The dialogue between metabolism and inflammation

As we highlighted in the previous chapters, a growing body of scientific evidence has progressively challenged the paradigm that hyperinsulinemia is exclusively a compensatory response to insulin resistance; likewise, as we will see with the following scientific evidence, hyperinsulinemia can also become the driver for the activation and maintenance of chronic inflammation.

In fact, a broad and systematic review published in 2021 contributed decisively to this shift in perspective. By analyzing experimental and clinical data, it emerges that, in the early stages of obesity and type 2 diabetes, hyperinsulinemia often precedes both hyperglycemia and overt insulin resistance. In this context, excess insulin does not act as a neutral signal, but as a powerful modulator of cellular signaling, capable of promoting chronic inflammation, metabolic dysfunction, acceleration of aging processes, and an increased oncological risk (Zhang et al., 2021) [1].

Supporting this view, experimental studies that isolate the effect of insulin from caloric excess provide particularly robust evidence. In a 2018 study, chronic hyperinsulinemia was induced in murine models (laboratory animals, mainly mice, used in scientific studies) through continuous insulin infusion, in the absence of an increase in energy intake. Under these conditions, hyperinsulinemia proved sufficient to induce adipose tissue inflammation, characterized by an increase in pro-inflammatory immune populations, an imbalance between M1 and M2 macrophages (immune system cells), and marked extracellular matrix deposition. Conversely, partial reduction of circulating insulin, even in the presence of obesity, attenuated inflammation and improved insulin sensitivity, identifying hyperinsulinemia as a direct causal factor of metabolic inflammation (Kumar et al., 2018) [2].

The pathogenetic role of hyperinsulinemia also clearly emerges outside of classically metabolic tissues. In 2022, it was demonstrated that hyperinsulinemia associated with insulin resistance promotes the pro-inflammatory activation of microglia, the main immune population of the central nervous system. In animal and cellular models, chronic exposure to insulin induces alterations in microglial energy metabolism, mitochondrial dysfunction (cellular structures that produce the energy necessary for the cell), and polarization toward a pro-inflammatory phenotype. These changes are closely associated with the development of neuroinflammation and cognitive deficits, suggesting a direct pathogenetic link between type 2 diabetes and neurodegenerative diseases (Yang et al., 2022) [3].

Further confirmation of this model emerges from a review published in 2022, which critically analyzes the interactions between hyperinsulinemia, inflammation, and metabolic dysfunction. In this context, macrophages are identified as a central node for integrating metabolic and immune signals. Macrophages express insulin receptors and respond directly to its signaling; however, chronic exposure to high concentrations of insulin compromises physiological regulatory function, favoring the loss of anti-inflammatory signals and the enhancement of pro-inflammatory responses. This imbalance contributes to the formation of immuno-metabolic vicious cycles that sustain and amplify the progression of metabolic diseases (Püschel et al., 2022) [4].

Taken together, these studies converge on a fundamental point: chronic hyperinsulinemia emerges as an active pathogenetic determinant, capable of triggering and sustaining systemic and tissue inflammation. Inflammation, therefore, does not appear as a secondary or subsequent event, but as a consistent consequence of aberrant and non-physiological insulin signaling. This conceptual framework constitutes the necessary prerequisite for analyzing, in the following sections, the 3 phases through which excess insulin is translated into inflammatory activation.

The first event is silent but fundamental: insulin stimulates the cell to increase the production of arachidonic acid, an omega-6 polyunsaturated fatty acid present in cell membranes. This occurs through the activation of SREBP-1c (a protein that activates genes involved in the production of fats by the cell), which enhances the enzymes necessary to build lipid substrates such as triglycerides and cholesterol (Matsuzaka et al., 2002) [5]. In other words, hyperinsulinemia does not yet generate inflammation, but it creates the conditions for it to become possible.

Once accumulated in cell membranes, arachidonic acid must be released to become biologically active. Here, a second decisive step comes into play: insulin activates the MAPK cascade (a chain of signals that transmits information within the cell), increases intracellular calcium, and activates cPLA₂α, the enzyme responsible for the release of free arachidonic acid (Gijón et al., 2000) [6].

