Par complex in cancer: a regulator of normal cell polarity joins the dark side

PARD3 drives tumorigenesis through activating Sonic Hedgehog signalling in tumour-initiating cells in liver cancer

BACKGROUND

Par-3 Family Cell Polarity Regulator (PARD3) is a cellular protein essential for asymmetric cell division and polarized growth. This study aimed to study the role of PARD3 in hepatic tumorigenesis.

METHODS

The essential role of PARD3 in mediating hepatic tumorigenesis was assessed in diet-induced spontaneous liver tumour and syngeneic tumour models. The mechanism of PARD3 was delineated by bulk and single-cell RNA sequencing. The clinical significance of PARD3 was identified by tissue array analysis.

RESULTS

PARD3 was overexpressed in tumour tissues and PARD3 overexpression was positively correlated with high tumour stage as well as the poor prognosis in patients. In models of spontaneous liver cancer induced by choline-deficient, amino acid-defined (CDAA) and methionine-choline-deficient (MCD) diets, upregulation of PARD3 was induced specifically at the tumorigenesis stage rather than other early stages of liver disease progression. Site-directed knockout of PARD3 using an adeno-associated virus 8 (AAV8)-delivered CRISPR/Cas9 single-guide RNA (sgRNA) plasmid blocked hepatic tumorigenesis, while PARD3 overexpression accelerated liver tumour progression. In particular, single-cell sequencing analysis suggested that PARD3 was enriched in primitive tumour cells and its overexpression enhanced tumour-initiating cell (TICs). Overexpression of PARD3 maintained the self-renewal ability of the CD133+ TIC population within hepatocellular carcinoma (HCC) cells and promoted the in vitro and in vivo tumorigenicity of CD133+ TICs. Transcriptome analysis revealed that Sonic Hedgehog (SHH) signalling was activated in PARD3-overexpressing CD133+ TICs. Mechanistically, PARD3 interacted with aPKC to further activate SHH signalling and downstream stemness-related genes. Suppression of SHH signalling and aPKC expression attenuated the in vitro and in vivo tumorigenicity of PARD3-overexpressing CD133+ TICs. Tissue array analysis revealed that PARD3 expression was positively associated with the phosphorylation of aPKC, SOX2 and Gli1 and that the combination of these markers could be used to stratify HCC patients into two clusters with different clinicopathological characteristics and overall survival prognoses. The natural compound berberine was selected as a potent suppressor of PARD3 expression and could be used as a preventive agent for liver cancer that completely blocks diet-induced hepatic tumorigenesis in a PARD3-dependent manner.

CONCLUSION

This study revealed PARD3 as a potential preventive target of liver tumorigenesis via TIC regulation.

Cell polarity proteins promote macropinocytosis in response to metabolic stress

Macropinocytosis has emerged as a nutrient-scavenging pathway that cancer cells exploit to survive the nutrient-deprived conditions of the tumor microenvironment. Cancer cells are especially reliant on glutamine for their survival, and in pancreatic ductal adenocarcinoma (PDAC) cells, glutamine deficiency can enhance the stimulation of macropinocytosis, allowing the cells to escape metabolic stress through the production of extracellular-protein-derived amino acids. Here, we identify the atypical protein kinase C (aPKC) enzymes, PKCζ and PKCι as novel regulators of macropinocytosis. In normal epithelial cells, aPKCs are known to regulate cell polarity in association with the scaffold proteins Par3 and Par6, controlling the function of several targets, including the Par1 kinases. In PDAC cells, we identify that each of these cell polarity proteins are required for glutamine stress-induced macropinocytosis. Mechanistically, we find that the aPKCs are regulated by EGFR signaling or by the transcription factor CREM to promote the relocation of Par3 to microtubules, facilitating macropinocytosis in a dynein-dependent manner. Importantly, we determine that cell fitness impairment caused by aPKC depletion is rescued by the restoration of macropinocytosis and that aPKCs support PDAC growth in vivo. These results identify a previously unappreciated role for cell polarity proteins in the regulation of macropinocytosis and provide a better understanding of the mechanistic underpinnings that control macropinocytic uptake in the context of metabolic stress.

Viral manipulation of cell polarity signalling

Cell polarity refers to the asymmetric distribution of biomacromolecules that enable the correct orientation of a cell in a particular direction. It is thus an essential component for appropriate tissue development and function. Viral infections can lead to dysregulation of polarity. This is associated with a poor prognosis due to viral interference with core cell polarity regulatory scaffolding proteins that often feature PDZ (PSD-95, DLG, and ZO-1) domains including Scrib, Dlg, Pals1, PatJ, Par3 and Par6. PDZ domains are also promiscuous, binding to several different partners through their C-terminal region which contain PDZ-binding motifs (PBM). Numerous viruses encode viral effector proteins that target cell polarity regulators for their benefit and include papillomaviruses, flaviviruses and coronaviruses. A better understanding of the mechanisms of action utilised by viral effector proteins to subvert host cell polarity sigalling will provide avenues for future therapeutic intervention, while at the same time enhance our understanding of cell polarity regulation and its role tissue homeostasis.

Hypoxia disrupt tight junctions and promote metastasis of oral squamous cell carcinoma via loss of par3

BACKGROUND

Oral squamous cell carcinoma (OSCC) is a highly malignant tumor that is frequently associated with lymph node metastasis, resulting in poor prognosis and survival in patients. In the tumor microenvironment, hypoxia plays an important role in regulating cellular responses such as progressive and rapid growth and metastasis. In these processes, tumor cells autonomously undergo diverse transitions and acquire functions. However, hypoxia-induced transition of OSCC and the involvement of hypoxia in OSCC metastasis remain unclear. Therefore, in this study, we aimed to elucidate the mechanism of hypoxia-induced OSCC metastasis and particularly, its impact on tight junctions (TJs).

METHODS

The expression of hypoxia-inducible factor 1-alpha (HIF-1α) was detected in tumor tissues and adjacent normal tissues from 29 patients with OSCC using reverse transcription quantitative real-time polymerase chain reaction (qRT-PCR), western blotting, and immunohistochemistry (IHC). The migration and invasion abilities of OSCC cell lines treated with small interfering (si)RNA targeting HIF-1α or cultured in hypoxic conditions were analyzed using Transwell assays. The effect of HIF-1α expression on in vivo tumor metastasis of OSCC cells was evaluated using lung metastasis model.

RESULTS

HIF-1α was overexpressed in patients with OSCC. OSCC metastasis was correlated with HIF-1α expression in OSCC tissues. Hypoxia increased the migration and invasion abilities of OSCC cell lines by regulating the expression and localization of partitioning-defective protein 3 (Par3) and TJs. Furthermore, HIF-1α silencing effectively decreased the invasion and migration abilities of OSCC cell lines and restored TJ expression and localization via Par3. The expression of HIF-1α was positively regulated the OSCC metastasis in vivo.

CONCLUSIONS

Hypoxia promotes OSCC metastasis by regulating the expression and localization of Par3 and TJ proteins. HIF-1α positively correlates to OSCC metastasis. Lastly, HIF-1α expression could regulate the expression of Par3 and TJs in OSCC. This finding may aid in elucidating the molecular mechanisms of OSCC metastasis and progression and developing new diagnostic and therapeutic approaches for OSCC metastasis.

Par6 Enhances Glioma Invasion by Activating MEK/ERK Pathway Through a LIN28/let-7d Positive Feedback Loop

The invasion of glioblastoma usually results in the recurrence and poor prognosis in patients with glioma. However, the underlying mechanisms involved in glioma invasion remains undefined. In this study, immunohistochemistry analyses of glioma specimens demonstrated that high expression of Par6 was positively correlated with malignancy and poor prognosis of patients with glioma. Par6-overexpressing glioma cells showed much more fibroblast-like morphology, suggesting that regulation of Par6 expression might be associated with tumor invasion in glioma cells. Further study indicated that Par6 overexpression subsequently increased CD44 and N-cadherin expression to enhance glioma invasion through activating MEK/ERK/STAT3 pathway, in vivo and in vitro. Moreover, we found that LIN28/let-7d axis was involved in this process via a positive feedback loop, suggesting that MEK/ERK/LIN28/let-7d/STAT3 cascade might be essential for Par6-mediated glioma invasion. Therefore, these data highlight the roles of Par6 in glioma invasion, and Par6 may serve as a potential therapeutic target for patients with glioma.

