Hormone Resistance in and : Insulin, , and FGF21

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Citation Flier, Jeffrey S. 2012. Hormone resistance in diabetes and obesity: insulin, leptin, and fgf21. The Yale Journal of and Medicine 85(3): 405-414.

Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:10436248

Terms of Use This article was downloaded from ’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA YALE JOURNAL OF BIOLOGY AND MEDICINE 85 (2012), pp.405-414. Copyright © 2012.

LEE E. FARR LECTURE

Hormone resistance in diabetes and obesity: insulin, Leptin, and FGF21

Jeffrey S. Flier

Dean of the Faculty of Medicine, Harvard University, , , Massachusetts

This an edited transcript of the Lee E. Farr Lecture given by Dr. Jeffrey Flier on May 8, 2012, at the culmination of the annual Student Research Day at the Yale School of Medicine. In this presentation, Dr. Flier discusses his and his wife’s research on insulin, leptin, and FGF21 in the context of his reflections upon his life’s work and his advice for young investigators.

introduction Chief Academic Officer for Beth Israel Deaconess Medical Center (BIDMC), a Dr. Jeffrey S. Flier was named the 21st Harvard teaching affiliate. Dean of the Faculty of Medicine at Harvard Flier was born in . He University on July 11, 2007. Flier, an en - received a BS from City College of New docrinologist and an authority on the mo - York in 1968 and an MD from Mount Sinai lecular causes of obesity and diabetes, is School of Medicine in 1972, graduating also the Caroline Shields Walker Professor with the Elster Award for Highest Aca - of Medicine at Harvard Medical School demic Standing. Following his residency (HMS). Previously, he served as HMS Fac - training in internal medicine at Mount Sinai ulty Dean for Academic Programs and Hospital from 1972 to 1974, Flier moved

To whom all correspondence should be addressed: Jeffrey S. Flier, MD, Dean of the Fac - ulty of Medicine, Harvard University, Harvard Medical School, 25 Shattuck St. – Gordon Hall, Room 111, Boston, MA 02115; Email: [email protected].

†Abbreviations: HMS, Harvard Medical School; BIDMC, Beth Israel Deaconess Medical Center; NIH, National Institutes of Health; NIDDM, non-insulin-dependent diabetes melli - tus; NAFLD, non-alcoholic fatty liver disease.

Keywords: Farr Lecture, insulin, leptin, FGF21, obesity, diabetes 405 406 Flier: Lee E. Farr lecture

