Structural basis of protein kinase C isoform function

Impact of deleterious missense PRKCI variants on structural and functional dynamics of protein

Protein kinase C iota (PKCɩ) is a novel protein containing 596 amino acids and is also a member of atypical kinase family. The role of PKCɩ has been explored in neurodegenerative diseases, neuroblastoma, ovarian and pancreatic cancers. Single nucleotide polymorphisms (SNPs) have not been studied in PKCɩ till date. The purpose of the current study is to scrutinize the deleterious missense variants in PKCɩ and determine the effect of these variants on stability and dynamics of the protein. The structure of protein PKCɩ was predicted for the first time and post translational modifications were determined. Genetic variants of PKCɩ were retrieved from ENSEMBL and only missense variants were further analyzed because of its linkage with diseases. The pathogenicity of missense variants, effect on structure and function of protein, association with cancer and conservancy of the protein residues were determined through computational approaches. It is observed that C1 and the pseudo substrate region has the highest number of pathogenic SNPs. Variations in the kinase domain of the protein are predicted to alter overall phosphorylation of the protein. Molecular dynamic simulations predicted noteworthy change in structural and functional dynamics of the protein because of these variants. The study revealed that nine deleterious variants can possibly contribute to malfunctioning of the protein and can be associated with diseases. This can be useful in diagnostics and developing therapeutics for diseases related to these polymorphisms.

Identification of novel inhibitors targeting TIRAP interactions with BTK and PKCδ in inflammation through an in silico approach

Advanced computational tools focusing on protein-protein interaction (PPI) based drug development is a powerful platform to accelerate the therapeutic development of small lead molecules and repurposed drugs. Toll/interleukin-1 receptor (TIR) domain-containing adapter protein (TIRAP) and its interactions with other proteins in macrophages signalling are crucial components of severe or persistent inflammation. TIRAP activation through Bruton's tyrosine kinase (BTK) and Protein Kinase C delta (PKCδ) is essential for downstream inflammatory signalling. We created homology-based structural models of BTK and PKCδ in MODELLER 9.24. TIRAP interactions with BTK and PKCδ in its non-phosphorylated and phosphorylated states were determined by multiple docking tools including HADDOCK 2.4, pyDockWEB and ClusPro 2.0. Food and Drug Administration (FDA)-approved drugs were virtually screened through Discovery Studio LibDock and Autodock Vina tools to target the common TIR domain residues of TIRAP, which interact with both BTK and PKC at the identified interfacial sites of the complexes. Four FDA-approved drugs were identified and found to have stable interactions over a range of 100 ns MD simulation timescales. These drugs block the interactions of both kinases with TIRAP in silico. Hence, these drugs have the potential to dampen downstream inflammatory signalling and inflammation-mediated disease.

Lysosomes: Multifunctional compartments ruled by a complex regulatory network

More than 50 years have passed since Nobel laureate Cristian de Duve described for the first time the presence of tiny subcellular compartments filled with hydrolytic enzymes: the lysosome. For a long time, lysosomes were deemed simple waste bags exerting a plethora of hydrolytic activities involved in the recycling of biopolymers, and lysosomal genes were considered to just be simple housekeeping genes, transcribed in a constitutive fashion. However, lysosomes are emerging as multifunctional signalling hubs involved in multiple aspects of cell biology, both under homeostatic and pathological conditions. Lysosomes are involved in the regulation of cell metabolism through the mTOR/TFEB axis. They are also key players in the regulation and onset of the immune response. Furthermore, it is becoming clear that lysosomal hydrolases can regulate several biological processes outside of the lysosome. They are also implicated in a complex communication network among subcellular compartments that involves intimate organelle‐to‐organelle contacts. Furthermore, lysosomal dysfunction is nowadays accepted as the causative event behind several human pathologies: low frequency inherited diseases, cancer, or neurodegenerative, metabolic, inflammatory, and autoimmune diseases. Recent advances in our knowledge of the complex biology of lysosomes have established them as promising therapeutic targets for the treatment of different pathologies. Although recent discoveries have started to highlight that lysosomes are controlled by a complex web of regulatory networks, which in some cases seem to be cell‐ and stimuli‐dependent, to harness the full potential of lysosomes as therapeutic targets, we need a deeper understanding of the little‐known signalling pathways regulating this subcellular compartment and its functions.

Role and Mechanism of PKC-δ for Cardiovascular Disease: Current Status and Perspective

Protein kinase C (PKC) is a protein kinase with important cellular functions. PKC-δ, a member of the novel PKC subfamily, has been well-documented over the years. Activation of PKC-δ plays an important regulatory role in myocardial ischemia/reperfusion (IRI) injury and myocardial fibrosis, and its activity and expression levels can regulate pathological cardiovascular diseases such as atherosclerosis, hypertension, cardiac hypertrophy, and heart failure. This article aims to review the structure and function of PKC-δ, summarize the current research regarding its activation mechanism and its role in cardiovascular disease, and provide novel insight into further research on the role of PKC-δ in cardiovascular diseases.

PKC-ζ Aggravates Doxorubicin-Induced Cardiotoxicity by Inhibiting Wnt/β-Catenin Signaling

Doxorubicin (Dox) is a chemotherapeutic drug used to treat a wide range of cancers, but its clinical application is limited due to its cardiotoxicity. Protein kinase C-ζ (PKC-ζ) is a serine/threonine kinase belonging to atypical protein kinase C (PKC) subfamily, and is activated by its phosphorylation. We and others have reported that PKC-ζ induced cardiac hypertrophy by activating the inflammatory signaling pathway. This study focused on whether PKC-ζ played an important role in Dox-induced cardiotoxicity. We found that PKC-ζ phosphorylation was increased by Dox treatment in vivo and in vitro. PKC-ζ overexpression exacerbated Dox-induced cardiotoxicity. Conversely, knockdown of PKC-ζ by siRNA relieved Dox-induced cardiotoxicity. Similar results were observed when PKC-ζ enzyme activity was inhibited by its pseudosubstrate inhibitor, Myristoylated. PKC-ζ interacted with β-catenin and inhibited Wnt/β-catenin signaling pathway. Activation of Wnt/β-catenin signaling by LiCl protected against Dox-induced cardiotoxicity. The Wnt/β-catenin inhibitor XAV-939 aggravated Dox-caused decline of β-catenin and cardiomyocyte apoptosis and mitochondrial damage. Moreover, activation of Wnt/β-catenin suppressed aggravation of Dox-induced cardiotoxicity due to PKC-ζ overexpression. Taken together, our study revealed that inhibition of PKC-ζ activity was a potential cardioprotective approach to preventing Dox-induced cardiac injury.

Connexin32 ameliorates epithelial-to-mesenchymal-transition in diabetic renal tubular via inhibiting NOX4

Renal tubulointerstitial fibrosis (RIF), characterized by epithelial-to-mesenchymal transition (EMT) of renal tubular epithelial cells (TECs), is the main cause of diabetic renal fibrosis. Oxidative stress plays a pivotal role in the development of diabetic RIF. Connexin32 (Cx32), prominently expressed in renal TECs, has emerged as an important player in the regulation of oxidative stress. However, the role of Cx32 in diabetic RIF has not been explored yet. Here, we showed that adenovirus-mediated Cx32 overexpression suppressed EMT to ameliorate RIF and renal function in STZ-induced diabetic mice, while knockout (KO) of Cx32 exacerbated RIF in diabetic mice. Moreover, overexpression of Cx32 inhibited EMT and the production of extra cellular matrix (ECM) in high glucose (HG) induced NRK-52E cells, whereas knockdown of Cx32 showed the opposite effects. Furthermore, we showed that NOX4, the main source of ROS in renal tubular, was down-regulated by Cx32. Mechanistically, Cx32 down-regulated the expression of PKC alpha in a carboxyl-terminal-dependent manner, thereby inhibiting the phosphorylation at Thr147 of p22phox triggered by PKC alpha, which ultimately repressed the formation of the p22phox-NOX4 complex to reduce the protein level of NOX4. Thus, we establish Cx32 as a novel target and confirm the protection mechanism in RIF.

