Regulation of cellular morphology by the Rous sarcoma virus src gene: analysis of fusiform mutants

Remembering Jan Svoboda: A Personal Reflection

The Czech scientist Jan Svoboda was a pioneer of Rous sarcoma virus (RSV). In the 1960s, before the discovery of reverse transcriptase, he demonstrated the long-term persistence of the viral genome in non-productive mammalian cells, and he supported the DNA provirus hypothesis of Howard Temin. He showed how the virus can be rescued in the infectious form and elucidated the replication-competent nature of the Prague strain of RSV later used for the identification of the src oncogene. His studies straddled molecular oncology and virology, and he remained an active contributor to the field until his death last year. Throughout the 50 years that I was privileged to know Svoboda as my mentor and friend, I admired his depth of scientific inquiry and his steadfast integrity in the face of political oppression.

Analysis of the suitability of calreticulin inducible HEK cells for adhesion studies: microscopical and biochemical comparisons

Calreticulin is a Ca(2+)-buffering ER chaperone that also modulates cell adhesiveness. In order to study the effect of calreticulin on the expression of adhesion-related genes, we created a calreticulin inducible Human Embryonic Kidney (HEK) 293 cell line. We found that fibronectin mRNA and both intra- and extra-cellular fibronectin protein levels increased following calreticulin induction. However, despite this increase in fibronectin, HEK293 cells did not assemble an extracellular fibrillar fibronectin matrix regardless of the level of calreticulin expression. Furthermore, HEK293 cells exhibited a poorly organized actin cytoskeleton, did not have clustered fibronectin receptors at the cell surface, and did not form focal contacts. This likely accounts for the lack of fibronectin matrix deposition by these cells regardless of calreticulin expression level. Vinculin abundance did not appreciably increase upon calreticulin induction and the level of active c-Src, a regulatory kinase of focal contacts, was found to be abundant and unregulated by calreticulin induction in these cells. The inability to form stable focal contacts and to commence fibronectin fibrillogenesis due to high c-Src activity may be responsible for the poor adhesive phenotype of HEK 293 cells. Thus, we show here that HEK293 cells are not suitable for microscopical studies of cell-substratum adhesions, but are best suited for biochemical studies.

Partial reversal of transformed fusiform phenotype by overexpression of calreticulin

Calreticulin, a Ca²⁺-storage and chaperone protein of the ER, has also been shown to affect cell adhesiveness. To examine the effects of differential expression of calreticulin on cellular adhesiveness, we used L fibroblast cell lines stably expressing either elevated or reduced amounts of full length, ER-targeted calreticulin. Overexpression of calreticulin correlates with an increase in adhesiveness of L fibroblasts such that these transformed cells acquire epithelioid morphology and form an epithelial-cell sheet when crowded. Functionally, the “reversal” of transformed phenotype in L fibroblasts differentially overexpressing calreticulin can be accounted for by changes in levels of expression of N-cadherin and vinculin. Structurally, however, although the form and extent of cell-cell contacts in L fibroblasts overexpressing calreticulin mimicked those in normal epithelia, electron microscopical examination revealed that cell-cell junctions formed by these transformed cells bore only superficial resemblance to those of normal epithelia in culture. Our data imply that overexpression of calreticulin, while partially reverses fusiform transformed phenotype is in itself insufficient to re-establish bona fide zonulae adherens in transformed fibroblasts.

