PKCη confers protection against apoptosis by inhibiting the pro-apoptotic JNK activity in MCF-7 cells

PKCeta Promotes Stress-Induced Autophagy and Senescence in Breast Cancer Cells, Presenting a Target for Therapy

The emergence of chemoresistance in neoplastic cells is one of the major obstacles in cancer therapy. Autophagy was recently reported as one of the mechanisms that promote chemoresistance in cancer cells by protecting against apoptosis and driving senescence. Thus, understanding the role of autophagy and its underlying signaling pathways is crucial for the development of new therapeutic strategies to overcome chemoresistance. We have previously reported that PKCη is a stress-induced kinase that confers resistance in breast cancer cells against chemotherapy by inducing senescence. Here, we show that PKCη promotes autophagy induced by ER and oxidative stress and facilitates the transition from autophagy to senescence. We demonstrate that PKCη knockdown reduces both the autophagic flux and markers of senescence. Additionally, using autophagy inhibitors such as chloroquine and 3-methyladenine, we show that PKCη and autophagy are required for establishing senescence in MCF-7 in response to oxidative stress. Different drugs used in the clinic are known to induce autophagy and senescence in breast cancer cells. Our study proposes PKCη as a target for therapeutic intervention, acting in synergy with autophagy-inducing drugs to overcome resistance and enhance cell death in breast cancer.

Unraveling the hidden role of a uORF-encoded peptide as a kinase inhibitor of PKCs

Approximately 40% of human messenger RNAs (mRNAs) contain upstream open reading frames (uORFs) in their 5' untranslated regions. Some of these uORF sequences, thought to attenuate scanning ribosomes or lead to mRNA degradation, were recently shown to be translated, although the function of the encoded peptides remains unknown. Here, we show a uORF-encoded peptide that exhibits kinase inhibitory functions. This uORF, upstream of the protein kinase C-eta (PKC-η) main ORF, encodes a peptide (uPEP2) containing the typical PKC pseudosubstrate motif present in all PKCs that autoinhibits their kinase activity. We show that uPEP2 directly binds to and selectively inhibits the catalytic activity of novel PKCs but not of classical or atypical PKCs. The endogenous deletion of uORF2 or its overexpression in MCF-7 cells revealed that the endogenously translated uPEP2 reduces the protein levels of PKC-η and other novel PKCs and restricts cell proliferation. Functionally, treatment of breast cancer cells with uPEP2 diminished cell survival and their migration and synergized with chemotherapy by interfering with the response to DNA damage. Furthermore, in a xenograft of MDA-MB-231 breast cancer tumor in mice models, uPEP2 suppressed tumor progression, invasion, and metastasis. Tumor histology showed reduced proliferation, enhanced cell death, and lower protein expression levels of novel PKCs along with diminished phosphorylation of PKC substrates. Hence, our study demonstrates that uORFs may encode biologically active peptides beyond their role as translation regulators of their downstream ORFs. Together, we point to a unique function of a uORF-encoded peptide as a kinase inhibitor, pertinent to cancer therapy.

MBD2 Mediates Septic AKI through Activation of PKCη/p38MAPK and the ERK1/2 Axis

Our previous study demonstrated that the methyl-CpG-binding domain protein 2 (MBD2) mediates vancomycin (VAN)-induced acute kidney injury (AKI). However, the role and regulation of MBD2 in septic AKI are unknown. Herein, MBD2 was induced by lipopolysaccharide (LPS) in Boston University mouse proximal tubules (BUMPTs) and mice. For both in vitro and in vivo experiments, we showed that inhibition of MBD2 by MBD2 small interfering RNA (siRNA) and MBD2-knockout (KO) substantially improved the survival rate and attenuated both LPS and cecal ligation and puncture (CLP)-induced AKI, renal cell apoptosis, and inflammatory factor production. Global genetic expression analyses and in vitro experiments suggest that the expression of protein kinase C eta (PKCη), caused by LPS, is markedly suppressed in MBD2-KO mice and MBD2 siRNA, respectively. Mechanistically, chromatin immunoprecipitation (ChIP) analysis indicates that MBD2 directly binds to promoter region CpG islands of PKCη via suppression of promoter methylation. Furthermore, PKCη siRNA improves the survival rate and attenuates LPS-induced BUMPT cell apoptosis and inflammatory factor production via inactivation of p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK)1/2, which were further verified by PKCη siRNA treatment in CLP-induced AKI. Finally, MBD2-KO mice exhibited CLP-induced renal cell apoptosis and inflammatory factor production by inactivation of PKCη/p38MAPK and ERK1/2 signaling. Taken together, the data indicate that MBD2 mediates septic-induced AKI through the activation of PKCη/p38MAPK and the ERK1/2 axis. MBD2 represents a potential target for treatment of septic AKI.

