Genetic lesions involved in temperature sensitivity of the src gene products of four Rous sarcoma virus mutants

mDia1 targets v-Src to the cell periphery and facilitates cell transformation, tumorigenesis, and invasion

The small GTPase Rho regulates cell morphogenesis through remodeling of the actin cytoskeleton. While Rho is overexpressed in many clinical cancers, the role of Rho signaling in oncogenesis remains unknown. mDia1 is a Rho effector producing straight actin filaments. Here we transduced mouse embryonic fibroblasts from mDia1-deficient mice with temperature-sensitive v-Src and examined the involvement and mechanism of the Rho-mDia1 pathway in Src-induced oncogenesis. We showed that in v-Src-transduced mDia1-deficient cells, formation of actin filaments is suppressed, and v-Src in the perinuclear region does not move to focal adhesions upon a temperature shift. Consequently, membrane translocation of v-Src, v-Src-induced morphological transformation, and podosome formation are all suppressed in mDia1-deficient cells with impaired tyrosine phosphorylation. mDia1-deficient cells show reduced transformation in vitro as examined by focus formation and colony formation in soft agar and exhibit suppressed tumorigenesis and invasion when implanted in nude mice in vivo. Given overexpression of c-Src in various cancers, these findings suggest that Rho-mDia1 signaling facilitates malignant transformation and invasion by manipulating the actin cytoskeleton and targeting Src to the cell periphery.

Roles of tyrosine residues 845, 892 and 922 in constitutive activation of murine FLT3 kinase domain mutant

FLT3 tyrosine kinase domain (TKD) mutations are detected in approximately 7% of acute myeloid leukemia patients, and suggested to correlate with poor prognosis and confer resistance to FLT3 inhibitors. To explore activation mechanism of FLT3 TKD mutation, we analysed critical tyrosine residues for the constitutive activation and downstream signaling of the mutant by generating a series of single Tyr --> Phe substitution mutant of all 22 cytoplasmic tyrosine residues of murine FLT3 TKD-mutant (mFLT3Asp838Val). Tyr845Phe, Tyr892Phe and Tyr922Phe substitutions suppressed the phosphorylation of mFLT3Asp838Val itself, the activation of Erk1/2, STAT3 and STAT5, and the factor-independent cell proliferation and survival. In contrast, these three Tyr --> Phe mutations partially suppressed but maintained the ligand-dependent activation and anti-apoptotic activity of wild-type FLT3, suggesting that these tyrosine residues were more critical for the constitutive activation and signaling of mFLT3Asp838Val. These three Tyr --> Phe mutations also inhibited the constitutive activation of other FLT3 mutants bearing internal tandem duplication, Asp838Tyr or Ile839del. The suppression of mFLT3Asp838Val activation and signaling by these substitutions was partially recovered by shifting the culture temperature from 37 to 33 degrees C, or by the introduction of Cdc37 and Hsp90. Taken together, Tyr845, Tyr892 and Tyr922 are the critical residues in mFLT3Asp838Val activation, possibly through stabilizing the active conformation of mFLT3Asp838Val.

v-Src rescues actin-based cytoskeletal architecture and cell motility and induces enhanced anchorage independence during oncogenic transformation of focal adhesion kinase-null fibroblasts

The ability of the focal adhesion kinase (FAK) to integrate signals from extracellular matrix and growth factor receptors requires the integrity of Tyr397, a major autophosphorylation site that mediates the Src homology 2-dependent binding of Src family kinases. However, the precise roles played by FAK in specific Src-induced pathways, especially as they relate to oncogenic transformation, remain unclear. Here, we investigate the role of FAK in v-Src-induced oncogenic transformation by transducing temperature-sensitive v-Src (ts72v-Src) into p53-null FAK+/+ or FAK-/- mouse embryo fibroblasts (MEF). At the permissive temperature (PT), ts72v-Src induced abundant tyrosine phosphorylation, morphological transformation and cytoskeletal rearrangement in FAK-/- MEF, including the restoration of cell polarity, typical focal adhesion complexes, and longitudinal F-actin stress fibers. v-Src rescued the haptotactic, linear directional, and invasive motility defects of FAK-/- cells to levels found in FAK+/+ or FAK+/+-[ts72v-Src] cells, and, in the case of monolayer wound healing motility, there was an enhancement. Src activation failed to increase the high basal tyrosine phosphorylation of the Crk-associated substrate, CAS, found in FAK-/- MEF, indicating that CAS phosphorylation alone is insufficient to induce motility in the absence of FAK- or v-Src-induced cytoskeletal remodeling. Compared with FAK+/+[ts72v-Src] controls, FAK-/-[ts72v-Src] clones exhibited 7-10-fold higher anchorage-independent proliferation that could not be attributed to variations in either v-Src protein level or stability. Re-expression of FAK diminished the colony-forming activities of FAK-/-[ts72v-Src] without altering ts72v-Src expression levels, suggesting that FAK attenuates Src-induced anchorage independence. Our data also indicate that the enhanced Pyk2 level found in FAK-/- MEF plays no role in v-Src-induced anchorage independence. Overall, our data indicate that FAK, although dispensable, attenuates v-Src-induced oncogenic transformation by modulating distinct signaling and cytoskeletal pathways.

Inhibition of Src by direct interaction with protein phosphatase 2A

In this study, we report that Src kinase is inhibited by protein phosphatase 2A (PP2A), a serine/threonine phosphatase. We carried out experiments in vitro using purified PP2A (AC dimer) and full-length v-Src or truncated forms of v-Src. The inhibition of v-Src by PP2A is concentration- and time-dependent. Addition of okadaic acid, a PP2A phosphatase inhibitor, abolished the PP2A-dependent inhibition of v-Src. When experiments were carried out at 4 degrees C under conditions where PP2A activity is inhibited, Src activity was unaffected by the presence of PP2A, suggesting that PP2A binding alone is insufficient to block Src activity. These results imply that PP2A activity is essential for inhibition of v-Src. We also demonstrate that PP2A binds to the catalytic and the regulatory domains of v-Src.

Protein phosphatase 2A interacts with the Src kinase substrate p130(CAS)

In this study, we report that the Src substrate Cas (p130 Crk-associated substrate) associates with protein phosphatase 2A (PP2A), a serine/threonine phosphatase. We investigated this interaction in cells expressing a temperature-sensitive mutant form of v-Src. v-Src activation (by shifting cells from the nonpermissive to the permissive temperature) led to an increase in the tyrosine phosphorylation of v-Src and Cas, as well as in the association between v-Src and Cas. v-Src has previously been shown to bind to PP2A and to phosphorylate the catalytic subunit of PP2A, resulting in inhibition of phosphatase activity. We found that the association between v-Src and PP2A decreased as cells were shifted to the permissive temperature. In contrast, the levels of PP2A that co-immunoprecipitated with Cas increased when v-Src was activated. We obtained similar results in pull-down experiments with immobilized Microcystin, a PP2A inhibitor. Serine/threonine phosphorylation of Cas has previously been shown to occur in a cell cycle regulated matter. Treatment of NIH3T3 cells with okadaic acid, a PP2A inhibitor, augments the serine/threonine phosphorylation of Cas that occurs at mitosis. Furthermore, PP2A dephosphorylates serine residues on Cas in vitro. Taken together, our results suggest that PP2A may be involved in the cell cycle-specific dephosphorylation of Cas.

