What is EPAC2 and when was it first discovered?
EPAC stands for exchange protein activated by cyclic AMP (cAMP). cAMP is an (among many) intracellular messenger molecule.
cAMP is generated when certain hormones stimulate a cell by binding to their receptor, which is at the outside surface of the cell. For example glucagon-like peptide-1 binds to its receptor on pancreatic ß-cells and stimulates cAMP production in the cell.
The main target of cAMP is an enzyme known as protein kinase A (PKA). For a long time PKA was the only known cAMP responsive protein. But in the 1990’s it was discovered that when PKA was inhibited, cAMP still had effects in pancreatic ß-cells. This effect was called the non-PKA effect of cAMP.
A convergence of studies done in three different laboratories around the world then found that there are cAMP responsive proteins, which are different from PKA. These were called exchange proteins.
Furthermore, exchange proteins activated by cAMP were discovered in all cells. EPAC2 is found mainly in the pancreatic islets of Langerhans and in the brain.
EPAC2 was previously thought to have a minor role in type 2 diabetes. What exactly was EPAC2 thought to do and what evidence was there for this minor role?
Following the considerations above, we now have in the pancreatic ß-cell two separate cAMP responsive signalling pathways. One is the cAMP-PKA branch. The other is the cAMP-EPAC2A branch.
Both are activated in ß-cells by the hormone glucagon like peptide-1. Both signaling branches are considered to increase insulin release from ß-cells when these are stimulated by increasing blood glucose levels.
Further studies of these two individual signalling branches indicated that the cAMP-PKA branch was very strong and necessary for the ß-cell to respond to cAMP stimulation. Therefore, the cAMP-PKA branch was considered to be the dominant signalling pathway and the cAMP-EPAC2A pathway was considered to be the “second fiddle”
A laboratory in Japan (Dr. S. Seino’s laboratory) then generated a mouse in which the EPAC2A gene was silenced and the mice did not have any EPAC2A in their pancreatic islets. These mice showed no major defect in their ability to handle glucose.
But the Japanese group found that the EPAC2A deficient mice (EPAC2A knockout mice) did not respond well to sulfonylurea class of drugs, which are used to help ß-cells from diabetic patients to secrete insulin.
When ß-cells fail, diabetics benefit from sulfonylurea drugs. Dr. Seino’s findings provided a clue that EPAC2A was important in improving ß-cell function in patients with type 2 diabetes.
How did your research into EPAC2 originate?
My laboratory had been studying the cAMP-PKA branch of glucagon-like peptide-1 signaling in pancreatic ß-cells. We had identified a protein called snapin, which is a target of PKA action. When PKA acts on snapin, snapin assembles a group of proteins, which then literally allow insulin to exit the ß-cells.
Insulin is normally stored in small vesicles (small bubbles) within the ß-cells. And snapin allows these bubbles to reach the outside surface of the ß-cells and to release insulin into the blood stream.
When we examined which proteins snapin was interacting with, we found EPAC2A to be a partner!
That led us to ask, what if EPAC2A would be missing form the complex that snapin assembles before insulin is released?
What did your research involve?
Drs. Seino and Shibasaki from Kobe University in Japan were very generous and provided us the EPAC2A knockout mice, which he had developed. Just like they had observed, EPAC2A knockout mice did not show any difficulties with insulin secretion in response to a glucose stimulus.
But then we looked at what happens when the EPAC2A knockout mice are treated with glucagon-like peptide-1 analogues (GLP-1 analogue), which are currently being used to treat type 2 diabetes.
We also looked at what happens when EPAC2A knockout mice are given a diet, which is high in calories and high in fat content – a diet, which may be similar to a diet of a human being who lives of fast food.
We wanted to know whether EPAC2A had a role when the ß-cells were challenged to ramp up their insulin secretion.
What role did your research find EPAC2 to have? Is this in addition to the previous minor role EPAC2 was thought to have in type 2 diabetes?
