Protein kinase C delta overexpressing transgenic mice are resistant to chemically but not to UV radiation-induced development of squamous cell carcinomas: a possible link to specific cytokines and cyclooxygenase-2

Protein kinase C signaling "in" and "to" the nucleus: Master kinases in transcriptional regulation

PKC is a multifunctional family of Ser-Thr kinases widely implicated in the regulation of fundamental cellular functions, including proliferation, polarity, motility, and differentiation. Notwithstanding their primary cytoplasmic localization and stringent activation by cell surface receptors, PKC isozymes impel prominent nuclear signaling ultimately impacting gene expression. While transcriptional regulation may be wielded by nuclear PKCs, it most often relies on cytoplasmic phosphorylation events that result in nuclear shuttling of PKC downstream effectors, including transcription factors. As expected from the unique coupling of PKC isozymes to signaling effector pathways, glaring disparities in gene activation/repression are observed upon targeting individual PKC family members. Notably, specific PKCs control the expression and activation of transcription factors implicated in cell cycle/mitogenesis, epithelial-to-mesenchymal transition and immune function. Additionally, PKCs isozymes tightly regulate transcription factors involved in stepwise differentiation of pluripotent stem cells toward specific epithelial, mesenchymal, and hematopoietic cell lineages. Aberrant PKC expression and/or activation in pathological conditions, such as in cancer, leads to profound alterations in gene expression, leading to an extensive rewiring of transcriptional networks associated with mitogenesis, invasiveness, stemness, and tumor microenvironment dysregulation. In this review, we outline the current understanding of PKC signaling "in" and "to" the nucleus, with significant focus on established paradigms of PKC-mediated transcriptional control. Dissecting these complexities would allow the identification of relevant molecular targets implicated in a wide spectrum of diseases.

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.

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.

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.

Equivocal, explicit and emergent actions of PKC isoforms in cancer

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

Functional proteomic analysis reveals roles for PKCδ in regulation of cell survival and cell death: Implications for cancer pathogenesis and therapy

Protein Kinase C-δ (PKCδ), regulates a broad group of biological functions and disease processes, including well-defined roles in immune function, cell survival and apoptosis. PKCδ primarily regulates apoptosis in normal tissues and non-transformed cells, and genetic disruption of the PRKCD gene in mice is protective in many diseases and tissue damage models. However pro-survival/pro-proliferative functions have also been described in some transformed cells and in mouse models of cancer. Recent evidence suggests that the contribution of PKCδ to specific cancers may depend in part on the oncogenic context of the tumor, consistent with its paradoxical role in cell survival and cell death. Here we will discuss what is currently known about biological functions of PKCδ and potential paradigms for PKCδ function in cancer. To further understand mechanisms of regulation by PKCδ, and to gain insight into the plasticity of PKCδ signaling, we have used functional proteomics to identify pathways that are dependent on PKCδ. Understanding how these distinct functions of PKCδ are regulated will be critical for the logical design of therapeutics to target this pathway.

Protein kinase C: perfectly balanced

Protein kinase C (PKC) isozymes belong to a family of Ser/Thr kinases whose activity is governed by reversible release of an autoinhibitory pseudosubstrate. For conventional and novel isozymes, this is effected by binding the lipid second messenger, diacylglycerol, but for atypical PKC isozymes, this is effected by binding protein scaffolds. PKC shot into the limelight following the discovery in the 1980s that the diacylglycerol-sensitive isozymes are "receptors" for the potent tumor-promoting phorbol esters. This set in place a concept that PKC isozymes are oncoproteins. Yet three decades of cancer clinical trials targeting PKC with inhibitors failed and, in some cases, worsened patient outcome. Emerging evidence from cancer-associated mutations and protein expression levels provide a reason: PKC isozymes generally function as tumor suppressors and their activity should be restored, not inhibited, in cancer therapies. And whereas not enough activity is associated with cancer, variants with enhanced activity are associated with degenerative diseases such as Alzheimer's disease. This review describes the tightly controlled mechanisms that ensure PKC activity is perfectly balanced and what happens when these controls are deregulated. PKC isozymes serve as a paradigm for the wisdom of Confucius: "to go beyond is as wrong as to fall short."