to the National Institutes of Health (NIH) as Endocrine Society and the Banting Lecture a Clinical Associate. In 1978, he joined the of the American Diabetes Association, its Faculty of Medicine at HMS, serving as highest scientific honor. In 2010, Flier was Chief of the Diabetes unit at Beth Israel awarded an Honorary Doctor of Science De - Hospital until 1990, when he was named gree from the University of Edinburgh and chief of the hospital’s Endocrine Division. last year was awarded the 2011 Rolf Luft In 2002, Flier was named Chief Aca - Award for Metabolic Research by the demic Officer of BIDMC, a newly created Karolinska Institute in Stockholm, Sweden. senior position responsible for research and We are delighted and honored that he academic programs. He worked with Beth delivered the 25th Lee E. Farr Lecture, and Israel Deaconess academic department his doing so is a testament to how meaning - chairs to ensure the quality and breadth of ful medical student research is to him. academic programs at the Medical Center, through which most of HMS students pass. Lee e. Farr Lecture He also served as the formal liaison to HMS, sitting on the Council of Academic Deans. It is a distinct pleasure and honor to be Flier is one of the country’s leading in - asked to give the Farr lecture, which has a vestigators in the areas of obesity and dia - tradition of excellent speakers and is espe - betes. His research has produced major cially meaningful for its focus on medical insights into the molecular mechanism of in - students’ research. My own research career sulin action, the molecular mechanisms of began when I was a medical student, and the in human disease, and the state of medical student research at HMS, molecular pathophysiology of obesity. He the school that I now lead, is something to was one of the first to demonstrate that diet- which I have given much attention. Re - induced obesity in rodents is associated with search is an aspect of medical education that increased leptin expression and leptin re - has been prominent at Yale for more than a sistance, and that short-term starvation is as - century. Harvard has always had many stu - sociated with decreased leptin expression dents who do research, but as of this past and blood levels. His proposal that leptin year, it is now a requirement for every en - level serves as a switch from the fed to the tering student to complete a scholarly proj - starved state has fundamentally shaped dis - ect, and this is an exciting initiative for the course in the field. school. Flier has authored more than 200 schol - This lecture will not focus in great de - arly papers and reviews and has held many tail on the most recent work in my labora - editorial positions, including Associate Edi - tory. Rather, my goal is to use some research tor of the Journal of Clinical Investigation , vignettes to illustrate aspects of my own ap - and has served on the Editorial Boards of proach to research over the years. I will Molecular , the Journal of begin with the start of my own research dur - Clinical Endocrinology and Metabolism , ing medical school, how I continued this at and the American Journal of Medicine . He the NIH and then through a long career on served on the Board of Consulting Editors the HMS faculty at Beth Israel and then of Science Magazine . BIDMC. I won’t speak much about being An elected member of the Institute of the dean of HMS, but I will speak about Medicine and a fellow of the American family and the bigger picture of “what it’s Academy of Arts and Sciences, Flier’s hon - all about.” ors also include the Eli Lilly Award of the I entered the Mount Sinai School of American Diabetes Association, the Berson Medicine in 1968, in the first entering class Lecture of the American Physiological So - of the school. I was one of 36 entering stu - ciety, and an Honorary Doctorate from the dents. One of the reasons I decided to go to University of Athens. He delivered the 2003 Mount Sinai was that I thought being part of Edwin B. Astwood Lecture Award from the this new school would be an adventure. I Flier: Lee E. Farr lecture 407

also was impressed with the people named had been Dr. Berson’s first student. At that as chairs of the major departments. As it time, the Roth group was leading the world turned out, working with two of them in par - in the identification and characterization of ticular influenced me greatly. cell surface receptors. At that time, the struc - The chair of Biochemistry, P.G. Kat - ture of such receptors was unknown, either soyannis, led a group that was the first to biochemically or genetically. Were they on synthesize insulin through peptide synthesis the membrane? Were they inside the cell? and then show it to be biologically active. What kind of molecules were they? How did He and his colleagues made an enormous they generate signals? Jesse Roth’s group number of insulin analogues with varying had terrific science and many smart people, properties that represented an important and the atmosphere was quite electric. body of work. I got to know him, and I be - I was very fortunate in my research at came interested in insulin during my first NIH, but during my first 6 months of re - few months at Mount Sinai. search, literally nothing worked. I had begun The chairman of Medicine was to think that I would leave for some further Solomon Berson. He became my personal clinical training when I had one opportunity hero. He and his research partner, Rosalyn to pursue a good idea, and everything just Yalow, developed the technique of radioim - sort of fell into place. A few months later, munoassay, and though he unfortunately that idea evolved in a paper in the journal died prematurely during my senior year of Science [1] in which we showed that some medical school, Dr. Yalow went on to win patients with extreme insulin resistance had the Nobel Prize in 1977 for their joint dis - autoantibodies against their insulin receptor. covery, which had many practical implica - The paper also showed for the first time that tions, notably providing the first ability to the receptor we identified with this antibody quantitate levels of insulin, growth hor - was truly critical for insulin’s action. We mone, and many other molecules. So I were able to show that the patient’s own worked with the person who first synthe - cells ― removed and studied ex vivo ― had sized insulin and the person who first meas - reduced insulin binding, and we could re - ured it. I was clearly destined for a career produce this by exposing normal cells to studying insulin. serum and eventually to purified im - I had other mentors. Dr. Kurt Hirschhorn munoglobulin. I also thought for one intense was an immunologist and the head of genet - weekend that perhaps similar antibodies ics, and I spent most afternoons working in might be the cause of type II diabetes. So I his lab and authored my first paper with him. located serum from patients with type II di - He thought that I might go into immunology, abetes to test, but alas, they did not seem to but I turned to endocrinology instead. The have these pathogenic antibodies to explain head of endocrinology was Dr. Dorothy the insulin resistance that was present. Krieger, and she was also a very important But what was truly fascinating was an - mentor to me. She came back to the field after other group of patients that had a very sim - having taken time away to raise children, and ilar clinical syndrome, but they did not have she became one of the leaders in understand - these antibodies. Many of these patients sub - ing ACTH and proopiomelanocortin. She was sequently proved to have mutations in the also a mentor to my wife, who was about four insulin receptor gene. That said, this was the years my junior in medical school. paper that got me started, and once I had As you can see, I had some great men - made this observation, I said, “This is what tors at Mount Sinai, and they fostered in me I am going to do for the rest of my life.” And a desire to become either an endocrinologist then the question was: “Where would I do or an immunologist or some fusion of the it?” two. Shortly before he died, Dr. Berson rec - I had the chance to choose possible fac - ommended that I work with Dr. Jesse Roth, ulty positions at Beth Israel in Boston, Co - the head of the diabetes branch at NIH who lumbia, and Yale, all of which were 408 Flier: Lee E. Farr lecture

