Protein kinase Cα suppresses Kras-mediated lung tumor formation through activation of a p38 MAPK-TGFβ signaling axis

Transcriptome analysis reveals PRKCA as a potential therapeutic target for overcoming cisplatin resistance in lung cancer through ferroptosis

Cisplatin-based chemotherapy is the current standard care for lung cancer patients; however, drug resistance frequently develops during treatment, thereby limiting therapeutic efficacy.The molecular mechanisms underlying cisplatin resistance remain elusive. In this study, we conducted an analysis of microarray data from the Gene Expression Omnibus (GEO) database under the accession numbers GSE21656, which encompassed expression profiling of cisplatin-resistant H460 (DDP-H460)and the parental cells (H460). Subsequently, we calculated the differentially expressed genes (DEGs) between DDP-H460 and H460. Gene Ontology (GO) enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of DEGs demonstrated significant impact on the Rap1, PI3K/AKT and MAPK signaling pathways. Moreover, protein and protein interaction (PPI) network analysis identified PRKCA, DET1, and UBE2N as hub genes that potentially contribute predominantly to cisplatin resistance. Ultimately, PRKCA was selected for validation due to its significant prognostic effect, which predicts unfavorable overall survival and disease-free survival in patients with lung cancer. Network analysis conducted on The Cancer Genome Atlas (TCGA) database revealed a strong gene-level correlation between PRKCA and TP53, CDKN2A, BYR2, TTN, KRAS, and PIK3CA; whereas at the protein level, it exhibited a high correlation with EGFR, Lck, Bcl2, and Syk. The in vitro experiments revealed that PRKCA was upregulated in the cisplatin-resistant A549 cells (DDP-A549), while knockdown of PRKCA increased DDP-A549 apoptosis upon cisplatin treatment. Moreover, we observed that PRKCA knockdown attenuated DDP-A549 proliferation, migration and invasion ability. Western blot analysis demonstrated that PRKCA knockdown downregulated phosphorylation of PI3K expression while upregulated the genes involved in ferroptosis signaling. In summary, our results elucidate the role of PRKCA in acquiring resistance to cisplatin and underscore its potential as a therapeutic target for cisplatin-resistant lung cancer.

Circular RNA in Non-small Cell Lung Carcinoma: Identification of Targets and New Treatment Modalities

Despite availability of several treatment options for non-small cell lung cancer (NSCLC), such as surgery, chemotherapy, radiation, targeted therapy and immunotherapy, the survival rate of patients for five years is in the range of 22%. Therefore, identification of new targets and treatment modalities for this disease is an important issue. In this context, we screened the PubMed database for up-regulated circular RNAs (circRNAs) which promote growth of NSCLC in preclinical models in vitro as well as in vivo xenograft models in immuno-compromised mice. This approach led to potential targets for further validation and inhibition with small molecules or antibody-derived entities. In case of preclinical validation, the corresponding circRNAs can be inhibited with small interfering RNAs (siRNA) or short hairpin RNAs (shRNA). The identified circRNAs act by sponging microRNAs (miRs) preventing cleavage of the mRNA of the corresponding targets. We identified nine circRNAs up-regulating transmembrane receptors, five circRNAs increasing expression of secreted proteins, nine circRNAs promoting expression of components of signaling pathways, six circRNAs involved in regulation of splicing and RNA processing, six circRNAs up-regulating actin-related and RNA processing components, seven circRNAs increasing the steady-state levels of transcription factors, two circRNAs increasing high-mobility group proteins, four circRNAs increasing components of the epigenetic modification system and three circRNAs up-regulating protein components of additional systems.

Protein Kinase C at the Crossroad of Mutations, Cancer, Targeted Therapy and Immune Response

The frequent PKC dysregulations observed in many tumors have made these enzymes natural targets for anticancer applications. Nevertheless, this considerable interest in the development of PKC modulators has not led to the expected therapeutic benefits, likely due to the complex biological activities regulated by PKC isoenzymes, often playing ambiguous and protective functions, further driven by the occurrence of mutations. The structure, regulation and functions of PKCs have been extensively covered in other publications. Herein, we focused on PKC alterations mostly associated with complete functional loss. We also addressed the modest yet encouraging results obtained targeting PKC in selected malignancies and the more frequent negative clinical outcomes. The reported observations advocate the need for more selective molecules and a better understanding of the involved pathways. Furthermore, we underlined the most relevant immune mechanisms controlled by PKC isoforms potentially impacting the immune checkpoint inhibitor blockade-mediated immune recovery. We believe that a comprehensive examination of the molecular features of the tumor microenvironment might improve clinical outcomes by tailoring PKC modulation. This approach can be further supported by the identification of potential response biomarkers, which may indicate patients who may benefit from the manipulation of distinctive PKC isoforms.

KRAS mutation-induced EndMT of brain arteriovenous malformation is mediated through the TGF-β/BMP-SMAD4 pathway

OBJECTIVE

Somatic KRAS mutations have been identified in the majority of brain arteriovenous malformations (bAVMs), and subsequent in vivo experiments have confirmed that KRAS mutation in endothelial cells (ECs) causes AVMs in mouse and zebrafish models. Our previous study demonstrated that the KRASG12D mutant independently induced the endothelial-mesenchymal transition (EndMT), which was reversed by treatment with the lipid-lowering drug lovastatin. However, the underlying mechanisms of action were unclear.

METHODS

We used human umbilical vein ECs (HUVECs) overexpressing the KRASG12D mutant for Western blotting, quantitative real-time PCR, and immunofluorescence and wound healing assays to evaluate the EndMT and determine the activation of downstream pathways. Knockdown of SMAD4 by RNA interference was performed to explore the role of SMAD4 in regulating the EndMT. BAVM ECs expressing the KRASG12D mutant were obtained to verify the SMAD4 function. Finally, we performed a coimmunoprecipitation assay to probe the mechanism by which lovastatin affects SMAD4.

RESULTS

HUVECs infected with KRASG12D adenovirus underwent the EndMT. Transforming growth factor beta (TGF-β) and bone morphogenetic protein (BMP) signalling pathways were activated in the KRASG12D-mutant HUVECs and ECs in bAVM tissue. Knocking down SMAD4 expression in both KRASG12D-mutant HUVECs and ECs in bAVM tissues inhibited the EndMT. Lovastatin attenuated the EndMT by downregulating p-SMAD2/3, p-SMAD1/5 and acetylated SMAD4 expression in KRASG12D-mutant HUVECs.

CONCLUSIONS

Our findings suggest that the KRASG12D mutant induces the EndMT by activating the ERK-TGF-β/BMP-SMAD4 signalling pathway and that lovastatin inhibits the EndMT by suppressing TGF-β/BMP pathway activation and SMAD4 acetylation.

