The interplay between Eps8 and IRSp53 contributes to Src-mediated transformation.

As an oncoprotein, Eps8 participates in v-Src-induced cellular transformation. To delineate the underlying mechanism, we conducted a yeast two-hybrid screening and identified IRSp53S, a protein critical in cell mobilization, as one of the Eps8-binding partners from a human brain cDNA library. The association was mediated by the multiple proline-rich regions of Eps8 and the C-terminal SH3-WWB containing domains of IRSp53S. In this study, we observed that Eps8 modulated the expression of IRSp53 in v-Src-transformed cells (IV5), raising the question of whether Eps8/IRSp53 interaction was crucial in carcinogenesis. To address this issue, we generated IV5-expressing irsp53 siRNA cells. Attenuation of IRSp53 reduced cell proliferation of IV5 in culture dish and tumor formation in mice, which could be partly rescued by ectopically expressed human IRSp53S. In addition, IRSp53 knockdown impaired activity of phosphatidylinositol 3-kinase (as reflected by Pi-Ser473 AKT) and Stat3 (as reflected by Pi-Tyr705 Stat3), and reduced cyclin D1 expression that culminated to impede G(1)-phase cell-cycle progression. Ectopically expressed human IRSp53S, but not its Eps8-binding defective mutants (that is, Delta363 and PPPDA), rescued these defects and partly restored cell proliferation. Remarkably, through activation of Src, EGF increased the formation of Eps8/IRSp53 complex and Stat3 activation in HeLa cells. With these results, we show for the first time that IRSp53, through its interaction with Eps8, not only affects cell migration but also dictates cellular growth in cancer cells.

Overexpression of p97Eps8 leads to cellular transformation: implication of pleckstrin homology domain in p97Eps8-mediated ERK activation.

Two isoforms of Eps8, p97Eps8 and p68Eps8, have been identified as the substrates for receptor tyrosine kinases. Our previous studies indicated that both tyrosyl phosphorylation and protein expression of Eps8 were elevated in v-Src transformed cells. In an attempt to examine the role played by p97Eps8 in tumorigenesis, we have first obtained cells overexpressing p97Eps8 and its pleckstrin homology (PH)-truncated variant. We then demonstrated that cells overexpressing p97Eps8 not only exhibited the ability of focus formation in cell culture but also promoted the tumor formation in mice as compared to controls. Furthermore, elevated serum-induced extracellular responsive kinase (ERK) activation was observed in p97Eps8 overexpressors. This enhanced ERK activation was sensitive to a MEK1 specific inhibitor PD98059 and was important for p97Eps8-mediated transformation, since transfection of vectors expressing dominant negative MEK1 and p97Eps8 abrogated focus formation by p97Eps8. In contrast, PH-truncated p97Eps8 failed to localize at the plasma membrane and that the truncated variant also did not elevate ERK activation and cellular transformation in response to serum stimulation. Our results thus indicated that: (i) the gene encoding p97Eps8 was an oncogene; (ii) p97Eps8-induced oncogenesis was partly mediated by ERK activation; and (iii) the PH domain of p97Eps8 was critical for its cellular localization, ERK activation and its ability to transform cells. Oncogene (2001) 20, 106 - 112.

Enhancement of tyrosyl phosphorylation and protein expression of eps8 by v-Src.

Two eps8 isoforms, p97eps8 and p68eps8, were previously identified as substrates for receptor tyrosine kinases. Analysis of eps8 phosphotyrosine content in v-Src transformed cells (IV5) revealed that both isoforms were highly tyrosyl phosphorylated and their readiness to be phosphorylated by Src in vitro further indicated that they were putative Src substrates as well. Indeed, the enhancement of tyrosyl phosphorylation of p97eps8 detected in cells coexpressing both p97eps8 and active Src relative to that in cells expressing p97eps8 alone supported our hypothesis. The existence of common phosphotryptic peptides between in vitro 32P-labeled p97eps8 and p68eps8 indicated that these two proteins shared the same Src-mediated sites. Further in vitro binding assays demonstrated that p68eps8 was the major eps8 isoforms that could be precipitated by bacterial fusion protein containing Src SH3. Interestingly, both p68eps8 and p97eps8 were preferentially expressed in v-Src transformed cells and the presence of p68eps8 appeared to depend on Src. Since p97eps8 has been implicated in mitogenesis and tumorigenesis, its readiness to be phosphorylated and induced by v-Src might attribute to v-Src-mediated transformation.

