Metabolic syndrome webinar

Metabolic Syndrome- Molecular Mechanisms of Insulin and Leptin Signaling (Part 2 of 3) webinar

The second of a three-part webinar series on metabolic mechanisms presented by Dr Khosrow Adeli 

This series is intended for faculty, students, and technologists involved in biological and molecular research.  This series would also be of interest if you would like to gain a better understand of the pathophysiology and molecular mechanisms of obesity and diabetes.

Watch webinar

Webinar topics:

  • Metabolic and insulin signaling pathways
  • Molecular basis of insulin resistance
  • Molecular basis of leptin resistance
  • Role of gut peptides in metabolic control and insulin sensitivity
  • Brain-gut-liver axis in control of metabolism and energy expenditure

About the presenter

Dr Khosrow Adeli is currently a Senior Associate Scientist in the program in Molecular Structure and Function at the Hospital for Sick Children in the University of Toronto.

He is also the head and a full professor of Clinical Biochemistry at the Hospital for Sick Children, the Departments of Biochemistry, and Laboratory Medicine & Pathobiology at the University of Toronto, all in Canada.

Webinar transcript 

Good day and welcome to the Molecular Mechanisms of Insulin Resistance in Type II Diabetes Webinar. 

Hello and thank you for joining us for today's webinar. It is my pleasure to introduce today's presenter, Dr Khosrow Adeli. Dr Adeli is currently a Senior Associate Scientist in the program in Molecular Structure and Function at the Hospital for Sick Children in the University of Toronto. He is also the head and full professor of Clinical Biochemistry at the Hospital for Sick Children, and the Departments of Biochemistry, and Laboratory Medicine & Pathobiology also all in Toronto. 

Joining Dr Adeli today is Sambhav Dave, the Senior Associate in the Business Development team, with a primary focus in Neuroscience and Signal Transduction research areas. As a reminder if you have any questions throughout this presentation, we invite you to submit them on the right hand side of your screen. Questions will be answered during the troubleshooting portion of this webinar. At this time, I'd like to hand the presentation over to Dr Adeli.

KA:    Thank you, Sarah, and thank you for those listening to this webinar. I'm pleased to present the second part of this series, and the first part was focused on epidemiology and counter physiology of the metabolic syndrome and type II diabetes, and obesity-related so-called metabolic disease. Today, I will be focusing on metabolic signaling cascades, particularly insulin and leptin signaling pathways. In the first webinar, we had introduced a concept of insulin resistance, but today I will be talking about how insulin resistance is developed; and how this is associated with another important metabolic issue or problem, and that is leptin resistance. So we will be talking about both insulin and leptin signaling cascades. 

Basically, there are a number of key pathways that regulate metabolism, but leptin and insulin signaling are really important. But there are others such as adiponectin signaling, nutrient signaling through other receptors, and a number of metabolites that are involved in the metabolic signaling cascades.

So today we'll discuss insulin signaling and leptin signaling pathways, we'll talk about the molecular basis of insulin resistance and leptin resistance, and then we'll try to talk about at the end how insulin and leptin signaling interacts to regulate appetite, food intake and metabolism. 

As we have discussed in the first webinar, insulin resistance is a major problem that develops, largely nowadays, because of over-consumption of calories and high-density foods, lack of exercise. But at the molecular level, this problem initiates in a number of tissues: the muscle, the liver, the adipose tissue as shown here, and these tissues are also referred to as insulin sensitive tissues. Resistance to insulin action leads to the pancreas secreting more insulin, leading to a transient hyperinsulinemic state. This can go on for years until the pancreas fails to produce an efficient amount of insulin to fight this insulin resistance. Therefore, now you have a drop in insulin levels due to inadequate secretion, and then blood sugar starts going up because of reduced insulin levels, and also insulin resistance; and that eventually leads to the development of type II diabetes. Now, to understand at the molecular level how these changes occur, we need to understand the insulin signaling pathway, which will be discussed. 