A mutant form of the rho protein can restore stress fibers and adhesion plaques in v-src transformed fibroblasts

The organization of polymerized actin in the mammalian cell is regulated by several members of the rho family. Three rho proteins, cdc42, rac and rho act in a cascade to organize the intracellular actin cytoskeleton. Rho proteins are involved in the formation of actin stress fibers and adhesion plaques in fibroblasts. During transformation of mammalian cells by oncogenes the cytoskeleton is rearranged and stress fibers and adhesion plaques are disintegrated. In this paper we investigate the function of the rho protein in RR1022 rat fibroblasts transformed by the Rous sarcoma virus. Two activated mutants of the rho protein, rho G14V and rho Q63L, and a dominant negative mutant, rho N1171, were stably transfected into RR1022 cells. The resulting cell lines were analysed for the organization of polymerized actin and adhesion plaques. Cells expressing rho Q63L, but not rho wt, rho G14V or rho N1171, showed an altered morphology. These cells displayed a flat, fibroblast like shape when compared with the RR1022 ancestor cells. Immunofluorescence analyses revealed that actin stress fibers and adhesion plaques were rearranged in these cells. We conclude from these data that an active rho protein can restore elements of the actin cytoskeleton in transformed cells by overriding the tyrosine kinase phosphorylation induced by the pp60(v-src).

Transformation and pp60v-src autophosphorylation correlate with SHC-GRB2 complex formation in rat and chicken cells expressing host-range and kinase-active, transformation-defective alleles of v-src

The biochemical properties of several pp60v-src substrates believed to participate in src-mediated transformation were examined in cells expressing a kinase-active, transformation-defective v-src allele (v-src-F172 delta/Y416F) and its parental allele, v-src-F172 delta, a host-range--dependent allele that transforms chicken cells to a fusiform morphology, but does not transform rat cells. Because pp60v-src-F172 delta is dependent on autophosphorylation for transforming ability, these alleles provide a unique opportunity to examine the role of pp60v-src autophosphorylation in regulating substrate interactions. Increased pp125FAK tyrosine phosphorylation and high levels of pp60v-src-associated phosphotidylinositol-3' kinase activity were detected specifically in chicken cells exhibiting round, refractile transformation but not in cells transformed to a fusiform morphology. Increased pp125FAK kinase activity, but not increased pp125FAK tyrosine-phosphorylation correlated with pp60v-src autophosphorylation and increased anchorage-independent growth. Thus, pp125FAK and PI3'K may participate in morphological transformation by v-src. Furthermore, association of phosphorylated SHC with the adapter GRB2 correlated with increased anchorage-independent growth (and autophosphorylation) in both rat and chicken cells independent of the morphological phenotype induced. Therefore, host-range dependence for transformation may be regulated through association of SHC with GRB2, thus implicating SHC as a crucial substrate for src-dependent transformation.

The protein tyrosine kinase p56lck regulates cell adhesion mediated by CD4 and major histocompatibility complex class II proteins

The CD4 protein is expressed on a subset of human T lymphocytes that recognize antigen in the context of major histocompatibility complex (MHC) class II molecules. Using Chinese hamster ovary (CHO) cells expressing human CD4, we have previously demonstrated that the CD4 protein can mediate cell adhesion by direct interaction with MHC class II molecules. In T lymphocytes, CD4 can also function as a signaling molecule, presumably through its intracellular association with p56lck, a member of the src family of protein tyrosine kinases. In the present report, we show that p56lck can affect cell adhesion mediated by CD4 and MHC class II molecules. The expression of wild-type p56lck in CHO-CD4 cells augments the binding of MHC class II+ B cells, whereas the expression of a mutant p56lck protein with elevated tyrosine kinase activity results in decreased binding of MHC class II+ B cells. Using site-specific mutants of p56lck, we demonstrate that the both the enzymatic activity of p56lck and its association with CD4 are required for this effect on CD4/MHC class II adhesion. Further, the binding of MHC class II+ B cells induces CD4 at the cell surface to become organized into structures resembling adhesions-type junctions. Both wild-type and mutant forms of p56lck influence CD4-mediated adhesion by regulating the formation of these structures. The wild-type lck protein enhances CD4/MHC class II adhesion by augmenting the formation of CD4-associated adherens junctions whereas the elevated tyrosine kinase activity of the mutant p56lck decreases CD4-mediated cell adhesion by preventing the formation of these structures.