TGR5 Activation Ameliorates Mitochondrial Homeostasis via Regulating the PKCδ/Drp1-HK2 Signaling in Diabetic Retinopathy

Background: Diabetic retinopathy (DR) is one of the most important microvascular diseases of diabetes. Our previous research demonstrated that bile acid G-protein-coupled membrane receptor (TGR5), a novel cell membrane receptor of bile acid, ameliorates the vascular endothelial cell dysfunction in DR. However, the precise mechanism leading to this alteration remains unknown. Thus, the mechanism of TGR5 in the progress of DR should be urgently explored.

Methods: In this study, we established high glucose (HG)-induced human retinal vascular endothelial cells (RMECs) and streptozotocin-induced DR rat in vitro and in vivo. The expression of TGR5 was interfered through the specific agonist or siRNA to study the effect of TGR5 on the function of endothelial cell in vitro. Western blot, immunofluorescence and fluorescent probes were used to explore how TGR5 regulated mitochondrial homeostasis and related molecular mechanism. The adeno-associated virus serotype 8-shTGR5 (AAV8-shTGR5) was performed to evaluate retinal dysfunction in vivo and further confirm the role of TGR5 in DR by HE staining, TUNEL staining, PAS staining and Evans Blue dye.

Results: We found that TGR5 activation alleviated HG-induced endothelial cell apoptosis by improving mitochondrial homeostasis. Additionally, TGR5 signaling reduced mitochondrial fission by suppressing the Ca²⁺-PKCδ/Drp1 signaling and enhanced mitophagy through the upregulation of the PINK1/Parkin signaling pathway. Furthermore, our result indicated that Drp1 inhibited mitophagy by facilitating the hexokinase (HK) 2 separation from the mitochondria and HK2-PINK1/Parkin signaling. In vivo, intraretinal microvascular abnormalities, including retinal vascular leakage, acellular capillaries and apoptosis, were poor in AAV8-shTGR5-treated group under DR, but this effect was reversed by pretreatment with the mitochondrial fission inhibitor Mdivi-1 or autophagy agonist Rapamycin.

Conclusion: Overall, our findings indicated that TGR5 inhibited mitochondrial fission and enhanced mitophagy in RMECs by regulating the PKCδ/Drp1-HK2 signaling pathway. These results revealed the molecular mechanisms underlying the protective effects of TGR5 and suggested that activation of TGR5 might be a potential therapeutic strategy for DR.

Insights into the potential of withanolides as Phosphodiesterase-4 (PDE4D) inhibitors

Medicinal herbs have been used as traditional medicines for centuries. The molecular mechanism of action of their bioactive molecules against various diseases or therapeutic targets is still being explored. Here, the active compounds (withanolides) of a well-known Indian medicinal herb, Ashwagandha (Withania somnifera), have been studied for their most potential therapeutic targets and their mechanism of action using ligand-based screening and receptor-based approaches. Ligand-based screening predicted the six top therapeutic targets, namely, Protein kinase C alpha (PRKCA), Protein kinase C delta (PRKCD), Protein kinase C epsilon (PRKCE), Androgenic Receptor (AR), Cycloxygenase-2 (PTGS-2) and Phosphodiesterase-4D (PDE4D). Further, when these predictions were validated using receptor-based studies, i.e. molecular docking, molecular dynamics simulation and free energy calculations, it was found that PDE4D was the most potent target for four withanolides, namely, Withaferin-A, 17-Hydroxywithaferin-A, 27-Hydroxywithanone and Withanolide-R. These compounds had a better binding affinity and similar interactions as that of an already known inhibitor (Zardaverine) of PDE4D. These results warrant further in-vitro and in-vivo investigations to examine their therapeutic potential as an inhibitor of PDE4D.Communicated by Ramaswamy H. Sarma.

Multiple subcellular localizations and functions of protein kinase Cδ in liver cancer

Protein kinase Cδ (PKCδ) is a member of the PKC family, and its implications have been reported in various biological and cancerous processes, including cell proliferation, cell death, tumor suppression, and tumor progression. In liver cancer cells, accumulating reports show the bi-functional regulation of PKCδ in cell death and survival. PKCδ function is defined by various factors, such as phosphorylation, catalytic domain cleavage, and subcellular localization. PKCδ has multiple intracellular distribution patterns, ranging from the cytosol to the nucleus. We recently found a unique extracellular localization of PKCδ in liver cancer and its growth factor-like function in liver cancer cells. In this review, we first discuss the structural features of PKCδ and then focus on the functional diversity of PKCδ based on its subcellular localization, such as the nucleus, cell surface, and extracellular space. These findings improve our knowledge of PKCδ involvement in the progression of liver cancer.

Role of Nox4 in High Calcium-Induced Renal Oxidative Stress Damage and Crystal Deposition

Aims: We aimed at exploring the role of nicotinamide adenine dinucleotide phosphate oxidase subunit 4 (Nox4) in the regulation of hypercalciuria-induced renal oxidative damage and crystal depositions. Results: High calcium activated Nox4 expression through protein kinase C (PKC). Downregulation of Nox4 expression attenuated hypercalciuria-induced osteoblast-associated protein expression, oxidative stress injury, and crystal deposition in rat kidneys of 1,25-dihydroxyvitamin D3 (VitD) urolithiasis model. Further, calcium-induced activation of mitogen-activated protein kinase (MAPK), overexpression of osteoblast-associated protein, oxidative stress injury, apoptosis, and calcium salt deposition in normal rat kidney epithelial-like (NRK-52E) cells were reversed by downregulating Nox4 expression but were enhanced by upregulating Nox4 expression in vitro. Moreover, calcium-induced increases of osteoblast-associated protein expression were attenuated by the c-Jun-N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) inhibitors. Innovation: Our results demonstrated the effect of Nox4 in the pathological process of kidney stones in in vitro and in vivo studies for the first time. Calcium aggravates renal oxidative stress injury and crystal deposition by activating the Nox4-related reactive oxygen species (ROS)-ERK/JNK pathway in the rat kidney. This study is expected to provide a new theoretical basis for the prevention and treatment of kidney stones. Conclusion: Nox4-derived ROS induced by calcium through PKC caused oxidative stress damage and apoptosis in renal tubular epithelial cells; in addition, Nox4-derived ROS induced by calcium mediated abnormal activation of the bone morphogenetic protein 2 (BMP2) signaling pathway through the MAPK signaling pathway, which induced renal tubular epithelial cells to transdifferentiate into osteoblast-like cells, resulting in the formation of a kidney stone. Antioxid. Redox Signal. 36, 15-38.

Diabetic Complications: An Update on Pathobiology and Therapeutic Strategies

Despite the advent of novel therapies which manage and control diabetes well, the increased risk of morbidity and mortality in diabetic subjects is associated with the devastating secondary complications it produces. Long-standing diabetes majorly drives cellular and molecular alterations, which eventually damage both small and large blood vessels. The complications are prevalent both in type I and type II diabetic subjects. The microvascular complications include diabetic neuropathy, diabetic nephropathy, diabetic retinopathy, while the macrovascular complications include diabetic heart disease and stroke. The current therapeutic strategy alleviates the complications to some extent but does not cure or prevent them. Also, the recent clinical trial outcomes in this field are disappointing. Success in the drug discovery of diabetic complications may be achieved by a better understanding of the underlying pathophysiology and by recognising the crucial factors contributing to the development and progression of the disease. In this review, we discuss the well-studied cellular mechanisms leading to the development and progression of diabetic complications. In addition, we also highlight the various therapeutic paradigms currently in clinical practice.

Chiral 2-phenyl-3-hydroxypropyl esters as PKC-alpha modulators: HPLC enantioseparation, NMR absolute configuration assignment, and molecular docking studies

Protein kinase C (PKC) isoforms play a pivotal role in the regulation of numerous cellular functions, making them extensively studied and highly attractive drug targets. In our previous work, we identified in racemate 1-2, based on the 2-benzyl-3-hydroxypropyl ester scaffold, two new potent and promising PKCα and PKCδ ligands, targeting the C1 domain of these two kinases. Herein, we report the resolution of the racemates by enantioselective semi-preparative HPLC. The attribution of the absolute configuration (AC) of homochirals 1 was performed by NMR, via methoxy-α-trifluoromethyl-α-phenylacetic acid derivatization (MTPA or Mosher's acid). Moreover, the match between the experimental and predicted electronic circular dichroism (ECD) spectra confirmed the assigned AC. These results proved that Mosher's esters can be properly exploited for the determination of the AC also for chiral primary alcohols. Lastly, homochiral 1 and 2 were assessed for binding affinity and functional activity against PKCα. No significative differences in the K_i of the enantiopure compounds was observed, thus suggesting that chirality does not seem to play a significant role in targeting PKC C1 domain. These results are in accordance with the molecular docking studies performed using a new homology model for the human PKCαC1B domain.