In the next slide, I also wanted to remind you that insulin resistance is a major risk factor for a number of major metabolic changes, but particularly development of type II diabetes, but also an important risk factor for cardiovascular disease and development of atherosclerosis. So how does the insulin signaling work, and how does the resistance develop? 

So the next slide gives you a depiction of an insulin signaling pathway. Now, this is actually a relatively simplified form of this cascade, and every day we're learning more about other factors that are involved in insulin signaling. But this gives you a good overview of two major pathways that are regulated by insulin: one is called the metabolic pathway, or the PI-3 kinase pathway; and the other is the mitogenic pathway, or the MAP kinase pathway. Insulin first binds to its receptor, which is present on the cell surface, and once it binds it triggers autophosphorylation of the receptor. That, in turn, leads to activation of a number of substrates, such as IRS-1, IRS-2; IRS is standing for Insulin Receptor Substrate. So there are a number of substrates that are activated, phosphorylated to this pathway. This leads to activation of a kinase; a very important kinase in metabolism called PI3-Kinase, standing for phosphoinositide or 3-Kinase. This, in turn, is actually a lipid kinase and it phosphorylates lipids on the plasma membrane, making a new or synthesizing a new phosphate or a lipid phosphatidylinositol lipid called PIP-3. This is a very important lipid, that actually is involved in signaling, to many other kinases, and the key kinase that through this pathway gets activated is Akt; and Akt is a very important kinase that regulates many other factors. As we will discuss in some detail, Akt regulates glucose transport as well as glycogen synthase; but, as well as, it's actually a growth factor and can trigger also many growth pathways.

This pathway, also the PIP-3, activates through a number of other kinases, activates mTOR; and mTOR is another important signaling molecule, that is actually a complex, that leads to activation of protein synthesis. So, as you can see, this insulin signaling cascade, at least on this arm, regulates a number of aspects of metabolism; both carbohydrate and synthesis carbohydrate metabolism, as well as protein synthesis and metabolism. I should also mention that this pathway regulates lipid metabolism, and this can be then regulated in insulin resistance, and that leads to this regulation of lipids and carbohydrate metabolism. We will discuss that in some detail later. I should mention that this pathway is also responsible for activating the mitogenic pathway. This is responsible for cell growth and division, and, therefore, insulin is also regarded as a growth factor, in the sense that it can signal growth and cell division, and therefore it's involved in many aspects of cell growth and differentiation.

The next slide, I give a little bit more detail about the first key step which is when insulin binds to the receptor, initially, the receptor is in the OFF switch type state. When insulin binds, this turns on the pathway, basically the receptor is turned ON and now you have an ON switch. This is due to autophosphorylation of this receptor, which then recruits IRS-1, IRS-2 and some of the other substrates. This, therefore, is the first step in turning on this pathway. Now, there are a number of phosphatases called PTPs, Protein Tyrosine Phosphatase. These are enzymes - very important enzymes - that actually can block this signal by turning off this receptor. This is done by removing the phosphate groups; these are phosphatases so they remove the phosphate groups, and they turn off the receptor. It's been shown that actually for insulin to act on this receptor, it actually has to transiently inhibit PTPs in order to allow for a signal to be generated, but as soon as this signal goes through, this pathway is inhibited. This is important, because insulin over-activation can cause all sorts of issues, including potentially-induced carcinogenesis, so, therefore, this pathway has to be very closely regulated.

On the next slide I show you basically the key domain that's involved in this signaling interaction, so when the insulin receptor is phosphorylated, in order to recruit adaptors such as IRS-1, it uses an important domain called the PTP domain, or phosphotyrosine containing peptides that can actually mediate this interaction, and lead to the interaction of these various molecules within the pathway. So this is an important domain that has been studied and actually crystallized, as shown here, and shown to be an important mechanism for the signaling cascade to go through. Now, there are a number of important mediators, or downstream mediators of the insulin receptor cascade.

