High Fructose Intake and Adipogenesis.

In modern societies, high fructose intake from sugar-sweetened beverages has contributed to obesity development. In the diet, sucrose and high fructose corn syrup are the main sources of fructose and can be metabolized in the intestine and transported into the systemic circulation. The liver can metabolize around 70% of fructose intake, while the remaining is metabolized by other tissues. Several tissues including adipose tissue express the main fructose transporter GLUT5. In vivo, chronic fructose intake promotes white adipose tissue accumulation through activating adipogenesis. In vitro experiments have also demonstrated that fructose alone induces adipogenesis by several mechanisms, including (1) triglycerides and very-low-density lipoprotein (VLDL) production by fructose metabolism, (2) the stimulation of glucocorticoid activation by increasing 11β-HSD1 activity, and (3) the promotion of reactive oxygen species (ROS) production through uric acid, NOX and XOR expression, mTORC1 signaling and Ang II induction. Moreover, it has been observed that fructose induces adipogenesis through increased ACE2 expression, which promotes high Ang-(1-7) levels, and through the inhibition of the thermogenic program by regulating Sirt1 and UCP1. Finally, microRNAs may also be involved in regulating adipogenesis in high fructose intake conditions. In this paper, we propose further directions for research in fructose participation in adipogenesis.

1. Introduction

Modern societies, especially those with a high incidence of obesity, are characterized by high fructose intake [1]. Most of this high fructose intake comes from the consumption of added sugar products [2,3]. Both sucrose (glucose-fructose disaccharide), or high fructose corn syrup (HFCS, free fructose and glucose at different concentrations) are primary sources of fructose in the diet.

There is a vast body of evidence of the harmful effects of excessive intake of fructose in the liver and other tissues [4,5,6]. However, the role of fructose in adipogenesis is less known. Therefore, the purpose of this review is to give some insight into the effects of fructose on white adipose tissue (WAT) cells, specifically on the adipogenic programs. In this regard, some studies have reported WAT expansion in response to high fructose intake [7,8]. In overweight and obese subjects, the consumption of 25% of daily energy requirement as fructose, provided as a sweetened beverage, for 10 weeks increased the visceral adiposity as compared to those who consumed an isocaloric glucose-sweetened beverage [8]. Supporting this notion, a 6-year prospective study in the Framingham’s Third Generation Cohort showed that those individuals who drink sugar-sweetened beverages (SSBs) on a daily basis had 29% more visceral WAT than the non-consumers [9]. Thus, high fructose consumption in the form of SSBs contributes to WAT expansion. Studies have long shown that fructose acute absorption can occur in white adipose tissue [10,11]. There is also evidence that fructose on its own induces the differentiation of adipocytes in vitro [12,13]. This has been shown in cell lines as well as in primary cultures [14,15].

The increased exposure of WAT and the cellular metabolic adaptations to chronic fructose intake target several mechanisms involved in the formation of new adipocyte cells. In the present review, we revisit how fructose induces glucocorticoids (GCs), activates the production of reactive oxygen species (ROS), and activates the renin–angiotensin system (RAS) and the induction of microRNAs (miRNAs) as an adipogenic mechanism. Finally, further directions in research are proposed.

2. Fructose Absorption to the Systemic Circulation

Humans, as well as other species, can directly metabolize and absorb fructose from different sources. Sucrose and other sugars are metabolized into glucose and fructose in the intestine [16]. Fructose is absorbed in the brush border from the lower part of the duodenum and jejunum by the glucose transporter type 5 (GLUT5, also named SLC2A5). Meanwhile, GLUT2 is located basolaterally, and translocates fructose into the circulation [17] (Figure 1).

Figure 1

Chronic fructose intake induces white adipose tissue (WAT) expansion. Chronic fructose intake increases fructose concentration in the peripheral venous blood, which may reach WAT. Fructose may promote adipogenesis through direct mechanisms such as uric acid and leptin resistance or indirect mechanisms such as glucocorticoids action, oxidative stress, the renin–angiotensin system (RAS) and the induction of microRNAs. WAT: White adipose tissue; GCs: Glucocorticoids; RAS: Renin–angiotensin system. Created with Biorender.com.