p50(Cdc37) can buffer the temperature-sensitive properties of a mutant of Hck

Genetic studies have previously revealed that Cdc37p is required for the catalytic competence of v-Src in yeast. We have reasoned that temperature-sensitive mutants of Src family kinases might be more sensitive to the cellular level of p50(Cdc37), the mammalian homolog of Cdc37p, than their wild-type counterpart, thus potentially providing a unique opportunity to elucidate the involvement of p50(Cdc37) in the folding and stabilization of Src family kinases. A temperature-sensitive mutant of a constitutively active form of Hck (i.e., tsHck499F) was created by mutating two amino acids within the kinase domain of Hck499F. Significantly, overexpression of p50(Cdc37) rescues the catalytic activity of tsHck499F at 33 degrees C, while partially buffering it against inactivation at higher temperatures (e.g., 37 and 39 degrees C). Hsp90 function is required for tsHck499F activity and its stabilization by p50(Cdc37), but overexpression of Hsp90 is not sufficient to stabilize tsHck499F. Overexpression of p50(Cdc37) promotes the association of tsHck499F with Hsp90, suggesting that the cellular level of p50(Cdc37) might be the rate-limiting step in the association of tsHck499F with Hsp90. A truncation mutant of p50(Cdc37) that cannot bind Hsp90 still has a limited capacity to rescue the catalytic activity of tsHck499F and promote its association with Hsp90. This is a particularly important observation, since it argues that rather than solely acting as a passive adapter protein to tether tsHck499F to Hsp90, p50(Cdc37) may also act allosterically to enhance the association of tsHck499F with Hsp90. The findings presented here might also have implications for our understanding of the evolution of protein kinases and tumor development.

Rap2 as a slowly responding molecular switch in the Rap1 signaling cascade

Rap2 is a member of the Ras family of GTPases and exhibits 60% identity to Rap1, but the function and regulation of Rap2 remain obscure. We found that, unlike the other Ras family proteins, the GTP-bound active form exceeded 50% of total Rap2 protein in adherent cells. Guanine nucleotide exchange factors (GEFs) for Rap1, C3G, Epac (or cyclic AMP [cAMP]-GEF), CalDAG-GEFI, PDZ-GEF1, and GFR efficiently increased the level of GTP-Rap2 both in 293T cells and in vitro. GTPase-activating proteins (GAPs) for Rap1, rap1GAPII and SPA-1, stimulated Rap2 GTPase, but with low efficiency. The half-life of GTP-Rap2 was significantly longer than that of GTP-Rap1 in 293T cells, indicating that low sensitivity to GAPs caused a high GTP/GDP ratio on Rap2. Rap2 bound to the Ras-binding domain of Raf and inhibited Ras-dependent activation of Elk1 transcription factor, as did Rap1. The level of GTP-Rap2 in rat 3Y1 fibroblasts was decreased by the expression of v-Src, and expression of a GTPase-deficient Rap2 mutant inhibited v-Src-dependent transformation of 3Y1 cells. Altogether, Rap2 is regulated by a similar set of GEFs and GAPs as Rap1 and functions as a slowly responding molecular switch in the Rap1 signaling cascade.

Down-regulation of the extracellular matrix protein SPARC in vSrc- and vJun-transformed chick embryo fibroblasts contributes to tumor formation in vivo

In vitro transformation of primary cultures of chick embryo fibroblasts by the membrane-bound vSrc or the nuclear vJun oncoproteins is correlated with a down-regulation of the secreted glycoprotein SPARC (also called BM-40 or osteonectin). This protein is a nonstructural component of the extracellular matrix that is thought to regulate cell-matrix interaction during development, wound repair, and carcinogenesis. Its precise function remains unclear. To estimate the contribution of SPARC down-regulation to the major aspects of the transformed phenotype, we have reexpressed this protein from a self-replicating retrovirus Rcas, designated R-SPARC, in the transformed cultures. These R-SPARC-infected cultures display the following main properties: (i) they accumulate the SPARC protein to a level identical to or only slightly higher than the level in normal chick embryo fibroblasts; (ii) they retain the main phenotypic properties characteristic of in vitro transformed cells, that is, altered morphology, capacity to grow in a reduced amount of serum, and capacity to develop colonies from single cells in agar; (iii) they display a clearly reduced capacity to develop local fibrosarcomas in vivo. Taken together, these data strongly suggest that down-regulation of SPARC contributes to the transformed phenotype triggered by vSrc and vJun in primary avian fibroblasts, by facilitating in vivo tumorigenesis.

Autocrine growth and anchorage independence: two complementing Jun-controlled genetic programs of cellular transformation

Cellular transformation can be achieved by constitutive activation of growth-regulatory signaling pathways, which, in turn, activate nuclear transcription factors thought to execute a transformation-specific program of gene expression. Members of the dimeric transcription factor family AP-1 are at the receiving end of such growth-regulating pathways and the viral form of the AP-1 subunit Jun establishes one important aspect of transformation in chick embryo fibroblasts (CEFs): enhanced growth in agar and in low serum. Enhanced Jun activity is likely to target several different genetic programs as Jun forms heterodimers with one of several members of the Fos and ATF2 subfamilies, resulting in transcription factors with different sequence specificities. To identify the programs relevant for transformation, we have reduced the complexity of AP-1 factors by constructing Jun bZip mutants that can efficiently dimerize and transactivate with only a restricted set of partner subunits. Upon introduction into CEFs, a Jun mutant selective for the Fos family induced anchorage-independent growth but no growth factor-independence. In contrast, a c-Jun mutant with preference for ATF2-like proteins caused growth factor-independence, but no growth in agar. Coexpression of both mutants reestablished the combined transformation program as induced by wild-type Jun. These data show that Jun-dependent cell transformation can be resolved into at least two distinct and independent processes, anchorage and growth factor independence, obviously triggered by two classes of Jun heterodimers likely regulating different sets of target genes.

NSAID-induced apoptosis in Rous sarcoma virus-transformed chicken embryo fibroblasts is dependent on v-src and c-myc and is inhibited by bcl-2

Mounting epidemiological and experimental evidence implicates non-steroidal antiinflammatory drugs as anti-tumorigenic agents. Our previous work showed that nonsteroidal antiinflammatory drug treatment of src-transformed chicken embryo fibroblasts caused apoptosis--a mechanism by which these drugs might exert their anti-tumorigenic effect. The present studies employ a sensitive technique for detecting single- and double-stranded DNA cleavage (the comet assay) to quantitate apoptosis. By this method pp60v-src, which antagonizes apoptosis in many cell systems, was found to induce apoptosis in 11-23% of serum-starved fibroblasts. However, treatment with diclofenac following pp60v-src activation produced a much stronger response beginning within 6 hours of treatment that resulted in 100% lethality. During cell death, cyclooxygenase-2 but not cyclooxygenase-1 mRNA was found to be uniformly increased by all apoptotic drugs tested. Examination of the expression of apoptosis-associated genes showed that c-rel and p53 (found in normal or v-src-transformed chicken embryo fibroblasts at moderate levels), and bcl-2 (present at an extremely low level) were largely unchanged by treatment with eight different nonsteroidal antiinflammatory drugs. However, overexpression of human bcl-2 inhibited diclofenac-mediated apoptosis by 90%, demonstrating directly that bcl-2 expression can regulate nonsteroidal antiinflammatory drug induction of cell death. The proto-oncogene c-myc is known to cause apoptosis in chicken embryo fibroblasts when artificially overexpressed in cells deprived of trophic factors. We found that nonsteroidal antiinflammatory drug treatment following pp60v-src activation persistently induced myc protein and mRNA by more than 20-fold above that evoked by pp60v-src activation alone. Moreover, transfection of antisense c-myc oligonucleotides reduced drug-induced myc expression by 80% and caused a concomitant 50% reduction in cell death. These findings suggest that nonsteroidal antiinflammatory drug-induced apoptosis proceeds through a src/myc dependent pathway which is negatively regulated by bcl-2.