This slide shows a number of important downstream molecules, particularly Fox01, SREBP1c, GSK-3 and AS160. I wanted to focus on these, because these are really important in regulating metabolism and in many cases they can be disregulated in obesity and diabetes. Now, Fox01 is a downstream of Akt, and if you recall, Akt can be activated by the insulin receptor signaling. Once Akt is activated it can actually regulate Fox01; Fox01 regulates gluconeogenesis, and glucose production in the liver and this is really important in regulating our blood sugar, and actually preventing diabetes. In insulin resistance, this pathway is disinhibited and therefore it leads to hepatic glucose overproduction. I'll talk about it a little bit more on the next slide. The other important molecule is SREBP1c, and this is a Sterol Response Element Binding Protein. It's actually a very important transcription factor that's regulated by insulin, and this is involved in activating genes involved in fatty acid synthesis and so-called lipogenesis, which is synthesizing new lipids and this is, therefore, important in lipid metabolism. Akt also regulates GSK-3, which is involving regulating glycogen synthesis and AS160, which is an important player in regulating glucose uptake by the liver and muscle, and other tissues. So just to remember these four important players. 

The next slide, a little bit more about Fox01; Fox-01, as I mentioned, is activated by Akt through phosphorylation. It's interesting that Fox01 is normally present in the nucleus, and it binds to important gene sequences, and activates important genes; genes such as glucose 6 phosphatase and PEPCK (Phosphoenolpyruvate Carboxykinase). These two enzymes are involved in gluconeogenesis, which is glucose synthesis and glucose production from the liver. This is really important in controlling of blood sugar and the development of diabetes, so Fox01 regulates those genes. Now, in the absence of insulin it activates, but when insulin is present, for example, after a meal, insulin is secreted and then activates the receptor. This, in turn, leads to Akt activation, and Akt leads to phosphorylation of the Fox01 in the nucleus. Now, Fox01 phosphorylation leads to exclusion of Fox01 from the nucleus, so Fox01 is brought out of the nucleus. This prevents activation of genes, so this is therefore an exclusion type regulation where insulin signaling leads to prevention of gene activation by Fox01. So this is a really important pathway in regulating glucose metabolism.

The next slide talks about another important player, and that is AS160. I mentioned that this is another important downstream mediator of insulin signaling. Now, what does this regulate? It actually regulates glucose uptake. Glucose uptake is really important, so normally our tissues are sensitive to insulin and they pick up glucose that comes from the diet, and they nicely absorb and metabolize glucose. For this to occur, you need a transporter called GLUT4; GLUT4 is present normally on the, it should come to the cell surface to help absorb glucose or take up glucose. But, normally, it's actually present in the trans-Golgi network, but it requires insulin activation of Akt which then, in turn, helps activate the transport of GLUT4 from the cell, inside the cell, to the cell surface. This is shown here as these red lines, and these are GLUT4 transporters, so they actually bind to glucose and absorb glucose. For this to occur properly, you require insulin activation and what insulin does is that it activates Akt, and Akt, in turn, phosphorylates this protein called AS160. AS160 is actually a Rab GAP, and GTP is activating protein, and by phosphorylation it prevents its action. Normally, it actually keeps these vesicles containing GLUT4 inside the cell, so phosphorylating AS160 actually prevents it from blocking this transport. So it's another way of excluding this AS160 from action, and preventing it from blocking activation. In summary, insulin, a true AS160 can allow GLUT4 transporters to be activated, brought to the cell surface so that glucose uptake can be activated and increase so that glucose can enter the cell. This is really important in ensuring appropriate glucose metabolism.

