Involvement of the N-terminal unique domain of Chk tyrosine kinase in Chk-induced tyrosine phosphorylation in the nucleus

Chk tyrosine kinase phosphorylates Src-family kinases and suppresses their kinase activity. We recently showed that Chk localizes to the nucleus as well as the cytoplasm and inhibits cell proliferation. In this study, we explored the role of the N-terminal unique domain of Chk in nuclear localization and Chk-induced tyrosine phosphorylation in the nucleus. In situ binding experiments showed that the N-terminal domain of Chk was associated with the nucleus and the nuclear matrix. The presence of the N-terminal domain of Chk led to a fourfold increase in cell population exhibiting Chk-induced tyrosine phosphorylation in the nucleus. Expression of Chk but not kinase-deficient Chk induced tyrosine phosphorylation of a variety of proteins ranging from 23 kDa to approximately 200 kDa, especially in Triton X-100-insoluble fraction that included chromatin and the nuclear matrix. Intriguingly, in situ subnuclear fractionations revealed that Chk induced tyrosine phosphorylation of proteins that were associated with the nuclear matrix. These results suggest that various unidentified substrates of Chk, besides Src-family kinases, may be present in the nucleus. Thus, our findings indicate that the importance of the N-terminal domain to Chk-induced tyrosine phosphorylation in the nucleus, implicating that these nuclear tyrosine-phosphorylated proteins may contribute to inhibition of cell proliferation.

Expression of c-Src and comparison of cytologic features in cherubism, central giant cell granuloma and giant cell tumors

Cherubism (CBM) and central giant cell granuloma (CGCG) of the jaw and giant cell tumor (GCT) of the long bone are clinically different diseases. Histologically, they are all multinucleated giant cell (MGC)-containing lesions. This study aims to evaluate the expression of c-Src and cytologic features in CBM, CGCG and GCT and to clarify whether there is a common mechanism underlying the formation of multi-nucleated giant cells (MGCs) in these lesions. Specimens and paraffin blocks were collected from patients with CBM (12 cases), CGCG (24 cases) and GCT (37 cases). Histomorpho-metric differences in MGCs were compared among the three types of lesions. The expression of c-Src by immunohistochemistry and in situ hybridization and the expression of TRAP by enzyme histochemical staining were examined. Expression of c-Src mRNA and protein, as well as TRAP staining, was detected in both MGCs and a fraction of mononuclear cells in all investigated lesions. There are no quantitative differences for cytologic features and c-Src expression among the lesions. The results suggested that CBM, CGCG and GCT have overlapping cytological features at the histological level, and c-Src may be involved in the formation of MGCs in the three different diseases.

Postnatal changes in the expression of p60c-Src in mouse testes

Src family non-receptor tyrosine kinases are involved in signaling pathways which mediate cell growth, differentiation, transformation and tissue remodeling in various organs. In an effort to elucidate functional involvement of p60c-Src (c-Src) in spermatogenesis, the postnatal changes in c-src mRNA and c-Src protein together with kinase activity and subcellular localization were examined in mouse testes. c-src mRNA levels in testes increased during the first 2 weeks of postnatal development (PND). Following a decrease at puberty (PND 28), the c-src mRNA levels re-increased at adulthood (PND 50). Src kinase activity of testes was low at PND 7 but sharply increased prepubertally (PND 15) and highest at adulthood. Upon Western blotting, the level of c-Src protein was the highest in prepubertal testes but rather decreased in adult testes at PND 50. In adult testes, ubiquitination of c-Src proteins was apparent compared with immature one at PND 7, suggesting active turnover of c-Src by ubiquitination. In immature testes, c-Src immunoreactivity was largely found in the cytoplasm of the Sertoli cells. By contrast, in pubertal and adult testes intense immunoreactivity was localized at the adluminal and basal cytoplasm of Sertoli cells bearing elongated spermatids and early germ cells, respectively. The immunoreactivity of c-Src in the Leydig cells was increased during pubertal development, suggesting the functional involvement of c-Src in differentiated adult Leydig cells. Throughout postnatal development, some spermatogonia and spermatocytes showed intensive c-Src immunoreactivity compared with other germ cells, suggesting a possible role of c-Src in germ cell death. Taken together, it is suggested that c-Src may participate in the remodeling of the seminiferous epithelia and functional differentiation of Leydig cells during the postnatal development of mouse testes.

[Effect of PP60c-Src on angiotensin II-induced signal transduction in rat vascular smooth muscle cells]

OBJECTIVE

To further clarify the mechanism of Ang II-induced intracellular signal transduction in vascular smooth muscle cells(VSMCs) proliferation by observing the effect of c-Src on Ang II-mediated MAPK activation and c-fos protein expressions in rat VSMCs.

METHODS

Aortic VSMCs from SD rats were cultured primarily and subcultured, which were transfected with anti-sense c-Src oligodeoxynucleotides(ODNs) wrapped with lipofectin to inhibit c-Src activity and protein production. Untransfected VSMCs were used as control, to observe the role of Ang II stimulation in MAPK activation and c-fos protein expression in VSMC. Protein immunoprecipitation and kinase phosphorylation were employed to measure c-Src kinase activity; MAPK kinase activity was assessed by the phosphorylation rate of the substrate MBP(Myelin Basic Protein); Western blot was used to assess the protein expression of c-Src and c-fos.

RESULTS

c-Src protein expressions in VSMC, which were transfected with different concentrations of anti-sense c-Src ODNs, were significantly decreased in a negative dose-effect manner (0.2 microm, 0.5 microm, 1.0 microm and 2.0 microm were 68.2%, 34.7%, 30.3% and 15.8% respectively compared with control). c-Src kinase activity was also obviously inhibited. Following stimulation of Ang II on VSMC transfected with anti-sense c-Src ODNs, the increase of c-Src activity was only 8.7% of control,the activity of MAPK only 1.6% compared with control, and the increase in c-fos protein expression 30.3% as control.

CONCLUSION

Ang II can induce c-Src activation and intracellular signal transduction in VSMC which depend on c-Src activation, indicating that c-Src is a pivotal signal factor in VSMC proliferation.

Neutral loss analysis in MALDI-MS using an ion-gate scanning mode

Matrix-assisted laser desorption/ionization reflector time-of-flight (MALDI-reTOF) and electrospray ionization (ESI) mass spectrometry (MS) have become essential tools for the characterization of peptides and proteins. Whereas ESI in combination with a triple quadrupole analyzer allows product ion, precursor ion, and neutral loss analyses, MALDI-reTOF instruments can only be used to record product ion spectra based on the in-source or postsource decay (PSD). We describe a new method to perform neutral loss analyses in MALDI-reTOF instruments in a manner that identifies posttranslationally modified peptides and furthermore retrieves sequence information from peptides. The method is based on the selection of ions in a small time interval to record only signals within the corresponding mass interval. By stepping the time interval through the complete mass range, we obtained a spectrum of stable ions by combining the signals of all individually recorded time intervals. This method furthermore permits PSD fragment ions to be identified, since they reach the detector earlier than the stable ions transmitted in the chosen time interval. The neutral loss analysis were calculated by correlating the PSD fragment ions to the corresponding parent ion detected in this time interval. Moreover, this MALDI-MS mode increased the number of detectable signals in complex peptide mixtures and the signal-to-noise ratio.

Regulation of pp60(c-src) synthesis in rat hippocampal slices by in vitro ischemia and glucocorticoid administration

Corticosteroids, released from the adrenal gland in response to stress, bind to receptors that act as transcription factors to alter gene expression and, subsequently, protein synthesis. Using [(35)S]-methionine-cysteine incorporation to measure protein synthesis in hippocampal slices incubated under ischemic conditions, synthesis of 60 kDa and 78 kDa proteins decreases 4 hr after in vivo administration of corticosterone to rats. The former protein has been identified by immunoblotting and immunoprecipitation to be the proto-oncogene, pp60(c-src). In the absence of prior glucocorticoid administration, ischemic conditions increase the amount of immunoreactive pp60(c-src) in membranes of hippocampal slices. Chronic exposure to elevated titers of glucocorticoids has been demonstrated to result in cell loss as well as in reduced neuronal plasticity and regeneration. Given the involvement of pp60(c-src) in synaptic plasticity and cell growth, glucocorticoid-mediated reduction in its synthesis is a potential molecular marker for stress-induced alterations in brain function.