Wogonin and alleviation of hyperglycemia via inhibition of DAG mediated PKC expression. A brief insight

Protein kinase C (PKC) is a family of protein kinase enzymes that can phosphorylate other proteins and influence their functions, such as signal transduction, cell survival, and death. Increased diacylglycerol (DAG) concentrations, which are typically observed raised in hyperglycemic situations such as diabetes mellitus, can also activate PKC enzymes (DM). On the other hand, PKC isomers have been shown to play an essential role in diabetes and many hyperglycemic complications, most importantly atherosclerosis and diabetic cardiomyopathy (DCM). As a result, blocking PKC activation via DAG can prevent hyperglycemia and related consequences, such as DCM. Wogonin is a herbal medicine which has anti-inflammatory properties, and investigations show that it scavenge oxidative radicals, attenuate nuclear factor-kappa B (NF-κB) activity, inhibit several essential cell cycle regulatory genes, block nitric oxide (NO) and suppress cyclooxygenase-2 (COX-2). Furthermore, several investigations show that wogonin also attenuates diacylglycerol DAG levels in diabetic mice. Since the DAG-PKC pathway is linked with hyperglycemia and its complications, Wogonin-mediated DAG-PKC attenuation can help treat hyperglycemia and its complications.

Receptor-specific Ca2+ oscillation patterns mediated by differential regulation of P2Y purinergic receptors in rat hepatocytes

Extracellular agonists linked to inositol-1,4,5-trisphosphate (IP₃) formation elicit cytosolic Ca²⁺ oscillations in many cell types, but despite a common signaling pathway, distinct agonist-specific Ca²⁺ spike patterns are observed. Using qPCR, we show that rat hepatocytes express multiple purinergic P2Y and P2X receptors (R). ADP acting through P2Y1R elicits narrow Ca²⁺ oscillations, whereas UTP acting through P2Y2R elicits broad Ca²⁺ oscillations, with composite patterns observed for ATP. P2XRs do not play a role at physiological agonist levels. The discrete Ca²⁺ signatures reflect differential effects of protein kinase C (PKC), which selectively modifies the falling phase of the Ca²⁺ spikes. Negative feedback by PKC limits the duration of P2Y1R-induced Ca²⁺ spikes in a manner that requires extracellular Ca²⁺. By contrast, P2Y2R is resistant to PKC negative feedback. Thus, the PKC leg of the bifurcated IP₃ signaling pathway shapes unique Ca²⁺ oscillation patterns that allows for distinct cellular responses to different agonists.

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Distinct stereotypic Ca²⁺ oscillations are elicited by P2Y1 and P2Y2 receptors

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P2X receptors do not contribute to the generation of Ca²⁺ oscillations

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Agonist-specific Ca²⁺ spike shapes reflect discrete modes of PKC negative feedback

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Bifurcation of IP₃/PKC signaling yields unique Ca²⁺ oscillation signatures

Activators and Inhibitors of Protein Kinase C (PKC): Their Applications in Clinical Trials

Protein kinase C (PKC), a family of phospholipid-dependent serine/threonine kinase, is classed into three subfamilies based on their structural and activation characteristics: conventional or classic PKC isozymes (cPKCs; α, βI, βII, and γ), novel or non-classic PKC isozymes (nPKCs; δ, ε, η, and θ), and atypical PKC isozymes (aPKCs; ζ, ι, and λ). PKC inhibitors and activators are used to understand PKC-mediated intracellular signaling pathways and for the diagnosis and treatment of various PKC-associated diseases, such as cancers, neurological diseases, cardiovascular diseases, and infections. Many clinical trials of PKC inhibitors in cancers showed no significant clinical benefits, meaning that there is a limitation to design a cancer therapeutic strategy targeting PKC alone. This review will focus on the activators and inhibitors of PKC and their applications in clinical trials.

PLCγ1/PKCθ Downstream Signaling Controls Cutaneous T-Cell Lymphoma Development And Progression

Developing mechanistic rationales can improve the clinical management of cutaneous T-cell lymphomas (CTCL). There is considerable genetic and biological evidence of a malignant network of signaling mechanisms, highly influenced by deregulated TCR/PLCγ1 activity, controlling the biology of these lesions. In addition, activated STAT3 is associated with clinical progression, although the alterations responsible for this have not been fully elucidated. Here we studied PLCγ1-dependent mechanisms that can mediate STAT3 activation and control tumor growth and progression. Downstream of PLCγ1, the pharmacological inhibition and genetic knockdown of PKCθ inhibited STAT3 activation, impaired proliferation, and promoted apoptosis in CTCL cells. A PKCθ-dependent transcriptome in MF/SS cells revealed potential effector genes controlling cytokine signaling, TP53, and actin cytoskeleton dynamics. Consistently, an in vivo chicken embryo model xenografted with MF cells showed that PKCθ blockage abrogates tumor growth and spread to distant organs. Finally, the expression of a number of PKCθ target genes, found in MF cells, significantly correlated with that of PRKCQ (PKCθ) in 81 human MF samples. In summary, PKCθ can play a central role in the activation of malignant CTCL mechanisms via multiple routes, including, but not restricted to, STAT3. These mechanisms may, in turn, serve as targets for specific therapies.

Vernicia fordii (Hemsl.) Airy Shaw extract stimulates insulin secretion in pancreatic β-cells and improves insulin sensitivity in diabetic mice

ETHNOPHARMACOLOGICAL RELEVANCE

Vernicia fordii (Hemsl.) Airy Shaw (V. fordii) is also known as the tung tree and its leaves and fruit are used as an oriental treatment for dyspepsia, edema, and skin diseases, which are known as diabetic complications.

AIM OF THE STUDY

In this study, we aimed to investigate the methanolic extract (VF5) of the leaves of V. fordii as an insulin secretagogue and its probable mechanism and verify the effect in HFD-fed mice.

MATERIALS AND METHODS

The insulin secretagogue activity of different doses of VF5 (0.1, 0.3 and 1.0 μg/ml) was assessed using in vitro insulin secretion assay and confirmed the anti-diabetic effect in mice fed HFD for 4 weeks with different doses of VF5 (10, 20 and 50 mg/kg oral) for another 6 weeks. Glbenclamide (30 mg/kg, oral) was used as positive control drug. The possible mechanisms were evaluated by using Gö6983 (10 μM), U73122 (10 μM) and nifedipine (10 μM). The major constituents of VF5 were analyzed by UPLC-QToF-MS and ¹H and ¹³C NMR spectroscopy.

RESULTS

UPLC-QToF-MS and NMR spectroscopy analysis indicated that one of the main active components of VF5 was tigliane-diterpene esters. VF5 functioned as an insulin secretagogue and enhanced mitochondria respiration and insulin homeostasis. We confirmed that VF5 preserved the β-cell and reduced the β-cell expansion which caused by metabolic stress under HFD. The antidiabetic role of VF5 in HFD fed mice was assessed by glucose tolerance test (GTT) and insulin tolerance test (ITT), fasting plasma insulin level, fasting blood glucose level, AKT signal in peripheral tissue in the absence of toxic effects. Mechanistically, insulinotropic effect of VF5 was mediated by activation of PKCα via intracellular Ca²⁺ influx and enhanced mitochondria function.

CONCLUSION

VF5 exhibits potent insulin secretagogue function and improves insulin sensitivity and protection of pancreatic β-cells from metabolic stress without toxicity. Taken together, our study suggests that VF5 could be potentially used for treating diabetes and metabolic diseases through improving β-cell function.