Impact of peptide permeation enhancer on tight junctions opening cellular mechanisms

The myristoylated pentapeptide, L-R5, contains an amino acid sequence of the zeta inhibitory peptide (ZIP) portion (pseudosubstrate) of protein kinase C zeta (PKC ζ). As PKC ζ is involved in the modulation of epithelial tight junctions (TJs) through the phosphorylation of TJ proteins, L-R5 was suggested to interact with the enzyme resulting in the enhancement of paracellular permeability. This study shows that L-R5 does not bind to the enzyme but interacts directly with TJ proteins. We show here that the binding of PKC ζ to occludin and its successive phosphorylation is prevented by L-R5, which leads to TJ disruption and enhanced epithelial permeability. Although L-R5 did not show any in vitro cytotoxicity, a proteomics study revealed that L-R5 interferes with other regulatory pathways, e.g., apoptosis and immune response. We suggest that structural modification of the peptide may increase the specificity TJ protein-peptide interaction.

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Microscale thermophoresis (MST) showed robust results for protein bindings.

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The competitivity of L-R5 peptide for the binding of occludin-PKC zeta was shown.

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Tight junctions proteins expression was decreased due to L-R5 peptide.

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Multiple other mechanisms can be explored to use L-R5 for other therapies.

Dissecting cell fate dynamics in pediatric glioblastoma through the lens of complex systems and cellular cybernetics

Cancers are complex dynamic ecosystems. Reductionist approaches to science are inadequate in characterizing their self-organized patterns and collective emergent behaviors. Since current approaches to single-cell analysis in cancer systems rely primarily on single time-point multiomics, many of the temporal features and causal adaptive behaviors in cancer dynamics are vastly ignored. As such, tools and concepts from the interdisciplinary paradigm of complex systems theory are introduced herein to decode the cellular cybernetics of cancer differentiation dynamics and behavioral patterns. An intuition for the attractors and complex networks underlying cancer processes such as cell fate decision-making, multiscale pattern formation systems, and epigenetic state-transitions is developed. The applications of complex systems physics in paving targeted therapies and causal pattern discovery in precision oncology are discussed. Pediatric high-grade gliomas are discussed as a model-system to demonstrate that cancers are complex adaptive systems, in which the emergence and selection of heterogeneous cellular states and phenotypic plasticity are driven by complex multiscale network dynamics. In specific, pediatric glioblastoma (GBM) is used as a proof-of-concept model to illustrate the applications of the complex systems framework in understanding GBM cell fate decisions and decoding their adaptive cellular dynamics. The scope of these tools in forecasting cancer cell fate dynamics in the emerging field of computational oncology and patient-centered systems medicine is highlighted.

Dual Function of Par3 in Tumorigenesis

Cell maintenance and the establishment of cell polarity involve complicated interactions among multiple protein complexes as well as the regulation of different signaling pathways. As an important cell polarity protein, Par3 is evolutionarily conserved and involved in tight junction formation as well as tumorigenesis. In this review, we aimed to explore the function of Par3 in tumorigenesis. Research has shown that Par3 exhibits dual functions in human cancers, both tumor-promoting and tumor-suppressive. Here, we focus on the activities of Par3 in different stages and types of tumors, aiming to offer a new perspective on the molecular mechanisms that regulate the functions of Par3 in tumor development. Tumor origin, tumor microenvironment, tumor type, cell density, cell–cell contact, and the synergistic effect of Par3 and other tumor-associated signaling pathways may be important reasons for the dual function of Par3. The important role of Par3 in mammalian tumorigenesis and potential signaling pathways is context dependent.

MicroRNAs as the critical regulators of cell migration and invasion in thyroid cancer

Thyroid cancer (TC) is one of the most frequent endocrine malignancies that is more common among females. Tumor recurrence is one of the most important clinical manifestations in differentiated TC which is associated with different factors including age, tumor size, and histological features. Various molecular processes such as genetic or epigenetic modifications and non-coding RNAs are also involved in TC progression and metastasis. The epithelial-to-mesenchymal transition (EMT) is an important biological process during tumor invasion and migration that affects the initiation and transformation of early-stage tumors into invasive malignancies. A combination of transcription factors, growth factors, signaling pathways, and epigenetic regulations affect the thyroid cell migration and EMT process. MicroRNAs (miRNAs) are important molecular factors involved in tumor metastasis by regulation of EMT-activating signaling pathways. Various miRNAs are involved in the signaling pathways associated with TC metastasis which can be used as diagnostic and therapeutic biomarkers. Since, the miRNAs are sensitive, specific, and non-invasive, they can be suggested as efficient and optimal biomarkers of tumor invasion and metastasis. In the present review, we have summarized all of the miRNAs which have been significantly involved in thyroid tumor cells migration and invasion. We also categorized all of the reported miRNAs based on their cellular processes to clarify the molecular role of miRNAs during thyroid tumor cell migration and invasion. This review paves the way of introducing a non-invasive diagnostic and prognostic panel of miRNAs in aggressive and metastatic TC patients.

Partitioning defective 6 homolog alpha (PARD6A) promotes epithelial–mesenchymal transition via integrin β1-ILK-SNAIL1 pathway in ovarian cancer

Partitioning-defective protein 6 (Par6) family proteins have been demonstrated to be closely associated with the occurrence and development of cancers. It is well accepted that dysregulation of epithelial–mesenchymal transition (EMT) greatly contributes to carcinogenesis and metastases of ovarian cancer. So far, the roles of Par6 in EMT of ovarian cancer are not clear. Functional experiments were carried out to study the roles of PARD6A in EMT of ovarian cancer in vitro and in vivo, and EMT pathways potentially affected by PARD6A expression were screened. We found that PARD6A was significantly highly expressed in tissues of ovarian cancer patients in III-IV stages, poorly differentiated or with lymphatic metastases versus I-II stages, moderately or well differentiated, or without lymphatic metastases, respectively. PARD6A knockdown suppressed EMT of SKOV3 and A2780 cells in vitro and ovarian cancer metastasis in vivo, while overexpression of PARD6A promoted EMT in HO8910 and OVCAR8 cells. It was indicated that PARD6A affected EMT of ovarian cancer cells through SNAIL1 signaling pathway and subsequently modulated the expression of VIMENTIN and E-cadherin, which was further confirmed by knockdown and overexpression of SNAIL1 experiments. PARD6A was also demonstrated to regulate expression of SNAIL1 by modulating integrin β1 and ILK proteins, specifically it was shown that the transcription of SNAIL1 was regulated by ILK in this study. In addition, expression of ILK in ovarian cancer tissues was demonstrated to be correlated with tumor stages and lymphatic metastases clinically. In this study, we identified a novel role of PARD6A as an inducer of cell migration and invasion, which is likely to play an important role in metastasis of ovarian cancer. The molecular pathways of EMT mediated by PARD6A-Integrin β1-ILK-SNAIL1 and finally implemented by E-cadherin and VIMENTIN may provide a novel strategy for drug development for ovarian cancer therapy in the near future.

Polarity protein Par3 sensitizes breast cancer to paclitaxel by promoting cell cycle arrest

PURPOSE

Paclitaxel, belongs to tubulin-binding agents (TBAs), shows a great efficacy against breast cancer via stabilizing microtubules. Drug resistance limits its clinical application. Here we aimed to explore a role of Polarity protein Par3 in improving paclitaxel effectiveness.

METHODS

Breast cancer specimens from 45 patients were collected to study the relationship between Par3 expression and paclitaxel efficacy. The Kaplan-Meier method was used for survival analysis. Cell viability was measured in breast cancer cells (SK-BR-3 and T-47D) with Par3 over-expression or knockdown. The flow cytometry assays were performed to measure cell apoptosis and cell cycle. BrdU incorporation assay and Hoechst 33,258 staining were performed to measure cell proliferation and cell apoptosis, respectively. Immunofluorescence was used to detect microtubule structures.

