Mammalian aPKC/Par polarity complex mediated regulation of epithelial division orientation and cell fate

Cell polarity changes in cancer initiation and progression

Cell polarity, which consists of the morphological, structural, and functional organization of cells along a defined axis, is a feature of healthy cells and tissues. In contrast, abnormal polarity is a hallmark of cancer cells. At the molecular level, key evolutionarily conserved proteins that control polarity establishment and maintenance in various contexts are frequently altered in cancer, but the relevance of these molecular alterations in the oncogenic processes is not always clear. Here, we summarize the recent findings, shedding new light on the involvement of polarity players in cancer development, and discuss the possibility of harnessing cell polarity changes to better predict, diagnose, and cure cancers.

A targeted genetic modifier screen in Drosophila uncovers vulnerabilities in a genetically complex model of colon cancer

Received on 16 January 2023; accepted on 21 February 2023Kinases are key regulators of cellular signal transduction pathways. Many diseases, including cancer, are associated with global alterations in protein phosphorylation networks. As a result, kinases are frequent targets of drug discovery efforts. However, target identification and assessment, a critical step in targeted drug discovery that involves identifying essential genetic mediators of disease phenotypes, can be challenging in complex, heterogeneous diseases like cancer, where multiple concurrent genomic alterations are common. Drosophila is a particularly useful genetic model system to identify novel regulators of biological processes through unbiased genetic screens. Here, we report 2 classic genetic modifier screens focusing on the Drosophila kinome to identify kinase regulators in 2 different backgrounds: KRAS TP53 PTEN APC, a multigenic cancer model that targets 4 genes recurrently mutated in human colon tumors and KRAS alone, a simpler model that targets one of the most frequently altered pathways in cancer. These screens identified hits unique to each model and one shared by both, emphasizing the importance of capturing the genetic complexity of human tumor genome landscapes in experimental models. Our follow-up analysis of 2 hits from the KRAS-only screen suggests that classical genetic modifier screens in heterozygous mutant backgrounds that result in a modest, nonlethal reduction in candidate gene activity in the context of a whole animal-a key goal of systemic drug treatment-may be a particularly useful approach to identify the most rate-limiting genetic vulnerabilities in disease models as ideal candidate drug targets.

Protein kinase Cλ/ι in cancer: a contextual balance of time and signals

Nononcogenic cancer drivers often impinge on complex signals that create new addictions and vulnerabilities. Protein kinase Cλ/ι (PKCλ/ι) suppresses tumorigenesis by blocking metabolic pathways that regulate fuel oxidation and create building blocks for the epigenetic control of cell differentiation. Reduced levels of PKCλ/ι unleash these pathways to promote tumorigenesis, but the simultaneous activation of the STING-driven interferon cascade prevents tumor initiation by triggering immunosurveillance mechanisms. However, depending on the context of other signaling pathways, such as WNT/β-catenin or PKCζ, and timing, PKCλ/ι deletion can promote or inhibit tumorigenesis. In this review, we discuss in detail the molecular and cellular underpinnings of PKCλ/ι functions in cancer with the perspective of the crosstalk between metabolism and inflammation in the tumor microenvironment.

Apoptosis and tissue thinning contribute to symmetric cell division in the developing mouse epidermis in a nonautonomous way

Mitotic spindle orientation (SO) is a conserved mechanism that governs cell fate and tissue morphogenesis. In the developing epidermis, a balance between self-renewing symmetric divisions and differentiative asymmetric divisions is necessary for normal development. While the cellular machinery that executes SO is well characterized, the extrinsic cues that guide it are poorly understood. Here, we identified the basal cell adhesion molecule (BCAM), a β1 integrin coreceptor, as a novel regulator of epidermal morphogenesis. In utero RNAi-mediated depletion of Bcam in the mouse embryo did not hinder β1 integrin distribution or cell adhesion and polarity. However, Bcam depletion promoted apoptosis, thinning of the epidermis, and symmetric cell division, and the defects were reversed by concomitant overexpression of the apoptosis inhibitor Xiap. Moreover, in mosaic epidermis, depletion of Bcam or Xiap induced symmetric divisions in neighboring wild-type cells. These results identify apoptosis and epidermal architecture as extrinsic cues that guide SO in the developing epidermis.

PRKCI Mediates Radiosensitivity via the Hedgehog/GLI1 Pathway in Cervical Cancer

Objective

Insensitivity to radiotherapy accounts for the majority of therapeutic failures in cervical cancer (CC) patients who undergo radical radiotherapy. We aimed to elucidate the molecular mechanisms underlying radiosensitivity to identify methods to improve the overall 5-year survival rate. The atypical protein kinase C iota (aPKCι) gene PRKCI exhibits tumor-specific copy number amplification (CNA) in CC. We investigated how PRKCI decreases radiosensitivity in CC and assessed the interplay between PRKCI and the Hedgehog (Hh)/GLI1 pathway in the present research.

Methods

The biological functions of PRKCI in CC radiosensitivity were explored through immunohistochemistry, colony formation, Cell Counting Kit-8 (CCK-8), cell cycle, apoptosis assays, and xenograft models. qRT-PCR, Western blotting analysis, and immunofluorescence assays were utilized to evaluate the interplay between PRKCI and the Hh/GLI1 pathway and its mechanism in PRKCI-decreased radiosensitivity in CC. Furthermore, the effect of auranofin (AF), a selective inhibitor of PKCι, on CC cells was explored through biochemical assays in vitro and in vivo.

