Discovery of Potent SOS1 PROTACs with Effective Antitumor Activities against NCI-H358 Tumor Cells In Vitro/In Vivo

Directly targeted KRAS inhibitors are now facing resistance problems, which might be partially solved by the combination of SOS1 inhibitors with KRAS inhibitors. However, this combination may still have some resistance mitigation potential. Comparatively, SOS1 PROTAC may have promising applications in addressing the drug resistance problem by degrading the SOS1 protein. Herein, we report the discovery of novel SOS1 PROTACs and their antitumor activity both in vitro and in vivo. In vitro studies demonstrated that degrader 4 had strong inhibitory effects on the proliferation of NCI-H358 cells with IC50 of 5 nM, together with significant degradation of SOS1 protein with DC50 of 13 nM. In the NCI-H358 xenograft model, degrader 4 exhibited significant antitumor activities with TGITV values of 58.8% at 30 mg/kg bid. The PK and safety profiles also supported degrader 4 for further studies as an effective tool compound.

PPDPF Promotes the Development of Mutant KRAS-Driven Pancreatic Ductal Adenocarcinoma by Regulating the GEF Activity of SOS1

The guanine nucleotide exchange factor (GEF) SOS1 catalyzes the exchange of GDP for GTP on RAS. However, regulation of the GEF activity remains elusive. Here, the authors report that PPDPF functions as an important regulator of SOS1. The expression of PPDPF is significantly increased in pancreatic ductal adenocarcinoma (PDAC), associated with poor prognosis and recurrence of PDAC patients. Overexpression of PPDPF promotes PDAC cell growth in vitro and in vivo, while PPDPF knockout exerts opposite effects. Pancreatic-specific deletion of PPDPF profoundly inhibits tumor development in KRASG12D -driven genetic mouse models of PDAC. PPDPF can bind GTP and transfer GTP to SOS1. Mutations of the GTP-binding sites severely impair the tumor-promoting effect of PPDPF. Consistently, mutations of the critical amino acids mediating SOS1-PPDPF interaction significantly impair the GEF activity of SOS1. Therefore, this study demonstrates a novel model of KRAS activation via PPDPF-SOS1 axis, and provides a promising therapeutic target for PDAC.

M-Ras/R-Ras3, a transforming ras protein regulated by Sos1, GRF1, and p120 Ras GTPase-activating protein, interacts with the putative Ras effector AF6

M-Ras is a Ras-related protein that shares approximately 55% identity with K-Ras and TC21. The M-Ras message was widely expressed but was most predominant in ovary and brain. Similarly to Ha-Ras, expression of mutationally activated M-Ras in NIH 3T3 mouse fibroblasts or C2 myoblasts resulted in cellular transformation or inhibition of differentiation, respectively. M-Ras only weakly activated extracellular signal-regulated kinase 2 (ERK2), but it cooperated with Raf, Rac, and Rho to induce transforming foci in NIH 3T3 cells, suggesting that M-Ras signaled via alternate pathways to these effectors. Although the mitogen-activated protein kinase/ERK kinase inhibitor, PD98059, blocked M-Ras-induced transformation, M-Ras was more effective than an activated mitogen-activated protein kinase/ERK kinase mutant at inducing focus formation. These data indicate that multiple pathways must contribute to M-Ras-induced transformation. M-Ras interacted poorly in a yeast two-hybrid assay with multiple Ras effectors, including c-Raf-1, A-Raf, B-Raf, phosphoinositol-3 kinase delta, RalGDS, and Rin1. Although M-Ras coimmunoprecipitated with AF6, a putative regulator of cell junction formation, overexpression of AF6 did not contribute to fibroblast transformation, suggesting the possibility of novel effector proteins. The M-Ras GTP/GDP cycle was sensitive to the Ras GEFs, Sos1, and GRF1 and to p120 Ras GAP. Together, these findings suggest that while M-Ras is regulated by similar upstream stimuli to Ha-Ras, novel targets may be responsible for its effects on cellular transformation and differentiation.

