The TWIST1 oncogene is a direct target of hypoxia-inducible factor-2alpha.

Hypoxia-inducible factors (HIFs) are highly conserved transcription factors that play a crucial role in oxygen homeostasis. Intratumoral hypoxia and genetic alterations lead to HIF activity, which is a hallmark of solid cancer and is associated with poor clinical outcome. HIF activity is regulated by an evolutionary conserved mechanism involving oxygen-dependent HIFalpha protein degradation. To identify novel components of the HIF pathway, we performed a genome-wide RNA interference screen in Caenorhabditis elegans, to suppress HIF-dependent phenotypes, like egg-laying defects and hypoxia survival. In addition to hif-1 (HIFalpha) and aha-1 (HIFbeta), we identified hlh-8, gska-3 and spe-8. The hlh-8 gene is homologous to the human oncogene TWIST1. We show that TWIST1 expression in human cancer cells is enhanced by hypoxia in a HIF-2alpha-dependent manner. Furthermore, intronic hypoxia response elements of TWIST1 are regulated by HIF-2alpha, but not HIF-1alpha. These results identify TWIST1 as a direct target gene of HIF-2alpha, which may provide insight into the acquired metastatic capacity of hypoxic tumors.

Hypoxia-inducible factor-1alpha expression requires PI 3-kinase activity and correlates with Akt1 phosphorylation in invasive breast carcinomas.

Hypoxia-inducible factor-1 alpha (HIF-1alpha) is the regulatory subunit of the heterodimeric transcription factor HIF-1 and the key factor in cellular response to low oxygen tension. Expression of HIF-1alpha protein is associated with poor patient survival and therapy resistance in many types of solid tumors. Insight into HIF-1alpha regulation in solid tumors is important for therapeutic strategies. In this study, we determined the pathophysiological relevance of HIF-1alpha regulation by the oncogenic phosphatidylinositol 3'-kinase (PI 3-kinase)/Akt signaling pathway. We modeled the physiology of hypoxic tumor regions by culturing carcinoma cells under low oxygen tension in the absence of serum. We observed that hypoxic induction of HIF-1alpha protein was decreased by serum deprivation. Overexpression of dominant-active Akt1 restored HIF-1alpha expression, whereas inhibition of PI 3-kinase activity reduced hypoxic HIF-1alpha protein levels to a similar extent as serum deprivation. Immunohistochemical analysis of 95 human breast cancers revealed that lack of Akt1 phosphorylation correlates with low HIF-1alpha levels. To our knowledge, this is the first reported comparison between HIF-1alpha expression and Akt phosphorylation in human carcinomas. We conclude that Akt activity is physiologically relevant for HIF-1alpha expression in breast cancer. This implies that HIF-1alpha function might be therapeutically targeted by inhibition of the PI 3-kinase/Akt pathway.

Regulating c-Ras function. cholesterol depletion affects caveolin association, GTP loading, and signaling.

Cholesterol-rich and caveolin-containing microdomains of the plasma membrane, termed "caveolae," have been implicated in signal transduction. However, the role of caveolae in regulating the Ras-MAP kinase cascade is incompletely understood. The mammalian Ras isoforms (H, N, and K) use different membrane anchors to attach to the plasma membrane and thereby may localize to functionally distinct microdomains, which might explain isoform-specific signaling. Here, we show that, in Cos epithelial cells, endogenous K-Ras colocalizes largely with caveolin, whereas N-Ras localizes to both caveolar and noncaveolar subdomains; H-Ras localization was below detection limits. We find that epidermal growth factor (EGF) activates N-Ras but fails to activate K-Ras in these cells. Extraction of cholesterol with methyl-beta-cyclodextrin disrupts complex formation between caveolin and K- and N-Ras and, strikingly, enables EGF to activate both K-Ras and N-Ras. While cholesterol depletion enhances GTP-loading on total c-Ras, activation of the downstream MEK-MAP kinase cascade by EGF and lysophosphatidic acid but not that by phorbol ester is inhibited. Thus, plasma membrane cholesterol is essential for negative regulation of c-Ras isoforms (complexed to caveolin), as well as for mitogenic signaling downstream of receptor-activated c-Ras.