RESULTS

Par3 expression was associated with good response of paclitaxel in breast cancer patients. Consistently, Par3 over-expression significantly sensitized breast cancer cells to paclitaxel by promoting cell apoptosis and reducing cell proliferation. In Par3 overexpressing cells upon paclitaxel treatment, we observed intensified cell cycle arrests at metaphase. Further exploration showed that Par3 over-expression stabilized microtubules of breast cancer cells in response to paclitaxel and resists to microtubules instability induced by nocodazole, a microtubule-depolymerizing agent.

CONCLUSION

Par3 facilitates polymeric forms of tubulin and stabilizes microtubule structure, which aggravates paclitaxel-induced delay at the metaphase-anaphase transition, leading to proliferation inhibition and apoptosis of breast cancer cells. Par3 has a potential role in sensitizing breast cancer cells to paclitaxel, which may provide a more precise assessment of individual treatment and novel therapeutic targets.

Role of RhoC in cancer cell migration

Migration is one of the five major behaviors of cells. Although RhoC-a classic member of the Rho gene family-was first identified in 1985, functional RhoC data have only been widely reported in recent years. Cell migration involves highly complex signaling mechanisms, in which RhoC plays an essential role. Cell migration regulated by RhoC-of which the most well-known function is its role in cancer metastasis-has been widely reported in breast, gastric, colon, bladder, prostate, lung, pancreatic, liver, and other cancers. Our review describes the role of RhoC in various types of cell migration. The classic two-dimensional cell migration cycle constitutes cell polarization, adhesion regulation, cell contraction and tail retraction, most of which are modulated by RhoC. In the three-dimensional cell migration model, amoeboid migration is the most classic and well-studied model. Here, RhoC modulates the formation of membrane vesicles by regulating myosin II, thereby affecting the rate and persistence of amoeba-like migration. To the best of our knowledge, this review is the first to describe the role of RhoC in all cell migration processes. We believe that understanding the detail of RhoC-regulated migration processes will help us better comprehend the mechanism of cancer metastasis. This will contribute to the study of anti-metastatic treatment approaches, aiding in the identification of new intervention targets for therapeutic or genetic transformational purposes.

Polarized endosome dynamics engage cytoplasmic Par-3 that recruits dynein during asymmetric cell division

In the developing embryos, the cortical polarity regulator Par-3 is critical for establishing Notch signaling asymmetry between daughter cells during asymmetric cell division (ACD). How cortically localized Par-3 establishes asymmetric Notch activity in the nucleus is not understood. Here, using in vivo time-lapse imaging of mitotic radial glia progenitors in the developing zebrafish forebrain, we uncover that during horizontal ACD along the anteroposterior embryonic axis, endosomes containing the Notch ligand DeltaD (Dld) move toward the cleavage plane and preferentially segregate into the posterior (subsequently basal) Notchhi daughter. This asymmetric segregation requires the activity of Par-3 and dynein motor complex. Using label retention expansion microscopy, we further detect Par-3 in the cytosol colocalizing the dynein light intermediate chain 1 (Dlic1) onto Dld endosomes. Par-3, Dlic1, and Dld are associated in protein complexes in vivo. Our data reveal an unanticipated mechanism by which cytoplasmic Par-3 directly polarizes Notch signaling components during ACD.

Investigations of mouse models during tumorigenesis revealed essential but distinct in vivo functions among the PKC family

PKC isozymes have been put in place as oncoproteins since the discovery that they can function as receptors for potent tumor-promoting phorbol esters in the 1980s. Despite nearly two decades of research, a clear in vivo proof of that concept was missing. The availability of so-called knock out mouse lines of individual PKC genes provided a tool to investigate isozyme specific in vivo functions in the context of tumor initiation, development and progression. This review aims to provide a limited overview of how the application of these mouse lines in combination with a cancer mouse model helped to understand PKC's in vivo function during tumorigenesis. The focus of this review will be on skin, colon and lung cancer.

Par6 regulates cell cycle progression through enhancement of Akt/PI3K/GSK-3β signaling pathway activation in glioma

As the key factor of the polarity protein complex, Par6 not only regulates polarization processes, but also plays important roles in tumor metastasis and progression in many epithelium malignancy tumors. Here, we showed that Par6 is an essential component in glioma tumorigenesis. Our results indicated the aberrant expression of Par6 in malignant glioma tissues and cell lines. We found that the regulation of Par6 expression induces cell proliferation and tumor growth in vivo and in vitro. Additionally, RNA-seq revealed the effects of Par6 were associated with cyclin D1-regulated cell cycle progression in glioma cells. Moreover, our results demonstrated that the regulation of Par6 can enhance the activation of Akt/PI3K signaling pathway, and subsequently upregulate the expression level of GSK-3β protein, which then regulate cyclin D1-mediated cell cycle regulation. Furthermore, we found that TGF-β-induced the upregulation of Par6 expression may be involved in this process. The pathological analysis confirmed the correlation between Par6 expression and the prognosis in human glioma tissues, suggesting the regulation of Par6 expression regulates glioma tumorigenesis and progression. Thus, our findings showed that Par6 might be a potential biomarker for the diagnosis and providing a therapeutic strategy for the treatment of malignant glioma.

Genetic alterations of epithelial polarity genes are associated with loss of polarity in invasive breast cancer

Breast cancer remains a leading cause of cancer-related death for women. The stepwise development of breast cancer through preinvasive to invasive disease is associated with progressive disruption of cellular and tissue organization. Apical-basal polarity is thought to be a barrier to breast cancer development, but the extent and potential mechanisms that contribute to disrupted polarity are incompletely understood. To investigate the cell polarity status of invasive breast cancers, we performed multiplex imaging of polarity markers on tissue cores from 432 patients from a spectrum of grades, stages and molecular subtypes. Apical-basal cell polarity was lost in 100% of cells in all cases studied, indicating that loss of epithelial polarity may be a universal feature of invasive breast cancer. We then analyzed genomic events from the TCGA dataset for an 18-gene set of core polarity genes. Coamplification of polarity genes with established breast oncogenes was found, which is consistent with functional cooperation within signaling amplicons. Gene-expression levels of several polarity genes were significantly associated with survival, and protein localization of Par6 correlated with higher grade, nodal metastasis and molecular subtype. Finally, multiple hotspot mutations in protein-protein interaction domains critical for cell polarity were identified. Our data indicate that genomic events likely contribute to pervasive disruption of epithelial polarity observed in invasive breast cancer.

Actin-binding domains mediate the distinct distribution of two Dictyostelium Talins through different affinities to specific subsets of actin filaments during directed cell migration

Although the distinct distribution of certain molecules along the anterior or posterior edge is essential for directed cell migration, the mechanisms to maintain asymmetric protein localization have not yet been fully elucidated. Here, we studied a mechanism for the distinct localizations of two Dictyostelium talin homologues, talin A and talin B, both of which play important roles in cell migration and adhesion. Using GFP fusion, we found that talin B, as well as its C-terminal actin-binding region, which consists of an I/LWEQ domain and a villin headpiece domain, was restricted to the leading edge of migrating cells. This is in sharp contrast to talin A and its C-terminal actin-binding domain, which co-localized with myosin II along the cell posterior cortex, as reported previously. Intriguingly, even in myosin II-null cells, talin A and its actin-binding domain displayed a specific distribution, co-localizing with stretched actin filaments. In contrast, talin B was excluded from regions rich in stretched actin filaments, although a certain amount of its actin-binding region alone was present in those areas. When cells were sucked by a micro-pipette, talin B was not detected in the retracting aspirated lobe where acto-myosin, talin A, and the actin-binding regions of talin A and talin B accumulated. Based on these results, we suggest that talin A predominantly interacts with actin filaments stretched by myosin II through its C-terminal actin-binding region, while the actin-binding region of talin B does not make such distinctions. Furthermore, talin B appears to have an additional, unidentified mechanism that excludes it from the region rich in stretched actin filaments. We propose that these actin-binding properties play important roles in the anterior and posterior enrichment of talin B and talin A, respectively, during directed cell migration.

