The process of cell division and cell proliferation is important for the growth and development of an organism, and organs undergo cellular evolution, renewal, or repair
[17][18]. To maintain bodily homeostasis, it is essential to sequentially control the molecular events that occur during the cell cycle. Dysregulation of these processes can lead to various diseases, including cancer and developmental deficiency. The mammalian cell cycle is composed of four phases: G
1 (the post-mitotic interphase), S (the DNA synthetic phase), G
2 (the post-synthetic phase), and M (the mitotic phase)
[19]. The G
0 phase is usually difficult to distinguish from G
1, for which reason it is usually termed the “G
0/G
1” phase. The cell cycle is regulated by CDKs and their associated cyclins at different stages (
Figure 1). Specifically, CDK4/6 complexes with cyclin D at the early G
1 phase; CDK2 complexes with cyclin E during the late G
1 phase, or the G
1/S phase transition, and complexes with cyclin A at the S phase; CDK1 complexes with cyclin A at the G
2 phase and cyclin B at the M phase
[20]. CDKs can be regulated by the intrinsic regulators, including p53, p21, p16, and cdc25. Epigenetic mechanisms play essential roles in cell cycle regulation, either through controlling the expression of cell-cycle-related genes—such as CDKs—or through the modification of chromatin condensation and controlling the related signaling proteins via histone marks (
Figure 1).
3. Epigenetic Regulation on Ciliogenesis
Cilia are microtubule-based structures that protrude from the cell surface. There are two types of cilia—motile, and non-motile cilia—serving motile and sensory functions, respectively
[21][22]. In animals, motile cilia are found on limited tissues, such as respiratory epithelium and choroid plexus epithelial cells
[23]. Non-motile cilia, also called primary cilia, can be found on nearly every cell type, and function as sensors for mechanical and chemical environmental cues that regulate cellular differentiation or division
[24][25]. In this review, we mainly focus on primary cilia, and the term “cilium/cilia” represents the primary cilium/cilia only, unless otherwise specified. In general, the cilium undergoes cycles of assembly and disassembly that are regulated tightly by a complicated protein network, with proteins trafficking from the base to the cilia tip (anterograde), and from the cilia tip back to the cytoplasm (retrograde), mediated by intraflagellar transport (IFT) complexes of the IFT-B and IFT-A families, driven by a kinesin-2 motor in the anterograde direction and a dynein motor in the retrograde direction, respectively
[26].
4. Crosstalk between the Cell Cycle and Ciliogenesis
With the knowledge of the process of ciliogenesis, from initiation and elongation to resorption, it is necessary to understand the crosstalk between the cell cycle and ciliogenesis. In general, the cilia assembly and disassembly cycle is closely associated with the cell cycle, as described above, which suggests that the cilia biogenesis process can be passively regulated by cell cycle progression. However, several studies have provided some new clues about the relationship between the primary cilia and the cell cycle. Firstly, studies have indicated that by the inhibition of several negative regulators of ciliogenesis, cilia formation was maintained in proliferating cells and the cell cycle was arrested, while overexpression of these factors inhibited cilia formation, even in serum-starved cells
[27]. It has been reported that trichoplein complexed with Aurora A (Aur A) blocks aberrant primary cilia assembly in proliferating cells. Overexpression of trichoplein inhibited primary cilia assembly in serum-starved cells, whereas knockdown of trichoplein induced primary cilia assembly, even in serum-contained medium, and induced G
0/G
1 arrest
[28], suggesting the independence of ciliogenesis from the cell cycle. The following findings also indicate that ciliogenesis can function as a regulator of the cell cycle and cell proliferation: (1) defects of the cilia or lack of cilia cause cellular proliferation abnormalities and diseases; (2) primary cilia sense urine flow, and are essential for the maintenance of the epithelial architecture in the kidneys; and (3) defects of primary cilia cause cystic kidney disease characterized by epithelial abnormalities, suggesting the roles of primary cilia in the regulation of the cell cycle and cell proliferation.
5. Cilia-Dependent Signaling Pathways in Cell Cycle Regulation, and the Association of Epigenetic Mechanisms with These Signaling Pathways
Cilia also function as regulatory switches to control diverse signaling pathways, including Hedgehog-, Wnt-, PDGFR-, Notch-, TGF-β-, mTOR-, and GPCR-associated signaling. All of these signaling pathways play crucial roles in various cellular processes, such as in organ and embryonic development, cardiac function, planar cell polarity, transactivation, differentiation, cell cycle progression, apoptosis, tissue homeostasis, and the immune response. Growing evidence demonstrates that these signaling pathways can be regulated by epigenetic mechanisms in human diseases.
6. Therapeutic Targets of Cell-Cycle- and Ciliogenesis-Associated Epigenetic Regulators
Increasing evidence exhibits the roles of epigenetic regulators in regulating the cell cycle and ciliogenesis during organ development, and in many proliferative diseases (such as cancers) and cilia-deficient diseases (termed as ciliopathies)
[29][30][31]. Thus, targeting abnormal epigenetic regulators might be an advanced therapeutic strategy.
The role of the histone methyltransferase EZH2 in driving transformation from benign to malignant melanoma has been associated with the regulation of both the cell cycle and ciliogenesis. On one hand, EZH2 suppresses the expression of ciliary proteins through its repressive histone mark H3K27me3, by binding to the promoters of many ciliary genes, including WDR19, IFT81, FUZ, etc. On the other hand, loss of cilia results in the inappropriate activation of cilia-dependent signaling. In A375 human melanoma cells, the enhanced WNT/β-catenin activity and nuclear accumulation is accompanied by loss of cilia, due to the absence of cilia-mediated spatial segregation. The involvement of EZH2 in this process was evidenced by the findings that increased EZH enhances WNT/β-catenin signaling in benign melanocytic cells, and inhibition of EZH2 with GSK503 induces cilia assembly in most melanoma cell lines. Blockade of EZH induces cilia assembly, which could be reversed by silencing ciliary genes
[31]. The EZH2 inhibitor GSK503 is a small molecule, which exerts its inhibition by competition with S-adenosylmethionine (SAM), the donor of the methyl group. There are several other EZH2 inhibitors—such as DZNep, EPZ005687, EI1, GSK126, UNC1999, GSK503, and EPZ-6438—which are all small molecules and function as SAM-competitive inhibitors. DZNep is one of the purine nucleoside analogs, which has been tested in the treatment of hematological malignancies and autoimmune diseases. EPZ005687 is a selective inhibitor of EZH2 rather than EZH1. EI1 mainly regulates and activates EZH2 target genes via the removal of repressive histone mark H3K27 methylation. GSK126 has been tested in a live failure murine model, in which it decreased inflammation response through the reduction of H3K27 methylation. UNC1999 functions similarly to GSK126. GSK503 has been tested in a human melanoma murine model. EPZ-6438 is an improved product of EPZ005687. Among those inhibitors, tazemetostat (EPZ-6438) has been approved for the treatment of epithelioid sarcoma
[32]. Whether those EZH2 inhibitors also affect ciliogenesis in different disease models needs to be further investigated.