Regulation of androgen receptor stability by the β1 Pix/STUB1 complex

The androgen receptor (AR) is essential in the development and differentiation of testes and male genitalia. AR expression is tightly regulated at the translational and posttranslational levels. AR posttranscriptional regulation is a major determinant of AR availability since AR is a direct target of E3 ubiquitin ligase STUB1. Our work indicated that the Rac/Cdc42 guanosine triphosphatase guanine nucleotide exchange factor, β1 Pix, enhanced AR levels after AR stimulation in HEK293 and HeLa cells. AR stimulation decreased AR ubiquitination which is accompanied by increased β1 Pix binding to AR. Ectopic expression of β1 Pix decreased AR ubiquitination in Tm4 and HEK293 cells. We demonstrated that the formation of a multimolecular complex comprised of AR/β1 Pix/STUB1 responded in a time-dependent manner to AR stimulation. β1 Pix binding dissociated STUB1 from AR and thus prevented STUB1 from catalyzing receptor ubiquitination. β1 Pix enhanced AR transcriptional activity and increased AR target gene expression. Irrespective of treatment, immunofluorescence analysis showed a strong nuclear colocalization of endogenous AR and endogenous βPix in Tm4 cells. However, using Tm4 cell fractionation, AR stimulation decreased βPix/AR association in the cytosolic fraction and increased binding of AR to βPix in the nuclear fraction. To support the role of β1 Pix in androgen regulation, we found that individuals lacking this gene have a significant increase in genitourinary malformations associated with androgen dysfunction. Our data indicate that β1 Pix is an important modulator of AR stability and ligand-dependent AR transcriptional activity. We propose that β1 Pix could serve as a promising therapeutic target to modulate AR signaling.

Muscarinic receptor agonist-induced βPix binding to β-catenin promotes colon neoplasia

M3 muscarinic receptors (M3R) modulate β-catenin signaling and colon neoplasia. CDC42/RAC guanine nucleotide exchange factor, βPix, binds to β-catenin in colon cancer cells, augmenting β-catenin transcriptional activity. Using in silico, in vitro, and in vivo approaches, we explored whether these actions are regulated by M3R. At the invasive fronts of murine and human colon cancers, we detected co-localized nuclear expression of βPix and β-catenin in stem cells overexpressing M3R. Using immunohistochemistry, immunoprecipitation, proximity ligand, and fluorescent cell sorting assays in human tissues and established and primary human colon cancer cell cultures, we detected time-dependent M3R agonist-induced cytoplasmic and nuclear association of βPix with β-catenin. βPix knockdown attenuated M3R agonist-induced human colon cancer cell proliferation, migration, invasion, and expression of PTGS2, the gene encoding cyclooxygenase-2, a key player in colon neoplasia. Overexpressing βPix dose-dependently augmented β-catenin binding to the transcription factor TCF4. In a murine model of sporadic colon cancer, advanced neoplasia was attenuated in conditional knockout mice with intestinal epithelial cell deficiency of βPix. Expression levels of β-catenin target genes and proteins relevant to colon neoplasia, including c-Myc and Ptgs2, were reduced in colon tumors from βPix-deficient conditional knockout mice. Targeting the M3R/βPix/β-catenin axis may have therapeutic potential.

Abstract P420: Nano-Triciribine Reduces SARS2-CoV-2 Infection By Sequestering ACE2 And The Novel Host Factor LDLR

Background: People with previous CVD hospitalized for COVID-19 have elevated death rate. We reported that patients with diabetes and HF higher protein levels of the low density lipoprotein receptor (LDLR). We hypothesized that LDLR is a novel host factor for the SARS-CoV-2-Spike (S2S) protein that may be regulated by the Akt inhibitor Triciribine (TCN), a drug being tested in Phase III studies for breast cancer. We also hypothesized that nano-formulation of Triciribine (NanoTriciribine; NTCN) would enhance its efficacy and allow for intranasal delivery.

Methods: Interactions between the recombinant proteins Spike-RBD (receptor binding domain), ACE2, LDLR and its ectodomains (EGFA-EFFB, C2-C5 and C2) were analyzed by binding assays and co-IP in HepG2, HK2, and 293T cells. Viral entry assays were performed with 2 S2S pseudoviruses using 293T cells + hACE2 and TMPRSS2 or Furin protease. The effect of NTCN or the LXR agonist GW-3965 on viral uptake (pseudotyped VSVΔG-GFP*S2S or chimera VSV-S2S-eGFP virus) was assessed. Akt, pAkt, ACE2, and LDLR levels were determined in 293T+hACE2 by flow cytometry. Assays were done in triplicates and 1-way-ANOVA with Tukey’s correction was used for statistics.