Emerging Themes in PDZ Domain Signaling: Structure, Function, and Inhibition

Post-synaptic density-95, disks-large and zonula occludens-1 (PDZ) domains are small globular protein-protein interaction domains widely conserved from yeast to humans. They are composed of ∼90 amino acids and form a classical two α-helical/six β-strand structure. The prototypical ligand is the C-terminus of partner proteins; however, they also bind internal peptide sequences. Recent findings indicate that PDZ domains also bind phosphatidylinositides and cholesterol. Through their ligand interactions, PDZ domain proteins are critical for cellular trafficking and the surface retention of various ion channels. In addition, PDZ proteins are essential for neuronal signaling, memory, and learning. PDZ proteins also contribute to cytoskeletal dynamics by mediating interactions critical for maintaining cell-cell junctions, cell polarity, and cell migration. Given their important biological roles, it is not surprising that their dysfunction can lead to multiple disease states. As such, PDZ domain-containing proteins have emerged as potential targets for the development of small molecular inhibitors as therapeutic agents. Recent data suggest that the critical binding function of PDZ domains in cell signaling is more than just glue, and their binding function can be regulated by phosphorylation or allosterically by other binding partners. These studies also provide a wealth of structural and biophysical data that are beginning to reveal the physical features that endow this small modular domain with a central role in cell signaling.

aPKC controls endothelial growth by modulating c-Myc via FoxO1 DNA-binding ability

Strict regulation of proliferation is vital for development, whereas unregulated cell proliferation is a fundamental characteristic of cancer. The polarity protein atypical protein kinase C lambda/iota (aPKCλ) is associated with cell proliferation through unknown mechanisms. In endothelial cells, suppression of aPKCλ impairs proliferation despite hyperactivated mitogenic signaling. Here we show that aPKCλ phosphorylates the DNA binding domain of forkhead box O1 (FoxO1) transcription factor, a gatekeeper of endothelial growth. Although mitogenic signaling excludes FoxO1 from the nucleus, consequently increasing c-Myc abundance and proliferation, aPKCλ controls c-Myc expression via FoxO1/miR-34c signaling without affecting its localization. We find this pathway is strongly activated in the malignant vascular sarcoma, angiosarcoma, and aPKC inhibition reduces c-Myc expression and proliferation of angiosarcoma cells. Moreover, FoxO1 phosphorylation at Ser218 and aPKC expression correlates with poor patient prognosis. Our findings may provide a potential therapeutic strategy for treatment of malignant cancers, like angiosarcoma.

The cell polarity regulator aPKC is associated with cell proliferation but the precise mechanism are unknown. Here, the authors find that aPKC lambda phosphorylates the FoxO1 transcription factor, a gatekeeper of endothelial growth, during both angiogenesis and angiosarcomas.

MicroRNA 483-3p targets Pard3 to potentiate TGF-β1-induced cell migration, invasion, and epithelial-mesenchymal transition in anaplastic thyroid cancer cells

Anaplastic thyroid cancer (ATC) is associated with poor prognosis and is often untreatable. MicroRNA 483-3p (miR-483) and partitioning-defective 3 (Pard3), a member of the Pard family, have functions and regulatory mechanisms in ATC. The abnormal regulation of miR-483 may play an important role in tumorigenesis, and Par3 is known to regulate cell polarity, cell migration, and cell division. Tumor proliferation promoted by the regulation of miRNA expression can be regulated in thyroid cancer by upregulating transforming growth factor-β1 (TGF-β1), which is thought to interact with Pard3. When compared with adjacent non-tumor tissues, we found that miR-483 was upregulated and Pard3 was downregulated in 80 thyroid tumor samples. Disease-free survival was decreased when expression of miR-483 was upregulated and Pard3 expression was downregulated. Cell growth, migration, and invasion were induced by overexpression of miR-483. However, knockdown of miR-483 resulted in a loss of cell invasion and viability, both in vitro and in vivo. The expression of Pard3 was increased by the inhibition of miR-483, but TGF-β1-induced cell migration and invasion were decreased by miR-483 inhibition. A dual-luciferase reporter assay determined that Pard3 expression was downregulated when targeted with miR-483. The epithelial–mesenchymal transition (EMT), as well as Tiam1-Rac signaling, was induced by TGF-β1, which was decreased by the overexpression of Pard3. Pard3 decreased the inhibition of EMT and Tiam-Rac1 signaling, which resulted from transfection of ATC cells with miR-483. Overall, the results showed that downregulation of Pard3 resulted in increased cell invasion and EMT in ATC, which was promoted by treatment with miR-483. These findings suggest novel therapeutic targets and treatment strategies for this disease.

PAR-3 controls endothelial planar polarity and vascular inflammation under laminar flow

Impaired cell polarity is a hallmark of diseased tissue. In the cardiovascular system, laminar blood flow induces endothelial planar cell polarity, represented by elongated cell shape and asymmetric distribution of intracellular organelles along the axis of blood flow. Disrupted endothelial planar polarity is considered to be pro-inflammatory, suggesting that the establishment of endothelial polarity elicits an anti-inflammatory response. However, a causative relationship between polarity and inflammatory responses has not been firmly established. Here, we find that a cell polarity protein, PAR-3, is an essential gatekeeper of GSK3β activity in response to laminar blood flow. We show that flow-induced spatial distribution of PAR-3/aPKCλ and aPKCλ/GSK3β complexes controls local GSK3β activity and thereby regulates endothelial planar polarity. The spatial information for GSK3β activation is essential for flow-dependent polarity to the flow axis, but is not necessary for flow-induced anti-inflammatory response. Our results shed light on a novel relationship between endothelial polarity and vascular homeostasis highlighting avenues for novel therapeutic strategies.

Roles of partitioning-defective protein 6 (Par6) and its complexes in the proliferation, migration and invasion of cancer cells

A pivotal regulator of cell polarity and homeostasis, partitioning-defective protein 6 (Par6) forms multicomponent complexes that not only regulate cell polarity and stabilize cell morphology, but have also been demonstrated to participate in the proliferation, migration and invasion of cancer cells. The transforming growth factor (TGF)-β and extracellular signal-regulated kinase (Erk) 1/2 pathways are the most thoroughly studied pathways involving Par6 in many cancers. Aurothiomalate has been used to disrupt the interaction between Par6 and atypical protein kinase C within the multicomponent complexes, and has been shown to effectively block transformed growth and metastasis in vitro and/or in vivo in a variety of cancers, including pancreatic, prostate and lung cancers, as well as alveolar rhabdomyosarcoma. It is likely that with further revelations regarding the critical roles of Par6 in cancer initiation, progression and metastasis, targeted therapies against Par6 will be discovered and prove effective preclinically, and hopefully clinically, in cancer treatment.

Downregulation of PKCζ/Pard3/Pard6b is responsible for lung adenocarcinoma cell EMT and invasion

Atypical protein kinase C ζ (PKCζ) forms an apico-basal polarity complex with Partitioning Defective (Pard) 3 and Pard6 to regulate normal epithelial cell apico-basolateral polarization. The dissociation of the PKCζ/Pard3/Pard6 complex is essential for the disassembly of the tight/adherens junction and epithelial-mesenchymal transition (EMT) that is critical for tumor spreading. Loss of cell polarity and epithelial organization is strongly correlated with malignancy and tumor progression in some other cancer types. However, it is unclear whether the PKCζ/Pard3/Pard6 complex plays a role in the progression of non-small-cell lung cancer (NSCLC). We found that hypoxia downregulated the PKCζ/Pard3/Pard6 complex, correlating with induction of lung cancer cell migration and invasion. Silencing of the PKCζ/Pard3/Pard6 polarity complex components induced lung cancer cell EMT, invasion, and colonization in vivo. Suppression of Pard3 was associated with altered expression of genes regulating wound healing, cell apoptosis/death and cell motility, and particularly upregulation of MAP3K1 and fibronectin which are known to contribute to lung cancer progression. Human lung adenocarcinoma tissues expressed less Pard6b and PKCζ than the adjacent normal tissues and in experimental mouse lung adenocarcinoma, the levels of Pard3 and PKCζ were also decreased. In addition, we showed that a methylation locus in the gene body of Pard3 is positively associated with the expression of Pard3 and that methylation of the Pard3 gene increased cellular sensitivity to carboplatin, a common chemotherapy drug. Suppression of Pard3 increased chemoresistance in lung cancer cells. Together, these results suggest that reduced expression of PKCζ/Pard3/Pard6 contributes to NSCLC EMT, invasion, and chemoresistance.