c-Src-mediated phosphorylation of the epidermal growth factor receptor on Tyr845 and Tyr1101 is associated with modulation of receptor function.

Accumulating evidence indicates that interactions between the epidermal growth factor receptor (EGFR) and the nonreceptor tyrosine kinase c-Src may contribute to an aggressive phenotype in multiple human tumors. Previous work from our laboratory demonstrated that murine fibroblasts which overexpress both these tyrosine kinases display synergistic increases in DNA synthesis, soft agar growth, and tumor formation in nude mice, and increased phosphorylation of the receptor substrates Shc and phospholipase gamma as compared with single overexpressors. These parameters correlated with the ability of c-Src and EGFR to form an EGF-dependent heterocomplex in vivo. Here we provide evidence that association between c-Src and EGFR can occur directly, as shown by receptor overlay experiments, and that it results in the appearance of two novel tyrosine phosphorylations on the receptor that are seen both in vitro and in vivo following EGF stimulation. Edman degradation analyses and co-migration of synthetic peptides with EGFR-derived tryptic phosphopeptides identify these sites as Tyr845 and Tyr1101. Tyr1101 lies within the carboxyl-terminal region of the EGFR among sites of receptor autophosphorylation, while Tyr845 resides in the catalytic domain, in a position analogous to Tyr416 of c-Src. Phosphorylation of Tyr416 and homologous residues in other tyrosine kinase receptors has been shown to be required for or to increase catalytic activity, suggesting that c-Src can influence EGFR activity by mediating phosphorylation of Tyr845. Indeed, EGF-induced phosphorylation of Tyr845 was increased in MDA468 human breast cancer cells engineered to overexpress c-Src as compared with parental MDA 468 cells. Furthermore, transient expression of a Y845F variant EGFR in murine fibroblasts resulted in an ablation of EGF-induced DNA synthesis to nonstimulated levels. Together, these data support the hypothesis that c-Src-mediated phosphorylation of EGFR Tyr845 is involved in regulation of receptor function, as well as in tumor progression.

Vanadate-dependent FAK activation is accomplished by the sustained FAK Tyr-576/577 phosphorylation.

Focal adhesion kinase (FAK) is a nonreceptor tyrosine kinase implicated in cell-matrix interaction and integrin signaling. It is well established that Tyr-397 is the FAK autophosphorylation site and Tyr-407, -576/577, -861, and -925 are the sites on murine FAK that are mediated by Src family kinases. To study how FAK is regulated by tyrosine phosphatase(s), cells overexpressing chicken FAK are treated with sodium vanadate. Both the phosphotyrosine content and the enzymatic activity of FAK are increased in response to vanadate. Interestingly, sustained FAK Tyr-576/577 and -863 phosphorylations are detected in vanadate-treated FAK overexpressors and are dependent on FAK autophosphorylation. Further analysis of sodium vanadate-treated FAK overexpressors reveals that the enhanced FAK kinase activity parallels its elevated Tyr-576/577 phosphorylation. Thus, we conclude that Src-mediated FAK phosphorylation is regulated by a tyrosine phosphatase(s) and may be of physioligical significance.

