Chronic stress and metabolic regulation

After understanding how chronic stress can profoundly alter the activity of the hypothalamic-pituitary-adrenal axis, the next step is to understand how this neuroendocrine hyperactivation can progressively transform into insulin resistance. In physiology, cortisol is an essential hormone for survival: during a dangerous situation, its task is to ensure the brain and muscles have rapid access to energy. To do this, it increases hepatic glucose production, reduces peripheral sugar consumption, and mobilizes fats and proteins as alternative energy sources. In the short term, this mechanism is advantageous; the problem emerges when cortisol remains elevated for weeks, months, or years. At that point, the body continues to behave as if it were constantly under threat, maintaining active metabolic processes that slowly end up compromising insulin sensitivity.

One of the first effects of chronic cortisol concerns the liver. Glucocorticoids stimulate hepatic gluconeogenesis, which is the production of glucose from amino acids and other energy substrates. Simultaneously, they reduce peripheral glucose utilization by muscles and interfere with normal insulin signaling. At the cellular level, cortisol reduces the translocation of the GLUT-4 transporter, alters the phosphorylation (activation process) of IRS-1, and compromises the PI3K/AKT pathway, one of the main intracellular systems through which insulin exerts its metabolic effects (Blaslov et al., 2018) [9]. It is as if insulin were continuing to knock on the cell door, but the signal were being progressively ignored.

This alteration can initially remain invisible to standard tests. The pancreas, in fact, tries to compensate by producing ever-increasing amounts of insulin. In an initial phase, the body still manages to maintain relatively normal glycemic values thanks to compensatory hyperinsulinemia. However, behind this apparent metabolic normality, the system is already working in overload. An experimental study conducted on healthy women treated with dexamethasone (a drug similar to cortisol that reduces inflammation) clearly showed this mechanism: after a few days of exposure to glucocorticoids, insulin sensitivity decreased drastically, while insulin secretion increased compensatorily. Although the pancreas managed to temporarily maintain normal peripheral glucose utilization, the liver continued to produce excess glucose, signaling persistent hepatic insulin resistance (Nicod et al., 2003) [10].

This finding is particularly important because it suggests that the liver may be one of the first organs to lose sensitivity to insulin during chronic hyperactivation of the HPA axis. Even in the presence of high insulin levels, hepatic glucose production is not adequately suppressed. In practice, the body continues to “manufacture sugar” even when it does not need it. In the long term, this mechanism contributes to chronic hyperinsulinemia, increased glycemia, and the progressive compensatory exhaustion of pancreatic β-cells.

In recent years, various studies have shown that these phenomena can also appear in apparently healthy, normal-weight individuals. A study conducted in Algiers observed that individuals with morning cortisol levels in the high end of the physiological range had significantly higher values of the HOMA-IR index, one of the main indicators of insulin resistance. Subjects with higher cortisol also showed greater abdominal obesity, insomnia, sedentary behavior, and a worse general lipid profile (Ould Bessi et al., 2025) [11]. This suggests that it is not necessary to reach extreme pathological levels of cortisol, as in Cushing's syndrome (a condition caused by prolonged exposure of the body to excessive levels of cortisol), to observe significant metabolic alterations. Even moderate but persistent chronic hyperactivation of the HPA axis can gradually contribute to the loss of insulin sensitivity.

A particularly interesting aspect is that insulin resistance is not exclusively a disease of obesity. For many years, it was thought that the main driver of insulin dysfunction was excess visceral adipose tissue; although obesity is certainly one of the most important factors, we now know that even lean individuals can develop significant insulin resistance. According to the hypothesis proposed by Blaslov and colleagues, in these subjects, the main pathological trigger could be neuroendocrine rather than adipocentric. Chronic stress, through the hyperactivation of the HPA axis and the persistent increase of insulin counter-regulatory hormones, could therefore represent one of the main hidden metabolic factors in normal-weight subjects (Blaslov et al., 2018) [9].

