Receptor regulation of phosphoinositidase C

An immobilized metal ion affinity adsorption and scintillation proximity assay for receptor-stimulated phosphoinositide hydrolysis

A novel approach to measuring receptor-stimulated phosphoinositide hydrolysis was developed based on the principles of immobilized metal ion affinity chromatography (IMAC) and scintillation proximity assay (SPA). Hard Lewis metal ions, such as Zr(4+), Ga(3+), Al(3+), Fe(3+), Lu(3+), and Sc(3+), were immobilized on SPA beads via metal chelate and utilized as affinity ligands to entrap inositol phosphates. [3H]Inositol phosphates bound to IMAC-SPA beads through the strong interaction of their phosphate group with the immobilized metal ions. The binding brought [3H]inositol phosphates in close proximity to the scintillant embedded in the SPA beads, thereby allowing the radioactivity to be quantified. Quantification of [3H]inositol phosphate production in cells preincubated with [3H]inositol provided a highly sensitive measurement of phosphoinositide hydrolysis. The utility of this approach was demonstrated in measuring the response mediated by the G-protein-coupled neurokinin NK1 receptor and the tyrosine kinase-linked platelet-derived growth factor (PDGF) receptor. Substance P stimulated phosphoinositide hydrolysis concentration-dependently in CHO cells expressing NK1 receptors with a maximal 12-fold increase in inositol phosphate production. Similarly, PDGF-BB stimulated a 5-fold increase in phosphoinositide hydrolysis in quiescent Swiss 3T3 cells. This new approach is highly sensitive, fast, simple, easily performed on 96-well plates, and amenable for high-throughput screening.

The direct interaction of phospholipase C-gamma 1 with phospholipase D2 is important for epidermal growth factor signaling

The epidermal growth factor (EGF) receptor has an important role in cellular proliferation, and the enzymatic activity of phospholipase C (PLC)-gamma1 is regarded to be critical for EGF-induced mitogenesis. In this study, we report for the first time a phospholipase complex composed of PLC-gamma1 and phospholipase D2 (PLD2). PLC-gamma1 is co-immunoprecipitated with PLD2 in COS-7 cells. The results of in vitro binding analysis and co-immunoprecipitation analysis in COS-7 cells show that the Src homology (SH) 3 domain of PLC-gamma1 binds to the proline-rich motif within the Phox homology (PX) domain of PLD2. The interaction between PLC-gamma1 and PLD2 is EGF stimulation-dependent and potentiates EGF-induced inositol 1,4,5-trisphosphate (IP(3)) formation and Ca(2+) increase. Mutating Pro-145 and Pro-148 within the PX domain of PLD2 to leucines disrupts the interaction between PLC-gamma1 and PLD2 and fails to potentiate EGF-induced IP(3) formation and Ca(2+) increase. However, neither PLD2 wild type nor PLD2 mutant affects the EGF-induced tyrosine phosphorylation of PLC-gamma1. These findings suggest that, upon EGF stimulation, PLC-gamma1 directly interacts with PLD2 and this interaction is important for PLC-gamma1 activity.

Localization of phospholipase C-gamma1 signaling in caveolae: importance in EGF-induced phosphoinositide hydrolysis but not in tyrosine phosphorylation

Upon epidermal growth factor treatment, phospholipase C-gamma1 (PLC-gamma1) translocates from cytosol to membrane where it is phosphorylated at tyrosine residues. Caveolae are small plasma membrane invaginations whose structural protein is caveolin. In this study, we show that the translocation of PLC-gamma1 and its tyrosine phosphorylation are localized in caveolae by caveolin-enriched low-density membrane (CM) preparation and immunostaining of cells. Pretreatment of cells with methyl-beta-cyclodextrin (MbetaCD), a chemical disrupting caveolae structure, inhibits the translocation of PLC-gamma1 to CM as well as phosphatidylinositol (PtdIns) turnover. However, MbetaCD shows no effect on tyrosine phosphorylation level of PLC-gamma1. Our findings suggest that, for proper signaling, PLC-gamma1 phosphorylation has to occur at PtdInsP(2)-enriched sites.