Results: RBD protein binds modestly to the human LDLR (EC 50 :10μM) and its C2-C5 ectodomain (EC 50 :13.8μM). Co-IP revealed a novel and strong LDLR-ACE2 interaction in several human cell lines. LDLR overexpression in human cells increased the uptake of VSVΔG-GFP*S2S (FC=2.32;p<0.001) and chimera virus (FC=.33; p<.0001). NTCN and TCN each reduced pAkt/Akt ratio. 1μM TCN or NTCN reduced LDLR (7.2%;p<.01 & 15.6%;p<0.0001) and ACE2 (32%;p<0.05 & 44.7%;p<.01) cell surface expression, respectively. 1μM NTCN or GW-3965 reduced S2S viral entry by 64.2% (p<.0001) and 40.7% (p<.01), respectively, confirming a role for LDLR in S2S infection. In hACE2tg mice, chimera VSV-S2S caused significant lung infection as measured by qPCR, GFP expression in proximal and distal lung airway epithelial cells, and electron microscopy. Intranasal delivery of NTCN was well tolerated.

Conclusions: LDLR enhanced S2S viral entry supporting the elevated COVID-19 susceptibility seen in patients with heart disease. NTCN is a promising candidate for prophylactic treatment against COVID-19.

Abstract MP257: Dual Actions Of β 2 Ar-agonism Confer Protection Against Heart Failure And Renal Dysfunction Via Inotropic And Lusitropic Effects And Normalized Cholesterol Homeostasis In A Mouse Model Of Alport Syndrome

Background: Col4a3 -/- Alport mice present a model of heart failure with preserved ejection fraction (HFpEF) secondary to chronic kidney disease (CKD) wherein etiological relationships have been established between hypertension, pulmonary edema, inflammation, cardiac hypertrophy and fibrosis, diastolic dysfunction and underlying abnormalities of elevated low-density lipoprotein receptor (LDLR) expression, excess LDL-cholesterol (LDL-C) accumulation, and mitochondrial dysfunction in renal tubules. HFpEF is characteristically unresponsive to pharmacological intervention. Here, we tested the hypothesis that selective β 2 -Adrenoceptor (β 2 AR) modulation with salbutamol, a short-acting β 2 AR agonist, could alleviate symptoms of CKD and simultaneously augment cardiac function. Secondarily, we investigated the mechanism of actions of such β 2 AR-mediated therapeutics on cardiac and renal functions.

Methods: Alport mice were injected intraperitoneally with salbutamol or DMSO vehicle as a single bolus of 200μg/dose in short-term studies or daily with 100 μg/dose for 2 weeks long-term. Cardiac and renal functions, cAMP levels, in vivo renal tubular LDL-C uptake and renal histology were evaluated post-injection. In vitro mechanistic studies were performed in HK-2, Alport dog smooth muscle and tubular epithelial cells differentiated from Alport patient-derived iPSCs. Protein-protein interactions were studied using co-immunoprecipitation experiments and LDL-C uptake was measured by live-cell imaging.

Results: Short-term, salbutamol improved renal function in parallel with decreased LDLR levels and reduced uptake of LDL-C into renal tubules. Long-term, cardiac diastolic function assessed by isovolumetric relaxation time (IVRT), filling pressures (E/E’), and myocardial performance index, and systolic function reflected by ejection fraction, stroke volume and cardiac output improved significantly in parallel with increased cardiac cAMP. Mechanistically, in the kidney, salbutamol activated IDOL and hence lysosomal ubiquitination and degradation of LDLR via a novel β 2 AR-mediated, cAMP-independent pathway involving the Rac1/Cdc42 β 1 PixGEF. β 1 Pix reversibly sequesters IDOL into a complex with LDLR, thereby blocking the degradation pathway. β 2 AR stimulation dissipates the complex reactivating IDOL-mediated LDLR degradation thereby re-establishing LDL-C homeostasis and renal function. Using flow cytometry in HEK293T cells, ectopic expression of bPix stabilized membrane LDLR, sensitive to IDOL- but not PCSK-mediated degradation.

Conclusions: β 2 AR agonism represents a potential treatment strategy to alleviate progression of CKD and heart failure associated with HFpEF phenogroup 3.

Targeted intestinal deletion of Rho guanine nucleotide exchange factor 7, βPIX, impairs enterocyte proliferation, villus maturation, and mucosal defenses in mice

Rho guanine nucleotide exchange factors (RhoGEFs) regulate Rho GTPase activity and cytoskeletal and cell adhesion dynamics. βPix, a CDC42/RAC family RhoGEF encoded by ARHGEF7, is reported to modulate human colon cancer cell proliferation and postwounding restitution of rat intestinal epithelial monolayers. We hypothesized that βPix plays a role in maintaining intestinal epithelial homeostasis. To test this hypothesis, we examined βPix distribution in the human and murine intestine and created mice with intestinal epithelial-selective βPix deletion [βPixflox/flox/Tg(villin-Cre); Arhgef7 CKO mice]. Using Arhgef7 conditional knockout (CKO) and control mice, we investigated the consequences of βPix deficiency in vivo on intestinal epithelial and enteroid development, dextran sodium sulfate-induced mucosal injury, and gut permeability. In normal human and murine intestines, we observed diffuse cytoplasmic and moderate nuclear βPix immunostaining in enterocytes. Arhgef7 CKO mice were viable and fertile, with normal gross intestinal architecture but reduced small intestinal villus height, villus-to-crypt ratio, and goblet cells; small intestinal crypt cells had reduced Ki67 staining, compatible with impaired cell proliferation. Enteroids derived from control mouse small intestine were viable for more than 20 passages, but those from Arhgef7 CKO mice did not survive beyond 24 h despite addition of Wnt proteins or conditioned media from normal enteroids. Adding a Rho kinase (ROCK) inhibitor partially rescued CKO enteroid development. Compared with littermate control mice, dextran sodium sulfate-treated βPix-deficient mice lost more weight and had greater impairment of intestinal barrier function, and more severe colonic mucosal injury. These findings reveal βPix expression is important for enterocyte development, intestinal homeostasis, and resistance to toxic injury.NEW & NOTEWORTHY To explore the role of βPix, a guanine nucleotide exchange factor encoded by ARHGEF7, in intestinal development and physiology, we created mice with intestinal epithelial cell Arhgef7/βPix deficiency. We found βPix essential for normal small intestinal epithelial cell proliferation, villus development, and mucosal resistance to injury. Moreover, Rho kinase signaling mediated developmental arrest observed in enteroids derived from βPix-deficient small intestinal crypts. Our studies provide insights into the role Arhgef7/βPix plays in intestinal epithelial homeostasis.