Transcriptional down regulation of the nov proto-oncogene in fibroblasts transformed by p60v-src

We have sought to identify genes whose expression is altered as a consequence of transformation by p60v-src. Using the mRNA differential display method, we have identified the nov proto-oncogene as one gene that is down regulated in chicken embryo fibroblasts (CEFs) transformed by p60v-src. nov transcripts were also found to be present at only very low levels in proliferating CEFs in comparison with quiescent CEFs. Serum stimulation of quiescent CEFs also resulted in a decline in the steady-state level of nov transcripts. Taken together, these findings suggest that the nov gene is expressed only in quiescent fibroblasts and that its down regulation may contribute to cellular transformation by the v-src oncogene. Down regulation of the nov gene appears to occur at both the transcriptional and posttranscriptional levels. Results obtained from experiments with a protein kinase inhibitor suggest that protein kinase C may be a key downstream effector in mediating the down regulation of nov transcripts in response to activation of p60src or serum stimulation. In addition, we found that transcription of an unknown gene is required for the decline in the steady-state level of nov transcripts in response to serum stimulation.

Nonsteroidal antiinflammatory drugs cause apoptosis and induce cyclooxygenases in chicken embryo fibroblasts

Programmed cell death (apoptosis) is an intrinsic part of organismal development and aging. Here we report that many nonsteroidal antiinflammatory drugs (NSAIDs) cause apoptosis when applied to v-src-transformed chicken embryo fibroblasts (CEFs). Cell death was characterized by morphological changes, the induction of tissue transglutaminase, and autodigestion of DNA. Dexamethasone, a repressor of cyclooxygenase (COX) 2, neither induced apoptosis nor altered the NSAID effect. Prostaglandin E2, the primary eicosanoid made by CEFs, also failed to inhibit apoptosis. Expression of the protooncogene bcl-2 is very low in CEFs and is not altered by NSAID treatment. In contrast, p20, a protein that may protect against apoptosis when fibroblasts enter G0 phase, was strongly repressed. The NSAID concentrations used here transiently inhibit COXs. Nevertheless, COX-1 and COX-2 mRNAs and COX-2 protein were induced. In some cell types, then, chronic NSAID treatment may lead to increased, rather than decreased, COX activity and, thus, exacerbate prostaglandin-mediated inflammatory effects. The COX-2 transcript is a partially spliced and nonfunctional form previously described. Thus, these findings suggest that COXs and their products play key roles in preventing apoptosis in CEFs and perhaps other cell types.

Isolation and characterization of a novel mitogenic regulatory gene, 322, which is transcriptionally suppressed in cells transformed by src and ras

In an attempt to isolate novel regulatory and/or tumor suppressor genes, we identified cDNAs whose abundance is low in NIH 3T3 cells and further decreased following the expression of the activated oncogene, v-src. The transcription of one such gene, 322, is suppressed at least 15-fold in src-, ras-, and fos-transformed cells and 3-fold in myc-transformed cells but is unaffected in raf-, mos-, or neu-transformed cells. Activation of a ts-v-src allele in confluent 3Y1 fibroblasts resulted in an initial increase in 322 mRNA levels after 1 to 2 h followed by a rapid decrease to suppressed levels after 4 to 8 h. Morphological transformation was not detected until 12 h later, indicating that the accumulation of 322 transcripts is regulated by v-src and not as a consequence of transformation. Addition of fetal calf serum to starved subconfluent NIH 3T3 or 3Y1 fibroblasts resulted in a similar biphasic regulation of 322, indicating that 322 transcription is responsive to mitogenic factors. Sequence analysis of a putative full-length 322 cDNA clone (5.4 kb) identified a large open reading frame (ORF) encoding a 148.1-kDa product. In vitro transcription and translation of the 322 cDNA from a T7 promoter resulted in a 207-kDa product whose electrophoretic mobility on a sodium dodecyl sulfate-polyacrylamide gel electrophoresis gel was unaffected by digestion with endoglycosidase F. The discrepancy in predicted versus measured molecular weights may result from the high percentage of acidic residues (roughly 20% Glu or Asp) in the 322 ORF product. Comparison of the 322 cDNA ORF with sequences in data banks indicates that this gene is novel. The 322 ORF product contains a potential Cys-1-His-3 Zn finger, at least five nuclear localization signals of the adenovirus E1a motif K(R/K)X(R/K), and alternating acidic and basic domains. Overexpression of the 322 cells resulted in the selection of rapidly growing cells which had lost the transduced 322 cDNA. Thus, 322 represent a novel src- and ras-regulated gene which encodes a potential regulator of mitogenesis and/or tumor suppressor.

Activation of YRP kinase by v-Src and protein kinase C-mediated signal transduction pathways

We have previously reported that a serine(threonine) protein kinase that phosphorylates histone H1 in vitro is activated by tyrosine phosphorylation in v-Src-transformed rat 3Y1 fibroblasts. We now refer to this kinase as YRP kinase, for tyrosine-regulated protein kinase. Since YRP kinase may play a role in mediating the growth-stimulatory and morphology-altering effects of v-Src, we have further examined the signal transduction involved in the activation of YRP kinase. Although YRP kinase is constitutively activated in fibroblasts transformed by v-Src, activation of protein kinase C was also found to lead to activation of YRP kinase. Activation of YRP kinase by protein kinase C was found to be potentiated by vanadate treatment or overexpression of c-Src. The activation of YRP kinase by v-Src, however, does not appear to be mediated by protein kinase C, suggesting that YRP kinase can be activated by two separate signal transduction pathways. Transformation of fibroblasts by v-Ras or v-Mil did not result in activation of YRP kinase, indicating that the MAP kinase pathway does not mediate the activation of YRP kinase by v-Src or protein kinase C.

Mutations in v-Src SH3 and catalytic domains that jointly confer temperature-sensitive transformation with minimal temperature-dependent changes in cellular tyrosine phosphorylation

We have analyzed two functionally significant amino acid alterations encoded by the temperature-sensitive (ts) v-src mutant of Rous sarcoma virus, LA32. The G-to-V change at residue 300 in the catalytic domain nonconditionally impairs morphological transformation, in vitro kinase activity, in vivo tyrosine phosphorylation, and the cytoskeletal association of v-Src while rendering anchorage- and serum-independent growth ts. The R-to-P mutation in the SH3 domain subtly enhances morphological transformation but has no phenotype if the catalytic domain is inactivated. In the presence of the G-300-to-V mutation, this SH3 domain lesion does not affect v-Src in vitro kinase activity and cytoskeletal association, but it nonconditionally enhances cellular tyrosine phosphorylation and restores morphological transformation at the permissive temperature only. This ability to induce a ts transformed morphology, in concert with nonconditional elevations of cellular phosphotyrosine, suggest that a subset of v-Src targets that are crucial to transformation may be affected in ts fashion by the SH3 mutation. Consistent with this, we find that the R-107-to-P mutation confers ts activity and tyrosine phosphorylation on the SH3-binding enzyme phosphatidylinositol 3'-kinase. Thus, both the SH3 and catalytic domain mutations in LA32 have some ts attributes and they cooperate in determining the mutant's behavior. The ts SH3 mutation is unique and offers the potential for deeper understanding of the function of this domain.

Ala-->Gly mutation in the putative catalytic loop confers temperature sensitivity on Ros, insulin receptor, and insulin-like growth factor I receptor protein-tyrosine kinases

Temperature-sensitive mutations in the avian sarcoma virus UR2 oncogene ros, encoding a receptor protein-tyrosine kinase (PTK), were identified. The Ala385-->Gly change mapping within the highly conserved RDLAARN motif in the Ros kinase domain was responsible for the temperature-sensitive phenotype. Based on the sequence homology of all known protein kinases and the crystalline structure of the cAMP-dependent protein kinase, this conserved region probably represents the PTK catalytic loop. The same mutation when introduced into the human insulin and insulin-like growth factor I receptors made these PTKs temperature sensitive in both biological function and kinase activity. Our results support the presumed catalytic role of this highly conserved sequence in PTKs. Due to its highly conserved nature, we predict that the same mutation would probably confer temperature sensitivity on other PTKs.

