Lipid metabolism disorders are recognized as key risk factors for atherosclerosis. Macrophages and vascular smooth muscle cells (VSMCs) engulf the accumulated lipoproteins [particularly oxidized low-density lipoprotein (ox-LDL)] in the damaged artery wall and finally transform into foam cells (25). At the same time, the accumulation of ox-LDL may result in the formation of lipid streaks and even lipid plaques, thus accelerating the development of atherosclerosis. Several studies have indicated that HOX-lncRNAs participate in lipid metabolism. Huang et al (26) reported that lncRNA HOXC cluster antisense RNA 1 (HOXC-AS1) expression is downregulated in carotid atherosclerosis. Subsequent research has shown that HOXC-AS1 overexpression partly blocks the cholesterol accumulation induced by ox-LDL in THP-1 macrophages (26), which may provide insight into the regulation of cholesterol homeostasis and suggests that HOXC-AS1 is a promising therapeutic target that governs atherosclerosis. Another study indicated that HOXB-AS3 regulates lipid metabolism in endometrial cancer (27). Gariani and Jornayvaz (28) revealed that HOTAIR knockdown decreases total cholesterol and triglyceride levels. Overall, these data preliminarily show that HOX-lncRNAs have a causal role in lipid metabolism.

The inflammatory response is well-known to have a predominant role in driving the pathogenesis of atherosclerosis (1). Several mechanisms underlying the inflammatory activation of endothelial cells (ECs), foam cell formation and foam cell secretion of inflammatory cytokines contribute to the development of atherosclerosis (29). The persistent activation of the inflammatory response leads to increased EC permeability and higher rates of lipid entry, thus fueling the development of atherosclerosis. Overwhelming findings have displayed the causal effect of HOX-lncRNAs in the inflammatory response. For instance, one of the most studied HOX cluster-embedded lncRNAs is HOTAIR, which resides between HOXC11 and HOXC12 in the HOXC cluster on human chromosome 12q13.13 (30). HOTAIR overexpression attenuates the expression of pro-inflammatory cytokines (TNF-α and IL-1β) and potentiates the expression of anti-inflammatory cytokines (IL-4 and IL-10) in Raw264.7 cells exposed to ox-LDL (31). Likewise, another study also reported that HOTAIR facilitates oxidative stress and the inflammatory response by sponging miR-330-5p in human macrophages in the presence of ox-LDL (32). In addition, Lu et al (33) found that HOTAIR expression was upregulated in an AMI group compared to that in its counterparts. Functionally, HOTAIR overexpression boosts the secretion of inflammatory cytokines, such as IL-6 and TNF-α, in H9C2 cells in the presence of a hypoxia stimulus (33). Nuclear factor-κB (NF-κB), as a major transcription factor involved in inflammatory responses, was first discovered in 1986 (34). NF-κB activators and NF-κB-mediated genes have been determined to participate directly or indirectly in the pathogenesis of atherosclerosis (35). HOTAIR modulates the inflammatory response and oxidative stress in H9C2 cells by affecting the NF-κB pathway, thus protecting cardiomyocytes (36). As a similar regulatory mechanism, it was also reported that lncRNA HOXA cluster antisense RNA 2 (HOXA-AS2)-mediated endothelium protection is partly attributed to inhibition of the NF-κB pathway (37). Another study showed that HOXA-AS3 can interact with NF-κB and positively regulate its activity by modulating the NF-κB inhibitor protein IκBα (38). HOXA11-AS knockdown markedly inhibits the expression of inflammation-related genes induced by TNF-α in VSMCs and by platelet-derived growth factor in vascular endothelial cells (VECs) (39). In addition, HOTAIR expression is upregulated in macrophages upon exposure to lipopolysaccharide (LPS). Furthermore, HOTAIR depletion diminishes NF-κB-mediated inflammatory gene and cytokine expression. Further mechanistic studies have indicated that HOTAIR knockdown leads to reduced expression of NF-κB target genes via a decrease in the recruitment of NF-κB and associated cofactors at the target gene promoters (40). The inflammatory response normally requires high energy and, hence, is closely related to glucose metabolism. One study showed that HOTAIR has key roles in the expression of glucose transporter isoform 1, which controls glucose uptake by macrophages (41). Ultimately, in LPS-induced H9C2 cells, HOTAIR promotes the inflammatory response and apoptosis by enhancing programmed cell death 4 stability. In vivo experiments verified that HOTAIR knockdown alleviates cardiac injury and the secretion of inflammatory factors caused by sepsis (42). These studies suggest that HOTAIR has a significant role in mediating inflammatory responses. In conclusion, HOX-lncRNAs have important roles in inflammatory responses through several mechanisms. Ongoing research and the increasing awareness of HOX-lncRNAs will help to elucidate the important role of HOX-lncRNAs in inflammation and inflammation-mediated atherosclerosis.

