Transduction of c-src coding and intron sequences by a transformation-defective deletion mutant of Rous sarcoma virus.

The mechanism of cellular src (c-src) transduction by a transformation-defective deletion mutant, td109, of Rous sarcoma virus was studied by sequence analysis of the recombinational junctions in three td109-derived recovered sarcoma viruses (rASVs). Our results show that two rASVs have been generated by recombination between td109 and c-src at the region between exons 1 and 2 defined previously. Significant homology between td109 and c-src sequences was present at the sites of recombination. The viral and c-src sequence junction of the third rASV was formed by splicing a cryptic donor site at the 5' region of env of td109 to exon 1 of c-src. Various lengths of c-src internal intron 1 sequences were incorporated into all three rASV genomes, which resulted from activation of potential splice donor and acceptor sites. The incorporated intron 1 sequences were absent in the c-src mRNA, excluding its being the precursor for recombination with td109 and implying that initial recombinations most likely took place at the DNA level. A potential splice acceptor site within the incorporated intron 1 sequences in two rASVs was activated and was used for the src mRNA synthesis in infected cells. The normal env mRNA splice acceptor site was used for src mRNA synthesis for the third rASV.     

An alternative non-tyrosine protein kinase product of the c-src gene in chicken skeletal muscle.

While the c-src locus is expressed as a 4.0-kilobase (kb) mRNA coding for pp60c-src in various chicken tissues, including embryonic muscle, it is expressed as a novel 3.0-kb mRNA in adult skeletal muscle. We have analyzed the primary structure of this alternatively transcribed and spliced c-src mRNA. The sequence revealed three open reading frames, with the previously defined c-src exons 1 through 5 or 6 comprising the third, on the 3' untranslated region of this 3-kb mRNA. The exons coding for the tyrosine kinase domain of pp60c-src were excluded. On the 5' side, 2 kb of sequence upstream from the previously defined exon 1 of the c-src gene was included in this mRNA. The start site for the 3-kb mRNA probably lies downstream of that for the 4-kb mRNA. The first reading frame of the 3.0-kb mRNA, called sur (for src upstream region), encoded a 24-kilodalton (kDa) protein product rich in cysteine and proline residues. In vitro analysis indicated that the 24-kDa sur protein was membrane associated. Antibodies to sur protein detected in vivo a 24-kDa muscle-specific protein which was developmentally regulated and corresponded to the switch from the 4-kb to the 3-kb c-src mRNA. A striking kinetic pattern of appearance of sur protein and disappearance of pp60c-src suggests that the expression of these two proteins is inversely related.     

pp60c-src is developmentally regulated in the neural retina

We have localized normal cellular pp60c-src in the developing chick neural retina by immunocytochemical staining using antisera raised against bacterially expressed pp60v-src, the src gene product of Rous sarcoma virus. pp60c-src was expressed in developing retinal neurons at the onset of differentiation. Expression of pp60c-src persisted in mature neuronal cells that were postmitotic, fully differentiated, and functional. pp60c-src immunoreactivity was localized within processes and cell bodies of ganglion neurons, processes of rods and cones, and in some but not all neurons of the inner nuclear layer. Protein kinase assays and Western transfer analyses identified the immunoreactive protein as pp60c-src, and confirmed that its expression occurs at the time the first neuronal cells in the retina differentiate. We conclude from these studies that pp60c-src is the product of a developmentally regulated gene that is more important in neuronal differentiation or function than cell proliferation.     

pp60c-src expression in transdifferentiating cultures of embryonic chick neural retina cells

Chick embryo neural retinal cells transdifferentiate extensively into lens cells when cultured in Eagle's MEM containing horse and fetal calf sera (FHMEM). Such cultures express elevated levels of pp60c-src-associated tyrosine kinase activity relative to parallel cultures prevented from transdifferentiating by the addition of supplementary glucose (FHGMEM) or replacement of MEM by medium 199 (F199). Northern blotting and in vitro translation studies suggest that c-src mRNA levels are only slightly higher in late transdifferentiating (FHMEM) cultures as compared to parallel blocked (FHGMEM or F199) cultures. By immunocytochemical staining, we show that pp60c-src protein is largely localized in cell groups undergoing conversion into lens (i.e. expressing delta crystallin) in late FHMEM cultures. Initial studies of pp60c-src in chick lens tissues during development indicate that higher kinase activity is found in the epithelial cells relative to mature lens fibres. Thus pp60c-src may be expressed both during the differentiation of lens cells in vivo and during the transdifferentiation of neural retina cells into lens in vitro.     