Transcriptional downregulation of the retina-specific QR1 gene by pp60v-src and identification of a novel v-src-responsive unit

The embryonic avian neuroretina (NR) is part of the central nervous system and is composed of various cell types: photoreceptors and neuronal and Müller (glial) cells. These cells are derived from proliferating neuroectodermal precursors which differentiate after terminal mitosis and become organized in cell strata. Proliferation of differentiating NR cells can be induced by infection with Rous sarcoma virus (RSV) and requires the expression of a functional v-src gene. To understand the mechanisms involved in the regulation of neural cell growth and differentiation, we studied the transcriptional regulation of QR1, a gene specifically expressed in postmitotic NR cells. Transcription of this gene is detected primarily in Müller cells and is strongly downregulated by the v-src gene product. Moreover, QR1 expression takes place only during the late phase of retinal development and is shut off abruptly at hatching. We have isolated a promoter region(s) of the QR1 gene that confers v-src responsiveness. By transfection of QR1-CAT constructs into quail NR cells infected with the temperature-sensitive mutant of RSV, PA101, we have identified a v-src-responsive region located between -1208 and -1161 upstream of the transcription initiation site. This sequence is able to form two DNA-protein complexes, C1 and C2. Formation of complex C2 is specifically induced in cells expressing an active v-src product, while formation of C1 is detected mainly in nonproliferating quail NR cells upon pp60v-src inactivation. C1 is also a target for regulation during development. We have identified the DNA binding site for the C1 complex, a repeated GCTGAC sequence, and shown that mutations in this element abolish binding of this factor as well as transcription of the gene at the nonpermissive temperature. Neither formation of C1 nor that of C2 seems to involve factors known to be targeted in the pp60v-src cascade. Our data suggest that C1 could be a novel target for both developmental control and oncogene-induced cell growth regulation.

Transcriptional downregulation of the retina-specific QR1 gene by pp60v-src and identification of a novel v-src-responsive unit

The embryonic avian neuroretina (NR) is part of the central nervous system and is composed of various cell types: photoreceptors and neuronal and Müller (glial) cells. These cells are derived from proliferating neuroectodermal precursors which differentiate after terminal mitosis and become organized in cell strata. Proliferation of differentiating NR cells can be induced by infection with Rous sarcoma virus (RSV) and requires the expression of a functional v-src gene. To understand the mechanisms involved in the regulation of neural cell growth and differentiation, we studied the transcriptional regulation of QR1, a gene specifically expressed in postmitotic NR cells. Transcription of this gene is detected primarily in Müller cells and is strongly downregulated by the v-src gene product. Moreover, QR1 expression takes place only during the late phase of retinal development and is shut off abruptly at hatching. We have isolated a promoter region(s) of the QR1 gene that confers v-src responsiveness. By transfection of QR1-CAT constructs into quail NR cells infected with the temperature-sensitive mutant of RSV, PA101, we have identified a v-src-responsive region located between -1208 and -1161 upstream of the transcription initiation site. This sequence is able to form two DNA-protein complexes, C1 and C2. Formation of complex C2 is specifically induced in cells expressing an active v-src product, while formation of C1 is detected mainly in nonproliferating quail NR cells upon pp60v-src inactivation. C1 is also a target for regulation during development. We have identified the DNA binding site for the C1 complex, a repeated GCTGAC sequence, and shown that mutations in this element abolish binding of this factor as well as transcription of the gene at the nonpermissive temperature. Neither formation of C1 nor that of C2 seems to involve factors known to be targeted in the pp60v-src cascade. Our data suggest that C1 could be a novel target for both developmental control and oncogene-induced cell growth regulation.

Creation and characterization of temperature-sensitive mutants of the lck tyrosine protein kinase

Temperature-sensitive mutants of the lck tyrosine protein kinase were created by the introduction of mutations known to cause temperature sensitivity of the v-src tyrosine protein kinase of Rous sarcoma virus. p56lck activated by mutation of the regulatory site of tyrosine phosphorylation, Tyr-505, to Phe transforms fibroblasts in culture. Mutations identical to those responsible for the temperature-sensitive phenotypes of the tsNY68 and tsNY72-4 v-src mutants rendered this activated lck gene temperature sensitive for both morphological transformation and induction of growth in soft agar. The mutant proteins were incapable of cellular transformation at the nonpermissive temperature in part because of failure of the lck protein to accumulate to normal levels. Morphological transformation of fibroblasts was detectable within 24 h of a shift of cells to the permissive temperature and was essentially complete in 48 to 72 h. These mutants should prove useful for the study of the function of the lck kinase in hematopoietic cells.

Small deletion in v-src SH3 domain of a transformation defective mutant of Rous sarcoma virus restores wild type transforming properties

RSV mutant virus PA101T was obtained while assaying the tumorigenicity of parental PA101 virus in chickens. PA101 is a transformation defective mutant of RSV which has a low src kinase activity. However, PA101 retained a temperature-sensitive ability to induce sustained proliferation of neuroretina cells. PA101T appeared as a wild-type phenotype revertant of PA101. Molecular cloning and sequencing of PA101T showed that this reversion is due to additional mutations in PA101 src gene. These mutations are a deletion eliminating three amino acids in the N-terminal region of SH3 domain and mutation of Ala 426 to Val. Analysis of the properties of chimeric src genes associating either half of PA101T with the complementary regions of PA101 or wild-type virus showed that the N-terminal moiety of PA101T src, which contains the deletion, confers wild-type transforming properties, whereas its C-terminal moiety, which contains single amino acid mutation, confers a partially temperature-sensitive phenotype. These results are consistent with other reports showing that mutations or deletions in this region of SH3 activate the transforming potential of c-src. They support the hypothesis that the N-terminal region of SH3 interacts with a cellular negative regulator of src activity.

Mutations in the SH3 domain of the src oncogene which decrease association of phosphatidylinositol 3'-kinase activity with pp60v-src and alter cellular morphology

To analyze the signaling pathways utilized in malignant transformation by pp60v-src, we have isolated and characterized src mutants which possess normal levels of protein tyrosine kinase activity but which cause only a partially transformed phenotype. Our hypothesis is that such mutants are partially defective for transformation because they are defective in their ability to activate specific components of the cellular signaling machinery while still activating others. In this communication, we report on the molecular and biochemical characterization of one such mutant, CU12 (D. D. Anderson, R. P. Beckmann, E. H. Harms, K. Nakamura, and M. J. Weber, J. Virol. 37:455-458, 1981). Cells infected with this mutant are capable of anchorage-independent growth, but rather than exhibiting the rounded and refractile morphology characteristic of wild-type-infected cells, they display an extremely elongated, fusiform morphology. The morphological properties of this mutant src could be accounted for entirely by a single mutation in the SH3 domain (lysine 106 to glutamate). Other mutations were constructed in this region by in vitro mutagenesis, both in a v-src and in an activated c-src background, and several of them also induced a fusiform morphology. All of the mutations inducing fusiform morphology also resulted in decreased association of pp60src with phosphatidylinositol 3'-kinase activity. In addition, association of pp60src with some tyrosine-phosphorylated proteins was altered. We propose that the SH3 domain participates (along with the SH2 domain) in the interaction of pp60src with cellular signaling proteins, and we speculate that the association with phosphatidylinositol 3'-kinase plays an important role in the regulation of cellular morphology.

