Lung inflammatory diseases are tightly linked with aberrant SREBP activity in respiratory epithelial cells and with dyslipidemia. SREBP2-mediated lipid metabolism promotion correlates with (COVID-19)-induced cytokine storm (16). The increased expression of SREBP-1 after respiratory virus infection (i.e., SARS-CoV), regulates the accumulation of lipid droplets, cell organelles involved in the amplification of inflammatory mediators’ production (17). Dysregulation of free fatty acids, cholesterol, and ceramides homeostasis in the lungs is associated with the pathogenesis and the progression of chronic obstructive pulmonary disease (COPD). Cigarette smoke–induced alveolar accumulation of lipids is an important event that triggers inflammation in alveolar macrophages (18). Several studies have demonstrated that ceramides, the central hub of sphingolipids metabolism, are elevated in COPD and contribute to chronic inflammation (19). Idiopathic pulmonary fibrosis (IPF) is a progressive lung disease characterized by inflammation and fibrosis. As recently reviewed by Suryadevara et al. (20), bioactive lipid mediators derived from fatty acids, glycerolipids, phospholipids, and sphingolipids exhibit pro- or anti-fibrotic/inflammatory effects in lung tissues of IPF patients. Acute Respiratory Distress Syndrome (ARDS) is a severe form of clinical acute lung injury (ALI), both sharing severe inflammatory conditions. Phospholipase A2 enzymes (sPLA2s), that generate free fatty acids and lysophospholipids from glycerophospholipids, are upregulated in different cell types in the ALI/ARDS lung compartment. Significant evidence demonstrates that in these pathological settings, PLA2s are functionally important as regulators of inflammatory signaling (21). Caused by several factors, pneumonia is the inflammation localized to the terminal airways, the alveoli, and the interstitium of the lungs. A pilot study conducted in 2017 during community-acquired pneumonia (22), reported that monounsaturated and polyunsaturated fatty acid accumulation elicits an inflammatory response during pneumonia progression. Cystic Fibrosis (CF) is an inherited recessive disease. CF is caused by mutations in the CF transmembrane conductance regulator gene, with most of the mortality given by the lung dysfunction (23), being inflammation an independent risk factor for disease progression. We previously demonstrated that CF bronchial epithelial cells accumulate the sphingolipid ceramide (24), which contributes to CF airways inflammation (25), as well as glycerophospholipids and lyso-glycerophospholipids, the latest considered pro-inflammatory molecules (26), cholesterols, cholesterol esters and triacylglycerols (27), compared to normal cells.

Senescence is a stress-response process characterized by the irreversible arrest of the cell cycle following DNA damage. Senescent cells develop a senescence-associated secretory phenotype (SASP) to maintain their status and to communicate within their microenvironment. SASP factors, both released as soluble molecules or within extracellular vesicles (EVs), have strong biological activities on neighboring cells, including the promotion of inflammation. Indeed, the study of SASP in all in vitro generated models demonstrated the presence of the proinflammatory interleukin-6 (IL-6), CXC chemokine ligand 8 (IL-8), and monocyte chemoattractant protein 1 (28). Even if senescence allows the maintaining of tissue homeostasis, an irreversible proliferation arrest might limit the regenerative capacity of the tissue, and the presence of a pro-inflammatory milieu might sustain the onset of pathological condition and age-related disorders. In pulmonary diseases, accelerated or premature aging emerged to be significant. The oxidative stress caused by cigarette smoke induces senescence in alveolar epithelial cells (AEC) and endothelial cells that accumulate in COPD lungs, while in IPF the shortening of the telomeres might be responsible for the senescent status of fibroblasts and AEC (29). Of note, AEC might be driven to senescence also after the serine/threonine kinase mTOR hyperactivation (30). The constitutive activation of mTOR in the lung epithelial cells of transgenic mice results in weakness tight junctions’ (TJ), that is a sign of injury in the lung epithelium. In parallel, also epithelial to mesenchymal transition (EMT) was enhanced, ultimately contributing to lung fibrosis. In this study, however, senescence was not observed, but the authors suggested that this might be due to differences in the timing of mTOR activation compared to previous studies (31). Moreover, the peculiar contribution of the AEC to local inflammation might be ascribed also to the release of EVs. Interestingly, it was recently demonstrated that AEC released EVs that differs on their miRNA content, depending on the apical or basolateral origin site of their release. EVs released from the apical side of epithelial cells are enriched in miRNAs enhancing the mTOR signaling pathways, probably being responsible for the TJ vulnerability and for the EMT, which can cause lung injury (32). Of note, senescence and lipid metabolism share many regulatory proteins, among which mTOR has a crucial role (33). Senescent cells have increased lipids uptake and accumulation in lipids droplets (34) and it has recently arisen that the regulation of specific lipid species plays a critical role in senescence contributing to the chronic inflammation associated with SASP (35). For example, the prostaglandin E2 (PGE₂), which is synthesized from the arachidonic acid, is expressed at higher levels in COPD fibroblasts compared to healthy controls and is responsible for the autocrine and paracrine spreading of senescence in neighboring cells, as well as of the induction of inflammatory response through the up-regulation of its receptors EP2 and EP4 in a cyclooxygenase 2 -dependent reactive oxygen species response (36). Notably, PGE₂ is released also by epithelial cells to relax airways smooth muscle cells, indicating that the roles of lipid molecules in the crosstalk between cells in the lung environment is complex and needs further investigation (37). Additionally, the signaling lipid leukotrienes, that are SASP components, are secreted by senescent fibroblasts and contribute to worsen IPF promoting fibrosis in lung (38). Finally, lysophosphatidylcholines (LPCs) have SASP activity, being expressed at high levels in senescent fibroblasts and being capable to induce the secretion of IL-8 and IL-6 (35).

