Bone is a highly adaptive and dynamic tissue in the body, mainly comprising cells of multiple lineages, type 1 collagen, and mineral. The integrity of the bone structure is maintained through continuously remodeling by osteoclasts and osteoblasts (Zaidi, 2007). Bone homeostasis is influenced by a variety of factors, with both general ageing and oestrogen deficiency causing imbalances in bone metabolism (Manolagas, 2010; Ceco et al., 2017). When bone homeostasis is dysregulated, the excessive bone loss causes osteoporosis (OP), which is characterized by bone microstructure degradation, decreased bone mineral density (BMD), and then increased risk of fracture (Siris et al., 2014). With the rapid development of the aging population, the incidence of OP is continually rising. More than 200 million people suffer from OP of different degrees worldwide (De Martinis et al., 2021). Fractures caused by OP have become a serious public health problem because of the associated high morbidity, disability, and mortality rates, which lead to the consumption of extensive social public health resources (Eastell et al., 2016; Ebeling et al., 2022).

Currently, m6A modification attracts great interest due to its important roles both in the various biological processes and pathogenesis of multitudinous diseases (Du et al., 2021). m6A modification, occurs in the adenosine base at the nitrogen-6 position of mRNAs and interferes with more than 7,000 mRNAs in mammalian transcriptomes (Dominissini et al., 2012; Zhou et al., 2016). Abnormal m6A modification is correlated with the proliferation, survival, and invasion of cancer cells (Zhang et al., 2016; Du et al., 2017; Cai et al., 2018; Chen et al., 2019; Lin et al., 2019).

Fat mass and obesity-associated protein (FTO) is the first identified m6A demethylase, which eliminates m6A modifications from RNAs (Jia et al., 2011). FTO was initially reported strongly related to polygenic obesity in humans (Dina et al., 2007; Zhao et al., 2014a). Then FTO is found partially located on nuclear speckles and FTO depletion remarkably increases the total m6A levels (Jia et al., 2011). Variation in FTO affects m6A methylation and thus disturbs physiological activities (Wang et al., 2020; Qing et al., 2021). Recently, m6A methylation has been reported to regulate bone metabolism via modulation of hormones, cytokines (Huang et al., 2021a). Given the essential roles of epigenetics in regulating bone homeostasis, FTO-mediated m6A methylation has been found to be associated with bone-related diseases (Wang et al., 2021). In this review, we summarized the influence of FTO-mediated m6A methylation in bone formation and resorption (Figure 1), which helps to explore the pathogenesis of osteoporosis and provide therapeutic strategies based on epigenetics.

The FTO gene was initially reported as an obesity-susceptibility gene that predisposes individuals to obesity (Dina et al., 2007; Peeters et al., 2008; Price et al., 2008). FTO gene is located on human chromosome 16 and regulates energy balance by controlling energy consumption (Fischer et al., 2009). Human FTO is about 400 kb in length and consists of 8 introns and 9 exons, encoding a variety of protein products. The FTO gene has high homology in mammalian species such as pigs, mice and other mammals (Stratigopoulos et al., 2008). The FTO protein belongs to the Fe(II)- and 2-oxoglutarate-dependent dioxygenases family (Gerken et al., 2007). Structural analysis revealed two domains of FTO: an amino–terminal domain (NTD) and a carboxy–terminal domain (CTD). The NTD is the core region of FTO demethylation activity, mainly consisting of a β-fold. While the CTD is composed largely of an α-helix, interacting with NTD to keep the stability of the protein (Han et al., 2010).

