Involvement of protein kinase Cdelta in contact-dependent inhibition of growth in human and murine fibroblasts

Cell-cell contacts prevent t-BuOOH-triggered ferroptosis and cellular damage in vitro by regulation of intracellular calcium

Tert-butyl hydroperoxide (t-BuOOH) is an organic hydroperoxide widely used as a model compound to induce oxidative stress. It leads to a plethora of cellular damage, including lipid peroxidation, DNA double-strand breaks (DNA DSBs), and breakdown of the mitochondrial membrane potential (MMP). We could show in several cell lines that t-BuOOH induces ferroptosis, triggered by iron-dependent lipid peroxidation. We have further revealed that not only t-BuOOH-mediated ferroptosis, but also DNA DSBs and loss of MMP are prevented by cell-cell contacts. The underlying mechanisms are not known. Here, we show in murine fibroblasts and a human colon carcinoma cell line that t-BuOOH (50 or 100 µM, resp.) causes an increase in intracellular Ca2+, and that this increase is key to lipid peroxidation and ferroptosis, DNA DSB formation and dissipation of the MMP. We further demonstrate that cell-cell contacts prevent t-BuOOH-mediated raise in intracellular Ca2+. Hence, we provide novel insights into the mechanism of t-BuOOH-triggered cellular damage including ferroptosis and propose a model in which cell-cell contacts control intracellular Ca2+ levels to prevent lipid peroxidation, DNA DSB-formation and loss of MMP. Since Ca2+ is a central player of toxicity in response to oxidative stress and is involved in various cell death pathways, our observations suggest a broad protective function of cell-cell contacts against a variety of exogenous toxicants.

Modelling the spatiotemporal dynamics of senescent cells in wound healing, chronic wounds, and fibrosis

Cellular senescence is known to drive age-related pathology through the senescence-associated secretory phenotype (SASP). However, it also plays important physiological roles such as cancer suppression, embryogenesis and wound healing. Wound healing is a tightly regulated process which when disrupted results in conditions such as fibrosis and chronic wounds. Senescent cells appear during the proliferation phase of the healing process where the SASP is involved in maintaining tissue homeostasis after damage. Interestingly, SASP composition and functionality was recently found to be temporally regulated, with distinct SASP profiles involved: a fibrogenic, followed by a fibrolytic SASP, which could have important implications for the role of senescent cells in wound healing. Given the number of factors at play a full understanding requires addressing the multiple levels of complexity, pertaining to the various cell behaviours, individually followed by investigating the interactions and influence each of these elements have on each other and the system as a whole. Here, a systems biology approach was adopted whereby a multi-scale model of wound healing that includes the dynamics of senescent cell behaviour and corresponding SASP composition within the wound microenvironment was developed. The model was built using the software CompuCell3D, which is based on a Cellular Potts modelling framework. We used an existing body of data on healthy wound healing to calibrate the model and validation was done on known disease conditions. The model provides understanding of the spatiotemporal dynamics of different senescent cell phenotypes and the roles they play within the wound healing process. The model also shows how an overall disruption of tissue-level coordination due to age-related changes results in different disease states including fibrosis and chronic wounds. Further specific data to increase model confidence could be used to explore senolytic treatments in wound disorders.

Ferroptosis Meets Cell-Cell Contacts

Ferroptosis is a regulated form of cell death characterized by iron dependency and increased lipid peroxidation. Initially assumed to be selectively induced in tumour cells, there is increasing evidence that ferroptosis plays an important role in pathophysiology and numerous cell types and tissues. Deregulated ferroptosis has been linked to human diseases, such as neurodegenerative diseases, cardiovascular disorders, and cancer. Along these lines, ferroptosis is a promising pathway to overcoming therapy resistance of cancer cells. It is therefore of utmost importance to understand the cellular signalling pathways and the molecular mechanisms underlying ferroptosis regulation, including context-specific effects mediated by the neighbouring cells through cell–cell contacts. Here, we give an overview on the molecular events and machinery linked to ferroptosis induction and commitment. We further summarize and discuss current knowledge about the role of cell–cell contacts, which differ in ferroptosis regulation between normal somatic cells and cancer cells. We present emerging concepts on the underlying mechanisms, address open questions, and discuss the possible impact of cell–cell contacts on exploiting ferroptosis in cancer therapy.

