A hydrophobic residue in the TALE homeodomain of PBX1 promotes epithelial-to-mesenchymal transition of gastric carcinoma

Comprehensive summary: the role of PBX1 in development and cancers

PBX1 is a transcription factor that can promote the occurrence of various tumors and play a reg-ulatory role in tumor growth, metastasis, invasion, and drug resistance. Furthermore, a variant generated by fusion of E2A and PBX1, E2A-PBX1, has been found in 25% of patients with childhood acute lymphoblastic leukemia. Thus, PBX1 is a potential therapeutic target for many cancers. Here, we describe the structure of PBX1 and E2A-PBX1 as well as the molecular mecha-nisms whereby these proteins promote tumorigenesis to provide future research directions for developing new treatments. We show that PBX1 and E2A-PBX1 induce the development of highly malignant and difficult-to-treat solid and blood tumors. The development of specific drugs against their targets may be a good therapeutic strategy for PBX1-related cancers. Furthermore, we strongly recommend E2A-PBX1 as one of the genes for prenatal screening to reduce the incidence of childhood hematological malignancies.

PBX1 as a novel master regulator in cancer: Its regulation, molecular biology, and therapeutic applications

PBX1 is a critical transcription factor at the top of various cell fate-determining pathways. In cancer, PBX1 stands at the crossroads of multiple oncogenic signaling pathways and mediates responses by recruiting a broad repertoire of downstream targets. Research thus far has corroborated the involvement of PBX1 in cancer proliferation, resisting apoptosis, tumor-associated neoangiogenesis, epithelial-mesenchymal transition (EMT) and metastasis, immune evasion, genome instability, and dysregulating cellular metabolism. Recently, our understanding of the functional regulation of the PBX1 protein has advanced, as increasing evidence has depicted a regulatory network consisting of transcriptional, post-transcriptional, and post-translational levels of control mechanisms. Furthermore, accumulating studies have supported the clinical utilization of PBX1 as a prognostic or therapeutic target in cancer. Preliminary results showed that PBX1 entails vast potential as a targetable master regulator in the treatment of cancer, particularly in those with high-risk features and resistance to other therapeutic strategies. In this review, we will explore the regulation, protein-protein interactions, molecular pathways, clinical application, and future challenges of PBX1.

PBX1: a TALE of two seasons-key roles during development and in cancer

Pre-B cell leukemia factor 1 (PBX1) is a Three Aminoacid Loop Extension (TALE) homeodomain-containing transcription factor playing crucial roles in organ pattering during embryogenesis, through the formation of nuclear complexes with other TALE class and/or homeobox proteins to regulate target genes. Its contribution to the development of several organs has been elucidated mainly through the study of murine knockout models. A crucial role for human development has been recently highlighted through the discovery of different de novo pathogenic PBX1 variants in children affected by developmental defects. In the adult, PBX1 is expressed in selected tissues such as in the brain, in the gastro-intestinal and urinary systems, or in hematopoietic stem and progenitor cells, while in other organs is barely detectable. When involved in the t(1;19) chromosomal translocation it acts as an oncogene, since the resulting fusion protein drives pre-B cell leukemia, due to the induction of target genes not normally targeted by the native protein. Its aberrant expression has been associated to tumor development, progression, or therapy-resistance as in breast cancer, ovarian cancer or myeloproliferative neoplasm (MPN). On the other hand, in colorectal cancer PBX1 functions as a tumor suppressor, highlighting its context-dependent role. We here discuss differences and analogies of PBX1 roles during embryonic development and in cancer, focusing mainly on the most recent discoveries.

Comprehensive overview of the role of PBX1 in mammalian kidneys

Pre-B-cell leukemia homeobox transcription factor 1 (PBX1) is a member of the TALE (three-amino acid loop extension) family and functions as a homeodomain transcription factor (TF). When dimerized with other TALE proteins, it can act as a pioneer factor and provide regulatory sequences via interaction with partners. In vertebrates, PBX1 is expressed during the blastula stage, and its germline variations in humans are interrelated with syndromic anomalies of the kidney, which plays an important role in hematopoiesis and immunity among vertebrates. Herein, we summarize the existing data on PBX1 functions and the impact of PBX1 on renal tumors, PBX1-deficient animal models, and blood vessels in mammalian kidneys. The data indicated that the interaction of PBX1 with different partners such as the HOX genes is responsible for abnormal proliferation and variation of the embryonic mesenchyme, while truncating variants were shown to cause milder phenotypes (mostly cryptorchidism and deafness). Although such interactions have been identified to be the cause of many defects in mammals, some phenotypic variations are yet to be understood. Thus, further research on the TALE family is required.