Socioecological factors influencing women's HIV risk in the United States: qualitative findings from the women's HIV SeroIncidence study (HPTN 064)

Background

We sought to understand the multilevel syndemic factors that are concurrently contributing to the HIV epidemic among women living in the US. We specifically examined community, network, dyadic, and individual factors to explain HIV vulnerability within a socioecological framework.

Methods

We gathered qualitative data (120 interviews and 31 focus groups) from a subset of women ages 18–44 years (N = 2,099) enrolled in the HPTN 064 HIV seroincidence estimation study across 10 US communities. We analyzed data from 4 diverse locations: Atlanta, New York City (the Bronx), Raleigh, and Washington, DC. Data were thematically coded using grounded theory methodology. Intercoder reliability was assessed to evaluate consistency of team-based coding practices.

Results

The following themes were identified at 4 levels including 1) exosystem (community): poverty prevalence, discrimination, gender imbalances, community violence, and housing challenges; 2) mesosystem (network): organizational social support and sexual concurrency; 3) microsystem (dyadic): sex exchange, interpersonal social support, intimate partner violence; and 4) individual: HIV/STI awareness, risk taking, and substance use. A strong theme emerged with over 80 % of responses linked to the fundamental role of financial insecurity underlying risk-taking behavioral pathways.

Conclusions

Multilevel syndemic factors contribute to women’s vulnerability to HIV in the US. Financial insecurity is a predominant theme, suggesting the need for tailored programming for women to reduce HIV risk.

Trial registration

Clinicaltrials.gov, NCT00995176

Multifunctional roles of PKCδ: Opportunities for targeted therapy in human disease

The serine-threonine protein kinase, protein kinase C-δ (PKCδ), is emerging as a bi-functional regulator of cell death and proliferation. Studies in PKCδ-/- mice have confirmed a pro-apoptotic role for this kinase in response to DNA damage and a tumor promoter role in some oncogenic contexts. In non-transformed cells, inhibition of PKCδ suppresses the release of cytochrome c and caspase activation, indicating a function upstream of apoptotic pathways. Data from PKCδ-/- mice demonstrate a role for PKCδ in the execution of DNA damage-induced and physiologic apoptosis. This has led to the important finding that inhibitors of PKCδ can be used therapeutically to reduce irradiation and chemotherapy-induced toxicity. By contrast, PKCδ is a tumor promoter in mouse models of mammary gland and lung cancer, and increased PKCδ expression is a negative prognostic indicator in Her2+ and other subtypes of human breast cancer. Understanding how these distinct functions of PKCδ are regulated is critical for the design of therapeutics to target this pathway. This review will discuss what is currently known about biological roles of PKCδ and prospects for targeting PKCδ in human disease.

Regulation of mitochondrial morphology by positive feedback interaction between PKCδ and Drp1 in vascular smooth muscle cell

Dynamin-related protein-1 (Drp1) plays a critical role in mitochondrial fission which allows cell proliferation and Mdivi-1, a specific small molecule Drp1 inhibitor, is revealed to attenuate proliferation. However, few molecular mechanisms-related to Drp1 under stimulus for restenosis or atherosclerosis have been investigated in vascular smooth muscle cells (vSMCs). Therefore, we hypothesized that Drp1 inhibition can prevent vascular restenosis and investigated its regulatory mechanism. Angiotensin II (Ang II) or hydrogen peroxide (H2 O2 )-induced proliferation and migration in SMCs were attenuated by down-regulation of Drp1 Ser 616 phosphorylation, which was demonstrated by in vitro assays for migration and proliferation. Excessive amounts of ROS production and changes in mitochondrial membrane potential were prevented by Drp1 inhibition under Ang II and H2 O2 . Under the Ang II stimulation, activated Drp1 interacted with PKCδ and then activated MEK1/2-ERK1/2 signaling cascade and MMP2, but not MMP9. Furthermore, in ex vivo aortic ring assay, inhibition of the Drp1 had significant anti-proliferative and -migration effects for vSMCs. A formation of vascular neointima in response to a rat carotid artery balloon injury was prevented by Drp1 inhibition, which shows a beneficial effect of Drp1 regulation in the pathologic vascular condition. Drp1-mediated SMC proliferation and migration can be prevented by mitochondrial division inhibitor (Mdivi-1) in in vitro, ex vivo and in vivo, and these results suggest the possibility that Drp1 can be a new therapeutic target for restenosis or atherosclerosis.

