The dysregulation of the ubiquitin-proteasome system will result in the abnormal accumulation and dysfunction of proteins, thus leading to severe diseases. Seven in absentia homolog 1 (Siah1), an E3 ubiquitin ligase, has attracted wide attention due to its varied functions in physiological and pathological conditions, and the numerous newly discovered Siah1 substrates. In cancer and nervous system diseases, the functions of Siah1 as a promoter or a suppressor of diseases are related to the change in cellular microenvironment and subcellular localization. At the same time, complex upstream regulations make Siah1 different from other E3 ubiquitin ligases. Understanding the molecular mechanism of Siah1 will help the study of various signaling pathways and benefit the therapeutic strategy of human diseases (e.g., cancer and nervous system diseases). In the present review, the functions and regulations of Siah1 are described. Moreover, novel substrates of Siah1 discovered in recent studies will be highlighted in cancer and nervous system diseases, providing ideas for future research and clinical targeted therapies using Siah1.
Ubiquitination is a key process of the post-translational modification of proteins, playing an important role in the stability of the intracellular environment, the proliferation and differentiation of cells, and a number of cellular functions (
Increasing attention has been focused on E3 ubiquitin ligases due to their unique functions compared with E1 and E2. E3 ubiquitin ligases regulate a range of cellular physiological processes, such as cell proliferation and differentiation, participate in DNA damage and repair, and control the cell cycle (
Currently, E3 ubiquitin ligases can be classified in three main types [i.e., RING E3s (~600 in humans), homologous to the E6AP carboxyl terminus (HECT) E3s (~30 in humans) and RING-between-RING (RBR) E3s (~12 in humans)] depending on the characteristic domains and the mechanism of ubiquitin transfer to the substrates (
The seven in absentia homolog (Siah) family of proteins, which belong to the RING E3s, are the mammalian homologs of the
Siah1 and Siah2 share high sequence similarity (86%) and presumably high structural homology. The difference between Siah1 and Siah2 is the additional amino acid sequence in the N terminal of Siah2 (
Siah family proteins usually consist of an N-terminal catalytic RING domain, two zinc finger domains and a C-terminal substrate-binding domain (SBD) that includes the first two zinc finger domains (
The characteristics of the SIPs have also been studied (
HCC is one of the most common malignancies worldwide (in 2020, there were 910,000 new cases of HCC worldwide, ranking sixth among all cancer types; The Cancer Genome Atlas,
Wnt/β-catenin signaling pathways are one of the key cascades regulating cell growth, cell development and differentiation of normal stem cells, and have also been tightly associated with cancers made up of several key proteins, including Wnt, β-catenin, AXIN1, adenomatous polyposis coli protein (APC) and glycogen synthase kinase-3β (GSK-3β) (
However, some studies have reported that Siah1 also promotes Wnt/β-catenin signaling pathways by inducing the ubiquitination and proteasomal degradation of AXIN1, suggesting the positive regulation of Siah1 in Wnt/β-catenin signaling pathways (
Acquired chemoresistance during long-term chemotherapy is one of the most important factors to limit the application of some chemotherapy drugs, such as doxorubicin (Dox), for the clinical treatment of patients with HCC (
Notably, Siah1 in HCC functions as a tumor suppressor protein and as an oncoprotein (
BC is the most common cancer in women and is considered the second leading cause of cancer-related death in women (in 2020, there were 2,260,000 new cases of BC worldwide, ranking first among all cancer types of women; The Cancer Genome Atlas,
The classification of BC is complex. Under the general trend of the development of precision medicine, oncologists prefer to classify breast cancer by molecular classification (
Chemotherapy is the basic treatment of BC, and has made marked progress over the last few decades, with the emergence of new beneficial treatment methods, such as neoadjuvant chemotherapy (
The dysregulation of the ubiquitin-proteasome system (UPS) is observed in solid tumors and leukemia (
