Long Non-coding RNAs Involved in Metabolic Alterations in Breast and Prostate Cancers

Breast and prostate cancers are the most prevalent cancers in females and males, respectively. These cancers exhibit sex hormone dependence and thus, hormonal therapies are used to treat these cancers. However, acquired resistance to hormone therapies is a major clinical problem. In addition, certain portions of these cancers initially exhibit hormone-independence due to the absence of sex hormone receptors. Therefore, precise and profound understanding of the cancer pathophysiology is required to develop novel clinical strategies against breast and prostate cancers. Metabolic reprogramming is currently recognized as one of the hallmarks of cancer, as exemplified by the alteration of glucose metabolism, oxidative phosphorylation, and lipid metabolism. Dysregulation of metabolic enzymes and their regulators such as kinases, transcription factors, and other signaling molecules contributes to metabolic alteration in cancer. Moreover, accumulating lines of evidence reveal that long non-coding RNAs (lncRNAs) regulate cancer development and progression by modulating metabolism. Understanding the mechanism and function of lncRNAs associated with cancer-specific metabolic alteration will therefore provide new knowledge for cancer diagnosis and treatment. This review provides an overview of recent studies regarding the role of lncRNAs in metabolism in breast and prostate cancers, with a focus on both sex hormone-dependent and -independent pathways.

Breast and prostate cancers are the most prevalent cancers in females and males, respectively. These cancers exhibit sex hormone dependence and thus, hormonal therapies are used to treat these cancers. However, acquired resistance to hormone therapies is a major clinical problem. In addition, certain portions of these cancers initially exhibit hormone-independence due to the absence of sex hormone receptors. Therefore, precise and profound understanding of the cancer pathophysiology is required to develop novel clinical strategies against breast and prostate cancers. Metabolic reprogramming is currently recognized as one of the hallmarks of cancer, as exemplified by the alteration of glucose metabolism, oxidative phosphorylation, and lipid metabolism. Dysregulation of metabolic enzymes and their regulators such as kinases, transcription factors, and other signaling molecules contributes to metabolic alteration in cancer. Moreover, accumulating lines of evidence reveal that long noncoding RNAs (lncRNAs) regulate cancer development and progression by modulating metabolism. Understanding the mechanism and function of lncRNAs associated with cancer-specific metabolic alteration will therefore provide new knowledge for cancer diagnosis and treatment. This review provides an overview of recent studies regarding the role of lncRNAs in metabolism in breast and prostate cancers, with a focus on both sex hormone-dependent and -independent pathways.

BREAST AND PROSTATE CANCERS
In developed countries, breast and prostate cancers are the most common malignancies in women and men, respectively (1). In both breast and prostate cancers, growth and survival are mainly controlled by sex steroid hormones, which include estrogens and androgens, respectively (2). These unique properties have been targeted in treating breast and prostate cancers with hormone therapy (3). However, during long-term hormone therapy, acquired resistance develops in many patients (4), and there are few effective treatments for hormone-refractory cancers. In addition, from the tumor diagnosis, breast cancers negative for estrogen receptor α (ERα) such as triplenegative breast cancer (TNBC) exhibit no response to hormone therapy (5). In the case of prostate cancer, most cancers are initially androgen-dependent and respond to hormone therapies; however, they eventually acquire endocrine therapy resistance and become castration-resistant prostate cancer (CRPC) (6). Thus, understanding both mechanisms of hormone dependence and hormone resistance is important for the development of diagnosis and treatment options of breast and prostate cancers.