In mice, upon low fructose concentration intake, it may be metabolized in the splanchnic organs. However, fructose overload increases its circulatory concentrations [18]. In humans, Francey and collaborators used marked fructose and demonstrated that nearly 70% of fructose was directly metabolized by the liver [19], although other tissues such as WAT may metabolize the remaining amount. Also, chronic fructose intake can increase its absorption, promoting GLUT5 gene overexpression in the intestine [20]. Thus, after high exposure, more fructose enters portal circulation and may escape the liver and enter into the systemic circulation.

The concentrations of systemic fructose, unlike glucose, fluctuate greatly, and very few studies have been conducted on portal circulation. In a study of healthy adults that ingested 30.4 g of fructose, 4.4 g of fructose reached the systemic circulation [19]. Another study demonstrated that intake of an HFCS-sweetened beverage containing 39.2 g of fructose and 28.8 g of glucose increased fructose concentration from 5.4 µM to 363.4 µM in peripheral venous blood [21]. Moreover, rats administered with 2 g/kg of a solution rich in fructose reached concentrations of 20 µM arterial and 146 µM peripheral blood. The same study found the highest fructose levels in the portal vein [22]. However, studies have shown that some amount of the fructose ingested may be excreted in the urine. Indeed, urinary fructose has been used as a reliable marker of sucrose and fructose intake. Campbell and collaborators demonstrated that 20.2 mg/L of urinary fructose was excreted by children and adolescents that consumed 75.7 g of fructose [23]. In another study, the fructose consumptions estimated in women and men were 117 g/day and 162 g/day, respectively. Meanwhile, the urinary fructose levels were 18.1 mg/day in men and 17.5 mg/day in women [24].

In addition, animal models have shown that some fructose may be reabsorbed in the kidney. Rats with chronic intake of 20% fructose (w/v) in drinking water demonstrated that proximal tubules reabsorbed fructose at a rate of 20 pmol/mm/min compared to 12.8 pmol/mm/min in control rats. The same study found that fructose consumption increased GLUT2, GLUT4 and GLUT5 expression in the proximal tubule [25]. In summary, this evidence clearly suggests that fructose reaches other metabolically active tissues, activating the expression of GLUT5 in tissues such as WAT.

3. Effects of Excessive Fructose Intake on WAT Accumulation

Chronic fructose intake has been demonstrated to promote WAT accumulation (Figure 1). Several studies have also indicated that high fructose intake in beverages induces WAT accumulation in human and rodent models. In adolescents, the consumption of 350 mL/day of HFCS-sweetened beverage induced insulin resistance and visceral WAT accumulation [26]. Also, adult subjects that consumed 75 g of fructose beverage had increased liver fat content, which positively correlated with WAT expansion [27]. On the contrary, opposite trends for visceral WAT accumulation were found in obese children given an isocaloric fructose restriction diet [28]. Furthermore, rats that consumed 60% (w/v) fructose in drinking water for nine weeks developed visceral WAT accumulation and elevated triglyceride levels [29]. Kovačević and collaborators found similar results in rats drinking a 10% fructose (w/v) solution for nine weeks [30]. Moreover, there are genes that modify how the organism responds to high fructose intake. Therefore, there is an association of at least 30 obesity-related genetic variants identified in genome-wide association studies (GWAS) with effects exacerbated by high SSB intake [31,32,33]. In sum, these studies indicate that excessive fructose consumption is an important inductor of WAT accumulation.

