Early response pattern analysis of the mitogenic pathway in lymphocytes and fibroblasts

The Role for Myc in Coordinating Glycolysis, Oxidative Phosphorylation, Glutaminolysis, and Fatty Acid Metabolism in Normal and Neoplastic Tissues

That cancer cells show patterns of metabolism different from normal cells has been known for over 50 years. Yet, it is only in the past decade or so that an appreciation of the benefits of these changes has begun to emerge. Altered cancer cell metabolism was initially attributed to defective mitochondria. However, we now realize that most cancers do not have mitochondrial mutations and that normal cells can transiently adopt cancer-like metabolism during periods of rapid proliferation. Indeed, an encompassing, albeit somewhat simplified, conceptual framework to explain both normal and cancer cell metabolism rests on several simple premises. First, the metabolic pathways used by cancer cells and their normal counterparts are the same. Second, normal quiescent cells use their metabolic pathways and the energy they generate largely to maintain cellular health and organelle turnover and, in some cases, to provide secreted products necessary for the survival of the intact organism. By contrast, undifferentiated cancer cells minimize the latter functions and devote their energy to producing the anabolic substrates necessary to maintain high rates of unremitting cellular proliferation. Third, as a result of the uncontrolled proliferation of cancer cells, a larger fraction of the metabolic intermediates normally used by quiescent cells purely as a source of energy are instead channeled into competing proliferation-focused and energy-consuming anabolic pathways. Fourth, cancer cell clones with the most plastic and rapidly adaptable metabolism will eventually outcompete their less well-adapted brethren during tumor progression and evolution. This attribute becomes increasingly important as tumors grow and as their individual cells compete in a constantly changing and inimical environment marked by nutrient, oxygen, and growth factor deficits. Here, we review some of the metabolic pathways whose importance has gained center stage for tumor growth, particularly those under the control of the c-Myc (Myc) oncoprotein. We discuss how these pathways differ functionally between quiescent and proliferating normal cells, how they are kidnapped and corrupted during the course of transformation, and consider potential therapeutic strategies that take advantage of common features of neoplastic and metabolic disorders.

The use of sea urchin eggs as a model to investigate the effects of crassolide, a diterpene isolated from a soft coral

Crassolide, a monocyclic diterpene isolated and purified from the soft coral Lobophytum crassum, inhibited the cell cleavage of sea urchin eggs without affecting fertilization. The effect was observed with concentrations above 2 x 10(-5)m in egg suspensions. Addition of crassolide between 5 and 40 min post-fertilization totally blocked the first cleavage, which in the control occurs 1 hr after fertilization. When added between 50 and 60 min post-fertilization, crassolide produced polynucleated cells in embryos. Crassolide did not affect the egg permeability to Na(+) and Ca(2+), but caused an increase of 0.2 units in the intracellular pH of fertilized eggs coupled with a proton efflux. Crassolide, which does not affect Ca(2+) influx or permeability at the level of storage in reticular vesicles, could be used as a negative control when analysing calcium changes in short-term toxicological studies. The relationship between the pH increase and the cell cleavage needs further investigation.

A comprehensive SAGE database for the analysis of gammadelta T cells

Gammadelta T cells have been conserved since the adaptive immune system arose, yet their importance is still unclear. In an attempt to compensate for the lack of a broad knowledge base of gammadelta T cells across species, global analyses of gammadelta T cell transcriptomes have been performed using serial analysis of gene expression (SAGE). Twelve new SAGE libraries were generated from the following bovine lymphocyte populations: magnetic bead-sorted blood gammadelta T cells, spleen gammadelta T cells and enriched spleen alphabeta T cells from a single calf, both rested and Con A/IL2 stimulated, and flow cytometry-sorted blood gammadelta and alphabeta T cells each either rested, Con A/IL2, or phorbol 12 myristate 13-acetate/ionomycin stimulated. These new libraries complement two earlier SAGE libraries of circulating gammadelta T cell subsets. These databases were analyzed using new web-based bioinformatic tools, which allow the user to rapidly compare gene expression patterns within these and other SAGE and standard expressed sequence tag libraries generated from different cell types and different species. These analyses revealed striking differences between blood and spleen gammadelta T cells and how these cells respond to mitogenic stimulation. These analyses also confirm previous studies that suggested that global gene expression in gammadelta and alphabeta T cells is quite similar; however, a 5-fold increase in gammadelta T cell-specific transcripts could be induced by Con A/IL2 stimulation. These new public databases provide additional resources for the annotation/analysis of global gene expression in gammadelta T cells, which will facilitate studies of the biology of this enigmatic lymphoid cell.

