Partial characterization of the mitogenic action of pp60v-src, the oncogenic protein product of the src gene of avian sarcoma virus

Translocation of Src kinase to the cell periphery is mediated by the actin cytoskeleton under the control of the Rho family of small G proteins

We have isolated Swiss 3T3 subclones that are resistant to the mitogenic and morphological transforming effects of v-Src as a consequence of aberrant translocation of the oncoprotein under low serum conditions. In chicken embryo and NIH 3T3 fibroblasts under similar conditions, v-Src rapidly translocates from the perinuclear region to the focal adhesions upon activation of the tyrosine kinase, resulting in downstream activation of activator protein-1 and mitogen-activated protein kinase, which are required for the mitogenic and transforming activity of the oncoprotein. Since serum deprivation induces cytoskeletal disorganization in Swiss 3T3, we examined whether regulators of the cytoskeleton play a role in the translocation of v-Src, and also c-Src, in response to biological stimuli. Actin stress fibers and translocation of active v-Src to focal adhesions in quiescent Swiss 3T3 cells were restored by microinjection of activated Rho A and by serum. Double labeling with anti-Src and phalloidin demonstrated that v-Src localized along the reformed actin filaments in a pattern that would be consistent with trafficking in complexes along the stress fibers to focal adhesions. Furthermore, treatment with the actin-disrupting drug cytochalasin D, but not the microtubule-disrupting drug nocodazole, prevented v-Src translocation. In addition to v-Src, we observed that PDGF-induced, Rac-mediated membrane ruffling was accompanied by translocation of c-Src from the cytoplasm to the plasma membrane, an effect that was also blocked by cytochalasin D. Thus, we conclude that translocation of Src from its site of synthesis to its site of action at the cell membrane requires an intact cytoskeletal network and that the small G proteins of the Rho family may specify the peripheral localization in focal adhesions or along the membrane, mediated by their effects on the cytoskeleton.

Cooperation of Src homology domains in the regulated binding of phosphatidylinositol 3-kinase. A role for the Src homology 2 domain

Fibroblasts transformed by the v-Src oncoprotein exhibit elevated activity of the enzyme phosphatidylinositol 3'-kinase (PI 3-kinase), which binds to, and is activated by, a wide range of receptor tyrosine kinases as well as v-Src and transforming polyoma middle T/c-Src complexes. Here we consider the role of the v-Src homology (SH) domains, SH3 and SH2, and the tyrosine kinase catalytic domain, in the stimulation of v-Src-associated PI 3-kinase activity in response to rapid activation of the oncoprotein. As shown by others, we find that the v-Src SH3 domain tightly binds the PI 3-kinase p85 regulatory subunit in normal growing chicken embryo fibroblasts. However, we also find that in transformed cells there is additional efficient binding of PI 3-kinase to the v-Src SH2 domain in a catalytically active form. Furthermore, the binding of p85 to the SH2 domain, which is almost undetectable in quiescent cells, is rapidly stimulated upon activation of temperature-sensitive v-Src and consequent cell cycle entry, demonstrating that binding is a target for regulation. We also show that v-Src-associated PI 3-kinase differs considerably from PDGF receptor-associated enzyme by a different mode of binding, a lack of substantial allosteric activation, and a dependence on the tyrosine kinase activity of v-Src. The rapidly induced binding and activation of PI 3-kinase thus provides sensitive regulation of recruitment of PI 3-kinase to its substrates and into other signaling complexes at the cell membrane, which involves all the Src homology domains.

