Regulatory Mechanism of lncRNAs in M1/M2 Macrophages Polarization in the Diseases of Different Etiology

Precise expression and regulation of genes in the immune system is important for organisms to produce strong immunity towards pathogens and limit autoimmunity. In recent years, an increasing number of studies has shown that long noncoding RNAs (lncRNAs) are closely related to immune function and can participate in regulating immune responses by regulating immune cell differentiation, development, and function. As immune cells, the polarization response of macrophages (Mφs) plays an important role in immune function and inflammation. LncRNAs can regulate the phenotypic polarization of Mφs to M1 or M2 through various mechanisms; promote pro-inflammatory or anti-inflammatory effects; and participate in the pathogenesis of cancers, inflammatory diseases, infections, metabolic diseases, and autoimmune diseases. In addition, it is important to explore the regulatory mechanisms of lncRNAs on the dynamic transition between different Mφs phenotypes. Thus, the regulatory role of lncRNAs in the polarization of Mφs and their mechanism are discussed in this review.


INTRODUCTION
Macrophages (Mjs) are vital antigen-presenting cells with high heterogeneity and plasticity in the human immune system (1). Mjs can induce multiple polarization processes and exert different functions depending on the conditions (2). The local cytokine environment can lead to polarization of Mjs. Therefore, according to their expression of cell surface markers, the production of specific factors, and biological activities, Mjs are classified into the M1-like and M2-like types (3). M1 and M2 polarization is a remarkable characteristic of Mjs involved in numerous biological processes. M1 Mjs are known as classical Mjs and are activated by bacterial lipopolysaccharide or interferon (IFN)-g. To produce and secrete higher levels of proinflammatory cytokines, including tumor necrosis factor (TNF)-a, interleukin (IL)-6, IL-12, and cyclooxygenase-21. M1 Mjs have robust antimicrobial functions and inhibit tumor growth (4). In M2 Mjs, signal transducer and activator of transcription 6 (STAT6) is activated by IL-4 receptor a (IL-4Ra) and M2 Mjs are polarized by Th2 cytokine IL-4/IL-13. M2 Mjs have anti-inflammatory and strong phagocytic abilities to eliminate apoptotic cells and are useful for chronic infection treatment and wound healing (5). Moreover, according to the different stimuli received, M2 Mjs can be further divided into four different types: M2a, M2b, M2c, and M2d (6) (See Figure 1).
Precise regulation of Mjs polarization is a key step in controlling immune system function, clearing pathogens, and defending against external evil. The evidence accumulated in the past decade shows that lncRNAs are widely expressed and plays a key role in gene regulation. LncRNAs are a group of non-coding RNAs >200 nucleotides in length and composed of molecules transcribed by different types of RNA polymerase II (7). LncRNAs could regulate chromatin function, change the stability and translation of cytoplasmic mRNAs, and interfere with the signaling pathways. These functions influence gene expression in different biological and pathological environments, such as immunoreactive diseases (8). In addition, lncRNAs are responsible for the integrity of nuclear structure and can regulate the expression of nearby genes or genes in other parts of the cell by interacting with proteins, RNAs and DNAs (7). LncRNAs are a potential biomarker (9) and act as crucial regulators of Mjs polarization in response to intracellular or extracellular stimulation. various lncRNAs exhibit different functions in Mjs infiltration and polarization and regulate the secretion of inflammatory cytokines released by Mjs (10). Recent studies indicated that lncRNAs could participate in M1 or M2 polarization and exert multifunctional roles in disease development (11) (See Tables 1, 2).
Here, we review the current understanding of lncRNA functions in Mjs polarization. Using different disease models, we reviewed that different lncRNAs regulate the polarization of Mjs via different signaling pathways and act on different targets; we also explain the mechanism of these effects.