The normal cellular form of prion protein modulates T cell responses

Expression of the normal form of prion protein (PrP(C)) has been reported on a wide range cells including lymphocytes and antigen presenting cells, however the functional role of PrP(C) remains to be fully elucidated. Here we report the effect of reintroducing the PrP gene into splenocytes derived from prion knockout (PrP 0/0) mice and comparing their responses with splenocytes lacking a functional PrP gene. Reintroduction of the PrP gene was carried out by transfecting cells with pC1PrPEH, a plasmid expressing mouse PrP. Following transfection, T cells demonstrated an increased capacity to proliferate in response to ConA and PMA/ionomycin compared to T cells lacking the functional PrP gene. A bioassay used to determine IL-2 levels indicated that the reintroduction of the PrP gene might enhance IL-2 expression in response to ConA. Levels of IFN-gamma produced also showed an increase following transfection with PrP expressing plasmid. A comparison between splenocytes derived from PrP 0/0 and PrP +/+ also demonstrated some differences in cytokine production and proliferation. Together these results show PrP(C) has an impact on the normal T cell activation and proliferation in response to mitogens and also potentially antigen responsiveness.

Effect of cocaine and morphine on neutral endopeptidase activity of human peripheral blood mononuclear cells cultured with lectins

We have tested the effect of alkaloids (cocaine, morphine) and enkephalins on neutral endopeptidase of peripheral blood mononuclear cells activated by lectins. When treated with concanavalin A and cocaine, peripheral blood mononuclear cells showed an enhanced activity (+110 per cent) of the membrane neutral endopeptidase, which was not related to the expression of the common acute lymphoblastic leukemia antigen at the cell surface, although both molecules have the identical amino acid sequence. Phytohemagglutinin-P, morphine and synthetic enkephalins did not induce the activity of neutral endopeptidase nor the expression of common acute lymphoblastic leukemia antigen. Our findings suggested that the drugs of abuse, cocaine and morphine, affected specific membrane constituents without altering proliferation, subcellular localization of membrane enzymes or the surface immune phenotype of peripheral blood mononuclear cells.

Stimulatory and inhibitory effects of 1,25-dihydroxyvitamin D3 on thymocyte mitogenesis induced by phorbol ester and calcium ionophore

Mouse medullary thymocytes have specific receptors for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). The mitogenic stimulation of these cells by phytohemagglutinin in the presence or absence of the phorbol ester TPA is inhibited by 1,25(OH)2D3. The calcium ionophore A23187 did not reverse the inhibition by 1,25(OH)2D3 of phytohemagglutinin. Stimulation of thymocytes with either TPA or A23187 alone did not result in proliferation. Co-stimulation of the thymocytes with TPA and A23187 induces cell proliferation. 1,25(OH)2D3 markedly enhanced the TPA and A23187-induced cell proliferation even when added 4 h after the initiation of the culture. In contrast, DNA synthesis by thymocytes incubated for 4 h in the presence of TPA and A23187 and then cultured in medium containing 1,25(OH)2D3 but in the absence of both TPA and A23187, was inhibited by 1,25(OH)2D3. The extent of inhibition was comparable to the inhibition of lectin-induced stimulation by the hormone. Using monoclonal antibodies to neutralize IL-2 and block IL-2 receptors we showed that 1,25(OH)2D3 enhanced the IL-2-independent component of the A23187- and TPA-induced mitogenesis.