Abstract 123: Osteopontin Regulates Adult Cardiomyocyte Division in a Mouse Model of Pressure Overload Induced Heart Failure

Background: Our previous work showed that pharmacological blockade of Osteopontin (OPN) signaling can prevent and reverse heart failure induced by pressure overload in a transverse aortic construction (TAC) mouse model. Surprisingly, OPN Knockout (KO) mice subjected to 3 month TAC had worse cardiac function and bigger hearts than wild type (WT) TAC mice, despite lack of cardiomyocyte hypertrophy. We hypothesized that OPN KO increased adult cardiomyocyte proliferation in TAC-induced heart failure.

Methods: Male C57Bl/6 (n=17) or OPN KO (n=11) mice were subject to TAC for 3 months. The protein levels of the mitosis marker H3P was quantified using immunofluorescence in paraffin-embedded myocardial sections. Myocytes were co-stained with WGA and MLC2 to count the number of myocytes per field. Adult primary cardiomyocytes from WT hearts were isolated and analyzed with co-imunoprecipitation (Co-IP) to study the interation between the regeneration factor YAP1, OPN and OPN-regulated proteins such as LDLR. For validation and mechanistic studies, more Co-IP experiments were performed in proliferative human liver HEPG2 cells. To study the effect of OPN blockade on YAP1 nuclear translocation, HEPG2 cells and human iPSC- derived cardiomyocytes (hiPS-CMs) were treated with an IgG or OPN blocking antibody for 24 hours followed by immunostaining for YAP1 and PITX2.

Results: Nuclear H3P normalized to myocyte count was significantly increased in OPN KO relative to WT TAC hearts (Fold Change = 1.4; p=0,04). Co-IP results revealed a novel interaction between OPN, LDLR and YAP1. Stimulation of β2 adrenergic receptor increased the formation of this multi-molecular complex in a time-dependent manner. Blockade of OPN by a monocolonal antibody for 24 hours caused nuclear localization of YAP1 and PITX2 in HEPG2 cells and hiPS-CMs.

Conclusion: OPN regulates the mitotic program in adult cardiomyocytes. Furthermore, the interaction of OPN with LDLR and YAP1 to form a new multi-molecular protein complex is regulated by β2-cAMP signaling pathway. Importantly, OPN regulates nuclear translocation of the regeneration factors YAP1 and PITX2, suggesting that OPN signaling may be important for adult cardiomyocyte division in TAC and potentially myocardial infarction and aging.

The Cdc42/Rac nucleotide exchange factor protein β1Pix (Pak-interacting exchange factor) modulates β-catenin transcriptional activity in colon cancer cells: evidence for direct interaction of β1PIX with β-catenin

Wnt/β-catenin signaling is highly regulated and critical for intestinal epithelial development and repair; aberrant β-catenin signaling is strongly associated with colon cancer. The small GTPase Rac1 regulates β-catenin nuclear translocation and signaling. Here we tested the hypothesis that β1Pix, a Cdc42/Rac guanine nucleotide exchange factor (GEF), regulates β-catenin-dependent transcriptional activity and cell function. We report the novel observations that β1Pix binds directly to β-catenin, an action requiring both the β1Pix DH and dimerization domains but not β1Pix GEF activity. In human colon cancer cells, activation of β-catenin signaling with LiCl decreased β1Pix/β-catenin association in the cytosol and increased nuclear binding of β-catenin to β1Pix. Nuclear association of β1Pix and β-catenin was independent of Rac1 expression and activation; down- and up-regulating Rac1 expression levels did not alter nuclear β1Pix/β-catenin association. Ectopic β1Pix expression enhanced LiCl-induced β-catenin transcriptional activity. Conversely, siRNA knockdown of β1Pix attenuated both LiCl-induced β-catenin transcriptional activity and colon cancer cell proliferation. Ectopic expression of β1Pix stimulated β-catenin transcriptional activity, whereas β1PixΔ(602-611), which is unable to bind β-catenin, had no effect. Altogether, these findings suggest that β1Pix functions as a transcriptional regulator of β-catenin signaling through direct interaction with β-catenin, an action that may be functionally relevant to colon cancer biology.

