Cholecystokinin-stimulated protein kinase C-delta kinase activation, tyrosine phosphorylation, and translocation are mediated by Src tyrosine kinases in pancreatic acinar cells

Cofilin activation in pancreatic acinar cells plays a pivotal convergent role for mediating CCK-stimulated enzyme secretion and growth

Introduction: The actin regulatory protein, cofilin plays a key signaling role in many cells for numerous cellular responses including in proliferation, development, motility, migration, secretion and growth. In the pancreas it is important in islet insulin secretion, growth of pancreatic cancer cells and in pancreatitis. However, there are no studies on its role or activation in pancreatic acinar cells. Methods: To address this question, we studied the ability of CCK to activate cofilin in pancreatic acinar cells, AR42J cells and CCK₁-R transfected Panc-1 cells, the signaling cascades involved and its effect on enzyme secretion and MAPK activation, a key mediator of pancreatic growth. Results: CCK (0.3 and 100 nM), TPA, carbachol, Bombesin, secretin and VIP decreased phospho-cofilin (i.e., activate cofilin) and both phospho-kinetic and inhibitor studies of cofilin, LIM kinase (LIMK) and Slingshot Protein Phosphatase (SSH1) demonstrated these conventional activators of cofilin were not involved. Serine phosphatases inhibitors (calyculin A and okadaic acid), however inhibited CCK/TPA-cofilin activation. Studies of various CCK-activated signaling cascades showed activation of PKC/PKD, Src, PAK4, JNK, ROCK mediated cofilin activation, but not PI3K, p38, or MEK. Furthermore, using both siRNA and cofilin inhibitors, cofilin activation was shown to be essential for CCK-mediated enzyme secretion and MAPK activation. Conclusion: These results support the conclusion that cofilin activation plays a pivotal convergent role for various cell signaling cascades in CCK mediated growth/enzyme secretion in pancreatic acini.

SILAC Phosphoproteomics Reveals Unique Signaling Circuits in CAR-T Cells and the Inhibition of B Cell-Activating Phosphorylation in Target Cells

Chimeric antigen receptor (CAR) is a single-pass transmembrane receptor designed to specifically target and eliminate cancers. While CARs prove highly efficacious against B cell malignancies, the intracellular signaling events which promote CAR T cell activity remain elusive. To gain further insight into both CAR T cell signaling and the potential signaling response of cells targeted by CAR, we analyzed phosphopeptides captured by two separate phosphoenrichment strategies from third generation CD19-CAR T cells cocultured with SILAC labeled Raji B cells by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Here, we report that CD19-CAR T cells upregulated several key phosphorylation events also observed in canonical T cell receptor (TCR) signaling, while Raji B cells exhibited a significant decrease in B cell receptor-signaling related phosphorylation events in response to coculture. Our data suggest that CD19-CAR stimulation activates a mixture of unique CD19-CAR-specific signaling pathways and canonical TCR signaling, while global phosphorylation in Raji B cells is reduced after association with the CD19-CAR T cells.

Signal Integration by Translocation and Phosphorylation of PKCδ in the B Cell Alternate Pathway

B cell signaling for activation via the BCR occurs as an isolated event only in vitro; in real life, BCR signaling takes place within a complex milieu that involves interactions with agents that trigger additional receptors. Chief among these is IL-4. We have shown that BCR signaling is reprogrammed by IL-4 receptor engagement and that this reprogramming involves creation of a new, signalosome-independent, Lyn-dependent alternate signaling pathway in B cells isolated from BALB/cByJ mice. A unique aspect of the alternate pathway is protein kinase Cδ (PKCδ) phosphorylation. In dissecting this pathway, we unexpectedly found that Lyn is associated with IL-4Rα, that IL-4 induces Lyn activation, and that Lyn immunoprecipitated from IL-4-treated B cells capably phosphorylates PKCδ in a cell-free system. However, PKCδ phosphorylation does not occur in the absence of BCR triggering in vivo. This raised the question of why IL-4 alone failed to produce PKCδ phosphorylation. We considered the possibility that Lyn and PKCδ may be spatially separated. As expected, before any treatment, Lyn is located primarily in the membrane fraction, whereas PKCδ is located mainly in the cytosol fraction. However, when anti-Ig follows IL-4 treatment, PKCδ is found in the membrane fraction and phosphorylated. This translocation of PKCδ to the membrane fraction is not affected by loss of Lyn, although PKCδ phosphorylation requires Lyn. Thus, PKCδ phosphorylation through the alternate pathway represents the result of signal integration, whereby neither IL-4 nor anti-Ig working alone produces this outcome, but together they achieve this result by Lyn activation (IL-4) and PKCδ translocation (IL-4 followed by anti-Ig).

Cholecystokinin (CCK) Regulation of Pancreatic Acinar Cells: Physiological Actions and Signal Transduction Mechanisms

Pancreatic acinar cells synthesize and secrete about 20 digestive enzymes and ancillary proteins with the processes that match the supply of these enzymes to their need in digestion being regulated by a number of hormones (CCK, secretin and insulin), neurotransmitters (acetylcholine and VIP) and growth factors (EGF and IGF). Of these regulators, one of the most important and best studied is the gastrointestinal hormone, cholecystokinin (CCK). Furthermore, the acinar cell has become a model for seven transmembrane, heterotrimeric G protein coupled receptors to regulate multiple processes by distinct signal transduction cascades. In this review, we briefly describe the chemistry and physiology of CCK and then consider the major physiological effects of CCK on pancreatic acinar cells. The majority of the review is devoted to the physiologic signaling pathways activated by CCK receptors and heterotrimeric G proteins and the functions they affect. The pathways covered include the traditional second messenger pathways PLC-IP3-Ca²⁺ , DAG-PKC, and AC-cAMP-PKA/EPAC that primarily relate to secretion. Then there are the protein-protein interaction pathways Akt-mTOR-S6K, the three major MAPK pathways (ERK, JNK, and p38 MAPK), and Ca²⁺ -calcineurin-NFAT pathways that primarily regulate non-secretory processes including biosynthesis and growth, and several miscellaneous pathways that include the Rho family small G proteins, PKD, FAK, and Src that may regulate both secretory and nonsecretory processes but are not as well understood. © 2019 American Physiological Society. Compr Physiol 9:535-564, 2019.

Cyclic AMP-dependent protein kinase A and EPAC mediate VIP and secretin stimulation of PAK4 and activation of Na+,K+-ATPase in pancreatic acinar cells

Rat pancreatic acinar cells possess only the p21-activated kinase (PAKs), PAK4 of the group II PAK, and it is activated by gastrointestinal hormones/neurotransmitters stimulating PLC and by a number of growth factors. However, little is known generally of cAMP agents causing PAK4 activation, and there are no studies with gastrointestinal hormones/neurotransmitters activating cAMP cascades. In the present study, we examined the ability of VIP and secretin, which stimulate cAMP generation in pancreatic acini, to stimulate PAK4 activation, the signaling cascades involved, and their possible role in activating sodium-potassium adenosine triphosphatase (Na⁺,K⁺-ATPase). PAK4 activation was compared with activation of the well-established cAMP target, cyclic AMP response element binding protein (CREB). Secretin-stimulated PAK4 activation was inhibited by KT-5720 and PKA Type II inhibitor (PKI), protein kinase A (PKA) inhibitors, whereas VIP activation was inhibited by ESI-09 and HJC0197, exchange protein directly activated by cAMP (EPAC) inhibitors. In contrast, both VIP/secretin-stimulated phosphorylation of CREB (pCREB) via EPAC activation; however, it was inhibited by the p44/42 inhibitor PD98059 and the p38 inhibitor SB202190. The specific EPAC agonist 8-CPT-2- O-Me-cAMP as well 8-Br-cAMP and forskolin stimulated PAK4 activation. Secretin/VIP activation of Na⁺,K⁺-ATPase, was inhibited by PAK4 inhibitors (PF-3758309, LCH-7749944). These results demonstrate PAK4 is activated in pancreatic acini by stimulation of both VIP-/secretin-preferring receptors, as is CREB. However, they differ in their signaling cascades. Furthermore, PAK4 activation is needed for Na⁺,K⁺ATPase activation, which mediates pancreatic fluid secretion. These results, coupled with recent studies reporting PAKs are involved in both pancreatitis/pancreatic cancer growth/enzyme secretion, show that PAK4, similar to PAK2, likely plays an important role in both pancreatic physiological/pathological responses. NEW & NOTEWORTHY Pancreatic acini possess only the group II p21-activated kinase, PAK4, which is activated by PLC-stimulating agents/growth factors and is important in enzyme-secretion/growth/pancreatitis. Little information exists on cAMP-activating agents stimulating group II PAKs. We studied ability/effect of cyclic AMP-stimulating agents [vasoactive intestinal polypeptide (VIP), secretin] on PAK4 activity in rat pancreatic-acini. Both VIP/secretin activated PAK4/CREB, but the cAMP signaling cascades differed for EPAC, MAPK, and PKA pathways. Both hormones require PAK4 activation to stimulate sodium-potassium adenosine triphosphatase activity. This study shows PAK4 plays an important role in VIP-/secretin-stimulated pancreatic fluid secretion and suggests it plays important roles in pancreatic acinar physiological/pathophysiological responses mediated by cAMP-activating agents.

c-Src regulates cargo transit via the Golgi in pancreatic acinar cells

The exocrine pancreatic acinar cell is unique for its rapid protein synthesis and packaging in zymogen granules (ZGs). However, while crucial to the pathogenesis of pancreatitis, the signaling involved in the transit of proteins via the Golgi is poorly understood in these cells. Noting the evidence of c-Src in regulating transit of cargo via the Golgi in other systems, we explored this in acinar cells. Stimulation of ZG formation with dexamethasone activated Src and increased the Golgi area in acinar cells. c-Src localized to the microsomes of acinar cells on immunofluorescence and subcellular fractionation. While other Src family members had no effect on the Golgi markers P115 and GM130, active c-Src increased the Golgi area these stained, extending them into the ER. Src inhibition reduced amylase staining outside the Golgi and increased it in a stack like Golgi morphology. In vivo pharmacologic inhibition or acinar specific genetic deletion of c-Src reduced ZG number and staining of amylase in ZGs along with increasing amylase retention in the microsomal fraction. Morphologically this was associated with smaller Golgi stacks, and dilation of the endoplasmic reticulum. Therefore the role c-Src regulated Golgi function, ZG formation and microsomal zymogen transit in acinar cells needs to be explored in pancreatitis.

