Conservatively managed patients with stage 5 chronic kidney disease--outcomes from a single center experience

BACKGROUND

Limited survival data are available on chronic kidney disease stage 5 (CKD 5) patients who opt for conservative management rather than dialysis.

AIM

To measure survival in such patients and investigate potential factors predicting survival.

DESIGN

Retrospective survival analysis of a cohort of conservatively managed CKD 5 patients from a single center.

METHODS

Survival was measured in 69 conservatively managed patients from the time they were first known to have CKD 5. Comorbidities, residual renal function and other laboratory parameters (calcium, phosphate, parathyroid hormone, albumin and hemoglobin) and blood pressure were recorded.

RESULTS

Overall median patient survival from the time of first known CKD 5 was 21 months. Patients known to a nephrologist before reaching CKD 5 survived longer (median 32 months) than those presenting with CKD 5 (15 months, P = 0.025). Serum albumin >35 g/l was associated with greater survival, but other biochemical parameters, comorbidity grade and age did not predict survival.

CONCLUSION

These survival data provide useful information for nephrologists counseling CKD 5 patients considering whether to pursue dialysis or conservative management. Risk factors that correlate with survival in the dialysis population may not predict survival in conservatively managed CKD 5 patients.

The management of CKD: a look into the future

The increasing global prevalence of chronic kidney disease (CKD) and end-stage renal disease with the associated spiraling cost has profound public health and economic implications. This has made slowing the progression of CKD, a major health-care priority. CKD is invariably characterized by progressive kidney fibrosis and at present, treatment aiming to slow the progression of CKD is limited to aggressive blood pressure control, with few therapies targeting the fibrotic process itself. In this review, we explore the potential of experimental therapeutic strategies, based on preventing or reversing the pathophysiologic steps of kidney remodeling that lead to fibrosis.

The role of geranylgeranylated proteins in human mesangial cell proliferation

The Rho family of guanine 5'-triphosphatases (GTPases) play a key role in regulating cell proliferation, tubulointerstitial fibrosis, and glomerular hemodynamics. The post-translational prenylation of RhoGTPases by the addition of a geranylgeranyl moiety is critical for cellular localization and signaling activity. This study investigates the effects of (i) inhibiting geranylgeranylation (GG) in human mesangial cell (HMC) proliferation and apoptosis, using GGTI 298, a specific inhibitor of GG and (ii) lovastatin, an HMG-coacetyl A-reductase inhibitor, which depletes the availability of prenylation substrates. HMC proliferation was assessed using an assay of viable cell number and measuring bromodeoxyuridine (BrdU) incorporation. Hoechst 33342 staining was used to determine apoptosis. Extracellular signal-regulated protein kinase (Erk)1/2 and Akt activation were analysed by Western blotting. Rho activation was determined using the Rhotekin pull-down assay. Immunocytochemistry was performed to study the effects on the actin cytoskeleton and RhoA localization. GGTI 298 (10-20 muM) and lovastatin (5-10 muM) potently inhibited platelet-derived growth factor and serum-stimulated HMC proliferation and induced apoptosis. These effects of lovastatin were attenuated by co-incubation with geranylgeranylpyrophosphate. C3 exoenzyme, a clostridial toxin that specifically targets Rho also inhibited BrdU incorporation and promoted apoptosis. GGTI 298 increased cytosolic expression of RhoA, prevented RhoA activation, and inhibited the activation of Erk1/2 and the survival protein Akt. GGTI 298, lovastatin, and C3 exoenzyme inhibit HMC proliferation and promote apoptosis. Inhibiting GG increases cytosolic RhoA expression, disrupts the actin cytoskeleton, and inhibits RhoA activation. These results suggest that targeting geranylgeranylated proteins with statins or GGTI 298 is a promising therapeutic strategy in human mesangioproliferative renal disease.

