Expression of Plakophilins (PKP1, PKP2, and PKP3) in breast cancers

Unveiling the Binding between the Armadillo-Repeat Domain of Plakophilin 1 and the Intrinsically Disordered Transcriptional Repressor RYBP

Plakophilin 1 (PKP1), a member of the p120ctn subfamily of the armadillo (ARM)-repeat-containing proteins, is an important structural component of cell-cell adhesion scaffolds although it can also be ubiquitously found in the cytoplasm and the nucleus. RYBP (RING 1A and YY1 binding protein) is a multifunctional intrinsically disordered protein (IDP) best described as a transcriptional regulator. Both proteins are involved in the development and metastasis of several types of tumors. We studied the binding of the armadillo domain of PKP1 (ARM-PKP1) with RYBP by using in cellulo methods, namely immunofluorescence (IF) and proximity ligation assay (PLA), and in vitro biophysical techniques, namely fluorescence, far-ultraviolet (far-UV) circular dichroism (CD), and isothermal titration calorimetry (ITC). We also characterized the binding of the two proteins by using in silico experiments. Our results showed that there was binding in tumor and non-tumoral cell lines. Binding in vitro between the two proteins was also monitored and found to occur with a dissociation constant in the low micromolar range (~10 μM). Finally, in silico experiments provided additional information on the possible structure of the binding complex, especially on the binding ARM-PKP1 hot-spot. Our findings suggest that RYBP might be a rescuer of the high expression of PKP1 in tumors, where it could decrease the epithelial-mesenchymal transition in some cancer cells.

FERMT1 promotes cell migration and invasion in non-small cell lung cancer via regulating PKP3-mediated activation of p38 MAPK signaling

BACKGROUND

Fermitin family member 1 (FERMT1) is highly expressed in many tumors and acts as an oncogene. Nonetheless, the precise function of FERMT1 in non-small cell lung cancer (NSCLC) has not been clearly elucidated.

METHODS

Bioinformatics software predicted the FERMT1 expression in NSCLC. Transwell assays facilitated the detection of NSCLC cell migration and invasion. Western blotting techniques were employed to detect the protein levels regulated by FERMT1.

RESULTS

FERMT1 exhibited high expression levels in NSCLC and was linked to the patients' poor prognosis, as determined by a variety of bioinformatics predictions combined with experimental verification. FERMT1 promoted the migration and invasion of NSCLC and regulated epithelial to mesenchymal transition (EMT) -related markers. Further studies showed that FERMT1 could up-regulate the expression level of plakophilin 3(PKP3). Further research has indicated that FERMT1 can promote cell migration and invasion via up-regulating PKP3 expression. By exploring downstream signaling pathways, we found that FERMT1 has the capability to activate the p38 mitogen-activated protein kinases (p38 MAPK) signaling pathway, and knocking down PKP3 can counteract the activation induced by FERMT1 overexpression.

CONCLUSIONS

FERMT1 was highly expressed in NSCLC and can activate the p38 MAPK signaling pathway through up-regulation of PKP3, thus promoting the invasion and migration of NSCLC.

Role and function of plakophilin 3 in cancer progression and skin disease

Plakophilin 3 (PKP3), a component of desmosome, is aberrantly expressed in many kinds of human diseases, especially in cancers. Through direct interaction, PKP3 binds with a series of desmosomal proteins, such as desmoglein, desmocollin, plakoglobin, and desmoplakin, to initiate desmosome aggregation, then promotes its stability. As PKP3 is mostly expressed in the skin, loss of PKP3 promotes the development of several skin diseases, such as paraneoplastic pemphigus, pemphigus vulgaris, and hypertrophic scar. Moreover, accumulated clinical data indicate that PKP3 dysregulates in diverse cancers, including breast, ovarian, colon, and lung cancers. Numerous lines of evidence have shown that PKP3 plays important roles in multiple cellular processes during cancer progression, including metastasis, invasion, tumor formation, autophagy, and proliferation. This review examines the diverse functions of PKP3 in regulating tumor formation and development in various types of cancers and summarizes its detailed mechanisms in the occurrence of skin diseases.

