Src kinase mediates the regulation of phospholipase C-gamma activity by glycosphingolipids

Sphingolipids and glycerophospholipids - The "ying and yang" of lipotoxicity in metabolic diseases

Sphingolipids in general and ceramides in particular, contribute to pathophysiological mechanisms by modifying signalling and metabolic pathways. Here, we present the available evidence for a bidirectional homeostatic crosstalk between sphingolipids and glycerophospholipids, whose dysregulation contributes to lipotoxicity induced metabolic stress. The initial evidence for this crosstalk originates from simulated models designed to investigate the biophysical properties of sphingolipids in plasma membrane representations. In this review, we reinterpret some of the original findings and conceptualise them as a sort of "ying/yang" interaction model of opposed/complementary forces, which is consistent with the current knowledge of lipid homeostasis and pathophysiology. We also propose that the dysregulation of the balance between sphingolipids and glycerophospholipids results in a lipotoxic insult relevant in the pathophysiology of common metabolic diseases, typically characterised by their increased ceramide/sphingosine pools.

Eliglustat tartrate, a prototypic glucosylceramide synthase inhibitor

Eliglustat tartrate is a highly specific inhibitor of glucosylceramide synthase, developed for the treatment glucosylceramide-based glycosphingolipidoses. Eliglustat is in late clinical development for Gaucher disease type 1. Phase II and III clinical trials have demonstrated clinical efficacy for eliglustat as a stand-alone agent for newly diagnosed patients that are naïve to prior therapy and for patients who have been previously treated with enzyme replacement therapy. Importantly, the reported toxicity of eliglustat has been limited. Eliglustat will be submitted for the US FDA and EMA review in late 2013. Several structurally unrelated glucosylceramide synthase inhibitors have been identified and are in various stages of development, some of which cross the blood-brain barrier. Targeting glucosylceramide synthesis is also a promising approach for the treatment of type 2 diabetes mellitus, autosomal dominant polycystic kidney disease and certain cancers.

Property-based design of a glucosylceramide synthase inhibitor that reduces glucosylceramide in the brain

Synthesis inhibition is the basis for the treatment of type 1 Gaucher disease by the glucosylceramide synthase (GCS) inhibitor eliglustat tartrate. However, the extended use of eliglustat and related compounds for the treatment of glycosphingolipid storage diseases with CNS manifestations is limited by the lack of brain penetration of this drug. Property modeling around the D-threo-1-phenyl-2-decanoylamino-3-morpholino-propanol (PDMP) pharmacophore was employed in a search for compounds of comparable activity against the GCS but lacking P-glycoprotein (MDR1) recognition. Modifications of the carboxamide N-acyl group were made to lower total polar surface area and rotatable bond number. Compounds were screened for inhibition of GCS in crude enzyme and whole cell assays and for MDR1 substrate recognition. One analog, 2-(2,3-dihydro-1H-inden-2-yl)-N-((1R,2R)-1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-1-hydroxy-3-(pyrrolidin-1-yl)propan-2-yl)acetamide (CCG-203586), was identified that inhibited GCS at low nanomolar concentrations with little to no apparent recognition by MDR1. Intraperitoneal administration of this compound to mice for 3 days resulted in a significant dose dependent decrease in brain glucosylceramide content, an effect not seen in mice dosed in parallel with eliglustat tartrate.

Evidence for calreticulin attenuation of cardiac hypertrophy induced by pressure overload and soluble agonists

While calreticulin has been shown to be critical for cardiac development, its role in cardiac pathology is unclear. Previous studies have shown the detrimental effects on the heart of sustained germline calreticulin overexpression, yet without calreticulin, the heart does not develop normally. Thus, carefully balanced calreticulin levels are required for the heart to develop and to function properly into adulthood. But what happens to calreticulin levels, and how is this regulated, during cardiac hypertrophy, during which the fetal gene program is reactivated, at least partially? Our working hypothesis was that c-Src, a kinase whose activity we previously found to be correlated with calreticulin expression, was involved with calreticulin in regulating the response to hypertrophic signals. Thus, we subjected adult mice to transverse aortic constriction to induce left ventricular hypertrophy. We found that aortic constriction caused calreticulin levels to increase, whereas those of c-Src fell with longer constriction time. We also examined the ability of embryonic stem cell-derived cardiomyocytes to respond to soluble hypertrophic agonists. Endothelin-1 treatment caused a significantly greater cell area increase of calreticulin-null cardiomyocytes, which had higher c-Src activity, compared with wild-type cells. c-Src inhibition abolished this difference. Greater c-Src activity may explain the efficacy with which calreticulin-null cells are able to induce the hypertrophic program, while cells containing calreticulin may be able to attenuate the hypertrophic response as a result of decreased c-Src activity. Thus, calreticulin may have a protective effect on the heart in the face of cardiac hypertrophy.

