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* Research Service, Carl T. Hayden Veterans Affairs Medical Center, Phoenix, AZ 85012; and
Respiratory Sciences, University of Arizona, Tucson, AZ 85721
| Abstract |
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, IL-1
, and IFN-
each at 5 ng/ml). Production of NO or iNOS was not detected in controls with or without erythromycin A. In the presence of cytomix, erythromycin A did not decrease NO, nitrite, iNOS protein, or mRNA production. In contrast, doxycycline caused a dose-dependent decrease in NO, nitrite, iNOS protein, and mRNA production in cytomix-treated cells. Doxycycline at 30 µg/ml produced a 90% decrease in nitrite and NO production and a 52% decrease in iNOS mRNA transcription compared with cytomix treatment alone. Actinomycin D treatment suggests that doxycycline decreases stability of iNOS mRNA in cytomix-treated cells. To determine a mechanism for the decrease in iNOS expression, NF-
B and AP-1 transcription regulatory systems and p38 MAPK were examined. Doxycycline treatment gave no statistically significant change in NF-
B activation but did decrease p38 MAPK protein in cytomix-treated cells by 50%, suggesting that p38 MAPK may be responsible for stabilization of iNOS mRNA. These results demonstrate that doxycycline decreases NO production from iNOS by destabilization of iNOS mRNA via decreased expression of p38 MAPK. | Introduction |
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Macrolides and tetracyclines are bacterial and fungal secondary metabolites that possess antibacterial action but also exhibit distinct and separate anti-inflammatory actions. These antibacterial compounds have been known to reduce inflammation associated with a variety of inflammatory diseases since the 1970s (4, 5). Among these compounds are erythromycin A and many of its derivatives such as azithromycin, clarithromycin, and roxithromycin, as well as tetracycline and several related compounds such as doxycycline. These compounds have exhibited anti-inflammatory activity against a variety of inflammatory airway disorders (5). However, concern has been raised about development of microbial antibiotic resistance limiting macrolide usefulness and suggesting a need for alternative anti-inflammatory therapies.
One marker of inflammation has been measurement of NO, a gas produced by lung epithelial cells, which has been shown to be elevated in several inflammatory airway conditions (6). We hypothesized that doxycycline possesses potent anti-inflammatory properties in murine lung epithelial cells, leading to a reduction of NO produced by inducible NO synthase (iNOS)3 and that this inhibition is due, at least in part, to decreased stabilization and expression of iNOS mRNA and its translation to form active iNOS enzyme. The effect of doxycycline on the NF-
B transcriptional regulatory system was examined as well as the effects of p38 MAPK on iNOS mRNA stability. The results confirm this hypothesis in vitro and suggest that doxycycline treatment may be an effective airway anti-inflammatory therapy in diseases such as asthma, chronic bronchitis, diffuse panbronchiolitis, and bronchiectasis.
| Materials and Methods |
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The clonal murine lung alveolar epithelial cell line LA4 was purchased from the American Type Culture Collection (7). LA4 cells were grown in 25-cm2 tissue culture flasks (Corning Costar) in Hams F-12 containing 10% FCS, 2 mM L-glutamine, and penicillin-streptomycin (100 U/ml, 100 mg/ml) until confluent. After washing with serum-free medium doxycycline, dissolved in distilled water, or erythromycin A, dissolved in DMSO, was added to confluent LA4 cultures for 3 h, and the cells were cultured for 18 h in serum-free Hams F-12 with or without cytomix (CM). CM is a combination of recombinant human TNF-
, human IL-1
, and murine IFN-
, each at 5 ng/ml concentration (all cytokines from R&D Systems), that is used to stimulate the expression of iNOS (8, 9). Concentrations of test chemicals ranged from 0.3 to 30 µg/ml for doxycycline and 0.3 to 60 µg/ml for erythromycin A based on reports of clinically relevant serum and tissue levels (10, 11). Serum levels of doxycycline are reported to be up to 79 µg/ml (depending on the individual) (10) and for erythromycin in the range of 1.9 to 6.5 µg/ml (11). Ampicillin (50 µg/ml) and sparsomycin (0.5 µg/ml) were used as control compounds. All control cultures contained the appropriate amount of carrier solvent alone.