The proto-oncogene c-src is involved in primordial follicle activation through the PI3K, PKC and MAPK signaling pathways

ABSTRACT: BACKGROUND: C-src is an evolutionarily conserved proto-oncogene that regulates cell proliferation, differentiation and apoptosis. In our previous studies, we have reported that another proto-oncogene, c-erbB2, plays an important role in primordial follicle activation and development. We also found that c-src was expressed in mammalian ovaries, but its functions in primordial follicle activation remain unclear. The objective of this study is to investigate the role and mechanism of c-src during the growth of primordial follicles. METHODS: Ovaries from 2-day-old rats were cultured in vitro for 8 days. Three c-src-targeting and one negative control siRNA were designed and used in the present study. PCR, Western blotting and primordial follicle development were assessed for the silencing efficiency of the lentivirus c-src siRNA and its effect on primordial follicle onset. The expression of c-src mRNA and protein in primordial follicle growth were examined using the PCR method and immunohistochemical staining. Furthermore, the MAPK inhibitor PD98059, the PKC inhibitor Calphostin and the PI3K inhibitor LY294002 were used to explore the possible signaling pathways of c-src in primordial folliculogenesis. RESULTS: The results showed that Src protein was distributed in the ooplasmic membrane and the granulosa cell membrane in the primordial follicles, and c-src expression level increased with the growth of primordial follicle. The c-src -targeting lentivirus siRNAs had a silencing effect on c-src mRNA and protein expression. Eight days after transfection of rat ovaries with c-src siRNA, the GFP fluorescence in frozen ovarian sections was clearly discernible under a fluorescence microscope, and its relative expression level was 5-fold higher than that in the control group. Furthermore, the c-src-targeting lentivirus siRNAs lowered its relative expression level 1.96 times. We also found that the development of cultured primordial follicles was completely arrested after c-src siRNA knockdown of c-src expression. Furthermore, our studies demonstrated that folliculogenesis onset was inhibited by Calphostin, PD98059 or LY294002 treatment,but none of them down-regulated c-src expression. In contrast, the expression levels of p-PKC, p-ERK1/2 and p-PI3K in the follicles were clearly decreased by c-src siRNA transfection. Correspondingly, both Calphostin and LY294002 treatment resulted in a decrease in the p-PKC level in follicles, but no change was observed in the PD98059 group. Finally, LY294002 treatment decreased the p-PI3K expression level in the follicles, but no changes were observed in the PD98059 and Calphostin groups. CONCLUSIONS: C-src plays an important role in regulating primordial follicle activation and growth via the PI3K-PKC- ERK1/2 pathway.     

Expression of c-src and c-abl in embryonal carcinoma cells and adult mouse tissues

We have studied the expression of c-src and c-abl proto-oncogenes in early mouse development using embryonal carcinoma (EC) cells as a model system, and compared this to the expression pattern in adult tissues. In all three EC lines tested (F9, PC13, and P19), c-src and c-abl mRNA can be detected. When F9 and PC13 are induced to differentiate they form endodermal cells characteristic of the early embryo, and we found no change in c-src or c-abl expression. In contrast, P19 cells showed increased levels of both mRNAs both mRNAs when induced to differentiate along the neural pathway by retinoic acid, whereas differentiation along the muscle pathway by dimethyl sulfoxide resulted in decreased levels of c-abl expression. These results are consistent with the idea that c-src and c-abl have important functions in the differentiation of the cell types of the later embryo, but not in those of the early embryo.     

Localization of caveolin-1 and c-src in mature and differentiating photoreceptors: raft proteins co-distribute with rhodopsin during development

Numerous biochemical and morphological studies have provided insight into the distribution pattern of caveolin-1 and the presence of membrane rafts in the vertebrate retina. To date however, studies have not addressed the localization profile of raft specific proteins during development. Therefore the purpose of our studies was to follow the localization pattern of caveolin-1, phospho-caveolin-1 and c-src in the developing retina and compare it to that observed in adults. Specific antibodies were used to visualize the distribution of caveolin-1, c-src, a kinase phosphorylating caveolin-1, and phospho-caveolin-1. The labeling pattern of this scaffolded complex was compared to those of rhodopsin and rhodopsin kinase. Samples were analyzed at various time points during postnatal development and compared to adult retinas. The immunocytochemical studies were complemented with immunoblots and immunoprecipitation studies. In the mature retina caveolin-1 and c-src localized mainly to the cell body and IS of photoreceptors, with only very weakly labeled OS. In contrast, phospho-caveolin-1 was only detectable in the OS of photoreceptors. During development we followed the expression and distribution profile of these proteins in a temporal sequence with special attention to the period when OS formation is most robust. Double labeling immunocytochemistry and immunoprecipitation showed rhodopsin to colocalize and co-immunoprecipitate with caveolin-1 and c-src. Individual punctate structures between the outer limiting membrane and the outer plexiform layer were seen at P10 to be labeled by both rhodopsin and caveolin-1 as well as by rhodopsin and c-src, respectively. These studies suggest that membrane raft specific proteins are co-distributed during development, thereby pointing to a role for such complexes in OS formation. In addition, the presence of small punctate structures containing caveolin-1, c-src and rhodopsin raise the possibility that these proteins may transport together to OS during development and that caveolin-1 exists predominantly in a phosphorylated form in the OS.     

