FSP27 and links to obesity and diabetes mellitus

Abstract

Purpose of review:

Obesity is a pandemic, yet preventable healthcare problem. Insulin resistance, diabetes mellitus, dyslipidemia, and cardiovascular complications are core manifestation of obesity. While adipose tissue is a primary site of energy storage, it is also an endocrine organ, secreting a large number of adipokines and cytokines. Nonetheless in obesity, the secretion of cytokines and free fatty acids increase significantly, and are associated with the degree of adiposity and insulin resistance. Fat specific protein 27 (FSP27) has emerged as one of the major proteins that promote physiological storage of fat in adipose tissue.

Recent findings:

Review of number of recent findings suggest that FSP27 play a crucial role in physiological storage of fat within the adipose tissue especially in humans. However, in disease conditions such as obesity, FSP27 may contribute to ectopic fat accumulation in non-adipose tissue.

Summary:

More studies are required to highlight the tissue specific role of FSP27, especially in humans.

Introduction

Obesity is a pandemic, yet preventable global healthcare problem that afflicts both adults and children [1–3]. The current trends predict that by 2030, more than 86% of US adults will either be overweight or obese [4]. Obesity is characterized by the accumulation of excess fat; abnormalities arise when spillover and/or ectopic accumulation of fat in non-adipose tissue, particularly in the intra abdominal visceral region and in the liver occurs [5–7]. Insulin resistance, diabetes mellitus (DM), dyslipidemia, and cardiovascular complications are the core manifestation of obesity [8–10]. However, the mechanisms that links obesity to these complications are not yet fully understood. Insulin resistance and DM afflict more than 415 million people and is a leading cause of morbidity and mortality worldwide [11]. Studies show that a state of insulin resistance, which exists in most obese subjects, represents a fundamental pathophysiological abnormality associated with a number of cardiovascular and metabolic dysfunctions in [12].

Metabolic action of insulin on the peripheral tissue is mediated by tissue specific actions, which involve complex molecular signaling, and rapid changes in protein phosphorylation and their biological action [13–15]. Diabetes mellitus, a chronic disorder that usually originates as insulin resistance, occurs with defects in either insulin production or action in the peripheral tissue, and unfavorably modifies whole body carbohydrate and fat metabolism [16]. The condition leads to chronic elevated free fatty acids (FFA), causing decreased glucose transport to the periphery, as well as increased fat breakdown, which subsequently leading to elevated hepatic glucose production, contributing to chronic high blood glucose [17–19].

Despite being a primary site for energy storage, adipose tissue is in fact is an endocrine organ, secreting a large number of hormones, adipokine, cytokines and FFA. Studies show that the secretion, especially of cytokines and FFA, increase significantly with both the degree of adiposity, and insulin resistance [20, 21]. Conversely, when the level of plasma FFA decreases, as in the case of an anti-lipolytic agent, peripheral insulin and glucose uptake improves.27 It appears that central/visceral adiposity is more lipolytic than subcutaneous fat, and does not respond easily to the anti-lipolytic action of insulin, which implies visceral fat is more detrimental in causing insulin resistance, and diabetes [22, 23].

Adipose tissue store fat as a triglyceride in large, often solitary cytoplasmic organelles called lipid droplets (LD), which composed of a core TG surrounded by a phospholipid monolayer in which numerous proteins are embedded[24, 25]. Fat Specific Protein 27 (FSP27), a lipid droplet-associated protein, has emerged as one of the major molecule that plays a role in physiological storage of fat [26, 27]. Initially identified in murine adipocytes, FSP27 is localize on the surface of lipid droplets and promote lipid droplet fusion, thus making LD large and unilocular [26, 28, 29]. In recent years, FSP27 has been identified as a regulator of lipolysis, a catabolic component of the fat metabolism that provides energy in metabolic needs [30] [31]. Despite their vital physiological importance in energy balance, increased FFA leading to impaired metabolism and signaling, as in case of obesity, can cause numerous impairments associated with lipotoxicity. This review will focus on recent progress on identifying role of FSP27 in obesity, insulin resistance and diabetes.