Separation of v-Src-induced mitogenesis and morphological transformation by inhibition of AP-1

v-Src activity results in both morphological transformation and reentry of quiescent chick embryo fibroblasts (CEF) into cell cycle. We have previously used temperature-sensitive v-Src mutants to show that enhanced activity of cellular AP-1 in the first few hours after activation of v-Src invariably precedes the biological consequences. Here we have investigated whether the early activation of AP-1 is essential for any or all of the v-Src responses by using a mutant c-Fos that comprises the leucine zipper and a disrupted basic region. Expression of the c-Fos mutant partially reduced cellular AP-1 activity in exponentially growing cells. However, in CEF that had been made quiescent by serum deprivation, v-Src-induced stimulation of AP-1 DNA binding activity was substantially reduced. In addition, quiescent CEF stably transfected with this mutant show an impaired mitogenic response to v-Src, indicating that the AP-1 stimulation is a necessary prerequisite for cell-cycle reentry. The ability of v-Src to morphologically transform quiescent CEF was not impaired by the inhibition of AP-1 stimulation, indicating that the mitogenic and morphological consequences of v-Src have distinguishable biochemical mediators. Focal adhesion kinase, a recently identified determinant of cell morphology, undergoes a gel mobility shift, characteristic of its hyperphosphorylated state, in response to v-Src activation in cells expressing the inhibitory AP-1 protein. This provides further evidence that the pathways that regulate morphological transformation are independent of AP-1.

A cell cycle and mutational analysis of anchorage-independent growth: cell adhesion and TGF-beta 1 control G1/S transit specifically

We have examined cell cycle control of anchorage-independent growth in nontransformed fibroblasts. In previous studies using G0-synchronized NRK and NIH-3T3 cells, we showed that anchorage-independent growth is regulated by an attachment-dependent transition at G1/S that resembles the START control point in the cell cycle of Saccharomyces cerevisiae. In the studies reported here, we have synchronized NRK and NIH-3T3 fibroblasts immediately after this attachment-dependent transition to determine if other portions of the fibroblast cell cycle are similarly regulated by adhesion. Our results show that S-, G2-, and M-phase progression proceed in the absence of attachment. Thus, we conclude that the adhesion requirement for proliferation of these cells can be explained in terms of the single START-like transition. In related studies, we show that TGF-beta 1 overrides the attachment-dependent transition in NRK and AKR-2B fibroblasts (lines in which TGF-beta 1 induces anchorage-independent growth), but not in NIH-3T3 or Balb/c 3T3 fibroblasts (lines in which TGF-beta 1 fails to induce anchorage-independent growth). These results show that (a) adhesion and TGF-beta 1 can have similar effects in stimulating cell cycle progression from G1 to S and (b) the differential effects of TGF-beta 1 on anchorage-independent growth of various fibroblast lines are directly reflected in the differential effects of the growth factor at G1/S. Finally, we have randomly mutagenized NRK fibroblasts to generate mutant lines that have lost their attachment/TGF-beta 1 requirement for G1/S transit while retaining their normal mitogen requirements for proliferation. These clones, which readily proliferate in mitogen-supplemented soft agar, appear non-transformed in monolayer: they are well spread, nonrefractile, and contact inhibited. The existence of this new fibroblast phenotype demonstrates (a) that the growth factor and adhesion/TGF-beta 1 requirements for cell cycle progression are genetically separable, (b) that the two major control points in the fibroblast cell cycle (G0/G1 and G1/S) are regulated by distinct extracellular signals, and (c) that the genes regulating anchorage-independent growth need not be involved in regulating contact inhibition, focus formation, or growth factor dependence.

Cell cycle-dependent gene expression in V point-arrested BALB/c-3T3 cells

Density-arrested BALB/c-3T3 cells stimulated to proliferate in an amino acid-deficient medium arrest in mid-G1 at a point termed the V point. Cells released from V point arrest require 6 hr to traverse late G1 and enter S phase. As data presented here show that mRNA synthesis is needed for 2-3 hr after release of cells from the V point, after which inhibition of mRNA synthesis does not prevent entry into S phase, we used this mid-G1 arrest protocol to analyze gene expression in late G1. We found that although stimulation of cells in amino acid-deficient medium did not inhibit the induction of genes expressed in early G1, genes normally expressed in late G1 were expressed only after release from the V point. The expression of late G1 genes in cells released from the V point was temporally similar, in respect to G1 location, as was seen in stimulation of quiescent G0 cells. As this protocol effectively divides gene expression into early (pre-V point) and late (post-V point) categories, it should be useful in studies of growth factor-modulated events that regulate traverse of late G1 and commitment to DNA synthesis. In addition, we used c-myb antisense oligonucleotides to show that c-myb expression, which occurs in late G1, is required for BALB/c-3T3 fibroblasts to traverse late G1 and initiate DNA synthesis.

