1. Introduction
Vanadium-based compounds are known to exert insulin-enhancing activity and have demonstrated their antidiabetic activity in vitro and in vivo [
1,
2,
3,
4]. Vanadium complexes demonstrate superior biological effects in contrast to salts. Specifically, bis(ethylmaltolato)oxidovanadium(IV) (BEOV) exhibited promising outcomes during Phase 1 and Phase 2 clinical trials; however, the compound was discontinued due to renal toxicity concerns [
5]. There is therefore a need to reduce toxicity and determine some of the beneficial effects of vanadium compounds.
Furthermore, vanadium-based compounds have biological effects on various aspects of carbohydrate and lipid metabolism, including glucose uptake, glycolysis and glycolytic enzymes, glucose oxidation, glucose production, glycogen synthesis, and lipogenesis, and they show significant activity in models of insulin resistance [
6]. The potential for V compounds to overcome insulin resistance could be considerable and is investigated in further detail in this work.
The animal model employed for diabetes
mellitus (DM) to investigate the vanadium compound effects is the STZ type I model—caused by decreasing and absent insulin secretion by pancreatic beta cells [
5,
6]. Nonetheless, DM2 constitutes approximately 95% of all cases and is distinguished by enduring insulin resistance (IR), leading to disruptions in protein, lipid, and carbohydrate metabolism [
7]. Animal models with high-fat diets and glucocorticoid administration have been used to test for insulin resistance induced in rodent subjects [
8].
Glucocorticoids (GCs) are steroid hormones produced and released by the adrenal cortex under hypothalamic–pituitary–adrenal axis regulation. They interact with the glucocorticoid receptor (GR), modulating several physiological functions [
8]. However, excessive levels of GCs can lead to hypertensive conditions, increased body fat mass, osteoporosis, depression, infection risk, muscle wasting, and hyperglycemia [
9].
The effects of GCs on glucose and lipid metabolism involve several mechanisms, such as insulin secretion inhibition from pancreatic beta cells, a reduction in glucose utilization, lipolysis, skeletal muscle proteolysis, and hepatic glucose production [
10]. In addition, the clinical use of GCs is typically associated with glucose, protein, and lipid disturbances that are reproducible in both rodents and humans [
11,
12,
13,
14].
Dexamethasone (DEX) is a non-selective synthetic glucocorticoid widely prescribed because of its anti-inflammatory, anti-allergic, and immunosuppressive properties. It is approximately 50 to 100-fold more potent than cortisol. However, when administered in excess, DEX induces side effects such as muscle catabolism, hyperphagia, increased adiposity, dyslipidemia and hypertriacylglycerolemia, and IR in vitro and in vivo [
10,
15,
16,
17,
18,
19,
20]. In addition, the long-term administration of DEX leads to the generation of free radicals such as superoxide, hydrogen peroxide, and hydroxyl radicals, which contribute to oxidative stress and deteriorate both insulin action and secretion, accelerating the consequent appearance of type 2 DM [
21]. The adverse metabolic effects of DEX treatment may be reversible upon rapid discontinuation [
22]. However, patients receiving GCs are often subjected to prolonged therapy, which may result in irreversible metabolic damage, such as IR, oxidative stress, and type 2 diabetes [
23]. Considering the diabetogenic effect of GC therapy, it is important to propose new therapies to prevent undesirable side effects of the acute and long-term use of synthetic GCs such as dexamethasone.
In recent years, GC-induced disorders have gained prominence due to the need for emergency clinical use during the COVID-19 pandemic. Glucocorticoids, especially DEX, were widely used in patients with severe acute respiratory syndrome (SARS) to interrupt the inflammatory cascades caused by the virus [
24,
25]. For these cases, the use of some oral medications showed a low efficiency in post-COVID glycemic control. Despite this, new alternatives for glycemic control in these patients have been studied using an insulin-resistant diabetic animal model induced via dexamethasone [
26]. In this context, oxidovanadium complexes have emerged as an efficient therapeutic proposal capable of controlling metabolic disorders associated with DM [
1,
2,
3,
4].
Recently, we evaluated the therapeutic potential of the oxidovanadium(IV) complex [V
IVO(octd)], containing a 3,6-dithio-1,8-octanediol ligand (octd), which was evaluated as an oral antidiabetic agent on STZ-induced diabetic Wistar rats [
27]. The proposed oxidovanadium complex has not been subjected to biological studies probing the effects associated with IR factors induced by acute exposure to dexamethasone. In this study, the biological effects of the [V
IVO(octd)] complex on dexamethasone-induced insulin resistance in female mice were evaluated.