Cell-cell contacts protect against t-BuOOH-induced cellular damage and ferroptosis in vitro

Ferroptosis is a recently discovered pathway of regulated necrosis dependent on iron and lipid peroxidation. It has gained broad attention since it is a promising approach to overcome resistance to apoptosis in cancer chemotherapy. We have recently identified tertiary-butyl hydroperoxide (t-BuOOH) as a novel inducer of ferroptosis. t-BuOOH is a widely used compound to induce oxidative stress in vitro. t-BuOOH induces lipid peroxidation and consequently ferroptosis in murine and human cell lines. t-BuOOH additionally results in a loss of mitochondrial membrane potential, formation of DNA double-strand breaks, and replication block. Here, we specifically address the question whether cell-cell contacts regulate t-BuOOH-induced ferroptosis and cellular damage. To this end, murine NIH3T3 or human HaCaT cells were seeded to confluence, but below their saturation density to allow the establishment of cell-cell contacts without inducing quiescence. Cells were then treated with t-BuOOH (50 or 200 µM, respectively). We revealed that cell-cell contacts reduce basal and t-BuOOH-triggered lipid peroxidation and consequently block ferroptosis. Similar results were obtained with the specific ferroptosis inducer erastin. Cell-cell contacts further protect against t-BuOOH-induced loss of mitochondrial membrane potential, and formation of DNA double-strand breaks. Interestingly, cell-cell contacts failed to prevent t-BuOOH-mediated replication block or formation of the oxidative base lesion 8-oxo-dG. Since evidence of protection against cell death was both (i) observed after treatment with hydrogen peroxide, methyl methanesulfonate or UV-C, and (ii) seen in several cell lines, we conclude that protection by cell-cell contacts is a widespread phenomenon. The impact of cell-cell contacts on toxicity might have important implications in cancer chemotherapy.

Sleeping Beauty and the Microenvironment Enchantment: Microenvironmental Regulation of the Proliferation-Quiescence Decision in Normal Tissues and in Cancer Development

Cells from prokaryota to the more complex metazoans cease proliferating at some point in their lives and enter a reversible, proliferative-dormant state termed quiescence. The appearance of quiescence in the course of evolution was essential to the acquisition of multicellular specialization and compartmentalization and is also a central aspect of tissue function and homeostasis. But what makes a cell cease proliferating even in the presence of nutrients, growth factors, and mitogens? And what makes some cells "wake up" when they should not, as is the case in cancer? Here, we summarize and discuss evidence showing how microenvironmental cues such as those originating from metabolism, extracellular matrix (ECM) composition and arrangement, neighboring cells and tissue architecture control the cellular proliferation-quiescence decision, and how this complex regulation is corrupted in cancer.

The Hippo-YAP signaling pathway and contact inhibition of growth

The Hippo-YAP pathway mediates the control of cell proliferation by contact inhibition as well as other attributes of the physical state of cells in tissues. Several mechanisms sense the spatial and physical organization of cells, and function through distinct upstream modules to stimulate Hippo-YAP signaling: adherens junction or cadherin-catenin complexes, epithelial polarity and tight junction complexes, the FAT-Dachsous morphogen pathway, as well as cell shape, actomyosin or mechanotransduction. Soluble extracellular factors also regulate Hippo pathway signaling, often inhibiting its activity. Indeed, the Hippo pathway mediates a reciprocal relationship between contact inhibition and mitogenic signaling. As a result, cells at the edges of a colony, a wound in a tissue or a tumor are more sensitive to ambient levels of growth factors and more likely to proliferate, migrate or differentiate through a YAP and/or TAZ-dependent process. Thus, the Hippo-YAP pathway senses and responds to the physical organization of cells in tissues and coordinates these physical cues with classic growth-factor-mediated signaling pathways. This Commentary is focused on the biological significance of Hippo-YAP signaling and how upstream regulatory modules of the pathway interact to produce biological outcomes.

