Direct visualization by FRET-FLIM of a putative mechanosome complex involving Src, Pyk2 and MBD2 in living MLO-Y4 cells

Earlier, we proposed the “mechanosome” concept as a testable model for understanding how mechanical stimuli detected by cell surface adhesion molecules are transmitted to modulate gene expression inside cells. Here, for the first time we document a putative mechanosome involving Src, Pyk2 and MBD2 in MLO-Y4 osteocytes with high spatial resolution using FRET-FLIM. Src-Pyk2 complexes were concentrated at the periphery of focal adhesions and the peri-nuclear region. Pyk2-MBD2 complexes were located primarily in the nucleus and peri-nuclear region. Lifetime measurements indicated that Src and MBD2 did not interact directly. Finally, mechanical stimulation by fluid flow induced apparent accumulation of Src-Pyk2 protein complexes in the peri-nuclear/nuclear region, consistent with the proposed behavior of a mechanosome in response to a mechanical stimulus.

Prognostic relevance of Src activation in stage II-III colon cancer

Src belongs to a family of cytoplasmic tyrosine kinases that play a key role in tumor initiation and progression. Src activation has been associated with a more aggressive neoplastic phenotype and induces resistance to platinum agents in preclinical models. The aim of our study was to assess the prognostic and/or predictive value of Src activation in patients with stage II-III colon cancer. pSrc expression was assessed in paraffin-embedded tumor samples by immunohistochemistry (phospho-Y418, ab4816; Abcam). Cases were classified by staining intensity in 4 categories: no staining (0), weak (1+), moderate (2+), and intense (3+) staining. A total of 487 patients were evaluated (240 stage II, 247 stage III), of whom 298 (61%) had received adjuvant chemotherapy. Staining was absent in 78 (16%), weak in 262 (54%), moderate in 103 (21%), and intense in 44 (9%). High pSrc expression was significantly associated with decreased 5-year disease-free survival (39% versus 63% for patients with high versus low pSrc expression; hazard ratio, 0.56; P=.005) and overall survival (58% versus 74%; hazard ratio, 0.55; P=.02). Multivariate analysis confirmed pSrc expression as a significant prognostic factor both for disease-free survival and overall survival, independent of age, sex, tumor stage, bowel obstruction/perforation, or adjuvant chemotherapy. These findings illustrate the relevance of Src activation in colon cancer biology, conferring a poor prognosis to patients with early stage colon cancer regardless of adjuvant chemotherapy. Our findings may help improve prognostic stratification of patients for clinical decisions and open new avenues for potential pharmacologic manipulation that may eventually improve patients' outcomes.

Src/STAT3 signaling pathways are involved in KAI1-induced downregulation of VEGF-C expression in pancreatic cancer

A previous study by our group demonstrated that overexpression of KAI1 was associated with lymphatic metastasis in pancreatic cancer. The present study further investigated the signaling pathways involved in KAI1‑induced downregulation of vascular endothelial growth factor C (VEGF‑C) and lymphatic metastasis in pancreatic cancer. Immunohistochemistry was performed to examine KAI1 and VEGF‑C expression in 28 surgically resected pancreatic cancer tissues. MIA PaCa‑2 and PCAN1 pancreatic cancer cell lines were transfected with KAI1 overexpression vector. VEGF‑C expression as well as phosphorylation of Src and signal transducer and activator of transcription (STAT)3 were assessed by western blot analysis. Furthermore, the signal transduction inhibitors PP2 and AG490 were used to block the Src and STAT3 signaling pathways, respectively. KAI1 was negatively correlated with VEGF‑C expression in pancreatic tumor samples. In MIA PaCa‑2 cells, VEGF‑C expression was more significantly inhibited by restoration of KAI1 than that in PCAN1 cells. In addition, Src and STAT3 phosphorylation was decreased by KAI1 in MIA PaCa‑2 cells. Of note, pre‑treatment with PP2 efficiently reversed the KAI1-induced enhancement of Src and STAT3 phosphorylation as well as VEGF‑C expression. Pre‑treatment with AG490 efficiently reversed the KAI1-induced enhancement of STAT3 phosphorylation and VEGF‑C expression, but had no effect on the upregulation of Src phosphorylation. The present study identified the involvement of Src/STAT3 signaling pathways in KAI1‑induced downregulation of VEGF‑C expression and suggested the implication of these pathways in lymphatic metastasis of pancreatic cancer.

CDCP1 identifies a CD146 negative subset of marrow fibroblasts involved with cytokine production

In vitro expanded bone marrow stromal cells contain at least two populations of fibroblasts, a CD146/MCAM positive population, previously reported to be critical for establishing the stem cell niche and a CD146-negative population that expresses CUB domain-containing protein 1 (CDCP1)/CD318. Immunohistochemistry of marrow biopsies shows that clusters of CDCP1+ cells are present in discrete areas distinct from areas of fibroblasts expressing CD146. Using a stromal cell line, HS5, which approximates primary CDCP1+ stromal cells, we show that binding of an activating antibody against CDCP1 results in tyrosine-phosphorylation of CDCP1, paralleled by phosphorylation of Src Family Kinases (SFKs) Protein Kinase C delta (PKC-δ). When CDCP1 expression is knocked-down by siRNA, the expression and secretion of myelopoietic cytokines is increased. These data suggest CDCP1 expression can be used to identify a subset of marrow fibroblasts functionally distinct from CD146+ fibroblasts. Furthermore the CDCP1 protein may contribute to the defining function of these cells by regulating cytokine expression.

SRC-family tyrosine kinases in oogenesis, oocyte maturation and fertilization: an evolutionary perspective

The oocyte is a highly specialized cell poised to respond to fertilization with a unique set of actions needed to recognize and incorporate a single sperm, complete meiosis, reprogram maternal and paternal genomes and assemble them into a unique zygotic genome, and finally initiate the mitotic cell cycle. Oocytes accomplish this diverse series of events through an array of signal transduction pathway components that include a characteristic collection of protein tyrosine kinases. The src-family protein kinases (SFKs) figure importantly in this signaling array and oocytes characteristically express certain SFKs at high levels to provide for the unique actions that the oocyte must perform. The SFKs typically exhibit a distinct pattern of subcellular localization in oocytes and perform critical functions in different subcellular compartments at different steps during oocyte maturation and fertilization. While many aspects of SFK signaling are conserved among oocytes from different species, significant differences exist in the extent to which src-family-mediated pathways are used by oocytes from species that fertilize externally vs those which are fertilized internally. The observation that several oocyte functions which require SFK signaling appear to represent common points of failure during assisted reproductive techniques in humans, highlights the importance of these signaling pathways for human reproductive health.

Phosphorylated c-Src is a novel predictor for recurrence in cervical squamous cell cancer patients

AIMS

The molecular mechanisms of the tumorigenesis and recurrence of cervical cancer are poorly understood. The objective of this study was to analyze the expression of phosphorylated c-Src (phospho-c-Src) and its clinical significance in human cervical cancer.

METHODS

The expression of phospho-c-Src was determined by immunohistochemistry in a total of 127 cervical specimens including 20 normal cervix tissues, 20 cases of carcinoma in situ of cervix (CIS), and 87 cases of cervical squamous cell carcinoma (CSCC).

RESULTS

The expression of phospho-Src in normal cervix, CIS, and CSCC increased gradually in ascending order (p=0.026). In addition, the expression of phospho-Src was correlated with overall (p=0.037) and recurrence (p=0.001) survival of cervical cancer. In multivariate Cox regression analysis, phospho-Src expression was an independent prognosis factor for recurrence-free survival (p=0.004).

CONCLUSION

Our present study suggests that Src signaling may play essential role in cervical cancer progression. Phospho-Src expression may be considered as a prognostic marker to predict recurrence in CSCC.