Genetic deletion of TNFα inhibits ultraviolet radiation-induced development of cutaneous squamous cell carcinomas in PKCε transgenic mice via inhibition of cell survival signals

Protein kinase C epsilon (PKCε), a Ca(2+)-independent phospholipid-dependent serine/threonine kinase, is among the six PKC isoforms (α, δ, ε, η, μ, ζ) expressed in both mouse and human skin. Epidermal PKCε level dictates the susceptibility of PKCε transgenic (TG) mice to the development of cutaneous squamous cell carcinomas (SCC) elicited either by repeated exposure to ultraviolet radiation (UVR) or by using the DMBA initiation-TPA (12-O-tetradecanoylphorbol-13-acetate) tumor promotion protocol (Wheeler,D.L. et al. (2004) Protein kinase C epsilon is an endogenous photosensitizer that enhances ultraviolet radiation-induced cutaneous damage and development of squamous cell carcinomas. Cancer Res., 64, 7756-7765). Histologically, SCC in TG mice, like human SCC, is poorly differentiated and metastatic. Our earlier studies to elucidate mechanisms of PKCε-mediated development of SCC, using either DMBA-TPA or UVR, indicated elevated release of cytokine TNFα. To determine whether TNFα is essential for the development of SCC in TG mice, we generated PKCε transgenic mice/TNFα-knockout (TG/TNFαKO) by crossbreeding TNFαKO with TG mice. We now present that deletion of TNFα in TG mice inhibited the development of SCC either by repeated UVR exposures or by the DMBA-TPA protocol. TG mice deficient in TNFα elicited both increase in SCC latency and decrease in SCC incidence. Inhibition of UVR-induced SCC development in TG/TNFαKO was accompanied by inhibition of (i) the expression levels of TNFα receptors TNFRI and TNFRII and cell proliferation marker ornithine decarboxylase and metastatic markers MMP7 and MMP9, (ii) the activation of transcription factors Stat3 and NF-kB and (iii) proliferation of hair follicle stem cells and epidermal hyperplasia. The results presented here provide the first genetic evidence that TNFα is linked to PKCε-mediated sensitivity to DMBA-TPA or UVR-induced development of cutaneous SCC.

Impact of Helicobacter pylori on the growth of hepatic orthotopic graft tumors in mice

Helicobacter pylori is a well-known causative organism of chronic gastric diseases and has been found in many hepatic carcinoma samples. To explore the expression of apoptosis-related proteins and carcinoma development in H. pylori-infected livers, we utilized BALB/cAnSlac mice to establish an H. pylori-infected model by oral inoculation and orthotopic grafts of hepatic tumors by H22 cells, respectively. We found that H. pylori colonies could not be cultured from all liver and tumor samples. However, its 16S rRNA was detectable in 85.3% of livers and 66.7% of tumors in the infected mice. Inflammatory cells were observed and thinly distributed in the lobule portions of the liver, and H. pylori mainly existed in the infected hepatic sinusoids and the necrotic areas of the infected tumors. No significant difference was found in liver to body weight ratio between the infected and uninfected. Moreover, the pathological tumor difference was unremarkable between the two. The proliferating cell nuclear antigen (PCNA) and Bcl-2-associated X protein (Bax) expression in the infected tumors was significantly higher and lower, respectively, than those of the uninfected tumors. However, no significant difference in Bcl-2 (B-cell lymphoma 2) expression existed. The results indicate that H. pylori found in the livers which were infected by H. pylori oral inoculation could contribute to the infiltration of inflammatory cells in livers. Although H. pylori has no significant impact on the liver to body weight ratio or tumor Bcl-2 expression, it may upregulate PCNA expression and downregulate Bax expression, respectively. All our findings show that H. pylori may promote proliferation and inhibit apoptosis of tumor cells.