Acute promyelocytic leukemia (APL) is one of the most characterized forms of acute myeloid leukemia (AML) (
Super elongation complexes (SECs) promote the transcription of normal and leukemia-associated gene expression (
GBM is the most common brain cancer (48%), with high tumor heterogeneity and poor survival time (median overall survival time, 12–14 months) in adults (
Additionally, some studies have suggested that the Siah1-homeodomain-interacting protein kinase 2 (HIPK2)-p53Ser46 axis plays a key role in the promotion of glioma progression (
Siah1 may also partially act as a tumor suppressor in GBM when it interacts with CacyBP/SIP. CacyBP/SIP inhibits the migration and invasion behaviors of GBM cells by activating Siah1-mediated ubiquitination and degradation of cytoplasmic p27/kip1 (a key transcription factor and an oncoprotein highly expressed in GBM tissues) (
Studies have shown that Siah1 promotes cancer progression only in GBM and HCC, and only when Siah1 is localized in the nucleus (
The functions of Siah1 to induce the proliferation of cancer cells may be due to increased protein levels of FBP-3 (
Siah1 also acts as a tumor suppressor in CRC and pancreatic carcinogenesis. The knock down of Siah1 by shRNA promotes HCT116/SW480 CRC cell proliferation and migration, and results in fast tumor growth and a markedly large tumor volume in nude mice. Mechanically, Siah1 represses the occurrence and development of CRC by promoting the ubiquitylation of AKT and inhibiting the activity of the MAPK, PI3K-AKT and Hippo pathways (
PD, one of the most common neurodegenerative diseases, is manifested by a series of movement disorders, such as static tremor, bradykinesia, myotonia, and postural and gait disorders (
Developmental delay is defined as the skills of a child in one or a number aspects, including physical, motor, socioemotional, speech and language, and cognitive development, being significantly slower than those of other children of the same age (
The overinhibition of the Wnt/β-catenin signaling pathways is one of the important pathogenesis factors of developmental delay (
In addition, Siah1 has recently been identified as an upstream regulator of Akt3 (Akt signaling is an important regulator of neural development) (
Neuronal damage includes a series of diseases, including spinal cord injury and cerebral ischemia reperfusion, and the common feature of these diseases is the excessive apoptosis of nerve cells (
Siah1 was previously considered to be only a neuroprotective factor (
AD, the most common chronic and irreversible neurodegenerative disease in the world, is characterized by impaired cognitive function and loss of self-care ability (
Siah1 functions as a tumor suppressor in the vast majority of tumors [e.g., BC (
The functions of a protein depend on a number of elements, including post-translational modification, cell types, cellular microenvironment and binding to other proteins. Siah1 was originally identified as a tumor-suppressing protein for BC (
As an E3 ubiquitin ligase, the most important function of Siah1 is the ubiquitination of substrates to promote their degradation or change their function (
The subcellular localization of Siah1 also determines the function of Siah1. The nuclear localization of Siah1 promotes the occurrence of cancer and nerve apoptosis, suggesting that the nuclear localization of Siah1 is a pathological phenomenon (
The oncoprotein functions of Siah1 in GBM seem to be closely related to the tumor hypoxic microenvironment (
The dysregulation of UPS is usually caused by the mutations of E3 ubiquitin ligase, such as SPOP (
Although numerous studies have focused on the upstream regulations of Siah1 (
The present review summarized the novel substrates and complex upstream regulations of Siah1, describes the functions of Siah1 as a tumor suppressor protein and an oncoprotein, and discusses the potential mechanisms of the different roles of Siah1 in the nervous system and cancer. In addition, the review focused on the effects of Siah1 nuclear localization and the special status of Siah1 (interacting with E2 alone or forming the ubiquitin-ligase complex). Moreover, it highlighted the clinical significance of Siah1 in human diseases. This review may provide inspiration for future Siah1 research.
The authors would like to thank Dr Yuqi Wang (West Lake University, Hangzhou, Zhejiang, China) for the kind help and good advice provided.
Not applicable.
