Dale G. Hoyt

Bleomycin: A pharmacologic tool in the study of the pathogenesis of interstitial pulmonary fibrosis

Bleomycin is a unique DNA-interactive antitumor agent that has become a popular tool in studies of the pathogenesis of interstitial pulmonary fibrosis. The biochemical and morphological changes seen in the lungs of many species after bleomycin simulate those seen in humans. The availability of these animal models of interstitial pulmonary fibrosis also provides the opportunity to investigate novel pharmacological approaches to preventing this disease.

Nitric oxide inhibits lipopolysaccharide-induced apoptosis in pulmonary artery endothelial cells

Our group recently reported that cultured sheep pulmonary artery endothelial cells (SPAECs) became resistant to lipopolysaccharide (LPS)-induced apoptosis several days after constitutive synthesis of nitric oxide (NO) after adenoviral (Ad) transfer of inducible NO synthase (iNOS) or exposure to the NO donor S-nitroso-N-acetylpenicillamine (SNAP) (E. Tzeng, Y.-M. Kim, B. R. Pitt, A. Lizonova, I. Kovesdi, and T. R. Billiar. Surgery 122: 255-263, 1997). In the present study, we confirmed this observation by establishing stable transfectants after retroviral gene transfer [replication-deficient retrovirus (DFG)] of human iNOS (DFG-iNOS) SPAECs and then used all three approaches (Ad, DFG, and SNAP) to determine underlying mechanisms of this phenomenon. Continuous endogenous production of NO in itself did not cause apoptosis as assessed by phase-contrast microscopy, nuclear morphology, and internucleosomal DNA fragmentation. Prolonged (72-96 h) synthesis of NO, however, after DFG- or replication-deficient adenovirus (Ad. CMV)-iNOS or SNAP (100 microM, 96 h) inhibited LPS-induced apoptosis. The kinetics of such protection suggested that NO may be inducing other gene products. Ad-mediated transfer of manganese superoxide dismutase (MnSOD) decreased the sensitivity of wild-type SPAECs to LPS-induced apoptosis. MnSOD, however, was not induced in an NG-monomethyl-L-arginine (L-NMMA)-sensitive time-dependent fashion after Ad.CMV-iNOS. Other inducible genes that may be affected by NO and that may protect against potential oxidant-mediated LPS-induced apoptosis including 70-kDa heat shock protein, heme oxygenase-1, metallothionein, and Bcl-2 also were not elevated in an L-NMMA-sensitive, time-dependent fashion. Although the candidate gene product underlying NO-induced protection remains unclear, we did note that prolonged synthesis of NO inhibited LPS-induced activation of an interleukin-1beta-converting enzyme-like cysteine protease (cysteine protease protein-32-like) in a dithiothreitol-sensitive fashion, suggesting that S-nitrosylation of an important downstream target of convergence of apoptotic signals may contribute to the sensitivity of SPAECs to LPS.Our group recently reported that cultured sheep pulmonary artery endothelial cells (SPAECs) became resistant to lipopolysaccharide (LPS)-induced apoptosis several days after constitutive synthesis of nitric oxide (NO) after adenoviral (Ad) transfer of inducible NO synthase (iNOS) or exposure to the NO donor S-nitroso- N-acetylpenicillamine (SNAP) (E. Tzeng, Y.-M. Kim, B. R. Pitt, A. Lizonova, I. Kovesdi, and T. R. Billiar. Surgery 122: 255-263, 1997). In the present study, we confirmed this observation by establishing stable transfectants after retroviral gene transfer [replication-deficient retrovirus (DFG)] of human iNOS (DFG-iNOS) SPAECs and then used all three approaches (Ad, DFG, and SNAP) to determine underlying mechanisms of this phenomenon. Continuous endogenous production of NO in itself did not cause apoptosis as assessed by phase-contrast microscopy, nuclear morphology, and internucleosomal DNA fragmentation. Prolonged (72-96 h) synthesis of NO, however, after DFG- or replication-deficient adenovirus (Ad.CMV)-iNOS or SNAP (100 μM, 96 h) inhibited LPS-induced apoptosis. The kinetics of such protection suggested that NO may be inducing other gene products. Ad-mediated transfer of manganese superoxide dismutase (MnSOD) decreased the sensitivity of wild-type SPAECs to LPS-induced apoptosis. MnSOD, however, was not induced in an N G-monomethyl-l-arginine (l-NMMA)-sensitive time-dependent fashion after Ad.CMV-iNOS. Other inducible genes that may be affected by NO and that may protect against potential oxidant-mediated LPS-induced apoptosis including 70-kDa heat shock protein, heme oxygenase-1, metallothionein, and Bcl-2 also were not elevated in an l-NMMA-sensitive, time-dependent fashion. Although the candidate gene product underlying NO-induced protection remains unclear, we did note that prolonged synthesis of NO inhibited LPS-induced activation of an interleukin-1β-converting enzyme-like cysteine protease (cysteine protease protein-32-like) in a dithiothreitol-sensitive fashion, suggesting that S-nitrosylation of an important downstream target of convergence of apoptotic signals may contribute to the sensitivity of SPAECs to LPS.

