Normally, microbial infection triggers macrophage polarization and the inflammatory response by recognizing PAMPs. Common infectious diseases are caused by bacterial, viral, and parasitic infections.

During bacterial infection, pattern recognition receptors (PRRs) are anchored on the surface of macrophages to recognize PAMPs of bacteria, such as toll-like receptor (TLR) 2, TLR4, TLR5, and TLR9, which are specific for lipopeptides, lipopolysaccharide (LPS), flagellin, and the low-methylated DNA sugar backbone, respectively (7, 8) ( Figure 2A ). PAMPs induce macrophages to express high-level costimulatory molecules, such as cluster of differentiation (CD) 40, CD80, CD86, and major histocompatibility complex-II (MHC-II), to perform antigen presentation (2) ( Figure 2A ). These activated macrophages also produce and secrete proinflammatory cytokines to promote the Th1 immune response (2) ( Figure 2A ). Some bacteria could decrease the level of M1 polarization for bacterial survival. Mycobacterium tuberculosis (M. tuberculosis) inhibits pro-inflammation and induces M2 polarization (9). In addition, we found that Mobile colistin resistance (mcr)-1/3 resistance enzyme-modified LPS decreases NF-κB activation and induces weaker macrophage response than native LPS in vitro. mcr-1/3 positive bacteria induce lower level macrophage inflammation and M1 polarization, further causing higher mortality in mice (10). Wu reported that promoting M2 polarization and decreasing M1 polarization ameliorate bacterium-induced acute lung injury (11).

Viral infection triggers a pro-inflammatory response that is similar to bacterial infection. TLR 3, 7, and 9 identify CpG motifs-dsRNA, ssRNA, and unmethylated DNA of viruses, respectively (12) ( Figure 2A ). Jiang found that simian immunodeficiency virus (SIV) induces M2 macrophage polarization (13). Hepatitis B virus (HBV) decreases the levels of IL-1β and IL-6 secretion by M1 macrophages, and increases the IL-10 secretion by M2 macrophages (14).

Parasite infection causes M2 polarization. TLR2, 7, 9, and 11 identify lipoprotein, CpG DNA, ssRNA, and profilin of parasites (15) ( Figure 2A ). Helminth infection induces a Th2 immune response to produce high levels of Arg-1, TGF-β, vascular endothelial growth factor (VEGF), YM1, and insulin-like growth factor-1 (IGF-1) to inhibit parasitic confinement and clearance (16) ( Figure 2A ). Protozoans induce anti-inflammatory responses to antagonize macrophages for protozoan survival (17).

Therefore, enhancing the proinflammatory response in the microenvironment can enhance bacterial and viral clearance, although this may cause more severe tissue damage. M2 anti-inflammation increases parasitic clearance. Those evidences suggest that regulating the balance of M1 and M2 activation in infectious disease could be a potential therapeutic intervention. It is of great significance to explore the molecular mechanism of the polarization balance between M1 and M2 macrophages to control microbial infection.

Abnormal macrophage polarization also affects noninfectious diseases. Aberrant M1 activation induces autoimmune disease and chronic inflammatory diseases. M2 overactivation is involved in fibrosis and tumors.

Glycolysis metabolizes glucose to produce pyruvate, lactate, and ATP (4). Although glycolysis only produces a small amount of ATP, it proceeds rapidly to provide energy for requiring rapid production of physiological activities (4). In response to LPS or PAMPs, the metabolism of macrophages switches toward anaerobic glycolysis, but IL-4 almost does not affect glycolysis (22). Hypoxia-inducible factor-1α (HIF-1α) is a transcription factor. In response to LPS stimulation, HIF-1α is upregulated and promotes glycolytic enzyme and proinflammatory cytokine expression (23, 24) ( Figure 3A ). Inhibition of macrophage glycolysis can reduce M1 polarization by facilitating the proteasome degradation of HIF-1α (25, 26). Pyruvate kinase (PK) is the rate-limiting enzyme of glycolysis. Enhancing glycolysis by increasing PKM2 phosphorylation or decreasing PKM2 ubiquitination can promote polarization of M1 macrophages, following an increased proinflammatory response to aggravate liver fibrosis (27) ( Figure 3A ). Annexin A5 promotes hepatic macrophage shifting from M1 to M2 by inhibiting PKM2 phosphorylation (28). Lactate (a product of anaerobic glycolysis and glutaminolysis)-derived acetylation in histone lysine residues directly induces transcription of M2-like genes such as Arg-1 at the late phase of M1 polarization with bacterial infection (29) ( Figure 3B ). Lactate inhibits the levels of M1 markers and the proportion of M1 macrophages in breast cancer tissues (30).