The tyrosine kinase inhibitor AMN107 (Nilotinib) exhibits off-target effects in lymphoblastic cell lines

The aminopyrimidine inhibitor AMN107 (Nilotinib) was rationally designed to antagonize the aberrant tyrosine kinase activity of Bcr-Abl-positive cells. We here evaluated, whether AMN107 is also able to induce apoptosis in Bcr-Abl-negative cells of lymphatic origin. The B-cell lines DOHH-2 and WSU-NHL and the T-cell lines Jurkat and HUT78 were incubated with increasing amounts of AMN107 corresponding to clinically achievable dosages. Subsequently, induced molecular changes were assessed by FACS analysis, Western blot, and enzyme activity assays. Although AMN107 exhibited only a minor apoptosis-inducing effect in the T-cell lines, it exerted a considerable, dose-dependent cytotoxicity in the B-cell lines. Using selective caspase-inhibitors, we show that apoptosis in responder cell lines critically relies on activation of caspase-6 and caspase-9. Cell lines sensitive and resistant towards AMN107 can be discriminated by their differential expression of Src-kinases. Although the AMN107-sensitive cell lines DOHH-2 and WSU-NHL exhibited low or no expression of the Src-kinases Lck, phosphorylated Lck, and Yes with a concomitant high expression of Hck, Lyn, and phosphorylated Lyn, the expression pattern of these kinases was inverse in the AMN107-resistant T-cell lines. In conclusion, this is the first report providing evidence that activity of AMN107 is not restricted to Bcr-Abl, c-Kit, or PDGFR-positive cells, but also extends to lymphatic cell lines of B-cell origin.

Tyrosine phosphorylation of β2-chimaerin by Src-family kinase negatively regulates its Rac-specific GAP activity

beta2-Chimaerin, an intracellular receptor for the second messenger diacylglycerol and phorbol esters, is a GTPase-activating protein (GAP) specific for Rac. beta2-Chimaerin negatively controls many Rac-dependent pathophysiological events including tumor development. However, the regulatory mechanism of beta2-chimaerin remains largely unknown. Here we report that beta2-chimaerin is tyrosine-phosphorylated by Src-family kinases (SFKs) upon cell stimulation with epidermal growth factor (EGF). Mutational analysis identified Tyr-21 in the N-terminal regulatory region as a major phosphorylation site. Intriguingly, the addition of SFK inhibitor and the replacement of Tyr-21 with Phe (Y21F) markedly enhanced Rac-GAP activity of beta2-chimaerin in EGF-treated cells. Moreover, the Y21F mutant inhibited integrin-dependent cell spreading, in which Rac1 plays a critical role, more strongly than wild-type beta2-chimaerin. These results suggest Tyr-21 phosphorylation as a novel, SFK-dependent mechanism that negatively regulates beta2-chimaerin Rac-GAP activity.

Nonreceptor tyrosine kinases in prostate cancer

BACKGROUND

Carcinoma of the prostate (CaP) is the most commonly diagnosed cancer in men in the United States. Signal transduction molecules such as tyrosine kinases play important roles in CaP. Src, a nonreceptor tyrosine kinase (NRTK) and the first proto-oncogene discovered is shown to participate in processes such as cell proliferation and migration in CaP. Underscoring NRTK's and, specifically, Src's importance in cancer is the recent approval by the US Food and Drug Administration of dasatinib, the first commercial Src inhibitor for clinical use in chronic myelogenous leukemia (CML). In this review we will focus on NRTKs and their roles in the biology of CaP.

MATERIALS AND METHODS

Publicly available literature from PubMed regarding the topic of members of NRTKs in CaP was searched and reviewed.

RESULTS

Src, FAK, JaK1/2, and ETK are involved in processes indispensable to the biology of CaP: cell growth, migration, invasion, angiogenesis, and apoptosis.

CONCLUSIONS

Src emerges as a common signaling and regulatory molecule in multiple biological processes in CaP. Src's relative importance in particular stages of CaP, however, required further definition. Continued investigation of NRTKs will increase our understanding of their biological function and potential role as new therapeutic targets.

Localized activation of Src-family protein kinases in the mouse egg

Recent studies in species that fertilize externally have demonstrated that fertilization triggers localized activation of Src-family protein kinases in the egg cortex. However, the requirement for Src-family kinases in activation of the mammalian egg is different from lower species and the objective of this study was to characterize changes in the distribution and activity of Src-family protein tyrosine kinases (PTKs) during zygotic development in the mouse. Immunofluorescence analysis of mouse oocytes and zygotes with an anti-phosphotyrosine antibody revealed that fertilization stimulated accumulation of P-Tyr-containing proteins in the egg cortex and that their abundance was elevated in the region overlying the MII spindle. In addition, the poles of the MII spindle exhibited elevated P-Tyr levels. As polar body extrusion progressed, P-Tyr-containing proteins were especially concentrated in the region of cortex adjacent to the maternal chromatin and the forming polar body. In contrast, P-Tyr labeling of the spindle poles eventually disappeared as meiosis II progressed to anaphase II. In approximately 24% of cases, the fertilizing sperm nucleus was associated with increased P-Tyr labeling in the overlying cortex and oolemma. To determine whether Src-family protein tyrosine kinases could be responsible for the observed changes in the distribution of P-Tyr containing proteins, an antibody to the activated form of Src-family PTKs was used to localize activated Src, Fyn or Yes. Activated Src-family kinases were found to be strongly associated with the meiotic spindle at all stages of meiosis II; however, no concentration of labeling was evident at the egg cortex. The absence of cortical Src-family PTK activity continued until the blastocyst stage when strong cortical activity became evident. At the pronuclear stage, activated Src-family PTKs became concentrated around the pronuclei in close association with the nuclear envelope. This pattern was unique to the earliest stages of development and disappeared by the eight cell stage. Functional studies using chemical inhibitors and a dominant-negative Fyn construct demonstrated that Src-family PTKs play an essential role in completion of meiosis II following fertilization and progression from the pronuclear stage into mitosis. These data suggest that while Src-family PTKs are not required for fertilization-induced calcium oscillations, they do play a critical role in development of the zygote. Furthermore, activation of these kinases in the mouse egg is limited to distinct regions and occurs at specific times after fertilization.

Protease-activated receptor-2 simultaneously directs beta-arrestin-1-dependent inhibition and Galphaq-dependent activation of phosphatidylinositol 3-kinase

Protease-activated receptor-2 (PAR-2) is a G-protein-coupled receptor (GPCR) activated upon proteolytic cleavage of its N-terminus by a number of serine proteases. We have previously reported that formation of a beta-arrestin-dependent signaling scaffold is required for PAR-2-stimulated activation of extracellular signal regulated kinases 1 and 2 and chemotaxis. beta-Arrestin-dependent pathways downstream of some GPCRs have been shown to function independently and sometimes in opposition to classic signaling through heterotrimeric G-proteins; however, this possibility has not been addressed with respect to PAR-2. Here we demonstrate that PAR-2 can increase PI3K activity through a Galphaq/Ca(2+)-dependent pathway involving PYK2 and a Src-family kinase, while inhibiting PI3K activity through a beta-arrestin-dependent mechanism, and that beta-arrestin-1 can directly associate with and inhibit the catalytic activity of p110alpha. Using size exclusion chromatography and co-immunoprecipitation, we demonstrate that the PI3K is recruited into a scaffolding complex containing PAR-2 and beta-arrestins. Inhibition of PI3K activity blocks PAR-2-stimulated chemotaxis, and beta-arrestin-1 colocalizes with p85 within the pseudopodia, suggesting that beta-arrestin-1 association with PI3K may spatially restrict its enzymatic activity and that this localized inhibition may be crucial for PAR-2-stimulated chemotaxis.

