Stimulation of DNA synthesis by transient exposure of cell cultures to TPA or polypeptide mitogens: induction of competence or incomplete removal?

Protein kinase C signaling "in" and "to" the nucleus: Master kinases in transcriptional regulation

PKC is a multifunctional family of Ser-Thr kinases widely implicated in the regulation of fundamental cellular functions, including proliferation, polarity, motility, and differentiation. Notwithstanding their primary cytoplasmic localization and stringent activation by cell surface receptors, PKC isozymes impel prominent nuclear signaling ultimately impacting gene expression. While transcriptional regulation may be wielded by nuclear PKCs, it most often relies on cytoplasmic phosphorylation events that result in nuclear shuttling of PKC downstream effectors, including transcription factors. As expected from the unique coupling of PKC isozymes to signaling effector pathways, glaring disparities in gene activation/repression are observed upon targeting individual PKC family members. Notably, specific PKCs control the expression and activation of transcription factors implicated in cell cycle/mitogenesis, epithelial-to-mesenchymal transition and immune function. Additionally, PKCs isozymes tightly regulate transcription factors involved in stepwise differentiation of pluripotent stem cells toward specific epithelial, mesenchymal, and hematopoietic cell lineages. Aberrant PKC expression and/or activation in pathological conditions, such as in cancer, leads to profound alterations in gene expression, leading to an extensive rewiring of transcriptional networks associated with mitogenesis, invasiveness, stemness, and tumor microenvironment dysregulation. In this review, we outline the current understanding of PKC signaling "in" and "to" the nucleus, with significant focus on established paradigms of PKC-mediated transcriptional control. Dissecting these complexities would allow the identification of relevant molecular targets implicated in a wide spectrum of diseases.

The PKC universe keeps expanding: From cancer initiation to metastasis

Classical and novel protein kinase C (PKC) isozymes (c/nPKCs), members of the PKC family that become activated by the lipid second messenger diacylglycerol (DAG) and phorbol esters, exert a myriad of cellular effects that impact proliferative and motile cellular responses. While c/nPKCs have been indisputably associated with tumor promotion, their roles exceed by far their sole involvement as promoter kinases. Indeed, this original dogma has been subsequently redefined by the introduction of several new concepts: the identification of tumor suppressing roles for c/nPKCs, and their participation in early and late stages of carcinogenesis. This review dives deep into the intricate roles of c/nPKCs in cancer initiation as well as in the different stages of the metastatic cascade, with great emphasis in their involvement in cancer cell motility via regulation of small Rho GTPases, the production of extracellular matrix (ECM)-degrading proteases, and the epithelial-to-mesenchymal transition (EMT) program required for the acquisition of highly invasive traits. Here, we highlight functional interplays between either PKCα or PKCε and mesenchymal features that may ultimately contribute to anticancer drug resistance in cellular and animal models. We also introduce the novel hypothesis that c/nPKCs may be implicated in the control of immune evasion through the regulation of immune checkpoint protein expression. In summary, dissecting the colossal complexity of c/nPKC signaling in the wide spectrum of cancer progression may bring new opportunities for the development of meaningful tools aiding for cancer prognosis and therapy.

Signalling mitogenesis in 3T3 cells: role of Ca2+-sensitive, phospholipid-dependent protein kinase

Understanding the molecular mechanisms that control cell proliferation requires the identification of the early signals important for initiating a mitogenic response. In this context, the activation of Ca2+-sensitive, phospholipid-dependent protein kinase (protein kinase C), which is stimulated by diacylglycerols and serves as a major phorbol ester receptor, may play an important part in signalling mitogenesis. This conclusion is based on two main lines of evidence. Firstly, activation of protein kinase C in intact quiescent fibroblasts is one of the earliest events elicited by a variety of growth-promoting agents including serum, platelet-derived growth factor (PDGF), vasopressin and bombesin, as judged by the increase in the phosphorylation of a cellular protein characterized by an Mr of 80 000 and a pI of 5. Secondly, the synthetic diacylglycerol, 1-oleoyl-2-acetylglycerol, which directly competes with [3H]phorbol dibutyrate for binding to specific receptors in intact 3T3 cells and rapidly stimulates protein kinase C in these cells, is a potent mitogen for Swiss 3T3 cells, acting synergistically with other growth factors. We propose that activation of protein kinase C may be one of the early signals that mediate the mitogenic effects of a variety of growth factors and peptide hormones in quiescent fibroblastic cells.

Sodium arsenite inhibits terminal differentiation of murine C3H 10T1/2 preadipocytes

Cancer represents an imbalance between cell proliferation and differentiation, two processes that are coordinately and antagonistically regulated. Aberrant cell proliferation is considered to be an important etiological factor in the development of arsenic-induced cancer, suggesting that arsenic also dysregulates differentiation. Based on evidence that arsenic modulates mitogenic events that antagonize the process of differentiation, this study addresses the hypothesis that sodium arsenite inhibits insulin/dexamethasone-induced differentiation of C3H 10T1/2 preadipocytes; it was further postulated that arsenic-treated cells retain mitogenic responsiveness under differentiating conditions. To test this hypothesis, the differentiation capacity of C3H 10T1/2 preadipocytes was examined in control cells and cells treated with sodium arsenite. Differentiation was assessed morphologically and quantified by Oil Red-O staining of accumulated lipids. The effect of long-term arsenic exposure on mitogenic competence was quantified by flow cytometry, [(3)H]thymidine incorporation, and cell counting under conditions favorable for adipocyte differentiation. Results indicate that arsenic inhibits morphological differentiation of wild-type C3H 10T1/2 preadipocytes. Short-term arsenic exposure inhibits differentiation in a dose-dependent manner, with arsenic concentrations > or = 3 microM producing a significant inhibition of dexamethasone/insulin-induced lipid accumulation. Furthermore, arsenic-treated cells exhibit an accentuated response to mitogenic stimulation under differentiating conditions. These data suggest that arsenic exposure results in the inhibition of cellular programming required for terminal differentiation of C3H 10T1/2 preadipocytes and that cells acquire mitogenic hyperresponsiveness. The ability of arsenic to dysregulate the balance between proliferation and differentiation is proposed to be one mechanism by which this metalloid causes cancer in humans.

Synergistic stimulation of DNA synthesis by bradykinin and vasopressin in Swiss 3T3 cells

Vasopressin and bradykinin bind to receptors coupled to GTP-binding proteins and rapidly induce polyphosphoinositide breakdown leading to Ca2+ mobilization and activation of protein kinase C. Both peptides are known to induce mitogenesis in the presence of growth factors that act through receptors with intrinsic tyrosine kinase activity. Surprisingly, addition of a combination of vasopressin and bradykinin to Swiss 3T3 cells synergistically stimulates DNA synthesis in the absence of any other growth factors. This effect is induced at nanomolar concentrations of the peptides and could be inhibited by addition of specific receptor antagonists or broad spectrum neuropeptide antagonists. Bradykinin, which stimulates transient activation of protein kinase C, induces DNA synthesis in synergy with substances that cause long-term activation of protein kinase C, like vasopressin or phorbol 12,13-dibutyrate. Down-regulation of protein kinase C inhibited the induction of mitogenesis by the combination of vasopressin and bradykinin, thus demonstrating the importance of long-term activation of this enzyme for DNA synthesis. Analysis of tyrosine phosphorylated proteins of M(r) = 110,000-130,000 and M(r) = 70,000-80,000 revealed a biphasic response after stimulation with bradykinin, whereas the response induced by vasopressin declined after the initial maximum. The combination of bradykinin with vasopressin caused an enhanced and prolonged increase in tyrosine phosphorylation of these proteins as compared with the individual peptides. Inhibition of tyrosine phosphorylation by tyrphostin was paralleled by inhibition of DNA synthesis. Together, these results demonstrate synergistic stimulation of DNA synthesis by bradykinin and vasopressin via prolonged stimulation of multiple signaling pathways and imply that the interactive effects of Ca(2+)-mobilizing peptides on mitogenesis may be more general than previously thought.

