Metabolic Disease and Adipose Tissue: Studies in Patients Undergoing Bariatric Surgery
ARC Directors and Co-Directors:
Overall goal and mission:
It has long been established that obese individuals are typically insulin resistant and predisposed to such disorders as type 2 diabetes, atherosclerotic cardiovascular disease, NAFLD and certain cancers. More recent studies suggest that inflammation in adipose tissue could be a key factor leading to these illnesses. Initial studies of bariatric surgery patients by the Ruderman laboratory established (1) that they could be subdivided into insulin resistant (75%) and insulin sensitive (25%) subgroups and (2) that these subgroups were distinguished by differences in AMPK activity, oxidative stress and the expression of inflammatory, mitochondrial and other genes in their adipose tissue. This, plus the fact that these patients can be studied post-operatively led us to expand our research on these patients and submit an ARC application. Also instrumental in this decision were (1) the interest of other investigators at BU (Drs. Apovian, Kandror and Puri) and Eastern Carolina University (a pioneering group in the bariatric surgery field) in joining us and (2) encouragement from the NIDDK to develop a multi-investigator program to extend our studies, for which they awarded us an R24 Seed Grant (2012-2013). Because of a shift in funding away from multi-investigator grants, in May 2013 the NIDDK requested that in June 2013, we submit two multi-investigator R01 secondary proposals, including one jointly with ECU, rather than a single PPG. For this reason the two projects (grants) from BU-BMC will be described separately. The participation of ECU will be primarily in Project 1.
Rationale and Background
Obesity and disorders associated with the metabolic syndrome such as Type 2 Diabetes continue to be an epidemic in developed countries. It is our goal to establish an ARC focused on understanding the relationship between metabolic disorders and Adipose Tissue Biology by studying adipose tissues from insulin sensitive and insulin resistant patients obtained during and after Bariatric Surgery.
With collective efforts of faculty representing different expertise and disciplines, we are posed to begin the development at BUMC of a unique center of knowledge in the field of adipose tissue biology pre- and -post bariatric surgery. An important feature of our proposal will be the cross talk and tight collaborations between basic and clinical researchers and clinicians, with expertise ranging from metabolic disease, to biochemistry to immunology to Biostatistics, and to surgery. We anticipate that our proposed project will entail additional faculty at BU and perhaps other institutions joining our group, with the intent of building one of a kind Evans Center-based ARC, with potential to lead to a national initiative/center focused on Metabolic Health and Bariatric Surgery. Such a future grand plan will require initial efforts in order to develop an infrastructure (research coordinator, biological cores, interacting investigators) for clinically characterizing the patients, as well as an array of fundamental studies of mechanisms. The ARC mechanism and the Evans Center platform and leadership provide ideal incubators for such a challenge.
Obesity, insulin resistance and the metabolic syndrome
Obesity in humans has been closely linked with the metabolic syndrome, a disorder characterized by insulin resistance and a predisposition to hyperglycemia, dyslipidemia, and hypertension. Patients with the metabolic syndrome are at increased risk for developing type 2 diabetes, atherosclerotic cardiovascular disease, NAFLD, cardiomyopathies, certain cancers and even Alzheimer’s disease. Over the past two decades it has been established that bariatric surgery diminishes hyperinsulemia and insulin resistance in morbidly obese patients and that this is associated with the reversal of diabetes, even prior to major weight loss. Furthermore, long term studies have revealed that such surgery also diminishes the incidence of other metabolic syndrome disorders (see above), as well as overall mortality. Despite this, the molecular mechanisms by which obesity produces insulin resistance and bariatric surgery reverses it are unclear. A related question is why approximately 25% of massively obese people are insulin sensitive and less predisposed to metabolic syndrome-associated diseases.