conducting very interesting research taking sion, acanthosis nigricans, which is a cuta - place in the field of diabetes. The person neous marker for this severe resistance. who recruited me to Beth Israel Hospital, There were three patients whom we referred more than anyone else, was Franklin Ep - to as “type B” patients who had receptor au - stein. Some people in this audience knew toantibodies, and there were three other pa - him exceptionally well [2,3]. He was a won - tients who did not have them. So we learned derful person, a leading nephrologist, and a that there were two new syndromes [4]. great physician, famous among other things The immune syndrome provided the for his Saturday rounds in which he would best evidence that this receptor was truly the round with an intern and a student. No one key receptor for insulin action, and then it who ever saw him in that context would ever provided, for a period a years, a unique tool. forget how he conducted these rounds. Using these antibodies, we studied the re - In my time on the HMS faculty, I pur - ceptor, explored aspects of the mechanism of sued a variety of research directions. I grad - action, undertook partial purification of the ually added academic administration and receptor, and used them to develop an im - leadership roles to my responsibilities. munoassay for the receptor. We also showed The theme of this lecture is to say a few that monovalent antibodies were antagonists things about the concept of hormone resist - and that bivalent antibodies were agonists, ance, because at this point, I am trying to un - and we did that early on in the field while derstand why it is that I kept on coming back that common principle of receptor dimeriza - to this subject in my work. The simplest an - tion and crosslinking was still unknown. swer is, “Because it exists!” It seems to be, When I came to Boston in 1978, among surprisingly, the key to the pathophysiology the things that I decided to do was to gather of many diseases, including diabetes and obe - the patients who did not have these autoan - sity, and by studying insulin resistance and tibodies, suspecting they would be the basis other forms of resistance, we have identified for a new line of discovery. I began to collect key facts regarding hormone action, physiol - these patients clinically and created cell ogy, and pathophysiology that might not have lines from them. It was not until 1985 that otherwise been discovered. I’ve studied three the insulin receptor was cloned ― consid - different molecules related to resistance. They ered a major breakthrough at the time ― and are insulin, leptin, and FGF21. it made the cover of Nature [5]. With that in - In the area of insulin, my research began formation, we looked at the patients we had with rare human syndromes of extreme in - and discovered insulin receptor mutations in sulin resistance, as I just discussed. These several. We characterized them, and for disorders were so-called “experiments of na - about 5 to 7 years, that was a significant por - ture,” and they were both immune and ge - tion of my work. netic. I also worked on common syndromes At various points, I thought that we including obesity and non-insulin-dependent would learn far more about the nature of in - diabetes mellitus (NIDDM). sulin signaling by working with these differ - When leptin was discovered, we were ent mutants, but it just did not work out that interested in aspects of leptin biology in rare way. We learned a great deal, but we reached mouse syndromes and common syndromes. a point where “the well ran dry.” There were Most recently, I’ve done some work with my few other molecules related to insulin sig - wife, Terry (Eleftheria Maratos-Flier), on naling that we could study along with the re - FGF21. ceptor. So where do we stand 23 years after the discovery of the genetic syndromes and Insulin 36 years after the autoantibodies? After that paper in Science [1], we pub - We know that a syndrome of severe in - lished a paper in the New England Journal herited insulin resistance with acanthosis of Medicine describing the clinical syn - nigricans (type A) exists. It turns out, in ret - dromes of insulin resistance with the skin le - rospect, that only about 15 percent have Flier: Lee E. Farr lecture 409

