Hakai, an E3-ligase for E-cadherin, stabilizes δ-catenin through Src kinase

Steroid sulfatase suppresses keratinization by inducing proteasomal degradation of E-cadherin via Hakai regulation

X-linked ichthyosis (XLI) is a genetic disorder characterized by a steroid sulfatase (STS) deficiency inducing excessive cholesterol sulfate accumulation and keratinization. Our study utilizes STS knockout mice to reproduce the hyperkeratinization typical of XLI, providing a valuable model for investigating the underlying mechanisms. From the experiment of STS-deficient keratinocytes using the CRISPR/Cas9 system, we observed upregulation of E-cadherin, which is associated with keratinocyte differentiation and stratification. This was accompanied by elevated levels of keratinization markers, including involucrin and loricrin. We also found an increased expression of SULT2B1, which converts cholesterol to cholesterol sulfate, further accelerating cholesterol sulfate accumulation. As a result, STS deficiency and cholesterol sulfate accumulation lead to decreased expression of Hakai, the ubiquitin E3 ligase for E-cadherin. With reduced Hakai, endocytosis and ubiquitin-mediated degradation of E-cadherin are inhibited, resulting in its stabilization. This stabilization of E-cadherin is accompanied by increased expression of involucrin and loricrin, which is suppressed when the N-terminal extracellular domain of E-cadherin, responsible for cell-cell adhesion, is genetically modified. We propose that inhibition of E-cadherin, genetic modification of the N-terminal extracellular domain, and treatment with miR-6766 targeting E-cadherin significantly reduce the expression of keratinization markers, suggesting a potential therapeutic approach. We further suggest that the increased expression of E-cadherin observed in keratinocytes with STS deficiency is regulated by Hakai, underscoring the central role of E-cadherin in the pathogenesis of XLI.

The role of celastrol in inflammation and diseases

Celastrol is one of the main active ingredients extracted from the plant Tripterygium wilfordii Hook F. A growing number of studies have shown that celastrol has various pharmacological effects, including anti-inflammation, anti-rheumatism, treatment of neurodegenerative diseases, and anti-tumor. This article systematically summarized the mechanism and role of celastrol in lipid metabolism and obesity, rheumatoid arthritis (RA), osteoarthritis (OA), gouty arthritis, inflammatory bowel disease, neurodegenerative diseases, and cancer and other diseases (such as diabetes, respiratory-related diseases, atherosclerosis, psoriasis, hearing loss, etc.). The celastrol played roles in inflammation response, cell apoptosis, autophagy, ferroptosis, and lipid metabolism mainly by acting on chondrocytes, macrophages, mitochondria, and endoplasmic reticulum (ER) through NF-κB, STAT, MAPK, TLR, PI3K-AKT-mTOR, and other signal pathways. This review could provide a reference for the clinical application and further development and utilization of celastrol.

Beyond destruction: emerging roles of the E3 ubiquitin ligase Hakai

Hakai protein (CBLL1 gene) was identified as an E3 ubiquitin ligase of E-cadherin complex, inducing its ubiquitination and degradation, thus inducing epithelial-to-mesenchymal transition. Most of the knowledge about the protein was associated to its E3 ubiquitin ligase canonical role. However, important recent published research has highlighted the noncanonical role of Hakai, independent of its E3 ubiquitin ligase activity, underscoring its involvement in the N6-methyladenosine (m6A) writer complex and its impact on the methylation of RNA. The involvement of Hakai in this mRNA modification process has renewed the relevance of this protein as an important contributor in cancer. Moreover, Hakai potential as a cancer biomarker and its prognostic value in malignant disease also emphasize its untapped potential in precision medicine, which would also be discussed in detail in our review. The development of the first small-molecule inhibitor that targets its atypical substrate binding domain is a promising step that could eventually lead to patient benefit, and we would cover its discovery and ongoing efforts toward its use in clinic.