Regulation of epidermal growth factor receptor expression and growth by protein kinase C and retinoic acid in LLC-PK1 cells

The importance of receptor expression and protein kinase C to epidermal growth factor (EGF)-induced cell proliferation was studied in LLC-PK1 kidney cells. These cells have both high- and low-affinity binding sites for EGF. Neither transforming growth factor-beta nor tumor necrosis factor altered EGF receptor expression. On the other hand, retinoic acid induced a concentration-dependent increase in EGF binding that was maximal at 1 mumol/L. One micromolar of retinoic acid increased EGF binding from 0.38 +/- 0.01 fmol/10(6) cells in controls to 1.10 +/- 0.03 fmol/10(6) in treated cells at 18 hours (n = 8, P < 0.001). The increase in binding was the result of an increase in the Bmax of the high-affinity receptor. The upregulation of the EGF receptor induced by retinoic acid was associated with enhanced EGF-induced growth promotion. A 45-minute incubation of cells with phorbol 12-myristate 13-acetate caused a concentration-dependent decrease in EGF binding that was prevented by a 40-hour, 2 mumol/L pre-exposure to phorbol 12-myristate 13-acetate; 10(-8) mol/L EGF also caused a downregulation of the EGF receptor that was not prevented by phorbol 12-myristate 13-acetate or retinoic acid. Downregulation of protein kinase C did not interfere in the capacity of EGF to induce growth in these cells. These studies demonstrate that EGF receptor upregulation plays an important role in the control of EGF-induced cell growth. Protein kinase C regulates EGF binding in these cells; however, it is not necessary for EGF-induced growth promotion or receptor downregulation.

The dephosphorylation characteristics of the receptors for epidermal growth factor and platelet-derived growth factor in Swiss 3T3 cell membranes suggest differential regulation of receptor signalling by endogenous protein-tyrosine phosphatases

Comparison of the phosphotyrosine-specific dephosphorylation of the autophosphorylated receptors for epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) in Swiss 3T3 cell membranes by the endogenous phosphatases revealed striking differences. EGF receptor dephosphorylation was clearly faster than PDGF receptor dephosphorylation and strongly inhibited by Triton X-100 and octylglucoside, whereas PDGF receptor dephosphorylation was to a lesser extent detergent-susceptible. PDGF receptor dephosphorylation was effectively inhibited by phenylarsineoxide, protamine and poly-lysine and partially by N-ethylmaleinimide, whereas EGF receptor dephosphorylation was not affected by these agents. We suggest that these differences in dephosphorylation of EGF and PDGF receptors are due to their differential interaction with membrane-associated protein-tyrosine phosphatases and important for differential regulation of receptor signalling.

Tumor necrosis factor-alpha production induced in T lymphocytes through the AIM/CD69 activation pathway

Human activation inducer molecule (AIM/CD69), a dimeric glycoprotein of 33 and 27 kDa, is the earliest inducible cell surface antigen expressed during lymphocyte activation, which has been also involved in lymphocyte proliferation. Although AIM is absent from peripheral blood resting lymphocytes, it is expressed by in vivo activated lymphocytes infiltrating sites of chronic inflammation in several pathologies, as well as by lymphocytes after in vitro activation with different stimuli. We have investigated the possibility that tumor necrosis factor-alpha (TNF-alpha) gene expression and protein secretion could be induced in peripheral blood T cells through the AIM/CD69 molecule. Anti-AIM monoclonal antibodies (mAb) were able to induce TNF-alpha secretion in T cells when protein kinase C (PKC) was simultaneously activated by treatment with phorbol esters. TNF-alpha secretion was detected at 24 h and peaked at day 3 upon T lymphocyte activation with anti-AIM mAb. Immunoprecipitation studies with an anti-TNF-alpha mAb from surface iodinated T cells activated through AIM, demonstrated that TNF-alpha first appeared as a cell surface molecular form of 26 kDa, which is subsequently released to the extracellular medium as the 17-kDa molecular form of TNF-alpha. AIM stimulation dramatically increased TNF-alpha mRNA levels, and this mRNA induction and subsequent TNF-alpha secretion were virtually abrogated by the immunosuppressive drug cyclosporin A. Taken together these results indicate that AIM constitutes a novel molecular pathway in T lymphocytes for induction of TNF-alpha, and suggest a relevant pathologic role for AIM+ lymphocytes located at sites of tissue injury in the pathogenesis of different chronic inflammatory diseases.

Phorbol-ester-mediated expression of the collagen type I pro-alpha 2 gene in mouse 3T3-L1 cells and its absence in a phorbol 12-myristate 13-acetate-non-responsive variant

We have previously isolated a phorbol 12-myristate 13-acetate (PMA)-non-responsive variant line (VT-1) from mouse 3T3-L1 cells [Shimizu, Fujiki, Sugimura & Shimizu (1986) Cancer Res. 46, 4027-4031]. Differential hybridization of cDNAs obtained from PMA-treated 3T3-L1 and VT-1 cells resulted in the isolation of a number of unique cDNA clones, including one with a high degree of sequence similarity to the type I pro-alpha 2 collagen gene (COL1A2) [Amagai, Inokuchi, Nishikawa, Shimizu & Shimizu (1989) Somat. Cell Mol. Genet. 15, 153-158]. Here we examined the expression of type I collagen pro-alpha 2 [pro-alpha 2(I)] mRNA and production of type I collagen in these two cell lines. In quiescent cells, the pro-alpha 2(I) steady-state mRNA levels were four times greater in 3T3-L1 cells than in VT-1 cells. PMA addition caused the steady-state levels of pro-alpha 2(I) mRNA to be six times greater in 3T3-L1 cells than in VT-1 cells, with a maximum at 30-60 min. The pro-alpha 2(I) protein levels in the extracellular matrix or culture media of 3T3-L1 cells were substantially elevated by PMA treatment, but no significant increase was detected in VT-1 cells. The correlation of collagen expression with a PMA-mediated mitogenic response suggests a new role for collagen as an early component of mitogenic signal transduction.

