Inhibition of chicken embryo lens differentiation and lens junction formation in culture by pp60v-src

The molecular mechanisms underlying lens fiber elongation

Lens fiber cells are highly elongated cells with complex membrane morphologies that are critical for the transparency of the ocular lens. Investigations into the molecular mechanisms underlying lens fiber cell elongation were first reported in the 1960s, however, our understanding of the process is still poor nearly 50 years later. This review summarizes what is currently hypothesized about the regulation of lens fiber cell elongation along with the available experimental evidence, and how this information relates to what is known about the regulation of cell shape/elongation in other cell types, particularly neurons.

Distinct roles for N-Cadherin linked c-Src and fyn kinases in lens development

BACKGROUND

Src family tyrosine kinases (SFKs) are often coincidently expressed but few studies have dissected their individual functions in the same cell during development. Using the classical embryonic lens as our model, we investigated SFK signaling in the regulation of both differentiation initiation and morphogenesis, and the distinct functions of c-Src and Fyn in these processes.

RESULTS

Blocking SFK activity with the highly specific inhibitor PP1 induced initiation of the lens differentiation program but blocked lens fiber cell elongation and organization into mini lens-like structures called lentoids. These dichotomous roles for SFK signaling were discovered to reflect distinct functions of c-Src and Fyn and their differentiation-state-specific recruitment to and action at N-cadherin junctions. c-Src was highly associated with the nascent N-cadherin junctions of undifferentiated lens epithelial cells. Its siRNA knockdown promoted N-cadherin junctional maturation, blocked proliferation, and induced lens cell differentiation. In contrast, Fyn was recruited to mature N-cadherin junctions of differentiating lens cells and siRNA knockdown suppressed differentiation-specific gene expression and blocked morphogenesis.

CONCLUSIONS

Through inhibition of N-cadherin junction maturation, c-Src promotes lens epithelial cell proliferation and the maintenance of the lens epithelial cell undifferentiated state, while Fyn, signaling downstream of mature N-cadherin junctions, promotes lens fiber cell morphogenesis.

Cellular processes of v-Src transformation revealed by gene profiling of primary cells--implications for human cancer

Background

Cell transformation by the Src tyrosine kinase is characterized by extensive changes in gene expression. In this study, we took advantage of several strains of the Rous sarcoma virus (RSV) to characterize the patterns of v-Src-dependent gene expression in two different primary cell types, namely chicken embryo fibroblasts (CEF) and chicken neuroretinal (CNR) cells. We identified a common set of v-Src regulated genes and assessed if their expression is associated with disease-free survival using several independent human tumor data sets.

Methods

CEF and CNR cells were infected with transforming, non-transforming, and temperature sensitive mutants of RSV to identify the patterns of gene expression in response to v-Src-transformation. Microarray analysis was used to measure changes in gene expression and to define a common set of v-Src regulated genes (CSR genes) in CEF and CNR cells. A clustering enrichment regime using the CSR genes and two independent breast tumor data-sets was used to identify a 42-gene aggressive tumor gene signature. The aggressive gene signature was tested for its prognostic value by conducting survival analyses on six additional tumor data sets.

Results

The analysis of CEF and CNR cells revealed that cell transformation by v-Src alters the expression of 6% of the protein coding genes of the genome. A common set of 175 v-Src regulated genes (CSR genes) was regulated in both CEF and CNR cells. Within the CSR gene set, a group of 42 v-Src inducible genes was associated with reduced disease- and metastasis-free survival in several independent patient cohorts with breast or lung cancer. Gene classes represented within this group include DNA replication, cell cycle, the DNA damage and stress responses, and blood vessel morphogenesis.

Conclusion

By studying the v-Src-dependent changes in gene expression in two types of primary cells, we identified a set of 42 inducible genes associated with poor prognosis in breast and lung cancer. The identification of these genes provides a set of biomarkers of aggressive tumor behavior and a framework for the study of cancer cells characterized by elevated Src kinase activity.

Cell cycle regulation in the developing lens

Regulation of cell proliferation is a critical aspect of the development of multicellular organisms. The ocular lens is an excellent model system in which to unravel the mechanisms controlling cell proliferation during development. In recent years, several cell cycle regulators have been shown to be essential for maintaining normal patterns of lens cell proliferation. Additionally, many growth factor signaling pathways and cell adhesion factors have been shown to have the capacity to regulate lens cell proliferation. Given this complexity, understanding the cross talk between these many signaling pathways and how they are coordinated are important directions for the future.