Comprehensive Analysis Identifies PKP3 Overexpression in Pancreatic Cancer Related to Unfavorable Prognosis

Plakophilin 3 (PKP3) affects cell signal transduction and cell adhesion and performs a crucial function in tumorigenesis. The current investigation evaluated the predictive significance and underlying processes of PKP3 within pancreatic cancer (PC) tissues. The assessment of differences in PKP3 expression was conducted through an analysis of RNA-seq data acquired from The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) databases. Additionally, clinical samples were collected to validate the findings. The predictive significance of PKP3 was investigated by analyzing survival data derived from TCGA and clinical specimens. PKP3's biological function was assessed via phenotypic experiments after the suppression of PKP3 expression within PC cells. Functional enrichment analysis, encompassing KEGG, GO, and GSEA, was employed to assess the underlying mechanism of PKP3. Immune infiltration analysis was conducted in the present investigation to determine the association between PKP3 and tumor-infiltrating immune cells (TICs). In PC tissues, PKP3 expression was abnormally upregulated and correlated with a negative prognosis in individuals with PC. PKP3 can promote the progression, migration, and invasive capacity of PC cells and is relevant to the regulation of the PI3K-Akt and MAPK signaling pathways. Immune infiltration analysis demonstrated that PKP3 impeded CD8+ T-cell infiltration and immune cytokine expression within the tumor microenvironment. The PKP3 protein was identified as a prospective independent predictive indicator and represents a viable approach for immunotherapy in the context of PC. PKP3 may impact prognosis by broadly inhibiting immune cell infiltration and promoting the activation of tumor-associated signaling pathways.

Plakophilin 3 facilitates G1/S phase transition and enhances proliferation by capturing RB protein in the cytoplasm and promoting EGFR signaling

Plakophilin 3 (PKP3) is a component of desmosomes and is frequently overexpressed in cancer. Using keratinocytes either lacking or overexpressing PKP3, we identify a signaling axis from ERK to the retinoblastoma (RB) protein and the E2F1 transcription factor that is controlled by PKP3. RB and E2F1 are key components controlling G1/S transition in the cell cycle. We show that PKP3 stimulates the activity of ERK and its target RSK1. This inhibits expression of the transcription factor RUNX3, a positive regulator of the CDK inhibitor CDKN1A/p21, which is also downregulated by PKP3. Elevated CDKN1A prevents RB phosphorylation and E2F1 target gene expression, leading to delayed S phase entry and reduced proliferation in PKP3-depleted cells. Elevated PKP3 expression not only increases ERK activity but also captures phosphorylated RB (phospho-RB) in the cytoplasm to promote E2F1 activity and cell-cycle progression. These data identify a mechanism by which PKP3 promotes proliferation and acts as an oncogene.

Plakophilin3 loss leads to increased adenoma formation and rectal prolapse in APCmin mice

Plakophilin3 (PKP3) loss leads to tumor progression and metastasis of colon cancer cells. The goal of this report was to determine if PKP3 loss led to increased disease progression in mice. We generated a colonocyte-specific knockout of PKP3 in APC^min mice, which led to increased adenoma formation, the formation of rectal prolapse, and a significant decrease in survival. The observed increase in rectal prolapse formation and decrease in survival correlated with an increase in the expression of Lipocalin2 (LCN2). Increased disease progression was observed even upon treatment with 5-fluorouracil (5FU). These results suggest that an increase in LCN2 expression might lead to therapy resistance and that LCN2 might serve as a potential therapeutic target in colorectal cancer.

