The Multifaceted Roles of TLE Proteins in Cancer and Immune Regulation

Introduction

The transcriptional corepressor family TLE (transducin-like enhancer of split) plays a crucial role in the repression of gene expression. These proteins do not directly bind to DNA but are recruited to the promoter regions of target genes by transcription factors (TFs) to regulate gene expression. The family of transcriptional corepressors includes CtBP, NCoR, SMRT, and TLE members. This article will explore the expression, and regulatory mechanisms of TLE family proteins in different tissues, and detail their roles in tumorigenesis and immune regulation, thereby providing new insights for future cancer treatment strategies.

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Structure of TLE Proteins

The TLE family in mammals comprises seven members (TLE1-7). TLE7 is minimally expressed, and its function remains largely unexplored, but it is included here for completeness. The family is divided into long and short types. TLE1-4 are long TLE members, characterized by a glutamine-enriched N-terminal Q domain and C-terminal WD repeats (WDRs). The WDR domain is evolutionarily conserved with 80% identity among TLE members and is crucial for interactions with transcription factors (TFs). The Q domain is necessary for the tetramerization of TLE proteins, which is essential for their transcriptional suppressive function. The central region, less conserved than Q and WDR domains, includes Gly-Pro-enriched (GP), CcN, and Ser-Pro-enriched (SP) domains, subject to regulatory posttranslational modifications such as phosphorylation.

Short-type TLE proteins include TLE5 (AES) and TLE6. TLE5 lacks the WDR domain and contains only Q- and C-terminal domains, structurally similar to the long-type TLE's GP domain. TLE6 retains the conserved WDR domain but lacks the Q, GP, CcN, and SP domains, exhibiting the least identity with other TLE members. TLE5 is considered a dominant negative isoform, as it likely inhibits the formation of functional TLE tetramers by binding the Q domain of long-type TLE proteins.

The expression profiles of TLE proteins are tissue-specific and regulated during development. Long-type TLE members are coexpressed in some epithelial tissues, and their expression levels are higher in developing, proliferating cells than in differentiated cells. TLE members exhibit complementary expression patterns in developing neurons and the pancreas, suggesting spatiotemporal regulation and cell context-specific activity.

Fig. 1 Protein structures, domains, and functions of human TLE family members. (Yu G., et al. 2022) 

TLEs in Tumorigenesis and Cancer Therapy

Tumor Suppressive Activity of TLEs

TLE proteins are dysregulated in various cancers, with some acting as tumor suppressors. TLE1 is highly expressed in synovial sarcoma and serves as a diagnostic marker. In blood cancers, TLE1 and TLE4 function as tumor suppressors. For instance, the AML1-ETO fusion protein drives acute myeloid leukemia (AML), but it requires the loss of TLE1/TLE4 to induce tumorigenesis. Overexpression of TLE1 or TLE4 inhibits lymphoma cell proliferation, while their knockdown promotes tumor growth in models. Epigenetic inactivation of TLE1 through hypermethylation is observed in blood tumors like AML and non-Hodgkin's lymphoma. In T-cell acute lymphoblastic leukemia (T-ALL), decreased TLE1 expression correlates with poor prognosis.

In solid tumors, high TLE1 levels are associated with a better prognosis in pancreatic cancer and inhibit tumor cell proliferation and migration. TLE1 is similarly upregulated in gastric cancer and associated with better patient outcomes. In liver cancer, TLE1 downregulation by miRNA-657 contributes to its tumor-suppressive role. TLE2 expression correlates with a favorable prognosis in pancreatic and bladder cancer, and TLE3's presence in triple-negative breast cancer predicts better outcomes. High RNF6 expression in colorectal cancer promotes tumor growth via TLE3 ubiquitination and degradation, whereas TLE5 inhibits metastasis by suppressing Notch and androgen receptor signaling in colorectal and prostate cancers.

Tumor-Promoting Activity of TLEs

Conversely, some TLE family members exhibit oncogenic properties in certain cancers. TLE1 transgenic mice develop lung cancer due to increased ERBB1 and ERBB2 expression. In lung adenocarcinomas, high TLE1 levels predict poor prognosis, partly due to induced E-cadherin suppression, epithelial-mesenchymal transition (EMT), and anoikis resistance. TLE1 overexpression supports ER target gene expression in breast cancer, enhancing tumor cell proliferation. Furthermore, TLE1's interaction with HES1 and subsequent Notch pathway activation contributes to glioblastoma progression.

TLE3 expression increases during melanoma progression, promoting cell proliferation and tumor growth. This protumorigenic activity can be blocked by HDAC inhibitors, suggesting TLE3-mediated Wnt signaling suppression in melanoma.