Muscarinic receptor agonists stimulate matrix metalloproteinase 1-dependent invasion of human colon cancer cells

Mammalian matrix metalloproteinases (MMPs) which degrade extracellular matrix facilitate colon cancer cell invasion into the bloodstream and extra-colonic tissues; in particular, MMP1 expression correlates strongly with advanced colon cancer stage, hematogenous metastasis and poor prognosis. Likewise, muscarinic receptor signaling plays an important role in colon cancer; muscarinic receptors are over-expressed in colon cancer compared to normal colon epithelial cells. Muscarinic receptor activation stimulates proliferation, migration and invasion of human colon cancer cells. In mouse intestinal neoplasia models genetic ablation of muscarinic receptors attenuates carcinogenesis. In the present work, we sought to link these observations by showing that MMP1 expression and activation plays a mechanistic role in muscarinic receptor agonist-induced colon cancer cell invasion. We show that acetylcholine, which robustly increases MMP1 expression, stimulates invasion of HT29 and H508 human colon cancer cells into human umbilical vein endothelial cell monolayers - this was abolished by pre-incubation with atropine, a non-selective muscarinic receptor inhibitor, and by pre-incubation with anti-MMP1 neutralizing antibody. Similar results were obtained using a Matrigel chamber assay and deoxycholyltaurine (DCT), an amidated dihydroxy bile acid associated with colon neoplasia in animal models and humans, and previously shown to interact functionally with muscarinic receptors. DCT treatment of human colon cancer cells resulted in time-dependent, 10-fold increased MMP1 expression, and DCT-induced cell invasion was also blocked by pre-treatment with anti-MMP1 antibody. This study contributes to understanding mechanisms underlying muscarinic receptor agonist-induced promotion of colon cancer and, more importantly, indicates that blocking MMP1 expression and activation has therapeutic promise to stop or retard colon cancer invasion and dissemination.

Muscarinic receptor agonists stimulate human colon cancer cell migration and invasion

Muscarinic receptors (CHRM) are overexpressed in colon cancer. To explore a role for muscarinic receptor signaling in colon cancer metastasis, we used human H508 and HT29 colon cancer cells that coexpress epidermal growth factor (ERBB) and CHRM3 receptors. In a wound closure model, following 8-h incubation of H508 cells with 100 μM ACh we observed a threefold increase in cell migration indistinguishable from the actions of epidermal growth factor (EGF). Atropine blocked the actions of ACh but not of EGF. In SNU-C4 colon cancer cells that express ERBB but not CHRM, EGF caused a threefold increase in migration; ACh had no effect. ACh-induced cell migration was attenuated by chemical inhibitors of ERBB1 activation, by anti-ERBB1 antibody, and by inhibitors of ERK and phosphatidylinositol 3-kinase (PI3K) signaling. Consistent with matrix metalloproteinase-7 (MMP7)-mediated release of an ERBB1 ligand, heparin binding epidermal growth factor-like growth factor (HBEGF), ACh-induced migration was inhibited by an MMP inhibitor and by anti-MMP7 and -HBEGF antibodies. ACh-induced cell migration was blocked by inhibiting RhoA and ROCK, key proteins that interact with the actin cytoskeleton. ACh-induced RhoA activation was attenuated by agents that inhibit ERBB1, ERK, and PI3K activation. Collectively, these findings indicate that ACh-induced cell migration is mediated by MMP7-mediated release of HBEGF, an ERBB ligand that activates ERBB1 and downstream ERK and PI3K signaling. In a cell invasion model, ACh-induced HT29 cell invasion was blocked by atropine. In concert with previous observations, these findings indicate that muscarinic receptor signaling plays a key role in colon cancer cell proliferation, survival, migration, and invasion.

Endothelin-1 inhibits the epithelial Na+ channel through betaPix/14-3-3/Nedd4-2

Epithelial Na+ channels (ENaCs) mediate sodium reabsorption in the cortical collecting duct (CCD), but the regulatory pathways that modulate the activity of these channels are incompletely understood. Here, we observed that endothelin-1 (ET-1) attenuates ENaC activity acutely by reducing the channel's open probability and chronically by decreasing the number of channels in the plasma membrane. To investigate whether beta1Pix, a signaling protein activated by ET-1, mediates ENaC activity, we reconstituted ENaC in CHO cells with or without coexpressed beta1Pix and found that beta1Pix negatively regulates ENaC. Knockdown of betaPix in native principal cells abolished the ET-1-induced decrease in ENaC channel number. Furthermore, we found that betaPix does not decrease ENaC activity through its guanine nucleotide exchange factor (GEF) activity for Rac1 and Cdc42. Instead, coexpression of beta1Pix mutant constructs revealed that beta1Pix affects ENaC activity through binding 14-3-3 proteins. Coimmunoprecipitation experiments supported a physical interaction between beta1Pix and 14-3-3beta in cultured principal cells. Coexpression of 14-3-3beta increased ENaC activity in CHO cells, but concomitant expression of beta1Pix attenuated this increase. Recruitment of 14-3-3beta by beta1Pix impaired the interaction of 14-3-3beta with the ubiquitin ligase Nedd4-2, thereby promoting ubiquitination and degradation of ENaC. Taken together, these results suggest that the inhibitory effects of chronic ET-1 on ENaC result from betaPix interacting with the 14-3-3/Nedd4-2 pathway.