LNCRNAS REGULATE MFS POLARIZATION IN VARIOUS CANCERS
Tumor-associated macrophages (TAMs), which are specialized phagocytic cells, play an important role in the pathology and pathogenesis of cancers and are responsible for modulating the tumor microenvironment (TME) (95) (See Table 1). The microenvironment signals exposed to Mjs are the key factors that determine the phenotype of Mjs; these microenvironment signals selectively regulate their functions, including M1 and M2 polarization (96). At present, TAMs are mainly divided into two subtypes (See Figure 2): M1 TAMs of classical activation pathway and M2 TAMs of alternative activation pathway. M1 TAMs have significant anti-tumor effect. In the TME, it can identify tumor cells and kill tumor cells through the corresponding signaling pathway. According to the different pathways of action, it can be divided into direct killing mechanism and antibody-dependent cytotoxicity (ADCC) mechanism (97). M2 TAMs can significantly promote tumor growth. First of all, M2 TAMs secrete cytokines such as IL-6 and CXCL-8 into the TME and directly promotes the growth of tumor cells (98). Secondly, M2 TAMs can also block the inducible nitric oxide synthase (iNOS) pathway, reduce the synthesis of NO, accelerate the production of polyamines, and then promote the proliferation of tumor cells (99). M2 TAMs also promote angiogenesis of tumor cells by secreting vascular endothelial growth factor (VEGF) and IL-17, and can release matrix metalloproteinase (MMP)-2, MMP-9 and other substances to degrade extracellular matrix, promote vascular endothelial migration and induce angiogenesis (100,101). M1 and M2 TAMs existed in all stages of the tumor, mainly M1 in the early stage and M2 in the middle and late stages of the tumor. With the progression of tumor, M1 is gradually polarized to M2, and the increase number of M2 TAMs also indicates a poor prognosis. Clinical studies have confirmed that M1 TAMs polarize to M2 TAMs induced by proliferator-activated receptor (PPAR)-g. The increase in the proportion of M2 TAMs are closely related to tumor cell proliferation and tumor tissue vascular density (MVD) (102). A new study has shown that the inhibition of nuclear factorkappa B (NF-kB) activity and the increase of STAT3 activity in Mjs represent a tumor-mediated mechanism that triggers M1/M2 deviation (103). Moreover, lncRNAs are increasingly appreciated as important regulators of gene expression (104), and multiple studies have showed (105,106) that lncRNAs have a significant role in the regulation of TAM polarization in TME and functions principally as diagnostic and therapeutic biomarkers. Because lncRNAs are highly stable, specific and easy to detect, and can be expressed abnormally under different pathophysiological conditions (107); it can be used as a regulatory molecule of gene expression, acting as oncogenes or tumor suppressors, thus playing a role and directly involved in the occurrence of cancer, apoptosis, cell migration and invasion (108,109). As a potential biomarker for diagnosis and prediction, lncRNAs have a wide range of functions in the occurrence and development of various human diseases, especially in the occurrence and development of many cancers.

Respiratory System Cancers
Lung cancer is the most common respiratory cancer, with nonsmall cell lung cancer (NSCLC) accountings for approximately 85% of all lung cancers as the most common cancer worldwide. Increasing evidence has shown that Mjs and lncRNAs are involved in the development of inflammation and malignant transformation of tumor cells. LncRNAs are also involved in the pathogenesis of lung cancer. Li et al. (40) used lncRNAGNAS-AS1, to explore the role of GNAS-AS1 in the development of NSCLC. These experimental studies showed that lncRNAGNAS-AS1 was highly expressed in M2 Mjs and NSCLC cell lines, which was significantly higher than that in M1 Mjs, and promoted the polarization of M2 Mjs and progression of NSCLC by directly inhibiting miR4319 and increasing the level of N-terminalEF-

Urinary System Cancers
Prostate cancer (PC) is one of the most common tumors in the urinary system, and methods for its diagnosis and treatment have rapidly progressed in recent years; however, therapeutic effects remain unsatisfactory (110). Recently, the therapeutic value of lncRNAs in regulating Mjs in the TME has been widely explored, and a Chinese research team has found (44) that lncRNA LINC00467 may be beneficial for treating PC. Additionally, the lncRNA LINC00467 mainly exists in the cytoplasm, and its expression is upregulated in PC tissues and cells to enhance polarization of the M2 phenotype via the miR-494-3p/STAT3 axis. While M2 polarization can activate the STAT3 pathway, enhance the tumor enhancement of M2, and thus promote the growth and migration of PC cells (44). Compared with PC, bladder cancer is among the most serious malignant tumors worldwide. Although the incidence of muscleinvasive bladder cancer (MIBC) is low, the overall mortality rate is high. Chen et al. (15) found that a new lncRNA named as LNMAT1 through sequencing of five MIBC organizations. Its expression in MIBC tissues was significantly upregulated and positively correlated with the intensity of Mjs infiltration around the tumors. Further experiments showed that overexpression of LNMAT1 in vivo activated Mjs polarization by mediating heterogeneous nuclear Ribonucleoprotein L (hnRNPL) to promote the transcription of its target gene, CCL2. Furthermore, this lncRNA can induce the expression of CCL2 in bladder cancer cells, upregulate CCL2 to stimulate TAMs to secrete VEGF-C, participate in lymphogenesis, promote lymphatic metastasis and lymphangiogenesis, and accelerate the invasiveness of bladder cancer cells. These findings elaborate that LNMAT1 regulates the polarization of TAMs and mediates lymphatic metastasis, revealing a carcinogenic role for LNMAT1 in bladder cancer progression. Differential genes of lncRNA LINC01140 and fibroblast growth factor 9 (FGF9) were found in another study of an online microarray expression profile (GSE77952) of MIBC (39). By the analysis of polymerase chain reaction (PCR) and enzyme linked immunosorbent assay (ELISA), revealed a positive correlation between the expression of LINC01140 and FGF9. FGF9 and LINC01140 were highly expressed in M2 Mjs; after knocking down FGF9 and LINC01140, the viability and invasive ability of MIBC cells were significantly inhibited, leading to a decrease in the M2 markers IL-10 and Arg1 and weakened M2 polarization. In addition, by using the competing endogenous RNA mechanism, it was found that inhibition of miR-140-5p significantly reversed the expression and effects of LINC01140 and FGF9, thus affecting M2 polarization. Finally, LINC01140, miR-140-5p and FGF9 form a lncRNA-miRNA-mRNA mechanism axis, which influences M2 polarization by regulating the TME, thereby affecting the progression of MIBC.