P21-activated kinase 4 in pancreatic acinar cells is activated by numerous gastrointestinal hormones/neurotransmitters and growth factors by novel signaling, and its activation stimulates secretory/growth cascades

p21-activated kinases (PAKs) are highly conserved serine/threonine protein kinases, which are divided into two groups: group-I (PAKs1-3) and group-II (PAKs4-6). In various tissues, Group-II PAKs play important roles in cytoskeletal dynamics and cell growth as well as neoplastic development/progression. However, little is known about Group-II PAK's role in a number of physiological events, including their ability to be activated by gastrointestinal (GI) hormones/neurotransmitters/growth factors (GFs). We used rat pancreatic acini to explore the ability of GI hormones/neurotransmitters/GFs to activate Group-II-PAKs and the signaling cascades involved. Only PAK4 was detected in pancreatic acini. PAK4 was activated by endothelin, secretagogues-stimulating phospholipase C (bombesin, CCK-8, and carbachol), by pancreatic GFs (insulin, insulin-like growth factor 1, hepatocyte growth factor, epidermal growth factor, basic fibroblast growth factor, and platelet-derived growth factor), and by postreceptor stimulants (12-O-tetradecanoylphobol-13-acetate and A23187 ). CCK-8 activation of PAK4 required both high- and low-affinity CCK1-receptor state activation. It was reduced by PKC-, Src-, p44/42-, or p38-inhibition but not with phosphatidylinositol 3-kinase-inhibitors and only minimally by thapsigargin. A protein kinase D (PKD)-inhibitor completely inhibited CCK-8-stimulated PKD-activation; however, stimulated PAK4 phosphorylation was only inhibited by 60%, demonstrating that it is both PKD-dependent and PKD-independent. PF-3758309 and LCH-7749944, inhibitors of PAK4, decreased CCK-8-stimulated PAK4 activation but not PAK2 activation. Each inhibited ERK1/2 activation and amylase release induced by CCK-8 or bombesin. These results show that PAK4 has an important role in modulating signal cascades activated by a number of GI hormones/neurotransmitters/GFs that have been shown to mediate both physiological/pathological responses in acinar cells. Therefore, in addition to the extensive studies on PAK4 in pancreatic cancer, PAK4 should also be considered an important signaling molecule for pancreatic acinar physiological responses and, in the future, should be investigated for a possible role in pancreatic acinar pathophysiological responses, such as in pancreatitis. NEW & NOTEWORTHY This study demonstrates that the only Group-II p21-activated kinase (PAK) in rat pancreatic acinar cells is PAK4, and thus differs from islets/pancreatic cancer. Both gastrointestinal hormones/neurotransmitters stimulating PLC and pancreatic growth factors activate PAK4. With cholecystokinin (CCK), activation is PKC-dependent/-independent, requires both CCK1-R affinity states, Src, p42/44, and p38 activation. PAK4 activation is required for CCK-mediated p42/44 activation/amylase release. These results show PAK4 plays an important role in mediating CCK physiological signal cascades and suggest it may be a target in pancreatic acinar diseases besides cancer.

Src kinases play a novel dual role in acute pancreatitis affecting severity but no role in stimulated enzyme secretion

In pancreatic acinar cells, the Src family of kinases (SFK) is involved in the activation of several signaling cascades that are implicated in mediating cellular processes (growth, cytoskeletal changes, apoptosis). However, the role of SFKs in various physiological responses such as enzyme secretion or in pathophysiological processes such as acute pancreatitis is either controversial, unknown, or incompletely understood. To address this, in this study, we investigated the role/mechanisms of SFKs in acute pancreatitis and enzyme release. Enzyme secretion was studied in rat dispersed pancreatic acini, in vitro acute-pancreatitis-like changes induced by supramaximal COOH-terminal octapeptide of cholecystokinin (CCK). SFK involvement assessed using the chemical SFK inhibitor (PP2) with its inactive control, 4-amino-7-phenylpyrazol[3,4-d]pyrimidine (PP3), under experimental conditions, markedly inhibiting SFK activation. In CCK-stimulated pancreatic acinar cells, activation occurred of trypsinogen, various MAP kinases (p42/44, JNK), transcription factors (signal transducer and activator of transcription-3, nuclear factor-κB, activator protein-1), caspases (3, 8, and 9) inducing apoptosis, LDH release reflective of necrosis, and various chemokines secreted (monocyte chemotactic protein-1, macrophage inflammatory protein-1α, regulated on activation, normal T cell expressed and secreted). All were inhibited by PP2, not by PP3, except caspase activation leading to apoptosis, which was increased, and trypsin activation, which was unaffected, as was CCK-induced amylase release. These results demonstrate SFK activation is playing a dual role in acute pancreatitis, inhibiting apoptosis and promoting necrosis as well as chemokine/cytokine release inducing inflammation, leading to more severe disease, as well as not affecting secretion. Thus, our studies indicate that SFK is a key mediator of inflammation and pancreatic acinar cell death in acute pancreatitis, suggesting it could be a potential therapeutic target in acute pancreatitis.

Novel small molecules targeting ciliary transport of Smoothened and oncogenic Hedgehog pathway activation

Trafficking of the G protein-coupled receptor (GPCR) Smoothened (Smo) to the primary cilium (PC) is a potential target to inhibit oncogenic Hh pathway activation in a large number of tumors. One drawback is the appearance of Smo mutations that resist drug treatment, which is a common reason for cancer treatment failure. Here, we undertook a high content screen with compounds in preclinical or clinical development and identified ten small molecules that prevent constitutive active mutant SmoM2 transport into PC for subsequent Hh pathway activation. Eight of the ten small molecules act through direct interference with the G protein-coupled receptor associated sorting protein 2 (Gprasp2)-SmoM2 ciliary targeting complex, whereas one antagonist of ionotropic receptors prevents intracellular trafficking of Smo to the PC. Together, these findings identify several compounds with the potential to treat drug-resistant SmoM2-driven cancer forms, but also reveal off-target effects of established drugs in the clinics.

The p21-activated kinase, PAK2, is important in the activation of numerous pancreatic acinar cell signaling cascades and in the onset of early pancreatitis events

In a recent study we explored Group-1-p21-activated kinases (GP.1-PAKs) in rat pancreatic acini. Only PAK2 was present; it was activated by gastrointestinal-hormones/neurotransmitters and growth factors in a PKC-, Src- and small-GTPase-mediated manner. PAK2 was required for enzyme-secretion and ERK/1-2-activation. In the present study we examined PAK2's role in CCK and TPA-activation of important distal signaling cascades mediating their physiological/pathophysiological effects and analyzed its role in pathophysiological processes important in early pancreatitis. In rat pancreatic acini, PAK2-inhibition by the specific, GP.1.PAK-inhibitor, IPA-3-suppressed cholecystokinin (CCK)/TPA-stimulated activation of focal-adhesion kinases and mitogen-activated protein-kinases. PAK2-inhibition reversed the dual stimulatory/inhibitory effect of CCK/TPA on the PI3K/Akt/GSK-3β pathway. However, its inhibition did not affect PKC activation. PAK2-inhibition protected acini from CCK-induced ROS-generation; caspase/trypsin-activation, important in early pancreatitis; as well as from cell-necrosis. Furthermore, PAK2-inhibition reduced proteolytic-activation of PAK-2p34, which is involved in programmed-cell-death. To ensure that the study did not only rely in the specificity of IPA-3 as a PAK inhibitor, we used two other approaches for PAK inhibition, FRAX597 a ATP-competitive-GP.1-PAKs-inhibitor and infection with a PAK2-dominant negative(DN)-Advirus. Those two approaches confirmed the results obtained with IPA-3. This study demonstrates that PAK2 is important in mediating CCK's effect on the activation of signaling-pathways known to mediate its physiological/pathophysiological responses including several cellular processes linked to the onset of pancreatitis. Our results suggest that PAK2 could be a new, important therapeutic target to consider for the treatment of diseases involving deregulation of pancreatic acinar cells.

Elucidation of the roles of the Src kinases in pancreatic acinar cell signaling

Recent studies report the Src-family kinases (SFK's) are important in a number of physiological and pathophysiological responses of pancreatic acinar cells (pancreatitis, growth, apoptosis); however, the role of SFKs in various signaling cascades important in mediating these cell functions is either not investigated or unclear. To address this we investigated the action of SFKs in these signaling cascades in rat pancreatic acini by modulating SFK activity using three methods: adenovirus-induced expression of an inactive dominant-negative CSK (Dn-CSK-Advirus) or wild-type CSK (Wt-CSK-Advirus), which activate or inhibit SFK, respectively, or using the chemical inhibitor, PP2, with its inactive control, PP3. CCK (0.3, 100 nM) and TPA (1 μM) activated SFK and altered the activation of FAK proteins (PYK2, p125(FAK)), adaptor proteins (p130(CAS), paxillin), MAPK (p42/44, JNK, p38), Shc, PKC (PKD, MARCKS), Akt but not GSK3-β. Changes in SFK activity by using the three methods of altering SFK activity affected CCK/TPAs activation of SFK, PYK2, p125(FAK), p130(CAS), Shc, paxillin, Akt but not p42/44, JNK, p38, PKC (PKD, MARCKS) or GSK3-β. With chemical inhibition the active SFK inhibitor, PP2, but not the inactive control analogue, PP3, showed these effects. For all stimulated changes pre-incubation with both adenoviruses showed similar effects to chemical inhibition of SFK activity. In conclusion, using three different approaches to altering Src activity allowed us to define fully for the first time the roles of SFKs in acinar cell signaling. Our results show that in pancreatic acinar cells, SFKs play a much wider role than previously reported in activating a number of important cellular signaling cascades shown to be important in mediating both acinar cell physiological and pathophysiological responses.

