Protein kinase D potentiates DNA synthesis and cell proliferation induced by bombesin, vasopressin, or phorbol esters in Swiss 3T3 cells

RNA interference reveals a differential role of FAK and Pyk2 in cell migration, leading edge formation and increase in focal adhesions induced by LPA in intestinal epithelial cells

In the gastrointestinal mucosa, cell migration plays a crucial role in the organization and maintenance of tissue integrity but the mechanisms involved remain incompletely understood. Here, we used small-interfering RNA (siRNA)-mediated depletion of focal adhesion kinase (FAK) protein to determine the role of FAK in wound-induced migration and cytoskeletal organization in the non-transformed intestinal epithelial cells IEC-6 and IEC-18 stimulated with the G protein-coupled receptors (GPCR) agonist lysophosphatidic acid (LPA). Treatment of these cells with FAK siRNA substantially reduced FAK expression, but did not affect the expression of proline-rich tyrosine kinase 2 (Pyk2). Knockdown of FAK protein significantly inhibited LPA-induced migration of both IEC-18 and IEC-6 cells. LPA induced reorganization of actin and microtubule cytoskeleton in the leading edge was largely inhibited in FAK siRNA-transfected IEC-18 cells. Interestingly, in contrast to the FAK-/- cells, which exhibit an increased number of prominent focal adhesions when plated on fibronectin, FAK knockdown IEC-18 cells exhibited dramatically decreased number of focal adhesions in response to both LPA and fibronectin as compared with the control cells. We also used siRNAs to knockdown Pyk2 expression without reducing FAK expression. Depletion of Pyk2 did not prevent LPA-induced migration or cytoskeletal reorganization in IEC-18 cells. In conclusion, our study shows that FAK plays a critical role in LPA-induced migration, cytoskeletal reorganization, and assembly of focal adhesions in intestinal epithelial cells whereas depletion of Pyk2 did not interfere with any of these responses elicited by LPA.

PKD at the crossroads of DAG and PKC signaling

Diacylglycerol (DAG) and its primary target protein kinase C (PKC) regulate many important cellular responses, yet the molecular mechanisms that control the specificity of DAG and PKC signaling are not fully understood. As such, targeting the PKC pathway for therapeutic purposes has been challenging. Protein kinase D (PKD), a novel DAG receptor, has been the subject of intense investigation in recent years. DAG regulates the intracellular localization of PKD and also activates PKD through PKC by phosphorylation. The PKC-PKD signaling cascade is crucial to PKD function in cells. Important discoveries have been made regarding the roles of PKD in cell growth, gene expression, survival, motility, protein trafficking and lymphocyte biology. This kinase is implicated in pathological processes such as cardiac hypertrophy, tumor cell proliferation and metastasis. Thus, PKD represents a novel therapeutic target for the DAG-PKC signaling network.

Protein kinase D distribution in normal human epidermis, basal cell carcinoma and psoriasis

BACKGROUND

Keratinocytes undergo a defined programme of proliferation and differentiation during normal stratification of the epidermis. Anomalies in the signalling pathways controlling this process probably contribute to the pathogenesis of hyperproliferative dermatological diseases, including psoriasis and basal cell carcinoma (BCC). We have previously proposed that protein kinase D (PKD) is a proproliferative signalling enzyme in keratinocytes and have speculated that abnormalities in its levels or regulation may contribute to hyperproliferative disorders of the skin.

OBJECTIVES

To determine if hyperproliferative human skin disorders are characterized by abnormal protein expression or distribution of PKD, normal human epidermis was compared with BCC and uninvolved and involved psoriatic epidermis.

METHODS

To examine protein expression, immunohistochemical analysis of human samples and Western blotting of neoplastic mouse keratinocytes was performed. Western analysis of neoplastic mouse cells using a phosphospecific PKD antibody allowed estimation of PKD activation status.

RESULTS

Normal human epidermis demonstrated predominant PKD protein expression in the stratum basalis, the proliferative epidermal compartment, with decreased relative expression throughout the suprabasal strata. Uninvolved psoriatic skin showed a similar pattern, but in contrast, psoriatic lesions demonstrated a diffuse distribution of PKD staining throughout all strata. The majority of BCCs examined showed significant PKD protein levels and, in those biopsies in which the levels could be compared, elevated PKD levels relative to normal epidermis. PKD levels and activation status were also increased in a neoplastic mouse keratinocyte cell line.

CONCLUSIONS

PKD was elevated or misdistributed in the hyperproliferative human skin disorders, BCC and psoriasis, as well as neoplastic mouse keratinocytes. We speculate that PKD exerts proproliferative and/or antidifferentiative effects in the epidermis, and that anomalous distribution and/or activation of PKD may be involved in precipitating or sustaining the disease process in BCC and psoriasis.

