Targeting of PKCα and ϵ in the pituitary: a highly regulated mechanism involving a GD(E)E motif of the V3 region

The complexities of PKCα signaling in cancer

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

Phosphorylation of E-cadherin at threonine 790 by protein kinase Cδ reduces β-catenin binding and suppresses the function of E-cadherin

Proper control of cell-cell adhesion is crucial for embryogenesis and tissue homeostasis. In this study, we show that protein kinase C (PKC)δ, a member of the novel PKC subfamily, localizes at cell-cell contacts of epithelial cells through its C2-like domain in an F-actin-dependent manner. Upon hepatocyte growth factor stimulation, PKCδ is phosphorylated and activated by Src, which then phosphorylates E-cadherin at Thr790. Phosphorylation of E-cadherin at Thr790 diminishes its interaction with β-catenin and impairs the homophilic interaction between the ectodomains of E-cadherin. The suppression of PKCδ by its dominant-negative mutants or specific short-hairpin RNA inhibits the disruption of cell-cell adhesions induced by hepatocyte growth factor. Elevated PKCδ expression in cancer cells is correlated with increased phosphorylation of E-cadherin at Thr790, reduced binding of E-cadherin to β-catenin, and poor homophilic interaction between E-cadherin. Analysis of surgical specimens confirmed that PKCδ is overexpressed in cervical cancer tissues, accompanied by increased phosphorylation of E-cadherin at Thr790. Together, our findings unveil a negative role for PKCδ in cell-cell adhesion through phosphorylation of E-cadherin.

The yin and yang of protein kinase C-theta (PKCθ): a novel drug target for selective immunosuppression

Protein kinase C-theta (PKCθ) is a protein kinase C (PKC) family member expressed predominantly in T lymphocytes, and extensive studies addressing its function have been conducted. PKCθ is the only T cell-expressed PKC that localizes selectively to the center of the immunological synapse (IS) following conventional T cell antigen stimulation, and this unique localization is essential for PKCθ-mediated downstream signaling. While playing a minor role in T cell development, early in vitro studies relying, among others, on the use of PKCθ-deficient (Prkcq(-/-)) T cells revealed that PKCθ is required for the activation and proliferation of mature T cells, reflecting its importance in activating the transcription factors nuclear factor kappa B, activator protein-1, and nuclear factor of activated T cells, as well as for the survival of activated T cells. Upon subsequent analysis of in vivo immune responses in Prkcq(-/-) mice, it became clear that PKCθ has a selective role in the immune system: it is required for experimental Th2- and Th17-mediated allergic and autoimmune diseases, respectively, and for alloimmune responses, but is dispensable for protective responses against pathogens and for graft-versus-leukemia responses. Surprisingly, PKCθ was recently found to be excluded from the IS of regulatory T cells and to negatively regulate their suppressive function. These attributes of PKCθ make it an attractive target for catalytic or allosteric inhibitors that are expected to selectively suppress harmful inflammatory and alloimmune responses without interfering with beneficial immunity to infections. Early progress in developing such drugs is being made, but additional studies on the role of PKCθ in the human immune system are urgently needed.

The biology of protein kinase C

This review gives a basic introduction to the biology of protein kinase C, one of the first calcium-dependent kinases to be discovered. We review the structure and function of protein kinase C, along with some of the substrates of individual isoforms. We then review strategies for inhibiting PKC in experimental systems and finally discuss the therapeutic potential of targeting PKC. Each aspect is covered in summary, with links to detailed resources where appropriate.