There are well-recognized indirect effects of high fructose intake promoting systemic factors that contribute to WAT expansion (Figure 1), including fructose-increased caloric intake mediated via leptin resistance [7] and the antagonism of glucagon-like peptide-1 receptor (GLP-1R) action in the brain [34], as well as hyperuricemia [1] and visceral WAT inflammation [35]. However, there is also evidence that suggests a more direct effect of fructose on WAT. Most of the evidence for the direct effect of fructose on WAT comes from studies made in vitro [15,36,37]. For example, GLUT5 expression and function play a role in adipocyte differentiation [15]. It has also been shown that fructose exposure increases GLUT5 expression in WAT [12]. These in vitro results suggest that fructose alone can induce adipogenesis. In adipocyte precursor cells (APCs) as well as in preadipocytes, fructose induces adipogenic programs. In rats, 10% fructose in drinking water increased the number of APCs and the percentage of differentiated adipocytes [14]. A few other studies have also shown that physiological levels of fructose (50–550 μM) can induce 3T3-L1 to differentiate [15]. In summary, fructose in the circulation can be responsible for the induction of WAT hyperplasia as well as adipocyte hypertrophy (Figure 1). Some of the mechanisms implicated are discussed further in this paper.

4. Fructose, Hypertriglyceridemia, Very-Low-Density Lipoproteins (VLDLs) and Adipogenesis

High fructose intake has been associated with hepatic de novo lipogenesis (DNL), hypertriglyceridemia and increased visceral fat mass. In overweight and obese humans, in which 25% of their caloric requirements were given as glucose- or fructose-sweetened beverages for 10 weeks, only subjects receiving fructose showed a marked increment of plasma lipids and lipoprotein concentrations as well as postprandial triglycerides (TG). It was shown that both increased DNL induced by fructose and reduced clearance contributed to the rise in postprandial TG [8]. In addition, only fructose intake was associated with a significant increment of visceral WAT, despite the fact that both glucose and fructose groups gained comparable body weight at the end of the follow-up [8]. Moreover, in healthy adults, the intake of meals high in fructose also increased postprandial DNL without changes in fatty acid oxidation; such effects were better observed in women [38]. Fructose can induce DNL since it stimulates the carbohydrate-responsive element-binding protein (ChREBP) [39] via uric acid production and protein phosphatase 2A [40]. Fructose also activates the transcription factor sterol regulatory element-binding protein 1c (SREBP-1c), independent of insulin secretion via PPAR- coactivator 1 [36]. These transcriptor factors play an important role in the synthesis of fatty acid and triglycerides [41]. Thus, high fructose intake alters lipid metabolism, which increases VLDL lipoproteins and TG levels in circulation. In addition, it has been suggested that the hydrolysis of TG-rich lipoproteins is a major source of adipogenic free fatty acid, which may promote WAT expansion in cases of obesity [42].

High fructose intake promotes an increase in postprandial triglycerides levels with the VLDL lipoprotein fraction in plasma from rats [43] and humans [38]. VLDL promotes adipogenesis in an ApoE-dependent manner in 3T3-L1 cells [44]. In another study, Zhang and collaborators found similar results, using lipoproteins from hypertriglyceridemic individuals in 3T3-L1 cells by increasing aP2 and PPARϒ mRNA expression [45] (Figure 2a). Interestingly, in the same study, 3T3-L1 cells stimulated with lipoproteins and insulin showed higher induced adipogenic program compared to lipoprotein or insulin treatment alone [45]. Therefore, the increased VLDL levels by fructose metabolism in the liver may be an indirect effect of the induction of adipogenic program in WAT by high fructose intake (Figure 2a).

Figure 2

Fructose induces adipogenesis through very-low-density lipoprotein (VLDL) production and glucocorticoid (GC) signalling. (A) High fructose intake increases triglycerides and VLDL production, which induce adipogenesis by increasing PPARγ and aP2 expression. (B) Fructose metabolism in adipocytes promotes glucocorticoid activation by inducting 11β-HSD1. The effects of GCs in adipocyte precursors cells induce the expression of adipogenic genes through a few mechanisms. VLDL: Very-low-density lipoprotein; GCs: Glucocorticoids; GR: Glucocorticoid receptor; NADPH: Nicotinamide adenosine dinucleotide phosphate; 11β-HSD1: 11-beta hydroxysteroid dehydrogenase type 1; CREB: cAMP response element-binding protein; cAMP: Cyclic adenosine monophosphate; PDE3BB: Cyclic-nucleotide phosphodiesterase 3B; HPI: Hexose-phosphate isomerase; H6PD: Hexose-6-phosphate dehydrogenase. Created with Biorender.com.