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.     

Retrovirus shuttle vector for study of kinase activities of pp60c-src synthesized in vitro and overproduced in vivo.

We have constructed a recombinant murine retrovirus which efficiently transduces avian pp60c-src into murine cells and which is easily rescued from infected cells in plasmid form. To characterize the virus, several randomly selected NIH 3T3 lines were isolated after infection with recombinant retroviral stocks. All lines overproduced avian pp60c-src and appeared morphologically normal. Immunoprecipitates made from these lines with antisera specific for pp60c-src were tested for their kinase activities in vitro. We find that both autokinase and enolase kinase activities increase proportionately with the level of pp60c-src in the immunoprecipitates. To further test the authenticity of the pp60c-src encoded by the retroviral vector, these analyses were repeated in the presence of polyomavirus middle T antigen. Avian pp60c-src was activated as a protein kinase, indicating that the virally encoded pp60c-src interacts normally with middle T antigen. Interestingly, by increasing the intracellular levels of pp60c-src 15-fold over normal endogenous levels, we were unable to obtain a proportionate increase in the amount of middle-T-antigen-pp60c-src complex. Finally, using the shuttle features designed into the vector, we have isolated the first fully processed cDNA encoding functional avian pp60c-src X pp60c-src synthesized in vitro with this cDNA had intrinsic protein kinase activity and no detectable phosphatidylinositol kinase activity.     

Quantitation of a src-like tyrosine protein kinase during fertilization of the sea urchin egg

Fertilization of the sea urchin egg is known to involve an increase in overall protein tyrosine kinase activity which precede the first cell division. In order to determine the types of tyrosine kinases that are involved in fertilization, we have used immunological and other criteria to identify a c-src related protein kinase in eggs of the sea urchin L. variegatus. Using an immune complex assay, we have measured the level of this c-src related protein kinase during fertilization and early embryonic development. Fertilization results in a decrease in the c-src kinase detectable by this technique suggesting that c-src does not contribute to the fertilization induced increase in protein tyrosine kinase activity.     

pp60c-src Kinase is in chick and human embryonic tissues

The normal cellular protein pp60c-src is a tyrosine-specific protein kinase that is homologous to the transforming protein of Rous sarcoma virus (RSV) but its function is unknown. The expression of pp60c-src in chick and human embryonic tissues was monitored by the immune complex protein kinase assay, Western transfer analysis, and immunocytochemical staining at the light microscope level. pp60c-src kinase was expressed in the head and trunk regions of the chick embryo at all stages of development examined; however, expression increased significantly during the major period of organogenesis (Hamburger and Hamilton stages 21 to 32). Western transfer analysis showed that the amount of pp60c-src protein increased in parallel with the increase in kinase activity. Highest levels of pp60c-src kinase were present in the neural tube, brain, and heart of the stage 32 chick embryo. Lower levels of activity were found in eye, limb bud, and liver. Immunocytochemical staining of the neural tube region and heart of the chick confirmed the results of biochemical analysis and showed immunoreactive pp60c-src distributed throughout the neural tube and heart. The distribution of pp60c-src kinase in human fetal tissues was similar to that in the chick embryo; elevated levels of pp60c-src kinase were present in cerebral cortex, spinal cord, and heart, but all other tissues examined expressed some pp60c-src kinase. The results of our studies suggest that pp60c-src plays a fundamental role in an aspect of cellular metabolism that is particularly important in electrogenic tissues.     

Cortisol receptors and inducibility of glutamine synthetase in embryonic retina

Glutamine synthetase (GS) is a marker enzyme for Müller glia cells in neural retina. In chick embryo retina GS begins to increase sharply on the 16th day of development, but can be precociously induced by premature supply of the inducer, cortisol, already on the 8th day. At this stage GS inducibility is low, but it increases progressively with embryonic age. We investigated whether there was a corresponding age-dependent increase of cortisol-binding molecules (cortisol receptors) and found that their level is highest in the early retina and decreases with development. In light of this inverse relationship, we examined whether functional characteristics of these receptors change with age, but detected no differences. In in vitro tests, receptors from older retina translocated cortisol into nuclei from young retina, and vice versa, with similar effectiveness. Also, cortisol receptors from liver cells (which differ from retina receptors) can translocate the hormone into retina nuclei, and vice versa. These findings indicate that translocation of cortisol receptors is neither tissue-specific or age-dependent, nor is it conditional on the total amount of receptors normally present in cells. Therefore, the age-dependent increase of GS inducibility in embryonic retina cannot be directly related to quantitative or functional differences of cortisol receptors and is evidently controlled primarily at the gene level. The very large amount of cortisol-binding molecules in early embryonic retina raises the possibility that they play some role in early differentiation of retina cells unrelated to hormone binding.     

