PI3-kinase-dependent activation of apoptotic machinery occurs on commitment of epidermal keratinocytes to terminal differentiation

Transglutaminase 3 regulates cutaneous squamous carcinoma differentiation and inhibits progression via PI3K-AKT signaling pathway-mediated Keratin 14 degradation

Cutaneous squamous carcinoma is the second most common epithelial malignancy, associated with significant morbidity, mortality, and economic burden. However, the mechanisms underlying cSCC remain poorly understood. In this study, we identified TGM3 as a novel cSCC tumor suppressor that acts via the PI3K-AKT axis. RT-qPCR, IHC and western blotting were employed to assess TGM3 levels. TGM3-overexpression/knockdown cSCC cell lines were utilized to detect TGM3's impact on epithelial differentiation as well as tumor cell proliferation, migration, and invasion in vitro. Additionally, subcutaneous xenograft tumor models were employed to examine the effect of TGM3 knockdown on tumor growth in vivo. Finally, molecular and biochemical approaches were employed to gain insight into the tumor-suppressing mechanisms of TGM3. TGM3 expression was increased in well-differentiated cSCC tumors, whereas it was decreased in poor-differentiated cSCC tumors. Loss of TGM3 is associated with poor differentiation and a high recurrence rate in patients with cSCC. TGM3 exhibited tumor-suppressing activity by regulating cell proliferation, migration, and invasion both in vitro and in vivo. As a novel cSCC tumor differentiation marker, TGM3 expression was positively correlated with cell differentiation. In addition, our results demonstrated an interaction between TGM3 and KRT14 that aids in the degradation of KRT14. TGM3 deficiency disrupts keratinocytes differentiation, and ultimately leads to tumorigenesis. Furthermore, RNA-sequence analysis revealed that loss of TGM3 enhanced EMT via the PI3K-AKT signaling pathway. Deguelin, a PI3K-AKT inhibitor, blocked cSCC tumor growth induced by TGM3 knockdown in vivo. Taken together, TGM3 inhibits cSCC tumor growth via PI3K-AKT signaling, which could also serve as a tumor differentiation marker and a potential therapeutic target for cSCC. Proposed model depicted the mechanism by which TGM3 suppress cSCC development. TGM3 reduces the phosphorylation level of AKT and degrades KRT14. In the epithelial cell layer, TGM3 exhibits a characteristic pattern of increasing expression from bottom to top, while KRT14 and pAKT are the opposite. Loss of TGM3 leads to reduced degradation of KRT14 and activation of pAKT, disrupting keratinocyte differentiation, and eventually resulting in the occurrence of low-differentiated cSCC.

Caspase activation in tumour-infiltrating lymphocytes is associated with lymph node metastasis in oral squamous cell carcinoma

Oral squamous cell carcinomas (OSCCs) are genetically heterogeneous and exhibit diverse stromal and immune microenvironments. Acquired resistance to standard chemo-, radio-, and targeted therapies remains a major hurdle in planning effective treatment modalities for OSCC patients. Since Caspase 8 (CASP8) is frequently mutated in OSCCs, we were interested to explore a potential interaction between tumour-infiltrating lymphocytes (TILs) and CASP8 activation using high-content image analysis of human tumour (n = 32) sections. Despite the lymphocyte-rich tumour microenvironment, we observed lower activation of CASP8 (0-10% of tumour area) and its downstream effector CASP3 (0-6%) in tumours than in normal oral epithelium. Conversely, we found apoptosis was high for all the lymphocyte subtypes examined (38-52% of lymphocytes within tumour islands). Tumours with higher Fas ligand (FasL) expression had a significantly higher proportion of cleaved CASP3/8 positive cytotoxic T cells within the tumour islands (p = 0.05), and this was associated with the presence of lymph node metastatic disease [odds ratio: 1.046, 95% confidence interval (1.002-1.091), p = 0.039]. Our finding of extensive activation of the extrinsic pathway of apoptosis in TILs, together with evidence of higher FasL in CASP8 mutated tumours, may be useful in predicting the course of disease in individual patients. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

pTINCR microprotein promotes epithelial differentiation and suppresses tumor growth through CDC42 SUMOylation and activation

