PDZ domain-containing 1 (PDZK1) protein regulates phospholipase C-ß3 (PLC-ß3)-specific activation of somatostatin by forming a ternary complex with PLC-ß3 and somatostatin receptors.

Phospholipase C-ß (PLC-ß) is a key molecule in G protein-coupled receptor (GPCR)-mediated signaling. Many studies have shown that the four PLC-ß subtypes have different physiological functions despite their similar structures. Because the PLC-ß subtypes possess different PDZ-binding motifs, they have the potential to interact with different PDZ proteins. In this study, we identified PDZ domain-containing 1 (PDZK1) as a PDZ protein that specifically interacts with PLC-ß3. To elucidate the functional roles of PDZK1, we next screened for potential interacting proteins of PDZK1 and identified the somatostatin receptors (SSTRs) as another protein that interacts with PDZK1. Through these interactions, PDZK1 assembles as a ternary complex with PLC-ß3 and SSTRs. Interestingly, the expression of PDZK1 and PLC-ß3, but not PLC-ß1, markedly potentiated SST-induced PLC activation. However, disruption of the ternary complex inhibited SST-induced PLC activation, which suggests that PDZK1-mediated complex formation is required for the specific activation of PLC-ß3 by SST. Consistent with this observation, the knockdown of PDZK1 or PLC-ß3, but not that of PLC-ß1, significantly inhibited SST-induced intracellular Ca(2+) mobilization, which further attenuated subsequent ERK1/2 phosphorylation. Taken together, our results strongly suggest that the formation of a complex between SSTRs, PDZK1, and PLC-ß3 is essential for the specific activation of PLC-ß3 and the subsequent physiologic responses by SST.

Activation of AMP-activated protein kinase is essential for lysophosphatidic acid-induced cell migration in ovarian cancer cells.

Lysophosphatidic acid (LPA) is a bioactive phospholipid that affects various biological functions, such as cell proliferation, migration, and survival, through LPA receptors. Among them, the motility of cancer cells is an especially important activity for invasion and metastasis. Recently, AMP-activated protein kinase (AMPK), an energy-sensing kinase, was shown to regulate cell migration. However, the specific role of AMPK in cancer cell migration is unknown. The present study investigated whether LPA could induce AMPK activation and whether this process was associated with cell migration in ovarian cancer cells. We found that LPA led to a striking increase in AMPK phosphorylation in pathways involving the phospholipase C-ß3 (PLC-ß3) and calcium/calmodulin-dependent protein kinase kinase ß (CaMKKß) in SKOV3 ovarian cancer cells. siRNA-mediated knockdown of AMPKα1, PLC-ß3, or (CaMKKß) impaired the stimulatory effects of LPA on cell migration. Furthermore, we found that knockdown of AMPKα1 abrogated LPA-induced activation of the small GTPase RhoA and ezrin/radixin/moesin proteins regulating membrane dynamics as membrane-cytoskeleton linkers. In ovarian cancer xenograft models, knockdown of AMPK significantly decreased peritoneal dissemination and lung metastasis. Taken together, our results suggest that activation of AMPK by LPA induces cell migration through the signaling pathway to cytoskeletal dynamics and increases tumor metastasis in ovarian cancer.

Phospholipase C-η1 is activated by intracellular Ca(2+) mobilization and enhances GPCRs/PLC/Ca(2+) signaling.

Phospholipase C-η1 (PLC-η1) is the most recently identified PLC isotype and is primarily expressed in nerve tissue. However, its functional role is unclear. In the present study, we report for the first time that PLC-η1 acts as a signal amplifier in G protein-coupled receptor (GPCR)-mediated PLC and Ca(2+) signaling. Short-hairpin RNA (shRNA)-mediated knockdown of endogenous PLC-η1 reduced lysophosphatidic acid (LPA)-, bradykinin (BK)-, and PACAP-induced PLC activity in mouse neuroblastoma Neuro2A (N2A) cells, indicating that PLC-η1 participates in GPCR-mediated PLC activation. Interestingly, ionomycin-induced PLC activity was significantly decreased by PLC-η1, but not PLC-η2, knockdown. In addition, we found that intracellular Ca(2+) source is enough for PLC-η1 activation. Furthermore, the IP(3) receptor inhibitor, 2-APB, inhibited LPA-induced PLC activity in control N2A cells, whereas this effect was not observed in PLC-η1 knockdown N2A cells, suggesting a pivotal role of intracellular Ca(2+) mobilization in PLC-η1 activation. Finally, we found that LPA-induced ERK1/2 phosphorylation and expression of the downstream target gene, krox-24, were significantly decreased by PLC-η1 knockdown, and these knockdown effects were abolished by 2-APB. Taken together, our results strongly suggest that PLC-η1 is activated via intracellular Ca(2+) mobilization from the ER, and therefore amplifies GPCR-mediated signaling.