LONG NON-CODING RNA (lncRNA)
Recent cancer research has focused on the roles and importance of lncRNAs in cancer development. Although up to 70% of the human genome is actively transcribed, only 2% of transcripts are translated into proteins (7). Transcripts that do not encode proteins are called non-coding RNAs, among them those with a length >200 nucleotides are categorized as lncRNAs (8). LncRNAs exert their function by regulating a variety of intracellular processes, for example, (1) recruiting epigenetic modifiers and transcription factors to the enhancer/promoter of the lncRNA target gene, (2) forming RNA-RNA binding with its target mRNA for mRNA degradation, (3) acting as a decoy, enabling avoidance of the associated protein from transcriptional regulatory regions, and (4) acting as a molecular sponge or as a competitive endogenous RNA (ceRNA) for microRNA (4,(9)(10)(11). In terms of the contribution of lncRNAs to cancer metabolism, characterization of lncRNAs associated with glucose metabolism has proceeded (12,13) probably because the Warburg effect with aerobic glycolysis has been considered as a central event for cancer metabolism. Recently, lncRNA functions in other metabolic pathways including mitochondrial oxidative phosphorylation (OXPHOS) and lipid biosynthesis have been reported. Moreover, it has been shown that lncRNAs also modulate hormone sensitivity and resistance in breast and prostate cancers (4,10,14). These findings suggest that the metabolism-associated lncRNAs would be new therapeutic targets for hormone-refractory breast and prostate cancers. Further intensive analyses will reveal the precise characteristics of these lncRNAs including their expression profiles, binding factors, and three-dimensional structures, using advanced technologies such as high-throughput sequencing, mass spectrometry, and bioinformatics. Based on the molecular aspects of lncRNAs, their expressions and functions could be modulated by nucleic acid therapy using antisense oligonucleotide and RNA interference technique, as well as by small-molecule drugs that bind to lncRNAs. Moreover, effector proteins of lncRNAs will be also potential targets for modulating the lncRNA-associated signaling (4,7). In addition, lncRNA delivery and lncRNA-inducing drugs should be important to modify the functions of these lncRNAs. While efforts are underway to improve the molecular technology and skills for analyzing lncRNAs, a comprehensive understanding for their roles in cancer metabolism remains to be elusive and further investigation will be required to apply these molecules for cancer therapy. In the present review, we provide an overview of the recent findings regarding lncRNA involvement in cancer metabolism, with a focus on breast and prostate cancers (Figure 1).

GLUCOSE METABOLISM AND lncRNA
Metabolic reprogramming is one of the hallmarks of cancer. Tumor cells preferentially use glycolysis for ATP production even in the presence of oxygen (15). However, the ATP production efficiency of glycolysis itself is not high compared with that of OXPHOS. Glycolysis-dependent cancer cells will be prone to undergo metabolic reprogramming as they eventually need to compensate for lower energy production efficiency by glycolysis due to high turnover of cell proliferation. As one strategy for the improvement of energy production efficiency, cancer cells often facilitate glucose uptake and usage by upregulating glucose transporters and glycolytic enzymes (16). For instance, the glucose transporter (GLUT) family, which is responsible for cellular glucose uptake, is upregulated in multiple cancer types (17). Hexokinases (HK), which convert glucose to glucose-6-phosphate (G6P) at the first step of glycolysis, are particularly activated in cancer cells wherein they function as a driver of glycolysis (18). Phosphofructokinase-2/fructose-bisphosphatase-2 (PFK2), which is responsible for the synthesis of fructose-2,6-bisphosphate, is also upregulated in various cancers (19). Pyruvate kinase catalyzes the final step of glycolysis to produce pyruvate (20), and pyruvate kinase M2 (PKM2) among the 4 pyruvate kinase isomers is assumed to play a potential role in tumorigenesis of several cancers including breast (21) and prostate cancer (22). Pyruvate dehydrogenase kinase (PDK) phosphorylates and inactivates the pyruvate dehydrogenase complex, which synthesizes acetyl coenzyme A as a critical substrate of tricarboxylic acid (TCA) cycle from pyruvate. Inhibition of pyruvate dehydrogenase complex activity by PDK-mediated phosphorylation is involved in cancer pathophysiology (23)(24)(25). Lactate dehydrogenase A (LDHA) is an enzyme that converts pyruvate to lactate, and its overexpression is also observed in many types of cancers (26). Oncogenes such as MYC proto-oncogene (MYC) and hypoxia-inducible factor 1A (HIF1α) have been also shown as master regulators for glucose metabolic alteration in cancer cells (27,28). For example, the expression of glycolytic enzymes such as HK2, PFK, and LDHA is cooperatively regulated by MYC and HIF1α (29)(30)(31). In the following sections, we will discuss several major lncRNAs involved in the metabolic pathways in breast and prostate cancers.
DU145 and PC-3 cells whereas the mechanism remains to be clarified (36).
Long Intergenic Non-coding RNA-p21 (lincRNA-p21) LincRNA-p21 is inducible by hypoxia or HIF1α function in breast cancer cells (4), and binds to HIF1α and its E3 ubiquitin ligase von Hippel-Lindau (VHL), thereby disrupting the VHL-HIF1α interaction. This dissociation attenuates VHL-mediated HIF1α ubiquitination and results in HIF1α stabilization as a positive feedback loop for HIF1α-dependent pathways. It is revealed that lincRNA-p21 promotes hypoxia-induced lactate production and glucose uptake in breast cancer cells through the upregulation of GLUT1 and LDHA (4). In contrast, lincRNA-p21 is often downregulated in prostate cancer, and lincRNA-p21 silencing activates PKM2 and glycolysis in LNCaP and DU145 prostate cancer cells (37). The differential effects of lincRNA-p21 on glycolysis may be attributed to a cancer type-specific metabolic pathway regulation between breast and prostate cancers.