Endothelin-1 induces p66Shc activation through EGF receptor transactivation: Role of beta(1)Pix/Galpha(i3) interaction

Endothelin-1 (ET-1) is a vasoconstrictor peptide known to be a potent mitogen for glomerular mesangial cells. We have shown that ET-1 stimulates the adaptor protein p66Shc through Rac/Cdc42 guanine nucleotide exchange factor beta(1)Pix. In this study, we demonstrate that ET-1-induced serine phosphorylation of p66Shc is mediated through Galpha(i3). Pertussis toxin treatment of cells induced a significant decrease in the interaction between beta(1)Pix and ET(A)-R, and an increase in the binding of Galpha(i3) and G(beta1) to beta(1)Pix. Activation of heterotrimeric G proteins by AlF(4)(-) resulted in an increase of Galpha(i3) binding to beta(1)Pix, which was significantly disrupted in cells expressing beta(1)Pix dimerization deficient mutant, beta(1)PixDelta (602-611). In cells expressing beta(1)PixDelta (602-611), ET-1-induced p66Shc activation was also significantly decreased. Specific inhibition of EGF receptor by AG1478 blocked ET-1-induced p66Shc activation and the binding of p66Shc and Galpha(i3) to beta(1)Pix. Inhibition of Erk1/2 blocked p66Shc activation induced by ET-1. Altogether, our results indicate that ET-1 activates p66Shc through EGF receptor transactivation, leading to the activation of Galpha(i3), beta(1)Pix and Erk1/2.

Endothelin-1 couples betaPix to p66Shc: role of betaPix in cell proliferation through FOXO3a phosphorylation and p27kip1 down-regulation independently of Akt

The phosphorylation of forkhead transcription factor FOXO3a by Akt is critical regulator of cell proliferation induced by serum. We show that endothelin-1 (ET-1) stimulation of primary human mesangial cells (HMCs) induces betaPix and p66Shc up-regulation, resulting in the formation of the betaPix/p66Shc complex. In transformed HMCs, ET-1 induces a biphasic phosphorylation of p66Shc and FOXO3a. The second phase leads to p27(kip1) down-regulation independently of Akt. Depletion of betaPix blocks the second phase of p66Shc and FOXO3a phosphorylation and prevents p27(kip1) down-regulation induced by ET-1. Depletion of either betaPix or p66Shc inhibits ET-1-induced cell proliferation. The expression of beta(1)Pix induces FOXO3a phosphorylation through activation of Rac1, ERK1/2, and p66Shc. Using either p66Shc- or Akt-depleted cells; we show that beta(1)Pix-induced FOXO3a phosphorylation requires p66Shc but not Akt. beta(1)Pix-induced p27(kip1) down-regulation was blocked by U0126 but not by wortmannin. Endogenous betaPix and FOXO3a are constitutively associated with endogenous p66Shc. FOXO3a and p66Shc binding requires beta(1)Pix homodimerization. Expression of beta(1)Pix homodimerization deficient mutant abrogates beta(1)Pix-induced p27(kip1) down-regulation and cell proliferation. Our results identify p66Shc and FOXO3a as novel partners of beta(1)Pix and represent the first direct evidence of beta(1)Pix in cell proliferation via Erk/p66Shc-dependent and Akt-independent mechanisms.

Endothelin 1 stimulates beta1Pix-dependent activation of Cdc42 through the G(salpha) pathway

Beta1Pix (PAK-interacting exchange factor) is a recently identified guanine nucleotide exchange factor (GEF) for the Rho family small G protein Cdc42/Rac. On stimulation with extracellular signals, GEFs induce the exchange of guanosine diphosphate to guanosine triphosphate, resulting in the activation of the small guanosine 5C-triphosphatases. This activation enables the signal to propagate to downstream effectors. Herein, we show that G(salpha) stimulation by cholera toxin increased Cdc42 activation by endothelin-1 (ET-1), whereas pertussis toxin had no effect. H-89, a protein kinase A (PKA) inhibitor, strongly inhibited Cdc42 activation by ET-1. Moreover, the overexpression of beta1Pix enhanced ET-1-induced Cdc42 activation. The essential role of beta1Pix in ET-1-induced Cdc42 activation was evidenced by the blocking of Cdc42 activation in cells expressing beta1Pix mutant lacking the ability to bind PAK (beta1Pix SH3m[W43K]) or mutant lacking GEF activity (beta1PixdeltaDH). The overexpression of mutant lacking the pleckstrin homology domain beta1PixdeltaPH, which is unable to bind phospholipids, had no effect on Cdc42 activation. These results demonstrate that beta1Pix, along with PKA, plays a crucial role in the regulation of Cdc42 activation by ET-1.