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

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

Molecular Signaling and Transcriptional Regulation of Histamine H1 Receptor Gene

Histamine-activated histamine H₁ receptor (H1R) signaling regulates many gene expressions, mainly through the protein kinase C (PKC)/extracellular signal-regulated kinases (ERK) signaling. Involvement of other signaling, including NF-κB, Wnt, RUNX-2, and Rho A signaling was also demonstrated. In addition, cAMP production through the activation of H1R signaling was reported. H1R gene itself is also up-regulated by the activation of H1R signaling with histamine. Here, we review our recent findings in the molecular signaling and transcriptional regulation of the H1R gene. Stimulation with histamine up-regulates H1R gene expression through the activation of H1R in HeLa cells. The PKCδ/ERK/poly(ADP)ribosyl transferase-1 (PARP-1) signaling was involved in this up-regulation. Heat shock protein 90 also plays an important role in regulating PKCδ translocation. Promoter analyses revealed the existence of two promoters in the human H1R gene in HeLa cells. H1R-activated H1R gene up-regulation in response to histamine was also observed in U373 astroglioma cells. However, this up-regulation was mediated not through the PKCδ signaling but possibly through the PKCα signaling. In addition, the promoter region responsible for histamine-induced H1R gene transcription in U373 cells was different from that of HeLa cells. These findings suggest that the molecular signaling and transcriptional regulation of the H1R gene are different between neuronal cells and non-neuronal cells.

IGFBP-1 hyperphosphorylation in response to nutrient deprivation is mediated by activation of protein kinase Cα (PKCα)

Fetal growth restriction (FGR) is associated with decreased nutrient availability and reduced insulin-line growth factor (IGF)-I bioavailability via increased IGF binding protein (IGFBP)-1 phosphorylation. While protein kinase C (PKC) is implicated in IGFBP-1 hyperphosphorylation in nutrient deprivation, the mechanisms remain unclear. We hypothesised that the interaction of PKCα with protein kinase CK2β and activation of PKCα under leucine deprivation (L0) mediate fetal hepatic IGFBP-1 hyperphosphorylation. Parallel Reaction Monitoring Mass Spectrometry (PRM-MS) followed by PKCα knockdown demonstrated the PKCα isoform interacts with IGFBP-1 and CK2β under L0. Pharmacological PKCα activation with phorbol 12-myristate 13-acetate (PMA) increased whereas inhibition with bisindolylmaleimide II (Bis II) decreased IGFBP-1 phosphorylation (Ser101/119/169, Ser98 + 101 and Ser169 + 174), respectively. Furthermore, PMA mimicked L0-induced PKCα translocation and IGFBP-1 expression. PKCα expression was increased in baboon fetal liver in FGR, providing biological relevance in vivo. In summary, we report a novel nutrient-sensitive mechanism for PKCα in mediating IGFBP-1 hyperphosphorylation in FGR.

Uncovering the 'sphinx' of sphingosine 1-phosphate signalling: from cellular events to organ morphogenesis

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid metabolite, functioning as a signalling molecule in diverse cellular processes. Over the past few decades, studies of S1P signalling have revealed that the physiological activity of S1P largely depends on S1P metabolizing enzymes, transporters and receptors on the plasma membrane, as well as on the intracellular proteins that S1P binds directly to. In addition to its roles in cancer signalling, immunity and inflammation, a large body of evidence has identified a close link of S1P signalling with organ morphogenesis. Here we discuss the vital role of S1P signalling in orchestrating various cellular events during organ morphogenesis through analysing each component along the extracellular and intracellular S1P signalling axes. For each component, we review advances in our understanding of S1P signalling and function from the upstream regulators to the downstream effectors and from cellular behaviours to tissue organization, primarily in the context of morphogenetic mechanisms. S1P-mediated vesicular trafficking is also discussed as a function independent of its signalling function. A picture emerges that reveals a multifaceted role of S1P-dependent pathways in the development and maintenance of organ structure and function.

Primary Cilia-Related Pathways Moderate the Development and Therapy Resistance of Glioblastoma

As microtubule-based structures, primary cilia are typically present on the cells during the G0 or G1-S/G2 phase of the cell cycle and are closely related to the development of the central nervous system. The presence or absence of this special organelle may regulate the central nervous system tumorigenesis (e.g., glioblastoma) and several degenerative diseases. Additionally, the development of primary cilia can be regulated by several pathways. Conversely, primary cilia are able to regulate a few signaling transduction pathways. Therefore, development of the central nervous system tumors in conjunction with abnormal cilia can be regulated by up- or downregulation of the pathways related to cilia and ciliogenesis. Here, we review some pathways related to ciliogenesis and tumorigenesis, aiming to provide a potential target for developing new therapies at genetic and molecular levels.

Protocatechuic acid attenuates isoproterenol-induced cardiac hypertrophy via downregulation of ROCK1-Sp1-PKCγ axis

Cardiac hypertrophy is an adaptive response of the myocardium to pressure overload or adrenergic agonists. Here, we investigated the protective effects and the regulatory mechanism of protocatechuic acid, a phenolic compound, using a mouse model of isoproterenol-induced cardiac hypertrophy. Our results demonstrated that protocatechuic acid treatment significantly downregulated the expression of cardiac hypertrophic markers (Nppa, Nppb, and Myh7), cardiomyocyte size, heart weight to body weight ratio, cross-sectional area, and thickness of left ventricular septum and posterior wall. This treatment also reduced the expression of isoproterenol-induced ROCK1, Sp1, and PKCγ both in vivo and in vitro. To investigate the mechanism, we performed knockdown and overexpression experiments. The knockdown of ROCK1, Sp1, or PKCγ decreased the isoproterenol-induced cell area and the expression of hypertrophic markers, while the overexpression of Sp1 or PKCγ increased the levels of hypertrophic markers. Protocatechuic acid treatment reversed these effects. Interestingly, the overexpression of Sp1 increased cell area and induced PKCγ expression. Furthermore, experiments using transcription inhibitor actinomycin D showed that ROCK1 and Sp1 suppression by protocatechuic acid was not regulated at the transcriptional level. Our results indicate that protocatechuic acid acts via the ROCK1/Sp1/PKCγ axis and therefore has promising therapeutic potential as a treatment for cardiac hypertrophy.

Identification of Activated Protein Kinase Cα (PKCα) in the Urine of Orthotopic Bladder Cancer Xenograft Model as a Potential Biomarker for the Diagnosis of Bladder Cancer

Bladder cancer has a high recurrence rate; therefore, frequent and effective monitoring is essential for disease management. Cystoscopy is considered the gold standard for the diagnosis and continuous monitoring of bladder cancer. However, cystoscopy is invasive and relatively expensive. Thus, there is a need for non-invasive, relatively inexpensive urinary biomarker-based diagnoses of bladder cancer. This study aimed to investigate the presence of activated protein kinase Cα (PKCα) in urine samples and the possibility of PKCα as a urinary biomarker for bladder cancer diagnosis. Activated PKCα was found to be present at higher levels in bladder cancer tissues than in normal bladder tissues. Furthermore, high levels of activated PKCα were observed in urine samples collected from orthotopic xenograft mice carrying human bladder cancer cells compared to urine samples from normal mice. These results suggest that activated PKCα can be used as a urinary biomarker to diagnose bladder cancer. To the best of our knowledge, this is the first report describing the presence of activated PKCα in the urine of orthotopic xenograft mice.

Blood glucose regulation in context of infection

The immune and endocrine systems collectively control homeostasis in the body. The endocrine system ensures that values of essential factors and nutrients such as glucose, electrolytes and vitamins are maintained within threshold values. The immune system resolves local disruptions in tissue homeostasis, caused by pathogens or malfunctioning cells. The immediate goals of these two systems do not always align. The immune system benefits from optimal access to nutrients for itself and restriction of nutrient availability to all other organs to limit pathogen replication. The endocrine system aims to ensure optimal nutrient access for all organs, limited only by the nutrients stores that the body has available. The actual state of homeostatic parameters such as blood glucose levels represents a careful balance based on regulatory signals from the immune and endocrine systems. This state is not static but continuously adjusted in response to changes in the current metabolic needs of the body, the amount of resources it has available and the level of threats it encounters. This balance is maintained by the ability of the immune and endocrine systems to interact and co-regulate systemic metabolism. In context of metabolic disease, this system is disrupted, which impairs functionality of both systems. The failure of the endocrine system to retain levels of nutrients such as glucose within threshold values impairs functionality of the immune system. In addition, metabolic stress of organs in context of obesity is perceived by the immune system as a disruption in local homeostasis, which it tries to resolve by the excretion of factors which further disrupt normal metabolic control. In this chapter, we will discuss how the immune and endocrine systems interact under homeostatic conditions and during infection with a focus on blood glucose regulation. In addition, we will discuss how this system fails in the context of metabolic disease.

