Characterization of a factor that stimulates hydrolysis of GTP bound to ras gene product p21 (GTPase-activating protein) and correlation of its activity to cell density

GTPase-activating protein Rasal1 associates with ZAP-70 of the TCR and negatively regulates T-cell tumor immunity

Immunotherapy involving checkpoint blockades of inhibitory co-receptors is effective in combating cancer. Despite this, the full range of mediators that inhibit T-cell activation and influence anti-tumor immunity is unclear. Here, we identify the GTPase-activating protein (GAP) Rasal1 as a novel TCR-ZAP-70 binding protein that negatively regulates T-cell activation and tumor immunity. Rasal1 inhibits via two pathways, the binding and inhibition of the kinase domain of ZAP-70, and GAP inhibition of the p21^ras-ERK pathway. It is expressed in activated CD4 + and CD8 + T-cells, and inhibits CD4 + T-cell responses to antigenic peptides presented by dendritic cells as well as CD4 + T-cell responses to peptide antigens in vivo. Furthermore, siRNA reduction of Rasal1 expression in T-cells shrinks B16 melanoma and EL-4 lymphoma tumors, concurrent with an increase in CD8 + tumor-infiltrating T-cells expressing granzyme B and interferon γ-1. Our findings identify ZAP-70-associated Rasal1 as a new negative regulator of T-cell activation and tumor immunity.

Activation of T cells in the tumor microenvironment can be inhibited through a variety of mechanisms. Here, the authors show that Rasal1, a GTPase-activating protein, binds and inhibits signaling downstream of the T Cell Receptor complex and that consistently, its reduced expression enhances anti-tumor T-cell responses in two syngeneic cancer mouse models.

The NF2 tumor suppressor merlin interacts with Ras and RasGAP, which may modulate Ras signaling

Inactivation of the tumor suppressor NF2/merlin underlies neurofibromatosis type 2 (NF2) and some sporadic tumors. Previous studies have established that merlin mediates contact inhibition of proliferation; however, the exact mechanisms remain obscure and multiple pathways have been implicated. We have previously reported that merlin inhibits Ras and Rac activity during contact inhibition, but how merlin regulates Ras activity has remained elusive. Here we demonstrate that merlin can directly interact with both Ras and p120RasGAP (also named RasGAP). While merlin does not increase the catalytic activity of RasGAP, the interactions with Ras and RasGAP may fine-tune Ras signaling. In vivo, loss of RasGAP in Schwann cells, unlike the loss of merlin, failed to promote tumorigenic growth in an orthotopic model. Therefore, modulation of Ras signaling through RasGAP likely contributes to, but is not sufficient to account for, merlin’s tumor suppressor activity. Our study provides new insight into the mechanisms of merlin-dependent Ras regulation and may have additional implications for merlin-dependent regulation of other small GTPases.

The small GTPase Rap1 promotes cell movement rather than stabilizes adhesion in epithelial cells responding to insulin-like growth factor I

The Ras-related GTPase Rap1 promotes cell adhesion and migration. Although the significance of Rap1 contribution to cell migration is increasingly being recognized, little is known about the biochemical mechanisms driving this process. In the present study, we discovered a previously unidentified regulatory role of insulin-like growth factor type I (IGF-I) receptor (IGF-IR) in CRK Src homology 3 (SH3)-binding guanine-nucleotide-releasing protein (C3G)-Rap1-fascin-actin axis promoting cell movement. We demonstrate that a burst of Rap1 activity, rather than presumed hyperactivation, is imperative for the onset of cell movement. We show that while autophosphorylated IGF-IR signals to C3G to activate Rap1, subsequent IGF-IR internalization promotes gradual inactivation of Rap1 by putative Rap1 GTPase-activating protein (GAP). Additionally, IGF-IR signalling recruits active Rap1 at sites of cell motile protrusions. C3G depletion prevents IGF-I-induced fascin accumulation at actin microspikes and blocks protrusions. In the absence of IGF-IR activity, the wild-type (WT) Rap1 and the constitutively active V12Rap1 mutant remain in cell-cell contacts. Forced inactivation of Rap1 signalling by overexpressing dominant negative N17Rap1, Rap1GAP or by silencing C3G has a detrimental effect on filamentous (F)-actin and cell adhesion irrespective of IGF-IR signalling. We conclude that the basal levels of Rap1 activity holds up cell adhesion, whereas sequential regulation of C3G and GAP by IGF-IR reverses the labile Rap1 function from supporting adhesion to promoting migration.