Subtype-specific role of phospholipase C-beta in bradykinin and LPA signaling through differential binding of different PDZ scaffold proteins.

Among phospholipase C (PLC) isozymes (beta, gamma, delta, epsilon, zeta and eta), PLC-beta plays a key role in G-protein coupled receptor (GPCR)-mediated signaling. PLC-beta subtypes are often overlapped in their distribution, but have unique knock-out phenotypes in organism, suggesting that each subtype may have the different role even within the same type of cells. In this study, we examined the possibility of the differential coupling of each PLC-beta subtype to GPCRs, and explored the molecular mechanism underlying the specificity. Firstly, we found that PLC-beta1 and PLC-beta 3 are activated by bradykinin (BK) or lysophosphatidic acid (LPA), respectively. BK-triggered phosphoinositides hydrolysis and subsequent Ca(2+) mobilization were abolished specifically by PLC-beta1 silencing, whereas LPA-triggered events were by PLC-beta 3 silencing. Secondly, we showed the evidence that PDZ scaffold proteins is a key mediator for the selective coupling between PLC-beta subtype and GPCR. We found PAR-3 mediates physical interaction between PLC-beta1 and BK receptor, while NHERF2 does between PLC-beta 3 and LPA(2) receptor. Consistently, the silencing of PAR-3 or NHERF2 blunted PLC signaling induced by BK or LPA respectively. Taken together, these data suggest that each subtype of PLC-beta is selectively coupled to GPCR via PDZ scaffold proteins in given cell types and plays differential role in the signaling of various GPCRs.

A myristoylated pseudosubstrate peptide of PKC-zeta induces degranulation in HMC-1 cells independently of PKC-zeta activity.

Mast cells play a central role in allergic disease and host defense against several pathogens through the release of various bioactive compounds via degranulation. In this study, we found that a myristoylated pseudosubstrate of PKC-zeta (zeta-PS; myristoyl-SIYRRGARRWRKL, a PKC-zeta inhibitor) regulates mast cell degranulation. zeta-PS increased [Ca+2]i level at nanomolar concentrations in a PKC-zeta activity-independent manner in HMC-1 cells. Moreover, zeta-PS-induced [Ca+2]i generation was completely abrogated by phospholipase C (PLC), IP3 receptor or Galpha i/o inhibitor and zeta-PS potently induced degranulation in HMC-1 cells which was significantly inhibited by pretreating PLC inhibitors or a calcium chelator. Therefore, our results suggest that zeta-PS can induce degranulation in HMC-1 cells by triggering the calcium signal via a PKC-zeta-independent but Galpha i/o, PLC and IP3-dependent pathways.

Inhibition of phospholipase C-beta1-mediated signaling by O-GlcNAc modification.

Here we report inhibition of phospholipase C-beta1 (PLC-beta1)-mediated signaling by post-translational glycosylation with beta-N-acetylglucosamine (O-GlcNAc modification). In C2C12 myoblasts, isoform-specific knock-down experiments using siRNA showed that activation of bradykinin (BK) receptor led to stimulation of PLC-beta1 and subsequent intracellular Ca2+ mobilization. In C2C12 myotubes, O-GlcNAc modification of PLC-beta1 was markedly enhanced in response to treatment with glucosamine (GlcNH2), an inhibitor of O-GlcNAase (PUGNAc) and hyperglycemia. This was associated with more than 50% inhibition of intracellular production of IP3 and Ca2+ mobilization in response to BK. Since the abundance of PLC-beta1 remained unchanged, these data suggest that O-GlcNAc modification of PLC-beta1 led to inhibition of its activity. Moreover, glucose uptake stimulated by BK was significantly blunted by treatment with PUGNAc. These data support the notion that O-GlcNAc modification negatively modulates the activity of PLC-beta1.

Phospholipase C-beta3 mediates the thrombin-induced Ca2+ response in glial cells.