Chronic stress also appears to exert profound effects on pancreatic β-cells, the cells responsible for insulin production. In overweight young Latin Americans, higher cortisol levels were associated not only with reduced insulin sensitivity but also with a lower compensatory capacity of the pancreas. In particular, cortisol correlated negatively with the acute insulin response to glucose and with the disposition index, a parameter that reflects the compensatory functionality of β-cells. This means that cortisol does not seem to be limited to making peripheral cells less sensitive to insulin, but could also progressively compromise the pancreas's ability to react adequately to the increase in insulin demand (Adam et al., 2010) [12].

This phenomenon is extremely relevant in the progression toward type 2 diabetes. In the initial phases, the pancreas manages to compensate for insulin resistance by increasing insulin secretion. Over time, however, chronic functional overload can favor β-cell deterioration. It is like constantly asking an engine to run beyond its limit: initially, it manages to maintain performance, but in the long run, structural failures begin.

Experimental evidence also shows that chronic stress does not only act on peripheral metabolism but can also directly alter brain metabolism. In murine models (laboratory animals, mainly mice) of chronic unpredictable stress, prolonged exposure to stressors significantly increased the HOMA-IR index and induced pathological alterations in insulin signaling in the hippocampus. In particular, increased inhibitory phosphorylation of IRS1 and activation of JNK, a kinase involved in neuronal insulin resistance, were observed. Simultaneously, the animals developed significant cognitive and memory deficits (Rong et al., 2024) [13].

Parallel to the endocrine imbalance, chronic stress also promotes significant production of reactive oxygen species. Oxidative stress appears to be one of the main mechanisms through which cortisol alters insulin sensitivity. In police officers exposed to high occupational stress, higher levels of urinary cortisol were associated with both markers of oxidative stress and an increase in the HOMA-IR index. Researchers observed that perceived stress correlated positively with an indicator of lipid peroxidation (damage to cellular fats caused by oxidation) and oxidative damage (Janczura et al., 2020) [14].

An experimental study on mice exposed to chronic unpredictable environmental stress showed the systemic consequences of this condition extremely clearly. After weeks of exposure to stress, the animals developed persistent hyperglycemia, increased HbA1c, an increase in the HOMA-IR index, and a significant reduction in insulin levels. Simultaneously, a marked reduction in antioxidant defenses, DNA damage, and diffuse degenerative alterations in the pancreas, liver, kidneys, and muscles were observed (Raghav et al., 2019) [15].

Even during pregnancy, cortisol appears to profoundly influence insulin homeostasis. In a cohort of over 500 Peruvian women, higher levels of cortisol in hair were associated with significant increases in fasting insulin and the HOMA-IR index. This suggests that chronic hyperactivation of the HPA axis can amplify physiological gestational insulin resistance, forcing pancreatic β-cells into further compensatory effort.

It is interesting to note that cortisol was not directly associated with the onset of overt gestational diabetes. This finding suggests that hyperactivation of the HPA axis can initially alter insulin metabolism without immediately producing clinically evident hyperglycemia. Once again, the concept of a long “silent” compensatory phase emerges, in which the body manages to temporarily maintain glycemic balance at the cost of increasing metabolic overload (Juvinao-Quintero et al., 2023) [16].

Overall, all this evidence converges toward a central concept: chronic stress is not just a psychological phenomenon, but a biological condition capable of profoundly modifying the insulin regulation of the entire organism. Through cortisol, catecholamines (substances that help the body and nervous system respond rapidly to stress and stimuli), oxidative stress, inflammation, and alterations in cellular signaling, chronic hyperactivation of the HPA axis appears to contribute progressively to the loss of insulin sensitivity, β-cell deterioration, and systemic metabolic dysfunction. In this view, insulin resistance no longer appears only as a consequence of caloric excess or obesity, but as the result of a complex interaction between metabolism, the brain, the endocrine system, and the psychological environment.

If insulin resistance represents one of the main metabolic consequences of chronic hyperactivation of cortisol, another fundamental target of glucocorticoids is the mTOR signaling pathway, one of the most important regulators of cell growth, protein synthesis, and energy anabolism. mTOR acts as a sort of “metabolic control center” capable of integrating information from nutrients, amino acids, cellular energy status, oxygen, and hormonal signals. When environmental conditions are favorable, mTOR promotes cell growth, protein synthesis, and energy storage. Conversely, in the presence of stress or energy deficiency, its activity is progressively suppressed to reduce the consumption of ATP (the cell's main immediate source of usable energy) and preserve cellular resources.