MicroRNA-132-mediated loss of p120RasGAP activates the endothelium to facilitate pathological angiogenesis

Although it is well established that tumors initiate an angiogenic switch, the molecular basis of this process remains incompletely understood. Here we show that the miRNA miR-132 acts as an angiogenic switch by targeting p120RasGAP in the endothelium and thereby inducing neovascularization. We identified miR-132 as a highly upregulated miRNA in a human embryonic stem cell model of vasculogenesis and found that miR-132 was highly expressed in the endothelium of human tumors and hemangiomas but was undetectable in normal endothelium. Ectopic expression of miR-132 in endothelial cells in vitro increased their proliferation and tube-forming capacity, whereas intraocular injection of an antagomir targeting miR-132, anti-miR-132, reduced postnatal retinal vascular development in mice. Among the top-ranking predicted targets of miR-132 was p120RasGAP, which we found to be expressed in normal but not tumor endothelium. Endothelial expression of miR-132 suppressed p120RasGAP expression and increased Ras activity, whereas a miRNA-resistant version of p120RasGAP reversed the vascular response induced by miR-132. Notably, administration of anti-miR-132 inhibited angiogenesis in wild-type mice but not in mice with an inducible deletion of Rasa1 (encoding p120RasGAP). Finally, vessel-targeted nanoparticle delivery of anti-miR-132 restored p120RasGAP expression in the tumor endothelium, suppressed angiogenesis and decreased tumor burden in an orthotopic xenograft mouse model of human breast carcinoma. We conclude that miR-132 acts as an angiogenic switch by suppressing endothelial p120RasGAP expression, leading to Ras activation and the induction of neovascularization, whereas the application of anti-miR-132 inhibits neovascularization by maintaining vessels in the resting state.

Neurofibromatosis type I tumor suppressor neurofibromin regulates neuronal differentiation via its GTPase-activating protein function toward Ras

Neurofibromin, the neurofibromatosis type 1 (NF1) gene product, contains a central domain homologous to a family of proteins known as Ras-GTPase-activating proteins (Ras-GAPs), which function as negative regulators of Ras. The loss of neurofibromin function has been thought to be implicated in the abnormal regulation of Ras in NF1-related pathogenesis. In this study, we found a novel role of neurofibromin in neuronal differentiation in conjunction with the regulation of Ras activity via its GAP-related domain (GRD) in neuronal cells. In PC12 cells, time-dependent increases in the GAP activity of cellular neurofibromin (NF1-GAP) were detected after NGF stimulation, which were correlated with the down-regulation of Ras activity during neurite elongation. Interestingly, the NF1-GAP increase was due to the induction of alternative splicing of NF1-GRD type I triggered by the NGF-induced Ras activation. Dominant-negative (DN) forms of NF1-GRD type I significantly inhibited the neurite extension of PC12 cells via regulation of the Ras state. NF1-GRD-DN also reduced axonal and dendritic branching/extension of rat embryonic hippocampal neurons. These results demonstrate that the mutual regulation of Ras and NF1-GAP is essential for normal neuronal differentiation and that abnormal regulation in neuronal cells may be implicated in NF1-related learning and memory disturbance.

SPAL, a Rap-specific GTPase activating protein, is present in the NMDA receptor-PSD-95 complex in the hippocampus

BACKGROUND

The PSD-95 family of proteins possesses multiple protein binding domains, including three PDZ domains, an SH3 domain, a HOOK domain and a guanylate kinase-like (GK) domain. The PSD-95 proteins function as scaffolding proteins that link ion channels such as the N-methyl-d-aspartate-receptors (NMDA-Rs) with cytoskeletal networks and signalling molecules, thereby controlling synaptic plasticity and learning.

RESULTS

We found that the PSD-95 family proteins interact via their GK domains with SPA-1-like protein (SPAL), a GTPase-activating protein (GAP) that is specific for Rap1. SPAL was contained within the NMDA-R-PSD-95 complex, and co-localized with PSD-95 and NMDA-R at the synapses in cultured hippocampal neurones. Furthermore, NMDA stimulation induced the dephosphorylation of SPAL in cultured hippocampal neurones.

CONCLUSION

Our findings suggest that SPAL may be involved in the NMDA-mediated organization of cytoskeletal networks and signal transduction.