Decreased nitric oxide bioavailability in a mouse model of Fabry disease

Fabry disease is a lysosomal storage disorder that results in an accumulation of globotriaosylceramide in vascular tissue secondary to a deficiency in alpha-galactosidase A. The glycolipid-associated vasculopathy results in strokes and cardiac disease, but the basis for these complications is poorly understood. Recent studies in the alpha-galactosidase A-knockout mouse suggested that a decrease in nitric oxide (NO) bioavailability may play a role in the abnormal thrombosis, atherogenesis, and vasorelaxation that are characteristic of these mice. To understand better the association between impaired NO bioavailability and glycolipid accumulation, we studied alpha-galactosidase A-knockout mice or primary cultures of their aortic endothelial cells. Treatment of knockout mice with a potent inhibitor of glucosylceramide synthase reversed accumulation of globotriaosylceramide but failed to normalize the defect in vasorelaxation. Basal and insulin-stimulated endothelial NO synthase (eNOS) activities in endothelial cells derived from knockout mice were lower than those observed from wild-type mice; normalization of glycolipid only partially reversed this reduction in eNOS activity. The loss of eNOS activity associated with a decrease in high molecular weight caveolin oligomers in endothelial cells and isolated caveolae, suggesting a role for glycolipids in caveolin assembly. Finally, concentrations of ortho-tyrosine and nitrotyrosine in knockout endothelial cells were markedly elevated compared with wild-type endothelial cells. These findings are consistent with a loss of NO bioavailability, associated with eNOS uncoupling, in the alpha-galactosidase A-knockout mouse.

Specific glycosphingolipids mediate epithelial-to-mesenchymal transition of human and mouse epithelial cell lines

Epithelial-to-mesenchymal cell transition (EMT) is a basic process in embryonic development and cancer progression. The present study demonstrates involvement of glycosphingolipids (GSLs) in the EMT process by using normal murine mammary gland NMuMG, human normal bladder HCV29, and human mammary carcinoma MCF7 cells. Treatment of these cells with D-threo-1-(3',4'-ethylenedioxy)phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol (EtDO-P4), the glucosylceramide (GlcCer) synthase inhibitor, which depletes all GSLs derived from GlcCer, (i) down-regulated expression of a major epithelial cell marker, E-cadherin; (ii) up-regulated expression of mesenchymal cell markers vimentin, fibronectin, and N-cadherin; (iii) enhanced haptotactic cell motility; and (iv) converted epithelial to fibroblastic morphology. These changes also were induced in these cell lines with TGF-beta, which is a well-documented EMT inducer. A close association between specific GSL changes and EMT processes induced by EtDO-P4 or TGF-beta is indicated by the following findings: (i) The enhanced cell motility of EtDO-P4-treated cells was abrogated by exogenous addition of GM2 or Gg4, but not GM1 or GM3, in all 3 cell lines. (ii) TGF-beta treatment caused changes in the GSL composition of cells. Notably, Gg4 or GM2 was depleted or reduced in NMuMG, and GM2 was reduced in HCV29. (iii) Exogenous addition of Gg4 inhibited TGF-beta-induced changes of morphology, motility, and levels of epithelial and mesenchymal markers. These observations indicate that specific GSLs play key roles in defining phenotypes associated with EMT and its reverse process (i.e., mesenchymal-to-epithelial transition).

Differential involvement of COX1 and COX2 in the vasculopathy associated with the alpha-galactosidase A-knockout mouse