Digestive System Cancers
Oral squamous cell carcinoma (OSCC) is a digestive system cancer that is also a common type of head and neck cancer. The occurrence and progress of OSCC are closely related to Mjs. M2 Mjs accelerate the progression of OSCC through a complex cross-talk mechanism and are regulated by lncRNAs. The NF-kB signaling plays an important role in tumor development. Ai et al. (42) found that deletion of lncRNA DCST1AS1 hindered the progression of OSCC by inhibiting the NF-kB signaling pathway. Its overexpression can cause M2 polarization to can promote the growth and differentiation of OSCC cells and accelerate the development of cancer by regulating the tumor immune environment. Another lncRNA, LBX1-AS1, is secreted by RBPJ-overexpression (RBPJ-OE) Mjs and belongs to the exocrine RNA. Its mechanism is mediated via the miR-182-5p/ FOXO3 pathway. As a competing endogenous RNA, LBX1-AS1 stimulates Mjs by acting on miR-182-5p and fork head box protein O3 (FOXO3), causing Mjs polarization and activating Mjs to regulate the development of tumor cells (49).
Gastric cancer (GC) is a common disease affecting the digestive system. In recent years, research on GC has mainly focused on the molecular mechanism and attempts to identify new therapeutic targets. LncRNAs and Mjs polarization were shown to be closely related to the development of GC. M1 polarization can kill tumor cells and inhibit tumor lymphangiogenesis and angiogenesis (111). Overexpression of lncRNA ANCR in Mjs significantly reduced the expression of the M1 polarization marker IL-1b and IL-6, thus inhibiting M1 polarization. Co-culture of Mjs with GC cells showing high expression of lncRNA ANCR resulted in significantly enhanced invasiveness and migration of GC cells. FOXO1 is the target gene of the lncRNA ANCR. On one hand, FOXO1 can promote Mjs to produce inflammatory factors and induce M1 polarization. In contrast, lncRNA ANCR accelerates the ubiquitination of FOXO1 and inhibits its expression. After downregulation of FOXO1, M1 polarization was also inhibited. Based on these results, overexpression of lncRNA ANCR can downregulate FOXO1, thus inhibiting M1 polarization and promoting the development of GC cells (12). Other lncRNA can affect GC progression by influencing M2 polarization. M2 Mjs can induce GC cells growth, and lncRNA HCG18 can positively regulate M2 polarization. Specifically, lncRNA HCG18 can bind to miR-875-3p and target krüppel-like factor4 (KLF4) regulation, as well as upregulate KLF4 by reducing the expression of miR-875-3p in Mjs, thereby promoting M2 polarization. M2 Mjs have antiinflammatory effects, secrete inflammatory cytokines to kill tumor cells, slow GC development, and play a role in immune surveillance (45).
