Embargo expired: 6-Nov-2014 7:00 AM EST
Source Newsroom: University of Alabama at Birmingham
• Read this story and see more at www.uab.edu/news
Nov. 9, 2014 - BIRMINGHAM, Ala. – In 2002, diabetes researcher Anath Shalev, M.D., asked a basic question: What gene in the insulin-producing islets of the human pancreas is most turned on by high levels of glucose, a hallmark of diabetes?
The answer has led the University of Alabama at Birmingham endocrinologist to discover new cellular pathways in beta cells of the islets, pathways that are a key to diabetes progression or protection. Those discoveries have now opened the door to the first human trial of a potential diabetes drug with a mode of action different from any current diabetes treatment.
Beta cells are critical in type 1 and type 2 diabetes. In both diseases, the cells are lost gradually due to programmed cell death (apoptosis); but the trigger for that programmed death was unknown. The loss of beta cells contributes to the progression of diabetes, a growing worldwide epidemic that affects more than 20 million in the U.S., making it the seventh leading cause of death and the source of complications like blindness and more than 40,000 lower limb amputations a year.
The beta-cell gene that responded to the high glucose in Shalev’s 2002 experiment produces TXNIP, a protein normally involved in controlling oxygen radicals in many types of cells but never known to be important in beta-cell biology. Its response to glucose was intriguing because TXNIP (thioredoxin-interacting protein) was already recognized as a regulator of thioredoxin. Overexpression of thioredoxin had previously been shown to prevent experimentally induced diabetes by inhibiting the programmed death of islet beta cells. Since TXNIP inhibits thioredoxin, and because Shalev had discovered that islet TXNIP was highly regulated by glucose, Shalev realized that TXNIP might have major implications for beta-cell biology.
Over the next dozen years, Shalev — who left the University of Wisconsin–Madison to head the UAB Comprehensive Diabetes Center in 2010 — set out to reveal how TXNIP acts in cells at the molecular level, knowing that an understanding of those molecular mechanisms might point to possible new diabetes treatments. The payoff has been substantial: Using cell cultures, mouse models and pancreatic islets isolated from humans, the Shalev lab team has shown that manipulating TXNIP levels up or down in beta cells could exacerbate or protect against experimental diabetes.
In 2005, the Shalev lab team found that beta-cell TXNIP levels are higher in mouse diabetes models, and that experimentally increasing TXNIP levels in rat beta cells in vitro led to increased programmed cell death, by means of a well-known trigger signal of apoptosis. The Shalev team also found that sugars in general, whether metabolized or not, turn the TXNIP gene on. This clue led them to a newly identified carbohydrate response element (ChoRE) in the TXNIP promoter that acts as a regulator of TXNIP.
In 2008, the Shalev laboratory team developed mice that had little or no TXNIP in their beta cells. These lower levels protected against experimental diabetes. The team also discovered that the lower levels sent a known signal that inhibited mitochondrial beta-cell death. Shalev wrote, “These results suggest that lowering beta-cell TXNIP production could serve as a novel strategy for the treatment of type 1 and type 2 diabetes by promoting endogenous beta-cell survival.”
In 2012, the Shalev group tested an already approved oral drug that they had earlier found to reduce levels of TXNIP in heart cells. The drug — verapamil — is a calcium channel blocker used primarily to treat high blood pressure, but also to treat migraine headaches. Shalev’s team found that exposing in vitro beta cells or isolated human islets to verapamil reduced TXNIP expression, and halted programmed apoptotic death of beta cells. Furthermore, mice that were fed verapamil in their drinking water were protected from experimentally induced diabetes, and verapamil rescued mice that already had diabetes. The verapamil mice had lower TXNIP levels and less programmed beta-cell death, as well as better levels of insulin. In those studies, the group also revealed how verapamil lowers TXNIP — the decreased intracellular level of calcium ions caused by verapamil led to phosphorylation of the ChoRE binding protein that normally responds to glucose to control TXNIP transcription at the ChoRE. This phosphorylation prevented the binding protein from entering the beta-cell nucleus and interacting with the TXNIP gene. Shalev noted that these verapamil results identified, for the first time, “… an effective pharmacological means … to inhibit pancreatic beta-cell expression of proapoptotic TXNIP, enhance beta-cell survival and function, and thereby prevent and even improve overt diabetes and shed light on the mechanisms involved.”