FAK phosphorylation at Ser-843 inhibits Tyr-397 phosphorylation, cell spreading and migration

Multiple stimuli promote the tyrosine phosphorylation and activation of focal adhesion kinase (FAK), which ultimately facilitates migration. Little is known about the effect of adhesion-dependent signals and cytoskeleton organization on the regulation of FAK phosphorylation at serine sites, or about the role of FAK serine phosphorylation in cell migration. Here, we show that FAK phosphorylation at Ser-843 is strikingly increased when adherent cells are removed from the substratum and held in suspension or by treatment of adherent cells with cytochalasin D, conditions that disrupt the F-actin cytoskeleton and promote focal adhesion disassembly. Notably, the increase in Ser-843 phosphorylation was accompanied by a concomitant sharp decrease in Tyr-397 phosphorylation. To further examine the cause-effect relationship between these two phosphorylation sites we generated Ser-843 phosphorylation-deficient and phosphorylation-mimicking FAK mutants. We found that mutation of Ser-843 to aspartic acid (FAK[S843D]) markedly decreased FAK Tyr-397 phosphorylation in integrin-stimulated cells. While the migratory defect of FAK-deficient fibroblasts was rescued by stable re-expression of WT FAK or FAK[S843A], stable re-expression of FAK[S843D] failed to restore the ability of the cells to migrate into the denuded area of a wound. Our results indicate that increased FAK phosphorylation at Ser-843 represses FAK phosphorylation at Tyr-397, thus suggesting a mechanism of cross-talk between these phosphorylation sites that could regulate FAK-mediated cell shape and migration.

Expression of the protein kinase D (PKD) family during mouse embryogenesis

The serine/threonine protein kinase D (PKD) family comprises of three members, PKD1 (PKCmu), PKD2 and PKD3 (PKCnu). Like the related C-type protein kinases (PKCs), PKDs are activated by diacylglycerol (DAG). PKDs have been implicated in numerous intracellular signaling pathways including vesicular transport, cell proliferation, survival, migration and immune responses. While experimental data on this recently discovered kinase family are starting to accumulate family member specific information is still sparse and only small effort has been taken to functionally differentiate the three PKDs. To address this issue we followed the expression patterns of PKD1, 2 and 3 during the development of the mouse embryo. Using specific probe sets for RT-PCR and in situ hybridization, we demonstrate shared and differential expression domains for the three PKD family members in both neuronal and non-neuronal tissues.

Activation of protein kinase D3 by signaling through Rac and the alpha subunits of the heterotrimeric G proteins G12 and G13

PKD is the founding member of a novel protein kinase family that also includes PKD2 and PKD3. PKD has been the focus of most studies up to date, but little is known about the mechanisms that mediate PKD3 activation. Here, we show that addition of aluminum fluoride to COS-7 cells cotransfected with PKD3 and Galpha13 or Galpha12 induced PKD3 activation, which was associated with a transient plasma membrane translocation of cytosolic PKD3. Treatment with Clostridium difficile toxin B blocked PKD3 activation induced by either bombesin or by aluminum fluoride-stimulated Galpha12/13 but did not affect Galphaq-induced PKD3 activation. Furthermore, PKD3 immunoprecipitated from cells cotransfected with a constitutively active Rac (RacV12) exhibited a marked increase in PKD3 basal catalytic activity. In contrast, cotransfection with active Rho (RhoQ63L), Cdc42 (Cdc42Q61L), or Ras (RasV12) did not promote PKD3 activation. Expression of either COOH-terminal dominant-negative fragment of Galpha13 or dominant negative Rac (Rac N17) attenuated bombesin-induced PKD3 activation. Treatment with protein kinase C (PKC) inhibitors prevented the increase in PKD3 activity induced by RacV12 and aluminum fluoride-stimulated Galpha12/13. The catalytic activation of PKD3 in response to RacV12, alpha12/13 signaling or bombesin correlated with Ser-731/Ser-735 phosphorylation in the activation loop of this enzyme. Our results indicate that Galpha12/13 and Rac are important components in the signal transduction pathways that mediate bombesin receptor-induced PKD3 activation.

Protein kinase D3 activation and phosphorylation by signaling through Gαq

PKD is the founding member of a novel protein kinase family that also includes PKD2 and PKD3. PKD has been the focus of most studies up to date, but little is known about the mechanisms that mediate PKD3 activation. Here, we demonstrate that PKD3 immunoprecipitated from COS-7 cells transfected with a constitutively active G alpha q subunit (alpha(q)Q209L) exhibited a marked increase in basal activity. Addition of aluminum fluoride to cells co-transfected with PKD3 and wild type G alpha(q) also induced PKD3 activation. G alpha(q)-mediated PKD3 activation is associated with persistent translocation of PKD3 from both cytosol and nucleus to plasma membrane. Expression of a COOH-terminal fragment of G alpha q that acts in a dominant-negative fashion attenuated PKD3 activation in response to bombesin receptor stimulation. Our results indicate that G alpha q activation is sufficient to stimulate sustained PKD3 activation and show that the endogenous G alpha q is a major component in the signaling pathway that mediates bombesin-induced PKD3 activation.

Regulation of protein kinase D during differentiation and proliferation of primary mouse keratinocytes

Diseased skin often exhibits a deregulated program of the keratinocyte maturation necessary for epidermal stratification and function. Protein kinase D (PKD), a serine/threonine kinase, is expressed in proliferating keratinocytes, and PKD activation occurs in response to mitogen stimulation in other cell types. We have proposed that PKD functions as a pro-proliferative and/or anti-differentiative signal in keratinocytes and hypothesized that differentiation inducers will downmodulate PKD to allow differentiation to proceed. Thus, changes in PKD levels, autophosphorylation, and activity were analyzed upon stimulation of differentiation and proliferation in primary mouse keratinocytes. Elevated extracellular calcium and acute 12-O-tetradecanoylphorbol-13-acetate (TPA) treatments induced differentiation and triggered a downmodulation of PKD levels, autophosphorylation at serine 916, and activity. Chronic TPA treatment stimulated proliferation and resulted in a recovery of PKD levels, autophosphorylation, and activity. Immunohistochemical analysis demonstrated PKD localization predominantly in the proliferative basal layer of mouse epidermis. Co-expression studies revealed a pro-proliferative, anti-differentiative effect of PKD on keratinocyte maturation as monitored by increased and decreased promoter activities of keratin 5, a proliferative marker, and involucrin, a differentiative marker, respectively. This work describes the inverse regulation of PKD during keratinocyte differentiation and proliferation and the pro-proliferative/anti-differentiative effects of PKD co-expression on keratinocyte maturation.