These results show that enhancing glycolysis can promote M1 polarization. Notably, HIF-1α and the rate-limiting enzymes are crucial in increasing M1 macrophage polarization ( Figure 3A ). Meanwhile, lactate inhibits the M1 marker and promotes M2 polarization ( Figure 3B ). This suggests that glycolysis is a target to regulate macrophage polarization.

As a branch of glycolysis, the PPP is a major source of NADPH and is required for ribonucleotide synthesis (31). The PPP is significantly upregulated in M1 macrophages, suggesting that the PPP plays an important role in proinflammatory polarization (3, 32, 33). Glucose-6-phosphate (G6P, an intermediate product of glycolysis) is oxidized to produce NADPH, which is the foundation of macrophage-killing activity and is involved in fatty acid biosynthesis and antioxidant defense mechanisms (34). Inhibiting PPP by the knockdown of 6-phosphogluconate dehydrogenase (PGD) can decrease macrophage-producing pro-inflammatory cytokines with LPS treatment (35) ( Figure 3A ). In addition, the carbohydrate kinase-like protein CARKL, which catalyzes an orphan reaction in PPP, can reprogram M1- and M2- polarization metabolism (36) ( Figures 3A, B ). CARKL reduces the production of proinflammatory cytokines (M1 phenotype) (36). CARKL gene overexpression in macrophages significantly inhibits intracellular ROS production and is more sensitive to M2 stimulation (37).

Those evidences suggest that the PPP is also involved in increasing proinflammatory cytokine production and M1 macrophage polarization.

Pyruvate in the cytoplasm is transported to the mitochondria and then used in the TCA cycle (also known as the Krebs cycle) (4). ( Figure 3A ). In M1 macrophages, isocitrate dehydrogenase (IDH, which catalyzes the conversion of isocitrate to α-ketoglutarate) breaks the TCA cycle (38), resulting in the accumulation of the intermediate products citrate, itaconate, and succinate, which participate in pro-inflammation (34, 38–44). HBV decreases the incomplete TCA cycle and drives M2 polarization (45). After DCA (an agonist of TCA) treatment, enhancing incomplete TCA can significantly reduce M2 marker expression (45). Inhibition of citrate synthase (CS)/pyruvate dehydrogenase complex hyperacetylation (PDH) in macrophages increases M2-like polarization (45). ( Figure 3A ). Citrate is exported from mitochondria and catabolized to produce acetyl-CoA to enhance glycolytic gene transcription by histone acetylation, which drives M1 activation (39) and inflammatory cytokine, nitric oxide (NO) and reactive oxygen species (ROS) production (40) ( Figure 3A ). Inhibition of the citrate carrier (CIC, which exports citrate from mitochondria) ensures oxidative metabolic flux in the TCA cycle (39). CIC inhibition decreases IL-1β and Nos 2 gene (M1 marker) expression, increases IL-10 and Arg-1 gene (M2 marker) expression, and promotes the switch from M1 to M2 (39). The accumulation of itaconate has been recognized as characteristic of inflammation and the killing activity of M1 macrophages (41). In M1 macrophages, itaconate is produced in mitochondria through aconitase decarboxylase 1 (ACOD1) (42) ( Figure 3A ). It has been shown that itaconate enhances innate antibacterial immunity (43). In addition, itaconate decreases IL-1β, IL-6, IL-12, and IL-18 levels by inhibiting succinate dehydrogenase-mediated oxidation of succinate after LPS treatment (44). Succinate has been reported to be involved in M1 activation after LPS stimuli (40). Succinate can stabilize HIF-1α and drive IL-1β secretion by promoting glycolysis (23) ( Figure 3A ).