3. Discussion
Recently, Lima and collaborators (2023) reported antidiabetic effects in STZ-induced diabetic rats of the sulfur–oxidovanadium(IV) complex—[V
IVO(octd)] [
27]. In the current study, in addition to demonstrating the antioxidant properties of the [V
IVO(octd)] complex was possible to confirm the fasting blood glucose levels reduction, as well as attenuate the metabolic disturbances associated with insulin resistance (IR), preserving the lean mass, and reduced fat depots in female mice. [V
IVO(octd)] complex also reduced the serum lipid profile and the hepatic lipid accumulation in DEXA-treated mice.
Since the 1970s, several oxidovanadium complexes have been discussed as insulin-enhancer agents with beneficial effects on glucose and lipid metabolism, in vitro and in vivo [
1,
2,
3,
4,
5,
6,
28,
29,
30,
31]. Among the various animal models of diabetes, the most widely used for vanadium compounds is streptozotocin (STZ) induced [
32,
33,
34,
35,
36]. However, the STZ-induced DM model more closely resembles type I diabetes, and type II diabetes corresponds to around 95% of the cases. In addition, type 2 DM is characterized by peripherical IR, mainly in the liver, white adipose, and skeletal muscle tissues. There have been a few papers related to the investigation of vanadium compounds using IR models, high-fat diets, and synthetic glucocorticoids, which are the conditions that most represent type 2 diabetic patients [
19,
20].
In this way, the present work verified the influence of the administration of an oxidovanadium complex, [V
IVO(octd)], on an animal model that consisted of mice with dexamethasone-induced IR, and it evaluated the complex’s potential therapeutical use as a novel antihyperglycemic agent. The dexamethasone-treated mice probably exhibited a reduction in corticosterone values due to the HPA axis inhibition, as suggested by adrenal gland weight (
Table 2). These results confirm that excess dexamethasone can induce energetic metabolism damage such as hyperglycemia, hyper-triacylglyceridemia, and IR in rodents [
10,
18]. In the current research, we used metabolic parameterization methods such as the TyG index, a validated indicator to demonstrate IR that has been compared with the gold standard technique of the “euglycemic–hyperinsulinemic” clamp test. It is well known in the literature that the administration of glucocorticoids induces a decrease in peripheral glucose disposal and increased liver gluconeogenesis, which is associated with damage in the insulin anabolic action [
9].
Interestingly, the [V
IVO(octd)] use reduced fasting glucose and TG levels, as well as calculated TyG index (
Figure 2), all for the animal model assayed here. Nonetheless, Barbera et al. (2001) [
20] showed that a treatment strategy based on the VOSO
4 did not block or attenuate DEXA-induced IR in rats. On the other hand, therapeutical strategies based on vanadium complexes are promising; for example, the use of the bis(alpha-furancarboxylato)oxidovanadium(IV) (BFOV) enhanced the action of insulin and completely prevented the development of insulin resistance induced by dexamethasone, modulating the gene expression of key proteins of the insulin cascade such as insulin receptor substrate 1 (IRS-1) and glucose transporter 4 (GLUT4) in 3T3-L1 adipocytes [
19]. Although we have not yet clarified the molecular mechanism involved in the anti-IR of [V
IVO(octd)] complex, it is reasonable to hypothesize that this vanadium-based compound activates the insulin pathway in metabolically important tissues such as the liver and skeletal muscle, which counteracts the anti-insulin effects of dexamethasone.
Based on previous speciation studies, it was possible to predict the real V species responsible for the biological effects described in this study [
28]. [V
(IV)O(octd)] is the major species at pH = 4; then, it undergoes partial speciation. On the other hand, at pH = 7 in an aqueous solution, the initial compound is less stable because hydrolysis/decomposition occurs from 0 h, originating the oxidation product of the [V
VO
2(octd)
–] complex, ligands and metals free, that are oxidized to the V
(V) species, V
1, V
2, V
4, and V
5, at pH = 7 and 24 h. Species of the biological system responsible for blood glucose and insulin-resistance reduction depend on time. Therefore, it is reasonable to hypothesize that [V
IVO(octd)] and its oxidation/decomposition products affect the antihyperglycemic and could activate the insulin pathway in metabolically important tissues such as the liver, adipose tissue, and skeletal muscle, which counteracts the anti-insulin action of DEXA.