Development and validation of a new tumor-based gene signature predicting prognosis of HBV/HCV-included resected hepatocellular carcinoma patients

BACKGROUND

Due to the phenotypic and molecular diversity of hepatocellular carcinomas (HCC), it is still a challenge to determine patients' prognosis. We aim to identify new prognostic markers for resected HCC patients.

METHODS

274 patients were retrospectively identified and samples collected from Zhongshan hospital, Fudan University. We analyzed the gene expression patterns of tumors and compared expression patterns with patient survival times. We identified a "9-gene signature" associated with survival by using the coefficient and regression formula of multivariate Cox model. This molecular signature was then validated in three patients cohorts from internal cohort (n = 69), TCGA (n = 369) and GEO dataset (n = 80).

RESULTS

We identified 9-gene signature consisting of ZC2HC1A, MARCKSL1, PTGS1, CDKN2B, CLEC10A, PRDX3, PRKCH, MPEG1 and LMO2. The 9-gene signature was used, combined with clinical parameters, to fit a multivariable Cox model to the training cohort (concordance index, ci = 0.85), which was successfully validated (ci = 0.86 for internal cohort; ci = 0.78 for in silico cohort). The signature showed improved performance compared with clinical parameters alone (ci = 0.70). Furthermore, the signature predicted patient prognosis than previous gene signatures more accurately. It was also used to stratify early-stage, HBV or HCV-infected patients into low and high-risk groups, leading to significant differences in survival in training and validation (P < 0.001).

CONCLUSIONS

The 9-gene signature, in which four were upregulated (ZC2HC1A, MARCKSL1, PTGS1, CDKN2B) and five (CLEC10A, PRDX3, PRKCH, MPEG1, LMO2) were downregulated in HCC with poor prognosis, stratified HCC patients into low and high risk group significantly in different clinical settings, including receiving adjuvant transarterial chemoembolization and especially in early stage disease. This new signature should be validated in prospective studies to stratify patients in clinical decisions.

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.

Superhero Rictor promotes cellular differentiation of mouse embryonic stem cells

Embryonic stem cells (ESCs) hold great promise for regenerative medicine. To harness the full therapeutic potential of ESCs, better understanding of the molecular mechanisms underlying the maintenance and differentiation of ESCs is required. Mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase that integrates growth factor receptor signaling with cellular growth and proliferation. Dysregulation of mTOR signaling has been linked to various human diseases including cancer and metabolic syndromes. However, little is known regarding the function of mTOR signaling in the regulation of ES cell differentiation. Here we report that Rictor, a key component of mTORC2, functions as a novel ES cell differentiation promoting factor. Mechanistically, Rictor is able to interact with Prkch and facilitate Prkch phosphorylation at Ser-642. Upon phosphorylation, Prkch promotes Klf4 phosphorylation and inhibits Klf4-dependent E-cadherin expression, thereafter leading to the ES cell differentiation. These findings reveal a novel Rictor-Prkch-Klf4 pathway that plays an important role in the regulation of ES cell differentiation.

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.