5. Fructose Induces Adipogenesis by GC Action

In addition to their anti-inflammatory effects, GCs also play a role in the induction of WAT expansion. For example, in Cushing’s syndrome, hypercortisolemia increases intraabdominal and subcutaneous fat mass [46,47]. Thus, chronic GC treatments induce weight gain and WAT expansion [48]. It has been long recognized that obesity and obesogenic factors increase endogenous GCs. Fructose intake increases GCs, such as cortisol in human serum and WAT corticosterone levels in rats and mice [49]. It has been suggested that fructose intake may act as a modifier factor on inflammatory mechanisms in WAT.

In WAT cells, fructose has direct actions (Figure 2b)—it is first transported by GLUT5 and then metabolized to fructose-6-phosphate (F6P) by hexokinase. Subsequently, F6P is transported into the ER lumen, where there is a luminal hexose-phosphate isomerase (HPI) that converts F6P into glucose-6-phosphate, which is a substrate of the hexose-6-phosphate dehydrogenase (H6PDH) that generates the nicotinamide adenosine dinucleotide phosphate (NADPH) necessary for 11-beta hydroxysteroid dehydrogenase type 1 (11β-HSD1) oxidoreductase activity [50]. 11β-HSD1 is an NADPH-dependent enzyme that works as a tissue-specific amplifier of glucocorticoid action, which contributes significantly to the regulation of intracellular glucocorticoids levels through its activation from inactive 11-keto forms [51].

Fructose consumption increases active GC levels through 11β-HSD1 activity in rat WAT [52]. In accordance with this, an increase of corticosterone in WAT and plasma accompanied total WAT gain in rats fed with 10% fructose (w/v) in drinking water [53]. The authors also reported that this finding was due to the high activity/expression of H6PDH and 11β-HSD1 in epididymal WAT [53]. In vitro studies of fructose-induced adipogenesis in 3T3-L1 cells showed an increase in 11β-HSD1 expression, which promoted high endogenous GC levels [12]. Interestingly, glucose exposure did not induce 11β-HSD1 expression. These results support the idea that fructose probably causes WAT expansion by increasing 11β-HSD1 activity that promotes adipogenesis (Figure 2b).

As mentioned above, fructose increases active GCs in adipocytes. The glucocorticoid receptor (GR) mediates GC actions in these cells. GR is a nuclear receptor that recognizes GCs that, in their active form, are translocated to the nucleus [54]. Dexamethasone (DEX) is a synthetic GC that is used in the differentiation of 3T3-L1 preadipocytes to mature adipocytes. In this regard, DEX-bound GR accelerates adipogenesis, increasing the expression of adipogenic transcription factors such as C/EBPβ, C/EBPδ, Klf5, Klf9, C/EBPα and PPARγ [55]. Also, DEX downregulates cyclic-nucleotide phosphodiesterase 3B, which in turn elevates cellular cAMP and promotes the activating protein kinase A (PKA) [56]. In preadipocytes, elevated cAMP levels have been suggested to transcriptionally activate C/EBPβ by cAMP response element-binding protein (CREBP) [56]. In fructose-fed rats, GR activation is promoted, and this process may enhance adipogenesis by inducing adipogenic genes [57,58]. In this scenario, it could be speculated that GCs activate GR in preadipocytes upon fructose consumption, because fructose increases active GC levels in WAT. This process would generate the increase in the concentration of cAMP, which in turn activates CREBP. The induction of adipogenic genes by chronically increased levels of GCs by persistent fructose exposure could in part explain the enhanced adipogenesis.