Results

We found that high PRKCI expression was responsible for decreased survival in CC. PRKCI was intimately associated with radiation-triggered alterations in proliferation, the cell cycle, apoptosis, and xenograft growth. The Hh/GLI1 pathway was activated when PRKCI expression was altered. PRKCI functions downstream of the Hh/GLI1 pathway to phosphorylate and activate the transcription factor GLI1. AF acts as a radiosensitizer and showed biological effects in vitro and in vivo.

Conclusions

PRKCI is a therapeutic target for regulating radiosensitivity in CC. This molecule regulates radiosensitivity by modulating GLI1 relocalization and phosphorylation in CC via the Hh/GLI1 pathway.

Collagen XVII deficiency alters epidermal patterning

Vertebrates exhibit patterned epidermis, exemplified by scales/interscales in mice tails and grooves/ridges on the human skin surface (microtopography). Although the role of spatiotemporal regulation of stem cells (SCs) has been implicated in this process, the mechanism underlying the development of such epidermal patterns is poorly understood. Here, we show that collagen XVII (COL17), a niche for epidermal SCs, helps stabilize epidermal patterns. Gene knockout and rescue experiments revealed that COL17 maintains the width of the murine tail scale epidermis independently of epidermal cell polarity. Skin regeneration after wounding was associated with slender scale epidermis, which was alleviated by overexpression of human COL17. COL17-negative skin in human junctional epidermolysis bullosa showed a distinct epidermal pattern from COL17-positive skin that resulted from revertant mosaicism. These results demonstrate that COL17 contributes to defining mouse tail scale shapes and human skin microtopography. Our study sheds light on the role of the SC niche in tissue pattern formation.

Insights Into Mechanisms of Oriented Division From Studies in 3D Cellular Models

In multicellular organisms, epithelial cells are key elements of tissue organization. In developing tissues, cellular proliferation and differentiation are under the tight regulation of morphogenetic programs, that ensure the correct organ formation and functioning. In these processes, mitotic rates and division orientation are crucial in regulating the velocity and the timing of the forming tissue. Division orientation, specified by mitotic spindle placement with respect to epithelial apico-basal polarity, controls not only the partitioning of cellular components but also the positioning of the daughter cells within the tissue, and hence the contacts that daughter cells retain with the surrounding microenvironment. Daughter cells positioning is important to determine signal sensing and fate, and therefore the final function of the developing organ. In this review, we will discuss recent discoveries regarding the mechanistics of planar divisions in mammalian epithelial cells, summarizing technologies and model systems used to study oriented cell divisions in vitro such as three-dimensional cysts of immortalized cells and intestinal organoids. We also highlight how misorientation is corrected in vivo and in vitro, and how it might contribute to the onset of pathological conditions.

The polarity protein Par3 coordinates positively self-renewal and negatively invasiveness in glioblastoma

Glioblastoma (GBM) is a brain malignancy characterized by invasiveness to the surrounding brain tissue and by stem-like cells, which propagate the tumor and may also regulate invasiveness. During brain development, polarity proteins, such as Par3, regulate asymmetric cell division of neuro-glial progenitors and neurite motility. We, therefore, studied the role of the Par3 protein (encoded by PARD3) in GBM. GBM patient transcriptomic data and patient-derived culture analysis indicated diverse levels of expression of PARD3 across and independent from subtypes. Multiplex immunolocalization in GBM tumors identified Par3 protein enrichment in SOX2-, CD133-, and NESTIN-positive (stem-like) cells. Analysis of GBM cultures of the three subtypes (proneural, classical, mesenchymal), revealed decreased gliomasphere forming capacity and enhanced invasiveness upon silencing Par3. GBM cultures with suppressed Par3 showed low expression of stemness (SOX2 and NESTIN) but higher expression of differentiation (GFAP) genes. Moreover, Par3 silencing reduced the expression of a set of genes encoding mitochondrial enzymes that generate ATP. Accordingly, silencing Par3 reduced ATP production and concomitantly increased reactive oxygen species. The latter was required for the enhanced migration observed upon silencing of Par3 as anti-oxidants blocked the enhanced migration. These findings support the notion that Par3 exerts homeostatic redox control, which could limit the tumor cell-derived pool of oxygen radicals, and thereby the tumorigenicity of GBM.

