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

A role for Src in signal relay by the platelet-derived growth factor alpha receptor

Previous studies have shown that Src is required for platelet-derived growth factor (PDGF)-dependent cell cycle progression in fibroblasts. Since fibroblasts usually express both PDGF receptors (PDGFRs), these findings suggested that Src was mandatory for signal relay by both the alpha and betaPDGFRs. In this study, we have focused on the role of Src in signal relay by the alphaPDGFR. In response to stimulation with PDGF-AA, which selectively engages the alphaPDGFR, Src family members (Src) associated with the alphaPDGFR and Src kinase were activated. A mutant receptor, in which tyrosines 572 and 574 were replaced with phenylalanine (F72/74), failed to efficiently associate with Src or activate Src. The wild type (WT) and F72/74 receptors induced the expression of c-myc and c-fos to comparable levels. Furthermore, an equivalent extent of PDGF-dependent soft agar growth was observed in cells expressing the WT or the F72/74 alphaPDGFR. Comparing the ability of these two receptors to initiate tyrosine phosphorylation of signaling molecules indicated that both receptors mediated phosphorylation of the receptor itself, phospholipase Cgamma 1, and SHP-2 to similar levels. In contrast, the F72/74 receptor triggered phosphorylation of Shc to 1 and 20% of the WT levels for the 55- and 46-kDa Shc isoforms, respectively. These findings indicate that after exposure of cells to PDGF-AA, Src stably associates with the alphaPDGFR, and Src activity is increased. Furthermore, Src is required for the PDGF-dependent phosphorylation of signaling molecules such as Shc. Finally, activation of Src during the G0/G1 transition does not appear to be required for latter cell cycle events such as induction of c-myc or cell proliferation.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

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.

Overexpression of cellular Src in fibroblasts enhances endocytic internalization of epidermal growth factor receptor

Previous studies have demonstrated a requirement for the nonreceptor tyrosine kinase, cellular Src (c-Src), in epidermal growth factor (EGF)-induced mitogenesis and a synergistic interaction between c-Src and EGF receptor (EGFR) in tumorigenesis. Although endocytic internalization of EGFR may be thought to attenuate EGF-stimulated signaling, recent evidence suggests that signaling through Ras can be amplified by repeated encounters of endosome-localized, receptor. Shc.Grb2.Sos complexes with the plasma membrane, where Ras resides almost exclusively. Based on these reports, we examined EGFR trafficking behavior in a set of single and double c-Src/EGFR C3H10T1/2 overexpressors to determine if c-Src affects basal receptor half-life, ligand-induced internalization, and/or recycling. Our results show that overexpression of c-Src causes no change in EGFR half-life but does produce an increase in the internalization rate constant of EGF.EGFR complexes when the endocytic apparatus is not stoichiometrically saturated; this effect of c-Src on EGFR endocytosis is negligible at high receptor occupancy in cells overexpressing the receptor. In neither case are EGFR recycling rate constants affected by c-Src. These data indicate a functional role for c-Src in receptor internalization, which in turn could alter some aspects of EGFR signaling related to mitogenesis and tumorigenesis.

Captopril inhibits pp60(c-src) tyrosine phosphorylation in cultured human mesangial cells

The present experiments were devoted to analyzing the mechanisms involved in the captopril-dependent inhibition of human mesangial cell proliferation. Studies were performed in cultured human mesangial cells incubated with captopril, an angiotensin II-converting enzyme inhibitor with antioxidant properties, lisinopril, a non-antioxidant angiotensin II-converting enzyme inhibitor, and tocopherol, a pure antioxidant. Both angiotensin II-converting enzyme inhibitors significantly inhibited fetal calf serum-induced [3H]thymidine uptake by human mesangial cells, in a dose- and time-dependent manner, an effect which was not observed with tocopherol. The antiproliferative effect of captopril and its ability to block tyrosine phosphorylation of human mesangial cells proteins were significantly greater than those of lisinopril. Moreover, captopril significantly prevented the fetal calf serum-induced tyrosine phosphorylation of pp60(c-src). The present results suggest that the antiproliferative ability of captopril does not completely depend on its angiotensin II-converting enzyme inhibitor properties, pointing to a possible interaction of the drug with the intracellular mechanisms responsible for the transmission of the proliferative signals.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes an Ah receptor-dependent and ARNT-independent increase in membrane levels and activity of p60(Src)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to affect various cellular activities including growth factor signal transduction, hormone responses, and cell differentiation. The purpose of this study was to examine more closely the very early effects of TCDD on protein tyrosine kinase activity, specifically p60(Src). We found that TCDD causes rapid changes in the plasma-microsomal membrane levels and activity of p60(Src) in Hepa 1c1c7, Hepa c4 cells, and SR3Y1 cells, a p60(v-Src) overexpressing cell line. Such cellular changes occur within 30 minutes after 10 nM TCDD treatment, as measured by Western blot analysis. TCDD's ability to increase p60(Src) levels was found to be: (1) dose-dependent, with an estimated EC(50) between 10(-10) and 10(-11) M TCDD; (2) Ah receptor (AhR)-dependent, since TCDD's effect was blocked by co-administration with 1 μM α-naphthoflavone, an AhR antagonist; and interestingly (3) ARNT-independent, since TCDD's effect was observed in Hepa c4 cells, an ARNT(-) mutant cell line. Since ARNT is a heterodimerization partner of the AhR required for binding of the ligand-activated AhR to dioxin-responsive elements on DNA in the nucleus to transactivate genes controlled by the AhR, an alternative mechanism for TCDD's action is discussed which does not require ARNT. Along with increased membrane levels of p60(Src), we observed a corresponding increase in the activity of a 60 kDa protein tyrosine kinase using two different kinase detection assays. This effect of TCDD was also found to be AhR-dependent, ARNT-independent, and independent of de novo protein synthesis since cycloheximide was unable to completely abolish TCDD's effect. The present findings provide a potentially important mechanism by which TCDD can alter cell growth and differentiation.