Butyrate blocks the accumulation of CDC2 mRNA in late G1 phase but inhibits both the early and late G1 progression in chemically transformed mouse fibroblasts BP-A31

Sodium butyrate (6 mM) blocks the resumption of the cell division cycle in serum-deprived chemically transformed Balb/c-3T3 mouse fibroblasts (BP-A31). The inhibition of G1 progression by sodium butyrate is not restricted to a specific mitogenic signaling pathway and is equally effective when tetradecanoyl phorbol acetate (TPA), insulin, or fetal calf serum (FCS) is used as inducer. The inhibitor acts in early as well as late G1 phase as indicated by experiments in which inhibitor was added and withdrawn at different times after restimulation of quiescent cells by FCS. At the gene expression level, sodium butyrate does not affect the inducibility of early cell cycle-related genes (c-myc, c-jun) while blocking the induction of cdc 2 mRNA, a late G1 marker. We conclude that sodium butyrate does not interfere with the growth factor signaling pathways regulating the (early) cell cycle-related gene expression. However, the presence of sodium butyrate early in G1 phase inhibits the cascade of events leading eventually to the expression of late G1-characteristic genes such as cdc2. The antimitogenic activity of sodium butyrate may be related to its interference with an (unknown) process involved in the "mitogenic" cascade.

Abnormal protein kinase C down regulation and reduced substrate levels in non-phorbol ester-responsive 3T3-TNR9 cells

The cell line TNR9 (E. Butler-Gralla and H. R. Herschman, J. Cell. Physiol. 107:59-67, 1981) in a Swiss 3T3 cell variant that expresses protein kinase C (PKC) but is mitogenically nonresponsive to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). We have found that PKCs purified from variant and parental cells are identical as judged by kinase activity, protease mapping, and column chromatography. We analyzed cellular levels and subcellular location of PKC in TPA-treated 3T3 and TNR9 cells via immunoprecipitation of [35S]methionine-labeled protein and assay of immune-complex PKC kinase activity. TNR9 cells grew to higher densities than parental 3T3 cells. TNR9 cells at maximal density did not down regulate PKC in response to long-term TPA treatment. We compared the 80-kilodalton (kDa) PKC substrate phosphorylation in 3T3 and TNR9 cells by using two-dimensional gels and found that TNR9 cells treated with TPA for 30 min contained only 10 to 15% as much 32Pi associated with the 80-kDa as did parental cells. The TNR9 80-kDa substrate was present at reduced levels compared with the parental-cell 80-kDa substrate as judged by immunoblot and silver staining. Thus, the loss of mitogenic responsiveness to TPA in TNR9 cells is accompanied by resistance to TPA-mediated down regulation of PKC and reduced phosphosubstrate levels.

Abnormal protein kinase C down regulation and reduced substrate levels in non-phorbol ester-responsive 3T3-TNR9 cells

The cell line TNR9 (E. Butler-Gralla and H. R. Herschman, J. Cell. Physiol. 107:59-67, 1981) in a Swiss 3T3 cell variant that expresses protein kinase C (PKC) but is mitogenically nonresponsive to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). We have found that PKCs purified from variant and parental cells are identical as judged by kinase activity, protease mapping, and column chromatography. We analyzed cellular levels and subcellular location of PKC in TPA-treated 3T3 and TNR9 cells via immunoprecipitation of [35S]methionine-labeled protein and assay of immune-complex PKC kinase activity. TNR9 cells grew to higher densities than parental 3T3 cells. TNR9 cells at maximal density did not down regulate PKC in response to long-term TPA treatment. We compared the 80-kilodalton (kDa) PKC substrate phosphorylation in 3T3 and TNR9 cells by using two-dimensional gels and found that TNR9 cells treated with TPA for 30 min contained only 10 to 15% as much 32Pi associated with the 80-kDa as did parental cells. The TNR9 80-kDa substrate was present at reduced levels compared with the parental-cell 80-kDa substrate as judged by immunoblot and silver staining. Thus, the loss of mitogenic responsiveness to TPA in TNR9 cells is accompanied by resistance to TPA-mediated down regulation of PKC and reduced phosphosubstrate levels.

Fibroblast growth factor-dependent mitogenic signal transduction pathway in chemically transformed mouse fibroblasts is similar to but distinct from that initiated by phorbol esters

Treatment of the quiescent, chemically transformed Balb/c mouse 3T3 cells (BP-A31) with fibroblast growth factor (FGF) leads to reinitiation of the cell division cycle in a large proportion of the cells. The characteristics of the mitogenic action of FGF closely resemble those of phorbol esters (activators of protein kinases type C) and differ from those of insulin (mediated by insulin-like growth factor 1 receptors). In particular, the effects of FGF as well as of phorbol-2-myristate-13-acetate (PMA), unlike the effects of insulin, are prevented by a low concentration (7.5 nM) of staurosporin (an efficient inhibitor of protein kinase C) as well as by 3-isobutyl-1-methyl xanthin (IBMX). Both FGF and PMA are good inducers of the accumulation of c-fos and c-jun mRNAs, whereas insulin has little effect. However, FGF was fully active (both as a mitogen and as inducer of c-fos mRNA accumulation) also in cells where the protein kinase C-mediated pathway had been downregulated by a long exposure to phorbol dibutyrate. We propose that the mitogenic effect of FGF does not require activation of protein kinase C, but that the subsequent events in the transduction pathways initiated by FGF and PMA, respectively, are (in part) coincident.

Expression and function of AIM, an activation inducer molecule of human lymphocytes, is dependent on the activation of protein kinase C

AIM is an activation inducer molecule selectively expressed by activated lymphocytes through which agonistic proliferative signals can be triggered. The relationship between the expression of AIM with the activation of protein kinase C (PKC) has been studied. Different activators of PKC such as the active phorbol esters, phorbol myristate acetate and phorbol dibutyrate, or the phorbol-related ester mezerein were able to induce AIM expression on peripheral blood lymphocytes as assessed by immunofluorescence flow cytometry. Moreover, the expression of this activation antigen was also induced by treatment of peripheral blood lymphocytes either with dioctanoyl-rac-glycerol, a synthetic analogue of diacylglycerol, the physiological mediator of PKC activation. Further indirect evidence that AIM expression was dependent on the activation of PKC was obtained by blockade of the induction of its expression in cells treated with H7, an inhibitor of PKC. The AIM expression can be detected as early as 3 h after addition of phorbol esters and it requires active RNA and protein synthesis. The activation of PKC appears to be also required in the proliferative response induced by anti-AIM monoclonal antibody (mAb) in conjunction with phorbol esters. Agents such as phorbol myristate acetate, phorbol dibutyrate or mezerein but not the inactive phorbol ester methyl-phorbol myristate acetate induced a high proliferation of peripheral blood lymphocytes in the presence of anti-AIM mAb. In addition, we have demonstrated that the anti-AIM mAb is not sufficient by itself to induce cellular proliferation once the AIM antigen is expressed at the cell surface, requiring the simultaneous stimulation of the PKC to trigger high proliferative responses. Furthermore, the anti-AIM mAb did not appear to exert its effect on proliferation by rapidly increasing the intracytoplasmic Ca2+ levels. Taken together all these results indicate that the expression and function of AIM antigen is dependent on the activation of PKC.