High expression of partitioning defective 3-like protein is associated with malignancy in colorectal cancer

Partitioning defective 3-like protein is a novel cell polarity protein. Recently, partitioning defective 3-like protein has been demonstrated with tumor-promoting function by disrupting tight junction, inhibiting tumor suppressor liver kinase B1, and maintaining mammary stem cells. For the first time, we studied partitioning defective 3-like protein expression in malignant colorectal cancer. We used immunohistochemistry scoring system to evaluate partitioning defective 3-like protein expression in 196 colorectal cancer tissues and 33 adjacent normal tissues. We found that colorectal cancer tissues had much stronger partitioning defective 3-like protein immunoreactivity than normal tissues, and colorectal cancer patients with positive partitioning defective 3-like protein expression were characterized with higher cancer stages, metastasis, poor tumor differentiation, larger tumor size, as well as high levels of colorectal cancer markers carcinoembryonic antigen and cancer antigen 19-9. Besides, partitioning defective 3-like protein overexpression was independently predictive of lower survival rate in colorectal cancer patients, even after adjusting the influence of cofactors. Moreover, we also found that partitioning defective 3-like protein was associated with rapid growing colorectal cancer, while knockdown of partitioning defective 3-like protein expression largely inhibited cancer cell proliferation. Our study provided the first evidence that partitioning defective 3-like protein was overexpressed in colorectal cancer and associated with disease malignancy. Also, partitioning defective 3-like protein may serve as a promising prognostic marker and a potential therapeutic target for colorectal cancer treatment. Further study is necessary to understand the regulatory mechanism of partitioning defective 3-like protein in colorectal cancer and the feasibility of its application in clinic.

The scaffold protein p140Cap limits ERBB2-mediated breast cancer progression interfering with Rac GTPase-controlled circuitries

The docking protein p140Cap negatively regulates tumour cell features. Its relevance on breast cancer patient survival, as well as its ability to counteract relevant cancer signalling pathways, are not fully understood. Here we report that in patients with ERBB2-amplified breast cancer, a p140Cap-positive status associates with a significantly lower probability of developing a distant event, and a clear difference in survival. p140Cap dampens ERBB2-positive tumour cell progression, impairing tumour onset and growth in the NeuT mouse model, and counteracting epithelial mesenchymal transition, resulting in decreased metastasis formation. One major mechanism is the ability of p140Cap to interfere with ERBB2-dependent activation of Rac GTPase-controlled circuitries. Our findings point to a specific role of p140Cap in curbing the aggressiveness of ERBB2-amplified breast cancers and suggest that, due to its ability to impinge on specific molecular pathways, p140Cap may represent a predictive biomarker of response to targeted anti-ERBB2 therapies.

p140Cap adaptor proteins interfere with adhesion and growth factor-dependent signalling in cancer cells but the mechanisms are unclear. Here the authors show that p140Cap interferes with ERBB2-dependent activation of Rac GTPase-controlled circuitries reducing metastasis and cancer progression.

Expression of Par3 polarity protein correlates with poor prognosis in ovarian cancer

BACKGROUND

Previous studies have shown that the cell polarity protein partitioning defective 3 (Par3) plays an essential role in the formation of tight junctions and definition of apical-basal polarity. Aberrant function of this protein has been reported to be involved in epithelial-mesenchymal transition (EMT) and cancer invasion. The aim of this study was to examine the functional mechanism of Par3 in ovarian cancer.

METHODS

First, we investigated the association between Par3 expression level and survival of 50 ovarian cancer patients. Next, we conducted an in vitro analysis of ovarian cancer cell lines, focusing on the cell line JHOC5, to investigate Par3 function. To investigate the function of Par3 in invasion, the IL-6/STAT3 pathway was analyzed upon Par3 knockdown with siRNA. The effect of siRNA treatment was assessed by qPCR, ELISA, and western blotting. Invasiveness and cell proliferation following treatment with siRNA against Par3 were investigated using Matrigel chamber, wound healing, and cell proliferation assays.

RESULTS

Expression array data for ovarian cancer patient samples revealed low Par3 expression was significantly associated with good prognosis. Univariate analysis of clinicopathological factors revealed significant association between high Par3 levels and peritoneal dissemination at the time of diagnosis. Knockdown of Par3 in JHOC5 cells suppressed cell invasiveness, migration, and cell proliferation with deregulation of IL-6/STAT3 activity.

CONCLUSION

Taken together, these results suggest that Par3 expression is likely involved in ovarian cancer progression, especially in peritoneal metastasis. The underlying mechanism may be that Par3 modulates IL-6 /STAT3 signaling. Here, we propose that the expression of Par3 in ovarian cancer may control disease outcome.

Loss of Par3 promotes lung adenocarcinoma metastasis through 14-3-3ζ protein

Partitioning defective protein 3 (Par3) can activate the Tiam1/Rac pathway to inhibit invasion and metastasis in many cancers; however, the role of Par3 in lung adenocarcinoma remains unknown. Here we show that Par3 is downregulated in lung adenocarcinoma tissues and is associated with higher rates of lymph node metastasis and recurrence. Our functional study demonstrated that knock-down of Par3 promoted lung adenocarcinoma cell growth, cell migration, tumor formation, and metastasis, all of which were effectively inhibited when 14-3-3ζ was silenced. We found that Par3 binded with 14-3-3ζ protein and also showed that Par3 abrogated the binding of 14-3-3ζ to Tiam1, which was responsible for Rac1 activation. Knock-down of 14-3-3ζ inhibited Tiam1/Rac-GTP activation and blocked the invasive behavior of cells lacking Par3. These data suggest that loss of Par3 promotes metastatic behavior in lung adenocarcinoma cells through 14-3-3ζ protein.

Cell adhesion and polarity in squamous cell carcinoma of the lung

Lung cancer is a deadly disease that can roughly be classified into three histopathological groups: lung adenocarcinomas, lung squamous cell carcinomas (LSCCs), and small cell carcinomas. These types of lung cancer are molecularly, phenotypically, and regionally diverse neoplasms, reflecting differences in their cells of origin. LSCCs commonly arise in the airway epithelium of a main or lobar bronchus, which is an important line of defence against the external environment. Furthermore, most LSCCs are characterized histopathologically by the presence of keratinization and/or intercellular bridges, consistent with the molecular features of these tumours, characterized by high levels of transcripts encoding keratins and proteins relevant to intercellular junctions and cell polarity. In this review, the relationships between the molecular features of LSCCs and the types of cell and epithelia of origin are discussed. Recurrent alterations in genes involved in intercellular adhesion and cell polarity in LSCCs are also reviewed, emphasizing the importance of the disruption of PAR3 and the PAR complex. Finally, the possible functional effects of these alterations on epithelial homeostasis, and how they contribute to the development of LSCC, are discussed.

PARD3 Inactivation in Lung Squamous Cell Carcinomas Impairs STAT3 and Promotes Malignant Invasion

Correct apicobasal polarization and intercellular adhesions are essential for the appropriate development of normal epithelia. Here, we investigated the contribution of the cell polarity regulator PARD3 to the development of lung squamous cell carcinomas (LSCC). Tumor-specific PARD3 alterations were found in 8% of LSCCs examined, placing PARD3 among the most common tumor suppressor genes in this malignancy. Most PAR3-mutant proteins exhibited a relative reduction in the ability to mediate formation of tight junctions and actin-based protrusions, bind atypical protein kinase C, activate RAC1, and activate STAT3 at cell confluence. Thus, PARD3 alterations prevented the formation of contacts between neighboring cells and the subsequent downstream signaling. Notably, reconstituting PAR3 activity in vivo reduced tumor-invasive and metastatic properties. Our findings define PARD3 as a recurrently inactivated cell polarity regulator in LSCC that affects tumor aggressiveness and metastasis.