Finally, the released arachidonic acid is converted into prostaglandin E₂, one of the central mediators of chronic inflammation, by the action of COX2 (an enzyme that contributes to the production of substances involved in inflammation) stimulated by excess insulin through the activation of the MAPK cascade. PGE₂ does not remain a simple end product: it becomes an active signal that turns on the EP4 receptor (a cellular receptor that receives the signal of prostaglandin E₂ (PGE₂)), reactivates EGFR (a cellular receptor that, when activated, stimulates cell growth and survival signals), and converges again on the PI3K–AKT–mTOR axis, closing a deep amplification circuit between inflammation and cell growth (Oshima et al., 2011) [7].

In the following paragraphs, we will go into more detail on each of the 3 phases described above to better explain all the biologically connected steps.

One of the first cellular responses to chronic insulin stimulation is the activation of the transcription factor SREBP-1c (Sterol Regulatory Element-Binding Protein-1c), which initiates the lipogenic program (the production of lipids such as triglycerides and cholesterol). This step represents a crucial node in converting insulin signaling into lipid remodeling and constitutes the first link between hyperinsulinemia and the production of bioactive lipid mediators.

Even in physiological models, insulin stimulation is capable of markedly increasing SREBP-1c expression in the liver, an effect that remains selectively conserved (maintained over time because it is useful or important) even under conditions of insulin resistance. In particular, it has been demonstrated that the activation of SREBP-1c and lipogenesis remains intact or is even enhanced.

From a molecular perspective, the insulin-mediated activation of SREBP-1c requires the integrity of the PI3K/AKT pathway and, specifically, the mTORC1 complex. Pharmacological inhibition of mTORC1 selectively blocks the induction of SREBP-1c by insulin, regardless of other metabolic effects of the hormone, demonstrating that lipogenesis represents a functional branch of insulin signaling (Li et al., 2010) [8].

In addition to transcriptional regulation, insulin also controls the proteolysis (the process of protein degradation by the body) of SREBP-1c, which is necessary for its translocation into the nucleus and the activation of target genes. It has been clarified that this second level of control also depends on mTORC1. This dual control—transcriptional and post-translational—makes SREBP-1c particularly sensitive to persistent insulin stimulation, as in the case of hyperinsulinemia (Owen et al., 2012) [9].

The functional importance of this axis has been further confirmed in cellular and animal models, in which AKT activation promotes cell growth and lipid accumulation exclusively when mTORC1 and SREBP activity is preserved. Blocking SREBP prevents the increase in lipid synthesis and limits cell expansion, demonstrating that insulin-induced lipogenesis is not an accessory effect, but a structural component of the insulin anabolic program (Porstmann et al., 2008) [10].

This mechanism is not confined to the liver. Immune cells such as monocytes also respond directly to insulin through the induction of SREBP-1c. It has been shown that insulin exposure increases the expression of SREBP-1c and key fatty acid desaturases (fundamental enzymes in lipid metabolism), suggesting that insulin can directly modulate cellular lipid composition and the availability of polyunsaturated fatty acids (linoleic acid, DGLA, arachidonic acid), which are precursors to inflammatory mediators, as we will see later (Arbo et al., 2011) [11].

Taken together, these findings indicate that the activation of SREBP-1c represents a primary and highly conserved response to insulin stimulation. Under conditions of chronic hyperinsulinemia, this lipogenic program loses its adaptive character and becomes persistent, predisposing the cellular environment to an increase in fatty acid synthesis and modification, an essential step for the subsequent phases of the inflammatory cascade.

The binding of insulin to its receptor does not exclusively activate the PI3K–AKT pathway responsible for classic metabolic effects, but simultaneously triggers a second signaling cascade: the MAPK-ERK pathway. This bifurcation of the insulin signal is a well-documented physiological event and represents a key step in translating the hormonal signal into non-traditional metabolic cellular responses.

Following the activation of the insulin receptor, the phosphorylation of IRS substrates (the addition of phosphate groups to IRS, which modifies its ability to transmit the insulin signal) triggers the sequential MAPK-ERK cascade: this branch of insulin signaling is functionally distinct from the AKT pathway, yet it is activated in a coordinated and temporally overlapping manner (Arkun, 2016) [18].

The activation of the MAPK-ERK pathway by insulin is not an accessory event, but rather a regulated and necessary step for the fine-tuning of the duration and intensity of the insulin signal. It has been demonstrated that the mediators of this cascade participate in feedback loops that modulate the activation of the insulin receptor in the plasma membrane, contributing to its activity on the cell membrane (Choi et al., 2019) [19].

