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Department of Endocrinology, William Harvey Research Institute, Barts and the London, Queen Marys School of Medicine and Dentistry, Queen Mary, University of London, London EC1A 7BE, United Kingdom
Address all correspondence and requests for reprints to: Dr. J. P. Hinson, Department of Endocrinology, Barts and the London, Queen Mary School of Medicine and Dentistry, Suite 12, Dominion House, Bartholomew Close, London EC1A 7BE, United Kingdom. E-mail: j.p.hinson{at}qmul.ac.uk.
| Abstract |
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| Introduction |
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LPS from Gram-negative bacteria (Escherichia coli LPS) is known to act though Toll-like receptors (TLRs), specifically through TLR4 (9). The TLRs are a receptor family, related to the IL-1 receptor, with multiple ligands and a range of signal transduction pathways (for review, see Refs. 9, 10, 11). Binding of LPS to TLRs is not simple. It appears that a range of other molecules is required for TLRs to recognize the LPS signal. These include CD14, MD-2, and LPS-binding protein, which are all important in the presentation of LPS and its recognition by the TLR (12). Recently, binding of LPS to adrenal cells has been reported (13), and the expression of TLRs has been described in the human adrenal (14).
It is clear that the TLRs signal through a range of different pathways to stimulate the release of cytokines, particularly members of the IL family (10). LPS action has also been linked to prostaglandin (PG) production in some tissues (15), and there is evidence that in intact rats, LPS administration causes up-regulation of COX-2 in the adrenal gland (16). It is not clear whether this is a direct action of LPS on the adrenal, however.
A previous study from our laboratory indicated that LPS exerted a direct action on human adrenocortical cells (8). The present study was designed to extend these findings, determine the dose-response relationship, and elucidate the time course and mechanism of this effect. These studies used the human adrenocortical cell line, NCI-H295R, which produces the normal range of human adrenal steroids (17). It has the advantage of being a pure adrenocortical cell preparation with no contaminating endothelial, medullary, or mast cells that could potentially influence the response to LPS.
| Materials and Methods |
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Crude LPS (E. coli 0127:B8) was obtained from Sigma-Aldrich Corp. A highly purified preparation of LPS (E. coli 0127:B8) was also purchased from Sigma-Aldrich Corp. (prepared by phenolic extraction and gel filtration chromatography; protein content, <1%; RNA, <1%). This is referred to as pure LPS. A stock solution of LPS dissolved in water was aliquoted and stored at 20 C. Lipid A (from E. coli) was dissolved in dimethylsulfoxide (1 mg/ml). Pam3Cys (EMC Microcollections, Tubingen, Germany) was dissolved in water (1 mg/ml). Indomethacin was dissolved in ethanol (10 mg/ml), and forskolin, SC560, and NS398 were each dissolved in dimethylsulfoxide (5 mg/ml). Stock solutions were diluted with cell culture medium to give the final required concentration. Control experiments were carried out with each of the solvents alone at the final concentration in the incubation. None had any effect on cortisol release.
Cell culture
H295R cells were routinely maintained in 75-cm2 tissue culture flasks in DMEM/Hams F-12 Nutrient Mix supplemented with 2% (wt/vol) Ultroser SF, 1% ITS (6.25 mg insulin, 6.25 mg transferrin, 6.25 mg selenium, and 5.35 mg linoleic acid) and 1% penicillin/streptomycin at 37 C under an atmosphere of 95% air/5% carbon dioxide. For all experiments, preincubations and incubations were carried out in serum-free medium which consisted of DMEM/Hams F-12 Nutrient Mix supplemented only with 1% penicillin/streptomycin. Cells were passaged 1:2 using a solution of trypsin (0.5 g/liter)/EDTA (0.2 g/liter) every 4 d.
Cell incubations
H295R cells were plated to a density of 500,000 cells/well in six-well plates and were serum-starved for 24 h before treatment. Cells were incubated in serum-free medium in the presence or absence of LPS for different time periods. The effects of the specific TLR agonists were investigated by incubating cells in the presence or absence of these agents for 24 h. Lipid A was used at 1 µg/ml, and Pam3Cys was used at 5 µg/ml. At these concentrations, lipid A is a specific TLR4 agonist (18), and Pam3Cys is a specific TLR2 agonist (19).
To investigate the effects of the cyclooxygenase (COX) inhibitors, indomethacin, SC 560, and NS398, experiments were carried out as described above, using forskolin (105 mol/liter) as a positive control. Cells were incubated for 24 h with either LPS or forskolin in the presence or absence of the inhibitor. Indomethacin was used at a concentration of 105 mol/liter, SC560 at 107 mol/liter, and NS398 at 5 x 105 mol/liter. At these concentrations, SC560 is a selective inhibitor of COX-1 (20), and NS398 is a selective inhibitor of COX-2 (21).
At the end of the incubation period, medium was removed from the cells and stored at 20 C until assayed. Cell viability was determined using a 3-[4,5-dimethlthiazol-2-yl]-2,5-diphenyltetrazolium bromide assay (Promega Corp., Madison, WI). None of the treatments used in this study had any effect on cell viability.
