A study in the June 15 issue of The New England Journal of Medicine (NEJM) reveals that elevated levels of a molecule called RBP4 (retinol binding protein 4) can foretell early stages in the development of insulin resistance, a major cause of type 2 diabetes as well as cardiovascular disease.
The new findings, led by researchers at Beth Israel Deaconess Medical Center (BIDMC), offer a potential new target for the development of anti-diabetic therapies to lower serum RBP4 levels as well as an early means of identifying individuals who are at risk of developing diabetes - before the onset of overt disease.
"Type 2 diabetes is a rapidly increasing epidemic in the Western world," explains senior author Barbara Kahn, MD, Chief of the Division of Diabetes, Endocrinology and Metabolism at BIDMC and Professor of Medicine at Harvard Medical School. "Since it is now occurring even in childhood, predictions indicate that it could shorten lifespan in the U.S. for the first time in more than a century."
Insulin resistance develops when the body's muscles, fat and liver cells lose the ability to respond to the hormone insulin. Because insulin is necessary to enable the body to take up sugar from blood and convert it into energy, this impairment results in a buildup of glucose in the bloodstream.
"Insulin resistance not only predisposes individuals to type 2 diabetes, it is also a major risk factor for cardiovascular disease," adds co-lead author Timothy Graham, MD, an investigator in the Kahn laboratory. "Unfortunately, in the clinical setting, it is often difficult to distinguish individuals with and without insulin resistance."
Last year, in a study conducted in animals, Kahn's laboratory made the discovery that RBP4, a protein secreted from fat, can cause insulin resistance. Prior to this, the molecule was recognized only for its role in the transport of vitamin A.
In this new research, Graham, together with co-lead author Qin Yang, MD, PhD, set out to determine whether levels of RBP4, as measured in blood, correlate with the presence or absence of insulin resistance. (Study subjects represented three separate cohorts from San Diego, California; Goteberg, Sweden; and Leipzig, Germany.)
They first studied individuals with either obesity, impaired glucose tolerance (a "pre-diabetic" state), or with type 2 diabetes, comparing the blood levels of RBP4 in these insulin-resistant subjects with levels found in non-obese healthy subjects. Their results showed that not only were RBP4 levels higher in all cases in which insulin resistance was high, but that elevated serum RBP4 was also closely associated with components of the metabolic syndrome, including increased body mass index, waist-to-hip ratio, serum triglyceride levels, and systolic blood pressure, as well as decreased levels of high-density lipoprotein (HDL), or good, cholesterol.