Src-dependent aprotein kinase C iota/lambda (aPKCiota/lambda) tyrosine phosphorylation is required for aPKCiota/lambda association with Rab2 and glyceraldehyde-3-phosphate dehydrogenase on pre-golgi intermediates

The small GTPase Rab2 is required for membrane transport between the endoplasmic reticulum (ER) and the Golgi complex. Rab2 associates with pre-Golgi intermediates (also termed vesicular tubular clusters; VTCs) that sort cargo to the anterograde pathway from recycling proteins retrieved to the ER. Our previous studies have shown that Rab2 stimulates atypical protein kinase C iota/lambda (aPKCiota/lambda) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) recruitment to VTCs. Both aPKCiota/lambda and GAPDH bind directly to Rab2 and aPKCiota/lambda and GAPDH interact. Based on the reports demonstrating aPKCiota-Src interaction and Src activity in the retrograde pathway (Golgi-ER), studies were initiated to learn whether Rab2 also promoted Src recruitment to VTCs. Using a quantitative membrane binding assay, we found that Rab2-stimulated Src membrane association in a dose-dependent manner. The recruited Src binds to aPKCiota/lambda and GAPDH on the membrane; however, Src does not interact with Rab2. The membrane-associated Src tyrosine phosphorylates aPKCiota/lambda on the VTC. To determine the consequence of aPKCiota/lambda tyrosine phosphorylation, the membrane binding assay was supplemented with the Src-specific tyrosine kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo(3,4-d)pyrimidine (PP2). Although Rab2, Src, and GAPDH recruitment was not affected, the Rab2-PP2-treated membranes contained a negligible amount of aPKCiota/lambda. Since Rab2 requires aPKCiota/lambda for the downstream recruitment of beta-coat protein (beta-COP) to VTCs, the Rab2-PP2-treated membranes were evaluated for the presence of beta-COP. Like aPKCiota/lambda, the membranes contained a negligible amount of beta-COP that was reflected by the drastic reduction in Rab2-dependent vesicle formation. These data suggest that Src-mediated tyrosine phosphorylation of aPKCiota/lambda facilitates aPKCiota/lambda association with Rab2-Src-GAPDH on VTCs, which is ultimately necessary for the downstream recruitment of beta-COP and release of Rab2-mediated retrograde-directed vesicles.

Implication of Gbetagamma proteins and c-SRC tyrosine kinase in parathyroid hormone-induced signal transduction in rat enterocytes

Parathyroid hormone (PTH) interacts in target tissues with a G protein-coupled receptor (GPCR) localized in the plasma membrane. Although activation of GPCR can elicit rapid stimulation of cellular protein tyrosine phosphorylation, the mechanism by which G proteins activate protein-tyrosine kinases is not completely understood. In the present work, we demonstrate that PTH rapidly increases the activity of non-receptor tyrosine kinase c-Src in rat intestinal cells (enterocytes). The response is biphasic, the early phase is fast and transient, peaking at 30 s (+120%), while the second phase progressively increases up to 5 min (+220%). The hormone activates c-Src in intestinal cells through fast changes in tyrosine phosphorylation of the enzyme. The first event in the activation of c-Src is the dephosphorylation of Tyr527 (which happens after a few seconds of PTH treatment), followed by a second event of activation with phosphorylation at Tyr416 (+twofold, 5 min). Removal of external Ca(2+) (EGTA, 0.5 mM) and chelation of intracellular Ca(2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetracetic acid acetoxymethyl ester (BAPTA) (5 microM) suppressed Tyr527 dephosphorylation and Tyr416 phosphorylation, indicating that Ca(2+) is an upstream activator of c-Src in enterocytes stimulated with PTH. The G protein subunits, Galphas and Gbeta, are associated with c-Src in basal conditions and this association increases two- to threefold in cells treated with PTH. Blocking of Gbeta subunits by preincubation of cells with a Gbeta antibody abolished hormone-dependent c-Src Tyr416 phosphorylation and ERK1/ERK2 activation. The results of this work indicate that PTH activates c-Src in intestinal cells through conformational changes via G proteins and calcium-dependent modulation of tyrosine phosphorylation of the enzyme, and that PTH receptor activation leads via Gbetagamma-c-Src to the phosphorylation of the MAP kinases, ERK1 and ERK2.

Requirements of Src family kinase during meiotic maturation in mouse oocyte

The Src family kinase (SFK) is important in normal cell cycle control. However, its role in meiotic maturation in mammalian has not been examined. We used confocal microscope immunofluorescence to examine the in vitro dynamics of the subcellular distribution of SFK during the mouse oocyte meiotic maturation and further evaluated the functions of SFK via biochemical analysis using a specific SFK pharmacological inhibitor, PP(2). Our results showed that nonphospho-SFK was absent in oocyte upon its release from follicle. Nonphospho-SFK appeared in cytoplasm 0.5 hr after the release of oocyte and translocated to germinal vesicle (GV) before germinal vesicle breakdown (GVBD). After GVBD, nonphospho-SFK colocated with condensed chromosomes. In occyte at metaphase I (MI) and telophase I, nonphospho-SFK accumulated in the cortex and the cleavage furrow respectively besides its existence in cytoplasm in both stages. In oocyte at metaphase II (MII), nonphospho-SFK concentrated at the aligned chromosomes. In contrast, phospho-SFK was absent in oocyte until 1 hr after its release from the follicle. Phospho-SFK accumulated in the GV, the cortex, and cytoplasm immediately prior to GVBD. After GVBD, phospho-SFK evenly distributed in oocyte. In oocyte at MII, phospho-SFK localized throughout the cytoplasm and under the egg member. When the SFK activity was inhibited, the oocyte failed to initiate GVBD, could not go into MII, and could not extrude the first polar body. Our results demonstrated that SFK is required for meiotic maturation in mouse oocyte.

[Identification of proteins interacting with adaptor protein Bam32]

AIM

To study the role of adaptor protein Bam32 in B cell antigen receptor (BCR) signaling cascades.

METHODS

Using full length Bam32 as bait, yeast two-hybrid technique was used to screen the protein that could interact with Bam32. The interaction was further confirmed by co-transfection of 293T cells and coimmunoprecipitation.

RESULTS

Protein tyrosine kinase Lyn was one of the strong positive clones identified by the yeast two-hybrid screening. This interaction was further confirmed in 293T cells by co-transfection and coimmunoprecipitation. By using specific anti-phosphotyrosine antibody, it was found that Bam32 could be phosphorylated by Lyn.

CONCLUSION

The interaction of Bam32 with Lyn leads to Bam32 phosphorylation, which might play an important role in activating downstream signaling molecules.

Expression pattern of intracellular leukocyte-associated proteins in primary mediastinal B cell lymphoma

Two microarray studies of mediastinal B cell lymphoma have shown that this disease has a distinct gene expression profile, and also that this is closest to the pattern seen in classical Hodgkin's disease. We reported previously an immunohistologic study in which the loss of intracellular B cell-associated signaling molecules in Reed-Sternberg cells was demonstrated, and in this study we have investigated the expression of the same components in more than 60 mediastinal B cell lymphomas. We report that these signaling molecules are frequently present, and in particular that Syk, BLNK and PLC-gamma2 (absent from Reed-Sternberg cells) are present in the majority of mediastinal B cell lymphomas. The overall pattern of B cell signaling molecules in this disease is therefore closer to that of diffuse large B cell lymphoma than to Hodgkin's disease, and is consistent with a common cell of origin as an explanation of the similar gene expression profiles.

Role of Yes kinase during early zebrafish development

We have identified the Yes kinase in zebrafish eggs and investigated its role in development of the zebrafish embryo. In situ hybridization as well as immunofluorescence techniques demonstrated that Yes kinase is maternally expressed and is localized to the cortical region of the unfertilized egg. Fertilization resulted in concentration of Yes kinase to the blastodisc where it continued to be localized to the blastoderm cells through cleavage, gastrulation, and later development. Yes kinase activity was found to decrease abruptly at fertilization, then increase progressively during epiboly, and was maintained at high levels throughout gastrulation. The role of Yes kinase in development was tested by treating embryos with chemical protein tyrosine kinase (PTK) inhibitors such as 4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo[3,4-d] pyrimidine (PP2) and by injection of antisense morpholinos. Both treatments resulted in the arrest of development at the beginning of the epiboly. Co-immunoprecipitation studies demonstrated that Yes kinase participates in a stable complex with focal adhesion kinase (FAK), which is phosphorylated in vitro. These results demonstrate that Yes kinase plays an important role in epiboly and indicate that Yes kinase participates in signaling by focal adhesion kinase during early development.