M1 and M2 mAChRs activate PDK1 and regulate PKC βI and ε and the exocytotic apparatus at the NMJ

Neuromuscular junctions (NMJ) regulate cholinergic exocytosis through the M₁ and M₂ muscarinic acetylcholine autoreceptors (mAChR), involving the crosstalk between receptors and downstream pathways. Protein kinase C (PKC) regulates neurotransmission but how it associates with the mAChRs remains unknown. Here, we investigate whether mAChRs recruit the classical PKCβI and the novel PKCε isoforms and modulate their priming by PDK1, translocation and activity on neurosecretion targets. We show that each M₁ and M₂ mAChR activates the master kinase PDK1 and promotes a particular priming of the presynaptic PKCβI and ε isoforms. M₁ recruits both primed-PKCs to the membrane and promotes Munc18-1, SNAP-25, and MARCKS phosphorylation. In contrast, M₂ downregulates PKCε through a PKA-dependent pathway, which inhibits Munc18-1 synthesis and PKC phosphorylation. In summary, our results discover a co-dependent balance between muscarinic autoreceptors which orchestrates the presynaptic PKC and their action on ACh release SNARE-SM mechanism. Altogether, this molecular signaling explains previous functional studies at the NMJ and guide toward potential therapeutic targets.

Potential Role of Protein Kinase C in the Pathophysiology of Diabetes-Associated Atherosclerosis

Diabetes mellitus is a metabolic syndrome that affects millions of people worldwide. Recent studies have demonstrated that protein kinase C (PKC) activation plays an important role in hyperglycemia-induced atherosclerosis. PKC activation is involved in several cellular responses such as the expression of various growth factors, activation of signaling pathways, and enhancement of oxidative stress in hyperglycemia. However, the role of PKC activation in pro-atherogenic and anti-atherogenic mechanisms remains controversial, especially under hyperglycemic condition. In this review, we discuss the role of different PKC isoforms in lipid regulation, oxidative stress, inflammatory response, and apoptosis. These intracellular events are linked to the pathogenesis of atherosclerosis in diabetes. PKC deletion or treatment with PKC inhibitors has been studied in the regulation of atherosclerotic plaque formation and evolution. Furthermore, some preclinical and clinical studies have indicated that PKCβ and PKCδ are potential targets for the treatment of diabetic vascular complications. The current review summarizes these multiple signaling pathways and cellular responses regulated by PKC activation and the potential therapeutic targets of PKC in diabetic complications.

PKCδ deficiency inhibits fetal development and is associated with heart elastic fiber hyperplasia and lung inflammation in adult PKCδ knockout mice

Protein kinase C-delta (PKCδ) has a caspase-3 recognition sequence in its structure, suggesting its involvement in apoptosis. In addition, PKCδ was recently reported to function as an anti-cancer factor. The generation of a PKCδ knockout mouse model indicated that PKCδ plays a role in B cell homeostasis. However, the Pkcrd gene, which is regulated through complex transcription, produces multiple proteins via alternative splicing. Since gene mutations can result in the loss of function of molecular species required for each tissue, in the present study, conditional PKCδ knockout mice lacking PKCδI, II, IV, V, VI, and VII were generated to enable tissue-specific deletion of PKCδ using a suitable Cre mouse. We generated PKCδ-null mice that lacked whole-body expression of PKCδ. PKCδ+/- parental mice gave birth to only 3.4% PKCδ-/- offsprings that deviated significantly from the expected Mendelian ratio (χ2(2) = 101.7, P < 0.001). Examination of mice on embryonic day 11.5 (E11.5) showed the proportion of PKCδ-/- mice implanted in the uterus in accordance with Mendelian rules; however, approximately 70% of the fetuses did not survive at E11.5. PKCδ-/- mice that survived until adulthood showed enlarged spleens, with some having cardiac and pulmonary abnormalities. Our findings suggest that the lack of PKCδ may have harmful effects on fetal development, and heart and lung functions after birth. Furthermore, our study provides a reference for future studies on PKCδ deficient mice that would elucidate the effects of the multiple protein variants in mice and decipher the roles of PKCδ in various diseases.

SUMO wrestling in the cellular dohyō: crosstalk between phosphorylation and SUMOylation of PKCδ regulates oxidative cell damage

The novel PKCδ isoform has been shown to mediate a pro-apoptotic function in response to oxidative stressors. Siman Gao and colleagues performed an in-depth biochemical and molecular analysis of factors that regulate PKCδ function. They demonstrated convincingly that PKCδ is regulated by an interplay between SUMOylation and phosphorylation. They also showed that these events are critical for hydrogen peroxide induced apoptosis, thus identifying potentially novel mechanisms that may be harnessed for cell protection.

mTOR-Rictor-EGFR axis in oncogenesis and diagnosis of glioblastoma multiforme

Glioblastoma multiforme (GBM) is one of the aggressive brain cancers with patients having less survival period upto 12-15 months. Mammalian target of rapamycin (mTOR) is a serine/threonine kinase, belongs to the phosphatidylinositol 3-kinases (PI3K) pathway and is involved in various cellular processes of cancer cells. Cancer metabolism is regulated by mTOR and its components. mTOR forms two complexes as mTORC1 and mTORC2. Studies have identified the key component of the mTORC2 complex, Rapamycin-insensitive companion of mammalian target of rapamycin (Rictor) plays a prominent role in the regulation of cancer cell proliferation and metabolism. Apart, growth factor receptor signaling such as epidermal growth factor signaling mediated by epidermal growth factor receptor (EGFR) regulates cancer-related processes. In EGFR signaling various other signaling cascades such as phosphatidyl-inositol 3-kinase (PI3K)/protein kinase B (Akt)/mammalian target of rapamycin (mTOR pathway) and Ras/Raf/mitogen-activated protein kinase/ERK kinase (MEK)/extracellular-signal-regulated kinase (ERK) -dependent signaling cross-talk each other. From various studies about GBM, it is very well established that Rictor and EGFR mediated signaling pathways majorly playing a pivotal role in chemoresistance and tumor aggressiveness. Recent studies have shown that non-coding RNAs such as microRNAs (miRs) and long non-coding RNAs (lncRNAs) regulate the EGFR and Rictor and sensitize the cells towards chemotherapeutic agents. Thus, understanding of microRNA mediated regulation of EGFR and Rictor will help in cancer prevention and management as well as a future therapy.

Progesterone Receptor Together with PKCα Expression as Prognostic Factors for Astrocytomas Malignancy

INTRODUCTION

Astrocytomas are the most common and aggressive primary brain tumors, and they are classified according to the degree of malignancy on a scale of I to IV, in which grade I is the least malignant and grade IV the highest. Many factors are related to astrocytomas progression as progesterone receptor (PR), whose transcriptional activity could be regulated by phosphorylation by protein kinase C alpha (PKCα) at the residue Ser400. Our aim was to investigate if PR phosphorylation together with PKCα expression could be used as a prognostic factor for astrocytomas malignancy.

METHODS

By immunofluorescence, we detected the content of PKCα, PR and its phosphorylation at Ser400 in 46 biopsies from Mexican patients with different astrocytoma malignancy grades; by bioinformatic tools using TCGA data, we evaluated the expression of PR and PKCα mRNA according to astrocytoma malignancy grades. For all statistical analyses, significance was p<0.05.

RESULTS

We detected a positive correlation between the tumor grade and the content of PKCα, PR and its phosphorylation at Ser400, as well as the intracellular colocalization of these proteins. Interestingly, using an in silico assay, we found that the PR and PKCα expression at mRNA level has an inverse ratio with astrocytomas tumor grade.

DISCUSSION

These results indicate that PR and its phosphorylation at Ser400 site, as well as PKCα and their colocalization, could be considered as possible malignancy biomarkers for astrocytomas grades I-IV.