Protein Never in Mitosis Gene A Interacting-1 (PIN1) regulates degradation of inducible nitric oxide synthase in endothelial cells.

The peptidyl-proline isomerase Protein Never in Mitosis Gene A Interacting-1 (PIN1) increases the level or activity of several transcription factors that can induce the inducible nitric oxide (NO) synthase (iNOS). PIN1 can also regulate mRNA and protein turnover. Here, the effect of depletion of PIN1 on induction of iNOS by Escherichia coli endotoxin (LPS) and interferon-gamma (IFNgamma) in murine aortic endothelial cells (MAEC) was determined. Suppression of PIN1 by 85% with small hairpin RNA enhanced the induction of NO and iNOS protein by LPS-IFNgamma. There was no effect on induction of iNOS mRNA, suggesting a posttranscriptional effect. The enhanced levels of iNOS protein were functionally significant since LPS-IFNgamma was cytotoxic to MAEC lacking PIN1 but not MAEC harboring an inactive control construct, and because cytotoxicity was blocked by the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester. Consistent with posttranscriptional action, knockdown of PIN1 increased the stability of iNOS protein in cycloheximide-treated cells. Furthermore, loss of iNOS was blocked by the calpain inhibitor carbobenzoxy-valinyl-phenylalaninal but not by the selective proteasome inhibitor epoxomicin. Immunoprecipitation indicated that PIN1 can interact with iNOS. Pull down of iNOS with a wild-type glutathione-S-transferase-PIN1 fusion protein, but not with a mutant of the amino terminal phospho-(serine/threonine)-proline binding WW domain of PIN1, indicated that this domain mediates interaction. The results suggest that PIN1 associates with iNOS and can limit its induction by facilitating calpain-mediated degradation in MAEC.

Base Excision Repair Proteins Are Required for Integrin-Mediated Suppression of Bleomycin-Induced DNA Breakage in Murine Lung Endothelial Cells

Engagement of integrin cell adhesion receptors suppresses bleomycin (BLM)-induced DNA strand breakage in endothelial cells. Previous investigation of cells from poly(ADP-ribose) polymerase (PARP)-1 knockout mice and with an inhibitor of the enzyme indicated that this facilitator of base excision repair (BER) is required for integrin-mediated suppression of DNA strand breakage. Here, small inhibitory RNA (siRNA) was used to assess the requirement for the BER proteins, DNA ligase III (Lig3) α, PARP-1, and X-ray repair complementing defective repair in Chinese hamster cells 1 (XRCC1), and for the long-patch BER ligase, DNA ligase I (Lig1), in integrin-mediated protection from BLM-induced DNA breakage. Murine lung endothelial cells (MLECs) were transfected with siRNA, treated with anti-β1 integrin antibody, and then BLM. 3′-OH in DNA and accumulation of phosphorylated histone H2AX (γH2AX), which reflects double-strand breakage, were measured. Integrin antibody inhibited the increases in 3′-OH caused by BLM in MLECs transfected with either control or Lig1 siRNA. However, after knockdown of Lig3α, PARP-1, or XRCC1, suppression of DNA breakage by integrin antibody was limited. BLM increased γH2AX levels, and integrin treatment inhibited this by 57 to 73% in MLECs transfected with control siRNA. Integrin engagement also inhibited increases in γH2AX in BLM-treated cells transfected with Lig1 siRNA. In contrast, Lig3α, PARP-1, and XRCC1 siRNAs prevented integrin-mediated inhibition of BLM-induced γH2AX levels. The results suggest that the BER proteins, Lig3α, PARP-1, and XRCC1, are required for integrin-mediated suppression of BLM-induced DNA breakage.