The third pathway I just wanted to briefly introduce, remember this important factor, called SREBP1c. This is a very important - again, another transcription factor that regulates not glucose metabolism so much, but lipid metabolism and lipogenesis; and this is also under the regulation of insulin signaling cascade. This is quite a complex pathway and I don't have time to go through details, but just briefly, glucose metabolism is regulated by insulin, so is lipid metabolism and this is one of many pathways where insulin activates a number of kinases, but particularly Akt. This leads to, eventually, activation of SREBP1c. This activation partly occurs through, for example, phosphorylation of GSK-3 which prevents its own inhibition of SREBP1c. The end result being that insulin activates SREBP1c. SREPB1c is a transcription factor that binds to the DNA elements or promoters on a number of genes, including genes involving the fatty acid synthesis, and basically fat production which is referred to as lipogenesis. So, therefore, insulin is a key regulator of this pathway as well.

I wanted to also introduce to you an important factor that's become a subject of great interest, and that is protein-tyrosine phosphatase 1B. This phosphatase is a member of a large family of phosphatases, which are involved in dephosphorylating the insulin receptor. It's actually present on the ER membrane, typically, and it's activated and brought to the insulin receptor where it dephosphorylates the insulin receptor, but it's also involved in dephosphorylating a number of receptors. This particular phosphatase has been implicated in the development of insulin resistance and type II diabetes, that's why we're talking about it.

This slide shows you that this PTP1B is a member of a large family of phosphatases. It's referred to as a non-transmembrane or receptor light type of PTP, and it's shown here as one of the members of a large family. All of these are important in values, aspects of metabolism and signaling, but PTP1B has particularly been found to be involved in regulating insulin signaling and, as you will see, even leptin signaling. 

This slide shows you a little bit more detail as to how it's involved. So PTP1B has been referred to as a master switch. I've already mentioned, and if we can focus on the right here, the insulin receptor, as we discussed, can be activated leading to phosphorylation of IRS-1 and other substrates. This eventually activates the PRT kinase. PTP1B, which I just introduced, is a phosphatase that has been shown to bind and dephosphorylate this IRS-1, but also can even dephosphorylate the insulin receptor itself. This, of course, blocks insulin action quite effectively. You could, therefore, refer to PTP1B as an inhibitor of insulin signaling. We will talk about this last slide later in this webinar, and that is regulation of leptin signaling. For now, if we could focus on the fact that PTP1B is an important inhibitor of the insulin receptor signaling, and this occurs by dephosphorylation.

It's been shown that PTP1B not only regulates insulin receptor signaling, but also regulates a number of other signaling cascades, including oncogenic signaling pathways such as EGF receptor signaling, IGF-1 receptor signaling, as well as cytokine signaling such as interferon gamma, and interferon alpha signaling cascades. So this is a very important regulator of multiple signaling cascades.

What has been found recently, is that PTP1B can be a very important cause of so-called insulin resistance, which we have discussed in some detail. So this describes basically, or explains the molecular basis of insulin resistance, at least under some conditions. Insulin resistance can be caused by other mechanisms, but this appears to be a key mechanism. That is high levels of PTP1B in the cell can lead to reduction in insulin receptor activation or phosphorylation; reduction in IRS-1, IRS-2 phosphorylation; reduction in activation of PI-3 kinase. This, of course, reduces Akt phosphorylation downstream. All of this can lead to so-called the phenomena of insulin resistance, or the molecular basis of insulin resistance in many cases.

This has been actually shown in vivo, both in animal models as well as humans. Actually, in ob/ob mice, and these are mice that are highly obese, and I'll talk about that when we talk about leptin signaling. But these mice are highly obese and it's been shown that the obese mice actually have high levels of PTP1B expression. Also in humans who are obese and diabetic, appear to have high expression of PTP1B. It's also been shown that polymorphisms in the gene expressing PTP1B actually can either, depending on the type of polymorphism they can either increase susceptibility to diabetes, or protect from development of diabetes. Most interestingly, in the late-nineties, more than ten years ago, they actually found that when you knock out PTP1B this actually protects mice from obesity and diabetes. So no matter how much you feed these animals, they are protected from these conditions, and they become resistant to a high-fat diet-induced insulin resistance.