Effect of gyromagnetic fields on human prostatic adenocarcinoma cells

PURPOSE

To investigate the biological effect of gyromagnetic fields (GMFs) on cell proliferation and apoptosis of human prostatic adenocarcinoma cells and explore the underlying mechanisms.

METHODS

PC-3 cells were grouped into normal control (NC) and GMF treatment groups. Cell proliferation was analyzed with kit-8 and Ki67 immunofluorescence staining, while cell apoptosis was analyzed with flow cytometry double staining of Annexin V-PE/7-AAD. The Akt and p38 MAPK/Caspase signaling pathways were analyzed by western blotting and immunofluorescence staining, and cell polarization was analyzed with PARD3.

RESULTS

Cell proliferation and activity of the Akt pathway were significantly decreased by the GMF, while cell apoptosis, activity of p38 MAPK, and PARD3-positive cell number were significantly increased in the GMF group compared to the NC group.

CONCLUSION

GMFs inhibit cell proliferation, induce apoptosis, and regulate tumor cell polarity conditions, potentially through down-regulating Akt, activating the p38 MAPK/Caspase pathway, and promoting PARD3 expression in PC-3 cells.

PKCζ Promotes Breast Cancer Invasion by Regulating Expression of E-cadherin and Zonula Occludens-1 (ZO-1) via NFκB-p65

Atypical Protein Kinase C zeta (PKCζ) forms Partitioning-defective (PAR) polarity complex for apico-basal distribution of membrane proteins essential to maintain normal cellular junctional complexes and tissue homeostasis. Consistently, tumor suppressive role of PKCζ has been established for multiple human cancers. However, recent studies also indicate pro-oncogenic function of PKCζ without firm understanding of detailed molecular mechanism. Here we report a possible mechanism of oncogenic PKCζ signaling in the context of breast cancer. We observed that depletion of PKCζ promotes epithelial morphology in mesenchymal-like MDA-MB-231 cells. The induction of epithelial morphology is associated with significant upregulation of adherens junction (AJ) protein E-cadherin and tight junction (TJ) protein Zonula Occludens-1 (ZO-1). Functionally, depletion of PKCζ significantly inhibits invasion and metastatic progression. Consistently, we observed higher expression and activation of PKCζ signaling in invasive and metastatic breast cancers compared to non-invasive diseases. Mechanistically, an oncogenic PKCζ– NFκB-p65 signaling node might be involved to suppress E-cadherin and ZO-1 expression and ectopic expression of a constitutively active form of NFκB-p65 (S536E-NFκB-p65) significantly rescues invasive potential of PKCζ-depleted breast cancer cells. Thus, our study discovered a PKCζ - NFκB-p65 signaling pathway might be involved to alter cellular junctional dynamics for breast cancer invasive progression.

NGL-2 Is a New Partner of PAR Complex in Axon Differentiation

Neuronal polarization is pivotal for neural network formation during brain development. Axon differentiation is a hallmark of initial neuronal polarization. Here, we report that the leucine-rich repeat-containing protein netrin-G ligand-2 (NGL-2) as a polarity regulator that localizes asymmetrically in rat hippocampal neurons and is required for differentiation of the future axon. NGL-2 was associated with PAR complex, and this interaction resulted in local stabilization of axonal microtubules. Further study showed that the C terminal of NGL-2 binds to the PDZ domain of PAR6, and NGL-2 interacts with PAR3 and atypical PKCζ (aPKCζ), with PAR6 acting as a bridge or modifier. Then, NGL-2 regulates the local stabilization of microtubules and promotes axon differentiation by the aPKCζ/microtubule affinity-regulating kinase 2 pathway. These findings reveal the critical role of NGL-2 in regulating axon differentiation in rat hippocampal neurons and reveal a novel partner of the PAR complex.

TGFβ-induced phosphorylation of Par6 promotes migration and invasion in prostate cancer cells

Background:

The Par complex – comprising partition-defective 6 (Par6), Par3, and atypical protein kinase C (aPKC) – is crucial for cell polarisation, the loss of which contributes to cancer progression. Transforming growth factor β (TGFβ)-induced phosphorylation of Par6 on the conserved serine 345 is implicated in epithelial-to-mesenchymal transition (EMT) in breast cancer. Here we investigated the importance of phosphorylated Par6 in prostate cancer.

Methods:

We generated a p-Par6³⁴⁵-specific antibody and verified its specificity in vitro. Endogenous p-Par6³⁴⁵ was analysed by immunoblotting in normal human prostate RWPE1 and prostate cancer (PC-3U) cells. Subcellular localisation of p-Par6³⁴⁵ in migrating TGFβ-treated PC-3U cells was analysed by confocal imaging. Invasion assays of TGFβ-treated PC-3U cells were performed. p-Par6 expression was immunohistochemically analysed in prostate cancer tissues.

Results:

TGFβ induced Par6 phosphorylation on Ser345 and its recruitment to the leading edge of the membrane ruffle in migrating PC-3U cells, where it colocalised with aPKCζ. The p-Par6–aPKCζ complex is important for cell migration and invasion, as interference with this complex prevented prostate cancer cell invasion. High levels of activated Par6 correlated with aggressive prostate cancer.

Conclusions:

Increased p-Par6Ser³⁴⁵ levels in aggressive prostate cancer tissues and cells suggest that it could be a useful novel biomarker for predicting prostate cancer progression.

Apico-basal polarity complex and cancer

Apico-basal polarity is a cardinal molecular feature of adult eukaryotic epithelial cells and appears to be involved in several key cellular processes including polarized cell migration and maintenance of tissue architecture. Epithelial cell polarity is maintained by three well-conserved polarity complexes, namely, PAR, Crumbs and SCRIB. The location and interaction between the components of these complexes defines distinct structural domains of epithelial cells. Establishment and maintenance of apico-basal polarity is regulated through various conserved cell signalling pathways including TGF beta, Integrin and WNT signalling. Loss of cell polarity is a hallmark for carcinoma, and its underlying molecular mechanism is beginning to emerge from studies on model organisms and cancer cell lines. Moreover, deregulated expression of apico-basal polarity complex components has been reported in human tumours. In this review, we provide an overview of the apico-basal polarity complexes and their regulation, their role in cell migration, and finally their involvement in carcinogenesis.

Actomyosin regulation and symmetry breaking in a model of polarization in the early Caenorhabditis elegans embryo : symmetry breaking in cell polarization

Polarization, whereby a cell defines a spatial axis by segregating specific determinants to distinct regions, is an essential and highly conserved biological process. The process of polarization is initiated by a cue that breaks an initially symmetric distribution of determinants, allowing for a spatially asymmetric redistribution. The nature of this cue is currently not well understood. Utilizing the conservation of polarization process and its determinants, we theoretically investigate the nature of the cue and the regulation of contractility that enables the establishment of polarity in early embryos of the nematode worm Caenorhabditis elegans (C. elegans). Our biologically based model, which consists of coupled partial differential equations, suggests that a biochemical but not mechanical cue is sufficient for symmetry breaking, and inhibition of contractile elements by specific determinants is needed for sustained spatial redistribution.

Human papillomavirus (HPV)-18 E6 oncoprotein interferes with the epithelial cell polarity Par3 protein

High-risk human papillomavirus (HPV) infection is the principal risk factor for the development of cervical cancer. The HPV E6 oncoprotein has the ability to target and interfere with several PSD-95/DLG/ZO-1 (PDZ) domain-containing proteins that are involved in the control of cell polarity. This function can be significant for E6 oncogenic activity because a deficiency in cell polarisation is a marker of tumour progression. The establishment and control of polarity in epithelial cells depend on the correct asymmetrical distribution of proteins and lipids at the cell borders and on specialised cell junctions. In this report, we have investigated the effects of HPV E6 protein on the polarity machinery, with a focus on the PDZ partitioning defective 3 (Par3) protein, which is a key component of tight junctions (TJ) and the polarity network. We demonstrate that E6 is able to bind and induce the mislocalisation of Par3 protein in a PDZ-dependent manner without significant reduction in Par3 protein levels. In addition, the high-risk HPV-18 E6 protein promotes a delay in TJ formation when analysed by calcium switch assays. Taken together, the data presented in this study contribute to our understanding of the molecular mechanism by which HPVs induce the loss of cell polarity, with potential implications for the development and progression of HPV-associated tumours.