Role for PKCβ in enhanced endothelin-1-induced pulmonary vasoconstrictor reactivity following intermittent hypoxia

Intermittent hypoxia (IH) resulting from sleep apnea causes both systemic and pulmonary hypertension. Enhanced endothelin-1 (ET-1)-induced vasoconstrictor reactivity is thought to play a central role in the systemic hypertensive response to IH. However, whether IH similarly increases pulmonary vasoreactivity and the signaling mechanisms involved are unknown. The objective of the present study was to test the hypothesis that IH augments ET-1-induced pulmonary vasoconstrictor reactivity through a PKCβ-dependent signaling pathway. Responses to ET-1 were assessed in endothelium-disrupted, pressurized pulmonary arteries (∼150 μm inner diameter) from eucapnic-IH [(E-IH) 3 min cycles, 5% O(2)-5% CO(2)/air flush, 7 h/day; 4 wk] and sham (air-cycled) rats. Arteries were loaded with fura-2 AM to monitor vascular smooth muscle (VSM) intracellular Ca(2+) concentration ([Ca(2+)](i)). E-IH increased vasoconstrictor reactivity without altering Ca(2+) responses, suggestive of myofilament Ca(2+) sensitization. Consistent with our hypothesis, inhibitors of both PKCα/β (myr-PKC) and PKCβ (LY-333-531) selectively decreased vasoconstriction to ET-1 in arteries from E-IH rats and normalized responses between groups, whereas Rho kinase (fasudil) and PKCδ (rottlerin) inhibition were without effect. Although E-IH did not alter arterial PKCα/β mRNA or protein expression, E-IH increased basal PKCβI/II membrane localization and caused ET-1-induced translocation of these isoforms away from the membrane fraction. We conclude that E-IH augments pulmonary vasoconstrictor reactivity to ET-1 through a novel PKCβ-dependent mechanism that is independent of altered PKC expression. These findings provide new insights into signaling mechanisms that contribute to vasoconstriction in the hypertensive pulmonary circulation.

Protein kinase C: poised to signal

Nestled at the tip of a branch of the kinome, protein kinase C (PKC) family members are poised to transduce signals emanating from the cell surface. Cell membranes provide the platform for PKC function, supporting the maturation of PKC through phosphorylation, its allosteric activation by binding specific lipids, and, ultimately, promoting the downregulation of the enzyme. These regulatory mechanisms precisely control the level of signaling-competent PKC in the cell. Disruption of this regulation results in pathophysiological states, most notably cancer, where PKC levels are often grossly altered. This review introduces the PKC family and then focuses on recent advances in understanding the cellular regulation of its diacylglycerol-regulated members.

A biologically active sequence of the laminin alpha2 large globular 1 domain promotes cell adhesion through syndecan-1 by inducing phosphorylation and membrane localization of protein kinase Cdelta

Laminin-2 promotes basement membrane assembly and peripheral myelinogenesis; however, a receptor-binding motif within laminin-2 and the downstream signaling pathways for motif-mediated cell adhesion have not been fully established. The human laminin-2 alpha2 chain cDNAs cloned from human keratinocytes and fibroblasts correspond to the laminin alpha2 chain variant sequence from the human brain. Individually expressed recombinant large globular (LG) 1 protein promotes cell adhesion and has heparin binding activities. Studies with synthetic peptides delineate the DLTIDDSYWYRI motif (Ln2-P3) within the LG1 as a major site for both heparin and cell binding. Cell adhesion to LG1 and Ln2-P3 is inhibited by treatment of heparitinase I and chondroitinase ABC. Syndecan-1 from PC12 cells binds to LG1 and Ln2-P3 and colocalizes with both molecules. Suppression of syndecan-1 with RNA interference inhibits cell adhesion to LG1 and Ln2-P3. The binding of syndecan-1 with LG1 and Ln2-P3 induces the recruitment of protein kinase Cdelta (PKCdelta) into the membrane and stimulates its tyrosine phosphorylation. A decrease in PKCdelta activity significantly reduces cell adhesion to LG1 and Ln2-P3. Taken together, these results indicate that the Ln2-P3 motif and LG1 domain, containing the motif, within the human laminin-2 alpha2 chain are major ligands for syndecan-1, which mediates cell adhesion through the PKCdelta signaling pathway.

