Biologists at Washington University in St. Louis have discovered a mechanism in blood vessels that opens the door for bone loss in such diseases as rheumatoid arthritis, periodontal disease, osteoporosis, tumor-associated bone loss, or artificial implant loosening.
Patricia Collin-Osdoby, Ph.D., research associate professor of biology in Arts & Sciences at Washington University, and Philip Osdoby, Ph.D., professor of biology in Arts & Sciences, and Linda Rothe, Washington University research associate, have for the first time shown that blood vessels at inflamed sites where bone loss is occurring create signals that set into motion a cascade of events leading to local bone destruction.
When an area of tissue in or near bone becomes inflamed, key molecules called cytokines are locally produced and increase in the bloodstream. Studying human tissue and cell samples, the Osdobys have shown that two key inflammatory cytokines, interleukin-1 (IL-1) and tumor necrosis factor (TNF), signal the endothelial cells of blood vessels and capillaries to make and display on their cell surface a molecule called RANKL. RANKL is the critical signal that tells the body to make and activate bone-degrading cells called osteoclasts. After osteoclasts take bone away, osteoblasts go back in and add new bone. Normally, this bone remodeling, which is associated with a blood vessel or capillary at such sites, is a carefully balanced process. However, in persons with inflammatory bone disease, osteoclasts out-number and out-work the bone-forming osteoblasts, leading to weakened bone matrix, bone loss, and an increased risk of fracture. The Osdobys believe that inflamed blood vessels beckon cells to the region, and then initiate their development into highly active bone-degrading osteoclasts.
The researchers also found that in this biochemical chain of events, the blood vessels themselves make an antagonist molecule, osteoprotegerin (OPG), which neutralizes RANKL activity. Although OPG is made in this process, it peaks early and RANKL gets the upper hand. This is aided by the fact that RANKL is tethered on the cell surface while OPG is a soluble molecule that can be carried away by the circulation.
Drug chemical companies are interested in RANKL as a target and OPG as a possible therapeutic or preventative molecule to eliminate excessive osteoclast formation and activity. It is the progressive and irreversible loss of bone and cartilage that is the most difficult to control and treat in rheumatoid arthritis, periodontal disease or cancer. Current anti-inflammatory or chemotherapeutic treatments are inadequate for this purpose. However, OPG injection prevents such bone and cartilage loss, without interfering with normal bone remodeling.
More significantly, the discovery that blood vessel cells themselves are initiators of this elaborate process could make drug delivery easier or more efficient. Rather than receiving a local injection, patients may be able to take an oral or systemic dose that goes directly into the blood stream and allows the drug to work immediately in the early stages of RANKL activity. This could prevent new areas of bone degradation from getting started and slow down those that have already begun.
"People in the past few years have been looking at the expression of RANKL and OPG in bone marrow stromal cells, osteoblasts and T cells, but nobody had looked in blood vessels," Collin-Osdoby said. "There is a growing appreciation that blood vessels do much more in the body than simply provide a physical barrier and a route to passively deliver nutrients and cells. Our findings show that blood vessels can play a key role in actively regulating bone remodeling and physiology."
According to Osdoby, the discovery opens up several options to eliminate or minimize bone loss in inflammatory-related diseases.
"Because we know that blood vessels overgrow and are activated to cause osteoclast formation in inflammatory disorders, researchers can begin to think of ways to dampen the formation of new blood vessels or capillaries, deactivate the osteoclasts, or neutralize the RANKL expressed," he said. "It's the osteoclasts that are directly responsible for the loss of bone, even though many other cell types, signals, and enzyme activities are produced and play a role. So, the most obvious approach to prevent such bone loss is to directly interfere with the formation of bone-degrading osteoclasts."
The research was published in the June, 2001 issue of the Journal of Biological Chemistry. The research was supported by the National Institutes of Health.
Such popular arthritis drugs as Vioxx, Celebrex, and Enbrel work on the principal of interrupting the work of the cytokines to ease inflammation. Vioxx and Celebrex interrupt a similar cascade of events by preventing IL-1 from generating prostaglandins at inflammatory sites. Enbrel suppresses the work of TNF. However, something more is needed to avoid bone and cartilage destruction. Collin-Osdoby said that the balance between the levels of RANKL and OPG produced is critical for determining the amount of osteoclast formation and bone breakdown that occurs. Previous studies have shown that the absence of RANKL in mice leads to too much bone formation (osteopetrosis) because osteoclasts are not formed, whereas the absence of OPG in mice leads to severe bone loss and osteoporosis. Interestingly, mice lacking the OPG gene also have calcified arteries, leading to diseased blood vessels.
"We've shown that the endothelial cells of blood vessels make their own OPG and may represent a major source for the levels of this factor found circulating in serum," she said. "In the case we've studied, the amount of OPG that endothelial cells make is regulated by inflammatory cytokines."
The finding that RANKL and OPG are produced by vascular endothelial cells also has broader implications for the role of blood vessels in organ development and immune regulation. This is because RANKL also is critical for the formation of lymph nodes, lymphocytes, and immune cell interactions, while OPG counteracts cell death signals (by TRAIL molecules) to which cancer cells are particularly sensitive.
In addition to further RANKL studies, the Osdobys also will be looking closely at OPG production in response to key regulators of endothelial cells because this molecule seems to provide some survival benefits for the endothelial cells themselves.
Journal
Journal of Biological Chemistry