Therefore, intact TCA can completely metabolize pyruvate, enhance oxidative phosphorylation, and promote macrophages to polarize into M2. Interrupted TCA can increase the pro-inflammatory response. These results suggest that the polarization and biological activity of macrophages can be regulated by affecting the integrity of the TCA cycle.

Nonlipid precursors (e.g., glucose-derived citrate) are transported to the cytosol and synthesized as Ac-CoA (the raw material for fatty acid synthesis) (46). When fatty acids are broken down, they produce Ac-CoA, FADH2, and NADH in the mitochondrial matrix (46). Fatty acid metabolism also provides energy for supporting the macrophage inflammatory response and polarization (47). FAS plays a vital role in energy source and prostaglandin biosynthesis in M1 macrophages (48), and malonyl-CoA promotes the pro-inflammatory response (49, 50) ( Figure 3A ). FAO (also known as β oxidation) in M2 macrophages can drive mitochondrial oxidative phosphorylation (OXPHOS) to produce energy. Inhibition of FAO prevents M2 polarization (51) ( Figure 3B ). However, other studies have shown that FAO is not necessary for M2 polarization upon IL-4 stimulation, and FAO may play a role in M1 activity (52–54), suggesting that the regulation of macrophage polarization by fatty acid metabolism may be more complicated than previously thought. Other functions of FAO are receiving increasing attention. Chandra reported that when FAO is inhibited in macrophages, the bacteria cannot survive, and the tissue bacterial load of mice decreases (55). Enhancing FAO exacerbates LPS-induced sepsis (56).

Consequently, fatty acid metabolism affects macrophage polarization. However, there are many contradictions in the literature. How fatty acid metabolism regulates polarization still needs to be further explored.

Amino acid metabolism usually refers to the synthesis of amino acids and their decomposition (4). Amino acid and their metabolic producers are important for the immune response (34). A deficiency of amino acid may affect the migration, division, and maturation of immune cells (34). Deficiency of glutamine increases M1-specific marker genes and decreases M2-specific markers (38) ( Figures 3A, B ). α-Ketoglutarate (α-KG) produced by glutamate decomposition promotes M2 polarization but inhibits M1 polarization (57) ( Figure 3B ). The limiting enzyme of tryptophan metabolism is indoleamine 2,3-dioxygenase (IDO) (34). Macrophages polarize into M2 with overexpression of IDO, and silencing IDO macrophages polarize into M1 (58) ( Figures 3A, B ). Two L-arginine catalytic enzymes, inducible nitric oxide synthase (iNOS) and Arg-1, are M1 and M2 macrophage markers, respectively. Arginine metabolism plays an essential role in macrophage inflammatory function (59). In response to proinflammatory stimuli (e.g., LPS, IFN-γ, and TNF), iNOS catabolizes arginine to produce NO. NO promotes M1 polarization and prevents M1 to M2 polarization (60, 61) ( Figure 3A ). In M2 macrophages, Arg-1 catabolizes arginine to produce ornithine and urea, and ornithine is catalyzed to produce spermidine. Furthermore, TCA and OXPHOS are promoted, which induces macrophage polarization to M2 (62) ( Figure 3B ). However, the two catabolic pathways of arginine are not independent of each other. In tumors, arginine is catabolized by both iNOS and Arg-1, which suggests that targeting arginine is a new strategy to regulate macrophage polarization (63).

These results indicate that amino acids, related enzymes, and their production can change the polarization of macrophages. Therefore, they can be used as targets to explore the mechanism of macrophage polarization.