Mouse models of nonmelanoma skin cancer

The skin is the largest organ of the mammalian body, made up of multiple layers, which include the epidermis, dermis, and subcutis (Alam and Ratner, N Engl J Med 344(13):975-983, 2001). The human interfollicular epidermis can be subdivided into five different layers: (1) stratum basale, (2) stratum spinosum, (3) stratum granulosum, (4) stratum lucidum, and (5) stratum corneum, all originating from basal keratinocytes by differentiation (Hameetman et al., BMC cancer 13:58, 2013; Ramirez et al., Differentiation 58(1):53-64, 1994). The epidermis is also able to generate different appendages: hair follicles (HF) and their associated sebaceous glands (Sibilia et al., Cell 102(2):211-220, 2000) as well as sweat glands (Luetteke et al., Genes Dev 8(4):399-413, 1994). The skin has important functions in several biological processes like environmental barrier, tissue regeneration, hair cycling, and wound repair. During these processes, stem cells from the interfollicular epidermis and from the hair follicle bulge are activated to renew the epidermis or hair. The epidermis and hair undergo continuous homeostatic regeneration and mutations, upon mutations which disturb the balance of homeostatic regeneration of epidermis and hair and lead to enhanced proliferation of keratinocytes, development of skin cancer is developed. Tumors that arise in the skin are mainly of three types: malignant melanoma, arising from melanocytes, basal cell carcinoma (BCC), and squamous cell carcinoma (SCC), the latter two both arising from keratinocytes or hair follicle cells. In this chapter, we will describe some genetically engineered mouse models (GEMM) that aim at modeling human BCC and SCC and their respective precancerous lesions. We will describe the experimental approaches used in our laboratory to analyze tumor-bearing mice focusing on methods necessary for the induction of tumor growth as well as for the molecular and histological analysis of tumor tissue.

The tumor promoting activity of the EP4 receptor for prostaglandin E2 in murine skin

To determine whether the EP4 receptor for prostaglandin E2 (PGE2) contributes to the tumor promoting activity of PGs in murine skin, EP4 over-expressing mice (BK5.EP4) were generated and subjected carcinogenesis protocols. An initiation/promotion protocol resulted in 25-fold more squamous cell carcinomas (SCCs) in the BK5.EP4 mice than wild type (WT) mice. An increase in SCCs also occurred following treatment with initiator alone or UV irradiation. The initiator dimethylbenz[a]anthracene caused cytotoxicity in BK5.EP4, but not WT mice, characterized by sloughing of the interfollicular epidermis, regeneration and subsequent SCC development. A comparison of transcriptomes between BK5.EP4 and WT mice treated with PGE2 showed a significant upregulation of a number of genes known to be associated with tumor development, supporting a pro-tumorigenic role for the EP4 receptor.

Molecular mechanisms of mouse skin tumor promotion

Multiple molecular mechanisms are involved in the promotion of skin carcinogenesis. Induction of sustained proliferation and epidermal hyperplasia by direct activation of mitotic signaling pathways or indirectly in response to chronic wounding and/or inflammation, or due to a block in terminal differentiation or resistance to apoptosis is necessary to allow clonal expansion of initiated cells with DNA mutations to form skin tumors. The mitotic pathways include activation of epidermal growth factor receptor and Ras/Raf/mitogen-activated protein kinase signaling. Chronic inflammation results in inflammatory cell secretion of growth factors and cytokines such as tumor necrosis factor-α and interleukins, as well as production of reactive oxygen species, all of which can stimulate proliferation. Persistent activation of these pathways leads to tumor promotion.

Sensitization of human pancreatic cancer cells harboring mutated K-ras to apoptosis

Pancreatic cancer is a devastating human malignancy and gain of functional mutations in K-ras oncogene is observed in 75%-90% of the patients. Studies have shown that oncogenic ras is not only able to promote cell growth or survival, but also apoptosis, depending upon circumstances. Using pancreatic cancer cell lines with or without expressing mutated K-ras, we demonstrated that the inhibition of endogenous PKC activity sensitized human pancreatic cancer cells (MIA and PANC-1) expressing mutated K-ras to apoptosis, which had no apoptotic effect on BxPC-3 pancreatic cancer cells that contain a normal Ras as well as human lung epithelial BAES-2B cells. In this apoptotic process, the level of ROS was increased and PUMA was upregulated in a p73-dependent fashion in MIA and PANC-1 cells. Subsequently, caspase-3 was cleaved. A full induction of apoptosis required the activation of both ROS- and p73-mediated pathways. The data suggest that PKC is a crucial factor that copes with aberrant K-ras to maintain the homeostasis of the pancreatic cancer cells harboring mutated K-ras. However, the suppression or loss of PKC disrupts the balance and initiates an apoptotic crisis, in which ROS and p73 appear the potential, key targets.

