p60(v-src) and serum control cell shape and apoptosis via distinct pathways in quail neuroretina cells

Pleiotropic action of AP-1 in v-Src-transformed cells

The activation of AP-1 is a hallmark of cell transformation by tyrosine kinases. In this study, we characterize the role of AP-1 proteins in the transformation of chicken embryo fibroblasts (CEF) by v-Src. In normal CEF, the expression of a dominant negative mutant of c-Jun (TAM67) induced senescence. In contrast, three distinct phenotypes were observed when TAM67 was expressed in v-Src-transformed CEF. While senescent cells were also present, the inhibition of AP-1 caused apoptosis in a fraction of the v-Src-transformed cells. In addition, cells containing lipid-rich vesicles accumulated, suggesting that a subpopulation of the v-Src-transformed cells underwent differentiation in response to the inhibition of AP-1. JunD and Fra-2 were the main components of this factor, while c-Jun accounted for a minor fraction of AP-1 in v-Src-transformed CEF. The downregulation of c-Jun expression by short hairpin RNA (shRNA) induced senescence in normal and v-Src-transformed cells. In contrast, a high incidence of apoptosis was caused by the downregulation of JunD, suggesting that it is required for the survival of v-Src-transformed CEF. Levels of the p53 tumor suppressor were elevated under conditions of JunD inhibition. Repression of p53 by shRNA enhanced the survival and anchorage-independent proliferation of v-Src-transformed CEF with JunD/AP-1 inhibition. The inhibition of Fra-2 had no visible phenotype in normal CEF but caused the appearance of lipid-rich vesicles in v-Src-transformed CEF. Therefore, AP-1 facilitated transformation by acting as a survival factor, by inhibiting premature entry into senescence, and by blocking the differentiation of v-Src-transformed CEF.

Stable expression of intracellular Notch suppresses v-Src-induced transformation in avian neural cells

Understanding how disruption of differentiation contributes to the cancer cell phenotype is required to identify alterations essential for malignant transformation and provide experimental basis for their correction. We investigated whether primary quail neuroretina cells, transformed by a conditional v-Src mutant (QNR/v-src(ts)), could revert to a normal phenotype, in response to the stable expression of constitutively active Notch1 intracellular domain (ICN). This model system was chosen because Notch signaling plays an instructive role in cell fate determination during NR development, and because the intrinsic capacity of QNR cultures to differentiate is blocked by v-Src. We report that stable ICN expression results in suppression of QNR/v-src(ts) cell transformation in the presence of an active oncoprotein. This phenotypic reversion coincides with a major switch in cell identity, as these undifferentiated cells acquire glial differentiation traits. Both changes appear to be mediated by CBF, a transcription factor that binds to ICN and activates target genes. Cells restored to a normal and differentiated phenotype have undergone changes in the functioning of signaling effectors, essentially regulating cell morphology and cytoskeleton organization. This dominant interference may be partially mediated by an autocrine/paracrine mechanism, as revertant cells secrete a factor(s), which inhibits transformation properties of QNR/v-src(ts) cells.

Modulation of Nr-13 antideath activity by peptide aptamers

Tumor cells are characterized by deregulated proliferation and resistance to proapoptotic stimuli. The Bcl-2 family of antiapoptotic proteins is overexpressed in a large number of chemoresistant tumors. Downregulation or inhibition of antiapoptotic proteins might result in the sensitization of cancer cells to chemotherapeutic agents. In the present study, we took advantage of the peptide aptamer strategy to target Nr-13, a Bcl-2 antiapoptotic protein involved in neoplastic transformation by the Rous sarcoma virus. We isolated peptide aptamers that behave as Nr-13 regulators, in vitro and in mammalian cells in culture. Some of these aptamers have potential proapoptotic activities. These data suggest that peptide aptamers targeting the Bcl-2 family of apoptosis inhibitors may be useful for the development of anticancer molecules.

The zebrafish bcl-2 homologue Nrz controls development during somitogenesis and gastrulation via apoptosis-dependent and -independent mechanisms

Although the role of the b-cell lymphoma (Bcl)-2 family of apoptosis inhibitors is well documented in tumor cells and tissue morphogenesis, their role during the early development of vertebrates is unknown. Here, we characterize Nrz, a new Bcl-2-related inhibitor of apoptosis in zebrafish. Nrz is a mitochondrial protein, antagonizing the death-accelerator Bax. The nrz gene is mainly expressed during gastrulation and somitogenesis. The knockdown of nrz with antisense morpholinos leads to alterations of the somites, correlated with an increase in apoptosis. In addition, earlier during development, in the zebrafish gastrula, nrz knockdown results in an increase of snail-1 expression at the margin and frequent gastrulation arrest at the shield stage, independently of apoptosis. Together these data suggest that Nrz, in addition to its effect on apoptosis, contributes to cell movements during gastrulation by negatively regulating the expression of Snail-1, a transcription factor that controls cell adhesion.