Involvement of the transcription factor FoxM1 in contact inhibition

Contact inhibition is a crucial mechanism regulating proliferation in vitro and in vivo. Although it is generally accepted that contact inhibition plays a pivotal role in maintaining tissue homeostasis, the molecular mechanisms of contact inhibition are still not fully understood. FoxM1 is known as a proliferation-associated transcription factor and is upregulated in many cancer types. Vice versa, anti-proliferative signals, such as TGF-β and differentiation signals decrease FoxM1 expression. Here we investigated the role of FoxM1 in contact inhibition in fibroblasts. We show that protein expression of FoxM1 is severely and rapidly downregulated upon contact inhibition, probably by inhibition of ERK activity, which then leads to decreased expression of cyclin A and polo-like kinase 1. Vice versa, ectopic expression of FoxM1 prevents the decrease in cyclin A and polo-like kinase 1 and causes a two-fold increase in saturation density indicating loss of contact inhibition. Hence, we show that downregulation of FoxM1 is required for contact inhibition by regulating expression of cyclin A and polo-like kinase 1.

Protein kinases and phosphatases in the control of cell fate

Protein phosphorylation controls many aspects of cell fate and is often deregulated in pathological conditions. Several recent findings have provided an intriguing insight into the spatial regulation of protein phosphorylation across different subcellular compartments and how this can be finely orchestrated by specific kinases and phosphatases. In this review, the focus will be placed on (i) the phosphoinositide 3-kinase (PI3K) pathway, specifically on the kinases Akt and mTOR and on the phosphatases PP2a and PTEN, and on (ii) the PKC family of serine/threonine kinases. We will look at general aspects of cell physiology controlled by these kinases and phosphatases, highlighting the signalling pathways that drive cell division, proliferation, and apoptosis.

PKCδ sensitizes neuroblastoma cells to L-buthionine-sulfoximine and etoposide inducing reactive oxygen species overproduction and DNA damage

Neuroblastoma is a type of pediatric cancer. The sensitivity of neuroblastoma (NB) cancer cells to chemotherapy and radiation is inhibited by the presence of antioxidants, such as glutathione (GSH), which is crucial in counteracting the endogenous production of reactive oxygen species (ROS). We have previously demonstrated that cells depleted of GSH undergo apoptosis via oxidative stress and Protein kinase C (PKC) δ activation. In the present study, we transfected PKCδ in NB cells resistant to oxidative death induced by L-buthionine-S,R-sulfoximine (BSO), a GSH-depleting agent. Cell responses, in terms of ROS production, apoptosis and DNA damage were evaluated. Moreover, PKCδ activation was monitored by analyzing the phosphorylation status of threonine 505 residue, carrying out PKC activity assay and investigating the subcellular localization of the kinase. The cell responses obtained in BSO-resistant cells were also compared with those obtained in BSO-sensitive cells subjected to the same experimental protocol. Our results demonstrate, for the first time, that PKCδ induces DNA oxidation and ROS overproduction leading to apoptosis of BSO-resistant NB cells and potentiates the cytotoxic effects induced by BSO in sensitive cells. Moreover, PKCδ overexpression enhances the sensitivity of NB cells to etoposide, a well-characterised drug, commonly used in neuroblastoma therapy. Altogether our data provide evidence of a pro-oxidant role of PKCδ that might be exploited to design new therapeutic strategies aimed at selective killing of cancer cells and overcoming drug resistance. However, it becomes evident that a more detailed understanding of ROS-mediated signaling in cancer cells is necessary for the development of redox-modulated therapeutic approaches.

The transcriptional programme of contact-inhibition

Proliferation of non-transformed cells is regulated by cell-cell contacts, which are referred to as contact-inhibition. Vice versa, transformed cells are characterised by a loss of contact-inhibition. Despite its generally accepted importance for cell-cycle control, little is known about the intracellular signalling pathways involved in contact-inhibition. Unravelling the molecular mechanisms of contact-inhibition and its loss during tumourigenesis will be an important step towards the identification of novel target genes in tumour diagnosis and treatment. To better understand the underlying molecular mechanisms we identified the transcriptional programme of contact-inhibition in NIH3T3 fibroblast using high-density microarrays. Setting the cut off: >or=1.5-fold, P <or= 0.05, 853 genes and 73 cDNA sequences were differentially expressed in confluent compared to exponentially growing cultures. Importing these data into GenMAPP software revealed a comprehensive list of cell-cycle regulatory genes mediating G0/G1 arrest, which was confirmed by RT-PCR and Western blot. In a narrow analysis (cut off: >or=2-fold, P <or= 0.002), we found 110 transcripts to be differentially expressed representing 107 genes and 3 cDNA sequences involved, for example, in proliferation, signal transduction, transcriptional regulation, cell adhesion and communication. Interestingly, the majority of genes was upregulated indicating that contact-inhibition is not a passive state, but actively induced. Furthermore, we confirmed differential expression of eight genes by semi-quantitative RT-PCR and identified the potential tumour suppressor transforming growth factor-beta (TGF-beta)-1-induced clone 22 (TSC-22; tgfb1i4) as a novel protein to be induced in contact-inhibited cells.

