Biomedical Frontiers: Fall 1997, Vol.4, No.2
Diabetes Research
THE RAGE IN DIABETES

No matter what the original cause of their disease--genetic susceptibility, infection, environmental factors--people with diabetes have one thing in common: an increased risk of complications, including cardiovascular disease and strokes, kidney failure, blindness, and poor wound healing. Now, a receptor first identified by CPMC researchers 1990 is proving to have a central role in many of these disorders and is offering hope for preventing, treating, and possibly even reversing the complications of diabetes.

AGE and its cell surface receptor, RAGE, are key components in diabetic complications.

When blood sugar levels are not optimally controlled, as happens in diabetes, a process called non-enzymatic glycation takes place. In a process similar to the browning of food when it cooks, lipids and proteins join together permanently to form molecules called advanced glycation endproducts, or AGEs. These AGEs accumulate in the blood vessel walls, skin, nerves, and virtually every tissue of the body.

AGEs affect cellular functions by binding to cell surface receptors known as Receptor for AGE, or RAGE. This interaction impairs the function of critical cells, eventually leading to the complications of diabetes, researchers believe. Drs. Ann Marie Schmidt, assistant professor of medicine and of surgery, and David Stern, professor of physiology and cellular biophysics in surgery, and collaborators provided the first definitive identification of RAGE in two studies published in 1992 in the Journal of Biological Chemistry.

When AGE binds to RAGE, endothelial cells that are normally tightly opposed form gaps through which blood components can leak.

In characterizing RAGE, the researchers found that its structure resembled that of immunoglobulins that ordinarily defend the body against infection. The RAGE gene has been mapped to chromosome 6 in the major histocompatibility complex, the site where many genes controlling the immune response are found. This suggests that RAGE also participates in immune and inflammatory disorders.

RAGE, Endothelial Cells, and Vascular Complications
Endothelial cells, which form the inner lining of blood vessels, express RAGE. In people with diabetes, the endothelium is bathed in AGEs from both the circulating blood and the vessel wall. The binding of these AGEs to endothelial RAGE disturbs critical vascular homeostatic functions. For instance, an important job of the endothelium is to maintain a barrier to prevent blood components from leaking into tissues. Following AGE binding to RAGE, the initially tightly opposed endothelial cells withdraw from each other, forming gaps through which fluid can leak. Working with Dr. Jean-Luc Wautier at the University of Paris, Drs. Schmidt and Stern demonstrated that blocking access of AGE to RAGE reversed the hyperpermeability of diabetic vessels (Journal of Clinical Investigation, Jan. 1996). Since increased leakage of blood vessels is an early step in vascular complications, the ability to reverse this defect could have important clinical implications.

The molecule sVCAM-1, present in the blood of people with diabetes, may serve as a marker for monitoring AGE-RAGE cellular disturbances. This image demonstrates a possible pathway through which AGE-RAGE interaction triggers intracellular events leading to expression of VCAM-1 and sVCAM-1.

One way that endothelial cells control the vascular micro-environment is by expression of cell surface molecules called cell adhesion molecules. These molecules selectively promote binding of certain classes of white blood cells to the vessel wall. For instance, in acute inflammation, endo-thelial cell adhesion molecules attract white blood cells known as polymorphonuclear leukocytes, which defend against infection.

In diabetes the situation is different. When AGEs bind to endothelial RAGE, the endothelial cells express Vascular Cell Adhesion Molecule-1 (VCAM-1), which recruits monocytes to the vessel wall (Journal of Clinical Investigation, Sept. 1995). Monocytes infiltrate the blood vessel wall, setting the stage for diffuse and accelerated atherosclerosis as they become activated, elaborating cytokines and growth factors. Diabetic blood vessels show an increased expression of VCAM-1 and monocyte infiltration in pathological studies.

In animal studies, sRAGE binds up AGE in diabetic tissues, preventing its interaction with RAGE.

In addition to intact cell surface VCAM-1, Drs. Schmidt and Stern have found that the endothelial cells also release a smaller molecule called sVCAM-1, which conceivably could serve as a marker for monitoring AGE-RAGE mediated cellular perturbation. Studies of people with diabetes at especially high risk for vascular complications have found elevated levels of sVCAM-1.

CPMC researchers have created a mouse model to study RAGE and accelerated atherosclerosis. "We are hopeful that we will eventually be able to use RAGE blockade therapy to prevent and treat complications of diabetes, especially in vascular disease," says Dr. Schmidt.

Before Insulin Therapy	After Insulin Therapy
The first patient to be treated with insulin at CPMC, in 1923. The insulin was received from Banting and Best in Toronto.

Other Complications
CPMC researchers also have implicated RAGE in other complications of diabetes. For instance, people with diabetes are prone to impaired wound healing, which can result in ulcers, infections, and, ultimately, amputations. Drs. David Chiu, June Wu, and Eric Rose in surgery, along with Dr. George Todd in vascular surgery and Drs. Schmidt and Stern, established a model of delayed wound healing in genetically diabetic mice. Applying a short form of RAGE (sRAGE) to wounds in these mice speeded healing. The researchers believe that sRAGE binds up AGEs in diabetic tissues, preventing their interaction with RAGE.

AGE-RAGE interaction is likely to contribute to other complications, from accelerated retinal disease (now under study in collaboration with Dr. Stanley Chang in the Eye Institute) to renal failure and serious periodontal disease. A study now under way in collaboration with Dr. Ira Lamster of the School of Dental and Oral Surgery is examining the effects of blocking RAGE on accelerated periodontal disease in diabetes.

Source: Diabetes in America, 2nd edition, NIH, 1995.

"RAGE appears to be closely intertwined with the pathogenesis of immune, inflammatory, neurodegenerative, and diabetic disorders," says Dr. Stern. "The clear and pressing challenge is to exploit our insights from the biology of RAGE to develop a new approach to the prevention and cure of diabetic complications."