Analysis of Multiple Human Tumor Cases Reveals the Carcinogenic Effects of PKP3

Plakophilins (PKPs) act as a key regulator of different signaling programs and control a variety of cellular processes ranging from transcription, protein synthesis, growth, proliferation, and tumor development. The function and possible mechanism of PKP3 in ovarian cancer (OC) remain unknown. It is extremely important to investigate the expression and prognostic values of PKP3, as well as their possible mechanisms, and immune infiltration in OC. Therefore, in this paper we explored the potential oncogenic role of PKP3 in 33 tumors based on The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) datasets. The result outcomes showed that PKP3 is highly expressed in most cancers, and the expression level and prognosis of PKP3 showed little significance in cancer patients. Moreover, oncologists have found that members of the plakophilin family have different degrees of abnormality in ovarian cancer. PKP3 played a key part in carcinogenesis and aggressiveness of OC as well as malignant biological activity and can be used as a biomarker for early diagnosis and prognosis evaluation in OC.

Disease-related blood-based differential methylation in cystic fibrosis and its representation in lung cancer revealed a regulatory locus in PKP3 in lung epithelial cells

Cystic fibrosis (CF) is a monogenic disease, characterized by massive chronic lung inflammation. The observed variability in clinical phenotypes in monozygotic CF twins is likely associated with the extent of inflammation. This study sought to investigate inflammation-related aberrant DNA methylation in CF twins and to determine to what extent acquired methylation changes may be associated with lung cancer.

Blood-based genome-wide DNA methylation analysis was performed to compare the DNA methylomes of monozygotic twins, from the European CF Twin and Sibling Study with various degrees of disease severity. Putatively inflammation-related and differentially methylated positions were selected from a large lung cancer case-control study and investigated in blood by targeted bisulphite next-generation-sequencing. An inflammation-related locus located in the Plakophilin-3 (PKP3) gene was functionally analysed regarding promoter and enhancer activity in presence and absence of methylation using luciferase reporter assays.

We confirmed in a unique cohort that monozygotic twins, even if clinically discordant, have only minor differences in global DNA methylation patterns and blood cell composition. Further, we determined the most differentially methylated positions, a high proportion of which are blood cell-type-specific, whereas others may be acquired and thus have potential relevance in the context of inflammation as lung cancer risk factors. We identified a sequence in the gene body of PKP3 which is hypermethylated in blood from CF twins with severe phenotype and highly variably methylated in lung cancer patients and controls, independent of known clinical parameters, and showed that this region exhibits methylation-dependent promoter activity in lung epithelial cells.

N6-methyladenosine-modified circIGF2BP3 inhibits CD8+ T-cell responses to facilitate tumor immune evasion by promoting the deubiquitination of PD-L1 in non-small cell lung cancer

Background

An in-depth understanding of immune evasion mechanisms in tumors is crucial to overcome resistance and enable innovative advances in immunotherapy. Circular RNAs (circRNAs) have been implicated in cancer progression. However, much remains unknown regarding whether circRNAs impact immune escape in non-small-cell lung carcinoma (NSCLC).

Methods

We performed bioinformatics analysis to profile and identify the circRNAs mediating immune evasion in NSCLC. A luciferase reporter assay, RNA immunoprecipitation (RIP), RNA pulldown assays and fluorescence in situ hybridization were performed to identify the interactions among circIGF2BP3, miR-328-3p, miR-3173-5p and plakophilin 3 (PKP3). In vitro T cell-mediated killing assays and in vivo syngeneic mouse models were used to investigate the functional roles of circIGF2BP3 and its downstream target PKP3 in antitumor immunity in NSCLC. The molecular mechanism of PKP3-induced PD-L1 upregulation was explored by immunoprecipitation, RIP, and ubiquitination assays.

Results

We demonstrated that circIGF2BP3 (hsa_circ_0079587) expression was increased in NSCLC and negatively correlated with CD8⁺ T cell infiltration. Functionally, elevated circIGF2BP3 inactivated cocultured T cells in vitro and compromised antitumor immunity in an immunocompetent mouse model, and this effect was dependent on CD8⁺ T cells. Mechanistically, METTL3 mediates the N⁶-methyladenosine (m⁶A) modification of circIGF2BP3 and promotes its circularization in a manner dependent on the m⁶A reader protein YTHDC1. circIGF2BP3 competitively upregulates PKP3 expression by sponging miR-328-3p and miR-3173-5p to compromise the cancer immune response. Furthermore, PKP3 engages with the RNA-binding protein FXR1 to stabilize OTUB1 mRNA, and OTUB1 elevates PD-L1 abundance by facilitating its deubiquitination. Tumor PD-L1 deletion completely blocked the impact of the circIGF2BP3/PKP3 axis on the CD8⁺ T cell response. The inhibition of circIGF2BP3/PKP3 enhanced the treatment efficacy of anti-PD-1 therapy in a Lewis lung carcinoma mouse model. Collectively, the PKP3/PD-L1 signature and the infiltrating CD8⁺ T cell status stratified NSCLC patients into different risk groups.