Signal transduction by nerve growth factor and fibroblast growth factor in PC12 cells requires a sequence of src and ras actions

We have investigated the roles of pp60c-src and p21c-ras proteins in transducing the nerve growth factor (NGF) and fibroblast growth factor (FGF) signals which promote the sympathetic neuronlike phenotype in PC12 cells. Neutralizing antibodies directed against either Src or Ras proteins were microinjected into fused PC12 cells. Each antibody both prevented and reversed NGF- or FGF-induced neurite growth, a prominent morphological marker for the neuronal phenotype. These data demonstrate the involvement of both pp60c-src and p21c-ras proteins in NGF and FGF actions in PC12 cells, and establish a physiological role for the pp60c-src tyrosine kinase in signal transduction pathways initiated by receptor tyrosine kinases in these cells. Additional microinjection experiments, using PC12 transfectants containing inducible v-src or ras oncogene activities, demonstrated a specific sequence of Src and Ras actions. Microinjection of anti-Ras antibody blocked v-src-induced neurite growth, but microinjection of anti-Src antibodies had no effect on ras oncogene-induced neurite growth. We propose that a cascade of Src and Ras actions, with Src acting first, is a significant feature of the signal transduction pathways for NGF and FGF. The Src-Ras cascade may define a functional cassette in the signal transduction pathways used by growth factors and other ligands whose receptors have diverse structures and whose range of actions on various cell types include mitogenesis and differentiation.

Induction of neurite outgrowth by v-src mimics critical aspects of nerve growth factor-induced differentiation

PC12 cells treated with nerve growth factor (NGF) or infected with Rous sarcoma virus differentiate into sympathetic, neuronlike cells. To compare the differentiation programs induced by NGF and v-src, we have established a PC12 cell line expressing a temperature-sensitive v-src protein. The v-src-expressing PC12 cell line was shown to elaborate neuritic processes in a temperature-inducible manner, indicating that the differentiation process was dependent on the activity of the v-src protein. Further characterization of this cell line, in comparison with NGF-treated PC12 cells, indicated that the events associated with neurite outgrowth induced by these two agents shared features but could be distinguished by others. Both NGF- and v-src-induced neurite outgrowths were reversible. In addition, NGF and v-src could prime PC12 cells for NGF-induced neurite outgrowth, and representative early and late NGF-responsive genes were also induced by v-src. However, unlike NGF-induced neurite growth, v-src-induced neurite outgrowth was not blocked at high cell density. A comparison of phosphotyrosine containing-protein profiles showed that v-src and NGF each increase tyrosine phosphorylation of multiple cellular proteins. There was overlap in substrates; however, both NGF-specific and v-src-specific tyrosine phosphorylations were observed. One protein which was found to be phosphorylated in both the NGF- and v-src-induced PC12 cells was phospholipase C-gamma 1. Taken together, these results suggest that v-src's ability to function as an inducing agent may be a consequence of its ability to mimic critical aspects of the NGF differentiation program and raise the possibility that Src-like tyrosine kinases are involved in mediating some of the events triggered by NGF.

Induction of neurite outgrowth by v-src mimics critical aspects of nerve growth factor-induced differentiation

PC12 cells treated with nerve growth factor (NGF) or infected with Rous sarcoma virus differentiate into sympathetic, neuronlike cells. To compare the differentiation programs induced by NGF and v-src, we have established a PC12 cell line expressing a temperature-sensitive v-src protein. The v-src-expressing PC12 cell line was shown to elaborate neuritic processes in a temperature-inducible manner, indicating that the differentiation process was dependent on the activity of the v-src protein. Further characterization of this cell line, in comparison with NGF-treated PC12 cells, indicated that the events associated with neurite outgrowth induced by these two agents shared features but could be distinguished by others. Both NGF- and v-src-induced neurite outgrowths were reversible. In addition, NGF and v-src could prime PC12 cells for NGF-induced neurite outgrowth, and representative early and late NGF-responsive genes were also induced by v-src. However, unlike NGF-induced neurite growth, v-src-induced neurite outgrowth was not blocked at high cell density. A comparison of phosphotyrosine containing-protein profiles showed that v-src and NGF each increase tyrosine phosphorylation of multiple cellular proteins. There was overlap in substrates; however, both NGF-specific and v-src-specific tyrosine phosphorylations were observed. One protein which was found to be phosphorylated in both the NGF- and v-src-induced PC12 cells was phospholipase C-gamma 1. Taken together, these results suggest that v-src's ability to function as an inducing agent may be a consequence of its ability to mimic critical aspects of the NGF differentiation program and raise the possibility that Src-like tyrosine kinases are involved in mediating some of the events triggered by NGF.

Analysis of cDNAs of the proto-oncogene c-src: heterogeneity in 5' exons and possible mechanism for the genesis of the 3' end of v-src

To further characterize the gene structure of the proto-oncogene c-src and the mechanism for the genesis of the v-src sequence in Rous sarcoma virus, we have analyzed genomic and cDNA copies of the chicken c-src gene. From a cDNA library of chicken embryo fibroblasts, we isolated and sequenced several overlapping cDNA clones covering the full length of the 4-kb c-src mRNA. The cDNA sequence contains a 1.84-kb sequence downstream from the 1.6-kb pp60c-src coding region. An open reading frame of 217 amino acids, called sdr (src downstream region), was found 105 nucleotides from the termination codon for pp60c-src. Within the 3' noncoding region, a 39-bp sequence corresponding to the 3' end of the RSV v-src was detected 660 bases downstream of the pp60c-src termination codon. The presence of this sequence in the c-src mRNA exon supports a model involving an RNA intermediate during transduction of the c-src sequence. The 5' region of the c-src cDNA was determined by analyzing several cDNA clones generated by conventional cloning methods and by polymerase chain reaction. Sequences of these chicken embryo fibroblast clones plus two c-src cDNA clones isolated from a brain cDNA library show that there is considerable heterogeneity in sequences upstream from the c-src coding sequence. Within this region, which contains at least 300 nucleotides upstream of the translational initiation site in exon 2, there exist at least two exons in each cDNA which fall into five cDNA classes. Four unique 5' exon sequences, designated exons UE1, UE2, UEX, and UEY, were observed. All of them are spliced to the previously characterized c-src exons 1 and 2 with the exception of type 2 cDNA. In type 2, the exon 1 is spliced to a novel downstream exon, designated exon 1a, which maps in the region of the c-src DNA defined previously as intron 1. Exon UE1 is rich in G+C content and is mapped at 7.8 kb upstream from exon 1. This exon is also present in the two cDNA clones from the brain cDNA library. Exon UE2 is located at 8.5 kb upstream from exon 1. The precise locations of exons UEX and UEY have not been determined, but both are more than 12 kb upstream from exon 1. The existence and exon arrangements of these 5' cDNAs were further confirmed by RNase protection assays and polymerase chain reactions using specific primers. Our findings indicate that the heterogeneity in the 5' sequences of the c-src mRNAs results from differential splicing and perhaps use of distinct initiation sites. All of these RNAs have the potential of coding for pp60c-src, since their 5' exons are all eventually joined to exon 2.

