Protein kinase Cδ inactivation inhibits cellular proliferation and decreases survival in human neuroendocrine tumors

The metabolomics of a protein kinase C delta (PKCδ) knock-out mouse model

INTRODUCTION

PKCδ is ubiquitously expressed in mammalian cells and its dysregulation plays a key role in the onset of several incurable diseases and metabolic disorders. However, much remains unknown about the metabolic pathways and disturbances induced by PKC deficiency, as well as the metabolic mechanisms involved.

OBJECTIVES

This study aims to use metabolomics to further characterize the function of PKC from a metabolomics standpoint, by comparing the full serum metabolic profiles of PKC deficient mice to those of wild-type mice.

METHODS

The serum metabolomes of PKCδ knock-out mice were compared to that of a wild-type strain using a GCxGC-TOFMS metabolomics research approach and various univariate and multivariate statistical analyses.

RESULTS

Thirty-seven serum metabolite markers best describing the difference between PKCδ knock-out and wild-type mice were identified based on a PCA power value > 0.9, a t-test p-value < 0.05, or an effect size > 1. XERp prediction was also done to accurately select the metabolite markers within the 2 sample groups. Of the metabolite markers identified, 78.4% (29/37) were elevated and 48.65% of these markers were fatty acids (18/37). It is clear that a total loss of PKCδ functionality results in an inhibition of glycolysis, the TCA cycle, and steroid synthesis, accompanied by upregulation of the pentose phosphate pathway, fatty acids oxidation, cholesterol transport/storage, single carbon and sulphur-containing amino acid synthesis, branched-chain amino acids (BCAA), ketogenesis, and an increased cell signalling via N-acetylglucosamine.

CONCLUSION

The charaterization of the dysregulated serum metabolites in this study, may represent an additional tool for the early detection and screening of PKCδ-deficiencies or abnormalities.

Differential roles and regulation of the protein kinases PAK4, PAK5 and PAK6 in melanoma cells

The protein kinases PAK4, PAK5 and PAK6 comprise a family of ohnologues. In multiple cancers including melanomas PAK5 most frequently carries non-synonymous mutations; PAK6 and PAK4 have fewer; and PAK4 is often amplified. To help interpret these genomic data, initially we compared the cellular regulation of the sister kinases and their roles in melanoma cells. In common with many ohnologue protein kinases, PAK4, PAK5 and PAK6 each have two 14-3-3-binding phosphosites of which phosphoSer99 is conserved. PAK4 localises to the leading edge of cells in response to phorbol ester-stimulated binding of 14-3-3 to phosphoSer99 and phosphoSer181, which are phosphorylated by two different PKCs or PKDs. These phosphorylations of PAK4 are essential for its phorbol ester-stimulated phosphorylation of downstream substrates. In contrast, 14-3-3 interacts with PAK5 in response to phorbol ester-stimulated phosphorylation of Ser99 and epidermal growth factor-stimulated phosphorylation of Ser288; whereas PAK6 docks onto 14-3-3 and is prevented from localising to cell–cell junctions when Ser133 is phosphorylated in response to cAMP-elevating agents via PKA and insulin-like growth factor 1 via PKB/Akt. Silencing of PAK4 impairs viability, migration and invasive behaviour of melanoma cells carrying BRAF^V600E or NRAS^Q61K mutations. These defects are rescued by ectopic expression of PAK4, more so by a 14-3-3-binding deficient PAK4, and barely by PAK5 or PAK6. Together these genomic, biochemical and cellular data suggest that the oncogenic properties of PAK4 are regulated by PKC–PKD signalling in melanoma, while PAK5 and PAK6 are dispensable in this cancer.

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.

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.