Conclusion

Our results reveal the function of circIGF2BP3 in causing immune escape from CD8⁺ T cell-mediated killing through a decrease in PD-L1 ubiquitination and subsequent proteasomal degradation by stabilizing OTUB1 mRNA in a PKP3-dependent manner. This work sheds light on a novel mechanism of PD-L1 regulation in NSCLC and provides a rationale to enhance the efficacy of anti-PD-1 treatment in NSCLC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12943-021-01398-4.

Increased expression of plakophilin 3 is associated with poor prognosis in ovarian cancer

Considering the essential role of plakophilin 3 (PKP3) in the maintenance cell-cell adhesion, dysregulation of PKP3 is involved in human diseases. This study aimed to explore the clinical significance of PKP3 in ovarian cancer. Immunohistochemistry was performed to examine the PKP3 expression in 157 cancer specimens from primary ovarian cancer patients. PKP3 was expressed in both the cytoplasm and nucleus. Eighty-one (51.6%) out of 157 ovarian cancer tissues showed PKP3 expression, while absent expression was observed in normal ovarian tissues. High PKP3 expression was associated with lymph node metastasis (LNM, P = .004) and advanced International Federation of Gynecology and Obstetrics (FIGO) stage (P = .013). Patients with high PKP3 expression had shorter overall survival (OS) than those with low PKP3 expression (60.2 months vs 74.2 months, P = .021). However, no association between PKP3 expression and progression-free survival (PFS) was observed (P = .790). Cox regression analysis indicated that PKP3 expression was an independently predictive factor for the OS of patient with ovarian cancer (adjusted HR = 1.601, 95%CI: 1.014-2.528, P = .043), especially those with FIGO stages III and IV disease (adjusted HR = 1.607, 95%CI: 1.006-2.567, P = .047). The gene expression profiling interactive analysis (GEPIA) databases also showed that PKP3 was upregulated in ovarian cancer (P < .001) and patients with high PKP3 expression had shorter OS (P = .004). In conclusion, our findings suggest that PKP3 is upregulated in ovarian cancer and is likely involved in the progression of ovarian cancer. PKP3 might therefore serve as a prognostic biomarker for patients with ovarian cancer.

Desmoglein 1 Regulates Invadopodia by Suppressing EGFR/Erk Signaling in an Erbin-Dependent Manner

Loss of the desmosomal cell-cell adhesion molecule, Desmoglein 1 (Dsg1), has been reported as an indicator of poor prognosis in head and neck squamous cell carcinomas (HNSCC) overexpressing epidermal growth factor receptor (EGFR). It has been well established that EGFR signaling promotes the formation of invadopodia, actin-based protrusions formed by cancer cells to facilitate invasion and metastasis, by activating pathways leading to actin polymerization and ultimately matrix degradation. We previously showed that Dsg1 downregulates EGFR/Erk signaling by interacting with the ErbB2-binding protein Erbin (ErbB2 Interacting Protein) to promote keratinocyte differentiation. Here, we provide evidence that restoring Dsg1 expression in cells derived from HNSCC suppresses invasion by decreasing the number of invadopodia and matrix degradation. Moreover, Dsg1 requires Erbin to downregulate EGFR/Erk signaling and to fully suppress invadopodia formation. Our findings indicate a novel role for Dsg1 in the regulation of invadopodia signaling and provide potential new targets for development of therapies to prevent invadopodia formation and therefore cancer invasion and metastasis. IMPLICATIONS: Our work exposes a new pathway by which a desmosomal cadherin called Dsg1, which is lost early in head and neck cancer progression, suppresses cancer cell invadopodia formation by scaffolding ErbB2 Interacting Protein and consequent attenuation of EGF/Erk signaling.

