PKCeta associates with cyclin E/cdk2/p21 complex, phosphorylates p21 and inhibits cdk2 kinase in keratinocytes

Hepatic stellate cell senescence in liver fibrosis: Characteristics, mechanisms and perspectives

Myofibroblasts play an important role in fibrogenesis. Hepatic stellate cells are the main precursors of myofibroblasts. Cellular senescence is the terminal cell fate in which proliferating cells undergo irreversible cell cycle arrest. Senescent hepatic stellate cells were identified in liver fibrosis. Senescent hepatic stellate cells display decreased collagen production and proliferation. Therefore, induction of senescence could be a protective mechanism against progression of liver fibrosis and the concept of therapy-induced senescence has been proposed to treat liver fibrosis. In this review, characteristics of senescent hepatic stellate cells and the essential signaling pathways involved in senescence are reviewed. Furthermore, the potential impact of senescent hepatic stellate cells on other liver cell types are discussed. Senescent cells are cleared by the immune system. The persistence of senescent cells can remodel the microenvironment and interact with inflammatory cells to induce aging-related dysfunction. Therefore, senolytics, a class of compounds that selectively induce death of senescent cells, were introduced as treatment to remove senescent cells and consequently decrease the disadvantageous effects of persisting senescent cells. The effects of senescent hepatic stellate cells in liver fibrosis need further investigation.

Effect of extracorporeal shock wave therapy on keratinocytes derived from human hypertrophic scars

Hypertrophic scars represent a common complication in burn patients. In addition to cosmetic defects, they may cause serious sensory abnormalities such as pain and itching, severe dysfunction depending on the site, and emotional disorders such as anxiety and depression. The present study aimed to identify the molecular mechanisms underlying the use of extracorporeal shock wave therapy in keratinocytes. Keratinocytes derived from hypertrophic scar tissue were cultured and expression of proliferation markers (keratin 5 and 14), activation markers (keratin 6 and 17), differentiation markers (keratin 1, 10, and involucrin), apoptosis factors (Bax, Bcl2, and Caspase 14), and proliferation/differentiation regulators (p21 and p27) was investigated to compared with that of those in keratinocytes derived from normal skin tissue. Scar-derived keratinocytes were treated with extracorporeal shock waves under 1000 impulses at 0.1, 0.2, and 0.3 mJ/mm². Shock waves altered the molecular pattern of proliferation, activation, differentiation, and apoptosis, as well as proliferation/ differentiation regulators, including Bax, Bcl2, ASK1, p21, p27, and Notch1. In summary, we show that extracorporeal shock wave therapy regulates the proliferation and differentiation of keratinocytes derived from hypertrophic scar to maintain normal epidermal integrity.

Subunits of the GPI transamidase complex localize to the endoplasmic reticulum and nuclear envelope in Drosophila

A total of 10-20% of plasma membrane proteins are anchored by glycosylphosphatidylinositol (GPI). GPI is attached to proteins by GPI transamidase (GPI-T), which contains five subunits named PIGK, PIGS, PIGT, PIGU, and GPAA1. We previously reported that PIGT localizes near the nucleus in Drosophila. However, localizations of the other four subunits remain unknown. Here, we show that a catalytic subunit of GPI-T, PIGK, mainly localizes to the endoplasmic reticulum (ER), while the other four subunits localize to the nuclear envelope (NE) and ER. The NE/ER localization ratio of PIGS differs between cell types and developmental stages. Our results suggest that GPI-T catalyzes GPI attachment in the ER and the other four subunits may have other unknown functions in the NE.

Equivocal, explicit and emergent actions of PKC isoforms in cancer

The maturing mutational landscape of cancer genomes, the development and application of clinical interventions and evolving insights into tumour-associated functions reveal unexpected features of the protein kinase C (PKC) family of serine/threonine protein kinases. These advances include recent work showing gain or loss-of-function mutations relating to driver or bystander roles, how conformational constraints and plasticity impact this class of proteins and how emergent cancer-associated properties may offer opportunities for intervention. The profound impact of the tumour microenvironment, reflected in the efficacy of immune checkpoint interventions, further prompts to incorporate PKC family actions and interventions in this ecosystem, informed by insights into the control of stromal and immune cell functions. Drugging PKC isoforms has offered much promise, but when and how is not obvious.

Long non-coding RNA FOXD2-AS1 promotes cell proliferation, metastasis and EMT in glioma by sponging miR-506-5p

Long non-coding RNA forkhead box D2 adjacent opposite strand RNA 1 (FOXD2-AS1) has emerged as a potential oncogene in several tumors. However, its biological function and potential regulatory mechanism in glioma have not been fully investigated to date. In the present study, RT-qPCR was conducted to detect the levels of FOXD2-AS1 and microRNA (miR)-506-5p, and western blot assays were performed to measure the expression of CDK2, cyclinE1, P21, matrix metalloproteinase (MMP)7, MMP9, N-cadherin, E-cadherin and vimentin in glioma cells. A luciferase reporter assay was performed to verify the direct targeting of miR-506-5p by FOXD2-AS1. Subsequently, cell viability was analyzed using the CCK-8 assay. Cell migration and invasion were analyzed using Transwell and wound healing assays, respectively. The results demonstrated that FOXD2-AS1 was significantly overexpressed in glioma cells, particularly in U251 cells. Knockdown of FOXD2-AS1 in glioma cells significantly inhibited cell proliferation, migration, invasion and epithelial–mesenchymal transition (EMT) and regulated the expression of CDK2, cyclinE1, P21, MMP7 and MMP9. Next, a possible mechanism for these results was explored, and it was observed that FOXD2-AS1 binds to and negatively regulates miR-506-5p, which is known to be a tumor-suppressor gene in certain human cancer types. Furthermore, overexpression of miR-506-5p significantly inhibited cell proliferation, migration, invasion and EMT, and these effects could be reversed by transfecting FOXD2-AS1 into the cells. In conclusion, our data suggested that FOXD2-AS1 contributed to glioma proliferation, metastasis and EMT via competitively binding to miR-506-5p. FOXD2-AS1 may be a promising target for therapy in patients with glioma.

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

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

The Enigmatic Protein Kinase C-eta

Protein kinase C (PKC), a multi-gene family, plays critical roles in signal transduction and cell regulation. Protein kinase C-eta (PKCη) is a unique member of the PKC family since its regulation is distinct from other PKC isozymes. PKCη was shown to regulate cell proliferation, differentiation and cell death. It was also shown to contribute to chemoresistance in several cancers. PKCη has been associated with several cancers, including renal cell carcinoma, glioblastoma, breast cancer, non-small cell lung cancer, and acute myeloid leukemia. However, mice lacking PKCη were more susceptible to tumor formation in a two-stage carcinogenesis model, and it is downregulated in hepatocellular carcinoma. Thus, the role of PKCη in cancer remains controversial. The purpose of this review article is to discuss how PKCη regulates various cellular processes that may contribute to its contrasting roles in cancer.

miR-24-3p Suppresses Malignant Behavior of Lacrimal Adenoid Cystic Carcinoma by Targeting PRKCH to Regulate p53/p21 Pathway

MicroRNA (miRNA) may function as an oncogene or a tumor suppressor in tumorigenesis. However, the mechanism of miRNAs in adenoid cystic carcinoma (ACC) is unclear. Here, we provide evidence that miR-24-3p was downreglated and functions as a tumor suppressor in human lacrimal adenoid cystic carcinoma by suppressing proliferation and migration/invasion while promoting apoptosis. miR-24-3p down-regulated protein kinase C eta (PRKCH) by binding to its untranslated region (3’UTR). PRKCH increased the of the cell growth and migration/invasion in ACC cells and suppressed the expression of p53 and p21 in both mRNA and protein level. The overexpression of miR-24-3p decreased its malignant phenotype. Ectopic expression of PRKCH counteracted the suppression of malignancy induced by miR-24-3p, as well as ectopic expression of miR-24-3p rescued the suppression of PRKCH in the p53/p21 pathway. These results suggest that miR-24-3p promotes the p53/p21 pathway by down-regulating PRKCH expression in lacrimal adenoid cystic carcinoma cells.