tsLA23-NRK cells need pp60v-src protein-tyrosine kinase activity in G2 phase to initiate mitosis in serum-free medium

Lowering the temperature from 41 to 36 degrees C stimulates quiescent tsLA23-NRK rat cells (infected with the tsLA23 mutant of the Rous sarcoma virus) in serum-free medium to resume cycling and initiate DNA replication by reactivating the tsLA23-RSV's abnormally thermolabile pp60v-src protein-tyrosine kinase. Inactivating the enzyme in these pp60v-src-stimulated cells by again raising the temperature to 41 degrees C after the cells had initiated DNA replication did not prevent the completion of DNA replication and entry into the G2 phase, but it stopped the initiation of mitosis. Adding serum at the time of the temperature increase replaced the lost pp60v-src activity and the cells were able to continue to mitosis. The G2-arrested cells at 41 degrees C were able to initiate mitosis when pp60v-src was reactivated again by lowering the temperature to 36 degrees C. These observations suggest that protein-tyrosine kinase activity is needed to initiate mitosis and that the tsLA23-NRK cell is a good model for studying the function of this kinase activity in the initiation of mitosis.

Transforming signals generated by the polyoma virus tumor antigens

The secretion of a threshold amount of SAGF can elicit one of the major features of the transformed phenotype, anchorage-independent growth. It appears that, like other cells, NIH 3T3 cells make and secrete SAGF, but not enough to enable them to proliferate in soft agar. The stream of signals from polyoma virus MTAg:pp60c-src:PI-kinase complexes attached to the inner surface of the cell membrane is not enough to raise SAGF secretion to the threshold level for agar growth, but these signals can powerfully enhance the responsiveness to added SAGF. Only when NIH 3T3 cells express all three polyoma T antigens do they secrete enough SAGF and become responsive enough to the factor to form colonies in soft agar. This is in contrast to F111 cells which produce adequate amounts of, and respond to, SAGF after MTAg expression alone. The reasons for this difference remain to be investigated. Obviously, factors expressed during the course of the spontaneous establishment are different for these two lines and they affect the cells' response to MTAg. Clearly, knowing how polyoma virus transforms cells will require a deeper understanding of the deregulation of membrane signaling enzymes by the viral MTAg and the identification and characterization of the autocrine SAGF's that mediate at least part of the neoplastic response to the viral signals.

Membrane protein kinase C activity rapidly increases in quiescent tsRSV-infected NRK cells upon reactivation of the mitogenic v-src protein kinase

The viral src protein kinase, pp60v-src, is a powerful intracellular mitogen which can initiate and maintain the proliferation of quiescent cells in the absence of any exogenous growth factors. In an attempt to understand how pp60v-src induces proliferation, we examined the early events in the G0 to G1 transition caused by the activation of a thermolabile v-src protein in quiescent, serum-starved tsRSV-transformed NRK cells. The reactivation of pp60v-src, in the absence of exogenous growth factors, triggered a rapid biphasic surge of membrane-associated protein kinase C (PKC) activity. Unlike TPA-stimulated PKC activity, the pp60v-src-induced increase in PKC was readily extracted from membranes by EGTA. The down-regulation of PKC activity in these quiescent cells by prolonged exposure to TPA strongly inhibited the ability of the reactivated v-src protein to stimulate DNA replication in serum-deficient medium, suggesting that PKC plays a role in the initial signal by which the viral enzyme induces the G0 to G1 transition in NRK cells.