EC dysfunction is a primary contributor, driven by pathological angiogenesis within the arterial wall, to the pathogenesis of atherosclerosis (43). Angiogenesis markedly affects plaque growth and the instability of atherosclerotic lesions. Important research has shown that HOTAIR is involved in angiogenesis, and its overexpression resulted in a substantial reduction in EC proliferation, migration and tube formation. Mechanistically, HOTAIR transcriptionally inhibits vascular endothelial growth factor (VEGF) via direct binding to its promoter (44). VEGF is a crucial factor associated with angiogenesis (45). Furthermore, a recent study demonstrated that HOTAIR knockdown enhances the migratory and tube formation abilities of oxygen-glucose deprivation/reperfusion-induced human brain microvascular ECs (46). In summary, these data demonstrate a HOTAIR-mediated anti-angiogenic effect, thus blocking the progression of atherosclerosis. However, the role of HOX-lncRNAs in angiogenesis remains unclear and further in-depth studies on this topic will improve the current understanding of angiogenesis relevant to atherosclerosis.

VSMCs and VECs are the main cell types involved in atherosclerosis, and their abnormal proliferation and apoptosis have a key role in the development and progression of atherosclerosis (47). Given that the development of atherosclerosis comprises the orchestrated interplay between ECs and SMCs, the following chapters outline the contribution of HOX-lncRNAs to cellular proliferation and apoptosis.

Dysfunction of the endothelial lining of blood vessels is a critical event in atherosclerosis initiation (48). As such, abnormal EC proliferation and apoptosis contribute to this disease (49). Mounting evidence indicates that HOX-lncRNAs are closely related to the proliferation and apoptosis of VSMCs and VECs, suggesting an important role for HOX-lncRNAs in the process of atherosclerosis. HOXA-AS3 expression is upregulated in human umbilical vein ECs (HUVECs) upon ox-LDL stimulation, and its depletion significantly suppresses the progression of atherosclerosis. Mechanistically, it acts as a competing endogenous (ceRNA) for miR-455-5p to decrease the protein level of p27^Kip1 (50), a cell cycle regulator first identified as a cyclin-dependent kinase antagonist (51). Similarly, the expression of HOXA11-AS is increased in both atherosclerotic mouse aortic tissue and ox-LDL-stimulated HUVECs. HOXA11-AS knockdown markedly blunts the ox-LDL-induced inhibitory effect on cell proliferation and diminished apoptosis in HUVECs, suggesting that HOXA11-AS sponges miR-515-5p to stimulate the expression of rho-associated coiled-coil containing protein kinase 1 (ROCK1), a direct target of miR-515-5p, thus contributing to atherosclerotic injury by directly regulating the miR-515-5p/ROCK1 axis (52). In summary, these observations indicate that HOX-lncRNAs affect the proliferation and apoptosis of VECs through miRNA sponging mechanisms to modulate their target genes. Beyond functioning as ceRNA machinery, HOX-lncRNAs can also modulate the proliferation and migration of ECs by regulating related signaling pathways. An increasing number of studies have shown that HOTAIR has a significant role in cardiovascular biology and diseases. Specifically, its expression is much lower in ECs from atherosclerotic plaques. Functional assays further indicated that HOTAIR facilitates proliferation and migration and suppresses apoptosis in ECs. Mechanistically, HOTAIR, activated by thymic stromal lymphopoietin, regulates the proliferation and migration of ECs via the PI3K/AKT-interferon regulatory factor 1 pathway (53). Finally, HOTTIP expression was reported to be higher in coronary artery disease tissues than in normal tissues, and its depletion was observed to block EC proliferation and migration. Further mechanistic studies indicated that HOTTIP may govern EC proliferation and migration via Wnt/β-catenin pathway activation (54). In addition, HOTAIR facilitates pulmonary VEC apoptosis via the DNMT1-mediated hypermethylation of the Bcl-2 promoter in chronic obstructive pulmonary disease (55). Overall, the increasing evidence presented here underscores the important contribution of HOX-lncRNAs to EC functions, particularly in human EC models.