PKCη is an anti-apoptotic kinase that predicts poor prognosis in breast and lung cancer

The successful treatment of cancer in a disseminated stage using chemotherapy is limited by the occurrence of drug resistance, often mediated by anti-apoptotic mechanisms. Thus the challenge is to pinpoint the underlying key factors and to develop therapies for their direct targeting. Protein kinase C (PKC) enzymes are promising candidates, as some PKCs were shown to be involved in regulation of apoptosis. Our studies and others have shown that PKCη is an anti-apoptotic kinase, able to confer protection on tumour cells against stress and chemotherapy. We have demonstrated that PKCη shuttles between the cytoplasm and the nucleus and that upon DNA damage is tethered at the nuclear membrane. The C1b domain mediates translocation of PKCη to the nuclear envelope and, similar to the full-length protein, could also confer protection against cell death. Furthermore, its localization in cell and nuclear membranes in breast cancer biopsies of neoadjuvant-treated breast cancer patients was an indicator for poor survival and a predictor for the effectiveness of treatment. PKCη is also a novel biomarker for poor prognosis in non-small-cell lung cancer (NSCLC). Thus PKCη presents a potential target for therapy where inhibition of its activity and/or translocation to membranes could interfere with the resistance to chemotherapy.

PKCη promotes senescence induced by oxidative stress and chemotherapy

Senescence is characterized by permanent cell-cycle arrest despite continued viability and metabolic activity, in conjunction with the secretion of a complex mixture of extracellular proteins and soluble factors known as the senescence-associated secretory phenotype (SASP). Cellular senescence has been shown to prevent the proliferation of potentially tumorigenic cells, and is thus generally considered a tumor suppressive process. However, some SASP components may act as pro-tumorigenic mediators on premalignant cells in the microenvironment. A limited number of studies indicated that protein kinase C (PKC) has a role in senescence, with different isoforms having opposing effects. It is therefore important to elucidate the functional role of specific PKCs in senescence. Here we show that PKCη, an epithelial specific and anti-apoptotic kinase, promotes senescence induced by oxidative stress and DNA damage. We further demonstrate that PKCη promotes senescence through its ability to upregulate the expression of the cell cycle inhibitors p21^Cip1 and p27^Kip1 and enhance transcription and secretion of interleukin-6 (IL-6). Moreover, we demonstrate that PKCη creates a positive loop for reinforcing senescence by increasing the transcription of both IL-6 and IL-6 receptor, whereas the expression of IL-8 is specifically suppressed by PKCη. Thus, the presence/absence of PKCη modulates major components of SASP. Furthermore, we show that the human polymorphic variant of PKCη, 374I, that exhibits higher kinase activity in comparison to WT-374V, is also more effective in IL-6 secretion, p21^Cip1 expression and the promotion of senescence, further supporting a role for PKCη in senescence. As there is now considerable interest in senescence activation/elimination to control tumor progression, it is first crucial to reveal the molecular regulators of senescence. This will improve our ability to develop new strategies to harness senescence as a potential cancer therapy in the future.

Protein kinase C involvement in cell cycle modulation

Protein kinases C (PKCs) are a family of serine/threonine kinases which act as key regulators in cell cycle progression and differentiation. Studies of the involvement of PKCs in cell proliferation showed that their role is dependent on cell models, cell cycle phases, timing of activation and localization. Indeed, PKCs can positively and negatively act on it, regulating entry, progression and exit from the cell cycle. In particular, the targets of PKCs resulted to be some of the key proteins involved in the cell cycle including cyclins, cyclin-dependent kinases (Cdks), Cip/Kip inhibitors and lamins. Several findings described roles for PKCs in the regulation of G1/S and G2/M checkpoints. As a matter of fact, data from independent laboratories demonstrated PKC-related modulations of cyclins D, leading to effects on the G1/S transition and differentiation of different cell lines. Moreover, interesting data were published on PKC-mediated phosphorylation of lamins. In addition, PKC isoenzymes can accumulate in the nuclei, attracted by different stimuli including diacylglycerol (DAG) fluctuations during cell cycle progression, and target lamins, leading to their disassembly at mitosis. In the present paper, we briefly review how PKCs could regulate cell proliferation and differentiation affecting different molecules related to cell cycle progression.

PKC signaling in glioblastoma

Glioblastoma Multiforme (GBM) is the most aggressive brain tumor characterized by intratumoral heterogeneity at cytopathological, genomic and transcriptional levels. Despite the efforts to develop new therapeutic strategies the median survival of GBM patients is 12-14 months. Results from large-scale gene expression profile studies confirmed that the genetic alterations in GBM affect pathways controlling cell cycle progression, cellular proliferation and survival and invasion ability, which may explain the difficulty to treat GBM patients. One of the signaling pathways that contribute to the aggressive behavior of glioma cells is the protein kinase C (PKC) pathway. PKC is a family of serine/threonine-specific protein kinases organized into three groups according the activating domains. Due to the variability of actions controlled by PKC isoforms, its contribution to the development of GBM is poorly understood. This review intends to highlight the contribution of PKC isoforms to proliferation, survival and invasive ability of glioma cells.

Serine 123 phosphorylation modulates p21 protein stability and activity by suppressing ubiquitin-independent proteasomal degradation

The cyclin-dependent kinase inhibitor p21(Waf1/Cip1) is a major regulator of the cell cycle and plays an important role in many cellular processes, including differentiation, stress response, apoptosis, and tumorigenesis. We previously cloned the gene encoding dog p21 and found that unlike its human ortholog, dog p21 is expressed as two isoforms, one high molecular mass band of 23 kDa and one low molecular mass band of 19 kDa. In the current study, we found that the high molecular mass band is phosphorylated, whereas the low molecular mass band is hypophosphorylated. Moreover, by generating multiple mutants of dog p21, we found that serine 123 and proline 124, which form a consensus site for proline-directed phosphorylation, are required for expression of the high molecular mass p21 isoform through phosphorylation at serine 123. Most importantly, we showed that serine 123 phosphorylation inhibits ubiquitin-independent proteasomal degradation of p21 protein and subsequently, prolongs p21 protein half-life and enhances the ability of p21 to suppress cell proliferation. Taken together, these data reveal that serine 123 phosphorylation modulates p21 protein stability and activity by suppressing ubiquitin-independent proteasomal degradation.

Involvement of distinct PKC gene products in T cell functions

It is well established that members of the protein kinase C (PKC) family seem to have important roles in T cells. Focusing on the physiological and non-redundant PKC functions established in primary mouse T cells via germline gene-targeting approaches, our current knowledge defines two particularly critical PKC gene products, PKCθ and PKCα, as the "flavor of PKC" in T cells that appear to have a positive role in signaling pathways that are necessary for full antigen receptor-mediated T cell activation ex vivo and T cell-mediated immunity in vivo. Consistently, in spite of the current dogma that PKCθ inhibition might be sufficient to achieve complete immunosuppressive effects, more recent results have indicated that the pharmacological inhibition of PKCθ, and additionally, at least PKCα, appears to be needed to provide a successful approach for the prevention of allograft rejection and treatment of autoimmune diseases.