IN CONCLUSION

(1) The nature and presence of the mitogenic signal determines whether 1,25(OH)2D3 enhances or inhibits thymocyte stimulation. (2) Both stimulatory and inhibitory actions of 1,25(OH)2D3 seem to take place at points distal to the initial increase in intracellular calcium or activation of protein kinase C.

Differential effects of the calcium ionophore A23187 and the phorbol ester PMA on lymphocyte proliferation

The effects of the phorbol ester, phorbol-12-myristate-13-acetate (PMA), and of the calcium ionophore A23187 on DNA synthesis in murine quiescent and mitogen-stimulated lymphocytes have been examined. Neither PMA nor A23187 had any mitogen effect on their own on quiescent lymphocytes. However, stimulation of cells sequentially with A23187 and then PMA resulted in a proliferative response in proportion to the duration of the exposure to A23187, and the sustained simultaneous presence of both agents was necessary for maximum proliferation. On the other hand, while short incubations with A23187 potentiated mitogen-induced DNA synthesis, prolonged exposure inhibited it. Furthermore, on lymphocytes stimulated with two T cell mitogens, the effects of A23187 and PMA depended on the proliferation-inducing mitogens and the responsiveness level induced by them. Therefore, while PMA and short pretreatments with A23187 had no effect on high intensity mitogenic responses, low intensity responses were significantly enhanced. These results demonstrate differential effects of A23187 and PMA on DNA synthesis that should be useful in studies on the mechanisms of activation of lymphocyte proliferation.

Association of cellular thiol redox status with mitogen-induced calcium mobilization and cell cycle progression in human fibroblasts

Human gingival fibroblast cultures were used to investigate the role of cellular thiol redox status in the mitogenic response. Increases in intracellular Ca2+ and cell cycle progression beyond G1 were followed as parameters of cellular mitogen-induced responses. Ethionine provided a G1 stage synchronization and altered the cellular redox poise as measured by the ratio NAD(P)H/NAD(P)+. Cultures harvested immediately after the 6 day ethionine low-serum synchronization showed a significant oxidation of their redox poise. Synchronized cultures, which were also glutathione (GSH) depleted, still showed an oxidized redox poise and significantly reduced GSH levels following a 24 hr incubation in drug-free, rich medium. Cellular reduced nicotinamide nucleotide levels correlated strongly (r = 0.995) with capacity to mobilize intracellular Ca2+ in response to basic fibroblast growth factor (bFGF). The sustained mitogenic response, as determined by cell cycle progression beyond G1, was also found to be interrelated with the cellular thiol redox status. Following a 24 hr recovery incubation in serum-rich medium, formerly synchronized cultures showed a rebound of their redox poise to a more reduced state and significant cell cycle progression beyond G1. In contrast, synchronized, GSH-depleted cultures did not progress and showed population distributions similar to those of cultures harvested immediately postsynchronization. Upon recovery of cellular GSH and reduced nicotinamide nucleotide levels, formerly GSH-depleted, growth-arrested cultures resumed cell cycle progression. The results suggest that the cellular response to specific mitogens is interrelated with the cellular thiol redox status. Cells that possess a thiol redox status below a threshold response point may have compromised Ca2+ sequestration and/or mobilization and therefore may be incapable of initiating the mitogen induced response cascade that culminates in cell cycle progression.