Glucocorticoids appear to exploit this very adaptive mechanism. Numerous studies have indeed demonstrated that dexamethasone (a synthetic corticosteroid drug) and corticosterone (a natural glucocorticoid hormone) are capable of directly repressing mTOR signaling, especially in skeletal muscle. In fact, as demonstrated by Wang and his collaborators, this mechanism occurred through the induction of REDD1 expression in skeletal muscle, and consequently, glucocorticoid treatment significantly reduced the phosphorylation of two of the main downstream targets of mTOR responsible for activating protein synthesis. In practice, cortisol seemed to “turn off” the cellular anabolic program (Wang et al., 2006) [17].

The most important aspect of the study, however, emerged when the researchers silenced REDD1 using interfering RNA (a natural cellular mechanism that acts as a genetic “switch”). In the absence of REDD1, dexamethasone lost much of its ability to inhibit mTOR. This finding demonstrated that REDD1 is not merely a marker of glucocorticoid stress, but an indispensable functional mediator of mTOR inhibition.

From a molecular perspective, as we saw in chapter 4 dedicated to the insulin pathway, REDD1 acts through the tuberous sclerosis complex TSC1/TSC2, consisting of hamartin and tuberin. This complex functions as a potent repressor of an enzyme that is among the main direct activators of mTORC1. Under physiological conditions, it keeps mTOR active; REDD1, on the other hand, promotes the activation of the TSC1/TSC2 complex, thereby turning off anabolic signaling. It is as if REDD1 activates a metabolic emergency brake capable of drastically slowing down cell growth in the presence of stress.

This adaptive role of REDD1 does not seem to be limited to pharmacological glucocorticoids. Even physiological conditions of energy stress, such as fasting and nutritional deprivation, induce a marked increase in its expression. In 2009, some researchers demonstrated that just 18 hours of food deprivation were sufficient to drastically increase REDD1 in rat skeletal muscle. Refeeding, by contrast, rapidly returned REDD1 to baseline levels and reactivated mTOR within less than an hour (McGhee et al., 2009) [18].

The study also revealed an extremely important aspect: the induction of REDD1 during fasting did not depend primarily on the reduction of insulin, but on the increase in glucocorticoids. Corticosterone levels were, in fact, closely correlated with REDD1 expression, while pharmacological blockade of the glucocorticoid receptor almost completely abolished the induction of the protein. This suggests that cortisol represents one of the main endocrine signals through which the body communicates to the muscle the need to reduce energy consumption during periods of nutritional deficiency.

From an evolutionary standpoint, this mechanism possesses an extremely efficient logic. Protein synthesis is one of the most biologically expensive processes in terms of energy. In conditions of caloric scarcity or chronic stress, maintaining high mTOR activity could rapidly compromise cellular energy reserves. REDD1 therefore seems to act as a “metabolic switch” that temporarily limits anabolic processes to favor survival.

This interpretation was further explored by some researchers in 2018. The authors of the study demonstrated that REDD1 does not act simply as a repressor of muscle anabolism, but as a true regulator of energy homeostasis. Mice lacking REDD1 indeed showed a hyperactive metabolism, with increased oxygen consumption and greater depletion of energy reserves during fasting, hypoxia, or intense exercise (Britto et al., 2018) [19].

Through the inhibition of the AKT/mTORC1 axis, REDD1 reduces protein synthesis, glycogen accumulation, and cellular energy consumption. In other words, cortisol, via REDD1, seems to temporarily transform the cell into an “energy-saving” mode.

This concept helps to understand an apparent biological paradox. In physiological conditions of energy stress, REDD1 can be protective because it prevents cellular energy collapse. However, when glucocorticoid activation becomes chronic, the same mechanism can progressively turn into a potent catabolic driver (a factor that favors the degradation of the body's molecules and reserves). Persistent inhibition of mTOR, in fact, reduces muscle protein synthesis, promotes atrophy (reduction in the volume and functionality of a tissue), loss of lean mass, and metabolic deterioration.