Phosphorylation of histone deacetylase 7 by protein kinase D mediates T cell receptor-induced Nur77 expression and apoptosis

The molecular basis of thymocyte negative selection, a crucial mechanism in establishing central tolerance, is not yet resolved. Histone deacetylases (HDACs) have emerged as key transcriptional regulators in several major developmental programs. Recently, we showed that the class IIa member, HDAC7, regulates negative selection by repressing expression of Nur77, an orphan nuclear receptor involved in antigen-induced apoptosis of thymocytes. Engagement of the T cell receptor (TCR) alleviates this repression through phosphorylation-dependent nuclear exclusion of HDAC7. However, the identity of the TCR-activated kinase that phosphorylates and inactivates HDAC7 was still unknown. Here, we demonstrate that TCR-induced nuclear export of HDAC7 and Nur77 expression is mediated by activation of protein kinase D (PKD). Indeed, active PKD stimulates HDAC7 nuclear export and Nur77 expression. In contrast, inhibition of PKD prevents TCR-mediated nuclear exclusion of HDAC7 and associated Nur77 activation. Furthermore, we show that HDAC7 is an interaction partner and a substrate for PKD. We identify four serine residues in the NH₂ terminus of HDAC7 as targets for PKD. More importantly, a mutant of HDAC7 specifically deficient in phosphorylation by PKD, inhibits TCR-mediated apoptosis of T cell hybridomas. These findings indicate that PKD is likely to play a key role in the signaling pathways controlling negative selection.

Bombesin and nutrients independently and additively regulate hormone release from GIP/Ins cells

Glucose-dependent insulinotropic polypeptide (GIP) regulates glucose homeostasis and high-fat diet-induced obesity and insulin resistance. Therefore, elucidating the mechanisms that regulate GIP release is important. GIP is produced by K cells, a specific subtype of small intestinal enteroendocrine (EE) cell. Bombesin-like peptides produced by enteric neurons and luminal nutrients stimulate GIP release in vivo. We previously showed that PMA, bombesin, meat hydrolysate, glyceraldehyde, and methylpyruvate increase hormone release from a GIP-producing EE cell line (GIP/Ins cells). Here we demonstrate that bombesin and nutrients additively stimulate hormone release from GIP/Ins cells. In various cell systems, bombesin and PMA regulate cell physiology by activating PKD signaling in a PKC-dependent fashion, whereas nutrients regulate cell physiology by inhibiting AMPK signaling. Western blot analyses of GIP/Ins cells using antibodies specific for activated and/or phosphorylated forms of PKD and AMPK and one substrate for each kinase revealed that bombesin and PMA, but not nutrients, activated PKC, but not PKD. Conversely, nutrients, but not bombesin or PMA, inhibited AMPK activity. Pharmacological studies showed that PKC inhibition blocked bombesin- and PMA-stimulated hormone release, but AMPK activation failed to suppress nutrient-stimulated hormone secretion. Forced expression of constitutively active vs. dominant negative PKDs or AMPKs failed to perturb bombesin- or nutrient-stimulated hormone release. Thus, in GIP/Ins cells, PKC regulates bombesin-stimulated hormone release, whereas nutrients may control hormone release by regulating the activity of AMPK-related kinases, rather than AMPK itself. These results strongly suggest that K cells in vivo independently respond to neuronal vs. nutritional stimuli via two distinct signaling pathways.

G protein-coupled receptor-mediated phosphorylation of the activation loop of protein kinase D: dependence on plasma membrane translocation and protein kinase Cepsilon

Protein kinase D (PKD) is a serine/threonine protein kinase activated by G protein-coupled receptor (GPCR) agonists through an incompletely characterized mechanism that includes its reversible plasma membrane translocation and activation loop phosphorylation via a protein kinase C (PKC)-dependent pathway. To gain a better understanding of the mechanism regulating the activation of PKD in response to GPCR stimulation, we investigated the role of its rapid plasma membrane translocation on its activation loop phosphorylation and identified the endogenous PKC isozyme that mediates that event in vivo. We had found that the activation loop of a PKD mutant, with reduced affinity for diacylglycerol and phorbol esters, was only phosphorylated upon its plasma membrane association. We also found that the activation loop phosphorylation and rapid plasma membrane dissociation of PKD were inhibited either by preventing the plasma membrane translocation of PKCepsilon, through abolition of its interaction with receptor for activated C kinase, or by suppressing the expression of PKCepsilon via specific small interfering RNAs. Thus, this study demonstrates that the plasma membrane translocation of PKD, in response to GPCR stimulation, is necessary for the PKCepsilon-mediated phosphorylation of the activation loop of PKD and that this event requires the translocation of both kinases to the plasma membrane. Based on these and previous results, we propose a model of GPCR-mediated PKD regulation that integrates its changes in distribution, catalytic activity, and multisite phosphorylation.

