I want to set the stage for the other 2 speakers, and let you know what we know about how beta cells become dysfunctional in the pathogenesis of type 2 diabetes.

This is almost a requisite slide when you talk about beta-cell dysfunction. It really does show, in a nice linear pattern, the pathogenesis of type 2 diabetes over time. When you do not exercise, as you become more obese, and as part of the normal aging process, insulin resistance increases. In response to increasing insulin resistance, your beta cell secretes more insulin and amylin; all this is the normal physiological state. What happens is that in those susceptible individuals beta-cell function begins to decline. We are going to go through the reasons why this may happen. This decline of beta-cell function -- in the face of usually severe but stable insulin resistance -- leads to first postprandial hyperglycemia (in most cases) and then fasting hyperglycemia. Our job is to try to figure out why this decline in beta-cell function is happening, what can we do about it when it occurs, or how can we prevent it..

An important feature is that beta cells do adapt.In response to eating a meal, beta cells secrete insulin. Counter-regulatory hormones go up when we sleep; growth hormone goes up, at least in younger people; cortisol goes up; and you need to secrete more insulin to overcome insulin resistance caused by these hormones. Stressors, such as infection, also result in increases in insulin secretion. These are the normal physiologic increases. Chronically, the enhanced function of these individual cells is maintained, but there is probably also an expansion of beta-cell mass to deal with a chronic increase in the demand on the beta cells.

From this slide you can see what happens when you become obese. In the early stages of this process, your glucose levels can be maintained through each meal. As you can see here, there are spikes of glucose. In normal and obese individuals, blood glucose levels stay relatively the same, but in the obese individual, there is some elevation of circulating insulin levels. This occurs dueto an increase in insulin secretion, such that you need more insulin secreted to overcome insulin resistance and maintain normal blood glucose. Thankfully, most individuals can maintain this high degree of compensatory insulin secretion throughout their life, so type 2 diabetes doesn't develop.

This is one of the very important concepts that Dr. Henry will talk about later. It is called "the disposition index." It gives you an idea of the relationship between insulin sensitivity and secretion -- how well your body can utilize insulin and the insulin secretion (the acute insulin response to an acute load of glucose). The important thing to realize is that this is a hyperbolic curve. This concept was developed by Richard Bergman and his colleagues; he received the Banting Award this year for his insights into this relationship between insulin sensitivity and insulin secretion. This hyperbolic curve tells us that, for the most part, as insulin sensitivity goes down or insulin resistance goes up, beta-cell function increases. But with severe insulin resistance, the beta cell has to start working very hard and produce large amounts of insulin.

This relationship tells us a few things. All you have to do is change your insulin sensitivity just a small amount when you are very insulin resistant, and you can remarkably decrease the amount of insulin secretion that you need to maintain normal blood glucose levels. So what we need to do is try to get people back on this normal curve, and try to get their insulin sensitivity improved so that their beta cells don't have to work so hard.

How do the beta cells adapt to changing insulin sensitivity? We think that there are a number of events going on. There may be an increase in neogenesis, or new beta-cell formation (replication), and an increase in the beta-cell mass, or hypertrophy, that may occur. We also know that we probably lose beta-cell mass through apoptosis and perhaps necrosis. There is actually some evidence that non-apoptotic and non-necrotic mechanisms may also be involved in the loss of beta cells, but it is the balance between neogenesis and replication and cell death that really determines our beta-cell mass. There is an increase in mass and an adaptation to the increase in demand for insulin; there is also a change in function of the individual beta cells.

There are multiple animal models that show a dynamic increase in the number of beta cells (mass) in response to insulin resistance; it is really just replication from existing beta cells. Recent data suggest that one of the main mechanisms to expand beta-cell mass is to increase replication. There is also evidence that neogenesis can occur; this is sort of the "holy grail" of those involved in beta-cell transplants -- at least in the adult stem-cell field -- to try to determine where the new beta cells come from. Is it purely from preexisting beta cells, or is it a robust neogenesis, or new beta-cell formation, occurring under the right conditions? Autopsy studies show an increase in beta-cell mass in obese compared towith lean individuals; I will show you some of these data. The increased mass is due to an increase in the number of beta cells (hyperplasia), but there is also hypertrophy. It is like exercising your skeletal muscle; you can also cause hypertrophy (or increase thesize of your beta cells). You can have more endoplasmic reticulum, more synthetic machinery. That is another way to increase your beta-cell secretory capacity.