Rottlerin acts as a therapeutic in primary open-angle glaucoma by targeting the trabecular meshwork via activation of Rap1 signaling

Primary open-angle glaucoma (POAG) is a leading cause of irreversible blindness worldwide, and elevated intraocular pressure (IOP) is a major risk factor. While IOP is mainly controlled by adjusting the outflow resistance in the trabecular meshwork (TM), drugs that act directly on the TM are rare. In this study, we discovered a novel compound and pathway that acts on the TM and decreases IOP by genomic, proteomic, and bioinformatic analyses of POAG-derived TMs and experimental validation. Overlapping differentially expressed genes of the TM between patients with POAG and normal controls from two independent gene expression profiles in public databases were analyzed and matched by using the Connectivity Map (CMap). Rottlerin was identified as a potential compound. Subsequent experiments confirmed that rottlerin reversed POAG phenotypes in vitro and that it decreased IOP and actin/extracellular matrix accumulation in vivo with no detectable ocular side effects. SwissTargetPrediction in combination with pathway analysis predicted that the effects of rottlerin may be mediated by activation of the Rap1 pathway. Finally, we confirmed that rottlerin upregulated Rap1 and the downstream PI3K/AKT pathway independent of the MAPK/ERK pathway in a dexamethasone-induced POAG cell model.

The Caspase-3/PKCδ/Akt/VEGF-A Signaling Pathway Mediates Tumor Repopulation during Radiotherapy

PURPOSE

Tumor repopulation is known as a major cause of treatment failure and/or tumor recurrence after radiotherapy. The underlying mechanism remains unclear. Our previous study demonstrated that irradiated apoptotic cells mediated tumor repopulation, in which caspase-3 played an important role. Herein, we investigated downstream effectors of caspase-3 involved in this process.

EXPERIMENTAL DESIGN

A dominant-negative protein kinase Cδ (DN_PKCδ) mutant that could not be cleaved by caspase-3 and therefore could not be activated by irradiation-induced apoptosis was constructed. DN_PKCδ stably transduced tumor cells were compared with wild-type tumor cells for their growth stimulation effects in in vitro and in vivo tumor repopulation models. Downstream effectors of caspase-3 and PKCδ were investigated. The role of PKCδ was further verified in human colorectal tumor specimens.

RESULTS

Inactivation of caspase-3 or caspase-7 attenuated tumor repopulation and weakened PKCδ cleavage. Both DN_PKCδ and PKCδ inhibitors restrained tumor repopulation both in vitro and in vivo. Phosphorylated Akt was attenuated in caspase-3-, caspase-7-, or PKCδ-inactivated tumor cells. Furthermore, expression of vascular endothelial growth factor (VEGF)-A but not hypoxia-inducible factor 1α (HIF1α) was decreased in PKCδ- or Akt-inactivated tumor cells. In addition, inhibition of p-Akt, HIF1α, VEGF-A, or VEGF-A receptor reduced tumor repopulation significantly. Finally, increased nuclear translocation of PKCδ in colorectal tumor specimens was associated with worse patient prognosis.

CONCLUSIONS

The caspase-3/PKCδ/Akt/VEGF-A axis is involved in tumor repopulation and could be exploited as a potential target to enhance the efficacy of radiotherapy.

Molecular Genetic Studies of Pancreatic Neuroendocrine Tumors: New Therapeutic Approaches

Pancreatic neuroendocrine tumors (PNETs) arise sporadically or as part of familial syndromes. Genetic studies of hereditary syndromes and whole exome sequencing analysis of sporadic NETs have revealed the roles of some genes involved in PNET tumorigenesis. The multiple endocrine neoplasia type 1 (MEN1) gene is most commonly mutated. Its encoded protein, menin, has roles in transcriptional regulation, genome stability, DNA repair, protein degradation, cell motility and adhesion, microRNA biogenesis, cell division, cell cycle control, and epigenetic regulation. Therapies targeting epigenetic regulation and MEN1 gene replacement have been reported to be effective in preclinical models.