We found that while EPAC2A was not required for a lean mouse on a normal diet to be able to handle blood sugar, EPAC2A was required for the ß-cells to ramp up their insulin release when the mice were challenged with a high fat diet or when the mice were treated with the GLP-1 analogue.
A newer class of drugs used in type 2 diabetes to stimulated ß-cell function are called GPR40 receptor activators. GPR40 activation acts of the cAMP signalling pathway.
We assumed that GPR40 activation would work just the same in normal and EPAC2A knockout mice. To our surprise, EPAC2A knockout mice responded less well to GPR40 activation as compared to normal control mice.
Thus we found that EPAC2A is not only important for GLP-1 – cAMP stimulated insulin secretion, but that EPAC2A is generally required for ß-cells to ramp up their insulin release.
Our studies also showed that EPAC2A plays a pivotal role in allowing the ß-cell to increase its calcium levels – this is absolutely required for insulin to be released from ß-cells.
In summary, in all situations that we looked at where –cells are held to ramp up insulin release, EPAC2A is required for the ß-cell to do that in full force.
What impact do you think your findings will have?
Our findings mainly point towards a role for EPAC2A in ß-cells when these are challenged to produce and release more insulin, as they are in obese and diabetic humans.
The findings indicate that the ß-cells has a mechanism to respond to the higher demand to release insulin. This mechanism involves EPAC2A – and without EPAC2A the ß-cell cannot ramp up insulin release when necessary.
Your research suggested that EPAC2 could provide an important new target for treatment to restore pancreatic cell function. Why do current diabetes treatments only halt disease progression and how could EPAC2 provide the ability to reverse the disease?
We know that in type 2 diabetes, insulin release is defective. If we could target EPAC2A with pharmacologic drugs, then we could potentially help the ß-cell ramp up the much needed insulin secretion. This way blood sugar levels could be controlled and the complications of diabetes (such as blindness, kidney failure) could be delayed or avoided.
We are not sure why in humans type 2 diabetes progresses despite treatment. We will need to examine what happens with EPAC2A in the islets of Langerhans of patients who develop type 2 diabetes mellitus. We will have to look at mouse models first.
Your research was carried out in mice. How far are your findings from human application?
We know that EPAC2A is present in human islets and that it fulfils a similar role in humans as it does in mice. The EPAC2A protein in humans and mice are quite similar.
The bottleneck is to find pharmacologic compounds, which will specifically activate EPAC2A. Do screen compounds with a reliable method is a long process. We still have a lot more work ahead.
What further research needs to be carried out on this topic and are there plans in place to do so?
These findings open up many avenues. One would be to try to find EPAC2A activators. This may be a potential new class of drugs, which would help humans with type 2 diabetes.
Despite our extensive studies, we still need to fully understand how EPAC2A functions at the molecular level. Some of these questions can not be tackled by my laboratory alone, and the information coming from other laboratories working in this field will be required for us to get a full understanding of the biology of EPAC2A function and to evaluate EPAC2A as a drug target.
My laboratory will continue to examine the interplay between cAMP-PKA and cAMP-EPAC branches in regulating insulin release
Where can readers find more information?
The world-wide web is a great resource. For an overview of the topic, I would recommend a review article by either Dr. Seino from be University in Japan or by Dr. Holz from State University of New York in Upstate Syracuse. Both are experts in the field of EPAC2A biology.
About Dr. Mehboob Hussain
Dr. Mehboob Hussain graduated from Medical School at the University of Zurich, Switzerland.
After training as an internist and endocrinologist in Switzerland, he moved to the Massachusetts General Hospital in and Harvard Medical School in Boston, USA where he completed a postdoctoral fellowship and held a junior faculty position.
Dr. Hussain has since held faculty positions at New York University and at the University of Chicago.
He is currently an associate professor in Pediatrics, Medicine and Biological Chemistry at Johns Hopkins University, where he directs an islet research laboratory, the cell biology core facility of the Diabetes Research and Training Center, and treats patients as an Internist.
Dr. Hussain is internationally known for his leading research in the biology of the pancreatic islets of Langerhans. His research is supported by multiple grants from the National Institutes of Health.