A selective inhibitor of the p110delta isoform of PI 3-kinase inhibits AML cell proliferation and survival and increases the cytotoxic effects of VP16

Current therapy for acute myeloid leukaemia (AML) is suboptimal with a high incidence of relapse. There is strong evidence that constitutive phosphoinositide 3-kinase (PI3K) activity plays a significant role in the pathophysiology of AML. PI3K products are derived from the activity of a number of PI3K catalytic isoforms (class I, II and III) but the relative contribution of these enzymes in AML remains unknown. As non-isoform-selective inhibitors of PI3K such as LY294002 may produce unwanted toxicity to normal tissues, we have investigated the role of the leukocyte-restricted p110delta PI3K isoform in 14 cases of AML. p110delta was detected in all cases whereas the expression levels of the other class I PI3Ks varied more widely, and were often undetectable. The p110delta-selective compound IC87114 inhibited constitutive phosphorylation of the PI3K target Akt/PKB and reduced cell number to a mean of 66+/-5% (range 14-88%). In eight cases, the combination of IC87114 and VP16 (a topoisomerase II inhibitor) was synergistic in reducing viable cell number, and was associated with a reduction in constitutive NF-kappaB activity. IC87114 did not have direct adverse effects or enhance the activity of VP16 on the proliferation and survival of normal haemopoietic progenitors. Overall, our results identify the p110delta isoform as a potential therapeutic target in AML and support a clinical approach to use isoform-selective over broad-spectrum PI3K inhibitors.

Distinct functions for Ras GTPases in the control of proliferation and apoptosis in mouse and human mesangial cells

In previous work, we have demonstrated that Ras GTPases regulate proliferation in a range of human renal cells. The present work compares human and mouse mesangial cell (HMC and MMC) responses to specific knockdown of Ras genes with antisense oligonucleotides (AS-oligos), and examines the role of the p21 (cip1) and p27 (kip1) cyclin-dependent kinase inhibitors in these responses in mouse cells. HMC and MMC were lipofectin transfected with ras-targeted AS-oligo at 200-400 nM for 18 h followed by growth of cells in 20% serum for 18-72 h. Cell proliferation was assessed with an MTS assay and bromodeoxyuridine (BrdU) uptake. Apoptosis was quantified using nuclear stain with Hoechst 33342 dye. In MMC, Ha-ras AS-oligo caused an increase in apoptosis from <2% to 10-15% of cells after 18 h in serum (P<0.01). Control, Ki-ras and N-ras AS-oligos had minimal effects on apoptosis. BrdU uptake studies showed that BrdU+ve MMC were increased by 20-40% (P<0.05) after Ha-ras AS-oligo at 24 h; other ras AS-oligos were inactive. HMC number was reduced by 40-80% (P<0.01) at 48-72 h by both Ha-ras and Ki-ras AS-oligos. These actions were associated with reductions in BrdU+ve cells. In HMC, the ras AS-oligos did not induce apoptosis. p21(-,-) MMC showed exaggerated apoptotic responses to Ha-Ras AS-oligo. In mouse cells, Ha-Ras expression appears necessary to prevent apoptotic cell death; Ras expression does not appear necessary for cells to progress through the cell cycle. In human cells, Ras does not appear necessary to prevent apoptosis but Ha-Ras and Ki-Ras appear to be required for cell cycle progression.

PI3-kinase/Akt is constitutively active in primary acute myeloid leukaemia cells and regulates survival and chemoresistance via NF-kB, MAPkinase and p53 pathways

The phosphoinositide 3-kinase (PI3-kinase) signalling pathway plays a key role in the regulation of cell survival and proliferation. We show that the PI3-kinase/Akt pathway is constitutively active in primary acute myeloid leukaemia (AML) cells and that blockade by the selective inhibitor LY294002 reduces survival of the total blast population (mean 52%). The ERK/MAPK module is also constitutively active and treatment with the MAPKK inhibitor U0126 reduces cell survival by 22%. In 10 of 18 samples, PI3-kinase contributes to MAPK activation as incubation with LY294002 leads to a marked reduction in its phosphorylation. PI3-kinase inhibition reduces survival of the CD34+38- AML progenitor subset by 44%, whereas MAPKK inhibition has little effect. Reporter assays in primary AML cells show that blocking PI3-kinase leads to a marked reduction of constitutive NF-kappaB activity and promotes p53-mediated transcription. This is associated with a synergistic interaction between LY294002 and Ara-C. An inducible activated form of Akt protects normal myeloid cells from Ara-C and etoposide-mediated apoptosis. These results show that blocking PI3-kinase has direct antileukaemic effects and potentiates the response to conventional cytotoxics via a number of targets including NF-kappaB, p53 and MAPK. Inhibitors of PI3-kinase and Akt may be useful in the treatment of AML.