Mitogenic activity of phorbol esters and insulin-like growth factor 1 in chemically transformed mouse fibroblasts BP-A31: independent effects and differential sensitivity to inhibition by 3-isobutyl-1-methyl xanthine

Mitogenic effects of agents activating either the protein kinase C (PDGF; phorbol esters) or the insulin-like growth factor 1 (IGF1)-receptor pathway were studied in quiescent chemically transformed mouse fibroblasts (BP-A31), by evaluating the rate of [3H]thymidine incorporation. Each of these pathways alone was found to be sufficient to sustain progression through the entire cell division cycle. The mitogenic activity of phorbol 12-myristate 13-acetate (PMA) but not that of insulin was blocked by staurosporine (an inhibitor of protein kinase C), in support of the notion that protein kinase C activation was required for the PMA-induced cell cycle progression. The mitogenic effects of PMA were potentiated by cycloheximide pretreatment, and they were abolished by 3-isobutyl-1-methyl xanthine (IBMX; a cyclic nucleotide phosphodiesterase inhibitor). PDGF (known to activate the phospholipase C-protein kinase C pathway) also displayed mitogenic activity in the cycloheximide-pretreated BP-A31 cells, and its effects were prevented by IBMX. In contrast, the mitogenic effects of insulin (at concentrations where it activates the IGF1 receptor) or of IGF1 neither were notably influenced by cycloheximide pretreatment nor were inhibited by IBMX (in the presence of IBMX, the onset of S-phase was delayed by several hours). The expression of the c-fos gene was absent at quiescence; its induction by growth factors was not proportional to their mitogenic potency. Thus, c-fos expression was strongly induced by PMA but only weakly by insulin. IBMX was a powerful inducer of c-fos gene expression but caused a decrease in the level of c-myc mRNA.

Dephosphorylation of the human T lymphocyte CD3 antigen

Previous studies demonstrated that activation of T lymphocytes by phorbol ester or mitogenic lectin leads to phosphorylation of Ser 126 of the CD3 antigen gamma chain, whereas treatment with ionomycin results in phosphorylation of both Ser 123 and 126 [Davies, A. A. et al. (1987) J. Biol. Chem. 262, 10918-10921]. In the present study, the dephosphorylation of Ser 123 and Ser 126 of the gamma chain was investigated. Phorbol-ester-induced phosphorylation of the gamma-chain Ser 126 in vivo was reversed following removal of phorbol ester. Dephosphorylation of both Ser 123 and 126 was also observed in vitro using the microsome fraction of T lymphocytes. In order to identify the phosphatases acting at these two sites, the immunoprecipitated gamma chain was used as substrate either following treatment with protein kinase C in vitro, in which case phosphorylation occurs mainly at Ser 123, or following in vivo phosphorylation of Ser 126. Purified oligomeric forms of the polycation-stimulated phosphatases were more effective in dephosphorylating both phosphorylated forms of the gamma chain compared with equivalent amounts of ATP,Mg2+-dependent phosphatases or calcineurin. By using phosphopeptide analogues of the CD3 gamma chain containing Ser 123 or Ser 126 as substrates (A3 and A6), it was shown that polycation-stimulated phosphatases selectively dephosphorylated Ser 123 compared to Ser 126. In order to determine which phosphatases dephosphorylate the gamma chain in microsomes, A3 and A6 were used as substrates for characterising phosphatases in microsomes from human T leukaemia Jurkat 6 cells. Three phosphopeptide phosphatases (250-400 kDa) co-eluted through five purification steps with three forms of polycation-stimulated phosphorylase phosphatase. The partially purified A3/A6 phosphopeptide phosphatases were insensitive to Ca2+, calmodulin and inhibitor-1, and dephosphorylated A3 preferentially compared with A6. A latent form of microsomal ATP,Mg2+-dependent phosphorylase phosphatase was stimulated 10-fold by trypsinisation, but did not dephosphorylate phosphopeptides A3 and A6. The results show that high-Mr forms of polycation-stimulated phosphatases are the only enzymes in human T leukaemia cell microsomes which dephosphorylate gamma chain phosphopeptides. The data point to an important role for polycation-stimulated phosphatases in regulating the phosphorylation state, and so function(s), of the CD3 antigen.

Colchicine acts as a progression factor to initiate DNA synthesis in quiescent Balb/c 3T3 cells

In quiescent Balb/c 3T3 cells, competence factors such as 12-O-tetradecanoylphorbol-13-acetate (TPA) and platelet-derived growth factor (PDGF) synergize with progression factors such as insulin to initiate DNA synthesis. In this study, we found that colchicine, a microtubule-disrupting agent, acted synergistically with TPA, but not with insulin, to induce the maximal stimulation of DNA synthesis. Colchicine also synergized with PDGF in the presence of epidermal growth factor to elicit nearly the optimal induction of DNA synthesis. Moreover, it acted synergistically with fibroblast growth factor, another competence factor. These results suggest that colchicine acts as a progression factor like insulin in quiescent Balb/c 3T3 cells.

Protein kinase C in the regenerating rat liver

Protein kinase C (PKC) activity in the soluble fraction of regenerating rat liver showed a biphasic decrease prior to the initiation of DNA synthesis. The first decrease was seen at 3-6 h after partial hepatectomy (PH). The second decrease occurred after 14 h and continued until 24 h, while DNA synthesis began 18-20 h and reached peak value 22 h after PH. It is suggested that the soluble PKC in regenerating rat liver is activated and bound to the plasma membrane prior to the initiation of DNA synthesis.

Biochemical events associated with inhibition of B-cell proliferation by phorbol diesters

Phorbol myristate acelate (PMA), a potent tumor promoter, has a variety of effects on cells of the immune system resulting in altered patterns of cell proliferation and differentiation. Although PMA is mitogenic or co-mitogenic for human lymphocytes and murine T-cells, it inhibits proliferation of murine B-cells stimulated by LPS or anti-Ig. PMA, however, does not inhibit the ability of LPS or anti-Ig to activate B-cells, as evidenced by increased Ia antigen expression and RNA synthesis. In the present studies it was shown that inhibition of DNA synthesis by PMA coincided with qualitative and quantitative changes in phosphorylated proteins. In particular, PMA treatment resulted in a unique profile of phosphoproteins independent of LPS or anti-Ig treatment. Inhibition of DNA synthesis occurred over a wide range of PMA concentrations. At concentrations up to 10(-9) M, inhibition of proliferation correlated with decreased phosphatidylinositol turnover and decreased intracellular Ca2+ levels, suggesting that PMA affects the phosphoinositide signal transduction pathway. However, at PMA concentrations less than 10(-10) M, inhibition of anti-Ig- and LPS-mediated proliferation occurred without inhibition of the phosphoinositide transduction signal. At these concentrations, PMA-induced inhibition of DNA synthesis was highly sensitive to recombinant IL-2. These data suggest that the antiproliferative effects of PMA on B-cells stimulated by LPS or anti-Ig may be mediated by two mechanisms. At high concentrations, PMA causes a feedback regulation of the phosphoinositide-dependent messenger system, while at lower concentrations, PMA alters the response to specific growth factors. Since PMA induces unique phosphoproteins and both of these events can be regulated by protein phosphorylation, it is possible that these unique phosphoproteins are responsible for the antiproliferative effects of PMA.