identified genetic etiology, despite strong could be the missing fat-cell-secreted protein efforts to find the molecular explanation whose absence could cause obesity. We now with deep sequencing. Almost all of worked very hard to see if that could possibly those identified mutations are of the insulin be true. A few months after that, we published receptor. The insulin receptor substrates two back-to-back papers in Science . The one have not been established as a cause. There that reported my portion of the work showed a couple of extremely rare, single case fam - that there was severely impaired adipsin ex - ilies, in which resistance appears to be due pression in genetic and acquired obesity [8]. to dominant negative Akt2 [6], but we still We then spent considerable time working with don’t have an adequate explanation for a company that we co-founded to give re - most patients with severe inherited insulin combinant adipsin to ob/ob mice to see if we resistance, and I never would have pre - could cure obesity. The bottom line was that dicted that to be true so many years after we did not have any effect on obesity, and to this research began. this day, we do not know what exactly is going And what about the case of insulin re - on with this adipocyte-secreted protein, which sistance in type II diabetes, an extremely im - we now know as “complement factor D.” It portant and prevalent disease? Insulin still remains an open question. The only last - resistance is common, develops early in the ing positive from that work is a tremendous course of the disorder, and appears to be collaboration and friendship with Bruce that strongly influenced by genetics. Amazingly, lasts to this day. I also became psychologically the genetic etiology of insulin resistance in prepared for the discovery of leptin when it type II diabetes remains largely unknown. was discovered by Jeff Friedman. After millions spent on sequencing of can - In 1995, leptin was known in the field didate genes and on genome-wide associa - as an anti-obesity hormone. The physiolog - tion studies, the canonical signaling ical feedback loop was thought to be that as pathways seem to be exonerated, and few, I you get fatter, your fat cells enlarge and lep - believe, predicted that outcome. So what are tin production goes up ― which it does ― the causative genes? We don’t know. What and then this somehow acts on the brain. about leptin? There was evidence that it had a very pow - erful action on the brain, and the conse - Leptin quence of acting on the brain would be to I had been interested, even as a fellow reduce food intake and increase energy ex - at the NIH, in the ob/ob mouse model. I penditure, causing weight loss in an elegant made a number of abortive attempts to do feedback loop. That’s the way it was imag - research related to them, but I decided that ined to work. someday I would work on these mice be - In fact, that construct was consistent, cause there had to be something profoundly not only with the initial mouse data, but with important to explain this recessive obesity. exciting human data. The first ob/ob human Then in December 1994, a paper by Jeff treated with leptin showed a dramatic effect Friedman [7], who has given an earlier Farr of leptin therapy [9]. Within 24 hours of re - Lecture and is a friend of mine, reported on ceiving leptin, the individual treated was eat - the cloning of the ob gene, demonstrating ing a normal amount of food and remarkable that it encoded a protein mainly expressed weight loss followed. These studies were in fat. This protein is essentially absent in conducted by Sadaf Farooqi and Stephen the ob/ob mouse due to a nonsense mutation. O’Rahilly. Steve had been a post doc in my This discovery changed everyone’s thinking lab at Beth Israel working on the genetics of about the field of energy balance. insulin resistance a few years earlier. As an aside, 7 or 8 years before that My lab devised a way to quantitate lep - paper, I worked on a molecule called adipsin tin, and we measured it in fat and lean mice, in collaboration with Bruce Spiegelman, and and we showed that there was an extremely for a while we were convinced that adipsin clear positive relationship between total 410 Flier: Lee E. Farr lecture