Growth factor withdrawal from primary human erythroid progenitors induces apoptosis through a pathway involving glycogen synthase kinase-3 and Bax

The prevention of apoptosis is a key function of growth factors in the regulation of erythropoiesis. This study examined the role of the constitutively active serine/threonine kinase glycogen synthase kinase-3 (GSK3), a target of the phosphoinositide-3-kinase (PI3K)/Akt pathway, in the regulation of apoptosis in primary human erythroid progenitors. GSK3 phosphorylation at its key regulatory residues S21 (alpha isoform) and S9 (beta isoform) was high in steady-state culture, disappeared on growth factor withdrawal, and returned in response to treatment of cells with either erythropoietin or stem cell factor. Phosphorylation correlated with a PI3K-dependent reduction of 25% to 30% in measured GSK3 activity. LY294002, a specific inhibitor of PI3K, induced apoptosis in growth factor-replete erythroid cells to a degree similar to growth factor deprivation, whereas the Mek1 inhibitor U0126 had no effect, implicating PI3K and not mitogen-activated protein kinase in survival signaling. Growth factor-deprived erythroblasts, which undergo apoptosis rapidly, were protected from apoptosis by both lithium chloride, a GSK3 selective inhibitor, and inhibition of caspase activity. However, the clonogenic potential of single cells, which more accurately reflects cell survival, was maintained by lithium chloride, but not by caspase inhibition. Furthermore, lithium chloride, but not caspase inhibition, prevented the appearance of the conformational form of Bax associated with apoptosis induction. In summary, GSK3 activity is suppressed by erythropoietin and stem cell factor in human erythroid progenitor cells, and increased GSK3 activity, brought about by growth factor withdrawal, may regulate commitment to cell death through a caspase-independent pathway that results in a conformational change in Bax.

The PI3 kinase, p38 SAP kinase, and NF-kappaB signal transduction pathways are involved in the survival and maturation of lipopolysaccharide-stimulated human monocyte-derived dendritic cells

As a dendritic cell (DC) matures, it becomes more potent as an antigen-presenting cell. This functional change is accompanied by a change in DC immunophenotype. The signal transduction events underlying this process are poorly characterized. In this study, we have investigated the signal transduction pathways involved in the lipopolysaccharide (LPS)-induced maturation of human monocyte-derived DCs (MoDCs) in vitro. We show that exposure of immature MoDCs to LPS activates the p38 stress-activated protein kinase (p38SAPK), extracellular signal-regulated protein kinase (ERK), phosphoinositide 3-OH kinase (PI3 kinase)/Akt, and nuclear factor (NF)-kappaB pathways. Studies using inhibitors demonstrate that PI3 kinase/Akt but not the other pathways are important in maintaining survival of LPS-stimulated MoDCs. Inhibiting p38SAPK prevented activation of the transcription factors ATF-2 and CREB and significantly reduced the LPS-induced up-regulation of CD80, CD83, and CD86, but did not have any significant effect on the LPS-induced changes in macropinocytosis or HLA-DR, CD40, and CD1a expression. Inhibiting the NF-kappaB pathway significantly reduced the LPS-induced up-regulation of HLA-DR as well as CD80, CD83, and CD86. Inhibiting the p38SAPK and NF-kappaB pathways simultaneously had variable effects depending on the cell surface marker studied. It thus appears that different aspects of LPS-induced MoDC maturation are regulated by different and sometimes overlapping pathways.

Prenylation inhibitors in renal disease

Members of the superfamily of Ras GTPase signalling proteins (monomeric G proteins) require post-translational carboxy-terminal prenylation to function. Prenylation is the covalent attachment of a hydrophobic prenyl group (either farnesyl or geranylgeranyl), which localises the GTPase to cell membranes. Ras proteins exert substantial control on cell proliferation and gene-transcription events, and prenylation inhibitors are now included in clinical trials for cancer. Many renal diseases are highly proliferative and are driven by a range of profibrotic cytokines. We hypothesise that inhibition of prenylation could be of substantial therapeutic benefit in such diseases, providing greater selectivity against abnormal cytokine-driven proliferation and fibrogenesis than current treatments available to nephrologists.

Resistance to the cytotoxic effects of tumor necrosis factor alpha can be overcome by inhibition of a FADD/caspase-dependent signaling pathway

Tumor necrosis factor (TNF) alpha initiates the activation of a pro-apoptotic pathway involving the recruitment of the death domain containing protein FADD and the subsequent activation of specific proteases (caspases). Many cells are resistant, however, to the cytotoxic effects of TNFalpha due to the concurrent activation of pro-survival pathways involving the transcription factor NFkappaB and TRAF2. Here we show that the TNFalpha-activated FADD/caspase pathway can also exert an unexpected pro-survival effect. Inhibition of this pathway in NIH3T3 fibroblasts or U937 leukemic cells by peptide caspase inhibitors or expression of dominant-negative FADD leads to rapid death following treatment with TNFalpha, whereas control cells are TNFalpha-resistant. FADD/caspase-inhibited cells die by a non-apoptotic mechanism caused by increased production of reactive oxygen species which precedes loss of the mitochondrial membrane potential. Cytotoxicity can be prevented by preincubation with antioxidants including reduced glutathione or by expression of a dominant-negative Rac GTP-binding protein. These results indicate that caspase activation in response to TNFalpha has anti-necrotic as well as pro-apoptotic effects and extend our understanding of the biological role of these proteases.