Endothelin 1 induces beta 1Pix translocation and Cdc42 activation via protein kinase A-dependent pathway

p21-activated kinase (Pak)-interacting exchange factor (Pix), a Rho family guanine nucleotide exchange factor (GEF), has been shown to co-localize with Pak and form activated Cdc42- and Rac1-driven focal complexes. In this study we have presented evidence that treatment of human mesangial cells (HMC) with endothelin 1 (ET-1) and stimulation of adenylate cyclase with either forskolin or with the cAMP analog 8-Br-cAMP activated the GTP loading of Cdc42. Transient expression of constitutively active G alpha(s) also stimulated Cdc42. In addition, overexpression of beta(1)Pix enhanced ET-1-induced Cdc42 activation, whereas the expression of beta(1)Pix SH3m(W43K), which lacks the ability to bind Pak, and beta(1)PixDHm(L238R/L239S), which lacks GEF activity, decreased ET-1-induced Cdc42 activation. Furthermore, ET-1 stimulation induced beta(1)Pix translocation to focal complexes. Interestingly, pretreatment of HMC with protein kinase A (PKA) inhibitors blocked both Cdc42 activation and beta(1)Pix translocation induced by ET-1, indicating the involvement of the PKA pathway. Through site-directed mutagenesis studies of consensus PKA phosphorylation sites and in vitro PKA kinase assay, we have shown that beta(1)Pix is phosphorylated by PKA. Using purified recombinant beta(1)Pix(wt) and beta(1)Pix mutants, we have identified Ser-516 and Thr-526 as the major phosphorylation sites by PKA. beta(1)Pix(S516A/T526A), in which both phosphorylation sites are replaced by alanine, blocks beta(1)Pix translocation and Cdc42 activation. Our results have provided evidence that stimulation of PKA pathway by ET-1 or cAMP analog results in beta(1)Pix phosphorylation, which in turn controls beta(1)Pix translocation to focal complexes and Cdc42 activation.

Activation of RBL-2H3 mast cells is dependent on tyrosine phosphorylation of phospholipase D2 by Fyn and Fgr

Both phospholipase D1 (PLD1) and PLD2 regulate degranulation when RBL-2H3 cells are stimulated via the immunoglobulin E receptor, Fc epsilon RI. However, the activation mechanism for PLD2 is unclear. As reported here, PLD2 but not PLD1 is phosphorylated through the Src kinases, Fyn and Fgr, and this phosphorylation appears to regulate PLD2 activation and degranulation. For example, only hemagglutinin-tagged PLD2 was tyrosine phosphorylated in antigen-stimulated cells that had been made to express HA-PLD1 and HA-PLD2. This phosphorylation was blocked by a Src kinase inhibitor or by small interfering RNAs directed against Fyn and Fgr and was enhanced by overexpression of Fyn and Fgr but not by other Src kinases. The phosphorylation and activity of PLD2 were further enhanced by the tyrosine phosphatase inhibitor, Na(3)VO(4). Mutation of PLD2 at tyrosines 11, 14, 165, or 470 partially impaired, and mutation of all tyrosines blocked, PLD2 phosphorylation and activation, although two of these mutations were detrimental to PLD2 function. PLD2 phosphorylation preceded degranulation, both events were equally sensitive to inhibition of Src kinase activity, and both were enhanced by coexpression of PLD2 and the Src kinases. The findings provide the first description of a mechanism for activation of PLD2 in a physiological setting and of a role for Fgr in Fc epsilon RI-mediated signaling.

The Rac/Cdc42 guanine nucleotide exchange factor beta1Pix enhances mastoparan-activated Gi-dependent pathway in mast cells

Carbachol stimulates granule exocytosis, phospholipase C (PLC), and phospholipase D (PLD) in RBL-2H3hm1 mast cells by a mechanism that involves Galphaq. However, mastoparan stimulates the same responses through Gi protein. Both Gi and Galphaq pathways are suppressed by Clostridium difficile toxin B, suggesting that Rac and Cdc42 small GTPases are also involved. Over-expression of beta1Pix, a guanine nucleotide exchange factor for Rac and Cdc42, enhances mastoparan-but not carbachol-induced hexosaminidase secretion and PLC and PLD activation. Furthermore, cells expressing beta1Pix exhibit elevated levels of mastoparan-stimulated IP3 production. Taken together, these findings implicate beta1Pix in regulating hexoasaminidase secretion and IP3 production in early stage upon mastoparan stimulation.

