Protein kinase C zeta isoform is critical for mitogenic signal transduction

Activation of Ca2+ influx by transforming Ha-ras

The effect of an induction of transforming Ha-ras on Ca2+ influx into NIH3T3 cells was studied employing Fura-2 quenching by Mn2+. The expression of transforming p21Ha-ras caused a significant increase in Mn2+ influx which was blocked by Cd2+, La3+, niguldipine and the Ca(2+)-channel blocker SK&F96365. This effect was specific for transforming Ha-ras and was not seen after overexpression of the Ha-ras proto-oncogene or v-mos. In addition to the enhanced Mn2+ influx, transforming p21Ha-ras elicited an increased efflux of the K(+)-congener 86Rb+ which was inhibitable by Ca(2+)-channel blockers and charybdotoxin, a selective inhibitor of high and intermediate conductance Ca(2+)-dependent K+ channels. Charybdotoxin did not reduce the increase in Mn2+ influx by ras, demonstrating that the activation of Ca(2+)-dependent K+ channels was not required for the sustained Mn2+/Ca2+ influx in the presence of transforming Ha-ras. In ras-expressing cells, the bradykinin-induced Mn2+ influx and charybdotoxin sensitive 86Rb+ efflux were markedly potentiated. The increase in the inositol- 1,4,5-trisphosphate and inositol-1,3,4,5-tetrakisphosphate levels by ras is not sufficient to explain the elevated Mn2+ influx. The mitogenic response to an expression of transforming Ha-ras was inhibited by the Ca(2+)-channel blockers not, however, by charybdotoxin. These data suggest the existence of an agonist-independent activation of a receptor- or second messenger-operated Ca2+ channel by transforming Ha-ras which is necessary for the mitogenic response to the activation of the oncogene.

The presence of phorbol ester responsive and non-responsive forms of the zeta isozyme of protein kinase C in mouse epidermal cells

The possible involvement of zeta isozyme of protein kinase C (PKC zeta) in phorbol ester-induced signal transduction was investigated in mouse epidermal cells. Western blot analysis of RESOURCE Q column chromatography eluates obtained from 105,000 g supernatants and particulate fractions of epidermal cells was performed using anti-PKC zeta specific antibody. Anti-PKC zeta antibody recognised proteins in low salt range corresponding to 25-125 mM NaCl (low salt-eluted PKC zeta; 1-PKC zeta) as well as high salt range corresponding to 175-300 mM NaCl (high salt-eluted PKC zeta; h-PKC zeta) in both subcellular fractions. 1-PKC zeta and h-PKC zeta were detected as a doublet protein of 79,000 and 85,000 M(r) in 105,000 g supernatants, but as a 79,000 M(r) protein in particulate fractions. Immunoprecipitated 1-PKC zeta and h-PKC zeta with anti-PKC zeta specific antibody possessed phosphatidylserine (PS)-dependent protein kinase activity, but neither 1-PKC zeta nor h-PKC zeta were further activated by 40 nM phorbol 12-myristate 13-acetate (PMA) in the presence of PS. Furthermore, 1-PKC zeta and h-PKC zeta can be autophosphorylated, indicating that both 1-PKC zeta and h-PKC zeta are PKC zeta. Treatment of intact epidermal cells with PMA or other PKC activators caused the apparent shift of 79,000 M(r) 1-PKC zeta to the 85,000 M(r) from in particulate fractions. Prolonged treatment of the cells with PMA induced the downregulation of both forms of 1-PKC zeta in particulate fractions. Under the same condition, 1-PKC zeta in 105,000 g supernatants and h-PKC zeta in both fractions did not respond to PMA. This apparent shift was reversible and the content ratio of 85,000 to 75,000 M(r) 1-PKC zeta was decreased by acid phosphatase treatment, indicating that the apparent shift results at least in part from phosphorylation of 79,000 M(r) 1-PKC zeta. Total activity of 1-PKC zeta was increased in association with the apparent shift from the 79,000 to 85,000 M(r) form in response to PMA treatment of intact epidermal cells. All of these results indicate that PKC zeta is present as multiple forms in mouse epidermal cells, and that especially 1-PKC zeta in particulate fractions play a significant role(s) in PMA-induced signal transduction in mouse epidermal cells.

The zeta protein kinase C isoform from the testis of the grey mullet Mugil cephalus with a specific reaction protein of M(r) 48,000 on oolemma

The zeta protein kinase C isoform (PKC-zeta) was purified from the testis of the grey mullet Mugil cephalus and has relative masses (M(r)) of 65,000 and 63,000. The subunits of PKC-zeta from spermatozoa degenerated to M(r) 58,000 and 53,000 after continuous freezing and thawing. Proteins of M(r) 48,000 on the oolemma of the grey mullet Mugil cephalus were found to be the reaction proteins of the PKC-zeta from spermatozoa.

Perturbation of the platelet-derived growth factor receptor signaling by dibutyryl-cAMP in human astrocytoma cells

It has been shown that cAMP may perturb the polypeptide growth factor-induced nuclear events. However, the possible interactions of the cAMP-protein kinase A (cAMP-PKA) and receptor tyrosine kinase pathways in the cytosol have not been fully elucidated. In this study, we use human astrocytoma cells as a model to investigate this issue. The results show that platelet-derived growth factor (PDGF)-induced receptor autophosphorylation in human astrocytoma cells is suppressed by dibutyryl-cAMP pretreatment and such suppression is not due to changes in the ligand-receptor binding properties. Further studies show that PDGF-induced tyrosine phosphorylation of phospholipase C-gamma 1 (PLC-gamma 1) and phosphatidylinositol 3-kinase (PI 3-kinase) are also suppressed in dibutyryl-cAMP-pretreated cells. The suppression of PLC-gamma 1 tyrosine phosphorylation was accompanied by a decreased production of water soluble inositol phosphates. In contrast, similar treatment with normal human astrocytes potentiates the tyrosine phosphorylation of PLC-gamma 1 and PI 3-kinase. The results indicate that cAMP can either negatively or positively modulate the PDGF receptor tyrosine kinase activity depending on the cell types examined.