Gap junction- and hemichannel-independent actions of connexins

Connexins have been known to be the protein building blocks of gap junctions and mediate cell-cell communication. In contrast to the conventional dogma, recent evidence suggests that in addition to forming gap junction channels, connexins possess gap junction-independent functions. One important gap junction-independent function for connexins is to serve as the major functional component for hemichannels, the un-apposed halves of gap junctions. Hemichannels, as independent functional units, play roles that are different from that of gap junctions in the cell. The other functions of connexins appear to be gap junction- and hemichannel-independent. Published studies implicate the latter functions of connexins in cell growth, differentiation, tumorigenicity, injury, and apoptosis, although the mechanistic aspects of these actions remain largely unknown. In this review, gap junction- and hemichannel-independent functions of connexins are summarized, and the molecular mechanisms underlying these connexin functions are speculated and discussed.

Expression of v-src in embryonic neural retina alters cell adhesion, inhibits histogenesis, and prevents induction of glutamine synthetase

Using Rous sarcoma virus as the vector, v-src or c-src genes were introduced into 6-day chicken embryo retina tissue in organ culture and their effects on retina development were investigated. Overexpression of c-src in many of the cells had no noticeable effect on retina development. In contrast, infection with v-src resulted in abnormal histogenesis and inhibition of differentiation. Although only a portion of the cells in infected tissue expressed the oncogene and displayed the transformation phenotype, the other cells were also hindered from becoming normally positioned and organized. Therefore, presence of oncogene-transformed cells within the tissue hindered organization and development of adjacent nontransformed cells. Failure of normal cell relationships impeded induction by cortisol of glutamine synthetase in Muller glia, which requires contact associations of the glia cells with neurons. The transformed cells tended to assemble into chaotic clusters, suggesting that their adhesiveness and contact affinities had become altered. This was confirmed by aggregation experiments with dissociated cells which showed that adhesiveness of transformed cells was greatly reduced and that they had lost the ability to cohere with nontransformed cells. In binary mixtures of transformed and nontransformed cells, the two sorted out into separate aggregates. Transformed cells formed loose clusters devoid of tissue architecture; aggregates of nontransformed cells became organized into retinotypic structures, and glutamine synthetase was inducible. Our findings suggest that the mechanisms of cell adhesion and cell affinities are a key target of v-src activity in infected cells and that modification of the cell surface may be a leading factor in other cellular changes characteristic of the v-src transformation phenotype.

Expression of v-src in embryonic neural retina alters cell adhesion, inhibits histogenesis, and prevents induction of glutamine synthetase

Using Rous sarcoma virus as the vector, v-src or c-src genes were introduced into 6-day chicken embryo retina tissue in organ culture and their effects on retina development were investigated. Overexpression of c-src in many of the cells had no noticeable effect on retina development. In contrast, infection with v-src resulted in abnormal histogenesis and inhibition of differentiation. Although only a portion of the cells in infected tissue expressed the oncogene and displayed the transformation phenotype, the other cells were also hindered from becoming normally positioned and organized. Therefore, presence of oncogene-transformed cells within the tissue hindered organization and development of adjacent nontransformed cells. Failure of normal cell relationships impeded induction by cortisol of glutamine synthetase in Muller glia, which requires contact associations of the glia cells with neurons. The transformed cells tended to assemble into chaotic clusters, suggesting that their adhesiveness and contact affinities had become altered. This was confirmed by aggregation experiments with dissociated cells which showed that adhesiveness of transformed cells was greatly reduced and that they had lost the ability to cohere with nontransformed cells. In binary mixtures of transformed and nontransformed cells, the two sorted out into separate aggregates. Transformed cells formed loose clusters devoid of tissue architecture; aggregates of nontransformed cells became organized into retinotypic structures, and glutamine synthetase was inducible. Our findings suggest that the mechanisms of cell adhesion and cell affinities are a key target of v-src activity in infected cells and that modification of the cell surface may be a leading factor in other cellular changes characteristic of the v-src transformation phenotype.