Functional Assessment of a New PBX1 Variant in a 46,XY Fetus with Severe Syndromic Difference of Sexual Development through CRISPR-Cas9 Gene Editing

Sexual development is a complex process relying on numerous genes. Disruptions in some of these genes are known to cause differences of sexual development (DSDs). Advances in genome sequencing allowed the discovery of new genes implicated in sexual development, such as PBX1. We present here a fetus with a new PBX1 NM_002585.3: c.320G>A,p.(Arg107Gln) variant, presenting with severe DSD along with renal and lung malformations. Using CRISPR-Cas9 gene editing on HEK293T cells, we generated a KD cell line for PBX1. The KD cell line showed reduced proliferation and adhesion properties compared with HEK293T cells. HEK293T and KD cells were then transfected plasmids coding either PBX1 WT or PBX1-320G>A (mutant). WT or mutant PBX1 overexpression rescued cell proliferation in both cell lines. RNA-seq analyses showed less than 30 differentially expressed genes, in ectopic mutant-PBX1-expressing cells compared with WT-PBX1. Among them, U2AF1, encoding a splicing factor subunit, is an interesting candidate. Overall, mutant PBX1 seems to have modest effects compared with WT PBX1 in our model. However, the recurrence of PBX1 Arg107 substitution in patients with closely related phenotypes calls for its impact in human diseases. Further functional studies are needed to explore its effects on cellular metabolism.

The TALE never ends: A comprehensive overview of the role of PBX1, a TALE transcription factor, in human developmental defects

PBX1 is a highly conserved atypical homeodomain transcription factor (TF) belonging to the TALE (three amino acid loop extension) family. Dimerized with other TALE proteins, it can interact with numerous partners and reach dozens of regulating sequences, suggesting its role as a pioneer factor. PBX1 is expressed throughout the embryonic stages (as early as the blastula stage) in vertebrates. In human, PBX1 germline variations are linked to syndromic renal anomalies (CAKUTHED). In this review, we summarized available data on PBX1 functions, PBX1-deficient animal models, and PBX1 germline variations in humans. Two types of genetic alterations were identified in PBX1 gene. PBX1 missense variations generate a severe phenotype including lung hypoplasia, cardiac malformations, and sexual development defects (DSDs). Conversely, truncating variants generate milder phenotypes (mainly cryptorchidism and deafness). We suggest that defects in PBX1 interactions with various partners, including proteins from the HOX (HOXA7, HOXA10, etc.), WNT (WNT9B, WNT3), and Polycomb (BMI1, EED) families are responsible for abnormal proliferation and differentiation of the embryonic mesenchyme. These alterations could explain most of the defects observed in humans. However, some phenotype variability (especially DSDs) remains poorly understood. Further studies are needed to explore the TALE family in greater depth.

The regulation of PBXs and their emerging role in cancer

Pre‐B‐cell leukaemia transcription factor (PBX) proteins are a subfamily of evolutionarily conserved, atypical homeodomain transcription factors that belong to the superfamily of three amino acid loop extension (TALE) homeodomain proteins. Members of the PBX family play crucial roles in regulating multiple pathophysiological processes, such as the development of organs, congenital cardiac defects and carcinogenesis. The dysregulation of PBXs has been shown to be closely associated with many diseases, particularly cancer. However, the detailed mechanisms of PBX dysregulation in cancer progression are still inconclusive. In this review, we summarize the recent advances in the structures, functions and regulatory mechanisms of PBXs, and discuss their underlying mechanisms in cancer progression. We also highlight the great potential of PBXs as biomarkers for the early diagnosis and prognostic evaluation of cancer as well as their therapeutic applications. The information reviewed here may expand researchers’ understanding of PBXs and could strengthen the clinical implication of PBXs in cancer treatment.