Role of FSP27 in cellular/murine models:

FSP27 was first identified in early 1990, as a target gene of C/EBP [32]. The functional aspect of FSP27 in adipocytes remained elusive until studies in cultured adipocytes, [28] and FSP27-null mice [33] [34], demonstrated that it is an integral part of LD protein and is required for the formation of a large unilocular LD in adipocytes. Although, the importance of FSP27 in mediating the formation of a LD in adipocytes is clear [29, 35], the physiological consequence of this activity is not certain. The reports of FSP27 being both a negative, and a positive contributor in development of obesity and insulin resistance have been described. For example, using a murine model, Nishino and colleagues demonstrated that shutting off FSP27 presumably from the whole body protects from diet-induced obesity presenting as a lean phenotype, although FSP27 was measured only in fat [33, 34]. Additionally, mice were protected from excess lipid storage and insulin resistance even in high fat diet. Recent studies using antisense oligonucleotides (ASOs) from Baldan’s laboratory also suggested that silencing of FSP27 throughout the whole body improves glycemic control, insulin resistance, and diminishes the progression of atherosclerosis in mouse models of obesity and atherosclerosis respectively under high fat diet conditions [36*, 37*], again suggesting a pathological role of FSP27. These results suggest that FSP27 contributes to the progression of obesity and negatively regulate insulin signaling, promoting obese phenotype.

On the other hand, the presence of multilocular LDs in FSP27 deficient adipocytes was reported to be associated with increased basal lipolysis [33, 38*], likely as a result of a substantial increase in the surface area that is accessible to lipases such as adipose triglyceride lipase (ATGL), leading to increased FFA release in the system without lipolytic demand or signaling. A cellular study using human adipocytes exhibited that FSP27 regulates lipolysis and release of FFA by interacting with rate limiting lipase ATGL [38*]. While silencing of FSP27 impaired insulin signaling, overexpression protected adipocytes from FFA mediated inulin resistance. With a similar notion, Gonzalez and colleagues demonstrated that adipocyte specific deletion of FSP27 augments basal lipolysis and promotes insulin-resistance under high fat conditions [39*]. In agreement, Zhou and colleagues demonstrated that deletion of FSP27 in leptin deficient mice that are obese, and challenged with prolonged high fat diet promoted insulin resistance, adipose tissue inflammation and fatty liver [40*]. Collectively, these studies suggest that FSP27 has a physiological role in storing fat within adipose tissue that prevents spillover and or ectopic accumulation in non-adipose tissue. Thus lack of the FSP27 in adipose tissue cause unwarranted breakdown of fat that will cause unfavorable health outcomes that are typically seen in obesity and insulin resistance.

Few important, including some recent murine studies (cellular, intact tissue, and in vivo) are summarized in table 1 that highlights opposing data from disruption/modulation of FSP27 in obesity and insulin resistance. Additionally, few studies listed in the table also depict impact in the liver that will be discussed below.

Table 1.

Summary of cellular studies and murine models

Manuscripts	Manipulations	Results/outcome
Nishino et. al. 2008 J Clin Invest	Murine in vivo: Knockdown of FSP27	Protected from diet induced obesity, insulin resistance, increased mitochondrial biogenesis
Gong et. al. 2011 J Cell Bio	Murine in vivo: Silencing of FSP27	Protected from diet induced obesity, insulin resistance, and whole-body glycemic control
Langhi et. al. 2017 J Lipid Res	Murine in vivo: Silencing of FSP27	Protected from diet induced hyper TG, reduced atherosclerosis lesion size
Rajamoorthi et. al. 2017 J Lipid Res	Murine in vivo: Ob/Ob, FSP27−/− mice	Decreased hepatic steatosis, insulin resistance, reduced adipose tissue inflammation
Zhou et. al. 2014 Nature communication	Murine in vivo: Adipose specific KO	Low fat mass and body weight. Increased energy expenditure, but impaired insulin sensitivity, hepatic steatosis, dyslipidemia
Tanaka et. al. 2015 J Biol Chem	Murine adipocyte: overexpression of FSP27	Protected from growth hormone induced insulin resistance
Sharma et. al. 2018 Bioscientifica	Healthy humans: growth hormones/saline infusion	Growth hormone mediated reduction in FSP27, increased lipolysis and FFA, insulin resistance
Sharma et. al. 2019 Am J Physiol Endocrinol Metab	Humans, hADSC: growth hormone induced defects	Growth hormone mediated reduction in FSP27, impaired insulin signaling, overexpression of FSP27 protective

