AFAP1‐AS1: A novel oncogenic long non‐coding RNA in human cancers

Long non‐coding RNAs (lncRNAs), a group of non‐protein‐coding RNAs with more than 200 nucleotides in length, are involved in multiple biological processes, such as the proliferation, apoptosis, migration and invasion. Moreover, numerous studies have shown that lncRNAs play important roles as oncogenes or tumour suppressor genes in human cancers. In this paper, we concentrate on actin filament‐associated protein 1‐antisense RNA 1 (AFAP1‐AS1), a well‐known long non‐coding RNA that is overexpressed in various tumour tissues and cell lines, including oesophageal cancer, pancreatic ductal adenocarcinoma, nasopharyngeal carcinoma, lung cancer, hepatocellular carcinoma, ovarian cancer, colorectal cancer, biliary tract cancer and gastric cancer. Moreover, high expression of AFAP1‐AS1 was associated with the clinicopathological features and cancer progression. In this review, we sum up the current studies on the characteristics of AFAP1‐AS1 in the biological function and mechanism of human cancers.


| INTRODUCTION
Nowadays, cancer is one of the major causes of death all over the world. 1,2 According to mortality data collected by the National Center for Health Statistics, it is estimated that 1 688 780 new cancer cases and 600 920 cancer deaths will take place in the United States in 2017. 3 In order to fight against cancer effectively, we should make a great effort to find more precise diagnostic biomarkers and effective therapeutic targets.
Recently, increasing evidence has shown that the non-coding portion of the genome has a crucial functional importance in both normal physiology and diseases. [4][5][6] Long non-coding RNAs (lncRNAs), a group of non-protein-coding RNAs with more than 200 nucleotides in length, play a vital role in regulating significant cellular functions, including cell proliferation, differentiation, apoptosis, invasion, metabolism, developmental timing and immune responses. [7][8][9][10][11][12] However, mutation of lncRNAs will cause cancer initiation and promote the metastasis of malignancy. [13][14][15] For instance, lncRNA metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) was first found in lung cancer metastasis. Many studies had demonstrated that lncRNA MALAT1 was overexpressed in various cancer, and it might act as a potential biomarker and therapeutic target in cancer treatment. [16][17][18] LncRNA MALAT1 might function as an oncogene through controlling alternative splicing process in breast cancer, influencing the expression of N-cadherin and E-cadherin in bladder cancer, combining with a multifunctional RNA-binding protein in colorectal cancer (CRC) and osteosarcoma. [19][20][21][22] LncRNA HOX antisense intergenic RNA (HOTAIR) induced cancer invasion and metastasis by regulating PRC2 target genes in breast cancer and epithelial-mesenchymal transition (EMT) programme in gastric cancer (GC). 23,24 LncRNA H19 was upregulated in GC and associated with miR-675, p53 and Isthmin1 that improved cells proliferation, migration and invasion. [25][26][27][28][29] Among so many cancerrelated lncRNAs, actin filament-associated protein 1-antisense RNA 1 (AFAP1-AS1) was initially discovered in oesophageal adenocarcinoma in 2013. 30 Then, numerous recent studies had focused on lncRNA AFAP1-AS1 and demonstrated that it was upregulated in many cancers and played an important role in tumour progression. A meta-analysis had shown that high expression of AFAP1-AS1 in human cancers was closely related to poor clinical outcome such as lymph node metastasis and distant metastasis. 31 Hence, we chose it as the main research object to summarize its characteristics in the biological function and mechanism of human cancers.

| IDENTIFICATION OF AFAP1-AS1
Actin filament-associated protein 1 (formerly AFAP-110), an actin cross-linking protein and a cSrc-binding partner, is a member of the AFAP family which includes AFAP1, AFAP1 like-1 and AFAP1 like-2/ XB-130. 32,33 There are 2 pleckstrin homology domains in AFAP1, and one of them involves a protein kinase C-binding site and carboxyterminal domains. 33,34 On the basis of multimerization associated with its leucine zipper and binding to actin filaments through its carboxy- can affect the expression of AFAP1. [36][37][38][39] Further experiments have demonstrated that AFAP1-AS1 was overexpressed in cancer tissues and cell lines, such as oesophageal cancer, pancreatic ductal adenocarcinoma (PDAC), nasopharyngeal carcinoma (NPC) and lung cancer. In addition, overexpression of AFAP1-AS1 was closely associated with tumour size, lymphatic metastasis, distant metastasis, tumournode-metastasis (TNM) stage and poor prognosis of cancer patients.