Distinct protease pathways control cell shape and apoptosis in v-src-transformed quail neuroretina cells

Intracellular proteases play key roles in cell differentiation, proliferation and apoptosis. In nerve cells, little is known about their relative contribution to the pathways which control cell physiology, including cell death. Neoplastic transformation of avian neuroretina cells by p60(v-src) tyrosine kinase results in dramatic morphological changes and deregulation of apoptosis. To identify the proteases involved in the cellular response to p60(v-src), we evaluated the effect of specific inhibitors of caspases, calpains and the proteasome on cell shape changes and apoptosis induced by p60(v-src) inactivation in quail neuroretina cells transformed by tsNY68, a thermosensitive strain of Rous sarcoma virus. We found that the ubiquitin-proteasome pathway is recruited early after p60(v-src) inactivation and is critical for morphological changes, whereas caspases are essential for cell death. This study provides evidence that distinct intracellular proteases are involved in the control of the morphology and fate of v-src-transformed cells.

Reduction of c-Src activity by substituted 5,7-diphenyl-pyrrolo[2,3-d]-pyrimidines induces osteoclast apoptosis in vivo and in vitro. Involvement of ERK1/2 pathway

We employed potent and selective c-Src inhibitors to investigate the functional and molecular consequences of inhibited c-Src tyrosine kinase activity in osteoclasts. These pyrrolopyrimidine derivatives reduced osteoclast numbers and induced osteoclast disruption in vivo. In vitro, they inhibited resorption pit formation and osteoclastogenesis, impaired adhesion ability and actin ring organization, and induced programmed cell death in mature osteoclasts. The cell death receptor Fas and p53 were insensitive to c-Src modulation. The expression of the cyclin-dependent kinase (CDK)-inhibitor p21WAF1/CIP1 was markedly reduced, but neither Bcl-2 nor Bcl-xL or Bax were modulated by c-Src inhibition. Caspase-9, and to a lesser extent caspase-3, but not caspase-8, were transiently cleaved (activated) by treatment with the c-Src inhibitors. c-Src inhibition stabilized p38 mitogen-activated protein kinase (MAPK), whereas the c-Jun N-terminal kinase (JNK) pathway did not appear to be modulated by our compounds. Most interestingly, transient extracellular signal regulated kinase (ERK1/2) dephosphorylation followed by sustained remarkable rephosphorylation overwhelming control levels was observed in response to c-Src inhibition. Blockade of ERK1/2 rephosphorylation by PD98059 reduced osteoclast nuclear disruption, suggesting the involvement of this pathway in apoptosis. Collectively, these data demonstrate that small pyrrolopyrimidine derivatives impair osteoclast function and induce cell damage suggestive of apoptosis in vivo and in vitro, with mechanisms presumably involving selective sustained ERK1/2 phosphorylation.

Rac1 function is required for Src-induced transformation. Evidence of a role for Tiam1 and Vav2 in Rac activation by Src

The proto-oncogene c-Src has been implicated in the development and progression of a number of human cancers including those of colon and breast. Accumulating evidence indicates that activated alleles of Src may induce cell transformation through Ras-ERK-dependent and -independent pathways. Here we show that Rac1 activity is strongly elevated in Src-transformed cells and that this small G protein is a critical component of the pathway connecting oncogenic Src with cell transformation. We further show that Vav2 and the ubiquitously expressed Rac1 guanine nucleotide exchange factor Tiam1 are phosphorylated in tyrosine residues in cells transfected with active and oncogenic Src. Moreover, phosphorylation of Tiam1 in cells treated with pervanadate, a potent inhibitor of tyrosine phosphatases, was partially inhibited by the Src inhibitor SU6656. Using truncated mutants of Tiam1, we demonstrate that multiple sites can be tyrosine-phosphorylated by Src. Furthermore, Tiam1 cooperated with Src to induce activation of Rac1 in vivo and the formation of membrane ruffles. Similarly, activation of JNK and the c-jun promoter by Src were also potently increased by Tiam1. Together, these results suggest that Vav2 and Tiam1 may act as downstream effectors of Src, thereby regulating Rac1-dependent pathways that participate in Src-induced cell transformation.

Carbazolequinone induction of caspase-dependent cell death in Src-overexpressing cells

We previously reported that RSV-transformed quail neuroretina cells (QNR-ts68) were highly resistant to apoptosis provoked by serum withdrawal, and that this property was due to v-Src kinase activity. The present study investigates the cytotoxic effect and the functional mechanism of carbazolequinone-mediated cell death in this system. QNR-ts68 cells were subjected to carbazolequinone treatment and both growth inhibition and cell death induction were examined using formazan assays. Cell death mechanism (both apoptosis and necrosis) was confirmed through phosphatidyl serine exposure and propidium iodide incorporation. Furthermore, the effect of active carbazolequinone was inhibited by a pan caspase inhibitor. Cytofluorimetric and immunofluorescence data demonstrated the activation of caspase-3 and the involvement of mitochondria. Therefore, this study clearly indicates that carbazolequinones could induce cell death in transformed cells displaying high levels of antiapoptotic tyrosine kinase activity. Further investigations would be necessary to elucidate the mechanisms by which these carbazolequinones act as antitumor agents.