MicroRNA-141-3p inhibits the progression of oral squamous cell carcinoma via targeting PBX1 through the JAK2/STAT3 pathway

Oral squamous cell carcinoma (OSCC), which is the most common epithelial malignant neoplasm in the head and neck, is characterized by local infiltration and metastasis of lymph nodes. The five-year survival rate of OSCC remains low despite the advances in clinical methods. miR-141-3p has been shown to activate or inhibit tumorigenesis. However, the effects of miR-141-3p on invasion and migration of OSCC remain unclear. The present study aimed to evaluate the effects of miR-141-3p on invasion, proliferation, and migration in oral squamous cell carcinoma (OSCC). Reverse transcription quantitative PCR, western blotting and immunohistochemistry were used to detect microRNA(miR)-141-3p and pre-B-cell leukaemia homeobox-1 (PBX1) expression in OSCC tissues and cell lines. The luciferase reporter assay was used to detect targets of miR-141-3p in OSCC. MTT, Transwell and wound healing assays were used to determine the cell proliferation and invasive and migratory abilities, respectively. Expression of constitutive phosphorylated (p)-Janus kinase 2 (JAK2) and p-signal transducer and activator of transcription 3 (STAT3) was detected using western blotting in tissues and cells. miR-141-3p expression was decreased in OSCC tissues and cells, while PBX1 protein expression was increased compared with non-cancerous controls. The result from the dual-luciferase reporter assay revealed that PBX1 was the direct target of miR-141-3p in OSCC tissues. Furthermore, miR-141-3p overexpression and PBX1 knockdown could reduce cell invasion, proliferation and migration, and inhibit the JAK2/STAT3 pathway; however, miR-141-3p downregulation had the opposite effects. In addition, silencing of PBX1 using small interfering RNA could weaken the effects of miR-141-3p inhibitor on JAK2/STAT3 pathway and cell progression in CAL27 cells. In summary, the findings from this study indicated that miR-141-3p upregulation could inhibit OSCC cell invasion, proliferation and migration, by targeting PBX1 via the JAK2/STAT3 pathway.

PBX1: a key character of the hallmarks of cancer

Pre-B-cell leukemia homeobox transcription factor 1 (PBX1) was first identified as part of a fusion protein resulting from the chromosomal translocation t(1;19) in pre-B cell acute lymphoblastic leukemias. Since then, PBX1 has been associated with important developmental programs, and its expression dysregulation has been related to multifactorial disorders, including cancer. As PBX1 overexpression in many cancers is correlated to poor prognosis, we sought to understand how this transcription factor contributes to carcinogenesis, and to organize PBX1's roles in the hallmarks of cancer. There is enough evidence to associate PBX1 with at least five hallmarks: sustaining proliferative signaling, activating invasion and metastasis, inducing angiogenesis, resisting cell death, and deregulating cellular energetics. The lack of studies investigating a possible role for PBX1 on the remaining hallmarks made it impossible to defend or refute its contribution on them. However, the functions of some of the PBX1's transcription targets indicate a potential engagement of PBX1 in the avoidance of immune destruction and in the tumor-promoting inflammation hallmarks. Interestingly, PBX1 might be a player in tumor suppression by activating the transcription of some DNA damage response genes. This is the first review organizing PBX1 roles into the hallmarks of cancer. Thus, we encourage future studies to uncover the PBX1's underlying mechanisms to promote carcinogenesis, for it is a promising diagnostic and prognostic biomarker, as well as a potential target in cancer treatment.

Development of small molecule inhibitors targeting PBX1 transcription signaling as a novel cancer therapeutic strategy

PBX1 is a transcription factor involved in diverse cellular functions including organ development, stem cell renewal, and tumorigenesis. PBX1 is localized at chr1q23.3, a frequently amplified chromosomal region, and it is overexpressed in many human malignancies. Cancer cells with elevated PBX1 signaling are particularly vulnerable to PBX1 withdrawal. We designed a series of small molecule compounds capable of docking to the interface between PBX1 and its cognate DNA target sequence. Among them, T417 is found to be a lead compound. In cell-based assays, T417 significantly suppressed self-renewal and proliferation of cancer cells expressing high levels of PBX1. T417 also re-sensitized platinum-resistant ovarian tumors to carboplatin. T417 did not affect healthy tissues likely due to their lower PBX1 expression levels. Therefore, targeting PBX-DNA interface can be a promising strategy for treating human tumors reliant on PBX1 for survival.

rs6426881 in the 3'-UTR of PBX1 is involved in breast and gastric cancers via altering the binding potential of miR-522-3p

BACKGROUND

Breast and gastric cancers are the most important diseases that lead to cancer death and social healthcare challenge. Overexpression of PBX1, a proto-oncogene, is correlated with the progression and metastasis of various cancers. For the first time, in this study the researchers evaluated the relationship between rs6426881, affecting miR-522-3p binding to the PBX1, with breast and gastric cancers.