Gap junction protein connexin-43 interacts directly with microtubules.

Gap junctions are specialized cell-cell junctions that mediate intercellular communication. They are composed of connexin proteins, which form transmembrane channels for small molecules [1, 2]. The C-terminal tail of connexin-43 (Cx43), the most widely expressed connexin member, has been implicated in the regulation of Cx43 channel gating by growth factors [3-5]. The Cx43 tail contains various protein interaction sites, but little is known about binding partners. To identify Cx43-interacting proteins, we performed pull-down experiments using the C-terminal tail of Cx43 fused to glutathione-S-transferase. We find that the Cx43 tail binds directly to tubulin and, like full-length Cx43, sediments with microtubules. Tubulin binding to Cx43 is specific in that it is not observed with three other connexins. We established that a 35-amino acid juxtamembrane region in the Cx43 tail, which contains a presumptive tubulin binding motif, is necessary and sufficient for microtubule binding. Immunofluorescence and immunoelectron microscopy studies reveal that microtubules extend to Cx43-based gap junctions in contacted cells. However, intact microtubules are dispensable for the regulation of Cx43 gap-junctional communication. Our findings suggest that, in addition to its well-established role as a channel-forming protein, Cx43 can anchor microtubule distal ends to gap junctions and thereby might influence the properties of microtubules in contacted cells.

Connexin-43 interactions with ZO-1 and alpha- and beta-tubulin.

Gap junctions are composed of connexins that form transmembrane channels between adjacent cells. The C-terminal tail of connexin-43 (Cx43), the most widely expressed connexin member, has been implicated in the regulation of Cx43 channel gating. Interestingly, channel-independent processes regulated by Cx43 have also been postulated. In our studies to elucidate the mechanism of Cx43 channel gating by growth factors and to explore additional functions of gap junctions, we have identified three interacting partners of the C-terminal tail of Cx43 (Cx43CT). (i) the c-Src tyrosine kinase, which phosphorylates Cx43CT and is involved in G protein-mediated inhibition of Cx43 gap junctional communication. (ii) the ZO-1 'scaffold' protein, which might recruit signaling proteins into Cx43-based gap junctions. (iii) microtubules (consisting of alpha/beta-tubulin dimers), which extend with their distal ends to Cx43-based gap junctions, suggesting that Cx43 gap junctions may play a novel role in regulating microtubule stability in contacted cells. Here we show that Cx43 binds alpha-tubulin equally well as beta-tubulin. In addition, we show that the second, but not the first, PDZ domain of ZO-1 binds directly to Cx43, and we confirm that the very C-terminal isoleucine residue of Cx43 is critical for ZO-1 binding.

Dynamin is required for the activation of mitogen-activated protein (MAP) kinase by MAP kinase kinase.

Internalization of activated receptors from the plasma membrane has been implicated in the activation of mitogen-activated protein (MAP) kinase. However, the mechanism whereby membrane trafficking may regulate mitogenic signaling remains unclear. Here we report that dominant-negative dynamin (K44A), an inhibitor of endocytic vesicle formation, abrogates MAP kinase activation in response to epidermal growth factor, lysophosphatidic acid, and protein kinase C-activating phorbol ester. In contrast, dynamin-K44A does not affect the activation of Ras, Raf, and MAP kinase kinase (MEK) by either agonist. Through immunofluorescence and subcellular fractionation studies, we find that activated MEK is present both at the plasma membrane and in intracellular vesicles but not in the cytosol. Our findings suggest that dynamin-regulated endocytosis of activated MEK, rather than activated receptors, is a critical event in the MAP kinase activation cascade.