body lipid and the leptin levels. Levels were and had excellent ideas about how to ap - greater at any given body fat in female mice proach the critical experiment. So we took than in male mice, a result that holds true in some normal mice and we fasted them, with every other subsequent study. We wrote a or without replacing leptin. We found that paper that claimed that leptin levels reflect fasting alone caused a long delay in estrus body lipid content in mice and that this pro - cycling, but when we gave leptin during the vided evidence for diet-induced resistance fast, that delay of estrus did not occur. And to leptin action; this is the earliest of 3,000 then we also did studies with thyroid and articles on PubMed if one searches “leptin other endocrine axes, and, though not as dra - resistance” [10]. On the human side ― in matic, there was a similar story, with leptin the New England Journal of Medicine paper limiting the effect. And the last line of our that followed ― you could also see a posi - paper was, “Given the high prevalence of tive relationship between leptin levels in apparent leptin resistance in obese rodents blood and body fat, and once again it and humans, the physiologic response to de - seemed that my research was gravitating to - creasing leptin concentration with starvation ward the phenomenon of resistance. may be the dominant role of this hormone.” This led to one of the great “ah-ha” mo - When we submitted this paper to Na - ments of my career. In 1995, I was teaching ture originally, it was rejected on the basis Harvard medical students in their basic me - of comments from two reviewers. Ironically, tabolism course. One of the things that I the first reviewer said that “it could not pos - taught was the physiology of insulin, and it sibly be true,” and the other one said, having two roles. When you eat, it is impor - “everyone already knows that it’s true.” In tant that insulin levels go up to prevent hy - the end, the paper was published, and it has perglycemia, and when you are not eating, been cited more than any I’ve written, about it’s extremely critical that insulin levels go 2,500 at the moment [11]. I’m also happy to down. If they don’t go down, you will die of say that Rex was recently made a full pro - hypoglycemia. So the fall in insulin during fessor of medicine at Penn, and he is an out - food restriction is as important ― if not standing investigator of whom I am very more important ― than the rise. That is proud. when I got to thinking that perhaps the same We were interested in discovering the concept might be true for leptin. Perhaps mechanisms for leptin resistance. We gave falling leptin was a starvation signal. The leptin to mice who were either on a typical idea would be that as you starve, the leptin low-fat diet or on a high-fat diet. We took levels go down ― which we showed to be out the hypothalamus after leptin adminis - the case ― and that would be a signal to the tration and looked to see if leptin was acti - brain to increase food intake and decrease vating signals in the hypothalamus of energy expenditure. Falling leptin might animals with the high fat diet, and it was not. also be an endocrine regulator, because we That indicated that there was leptin resist - know that when you starve, reproduction is ance at the level of signaling caused by high suppressed and various other endocrine axes fat diets in these obese mice. But what was are suppressed. the molecular basis of this leptin resistance? So we did an experiment to test that hy - There was an issue of Nature in which three pothesis. I had a wonderful endocrine fellow papers came out describing the suppressors at the time, Rex Ahima, who had trained in of cytokine-signaling-family of proteins neuroendocrinology before he came to our [12]. Since leptin is a member of the broad endocrine program. I had tried to sell this cytokine family of proteins, we made probes project to three different fellows before Rex. for all the SOCS-family proteins that we They had all been nice, smart people, but knew of at the time and found that only one, none of them were motivated to take on this SOCS3, was activated by leptin, and it was experiment. When I mentioned this project activated precisely in the medial basal hy - to Rex Ahima, he was immediately excited pothalamus, where leptin has important ac - Flier: Lee E. Farr lecture 411

