFA). Cell viability was assessed by trypan blue exclusion and apoptosis was demonstrated by morphology, cell death ELISA, and DNA ladder formation. Additionally, AM were characterized according to RFD1(+) (immune stimulatory macrophages) and RFD1(+)7(+) (suppressor macrophages) phenotypes by flow cytometry. ROFA particles caused AM necrosis at concentrations as low as 10 microg/ml, urban particles had no effect except at 200 microg/ml, and MSH had no effect at 200 microg/ml. ROFA (25 microg/ml) and particles #1648 or #1649 (100 microg/ml) caused apoptosis of AM by all three criteria, but 200 microg/ml MSH had no effect. Finally, 25 microg/ml ROFA and 100 microg/ml particles #1648 or #1649 up regulated the expression of the RFD1(+) AM phenotype, while only ROFA decreased the RFD1(+)7(+) phenotype. Consequently, ROFA and urban particles can induce apoptosis of human AM and increase the ratio of AM phenotypes toward a higher immune active state (i.e., increased RFD1(+):RFD1(+)7(+) ratio). Ifurban particles cause similar changes in vivo, this could result in lung inflammation and possible increased pulmonary and cardiovascular disease.

Asbestos and silica-induced changes in human alveolar macrophage phenotype.

The mechanism by which fibrogenic particulates induce inflammation that can progress to lung fibrosis is uncertain. The alveolar macrophage (AM) has been implicated in the inflammatory process because of its function and reported release of inflammatory mediators when isolated from fibrotic patients. It has been recently shown that fibrogenic, but not nonfibrogenic, particulates are highly potent in inducing apoptosis of human AM. In this study, we tested the hypothesis that fibrogenic particulates could shift the phenotypic ratio of human AM to a more inflammatory condition. The macrophage phenotypes were characterized by flow cytometry targeting the RFD1 and RFD7 epitopes. Results demonstrated that chrysotile and crocidolite asbestos, as well as crystalline silica, but not titanium dioxide or wollastonite, increased the RFD1+ phenotype (inducer or immune activator macrophages) and decreased the RFD1+ RFD7+ phenotype (suppressor macrophages). These results provide a mechanistic explanation that may link apoptosis (namely, suppressor macrophages) to a shift in the ratio of macrophage phenotypes that could initiate lung inflammation.

Acrolein-induced cell death in human alveolar macrophages.

Acrolein is an environmental air pollutant that is known to suppress respiratory host defense against infections. The mechanism of the decrease in host defense is not yet clear. In this study, the effects of acrolein on human alveolar macrophages and their function were examined. Acrolein caused dose-dependent cytotoxicity to alveolar macrophages as demonstrated by the induction of apoptosis and necrosis. In addition, at lower doses, acrolein caused induction of heme oxygenase 1 protein; however, stress protein 72 (SP72) was not induced. These findings demonstrated that acrolein caused a dose-dependent selective induction of a stress response, apoptosis, and necrosis in human alveolar macrophages. Macrophage function was assessed by release of cytokines in response to endotoxin stimulation. Acrolein caused a dose-dependent inhibition of release of IL-1beta, TNF-alpha, and IL-12. The inhibition of cytokine release and cytotoxicity to alveolar macrophages may in part be responsible for acrolein-induced immunosuppression of the lung.

4-Hydroxynonenal inhibits interleukin-1 beta converting enzyme.

Lipid peroxidation results from the interaction of reactive oxygen species and polyunsaturated fatty acids. Metabolites generated from oxidative stress play an important role in the pathogenesis of a variety of diseases and biologic processes. One such product generated from lipid peroxidation in 4-hydroxynonenal (HNE). HNE is thiol reactive and exhibits numerous cellular effects. In this study, the inhibition of the cysteine protease, interleukin-1 beta (IL-1 beta) converting enzyme (ICE), by HNE in human blood mononuclear cells was investigated. HNE blocked the release of lipopolysaccharide (LPS)-stimulated IL-1 beta (EC50 5 microM) and IL-10 (EC50 2 microM) in a dose-dependent manner and, to a lesser extent, tumor necrosis factor-alpha (TNF-alpha) (EC50 15 microM) release. However, LPS-stimulated elevation of intracellular proIL-1 beta levels was not affected by HNE treatment. HNE inhibited ICE activity in lysed cells in a similar dose-dependent manner, measured by hydrolysis of the fluorogenic substrate YVAD-AMC and recombinant proIL-1 beta. To confirm that the inhibition of ICE activity by HNE was not an indirect effect, ICE activity was examined using purified recombinant human ICE (rHu-ICE). HNE inhibited rHu-ICE activity in a dose-dependent manner. Thus, low levels of HNE can suppress mononuclear cell release of IL-1 beta, probably by interacting with the active site cysteine of ICE. These results have implications for modulating mononuclear cell function during oxidative stress conditions.