Modulation of the immune response and tumor growth by activated Ras

As a result of its transforming abilities, activated Ras is expressed in a great number of cancers. The ras mutation frequency varies between 95% in pancreatic cancer and 5% in breast cancer. In leukemia, the highest frequency (30%) is found in acute myeloid leukemia. The presence of ras mutations has been correlated with a poor prognosis and negative clinical outcome. This suggests that mutated Ras activates mechanisms, which favor tumor growth, enhance the metastatic capacity of tumors or modulate tumor-specific immune responses. Several new functions of Ras, such as downregulation of major histocompatibility complex molecules, upregulation of certain cytokines, growth factors and degradative enzymes have been uncovered in the last decade. Additionally, mutated Ras can also serve as a primary target for the development of immunotherapy or drug therapy. This review will discuss the mechanisms by which Ras expressing tumors are able to evade destruction by the immune system and enhance their growth and metastatic potential. It will further elaborate on the attempts to develop successful immunotherapy and drug therapy targeting Ras expressing tumors.

The solution structure of the pleckstrin homology domain of human SOS1. A possible structural role for the sequential association of diffuse B cell lymphoma and pleckstrin homology domains

A large subset of pleckstrin homology (PH) domains are immediately to the C terminus of diffuse B cell lymphoma (Dbl) homology (DbH) domains. Dbl domains are generally considered to be GTPase-exchange factors; many are proto-oncogenes. PH domains appear to function as membrane-recruitment factors, or have specific protein-protein interactions. Since dual domain (DbH/PH) constructs are known to have significant properties in other pathways, it is possible that a defined interdomain relationship is required for DbH/PH function. We determined the solution structure of the human SOS1 PH domain for a construct partially extended into the preceding DbH domain. There are specific structural contacts between the PH and the vestigial DbH domain. This appears to involve structural elements common to this subfamily of PH domains, and to DbH domains. The human SOS1 PH domain binds to inositol 1,4,5-triphosphate with a approximately 60 mu M affinity. Using chemical shift titration, the binding site is identified to be essentially identical to that observed crystallographically for the inositol 1,4,5-triphosphate complex with the PH domain of phospholipase Cdelta. This site may serve as an interdomain regulator of DbH or other domains' functions. While the overall fold of the human SOS1 PH domain is similar to other PH domains, the size and position of the intrastrand loops and the presence of an N-terminal alpha-helix of the vestigial DbH domain suggest that the subfamily of PH domains associated with DbH domains may be a well defined structural group in which the PH domain is a membrane recruiter and modulator.

IL-6 triggers cell growth via the Ras-dependent mitogen-activated protein kinase cascade

IL-6 mediates growth of some human multiple myeloma (MM) cells and IL-6-dependent cell lines. Although three IL-6 signaling pathways (STAT1, STAT3, and Ras-dependent MAPK cascade) have been reported, cascades mediating IL-6-triggered growth of MM cells and cell lines are not defined. In this study, we therefore characterized IL-6 signaling cascades in MM cell lines, MM patient cells, and IL-6-dependent B9 cells to determine which pathway mediates IL-6-dependent growth. IL-6 induced phosphorylation of JAK kinases and gp130, regardless of the proliferative response of MM cells to this growth factor. Accordingly, we next examined downstream IL-6 signaling via the STAT3, STAT1, and Ras-dependent mitogen-activated protein kinase (MAPK) cascades. IL-6 triggered phosphorylation of STAT1 and/or STAT3 in MM cells independent of their proliferative response to IL-6. In contrast, IL-6 induced phosphorylation of Shc and its association with Sos1, as well as phosphorylation of MAPK, only in MM cells and B9 cells that proliferated in response to IL-6. Moreover, MAPK antisense, but not sense, oligonucleotide inhibited IL-6-induced proliferation of these cells. These data suggest that STAT1 and/or STAT3 activation may occur independently of the proliferative response to IL-6, and that activation of the MAPK cascade is an important distal pathway for IL-6-mediated growth.