A 20-amino acid module of protein kinase C{epsilon} involved in translocation and selective targeting at cell-cell contacts

In the pituitary gland, activated protein kinase C (PKC) isoforms accumulate either selectively at the cell-cell contact (alpha and epsilon) or at the entire plasma membrane (beta1 and delta). The molecular mechanisms underlying these various subcellular locations are not known. Here, we demonstrate the existence within PKCepsilon of a cell-cell contact targeting sequence (3CTS) that, upon stimulation, is capable of targeting PKCdelta, chimerin-alpha1, and the PKCepsilon C1 domain to the cell-cell contact. We show that this selective targeting of PKCepsilon is lost upon overexpression of 3CTS fused to a (R-Ahx-R)(4) (where Ahx is 6-aminohexanoic acid) vectorization peptide, reflecting a dominant-negative effect of the overexpressed 3CTS on targeting selectivity. 3CTS contains a putative amphipathic alpha-helix, a 14-3-3-binding site, and the Glu-374 amino acid, involved in targeting selectivity. We show that the integrity of the alpha-helix is important for translocation but that 14-3-3 is not involved in targeting selectivity. However, PKCepsilon translocation is increased when PKCepsilon/14-3-3 interaction is abolished, suggesting that phorbol 12-myristate 13-acetate activation may initiate two sets of PKCepsilon functions, those depending on 14-3-3 and those depending on translocation to cell-cell contacts. Thus, 3CTS is involved in the modulation of translocation via its 14-3-3-binding site, in cytoplasmic desequestration via the alpha-helix, and in selective PKCepsilon targeting at the cell-cell contact via Glu-374.

Functional suppression of E-cadherin by protein kinase Cdelta

Protein kinase C (PKC) delta, a member of the novel PKC subfamily, has been shown to have an important role in cell proliferation, differentiation, apoptosis and cell motility. In this study, we investigated the effect of green fluorescent protein (GFP)-PKCdelta and GFP-PKCalpha on cell-cell junctions of Madin-Darby canine kidney (MDCK) cells and found that only GFP-PKCdelta suppressed the homophilic interactions between the ectodomains of E-cadherins, accompanied by a weaker cell-cell adhesion. The kinase-deficient mutant of GFP-PKCdelta retained its localization at cell-cell junctions but failed to suppress the function of E-cadherin. In addition, we demonstrated that the hinge region (residues 280-347) that links the regulatory domain and the catalytic domain of PKCdelta is essential for both its kinase activity and the targeting of cell-cell junctions. A PKCdelta mutant with the deletion of amino acids 280-323 within the hinge region, which is catalytically active but defective in the targeting of cell-cell junctions, failed to suppress the function of E-cadherin. Moreover, expression of GFP-PKCdelta in MDCK cells expedited the detachment of cells from their neighbors and facilitated cell scatter induced by hepatocyte growth factor. By contrast, the GFP-PKCdelta mutants including the kinase-deficient mutant and the truncated mutant lacking residues 280-323 suppressed hepatocyte-growth-factor-induced cell scattering. Finally, siRNA-mediated knockdown of endogenous PKCdelta in MDCK cells was found to delay the onset of cell-cell detachment and cell scattering induced by hepatocyte growth factor. Taken together, our results demonstrate that the catalytic activity of PKCdelta and its localization to cell-cell junctions are necessary for PKCdelta to suppress the function of E-cadherin, which thereby facilitates scattering of epithelial cells in response to extracellular cues.