A previous study by the same group had already demonstrated this phenomenon extremely clearly: in “wild-type” mice treated with dexamethasone, a marked reduction in muscle mass and skeletal fiber size was observed, accompanied by strong suppression of mTORC1 signaling. Conversely, mice lacking REDD1 were almost completely protected from glucocorticoid-induced atrophy (Britto et al., 2014) [20].

The absence of REDD1 effectively prevented the reduction of protein synthesis and the functional inhibition of mTORC1.

The inhibition of mTOR by glucocorticoids involves not only muscle metabolism but also the regulation of cellular autophagy. In lymphocytes treated with dexamethasone, the increase in REDD1 indeed induces significant activation of the autophagic program through the suppression of mTOR. Autophagy represents a highly conserved catabolic process that allows the cell to degrade damaged or unnecessary intracellular components to recover nutrients and energy.

In this context, REDD1 acts once again as a mediator of stress. Silencing REDD1 attenuates mTOR inhibition and significantly decreases autophagic flux. At the same time, cell death increases, suggesting that glucocorticoid-induced autophagy initially represents an adaptive survival mechanism (Molitoris et al., 2011) [21].

This aspect is particularly interesting because it shows how cortisol does not act simply by “destroying” tissues, but by inducing biological programs of adaptation to energy stress. In the short term, such mechanisms can be advantageous; the problem emerges when the stimulation becomes chronic and persistent.

The relationship between REDD1, energy stress, and mTOR had already been intuited by a study published in 2005. The authors demonstrated that REDD1 was strongly induced during energy depletion. In cells lacking REDD1, the adaptive response to metabolic stress was profoundly compromised, maintaining active protein synthesis in a critical metabolic situation (Sofer et al., 2005) [22].

Overall, all this evidence suggests that cortisol exerts much of its catabolic effects through the modulation of REDD1 and the suppression of the mTOR pathway. In acute conditions, this biological program represents a sophisticated survival strategy capable of reducing energy expenditure and preserving ATP during stress, fasting, or hypoxia. However, when the activation of the HPA axis becomes chronic, the persistent inhibition of mTOR can progressively contribute to muscle atrophy, loss of lean mass, reduced protein synthesis, metabolic alterations, and systemic functional deterioration.

If mTOR represents one of the main regulators of cellular anabolism, AKT acts as the major upstream coordinator of this system. The relationship between AKT and mTOR is so close that, in many biological contexts, the two pathways are described as a single functional network. In reality, as we saw in chapter 4, which was dedicated to the insulin pathway, the relationship between these proteins is much more complex: AKT activates mTOR, but at the same time, mTOR can regulate AKT through sophisticated reciprocal feedback mechanisms. This continuous dialogue allows the cell to integrate information from nutrients, insulin, growth factors, and energy status, modulating growth, survival, metabolism, and protein synthesis.

AKT, also known as Protein Kinase B (PKB), represents one of the main intracellular effectors of the PI3K pathway. Under physiological conditions, stimulation by insulin or growth factors activates PI3K, which, through a phospholipid (a type of fat that forms the basic structure of cell membranes) fundamental to cellular signaling, acts as a recruitment platform for AKT on the cell membrane, allowing for its activation (Liu et al., 2015) [23].

Full activation of AKT requires two distinct phosphorylation events (the addition of a phosphate that can activate, deactivate, or modify the functioning of a protein), but it is the second step that has revolutionized the biological understanding of mTOR. For many years, mTOR was considered almost exclusively a downstream regulator of AKT; today, however, we know that one of its complexes, mTORC2, is essential for the full activation of AKT itself (Kim et al., 2021) [24].

mTOR exists in two distinct multiprotein complexes: mTORC1, primarily involved in protein synthesis and cell growth, and mTORC2, involved in cell survival, cytoskeleton organization, and, above all, the activation of AKT (Jhanwar-Uniyal et al., 2019) [25].

