Lin28/let-7 Axis
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This entry is adapted from the peer-reviewed paper 10.3390/cancers14194640

An RNA-binding protein, Lin28, in regulating cancer cell stemness to drive tumour progression. Lin28 blocks the synthesis of let-7, a tumour-suppressor microRNA, and acts as a global regulator of cell differentiation and proliferation. Lin28also targets messenger RNAs and regulates protein translation. 

Lin28 let-7 metastasis metabolism epithelial-to-mesenchymal transition

1. Introduction

Cancers such as prostate cancer (PC) contain heterogeneous cell populations and, therefore, respond to anticancer treatments differently [1]. This heterogeneity is reflected not only in genomics and transcriptomics but also RNA processing re-programming that can give rise to tumour cells with various co-existing phenotypes and histological presentations [2][3][4]. When a targeted therapy is applied to a tumour, it will be effective in suppressing some populations of tumour cells and, at the same time, may provide an opportunity to allow other cell populations to thrive and eventually develop into therapy-resistant tumours. One good example is how androgen receptor (AR)-targeted therapy in metastatic PC can result in short-term tumour suppression effects but also induces castrate-resistant PC (CRPC) in the long term that presents an AR-indifferent phenotype containing weakly expressed AR and AR-regulated PSA, cancer stem cell (CSC) markers (e.g., CD44, CD133, BMI1, EZH2), and even AR negative neuroendocrine PC (NEPC) markers (e.g., CHGA, SYP) [5][6][7]. Several studies using patient tumour samples, patient-derived xenograft (PDX), and even cell models have demonstrated that there exist landscape switches in phenotypes between primary and therapy-resistant tumour cells, indicating that therapy-resistant tumours are under the control of different oncogenic signal networks than primary tumours [8][9][10]. These findings underscore that tumour heterogeneity is an inherent challenge for targeted therapies, and the identification of the driver genes of therapy-resistant tumours could inform the design of more effective treatments for cancer patients.
Cancer stem cells (CSCs) are an important component of tumour heterogeneity and have been proposed to relate to therapy-resistant tumour progression through two possible mechanisms. One is that a low number of CSCs pre-exist in primary tumours [11]. They are surrounded by a large number of relatively fast-growing non-CSC cells and remain in a quiescence/dormancy state. While therapy-induced stress is efficient to abolish non-CSCs, it can activate CSCs to give rise to tumour cells that are resistant to therapies. Single-cell sequencing of patient PC samples has shown that CRPC cells with characteristic CSC and basal cell phenotypes pre-exist in primary PC [12]. These cells express high levels of EZH2 and SYP and low levels of AR and AR-regulated PCA3 and have Rb1 loss, all of which are consistent with the molecular profile of classic late-stage NEPC [12]. One lab also reported that the NEPC driver gene, SRRM4, is expressed in 16% of hormone-naïve primary tumours, and SRRM4-positive tumour cells increased to ~30% of tumours under AR target therapy, supporting the idea that CRPC tumour cells are pre-composite in untreated PC and become prevalent upon anticancer treatment [13]. The other mechanism is that PC cells with specific genomic features can acquire CSC phenotypes upon exposure to therapy-induced stress [14][15]. Androgen receptor (AR)-mediated signalling is essential to maintain the luminal epithelial phenotype of prostate adenocarcinoma. However, it has been reported in transgenic mouse models that, when AR inhibitors were used to treat PC cells with TP53 and RB1 gene loss, these cells gained CSC, epithelial-to-mesenchymal, basal, and neuroendocrine phenotypes through the action of transcriptional factors, such as SOX2, and, later, developed into NEPC [15]. In vitro LNCaP PC cells were reported to undergo transdifferentiation to gain CSC phenotypes when treated with AR inhibitors [14]. Gain of function of SRRM4 in DU145 cells that had TP53 and RB1 gene disruptions, but not LNCaP cells, which were TP53- and RB1-intact, induced a classic CSC gene signature, primarily driven by the Lin28–SOX2 axis [16]. These findings indicate that prostate adenocarcinoma cells are plastic and can alter their phenotypes to cope with therapy-induced stress, during which process the CSC gene network plays a key role.

2. Lin28/let-7 axis

2.1. Lin28 Structure and Function

There are two paralogs of Lin28 protein in vertebrates, Lin28A and Lin28B [17]. Their structures share high homology in protein sequences, except that Lin28B has a longer C-terminus containing the nuclear localization signal (NLS) and nucleolar localization signal (NoLS) [17][18]. Indeed, various studies have confirmed that the cellular localization of the two isoforms is different. Lin28B can be localized throughout the cells, even inside the nucleolus, while Lin28A is predominantly localized at the cytosol [17][18]. Both paralogs contain highly conserved RNA binding regions: the N terminal cold-shock domain (CSD) and a C terminal cysteine–cysteine–histidine–cystine (CCHC)-conserved zinc knuckle domain (ZKD) [17]. Lin28A and B bind the immature forms of let-7 microRNA family members and prevent them from being synthesized. Lin28A and Lin28B are mutually exclusive in expression in human cancer cell lines [19]. For instance, Piskounova et al. [19] confirmed that only Lin28A was detected in MES and IGROVE1, while only Lin28B was detected in HEK293, H1299, HepG2, and K562 cell lines. However, a minority of cancers, including ovarian cancer, germ cell tumours, and teratomas, express both paralogs of Lin28 [20][21][22].