Long Intergenic Non-coding RNA for Kinase Activation (LINK-A)
LINK-A has been identified as a cytoplasmic lncRNA that is preferentially expressed in TNBC (38). LINK-A binds with both breast tumor kinase (BRK) and leucine rich-repeat kinase 2 (LRRK2), and this process is stimulated by the signaling from heparin-binding epidermal growth factor (HB-EGF)-triggered epidermal growth factor receptor (EGFR) and transmembrane glycoprotein non-metastatic melanoma protein B (GPNMB) complex formation. BRK phosphorylates the Tyr 565 of HIF1α, which interferes with the Pro 564 hydroxylationmediated HIF1α degradation, whereas LRRK2 phosphorylates HIF1α Ser 797 which enhances HIF1α transcriptional activity. These HIF1α phosphorylation events facilitate glycolysis reprogramming, i.e., glucose uptake and lactate production, in TNBC (38).

Prostate Cancer Gene Expression Marker 1 (PCGEM1)
PCGEM1 was originally identified as an androgen-induced prostate-specific lncRNA. Its overexpression is highly associated with prostate tumors, and it promotes cancer cell proliferation (39,40). In addition to hormonal regulation, PCGEM1 functions as a coactivator of MYC and is estimated to induce expression of glycolytic enzymes such as HK2, glucose-6phosphate dehydrogenase (G6PD), and LDHA (41). PCGEM1 overexpression increases glucose uptake and lactate production along with the pentose phosphate shunt to provide a sufficient supply of nucleotides and lipids in prostate cancer.
FGF13 Antisense RNA 1 (FGF13-AS1) FGF13-AS1 binds with an RNA-binding protein, insulin-like growth factor 2 mRNA binding protein (IGF2BP), which interacts with the MYC mRNA (42). Binding of FGF13-AS1 with IGF2BP disrupts the interaction between IGF2BP and MYC mRNAs, leading to a decrease in the half-life of MYC mRNA. FGF13-AS1 overexpression decreases glucose consumption and lactate production in TNBC subtype MDA-MB-231 cells, while FGF13-AS1 depletion increases them in luminal subtype MCF-7 cells. These results indicate that FGF13-AS1 suppresses the glucose metabolism in these breast cancer subtypes. FGF13-AS1 also suppresses the spheroid formation and stemness properties of these breast cancer subtypes (42).