Cell viability assays
Cell viability was determined by assay for release of lactate dehydrogenase (LDH) into cell culture supernatants using a commercially available LDH assay kit (Sigma-Aldrich).
Determination of NO concentration in culture flask head space and nitrite in culture flask supernatants
After 18 h of stimulation with CM, culture flasks were tightly sealed and incubated at 37°C. After 3 h head space gas was withdrawn using a needle and syringe. NO concentrations were measured using a NO chemiluminescence analyzer (Sievers) with a detection sensitivity of <1 parts per billion (ppb). The analyzer was calibrated daily according to the manufacturers directions using a gas of known NO concentration. Nitrite concentrations of the cell culture supernatant were determined spectrophotometrically using the Griess reaction (12).
Protein determination
Protein concentrations were determined spectrophotometrically using the Bradford assay (13).
Cell lysis, SDS-PAGE, and Western blotting
Cell extracts were prepared for Western blotting using a modification of previously described methods (14). Cells were lysed using 10 mM Tris-HCl (pH 7.4) containing 1% SDS and 1 mM EDTA and a premade mixture of protease inhibitors (Sigma-Aldrich). Cell lysates were clarified by centrifugation before electrophoresis. SDS-PAGE was performed using a 4% T acrylamide stacking gel and a 7.5% T acrylamide separating gel for iNOS or a 10% T separating gel of p38 MAPK (15). After electrophoresis was complete, the separated proteins were transferred to a polyvinylidene difluoride membrane for further analysis (16). Transfer was performed using a Tris-HCl (25 mM), glycine (192 mM), and methanol (20% v/v) buffer system. The transfer was performed at 4°C for 16 h at 30 V and then for 30 min at 60 V. Abs for iNOS (Transduction Laboratories) or p38 MAPK (Santa Cruz Biotechnology) were used to immunochemically detect iNOS or p38 MAPK protein on the transfer membranes using goat anti-rabbit IgG alkaline phosphatase as the secondary Ab-enzyme conjugate and NBT/5-bromo-4-chloro-3-indolyl phosphate phosphatase substrate tablets (Sigma-Aldrich) in 0.1 M Tris-HCl (pH 9.1) as the substrate system. Five micrograms of Ab were used at each step during the development procedures. The transfer membranes were blocked with BLOTTO (20 mM Tris base, 180 mM NaCl, 4% nonfat dry milk, 0.02% NaN3) to minimize nonspecific binding.
RNA isolation and RT-PCR
iNOS mRNA was analyzed by RT-PCR. Total cellular RNA was extracted from adherent cells using a modification of the methods of Chomczynski and Sacchi (17). The RNA was reverse transcribed using a commercially available RT-PCR kit (Promega) using the sense and antisense primers (Table I) added at 0.2 µM final concentration. Using a PerkinElmer model 480 thermal cycler, reverse transcription was performed on total cellular RNA at 48°C for 45 min. PCR for iNOS was performed in the same sample tubes at 94°C for 2 min followed by 28 cycles consisting of 94°C for 45 s, 60°C for 45 s, 72°C for 2 min, followed by 72°C for an additional 7 min.
-Actin (primers shown in Table I) was used as a "housekeeping gene" and RT-PCR was performed in a similar manner as for iNOS except for an annealing temperature of 50°C. The DNA fragments were separated by electrophoresis on a 2% agarose gel in TAE buffer (40 mM Tris base, 20 mM acetic acid, 2 mM Na2 EDTA, pH 8.5) and then analyzed and quantitated by densitometry. Increasing PCR cycles gave increasing amounts of DNA through a fairly broad range with linearity obtained from about cycles 2432, beginning with 1 µg of DNA (r = 0.99 for iNOS and
-actin).
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LA4 cells were treated with CM or CM plus doxycycline for 18 h as described above and then treated with actinomycin D (10 µg/ml) to stop transcription. RNA was isolated from cultures at various times. RT-PCR was used to determine the amount of iNOS mRNA present at each time.