Hakai, a novel Runx2 interacting protein, augments osteoblast differentiation by rescuing Runx2 from Smurf2-mediated proteasome degradation

Runt-related transcription factor 2 (Runx2) is a key regulator of osteoblast differentiation and bone formation. In Runx2-deficient embryos, skeletal development ceases at the cartilage anlage stage. These embryos die of respiratory failure upon birth and display a complete absence of bone and cartilage mineralization. Here, we identified Hakai, a type of E3 ubiquitin ligase as a potential Runx2 interacting partner through affinity pulldown-based proteomic approach. Subsequently, we observed that similar to Runx2, Hakai was downregulated in osteopenic ovariectomized rats, suggesting its involvement in bone formation. Consistent with this observation, Hakai overexpression significantly enhanced osteoblast differentiation in mesenchyme-like C3H10T1/2 as well as primary rat calvaria osteoblast (RCO) cells in vitro. Conversely, overexpression of a catalytically inactive Hakai mutant (C109A) exhibited minimal to no effect, whereas Hakai depletion markedly reduced endogenous Runx2 levels and impaired osteogenic differentiation in both C3H10T1/2 and RCOs. Mechanistically, Hakai physically interacts with Runx2 and enhances its protein turnover by rescuing it from Smad ubiquitination regulatory factor 2 (Smurf2)-mediated proteasome degradation. Wild-type Hakai but not Hakai-C109A inhibited Smurf2 protein levels through proteasome-mediated degradation. These findings underscore Hakai's functional role in bone formation, primarily through its positive modulation of Runx2 protein turnover by protecting it from Smurf2-mediated ubiquitin-proteasomal degradation. Collectively, our results demonstrate Hakai as a promising novel therapeutic target for osteoporosis.

Inhibition of δ-catenin palmitoylation slows the progression of prostate cancer

Prostate cancer (PCa) is the second leading cause of death in males. It has been reported that δ-catenin expression is upregulated during the late stage of prostate cancer. Palmitoylation promotes protein transport to the cytomembrane and regulates protein localization and function. However, the effect of δ-catenin palmitoylation on the regulation of cancer remains unknown. In this study, we utilized prostate cancer cells overexpressing mutant δ-catenin (J6A cells) to induce a depalmitoylation phenotype and investigate its effect on prostate cancer. Our results indicated that depalmitoylation of δ-catenin not only reduced its membrane expression but also promoted its degradation in the cytoplasm, resulting in a decrease in the effect of EGFR and E-cadherin signaling. Consequently, depalmitoylation of δ-catenin reduced the proliferation and metastasis of prostate cancer cells. Our findings provide novel insights into potential therapeutic strategies for controlling the progression of prostate cancer through palmitoylation-based targeting of δ-catenin.

Protein degradation: expanding the toolbox to restrain cancer drug resistance

Despite significant progress in clinical management, drug resistance remains a major obstacle. Recent research based on protein degradation to restrain drug resistance has attracted wide attention, and several therapeutic strategies such as inhibition of proteasome with bortezomib and proteolysis-targeting chimeric have been developed. Compared with intervention at the transcriptional level, targeting the degradation process seems to be a more rapid and direct strategy. Proteasomal proteolysis and lysosomal proteolysis are the most critical quality control systems responsible for the degradation of proteins or organelles. Although proteasomal and lysosomal inhibitors (e.g., bortezomib and chloroquine) have achieved certain improvements in some clinical application scenarios, their routine application in practice is still a long way off, which is due to the lack of precise targeting capabilities and inevitable side effects. In-depth studies on the regulatory mechanism of critical protein degradation regulators, including E3 ubiquitin ligases, deubiquitylating enzymes (DUBs), and chaperones, are expected to provide precise clues for developing targeting strategies and reducing side effects. Here, we discuss the underlying mechanisms of protein degradation in regulating drug efflux, drug metabolism, DNA repair, drug target alteration, downstream bypass signaling, sustaining of stemness, and tumor microenvironment remodeling to delineate the functional roles of protein degradation in drug resistance. We also highlight specific E3 ligases, DUBs, and chaperones, discussing possible strategies modulating protein degradation to target cancer drug resistance. A systematic summary of the molecular basis by which protein degradation regulates tumor drug resistance will help facilitate the development of appropriate clinical strategies.

The Role of RKIP in the Regulation of EMT in the Tumor Microenvironment

Simple Summary

Raf kinase inhibitor protein (RKIP) expression in cancer cells is significantly reduced and promoting cancer cells growth and invasiveness. Overexpresssion of RKIP has been reported to mediate pleiotropic anti-cancer activities including the inhibition of survival signaling pathways, sensitization to cell death by cytotoxic drugs, inhibition of invasion, EMT and metastasis. The molecular mechanism by which RKIP inhibits EMT is not clear. In this review, we have examined how RKIP inhibits the selected EMT gene products (Snail, vimentin, N-cadherin, laminin alpha) and found that it involves signaling cross-talks between RKIP and each of the EMT gene products. These findings were validated by bioinformatic analyses demonstrating in various human cancers a negative correlation between the expression of RKIP and the expression of the EMT gene products. These findings suggest that targeting RKIP induction in cancer cells will result in multiple hits by inhibiting tumor growth, metastasis and reversal of chemo-immuno resistance.