Urothelial Carcinoma Associated 1 (UCA1)
UCA1 participates in glucose metabolism in several cancer types (43)(44)(45)(46). Particularly, in tumor suppressor Merlin-deficient breast cancer cells, upregulated UCA1 stimulates glucose consumption and lactate production through the activation of serine/threonine kinase AKT and signal transducer and activator of transcription 3 (STAT3) with a simultaneous increase in HK2 expression (47). Moreover, UCA1 has been implicated in hormone therapy resistance in breast cancer cells as it sponges miR-18a, leading to HIF1α activation or inhibition of mTOR signaling (48,49), suggesting its multiple functions in breast cancer metabolism.

Maternally Expressed 3 (MEG3)
As shown in other type of cancer (50), the lncRNA MEG3 is downregulated in breast cancer tissues (51). MEG3 acts as a molecular sponge of miR-21, resulting in the repression of HK2 protein levels and the glycolytic pathway (i.e., glucose consumption and lactate production) in both MCF-7 and MDA-MB-231 cells (51).

YIYA
YIYA is an lncRNA expressed in approximately 40% of clinical breast cancer tumors and is associated with poor recurrencefree survival of the patients (52). This report also indicates that YIYA binds with cyclin-dependent kinase 6 (CDK6) to enhance CDK6-dependent phosphorylation of PFK2. In breast cancer cells, YIYA stimulates cell proliferation with an increase in the glycolytic pathway (i.e., elevation of glucose consumption and lactate production) (52).

Breast Cancer Anti-Estrogen Resistance 4 (BCAR4)
BCAR4 has been identified as a TNBC-upregulated lncRNA that is essential for breast cancer metastasis (53). This report indicates that BCAR4 associates with Smad nuclear-interacting protein 1 (SNIP1) and serine/threonine-protein phosphatase 1 regulatory subunit 10 (PPP1R10/PNUTS) to promote the transcriptional signaling by non-canonical hedgehog signal mediator gliomaassociated oncogene homolog 2 (GLI2) by discharging the inhibitory role of SNIP1 on p300 histone acetyltransferase activity. More recently, it was demonstrated that BCAR4 is upregulated by the Hippo pathway downstream effector, Yesassociated protein (YAP), and modulates Hedgehog signaling to activate transcription of HK2 and PFK2 in TNBC cells (54). Notably, BCAR4 induces glucose uptake and lactate production in these cells. Furthermore, high expression of both BCAR4 and YAP is associated with poor survival of patients with breast cancer, suggesting a critical role for the YAP-BCAR4-glycolysis axis in this disease.