Protein kinase C and a viral K-RAS protein cooperatively enhance the response of adenylate cyclase to stimulators

The protein kinase C stimulator TPA (12-O-tetradecanoyl phorbol-13-acetate) enhanced the responsiveness of adenylate cyclase to IPR (isoproterenol) and PGE1 (prostaglandin E1) in quiescent tsKSV-NRK cells at the nonpermissive 41 degrees C. Reactivating the thermolabile mitogenic/oncogenic K-ras protein in tsKSV-NRK cells by dropping the temperature to 36 degrees C also enhanced the responsiveness of adenylate cyclase to IPR and PGE1. The enhancement was transient and peaked at 6 hours after the temperature shift. This enhanced responsiveness was specifically due to the reactivated viral K-ras protein rather than the temperature shift because the same temperature shift did not affect adenylate cyclase responsiveness in uninfected NRK cells, nor was it a result of the mitogenic stimulus since reacting the mitogenic pp60v-src protein in tsASV-NRK cells did not affect adenylate cyclase responsiveness. The increased responsiveness of adenylate cyclase at 6 hours after the temperature shift was not a result of elevated membrane-associated PKC activity. However, the reactivated viral K-ras protein strongly increased the stimulability of membrane-associated PKC by TPA and it further increased TPA's ability to enhance the responsiveness of adenylate cyclase to IPR and PGE1. Thus, a viral K-ras protein and membrane-associated protein kinase C can cooperate to increase the responsiveness of adenylate cyclase to agonists.

Viral p21 Ki-RAS protein: a potent intracellular mitogen that stimulates adenylate cyclase activity in early G1 phase of cultured rat cells

Rat kidney (NRK) cells infected with a temperature-sensitive mutant of the Kirsten sarcoma virus were arrested in the G0/G1 phase of their cell cycle by incubation in serum-deficient medium at a p21-inactivating temperature of 41 degrees C. These quiescent ts K-NRK cells were then stimulated to transit G1 and initiate DNA replication by lowering the temperature to 36 degrees C, which rapidly reactivated p21. Reactivating the viral Ki-RAS protein by temperature shift led to an increase in adenylate cyclase activity in early G1 phase. The Ki-RAS protein increased the sensitivity of adenylate cyclase to guanyl nucleotides by a mechanism that seemed to involve inactivation of the enzyme's inhibitory G1 regulatory protein.

Characterization of G1 transit induced by the mitogenic-oncogenic viral Ki-ras gene product

NRK rat kidney cells infected with a temperature-sensitive mutant of the Kirsten sarcoma virus (ts371) were transformed at 36 degrees C but were phenotypically nontransformed at 41 degrees C because of the abnormal thermolability of the oncogenic 21-kilodalton product of the viral Ki-ras gene. Thus tsK-NRK cells were rendered quiescent in a G0-G1 state by a 48-h incubation in serum-free medium at the nonpermissive, p21-inactivating temperature of 41 degrees C. The serum-starved cells could then be stimulated to transit G1 either as nontransformed cells by adding serum at 41 degrees C or as transformed cells by lowering the temperature to a p21-activating 36 degrees C. The viral p21 protein was as effective as serum in stimulating tsK-NRK cells to transit G1 and to start replicating DNA. While p21 effectively stimulated cells to transit G1 even in unconditioned, serum-free medium, they still needed cell-derived conditioning factors to subsequently divide. The p21 protein also enabled the cells to transit G1 in spite of an extracellular Ca2+ deficiency that inhibited the G1 transit of serum-stimulated cells. p21 activity was needed to stimulate both early and late G1 events. In contrast to serum, p21 did not stimulate total RNA or protein synthesis, but some RNA and protein synthesis must have been needed for the p21-driven G1 transit because it could be stopped by actinomycin D or cycloheximide.

Calcium, cyclic AMP and protein kinase C--partners in mitogenesis

Evidence is steadily mounting that the proto-oncogenes, whose products organize and start the programs that drive normal eukaryotic cells through their chromosome replication/mitosis cycles, are transiently stimulated by sequential signals from a multi-purpose, receptor-operated mechanism (consisting of internal surges of Ca2+ and bursts of protein kinase C activity resulting from phosphatidylinositol 4,5-bisphosphate breakdown and the opening of membrane Ca2+ channels induced by receptor-associated tyrosine-protein kinase activity) and bursts of cyclic AMP-dependent kinase activity. The bypassing or subversion of the receptor-operated Ca2+/phospholipid breakdown/protein kinase C signalling mechanism is probably the basis of the freeing of cell proliferation from external controls that characterizes all neoplastic transformations.