Activation of phospholipase C-beta1 via Galphaq/11 during calcium mobilization by calcitonin gene-related peptide

Interaction of calcitonin gene-related peptide (CGRP) with its receptors leads to stimulation of adenylyl cyclase and/or phospholipase C (PLC). While regulation of adenylyl cyclase is thought to involve the G-protein Gs, it is not known whether activation of PLC results from coupling the receptor to Gq family proteins or whether beta gamma subunits released from receptor-activated Gs activate PLC. We used human bone cells OHS-4 bearing CGRP receptors in which CGRP activates only the PLC signaling pathway to determine how CGRP acts. CGRP increased the concentration of intracellular calcium ([Ca2+]i) within 5 s via a Ca2+ influx through voltage-gated calcium channels and by mobilizing calcium from the endoplasmic reticulum. The activation of effectors, like PLC coupled to G-proteins, is the early event in the pathway leading to inositol 1,4,5-trisphosphate formation, which is responsible for Ca2+ mobilization. Western blotting demonstrated a range of PLC-beta isoforms (beta1, beta3, beta4, but not beta2) and G-proteins (Galphaq/11 and Galphas). Only phospholipase C-beta1 is involved in the mobilization of Ca2+ from the endoplasmic reticulum of Fura-2-loaded confluent OHS-4 cells and the formation of inositol 1,4,5-trisphosphate by CGRP; PLC-gamma have no effect. Activation of PLC-beta1 by CGRP involves the Galphaq/11 subunit, which is insensitive to pertussis toxin, but not Gbeta gamma subunits. We therefore believe that CGRP causes the activation of two separate G-proteins.

Phospholipase C beta and membrane action of calcitriol and estradiol

We have shown that estrogens and calcitriol, the hormonally active form of vitamin D, increase the concentration of intracellular calcium ([Ca2+]i) within 5 s by mobilizing calcium from the endoplasmic reticulum and the formation of inositol 1,4, 5-trisphosphate and diacylglycerol. Because the activation of effectors as phospholipase C (PLC) coupled to G-proteins is the early event in the signal transduction pathway leading to the inositol 1,4,5-trisphosphate formation and to [Ca2+]i increase, we described different PLC isoforms (beta1, beta2, gamma1, and gamma2, but not beta4) in female rat osteoblasts using Western immunoblotting. The data showed that phospholipase C beta was involved in the mobilization of Ca2+ from the endoplasmic reticulum of Fura-2-loaded confluent osteoblasts by calcitriol and 17beta estradiol, and PLC gamma was ineffective. The data also showed that only a PLC beta1 linked to a Pertussis toxin-insensitive G-protein and a PLC beta2 coupled to a Pertussis toxin-sensitive G-protein are involved in the effects of calcitriol and 17beta estradiol on the mobilization of Ca2+ from intracellular Ca2+ stores. In conclusion, these results may be an important step toward understanding membrane effects of these steroids and may be an additional argument in favor of membrane receptors to steroid hormones.

Concentration of enzyme-dependent activation of PLC-beta 1 and PLC-beta 2 by G alpha 11 and beta gamma-subunits