Role of protein kinase C-η in cigarette smoke extract-induced apoptosis in MRC-5-cells

Cigarette smoke (CS) is a major risk factor for emphysema, which causes cell death in structural cells of the lung by mechanisms that are still not completely understood. We demonstrated previously that CS extract (CSE) induces caspase activation in MRC-5 human lung fibroblasts, activated protein kinase C-η (PKC-η), and translocated PKC-η from the cytosol to the membrane. The objective of this study was to investigate the involvement of PKC-η activation in a CSE-induced extrinsic apoptotic pathway. We determined that CSE increases expression of caspase 3 and 8 cleavage in MRC-5 cells and overexpression of PKC-η significantly increased expression of caspase 3 and 8 cleavage compared with control LacZ-infected cells. In contrast, dominant negative (dn) PKC-η inhibited apoptosis in MRC-5 cells exposed to CSE and decreased expression of caspase 3 and 8 compared with control cells. Exposure to 10% CSE for >8 h significantly increased lactate dehydrogenase release in PKC-η-infected cells compared with LacZ-infected cells. Additionally, PKC-η-infected cells had an increased number of Hoechst 33342 stained nuclei compared with LacZ-infected cells, while dn PKC-η-infected cells exhibited fewer morphological changes than LacZ-infected cells under phase-contrast microscopy. In conclusion, PKC-η activation plays a pro-apoptotic role in CSE-induced extrinsic apoptotic pathway in MRC-5 cells. These results suggest that modulation of PKC-η may be a useful tool for regulating the extrinsic apoptosis of MRC-5 cells by CSE and may have therapeutic potential in the treatment of CS-induced lung injury.

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 Regulates the TGFβ3-induced Chondevrepogenic Differentiation of Human Mesenchymal Stem Cell

Transforming growth factor (TGF) family is well known to induce the chondevrepogenic differentiation of mesenchymal stem cells (MSC). However, the precise signal transduction pathways and underlying factors are not well known. Thus the present study aims to evaluate the possible role of C2 domain in the chondevrepogenic differentiation of human mesenchymal stem cells. To this end, 145 C2 domains in the adenovirus were individually transfected to hMSC, and morphological changes were examined. Among 145 C2 domains, C2 domain of protein kinase C eta (PKCη) was selected as a possible chondevrepogenic differentiation factor for hMSC. To confirm this possibility, we treated TGFβ3, a well known chondevrepogenic differentiation factor of hMSC, and examined the increased-expression of glycosaminoglycan (GAG), collagen type II (COL II) as well as PKCη using PT-PCR, immunocytochemistry and Western blot analysis. To further evaluation of C2 domain of PKCη, we examined morphological changes, expressions of GAG and COL II after transfection of PKCη -C2 domain in hMSC. Overexpression of PKCη-C2 domain induced morphological change and increased GAG and COL II expressions. The present results demonstrate that PKCη involves in the TGF-β3-induced chondevrepogenic differentiation of hMSC, and C2 domain of PKCη has important role in this process.

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.

The unique protein kinase Cη: implications for breast cancer (review)

Deregulation of key signal transduction pathways that govern important cellular processes leads to cancer. The development of effective therapeutics for cancer warrants a comprehensive understanding of the signaling pathways that are deregulated in cancer. The protein kinase C (PKC) family has served as an attractive target for cancer therapy for decades owing to its crucial roles in several cellular processes. PKCη is a novel member of the PKC family that plays critical roles in various cellular processes such as growth, proliferation, differentiation and cell death. The regulation of PKCη appears to be unique compared to other PKC isozymes, and there are conflicting reports regarding its role in cancer. This review focuses on the unique aspects of PKCη in terms of its structure, regulation and subcellular distribution and speculates on how these features could account for its distinct functions. We have also discussed the functional implications of PKCη in cancer with particular emphasis on breast cancer.

Protein Kinase C (PKC) Isozymes and Cancer

Protein kinase C (PKC) is a family of phospholipid-dependent serine/threonine kinases, which can be further classified into three PKC isozymes subfamilies: conventional or classic, novel or nonclassic, and atypical. PKC isozymes are known to be involved in cell proliferation, survival, invasion, migration, apoptosis, angiogenesis, and drug resistance. Because of their key roles in cell signaling, PKC isozymes also have the potential to be promising therapeutic targets for several diseases, such as cardiovascular diseases, immune and inflammatory diseases, neurological diseases, metabolic disorders, and multiple types of cancer. This review primarily focuses on the activation, mechanism, and function of PKC isozymes during cancer development and progression.