Modulation of intercellular adherens-type junctions and tyrosine phosphorylation of their components in RSV-transformed cultured chick lens cells

Transformation of cultured chick lens epithelial cells with a temperature-sensitive mutant of Rous sarcoma virus (tsRSV) leads to radical changes in cell shape and interactions. When cultured at the restrictive temperature (42 degrees C), the transformed cells largely retained epithelial morphology and intercellular adherens junctions (AJ), whereas on switch to the permissive temperature (37 degrees C) they rapidly became fibroblastoid, their AJ deteriorated, and cell adhesion molecules (A-CAM) (N-cadherin) largely disappeared from intercellular contact sites. The microfilament system that was primarily associated with these junctions was markedly rearranged on shift to 37 degrees C and remained associated mainly with cell-substrate focal contacts. These apparent changes in intercellular AJ were not accompanied by significant alterations in the cellular content of several junction-associated molecules, including A-CAM, vinculin, and talin. Immunolabeling with phosphotyrosine-specific antibodies indicated that both cell-substrate and intercellular AJ were the major cellular targets for the pp60v-src tyrosine-specific protein kinase. It was further shown that intercellular AJ components serve as substrates to tyrosine kinases also in nontransformed lens cells, because the addition of a combination of vanadate and H2O2--which are potent inhibitors of protein tyrosine phosphatases--leads to a remarkable accumulation of immunoreactive phosphotyrosine-containing proteins in these junctions. This finding suggests that intercellular junctions are major sites of action of protein tyrosine kinases and that protein tyrosine phosphatases play a major role in the regulation of phosphotyrosine levels in AJ of both normal and RSV-transformed cells.

Tyrosine phosphorylation of a c-Src-like protein is increased in membranes of CD4- CD8- T lymphocytes from lpr/lpr mice

Mice homozygous for the autosomal recessive lpr gene have a disorder that results in autoimmunity and massive accumulation of T lymphocytes lacking CD4 and CD8 surface markers. These abnormal T cells exhibit constitutive tyrosine phosphorylation of a component of the CD3-T-cell receptor complex. We compared membrane tyrosine phosphorylation in lpr/lpr CD4- CD8- T cells and control T cells, lpr membranes exhibited a 7.3-fold increase (n = 16) in tyrosine phosphorylation of a 60-kilodalton protein. The increase was correlated with the Lpr but not the CD4- CD8- phenotype in that p60 phosphorylation was not increased in membranes from normal CD4- CD8- thymocytes. To identify the p60 in lpr cells, we examined the activity of several T-cell tyrosine-specific protein kinases. p56lck phosphorylation was only slightly increased in lpr membranes (2.2-fold; n = 16). Phorbol ester treatment of intact T cells before membrane isolation caused p56lck to migrate as pp60lck; however, pp60lck could be clearly distinguished from the pp60 in lpr cells by two-dimensional gel electrophoresis. The pp60 from lpr cells exhibited several isoforms at pH approximately 6.3 to 6.5. Although on two-dimensional gels pp60c-src had a pI (6.4 to 6.8) within a similar region, p60c-src mRNA, protein, and kinase activities were not increased in lpr cells. In addition, staphylococcal V8 proteolytic cleavage of the lpr pp60 isolated on two-dimensional gels yielded two major fragments, a pattern distinct from that of pp60c-src. However, by using an antiserum against the C-terminal sequence of c-Src and other related kinases, including p59fyn, the pp60 could be immunoprecipitated in greater amounts from lpr than from control T cells. When pp59(fyn) was selectively immunoprecipitated from T-cell membranes with specific antisera, its molecular weight, proteolytic cleavage pattern, and behavior on two-dimensional gels were identical to those of the pp60 from lpr cells. We conclude that p59(fyn) phosphorylation is increased in membranes from lpr/lpr CD4(-) CD8(-) T cells and that the increase is correlated with constitutive tyrosine phosphorylation and perhaps with the expansion of this unusual T-cell population.

Early activation of endogenous pp60src kinase activity during neuronal differentiation of cultured human neuroblastoma cells

The proto-oncogene product pp60c-src is a tyrosine-specific kinase with a still unresolved cellular function. High levels of pp60c-src in neurons and the existence of a neuronal pp60c-src variant, pp60c-srcN, suggest participation in the progress or maintenance of the differentiated phenotype of neurons. We have previously reported that phorbol esters, e.g., 12-O-tetradecanoylphorbol-13-acetate (TPA), stimulate human SH-SY5Y neuroblastoma cells to neuronal differentiation, as monitored by morphological, biochemical, and functional differentiation markers. In this report, we describe activation of the pp60src (pp60c-src and pp60c-srcN) kinase activity observed at 6 h after induction of SH-SY5Y cells with TPA. This phenomenon coincides in time with neurite outgrowth, formation of growth cone-like structures, and an increase of GAP43 mRNA expression, which are the earliest indications of neuronal differentiation in these cells. The highest specific src kinase activity (a three- to fourfold increase 4 days after induction) was noted in cells treated with 16 nM TPA; this concentration is optimal for development of the TPA-induced neuronal phenotype. During differentiation, there was no alteration in the 1:1 ratio of pp60c-src to pp60c-srcN found in untreated SH-SY5Y cells. V8 protease and trypsin phosphopeptide mapping of pp60src from in vivo 32P-labeled cells showed that the overall phosphorylation of pp60src was higher in differentiated than in untreated cells, mainly because of an intense serine 12 phosphorylation. Tyrosine 416 phosphorylation was not detectable in either cell type, and no change during differentiation in tyrosine 527 phosphorylation was observed.