Role of FSP27 in human tissue

Studies in primary human cells and in an intact human tissue demonstrate the presence of FSP27 protein in humans. Unlike murine data, there seems to be a consensus among the results that are published in past ten years, implying a physiological role of FSP27. Studies in human adipocytes demonstrated that depletion of FSP27 using siRNA impaired insulin sensitivity and overexpression of FSP27 protected from FFA induced insulin resistance [38]. Additionally, homozygous mutation of FSP27 in human patient resulted in a significant reduction in total fat mass within the adipose tissue, creating a lean phenotype, but the overall systemic consequences were unfavorable [41]. Biopsy samples from subcutaneous fat demonstrated the presence of multilocular LD, as in FSP27 deficient mice but the patient exhibited partial lipodystrophy with ectopic lipid accumulation in the liver, dyslipidemia, and insulin resistance. Reports on more than 80 obese subjects show that the level of FSP27 in visceral fat was significantly lower than that of lean counterparts, which was not the case between the subcutaneous fat within same subjects [42**]. Visceral fat, especially of obese subjects, is associated with several abnormalities including insulin resistance [14, 43, 44], and believed to be a major contributor of FFA (basal) into the circulating system [45, 46], thus it is plausible to hypothesize that having less of the FSP27 in visceral depot may contribute to increased circulating FFA leading to insulin resistance [47, 48]. Additionally, gene expression of FSP27 in visceral adipose tissue was negatively correlated with body mass index, fat mass, fasting glucose, insulin, and HOMA IR in obese patients, suggesting a functional role of FSP27 in obesity and insulin resistance. Despite less distinct links between subcutaneous fat and above-mentioned abnormalities within the same patients, gene and protein levels of FSP27 were increased substantially after weight loss among patients. Consistent with these studies, earlier results also demonstrated that FSP27 levels are positively associated with insulin sensitivity in obese subjects [31].

Growth hormone (GH) plays a vital role in lipid mobilization and utilization, especially during fasting. Unregulated surplus of GH production has been linked with adverse health outcomes; similar to that of FFA, a sustained increase in GH induced lipolysis is causally linked to insulin resistance[49, 50]. Recent study of GH infused healthy subjects demonstrated that levels of FSP27 in subcutaneous fat were significantly reduced compared to that of saline infused counterparts [51]. Moreover, at the cellular level the decreased in FSP27 was accompanied by increased basal lipolysis and impaired insulin signaling [52*]. The overexpression of FSP27 reversed these abnormalities. Collectively, these studies from intact human tissue and adipocytes suggest a physiological role of FSP27 in lipid storage, mobilization, and insulin sensitivity, and by promoting lipid storage as TG within the adipose tissue, FSP27 is inhibiting systemic FFA spillover.

Summary of human studies, including some recent findings, are listed in table 2. Data indicate that disruption of FSP27 in adipose tissue instigate the systemic pathological consequence of obesity and insulin resistance.

Table 2.

Summary human studies

Manuscripts	Manipulations	Results/outcome
Rubio et. al. 2009 EMBO Mol Med	Human: Homozygous mutation of FSP27	Lipodystrophy, insulin resistance, diabetes
Hall et. al. 2010 Obesity	Humans: Weight-loss surgery	The liver FSP27 positively correlated with degree of obesity, levels of FSP27 goes down with weight-loss
Moreno-Navarrete et. al. 2013 Int. J. Obes	Humans: Lean and obese	Low FSP27 in obesity vs. lean, low FSP27 in visceral than in subcutaneous fat, negatively correlated with blood parameters
Grahn et. al. 2014 J Biol Chem	Human adipocytes: Silencing/overexpression of FSP27	Silencing of FSP27 impaired insulin signaling, overexpression protected from FFA induced insulin resistance
Sharma et. al. 2018 Bioscientifica	Murine adipocyte: overexpression of FSP27	Protects from growth hormone induced insulin resistance
Sharma et. al. 2019 Am J Physiol Endocrinol Metab	Healthy humans: growth hormones/saline infusion	Growth hormone mediated reduction in FSP27, increased lipolysis and FFA, insulin resistance

Role of FSP27 in the liver

The liver plays an important role in whole body metabolism by maintaining normoglycemia, and regulating lipid storage and release [19]. Nonetheless lipid accumulation in the liver is one of the hallmarks of obesity, which is key in inducing insulin resistance and diabetes. Physiologically, during an early stage of fasting, the liver produces glucose mainly through glycogenolysis (carbohydrate breakdown) and during the later stage by gluconeogenesis (fat breakdown). The liver uses FFA that originates from adipose tissue as a source of energy to support the latter process. Although, named as “fat-specific”, studies show expression of FSP27 in the liver, although levels appeared to increase in disease conditions and correlates with lipid accumulation [53, 54*].

Murine models of hepatic steatosis demonstrated a significant increase in levels of FSP27 in the liver [55, 56]. Additionally, it was speculated that hepatic steatosis that was present in leptin deficient mice was promoted by increase in FSP27 in the liver, as deletion of FSP27 ameliorates hepatic steatosis in these mice. In recent study of diet-induced obese mice with steatohepatitis, silencing of FSP27 using antisense oligonucleotide alone, and in combination with fenofibrate, a PPARα agonist, reversed hepatic steatosis [57*]. The liver samples from mouse model of alcoholic steatohepatitis and humans with alcoholic hepatitis demonstrated substantial increase in amount of FSP27 in the tissue compared to healthy counterparts [54]. Additionally, level of expression was associated with degree of steatosis, which was positively correlated with severity of disease.