Differential regulation of PLC-beta 1 and -beta 2 by the G-protein alpha-subunit, G alpha 11, and by G-protein beta gamma-subunits was studied utilizing recombinant PLC-beta 1 and -beta 2. Rat PLC-beta 1 and human PLC-beta 2 were purified after recombinant baculovirus-mediated expression in Sf9 cells. The catalytic properties of the purified recombinant isoenzymes were directly compared to PLC-beta 1 purified from bovine brain and PLC-beta 2 partially purified from HL60 polymorphonuclear neutrophils. The recombinant isoenzymes were indistinguishable from the native isoenzymes with respect to dependence of reaction velocity on bulk PtdIns(4,5)P2 substrate concentration, pH, and free Ca2+ concentration. Marked AlF(4-)-dependent activation was observed upon reconstitution of rPLC-beta 1 with the G-protein alpha-subunit, G alpha 11. Activation occurred with a concentration dependence on G alpha 11 for activation and elevation in reaction velocity that was similar to that of native PLC-beta 1. In contrast, G alpha 11 promoted only a small elevation in the catalytic rate of recombinant PLC-beta 2, which was also typical of the native isoenzyme. Maximal reaction rates with respect to PLC-beta isoenzyme concentration were achieved and indicated that rPLC-beta 2 required 10-fold greater concentrations of both G alpha 11 and of rPLC-beta 2 for activation than did rPLC-beta 1. rPLC-beta 1 and rPLC-beta 2 were also differentially regulated by beta gamma-subunits. This differential activation was not the result of different concentration dependencies on beta gamma-subunit for activation, but rather, the result of the greater degree to which the catalytic rate of PLC-beta 2 was elevated by beta gamma-subunits when compared to PLC-beta 1.

Phosphoinositide hydrolysis by phospholipase C modulated by multivalent cations La(3+), Al(3+), neomycin, polyamines, and melittin

Second messenger production from phosphoinositide hydrolysis is regulated by different pathways, such as G-proteins or tyrosine phosphorylation of phosphoinositide phospholipase C (PI-PLC). Another means of altering the activity of PI-PLC is through cation interaction with the phosphoinositide substrate. A variety of organic and inorganic multi-valent cations were examined for their effects on the activity of purified PI-PLC delta. Surprisingly, the cations produced both stimulation and inhibition of PI-PLC catalyzed phosphoinositide hydrolysis, depending on the substrate and the ion to phosphoinositide stoichiometry. These data support the hypothesis that ionic complexes with phosphoinositides may alter their hydrolysis by PI-PLC.

Regulation of cell proliferation and growth by angiotensin II

The peptide hormone angiotensin II (AngII) has clearly defined physiologic roles as a regulator of vasomotor tone and fluid homeostasis. In addition AngII has trophic or mitogenic effects on a variety of target tissues, including vascular smooth muscle and adrenal cells. More recent data indicate that AngII exhibits many characteristics of the 'classical' peptide growth factors such as EGF/TGF alpha, PDGF and IGF-1. These include the capacity for local generation ('autocrine or paracrine' action) and the ability to stimulate tyrosine phosphorylation, to activate MAP kinases and to increase expression of nuclear proto-oncogenes. The type 1 AngII receptor, which is responsible for all known physiologic actions of AngII, has been cloned. Activation of this receptor leads to elevated phosphoinositide hydrolysis, mobilization of intracellular Ca2+ and diacylglycerol, and activation of Ca2+/calmodulin and Ca2+/phospholipid-dependent Ser/Thr kinases, as well as Ca2+ regulated tyrosine kinases. The existence of other AngII receptor subtypes has been postulated, but the function(s) of these sites remains unclear. In vascular smooth muscle, AngII can promote cellular hypertrophy and/or hyperplasia, depending in part on the patterns of induction of secondary factors that are known to stimulate (PDGF, IGF-1, basic FGF) or inhibit (TGF-beta) mitosis. Together, these findings have suggested that AngII plays important roles in both the normal development and pathophysiology of vascular, cardiac, renal and central nervous system tissues.

G proteins in aortic endothelial cells and bradykinin-induced formation of nitric oxide

In bovine aortic endothelial cells (BAEC), pertussis toxin (PTx) ADP-ribosylated two major substrates with apparent molecular masses of 40 and 41 kDa, whereas cholera toxin (CTx) ADP-ribosylated two other substrates of 44 and 50 kDa. [alpha-32P]GTP bound to three bands in the 22-27 kDa range. Immunoblot analysis revealed the simultaneous presence of G alpha i1, G alpha i2, G alpha i3, G alpha q or G alpha 11 and of different forms of G alpha s but did not detect significant levels of G alpha 0. Bradykinin caused a 9-fold increase in intracellular cyclic GMP level in BAEC (measured as an index of NO production). Preincubation of BAEC with CTx, but not with PTx, inhibited bradykinin-dependent production of cyclic GMP. These results show that G alpha s, G alpha q or alpha 11, Gi and small GTP-binding proteins are present in BAEC and suggest that a CTx-sensitive G-protein (possibly either small G-protein, G alpha q or G alpha 11) could be associated with the bradykinin-mediated NO formation.

