|More than 12 years of research led Anath Shalev, M.D. (right, with Junqin Chen, Ph.D.) from a basic discovery to the first human trial of a new type of diabetes drug.|
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 UAB 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. (Learn more about the trial, which will begin in early 2015, in this story.)
|Anath Shalev explains verapamil's protective effects against diabetes, and a new human clinical trial of the drug at UAB, in this video.|
From Molecular Mechanisms to New TreatmentsThe beta-cell gene that responded to the high glucose in Shalev’s 2002 experiment produces TXNIP (pronounced "ticks-nip"), 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.
Details about the research journey show the incremental steps that basic science takes, and how those connected steps sometimes lead to potential clinical impacts.
Controlling TXNIP to Treat 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 lab 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.”
An Approved Drug Offers Protection
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.”
Another Role for TXNIP, Another Drug Target?
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.
TXNIP's Vicious Cycle
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.”
First Human Trial
|Get a quick overview of the science behind UAB's verapamil |
trial in this animation
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. [Learn more about this work, and other high-potential projects in the Alabama Drug Discovery Alliance, in a new feature from UAB Magazine.]
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.
— Jeff Hansen