Spatio-temporal images of growth-factor-induced activation of Ras and Rap1

G proteins of the Ras family function as molecular switches in many signalling cascades; however, little is known about where they become activated in living cells. Here we use FRET (fluorescent resonance energy transfer)-based sensors to report on the spatio-temporal images of growth-factor-induced activation of Ras and Rap1. Epidermal growth factor activated Ras at the peripheral plasma membrane and Rap1 at the intracellular perinuclear region of COS-1 cells. In PC12 cells, nerve growth factor-induced activation of Ras was initiated at the plasma membrane and transmitted to the whole cell body. After three hours, high Ras activity was observed at the extending neurites. By using the FRAP (fluorescence recovery after photobleaching) technique, we found that Ras at the neurites turned over rapidly; therefore, the sustained Ras activity at neurites was due to high GTP/GDP exchange rate and/or low GTPase activity, but not to the retention of the active Ras. These observations may resolve long-standing questions as to how Ras and Rap1 induce different cellular responses and how the signals for differentiation and survival are distinguished by neuronal cells.

Effect of growth factor on GTPase-activating protein (Ras GAP) in Chinese hamster ovary cells

GTPase-activating proteins (GAPs) stimulate the hydrolysis of GTP bound to small G-proteins and regulate the signal transduction pathway. Changes in the expression of p21-Ras p120-GAP induced by growth factor treatment were examined in cultured Chinese hamster ovary (CHO) and human choriocarcinoma (BeWo) cells. Expression of p120-GAP and GAP activity were measured. Fetal bovine serum induced a significant increased level of GAP in CHO cells, but did not increase GAP in BeWo cells. The results suggest that growth factors affect Ras GAP expression in CHO cells, while they do not in other cells such as BeWo cells.

Expression of Ras GTPase-activating protein (GAP) in human normal chorionic villi and hydatidiform mole

Ras GTPase-activating protein (GAP), an important downregulator of Ras activity, has previously been shown to be abundant in human placenta. The expression of p120 and p100 isoforms of GAP in human normal chorionic villi (n=5) and hydatidiform mole (n=5) was investigated to clarify the involvement of Ras GAP in the growth of chorionic villi in the first trimester of pregnancy. Immunoblot analysis revealed that both p120- and p100-GAP isoforms were remarkably less expressed in mole villi than in normal chorionic villi. The expression of p100-GAP significantly reduced in comparison with that of pl20-GAP in mole villi. Northern blot analysis showed that the amount of GAP mRNA reduced in hydatidiform mole less than one-third of that in normal chorionic villi. The GAP activity, measured by the effect of tissue extract on the hydrolysis of Ras-bound GTP, was significantly lower in hydatidiform mole than in normal chorionic villi. These results suggest that Ras GAP may play an important role in the normal growth and differentiation of human chorionic villi in the first trimester.

Compensatory modulation of GAP activity in response to oncogenic stimulation

GAP is a key negative regulator of the receptor tyrosine kinase (RTK) signal transduction pathway. The purpose of this study was to determine if expression or activity of GAP is modulated by hyperstimulation of the RTK pathway. It was found that cells forced to express wild-type Ha-ras, viral Ha-ras, or v-src exhibit increased GAP activity as compared to control cells. In addition, a novel GAP isoform appears in all ras-expressing NIH3T3 cell clones. These data indicate that there is compensatory regulation of GAP in response to an increase in RTK pathway activity.

Ras-GTP regulation is not altered in cultured melanocytes with reduced levels of neurofibromin derived from patients with neurofibromatosis 1 (NF1)

As derivatives of the neural crest, epidermal melanocytes are supposed to be clinically affected by NF1 gene defects. The NF1 gene shares sequence homology with the p120 GTPase activating protein (p120-GAP) and neurofibromin has been shown to participate in Ras-regulation. By immunoprecipitation and Western blotting, neurofibromin was found to be expressed in melanocytes from the unaffected skin and café au lait macules of NF1 patients, but the intensity of the neurofibromin band was decreased compared to control cultures. The Ras-GTP/Ras-GDP ratios of NF1 derived melanocyte cultures were comparable to those derived from healthy donors. Furthermore, the total GAP-activity of cell lysates was not altered in NF1 melanocyte cultures compared to controls. However, lysates of proliferating melanocytes, both from NF1 patients and from healthy donors, showed an about 2-fold higher GAP-activity than poorly growing cells. Neurofibromin contributed approximately one third of total GAP-activity, in both control and NF1 melanocytes, indicating that it is not the major regulator of Ras in these cells. These results suggest that the function of neurofibromin in melanocytes is not limited to regulation of Ras activity.