Silencing of PKCη induces cycle arrest of EBV(+) B lymphoma cells by upregulating expression of p38-MAPK/TAp73/GADD45α and increases susceptibility to chemotherapeutic agents

PKCη is involved in proliferation, differentiation, and drug resistance. However, PKCη function in EBV(+) B lymphoma remains poorly understood. Gene silencing of PKCη through siRNA knockdown inhibited cellular proliferation, induced cell cycle arrest in G0/G1 and G2/M phases, and sensitized cells to chemotherapeutic drugs. Upon PKCη knockdown, expression levels of p21, GADD45α, and TAp73 were all increased, whereas expression levels of CDK2, CDK4, CDK6, cyclin E, cyclin B1, and cdc2 were all downregulated. PKCη silencing also activated p38-MAPK, which in turn contributed to the expression of cell cycle arrest-related molecules. These results suggest that siRNA-mediated silencing of PKCη can be a potent tool to complement existing chemotherapy regimens for treating EBV(+) B lymphoma.

Docosahexaenoic acid and tetracyclines as promising neuroprotective compounds with poly(ADP-ribose) polymerase inhibitory activities for oxidative/genotoxic stress treatment

The human genome is exposed to oxidative/genotoxic stress by several endogenous and exogenous compounds. These events evoke DNA damage and activate poly(ADP-ribose) polymerase-1 (PARP-1), the key enzyme involved in DNA repair. The massive stress and over-activation of this DNA-bound enzyme can be responsible for an energy crisis and neuronal death. The last data indicated that product of PARP-1, i.e. poly(ADP-ribose) (PAR), acts as a signalling molecule and plays a significant role in nucleus-mitochondria cross-talk. PAR translocated to the mitochondria can be involved in mitochondrial permeability, the release of an apoptosis-inducing factor (AIF). Its translocation into the nucleus leads to chromatin condensation, fragmentation and cell death. The exact mechanism of this novel death pathway has not yet fully been understood. In this study the relationship between AIF and PARP/PAR in death signalling in the neuronal cell line (HT22) subjected to oxidative/genotoxic stress evoked by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was explored. The neuroprotective influence of docosahexaenoic acid (DHA), major dietary ω-3 long-chain polyunsaturated fatty acids as well as the action of tetracyclines, the novel suppressors of PARP-1, were examined. The effect of these all compounds was compared with specific PARP-1 inhibitors. The oxidative/genotoxic stress evoked by MNNG enhanced the level of PAR in a time-dependent manner with a concomitant significant decrease in the mitochondrial AIF protein level. Moreover, the down-regulation of the anti-apoptotic proteins (Bcl-2 and Bcl-xL) and the up-regulation of the Bax pro-apoptotic protein were presented. In these conditions massive HT22 cell death was observed. Both PARP-1 inhibitors: 3-aminobenzamide (3-AB) and PJ 34, tetracycline: doxocycline and minocycline, as well as DHA protected the cells against PAR formation and AIF translocation. Moreover, all of these compounds enhanced Bcl-xL gene expression and protected the cells against MNNG-induced death. Our data show that both DHA and tetracyclines offer a novel neuroprotective strategy for oxidative/genotoxic stress treatment.

Regulation of eukaryotic gene expression by the untranslated gene regions and other non-coding elements

There is now compelling evidence that the complexity of higher organisms correlates with the relative amount of non-coding RNA rather than the number of protein-coding genes. Previously dismissed as "junk DNA", it is the non-coding regions of the genome that are responsible for regulation, facilitating complex temporal and spatial gene expression through the combinatorial effect of numerous mechanisms and interactions working together to fine-tune gene expression. The major regions involved in regulation of a particular gene are the 5' and 3' untranslated regions and introns. In addition, pervasive transcription of complex genomes produces a variety of non-coding transcripts that interact with these regions and contribute to regulation. This review discusses recent insights into the regulatory roles of the untranslated gene regions and non-coding RNAs in the control of complex gene expression, as well as the implications of this in terms of organism complexity and evolution.