The human transcriptome contains thousands of small open reading frames (sORFs) that encode microproteins whose functions remain largely unexplored. Here, we show that TINCR lncRNA encodes pTINCR, an evolutionary conserved ubiquitin-like protein (UBL) expressed in many epithelia and upregulated upon differentiation and under cellular stress. By gain- and loss-of-function studies, we demonstrate that pTINCR is a key inducer of epithelial differentiation in vitro and in vivo. Interestingly, low expression of TINCR associates with worse prognosis in several epithelial cancers, and pTINCR overexpression reduces malignancy in patient-derived xenografts. At the molecular level, pTINCR binds to SUMO through its SUMO interacting motif (SIM) and to CDC42, a Rho-GTPase critical for actin cytoskeleton remodeling and epithelial differentiation. Moreover, pTINCR increases CDC42 SUMOylation and promotes its activation, triggering a pro-differentiation cascade. Our findings suggest that the microproteome is a source of new regulators of cell identity relevant for cancer.

Mammalian Epidermis: A Compendium of Lipid Functionality

Mammalian epidermis is a striking example of the role of lipids in tissue biology. In this stratified epithelium, highly specialized structures are formed that leverage the hydrophobic properties of lipids to form an impermeable barrier and protect the humid internal environment of the body from the dry outside. This is achieved through tightly regulated lipid synthesis that generates the molecular species unique to the tissue. Beyond their fundamental structural role, lipids are involved in the active protection of the body from external insults. Lipid species present on the surface of the body possess antimicrobial activity and directly contribute to shaping the commensal microbiota. Lipids belonging to a variety of classes are also involved in the signaling events that modulate the immune responses to environmental stress as well as differentiation of the epidermal keratinocytes themselves. Recently, high-resolution methods are beginning to provide evidence for the involvement of newly identified specific lipid molecules in the regulation of epidermal homeostasis. In this review we give an overview of the wide range of biological functions of mammalian epidermal lipids.

Unlocking the Mechanisms of Cutaneous Adverse Drug Reactions: Activation of the Phosphatidylinositol 3-Kinase/Protein Kinase B Pathway by EGFR Inhibitors Triggers Keratinocyte Differentiation and Polarization of Epidermal Immune Responses

EGFR inhibitors used in oncology therapy modify the keratinocyte differentiation processes, impairing proper skin barrier formation and leading to cutaneous adverse drug reactions. To uncover the molecular signatures associated with cutaneous adverse drug reactions, we applied phosphoproteomic and transcriptomic assays on reconstructed human epidermis tissues exposed to a therapeutically relevant concentration of afatinib, a second-generation EGFR inhibitor. After drug exposure, we observed activation of the phosphatidylinositol 3-kinase/protein kinase B pathway associated with an increased expression of gene families involved in keratinocyte differentiation, senescence, oxidative stress, and alterations in the epidermal immune-related markers. Furthermore, our results show that afatinib may interfere with vitamin D3 metabolism, acting via CYP27A1 and CYP24A1 to regulate calcium concentration through the phosphatidylinositol 3-kinase/protein kinase B pathway. Consequently, basal layer keratinocytes switch from a pro-proliferating to a prodifferentiative program, characterized by upregulation of biomarkers associated with increased keratinization, cornification, T helper type 2 response, and decreased innate immunity. Such effects may increase skin susceptibility to cutaneous penetration of irritants and pathogens. Taken together, these findings demonstrate a molecular mechanism of EGFR inhibitor–induced cutaneous adverse drug reactions.