The molecular basis of interstitial pulmonary fibrosis caused by antineoplastic agents

Pulmonary toxicity is an important, proliferation-independent, undesirable aspect of anticancer chemotherapy, which can ultimately develop into lethal interstitial pulmonary fibrosis. The loss of lung function, which defines pulmonary fibrosis, is due to pathological alterations in the content or composition of extracellular matrix components such as collagen, fibronectin, glycosaminoglycans and elastin (1). Biochemical studies suggest that the alteration in matrix is due primarily to an increase in extracellular matrix deposition. The altered synthesis of matrix may reflect an expansion of a cell population (e.g. fibroblasts) (15, 20). New matrix-producing cells also may be recruited into the lungs by the chemotactic activities of polypeptides (15). I n addition, the constitutive levels of matrix synthesis by the resident cell population could be altered. There is increasing evidence that endogenous growth factors and cytokines participate in the remodeling of the interstitial regions of the lungs (20). One focus of our research has been to identify these factors and the mechanisms that regulate altered collagen and extracellular matrix deposition. We have studied the pulmonary toxicity of the DNA interactive anticancer agents, bleomycin (BLM) and cyclophosphamide (CYC). Wh en administered to animals, they cause a reproducible fibrosis that has great similarity to the final fibrotic lesion seen in humans (4, 8, 16). We and others (3, 4, 7, 8, 16) have identified inbred murine strain differences in the pulmonary responsiveness to both BLM and CYC. Thus, C57B1/6 mice are much more sensitive to BLM-induced fibrosis compared to BALB/c mice. Conversely, BALB/c mice are more sensitive to CYC-induced fibrosis than C57B1/6 mice. The differences in strain sensitivity to these fibrogenic agents may reflect differences in drug metabolizing enzymes in the lungs (3, 7). In the case of BLM, the resistant strains have higher levels of the inactivating enzyme, bleomycin hydrolase, which has recently been identified as a member of the thiol protease family (17). Th e availability of murine strains with different sensitivities to fibrogenic drugs provides an important tool for identifying essential prefibrotic factors of fibrosis. The accumulation of pulmonary collagen in C57B1/6N mice after S.C. BLM infusion is preceded by sequential increases in total pulmonary fibronectin, ~(~111 procollagen and sl,I procollagen mRNA (7), as well as increases in a,IV procollagen and a,IV procollagen mRNA levels. Other laboratories have observed similar results with fibronectin, a,111 procollagen and a,1 procollagen mRNA in rats after BLM (2, 9, 13, 14). In our studies, however. these increases are largely confined to the sensitive C57B1/6N mice and are

Acute pneumocyte injury, poly(ADP-ribose) polymerase activity, and pyridine nucleotide levels after in vitro exposure of murine lung slices to cyclophosphamide