Analysis of cDNAs of the proto-oncogene c-src: heterogeneity in 5' exons and possible mechanism for the genesis of the 3' end of v-src

To further characterize the gene structure of the proto-oncogene c-src and the mechanism for the genesis of the v-src sequence in Rous sarcoma virus, we have analyzed genomic and cDNA copies of the chicken c-src gene. From a cDNA library of chicken embryo fibroblasts, we isolated and sequenced several overlapping cDNA clones covering the full length of the 4-kb c-src mRNA. The cDNA sequence contains a 1.84-kb sequence downstream from the 1.6-kb pp60c-src coding region. An open reading frame of 217 amino acids, called sdr (src downstream region), was found 105 nucleotides from the termination codon for pp60c-src. Within the 3' noncoding region, a 39-bp sequence corresponding to the 3' end of the RSV v-src was detected 660 bases downstream of the pp60c-src termination codon. The presence of this sequence in the c-src mRNA exon supports a model involving an RNA intermediate during transduction of the c-src sequence. The 5' region of the c-src cDNA was determined by analyzing several cDNA clones generated by conventional cloning methods and by polymerase chain reaction. Sequences of these chicken embryo fibroblast clones plus two c-src cDNA clones isolated from a brain cDNA library show that there is considerable heterogeneity in sequences upstream from the c-src coding sequence. Within this region, which contains at least 300 nucleotides upstream of the translational initiation site in exon 2, there exist at least two exons in each cDNA which fall into five cDNA classes. Four unique 5' exon sequences, designated exons UE1, UE2, UEX, and UEY, were observed. All of them are spliced to the previously characterized c-src exons 1 and 2 with the exception of type 2 cDNA. In type 2, the exon 1 is spliced to a novel downstream exon, designated exon 1a, which maps in the region of the c-src DNA defined previously as intron 1. Exon UE1 is rich in G+C content and is mapped at 7.8 kb upstream from exon 1. This exon is also present in the two cDNA clones from the brain cDNA library. Exon UE2 is located at 8.5 kb upstream from exon 1. The precise locations of exons UEX and UEY have not been determined, but both are more than 12 kb upstream from exon 1. The existence and exon arrangements of these 5' cDNAs were further confirmed by RNase protection assays and polymerase chain reactions using specific primers. Our findings indicate that the heterogeneity in the 5' sequences of the c-src mRNAs results from differential splicing and perhaps use of distinct initiation sites. All of these RNAs have the potential of coding for pp60c-src, since their 5' exons are all eventually joined to exon 2.

Transcription of a quail gene expressed in embryonic retinal cells is shut off sharply at hatching

The avian neuroretina (NR) is part of the central nervous system and is composed of photoreceptors, neuronal cells, and Müller (glial) cells. These cells are derived from proliferating neuroectodermal precursors that differentiate after terminal mitosis and become organized in cell strata. Genes that are specifically expressed at the various stages of retinal development are presently unknown. We have isolated a quail (Coturnix coturnix japonica) cDNA clone, named QR1, encoding a 676-amino acid protein whose carboxyl-terminal portion shows significant similarity to those of the extracellular glycoprotein osteonectin/SPARC/BM40 and of the recently described SC1 protein. The QR1 cDNA identifies a mRNA detected in NR but not in other embryonic tissues examined. The levels of this mRNA are markedly reduced when nondividing NR cells are induced to proliferate by the v-src oncogene. QR1 expression in NR is limited to the middle portion of the inner nuclear layer, a localization that essentially corresponds to that of Müller cells. Transcription of QR1 takes place only during the late phase of retinal development and is shut off sharply at hatching. Signals that regulate this unique pattern of expression appear to originate within the NR, since the QR1 mRNA is transcribed in cultured NR cells and is shut off also in vitro at a time coinciding with hatching.

The v-src inducible gene 9E3/pCEF4 is regulated by both its promoter upstream sequence and its 3' untranslated region

The 9E3/pCEF4 mRNA is strongly induced in Rous sarcoma virus-transformed chicken embryo fibroblasts when compared to untransformed cells. To identify cis-acting transcriptional elements that confer inducibility by v-src, we isolated the 9E3 promoter upstream region. We found that 1.53 kilobases upstream of the transcriptional start site, when placed in front of a reporter gene, conferred a small degree of inducibility by v-src, in both transient and stable transfections. Two potential AP-1 sites were identified in the 9E3 promoter. AP-1 elements have been implicated previously in mediating a transcriptional response to v-src in fibroblast cell lines. These elements alone do not confer a significant inducibility by v-src in primary chicken embryo fibroblasts. Since the 9E3 mRNA is stabilized in transformed cells, we replaced the 3' untranslated region of the reporter gene with the 9E3 3' untranslated region and found this construct to be strongly responsive to stimulation by v-src. In addition, the 9E3 3' untranslated region increased the response to serum and the tumor promoter phorbol 12-myristate 13-acetate. This suggests that a posttranscriptional mechanism plays a major role in the induction of 9E3 expression.

Expression of 9E3 mRNA is associated with mitogenicity, phosphorylation, and morphological alteration in chicken embryo fibroblasts

Transformation of chicken embryo fibroblasts (CEF) with viruses encoding src, ros, yes, and fps as well as ras, mos, middle T, erbA and erbB, myc, and crk stimulated 9E3 mRNA expression. Treatment of CEF with agents that modulate cell shape or attachment to the substratum caused an increase in 9E3 mRNA without an increase in tyrosine phosphorylation. 9E3 mRNA was also increased in CEF in response to several agents which modulate phosphorylation, including phorbol myristic acetate, vanadate, and okadaic acid, which suggests that the rapid induction of 9E3 mRNA expression in CEF by the src protein occurs downstream of morphological or phosphorylation events.

Down regulation by p60v-src of genes specifically expressed and developmentally regulated in postmitotic quail neuroretina cells

The avian neuroretina (NR) is composed of photoreceptors and different neurons that are derived from proliferating precursor cells. Neuronal differentiation takes place after terminal mitosis. We have previously shown that differentiating NR cells can be induced to proliferate by infection with Rous sarcoma virus (RSV) and that cell multiplication requires expression of a functional v-src gene. We speculated that the quiescence of NR cells could be determined by specific genes. Cell proliferation could then result from the negative regulation of these genes by the v-src protein. By differential hybridization of a cDNA library, we isolated eight clones corresponding to genes expressed in postmitotic NR cells from 13-day-old quail embryos, transcriptional levels of which are significantly reduced in NR cells induced to proliferate by tsNY68, an RSV mutant with temperature-sensitive mitogenic activity. Partial sequencing analysis indicated that one RNA encoded the calmodulin gene, whereas the other seven showed no similarity to known sequences. By using v-src mutants that induce NR cell proliferation in the absence of transformation, we showed that transcription of six genes was negatively regulated by the v-src protein and that of four genes was correlated with NR cell quiescence. We also report that a subset of genes are specifically transcribed in neural cells and developmentally regulated in the NR. These results indicate that the v-src protein regulates expression of genes likely to play a role in the control of neural cell growth or differentiation.

Expression of 9E3 mRNA is associated with mitogenicity, phosphorylation, and morphological alteration in chicken embryo fibroblasts

Transformation of chicken embryo fibroblasts (CEF) with viruses encoding src, ros, yes, and fps as well as ras, mos, middle T, erbA and erbB, myc, and crk stimulated 9E3 mRNA expression. Treatment of CEF with agents that modulate cell shape or attachment to the substratum caused an increase in 9E3 mRNA without an increase in tyrosine phosphorylation. 9E3 mRNA was also increased in CEF in response to several agents which modulate phosphorylation, including phorbol myristic acetate, vanadate, and okadaic acid, which suggests that the rapid induction of 9E3 mRNA expression in CEF by the src protein occurs downstream of morphological or phosphorylation events.

Sequence and functional differences between Schmidt-Ruppin D and Schmidt-Ruppin A strains of pp60v-src

We show that Schmidt-Ruppin D pp60v-src kinase activity is reduced by a mutation previously shown to be associated with Schmidt-Ruppin A pp60v-src temperature sensitivity and that its reduced transforming activity is associated with a conformational change in the SH3 region. The evolutionary relationship of seven v-src strains was studied by using parsimony analysis.

