ReviewInvolvement of F-BOX proteins in progression and development of human malignancies
Introduction
Timely and spatial regulated mechanism based on abundance and destruction of multi-protein molecular machines, which regulate fundamental cellular functions, is critical for maintaining systemic homeostasis. Ubiquitin Proteasome System (UPS) is an evolutionary conserved protein degradation mechanism that is involved in various physiological responses like cell cycle control, DNA replication, transcription, cell signaling, as well as DNA repair and cell death [1], [2], [3]. Dysregulation of ubiquitin-dependent proteolysis has been broadly suggested in progression and development of cancer, as the UPS plays critical roles by catalyzing many governing proteins in key signaling pathways throughout the cell [4], [5], [6]. Polyubiquitin formation is carried out in three subsequent strides by the various genes of ubiquitin activating (E1), ubiquitin-conjugating (E2) and ubiquitin ligase (E3) enzymes [7]. In the first step entire cellular pool of ubiquitin (Ub) is activated by the E1 enzyme by the creation of thioester linkage between Ub and the cysteine residues of E1. Then, activated Ub is conjugated to an active cysteine side chain of ubiquitin-carrier protein, the E2 enzyme. In the third step catalyzed by E3 enzyme, the C-terminus of ubiquitin moities are linked through an amide-isopeptide to a ɛ-amino group of the lysine residue in the substrate protein (Fig. 1). This kind of modification to a substrate through sequential conjugation of ubiquitin on the lysine residue at position 48, leads to protein degradation [8].
Although inhibition of E1 enzyme has been proposed as a strategy for anti-cancer therapy [9], there is no direct evidence that the two discovered human E1 activating enzymes are involved in carcinogenesis, and only a few reports from 38 known E2 conjugating enzymes are pointing out their role in malignant transformation [8], [9]. In contrast, the human genome possibly encodes up to 1000 E3 ligases and many of them have been broadly implicated in the development of cancer, mostly due to the fact that a high variability of different E3 enzymes is a key feature responsible for substrate specificity [10], [11]. From the two primary classes of ubiquitin ligases, HECT domain (Homologous to E6-AP Carboxy Terminus-type E3s) and RING domain (Really Interesting New Gene-type E3s), the later one constitutes the largest superfamily of more than 600 E3s in mammalian cells extensively studied over past years [12], [13]. Perhaps the best characterized RING-type E3s that occur are multisubunit assemblies containing cullin-family scaffold proteins (Cullin-RING Ligases – CRLs). A prototypical example of E3 enzyme based on the cullin scaffold is the SCF ubiquitin ligase complex. This E3 ligase is composed of three primary and invariable components – SKP1 (S-phase kinase-associated protein-1), CUL1 (Cullin), and RBX1/ROC1 and an FBP, a variable element capable of substrate selection [14], [15]. Among various regulatory proteins, FBPs are essential functional components of UPS. FBPs mainly operate by regulating substrates in multiple pathways that are critical for the biological functions such as cell division, cell growth, cell survival and death, signaling responses, and development and differentiation [16], [17], [18]. Scientific evidence gathered over time strongly argues that any abnormality in FBPs results in cancer by either genomic instability or by uncontrolled proliferation. FBPs are extensively studied in perspective of cell proliferation even though FBPs are known to be involved in varied biological pathways. As cell proliferation is the primary target for any cancer study, therefore the role of FBP has been widely seen in the context of cancer biology [19]. The members of FBPs like FBXW7, SKP2, and βTrCP, have been thoroughly characterized in terms of their biological functions [20], [21]. Keeping in mind the role played by FBPs, it becomes immensely important to study the role of other members of FBPs in cancer as well as in other diseases. Therefore, this review focuses mainly on the well characterized FBPs that are playing an important role in the pathology of human malignancies.
Section snippets
The FBPs families
FBPs are the important class of proteins that are involved in cellular functions such as cell division and cell signaling. They derive their name from the presence of at least one F-box domain, which was primarily discovered in cyclin F as a characteristic 50-amino-acid region [22] (Fig. 2). Other homology domains, in addition to F-box domain, have been found in FBPs and are responsible for the substrate recognition and binding. Based on the types of the substrate-interaction domains, 69 FBPs
FBXW7
FBXW7 is a member of FBP family that displays a significant tumor suppressive role in many malignancies. It plays the important role in various cellular and physiological functions such as in cell cycle progression, cell growth, apoptosis, invasion and drug resistance [28]. FBXW7 prevents tumor development by facilitating the destruction of oncogenic proteins via involving ubiquitination and proteasomal pathways [28]. FBXW7 mediated tumor suppressive effects occurs by negative regulation of
SKP2
The SKP2 is an SCF ubiquitin ligase critical for cell cycle progression and cell proliferation. SKP2 is found to be the rate-limiting element of the SCF protein complex that degrades tumor suppressor protein p27 via ubiquitylation [56], [57], [58]. SKP2 recognizes phosphorylated p27Kip1 in S-phase with the cooperation of SKP1 and leads to its degradation along with the other tumor suppressor genes, thus considered as an oncogene. There are other numerous studies showing the participation of
FBPs: defined classes
Among all the FBPs discovered, only SKP2, FBXW7 and β-TrCP are well studied and characterized with respect to their substrates as well as their functionality. There are still a large number of FBPs that have been identified but yet to be characterized in detail.