This distinction is fundamental because the two complexes respond to different signals and perform complementary but non-overlapping biological functions.

One of the most important studies in understanding the activation of mTORC2 was published in 2015. The authors demonstrated that a phospholipid produced by PI3K does not merely recruit AKT but directly activates mTORC2 through a protein domain (Liu et al., 2015) [23].

The final result is the phosphorylation of AKT: this mechanism clarifies how PI3K, AKT, and mTORC2 are not separate pathways, but deeply integrated components of the same metabolic network.

An even more interesting aspect concerns the spatial localization of AKT activation. For a long time, it was believed that the entire activation of the PI3K/AKT pathway occurred exclusively on the plasma membrane. However, more recent studies have shown that the full maturation of the signal requires specific intracellular platforms.

In 2021, some researchers demonstrated that early endosomes (vesicular organelles present in the cytoplasm of cells) represent true signaling hubs, essential for the phosphorylation of AKT by mTORC2: indeed, the study highlights how the components of mTORC2 accumulate on early endosomes (Kim et al., 2021) [24].

All of this allows for the correct assembly of mTORC2 and the subsequent phosphorylation of AKT. In practice, endosomes function as dynamic platforms that amplify and stabilize the PI3K/AKT pathway signal, favoring cell survival, growth, and persistence. This model suggests that AKT activation is not a static event, but a process spatially organized into different cellular compartments (Kim et al., 2021) [24].

In practice, AKT represents the main intracellular interpreter of the insulin signal.

The metabolic role of mTORC2 is particularly evident in the liver. In 2012, some researchers demonstrated that the hepatic mTORC2 complex directly controls glycolysis and lipogenesis through AKT (two fundamental and interconnected metabolic processes that the body uses to manage and store energy) (Hagiwara et al., 2012) [26]. In the absence of mTORC2, the liver loses much of its ability to activate insulin-induced metabolic programs.

This means that mTORC2 is not simply an “accessory activator” of AKT, but an essential regulator of glucose and lipid metabolism.

Studies by some researchers showed that rapamycin, if administered for a long time, does not just inhibit mTORC1 but also progressively compromises the assembly of mTORC2. The consequence is a reduction in AKT phosphorylation and a progressive loss of insulin sensitivity (Sarbassov et al., 2006) [28].

Other researchers confirmed this phenomenon in mouse cell structures, showing that rapamycin exerts a biphasic effect, namely an initial improvement in insulin sensitivity followed by severe insulin resistance after chronic exposure (Ye et al., 2012) [27].

The most important finding of the study was that silencing a specific component of mTORC2 reproduced almost perfectly the insulin resistance induced by rapamycin. Conversely, silencing mTORC1 primarily altered mitochondrial function without significantly compromising the insulin response.

This demonstrates that mTORC1 is primarily involved in anabolism and energy biogenesis, while mTORC2 is fundamental for maintaining insulin sensitivity through AKT. In other words, chronically blocking mTORC2 is equivalent to “turning off” an essential part of the insulin signal.

This discovery has profoundly changed the biological understanding of rapamycin and the mTOR pathway. For many years, it was thought that inhibiting mTOR represented a universally beneficial strategy; today, we know that the indiscriminate inhibition of mTORC2 can have significant metabolic consequences.

For example, in glioblastoma (the most frequent and aggressive primary brain tumor in adults), hyperactivation of the PI3K/AKT/mTOR pathway is one of the main genetic alterations that drive the origin and growth of a tumor. In these diseases: PI3K activates AKT and mTORC2, AKT promotes tumor growth, survival, and metabolism, and mTORC1 supports protein synthesis and cell proliferation. The two mTOR complexes are therefore reciprocally interconnected: mTORC2 activates AKT, AKT activates mTORC1, and the latter modulates mTORC2 via feedback (Jhanwar-Uniyal et al., 2019) [25].