Notch signaling pathway: a comprehensive prognostic and gene expression profile analysis in breast cancer

Breast cancer is a complex disease exhibiting a great degree of heterogeneity due to different molecular subtypes. Notch signaling regulates the differentiation of breast epithelial cells during normal development and plays a crucial role in breast cancer progression through the abnormal expression of the Notch up-and down-stream effectors. To date, there are only a few patient-centered clinical studies using datasets characterizing the role of Notch signaling pathway regulators in breast cancer; thus, we investigate the role and functionality of these factors in different subtypes using publicly available databases containing records from large studies. High-throughput genomic data and clinical information extracted from TCGA were analyzed. We performed Kaplan-Meier survival and differential gene expression analyses using the HALLMARK_NOTCH_SIGNALING gene set. To determine if epigenetic regulation of the Notch regulators contributes to their expression, we analyzed methylation levels of these factors using the TCGA HumanMethylation450 Array data. Notch receptors and ligands expression is generally associated with the tumor subtype, grade, and stage. Furthermore, we showed gene expression levels of most Notch factors were associated with DNA methylation rate. Modulating the expression levels of Notch receptors and effectors can be a potential therapeutic approach for breast cancer. As we outline herein, elucidating the novel prognostic and regulatory roles of Notch implicate this pathway as an essential mediator controlling breast cancer progression.

Protein Kinase C (PKC) Isozymes as Diagnostic and Prognostic Biomarkers and Therapeutic Targets for Cancer

Protein kinase C (PKC) is a large family of calcium- and phospholipid-dependent serine/threonine kinases that consists of at least 11 isozymes. Based on their structural characteristics and mode of activation, the PKC family is classified into three subfamilies: conventional or classic (cPKCs; α, βI, βII, and γ), novel or non-classic (nPKCs; δ, ε, η, and θ), and atypical (aPKCs; ζ, ι, and λ) (PKCλ is the mouse homolog of PKCι) PKC isozymes. PKC isozymes play important roles in proliferation, differentiation, survival, migration, invasion, apoptosis, and anticancer drug resistance in cancer cells. Several studies have shown a positive relationship between PKC isozymes and poor disease-free survival, poor survival following anticancer drug treatment, and increased recurrence. Furthermore, a higher level of PKC activation has been reported in cancer tissues compared to that in normal tissues. These data suggest that PKC isozymes represent potential diagnostic and prognostic biomarkers and therapeutic targets for cancer. This review summarizes the current knowledge and discusses the potential of PKC isozymes as biomarkers in the diagnosis, prognosis, and treatment of cancers.

PROTEIN KINASE C ALPHA IS A CENTRAL NODE FOR TUMORIGENIC TRANSCRIPTIONAL NETWORKS IN HUMAN PROSTATE CANCER

Aberrant expression of protein kinase C (PKC) isozymes is a hallmark of cancer. The different members of the PKC family control cellular events associated with cancer development and progression. Whereas the classical/conventional PKCα isozyme has been linked to tumor suppression in most cancer types, here we demonstrate that this kinase is required for the mitogenic activity of aggressive human prostate cancer cells displaying aberrantly high PKCα expression. Immunohistochemical analysis showed abnormal up-regulation of PKCα in human primary prostate tumors. Interestingly, silencing PKCα expression from aggressive prostate cancer cells impairs cell cycle progression, proliferation and invasion, as well as their tumorigenic activity in a mouse xenograft model. Mechanistic analysis revealed that PKCα exerts a profound control of gene expression, particularly over genes and transcriptional networks associated with cell cycle progression and E2F transcription factors. PKCα RNAi depletion from PC3 prostate cancer cells led to a reduction in the expression of pro-inflammatory cytokine and epithelial-to-mesenchymal transition (EMT) genes, as well as a prominent down-regulation of the immune checkpoint ligand PD-L1. This PKCα-dependent gene expression profile was corroborated in silico using human prostate cancer databases. Our studies established PKCα as a multifunctional kinase that plays pleiotropic roles in prostate cancer, particularly by controlling genetic networks associated with tumor growth and progression. The identification of PKCα as a pro-tumorigenic kinase in human prostate cancer provides strong rationale for the development of therapeutic approaches towards targeting PKCα or its effectors.

PKC signal amplification suppresses non-small cell lung cancer growth by promoting p21 expression and phosphorylation

Protein kinase C (PKC) activation was previously associated with oncogenic features. However, small molecule inhibitors targeting PKC have so far proved ineffective in a number of clinical trials for cancer treatment. Recent progresses have revealed that most PKC mutations detected in diverse cancers actually lead to loss-of-function, thus suggesting the tumor-suppressive roles of PKC proteins. Unfortunately, the development of chemicals to enhance PKC activity is lagging behind relative to its small molecular inhibitors. Here, we report that a bisindolylmaleimide derivative (3,4-bis(1-(prop-2-ynyl)-1H-indol-3-yl)-1 H-pyrrole-2,5-dione, BD-15) significantly inhibited cell growth in non-small cell lung cancer (NSCLC). Mechanistically, BD-15 treatment resulted in markedly enhanced phosphorylation of PKC substrates and led to cell cycle arrest in G2/M. Further, BD-15 treatment upregulated p21 protein levels and enhanced p21 phosphorylation. BD-15 also promoted caspase3 cleavage and triggered cellular apoptosis. In xenograft mouse models, BD-15 exerted anti-tumor effects to suppress in vivo tumor formation. Collectively, our findings revealed the tumor-suppressive roles of BD-15 through enhancing PKC signaling and thus leading to upregulation of p21 expression and phosphorylation.

PKCα and PKCδ: Friends and Rivals

PKC comprises a large family of serine/threonine kinases that share a requirement for allosteric activation by lipids. While PKC isoforms have significant homology, functional divergence is evident among subfamilies and between individual PKC isoforms within a subfamily. Here, we highlight these differences by comparing the regulation and function of representative PKC isoforms from the conventional (PKCα) and novel (PKCδ) subfamilies. We discuss how unique structural features of PKCα and PKCδ underlie differences in activation and highlight the similar, divergent, and even opposing biological functions of these kinases. We also consider how PKCα and PKCδ can contribute to pathophysiological conditions and discuss challenges to targeting these kinases therapeutically.