Distinctive roles of unsaturated and saturated fatty acids in hyperlipidemic pancreatitis

AIM: To investigate how the saturated and unsaturated fatty acid composition influences the susceptibility of developing acute pancreatitis.

METHODS: Primary pancreatic acinar cells were treated with low and high concentrations of different saturated and unsaturated fatty acids, and changes in the cytosolic Ca²⁺ signal and the expression of protein kinase C (PKC) were measured after treatment.

RESULTS: Unsaturated fatty acids at high concentrations, including oleic acid, linoleic acid, palmitoleic acid, docosahexaenoic acid, and arachidonic acid, induced a persistent rise in cytosolic Ca²⁺ concentrations in acinar cells. Unsaturated fatty acids at low concentrations and saturated fatty acids, including palmitic acid, stearic acid, and triglycerides, at low and high concentrations were unable to induce a rise in Ca²⁺ concentrations in acinar cells. Unsaturated fatty acids at high concentrations but not saturated fatty acids induced intra-acinar cell trypsin activation and cell damage and increased PKC expression.

CONCLUSION: At sufficiently high concentrations, unsaturated fatty acids were able to induce acinar cells injury and promote the development of pancreatitis. Unsaturated fatty acids may play a distinctive role in the pathogenesis of pancreatitis through the activation of PKC family members.

Gastrointestinal hormones/neurotransmitters and growth factors can activate P21 activated kinase 2 in pancreatic acinar cells by novel mechanisms

P-21-activated kinases (PAKs) are serine/threonine kinases comprising six isoforms divided in two groups, group-I (PAK1-3)/group-II (PAK4-6) which play important roles in cell cytoskeletal dynamics, survival, secretion and proliferation and are activated by diverse stimuli. However, little is known about PAKs ability to be activated by gastrointestinal (GI) hormones/neurotransmitters/growth-factors. We used rat pancreatic acini to explore the ability of GI-hormones/neurotransmitters/growth-factors to activate Group-I-PAKs and the signaling cascades involved. Only PAK2 was present in acini. PAK2 was activated by some pancreatic growth-factors [EGF, PDGF, bFGF], by secretagogues activating phospholipase-C (PLC) [CCK, carbachol, bombesin] and by post-receptor stimulants activating PKC [TPA], but not agents only mobilizing cellular calcium or increasing cyclic AMP. CCK-activation of PAK2 required both high- and low-affinity-CCK1-receptor-state activation. It was partially reduced by PKC- or Src-inhibition, but not with PI3K-inhibitors (wortmannin, LY294002) or thapsigargin. IPA-3, which prevents PAK2 binding to small-GTPases partially inhibited PAK2-activation, as well as reduced CCK-induced ERK1/2 activation and amylase release induced by CCK or bombesin. This study demonstrates pancreatic acini, possess only one Group-I-PAK, PAK2. CCK and other GI-hormones/neurotransmitters/growth-factors activate PAK2 via small GTPases (CDC42/Rac1), PKC and SFK but not cytosolic calcium or PI3K. CCK-activation of PAK2 showed several novel features being dependent on both receptor-activation states, having PLC- and PKC-dependent/independent components and small-GTPase-dependent/independent components. These results show that PAK2 is important in signaling cascades activated by numerous pancreatic stimuli which mediate their various physiological/pathophysiological responses and thus could be a promising target for the development of therapies in some pancreatic disorders such as pancreatitis.

Role of VEGF receptors in normal and psoriatic human keratinocytes: evidence from irradiation with different UV sources

Vascular endothelial growth factor (VEGF) promotes angiogenesis and plays important roles both in physiological and pathological conditions. VEGF receptors (VEGFRs) are high-affinity receptors for VEGF and are originally considered specific to endothelial cells. We previously reported that VEGFRs were also constitutively expressed in normal human keratinocytes and overexpressed in psoriatic epidermis. In addition, UVB can activate VEGFRs in normal keratinocytes, and the activated VEGFR-2 signaling is involved in the pro-survival mechanism. Here, we show that VEGFRs were also upregulated and activated by UVA in normal human keratinocytes via PKC, and interestingly, both the activated VEGFR-1 and VEGFR-2 protected against UVA-induced cell death. As VEGFRs were over-expressed in psoriatic epidermis, we further investigated whether narrowband UVB (NB-UVB) phototherapy or topical halomethasone monohydrate 0.05% cream could affect their expression. Surprisingly, the over-expressed VEGFRs in psoriatic epidermis were significantly attenuated by both treatments. During NB-UVB therapy, VEGFRs declined first in the basal, and then gradually in the upper psoriatic epidermis. VEGFRs were activated in psoriatic epidermis, their activation was enhanced by NB-UVB, but turned undetectable after whole therapy. This process was quite different from that by halomethasone, in which VEGFRs and phospho-VEGFRs decreased in a gradual, homogeneous manner. Our findings further suggest that UV-induced activation of VEGFRs serves as a pro-survival signal for keratinocytes. In addition, VEGFRs may be involved in the pathological process of psoriasis, and UV phototherapy is effective for psoriasis by directly modulating the expression of VEGFRs.

β-Adrenergic receptors stimulate interleukin-6 production through Epac-dependent activation of PKCδ/p38 MAPK signalling in neonatal mouse cardiac fibroblasts

BACKGROUND AND PURPOSE IL-6 plays crucial roles in cardiac hypertrophy, cardiac fibrosis and heart failure. Activation of β-adrenoceptors induced IL-6 production in neonatal mouse cardiac fibroblasts (NMCFs) through a G(s) /adenylate cyclase/cAMP/p38 MAPK pathway but independent of PKA. However, how cAMP activates p38 MAPK is still not defined. In this study, we have assessed the role of the exchange protein directly activated by cAMP (Epac) and PKCδ in p38 MAPK activation and IL-6 production by stimulated by the β-adrenoceptor agonist isoprenaline in NMCFs. EXPERIMENTAL APPROACH The IL-6 concentration in cell culture supernatants was measured by ELISA. The levels of phosphorylated and total p38 MAPK and PKCδ were determined by Western blot analysis. The translocation of PKCδ was determined by immunoblotting the soluble and particulate fractions. Expression of Epac1 or PKCδ was knocked down by the corresponding, adenovirus-mediated, small hairpin RNA (shRNA). RESULTS In NMCFs, activation of β-adrenoceptors enhanced PKCδ phosphorylation and translocation. Furthermore, knock-down of the PKCδ isoform using an adenovirus-mediated shRNA markedly down-regulated IL-6 induction by NMCFs stimulated with isoprenaline. Moreover, knock-down of Epac1 confirmed that Epac1 was upstream of PKCδ in IL-6 production. Additionally, both Epac1 and PKCδ mediated the p38 MAPK activation induced by isoprenaline. CONCLUSIONS AND IMPLICATIONS β-Adrenoceptor agonists activate a cAMP/Epac/PKCδ/p38 MAPK pathway to produce IL-6 in NMCFs. This study identifies Epac as the link between cAMP and p38 MAPK signalling pathways and demonstrates that PKCδ can function as a novel downstream effector of this β-adrenoceptor/cAMP/Epac pathway.

Protein kinase Cδ differentially regulates cAMP-dependent translocation of NTCP and MRP2 to the plasma membrane

Cyclic AMP stimulates translocation of Na(+)/taurocholate cotransporting polypeptide (NTCP) from the cytosol to the sinusoidal membrane and multidrug resistance-associated protein 2 (MRP2) to the canalicular membrane. A recent study suggested that protein kinase Cδ (PKCδ) may mediate cAMP-induced translocation of Ntcp and Mrp2. In addition, cAMP has been shown to stimulate NTCP translocation in part via Rab4. The aim of this study was to determine whether cAMP-induced translocation of NTCP and MRP2 require kinase activity of PKCδ and to test the hypothesis that cAMP-induced activation of Rab4 is mediated via PKCδ. Studies were conducted in HuH-NTCP cells (HuH-7 cells stably transfected with NTCP). Transfection of cells with wild-type PKCδ increased plasma membrane PKCδ and NTCP and increased Rab4 activity. Paradoxically, overexpression of kinase-dead dominant-negative PKCδ also increased plasma membrane PKCδ and NTCP as well as Rab4 activity. Similar results were obtained in PKCδ knockdown experiments, despite a decrease in total PKCδ. These results raised the possibility that plasma membrane localization rather than kinase activity of PKCδ is necessary for NTCP translocation and Rab4 activity. This hypothesis was supported by results showing that rottlerin, which has previously been shown to inhibit cAMP-induced membrane translocation of PKCδ and NTCP, inhibited cAMP-induced Rab4 activity. In addition, LY294002 (a phosphoinositide-3-kinase inhibitor), which has been shown to inhibit cAMP-induced NTCP translocation, also inhibited cAMP-induced PKCδ translocation. In contrast to the results with NTCP, cAMP-induced MRP2 translocation was inhibited in cells transfected with DN-PKCδ and small interfering RNA PKCδ. Taken together, these results suggest that the plasma membrane localization rather than kinase activity of PKCδ plays an important role in cAMP-induced NTCP translocation and Rab4 activity, whereas the kinase activity of PKCδ is necessary for cAMP-induced MRP2 translocation.