Cyclophosphamide (CYC) is a metabolically activated, DNA-alkylating, antitumor agent that causes pulmonary fibrosis. BALB/cN (B) mice are sensitive and C57Bl/6N (C) mice are resistant to CYC-induced fibrosis. Pulmonary bioactivation may contribute to strain sensitivity. Therefore, we tested the intrinsic susceptibility of murine lung slices to cell injury by direct exposure to CYC for 2-8 hr. Injury was measured by release of lactate dehydrogenase (LDH). DNA damage activates the nuclear enzyme poly(ADP-ribose) polymerase (PAP, EC 2.4.2.30), causing depletion of its substrate, NAD. NAD can also be decreased by phosphorylation to NADP, as seen with oxidative stress. Depletion of NAD can lead to loss of ATP. Thus, we measured LDH release, PAP activation, NAD, NADP and ATP in slices incubated with or without the PAP-inhibitor, 3-aminobenzamide (3-AB). CYC (0.1 to 1.0 mg/mL for 4-8 hr) caused LDH release in slices from both murine strains, but LDH release was significantly greater in B lung slices than in C slices. After an 8-hr incubation 63.9 +/- 3.7% (mean +/- SEM) of total LDH was released from B lung slices with 1.0 mg CYC/mL, whereas only 45.8 +/- 2.6% was released from C lung slices (P < 0.05). 3-AB reduced LDH release to 44.7 +/- 2.4% in B slices and 28.1 +/- 2.0% in C slices (P < 0.05 vs CYC only). PAP activity in nuclei isolated from CYC-treated B lung slices was increased 2- to 4-fold after 2 hr of incubation with 0.5 and 1.0 mg CYC/mL. PAP activation was delayed and reduced with incubation in 3-AB. PAP was activated 2-fold in nuclei from C slices treated with 0.5 mg CYC/mL for 2 hr. NAD was decreased at 2 and 4 hr in B slices treated with 0.5 and 1.0 mg CYC/mL, and at 4 hr with 0.1 mg CYC/mL. NAD depletion occurred only at 4 hr in the resistant C slices treated with 1.0 mg CYC/mL. CYC increased NADP by a similar extent in B and C lung slices. In B slices, NAD losses were approximately 4 times the increases in NADP. CYC did not decrease ATP in B slices and ATP dropped 25% only after 4 hr in the resistant C slices. We conclude that CYC is directly toxic to lung tissue and observe that strain sensitivity in vitro mirrors the sensitivity to fibrosis in vivo. PAP activation and oxidative stress may contribute to this toxicity.

NAD depletion after in vitro exposure of murine lung slices to bleomycin

Bleomycin (BLM), a DNA-cleaving, antitumor antibiotic, causes pulmonary fibrosis. It also causes cell injury and activates the nuclear enzyme poly(ADP-ribose) polymerase (PAP; EC 2.4.2.30) in lung slices exposed to the drug in vitro. 3-Aminobenzamide (3-AB), a PAP inhibitor, prevents enzyme activation and cell injury. We have examined the potential role of ATP and NAD depletion in injury of BLM-sensitive C57B1/6N and -resistant BALB/cN murine lung slices treated with BLM or deprived of glucose, the major metabolic substrate of lung. Lung slices either were treated for 45 min with injurious concentrations of BLM (10-500 micrograms/mL) or were incubated without glucose, in the presence or absence of 2.5 mM 3-AB. Only the highest concentration of BLM, 500 micrograms/mL, caused any ATP depletion, and this 35% decrease was transient, occurring at 220 min in C57B1/6N slices. In contrast, glucose deprivation caused 50-70% ATP depletion in slices from both strains. BLM alone at 100 and 500 micrograms/mL caused a sustained 30-70% NAD depletion from 75 min through 400 min in C57B1/6N mouse lung slices. In the resistant BALB/cN lung slices, NAD depletion by BLM was only seen at 400 min. 3-AB almost completely antagonized NAD depletion in slices from both strains. In contrast to BLM, glucose deprivation did not decrease NAD levels unless 3-AB was present in C57B1/6N slices. Thus, ATP depletion may play a role in the injurious effects of glucose deprivation, but does not appear to be a major factor in pneumocyte injury caused by BLM. NAD depletion or other effects of PAP activation appear to account for the strain-selective, injurious effect of BLM on lung tissue.