Another molecule that plays a role in adipogenesis which is regulated by GCs is Lipin-1 (Figure 2B). This molecule participates in the accumulation of triglycerides in adipocytes. Lipin-1 is a phosphatidic acid phosphatase-1 that catalyzes the conversion of phosphatidate to diacylglycerol (the immediate precursor of triacylglycerol) [59]. In addition, DEX also induces an increase of Lipin-1 expression during the differentiation of 3T3-L1 preadipocytes [60]. In rats fed 60% fructose in drinking water, Lipin-1 expression was induced in the microsomal fraction from visceral WAT [29]. Also, Lipin-1 has been considered as an upstream factor that controls adipogenesis-related genes and regulates a positive feedback loop between PPARγ and C/EBPα in 3T3-L1 preadipocytes [61,62,63,64]. Conversely, in Lipin-1-deficient mice, a reduced WAT mass with an absence of mature adipocytes is found [64]. Therefore, it is possible that fructose consumption-induced GCs could promote adipocyte differentiation through the induction of Lipin-1 expression (Figure 2b). However, studies that explore the participation of Lipin-1 in adipogenesis induced by fructose are still lacking.

6. Reactive Oxygen Species and Fructose

Reactive oxygen species (ROS) are oxygen-derived small molecules, generated during physiological and pathological processes [65]. NADPH oxidases (NOX) and xanthine oxidase (XOR) as well as other enzymes participate in ROS production [66]. In addition, several studies indicate that ROS production induces signaling pathways regulating survival, apoptosis, proliferation, and differentiation factors [67,68]. In 3T3-L1 cells adipogenesis, ROS levels are increased. This extracellular oxidizing environmental status accelerates adipogenesis by increasing lipid accumulation and the expression of the adipogenic gene program [69]. Tormos and collaborators also found that lipogenesis and adipogenesis mitochondrial metabolism and ROS generation were dependent on mTORC1 signaling in human mesenchymal stem cells (MSC) [70]. Moreover, ROS inhibition leads to a decrease in the differentiation to mature adipocytes in 3T3-L1, and equine adipose-derived stromal cells [71] and antioxidant treatment showed similar results in 3T3-L1 cells [72]. These results suggest that an oxidizing environment produced by mitochondrial metabolism and ROS play a crucial role in promoting adipocyte differentiation.

Interestingly, fructose exposure increases ROS levels in several tissues such as WAT [73,74,75,76,77] (Figure 3). The oxidizing environment in WAT is a mechanism that sustains the obesity phenotype [78,79,80]. Moreover, fructose can induce more ROS production than glucose [81]. In human MSCs, fructose increases isoprostane and heme contents. Conversely, cobalt protoporphyrin, a heme oxygenase-1 (HO-1) inducer, decreases isoprostane and heme levels, thus also decreasing adipogenesis. In addition, HO-1 decreases the expression of β-catenin and Wnt10b (Wnt Family Member 10B), which are inhibitors of adipogenesis [82]. Therefore, some evidence indicates that ROS production may participate in the induction of adipogenesis by fructose consumption (Figure 3).

Figure 3

Fructose-induced reactive oxygen species (ROS) production promotes adipogenesis. Fructose metabolism induces adipogenesis through increasing ROS production. ROS may be induced by uric acid, NADPH oxidase activity, mTORC1 signaling and AT1 receptor signaling. ROS: Reactive oxygen species; NADPH: Nicotinamide adenosine dinucleotide phosphate; XOR: Xanthine oxidase; FAS: Fatty acid synthase; C/EBPα: CCAAT-enhancer-binding protein α; PPARϒ: Peroxisome proliferator-activated receptor ϒ. Created with Biorender.com.