Loss of protein kinase C delta gene expression in human squamous cell carcinomas: a laser capture microdissection study

Protein kinase C delta (PKC-delta) protein levels are frequently low in chemically and UV-induced mouse skin tumors as well as in human cutaneous squamous cell carcinomas (SCCs). Furthermore, overexpression of PKC-delta in human SCC lines and mouse epidermis is sufficient to induce apoptosis and suppress tumorigenicity, making PKC-delta a potential tumor suppressor gene for SCCs. Here we report that PKC-delta is lost in human SCCs at the transcriptional level. We used laser capture microdissection to isolate cells from three normal human epidermis and 14 human SCCs with low PKC-delta protein. Analysis by quantitative reverse transcription-PCR revealed that PKC-delta RNA was reduced an average of 90% in the SCCs tested, consistent with PKC-delta down-regulation at the protein level. Analysis of DNA from nine of the same tumors revealed that PKC-delta gene was deleted in only one tumor. In addition, Ras-transformed human keratinocytes, which have selective down-regulation of PKC-delta at both protein and mRNA levels, had significantly repressed human PKC-delta promoter activity. Together, these results indicate that PKC-delta gene expression is suppressed in human SCCs, probably via transcription repression. Our results have implications for the development of topical therapeutic strategies to trigger the re-expression of pro-apoptotic PKC-delta to induce apoptosis in SCCs.

The protein kinase Cdelta catalytic fragment is critical for maintenance of the G2/M DNA damage checkpoint

Protein kinase Cdelta (PKCdelta) is an essential component of the intrinsic apoptotic program. Following DNA damage, such as exposure to UV radiation, PKCdelta is cleaved in a caspase-dependent manner, generating a constitutively active catalytic fragment (PKCdelta-cat), which is necessary and sufficient for keratinocyte apoptosis. We found that in addition to inducing apoptosis, expression of PKCdelta-cat caused a pronounced G(2)/M cell cycle arrest in both primary human keratinocytes and immortalized HaCaT cells. Consistent with a G(2)/M arrest, PKCdelta-cat induced phosphorylation of Cdk1 (Tyr(15)), a critical event in the G(2)/M checkpoint. Treatment with the ATM/ATR inhibitor caffeine was unable to prevent PKCdelta-cat-induced G(2)/M arrest, suggesting that PKCdelta-cat is functioning downstream of ATM/ATR in the G(2)/M checkpoint. To better understand the role of PKCdelta and PKCdelta-cat in the cell cycle response to DNA damage, we exposed wild-type and PKCdelta null mouse embryonic fibroblasts (MEFs) to UV radiation. Wild-type MEFs underwent a pronounced G(2)/M arrest, Cdk1 phosphorylation, and induction of apoptosis following UV exposure, whereas PKCdelta null MEFs were resistant to these effects. Expression of PKCdelta-green fluorescent protein, but not caspase-resistant or kinase-inactive PKCdelta, was able to restore G(2)/M checkpoint integrity in PKCdelta null MEFs. The function of PKCdelta in the DNA damage-induced G(2)/M cell cycle checkpoint may be a critical component of its tumor suppressor function.

Inhibitory effects of oligonol on phorbol ester-induced tumor promotion and COX-2 expression in mouse skin: NF-kappaB and C/EBP as potential targets

Plant polyphenols possess anti-oxidant and anti-inflammatory activities and are hence potential candidates for preventing cancer. The present study was aimed at evaluating the anti-inflammatory and anti-tumor promoting activity of oligonol, a formulation of catechin-type oligomers, in mouse skin stimulated with a proto-type tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Pretreatment of mouse skin with oligonol significantly inhibited TPA-induced expression of cyclooxygenase-2 (COX-2). Oligonol diminished nuclear translocation and DNA binding of nuclear factor-kappaB (NF-kappaB) via blockade of phosphorylation and subsequent degradation of IkappaB alpha in TPA-treated mouse skin. Moreover, oligonol suppressed TPA-induced DNA binding of CCAAT/enhancer-binding protein (C/EBP) in mouse skin. Oligonol pretreatment also attenuated the phosphorylation and/or catalytic activities of extracellular signal-regulated protein kinase-1/2 (ERK1/2) and p38 mitogen-activated protein (MAP) kinase. Moreover, p38 MAP kinase inhibitor SB203580, but not the MEK inhibitor U0126, negated TPA-induced DNA binding of C/EBP. In addition, oligonol reduced the incidence and the multiplicity of papillomas and squamous cell carcinomas in 7,12-dimethylbenz[a]anthracene (DMBA)-initiated and TPA-promoted mouse skin, and prolonged the survival of tumor-bearing mice. Pretreatment with oligonol diminished the levels of proliferating cell nuclear antigen and expression of COX-2 in papillomas and carcinomas, respectively, as compared to DMBA plus TPA treatment alone. Taken together, the above findings suggest that oligonol inhibits TPA-induced COX-2 expression by blocking the activation of NF-kappaB and C/EBP via modulation of MAP kinases and suppresses chemically induced mouse skin tumorigenesis.