Pathogenesis of insulin resistance in the setting of obesity
It is widely held, based on studies in experimental animals and, to some extent, humans, that insulin resistance in the setting of obesity results from the saturation of lipid storage capacity and the subsequent development of inflammation in adipose tissue. The latter has been attributed at least in part to the infiltration of adipose tissue with macrophages, lymphocytes and probably polymorphonuclear cells that originate in bone marrow. It has also been proposed that such inflammation in adipose tissue alters the release of adipokines (e.g., increased TNF-α and decreased adiponectin) and that this secondarily causes inflammation and insulin resistance in other tissues (e.g., liver, muscle). The initial inflammation in adipose tissue is generally attributed to the generation of chemokines and adhesion molecules that attract pro-inflammatory cells from the circulation and ultimately lead to an increase in the abundance of type 1 macrophages and certain lymphocytes. As already noted, the above-mentioned events do not occur and/or are much less severe in massively obese individuals who are insulin sensitive than in comparably obese people who are insulin resistant. It is our working hypothesis, based on recent studies, that differences in the regulation of the fuel sensing enzyme AMP-activated protein kinase (AMPK) (Gauthier et al 2011; Xu et al 2012) and the histone protein deacetylase, SIRT1 (Kloting 2010; Xu 2012), a redox sensor implicated in the increase in longevity caused by caloric restriction, may, at least partially, account for this difference. However, we wish to emphasize that, as indicated below, we are open to the possibility that other factors, such as inflammation and/or an increase in chemokines, that attract inflammatory cells could be a primary event.
Studies in intact rodents and cultured cells have demonstrated that AMPK and SIRT1 are part of a more complex network of molecules that regulate cellular and systemic events. In addition, it has become increasingly clear that they occupy this role both by regulating each other, and phosphorylating and deacetylating specific enzymes, transcriptional activators (e.g. FOX0s, p53) and co-activators (PGCIα), and signaling molecules such as eNOS and mTOR (Ruderman AJP 2010). In keeping with their key role, the activity of AMPK, and where studied SIRT1, is diminished in tissues of many obese and insulin-resistant rodents (dietary and genetic) and treatments that activate them directly or indirectly (e.g. exercise, AICAR, calorie restriction) can prevent diabetes, atherosclerosis and insulin resistance in these animals. Furthermore, AMPK and SIRT1 activation have been shown to diminish the inflammation, oxidative stress and mitochondrial dysfunction caused by gluco- and lipotoxicity and inflammatory cytokines and decreases in SIRT1 and AMPK activity have the opposite effect (Ruderman AJP 2010) in many cell types including macrophages and endothelial cells. It must be noted, however, that two of the abnormalities diminished by AMPK and SIRT1, inflammation and oxidative stress, can in some situations downregulate AMPK activity. Adding to the complexity, an anti-inflammatory agent, salsalate, that has been shown to diminish insulin resistance in both humans and experimental animals with type 2 diabetes and/or obesity, has recently been shown to activate AMPK.
Studies in adipose tissue of obese humans
Despite the abundant evidence linking AMPK and SIRT dysregulation to the maintenance of insulin sensitivity in cultured cells and experimental animals, little is known about their roles in governing insulin sensitivity in humans. Likewise, the role of inflammation is unclear. Since insulin resistance in humans has been described by some investigators (e.g., G.I. Schulman), even in the absence of inflammation. Furthermore, most of our existing knowledge is based on studies in skeletal muscle. Essentially it has been established that both AMPK and SIRT1 are activated in human muscle by exercise, which also increases insulin sensitivity. On the other hand, studies of these enzymes in muscle of patients with obesity and/or type 2 diabetes to date have not yielded consistent results. This caused us to turn our attention to adipose tissue, in which insulin resistance, inflammation, oxidative stress and macrophage infiltration had already been described in obese humans.
We chose to study patients undergoing bariatric surgery for massive obesity for a number of reasons, including the following:
(1) As a group, such patients have many of the abnormalities described above in their adipose tissue. Furthermore, several distinct fat depots can be obtained for analysis at the time of surgery;
(2) Bariatric surgery diminishes insulin resistance and subsequently oxidative stress and obesity in these individuals;
(3) It also reverses diabetes (80% of patients) and diminishes the incidence of coronary heart disease, NAFLD , certain cancers and other disorders;
(4) Approximately 75% of patients undergoing bariatric surgery are insulin resistant; however, the other 25% appear to be insulin sensitive; and
(5) Our previous studies had established that AMPK is activated in cultured 3T3L1 adipocytes when lipolysis is increased, and that failure of this to occur (due to pharmacological or genetic treatments) results in oxidative stress and increased adhesion of mononuclear cells. This in turn led to a small study in which we observed that AMPK activity is decreased in both visceral and subcutaneous adipose tissue (taken during bypass surgery) of massively obese individuals who are insulin resistant compared to equally obese patients who are insulin sensitive. Likewise, others had observed decreased SIRT1 mRNA in adipose tissue of such patients (Kloting 2010).