Molekularne mechanizmy działania czynnika transkrypcyjnego FOXN1 w skórze

Artykuł jest przeglądem wyników badań dotyczących funkcji i mechanizmu działania czynnika transkrypcyjnego FOXN1. Lokalizacja FOXN1 u przedstawicieli wszystkich ssaków ogranicza się do nabłonka tylko dwóch organów: skóry i grasicy. W skórze FOXN1 stymuluje różnicowanie się keratynocytów, reguluje proces pigmentacji i bierze udział w rozwoju włosów. W skórze objętej urazem FOXN1 jest zaangażowany w bliznowy proces gojenia poprzez udział w reepitelializacji oraz w procesie przejścia epitelialno-mezenchymalnego (epithelial-mesenchymal transition; EMT). Pozbawione aktywnego czynnika transkrypcyjnego FOXN1 dorosłe myszy (Foxn1-/-) goją urazy skórne w unikalnym, charakterystycznym jedynie dla płodów ssaków, procesie bezbliznowej (scar-free) regeneracji. Analiza porównawcza transkryptomów skóry: dorosłych myszy Foxn1-/- oraz skóry płodów myszy (14. dzień rozwoju płodowego) wykazała istotne podobieństwa w ekspresji genów związanych przede wszystkim z przebudową tkanek, budową cytoszkieletu, gojeniem urazów, odpowiedzią immunologiczną oraz różnicowaniem. Wyniki te wskazują, iż FOXN1 może być głównym elementem szlaku sygnałowego na drodze tzw. punktu tranzycyjnego czyli przejścia z etapu gojenia bezbliznowego (płodowego) do bliznowego (dorosłego) w trakcie rozwoju płodowego.

Trichohyalin-like 1 protein plays a crucial role in proliferation and anti-apoptosis of normal human keratinocytes and squamous cell carcinoma cells

Epidermal differentiation is a complex process that requires the regulated and sequential expression of various genes. Most fused-type S100 proteins are expressed in the granular layer and it is hypothesized that these proteins may be associated with cornification and barrier formation. We previously identified a member of the fused-type S100 proteins, Trichohyalin-like 1 (TCHHL1) protein. TCHHL1 is distributed in the basal layer of the normal epidermis. Furthermore, the expression is markedly increased in cancerous/non-cancerous skin samples with the hyperproliferation of keratinocytes. We herein examined the role of TCHHL1 in normal human keratinocytes (NHKs) and squamous cell carcinoma (SCC). The knockdown of TCHHL1 by transfection with TCHHL1 siRNA significantly inhibited proliferation and induced the early apoptosis of NHKs. In TCHHL1-knockdown NHKs, the level of extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation was markedly decreased. In addition, the slight inhibition of v-akt murine thymoma viral oncogene homolog (AKT) phosphorylation and upregulation of forkhead box-containing protein O1(FOXO1), B-cell lymphoma2 (BCL2) and Bcl2-like protein 11 (BCL2L11) was observed. Skin-equivalent models built by TCHHL1-knockdown NHKs showed a markedly hypoplastic epidermis. These findings highlight that TCHHL1 plays an important role in homeostasis of the normal epidermis. TCHHL1 was expressed in the growing cells of cutaneous SCC; therefore, we next examined an association with the cell growth in HSC-1 cells (a human SCC line). In HSC-1 cells, the knockdown of TCHHL1 also suppressed cell proliferation and induced apoptosis. These cells showed an inhibition of phosphorylation of ERK1/2, AKT and signal transducers and activator of transcription 3, and the significant upregulation of FOXO1, BCL2, and BCL2L11. Accordingly, TCHHL1 is associated with survival of cutaneous SCC. In addition, we hypothesize that TCHHL1 may be a novel therapeutic target in cutaneous SCC.

Single-Cell ID-seq Reveals Dynamic BMP Pathway Activation Upstream of the MAF/MAFB-Program in Epidermal Differentiation

Epidermal homeostasis requires balanced and coordinated adult stem cell renewal and differentiation. These processes are controlled by both extracellular signaling and by cell intrinsic transcription regulatory networks, yet how these control mechanisms are integrated to achieve this is unclear. Here, we developed single-cell Immuno-Detection by sequencing (scID-seq) and simultaneously measured 69 proteins (including 34 phosphorylated epitopes) at single-cell resolution to study the activation state of signaling pathways during human epidermal differentiation. Computational pseudo-timing inference revealed dynamic activation of the JAK-STAT, WNT, and BMP pathways along the epidermal differentiation trajectory. We found that during differentiation, cells start producing BMP2-ligands and activate the canonical intracellular effectors SMAD1/5/9. Mechanistically, the BMP pathway is responsible for activating the MAF/MAFB/ZNF750 transcription factor network to drive late-stage epidermal differentiation. Our work indicates that incorporating signaling pathway activation into this transcription regulatory network enables coordination of transcription programs during epidermal differentiation.