Protein kinase Cϵ activity induces anti-inflammatory and anti-apoptotic genes via an ERK1/2- and NF-κB-dependent pathway to enhance vascular protection

Vascular endothelial injury predisposes to endothelial dysfunction and atherogenesis. We have investigated the hypothesis that PKCϵ (protein kinase Cϵ) is an important upstream regulator of cytoprotective pathways in vascular ECs (endothelial cells). Depletion of PKCϵ in human ECs reduced expression of the cytoprotective genes A1, A20 and Bcl-2. Conversely, constitutively active PKCϵ expressed in human ECs increased mRNA and protein levels of these cytoprotective genes, with up-regulation dependent upon ERK1/2 (extracellular-signal-regulated kinase 1/2) activation. Furthermore, inhibition of NF-κB (nuclear factor κB) by the pharmacological antagonist BAY 11-7085 or an IκB (inhibitor of NF-κB) SuperRepressor prevented cytoprotective gene induction. Activation of PKCϵ enhanced p65 NF-κB DNA binding and elevated NF-κB transcriptional activity. Importantly, although NF-κB activation by PKCϵ induced cytoprotective genes, it did not up-regulate pro-inflammatory NF-κB targets [E-selectin, VCAM-1 (vascular cell adhesion molecule 1) and ICAM-1 (intercellular adhesion molecule 1)]. Indeed, PKCϵ exhibited cytoprotective and anti-inflammatory actions, including inhibition of TNFα (tumour necrosis factor α)-induced JNK (c-Jun N-terminal kinase) phosphorylation and ICAM-1 up-regulation, a response attenuated by depletion of A20. Thus we conclude that PKCϵ plays an essential role in endothelial homoeostasis, acting as an upstream co-ordinator of gene expression through activation of ERK1/2, inhibition of JNK and diversion of the NF-κB pathway to cytoprotective gene induction, and propose that PKCϵ represents a novel therapeutic target for endothelial dysfunction.

Localization of PKCη in cell membranes as a predictor for breast cancer response to treatment

BACKGROUND

Successful treatment of breast cancer is frequently limited by the resistance of tumors to chemotherapy. Recent studies suggested a role for protein kinase C (PKC) in the resistance to chemotherapy. Here we used retrospective analysis of breast cancer biopsies of neoadjuvantly treated patients to investigate the correlation of PKC expression with aggressiveness and resistance to chemotherapy.

PATIENTS AND METHODS

Our cohort (n = 25) included patients with advanced and aggressive breast cancers, who underwent neoadjuvant therapy with the CAF regimen (cyclophosphamide, doxorubicin, fluorouracil). Core biopsies (pre-chemotherapy) and surgical biopsies of primary tumors and lymph node metastases (post-chemotherapy) were scored for PKCeta (PKCh) and PKCepsilon (PKCe) expression in the cytoplasm, cell membrane, nuclear membrane, and the nucleus.

RESULTS

Our results showed increased expression of PKCh (not PKCe) in the cytoplasm and cell membranes of post-chemotherapy biopsies (p = 0.03). PKCh presence in cell membranes, indicating activation, was in correlation with poor survival (p = 0.007).

CONCLUSION

PKCh staining in cell and nuclear membranes is an indicator for poor survival and a predictor for the effectiveness of neoadjuvant treatment. Other avenues of treatment should be considered for these patients. PKCh presents a target for therapy where inhibition of its activity and/or translocation to membranes could interfere with the resistance to chemotherapy.

PKCη is a negative regulator of AKT inhibiting the IGF-I induced proliferation

The PI3K-AKT pathway is frequently activated in human cancers, including breast cancer, and its activation appears to be critical for tumor maintenance. Some malignant cells are dependent on activated AKT for their survival; tumors exhibiting elevated AKT activity show sensitivity to its inhibition, providing an Achilles heel for their treatment. Here we show that the PKCη isoform is a negative regulator of the AKT signaling pathway. The IGF-I induced phosphorylation on Ser473 of AKT was inhibited by the PKCη-induced expression in MCF-7 breast adenocarcinoma cancer cells. This was further confirmed in shRNA PKCη-knocked-down MCF-7 cells, demonstrating elevated phosphorylation on AKT Ser473. While PKCη exhibited negative regulation on AKT phosphorylation it did not alter the IGF-I induced ERK phosphorylation. However, it enhanced ERK phosphorylation when stimulated by PDGF. Moreover, its effects on IGF-I/AKT and PDGF/ERK pathways were in correlation with cell proliferation. We further show that both PKCη and IGF-I confer protection against UV-induced apoptosis and cell death having additive effects. Although the protective effect of IGF-I involved activation of AKT, it was not affected by PKCη expression, suggesting that PKCη acts through a different route to increase cell survival. Hence, our studies show that PKCη provides negative control on AKT pathway leading to reduced cell proliferation, and further suggest that its presence/absence in breast cancer cells will affect cell death, which could be of therapeutic value.