Otic Neurogenesis in Xenopus laevis: Proliferation, Differentiation, and the Role of Eya1

Using immunostaining and confocal microscopy, we here provide the first detailed description of otic neurogenesis in Xenopus laevis. We show that the otic vesicle comprises a pseudostratified epithelium with apicobasal polarity (apical enrichment of Par3, aPKC, phosphorylated Myosin light chain, N-cadherin) and interkinetic nuclear migration (apical localization of mitotic, pH3-positive cells). A Sox3-immunopositive neurosensory area in the ventromedial otic vesicle gives rise to neuroblasts, which delaminate through breaches in the basal lamina between stages 26/27 and 39. Delaminated cells congregate to form the vestibulocochlear ganglion, whose peripheral cells continue to proliferate (as judged by EdU incorporation), while central cells differentiate into Islet1/2-immunopositive neurons from stage 29 on and send out neurites at stage 31. The central part of the neurosensory area retains Sox3 but stops proliferating from stage 33, forming the first sensory areas (utricular/saccular maculae). The phosphatase and transcriptional coactivator Eya1 has previously been shown to play a central role for otic neurogenesis but the underlying mechanism is poorly understood. Using an antibody specifically raised against Xenopus Eya1, we characterize the subcellular localization of Eya1 proteins, their levels of expression as well as their distribution in relation to progenitor and neuronal differentiation markers during otic neurogenesis. We show that Eya1 protein localizes to both nuclei and cytoplasm in the otic epithelium, with levels of nuclear Eya1 declining in differentiating (Islet1/2+) vestibulocochlear ganglion neurons and in the developing sensory areas. Morpholino-based knockdown of Eya1 leads to reduction of proliferating, Sox3- and Islet1/2-immunopositive cells, redistribution of cell polarity proteins and loss of N-cadherin suggesting that Eya1 is required for maintenance of epithelial cells with apicobasal polarity, progenitor proliferation and neuronal differentiation during otic neurogenesis.

Cyclin A2/cyclin-dependent kinase 1-dependent phosphorylation of Top2a is required for S phase entry during retinal development in zebrafish

Cyclin-dependent kinase 1 (CDK1) plays an essential role in cell cycle regulation. However, as mouse Cdk1 embryos die early, the role of CDK1 in regulating the cell cycle and embryo development remains unclear. Here, we showed that zebrafish cdk1^-/- embryos exhibit severe microphthalmia accompanied by multiple defects in S phase entry, M phase progression, and cell differentiation but not in interkinetic nuclear migration. We identified Top2a as a potential downstream target and cyclin A2 and cyclin B1 as partners of Cdk1 in cell cycle regulation via an in silico analysis. While depletion of either cyclin A2 or Top2a led to the decreased S phase entry in zebrafish retinal cells, the depletion of cyclin B1 led to M phase arrest. Moreover, phosphorylation of Top2a at serine 1213 (S1213) was nearly abolished in both cdk1 and ccna2 mutants, but not in ccnb1 mutants. Furthermore, overexpression of TOP2A^S1213D, the phosphomimetic form of human TOP2A, rescued S phase entry and alleviated the microphthalmia defects in both cdk1^-/- and ccna2^-/- embryos. Taken together, our data suggest that Cdk1 interacts with cyclin A2 to regulate S phase entry partially through Top2a phosphorylation and interacts with cyclin B1 to regulate M phase progression.

Identification of Key Phospholipids That Bind and Activate Atypical PKCs

PKCζ and PKCι/λ form the atypical protein kinase C subgroup, characterised by a lack of regulation by calcium and the neutral lipid diacylglycerol. To better understand the regulation of these kinases, we systematically explored their interactions with various purified phospholipids using the lipid overlay assays, followed by kinase activity assays to evaluate the lipid effects on their enzymatic activity. We observed that both PKCζ and PKCι interact with phosphatidic acid and phosphatidylserine. Conversely, PKCι is unique in binding also to phosphatidylinositol-monophosphates (e.g., phosphatidylinositol 3-phosphate, 4-phosphate, and 5-phosphate). Moreover, we observed that phosphatidylinositol 4-phosphate specifically activates PKCι, while both isoforms are responsive to phosphatidic acid and phosphatidylserine. Overall, our results suggest that atypical Protein kinase C (PKC) localisation and activity are regulated by membrane lipids distinct from those involved in conventional PKCs and unveil a specific regulation of PKCι by phosphatidylinositol-monophosphates.

An overview of signaling pathways regulating YAP/TAZ activity

YAP and TAZ are ubiquitously expressed homologous proteins originally identified as penultimate effectors of the Hippo signaling pathway, which plays a key role in maintaining mammalian tissue/organ size. Presently, it is known that YAP/TAZ also interact with various non-Hippo signaling pathways, and have diverse roles in multiple biological processes, including cell proliferation, tissue regeneration, cell lineage fate determination, tumorigenesis, and mechanosensing. In this review, we first examine the various microenvironmental cues and signaling pathways that regulate YAP/TAZ activation, through the Hippo and non-Hippo signaling pathways. This is followed by a brief summary of the interactions of YAP/TAZ with TEAD1-4 and a diverse array of other non-TEAD transcription factors. Finally, we offer a critical perspective on how increasing knowledge of the regulatory mechanisms of YAP/TAZ signaling might open the door to novel therapeutic applications in the interrelated fields of biomaterials, tissue engineering, regenerative medicine and synthetic biology.