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.

Association of renal cell carcinoma antigen, disialylgalactosylgloboside, with c-Src and Rho A in clustered domains at the surface membrane

Disialylgalactosylgloboside (DSGG), defined by monoclonal antibody RM2, is a renal cell carcinoma (RCC)-associated antigen which mediates adhesion of RCC TOS-1 cells to certain lung tissue target cells. This adhesion process may initiate preferential lung metastasis of RCC. Ganglioside GM3 is a B16 melanoma-associated antigen which similarly adheres to target cells and promotes consequent metastasis. In view of the close association of GM3-enriched microdomain with transducer molecules c-Src, Rho A, and FAK in B16 cells, we investigated the organizational status of DSGG in RCC cell line TOS-1, with the following results: i) DSGG, but not monosialylgalactosylgloboside, showed extensive clustering at the TOS-1 cell surface; ii) a low-density membrane fraction isolated from TOS-1 cells contained >95% of cellular DSGG, although protein content in this fraction was <1% of total cellular protein; iii) this fraction contained c-Src, Rho A, and FAK, but not H-Ras; iv) c-Src and Rho A were co-immunoprecipitated with DSGG through anti-DSGG mAb RM2 (IgM) affixed to a column. These observations indicate that DSGG is clustered in RCC, as typified by TOS-1 cells, to form a microdomain in which it is closely associated with c-Src, Rho A, and FAK, and may constitute a functional unit as has been observed for GM3 with transducer molecules in B16 cells. The functional organization of such units may be essential in determining malignant properties of RCC cells.

Vascular endothelial growth factor induces activation and subcellular translocation of focal adhesion kinase (p125FAK) in cultured rat cardiac myocytes

Vascular endothelial growth factor (VEGF) has been proposed to be among the candidate factors with the most potential to play a role in ischemia-induced collateral vessel formation. Recently, we found that VEGF activated the mitogen-activated protein kinase cascade in cultured rat cardiac myocytes. To elucidate how VEGF affects adhesive interaction of cardiac myocytes with the extracellular matrix (ECM), one of the important cell functions, we investigated the molecular mechanism of activation of focal adhesion-related proteins, especially focal adhesion kinase (p125(FAK)), in cultured rat cardiac myocytes. We found that the 2 VEGF receptors, KDR/Flk-1 and Flt-1, were expressed in cardiac myocytes and that KDR/Flk-1 was significantly tyrosine phosphorylated on VEGF stimulation. VEGF induced tyrosine phosphorylation and activation of p125(FAK) as well as tyrosine phosphorylation of paxillin; this was accompanied by subcellular translocation of p125(FAK) from perinuclear sites to the focal adhesions. This VEGF-induced activation of p125(FAK) was inhibited partially by the tyrosine kinase inhibitors genistein and tyrphostin. Activation of p125(FAK) was accompanied by its increased association with adapter proteins GRB2, Shc, and nonreceptor type tyrosine kinase p60(c-src). Furthermore, we confirmed that VEGF induced a significant increase in adhesive interaction between cardiac myocytes and ECM using an electric cell-substrate impedance sensor. These results strongly suggest that p125(FAK) is one of the most important components in VEGF-induced signaling in cardiac myocytes, playing a critical role in adhesive interaction between cardiac myocytes and ECM.

Overexpression and activation of the tyrosine kinase Src in human pancreatic carcinoma

Src family tyrosine kinases participate in the regulation of cell adhesion, cell growth and differentiation. Here, we examine for the first time the potential role of Src for growth regulation of human pancreatic carcinoma cells. By immunohistochemical analysis, Src was overexpressed in 13/13 pancreatic carcinoma tissue but not in 6 normal pancreatic tissue specimen. In Western blots of total cellular extracts, Src protein expression was elevated in 14/17 carcinoma cell lines as compared to normal pancreas or cultured human pancreatic duct cells. Kinase activity was only detectable in cancer cells and did not correlate with the amount of kinase protein or with the expression of the regulatory kinase Csk, indicating that Src is not regulated through protein expression or through expression of Csk. The Src-specific tyrosine kinase inhibitor herbimycin A decreased cell growth in a dose-dependent manner. We suggest that Src family kinases participate in growth regulation of pancreatic cancer cells.

Epidermal growth factor-induced activation and translocation of c-Src to the cytoskeleton depends on the actin binding domain of the EGF-receptor

In the epidermal growth factor (EGF)-receptor signal transduction cascade, the non-receptor tyrosine kinase c-Src has been demonstrated to become activated upon EGF stimulation. In this paper we show that c-Src associates with the cytoskeleton and co-isolates with actin filaments upon EGF treatment of NIH-3T3 cells transfected with the EGF receptor. Immunofluorescence studies using CLSM show colocalization of F-actin and endogenous c-Src predominantly around endosomes and not on stress fibers and cell-cell contacts. Stimulation of EGF receptor-transfected NIH-3T3 cells with EGF induces an activation and translocation of c-Src to the cytoskeleton. These processes depend upon the presence of the actin binding domain of the EGF-receptor since in cells that express EGF-receptors lacking this domain, EGF fails to induce an activation and translocation to the cytoskeleton of c-Src. These data suggest a role for the actin binding domain of the EGF-receptor in the translocation of c-Src.

Substrate recognition by osteoclast precursors induces C-src/microtubule association

The osteoclast is distinguished from other macrophage polykaryons by its polarization, a feature induced by substrate recognition. The most striking component of the polarized osteoclast is its ruffled membrane, probably reflecting insertion of intracellular vesicles into the bone apposed plasmalemma. The failure of osteoclasts in c-src-/- osteopetrotic mice to form ruffled membranes indicates pp60(c-src) (c-src) is essential to osteoclast polarization. Interestingly, c-src itself is a vesicular protein that targets the ruffled membrane. This being the case, we hypothesized that matrix recognition by osteoclasts, and their precursors, induces c-src to associate with microtubules that traffic proteins to the cell surface. We find abundant c-src associates with tubulin immunoprecipitated from avian marrow macrophages (osteoclast precursors) maintained in the adherent, but not nonadherent, state. Since the two proteins colocalize only within adherent avian osteoclast-like cells examined by double antibody immunoconfocal microscopy, c-src/tubulin association reflects an authentic intracellular event. C-src/tubulin association is evident within 90 min of cell-substrate recognition, and the event does not reflect increased expression of either protein. In vitro kinase assay demonstrates tubulin-associated c-src is enzymatically active, phosphorylating itself as well as exogenous substrate. The increase in microtubule-associated kinase activity attending adhesion mirrors tubulin-bound c-src and does not reflect enhanced specific activity. The fact that microtubule-dissociating drugs, as well as cold, prevent adherence-induced c-src/tubulin association indicates the protooncogene complexes primarily, if not exclusively, with polymerized tubulin. Association of the two proteins does not depend upon protein tyrosine phosphorylation and is substrate specific, as it is induced by vitronectin and fibronectin but not type 1 collagen. Finally, consistent with cotransport of c-src and the osteoclast vacuolar proton pump to the polarized plasmalemma, the H+-ATPase decorates microtubules in a manner similar to the protooncogene, specifically coimmunoprecipitates with c-src from the osteoclast light Golgi membrane fraction, and is present, with c-src, in preparations enriched with acidifying vesicles reconstituted from the osteoclast ruffled membrane.