Centriolar Protein C2cd3 Is Required for Craniofacial Development

The primary cilium is a ubiquitous, microtubule-based cellular organelle. Primary cilia dysfunction results in a group of disorders termed ciliopathies. C2 domain containing 3 centriole elongation regulator (C2cd3), encodes a centriolar protein essential for ciliogenesis. Mutations in human C2CD3 are associated with the human ciliopathy Oral-Facial-Digital syndrome type 14 (OFD14). In order to better understand the etiology of ciliopathies including OFD14, we generated numerous murine models targeting C2cd3. Initial analysis revealed several tissue-specific isoforms of C2cd3, and while the loss of C2cd3 has previously been reported to result in exencephaly, tight mesencephalic flexure, pericardial edema, abnormal heart looping and a twisted body axis, further analysis revealed that genetic background may also contribute to phenotypic variation. Additional analyses of a conditional allelic series targeting C-terminal PKC-C2 domains or the N-terminal C2CD3N-C2 domain of C2cd3 revealed a variable degree of phenotypic severity, suggesting that while the N-terminal C2CD3N-C2 domain was critical for early embryonic development as a whole, there was also a craniofacial specific role for the C2CD3N-C2 domains. Together, through generation of novel models and evaluation of C2cd3 expression, these data provide valuable insight into mechanisms of pathology for craniofacial ciliopathies that can be further explored in the future.

The interplay between SUMOylation and phosphorylation of PKCδ facilitates oxidative stress-induced apoptosis

Although the increase in the number of identified posttranslational modifications (PTMs) has substantially improved our knowledge about substrate site specificity of single PTMs, the fact that different types of PTMs can crosstalk and act in concert to exert important regulatory mechanisms for protein function has not gained much attention. Here, we show that protein kinase Cδ (PKCδ) is SUMOylated at lysine 473 in its C-terminal catalytic domain, and the SUMOylation increases PKCδ stability by repressing its ubiquitination. In addition, we uncover a functional interplay between the phosphorylation and SUMOylation of PKCδ, which can strengthen each other through recruiting SUMO E2/E3 ligases and the PKCδ kinase, respectively, to the PKCδ complexes. We identified PIAS2β as the SUMO E3 ligase of PKCδ. More importantly, by enhancing PKCδ protein stability and its phosphorylation through an interdependent interplay of the PTMs, the SUMOylation of PKCδ promotes apoptotic cell death induced by H₂ O₂ . We conclude that SUMOylation represents an important regulatory mechanism of PKCδ PTMs for the kinase's function in oxidative cell damage.

Protein Kinase C as a Therapeutic Target in Non-Small Cell Lung Cancer

Driver-directed therapeutics have revolutionized cancer treatment, presenting similar or better efficacy compared to traditional chemotherapy and substantially improving quality of life. Despite significant advances, targeted therapy is greatly limited by resistance acquisition, which emerges in nearly all patients receiving treatment. As a result, identifying the molecular modulators of resistance is of great interest. Recent work has implicated protein kinase C (PKC) isozymes as mediators of drug resistance in non-small cell lung cancer (NSCLC). Importantly, previous findings on PKC have implicated this family of enzymes in both tumor-promotive and tumor-suppressive biology in various tissues. Here, we review the biological role of PKC isozymes in NSCLC through extensive analysis of cell-line-based studies to better understand the rationale for PKC inhibition. PKC isoforms α, ε, η, ι, ζ upregulation has been reported in lung cancer, and overexpression correlates with worse prognosis in NSCLC patients. Most importantly, PKC isozymes have been established as mediators of resistance to tyrosine kinase inhibitors in NSCLC. Unfortunately, however, PKC-directed therapeutics have yielded unsatisfactory results, likely due to a lack of specific evaluation for PKC. To achieve satisfactory results in clinical trials, predictive biomarkers of PKC activity must be established and screened for prior to patient enrollment. Furthermore, tandem inhibition of PKC and molecular drivers may be a potential therapeutic strategy to prevent the emergence of resistance in NSCLC.

mTOR regulates GPVI-mediated platelet activation

Background

Due to mTOR (mammalian/mechanistic target of rapamycin) gene-loss mice die during embryonic development, the role of mTOR in platelets has not been evaluated using gene knockout technology.

Methods

A mouse model with megakaryocyte/platelet-specific deletion of mTOR was established, and be used to evaluate the role of mTOR in platelet activation and thrombus formation.

Results

mTOR^−/− platelets were deficient in thrombus formation when grown on low-concentration collagen-coated surfaces; however, no deficiency in thrombus formation was observed when mTOR^−/− platelets were perfused on higher concentration collagen-coated surfaces. In FeCl₃-induced mouse mesenteric arteriole thrombosis models, wild-type (WT) and mTOR^−/− mice displayed significantly different responses to low-extent injury with respect to the ratio of occluded mice, especially within the first 40 min. Additionally, mTOR^−/− platelets displayed reduced aggregation and dense granule secretion (ATP release) in response to low doses of the glycoprotein VI (GPVI) agonist collagen related peptide (CRP) and the protease-activated receptor-4 (PAR4) agonist GYPGKF-NH₂; these deficiencies were overcame by stimulation with higher concentration agonists, suggesting dose dependence of the response. At low doses of GPVI or PAR agonist, the activation of α_IIbβ₃ in mTOR^−/− platelets was reduced. Moreover, stimulation of mTOR^−/− platelets with low-dose CRP attenuated the phosphorylation of S6K1, S6 and Akt Ser473, and increased the phosphorylation of PKCδ Thr505 and PKCε Ser729. Using isoform-specific inhibitors of PKCs (δ, ɛ, and α/β), we established that PKCδ/ɛ, and especially PKCδ but not PKCα/β or PKCθ, may be involved in low-dose GPVI-mediated/mTOR-dependent signaling.

Conclusion

These observations indicate that mTOR plays an important role in GPVI-dependent platelet activation and thrombus formation.

6,6'-Dihydroxythiobinupharidine (DTBN) Purified from Nuphar lutea Leaves Is an Inhibitor of Protein Kinase C Catalytic Activity

Water lily (Nuphar) bioactive extracts have been widely used in traditional medicine owing to their multiple applications against human ailments. Phyto-active Nuphar extracts and their purified and synthetic derivatives have attracted the attention of ethnobotanists and biochemists. Here, we report that 6,6'-dihydroxythiobinupharidine (DTBN), purified from extracts of Nuphar lutea (L.) Sm. leaves, is an effective inhibitor of the kinase activity of members of the protein kinase C (PKC) family using in vitro and in silico approaches. We demonstrate that members of the conventional subfamily of PKCs, PKCα and PKCγ, were more sensitive to DTBN inhibition as compared to novel or atypical PKCs. Molecular docking analysis demonstrated the interaction of DTBN, with the kinase domain of PKCs depicting the best affinity towards conventional PKCs, in accordance with our in vitro kinase activity data. The current study reveals novel targets for DTBN activity, functioning as an inhibitor for PKCs kinase activity. Thus, this and other data indicate that DTBN modulates key cellular signal transduction pathways relevant to disease biology, including cancer.

PKC98E Regulates Odorant Responses in Drosophila melanogaster

Drosophila odorant receptors (Ors) are ligand gated ion channels composed of a common receptor subunit Or co-receptor (ORCO) and one of 62 "tuning" receptor subunits that confer odorant specificity to olfactory neuron responses. Like other sensory systems studied to date, exposing Drosophila olfactory neurons to activating ligands results in reduced responses to subsequent exposures through a process called desensitization. We recently showed that phosphorylation of serine 289 on the common Or subunit ORCO is required for normal peak olfactory neuron responses. Dephosphorylation of this residue occurs on prolonged odorant exposure, and underlies the slow modulation of olfactory neuron responses we term "slow desensitization." Slow desensitization results in the reduction of peak olfactory neuron responses and flattening of dose-response curves, implicating changes in ORCO^S289 phosphorylation state as an important modulator of olfactory neuron responses. Here, we report the identification of the primary kinase responsible for ORCO^S289 phosphorylation, PKC98E. Antiserum localizes the kinase to the dendrites of the olfactory neurons. Deletion of the kinase from olfactory neurons in the naive state (the absence of prolonged odor exposure) reduces ORCO^S289 phosphorylation and reduces peak odorant responses without altering receptor localization or expression levels. Genetic rescue with a PKC98E predicted to be constitutively active restores ORCO S289 phosphorylation and olfactory neuron sensitivity to the PKC98E mutants in the naive state. However, the dominant kinase is defective for slow desensitization. Together, these findings reveal that PKC98E is an important regulator of ORCO receptors and olfactory neuron function.SIGNIFICANCE STATEMENT We have identified PKC98E as the kinase responsible for phosphorylation of the odorant receptor co-receptor (ORCO) at S289 that is required for normal odorant response kinetics of olfactory neurons. This is a significant step toward revealing the enzymology underlying the regulation of odorant response regulation in insects.