Thrombin Induces COX-2 and PGE2 Expression via PAR1/PKCalpha/MAPK-Dependent NF-kappaB Activation in Human Tracheal Smooth Muscle Cells

The inflammation of the airway and lung could be triggered by upregulation cyclooxygenase (COX)-2 and prostaglandin E₂ (PGE₂) induced by various proinflammatory factors. COX-2 induction by thrombin has been shown to play a vital role in various inflammatory diseases. However, in human tracheal smooth muscle cells (HTSMCs), how thrombin enhanced the levels of COX-2/PGE₂ is not completely characterized. Thus, in this study, the levels of COX-2 expression and PGE₂ synthesis induced by thrombin were determined by Western blot, promoter-reporter assay, real-time PCR, and ELISA kit. The various signaling components involved in the thrombin-mediated responses were differentiated by transfection with siRNAs and selective pharmacological inhibitors. The role of NF-κB was assessed by a chromatin immunoprecipitation (ChIP) assay, immunofluorescent staining, as well as Western blot. Our results verified that thrombin markedly triggered PGE₂ secretion via COX-2 upregulation which were diminished by the inhibitor of thrombin (PPACK), PAR1 (SCH79797), G_i/o protein (GPA2), G_q protein (GPA2A), PKCα (Gö6976), p38 MAPK (SB202190), JNK1/2 (SP600125), MEK1/2 (U0126), or NF-κB (helenalin) and transfection with siRNA of PAR1, G_qα, G_iα, PKCα, JNK2, p38, p42, or p65. Moreover, thrombin induced PAR1-dependent PKCα phosphorylation in HTSMCs. We also observed that thrombin induced p38 MAPK, JNK1/2, and p42/p44 MAPK activation through a PAR1/PKCα pathway. Thrombin promoted phosphorylation of NF-κB p65, leading to nuclear translocation and binding to the COX-2 promoter element to enhance promoter activity, which was reduced by Gö6976, SP600125, SB202190, or U0126. These findings supported that COX-2/PGE₂ expression triggered by thrombin was engaged in PAR1/G_q or G_i/o/PKCα/MAPK-dependent NF-κB activation in HTSMCs.

Overarching roles of diacylglycerol signaling in cancer development and antitumor immunity

Diacylglycerol (DAG) is a lipid second messenger that is generated in response to extracellular stimuli and channels intracellular signals that affect mammalian cell proliferation, survival, and motility. DAG exerts a myriad of biological functions through protein kinase C (PKC) and other effectors, such as protein kinase D (PKD) isozymes and small GTPase-regulating proteins (such as RasGRPs). Imbalances in the fine-tuned homeostasis between DAG generation by phospholipase C (PLC) enzymes and termination by DAG kinases (DGKs), as well as dysregulation in the activity or abundance of DAG effectors, have been widely associated with tumor initiation, progression, and metastasis. DAG is also a key orchestrator of T cell function and thus plays a major role in tumor immunosurveillance. In addition, DAG pathways shape the tumor ecosystem by arbitrating the complex, dynamic interaction between cancer cells and the immune landscape, hence representing powerful modifiers of immune checkpoint and adoptive T cell-directed immunotherapy. Exploiting the wide spectrum of DAG signals from an integrated perspective could underscore meaningful advances in targeted cancer therapy.

The deubiquitinase USP16 functions as an oncogenic factor in K-RAS-driven lung tumorigenesis

K-RAS mutation and molecular alterations of its surrogates function essentially in lung tumorigenesis and malignant progression. However, it remains elusive how tumor-promoting and deleterious events downstream of K-RAS signaling are coordinated in lung tumorigenesis. Here, we show that USP16, a deubiquitinase involved in various biological processes, functions as a promoter for the development of K-RAS-driven lung tumor. Usp16 deletion significantly attenuates K-rasG12D-mutation-induced lung tumorigenesis in mice. USP16 upregulation upon RAS activation averts reactive oxygen species (ROS)-induced p38 activation that would otherwise detrimentally influence the survival and proliferation of tumor cells. In addition, USP16 interacts with and deubiquitinates JAK1, and thereby promoting lung tumor growth by augmenting JAK1 signaling. Therefore, our results reveal that USP16 functions critically in the K-RAS-driven lung tumorigenesis through modulating the strength of p38 and JAK1 signaling.

The complexities of PKCα signaling in cancer

Protein kinase C α (PKCα) is a ubiquitously expressed member of the PKC family of serine/threonine kinases with diverse functions in normal and neoplastic cells. Early studies identified anti-proliferative and differentiation-inducing functions for PKCα in some normal tissues (e.g., regenerating epithelia) and pro-proliferative effects in others (e.g., cells of the hematopoietic system, smooth muscle cells). Additional well documented roles of PKCα signaling in normal cells include regulation of the cytoskeleton, cell adhesion, and cell migration, and PKCα can function as a survival factor in many contexts. While a majority of tumors lose expression of PKCα, others display aberrant overexpression of the enzyme. Cancer-related mutations in PKCα are uncommon, but rare examples of driver mutations have been detected in certain cancer types (e. g., choroid gliomas). Here we review the role of PKCα in various cancers, describe mechanisms by which PKCα affects cancer-related cell functions, and discuss how the diverse functions of PKCα contribute to tumor suppressive and tumor promoting activities of the enzyme. We end the discussion by addressing mutations and expression of PKCα in tumors and the clinical relevance of these findings.

PHLPPing the balance: restoration of protein kinase C in cancer

Protein kinase signalling, which transduces external messages to mediate cellular growth and metabolism, is frequently deregulated in human disease, and specifically in cancer. As such, there are 77 kinase inhibitors currently approved for the treatment of human disease by the FDA. Due to their historical association as the receptors for the tumour-promoting phorbol esters, PKC isozymes were initially targeted as oncogenes in cancer. However, a meta-analysis of clinical trials with PKC inhibitors in combination with chemotherapy revealed that these treatments were not advantageous, and instead resulted in poorer outcomes and greater adverse effects. More recent studies suggest that instead of inhibiting PKC, therapies should aim to restore PKC function in cancer: cancer-associated PKC mutations are generally loss-of-function and high PKC protein is protective in many cancers, including most notably KRAS-driven cancers. These recent findings have reframed PKC as having a tumour suppressive function. This review focusses on a potential new mechanism of restoring PKC function in cancer - through targeting of its negative regulator, the Ser/Thr protein phosphatase PHLPP. This phosphatase regulates PKC steady-state levels by regulating the phosphorylation of a key site, the hydrophobic motif, whose phosphorylation is necessary for the stability of the enzyme. We also consider whether the phosphorylation of the potent oncogene KRAS provides a mechanism by which high PKC expression may be protective in KRAS-driven human cancers.