Raf-1 kinase inhibitory protein (RKIP) mediates ethanol-induced sensitization of secretagogue signaling in pancreatic acinar cells

Excessive alcohol consumption is associated with most cases of chronic pancreatitis, a progressive necrotizing inflammatory disease that can result in pancreatic insufficiency due to acinar atrophy and fibrosis and an increased risk of pancreatic cancer. At a cellular level acute alcohol exposure can sensitize pancreatic acinar cells to secretagogue stimulation, resulting in dysregulation of intracellular Ca(2+) homeostasis and premature digestive enzyme activation; however, the molecular mechanisms by which ethanol exerts these toxic effects have remained undefined. In this study we identify Raf-1 kinase inhibitory protein as an essential mediator of ethanol-induced sensitization of cholecystokinin- and carbachol-regulated Ca(2+) signaling in pancreatic acinar cells. We show that exposure of rodent acinar cells to ethanol induces protein kinase C-dependent Raf-1 kinase inhibitory protein phosphorylation, sensitization of cholecystokinin-stimulated Ca(2+) signaling, and potentiation of both basal and cholecystokinin-stimulated extracellular signal-regulated kinase activation. Furthermore, we show that either suppression of Raf-1 kinase inhibitory protein expression using short hairpin RNA or gene ablation prevented the sensitizing effects of ethanol on cholecystokinin- and carbachol-stimulated Ca(2+) signaling and intracellular chymotrypsin activation in pancreatic acinar cells, suggesting that the modulation of Raf-1 inhibitory protein expression may have future therapeutic utility in the prevention or treatment of alcohol-associated pancreatitis.

The Src kinase Yes is activated in pancreatic acinar cells by gastrointestinal hormones/neurotransmitters, but not pancreatic growth factors, which stimulate its association with numerous other signaling molecules

For growth factors, cytokines, G-protein-coupled receptors and numerous other stimuli, the Src Family of kinases (SFK) play a central signaling role. SFKs also play an important role in pancreatic acinar cell function including metabolism, secretion, endocytosis, growth and cytoskeletal integrity, although the specific SFKs involved are not fully known. In the present study we used specific antibodies for the SFK, Yes, to determine its presence, activation by pancreatic secretagogues or growth factors, and interaction with cellular signaling cascades mediated by CCK in which Yes participates in to cause acinar cell responses. Yes was identified in acini and secretagogues known to activate phospholipase C (PLC) [CCK, carbachol, bombesin] as well as post-receptor stimulants activating PKC [TPA] or mobilizing cellular calcium [thapsigargin/calcium ionophore (A23187)] each activated Yes. Secretin, which activates adenylate cyclase did not stimulate Yes, nor did pancreatic growth factors. CCK activation of Yes required both high- and low-affinity CCK(1)-receptor states. TPA-/CCK-stimulated Yes activation was completely inhibited by thapsigargin and the PKC inhibitor, GF109203X. CCK/TPA stimulated the association of Yes with focal adhesion kinases (Pyk2, FAK) and its autophosphorylated forms (pY397FAK, pY402Pyk2). Moreover, CCK/TPA stimulated Yes interacted with a number of other signaling proteins, including Shc, PKD, p130(Cas), PI3K and PTEN. This study demonstrates that in rat pancreatic acini, the SFK member Yes is expressed and activated by CCK and other gastrointestinal hormones/neurotransmitters. Because its activation results in the direct activation of many cellular signaling cascades that have been shown to mediate CCK's effect in acinar cell function our results suggest that it is one of the important pancreatic SFKs mediating these effects.

Regulated binding of importin-α to protein kinase Cδ in response to apoptotic signals facilitates nuclear import

PKCδ translocates into the nucleus in response to apoptotic agents and functions as a potent cell death signal. Cytoplasmic retention of PKCδ and its transport into the nucleus are essential for cell homeostasis, but how these processes are regulated is poorly understood. We show that PKCδ resides in the cytoplasm in a conformation that precludes binding of importin-α. A structural model of PKCδ in the inactive state suggests that the nuclear localization sequence (NLS) is prevented from binding to importin-α through intramolecular contacts between the C2 and catalytic domains. We have previously shown that PKCδ is phosphorylated on specific tyrosine residues in response to apoptotic agents. Here, we show that phosphorylation of PKCδ at Tyr-64 and Tyr-155 results in a conformational change that allows exposure of the NLS and binding of importin-α. In addition, Hsp90 binds to PKCδ with similar kinetics as importin-α and is required for the interaction of importin-α with the NLS. Finally, we elucidate a role for a conserved PPxxP motif, which overlaps the NLS, in nuclear exclusion of PKCδ. Mutagenesis of the conserved prolines to alanines enhanced importin-α binding to PKCδ and induced its nuclear import in resting cells. Thus, the PPxxP motif is important for maintaining a conformation that facilitates cytosplasmic retention of PKCδ. Taken together, this study establishes a novel mechanism that retains PKCδ in the cytoplasm of resting cells and regulates its nuclear import in response to apoptotic stimuli.

PKCθ activation in pancreatic acinar cells by gastrointestinal hormones/neurotransmitters and growth factors is needed for stimulation of numerous important cellular signaling cascades

The novel PKCθ isoform is highly expressed in T-cells, brain and skeletal muscle and originally thought to have a restricted distribution. It has been extensively studied in T-cells and shown to be important for apoptosis, T-cell activation and proliferation. Recent studies showed its presence in other tissues and importance in insulin signaling, lung surfactant secretion, intestinal barrier permeability, platelet and mast-cell functions. However, little information is available for PKCθ activation by gastrointestinal (GI) hormones/neurotransmitters and growth factors. In the present study we used rat pancreatic acinar cells to explore their ability to activate PKCθ and the possible interactions with important cellular mediators of their actions. Particular attention was paid to cholecystokinin (CCK), a physiological regulator of pancreatic function and important in pathological processes affecting acinar function, like pancreatitis. PKCθ-protein/mRNA was present in the pancreatic acini, and T538-PKCθ phosphorylation/activation was stimulated only by hormones/neurotransmitters activating phospholipase C. PKCθ was activated in time- and dose-related manner by CCK, mediated 30% by high-affinity CCK(A)-receptor activation. CCK stimulated PKCθ translocation from cytosol to membrane. PKCθ inhibition (by pseudostrate-inhibitor or dominant negative) inhibited CCK- and TPA-stimulation of PKD, Src, RafC, PYK2, p125(FAK) and IKKα/β, but not basal/stimulated enzyme secretion. Also CCK- and TPA-induced PKCθ activation produced an increment in PKCθ's direct association with AKT, RafA, RafC and Lyn. These results show for the first time the PKCθ presence in pancreatic acinar cells, its activation by some GI hormones/neurotransmitters and involvement in important cell signaling pathways mediating physiological responses (enzyme secretion, proliferation, apoptosis, cytokine expression, and pathological responses like pancreatitis and cancer growth).

PKC δ mediates pro-inflammatory responses in a mouse model of caerulein-induced acute pancreatitis

Acute pancreatitis is an inflammatory disorder of the pancreas. Protein kinase C (PKC) δ plays an important role in mediating chemokine production in mouse pancreatic acinar cells. This study aims to investigate the role of PKC δ in the pathogenesis of acute pancreatitis and to explore the mechanisms through which PKC δ mediates pro-inflammatory signaling. Acute pancreatitis was induced in mice by ten hourly intraperitoneal injections of caerulein. PKC δ translocation inhibitor peptide (δV1-1) at a dose of 1.0 mg/kg or Tat (carrier peptide) at a dose of 1.0 mg/kg was administered to mice either 1 h before or 1 h after the first caerulein injection. One hour after the last caerulein injection, the mice were killed and pancreas, lungs, and blood were collected. Prophylactic and therapeutic treatment with δV1-1 attenuated caerulein-induced plasma amylase levels and pancreatic edema. Treatment with δV1-1 decreased myeloperoxidase activity and monocyte chemotactic protein-1 levels in both pancreas and plasma. PKC δ mediated acute pancreatitis by activating pancreatic nuclear factor κB, activator protein-1, and mitogen-activated protein kinases. Moreover, blockade of PKC δ attenuated lung myeloperoxidase activity and edema. Histological examination of pancreatic and lung sections confirmed protection against acute pancreatitis. Treatment with Tat had no protective effect on acute pancreatitis. Blockade of PKC δ represents a promising prophylactic and/or therapeutic tool for the treatment of acute pancreatitis.

Effects of free fatty acid elevation on hepatic insulin resistance and hepatic oxidative stress

AIM: To investigate the effects of long-term elevation of free fatty acids (FFA) on hepatic insulin resistance and hepatic oxidative stress.

METHODS: A total of 32 Wistar rats were divided into two groups and given an intravenous infusion of intralipid plus heparin (IH) and saline (SAL), respectively. Two hours before the end of the infusion, a hyperinsulinemic-euglycemic clamp was performed to examine the ability of FFA to induce hepatic insulin resistance. After the clamp, liver tissue samples were taken to determine the protein carbonyl content by DNPH (2,4-dinitrophenylhydrazine) colorimetry and the content of protein kinase C-δ by Western blot.

RESULTS: Compared to SAL infusion, IH infusion resulted in an increase in plasma FFA level by 4.3 folds (P < 0.001), hepatic protein carbonyl content by 3.2 folds, and the membrane/cytosol ratio of PKC-δ by 4.0 folds (P < 0.001).

CONCLUSION: Long-term elevation of FFA induces elevation of hepatic protein carbonyl content, PKC-δ translocation and insulin resistance, suggesting that PKC-δ plays an important role in FFA-induced hepatic insulin resistance.