Gi-mediated tyrosine phosphorylation of Grb2 (growth-factor-receptor-bound protein 2)-bound dynamin-II by lysophosphatidic acid.

Lysophosphatidic acid (LPA) is the prototypic G-protein-coupled receptor agonist that activates the Ras-mitogen-activated protein (MAP) kinase cascade through pertussis toxin (PTX)-sensitive Gi and enhanced tyrosine kinase activity. We recently detected a 100 kDa protein (p100) that binds to the C-terminal SH3 domain of growth-factor-receptor-bound protein 2 (Grb2) and becomes tyrosine phosphorylated in a PTX-sensitive manner in LPA-treated Rat-1 cells [Kranenburg, Verlaan, Hordijk and Moolenaar (1997) EMBO J. 16, 3097-3105]. Through glutathione S-transferase-Grb2 affinity purification and microsequencing, we have now identified p100 as dynamin-II, a GTPase that regulates clathrin-mediated endocytosis. We show that in Rat-1 cells, Grb2-bound dynamin-II is rapidly tyrosine phosphorylated in response to LPA in a PTX-sensitive manner. Thus, tyrosine phosphorylation of Grb2-bound dynamin-II may be a critical event in Gi-mediated activation of the Ras-MAP kinase cascade in fibroblasts.

Sphingosine 1-phosphate signalling through the G-protein-coupled receptor Edg-1.

Sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) are structurally related lipid mediators that act on distinct G-protein-coupled receptors to evoke similar responses, including Ca2+ mobilization, adenylate cyclase inhibition, and mitogen-activated protein (MAP) kinase activation. However, little is still known about the respective receptors. A recently cloned putative LPA receptor (Vzg-1/Edg-2) is similar to an orphan Gi-coupled receptor termed Edg-1. Here we show that expression of Edg-1 in Sf9 and COS-7 cells results in inhibition of adenylate cyclase and activation of MAP kinase (Gi-mediated), but not Ca2+ mobilization, in response to S1P. These responses are specific in that (i) S1P action is not mimicked by LPA, and (ii) Vzg-1/Edg-2 cannot substitute for Edg-1. Thus the Edg-1 receptor is capable of mediating a subset of the cellular responses to S1P.

Gi-mediated activation of the Ras/MAP kinase pathway involves a 100 kDa tyrosine-phosphorylated Grb2 SH3 binding protein, but not Src nor Shc.

Mitogenic G protein-coupled receptors, such as those for lysophosphatidic acid (LPA) and thrombin, activate the Ras/MAP kinase pathway via pertussis toxin (PTX)-sensitive Gi, tyrosine kinase activity and recruitment of Grb2, which targets guanine nucleotide exchange activity to Ras. Little is known about the tyrosine phosphorylations involved, although Src activation and Shc phosphorylation are thought to be critical. We find that agonist-induced Src activation in Rat-1 cells is not mediated by Gi and shows no correlation with Ras/MAP kinase activation. Furthermore, LPA-induced tyrosine phosphorylation of Shc is PTX-insensitive and Ca2+-dependent in COS cells, but undetectable in Rat-1 cells. Expression of dominant-negative Src or Shc does not affect MAP kinase activation by LPA. Thus, Gi-mediated Ras/MAP kinase activation in fibroblasts and COS cells involves neither Src nor Shc. Instead, we detect a 100 kDa tyrosine-phosphorylated protein (p100) that binds to the C-terminal SH3 domain of Grb2 in a strictly Gi- and agonist-dependent manner. Tyrosine kinase inhibitors and wortmannin, a phosphatidylinositol (PI) 3-kinase inhibitor, prevent p100-Grb2 complex formation and MAP kinase activation by LPA. Our results suggest that the p100-Grb2 complex, together with an upstream non-Src tyrosine kinase and PI 3-kinase, couples Gi to Ras/MAP kinase activation, while Src and Shc act in a different pathway.

cAMP abrogates the p21ras-mitogen-activated protein kinase pathway in fibroblasts.