Interaction of the nuclear GTP-binding protein Ran with its regulatory proteins RCC1 and RanGAP1

The guanine nucleotide dissociation and GTPase reactions of Ran, a Ras-related nuclear protein, have been investigated using different fluorescence techniques to determine how these reactions are stimulated by the guanine nucleotide exchange factor RCC1 and the other regulatory protein, RanGAP1 (GTPase-activating protein). The intrinsic GTPase of Ran is one-tenth of the rate of p21ras and is even lower in the Ran(Q69L) mutant. Under saturating conditions the rate constant for the RanGAP1 stimulated GTPase reaction is 2.1 s-1 at 25 degrees C, which is a 10(5)-fold stimulation, whereas RanGAP1 has no effect on Ran(Q69L). The intrinsic guanine nucleotide dissociation rates of Ran are also very low and are likewise increased 10(5)-fold by the exchange factor RCC1. Methods to describe the reaction kinetically are presented. The Ran(T24N) mutant, which is analogous to the S17N mutant of p21ras, has decreased relative affinities for both GDP/GTP and favors GDP binding. However, it was found to interact almost normally with RCC1. The combination of these properties leads to stabilization of the Ran(T24N)-RCC1 complex and may result in vivo in depletion of RCC1 available for stimulating guanine nucleotide exchange.

Modulation of myogenic determination factor 1 expression by cell-cell contact

Myogenic determination factor 1 (MyoD1) expression is modulated by a variety of agents including growth factors and activated cellular proto-oncogenes. However, little is known about the effect of cell-cell contact, which increases as myoblasts terminally differentiate, on the control of MyoD1 expression. Steady-state levels of MyoD1 transcripts decline over a 6-12 hour time period when myoblasts plated at a single cell density are incubated in media supplemented with 0.2% serum; by 48 hours MyoD1 mRNA levels have returned to the initial basal level. The decline in MyoD1 transcripts is diminished, but not prevented in myoblasts which maintain cell-cell contacts (at least 50% of cells with two or more sites of contact). MyoD1 transcript levels do not change if single cell cultures are maintained in 10% serum or are cocultured with fibroblasts. Analysis of conditioned media revealed that myoblasts plated at the single cell density or at a density which allowed multiple sites of cell-cell contact are not producing an activity(s) responsible for modulating MyoD1 mRNA levels. The changes in MyoD1 expression are mediated at the transcriptional level. Thus changes in the degree of cell-cell contact in cultures of myogenically determined cell lines effect changes in MyoD1 gene expression. Consequently when the influence of cytokines or other pharmacological agents on commitment to terminal myogenic differentiation is examined, the degree of cell-cell contact within the culture system may affect the response elicited.

Differential contribution of M(r) 120 kDa rasGTPase-activating protein and neurofibromatosis type 1 gene product during the transition from growth phase to arrested state in human fibroblasts accompanied by a unique rasGTPase-activating activity

Using octyl glucoside-solubilized cell extracts from human fibroblasts during growth phase to G0/G1 arrest state, we found that while the number of M(r) 120 kDa rasGTPase-activating protein (p120GAP) molecules per cell decreases to half its original levels, the amount of neurofibromatosis type 1 gene product (NF1, a neurofibromin) remains constant during the transition. The contribution of p120GAP to the total rasGTPase-activating (rasGA) activity in growing cells was found to be larger than that observed in arrested cells (84% vs 53%). On the other hand, NF1 contributes less than 15% of the total rasGA activity in either extract. These results indicate that the qualitative changes occur in the contributors to rasGA activity during transition. They also suggest that a unique rasGA activity exists in the arrested cells, which was obtained separately from both p120GAP and NF1 by heparin-Sepharose column chromatography.

Antibody against neurofibromatosis type 1 gene product reacts with a triton-insoluble GTPase activating protein toward ras p21

Cellular fractionation of GTPase activating protein (GAP) activity using bovine cerebral cortex revealed that about half of GAP activity was found in membrane fraction. GAP activity of membrane was not solubilized with 0.5% (v/v) triton X-100 and was immunoprecipitated with antibody against carboxy-terminus of neurofibromatosis type 1 (NF1) gene product. In contrast, soluble GAP activity was precipitated with antibody against GAP but not with anti-NF1. These results suggest that NF1 gene product is a GTPase activating protein toward ras p21 with completely different intracellular distribution from that of GAP.