Others report that liver steatosis is a secondary effect of deletion/depletion of FSP27 in adipose tissue, showing adverse systemic consequences of not having FSP27 for proper storage of TG [39, 40]. In high fat-fed leptin and FSP27 deficient mice, despite exhibiting a lean phenotype, and protected from diet-induced obesity, adipose tissue specific deletion of FSP27 resulted in excess overflow of fat, which accumulated in hepatocytes causing systemic insulin resistance. Thus the livers of these mice presented with increased FSP27, increased triglyceride accumulation and severe steatosis. Although, these tissue specific results appeared to be counterintuitive, the responses were possibly physiological. For instance, the shutdown of storing capacity of fat, by deleting FSP27 in adipose tissue, pushed the pathological accumulation of lipid in the liver, which in turn triggered the increase in hepatic FSP27 expression. Additionally, deletion of FSP27 in the whole body vs. in adipose tissue specifically appears to have a differential effect in whole body physiology.

Although few, studies show that hepatic FSP27 levels are also regulated by nutritional status. During the initial stages of fasting (6–15 h fasting), FSP27 was induced at the maximum level, while during the latter phase of fasting (24 h fasting), its expression was decreased [58], with the initial increase in FSP27 possibly triggered by glucagon. Such a transient change in FSP27 level during fasting may reflect the ability of the liver to adapt in preparation of future energy needs, and store energy efficiently in preparation for further needs [59]. In contrast, in a state of obesity, delivery of energy to the liver is not out of needs but rather is a spillover; the liver is then forced to store excess TG, which upregulates FSP27. While there are very few reports of the liver specific expression of FSP27 in human obesity, but Hall and colleagues revealed that the liver of obese humans expressed high level of FSP27 and levels were decreases after significant weight loss after bariatric surgery [53]. On the other hand mutation of FSP27 in human fat has been shown to lead to lipodystrophy, ectopic fat accumulation in the liver, dyslipidemia and insulin-resistant/diabetes [41], suggesting tissue specificity on its expression and function.

Collectively, these studies indicate that, under physiological state the liver may express minimal amount of FSP27, and the level goes up as a part of normal physiology as in the case of initial starvation, and initiate a transient storage of TG in preparation for future long term (more than 24 hours) fasting. However, significant overexpression of FSP27 in the liver indicates a pathological state, as in the case of obesity, steatohepatitis, steatosis, and alcoholic hepatitis. The possibility of significant increase of FSP27 in the liver due to the secondary effect of adipose specific disruption of FSP27 also exists. The disruption of adipose specific FSP27 forces the liver to store fat in hepatocytes causing steatosis. Additionally, there are few reports suggesting that the isoforms of FSP27 that are present in adipose tissue and in the liver are distinct [60]. Adipocytes primarily express FSP27α isoform and FSP27β tends to be expressed more in the liver, which has an additional 10 amino acids at the N-terminal [61*]. Additionally, a specific protein regulates each isoform; peroxisome proliferator-activated receptor gamma (PPARγ) regulates adipose specific FSP27α and cyclic-AMP response element binding protein (CREB) regulates FSP27β [60]. While some reports of suggesting that PPARα also regulates FSP27β, suggesting a tissue specific pathophysiological regulation of FSP27 that needs to be explored further, especially in human tissue and at the cellular level.

Conclusion:

Adipose tissue has a central role in regulating systemic energy stores, and providing energy during metabolic needs. The notion is that adipocytes can protect other tissues, for example muscles and the liver, from the deleterious effects of excess circulating FFA, by esterifying FFA into TG and sequestering them within lipid droplets within the adipose tissue. Obesity contributes to a surplus of lipids in adipose tissue resulting in ectopic deposition of fat in non-adipose tissue and circulating FFA, which culminates in complications including insulin resistance and fatty the liver. In recent years, it has become increasingly clear that fat storage and mobilization is tightly regulated process. FSP27 is a LD protein, which promotes fat storage, and inhibits unwarranted breakdown of fat without an external stimuli. While opposing data exists regarding the contribution of FSP27 in the pathophysiology of obesity in mouse and cellular studies; human data indicate that FSP27 plays a physiological role in storing lipids in human fat, thus less of FSP27, as in visceral obesity, contributes to pathological conditions such as in insulin resistance. Additionally, data suggests that the presence of FSP27 in the liver is pathological in both mice and human and could be secondary to decline of FSP27 in adipose tissue. It is also likely that FSP27 may have tissue specific roles, by controlling different isoforms, as it has been shown that FSP27 is also present in muscles, and our unpublished data suggests that it is present in vascular beds. Clearly more data is required to shed some lights in the role FSP27 and how it plays in the pathophysiology of obesity and related complications especially in humans.