American Association for Clinical Chemistry
Better health through laboratory medicine
June 29, 2006



In This Issue . . .

Targeting the Early Days of Type 2 Diabetes by Measuring RBP4
Julie L McDowell

 Insulin resistance is one of the primary causes of type 2 diabetes, as well as a major risk factor for cardiovascular disease. Identifying a biomarker for insulin resistance would be a boon for clinicians, as it would offer an opportunity to intervene in the early stages of the condition, before the onset of type 2 diabetes. This issue of Strategies looks at recent research published in the New England Journal of Medicine indicating that retinol binding protein 4 (RBP4) has the potential to be a biomarker for insulin resistance, as well as cardiovascular disease.

Currently, clinicians evaluate patients for insulin resistance by measuring patients' fasting insulin levels or a constellation of other parameters (serum lipids, inflammatory markers, etc) that are not specific for insulin resistance. However, fasting insulin measurement is a clinical snapshot of a moment in time and is not highly sensitive. Furthermore, a threshold for insulin resistance has not been standardized. More reliable measurements such as euglycemic clamp studies can be used for research studies but are not feasible for clinical practice, according to Barbara K. Kahn, MD, Chief of the Division of Diabetes, Endocrinology, and Metabolism at Beth Israel Deaconess Medical Center (BIDMC) in Boston and Professor of Medicine at Harvard Medical School . She is also the senior author of a study on RBP4 and insulin resistance in the June 15 th issue of the New England Journal of Medicine (NEJM) [2006;354:2552-63]. “Insulin levels can vary greatly over the course of a day,” she explained. “Fasting insulin tests are okay, but they are not absolutely diagnostic, because of variability from day to day and lack of standardization.”

Other than this test, insulin resistance is evaluated through a constellation of factors, including serum lipid levels. “Many insulin resistant people—not all, but many—will have elevated triglycerides, along with low HDL and high LDL levels,” said Kahn. “But then we're really looking at metabolic syndrome, not insulin resistance per se.”

Research from Kahn's lab suggests RBP4 could be a good biomarker for insulin resistance. This protein, secreted by fat cells called adipocytes, is the only transport protein for retinol or vitamin A in the body's circulatory system. In previous research, Kahn and her co-authors found that elevated levels of RBP4 correlated with insulin resistance in mice. In this recent study, the researchers wanted to see how RBP4 levels correlated with insulin resistance in humans, and if an intervention that improved insulin sensitivity also lowered serum RBP4 levels. “If we found that RBP4 could be used as a marker, or a therapeutic target, then clinicians could intervene early and try to prevent the onset of diabetes,” said Kahn.

The BIDMC researchers looked at three groups of patients. The first groups came from a VA hospital in San Diego ; they were lean (controls), obese without diabetes, or obese with type 2 diabetes. The second group of patients was from Leipzig , Germany , and were newly diagnosed with either impaired glucose tolerance (IGT) or type 2 diabetes. People with impaired glucose tolerance are known to have a high risk for developing diabetes and cardiovascular disease. The third group of patients had a family history of diabetes, but were lean and didn't have any overt disease. “Everybody in this third group had one or two first degree relatives with type 2 diabetes,” said Kahn. “We wanted to know whether we could see the association with insulin resistance even in lean people who have a high genetic risk for developing type 2 diabetes, and if so, how early could we see it? Could we see it before there's even an abnormality on the glucose tolerance test?”

The insulin resistant people in all of the groups had elevated levels of RBP4, leading the researchers to conclude that this protein is highly correlated with insulin resistance in lean people; obese-non-diabetic patients; obese people with impaired glucose tolerance patients; and obese-diabetic patients. In addition, when exercise was introduced, there was a reduction in RBP4 in those patients whose insulin resistance improved. “It looks like, unequivocally, RBP4 is an excellent marker for insulin resistance and probably also for these other components of the metabolic syndrome, which includes the cardiovascular risk factors, and it looks like it predicts quite early, before there is any overt disease,” said Kahn.

RBP4's Role

While Kahn and her research team may have established a correlation between insulin resistance and RBP4, the study did not attempt to determine whether RBP4 has a causal role in the pathogenesis of insulin resistance and type 2 diabetes, said an accompanying NEJM editorial by Kenneth Polonsky, MD, Professor of Cell Biology and Physiology, Department of Medicine, Washington University School of Medicine, St. Louis, Mo. Future research needs to explore this causal question, as well as questions about the signal pathways activated or inhibited by RBP4 that could affect insulin action and the biologic action of RBP4 in relation to insulin resistance and diabetes. Proposed Role of RBP4 in the Pathogenesis ofInsulin Resistance and Glucose Intolerance click on thumbnail to see larger imagePolonsky, K. S. N Engl J Med 2006;354:2596-2598Copyright © 2006 Massachusetts Medical Society. All rights reserved.

Proposed Role of RBP4 in the Pathogenesis of Insulin Resistance and Glucose Intolerance

click on thumbnail to see larger image

Polonsky, K. S. N Engl J Med 2006;354:2596-2598 Copyright © 2006 Massachusetts Medical Society.
All rights reserved

Kahn acknowledges that the causal role of RBP4 has only been shown in mice. “Whether or not RBP4 is a cause, we don't know in humans,” she explained. “But in mice, we know that if we raise the levels of RBP4 we can make them insulin resistant.”

Kahn and Tim Graham, co-first author on the NEJM paper, are also concerned about the RBP4 detection capabilities of the commercial assays currently on the market, none of which is absolutely reliable for quantitating elevation of RBP4. Kahn's team used a commercially available ELISA, but the manufacturer has since changed the plate coating procedure, the incubation times, and other critical aspects of the assay, she explained. The team has also tested several other kits on the market. The primary problem is that most RBP4 assays were developed to assess vitamin A status, and so were optimized in the low ranges to diagnosis vitamin A deficiency which is generally associated with low RBP4 levels. Therefore, Graham and Kahn are calling on the clinical lab community to develop RBP4 assays which are linear over a larger dynamic range. “ The current assays were not developed with enough dynamic range to accurately quantitate the range of concentrations that we see,” added Kahn. “The assays need to be modified and right now the only reliable way to see these kind of elevations is by quantitative Western blotting using full-length recombinant RBP4 standards.” Julie McDowell is the Editor of Strategies. She can be reached by email.