Down regulation by p60v-src of genes specifically expressed and developmentally regulated in postmitotic quail neuroretina cells

The avian neuroretina (NR) is composed of photoreceptors and different neurons that are derived from proliferating precursor cells. Neuronal differentiation takes place after terminal mitosis. We have previously shown that differentiating NR cells can be induced to proliferate by infection with Rous sarcoma virus (RSV) and that cell multiplication requires expression of a functional v-src gene. We speculated that the quiescence of NR cells could be determined by specific genes. Cell proliferation could then result from the negative regulation of these genes by the v-src protein. By differential hybridization of a cDNA library, we isolated eight clones corresponding to genes expressed in postmitotic NR cells from 13-day-old quail embryos, transcriptional levels of which are significantly reduced in NR cells induced to proliferate by tsNY68, an RSV mutant with temperature-sensitive mitogenic activity. Partial sequencing analysis indicated that one RNA encoded the calmodulin gene, whereas the other seven showed no similarity to known sequences. By using v-src mutants that induce NR cell proliferation in the absence of transformation, we showed that transcription of six genes was negatively regulated by the v-src protein and that of four genes was correlated with NR cell quiescence. We also report that a subset of genes are specifically transcribed in neural cells and developmentally regulated in the NR. These results indicate that the v-src protein regulates expression of genes likely to play a role in the control of neural cell growth or differentiation.

Evidence that v-src and v-fps gene products use a protein kinase C-mediated pathway to induce expression of a transformation-related gene

Induction of the transformation-related gene 9E3 by the v-src and v-fps gene products (v-Src and v-Fps) is blocked in chicken embryo fibroblasts depleted of protein kinase C (PKC). PKC agonists induce 9E3 gene expression. Protein kinase inhibitors block v-Src- and v-Fps-induced 9E3 gene expression with the same dose-response curves seen for PKC agonist-induced 9E3 gene expression. These data suggest that v-Src and v-Fps use a PKC-mediated signal-transduction pathway to induce expression of the transformation-related 9E3 gene. Consistent with this hypothesis, we find that both v-Src and v-Fps rapidly induce phosphorylation of a 67-kDa PKC substrate.

Phosphatidylinositol kinase activity associates with viral p60src protein

Immunoprecipitates of p60v-src proteins from chicken embryo fibroblasts infected with Rous sarcoma virus were assayed for phosphatidylinositol (PI) kinase activity in the absence of detergents. The product of the PI kinase reaction, phosphatidylinositol monophosphate (PIP), migrated slightly slower than did the authentic phosphatidylinositol-4-monophosphate marker in thin-layer chromatography and was indistinguishable from phosphatidylinositol-3-monophosphate produced by PI kinase type I. Furthermore, the deacylated product comigrated with glycerophosphoinositol-3-phosphate in high-performance liquid chromatography. Both sucrose gradient fractionation and the heat stability of PI kinase activity from cells infected with temperature-sensitive mutants suggest that the PI kinase activity is not intrinsic to p60v-src but is a property of another molecule complexed with p60v-src. All transforming variants of p60src were associated with PI kinase activity, whereas this enzyme activity was hardly detectable in immunoprecipitates from cells infected with nontransforming viruses encoding p60c-src or an enzymatically inactive variant. However, PI kinase activity was found in p60src immunoprecipitates from cells infected with nonmyristylated, nontransforming mutants as well as temperature-sensitive mutants at the nonpermissive temperature, which indicated that simple association of PI kinase activity with p60src is not sufficient for cell transformation.

Phosphatidylinositol kinase activity associates with viral p60src protein

Immunoprecipitates of p60v-src proteins from chicken embryo fibroblasts infected with Rous sarcoma virus were assayed for phosphatidylinositol (PI) kinase activity in the absence of detergents. The product of the PI kinase reaction, phosphatidylinositol monophosphate (PIP), migrated slightly slower than did the authentic phosphatidylinositol-4-monophosphate marker in thin-layer chromatography and was indistinguishable from phosphatidylinositol-3-monophosphate produced by PI kinase type I. Furthermore, the deacylated product comigrated with glycerophosphoinositol-3-phosphate in high-performance liquid chromatography. Both sucrose gradient fractionation and the heat stability of PI kinase activity from cells infected with temperature-sensitive mutants suggest that the PI kinase activity is not intrinsic to p60v-src but is a property of another molecule complexed with p60v-src. All transforming variants of p60src were associated with PI kinase activity, whereas this enzyme activity was hardly detectable in immunoprecipitates from cells infected with nontransforming viruses encoding p60c-src or an enzymatically inactive variant. However, PI kinase activity was found in p60src immunoprecipitates from cells infected with nonmyristylated, nontransforming mutants as well as temperature-sensitive mutants at the nonpermissive temperature, which indicated that simple association of PI kinase activity with p60src is not sufficient for cell transformation.

Phosphorylation of cellular proteins in Rous sarcoma virus-infected cells: analysis by use of anti-phosphotyrosine antibodies

The protein substrates for the tyrosine protein kinases in cells transformed by avian sarcoma viruses were analyzed by gel electrophoresis in combination with immunoblotting or immunoprecipitation by antibodies against phosphotyrosine. We found that greater than 90% of phosphotyrosine-containing cellular proteins can be immunoprecipitated by these antibodies. The level of phosphotyrosine-containing cellular proteins detectable by this method markedly increased upon transformation with Rous sarcoma virus, and more than 20 distinct bands of such proteins were found in lysates of Rous sarcoma virus-transformed cells. Most of these phosphotyrosine-containing proteins had not been identified by other methods, and their presence appeared to correlate with morphological transformation in cells infected with various Rous sarcoma virus mutants and Y73, PRCII, and Fujinami sarcoma viruses. However, considerably different patterns were obtained with cells infected with nontransforming Rous sarcoma virus mutants that encode nonmyristylated src kinases, indicating that most substrates that correlate with transformation can only be recognized by p60v-src associated with the plasma membrane.

Identification of a phorbol ester-repressible v-src-inducible gene

Chicken embryo fibroblasts (CEF) infected with a temperature-sensitive Rous sarcoma virus (RSV) mutant, tsNY72-4, express a set of pp60v-src-induced RNAs soon after shift to the permissive temperature. By subtractive and differential screening, we have cloned 12 of these sequences, 2 of which were c-fos and krox-24. Serum induced all the v-src-inducible genes tested, suggesting that these genes serve roles in normal cell division and are not specific to transformation per se. Significantly, however, v-src produced prolonged, and in some cases kinetically complex, patterns of induction compared to serum. For most of the clones, phorbol 12-tetradecanoate 13-acetate (TPA) induced mRNAs with kinetics similar to that of serum. However, one clone (CEF-4) was expressed in a biphasic manner. Another (CEF-10) was repressed by TPA at 1 hr, after which this mRNA was permanently induced. The pattern of repression-induction of CEF-10 mRNA is the inverse of protein kinase C (PKC) activity in the cell, suggesting that PKC actively represses this gene. In vivo expression of CEF-10 mRNA is restricted predominantly to the lung. A full-length CEF-10 cDNA encodes a 41-kDa protein that has an amino-terminal signal peptide for secretion, contains a markedly high number of cysteine residues, and shows no sequence similarity to known proteins.

A mutation in v-src that removes a single conserved residue in the SH-2 domain of pp60v-src restricts transformation in a host-dependent manner

The v-src oncogene of Rous sarcoma virus (RSV) is able to transform both avian and mammalian cells, but the mutant allele v-src-L displays a host range dependence for transformation, transforming chicken but not rat cells with wild-type efficiency. This host range restriction can be detected by measuring growth in low serum, saturation density, and anchorage independent growth. In addition, rat cells expressing v-src-L do not form tumors in syngeneic rats or nude mice, but RSV carrying the mutant allele causes tumors in chicks, although at a reduced efficiency and with increased latency. To determine the lesion responsible for this phenotype, we sequenced the entire v-src gene from the parental B77 strain of RSV, as well as the mutant allele. v-src-L is missing 3 nucleotides present in the wild-type parent, RSV B31, eliminating Phe-172, an invariant residue in a conserved region of src-related proteins known as SH-2. The kinase activity of pp60v-src-L was indistinguishable from that of the wild type in chicken cells but was significantly reduced in rat cells as assayed by an in vitro immune complex assay; in vivo phosphorylation of one specific substrate, p36 (calpactin I heavy chain); and total phosphotyrosine-containing proteins. In addition, the pattern of phosphotyrosine-containing proteins in rat cells was qualitatively different when cells containing pp60v-src-L were compared with cells with wild-type pp60v-src, even though both pp60v-src proteins were membrane associated. The data are consistent with a role for the SH-2 region in substrate specificity.