Effect of doxycycline on NF-
B
The effect of doxycycline on the NF-
B transcription regulatory system was determined using a commercially available ELISA kit (Active Motif). The primary Ab used in this kit detects an epitope of p50/p65 that is accessible only when NF-
B is activated and bound to its consensus DNA-binding sequence. Cell extracts were prepared according to manufacturers instructions. LA4 cells were treated with 30 µg/ml doxycycline for 3 h followed by addition of CM for 1 h before preparation of cell extracts for NF-
B analysis. For each analysis each well contained 5 µg protein.
Statistical analysis
Data were analyzed by one-way ANOVA with Fishers protected least significant difference (Fishers PLSD). In all cases, a p < 0.05 was considered to be significant. The data are expressed as mean ± SD.
| Results |
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Doxycycline, but not erythromycin A, decreased NO production in CM (5 ng/ml each of IL-1
, TNF-
, and IFN-
)-stimulated LA4 cells (Table II). Doxycycline markedly inhibited NO production at the highest concentration used. NO production was also inhibited at the lower doxycycline concentrations, in a concentration-dependent manner, but the decrease was not as dramatic. The differences in NO production between CM-treated and CM plus 3 µg/ml, 10 µg/ml, or 30 µg/ml doxycycline was statistically significant (p < 0.05). The difference with CM plus 0.3 µg/ml doxycycline was not statistically significant (p > 0.05). In contrast, erythromycin A exhibited little inhibition of NO production at all concentrations tested. Assays for NO and nitrite were performed on at least two sets of samples in triplicate.
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50% of the amount in the CM control. At 3 mg/ml doxycycline, the iNOS protein is roughly the same as in the CM control. No further decrease in iNOS protein is seen when the doxycycline concentration is decreased to 0.3 µg/ml (data not shown). In contrast, erythromycin A at the concentrations tested did not decrease iNOS protein production (Fig. 2).
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-Actin was used as a control. Expression of the
-actin gene and production of its mRNA was not altered during any of the treatments discussed (data not shown). At a concentration of 30 µg/ml doxycycline, a 50% decrease of iNOS mRNA production in CM-stimulated cells compared with CM alone was observed (Fig. 3). Lower concentrations of doxycycline did not significantly alter iNOS mRNA production. Although there was some variability in iNOS mRNA detected by RT-PCR, CM-induced iNOS mRNA was clearly lowered by doxycycline at 30 µg/ml (Fig. 3, p < 0.05 when compared by scanning densitometry).
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-actin gene and production of its mRNA was not altered during any of the treatments discussed (data not shown). Erythromycin A at a concentration of 60 µg/ml did not alter iNOS mRNA expression or production of nitrite (data not shown). In addition, ampicillin (50 µg/ml) and sparsomycin (0.5 µg/ml), both used as control compounds, did not alter iNOS mRNA expression or the production of nitrite (data not shown). Concentrations of sparsomycin greater than 0.5 µg/ml were cytotoxic.
Doxycycline treatment does not result in a statistically significant decrease in NF-
B in CM-stimulated cells with respect to CM treatment alone (Fig. 5). In a companion experiment, treatment of LA4 cells with erythromycin A results in a statistically significant decrease in NF-
B after CM treatment (data not shown), but there is no effect on iNOS mRNA transcription.
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| Discussion |
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Analysis of the NF-
B transcription regulatory signaling pathway suggests that it may not play a significant role in attenuating expression of iNOS mRNA after exposure to doxycycline (Fig. 5). The primary Ab in the NF-
B ELISA kit (Active Motif) that was used detects an epitope of p50/p65 that is accessible only when NF-
B is activated and bound to its consensus DNA-binding sequence. This would provide a reliable measure of the degree of NF-
B activation in the presence or absence of doxycycline in CM-treated cells.
The p38 MAPK pathway plays an important role in the inflammatory response (20). A primary function of p38 MAPK is to regulate the stability of inflammatory protein mRNAs such as MCP-1 (21), GM-CSF and vascular endothelial growth factor (22, 23), TNF-
, IL-1, cyclooxygenase 2, IL-6, IL-8 (24, 25, 26, 27), and iNOS (28). The p38 MAPK pathway influences stability of various mRNAs by promoting translation via AU-rich elements in the 3'-untranslated region of these molecules (24, 29). Doxycycline decreases the amount of p38 MAPK protein produced in CM-treated cells (Fig. 6). In addition, doxycycline appears to decrease the stability of iNOS mRNA in CM-treated cells (Fig. 7). These data suggest that the doxycycline-induced decrease in p38 MAPK protein may be responsible for the observed decrease in iNOS mRNA stability.