Regulation of dog thyroid epithelial cell cycle by forskolin, an adenylate cyclase activator

Dog thyroid epithelial follicular cells in primary culture are quiescent in an insulin-supplemented serum-free medium. They are induced, after a 16- to 20-h prereplicative phase, to synthesize DNA upon stimulation by forskolin, a general adenylate cyclase activator that mimics all the effects of thyrotropin in these cells. The characteristics of adenylate cyclase activation by forskolin make this drug a convenient tool to enhance cellular cyclic AMP levels for well-defined periods of the cell cycle, allowing determination of which parts of the prereplicative phase are controlled by cyclic AMP. We observe that induction of DNA synthesis by forskolin requires its continuous presence for most of the prereplicative phase until a point that little precedes the initiation of DNA replication. Before this point, interruptions in forskolin presence as short as 2 h delay the onset of DNA synthesis, indicating a rapid regression of the cells to an earlier part of G1 from which they can be rescued by forskolin readdition. Similar delays in the onset of S phase are also induced by reversible protein synthesis inhibitions using pulses of cycloheximide. These data suggest that in dog thyrocytes elevated cyclic AMP levels stimulate the progression into G1 phase until a late commitment point before DNA synthesis. This progression depends on peculiarly labile cyclic AMP-stimulated events which might well be the induction by cyclic AMP of the synthesis of labile proteins.

Identification of a signal-transduction pathway shared by haematopoietic growth factors with diverse biological specificity

The haematopoietic growth factors multi-colony-stimulating factor, granulocyte/macrophage colony-stimulating factor, granulocyte colony-stimulating factor and interleukin 2 specifically control the production and proliferation of distinct leucocyte series. Each growth factor acts on a unique surface receptor associated with an appropriate signal-transduction apparatus. In this report we identify a 68 kDa substrate which is phosphorylated after stimulation of different cell types with multi-colony-stimulating factor, granulocyte colony-stimulating factor and interleukin 2. The 68 kDa substrate is also phosphorylated in each cell line stimulated with synthetic diacylglycerol, a direct activator of protein kinase C. Interestingly, granulocyte/macrophage colony-stimulating factor does not induce phosphorylation of the 68 kDa molecule. The 68 kDa molecule that is phosphorylated after stimulation with each ligand yielded similar peptide maps after chymotryptic digestion; furthermore, the substrate was always phosphorylated on threonine residues. Phosphorylation of the same residues in the 68 kDa substrate suggests that activation of protein kinase C is one common signal-transduction event associated with the action of multi-colony-stimulating factor, granulocyte colony-stimulating factor and interleukin 2.

Identity of common phosphoprotein substrates stimulated by interleukin 2 and diacylglycerol suggests a role of protein kinase C for IL 2 signal transduction

Interleukin 2 (IL 2) is a polypeptide growth factor essential for the proliferation and differentiation of T lymphocytes, large granulocytic lymphocytes, and, potentially, cells of the antibody-producing lineage, B lymphocytes. Many of the biological properties of IL 2 may be mimicked or potentiated by a potent class of tumor promoters, phorbol esters. Phorbol esters have recently been shown to associate with and activate a unique phospholipid/Ca2+-dependent phosphotransferase, protein kinase C (PK-C). Utilizing two-dimensional gel electrophoresis, we have compared the IL 2 and diacylglycerol-induced protein phosphorylation patterns of several IL 2-dependent murine cell lines. Both IL 2 and synthetic diacylglycerol, 1-oleyl-2-acetylglycerol (OAG), stimulated phosphorylation of a number of protein substrates in intact cells compared to unstimulated controls. Three groups of substrates were identified; the first showed increased phosphorylation following stimulation with either IL 2 or OAG, while the second and third groups showed increased phosphorylation following stimulation with IL 2 but not OAG, and with OAG but not IL 2, respectively. Here, we characterize the kinetics of phosphorylation of one cellular substrate, p68, which appears to be phosphorylated in response to direct activators of PK-C or lymphoid or myeloid growth factors in their respective lineage cell lines. The observation that IL 2 also stimulates a unique series of phosphoproteins in addition to those induced by direct PK-C activators suggests that IL 2 may initiate additional protein kinase activities, unrelated to PK-C, which may also be critical for the ligand-receptor signal transduction process regulating growth and gene expression.

Bombesin induction of c-fos and c-myc proto-oncogenes in Swiss 3T3 cells: significance for the mitogenic response

Bombesin is a potent mitogen for Swiss 3T3 cells and acts synergistically with insulin and other growth factors. We show here that addition of bombesin to quiescent Swiss 3T3 cells causes a striking increase in the levels of c-fos and c-myc mRNAs. Enhanced expression of c-fos (122 +/- 14-fold) occurred within minutes of peptide addition followed by increased expression of c-myc (82 +/- 16-fold). The concentrations of peptide required for half-maximal increase in the levels of c-fos and c-myc mRNAs were 1.0 and 0.9 nM, respectively. The peptide [D-Arg1, D-Pro2, D-Trp7,9, Leu11] substance P which inhibits the binding of bombesin to its receptor and bombesin-stimulated DNA synthesis in Swiss 3T3 cells blocked the increase in c-fos and c-myc mRNA levels promoted by bombesin. Down-regulation of protein kinase C by long-term exposure to phorbol esters prevented c-fos and c-myc induction by bombesin. This and other results indicate that the induction of these proto-oncogenes by bombesin could be mediated by the coordinated effects of protein kinase C activation and Ca2+ mobilization. The marked synergistic effect between bombesin and insulin was used to assess whether the increase in the induction of c-fos and c-myc is an obligatory event in cell activation. In the presence of insulin, bombesin stimulated DNA synthesis at subnanomolar concentrations but had only a small effect on c-fos and c-myc mRNA levels. This apparent dissociation of mitogenesis from proto-oncogene induction was even more dramatic in 3T3 cells with down-regulated protein kinase C. In these cells bombesin stimulated DNA synthesis in the presence of insulin but failed to enhance c-fos and c-myc mRNA levels at comparable concentrations. Thus, the induction of c-fos and c-myc may be a necessary step in the mitogenic response initiated by ligands that act through activation of protein kinase C but the expression of these proto-oncogenes may not be an obligatory event in the stimulation of mitogenesis in 3T3 cells by mitogens that utilise other signalling pathways.

Thyrotropin-releasing hormone activates Na+/H+ exchange in rat pituitary cells

The effects of thyrotropin-releasing hormone (TRH) and 12-O-tetradecanoylphorbol 13-acetate (TPA) on cytosolic pH (pHi) were studied on GH4C1 pituitary cells loaded with the fluorescent pH indicator bis(carboxyethyl)carboxyfluorescein (BCECF) and the fluorescent Ca2+ indicator quin2. TRH, which was minimally effective at around 10(-9) M, and TPA, 100 nM, produced very small elevations in pHi of about 0.05 pH units from the normal basal resting pHi of GH4C1 cells of around 7.05. The effects were more marked after acid-loading the cells using 1 micrograms of nigericin/ml. Preincubation with amiloride or replacing the extracellular Na+ with choline+ completely blocked the elevations stimulated by TRH or TPA, consistent with an activation of the Na+/H+ antiport mechanism. The effects were completely independent of the cytoplasmic free calcium concentration ([Ca2+]i). The calcium ionophore ionomycin produced an elevation in [Ca2+]i with no concomitant effect on pHi, and amiloride, although completely inhibiting the pH change stimulated by TRH, failed to affect the initial stimulated [Ca2+]i transient. Although the data are consistent with an elevation in pHi by TRH which is caused by stimulation of a protein kinase C and subsequent activation of the antiporter, the rapidity of the onset of the pHi response to TRH could not be mimicked by a combination of TPA and ionomycin. These results, together with previous findings which show that secretion can be mimicked by TPA and ionomycin, suggest that TRH-stimulated Na+/H+ exchange plays no part in the acute stimulation of secretion, but that TRH increases the pH-sensitivity of the antiport system during increased synthesis of prolactin and growth hormone.