A new member of the amphiphysin family connecting endocytosis and signal transduction pathways

Src homology 3 (SH3) domains are conserved modules which participate in protein interaction by recognizing proline-rich motifs on target molecules. To identify new SH3-containing proteins, we performed a two-hybrid screen with a proline-rich region of human SOS-1. One of the specific SOS-1 interacting clones that were isolated from a mouse brain cDNA library defines a new protein that was named amphiphysin 2 because of its homology to the previously reported amphiphysin. Amphiphysin 2 is expressed in a number of mouse tissues through multiple RNA transcripts. Here, we report the amino acid sequence of a brain form of amphiphysin 2 (BRAMP2) encoded by a 2. 5-kilobase mRNA. BRAMP2 associates in vitro with elements of the endocytosis machinery such as alpha-adaptin and dynamin. On a biosensor surface, the BRAMP2/dynamin interaction appeared to be direct and partly dependent on a proline-rich sequence of dynamin. Association with dynamin was also observed in PC12 cells after cell stimulation with nerve growth factor, suggesting that amphiphysin 2 may be connected to receptor-dependent signaling pathways. This hypothesis is strengthened by the ability of BRAMP2 to interact with the p21(ras) exchange factor SOS, in vitro, as a possible point of interconnection between the endocytic and signaling pathways.

A simple improvement in expression cloning

Expression cloning is an effective approach for isolating genes encoding proteins that associate with a target species. Several molecules have been isolated by expression cloning, including CRE-BP1 associating with Jun (Macgregor et al., 1990); Grb1, identical to p85 PI3-kinase, with the EGF receptor (Skolnik et al., 1991); and Max with Myc (Blackwood and Eisenman, 1991). Expression cloning involves induction of proteins from a lambda gt11 cDNA expression library and screening the proteins on nitrocellulose membranes using a peptide probe (Macgregor et al., 1990). With this method, we previously isolated an Lck tyrosine kinase-associated protein, LckBP1, which is identical to HS1 (Kitamura et al., 1989, 1995; Takemoto et al., 1995). In those experiments, we used a glutathione S-transferase (GST)-Lck SH3 domain fusion protein as a probe, followed by detection of the complex with anti-GST polyclonal antibody. Whereas the ease of obtaining the fusion construct and high-titer anti-GST polyclonal antibody represented clear advantages, the system suffered from high background and low sensitivity. Here we show that pretreatment of nitrocellulose filters with NaDodSO4 reduces background and, in turn, increases sensitivity.

High affinity binding of the pleckstrin homology domain of mSos1 to phosphatidylinositol (4,5)-bisphosphate

mSos1 has been implicated in coupling mammalian tyrosine kinases to the Ras GTPase. Because activation of Ras induced by growth factor stimulation likely requires the localization of mSos1 to the plasma membrane, we have investigated the possibility that the PH domain of mSos1 might mediate an interaction of mSos1 with phospholipid membranes. A glutathione S-transferase fusion protein containing the pleckstrin homology (PH) domain of mSos1 bound specifically and tightly to phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) with a Kd of 1.8 +/- 0.4 microM. This interaction was saturable and was competed away with the soluble head group of PI(4,5)P2, inositol 1,4, 5-triphosphate. Substitution of Arg452 within the PH domain with Ala had only a slight effect on binding to PI(4,5)P2, whereas substitution of Arg459 severely compromised the ability of the mSos1 PH domain to bind to PI(4,5)P2 containing vesicles. Purified full-length mSos1 and mSos1 complexed with Grb2 were also tested for binding to various phosphoinositol derivatives and demonstrated a specific interaction with PI(4,5)P2, although these interactions were weaker (Kd = approximately 53 and approximately 69 microM, respectively) than that of the PH domain alone. These findings suggest that the PH domain of mSos1 can interact in vitro with phospholipid vesicles containing PI(4,5)P2 and that this interaction is facilitated by the ionic interaction of Arg459 with the negatively charged head group of PI(4,5)P2. The association of the mSos1 PH domain with phospholipid may therefore play a role in regulating the function of this enzyme in vivo.