Hepatocellular carcinoma (HCC) is a common tumor of the digestive system with a high incidence in Asia. M2 Mjs can promote tumor angiogenesis and induce cancer cell metastasis, but the mechanism of Mjs polarization is unclear. Four studies have explained the role of this mechanism in HCC development through the regulation of different lncRNAs on M2 polarization. It has been found (26) that M2 Mjs can promote human umbilical vein endothelial cell angiogenesis when M2-THP-1 cells are co-cultured with these cells. In mouse model of H22 tumor cells, lncRNA CENDE downregulated the expression of CD163, STAT-6, and VEGF by promoting M2 polarization, thus indirectly regulating tumor angiogenesis. In a study of SMCC-7721 cells, the supernatant of TAMs significantly promoted cell invasiveness. The expression of lncRNA GAS5 is downregulated in M2 Mjs and TAMs, which has been shown to be related to Mjs polarization in the progression of HCC. This lncRNA plays a regulatory role by regulating the expression of the target protein phosphatase and tensin homolog (PTEN) to inhibit M2 polarization of TAMs in SMCC-7721 cells (22). M2 polarization can also interact with the miR-372-3p/TLR4 axis, thus affecting the delivery of lncRNA PART1 into liver cancer tissues and cells via tumor-derived extracellular vesicles, inducing the upregulation of PART1 and Toll-like receptor 4 (TLR4) and downregulation of miR-372-3p, and promoting the proliferation and invasion of liver cancer cells. Additionally, miR-372-3p can bind to PART1 and negatively regulate TLR4, act on extracellular vesicles, and induce M2 polarization in HCC tissues, thus promoting the occurrence of HCC (23). The microRNA-15a-5p/CXCL17 axis is another regulatory pathway of M2 polarization in HCC. A previous study showed (24) that lncRNA DLX6-AS1, miR-15a-5p, and CXCL17 were highly expressed in HCC cells, and exosomes were isolated from HCC cells and co-cultured with M2 Mjs. The results showed that extracellular ubiquitin induced and intracellular HCC-exosomes transported DLX6-AS1 to Mjs, targeted binding to miR-15a-5p, and regulated CXCL17 expression to stimulate M2 polarization. In addition, Wnt ligands in tumor cells can stimulate M2 polarization, which can jointly accelerate the development and metastasis of HCC cells in vivo and in vitro. Furthermore, numerous studies have shown that different lncRNAs (lncRNA COX-2, lncRNA TUC339, and lncRNA Ma301) induce M1 and M2 polarization in the tumor microenvironment through different action targets (E-cadherin, caprin-1) and pathways (Akt/Erk1pathway), decrease the levels of iNOS and TNF-a in M1 Mjs, and increase the levels of Arg-1 and Fizz-1 in M2 Mjs. LncRNAs also promotes the production of various inflammatory factors, chemokines and exocrine, thus acting on the immune response of tumors and inhibiting pathological processes such as immune escape and invasion of HCC. These studies provides a new theoretical basis for treating HCC (46)(47)(48).
With the aging of the population and poor lifestyle, the global morbidity and mortality of colorectal cancer (CRC) are increasing each year. In epigenetic studies, lncRNAs were confirmed to be closely related to the pathogenesis of CRC. For example, lncRNA RP4 functions in CRC by binding to miR7-5p (112). Previous studies also showed that Mjs are involved in the pathogenesis of numerous cancers. Is there a regulatory relationship between lncRNAs and Mjs that interfere with the progression of CRC? Related studies found that lncRNAs HLA-F-AS1, NBR2, PTTG3P, and RPPH1 induce the polarization response of tumor-associated Mjs by acting on the corresponding targets or pathways, thus participating in the pathological development of CRC. Mechanistically, lncRNA MIR155HG (31) and lncRNA HLA-F-AS1 (27) can competitively bind with miR-650 and miR-375, reverse-regulate the expression of miR-650 and miR-375, and indirectly target annexin A2 (ANXA2) and profilin 1 (PFN1). Inhibiting of miR-650 and miR-375 promoted the expression of ANXA2 and PFN1 in CRC-derived extracellular vesicles. This mechanism further stimulated increased M2 Mjs marker expression, induced M2 Mjs phenotypic polarization, and accelerated CRC cells invasion. In contrast, lncRNA NBR2 can activate M1 polarization, inhibit M2 polarization, and regulate the polarization of TAMs to promote the release of inflammatory factors such as TNF-a, IL1-b, and IL-10, thus acting on the TME. However, after knockout of the lncRNA NBR2 gene, these effects were reversed (28). A similar process occurs with lncRNA PTTG3P (29). Under the action of hypoxia-inducible factor-1-a (HIF-1a), the hypoxia environment induces the overexpression of PTTG3P to promotes M2 polarization and stimulates tumor development. In addition, exosomes can mediate Mjs polarization. Liang et al. (30) found that the expression of lncRNA RPPH1 is upregulated in CRC, which is related to poor prognosis. In terms of the mechanism, RPPH1 binds to b-III tubulin and prevents its ubiquitination, which induces the transformation of epithelial-mesenchymal transition and accelerates the metastasis of cancer cells. Further studies showed that RPPH1 is closely related to exosomes, which can transfer into Mjs by exosomes to mediate M2 polarization and promote tumor growth and migration.