Caspase-mediated proteolysis and activation of protein kinase Cdelta plays a central role in neutrophil apoptosis

Neutrophils undergo constitutive apoptosis when aged ex vivo. Recent studies have indicated roles for Fas/CD95 and the nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase system in this process. We have investigated the role of protein kinase C (PKC) in neutrophil death. We show that there is proteolysis and activation of the novel isoform PKCdelta in aged neutrophils and that this process is accelerated by the addition of an agonistic Fas antibody. PKCdelta proteolysis occurs before the onset of any detectable features of apoptosis and pharmacologic inhibition of this enzyme inhibits neutrophil apoptosis. PKCdelta cleavage and activation is dependent on caspase-8/FADD-like interleukin-1beta converting enzyme (FLICE)-mediated processing of caspase-3/CPP32. Neutrophil survival is prolonged by the addition of broad spectrum (BD.fmk) or caspase-8 targeted (zIETD.fmk) peptide caspase inhibitors. Inhibition of PKCdelta does not prevent apoptosis triggered by factor withdrawal in immature hematopoietic cells, including normal human CD34(+) progenitors indicating that within a given lineage, the mechanisms of apoptosis may be differentiation-stage-specific. Ex vivo aging of neutrophils leads to the increasing production of reactive oxygen species and this is attenuated in cells treated with either caspase or PKCdelta inhibitors. Proteolytically activated PKCdelta acts as a molecular link between the Fas/CD95 receptor and the NADPH-oxidase system and plays a central role in regulating the process of neutrophil apoptosis.

Stent placement for treatment of mesenteric artery stenoses or occlusions

PURPOSE

To evaluate stent placement in the treatment of mesenteric ischemia.

PATIENTS AND METHODS

Twelve patients (eight women, four men; mean age, 63 years) with chronic mesenteric ischemia underwent stent placement for stenoses or occlusions during a 5.5-year period. Nine patients with 10 stenoses (three celiac arteries, seven superior mesenteric arteries) and three patients with three chronic occlusions (two superior mesenteric arteries, one aortosuperior mesenteric artery bypass graft) were treated.

RESULTS

Initial technical success was achieved in 11 of the 12 patients (92%), including all three patients with chronic occlusions. There were no technical complications. There was one postprocedural death (<30 days) due to bowel ischemia and infarction, despite a technically successful procedure. Clinical follow-up was available in all 12 patients, with a mean follow-up of 15.7 months (range, 0-38.5 months). Primary and primary-assisted patency up to 18 months was 74% (standard error [SE], 13%) and 83% (SE, 11%), respectively. Secondary patency was 83% (SE, 11%) at 3 years. All three patients (100%) with chronic occlusions had relief of clinical signs and symptoms at a mean follow-up of 22 months (range, 13-38.5 months).

CONCLUSIONS

Stent placement is safe and clinically effective as an adjunctive therapy to angioplasty or as a primary method of treatment for chronic mesenteric ischemia in patients with focal visceral artery stenoses or occlusions.

Phosphoinositide 3-kinase induces scattering and tubulogenesis in epithelial cells through a novel pathway