Platelet-derived growth factor-induced alterations in vinculin distribution in porcine vascular smooth muscle cells

Exposure of porcine vascular smooth muscle cells to platelet-derived growth factor (PDGF; 18-180 ng/ml) but not epidermal growth factor (EGF; 30 ng/ml), somatomedin C (SmC; 30 ng/ml), or insulin (10 microM), results in a rapid, reversible, time- and concentration-dependent disappearance of vinculin staining in adhesion plaques; actin-containing stress fibers also become disrupted following exposure of cells to PDGF. Disappearance of vinculin staining from adhesion plaques is also caused by 12-O-tetradecanoyl-phorbol-13-acetate (TPA; 200-400 nM), though the time course of the disappearance of vinculin staining under these conditions takes longer than in cells exposed to PDGF. The PDGF-induced removal of vinculin from adhesion plaques was inhibited in a concentration-dependent fashion by 8-(N,N-diethylamino) octyl-3,4,5-trimethoxybenzoate (TMB-8; 0.25-4 microM) and leupepetin (2-300 microM), and by n-alpha-tosyl-L-lysine chloromethylketone (TLCK; 100 microM) and trifluoperazine (TFP; 2.5 microM). Addition of PDGF to vascular smooth muscle cells caused a rapid, transient increase in cytosolic free calcium, from a basal resting level of 146 +/- 6.9 nM (SEM, n = 62) to 414 +/- 34 nM (SEM, n = 22) as determined using the calcium-sensitive indicator Fura-2 and Digitized Video Microscopy. This increase in cellular calcium preceded the disappearance of vinculin from adhesion plaques and was partially blocked by pretreatment of cells with TMB-8 but not leupeptin. This rise in cytosolic free calcium was found to occur in approximately 80% of the sample population and displayed both spatial and temporal subcellular heterogeneity. Exposure of cells to TPA (100 nM) did not result in a change in cytosolic free calcium. Both PDGF (20 ng/ml) and TPA (100 nM) caused cytosolic alkalinization which occurred after PDGF-induced disruption of vinculin from adhesion plaques, as determined using the pH-sensitive indicator BCECF and Digitized Video Microscopy. PDGF stimulated DNA synthesis and vinculin disruption in a similar dose-dependent fashion. Both could be inhibited by leupeptin or TMB-8. These results suggest that 1) exposure of vascular smooth muscle cells to PDGF is associated with the disruption of vinculin from adhesion plaques, 2) PDGF-induced vinculin disruption is regulated by an increase in cytosolic calcium (but not cytosolic alkalinization), and involves proteolysis; 3) activation of protein kinase C also causes vinculin removal from adhesion plaques but by a calcium-independent mechanism, and 4) the cellular response to PDGF-stimulated increases in cytosolic free calcium is heterogeneous.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition by phorbol esters of antiimmunoglobulin-induced calcium signalling and B-cell activation

The inhibitory effect of phorbol dibutyrate (PDB) on B-cell stimulation was evaluated using a model in which activation is induced by modest doses of antiimmunoglobulin antibody (anti-Ig) and progression to DNA synthesis is induced by cytochalasin. PDB preferentially inhibited anti-Ig-induced activation and did so during brief (2 hr) preincubation with anti-Ig. Activation was inhibited whether PDB was added before or shortly after anti-Ig. Since activation for cytochalasin responsiveness appears to be mediated by Ca2+, the effect of PDB on the anti-Ig-induced rise in intracellular Ca2+ was evaluated. PDB (and other phorbol esters that activate protein kinase C) inhibited the rise in Ca2+ normally associated with anti-Ig treatment; moreover, PDB reversed an established anti-Ig-induced Ca2+ response. These data suggest that phorbol esters inhibit B-cell activation by interfering with the elevated levels of intracellular Ca2+ produced by cross-linking of surface immunoglobulin by anti-Ig. This could represent a "feedback inhibition" type of response, but it remains to be seen if this occurs under physiological conditions of protein kinase C activation.

Rapid dephosphorylation of a Mr 80,000 protein, a specific substrate of protein kinase C upon removal of phorbol esters, bombesin and vasopressin

The mitogens phorbol 12,13-dibutyrate, bombesin and vasopressin stimulate the phosphorylation of an acidic Mr 80,000 cellular protein, a specific substrate of protein kinase C, in intact Swiss 3T3 cells. Phosphorylation of this substrate was rapidly reversed upon the removal of each of these agents. Dephosphorylation occurred with a similar half-life in each of the cases studied (2.2, 1.5 and 2 minutes for phorbol 12,13-dibutyrate, bombesin and vasopressin respectively) and agreed closely with the dissociation of the ligands from their specific high-affinity binding sites in Swiss 3T3 cells.

Early signals in the mitogenic response

Polypeptide growth factors, regulatory peptides, and a variety of pharmacological agents acting alone or synergistically induce mitogenesis in cultured fibroblasts. The early signals in the membrane, cytosol, and nucleus promoted by these extracellular factors, together with their mitogenic effectiveness, are integrated in a unified hypothesis for the regulation of fibroblast growth.

Vasopressin rapidly stimulates protein kinase C in quiescent Swiss 3T3 cells

Addition of vasopressin to quiescent cultures of Swiss 3T3 cells caused a rapid increase in the phosphorylation of an acidic molecular weight 80,000 cellular protein (termed 80K). The effect was concentration- and time-dependent; enhancement in 80K phosphorylation could be detected as early as 30 sec after the addition of the hormone. Recently, a rapid increase in the phosphorylation of an 80K cellular protein following treatment with phorbol esters or diacylglycerol has been shown to reflect the activation of protein kinase C in intact Swiss 3T3 cells. Here we show that the 80K phosphoproteins generated in response to vasopressin and phorbol 12,13-dibutyrate (PBt2) were identical as judged by one- and two-dimensional polyacrylamide gel electrophoresis (PAGE) and peptide mapping following partial proteolysis with Staphylococcus aureus V8 protease. In addition, prolonged pretreatment of 3T3 cells with PBt2 which leads to the disappearance of protein kinase C activity blocked the ability of vasopressin to stimulate the phosphorylation of 80K. The effect of vasopressin on 80K phosphorylation and mitogenesis was selectively blocked by the vasopressin antagonist (Pmp1-O-Me-Tyr2-Arg8) vasopressin suggesting that these responses are mediated by its specific receptor in these cells. The removal of vasopressin leads to dephosphorylation (within minutes) of the 80K phosphoprotein. We conclude that vasopressin rapidly stimulates protein kinase C activity in intact 3T3 cells.