Isolation and characterization of lipid rafts with different properties from RBL-2H3 (rat basophilic leukaemia) cells

Lipid rafts are plasma-membrane microdomains that are enriched in certain lipids (sphingolipids, glycosphingolipids and cholesterol), as well as in lipid-modified proteins. Rafts appear to exist in the liquid-ordered phase, which contributes to their partitioning from the surrounding liquid-disordered glycerophospholipid environment. DRM (detergent-resistant membrane) fractions isolated from cells are believed to represent coalesced lipid rafts. We have employed extraction using two different non-ionic detergents, Brij-96 and Triton X-100, to isolate detergent-resistant lipid rafts from rat basophilic leukaemia cell line RBL-2H3, and compared their properties with each other and with plasma-membrane vesicles. DRM fractions were isolated as sealed unilamellar vesicles of similar size (135-170 nm diameter), using either sucrose-density-gradient sedimentation or gel-filtration chromatography. Lipid rafts isolated using Brij-96 and Triton X-100 differed in density, protein content and the distribution between high- and low-density fractions of the known raft constituents, Thy-1, and the non-receptor protein tyrosine kinases, Yes and Lyn. Lyn was found in the raft microdomains in predominantly phosphorylated form. The level of enrichment of the protein constituents of the isolated lipid rafts seemed to depend on the ratio of cell lipid/protein to detergent. As indicated by reactivity with anti-Thy-1 antibodies, lipid rafts prepared using Brij-96 appeared to consist of vesicles with primarily right-side-out orientation. Both Brij-96 and Triton X-100 appear to isolate detergent-insoluble raft microdomains from the rat basophilic leukaemia cell line RBL-2H3, but the observed differences suggest that either the detergents themselves play a role in determining the physicochemical characteristics of the resulting DRM fractions, or different subsets of rafts are isolated by the two detergents.

Western blot analysis of Src kinase assays using peptide substrates ligated to a carrier protein

We have applied intein-mediated peptide ligation (IPL) to the use of peptide substrates for kinase assays and subsequent Western blot analysis. IPL allows for the efficient ligation of a synthetic peptide with an N-terminal cysteine residue to an intein-generated carrier protein containing a cysteine reactive C-terminal thioester through a native peptide bond. A distinct advantage of this procedure is that each carrier protein molecule ligates only one peptide, ensuring that the ligation product forms a sharp band on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). We demonstrate the effectiveness of this approach by mutational analysis of peptide substrates derived from human cyclin-dependent kinase, Cdc2, which contains a phosphorylation site of human c-Src protein tyrosine kinase.

Intracellular signalling molecules as immunohistochemical markers of normal and neoplastic human leucocytes in routine biopsy samples

We have investigated whether intracellular signal transduction molecules can be used as immunohistological markers of normal and neoplastic human leucocytes in routine tissue sections. We obtained selective labelling of white cells for eight such molecules (the 'linker' molecules SLP-76 and BLNK, the Src family kinases Lyn, Fyn, Syk and Hck, and the phospholipases PLC-gamma1 and PLC-gamma2). Antibodies to SLP-76 and PLC-gamma1 selectively labelled T cells, and antibodies to BLNK, Lyn, Fyn, Syk and PLC-gamma2 labelled B cells (although Fyn immunostaining was restricted to mantle zone B cells). Antibodies to the Syk and Hck kinases labelled probable thymocyte precursors at the periphery of the thymic cortex. In addition to lymphoid cells, several other leucocyte types were immunostained (e.g. SLP-76, Lyn, Syk and Hck were found in megakaryocytes, myeloid cells and/or macrophages, and PLC-gamma2 was detected in arterial endothelium). SLP-76 and PLC-gamma1 were found in most T-cell lymphomas studied, and some B-cell lymphomas were also positive for PLC-gamma1 (e.g. diffuse large cell and Burkitt's lymphoma). The five B cell-associated markers were found in most B-cell non-Hodgkin's lymphomas, although some diffuse large B-cell lymphomas were negative (e.g. for Lyn) and anti-Fyn tended not to stain small B-cell neoplasms. The observation that a range of leucocyte signalling molecules can be detected in routine biopsies offers new possibilities for studying normal and neoplastic human white cells in diagnostic tissue samples.

Identification and activation of Src family kinases in primary megakaryocytes

OBJECTIVES

We have recently shown that the Src family of tyrosine kinases (SFKs) are activated by TPO stimulation in both primary megakaryocytic progenitors and a hematopoietic cells line (BaF3) expressing the TPO receptor (Mpl). In this study, we examine which of the eight Src family members are expressed in primary megakaryocytes (MKs) and determine which of these become activated in response to TPO.

MATERIALS AND METHODS

High-density oligonucleotide microarrays were used to compare the gene expression profiles of Src kinases from undifferentiated hematopoietic progenitors (CD34+/CD38(lo)) and after in vitro megakaryocytic differentiation. Western blot analysis of lysates from purified, mature murine MKs identified which of SFKs are present. Finally, in vitro kinase assays determined which of the SFKs in primary MKs are activated by TPO stimulation.

RESULTS

Array profiles demonstrate that Fyn, Lyn, Fgr, Hck, Src, and Yes are all expressed in cultured human MKs (Fyn, Lyn>Src, Yes, Fgr, Hck). Similarly, Western blots of murine MKs identified the same six SFKs (Fyn, Fgr, Hck, Lyn, Src, and Yes). Of these, only Fyn and Lyn demonstrate increased kinase activity after TPO stimulation. Interestingly, gene expression analysis indicates that, among the SFKs, Fyn expression is uniquely upregulated during MK development.

CONCLUSION

These results provide the first direct evidence that two Src kinases are activated in primary MKs, Fyn and Lyn. The fact that only Fyn expression is significantly upregulated during MK differentiation suggests variable gene regulation. Specificity of the TPO signaling cascade is demonstrated by the selective activation of Fyn and Lyn.

Regulation of 17-AAG-induced apoptosis: role of Bcl-2, Bcl-XL, and Bax downstream of 17-AAG-mediated down-regulation of Akt, Raf-1, and Src kinases

17-allylamino-demethoxy geldanamycin (17-AAG) inhibits the chaperone function of heat shock protein-90 (Hsp-90) and promotes the proteasomal degradation of its misfolded client proteins. Here, we demonstrate that treatment of the human acute myeloid leukemia HL-60 cells with 17-AAG attenuates the intracellular levels of a number of Hsp-90 client proteins, including Akt, c-Raf-1, and c-Src. Also, 17-AAG induced the mitochondrial release and cytosolic accumulation of cytochrome c (cyt c) and second mitochondria-derived activator of caspases (Smac)/DIABLO, resulting in the activation of caspase-9 and caspase-3 and apoptosis. Treatment with 17-AAG triggered the B-cell lymphoma-2 (Bcl-2)-associated X protein (Bax) conformational change associated with apoptosis, while Bax-deficient cells were resistant to 17-AAG-induced apoptosis. In addition, in HL-60/Bcl-2 and HL-60/Bcl-xL cells, which ectopically express Bcl-2 and Bcl-xL respectively, 17-AAG-induced Bax conformational change, cytosolic accumulation of cyt c and Smac/DIABLO, and apoptosis were markedly inhibited. Although the rate of 17-AAG-mediated decline in Akt, c-Raf-1, and c-Src levels was blunted, the total decline was not compromised in HL-60/Bcl-2 and HL-60/Bcl-xL cells. Cotreatment with HA14-1, a nonpeptidic ligand that can bind and inhibit the antiapoptotic activity of Bcl-2, significantly overcame the resistance to 17-AAG-induced apoptosis in HL-60/Bcl-2 cells. Together, these findings indicate that although 17-AAG treatment causes the levels of a number of survival-signaling protein kinases to decline, the downstream engagement of the mitochondrial pathway of apoptosis is regulated by the activity of the Bcl-2 family of proteins. Also, neutralizing the antiapoptotic effect of Bcl-2 would further enhance the antileukemia activity of 17-AAG.

Src family kinases and nitric oxide production are required for hepatocyte growth factor-stimulated endothelial cell growth

Hepatocyte growth factor (HGF) is a potent mitogen for vascular endothelial cells (EC); however, signal transduction pathways for HGF-stimulated EC growth remain unclear. In the present study we investigated the role of Src family kinases and nitric oxide (NO) in HGF-stimulated EC growth. Human umbilical vein endothelial cells (HUVEC) were stimulated with HGF and NO was measured by an NOx analyzing HPLC system. Activation of ERK1/2 and p38 MAPK was assessed by Western blot. NO production in HUVEC increased 1.8-fold by HGF. A Src family kinases inhibitor PP1 inhibited HGF-stimulated NO production by 71%. HUVEC growth increased 1.9-fold in cell number by HGF. PP1 and Nitro-L-arginine methylester (L-NAME) inhibited HGF-stimulated HUVEC growth by 51 and by 71%. ERK1/2 and p38 MAPK were phosphorylated by HGF and a MEK inhibitor PD98059 and a p38 MAPK inhibitor SB203580 inhibited HGF-stimulated HUVEC growth by 66% and by 58%; however, HGF-induced phosphorylation of ERK1/2 and p38 MAPK was not inhibited by L-NAME, indicating that NO is not an upstream activator of ERK1/2 and p38 MAPK. These findings demonstrated that Src family kinases regulate HGF-stimulated NO production in HUVEC and that HGF stimulates HUVEC growth through NO-dependent and NO-independent pathways.