The aryl hydrocarbon receptor (AhR) in the regulation of cell-cell contact and tumor growth

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor, which is activated by a large group of environmental pollutants including polycyclic aromatic hydrocarbons, dioxins and planar polychlorinated biphenyls. Ligand binding leads to dimerization of the AhR with aryl hydrocarbon receptor nuclear translocator and transcriptional activation of several xenobiotic phase I and phase II metabolizing enzymes, such as cytochrome P4501A1 and glutathione-S-transferase, respectively. Since phase I enzymes convert inert carcinogens to active genotoxins, the AhR plays a key role in tumor initiation. Besides this classical route, the AhR mediates tumor promotion and recent evidence suggests that the AhR also plays a role in tumor progression. To date, no mechanistic link could be established between the canonical pathway involving xenobiotic metabolism and AhR-dependent tumor promotion and progression. A hallmark of tumor promotion is unbalanced proliferation, whereas tumor progression is characterized by dedifferentiation, increased motility and metastasis of tumor cells. Tumor progression and presumably also tumor promotion are triggered by loss of cell-cell contact. Cell-cell contact is known to be a critical regulator of proliferation, differentiation and cell motility in vitro and in vivo. Increasing evidence suggests that activation of the AhR may lead to deregulation of cell-cell contact, thereby inducing unbalanced proliferation, dedifferentiation and enhanced motility. In line with this is the finding of increased AhR expression and malignancy in some animal and human cancers. Here, we summarize our current knowledge on non-canonical AhR-driven pathways being involved in deregulation of cell-cell contact and discuss the data with respect to tumor initiation, promotion and progression.

Neutral sphingomyelinase 2 (nSMase2) is a phosphoprotein regulated by calcineurin (PP2B)

We previously reported that exposure of human airway epithelial cells to oxidative stress increased ceramide generation via specific activation of neutral sphingomyelinase2 (nSMase2). Here we show that nSMase2 is a phosphoprotein exclusively phosphorylated at serine residues. The level of nSMase2 phosphorylation can be modulated by treatment with anisomycin or phorbol 12-myristate 13-acetate (PMA/12-O-tetradecanoylphorbol-13-acetate), suggesting that p38 mitogen-activated protein kinase (MAPK) and protein kinases Cs are upstream of nSMase2 phosphorylation. Oxidative stress enhances both the activity and phosphorylation of nSMase2. Strikingly, we show here that nSMase2 is bound directly by the phosphatase calcineurin (CaN), which acts as an on/off switch for nSMase2 phosphorylation in the presence or absence of oxidative stress. Specifically, CaN is being inhibited/degraded and therefore does not bind nSMase2 under oxidative stress, and a mutant nSMase2 that lacks the CaN binding site exhibits constitutively elevated phosphorylation and increased activity relative to wild type nSMase2. Importantly, the phosphorylation and activity of the mutant no longer responds to oxidative stress, confirming that CaN is the critical link that allows oxidative stress to modulate nSMase2 phosphorylation and function.

Sparse plating increases the heterogeneity of proliferative potential of fibroblasts

High heterogeneity of proliferative potential in the cultures of diploid human fibroblasts was reported in many studies. It was generally believed that the heterogeneity of proliferative potential of human fibroblasts reflects the unevenness of their senescence. However we show here that immortalized (telomerized) human fibroblasts obey the same rule. Up to 50% of these cells rapidly ceased to proliferate when plated at low density in contrast to usual conditions of mass culture where at least 98% of these cells keep on proliferating. Initially, we proposed that the appearance of non-dividing or slow-dividing cells in low-density cell culture experiments could be caused by cell damage due to the experimental setup. Indeed, lowering of oxygen level and addition of conditioned medium improved colony formation, but there were a large number of non-proliferating cells (13-20%). When we sparsely plated cells on a feeder layer of cells of certain density, the portion of non-proliferating cells decreased to 2%, i.e. became the same as in mass culture. Thus, the heterogeneity of proliferative potential is partially a result of the adverse effect of low cell density.