Reproductive System Cancers
Breast cancer occurs following uncontrolled proliferation of breast epithelial cells via the action of various carcinogenic factors and is considered as a reproductive system cancer. Modern studies showed that the lncRNA-miRNA-mRNA axis plays an important role in cancer progression. The lncRNA BCRT1/miR-1303/PTBP3 and lncRNA Xist/miR-101-3p/KLF6/C/EBPa can regulate the progression of breast cancer. In the lncRNA BCRT1/miR-1303/PTBP3 axis (18), polypyrimidine tract binding protein 3 (PTBP3) promotes breast cancer, and the expression of lncRNA BCRT1 in breast cancer tissues is significantly increased, which is related to the poor prognosis of patients. In the mechanism of action, lncRNA BCRT1 can bind to miR-1303 and act on PTBP3 to inhibit its expression. After overexpression, lncRNA BCRT1 significantly promotes M2 polarization mediated by exosomes. After M2 polarization, inflammatory factors such as TNF-a and IL-6 are produced, which promote cancer in tumor microenvironment; this effect is more obvious in hypoxic environments. Phenotypic transformation of TAMs mediates tumor immune escape. Zhao et al. (50) found that the expression of lncRNA Xist and C/EBPa was upregulated in M1 Mjs, whereas the expression of miR-101-3p was downregulated. LncRNA Xist knockout inhibited the expression of C/EBPa and KLF6 and induced the transformation from M1 to M2, thus promoting the proliferation and migration of breast cancer cells. Therefore, lncRNA Xist competes with miR-101-3p to regulate the expression of C/EBPa and KLF6 to mediate Mjs polarization, promote the expression of lncRNA Xist in M1 Mjs and inhibit the expression of miR-101-3p in M2 Mjs, thus inhibiting the progression of breast cancer. TAMs reprogram their specific functional phenotype in the tumor environment to promote cancer progression and metastasis. Zhou et al. (16) showed that lncRNA p21 was significantly upregulated in Mjs. Knockout of P21 led to mouse double minute 2 (MDM2)-induced proteasomedependent degradation of p53 and activated the NF-kB/STAT3 pathway, which promoted Mjs polarization to pro-inflammatory M1 and accelerated the progression of breast cancer. These results show that lncRNA p21 plays a key role in regulating the function of TAMs and can be a new therapeutic target for breast cancer.
In addition, after Zong (19) successfully induced M2 polarization by IL-4/IL-13 experimentally, the RNA level of lncRNA SNHG1 determined using by qRT-PCR was increased, indicating that this lncRNA is involved in M2 polarization.

Other Systemic Cancers
In addition to the above common cancers, Mjs polarization is involved in the pathological mechanisms of osteosarcoma and brain tumors. Osteosarcoma is common in children and adolescents. The lncRNA LOC100129620 can promote the proliferation and migration of osteosarcoma cells in vivo (33) by binding to miR-335-3p and regulating cyclin-dependent kinases 6 (CDK6), thus promoting angiogenesis and Mjs polarization. In contrast, LOC100129620 gene knockout decreased the infiltration and M2 TAMs polarization, and inhibited the development of osteosarcoma. In glioblastoma multiforme, Zhang found (38) that coagulation factor X (FX) was highly expressed and positively correlated with the concentration of TAMs. FX is secreted in the tumor microenvironment, has a strong chemotactic effect on Mjs, promotes the phosphorylation and activation of ERK1/2 and AKT in Mjs, and stimulates Mjs to M2 polarization. In addition, FX was identified as the target gene of miR-338-3p. The lncRNA CASC2c interacts with miR-338-3p and binds with FX to jointly inhibit the expression of FX and finally inhibit M2 polarization.
In summary, lncRNAs and Mjs play important role in multisystem cancers such as the respiratory, digestive, and reproductive systems. Through the analysis of the results of the above studies, it can be found that the mechanism of lncRNAs regulating Mjs polarization is mainly through the lncRNA-miRNA-mRNA axis. A large number of animal experiments and clinical trials have confirmed that M2 TAMs play an important role in tumor growth, invasion and metastasis, while M1 TAMs can inhibit tumor formation. It is also found that M2 polarization occurs more frequently and the mechanism of action is more complex than that of M1 in the occurrence and development of cancers. The presence of a large number of M2 Mjs in tumor tissue indicates a poor prognosis, which has a positive reference value for the clinical treatment of M2 Mjs. In the TME, TAMs produce a polarization response, secrete inflammatory factors and chemokines, regulate immune responses, and participate in the occurrence and development of cancers, which is important in cancer pathogenesis. Therefore, blocking Mjs from reaching tumor tissue, clearing Mjs and inhibiting the activation or repolarization of TAMs in TME has become a hot issue in the field of tumor immune research. Based on the understanding of the function of TAMs and the relationship between M1 and M2 Mjs, reversing the polarization of M1 to M2 Mjs is likely to be a key target for future tumor therapy. However, tumor treatment is a systematic project, and the effect of any single treatment is very limited. Regulating Mjs polarization through lncRNAs acting on different targets and pathways can provide new hope for the treatment of many types of cancers. Through the study of these lncRNAs in vivo and in vitro, TAMs can be polarized into proinflammatory or anti-inflammatory state, so as to develop new strategies to change the phenotype of Mjs. These cell therapies can be further combined with currently available chemotherapy and radiotherapy options to improve therapeutic effectiveness (113).