Gastrointestinal growth factors and hormones have divergent effects on Akt activation

Akt is a central regulator of apoptosis, cell growth and survival. Growth factors and some G-protein-coupled receptors (GPCR) regulate Akt. Whereas growth-factor activation of Akt has been extensively studied, the regulation of Akt by GPCR's, especially gastrointestinal hormones/neurotransmitters, remains unclear. To address this area, in this study the effects of GI growth factors and hormones/neurotransmitters were investigated in rat pancreatic acinar cells which are high responsive to these agents. Pancreatic acini expressed Akt and 5 of 7 known pancreatic growth-factors stimulate Akt phosphorylation (T308, S473) and translocation. These effects are mediated by p85 phosphorylation and activation of PI3K. GI hormones increasing intracellular cAMP had similar effects. However, GI-hormones/neurotransmitters [CCK, bombesin, carbachol] activating phospholipase C (PLC) inhibited basal and growth-factor-stimulated Akt activation. Detailed studies with CCK, which has both physiological and pathophysiological effects on pancreatic acinar cells at different concentrations, demonstrated CCK has a biphasic effect: at low concentrations (pM) stimulating Akt by a Src-dependent mechanism and at higher concentrations (nM) inhibited basal and stimulated Akt translocation, phosphorylation and activation, by de-phosphorylating p85 resulting in decreasing PI3K activity. This effect required activation of both limbs of the PLC-pathway and a protein tyrosine phosphatase, but was not mediated by p44/42 MAPK, Src or activation of a serine phosphatase. Akt inhibition by CCK was also found in vivo and in Panc-1 cancer cells where it inhibited serum-mediated rescue from apoptosis. These results demonstrate that GI growth factors as well as gastrointestinal hormones/neurotransmitters with different cellular basis of action can all regulate Akt phosphorylation in pancreatic acinar cells. This regulation is complex with phospholipase C agents such as CCK, because both stimulatory and inhibitory effects can be seen, which are mediated by different mechanisms.

PKD3 is the predominant protein kinase D isoform in mouse exocrine pancreas and promotes hormone-induced amylase secretion

The protein kinase D (PKD) family of serine/threonine kinases, which can be activated by gastrointestinal hormones, consists of three distinct isoforms that modulate a variety of cellular processes including intracellular protein transport as well as constitutive and regulated secretion. Although isoform-specific functions have been identified in a variety of cell lines, the expression and function of PKD isoforms in normal, differentiated secretory tissues is unknown. Here, we demonstrate that PKD isoforms are differentially expressed in the exocrine and endocrine cells of the pancreas. Specifically, PKD3 is the predominant isoform expressed in exocrine cells of the mouse and human pancreas, whereas PKD1 and PKD2 are more abundantly expressed in the pancreatic islets. Within isolated mouse pancreatic acinar cells, PKD3 undergoes rapid membrane translocation, trans-activating phosphorylation, and kinase activation after gastrointestinal hormone or cholinergic stimulation. PKD phosphorylation in pancreatic acinar cells occurs viaaCa2+-independent, diacylglycerol- and protein kinase C-dependent mechanism. PKD phosphorylation can also be induced by physiologic concentrations of secretagogues and by in vivo stimulation of the pancreas. Furthermore, activation of PKD3 potentiates MEK/ERK/RSK (RSK, ribosomal S6 kinase) signaling and significantly enhances cholecystokinin-mediated pancreatic amylase secretion. These findings reveal a novel distinction between the exocrine and endocrine cells of the pancreas and further identify PKD3 as a signaling molecule that promotes hormone-stimulated amylase secretion.

Modulation of PKCdelta signaling alters the shear stress-mediated increases in endothelial nitric oxide synthase transcription: role of STAT3

We have previously shown that the regulation of endothelial nitric oxide synthase (eNOS) in endothelial cells isolated from fetal lamb under static conditions is positively regulated by PKCdelta. In this study, we explore the role of PKCdelta in regulating shear-induced upregulation of eNOS. We found that shear caused a decrease in PKCdelta activation. Modulation of PKCdelta before shear with a dominant negative mutant of PKCdelta (DN PKCdelta) or bryostatin (a known PKCdelta activator) demonstrated that PKCdelta inhibition potentiates the shear-mediated increases in eNOS expression and activity, while PKCdelta activation inhibited these events. To gain insight into the mechanism by which PKCdelta inhibits shear-induced eNOS expression, we examined activation of STAT3, a known target for PKCdelta phosphorylation. We found that shear decreased the phosphorylation of STAT3. Further the transfection of cells with DN PKCdelta reduced, while PKCdelta activation enhanced, STAT3 phosphorylation in the presence of shear. Transfection of cells with a dominant negative mutant of STAT3 enhanced eNOS promoter activity and nitric oxide production in response to shear. Finally, we found that mutating the STAT3 binding site sequence within the eNOS promoter increased promoter activity in response to shear and that this was no longer inhibited by bryostatin. In conclusion, shear decreases PKCdelta activity and, subsequently, reduces STAT3 binding to the eNOS promoter. This signaling pathway plays a previously unidentified role in the regulation of eNOS expression by shear stress.

Down-regulation of caspase-2 by rottlerin via protein kinase C-delta-independent pathway

Protein kinase C-delta (PKC delta) plays an important role in DNA damage-induced apoptosis. We have previously shown that the PKC delta inhibitor rottlerin protects against cisplatin-induced apoptosis acting upstream of caspase-9. In the present study, we have investigated if rottlerin regulates caspase-2 activation. Knockdown of caspase-2 by siRNA inhibited processing of apical caspase-9 and caspase-8, whereas depletion of caspase-9 had little effect on caspase-2 processing. Rottlerin inhibited activation and processing of caspase-9 and caspase-8 and cleavage of poly(ADP)ribose polymerase. We made a novel observation that rottlerin induced down-regulation of caspase-2 but not of caspase-3, caspase-7, caspase-8, or caspase-9. Pharmacologic inhibitors of PKC, such as Gö 6983 and bisindolylmaleimide, or depletion of PKC delta by siRNA had no effect on the down-regulation of caspase-2 by rottlerin. The proteasome inhibitor MG132 reversed caspase-2 down-regulation by rottlerin, whereas calpain inhibitor had no effect. These results suggest that rottlerin induces down-regulation of caspase-2 via PKC delta-independent but ubiquitin proteasome-mediated pathway. Furthermore, down-regulation of caspase-2 by rottlerin can explain its antiapoptotic function during DNA damage-induced apoptosis.

The novel protein kinase C isoforms -delta and -epsilon modulate caerulein-induced zymogen activation in pancreatic acinar cells

Isoforms of protein kinase C (PKC) have been shown to modulate some cellular responses such as pathological secretion and generation of inflammatory mediators during acute pancreatitis (AP). We propose that PKC also participates in premature zymogen activation within the pancreatic acinar cell, a key event in the initiation of AP. This hypothesis was examined in in vivo and cellular models of caerulein-induced AP using PKC activators and inhibitors. Phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA, 200 nM), a known activator of PKC, enhanced zymogen activation at both 0.1 nM and 100 nM caerulein, concentrations which mimic physiological and supraphysiological effects of the hormone cholecystokinin, respectively, in preparations of pancreatic acinar cells. Isoform-specific PKC inhibitors for PKC-delta and PKC-epsilon reduced supraphysiological caerulein-induced zymogen activation. Using a cell-free reconstitution system, we showed that inhibition of PKC-delta and -epsilon, reduced zymogen activation in both zymogen granule-enriched and microsomal fractions. In dispersed acinar cells, 100 nM caerulein stimulation caused PKC-delta and -epsilon isoform translocation to microsomal membranes using cell fractionation and immunoblot analysis. PKC translocation was confirmed with in vivo studies and immunofluorescence microscopy in pancreatic tissues from rats treated with or without 100 nM caerulein. PKC-epsilon redistributed from an apical to a supranuclear region following caerulein administration. The signal for PKC-epsilon overlapped with granule membrane protein, GRAMP-92, an endosomal/lysosomal marker, in a supranuclear region where zymogen activation takes place. These results indicate that PKC-delta and -epsilon isoforms translocate to specific acinar cell compartments and modulate zymogen activation.

Protein kinase C in the pancreatic acinar cell

Pathological activation of selective signaling molecules within the pancreatic acinar cell mediates the development of acute pancreatitis. Some of the key early acinar cell events include activation of proteases, inhibition of apical secretion, and elaboration of inflammatory mediators. Previous studies have shown that supraphysiological concentrations of cholecystokinin (CCK) that can cause pancreatitis in vivo, also initiate these pathological responses in dispersed groups of acinar cells (acini). Protein kinase C (PKC) regulates many cellular events and a role for this family of signaling molecules has been described in some of the pathological responses of pancreatitis. Notably, ethanol can activate specific PKC isoforms and sensitize the acinar cells to the pathological effects of CCK. Our preliminary studies in isolated pancreatic acini and a cell-free reconstitution system suggest that PKC can mediate protease activation in the acinar cell. These findings may be relevant to the pathogenesis of pancreatitis from alcohol and other etiologies.

Function of non-visual arrestins in signaling and endocytosis of the gastrin-releasing peptide receptor (GRP receptor)

Little is known about the role of arrestins in gastrointestinal hormone/neurotransmitter receptor endocytosis. With other G protein-coupled receptors, arrestins induce G protein-uncoupling and receptor endocytosis. In this study, we used arrestin wild-type and dominant-negative mutant constructs to analyze the arrestin dependence of endocytosis and desensitization of the gastrin-releasing peptide receptor (GRP-R). Co-expression of the GRP-R with wild-type arrestin2 and arrestin3 increased not only GRP-R endocytosis but also GRP-R desensitization in arrestin-overexpressing cells. Co-expression of the dominant-negative mutants V53D-arrestin2 or V54D-arrestin3 reduced GRP-R endocytosis. Notably, different trafficking routes for agonist-activated GRP-R-arrestin2 and GRP-R-arrestin3 complexes were found. Arrestin3 internalizes with GRP-R to intracellular vesicles, arrestin2 splits from the GRP-R and localizes to the cell membrane. Also, the recycling pathway of the GRP-R was different if co-expressed with arrestin2 or arrestin3. Using different GRP-R mutants, the C-terminus and the 2nd intracellular loop of the GRP-R were found to be important for the GRP-R-arrestin interaction and for the difference in GRP receptor trafficking with the two arrestin subtypes. Our results show that both non-visual arrestins play an important role in GRP-R internalization and desensitization.