Exploring the five different genes associated with PKCα in bladder cancer based on gene expression microarray

Much progress has been made in understanding the mechanism of bladder cancer (BC) progression. Protein kinase C‐α (PKCα) is overexpressed in many kinds of cancers. Additionally, PKCα is considered an oncogene that regulates proliferation, invasion, migration, apoptosis and cell cycle in multiple cancers. However, the mechanism underlying how these cellular processes are regulated by PKCα remains unknown. In the present study, we used PKCα siRNA to knock down PKCα gene expression and found that down‐regulation of PKCα could significantly inhibit cell proliferation, migration and invasion and induce apoptosis and G1/S cell cycle arrest in vitro. Overexpression of PKCα promotes tumour growth in vivo. We applied cDNA microarray technology to detect the differential gene expression in J82 cells with PKCα knockdown and found that five key genes (BIRC2, BIRC3, CDK4, TRAF1 and BMP4) were involved in proliferation and apoptosis according to GO analysis and pathway analyses. Correlation analysis revealed a moderate positive correlation between PKCα expression and the expression of five downstream genes. BIRC2 and BIRC3 inhibit apoptosis, whereas CDK4, TRAF1 and BMP4 promote proliferation. Essentially, all five of these target genes participated in proliferation, and apoptosis was regulated by PKCα via the NF‐kB signalling pathway.

Genes and pathways involved in senescence bypass identified by functional genetic screens

Cellular senescence is a state of stable and irreversible cell cycle arrest with active metabolism, that normal cells undergo after a finite number of divisions (Hayflick limit). Senescence can be triggered by intrinsic and/or extrinsic stimuli including telomere shortening at the end of a cell's lifespan (telomere-initiated senescence) and in response to oxidative, genotoxic or oncogenic stresses (stress-induced premature senescence). Several effector mechanisms have been proposed to explain senescence programmes in diploid cells, including the induction of DNA damage responses, a senescence-associated secretory phenotype and epigenetic changes. Senescent cells display senescence-associated-β-galactosidase activity and undergo chromatin remodeling resulting in heterochromatinisation. Senescence is established by the pRb and p53 tumour suppressor networks. Senescence has been detected in in vitro cellular settings and in premalignant, but not malignant lesions in mice and humans expressing mutant oncogenes. Despite oncogene-induced senescence, which is believed to be a cancer initiating barrier and other tumour suppressive mechanisms, benign cancers may still develop into malignancies by bypassing senescence. Here, we summarise the functional genetic screens that have identified genes, uncovered pathways and characterised mechanisms involved in senescence evasion. These include cell cycle regulators and tumour suppressor pathways, DNA damage response pathways, epigenetic regulators, SASP components and noncoding RNAs.

The KRAS/Lin28B axis maintains stemness of pancreatic cancer cells via the let-7i/TET3 pathway

Increasing evidence demonstrates that Lin28B plays critical roles in numerous biological processes including cell proliferation and stemness maintenance. However, the molecular mechanisms underlying Lin28B nuclear translocation remain poorly understood. Here, we found for the first time that KRAS promoted Lin28B nuclear translocation through PKCβ, which directly bound to and phosphorylated Lin28B at S243. Firstly, we observed that Lin28B was upregulated in pancreatic cancer, contributing to cellular migration and proliferation. Furthermore, nuclear Lin28B upregulated TET3 messenger RNA and protein levels by blocking the production of mature let‐7i. Subsequently, increased TET3 expression could also promote the expression of Lin28B, thereby forming a Lin28B/let‐7i/TET3 feedback loop. Our results suggest that the KRAS/Lin28B axis drives the let‐7i/TET3 pathway to maintain the stemness of pancreatic cancer cells. These findings illuminate the distinct mechanism of Lin28B nuclear translocation and its important roles in KRAS‐driven pancreatic cancer, and have important implications for development of novel therapeutic strategies for this cancer.

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.

PKC Is Required for KRAS-Driven Lung Tumorigenesis

Non-small cell lung cancer (NSCLC) is the most frequent subtype of lung cancer and remains a highly lethal malignancy and one of the leading causes of cancer-related deaths worldwide. Mutant KRAS is the prevailing oncogenic driver of lung adenocarcinoma, the most common histologic form of NSCLC. In this study, we examined the role of PKCϵ, an oncogenic kinase highly expressed in NSCLC and other cancers, in KRAS-driven tumorigenesis. Database analysis revealed an association between PKCϵ expression and poor outcome in patients with lung adenocarcinoma specifically harboring KRAS mutations. A PKCϵ-deficient, conditionally activatable allele of oncogenic Kras (LSL-Kras^G12D ;PKCϵ^-/- mice) demonstrated the requirement of PKCϵ for Kras-driven lung tumorigenesis in vivo, which was consistent with impaired transformed growth reported in PKCϵ-deficient KRAS-dependent NSCLC cells. Moreover, PKCϵ-knockout mice were found to be less susceptible to lung tumorigenesis induced by benzo[a]pyrene, a carcinogen that induces mutations in Kras. Mechanistic analysis using RNA sequencing revealed little overlap for PKCϵ and KRAS in the control of genes and biological pathways relevant in NSCLC, suggesting that a permissive role of PKCϵ in KRAS-driven lung tumorigenesis may involve nonredundant mechanisms. Our results thus, highlight the relevance and potential of targeting PKCϵ for lung cancer therapeutics. SIGNIFICANCE: These findings demonstrate that KRAS-mediated tumorigenesis requires PKCϵ expression and highlight the potential for developing PKCϵ-targeted therapies for oncogenic RAS-driven malignancies.

Knockdown of circPRKCA Restrained Cell Growth, Migration, and Invasion of NSCLC Cells Both in vitro and in vivo via Regulating miR-330-5p/PDK1/AKT Pathway

Background

Protein kinase Cα (PRKCA) is an oncogene in multiple cancers including non-small-cell lung cancer (NSCLC) and can be transcribed into a number of circular PRKCAs (circPRKCAs). Here, we aimed to elaborate the role and mechanism of circPRKCA_024 (circPRKCA) in malignant progression of NSCLC.

Methods

Expression of circPRKCA, miRNA (miR)-330-5p and 3-phosphoinositide-dependent protein kinase-1 (PDK1) was measured by real-time quantitative PCR and Western blotting, and their relationship was testified by dual-luciferase reporter assay, RNA immunoprecipitation, and RNA pull-down assay. Cell behaviors were evaluated by cell counting kit (CCK)-8, flow cytometry, and transwell assays. AKT activity was confirmed by Western blotting. Xenograft experiment assessed tumor growth.

Results

Expression of circPRKCA and PDK1 was upregulated, and miR-330-5p was downregulated in NSCLC tissues and cell lines. High circPRKCA was correlated with TNM stage and lymph node metastasis of NSCLC patients. Silencing circPRKCA could suppress cell viability, migration, and invasion in A549 and H1299 cells, accompanied with apoptosis rate promotion. Moreover, circPRKCA knockdown retarded tumor growth of A549 cells in vivo. Molecularly, miR-330-5p was sponged by circPRKCA, and PDK1 was a target of miR-330-5p. Inhibiting miR-330-5p could attenuate the suppression of circPRKCA knockdown on cell growth, migration, and invasion; contrarily, promoting miR-330-5p caused inhibition on those cell behaviors by downregulating PDK1. Analogously, AKT activity was suppressed by circPRKCA downregulation and miR-330-5p upregulation in NSCLC cells both in vitro and in vivo.