Oncology drug discovery applications using the FMAT 8100 HTS system

High-throughput screening (HTS) for potential anticancer agents requires a broad portfolio of assay platforms that may include kinase enzyme assays, protein-protein binding assays, and functional cell-based apoptosis assays. The authors have explored the use of fluorometric microvolume assay technology (the FMAT 8100 HTS System) in three distinct homogeneous HTS assays: (1). a Src tyrosine kinase enzyme assay, (2). a Grb2-SH2 protein-peptide interaction assay, and (3). an annexin V binding apoptosis assay. Data obtained from all three assays suggest that the FMAT system should facilitate the implementation of homogeneous assays for a wide variety of molecular targeted and cell-based screens.

Lyn and Syk kinases are sequentially engaged in phagocytosis mediated by Fc gamma R

Recent data indicate that phagocytosis mediated by FcgammaRs is controlled by the Src and Syk families of protein tyrosine kinases. In this study, we demonstrate a sequential involvement of Lyn and Syk in the phagocytosis of IgG-coated particles. The particles isolated at the stage of their binding to FcgammaRs (4 degrees C) were accompanied by high amounts of Lyn, in addition to the signaling gamma-chain of FcgammaRs. Simultaneously, the particle binding induced rapid tyrosine phosphorylation of numerous proteins. During synchronized internalization of the particles induced by shifting the cell to 37 degrees C, Syk kinase and Src homology 2-containing tyrosine phosphatase-1 (SHP-1) were associated with the formed phagosomes. At this step, most of the proteins were dephosphorylated, although some underwent further tyrosine phosphorylation. Quantitative immunoelectron microscopy studies confirmed that Lyn accumulated under the plasma membrane beneath the bound particles. High amounts of the gamma-chain and tyrosine-phosphorylated proteins were also observed under the bound particles. When the particles were internalized, the gamma-chain was still detected in the region of the phagosomes, while amounts of Lyn were markedly reduced. In contrast, the vicinity of the phagosomes was heavily decorated with anti-Syk and anti-SHP-1 Abs. The local level of protein tyrosine phosphorylation was reduced. The data indicate that the accumulation of Lyn during the binding of IgG-coated particles to FcgammaRs correlated with strong tyrosine phosphorylation of numerous proteins, suggesting an initiating role for Lyn in protein phosphorylation at the onset of the phagocytosis. Syk kinase and SHP-1 phosphatase are mainly engaged at the stage of particle internalization.

FcepsilonRI signaling observed from the inside of the mast cell membrane

Crosslinking the high affinity IgE receptor, FcrepsilonRI, on basophils and mast cells initiates cascades of biochemical events leading to degranulation, membrane ruffling and other physiological responses. Downstream of FcepsilonRI and its coupled tyrosine kinases, Lyn and Syk, scores of different proteins and lipids are implicated in these signaling cascades and new players are being identified continuously. Here, we use immunogold probes to label receptors and signaling proteins on the cytoplasmic face of membrane sheets prepared from RBL-2H3 mast cells and transmission electron microscopy to examine their distributions in relationship to each other and to features of the membrane. New topographical data are integrated with existing knowledge of the biochemistry of FcepsilonRI signaling and of cell shape during signaling to implicate at least two distinct membrane domains in FcepsilonRI signaling. "Primary signaling domains", also called osmiophilic patches, are recognized by their dark staining with osmium, adjacency to coated pits (previously mapped to planar membrane between lamellae) and by the characteristic presence of receptor, Syk and PLCgamma2, but not Lyn. "Secondary signaling domains" are characterized by the presence of large elliptical linker for activation of T cells (LAT) rafts and of PLCgamma1 (previously mapped to lamellae) but not receptor. The signaling proteins, Vav, Grb2, Cbl and Gab2, and the endocytic proteins, AP2 and clathrin, all map to the primary domains, while the p85 regulatory subunit of phosphatidylinositol 3 (PI 3)-kinase maps to both domains. Recognition that FcepsilonRI signaling is controlled not only by which chemical species are available for interaction, but also by where the interactions occur, may provide new opportunities for the modeling of signaling cascades and new targets for the development of drugs to treat allergies and asthma.

Lipid rafts in mast cell signaling

Lipid rafts are defined as plasma membrane microdomains enriched with glycosphingolipids and cholesterol which render them insoluble in non-ionic detergents. Many surface receptors are constitutively or inducibly associated with lipid rafts, and it has been suggested that the rafts function as platforms regulating the induction of signaling pathways. The signaling capacity of lipid rafts has been extensively studied in rat basophilic leukemia cells. An aggregation of lipid raft components, such as glycosylphosphatidylinositol (GPI)-anchored glycoproteins (Thy-1 or TEC-21), triggers cell activation events which are similar to, but not identical with activation via the high-affinity IgE receptor (FcepsilonRI). Although FcepsilonRI in resting cells is not associated with lipid rafts, its aggregation induces a weak association with rafts and subsequent activation events. The properties of lipid rafts as well as the molecular mechanisms of their involvement in signal transduction are poorly understood. This review presents a critical analysis of recent results on structure-function relationship of lipid rafts and their regulatory role in signal transduction in mast cells.

Lactosylceramide-enriched glycosphingolipid signaling domain mediates superoxide generation from human neutrophils

This study is focused on the functional significance of neutrophil lactosylceramide (LacCer)-enriched microdomains, which are involved in the initiation of a signal transduction pathway leading to superoxide generation. Treatment of neutrophils with anti-LacCer antibody, T5A7 or Huly-m13, induced superoxide generation from the cells, which was blocked by PP1, a Src kinase inhibitor; wortmannin, a phosphatidylinositol-3 kinase inhibitor; SB203580, a p38 mitogen-activated protein kinase (MAPK) inhibitor; and H7, an inhibitor for protein kinase C. When promyelocytic leukemia HL-60 cells were differentiated into neutrophilic lineage by dimethyl sulfoxide (DMSO) treatment, they acquired superoxide-generating activity but did not respond to anti-LacCer antibodies. Density gradient centrifugation revealed that LacCer and Lyn were recovered in detergent-insoluble membrane (DIM) of neutrophils and DMSO-treated HL-60 cells. However, immunoprecipitation experiments indicated that LacCer was associated with Lyn in neutrophils but not in DMSO-treated HL-60 cells. Interestingly, T5A7 induced the phosphorylation of Lyn in neutrophils but not in DMSO-treated HL-60 cells. Moreover, T5A7 induced the phosphorylation of p38 MAPK in neutrophils. T5A7-induced Lyn phosphorylation in neutrophil DIM fraction was significantly enhanced by cholesterol depletion or sequestration with methyl-beta-cyclodextrin or nystatin. Collectively, these data suggest that neutrophils are characterized by the presence of cell surface LacCer-enriched glycosphingolipid signaling domain coupled with Lyn and that the ligand binding to LacCer induces the activation of Lyn, which may be suppressibly regulated by cholesterol, leading to superoxide generation through the phosphatidylinositol-3 kinase-, p38 MAPK-, and protein kinase C-dependent signal transduction pathway.

Characterization and location of Src-dependent tyrosine phosphorylation in rat brain mitochondria

Analysis of protein phosphorylation in highly purified rat brain mitochondria revealed the presence of several alkali-stable phosphoproteins whose phosphorylation markedly increases upon treatment with peroxovanadate and Mn(2+), a property indicating tyrosine phosphorylation. These include three prominent bands, with apparent sizes of 50, 60, and 75 kDa, which are detectable by anti-phosphotyrosine. Tyrosine phosphorylation disappears when mitochondria are treated with PP2, an inhibitor of the Src kinase family, suggesting the presence of members of this family in rat brain mitochondria. Immunoblotting and immunoprecipitation assays of mitochondrial lysates confirmed the presence of Fyn, Src and Lyn kinases, as well as Csk, a protein kinase which negatively controls the activity of the Src kinase family. Results show that tyrosine-phosphorylated proteins are membrane-bound and that they are located on the inner surface of the outer membrane and/or the external surface of the inner membrane. Instead, Src tyrosine kinases are mainly located in the intermembrane space - in particular, as revealed by immunogold experiments for Lyn kinase, in the cristal lumen. Rat brain mitochondria were also found to possess a marked level of tyrosine phosphatase activity, strongly inhibited by peroxovanadate.