This is one example. In a mouse fed a 10% vs a 45% fat diet, there is a marked expansion in beta cells. There is a very large increase in the number of beta cells and in the size of beta cells. Down here is the islet area; here is the beta-cell diameter in response. This does not take too long in the animal; about 6 weeks of feeding will cause this marked increase in beta-cell mass.

Autopsy studies indicate that there is an increase in beta-cell mass in obese people. A study by Kloppel, about 20 years ago, and more recently by Peter Butler and colleagues looked at lean vs obese individuals without diabetes. These studies indicated that there was an increase in the area of the islets that beta cells occupy; close to a 50% increase in obese individuals. One of the difficult areas to study in humans is to follow beta-cell mass longitudinally, because we cannot do longitudinal studies from autopsy specimens. What we need to develop is a noninvasive way to measure beta-cell mass over time.

The failure of the beta cell to compensate adequately for the presence of insulin resistance really underlies the development of type 2 diabetes. If you cannot match your insulin resistance with your beta-cell function, that is when problems with hyperglycemia begin to develop. We go through a prediabetic phase; you know it as impaired glucose tolerance, which is a sign of failing beta cells. We know it is a linear process; it occurs and progresses with time. It is important to remember that most obese individuals have adequate beta-cell compensation to the insulin resistance of obesity. The proportion that is unable to adequately compensate may be increasing slightly, but about 10% to 20% of people with significant weight problems and other risk factors will become diabetic. Beta-cell failure is probably what underlies the majority of type 2 diabetes that we see, and it likely has a genetic basis. It is the genetics of beta-cell failure that scientists in this area are trying tounderstand.

This slide depicts a stepwise increase in plasma glucose during progressive glucose intolerance, and the corresponding insulin responses during a 75-g oral glucose tolerance test. Subjects with impaired glucose tolerance are able to mount an elevated insulin secretory response and prevent development of hyperglycemia. Type 2 diabetic patients have an increase in glucose levels because they have a fall-off in the ability to secrete adequate amounts of insulin for the prevailing degree of insulin resistance.

Several years ago, Ralph DeFronzo and colleagues did a very interesting study that looked at beta-cell function in nondiabetic subjects, and those with IGT impaired glucose tolerance and type 2 diabetes. They compared the glucose area under the curve (AUC) (how high the glucose rose) in individuals with impaired glucose tolerance or with type 2 diabetes. What they looked at was a range of impaired glucose tolerance; they assessed people with glucose AUC between 140 mg/dL and 160 mg/dL, 160 mg/dL and 180 mg/dL, and 180 mg/dL and ≤ 200 mg/dL. These people, by definition, had impaired glucose tolerance (2-hour PG ≥ 1plasma glucose ≥ 40 mg/dL and < 200 mg/dL). Their glucose AUC (how much glucose they still had in their blood after an acute glucose challenge) was increased. They showed that with progressive increases in glucose AUC, there is a progressive fall-off in the insulin AUC, so that for in those people with blood glucose valuesin the upperrange, there is a relative fall-off in the insulin secretion. Then as one progresses to type 2 diabetes, in the different quartiles, there is a progressive fall-off in beta-cell function. So the dysfunction, or the failure to achieve adequate compensatory function, happens early in the development of IGT. Subsequently, there is a progressive fall-off in beta-cell function.

