'Fail safe' mechanism that helps keep inflammation in check
La Jolla, CA – Shutting down a master activator of the body's inflammatory response – which is the goal of several experimental drugs now in development for the treatment of arthritis – may create even more inflammation with its associated pain and swelling in the body.
The culprit is one of the two subunits that make up the IKK protein complex, which is known to regulate several biological processes including inflammation. While studying the basic function of the IKK complex in zebrafish and mice, researchers at the Salk Institute for Biological Studies discovered that one subunit reigns in the activity of the other instead of boosting it, as was widely believed before.
The findings are reported in two independent papers: the zebrafish study was published in the July 26 issue of Current Biology, the mouse study will be available in the early online edition of the Proceedings of the National Academy of Science this week.
"Inflammation plays a central role in many diseases, cancer, arteriosclerosis, rheumatoid arthritis, infections, in fact, inflammation is a major cause of human misery," says Inder Verma, professor in the Laboratory of Genetics and lead scientist of both studies. "It is important to understand how inflammation is regulated because without knowing the mechanism of inflammation we won't be able to resolve it," he adds.
"IKKs are one of the most heavily sought after drug targets today, because pharmaceutical companies want safe and effective medications to treat the pain of arthritis, now that the safety of COX-2 inhibitors, like Celebrex and Vioxx, have been called into question," says Salk researcher Vinay Tergaonkar who was involved in the zebrafish study. "But this work shows that designing a generic inhibitor of IKK could potentially cause more harm than it might help."
The IKK complex is a set of very similar kinases, known as IKK1 and IKK2. Kinases are molecules that are essential to turning on or off other proteins, and the IKK complex is believed to control the function of nuclear factor-kappa B (NF-kB). NF-kB is a powerful protein, which is essential for many cellular processes, including embryonic development and triggering an immune response against invading microbes. But, when overly active, NF-kB can help promote several disorders, including arthritis, in which inflammation spirals out of control.
Because NF-kB is also involved in embryonic development, Verma and his team got together with Juan Carlos Belmonte`s laboratory at Salk and turned to zebra fish to determine what would happen if IKK1 was deleted. (The embryos of zebra fish are fast growing and transparent, making it easy for scientists to follow their growth.) The researchers found embryos without IKK1 had structural malformations that they determined were due to increased, rather than decreased NF-kB activity.
"We saw there was no brake on NF-kB activity in these fishes and that clearly surprised us," says Tergaonkar a researcher in the Laboratory of Genetics.
The investigators theorize that IKK2 is the subunit that is critical to activating NF-kB, and that IKK1 operates as a 'fail safe system' to keep IKK2 in check.
"Our study offers not only a better understanding of how NF-kB pathway regulates early embryogenesis, but also how careful we have to be regarding the use of IKK inhibitors at the clinical level," says the Current Biology paper's first author, Ricardo Correa, formerly a scientist in Verma's Laboratory of Genetics and now a research fellow at University College London.
Intrigued by their results in zebrafish, the researchers then asked whether IKK1 has the same function in mammals and, when rendered inactive, would let inflammation run amok. They studied macrophages, a type of white blood cell, since these cells play a central role in acute inflammatory responses. When the body is attacked by microbes, macrophages release a flurry of chemical signals that trigger inflammation. In addition, they put other parts of the immune system on alert and function as a clean-up crew, recognizing invading pathogens and gobbling them up, a process known as phagocytosis.
The researchers isolated macrophages from mice in which they had deleted IKK1 and analyzed their reaction to bacteria. "We saw what you would expect from uncontrolled IKK2 activity," says Qiutang Li, first author of the mouse study. "More phagocytosis, more activation of other immune cells, and increased secretion of inflammation triggers."
"When you send out an army to combat invading germs, you need to be able to call it back when its job is done," says Verma. In any case, the researchers agree that these basic research findings provide an early warning to pharmaceutical drug developers.
Salk scientists who contributed to the zebrafish study include Ricardo G. Correa, Takaaki Matsui, Vinay Tergaonkar, Concepcion Rodriguez-Esteban, Juan Carlos Izpisua-Belmonte, and Inder M. Verma. For the mouse study, Qiutang Li, Qingxian Lu, Virginie Bottero, Gabriela Estepa, and Inder Verma collaborated with Lisa Morrison and Frank Mercurio at Celgene in San Diego.
The Salk Institute for Biological Studies in La Jolla, California, is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health and the training of future generations of researchers. Jonas Salk, M.D., whose polio vaccine all but eradicated the crippling disease poliomyelitis in 1955, founded the Institute in 1960 on land donated by the City of San Diego and with the financial support of the March of Dimes.