Ca2+-dependent cell surface protein phosphorylation may be involved in the initiation of DNA synthesis

Incubating T51B rat liver cells in Ca2+-deficient, serum-rich medium containing only 0.02 mM Ca2+ strikingly decreased the phosphorylation of several trypsin-removable cell surface proteins and arrested the cells in late G1 phase. Raising the Ca2+ concentration in the Ca2+-deficient medium from 0.02 mM to 0.5 mM or adding 80 nM TPA (12-O-tetradecanoyl-phorbol-13-acetate), a protein kinase C activator, stimulated the phosphorylation of a certain set of surface proteins within 5 min and the initiation of DNA replication within the next 2 hr. By contrast, incubation in the same Ca2+-deficient medium, which does not affect the proliferation of neoplastic T51B-261B cells, did not reduce the phosphorylation of cell surface proteins. These observations suggest that the stimulation of a Ca2+-dependent protein kinase (possibly protein kinase C) directly or indirectly phosphorylates certain cell surface proteins that might be part of the mechanism that triggers the Ca2+-dependent G1----S transition of normal cells. They also suggest that an alteration of this Ca2+-dependent protein kinase might be the reason for neoplastic cells being able to proliferate in the face of an external Ca2+ shortage that would stop the proliferation of normal cells.

Characterization of G1 transit induced by the mitogenic-oncogenic viral Ki-ras gene product

NRK rat kidney cells infected with a temperature-sensitive mutant of the Kirsten sarcoma virus (ts371) were transformed at 36 degrees C but were phenotypically nontransformed at 41 degrees C because of the abnormal thermolability of the oncogenic 21-kilodalton product of the viral Ki-ras gene. Thus tsK-NRK cells were rendered quiescent in a G0-G1 state by a 48-h incubation in serum-free medium at the nonpermissive, p21-inactivating temperature of 41 degrees C. The serum-starved cells could then be stimulated to transit G1 either as nontransformed cells by adding serum at 41 degrees C or as transformed cells by lowering the temperature to a p21-activating 36 degrees C. The viral p21 protein was as effective as serum in stimulating tsK-NRK cells to transit G1 and to start replicating DNA. While p21 effectively stimulated cells to transit G1 even in unconditioned, serum-free medium, they still needed cell-derived conditioning factors to subsequently divide. The p21 protein also enabled the cells to transit G1 in spite of an extracellular Ca2+ deficiency that inhibited the G1 transit of serum-stimulated cells. p21 activity was needed to stimulate both early and late G1 events. In contrast to serum, p21 did not stimulate total RNA or protein synthesis, but some RNA and protein synthesis must have been needed for the p21-driven G1 transit because it could be stopped by actinomycin D or cycloheximide.

The mitogenic/oncogenic p21 Ki-RAS protein stimulates adenylate cyclase activity early in the G1 phase of NRK rat kidney cells

tsK-NRK rat cells infected with a temperature-sensitive mutant of the Kirsten murine sarcoma virus were arrested in the G0/G1 phase of their cell cycle by incubation in serum-deficient medium at a temperature (41 degrees C) which inactivates the virus' abnormally thermolabile mitogenic/oncogenic 21 kDa (p 21) RAS protein product. Reactivating the viral RAS protein by lowering the temperature to a permissive 36 degrees C rapidly (within 1 hour) stimulated adenylate cyclase, sensitized the enzyme to stimulation by GTP and forskolin and caused the tsK-NRK cells to transit G1 and start replicating their DNA about 10 hours later. The 41 degrees C----36 degrees C shift did not affect adenylate cyclase or stimulate G1 transit in uninfected NRK cells. Thus, an oncogenic viral RAS protein was able to stimulate adenylate cyclase and G1 transit in a mammalian cell just as other RAS proteins appear to do in yeast cells.