Activation of mitogen-activated protein kinases and p90 ribosomal S6 kinase in failing human hearts with dilated cardiomyopathy

OBJECTIVE

A new member of the MAP kinase family, big MAP kinase-1 (BMK1), has been recently identified to promote cell growth and attenuate apoptosis. P90 ribosomal S6 kinase (p90RSK), one of the potentially important substrates of extracellular signal regulated kinase (ERK), regulates gene expression in part via phosphorylation of CREB and the Na(+)/H(+) exchanger. Recently, we have demonstrated that the activity of BMK1, Src (the upstream regulator of BMK1) and p90RSK was increased in hypertrophied myocardium induced by pressure-overload in the guinea pig. However, the abundance and activity of these kinases in human hearts are unknown.

METHODS

In addition to the three classical MAP kinases (ERK, p38 kinase, and c-Jun NH(2)-terminal kinase (JNK)), we examined the protein expression and activity of Src, BMK1, and p90RSK in explanted hearts from patients with dilated cardiomyopathy (n=9). Normal donor hearts, which were not suitable for transplant for technical reasons, were used as controls (n=5).

RESULTS

There were no significant differences in the levels of protein expression of these kinases between normal and failing hearts. ERK1/2 and p90RSK were activated in heart failure compared to control (P<0.01 and P<0.03, respectively), while the activity of p38 kinase was decreased (P<0.05) and the activity of JNK was unchanged in heart failure. By contrast, the activities of Src and BMK1 were significantly reduced in end-stage heart failure compared to normal donor hearts (P<0.05).

CONCLUSION

These data suggest that multiple MAP kinases, p90RSK, and Src are differentially regulated in human failing myocardium of patients with idiopathic dilated cardiomyopathy and may be involved in the pathogenesis of this complex disease.

Multiplexed enzyme assays in capillary electrophoretic single-use microfluidic devices

We describe a method of performing multiple enzyme assays in a single reaction vessel. The resolving power of capillary electrophoresis enables several enzyme assays to be analyzed at high speed in microfluidic arrays. Multiplexed measurement can increase throughput significantly without requiring highly dense microfluidic arrays. Enzyme assays in a multiplexed format for selected kinases in this work show essentially identical performance to assays performed individually. This establishes an approach for screening one compound against multiple enzyme targets simultaneously. Another potential application for performing multiplexed enzyme assay is to study protein-protein (especially enzyme-enzyme) interaction by monitoring the enzymatic activity changes.

Effects of src kinase and TGFbeta1 on the differentiation and morphogenesis of MDCK cells grown in three-dimensional collagen and Matrigel environments

This study attempted to analyse in detail the effect of src kinase on the growth and differentiation of MDCK cells in different extracellular matrix (ECM) environments. A method was developed to label the membrane proteins in situ and the distribution of cytoskeletal and junctional proteins was visualized in three-dimensional cell complexes, using optical sections generated by confocal microscopy. Independently of the ECM, non-transformed MDCK cells formed differentiated cell cysts with one or a few lumina, with the apical side facing the lumen; ZO-1 was expressed at the tight junctions close to the apical side and beta-catenin, E-cadherin and fodrin along the entire lateral walls. The phenotype of src kinase activated MDCK cells was strongly dependent on the ECM and varied from an irregular cluster in collagen I, to tubular structures in laminin or proteoglycans, and finally to a polarized cell cyst in Matrigel. In collagen I, E-cadherin and beta-catenin were seen partially along the lateral walls and partially in the cytoplasm of src-transformed MDCK cells; fodrin was released into the cytoplasm and ZO-1 was not visualized. When the src-transformed cells were cultivated in Matrigel, their junctional proteins were recruited to the cell membranes and ZO-1 reappeared at the apical face. Thus, the components of Matrigel could overcome the deleterious effect of src on the polarity of MDCK cells. TGFbeta1, together with its receptors and other soluble factors in Matrigel, were responsible for the induction of differentiation. The results show that tyrosine phosphorylation sensitizes the epithelial MDCK cells to ECM and TGFbeta1.

GP2, a GPI-anchored protein in the apical plasma membrane of the pancreatic acinar cell, co-immunoprecipitates with src kinases and caveolin

We previously showed that endocytosis at the apical plasma membrane (APM) of the pancreatic acinar cell is activated by the cleavage of GP2, a GPI-linked protein, from the apical cell surface. This endocytic process, as measured by horseradish peroxidase uptake into pancreatic acinar cells, is blocked by the tyrosine kinase inhibitors genistein and tyrphostin B42 as well as by disruption of actin filaments with cytochalasin. This suggests that the cleavage of GP2 from the cell membrane may activate endocytosis through a tyrosine kinase-regulated pathway. However, the mechanism by which GP2 and tyrosine kinases act together to activate endocytosis at the APM remains unknown. In this study, we demonstrate that pp60, p62yes, caveolin, and annexin, which have previously been implicated in endocytosis in other cell lines, were present in high abundance in GPI-enriched membranes by Western blot analysis. pp60, p62yes, and caveolin all co-immunoprecipitated with GP2 except annexin. An 85-kDa protein whose tyrosine-dependent phosphorylation is correlated with the activation of endocytosis in intact acinar cells also was present in these immunoprecipitates. This suggests that in pancreatic acini, GP2 may exist in a complex with src kinases, caveolin, and an 85-kDa phosphorylated substrate to regulate endocytosis at the APM.

Bombesin modulates the association of Src with a nuclear 110-kd protein expressed in dividing prostate cells

Polyclonal antibodies produced against a peptide derived from the Fer tyrosine kinase sequence also specifically recognized a 110-kd protein (p110) up-regulated in dividing versus resting dog prostate epithelial cells in vitro. In vivo in the dog prostate, p110 expression was detected when basal cell metaplasia was induced by estrogens after castration but not when renewing the differentiated epithelium with androgens. It was also detected in extracts from the human prostatic carcinoma cell lines LNCaP, DU145, and PC-3, and from 6 out of 11 human prostate cancer tissues analyzed, but not from normal or hyperplastic glands. The tyrosine kinase Src was shown by coimmunoprecipitation to associate with p110, and this interaction was positively modulated by bombesin stimulation of PC-3 cells. However, p110 was not tyrosine phosphorylated. Moreover, it was mainly distributed in the nuclear fraction. This nuclear p110 protein, expressed in dividing prostate epithelial cells and in human prostate cancer cells and tissues, could thus be a downstream mediator of bombesin-signaling pathways, acting via its association with Src.

Review: perinucleolar structures

Several perinucleolar structures have been described in recent years. This review primarily summarizes recent studies regarding two of these structures, the perinucleolar compartment (PNC) and the Sam68 nuclear body (SNB). A number of studies have explored their ultrastructure and molecular components. Despite their different nuclear localizations, PNCs and SNBs share some common characteristics. They both are enriched with RNA binding proteins and nucleic acids and are predominantly localized to the periphery of the nucleolus. They are observed mostly in transformed cells, although prevalence differs among different cell types and cell lines. Their structural integrity is influenced by the transcriptional state of the cell. However, the functions of both the PNC and the SNB remain unknown. In addition to the PNC and SNB, a perinucleolar structure immunolabeled with an antibody to hnRNP L will be discussed.

