NIAMS study suggests novel therapeutic target for TRAPS, other inflammatory diseases
BETHESDA, Md.—Scientists at the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), a component of the National Institutes of Health (NIH), have shown that molecules called reactive oxygen species (ROS) produced by mitochondria in cells play a role in TNF receptor-associated periodic syndrome (TRAPS), a rare inherited disorder in which uncontrolled inflammation damages the body's tissues. Further, by blocking these molecules, the scientists say they have been able to reduce inflammation in TRAPS, and this method could be used to treat other inflammatory diseases.
According to the scientists, their findings suggest that mitochondrial ROS may be a novel therapeutic target for TRAPS, which is characterized clinically by recurrent fevers with abdominal pain, migratory rash and inflammation, as well as other related disorders such as the cryopyrin-associated periodic syndromes (CAPS), and diseases which contain an inflammatory component, such as rheumatoid arthritis.
"ROS have been implicated in a variety of conditions, including cancer and atherosclerosis," explains Dr. Richard Siegel, NIAMS autoimmunity branch chief and acting clinical director. "In addition, it has previously been shown that they can activate a number of key molecules which signal for inflammation."
The gene for TRAPS was discovered by Dr. Daniel Kastner's research group in 1999. With a background in the area of the TNF superfamily of cytokines, Siegel has been working with Kastner on the gene's mechanisms. Their collaboration has resulted in a number of discoveries about the function of the mutant TNFR1 receptor and the pathogenesis of disease in TRAPS.
"We have previously shown that TRAPS- associated mutant receptors do not bind TNF, do not traffic to the cell surface and instead are retained at high levels in the endoplasmic reticulum in cells of TRAPS patients and mice with mutations homologous to two TRAPS-associated mutants (TRAPS "knock-in" mice)," Siegel says. "ROS can be produced within cells by a variety of sources, including enzymes and mitochondria, but previously, it was not clear which source of ROS was important for intracellular signaling that promotes the production of intracellular cytokines."
Siegel's team hypothesized that ROS play a role in normal inflammatory cytokine production as well as the increased cytokine responsiveness to LPS in TRAPS. They initially ruled out NADPH oxidases, which are well-known sources of ROS in inflammatory cells, and then turned their attention to mitochondria, partly based on the finding that the antioxidant drugs Diphenyliodonium (DPI) exert their anti-inflammatory effects at levels which block mitochondrial respiration, which are 100-fold higher than the doses that block NADPH oxidases.
"Once we made this observation, we hypothesized that cells in TRAPS may be predisposed to inflammation because the mutant receptor drives increased mitochondrial respiration," Siegel says.
Using standard cell and molecular biology techniques, Siegel and his colleagues found that ROS generated by mitochondrial respiration are important for normal lipopolysaccharide (LPS)-driven production of a number of pro-inflammatory cytokines and for the enhanced responsiveness to LPS seen in cells from patients with tumor necrosis factor receptor-associated periodic syndrome (TRAPS).
"This highlights the interplay between cellular metabolism and the inflammatory response, suggesting that these processes are capable of regulating one another," Siegel says. "With such a rare disease, this is a unique environment in which hypotheses can be rapidly tested in patient cells, and ultimately in investigational drug trials we hope will stem from this study."
As far as producing a new therapeutic approach to treating TRAPS and other inflammatory disorders, Siegel cautions that the efficacy of antioxidants in inflammation will have to be studied in controlled clinical trials.
"Although drugs that work in cells and mice do not always translate into humans, these studies provide a new avenue for future investigation," he says.
Siegel's team will next explore how the mutant TNFR1 receptor leads to a phenotype of enhanced mitochondrial function, as well as whether mitochondrial ROS may play a role in the pathophysiology of other inflammatory conditions.
"We are also contemplating a clinical trial of antioxidants that would be active against mitochondrial ROS production as adjunctive therapy for TRAPS," Siegel says. "Current therapies with anti-inflammatory drugs (e.g., steroids, NSAIDS), TNF and IL-1 inhibtors do not fully block symptoms of this disease. Although TRAPS is a rare disease, we believe that because they are genetically and clinically so homogeneous, testing antioxidants in this setting would allow us to see if these agents work in this disease as a proof-of-concept, but allow us to study the more general question of the role of mitochondrial ROS in inflammatory disease in a relatively homogeneous group of patients."
The study, "Mitochondrial reactive oxygen species promote production of proinflammatory cytokines and are elevated in TNFR1-associated periodic syndrome (TRAPS)," appeared last month in Journal of Experimental Medicine.
Siegel's co-authors included: Ariel C. Bulua, Anna Simon, Ravikanth Maddipati, Martin Pelletier, Heiyoung Park, Kye-Young Kim, Michael N. Sack and Daniel L. Kastner. The study was supported by intramural research funding from the NIAMS. Bulua was supported by the NIH Clinical Research Training Program, a public–private partnership between the Foundation for the NIH and Pfizer Inc.