K-Ras stabilization by estrogen via PKCδ is involved in endometrial tumorigenesis

Estrogens are considered as a major risk factor of endometrial cancer. In this study, we identified a mechanism of tumorigenesis in which K-Ras protein is stabilized via estrogen signaling through the ER-α36 receptor. PKCδ was shown to stabilize K-Ras specifically via estrogen signaling. Estrogens stabilize K-Ras via inhibition of polyubiquitylation-dependent proteasomal degradation. Estrogen-induced cellular transformation was abolished by either K-Ras or PKCδ knockdown. The role of PKCδ in estrogen-induced tumorigenesis was confirmed in a mouse xenograft model by reduction of tumors after treatment with rottlerin, a PKCδ inhibitor. Finally, levels of PKCδ correlated with that of Ras in human endometrial tumor tissues. Stabilization of K-Ras by estrogen signaling involving PKCδ up-regulation provides a potential therapeutic approach for treatment of endometrial cancer.

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.

A Rare Case of Diffuse Idiopathic Pulmonary Neuroendocrine Cell Hyperplasia

Diffuse idiopathic pulmonary neuroendocrine cell hyperplasia (DIPNECH) is a rare clinical condition with only about 100 cases reported in the literature. It is characterized by primary hyperplasia of pulmonary neuroendocrine cells (PNECs) which are specialized epithelial cells located throughout the entire respiratory tract, from the trachea to the terminal airways. DIPNECH appears in various forms that include diffuse proliferation of scattered neuroendocrine cells, small nodules, or a linear proliferation. It is usually seen in middle-aged, nonsmoking women with symptoms of cough, dyspnea, and wheezing. We present a 45-year-old, nonsmoking woman who presented with symptoms of DIPNECH associated with bilateral pulmonary nodules and left hilar adenopathy. Of interest, DIPNECH in our patient was associated with metastatic pulmonary carcinoids, papillary carcinoma of the left breast, oncocytoma and angiomyolipoma of her left kidney, and cortical nodules suggestive of tuberous sclerosis. She had video assisted thoracoscopic surgery (VATS), modified radical mastectomy with reconstruction, and radical nephrectomy. She is currently symptom-free most of the time with over two years of follow-up.

Targeting protein kinase C subtypes in pancreatic cancer

In preclinical studies, protein kinase C (PKC) enzymes have been implicated in regulating many aspects of pancreatic cancer development and progression. However, clinical Phase I or Phase II trials with compounds targeting classical PKC isoforms were not successful. Recent studies implicate that mainly atypical and novel PKC enzymes regulate oncogenic signaling pathways in pancreatic cancer. Members of these two subgroups converge signaling induced by mutant Kras, growth factors and inflammatory cytokines. Different approaches for the development of inhibitors for atypical PKC and novel PKC have been described; and new compounds include allosteric inhibitors and inhibitors that block ATP binding.

Protein kinase Cδ is a therapeutic target in malignant melanoma with NRAS mutation

NRAS is the second most frequently mutated gene in melanoma. Previous reports have demonstrated the sensitivity of cancer cell lines carrying KRAS mutations to apoptosis initiated by inhibition of protein kinase Cδ (PKCδ). Here, we report that PKCδ inhibition is cytotoxic in melanomas with primary NRAS mutations. Novel small-molecule inhibitors of PKCδ were designed as chimeric hybrids of two naturally occurring PKCδ inhibitors, staurosporine and rottlerin. The specific hypothesis interrogated and validated is that combining two domains of two naturally occurring PKCδ inhibitors into a chimeric or hybrid structure retains biochemical and biological activity and improves PKCδ isozyme selectivity. We have devised a potentially general synthetic protocol to make these chimeric species using Molander trifluorborate coupling chemistry. Inhibition of PKCδ, by siRNA or small molecule inhibitors, suppressed the growth of multiple melanoma cell lines carrying NRAS mutations, mediated via caspase-dependent apoptosis. Following PKCδ inhibition, the stress-responsive JNK pathway was activated, leading to the activation of H2AX. Consistent with recent reports on the apoptotic role of phospho-H2AX, knockdown of H2AX prior to PKCδ inhibition mitigated the induction of caspase-dependent apoptosis. Furthermore, PKCδ inhibition effectively induced cytotoxicity in BRAF mutant melanoma cell lines that had evolved resistance to a BRAF inhibitor, suggesting the potential clinical application of targeting PKCδ in patients who have relapsed following treatment with BRAF inhibitors. Taken together, the present work demonstrates that inhibition of PKCδ by novel small molecule inhibitors causes caspase-dependent apoptosis mediated via the JNK-H2AX pathway in melanomas with NRAS mutations or BRAF inhibitor resistance.