XJ conceived the present study. HZ drafted the manuscript. YG, JW and MY made substantial contributions to the interpretation and drafting of the study, and revised the manuscript critically for important intellectual content. All authors read and approved the final manuscript. Data authentication is not applicable.
Not applicable.
Not applicable.
The authors declare that they have no competing interests.
(A) The regulations of Siah1 vary greatly. At the transcriptional regulation level, p53, p21, E2F1 and HIF-1α trans-activate the transcription of Siah1. At the translational regulation level, microRNAs and lncRNAs inhibit the translation of Siah1 mRNA. At the post-translational regulation level, ASK1 induces the phosphorylation of Siah1. Overexpression of CacyBP/SIP promotes the interaction between Siah1 and cytoplasmic p27, which in turn increases the ubiquitination and degradation of cytoplasmic p27. Ubiquitin conjugase UbcH8 interacts with Siah1 to form a complex to ensure the functions of Siah1. HCF1 and HCF2 antagonize the E3 ligase activity of Siah1 through binding and blocking the substrate-binding domain. The AFF4-ELL2 interaction sequesters ELL2 away from Siah1, thereby inhibiting Siah1 ubiquitination of ELL2. (B) Siah1 consists of a N-terminal catalytic RING domain, two zinc finger domains and a C-terminal substrate binding domain that includes the first two zinc finger domains. A consensus Pro-X-Ala-X–Val-X-Pro (VxP, core sequence; where X is not conserved) motif is common to a family of SIPs (for example, CacyBP/SIP, Bim, PHD3, AXIN, HIPK2). Compared with Siah1, Siah2 has an additional amino acid sequence (~40 amino acids) at the N terminal. (C) Siah1 can interact with E2 ubiquitin-conjugating enzyme alone or become an essential part of the ubiquitin-ligase complex, which includes CacyBP/SIP, SKP1, TBL or EBI and Siah1. (D) β-catenin, AXIN1, APC and GSK-3β form a degradation complex without Wnt signaling, inducing the phosphorylation of β-catenin and finally leading to the degradation of β-catenin through the ubiquitin-proteasome pathway. The degradation complex is destroyed in response to Wnt signaling, releasing β-catenin, and thus activating transcription of downstream genes to promote the proliferation and survival of cells. Siah1 also induces the ubiquitination and proteasomal degradation of AXIN1 to promote the Wnt/β-catenin signaling pathways with Wnt signaling. TRAF4 protects β-catenin from Siah1-mediated degradation by competing with β-catenin for binding to Siah1 and replacing it for degradation. (E) The low protein level of Siah1 induces the degradation of dissociative ELL2 to prevent the formation of new SECs. The high protein level of Siah1 degrades all SECs. Siah1, seven in absentia homolog family proteins 1; Jab1, c-Jun activation domain-binding protein 1; ASK1, apoptosis signal-regulating kinase 1; UBCH8, ubiquitin conjugating enzyme human 8; Ub, ubiquitin; HCF1/2, host cell factor 1/2; P-TEFb, positive transcription elongation factor b; TRAF4, TNF receptor-associated factor 4; APC, adenomatous polyposis coli protein; GSK-3β, glycogen synthase kinase-3β; ELL2, elongation factor for RNA polymerase II 2; AFF4, AF4/FMR2 family member 4; miR, microRNA; lncRNA, long non-coding RNA; CacyBP, calcyclin-binding protein; Bim, Bcl-2-interacting mediator of cell death; PHD3, prolyl-hydroxylase protein 3; HIPK2, homeodomain-interacting protein kinase 2; SIP, Siah1-interacting protein; TBL/EBI, F-box-like/WD repeat-containing protein TBL1 or EBI.