Identification of heterogeneous ribonucleoprotein A1 as a novel substrate for protein kinase C zeta

The zeta isoform of protein kinase C (zeta PKC) has been shown to be an important step in mitogenic signal transduction. Using a yeast interaction screen to search for potential novel substrates of zeta PKC, we identified the heterogeneous ribonucleoprotein A1 (hnRNPA1). This protein specifically interacts with the catalytic domain of zeta PKC but not with its regulatory region or with the full-length protein, or with a kinase-defective mutant of the zeta PKC catalytic domain. In addition, no interaction was detected with other kinases such as Raf-1 or Mos, that, like zeta PKC, are critically involved in signal transduction, or with the catalytic domain of epsilon PKC, which is the PKC isotype with the highest homology to zeta PKC. hnRNPA1 is directly phosphorylated by both recombinant and native zeta PKC, and this phosphorylation is increased when zeta PKC is immunoprecipitated from mitogen-activated fibroblasts. As an additional control, hnRNPA1 is not phosphorylated appreciably by catalytic epsilon PKC or by a mixture of highly purified classical PKC isotypes maximally activated by phosphatidylserine and Ca2+. Treatment of quiescent cell cultures with a potent mitogen such as platelet-derived growth factor promotes a significant phosphorylation of hnRNPA1 in vivo that is impaired by expression of a dominant negative mutant of zeta PKC. Furthermore, expression of a catalytically active zeta PKC mutant phosphorylates hnRNPA1 in vivo. These findings suggest that zeta PKC could be critically involved in a novel pathway that connects membrane signaling to nuclear regulatory events, at the level of RNA transport and processing. Results also shown here by using different zeta PKC mutants suggesting the control of the cytoplasmic localization of hnRNPA1 by zeta PKC. Also of potential functional relevance are the results demonstrating that the phosphorylation by zeta PKC severely impairs both hnRNPA1 RNA binding and its ability to promote strand annealing in vitro.

Identification of the major phosphorylation sites for protein kinase C in kit/stem cell factor receptor in vitro and in intact cells

The c-kit-encoded tyrosine kinase receptor for stem cell factor (Kit/SCFR) is crucial for the development of hematopoietic cells, melanoblasts, and germ cells. Ligand stimulation of Kit/SCFR leads to receptor dimerization and autophosphorylation on tyrosine residues. We recently showed, that protein kinase C (PKC) acts in an SCF-stimulated negative feedback loop, which controls Kit/SCFR tyrosine kinase activity and modulates the cellular responses to SCF (Blume-Jensen, P., Siegbahn, A., Stabel, S., Heldin, C.-H., and Rönnstrand, L. (1993) EMBO J. 12, 4199-4209). We present here the identification of the major phosphorylation sites for PKC in Kit/SCFR. Two serine residues in the kinase insert, Ser-741 and Ser-746, are PKC-dependent phosphorylation sites in vivo and account for all phosphorylation by PKC in vitro. Together they comprise more than 60% of the total SCF-stimulated receptor phosphorylation in living cells and 85-90% of its phosphorylation in resting cells. Two additional serine residues, Ser-821 close to the major tyrosine autophosphorylation site in the kinase domain and Ser-959 in the carboxyl terminus are SCF-stimulated PKC-dependent phosphorylation sites. However, they are not phosphorylated directly by PKC-alpha in vitro. Both specific receptor tyrosine autophosphorylation and specific receptor-associated phosphatidylinositide 3'-kinase activity was increased approximately 2-fold in response to SCF in PAE cells stably expressing Kit/SCFR(S741A/S746A). Furthermore, the kinase activity of Kit/SCFR(S741A/S746A) toward an exogenous substrate was increased, which was reflected as a decreased Km and an increased Vmax, in accordance with the negative regulatory role of PKC on Kit/SCFR signaling.

Molecular characterization of a novel transcription factor that controls stromelysin expression

Stromelysins, which are the metalloproteinases with the widest substrate specificities, play a critical role in tumor invasion and metastasis. We have previously reported an element (SPRE) of the stromelysin promoter located between nucleotides -1221 and -1203 that is necessary and sufficient for the control of stromelysin gene expression by mitogenic activation, which induces a nuclear activity that binds to this sequence. Using a concatenated probe with several copies of this element to screen a lambda gt11 cDNA expression library from mouse Swiss 3T3 fibroblasts, we report here the molecular cloning of a cDNA coding for a novel protein (SPBP) of 937 amino acids that binds to this element and has several features of a transcription factor, such as a putative leucine zipper region, a nuclear localization signal, and a basic domain with homology to the DNA-binding domains of Fos and Jun. Evidence that SPBP is at least a critical component of the mitogen-induced SPRE nuclear binding activity is presented here. Furthermore, the transfection of an expression plasmid for SPBP transactivates reporter chloramphenicol acetyltransferase plasmids containing either the full-length stromelysin promoter or a single copy of the SPRE cloned upstream of the herpes simplex virus thymidine kinase minimal promoter. Therefore, the results presented here identify a novel transcription factor critically involved in the control of stromelysin expression.

Analysis of the role of protein kinase C-alpha, -epsilon, and -zeta in T cell activation

T cells express multiple isotypes of protein kinase C (PKC) and although it is well accepted that PKCs have an important role in T cell activation, little is known about the function of individual PKC isotypes. To address this issue, mutationally active PKC-alpha, -epsilon, or -zeta have been transfected into T cells and the consequences for T cell activation determined. p21ras plays an essential role in T cell activation. Accordingly, the effects of the constitutively active PKCs were compared to the effects of mutationally activated p21ras. The data indicate that PKC-epsilon and, to a lesser extent PKC-alpha but not -zeta, can regulate the transcription factors AP-1 and nuclear factor of activated T cells (NF-AT-1). The ability of PKC-epsilon to induce transactivation of NF-AT-1 and AP-1 was similar to the stimulatory effect of a constitutively activated p21ras. PKC-epsilon, but not PKC-alpha nor activated p21ras, was able to induce NF-KB activity. Phorbol esters induce expression of CD69 whereas none of the activated PKC isotypes tested were able to have this effect. Activated Src and p21ras were able to induce CD69 expression. These results indicate selective functions for different PKC isotypes in T cells. Moreover, the data comparing the effects of activated Ras and PKC mutants suggest that PKC-alpha, p21ras, and PKC-epsilon are not positioned linearly on a single signal transduction pathway.

Inhibition of ras p21 membrane localization and modulation of protein kinase C isozyme expression during regression of chemical carcinogen-induced murine skin tumors by lovastatin

We investigated the ras p21 membrane localization and the expression and activation of protein kinase C (PKC) isozymes in activated ras oncogene-containing tumors and assessed whether these events were related to tumor growth. We used 7,12-dimethylbenz[a]anthracene-initiated and 12-O-tetradecanoylphorbol-13-acetate-promoted SENCAR mouse skin tumors, which were shown to contain Ha-ras oncogene activated by point mutation at codon 61, as an in vivo model for these studies. Compared with levels in epidermis, highly elevated levels of membrane-bound Ha-ras p21 were observed in growing tumors, which also showed strong expression and membrane translocation of PKC zeta and beta II and weak expression of PCK alpha. However, when ras p21 membrane localization was blocked in vivo in growing tumors by lovastatin, opposite results were evident. Compared with saline-treated animals, in which tumor growth continued, lovastatin-treated animals had significantly inhibited tumor growth, which led to tumor regression with concomitant inhibition of Ha-ras p21 membrane localization. These regressing tumors from lovastatin-treated animals also showed a decrease in the expression and membrane translocation of PKC zeta and beta II but increased expression of PKC alpha. Taken together, our results indicate that ras p21 membrane localization and the expression and activation of PKC zeta, beta II, and alpha may be the critical events in the regulation of the growth of tumors that contain activated ras oncogenes.