Hepatocyte growth factor/scatter factor overexpression induces growth, abnormal development, and tumor formation in transgenic mouse livers

To investigate the in vivo role of hepatocyte growth factor/scatter factor (HGF/SF) in liver function, we generated transgenic mice using a mouse HGF/SF cDNA under the control of the mouse metallothionein gene promoter and 5'/3' flanking sequences. In adult HGF/SF transgenic mice, liver weight as a percentage of total body weight was at least twice that of wild-type mice. Comparison of transgenic and control liver morphology revealed dramatic heterogeneity in the size and appearance of hepatocytes as a distinctive feature of HGF/SF overexpression. Transgenic livers exhibited a significant increase in the number of small hepatocytes with a 2N DNA content, accounting for the observed increase in liver mass. The DNA labeling index of hepatocytes increased 11-fold at 4 weeks of age, when liver enlargement first became apparent, and was still elevated about 5-fold in adult HGF/SF transgenic mice. Moreover, hepatocytes isolated by perfusion of transgenic livers doubled every 2 days in culture, whereas little or no growth was observed with isolated control hepatocytes. The mechanistic basis of hepatocyte proliferation was elucidated as the chronic activation of the c-met proto-oncogene product. Met and substrates such as phosphatidylinositol 3-kinase, Src homology and collagen-like, pp60c-src, focal adhesion kinase p125FAK, and paxillin were associated with tyrosine-phosphorylated complexes in a hepatocyte cell line established from the transgenic liver. This proliferative stimulus triggered the formation of hepatocellular adenomas and/or carcinomas in most transgenic mice > or = 1.5 years of age. Finally, the rate of transgenic mouse liver regeneration was increased 3-fold over control livers following partial hepatectomy.

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.

Regulation of cell-cell contacts in developing Drosophila eyes by Dsrc41, a new, close relative of vertebrate c-src

In Drosophila, Dsrc64 is considered a unique ortholog of the vertebrate c-src; however, we show evidence to the contrary. The closest relative of vertebrate c-src so far found in Drosophila is not Dsrc64, but Dsrc41, a gene identified for the first time here. In contrast to Dsrc64, overexpression of wild-type Dsrc41 caused little or no appreciable phenotypic change in Drosophila. Both gain-of-function and dominant-negative mutations of Dsrc41 caused the formation of supernumerary R7-type neurons, suppressible by one-dose reduction of boss, sev, Ras1, or other genes involved in the Sev pathway. Dominant-negative mutant phenotypes were suppressed and enhanced, respectively, by increasing and decreasing the copy number of wild-type Dsrc41. Colocalization of Dsrc41 protein, actin fibers and DE-cadherin, and Dsrc41-dependent disorganization of actin fibers and putative adherens junctions in precluster cells suggested that Dsrc41 may be involved in the regulation of cytoskeleton organization and cell-cell contacts in developing ommatidia.

Serum-starvation induces the extracellular appearance of FGF-1

Autocrine/paracrine stimulation of cell growth by members of the fibroblast growth factor (FGF) family of polypeptides is dependent upon extracellular interactions with specific high affinity receptors at the cell surface. Acidic FGF (FGF-1) lacks a classical signal sequence for secretion, suggesting that intrinsic levels of this mitogen may not stimulate cell growth and utilizes a non-classical pathway to gain access to the extracellular compartment. To evaluate the biological potential of intracellular FGF-1 more rigorously, human cDNA sequences for the growth factor were introduced into primary murine embryonic fibroblasts using retrovirally mediated gene transfer. Heparin affinity, Western analysis, mitogenic assays, in situ immunohistochemical techniques, induction of tyrosine phosphorylation and antibody inhibition studies were used to demonstrate functionality of the FGF-1 transgene in this experimental model. Under normal culture conditions, cells constitutively expressing intracellular FGF-1 exhibited a slight growth advantage. In contrast, when maintained in reduced serum, these cells adopted a transformed phenotype and demonstrated an enhanced growth potential, induction of FGF-specific phosphotyrosyl proteins and the nuclear association of the growth factor. Analysis of the conditioned media from these stressed cells indicated that serum starvation induces the secretion of FGF-1 as latent high molecular mass complexes requiring reducing agents to activate its full biological potential.

Osteopontin activation of c-src in human melanoma cells requires the cytoplasmic domain of the integrin alpha v-subunit

In human melanoma cells, expression of the alpha v beta 3 integrin is correlated with the metastatic potential. The expression of osteopontin (OPN or OP), a protein ligand for the integrin alpha v beta 3, also correlates with metastatic potential of some tumors. Analysis of signal transduction, stimulated by OPN/alpha v beta 3 in human melanoma cells (M21), revealed activation of pp60c-src associated with the integrin. pp60c-src stimulation by OPN was dose dependent, and it was inhibited in vitro by a tyrosine kinase inhibitor, herbimycin-A. To determine the need for the cytoplasmic domain of the alpha v-subunit, in the association of pp60c-src with alpha v beta 3, a cell line expressing truncated alpha v was studied. M21-L cells lacked alpha v expression but stably transfected with complementary DNAs encoding alpha v full length protein alpha v 1018 or alpha v 995 (lacking 23 carboxyl-terminal amino acids), and a fibroblast cell line (FG) expressing alpha v beta 5 but not alpha v beta 3, were used. Western analysis and immune complex kinase assays of anti- alpha v immunoprecipitates demonstrated that M21-L/alpha v995 cells did not exhibit pp60c-src association with alpha v, whereas the alpha v1018 complementary DNA transfected cells and FG cells had pp60c-src associated with the alpha v integrins. Immunofluorescence analysis revealed pp60c-src, alpha v beta 3 integrin, and actin distribution along the plasma membrane of M21 cells. 35S-labeling of cells and analysis of complexes immunoprecipitated by a monoclonal antibody against alpha v beta 3 demonstrated association of actin with the immune complexes. We conclude that OPN stimulates pp60c-src kinase activity associated with the alpha v beta 3 integrin and that the association requires the cytoplasmic tail of the alpha v chain.

A novel monoclonal antibody to N-myristoyl glycine moiety found a new N-myristoylated HIV-1 p28gag protein in HIV-1-infected cells

A novel monoclonal antibody was raised against a synthetic N-myristoyl glycine that is characteristic of all N-myristoylated proteins. The immunoreaction suppressed in the presence of hemocyanin as well as albumin conjugated with N-myristoyl glycine and other N-myristoyl glycyl peptides, while underivatized and myristoyl amino acid proteins or various fatty acids other myristic acid exerted no effect. The antibody specifically reacted with N-myristoylated pp60c-src in human colon adenocarcinoma cells, N-myristoylated pp60v-src in Rous sarcoma virus-infected cells, and N-myristoylated Gag precursor protein Pr55gag in HIV-1 producing cells. Furthermore, the antibody immunoreacted with a new N-myristoylated p28gag derived from HIV-1 gag protein. The antibody is shown to be a very useful tool for identification of N-myristoylated proteins.

Histochemically demonstrable protein tyrosine phosphatase in human breast and colorectal cancer: large decrease in its activity in colorectal cancer suggests a tumor suppressor role in colorectal mucosal cells

Many oncogene products and growth factor receptors are protein tyrosine kinases, and exert their cellular effects by the phosphorylation of tyrosyl residues of effector proteins. The balance and dynamic renewal of phosphotyrosine proteins are also regulated by protein tyrosine phosphatases (PTPs), whose inhibition under experimental conditions causes cellular proliferation and transformation, with a concomitant increase in phosphotyrosine protein content. Inverse effects are obtained by increasing PTP activity. On the basis of these effects, PTPs might also function as tumor suppressors in human tissues. This possibility was further investigated here by demonstrating PTP and phosphotyrosine protein content with histochemical techniques. In normal human breast tissue PTP activity was low and in the majority of breast cancers the activity was increased and exhibited great variation between different cases. When the relationship of phosphotyrosine protein content with PTP was evaluated, no inverse dependence was detected, suggesting that in human breast tissue and cancer PTP may not show tumor suppressor activity. In normal colorectal mucosae PTP activity was high, while in all colorectal cancers it was very low, constituting only 14% of the activity present in normal mucosal cells. The great drop in PTP activity together with reported alterations in a gene encoding a PTP and in a chromosome containing a PTP gene in colorectal cancer strongly suggest that PTP may function as a tumor suppressor in human colorectal mucosae. The decrease in PTP activity may be one factor stimulating or causing neoplastic proliferation in multistep colorectal carcinogenesis.