Protein kinase C family: on the crossroads of cell signaling in skin and tumor epithelium

The protein kinase C (PKC) family represents a large group of phospholipid dependent enzymes catalyzing the covalent transfer of phosphate from ATP to serine and threonine residues of proteins. Phosphorylation of the substrate proteins induces a conformational change resulting in modification of their functional properties. The PKC family consists of at least ten members, divided into three subgroups: classical PKCs (alpha, betaI, betaII, gamma), novel PKCs (delta, epsilon, eta, theta), and atypical PKCs (zeta, iota/lambda). The specific cofactor requirements, tissue distribution, and cellular compartmentalization suggest differential functions and fine tuning of specific signaling cascades for each isoform. Thus, specific stimuli can lead to differential responses via isoform specific PKC signaling regulated by their expression, localization, and phosphorylation status in particular biological settings. PKC isoforms are activated by a variety of extracellular signals and, in turn, modify the activities of cellular proteins including receptors, enzymes, cytoskeletal proteins, and transcription factors. Accordingly, the PKC family plays a central role in cellular signal processing. Accumulating data suggest that various PKC isoforms participate in the regulation of cell proliferation, differentiation, survival and death. These findings have enabled identification of abnormalities in PKC isoform function, as they occur in several cancers. Specifically, the initiation of squamous cell carcinoma formation and progression to the malignant phenotype was found to be associated with distinct changes in PKC expression, activation, distribution, and phosphorylation. These studies were recently further extended to transgenic and knockout animals, which allowed a more direct analysis of individual PKC functions. Accordingly, this review is focused on the involvement of PKC in physiology and pathology of the skin.

Curcumin-induced GADD153 upregulation: Modulation by glutathione

As we reported previously, GADD153 is upregulated in colon cancer cells exposed to curcumin. In the present study, we ascertained the involvement of glutathione and certain sulfhydryl enzymes associated with signal transduction in mediating the effect of curcumin on GADD153. Curcumin-induced GADD153 gene upregulation was attenuated by reduced glutathione (GSH) or N-acetylcysteine (NAC) and potentiated by the glutathione synthesis inhibitor, L-buthionine-(S,R)-sulfoximine (BSO). Additionally, GSH and NAC decreased the intracellular content of curcumin. Conversely, curcumin decreased intracellular glutathione and also increased the formation of reactive oxygen species (ROS) in cells, but either GSH or NAC prevented both of these effects of curcumin. In affecting the thiol redox status, curcumin caused activation of certain sulfhydryl enzymes involved in signal transduction linked to GADD153 expression. Curcumin increased the expression of the phosphorylated forms of PTK, PDK1, and PKC-delta, which was attenuated by either GSH or NAC and potentiated by BSO. Furthermore, selective inhibitors of PI3K and PKC-delta attenuated curcumin-induced GADD153 upregulation. Collectively, these findings suggest that a regulatory thiol redox-sensitive signaling cascade exists in the molecular pathway leading to induction of GADD153 expression as caused by curcumin.

CCK causes PKD1 activation in pancreatic acini by signaling through PKC-delta and PKC-independent pathways

Protein kinase D1 (PKD1) is involved in cellular processes including protein secretion, proliferation and apoptosis. Studies suggest PKD1 is activated by various stimulants including gastrointestinal (GI) hormones/neurotransmitters and growth factors in a protein kinase C (PKC)-dependent pathway. However, little is known about the mechanisms of PKD1 activation in physiologic GI tissues. We explored PKD1 activation by GI hormones/neurotransmitters and growth factors and the mediators involved in rat pancreatic acini. Only hormones/neurotransmitters activating phospholipase C caused PKD1 phosphorylation (S916, S744/748). CCK activated PKD1 and caused a time- and dose-dependent increase in serine phosphorylation by activation of high- and low-affinity CCK(A) receptor states. Inhibition of CCK-stimulated increases in phospholipase C, PKC activity or intracellular calcium decreased PKD1 S916 phosphorylation by 56%, 62% and 96%, respectively. PKC inhibitors GF109203X/Go6976/Go6983/PKC-zeta pseudosubstrate caused a 62/43/49/0% inhibition of PKD1 S916 phosphorylation and an 87/13/82/0% inhibition of PKD1 S744/748 phosphorylation. Expression of dominant negative PKC-delta, but not PKC-epsilon, or treatment with PKC-delta translocation inhibitor caused marked inhibition of PKD phosphorylation. Inhibition of Src/PI3K/MAPK/tyrosine phosphorylation had no effect. In unstimulated cells, PKD1 was mostly located in the cytoplasm. CCK stimulated translocation of total and phosphorylated PKD1 to the membrane. These results demonstrate that CCK(A) receptor activation leads to PKD activation by signaling through PKC-dependent and PKC-independent pathways.

Gastrointestinal Hormones Cause Rapid c-Met Receptor Down-regulation by a Novel Mechanism Involving Clathrin-mediated Endocytosis and a Lysosome-dependent Mechanism

The activated c-Met receptor has potent effects on normal tissues and tumors. c-Met levels are regulated by hepatocyte growth factor (HGF); however, it is unknown if they can be regulated by gastrointestinal (GI) hormones. c-Met is found in many GI tissues/tumors that possess GI hormone receptors. We studied the effect of GI hormones on c-Met in rat pancreatic acini, which possess both receptors. CCK-8, carbachol, and bombesin, but not VIP/secretin, decreased c-Met. CCK-8 caused rapid and potent c-Met down-regulation and abolished HGF-induced c-Met and Gab1 tyrosine phosphorylation, while stimulating c-Met serine phosphorylation. The effect of cholecystokinin (CCK) was also seen in intact acini using immunofluorescence, in a biotinylated fraction representing membrane proteins, in single acinar cells, in Panc-1 tumor cells, and in vivo in rats injected with CCK. CCK-8 did not decrease cell viability or overall responsiveness. GF109203X, thapsigargin, or their combination partially reversed the effect of CCK-8. In contrast to HGF-induced c-Met down-regulation, the effect of CCK was decreased by a lysosome inhibitor (concanamycin) but not the proteasome inhibitor lactacystin. Inhibitors of clathrin-mediated endocytosis blocked the effect of CCK. HGF but not CCK-8 caused c-Met ubiquitination. These results show CCK and other GI hormones can cause rapid c-Met down-regulation, which occurs by a novel mechanism. These results could be important for c-Met regulation in normal as well as in neoplastic tissue in the GI tract.

Involvement of JAK2 and Src kinase tyrosine phosphorylation in human growth hormone-stimulated increases in cytosolic free Ca2+and insulin secretion

We previously reported that human growth hormone (hGH) increases cytoplasmic Ca2+concentration ([Ca2+]i) and proliferation in pancreatic β-cells (Sjöholm Å, Zhang Q, Welsh N, Hansson A, Larsson O, Tally M, and Berggren PO. J Biol Chem 275: 21033–21040, 2000) and that the hGH-induced rise in [Ca2+]iinvolves Ca2+-induced Ca2+release facilitated by tyrosine phosphorylation of ryanodine receptors (Zhang Q, Kohler M, Yang SN, Zhang F, Larsson O, and Berggren PO. Mol Endocrinol 18: 1658–1669, 2004). Here we investigated the tyrosine kinases that convey the hGH-induced rise in [Ca2+]iand insulin release in BRIN-BD11 β-cells. hGH caused tyrosine phosphorylation of Janus kinase (JAK)2 and c-Src, events inhibited by the JAK2 inhibitor AG490 or the Src kinase inhibitor PP2. Although hGH-stimulated rises in [Ca2+]iand insulin secretion were completely abolished by AG490 and JAK2 inhibitor II, the inhibitors had no effect on insulin secretion stimulated by a high K+concentration. Similarly, Src kinase inhibitor-1 and PP2, but not its inactive analog PP3, suppressed [Ca2+]ielevation and completely abolished insulin secretion stimulated by hGH but did not affect responses to K+. Ovine prolactin increased [Ca2+]iand insulin secretion to a similar extent as hGH, effects prevented by the JAK2 and Src kinase inhibitors. In contrast, bovine GH evoked a rise in [Ca2+]ibut did not stimulate insulin secretion. Neither JAK2 nor Src kinase inhibitors influenced the effect of bovine GH on [Ca2+]i. Our study indicates that hGH stimulates rise in [Ca2+]iand insulin secretion mainly through activation of the prolactin receptor and JAK2 and Src kinases in rat insulin-secreting cells.

Phorbol 12-Myristate 13-Acetate Protects against Tumor Necrosis Factor (TNF)-Induced Necrotic Cell Death by Modulating the Recruitment of TNF Receptor 1-Associated Death Domain and Receptor-Interacting Protein into the TNF Receptor 1 Signaling Complex: Implication for the Regulatory Role of Protein Kinase C

Protein kinase C (PKC) triggers cellular signals that regulate proliferation or death in a cell- and stimulus-specific manner. Although previous studies have demonstrated that activation of PKC with phorbol 12-myristate 13-acetate (PMA) protects cells from apoptosis induced by a number of mechanisms, including death receptor ligation, little is known about the effect or mechanism of PMA in the necrotic cell death. Here, we demonstrate that PMA-mediated activation of PKC protects against tumor necrosis factor (TNF)-induced necrosis by disrupting formation of the TNF receptor (TNFR)1 signaling complex. Pretreatment with PMA protected L929 cells from TNF-induced necrotic cell death in a PKC-dependent manner, but it did not protect against DNA-damaging agents, including doxorubicin (Adriamycin) and camptothecin. Analysis of the upstream signaling events affected by PMA revealed that it markedly inhibited the TNF-induced recruitment of TNFR1-associated death domain protein (TRADD) and receptor-interacting protein (RIP) to TNFR1, subsequently inhibiting TNF-induced activation of nuclear factor-kappaB and c-Jun NH2-terminal kinase (JNK). However, JNK inhibitors do not significantly affect TNF-induced necrosis, suggesting that the inhibition of JNK activation by PMA is not part of the antinecrotic mechanism. In addition, PMA acted as an antagonist of TNF-induced reactive oxygen species (ROS) production, thereby suppressing activation of ROS-mediated poly(ADP-ribose)polymerase (PARP), and thus inhibiting necrotic cell death. Furthermore, during TNF-induced necrosis, PARP was significantly activated in wild-type mouse embryonic fibroblast (MEF) cells but not in RIP-/- or TNFR-associated factor 2-/-MEF cells. Taken together, these results suggest that PKC activation ensures effective shutdown of the death receptor-mediated necrotic cell death pathway by modulating formation of the death receptor signaling complex.