Type XVII collagen interacts with the aPKC-PAR complex and maintains epidermal cell polarity

Type XVII collagen (COL17) is a transmembrane protein expressed in the basal epidermis. COL17 serves as a niche for epidermal stem cells, and although its reduction has been implicated in altering cell polarity and ageing of the epidermis, it is unknown how COL17 affects epidermal cell polarity. Here, we uncovered COL17 as a binding partner of the aPKC-PAR complex, which is a key regulating factor of cell polarity. Immunoprecipitation-immunoblot assay and protein-protein binding assay revealed that COL17 interacts with aPKC and PAR3. COL17 deficiency or epidermis-specific aPKCλ deletion destabilized PAR3 distribution in the epidermis, while aPKCζ knockout did not. Asymmetrical cell division was pronounced in COL17-null neonatal paw epidermis. These results show that COL17 is pivotal for maintaining epidermal cell polarity. Our study highlights the previously unrecognized role of COL17 in the basal keratinocytes.

Recurrent copy number gains drive PKCι expression and PKCι-dependent oncogenic signaling in human cancers

PRKCI is frequently overexpressed in multiple human cancers, and PKCι expression is often prognostic for poor patient survival, indicating that elevated PKCι broadly plays an oncogenic role in the cancer phenotype. PKCι drives multiple oncogenic signaling pathways involved in transformed growth, and transgenic mouse models have revealed that PKCι is a critical oncogenic driver in both lung and ovarian cancers. We now report that recurrent 3q26 copy number gain (CNG) is the predominant genetic driver of PRKCI mRNA expression in all major human cancer types exhibiting such CNGs. In addition to PRKCI, CNG at 3q26 leads to coordinate CNGs of ECT2 and SOX2, two additional 3q26 genes that collaborate with PRKCI to drive oncogenic signaling and tumor initiation in lung squamous cell carcinoma. Interestingly however, whereas 3q26 CNG is a strong driver of PRKCI mRNA expression across all tumor types examined, it has differential effects on ECT2 and SOX2 mRNA expression. In some tumors types, particularly those with squamous histology, all three 3q26 oncogenes are coordinately overexpressed as a consequence of 3q26 CNG, whereas in other cancers only PRKCI and ECT2 mRNA are coordinately overexpressed. This distinct pattern of expression of 3q26 genes corresponds to differences in genomic signatures reflective of activation of specific PKCι oncogenic signaling pathways. In addition to highly prevalent CNG, some tumor types exhibit monoallelic loss of PRKCI. Interestingly, many tumors harboring monoallelic loss of PRKCI express significantly lower PRKCI mRNA and exhibit evidence of WNT/β-catenin signaling pathway activation, which we previously characterized as a major oncogenic pathway in a newly described, PKCι-independent molecular subtype of lung adenocarcinoma. Finally, we show that CNG-driven activation of PKCι oncogenic signaling predicts poor patient survival in many major cancer types. We conclude that CNG and monoallelic loss are the major determinants of tumor PRKCI mRNA expression across virtually all tumor types, but that tumor-type specific mechanisms determine whether these copy number alterations also drive expression of the collaborating 3q26 oncogenes ECT2 and SOX2, and the oncogenic PKCι signaling pathways activated through the collaborative action of these genes. Our analysis may be useful in identifying tumor-specific predictive biomarkers and effective PKCι-targeted therapeutic strategies in the multitude of human cancers harboring genetic activation of PRKCI.

Formation and contraction of multicellular actomyosin cables facilitate lens placode invagination

Embryonic morphogenesis relies on the intrinsic ability of cells, often through remodeling the cytoskeleton, to shape epithelial tissues during development. Epithelial invagination is an example of morphogenesis that depends on this remodeling but the cellular mechanisms driving arrangement of cytoskeletal elements needed for tissue deformation remain incompletely characterized. To elucidate these mechanisms, live fluorescent microscopy and immunohistochemistry on fixed specimens were performed on chick and mouse lens placodes. This analysis revealed the formation of peripherally localized, circumferentially orientated and aligned junctions enriched in F-actin and MyoIIB. Once formed, the aligned junctions contract in a Rho-kinase and non-muscle myosin dependent manner. Further molecular characterization of these junctions revealed a Rho-kinase dependent accumulation of Arhgef11, a RhoA-specific guanine exchange factor known to regulate the formation of actomyosin cables and junctional contraction. In contrast, the localization of the Par-complex protein Par3, was reduced in these circumferentially orientated junctions. In an effort to determine if Par3 plays a negative role in MyoIIB accumulation, Par3-deficient mouse embryos were analyzed which not only revealed an increase in bicellular junctional accumulation of MyoIIB, but also a reduction of Arhgef11. Together, these results highlight the importance of the formation of the multicellular actomyosin cables that appear essential to the initiation of epithelial invagination and implicate the potential role of Arhgef11 and Par3 in their contraction and formation.

The Dual Roles of the Atypical Protein Kinase Cs in Cancer

Atypical protein kinase C (aPKC) isozymes, PKCλ/ι and PKCζ, are now considered fundamental regulators of tumorigenesis. However, the specific separation of functions that determine their different roles in cancer is still being unraveled. Both aPKCs have pleiotropic context-dependent functions that can translate into tumor-promoter or -suppressive functions. Here, we review early and more recent literature to discuss how the different tumor types, and their microenvironments, might account for the selective signaling of each aPKC isotype. This is of clinical relevance because a better understanding of the roles of these kinases is essential for the design of new anti-cancer treatments.

