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Hepcidin is elevated in mice injected with Mycoplasma arthritidis
© Koening et al; licensee BioMed Central Ltd. 2009
Received: 3 June 2009
Accepted: 24 November 2009
Published: 24 November 2009
Mycoplasma arthritidis causes arthritis in specific mouse strains. M. arthritidis mitogen (MAM), a superantigen produced by M. arthritidis, activates T cells by forming a complex between the major histocompatability complex II on antigen presenting cells and the T cell receptor on CD4+ T lymphocytes. The MAM superantigen is also known to interact with Toll-like receptors (TLR) 2 and 4. Hepcidin, an iron regulator protein, is upregulated by TLR4, IL-6, and IL-1. In this study, we evaluated serum hepcidin, transferrin saturation, ferritin, IL-6, IL-1, and hemoglobin levels in M. arthritidis injected C3H/HeJ (TLR2+/+, TLR4-/-) mice and C3H/HeSnJ (TLR2+/+, TLR4+/+) mice over a 21 day period. C3H/HeJ mice have a defective TLR4 and an inability to produce IL-6. We also measured arthritis severity in these mice and the amount of hepcidin transcripts produced by the liver and spleen. C3H/HeJ mice developed a more severe arthritis than that of C3H/HeSnJ mice. Both mice had an increase in serum hepcidin within three days after infection. Hepcidin levels were greater in C3H/HeJ mice despite a nonfunctioning TLR4 and low serum levels of IL-6. Splenic hepcidin production in C3H/HeJ mice was delayed compared to C3H/HeSnJ mice. Unlike C3H/HeSnJ mice, C3H/HeJ mice did not develop a significant rise in serum IL-6 levels but did develop a significant increase in IL-1β during the first ten days after injection. Both mice had an increase in serum ferritin but a decrease in serum transferrin saturation. In conclusion, serum hepcidin regulation in C3H/HeJ mice does not appear to be solely dependent upon TLR4 or IL-6.
Mycoplasma arthritidis (M. arthritidis) is a rodent pathogen that causes arthritis and a toxic shock-like syndrome in C3H mice. M. arthritidis injected mice have been used as a mouse model of human inflammatory arthritis for more than 30 years. Much of the disease phenotype and outcomes are influenced by the M. arthritidis mitogen (MAM) superantigen produced by the organism. MAM superantigen activates T cells by forming a complex between the major histocompatability complex (MHC) II molecule on antigen presenting cells and the Vβ chain segments of the T cell receptor (TCR) on CD4+T cells . MAM is a unique superantigen in that it also interacts with Toll like receptor (TLR) 4 and 2 found on cells of the innate immune system . C3H/HeJ mice are particularly susceptible to the effects of the MAM superantigen. Compared to C3H/HeSnJ mice, C3H/HeJ mice have a mutant lpsd gene that leads to a hypofunctional TLR4 . Macrophages from C3H/HeJ mice upregulate the number of cell surface TLR2 when exposed to the MAM superantigen . Similarly, C3H/HeJ injected mice have a type 1 cytokine profile (IL-2, interferon-γ, and tumor necrosis factor α) compared to inoculated C3H/HeSnJ mice that have a type 2 cytokine profile (IL-4, IL-6, and IL-10) .
Inflammation also alters iron metabolism. Hepcidin, an iron regulatory protein, is produced by hepatocytes and macrophages in response to proinflammatory stimuli. Hepcidin binds to and down-regulates ferroportin, the only known cellular iron exporter, found on the plasma membrane of macrophages, hepatocytes, enterocytes, and syncytial trophoblasts . Iron accumulates in cells that lack plasma membrane ferroportin, which leads to lower amounts of circulating iron available for erythropoiesis. Anemia caused by the upregulation of hepcidin in subjects with inflammation is known as the anemia of inflammation (AI). Little is known regarding the regulation of hepcidin in inflammatory states. Investigators have shown TLR4 activation with lipopolysaccharide (LPS) leads to the upregulation of hepcidin [5, 6]. Mice injected with Borrelia burgdorferi develop severe arthritis and increased serum levels of hepcidin. A primary mediator of this response is the activation of TLR2 by B. burgdorferi on bone marrow macrophages of infected mice . Hepcidin transcription is also upregulated by cytokines such as IL-1  and IL-6. IL-6 increases hepcidin expression through activation of the JAK/STAT3 pathway [9–11]. To determine if hepcidin could be expressed independent of TLR4 activation, we measured serum hepcidin levels in C3H/HeJ mice (TLR2+/+, TLR4-/-) and compared the values to C3H/HeSnJ mice (TLR2+/+, TLR4+/+) after infection with M. arthritidis. We found that hepcidin levels were increased in both mouse strains and hepcidin regulation was independent of TLR4 and IL-6.