Autocrine angiotensin system regulation of bovine aortic endothelial cell migration and plasminogen activator involves modulation of proto-oncogene pp60c-src expression

Rapid endothelial cell migration and inhibition of thrombosis are critical for the resolution of denudation injuries to the vessel wall. Inhibition of the endothelial cell autocrine angiotensin system, with either the angiotensin-converting enzyme inhibitor lisinopril or the angiotensin II receptor antagonist sar1, ile8-angiotensin II, leads to increased endothelial cell migration and urokinase-like plasminogen activator (u-PA) activity (Bell, L., and J. A. Madri. 1990. Am. J. Pathol. 137:7-12). Inhibition of the autocrine angiotensin system with the converting-enzyme inhibitor or the receptor antagonist also leads to increased expression of the proto-oncogene c-src: pp60c-src mRNA increased 7-11-fold, c-src protein 3-fold, and c-src kinase activity 2-3-fold. Endothelial cell expression of c-src was constitutively elevated after stable infection with a retroviral vector containing the c-src coding sequence. Constitutively increased c-src kinase activity reconstituted the increases in migration and u-PA observed with angiotensin system interruption. Antisera to bovine u-PA blocked the increase in migration associated with increased c-src expression. These data suggest that increases in endothelial cell migration and plasminogen activator after angiotensin system inhibition are at least partially pp60c-src mediated. Elevated c-src expression with angiotensin system inhibition may act to enhance intimal wound closure and to reduce luminal thrombogenicity in vivo.

Deletions in the SH2 domain of p60v-src prevent association with the detergent-insoluble cellular matrix

p60v-src has been shown to associate with a detergent-insoluble cellular matrix containing cytoskeletal proteins, but p60c-src does not bind to this matrix. We analyzed the association of mutant src proteins with the matrix and found that mutants which lack an amino-terminal portion (residues 149 to 169) of the SH2 domain cannot bind to the matrix. Neither the SH3 region nor other portions of the SH2 region were required for association. We also tested protein kinase-defective mutants and chimeras of p60v-src and p60c-src. We found a strong correlation between the kinase activity of p60src and its association with the detergent-insoluble matrix. Double infection of kinase-defective and kinase-active mutants did not result in matrix binding of the kinase-defective src proteins. We also found that Tyr-416, the major site of autophosphorylation in p60v-src, was not required for matrix association.

Deletions in the SH2 domain of p60v-src prevent association with the detergent-insoluble cellular matrix

p60v-src has been shown to associate with a detergent-insoluble cellular matrix containing cytoskeletal proteins, but p60c-src does not bind to this matrix. We analyzed the association of mutant src proteins with the matrix and found that mutants which lack an amino-terminal portion (residues 149 to 169) of the SH2 domain cannot bind to the matrix. Neither the SH3 region nor other portions of the SH2 region were required for association. We also tested protein kinase-defective mutants and chimeras of p60v-src and p60c-src. We found a strong correlation between the kinase activity of p60src and its association with the detergent-insoluble matrix. Double infection of kinase-defective and kinase-active mutants did not result in matrix binding of the kinase-defective src proteins. We also found that Tyr-416, the major site of autophosphorylation in p60v-src, was not required for matrix association.

Abelson murine leukemia virus induces platelet-derived growth factor-independent fibroblast growth: correlation with kinase activity and dissociation from full morphologic transformation

Abelson murine leukemia virus (A-MuLV) encodes a single protein product, a tyrosine-specific protein kinase, whose activity is necessary for cell transformation by this retrovirus. Using a defined medium culture system, we demonstrate that transformation of NIH 3T3 fibroblasts by A-MuLV abrogates their normal requirement for platelet-derived growth factor (PDGF) for cell growth. Analysis of constructed insertional mutant viruses revealed an absolute correlation between A-MuLV-encoded tyrosine kinase activity and PDGF-independent fibroblast growth. Sequences of the provirus not required for kinase activity appeared unnecessary for abrogating the fibroblast requirement for PDGF. Conversely, sequences required for kinase activity appeared necessary, suggesting that induction of PDGF-independent fibroblast growth, like cell transformation, is a function of this tyrosine kinase. Fibroblasts transformed by a partially transformation-defective mutant demonstrated incomplete morphological transformation but were still independent of PDGF for growth. Thus, the processes of full morphological transformation and growth factor independence can be partially dissociated.