NIH-3T3 cells transformed with a ras oncogene exhibit a protein kinase C-mediated inhibition of agonist-stimulated Ca2+ inflow

1. The ability of bombesin or platelet-derived growth factor (PDGF) to stimulate Ca2+ inflow (assessed by measuring changes in the intracellular free Ca2+ concentration in cells loaded with fura-2) in NIH-3T3 cells transformed with the EJ/T24-Ha-ras-1 oncogene is inhibited when compared with the action of the agonists on wild-type cells. 2. The effects of transformation with the ras oncogene are associated with complete inhibition of the ability of bombesin to release Ca2+ from intracellular stores, a substantial decrease in the number of bombesin receptors, no change in the ability of foetal calf serum or ionomycin to release Ca2+ from intracellular stores and the activation of protein kinase C. 3. The effects of transformation with the H-ras oncogene on the ability of bombesin or PDGF to stimulate Ca2+ inflow were mimicked by a 30 min exposure of wild-type cells to phorbol dibutyrate. This action of phorbol dibutyrate was completely blocked by prior treatment of wild-type cells for 24 h with the phorbol ester. 4. It is concluded that one of the actions of the H-ras oncogene in fibroblasts is to inhibit agonist-stimulated Ca2+ inflow by a mechanism which involves the activation of protein kinase C.

12-O-tetradecanoylphorbol 13-acetate potentiates the action of cAMP in inducing DNA cleavage in thymocytes

12-O-Tetradecanoylphorbol 13-acetate (TPA) potentiated the action of cAMP in DNA cleavage in thymocytes induced by a low concentration of adenosine receptor-site agonists such as adenosine, 2-chloroadenosine and forskolin. The enhancement of DNA cleavage by TPA was also observed in dibutyryl cAMP-treated thymocytes. On the other hand, TPA suppressed accumulation of cAMP by the adenosine receptor-site agonists. These results suggest that activation of protein kinase C inhibits cAMP production, but stimulates cAMP-triggered process to induce DNA cleavage and death of thymocytes.

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.

Effects of argon laser radiation on aortic endothelial cells: early membrane changes and proliferative response

Membrane fluidity, transmembrane signaling responses, and proliferative characteristics of endothelial cells were studied to characterize biochemical and molecular changes after treatment with argon laser energy. Bovine aortic endothelial cells grown in monolayers were irradiated at 50, 100, and 200 J with an argon laser (wavelength, 488 and 514 nm). Proliferation, assayed by [3H]thymidine incorporation, was measured daily for 6 days. An initial lag phase was observed for irradiated cells when compared to nonirradiated controls (P less than 0.03), with eventual recovery by the third day. Membrane fluidity, determined by fluorescence anisotropy, was measured 1 hr after irradiation. A decrease in static rotational motion of 1,6-diphenyl-1,3,5-hexatriene (DPH) was noted in irradiated versus nonirradiated cells indicating a decrease in membrane fluidity (P less than 0.02). Dynamic studies of intracellular calcium and pH flux utilizing fluorescent probes demonstrated a preserved response to mitogenic stimulation. An increase in intracellular Ca2+ with a concomitant alkalinization of the intracellular milieu was observed in irradiated and non-irradiated cells in response to stimulation with endothelial cell growth factor (ECGF). These responses resemble those characterized for other mitogens. Argon laser energy applied to aortic endothelial cells decreases membrane fluidity early after irradiation. These alterations probably cause the initial lag observed in their proliferative response; however, the capacity to respond to exogenous mitogenic stimulation is maintained.

Effects of cadmium on lymphocyte activation

The effects of cadmium (Cd) on phytohemoagglutinin or phorbol myristate acetate-induced lymphocyte activation were investigated and a dose-dependent inhibition of cell proliferation was found. Kinetic studies revealed that the Cd-sensitive step is an early event of T cell stimulation. Failure of IL2 secretion and reduction of IL2 receptor expression in the Cd-treated cells are also reported. Regardless of which mechanism is responsible for Cd effects, our studies show that the inhibition of lymphocyte activation is associated with reduced [3H]phorbol dibutyrate binding to Ca2+-phospholipid-dependent protein kinase and altered breakdown of phosphatidylinositols. Thus, Cd interferes with two biochemical events which play a critical role in lymphocyte signal transduction and activation.