Comparison of ability of protein kinase C inhibitors to arrest cell growth and to alter cellular protein kinase C localisation

Inhibitors of protein kinase C (PKC) such as the staurosporine analogues UCN-01 and CGP 41251 possess antineoplastic properties, but the mechanism of their cytostatic action is not understood. We tested the hypothesis that the ability of these compounds to arrest growth is intrinsically linked with their propensity to inhibit PKC. Compounds with varying degrees of potency and specificity for PKC were investigated in A549 and MCF-7 carcinoma cells. When the log values of drug concentration which arrested cell growth by 50% (IC50) were plotted against the logs of the IC50 values for inhibition of cytosolic PKC activity, two groups of compound could be distinguished. The group which comprised the more potent inhibitors of enzyme activity (calphostin C, staurosporine and its analogues UCN-01, RO 31-8220, CGP 41251) were the stronger growth inhibitors, whereas the weaker enzyme inhibitors (trimethylsphingosine, miltefosine, NPC-15437, H-7, H-7I) affected proliferation less potently. GF 109203X was exceptional in that it inhibited PKC with an IC50 in the 10(-8) M range, yet was only weakly cytostatic. To substantiate the role of PKC in the growth inhibition caused by these agents, cells were depleted of PKC by incubation with bryostatin 1 (1 microM). The susceptibility of these enzyme-depleted cells towards growth arrest induced by staurosporine, RO 31-8220, UCN-01 or H-7 was studied. The drug concentrations which inhibited incorporation of [3H]thymidine into PKC-depleted A549 cells by 50% were slightly, but not significantly, lower than significantly, lower than those observed in control cells. These results suggest that PKC is unlikely to play a direct role in the arrest of the growth of A549 and MCF-7 cells mediated by these agents. Staurosporine is not only a strong inhibitor of PKC but also mimics activators of this enzyme in that it elicits the cellular redistribution of certain PKC isoenzymes. The ability of kinase inhibitors other than staurosporine to exert a similar effect was investigated. Calphostin C, H-7, H-7I, miltefosine, staurosporine, UCN-01, RO 31-8220, CGP 41251 or GF 109203X were incubated for 30 min with A549 cells in the absence or presence of the PKC activator 12-O-tetradecanoyl phorbol-13-acetate. The subcellular distribution of PKC-alpha-, -epsilon and -zeta was measured by Western blot analysis. None of the agents affected PKC-alpha or -zeta.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential localization of protein kinase C isozymes in U937 cells: evidence for distinct isozyme functions during monocyte differentiation

U937 human promonocytic leukemia cells express PKC isozymes beta 1, beta 2, epsilon and zeta. Indirect immunocytofluorescence using affinity-purified PKC-specific antibodies indicates that each of the endogenous PKC isozymes in U937 cells display a unique compartmentalization within the intact cell. PKC-beta 1 is distributed between two identifiable pools: a cytoplasmic pool which redistributes to the plasma membrane upon activation with acute phorbol ester-treatment, and a membrane-bound pool associated with intracellular vesicles containing beta 2-integrin adhesion molecules, cd11b and cd11c. The vesicle-associated PKC-beta 1 translocates with the secretory granules to the plasma membrane upon agonist-stimulated activation. PKC-beta 2 is associated with the microtubule cytoskeleton in resting cells. PKC overlay assays indicate that PKC-beta 2 binds to proteins associated with microtubules, and not directly to tubulin. PKC-epsilon is associated with filamentous structures in resting cells and redistributes to the perinuclear region upon activation with phorbol esters. In differentiated U937 cells, PKC-beta 1 remains associated with vesicles translocating from the trans-Golgi region to the plasma membrane and PKC-epsilon is primarily associated with perinuclear and plasma membranes. PKC-zeta, which does not respond to phorbol ester treatment, is primarily cytosolic in undifferentiated cells and accumulates in the nucleus of differentiated cells blocked in the G2 phase of the cell cycle. The data clearly demonstrate that individual PKCs localize to different subcellular compartments and promote the hypothesis that PKC subcellular localization is indicative of unique functions for individual PKC isozymes.

Expression of protein kinase C isoforms in renal tissue

PKC plays a central role for the regulation of renal function. PKC consists of a family of isoenzymes. By employing Northern blot techniques we have demonstrated that mRNA transcripts for the classical Ca(2+)-dependent, diacylglycerol-activated isoform alpha, the novel, Ca(2+)-independent isoform delta and the atypical isoform zeta are abundantly expressed in the rat kidney. The novel PKC-epsilon was weakly expressed. The classical PKCs beta I, beta II and gamma could not be detected. The mRNA expression of PKC-delta and -zeta increased with age. The intrarenal localization of PKC-alpha, -delta and -zeta isoforms were studied in the adult kidney using in situ hybridization. In the cortex, the PKC-alpha isoform showed the strongest hybridization signal. PKC alpha, delta and zeta were all distributed in the outer medulla. The PKC-alpha probe detected particularly strong signal in the outer stripe of the outer medulla. Western blot confirmed the presence of the PKC-alpha, -delta and -zeta enzymes in renal tissue. The results show cell-specific and developmentally-dependent expression of three types of PKC isoforms with different responses to diacylglycerol and calcium. The developmental increase of both PKC-delta and PKC-zeta suggests a specific role for these isoforms for the functional regulation of the mature kidney.

Thyrotropin-induced mitogenesis is Ras dependent but appears to bypass the Raf-dependent cytoplasmic kinase cascade

Cellular growth control requires the coordination and integration of multiple signaling pathways which are likely to be activated concomitantly. Mitogenic signaling initiated by thyrotropin (TSH) in thyroid cells seems to require two distinct signaling pathways, a cyclic AMP (cAMP)-dependent signaling pathway and a Ras-dependent pathway. This is a paradox, since activated cAMP-dependent protein kinase disrupts Ras-dependent signaling induced by growth factors such as epidermal growth factor and platelet-derived growth factor. This inhibition may occur by preventing Raf-1 protein kinase from binding to Ras, an event thought to be necessary for the activation of Raf-1 and the subsequent activation of the mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK) kinases (MEKs) and MAP kinase (MAPK)/ERKs. Here we report that serum-stimulated hyperphosphorylation of Raf-1 was inhibited by TSH treatment of Wistar rat thyroid cells, indicating that in this cell line, as in other cell types, increases in intracellular cAMP levels inhibit activation of downstream kinases targeted by Ras. Ras-stimulated expression of genes containing AP-1 promoter elements was similarly inhibited by TSH. On the other hand, stimulation of thyroid cells with TSH resulted in stimulation of DNA synthesis which was Ras dependent but both Raf-1 and MEK independent. We also show that Ras-stimulated DNA synthesis required the use of this kinase cascade in untreated quiescent cells but not in TSH-treated cells. These data suggest that in TSH-treated thyroid cells, Ras might be able to signal through effectors other than the well-studied cytoplasmic kinase cascade.