Exploitation of protein kinase C: a useful target for cancer therapy

Protein kinase C is a family of serine/threonine kinases. The PKC family is made up of at least 12 isozymes, which have a role in cell proliferation, differentiation, angiogenesis, and apoptosis. Activation of PKC isozyme is dependent on tyrosine-kinase receptors and G-protein-coupled receptors. PKC isozymes regulate multiple signaling pathways including PI3-K/Akt, MAPK, and GSK-3beta. PKC isozymes have variable roles in tumor biology which in part depend on the cell type and intracellular localization. PKC isozymes are commonly dysregulated in the cancer of the prostate, breast, colon, pancreatic, liver, and kidney. Currently, several classes of PKC inhibitors are being evaluated in clinical trials and several challenges in targeting PKC isozymes have been recently identified. In conclusion, PKC remains a promising target for cancer prevention and therapy.

Counter regulatory effects of PKCbetaII and PKCdelta on coronary endothelial permeability

OBJECTIVE

The aim of this study was to examine the endothelial distribution and activity of selected PKC isoforms in coronary vessels with respect to their functional impact on endothelial permeability under the experimental conditions relevant to diabetes.

METHODS AND RESULTS

En face immunohistochemistry demonstrated a significant increase of PKC(betaII) and decrease of PKCdelta expression in coronary arterial endothelium of Zucker diabetic rats. To test whether changes in PKC expression alter endothelial barrier properties, we measured the transcellular electric resistance in human coronary microvascular endothelial monolayers and found that either PKC(betaII) overexpression or PKCdelta inhibition disrupted the cell-cell adhesive barrier. Three-dimensional fluorescence microscopy revealed that hyperpermeability was caused by altered PKC activity in association with distinct translocation of PKC(betaII) to the cell-cell junction and PKCdelta localization to the cytosol. Further analyses in fractionated endothelial lysates confirmed the differential redistribution of these isozymes. Additionally, FRET analysis of PKC subcellular dynamics demonstrated a high PKC(betaII) activity at the cell surface and junction, whereas PKCdelta activity is concentrated in intracellular membrane organelles.

CONCLUSIONS

Taken together, these data suggest that PKC(betaII) and PKCdelta counter-regulate coronary endothelial barrier properties by targeting distinctive subcellular sites. Imbalanced PKC(betaII)/PKCdelta expression and activity may contribute to endothelial hyperpermeability and coronary dysfunction in diabetes.

Protein kinase C delta regulates airway mucin secretion via phosphorylation of MARCKS protein

Mucin hypersecretion is a major pathological feature of many respiratory diseases, yet cellular mechanisms regulating secretion of mucin have not been fully elucidated. Previously, we reported that mucin hypersecretion induced by human neutrophil elastase involves activation of protein kinase C (PKC), specifically the delta-isoform (PKC delta). Here, we further investigated the role of PKC delta in mucin hypersecretion using both primary human bronchial epithelial cells and the human bronchial epithelial 1 cell line as in vitro model systems. Phorbol-12-myristate-13-acetate (PMA)-induced mucin hypersecretion was significantly attenuated by rottlerin, a PKC delta-selective inhibitor. Rottlerin also reduced PMA- or human neutrophil elastase-induced phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS) protein in these cells. Both secretion and MARCKS phosphorylation were significantly enhanced by the PKC delta activator bryostatin 1. A dominant-negative PKC delta construct (pEGFP-N1/PKC delta K376R) transfected into human bronchial epithelial 1 cells significantly attenuated both PMA-induced mucin secretion and phosphorylation of MARCKS, whereas transfection of a wild-type construct increased PKC delta and enhanced mucin secretion and MARCKS phosphorylation. Similar transfections of a dominant-negative or wild-type PKC epsilon construct did not affect either mucin secretion or MARCKS phosphorylation. The results suggest that PKC delta plays an important role in mucin secretion by airway epithelium via regulation of MARCKS phosphorylation.