Oxidative Stress Induces Protein Kinase C-mediated Activation Loop Phosphorylation and Nuclear Redistribution of Protein Kinase D

Oxidative stress induced by cell treatments with H(2)O(2) activates protein kinase D (PKD) via a protein kinase C (PKC)-dependent signal transduction pathway (Waldron, R. T., and Rozengurt, E. (2000) J. Biol. Chem. 275, 17114-17121). Here we show that oxidative stress induces PKC-dependent activation loop Ser(744) and Ser(748) phosphorylation to mediate dose- and time-dependent activation of PKD, both endogenously expressed in Swiss 3T3 cells and stably overexpressed in Swiss 3T3-GFP.PKD cells. Although oxidative stress induced PKD activation loop phosphorylation and activation with identical kinetics, both were dose-dependently blocked by preincubation of cells with selective inhibitors of PKC (GF109203X and Gö6983) or c-Src (PP2). Inhibition of Src tyrosine kinase activity eliminated oxidative stress-induced direct PKD tyrosine phosphorylation, but only partially attenuated activation loop phosphorylation and activation. Mutation of a putative tyrosine phosphorylation site on PKD, Tyr(469) to phenylalanine, had no effect on its activation by oxidative stress in transfected COS-7 cells. Similarly, a mutant with Tyr(469) replaced by aspartic acid had increased basal activity but was also further activated by oxidative stress. Thus, PKD tyrosine phosphorylation at this site neither produced full activation by itself nor was required for oxidative stress-induced activation mediated by activation loop phosphorylation. In addition to PKD activation, activation loop phosphorylation in response to oxidative stress also redistributed activated PKD to cell nuclei, as revealed by PKD indirect immunofluorescence, imaging of a PKD-green fluorescent protein fusion construct (GFP-PKD), and analysis of nuclear pellets. Cell preincubation with Gö6983 strongly diminished H(2)O(2)-induced nuclear relocalization of GFP-PKD. Taken together, these results indicate that PKC-mediated PKD Ser(744) and Ser(748) phosphorylation induced by oxidative stress integrates PKD activation with redistribution to the nucleus.

Lipid raft disruption triggers protein kinase C and Src-dependent protein kinase D activation and Kidins220 phosphorylation in neuronal cells

Kidins220 (kinase D-interacting substrate of 220 kDa) is a novel neurospecific protein recently cloned as the first substrate for the Ser/Thr kinase protein kinase D (PKD). Herein we report that Kidins220 is constitutively associated to lipid rafts in PC12 cells, rat primary cortical neurons, and brain synaptosomes. Immunocytochemistry and confocal microscopy together with sucrose gradient fractionation show co-localization of Kidins220 and lipid raft-associated proteins. In addition, cholesterol depletion of cell membranes with methyl-beta-cyclodextrin dramatically alters Kidins220 localization and detergent solubility. By studying the putative involvement of lipid rafts in PKD activation and signaling we have found that active PKD partitions in lipid raft fractions after sucrose gradient centrifugation and that green fluorescent protein-PKD translocates to lipid raft microdomains at the plasma membrane after phorbol ester treatment. Strikingly, lipid rafts disruption by methyl-beta-cyclodextrin delays green fluorescent protein-PKD translocation, as determined by live cell confocal microscopy, and activates PKD, increasing Kidins220 phosphorylation on Ser(919) by a mechanism involving PKCepsilon and the small soluble tyrosine kinase Src. Collectively, these results reveal the importance of lipid rafts on PKD activation, translocation, and downstream signaling to its substrate Kidins220.

Gastrin-Releasing Peptide (GRP) Analogues for Cancer Imaging

Small neuropeptides, labeled with gamma- and/or beta-emitting radionuclides, are currently being investigated for their ability to bind to cell-surface receptors, overexpressed in a wide variety of malignant tissues being, thus, potentially useful for radionuclide detection and/or therapy for tumors. Particular attention has been focused on the amphibian peptide, bombesin (BN), and the molecularly related gastrin-releasing peptide (GRP). These peptides act as neurotransmitters and endocrine cancer cell-growth factors on normal tissues as well as on neoplastic cells of various origin. In recent investigations, modification of the native peptide structure has been attempted in order to obtain derivatives, which might easily be labeled with radionuclides. Thus, iodinated (I-125) BN derivatives, as well as Indium (In-111) labeled BN analogs are currently being investigated, presenting satisfactory tumor localization. Also, some new BN analogs containing a 6-carbon linker have been prepared and labeled with Rhenium-188, resulting in positive in vitro binding to prostate cancer cells. More recent studies refer to the Technetium-99m labeling of BN, performed either directly, after attaching proper technetium-chelating groups onto the BN sequence, or indirectly, by coupling BN to a preformed 99mTc-tagging ligand. Both types of conjugates were found to have a high in vitro affinity for cells with BN receptors, also presenting satisfactory in vivo uptake in experimental tumor models. Pilot clinical studies of a new BN-derived, 99mTc-labeled pentadecapeptide indicated significant uptake by breast cancer and invaded lymph nodes, as well as by prostate cancer, small-cell lung carcinoma, gastro-entero-pancreatic tumors, and others, Further studies of this new GRP derivative, as well as of other new BN-like peptides, are intensively performed internationally today.