The lysosomal storage disorder Fabry disease is characterized by excessive globotriaosylceramide (Gb3) accumulation in major organs such as the heart and kidney. Defective lysosomal alpha-galactosidase A (Gla) is responsible for excessive Gb3 accumulation, and one cell sensitive to the effects of Gb3 accumulation is vascular endothelium. Endothelial dysfunction is associated with Fabry disease and excessive cellular Gb3. We previously demonstrated that excessive vascular Gb3 in a mouse model of Fabry disease, the Gla-knockout (Gla(-/0)) mouse, results in abnormal vascular function, which includes abnormal endothelium-dependent contractions, a vascular phenomenon known to involve cyclooxygenase (COX). Therefore, we hypothesized that the vasculopathy in the Gla knockout mouse may be due to a vasoactive COX-derived product. To test this hypothesis, vascular reactivity experiments were performed in aortic rings from wild-type (Gla(+/0)) and Gla(-/0) mice in the presence and absence of specific and nonspecific COX inhibitors. Specific inhibition of COX1 or COX2 in endothelium-intact rings from Gla(-/0) mice decreased overall phenylephrine contractility compared with untreated Gla(-/0) rings, whereas COX inhibitors had no effect on contractility in endothelium-denuded rings. Nonspecific inhibition of COX with indomethacin (10 micromol/l) or COX1 inhibition with valeryl salicylate (3 mmol/l) improved endothelial function in rings from Gla(-/0) mice, but COX2 inhibition with NS-398 (1 micromol/l) further increased endothelial dysfunction in rings from Gla(-/0) mice. These results suggest that, in the Gla(-/0) mice, COX1 and COX2 activity are increased and localized in the endothelium, producing vasopressor and vasorelaxant products, which contribute to the Fabry-related vasculopathy.

Vascular dysfunction in the alpha-galactosidase A-knockout mouse is an endothelial cell-, plasma membrane-based defect

Fabry disease results from an X-linked mutation in the lysosomal alpha-galactosidase A (Gla) gene. Defective Gla results in multi-organ accumulation of neutral glycosphingolipids (GSLs), especially in the vascular endothelium, with the major GSL accumulated being globotriaosylceramide (Gb3). Excessive endothelial Gb3 accumulation is associated with increased thrombosis, atherogenesis and endothelial dysfunction. However, the mechanism(s) by which endothelial dysfunction occurs is unclear. The purpose of the present study was to further characterize the vasculopathy associated with a murine model of Fabry disease. Vascular reactivity was performed in vessels from wild-type (Gla(+/0)) and Gla-knockout (Gla(-/0)) mice. Conscious blood pressure and heart rate were measured in Gla(+/0) and Gla(-/0) mice by telemetry. The present study demonstrates that vascular smooth muscle (VSM) contractions to phenylephrine and serotonin, but not to U46619, were blunted in Gla(-/0) mice. Endothelium-dependent contraction and receptor-mediated endothelium-dependent relaxation to acetylcholine were significantly attenuated in vessels from Gla(-/0) mice. However, receptor-independent endothelium-dependent relaxation to the calcium ionophore ionomycin remained intact in vessels from Gla(-/0) mice. Furthermore, VSM reactivity was normal in aortas from Gla(-/0) mice in the absence of endothelium. These changes in vascular function were observed without changes in whole-animal blood pressure or heart rate. These results suggest that the vasculopathy associated with Fabry disease is localized to the endothelium, despite the accumulation of GSLs throughout the vasculature.

Mapping mechanisms and charting the time course of premature cell senescence and apoptosis: lysosomal dysfunction and ganglioside accumulation in endothelial cells

Endothelial cells subjected to glycated collagen I develop premature senescence within 3-5 days, as revealed by increased senescence-associated beta-galactosidase activity, decreased proliferation, and an increase in cell size. Here, we analyzed the time course and possible mechanisms of this process. Lysosomal integrity studies revealed a rapid collapse of pH gradient and lysosomal permeabilization, detectable after 30 min, and preceded by the increased production of reactive oxygen species. Measurement of mitochondrial membrane potential after application of glycated collagen demonstrated that depolarization was delayed by 4 h compared with changes in lysosomal pH and permeability. Based on the above findings of lysosomal permeabilization, we hypothesized that the reduced activity of senescence-associated beta-galactosidase could be responsible for the cellular accumulation of gangliosides, previously shown to induce cell senescence. After 5 days of exposure to glycated collagen, there was an increase in the levels of gangliosides GM3, GD1b, and GT1b, coincident with development of cell senescence. Treatment of endothelial cells with d-threo-EtDOP4, an inhibitor of glucosylceramide synthase, inhibited apoptosis, but not the development of senescence. In conclusion, collagen I modified by advanced glycation initially induces apoptosis of human umbilical vein endothelial cells. This process is initiated by the collapse of lysosomal pH and an increase in lysosomal permeability, with the subsequent mitochondrial depolarization and accumulation of gangliosides. Blockade of ganglioside synthesis suppresses apoptosis, but not senescence, which develops after 3 days of exposure to glycated collagen. These data imply a critical role for lysosomal permeabilization in triggering apoptosis of endothelial cells exposed to the diabetic milieu.