TCDD deregulates contact inhibition in rat liver oval cells via Ah receptor, JunD and cyclin A

The aryl hydrocarbon receptor (AhR) is a transcription factor involved in physiological processes, but also mediates most, if not all, toxic responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Activation of the AhR by TCDD leads to its dimerization with aryl hydrocarbon nuclear translocator (ARNT) and transcriptional activation of several phase I and II metabolizing enzymes. However, this classical signalling pathway so far failed to explain the pleiotropic hazardous effects of TCDD, such as developmental toxicity and tumour promotion. Thus, there is an urgent need to define genetic programmes orchestrated by AhR to unravel its role in physiology and toxicology. Here we show that TCDD treatment of rat liver oval cells leads to induction of the transcription factor JunD, resulting in transcriptional upregulation of the proto-oncogene cyclin A which finally triggers a release from contact inhibition. Ectopic expression of cyclin A in confluent cultures overcomes G(1) arrest, indicating that increased cyclin A levels are indeed sufficient to bypass contact inhibition. Functional interference with AhR-, but not with ARNT, abolished TCDD-induced increase in JunD and cyclin A and prevented loss of contact inhibition. In summary, we have discovered a novel AhR-dependent and probably ARNT-independent signalling pathway involving JunD and cyclin A, which mediates TCDD-induced deregulation of cell cycle control.

Protein kinase C family: on the crossroads of cell signaling in skin and tumor epithelium

The protein kinase C (PKC) family represents a large group of phospholipid dependent enzymes catalyzing the covalent transfer of phosphate from ATP to serine and threonine residues of proteins. Phosphorylation of the substrate proteins induces a conformational change resulting in modification of their functional properties. The PKC family consists of at least ten members, divided into three subgroups: classical PKCs (alpha, betaI, betaII, gamma), novel PKCs (delta, epsilon, eta, theta), and atypical PKCs (zeta, iota/lambda). The specific cofactor requirements, tissue distribution, and cellular compartmentalization suggest differential functions and fine tuning of specific signaling cascades for each isoform. Thus, specific stimuli can lead to differential responses via isoform specific PKC signaling regulated by their expression, localization, and phosphorylation status in particular biological settings. PKC isoforms are activated by a variety of extracellular signals and, in turn, modify the activities of cellular proteins including receptors, enzymes, cytoskeletal proteins, and transcription factors. Accordingly, the PKC family plays a central role in cellular signal processing. Accumulating data suggest that various PKC isoforms participate in the regulation of cell proliferation, differentiation, survival and death. These findings have enabled identification of abnormalities in PKC isoform function, as they occur in several cancers. Specifically, the initiation of squamous cell carcinoma formation and progression to the malignant phenotype was found to be associated with distinct changes in PKC expression, activation, distribution, and phosphorylation. These studies were recently further extended to transgenic and knockout animals, which allowed a more direct analysis of individual PKC functions. Accordingly, this review is focused on the involvement of PKC in physiology and pathology of the skin.

Protein kinase C and other diacylglycerol effectors in cancer

Almost three decades after the discovery of protein kinase C (PKC), we still have only a partial understanding of how this family of serine/threonine kinases is involved in tumour promotion. PKC isozymes - effectors of diacylglycerol (DAG) and the main targets of phorbol-ester tumour promoters - have important roles in cell-cycle regulation, cellular survival, malignant transformation and apoptosis. How do PKC isozymes regulate these diverse cellular processes and what are their contributions to carcinogenesis? Moreover, what is the contribution of all phorbol-ester effectors, which include PKCs and small G-protein regulators? We now face the challenge of dissecting the relative contribution of each DAG signal to cancer progression.