7. Uric Acid Resulting from Fructose Metabolism as a Mediator of Adipogenesis

Hyperuricemia (HU) is a common alteration that rises to more than 20% in some populations [83]. Obesity is closely related to hyperuricemia; in particular, visceral fat deposits are positively associated with increased uric acid production [84]. Therefore, the visceral adiposity index (which composes waist circumference, body mass index, triglycerides and high-density lipoprotein cholesterol) also correlates positively with HU in obese and even in metabolically healthy subjects [85,86].

Uric acid (UA) is a byproduct of fructose when it is phosphorylated by ketohexokinase (KHK). As KHK does not have a negative feedback mechanism, all the fructose presented is phosphorylated until ATP exhaustion [87] (Figure 3). As ADP and inorganic phosphorous accumulates inside the cell, the purine degradation pathway is activated, thus significantly increasing the intracellular uric acid concentrations [87]. This process occurs in hepatocytes, enterocytes, muscle cells and proximal tubule renal cells. In adipocytes, there is some indirect evidence of the activity of this pathway [88]. For example, in MSC-derived adipocytes, incubation with 500 mM fructose significantly increased xanthine oxidase expression as well as uric acid concentrations. Furthermore, adipogenesis was increased by 50%. Moreover, incubation with UA (5 mg/dL) also increased adipogenesis by 50%, augmented the number of large lipid droplets, and decreased the number of small droplets compared to the control. In addition, UA also increased the mRNA of C/EBPα and PPARγ, known mediators of adipocyte hyperplasia [88]. XOR is an enzyme that catalyzes the catabolism of purines such as xanthine and hypoxanthine to uric acid [89]. XOR expression in WAT tissue is abundant, and in obese mice, the expression and activity of this enzyme are higher [90]. Cultured adipocytes (primary mature adipocytes and 3T3-L1 cells) also produced and secreted uric acid into the culture medium. Those effects were inhibited by blocking XOR with febuxostat [90]. Thus, there is a possible link between fructose and the induction of adipogenesis via uric acid (Figure 3).

Two mechanisms have been described so far to explain how UA may induce adipogenesis. (1) UA acts on adipocytes to increase ROS production through NADPH oxidase activation [88,91]. Nox4 is a major isoform of NADPH oxidase in adipocytes, which is activated upon UA exposure. The increase in intracellular oxidative stress induces the production and secretion of monocyte chemotactic protein 1 (MCP-1), a mediator of the proinflammatory state in adipocytes in obesity [91,92,93]. However, such an effect was blocked by Nox4. In obese mice, treatment with allopurinol reduced the production of MCP-1 in WAT, thus reducing macrophage infiltration. Kanda and collaborators also found that ROS were produced by Nox4-mediated adipocyte differentiation in 10T1/2 cells [94]. 2) UA upregulates the RAS in WAT. How the RAS is associated with adipogenesis is discussed in the next section.

8. RAS, Fructose and Adipogenesis

The RAS in visceral fat may play a role in WAT expansion by excessive fructose intake. Two studies showed that a fructose-rich diet increases Angiotensin II (Ang II) levels in plasma and WAT [95,96]. The RAS is also associated with increased visceral WAT [97]. Moreover, RAS inhibitors such as Temocapril, Olmesartan, Irbesartan and Aliskiren led to reduced Ang II levels in WAT as well as reduced visceral fat pad weight and adipocyte size in fructose-fed rats [98,99]. These studies suggest that the RAS may play a role in regulating visceral adiposity in fructose consumption.

On the other hand, the RAS may also contribute to ROS production in WAT. Ang II plays a role in ROS production through activating AT1 receptor, which promotes increased NOX expression [97,100] (Figure 3). Moreover, AT1 receptor is increased in WAT in fructose-fed rats [101]. As explained above, ROS production induces adipogenesis. Thus, the RAS may be in part responsible for ROS production and the induction of adipogenesis in WAT. However, it is necessary to clarify whether the RAS contributes to ROS production to induce adipogenesis in high fructose intake scenarios.