Plakophilin3 loss leads to an increase in lipocalin2 expression, which is required for tumour formation

An increase in tumour formation and metastasis are observed upon plakophilin3 (PKP3) loss. To identify pathways downstream of PKP3 loss that are required for increased tumour formation, a gene expression analysis was performed, which demonstrated that the expression of lipocalin2 (LCN2) was elevated upon PKP3 loss and this is consistent with expression data from human tumour samples suggesting that PKP3 loss correlates with an increase in LCN2 expression. PKP3 loss leads to an increase in invasion, tumour formation and metastasis and these phenotypes were dependent on the increase in LCN2 expression. The increased LCN2 expression was due to an increase in the activation of p38 MAPK in the HCT116 derived PKP3 knockdown clones as LCN2 expression decreased upon inhibition of p38 MAPK. The phosphorylated active form of p38 MAPK is translocated to the nucleus upon PKP3 loss and is dependent on complex formation between p38 MAPK and PKP3. WT PKP3 inhibits LCN2 reporter activity in PKP3 knockdown cells but a PKP3 mutant that fails to form a complex with p38 MAPK cannot suppress LCN2 promoter activity. Further, LCN2 expression is decreased upon loss of p38β, but not p38α, in the PKP3 knockdown cells. These results suggest that PKP3 loss leads to an increase in the nuclear translocation of p38 MAPK and p38β MAPK is required for the increase in LCN2 expression.

Up-regulation of plakophilin-2 is correlated with the progression of glioma

Glioma is the most common type of primary brain tumor in the CNS. Due to its poor prognosis and high mortality rates, it is urgent to find out more effective therapies. Plakophilin-2 (PKP2) is a widespread desmosomal plaque protein. Recently, the important roles of PKP2 in the proliferation and migration of cancer cells and tumor progression has been shown. However, the expression and potential function of PKP2 in glioma was still unclear. In this study, we demonstrated that PKP2 protein expression level was increased in glioma tissues compared with normal brain tissues, and its level was significantly associated with the Ki-67 expression and WHO grade by Western blot analysis and immunohistochemistry. Clinically, high PKP2 expression was tightly related to poor prognosis of glioma patients. Interestingly, we found that down-regulated PKP2 expression was shown to inhibit the migration of cells in glioma. Moreover, cell counting kit (CCK)-8 and colony formation analyses proved that reduced expression of PKP2 could weaken glioma cell proliferation. Taken together, these data uncover a potential role for PKP2 in the pathogenic process of glioma, suggesting that PKP2 may be a promising therapeutic target of glioma.

Human melanomas and ovarian cancers overexpressing mechanical barrier molecule genes lack immune signatures and have increased patient mortality risk

We have identified eight genes whose expression in human melanoma metastases and ovarian cancers is associated with a lack of Th1 immune signatures. They encode molecules with mechanical barrier function in the skin and other normal tissues and include filaggrin (FLG), tumor-associated calcium signal transducer 2 (TACSTD2), and six desmosomal proteins (DST, DSC3, DSP, PPL, PKP3, and JUP). This association has been validated in an independent series of 114 melanoma metastases. In these, DST expression alone is sufficient to identify melanomas without immune signatures, while FLG and the other six putative barrier molecules are overexpressed in a different subset of melanomas lacking immune signatures. Similar associations have been identified in a set of 186 ovarian cancers. RNA-seq data from 471 melanomas and 307 ovarian cancers in the TCGA database further support these findings and also reveal that overexpression of barrier molecules is strongly associated with early patient mortality for melanoma (p = 0.0002) and for ovarian cancer (p < 0.01). Interestingly, this association persists for FLG for melanoma (p = 0.012) and ovarian cancer (p = 0.006), whereas DST overexpression is negatively associated with CD8⁺ gene expression, but not with patient survival. Thus, overexpression of FLG or DST identifies two distinct patient populations with low immune cell infiltration in these cancers, but with different prognostic implications for each. These data raise the possibility that molecules with mechanical barrier function in skin and other tissues may be used by cancer cells to protect them from immune cell infiltration and immune-mediated destruction.