Evidence for the involvement of a Src-related tyrosine kinase in Xenopus egg activation

Recently, we have purified a Src-related tyrosine kinase, named Xenopus tyrosine kinase (Xyk), from oocytes of Xenopus laevis and found that the enzyme is activated within 1 min following fertilization [Sato et al. (1996) J. Biol. Chem. 271, 13250-13257]. A concomitant translocation of a part of the activated enzyme from the membrane fraction to the cytosolic fraction was also observed. In the present study, we show that parthenogenetic egg activation by a synthetic RGDS peptide [Y. Iwao and T. Fujimura, T. (1996) Dev. Biol. 177, 558-567], an integrin-interacting peptide, but not by electrical shock or the calcium ionophore A23187 causes the kinase activation, tyrosine phosphorylation, and translocation of Xyk. A synthetic tyrosine kinase-specific inhibitor peptide was employed to analyze the importance of the Xyk activity in egg activation. We found that the peptide inhibits the kinase activity of purified Xyk at IC50 of 8 microM. Further, egg activation induced by sperm or RGDS peptide but not by A23187 was inhibited by microinjection of the peptide. In the peptide-microinjected eggs, penetration of the sperm nucleus into the egg cytoplasm and meiotic resumption in the egg were blocked. Indirect immunofluorescence study demonstrates that Xyk is exclusively localized to the cortex of Xenopus eggs, indicating that Xyk can function in close proximity to the sperm-egg or RGDS peptide-egg interaction site. Taken together, these data suggest that the tyrosine kinase Xyk plays an important role in the early events of Xenopus egg activation in a manner independent or upstream of calcium signaling.

SHP-1 deficiency in B-lineage cells is associated with heightened lyn protein expression and increased lyn kinase activity

SHP-1 protein tyrosine phosphatase is a critical regulator of signal transduction in hematopoietic cells. In the present study, we derived two pre-B cell lines, PBCL-1 and PBCL-2, from normal and SHP-1-deficient motheaten mice, respectively, and characterized hyperphosphorylated proteins in PBCL-2 cells to identify SHP-1-regulated molecules. Two proteins of 56 and 53 kDa (p56/p53) in PBCL-2 cells showed heightened phosphorylation (3- to 6-fold) in comparison with those in PBCL-1. p56/p53 were identified as the two forms of the lyn protein tyrosine kinase (p56/p53lyn), which showed increased kinase activity in PBCL-2 cells. Interestingly, the protein levels of p56/53lyn were found to be 3- to 6-fold higher in PBCL-2 cells than those in PBCL-1, whereas the transcript levels of lyn in the two cell lines were comparable. A modest increase in p56/53lyn protein expression was also detected in primary spleen B cells of motheaten mice. Thus SHP-1 deficiency in B-lineage cells, especially pre-B cells, is associated with increased lyn protein expression and kinase activity. These data indicate a role for SHP-1 in regulating lyn through a post-transcriptional mechanism.

MAL, a novel integral membrane protein of human T lymphocytes, associates with glycosylphosphatidylinositol-anchored proteins and Src-like tyrosine kinases

A large fraction of glycosylphosphatidylinositol (GPI)-anchored proteins and Src-like kinases are confined to glycolipid-enriched membrane (GEM) microdomains. The particular membrane topology of GPI-anchored proteins has led to the postulation of the existence of integral membrane proteins linking extracellular stimuli with cytosolic machinery for endocytosis and signaling. The human MAL cDNA was identified during a search for novel genes differentially expressed during T cell development, and encodes a multispanning membrane protein displaying lipid-like properties. To address the biochemical characterization of endogenous MAL and to analyze its possible association with other proteins, we have generated a monoclonal antibody (mAb) specific to the MAL molecule. Using this mAb, we have identified MAL in GEM microdomains of both the HPB-ALL T cell line and human peripheral blood lymphocytes. Co-immunoprecipitation experiments with antibodies to the MAL molecule or to the GPI-anchored CD59 antigen indicated specific association of MAL with GPI-anchored proteins and Src-like tyrosine kinases. In addition, both MAL and the Src-like kinase Lck were identified in GEM obtained from an endosomal-enriched membrane fraction. These features of MAL closely match some of the properties expected for the hypothetical integral membrane linker proteins acting in specialized GEM-mediated functions.

Stimulation of the high-affinity IgE receptor results in the tyrosine phosphorylation of a 60 kD protein which is associated with the protein-tyrosine kinase, Csk

The protein tyrosine kinase Csk downregulates the activity of the Src family of kinases and has a negative effect on signal transduction through several Src kinase-associated receptors. Because the Src-family kinase Lyn plays a pivotal role in FcepsilonRI-mediated cellular activation, we examined whether Csk is involved in FcepsilonRI signaling events. Using anti-Csk antibodies and recombinant fusion proteins we detected a single tyrosine-phosphorylated protein of 60 kD (herein referred to as 'p60') that associates with the SH2 domain of Csk after stimulation of the FcepsilonRI. p60 phosphorylation reached a maximum within one minute and remained constant while the receptors were aggregated; disaggregation of the receptors resulted in rapid dephosphorylation of p60. The phosphorylation of p60 was only detected after activation by IgE and antigen and not by stimulation with PMA and/or ionomycin. Phosphorylated p60 was associated entirely with the membrane fraction of the cells. A considerable fraction of Csk was associated with the membrane in both unstimulated and stimulated cells, this fraction did not change upon activation. p60 coprecipitated with Csk from both unstimulated and FcepsilonRI stimulated cells and was phosphorylated by the immunocomplex. Total kinase activity of Csk immunoprecipitates increased upon FcepsilonRI stimulation. p60 did not react with antibodies to a number of known signaling molecules, including the recently cloned, GAP-associated protein, p62dok. Our data demonstrate that Csk associates with a membrane-anchored protein complex that is directly involved in FcepsilonRI signal transduction.

A close association of GM3 with c-Src and Rho in GM3-enriched microdomains at the B16 melanoma cell surface membrane: a preliminary note

B16 melanoma is characterized by high content of GM3 ganglioside, which has been recognized as a melanoma-associated antigen defined by specific monoclonal antibodies. We report now that GM3 is present predominantly (>90%) in the 1% Triton X-100-insoluble, low-density microvesicular fraction ("detergent-insoluble glycosphingolipid-enriched microdomain"; DIGEM) separated on sucrose density-gradient centrifugation. Associated with DIGEM, many signal transducer molecules such as c-Src, FAK, and the low-molecular-weight G-proteins Rho A and H-Ras were also found. Rho A and FAK were found in part, and PLC-beta2 and G alphas were found exclusively, in the high-density fraction. Immunoprecipitation of GM3 present in DIGEM by anti-GM3 antibody DH2, followed by Western blotting, revealed co-precipitation of Rho A and c-Src with GM3. These findings suggest (i) a specific organization of GM3 in close association with Rho A and c-Src within DIGEM at the melanoma cell surface; and (ii) such organizational units may be directly involved in signal transduction, in which glycosphingolipids receive signals which are subsequently transduced by associated transducer molecules.

Intrinsic signals in the unique domain target p56(lck) to the plasma membrane independently of CD4

In T lymphocytes, the Src-family protein tyrosine kinase p56(lck) (Lck) is mostly associated with the cytoplasmic face of the plasma membrane. To determine how this distribution is achieved, we analyzed the location of Lck in lymphoid and in transfected nonlymphoid cells by immunofluorescence. We found that in T cells Lck was targeted correctly, independently of the cell surface proteins CD4 and CD8 with which it interacts. Similarly, in transfected NIH-3T3 fibroblasts, Lck was localized at the plasma membrane, indicating that T cell-specific proteins are not required for targeting. Some variation in subcellular distribution was observed when Lck was expressed in HeLa and MDCK cells. In these cells, Lck associated with both the plasma membrane and the Golgi apparatus, while subsequent expression of CD4 resulted in the loss of Golgi-associated staining. Together, these data indicate that Lck contains intrinsic signals for targeting to the plasma membrane. Furthermore, delivery to this site may be achieved via association with exocytic transport vesicles. A mutant Lck molecule in which the palmitoylation site at cysteine 5 was changed to lysine (LC2) localized to the plasma membrane and the Golgi region in NIH3T3 cells. However, the localization of a mutant in which the palmitoylation site at cysteine 3 was changed to serine (LC1) was indistinguishable from wild-type Lck. Chimeras composed of only the unique domain of Lck linked to either c-Src or the green fluorescent protein similarly localized to the plasma membrane of NIH-3T3 cells. Thus, the targeting of Lck appears to be determined primarily by its unique domain and may be influenced by the use of different palmitoylation sites.

p53lyn and p56lyn: a new signaling pathway in human endometrium and endometrial adenocarcinomas

OBJECTIVE

To identify specific tyrosine kinases that are involved in endometrial signaling and to study their in vivo expression in normal and abnormal endometrium. We hypothesized that proteins that are differentially expressed would be more likely to be important in regulated cellular events.