As we have shown before, glycolysis, amino acid metabolism, the TCA cycle, and FAO regulate M1 polarization. Therefore, we focused on these metabolic pathways to explore the signaling pathways affecting M1 metabolism.

In glycolysis, there is a critical transcription factor, HIF-1α, which transcribes the key glycolysis kinases hexokinase 1/2 (HK1/2), glucose transporters 1/3 (GLUT1/3), lactate dehydrogenase (LDHA) and PKM (64). Therefore, controlling the expression and activation of HIF-1α is a strategy for affecting glycolysis. In breast cancer cells, responses to ROS, PI3K and AKT are activated and further promote HIF-1α transcription of HK2, thereby enhancing macrophage glycolysis and enhancing tumor survival (65). A similar process also exists in macrophages. In macrophages of mice with E. coli-induced sepsis, triggering receptors expressed on myeloid cells-1 (TREM-1), PI3K, AKT, and mTOR are activated in a cascade (66). Then, HIF-1α translocates into the nucleus and initiates the transcription of GLUT1, HK2, and LDHA to enhance glycolysis, control the activation of NOD-like receptor thermal protein domain associated protein 3, and affect the secretion of inflammatory factors in macrophages (66) ( Figure 4A ). In addition, the PI3K-AKT-mTOR pathway is also activated to regulate HIF-1α by growth factors (64). MEK/ERK, downstream of growth factors and LPS, can also restrict glycolysis through HIF-1α (67, 68) ( Figure 4A ). With LPS induction, another classical inflammatory pathway, nuclear factor kappa-B (NF-κB), is also involved in regulating the upregulation of glycolysis (69). In macrophages, inhibition of NF-κB activation damages glycolysis activity ( Figure 4A ). As a transcriptional regulator, NF-κB can participate in the transcriptional regulation of HK2 in CD8⁺ T cells, which is also a way for NF-κB to inhibit glycolysis (70). This suggests that PI3K-AKT-mTOR, MEK/ERK, and NF-κB promote M1 polarization by enhancing glycolysis.

In macrophages, signal transducer and activator of transcription (STAT) regulates L-arginine metabolism. Under IFN-γ treatment, Janus kinase (JAK)/STAT1 is phosphorylated to transcript Nos2 mRNA and express iNOS for L-arginine metabolism (71, 72) ( Figure 4A ). Furthermore, in the gastric cancer microenvironment, the SLC2A3-STAT3-SLC2A3 feedback loop promotes macrophage phenotype transition by phosphorylating downstream glycolytic targeting genes (73). This suggests that the STAT family plays an important role in the macrophage metabolism regulatory mechanism. When bacteria infect macrophages, immune-responsive gene 1 (IRG1), which is needed for the decarboxylation of cis-aconitate to form itaconate in the TCA cycle, is upregulated, therefore inducing itaconate accumulation to damage microbes (74, 75) ( Figure 4A ). However, prolonged excessive accumulation of itaconate leads to decreased sensitivity of M1 macrophages (75). In addition, IRG1 encodes an emerging IFN regulatory protein (IRP) with antimicrobial activity (74), and both JAK/STAT and NF-κB are upstream of IRG1 ( Figure 4A ). In a zebrafish bacterial infection model, the glucocorticoid receptors and JAK/STAT signaling pathways cooperatively regulate IRG1 expression (76). Therefore, IRG may be used as new targets to explore new antibacterial mechanisms and develop new antibacterial drugs in macrophages. Those evidences indicate JAK/STAT, which is related to the TCA cycle, increases M1 polarization. NF-κB affects the TCA cycle to promote M1 polarization.

In response to LPS stimuli, the Notch pathway is activated to initiate transcription of the Nos2 and pyruvate dehydrogenase phosphatase 1 (Pdp1) genes to induce concurrent glucose flux to the TCA cycle for M1 activation (77) ( Figure 4A ). In M1, a large amount of NO damages the mitochondrial respiratory chain (78). In cells lacking NO, citrate is significantly decreased, and itaconate and succinate are increased (78). These results indicate that NO is involved in regulating metabolic processes such as the TCA cycle in M1 cells and may participate in proinflammatory and antibacterial processes through metabolism ( Figure 4A ).