Type V collagen and protein kinase C η down-regulation in 8701-BC breast cancer cells

We previously reported that ductal infiltrating carcinomas (d.i.c.) of the human breast display profound modifications of the stromal architecture, associated with anomalous collagen composition. Among the major alterations observed in the interstitial collagen, the relative increase of type V collagen content was detected. When type V collagen was used as an "in vitro" substrate for 8701-BC d.i.c. cells, it appeared able to restrain cell growth, inhibit cell motility and invasion "in vitro", and modify the expression levels of genes coding for apoptosis factors, caspases and stress response proteins. In the present paper we demonstrate that type V collagen induces the down-regulation of protein kinase C η, an event that may be, at least in part, responsible of the previously-reported modifications of cell morphology and growth rate, and that appears to be involved in the already-observed changes of expression levels of genes encoding for anti- (Bcl-2) and pro-apoptotic factors (Bad, Dapk, Bcl-Xs) and enzymes (caspase 5 and 8).

Protein kinase C (PKC) delta suppresses keratinocyte proliferation by increasing p21(Cip1) level by a KLF4 transcription factor-dependent mechanism

PKCδ increases keratinocyte differentiation and suppresses keratinocyte proliferation and survival. However, the mechanism of proliferation suppression is not well understood. The present studies show that PKCδ overexpression increases p21(Cip1) mRNA and protein level and promoter activity and that treatment with dominant-negative PKCδ, PKCδ-siRNA, or rottlerin inhibits promoter activation. Analysis of the p21(Cip1) promoter upstream regulatory region reveals three DNA segments that mediate PKCδ-dependent promoter activation. The PKCδ response element most proximal to the transcription start site encodes six GC-rich DNA elements. Mutation of these sites results in a loss of PKCδ-dependent promoter activation. Gel mobility supershift and chromatin immunoprecipitation reveal that these DNA elements bind the Kruppel-like transcription factor KLF4. PKCδ increases KLF4 mRNA and protein level and KLF4 binding to the GC-rich elements in the p21(Cip1) proximal promoter. In addition, KLF4-siRNA inhibits PKCδ-dependent p21(Cip1) promoter activity. PKCδ increases KLF4 expression leading to enhanced KLF4 interaction with the GC-rich elements in the p21(Cip1) promoter to activate transcription.

DNA damage targets PKCη to the nuclear membrane via its C1b domain

Translocation to cellular membranes is one of the hallmarks of PKC activation, occurring as a result of the generation of lipid secondary messengers in target membrane compartments. The activation-induced translocation of PKCs and binding to membranes is largely directed by their regulatory domains. We have previously reported that PKCη, a member of the novel subfamily and an epithelial specific isoform, is localized at the cytoplasm and ER/Golgi and is translocated to the plasma membrane and the nuclear envelope upon short-term activation by PMA. Here we show that PKCη is shuttling between the cytoplasm and the nucleus and that upon etoposide induced DNA damage is tethered at the nuclear envelope. Although PKCη expression and its phosphorylation on the hydrophobic motif (Ser675) are increased by etoposide, this phosphorylation is not required for its accumulation at the nuclear envelope. Moreover, we demonstrate that the C1b domain is sufficient for translocation to the nuclear envelope. We further show that, similar to full-length PKCη, the C1b domain could also confer protection against etoposide-induced cell death. Our studies demonstrate translocation of PKCη to the nuclear envelope, and suggest that its spatial regulation could be important for its cellular functions including effects on cell death.

PRKCH gene polymorphism is associated with the risk of severe gastric atrophy

BACKGROUND

Individuals infected with Helicobacter pylori do not necessarily develop gastric atrophy (GA) and gastric cancer (GC). Several factors, including genetic polymorphism, can regulate the development of GA and GC. A G/A single nucleotide polymorphism (rs3783799) of the PRKCH gene, which encodes the eta isozyme of protein kinase C (PKCeta), has been reported to be a tag single nucleotide polymorphism (SNP) of the PRKCH gene linked to a functional 1425G/A SNP in exon 9 (rs2230500). To elucidate its applicability in the development of GA and GC, this study aimed to investigate the associations of the PRKCH polymorphism with the risks of GA and GC.

METHODS

The subjects consisted of 583 patients (cases) from first-visit outpatients at Aichi Cancer Center Hospital, aged 27 to 80 years, who were diagnosed as having GC from 2001 to 2005, and 1742 controls, frequency-matched for age and sex. Anti-H. pylori IgG antibodies and pepsinogens (PGs) in serum were measured for 1638 controls.

RESULTS

Of the 1638 controls, 57.3% were seropositive and 33.0% had GA (PG1 < or = 70 ng/dl and PG1/PG2 < or = 3). When compared to the seronegative controls without GA, the AA genotype was significantly associated with severe GA (PG1 < or = 30 ng/dl and PG1/PG2 < or = 2); odds ratio (OR), 2.37 (95% confidence interval, 1.11-5.05) relative to the GG genotype. The genotype was not associated with the risk of GC.

CONCLUSION

This was the first study to examine the associations of the PRKCH polymorphism with GA and GC, and suggested that the AA genotype, relative to the G/G genotype, may be a higher risk genotype for severe GA.

Pim-2 phosphorylation of p21(Cip1/WAF1) enhances its stability and inhibits cell proliferation in HCT116 cells

Pim-2 kinase is one of the three highly conserved Pim family members which are known to be involved in cell survival and cell proliferation. Here we demonstrate that like Pim-1, Pim-2 also phosphorylates the cell cycle inhibitor p21(Cip1/WAF1) (p21) on Thr145 in vitro and in vivo. Overexpression of Pim-2 in HCT116 cells leads to the increased stability of p21 and results in enhanced levels of both exogenous and endogenous p21 proteins. Knockdown of Pim-2 expression via siRNA results in reduced level of endogenous p21, indicating that like Pim-1, Pim-2 is another legitimate p21 kinase. However, Pim-2 has no influence on the nuclear localization of p21 in HCT116 cells. In addition, Pim-2 is able to arrest the cell cycle at G1/S phase and inhibit cell proliferation through phosphorylation of p21 in HCT116 cells. These data suggest that Pim-2 phosphorylation of p21 enhances p21's stability and inhibits cell proliferation in HCT116 cells.

Protein kinase C delta and eta differently regulate the expression of loricrin and Jun family proteins in human keratinocytes

Barrier function of the epidermis is maintained by precise expression of keratinocyte-specific structural proteins to form the cornified cell envelope (CE). Loricrin, a major component of the CE, is expressed at the late stage of keratinocyte differentiation. In this study, we reveal the isoform-specific function of protein kinase C (PKC) in the regulation of loricrin expression. Both PKCdelta and PKCeta have been recognized as differentiation-promoting isoforms. However, loricrin expression was inversely controlled by PKCdelta and PKCeta in cultured keratinocytes and 3D skin culture; i.e. loricrin expression was decreased by PKCdelta and increased by PKCeta. To clarify the mechanisms that PKCdelta and PKCeta oppositely regulate the loricrin expression, we examined the expression of activator protein-1 (AP-1) family proteins, which modulate the transcription of loricrin and are downstream molecules of PKC. PKCdelta decreased c-Jun expression, whereas PKCeta increased JunD, which are positive regulators of loricrin transcription. These findings suggest that inverse effects of PKCdelta and PKCeta on loricrin expression attributes to the expression of c-Jun and JunD.