PDK1 Regulates Transition Period of Apical Progenitors to Basal Progenitors by Controlling Asymmetric Cell Division

The six-layered neocortex consists of diverse neuron subtypes. Deeper-layer neurons originate from apical progenitors (APs), while upper-layer neurons are mainly produced by basal progenitors (BPs), which are derivatives of APs. As development proceeds, an AP generates two daughter cells that comprise an AP and a deeper-layer neuron or a BP. How the transition of APs to BPs is spatiotemporally regulated is a fundamental question. Here, we report that conditional deletion of phoshpoinositide-dependent protein kinase 1 (PDK1) in mouse developing cortex achieved by crossing Emx1Cre line with Pdk1fl/fl leads to a delayed transition of APs to BPs and subsequently causes an increased output of deeper-layer neurons. We demonstrate that PDK1 is involved in the modulation of the aPKC-Par3 complex and further regulates the asymmetric cell division (ACD). We also find Hes1, a downstream effecter of Notch signal pathway is obviously upregulated. Knockdown of Hes1 or treatment with Notch signal inhibitor DAPT recovers the ACD defect in the Pdk1 cKO. Thus, we have identified a novel function of PDK1 in controlling the transition of APs to BPs.

PKCζ and JNK signaling regulate radiation-induced compensatory proliferation in parotid salivary glands

Radiotherapy is a common treatment option for head and neck cancer patients; however, the surrounding healthy salivary glands are often incidentally irradiated during the process. As a result, patients often experience persistent xerostomia and hyposalivation, which deceases their quality of life. Clinically, there is currently no standard of care available to restore salivary function. Repair of epithelial wounds involves cellular proliferation and establishment of polarity in order to regenerate the tissue. This process is partially mediated by protein kinase C zeta (PKCζ), an apical polarity regulator; however, its role following radiation damage is not completely understood. Using an in vivo radiation model, we show a significant decrease in active PKCζ in irradiated murine parotid glands, which correlates with increased proliferation that is sustained through 30 days post-irradiation. Additionally, salivary glands in PKCζ null mice show increased basal proliferation which radiation treatment did not further potentiate. Radiation damage also activates Jun N-terminal kinase (JNK), a proliferation-inducing mitogen-activated protein kinase normally inhibited by PKCζ. In both a PKCζ null mouse model and in primary salivary gland cell cultures treated with a PKCζ inhibitor, there was increased JNK activity and production of downstream proliferative transcripts. Collectively, these findings provide a potential molecular link by which PKCζ suppression following radiation damage promotes JNK activation and radiation-induced compensatory proliferation in the salivary gland.

aPKCζ-dependent Repression of Yap is Necessary for Functional Restoration of Irradiated Salivary Glands with IGF-1

Xerostomia and salivary hypofunction often result as a consequence of radiation therapy for head and neck cancers, which are diagnosed in roughly 60,000 individuals every year in the U.S. Due to the lack of effective treatments for radiation-induced salivary hypofunction, stem cell-based therapies have been suggested to regenerate the irradiated salivary glands. Pharmacologically, restoration of salivary gland function has been accomplished in mice by administering IGF-1 shortly after radiation treatment, but it is not known if salivary stem and progenitor cells play a role. We show that radiation inactivates aPKCζ and promotes nuclear redistribution of Yap in a population of label-retaining cells in the acinar compartment of the parotid gland (PG)- which comprises a heterogeneous pool of salivary progenitors. Administration of IGF-1 post-radiation maintains activation of aPKCζ and partially rescues Yap's cellular localization in label retaining cells, while restoring salivary function. Finally, IGF-1 fails to restore saliva production in mice lacking aPKCζ, demonstrating the importance of the kinase as a potential therapeutic target.

Molecular components and polarity of radial glial cells during cerebral cortex development

Originating from ectodermal epithelium, radial glial cells (RGCs) retain apico-basolateral polarity and comprise a pseudostratified epithelial layer in the developing cerebral cortex. The apical endfeet of the RGCs faces the fluid-filled ventricles, while the basal processes extend across the entire cortical span towards the pial surface. RGC functions are largely dependent on this polarized structure and the molecular components that define it. In this review, we will dissect existing molecular evidence on RGC polarity establishment and during cerebral cortex development and provide our perspective on the remaining key questions.

Brief Report: Interleukin-17A-Dependent Asymmetric Stem Cell Divisions Are Increased in Human Psoriasis: A Mechanism Underlying Benign Hyperproliferation

The balance between asymmetric and symmetric stem cell (SC) divisions is key to tissue homeostasis, and dysregulation of this balance has been shown in cancers. We hypothesized that the balance between asymmetric cell divisions (ACDs) and symmetric cell divisions (SCDs) would be dysregulated in the benign hyperproliferation of psoriasis. We found that, while SCDs were increased in squamous cell carcinoma (SCC) (human and murine), ACDs were increased in the benign hyperproliferation of psoriasis (human and murine). Furthermore, while sonic hedgehog (linked to human cancer) and pifithrinα (p53 inhibitor) promoted SCDs, interleukin (IL)-1α and amphiregulin (associated with benign epidermal hyperproliferation) promoted ACDs. While there was dysregulation of the ACD:SCD ratio, no change in SC frequency was detected in epidermis from psoriasis patients, or in human keratinocytes treated with IL-1α or amphiregulin. We investigated the mechanism whereby immune alterations of psoriasis result in ACDs. IL17 inhibitors are effective new therapies for psoriasis. We found that IL17A increased ACDs in human keratinocytes. Additionally, studies in the imiquimod-induced psoriasis-like mouse model revealed that ACDs in psoriasis are IL17A-dependent. In summary, our studies suggest an association between benign hyperproliferation and increased ACDs. This work begins to elucidate the mechanisms by which immune alteration can induce keratinocyte hyperproliferation. Altogether, this work affirms that a finely tuned balance of ACDs and SCDs is important and that manipulating this balance may constitute an effective treatment strategy for hyperproliferative diseases. Stem Cells 2017;35:2001-2007.