PI-specific phospholipase C "alpha" from sheep seminal vesicles is a proteolytic fragment of PI-PLC delta

Phosphatidylinositide-specific phospholipase C enzymes (PLCs) catalyze the conversion of the phosphoinositides to biologically important signal transducing molecules. These enzymes may be grouped into "families" which share similar structures and modes of regulation. The existence of a structurally distinct family of PLC termed "alpha" has been recently called into question. In the current paper we show by immunoblotting experiments that PLC "alpha" from sheep seminal vesicles is recognized by monoclonal antibodies raised against the delta 1 isoform of bovine brain PLC, and appears to be derived from a higher molecular weight band at 85 kDa. We also show that antibodies raised against PLC alpha efficiently immunoprecipitate the 85-kDa PLC delta 1 isoform from bovine brain and Chinese hamster lung fibroblasts. These data provide strong evidence that the PLC alpha from sheep seminal vesicles is a proteolytic fragment of PLC delta 1. Thus, there is still no conclusive evidence for a separate "alpha" class of PLC.

Calcium-dependent increase in tyrosine kinase activity stimulated by angiotensin II

The cellular effects of numerous hormones and neurotransmitters, including the vasoactive agents angiotensin II (AngII) and [Arg8]vasopressin, are mediated in part by protein-serine threonine kinases activated by increase of cytosolic Ca2+ concentration. In this study, we have tested the ability of Ca(2+)-mobilizing agents to activate cellular tyrosine kinases. Treatment of intact GN4 liver epithelial cells with AngII rapidly (less than or equal to 15 sec) increased tyrosine kinase activity measured either in unfractionated cell lysates or in anti-phosphotyrosine immune complexes from detergent-solubilized cells. Increased phosphorylation of the exogenous substrate poly(Glu80Tyr20) (3- to 4-fold over control) by immunoprecipitated kinases closely paralleled the time- and dose-dependence of the appearance of tyrosine phosphoproteins in intact cells. This effect of AngII was mimicked by thapsigargin, a Ca(2+)-elevating tumor promoter. The ability of AngII, but not epidermal growth factor, to increase tyrosine kinase activity was blocked in cells loaded with the Ca2+ chelator bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid. Dephosphorylation of immunoprecipitated proteins by tyrosine phosphatase treatment was accompanied by a 60-70% loss in in vitro kinase activity, suggesting that the AngII-sensitive kinase(s) are activated by phosphorylation in intact cells. These findings demonstrate a link between two widely occurring signaling pathways, the tyrosine kinases and the Ca2+ second-messenger system, and suggest the possible involvement of Ca(2+)-activated tyrosine kinases in the endocrine actions of AngII and [Arg8]vasopressin.

In vivo formation of polyphosphoinositide isomers and association with progression of murine polycystic kidney disease

Polyphosphoinositide isomers have been demonstrated to be important mediators of cell proliferation in vitro. The present study demonstrates, for the first time, the in vivo formation of the novel isomer, phosphatidylinositol(3)phosphate, in the kidney and liver of intact animals following intraperitoneal administration of [3H]myo-inositol. The formation of renal [3H]phosphatidylinositol(3)phosphate relative to total [3H]phosphatidylinositol-phosphate was positively correlated with cyst proliferation and renal enlargement in a murine model of polycystic kidney disease. Furthermore, despite no difference in the formation of renal [3H]phosphatidylinositol(4)phosphate, a markedly lower accumulation (by 48%) of [3H]phosphatidylinositol(4,5)bisphosphate was observed in the diseased animals as compared to controls. These results indicate that further studies on the in vivo formation of specific polyphosphoinositide isomers in disease states characterized by abnormal growth and oncogene expression are warranted.