The mechanism by which cAMP inhibits growth factor-induced DNA synthesis in fibroblasts is not understood. Here we show that in Rat-1 fibroblasts, cAMP-raising agents inhibit p21ras-mediated mitogen-activated protein (MAP) kinase activation induced by either epidermal growth factor or lysophosphatidic acid. Under the same conditions, however, epidermal growth factor- or lysophosphatidic acid-induced protein tyrosine phosphorylation, Ca2+ mobilization, and activation of Na+/H+ exchange are not attenuated. In ras-transformed Rat-1 cells, 8-bromo-cAMP rapidly deactivates constitutively active MAP kinase without reducing p21ras.GTP levels; long term 8-bromo-cAMP treatment of these cells leads to growth arrest and reversion of the transformed phenotype. These results show that elevation of intracellular cAMP levels abrogates the p21ras MAP kinase pathway at a step downstream of p21ras activation. This finding provides a molecular basis for the growth-inhibitory action of cAMP in normal and transformed fibroblasts.

Protein tyrosine phosphorylation induced by lysophosphatidic acid in Rat-1 fibroblasts. Evidence that phosphorylation of map kinase is mediated by the Gi-p21ras pathway.

Lysophosphatidic acid (LPA) is a platelet-derived phospholipid that serves as a mitogen for fibroblasts. LPA activates its own G protein-coupled receptor(s) leading to stimulation of phospholipase C and inhibition of adenylate cyclase. Furthermore, LPA rapidly activates p21ras through a pertussis toxin-sensitive pathway. In this study, we have examined LPA-induced protein tyrosine phosphorylation in Rat-1 fibroblasts. LPA action was compared with that of endothelin, which is a stronger activator of phospholipase C than LPA but fails to activate p21ras and to stimulate DNA synthesis in these cells. LPA and, more effectively, endothelin rapidly stimulate tyrosine phosphorylation of proteins of 110-130, 95, and 65-75 kDa. The effect of LPA is dose- and time-dependent, being half-maximal at 3-30 nM and peaking after 2-5 min. Among the 110-130-kDa group of phosphotyrosyl proteins is the 125-kDa "focal adhesion kinase" (p125FAK) but not the 120-kDa p21ras GTPase-activating protein. Furthermore, LPA, like epidermal growth factor, causes tyrosine phosphorylation and activation of the p42/p44 mitogen-activated protein (MAP) kinases, paralleling p21ras activation. In contrast, endothelin fails to phosphorylate MAP kinase. Treatment of the cells with pertussis toxin blocks LPA-induced MAP kinase phosphorylation without affecting the other tyrosine phosphorylations. The kinase inhibitor staurosporine (1 microM) blocks LPA-induced, but not epidermal growth factor-induced, activation of p21ras and MAP kinase, consistent with an intermediate protein kinase linking the LPA receptor to p21ras activation. The results support a model in which LPA-induced phosphorylation of MAP kinase is mediated by p21ras, and tyrosine phosphorylation of the other substrates, including p125FAK, is associated with phospholipase C activation.

Second messenger modulation of epidermal growth factor receptor function does not occur at the level of receptor dimerization.

Epidermal growth factor (EGF)-induced receptor dimerization may provide a mechanism for activation of the receptor protein tyrosine kinase and for initiation of post-receptor signalling pathways. We have examined whether second messengers and agents that modulate EGF receptor function act at the level of receptor dimerization. Both the Ca2+ ionophore ionomycin and the tumour promotor tetradecanoylphorbol acetate (TPA), added shortly before EGF, inhibit EGF receptor protein tyrosine kinase activity in intact cells. In permeabilized cells, elevation of Ca2+ similarly inhibits EGF receptor function. The inhibitory effect of Ca2+, unlike that of TPA, appears not to be dependent on protein kinase C activity. Neither ionomycin nor phorbol ester affects EGF-induced receptor dimerization, as shown by cross-linking and immunoblotting techniques, although the phosphotyrosine content of both monomeric and dimeric receptors is strongly decreased. Furthermore, we show that EGF receptor dimerization is not affected by increases in cyclic AMP or intracellular pH, nor by changes in transmembrane potential, medium osmolarity or the glycosylation state of the receptor. These result suggest that modulation of EGF receptor function occurs at a step other than receptor dimerization.