Fas-signaling and effects on receptor tyrosine kinase signal transduction in human breast epithelial cells

Fas-mediated cell death was examined in MCF-10AT preneoplastic human breast epithelial cells. Treatment with anti-Fas for 48 h induced apoptosis with cells exhibiting typical apoptotic features including loss of cell contact, condensation of chromatin, and increased staining of the nuclear membrane. DNA fragmentation occurred in response to anti-Fas treatment. Anti-Fas treatment resulted in decreased p53 protein levels, while bcl-2 and bax protein levels remained unaffected. Cells treated with anti-Fas also exhibited increased tyrosine phosphorylation of the c-met growth factor receptor tyrosine kinase. Immunoprecipitation experiments demonstrated that Fas associated with c-erbB2 and c-met in untreated cells. Treatment with anti-Fas, however, significantly decreased Fas-c-erbB2 and Fas-c-met association. Anti-Fas treatment of these cells caused a significant decrease in p120-GAP levels, ERK-1 levels, and phosphorylation, as well as Grb2-Sosl and MEK-1-ERK-1 association. These results show that Fas-signaling exerted a suppressive effect on p53 levels and on downstream effectors of receptor tyrosine kinase signal transduction, thereby ensuring cell death.

TC21 causes transformation by Raf-independent signaling pathways

Although the Ras-related protein TC21/R-Ras2 has only 55% amino acid identity with Ras proteins, mutated forms of TC21 exhibit the same potent transforming activity as constitutively activated forms of Ras. Therefore, like Ras, TC21 may activate signaling pathways that control normal cell growth and differentiation. To address this possibility, we determined if regulators and effectors of Ras are also important for controlling TC21 activity. First, we determined that Ras guanine nucleotide exchange factors (SOS1 and RasGRF/CDC25) synergistically enhanced wild-type TC21 activity in vivo and that Ras GTPase-activating proteins (GAPs; p120-GAP and NF1-GAP) stimulated wild-type TC21 GTP hydrolysis in vitro. Thus, extracellular signals that activate Ras via SOS1 activation may cause coordinate activation of Ras and TC21. Second, we determined if Raf kinases were effectors for TC21 transformation. Unexpectedly, yeast two-hybrid binding analyses showed that although both Ras and TC21 could interact with the isolated Ras-binding domain of Raf-1, only Ras interacted with full-length Raf-1, A-Raf, or B-Raf. Consistent with this observation, we found that Ras- but not TC21-transformed NIH 3T3 cells possessed constitutively elevated Raf-1 and B-Raf kinase activity. Thus, Raf kinases are effectors for Ras, but not TC21, signaling and transformation. We conclude that common upstream signals cause activation of Ras and TC21, but activated TC21 controls cell growth via distinct Raf-independent downstream signaling pathways.

Isolated Sos1 PH domain exhibits germinal vesicle breakdown-inducing activity in Xenopus oocytes

Purified, bacterially expressed PH domains of Sos1, IRS-1, betaARK, and PLCdelta1 were analyzed functionally by means of microinjection into full grown, stage VI Xenopus laevis oocytes. Whereas the PH domains from IRS-1, betaARK, or PLCdelta1 did not show any effect in the oocytes, injection of the purified Sos1 PH domain resulted in induction of significant rates of germinal vesicle breakdown and meiotic maturation. Furthermore, the Sos1 PH domain exhibited also significant synergy with insulin or coinjected normal Ras protein in induction of germinal vesicle breakdown, although it did not affect the rate of progesterone-induced maturation. These results suggest that purified, isolated PH domains retain, at least in part, their functional specificity and that Xenopus oocytes may constitute a useful biological system to analyze the functional role of the Sos1 PH domain in Ras signaling pathways.

Characterization of Grb2-binding proteins in human platelets activated by Fc gamma RIIA cross-linking

Glutathione-S-transferase (GST)-Grb2 fusion proteins have been used to identify the potential role of Grb2-binding proteins in platelet activation by the platelet low-affinity IgG receptor, Fc gamma RIIA. Two tyrosine phosphoproteins of 38 and 63 kD bind to the SH2 domain of Grb2 following Fc gamma RIIA stimulation of platelets. Both are located in the particulate fraction following platelet activation and are also able to bind to a GST-construct containing the SH2 and SH3 domains of phospholipase C gamma 1. p38 also forms a complex with the tyrosine kinase csk in stimulated cells and is a substrate for the kinase. The SH3 domains of Grb2 form a stable complex with SOS1 and two proteins of 75 kD and 120 kD, which undergo tyrosine phosphorylation in Fc gamma RIIA stimulated cells. The 75-kD protein is recognized by antibodies to SLP-76, which has recently been isolated from T cells and sequenced. Tyrosine phosphorylation of p38 and p63 is also observed in platelets stimulated by the tyrosine kinase-linked receptor agonist collagen and by the G protein-coupled receptor agonist thrombin, although phosphorylation of SLP-76 is only observed in collagen-stimulated platelets. p38 and p63 may provide a docking site for Grb2, thereby linking Grb2 SH3-binding proteins SOS1, SLP-76, and p120 to downstream signalling events.