Overall, the relationship between AKT and mTOR represents one of the most sophisticated metabolic integration systems in the entire organism. AKT activates cellular anabolic programs, while mTOR coordinates the energy availability necessary to support them. mTORC2, in turn, ensures the full activation of AKT, creating a reciprocal regulatory circuit that controls growth, survival, glucose and lipid metabolism, protein synthesis, and insulin response.

Following the activation of AKT by the PI3K–mTORC2 cascade, the insulin signal enters one of its most decisive phases: the direct control of glucose entry into the cell. It is precisely at this point that metabolism ceases to be an abstract concept and translates into a concrete physical event: the movement of GLUT4 (the most well-known and studied glucose transporters) toward the cell membrane. Without this step, glucose remains in the blood; with this step, however, it can finally be absorbed and utilized by peripheral tissues.

The main molecular system responsible for this process is now identified as the AKT–AS160–GLUT4 axis, considered one of the most important networks for overall glucose homeostasis (maintaining glycemia within physiological values). This axis represents the meeting point between insulin signaling, vesicular trafficking, and cellular metabolism. When the system functions correctly, the body manages to maintain efficient glucose handling; when the network becomes progressively impaired, the first signs of insulin resistance begin to appear (Sharma and Dey, 2021) [29].

Under physiological conditions, insulin binds to its receptor, activating the IRS–PI3K–AKT cascade. Once phosphorylated and fully active, AKT begins to transmit the metabolic signal to various intracellular targets. Among these, one of the most important is AS160, a protein that directly controls the trafficking of vesicles containing GLUT4.

In basal conditions, GLUT4 remains almost completely sequestered inside the cell. In fact, over 95% of the transporter is stored in specialized intracellular compartments called GLUT4 Storage Vesicles. AS160 functions as a sort of molecular brake: it keeps specific proteins responsible for the mobilization of these vesicles inactive, thereby preventing GLUT4 from reaching the plasma membrane.

When insulin activates AKT, the latter phosphorylates AS160; at this point, the GLUT4 vesicles can begin their movement toward the cell membrane. It is an extremely dynamic and coordinated process, almost comparable to a sophisticated intracellular logistics system: the vesicles are mobilized, transported along the cytoskeleton (the internal structure that supports, organizes, and enables cell movement), anchored near the membrane, and finally fused with it, allowing the exposure of GLUT4 on the cell surface.

The final result is an increase in glucose entry into peripheral tissues, especially in skeletal muscle and adipose tissue, the two main organs responsible for post-prandial glucose disposal (Sharma and Dey, 2021) [29].

Subsequent experimental evidence further consolidated this concept. It was observed that the functional inhibition of AKT using antibodies or peptides drastically reduced insulin-induced GLUT4 translocation (Hill et al., 1999) [30]. In parallel, some researchers demonstrated that constitutively active forms of AKT were sufficient to promote GLUT4 translocation even in the absence of insulin. Conversely, a dominant-negative form of AKT significantly compromised the insulin response (Cong et al., 1997) [31].

These studies definitively clarified that AKT does not represent a simple accessory element of insulin signaling, but one of its main central nodes. In practice, without adequate AKT activation, GLUT4 trafficking becomes profoundly inefficient.

Some researchers obtained similar results using a constitutively active form of AKT in adipocytes. Persistent activation of the AKT kinase induced marked GLUT4 translocation, increased glucose uptake, and stimulated lipogenesis (Kohn et al., 1996) [32].

One of the most elegant studies on the role of AKT was published in 2008: the authors developed a sophisticated experimental system capable of rapidly activating AKT independently of the insulin receptor. Activation of the kinase produced the phosphorylation of AS160, the translocation of GLUT4, and a significant increase in glucose uptake within minutes (Ng et al., 2008) [33]. In practice, AKT was able to replicate most of the metabolic effects of insulin.

Overall, the AKT–AS160–GLUT4 axis represents the true operational heart of cellular glucose management. AKT translates the insulin signal into a concrete metabolic program, AS160 regulates vesicle trafficking, and GLUT4 finally allows glucose entry into the cell. When this intracellular communication is progressively interrupted, the main metabolic alterations that characterize insulin resistance and type 2 diabetes begin to appear.