T-cell antigen receptor-induced signal-transduction pathways--activation and function of protein kinases C in T lymphocytes

CONTENTS. T-cell activation--Structure of the T-cell antigen receptor--Modular organisation of the T-cell antigen receptor--T-cell antigen receptor-coupled signaling pathways: Activation of protein-tyrosine kinase by the T-cell antigen receptor; Signal transduction in lymphoid cells involves several protein-tyrosine kinases in parallel; Regulation of T-cell antigen receptor signaling by the phosphoprotein phosphatase CD45--Consequences of T-cell antigen receptor-induced tyrosine phosphorylation: Activation of phosphoinositol-lipid-turnover pathways--Activation of phospholipase C-gamma-1: p59fyn or p56lck?--G-protein motif of CD3-gamma: relevance for signal transduction--Association of lipid kinase with the T-cell antigen receptor--Intracellular signaling by phospholipid metabolites and calcium: activation of protein kinase C--Protein kinase C isoenzymes--Heterogenity of protein kinase C and mode of activation--Phospholipid-derived mediators in activation of protein kinase C in T-cells--Role of phospholipase D metabolites in activation of protein kinase C--Polyunsaturated fatty acids and lysophosphatidylcholine as activators of protein kinase C--Potein kinase C and p21ras function in interdependent and distinct signaling pathways during T-cell activation--Raf-1 kinase: regulator or target of protein kinase C?--Summary and perspectives.

Calcium, calmodulin and cell cycle progression

Proliferation of mammalian cells both in vivo and in vitro is dependent upon physiological concentrations of extracellular Ca2+. Growth factor stimulation of quiescent cells at the G0/G1 border usually results in a rapid mobilization of Ca2+ from both intra- and extracellular pools. However, Ca2+ influx is also required for later phases of cell cycle transition, especially in the late G1 phase for initiation of DNA synthesis. Available evidence indicates that calmodulin plays the major and essential roles in the Ca(2+)-dependent regulation of cell proliferation. Ca2+ and calmodulin act at multiple points in the cell cycle, including the initiation of the S phase and both initiation and completion of the M phase. Ca2+ and calmodulin stimulate the expression of genes involved in the cell cycle progression, leading to activation of cyclin-dependent kinases p33cdk2 and p34cdc2. Ca2+ and calmodulin are also involved in activation of enzymes participating in nucleotide metabolism and DNA replication, as well as nuclear envelope breakdown and cytokinesis. Ca2+/calmodulin-dependent protein kinase II and protein phosphatase calcineurin are both involved in the Ca2+ and calmodulin-mediated signalling of growth regulation. As compared to normal cells, growth of transformed cells is independent of extracellular Ca2+ and much less sensitive to calmodulin antagonists, suggesting the existence of derangements in the Ca2+ and calmodulin-mediated growth regulation mechanisms.

Activation of pp70/85 S6 kinases in interleukin-2-responsive lymphoid cells is mediated by phosphatidylinositol 3-kinase and inhibited by cyclic AMP

Activation of phosphatidylinositol 3-kinase (PI3K) and activation of the 70/85-kDa S6 protein kinases (alpha II and alpha I isoforms, referred to collectively as pp70S6k) have been independently linked to the regulation of cell proliferation. We demonstrate that these kinases lie on the same signalling pathway and that PI3K mediates the activation of pp70 by the cytokine interleukin-2 (IL-2). We also show that the activation of pp70S6k can be blocked at different points along the signalling pathway by using specific inhibitors of T-cell proliferation. Inhibition of PI3K activity with structurally unrelated but highly specific PI3K inhibitors (wortmannin or LY294002) results in inhibition of IL-2-dependent but not phorbol ester (conventional protein kinase C [cPKC])-dependent pp70S6k activation. The T-cell immunosuppressant rapamycin potently antagonizes IL-2-(PI3K)- and phorbol ester (cPKC)-mediated activation of pp70S6k. Thus, wortmannin and rapamycin antagonize IL-2-mediated activation of pp70S6k at distinct points along the PI3K-regulated signalling pathway, or rapamycin antagonizes another pathway required for pp70S6k activity. Agents that raise the concentration of intracellular cyclic AMP (cAMP) and activate cAMP-dependent protein kinase (PKA) also inhibit IL-2-dependent activation of pp70S6k. In this case, inhibition appears to occur at least two points in this signalling path. Like rapamycin, PKA appears to act downstream of cPKC-mediated pp70S6k activation, and like wortmannin, PKA antagonizes IL-2-dependent activation of PI3K. The results with rapamycin and wortmannin are of added interest since the yeast and mammalian rapamycin targets resemble PI3K in the catalytic domain.

Changes in calcium influx affect the differentiation of murine erythroleukaemia cells

As indicated by direct evidence, obtained by altering the cell-membrane permeability for Ca2+ in murine erythroleukaemia (MEL) cells, calpain is the triggering factor which connects fluctuations of the intracellular Ca2+ concentrations to the decay of protein kinase C (PKC), as well as to the kinetics of cell differentiation induced by hexamethylenebisacetamide. Cell exposure to verapamil caused a profound decrease in the rate of PKC down-regulation and a slower initial rate of accumulation of mature erythroid cells, whereas addition of the Ca2+ ionophore A23187 produced opposite effects. The high susceptibility of PKC-delta to calpain degradation, at concentrations of Ca2+ much lower than those required for degradation of the other PKC isoforms, may be explained by the finding that this kinase isoform is predominantly associated with the cell membrane. The different cellular localizations, as well as the different susceptibilities to calpain digestion, further support the hypothesis that in MEL cells the various PKC isoforms play distinct biological functions that are critical for the maintenance of the undifferentiated state of the cell and for its commitment to terminal erythroid differentiation.