From a functional perspective, the MAPK-ERK pathway represents an integration axis between nutritional, hormonal, and mitogenic signals (effects that promote cell multiplication). Under physiological conditions, its transient activation contributes to metabolic adaptation and the regulation of cellular gene expression. However, when insulin stimulation becomes chronic, as in hyperinsulinemia, the repeated or prolonged activation of MAPK-ERK can take on a qualitatively different significance, favoring cellular programs associated with metabolic stress and inflammation (Gehart et al., 2010) [20].

It is important to emphasize that this pathway does not simply act as an amplifier of the insulin signal, but modulates the cell's sensitivity to insulin itself. In vivo studies have shown that adequate activity of the MAPK-ERK cascade is necessary to maintain a normal insulin response, while its chronic alteration contributes to the development of insulin resistance through transcriptional regulation and negative feedback mechanisms (Zhang et al., 2011) [21].

Overall, the activation of the MAPK-ERK pathway represents the first step of the second phase of the process, which, through this cascade, directs the insulin signal toward cellular programs capable of modulating enzymes, membrane lipids, and inflammatory responses, creating the molecular foundation for all subsequent events.

Once insulin has made free arachidonic acid available through the activation of cPLA²α, the cell enters a new biochemical state: the lipid substrate AA is now present in the cytoplasm and can be immediately channeled toward the synthesis of eicosanoids. However, for arachidonic acid to be effectively transformed into prostaglandins, a second fundamental requirement is necessary: the induction of COX-2 (cyclooxygenase-2 enzyme), the main catalyst for the biosynthesis of PGE² (prostaglandin E²).

COX-2 represents the rate-limiting step of the lipid inflammatory response: under basal conditions, its expression is low or absent in most tissues, but it is rapidly induced by insulin signals through MAPK-dependent intracellular transduction pathways.

It has been demonstrated that the MAPK cascade is sufficient to cause a direct increase in COX-2 expression and a parallel increase in PGE² production. The activation of these mediators induces COX-2 in a manner entirely dependent on the phosphorylation of the MAPK-ERK pathway, and pharmacological inhibition of the latter completely abolishes both the expression of the enzyme and the synthesis of prostaglandins (Guan et al., 1998) [30].

These results clarify a crucial point: MAPK-ERK signaling does not act only upstream of arachidonic acid release, but also controls the cell's ability to transform it into inflammatory mediators through the induction of COX-2.

The importance of the MAPK-ERK pathway in the regulation of COX-2 has also been confirmed in pathological contexts characterized by persistent inflammation and pro-tumor microenvironments. It has been observed that sustained activation of the MAPK-ERK protein stabilizes COX-2 mRNA and amplifies its expression, producing an enhanced and lasting pro-inflammatory response (Park and Kwon, 2011) [31].

In the specific context of insulin signaling, this connection takes on direct significance. It has been shown that insulin can activate resident immune cells, such as microglia (a type of cell responsible for the primary active immune defense in the central nervous system), inducing a persistent increase in COX-2 expression. This finding indicates that insulin not only mobilizes the lipid substrate but can also initiate the induction of the enzymatic apparatus necessary for its conversion into pro-inflammatory prostaglandins (Haas et al., 2020) [32].

At the transcriptional level (the regulation during which DNA is transcribed into messenger RNA, mRNA), COX-2 expression is regulated by the activation of MAPK-ERK signals. In human cell models, selective inhibition of MAPK-ERK significantly reduces the induction of COX-2, confirming that this kinase represents one of the main functional nodes in the regulation of the enzyme (Garcia-Garcia et al., 2012) [33].

Overall, this first sub-phase of the final sequence shows that insulin, through the activation of the MAPK-ERK pathway, is capable of inducing COX-2, creating the enzymatic conditions necessary for the released arachidonic acid to be channeled toward the synthesis of prostaglandin E². In this way, the chronic insulin signal completes the transition from a physiological metabolic state to a pro-inflammatory state, predisposing the cell to the production of active inflammatory mediators, as we will see in the following paragraph.

This phase represents a crucial step because it shows how an inflammatory lipid mediator such as PGE², produced downstream of hyperinsulinemia, can activate cellular programs of growth, survival, and inflammatory amplification through interactions with EGFR (a membrane receptor that binds the epidermal growth factor).