(A) Siah1 mediates the ubiquitination of PHD3 and induces the degradation of PHD3, increasing the abundance of HIF-1α and promoting cells to adapt to hypoxia. HIF-1α trans-activates the transcription of Siah1 by coordinating key histone modifications on the Siah1 promoter to continuously increase HIF-1α expression and form a positive feedback loop. (B) Siah1 targets HIPK2 for poly-ubiquitination and proteasomal degradation, thereby inhibiting the phosphorylation of p53 at Ser46 and preventing cell apoptosis. p53 continues to activate the transcription of Siah1 and forms a positive feedback loop. The initiation of this positive feedback mechanism may be mediated by HIF-1α under hypoxic stress. (C) Under cell stress, GAPDH translocates to the nucleus in a Siah1-dependent manner upon glutamate stimulation and stabilizes Siah1 to facilitate degradation of nuclear proteins by Siah1, resulting in cell apoptosis and neuronal damage. (D) Siah1 functions as a tumor suppressor in the vast majority of tumors (breast cancer, hepatocellular cancer, leukemia, colorectal cancer and osteosarcoma). In some cancer types, such as glioblastoma and a part of hepatocellular carcinoma (where Siah1 is localized to the nucleus), Siah1 functions as an oncoprotein. Siah1, seven in absentia homolog family proteins 1; PHD3, prolyl-hydroxylase proteins 3; HIF-1α, hypoxia-inducible factor 1α; pVHL, von Hippel-Lindau disease tumor suppressor protein; HIPK2, homeodomain-interacting protein kinase 2; p53, tumor suppressor p53; P, phosphate group; GAPDH, glyceraldehyde 3-phosphate dehydrogenase.
Regulations of Siah1 ubiquitin ligase.
Level of regulation | Regulator | Mode of regulation |
---|---|---|
Transcriptional regulation | p53 | p53 acts directly on |
Jab1 | Jab1 inhibit the expression of p53 to suppress the transcription of |
|
HIF-1α | HIF-1α trans-activates the transcription of |
|
p21 | The transcription of |
|
E2F1 | E2F1 can activate transcription from the |
|
Translational regulation | miR-135a, miR-424, miR-944, miR-299-5p, miR-15b-5p, miR-107 | MicroRNAs inhibit the translation of |
lncRNA RP11, hnRNPA2B1 | RP11 directly binds to the CDS of |
|
Post-translational regulation | ASK1 | Phosphorylation of Siah1 by ASK1 triggers GAPDH-Siah1 stress signaling. |
CacyBP/SIP | Overexpression of CacyBP/SIP promotes the interaction between Siah1 and cytoplasmic p27, which in turn increases the ubiquitination and degradation of cytoplasmic p27. | |
HCF1/2 | HCF1 and HCF2 antagonize the E3 ligase activity of Siah1 by binding and blocking the substrate-binding domain. | |
AFF4 | The AFF4-ELL2 interaction sequesters ELL2 away from Siah1 thereby inhibiting Siah1 ubiquitination of ELL2. | |
UBCH8 | Ubiquitin conjugase UbcH8 interacts with Siah1 to form a complex to ensure the function of Siah1. |
p53, tumor suppressor p53; Jab1, c-Jun activation domain-binding protein 1; HIF-1α, hypoxia-inducible factor 1α; p21, cyclin-dependent kinase inhibitor p21; E2F1, E2F transcription factor 1; CDS, coding sequence; ASK1, apoptosis signal-regulating kinase 1; CacyBP/SIP, calcyclin-binding protein/Siah-1-interacting protein; HCF1/2, host cell factor 1/2; AFF4, AF4/FMR2 family member 4; UBCH8, ubiquitin conjugating enzyme human 8; miR, microRNA; lncRNA, long non-coding RNA.
SIPs of Siah1 ubiquitin ligase in human diseases.