The final step of this pathophysiological pathway inevitably leads to GLUT4, the main insulin-dependent glucose transporter in skeletal muscle and adipose tissue. If insulin represents the signal and AKT the intracellular transmission system, GLUT4 constitutes the true final executor of the entire metabolic response. It is, in fact, through this transporter that glucose can actually enter the cell. When the system is functioning correctly, insulin rapidly stimulates the translocation of GLUT4 to the cell membrane after a meal, allowing glucose to be removed from the blood and used as metabolic fuel. When GLUT4 can no longer reach the membrane correctly or loses its functional efficiency, one of the central defects of insulin resistance begins to appear.

For many years, it was thought that insulin resistance primarily stemmed from a global defect in insulin signaling. Today, we know the situation is much more complex. Numerous lines of evidence demonstrate that, in many cases, the upstream insulin cascade may remain relatively preserved while the true defect emerges in the terminal phases of intracellular GLUT4 trafficking (van Gerwen et al., 2023) [34]. In other words, the insulin signal may still be partially transmitted, but the transporter can no longer correctly complete its journey to the plasma membrane.

This concept was confirmed in a particularly elegant way by the study by Judge and colleagues, who developed a high-sensitivity imaging technique capable of directly observing the translocation of endogenous GLUT4 in adult muscle fibers without resorting to artificial constructs or overexpressed proteins. Using different models of insulin resistance, including chronic insulin exposure, treatment with palmitate (a saturated fatty acid used by the body as an energy source), and diet-induced obesity with high lipid content, the authors observed an extremely interesting finding: GLUT4 trafficking was selectively compromised, while other insulin-dependent pathways remained relatively preserved (Judge et al., 2025) [35].

This means that insulin resistance does not necessarily represent a complete “blackout” of insulin signaling, but can take the form of a highly selective defect in the glucose transport system. It is as if the power grid were still functioning, but the main glucose entry gate remained blocked.

As we saw in the previous paragraph, GLUT4 Storage Vesicles, or GSVs, represent highly specialized compartments that function as true “strategic depots” for GLUT4. Under basal conditions, most of the transporter remains sequestered within these vesicles; when the insulin signal arrives, the GSVs are rapidly mobilized toward the cell membrane through the coordinated activation of AKT, AS160, and cytoskeleton-dependent molecular motors (a set of cellular structures that form a three-dimensional network of tubules and filaments) (Drobiova et al., 2025) [36]. The system resembles a complex urban transport network: the vesicles must be correctly assembled, routed, transported, and finally fused with precision into the right area of the cell membrane.

It is precisely this extraordinary complexity that makes the system vulnerable. Even a minimal alteration in intracellular trafficking is enough to compromise the entire glucose uptake capacity. In insulin resistance, in fact, defects in GSV formation, alterations in GLUT4 sorting, and disorganization of the cytoskeleton have been observed (van Gerwen et al., 2023) [34] (Drobiova et al., 2025) [36].

The role of skeletal muscle also appears fundamental. Some researchers demonstrated that the selective inactivation of GLUT4 in muscle causes severe insulin resistance and significant glucose intolerance at an early age. Insulin-stimulated glucose uptake was almost completely abolished, confirming that GLUT4-mediated glucose transport in skeletal muscle represents one of the main determinants of systemic glycemic homeostasis (Zisman et al., 2000) [37].

In parallel, other researchers showed that adipocyte overexpression of GLUT4 was sufficient to reverse systemic insulin resistance even in the presence of a persistent muscle defect in glucose uptake (Carvalho et al., 2005) [38]. This study profoundly changed the classic view of metabolism, demonstrating that adipose tissue is not a simple energy depot but a true endocrine organ capable of controlling the insulin sensitivity of the entire organism.

Overall, all this evidence converges toward a central concept: GLUT4 represents the true operational terminal point of insulin action. The simple activation of the insulin cascade is not enough if the transporter cannot be correctly assembled, mobilized, fused, and distributed in the plasma membrane. Insulin resistance therefore appears less and less as a single molecular defect and more and more as a complex disorganization of the entire intracellular architecture responsible for glucose trafficking.