Protein kinase D potentiates DNA synthesis induced by Gq-coupled receptors by increasing the duration of ERK signaling in swiss 3T3 cells

Protein kinase D (PKD) potentiates cellular DNA synthesis in response to G protein-coupled receptor (GPCR) agonists but the mechanism(s) involved has not been elucidated. Here, we examined whether PKD overexpression in Swiss 3T3 cells regulates the activation/inactivation kinetics of the extracellular-regulated protein kinase (ERK) in response to the mitogenic GPCR agonists bombesin and vasopressin. Addition of bombesin or vasopressin to Swiss 3T3 cells overexpressing PKD induced a striking increase in the duration of MEK/ERK/RSK activation as compared with cultures of either control Swiss 3T3 cells or Swiss 3T3 cells expressing a kinase-inactive PKD mutant. In contrast, the duration of ERK activation in response to epidermal growth factor, which acts via protein kinase C/PKD-independent pathways, was not increased. Furthermore, bombesin or vasopressin promoted a striking increase in phosphorylation (at Ser-374) and accumulation of c-Fos (the c-fos proto-oncogene product) in Swiss 3T3 cells overexpressing wild-type (but not kinase-inactive) PKD. Inhibition of the sustained phase of ERK/RSK activation abrogated the increase in c-Fos accumulation and DNA synthesis induced by bombesin or vasopressin in PKD-overexpressing cells. Our results demonstrate that PKD selectively potentiates mitogenesis induced by bombesin or vasopressin in Swiss 3T3 cells by increasing the duration of MEK/ERK/RSK signaling.

PKCmu prevents CD95-mediated apoptosis and enhances proliferation in pancreatic tumour cells

Loss of growth control and a marked resistance to apoptosis are considered major mechanisms driving tumour progression. Protein kinases C (PKC) have been shown to be important in the regulation of proliferation and apoptosis. In this report, we investigated the role of the PKC-like kinase PKCmu in the control of these processes in pancreatic adenocarcinoma cells. We demonstrate that in these cells, PKCmu expression strongly correlates with resistance to CD95-induced apoptosis. Inhibition of PKCmu with Goe6983 sensitized resistant cells to CD95-induced apoptosis. In CD95-sensitive Colo357 cells, forced overexpression of PKCmu strongly reduced CD95-mediated apoptosis, an effect that could be reversed by pretreatment with Goe6983. In addition, PKCmu overexpression led to a strongly enhanced cell growth and to a significant increase of telomerase activity. In an attempt to identify the signalling pathways affected by PKCmu, we identified the antiapoptotic proteins c-FLIPL and survivin to be strongly upregulated in PKCmu overexpressing cells. Immunohistochemical analysis of pancreatic tumour tissue of 48 patients and 10 normal pancreatic tissues revealed marked overexpression of PKCmu in tumours. In conclusion, we showed that PKCmu controls proliferative, as well as anti-apoptotic, signalling pathways and therefore plays an important role in acquiring the malignant phenotype of pancreatic tumours.

Requirement of protein kinase C micro activation and calpain-mediated proteolysis for arachidonic acid-stimulated adhesion of MDA-MB-435 human mammary carcinoma cells to collagen type IV

Arachidonic acid (AA) stimulation of adhesion of human metastatic breast carcinoma cells to collagen type IV depends on the protein kinase C (PKC) pathway(s) and is associated with the translocation of PKC mu from the cytoplasm to the membrane. In the present study, we have further explored the role of PKC mu in AA-stimulated adhesion. PKC mu activation site serines 738/742 and autophosphorylation site serine 910 are rapidly phosphorylated, and in vitro PKC mu kinase activity is enhanced in response to AA treatment. Inhibition of PKC mu activation blocks AA-stimulated adhesion. A phosphorylated, truncated species of PKC mu was detected in AA-treated cells. This 77-kDa protein contains the kinase domain but lacks a significant portion of the regulatory domains. Inhibition of calpain protease activity blocks generation of the truncated protein, promotes accumulation of the activated, full-length protein in the membrane, and blocks the AA-mediated increase in adhesion. p38 MAPK activity is also required for AA-stimulated adhesion. Activation of PKC mu and p38 are independent events. However, inhibition of p38 activity reduces calpain-mediated proteolysis of PKC mu and in vivo calpain activity, suggesting a role for p38 in regulation of calpain activity and a point for cross-talk between the PKC and MAPK pathways. These results support the hypothesis that AA stimulates activation of PKC mu, which is cleaved by calpain at the cell membrane. The resulting truncated kinase, as well as the full-length kinase, may be required for increased cell adhesion to collagen type IV. Additionally, these studies present the first evidence for calpain cleavage of a non-structural protein leading to the promotion of tumor cell adhesion.

Vasopressin-induced intracellular redistribution of protein kinase D in intestinal epithelial cells

The spatio-temporal changes of signaling molecules in response to G protein-coupled receptors (GPCR) stimulation is a poorly understood process in intestinal epithelial cells. Here we investigate the dynamic mechanisms associated with GPCR signaling in living rat intestinal epithelial cells by characterizing the intracellular translocation of protein kinase D (PKD), a serine/threonine protein kinase involved in mitogenic signaling in intestinal epithelial cells. Analysis of the intracellular steady-state distribution of green fluorescent protein (GFP)-tagged PKD indicated that in non-stimulated IEC-18 cells, GFP-PKD is predominantly cytoplasmic. However, cell stimulation with the GPCR agonist vasopressin induces a rapid translocation of GFP-PKD from the cytosol to the plasma membrane that is accompanied by its activation via protein kinase C (PKC)-mediated process and posterior plasma membrane dissociation. Subsequently, active PKD is imported into the nuclei where it transiently accumulates before being exported into the cytosol by a mechanism that requires a competent Crm1 nuclear export pathway. These findings provide evidence for a mechanism by which PKC coordinates in intestinal epithelial cells the translocation and activation of PKD in response to vasopressin-induced GPCR activation.