The next slide actually shows you one of the slides from this publication, showing you that this is the wild-type mice; males on the left and females on the right. You can see that wild-type mice under a high-fat diet increase weight, and these are up to ten weeks of feeding you could see a significant rise in body weight, but this is attenuated quite significantly when you knock out PTP1B. The two lines: one is homozygote form of these mice of genotype, the homozygous genotype; and one is a heterozygous genotype. Meaning that you are either knocking out two copies of this gene or one ileal of this gene. Either way, they become resistant to a high fat-induced weight gain and they remain firmly lean on a high-fat diet. This is also true in females, suggesting that this gene can be very important in actually regulating body weight, but also these animals were shown to be highly insulin-sensitive, and have no insulin resistance on a high-fat diet. So this highlights the importance of PTP1B.

Another important experiment was done later, where they actually bred them across the mice that were knocked out in PTP1B, with mice that were obese, the so-called ob/ob background. These are mice that are highly obese, but what they did is that when they crossed these with PTP1B knock out mice, and they developed a new line that were ob/ob background, but with a knockout of PTP1B, this led to a significant reduction in weight gain, a reduction in adipose tissue or fat tissue, and an increase in metabolic rate, and these animals became more insulin sensitive. Again, these experiments clearly highlight that PTP1B could be a very important player in the development of insulin resistance, and this has actually led to a number of pharmaceutical companies developing drugs, trying to develop inhibitors of PTP1B as potential drugs for treatment of obesity and type II diabetes. There have been a number of challenges with these new drugs, and they haven't really made it to phase three clinical trials, and so the reason is that it's very difficult to develop very specific inhibitors, because there is a large family of these phosphatases and it's difficult to ensure a specificity of your drug. So there have been challenges, but the concept is definitely very interesting.

I wanted to now switch to leptin and describe briefly what leptin signaling is, and how later we'll discuss and end this webinar by talking about leptin interaction with insulin signaling. Leptin is a product of the so-called obese gene, it's a fairly small peptide of 167 amino acids. It's actually similar in structure to other cytokines, such as IL-6, or hormones, such as human growth hormone. It's secreted by the adipose tissue, largely, but also a number of other tissues have been shown to produce leptin, but the major source is the white adipose tissue which is basically fat cells. These cells secrete leptin, and leptin actually largely acts on the CNS or the central nervous system, but it can signal in other tissues and there is evidence that it signals in the liver, for example, but CNS is a major site of action.

Leptin, as I mentioned, is produced by the adipose tissue, and adipose tissue is where we store our triglyceride or dietary fat, or endogenously-synthesized fat. These are very important and initially this tissue was thought to be just a storage organ, but now we know that it's really important in producing a number of hormones and peptides that regulate metabolism, and one of them being leptin.

This slide here lists a number of hormones and cytokines produced by the adipose tissue; leptin being one of many. Another really important one is TNF alpha, which actually itself can induce insulin resistance, and it's involved in regulating immune function as well, so as well as a number of others, but today we'll focus on the leptin itself.

What led to the discovery of leptin was the initial discovery at the Jackson Labs in the US in Bar Harbor, Maine, where they found in the lab two various breeding experiments. A mice model that they referred to as the fat mouse, and this mouse, as you can see on the left, at 21 days of age is already larger than its wild-type litter mate. But then at ten months of age it's extremely obese, and more than double the size of its control litter mate. So this mutation was referred to as the fat mouse, and then they're also referred to as the ob/ob mice; ob standing for obese. 

This eventually led to investigators trying to find out what's wrong with these mice, and this led to a landmark publication in '94 by Jeffrey Friedman and his group discovering that these mice that are ob/ob or fat mouse phenotype, actually have a mutation in the gene called leptin. The leptin was then studied, and since then has been studied in great detail.