Atypical PKC and Notch Inhibition Differentially Modulate Cortical Interneuron Subclass Fate from Embryonic Stem Cells

Recent studies indicate that the location of neurogenesis within the medial ganglionic eminence (MGE) critically influences the fate determination of cortical interneuron subgroups, with parvalbumin (Pv) interneurons originating from subventricular zone divisions and somatostatin (Sst) interneurons primarily arising from apical divisions. The aPKC-CBP and Notch signaling pathways regulate the transition from apical to basal progenitor and their differentiation into post-mitotic neurons. We find that aPKC inhibition enhances intermediate neurogenesis from stem cell-derived MGE progenitors, resulting in a markedly increased ratio of Pv- to Sst-expressing interneurons. Conversely, inhibition of Notch signaling enriches for Sst subtypes at the expense of Pv fates. These findings confirm that the mode of neurogenesis influences the fate of MGE-derived interneurons and provide a means of further enrichment for the generation of specific interneuron subgroups from pluripotent stem cells.

Protein Kinases C-Mediated Regulations of Drug Transporter Activity, Localization and Expression

Drug transporters are now recognized as major actors in pharmacokinetics, involved notably in drug–drug interactions and drug adverse effects. Factors that govern their activity, localization and expression are therefore important to consider. In the present review, the implications of protein kinases C (PKCs) in transporter regulations are summarized and discussed. Both solute carrier (SLC) and ATP-binding cassette (ABC) drug transporters can be regulated by PKCs-related signaling pathways. PKCs thus target activity, membrane localization and/or expression level of major influx and efflux drug transporters, in various normal and pathological types of cells and tissues, often in a PKC isoform-specific manner. PKCs are notably implicated in membrane insertion of bile acid transporters in liver and, in this way, are thought to contribute to cholestatic or choleretic effects of endogenous compounds or drugs. The exact clinical relevance of PKCs-related regulation of drug transporters in terms of drug resistance, pharmacokinetics, drug–drug interactions and drug toxicity remains however to be precisely determined. This issue is likely important to consider in the context of the development of new drugs targeting PKCs-mediated signaling pathways, for treating notably cancers, diabetes or psychiatric disorders.

Multiscale View of Cytoskeletal Mechanoregulation of Cell and Tissue Polarity

The ability of cells to generate, maintain, and repair tissues with complex architecture, in which distinct cells function as coherent units, relies on polarity cues. Polarity can be described as an asymmetry along a defined axis, manifested at the molecular, structural, and functional levels. Several types of cell and tissue polarities were described in the literature, including front-back, apical-basal, anterior-posterior, and left-right polarity. Extensive research provided insights into the specific regulators of each polarization process, as well as into generic elements that affect all types of polarities. The actin cytoskeleton and the associated adhesion structures are major regulators of most, if not all, known forms of polarity. Actin filaments exhibit intrinsic polarity and their ability to bind many proteins including the mechanosensitive adhesion and motor proteins, such as myosins, play key roles in cell polarization. The actin cytoskeleton can generate mechanical forces and together with the associated adhesions, probe the mechanical, structural, and chemical properties of the environment, and transmit signals that impact numerous biological processes, including cell polarity. In this article we highlight novel mechanisms whereby the mechanical forces and actin-adhesion complexes regulate cell and tissue polarity in a variety of natural and experimental systems.

14-3-3ζ promotes lung cancer cell invasion by increasing the Snail protein expression through atypical protein kinase C (aPKC)/NF-κB signaling

14-3-3ζ has been identified as a putative oncogene in several cancers, including non-small cell lung cancer (NSCLC). However, the mechanisms underlying its functions remain undefined. In this study, we show that overexpression of 14-3-3ζ was frequently detected in lung adenocarcinoma (LuAC) tissues and was significantly associated with lymph node metastasis and poor outcome. Functional studies demonstrated that 14-3-3ζ promoted migration and invasion in A549 cells, both of which were effectively inhibited when 14-3-3ζ was silenced with short hairpin RNA (shRNA). Furthermore, 14-3-3ζ-mediated invasion of cancer cells was found to upregulate Snail through the activation of atypical protein kinase C (aPKC). Activation of aPKCζ mediates this effect by stimulating NF-κB signaling. Our results identify a specific pathway by which 14-3-3ζ induces tumor invasion and provide insight into potential therapeutic approaches to target 14-3-3ζ-associated lung adenocarcinoma.