tions. We ultimately showed that it is in leptin also confers limited protection against those cells that have leptin receptors that excessive energy storage as leptin levels SOCS3 mRNA is induced, and then we rise. That’s the hypothesis that I am fond of, showed that if you co-express SOCS3 with but if true, it would limit the therapeutic re - a leptin-responsive system that you block sponse to leptin, and as you probably know, leptin signaling. So now we had a molecule there has been little success in leptin’s ther - that was induced by leptin, and that would apeutic application despite the efforts of sev - cause leptin resistance, and we spent 5 years eral companies. working on that, trying to understand it. The To summarize, the genetic causes of lep - key question was, would SOCS3 deficiency tin resistance are limited to rare leptin recep - actually protect against obesity? tor mutations that cause severe congenital We were able to rapidly obtain some obesity, and I believe that identifying the mo - SOCS3 knockout mice. In the homozygous lecular mechanisms responsible for leptin re - form, SOCS deficiency is embryonic lethal, sistance in typical obesity should be a high and so we studied heterozygous knockout priority in obesity and diabetes research. mice and we found what we were hoping to FGF21 find, that is, at a very low dose of leptin the mice with only one SOCS3 allele had a bet - Though this may seem an odd segue, I ter response to leptin administration than the sense there may be some working couples wild type mice and these haplo-insufficient here who may benefit from this, so I would mice also had a greater loss of triglyceride like to answer the question of what it is like mass in response to leptin. In further stud - to actually collaborate with your wife as a ies, we asked, “Are SOCS3 haplo-insuffi - scientist for 34 years. Terry and I had always cient mouse protected against obesity?” And been advised that we should never work to - the answer was yes, they are. So, whereas a gether, but we worked together on many dif - normal mouse on a high-fat, high-sugar diet ferent topics and have authored 29 papers will gain weight, the SOCS3 haplo-insuffi - together on a variety of topics, and we have cient mouse gained less weight. They also actually found that our work has been syn - did not increase food intake or show a rise in ergistic with each of us making significant glucose as did the wild type mouse. contributions. As my administrative respon - And then even more interestingly, it sibilities have increased, my primary scien - turned out that SOCS3 is also an antagonist tific contribution has decreased, and I will of insulin signaling. So SOCS3 is actually a close this lecture with some remarks about very good candidate to be one of the major the work related to FGF21 that has been pre - mediators of both leptin and insulin resist - dominantly hers. ance. The problem with leptin resistance FGF21 is a new and exciting metabolic being localized primarily to the brain is that hormone. It is a member of the FGF gene we cannot readily sample the hypothalamus family; it is dominantly, but not exclusively, of humans. So we cannot readily prove that expressed in the liver. It acts through a sub - this mechanism occurs in the medial basal set of FGF receptors. It requires a co-recep - hypothalamus of humans. tor, beta klotho. So leptin resistance characterizes typi - FGF21 gained scientific attention after cal obesity in humans. It most often is ac - researchers at Eli Lilly published a paper quired and rarely is genetic. It’s selective, so showing both in vitro and in vivo stimulation that even within the central nervous system, by FGF21 of glucose uptake into fat cells by leptin’s reproductive signal remains largely inducing glucose transporter synthesis [13]. intact while the effect on body weight is di - FGF21 is an interesting candidate for the minished. This may be evolutionarily ad - treatment of non-insulin-dependent diabetes vantageous in that leptin’s primary role may mellitus (NIDDM). be in the transition from the fed to the We entered the field as a result of starved state as leptin levels decrease, while Terry’s studies that aimed to understand the 412 Flier: Lee E. Farr lecture

