The biological function of m6A demethylase ALKBH5 and its role in human disease

Chemical modifications of nucleobases were crucial for inducing changes in protein translation and regulating certain signaling pathways, which subsequently modulated kinds of biological processes. N6-methyladenosine (m6A) modification was first identified in mRNA-enriched RNA fractions in 1974, without any special attention [1]. m6A refered to methylation of the N6 position of adenosine base, which was one of the most prevalent and abundant internal modification in mammalian mRNAs and eukaryotes [2]. In recent years, with the application of high-throughput sequencing for detecting m6A, the understanding of the inside regulatory mechanisms came to light. m6A modification had been proved to be enriched in near stop codon and 3′untranslated terminal region (UTR) and translate near 5′UTR in a cap-independent manner [3]. m6A RNA modification of various RNAs, including messenger RNAs (mRNAs) [4], microRNAs (miRNAs) [5] and long non-coding RNAs (lncRNAs) [6], was a dynamic and reversible posttranscriptional modification process maintained by three different types of protein complex, including m6A readers, writers, and erasers [7]. This dynamic and reversible nature of the m6A modification made it pivotal in the rapid cellular communication. Recently, substantial progresses had been made in understanding m6A modifications in RNA metabolism, mRNA stability and splicing, translation efficiency, nuclear export, as well as alternative polyadenylation [8, 9].

Human AlkB homolog H5 (ALKBH5), was a famous m6A demethylase (also called m6A eraser), which had spawned significant biological and pharmacological interest among researchers [10]. Recently, the effects of ALKBH5 on many biological processes had been demonstrated, including proliferation [11], invasion [12], metastasis [13], ossification [14] and so on. In addition, ALKBH5 was also involved in kinds of cancers or non-cancers, such as glioblastoma [15], pancreatic cancer [16], colon cancer [17], breast cancer [18], gastric cancer [19], lung cancer [20], ovarian cancer [21], diabetes [22] and reproductive system diseases [23] (Figs. 1, 2).

In the field of cancer research, the underlying mechanisms of ALKBH5 in human cancers were not only unclear but also controversial. Up to date, the expression of ALKBH5 was up-regulated or down-regulated in various cancers, and played an oncogenic or tumor suppressive role in breast cancer, gastric cancer, colon cancer and so on [17, 19, 24]. The potential mechanisms were involved with long non-coding RNAs, miRNAs, mRNA and so on [19, 21, 24]. This kind of communication between ALKBH5 and different RNAs was importantly related with cancer cell proliferation, apoptosis, death, survival, migration, invasion, metastasis and so on [25, 26] (Table 1).

In the field of human non-cancer research, ALKBH5 was found to be dysregulated in human reproductive system diseases and osteogenic progression [27, 28]. For example, ALKBH5 acted to influence spermatogenesis and trophoblast invasion, thus affecting human reproductive system diseases [23, 29]. In addition, ALKBH5 also played an important role in the ossification of the ligamentum flavum cells and osteogenic progression via some famous signaling pathways, such as AKT and NF-κB signaling pathway [14, 28].

In this study, we summarized the recent advances made in relation to ALKBH5 dysregulation and its biological role coupled with the underlying mechanisms in various human diseases.

ALKBH5 was one of nine members of the AlkB family, a ferrous iron- and 2-oxoglutarate-dependent nucleic acid oxygenase (NAOX) and had been reported to catalyze the demethylation of m6A in RNA [30]. Zhou et al. [30] and Aik et al. [31] identified the crystal structure of ALKBH5 and revealed conserved residues important for recognition and demethylation mechanisms. Chu et al. [32] and Feng et al. [33] respectively identified the crystal structure of zebrafish ALKBH5 (fALKBH5) and human ALKBH5, which promoted the understanding of the substrate recognition specificity of ALKBH5 and offered a foundation for the development of inhibitors selective for NAOX. In addition, Xu et al. [34] affirmed the m6A binding pocket of ALKBH5 and the key residues related with m6A recognition through mutagenesis and isothermal titration calorimetry binding experiments. Furthermore, m6A served as a ‘conformational marker’, which induced different conformational outcomes in RNAs depending on sequence context and critically impacted its interactions with ALKBH5 [35]. In detail, it was found that the DSBH domain of ALKBH5 indispensablely interacted with ATP domain of DDX3, which was a member of the family of DEAD-box RNA helicases and took a great part in key biological processes such as cell cycle, apoptosis, and RNA metabolism [36]. Purslow et al. [37] further investigated the structure and dynamics of ALKBH5 in solution and offered the first atomic-resolution model of an AlkB protein in its disordered conformational state. There was also a great connection between single-nucleotide polymorphisms (SNPs) on the two RNA demethylases FTO and ALKBH5 [38]. At the same time, he reported the backbone ¹H, ¹⁵N, ¹³C chemical shift assignment of a fully active, 26 kDa construct of ALKBH5 [39].

In the majority of cancer research, the role of ALKBH5 in human cancers was controversial. It had been found to be upregulated or downregulated, and to play an oncogenic or tumor suppressive role in different kinds of cancers. We will discuss the different roles of ALKBH5 in various cancers thereinafter, including breast cancer, lung cancer, pancreatic cancer, glioma and so on.

ALKBH5 was also connected with human non-cancer, such as reproductive system diseases and the process of ossification.

The data described in this review suggested the controversial expression patterns and functions of ALKBH5. The mechanisms of ALKBH5 were involved in human cancers and non-cancers. On the one hand, ALKBH5 was up-regulated in kinds of cancer tissues and cell lines, such as breast cancer, lung cancer and epithelial ovarian cancer, and played an oncogenic role in tumor progression. On the other hand, other researchers indicated the opposite conclusions about the expression level and the role of ALKBH5 in pancreatic cancer, colon cancer and clear cell renal cell carcinoma. ALKBH5 regulated cancer cell proliferation, migration, invasion, tumor progression, metastasis, tumorigenesis and chemoresistance through modulating m6A methylation. These effects were orchestrated through multiple mechanisms, such as long non-coding RNAs, cancer stem cells self-renewal and autophagy. ALKBH5 could demethylate lncRNAs, enhance cancer cells self-renewal or regulate autophagy in cancers. Furthermore, ALKBH5 played a great role in human non-cancers, such as reproductive system diseases. Additionally, ALKBH5 had great potential for clinical application by acting as a new prognostic target. However, further researches are still needed to illuminate the exact details of ALKBH5 expression and its mechanisms in human cancers and non-cancers.