2.2. Let-7 Biogenesis

Let-7 miRNAs are key regulators of embryonic development and cancer progression. They are synthesized through several steps (Figure 1). First, the primary miRNA transcripts of let-7s (pri-let-7s) are transcribed from the MIRLET7 genes by RNA polymerase II [23][24]. These transcripts form structures of hairpin stem loops consisting of a stem region, and they have a characteristic pre-element (preE) bulge and a preE loop [24][25]. Second, pri-let-7 is cleaved at the stem-loop structure of the preE by Drosha (a nuclear-localized RNase III) and Pasha (double-stranded RNA binding protein) into a shorter (60–80 nts) pre-let-7 hairpin structure [24]. The pre-miRNA is then exported out of the nucleus by exportin-5 [26]. Finally, the preE element of pre-let-7 is cleaved by Dicer to produce a mature let-7 miRNA duplex with a 3’ overhang in the cytoplasm [25]. Some members of the let-7 family (pre-let-7a-2, -7c, and -7e) contain a bulged adenosine/uridine residue at the 3’ end that impairs them from being recognized by cytoplasmic Dicer complex [27][28]. In these cases, a terminal uridylyl transferase (TUT2, TUT4, or TUT7) will mono-uridylate the 3’ overhang of pre-let-7 to allow Dicer cleavage [27][28][29]. At the cytoplasm, HIV-1 TAR RNA-binding protein (TRBP) interacts with Dicer and recruits argonaute protein (i.e., AGO2) to form RNA-induced silencing complex (RISC) [30]. One strand of the mature let-7 duplex is preferentially incorporated into the active parts of the RISC [18][31]. Besides the conventional described maturation process, Drosha/Dicer-independent biogenesis of let-7 microRNA also exists [32][33].
Cancers 14 04640 g001
Figure 1. The biogenesis of let-7 microRNA. In the absence of Lin28B, pri-let-7s are cleaved into pre-let-7 miRNAs and then converted to mature let-7 with the aid of Dicer in both cytoplasm and nucleus. The stages at which Lin28A and/or B inhibit let-7 maturation are marked. The lower right square shows the structure of pri-let-7. The upper right square demonstrates the site where Dicer cleaves pre-let-7. The left square shows the bipartite binding of Lin28A/B to pri/pre-let-7 members. Created with BioRender.com.

2.3. Lin28 Regulates let-7 Maturation

As shown in Figure 1, two paralogs of Lin28 bind to let-7 to prevent it from being further processed [34][35]. The two conserved domains, CSD and ZKD, have a high affinity to let-7 precursors at the terminal loop region of preE [25]. Specifically, ZKD binds to the 3’ end of the preE bulge containing the GGAG motif, while CSD preferentially binds to the preE loop with the NGNGAYNNN motif [25]. Mayr et al. [36] showed that CSD binds to microRNA first and then induces a conformational change in the preE, making it available for ZKD binding. Although CSD has a higher affinity to miRNA, ZKD binding to the sequence is required and sufficient to recruit TUTase enzymes and induce oligouridylation [37]. In the cytosol, the Lin28B CSD recognizes the preE loop of pre-let-7 while allowing ZKD to bind with the conserved motif preE bulge within let-7 (GGAG) [17][27]. ZKD binding to miRNA blocks the Dicer cleavage site, which is proximal to the GGAG-conserved preE bulge, and prevents pre-let-7 maturation [17]. It is worth mentioning that some members of the let-7 family can escape Lin28-mediated inhibition [38]. For instance, Triboulet et al. [38] confirmed that human let-7a-3 and its murine ortholog, mouse let-7c-2, bypass Lin28A-mediated repression. However, these pathways must be less common because most of the studies confirm that let-7 levels are tightly regulated by Lin28A and Lin28B.
Lin28A and Lin28B inhibit let-7 maturation in distinct cellular compartments [35]. Lin28A is mostly found in the cytosol while Lin28B is mostly found in the nucleus, as Lin28B has an NLS [19]. In the nucleus, RNA-binding protein Musashi (MSI1) induces Lin28B nuclear localization and aids ZKD recognition of the terminal loop of pri-let-7, inhibiting Drosha cleavage [35][39]. Despite the different localization of Lin28A and B in cells, some other studies have proposed that the localization of the Lin28A and B paralogs might be cell cycle-dependent [40]. Furthermore, Piskounova et al. [19] found that, while Lin28A requires TUTase to inhibit let-7 maturation, Lin28B acts in a TUTase-independent manner, recruiting DGCR8 microprocessor instead.

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Subjects: Biochemistry & Molecular Biology
Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Zhuohui Lin
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Mariia Radaeva
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Artem Cherkasov
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Xuesen Dong
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Update Date: 13 Oct 2022
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