Src family protein tyrosine kinases: cooperating with growth factor and adhesion signaling pathways

The Src family of protein tyrosine kinases functionally interacts with several receptor and nonreceptor protein tyrosine kinases. Recent developments show that Src family kinases may cooperate with the epidermal growth factor and platelet-derived growth factor receptors and the integrin-linked focal adhesion kinase to diversify signals that regulate growth and cell movement.

The epidermal growth factor receptor in breast cancer

The epidermal growth factor receptor (EGFR)3 is a 170 kD transmembrane tyrosine kinase activated by several ligands. It is required for normal mammary development and lactation and is aberrantly expressed in approximately 40% of breast carcinomas, particularly those with a poor prognostic phenotype. Since EGF receptor levels are elevated in a high proportion of many tumor types its potential as a therapy target is being investigated using the EGF receptor to target toxins, as well as drugs that interfere with signaling and anti-receptor antibodies. These approaches are likely to be most effective when used in the adjuvant situation in combination with chemotherapy.

Brk, a breast tumor-derived non-receptor protein-tyrosine kinase, sensitizes mammary epithelial cells to epidermal growth factor

brk (breast tumor kinase) shows homology to the src family of non-receptor protein-tyrosine kinases and is expressed in breast carcinomas. In order to investigate the role of brk in breast tumor development, we have examined the growth and transformation properties of human mammary epithelial cells engineered to overexpress Brk. Interestingly, like c-Src, overexpression of Brk leads to sensitization to EGF, and also results in a partially transformed phenotype. Further investigation of the latter activity was attempted by mutational analysis, targeting key residues known to affect tyrosine kinase activity in Src-like kinases. Mutation of amino acid residue Lys-219 to Met, by analogy to Src, abolished both kinase activity and transformation capacity. Mutation of amino acid residue Tyr-447 to Phe, however, resulted in a decrease in transforming potential without affecting kinase activity. These results suggest that while Src and Brk share some functional properties, they act differently during transformation. These differences are discussed in the context of the mechanisms underlying breast cancer development.

Identification of the major autophosphorylation sites of Nyk/Mer, an NCAM-related receptor tyrosine kinase

Nyk/Mer receptor tyrosine kinase is a new member of the Ufo/Axl tyrosine kinase family and is characterized by its neural cell adhesion molecule-like extracellular domain. By using a vaccinia virus expression system to express a constitutively activated form of Nyk, we identified the major sites of Nyk autophosphorylation in tryptic peptide IY749SGDY753Y754R. Tyr-749, Tyr-753, and Tyr-754 in this peptide lie in the activation loop of the kinase domain. We also studied a series of Nyk mutants in which the three tyrosine residues were replaced individually, in pairs, or all together by phenylalanine. Single mutations of Tyr-749 or Tyr-753 to phenylalanine reduced Nyk kinase activity toward exogenous substrate to 39 or 10% of that of the wild type Nyk, respectively, whereas the Tyr-754 mutant is completely inactive. All of the double and triple Tyr-Phe mutants reduced Nyk kinase activity to a level below the background. Similar results were obtained when Nyk autophosphorylation levels were examined. Our studies suggest that full activity of Nyk/Mer kinase requires phosphorylation of all three tyrosine residues in the kinase domain (Tyr-749, Tyr-753, and Tyr-754) and that Nyk kinase activity is modulated by the level of autophosphorylation in the kinase domain. Given the highly conserved nature of this region among the Ufo/Axl receptor family members, the information presented in this report may provide insight to the biochemical properties of other members of this family.

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.