Different from LPS, CD40 signaling can regulate the conversion of glutamine and lactate to affect the NAD/NADH ratio by AMPK, thereby reprogramming the FAO-induced proinflammatory response and antitumorigenic activation (79) ( Figure 4A ).

Therefore, PI3K-AKT-mTOR, MEK/ERK, and NF-κB promote M1 polarization by enhancing glycolysis. JAK/STAT regulates arginine metabolism by iNOS expression to increase M1 polarization. NF-κB plays a proinflammatory role in the TCA cycle. Notch increases M1 polarization by the TCA cycle. CD40-AMPK reprograms the FAO-induced proinflammatory response. In summary, PI3K-AKT-mTOR, MEK/ERK, NF-κB, JAK/STAT, Notch, CD40-AMPK, and NO-related pathways are involved in the metabolic regulation of macrophage M1 polarization ( Figure 4A ).

After reviewing the literature, we found that FAS, FAO, and arginine metabolism play central roles in regulating M2 polarization. Therefore, we focused on exploring the signaling pathways that affect M2 metabolism.

PPARγ, a hallmark of M2 macrophages, is involved in lipid metabolism by upregulating CD36 (fatty acid transporter) to promote fatty acid uptake and FAO (80) ( Figure 4B ). PPARα is also involved in regulating macrophage metabolism. Li reported that in DSS-induced ulcerative colitis (UC), activation of AMPK-PPARα can promote β-oxidation, decrease de novo lipogenesis, and promote M2 polarization of macrophages (81) ( Figure 4B ). However, this study did not clearly show that AMPK-PPARα changes occurred in macrophages. However, based on previous reports that AMPK, PPARα, and FAO are upregulated under the induction of IL-4+IL-13 (51, 81), we speculated that a similar process would occur in macrophages. With IL-4 or MCSF induction, IRF4 is transcribed to promote FAO and OXPHOS by inducing glucose metabolism. The study also reported that STAT6 and mTORC2 are the pathways regulated upstream of IRF4 (82) ( Figure 4B ). In summary, PPARγ, AMPK-PPARα, and PI3K-AKT-mTORC2 increase M2 polarization by enhancing FAO.

STAT6 also regulates FAS. In response to IL-4, the anabolic transcription factor sterol regulatory element binding protein 1 (SREBP1) is activated by STAT6 to trigger de novo FAS and then promotes macrophage M2 activation (83) ( Figure 4B ). A study established that AKT-mTORC1 is also involved in FAS by activating ATP citratelyase (ACLY, catalyze citrate to produce acetyl-CoA) to promote acetyl-CoA derived acetylation of histone and M2 gene transcription (84) ( Figure 4B ). Under IL-4 stimuli, STAT6 is activated to produce Arg-1 for L-arginine metabolism (71) ( Figure 4B ). In summary, STAT6 and AKT-mTORC1 promote M2 polarization by FAS, and STAT6 also affects arginine metabolism for M2 polarization.

PPARγ, AMPK-PPARα, and PI3K-AKT-mTORC2 increase M2 polarization by regulating FAO. STAT6 promotes M2 polarization through FAO and arginine metabolism. AKT-mTORC1 increases M2 polarization by FAS. This suggests that these pathways are targets to control M2 macrophage polarization by affecting metabolism ( Figure 4B ).

It is worth noting that these metabolic and cell signaling pathways always crosstalk each other. Therefore, we speculated that there might be a pathway that could bring these complex regulatory mechanisms together.