People seem to have different patterns of how fast their beta cells deteriorate. One of the interesting points to address is: Can we predict who is going to go from impaired glucose tolerance to type 2 diabetes early, or who may maintain beta-cell function? This is a study out of Italy, from Ele Ferrannini's lab, that shows that they can define people who stay with normal glucose tolerance over time. Some people maintain normal glucose tolerance for about 3 years and then become diabetic; others become diabetic fairly early. Understanding how and why beta cells deteriorate is really important, because if we are going to try to prevent diabetes, it would be important to know who is going to do so early, and who you might have time to try other interventions in.

There are a few features that we find in people who are developing type 2 diabetes. First is the concept of decreased first-phase insulin secretion. First-phase insulin secretion exerts dominant control overof glucose levels immediately after a meal is eaten; it controls postprandial blood glucose. This loss of acute or first-phase insulin secretion contributes to glucose intolerance, because once your blood sugar starts rising, the consequences of glucose and free fatty acid toxicity begin as well. In normal, healthy subjects, the loss of first-phase response causes glucose intolerance. In type 2 diabetes, restoring first-phase insulin secretion improves glucose tolerance and reduces free fatty acid levels.

There is a direct correlation between your fasting plasma glucose and how much first-phase insulin secretion (how well you can secrete insulin) occurs. If you have normal glucose tolerance and a normal fasting glucose, you can have a nice spike of insulin secreted; even with a little impairment, you have a reasonable spike. However, as your blood glucose starts to go up -- particularly as your fasting glucose goes up -- you can see a marked loss of first-phase insulin secretion. The thought is that this may occur because of insulin resistance -- insulin is being secreted as rapidly as the beta cells can work, but there are insufficient stored insulin granules to be released immediately upon eating.

Why does this happen? Why do we lose beta-cell function with time? We know that there is potentially a reduction in beta-cell mass, and a reduction of secretion from each beta cell -- so, intrinsic defects of the beta cells and reduction in the beta-cell mass are likely involved. In addition, there are a number of potential mediators that may be operative.

The study looks at the islet cell mass and function (glucose-stimulated insulin release) in people with type 2 diabetes. Compared with control individuals with normal glucose tolerance, beta-cell mass is down in type 2 diabetes. Glucose-stimulated insulin release (GSIR) -- how well insulin is secreted for a given mass of islets provides information about the intrinsic function of the beta cells that remain. This is a sort of ratio of how many beta cells you have and how well they work. Both mass and intrinsic function are impaired in individuals with type 2 diabetes.

Why and how does glucolipotoxity happen? I am going to review how high glucose and free fatty acids may impair insulin secretion. The bottom line is that we do not really know why this happens. It is clear that it is associated with increased free fatty acid levels overworking the beta cell. One of the leading theories is this concept of glucolipotoxicity. There is a glucotoxicity field, and there is a lipotoxicity field; I try to bring everybody together and just call it glucolipotoxicity.

The thought is that it is the combined increased flux of free fatty acids and increased flux of glucose into the beta cell that has detrimental consequences. If you chronically overeat, there are always going to be nutrients going across your beta cell; it is going to have to work really hard. This combined increase of 2 nutrients -- which can only be metabolized in the mitochondria -- really causes a change in the selectivity or the partitioning of nutrients once they enter the cell. So if you have a lot of free fatty acid entering the cell, it really inhibits proper glucose utilization in the mitochondria.

Additionally, it is thought that these lipids are disposed of in a different way. When fat gets in the beta cells, lipid intermediates are generated that cause abnormal signaling and contribute to the intrinsic dysfunction, the intrinsic secretory capacity, of the beta cell. This may cause an increase in reactive oxygen species. When you have this competition, it seems that the mitochondria become a little less well coupled. You generate peroxides and other substances, which may cause damage. This also can generate nitric oxide and other highly reactive species, which can damage the cell and then lead to this beta-cell death.

There is also the thought that there may be a change in cytokines. These inflammatory cytokines may be released locally. Some people think that macrophages may infiltrate beta cells when they start becoming damaged. This causes cytokine release from the macrophages, which may cause intrinsic dysfunction. The cytokines may alter the ability of the beta cell to proliferate.