Increased intracellular cyclic AMP levels suppress the mitogenic responses of human astrocytoma cells to growth factors

It has been shown that the intracellular cAMP levels were decreased in human malignant astrocytomas. On the other hand, various growth factors and their receptors were found to be overexpressed in these tumors. It is therefore intriguing as to whether there is interplay between the two phenomena in the modulation of the astrocytoma cell growth. In a basal medium consisting of 75% DMEM, 25% Ham's F-12 supplemented with 2% FBS, we show that the mitogenic effects of platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), and epidermal growth factor (EGF) on human astrocytoma cells were suppressed by dibutyryl-cAMP. Dibutyryl-cAMP alone neither potentiated nor inhibited the tumor cell growth. Further studies show that PDGF-induced receptor autophosphorylation in human astrocytoma cells is suppressed by increased intracellular cAMP levels as measured by immunoprecipitation with anti-PDGF receptor and antiphosphotyrosine antibodies. Our results indicate that there is antagonistic interplay between the receptor tyrosine kinase pathway and cAMP-dependent protein kinase pathway in the control of the malignantly transformed glial cells. A reduced cAMP level seen in many human astrocytoma cells may favor their response to growth factor mitogenesis.

Localization of non-conventional protein kinase C isoforms in bovine brain cell nuclei

Using Western blotting and immunofluorescence microscopy we detected the protein kinase C isoforms delta, epsilon and zeta in isolated cell nuclei from bovine cerebral cortex. Both protein kinase C (PKC) delta and PKC epsilon are present in higher concentrations in neuronal than in glial nuclei and are located inside the nucleus and at the nuclear envelope. There they give a punctate staining in immunofluorescence microscopy. PKC zeta is also present both in the nucleoplasm and at the nuclear envelope. PKC eta could not be detected in the cell nuclei and, even in the homogenate of cerebral cortex, this isoform is present only in very low concentrations. The antibody against PKC eta bound strongly to a nucleoplasmic protein with an apparent molecular mass of 99 kDa. The localization of non-conventional PKC isoforms at the cell nucleus strongly indicates that these isoforms are directly involved in the regulation of nuclear processes.

Signal transduction by the antigen receptors of B and T lymphocytes

B and T lymphocytes of the immune system recognize and destroy invading microorganisms but are tolerant to the cells and tissues of one's own body. The basis for this self/non-self-discrimination is the clonal nature of the B and T cell antigen receptors. Each lymphocyte has antigen receptors with a single unique antigen specificity. Multiple mechanisms ensure that self-reactive lymphocytes are eliminated or silenced whereas lymphocytes directed against foreign antigens are activated only when the appropriate antigen is present. The key element in these processes is the ability of the antigen receptors to transmit signals to the interior of the lymphocyte when they bind the antigen for which they are specific. Whether these signals lead to activation, tolerance, or cell death is dependent on the maturation state of the lymphocytes as well as on signals from other receptors. We review the role of antigen receptor signaling in the development and activation of B and T lymphocytes and also describe the biochemical signaling mechanisms employed by these receptors. In addition, we discuss how signal transduction pathways activated by the antigen receptors may alter gene expression, regulate the cell cycle, and induce or prevent programmed cell death.

Progesterone-induced phospholipid N-methylation and sphingomyelin synthesis in the amphibian oocyte plasma membrane: a second source of the 1,2-diacylglycerol second messenger associated with the G2/M transition

The effects of progesterone and GTP gamma S on phospholipid N-methylation and sphingomyelin synthesis were studied in plasma-vitelline membranes isolated from amphibian (Rana pipiens) oocytes. Plasma-vitelline membranes were preincubated with S-adenosyl-L-[methyl-3H]methionine for 2 min at 20 degrees C and total phospholipids extracted at 0, 15, 30 and 60 s after addition of progesterone and/or GTP gamma S. Progesterone levels (3 microM) that induce meiosis in the intact oocyte stimulated [3H-methyl]incorporation into phosphatidylmonomethylethanolamine (PME) 9-10-fold over the first 60 s, with smaller increases in phosphatidyldimethylethanolamine (PDE) and phosphatidylcholine (PC). [methyl-3H] labeling of sphingomyelin (SM) rises after 30 s, approaching that of [methyl-3H]PME by 60 s. 17 beta-Estradiol, a noninducer of meiosis, was inactive. When oocytes were prelabeled with [3H]palmitic acid, it was found that a fall in [3H]ceramide coincides with the transient increase in [3H]SM, indicating that the end product of N-methylation (PC) undergoes a transfer reaction with ceramide to form SM and 1,2-DG. GTP gamma S levels previously reported to stimulate PC-specific phospholipase C activity in oocyte plasma membranes (5 microM) also stimulated both [methyl-3H]PME and [methyl-3H]SM formation. An inhibitor of phospholipid N-methylation, 2-(methyl-amino)ethanol, blocked stimulation of [methyl-3H]SM synthesis by both progesterone and GTP gamma S as well as induction of meiosis by progesterone. Progesterone thus acts at the oocyte plasma membrane to stimulate PE N-methyltransferase and SM synthase. The finding that GTP gamma S mimics progesterone suggests that N-methyltransferase is mediated by G-protein(s). The transient increase in 1,2-DG which we had previously reported to occur within 1-2 min following progesterone stimulation of the Rana oocyte appears to arise from PC by two different pathways: SM synthesis and hydrolysis of PC by phospholipase C.

Protein kinase C expression and translocation in dispersed chief cells from guinea-pig stomach

The protein kinase C (PKC) family of enzymes is comprised of at least nine isoforms that vary with respect to co-factor dependence, cellular distribution and substrate specificity. Using specific antibodies for alpha, beta, gamma, delta, epsilon, zeta and eta PKC isoforms, and Western blot analysis, we found that alpha and zeta PKC are expressed in gastric chief cells. We then used these methods to examine the effects of carbamylcholine, a cholinergic agonist that increases cellular calcium and diacylglycerol concentrations, and PMA, a phorbol ester that activates PKC, on the subcellular distribution of these isoforms. Carbamylcholine and PMA caused an increase in membrane-associated alpha PKC, but did not alter the subcellular distribution of zeta PKC. Comparison of the dose-response curves for carbamylcholine-induced pepsinogen secretion and alpha PKC membrane-association indicates that PKC translocation is not required for carbamylcholine-induced secretion. Nevertheless, maximal carbachol-induced secretion occurs at concentrations that also cause translocation of the alpha isoform. Whereas treatment of chief cells with PMA (300 nM) for 4 h down-regulated levels of alpha PKC by 61%, there was no change in the levels of zeta PKC. Separation of the two PKC isoforms in chief cell lysates by DEAE-column chromatography revealed that kinase activity in fractions containing the alpha isoform was increased more than 3-fold by calcium and lipids. In contrast, kinase activity in fractions containing the zeta isoform was not altered. In gastric chief cells, translocation and activation of alpha PKC occurs in response to agonist-induced increases in calcium and diacylglycerol. Zeta PKC may be involved in the regulation of basal pepsinogen secretion.