Protein kinase D: a family affair

The protein kinase D family of enzymes consists of three isoforms: PKD1/PKCmu PKD2 and PKD3/PKCnu. They all share a similar architecture with regulatory sub-domains that play specific roles in the activation, translocation and function of the enzymes. The PKD enzymes have recently been implicated in very diverse cellular functions, including Golgi organization and plasma membrane directed transport, metastasis, immune responses, apoptosis and cell proliferation.

ANG II stimulates PKC-dependent ERK activation, DNA synthesis, and cell division in intestinal epithelial cells

PKC, a major target for the tumor-promoting phorbol esters, has been implicated in the signal transduction pathways that mediate important functions in intestinal epithelial cells, including proliferation and carcinogenesis. With the use of IEC-18 cells arrested in G0/G1, addition of phorbol esters resulted in a modest increase in [3H]thymidine incorporation and a slight shift toward the S and G2/M phases of the cell cycle, whereas the combination of EGF and phorbol 12,13-dibutyrate (PDB) synergistically stimulated DNA synthesis. To investigate the effects of receptor-mediated PKC activation on mitogenesis, we demonstrated that ANG II induced ERK activation, a response completely blocked by pretreatment with mitogen/extracellular signal-regulated kinase inhibitors or specific PKC inhibitors. Furthermore, ANG II stimulated an over threefold increase in [3H]thymidine incorporation that was corroborated by flow cytometric analysis of the cell cycle to levels comparable to that achieved by the combination of EGF and PDB. Taken together, our results indicate that receptor-mediated PKC activation, as induced by ANG II, transduces mitogenic signals leading to DNA synthesis and cell proliferation in IEC-18 cells.

Protein kinase C nu/protein kinase D3 nuclear localization, catalytic activation, and intracellular redistribution in response to G protein-coupled receptor agonists

The protein kinase D (PKD) family consists of three serine/threonine kinases: PKC micro/PKD, PKD2, and PKCnu/PKD3. Whereas PKD has been the focus of most studies, virtually nothing is known about the effect of G protein-coupled receptor agonists (GPCR) on the regulatory properties and intracellular distribution of PKD3. Consequently, we examined the mechanism that mediates its activation and intracellular distribution. GPCR agonists induced a rapid activation of PKD3 by a protein kinase C (PKC)-dependent pathway that leads to the phosphorylation of the activation loop of PKD3. Comparison of the steady-state distribution of endogenous or tagged PKD3 versus PKD and PKD2 in unstimulated cells indicated that whereas PKD and PKD2 are predominantly cytoplasmic, PKD3 is present both in the nucleus and cytoplasm. This distribution of PKD3 results from its continuous shuttling between both compartments by a mechanism that requires a nuclear import receptor and a competent CRM1-nuclear export pathway. Cell stimulation with the GPCR agonist neurotensin induced a rapid and reversible plasma membrane translocation of PKD3 that is PKC-dependent. Interestingly, the nuclear accumulation of PKD3 can be dramatically enhanced in response to its activation. Thus, this study demonstrates that the intracellular distribution of PKD isoenzymes are distinct, and suggests that their signaling properties are regulated by differential localization.

Growth-promoting effect of muscarinic acetylcholine receptors in colon cancer cells

PURPOSE

G-protein-coupled receptors are known to mediate cell growth via divergent signaling pathways. It has been reported that colon cancer cells express muscarinic acetylcholine receptor (mAChR) although their functional role is largely unknown. The aim of this study is to elucidate possible mechanisms responsible for the growth-promoting effect of mAChRs in colon cancer cells by using colon cancer cell line T84.

METHODS

Carbachol, a stable mAChR agonist, dose-dependently induced cell growth with a maximal effect observed at 100 microM, equipotent with 1 nM EGF. 4-DAMP, a specific antagonist of subtype 3 mAChR, inhibited the stimulatory effect by carbachol, suggesting that the growth-promoting effect was receptor-mediated. Carbachol also dose-dependently stimulated extracellular signal-regulated protein kinase (ERK) activation. This effect was inhibited by PD98059, an inhibitor of extracellular signal-regulated protein kinase kinase, which also blocked carbachol activation of cell proliferation, indicating that the p21Ras-ERK pathway is an important signaling cascade in the mitogenic effect. To investigate how mAChR activated the p21Ras-ERK pathway, transactivation of epidermal growth factor receptor (EGFR) was examined.

RESULTS

Carbachol induced tyrosine phosphorylation of EGFR, which was abolished by an EGFR tyrosine kinase inhibitor AG1478. Transactivation by carbachol was also abrogated by a metalloproteinases (MMPs) inhibitor GM6001 or an EGFR-blocking antibody (LA-1), suggesting that binding of EGFR ligand(s) produced by MMPs may initiate transactivation in a manner dependent on EGFR tyrosine kinase. The tyrosine-phosphorylated EGFR was immunoprecipitated together with GRB2 and tyrosine-phosphorylated Shc, indicating that transactivated EGFR is able to generate downstream signals. AG 1478 and LA-1 inhibited carbachol stimulation of cell growth.

CONCLUSIONS

Taken together, our results indicate that the growth-promoting effect of subtype 3 mAChR in colon cancer cells may depend on transactivated EGFR-ERK pathways. EGFR not only receives external stimuli but also serves as a scaffold for downstream signaling molecules.