Protein kinase C-δ inactivation inhibits the proliferation and survival of cancer stem cells in culture and in vivo

Background

A subpopulation of tumor cells with distinct stem-like properties (cancer stem-like cells, CSCs) may be responsible for tumor initiation, invasive growth, and possibly dissemination to distant organ sites. CSCs exhibit a spectrum of biological, biochemical, and molecular features that are consistent with a stem-like phenotype, including growth as non-adherent spheres (clonogenic potential), ability to form a new tumor in xenograft assays, unlimited self-renewal, and the capacity for multipotency and lineage-specific differentiation. PKCδ is a novel class serine/threonine kinase of the PKC family, and functions in a number of cellular activities including cell proliferation, survival or apoptosis. PKCδ has previously been validated as a synthetic lethal target in cancer cells of multiple types with aberrant activation of Ras signaling, using both genetic (shRNA and dominant-negative PKCδ mutants) and small molecule inhibitors. In contrast, PKCδ is not required for the proliferation or survival of normal cells, suggesting the potential tumor-specificity of a PKCδ-targeted approach.

Methods

shRNA knockdown was used validate PKCδ as a target in primary cancer stem cell lines and stem-like cells derived from human tumor cell lines, including breast, pancreatic, prostate and melanoma tumor cells. Novel and potent small molecule PKCδ inhibitors were employed in assays monitoring apoptosis, proliferation and clonogenic capacity of these cancer stem-like populations. Significant differences among data sets were determined using two-tailed Student’s t tests or ANOVA.

Results

We demonstrate that CSC-like populations derived from multiple types of human primary tumors, from human cancer cell lines, and from transformed human cells, require PKCδ activity and are susceptible to agents which deplete PKCδ protein or activity. Inhibition of PKCδ by specific genetic strategies (shRNA) or by novel small molecule inhibitors is growth inhibitory and cytotoxic to multiple types of human CSCs in culture. PKCδ inhibition efficiently prevents tumor sphere outgrowth from tumor cell cultures, with exposure times as short as six hours. Small-molecule PKCδ inhibitors also inhibit human CSC growth in vivo in a mouse xenograft model.

Conclusions

These findings suggest that the novel PKC isozyme PKCδ may represent a new molecular target for cancer stem cell populations.

Targeting the RAS oncogene

INTRODUCTION

The Ras proteins (K-Ras, N-Ras, and H-Ras) are GTPases that function as molecular switches for a variety of critical cellular activities and their function is tightly and temporally regulated in normal cells. Oncogenic mutations in the RAS genes, which create constitutively-active Ras proteins, can result in uncontrolled proliferation or survival in tumor cells.

AREAS COVERED

The paper discusses three therapeutic approaches targeting the Ras pathway in cancer: i) Ras itself, ii) Ras downstream pathways, and iii) synthetic lethality. The most adopted approach is targeting Ras downstream signaling, and specifically the PI3K-AKT-mTOR and Raf-MEK pathways, as they are frequently major oncogenic drivers in cancers with high Ras signaling. Although direct targeting of Ras has not been successful clinically, newer approaches being investigated in preclinical studies, such as RNA interference-based and synthetic lethal approaches, promise great potential for clinical application.

EXPERT OPINION

The challenges of current and emerging therapeutics include the lack of "tumor specificity" and their limitation to those cancers which are "dependent" on aberrant Ras signaling for survival. While the newer approaches have the potential to overcome these limitations, they also highlight the importance of robust preclinical studies and bidirectional translational research for successful clinical development of Ras-related targeted therapies.