OXPHOS WITH lncRNA
As aerobic glycolysis has been assumed as a major energy resource for cancer metabolism, mitochondrial OXPHOS has been rather considered as a second-grade metabolic pathway in cancers. Nevertheless, recent lines of evidence indicate that OXPHOS is also crucial for energy metabolism in cancers (55). The OXPHOS system comprises five enzyme complexes embedded in the inner mitochondrial membrane: complex I (NADH dehydrogenase), complex II (succinate ubiquinone oxidoreductase), complex III (ubiquinol cytochrome c oxidoreductase), complex IV (cytochrome c oxidoreductase), and complex V (ATP synthase).
While the contributions of glycolysis and OXPHOS to ATP biosynthesis vary among cell types as shown by a metaanalysis, the contribution ratios of OXPHOS were almost equally ∼80% in both normal cells and cancer cells (56). Moreover, it has been revealed that OXPHOS rather remains the most significant source of energy production in tumors, suggesting that mitochondrial respiration is not generally impaired in cancer cells (57).
Several studies indicate that OXPHOS expression and function are both upregulated in breast and prostate cancers. Upregulation of OXPHOS gene expression and enzyme activity is detected in patients with breast cancer (58). In tumor suppressor RB1-deficient TNBC, OXPHOS is highly upregulated to promote cancer cell proliferation (59,60). Moreover, cytochrome c oxidase subunit 7a-related polypeptide (COX7RP), which was originally identified as an estrogen-responsive gene, promotes mitochondrial respiration and cell proliferation in breast cancer cells by accelerating the formation of mitochondrial supercomplexes, which contributes to efficient ATP synthesis by assembling complexes I, III, and IV (61).
It has been recently shown that a compound-induced mitochondrial fragmentation leading to the disruption of OXPHOS and ATP synthesis inhibits the proliferation of prostate cancer PC3 cells (62). An integrated analysis of RNA-sequencing datasets and proteome data indicates that OXPHOS-related genes are upregulated in prostate cancer and negatively associated with STAT3 expression (63). Another integrated analysis elucidates a preservative role of MYC on OXPHOS function in prostate cancer tumorigenesis (64). In this report, MYC knockdown in PC3 cells reduced the levels of oxidative phosphorylation and TCA cycle metabolites along with decreased oxygen consumption rates (OCR), leading to diminished ATP production. Docetaxelresistant PC3 cells show a metabolic shift from glycolysis toward mitochondrial respiration (65). Interestingly, RNA-seq analysis of prostate tissues shows remarkable enrichment of OXPHOS-related genes in CRPC (66). Moreover, a study analyzing substrate-specific OXPHOS capacities using primary human prostate tissues revealed that the malignant tissues exhibits a significant metabolic shift toward higher succinate oxidation by complex II, particularly in high-grade prostate tumors (67).
OXPHOS upregulation, however, is rather limited to particular cancer subtypes (68)(69)(70). In addition, OXPHOS activity can be coordinated by many factors including mitochondrial DNA content, expression levels of respiration complexes subunits and regulatory genes, and the cellular environment. Up to date, few studies have been performed for the analysis of OXPHOS activity in the association with lncRNAs. Further analysis will clarify the role of lncRNA on OXPHOS in different cancer types, including breast and prostate cancers.
Integrated analysis of the mitochondrial proteome and the gene expression dataset showed that the lncRNA small nucleolar RNA host gene 3 (SNHG3) is related to ovarian cancer survival as well as the expression of energy metabolism-related genes such as complex III subunit ubiquinol-cytochrome c reductase hinge protein (UQCRH) (71). In silico analysis speculates that SNHG3 can function by sponging miR-186-5p. In gastric cancer tissues, expression levels of lncRNA MIF-AS1 (lncMIF-AS1) and complex IV subunit NADH dehydrogenase1 alpha subcomplex 4 (NDUFA4) are higher compared with those in non-cancerous tissues (72). Mechanistically, lncMIF-AS1 sponges miR-212-5p to activate NDUFA4 expression. lncMIF-AS1 overexpression promotes proliferation and decreases apoptosis of gastric cancer cells through the activation of oxygen consumption and ATP production. The lncRNA cytoplasmic endogenous regulator of oxidative phosphorylation 1 (Cerox1) sponges miR-488-3p and elevates the expression of several subunits of complex I in mouse neuroblastoma cells (73).