Caveolin-associated Accumulation of Globotriaosylceramide in the Vascular Endothelium of α-Galactosidase A Null Mice

Cardiovascular complications, including stroke and myocardial infarction, result in premature mortality in patients with Fabry disease, an X-linked deficiency of alpha-galactosidase A (alpha-Gal A). The enzymatic defect results in the deposition of globotriaosylceramide (Gb3) in the vascular endothelium. To better understand the underlying pathogenesis of Fabry disease, the caveolar lipid content of primary cultured mouse aortic endothelial cells isolated from alpha-Gal A null mice was measured. Lipid mass analysis revealed that the excessive Gb3 in cultured alpha-Gal A-deficient mouse aortic endothelial cells accumulated in endothelial plasma membrane caveolar fractions. The levels of glucosylceramide and lactosylceramide increased in parallel with Gb3 levels in an age-dependent manner, whereas globotetraosylceramide (Gb4) levels reached maximal levels by 6 months of age and then rapidly decreased at older ages. The levels of cholesterol enriched in caveolar membranes declined in parallel with the progressive deposition of Gb3. Depleting Gb3 with recombinant human alpha-Gal A protein or d-threo-ethylenedioxyphenyl-P4, an inhibitor of glucosylceramide synthase, restored cholesterol in cultured alpha-Gal A-deficient mouse aortic endothelial cell caveolae. By contrast, recombinant human alpha-Gal A was less effective in normalizing the cholesterol content. These results demonstrate the caveolar accumulation of glycosphingolipids in an in vitro model of a lysosomal storage disease and raise the possibility that dynamic changes in the composition of plasma membrane lipid microdomains may mediate the endothelial dysfunction seen in Fabry disease.

Regulation of c-Src activity in glutamate-induced neurodegeneration

c-Src is heavily expressed in the brain and in human neural tissues. Our pursuit for characterization of the neuroprotective mechanisms of tocotrienols led to the first evidence demonstrating that rapid c-Src activation plays a central role in executing glutamate-induced neurodegeneration. It is now known that Src deficiency or blockade of Src activity in mice provides cerebral protection following stroke. Here, we sought to examine the mechanisms that regulate inducible c-Src activity in glutamate-challenged HT4 neural cells and primary cortical neurons. Knockdown of c-Src protected cells against glutamate-induced loss of viability. Consistently, microinjection of siRNA against c-Src protected cells against glutamate. Using overexpression and knockdown approaches, we noted that SHP-1 may be implicated in glutamate-induced c-Src activation. Following such activation, Cbp and caveolin-1 were phosphorylated and associated with Csk. Csk was translocated to the membrane where it down-regulated glutamate-induced c-Src activity by catalyzing the inhibitory phosphorylation of a tyrosine residue in c-Src. Findings of this study present a new paradigm that addresses the regulation of c-Src under neurodegenerative conditions.

The modulation of apoptosis by cyclic AMP involves Akt and epidermal growth factor receptor

Adenosine 3',5'-cyclic monophosphate (cAMP) and transforming growth factor-beta are important regulators of many biological processes. In this study we investigated the effect and its potential mechanism of cAMP on transforming growth factor-beta1- and serum deprivation-induced apoptosis in Mv1Lu cells. Transforming growth factor-beta1 treatment or serum deprivation induces apoptotic response in Mv1Lu cells. Forskolin, a cAMP-elevating agent, or 8-Bromo-cAMP (8-B-cAMP), a cell permeable cAMP analogue, inhibited the cell proliferation and markedly enhanced apoptosis induced by transforming growth factor-beta1, but completely suppressed serum deprivation-induced apoptosis. Furthermore, forskolin decreased the Akt phosphorylation, and the inhibition of phosphatidylinositol-3 kinase by LY294002 sensitized Mv1Lu cells to transforming growth factor-beta1-induced apoptosis. In addition, forskolin treatment induced tyrosine phosphorylation of epidermal growth factor receptor. Inhibition of epidermal growth factor receptor by specific inhibitor PD153035 blocked the cAMP-mediated suppression of serum deprivation-induced apoptosis. The results indicate that cAMP exerts its opposite effects in transforming growth factor-beta1- and serum deprivation-induced apoptosis via a mechanism involving the modulation of signaling components of phosphatidylinositol-3-kinase/Akt and epidermal growth factor receptor in Mv1Lu cells.