Conclusion

Depleting circPRKCA inhibited PDK1 to suppress NSCLC cell malignant behaviors through miR-330-5p/PDK1/AKT pathway.

The PKC universe keeps expanding: From cancer initiation to metastasis

Classical and novel protein kinase C (PKC) isozymes (c/nPKCs), members of the PKC family that become activated by the lipid second messenger diacylglycerol (DAG) and phorbol esters, exert a myriad of cellular effects that impact proliferative and motile cellular responses. While c/nPKCs have been indisputably associated with tumor promotion, their roles exceed by far their sole involvement as promoter kinases. Indeed, this original dogma has been subsequently redefined by the introduction of several new concepts: the identification of tumor suppressing roles for c/nPKCs, and their participation in early and late stages of carcinogenesis. This review dives deep into the intricate roles of c/nPKCs in cancer initiation as well as in the different stages of the metastatic cascade, with great emphasis in their involvement in cancer cell motility via regulation of small Rho GTPases, the production of extracellular matrix (ECM)-degrading proteases, and the epithelial-to-mesenchymal transition (EMT) program required for the acquisition of highly invasive traits. Here, we highlight functional interplays between either PKCα or PKCε and mesenchymal features that may ultimately contribute to anticancer drug resistance in cellular and animal models. We also introduce the novel hypothesis that c/nPKCs may be implicated in the control of immune evasion through the regulation of immune checkpoint protein expression. In summary, dissecting the colossal complexity of c/nPKC signaling in the wide spectrum of cancer progression may bring new opportunities for the development of meaningful tools aiding for cancer prognosis and therapy.

MED13L integrates Mediator-regulated epigenetic control into lung cancer radiosensitivity

To date, efforts to improve non-small-cell lung cancer (NSCLC) outcomes with increased radiation dose have not been successful. Identification of novel druggable targets that are capable to modulate NSCLC radiosensitivity may provide a way forward. Mediator complex is implicated in gene expression control, but it remains unclear how Mediator dysfunction is involved in cancer radiotherapy.

Methods: The biologic functions of miR-4497, MED13L and PRKCA in NSCLC radiosensitivity were examined through biochemical assays including gene expression profilling, cell proliferation assay, colony formation assay, wound healing assay, transwell assay, dual luciferase reporter assay, xenograft models, immunoprecipitation, and chromatin immunoprecipitation sequencing. Clinical implications of miR-4497, MED13L and PRKCA in radiosensitivity were evaluated in NSCLC patients treated with concurrent chemoradiotherapy or radiotherapy alone.

Results: We found that radiation can trigger disassemble of Mediator complex via silencing of MED13L by miR-4497 in NSCLC. Although not interrupting structure integrity of the core Mediator or the CDK8 kinase module, suppression of MED13L attenuated their physical interactions and reduced recruitment of acetyltransferase P300 to chromatin via Mediator. Silencing of MED13L therefore diminishes global H3K27ac signals written by P300, activities of enhancer and/or promoters and expression of multiple oncogenes, especially PRKCA. Inhibition of PRKCA expression potentiates the killing effect of radiotherapy in vitro and in vivo. Remarkably, high PRKCA expression in NSCLC tissues is correlated with poor prognosis of patients received radiotherapy.

Conclusions: Our study linking PRKCA to radiosensitivity through a novel mechanism may enable the rational targeting of PRKCA to unlock therapeutic potentials of NSCLC.

Downregulation of hsa_circ_0007580 inhibits non-small cell lung cancer tumorigenesis by reducing miR-545-3p sponging

Non-small cell lung cancer (NSCLC) is a highly malignant tumor. Many circular RNAs (circRNAs) are reportedly in regulating the progression of NSCLC. To identify potential therapeutic targets for NSCLC, we conducted a bioinformatics analysis of circRNAs differentially expressed between NSCLC tissues and adjacent normal tissues. Hsa_circ_0007580 was upregulated in NSCLC tumor tissues, and the expression of its host gene (protein kinase Ca) correlated negatively with overall survival. Short-hairpin RNAs were used to knock down hsa_circ_0007580 in NSCLC cells, and gene and protein levels were measured with qRT-PCR and Western blotting, respectively. NSCLC cell proliferation, migration and apoptosis were evaluated with CCK-8 assays, Ki-67 staining, Transwell assays and flow cytometry, respectively. Knocking down hsa_circ_0007580 inhibited proliferation and invasion by NSCLC cells and induced their apoptosis. Dual luciferase reporter assays indicated that miR-545-3p can bind to hsa_circ_0007580 (suggesting that hsa_circ_0007580 sponges miR-545-3p) and to protein kinase Ca (suggesting that miR-545-3p directly inhibits this gene). In a xenograft tumor model, downregulating hsa_circ_0007580 inhibited NSCLC tumorigenesis by inactivating p38/mitogen-activated protein kinase signaling. Thus, silencing hsa_circ_0007580 notably inhibited NSCLC progression in vitro and in vivo, suggesting this circRNA could be a novel treatment target for NSCLC.

Resveratrol attenuates diabetes-associated cell centrosome amplification via inhibiting the PKCα-p38 to c-myc/c-jun pathway

Type 2 diabetes increases the risk for cancer. Centrosome amplification can initiate tumorigenesis. We have described that type 2 diabetes increases the centrosome amplification of peripheral blood mononuclear cells, with high glucose, insulin, and palmitic acid as the triggers, which suggests that centrosome amplification is a candidate biological mechanism linking diabetes to cancer. In this study, we aimed to further investigate the signaling pathways of the diabetes-associated centrosome amplification and to examine whether and how resveratrol inhibits the centrosome amplification. The results showed that treatment with high glucose, insulin, and palmitic acid, alone or in combination, could increase the protein levels of phospho-protein kinase C alpha (p-PKCα), phospho-p38 mitogen-activated protein kinases (p-p38), c-myc, and c-jun, as well as the mRNA levels of c-myc and c-jun. PKCα inhibitor could inhibit the treatment-induced increase in the protein levels of p-p38, c-myc, and c-jun. Inhibitor or siRNA of p38 was also able to inhibit the treatment-induced increase in the levels of p-p38, c-myc, and c-jun. Meanwhile, knockdown of c-myc or c-jun did not alter the treatment-induced increase in the phosphorylation of PKCα or p38. Importantly, inhibition of the phosphorylation of PKCα or p38 and knockdown of c-myc or c-jun could attenuate the centrosome amplification. In diabetic mice, the levels of p-PKCα, p-p38, c-myc, and c-jun were all increased in the colon tissues. Interestingly, resveratrol, but not metformin, was able to attenuate the treatment-induced increase in the levels of p-PKCα, p-p38, c-myc, and c-jun, as well as the centrosome amplification. In conclusion, our results suggest that PKCα-p38 to c-myc/c-jun is the signaling pathway of the diabetes-associated centrosome amplification, and resveratrol attenuates the centrosome amplification by inhibiting this signaling pathway.