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scID-seq allows quantification of 70 (phospho-)proteins at single-cell level

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Pseudo-time inference reveals signaling dynamics during epidermal differentiation

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BMP signaling drives a late differentiation transcription program

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BMP signaling activates the MAF/MAFB/ZNF750 transcription factor network

Oncogenic activation of PI3K induces progenitor cell differentiation to suppress epidermal growth

Oncogenic lesions are surprisingly common in morphologically and functionally normal human skin, however, the cellular and molecular mechanisms that suppress their cancer-driving potential to maintain tissue homeostasis are unknown. By employing assays for direct and quantitative assessment of cell fate choices in vivo, we show that oncogenic activation of PI3K/AKT, the most commonly activated oncogenic pathway in cancer, promotes differentiation and cell-cycle exit of epidermal progenitors. As a result, oncogenic PI3K/AKT activated epidermis exhibits growth disadvantage even though its cells are more proliferative. To uncover the underlying mechanism behind oncogene-induced differentiation, we conduct a series of genetic screens in vivo, and identify an AKT substrate SH3RF1 as a specific promoter of epidermal differentiation that has no effect on proliferation. Our study provides evidence for a direct, cell autonomous mechanism that can suppresses progenitor cell renewal and block clonal expansion of epidermal cells bearing a common and activating mutation in Pik3ca.

Stem Cell Expansion and Fate Decision on Liquid Substrates Are Regulated by Self-Assembled Nanosheets

The culture of adherent cells is overwhelmingly relying on the use of solid substrates to support cell adhesion. Indeed, it is typically thought that relatively strong bulk mechanical properties (bulk moduli in the range of kPa to GPa) are essential to promote cell adhesion and, in turn, regulate cell expansion and fate decision. In this report, we show that adherent stem cells such as mesenchymal stem cells and primary keratinocytes can be cultured at the surface of liquid substrates and that this phenomenon is mediated by the assembly of polymer nanosheets at the liquid-liquid interface. We use interfacial rheology to quantify this assembly and demonstrate the strong mechanical properties of such nanosheets. Importantly, we show that cell adhesion to such quasi-2D materials is mediated by the classical integrin/acto-myosin machinery, despite the absence of bulk mechanical properties of the underlying liquid substrate. Finally, we show that stem cell proliferation and fate decision are also regulated by the mechanical properties of these self-assembled protein nanosheets. Liquid substrates offer attractive features for the culture of adherent cells and stem cells, and the development of novel stem cell technologies, such as liquid-liquid systems, are particularly well-adapted to automated parallel processing and scale up.

Activation of S6 signaling is associated with cell survival and multinucleation in hyperplastic skin after epidermal loss of AURORA-A Kinase

The role of mitosis in the progression of precancerous skin remains poorly understood. To address this question, we deleted the mitotic Kinase Aurora-A (Aur-A) in hyperplastic mutant p53 mouse skin as an experimental tool to study the G2/M transition in precancerous keratinocytes and AUR-A's role in this process. Epidermal Aur-A deletion (Aur-A^epiΔ) led to marked keratinocyte enlargement, pleomorphism, multinucleation, and  attenuated induction of cell death. This phenotype was characteristic of slippage after a stalled mitosis. We also observed altered or impaired epidermal differentiation, indicative of a partial skin barrier defect. The upregulation of mTOR/PI3K signaling was implicated as a mechanism by which keratinocytes may evade cell death after AUR-A deficiency. This was evidenced by the ectopic expression of the pathway readout, p-S6, in the basal layer of Aur-A^epiΔ skin and its mitotic upregulation in isolated keratinocytes. We further tested whether our findings were extended to skin carcinoma cells. The chemical inhibition of AUR-A led to a similar mitotic delay, polyploidy/multinucleation, and attenuated cell death in skin cancer cell lines. Moreover, inhibition of mTOR/PI3K signaling ameliorated the effects caused by the deficiency of AUR-A activity but was also associated with the persistence of mitotic p-S6 detection in surviving cancer cells. These results show the induction of multinucleation/polyploidy may be a compensatory state in keratinocytes that allows for cellular survival and maintenance of partial barrier function in face of aberrant cell division or differentiation. Moreover, mTOR/PI3K signaling is active in the mitosis of hyperplastic keratinocytes expressing mutant p53 and is further enhanced by stalled mitosis, indicating a potential resistance mechanism to the use of anti-mitotic drugs in the treatment of skin cancers.