Once synthesized from arachidonic acid via the COX-2 axis, PGE² does not simply act as a terminal product of inflammation, but becomes a true active signal. PGE² exerts its effects by binding to four receptors (EP1/2/3/4), among which the EP4 receptor emerges as one of the main mediators of proliferative and inflammatory responses.

The binding of PGE² to EP4 triggers a complex intracellular cascade, involving transduction mechanisms capable of activating the EGFR receptor. This event, known as transactivation (indirect activation of another cell signaling pathway), represents a fundamental amplification point in the biology of chronic inflammation.

It has been directly demonstrated that PGE²–EP4 signaling is capable of activating EGFR in gastric tumors through an increase in EGFR expression. Selective inhibition of EP4 completely suppresses this induction, demonstrating that EP4 constitutes the essential link between PGE² and EGFR in vivo (Oshima et al., 2011) [7].

Alongside this indirect mechanism, there are also faster forms of intracellular transactivation. It has been observed that PGE² can induce EGFR phosphorylation and AKT activation within minutes. EGFR inhibitors completely abolished PGE²-induced cell migration, highlighting that EGFR activation represents an obligatory early event in the cellular response to prostaglandin (Buchanan et al., 2003) [38].

This axis is also particularly well characterized in non-tumor cells. In cardiac myocytes (specialized cells that make up the myocardium), PGE² induces cellular hypertrophy through an EP4-dependent mechanism, which involves the transactivation of EGFR. Antagonism of EP4 or inhibition of EGFR completely abolishes PGE²-induced protein synthesis, demonstrating an EP4 → EGFR causal sequence (Mendez and LaPointe, 2005) [39].

At the molecular level, it has been shown that stimulation of the EP4 receptor by PGE² induces EGFR transactivation. Disruption of this complex drastically reduces cell migration and metastatic capacity, indicating that EP4 is not just a receptor, but an organizing center for pro-oncogenic inflammatory signaling (Buchanan et al., 2006) [40].

Taken together, these findings indicate that the PGE² produced downstream of hyperinsulinemia does not represent a simple terminal mediator, but a functional activator of EP4 capable of transactivating EGFR and thus amplifying the AKT signal and all pro-inflammatory and proliferative cellular programs. This step completes the transformation of the chronic insulin signal into a self-sustaining inflammatory network, which no longer involves only lipid metabolism, but also cell growth, migration, and pathological remodeling, through the PI3K-AKT-mTOR insulin pathway.

Following the binding of PGE² to the EP4 receptor and the activation of the EGFR receptor, the cell enters a qualitatively new phase: an intracellular inflammatory lipid signal is transformed into a program of growth, survival, and metabolic remodeling.

As we have seen, EGFR therefore represents one of the activation nodes of the PI3K–AKT-mTOR pathway, the central axis of insulin's metabolic action. In this way, inflammation derived from arachidonic acid converges directly on the same molecular effectors as hyperinsulinemia, establishing a powerful amplification circuit.

As we saw in the previous paragraph, it has been demonstrated that PGE², through EP4, induces rapid phosphorylation of EGFR and, immediately downstream, the activation of AKT. Pharmacological inhibition of EGFR completely abolishes the AKT activation induced by PGE², demonstrating that EGFR is an obligatory step in the link between prostaglandin signaling and the PI3K/AKT pathway (Buchanan et al., 2003) [38].

This observation is crucial: PGE² does not act only as an inflammatory mediator, but as an upstream activator of the very same pathway architecture that insulin uses physiologically.

Even more specific evidence was provided by demonstrating that EP4, but not EP2, is capable of selectively activating the PI3K/ERK cascade and inducing early growth genes such as EGR-1 (a protein that controls the activity of certain genes). Inhibition of PI3K completely eliminates the MAPK-ERK response, confirming that EP4 is functionally coupled to the PI3K pathway as the dominant branch of its signaling (Fujino et al., 2003) [41].

This property distinguishes EP4 as a receptor capable of transforming an inflammatory signal into a PI3K-dependent proliferative impulse.

In colon carcinoma, stimulation of the EP4 receptor fully restores cell proliferation even when endogenous PGE² synthesis is blocked by COX inhibitors. This effect is dependent on the activation of the PI3K/ERK cascade, demonstrating that PGE² is sufficient to activate proliferative pathways through EP4 and PI3K (Pozzi et al., 2004) [42].