Essential Role of Polarity Protein Par3 for Epidermal Homeostasis through Regulation of Barrier Function, Keratinocyte Differentiation, and Stem Cell Maintenance

Partitioning-defective (Par) proteins contribute to multiprotein complexes that drive cell polarity and fate in invertebrates. Of these, the ternary Par3-atypical protein kinase C-Par6 polarity complex mediates asymmetry in various systems, whereas Par3 and aPKC/Par6 can also act independently. aPKC-λ has recently been implicated in epidermal differentiation and stem cell fate; however, whether Par3 contributes to the homeostasis of adult stratified epithelia is currently unknown. Here, we provide functional evidence that epidermal Par3 loss disturbed the inside-out skin barrier, coinciding with altered expression and localization of principle tight junction components, and that epidermal differentiation and thickness were increased. Moreover, Par3 inactivation caused an initial expansion and later decline of hair follicle bulge stem cells, accompanied by an enrichment of committed progenitors, formation of hypertrophic sebaceous glands, and increased epidermal differentiation, suggesting aberrant cell fate decisions. Importantly, and opposite to aPKCλ deletion, Par3 loss did not enhance perpendicular cell divisions. Instead, in Par3-deficient hair follicles, spindles were shifted toward planar orientation, indicating that abnormal differentiation after Par3 inactivation is unlikely to be attributed to increased perpendicular spindle orientation. Collectively, mammalian Par3 controls the epidermal barrier, differentiation, and stem cell maintenance in the pilosebaceous unit, which are all essential for the homeostasis of an important barrier-forming epithelium.

Dual function of partitioning-defective 3 in the regulation of YAP phosphorylation and activation

Partitioning-defective 3 (Par3), a key component of the evolutionarily conserved polarity PAR complex (Par3/Par6/aPKC), controls cell polarity and contributes to cell migration, proliferation and tumor development. Emerging evidence indicates that cell polarity proteins function as upstream modulators that regulate the Hippo pathway. However, little is known about Par3’s involvement in the Hippo pathway. Here, we find Par3 and YAP dynamically co-localize in different subcellular compartments; that is, the membrane, cytoplasm and nucleus, in a cell-density-dependent manner. Interestingly, Par3 knockdown promotes YAP phosphorylation, leading to a significant impairment of YAP nuclear translocation at low cell density, but not at high density, in MDCK cells. Furthermore, via its third PDZ domain, Par3 directly binds to the PDZ-binding motif of YAP. The interaction is required for regulating YAP phosphorylation and nuclear localization. Mechanistically, Par3, as a scaffold protein, associates with LATS1 and protein phosphatase 1, α subunit (PP1A) in the cytoplasm and nucleus. Par3 promotes the dephosphorylation of LATS1 and YAP, thus enhancing YAP activation and cell proliferation. Strikingly, we also find that under the condition of PP1A knockdown, Par3 expression promotes YAP hyperphosphorylation, leading to the suppression of YAP activity and its downstream targets. Par3 expression results in differential effects on YAP phosphorylation and activation in different tumor cell lines. These findings indicate that Par3 may have a dual role in regulating the activation of the Hippo pathway, in a manner possibly dependent on cellular context or cell type in response to cell–cell contact and cell polarity signals.

PDK1 Is a Regulator of Epidermal Differentiation that Activates and Organizes Asymmetric Cell Division

Asymmetric cell division (ACD) in a perpendicular orientation promotes cell differentiation and organizes the stratified epithelium. However, the upstream cues regulating ACD have not been identified. Here, we report that phosphoinositide-dependent kinase 1 (PDK1) plays a critical role in establishing ACD in the epithelium. Production of phosphatidyl inositol triphosphate (PIP3) is localized to the apical side of basal cells. Asymmetric recruitment of atypical protein kinase C (aPKC) and partitioning defective (PAR) 3 is impaired in PDK1 conditional knockout (CKO) epidermis. PDK1(CKO) keratinocytes do not undergo calcium-induced activation of aPKC or IGF1-induced activation of AKT and fail to differentiate. PDK1(CKO) epidermis shows decreased expression of Notch, a downstream effector of ACD, and restoration of Notch rescues defective expression of differentiation-induced Notch targets in vitro. We therefore propose that PDK1 signaling regulates the basal-to-suprabasal switch in developing epidermis by acting as both an activator and organizer of ACD and the Notch-dependent differentiation program.

Molecular Control of Atypical Protein Kinase C: Tipping the Balance between Self-Renewal and Differentiation

Complex organisms are faced with the challenge of generating and maintaining diverse cell types, ranging from simple epithelia to neurons and motile immune cells [1-3]. To meet this challenge, a complex set of regulatory pathways controls nearly every aspect of cell growth and function, including genetic and epigenetic programming, cytoskeleton dynamics, and protein trafficking. The far reach of cell fate specification pathways makes it particularly catastrophic when they malfunction, both during development and for tissue homeostasis in adult organisms. Furthermore, the therapeutic promise of stem cells derives from their ability to deftly navigate the multitude of pathways that control cell fate [4]. How the molecular components making up these pathways function to specify cell fate is beginning to become clear. Work from diverse systems suggests that the atypical Protein Kinase C (aPKC) is a key regulator of cell fate decisions in metazoans [5-7]. Here, we examine some of the diverse physiological outcomes of aPKC's function in differentiation, along with the molecular pathways that control aPKC and those that are responsive to changes in its catalytic activity.