Stimulation of DNA synthesis in quiescent rabbit chondrocytes in culture by limited exposure to somatomedin-like growth factors

Cartilage-derived factor (CDF), extracted from fetal bovine cartilage, and multiplication-stimulating activity (MSA) stimulated DNA synthesis in quiescent rabbit costal chondrocytes in culture under serum-free conditions. As described previously, when added in the presence of fibroblast growth factor (FGF) or epidermal growth factor (EGF) a somatomedin-like growth factor, CDF or MSA, synergistically stimulated DNA synthesis in the cultured chondrocytes. The present study showed that exposure of the cells to MSA or CDF for only the initial 5 h was sufficient for transmission of their full stimulatory effect. Furthermore, the limited exposure did not alter the time course of stimulation of DNA synthesis: [3H]thymidine incorporation into DNA began to increase after 16 h and reached a maximum after 24 h. In contrast to the somatomedin-like growth factors, FGF and EGF were required continuously in the culture medium during traverse of the entire G1 phase for stimulation of DNA synthesis, and the mitogenic effects of FGF and EGF in cultured chondrocytes were stronger than those of CDF and MSA. Synergistic stimulation of DNA synthesis by CDF or MSA in the presence of FGF or EGF could be observed as long as FGF or EGF was continuously present, even when CDF or MSA was withdrawn after the first 5 h of culture. These findings suggest that, in contrast to FGF and EGF, somatomedin-like growth factors affect an early distinct stage in the G1 phase of chondrocytes.

Early events elicited by bombesin and structurally related peptides in quiescent Swiss 3T3 cells. I. Activation of protein kinase C and inhibition of epidermal growth factor binding

Addition of bombesin to quiescent cultures of Swiss 3T3 cells caused a rapid increase in the phosphorylation of an Mr 80,000 cellular protein (designated 80k). The effect was both concentration and time dependent; enhancement in 80k phosphorylation could be detected as early as 10 s after the addition of peptide. Recently, a rapid increase in the phosphorylation of an 80k cellular protein after treatment with phorbol esters or diacylglycerol has been shown to reflect the activation of protein kinase C in intact fibroblasts (Rozengurt, E., A. Rodriguez-Pena, and K. A. Smith, 1983, Proc. Natl. Acad. Sci. USA., 80:7244-7248; Rozengurt, E., A. Rodriguez-Pena, M. Coombs, and J. Sinnett-Smith, 1984, Proc. Natl. Acad. Sci. USA., 81:5748-5752). The 80k phosphoproteins generated in response to bombesin and to phorbol 12,13-dibutyrate were identical as judged by one- and two-dimensional PAGE and by peptide mapping after partial proteolysis with Staphylococcus aureus V8 protease. In addition, prolonged pretreatment of 3T3 cells with phorbol 12,13-dibutyrate, which leads to the disappearance of protein kinase C activity, blocked the ability of bombesin to stimulate 80k. Bombesin also caused a rapid (1 min) inhibition of 125I-labeled epidermal growth factor (125I-EGF) binding to Swiss 3T3 cells. The inhibition was both concentration and temperature dependent and resulted from a marked decrease in the affinity of the EGF receptor for its ligand. Peptides structurally related to bombesin, including gastrin-releasing peptide, also stimulated 80k phosphorylation and inhibited 125I-EGF binding; both effects were selectively blocked by a novel bombesin antagonist. These results strongly suggest that these responses are mediated by specific high-affinity receptors that recognize the peptides of the bombesin family in Swiss 3T3 cells. While an increase in cytosolic Ca2+ concentration does not mediate the bombesin inhibition of 125I-EGF binding, the activation of protein kinase C in intact Swiss 3T3 cells by peptides of the bombesin family may lead to rapid inhibition of the binding of 125I-EGF to its cellular receptor.

Early events elicited by bombesin and structurally related peptides in quiescent Swiss 3T3 cells. II. Changes in Na+ and Ca2+ fluxes, Na+/K+ pump activity, and intracellular pH

The amphibian tetradecapeptide, bombesin, and structurally related peptides caused a marked increase in ouabain-sensitive 86Rb+ uptake (a measure of Na+/K+ pump activity) in quiescent Swiss 3T3 cells. This effect occurred within seconds after the addition of the peptide and appeared to be mediated by an increase in Na+ entry into the cells. The effect of bombesin on Na+ entry and Na+/K+ pump activity was concentration dependent with half-maximal stimulation occurring at 0.3-0.4 nM. The structurally related peptides litorin, gastrin-releasing peptide, and neuromedin B also stimulated ouabain-sensitive 86Rb+ uptake; the relative potencies of these peptides in stimulating the Na+/K+ pump were comparable to their potencies in increasing DNA synthesis (Zachary, I., and E. Rozengurt, 1985, Proc. Natl. Acad. Sci. USA., 82:7616-7620). Bombesin increased Na+ influx, at least in part, through an Na+/H+ antiport. The peptide augmented intracellular pH and this effect was abolished in the absence of extracellular Na+. In addition to monovalent ion transport, bombesin and the structurally related peptides rapidly increased the efflux of 45Ca2+ from quiescent Swiss 3T3 cells. This Ca2+ came from an intracellular pool and the efflux was associated with a 50% decrease in total intracellular Ca2+. The peptides also caused a rapid increase in cytosolic free calcium concentration. Prolonged pretreatment of Swiss 3T3 cells with phorbol dibutyrate, which causes a loss of protein kinase C activity (Rodriguez-Pena, A., and E. Rozengurt, 1984, Biochem. Biophys. Res. Commun., 120:1053-1059), greatly decreased the stimulation of 86Rb+ uptake and Na+ entry by bombesin implicating this phosphotransferase system in the mediation of part of these responses to bombesin. Since some activation of monovalent ion transport by bombesin was seen in phorbol dibutyrate-pretreated cells, it is likely that the peptide also stimulates monovalent ion transport by a second mechanism.

Phorbol esters and diacylglycerol inhibit vasopressin-induced increases in cytoplasmic-free Ca2+ and 45Ca2+ efflux in Swiss 3T3 cells

Vasopressin increased intracellular free calcium concentration [Ca2+]i in quin-2-loaded quiescent Swiss 3T3 cells. This effect of vasopressin was rapidly inhibited by biologically active tumour promoters including phorbol dibutyrate (PBt2) and by the synthetic diacylglycerol 1-oleoyl-2-acetyl-glycerol (OAG). Prolonged pretreatment of Swiss 3T3 cells with PBt2 causes a loss of protein kinase C activity (Rodriguez-Pena & Rozengurt, Biochem biophys res commun 120 (1984) 1053) [28]. This pretreatment abolished the inhibition by PBt2 or OAG of vasopressin-mediated increases in [Ca2+]i. Vasopressin also stimulated 45Ca2+ efflux from cells pre-loaded with the isotope. This effect of the hormone was also inhibited by PBt2. Prolonged pretreatment with PBt2 prevented the inhibition of vasopressin-stimulated 45Ca2+ release by PBt2. Thus, protein kinase C stimulation inhibits vasopressin-mediated increases in [Ca2+]i and 45Ca2+ efflux apparently by blocking the increased release of Ca2+ from an intracellular store caused by the hormone. These findings suggest that activation of protein kinase C may act as a feedback inhibitor to modulate ligand-mediated increases in [Ca2+]i.

Modulation of cytosolic protein kinase C activity by ferricyanide: priming event seems transmembrane redox signalling. A study on transformed C3H/10T1/2 cells in culture

Transformed 3T3/10T1/2 cultured cells incubated with ferricyanide caused a decrease of 2 mM EDTA extractable cytosolic protein kinase C activity in 2 min, whereas 5 or 20 min ferricyanide treatment reverted the enzyme activity to that observed without ferricyanide. The ferricyanide effect in 2 min was abolished by amiloride and sustained by ouabain. Thus, deactivation-activation of cytosolic protein kinase C is attributed to an unknown signal generation during H+ accumulation coupled with the Na+/H+ exchange phase. In this mechanism the priming event concerns the transmembrane redox process shedding H+ into the cell interior while impermeant ferricyanide acts as a unique electron acceptor.