Ang II undergoes hydrolysis by the angiotensin-converting enzyme (ACE) [102]; in particular, ACE2 is the crucial enzyme that generates Ang-(1–7) by the hydrolysis of Ang II [103]. Ang-(1–7) induces the oppose response compared to that of Ang II through its specific receptor (Mas) [104]. Both Ang-(1–7) and ACE2 were upregulated in WAT in sucrose-fed rats [105]. Ang-(1–7) and Mas receptor were also upregulated in 3T3-L1 cells [106]. However, Mas-KO 3T3-L1 cells showed decreased adipogenic markers, such as PPARγ, FAS, ACC, and aP2 [106]. As explained, Ang-(1–7) and Mas receptor may regulate adipocyte differentiation (Figure 4A). However, it is still necessary to study their participation in the adipogenesis process in fructose intake in humans.

Figure 4

Fructose-induced adipogenesis through Ang-(1–7) and inhibition of thermogenesis. (A) Fructose metabolism promotes the conversion from Ang-II to Ang-(1–7) by increasing ACE2 activity. Mas receptor signaling induces the expression of adipogenic genes. (B) Sirt1 regulates the thermogenic program by inhibiting PPARγ and activating Wnt/β-catenin signalling. High fructose intake inhibits the thermogenic program by downregulating Sirt1 and UCP1 in WAT. ACE2: Angiotensin-converting enzyme 2; FAS: Fatty acid synthase; PPARϒ: Peroxisome proliferator-activated receptor ϒ; ACC: Acetyl-CoA carboxylase; aP2: Adipocyte protein 2; Sirt1: Sirtuin 1; UCP1: Uncoupling protein 1; Wnt: Wingless e Int. Created with Biorender.com.

9. Fructose, Sirtuin1, UCP1 and Thermogenesis

Sirtuin1 (Sirt1) is a member of the sirtuin family of NAD+-dependent protein deacetylases. Sirt1 participates as a cellular energy sensor and a mediator of caloric restriction. Also, Sirt1 regulates glucose and fat metabolism and plays a role in the adipogenic program [107,108,109,110]. Clinical studies have shown that mRNA expression of Sirt1 decreases in WAT in overweight and obese subjects [111,112]. In addition, in fructose-fed mice, Sirt1 protein levels in WAT are decreased [113]. However, Pektas and collaborators did not find changes in Sirt1 mRNA expression in WAT [77]. In mature adipocytes, Sirt1 promotes lipolysis and inhibits the adipogenic program in preadipocytes by repressing PPARγ activity [114]. Also, Sirt1 inhibits adipogenesis by activating Wnt/β-catenin signaling [110,115]. Moreover, delphinidin-3-O-β-glucoside, an anthocyanin with anti-adipogenic activity, promotes an increase in protein Sirt1 levels and downregulates the expression of adipogenic and lipogenic markers in 3T3-L1 preadipocytes [116]. Therefore, high fructose intake may induce adipogenesis by downregulating Sirt1 expression (Figure 4B). However, more studies are needed to elucidate whether fructose induces differentiation to adipocytes through direct regulation by Sirt1.

On the other hand, Sirt1 also plays a role in the induction of a thermogenic gene program in adipocytes. Sirt1 induces a brown adipocyte-like phenotype in white adipocytes by deacetylating PPARγ [117]. For example, in marrow adipose tissue (AT) from 129/Sv Sirt1 haploin-sufficient (Sirt1Δ/+) mice, decreased expression of thermogenic genes was observed. In the same study, the inhibition of Sirt1 by EX527 downregulated UCP1 in C3HT101/2 cells [118]. In fructose-fed mice, uncoupling protein 1 (UCP1) mRNA expression was decreased in subcutaneous WAT [119,120]. On the other hand, it has been demonstrated that fructose-fed rats experienced weight brown AT (BAT) gain [121,122], enlarging intracellular lipid droplets [123]. This lipid accumulation could be in part due to a decrease in UCP1 expression because, in mice fed a high-fat and high-fructose diet, UCP1 mRNA expression in BAT was decreased [124]. These results indicate that high fructose intake may downregulate the thermogenic program in WAT (Figure 4B). Therefore, high fructose consumption may inhibit browning fate by promoting whitening fate in AT.