Phosphatidic acid activation of protein kinase C-zeta overexpressed in COS cells: comparison with other protein kinase C isotypes and other acidic lipids

Phosphatidic acid (PA) is produced rapidly in agonist-stimulated cells, but the physiological function of this PA is unknown. We have examined the effects of PA on distinct isoforms of protein kinase C (PKC) using a new cell-free assay system. Addition of PA to cytosol from COS cells overexpressing PKC-alpha, -epsilon or -zeta differentially-activated all three isotypes, as shown by PKC autophosphorylation, and prominent phosphorylation of multiple endogenous substrates. In the absence of Ca2+, the diacylglycerol-insensitive zeta-isotype of PKC was most strongly activated by both PA and bisPA, a newly identified product of activated phospholipase D, with each lipid inducing its own profile of protein phosphorylation. BisPA was also a strong activator of PKC-epsilon, but a weak activator of PKC-alpha. Ca2+, at > or = 0.1 microM, inhibited PA and bisPA activation of PKC-zeta, but did not affect PKC-epsilon activation. In contrast, PKC-alpha was strongly activated by PA only in the presence of Ca2+. BisPA-induced phosphorylations mediated by PKC-zeta could be mimicked in part by other acidic phospholipids and unsaturated fatty acids. PA activation of PKC-zeta was unique in that PA not only stimulated PKC-zeta-mediated phosphorylation of distinctive substrates, but also caused an upward shift in electrophoretic mobility of PKC-zeta, which was not observed with other acidic lipids or with PKC-alpha or -epsilon. We have presented evidence that this mobility shift is not caused by PKC-zeta autophosphorylation, but it coincides with physical binding of PA to PKC-zeta. These results suggest that in cells stimulated under conditions where intracellular Ca2+ is at (or has returned to) basal level, PA may be a physiological activator of PKC-zeta.

IL-5 receptor-mediated tyrosine phosphorylation of SH2/SH3-containing proteins and activation of Bruton's tyrosine and Janus 2 kinases

Interleukin 5 (IL-5) induces proliferation and differentiation of B cells and eosinophils by interacting with its receptor (IL-5R) which consists of two distinct polypeptide chains, alpha and beta (beta c). Although both IL-5R alpha and beta c lack a kinase catalytic domain, IL-5 is capable of inducing tyrosine phosphorylation of cellular proteins. We investigated the role of IL-5R alpha in tyrosine phosphorylation of molecules involved in IL-5 signal transduction, using an IL-5-dependent early B cell line, Y16 and transfectants expressing intact or mutant IL-5R alpha together with intact beta c. The results revealed that the transfectants expressing truncated IL-5R alpha, which entirely lacks a cytoplasmic domain, together with beta c, showed neither protein-tyrosine phosphorylation nor proliferation in response to IL-5. This confirms that IL-5R alpha plays a critical role in protein-tyrosine phosphorylation which triggers cell growth. IL-5 stimulation results in rapid tyrosine phosphorylation of beta c and proteins containing Src homology 2 (SH2) and/or SH3 domains such as phosphatidyl-inositol-3 kinase, Shc, Vav, and HS1, suggesting their involvement in IL-5-mediated signal transduction. IL-5 stimulation significantly enhanced activities of Janus 2 and B cell-specific Bruton's tyrosine kinases (JAK2 and Btk) and increased the tyrosine phosphorylation of JAK2 kinase. These results and recent data on signaling of growth factors taken together, multiple biochemical pathways driven by tyrosine kinases such as JAK2 and Btk are involved in IL-5 signal transduction.

Activation of protein kinase C by phorbol esters disrupts the tegument of Schistosoma mansoni

The tegument of the human parasite Schistosoma mansoni is critical for parasite survival within the mammalian host. The role of protein kinase C (PKC), a major effector molecule in the phosphoinositide pathway, in maintaining the structural organization of this syncytial layer was examined in adult worms. Phorbol 12-myristate, 13-acetate (PMA) and phorbol 12,13-dibutyrate (PDB), phorbol esters that activate PKC, induced formation of surface vesicles as determined by light and scanning electron microscopy. Similar results were seen with sn-2-dioctanoyl-glycerol, a synthetic analogue of diacylglycerol. No effect was seen in parasites incubated with 4-alpha-phorbol ester or alpha isomers of PMA or PDB, compounds that do not activate PKC. Vesicle formation was reversible in parasites treated with sn-2-dioctanoyl-glycerol but not with phorbol esters. The tegument of male worms was more sensitive to the effect of phorbol esters than females. Transmission electron microscopy revealed vacuolization of the tegument. These data suggest that signal transduction pathways may have a critical role in the maintenance of the structural integrity of the tegument of parasitic helminths.

Association of elevated levels of protein kinase C-zeta mRNA and protein with murine B-lymphocytic neoplasia

Expression of mRNA for protein kinase C (PKC)-alpha, -beta, -gamma, -delta, -epsilon, -zeta, and -eta has been shown, by polymerase chain reaction-generated isozyme-specific probes, to be cell-type -and differentiation-stage-specific in mouse hemopoietic cells. Recently, we cloned a 2.2-kb mouse PKC -zeta cDNA. In this study, we used the nearly full-length cDNA PKC-zeta probe to demonstrate that expression of PKC-zeta was significantly elevated in lymphocytic neoplasms at both the mRNA and protein levels. Normal brain, kidney, and liver contain 2.4- and 4.4-kb mRNAs, whereas normal lymphoid organs (spleen, thymus, and lymph nodes) express barely detectable amounts of PKC-zeta. These vanishingly small levels of PKC-zeta mRNA did not increase when polyclonal spleen B-cell proliferation and differentiation were induced in vivo with anti-immunoglobulin D antiserum or in vitro with lipopolysaccharide. In contrast, 2.4-kb transcripts of PKC-zeta are abundant in virtually all neoplastic B-lymphocytic cell lines. Furthermore, additional transcripts of a novel size, about 7 and 8 kb, were found in several mature B-cell lymphomas and plasma cell tumors. Western blot analysis of protein extracts from normal B cells and hemopoietic tumors confirmed that these quantitative differences in PKC-zeta mRNA also exist at the protein level. That is, only trace amounts of PKC-zeta protein were detectable in pro-B cells and pre-B cells, but abundant amounts of this isoform were found in protein extracts from most B-cell lymphomas and plasma cell tumors. These findings suggest that this atypical member of the PKC multigene family participate in the multistep process of malignant transformation of lymphocytes.