Epidermal growth factor-induced phosphoinositide hydrolysis in permeabilized 3T3 cells: lack of guanosine triphosphate dependence and inhibition by tyrosine-containing peptides.

The possible involvement of a stimulatory guanosine triphosphate (GTP)-binding (G) protein in epidermal growth factor (EGF)-induced phosphoinositide hydrolysis has been investigated in permeabilized NIH-3T3 cells expressing the human EGF receptor. The mitogenic phospholipid lysophosphatidate (LPA), a potent inducer of phosphoinositide hydrolysis, was used as a control stimulus. In intact cells, pertussis toxin partially inhibits the LPA-induced formation of inositol phosphates, but has no effect on the response to EGF. In cells permeabilized with streptolysin-O, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) dramatically increases the initial rate of inositol phosphate formation induced by LPA. In contrast, activation of phospholipase C (PLC) by EGF occurs in a GTP-independent manner. Guanine 5'-O-(2-thiodiphosphate) (GDP beta S) which keeps G proteins in their inactive state, blocks the stimulation by LPA and GTP gamma S, but fails to affect the EGF-induced response. Tyrosine-containing substrate peptides, when added to permeabilized cells, inhibit EGF-induced phosphoinositide hydrolysis without interfering with the response to LPA and GTP gamma S. These data suggest that the EGF receptor does not utilize an intermediary G protein to activate PLC and that receptor-mediated activation of effector systems can be inhibited by exogenous substrate peptides.

Histamine as a growth factor and chemoattractant for human carcinoma and melanoma cells: action through Ca2(+)-mobilizing H1 receptors.

Histamine receptors are present on the surface of various normal and tumor-derived cell types, where their biological function is incompletely understood. Here we report that histamine not only stimulates cell proliferation under serum-free conditions, but also is chemotactic for human carcinoma (Hela and A431) and melanoma (A875) cells expressing H1 type receptors. Histamine was found to be a potent activator of phospholipase C, leading to polyphosphoinositide hydrolysis and subsequent intracellular Ca2+ mobilization. In addition, histamine also causes the protein kinase C-mediated activation of Na+/H+ exchange, as evidenced by an amiloride-sensitive rise in cytoplasmic pH. All histamine-induced responses, including chemotaxis and DNA synthesis, are completely inhibited by the H1 receptor antagonist pyrilamine, but not by cimetidine, an inhibitor of histamine H2 type receptors. Our results suggest that histamine may have a previously unrecognized role in the migration and proliferation of cells expressing H1 receptors.

High-affinity epidermal growth factor binding is specifically reduced by a monoclonal antibody, and appears necessary for early responses.