Phosphorylation of protein kinase Cδ Tyr311 positively regulates thromboxane generation in platelets

Platelets are key mediators of physiological hemostasis and pathological thrombosis, whose function must be carefully balanced by signaling downstream of receptors such as protease-activated receptor (PAR)4. Protein kinase C (PKC) is known to regulate various aspects of platelet function. For instance, PKCδ is known to regulate dense granule secretion, which is important for platelet activation. However, the mechanism by which PKCδ regulates this process as well as other facets of platelet activity is unknown. We speculated that the way PKCδ regulates platelet function may be because of the phosphorylation of tyrosine residues on PKCδ. We investigated phosphorylation of PKCδ following glycoprotein VI-mediated and PAR4-mediated platelet activation and found that Y311 is selectively phosphorylated when PAR4 is activated in human platelets. Therefore, we generated PKCδ Y311F knock-in mice, which are viable and have no gross abnormalities. However, PKCδY311F mice have significantly enhanced tail-bleeding times compared with WT littermate controls, which means hemostasis is interrupted. Furthermore, PKCδY311F mice exhibit longer time to carotid artery occlusion compared with WT control using a ferric chloride in vivo thrombosis model, indicating that the phosphorylation of PKCδ Y311 is prothrombotic. Washed platelets from PKCδY311F mice have reduced reactivity after stimulation with a PAR-4 agonist indicating its importance in platelet signaling. The phenotype observed in Y311F mouse platelets is because of reduced thromboxane generation, as an inhibitor of thromboxane generation equalizes the PKCδY311F platelet response to that of WT. Therefore, phosphorylation of PKCδ on Y311 is important for regulation of platelet function and specifically thromboxane generation, which reinforces platelet activation.

LPA1 Receptor Promotes Progesterone Receptor Phosphorylation through PKCα in Human Glioblastoma Cells

Lysophosphatidic acid (LPA) induces a wide range of cellular processes and its signaling is increased in several cancers including glioblastoma (GBM), a high-grade astrocytoma, which is the most common malignant brain tumor. LPA1 receptor is expressed in GBM cells and its signaling pathways activate protein kinases C (PKCs). A downstream target of PKC, involved in GBM progression, is the intracellular progesterone receptor (PR), which can be phosphorylated by this enzyme, increasing its transcriptional activity. Interestingly, in GBM cells, PKCα isotype translocates to the nucleus after LPA stimulation, resulting in an increase in PR phosphorylation. In this study, we determined that LPA1 receptor activation induces protein-protein interaction between PKCα and PR in human GBM cells; this interaction increased PR phosphorylation in serine400. Moreover, LPA treatment augmented VEGF transcription, a known PR target. This effect was blocked by the PR selective modulator RU486; also, the activation of LPA1/PR signaling promoted migration of GBM cells. Interestingly, using TCGA data base, we found that mRNA expression of LPAR1 increases according to tumor malignancy and correlates with a lower survival in grade III astrocytomas. These results suggest that LPA1/PR pathway regulates GBM progression.

Target specific tight junction modulators

Intercellular tight junctions represent a formidable barrier against paracellular drug absorption at epithelia (e.g., nasal, intestinal) and the endothelium (e.g., blood-brain barrier). In order to enhance paracellular transport of drugs and increase their bioavailability and organ deposition, active excipients modulating tight junctions have been applied. First-generation of permeation enhancers (PEs) acted by unspecific interactions, while recently developed PEs address specific physiological mechanisms. Such target specific tight junction modulators (TJMs) have the advantage of a defined specific mechanism of action. To date, merely a few of these novel active excipients has entered into clinical trials, as their lack in safety and efficiency in vivo often impedes their commercialisation. A stronger focus on the development of such active excipients would result in an economic and therapeutic improvement of current and future drugs.

The Calcium Signaling Mechanisms in Arterial Smooth Muscle and Endothelial Cells

The contractile state of resistance arteries and arterioles is a crucial determinant of blood pressure and blood flow. Physiological regulation of arterial contractility requires constant communication between endothelial and smooth muscle cells. Various Ca²⁺ signals and Ca²⁺ -sensitive targets ensure dynamic control of intercellular communications in the vascular wall. The functional effect of a Ca²⁺ signal on arterial contractility depends on the type of Ca²⁺ -sensitive target engaged by that signal. Recent studies using advanced imaging methods have identified the spatiotemporal signatures of individual Ca²⁺ signals that control arterial and arteriolar contractility. Broadly speaking, intracellular Ca²⁺ is increased by ion channels and transporters on the plasma membrane and endoplasmic reticular membrane. Physiological roles for many vascular Ca²⁺ signals have already been confirmed, while further investigation is needed for other Ca²⁺ signals. This article focuses on endothelial and smooth muscle Ca²⁺ signaling mechanisms in resistance arteries and arterioles. We discuss the Ca²⁺ entry pathways at the plasma membrane, Ca²⁺ release signals from the intracellular stores, the functional and physiological relevance of Ca²⁺ signals, and their regulatory mechanisms. Finally, we describe the contribution of abnormal endothelial and smooth muscle Ca²⁺ signals to the pathogenesis of vascular disorders. © 2021 American Physiological Society. Compr Physiol 11:1831-1869, 2021.

Oncogenic N-Ras Mitigates Oxidative Stress-Induced Apoptosis of Hematopoietic Stem Cells

Leukemic relapse is believed to be driven by transformed hematopoietic stem cells (HSC) that harbor oncogenic mutations or have lost tumor suppressor function. Recent comprehensive sequencing studies have shown that mutations predicted to activate Ras signaling are highly prevalent in hematologic malignancies and, notably, in refractory and relapsed cases. To better understand what drives this clinical phenomenon, we expressed oncogenic NrasG12D within the hematopoietic system in mice and interrogated its effects on HSC survival. N-RasG12D conferred a survival benefit to HSCs and progenitors following metabolic and genotoxic stress. This effect was limited to HSCs and early progenitors and was independent of autophagy and cell proliferation. N-RasG12D-mediated HSC survival was not affected by inhibition of canonical Ras effectors such as MEK and PI3K. However, inhibition of the noncanonical Ras effector pathway protein kinase C (PKC) ameliorated the protective effects of N-RasG12D. Mechanistically, N-RasG12D lowered levels of reactive oxygen species (ROS), which correlated with reduced mitochondrial membrane potential and ATP levels. Inhibition of PKC restored the levels of ROS to that of control HSCs and abrogated the protective effects granted by N-RasG12D. Thus, N-RasG12D activation within HSCs promotes cell survival through the mitigation of ROS, and targeting this mechanism may represent a viable strategy to induce apoptosis during malignant transformation of HSCs. SIGNIFICANCE: Targeting oncogenic N-Ras-mediated reduction of ROS in hematopoietic stem cells through inhibition of the noncanonical Ras effector PKC may serve as a novel strategy for treatment of leukemia and other Ras-mutated cancers.

Phosphoproteomic identification of vasopressin-regulated protein kinases in collecting duct cells

BACKGROUND AND PURPOSE

The peptide hormone vasopressin regulates water transport in the renal collecting duct largely via the V2 receptor, which triggers a cAMP-mediated activation of a PKA-dependent signalling network. The protein kinases downstream from PKA have not been fully identified or mapped to regulated phosphoproteins.

EXPERIMENTAL APPROACH

We carried out systems-level analysis of large-scale phosphoproteomic data quantifying vasopressin-induced changes in phosphorylation in aquaporin-2-expressing cultured collecting duct (mpkCCD) cells. Quantification was done using stable isotope labelling (SILAC method).

KEY RESULTS

Six hundred forty phosphopeptides were quantified. Stringent statistical analysis identified significant changes in response to vasopressin in 429 of these phosphopeptides. The corresponding phosphoproteins were mapped to known vasopressin-regulated cellular processes. The vasopressin-regulated sites were classified according to the sequences surrounding the phosphorylated amino acids giving 11 groups. Among the vasopressin-regulated phosphoproteins were 25 distinct protein kinases. Among these, six plus PKA appeared to account for phosphorylation of about 81% of the 313 vasopressin-regulated phosphorylation sites. The six downstream kinases were salt-inducible kinase 2 (Sik2), cyclin-dependent kinase 18 (Cdk18), calmodulin-dependent kinase kinase 2 (Camkk2), protein kinase D2 (Prkd2), mitogen-activated kinase 3 (Mapk3) and myosin light chain kinase (Mylk).