In view of the deep interplay between dysregulated lipid metabolism and chronic inflammatory condition that characterizes several lung diseases, restoring lipid homeostasis could represent a novel therapeutic approach. Herein, there are examples of preclinical studies focusing on the potential anti-inflammatory efficacy of compounds targeting lipid pathway in pulmonary diseases (Table 1). Treatment with A922500, a pharmacological inhibitor of acyl-CoA:diacylglycerol acyltransferase-1, inhibits lipid droplets biogenesis triggered by SARS-CoV-2 infection in A549 human epithelial cells and in primary human monocytes. Thereon, the synthesis of pro-inflammatory lipid production and cytokinesis is downregulated (58). Interestingly, we were able to also reduce glycerol- and cholesterol-based lipids, to promote fatty acids oxidation, to reduce inflammation in in vitro model of CF by reducing ceramide accumulation with Myriocin, the inhibitor of the rate-limiting step in sphingolipid biosynthesis (27). Finally, in an in vivo model, the bleomycin-induced lung fibrosis and inflammation in rats was attenuated by AK106-001616 that inhibits the cytosolic PLA2 responsible for the generation of pro-inflammatory eicosanoids (59).

Lymphangioleiomyomatosis (LAM) cells have a metabolic signature of increased fatty acid uptake and synthesis, a characteristic in common with cancer cells (60), making altered lipid species in LAM both disease relevant biomarkers and potential therapeutic targets.

For instance, in the perspective of a LAM immunotherapy, it was demonstrated that LAM lung sections of both patients and Tsc2^–/– mice have a strong positivity to the ganglioside D3 (GD3), whose expression is a characteristic that LAM cells maintain also after multiple passages in vitro (61). Interestingly, LAM patients have lower anti-GD3 antibodies in their serum compared to healthy controls and the in vitro treatment of LAM cells with commercial anti-GD3 antibodies plus human complement induce death in the 12–42% of the population. This might suggest an approach similar to melanoma, in which the targeting of GD3 by antibodies reduces tumor growth through the activation of the natural killer T cells (62).

Even if GD3 expression in LAM cells and lipids accumulation in the renal angiomyolipomas of LAM patients can be considered a consequence of mTOR deregulation (63), emerging researches demonstrate the existence of uncovered TSC1/2-dependent-mTORC1-independent regulation of lipids metabolism that may contribute to the LAM pathogenesis and progression.