L1, N-cadherin, and laminin induce distinct distribution patterns of cytoskeletal elements in growth cones

Growth cones are highly motile extensions of growing neuronal processes that have a well-characterized cytoskeleton, which is necessary for motility and directed growth. In culture, neurons have been observed to extend processes on a variety of substrates made of cell adhesion molecules (CAMs) or extracellular matrix molecules (ECM molecules). We have previously shown that retinal ganglion cell (RGC) growth cones exhibit characteristic morphologies dependent on the substrate on which they are grown (Payne et al., 1992: Cell Motil. Cytoskel. 21:65-73). Upon contact with a sharp border between two substrates, the growth cones display rapid changes in morphology (Burden-Gulley et al., 1995: J. Neurosci. 15:4370-4381) that may result from extensive restructuring of the cytoskeleton. In the present study, immunocytochemical methods were used to examine the distribution of three cytoskeletal elements in RGC growth cones growing on L1, N-cadherin, or laminin as well as on dishes coated with alternating lanes of these substrates. Distinct distribution patterns of f-actin, microtubules (MTs), and neurofilaments (NFs) were observed in growth cones growing on individual substrates. At border regions between two substrates, growth cones with extensive lamellipodial contact with the second substrate were observed to have f-actin and MT distribution patterns appropriate for the new substrate encountered. Contact via filopodia alone did not evoke this change. Redistribution of NFs was observed only after the majority of the growth cone had crossed onto the second substrate. These results suggest that actin and MTs, but probably not NFs, are directly influenced by CAMs and ECM molecules to produce changes in growth cone morphology. The distribution of two members of the protein tyrosine kinase family, pp60arc and p59fyn, and phosphorylated tyrosine residues was also examined. No differences were observed in the distribution patterns of the kinases and phosphorylated tyrosine residues in growth cones on any of the substrate molecules tested. In addition, the distribution patterns were unchanged in growth cones that contacted and crossed borders between two substrates. These results suggest that redistribution of pp60arc or p59fyn is not required to produce alterations in growth cone morphology induced by contact with L1, N-cadherin or laminin.

Convergence of integrin and growth factor receptor signaling pathways within the focal adhesion complex

Extracellular matrix controls capillary endothelial cell sensitivity to soluble mitogens by binding integrin receptors and thereby activating a chemical signaling response that rapidly integrates with growth factor-induced signaling mechanisms. Here we report that in addition to integrins, growth factor receptors and multiple molecules that transduce signals conveyed by both types of receptors are immobilized on the cytoskeleton (CSK) and spatially integrated within the focal adhesion complex (FAC) at the site of integrin binding. FACs were rapidly induced in round cells and physically isolated from the remainder of the CSK after detergent-extraction using magnetic microbeads coated with fibronectin or a synthetic RGD-containing peptide. Immunofluorescence microscopy revealed that multiple signaling molecules (e.g., pp60c-src, pp125FAK, phosphatidylinositol-3-kinase, phospholipase C-gamma, and Na+/H+ antiporter) involved in both integrin and growth factor receptor signaling pathways became associated with the CSK framework of the FAC within 15 min after binding to beads coated with integrin ligands. Recruitment of tyrosine kinases to the FAC was also accompanied by a local increase in tyrosine phosphorylation, as indicated by enhanced phosphotyrosine staining at the site of integrin binding. In contrast, neither recruitment of signaling molecules nor increased phosphotyrosine staining was observed when cells bound to beads coated with a control ligand (acetylated low density lipoprotein) that ligates transmembrane scavenger receptors, but does not induce FAC formation. Western blot analysis confirmed that FACs isolated using RGD-beads were enriched for pp60c-src, pp125FAK, phospholipase C-gamma, and the Na+/H+ antiporter when compared with intact CSK or basal cell surface preparations that retained lipid bilayer. Isolated FACs were also greatly enriched for the high affinity fibroblast growth factor receptor flg. Most importantly, isolated FACs continued to exhibit multiple chemical signaling activities in vitro, including protein tyrosine kinase activities (pp60c-src and pp125FAK) as well as the ability to undergo multiple sequential steps in the inositol lipid synthesis cascade. These data suggest that many of the chemical signaling events that are induced by integrins and growth factor receptors in capillary cells may effectively function in a "solid-state" on insoluble CSK scaffolds within the FAC and that the FAC may represent a major site for signal integration between these two regulatory pathways. Future investigations into the biochemical and biophysical basis of signal transduction may be facilitated by this method, which results in isolation of FACs that retain the CSK framework as well as multiple associated chemical signaling activities.

pp60c-src expression in rat spermatogenesis

A fractionation of the membrane extract of rat testes revealed the existence of pp60c-src kinase activity. The expression of pp60c-src was examined in the developing rat testes. The immunecomplex kinase assay using a monoclonal antibody specific to pp60c-src (mAb327) showed that the expression of pp60c-src kinase activity increased during the development of rat testes and declined in the adult. The increase in pp60c-src kinase activity observed during the development of rat testes was accompanied with an increase in the amount of pp60c-src protein. The peak period in the increase of pp60c-src kinase activity well coincided with the timing, when the spermatogenesis by meiosis just began. The immunohistochemical staining of pp60c-src in rat testes demonstrated that pp60c-src is most abundantly expressed in the spermatids which are the spermatogenic cells in the post-meiotic phase of the spermatogenesis. These findings strongly suggest that pp60c-src is a developmentally regulated gene product which is involved in rat spermatogenesis.

Identification of a hormonally regulated protein tyrosine phosphatase associated with bone and testicular differentiation

Absence of the tyrosine kinase activity of c-src and c-fms results in impairment of bone remodeling. Such dysfunction underscores the importance of tyrosine phosphorylation, yet the role of protein tyrosine phosphatases in bone metabolism remains unexamined. We have isolated the cDNA for a novel receptor-like tyrosine phosphatase expressed in bone and testis named osteotesticular protein tyrosine phosphatase (OST-PTP). The deduced 1711-residue protein possesses an extracellular domain with 10 fibronectin type III repeats and a cytoplasmic region with two catalytic domains. In primary rat osteoblasts, the 5.8-kilobase OST-PTP transcript is up-regulated in differentiating cultures and down-regulated in late stage mineralizing cultures. In addition, a presumed alternate transcript of 4.8-5.0 kilobases, which may lack PTP domains, is present in proliferating osteoblasts, but not detectable at other stages. Parathyroid hormone, a modulator of bone function, as well as cyclic AMP analogues, increase OST-PTP mRNA 5-8-fold in UMR 106 cells. In situ hybridization of adult rat testis revealed stage-specific expression of OST-PTP. OST-PTP may function in signaling pathways during bone remodeling, as well as serve a broader role in cell interactions associated with differentiation in bone and testis.

Increased expression of pp60c-src in gastric mucosa of aged rats

The relationship between proliferative activity and the expression of pp60c-src in gastric mucosa (oxyntic gland area) of young (4-month) and aged (24-month) Fischer 344 rats was examined. Gastric mucosal proliferative activity, as assessed by 5-bromo-2'-deoxyuridine (BrdU) immunoreactive cells, was found to be 115% (p < .001) higher in aged than in young rats. This was associated with a 47% rise (p < .025) in overall tyrosine kinase (Tyr-k) activity and a 5-7-fold increase in autophosphorylation of four prominent protein bands with M(r) of 40, 55, 60, and 70 kDa in gastric mucosal membranes. A similar phenomenon was also observed for Tyr-k activity of pp60c-src in that the aged rats revealed a 69% (p < .025) higher enzyme activity and a 5-fold rise in the extent of autophosphorylation of this protein when compared with the corresponding values from young animals. Increased Tyr-k activity of pp60c-src in the gastric mucosa of aged rats could in part be due to higher levels of this protein because the relative concentration of pp60c-src, as assessed by Western blot analysis, showed a 2-5-fold increase over the young animals. In addition, the relative concentration of c-src mRNA in the gastric mucosa of aged rats was also found to be 5-6-fold higher than in young animals. We suggest that pp60c-src may play a role in regulating gastric mucosal proliferative processes in the gastric mucosa of aged rats.