10. MicroRNAs Implication in Fructose-Induced Adipogenesis

MicroRNAs (miRNAs) are involved in regulating several biological processes such as inflammation, metabolism, proliferation and differentiation, as well as adipogenesis [125]. miRNAs are short noncoding RNAs of ~22 nucleotides that bind to the 3′-UTR or 5′-UTR of target mRNA, leading to mRNA degradation or translational repression [125]. In humans, miRNAs regulate several genes involved in adipogenesis and obesity [126]. Also, miRNAs control the adipogenic gene expression program [127]. Few studies have established that the expression and function of several miRNAs may be involved in fructose overconsumption and lipogenic programs [128]. Moreover, to our knowledge, there are no studies investigating the role of miRNAs as well as other non-coding regulatory RNAs in fructose-induced adipogenesis. Therefore, in this section we provide a rationale for the implication of some miRNAs being related to adipogenesis caused by high fructose intake.

Some of the miRNAs that can be implicated in the induction of adipogenesis by high fructose intake are miR-206, miR-33, miR-122, miR-378a, miR-21, and miR-223. miR-206 negatively regulates adipogenesis. This miRNA acts on the c-Met/PI3K/Akt pathway in 3T3-L1 preadipocytes [129]. Fructose-fed rats showed decreased miR-206 expression in the renal cortex and podocytes [130]. On the other hand, mice deficient in miR-33 have greater WAT and adipocyte precursor cell numbers [131]. High fructose intake in these mice downregulates miR-33 expression in livers [128]. miR-122 is another candidate miRNA because it is downregulated in the liver in fructose-fed rats [132]. This miRNA suppresses PPARγ expression in 3T3-L1 preadipocytes [133]. Induction of miR-378a can also be involved. This miRNA promotes adipogenesis because it targets mitogen-activated protein kinase 1 [134]. Fructose also upregulates miR-378a in mouse livers [128]. Nevertheless, it is necessary to test if these miRNAs work similarly in WAT.

Considering that extracellular miRNA actions have been reported in several conditions, miR-21 and miR-223 were found to be upregulated in plasma microvesicles [135]. miR-223 promotes the differentiation of MSCs to adipocytes by regulating fibroblast growth factor receptor 2 (FGFR2) expression [136]. miR-21 also regulates adipogenesis in human WAT-derived MSCs. It modulates transforming growth factor beta (TGF-β) signaling [137] and activator protein 1 (AP-1) expression [138]. As explained above, fructose can modify miRNA expression; however, there are no studies demonstrating how fructose could modify either expression or the activity of miRNAs in adipogenesis. Studies explaining the role of circulating miRNAs in the condition of adipogenesis modulated by high fructose intake are needed.

Studies that estimate the amount of fructose or its immediate metabolites that may enter into different types of WAT are needed.

The acute metabolic and endocrine effects of high fructose intake on WAT from healthy and metabolically compromised individuals should be assessed.

The role of fructose in the adipogenesis of newly discovered adipocyte subsets should be interesting to investigate.

The reversibility of many high fructose intake-induced phenotypes should be studied.

Studies with modest fructose intake, especially from sources other than processed food, such as fruits, are lacking.

12. Conclusions

In conclusion, high fructose intake has an important role in the current obesity epidemic. The fructose contained in beverages is likely the culprit leading to its excessive intake. The capacity of AT to cope with this nutrient overload is exceeded. From the studies using fructose in water or those that directly employed SSBs, it has been demonstrated that the high intake of this sugar may disturb several functions in adipocytes. Although there are many statements on how much total sugar intake is adequate (around 10% of the total caloric intake or less, according to the World Health Organization), there is no clarity in terms of free fructose intake.