In the first systematic study of the LAM lipidome, high levels of four LPCs (C16:0, C18:0, C18:1, and C20:4) were found in the plasma of women with LAM compared with healthy controls, suggesting novel potential biomarkers (64). LPCs are bioactive lipids generated by the activity of phospholipase A (PLA) on the precursor PC (9) and preclinical models of lung cancer showed that those molecules might have a direct role in tumor angiogenesis (65), a condition that, together with lymphangiogenesis, is characteristic also in pulmonary and extrapulmonary LAM (49). Interestingly, the increase of LPCs were also observed in Tsc2^–/– mouse embryonic fibroblasts (MEFs), consistently with the hypothesis that tuberin loss might enhance several phospholipid and neutral lipid species. The treatment with the mTOR inhibitors rapamycin or torin1, or the down-regulation of SREBP1 does not suppress LPCs, while the down-regulation of specific PLAs decreases selectively the proliferation of Tsc2^–/– MEFs compared with Tsc2^+/+ MEFs (58). Among PLAs, the accumulation of the PLA2G16 isoform was observed in the lung nodules and in the renal angiomyolipomas of LAM patients, and it was demonstrated that the presence of tuberin negatively regulates PLA2G16 overexpression both in vitro and in vivo, affecting also the production of prostaglandins, that are critical mediators of chronic inflammation and cancer progression. In the same study, Tsc2^–/– MEFs treated with rapamycin or torin1 did not change the expression of PLA2G16, while the PLA inhibitor methyl arachidonyl fluorophosphonate (MAFP) reduced the growth and induced apoptosis on tuberin-deficient cells derived from LAM patient. Interestingly, the tuberin induced-expression in these cells prevents the effects of MAFP treatment (66).

Since mTORC1 hyperactivation causes the reduction of autophagy and induces a metabolic reprogramming of tuberin-deficient cells, a novel therapeutic approach was proposed in a recent study which demonstrates that Tsc2^–/– MEFs are sensitive to the treatment with rintaserin in combination with chloroquine, while the impact of those two drugs on Tsc2^+/+ MEFs is minimal (67). Rintaserin is an inhibitor of the diacylglycerol kinase alpha (DGKA), that induces the formation of the phosphatidic acid from diacylglycerol, ultimately regulating the homeostasis of cell membranes (68). Tsc2^–/– MEFs increased the expression and the activity of DGKA, with a consequent 5-fold higher accumulation of phosphatidic acid compared to Tsc2^+/+ MEFs and a higher capability to uptake nutrients from the extracellular space through macropinocytosis. The treatment with rintaserin blocks macropinocytosis and leads to the accumulation of diacylglycerol, reprogramming the phospholipid metabolism by reducing the storage of lipids in droplets and enhancing the synthesis of phospholipids (67). Interestingly, high levels of DGKA, DGKD, DGKQ, and DGKZ were found in the angiomyolipomas of TSC patients, that often develop LAM as pulmonary manifestation (69). In vivo, the injection of tuberin-deficient cells with the downregulation of DGKA does not cause the enlargement of lung alveoli observed in preclinical models of LAM (67, 70).

Taken together, these studies indicate that targeting of key metabolic pathways in lipids synthesis might be novel therapeutic approaches for LAM.

Moreover, lipids can be relevant biomarkers to understand LAM pathogenesis and progression. Indeed, the first evaluation of the LAM serum metabolome showed that there are differences in the metabolic profile when LAM patients were stratified according to menopausal status, lung function, disease burden and disease activity (15). These metabolic abnormalities involved almost exclusively sphingolipids, phospholipids and acylcarnitine fatty acids, reflecting metabolic processes downstream of mTOR. Remarkably, the observation that FEV₁ loss, used as measure of airflow limitation, was related to sphingolipid and acylcarnitine fatty acid metabolism, might reflect the extent of lung parenchyma disruption, resulting in the loss of elastic recoil. In the same way, the airflow obstruction in patients with COPD is associated with glycerophospholipids and sphingolipids metabolites, suggesting a common mechanism involving mTOR hyperactivation (71). Moreover, uncovered changes in glycerophospholipids were observed in patients with sporadic LAM compared with LAM/TSC ones after rapamycin treatment (15).

Finally, the altered lipid metabolism in LAM cells can be exploited to identify novel metabolic imaging biomarkers for the disease. Indeed, the previously reported enhancing in PC levels and neutral lipids in LAM cells (64) allowed to test the in vivo uptake of [¹⁸F]fluorocholine (FCH) and [¹⁸F]fluoroacetate (FACE) to detect tuberin-deficient cells in solid tumors and to monitor the response to rapamycin through dynamic PET (72). [¹⁸F]FCH uptake, but not [¹⁸F]FACE uptake in tuberin-deficient xenografts was rapamycin-sensitive and in vitro study on tuberin-deficient cells indicated that this difference might be due to an accumulation of these two compounds in two different cells compartment. In fact, [¹⁸F]FCH is mainly incorporated into lipids, while [¹⁸F]FACE enters into mitochondria and can be used as marker of mitochondrial activity, which is not suppressed by rapamycin, confirming a specific metabolic feature of tuberin-deficiency in LAM cells (73).