A specific microdomain ("glycosynapse 3") controls phenotypic conversion and reversion of bladder cancer cells through GM3-mediated interaction of alpha3beta1 integrin with CD9

Cell motility is highly dependent on the organization and function of microdomains composed of integrin, proteolipid/tetraspanin CD9, and ganglioside (Ono, M., Handa, K., Sonnino, S., Withers, D. A., Nagai, H., and Hakomori, S. (2001) Biochemistry 40, 6414-6421; Kawakami, Y., Kawakami, K., Steelant, W. F. A., Ono, M., Baek, R. C., Handa, K., Withers, D. A., and Hakomori, S. (2002) J. Biol. Chem. 277, 34349-34358), later termed "glycosynapse 3" (Hakomori, S., and Handa, K. (2002) FEBS Lett. 531, 88-92, 2002). Human bladder cancer cell lines KK47 (noninvasive and nonmetastatic) and YTS1 (highly invasive and metastatic), both derived from transitional bladder epithelia, are very similar in terms of integrin composition and levels of tetraspanin CD9. Tetraspanin CD82 is absent in both. The major difference is in the level of ganglioside GM3, which is several times higher in KK47 than in YTS1. We now report that the GM3 level reflects glycosynapse function as follows: (i) a stronger interaction of integrin alpha3 with CD9 in KK47 than in YTS1; (ii) conversion of benign, low motility KK47 to invasive, high motility cells by depletion of GM3 by P4 (D-threo-1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol) treatment or by knockdown of CD9 by the RNA interference method; (iii) reversion of high motility YTS1 to low motility phenotype like that of KK47 by exogenous GM3 addition, whereby the alpha3-to-CD9 interaction was enhanced; (iv) low GM3 level activated c-Src in YTS1 or in P4-treated KK47, and high GM3 level by exogenous addition caused Csk translocation into glycosynapse, with subsequent inhibition of c-Src activation; (v) inhibition of c-Src by "PP2" in YTS1 greatly reduced cell motility. Thus, GM3 in glycosynapse 3 plays a dual role in defining glycosynapse 3 function. One is by modulating the interaction of alpha3 with CD9; the other is by activating or inhibiting the c-Src activity, possibly through Csk translocation. High GM3 level decreases tumor cell motility/invasiveness, whereas low GM3 level enhances tumor cell motility/invasiveness. Oncogenic transformation and its reversion can be explained through the difference in glycosynapse organization.

An in vitro model of Fabry disease

Fabry disease is an X-linked inherited loss of alpha-galactosidase A (alpha-Gal A). Affected patients experience complications that include neuropathy, renal failure, and cardiovascular disease. Although the genetic and biochemical basis of this sphingolipidosis is well studied, the basis for the vascular disease remains poorly understood. In an attempt to create a suitable in vitro model of this disease, conditions for the growth of primary cultures of aortic endothelial cells from wild-type and alpha-Gal A -/0 mice were established. The cultured cells demonstrated CD-31 expression by flow cytometry and LDL binding by immunofluorescence. The glycolipid expression patterns were compared between wild-type and alpha-Gal A null cells. Importantly, cells from alpha-Gal A -/0 mice but not alpha-Gal A +/0 mice expressed high levels of the globo-series glycosphingolipid globotriaosylceramide (Gb3). The age-dependent elevation in Gb3 was measured. By 4 mo of age, alpha-Gal A -/0 mouse aortic endothelial cells achieved their peak Gb3 levels. The ability to lower Gb3 levels pharmacologically was assessed next. The glucosylceramide synthase inhibitor ethylenedioxyphenyl-P4 significantly lowered but did not eliminate Gb3 levels by 96 h of treatment. Gb3 synthesis was completely blocked as measured by [14C]galactose labeling. Recombinant alpha-Gal A more significantly lowered Gb3 levels by 48 h but had a more limited effect on de novo synthesis. Together, both agents eliminated detectable Gb3. In summary, primary cultures of aortic endothelial cells from Fabry mice retain the phenotype of elevated globo-series glycosphingolipids. These cells provide a useful model for comparing pharmacologic agents used for glycolipid reduction.