What's wrong with these mice? These mice basically are leptin-deficient. The ob/ob mice are leptin-deficient and they've been referred to as a genetic model of obesity. The reason they're deficient is that they have a non-sense mutation in the leptin gene, and this leads to a lack of the satiety factor, and therefore these animals are constantly hungry and are hyperphagic. This leads to a significant, of course, amount of weight gain and insulin resistance, so they're morbidly obese and insulin resistant. There is also, as I already mentioned, this high food intake, increased body weight, there is also a reduction in energy expenditure. There has been a study showing that they have lower thyroid hormone levels, higher insulin levels due to insulin resistance, increased corticosterone and reduction in gonadotropins, such as LH, FSH or SDG and some other hormones, so these tend to be lower. So these animals have a number of major metabolic changes.

Now, how does leptin work? Leptin actually binds to an important receptor called a leptin receptor. We know that leptin receptor has a number of isoforms, and there is a long form and there is a shorter form, and there have been a number of different forms described in the literature. It's also been postulated that the leptin receptor binds to a leptin co-receptor on the cell surface. This slide nicely shows this particular receptor, and that it's typically found in different regions of this brain, but also found in other tissues. The long form, as I mentioned, is the one that's involved in particularly regulation of satiety and food intake. The receptor is actually when it's activated by the leptin binding, it leads to activation of a pathway called a JAK/STAT pathway. JAK2 is a kinase and similar to insulin activating downstream kinases, and leptin here is activating a downstream kinase called JAK2; JAK2, in turn, recruits a transcription factor called STAT3. There are a number of others involved, but for simplicity I'm going to just focus on STAT3. STAT3 is a major adaptor, or actually it's a transcription factor that's activated by JAK2 kinase, and this leads to a so-called leptin signal, and the leptin signaling cascade. 

This slide gives you additional information on this signaling mechanism. As I mentioned, the long form of this receptor is particularly vital in regulating obesity, and this tends to be present especially in the brain, in the hypothalamic region and is the main site of leptin action. There's a mouse model called the db/db mice; db standing for diabetes. So these animals are actually - these mice are diabetic, and they've been shown that the reason they're diabetic is because of the mutation in the leptin receptor. So when this long form of the receptor is mutant, you actually develop full-blown type II diabetes. Also mentioned here is that there is a key signaling cascade, which I've already mentioned the JAK/STAT pathway that regulates signaling. There are two very important inhibitors of this pathway, which I will discuss towards the end of the webinar, SOCS3 and PTP1B. 

I've mentioned previously JAK2 is the kinase that's activated by the leptin receptor, and that leads to recruitment and activation of the STAT3. The STAT3, in turn, goes into the nucleus and it binds to a number of genes, and this is particularly happening in the hypothalamus and that we don't know all of the mechanisms, but this activation of the STAT3 transcription factor is key in exerting the leptin signal, and leptin action. 

The next slide, I show you how this STAT3 activation actually leads to a change in satiety. So, under basal conditions in the absence of leptin, normally there are two sets of neurons that regulate appetite and food intake; a set of neurons called POMC neurons, and another called NPY neurons. These, typically, without leptin, POMC neurons are not activated and whereas NPY neurons are, and this leads to an orectic signal or a hunger signal, so we become hungry and want to eat. But as soon as leptin is introduced to the hypothalamic region, this leads to a STAT3 activation and the STAT3 activation leads to activation of these POMC neurons, and it blocks actually the NPY neuronic pathway. This combination of these actions lead to a satiety signal reducing our need to consume food. Therefore, this pathway is very important in regulating our food uptake and intake, and reduces appetite. 

I should mention that the leptin signal is not the only signal that's involved in regulating food intake, there are a number of other signals as shown in this slide, such as TYY and CCK, insulin itself, ghrelin, which is another hormone that's been described generally that's secreted by the stomach, and important in inducing hunger. So leptin is not the only important regulator, the fact of leptin is actually integrated with a number of other signals, so this is what I wanted to highlight in this slide.