LNCRNAS REGULATE MFS POLARIZATION IN INFLAMMATORY DISEASES
In addition to resisting pathogens, Mjs play a key role in eliminating inflammation and repairing damaged tissues. The polarization response of Mjs is essential for immunity and dynamic balance. In the early stage of tissue injury, Mjs are activated to produce a pro-inflammatory effect, thereby removing cell fragments and pathogens (114). During this phase of transformation, Mjs change their phenotype and play a key role in inflammation regression and tissue repair (115).
Osteoarthritis (OA) is a joint degenerative disease that often occurs in the elderly, and affects joint activity and function. Currently, there is no efficient and standardized treatment. A large number of studies has shown that many inflammatory cells and inflammatory factors, such as IL-6, IL10, TNF-a, are involved in the pathogenesis of OA (116). For example, M1 Mjs secrete proinflammatory cytokines. Under the influence of the inflammatory microenvironment, M1 Mjs are activated and secrete high levels of inflammatory mediators, infiltrating synovial tissues and chondrocytes, leading to apoptosis and accelerating the development of OA, which is considered as a key factors regulating the pathogenesis of OA (117). The results of a previous study suggested (58) that osteopontin (OPN) affect M1 polarization. Overexpression of OPN can promote M1 Mjs polarization, produce pro-inflammatory cytokines and degrading enzymes, and act on chondrocytes to exert a cytotoxic effect. Further experiments showed that lncRNA XIST acts on OPN indirectly. It binds to miR-376c-5p competitively through the competing endogenous RNA mechanism, thus weakening the inhibition of miR-376c-5p on OPN, and then affects M1 polarization. In addition, Xist gene knockout affects M1 Mjs and chondrocyte apoptosis. The effect of the lncRNA XIST/miR-376c-5p/OPN axis on M1 Mjs and the inflammatory microenvironment may be a new mechanism of apoptosis in OA chondrocytes. A similar mechanism was found in the lncRNA IGHCg1 (90). This lncRNA is mainly located in the cytoplasm of Mjs and competitively binds to miR-6891-3p via the NF-kB signaling pathway, and regulates TLR4 to inhibit Mjs proliferation and inflammation, ultimately affecting OA development.
M1 and M2 polarization play important roles in lung inflammation and injury (118). Targeting Mjs polarization and affecting the inflammatory microenvironment may be useful for treating pneumonia. LncRNA GAS5 plays a key role in cell proliferation, invasion, apoptosis, and Mjs polarization (119). In a study of childhood pneumonia, Chi et al. (53), showed that overexpression of lncRNA GAS5 inhibits JAK2/STAT3 signal transduction, which promotes M1 polarization. The specific mechanism involves the miRNA-mRNA action axis: GAS5 combined with miR-455-5p can promote the expression of SOCS3, and then inhibit the JAK2/STAT3 pathway, thus promoting phenotypic polarization of Mjs from M2 to M1. These findings have potential for the development of therapeutic strategies against pneumonia. The mechanisms by which lncRNAs regulate M1 or M2 polarization in the pneumonia microenvironment are similar; for example, lncRNA p21 and lncRNA NKILA promote M2 polarization through NF-kB signaling pathway (54,64). Moreover, through the PI3K/AKT pathway, lncRNA Gm16410 reduces the expression of TNF-a in Mjs, inhibits inflammation, and stimulates polarization of Mjs induced by PM2.5, which participates in the activation of Mjs and process of inflammation (78). In addition, these effects have been described in numerous studies.