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.

Abelson murine leukemia virus induces platelet-derived growth factor-independent fibroblast growth: correlation with kinase activity and dissociation from full morphologic transformation

Abelson murine leukemia virus (A-MuLV) encodes a single protein product, a tyrosine-specific protein kinase, whose activity is necessary for cell transformation by this retrovirus. Using a defined medium culture system, we demonstrate that transformation of NIH 3T3 fibroblasts by A-MuLV abrogates their normal requirement for platelet-derived growth factor (PDGF) for cell growth. Analysis of constructed insertional mutant viruses revealed an absolute correlation between A-MuLV-encoded tyrosine kinase activity and PDGF-independent fibroblast growth. Sequences of the provirus not required for kinase activity appeared unnecessary for abrogating the fibroblast requirement for PDGF. Conversely, sequences required for kinase activity appeared necessary, suggesting that induction of PDGF-independent fibroblast growth, like cell transformation, is a function of this tyrosine kinase. Fibroblasts transformed by a partially transformation-defective mutant demonstrated incomplete morphological transformation but were still independent of PDGF for growth. Thus, the processes of full morphological transformation and growth factor independence can be partially dissociated.

Phosphorylation of talin at tyrosine in Rous sarcoma virus-transformed cells

The cytoskeletal protein talin was found to undergo enhanced phosphorylation at tyrosine residues in chicken embryo fibroblasts following transformation by Rous sarcoma virus. An increase in the tyrosine phosphorylation of talin was also observed within 6 h in cells infected by the temperature-sensitive mutant tsNY68 after a shift from the nonpermissive to the permissive temperature. The overall extent of phosphorylation was 0.07 mol of phosphate per mol of talin and was not appreciably altered by transformation. In uninfected cells talin was shown to be phosphorylated at multiple sites by tryptic peptide mapping. Following transformation most of these sites remained phosphorylated, to the same or to a lesser extent, while novel, phosphotyrosine-containing phosphopeptides appeared. Talin was phosphorylated at tyrosine in cells infected by Rous sarcoma virus mutants which induce altered or partial transformation morphologies; thus the increased phosphorylation of talin at tyrosine occurred irrespective of the morphology induced. Transformation by Y73 also induced elevated levels of phosphotyrosine in talin, whereas transformation by the avian erythroblastosis and Fujinami sarcoma viruses did not.

Expression of H-ras correlates with metastatic potential: evidence for direct regulation of the metastatic phenotype in 10T1/2 and NIH 3T3 cells

Using three independent approaches, we studied the effects of H-ras on metastasis formation. Analysis of five in vitro-ras-transfected 10T1/2 clones with either flat or refractile morphologies revealed a relationship between metastatic potential, H-ras expression, and anchorage-independent growth. Four metastatic variants derived from a poorly metastatic, low-H-ras-expressing line all expressed high levels of H-ras RNA and grew efficiently in soft agar. Activation of H-ras expression in the metastatic tumors had occurred through amplification and rearrangement of H-ras sequences. In addition, preinduction of p21 synthesis in NIH 3T3 line 433, which contains v-H-ras under transcriptional control of the glucocorticoid-sensitive mouse mammary tumor virus long terminal repeat, significantly increased metastatic efficiency. Glucocorticoid treatment of normal or pEJ-transformed NIH 3T3 cells did not affect metastatic potential. These data reveal a direct relationship between ras expression and metastasis formation and suggest that metastatic and transformed phenotypes may be coregulated in ras-transformed 10T1/2 and NIH 3T3 cells.