Translational control of protein kinase Ceta by two upstream open reading frames

Protein kinase C (PKC) represents a family of serine/threonine kinases that play a central role in the regulation of cell growth, differentiation, and transformation. Posttranslational control of the PKC isoforms and their activation have been extensively studied; however, not much is known about their translational regulation. Here we report that the expression of one of the PKC isoforms, PKCeta, is regulated at the translational level both under normal growth conditions and during stress imposed by amino acid starvation, the latter causing a marked increase in its protein levels. The 5' untranslated region (5' UTR) of PKCeta is unusually long and GC rich, characteristic of many oncogenes and growth regulatory genes. We have identified two conserved upstream open reading frames (uORFs) in its 5' UTR and show their effect in suppressing the expression of PKCeta in MCF-7 growing cells. While the two uORFs function as repressive elements that maintain low basal levels of PKCeta in growing cells, they are required for its enhanced expression upon amino acid starvation. We show that the translational regulation during stress involves leaky scanning and is dependent on eIF-2alpha phosphorylation by GCN2. Our work further suggests that translational regulation could provide an additional level for controlling the expression of PKC family members, being more common than currently recognized.

Activation of protein kinase C{eta} by type I interferons

Type I interferons (IFNs) are cytokines with diverse biological properties, including antiviral, growth inhibitory, and immunomodulatory effects. Although several signaling pathways are activated during engagement of the type I IFN receptor and participate in the induction of IFN responses, the mechanisms of generation of specific signals for distinct biological effects remain to be elucidated. We provide evidence that a novel member of the protein kinase C (PKC) family of proteins is rapidly phosphorylated and activated during engagement of the type I IFN receptor. In contrast to other members of the PKC family that are also regulated by IFN receptors, PKCeta does not regulate IFN-inducible transcription of interferon-stimulated genes or generation of antiviral responses. However, its function promotes cell cycle arrest and is essential for the generation of the suppressive effects of IFNalpha on normal and leukemic human myeloid (colony-forming unit-granulocyte macrophage) bone marrow progenitors. Altogether, our studies establish PKCeta as a unique element in IFN signaling that plays a key and essential role in the generation of the regulatory effects of type I IFNs on normal and leukemic hematopoiesis.

S-Phase-specific activation of PKC alpha induces senescence in non-small cell lung cancer cells

Protein kinase C (PKC) has been widely implicated in positive and negative control of cell proliferation. We have recently shown that treatment of non-small cell lung cancer (NSCLC) cells with phorbol 12-myristate 13-acetate (PMA) during G1 phase inhibits the progression into S phase, an effect mediated by PKC delta-induced up-regulation of the cell cycle inhibitor p21 Cip1. However, PMA treatment in asynchronously growing NSCLC cells leads to accumulation of cells in G2/M. Studies in post-G1 phases revealed that PMA induced an irreversible G2/M cell cycle arrest in NSCLC cells and conferred morphological and biochemical features of senescence, including elevated SA-beta-Gal activity and reduced telomerase activity. Remarkably, this effect was phase-specific, as it occurred only when PKC was activated in S, but not in G1, phase. Mechanistic analysis revealed a crucial role for the classical PKC alpha isozyme as mediator of the G2/M arrest and senescence, as well as for inducing p21(Cip1) an obligatory event for conferring the senescence phenotype. In addition to the unappreciated role of PKC isozymes, and specifically PKC alpha, in senescence, our data introduce the paradigm that discrete PKCs trigger distinctive responses when activated in different phases of the cell cycle via a common mechanism that involves p21 Cip1 up-regulation.

Protein Kinase Cε Interacts with Signal Transducers and Activators of Transcription 3 (Stat3), Phosphorylates Stat3Ser727, and Regulates Its Constitutive Activation in Prostate Cancer

Prostate cancer is the most common type of cancer in men and ranks second only to lung cancer in cancer-related deaths. The management of locally advanced prostate cancer is difficult because the cancer often becomes hormone insensitive and unresponsive to current chemotherapeutic agents. Knowledge about the regulatory molecules involved in the transformation to androgen-independent prostate cancer is essential for the rational design of agents to prevent and treat prostate cancer. Protein kinase Cepsilon (PKCepsilon), a member of the novel PKC subfamily, is linked to the development of androgen-independent prostate cancer. PKCepsilon expression levels, as determined by immunohistochemistry of human prostate cancer tissue microarrays, correlated with the aggressiveness of prostate cancer. The mechanism by which PKCepsilon mediates progression to prostate cancer remains elusive. We present here for the first time that signal transducers and activators of transcription 3 (Stat3), which is constitutively activated in a wide variety of human cancers, including prostate cancer, interacts with PKCepsilon. The interaction of PKCepsilon with Stat3 was observed in human prostate cancer, human prostate cancer cell lines (LNCaP, DU145, PC3, and CW22rv1), and prostate cancer that developed in transgenic adenocarcinoma of mouse prostate mice. In reciprocal immunoprecipitation/blotting experiments, prostatic Stat3 coimmunoprecipitated with PKCepsilon. Localization of PKCepsilon with Stat3 was confirmed by double immunofluorescence staining. The interaction of PKCepsilon with Stat3 was PKCepsilon isoform specific. Inhibition of PKCepsilon protein expression in DU145 cells using specific PKCepsilon small interfering RNA (a) inhibited Stat3Ser727 phosphorylation, (b) decreased both Stat3 DNA-binding and transcriptional activity, and (c) decreased DU145 cell invasion. These results indicate that PKCepsilon activation is essential for constitutive activation of Stat3 and prostate cancer progression.

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 epsilon is overexpressed in primary human non-small cell lung cancers and functionally required for proliferation of non-small cell lung cancer cells in a p21/Cip1-dependent manner

The protein kinase C (PKC) family of proteins plays important roles in growth regulation and is implicated in tumorigenesis. It has become clear that the role of PKC in tumorigenesis is cell context dependent and/or isoform specific. In this study, we showed for the first time by immunohistochemistry that overexpression of PKC epsilon was detected in the vast majority (>90%) of primary human non-small cell lung cancers (NSCLC) compared with normal lung epithelium. Inhibition of the PKC epsilon pathway using a kinase-inactive, dominant-negative PKC epsilon, PKC epsilon(KR), led to a significant inhibition of proliferation and anchorage-independent growth of human NSCLC cells in a p53-independent manner. This was accompanied by a specific induction of the cyclin-dependent kinase (cdk) inhibitor p21/Cip1 but not p27/Kip1. In response to serum stimulation, PKC epsilon(KR)-expressing cells showed a prolonged G(1)-S transition and delayed and reduced activation of cdk2 complexes, which was likely attributed to the increased binding of p21/Cip1 to cdk2. Furthermore, inhibition of PKC epsilon function either by expressing PKC epsilon(KR) or by small interfering RNA (siRNA)-mediated gene knockdown resulted in c-Myc down-regulation, which, in turn, regulated p21/Cip1 expression. Knockdown of PKC epsilon or c-Myc expression using siRNA led to induction of p21/Cip1 and attenuation of G(1)-S transition in NSCLC cells. Using p21(+/+) and p21(-/-) HCT116 isogenic cell lines, we further showed that growth inhibition by PKC epsilon(KR) required the function of p21/Cip1. Collectively, these results reveal an important role for PKC epsilon signaling in lung cancer and suggest that one potential mechanism by which PKC epsilon exerts its oncogenic activity is through deregulation of the cell cycle via a p21/Cip1-dependent mechanism.