Potentiation of epidermal growth factor receptor-mediated oncogenesis by c-Src: implications for the etiology of multiple human cancers.

c-Src is a nontransforming tyrosine kinase that participates in signaling events mediated by a variety of polypeptide growth factor receptors, including the epidermal growth factor receptor (EGFR). Overexpression and continual ligand stimulation of the EGFR results in morphological transformation of cells in vitro and tumor development in vivo. Elevated levels of c-Src and the EGFR are found in a variety of human malignancies, raising the question of whether c-Src can functionally cooperate with the EGFR during tumorigenesis. To address this issue, we generated c-Src/EGFR double overexpressors and compared their proliferative and biochemical characteristics to those of single overexpressors and control cells. We found that in cells expressing high levels of receptor, c-Src potentiated DNA synthesis, growth in soft agar, and tumor formation in nude mice. Growth potentiation was associated with the formation of a heterocomplex between c-Src and activated EGFR, the appearance of a distinct tyrosyl phosphorylation on the receptor, and an enhancement of receptor substrate phosphorylation. These findings indicate that c-Src is capable of potentiating receptor-mediated tumorigenesis and suggest that synergism between c-Src and the EGFR may contribute to a more aggressive phenotype in multiple human tumors.

A protein that is highly related to GTPase-activating protein-associated p62 complexes with phospholipase C gamma.

p62 is a highly tyrosyl phosphorylated protein that was first identified in immunoprecipitates of the GTPase-activating protein (GAP) of p21ras from cells transformed by oncogenic nonreceptor tyrosine kinases or stimulated through tyrosine kinase receptors (C. Ellis, M. Moran, F. McCormick, and T. Pawson, Nature 343:377-381, 1991). In this article we describe a highly related 62-kDa protein that becomes tyrosyl phosphorylated and associated with phospholipase C gamma (PLC gamma) in C3H10T1/2 cells stimulated with epidermal growth factor (EGF) or transformed by v-src. GAP-associated and PLC gamma-associated p62 comigrated in one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis and exhibited nearly identical phosphotryptic peptide patterns. That the association of p62 with PLC gamma was direct and not mediated through binding of GAP-p62 to PLC gamma or to the EGF receptor (and coprecipitation of the receptor with PLC gamma) was demonstrated by (i) the inability to detect GAP in PLC gamma immunocomplexes or PLC gamma in GAP immunocomplexes, (ii) the association of p62 with PLC gamma in v-src-transformed cells in the absence of EGF stimulation, and (iii) in vitro solution binding and direct blotting of p62 with a glutathione S-transferase fusion protein containing the Src homology 2 (SH2) domains of PLC gamma. Unlike GAP, whose N-terminal SH2 mediates the interaction between GAP and p62, PLC gamma was found to require both its N- and C-terminal SH2 regions for p62 binding. These studies demonstrate that a protein identical to or highly related to GAP-associated p62 binds PLC gamma and suggest a means by which "cross-talk" between PLC gamma- and GAP-mediated signalling may occur.

Identification and characterization of a cytoskeleton-associated, epidermal growth factor sensitive pp60c-src substrate.

In studies aimed at identifying and characterizing pp60c-src substrates that participate in the enhanced mitogenic response to epidermal growth factor (EGF) observed in murine C3H10T1/2 fibroblasts overexpressing c-src, we have identified a 75-kDa protein (p75) whose properties are consistent with those expected of such a substrate. We present evidence to show that p75 is immunologically related to a recently described, cytoskeleton-associated, pp60v-src substrate [Wu et al. (1991). Mol. Cell. Biol., 11, 5113-5124), and that its phosphotyrosine content is increased cooperatively by c-src overexpression and EGF stimulation. p75 is rapidly (within 2 min) phosphorylated on tyrosine upon EGF treatment and undergoes a second, prolonged phase of tyrosyl phosphorylation from 7 to 21 h after EGF addition, suggesting that tyrosyl phosphorylation of p75 is important for late as well as early events following EGF receptor activation. Enhanced tyrosyl phosphorylation of p75 is also seen when cells overexpressing c-src are treated with platelet-derived growth factor (PDGF), but significantly less phosphorylation is observed with insulin and fibroblast growth factor (FGF). Both basal and EGF-induced tyrosyl phosphorylation of p75 are reduced in cells overexpressing mutated forms of c-src (unmyristylated, or kinase deficient) as compared with wild-type c-src overexpressers, indicating the dependence of the enhanced tyrosyl phosphorylation on membrane-associated, enzymatically active pp60c-src. In cellular fractionation experiments p75 partitions with the cytosol, while immunofluorescence studies reveal a striking colocalization with pp60c-src at the plasma membrane and in the perinuclear region. Partial co-staining of p75 and actin occurs at the cell's periphery. These data provide evidence for p75 being a direct substrate of pp60c-src. The possible role of p75 in the enhanced response to EGF seen in c-src overexpressers is discussed.