Asbestos induces apoptosis in human alveolar macrophages.

Asbestos refers to a group of fibrous minerals implicated in the development of several lung diseases, including fibrosis (asbestosis), cancer, and malignant mesothelioma. Although major health risks exist in occupationally exposed individuals, low-level exposures of asbestos may still contribute to health problems. The mechanism by which asbestos causes lung disease is not clearly understood but has been proposed to involve the alveolar macrophage (AM). We propose that asbestos induces apoptosis of AM, resulting in the development of an inflammatory state. In this study, we examined two forms of asbestos, chrysotile (CHR) and crocidolite (CRO), along with a control fiber, wollastonite (WOL), to characterize their relative cytotoxicity and ability to stimulate apoptosis in vitro. AM were cultured for 24 h with these particulates and examined for cell viability (trypan blue exclusion) and apoptosis (morphology, levels of cytosolic oligonucleosomal DNA fragments, and DNA ladder). In the absence of a decrease in cell viability, both CHR and CRO produced changes in cell morphology consistent with apoptosis. In addition, levels of cytoplasmic oligonucleosomal DNA (Cell Death Detection enzyme-linked immunosorbent assay) were significantly enhanced for CHR (3-25 micrograms/ml) and CRO (25-75 micrograms/ml) in a dose-dependent manner (a process that was inhibitable by 10 microM Z-Val-Ala-Asp fluoromethyl ketone, an interleukin-converting enzyme inhibitor). In contrast, WOL (up to 400 micrograms/ml) produced no significant DNA fragmentation in a 24-h culture. Neither CHR nor CRO caused DNA ladder formation in 24-h cell cultures. However, in 48-h cell cultures, both CHR- and CRO-exposed cells, but not WOL, resulted in the formation of DNA ladders characteristic of apoptosis. In summary, these results suggest that, unlike nonfibrogenic particulates, low doses of asbestos fibers cause apoptosis in cultured human AM that may be an early step in the development of lung fibrosis.

Silica-induced apoptosis mediated via scavenger receptor in human alveolar macrophages.

Exposure to silica dust can result in lung inflammation that may progress to fibrosis, for which there is no effective clinical treatment. The mechanisms involved in the development of pulmonary silicosis have not been well defined; however, most current evidence implicates a central role for alveolar macrophages (AM) in this process. We propose that the fibrotic potential of a particulate depends upon its ability to cause apoptosis in AM. In this study, human AM were treated with fibrogenic, poorly fibrogenic, and nonfibrogenic model particulates, such as silica (133 micrograms/ml), amorphous silica (80 micrograms/ml), and titanium dioxide (60 micrograms/ml), respectively. Cell were treated with these particulates in vitro for 6 and 24 hr and examined for apoptosis by morphological analysis, DNA fragmentation, and levels of cytosolic histone-bound DNA fragments (cell death ELISA assays). Treatment with silica resulted in morphological changes typical of apoptotic cells, enhanced DNA fragmentation (a characteristic feature of programmed cell death), and significant alveolar macrophage apoptosis as observed by cell death ELISA assays. In contrast, amorphous silica and titanium dioxide demonstrated no significant apoptotic potential. To elucidate the possible mechanism by which silica causes apoptosis, we investigated the role of the scavenger receptor (SR) in silica-induced apoptosis. Cells were pretreated with and without SR ligand binding inhibitor, polyinosinic acid (poly(I), 500 micrograms/ml), for 10 min prior to silica treatment. Pretreatment with poly(I) resulted in complete inhibition of silica-induced apoptosis as measured by cell death ELISA. Further, we examined the involvement of interleukin-converting enzyme (ICE) in silica-mediated apoptosis using an ICE inhibitor, Z-Val-Ala-Asp-fluoromethyl ketone. Z-Val-Ala-Asp-fluoromethyl ketone inhibited silica-induced apoptosis and IL-1 beta release. These results suggest that fibrogenic particulates, such as silica, caused apoptosis of alveolar macrophages and that this apoptotic potential of fibrogenic particulates may be a critical factor in initiating an inflammatory response resulting in fibrosis. Additionally, silica-induced apoptosis of alveolar macrophages may be due to the interaction of silica particulates with the SR, initiating one or a number of signaling pathways involving ICE, ultimately leading to apoptosis.