Involvement of deterioration in S100C/A11-mediated pathway in resistance of human squamous cancer cell lines to TGFβ-induced growth suppression

Recently, we demonstrated that S100C/A11 comprises an essential pathway for growth suppression by TGFbeta in normal human keratinocytes. Nuclear transfer of S100C/A11 was a hallmark of the activation of the process. In the present study, we examined the possible deterioration in the pathway in human squamous cancer cell lines, focusing on intracellular localization of S100C/A11 and its functional partners Smad3 and Smad4. All four human squamous cancer cell lines examined (A431, BSCC-93, DJM-1, and HSC-5) were resistant to growth suppression by TGFbeta. In BSCC-93, DJM-1, and HSC-5 cells exposed to TGFbeta, S100C/A11 was not transferred to the nuclei, and p21(WAF1) was not induced. Overexpression of nucleus-targeted S100C/A11 partially recovered induction of p21(WAF1) and p15(INK4B) and growth suppression by TGFbeta1 in these cells. These results indicate that the deterioration in the S100C/A11-mediated pathway conferred upon the cancer cell lines resistance to TGFbeta. In A431 cells, S100C/A11, Smad3, and Smad4 were simultaneously transferred to the nuclei, and p21(WAF1) was induced upon exposure to TGFbeta. We provide evidence to indicate that refractoriness of A431 cells to TGFbeta was probably because the amount of p21(WAF1) induced by TGFbeta was insufficient to counteract cyclin A, which is highly overexpressed in A431 cells. Thus, the newly found S100C/A11-mediated pathway is at least partly involved in conferring upon human squamous cell cancers resistant to TGFbeta-induced growth suppression, which is considered to play a critical role for the initiation and progression of many human cancers.

The role of proline-rich protein tyrosine kinase 2 in differentiation-dependent signaling in human epidermal keratinocytes

Non-receptor tyrosine kinase proline-rich protein tyrosine kinase 2 (Pyk2) functions as an integrator of multiple signaling pathways involved in the regulation of fundamental cellular processes. Pyk2 expression, regulation, and functions in skin have not been examined. Here we investigated the expression and subcellular localization of Pyk2 in human epidermis and in primary human keratinocytes, and studied the mechanisms of Pyk2 activation by differentiation-inducing stimuli, and the role of Pyk2 as a regulator of keratinocyte differentiation. We demonstrate that Pyk2 is abundantly expressed in skin keratinocytes. Notably, the endogenous Pyk2 protein is predominantly localized in keratinocyte nuclei throughout all layers of healthy human epidermis, and in cultured human keratinocytes. Pyk2 is activated by treatment with keratinocyte-differentiating agents, 12-O-tetradecanoylphorbol-13-acetate and calcium via a mechanism that requires intracellular calcium release and functional protein kinase C (PKC) and Src activities. Particularly, differentiation-promoting PKC delta and PKC eta elicit Pyk2 activation. Our data show that Pyk2 increases promoter activity and endogenous protein levels of involucrin, a marker of keratinocyte terminal differentiation. This regulation is associated with increased expression of Fra-1 and JunD, activator protein-1 transcription factors known to be required for involucrin expression. Altogether, these results provide insights into Pyk2 signaling in epidermis and reveal a novel role for Pyk2 in regulation of keratinocyte differentiation.

E2F1 stability is regulated by a novel-PKC/p38beta MAP kinase signaling pathway during keratinocyte differentiation

E2F transcription factors regulate proliferation, differentiation, DNA repair and apoptosis. Tight E2F regulation is crucial for epidermal formation and regeneration. However, virtually nothing is known about the molecular events modulating E2F during epidermal keratinocyte differentiation. Elucidation of these events is essential to understand epidermal morphogenesis, transformation and repair. Here we show that, in differentiating keratinocytes, Ca(2+)-induced protein kinase C (PKC) activation downregulates E2F1 protein levels. Further, we have identified PKC delta and eta as those isoforms specifically involved in induction of E2F1 proteasomal degradation. We also demonstrate that E2F1 downregulation by novel PKC isozymes requires activation of p38beta mitogen-activated protein kinase (MAPK). This is the first example of regulation in the E2F transcription factor family by activation of PKC and MAPK in the context of biologically significant differentiation stimuli in epithelia.

PKCeta is localized in the Golgi, ER and nuclear envelope and translocates to the nuclear envelope upon PMA activation and serum-starvation: C1b domain and the pseudosubstrate containing fragment target PKCeta to the Golgi and the nuclear envelope

Protein kinase C (PKC) represents a family of serin/threonine kinases, playing a central role in the regulation of cell growth, differentiation and transformation. These enzymes differ in their primary structure, biochemical properties, tissue distribution and subcellular localization. The specific cellular functions of PKC isoforms are largely controlled by their localization. PKCeta, a member of the novel subfamily, is expressed predominantly in epithelial tissues. However, not much is known with respect to its mechanism of activation and regulation. Our recent studies suggest its role in cell cycle control. Here we show that PKCeta is localized at the Golgi apparatus, ER and the nuclear envelope. Furthermore, using GFP-fusion proteins of the different functional domains of PKCeta we deciphered the specific structural domains of the protein responsible for its apparent localization. We show that the cysteine-rich repeat C1b is responsible for its Golgi localization, while for its presence at the ER/nuclear envelope the pseudosubstrate containing fragment coupled to the C1 domain is required. In response to short-term activation by PMA we show translocation of PKCeta to the plasma membrane and the nuclear envelope. We demonstrate that the C1b is sufficient for its translocation to the plasma membrane. Interestingly, accumulation of PKCeta at the nuclear envelope also occurred in response to serum-starvation. It should be noted that interaction of PKCeta with the cyclin E/Cdk2 complex at the perinuclear region was recently reported by us in response to serum-starvation. Thus, our studies demonstrate translocation of PKCeta to the nuclear envelope, and suggest that the spatial regulation of PKCeta could be important for its cellular functions including effects on cell cycle control and involvement in tumor promotion.

Binding of calmodulin to the carboxy-terminal region of p21 induces nuclear accumulation via inhibition of protein kinase C-mediated phosphorylation of Ser153

Intracellular localization plays an important role in the functional regulation of the cell cycle inhibitor p21. We have previously shown that calmodulin binds to p21 and that calmodulin is essential for the nuclear accumulation of p21. Here, we analyze the mechanism of this regulation. We show that calmodulin inhibits in vitro phosphorylation of p21 by protein kinase C (PKC) and that this inhibition is dependent upon calmodulin binding to p21. Two-dimensional electrophoresis analysis of cells expressing the p21 wild type or p21S153A, a nonphosphorylatable mutant of p21 at position 153, indicates that Ser153 of p21 is a phosphorylable residue in vivo. Furthermore, Western blot analysis using phospho-Ser153-specific antibodies indicates that Ser153 phosphorylation in vivo is induced when PKC is activated and calmodulin is inhibited. The mutation of Ser153 to aspartate, a pseudophosphorylated residue, inhibits the nuclear accumulation of p21. Finally, whereas wild-type p21 translocates to the cytoplasm after PKC activation in the presence of calmodulin inhibitors, p21 carrying a nonphosphorylatable residue at position 153 remains in the nucleus. We propose that calmodulin binding to p21 prevents its phosphorylation by PKC at Ser153 and consequently allows its nuclear localization. When phosphorylated at Ser153, p21 is located at the cytoplasm and disrupts stress fibers.

The repression of E2F-1 is critical for the activity of Minerval against cancer

The recently discovered anticancer drug Minerval (2-hydroxy-9-cis-octadecenoic acid) is a synthetic fatty acid that modifies the structure of the membrane. This restructuring facilitates the recruitment of protein kinase C (PKC) alpha to membranes and is associated with the antineoplastic activity of Minerval in cellular and animal models of cancer. Minerval is a derivative of oleic acid (OA) with an enhanced antiproliferative activity in human cancer cells and animal models of cancer, which is associated with PKCalpha activation and p21(CIP) overexpression. However, the signaling cascades involved in its pharmacological activity remain largely unknown. Here, we showed that this drug induced cell cycle arrest before entry into S phase, human lung adenocarcinoma (A549) cells accumulating in the G0/G1 phase. This cell cycle arrest was associated with a marked decrease in the expression of E2F-1. This transcription factor activates several cell cycle-related genes, and, accordingly, the expression of certain cyclins and cyclin-dependent kinases (cdks) was markedly lower upon exposure to Minerval. The reduced availability of these kinase heterodimers was associated with reduced phosphorylation of the retinoblastoma protein (pRb) observed after drug treatment. Significantly, hypophosphorylated pRb remains bound to E2F-1 and maintains this transcription factor inactive. The modulation of these antiproliferative mechanisms by Minerval explains its anticancer potency, through a new therapeutic strategy that can be used to develop new antitumor drugs. On the other hand, apoptosis did not seem to be involved in its pharmacological mechanism. Interestingly, whereas the changes induced by OA were only modest, they may reflect the beneficial effects of high olive oil intake against cancer.