Roles of phosphatidylinositol 3-kinase and p38 mitogen-activated protein kinase in the regulation of protein kinase C-alpha activation in interferon-gamma-stimulated macrophages

Members of the protein kinase C (PKC) family are activated by interferon-gamma (IFN-gamma) and modulate IFN-gamma-induced cellular responses by regulating the activity of transcription factors. We previously reported that PKC-alpha enhances the ability of IFN regulatory factor-1 to transactivate the class II transactivator (CIITA) promoter IV in IFN-gamma-stimulated macrophages. In addition, we showed that IFN-gamma induces the nuclear translocation of PKC-alpha but the mechanisms for this remain to be elucidated. In this study, we sought to identify signalling pathways involved in IFN-gamma-induced activation of PKC-alpha and to characterize their potential roles in modulating IFN-gamma-induced responses in macrophages. IFN-gamma-mediated nuclear translocation of PKC-alpha was a Janus activated kinase 2 (JAK2)-independent process, which required phosphatidylinositol 3-kinase (PI3K) and p38 mitogen-activated protein kinase (MAPK). However, PKC-alpha phosphorylation was independent of PI3K and p38 MAPK, indicating that IFN-gamma-induced phosphorylation and nuclear translocation of PKC-alpha are mediated by distinct mechanisms. In addition, inhibition of PI3K, but not of p38 MAPK, strongly impaired IFN-gamma-induced CIITA and MHC II gene expression. Finally, PKC-alpha associated with signal transducer and activator of transcription 1 (STAT1) and was required for the phosphorylation of STAT1 on serine 727 in IFN-gamma-stimulated macrophages. Taken together, our data indicate that PI3K and p38 MAPK modulate IFN-gamma-stimulated PKC-alpha nuclear translocation independently of JAK2 activity and that both PI3K and PKC-alpha are required for type IV CIITA and MHC II gene expression in IFN-gamma-stimulated macrophages.

Structural basis of protein kinase C isoform function

Protein kinase C (PKC) isoforms comprise a family of lipid-activated enzymes that have been implicated in a wide range of cellular functions. PKCs are modular enzymes comprised of a regulatory domain (that contains the membrane-targeting motifs that respond to lipid cofactors, and in the case of some PKCs calcium) and a relatively conserved catalytic domain that binds ATP and substrates. These enzymes are coexpressed and respond to similar stimulatory agonists in many cell types. However, there is growing evidence that individual PKC isoforms subserve unique (and in some cases opposing) functions in cells, at least in part as a result of isoform-specific subcellular compartmentalization patterns, protein-protein interactions, and posttranslational modifications that influence catalytic function. This review focuses on the structural basis for differences in lipid cofactor responsiveness for individual PKC isoforms, the regulatory phosphorylations that control the normal maturation, activation, signaling function, and downregulation of these enzymes, and the intra-/intermolecular interactions that control PKC isoform activation and subcellular targeting in cells. A detailed understanding of the unique molecular features that underlie isoform-specific posttranslational modification patterns, protein-protein interactions, and subcellular targeting (i.e., that impart functional specificity) should provide the basis for the design of novel PKC isoform-specific activator or inhibitor compounds that can achieve therapeutically useful changes in PKC signaling in cells.

Activation of PKC epsilon induces lactotroph proliferation through ERK1/2 in response to phorbol ester

The aim of this investigation was to contribute to current knowledge about intracellular mechanisms that are involved in lactotroph cell proliferation, by evaluating the role of PKCalpha, PKCepsilon and extracellular-signal regulated kinase (ERK) 1/2 in response to phorbol 12-myristate13-acetate (PMA). In primary pituitary cultures, the activation of protein kinase C (PKC) by PMA for 15 min stimulated lactotroph proliferation; whereas a prolonged activation for 3-8h diminished this proliferative effect. The use of PMA for 15 min-activated PKCepsilon and ERK1/2, whereas incubation with PMA for 3 h induced PKCalpha activation and attenuated the PMA-triggered phosphorylation of ERK1/2. The following inhibitors: PKCs (bisindolylmaleimide I), PKCepsilon (epsilonV1 peptide) and ERK1/2 (PD98059) prevented the mitogenic activity induced by PMA for 15 min. Lactotroph cells stimulated with PMA for 15 min showed a translocation of PKCepsilon to membrane compartment and nucleus. These results thus establish that PKCepsilon plays an essential role in the lactotroph proliferation induced by PMA by triggering signals that involve ERK1/2 activation.