Simvastatin Mitigates Apoptosis and Transforming Growth Factor-Beta Upregulation in Stretch-Induced Endothelial Cells

Portal hypertension is a common clinical symptom of digestive disorders. With an increase in portal pressure, the portal vein will continue to dilate. We aimed to determine whether continuous stretch induced by portal hypertension may impair the function of endothelial cells (ECs) in the portal vein and aggravate the progress of portal hypertension and explore its mechanism. ECs were cultured on an elastic silicone membrane and subjected to continuous uniaxial stretch. Apoptosis and expression of TGF-β in ECs under stretch were measured. We found that sustained stretch induced the apoptosis of ECs in a stretch length-dependent manner. Compared with the control, continuous stretch increased the nicotinamide adenine dinucleotide phosphate oxidase 2 (NOX2) expression and damaged the mitochondria, resulting in an evident increase in reactive oxygen species (ROS) levels; pretreatment with gp91ds-tat or MitoTEMPO decreased the ROS level in the intracellular levels. N-acetyl-cysteine (NAC) treatment before stretch not only reduced ROS levels but also mitigated the apoptosis of ECs; simvastatin had similar effects through targeting NOX2 and mitochondria. During the stretch, the phosphorylation of p38 mitogen-activated protein kinase (P38MAPK), c-Jun N-terminal kinase (JNK), and nuclear factor-kappa B (NF-κB) was obviously increased; pretreatment with P38MAPK or JNK inhibitors decreased the phosphorylation of NF-κB and TGF-β expression. Pyrrolidine dithiocarbamate (PDTC) treatment before stretch also reduced TGF-β expression. After pretreatment with NAC, the phosphorylation of P38MAPK, JNK, and NF-κB and TGF-β expressions in ECs under stretch was suppressed; similar results were observed in simvastatin-treated ECs. This study demonstrated that simvastatin could mitigate EC apoptosis and TGF-β upregulation induced by continuous stretch by reducing the level of ROS.

Discovery of a novel protein kinase C activator from Croton tiglium for inhibition of non-small cell lung cancer

BACKGROUND

The incidence of non-small cell lung cancer (NSCLC) accounts for approximately 85-90% of lung cancer, which has been shown to be challenging for treatment owing to poorly understanding of pathological mechanisms. Natural products serve as a source of almost all pharmaceutical preparations or offer guidance for those chemicals that have entered clinical trials, especially in NSCLC.

PURPOSE

We investigated the effect of B10G5, a natural products isolated from the Croton tiglium, in human non-small cell lung canceras as a protein kinase C (PKC) activator.

METHODS

The cell viability assay was evaluated by the MTT assay. The apoptosis and cell cycle distribution were assessed by flow cytometry. Reactive oxygen species (ROS) production was determined by using the fluorescent probe DCFDA. Cell migration ability of H1975 cells was analyzed by using the wound healing assay. The inhibiting effect of B10G5 against the phosphorylation level of the substrate by PKCs was assessed by using homogeneous time-resolved fluorescence (HTRF) technology. The correlation between PKCs and overall survival (OS) of Lung Adenocarcinoma (LUAD) patients was analysis by TCGA portal. The binding mode between B10G5 and the PKC isoforms was explored by molecular docking. Protein expression was detected by western blotting analysis.

RESULTS

B10G5 suppressed cell proliferation and colony formation, as well as migration ability of NSCLC cells, without significant toxic effect on normal lung cells. B10G5 induced the cell apoptosis through the development of PARP cleavage, which is evidenced by means of the production of mitochondrial ROS. In addition, the B10G5 inhibitory effect was also related to the cell cycle arrest at G2/M phase. Mechanistically, molecular modelling technology suggested that the potential target of B10G5 was associated with PKC family. In vitro PKC kinase assay indicated that B10G5 effectively activated the PKC activity. Western blotting data revealed that B10G5 upregulated PKC to activate PKC-mediated RAF/MEK/ERK pathway.

CONCLUSION

Our results showed that B10G5, a naturally occurring phorbol ester, considered to be a potential and a valuable therapeutic chemical in the treatment of NSCLC.

PKCα is required for Akt-mTORC1 activation in non-small cell lung carcinoma (NSCLC) with EGFR mutation

Mutational activation of the epidermal growth factor receptor (EGFR) is a major player in the pathogenesis of non-small cell lung cancer (NSCLC). NSCLC patients with constitutively active EGFR mutations (mEGFR) eventually develop drug resistance against EGFR tyrosine-kinase inhibitors (TKIs); therefore, better understandings of key components of mEGFR signaling are required. Here, we initially observed aberrantly high expression of protein kinase Cα (PKCα) in lung adenocarcinomas, especially those with mEGFR, and proceeded to examine the role of PKCα in the regulation of the signaling pathways downstream of mutant EGFR (mtEGFR). The results showed that NSCLC cell lines with constitutively active EGFR mutations tend to have very or moderately high PKCα levels. Furthermore, PKCα was constitutively activated in HCC827 and H4006 cells which have an EGFR deletion mutation in exon 19. Interestingly, mtEGFR was not required for the induction of PKCα at protein and message levels, suggesting that the increased levels of PKCα are due to independent selection. Whereas, mtEGFR activity was required for robust activation of PKCα. Loss of functions studies revealed that the NSCLC cells rely heavily on PKCα for the activation of the mTORC1 signaling pathway. Unexpectedly, the results demonstrated that PKCα was required for activation of Akt upstream of mTOR but only in cells with the mtEGFR and with the increased expression of PKCα. Functionally, inhibition of PKCα in HCC827 led to caspase-3-dependent apoptosis and a significant decrease in cell survival in response to cellular stress induced by serum starvation. In summary, the results identified important roles of PKCα in regulating mTORC1 activity in lung cancer cells, whereby a primary switching occurs from PKCα-independent to PKCα-dependent signaling in the presence of mEGFR. The results present PKCα as a potential synergistic target of personalized treatment for NSCLC with constitutively active mutant forms of EGFR and constitutively active PKCα.