Involvement of the switch 2 domain of Ras in its interaction with guanine nucleotide exchange factors

While Ras proteins are activated by stimulated GDP release, which enables acquisition of the active GTP-bound state, little is known about how guanine nucleotide exchange factors (GEFs) interact with Ras to promote this exchange reaction. Here we report that mutations within the switch 2 domain of Ras (residues 62-69) inhibit activation of Ras by the mammalian GEFs, Sos1, and GRF/CDC25Mm. While mutations in the 62-69 region blocked upstream activation of Ras, they did not disrupt Ras effector functions, including transcriptional activation and transformation of NIH 3T3 cells. Biochemical analysis indicated that the loss of GEF responsiveness of a Ras(69N) mutant was due to a loss of GEF binding, with no change in intrinsic nucleotide exchange activity. Furthermore, structural analysis of Ras(69N) using NMR spectroscopy indicated that mutation of residue 69 had a very localized effect on Ras structure that was limited to alpha-helix 2 of the switch 2 domain. Together, these results suggest that the switch 2 domain of Ras forms a direct interaction with GEFs.

Sos1 rapidly associates with Grb2 and is hypophosphorylated when complexed with the EGF receptor after EGF stimulation

The Son of sevenless (Sos) protein, a guanine nucleotide exchange factor for ras proteins, appears to play a central role in signalling between protein tyrosine kinase receptors and ras. The C-terminal region of Sos binds an adaptor protein, Grb2, which in turn binds to activated receptors including the EGF receptor (EGFR). Although the Sos protein is rapidly phosphorylated following cytokine stimulation, there is no evidence that this alters the enzymatic activity of Sos for ras proteins. Therefore, we investigated whether the ability of Sos1 to form complexes with Grb2 and with the EGF receptor (EGFR) changes following EGF stimulation, as a possible mechanism for regulating Sos activity. In contrast to earlier findings, we find that both the association and dissociation of Sos1 with Grb2 is responsive to EGF. Whilst the association of Sos1 and Grb2 following EGF stimulation is not cell type specific, we find that it is dependent on cell density and that the response to EGF differs to that induced by NGF. We find that following EGF stimulation, the Sos1 protein associated with the EGFR is markedly less phosphorylated than the majority of the Sos1 within the cell and there was reduced binding of Grb2 with phosphorylated Sos1 protein in a direct binding assay. A time course analysis showed that Sos1 dissociates from the EGFR more rapidly than does Grb2 following EGF stimulation. Collectively our findings are consistent with the notion that the phosphorylation of Sos1 affects its ability to complex with the EGFR and Grb2.

Expression of alternative forms of Ras exchange factors GRF and SOS1 in different human tissues and cell lines

DNA probes and antibodies specific for different coding regions of human SOS1 and GRF genes were used to screen expression of these genes in a variety of adult and fetal human tissues and cell lines. Despite previous reports of the exclusive expression of hGRF RNA in brain, we also observed expression of this gene in various other tissues including lung and pancreas, as well as several tumor cell lines. At least three different hGRF mRNA transcripts were observed depending on the probe used, with the larger transcripts being detected by probes corresponding to the 5' end of the gene while smaller transcripts were detected by probes corresponding to the 3' end. Expression of hSOS1-related transcripts was more ubiquitous and homogeneous than with hGRF, with similar levels of specific transcripts being detected in most tissues and cell fines tested. Three to five different transcripts were detected in human tissues when using probes for the 5' end and middle regions of this gene, whereas only two were detected with probes corresponding to the 3' end. Screening of multiple human tumor cell lines showed ubiquitous expression of three specific transcripts, although the level and ratio of each of these transcripts varied widely among individual cell lines. Consistent with the variety of transcripts detected, several protein forms were also identified in Western immunoblots with antisera raised against specific domains of hSOS1 and human Ras-GRF gene products. Fluorescence in situ chromosomal hybridization suggested that, in both cases, the multiple forms arise from single chromosomal loci. The heterogeneity of hGRF and hSOS1 gene products detected (which appear to retain in most cases a functional catalytic domain), suggests that differentially expressed, alternatively spliced hSOS1 and hGRF forms may contribute to fine regulation of Ras activation in different tissues or at different stages of development.

