Researchers at Joslin Diabetes Center have investigated PKC-beta - a critical enzyme implicated in the devastating complications of type 1 and type 2 diabetes - for more than two decades.
Their latest research, presented at the American Diabetes Association's (ADA) 65th Scientific Sessions in San Diego, Calif., confirms the link between hyperglycemia (high blood glucose), overexpression of PKC-beta 2 and kidney disease.
"The significance of this study is that we found strong evidence linking chronic activation of a specific form of the PKC enzyme - beta 2 - to the abnormal kidney changes and oxidative stress seen in diabetes," said George L. King, M.D., the study's lead author, Joslin's Director of Research, Head of the Section on Vascular Cell Biology, and a Professor of Medicine at Harvard Medical School. Other investigators in the study included previous Joslin fellows Yutaka Yasuda, M.D., Ph.D., and Noriko Takahara, M.D., as well as Timothy S. Kern, Ph.D., of Case Western Reserve Medical School, Cleveland, Ohio.
Protein kinase C (PKC) is an enzyme essential to the normal functions of the cell and the body. The PKC family of enzymes, which helps regulate many blood vessel functions, comprises about a dozen different molecular forms, or isoforms, including PKC-alpha, PKC-beta 1, PKC-beta 2 and PKC-delta.
In this study, Dr. King and his colleagues proposed that chronic activation or overexpression of the PKC-beta 2 isoform plays an important role in the progression of diabetic kidney disease. To test this hypothesis, researchers used genetic engineering techniques to develop mice that expressed three times the normal amount of PKC-beta 2 in tiny blood vessels.
"Using transgenic mice, we targeted the specific isoform, PKC-beta 2, to the blood vessels to test our hypothesis," said Dr. King. "By manipulating the gene that makes this isoform, we created mice that overexpressed only PKC-beta 2 which mimics the effects of high glucose levels and diabetes."
Dr. King and his team compared these transgenic "overexpressors" with normal mice - in both diabetic and nondiabetic mice models - using conventional screening and diagnostic tests to assess the progression of kidney disease in each model.
In diabetes, hyperglycemia overactivates PKC-beta and gradually damages the microvessels of the kidney. Over time, tiny capillaries known as glomeruli become so porous they can no longer adequately filter waste from the blood, and instead allow large proteins such as albumin to pass into the urine (an early sign of kidney damage).
In this study, albumin levels were tenfold higher in overexpressors of PKC-beta 2 than in normal mice after six weeks. At six months of age, although the two groups of mice did not differ in filtration rate, 24 percent of the glomeruli of the overexpressors showed mesangial expansion (enlargement of specialized kidney cells, another measure of kidney disease) compared with only 8.9 percent of normal mice. In the diabetic models, 49 percent of the glomeruli of overexpressors showed mesangial expansion compared with 32 percent of the normal-expressing mice. Finally, oxidative stress - the accumulation of destructive molecules, such as free radicals - was three times higher in the nondiabetic PKC beta 2 transgenic mice than in normal mice.
These findings demonstrate that chronic activation of the PKC-beta 2 isoform, induced by hyperglycemia, is at least partially responsible for the development of abnormal changes in the kidney and the increase in oxidative stress associated with diabetes.
Dr. King and his team were the first to propose, in the late 1980s, that PKC is the major signaling pathway stimulated by hyperglycemia, the hallmark of diabetes. Since then, dozens of studies in the King Laboratory have investigated the role of PKC in blood vessel damage of the eye, kidney, heart and large arteries - organs that can deteriorate over time in people with diabetes, leading to complications such as blindness, heart disease and kidney failure.
In the early 1990s, Dr. King began working with other scientists to design a chemical inhibitor that would block PKC and therefore delay the onset of symptoms or prevent diabetic kidney disease and other complications altogether. To learn more about Joslin Diabetes Center's leadership role in PKC research, please click on the following link to background information on Joslin's Website.