Hepatocyte growth factor/scatter factor (HGF/SF) treatment of the Madin-Darby canine kidney epithelial cell line causes scattering of cells grown in monolayer culture and the formation of branching tubules by cells grown in collagen gels. HGF/SF causes prolonged activation of both the mitogen-activated protein (MAP) kinase extracellular signal-regulated kinase 2 (ERK2) and the phosphoinositide 3-OH kinase (PI 3-kinase) target protein kinase B (PKB)/Akt; inhibition of either the MAP kinase pathway by the MAP kinase/ERK kinase inhibitor PD98059 or the PI 3-kinase pathway by LY294002 blocks HGF/SF induction of scattering, although in morphologically distinct ways. Expression of constitutively activated PI 3-kinase, Ras, or R-Ras will cause scattering, but activated Raf will not, indicating that activation of the MAP kinase pathway is not sufficient for this response. Downstream of PI 3-kinase, activated PKB/Akt and Rac are both unable to induce scattering, implicating a novel pathway. Scattering induced by Ras or PI 3-kinase is sensitive to PD98059, as well as to LY294002, suggesting that basal MAP kinase activity is required, but not sufficient, for the scattering response. Induction of MDCK cell tubulogenesis in collagen gels by HGF/SF is inhibited by PD98059; expression of activated Ras and Raf causes disorganized growth in this system, but activated PI 3-kinase or R-Ras causes branching tubule formation similar to that seen with HGF/SF treatment. These data indicate that multiple signaling pathways acting downstream of Met and Ras are needed for these morphological effects; scattering is induced primarily by the PI 3-kinase pathway, which acts through effectors other than PKB/Akt or Rac and requires at least basal MAP kinase function. Elevated PI 3-kinase activity induces tubulogenesis, but total inhibition and excess activation of the MAP kinase pathway both oppose this effect.

A truncated isoform of the human beta chain common to the receptors for granulocyte-macrophage colony-stimulating factor, interleukin-3 (IL-3), and IL-5 with increased mRNA expression in some patients with acute leukemia

We report here a naturally occurring isoform of the human beta chain common to the receptors for granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 (GMRbetaC) with a truncated intracytoplasmic tail caused by deletion of a 104-bp exon in the membrane-proximal region of the chain. This beta intracytoplasmic truncated chain (betaIT) has a predicted tail of 46 amino acids, instead of 432 for betaC, with 23 amino acids in common with betaC and then a new sequence of 23 amino acids. In primary myeloid cells, betaIT comprised approximately 20% of the total beta chain message, but was increased up to 90% of total in blast cells from a significant proportion of patients with acute leukemia. Specific anti-betaIT antibodies demonstrated its presence in primary myeloid cells and cell lines. Coexpression of betaIT converted low-affinity GMRalpha chains (KD 2.5 nmol/L) to higher-affinity alphabeta complexes (KD 200 pmol/L). These could bind JAK2 that was tyrosine-phosphorylated by stimulation with GM-CSF. betaIT did not support GM-CSF-induced proliferation when cotransfected with GMRalpha into CTLL-2 cells. Therefore, it may interfere with the signal-transducing properties of the betaC chain and play a role in the pathogenesis of leukemia.

Lack of correlation between activation of Jun-NH2-terminal kinase and induction of apoptosis after detachment of epithelial cells

Detachment of epithelial cells from the extracellular matrix leads to induction of programmed cell death, a process that has been termed "anoikis." It has been reported recently that detachment of MDCK cells from matrix results in activation of Jun-NH2-terminal kinases (JNKs) and speculated that these stress activated protein kinases play a causal role in the induction of anoikis (Frisch, S.M., K. Vuori, D. Kelaita, and S. Sicks. 1996. J. Cell Biol. 135:1377-1382). We report here that although JNK is activated by detachment of normal MDCK cells, study of cell lines expressing activated signaling proteins usually controlled by Ras shows that stimulation of JNK fails to correlate with induction of anoikis. Activated phosphoinositide 3-OH kinase and activated PKB/Akt protect MDCK cells from detachment-induced apoptosis without suppressing JNK activation. Conversely, activated Raf and dominant negative SEK1, a JNK kinase, attenuate detachment-induced JNK activation without protecting from apoptosis. zVAD-fmk, a peptide inhibitor of caspases, prevents MDCK cell anoikis without affecting JNK activation. p38, a related stress-activated kinase, is also stimulated by detachment from matrix, but inhibition of this kinase with SB 203580 does not protect from anoikis. It is therefore unlikely that either JNK or p38 play a direct role in detachment-induced programmed cell death in epithelial cells.