Structural and functional analysis of the murine adenosine deaminase gene.

We describe the structural and functional analysis of cosmid clones that span the entire murine adenosine deaminase gene. Functional analysis indicated that these clones are capable of encoding murine adenosine deaminase activity when introduced into human cell lines. Structural analysis revealed that the gene consists of 12 exons distributed over approximately 25 kb. The exact size of each exon and the sequence of each exon/intron junction were determined. The results show that the 1056-nucleotide open reading frame for adenosine deaminase extends from exon 1 to exon 11, and that exon 12 contains untranslated sequences only. During the course of these investigations, we discovered that a gene encoding an abundant 1.3-kb polyadenylated transcript overlaps the 3' end of the murine adenosine deaminase gene and is transcribed from the opposite strand.

Identification of transcription stop sites at the 5' and 3' ends of the murine adenosine deaminase gene.

We report here the identification and nucleotide sequence of two transcription termination regions associated with the murine adenosine deaminase gene. One region is situated within or very near exon one and in mouse fibroblasts accounts for more than a 50-fold drop in the abundance of nascent transcripts. This termination region is believed to be involved in the regulation of adenosine deaminase gene transcription. The other termination region is located approximately 3.5 kilobases beyond the major polyadenylation site and defines the 3' boundary of the adenosine deaminase transcription unit.

Sequence requirements for transcriptional arrest in exon 1 of the murine adenosine deaminase gene.

We have previously shown that a transcription arrest site near the 5' end of the murine adenosine deaminase (ADA) gene is significantly involved in the regulation of ADA gene expression. To facilitate the analysis of this transcription arrest site, we have analyzed the transcription products from cloned ADA gene fragments injected into Xenopus laevis oocytes. When genomic fragments spanning the 5' end of the ADA gene were injected into oocytes, a 96-nucleotide (nt) ADA RNA was the major transcription product. The 5' end of this RNA mapped to the transcription initiation site for the ADA gene, and its 3' terminus mapped 7 nt downstream of the translation initiation codon within exon 1. A 300-base-pair fragment of genomic DNA spanning the 5' end of the ADA gene was sufficient to generate the 96-nt transcript which accounted for approximately one-half of the transcription products from injected templates. Deletion of a segment of approximately 65 base pairs, located immediately downstream of the 3' terminus of the 96-nt transcript, resulted in a substantial reduction in the synthesis of the 96-nt transcript and a corresponding increase in the production of larger transcripts. These studies show that the transcriptional apparatus of X. laevis oocytes responds to the transcription arrest site associated with exon 1 of the murine ADA gene and that oocyte injections provide a convenient functional assay for additional mechanistic studies.

Adenosine deaminase gene expression. Tissue-dependent regulation of transcriptional elongation.

The mechanism by which the expression of adenosine deaminase (ADA, EC 3.5.4.4) is regulated in murine tissues was analyzed. Elevated levels of the specific activity of this enzyme appeared to correlate directly with steady state levels of ADA mRNA. Transcriptional studies using nuclear run-on experiments revealed a pronounced asymmetric distribution of nascent transcripts across the ADA genome. The ability to detect nascent transcripts downstream of the promoter proximal region varied among tissues and correlated with tissue levels of ADA mRNA and enzymatic activity. These results indicate that processes affecting the elongation of nascent transcripts into completed primary transcripts play a role in tissue-specific expression of ADA.