p38alpha MAPK is required for contact inhibition

Proliferation of nontransformed cells is regulated by cell-cell contacts, which are referred to as contact-inhibition. Despite its generally accepted importance for cell cycle control, knowledge about the intracellular signalling pathways involved in contact inhibition is scarce. In the present work we show that p38alpha mitogen-activated protein kinase (MAPK) is involved in the growth-inhibitory signalling cascade of contact inhibition in fibroblasts. p38alpha activity is increased in confluent cultures of human fibroblasts compared to proliferating cultures. Time course studies show a sustained activation of p38alpha in response to cell-cell contacts in contrast to a transient activation after serum stimulation. The induction of contact inhibition by addition of glutaraldehyde-fixed cells is impaired by pharmacological inhibition of p38 as well as in p38alpha-/- fibroblasts. Further evidence for a central role of p38alpha in contact inhibition comes from the observation that p38alpha-/- fibroblasts show a higher saturation density compared to wild-type (wt) fibroblasts, which is reversed by reconstituted expression of p38alpha. In agreement with a defect in contact inhibition, p27(Kip1) accumulation is impaired in p38alpha-/- fibroblasts compared to wt fibroblasts. Hence, our work shows a new role for p38alpha in contact inhibition and provides a mechanistic basis for the recently proposed tumour suppressive function of this MAPK pathway.

The enigmatic protein kinase Cdelta: complex roles in cell proliferation and survival

Protein kinase Cdelta (PKCdelta) has been implicated both as a tumor suppressor and a positive regulator of cell cycle progression. PKCdelta has also been reported to positively and negatively regulate apoptotic programs. This has led to conflicting hypotheses on the role of PKCdelta in the control of cell proliferation and survival. Surprisingly, PKCdelta mice develop normally and are fertile, indicating that PKCdelta is not critical for normal cell proliferation during development. However, PKCdelta may play important roles in neoplastic cell proliferation. In this review, we have summarized the apparent multifunctional properties of this enigmatic protein with regard to its role in the regulation of cell cycle progression and cell survival. It is proposed that PKCdelta has both tumor suppressor and proliferation capabilities that can be recruited as a backup kinase for both gatekeeper tumor suppression and as an activator of the Ras/Raf/MEK/MAP kinase signaling pathway in cell proliferation.

Inhibition of human p53 basal transcription by down-regulation of protein kinase Cdelta

In response to DNA damage, signal transduction pathways are activated that result in the increase of p53 protein levels, leading to either growth arrest or apoptosis. Protein kinase C (PKC) delta has been implicated as a tumor suppressor that is down-regulated by tumor-promoting phorbol esters in both mouse skin and cell culture models. We report here that the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate prevents DNA damage-induced up-regulation of p53 by down-regulating PKC delta. Regulation of p53 in response to stress most commonly occurs by preventing ubiquitination and degradation of the p53 protein. Surprisingly, suppression of p53 expression by inhibition of PKC delta was caused by the inhibition of p53 synthesis, not increased degradation of p53 protein. Inhibiting PKC delta blocked both basal transcription of the human p53 gene and initiation of transcription from the human p53 promoter. Therefore, the tumor-suppressing effects of PKC delta are mediated at least in part through activating p53 transcription.

Synthesis of a simplified bryostatin C-ring analogue that binds to the CRD2 of human PKC-alpha and construction of a novel BC-analogue by an unusual Julia olefination process

[structure: see text] The synthesis of two truncated bryostatin analogues 2 and 3 is described. High-field NMR measurements on the C-ring analogue 3 in C(2)H(3)CN containing 25% (2)H(2)O have shown that it binds to the CRD2 of human PKC-alpha at virtually the same position as phorbol-13-acetate (PA) and bryostatin 1 (1). NMR titration studies have also revealed that 3 binds to the CRD2 with a potency similar in magnitude to PA but much less potently than 1.

Phosphorylation is required for PMA- and cell-cycle-induced degradation of protein kinase Cdelta

Classical and novel protein kinase C (PKC) isoforms are down-regulated as a result of chronic activation by certain tumour promoters and physiological stimuli; however, the mechanisms leading to down-regulation are not fully understood. In the present study, we have studied the PMA ('TPA')-induced degradation of PKCdelta in NIH 3T3 cells under culture conditions where PKCdelta displays cell-cycle-dependent down-regulation. In contrast with previous studies, a hyperphosphorylated form of this PKC isoform, promoted by calyculin A, was rapidly degraded in PMA-treated cells. Similarly, the presence of calyculin A enhanced the down-regulation of PKCdelta observed on G(1)/S-phase progression through the cell cycle. Analysis of phosphorylation-site mutants indicated that the T-loop Thr(505) phosphorylation site was critical for induced degradation.