Abstract

The Raf Kinase Inhibitor Protein (RKIP) is a unique gene product that directly inhibits the Raf/Mek/Erk and NF-kB pathways in cancer cells and resulting in the inhibition of cell proliferation, viability, EMT, and metastasis. Additionally, RKIP is involved in the regulation of cancer cell resistance to both chemotherapy and immunotherapy. The low expression of RKIP expression in many cancer types is responsible, in part, for the pathogenesis of cancer and its multiple properties. The inhibition of EMT and metastasis by RKIP led to its classification as a tumor suppressor. However, the mechanism by which RKIP mediates its inhibitory effects on EMT and metastases was not clear. We have proposed that one mechanism involves the negative regulation by RKIP of the expression of various gene products that mediate the mesenchymal phenotype as well as the positive regulation of gene products that mediate the epithelial phenotype via signaling cross talks between RKIP and each gene product. We examined several EMT mesenchymal gene products such as Snail, vimentin, N-cadherin, laminin and EPCAM and epithelial gene products such as E-cadherin and laminin. We have found that indeed these negative and positive correlations were detected in the signaling cross-talks. In addition, we have also examined bioinformatic data sets on different human cancers and the findings corroborated, in large part, the findings observed in the signaling cross-talks with few exceptions in some cancer types. The overall findings support the underlying mechanism by which the tumor suppressor RKIP regulates the expression of gene products involved in EMT and metastasis. Hence, the development of agent that can selectively induce RKIP expression in cancers with low expressions should result in the activation of the pleiotropic anti-cancer activities of RKIP and resulting in multiple effects including inhibition of tumor cell proliferation, EMT, metastasis and sensitization of resistant tumor cells to respond to both chemotherapeutics and immunotherapeutics.

The Cross-Talk Between EGFR and E-Cadherin

Epidermal growth factor receptor (EGFR) and adhesion protein E-cadherin are major regulators of proliferation and differentiation in epithelial cells. Consistently, defects in both EGFR and E-cadherin-mediated intercellular adhesion are linked to various malignancies. These defects in either are further exacerbated by the reciprocal interactions between the two transmembrane proteins. On the one hand, EGFR can destabilize E-cadherin adhesion by increasing E-cadherin endocytosis, modifying its interactions with cytoskeleton and decreasing its expression, thus promoting tumorigenesis. On the other hand, E-cadherin regulates EGFR localization and tunes its activity. As a result, loss and mutations of E-cadherin promote cancer cell invasion due to uncontrolled activation of EGFR, which displays enhanced surface motility and changes in endocytosis. In this minireview, we discuss the molecular and cellular mechanisms of the cross-talk between E-cadherin and EGFR, highlighting emerging evidence for the role of endocytosis in this feedback, as well as its relevance to tissue morphogenesis, homeostasis and cancer progression.

CBLL1 is highly expressed in non-small cell lung cancer and promotes cell proliferation and invasion

Background

Studies have shown that E3 ubiquitin ligase CBLL1 plays multiple roles in development and tumorigenesis. CBLL1 is over‐expressed in colon cancer and associated with cancer cell proliferation. While, the overexpression of CBLL1 inhibited the estrogenic dependent cell proliferation and migration in ER alpha dependent breast cancer cell MCF‐7.

Methods

We used an immunohistochemical method to detect CBLL1 expression in human NSCLC and corresponding normal lung tissues and analyzed its relationship with clinicopathological parameters. Moreover, we investigated the role of CBLL1 in NSCLC cell behavior by inhibiting its expression in A549 and H1299 cells.

Results

In this study, we found that CBLL1 was frequently upregulated in non‐small lung cancer (NSCLC) tissues compared to the adjacent nontumor tissues. We found that the high expression of CBLL1 was associated with the tumor size in NSCLC tissues. It has been recently reported that CBLL1 promotes cell proliferation and invasion in A549 and H460 cells. Our results confirmed that CBLL1 promoted the proliferation by promoting G1/S cell cycle transition in NSCLCs cells. Moreover, CBLL1 knockdown inhibited cell invasion via increased E‐cadherin protein expression, and decreased expression of MMP2 and MMP9 in NSCLC cell lines. The protein expression of E‐cadherin was increased after CBLL1 depletion while the E‐cadherin mRNA was not affected after knockdown of the endogenous CBLL1.

Conclusion

These results provide important insights for using CBLL1 as an oncogenic marker gene in the development and progression of non‐small cell lung cancer.

