Selective secretion of protein kinase C isozymes by thrombin-stimulated human platelets

Genetic Analysis of Platelet-Related Genes in Hepatocellular Carcinoma Reveals a Novel Prognostic Signature and Determines PRKCD as the Potential Molecular Bridge

BACKGROUND

Hepatocellular carcinoma (HCC) belongs to a representative lethality gastrointestinal malignancy, and comprehensive management of HCC remains intractable at present on account of its invasive biological feature that is easy to relapse and early metastasis. The intimate connection between platelets and tumor progression has been widely reported, and platelet-related indicators are also used in the clinical practice of carcinoma. This work is designed to investigate the significance of platelet-related genes in the prognostic prediction of patients with HCC and their potential role in the cross-talk between HCC cells and platelets in the tumor microenvironment.

METHODS

By integrating the RNA-seq data and clinicopathological information of HCC patients, we extracted prognosis-associated platelet-related genes based on the univariate cox analysis and further established a relevant prognostic signature via the lasso cox regression analysis, and two independent HCC cohorts were used as external validation. Multiple bioinformatics methods were utilized to explore the underlying functional discrepancy between different risk groups classified by the risk model. And in vitro proliferation, invasion, and migration assays were conducted to investigate the effect of platelet stimulation on HCC cells' viability and motility, and flow cytometric analysis was exerted to demonstrate the influence of HCC cells on platelet activation.

RESULTS

A novel platelet-related risk model was developed and patients both in the training and testing cohorts were divided into distinct risk subgroups according to the median risk score. It was observed that the high-risk status was closely associated with poor prognosis and worse clinicopathological parameters. Meanwhile, an obvious discrepancy in the constitution of the immune microenvironment also indicated that distinct immune status might be a potential determinant affecting prognosis as well as immunotherapy reactiveness. Moreover, in vitro experiments demonstrated that PRKCD could act as a molecular bridge between tumor cells and platelets, which could either participate in regulating tumor malignant phenotype or mediating platelet activation.

CONCLUSIONS

In brief, this work reveals a novel platelet-related risk signature for prognostic evaluation of HCC patients and confirms that PRKCD is a key messenger in HCC cell-platelet interaction and plays a crucial role in mediating platelet-induced tumor progression.

The secreted tyrosine kinase VLK is essential for normal platelet activation and thrombus formation

Tyrosine phosphorylation of extracellular proteins is observed in cell cultures and in vivo, but little is known about the functional roles of tyrosine phosphorylation of extracellular proteins. Vertebrate lonesome kinase (VLK) is a broadly expressed secretory pathway tyrosine kinase present in platelet α-granules. It is released from platelets upon activation and phosphorylates substrates extracellularly. Its role in platelet function, however, has not been previously studied. In human platelets, we identified phosphorylated tyrosines mapped to luminal or extracellular domains of transmembrane and secreted proteins implicated in the regulation of platelet activation. To determine the role of VLK in extracellular tyrosine phosphorylation and platelet function, we generated mice with a megakaryocyte/platelet-specific deficiency of VLK. Platelets from these mice are normal in abundance and morphology but have significant changes in function both in vitro and in vivo. Resting and thrombin-stimulated VLK-deficient platelets exhibit a significant decrease in several tyrosine phosphobands. Results of functional testing of VLK-deficient platelets show decreased protease-activated receptor 4-mediated and collagen-mediated platelet aggregation but normal responses to adenosine 5'-diphosphate. Dense granule and α-granule release are reduced in these platelets. Furthermore, VLK-deficient platelets exhibit decreased protease-activated receptor 4-mediated Akt (S473) and Erk1/2 (T202/Y204) phosphorylation, indicating altered proximal signaling. In vivo, mice lacking VLK in megakaryocytes/platelets display strongly reduced platelet accumulation and fibrin formation after laser-induced injury of cremaster arterioles compared with control mice but with normal bleeding times. These studies show that the secretory pathway tyrosine kinase VLK is critical for stimulus-dependent platelet activation and thrombus formation, providing the first evidence that a secreted protein kinase is required for normal platelet function.