PKC-eta mediates glioblastoma cell proliferation through the Akt and mTOR signaling pathways

We previously demonstrated that protein kinase C-eta (PKC-eta) mediates a phorbol 12-myristate-13-acetate (PMA)-induced proliferative response in human glioblastoma (GBM) cells. In this report, we show that PMA-stimulated activation of PKC-eta in U-251 GBM cells resulted in activation of both Akt and the mammalian target of rapamycin (mTOR) signaling pathways and an increase in cell proliferation. Expression of a kinase dead PKC-eta (PKC-etaKR) construct reduced the basal and PMA-evoked proliferation of PKC-eta-expressing U-251 GBM cells, as well as abrogated the PMA-induced activation of Akt, mTOR, and the mTOR targets 4E-BP1 and STAT-3. Treatment of cells with the PI-3 kinase inhibitor LY294002 (10 muM) or the mTOR inhibitor rapamycin (10 nM) also reduced PMA-induced proliferation and cell-cycle progression. Expression of a constitutively active PKC-eta (PKC-etaDeltaNPS) construct in a GBM cell line with no endogenous PKC-eta (U-1242) also provided evidence that PKC-eta targets the Akt and mTOR signaling pathways. Moreover, activation of 4E-BP1 and STAT-3 in both PMA-treated U-251 and PKC-etaDeltaNPS-expressing U-1242 GBM cells was inhibited by rapamycin. However, activation of Akt, but not mTOR was inhibited by the PI-3 kinase inhibitor LY294002. This study identifies Akt and mTOR as downstream targets of PKC-eta that are involved in GBM cell proliferation.

Signalling in the epidermis: the E2F cell cycle regulatory pathway in epidermal morphogenesis, regeneration and transformation

The epidermis is the outermost layer in the skin, and it is the first line of defence against the environment. The epidermis also provides a barrier against loss of fluids and electrolytes, which is crucial for life. Essential in the maintenance of this tissue is its ability to continually self-renew and regenerate after injury. These two characteristics are critically dependent on the ability of the principal epidermal cell type, the keratinocyte, to proliferate and to respond to differentiation cues. Indeed, the epidermis is a multilayered tissue composed of keratinocyte stem cells and their differentiated progeny. Central for the control of cell proliferation is the E2F transcription factor regulatory network. This signaling network also includes cyclins, cdk, cdk inhibitors and the retinoblastoma (pRb) family of proteins. The biological importance of the E2F/pRb pathway is emphasized by the fact that a majority of human tumours exhibit alterations that disrupt the ability of pRb proteins to inhibit E2F, leading to permanent activation of the latter. Further, E2F is essential for normal epidermal regeneration after injury. Other member of the E2F signaling pathway are also involved in epidermal development and pathophysiology. Thus, whereas the pRb family of proteins is essential for epidermal morphogenesis, abnormal regulation of cyclins and E2F proteins results in tumorgenesis in this tissue. In this review, we discuss the role of each member of this important growth regulatory network in epidermal formation, homeostasis and carcinogenesis.

Membrane structure modulation, protein kinase C alpha activation, and anticancer activity of minerval

Most drugs currently used for human therapy interact with proteins, altering their activity to modulate the pathological cell physiology. In contrast, 2-hydroxy-9-cis-octadecenoic acid (Minerval) was designed to modify the lipid organization of the membrane. Its structure was deduced following the guidelines of the mechanism of action previously proposed by us for certain antitumor drugs. The antiproliferative activity of Minerval supports the above-mentioned hypothesis. This molecule augments the propensity of membrane lipids to organize into nonlamellar (hexagonal H(II)) phases, promoting the subsequent recruitment of protein kinase C (PKC) to the cell membrane. The binding of the enzyme to membranes was marked and significantly elevated by Minerval in model (liposomes) and cell (A549) membranes and in heart membranes from animals treated with this drug. In addition, Minerval induced increased PKCalpha expression (mRNA and protein levels) in A549 cells. This drug also induced PKC activation, which led to a p53-independent increase in p21(CIP) expression, followed by a decrease in the cellular concentrations of cyclins A, B, and D3 and cdk2. These molecular changes impaired the cell cycle progression of A549 cells. At the cellular and physiological level, administration of Minerval inhibited the growth of cancer cells and exerted antitumor effects in animal models of cancer without apparent histological toxicity. The present results support the potential use of Minerval and related compounds in the treatment of tumor pathologies.

Protein kinase Cdelta regulates keratinocyte death and survival by regulating activity and subcellular localization of a p38delta-extracellular signal-regulated kinase 1/2 complex

Protein kinase Cdelta (PKCdelta) is an important regulator of apoptosis in epidermal keratinocytes. However, little information is available regarding the downstream kinases that mediate PKCdelta-dependent keratinocyte death. This study implicates p38delta mitogen-activated protein kinase (MAPK) as a downstream carrier of the PKCdelta-dependent death signal. We show that coexpression of PKCdelta with p38delta produces profound apoptosis-like morphological changes. These morphological changes are associated with increased sub-G(1) cell population, cytochrome c release, loss of mitochondrial membrane potential, caspase activation, and PARP cleavage. This death response is specific for the combination of PKCdelta and p38delta and is not produced by replacing PKCdelta with PKCalpha or p38delta with p38alpha. A constitutively active form of MEK6, an upstream activator of p38delta, can also produce cell death when coupled with p38delta. In addition, concurrent p38delta activation and extracellular signal-regulated kinase 1/2 (ERK1/2) inactivation are required for apoptosis. Regarding this inverse regulation, we describe a p38delta-ERK1/2 complex that may coordinate these changes in activity. We further show that this p38delta-ERK1/2 complex relocates into the nucleus in response to PKCdelta expression. This regulation appears to be physiological, since H(2)O(2), a known inducer of keratinocyte apoptosis, promotes identical PKCdelta and p38delta-ERK1/2 activity changes, leading to similar morphological changes.

Down-regulation of protein kinase Ceta by antisense oligonucleotides sensitises A549 lung cancer cells to vincristine and paclitaxel

Previous studies point to protein kinase C (PKC) isozyme eta as a resistance factor in cancer cells. Therefore, we investigated whether down-regulation of PKCeta with second generation antisense oligonucleotides (ODNs) would sensitise A549 human lung carcinoma cells to cytostatics. The effects were compared to the outcome of Bcl-xL down-regulation. Upon treatment with antisense ODNs, PKCeta and Bcl-xL were both significantly reduced on mRNA and protein level. Down-regulation of either PKCeta or Bcl-xL in combination with vincristine or paclitaxel resulted in a significant increase in caspase-3 activity compared to that in the control oligonucleotide treated cells. In addition, PKCeta down-regulation augmented vincristine-induced dissipation of mitochondrial transmembrane potential. In conclusion, these results confirm that PKCeta might represent a considerable resistance factor and an interesting target to improve anticancer chemotherapy.