Stimulation of growth factor receptor signal transduction by activation of voltage-sensitive calcium channels

To understand the mechanisms by which electrical activity may generate long-term responses in the nervous system, we examined how activation of voltage-sensitive calcium channels (VSCCs) can stimulate the Ras/mitogen-activated protein kinase (MAPK) signaling pathway. Calcium influx through L-type VSCCs leads to tyrosine phosphorylation of the adaptor protein Shc and its association with the adaptor protein Grb2, which is bound to the guanine nucleotide exchange factor Sos1. In response to calcium influx, Shc, Grb2, and Sos1 inducibly associate with a 180-kDa tyrosine-phosphorylated protein, which was determined to be the epidermal growth factor receptor (EGFR). Calcium influx induces tyrosine phosphorylation of the EGFR to levels that can activate the MAPK signaling pathway. Thus, ion channel activation stimulates growth factor receptor signal transduction.

Downregulation of the Ras activation pathway by MAP kinase phosphorylation of Sos

Formation of a complex of the nucleotide exchange factor Sos, the SH2 and SH3 containing adaptor protein Grb2/Sem-5 and tyrosine phosphorylated EGF receptor and Shc has been implicated in the activation of Ras by epidermal growth factor (EGF) in fibroblasts: related mechanisms for activation of Ras operate in other cell types. An increase in the apparent molecular weight of Sos has been reported to occur after several minutes of receptor stimulation due to phosphorylation by mitogen-activated protein (MAP) kinases. We report here that treatment of human peripheral blood T lymphoblasts with phorbol esters causes a similar shift in mobility of Sos. This modification of Sos does not alter its ability to bind Grb2, but correlates with strong inhibition of the binding of the Sos/Grb2 complex to tyrosine phosphorylated sequences, either a tyrosine phosphopeptide in cell lysates or p36 in intact cells. This effect, along with the mobility shift of Sos, can be mimicked in vitro by phosphorylation of Sos by the mitogen-activated protein kinase, ERK1. A novel negative feedback mechanism therefore exists whereby activation of MAP kinases through Ras results in the uncoupling of the Sos/Grb2 complex from tyrosine kinase substrates without blocking the interaction of Sos with Grb2.

Quantitative analysis of Grb2-Sos1 interaction: the N-terminal SH3 domain of Grb2 mediates affinity

Grb2 is an adaptor protein that links receptor and cytoplasmic tyrosine kinases to the Ras signalling pathway by binding the Ras-specific guanine nucleotide exchange factor, Sos1, through its SH3 domains. The Grb2-SH3 domain binding has been localized to the carboxy-terminal two hundred amino acids of Sos1 (Sos1-c). By using real time biospecific interaction analysis (BIAcore), we studied the kinetic parameters and binding affinity of the Grb2-Sos1-c interaction. The binding of Grb2 to Sos1-c is a high affinity interaction with a moderate association rate (9.45 x 10(4) per M per s), a slow dissociation rate (13.8 x 10(-5) s), and an affinity constant of 1.48 nM. BIAcore measurements on isolated N-terminal and C-terminal SH3 domains (NSH3 and CSH3) further indicate that the high affinity Grb2-Sos1-c interaction is primarily mediated through the NSH3 domain (Kd = 1.68 nM). The CSH3 domain shows substantially reduced binding to Sos1-c in these measurements. Inhibition studies with BIAcore using proline rich peptides derived from the C-terminus of Sos1 show that there is a single major binding site for Grb2 in Sos1. This binding site is contained within the peptide N20, which corresponds to amino acids 1143-1162 of Sos1. This peptide completely blocks the Grb2-Sos1-c and NSH3-Sos1-c interactions with IC50 values of 8 microM and 4 microM respectively. The discrete interaction between the NSH3 domain and the N20 peptide may be amenable for drug discovery through screening or peptidomimetic approaches.