Parathyroid hormone stimulates protein kinase C but not adenylate cyclase in mouse epidermal keratinocytes

Intact human parathyroid hormone, hPTH [1-84], and the hPTH [1-34] fragment stimulated membrane-associated protein kinase C (PKC) activity in immortalized (but still differentiation-competent) murine BALB/MK-2 skin keratinocytes. Unexpectedly, the hormone and its fragment did not stimulate adenylate cyclase. The failure of PTH to stimulate adenylate cyclase activity was not due to the lack of a functioning receptor-cyclase coupling mechanism because the cells were stimulated to synthesize cyclic adenosine monophosphate (cyclic AMP) by the beta-adrenergic drug isoproterenol. Thus, skin keratinocytes seem to have an unconventional PTH receptor that is coupled to a PKC-activating mechanism but not to adenylate cyclase. These observations suggest that normal and neoplastic skin keratinocytes respond to the PTH-related peptide that they make and secrete.

Heparin and other polyanions uncouple alpha 1-adrenoceptors from G-proteins

Several polyanionic compounds antagonize the interaction between receptors and the G-proteins that regulate adenylate cyclase or K+ channels, possibly by binding to a basic stretch of the receptor that is proposed to mediate its interaction with the G-proteins. We have studied the effects of polyanions on the interaction between the liver alpha 1-adrenoceptor and the G-protein through which it stimulates polyphosphoinositide turnover. Heparin [concn. causing 50% of maximal effect (EC50) = 0.5 microM], Trypan Blue (EC50 7.1 microM) or suramin (EC50 2.1 microM) prevented formation of the high-affinity adrenaline-receptor-G-protein complex without affecting antagonist binding. After alkaline treatment of the membranes, previously reported to cause G-protein removal, binding of agonists was insensitive to both guanine nucleotides and heparin. We conclude that these polyanions uncouple the alpha 1-adrenoceptor from its G-protein, suggesting that similar coupling mechanisms may underlie receptor activation of the G-proteins that activate polyphosphoinositide hydrolysis and those that regulate adenylate cyclase. This action of heparin severely limits its utility as a selective antagonist of the Ins(1,4,5)P3 receptor in intact cells.

Phospholipase C-beta 1 is regulated by a pertussis toxin-insensitive G-protein

Regulation of phospholipase C (PLC) by receptors is mediated either through protein tyrosine phosphorylation or by activation of GTP-binding proteins (Gp). For the latter, pertussis toxin (PT)-sensitive and -insensitive pathways have been described, indicating PLC regulation by at least two types of G-proteins. The identity of PLC isoenzymes which are regulated by either type of Gp remains to be determined. Thyrotropin-releasing hormone stimulates a PLC in GH3 cells via a PT-insensitive Gp. Reconstitution methods for the assay of the GH3-cell Gp were developed. Previously, the membrane PLC was found to be reversibly extracted from membranes by high salt and to be activated by guanosine 5'-[gamma-thio]triphosphate (GTP[S]) only when membrane-associated, suggesting that Gp was retained in salt-extracted membranes. In the present work, Gp was cholate-solubilized from PLC-deficient membranes and incorporated into phospholipid vesicles, which were found to confer GTP[S]- and AlF4(-)-stimulated activity on a solubilized membrane PLC. The reconstitution provided a direct assay for the GH3-cell Gp which was shown to be distinct from Gi, Go and Gs proteins by immunodepletion studies. Incorporation of G-protein beta-gamma subunits into phospholipid vesicles with Gp inhibited GTP[S]-stimulated activity in the reconstitution. The results indicated that Gp is a heterotrimeric G-protein with the properties expected for the PT-insensitive GH3-cell Gp protein. PLC-beta 1 was fully purified and shown to be regulated by Gp in the reconstitution. In contrast, PT-sensitive G-proteins failed to affect the activity of PLC-beta 1. The results indicate (1) that a PT-insensitive Gp regulates PLC-beta 1 and (2) that PT-sensitive and -insensitive pathways of PLC regulation employ different PLC isoenzymes as well as different G-proteins.