Nearly 16 million Americans may ultimately benefit from research currently being carried out in southeastern Connecticut. They are individuals living with diabetes mellitus.
Diabetes is a chronic disease that affects the body's ability to produce or use the hormone insulin. This very intrusive disease, afflicting newborns, teenagers, and adults, requires constant monitoring.
It is also a very complex disease. Type I diabetes mellitus, previously called juvenile onset or insulin-dependent diabetes, is an autoimmune disorder. The insulin-producing beta cells in the islets of Langerhans of the pancreas are destroyed by the body's own immune system for reasons not yet fully understood. About half of the 800,000 patients with type I disease are under 20 years of age. Producing little or no natural insulin, these diabetics must inject insulin several times a day and closely monitor their blood glucose levels. Too little insulin allows glucose to accumulate in the bloodstream instead of being used by the cells for energy; this can result in hyperglycemia and acidosis with vomiting, rapid breathing, and drowsiness. Too much insulin lowers blood glucose levels too much-a condition called hypoglycemia-with possible disorientation, aggressiveness, and unconsciousness.
About 90-95% of diabetics have type II, once called non-insulin-dependent or late-onset diabetes mellitus. These individuals usually produce insulin, but either in too small quantities or their bodies show resistance to the hormone, or both. Either way, glucose accumulates in the blood. Many type II diabetics can control their glucose levels with diet, exercise, and medication to increase natural insulin production or action, but about 40% will need one or more daily insulin injections.
As difficult as diabetes is to live with, its complications are even more devastating-physically, emotionally, and financially. The most common complications include:
The federal government estimates that caring for these complications and the underlying diabetes that leads to them accounts for one in four Medicare dollars and one in seven dollars spent on health care annually in the United States.
The complexity of the disease and its complications means that research must explore a range of topics and involve diverse disciplines. The National Institutes of Health (NIH) are funding 13 Diabetes Endocrinology Research Centers, one of which is located at the Yale University School of Medicine in New Haven. Supported by a 5-year, $6 million grant, the Yale center provides the infrastructure needed to carry out research on the causes, improved treatments, complications, and prevention of diabetes. The center involves about 100 professionals from 16 departments, including molecular biologists, cell biologists, animal geneticists, and others.
Robert S. Sherwin, C.N.H. Long Professor of Internal Medicine, is the center's director.
"One of Yale's strengths is that in general it tends to encourage interactions in ways that are not always seen in other institutions," says Dr. Sherwin. "There is a tradition of collaboration. I think this is very important in a disease like diabetes, which is very complex and needs an interdisciplinary approach."
Specific projects carried out by the center are funded by additional grants from the NIH, Juvenile Diabetes Foundation International, American Diabetes Association, the Howard Hughes Medical Institute, and others. During the period from 1991 through 1996, support from these sources increased 100%.
While a cure is the ultimate goal for much of the research, this will not be achieved without a clear understanding of the causes. Among the Yale groups studying the cause of type II diabetes is one headed by Gerald I. Shulman, professor of medicine and cellular and molecular physiology and associate director of the center.
According to Dr. Shulman, insulin resistance is the best predictor of developing type II diabetes. "If we can understand and prevent insulin resistance, we may be able to prevent diabetes. If we are able to reverse the resistance, we may be able to treat diabetes effectively."
Two organs are primarily involved in the pathogenesis of resistance, the liver and the muscles. Dr. Shulman's group has identified a three-step process whereby glucose in the bloodstream is taken into the muscle cell and converted to glycogen. In individuals with type II diabetes, this glycogen synthesis is impaired. Using a noninvasive technique called nuclear magnetic resonance (NMR) spectroscopy that uses stable nonradioactive isotopes to provide biochemical information, the researchers have narrowed the defect to step one (glucose transport through the cell membrane) or step two (phosphorylation, the addition of a phosphate molecule to the glucose). Recently, they completed animal studies on a technique that will allow them to measure intracellular free glucose (step one); once human studies are complete, they should know whether step one or two is defective.
This group is also trying to understand the effect of exercise and fat on glucose synthesis. One study called for non-diabetic individuals who showed insulin resistance and had a family history of type II diabetes to exercise for six weeks. NMR spectroscopy found that these individuals had 43% greater insulin sensitivity after exercise because of a twofold increase in glycogen synthesis in muscle. The exercise appears to reverse the defect in glucose transport or phosphorylation, although not to normal levels.
As an autoimmune disease, type I diabetes calls for entirely different approaches to the cause. Charles A. Janeway, Jr., professor of immunobiology and an investigator in the Howard Hughes Medical Institute at Yale, is directing a program project to study the immunology of diabetes in mice.
"Destruction of beta cells [in the pancreas] characterizes type I diabetes," says Dr. Janeway. The immediate goal of his research has been to establish how this destruction takes place. In studies on mice reported earlier this year, Dr. Janeway's group identified a protein called Fas, produced by beta cells when they are approached by T lymphocytes. The T cells express a molecule called Fas ligand, which binds to the beta cell Fas and triggers the cell's destruction. The T-cells also express Fas itself.
"If we could equip the beta cells with the Fas ligand gene," hypothesizes Dr. Janeway, "then we could reverse this process." That is, when the T lymphocytes approach these new beta cells, the Fas ligand would bind to the T cells and kill them.
Dr. Janeway's group is now working to link up two genes in mice, one with a mutant Fas and one with Fas ligand and express them in the beta cells. Mice with these genes should have beta cells that would be resistant to spontaneous diabetes and to T-cell clones that express the Fas ligand.
Once the mouse-testing phase is completed within five years, the next phase would be to create a comparable human gene sequence and transplant it into a larger animal such as a sheep or pig. Once the researchers are able to establish that this system works with human genes, eventually transplants would be carried out in patients with type I diabetes.