We have tested the effects of an mAb directed against the protein core of the extracellular domain of the human EGF receptor (mAb108), on the binding of EGF, and on the early responses of cells to EGF presentation. We used NIH 3T3 cells devoid of murine EGF receptor, transfected with a cDNA encoding the full-length human EGF receptor gene, and fully responsive to EGF. The binding to saturation of mAb108 to the surface of these cells at 4 degrees C and at other temperatures specifically reduced high-affinity binding of EGF, but did not change the dissociation constant or the estimated number of binding sites for low-affinity binding of EGF. The kinetics of EGF binding to the transfected cells were measured to determine the effects of the mAb on the initial rate of EGF binding at 37 degrees C. Interestingly, high-affinity EGF receptor bound EGF with an intrinsic on-rate constant 40-fold higher (9.8 x 10(6) M-1.s-1) than did low-affinity receptor (2.5 x 10(5) M-1.s-1), whereas the off-rate constants, measured at 4 degrees C were similar. Cells treated with the mAb or with phorbol myristate acetate displayed single on-rate constants similar to that for the low-affinity receptors. At low doses of EGF ranging from 0.4 to 1.2 nM, pretreatment of cells with mAb108 inhibited by 50-100% all of the early responses tested, including stimulation of tyrosine-specific phosphorylation of the EGF receptor, turnover of phosphatidyl inositol, elevation of cytoplasmic pH, and release of Ca2+ from intracellular stores. At saturating doses of EGF (20 nM) the inhibition of these early responses by prebinding of mAb108 was overcome. On the basis of these results, we propose that the high-affinity EGF receptors are necessary for EGF receptor signal transduction.

Epidermal-growth-factor-induced formation of inositol phosphates in human A431 cells. Differences from the effect of bradykinin.

In human A431 epidermoid carcinoma cells, epidermal growth factor (EGF) rapidly stimulates the breakdown of inositol phospholipids and raises cytoplasmic free [Ca2+]. In this paper, we investigate the action of EGF on inositol phosphate metabolism, and we compare it with the previously described effects of bradykinin on the same cell system [Tilly, van Paridon, Verlaan, Wirtz, de Laat & Moolenaar (1987) Biochem. J. 244, 129-135]. In cells prelabelled with [3H]inositol, EGF slowly but persistently (for at least 30 min) stimulates the formation of [3H]inositol phosphates, whereas bradykinin causes an immediate but transient release of inositol phosphates, which lasts for only a few minutes. The EGF effect is additive to bradykinin stimulation and does not require extracellular Ca2+. In contrast, inositol phosphate formation induced by Ca2+-ionophore A23187 has an absolute requirement for external Ca2+. Treatment of the cells with 12-O-tetradecanoylphorbol 13-acetate completely abolishes the response to EGF and to sub-optimal doses of bradykinin, suggesting a negative-feedback function of protein kinase C. Pretreatment of the cells with pertussis toxin has no effect on inositol phosphate formation induced by either EGF or bradykinin. Unlike bradykinin, EGF stimulates very little accumulation of inositol 1,4,5-trisphosphate, with only a small and rather variable release of Ca2+ from intracellular stores. EGF rapidly but transiently increases inositol 1,3,4-trisphosphate and 1,3,4,5-tetrakisphosphate, but the effects are much smaller than those of bradykinin. In addition, EGF increases both inositol mono- and bis-phosphate. At 10 min after EGF addition, inositol monophosphate, unlike the other inositol phosphates, is still increasing. It is concluded that the EGF-dependent pattern of stimulation is different from that observed in bradykinin-stimulated A431 cells, suggesting that there are separate mechanisms of inositol-lipid hydrolysis involved.

A point mutation at the ATP-binding site of the EGF-receptor abolishes signal transduction.

The EGF-receptor (EGF-R) is a transmembrane glycoprotein with intrinsic protein tyrosine kinase (TK) activity. To explore the importance of the receptor TK in the action of EGF, we have used transfected NIH-3T3 cells expressing either the normal human EGF-R or a receptor mutated at Lys721, a key residue in the presumed ATP-binding region. The wild-type receptor responds to EGF by causing inositol phosphate formation, Ca2+ influx, activation of Na+/H+ exchange and DNA synthesis. In contrast, the TK-deficient mutant receptor fails to evoke any of these responses. It is concluded that activation of the receptor TK is a crucial signal that initiates the multiple post-receptor effects of EGF leading to DNA synthesis. Furthermore, the results suggest that tyrosine phosphorylation plays a role in the activation of the phosphoinositide signalling system.