In 2013, TXNIP was shown to play another crucial role in beta-cell biology when the Shalev laboratory team discovered that high levels of TXNIP directly blocked insulin production in beta cells, acting through a newly identified pathway. TXNIP, they found, induced a microRNA called miR-204, which in turn down-regulated the MAFA transcription factor involved in promoting transcription of the insulin gene.
This means that miR-204 may offer another target for a future RNA drug, an area that is currently also being actively pursued by the Shalev lab. MicroRNAs, with 20 to 24 noncoding nucleotides, have rapidly gained prominence as regulators of gene expression in health and disease. Researchers are beginning to explore whether silencing targeted microRNAs may lead to a treatment for cancers or other diseases.
This year Shalev reported that TXNIP — surprisingly — can induce its own transcription. Her UAB research team found that TXNIP does this by affecting the same ChoRE binding protein (ChREBP) that was previously found to be key in the response to the drug verapamil. The researchers experimentally elevated TXNIP levels in beta cells and found this caused decreased phosphorylation of ChREBP, which led to its increased entry into the nucleus and its increased binding to the TXNIP promoter to boost transcription. This creates a harmful positive-feedback loop.
“… these findings support the notion,” Shalev wrote in this 2014 paper, “that TXNIP levels rise over time, not only as a result of elevated blood glucose levels and/or endoplasmic reticulum stress, but also as part of a vicious cycle by which increased TXNIP levels lead to more TXNIP expression and thereby amplify the associated detrimental effects on beta-cell biology including oxidative stress, inflammation, and ultimately beta-cell death and disease progression.”
The story doesn’t end here. Shalev’s long trail of laboratory research has now led to the first human trial to see if verapamil has an effect in patients who have developed type 1 diabetes within the previous three months. Adult volunteers, ages 19-45, will be treated with verapamil or a placebo for one year, as their insulin and blood glucose levels are continuously monitored. The three-year, $2.1 million trial will be conducted by the UAB Comprehensive Diabetes Center with funding from JDRF, the largest charitable supporter of type 1 diabetes research.
Meanwhile, a UAB partnership with the Southern Research Institute — called the Alabama Drug Discovery Alliance — is already working to develop small therapeutic molecules that mimic the diabetes-protecting effect produced by verapamil and inhibit TXNIP, but have a greater selectivity and efficacy.
So Shalev’s simple question — what gene in insulin-producing beta cells is most turned on by glucose? — has thus led the research out of her laboratory to possible new drugs, acting against a novel target to alleviate or reverse diabetes.
About the UAB Comprehensive Diabetes Center
The University of Alabama at Birmingham Comprehensive Diabetes Center is a university-wide interdisciplinary research center comprising approximately 190 faculty from 10 different schools and multiple departments. It is the umbrella for various research projects, programs and awards, including the Diabetes Research and Training Center, one of only seven centers nationwide funded by the National Institutes of Health. Faculty at the Comprehensive Diabetes Center conduct cutting-edge research into the causes and consequences of diabetes and work collaboratively toward the discovery of better treatment approaches and a cure for diabetes. The center also offers educational services with conferences, seminars and training opportunities for scientists, clinicians and the public, and provides a number of state-of-the-art diabetes specialty clinics including a Multidisciplinary Comprehensive Diabetes Clinic to meet the needs of patients with diabetes.
Known for its innovative and interdisciplinary approach to education at both the graduate and undergraduate levels, UAB is an internationally renowned research university and academic medical center and the state of Alabama’s largest employer, with some 23,000 employees and an economic impact exceeding $5 billion annually on the state. The five pillars of UAB’s mission deliver knowledge that will change your world: the education of students, who are exposed to multidisciplinary learning and a new world of diversity; research, the creation of new knowledge; patient care, the outcome of ‘bench-to-bedside’ translational knowledge; service to the community at home and around the globe, from free clinics in local neighborhoods to the transformational experience of the arts; and the economic development of Birmingham and Alabama. Learn more at www.uab.edu.
EDITOR’S NOTE: The University of Alabama at Birmingham is a separate, independent institution from the University of Alabama, which is located in Tuscaloosa. Please use University of Alabama at Birmingham on first reference and UAB on all subsequent references.