4-Hydroxynonenal mimics ozone-induced modulation of macrophage function ex vivo.

Ozone is a ubiquitous pollutant that can cause acute pulmonary inflammation, cellular injury and may contribute to the development or exacerbation of chronic lung diseases. Despite much research, the effects of ozone on humans and potential cellular mechanisms of injury are still uncertain. However, ozone has been reported to increase the formation of aldehydes that could react with cellular proteins. Therefore, the purpose of these studies was to determine whether 4-hydroxynonenal (HNE), a previously unidentified aldehyde product of ozone exposure, is formed in human subjects exposed to ozone, and whether the response of human alveolar macrophages (AM) following a 1-h exposure to 0.25 ppm ozone with moderate exercise could be mimicked by in vitro incubation of AM with HNE. Western analysis demonstrated increased HNE protein adducts in airway fluid and alveolar macrophages after ozone exposure. AM were examined for endotoxin (lipopolysaccharide [LPS])-stimulated interleukin-1 beta (IL-1 beta) release and expression of heat shock protein 72 (HSP72). Immediately after ozone exposure there was no change in HSP72, but a 5-fold increase occurred 4 h after exposure. By 18 h after exposure, HSP72 levels decreased to below comparable air-exposed levels. Immediately after ozone exposure there was no effect on IL-1 beta release stimulated by LPS. However, IL-1 beta release stimulated by LPS was significantly inhibited 4 h after ozone exposure. By 18 h after ozone exposure, IL-1 beta release stimulated by LPS returned to normal. Incubation of human AM in vitro with HNE induced HSP72 and blocked LPS-stimulated IL-1 beta release possibly by inhibiting interleukin converting enzyme. Consequently, the in vitro results and demonstration of HNE protein adducts following ozone exposure are consistent with HNE being involved in this process in vivo and suggest that the cellular toxic effects of ozone could be a result of thiol reactive aldehydes produced by ozone.

4-Hydroxynonenal-induced cell death in murine alveolar macrophages.

Oxidative stress is known to cause apoptosis in many cell types, yet the mechanism of oxidative stress-induced apoptosis is not clear. Oxidative stress has been described to cause peroxidation of polyunsaturated fatty acids. 4-Hydroxynonenal (HNE) is a diffusible product of lipid peroxidation and has been shown to be toxic to cells. In this study, the effects of HNE on isolated alveolar macrophages (AM) from two murine strains (C3H/HeJ and C57BL/6J) were examined. HNE induced the formation of protein adducts in AMs from both strains of mice in a dose-dependent manner, and the amounts of HNE-protein adducts formed in cells from both strains were very similar. In the HNE dose range from 1 to 100 microM, AMs from both strains had very little necrosis as shown by trypan blue staining. However, AMs from both C3H/HeJ and C57BL/6J mice had extensive apoptosis at 100 microM HNE, but little or no apoptosis at 25 microM HNE. Furthermore, AMs from C57BL/6J mice had significant apoptosis at 50 microM HNE while AMs from C3H/HeJ mice had no significant apoptosis at this dose. At low doses of HNE (10 to 25 microM), there was induction of heme oxygenase 1. The data indicated that HNE induces apoptosis in murine macrophages, and cells from different strains of mice have different sensitivities to the HNE-induced apoptosis. The cause of the difference in susceptibility is not known, but it is possible that different stress response and/or apoptosis-regulating proteins may be in part responsible. Our observation that a product of lipid peroxidation causes apoptosis suggested that it might be a mediator for oxidative stress-induced apoptosis.

Bleomycin induces apoptosis in human alveolar macrophages.