Suppression of c-ras transformation by GTPase-activating protein

The ras genes are required for normal cell growth and mediate transformation by oncogenes encoding protein tyrosine kinases. Normal ras can transform cells in vitro and in vivo, but mutationally activated ras does so much more efficiently, and highly transforming mutant versions of ras have been isolated from a variety of human and animal tumours. The ras genes encode membrane-associated, guanine nucleotide-binding proteins that are active when GTP is bound and inactive when GDP is bound. The slow intrinsic GTPase activity of normal mammalian Ras proteins can be greatly accelerated by the GTPase-activating protein (GAP), which is predominantly cytoplasmic. This activity of GAP, which can increase with cell density in contact-inhibited cells, suggests that it functions as a negative, upstream regulator of ras. Other studies, however, show that GAP interacts with a region of ras-encoded protein implicated in ras effector function, which raises the possibility that GAP might also be a downstream target of ras. Mutationally activated ras-encoded proteins also interact with GAP, although they are resistant to its catalytic activity. In an attempt to define the role of GAP in ras-mediated transformation, we examined the effects on transformation of normal or mutant ras when cells overexpress GAP. We found that GAP suppresses transformation of NIH 3T3 cells by normal Ha-ras (c-ras) but does not inhibit transformation by activated Ha-ras (v-ras). These results support the hypothesis that GAP functions as a negative regulator of normal ras and make it unlikely that GAP alone is the ras target.

GTPase activating proteins for the smg-21 GTP-binding protein having the same effector domain as the ras proteins in human platelets

Two proteins stimulating the GTPase activity of the smg-21 GTP-binding protein (smg p21) having the same effector domain as the ras proteins (ras p21s) are partially purified from the cytosol fraction of human platelets. These proteins, designated as smg p21 GTPase activating protein (GAP) 1 and 2, do not stimulate the GTPase activity of c-Ha-ras p21. The GAP activity for c-Ha-ras p21 is also detected in the cytosol fraction of human platelets. smg p21 GAP1 and 2 are separated from c-Ha-ras p21 GAP by column chromatographies. The activity of smg p21 GAP1 and 2 is killed by tryptic digestion or heat boiling. The Mr values of smg p21 GAP1 and 2 are similar and are estimated to be 2.5-3.5 x 10(5) by gel filtration analysis. These results indicate that there are two GAPs for smg p21 in addition to a GAP for c-Ha-ras p21 in human platelets.

Oncogenes, protooncogenes, and signal transduction: toward a unified theory?

This paper has reviewed, in a broad sense, the potential involvement of the oncogenes and their progenitors, the protooncogenes, in signal transduction pathways. The membrane-associated oncogene products appear to be connected with the generation and/or regulation of secondary messengers, particularly those associated with Ca2+/phospholipid-dependent activation of the serine/threonine kinase protein kinase C. Activation of transmembrane receptors, either through binding their native ligand or through point mutations that lead to constitutive expression, results in the expression of their intrinsic tyrosine-specific protein kinases. In PDGF-stimulated cells, this results in the increased turnover of phosphatidylinositols and the subsequent release of IP3 (Habenicht et al., 1981; Berridge et al., 1984). This coincides with activation of a PI kinase activity (Kaplan et al., 1987). Likewise, the fms product, which is the receptor for CSF-1, induces a guanine nucleotide-dependent activation of phospholipase C (Jackowski et al., 1986). Receptor functions are potentially regulated through differential binding of ligands (as proposed with PDGF), through interactions with other receptors, and through the "feedback" regulation mediated by protein kinase C. PDGF stimulation leads to modulation of the EGF receptor through protein kinase C (Bowen-Pope et al., 1983; Collins et al., 1983; Davis and Czech, 1985). Similarly, the neu product becomes phosphorylated on tyrosine residues following treatment of cells with EGF, although the neu protein does not bind EGF itself (King et al., 1988; Stern and Kamps, 1988). The tyrosine kinases of the src family are not receptors themselves, although they may mediate specific receptor-generated signals. The clck product is physically and functionally associated with the T-cell receptors CD4 and CD8, and becomes active upon specific stimulation of cells expressing those markers (Veillette et al., 1988a,b). The precise physiological role of the src family products has not been established, but their kinase activity is intrinsic to that function. The v- and c-src products are hyperphosphorylated during mitosis (Chackalaparampil and Shalloway, 1988), which correlates with periods of reduced cell-to-cell adhesion and communication (Warren and Nelson, 1987; Azarnia et al., 1988). Furthermore, pp60c-src is associated with a PI kinase activity when complexed with MTAg of polyoma virus, suggesting a function in stimulating increased turnover of the phosphatidylinositols (Heber and Courtneidge, 1987; Kaplan et al., 1987).(ABSTRACT TRUNCATED AT 400 WORDS)