The Human Platelet as an Innate Immune Cell: Interactions Between Activated Platelets and the Complement System

Platelets play an essential role in maintaining homeostasis in the circulatory system after an injury by forming a platelet thrombus, but they also occupy a central node in the intravascular innate immune system. This concept is supported by their extensive interactions with immune cells and the cascade systems of the blood. In this review we discuss the close relationship between platelets and the complement system and the role of these interactions during thromboinflammation. Platelets are protected from complement-mediated damage by soluble and membrane-expressed complement regulators, but they bind several complement components on their surfaces and trigger complement activation in the fluid phase. Furthermore, localized complement activation may enhance the procoagulant responses of platelets through the generation of procoagulant microparticles by insertion of sublytic amounts of C5b9 into the platelet membrane. We also highlight the role of post-translational protein modifications in regulating the complement system and the critical role of platelets in driving these reactions. In particular, modification of disulfide bonds by thiol isomerases and protein phosphorylation by extracellular kinases have emerged as important mechanisms to fine-tune complement activity in the platelet microenvironment. Lastly, we describe disorders with perturbed complement activation where part of the clinical presentation includes uncontrolled platelet activation that results in thrombocytopenia, and illustrate how complement-targeting drugs are alleviating the prothrombotic phenotype in these patients. Based on these clinical observations, we discuss the role of limited complement activation in enhancing platelet activation and consider how these drugs may provide opportunities for further dissecting the complex interactions between complement and platelets.

Phosphorylated fibronectin enhances cell attachment and upregulates mechanical cell functions

A large number of extracellular matrix proteins have been found in phosphorylated states, yet little is known about how the phosphorylation of extracellular matrix proteins might affect cell functions. We thus tested the hypothesis whether the phosphorylation of fibronectin, a major adhesion protein, affects cell behavior. Controlled in vitro phosphorylation of fibronectin by a casein kinase II (CKII) significantly upregulated cell traction forces and total strain energy generated by fibroblasts on nanopillar arrays, and consequently other elementary cell functions including cell spreading and metabolic activity. Mass spectrometry of plasma fibronectin from healthy human donors then identified a constitutively phosphorylated site in the C-terminus, and numerous other residues that became phosphorylated by the CKII kinase in vitro. Our findings open up novel strategies for translational applications including targeting diseased ECM, or to develop assays that probe the phosphorylation state of the ECM or blood as potential cancer markers.

Extracellular Protein Phosphorylation, the Neglected Side of the Modification

The very existence of extracellular phosphorylation has been questioned for a long time, although casein phosphorylation was discovered a century ago. In addition, several modification sites localized on secreted proteins or on extracellular or lumenal domains of transmembrane proteins have been catalogued in large scale phosphorylation analyses, though in most such studies this aspect of cellular localization was not considered. Our review presents examples when additional analyses were performed on already public data sets that revealed a wealth of information about this "neglected side" of the modification. We also sum up accumulated knowledge about extracellular phosphorylation, including the discovery of Golgi-residing kinases and the special difficulties encountered in targeted analyses. We hope future phosphorylation studies will not ignore the existence of phosphorylation outside of the cell, and further discoveries will shed more light on its biological role.

PKCs in thrombus formation

The protein kinase C (PKC) family has been implicated in several physiological processes regulating platelet activation. Each isoform of PKC expressed on platelets, may have a positive and/or negative role depending on the nature and concentration of the agonist. Mice lacking PKCα show much reduced thrombus formation in vivo, while PKCθ(-/-) showed inhibition of aggregation in response to PAR4. On the other hand, PKCδ by associating with Fyn, inhibits platelet aggregation. In addition, PKCβ by interacting with its receptor RACK1 has been implicated in the primary phases of signaling via the αIIbβ3 and finally PKCɛ appears to be involved in platelet function downstream GPVI. The present review discusses the latest observations relevant to the role of individual PKC isoforms in platelet activation and thrombus formation.

Ecto-protein kinases and phosphatases: an emerging field for translational medicine

Progress in translational research has led to effective new treatments of a large number of diseases. Despite this progress, diseases including cancer and cardiovascular disorders still are at the top in death statistics and disorders such as osteoporosis and osteoarthritis represent an increasing disease burden in the aging population. Novel strategies in research are needed more than ever to overcome such diseases. The growing field of extracellular protein phosphorylation provides excellent opportunities to make major discoveries of disease mechanisms that can lead to novel therapies. Reversible phosphorylation/dephosphorylation of sites in the extracellular domains of matrix, cell-surface and trans-membrane proteins is emerging as a critical regulatory mechanism in health and disease. Moreover, a new concept is emerging from studies of extracellular protein phosphorylation: in cells where ATP is stored within secretory vesicles and released by exocytosis upon cell-stimulation, phosphorylation of extracellular proteins can operate as a messenger operating uniquely in signaling pathways responsible for long-term cellular adaptation. Here, we highlight new concepts that arise from this research, and discuss translation of the findings into clinical applications such as development of diagnostic disease markers and next-generation drugs.