Effect of ganglioside and tetraspanins in microdomains on interaction of integrins with fibroblast growth factor receptor

The functional interaction ("cross-talk") of integrins with growth factor receptors has become increasingly clear as a basic mechanism in cell biology, defining cell growth, adhesion, and motility. However, no studies have addressed the microdomains in which such interaction takes place nor the effect of gangliosides and tetraspanins (TSPs) on such interaction. Growth of human embryonal WI38 fibroblasts is highly dependent on fibroblast growth factor (FGF) and its receptor (FGFR), stably associated with ganglioside GM3 and TSPs CD9 and CD81 in the ganglioside-enriched microdomain. Adhesion and motility of these cells are mediated by laminin-5 ((LN5) and fibronectin (FN) through alpha3beta1 and alpha5beta1 integrin receptors, respectively. When WI38 cells or its transformant VA13 cells were adhered to LN5 or FN, alpha3beta1 or alpha5beta1 were stimulated, giving rise to signaling to activate FGFR through tyrosine phosphorylation and inducing cell proliferation under serum-free conditions without FGF addition. Types and intensity of signaling during the time course differed significantly depending on the type of integrin stimulated (alpha3beta1 versus alpha5beta1), and on cell type (WI38 versus VA13). Such effect of cross-talk between integrins and FGFR was influenced strongly by the change of GM3 and TSPs. (i) GM3 depletion by P4 caused enhanced tyrosine phosphorylation of FGFR and Akt followed by MAPK activation, without significant change of ceramide level. GM3 depletion also caused enhanced co-immunoprecipitation of FGFR with alpha3/alpha5/beta1 and of these integrins with CD9/CD81. (ii) LN5- or FN-dependent proliferation of both WI38 and VA13 was strongly enhanced by GM3 depletion and by CD9/CD81 knockdown by siRNA. Thus, integrin-FGFR cross-talk is strongly influenced by GM3 and/or TSPs within the ganglioside-enriched microdomain.

MUC1 Initiates a Calcium Signal after Ligation by Intercellular Adhesion Molecule-1

The MUC1 mucin is normally restricted to the apical surface of breast epithelial cells. In tumors, it is frequently overexpressed and underglycosylated. The MUC1 peptide core mediates firm adhesion of tumor cells to adjacent cells via binding to intercellular adhesion molecule-1 (ICAM-1). There is increasing evidence that MUC1 is involved in signaling, with current reports focusing on phosphorylation of the MUC1 cytoplasmic tail after indirect or artificial modes of stimulation. ICAM-1 is the only known direct ligand of the MUC1 extracellular domain. The data presented herein show that MUC1 expressed on the surface of breast cancer cell lines or transfected 293T cells can initiate a calcium-based oscillatory signal on contact with ICAM-1-transfected NIH 3T3 cells, and we present a novel method of quantifying and comparing calcium oscillations. The MUC1-induced signal appears to be distinct from those previously described, and may involve a Src family kinase, phosphoinositol 3-kinase, phospholipase C, and lipid rafts, but not mitogen-activated protein kinase. As calcium signaling has been associated with cytoskeletal change and motility, it is possible that the functions of MUC1 include heterotypic cell-cell adhesion followed by a calcium-based promigratory signal within tumor cells, thus facilitating metastasis.

GIT1 mediates Src-dependent activation of phospholipase Cgamma by angiotensin II and epidermal growth factor

Critical events for vasoconstrictor and growth factor signal transduction include stimulation of phospholipase Cgamma (PLCgamma) and elevation of intracellular calcium. c-Src has been proposed as a common mediator for these signals activated by both G protein-coupled receptors (GPCRs) and tyrosine kinase-coupled receptors (TKRs). Here we show that the GPCR kinase-interacting protein-1 (GIT1) is a substrate for c-Src that undergoes tyrosine phosphorylation in response to angiotensin II (AngII) and EGF in vascular smooth muscle and 293 cells. GIT1 associates with PLCgamma via the PLCgamma Src homology 2 and 3 domains constitutively, and the interaction is unaltered by AngII and EGF. GIT1 interaction with PLCgamma is required for PLCgamma activation based on inhibition of tyrosine phosphorylation and calcium mobilization after GIT1 knockdown with antisense GIT1 oligonucleotides. GIT1 interacts with PLCgamma via a novel Spa homology domain (SHD) and a coiled-coil domain. Deletion mutation analysis showed that GIT1(SHD) is required for AngII- and EGF-mediated PLCgamma activation (measured by phosphorylation of Tyr783 and inositol 1,4,5-trisphosphate formation). We propose that GIT1 is a novel regulator of PLCgamma function that mediates PLCgamma activation by c-Src and integrates signal transduction by GPCRs and TKRs.