FTO is found partly located on nuclear speckles and widely expressed in fetal and adult tissues (Jia et al., 2011). The expression level of Fto mRNA is the highest in white adipose tissue and specific brain regions across mouse tissues. Moderate levels of Fto expression were revealed in calvaria bones, bone marrow and femur, while low expression was observed in skeletal muscle and heart (Zhang et al., 2019). FTO is the first classified m6A demethylase, which performs methylation modification at the N6 position of adenosine (Dominissini et al., 2012; Meyer et al., 2012) and interferes with genome-wide m6A demethylation (Wei et al., 2018). It is the most widespread internal chemical modification affects over 7,000 mRNAs in mammalian transcriptomes (Zhou et al., 2016). The functions of m6A are regulated by the interplay of methyltransferases (“writers”), binding proteins (“readers”), and demethylases (“erasers”) (Panneerdoss et al., 2018; Shi et al., 2019). The m6A writers methyltransferase-like 3 (METTL3), methyltransferase-like 14 (METTL14), and Wilms’s tumor 1-associating protein (WTAP) transfer methyl groups from reactive methyl compounds onto the N-6 position of adenosine (Ping et al., 2014). The m6A erasers FTO and AlkB homolog 5 (ALKHB5) can reversibly detach the methyl groups of m6A off RNAs (Jia et al., 2011; Zheng et al., 2013). FTO can oxidize m6A to N6-hydroxymethyl adenosine (hm6A) as an oxidation intermediate through intermediate modification of 2-oxoglutarate-dependent dioxygenase and ferrous ions. Then FTO performs further oxidation on the metastable hm6A in the same way to form an unprecedented N6-formyladenosine (f6A). Under physiological conditions, hm6A and f6A have a half-life of ∼3 h in an aqueous solution and then finally decompose into adenine (A) (Fu et al., 2013). After FTO removes the methyl group of m6A, the original sites of m6A methylation cannot be identified by “reader” proteins, which affects multiple physiological and pathological processes (Li et al., 2022; Yang et al., 2022). Abnormal expression and function of FTO are related to the occurrence of bone metabolic disorders. Therefore, this review mainly focuses on the role of FTO in bone metabolism and its regulatory mechanism.

Bone metabolism is a dynamic process of bone formation and resorption, in which multiple growth factors and signaling pathways are implicated (Clezardin et al., 2021). Osteogenesis that begins from bone marrow mesenchymal stem cells (BMSCs) to osteoblasts is extremely important in skeleton formation and regeneration. The complex correlation of m6A modification with osteogenesis has been consecutively explored. Growing evidence had indicated that FTO-mediated m6A modification is a novel regulator in regulating bone cells (Table 1).

Osteoclasts, bone-resorbing cells, which originate from myeloid precursors, undergo several stages of differentiation (Jacome-Galarza et al., 2019). Under physiological conditions, bone remodeling consists of osteoblast-mediated bone formation and osteoclast-mediated bone resorption. The pathological cause of osteoporosis is a disturbance in these two balanced processes (Durdan et al., 2022). A study indicated FTO expression was upregulated in bone marrow monocytes (BMMs) from OVX mice. FTO overexpression facilitated RANKL-induced binding of NF-κB to NFATc1 promoter and then promoted osteoclast differentiation in RAW264.7 and BMMs cells. FTO knockdown through periosteal injection of lentiviral shRNA-FTO inhibited osteoclastogenesis and bone resorption in osteoporotic mice (Zhuang et al., 2021). NF-κB, a downstream factor of RANKL/RANK signaling pathway, is essential in mediating the differentiation and maturation of osteoclasts (Park et al., 2017). Another research also showed that FTO depletion restrained the NF-κB signaling pathway via reducing the phosphorylation levels of p65, IKKα/β and IκBα in RAW264.7 cells and bone-marrow-derived macrophages (Gu et al., 2020). In addition, silencing FTO also markedly raised the expressions of pro-inflammatory markers (TNF-α and iNOS) in macrophages (Hu et al., 2019). Due to its important roles in osteoclast-specific genes, FTO is a promising therapeutic target on osteoporosis caused by abnormal bone resorption.

FTO is functionally essential in physiological processes, and its dysfunction has been related to various human diseases. Therefore, exploiting small-molecule modulators to utilize their biological function and excavate the pathogenic mechanisms are warranted. Flavonoids are polyphenols with high polarity and poor water solubility, which are difficult to be absorbed. Flavonoids can eliminate free radicals and play the role as antioxidants (Hernandez et al., 2009; Zhang et al., 2020b). Several studies have reported the role of flavonoids in improving bone health due to their anti-inflammatory and antioxidant properties (Preethi Soundarya et al., 2018; Ramesh et al., 2021). Flavonoids contain hydroxyl groups which readily form hydrogen bonds with amino acid residues, whereas low polarity compounds tend to bind to hydrophobic residues of FTO proteins. Chen et al. found the natural product Rhein has strong interaction with FTO. Rhein is a naturally existing anthraquinone with multifarious pharmacological effect (Cheng et al., 2021), which has been classified to bind with FTO and abolishes its demethylase activity (Chen et al., 2012). Rhein regulates m6A methylome rearrangement and represses adipogenic differentiation in a dose-dependent manner (Huang et al., 2021b). But Rhein can concurrently combine with FTO or alkB (Li et al., 2016). Implementing a method for selective restraint of FTO over ALKBH5 remains important. Besides, Rhein and its derivatives have been reported to have anti-osteoclastogenesis activity, inhibiting the TRAP activity and osteoclast differentiation (Xu et al., 2016; Jiang et al., 2019).