Mastoparan selectively activates phospholipase D2 in cell membranes

Both known isoforms of phospholipase (PL) D, PLD1 and PLD2, require phosphatidylinositol 4,5-bisphosphate for activity. However, PLD2 is fully active in the presence of this phospholipid, whereas PLD1 activation is dependent on additional factors such as ADP-ribosylation factor-1 (ARF-1) and protein kinase Calpha. We find that mastoparan, an activator of G(i) and mast cells, stimulates an intrinsic PLD activity, most likely PLD2, in fractions enriched in plasma membranes from rat basophilic leukemia 2H3 mast cells. Overexpression of PLD2, but not of PLD1, results in a large increase in the mastoparan-inducible PLD activity in membrane fractions, particularly those enriched in plasma membranes. As in previous studies, expressed PLD2 is localized primarily in the plasma membrane and PLD1 in granule membranes. Studies with pertussis toxin and other agents indicate that mastoparan stimulates PLD2 independently of G(i), ARF-1, protein kinase C, and calcium. Kinetic studies indicate that mastoparan interacts synergistically with phosphatidylinositol 4,5-bisphosphate and that oleate, itself a weak stimulant of PLD2 at low concentrations, is a competitive inhibitor of mastoparan stimulation of PLD2. Therefore, mastoparan may be useful for investigating the regulation of PLD2, particularly in view of the well studied molecular interactions of mastoparan with certain other strategic signaling proteins.

Serine/threonine protein kinases synergistically regulate phospholipase D1 and 2 and secretion in RBL-2H3 mast cells

The role of phospholipase (PL) D in secretion was examined in RBL-2H3 mast cells which contain both PLD1 and 2. The effects of pharmacologic stimulants and inhibitors of Ca(2+)/calmodulin-dependent kinase II, protein kinase C, and protein kinase A suggested that all three kinases synergistically stimulate PLD and, when associated with a calcium signal, secretion as well to indicate a possible linkage between these two events. Overexpression of either PLD1 or 2 markedly enhanced the activation of PLD by pharmacologic stimulants as well as antigen and both isoforms thus appear co-ordinately regulated. As the expressed PLD1 was associated with secretory granules and PLD2 with the plasma membrane, the two isoforms may serve distinct but complementary functions in secretion.

Regulation of phospholipase D and secretion in mast cells by protein kinase A and other protein kinases

Functions attributed to phospholipase (PL) D include the regulation of intracellular trafficking of Golgi-derived vesicles and secretion of granules from mast cells. We have reported that activation of PLD and secretion in a rat mast cell (RBL-2H3) line is substantially enhanced by cholera toxin, a known activator of protein kinase (PK) A. Here we review the evidence that (1) the synergistic interactions of cholera toxin and other pharmacological agents on mast cell secretion are attributable to the synergistic activation of PLD via PKA, CaM kinase II, and PKC and (2) both PLD1 and PLD2 participate in this process. For example, treatment with cholera toxin, thapsigargin, and phorbol 12-myristate 13-acetate (which activate PKA, CaM kinase II, and PKC, respectively) exhibit synergy in the stimulation of both PLD and secretion. These kinases and PLD are likely confined to membrane components, as similar synergistic interactions could be demonstrated in permeabilized cells. The regulation of PLD and secretion by these kinases is also apparent from studies of inhibitors of PKA and other kinases. Also, by overexpression of either PLD1 or PLD2 it is apparent that both isoforms respond to the same stimuli as endogenous PLD, although PLD1 is largely associated with secretory granules and PLD2 with plasma membrane. The studies reveal interesting differences in the regulation of the translocation of granules (regulated by PKA) and the fusion of these granules with the plasma membrane (regulated by Ca(2+) and PKC). The pathological/physiological implications of the regulation of PLD by PKA require further evaluation in other cell systems.

Compound 48/80 activates mast cell phospholipase D via heterotrimeric GTP-binding proteins

Previous studies have indicated the presence of a cholera toxin-sensitive phospholipase D (PLD) in cultured RBL-2H3 mast cells that is synergistically activated via calcium, protein kinase C, and another unidentified signal. Here we identify a third potential signal for activation transduced by a pertussis toxin-sensitive trimeric GTP-binding protein, most likely via G(i2) or G(i3). Quercetin-treated RBL-2H3 cells in which expression of G(alphai2) and G(alphai3) is enhanced more than 7-fold respond to the G(i) stimulant compound 48/80 with the activation of PLD, a transient activation of phospholipase C, and enhanced membrane GTPase activity. The activation of PLD was blocked in pertussis toxin-treated cells and, as with other stimulants of PLD, was enhanced in cholera toxin-treated cells. The PLD response to compound 48/80 was only partially inhibited by calcium deprivation and inhibition of protein kinase C to indicate a component of the response that was independent of calcium, protein kinase C, and, presumably, phospholipase C. Based on these and other data, we hypothesized that betagamma-subunits, released from G(i2) or G(i3) by compound 48/80 or from G(s) by cholera toxin, provide an additional signal for the activation of PLD. Consistent with this hypothesis, recombinant G(beta2gamma2) subunits, but not G(alphai-3) subunits, at concentrations of 50 to 300 nM markedly synergized PLD activation by compound 48/80 in permeabilized RBL-2H3 cells.