TLEs and Resistance to Cancer Therapy

TLE levels affect cancer cells' responses to therapies. In prostate cancer, TLE3 deficiency correlates with resistance to androgen receptor inhibitors. Pancreatic cancer cells with increased TLE2 expression show greater sensitivity to gemcitabine. In AML, TLE4 loss induces chemotherapy resistance via Wnt pathway activation. TLE3 expression enhances taxane sensitivity in ovarian and certain breast cancers and correlates with tamoxifen sensitivity in breast cancer cells. TLE3 overexpression also enhances non-small cell lung cancer sensitivity to taxanes. These findings underscore the need for further investigation into TLE-mediated therapy resistance mechanisms.

TLEs in Signaling Pathways: Tumorigenesis and Immune Regulation

Notch Signaling

Notch signaling governs many physiological processes, including development, immune regulation, and cancer. TLEs regulate Notch signaling by interacting with HES1 and HDACs, forming suppressive complexes that inhibit gene transcription. TLE5 recruits HDAC3 to Notch target gene promoters, particularly in colorectal cancer, forming unique complexes with TLE1.

Wnt Signaling

Wnt signaling is essential for development and is implicated in many cancers, especially colorectal cancer. TLEs regulate Wnt target gene expression by forming complexes with TCF/LEF1 family proteins. Wnt pathway activation leads to dissociation of the β-catenin destruction complex, allowing β-catenin to block TLEs' inhibitory effects. Ubiquitination and subsequent degradation of TLEs, often mediated by E3 ubiquitin ligases like UBR5, RNF6, and XIAP, relieves Wnt signaling inhibition and promotes tumorigenesis.

EGFR/MAPK Signaling

The antagonism between EGFR and Notch signaling pathways involves Gro/TLE proteins in Drosophila, a mechanism conserved in mammals. EGFR activation phosphorylates Gro, reducing its suppressive activity and allowing gene expression. In tumors with constitutive MAPK activation, such as pancreatic and lung cancer, TLE1 phosphorylation and nuclear translocation regulation need further investigation.

NF-κB Signaling

TLEs modulate NF-κB signaling, crucial for inflammation and cancer progression. TLE1 interacts with NOD2, inhibiting NF-κB signaling, and mutations or reduced expression of TLE1 are linked to inflammatory diseases and cancer. TLE5 inhibits NF-κB by binding to p65, whereas TLE1, through interactions with Sirt1, suppresses NF-κB signaling independent of Sirt1's deacetylase activity.

ER/AR Receptor Signaling

TLEs also regulate estrogen receptor (ER) and androgen receptor (AR) signaling pathways, vital for cancer cell survival. TLE3 complexes with FOXA1 and HDAC2 suppress basal ER/AR target gene expression. Upon ligand stimulation, TLE1 facilitates ER/AR target gene transcription. TLE5 inhibits DNA binding by AR, thus suppressing AR target gene expression and reducing prostate cancer progression and metastasis.

Crosstalk Among TLE-Regulated Pathways in Tumorigenesis

TLEs play critical roles in cell fate decisions during development and cancer progression. Notch signaling, modulated by TLE interactions, can lead to either pro- or antitumorigenic effects depending on the cellular context. In lung tissue, Notch activation influences differentiation, contributing to either adenocarcinoma or small-cell lung cancer outcomes. TLE1 transgenic mice developing spontaneous lung cancer highlight the potential interaction between TLE1 and Notch signaling.

Notch signaling's tumor-suppressive or oncogenic roles vary with tissue type. In AML, Notch reactivation is a potential therapeutic strategy, while in T-ALL, continuous Notch activation supports cancer cell survival. TLE1/4 deletion synergizes with oncogenic events to promote AML by disrupting lineage choice, whereas in T-ALL, low TLE1 expression associates with poorer survival.

Perspectives on Regulating TLEs for Cancer Therapy

Targeted Therapy

TLEs' regulation of multiple signaling pathways implicates them in cancer therapy resistance mechanisms. EGFR pathway activation reduces Gro/TLE suppressive activity, suggesting that targeting TLE phosphorylation or relevant ubiquitin ligases could enhance cancer therapy effectiveness. Given the feedback activation of Notch and Wnt pathways in resistance to EGFR inhibitors, examining TLE roles in these pathways is warranted.

Immunotherapy

TLEs' regulation of immune responses, particularly in macrophages and T cells, suggests targeting TLEs may enhance immunotherapy. Suppressing TLE1 could enhance M1 macrophage differentiation and NF-κB activation, potentially improving tumor immune surveillance and inhibiting cancer progression.

Conclusion

TLE family proteins play multifaceted roles in gene regulation, development, tumorigenesis, and immune responses. Understanding their regulation and function is crucial for developing targeted therapies. Future research should focus on elucidating TLE-mediated mechanisms in cancer and immune regulation, exploring conditional knockout models, and developing specific TLE inhibitors to modulate their activity for therapeutic benefit.

References

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4. Hu S., et al. TLE2 is associated with favorable prognosis and regulates cell growth and gemcitabine sensitivity in pancreatic cancer. Annals of Translational Medicine. 2020, 8 (16).
5. Brunquell C., et al. TLE1 is an anoikis regulator and is downregulated by Bit1 in breast cancer cells. Molecular Cancer Research. 2012, 10 (11): 1482-95.