How does the actual STAT3 signaling lead to changes in appetite or food intake? There have been a number of studies done, and this slide shows one of the studies showing the potential involvement of the potassium KATP channels. Potassium KATP channels appear - need to be activated in order for us to suppress our appetite, basically, and this occurs in neurons. This involves the phosphorylation event typically actually activated with PI3 kinase, which is also not only activated by insulin, but also it can be activated by leptin. This leads to production of this lipid called PIP3 (phosphatidylinositol trisphosphate), and this, in turn, has been shown to lead to a remodeling of actin, F-actin turning into G-actin. This leads to opening of the potassium channel, and that's been suggested to be important in regulating appetite. 

Now that I've introduced both insulin and leptin signaling cascades, I want to finish by talking about how they are integrated. So these signaling pathways don't work in isolation, and there is significant interaction. This slide shows you that when the insulin receptor is triggered and leptin receptor is triggered, they actually can work in harmony to regulate our food intake. This occurs by activation of the POMC neuron and the inhibition of the NPY, AGRP neurons. So this is an integrated regulation by the two signaling pathways, and involves a number of downstream molecules. I've already mentioned STAT3 in the case of leptin signaling, and we have talked about Fox01 as being a downstream molecule of the insulin receptor. So this integration is really important in regulating food intake, but, as I mentioned, there are a number of other players such as some of the peptides, such as ghrelin and as well as things such as NPYY, as well as GLP-1. 

This slide shows you how the two are integrated. Again, when leptin and insulin are present, you basically reduce food intake and induce a satiety signal, so they both actually induce a satiety signal and both can reduce appetite. A mechanism being similar to what I've already described in terms of activating POMC neurons and preventing the NPY/AGRP neurons.

To end, I wanted to talk about leptin resistance, so we have already talked about insulin resistance, but there's been evidence for the last decade that there is significant leptin resistance that's found in people with obesity and type II diabetes. This appears to be due to either a mutation in the leptin receptor, or some other downstream inhibition of the receptor signaling that leads to a resistance to leptin action; this leads to a number of metabolic changes. This slide shows you some of the key causes. What I wanted to talk about are these two players, SOCS3 and PTP1B to end the webinar, but there are a number of other players as well, but these have been studied quite extensively and are thought to be major players.

So SOCS3 is actually a key regulator that has been found to inhibit leptin signaling, and cause so-called leptin resistance. This is found to be the mechanism that SOCS3 appears to trigger recruitment of the receptor and some of its downstream molecules, such as STAT3 to a degradated mechanism, particularly the proteasome of degradation. So SOCS3 conducts a sort of E3 ubiquitin ligase complex to the receptor. Ubiquitin ligases are involved in tagging proteins for degradation, they're very important in degradative mechanisms involving the proteasome. SOCS3, by doing this, by inducing degradation of a number of the receptor itself and potentially STAT3 and JAK2, can block this signal, so this, of course, can induce insulin resistance.

Another mechanism, which I briefly mentioned before, is the PTP1B itself. PTP1B we talked about as a player in insulin resistance, but it's been shown by many studies that PTP1B can also make cells and neurons resistant to leptin action, and therefore induce leptin resistance. This occurs by PTP1B dephosphorylating or removing phosphates from the JAK2 kinase, and preventing JAK to activation. That, of course, prevents the STAT3 activation and leptin signal cannot go through.

Actually, this experiment has been done by a number of groups, but this is one paper I wanted to show at the end, where they studied leptin ob/ob mice. These are animals that are deficient in leptin, and when they actually bred them with the PTP1B, and I had mentioned that earlier, they actually saw a significant reduction in weight gain. So leptin-deficient animals were made leaner by basically taking out their PTP1B chain, or knocking out PTP1B chain. So, basically PTP1B is a regulator of leptin signaling and this is important in development of obesity.