A role for PKCtheta in outside-in alpha(IIb)beta3 signaling

Fibrinogen binding to platelets triggers alpha(IIb)beta3-dependent outside-in signals that promote actin rearrangements and cell spreading. Studies with chemical inhibitors or activators have implicated protein kinase C (PKC) in alpha(IIb)beta3 function. However, the role of individual PKC isoforms is poorly understood. Biochemical and genetic approaches were used to determine whether PKCtheta is involved in alpha(IIb)beta3 signaling. PKCtheta was constitutively associated with alpha(IIb)beta3 in human and murine platelets. Fibrinogen binding to alpha(IIb)beta3 stimulated the association of PKCtheta with tyrosine kinases Btk and Syk, and tyrosine phosphorylation of PKCtheta, Btk and the actin regulator, Wiskott-Aldrich syndrome protein (WASP). Mouse platelets deficient in PKCtheta or Btk failed to spread on fibrinogen. Furthermore, PKCtheta was required for phosphorylation of WASP-interacting protein on Ser-488, an event that has been linked to WASP activation of the Arp2/3 complex and actin polymerization in lymphocytes. Neither PKCtheta nor Btk were required for agonist-induced inside-out signaling and fibrinogen binding to alpha(IIb)beta3. Thus, PKCtheta is a newly identified, essential member of a dynamic outside-in signaling complex that includes Btk and that couples alpha(IIb)beta3 to the actin cytoskeleton.

Regulation of outside-in signaling in platelets by integrin-associated protein kinase C beta

Studies with inhibitors have implicated protein kinase C (PKC) in the adhesive functions of integrin alpha(IIb)beta(3) in platelets, but the responsible PKC isoforms and mechanisms are unknown. Alpha(IIb)beta(3) interacts directly with tyrosine kinases c-Src and Syk. Therefore, we asked whether alpha(IIb)beta(3) might also interact with PKC. Of the several PKC isoforms expressed in platelets, only PKC beta co-immunoprecipitated with alpha(IIb)beta(3) in response to the interaction of platelets with soluble or immobilized fibrinogen. PKC beta recruitment to alpha(IIb)beta(3) was accompanied by a 9-fold increase in PKC activity in alpha(IIb)beta(3) immunoprecipitates. RACK1, an intracellular adapter for activated PKC beta, also co-immunoprecipitated with alpha(IIb)beta(3), but in this case, the interaction was constitutive. Broad spectrum PKC inhibitors blocked both PKC beta recruitment to alpha(IIb)beta(3) and the spread of platelets on fibrinogen. Similarly, mouse platelets that are genetically deficient in PKC beta spread poorly on fibrinogen, despite normal agonist-induced fibrinogen binding. In a Chinese hamster ovary cell model system, adhesion to fibrinogen caused green fluorescent protein-PKC beta I to associate with alpha(IIb)beta(3) and to co-localize with it at lamellipodial edges. These responses, as well as Chinese hamster ovary cell migration on fibrinogen, were blocked by the deletion of the beta(3) cytoplasmic tail or by co-expression of a RACK1 mutant incapable of binding to beta(3). These studies demonstrate that the interaction of alpha(IIb)beta(3) with activated PKC beta is regulated by integrin occupancy and can be mediated by RACK1 and that the interaction is required for platelet spreading triggered through alpha(IIb)beta(3). Furthermore, the studies extend the concept of alpha(IIb)beta(3) as a scaffold for multiple protein kinases that regulate the platelet actin cytoskeleton.

Identification of a binding site for the anti-inflammatory tripeptide feG

The mechanism of action of feG, an anti-inflammatory peptide, was explored using data mining, molecular modeling, and enzymatic techniques. The molecular coordinates of protein kinase A (PKA) were used to create six virtual isoforms of protein kinase C (PKCalpha, betaI, betaII, delta, iota, and zeta). With in silico techniques a binding site for feG was identified on PKCbetaI that correlated significantly with a biological activity, the inhibition of intestinal anaphylaxis. Since feG selectively increased the binding of a PKCbetaI antibody, it is proposed that this peptide inhibits the reassociation of the hydrophobic tail of PKCbetaI with its binding site and prevents the enzyme from assuming an inactive conformation.