| Oesophageal cancer
Oesophageal cancer is the eighth most frequent types of cancer and is the sixth leading cause of tumour-related death all over the world. 59,60 There are 2 primary histological subtypes of oesophageal cancer, including oesophageal adenocarcinoma (OAC) and oesophageal squamous cell carcinoma (OSCC). 61,62 OSCC accounts for more than 95% of oesophageal cancer. 63 OAC is one of the fastest growing cancers in the Western world, while OSCC is the main subtype of oesophageal cancer in Asia. 3,64 Although different kinds of treatment have been developed, including chemotherapy, radiotherapy and surgery, the long-term survival rate of oesophageal cancer is still extremely low. 64,65 Because of the rising morbidity and poor prognosis of oesophageal cancer patients, it is urgent to look for new tumour markers and therapeutic targets for early diagnosis and advanced treatment of oesophageal cancer patients. Subsequent studies demonstrated that AFAP1-AS1 was increased in OSCC, and high expression of AFAP1-AS1 was closely associated with tumour size, tumour depth, lymphatic metastasis, distant metastasis and TNM stage. Moreover, those OSCC patients with increased AFAP1-AS1 level have shorter progression-free survival and overall survival. Overexpression of AFAP1-AS1 will lead to tumour resistance to radiotherapy and chemotherapy in OSCC patients who received definitive chemoradiotherapy. Furthermore, knockdown of AFAP1-AS1 in OSCC cells suppressed cell proliferation and colony formation and induced cell apoptosis. 40,41 Therefore, AFAP1-AS1 may work as a novel prognostic marker and potential therapeutic target for oesophageal cancer.

| Pancreatic ductal adenocarcinoma
Pancreatic ductal adenocarcinoma, one of the most aggressive solid malignancies, is the fourth leading cause of cancer-related deaths all over the world. 66,67 PDAC is characterized by a fatal disease with early metastasis and resistance to chemotherapy and radiation therapy. 7,68 Although the study of PDAC has made rapid progress in the last decades, the 5-year survival rate of PDAC patients is still only around 5%-7%. 69,70 Therefore, it is crucial to identify reliable biomarkers for early diagnosis of PDAC patients. including E-cadherin, N-cadherin, vimentin, Slug and Snail1. As we know, EMT is deemed to be the essential process of cancer progression, enhancing tumour migration, invasion and metastasis. [71][72][73][74] During the EMT process, epithelial cells lose epithelial status, apico-basal polarity and cell-cell adhesion so as to transform into mesenchymal cells. [74][75][76] In order to distinguish diverse functions, EMT is classified into 3 types: primary/type 1, secondary/type 2 and tertiary/type 3. Type 1 is associated with implantation, embryogenesis and organogenesis. Type 2 takes part in wound healing, tissue regeneration and organ development. Type 3 promotes tumour metastasis. [77][78][79] During cancer progression and metastasis, the expression of some EMT-related genes is changed, such as mesenchymal genes (fibronectin, N-cadherin and vimentin) are increased while epithelial genes (E-cadherin and ZO-1) are decreased. 80,81 Ye et al 42 also found that inhibition of AFAP1-AS1 reduced PDAC cell tumorigenicity in nude mice. However, amplification of AFAP1-AS1 produced opposite effects ( Figure 2). In conclusion, AFAP1-AS1 has potential value as a prognostic biomarker and therapeutic target in PDAC. T A B L E 1 Overexpression of actin filament-associated protein 1-antisense RNA 1 (AFAP1-AS1) is associated with clinicopathological features

| Nasopharyngeal carcinoma
Nasopharyngeal carcinoma, a unique disease to Southeast Asia, is associated with the Epstein-Barr virus (EBV). 82,83 About 75%-90% of NPC cases are diagnosed at advanced stages due to the non-specific symptoms at an early stage and poor accessibility for physical examination. 84,85 The main clinical treatment of NPC is radiotherapy over the past few decades, but many patients finally die because of recurrence and distant metastasis. 86,87 Therefore, it is essential to find therapeutic targets and prognostic biomarkers for

| Lung cancer
Lung cancer is the leading cause of cancer-related mortality all over the world. 88

| Ovarian cancer
Ovarian cancer (OC) is the third most widespread carcinoma of the female reproductive system. 104 Despite the advances in surgery, diagnostic method and new chemotherapy, OC mortality rate is still high because most patients are diagnosed at an advanced stage. [105][106][107] Therefore, it is exceedingly important to study its molecular mechanisms.
Yang et al 51  Therefore, their results indicate that AFAP1-AS1 can serve as a novel oncogene and therapeutic target for OC.

| Colorectal cancer
Colorectal cancer is the third most commonly diagnosed cancer and the second leading cause of cancer death worldwide. 104,108 Although advanced treatments, involving the combination of surgery, radiation therapy, chemotherapy and targeted therapy, are utilized to improve the prognosis of CRC patients, the recurrence and metastasis of CRC are still unavoidable. 109,110 The incidence and mortality of CRC will reduce by screening CRC from curable early stage, so we need to find a novel diagnostic and prognostic indicator for CRC.
Some experimental results proved that AFAP1-AS1 was aberrantly overexpressed in CRC tissues and cells lines, and overexpression of AFAP1-AS1 predicted poor prognosis of CRC patients. 52  Taken together, these results suggest that AFAP1-AS1 produces oncogenic effects in BTC and may become an effective diagnostic and therapeutic target for BTC.

| Gastric cancer
Gastric cancer, the fourth most commonly diagnosed cancer, is one of the major causes of cancer-related death all over the world. 115,116 Although surgery and chemotherapy for GC have made great progress, GC patients at an advanced stage remain a poor prognosis, having an extremely low 5-year survival rate. 117,118 Thus, it is urgent to find a new biomarkers for diagnosis and prognosis of GC.
Guo et al 58 reported that AFAP1-AS1 was overexpressed in GC tissues and cells compared with corresponding normal counterparts.