Distinctive requirement of PKCε in the control of Rho GTPases in epithelial and mesenchymally transformed lung cancer cells

Diacylglycerol (DAG)/phorbol ester-regulated protein kinase C (PKC) isozymes have been widely linked to tumor promotion and the development of a metastatic phenotype. PKCε, an oncogenic member of the PKC family, is abnormally overexpressed in lung cancer and other cancer types. This kinase plays significant roles in proliferation, survival and migration; however its role in epithelial-to-mesenchymal transition (EMT) has been scarcely studied. Silencing experiments in non-small lung cancer (NSCLC) cells revealed that PKCε or other DAG-regulated PKCs (PKCα and PKCδ) were dispensable for the acquisition of a mesenchymal phenotype induced by transforming growth factor beta (TGF-β). Unexpectedly, we found a nearly complete down-regulation of PKCε expression in TGF-β-mesenchymally transformed NSCLC cells. PMA and AJH-836 (a DAG-mimetic that preferentially activates PKCε) promote ruffle formation in NSCLC cells via Rac1, however they fail to induce these morphological changes in TGF-β-mesenchymally transformed cells despite their elevated Rac1 activity. Several Rac Guanine nucleotide Exchange-Factors (Rac-GEFs) were also up-regulated in TGF-β-treated NSCLC cells, including Trio and Tiam2, which were required for cell motility. Lastly, we found that silencing or inhibiting PKCε enhances RhoA activity and stress fiber formation, a phenotype also observed in TGF-β-transformed cells. Our studies established a distinctive involvement of PKCε in epithelial and mesenchymal NSCLC cells, and identified a complex interplay between PKCε and small GTPases that contributes to regulation of NSCLC cell morphology and motile activity.

Long non-coding RNA ZEB1-AS1 regulates miR-200b/FSCN1 signaling and enhances migration and invasion induced by TGF-β1 in bladder cancer cells

Background

The effect of competing endogenous RNA (ceRNA) can regulate gene expression by competitively binding microRNAs. Fascin-1 (FSCN1) plays an important role in the regulation of cellular migration and invasion during tumor progression, but how its regulatory mechanism works through the ceRNA effect is still unclear in bladder cancer (BLCA).

Methods

The role of fascin-1, miR-200b, and ZEB1-AS1 in BLCA was investigated in vitro and in vivo. The interaction between fascin-1, miR-200b, and ZEB1-AS1 was identified using bioinformatics analysis, luciferase activity assays, RNA-binding protein immunoprecipitation (RIP), quantitative PCR, and western blotting. Loss (or gain)-of-function experiments were performed to investigate the biological roles of miR-200b and ZEB1-AS1 on migration, invasion, proliferation, cell apoptosis, and cell cycle.

Results

ZEB1-AS1 functions as a competing endogenous RNA in BLCA to regulate the expression of fascin-1 through miR-200b. Moreover, the oncogenic long non-coding RNA ZEB1-AS1 was highly expressed in BLCA and positively correlated with high tumor grade, high TNM stage, and reduced survival of patients with BLCA. Moreover, ZEB1-AS1 downregulated the expression of miR-200b, promoted migration, invasion, and proliferation, and inhibited apoptosis in BLCA. Furthermore, we found TGF-β1 induced migration and invasion in BLCA by regulating the ZEB1-AS1/miR-200b/FSCN1 axis.

Conclusion

The observations in this study identify an important regulatory mechanism of fascin-1 in BLCA, and the TGF-β1/ZEB1-AS1/miR-200b/FSCN1 axis may serve as a potential target for cancer therapeutic purposes.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1102-6) contains supplementary material, which is available to authorized users.

Crosstalk between PKCα and PI3K/AKT Signaling Is Tumor Suppressive in the Endometrium

Protein kinase C (PKC) isozymes are commonly recognized as oncoproteins based on their activation by tumor-promoting phorbol esters. However, accumulating evidence indicates that PKCs can be inhibitory in some cancers, with recent findings propelling a shift in focus to understanding tumor suppressive functions of these enzymes. Here, we report that PKCα acts as a tumor suppressor in PI3K/AKT-driven endometrial cancer. Transcriptional suppression of PKCα is observed in human endometrial tumors in association with aggressive disease and poor prognosis. In murine models, loss of PKCα is rate limiting for endometrial tumor initiation. PKCα tumor suppression involves PP2A-family-dependent inactivation of AKT, which can occur even in the context of genetic hyperactivation of PI3K/AKT signaling by coincident mutations in PTEN, PIK3CA, and/or PIK3R1. Together, our data point to PKCα as a crucial tumor suppressor in the endometrium, with deregulation of a PKCα→PP2A/PP2A-like phosphatase signaling axis contributing to robust AKT activation and enhanced endometrial tumorigenesis.

Protein kinase C as a tumor suppressor

Protein kinase C (PKC) has historically been considered an oncoprotein. This stems in large part from the discovery in the early 1980s that PKC is directly activated by tumor-promoting phorbol esters. Yet three decades of clinical trials using PKC inhibitors in cancer therapies not only failed, but in some cases worsened patient outcome. Why has targeting PKC in cancer eluded successful therapies? Recent studies looking at the disease for insight provide an explanation: cancer-associated mutations in PKC are generally loss-of-function (LOF), supporting an unexpected function as tumor suppressors. And, contrasting with LOF mutations in cancer, germline mutations that enhance the activity of some PKC isozymes are associated with degenerative diseases such as Alzheimer's disease. This review provides a background on the diverse mechanisms that ensure PKC is only active when, where, and for the appropriate duration needed and summarizes recent findings converging on a paradigm reversal: PKC family members generally function by suppressing, rather than promoting, survival signaling.

Protein kinase C in cancer: The top five unanswered questions

Few kinases have been studied as extensively as protein kinase C (PKC), particularly in the context of cancer. As major cellular targets for the phorbol ester tumor promoters and diacylglycerol (DAG), a second messenger generated by stimulation of membrane receptors, PKC isozymes play major roles in the control of signaling pathways associated with proliferation, migration, invasion, tumorigenesis, and metastasis. However, despite decades of research, fundamental questions remain to be answered or are the subject of intense controversy. Primary among these unresolved issues are the role of PKC isozymes as either tumor promoter or tumor suppressor kinases and the incomplete understanding on isozyme-specific substrates and effectors. The involvement of PKC isozymes in cancer progression needs to be reassessed in the context of specific oncogenic and tumor suppressing alterations. In addition, there are still major hurdles in addressing isozyme-specific function due to the limited specificity of most pharmacological PKC modulators and the lack of validated predictive biomarkers for response, which impacts the translation of these agents to the clinic. In this review we focus on key controversial issues and upcoming challenges, with the expectation that understanding the intricacies of PKC function will help fulfill the yet unsuccessful promise of targeting PKCs for cancer therapeutics.