Assays
Cortisol was measured by RIA, using an antibody obtained from Bioclin (Cardiff, UK); labeled cortisol was purchased from Amersham Biosciences (Little Chalfont, UK), and the protocol provided was followed. The sensitivity of the assay was 5 pmol/ml. The intraassay variation was 5.2%, and the interassay variation was 7.5%. PGE2 was measured by RIA using a kit provided by Dr. Robert Abayasekara (Royal Veterinary College, London, UK) (22). The limit of detection of the assay was 2 pg/ml PGE2, and intra- and interassay coefficients of variation were 3.5% and 6.7%, respectively. Aldosterone was measured using an in-house assay (23). IL-6 was measured using a specific enzyme immunoassay kit purchased from IDS (Tyne & Wear, UK).
mRNA analysis
RNA was extracted from H295R cells using a QuickPrep Micro mRNA Purification Kit (Amersham Biosciences). The RNA obtained was treated with deoxyribonuclease to ensure that no genomic DNA was present, then subjected to first strand DNA synthesis using the protocol in the kit. The samples were heat inactivated (90 C for 5 min) to denature any RNA-cDNA duplex that had formed and also to inactivate the reverse transcriptase.
Five microliters (equivalent to 10 ng total RNA) were subjected to PCR in a 50-µl reaction volume containing 37.5 µl diethylpyrocarbonate (DEPC)-water, 5 µl 10x PCR buffer, 1 µl deoxy-NTP, 0.5 µl sense primer, 0.5 µl antisense primer, and 0.5 µl Taq polymerase under the following conditions: one cycle of denaturation at 94 C for 5 min and 35 cycles of denaturation at 94 C for 1 min, 1-min primer annealing at the calculated temperature (see Table 1
), and 1-min primer extension at 72 C. Ten microliters of the PCR products were electrophoresed through 2% agarose gels, stained with ethidium bromide, and viewed by UV light. The primers used are listed in Table 1
. The use of these primers for Toll-2 and Toll-4 has previously been reported (24). Glyceraldehyde-3-phosphate dehydrogenase was used to test the purity of the cDNA. The positive control was mRNA obtained from THP-1 cells, which express all members of the TLR family of receptors (25), and the negative control was DEPC (0.1%, vol/vol)-water. PCR products were sequenced to confirm their identity.
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| Results |
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| Discussion |
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It is known that endotoxin can act through a variety of signaling molecules, including the TLRs. This study has demonstrated the expression of genes encoding TLR2 and TLR4, and the use of specific ligands for these receptors provides indirect evidence to suggest that both receptor types may be functional in these cells. However, the discrepancy in magnitude of response between the effects of the specific ligands and the effects of crude LPS suggest that this response may be mediated by either a different receptor or an additive or cooperative action of the two receptor subtypes. This is the subject of an ongoing study in our laboratory.
The dose-dependency of the response to LPS was very similar to that previously reported in other cell types (15), with a threshold of 5 ng/ml. The maximally effective concentration in the present study was found to be 10 ng/ml, although the effects of higher concentrations were not investigated in the study by Wang and co-workers (15). The dose-response characteristics of the specific TLR ligands were not investigated in the present study, although it was established that Pam3Cys was toxic to these cells at concentrations higher than that used in this study (5 µg/ml; data not shown).
The time course of the response to LPS was noteworthy. Forskolin was used to stimulate the cAMP-dependent pathway, because these cells are relatively unresponsive to ACTH (17). The cortisol response to forskolin was evident at 4 h, but there was no increase in cortisol in response to LPS until 8 h. Previous studies using human subjects in vivo have shown a two-stage cortisol response to LPS administration (27). The first part of the response was seen about 34 h after LPS administration and coincided with the peak ACTH response. Both cortisol and ACTH returned to basal levels by 8 h, and cortisol, but not ACTH, then increased again, with a peak about 1112 h after LPS administration. In light of the present results, it seems likely that this second phase of the cortisol response may be due to a direct action of LPS on the adrenal cortex.
Endotoxin, but not forskolin, significantly stimulated PG synthesis, and a comparison of the time course of response revealed that the increase in PG release preceded the increase in cortisol secretion. The use of inhibitors of PG biosynthesis suggested that formation of PGs is an obligatory step in the adrenal response to LPS stimulation. Previous studies have implicated adrenal COX-2 in the hypothalamo-pituitary-adrenal response to endotoxin, because treatment of intact rats with endotoxin caused an increase in adrenal COX-2 expression (16). However, the finding that this effect was blocked by dexamethasone suggests that the change in adrenal COX-2 may have been a result of activation of the whole axis, rather than a direct effect of endotoxin on the adrenal gland. The results of the present study suggest that LPS may act directly on the human adrenal cortex to stimulate COX-2.
In other tissues, LPS acts to stimulate cytokine production. In the adrenal gland, production of PGE2 appears to be simply an intermediary in the signal transduction pathway leading to increased cortisol biosynthesis. It has previously been suggested that IL-6 may have a role in the rodent adrenocortical response to LPS, because IL-6 knockout mice had an attenuated glucocorticoid response (28). However, IL-6 release in this study was below the assay detection limit; thus, it was not possible to determine whether LPS altered adrenal IL-6 secretion. It is possible that the adrenal effect of LPS is a late part of the physiological response to infection, and in this regard it is probably significant that the adrenal response to LPS stimulation is much slower than the response to forskolin.
In conclusion, these studies demonstrate a direct stimulatory effect of bacterial endotoxin on cortisol secretion by human adrenal cells. This provides additional evidence for immune-adrenal interactions and may be a significant part of the physiological response to infection.
| Footnotes |
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First Published Online November 24, 2004
Abbreviations: COX, Cyclooxygenase; DEPC, diethylpyrocarbonate; HPA, hypothalamo-pituitary-adrenal; LPS lipopolysaccharide; PGE2, prostaglandin E2; TLR, Toll-like receptor.
Received July 9, 2004.
Accepted for publication November 19, 2004.
| References |
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-melanocyte-stimulating hormone (
-MSH) on rat adrenal zona glomerulosa. J Mol Endocrinol 9:4754[Abstract]
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