Tyrosine Phosphorylation of the Pioneer Transcription Factor FoxA1 Promotes Activation of Estrogen Signaling

FOXA1 O-GlcNAcylation-mediated transcriptional switch governs metastasis capacity in breast cancer

FOXA1, a transcription factor involved in epigenetic reprogramming, is crucial for breast cancer progression. However, the mechanisms by which FOXA1 achieves its oncogenic functions remain elusive. Here, we demonstrate that the O-linked β-N-acetylglucosamine modification (O-GlcNAcylation) of FOXA1 promotes breast cancer metastasis by orchestrating the transcription of numerous metastasis regulators. O-GlcNAcylation at Thr432, Ser441, and Ser443 regulates the stability of FOXA1 and promotes its assembly with chromatin. O-GlcNAcylation shapes the FOXA1 interactome, especially triggering the recruitment of the transcriptional repressor methyl-CpG binding protein 2 and consequently stimulating FOXA1 chromatin-binding sites to switch to chromatin loci of adhesion-related genes, including EPB41L3 and COL9A2. Site-specific depletion of O-GlcNAcylation on FOXA1 affects the expression of various downstream genes and thus inhibits breast cancer proliferation and metastasis both in vitro and in vivo. Our data establish the importance of aberrant FOXA1 O-GlcNAcylation in breast cancer progression and indicate that targeting O-GlcNAcylation is a therapeutic strategy for metastatic breast cancer.

[Novel Tyrosine Phosphorylation Signals in the Nucleus and on Mitotic Spindle Fibers and Lysosomes Revealed by Strong Inhibition of Tyrosine Dephosphorylation]

Protein-tyrosine phosphorylation is one of the posttranslational modifications and plays critical roles in regulating a wide variety of cellular processes, such as cell proliferation, differentiation, adhesion, migration, survival, and apoptosis. Protein-tyrosine phosphorylation is reversibly regulated by protein-tyrosine kinases and protein-tyrosine phosphatases. Strong inhibition of protein-tyrosine phosphatase activities is required to undoubtedly detect tyrosine phosphorylation. Our extremely careful usage of Na₃VO₄, a potent protein-tyrosine phosphatase inhibitor, has revealed not only the different intracellular trafficking pathways of Src-family tyrosine kinase members but also novel tyrosine phosphorylation signals in the nucleus and on mitotic spindle fibers and lysosomes. Furthermore, despite that the first identified oncogene product v-Src is generally believed to induce transformation through continuous stimulation of proliferation signaling by its strong tyrosine kinase activity, v-Src-driven transformation was found to be caused not by continuous proliferation signaling but by v-Src tyrosine kinase activity-dependent stochastic genome alterations. Here, I summarize our findings regarding novel tyrosine phosphorylation signaling in a spatiotemporal sense and highlight the significance of the roles of tyrosine phosphorylation in transcriptional regulation inside the nucleus and chromosome dynamics.

Pioneer of prostate cancer: past, present and the future of FOXA1

Prostate cancer is the most commonly diagnosed non-cutaneous cancers in North American men. While androgen deprivation has remained as the cornerstone of prostate cancer treatment, resistance ensues leading to lethal disease. Forkhead box A1 (FOXA1) encodes a pioneer factor that induces open chromatin conformation to allow the binding of other transcription factors. Through direct interactions with the Androgen Receptor (AR), FOXA1 helps to shape AR signaling that drives the growth and survival of normal prostate and prostate cancer cells. FOXA1 also possesses an AR-independent role of regulating epithelial-to-mesenchymal transition (EMT). In prostate cancer, mutations converge onto the coding sequence and cis-regulatory elements (CREs) of FOXA1, leading to functional alterations. In addition, FOXA1 activity in prostate cancer can be modulated post-translationally through various mechanisms such as LSD1-mediated protein demethylation. In this review, we describe the latest discoveries related to the function and regulation of FOXA1 in prostate cancer, pointing to their relevance to guide future clinical interventions.

PBX1 is a valuable prognostic biomarker for patients with breast cancer

Pre-B-cell leukemia transcription factor (PBX) proteins have important roles in the development of numerous organs. To date, four members of the PBX family have been identified to be involved in human cancer but little is known about their expression patterns and precise functions in breast cancer (BC) progression. The aim of the present study was to determine whether they have the potential to be prognostic biomarkers in patients with BC. The expression patterns of PBXs were evaluated using Oncomine, Cancer Cell Line Encyclopedia and Gene expression-based Outcome for Breast cancer Online algorithm analyses. The prognostic value of PBX1 was determined by Kaplan-Meier plotter analysis. It was observed that, among all PBX family members, only PBX1 was significantly upregulated in BC vs. normal tissues. Meta-analysis in the Oncomine database revealed that PBX1 was significantly upregulated in invasive breast carcinoma stroma, ductal breast carcinoma, invasive lobular breast carcinoma, invasive mixed breast carcinoma and male breast carcinoma compared with normal tissues. In addition, PBX1 was significantly correlated with forkhead box protein A1. Subtype analysis indicated that PBX1 overexpression was associated with luminal-like and hormone receptor-sensitive subtypes. In the survival analysis, a high expression level of PBX1 was associated with poor prognosis of patients with estrogen receptor (ER)-positive, luminal A and luminal B subtypes of BC. The results of the present study indicate that PBX1 may serve as a specific biomarker and essential prognostic factor for ER-positive, luminal A and luminal B subtypes of BC.