Elevated c-Src tyrosine kinase activity in premalignant epithelia of ulcerative colitis

Ulcerative colitis (UC) is a chronic inflammatory disease of the colon with a high incidence of colon cancer. Dysplasia is a precursor to carcinoma and a predictor of malignant potential; epithelia containing high-grade or severe dysplasia is most likely to develop cancer. The cellular oncogene c-src and its viral homologue v-src (the transforming gene of Rous sarcoma virus) encode 60-kD cytoplasmic, membrane-associated protein tyrosine kinases. For the viral protein or transforming mutants of the cellular protein (Src), a close correlation exists between elevated tyrosine kinase activity and malignant transformation of cells. Previously, we and others observed elevated Src activity in sporadic colon carcinomas and benign adenomas at greatest risk for developing cancer (those with large size, villous architecture, and/or severe dysplasia). Here we report that Src activity and protein abundance are also elevated in neoplastic UC epithelia. Activity is highest in malignant and severely dysplastic epithelia, and 6-10-fold higher in mildly dysplastic than in nondysplastic epithelia. Thus, Src activity is elevated in premalignant UC epithelia, which is at greatest risk for developing cancer. The data suggest that activation of the src proto-oncogene is an early event in the genesis of UC colon cancer.

Localisation of pp60c-src to the surface membrane of human platelets

In platelets, pp60c-src has been variously localised to cytosol, dense bodies, or plasma membrane and surface-connected canalicular membranes. We have investigated the localisation of pp60c-src in highly purified human platelet subcellular fractions isolated using density gradient centrifugation and continuous flow electrophoresis followed by quantitative immunoblotting. We have obtained fractions enriched in surface membranes and alpha granule membranes as assessed by the presence of the marker proteins gpIb, gpIIb, thrombospondin, fibrinogen and fibronectin. Quantitative immunoblotting demonstrated that most of the cellular pp60c-src co-purified with markers for surface membrane components and that unlike the situation in fibroblasts, platelet pp60c-src did not associate with non-granule intracellular membranes such as Golgi, endoplasmic reticulum and endosomes. A small subpopulation of the total cellular pp60c-src co-purified with granule components or dense body markers, however this occurred in proportion to the presence of surface membrane markers. Our results demonstrate that the majority of platelet pp60c-src is located on the cytoplasmic face of the plasma membrane in human platelets.

c-Yes tyrosine kinase activity in human colon carcinoma

To examine the role of Src-related proteins in human colon carcinoma we measured the tyrosine kinase activity of pp60c-src (Src), p62c-yes (Yes), p56lck (Lck), p59fyn (Fyn), p59hck (Hck), p56lyn (Lyn) and p55c-fgr (Fgr) from colonic cells. Yes activity, similar to that of Src, was 10-20 fold higher in three of five colon carcinoma cell lines and fivefold higher in 10 of 21 primary colon cancers than that in normal colonic cells. Lck activity was present in COLO 205 cells, otherwise Lck, Fyn, Hck, Lyn and Fgr activities were not detected in any of the carcinoma cell lines or cancers tested. Increased Yes activity, like that of Src, was due mostly to increased protein levels and not to an apparent decrease in phosphorylation of Tyr 537, the major mechanisms known to deregulate enzymatic activity. Only those colon carcinoma cell lines with elevated Src and/or Yes tyrosine kinase activity as measured in vitro had elevated levels of three tyrosine-phosphorylated proteins as measured in vivo. Thus, colon carcinoma cells contain active tyrosine kinases and/or inactive tyrosine phosphatases not present in normal colonic cells, and Src and Yes appear to be active kinases in the carcinoma cells. These data, together with those demonstrating decreased Src activity in fully differentiated enterocytes, suggest that down regulation of Src-related tyrosine kinases is important for differentiation, and/or deregulation of the kinases is important for growth and transformation of intestinal epithelial cells.

Elevated expression of protein tyrosine kinase c-Yes, but not c-Src, in human malignant melanoma

The c-yes proto-oncogene encodes a protein tyrosine kinase, p62c-yes (c-Yes) that belongs to the Src family of non-receptor type protein tyrosine kinases. We compared the levels of c-Yes kinase activity and protein by immune complex kinase assays and immune blot analysis in 20 human melanoma and 10 human melanocyte cell lines. Results show that the average kinase activity of c-Yes in most melanoma cell lines is 5-10-fold higher than that in melanocyte cell lines. The protein level of c-Yes in these melanoma cell lines is correspondingly higher than that in melanocytes. The increase in c-Yes kinase activity is most likely attributable to the elevated protein level because single-strand conformational polymorphism of all structural and functional domains detected no mutations in any of the c-yes coding regions. Subcellular fractionation analysis indicated that c-Yes localizes to the plasma membrane, perinuclear and cytosolic compartments while c-Src predominantly associates with plasma membranes. In melanoma cells in which an elevated level of c-Yes is observed, a protein of 39 kD is heavily phosphorylated on tyrosine. This protein is only observed in melanoma cells and not in melanocytes, suggesting a perturbed signaling pathway in melanoma cells that results in abnormal tyrosine phosphorylation of cellular proteins. These data suggest that derangement of expression of the c-Yes tyrosine kinase may have a role in the malignant progression of the human melanocyte.

Immediate and transient stimulation of protein tyrosine phosphorylation by estradiol in MCF-7 cells

Estradiol stimulates protein phosphorylation on tyrosine in human breast cancer MCF-7 cells under conditions of estradiol-stimulated cell growth. The stimulatory effect of estradiol has been observed by 32P-labeling of cells followed by purification of proteins using antiphosphotyrosine antibody coupled to agarose and confirmed by immunoblotting analysis with antiphosphotyrosine antibody. This stimulation is immediate (maximal in 10 s) and transient. In addition, it is receptor-mediated since estradiol stimulation is prevented by two well-known antiestrogens, OH-Tamoxifen and ICI 164,384. Estradiol fails to stimulate tyrosine protein phosphorylation of Cos cells which do not express the estradiol receptor. Two substrates of the estrogen stimulated phosphorylation on tyrosine with approximate mol wt of 55 and 60 kDa interact with a polyclonal antibody raised against amino acids 527-533 of pp60c-src (anti-cst.1 antibody). Tyrosine kinase activity of immunoprecipitates made using either anti cst.1 antibody or the monoclonal 327 antibody specific for pp60c-src shows that kinase(s) strongly related to pp60c-src are immediately and transiently stimulated by estradiol treatment of cells. The present findings provide the first demonstration that a steroid hormone rapidly stimulates tyrosine phosphorylation of target cells and induces functional modifications of substrates of this phosphorylation. These modifications might initiate the estradiol action on cell growth.

Malignant osteopetrosis: c-src kinase is not reduced in fibroblasts

Targeted disruption of the c-src gene leads to a severe form of osteopetrosis in mice [2]. As the c-src gene is expressed in all tissues and cells tested, we have analyzed fibroblasts from three individuals with malignant, congenital osteopetrosis for the expression of c-src at the protein level. No differences could be detected in c-src protein and c-src kinase activity levels between fibroblasts from healthy controls and affected individuals. Thus, impairment of c-src function as an etiological factor in human osteopetrosis appears unlikely in the individuals investigated.