Cellular proteins binding to the first Src homology 3 (SH3) domain of the proto-oncogene product c-Crk indicate Crk-specific signaling pathways

The widely expressed c-Crk protein, composed of one SH2 and two SH3 domains, lacks an apparent catalytic domain, suggesting that it functions through the formation of specific complexes with other proteins. Bacterially expressed c-Crk formed in vitro highly stable complexes via the first SH3 domain [SH3(N)]. Most prominent were a 185 kDa protein of unknown identity (p185), Sos- immunoreactive bands of 170 kDa (p170) and 145 to 155 kDa bands, corresponding to the recently cloned C3G protein. p170 also bound to Ash/Grb2 and Nck while p185 and C3G bound only to Crk. Additional Crk binding proteins were found in hematopoietic cells, particularly the myeloid-monocytic lineage. The protein binding properties of Crk were subsequently compared to CRKL, the product of a homologous but distinct gene, and found to be very similar. The binding of two guanine nucleotide exchange factors, Sos and C3G, to Crk and CRKL indicates that Ras or related proteins likely play a role in signaling through Crk family proteins.

B cell antigen receptor cross-linking induces tyrosine phosphorylation and membrane translocation of a multimeric Shc complex that is augmented by CD19 co-ligation

The SH2 domain-containing transforming Shc protein has been implicated in mitogenic signaling via several surface receptors through p21ras. Following tyrosine phosphorylation by either receptor or non-receptor tyrosine kinases, Shc may interact with the adaptor protein Grb2, which is linked to Sos1, a guanine nucleotide exchange factor for human ras. Ligation of the antigen receptor complex on B cells (BCR) is known to activate various intracellular signaling pathways, which may accumulate in mitogenic responses. With respect to the initial steps, the activation of BCR-associated non-receptor tyrosine kinases appears to be indispensible. In this report we show that Shc proteins become tyrosine phosphorylated after BCR ligation on both transformed and normal human B cells. This is accompanied by the association of Shc with Grb2 proteins and a yet unidentified 145-kDa tyrosine phosphorylated protein. Subcellular fractionation revealed that this activation-induced multimeric Shc complex rapidly translocates towards the plasma membrane. Co-ligation of the BCR with the CD19 molecule results in a marked increase of these events, whereas CD19 cross-linking alone does not induce Shc tyrosine phosphorylation or translocation. Thus, in B cells the Shc complex may represent a molecular junction between the BCR and the mitogenic p21ras cascade.

Shc, Grb2, Sos1, and a 150-kilodalton tyrosine-phosphorylated protein form complexes with Fms in hematopoietic cells

Fms, the macrophage colony-stimulating factor (M-CSF) receptor, is normally expressed in myeloid cells and initiates signals for both growth and development along the monocyte/macrophage lineage. We have examined Fms signal transduction pathways in the murine myeloid progenitor cell line FDC-P1. M-CSF stimulation of FDC-P1 cells expressing exogenous Fms resulted in tyrosine phosphorylation of a variety of cellular proteins in addition to Fms. M-CSF stimulation also resulted in Fms association with two of these tyrosine-phosphorylated proteins, one of which was identified as the 55-kDa Shc, which is shown in other systems to be involved in growth stimulation, and the other was a previously uncharacterized 150-kDa protein (p150). Fms also formed complexes with Grb2 and Sos1, and neither contained phosphotyrosine. Whereas both Grb2 and Sos1 complexed with Fms only after M-CSF stimulation, the amount of Sos1 complexed with Grb2 was not M-CSF dependent. Shc coimmunoprecipitated Sos1, Grb2, and tyrosine-phosphorylated p150, while Grb2 immunoprecipitates contained mainly phosphorylated p150, Fms, Shc, and Sos1. Shc interacted with tyrosine-phosphorylated p150 via its SH2 domain, and the Grb2 SH2 domain likewise bound tyrosine-phosphorylated Fms and p150. Analysis of Fms mutated at each of four tyrosine autophosphorylation sites indicated that none of these sites dramatically affected p150 phosphorylation or its association with Shc and Grb2. M-CSF stimulation of fibroblast cell lines expressing exogenous murine Fms did not phosphorylate p150, and this protein was not detected either in cell lysates or in Grb2 or Shc immunoprecipitates. The p150 protein is not related to known signal transduction molecules and may be myeloid cell specific. These results suggest that M-CSF stimulation of myeloid cells could activate Ras through the nucleotide exchange factor Sos1 by Grb2 binding to either Fms, Shc, or p150 and that Fms signal transduction in myeloid cells differs from that in fibroblasts.