78-kilodalton glucose-regulated protein is induced in Rous sarcoma virus-transformed cells independently of glucose deprivation

To identify mRNAs with altered expression in Rous sarcoma virus (RSV)-transformed cells, we screened a chicken embryo fibroblast (CEF) cDNA library by differential hybridization. One clone, designated R1H, showed markedly elevated mRNA expression in RSV-transformed cells. Nucleotide sequence analysis indicated that R1H mRNA encodes 78-kilodalton glucose-regulated protein (GRP78). Chicken GRP78 was found to be very highly conserved in comparison with rat GRP78 (96% identity between chicken and rat amino acid sequences). In contrast to previous observations, we found that GRP78 was induced in RSV-transformed cells in the absence of glucose deprivation. When cells were grown in glucose-supplemented medium, the level of GRP78 mRNA was approximately fivefold higher in RSV-transformed CEF than in transformation-defective virus-infected or uninfected CEF. Similar changes in GRP78 protein content were also found. Using a temperature-sensitive mutant of RSV and supplemental glucose, we found a gradual increase in the level of GRP78 mRNA beginning at 4 h after shiftdown to permissive temperature. Uridine supplementation did not block the induction seen in CEF infected with a temperature-sensitive mutant. These results indicate that GRP78 is induced by p60v-src in the absence of glucose deprivation.

Phosphorylation of cellular proteins in Rous sarcoma virus-infected cells: analysis by use of anti-phosphotyrosine antibodies

The protein substrates for the tyrosine protein kinases in cells transformed by avian sarcoma viruses were analyzed by gel electrophoresis in combination with immunoblotting or immunoprecipitation by antibodies against phosphotyrosine. We found that greater than 90% of phosphotyrosine-containing cellular proteins can be immunoprecipitated by these antibodies. The level of phosphotyrosine-containing cellular proteins detectable by this method markedly increased upon transformation with Rous sarcoma virus, and more than 20 distinct bands of such proteins were found in lysates of Rous sarcoma virus-transformed cells. Most of these phosphotyrosine-containing proteins had not been identified by other methods, and their presence appeared to correlate with morphological transformation in cells infected with various Rous sarcoma virus mutants and Y73, PRCII, and Fujinami sarcoma viruses. However, considerably different patterns were obtained with cells infected with nontransforming Rous sarcoma virus mutants that encode nonmyristylated src kinases, indicating that most substrates that correlate with transformation can only be recognized by p60v-src associated with the plasma membrane.

78-kilodalton glucose-regulated protein is induced in Rous sarcoma virus-transformed cells independently of glucose deprivation

To identify mRNAs with altered expression in Rous sarcoma virus (RSV)-transformed cells, we screened a chicken embryo fibroblast (CEF) cDNA library by differential hybridization. One clone, designated R1H, showed markedly elevated mRNA expression in RSV-transformed cells. Nucleotide sequence analysis indicated that R1H mRNA encodes 78-kilodalton glucose-regulated protein (GRP78). Chicken GRP78 was found to be very highly conserved in comparison with rat GRP78 (96% identity between chicken and rat amino acid sequences). In contrast to previous observations, we found that GRP78 was induced in RSV-transformed cells in the absence of glucose deprivation. When cells were grown in glucose-supplemented medium, the level of GRP78 mRNA was approximately fivefold higher in RSV-transformed CEF than in transformation-defective virus-infected or uninfected CEF. Similar changes in GRP78 protein content were also found. Using a temperature-sensitive mutant of RSV and supplemental glucose, we found a gradual increase in the level of GRP78 mRNA beginning at 4 h after shiftdown to permissive temperature. Uridine supplementation did not block the induction seen in CEF infected with a temperature-sensitive mutant. These results indicate that GRP78 is induced by p60v-src in the absence of glucose deprivation.

A single point mutation has pleiotropic effects on pp60v-src function

The Rous sarcoma virus mutant tsLA29 encodes a pp60v-src molecule that is temperature sensitive for both tyrosine kinase activity and its ability to locate at the cell periphery. The defect in localization appears to be due to a perturbation in events following complex dissociation, since the mutant enzyme shows a rapidly reversible association with the cytoskeleton when shifted between permissive and restrictive temperatures. Although tsLA29 pp60v-src differs from the wild type at three amino acid residues, studies with chimeric proteins show that only one of the mutations, an alanine-for-proline substitution at residue 507, accounts for all the temperature-sensitive characteristics. Moreover, a single second site mutation, at residue 427, can restore the wild phenotype. Cells infected with a chimeric virus encoding only the alanine substitution at position 507 have a conspicuously fusiform morphology, suggesting that this mutation also has subtle effects on pp60v-src function that are apparently compensated for by the other mutations in native tsLA29.

A novel viral oncogene with structural similarity to phospholipase C

Numerous oncogenes have been isolated from acutely transforming retroviruses. To date, the products of these viral oncogenes have been protein kinases, nuclear proteins, growth factors, or GTP-binding proteins. We have cloned the previously uncharacterized avian sarcoma virus CT10 and sequenced its genome. This virus encodes a protein, p47gag-crk, that has blocks of sequence similarity to the amino-terminal, non-catalytic region of the non-receptor class of tyrosine kinases. In addition, the structure of p47gag-crk has striking similarity to a 180-amino acid region of bovine brain phospholipase C. Biochemical data suggest that p47gag-crk activates one or several endogenous tyrosine kinases.

Avian sarcoma viruses

Twelve independent isolates of avian sarcoma viruses (ASVs) can be divided into four groups according to the transforming genes harbored in the viral genomes. The first group is represented by viruses containing the transforming sequence, src, inserted in the viral genome as an independent gene; the other three groups of viruses contain transforming genes fps, yes or ros fused to various length of the truncated structural gene gag. These transforming sequences have been obtained by avian retroviruses from chicken cellular DNA by recombination. The src-containing viruses code for an independent polypeptide, p60src; and the representative fps, yes and ros-containing ASVs code for P140/130gag-fps, P90gag-yes and P68gag-ros fusion polypeptides respectively. All of these transforming proteins are associated with the tyrosine-specific protein kinase activity capable of autophosphorylation and phosphorylating certain foreign substrates. p60src and P68gag-ros are integral cellular membrane proteins and P140/130gag-fps and P90gag-yes are only loosely associated with the plasma membrane. Cells transformed by ASVs contain many newly phosphorylated proteins and in most cases have an elevated level of total phosphotyrosine. However, no definitive correlation between phosphorylation of a particular substrate and transformation has been established except that a marked increase of the tyrosine phosphorylation of a 34,000 to 37,000 dalton protein is observed in most ASV transformed cells. The kinase activity of ASV transforming proteins appears to be essential, but not sufficient for transformation. The N-terminal domain of p60src required for myristylation and membrane binding is also crucial for transformation. By contrast, the gag portion of the FSV P130gag-fps is dispensable for in vitro transformation and removal of it has only an attenuating effect on in vivo tumorigenicity. The products of cellular src, fps and yes proto-oncogenes have been identified and shown to also have tyrosine-specific protein kinase activity. The transforming potential of c-src and c-fps has been studied and shown that certain structural changes are necessary to convert them into transforming genes. Among the cellular proto-oncogenes related to the four ASV transforming genes, c-ros most likely codes for a growth factor receptor-like molecule. It is possible that the oncogene products of ASVs act through certain membrane receptor(s) or enzyme(s), such as protein kinase C, in the process of cell transformation.