Granulocyte-macrophage colony stimulating factor receptor alpha and beta chain complexes can form both high and intermediate affinity functional receptors

Scatchard analysis of primary human haemopoietic cells using iodinated GM-CSF suggests that there are low, intermediate and high affinity classes of the GM-CSF receptor. To investigate the molecular basis of this, we generated a clone of transfected NIH3T3 cells that constitutively expressed the human granulocyte-macrophage colony stimulating factor receptor (GM-CSF R) beta chain and inducibly expressed the human GM-CSF R alpha chain. In the cells fully induced to express the alpha chain the overall level of expression of the alpha and beta chains at the cell surface was comparable with that found in primary haemopoietic cells and cell lines. When cells were partially induced to express the alpha chain, the alpha:beta ratio determined by antibody binding was approximately 1:1 and Scatchard analysis revealed a single class of intermediate affinity receptors (Kd = 614+/-88 pM). In cells with fully induced alpha chain expression, the alpha:beta ratio was approximately 3:1 and there was a switch to a dual high and low affinity receptor with K(d)s of 67+/-32 pM and 1.7+/-0.56 nM respectively. The change from intermediate to high affinity was not associated with changes in alphabeta stoichiometry as detected by cross-linking with radiolabelled GM-CSF and gel electrophoresis. Both the high and intermediate affinity receptors were able to activate the STAT 5 and the MAP kinase pathways, although there was a difference in the ligand dose-response curves which was compatible with the different affinities of the receptors. It is proposed that the switch from an intermediate to high affinity receptor was due to the availability of free alpha chains presenting ligand to the alphabeta chain complexes at the surface of the cell membrane.

Matrix adhesion and Ras transformation both activate a phosphoinositide 3-OH kinase and protein kinase B/Akt cellular survival pathway

Upon detachment from the extracellular matrix, epithelial cells enter into programmed cell death, a phenomenon known as anoikis, ensuring that they are unable to survive in an inappropriate location. Activated ras oncogenes protect cells from this form of apoptosis. The nature of the survival signals activated by integrin engagement and usurped by oncogenic Ras are unknown: here we show that in both cases phosphoinositide 3-OH kinase (PI 3-kinase), but not Raf, mediates this protection, acting through protein kinase B/Akt (PKB/Akt). Constitutively activated PI 3-kinase or PKB/Akt block anoikis, while inhibition of PI 3-kinase abrogates protection by Ras, but not PKB/Akt. Inhibition of either PI 3-kinase or PKB/Akt induces apoptosis in adherent epithelial cells. Attachment of cells to matrix leads to rapid elevation of the levels of PI 3-kinase lipid products and PKB/Akt activity, both of which remain high in Ras-transformed cells even in suspension. PI 3-kinase acting through PKB/Akt is therefore implicated as a key mediator of the aberrant survival of Ras-transformed epithelial cells in the absence of attachment, and mediates matrix-induced survival of normal epithelial cells.

Role of phosphoinositide 3-OH kinase in cell transformation and control of the actin cytoskeleton by Ras

The pathways by which mammalian Ras proteins induce cortical actin rearrangement and cause cellular transformation are investigated using partial loss of function mutants of Ras and activated and inhibitory forms of various postulated target enzymes for Ras. Efficient transformation by Ras requires activation of other direct effectors in addition to the MAP kinase kinase kinase Raf and is inhibited by inactivation of the PI 3-kinase pathway. Actin rearrangement correlates with the ability of Ras mutants to activate PI 3-kinase. Inhibition of PI 3-kinase activity blocks Ras induction of membrane ruffling, while activated PI 3-kinase is sufficient to induce membrane ruffling, acting through Rac. The ability of activated Ras to stimulate PI 3-kinase in addition to Raf is therefore important in Ras transformation of mammalian cells and essential in Ras-induced cytoskeletal reorganization.

Networks of interaction of p120cbl and p130cas with Crk and Grb2 adaptor proteins

P120cbl, the product of the c-cbl proto-oncogene, has previously been shown to become tyrosine phosphorylated following EGF stimulation of cells, and to bind constitutively to the SH3 domain of the adaptor protein Grb2. Here we show that another adaptor protein, Crk, binds through its SH2 domain to tyrosine phosphorylated p120cbl. In addition, Crk becomes phosphorylated on tyrosine and serine following EGF treatment of PC12 and other cell lines. In unstimulated cells, while Grb2 is not bound to any tyrosine phosphoprotein, Crk is bound via its SH2 domain to tyrosine phosphorylated p130cas, the Crk-associated v-Src substrate. Following EGF treatment, Crk dissociates from p130cas, possibly due to a higher affinity of Crk SH2 for p120cbl compared with p130cas. Interaction between Grb2 and p120cbl increases threefold following EGF treatment of cells; in vitro, this induction of Grb2 association with unphosphorylated p120cbl can be mimicked by the addition of tyrosine phosphorylated Shc, suggesting a transfer of information between the SH2 and SH3 domains of Grb2. These data indicate that adaptor proteins can exchange binding partners in response to stimuli, and that different adaptor proteins can bind to the same partners by different mechanisms.