The Rac-GAP Bcr is a novel regulator of the Par complex that controls cell polarity

The Par complex (Par3, Par6, and PKCζ) controls cell polarity, which is essential for many biological processes. Here we identify the Rac1 GTPase-activating protein Bcr as an integral member of the Par complex that regulates polarized cell migration by locally restricting both Rac1 and PKCζ function.

Cell polarization is essential for many biological processes, including directed cell migration, and loss of polarity contributes to pathological conditions such as cancer. The Par complex (Par3, Par6, and PKCζ) controls cell polarity in part by recruiting the Rac-specific guanine nucleotide exchange factor T-lymphoma invasion and metastasis 1 (Tiam1) to specialized cellular sites, where Tiam1 promotes local Rac1 activation and cytoskeletal remodeling. However, the mechanisms that restrict Par-Tiam1 complex activity to the leading edge to maintain cell polarity during migration remain unclear. We identify the Rac-specific GTPase-activating protein (GAP) breakpoint cluster region protein (Bcr) as a novel regulator of the Par-Tiam1 complex. We show that Bcr interacts with members of the Par complex and inhibits both Rac1 and PKCζ signaling. Loss of Bcr results in faster, more random migration and striking polarity defects in astrocytes. These polarity defects are rescued by reducing PKCζ activity or by expressing full-length Bcr, but not an N-terminal deletion mutant or the homologous Rac-GAP, Abr, both of which fail to associate with the Par complex. These results demonstrate that Bcr is an integral member of the Par-Tiam1 complex that controls polarized cell migration by locally restricting both Rac1 and PKCζ function.

Overexpression of Numb suppresses tumor cell growth and enhances sensitivity to cisplatin in epithelioid malignant pleural mesothelioma

Malignant pleural mesothelioma (MPM) is a highly aggressive and conventional treatment-resistant tumor with a dismal prognosis. Among the three histological subtypes of MPM, the epithelioid is the most common type. Numb is considered as a tumor suppressor playing a critical role in controlling asymmetric cell division, maintenance of stem cell compartments, ubiquitination of specific substrates and regulating Notch-, Hedgehog- and TP53-activated pathways. The present study was designed to analyze the role of Numb in epithelioid MPM. We investigated the expression of Numb in 39 epithelioid MPM and 22 normal pleural tissues by immunohistochemistry. Furthermore, we overexpressed Numb in NCI-H2452, an epithelioid human MPM cell line, and investigated the effect of Numb overexpression on the proliferation, apoptosis and sensitivity to cisplatin in cells. The expression of Numb was significantly lower in MPM compared to the control group and Numb had an inverse correlation with the ki-67 labeling index. Loss of Numb expression was associated with poor prognosis in epithelioid MPM. Overexpression of Numb in NCI-H2452 cells significantly inhibited proliferation, promoted apoptosis and enhanced sensitivity to cisplatin. Moreover, Numb overexpression activated caspase-9 and caspase-3 through release of cytochrome c as well as downregulation of XIAP and survivin. We speculate that cytochrome c/caspase signaling is a possible mechanism through which Numb enhances the apoptosis of NCI-H2452 cells. These results suggest that Numb may be involved in epithelioid MPM development, and its upregulation may confer sensitivity to cisplatin, suggesting potential therapeutic options for MPM.

More targets, more pathways and more clues for mutant p53

Mutations in the transcription factor p53 are among the most common genetic alterations in human cancer, and missense p53 mutations in cancer cells can lead to aggressive phenotypes. So far, only few studies investigated transcriptional reprogramming under mutant p53 expression as a means to identify deregulated targets and pathways. A review of the literature was carried out focusing on mutant p53-dependent transcriptome changes with the aims of (i) verifying whether different p53 mutations can be equivalent for their effects, or whether there is a mutation-specific transcriptional reprogramming of target genes, (ii) understanding what is the main mechanism at the basis of upregulation or downregulation of gene expression under the p53 mutant background, (iii) identifying novel candidate target genes of WT and/or mutant p53 and (iv) defining cellular pathways affected by the mutant p53-dependent gene expression reprogramming. Nearly 600 genes were consistently found upregulated or downregulated upon ectopic expression of mutant p53, regardless of the specific p53 mutation studied. Promoter analysis and the use of ChIP-seq data indicate that, for most genes, the expression changes could be ascribed to a loss both of WT p53 transcriptional activation and repressor functions. Pathway analysis indicated changes in the metabolism/catabolism of amino acids such as aspartate, glutamate, arginine and proline. Novel p53 candidate target genes were also identified, including ARID3B, ARNT2, CLMN, FADS1, FTH1, KPNA2, LPHN2, PARD6B, PDE4C, PIAS2, PRPF40A, PYGL and RHOBTB2, involved in the metabolism, xenobiotic responses and cell differentiation.

Aberrant upregulation of LRRC1 contributes to human hepatocellular carcinoma

Loss of apico-basal polarity often results in a malignant phenotype in epithelial tissues. Aberrant expression of polarity mediator proteins is closely associated with this process. LRRC1/LANO, a putative cell polarity regulator, was previously screened from our gene expression profiling in which its expression was significantly upregulated in hepatocellular carcinoma (HCC). In the present study, we provide evidences that LRRC1 plays a potential oncogenic function in HCC. Consistent with the microarray data, quantitative real-time PCR results showed LRRC1 was aberrantly upregulated in 37/56 (66.1 %, more than twofolds) of HCC specimens compared with adjacent non-cancerous livers. Furthermore, the cellular expression of LRRC1 in all HCC cell lines examined exhibited much higher level than that in normal adult liver tissue. Functional analyses revealed that overexpression of LRRC1 promoted, while knockdown of LRRC1 by RNA interference inhibited the growth and colony formation of HCC cells. Importantly, enhanced expression of LRRC1 conferred NIH3T3 cells the ability of cell transformation. In a xenograft tumor model, we found LRRC1 overexpression increased the tumorigenicity of HCC cells. Thus, our collective findings suggest that LRRC1 contributes to HCC development, and may be a potential target for therapeutic intervention in this disease.

Loss of Par3 promotes breast cancer metastasis by compromising cell-cell cohesion

The mechanisms by which tumour cells metastasize and the role that cell polarity proteins play in this process are not well understood. We report that partitioning defective protein 3 (Par3) is dysregulated in metastasis in human breast cancer, and is associated with a higher tumour grade and ErbB2-positive status. Downregulation of Par3 cooperated with ErbB2 to induce cell invasion and metastasis in vivo. Interestingly, the metastatic behaviour was not associated with an overt mesenchymal phenotype. However, loss of Par3 inhibited E-cadherin junction stability, disrupted membrane and actin dynamics at cell-cell junctions and decreased cell-cell cohesion in a manner dependent on the Tiam1/Rac-GTP pathway. Inhibition of this pathway restored E-cadherin junction stability and blocked invasive behaviour of cells lacking Par3, suggesting that loss of Par3 promotes metastatic behaviour of ErbB2-induced tumour epithelial cells by decreasing cell-cell cohesion.

Polarity in mammalian epithelial morphogenesis

Cell polarity is fundamental for the architecture and function of epithelial tissues. Epithelial polarization requires the intervention of several fundamental cell processes, whose integration in space and time is only starting to be elucidated. To understand what governs the building of epithelial tissues during development, it is essential to consider the polarization process in the context of the whole tissue. To this end, the development of three-dimensional organotypic cell culture models has brought new insights into the mechanisms underlying the establishment and maintenance of higher-order epithelial tissue architecture, and in the dynamic remodeling of cell polarity that often occurs during development of epithelial organs. Here we discuss some important aspects of mammalian epithelial morphogenesis, from the establishment of cell polarity to epithelial tissue generation.