Disease type | Substrate | Function of the SIPs | Degradation of the substrate | Physiological evidence (cell or animal model) |
---|---|---|---|---|
Cancer | ||||
Breast cancer | JNK | Promotion of cell apoptosis and inhibition of MAPK signaling pathways. | No | The inhibition of Siah1 expression promotes human breast cancer cell proliferation, colony formation, migration and invasion, and inhibits apoptosis ( |
Bim | DNA-binding transcription factor activity. | No | ||
TRAF4 | Protection of β-catenin from Siah1-mediated degradation and leading chemotherapy resistance. | Yes | ||
Glioma | HIPK2 | Response to DNA damage and promotion of cells apoptosis. | Yes | The knockdown of Siah1 by shRNA severely suppresses the migration and invasion of human glioma U251 cells under hypoxia, while overexpression of Siah1 promotes it. However, when Siah1 interacts with CacyBP/SIP, the overexpression of Siah1 suppresses the migration and invasion of human glioma U251 and U87 cells ( |
PHD3 | Degradation of the HIF-1α. | Yes | ||
CacyBP/SIP | The part of Siah1 ubiquitin-ligase complexes. | No | ||
p27/kip1 | Negative regulation of the cell cycle. | Yes | ||
Hepatocellular carcinoma | Axin, β-catenin | Cell migration and cell differentiation. | Yes | The overexpression of Siah1 induces growth arrest and apoptosis in HepG2, SNU475 and Huh7 cells ( |
ZEB1 | Epithelial-mesenchymal transition. | Yes | ||
Leukemogenesis | PML-RARa | The fusion protein of leukemia. | Yes | In the murine myeloblastic cell line M1 (generated from a spontaneous leukemia), expression of a stably introduced temperature-sensitive mutant of the tumor suppressor p53 activates the |
ELL2 | An elongation factor that modulates gene expression. | Yes | ||
AML1-ETO | The fusion protein of leukemia. | Yes | ||
AF4-MLL | ||||
Colorectal cancer | AKT, YAP | Inhibition of cells apoptosis | Yes | The knockdown of Siah1 by shRNA promotes HCT116/SW480 colorectal cancer cell proliferation and migration, and results in faster tumor growth and a markedly larger tumor volume in nude mice ( |
ZEB1 | Epithelial-mesenchymal transition. | Yes | ||
Osteosarcoma | ZEB1 | Epithelial-mesenchymal transition. | Yes | None. |
Nervous system diseases | ||||
Development delay | Axin | Neuronal development and cell differentiation. | Yes | The development of skin and hair follicle development in the angora rabbit is affected by the level of Siah1 protein ( |
Akt3 | Neuronal development. | Yes | ||
Neuronal damage | GAPDH | Glycolysis and promotion of cell apoptosis. | No | Siah1 is upregulated after spinal cord injury in adult rats ( |
Parkinson's disease | α-synuclein | The development of Parkinson's disease and the formation of LBs. | No | Inhibition of Siah1 by siRNA increases cell proliferation and inhibits apoptosis in SH-SY5Y neuroblastoma cells ( |
Synphilin-1 | The development of Parkinson's disease and the formation of LBs. | Yes | ||
Alzheimer's disease | CacyBP/SIP | Part of the Siah1 ubiquitin-ligase complexes and de-phosphorylation of tau protein. | No | In tau transgenic mice, localization of CacyBP/SIP and Siah1 is similar to that observed for patients with Alzheimer's disease ( |
JNK, c-Jun N-terminal kinase; Bim, Bcl-2-interacting mediator of cell death; TRAF4, TNF receptor-associated factor 4; HIPK2, homeodomain-interacting protein kinase 2; PHD3, prolyl-hydroxylase proteins 3; CacyBP/SIP, calcyclin-binding protein/Siah1 interacting protein; p27/kip1, p27 kinase inhibitory protein 1; ZEB1, zinc finger E-box-binding homeobox 1; ELL2, elongation factor for RNA polymerase II 2; PML-RARα, t(15;17)(q24;q21), generating the promyelocytic leukemia-retinoic acid receptor α fusion protein; AML1-ETO, t(8;21)(q22;q22), RUNX family transcription factor 1 fusion protein; AF4-MLL, t(4;11)(q21;q23), ectopic activator of transcript initiation fusion protein; AKT, targeting protein kinase; YAP, transcriptional coactivator YAP1; AKT3, targeting protein kinase-b3; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; LBs, Lewy bodies; siRNA, small interfering RNA; shRNA, small hairpin RNA.