Activation of C-Kinase .ETA. through Its Cholesterol-3-sulfate-Dependent Phosphorylation by Casein Kinase I in Vitro

The physiological correlation between casein kinase I (CK-I) and an isoform eta of protein kinase C (C-kinase eta) was investigated in vitro, since it has been reported that (i) cholesterol-3-sulfate (CH-3S) effectively activates C-kinase eta rather than the other isoforms (C-kinase epsilon and C-kinase delta) in vitro; and (ii) CK-I efficiently phosphorylates CH-3S-binding proteins, such as high mobility group protein 1 (HMG1), in the presence of CH-3S in vitro. We found that (i) CK-I phosphorylated Thr in preference to Ser on recombinant human C-kinase isoform eta (rhC-kinase eta) in the presence of CH-3S; (ii) this phosphorylation was selectively inhibited by CK-I-7 (a CK-I inhibitor); and (iii) the activity (phosphorylation of protamine sulfate) of rhC-kinase eta was approx. 3.2-fold stimulated by its full phosphorylation by CK-I in the presence of 3 microM CH-3S. These results suggest that CK-I is a protein kinase responsible for the activation of rhC-kinase eta in the presence of CH-3S in vitro.

Thyroid hormones regulate DNA-synthesis and cell-cycle proteins by activation of PKC? and p42/44 MAPK in chick embryo hepatocytes

The molecular mechanism by which thyroid hormones exert their effects on cell growth is still unknown. In this study, we used chick embryo hepatocytes at different stages of development as a model to investigate the effect of the two thyroid hormones, T3 and T4, and of their metabolite T2, on the control of cell proliferation. We observed that T2 provokes increase of DNA-synthesis as well as T3 and T4, independently of developmental stage. We found that this stimulatory effect on the S phase is reverted by specific inhibitors of protein kinase C (PKC) and p42/44 mitogen-activated protein kinase (p42/44 MAPK), Ro 31-8220 or PD 98059. Furthermore, the treatment with thyroid hormones induces the activation of PKCalpha and p42/44 MAPK, suggesting their role as possible downstream mediators of cell response mediated by thyroid hormones. The increase of DNA-synthesis is well correlated with the increased levels of cyclin D1 and cdk4 that control the G1 phase, and also with the activities of cell-cycle proteins involved in the G1 to S phase progression, such as cyclin E/A-cdk2 complexes. Interestingly, the activity of cyclin-cdk2 complexes is strongly repressed in the presence of PKC and p42/44 MAPK inhibitors. In conclusion, we demonstrated that the thyroid hormones could modulate different signaling pathways that are able to control cell-cycle progression, mainly during G1/S transition.

Novel PKCeta is required to activate replicative functions of the major nonstructural protein NS1 of minute virus of mice

The multifunctional protein NS1 of minute virus of mice (MVMp) is posttranslationally modified and at least in part regulated by phosphorylation. The atypical lambda isoform of protein kinase C (PKClambda) phosphorylates residues T435 and S473 in vitro and in vivo, leading directly to an activation of NS1 helicase function, but it is insufficient to activate NS1 for rolling circle replication. The present study identifies an additional cellular protein kinase phosphorylating and regulating NS1 activities. We show in vitro that the recombinant novel PKCeta phosphorylates NS1 and in consequence is able to activate the viral polypeptide in concert with PKClambda for rolling circle replication. Moreover, this role of PKCeta was confirmed in vivo. We thereby created stably transfected A9 mouse fibroblasts, a typical MVMp-permissive host cell line with Flag-tagged constitutively active or inactive PKCeta mutants, in order to alter the activity of the NS1 regulating kinase. Indeed, tryptic phosphopeptide analyses of metabolically (32)P-labeled NS1 expressed in the presence of a dominant-negative mutant, PKCetaDN, showed a lack of distinct NS1 phosphorylation events. This correlates with impaired synthesis of viral DNA replication intermediates, as detected by Southern blotting at the level of the whole cell population and by BrdU incorporation at the single-cell level. Remarkably, MVM infection triggers an accumulation of endogenous PKCeta in the nuclear periphery, suggesting that besides being a target for PKCeta, parvovirus infections may also affect the regulation of this NS1 regulating kinase. Altogether, our results underline the tight interconnection between PKC-mediated signaling and the parvoviral life cycle.

FAK overexpression upregulates cyclin D3 and enhances cell proliferation via the PKC and PI3-kinase-Akt pathways

We previously demonstrated that FAK-transfected HL-60 (HL-60/FAK) cells exhibit anti-apoptotic capacity. Here, we report that HL-60/FAK cells proliferate much faster than vector-transfected control (HL-60/Vect) cells with a 1.5-fold faster doubling time. This observation prompted us to investigate the mechanism of how HL-60/FAK cells augment cell proliferation. Since a protein kinase C (PKC) inhibitor, chelerythrine, or a PI3-kinase inhibitor, LY294002, suppressed cell proliferation effectively, both PKC and PI-3-kinase pathways are presumed to be involved in the cell proliferation. Among cyclins and CDKs, cyclin D3 expression was particularly prominent in the HL-60/FAK cells. Among PKC family, particularly PKCalpha, beta and eta isoforms were activated and directly associated with FAK in HL-60/FAK cells. We assumed that FAK activates PKC and PI3-kinase-Akt pathway, which resulted in marked induction of cyclin D3 expression and CDK activity.

PKCeta associates with cyclin E/Cdk2 complex in serum-starved MCF-7 and NIH-3T3 cells

Protein kinase C (PKC) encodes a family of enzymes implicated in cellular differentiation, growth control, and tumor promotion. However, very little is known with respect to the molecular mechanisms that link protein kinase C to cell cycle control. Here we report that PKCeta associates with the cyclin E/Cdk2 complex. This is shown for the ectopically overexpressed PKCeta in NIH-3T3 cells, the inducibly expressed PKCeta in MCF-7 cells (under control of the tetracycline-responsive promoter), and the endogenously expressed PKCeta in mouse mammary epithelial HC11 cells. Subcellular cell fractionation experiments revealed that the complex with cyclin E is formed mostly in the nuclear fractions, although in these cells PKCeta is predominantly expressed in the cytosolic fractions. The complex of PKCeta and cyclin E was studied at various phases of the cell cycle, in serum-starved quiescent cells and in cells stimulated with serum to reenter the cell cycle. Interestingly, the interaction between PKCeta and cyclin E was most prominent in serum-starved cells and was disintegrated when cells entered the cells cycle. Immunofluorescence staining demonstrated that in serum-starved cells PKCeta is concentrated at the perinuclear zone, which is also the site of its colocalization with cyclin E. Colocalization of PKCeta and cyclin E in the perinuclear region was observed in serum-starved cells, and less in proliferating cells. These experiments suggest that the interaction between PKCeta and cyclin E is carefully regulated, and is correlated with the inactivated form of the cyclin E/Cdk2 complex. Thus, our studies support an important link between PKC and cell cycle control.

p38 Mitogen-activated protein kinases on the body surface--a function for p38 delta

The p38 family of mitogen-activated protein kinases includes p38 alpha (SAPK2a, CSBP), p38 beta (SAPK2b), p38 delta (SAPK4), and p38 gamma (SAPK3/ERK6). p38 alpha and p38 beta are widely expressed p38 isoforms that are involved in regulation of cell proliferation, differentiation, development, and response to stress. Relatively less is known regarding the function of the p38 delta isoform. In this review, we discuss the role of the p38 alpha, p38 beta, and p38 gamma isoforms and then present recent findings that define a role for p38 delta as a regulator of differentiation-dependent gene expression in keratinocytes.