Phospholipase C-beta (PLC-beta) hydrolyses phosphatidylinositol 4,5-bisphosphate and generates inositol 1,4,5-trisphosphate in response to activation of various G protein-coupled receptors (GPCRs). Using glial cells from knock-out mice lacking either PLC-beta1 [PLC-beta1 (-/-)] or PLC-beta3 [PLC-beta3 (-/-)], we examined which isotype of PLC-beta participated in the cellular signaling events triggered by thrombin. Generation of inositol phosphates (IPs) was enhanced by thrombin in PLC-beta1 (-/-) cells, but was negligible in PLC-beta3 (-/-) cells. Expression of PLC-beta3 in PLC-beta3 (-/-) cells resulted in an increase in pertussis toxin (PTx)-sensitive IPs in response to thrombin as well as to PAR1-specific peptide, while expression of PLC-beta1 in PLC-beta1 (-/-) cells did not have any effect on IP generation. The thrombin-induced [Ca2+]i increase was delayed and attenuated in PLC-beta3 (-/-) cells, but normal in PLC-beta1 (-/-) cells. Pertussis toxin evoked a delayed [Ca2+]i increase in PLC-beta3 (-/-) cells as well as in PLC-beta1 (-/-) cells. These results suggest that activation of PLC-beta3 by pertussis toxin-sensitive G proteins is responsible for the transient [Ca2+]i increase in response to thrombin, whereas the delayed [Ca2+]i increase may be due to activation of some other PLC, such as PLC-beta4, acting via PTx-insensitive G proteins.

Lysophosphatidic acid induces exocytic trafficking of Na(+)/H(+) exchanger 3 by E3KARP-dependent activation of phospholipase C.

Lysophosphatidic acid (LPA) stimulates Na(+)/H(+) exchanger 3 (NHE3) activity in opossum kidney proximal tubule (OK) cells by increasing the apical membrane amount of NHE3. This occurs by stimulation of exocytic trafficking of NHE3 to the apical plasma membrane by an E3KARP-dependent mechanism. However, it is still unclear how E3KARP leads to the LPA-induced exocytosis of NHE3. In the current study, we demonstrate that stable expression of exogenous E3KARP increases LPA-induced phospholipase C (PLC) activation and subsequent elevation of intracellular Ca(2+) in opossum kidney proximal tubule (OK) cells. Pretreatment with U73122, a PLC inhibitor, prevented the LPA-induced NHE3 activation and the exocytic trafficking of NHE3. To understand how the elevation of intracellular Ca(2+) leads to the stimulation of NHE3, we pretreated OK cells with BAPTA-AM, an intracellular Ca(2+) chelator. BAPTA-AM completely blocked the LPA-induced increase of NHE3 activity and surface NHE3 amount by decreasing the LPA-induced exocytic trafficking of NHE3. Pretreatment with GF109203X, a PKC inhibitor, did not affect the percent of LPA-induced NHE3 activation and increase of surface NHE3 amount. From these results, we suggest that E3KARP plays a necessary role in LPA-induced PLC activation, and that PLC-dependent elevation of intracellular Ca(2+) but not PKC activation is necessary for the LPA-induced increase of NHE3 exocytosis.

NHERF2 specifically interacts with LPA2 receptor and defines the specificity and efficiency of receptor-mediated phospholipase C-beta3 activation.

Lysophosphatidic acid (LPA) activates a family of cognate G protein-coupled receptors and is involved in various pathophysiological processes. However, it is not clearly understood how these LPA receptors are specifically coupled to their downstream signaling molecules. This study found that LPA(2), but not the other LPA receptor isoforms, specifically interacts with Na(+)/H(+) exchanger regulatory factor2 (NHERF2). In addition, the interaction between them requires the C-terminal PDZ domain-binding motif of LPA(2) and the second PDZ domain of NHERF2. Moreover, the stable expression of NHERF2 potentiated LPA-induced phospholipase C-beta (PLC-beta) activation, which was markedly attenuated by either a mutation in the PDZ-binding motif of LPA(2) or by the gene silencing of NHERF2. Using its second PDZ domain, NHERF2 was found to indirectly link LPA(2) to PLC-beta3 to form a complex, and the other PLC-beta isozymes were not included in the protein complex. Consistently, LPA(2)-mediated PLC-beta activation was specifically inhibited by the gene silencing of PLC-beta3. In addition, NHERF2 increases LPA-induced ERK activation, which is followed by cyclooxygenase-2 induction via a PLC-dependent pathway. Overall, the results suggest that a ternary complex composed of LPA(2), NHERF2, and PLC-beta3 may play a key role in the LPA(2)-mediated PLC-beta signaling pathway.