Regulation of inflammatory gene transcription represents a complex interplay between transcription factors and other factors mediating stability of mRNA. Previous studies suggest that NF-
B plays an important role in regulating iNOS transcription (30). However, our results suggest that NF-
B plays at best a minor role in the doxycycline-modulated decrease in iNOS-catalyzed NO production in murine LA4 cells. It seems likely that the decrease in iNOS mRNA and protein that is observed upon doxycycline treatment is due to decreased stability of iNOS mRNA that is a function of a concomitant decrease in p38 MAPK. This concept is supported by the recent work of Fechir et al. (31) that demonstrates that inhibition of p38 MAPK decreases stability of iNOS mRNA, ultimately leading to decreased iNOS expression. The effects of erythromycin on p38 MAPK have been previously reported elsewhere (32, 33).
Tetracycline and similar antibiotics such as doxycycline have long been known to effectively decrease inflammation in rheumatoid arthritis (34, 35). The present results support the possibility that doxycycline may have potential as an anti-inflammatory treatment for inflammatory lung diseases such as asthma, diffuse panbronchiolitis, chronic bronchitis, and bronchiectasis.
Antibiotics such as the macrolides (erythromycin A and derivatives) (36, 37) and tetracycline derivatives such as doxycycline (38, 39, 40) have been successfully used to treatment inflammation in a variety of clinical settings. However, successful treatment of inflammation requires long-term drug therapy which includes the possibility of the induction of resistance to antibiotics, such as erythromycin A, in potentially pathogenic bacteria. The possible increase in antibiotic-resistant Mycobacterium spp. (41, 42)., Nocardia spp. (43, 44), and other bacterial pathogens is a serious problem that is often encountered when treating inflammatory lung diseases with erythromycin-type antibiotics. Therefore, other potential anti-inflammatory therapies which might not induce resistance, such as doxycycline, would be beneficial for treatment of chronic inflammatory lung diseases. The data presented in this article suggest that doxycycline may be useful as an antiinflammatory therapy in inflammatory lung diseases where emergence of resistance to doxycycline may not be a serious clinical problem.
In addition to its prominent role in tissue damage during chronic inflammation, NO is also believed to have roles in which its reduction would be detrimental, such as in the defense against microbial pathogens (45). iNOS has been demonstrated to be an important antimicrobial defense mechanism (46). The NO produced can react with superoxide produced by macrophages or neutrophils, forming peroxynitrite (45), which can damage a variety of microbial targets including lipids, heme clusters, aromatic amino acids, thiols, and DNA (47).
A reduction in iNOS expression and NO production is also seen in cystic fibrosis (48). This observed decrease in iNOS expression and NO production may be, in part, responsible for the continued infection and colonization of the cystic fibrosis lung by Pseudomonas aeruginosa (49).
In support of the clinical use of antibiotics as anti-inflammatory agents, recent studies have shown that chronic administration of azithromycin is beneficial in the obstructive lung disease, cystic fibrosis (50). Although the mechanism is unclear, azithromycin is an ineffective antibiotic against bacteria which infect the lungs of cystic fibrosis patients, indicating a presumed anti-inflammatory effect. In conclusion, we have shown that doxycycline reduces cytokine-induced NO production from murine lung epithelial cells. These results suggest that doxycycline may provide a potential benefit in the management of in vivo inflammation in certain chronic pulmonary diseases.
| Disclosures |
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| Footnotes |
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1 This research is funded by a Veterans Affairs Merit Review Grant. ![]()
2 Address correspondence and reprint requests to Dr. Jeffrey C. Hoyt, Carl T. Hayden Veterans Affairs Medical Center, Research (RS/151), Building 27, 650 East Indian School Road, Phoenix, AZ 85012, E-mail address: jeff.hoyt2{at}med.va.gov ![]()
3 Abbreviations used in this paper: iNOS, inducible or type II nitric oxide synthase; CM, cytomix; LDH, lactic dehydrogenase. ![]()
Received for publication May 17, 2005. Accepted for publication October 13, 2005.
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