Bleomycin (BLM) is an effective antineoplastic drug; however, cumulative dosage is often associated with inflammation that can progress to pulmonary fibrosis. The mechanisms by which this occurs are not understood, but they have been proposed to involve the alveolar macrophage (AM). In this study, we examined the in vitro effects of BLM on human AM cytotoxicity and the role of heat shock proteins (HSP or stress proteins) in this process. Although BLM did not cause marked necrosis, it caused significant DNA fragmentation detected by in situ DNA labeling and confirmed by BLM-induced DNA ladder formation after 24 h. The DNA fragmentation was significantly blocked by 10 and 50 microM ZnCl2, suggesting that BLM was inducing apoptosis. BLM did not alter intracellular protooncogene bcl-2 or glutathione levels. However, BLM significantly (50%) blocked HSP-72 expression by 4 h during a mild heat stress (39.8 degrees C). This inhibition occurs without affecting mRNA levels (in situ hybridization) for HSP-72 or overall protein synthesis ([35S]methionine incorporation), suggesting that BLM is blocking the stress response relatively specifically and post-transcriptionally. In summary, these results suggest that BLM causes apoptosis in human AM in vitro that is preceded by the inhibition of HSP-72 induction that appears to be caused by a posttranscriptional mechanism.

Mechanisms associated with human alveolar macrophage stimulation by particulates.

Asbestos and silica are well-known fibrogenic dusts. However, there is no comprehensive understanding of the molecular and cellular events that lead to fibrosis as a consequence of asbestos or silica inhalation. Previous studies have shown that asbestos stimulates superoxide anion production in alveolar macrophages through the phospholipase C/protein kinase C pathway. In contrast, silica does not appear to activate this pathway nor stimulate superoxide anion production, but silica does stimulate cytokine release by some undetermined pathway. Therefore, using human alveolar macrophages isolated from normal healthy volunteers, we evaluated the potential involvement of intracellular calcium and tyrosine kinases as potential signal transduction pathways. In the absence of serum, crystalline silica, and to a lesser extent amorphous silica, caused a rapid and dose-dependent elevation of intracellular calcium coming from the extracellular space. However, in the presence of serum, which is required for silica-stimulated cytokine release, neither form of silica caused noticeable elevation of intracellular calcium. Silica, however, did increase the extent of tyrosine phosphorylation, most notably of proteins at approximately 46 and 50 kDa, suggesting activation of a tyrosine kinase pathway. Preincubation of alveolar macrophages for 24 hr with silica-primed human alveolar macrophages for enhanced interleukin-1 beta (IL-1 beta) release stimulated by endotoxin (LPS) that was dose dependent. The enhanced LPS-stimulated release of IL-1 beta correlated with enhanced mitogen-activated protein kinase activity. Taken together, these results indicate that a tyrosine kinase pathway is activated during silica stimulation of human alveolar macrophages.

The in vivo effects of rhIL-1 alpha therapy on human monocyte activity.

Pleiotropic cytokines such as interleukin-1 alpha (IL-1 alpha) have multiple effects on peripheral blood monocytes (PBMs). This study examined the ability of in vivo recombinant human IL-1 alpha (rhIL-1 alpha) therapy to enhance clinically important monocyte functions in ovarian cancer patients prior to chemotherapy. After 4 days of continuous infusion, in vivo rhIL-1 alpha therapy amplified both the number and activity of PBMs. Therapy with rhIL-1 alpha increased the number of PBMs sixfold. These monocytes had a significantly increased ability to produce superoxide anion in response to phorbol 12,13-dibutyrate stimulation. Their ability to secrete spontaneously the immunomodulatory cytokines IL-1 alpha and IL-1 beta was significantly increased, but their ability to secrete tumor necrosis factor alpha (TNF-alpha) was not significantly elevated. These effects of rhIL-1 alpha infusion on cytokine secretion by PBMs appear to be related to rhIL-1 alpha-induced increases in the mRNA levels for these cytokines. In contrast, rhIL-1 alpha therapy did not significantly alter PBM response to lipopolysaccharide (10 micrograms/ml). In summary, infused rhIL-1 alpha, in addition to its use as a myeloprotective agent, has enhancing effects on the number and activity of PBMs. The effects of rhIL-1 alpha infusion on PBM function demonstrated here should at least transiently increase the ability of monocytes to combat infection and enhance host immune response.