The Src family kinase, Lyn, is activated in pancreatic acinar cells by gastrointestinal hormones/neurotransmitters and growth factors which stimulate its association with numerous other signaling molecules

Src family kinases (SFK) play a central signaling role for growth factors, cytokines, G-protein-coupled receptors and other stimuli. SFKs play important roles in pancreatic acinar cell secretion, endocytosis, growth, cytoskeletal integrity and apoptosis, although little is known of the specific SFKs involved. In this study we demonstrate the SFK, Lyn, is present in rat pancreatic acini and investigate its activation/signaling. Ca(2+)-mobilizing agents, cAMP-mobilizing agents and pancreatic growth factors activated Lyn. CCK, a physiological regulator of pancreatic function, rapidly activated Lyn. The specific SFK inhibitor, PP2, decreased Lyn activation; however, the inactive analogue, PP3, had no effect. Inhibition of CCK-stimulated changes in [Ca(2+)](i) decreased Lyn activation by 55%; GFX, a PKC inhibitor by 36%; and the combination by 95%. CCK activation of Lyn required stimulation of high and low affinity CCK(A) receptor states. CCK stimulated an association of Lyn with PKC-delta, Shc, p125(FAK) and PYK2 as well as with their autophosphorylated forms, but not with Cbl, p85, p130(CAS) or ERK 1/2. These results show Lyn is activated by diverse pancreatic stimulants. CCK's activation of Lyn is likely an important mediator of its ability to cause tyrosine phosphorylation of numerous important cellular mediators such as p125(FAK), PYK2, PKC-delta and Shc, which play central roles in CCK's effects on acinar cell function.

6-hydroxydopamine-induced apoptosis is mediated via extracellular auto-oxidation and caspase 3-dependent activation of protein kinase Cdelta

6-Hydroxydopamine is a neurotoxin commonly used to lesion dopaminergic pathways and generate experimental models for Parkinson disease, however, the cellular mechanism of 6-hydroxydopamine-induced neurodegeneration is not well defined. In this study we have explored how 6-hydroxydopamine neurotoxicity is initiated. We have also investigated downstream signaling pathways activated in response to 6-hydroxydopamine, using a neuronal-like, catecholaminergic cell line (PC12 cells) as an in vitro model system. We have shown that 6-hydroxydopamine neurotoxicity is initiated via extracellular auto-oxidation and the induction of oxidative stress from the oxidative products generated. Neurotoxicity is completely attenuated by preincubation with catalase, suggesting that hydrogen peroxide, at least in part, evokes neuronal cell death in this model. 6-Hydroxydopamine does not initiate toxicity by dopamine transporter-mediated uptake into PC12 cells, because both GBR-12909 and nisoxetine (inhibitors of dopamine and noradrenaline transporters, respectively) failed to reduce toxicity. 6-Hydroxydopamine has previously been shown to induce both apoptotic and necrotic cell-death mechanisms. In this study oxidative stress initiated by 6-hydroxydopamine caused mitochondrial dysfunction, activation of caspases 3/7, nuclear fragmentation, and apoptosis. We have shown that, in this model, proteolytic activation of the proapoptotic protein kinase Cdelta (PKCdelta) is a key mediator of 6-hydroxydopamine-induced cell death. 6-Hydroxydopamine induces caspase 3-dependent cleavage of full-length PKCdelta (79 kDa) to yield a catalytic fragment (41 kDa). Inhibition of PKCdelta (with rottlerin or via RNA interference-mediated gene suppression) ameliorates the neurotoxicity evoked by 6-hydroxydopamine, implicating this kinase in 6-hydroxydopamine-induced neurotoxicity and Parkinsonian neurodegeneration.

Rottlerin inhibits stimulated enzymatic secretion and several intracellular signaling transduction pathways in pancreatic acinar cells by a non-PKC-delta-dependent mechanism

Protein kinase C-delta (PKC-delta) becomes activated in pancreatic acini in response to cholecystokinin (CCK) and plays a pivotal role in the exocrine pancreatic secretion. Rottlerin, a polyphenolic compound, has been widely used as a potent and specific PKC-delta inhibitor. However, some recent studies showed that rottlerin was not effective in inhibiting PKCdelta activity in vitro and that may display unspecific effects. The aims of this work were to investigate the specificity of rottlerin as an inhibitor of PKC-delta activity in intact cells and to elucidate the biochemical causes of its unspecificity. Preincubation of pancreatic acini with rottlerin (6 microM) inhibited CCK-stimulated translocation, tyrosine phosphorylation (TyrP) and activation of PKC-delta in pancreatic acini in a time-dependent manner. Rottlerin inhibited amylase secretion stimulated by both PKC-dependent pathways (CCK, bombesin, carbachol, TPA) and also by PKC-independent pathways (secretin, VIP, cAMP analogue). CCK-stimulation of MAPK activation and p125(FAK) TyrP which are mediated by PKC-dependent and -independent pathways were also inhibited by rottlerin. Moreover, rottlerin rapidly depleted ATP content in pancreatic acini in a similar way as the mitochondrial uncouplers CCCP and FCCP. All studied inhibitory effects of rottlerin in pancreatic acini were mimicked by FCCP (agonists-stimulated amylase secretion, p125(FAK) TyrP, MAPK activation and PKC-delta TyrP and translocation). Finally, rottlerin as well as FCCP display a potent inhibitory effect on the activation of other PKC isoforms present in pancreatic acini. Our results suggest that rottlerin effects in pancreatic acini are not due to a specific PKC-delta blockade, but likely due to its negative effect on acini energy resulting in ATP depletion. Therefore, to study the role of PKC-delta in cellular processes using rottlerin it is essential to keep in mind that may deplete ATP levels and inhibit different PKC isoforms. Our results give reasons for a more careful choice of rottlerin for PKC-delta investigation.

Distinct signaling pathways for induction of type II NOS by IFNγ and LPS in BV-2 microglial cells

Nitric oxide (NO) release upon microglial cell activation has been implicated in the tissue injury and cell death in many neurodegenerative diseases. Recent studies have indicated the ability of interferon-gamma (IFNgamma) and lipopolysaccharides (LPS) to independently induce type II nitric oxide synthase (iNOS) expression and NO production in BV-2 microglial cells. However, a detailed comparison between the signaling pathways activating iNOS by these two agents has not been accomplished. Analysis of PKC isoforms revealed mainly the presence of PKCdelta, iota and lambda in BV-2 cells. Although both IFNgamma and LPS could specifically enhance the tyrosine phosphorylation of PKCdelta, treatment with IFNgamma induced a steady increase of phospho-PKCdelta for up to 1h, whereas treatment with LPS elevated phospho-PKCdelta levels only transiently, with peak activity at 5 min. Rottlerin, a specific inhibitor for PKCdelta, dose-dependently inhibited IFNgamma- and LPS-induced NO production. Despite the common involvement of PKCdelta, IFNgamma- but not LPS-induced NO production involved extracellular signal-regulated kinases (ERK1/2) cascade and IFNgamma-induced phosphorylation of ERK1/2 was mediated through PKC. On the other hand, LPS- but not IFNgamma-induced NO production was through stimulation of NF-kappaB activation and nuclear translocation to interact with DNA. These results demonstrated distinct signaling pathways for induction of iNOS by IFNgamma and LPS in BV-2 microglial cells.

Signal transduction through substance P receptor in human glioblastoma cells: roles for Src and PKCdelta

Substance P receptor (SPR), a G protein-coupled receptor (GPCR), is found in human glioblastomas, and has been implicated in their growth. Consistent with a role for SPR in cell growth, activation of SPR in U373 MG human glioblastoma cells leads to the phosphorylation of mitogen-activated protein kinases [extracellular signal-regulated kinase 1 and 2 (ERK1/2)] and stimulation of cell proliferation. The purpose of the present study was to elucidate the pathway through which these actions occur. Using either the epidermal growth factor receptor (EGFR) kinase inhibitor, AG 1478, or a small-interfering RNA (siRNA) directed against human EGFR, we found that transactivation of EGFR by SPR is only marginally involved in SP-dependent ERK1/2 phosphorylation. Src, however, is shown to be a major component of SPR signaling because the Src kinase inhibitor, PP2, and a kinase-dead Src mutant both inhibit SP-dependent ERK1/2 phosphorylation. We also report that SPR stimulates the phosphorylation of protein kinase Cdelta(PKCdelta), and that this stimulation is blocked by PP2. SP-dependent ERK1/2 phosphorylation is also blocked by rottlerin, a PKCdelta inhibitor, and the calcium scavenger, BAPTA/AM. Finally, rottlerin and PP2 were both found to inhibit the growth of several glioblastoma cell lines, underscoring the potential of these agents to block glioblastoma growth.

Induction of ornithine decarboxylase in T/C-28a2 chondrocytes by lysophosphatidic acid: signaling pathway and inhibition of cell proliferation

Among several extracellular messengers tested, lysophosphatidic acid (LPA) was able to cause the most marked induction of ornithine decarboxylase (ODC) in serum-starved human T/C-28a2 chondrocytes. LPA also induced the activation/phosphorylation of Src, Akt and p44/42 MAPK, and the translocation of PKC-delta from cytosol to membrane coupled to its tyrosine phosphorylation. Experiments with selective signaling inhibitors indicate that LPA leads to Src activation through Gi protein-coupled receptors. In turn Src can activate PI3K and PKC-delta, and all these signaling proteins are required for ODC induction. In conclusion these results show that chondrocytes may be a novel target for LPA action. However, although LPA is considered a mitogen for several cell types and ODC induction is generally correlated to cell growth, LPA was not able to stimulate chondrocyte growth, but rather exerted an anti-proliferative effect.