Downstream of AKT, as we have extensively seen previously in the insulin pathway chapter, one of the most relevant effectors is the mTOR complex, a central regulator of cell growth and biosynthetic metabolism. In pathological conditions, this PI3K/AKT/mTOR axis constitutes one of the main bridges between chronic inflammation and tumor progression.

In mouse models of intestinal tumorigenesis, the selective deletion of EP4 in myeloid cells (bone marrow cells) drastically reduces the formation of adenomas and is associated with a marked suppression of mTOR and MAPK-ERK activation in the tumor microenvironment. This data demonstrates that PGE²–EP4 signaling is an upstream driver of mTOR activation in chronic inflammatory contexts (Chang et al., 2015) [43].

A 2020 study analyzes the role of prostaglandin E2 (PGE2) and its EP4 receptor in tumor progression. In tumors, increased COX-2 activity promotes the production of PGE2, which through EP4 can activate signaling pathways such as PI3K/AKT and MAPK-ERK, supporting proliferation, migration, invasion, and metastasis. EP4 also contributes to therapy resistance and the suppression of the anti-tumor immune response. The authors therefore highlight how the pharmacological blockade of EP4 represents a possible therapeutic strategy, especially in combination with chemotherapy and immunotherapy (Ching et al., 2020) [44].

Overall, this final phase closes the causal sequence initiated by hyperinsulinemia and finally converges on mTOR, transforming an inflammatory signal into a self-sustaining anabolic and proliferative program.

This represents one of the most powerful mechanisms through which hyperinsulinemia can trigger chronic inflammation and the progression of metabolic and degenerative diseases.

The cell that centrally activates the inflammatory response we have just examined in detail, step by step, following all the processes of the pathway, is the macrophage, a "sentinel" of the innate immune system present in tissues. The macrophage, as we will see more clearly in the next chapter dedicated entirely to the immune system, is a key cell in inflammation. Under normal conditions, it is activated only when necessary. However, when the mTOR pathway is persistently stimulated, the macrophage can enter a state of chronic hyperactivation.

In the macrophage, mTOR functions as a true "metabolic switch": it integrates signals of energy and nutrient availability and translates them into a more "aggressive" cellular program capable of sustaining an intense inflammatory response. In practical terms, it is as if the macrophage were "conditioned" to produce energy and substrates more rapidly, which are useful for synthesizing cytokines, enzymes, and inflammatory mediators. In summary, when mTOR remains chronically "on," the macrophage is not just more active: it is metabolically predisposed to sustain pro-inflammatory production and, at the same time, is less efficient at turning off and clearing the intracellular signals that fuel inflammation. (Weichhart et al., 2015) [45]

The decisive step, however, is what happens downstream of the hyperactive macrophage: once the production of cytokines and chemokines (small proteins used by cells, especially immune cells, to communicate with each other) increases, a cascade of immune system activation is triggered. The pro-inflammatory cytokines released (such as TNF-α, IL-1β, and IL-6) induce an "alarm" response in tissues, which includes an increase in the expression of endothelial adhesion molecules (proteins fundamental for mediating the interaction and migration of leukocytes from the blood into tissues during inflammation). In practice, the endothelium (the tissue lining the inside of blood vessels) is made more "sticky" and permissive to the passage of immune cells: this is one of the steps by which local inflammation becomes an organized process, populated by effector cells (Zhang et al., 2011) [46].

Simultaneously, the macrophage produces chemokines that guide the recruitment of other innate immune cells (monocytes, neutrophils) and contribute to maintaining and amplifying the inflammatory microenvironment. This is the moment when the response ceases to be the isolated behavior of a single cell and becomes an event extended to the entire tissue: more signals produced by the macrophage, more cells recruited, more mediators released, with a circuit effect that tends to be self-sustaining (Duque and Descoteaux, 2014) [47].

Finally, the transition toward adaptive immunity (the second line of defense of the immune system, which develops after contact with specific pathogens) is facilitated when antigen-presenting cells (substances recognized by the immune system), such as macrophages, increase the production of cytokines capable of directing the activation of T lymphocytes; in this context, mTOR is described as a regulator of immune cell function and their ability to stimulate lymphocytes, acting as a bridge between immunometabolism and the lymphocyte response (Sukhbaatar et al., 2016) [48].