Dexamethasone administration generates decreases in both body weight gain and food intake, as illustrated in
Table 1. The observed decline in energy intake does not appear to be the primary determinant for reduced body weight gain in rats subjected to exogenous glucocorticoid treatment [
9,
10,
37]. It is well described in the literature that GC treatment increases plasma insulin and leptin levels, hormones involved in the anorexigenic response in the hypothalamus [
37,
38]. Nevertheless, excess glucocorticoids produce a negative nitrogen balance, which may contribute to body weight loss [
39,
40]. Aru et al. (2018) showed similar results with a DEXA-induced reduction in the weight of fast-twitch muscle without any alteration in the weight of slow-twitch muscle [
41]. The [V
IVO(octd)] administration counteracted the catabolic effect in skeletal muscle caused by dexamethasone in mice. Also, the proposed vanadium-based compound spared skeletal muscle protein, which may explain, at least in part, the body weight gain in mice treated with [V
IVO(octd)] complex. This effect resulted in an adsorption improvement of insulin, that was induced by the proposed oxidovanadium complex.
It is well established that glucocorticoids promote adiposity, triglyceride synthesis, and adipose tissue hypertrophy [
18,
22,
26]. These aspects were reinforced in this work, whose results showed that retroperitoneal adipose tissue (RETRO) weight increased after 14 days of exposure to DEXA (
Table 2). The effect of glucocorticoids on adipose tissue is not entirely understood and is still controversial. For example, Aru et al. (2018) reported reduced fat body mass in 22-month-old rats overexposed to dexamethasone for 10 days [
41]. On the other hand, other research has shown that synthetic glucocorticoids induce adipose tissue gain [
18,
22,
42]. Ferreira et al. (2017) showed that despite impairing insulin-stimulated glucose uptake in both RETRO and epidydimal adipose tissue, dexamethasone increased adipogenesis, glyceroneogenesis, and phosphoenolpyruvate carboxykinase (PEPCK-C) activity in RETRO tissue, accompanied by a reduction in AKT phosphorylation, which suggests IR [
18]. Concerning the [V
IVO(octd)] complex, which blocked the adipogenic effect of DEXA in RETRO adipose tissue, the current proposed therapeutic prototype did not present the usual undesirable effects of antidiabetic pharmacotherapy such as adipose tissue gain and lipodystrophy.
The vanadium complex may contribute to differentially reestablishing insulin sensitivity among body tissues to reduce blood glucose levels with concomitant adipogenesis reduction. GCs may also increase lipolysis ex vivo and in vivo, which may be detected by increased glycerol levels, associated with IR [
18]. Glycerol is a lipolysis-derived substrate that may be used for glucose hepatic production [
43]. Accordingly, [V
IVO(octd)] reduced the availability of this gluconeogenesis substrate (
Table 3), which could justify the reduction in hyperglycemia in the DEXA-treated mice. In other words, this sulfur–oxidovanadium compound may counteract the hyperglycemic side effect of dexamethasone treatment because it reduces the gluconeogenic substrates, such as amino acids and glycerol, in the liver.
It has been demonstrated that glucocorticoids stimulate the storage of glucose as glycogen in the liver, including in fasting conditions, and it has been associated with GC-stimulated glycogen synthase (GS) and/or the inhibition of glycogen phosphorylase (GP) activity [
44]. Fasting-induced depletion in liver glycogen levels in adrenalectomized rats is blocked by dexamethasone administration [
45]. Furthermore, Praestholm et al. (2021) demonstrated that activating intracellular glucocorticoid receptors (GRs) is essential to hepatic glucose uptake and liver glycogen storage [
46]. In agreement with this, GRs directly regulate the expression of glucose kinase, a key enzyme to glucose utilization and storage in the liver. In addition, insulin and glucocorticoids have a synergic effect on GS activity and glycogen content in different physiological conditions [
46]. Previous research has demonstrated that DEXA increases hepatic glycogen [
22]. Our results showed that [V
IVO(octd)] and metformin, an insulin-sensitizing drug used as a control, reduced the glycogen storage induced by DEXA in fasting mice. Considering that DEXA induces hyperinsulinemia in mice [
22,
42], the present vanadium-based compound attenuated the effect of GCs on glycogen storage. It is possible that [V
IVO(octd)] improves insulin sensitivity in the liver and improves the glycogen turnover in DEXA-treated mice.