Early activation of endogenous pp60src kinase activity during neuronal differentiation of cultured human neuroblastoma cells

The proto-oncogene product pp60c-src is a tyrosine-specific kinase with a still unresolved cellular function. High levels of pp60c-src in neurons and the existence of a neuronal pp60c-src variant, pp60c-srcN, suggest participation in the progress or maintenance of the differentiated phenotype of neurons. We have previously reported that phorbol esters, e.g., 12-O-tetradecanoylphorbol-13-acetate (TPA), stimulate human SH-SY5Y neuroblastoma cells to neuronal differentiation, as monitored by morphological, biochemical, and functional differentiation markers. In this report, we describe activation of the pp60src (pp60c-src and pp60c-srcN) kinase activity observed at 6 h after induction of SH-SY5Y cells with TPA. This phenomenon coincides in time with neurite outgrowth, formation of growth cone-like structures, and an increase of GAP43 mRNA expression, which are the earliest indications of neuronal differentiation in these cells. The highest specific src kinase activity (a three- to fourfold increase 4 days after induction) was noted in cells treated with 16 nM TPA; this concentration is optimal for development of the TPA-induced neuronal phenotype. During differentiation, there was no alteration in the 1:1 ratio of pp60c-src to pp60c-srcN found in untreated SH-SY5Y cells. V8 protease and trypsin phosphopeptide mapping of pp60src from in vivo 32P-labeled cells showed that the overall phosphorylation of pp60src was higher in differentiated than in untreated cells, mainly because of an intense serine 12 phosphorylation. Tyrosine 416 phosphorylation was not detectable in either cell type, and no change during differentiation in tyrosine 527 phosphorylation was observed.

Tyrosine phosphorylation of a c-Src-like protein is increased in membranes of CD4- CD8- T lymphocytes from lpr/lpr mice

Mice homozygous for the autosomal recessive lpr gene have a disorder that results in autoimmunity and massive accumulation of T lymphocytes lacking CD4 and CD8 surface markers. These abnormal T cells exhibit constitutive tyrosine phosphorylation of a component of the CD3-T-cell receptor complex. We compared membrane tyrosine phosphorylation in lpr/lpr CD4- CD8- T cells and control T cells, lpr membranes exhibited a 7.3-fold increase (n = 16) in tyrosine phosphorylation of a 60-kilodalton protein. The increase was correlated with the Lpr but not the CD4- CD8- phenotype in that p60 phosphorylation was not increased in membranes from normal CD4- CD8- thymocytes. To identify the p60 in lpr cells, we examined the activity of several T-cell tyrosine-specific protein kinases. p56lck phosphorylation was only slightly increased in lpr membranes (2.2-fold; n = 16). Phorbol ester treatment of intact T cells before membrane isolation caused p56lck to migrate as pp60lck; however, pp60lck could be clearly distinguished from the pp60 in lpr cells by two-dimensional gel electrophoresis. The pp60 from lpr cells exhibited several isoforms at pH approximately 6.3 to 6.5. Although on two-dimensional gels pp60c-src had a pI (6.4 to 6.8) within a similar region, p60c-src mRNA, protein, and kinase activities were not increased in lpr cells. In addition, staphylococcal V8 proteolytic cleavage of the lpr pp60 isolated on two-dimensional gels yielded two major fragments, a pattern distinct from that of pp60c-src. However, by using an antiserum against the C-terminal sequence of c-Src and other related kinases, including p59fyn, the pp60 could be immunoprecipitated in greater amounts from lpr than from control T cells. When pp59(fyn) was selectively immunoprecipitated from T-cell membranes with specific antisera, its molecular weight, proteolytic cleavage pattern, and behavior on two-dimensional gels were identical to those of the pp60 from lpr cells. We conclude that p59(fyn) phosphorylation is increased in membranes from lpr/lpr CD4(-) CD8(-) T cells and that the increase is correlated with constitutive tyrosine phosphorylation and perhaps with the expansion of this unusual T-cell population.