The selective induction of a small number of proteins during G1 transit results from the mitogenic action of pp60v-src in tsASV-infected rat cells

Since the mitogenic/oncogenic pp60v-src product of the avian sarcoma virus (ASV) mutant, tsLA23, is abnormally thermolabile, tsLA23-NRK cells were phenotypically nontransformed at 40 degrees C and were consequently rendered quiescent by serum deprivation at this temperature. These serum-deprived cells were stimulated to transit G1 either as transformed cells by simply dropping the temperature to a pp60v-src -activating 36 degrees C, or as nontransformed cells by adding serum at 40 degrees C. Serum stimulation rapidly increased total protein synthesis in these cells and over 100 changes in cellular proteins (resolved by two-dimensional gel electrophoresis) occurred during G1 transit. By contrast, pp60v-src-activation did not increase total protein synthesis and only six proteins (18.5-44 kD) were clearly seen to appear or increase when quiescent cells were stimulated to transit G1 by activating pp60v-src. Three of these six pp60v-src- induced proteins also appeared or accumulated during the G1 transit of serum-stimulated cells. The appearance and/or accumulation of the six proteins and the subsequent initiation of DNA replication may have resulted from pp60v-src stimulating only a small number of critical cellular genes because both the protein changes and DNA replication were completely suppressed by the transcription inhibitor actinomycin D.

Transforming NRK cells with avian sarcoma virus reduces the extracellular Ca2+ requirement without affecting the calcicalmodulin requirement for the G1/S transition

NRK rat cells infected with a transformation-defective, temperature-sensitive (ts) mutant of the avian sarcoma virus could not proliferate in Ca2+-deficient medium at a nonpermissive temperature (40 degrees C) that inactivated the viral pp60v-src-transforming product and rendered the cells phenotypically untransformed. However, these arrested cells were stimulated to initiate DNA replication with little or no delay while still in the Ca2+-deficient medium, either by adding Ca2+ or calmodulin at 40 degrees C or by reducing the temperature to 36 degrees C which restored the transformed phenotype by rapidly reactivating pp60v-src. The G1/S transition triggered by restoring the transformed phenotype was suppressed by three different anticalmodulin drugs (R24571, trifluoperazine, W7). The suppression by one of these drugs, trifluoperazine, was overcome by adding calmodulin. Thus, neoplastic transformation by the avian sarcoma virus sharply reduces the extracellular Ca2+ requirement for the initiation of DNA replication without bypassing a calcicalmodulin-dependent mechanism also needed for the G1/S transition.

Microinjected ras family oncogenes stimulate DNA synthesis in quiescent mammalian cells

Oncogenes of the ras family stimulate DNA synthesis when microinjected into quiescent mouse and hamster fibroblasts, as detected by in situ autoradiography. The molecularly cloned genomes of Harvey and Kirsten sarcoma viruses, the cloned Harvey ras gene, and the product of the v-ras gene, the p21v-rasH protein, stimulate DNA synthesis in quiescent cells. This stimulation is comparable to the stimulatory activity of the microinjected SV40 T-antigen-coding gene. The demonstration that these oncogenes can stimulate transient DNA synthesis in quiescent cells is relevant to understanding the mechanism by which these genes are able to transform cells in vitro and induce tumors in animals.

The mitogenic activity of pp60v-src, the oncogenic protein product of the src gene of avian sarcoma virus, is independent of external serum growth factors

The oncogenic pp60v-src product of ASV (avian sarcoma virus) is shown to be a potent endogenous mitogen, which, unlike mitogens such as PDGF (platelet derived growth factor), is able to stimulate host cell proliferation without the help of other growth factors. Thus, NRK rat cells, infected with a temperature-sensitive ASV mutant which produces an abnormally thermolabile pp60v-src, became proliferatively quiescent at a pp60v-src-inactivating 40 degrees C in medium containing either 0.2% calf serum or no serum at all. Adding PDGF stimulated the quiescent tsASV-NRK cells at 40 degrees C to initiate DNA replication in medium containing 0.2% serum, but not in serum-free medium. By contrast, activating internal pp60v-src by dropping the temperature to a permissive 36 degrees C stimulated these quiescent cells to transit G1, initiate DNA replication and to enter mitosis even in serum-free medium. Thus, relative to PDGF, endogenous pp60v-src behaves as a complete mitogen.