Mechanism of persistent protein kinase D1 translocation and activation

The specificity of many signal transduction pathways relies on the spatiotemporal features of each signaling step. G protein-coupled receptor-mediated activation of protein kinases leads to diverse cellular effects. Upon receptor activation, PKD1 and several C-type protein kinases (PKCs), translocate to the plasma membrane and become catalytically active. Here we show that, unlike PKCs, PKD1 remains active at the membrane for hours. The two DAG binding C1 domains of PKD1 have distinct functional roles in targeting and maintaining PKD1 at the plasma membrane. C1A achieves fast, maximal, and reversible translocation, while C1B translocates partially, but persistently, to the plasma membrane. The persistent localization requires the C1B domain of PKD1, which binds Galphaq. We incorporate the kinetics of PKD1 translocation into a three-state model that suggests how PKD1 binding to DAG and Galphaq uniquely encodes frequency-dependent PKD1 signaling.

Arginine vasopressin inhibition of cyclin D1 gene expression blocks the cell cycle and cell proliferation in the mouse Y1 adrenocortical tumor cell line

Arginine vasopressin (AVP) is a nonapeptide long known as an endocrine and paracrine regulator of important systemic functions, namely, vasoconstriction, gluconeogenesis, corticosteroidogenesis, and excretion of water and urea. Here we report, for the first time, that AVP specifically inhibits expression of the cyclin D1 gene, leading to cell cycle blockage and halting cell proliferation. In G0/G1-arrested mouse Y1 adrenocortical tumor cells, maintained in serum-free medium (SFM), AVP mimics FGF2, promoting rapid ERK1/2 activation (5 min) followed by c-Fos protein induction (2 h). PKC inhibitor Go6983 and PI3K inhibitors wortmannin and LY294002 all inhibit ERK1/2 activation by AVP, but not by FGF2. Thus, AVP and FGF2 concur to activate ERK1/2 by different regulatory pathways. However, AVP is not a mitogenic factor for Y1 cells. On the contrary, AVP strongly antagonizes FGF2 late induction (2-5 h) of the cyclin D1 gene, down-regulating both cyclin D1 mRNA and protein. AVP inhibition of cyclin D1 expression is sufficient to block G1 phase progression and cell entry into the S phase, monitored by BrdU nuclear labeling. In addition, AVP completely inhibits proliferation of Y1 cells in 10% fetal calf serum (10% FCS) medium. On the other hand, ectopic expression of the cyclin D1 protein renders Y1 cells resistant to AVP for both entry into the S phase in SFM and continuous proliferation in 10% FCS medium. In conclusion, inhibition of cyclin D1 expression by AVP is an efficient mechanism of cell cycle blockage and consequent proliferation inhibition in Y1 adrenocortical cells.

Protein kinase C phosphorylates protein kinase D activation loop Ser744 and Ser748 and releases autoinhibition by the pleckstrin homology domain

Persistent activation of protein kinase D (PKD) via protein kinase C (PKC)-mediated signal transduction is accompanied by phosphorylation at Ser(744) and Ser(748) located in the catalytic domain activation loop, but whether PKC isoforms directly phosphorylate these residues, induce PKD autophosphorylation, or recruit intermediate upstream kinase(s) is unclear. Here, we explore the mechanism whereby PKC activates PKD in response to cellular stimuli. We first assessed in vitro PKC-PKD transphosphorylation and PKD activation. A PKD738-753 activation loop peptide was well phosphorylated by immunoprecipitated PKC isoforms, consistent with similarities between the loop and their known substrate specificities. A similar peptide with glutamic acid replacing Ser(748) was preferentially phosphorylated by PKCepsilon, suggesting that PKD containing phosphate at Ser(748) is rapidly targeted by this isoform at Ser(744). When incubated in the presence of phosphatidylserine, phorbol 12,13-dibutyrate and ATP, intact PKD slowly autophosphorylated in the activation loop but only at Ser(748). In contrast, addition of purified PKCepsilon to the incubation mixture induced rapid Ser(744) and Ser(748) phosphorylation, concomitant with persistent 2-3-fold increases in PKD activity, measured using reimmunoprecipitated PKD to phosphorylate an exogenous peptide, syntide-2. We also further examined pleckstrin homology domain-mediated PKD regulation to determine its relationship with activation loop phosphorylation. The high constitutive activity of the pleckstrin homology (PH) domain deletion mutant PKD-deltaPH was not abrogated by mutation of Ser(744) and Ser(748) to alanines, suggesting that one function of activation loop phosphorylation in the PKD activation mechanism is to relieve autoinhibition by the PH domain. These studies provide evidence of a direct PKCepsilon-PKD phosphorylation cascade and provide additional insight into the activation mechanism.

Putative conventional protein kinase C inhibitor Gödecke 6976 [12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole] stimulates transglutaminase activity in primary mouse epidermal keratinocytes