Fibronectin expression correlates with U937 cell adhesion to migrating bovine aortic endothelial cells in vitro

A blood vessel's response to denudation injury will determine its final luminal diameter as well as its function. The synthesis, deposition, and remodeling of extracellular matrix components and migration by vascular endothelial cells are major factors in determining luminal diameter, cellular proliferative and migratory responses, and mononuclear cell adhesion at sites of injury. Previously, we have shown that after in vivo and in vitro denudation injury, endothelial cell migration is dramatically influenced by the amount of fibronectin synthesized and deposited by the responding endothelial cell population. The aim of this study was to elucidate the roles of fibronectin in modulating mononuclear cell adhesion to the endothelial cell population during in vitro migration. In this report we demonstrate that U937 cell binding to the migrating fronts of endothelial cell monolayers is modulated by the amount of fibronectin synthesized and deposited by the endothelial cells. Agents which increase fibronectin deposition, such as transforming growth factor-beta 1, elicit greater U937 cell adhesion. Manipulations that decrease fibronectin deposition, such as transfection and overexpression of pp60c-src proto-oncogene in endothelial cells, reduce U937 cell adhesion. These results suggest that changes in endothelial cell extracellular matrix synthesis and deposition modulate, in part, the adhesive properties of the vessel wall after injury. In turn, the intensity and duration of mononuclear cell adhesion at sites of vessel wall injury determines, in part, the vessel wall response.

Neuronal pp60c-src(+) in the developing chick spinal cord as revealed with anti-hexapeptide antibody

Polyclonal antibody was raised in rabbits against a synthetic hexapeptide R-K-V-D-V-R corresponding to a unique amino acid sequence of the neuron-specific c-src gene product pp60c-src(+). The antibody was purified by affinity chromatography. A single band with an apparent molecular mass of 60 kDa was recognized when the supernatant of homogenates of brain and spinal cord from chick embryos and chicks was probed with the affinity purified anti-hexapeptide antibody after SDS-polyacrylamide gel electrophoresis followed by Western blotting. Specificity of the antibody was further characterized by autophosphorylation assay of immunoprecipitate in comparison with the monoclonal antibody 327. Immunocytochemical studies by light microscopy revealed that pp60c-src(+) was localized in flake-like aggregates in neuronal cell bodies of the spinal cord in 7-15-day-incubated chick embryos and newly hatched chicks. Developing spinal ganglia and muscle cells were also immunoreactive at early developmental stages. By electron microscopy, the reaction product was observed mainly in two regions. One region was at polysomes and along the membranes of the rough endoplasmic reticulum. The other region was along the neuronal plasma membrane--at subsurface cisterns and at synapses. At synapses, the postsynaptic density, presynaptic membrane and synaptic vesicle membranes were immunostained. Immunoreactivity at synapses were more frequently observed at earlier stages than at later stages of development. These findings suggest that pp60c-src(+) is actively produced in developing neurons and has some important roles in synaptogenesis. In mature synapses, pp60c-src(+) may be involved in the interaction of synaptic vesicles with the presynaptic membrane.

Osteoclasts express high levels of pp60c-src in association with intracellular membranes

Deletion of the c-src gene in transgenic mice by homologous recombination leads to osteopetrosis, a skeletal defect characterized by markedly deficient bone resorption (Soriano, P., C. Montgomery, R. Geske, and A. Bradley. 1991. Cell. 64:693-702), demonstrating a critical functional role of pp60c-src in osteoclast activity. Since decreased bone resorption could result from a defect either within the osteoclast or within other cells present in its environment, indirectly affecting osteoclast functions, we determined which cell(s) in bone expressed high levels of pp60c-src Measuring pp60c-src protein and kinase activities in osteoclasts and immunolocalizing pp60c-src in bone, we find that expression of pp60c-src is nearly as high in osteoclasts as in brain and platelets. In contrast, other bone cells contain only very low levels of the protein. In addition, expression of the c-src gene product increases when bone marrow cells are induced to express an osteoclast-like phenotype by 1,25-dihydroxy-vitamin D3, further suggesting that high expression of pp60c-src is part of the osteoclast phenotype. Three other src-like kinases, c-fyn, c-yes, and c-lyn, are also expressed in osteoclasts at ratios to pp60c-src similar to what is found in platelets. These src-related proteins do not, however, compensate for the absence of pp60c-src in the src- mice, thereby suggesting that pp60c-src may have a specific function in osteoclasts. Although further work is necessary to elucidate what the critical role of pp60c-src in osteoclasts is, our observation that the protein is associated mostly with the membranes of intracellular organelles suggests the possibility that this role might be at least in part related to the targeting or fusion of membrane vesicles.

Signal transduction by the cytoplasmic domains of Fc epsilon RI-gamma and TCR-zeta in rat basophilic leukemia cells

The gamma subunit of the high affinity IgE receptor, Fc epsilon RI, is a member of a family of proteins which form disulfide-linked dimers. This family also includes the zeta- and eta-chains of the T cell receptor. Engagement of Fc epsilon RI activates src-related protein tyrosine kinases in basophils and mast cells. However, the role of individual subunits of Fc epsilon RI in this activation is still not known. In an effort to determine the function of Fc epsilon RI-gamma, we used chimeric proteins containing the extracellular and transmembrane domains of the alpha chain of the human interleukin 2 receptor (Tac) and the cytoplasmic domains of either T cell receptor-zeta or Fc epsilon RI-gamma. We show that while cross-linking of the Tac chimeras in the rat basophilic leukemia cell line RBL-2H3 resulted in the tyrosine phosphorylation of a subset of proteins and a portion of the degranulation normally observed after Fc epsilon RI-mediated stimulation, no detectable activation of p56lyn or pp60c-src was observed. In contrast, an apparent transient deactivation of these two kinases was observed after Tac chimera cross-linking. These observations suggest that Fc epsilon RI-gamma is responsible for some, but not all, of the signaling that occurs after engagement of its receptor, and that other receptor subunits may also play important roles in this signaling process.

Differential pp60c-src activity in well and poorly differentiated human colon carcinomas and cell lines

The results presented in this report demonstrate increased pp60c-src kinase activity associated with moderate to well differentiated colon tumors, corroborating previous observations by other groups. Extension of this analysis to include a small number of poorly differentiated colon carcinomas revealed src kinase activity comparable to that observed in normal colonic mucosa, considerably less than that observed in moderate/well differentiated lesions. Correlations of src kinase activity with differentiation was confirmed within a panel of colon cell lines where increased activity, associated with moderate/well differentiated lines, was accompanied by increased expression of pp60c-src protein. Use of an antiphosphotyrosine antibody in immunoprecipitation revealed the presence of novel phosphotyrosyl cellular substrates in human colon cell lines displaying elevated pp60c-src kinase activity. These observations suggest a role for the src protooncogene in colonic differentiation pathways.

Retinoic acid receptor alpha suppresses polyomavirus transformation and c-fos expression in rat fibroblasts

To explore the molecular mechanisms by which retinoic acid inhibits oncogenic transformation, we have examined the effects of retinoic acid on the polyomavirus-induced transformation of rat fibroblasts. Treatment of rat F111 fibroblasts with high concentrations of retinoic acid (10(-6) M) partially inhibited the ability of polyomavirus to induce dense focus formation (50-70%). This effect was not secondary to a retinoic acid-dependent block of cellular proliferation. To address the role of the retinoic acid receptor (RAR-alpha) in mediating the transformation-inhibitory effect of retinoic acid, we have overexpressed either RAR-alpha or cellular retinoic acid-binding protein I (CRABP) cDNAs in F111 cells. Introduction of a CRABP I expression vector did not alter the responsiveness of F111 cells to retinoic acid in any detectable fashion. In contrast, overexpression of RAR-alpha increased the sensitivity of F111 cells to the transformation-inhibitory action of retinoic acid by 10- to 100-fold. At high concentrations, retinoic acid inhibited transformation of F111-RAR cells by polyomavirus by about 90%. At near physiological concentrations, retinoic acid inhibited transformation by 25-50% in F111-RAR cells but not in control cells. Retinoic acid did not inhibit either the synthesis of polyoma middle T (mT) or pp60c-src, the cellular target for mT action, or the formation of mT:pp60c-src:PI-3 kinase (phosphatidylinositol-3 kinase) complexes. Therefore, RAR-alpha was not acting as a negative regulator of expression of either the polyomavirus middle T oncogene or the cellular proto-oncogene, c-src. It seems likely that RAR-alpha regulates the expression of cellular genes whose products interact in some way with mT-regulated signaling pathways, leading to a ligand-dependent suppression of polyoma transformation. In addition, RAR-alpha overexpression selectively inhibits the serum-stimulated expression of the c-fos gene, but does not affect the expression of a number of other serum- and polyomavirus-inducible genes including c-jun, junB, c-myc and actin.