CONCLUSION AND IMPLICATIONS

In V2 receptor-mediated signalling, PKA is at the head of a complex network that includes at least six downstream vasopressin-regulated protein kinases that are prime targets for future study. The extensive phosphoproteomic data reported in this study are provided as a web-based data resource for future studies of GPCRs.

Short peptide sequence enhances epithelial permeability through interaction with protein kinase C

We have identified a short peptide sequence (L-R5) acting as partial inhibitor of intracellular protein kinase C, capable of tight junction modulation in terms of reversible and non-toxic drug permeation enhancement. L-R5 is a pentapeptide with a cell-penetrating group at the N-terminus and of the sequence myristoyl-ARRWR. Apically applied in vitro, L-R5 transiently increased epithelial permeability within minutes, enhancing apical-to-basolateral (AB) transport of 4-kDa dextran and BCS class III drug naloxone. L-R5 was shown to be stable and effective at 37°C over a period of 24 hours. L-R5 was shown to be non-cytotoxic in consecutive exposure studies on primary human nasal epithelial cells by LDH release assay and ciliary beating frequency test. Finally, L-R5 by itself showed very low diffusion across epithelial monolayers, which is of advantage with regard to its expected negligible systemic bioavailability and side effects. Taken together, these data demonstrate the potential of short peptide partial inhibitor L-R5 to enhance the epithelial paracellular permeability via a reversible mechanism, and in a non-toxic manner.

Differentiation of HL-60 cells in serum-free hematopoietic cell media enhances the production of neutrophil extracellular traps

Neutrophil extracellular traps (NETs) are web-like structures made of chromatin and have been identified to have a role in the host's immune defense. Differentiated human promyelocytic leukemia HL-60 cells (dHL-60) have been used to study the mechanisms of NETs formation, as neutrophils have a short lifespan that limits their use. However, dHL-60 cells are inefficient at generating NETs and therefore are not ideal replacements for neutrophils in studying of NET formation. In the present study, the optimal cell culture conditions and differentiation time that result in the most effective release of NETs from dHL-60 cells upon stimulation were determined. HL-60 cells were cultured in serum (FBS) or serum-free (X-VIVO) medium and differentiated using all-trans retinoic acid (ATRA) or dimethyl sulfoxide (DMSO). dHL-60 cells were stimulated with phorbol 12-myristate 13-acetate (PMA) or Ca²⁺ ionophore (CI). Cell differentiation and apoptosis, as well as the formation of reactive oxygen species (ROS) and citrullinated histone H3 (citH3) were analyzed using flow cytometry. NETs were visualized using fluorescence microscopy and NET quantification was performed using PicoGreen. Induction of HL-60 cells for five days produced the best results in terms of differentiation markers and cell viability. Both ATRA- and DMSO-induced dHL-60 cells were able to release NETs upon PMA and CI stimulation; dHL-60 cells in serum-free medium produced more NETs than those in serum-containing medium. DMSO-dHL-60 (X-VIVO) cells were most efficient at producing NETs and ROS upon stimulation with PMA, while ATRA-dHL-60 (X-VIVO) cells were most efficient at producing NETs and citH3 upon stimulation with CI. It was concluded that DMSO-dHL-60 (X-VIVO) may be a model for the study of ROS-high NETosis and ATRA-dHL-60 (X-VIVO) may be suitable for ROS-low NETosis.

The Emerging Function of PKCtheta in Cancer

Protein Kinase C theta (PKCθ) is a serine/threonine kinase that belongs to the novel PKC subfamily. In normal tissue, its expression is restricted to skeletal muscle cells, platelets and T lymphocytes in which PKCθ controls several essential cellular processes such as survival, proliferation and differentiation. Particularly, PKCθ has been extensively studied for its role in the immune system where its translocation to the immunological synapse plays a critical role in T cell activation. Beyond its physiological role in immune responses, increasing evidence implicates PKCθ in the pathology of various diseases, especially autoimmune disorders and cancers. In this review, we discuss the implication of PKCθ in various types of cancers and the PKCθ-mediated signaling events controlling cancer initiation and progression. In these types of cancers, the high PKCθ expression leads to aberrant cell proliferation, migration and invasion resulting in malignant phenotype. The recent development and application of PKCθ inhibitors in the context of autoimmune diseases could benefit the emergence of treatment for cancers in which PKCθ has been implicated.

Distinct Regulation of Cardiac Fibroblast Proliferation and Transdifferentiation by Classical and Novel Protein Kinase C Isoforms: Possible Implications for New Antifibrotic Therapies

Cardiac fibrosis is characterized by accumulation and activation of fibroblasts and excessive production of extracellular matrix, which results in myocardial stiffening and eventually leads to heart failure. Although previous work suggests that protein kinase C (PKC) isoforms play a role in cardiac fibrosis and remodeling, the results are conflicting. Moreover, the potential of targeting PKC with pharmacological tools to inhibit pathologic fibrosis has not been fully evaluated. Here we investigated the effects of selected PKC agonists and inhibitors on cardiac fibroblast (CF) phenotype, proliferation, and gene expression using primary adult mouse CFs, which spontaneously transdifferentiate into myofibroblasts in culture. A 48-hour exposure to the potent PKC activator phorbol 12-myristate 13-acetate (PMA) at 10 nM concentration reduced the intensity of α-smooth muscle actin staining by 56% and periostin mRNA levels by 60% compared with control. The decreases were inhibited with the pan-PKC inhibitor Gö6983 and the inhibitor of classical PKC isoforms Gö6976, suggesting that classical PKCs regulate CF transdifferentiation. PMA also induced a 33% decrease in 5-bromo-2'-deoxyuridine-positive CFs, which was inhibited with Gö6983 but not with Gö6976, indicating that novel PKC isoforms (nPKCs) regulate CF proliferation. Moreover, PMA downregulated the expression of collagen-encoding genes Col1a1 and Col3a1 nPKC-dependently, showing that PKC activation attenuates matrix synthesis in CFs. The partial PKC agonist isophthalate derivative bis(1-ethylpentyl) 5-(hydroxymethyl)isophthalate induced parallel changes in phenotype, cell cycle activity, and gene expression. In conclusion, our results reveal distinct PKC-dependent regulation of CF transdifferentiation and proliferation and suggest that PKC agonists exhibit potential as an antifibrotic treatment. SIGNIFICANCE STATEMENT: Cardiac fibrosis is a pathological process that contributes to the development of heart failure. The molecular mechanisms regulating fibrosis in the heart are, however, not fully understood, which hinders the development of new therapies. Here, we demonstrate that classical and novel protein kinase C (PKC) isoforms distinctly regulate cardiac fibroblast transdifferentiation and proliferation, the two central processes in fibrosis. Our results indicate that pharmacological PKC activation may be a promising strategy to inhibit myocardial fibrosis.

Dysregulation of Protein Kinase C in Adult Depression and Suicide: Evidence From Postmortem Brain Studies

BACKGROUND

Several lines of evidence suggest the abnormalities of protein kinase C (PKC) signaling system in mood disorders and suicide based primarily on the studies of PKC and its isozymes in the platelets and postmortem brain of depressed and suicidal subjects. In this study, we examined the role of PKC isozymes in depression and suicide.

METHODS

We determined the protein and mRNA expression of various PKC isozymes in the prefrontal cortical region (Brodmann area 9) in 24 normal control subjects, 24 depressed suicide (DS) subjects, and 12 depressed nonsuicide (DNS) subjects. The levels of mRNA in the prefrontal cortex were determined by quantitative real-time reverse transcription PCR, and the protein expression was determined by western blotting.

RESULTS

We observed a significant decrease in mRNA expression of PKCα, PKCβI, PKCδ, and PKCε and decreased protein expression in either the membrane or the cytosol fraction of PKC isozymes PKCα, PKCβI, PKCβII, and PKCδ in DS and DNS subjects compared with normal control subjects.

CONCLUSIONS

The current study provides detailed evidence of specific dysregulation of certain PKC isozymes in the postmortem brain of DS and DNS subjects and further supports earlier evidence for the role of PKC in the platelets and brain of the adult and teenage depressed and suicidal population. This comprehensive study may lead to further knowledge of the involvement of PKC in the pathophysiology of depression and suicide.