Keratinocyte survival, differentiation, and death: many roads lead to mitogen-activated protein kinase

The epidermis is a dynamic and continually renewing surface that provides and maintains a life-sustaining interface with the environment. The epidermal keratinocyte, the major cell type of the epidermis, undergoes a complex and carefully choreographed program of differentiation. This process requires a balance between keratinocyte proliferation, differentiation, and apoptosis. This overview will concentrate on cascades that regulate the balance between keratinocyte cell proliferation and survival, and apoptosis and cell differentiation, with a particular emphasis on the role of the mitogen-activated protein kinase cascades. A summary of the literature suggests that extracellular regulated kinases function to promote keratinocyte proliferation and survival, whereas p38 mitogen-activated protein kinase functions to promote differentiation and apoptosis.

Activation of protein kinase C triggers irreversible cell cycle withdrawal in human keratinocytes

Irreversible cell cycle withdrawal occurs as normal keratinocytes detach from the basement membrane and initiate their terminal differentiation program. To investigate which signaling pathways regulate this permanent cell cycle withdrawal, we added inhibitors of kinases implicated in integrin signaling and keratinocyte differentiation to normal human keratinocytes induced to differentiate in suspension culture, and assayed the growth capacity of the recovered cells. Keratinocytes suspended in methylcellulose for 24 h underwent approximately 1000-fold loss of proliferative capacity. Of the kinase inhibitors tested, only the protein kinase C inhibitor Bisindolylmaleimide I (GF109203X) caused dramatic protection from loss of growth potential. Direct activation of protein kinase C by 12-O-tetradecanoyl-phorbol-13-acetate was also sufficient to trigger irreversible growth arrest. Protein kinase C inhibitors selective for protein kinase Calpha, the only Ca2+-dependent protein kinase C isoform in keratinocytes, protected keratinocytes from suspension-induced cell cycle withdrawal. Consistent with this finding, we measured a specific induction of Ca2+-dependent protein kinase C activity 2-3 h after keratinocytes were placed into suspension culture. Furthermore, protein kinase Calpha was strongly localized to cell membranes in the suprabasal keratinocytes of human epidermis, suggesting translocation and activation in vivo. Coordinated changes in cell cycle regulators (p21, p27, pRb, p107, p130) consistent with cells exiting the cell cycle were observed in suspended keratinocytes, and these changes were blocked by protein kinase C inhibition. These results indicate that the loss of cell matrix adhesion triggers protein kinase C activation, which is an early event required for cell cycle withdrawal of terminally differentiating normal human keratinocytes.

Calcium-dependent involucrin expression is inversely regulated by protein kinase C (PKC)alpha and PKCdelta

Calcium is an important physiologic regulator of keratinocyte function that may regulate keratinocyte differentiation via modulation of protein kinase C (PKC) activity. PKCalpha and PKCdelta are two PKC isoforms that are expressed at high levels in keratinocytes. In the present study, we examine the effect of PKCdelta and PKCalpha on calcium-dependent keratinocyte differentiation as measured by effects on involucrin (hINV) gene expression. Our studies indicate that calcium increases hINV promoter activity and endogenous hINV gene expression. This response requires PKCdelta, as evidenced by the observation that treatment with dominant-negative PKCdelta inhibits calcium-dependent hINV promoter activity, whereas wild type PKCdelta increases activity. PKCalpha, in contrast, inhibits calcium-dependent hINV promoter activation, a finding that is consistent with the ability of dominant-negative PKCalpha and the PKCalpha inhibitor, Go6976, to increase hINV gene expression. The calcium-dependent regulatory response is mediated by an AP1 transcription factor-binding site located within the hINV promoter distal regulatory region that is also required for PKCdelta-dependent regulation; moreover, both calcium and PKCdelta produce similar, but not identical, changes in AP1 factor expression. A key question is whether calcium directly influences PKC isoform function. Our studies show that calcium does not regulate PKCalpha or delta levels or cause a marked redistribution to membranes. However, tyrosine phosphorylation of PKCdelta is markedly increased following calcium treatment. These findings suggest that PKCalpha and PKCdelta are required for, and modulate, calcium-dependent keratinocyte differentiation in opposing directions.

Perspectives for cancer therapies with cdk2 inhibitors

Modern anticancer strategies are designed against specific molecular targets with the goal of sparing normal, non-neoplastic tissues. Choosing specific molecular targets, however, is problematic. Cdk2 (Cyclin dependent kinase 2, cell division kinase 2, p33) is an important candidate target for therapeutic intervention. Phosphorylation of retinoblastoma protein (pRb) by Cdk2 is the penultimate step in the transition from G1 to S phase. Inhibition of this step could potentially result in inhibition of proliferation, cytostasis and possibly apoptosis in human tumors. Cdk2 also plays a critical role in the transition through S phase and the S to G2 transition as well. Inhibitors of the cyclin dependent kinases, such as flavopiridol and UCN-01, are currently in clinical trials. While demonstrating clinical activity, neither acts specifically against Cdk2. Other more specific Cdk2 inhibitors are currently in preclinical development. Further studies to explore the therapeutic worth of such agents are warranted.

PKCtheta II, a new isoform of protein kinase C specifically expressed in the seminiferous tubules of mouse testis

Protein kinase C (PKC) theta, a Ca(2+)-independent isoform of PKC, has been known to be expressed in skeletal muscle and T cells. In the present study, we isolated and characterized a smaller transcript expressed in the mouse testis, the cDNA of which is referred hereafter as PKCthetaII and the original PKCtheta as PKCthetaI. The cDNA clone of PKCthetaII has 2184 base pairs and 464 amino acids in the possible open reading frame, consisting of the 5' unique sequence of 20 amino acids and the PKCthetaI sequence of 444 amino acids. Genomic DNA analysis revealed that transcription of PKCthetaII is initiated from the PKCthetaII-specific exon, which is located between exons 7 and 8 of the PKCtheta gene, indicating that alternative splicing is the mechanism by which PKCthetaII is generated. PKCthetaII is expressed exclusively in the testis in an age-dependent manner with sexual maturation. In situ hybridization and reverse transcription-polymerase chain reaction of microdissected tissues clearly demonstrated that PKCthetaII is expressed in the seminiferous tubules of the mouse testis. Consistent with its molecular structure lacking the C1 regulatory domain, PKCthetaII is constitutively active as determined by an in vitro kinase assay, being independent of PKC activators, e.g. phosphatidylserine and phorbol ester. PKCthetaII may play a crucial role in spermatogenesis or some related function of the testis.

Altered expression and localization of PKC eta in human breast tumors

Protein kinase C (PKC) eta is a PKC isoform whose upregulation is associated with differentiation in many epithelial tissues, including the rat mammary gland. The purpose of this study was to examine whether PKC eta is altered, in expression or localization, in human breast cancer. Paraffin sections of 49 in situ breast lesions, 29 invasive breast tumors, and nine normal breast biopsies were examined for PKC eta expression by immunohistochemistry. Adjacent regions of normal epithelium, and in situ lesions that were present adjacent to invasive lesions were also analyzed. In normal epithelium, regardless of the presence of adjacent in situ or invasive lesions, PKC eta was present in the cytoplasm of the luminal epithelium, and increased in areas of normal lobular development, similar to normal rat mammary gland. PKC eta staining intensity was homogeneous in normal lobules, but heterogeneous in in situ and invasive lesions, being focally increased in cells with aberrant nuclear morphology. In situ lesions were similar to adjacent normal epithelium in average staining intensity, regardless of whether invasion was also present. However, the invasive lesions themselves were significantly decreased in staining intensity compared to adjacent in situ lesions. In addition, 75% of invasive breast cancer lesions showed decreased staining relative to adjacent normal epithelium, compared to 37% of in situ lesions. The invasive tumors which possessed high PKC eta staining were associated with positive lymph node status. These results demonstrate that quantitative and qualitative alterations in PKC eta occur in human breast cancers.