The Hippo pathway and glycolysis regulate M1 polarization (22, 86–90). However, no literature directly suggests that Hippo regulates macrophage polarization by glycolysis. These studies have reported that MST1 and YAP affect glycolysis in tumors, hepatocytes, cardiomyocytes, and chondrocytes (93–96). Activation of MST1 inhibits glycolysis by reducing the expression of GLUT1 and C-MYC in tumors, while YAP has a positive correlation with C-MYC, GLUT3, HK2, and PFKB3 to increase glycolysis in hepatocytes (93, 94). In cardiomyocytes, YAP promotes glycolysis by upregulating GLUT1 (95). In chondrocytes, YAP also plays a positive role in glycolysis (96). Therefore, YAP and MST1/2 may play roles in glycolysis to regulate macrophage polarization. But that needs to be further proven using experimental proof.

That reports that HIF-1α, MEK/ERK, PI3K-AKT-mTOR, and NF-κB regulate M1 polarization by glycolysis (64–70). Many studies show the Hippo pathway is in crosstalk with these pathways (96, 98–100). Therefore, Hippo, which is in crosstalk with these pathways, may be involved in the regulation of macrophage glycolysis. Next, we will describe specifically how Hippo interacts with these pathways in other cells and macrophages.

First, we summarized the crosstalk between Hippo and these pathways in cells other than macrophages. An important molecule is HIF-1α in glycolysis. It has already been mentioned in many studies that MST1 and YAP are involved in the regulation of HIF-1α in granulocyte progenitor cells, chondrocytes, and hepatocellular carcinoma cells (96, 98, 99). In granulocyte progenitor cells, deficient of MST1 increases glycolysis by enhancing mTOR and HIF-1α (98). During chondrogenesis, YAP binds to HIF-1α to form a complex that promotes glycolysis and chondrogenic differentiation (96). In hepatocellular carcinoma cells, in response to hypoxia, nuclear YAP binds to HIF-1α to maintain the stability of HIF-1α and promote the transcription of glycolytic genes (99). In addition, MEK/ERK also regulates glycolysis (68, 69). YAP inhibits the MEK/ERK pathway in initial segment epithelial cells. MST1/2 at least partially regulates initial segment differentiation by repressing YAP (106). In Jurkat T cells, MEK/ERK can activate MST1, caspase-3/7, and caspase-8 to enhance MST1 proteolytic cleavage (107). In tumor cells, the AKT pathway directly inhibits MST2 activity by promoting MST2/Raf-1 interaction (108). In HEK293 cells, AKT can block MST1 activation by phosphorylating MST1 at S387 (109). Cinar reported that MST1 is cleaved by caspase, and mature MST1 and cleavage products are inhibited by AKT in human prostate cancer cells (110). In idiopathic pulmonary fibrosis, YAP and PI3K-AKT-mTOR interact with each other to regulate the proliferation, migration, and polarity of abnormal cells (111). Therefore, AKT inhibits the activation of MST1/2, and YAP interacts with PI3K-AKT-mTOR. In addition, deficiency of MST1 in granulocyte progenitor cells upregulates glycolysis and downregulates OXPHOS by mTOR and HIF-1α for cell proliferation and differentiation (98). Therefore, according to these evidences, we speculate that Hippo may play a role through crosstalk with HIF-1α and MEK/ERK, and PI3K-AKT-mTOR in promoting glycolysis and the inflammatory response of macrophages.

Next, we summarized the crosstalk between Hippo and these pathways in macrophages. Only a small amount of literature has shown that YAP interacts with NF-κB (100). In M1-type macrophages, YAP enhances the proinflammatory response by binding to NF-κB p65 and increasing the accumulation of p65 in the nucleus (100). Meanwhile, NF-κB plays a role in glycolysis (69). Therefore, we suggest that YAP crosstalk with NF-κB may play a role in glycolysis in macrophages. And it still needs further verification.

Therefore, it can be speculated that YAP affects glycolysis and M1 polarization by p65, HIF-1α, and PI3K-AKT-mTOR ( Figure 5 ). MST1 may regulate polarization by glycolysis, which is regulated by AKT ( Figure 5 ). However, MEK/ERK function is inconsistent with expectations in glycolysis and M1 polarization as an unknown mechanism. It is worth noting that the above is only our conjecture. However, the role of Hippo in the regulation of glycolysis in macrophages deserves further study.