Identification of the O-GalNAcylation site(s) on FOXA1 catalyzed by ppGalNAc-T2 enzyme in vitro

FOXA1 functions as a pioneer factor of transcriptional regulation that binds to specific sites in the chromatin and recruits other transcription factors, promoting the initiation of gene transcription and mediating the regulation of downstream target gene expression. FOXA1 was reported to facilitate or reprogram ERα binding, thus playing a key function in breast cancer progression. Our previous results indicated that the O-linked N-acetylgalactosamine (O-GalNAc) modification of FOXA1 plays a potentially significant role in the ERα transcription network. However, further investigations are needed to identify the specific mechanism of modification and the specific glycosylation sites on FOXA1. In this study, we first suggested that FOXA1 could be O-GalNAcylated by ppGalNAc-T2 in vitro. By dividing and expressing recombinant FOXA1 as three segments, two O-GalNAcylation sites were found on FOXA1, both located at the C-terminal of the protein. Then, synthesized peptides, including the predicted O-GalNAc sites in the C-terminus of FOXA1, were used in a vitro reaction, and peptides mutated at the predicted O-GalNAc sites were employed as controls. Through an ESI-MS assay, S354 and S355 were identified as probable O-GalNAcylation sites on FOXA1. Additionally, we performed ESI-ETD-MS/MS analysis of the full-length O-GalNAcylated FOXA1 protein and identified S355 as the O-GalNAc modification site on FOXA1, consistent with the peptide reaction. In conclusion, our results demonstrated that FOXA1 can be O-GalNAcylated by ppGalNAc-T2 at S355 in vitro. These results will provide new insights for studying the role of O-GalNAcylation in the development of breast cancer.

Role for tyrosine phosphorylation of SUV39H1 histone methyltransferase in enhanced trimethylation of histone H3K9 via neuregulin-1/ErbB4 nuclear signaling

Protein-tyrosine kinases transmit signals by phosphorylating their substrates in diverse cellular events. The receptor-type tyrosine kinase ErbB4, a member of the epidermal growth factor receptor subfamily, is activated and proteolytically cleaved upon ligand stimulation, and the cleaved ErbB4 intracellular domain (4ICD) is released into the cytoplasm and the nucleus. We previously showed that generation of nuclear 4ICD by neuregulin-1 (NRG-1) stimulation enhances the levels of trimethylation of histone H3 at lysine 9 (H3K9me3). However, it remains unclear how nuclear 4ICD enhances H3K9me3 levels. Here we show that the histone H3K9 methyltransferase SUV39H1 associates with NRG-1/ErbB4-mediated H3K9me3. Knockdown of SUV39H1 blocked NRG-1-mediated enhancement of the levels of H3K9me3. Nuclear 4ICD was found to phosphorylate SUV39H1 primarily at Tyr-297, -303, and -308 that are conserved among humans, mice, and flies. Furthermore, knockdown-rescue experiments showed that the unphosphorylatable SUV39H1 mutant (3 YF) was incapable of enhancing the levels of H3K9me3 upon NRG-1 stimulation. These results suggest that nuclear ErbB4 enhances H3K9me3 levels through tyrosine phosphorylation of SUV39H1 in NRG-1/ErbB4 signal-mediated chromatin remodeling.

Desuppression of TGF-β signaling via nuclear c-Abl-mediated phosphorylation of TIF1γ/TRIM33 at Tyr-524, -610, and -1048

Protein-tyrosine kinases regulate a broad range of intracellular processes occurring primarily just beneath the plasma membrane. With the greatest care to prevent dephosphorylation, we have shown that nuclear tyrosine phosphorylation regulates global chromatin structural states. However, the roles for tyrosine phosphorylation in the nucleus are poorly understood. Here we identify transcriptional intermediary factor 1-γ (TIF1γ/TRIM33/Ectodermin), which suppresses transforming growth factor-β (TGF-β) signaling through the association with Smad2/3 transcription factor, as a new nuclear substrate of c-Abl tyrosine kinase. Replacement of the three tyrosine residues Tyr-524, -610, and -1048 with phenylalanine (3YF) inhibits c-Abl-mediated phosphorylation of TIF1γ and enhances TIF1γ's association with Smad3. Importantly, knockdown-rescue experiments show that 3YF strengthens TIF1γ's ability to suppress TGF-β signaling. Intriguingly, activation of c-Abl by epidermal growth factor (EGF) induces desuppression of TGF-β signaling via enhancing the tyrosine phosphorylation level of TIF1γ. TGF-β together with EGF synergistically provokes desuppressive responses of epithelial-to-mesenchymal transition through tyrosine phosphorylation of TIF1γ. These results suggest that nuclear c-Abl-mediated tyrosine phosphorylation of TIF1γ has a desuppressive role in TGF-β-Smad2/3 signaling.