METHODS

Complementary DNA libraries, constructed from human secretory (n = 5) and proliferative (n = 5) endometrial specimens, were screened with a polyclonal anti-phosphotyrosine antibody. Positive clones were sequenced and screened for differential expression using immunoblotting and Northern analysis of samples from proliferative and secretory endometrium. The expression of one identified clone, lyn, a Src family member, was characterized further with Western and Northern blot analyses and immunolocalization.

RESULTS

One protein identified by the above method was lyn, a member of the src family of protein tyrosine kinases, never before described in the human endometrium. Western blot analysis revealed two forms of lyn protein, p53lyn and p56lyn, that were most abundant in the late secretory phase. Immunohistochemistry demonstrated uniform protein expression by all cells in normal glandular epithelium and suggested a correlation between lyn protein expression and cell differentiation for human endometrial adenocarcinomas, with markedly-elevated levels noted in poorly differentiated adenocarcinomas compared with well-differentiated tumors (n = 3). Northern hybridization confirmed the presence of the expected 3.5-kb lyn transcript in normal and abnormal endometrium.

CONCLUSIONS

Our data demonstrate that human cDNA libraries created from different phases of the menstrual cycle can be screened successfully using anti-phosphotyrosine antibodies to identify differentially expressed protein tyrosine kinases. Although p53lyn and p56lyn expression has been thought of as a predominantly lymphoid-specific tyrosine kinase, we show prominent expression of lyn protein and mRNA by normal and malignant epithelium of the human endometrium, suggesting a role in endometrial signaling and human reproduction.

A chemiluminescent microtiter plate assay for sensitive detection of protein kinase activity

A chemiluminescent protein kinase assay using biotinylated substrate peptides captured on a streptavidin-coated microtiter plate and monoclonal antibodies to detect their phosphorylation is described. Assay conditions were optimized and validated for sensitive measurement of protein kinase A, protein kinase C, Ca2+/calmodulin-dependent protein kinase II (CAM-KII), receptor interacting protein, and src activities. The newly developed chemiluminescent assay has several advantages over currently used radioactive or colorimetric methods. It is highly sensitive at low enzyme and substrate concentrations and high, close to physiological ATP levels. It is fast, simple to perform and amenable to automation and high-throughput drug screening. The assay is also robust, exhibiting minimum interference from solvents and test substances from various sources. Overall, among the presently available methods for the detection of protein kinase activity, chemiluminescence was found to provide the highest sensitivity under conditions most closely mimicking the intracellular environment. This assay is expected to be useful in both academic and industrial laboratories, especially in identifying novel classes of protein kinase inhibitors.

Expression of a novel form of the p56lck protooncogene in rat cerebellar granular neurons

The src family protein tyrosine kinases (PTKs) are nonreceptor kinases. Some PTKs of this family are ubiquitously expressed, whereas others have a more restricted expression, as in neurons. Lymphoid cell kinase (lck) p56lck is highly expressed in tissues of lymphoid origin and believed to be specific for hematopoietic cells. Reports suggesting that CD4 is expressed in neurons prompted us to analyze the possibility that p56lck is also expressed in these cells. By western blot and immunoprecipitations using anti-lck antibody, an lck-like protein was detected in lysates from primary cultures of rat cerebellar granular neurons. This 56-kDa phosphoprotein was autophosphorylated in vitro and also phosphorylated enolase, similarly to p56lck. It was shown to be located actually in the neurons by immunocytofluorescence. Partial proteolysis mapping showed that the 56-kDa phosphoprotein had a peptide pattern very similar to the p56lck protein. Retrotranscription-PCR allowed the detection of an lck RNA in the neurons. The lck kinase domain was completely identical to the lymphocyte lck kinase domain, but the 5' end was modified in the neurons. These results show that p56lck is not lymphoid specific as is widely believed; its expression in neurons might underlie the toxicity of the HIV glycoprotein gp120 to neurons.

Impaired signaling in alloantigen-specific CD8+ T cells tolerized in vivo: employing a model of Ld-specific TCR transgenic mice transplanted with allogenic hearts under the cover of a short-term rapamycin treatment

Peripheral tolerance of T cells is necessary because thymic deletion is not complete, and tissue-specific Ags exist outside the thymus. We have reported that persistent Ag is required to maintain peripheral tolerance in vivo. We suspect that the TCR signaling pathway in in vivo tolerized cells is compromised due to continuous exposure to the Ags. In this study, the TCR signaling events in these cells were investigated using TCR transgenic mice (2C mice) whose T cells are predominantly Ld alloantigen-specific CD8 cells. The 2C mice were thymectomized and then rendered tolerant to Ld Ag by allogenic heart transplantation plus short-term treatment with rapamycin. We found that 1) the in vivo tolerized CD8 cells have compromised intracellular Ca2+ flux upon mitogen stimulation; and 2) their cellular tyrosine proteins fail to be phosphorylated properly upon TCR cross-linking. These results indicate that the signaling pathway in the in vivo tolerized CD8 cells is indeed defective. We also found that 1) the tolerized CD8 cells have no characteristic surface markers; and 2) the allograft is probably the place where the rejection response is initiated according to the appearance of an early activation marker of T cells on graft-infiltrating cells.

The Lck tyrosine kinase is expressed in brain neurons

The lck gene product, p56lck, is a member of the src-related family of protein tyrosine kinases. It is known as lymphocyte specific and involved in thymocyte development and in the immune response mediated by the T cell receptor. We report that the lck gene is also expressed in adult mouse CNS and that brain p56lck is similar to the thymus protein. In situ hybridization and immunohistochemistry show that the lck gene is expressed in neurons throughout the brain in distinct regions, including hippocampus and cerebellum. In primary cultures from fetal mouse brain, neuronal cells are immunoreactive to Lck antiserum. This suggests that the lck gene product might be involved in a new signal transduction pathway in mouse brain.

Biochemical isolation of a membrane microdomain from resting platelets highly enriched in the plasma membrane glycoprotein CD36

Here we describe the isolation and characterization of a Triton X-100-insoluble fraction isolated from lysates of platelets by flotation in sucrose gradients. Transmission electron microscopy of the insoluble material revealed a heterogeneous population of vesicles ranging in size from 20 to 1000 nm, and Western blot analyses of platelet lysates for the caveolae structural coat protein, caveolin/VIP21, were negative. Biochemical characterization of the Triton X-100-insoluble fraction showed it to be cholesterol-rich, greatly and specifically enriched in the plasma membrane glycoprotein CD36, and also to contain Src and the Src-related kinase, Lyn. CD36 within this fraction is shown to be palmitoylated, but the fraction itself is not generally enriched in palmitoylated platelet proteins. These results suggest that this fraction represents caveolin-negative, CD36-rich microdomains in the resting platelet membrane. CD36 can form associations with certain Src-related kinases and can signal to activate platelets. These results suggest the possibility that such microdomains are implicated in platelet activation.

HIV infection--induced posttranslational modification of T cell signaling molecules associated with disease progression

In attempt to elucidate the mechanism of the HIV infection induced T cell unresponsiveness, we studied signal-transducing molecules proximal to the T cell receptor (TCR) in T lymphocytes of HIV-infected individuals. Total amounts of protein tyrosine kinases (PTKs) Lck, Fyn, and ZAP-70 and the zeta chain of the TCR were found significantly decreased in T cells of symptomatic/AIDS patients as well as in T cells of individuals in acute and early asymptomatic stages of HIV infection. Unexpectedly, the detection of Lck, Fyn, and ZAP-70 was reversed after the treatment of cell lysates with dithiothreitol. This suggests that PTKs Lck, Fyn, and ZAP-70 were modified by a mechanism altering the status of sulfhydryl groups. Moreover, this mechanism seems to affect selectively T cells of HIV infected patients since B cell PTKs Syk and Lyn were detected structurally and functionally intact. Interestingly, similar alterations of signaling molecules were not detected in T cells of HIV-infected long-term asymptomatic individuals. Modification of T cell PTKs may thus underlie the HIV-induced impairment of lymphocyte function and may potentially predict disease progression.