Foxn1 in Skin Development, Homeostasis and Wound Healing

Intensive research effort has focused on cellular and molecular mechanisms that regulate skin biology, including the phenomenon of scar-free skin healing during foetal life. Transcription factors are the key molecules that tune gene expression and either promote or suppress gene transcription. The epidermis is the source of transcription factors that regulate many functions of epidermal cells such as proliferation, differentiation, apoptosis, and migration. Furthermore, the activation of epidermal transcription factors also causes changes in the dermal compartment of the skin. This review focuses on the transcription factor Foxn1 and its role in skin biology. The regulatory function of Foxn1 in the skin relates to physiological (development and homeostasis) and pathological (skin wound healing) conditions. In particular, the pivotal role of Foxn1 in skin development and the acquisition of the adult skin phenotype, which coincides with losing the ability of scar-free healing, is discussed. Thus, genetic manipulations with Foxn1 expression, specifically those introducing conditional Foxn1 silencing in a Foxn1+/+ organism or its knock-in in a Foxn1−/− model, may provide future perspectives for regenerative medicine.

A protein phosphatase network controls the temporal and spatial dynamics of differentiation commitment in human epidermis

Epidermal homeostasis depends on a balance between stem cell renewal and terminal differentiation. The transition between the two cell states, termed commitment, is poorly understood. Here, we characterise commitment by integrating transcriptomic and proteomic data from disaggregated primary human keratinocytes held in suspension to induce differentiation. Cell detachment induces several protein phosphatases, five of which - DUSP6, PPTC7, PTPN1, PTPN13 and PPP3CA – promote differentiation by negatively regulating ERK MAPK and positively regulating AP1 transcription factors. Conversely, DUSP10 expression antagonises commitment. The phosphatases form a dynamic network of transient positive and negative interactions that change over time, with DUSP6 predominating at commitment. Boolean network modelling identifies a mandatory switch between two stable states (stem and differentiated) via an unstable (committed) state. Phosphatase expression is also spatially regulated in vivo and in vitro. We conclude that an auto-regulatory phosphatase network maintains epidermal homeostasis by controlling the onset and duration of commitment.

Licochalcone B induces apoptosis of human oral squamous cell carcinoma through the extrinsic- and intrinsic-signaling pathways

Licochalcone B (Lico B), which belongs to the retrochalcone family, is isolated from the roots of Chinese licorice. Lico B has been reported to have several other useful pharmacological properties, such as anti-inflammatory, antibacterial, antioxidant, antiulcer, anticancer, and anti-metastasis activities. We elucidated the underlying mechanism by which Lico B can induce apoptosis in oral squamous cell carcinoma (OSCC). Our results showed that exposure of OSCC cells (HN22 and HSC4) to Lico B significantly inhibited cell proliferation in a time- and concentration-dependent manner. Lico B caused cell cycle arrest at G1 phase along with downregulation of cyclin D1 and upregulation of p21 and p27 proteins. Lico B also facilitated the diffusion of phospholipid phosphatidylserine (PS) from inner to outer leaflets of the plasma membrane with chromatin condensation, DNA fragmentation, accumulated sub-G1 population in a concentration-dependent manner. Moreover, Lico B promoted the generation of reactive oxygen species (ROS), which, in turn, can induce CHOP, death receptor (DR) 4 and DR5. Lico B treatment induced downregulation of anti-apoptotic proteins (Bid and Bcl-xl and Mcl-1), and up-regulation of pro-apoptotic protein (Bax). Lico B also led to the loss of mitochondrial membrane potential (MMP), resulting in cytochrome c release. As can be expected from the above results, the apoptotic protease activating factor-1 (Apaf-1) and survivin were oppositely expressed in favor of apoptotic cell death. This notion was supported by the fact that Lico B activated multi-caspases with cleavage of poly (ADP-ribose) polymerase (PARP) protein. Therefore, it is suggested that Lico B is a promising drug for the treatment of human oral cancer via the induction of apoptotic cell death.