Membrane-targeting potentiates guanine nucleotide exchange factor CDC25 and SOS1 activation of Ras transforming activity

Growth factor-triggered activation of Ras proteins is believed to be mediated by guanine nucleotide exchange factors (CDC25/GRF and SOS1/2) that promote formation of the active Ras GTP-bound state. Although the mechanism(s) of guanine nucleotide exchange factor regulation is unclear, recent studies suggest that translocation of SOS1 to the plasma membrane, where Ras is located, might be responsible for Ras activation. To evaluate this model, we generated constructs that encode the catalytic domains of human CDC25 or mouse SOS1, either alone (designated cCDC25 and cSOS1, respectively) or terminating in the carboxyl-terminal CAAX membrane-targeting sequence from K-Ras4B (designated cCDC25-CAAX and cSOS1-CAAX, respectively; in CAAX, C is Cys, A is an aliphatic amino acid, and X is Ser or Met). We then compared the transforming potential of cCDC25 and cSOS1 with their membrane-targeted counterparts. We observed that addition of the Ras plasma membrane-targeting sequence to the catalytic domains of CDC25 and SOS1 greatly enhanced their focus-forming activity (10- to 50-fold) in NIH 3T3 transfection assays. Similarly, we observed that the membrane-targeted versions showed a 5- to 10-fold enhanced ability to induce transcriptional activation from the Ets/AP-1 Ras-responsive element. Furthermore, whereas cells that stably expressed cCDC25 or cSOS1 exhibited the same morphologies as untransformed NIH 3T3 cells, cells expressing cCDC25-CAAX or cSOS1-CAAX displayed transformed morphologies that were indistinguishable from the elongated and refractile morphology of oncogenic Ras-transformed cells. Thus, these results suggest that membrane translocation alone is sufficient to potentiate guanine nucleotide exchange factor activation of Ras.

The transmembrane domain of the large subunit of HSV-2 ribonucleotide reductase (ICP10) is required for protein kinase activity and transformation-related signaling pathways that result in ras activation

The large subunit of Herpes simplex virus type 2 ribonucleotide reductase (ICP10) is a chimera consisting of a Ser/Thr protein kinase (PK) with features of a transmembrane (TM) helical segment localized at the amino terminus, and the RR1 domain localized at the carboxy terminus. To elucidate the role of the TM segment in ICP10-mediated transformation we established cell lines that constitutively express ICP10 (JHLa1) or its TM deleted mutant p139TM (JHL15). ICP10 was associated with purified JHLa1 plasma membranes. Membrane immunofluorescence and FACS analysis with antibodies to synthetic peptides located upstream and downstream of the TM indicated that ICP10 is a membrane-spanning protein. p139TM was not associated with JHL15 plasma membranes. ICP10 kinase activity was detected in JHLa1 but not JHL15 cells as determined by immunocomplex kinase assays and metabolic labeling. JHLa1 cells displayed anchorage-independent growth whereas JHL15 cells and JHL9 cells that express a mutant deleted in the PK catalytic domain were negative. ras-GTPase activating protein (ras-GAP) was phosphorylated in JHLa1 but not JHL15 cells and GTPase activity was lower in JHLa1 than JHL15 cells. Furthermore, ICP10 but not p139TM bound the guanine nucleotide releasing factor son of sevenless 1 (Sos1) and ras-GTP (activated ras) was higher in JHLa1 than JHL15 cells. The data suggest that ICP10 constitutively increases ras activity, and its TM segment plays a critical role in transformation-related signaling pathways.