Association of neuronal pp60c-src with growth cone glycoproteins of rat brain

Tyrosine phosphorylation and protein tyrosine kinase (PTK) activity in the growth cone membrane-associated glycoprotein (GCGP) fraction of 1-day-old rat brain were examined. Using immunoblotting and immunoprecipitation techniques, pp60c-src was identified as one of the major PTKs associated with GCGPs. Furthermore, only GCGP-associated src that was also tyrosine phosphorylated was active. Immunoprecipitation experiments using various src antibodies revealed that pp60c-src contributed partially to the PTK activity detected in GCGPs, and that it is associated with several proteins of Mr 140 K, 120 K, 85 K and 50 K. This association of src protein with GCGPs was specific, and another src family member p59fyn, which is also abundant in the brain, did not exhibit such an association. In addition to pp60c-src, the GCGP fraction contained several major phosphotyrosine-containing proteins of Mr 140 K, and a 97/90 K doublet that corresponded to the beta subunits of IGF-I/insulin receptors. These studies show that pp60c-src associated with GCGPs is an active PTK that could be involved in neuronal growth and development, transmembrane signalling, and in recognition and/or adhesive events.

Characterization of protein tyrosine kinases from human breast cancer: involvement of the c-src oncogene product

Tyrosine phosphorylation is an important regulatory mechanism in response to the action of growth factors and oncogenes. Since many oncogenes code for tyrosine kinases, increased or altered oncogene expression may be reflected in increased tyrosine kinase activity. In a recent study (Hennipman et al., Cancer Res., 49: 516-521, 1989), we found that the tyrosine kinase activity of the cytosolic and membrane fractions of malignant human breast tissue was significantly higher compared to the benign or the normal breast tissue. Moreover, the increase in the cytosolic fractions was found to be of prognostic value. In the present study we determined the protein tyrosine kinase (PTK) activity of another 72 breast cancer specimens, and it could be shown again that the PTK activity in all 72 of these tumors was elevated compared to normal controls. We characterized these cytosolic PTKs by anion exchange chromatography using fast protein liquid chromatography, and it could be shown that at least two different forms of PTK exist. Using antibodies against a number of known oncogene products, we could determine that at least 70% of the PTK activity in the cytosol originated from the presence of the c-src oncogene product. Both of the PTK activity peaks seen in the fast protein liquid chromatography patterns could be precipitated with the anti-Src antibody. Furthermore, using the MCF-7 breast cancer cell line, it could be shown that the antibody against c-src also precipitated a part of the cytosolic PTK activity. In normal human peripheral lymphocytes, no precipitation of the cytosolic and membrane PTK activity could be achieved using the anti-Src antibody. Inasmuch as the cytosolic PTK activity parallels the malignancy in breast tumors (Hennipman et al., Cancer Res., 49: 516-521, 1989), and the majority of this activity is precipitated by anti-Src antibodies, the c-src protooncogene may play a key role in the manifestation of breast cancer.

Association of p60c-src with endosomal membranes in mammalian fibroblasts

We have examined the subcellular localization of p60c-src in mammalian fibroblasts. Analysis of indirect immunofluorescence by three-dimensional optical sectioning microscopy revealed a granular cytoplasmic staining that co-localized with the microtubule organizing center. Immunofluorescence experiments with antibodies against a number of membrane markers demonstrated a striking co-localization between p60c-src and the cation-dependent mannose-6-phosphate receptor (CI-MPR), a marker that identifies endosomes. Both p60c-src and the CI-MPR were found to cluster at the spindle poles throughout mitosis. In addition, treatment of interphase and mitotic cells with brefeldin A resulted in a clustering of p60c-src and CI-MPR at a peri-centriolar position. Biochemical fractionation of cellular membranes showed that a major proportion of p60c-src co-enriched with endocytic membranes. Treatment of membranes containing HRP to alter their apparent density also altered the density of p60c-src-containing membranes. Similar density shift experiments with total cellular membranes revealed that the majority of membrane-associated p60c-src in the cell is associated with endosomes, while very little is associated with plasma membranes. These results support a role for p60c-src in the regulation of endosomal membranes and protein trafficking.

Identification and developmental expression of Src+ mRNAs in Xenopus laevis

Alternative splicing of src mRNA has been demonstrated in several vertebrate species to yield a neuron-specific form of src protein termed pp60+. The function of pp60+ is unknown. The early developmental expression pattern of src+ RNA has not been previously examined. We have identified and characterized src+ transcripts corresponding to the two src genes in Xenopus laevis using a reverse transcription/polymerase chain reaction (RT/PCR) method. Both Xenopus pp60+ proteins have a 5-amino-acid insert in contrast to the 6-amino-acid insert in fish, birds, and mammals. Src+ mRNA first appears in neural plate stage Xenopus embryos, after neural induction signaling events but prior to neural differentiation. Analysis of dissected neural plate stage embryos showed that src+ mRNAs are localized to the neural plate. These findings suggest that pp60+ may play a role in elaboration of neuron structure.

Specific association of the proto-oncogene product pp60c-src with an intracellular organelle, the PC12 synaptic vesicle

The protein product of the proto-oncogene c-src is a membrane-associated tyrosine kinase of unknown function. Identification of pp60c-src target membranes may elucidate the function of the c-src protein. The available evidence indicates that pp60c-src associates with distinct membranes within single cell types and has different distributions in different cell types. Our experiments demonstrate targeting of pp60c-src to an isolatable and biochemically identified membrane fraction in the neuroendocrine cell line PC12. The c-src protein was found to be specifically associated with synaptic vesicles since: (a) the pp60c-src immunofluorescent pattern overlapped with a synaptic vesicle marker, synaptophysin; (b) a significant proportion (44%) of the pp60c-src from PC12 but not fibroblast postnuclear supernatants was recovered in a small vesicle fraction; (c) an anti-synaptophysin cytoplasmic domain antibody immunodepleted all of the pp60c-src vesicles in this fraction, and (d) pp60c-src copurified during a 100-fold purification of PC12 synaptic vesicles. These results suggest a role for the c-src protein in the regulation of synaptic vesicle function.

Development of solid-phase enzyme-linked immunosorbent assays for the determination of epidermal growth factor receptor and pp60c-src tyrosine protein kinase activity

Solid-phase ELISAs for the determination of EGF receptor (EGF-R) and pp60c-src tyrosine protein kinase activity are described. The methods were developed and optimized using purified recombinant EGF-R intracellular domain (ICD) and pp60c-src tyrosine protein kinases. A standardized assay that utilizes poly (GluNa-Tyr)4:1 as substrate and a monoclonal antiphosphotyrosine antibody for detection is described. Assay conditions for both enzymes were optimized with respect to substrate and ELISA plate-coating condition, divalent metal ion preferences, enzyme concentration, apparent kinetic constants for ATP, and reaction linearity. Following standardization, a number of reference tyrosine protein kinase inhibitors were tested in the ELISAs and compared to results obtained using solution-phase radioactive tyrosine protein kinase assays, which are based on the transfer of 32P from [gamma-32P]ATP to synthetic substrate. To enable a comprehensive comparison, IC50 values obtained in the ELISA were compared with values obtained in radioactive assays using both the holo-EGF-R and EGF-R ICD kinases. No substantial qualitative differences between these assays were seen. For many routine tyrosine protein kinase assays, semiquantitative or qualitative measurement of TPK activity is adequate. For such purposes, the ELISAs would be an attractive alternative to radioactive assays.