Frequently rearranged and overexpressed δ-catenin is responsible for low sensitivity of prostate cancer cells to androgen receptor and β-catenin antagonists

The mechanism of prostate cancer (PCa) progression towards the hormone refractory state remains poorly understood. Treatment options for such patients are limited and present a major clinical challenge. Previously, δ-catenin was reported to promote PCa cell growth in vitro and its increased level is associated with PCa progression in vivo. In this study we show that re-arrangements at Catenin Delta 2 (CTNND2) locus, including gene duplications, are very common in clinically significant PCa and may underlie δ-catenin overexpression. We find that δ-catenin in PCa cells exists in a complex with E-cadherin, p120, and α- and β-catenin. Increased expression of δ-catenin leads to its further stabilization as well as upregulation and stabilization of its binding partners. Resistant to degradation and overexpressed δ-catenin isoform activates Wnt signaling pathway by increasing the level of nuclear β-catenin and subsequent stimulation of Tcf/Lef transcription targets. Evaluation of responses to treatments, with androgen receptor (AR) antagonist and β-catenin inhibitors revealed that cells with high levels of δ-catenin are more resistant to killing with single agent treatment than matched control cells. We show that combination treatment targeting both AR and β-catenin networks is more effective in suppressing tumor growth than targeting a single network. In conclusion, targeting clinically significant PCa with high levels of δ–catenin with anti-androgen and anti β-catenin combination therapy may prevent progression of the disease to a castration-resistant state and, thus, represents a promising therapeutic strategy.

A central role for cadherin signaling in cancer

Cadherins are homophilic adhesion molecules with important functions in cell-cell adhesion, tissue morphogenesis, and cancer. In epithelial cells, E-cadherin accumulates at areas of cell-cell contact, coalesces into macromolecular complexes to form the adherens junctions (AJs), and associates via accessory partners with a subcortical ring of actin to form the apical zonula adherens (ZA). As a master regulator of the epithelial phenotype, E-cadherin is essential for the overall maintenance and homeostasis of polarized epithelial monolayers. Its expression is regulated by a host of genetic and epigenetic mechanisms related to cancer, and its function is modulated by mechanical forces at the junctions, by direct binding and phosphorylation of accessory proteins collectively termed catenins, by endocytosis, recycling and degradation, as well as, by multiple signaling pathways and developmental processes, like the epithelial to mesenchymal transition (EMT). Nuclear signaling mediated by the cadherin associated proteins β-catenin and p120 promotes growth, migration and pluripotency. Receptor tyrosine kinase, PI3K/AKT, Rho GTPase, and HIPPO signaling, are all regulated by E-cadherin mediated cell-cell adhesion. Finally, the recruitment of the microprocessor complex to the ZA by PLEKHA7, and the subsequent regulation of a small subset of miRNAs provide an additional mechanism by which the state of epithelial cell-cell adhesion affects translation of target genes to maintain the homeostasis of polarized epithelial monolayers. Collectively, the data indicate that loss of E-cadherin function, especially at the ZA, is a common and crucial step in cancer progression.

Proteomic Analysis of the E3 Ubiquitin-Ligase Hakai Highlights a Role in Plasticity of the Cytoskeleton Dynamics and in the Proteasome System

Carcinoma, the most common type of cancer, arises from epithelial cells. The transition from adenoma to carcinoma is associated with the loss of E-cadherin and, in consequence, the disruption of cell-cell contacts. E-cadherin is a tumor suppressor, and it is down-regulated during epithelial-to-mesenchymal transition (EMT); indeed, its loss is a predictor of poor prognosis. Hakai is an E3 ubiquitin-ligase protein that mediates E-cadherin ubiquitination, endocytosis and finally degradation, leading the alterations of cell-cell contacts. Although E-cadherin is the most established substrate for Hakai activity, other regulated molecular targets for Hakai may be involved in cancer cell plasticity during tumor progression. In this work we employed an iTRAQ approach to explore novel molecular pathways involved in Hakai-driven EMT during tumor progression. Our results show that Hakai may have an important influence on cytoskeleton-related proteins, extracellular exosome-associated proteins, RNA-related proteins and proteins involved in metabolism. Moreover, a profound decreased expression in several proteasome subunits during Hakai-driven EMT was highlighted. Since proteasome inhibitors are becoming increasingly used in cancer treatment, our findings suggest that the E3 ubiquitin-ligase, such as Hakai, may be a better target than proteasome for using novel specific inhibitors in tumor subtypes that follow EMT.