Identification of the cellular mechanisms responsible for platelet-derived growth factor induced alterations in cytoplasmic vinculin distribution

Exposure of quiescent density arrested BALB/c-3T3 cells (clone A31) to platelet-derived growth factor (PDGF; 6-12 ng/ml) results in a rapid, reversible, time- and dose-dependent removal of vinculin from adhesion plaques (Herman and Pledger, 1985). Potential cellular mechanisms involved in PDGF-induced removal of vinculin from adhesion plaques were examined. Removal of vinculin from adhesion plaques following exposure of cells to PDGF was temperature dependent, occurred in many fibroblast cell lines, and could be mimicked by 12-tetradecanoyl phorbol-13-acetate (TPA; 5-125 nM) or melittin (0.35 microM). Unlike the effect of PDGF, TPA- or melittin-induced vinculin disruption was not reversible. The removal of vinculin from adhesion plaques was inhibited by trifluoroperazine (TFP; 2.5 microM). 8-(N,N-diethylamino) octyl-3,4,5-trimethoxy benzoate (TMB-8; 1.0 microM), mepacrine (220 microM), n-alpha-p-tosyl-L-lysine chloromethylketone (TLCK; 100 microM), phenylmethoxysulphonylfluoride (PMSF; 500 microM), and epsilon-aminocaproic acid (epsilon-ACA; 100 microM); however, amiloride (100 microM), A23187 (20 microM), and chloroquine (1 mM) were unable to inhibit this effect. Melittin disruption of vinculin was inhibited by (in order of decreasing effectiveness) mepacrine greater than TMB-8 greater than TFP greater than leupeptin greater than PMSF, whereas A23187 and amiloride had no effect. The return of vinculin to adhesion plaques following PDGF treatment required de novo mRNA transcription and protein synthesis and was associated with PDGF-stimulated synthesis of vinculin. The observation that both PDGF- and melittin-induced removal of vinculin from adhesion plaques is inhibited by mepacrine suggests that phospholipase activation may be an early and important step in PDGF-induced disruption of vinculin from adhesion plaques. In addition, TFP, TMB-8 and protease inhibitor inhibition of both the PDGF and melittin effects on vinculin distribution, coupled with the finding that TPA can mimic the PDGF or melittin response, suggests that Ca2+, calmodulin, protein kinase C, and/or proteolysis may play an important role(s) in the removal of vinculin from adhesion plaques following PDGF addition. The lack of effect of A23187 addition on vinculin distribution suggests that alterations in cellular Ca2+ is necessary but not sufficient for vinculin removal from adhesion plaques.

The antiproliferative effect of interferon and the mitogenic activity of growth factors are independent cell cycle events. Studies with vascular smooth muscle cells and endothelial cells

We studied the antagonistic effects of interferon (IFN) and growth factors in G0/G1-arrested normal bovine aortic smooth muscle cells (SMC) which were stimulated by serum, or purified platelet derived growth factor (PDGF), supplemented with plasma-derived serum (PDS). The growth response, measured as [3H]thymidine incorporation into DNA, was dependent on the concentration of the mitogen. Human IFN alpha, recombinant human IFN alpha 2, or a crude bovine-IFN preparation prepared from virus-infected bovine aortic endothelial cells, inhibited SMC growth induced by either serum or PDGF with PDS. The extent of IFN inhibition was inversely related to the concentration of the mitogenic stimulus. We also investigated whether IFN inhibited the early events in G1 phase, stimulated by the competence factor PDGF, or the progression of the cell into the S phase induced by PDS. The results indicated that IFN inhibited these two stages of the G1 phase independently. In addition, we investigated the antiproliferative effect of IFN on bovine aortic endothelial cells (BAEC), which do not respond to PDGF but to the mitogenic activity of fibroblast growth factor (FGF). IFN inhibited the mitogenic activity of FGF in a dose-dependent manner. The results indicate that the anti-proliferative activity of IFN and the mitogenic effects of different growth factors are independent.

Stimulation of Na+/H+ antiport activity by epidermal growth factor and insulin occurs without activation of protein kinase C

Addition of phorbol 12, 13 dibutyrate (PBt2) or a combination of epidermal growth factor (EGF) and insulin to quiescent cultures of Swiss 3T3 cells increased intracellular pH in a Na+-dependent fashion. In contrast to PBt2, EGF plus insulin failed to stimulate protein kinase C and elicited the ionic response in cells lacking this enzyme. We suggest that the stimulation of the Na+/H+ antiport in Swiss 3T3 cells is mediated by at least two separate pathways, only one of which is dependent upon the activation of protein kinase C.

Diacylglycerol treatment rapidly decreases the affinity of the epidermal growth factor receptors of Swiss 3T3 cells

The synthetic diacylglycerol 1-oleoyl-2-acetyl glycerol (OAG) and phorbol esters activate protein kinase C in intact cells. We report here that OAG inhibits the binding of 125I-labelled epidermal growth factor (125I-EGF) to Swiss 3T3 cells. The inhibition was detected as early as 1 min after treatment at 37 degrees C and persisted for at least 120 min. The effect of OAG was reversed upon removal of this diacylglycerol. Detailed Scatchard analysis of 125I-EGF binding to Swiss 3T3 cells at 4 degrees C after a 1 h incubation with a saturating dose of OAG at 37 degrees C, demonstrates that this OAG pretreatment does not change the apparent number of EGF receptors but causes a marked decrease in their apparent affinity for the ligand. Prolonged treatment (40 h) of the cells with phorbol dibutyrate (PBt2) which causes a marked decrease in the number of phorbol ester binding sites and in the activity of protein kinase C, prevented the inhibition of 125I-EGF binding by both PBt2 and OAG. The results support the possibility that protein kinase C plays a role in the transmodulation of the EGF receptor in intact cells.

Mitogens trigger a calcium-independent signal for proliferation in phorbol-ester-treated lymphocytes

The activation of T lymphocytes by mitogens requires at least two signals; the first, delivered to T cells by a mitogen in conjunction with accessory cells (monocytes/macrophages), leads to the generation of the second signal, interleukin-2 (IL-2). The first signal also induces the expression of IL-2 receptors on the surface of a subpopulation of T cells; binding of IL-2 to its receptor then initiates a cascade of events culminating in DNA synthesis by these cells. Certain compounds act synergistically with mitogens in promoting T-cell proliferation by substituting for the activities of interacting cells or their products. For example, the phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) has been shown to restore the ability of macrophage-depleted T-cell populations to respond to mitogenic lectins. Transmembrane fluxes of calcium, leading to increased free cytosolic calcium concentrations ([Ca2+]), have been demonstrated following mitogen binding to lymphocytes and have been implicated in the initiation of cell proliferation. We show here that the effect of TPA on lymphocyte proliferation occurs in the absence of extracellular Ca2+ or detectable changes in [Ca2+]i, but only in the presence of mitogens. This suggests that in cells which have been incubated with the phorbol ester, mitogens can induce proliferation by a calcium-independent signal.