Biodegradable Gelatin Microcarriers Facilitate Re-Epithelialization of Human Cutaneous Wounds - An In Vitro Study in Human Skin

The possibility to use a suspended tridimensional matrix as scaffolding for re-epithelialization of in vitro cutaneous wounds was investigated with the aid of a human in vitro wound healing model based on viable full thickness skin. Macroporous gelatin microcarriers, CultiSpher-S, were applied to in vitro wounds and cultured for 21 days. Tissue sections showed incorporation of wound edge keratinocytes into the microcarriers and thicker neoepidermis in wounds treated with microcarriers. Thickness of the neoepidermis was measured digitally, using immunohistochemical staining of keratins as epithelial demarcation. Air-lifting of wounds enhanced stratification in control wounds as well as wounds with CultiSpher-S. Immunohistochemical staining revealed expression of keratin 5, keratin 10, and laminin 5 in the neoepidermal component. We conclude that the CultiSpher-S microcarriers can function as tissue guiding scaffold for re-epithelialization of cutaneous wounds.

The nanoscale geometrical maturation of focal adhesions controls stem cell differentiation and mechanotransduction

We show that the nanoscale adhesion geometry controls the spreading and differentiation of epidermal stem cells. We find that cells respond to such hard nanopatterns similarly to their behavior on soft hydrogels. Cellular responses were seen to stem from local changes in diffusion dynamics of the adapter protein vinculin and associated impaired mechanotransduction rather than impaired recruitment of proteins involved in focal adhesion formation.

Actin filament dynamics impacts keratinocyte stem cell maintenance

Cultured human epidermal keratinocyte stem cells (holoclones) are crucial for regenerative medicine for burns and genetic disorders. In serial culture, holoclones progressively lose their proliferative capacity to become transient amplifying cells with limited growth (paraclones), a phenomenon termed clonal conversion. Although it negatively impacts the culture lifespan and the success of cell transplantation, little is known on the molecular mechanism underlying clonal conversion. Here, we show that holoclones and paraclones differ in their actin filament organization, with actin bundles distributed radially in holoclones and circumferentially in paraclones. Moreover, actin organization sets the stage for a differing response to epidermal growth factor (EGF), since EGF signalling induces a rapid expansion of colony size in holoclones and a significant reduction in paraclones. Furthermore, inhibition of PI3K or Rac1 in holoclones results in the reorganization of actin filaments in a pattern that is similar to that of paraclones. Importantly, continuous Rac1 inhibition in holoclones results in clonal conversion and reduction of growth potential. Together, our data connect loss of stem cells to EGF-induced colony dynamics governed by Rac1.

Keratinocytes propagated in serum-free, feeder-free culture conditions fail to form stratified epidermis in a reconstituted skin model

Primary human epidermal stem cells isolated from skin tissues and subsequently expanded in tissue culture are used for human therapeutic use to reconstitute skin on patients and to generate artificial skin in culture for academic and commercial research. Classically, epidermal cells, known as keratinocytes, required fibroblast feeder support and serum-containing media for serial propagation. In alignment with global efforts to remove potential animal contaminants, many serum-free, feeder-free culture methods have been developed that support derivation and growth of these cells in 2-dimensional culture. Here we show that keratinocytes grown continually in serum-free and feeder-free conditions were unable to form into a stratified, mature epidermis in a skin equivalent model. This is not due to loss of cell potential as keratinocytes propagated in serum-free, feeder-free conditions retain their ability to form stratified epidermis when re-introduced to classic serum-containing media. Extracellular calcium supplementation failed to improve epidermis development. In contrast, the addition of serum to commercial, growth media developed for serum-free expansion of keratinocytes facilitated 3-dimensional stratification in our skin equivalent model. Moreover, the addition of heat-inactivated serum improved the epidermis structure and thickness, suggesting that serum contains factors that both aid and inhibit stratification.

PIK3CA-mediated PI3-kinase signalling is essential for HPV-induced transformation in vitro

BACKGROUND

High-risk human papillomavirus (hrHPV) infections are causally related to cervical cancer development. The additional (epi)genetic alterations driving malignant transformation of hrHPV-infected cells however, are not yet fully elucidated. In this study we experimentally assessed the role of the PI3-kinase pathway and its regulator PIK3CA, which is frequently altered in cervical cancer, in HPV-induced transformation.