molecular physiology of the very low car - by ketogenic diets. After we found that bohydrate, high-fat, so called “ketogenic” FGF21 expression in the liver was physio - diet that produces weight loss in a manner logically regulated by simple starvation, we that she felt required molecular explanation. made an adenoviral anti-sense to inject into We all know about the Atkins diet, much de - the circulation and knock down FGF21 in bated and much discussed, and we know that liver, and when we put those mice on keto - it works, but no one had done an Atkins-like genic diet, we saw a massive increase of fat mouse model to try to understand energy in the liver. FGF21 appeared to be necessary balance and the molecular basis of the diet, for the adaptive increased burning of fat. so that’s what Terry did with four groups of Then we were fortunate to collaborate normal lab mice. with Eli Lilly, which 3 years earlier had One group was fed regular chow, one made FGF21 knockout mice. As we knew, the typical high-fat, high-sucrose diet, and if we put the wild-type mice on a ketogenic one the ketogenic diet, which is a specially diet, they lost weight. If we put the FGF21 made diet that has high-fat and low-carbo - knockout mice on ketogenic diet, however, hydrate. The fourth group was fed normal they actually gained weight. They also had chow but was calorically restricted such that increased food intake, increased body fat, it would lose the same amount of weight as and all the hallmarks of hepatic steatosis. In the group fed the ketogenic diet. The first this way, we had found that FGF21 is an es - three groups, ketogenic diet group included, sential molecular mediator of the weight loss ate exactly the same number of calories; the associated with a ketogenic diet. This was calorie-restricted group ate 30 percent fewer quite exciting to us. calories [14]. We have gone on more recently to She found that though the mice on the ke - human studies and found that people with togenic diet ate as many calories as the mice non-alcoholic fatty liver disease (NAFLD) eating a high-fat diet or regular chow, they have higher levels of FGF21 in their blood. dramatically lost weight while appearing to be To recapitulate, we now know that FGF21 otherwise healthy. They lost as much weight has a role in lipid metabolism, and this arose as mice eating normal chow but 30 percent from studies of the ketogenic diet in the fewer calories. This was a striking observa - manner that I have described. Suppression tion and indicated that the consumption of the of FGF21 in the liver using adenovirus or ketogenic diet induces a unique metabolic systemically through gene knockouts has state and weight loss with unchanged caloric major phenotypes, as I’ve described. In re - intake, and this has now been repeated three or cent work, we show that obese humans with four times and many physiological correlates NAFLD have paradoxically increased ex - have been investigated and published. These pression levels of FGF21. One would think, ketogenic diet mice have very low insulin lev - given the studies of FGF21 knockout mice els, ketosis, increased energy expenditure, in - above, that high FGF21 levels should be creased insulin sensitivity, and a mild fatty protective against steatosis. These patients liver, but not a fatty liver of the variety that have high levels of FGF21, but they also would be worrisome. have fatty liver disease. This new finding So then the question was: What is the brings us back again to a theme that has been molecular explanation? We did not feel that recurring throughout my career: Is there also there was an adequate explanation based on resistance to FGF21? The answer seems to changes in the levels of known hormones. be, yes, indeed these patients are FGF21 re - So we performed transcriptional analysis by sistant [15]. gene arrays on the mildly fatty liver, and we The study of this novel hepatic hormone found that changes in the levels of one gene is very exciting. We know that its expression were striking. That gene was FGF21. in the liver is regulated by the composition We found that FGF21 expression was and volume of the diet, and it has actions re - markedly and selectively induced in the liver lated to lipid oxidation and adipocyte glucose Flier: Lee E. Farr lecture 413