Neural tissues express high levels of the cellular src gene product pp60c-src.

Chicken embryo tissues were examined for the expression of pp60c-src, the normal cellular homolog of the transforming protein of Rous sarcoma virus. Three assays, including a solid-phase radioimmunoassay, a competitive radioimmunoprecipitation assay, and an immune complex protein kinase assay, were employed. Elevated levels of pp60c-src were detected in lysates from several neural tissues, including brain, retina, and spinal ganglia. Other tissues contained 8- to 10-fold-lower levels of pp60c-src, levels comparable to those found in chicken embryo fibroblasts. Expression of pp60c-src in brain tissues was also shown to vary with the developmental stage of the embryo.     

Analysis of pp60c-src protein kinase activity in human tumor cell lines and tissues

We have evaluated the level of pp60c-src protein kinase activity in a variety of human tumor tissues and human tumor cell lines, and have estimated the abundance of the c-src protein in several of these tissues and cell lines. All cell lines derived from tumors of neuroectodermal origin that express a neural phenotype were found to possess c-src molecules with high levels of tyrosine-specific protein kinase activity. In contrast, cell lines derived from tumors of neuroectodermal origin that do not express neural characteristics, such as glioblastomas and melanomas, were found to have pp60c-src molecules with low levels of protein kinase activity. A similar pattern was observed when we analyzed the activity of c-src molecules extracted directly from corresponding tumor tissues. Analysis of human tumor cell lines derived from tissues other than those of neuroectodermal origin revealed that pp60c-src protein kinase activity was low in most cases. Exceptions to this observation were all rhabdomyosarcoma, osteogenic sarcoma, Ewing's sarcoma, and colon carcinoma lines tested. Comparison of pp60c-src kinase activity in normal skeletal muscle and rhabdomyosarcoma tissue and in normal breast tissue and breast adenocarcinoma tissue revealed that pp60c-src kinase activity was specifically elevated in the tumor tissues in both cases. However, the amount of pp60c-src protein in both normal and tumor tissues was found to be similar. These observations suggest that increases in the specific activity of the pp60c-src phosphotransferase in some rhabdomyosarcomas and breast carcinomas may be a characteristic acquired during the malignant transformation of the cells that is retained in cell lines established from these tumors.     

Biological and biochemical properties of the c-src+ gene product overexpressed in chicken embryo fibroblasts.

The c-src protein isolated from neuronal cells (pp60c-src+) displays a higher level of protein kinase activity than does pp60c-src from nonneural tissues. There are two structural alterations present in the amino-terminal half of pp60c-src+ expressed in neurons which could contribute to the enhanced activity of this form of pp60c-src: (i) a hexapeptide insert located at amino acid 114 of avian pp60c-src+ and (ii) a novel site(s) of serine phosphorylation. We characterized pp60c-src+ expressed in a nonneuronal cell type to identify factors that regulate the activity of the c-src+ protein and the importance of the neuronal environment on this regulation. The c-src+ protein overexpressed in chicken embryo fibroblasts (CEFs) displayed higher kinase activity than did pp60c-src. The major sites of phosphorylation of the c-src+ protein were Ser-17 and Tyr-527. The unique site(s) of serine phosphorylation originally identified in pp60c-src+ expressed in neurons was not detected in the c-src+ protein overexpressed in CEFs. Therefore, the hexapeptide insert is sufficient to cause an elevation in the tyrosine protein kinase activity of pp60c-src+. Our data also indicate that CEFs infected with the Rous sarcoma virus (RSV)c-src+ display phenotypic changes that distinguish them from cultures producing pp60c-src and that pp60c-src+-expressing cells are better able to grow in an anchorage-independent manner. The level of total cellular tyrosine phosphorylation in RSVc-src+-infected cultures was moderately higher than the level observed in cultures infected with RSVc-src. This level was not as pronounced as that observed in cells infected with RSVv-src or oncogenic variants of RSVc-src. Thus, pp60c-src+ could be considered a partially activated c-src variant protein much like other c-src proteins that contain mutations in the amino-terminal domain.