Regulation of cadmium-induced apoptosis by PKCδ in U937 human promonocytic cells

Pulse treatment with cadmium chloride followed by recovery caused apoptosis in U937 human promonocytic cells. In addition, the treatment-induced PKCdelta translocation from cytosol to membrane fraction, which was already detected at 30 min of treatment; and also caused PKCdelta cleavage to give a 41-kDa fragment, which was detected at 3-6 h of recovery, concomitantly with the execution of apoptosis. All these effects were reduced by the PKCdelta-specific inhibitor rottlerin. By contrast, rottlerin did not prevent the cadmium-provoked stimulation of the stress response (as measured by HSP70 expression), nor inhibited the generation of apoptosis by heat-shock, which failed to cause PKCdelta translocation. Cadmium chloride rapidly induced p38(MAPK) activation, which was not affected by rottlerin. By contrast, the p38(MAPK) inhibitor SB203580 reduced PKCdelta translocation and cleavage, indicating that p38(MAPK) activation precedes and regulates PKCdelta activation. It is concluded that PKCdelta mediates apoptosis induction by cadmium ions via early membrane translocation, and also possibly through late kinase proteolytic cleavage and phosphorylation on tyrosine residues.

Identification of key amino acids in the gastrin-releasing peptide receptor (GRPR) responsible for high affinity binding of gastrin-releasing peptide (GRP)

The bombesin (Bn) receptor family includes the gastrin-releasing peptide (GRPR) and neuromedin B (NMBR) receptors, Bn receptor subtype 3 (BRS-3) and Bn receptor subtype 4 (BB(4)). They share 50% homology, yet their affinities for gastrin-releasing peptide (GRP) differ. The determinants of GRP high affinity for GRPR and BB(4), and low affinity for BRS-3 are largely unknown. To address this question we made an analysis of structural homologies in Bn receptor members correlated with their affinities for GRP to develop criteria to identify amino acids important for GRP selectivity. Fourteen differences were identified and each was mutated singly in GRPR to that found in hBRS-3. Eleven mutants had a loss of GRP affinity. Furthermore, three of four amino acids in the GRPR selected used a similar approach and previously reported to be important for high affinity Bn binding, were important for GRP affinity. Some GRPR mutants containing combinations of these mutations had greater decreases in GRP affinity than any single mutation. Particularly important for GRP selectivity were K101, Q121, A198, P199, S293, R288, T297 in GRPR. These results were confirmed by making the reverse mutations in BRS-3 to make GRP gain of affinity mutants. Modeling studies demonstrated a number of the important amino acids had side-chains oriented inward and within 6A of the binding pocket. These results demonstrated this approach could identify amino acids needed for GRP affinity and complemented results from chimera/mutagenesis studies by identifying which differences in the extracellular domains of Bn receptors were important for GRP affinity.

Induction of polyploidization in leukemic cell lines and primary bone marrow by Src kinase inhibitor SU6656

Megakaryocytes (MKs) undergo successive rounds of endomitosis during differentiation, resulting in polyploidy (typically, 16-64N). Previous studies have demonstrated that this occurs through an interruption of normal cell cycle progression during anaphase. However, the molecular mechanism(s) controlling this unique process is undefined. In the present report, we examine the effect of an Src kinase inhibitor, SU6656, on thrombopoietin (TPO)-induced growth and differentiation. Remarkably, when SU6656 (2.5 microM) was added to a megakaryocytic cell line, UT-7/TPO, the cells ceased cell division but continued to accumulate DNA by endomitosis. During this interval, CD41 and CD61 expression on the cell surface increased. Similar effects on polyploidization and MK differentiation were seen with expanded primary MKs, bone marrow from 2 patients with myelodysplastic syndrome, and other cell lines with MK potential. Our data suggest that SU6656 might be useful as a differentiation-inducing agent for MKs and is an important tool for understanding the molecular basis of MK endomitosis.

PKC-interacting proteins: from function to pharmacology

Protein kinase C (PKC) is a ubiquitously expressed family of kinases that have key roles in regulating multiple cellular activities. The activity of this family is controlled tightly by several molecular mechanisms, including interaction with binding-partner proteins. These PKC-interacting proteins (C-KIPs) confer specificity for individual PKC isoforms by regulating the activity and cellular localization of PKC isoforms and, subsequently, the ability of these isoforms to specifically regulate cellular functional events. Although many C-KIPs have been identified by genome and proteome-mining approaches, it is important to address the specificity and function of the interactions in greater detail because they might form novel drug targets. In this article, we review recent work on C-KIPs and the implications for pharmacological and therapeutic development.

PKC-delta and -epsilon regulate NF-kappaB activation induced by cholecystokinin and TNF-alpha in pancreatic acinar cells

Although NF-kappaB plays an important role in pancreatitis, mechanisms underlying its activation remain unclear. We investigated the signaling pathways mediating NF-kappaB activation in pancreatic acinar cells induced by high-dose cholecystokinin-8 (CCK-8), which causes pancreatitis in rodent models, and TNF-alpha, which contributes to inflammatory responses of pancreatitis, especially the role of PKC isoforms. We determined subcellular distribution and kinase activities of PKC isoforms and NF-kappaB activation in dispersed rat pancreatic acini. We applied isoform-specific, cell-permeable peptide inhibitors to assess the role of individual PKC isoforms in NF-kappaB activation. Both CCK-8 and TNF-alpha activated the novel isoforms PKC-delta and -epsilon and the atypical isoform PKC-zeta but not the conventional isoform PKC-alpha. Inhibition of the novel PKC isoforms but not the conventional or the atypical isoform resulted in the prevention of NF-kappaB activation induced by CCK-8 and TNF-alpha. NF-kappaB activation by CCK-8 and TNF-alpha required translocation but not tyrosine phosphorylation of PKC-delta. Activation of PKC-delta, PKC-epsilon, and NF-kappaB with CCK-8 involved both phosphatidylinositol-specific PLC and phosphatidylcholine (PC)-specific PLC, whereas with TNF-alpha they only required PC-specific PLC for activation. Results indicate that CCK-8 and TNF-alpha initiate NF-kappaB activation by different PLC pathways that converge at the novel PKCs (delta and epsilon) to mediate NF-kappaB activation in pancreatic acinar cells. These findings suggest a key role for the novel PKCs in pancreatitis.

Activation of Vascular Endothelial Growth Factor Receptor-3 and Its Downstream Signaling Promote Cell Survival under Oxidative Stress

Reactive oxygen species (ROS) mediate cell damage and have been implicated in the pathogenesis of diseases that involve endothelial injury. Cells possess antioxidant systems, including intracellular antioxidants and ROS scavenging enzymes, that control the redox state and prevent cell damage. In addition to intracellular antioxidants, certain growth factor receptors can be activated under oxidative stress and trigger downstream cell survival signaling cascades. Vascular endothelial growth factor receptor-3 (VEGFR-3) is a primary modulator of lymphatic endothelial proliferation and survival. Here, we provide evidence that activation of VEGFR-3 signaling in response to hydrogen peroxide (H(2)O(2)) promotes endothelial cell survival. Treatment with H(2)O(2) induced the tyrosine phosphorylation of VEGFR-3 and its association with the signaling adaptor proteins Shc, growth factor receptor binding protein 2, Sos, p85, SHP-2, and phospholipase C-gamma. Of note, a hereditary lymphoedema-linked mutant of VEGFR-3 was not phosphorylated by H(2)O(2) treatment. Isoforms of protein kinase C (PKC), alpha and delta, were also tyrosine-phosphorylated after H(2)O(2) stimulation. However, only the delta isoform of PKC was required for H(2)O(2)-induced phosphorylation of VEGFR-3. The tyrosine phosphorylation of VEGFR-3 or isoforms of PKC was completely inhibited by treatment with 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine, a specific inhibitor for Src family kinases, indicating that Src family kinases are upstream of PKC and VEGFR-3. Furthermore, expression of the wild-type but not the lymphoedema-linked mutant form of VEGFR-3 in porcine artery endothelial cells significantly enhanced the activation of Akt after H(2)O(2) stimulation. Consistent with these biochemical changes, we observed that expression and activation of the wild-type but not the mutant form of VEGFR-3 inhibited H(2)O(2)-induced apoptosis. These studies suggest that VEGFR-3 protects against oxidative damage in endothelial cells, and that patients with hereditary lymphoedema may be susceptible to ROS-induced cell damage.

Protein kinase Cdelta selectively regulates protein kinase D-dependent activation of NF-kappaB in oxidative stress signaling

Protein kinase D (PKD) participates in activation of the transcription factor NF-kappaB (nuclear factor kappaB) in cells exposed to oxidative stress, leading to increased cellular survival. We previously demonstrated that phosphorylation of PKD at Tyr463 in the PH (pleckstrin homology) domain is mediated by the Src-Abl pathway and that it is necessary for PKD activation and subsequent NF-kappaB induction. Here we show that activation of PKD in response to oxidative stress requires two sequential signaling events, i.e., phosphorylation of Tyr463 by Abl, which in turn promotes a second step, phosphorylation of the PKD activation loop (Ser738/Ser742). We show that this is mediated by PKCdelta (protein kinase Cdelta), a kinase that is activated by Src in response to oxidative stress. We also show that other PKCs, including PKCepsilon and PKCzeta, do not participate in PKD activation or NF-kappaB induction. We propose a model in which two coordinated signaling events are required for PKD activation. Tyrosine phosphorylation in the PH domain at Tyr463, mediated by the Src-Abl pathway, which in turn facilitates the phosphorylation of Ser738/Ser742 in the activation loop, mediated by the Src-PKCdelta pathway. Once active, the signal is relayed to the activation of NF-kappaB in oxidative stress responses.