Protein kinase Cepsilon interacts with Stat3 and regulates its activation that is essential for the development of skin cancer

Protein kinase C (PKC) represents a large family of phosphatidylserine (PS)-dependent serine/threonine protein kinases. At least six PKC isoforms (alpha, delta, epsilon, eta, micro, and zeta) are expressed in epidermis. PKC is a major intracellular receptor for 12-O-tetradecanoylphorbol-13-acetate (TPA) and is also activated by a variety of stress factors including ultraviolet radiation (UVR). PKC isozymes (alpha, delta, epsilon, and eta), exhibit specificities to the development of skin cancer. PKCepsilon, a calcium-insensitive PKC isoform, is linked to the development of squamous cell carcinoma (SCC) elicited either by the 7,12-Dimethylbenzanthracene (DMBA)-TPA protocol or by repeated exposures to UVR. PKCepsilon overexpressing transgenic mice, when treated either with TPA or exposed to UVR, elicit similar responses such as inhibition of apoptosis, promotion of cell survival, and development of SCC. PKCepsilon overexpression increases Stat3 activation after either TPA treatment or UVR exposure. Both PKCepsilon and signal transducers and activators of transcription-3 (Stat3) are implicated in the development of SCC. However, the link between PKCepsilon and Stat3 remains elusive. We found that PKCepsilon interacts with Stat3. PKCepsilon interaction with Stat3 was dependent upon UVR treatment. In reciprocal immunoprecipitation/blotting experiments, Stat3 coimmunoprecipitated with PKCepsilon. Colocalization of PKCepsilon with Stat3 was confirmed by double immunofluorescence staining. PKCepsilon interaction with Stat3 was PKCepsilon isoform specific and was not observed with other protein kinases. As observed in vitro with immunocomplex kinase assay with immunopurified PKCepsilon and Stat3, PKCepsilon phosphorylated Stat3 at the serine 727 residue. PKCepsilon depletion prevented Stat3Ser727 phosphorylation, Stat3 DNA binding, and transcriptional activity. The results presented indicate that PKCepsilon mediates Stat3 activation.

Protein kinase C delta enhances proliferation and survival of murine mammary cells

Protein kinase C (PKC) delta, a member of the novel family of PKC serine-threonine kinases, has been implicated in negative regulation of proliferation and apoptosis in a large number of cell types, including breast cancer cell lines, and postulated as a tumor suppressor gene. In this study we show that in murine NMuMG mammary cells PKCdelta promotes a mitogenic response. Overexpression of PKCdelta in NMuMG cells leads to a significant increase in [3H]-tymidine incorporation and cell proliferation, as well as enhanced extracellular signal-regulated kinase (ERK)-mitogen-activated protein kinase (MAPK) activation. Activation of PKCdelta with a phorbol ester leads to elevated cyclin D1 expression and an hyperphosphorylated Rb state. Surprisingly, ectopic expression of PKCdelta conferred anchorage-independent growth capacity to NMuMG cells. PKCdelta overexpressors showed enhanced resistance to apoptotic stimuli, such as serum deprivation or doxorubicin treatment, an effect that correlates with hyperactivation of the Akt survival pathway. Our results provide evidence for a role of PKCdelta as a positive modulator of proliferative and survival signals in immortalized mammary cells. The fact that PKCdelta exerts differential responses depending on the cell context not only highlights the necessity to carefully understand the signaling events controlled by this PKC in each cell type but also suggests that we should be cautious in considering this kinase a target for cancer therapy.

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

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