Desmosomes in acquired disease

Desmosomes are cell-cell junctions that mediate adhesion and couple the intermediate filament cytoskeleton to sites of cell-cell contact. This architectural arrangement integrates adhesion and cytoskeletal elements of adjacent cells. The importance of this robust adhesion system is evident in numerous human diseases, both inherited and acquired, which occur when desmosome function is compromised. This review focuses on autoimmune and infectious diseases that impair desmosome function. In addition, we discuss emerging evidence that desmosomal genes are often misregulated in cancer. The emphasis of our discussion is placed on the way in which human diseases can inform our understanding of basic desmosome biology and in turn, the means by which fundamental advances in the cell biology of desmosomes might lead to new treatments for acquired diseases of the desmosome.

Desmosome regulation and signaling in disease

Desmosomes are cell-cell adhesive organelles with a well-known role in forming strong intercellular adhesion during embryogenesis and in adult tissues subject to mechanical stress, such as the heart and skin. More recently, desmosome components have also emerged as cell signaling regulators. Loss of expression or interference with the function of desmosome molecules results in diseases of the heart and skin and contributes to cancer progression. However, the underlying molecular mechanisms that result in inherited and acquired disorders remain poorly understood. To address this question, researchers are directing their studies towards determining the functions that occur inside and outside of the junctions and the extent to which functions are adhesion-dependent or independent. This review focuses on recent discoveries that provide insights into the role of desmosomes and desmosome components in cell signaling and disease; wherever possible, we address molecular functions within and outside of the adhesive structure.

c-Src mediated tyrosine phosphorylation of plakophilin 3 as a new mechanism to control desmosome composition in cells exposed to oxidative stress

Plakophilins (PKP1 to PKP3) are essential for the structure and function of desmosomal junctions as demonstrated by the severe skin defects observed as a result of loss-of-function mutations in mice and men. PKPs play additional roles in cell signaling processes, such as those controlling the cellular stress response and cell proliferation. A key post-translational process controlling PKP function is phosphorylation. We have discovered that reactive oxygen species (ROS) trigger the c-Src kinase-mediated tyrosine (Tyr)-195 phosphorylation of PKP3. This modification is associated with a change in the subcellular distribution of the protein. Specifically, PKP3 bearing phospho-Tyr-195 is released from the desmosomes, suggesting that phospho-Tyr-195 is relevant for the control of desmosome disassembly and function, at least in cells exposed to ROS. Tyr-195 phosphorylation is transient under normal physiological conditions and seems to be strictly regulated, as the activation of particular growth factor receptors results in a modification at this site only when tyrosine phosphatases are inactivated by pervanadate. We have identified Tyr-195 of PKP3 as a phosphorylation target of epidermal growth factor receptor signaling. Interestingly, this PKP3 phosphorylation also occurs in certain poorly differentiated adenocarcinomas of the prostate, suggesting a possible role in tumor progression. Our study thus identifies a new mechanism controlling PKP3 and hence desmosome function in epithelial cells.

Plakophilin-3 catenin associates with the ETV1/ER81 transcription factor to positively modulate gene activity