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 Cη activates NF-κB in response to camptothecin-induced DNA damage

The nuclear factor κB (NF-κB) family of transcription factors participates in the regulation of genes involved in innate- and adaptive-immune responses, cell death and inflammation. The involvement of the Protein kinase C (PKC) family in the regulation of NF-κB in inflammation and immune-related signaling has been extensively studied. However, not much is known on the role of PKC in NF-κB regulation in response to DNA damage. Here we demonstrate for the first time that PKC-eta (PKCη) regulates NF-κB upstream signaling by activating the IκB kinase (IKK) and the degradation of IκB. Furthermore, PKCη enhances the nuclear translocation and transactivation of NF-κB under non-stressed conditions and in response to the anticancer drug camptothecin. We and others have previously shown that PKCη confers protection against DNA damage-induced apoptosis. Our present study suggests that PKCη is involved in NF-κB signaling leading to drug resistance.

PKCη promotes a proliferation to differentiation switch in keratinocytes via upregulation of p27Kip1 mRNA through suppression of JNK/c-Jun signaling under stress conditions

To maintain epidermal homeostasis, the balance between keratinocyte proliferation and differentiation is tightly controlled. However, the molecular mechanisms underlying this balance remain unclear. In 3D organotypic coculture with mouse keratinocytes and fibroblasts, the thickness of stratified cell layers was prolonged, and growth arrest and terminal differentiation were delayed when PKCη-null keratinocytes were used. Re-expression of PKCη in PKCη-null keratinocytes restored stratified cell layer thickness, growth arrest and terminal differentiation. We show that in 3D cocultured PKCη-null keratinocytes, p27^Kip1 mRNA was downregulated, whereas JNK/c-Jun signaling was enhanced. Furthermore, inhibition of JNK/c-Jun signaling in PKCη-null keratinocytes led to upregulation of p27^Kip1 mRNA, and to thinner stratified cell layers. Collectively, our findings indicate that PKCη upregulates p27^Kip1 mRNA through suppression of JNK/c-Jun signaling. This results in promoting a proliferation to differentiation switch in keratinocytes.

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.

Cancer/testis antigen CAGE exerts negative regulation on p53 expression through HDAC2 and confers resistance to anti-cancer drugs

The role of the cancer/testis antigen CAGE in drug resistance was investigated. The drug-resistant human melanoma Malme3M (Malme3M(R)) and the human hepatic cancer cell line SNU387 (SNU387(R)) showed in vivo drug resistance and CAGE induction. Induction of CAGE resulted from decreased expression and thereby displacement of DNA methyltransferase 1(DNMT1) from CAGE promoter sequences. Various drugs induce expression of CAGE by decreasing expression of DNMT1, and hypomethylation of CAGE was correlated with the increased expression of CAGE. Down-regulation of CAGE in these cell lines decreased invasion and enhanced drug sensitivity resulting from increased apoptosis. Down-regulation of CAGE also led to decreased anchorage-independent growth. Down-regulation of CAGE led to increased expression of p53, suggesting that CAGE may act as a negative regulator of p53. Down-regulation of p53 enhanced resistance to drugs and prevented drugs from exerting apoptotic effects. In SNU387(R) cells, CAGE induced the interaction between histone deacetylase 2 (HDAC2) and Snail, which exerted a negative effect on p53 expression. Chromatin immunoprecipitation assay showed that CAGE, through interaction with HDAC2, exerted a negative effect on p53 expression in Malme3M(R) cells. These results suggest that CAGE confers drug resistance by regulating expression of p53 through HDAC2. Taken together, these results show the potential value of CAGE as a target for the development of cancer therapeutics.

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.