Protein kinase C ε phosphorylates keratin 8 at Ser8 and Ser23 in GH4C1 cells stimulated by thyrotropin-releasing hormone

Protein kinase C epsilon (PKCepsilon) is activated by thyrotropin-releasing hormone (TRH), a regulator of pituitary function in rat pituitary GH(4)C(1) cells. We analyzed the downstream mechanism after PKCepsilon activation. Exposure of GH(4)C(1) cells to TRH or a phorbol ester increased the phosphorylation of three p52 proteins (p52a, p52b and p52c) and decreased the phosphorylation of destrin and cofilin. GF109203X, an inhibitor of protein kinases including PKC, inhibited phosphorylation of the p52 proteins by TRH stimulation. Peptide mapping, amino-acid sequencing, and immunochemical studies indicated that p52a, p52b, and p52c are the differentially phosphorylated isoforms of keratin 8 (K8), an intermediate filament protein. The unphosphorylated K8 (p52n) localized exclusively to the cytoskeleton, whereas the phosphorylated forms (especially p52c), which are increased in TRH-stimulated cells, localized mainly to the cytosol. K8 phosphorylation was enhanced in PKCepsilon-overexpressing clones, and purified recombinant PKCepsilon directly phosphorylated K8 with a profile similar to that observed in TRH-stimulated cells. PKCepsilon and K8 colocalized near the nucleus under basal conditions and were concentrated in the cell periphery and cell-cell contact area after TRH stimulation. MS analyses of phospho-K8 and K8-synthesized peptide (amino acids 1-53) showed that PKCepsilon phosphorylates Ser8 and Ser23 of K8. Phosphorylation of these sites is enhanced in TRH-stimulated cells and PKCepsilon-overexpressing cells, as assessed by immunoblotting using antibodies to phospho-K8. These results suggest that K8 is a physiological substrate for PKCepsilon, and the phosphorylation at Ser8 and Ser23 transduces, at least in part, TRH-PKCepsilon signaling in pituitary cells.

Insight into intra- and inter-molecular interactions of PKC: design of specific modulators of kinase function

Protein kinase C (PKC) is a family of kinases that are critical in many cellular events. These enzymes are activated by lipid-derived second messengers, are dependent on binding to negatively charged phospholipids and some members also require calcium to attain full activation. The interaction with lipids and calcium activators is mediated by binding to the regulatory domains C1 and C2. In addition, many protein-protein interactions between PKC and other proteins have been described. These include interactions with adaptor proteins, substrates and cytoskeletal elements. Regulation of the interactions between PKC, small molecules and other proteins is essential for signal transduction to occur. Finally, a number of auto-inhibitory intra-molecular protein-protein interactions have also been identified in PKC. This chapter focuses on mapping the sites for many of these inter- and intra-molecular interactions and how this information may be used to generate selective inhibitors and activators of PKC signaling.

Phosphorylation of adducin by protein kinase Cdelta promotes cell motility

Protein kinase Cdelta (PKCdelta) has been implicated to play a crucial role in cell proliferation, differentiation and apoptosis. In this study, we have investigated the role of PKCdelta in cell motility using Madin-Darby canine kidney cells. Overexpression of PKCdelta promoted membrane protrusions, concomitant with increased cell motility. By contrast, suppression of PKCdelta expression by RNA interference inhibited cell motility. Moreover, a fraction of PKCdelta was detected at the edge of membrane protrusions in which it colocalized with adducin, a membrane skeletal protein whose phosphorylation state is important for remodeling of the cortical actin cytoskeleton. Elevated expression of PKCdelta correlated with increased phosphorylation of adducin at Ser726 in intact cells. In vitro, PKCdelta, but not PKCalpha, directly phosphorylated the Ser726 of adducin. Finally, we demonstrated that overexpression of both adducin and PKCdelta could generate a synergistic effect on promoting cell spreading and cell migration. Our results support a positive role for PKCdelta in cell motility and strongly suggest a link between PKCdelta activity, adducin phosphorylation and cell motility.