So the last slide just tries to summarize that we have talked about leptin signaling and insulin signaling being really important in regulating our central mechanisms involved in food intake and appetite, and many aspects of metabolism. But I wanted to highlight the fact that there are other satiety signals, and those tend to be things such as peptides and I've already mentioned CCK, but also a number of other peptides. There are also a number of neuronal pathways, especially things such as enteric neurons and vagal signaling pathways, or vagal neuronal pathways. So there is a significant complexity to this whole regulation, and leptin and insulin are not the only players - I want to emphasize that - but they're clearly very important players in this pathway. So thank you very much, that concludes my webinar. If there are any questions, and I have prepared a couple of questions myself, common questions that are normally asked when I give these lectures to different universities, I can go through those. Sarah are there any questions?

No, professor, please feel free to answer the questions.

KA:    One of the common questions I'm asked normally when presenting on leptin, is how common is leptin deficiency, how common is leptin resistance? Leptin deficiency is actually very uncommon, it's very rare, fortunately, but there are a number of families, human families identified that have leptin deficiency due to a mutation in their leptin gene. Children of these families tend to be extremely obese, and there has actually been studies showing that if these children are treated with leptin or injected with leptin chronically over time, this resolves and significantly reduces the weight gain, and they become leaner and this prevents other complications such as diabetes that can develop. So leptin has been used as a drug to treat very overt obesity in leptin-deficient children, but this is a very rare condition. But much more common is the leptin resistant state, which is very common. Anyone really overweight and with obesity, and diabetes can potentially have leptin resistance; and leptin resistance may have contributed to their weight gain and to their development of diabetes. So this is much more common, as we know weight gain and obesity are very common, so, yes, leptin resistance is much more common than leptin deficiency.

Another question that I'm typically asked is in relation to insulin, and how we know that insulin definitely regulates carbohydrate metabolism, but what is the relationship between insulin and lipid metabolism? It appears to be more complex than initially thought. Insulin actually does activate lipid synthesis, so it can increase fatty acid synthesis and lipogenesis. But so is insulin resistance, so that's the complexity - how can we have increased lipogenesis with both insulin signaling and lack of insulin signaling? That has been rather controversial, but the mechanism appears to be that normally insulin does increase lipid synthesis, because it wants to use some of the fuel coming from glucose and other pathways, and help store that energy as fat. But this occurs in a very highly-regulated manner, but it's much more exasperated when there is insulin resistance. When there is insulin resistance, insulin pathway is inhibited, this, in turn, turns on a number of other pathways that exasperate the lipogenic signal. So insulin itself can be lipogenic, but lack of insulin can be worse in many ways in increasing lipid production and storage, leading to conditions such as fatty liver and increase the amount of adipose tissue stored in the body.

Perhaps the last question, the third one that I commonly find being asked, is this interaction between insulin and leptin, and which one becomes insulin resistant, first? Does insulin resistance come first or leptin resistance? Well, we really don't know at this time, but based on some of the studies done, it appears that both can occur simultaneously, but depending on the mechanism leading to metabolic disease it can involve one or both pathways. But insulin resistance is known to be induced, for example, by overconsumption of calories, leading to normally the liver first becoming insulin resistant and developing hepatic insulin resistance. This eventually can lead to increased accumulation of fat, and that can potentially lead to leptin resistance. The reverse can occur as well, where leptin resistance can potentially initiate first. So the answer is we don't really know exactly which one comes first. Most likely they both occur in most conditions of obesity and type II diabetes. 

So that I think provides answers to some of the common questions, so thank you for listening to this webinar. The third webinar, and the final one will focus on animal models of obesity and diabetes, both talking about genetic models, as well as dietary models and how they are used in basic research, but also in pharmaceutical and biotech research. So that will be the focus of the last webinar. Thank you.