Jacob J Lemon and Manisha Gupte*
Department of Biology, Austin Peay State University, Clarksville, TN 37040
*Corresponding author: Manisha Gupte, Department of Biology, Austin Peay State University, Clarksville, TN 37040
Submission: March 07, 2022; Published: April 29, 2022
ISSN 2637-8019Volume3 Issue3
Metabolic Syndrome which encompasses of hypertension, hyperlipidemia, insulin resistance, and obesity increases the risk of diseases such as cardiovascular diseases, Type 2 Diabetes Mellitus (T2DM), and cancer to list a few. The mechanisms leading to this syndrome have not been fully understood due to the multifaceted pathologies associated with this syndrome. While numerous molecules are being investigated in the etiology of metabolic syndrome, a number of studies have looked at the association of Glycogen Synthase Kinase-3 (GSK-3) and metabolic syndrome given the data that GSK-3 activity is increased with obesity and T2DM. The GSK-3 family consists of two isoforms, alpha (α) and beta (β). GSK-3α/β expression and activity has been found to be significantly elevated in muscle biopsies of T2DM patients and administration of GSK-3 inhibitors to rodent models of obesity and T2DM have improved insulin sensitivity and glucose homeostasis. Thus, GSK-3 is a promising therapeutic target for the management of metabolic diseases including T2DM. Numerous studies using either a pharmacological approach or animal models have investigated the role of GSK-3 in obesity-associated metabolic perturbations including glucose clearance However, this has been a daunting task given its ubiquitous expression, complex signaling, and the two very similar isoforms with some unique as well as redundant functions. Hence, as yet, the precise molecular mechanisms via which GSK-3 causes metabolic perturbations including obesity-induced glucose intolerance, diabetic cardiomyopathy, or cardiac dysfunction are yet unknown.
Obesity has reached epidemic proportions worldwide and is associated with increased incidence of numerous pathologies including hypertension, Type 2 Diabetes Mellitus (T2DM), and heart failure to list a few [1]. Thus, current available therapies to counter the onslaught of metabolic diseases associated with obesity are inadequate, hence a need to identify new molecular targets to combat obesity-associated pathologies. Numerous studies in humans and animal models have implicated Glycogen Synthase Kinase-3 (GSK-3), a ubiquitously expressed serine threonine kinase in glucose intolerance and peripheral insulin resistance which are hallmarks of Type 2 Diabetes Mellitus (T2DM) [2-6]. Identified in 1980 it was shown to regulate Glycogen Synthase (GS), a rate limiting enzyme of glycogen synthesis [7]. However, now it is becoming clear that GSK-3 is critical for the regulation of many other signaling pathways of glucose homeostasis [8-11]. GSK-3 has two isoforms: α and β which share 98% sequence homology in their kinase domain but differ in their N and C terminal [12]. Overall, the two isoforms are 85% identical and hence it is not surprising that a number of studies indicate a distinct and overlapping effects of the two GSK-3 isoforms in various pathologies. Another interesting feature of this kinase is that it is constitutively active in unstimulated state and becomes inactive by phosphorylation in response to external stimuli including insulin [12]. Numerous studies using either a pharmacological approach or animal models have been conducted to determine the role of GSK-3 in obesity-associated metabolic perturbations including glucose clearance [13-28]. However, this has been a daunting task given its ubiquitous expression, complex signaling, and the two very similar isoforms. As yet, we do not know the precise molecular mechanisms via which GSK-3 causes metabolic perturbations including obesity-induced glucose intolerance, diabetic cardiomyopathy, or cardiac dysfunction. Additionally, the role of specific GSK-3 isoforms (α and β) in peripheral tissues critical for glucose homeostasis is yet unclear. Hence numerous studies including those from our group are investigating the role and regulation of specific GSK-3 isoforms in tissues such as heart, liver, skeletal muscle, pancreas, and adipose tissue in High-Fat Diet (HFD) induced glucose intolerance, a hallmark of T2DM.
Regulation of GSK-3 in Heart
Heart metabolic pathophysiology has been linked to increased lipid accumulation and higher saturated fatty acids found in HFD, which increases the risk of Heart Failure (HF) and cardiovascular disease (CVD). The most common form of HF is Cardiomyopathy, where obesity induces lipotoxic cardiomyopathy by causing lipid accumulation in Cardiomyocytes (CM) leading to cardiac dysfunction [29]. CVD and HF are associated with abnormal GSK- 3 activity, such as regulating cardiomyopathy and angiogenesis. For instance, pigs induced with metabolic syndrome symptoms and cardiovascular ischemia while on a high-fat diet demonstrated elevated myocardial perfusions ratios and capillary and arterolic density after using the GSK-3ß inhibitor IM-12 [30]. Furthermore, in obesity-related cardiac dysfunction, GSK-3α mediates lipid accumulation in the heart by stimulating fatty acid uptake and storage by phosphorylating the nuclear receptor peroxisome proliferative-activated receptor alpha (PPARα) [31]. GSK-3ß’s role in obesity-induced cardiac dysfunction has been studied using CreloxP genetic recombination in mouse cardiomyocytes, which shows contrasting phenotypes whether the deletion occurs before or after establishing chronic obesity. On a control diet, CM-specific GSK- 3β-KO mice exhibit no alteration in cardiac function. Interestingly, CM-GSK-3α compensated for the loss of CM-GSK-3β, as evident by significantly reduced GSK-3αs21 phosphorylation (activation) resulting in a preserved canonical β-catenin ubiquitination pathway and cardiac function. However, this protective compensatory mechanism is lost with HFD, leading to excessive accumulation of β-catenin in HFD-fed CM-GSK-3β-KO hearts, resulting in adverse ventricular remodeling and cardiac dysfunction. These results suggest that cardiac GSK-3β is crucial to protect against obesityinduced adverse ventricular remodeling and cardiac dysfunction [32]. In stark contrast to the developing obesity model, deleting CM-GSK-3β in obese animals did not adversely affect the GSK-3αS21 phosphorylation (activity) and maintained canonical β-catenin degradation pathway and cardiac function. Importantly, deleting GSK-3β in CMs improved glucose clearance in obese CM-GSK-3β KO animals compared to the controls [33].