The proliferation and migration of VSMCs are key events in the progression of atherosclerotic lesions and restenosis. Several studies have investigated the potential functions of HOX-lncRNAs in VSMC proliferation and migration. HOTTIP is a well-characterized HOX-lncRNA that is induced in human aortic VSMCs (HAVSMCs) in response to ox-LDL and in the sera of patients with atherosclerosis. Functionally, HOTTIP knockdown suppresses the ox-LDL-induced proliferation and migration of HAVSMCs. Mechanistically, HOTTIP depletion blocks cell proliferation and migration through the regulation of the miR-490-3p/HMGB1 and PI3K-AKT pathways in ox-LDL-induced HAVSMCs (56). Given the recent interest in the interplay between proliferative pathways and atherosclerosis development, HOTAIR may be a prospective target for atherosclerosis interventions. Xue et al (57) revealed that HOTAIR overexpression facilitates viability and suppresses apoptosis in VSMCs. Mechanistically, HOTAIR serves as a ceRNA for miR-130b-3p to diminish the expression of peroxisome proliferators-activated receptors α (57). In addition, HOTAIR binds to and negatively regulates miR-148b-3p, leading to increased VSMC viability and migration (58). The lncRNA HOXA-AS2, located between and antisense to human HOXA3 and HOXA4, has been reported to potentiate the proliferative and migratory abilities and decrease the apoptosis of VSMCs by absorbing miR-877-3p (59). Recently, HOXA-AS2 was found to be highly expressed in thoracic aortic aneurysm (TAA) tissues. HOXA-AS2 upregulates the expression of NHS like 3 by targeting miR-520d-3p/IGF2BP3 to drive VSMC growth in TAA (60). Therefore, HOX lncRNAs are important regulators of VSMC proliferation and migration. However, further investigations are necessary to determine the significance of HOX-lncRNAs in regulating VSMC proliferation.

Cardiac hypertrophy is an initial adaptive response to various stresses, including pressure or volume overload, which lowers the increase in the wall tension and aids in maintaining cardiac output. This adaptive process is beneficial and can ameliorate cardiac functions; however, persistent exposure of the heart to increased workload leads to impaired blood flow, resulting in relative hypoxia and the subsequent loss of cardiomyocytes, ultimately causing HF. Cardiac fibrosis is closely associated with numerous heart diseases, such as chronic HF, MI, malignant arrhythmia and sudden cardiac death, and is a key pathological feature of cardiac remodeling.

Lai et al (61) reported that HOTAIR expression is downregulated in heart tissues from transverse aortic constriction-operated mice in vivo and in cultures treated with Ang-II in vitro. Its overexpression reduces the cell surface area and expression of the hypertrophic markers atrial natriuretic peptide, brain natriuretic peptide and β-myosin heavy chain in response to Ang-II. Regarding its molecular mechanisms in cardiac hypertrophy, HOTAIR may act as a ceRNA for miR-19, thereby modulating its target PTEN and playing an important role in inhibiting cardiac hypertrophy progression (61). Therefore, HOTAIR may have a key role in the regulation of cardiac hypertrophy. Ang-II upregulates HOTAIR expression in cardiac fibroblasts (CFs), which promotes cell proliferation and inhibits apoptosis. The mechanism by which HOTAIR regulates myocardial fibrosis may be related to activation of the Wnt signaling pathway by targeting uRI1 prefoldin like chaperone (62). Similarly, Tan et al (63) found that HOTAIR silencing significantly diminishes Ang-II-induced proliferation, migration and fibrosis in fibroblasts. Moreover, HOTAIR knockdown was found to markedly inhibit fibrosis in the heart tissues of atrial fibrillation (AF)-model mice via the regulation of Wnt signaling. Recently, Jiang et al (64) showed that HOTAIR functions as a ceRNA of miR-124 to promote myocardial fibrosis. Therefore, HOTAIR may have a role in cardiac fibrosis. In addition, another study showed that HOXA11-AS overexpression in CFs enhances the expression of transforming growth factor β1, a classic and powerful fibrogenic pathway mediator pertinent to cardiac fibrosis, thus contributing to cardiac fibrosis progression (65). Collectively, HOX-lncRNAs, such as HOTAIR and HOXA11-AS, appear to have prominent roles in regulating cardiac hypertrophy and fibrosis.