A spatiotemporally coordinated cascade of protein kinase C activation controls isoform-selective translocation

In pituitary GH3B6 cells, signaling involving the protein kinase C (PKC) multigene family can self-organize into a spatiotemporally coordinated cascade of isoform activation. Indeed, thyrotropin-releasing hormone (TRH) receptor activation sequentially activated green fluorescent protein (GFP)-tagged or endogenous PKCbeta1, PKCalpha, PKCepsilon, and PKCdelta, resulting in their accumulation at the entire plasma membrane (PKCbeta and -delta) or selectively at the cell-cell contacts (PKCalpha and -epsilon). The duration of activation ranged from 20 s for PKCalpha to 20 min for PKCepsilon. PKCalpha and -epsilon selective localization was lost in the presence of Gö6976, suggesting that accumulation at cell-cell contacts is dependent on the activity of a conventional PKC. Constitutively active, dominant-negative PKCs and small interfering RNAs showed that PKCalpha localization is controlled by PKCbeta1 activity and is calcium independent, while PKCepsilon localization is dependent on PKCalpha activity. PKCdelta was independent of the cascade linking PKCbeta1, -alpha, and -epsilon. Furthermore, PKCalpha, but not PKCepsilon, is involved in the TRH-induced beta-catenin relocation at cell-cell contacts, suggesting that PKCepsilon is not the unique functional effector of the cascade. Thus, TRH receptor activation results in PKCbeta1 activation, which in turn initiates a calcium-independent but PKCbeta1 activity-dependent sequential translocation of PKCalpha and -epsilon. These results challenge the current understanding of PKC signaling and raise the question of a functional dependence between isoforms.

The link between PKCalpha regulation and cellular transformation

Protein kinase Calpha (PKCalpha) is a serine/threonine protein kinase that has been implicated in the regulation of a variety of cellular functions such as proliferation, differentiation and apoptosis in response to a diverse range of stimuli. In order to execute these biological events PKCalpha activity is modulated by, and functionally interacts with, a number of proto-oncogenes, therefore it is perhaps unsurprising that dysregulation of PKCalpha is associated with a diverse range of cancers. Recently, PKCalpha has become a target for a number of anti-cancer therapies. The purpose of this review is to describe how PKCalpha regulates key biological events, to gain an insight into how PKCalpha-mediated cellular transformation may occur. In this way, it may be possible to design therapeutic tools to combat cancers specifically associated with PKCalpha dysfunction.

Tumor cell-mediated induction of the stromal factor stromelysin-3 requires heterotypic cell contact-dependent activation of specific protein kinase C isoforms

Stromelysin-3 (ST3, MMP-11) has been shown to be strongly overexpressed in stromal fibroblasts of most invasive human carcinomas. However, the molecular mechanisms leading to ST3 expression in nonmalignant fibroblasts remain unknown. The aim of the present study was to analyze the signaling pathways activated in normal pulmonary fibroblasts after their interaction with non-small cell lung cancer (NSCLC) cells and leading to ST3 expression. The use of selective signaling pathway inhibitors showed that conventional and novel protein kinase Cs (PKC) were required for ST3 induction, whereas Src kinases exerted a negative control. We observed by both conventional and real time confocal microscopy that green fluorescent protein-tagged PKCalpha and PKCepsilon, but not PKCdelta, transfected in fibroblasts, accumulate selectively at the cell-cell contacts between fibroblasts and tumor cells. In agreement, RNAi-mediated depletion of PKCalpha and PKCepsilon, but not PKCdelta significantly decreased co-culture-dependent ST3 production. Finally, a tetracycline-inducible expression model allowed us to confirm the central role of these PKC isoforms and the negative regulatory function of c-Src in the control of ST3 expression. Altogether, our data emphasize signaling changes occurring in the tumor microenvironment that may define new stromal targets for therapeutic intervention.