Low expression of PKCα and high expression of KRAS predict poor prognosis in patients with colorectal cancer

The current study aimed to determine the association between protein kinase Cα (PKCα) and Kirsten rat sarcoma viral oncogene homolog (KRAS) expression and the response to folinic acid, 5-fluorouracil and oxaliplatin (FOLFOX regimen) in patients with colorectal cancer (CRC). The protein levels of PKCα and KRAS were analyzed by immunohistochemistry in tissue samples from patients with CRC and in non-cancerous tissues, including 152 cases of colorectal adenocarcinoma, 30 cases of colorectal adenoma and 20 normal colonic mucosa samples. The association between PKCα and KRAS expression and clinicopathological features was analyzed. The rates of positive PKCα protein expression in patients with poorly, moderately and well-differentiated adenocarcinoma were 16.7% (6/36), 40.0% (24/60), and 57.1% (32/56), respectively (P<0.013). The rate of positive KRAS expression in CRC patients was significantly higher than in patients with colon adenoma and normal colon mucosa (P<0.001). Expression levels of KRAS were associated with the degree of differentiation of CRC (P<0.001). Expression of PKCα was negatively correlated with KRAS expression in CRC tissues. The mean progression-free survival (PFS) times in patients with high and low expression of PKCα were 43.9 and 38.8 months, respectively (P<0.001). The mean PFS times were 38.5 and 45.5 months in patients with high and low expression of KRAS, respectively (P=0.001). In conclusion, low PKCα and high KRAS expression predicted relatively poor prognosis in patients with CRC.

MT-4 suppresses resistant ovarian cancer growth through targeting tubulin and HSP27

OBJECTIVE

In this study, the anticancer mechanisms of MT-4 were examined in A2780 and multidrug-resistant NCI-ADR/res human ovarian cancer cell lines.

METHODS

To evaluate the activity of MT-4, we performed in vitro cell viability and cell cycle assays and in vivo xenograft assays. Immunoblotting analysis was carried out to evaluate the effect of MT-4 on ovarian cancer. Tubulin polymerization was determined using a tubulin binding assay.

RESULTS

MT-4 (2-Methoxy-5-[2-(3,4,5-trimethoxy-phenyl)-ethyl]-phenol), a derivative of moscatilin, can inhibit both sensitive A2780 and multidrug-resistant NCI-ADR/res cell growth and viability. MT-4 inhibited tubulin polymerization to induce G2/M arrest followed by caspase-mediated apoptosis. Further studies indicated that MT-4 is not a substrate of P-glycoprotein (p-gp). MT-4 also caused G2/M cell cycle arrest, accompanied by the upregulation of cyclin B, p-Thr161 Cdc2/p34, polo-like kinase 1 (PLK1), Aurora kinase B, and phospho-Ser10-histone H3 protein levels. In addition, we found that p38 MAPK pathway activation was involved in MT-4-induced apoptosis. Most importantly, MT-4 also decreased heat shock protein 27 expression and reduced its interaction with caspase-3, which inured cancer cells to chemotherapy resistance. Treatment of cells with SB203580 or overexpression of dominant negative (DN)-p38 or wild-type HSP27 reduced PARP cleavage caused by MT-4. MT-4 induced apoptosis through regulation of p38 and HSP27. Our xenograft models also show the in vivo efficacy of MT-4. MT-4 inhibited both A2780 and NCI-ADR/res cell growth in vitro and in vivo.

CONCLUSION

These findings indicate that MT-4 could be a potential lead compound for the treatment of multidrug-resistant ovarian cancer.

Protein kinase Cα mediates erlotinib resistance in lung cancer cells

Overexpression and mutational activation of the epidermal growth factor receptor (EGFR) plays an important role in the pathogenesis of non-small cell lung cancer (NSCLC). EGFR tyrosine-kinase inhibitors (TKIs) are given as a primary therapy for advanced patients with EGFR-activating mutations; however, the majority of these tumors relapse and patients eventually develop resistance to TKIs. To address a potential role of protein kinase C (PKC) isozymes in the resistance to TKIs, we used the isogenic NSCLC H1650 cell line and its erlotinib-resistant derivative H1650-M3, a cell line that displays a mesenchymal-like morphology driven by transforming growth factor-β signaling. We found that H1650-M3 cells display remarkable PKCα upregulation and PKCδ downregulation. Notably, silencing PKCα from H1650-M3 cells using RNA interference caused a significant reduction in the expression of epithelial-to-mesenchymal transition (EMT) markers vimentin, Zeb2, Snail, and Twist. Moreover, pharmacological inhibition or PKCα RNA interference depletion and PKCδ restoring sensitized H1650-M3 cells to erlotinib. Whereas ectopic overexpression of PKCα in parental H1650 cells was not sufficient to alter the expression of EMT genes or to confer resistance to erlotinib, it caused downregulation of PKCδ expression, suggesting a unidirectional crosstalk. Finally, mechanistic studies revealed that PKCα upregulation in H1650-M3 cells is driven by transforming growth factor-β. Our results identified important roles for specific PKC isozymes in erlotinib resistance and EMT in lung cancer cells, and highlight PKCα as a potential target for lung cancer treatment.

Cancer-associated protein kinase C mutations reveal kinase's role as tumor suppressor

Protein kinase C (PKC) isozymes have remained elusive cancer targets despite the unambiguous tumor promoting function of their potent ligands, phorbol esters, and the prevalence of their mutations. We analyzed 8% of PKC mutations identified in human cancers and found that, surprisingly, most were loss of function and none were activating. Loss-of-function mutations occurred in all PKC subgroups and impeded second-messenger binding, phosphorylation, or catalysis. Correction of a loss-of-function PKCβ mutation by CRISPR-mediated genome editing in a patient-derived colon cancer cell line suppressed anchorage-independent growth and reduced tumor growth in a xenograft model. Hemizygous deletion promoted anchorage-independent growth, revealing that PKCβ is haploinsufficient for tumor suppression. Several mutations were dominant negative, suppressing global PKC signaling output, and bioinformatic analysis suggested that PKC mutations cooperate with co-occurring mutations in cancer drivers. These data establish that PKC isozymes generally function as tumor suppressors, indicating that therapies should focus on restoring, not inhibiting, PKC activity.

Protein kinase C and cancer: what we know and what we do not

Since their discovery in the late 1970s, protein kinase C (PKC) isozymes represent one of the most extensively studied signaling kinases. PKCs signal through multiple pathways and control the expression of genes relevant for cell cycle progression, tumorigenesis and metastatic dissemination. Despite the vast amount of information concerning the mechanisms that control PKC activation and function in cellular models, the relevance of individual PKC isozymes in the progression of human cancer is still a matter of controversy. Although the expression of PKC isozymes is altered in multiple cancer types, the causal relationship between such changes and the initiation and progression of the disease remains poorly defined. Animal models developed in the last years helped to better understand the involvement of individual PKCs in various cancer types and in the context of specific oncogenic alterations. Unraveling the enormous complexity in the mechanisms by which PKC isozymes have an impact on tumorigenesis and metastasis is key for reassessing their potential as pharmacological targets for cancer treatment.

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.