Mjs have also been widely reported to be involved in the pathogenesis of cardiovascular inflammatory diseases. Viral myocarditis is a type of heart inflammatory disease caused by viruses, that can lead to heart failure in severe cases. There is increasing evidence that Mjs polarization plays an important role in coxsackievirus B3-induced viral myocarditis. Zhang et al. (67) found that lncRNA AK085865 can increase the susceptibility to viral myocarditis by regulating the Mjs polarization. It specifically binds to interleukin enhancer-binding factor (ILF2) and promotes M2 Mj polarization by negatively regulating the ILF2-ILF3 complex-mediated miRNA-192 pathway. Overexpression of miRNA-192 can also effectively stimulate the transformation of myocardial infiltrating Mjs to the M2 phenotype. This reveals the key mechanism by which lncRNA AK085865 regulates Mjs polarization and act on myocarditis. LncRNA MEG3 participates in the regulation of M1/M2 Mjs via another pathway (68); it regulates tumor necrosis factor receptor-associated factor 6 (TRAF6) expression by binding to miR-233. Inhibitory MEG3 can downregulate the NF-kB pathway; inhibited the expression of iNOS, TNF-a, and CD86; reduced the expression of M1 Mjs; promoted the expression of Arg-1 and Fizz-1; increased the M2 polarization, and finally reduced cardiomyocyte injury.
In traumatic diseases, lncRNA GBP9 regulates Mjs polarization and reduces the inflammatory response after spinal cord injury through corresponding pathways. Studies have shown that after spinal cord injury, Mjs first differentiate into M1 Mjs under the stimulation of lipopolysaccharide and TNF-a, and TLRs on the cell surface are activated, which induces the recruitment of downstream protein myeloid differentiation factor 88, activates NF-kB, JAK-STAT, and other pathways; and promotes M1 to release TNF-a, CCL8, and other molecules (57,120).
Moreover, M2 Mjs surface receptors that bind to IL-4 and IL-13 can promote the phosphorylation of STAT6 and stimulate M2 Mjs. M2 Mjs can highly express IL-10, transforming growth factor (TGF)-b and neurotrophic factors, which can inhibit the nerve cell apoptosis and pro-inflammatory effects of M1, as well as promote nerve tissue repair (121). Zhou et al. found (57) that lncRNA GBP9 competitively binds to miR-34a to eliminate the inhibition of suppressor of cytokine signaling-3 (SOCS3), thus regulating STAT1/STAT6 signaling and Mjs polarization. After M1 polarization, M1 showed stronger phagocytosis and antigen presentation ability. In M1 Mjs, lncRNA GBP9 overexpression significantly decreased the expression of p-STAT1, SOCS3, IL-6, and IL-12, whereas in M2 Mjs, the same overexpression increased the concentration of SOCS3 and decreased the production of p-STAT6, IL-10, and TGF-b1, indicating that lncRNA GBP9 overexpression promoted M1 polarization. In contrast, after silencing lncRNA GBP9, the M2 polarization response was enhanced, nerve repair factor was secreted, and spinal cord injury was improved.  (92) showed that the expression of lncRNA SAF was significantly upregulated in HIV-1-infected Mjs. Downregulation of SAF can increase the activity of caspase-3/7 in virus-infected Mjs. This caspaseinduced apoptosis occurs only in HIV-1-infected Mjs, but not in other cells, resulting in a significant reduction of HIV-1 virus in Mjs. Therefore, targeting of lncRNA SAF is a potential method for inducing the death of Mjs infected by HIV-1.

LNCRNAS REGULATE MFS POLARIZATION IN METABOLIC DISEASES
Recent studies revealed that the mechanism by which lncRNAs regulate Mjs are important in the development of metabolic diseases, such as diabetes. Diabetes mellitus is related to insulin resistance. It can promote inflammatory responses and induce neurovascular complications through the proliferation, polarization, and dysfunction of Mjs (123). Das et al. (86) studied the role of lncRNA Dnm3os in diabetes. The results showed that in Mjs, a decrease in nucleolin and overexpression of Dnm3os changed the modification of intracellular histone, enhanced the promoter H3K9ac, recruited histone acetyltransferase, and targeted upregulated the genes related to inflammation and immune response such as NKx3-2 AP1, STAT, and IRF1, which promoted phagocytosis, whereas knockout of Dnm3os weakened these responses. In addition, nucleolin gene knockout increased the expression of proinflammatory factor IL-6 in Mjs, which is further enhanced by lncRNA Dnm3os-regulated Mjs. Diabetic peripheral neuropathy (DPN) is the main complication of type 2 diabetes mellitus. It has been reported that the expression of M1 Mjs in patients with DPN is significantly increased and mediates the secretion of inflammatory factors such as IL-1 and IL-6, which promotes the occurrence and development of DPN (4). Further studies (52) showed that in DPN, lncRNA HCG18 competitively binds to miR-146a and regulates TNF receptor associated factor 6 (TRAF6) to promote M1 polarization and inflammatory factor secretion. MiR-146a can negatively regulate the expression of TRAF6, whereas inhibition of TRAF6 can reverse the promoting effect of HCG18 on M1 Mjs. HCG18 stimulates Mj activation via the TRAF6/NF-kB pathway. Therefore, lncRNA HCG18 can promote the proliferation of M1 Mjs in DPN and accelerate the development of disease by regulating the miR-146a/TRAF6 axis.