Inhibition of GTPase activity of Gi proteins and decreased agonist affinity at M2 muscarinic acetylcholine receptors by spermine and methoctramine

1. The effects of spermine and methoctramine, a selective M2 muscarinic receptor antagonist, were studied on the high-affinity GTPase activity of G proteins, and on ligand binding to M2 muscarinic receptors in pig heart sarcolemma. 2. The spontaneous GTP hydrolysis by pig heart sarcolemma and its stimulation by mastoparan or carbachol were prevented by pertussis toxin and inhibited by methoctramine (IC50s: 21, 13 and 0.005 microM, respectively), and spermine (IC50s: 967, 278 and 11 microM). Spermine and methoctramine also inhibited spontaneous GTP hydrolysis by rat peritoneal mast cell membranes which do not respond to carbachol. 3. The neutral muscarinic antagonists, AF-DX 116 and atropine, did not modify the inhibitory effect of high concentrations of methoctramine, indicating that this effect was not related to the antagonist binding site of muscarinic receptors. We suggest that methoctramine behaves as a receptor antagonist at nanomolar concentrations and interacts with G proteins at micromolar concentrations. 4. Spermine did not modify the binding of the tritiated muscarinic antagonist [3H]-NMS, but decreased the binding of the agonist [3H]-Oxo-M. Spermine elicited a rightward shift of the carbachol/[3H]-NMS binding isotherm with a decrease in the proportion of sites with high-affinity for carbachol, suggesting that polyamines uncouple Gi proteins from receptors. 5. The inhibition of GTPase activity by polyamines, preventing the re-association of alpha and betagamma subunits of Gi proteins, might sustain the regulatory effect of Gi subunits on downstream effectors. The level of intracellular polyamines might be important for the control of the transduction of extracellular signals through Gi protein-coupled receptors.

Drugs interacting with G protein alpha subunits: selectivity and perspectives

Extracellular signal molecules as diverse as hormones, neurotransmitters and photons use a signal transduction pathway involving a receptor, a G protein and effectors. Compounds that interact directly with G proteins can mimic the receptor-G protein interaction or can block the activation of G proteins by receptors. Several binding sites exist on the G alpha protein that may be exploited for the design of synthetic stimulatory or inhibitory ligands. The effector binding site is regulated by endogenous proteins and appears to be a target for selective exogenous ligands. The GTP binding site presents a large homology within the G protein families and therefore the nucleotide analogs might not be considered as a tool to discriminate between the G protein subclasses. In contrast, different experimental strategies have substantiated the specificity in the interaction between a receptor and a G protein, the receptor binding site of G proteins should be considered as potential drug targets. Drugs interfering with this site such as mastoparan and related peptides, GPAnt-2 and suramin, are lead compounds in the design of selective G protein antagonists. Benzalkonium chloride and methoctramine have agonist or antagonist properties, depending on G protein subtypes. Such compounds would be very useful to delineate the functions of G proteins and G protein-coupled receptors, to understand some side effects of drugs used in therapy and to develop new therapeutic agents.

The M2 muscarinic receptor antagonist methoctramine activates mast cells via pertussis toxin-sensitive G proteins

Methoctramine, a selective M2 muscarinic cholinergic receptor antagonist, has been reported to activate phosphoinositide breakdown at high concentrations. Its polyamine structure suggests a putative activation of guanine nucleotide-binding proteins (G proteins). Incubation of methoctramine with rat peritoneal mast cells resulted in a dose-dependent noncytotoxic histamine release, with an EC50 of 20 microM and a maximum effect at 1 mM. Atropine, pirenzepine and HHSiD neither inhibited methoctramine-induced histamine release nor stimulated histamine release. Histamine release and inositol phosphates generation induced by methoctramine were both inhibited by pertussis toxin pretreatment. Benzalkonium chloride, a selective inhibitor of histamine secretion induced by basic secretagogues, inhibited the secretory response to methoctramine. [p-Glu5, D-Trp7,9,l0]-SPs5-11 (GPAnt-2), a well-characterized antagonist of G proteins, blocked the methoctramine-induced histamine release when the antagonist was allowed to reach its intracellular target by streptolysin O-permeabilization. The response to methoctramine was prevented by the hydrolysis of sialic acid residues of the cell surface by neuraminidase. The response of mast cells was restored by permeabilization of the plasma membrane. These results demonstrate that methoctramine, following its entry into the cell and the involvement of pertussis toxin-sensitive G proteins, activates phosphoinositide hydrolysis leading to mast cell exocytosis.

Substance P-related inhibitors of mast cell exocytosis act on G-proteins or on the cell surface

[p-Glu5,D-Trp(7,9,10)]substance P-(5-11) inhibited mastoparan-stimulated GTPase activity in homogenized rat peritoneal mast cells and decreased histamine secretion induced by mastoparan from streptolysin O-permeabilized mast cells (IC50 of about 30 microM), but not from intact cells. In contrast, [D-Pro4,D-Trp(7,9,10)]substance P-(4-11) inhibited the secretion from intact cells (IC50 of about 10 microM) but had no effect on histamine secretion from permeabilized cells, suggesting that this peptide exerts its inhibitory effect on the plasma membrane, whereas [p-Glu5,D-Trp(7,9,10)]substance P-(5-11) interacts with G proteins. Pretreatment of mast cells with neuraminidase led to an inhibition of the secretory response to mastoparan and related triggers. This response was restored following cell permeabilization, demonstrating the role of the cell surface on the entry of mastoparan and related triggers and on their ability to reach G proteins sensitive to pertussis toxin and [p-Glu5,D-Trp(7,9,10)]substance P-(5-11).