The Hippo pathway and fatty acid metabolism regulate M2 polarization (49–54, 86–90). Deletion of YAP/TAZ in brown adipose tissue decreases the oxygen consumption rate and increases body fat content, suggesting that YAP/TAZ plays a role in lipid metabolism (97). They report that PPARγ and AMPK regulate M2 polarization by fatty acid metabolism (80, 81). But no study directly suggests that Hippo is crosstalk with PPARγ and AMPK in macrophages. Most of these literatures focused on epithelial stem cells, adipocytes, and tumor cells (101–104).

To be specific, activation of PPARγ inhibits WB-F344 (rat liver epithelial stem cell) proliferation. It induces cell cycle arrest by regulating the Hippo pathway (including promoting phosphorylation of MST1 and LATS2 and inhibiting nuclear translocation of YAP) (101). In turn, in adipocytes, YAP1 deficiency significantly increases the expression of PPARγ to affect the proliferation and differentiation of ovine preadipocytes (102). AMPK also modulates the activity of the Hippo pathway for energy homeostasis. During glucose starvation, AMPK is activated to phosphorylate YAP in tumor cells (103). Mo also found the same effect in response to energy stress (104). On the one hand, AMPK can phosphorylate LATS1/2 to inhibit YAP. On the other hand, AMPK can directly phosphorylate YAP Ser 94 to inhibit the binding of YAP to TEADs (104). Both PPARγ (a marker for M2) and AMPK-PPARα promote FAO. Therefore, we can speculate that, in the FAO-mediated M2 polarization, Hippo plays a role through crosstalk with PPARγ and AMPK ( Figure 5 ).

YAP is inhibited by the PI3K-AKT/β-catenin pathway with IL-4+IL-13 treatment in macrophages (86), and AKT is related to fatty acid metabolism in M2 macrophages (112). Unfortunately, we did not find more evidence about the regulation of fatty acid metabolism by Hippo crosstalk with AKT. The regulation of fatty acid metabolism by Hippo in macrophage polarization is only speculated by us based on the study of other cells, and whether there is such an association still needs to be further verified.

The Hippo pathway, TCA, and arginine metabolism regulate macrophage polarization (45, 59–63, 86–91). The Notch pathway promotes the accumulation of TCA intermediates and iNOS (74, 75), and an incomplete TCA cycle and iNOS promote macrophage polarization into M1 macrophages (77). In Kupffer cells, the Notch pathway increases Yap gene expression, and YAP can upregulate Notch ligand gene expression involved in macrophage polarization (90). Based on a previous summary, we found that YAP can also promote the TCA cycle through NF-κB in macrophages (74, 100) ( Figure 5 ). In addition, in hepatocellular carcinoma, there also is a positive feedback loop between YAP and the Notch pathway (105). These studies suggest that YAP may play a role in M1 polarization by the Notch pathway-mediated accumulation of TCA intermediates and iNOS ( Figure 5 ).

In summary, we can make the following assumptions: YAP promotes M1 polarization by glycolysis, incomplete TCA cycle, and arginine metabolism. The involved pathways may be NF-κB, HIF-1α, PI3K-AKT-mTOR, and Notch ( Figure 5 ). YAP inhibits M2 polarization by FAO, and the involved pathways may be the PPARγ-, AMPK-, and PI3K-AKT-related pathways ( Figure 5 ). In addition, MST1/2 plays a role in macrophage polarization by glycolysis and FAO, which may be regulated by AKT and PPARγ, respectively ( Figure 5 ).

Based on the above evidence, there is obvious crosstalk between the Hippo pathway and other pathways that regulate macrophage metabolism, which suggests that the Hippo pathway is a potential target for exploring the mechanism of macrophage metabolism and polarization.