LNCRNAS REGULATE MFS POLARIZATION IN AUTOIMMUNE DISEASES
Rheumatoid arthritis (RA) is a common autoimmune disease with a complex pathogenesis, and its incidence is increasing. In RA, TNF-a produced by M1 Mjs can trigger synovial cells to produce cytokines, leading to the development of arthritis. Infiltrating Mjs in the joint synovium plays an important role in the pathogenesis of RA, and the degree of infiltration is related to the activity of the disease and degree of joint erosion (124). A study showed (61) that silencing of lncRNA H19 altered the expression of lipopolysaccharide in Mjs from patients with RA, induced M1 polarization, and reduced the expression of factors such as IL-6 and CXCL10. However, overexpression of H19 showed the opposite effects by promoting M1 polarization and aggravating arthritis in mice. In practice, lncRNA H19 is highly expressed in patients with RA. Increased expression of KDM6A, promotes M1 polarization and aggravates arthritis. Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) characterized by extensive inflammation, demyelination and axonal loss (125,126). Lesions often occur in the brain and spinal cord. Due to unknown etiology and complex pathophysiological mechanisms, the etiology and pathogenesis of MS have not been fully elucidated (127,128). Microglia can be divided into M1 and M2 functional subtypes; they play a central role in inflammatory process and neuronal destruction in MS (129). It has been found that M2 polarized microglia can promote neuronal survival, neurite growth and oligodendrocyte progenitor cell (OPC) differentiation (130,131). In epigenetics, lncRNA Gas5 is an epigenetic regulator of microglial polarization. It is highly expressed in microglia of patients with MS and can inhibit M2 polarization of microglia. The specific mechanism is that Gas5 suppresses the transcription of TRF4, a key factor regulating M2 polarization, through recruiting the polycomb repressive complex 2 (PRC2), thus inhibiting M2 polarization. Therefore, this study suggests that lncRNA Gas5 may be a promising target for the treatment of MS (76).

CONCLUSION
Mjs constantly convert between two extreme polarization states, M1 and M2 Mjs. Mjs are involved in the pathogenesis and progression of many diseases. Identifying the key regulators of Mjs polarization is important for developing treatments for diseases. An increasing number of lncRNAs has been identified in the process of Mj polarization. There is a close relationship between lncRNAs and Mjs, which play an important regulatory role in Mjs. Their multifunctional capabilities and heterogeneous working mechanisms make them potential targets for treating diseases. In conclusion, lncRNAs exhibit tremendous effects on Mjs differentiation and polarization. The interaction between lncRNAs and Mjs is involved in the development of multiple systemic diseases and provides useful targets for the interference of macrophage involved disorders.
However, compared with the study of small molecule RNA, lncRNAs are still in the initial stage, and their function and regulation mechanism still need to be further clarified. In order to fully realize the therapeutic value of lncRNAs and the development of corresponding therapeutic drugs, there are still some difficulties to be overcome. First of all, we need to improve the method of transporting lncRNAs to specific areas. Because of the long length of lncRNA, it is difficult to deliver full-length lncRNAs, so we need an accurate and efficient means of transport to ensure the stability of lncRNAs. Secondly, we need to establish a complete, detailed and specific lncRNAs database to record the specific regulatory cell types and target genes of each lncRNAs. Finally, we need to solve the difficulty of finding homologous lncRNA. Especially in animal experiments, due to the epigenetic differences between animals and humans, the human lncRNAs may not exist in animals. This requires techniques such as gene knockout to build animal models, which makes the experiment more difficult. In recent years, with the deepening of research, the experimental techniques related to lncRNAs have been continuously developed, an increasing number of lncRNAs have been found, and play an important role in the occurrence and development of various diseases, and can be used as genes related to diagnosis and treatment. In particular, there is an increasing number of studies on lncRNAs regulating Mjs polarization and participating in the development of various diseases, which provides a new research content for future research. On the basis of fully understanding the mechanism of action, appropriate genetic engineering drugs are developed to achieve the purpose of prevention, diagnosis and treatment.

AUTHOR CONTRIBUTIONS
PJ wrote the manuscript. XL conceived the study. All authors have reviewed and approved the final version of the manuscript.