The FTO protein contains two-domain of 2-OG- and Fe(II)-dependent dioxygenases, showing -methyladenosine demethylase activity. A set of diverse 2-oxoglutarate (2-OG) analogs are selective inhibition of FTO. Studies have found that mice treated with IOX3, an effective inhibitor of FTO demethylase activity, showed significant reductions in BMD and BMC (McMurray et al., 2015). But IOX3 was also an inhibitor of other 2OG oxygenases, compounds designed to selectively inhibit FTO demethylase activity still needed further exploration. Aik et al. (2013) showed sodium oxalate restrained the FTO expression, and distinguished two broad-spectrum 2-OG oxygenase inhibitors: pyridine-2,4-dicarboxylate and NOG. Then FTO inhibitor R-enantiomer of 2-hydroxyglutarate (R-2HG) was found based upon the structure of NOG, which is a tumor metabolite generated by mutant isocitrate dehydrogenase (Su et al., 2018). The R-2HG dysregulated the osteogenic differentiation of MSCs via reducing the expression of early (IBSP and LPL) and late (BGLAP and Osterix) osteoblast differentiation-related genes (Liu et al., 2021). The oncometabolite R-2HG in AML cells stimulated epigenetic alteration and halted hematopoietic differentiation. Mechanically, R-2HG upregulated NF-κB-dependent genes in BMSC which promoted chemoresistance and proliferation of AML Cells (Chen et al., 2016). Meclofenamic acid (MA) is an anti-inflammatory drug that has been discovered as a selective inhibitor of FTO, which contends with FTO for the binding to the m6A-containing nucleic acid (Huang et al., 2015). Based on the structure of MA, the application of two MA-derived inhibitors, FB23 and FB23-2, impaired the osteogenesis of human MSCs (Chen et al., 2022). As shown in Figure 2, FTO inhibitors are classified according to their different structures. The binding sites and binding capacities of FTO inhibitors are different due to diverse structures. Given the functional role of FTO in the regulation of skeletal development and bone homeostasis, small molecule compounds targeting FTO can be developed to treat osteoporosis under the precise control of m6A modifications.

FTO-mediated m6A methylation is emerging as a novel target of bone development and bone metabolism. But the current comprehension of FTO-mediated m6A methylation in bone metabolism is still restricted. First, m6A methylation is a dynamic process, which requires multiple m6A methylation regulatory factors to work together in coordination. How the m6A methylation regulator coordinates and functions need to be further explored. Second, the molecular basis of FTO-mediated m6A methylation in bone resorption is not well elucidated. The effect of FTO on bone formation in different mouse models is still controversial. As discussed above, knockdown or overexpression of FTO in BMSC leads to a reduction in osteogenic differentiation. Inhibition of FTO in OVX mice promoted bone formation and alleviated osteoporosis. But FTO knockout prevented the skeleton from maintaining normal BMD and BMC. Thus, FTO may play a dual role in bone metabolism, requiring a certain level to maintain bone homeostasis, and the critical point of this process is not completely clear. And the role of FTO in osteoporosis in the current study is mainly focused on animal experiments, with fewer research in the population with osteoporosis. From the animal studies, it may be a potential biomarker for osteoporosis, but it needs to be further validated in clinical research on a larger population. Furthermore, small-molecule inhibitors of FTO have been identified and helped to understand its biological functions in multiple biological processes. Therefore, combining the precise regulation of m6A methylation in bone metabolism with small molecule inhibitors targeting FTO will provide new perspectives and treatment options for osteoporosis.