METHODS AND RESULTS

The Microarray analysis was performed for finding the relative expression level of PBX1 and hsa-miR-522-3p, based on high throughput experiments. The GSE54397, GSE112369, GSE10810, GSE241585.ER, GSE24185.PR, GSE68373, and GSE38167 datasets were analyzed. A case-control study was carried out in 123 Iranian suffering from breast cancer and 132 participants as control samples as well as 130 people suffering from gastric cancer and 54 people as control group members. SNP rs6426881 in the 3'-UTR of PBX1 was genotyped by the High-Resolution Melting (HRM) method. Association analysis revealed that rs6426881 is correlated with Estrogen and Progesterone receptors, grade, and stage of breast cancer. Furthermore, a significant relationship was observed between the genotypes and blood groups in gastric cancer, while the distribution of alleles was significantly related to smoking, status of the primary tumor, and metastasis (Chi-Square P < 0.05). Finally, Bioinformatics analyses suggested that rs6426881 contains binding sites for miR-522-3p in the 3'-UTR of PBX1 transcript. The finding suggested that TT genotype is associated with poor prognosis in breast and gastric cancer.

CONCLUSIONS

The rs6426881 T allele at PBX1 3'-UT is significantly related to breast and gastric cancers by altering the regulatory affinity of miR-522-3p to PBX1 3'-UTR and may be suggested as a novel prognostic biomarker for the diseases.

Coexpression of HOXA6 and PBX2 promotes metastasis in gastric cancer

HOXA6 gene plays a role of the oncogene in various cancers. Nonetheless, its effect on gastric cancer (GC) occurrence and development is still unclear. We analysed whether HOXA6 interacts with the PBX2 protein using the STRING database. The molecular mechanism by which HOXA6 synergizes with PBX2 in GC metastasis is not fully understood. Here, we found that the expression of HOXA6 was increased in GC tissues and cell lines. The upregulation of HOXA6 was closely associated with differentiation, lymph node metastasis, AJCC stage, TNM stage, and poor survival outcome in GC patients based on tissue microarray (TMA) data. Moreover, the overexpression of HOXA6 promoted, whereas siRNA-mediated repression of HOXA6 inhibited, the cell proliferation, migration, and invasion of GC cells. Furthermore, HOXA6 could physically interact with and stabilize PBX2. In addition, HOXA6 and PBX2 expression was positively correlated in GC cells and tissue. HOXA6 and PBX2 suppression in GC cells also led to decreased migration and invasion potential in vitro. In vivo, HOXA6 was shown to cooperate with PBX2 to enhance cell metastasis via orthotopic implantation. These data indicate that HOXA6 promotes cell proliferation, migration, and invasion and that the HOXA6-PBX2 axis may be a useful biomarker for disease progression in GC.

Inhibition of the deubiquitinase USP9x induces pre-B cell homeobox 1 (PBX1) degradation and thereby stimulates prostate cancer cell apoptosis

Chemoresistance is a leading obstacle in effective management of advanced prostate cancer (PCa). A better understanding of the molecular mechanisms involved in PCa chemoresistance could improve treatment of patients with PCa. In the present study, using immune histochemical, chemistry, and precipitation assays with cells from individuals with benign or malignant prostate cancer or established PCa cell lines, we found that the oncogenic transcription factor pre-B cell leukemia homeobox-1 (PBX1) promotes PCa cell proliferation and confers to resistance against common anti-cancer drugs such as doxorubicin and cisplatin. We observed that genetic PBX1 knockdown abrogates this resistance, and further experiments revealed that PBX1 stability was modulated by the ubiquitin-proteasomal pathway. To directly probe the impact of this pathway on PBX1 activity, we screened for PBX1-specific deubiquitinases (Dubs) and found that ubiquitin-specific peptidase 9 X-linked (USP9x) interacted with and stabilized the PBX1 protein by attenuating its Lys-48-linked polyubiquitination. Moreover, the USP9x inhibitor WP1130 markedly induced PBX1 degradation and promoted PCa cell apoptosis. The results in this study indicate that PBX1 confers to PCa chemoresistance and identify USP9x as a Dub of PBX1. We concluded that targeting the USP9x/PBX1 axis could be a potential therapeutic strategy for managing advanced prostate cancer.

Molecular alterations of cancer cell and tumour microenvironment in metastatic gastric cancer

The term metastasis is widely used to describe the endpoint of the process by which tumour cells spread from the primary location to an anatomically distant site. Achieving successful dissemination is dependent not only on the molecular alterations of the cancer cells themselves, but also on the microenvironment through which they encounter. Here, we reviewed the molecular alterations of metastatic gastric cancer (GC) as it reflects a large proportion of GC patients currently seen in clinic. We hope that further exploration and understanding of the multistep metastatic cascade will yield novel therapeutic targets that will lead to better patient outcomes.