Breast cancer
Malignancy of the breast is the most common cancer in female worldwide and second most common cause of cancer-related mortality after pulmonary disease [96]. Despite the introduction of newer useful therapeutic modalities in the management of breast cancer, it remains a fundamental cause of cancer morbidity and mortality. This is due to either development of resistance to newer drugs or relapse after initial response leading to tumor progression, metastasis and finally resulting in
FBPs and other cancers
The role of FBPs has been reported in many other cancers such as pancreatic cancer, hepatocellular carcinoma, thyroid cancer, ovarian cancer and renal cell carcinoma. In these cancers, the study about FBPs has been very modest and here we review most of the related literature. Despite recent improvements, overall survival for advanced adenocarcinoma of the pancreas continues to be poor. Previous studies reported the mutational status of BRAF and FBXW7 in relation to two distinct subsets of
FBPs as candidate for therapeutic target
As many studies have established the role of FBPs as tumor-suppressors or oncogenic functions, there is a precedent to explore FBPs as therapeutic targets for the prevention and treatment of various malignancies. Extensive studies establishing the role of SKP2 in different cancers makes it a promising therapeutic target. High-throughput screening identified many small molecules acting as SKP2 inhibitors which mainly block SKP2 E3 ligase activity [87], [206]. These small molecule inhibitors have
Conclusion and future perspective
The current understanding demonstrates that FBPs play essential role in tumorigenesis mainly by regulating the substrate turnover which could be both in E3 ligase activity-dependent or activity-independent manner [210], [211], [212]. Four FBPs, namely FBXW7, SKP2, β-TRCP1 and β-TRCP2, have been studied among the pool of 69 FBPs identified. Little is known of the remaining members of FBP family. Interestingly, the inhibitors of SKP2 have been shown to have therapeutic potential. This should
Conflict of interest
No conflicts of interest exist.
Funding
The authors declare that they have no competing financial interest.
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2021, Drug Discovery TodayCitation Excerpt :There are 69 FBPs encoded by the human genome, which can be divided into three categories based on their specific substrate recognition domains: F-box and WD40 domain (FBXW) category, comprising 12 proteins (e.g., β-TRCP1, FBXW7, and β-TRCP2); the 21 F-box and Leu-rich repeat (FBXL) family members (e.g., SKP2) containing leucine-rich repeat domains; and 36 F-box only (FBXO) proteins with different domains, including zinc-finger, proline-rich domains, and Sec7.9 These proteins regulate many biological processes, such as DNA replication, transcription, cell differentiation, and cell death, via ubiquitylation-mediated degradation of target proteins.10 Accumulating results indicate the vital role of FBPs in the pathogenesis of different human diseases, including cancer, and, indeed, the deregulated expression of these proteins is often detected at preclinical and clinical levels.7,10
Cyclin D degradation by E3 ligases in cancer progression and treatment
2020, Seminars in Cancer BiologyCitation Excerpt :β-TrCP is a WD40-repeat containing F-box protein. It regulates both physiological and pathological processes such as cell cycle progression, cell proliferation, signal transduction, and tumor development through targeting its substrates for ubiquitylation and degradation [122]. β-TrCP recognizes its substrates in a phosphorylation-dependent manner, through which a specific binding motif known as degron is phosphorylated at Ser or Thr, for example, DSGxxS sequence or its variants like DSG/DDG/EEG/SSG/TSGxxS/E/D [123].
Depletion of UBE2C reduces ovarian cancer malignancy and reverses cisplatin resistance via downregulating CDK1
2020, Biochemical and Biophysical Research CommunicationsCitation Excerpt :Additionally, the Kaplan Meier-plotter analysis showed that high UBE2C expression was associated with poor clinical outcomes in ovarian cancer. The ubiquitin-proteasome system (UPS), composed of the ubiquitin-activating E1 enzyme, the ubiquitin-conjugating E2 enzyme, ubiquitin E3 ligases, and proteasome, is essential for cell cycle regulation [27]. As one important member of the E2 family, UBE2C has been proven to be involved in cell cycle, and several studies have shown that depletion of UBE2C blocked cells at the G2 phase [28–30].
Ubiquitin ligase subunit FBXO9 inhibits V-ATPase assembly and impedes lung cancer metastasis
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