LIPID METABOLISM WITH lncRNA
Altered regulation in the synthesis and utilization of lipids is also a hallmark of cancers as it meets the high energy demand required for cancer proliferation and survival. In contrast to normal tissues and cells, malignant tumors prefer to synthesize lipids through the de novo pathway (74,75). Increased expression or abnormal activity of key lipogenic enzymes such as fatty acid synthase (FASN) and acetyl coenzyme A (acetyl-CoA) carboxylase is often attributed to the high growth rate and lipogenic phenotype of tumor cells (76)(77)(78)(79)(80). Moreover, carnitine palmitoyl transferase (CPT) 1 and 2, which are rate-limiting enzymes involved in mitochondrial fatty acid transportation, play crucial roles in increasing fatty acid oxidation required for the cellular fuel demands of breast cancer cells (81). CPT1A/CPT2 were highly expressed in recurrent human breast cancers and are associated with poor prognosis. Moreover, increased fatty acid oxidation has also been suggested as a source of energy (82), along with increased steroidogenesis by upregulation of Acyl-CoA synthetase long-chain family member (ACSL) 3 (ACSL3) in prostate cancer (83). ACSL4, as well as ACSL3, which is an enzyme that converts fatty acids to acyl-CoA, is also involved in the loss of androgen sensitivity and acquisition of castration resistance, leading to cancer growth and invasion in prostate cancer (83)(84)(85).We mention several lncRNAs involved in breast and prostate cancers in the following sections.

Nuclear Enriched Abundant Transcript 1 (NEAT1)
In breast and prostate cancers, NEAT1 stimulates the TCA cycle by promoting the use of free fatty acids as fuel. NEAT1 upregulates CPT1A expression by inhibiting miR-107 to promote the progression of breast cancer cells (86). CPT1A synthesizes acylcarnitines, which are transported from the cytosol into the mitochondria, and their acyl groups are metabolized through the TCA cycle. NEAT1 also affects ACSL4 expression by competitively sponging both miR-34a-5p and miR-204-5p in prostate cancer (87). The alteration of ACSL4 expression is essential in the development and progression of breast and prostate cancers: ER expression is inversely correlated with ACSL4 expression in breast cancer (84), and ACSL4 is also involved in the loss of steroid hormone sensitivity and the acquisition of castration resistance in prostate cancer (85,88). Recent findings indicate that NEAT1 affects mitochondrial dynamics and functions by regulating the sequestration of mRNAs encoding mitochondrial proteins in nuclear bodies called paraspeckles (89,90).

PCGEM1
In addition to regulating glucose metabolism as described above, PCGEM1 widely regulates metabolic gene expression, including lipid metabolism in prostate cancer cells (41). The expression levels of several enzymes involved in lipid biosynthesis, such as those of FASN and acetyl-CoA carboxylase alpha (ACACA), are decreased by PCGEM1 knockdown. PCGEM1-overexpressing prostate cancer cells showed an increase in the cellular level of citrate, indicating enhanced fatty acid synthesis.

Steroid Receptor RNA Activator (SRA)
SRA was originally identified as a lncRNA that coactivates steroid hormone receptor transcriptional activity by associating with steroid receptor coactivator-containing complexes (91). Furthermore, SRA has also been revealed to interact with other transcription factors (92,93). SRA overexpression has been found in various tumors including prostate cancer (91,94). Both androgen receptor-dependent and -independent mechanisms are involved in SRA-mediated prostate cancer progression (95,96). SRA expression is also significantly upregulated in breast cancer tissues (97). In MCF-7 breast cancer cells, SRA silencing decreased mRNA levels of insulin-induced gene 1 protein (INSIG1) and cholesterol transporter ATP-binding cassette transporter ABCA1, suggesting its role in lipids/cholesterol homeostasis (98,99).

CONCLUSION
In this review, we summarized the functions and mechanisms of cancer metabolism-related lncRNAs especially in breast and prostate cancers ( Table 1). We consider that these lncRNA functions may play critical roles in cancer pathophysiology from the viewpoint of their contribution to metabolic reprogramming. Moreover, the functions of cancer metabolism-related lncRNAs will be modulated by hormone status in breast and prostate cancers. Elucidating the mechanisms underlying metabolic alterations regulated by lncRNAs will lead to the development of new diagnostic and therapeutic options for both breast and prostate cancers.

AUTHOR CONTRIBUTIONS
SK, TT, KI, KH-I, and SI: conception, providing the data and design. SK and TT: manuscript writing. KI, KH-I, and SI: conception and final approval of manuscript. All authors contributed to the article and approved the submitted version.