Much data in the literature suggest a role for protein kinase C (PKC) in regulating keratinocyte proliferation and differentiation. Nevertheless, the exact role of this family of isoenzymes is unclear, since PKC agonists (e.g., phorbol esters) are known to stimulate expression of both proliferative and differentiative markers in keratinocytes. Similarly, PKC inhibitors have been demonstrated both to inhibit [2-[1-3(aminopropyl)indol-3-yl]-3(1-methyl-1H-indol-3-yl)maleimide, acetate (Ro 31-7549) and 3-[1-[3-(amidinothio)propyl-1H-indol-3-(1-methyl-1H-indol-3yl) maleimide (Ro 31-8220)] and to induce (staurosporine) keratinocyte differentiation. In this study, we examined the role of the PKC inhibitor, Gödecke 6976 (Gö6976) [12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo (3,4-c)-carbazole], on keratinocyte proliferation, as measured by DNA synthesis, and differentiation, as monitored by transglutaminase activity. This compound is reported to be selective for the conventional PKC isoforms, of which keratinocytes express only PKCalpha, and for protein kinase D (PKD; also known as PKCmu). We report that Gö6976 stimulated transglutaminase activity. Consistent with this effect, Gö6976 also potently inhibited [(3)H]thymidine incorporation (a half-maximal inhibitory concentration of approximately 0.1 microM). In addition, Gö6976 (1 microM) was able to enhance the stimulation of transglutaminase activity by 1,25-dihydroxyvitamin D(3) but had no effect on D(3)-induced expression of keratin-1. Conversely, Gö6983 [2-[1-(3-dimethylaminopropy)-5-methoxyindol-3-yl]-3-(1H-indol-3-yl)maleimide], a similar compound that also selectively inhibits conventional PKCalpha, but not PKD, had little or no effect on DNA synthesis or transglutaminase activity (up to 1 microM). The effect of Gö6976 was not due to cytotoxicity as its effect on thymidine incorporation was largely reversible, and its stimulation of transglutaminase activity could be inhibited by another general PKC inhibitor, bisindolylmaleimide I. Therefore, our results suggest a proproliferative, antidifferentiative role for PKD in epidermal maturation.

Technetium labeled bombesin-like peptide: preliminary report on breast cancer uptake in patients

Bombesin-like peptides are neurotransmitters and cancer growth factors. Several tumors, breast cancer among them, show one or more than one of the three known bombesin receptors. We have synthesized and labeled with technetium 99m a new pentadecapeptide, analogue to the leu13 amphibian bombesin (99mTc BN). Labeling yield was 83 +/- 4%. Prone Scintimammography was performed on five patients affected by breast cancers (T categorization: two T1b and three T1c), after injecting 0.7 mg, 185 to 296 MBq (5 to 8 mCi) of the peptide. Total body scan did not show free technetium biodistribution. No adverse reaction was observed. Prone Scintimammography with 99mTc Sestamibi (99mTc SM) was also performed few days later. 99mTc BN detected all 5 cancers, whereas 99mTc SM only four: all the T1c and one T1b cancer. Two of them showed axillary node invasion that was detected by both the radiotracers. A fibroadenoma present on contralateral breast to the one with cancer, was not detected neither by 99mTc SM nor by 99mTc BN. Tumor/breast normal tissue ratio (T/B) was constantly higher with 99mTc BN than with 99mTc SM. Maximal T/B was measured as 1.79 with 99mTc SM and 2.25 with 99mTc BN 5 min after fast i.v. administration. In conclusion our 99mTc BN is taken up by primary breast cancer showing higher T/B than 99mTc SM (p < 0.01). In our limited scale, 99mTc BN appears to be safe and, in our limited scale, even more accurate than 99mTc SM for detecting breast cancer.

Neuropeptides as growth factors for normal and cancerous cells

Neuropeptides are molecular messengers that regulate multiple functions in the central nervous system and in the periphery via G-protein-coupled receptors. These signaling peptides have also been identified as potent cellular growth factors for normal cells and they participate in autocrine/paracrine stimulation of tumor cell proliferation and migration. Recent studies on the signaling pathways activated by mitogenic neuropeptides revealed previously unsuspected connections and complexities, including the realization that these receptors not only stimulate the synthesis of conventional second messengers but also induce tyrosine phosphorylation cascades. A major task for the future will be to identify all the contributing molecules, define their functional importance and elucidate the spatiotemporal relationships of this complicated signaling network. As our understanding of the role of neuropeptides in cancer increases, novel possibilities for translational research are emerging for improving the diagnosis and treatment of the disease.

Protein kinase D: an intracellular traffic regulator on the move

Recent research has identified protein kinase D (PKD, also called PKCmu) as a serine/threonine kinase with potentially important roles in growth factor signaling as well as in stress-induced signaling. Moreover, PKD has emerged as an important regulator of plasma membrane enzymes and receptors, in some cases mediating cross-talk between different signaling systems. The recent discovery of two additional kinases belonging to the PKD family and the plethora of proteins that interact with PKD point to a multifaceted regulation and a multifunctional role for these enzymes, with functions in processes as diverse as cell proliferation, apoptosis, immune cell regulation, tumor cell invasion and regulation of Golgi vesicle fission.

Regulated nucleocytoplasmic transport of protein kinase D in response to G protein-coupled receptor activation

Protein kinase D (PKD)/protein kinase C mu is a serine/threonine protein kinase activated by growth factors, antigen-receptor engagement, and G protein-coupled receptor (GPCR) agonists via a phosphorylation-dependent mechanism that requires protein kinase C (PKC) activity. In order to investigate the dynamic mechanisms associated with GPCR signaling, the intracellular distribution of PKD was analyzed in live cells by imaging fluorescent protein-tagged PKD and in fixed cells by immunocytochemistry. We found that PKD shuttled between the cytoplasm and the nucleus in both fibroblasts and epithelial cells. Cell stimulation with mitogenic GPCR agonists that activate PKD induced a transient nuclear accumulation of PKD that was prevented by inhibiting PKC activity. The nuclear import of PKD requires its cys2 domain in conjunction with a nuclear import receptor, while its nuclear export requires its pleckstrin homology domain and a competent Crm1-dependent nuclear export pathway. This study thus characterizes the regulated nuclear transport of a signaling molecule in response to mitogenic GPCR agonists and positions PKD as a serine kinase whose kinase activity and intracellular localization is coordinated by PKC.