The studies to date have shown that "we can potentially engineer beta cells that will resist destruction by normal lymphocytes," emphasizes Dr. Janeway.
While these and other researchers continue to search for the cause and a potential cure, others are working to find better ways to care for diabetic patients. William V. Tamborlane, Jr., professor of pediatrics and director of the Children's Diabetes Clinic at Yale, was the principal investigator at Yale for the Diabetes Control and Complications Trial, an NIH project that studied more than 1,400 people with type I diabetes. One of the study's most important conclusions, reported within the past three years, was that if patients closely monitored their blood glucose levels and aggressively carried out their insulin therapy, they could delay or prevent the onset of complications. (However, these patients also had increased incidence of hypoglycemia compared to patients whose diabetes was cared for in the traditional manner.)
Drs. Tamborlane and Sherwin are now investigating the mechanisms behind this increased incidence of hypoglycemia. Apparently, diabetics have a defective mechanism for counteracting hypoglycemia. The Yale researchers have focused their attention on the brain's response to glucose because brain tissue is the only type that must use only glucose (and not also fat) for energy. An area called the ventromedial hypothalamus has been identified as the control center; work continues on delineating just how the brain metabolizes glucose. Helping children, especially teenagers, cope with such an intrusive disease is another area of interest for Dr. Tamborlane and his associates. Teenagers have significant difficulties in regulating their insulin, and one question has been just how much is physiological and how much is psychosocial.
"There are dramatic hormonal changes in adolescence-not just gonadal hormones but also growth hormone levels become very high," notes Dr. Tamborlane.
What role this might play is still unclear, but what has been found is that while teenagers produce proportionally higher levels of insulin than adults, their bodies don't respond as well to it, even in nondiabetic children. While research continues on insulin metabolism in this age group, Margaret Gray, of Yale's School of Nursing, is carrying out a study in which teenage diabetics work closely with a "trainer" to develop special coping skills.
Eliminating the need to self-inject insulin would be a significant improvement in the life of most diabetics, especially those with type I. Dr. Sherwin's group is working with Pfizer Pharmaceuticals Group in Groton to test inhaled insulin. A handheld device, developed by Inhale Therapeutic Systems of Palo Alto, CA, blows a premeasured dose of powdered insulin into a cloud of tiny particles. The patient takes a slow deep breath and inhales the insulin into the lungs, where it is transferred to the bloodstream.
The device is currently in clinical trials. These must demonstrate not only the drug's efficacy in this form, notes Pfizer group director Thomas A. Beyer, but also what the long-term effects are of delivering insulin in the lung. Also, the form of insulin being used is short-acting, taken by diabetics just before meals.
"Many patients will still need basal [long-acting] insulin," says Dr. Beyer, "so the challenge is to find not only the delivery system, but also the time of delivery."
"Patients who develop type II diabetes are initially told to exercise and lose weight, but many don't and so are prescribed oral agents to help lower their blood sugar," adds Ralph Stevenson, assistant director for metabolic diseases at Pfizer. "However, within 10 years, 50% will experience failure of their medication to control their blood sugar and need injections of insulin. These people could significantly benefit from inhaled insulin therapy to help them control their blood sugar effectively."
Pfizer is also working from an entirely different angle to prevent one of the disease's major complications: nerve damage. Currently in phase III clinical trials, Alond is an aldosereductase inhibitor expected to be approved within the next three to five years. The enzyme's function in normal tissue is unknown. However, animal experiments have shown that inappropriate metabolism of glucose by aldose reductase in hyperglycemia leads to a chain of metabolic changes that ultimately result in nerve damage. By inhibiting the enzyme, Alond blocks these changes. The drug is expected to complete clinical testing within the next two to three years.
"There has never been a drug approved and marketed for diabetic complications in the United States," emphasizes Dr. Stevenson. "The odds of getting a diabetic drug onto the market are even worse than the 1 in 8 usually cited for new pharmaceuticals." In fact, Alond is the fifth candidate in this area that Pfizer has developed and the fourth to go into clinical trials. Yet none of the earlier drugs was found marketable for a variety of reasons.
Pfizer and Yale are both also attacking diabetes prevention, albeit from different perspectives. Dr. Tamborlane is taking part in the NIH Diabetes Prevention Trial 1. Once again, the research is based on what we now know about the role of T lymphocytes in destroying beta cells in type I diabetes. Research has shown that insulin secreted by the beta cells produces an insulin gradient which attracts the T-cells and thus triggers the immune response in the pancreas. The NIH trial involves giving individuals with moderate or high risk of developing diabetes small doses of insulin. The hope is that the T cells will be diverted by the injected insulin and thus will not be attracted to and destroy the beta cells. Thus, although these individuals would have to take some insulin, they would not develop diabetes, would need to take less insulin than if they became diabetic, and perhaps most importantly, would be spared the significant complications.
Pfizer hopes to prevent type II diabetes in many individuals by controlling obesity, which promotes resistance to insulin by as-yet unknown mechanisms. Individuals who are more than 20% over the normal range on the Metropolitan Life weight tables are particularly prone to develop type II diabetes. Pfizer is in the early stage of development of two compounds: one is a neuropeptide designed to block the brain's signal for hunger, and the other a beta adenergic agonist to induce energy expenditure.
There is even work that may someday result in a vaccine to protect everyone against developing diabetes. Drs. Janeway and Sherwin have created a clone T cell line that produces a cytokine that somehow keeps attacking cells away from beta cells. It will take many years, but once this mechanism is understood and a technique for inducing these cytokines in humans is found, a vaccine could be developed for use first with individuals at high risk of developing diabetes and then with the general population.--- J. Lynne Dodson, freelance medical writer.
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