Ca2+-dependent inhibition of NHE3 requires PKC alpha which binds to E3KARP to decrease surface NHE3 containing plasma membrane complexes.

The intestinal brush border (BB) Na+/H+ exchanger isoform 3 (NHE3) is acutely inhibited by elevation in the concentration of free intracellular Ca2+ ([Ca2+]i) by the cholinergic agonist carbachol and Ca2+ ionophores in a protein kinase C (PKC)-dependent manner. We previously showed that elevating [Ca2+]i with ionomycin rapidly inhibited NHE3 activity and decreased the amount of NHE3 on the plasma membrane in a manner that depended on the presence of the PDZ domain-containing protein E3KARP (NHE3 kinase A regulatory protein, also called NHERF2). The current studies were performed in PS120 fibroblasts (NHE-null cell line) stably transfected with NHE3 and E3KARP to probe the mechanism of PKC involvement in Ca2+ regulation of NHE3. Pretreatment with the general PKC inhibitor, GF109203X prevented ionomycin inhibition of NHE3 without altering basal NHE3 activity. Similarly, the Ca2+-mediated inhibition of NHE3 activity was blocked after pretreatment with the conventional PKC inhibitor Gö-6976 and a specific PKCalpha pseudosubstrate-derived inhibitor peptide. [Ca2+]i elevation caused translocation of PKCalpha from cytosol to membrane. PKCalpha bound to the PDZ1 domain of GST-E3KARP in vitro in a Ca2+-dependent manner. PKCalpha and E3KARP coimmunoprecipitated from cell lysates; this occurred to a lesser extent at basal [Ca2+]i and was increased with ionomycin exposure. Biotinylation studies demonstrated that [Ca2+]i elevation induced oligomerization of NHE3 in total lysates and decreased the amount of plasma membrane NHE3. Treatment with PKC inhibitors did not affect the oligomerization of NHE3 but did prevent the decrease in surface amount of NHE3. These results suggest that PKCalpha is not necessary for the Ca2+-dependent formation of the NHE3 plasma membrane complex, although it is necessary for decreasing the membrane amounts of NHE3, probably by stimulating NHE3 endocytosis.

Lysophosphatidic acid stimulates brush border Na+/H+ exchanger 3 (NHE3) activity by increasing its exocytosis by an NHE3 kinase A regulatory protein-dependent mechanism.

Na(+)/H(+) exchanger 3 (NHE3) kinase A regulatory protein (E3KARP) has been implicated in cAMP- and Ca(2+)-dependent inhibition of NHE3. In the current study, a new role of E3KARP is demonstrated in the stimulation of NHE3 activity. Lysophosphatidic acid (LPA) is a mediator of the restitution phase of inflammation but has not been studied for effects on sodium absorption. LPA has no effect on NHE3 activity in opossum kidney (OK) proximal tubule cells, which lack expression of endogenous E3KARP. However, in OK cells exogenously expressing E3KARP, LPA stimulated NHE3 activity. Consistent with the stimulatory effect on NHE3 activity, LPA treatment increased the surface NHE3 amount, which occurred by accelerating exocytic trafficking (endocytic recycling) to the apical plasma membrane. These LPA effects only occurred in OK cells transfected with E3KARP. The LPA-induced increases of NHE3 activity, surface NHE3 amounts, and exocytosis were completely inhibited by pretreatment with the PI 3-kinase inhibitor, LY294002. LPA stimulation of the phosphorylation of Akt was used as an assay for PI 3-kinase activity. LY294002 completely prevented the LPA-induced increase in Akt phosphorylation, which is consistent with the inhibitory effect of LY294002 on the LPA stimulation of NHE3 activity. The LPA-induced phosphorylation of Akt was the same in OK cells with and without E3KARP. These results show that LPA stimulates NHE3 in the apical surface of OK cells by a mechanism that is dependent on both E3KARP and PI 3-kinase. This is the first demonstration that rapid stimulation of NHE3 activity is dependent on an apical membrane PDZ domain protein.