It is noteworthy that while GCs induce IR in the gluconeogenic pathway by upregulating the gene expression of key enzymes, such as phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6-P), the lipogenic effect of insulin remains preserved [
26]. Compensatory hyperinsulinemia occurs in response to GC-induced high blood glucose levels, which stimulate de novo lipogenesis (DNL) and subsequent lipid accumulation in the liver. In the current research, impairments in serum lipid profiles were associated with the atherogenic index increase in GC-treated mice (
Table 3). Furthermore, the GC treatment (DEXA) caused the imbalance of hepatic lipid homeostasis between the acquisition and removal of TG/fatty acid (
Table 4). The liver lipids machinery (synthesis and export) for the other tissues is complex and involves several key enzymes for lipid uptake, DNL, fatty acid β-oxidation, and liver lipid export [
26].
In this regard, GC action is not only affected by circulating serum levels or densities of GRs but also by tissue-specific GC-activating enzyme 11 β-hydroxysteroid dehydrogenase type 1 (11 β-HSD-1) or deactivating enzyme 11 β-hydroxysteroid dehydrogenase type 2 (11 β-HSD-2) [
8,
47]. In the present study, [V
IVO(octd)] administration reduced the effects of dexamethasone on lipid accumulation in the liver and the serum lipid profile; previously, 11β-HSD-1 knockout mice ameliorated glucose tolerance and reduced gluconeogenic gene expression during fasting [
47]. On the other hand, the overexpression of this limiting enzyme for activating GCs in adipose tissue was shown to cause glucose intolerance, IR, and moderate obesity in mice [
48]. However, we did not evaluate the expression of GRs and 11β-HSD-1, so it is possible to suggest that [V
IVO(octd)] complex not only improved insulin resistance by the classical mechanisms but also probably modified the GC signaling and availability for tissues. This hypothesis becomes even more attractive when we compare the effects of [V
IVO(octd)] complex on DM1 rats [
27] and dexamethasone-induced IR mice.
One of the most important reasons for the undesirable effects of treatment using DEXA is that it induces the overproduction of reactive nitrogen (RNS) and oxygen species (ROS), causing redox imbalance and leading to cellular damage [
49]. DEXA promoted an increase in oxidative stress biomarkers, which was counteracted by the administration of the sulfur–oxidovanadium(IV) complex (
Table 5). The antioxidant effect of some vanadium compounds is still controversial. Some vanadium-based compounds exacerbate allergic airway inflammation by triggering reactive oxidative stress [
50]. Polyoxovanadate, for example, induces severe toxicity in mice, at least in part by increasing oxidative stress, as seen in both an elevation of MDA levels and a reduction in the GSH/GSSG ratio [
51]. The design of different ligands in the coordination sphere of vanadium-based complexes has allowed for more bioactive compound building (antioxidant, hepato-protective, and antidiabetic) with reduced side effects. This enables their therapeutic uses in several diseases, including cardiovascular diseases (CVDs), obesity, and IR [
52]. The data suggest that [V
IVO(octd)] may modulate the cellular enzymatic apparatus to protect against the oxidative stress induced by synthetic GCs such as dexamethasone.
In addition, a primary consequence of a high blood glucose level is a non-enzymatic glycation reaction, which produces advanced-glycation end products (AGEs) and induces glucose-oxidative damage. Serum fructosamine is a glycated protein that reflects average glycemia over the previous 2–3 weeks [
53]. Here, dexamethasone elevated glucose-oxidative stress, as seen in the fructosamine higher levels in mice. However, the sulfur–oxidovanadium(IV) complex treatment reduced this AGE production. These results confirmed that the proposed vanadium-based compound increased glucose metabolism and prevented glucose-oxidative stress in IR mice.
Modifications of the structures of vanadium compounds have been used to reduce their toxicity and increase their chemical stability and bioavailability in living organisms. In this case, an ionophore ligand, with an S
2O
2 coordination mode, could contribute to the biological effects reported in this study. However, biodistribution and biotransformation studies of [V
IVO(octd)] must still be conducted; this compound has no toxicity in Wistar rats [
27]. In the future, it will be promising to use [V
IVO(octd)] as a potential therapeutical agent to protect patients against the metabolic side effects of synthetic GCs, mainly dyslipidemia, oxidative stress, and IR.