METHODS

Cervical carcinomas and ectocervical controls were assessed for PIK3CA mRNA and protein expression by quantitative RT-PCR and immunohistochemical staining, respectively. A longitudinal in vitro model system of hrHPV-transfected keratinocytes, representing the immortal and anchorage independent phenotype, was assayed for PI3-kinase activation and function using chemical pathway inhibition i.e. LY294002 treatment, and PIK3CA RNA interference. Phenotypes examined included cellular viability, migration, anchorage independent growth and differentiation. mRNA expression of hTERT and HPV16 E6E7 were studied using quantitative RT-PCR and Northern blotting.

RESULTS

Cervical carcinomas showed significant overexpression of PIK3CA compared to controls. During HPV-induced transformation in vitro, expression of the catalytic subunit PIK3CA as well as activation of downstream effector PKB/AKT progressively increased in parallel. Inhibition of PI3-kinase signalling in HPV16-transfected keratinocytes by chemical interference or siRNA-mediated silencing of PIK3CA resulted in a decreased phosphorylation of PKB/AKT. Moreover, blockage of PI3-kinase resulted in reduced cellular viability, migration, and anchorage independent growth. These properties were accompanied with a downregulation of HPV16E7 and hTERT mRNA expression. In organotypic raft cultures of HPV16- and HPV18-immortalized cells, phosphorylated PKB/AKT was primarily seen in differentiated cells staining positive for cytokeratin 10 (CK10). Upon PI3-kinase signalling inhibition, there was a severe impairment in epithelial tissue development as well as a dramatic reduction in p-PKB/AKT and CK10.

CONCLUSION

The present data indicate that activation of the PI3-kinase/PKB/AKT pathway through PIK3CA regulates various transformed phenotypes as well as growth and differentiation of HPV-immortalized cells and may therefore play a pivotal role in HPV-induced carcinogenesis.

Rac1 deletion causes thymic atrophy

The thymic stroma supports T lymphocyte development and consists of an epithelium maintained by thymic epithelial progenitors. The molecular pathways that govern epithelial homeostasis are poorly understood. Here we demonstrate that deletion of Rac1 in Keratin 5/Keratin 14 expressing embryonic and adult thymic epithelial cells leads to loss of the thymic epithelial compartment. Rac1 deletion led to an increase in c-Myc expression and a generalized increase in apoptosis associated with a decrease in thymic epithelial proliferation. Our results suggest Rac1 maintains the epithelial population, and equilibrium between Rac1 and c-Myc may control proliferation, apoptosis and maturation of the thymic epithelial compartment. Understanding thymic epithelial maintenance is a step toward the dual goals of in vitro thymic epithelial cell culture and T cell differentiation, and the clinical repair of thymic damage from graft-versus-host-disease, chemotherapy or irradiation.

Akt-dependent Pp2a activity is required for epidermal barrier formation during late embryonic development

Acquisition of epidermal barrier function occurs late in mouse gestation. Several days before birth a wave of barrier acquisition sweeps across murine fetal skin, converging on dorsal and ventral midlines. We investigated the molecular pathways active during epidermal barrier formation. Akt signaling increased as the barrier wave crossed epidermis and Jun was transiently dephosphorylated. Inhibitor experiments on embryonic explants showed that the dephosphorylation of Jun was dependent on both Akt and protein phosphatase 2A (Pp2a). Inhibition of Pp2a and Akt signaling also caused defects in epidermal barrier formation. These data are compatible with a model for developmental barrier acquisition mediated by Pp2a regulation of Jun dephosphorylation, downstream of Akt signaling. Support for this model was provided by siRNA-mediated knockdown of Ppp2r2a (Pr55alpha or B55alpha), a regulatory subunit of Pp2a expressed in an Akt-dependent manner in epidermis during barrier formation. Ppp2r2a reduction caused significant increase in Jun phosphorylation and interfered with the acquisition of barrier function, with barrier acquisition being restored by inhibition of Jun phosphorylation. Our data provide strong evidence that Ppp2r2a is a regulatory subunit of Pp2a that targets this phosphatase to Jun, and that Pp2a action is necessary for barrier formation. We therefore describe a novel Akt-dependent Pp2a activity that acts at least partly through Jun to affect initial barrier formation during late embryonic epidermal development.