Spatial regulation of VEGF receptor endocytosis in angiogenesis

Activities as diverse as migration, proliferation and patterning occur simultaneously and in a coordinated fashion during tissue morphogenesis. In the growing vasculature, the formation of motile, invasive and filopodia-carrying endothelial sprouts is balanced with the stabilization of blood-transporting vessels. Here, we show that sprouting endothelial cells in the retina have high rates of VEGF uptake, VEGF receptor endocytosis and turnover. These internalization processes are opposed by atypical protein kinase C activity in more stable and mature vessels. aPKC phosphorylates Dab2, a clathrin-associated sorting protein that, together with the transmembrane protein ephrin-B2 and the cell polarity regulator PAR-3, enables VEGF receptor endocytosis and downstream signal transduction. Accordingly, VEGF receptor internalization and the angiogenic growth of vascular beds are defective in loss-of-function mice lacking key components of this regulatory pathway. Our work uncovers how vessel growth is dynamically controlled by local VEGF receptor endocytosis and the activity of cell polarity proteins.

A novel interaction between hScrib and PP1γ downregulates ERK signaling and suppresses oncogene-induced cell transformation

Previous studies have shown that the cell polarity regulator hScrib interacts with, and consequently controls, the ERK signaling pathway. This interaction occurs through two well-conserved Kinase Interacting Motifs, which allow hScrib to bind ERK1 directly, resulting in a reduction in the levels of phospho-ERK. This suggests that hScrib might recruit a phosphatase to regulate this signaling pathway. Using a proteomic approach we now show that Protein Phosphatase 1γ (PP1γ) is a major interacting partner of hScrib. This interaction is direct and occurs through a conserved PP1γ interaction motif on the hScrib protein, and this interaction appears to be required for hScrib's ability to downregulate ERK phosphorylation. In addition, hScrib also controls the pattern of PP1γ localization, where loss of hScrib enhances the nuclear translocation of PP1γ. Furthermore, we also show that the ability of hScrib to interact with PP1γ is important for the ability of hScrib to suppress oncogene-induced transformation of primary rodent cells. Taken together, these results demonstrate that hScrib acts as a scaffold to integrate the control of the PP1γ and ERK signaling pathways and explains how disruption of hScrib localisation can contribute towards the development of human malignancy.

The ability to induce microtubule acetylation is a general feature of formin proteins

Cytoplasmic microtubules exist as distinct dynamic and stable populations within the cell. Stable microtubules direct and maintain cell polarity and it is thought that their stabilization is dependent on coordinative organization between the microtubule network and the actin cytoskeleton. A growing body of work suggests that some members of the formin family of actin remodeling proteins also regulate microtubule organization and stability. For example, we showed previously that expression of the novel formin INF1 is sufficient to induce microtubule stabilization and tubulin acetylation, but not tubulin detyrosination. An important issue with respect to the relationship between formins and microtubules is the determination of which formin domains mediate microtubule stabilization. INF1 has a distinct microtubule-binding domain at its C-terminus and the endogenous INF1 protein is associated with the microtubule network. Surprisingly, the INF1 microtubule-binding domain is not essential for INF1-induced microtubule acetylation. We show here that expression of the isolated FH1 + FH2 functional unit of INF1 is sufficient to induce microtubule acetylation independent of the INF1 microtubule-binding domain. It is not yet clear whether or not microtubule stabilization is a general property of all mammalian formins; therefore we expressed constitutively active derivatives of thirteen of the fifteen mammalian formin proteins in HeLa and NIH3T3 cells and measured their effects on stress fiber formation, MT organization and MT acetylation. We found that expression of the FH1 + FH2 unit of the majority of mammalian formins is sufficient to induce microtubule acetylation. Our results suggest that the regulation of microtubule acetylation is likely a general formin activity and that the FH2 should be thought of as a dual-function domain capable of regulating both actin and microtubule networks.

Hugl1 and Hugl2 in mammary epithelial cells: polarity, proliferation, and differentiation

Loss of epithelial polarity is described as a hallmark of epithelial cancer. To determine the role of Hugl1 and Hugl2 expression in the breast, we investigated their localization in human mammary duct tissue and the effects of expression modulation in normal and cancer cell lines on polarity, proliferation and differentiation. Expression of Hugl1 and Hugl2 was silenced in both MCF10A cells and Human Mammary Epithelial Cells and cell lines were grown in 2-D on plastic and in 3-D in Matrigel to form acini. Cells in monolayer were compared for proliferative and phenotypic changes while acini were examined for differences in size, ability to form a hollow lumen, nuclear size and shape, and localization of key domain-specific proteins as a measure of polarity. We detected overlapping but distinct localization of Hugl1 and Hugl2 in the human mammary gland, with Hugl1 expressed in both luminal and myoepithelium and Hugl2 largely restricted to myoepithelium. On a plastic surface, loss of Hugl1 or Hugl2 in normal epithelium induced a mesenchymal phenotype, and these cells formed large cellular masses when grown in Matrigel. In addition, loss of Hugl1 or Hugl2 expression in MCF10A cells resulted in increased proliferation on Matrigel, while gain of Hugl1 expression in tumor cells suppressed proliferation. Loss of polarity was also observed with knockdown of either Hugl1 or Hugl2, with cells growing in Matrigel appearing as a multilayered epithelium, with randomly oriented Golgi and multiple enlarged nuclei. Furthermore, Hugl1 knock down resulted in a loss of membrane identity and the development of cellular asymmetries in Human Mammary Epithelial Cells. Overall, these data demonstrate an essential role for both Hugl1 and Hugl2 in the maintenance of breast epithelial polarity and differentiated cell morphology, as well as growth control.

PAR6B is required for tight junction formation and activated PKCζ localization in breast cancer

Dysregulation of mechanisms that govern the control of epithelial cell polarity, morphology and plasticity are emerging as key processes in tumor progression. In this study we report amplification and overexpression of PAR6B, an essential component in epithelial cell tight junction (TJ) formation and maintenance of apico-basal polarity, in breast cancer cell lines. Analysis of chromosome 20q13.13 in 11 breast cancer cell lines by fluorescence in situ hybridization (FISH) identified a novel small amplicon centered at PARD6B in 5 cell lines, with copy number ranging from 7 to 27. The presence of the PARD6B amplicon correlated with PARD6B transcript and PAR6B protein abundance. Expression of related isoforms PARD6A and PARD6G were detectable at significantly lower levels. PARD6B overexpression correlated with TJ network formation in cultured cell monolayers. SiRNA-mediated inhibition of PAR6B in MCF7 resulted in loss of TJ assembly and membrane localization of atypical PKCζ (aPKC), but did not affect adherens junction formation. SiRNA-mediated inhibition of CDC42 in MCF7 also resulted in loss of TJ networks, confirming the requirement of a complete PAR6-aPKC-CDC42-PAR3 complex to activate and stabilize TJs. Immunohistochemical analysis of PAR6B expression on breast tumor microarrays indicated exquisite epithelial cell-specificity. Few quantitative differences in staining were observed between normal epithelium and adjacent tumor margins. However staining appeared reduced and cytoplasmic in more poorly differentiated tumors. We propose that quantitative imbalances in the components of pathways governing normal epithelial cell polarity arising from gain or loss of function may radically alter epithelial cell architecture and contribute to tumor progression.

Anterior PAR proteins function during cytokinesis and maintain DYN-1 at the cleavage furrow in Caenorhabditis elegans

PAR proteins are key regulators of cellular polarity and have links to the endocytic machinery and the actin cytoskeleton. Our data suggest a unique role for PAR proteins in cytokinesis. We have found that at the onset of cytokinesis, anterior PAR-6 and posterior PAR-2 proteins are redistributed to the furrow membrane in a temporal and spatial manner. PAR-6 and PAR-2 localize to the furrow membrane during ingression but PAR-2-GFP is distinct in that it is excluded from the extreme tip of the furrow. Once the midbody has formed, PAR-2-GFP becomes restricted to the midbody region (the midbody plus the membrane flanking it). Depletion of both anterior PAR proteins, PAR-3 and PAR-6, led to an increase in multinucleate embryos, suggesting that the anterior PAR proteins are necessary during cytokinesis and that PAR-3 and PAR-6 function in cytokinesis may be partially redundant. Lastly, anterior PAR proteins play a role in the maintenance of DYN-1 in the cleavage furrow. Our data indicate that the PAR proteins are involved in the events that occur during cytokinesis and may play a role in promoting the membrane trafficking and remodeling events that occur during this time.