Growth inhibition of human colorectal carcinoma cell lines by tumor necrosis factor-alpha correlates with reduced activity of pp60c-src

One molecular alteration that has been observed in the majority of colorectal carcinomas is the activation of pp60c-src kinase. To address the role of pp60c-src in growth control of colon carcinoma cell lines, the effects of the biologic response modifier, tumor necrosis factor (TNF)-alpha were studied on the established HT29 colorectal carcinoma cell line and two clonal variants derived from the parental line. In one clone, HT29-A34, approximately 55% growth inhibition was observed following a 120-h incubation period with 10(3) U/ml TNF. In this TNF-sensitive cell line, pp60c-src immune complex kinase activity was reduced 3.9-fold accompanied by only a slight reduction in pp60c-src protein levels. Growth inhibition and decreased pp60c-src kinase activity correlated in a dose-dependent manner. The HT29-A14 TNF-resistant clone was not growth-inhibited by TNF, and no changes in pp60c-src were observed following treatment. Growth inhibition also correlated with reduced pp60c-src kinase specific activity in a number of other established colon carcinoma cell lines that were TNF-sensitive. In TNF-resistant colon carcinoma cell lines, pp60c-src kinase activity and levels remained unchanged. When changes in specific activity of pp60c-src were observed in sensitive cells, they occurred after decreased [3H]-thymidine uptake was observed. Therefore, these changes in pp60c-src activity are not the earliest event in TNF-induced growth inhibition. Nevertheless, our results suggest that modulation of pp60c-src kinase activity may be important in growth control of colorectal carcinoma cell lines.

Expression of pp60c-src in human small cell and non-small cell lung carcinomas

c-src protein was found in 60% of lung carcinomas (20 of 33 cases or primary tumours) by immunoblotting with a monoclonal antibody (Mab 327) and immunohistochemistry with serum from rabbits bearing tumours induced by Rous sarcoma virus. src protein expression was assessed in 4 small cell lung carcinomas and in an atypical carcinoid of neuroendocrine origin. However, pp60c-src was also found in non-small cell lung carcinomas: in 60-80% of adenocarcinomas and bronchiolo-alveolar cancers and in 50% of squamous cell carcinomas. In the squamous cell carcinomas, src protein was expressed more frequently in poorly differentiated than in well and moderately differentiated carcinomas. Expression of pp60c-src was not found in epithelial cells of histologically unchanged lung tissues. These results show that pp60c-src may be activated in human lung carcinomas of different histopathological types.

An assay for acidic peptide substrates of protein kinases

The assay of acidic peptides as substrates for protein kinases has not been as easy to perform as testing basic peptides or polypeptides. We have developed a simple, rapid, and cost-effective procedure that allows the design and testing of potential peptide substrates without the constraints imposed by the phosphocellulose filter paper method (the need to incorporate positively charged residues into the peptide sequence). The technique combines the chelation of 32Pi by acid molybdate with PEI-cellulose chromatography. In this way the migration of 32P-labeled Pi, ATP, and protein are impeded while phosphopeptide is eluted in 1.5 ml from a 0.25-ml disposable column. In order to validate the assay we used two angiotensin II analogues as peptide substrates for the protein tyrosine kinase pp60c-src. The assay results using the new procedure were compared to those of the phosphocellulose filter paper technique. We also demonstrated the use of this method to test linear and cyclic peptides that could not be assayed with the phosphocellulose paper technique. This assay will aid those who are attempting to determine the substrate specificity of protein kinases.

Protein myristoylation in human mononclear phagocytes: modulation by interferon-gamma and tumor necrosis factor-alpha

Labelling of cells with [3H]myristic acid and analysis of labelled proteins by SDS-PAGE and fluorography, enabled the identification of a limited number of myristoylated proteins in human monocytes and monocyte-derived macrophages. In human monocytes, cultivated for one to three days, major myristoylated proteins observed were of 18 kDa, 44 kDa, 60–62 kDa, 90 kDa, and a doublet of 38–40 kDa. Differentiation of monocytes to macrophages by in vitro cultivation was accompanied by a selective decrease in the 60–62 kDa protein. Cultivation of the cells in the presence of the macrophage-activating cytokines interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha), prevented the decrease in the expression of the 60–62 kDa myristoylated protein. The effect of cytokines was observed when monocytes were treated with IFN-gamma or TNF-alpha for 24 or 48 h and protein myristoylation analyzed at day four of culture. Maintenance of monocytes in culture for up to nine days in the presence of cytokines prevented the decrease in the expression of the 60–62 kDa myristoylated protein. IFN-gamma had additional effects on myristoylation of macrophage proteins. Treatment of monocytes with IFN-gamma for a few hours caused the induction of a 66 kDa protein. Induction of this myristoylated protein by IFN-gamma was time-dependent and peaked at six hours. Analysis of the subcellular distribution of the 66 kDa protein induced by IFN-gamma showed that, analogously to other myristoylated proteins, most of it was associated with cell membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Signal transduction by nerve growth factor and fibroblast growth factor in PC12 cells requires a sequence of src and ras actions

We have investigated the roles of pp60c-src and p21c-ras proteins in transducing the nerve growth factor (NGF) and fibroblast growth factor (FGF) signals which promote the sympathetic neuronlike phenotype in PC12 cells. Neutralizing antibodies directed against either Src or Ras proteins were microinjected into fused PC12 cells. Each antibody both prevented and reversed NGF- or FGF-induced neurite growth, a prominent morphological marker for the neuronal phenotype. These data demonstrate the involvement of both pp60c-src and p21c-ras proteins in NGF and FGF actions in PC12 cells, and establish a physiological role for the pp60c-src tyrosine kinase in signal transduction pathways initiated by receptor tyrosine kinases in these cells. Additional microinjection experiments, using PC12 transfectants containing inducible v-src or ras oncogene activities, demonstrated a specific sequence of Src and Ras actions. Microinjection of anti-Ras antibody blocked v-src-induced neurite growth, but microinjection of anti-Src antibodies had no effect on ras oncogene-induced neurite growth. We propose that a cascade of Src and Ras actions, with Src acting first, is a significant feature of the signal transduction pathways for NGF and FGF. The Src-Ras cascade may define a functional cassette in the signal transduction pathways used by growth factors and other ligands whose receptors have diverse structures and whose range of actions on various cell types include mitogenesis and differentiation.

The v-src oncogene blocks the differentiation of a murine myeloid progenitor cell line and induces a tumorigenic phenotype

We have investigated the ability of the v-src oncogene to block the differentiation of the murine myeloid progenitor cell line 32D cl3. In response to granulocyte-colony stimulating factor (G-CSF), 32D cl3 cells are induced to differentiate into mature granulocytes (Valtieri et al., 1987). In contrast, no differentiation was observed following G-CSF treatment of 32D cl3 cells infected with a murine retrovirus carrying the wild-type v-src oncogene. Furthermore, cells infected with a v-src temperature-sensitive (ts) mutant did not differentiate at the permissive temperature, however, at the nonpermissive temperature G-CSF induced granulocytic differentiation. Differentiation of 32D cl3 cells infected with ts WP31A (ts LA31A src gene inserted into amphotropic murine leukemia virus 4070A; Anderson et al., 1987) occurred with the same kinetics as uninfected 32D cl3 cells. Temperature-shift experiments indicate that after 72 hours of treatment with G-CSF at the nonpermissive temperature, approximately half of the 32D cl3 cells infected with ts WP31A virus become committed to differentiation. Prior to that time, activation of v-src by shifting the cells to the permissive temperature resulted in the presence of only undifferentiated blast cells after six days in culture. In contrast to normal 32D cl3 cells, cells infected with the wild-type v-src were tumorigenic when injected into nu/nu Swiss mice. Lesions appeared in the spleen, liver, kidney, lungs and lymph nodes following subcutaneous injection. Growth factor-independent cells were recovered from the tumor, spleen, bone marrow and a lymph node of tumor-bearing nude mouse. Analysis of the proviral integration site by inverse polymerase chain reaction (PCR) demonstrated that the tumor cells were of donor cell origin.