The Truncated Isoform of the Receptor Tyrosine Kinase ALK Generated by Alternative Transcription Initiation (ALKATI) Induces Chromatin Structural Changes in the Nucleus in a Kinase Activity-Dependent Manner

Anaplastic lymphoma kinase (ALK) is a receptor-type tyrosine kinase that promotes cell growth upon stimulation with ligands such as midkine and pleiotrophin. Recently, a truncated isoform of ALK was identified in a variety of tumors. This isoform is expressed from a novel ALK transcript initiated from a de novo alternative transcription initiation (ATI) site in ALK intron 19 (referred to as ALK^ATI). ALK^ATI, which consists of only the intracellular kinase domain, localizes to the nucleus as well as the cytoplasm. However, its nuclear role is unknown. In this study, we determined that ALK^ATI promoted chromatin structural changes in the nucleus in a kinase activity-dependent manner. We found that expression of ALK^ATI increased the level of the heterochromatin marker Lys9 tri-methylated histone H3. In addition, we demonstrated that ALK^ATI phosphorylated the nuclear protein A-kinase anchoring protein 8 (AKAP8) and altered its subcellular localization from the insoluble fraction to the soluble fraction. These results suggest that ALK^ATI induces chromatin structural changes and heterochromatinization through phosphorylation of AKAP8 in the nucleus.

Down-regulation of Forkhead box protein A1 (FOXA1) leads to cancer stem cell-like properties in tamoxifen-resistant breast cancer cells through induction of interleukin-6

The selective estrogen receptor (ER) modulator tamoxifen inhibits ER signaling in breast cancer cells, and it is used for the treatment of ER-positive breast cancer. However, this type of cancer often acquires resistance to tamoxifen, and a better understanding of the molecular mechanisms underlying tamoxifen resistance is required. In this study, we established tamoxifen-resistant (TAM-R) breast cancer cells by long-term tamoxifen treatment of ER-positive breast cancer MCF7 cells. In TAM-R cells, expression of not only ERα, a major form of ER in breast cancer, but also its transcriptional partner forkhead box protein A1 (FOXA1) was found to be reduced. In contrast, activation of the transcription factor nuclear factor-κB (NF-κB) and expression of its target IL6 were increased in these cells. Stable expression of FOXA1, but not ERα, reduced the expression of IL6 in the FOXA1- and ERα-negative breast cancer MDA-MB-231 cells and TAM-R cells, without affecting the activation of the NF-κB signaling pathways. Conversely, FOXA1 knockdown induced IL6 expression in MCF7 cells. Chromatin immunoprecipitation assays revealed that FOXA1 bound to the promoter region of IL6 and repressed recruitment of the NF-κB complex to this region. TAM-R cells were found to have high mammosphere-forming activity, characteristics of cancer stem cells, and this activity was suppressed by NF-κB and IL6 signaling inhibitors. Taken together, these results suggest that FOXA1 suppresses expression of IL6 through inhibition of NF-κB recruitment to the IL6 promoter in an ERα-independent manner and that reduction in FOXA1 expression induces IL6 expression and contributes to cancer stem cell-like properties in TAM-R cells.

Src Acts as an Effector for Ku70-dependent Suppression of Apoptosis through Phosphorylation of Ku70 at Tyr-530

Src-family tyrosine kinases are widely expressed in many cell types and participate in a variety of signal transduction pathways. Despite the significance of Src in suppression of apoptosis, its mechanism remains poorly understood. Here we show that Src acts as an effector for Ku70-dependent suppression of apoptosis. Inhibition of endogenous Src activity promotes UV-induced apoptosis, which is impaired by Ku70 knockdown. Src phosphorylates Ku70 at Tyr-530, being close to the possible acetylation sites involved in promotion of apoptosis. Src-mediated phosphorylation of Ku70 at Tyr-530 decreases acetylation of Ku70, whereas Src inhibition augments acetylation of Ku70. Importantly, knockdown-rescue experiments with stable Ku70 knockdown cells show that the nonphosphorylatable Y530F mutant of Ku70 reduces the ability of Ku70 to suppress apoptosis accompanied by augmentation of Ku70 acetylation. Our results reveal that Src plays a protective role against hyperactive apoptotic cell death by reducing apoptotic susceptibility through phosphorylation of Ku70 at Tyr-530.