A total of 36 female mice (10 weeks), 18 C3H/HeJ (TLR2+/+, TLR4-/-) and 18 C3H/HeSnJ (TLR2+/+, TLR4+/+), were injected with M. arthritidis in accordance with the University of Utah Animal Resource Center as described previously . Mice were followed for a total of 21 days. Mice were evaluated for arthritis and toxicity as described previously immediately after injection and three, seven, ten, fourteen, and twenty-one days after injection . Three mice from each group were sacrificed under anesthesia on the days of arthritis scoring. Blood was collected by cardiac puncture and serum levels of hepcidin, ferritin, and transferrin saturation were measured as described previously [13, 14]. Serum IL-6 and IL-1β levels were assayed using mouse IL-1β and IL-6 ELISA Ready-SET-Go according to the manufacture's instructions (eBioscience, San Diego, CA). Hemoglobin (g/dl) values were measured in both strains of mice in the University of Utah Division of Hematology immediately after infection and then three, 10, and 21 days after infection. Livers and spleens were isolated from each mouse and homogenized. Total RNA extraction was performed using RNeasy (Qiagen, Valencia, CA) according to the manufacturer's instructions. Fifty nanograms of mRNA were used for RT-PCR1 Step according to the manufacturer's instructions (Invitrogen, Carlsbad, CA). The primer sequences used for RT-PCR were HAMP (forward) 5'AGAGCTGCAGCCTTTGCAC3', HAMP (reverse) 5'GAAGATGCAGATGGGGAAGT3', and actin (forward) 5'GACGGCCAAGTCATCACTATTG3', actin (reverse) 5'CCACAGGATTCCATACCCAAGA3'.
Results are reported as mean values ± standard deviation (SD).
Mean hemoglobin values ± SD were measured in C3H/HeJ and C3H/HeSnJ mice after injection with M. arthritidis.
14.425 ± 0.424
10.825 ± 7.141
13.200 ± 0.353
10.625 ± 0.565
10.875 ± 0.919
10.775 ± 2.404
12.275 ± 0.636
10.200 ± 3.670
Hepcidin is an important iron regulatory protein that when overexpressed may lead to hypoferremia and anemia. Systemic inflammation increases the levels of circulating hepcidin that binds to and degrades the cell membrane receptor ferroportin. By degrading ferroportin, iron can not be secreted into the plasma and hypoferremia develops. Extended periods of low serum iron decreases erythropoiesis and may lead to anemia. The mechanisms that lead to increased hepcidin expression vary among different inflammatory diseases. Hepcidin transcription may occur secondary to activation of TLR [5–7] or through increased expression of IL-6 or IL-1 [8–11]. Our results show that hepcidin secretion is affected by the presence of TLR4 and TLR2. Most notably, the absence of a functioning TLR4 in C3H/HeJ mice allows for unimpeded TLR2 activation in response to the MAM superantigen . Unimpeded TLR2 activation may lead to the overexpression of hepcidin in the infected C3H/HeJ mice. Previous studies indicate that IL-6, a proinflammatory cytokine, is a major inducer of hepcidin transcription. Our studies show that C3H/HeJ mice express low serum levels of IL-6 but have high levels of IL-1β, a cytokine also known to induce hepcidin transcription. Splenic macrophages activated by TLR appear to be important sources of hepcidin. Our results show that mice livers and spleens have different expression patterns of hepcidin mRNA. Hepcidin secretion from the spleens of C3H/HeSnJ mice and from the livers of both C3H/HeSnJ and C3H/HeJ mice occurs shortly after injection of M. arthritidis. However, splenic expression of hepcidin transcripts in C3H/HeJ mice is not detectable until several days after injection. The amount of serum hepcidin contributed by the spleen versus the liver is not known but we speculate splenic hepcidin production contributes a great deal to the serum levels of hepcidin seen in our experiments. Furthermore, both the presence and absence of TLR4 affects hepcidin secretion in response to inflammatory agents.
Our results show that C3H/HeJ mice have higher levels of serum hepcidin and ferritin than C3H/HeSnJ mice and that both strains have low transferrin saturation values 21 days after infection. Hemoglobin values were lower through out the experiment in the C3H/HeSnJ mice compared to the C3H/HeJ mice. We speculate the lower hemoglobin values may be specific for this strain but can not rule out that they may be due to an immediate reaction to the infection. The hemoglobin values for the C3H/HeJ mice were lower at the end of the experiment than at the beginning, but this was not seen in the C3H/HeSnJ mice. We suspect 21 days is not enough time to see a significant decline in hemoglobin values and if these mice were followed for a longer period of time, the serum hemoglobin values would decline further. We also speculate that the variation of hemoglobin values in both strains of mice at each time point represents the diverse systemic responses that can be seen in these mice after infection with M. arthritidis.
Limitations to our study include its small sample size and short period of followup. Furthermore, the sickest mice were sacrificed at each time point. Selecting the sickest mice for sacrifice gives a false impression that arthritis improves with time and makes it difficult to calculate statistical differences at each time point. It also allows for healthier mice to be analyzed later in the study and may be another reason why hemoglobin values had not declined further in both strains of mice by the end of the study.
In conclusion, serum hepcidin regulation in states of inflammation appears more complex than originally thought. Serum hepcidin may be upregulated independently of IL-6 and TLR4 activation. Splenic and liver hepcidin regulation is controlled by different mechanisms. TLR2 appears important in the regulation of hepcidin in M. arthritidis infected mice, but further work is needed to determine the exact mechanism of hepcidin expression in these mice.
This work was supported by the NIH Center of Excellence in Molecular Hematology grant SP30 DK072437 and an NIH grant DK070947 to JK. Dr. Koening is supported by the Public Health Services research grant numbers UL1-RR025764 and C06-RR11234 from the National Center for Research Resources and Dr. Mu is supported by a grant from the Nora Eccles Treadwell Foundation. The authors would like to thank Dr. Gerald Spangrude for measuring hemoglobin values.
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