Protein Kinase Cι Drives a NOTCH3-dependent Stem-like Phenotype in Mutant KRAS Lung Adenocarcinoma

We report that the protein kinase Cι (PKCι) oncogene controls expression of NOTCH3, a key driver of stemness, in KRAS-mediated lung adenocarcinoma (LADC). PKCι activates NOTCH3 expression by phosphorylating the ELF3 transcription factor and driving ELF3 occupancy on the NOTCH3 promoter. PKCι-ELF3-NOTCH3 signaling controls the tumor-initiating cell phenotype by regulating asymmetric cell division, a process necessary for tumor initiation and maintenance. Primary LADC tumors exhibit PKCι-ELF3-NOTCH3 signaling, and combined pharmacologic blockade of PKCι and NOTCH synergistically inhibits tumorigenic behavior in vitro and LADC growth in vivo demonstrating the therapeutic potential of PKCι-ELF3-NOTCH3 signal inhibition to more effectively treat KRAS LADC.

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.

The role of adherens junctions in the developing neocortex

The disproportional enlargement of the neocortex through evolution has been instrumental in the success of vertebrates, in particular mammals. The neocortex is a multilayered sheet of neurons generated from a simple proliferative neuroepithelium through a myriad of mechanisms with substantial evolutionary conservation. This developing neuroepithelium is populated by progenitors that can generate additional progenitors as well as post-mitotic neurons. Subtle alterations in the production of progenitors vs. differentiated cells during development can result in dramatic differences in neocortical size. This review article will examine how cadherin adhesion proteins, in particular α-catenin and N-cadherin, function in regulating the neural progenitor microenvironment, cell proliferation, and differentiation in cortical development.

The chromosome 3q26 OncCassette: A multigenic driver of human cancer

Recurrent copy number variations (CNVs) are genetic alterations commonly observed in human tumors. One of the most frequent CNVs in human tumors involves copy number gains (CNGs) at chromosome 3q26, which is estimated to occur in >20% of human tumors. The high prevalence and frequent occurrence of 3q26 CNG suggest that it drives the biology of tumors harboring this genetic alteration. The chromosomal region subject to CNG (the 3q26 amplicon) spans from chromosome 3q26 to q29, a region containing ∼200 protein-encoding genes. The large number of genes within the amplicon makes it difficult to identify relevant oncogenic target(s). Whereas a number of genes in this region have been linked to the transformed phenotype, recent studies indicate a high level of cooperativity among a subset of frequently amplified 3q26 genes. Here we use a novel bioinformatics approach to identify potential driver genes within the recurrent 3q26 amplicon in lung squamous cell carcinoma (LSCC). Our analysis reveals a set of 35 3q26 amplicon genes that are coordinately amplified and overexpressed in human LSCC tumors, and that also map to a major LSCC susceptibility locus identified on mouse chromosome 3 that is syntenic with human chromosome 3q26. Pathway analysis reveals that 21 of these genes exist within a single predicted network module. Four 3q26 genes, SOX2, ECT2, PRKCI and PI3KCA occupy the hub of this network module and serve as nodal genes around which the network is organized. Integration of available genetic, genomic, biochemical and functional data demonstrates that SOX2, ECT2, PRKCI and PIK3CA are cooperating oncogenes that function within an integrated cell signaling network that drives a highly aggressive, stem-like phenotype in LSCC tumors harboring 3q26 amplification. Based on the high level of genomic, genetic, biochemical and functional integration amongst these 4 3q26 nodal genes, we propose that they are the key oncogenic targets of the 3q26 amplicon and together define a "3q26 OncCassette" that mediates 3q26 CNG-driven tumorigenesis. Genomic analysis indicates that the 3q26 OncCassette also operates in other major tumor types that exhibit frequent 3q26 CNGs, including head and neck squamous cell carcinoma (HNSCC), ovarian serous cancer and cervical cancer. Finally, we discuss how the 3q26 OncCassette represents a tractable target for development of novel therapeutic intervention strategies that hold promise for improving treatment of 3q26-driven cancers.

Getting to the core of cadherin complex function in Caenorhabditis elegans

The classic cadherin-catenin complex (CCC) mediates cell-cell adhesion in metazoans. Although substantial insights have been gained by studying the CCC in vertebrate tissue culture, analyzing requirements for and regulation of the CCC in vertebrates remains challenging. Caenorhabditis elegans is a powerful system for connecting the molecular details of CCC function with functional requirements in a living organism. Recent data, using an “angstroms to embryos” approach, have elucidated functions for key residues, conserved across all metazoans, that mediate cadherin/β-catenin binding. Other recent work reveals a novel, potentially ancestral, role for the C. elegans p120ctn homologue in regulating polarization of blastomeres in the early embryo via Cdc42 and the partitioning-defective (PAR)/atypical protein kinase C (aPKC) complex. Finally, recent work suggests that the CCC is trafficked to the cell surface via the clathrin adaptor protein complex 1 (AP-1) in surprising ways. These studies continue to underscore the value of C. elegans as a model system for identifying conserved molecular mechanisms involving the CCC.