The identification of four protein kinase C isoforms in human glioblastoma cell lines: PKC alpha, gamma, epsilon, and zeta

Levels of protein kinase C (PKC) isoforms in eight human glioblastoma cell lines and two normal human glial cell cultures were determined. Earlier studies identified PKC-alpha and PKC-gamma in these cell lines but PKC-beta was not present. In this study, PKC-epsilon and PKC-zeta are demonstrated immunologically in these cell lines and also in two normal human glial cell cultures. Protein kinase C-delta was not present. When levels of the four isoforms in the tumor cells were compared to levels in the normal cells, no increase was observed in PKC-alpha or PKC-gamma, but PKC-epsilon was elevated three to 30 times in six of the eight tumors, and PKC-zeta was elevated approximately two times in all of the tumors. Incubation of cell line A172 with phorbol ester for 6 hours resulted in a 48-fold maximum increase in the nuclear PKC-epsilon and a sevenfold increase in the plasma membrane fraction with no change in the cytoplasmic fraction. A similar incubation for 4 hours produced a 0.5- to onefold increase of PKC-zeta in cytoplasmic, nuclear, and plasma membrane fractions. Other researchers have shown that overexpression of PKC-epsilon in fibroblasts results in tumorigenesis, and that blocking PKC-zeta function inhibits deoxyribonucleic acid synthesis. These data suggest that alteration in the expression of PKC-epsilon and PKC-zeta could be a factor in the conversion of normal glial cells to glioblastomas.

Redistribution of protein kinase C isoforms in association with vascular hypertrophy of rat aorta

Freshly enzymatically isolated cells from the aorta of a rat model of vascular hypertrophy were used to investigate the role of protein kinase C (PKC) isoforms during a physiologically relevant growth response. With the combination of immunofluorescence with digital imaging microscopy, PKC isoforms alpha, delta, and zeta were found to be present in single cells from control and hypertrophied rat aortas. The alpha- and zeta-isoforms were distributed in the cytoplasm of control cells; however, in hypertrophied cells, sustained translocations of alpha-PKC to the surface membrane and zeta-PKC to the intranuclear area were seen. delta-PKC was concentrated in the perinuclear area in control cells but appeared to translocate to a more diffuse localization in the cytosol of hypertrophied cells. Staining of mitochondria with rhodamine 123 indicated some similarity in the spatial distribution compared with that of delta-PKC. In control cells, translocation of isoforms alpha and delta was activated by phenylephrine or 12-deoxyphorbol 13-isobutyrate 20-acetate. Agonist stimulation produced translocation of no isoforms in the hypertrophied cells. These results indicate that isoform-specific spatial distribution and translocation of PKC occur in association with the growth response of vascular hypertrophy.

The phosphatidylinositol 3-kinase alpha is required for DNA synthesis induced by some, but not all, growth factors

The phosphatidylinositol 3-kinase (PI 3-K) becomes activated when quiescent cells are stimulated with a variety of growth factors. We have microinjected antibodies specific for the p110 alpha subunit of the PI 3-K into quiescent fibroblasts and tested their effect on the ability of growth factors to stimulate exit from quiescence and entry into S phase. The antibodies inhibited platelet-derived growth factor-induced DNA synthesis, a result in keeping with previous studies using mutant platelet-derived growth factor receptors. Interestingly, functional PI 3-K was required for the first 6 hr of G1--i.e., until approximately 4 hr before the point at which the cells were committed to make DNA. A second tyrosine kinase receptor, the epidermal growth factor receptor, also required the PI 3-K for efficient signaling. However, colony-stimulating factor 1 (whose receptor is highly related to the platelet-derived growth factor receptor) could induce DNA synthesis in the absence of active PI 3-K, as could two growth factors (bombesin and lysophosphatidic acid) whose receptors are functionally coupled to G proteins. These data, therefore, demonstrate that some, but not all, growth factors require functional PI 3-K.

Aluminum-induced DNA synthesis in osteoblasts: mediation by a G-protein coupled cation sensing mechanism

Aluminum (Al3+) stimulates de novo bone formation in dogs and is a potent stimulus for DNA synthesis in non-transformed osteoblasts in vitro. The recent identification of a G-protein coupled cation-sensing receptor (BoPCaR), which is activated by polyvalent agonists [e.g., gadolinium (Gd3+) > neomycin > calcium (Ca2+)], suggests that a similar physiologically important cation sensing receptor may be present in osteoblasts and pharmacologically activated by Al3+. To evaluate that possibility, we assessed whether known BoPCaR agonists stimulate DNA synthesis in MC3T3-E1 osteoblasts and examined the additive effects of Al3+ and BoPCaR agonists on DNA synthesis in MC3T3-E1 osteoblast-like cells. We found that Al3+, Gd3+, neomycin, and Ca2+ stimulated DNA synthesis in a dose-dependent fashion, achieving 50% effective extracellular concentrations (EC50) of 10 microM, 30 microM, 60 microM, and 2.5 mM, respectively. Al3+ displayed non-additive effects on DNA synthesis with the BoPCaR agonists as well as an unrelated G-protein coupled receptor agonist, PGF2 alpha, suggesting shared mechanisms of action. In contrast, the receptor tyrosine kinase agonist, IGF-I (10 eta g/ml), displayed additive proliferative effects when combined with AlCl3, indicating distinct signalling pathways. AlCl3 (25 microM) induced DAG levels 2-fold and the phosphorylation of the myristoylated alanine-rich C kinase (MARCKS) substrate 4-fold, but did not increase intracellular calcium concentrations. Down-regulation of PKC by pre-treatment with phorbol 12-myristate 13-acetate as well as PKC inhibition by H-7 and staurosporine blocked Al(3+)-induced DNA synthesis. Finally, Al3+, Gd3+, neomycin, and Ca2+ activated G-proteins in osteoblast membranes as evidenced by increased covalent binding of [32P]-GTP-azidoanilide to putative G alpha subunits. Our findings suggest that Al3+ stimulates DNA synthesis in osteoblasts through a cation sensing mechanism coupled to G-protein activation and signalling cascades involving DAG and PKC-dependent pathways.