Regulation of GSK-3 in Skeletal Muscle
Numerous studies have indicated a role of GSK-3 in insulinresponsive peripheral tissue such as Skeletal Muscle (SM) with total GSK-3 activity elevated in T2DM human skeletal muscle [13]. Importantly, GSK-3 inhibition in skeletal muscle of insulin resistant ZDF rats enhanced insulin action on glucose transport, oral glucose tolerance, whole body insulin sensitivity, and IRS-1-dependent insulin signaling [5,17]. Tissue and isoform specific experimentation reveals skeletal muscle specific GSK-3β KO mice, in contrast to the liver-deleted animals, display improved glucose tolerance that is coupled with enhanced insulin-stimulated glycogen synthase regulation and glycogen deposition [11]. In addition, in human non-diabetic skeletal muscle, siRNA against GSK-3ß led to 60-70% reduction in expression and increased glycogen synthase activity in absence of insulin and increased insulin action [34]. Unlike GSK-3α, GSK-3ß directly regulates both GS activity in the absence of added insulin and through control of insulin action [34]. Furthermore, mice fed a HFD and given a GSK-3ß inhibitor had higher glucose infusion rates, GS activity ratios and net glycogen synthesis, higher plasma glucose disappearance, improved peripheral insulin sensitivity, and lower endogenous glucose production compared to HF only [35].
Regulation of GSK-3 in Pancreas
Pancreatic activity and GSK-3’s role in T2DM are interconnected when understanding glucose tolerance and insulin effectiveness. Insulin is generated by the ß-cells found in the pancreas, and this secreted peptide hormone enters the bloodstream to lower blood glucose levels under normal conditions. When insulin resistance becomes chronic, partial ß-cell mass reduction in the pancreas occurs, which is a hallmark sign in T2DM patients [36]. Previous research shows GSK-3ß deletion in pancreatic ß-cell mice increased ß-cell mass leading to higher insulin levels and improved glucose tolerance even when fed a HFD [10]. Similarly, using isolated, human adult pancreatic islets, GSK-3 inhibition enhanced pancreatic ß-cell proliferation by reducing p27 expression, which is an important cell cycle regulatory protein to halt cell division [36].
Regulation of GSK-3 in Adipose Tissue
Obesity is associated with excessive adipose accumulation that can lead to worsening pathologies. Glucose intolerance and insulin resistance arising from obesity can lead to T2DM, but how GSK-3 is involved in these conditions is still being considered. Mice fed a HFD exhibit an increase in GSK-3 activity in adipose tissue and a rapid increase in plasma blood glucose levels, indicating that excessive adipose accumulation can lead to glucose intolerance and insulin resistance [6]. Weight-loss on the other hand is demonstrated to reduce both GSK-3 isoform expressions in adipose tissue [37]. Current understanding in lipid accumulation can be seen with mice exhibiting hypercortisolism from excess glucocorticoids and GSK-3 levels. Elevated adipose GSK3β and H6pdh expression were seen to contribute to 11ß-HSD1 mediating hypercortisolism associated with visceral adiposity [38].
Systemic Regulation of GSK-3
The above studies have utilized non-isoform specific inhibition or tissue-specific genetic models to investigate the role of GSK-3 in obesity-associated pathologies, but these are of limited value to predict the clinical outcome of systemic inhibition. To investigate the isoform specific role of GSK-3 in HFD-induced metabolic perturbations, we created a novel global conditional GSK-3-KO mouse model that allowed us to delete the gene globally in an isoform-specific and temporal manner. On an HFD, GSK-3α- KO mice had a significantly lower body weight and modest improvement in glucose tolerance compared to their littermate controls. In contrast, GSK-3β-deletion-mediated improved glucose tolerance was evident much earlier in the timeline and extended up to 12 weeks post- HFD. However, this protective effect was blunted after chronic HFD (16 weeks) when GSK-3β KO mice had a significantly higher body weight compared to controls. Importantly, GSK-3β KO mice on a control diet maintained significant improvement in glucose tolerance even after 16 weeks. In summary, our novel mouse models allowed us to delineate the isoform-specific role of GSK-3 in obesity and glucose tolerance and indicates the importance of maintaining a healthy weight in patients receiving lithium therapy, which is thought to work by GSK-3 inhibition mechanisms [39].
GSK-3 is a critical enzyme that has shed light into further understanding metabolic pathologies that underlie metabolic syndrome and related diseases. Many studies have examined whole enzyme inhibition to demonstrate its roles in glucose disposal, insulin sensitivity, and cardiac physiology [40]. Genetically deleting or enzymatically inhibiting each isoform has brought forth isoformspecific roles of GSK-3 in specific tissues and organs, such as heart, adipose, skeletal muscle, and pancreas. Systemic GSK-3 inhibition is a crucial advancement in delineating the isoform-specific roles of GSK-3 in glucose metabolism under a HFD, but homozygous genetic deletion is different from the level of inhibition seen with pharmacological agents. Therefore, future studies examining systemic heterozygous deletion in animal models are warranted (Table 1).
Table 1:
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