uptake. There are many other issues currently sault, as its funding sources are being threat - under investigation in our lab and elsewhere, ened, but I do believe that it is still a won - and now it seems that FGF21 also has an ac - derful career choice. Second, I would say tion in the brain. It is also a regulator of that for anyone wishing to do research over brown adipose tissue thermogenesis. Terry the course of a career, you should take ad - and Bruce Spiegelman have just published a vantage of the diversity ― and the surprises paper showing that FGF21 induces brown fat ― that will emerge over a lifetime and ca - activation in white fat depots and promotes reer. It may seem at the beginning that it will weight loss [16]. There also seem to be be a simple path. One may think, “I'm going mechanisms for FGF21 action to limit he - to do this, and then I'm going to keep doing patic lipid accumulation and toxicity, fibro - it,” but it almost never works out that way. I sis, and inflammation, and we are trying to know people, friends of mine, who have understand how this occurs, and we are try - been extremely successful in that way, they ing to understand the mechanism for FGF21 just go deeper and deeper and deeper on the resistance. The current state is: FGF21 action same problem, and that is great if you can and resistance may be important factors in do it. But I also know many people who the pathophysiology of obesity, Type II dia - have moved from one important problem to betes, and NAFLD. And it is a potential another and even from one field to another novel therapy for diabetes and NAFLD, and and they can also be very, very successful. this is being explored by several pharmaceu - More often than not, there will be surprises tical companies. rather than a straightforward path. Third, whatever your area ― whether it Hormone Resistance, in Summary is biomedical bench science, or social sci - Hormone resistance seems to be at the ence, or policy ― remain alert for ideas, core of the pathophysiology of obesity and connections, and opportunities that suggest a Type II diabetes, and insulin, leptin, and new direction. They are out there, but you FGF21 are certainly involved. Well-delin - should expect some negative response be - eated genetic causes exist, at least for insulin cause you will always get some negative re - and leptin resistance, but these are rare, and sponses to your best ideas, and our job is to the genetic and molecular etiology in the vast not be discouraged inappropriately. majority of patients with common syndromes Fourth, it is really important to have remains uncertain despite much effort. I find mentors and role models, and I think your this very disappointing given the advances in program here at Yale has been built upon that fundamental understanding in the field, espe - thesis for over a hundred years. The only cially for insulin signaling over the past 30 thing that I would counsel is this: Do not years. We have illuminated the black box seek to imitate your mentors. I know people such that we know an extraordinary amount who walk like, talk like, and have body tics about insulin signaling; we know dozens of like their mentors. Body tics are not usually molecules that are modulated up and down a good thing to emulate. Take elements, but and are phosphorylated and dephosphory - do not take the whole package. And then the lated, but we still don’t know the genetic final point ― though it might sound a little cause for insulin resistance in Type II dia - sappy ― in the end, over a lifetime, happi - betes. This does suggest, however, that major ness is the key, and this may not always cor - breakthroughs remain to be made, so I would relate with professional success. You may be counsel investigators to remain in this ex - in many situations where you will be needing tremely important field. This disease is as im - to make choices that balance your personal portant as it ever was, and many of the big happiness and that of your family with your questions have yet to be answered. profession, and there are people who make Allow me to end with a few reflections. the wrong decisions every day when they are The first: Medicine is and will remain an ex - confronted by these choices, but striking the traordinary profession. Research is under as - proper balance is what life is all about. 414 Flier: Lee E. Farr lecture reFerences netic and acquired obesity. Science. 1987;237(4813):405-8. 1. Flier JS, Kahn CR, Roth J, Bar RS. Antibod - 9. Farooqi IS, O’Rahilly S. Leptin: a pivotal ies that impair insulin receptor binding in an regulator of human energy homeostasis. Am unusual diabetic syndrome with severe insulin J Clin Nutr. 2009;89(3):980S-4S. resistance. Science. 1975;190(4209):63-5. 10. Frederich RC, Hamann A, Anderson S, Löll - 2. Forrest JN, Franklin H. Epstein: reminis - mann B, Lowell BB, Flier JS. Leptin Levels cences of a brilliant physician-scientist and Reflect Body Lipid Content in Mice: Evidence master clinician. J Am Soc Nephrol. for Diet-Induced Resistance to Leptin Action. 2009;20(8):1651-3. Nat Med. 1995;1:1311-4. 3. Forrest JN, Cohen AD, Torretti J, Himmel - 11. Ahima RS, Prabakaran D, Mantzoros C, Qu hoch JM, Epstein FH. On the mechanism of D, Lowell B, Maratos-Flier E, et al. Role of lithium-induced diabetes insipidus in man and leptin in the neuroendocrine response to fast - the rat. J Clin Invest. 1974;53(4):1115-23. ing. Nature. 1996;382(6588):250-2. 4. Kahn CR, Flier JS, Bar RS, Archer JA, Gor - 12. 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