I/R injury is a major cause of the necrotic, apoptotic and autophagic death of cardiomyocytes. In recent years, studies have shown that HOX-lncRNAs have a protective effect against myocardial I/R injury. For instance, Sun and Hu (66) found that HOTAIR expression is upregulated in mice after I/R and in H₂O₂-induced H9c2 cells. Overexpression of HOTAIR markedly suppressed viability and increased lactate dehydrogenase release and caspase-3 activity in H9c2 cells treated with H₂O₂, and further mechanistic studies implied that HOTAIR aggravates myocardial I/R injury by sponging miR-126 to upregulate serine and arginine rich splicing factor 1 expression (66). Furthermore, propofol (PPF) has certain protective effects against myocardial I/R injury (67). Chen et al (68) showed that PPF pretreatment markedly upregulated HOTAIR expression and that HOTAIR knockdown partially reversed the protective effects of PPF on myocardial I/R injury. Likewise, HOTAIR, as a ceRNA, affects myocardial I/R injury via a HOTAIR/miRNA-1/connexin-43 axis (69). In conclusion, these data suggest that HOTAIR is of paramount importance for myocardial I/R injury. In addition to HOTAIR, expression of HOXD antisense growth-associated lncRNA (HAGLR), also known as HOXD-AS1 and Mdgt, whose gene is located in the intergenic region between HOXD1 and HOXD3, has been reported to be significantly increased upon I/R in vivo or hypoxia-reoxygenation in vitro. HAGLR knockdown attenuated myocardial I/R injury, and mechanistically, HAGLR increases myocardial I/R injury by inhibiting miR-133a-3p, thereby promoting MAPK1 expression (70). Taken together, HOTAIR and HAGLR may be therapeutic targets for the treatment of myocardial I/R injury and much work is needed to fully understand how HOX-lncRNAs coordinate myocardial I/R injury.

Cardiomyocyte apoptosis, or programmed cell death, is a pivotal pathological manifestation of I/R injury and the primary cause of cardiac dysfunction (71). Therefore, an in-depth understanding of the mechanism underlying cardiomyocyte apoptosis is key to preventing myocardial injury and treating heart disease. Recently, HOX-lncRNAs were reported to be involved in cardiomyocyte apoptosis. For instance, HOTAIR, which is significantly downregulated in the ischemic myocardium of rats, inhibits H9c2 cell apoptosis induced by H₂O₂ in vitro. Mechanistically, HOTAIR binds to miR-130a-3p and acts as a sponge to block its functions in I/R injury (72). Similar results were obtained when determining the role of HOTAIR in AMI. Its expression was found to be substantially downregulated in the serum of patients with AMI and in mice. Furthermore, HOTAIR overexpression was found to promote H9c2 cell viability and inhibit apoptosis under hypoxic conditions in vitro. The mechanism was attributed to miR-206 binding fibronectin 1 (73). In addition, HOTTIP levels were determined to be significantly upregulated in the ischemic myocardium of mice with MI and in hypoxia-induced cardiomyocytes. Furthermore, HOTTIP functions as a ceRNA of miR-92a-2 to enhance c-Met expression and is involved in the modulation of cardiomyocyte growth and apoptosis, indicating that it is a potential therapeutic target for AMI (19). Ongoing investigations are needed to further determine their relevant roles.

Essential hypertension is a common and frequently occurring disease and a risk factor for various cardiovascular and cerebrovascular diseases. In recent years, the search for ncRNAs has attracted the attention of many researchers. According to reports, increasing studies have shown that lncRNAs have important roles in the occurrence and development of essential hypertension (74-78). As a special type of lncRNA, the roles of HOX-lncRNAs in essential hypertension have rarely been studied. However, studies have shown that HOTAIR expression is significantly downregulated in stroke patients with hypertension compared to that in stroke patients without hypertension (79), suggesting that it may be closely related to the occurrence and development of hypertension and that it could be used as a biomarker for the diagnosis and treatment of this condition.

Pulmonary arterial hypertension (PAH) is a vascular remodeling disease characterized by vasoconstriction and progressive obliteration of the distal pulmonary arteries, leading to elevated pulmonary pressure and eventually right ventricular HF (80,81). Pulmonary vascular remodeling is characterized by the excessive proliferation of pulmonary arterial smooth muscle cells (PASMCs) and the dysfunction of pulmonary arterial ECs (82). Therefore, it is important to explore the cellular mechanisms underlying the effects of HOX-lncRNAs in pulmonary VSMCs and in the pathogenesis of pulmonary hypertension. HOXA-AS3 is a novel lncRNA that is transcribed from the HOXA cluster. A previous study showed increased HOXA-AS3 levels in human (H)PASMCs exposed to hypoxia. Furthermore, its knockdown decreased growth and migration and induced apoptosis in HPASMCs. The mechanism associated with the effects of HOXA-AS3 was also examined; it was found to upregulate phosphodiesterase 5A expression by sponging miR-675-3p (83). Another study showed that HOXA-AS3 knockdown represses PASMC proliferation in vitro (84), thus providing a potential novel strategy for the treatment of PAH.