Members of the plakophilin-catenin sub-family (Pkp-1, -2, and -3) facilitate the linkage of desmosome junctional components to each other (e.g. desmosomal cadherins to desmoplakin) and the intermediate-filament cytoskeleton. Pkps also contribute to desmosomal stabilization and the trafficking of its components. The functions of Pkps outside of the desmosome are less well studied, despite evidence suggesting their roles in mRNA regulation, small-GTPase modulation (e.g. mid-body scission) during cell division, and cell survival following DNA damage. Pkp-catenins are further believed to have roles in the nucleus given their nuclear localization in some contexts and the known nuclear roles of structurally related catenins, such as beta-catenin and p120-catenin. Further, Pkp-catenin activities in the nuclear compartment have become of increased interest with the identification of interactions between Pkp2-catenin and RNA Pol III and Pkp1 with single-stranded DNA. Consistent with earlier reports suggesting possible nuclear roles in development, we previously demonstrated prominent nuclear localization of Pkp3 in Xenopus naïve ectoderm (“animal cap”) cells and recently resolved a similar localization in mouse embryonic stem cells. Here, we report the association and positive functional interaction of Pkp3 with a transcription factor, Ets variant gene 1 (ETV1), which has critical roles in neural development and prominent roles in human genetic disease. Our results are the first to report the interaction of a sequence-specific transcription factor with any Pkp. Using Xenopus laevis embryos and mammalian cells, we provide evidence for the Pkp3:ETV1 complex on both biochemical and functional levels.

Plakophilin-associated RNA-binding proteins in prostate cancer and their implications in tumor progression and metastasis

Both plakophilins (PKP) 1 and 3 play a role in the progression of prostate cancer. The RNA-binding proteins (RBPs) GAP-SH3-binding protein (G3BP), fragile-X-related protein 1 (FXR1), poly(A)-binding protein, cytoplasmic 1 (PABPC1), and up-frameshift factor 1 (UPF1) are associated with PKP3. All these RBPs have an impact on RNA metabolism. Until recently, the PKP-associated RBPs have not been analyzed in prostate cancer. In the current study, we showed by affinity purification that the PKP3-associated RBPs were also binding partners of PKP1. We examined the expression of PKP1/3-associated RBPs and PKP1/3 in prostate cell lines, tumor-free prostate, and 136 prostatic adenocarcinomas by immunofluorescence and immunoblot. All four RBPs G3BP, FXR1, UPF1, and PABPC1 were expressed in the glandular epithelium of the normal prostate. PKP1 and FXR1 were strongly reduced in tumor tissues with Gleason score >7 and diminished expression of PKP1 and FXR1 also appeared to be associated with a metastatic phenotype. Additionally, the predominant nuclear localization of UPF1 in normal glandular cells and low grade tumors was switched to a more cytoplasmic pattern in carcinomas with Gleason score >7. Our findings suggest that PKP1 and FXR1 may have a tumor-suppressive function and are downregulated in more aggressive tumors. Collectively, PKP1/3-associated RBPs FXR1 and UPF1 may have a functional role in prostate cancer progression and metastasis and highlight the potential importance of posttranscriptional regulation of gene expression and nonsense-mediated decay in cancer.

Plakophilin-3 is required for late embryonic amphibian development, exhibiting roles in ectodermal and neural tissues

The p120-catenin family has undergone a significant expansion during the evolution of vertebrates, resulting in varied functions that have yet to be discerned or fully characterized. Likewise, members of the plakophilins, a related catenin subfamily, are found throughout the cell with little known about their functions outside the desmosomal plaque. While the plakophilin-3 (Pkp3) knockout mouse resulted in skin defects, we find larger, including lethal effects following its depletion in Xenopus. Pkp3, unlike some other characterized catenins in amphibians, does not have significant maternal deposits of mRNA. However, during embryogenesis, two Pkp3 protein products whose temporal expression is partially complimentary become expressed. Only the smaller of these products is found in adult Xenopus tissues, with an expression pattern exhibiting distinctions as well as overlaps with those observed in mammalian studies. We determined that Xenopus Pkp3 depletion causes a skin fragility phenotype in keeping with the mouse knockout, but more novel, Xenopus tailbud embryos are hyposensitive to touch even in embryos lacking outward discernable phenotypes, and we additionally resolved disruptions in certain peripheral neural structures, altered establishment and migration of neural crest, and defects in ectodermal multiciliated cells. The use of two distinct morpholinos, as well as rescue approaches, indicated the specificity of these effects. Our results point to the requirement of Pkp3 in amphibian embryogenesis, with functional roles in a number of tissue types.