Expression of H-ras correlates with metastatic potential: evidence for direct regulation of the metastatic phenotype in 10T1/2 and NIH 3T3 cells

Using three independent approaches, we studied the effects of H-ras on metastasis formation. Analysis of five in vitro-ras-transfected 10T1/2 clones with either flat or refractile morphologies revealed a relationship between metastatic potential, H-ras expression, and anchorage-independent growth. Four metastatic variants derived from a poorly metastatic, low-H-ras-expressing line all expressed high levels of H-ras RNA and grew efficiently in soft agar. Activation of H-ras expression in the metastatic tumors had occurred through amplification and rearrangement of H-ras sequences. In addition, preinduction of p21 synthesis in NIH 3T3 line 433, which contains v-H-ras under transcriptional control of the glucocorticoid-sensitive mouse mammary tumor virus long terminal repeat, significantly increased metastatic efficiency. Glucocorticoid treatment of normal or pEJ-transformed NIH 3T3 cells did not affect metastatic potential. These data reveal a direct relationship between ras expression and metastasis formation and suggest that metastatic and transformed phenotypes may be coregulated in ras-transformed 10T1/2 and NIH 3T3 cells.

Phosphorylation of talin at tyrosine in Rous sarcoma virus-transformed cells

The cytoskeletal protein talin was found to undergo enhanced phosphorylation at tyrosine residues in chicken embryo fibroblasts following transformation by Rous sarcoma virus. An increase in the tyrosine phosphorylation of talin was also observed within 6 h in cells infected by the temperature-sensitive mutant tsNY68 after a shift from the nonpermissive to the permissive temperature. The overall extent of phosphorylation was 0.07 mol of phosphate per mol of talin and was not appreciably altered by transformation. In uninfected cells talin was shown to be phosphorylated at multiple sites by tryptic peptide mapping. Following transformation most of these sites remained phosphorylated, to the same or to a lesser extent, while novel, phosphotyrosine-containing phosphopeptides appeared. Talin was phosphorylated at tyrosine in cells infected by Rous sarcoma virus mutants which induce altered or partial transformation morphologies; thus the increased phosphorylation of talin at tyrosine occurred irrespective of the morphology induced. Transformation by Y73 also induced elevated levels of phosphotyrosine in talin, whereas transformation by the avian erythroblastosis and Fujinami sarcoma viruses did not.

Phosphorylation of the fibronectin receptor complex in cells transformed by oncogenes that encode tyrosine kinases

The fibronectin (FN) receptor in avian cells has been characterized previously as a complex of three membrane glycoproteins of about Mr 160,000, Mr 140,000, and Mr 120,000 (simply termed protein band 1, band 2, and band 3, respectively). Monoclonal antibodies to the band 3 protein of the complex prevent FN and laminin binding both in vivo and in vitro and enable the detection of the receptor proteins in the plasma membrane and in adhesion plaques. Association of the FN receptor proteins with the adhesion-plaque protein talin also has been reported. We now find that the band 2 and band 3 proteins in the complex are phosphorylated in Rous sarcoma virus-transformed chicken cells but not in normal chicken cells. Phosphorylation occurs predominantly on tyrosine and is accompanied by a reorganization of the receptor complex in the membrane of the transformed cells. Whereas normal cells contain the FN receptor in focal contacts and cellular processes between cells, v-src-transformed cells exhibit a more diffuse distribution of this receptor. In addition to the viral v-src oncogene, cells transformed by other avian oncogenes that also encode tyrosine kinases (v-fps, v-erbB, and v-yes) also express the receptor complex proteins in the phosphorylated state regardless of whether the transforming protein is detectable in adhesion plaques. These results suggest that the altered FN and laminin receptor proteins may contribute to the transformed phenotype, but their significance and role in the transformed state remain to be established.