One thing we know is that you need intact insulin signaling -- I am going to show some information later about insulin signaling -- that the beta cell may actually become insulin resistant. It is kind of odd to think that the cell that actually secretes insulin also has insulin receptors. It may be a very important thing that there is normal insulin signaling in insulin-producing cells to maintain beta cells. Therefore, it may be that changes in insulin signaling contribute to beta-cell dysfunction.

In the end, there are functional changes, and an imbalance of the regeneration of new cells and the death of those cells that result in the decrease of islet mass. If you start killing your neighbor cells, you are going to have to do more work; that just puts more and more stress on those remaining cells. So this may be a progressive fall-off that you can compensate for, and for a long time, but once you start falling off your beta-cell mass, you tend to fall off very rapidly, because anybody that is left is working a whole lot harder.

This is a schematic of what I talked about. It is thought that the excess glucose and free fatty acids coming into the beta cells generate intermediates that may change how glucose and fatty acids are metabolized. This causes the buildup of these intermediates, including long-chain acyl-(coenzyme A) CoA and ceramides; things like that do cause oxidative stress and dysfunction. This causes beta-cell dysfunction and ultimately beta-cell death.

I also want to talk about new information that has recently become available. It is now known that there needs to be a nice relationship between the endothelium of the beta cell and the beta cell itself. A recent paper by Johansson and colleagues shows that if you take endothelial cells from the islet, and you incubate them in the right kind of media and then put them back on the beta cells, you can increase the proliferation. Shown here is normal media, and media that was preincubated with endothelial cells; you can see that there is a marked increase in replication of the beta cells in culture.

This suggests that we need to start thinking about these interrelationships of beta cells. We found that in animal models, there is usually a nice fenestrated membrane -- the endothelium that lines beta cells. There are little holes that insulin can go through. Even before an animal becomes diabetic, there is dysfunction in the beta cells. You can see there are lipid droplets in the beta cells (shown by the little stars). They are part of the endoplasmic reticulum. One of the things that we find is a markedly thickened endothelium. Vascular endothelial growth factor (VEGF), one of the most predominant proteins that the beta cell secretes (besides amylin and insulin), maintains this fenestration. A disruption in "talk" between the endothelium and the beta cell can contribute to this dysfunction.

We also talked earlier about insulin sensitivity or insulin resistance in the beta cell. If you look at this bottom panel, this is an age-related process. If you knock out a protein called insulin receptor substrate 2 (IRS2), which is part of the insulin signaling pathway -- in essence making the beta cells very insulin-resistant -- there is a diminution in the mass of insulin staining cells (the green here is insulin staining.) As these animals age, they have a decrease in the number of beta cells, and glucose tolerance becomes abnormal. If you give exenatide (a glucagon-like peptide-1 analog), which we are going to talk about later, to a normal animal, you can increase beta-cell mass. If you inhibit insulin signaling and you make the beta cell insulin-resistant, you can see that you inhibit, or at least markedly attenuate, the ability of exenatide to maintain or increase beta-cell mass.

So what we are talking about is that there is cross-talk between different signaling systems. There is cross-talk between the different mediators and different cell types, which really contributes to this dysfunction of the beta cell. I think that understanding more of the molecular biology of these proceses is going to be very important.

I am going to end by summarizing that beta-cell failure really is a progressive disease. We talk about impaired glucose tolerance and about diabetes; I think it is really splitting hairs. This is really a very progressive disease; in those genetically-susceptible individuals -- those people who are going to get type 2 diabetes -- it is a progressive thing. It is both a functional defect and a defect in beta-cell mass. This is an important concept. Remember, if you start killing off your neighbors, you have to work harder. What we really need to do is either increase the neighborhood population or make these guys work less hard. The pathogenesis of beta-cell failure is really a response to increased flux of glucose and fatty acids. Perhaps this increase in glucose and fatty acids causes insulin resistance, causes a disruption in changes intrinsic to the beta cell, but also how it reacts to its environment, and how it interacts with neighboring cells.