Inositol phosphate metabolism in bradykinin-stimulated human A431 carcinoma cells. Relationship to calcium signalling.

Stimulation of human A431 epidermoid carcinoma cells by bradykinin causes a very rapid release of inositol phosphates and a transient rise in cytoplasmic free Ca2+ concentration ([Ca2+]i). Bradykinin-induced inositol phosphate formation is half-maximal at a concentration of 4 nM and is not affected by pertussis toxin. H.p.l.c. analysis of the various inositol phosphates shows an immediate but transient accumulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], which reaches a peak value of approx. 10 times the basal level within 15 s and slightly precedes the rise in [Ca2+]i, both parameters changing in parallel. After a lag period, bradykinin also induces a massive accumulation of Ins(1,3,4)P3 and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. Our data support the view that part of the newly formed Ins(1,4,5)P3 is converted into Ins(1,3,4)P3 phosphorylation/dephosphorylation with Ins(1,3,4,5)P4 as intermediate. Furthermore, A431 cells were found to contain strikingly high basal levels of two other inositol phosphates, presumably inositol pentakisphosphate (InsP5) and inositol hexakisphosphate (InsP6), representing more than 50% of the total 3H radioactivity incorporated into inositol phosphates. The presumptive InsP5 and InsP6 are only slightly affected by bradykinin. Although Ins(1,3,4)P3 and InsP4 could function as second messengers, our results suggest that, unlike Ins(1,4,5)P3, neither Ins(1,3,4)P3 nor InsP4 are involved in Ca2+ mobilization.

Expression of pp60v-src alters the ionic permeability of the plasma membrane in rat cells.

The transmembrane potential of Rous sarcoma virus (RSV)-infected Rat-1 cells, expressing the pp60v-src protein kinase, is markedly less negative (by approximately 30 mV) than that of their normal counterparts. By contrast, the membrane potential of Rat-1 cells infected with Kirsten sarcoma virus is virtually unaltered. The RSV-induced membrane depolarization is shown to be due to a severalfold increase in the cation permeability ratio (PNa/PK) of the plasma membrane. When cells infected with a temperature-sensitive mutant of RSV (ts LA29), encoding a src protein with heat-labile kinase activity, are shifted from the nonpermissive to the permissive temperature, a rapid and sustained membrane depolarization is observed. Conversely, thermal inactivation of the ts LA29 pp60v-src kinase activity rapidly restores the membrane potential to near normal levels. Addition of epidermal growth factor, platelet-derived growth factor, or insulin to uninfected cells fails to cause a detectable change in membrane potential. We conclude that, unlike growth factor receptor tyrosine kinases, pp60v-src can induce, either directly or indirectly, a major change in the membrane permeability to monovalent cations.

Growth factor-like action of phosphatidic acid.

Phosphatidic acid (PA), an intriguing phospholipid that is rapidly produced during receptor-stimulated breakdown of phosphoinositides, has often been proposed to function as a Ca2+ ionophore in activated cells. The PA-ionophore hypothesis is supported by the fact that exogenously applied PA stimulates Ca2+ uptake in various cells and can evoke Ca2+-mediated physiological responses, but it is not known whether PA accumulation affects cytoplasmic free Ca2+ concentration ([Ca2+]i). Here we report that PA elicits a transient rise in [Ca2+]i in cultured cells, not by stimulating Ca2+ influx, but, surprisingly, by releasing Ca2+ from intracellular stores. We further show that PA evokes growth factor-like effects in that it raises cytoplasmic pH, induces expression of the c-fos and c-myc proto-oncogenes and stimulates DNA synthesis. Our results indicate that, unlike an ionophore, PA acts by triggering the hydrolysis of phosphoinositides, with consequent formation of second messengers such as inositol trisphosphate signalling Cai2+ release. Furthermore, our data strengthen the notion that any Ca2+-mobilizing stimulus acting through phospholipase C may ultimately function as a growth factor.