Translocation and downregulation of protein kinase C isoenzymes-alpha and -epsilon by phorbol ester and bryostatin-1 in human keratinocytes and fibroblasts

Protein kinase C isoenzymes can be subdivided into two classes, based on their requirement for calcium. Protein kinase C-alpha, beta I, -beta II, and -gamma are calcium dependent, whereas protein kinase C-gamma, -epsilon, -zeta, -eta, and -theta are calcium independent. We have examined the expression, translocation, downregulation, and activation of calcium-dependent and -independent protein kinase C isoenzymes in human skin keratinocytes and fibroblasts. Human keratinocytes and fibroblasts expressed protein kinase C-alpha, -delta, -epsilon, and -zeta mRNA and protein, whereas protein kinase C-eta (L) was detected only in keratinocytes. Protein kinase C-beta I, -beta II, -gamma, and -theta were not detected in either cell type. The protein kinase C activators 12-0-tetradecanoylphorbol 13-acetate and bryostatin-1 (50 nM, for 5 min) induced translocation of protein kinase C-alpha and -epsilon cytosol to membrane in both keratinocytes and fibroblasts. 12-0-tetradecanoylphorbol 13-acetate and bryostatin-1, for 18 h, induced complete downregulation (i.e., loss) of protein kinase C-alpha and -epsilon in keratinocytes, but only partial downregulation was observed in fibroblasts. The subcellular distribution of protein kinase C-delta, -zeta or protein kinase C-eta, in keratinocytes or fibroblasts, did not change in response to 12-0-tetradecanoylphorbol 13-acetate or bryostatin-1. These data indicate differential expression, subcellular distribution, and regulation of protein kinase C isoenzymes in human skin cells.

Phosphatidylinositol 3-kinase

Currently, a central question in biology is how signals from the cell surface modulate intracellular processes. In recent years phosphoinositides have been shown to play a key role in signal transduction. Two phosphoinositide pathways have been characterized, to date. In the canonical phosphoinositide turnover pathway, activation of phosphatidylinositol-specific phospholipase C results in the hydrolysis of phosphatidylinositol 4,5-bisphosphate and the generation of two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. The 3-phosphoinositide pathway involves protein-tyrosine kinase-mediated recruitment and activation of phosphatidylinositol 3-kinase, resulting in the production of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. The 3-phosphoinositides are not substrates of any known phospholipase C, are not components of the canonical phosphoinositide turnover pathway, and may themselves act as intracellular mediators. The 3-phosphoinositide pathway has been implicated in growth factor-dependent mitogenesis, membrane ruffling and glucose uptake. Furthermore the homology of the yeast vps34 with the mammalian phosphatidylinositol 3-kinase has suggested a role for this pathway in vesicular trafficking. In this review the different mechanisms employed by protein-tyrosine kinases to activate phosphatidylinositol 3-kinase, and its involvement in the signaling cascade initiated by tyrosine phosphorylation, are examined.

Effect of a null mutation of the insulin-like growth factor I receptor gene on growth and transformation of mouse embryo fibroblasts

Fibroblast cell lines, designated R- and W cells, were generated, respectively, from mouse embryos homozygous for a targeted disruption of the Igf1r gene, encoding the type 1 insulin-like growth factor receptor, and from their wild-type littermates. W cells grow normally in serum-free medium supplemented with various combinations of purified growth factors, while pre- and postcrisis R- cells cannot grow, as they are arrested before entering the S phase. R- cells are able to grow in 10% serum, albeit more slowly than W cells, and with all phases of the cell cycle being elongated. An activated Ha-ras expressed from a stably transfected plasmid is unable to overcome the inability of R- cells to grow in serum-free medium supplemented with purified clones. Nevertheless, even in the presence of serum, R- cells stably transfected with Ha-ras, alone or in combination with simian virus 40 large T antigen, fail to form colonies in soft agar. Reintroduction into R- cells (or their derivatives) of a plasmid expressing the human insulin-like growth factor I receptor RNA and protein restores their ability to grow with purified growth factors or in soft agar. The signaling pathways participating in cell growth and transformation are discussed on the basis of these results.

Effect of a null mutation of the insulin-like growth factor I receptor gene on growth and transformation of mouse embryo fibroblasts

Fibroblast cell lines, designated R- and W cells, were generated, respectively, from mouse embryos homozygous for a targeted disruption of the Igf1r gene, encoding the type 1 insulin-like growth factor receptor, and from their wild-type littermates. W cells grow normally in serum-free medium supplemented with various combinations of purified growth factors, while pre- and postcrisis R- cells cannot grow, as they are arrested before entering the S phase. R- cells are able to grow in 10% serum, albeit more slowly than W cells, and with all phases of the cell cycle being elongated. An activated Ha-ras expressed from a stably transfected plasmid is unable to overcome the inability of R- cells to grow in serum-free medium supplemented with purified clones. Nevertheless, even in the presence of serum, R- cells stably transfected with Ha-ras, alone or in combination with simian virus 40 large T antigen, fail to form colonies in soft agar. Reintroduction into R- cells (or their derivatives) of a plasmid expressing the human insulin-like growth factor I receptor RNA and protein restores their ability to grow with purified growth factors or in soft agar. The signaling pathways participating in cell growth and transformation are discussed on the basis of these results.

Antipeptide antibodies directed against the C-terminus of protein kinase C zeta (PKC zeta) react with a Ca(2+)- and TPA-sensitive PKC in HT-29 human intestinal epithelial cells

We have studied the PKC isoforms present in HT-29 M6 colon cancer cells, the differentiation of which to mucus-secreting cells is blocked by TPA. In addition to a major 72 kDa band, a 77 kDa PKC isoform was recognized by two different antibodies raised against a C-terminus-specific peptide for the TPA-insensitive isoform, PKC zeta. By different criteria (association to the membrane, down-regulation, PKC activity in immunoprecipitates) we conclude that, contrary to the 72 kDa band, the 77 kDa band corresponds to a Ca(2+)- and TPA-sensitive PKC. These results suggest that antipeptide antibodies directed against the C-terminus of PKC zeta react in human cells with a member of the conventional PKC subfamily besides PKC zeta. Therefore, the data indicating that PKC zeta is sensitive to different agents in various cell lines should be carefully re-evaluated.

Developmental expression of the protein kinase C family in rat hippocampus

Protein kinase C (PKC) is a heterogeneous family of ten or more isoforms which plays an important role in neuronal signal transduction. Isoforms from all subclasses are prominently expressed in the rat hippocampus, as demonstrated by immunoblot with isozyme-specific antisera: Ca(2+)-dependent (alpha, beta I, beta II and gamma), Ca(2+)-independent (delta, epsilon and a newly characterized PKC related to eta) and atypical (zeta). In addition, the zeta isoform is also found as the free, constitutively active catalytic domain, protein kinase M zeta (PKM zeta). Two distinct patterns of expression of PKC isozymes in rat hippocampus are found during development from E18 to P28. PKC zeta, PKM zeta and PKC delta are present at birth and their expression does not increase postnatally. In contrast, the other isoforms are expressed only at low levels at birth and then increase in the first 4 weeks postnatally. These two patterns of expression suggest distinct functions for PKC isozymes during development.

Messenger molecules derived from membrane lipids

Significant advances have been made recently concerning mechanisms involved in the regulation of cell calcium levels. The mechanisms and physiological significance of agonist-induced phosphatidylcholine hydrolysis are also becoming clearer.