Prostaglandins inhibit proliferation of the murine P815 mastocytoma by decreasing cytoplasmic free calcium levels [( Ca+2]i)

Prostaglandins inhibit the proliferation of the murine P815 mastocytoma. The mechanism of this antitumour activity remains undefined. In several cell systems, the action of PGs is inhibited at the cell surface receptor by pertussis toxin likely through regulatory G proteins involved in the inhibition of adenyl cyclase or activation of phospholipase C. We therefore determined the effect of prostaglandins on the biochemical consequences of activation of these pathways; i.e. concentrations of cyclic AMP (cAMP) and cytosolic free Ca+2 concentrations [( Ca/2]i) respectively. PGD2 (6 ug/mL), PGE1 (10 ug/mL) and PGB1 (50 ug/mL) maximally inhibited (3H)-thymidine incorporation to DNA. PGF2 alpha did not affect DNA synthesis. PGE1 (10 ug/mL) induced a three fold increase in cAMP concentrations. In contrast, the other prostaglandins did not alter cAMP concentrations. Maximal growth inhibitory doses of PGD2, PGE1 and PGB1 decrease [Ca+2]i, as measured by the fluorescence of Indo-1, from 320 +/- 5 nM to 172 +/- 20 nM, 161 +/- 12 nM, and 151 +/- 18 nM respectively. PGF2 alpha did not alter [Ca+2]i. Therefore, in contrast to the effects on cAMP, the decrease in [Ca+2]i was concordant with the inhibition of DNA synthesis. This suggests that PGs may inhibit proliferation through decreasing [Ca+2]i in the P815 mastocytoma.

C-fos gene activation in murine thymocytes by a mechanism independent of protein kinase C or a Ca2+ signal

The accumulation of c-fos mRNA in mouse thymocytes was compared when the cells were stimulated by concanavalin A (Con A), the Ca2+ ionophore A23187 or the phorbol ester, TPA, either separately or by combinations of these mitogens. The c-fos response to mitogenic concentrations of Con A could not be attributed either to the rise in [Ca2+]i it induces or to activation of protein kinase C. Thus, although Con A causes the breakdown of phosphatidylinositol 4,5-bisphosphate in these cells, neither of the signals which can be generated by this response was responsible for the activation of the c-fos gene by Con A. This implies that some other unidentified signal generated by Con A activates the c-fos gene.

Phosphoinositide metabolism and the calcium response to concanavalin A in S49 T-lymphoma cells. A comparison with thymocytes

Comparisons were made between transformed S49 T-lymphoma cells and normal murine thymocytes in their polyphosphoinositides, inositol polyphosphates and cytosolic free calcium concentrations ([Ca2+]i), and the effects of the T-cell mitogen concanavalin A (Con A) on these properties. 1. The ratios of the polyphosphoinositides to phosphatidylinositol in both exponential-phase S49 cells and mitogen-stimulated thymocytes (G1 phase) were greater than in quiescent (G0-phase) thymocytes. 2. In response to Con A, the amount of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) in S49 cells decreased slightly (17% in 30 min), and this was sufficient to account for the small amounts of inositol phosphates that accumulated. In contrast, it has been shown previously that Con A stimulates a rapid resynthesis of PtdInsP2 in thymocytes and the amounts of inositol phosphates released rapidly exceed the steady-state amount of the PtdInsP2 precursor [Taylor, Metcalfe, Hesketh, Smith & Moore (1984) Nature (London) 312, 462-465]. 3. The [Ca2+]i did not differ significantly in S49 cells and thymocytes before the addition of Con A, and the increases in [Ca2+]i in response to Con A were similar in both types of cell. 4. The [Ca2+]i increase in response to Con A was inhibited by similar concentrations of intracellular cyclic AMP (2-10 microM) in S49 cells and thymocytes, suggesting that similar regulatory mechanisms act on this response in both types of cell. The data demonstrate that the basal [Ca2+]i and phosphoinositide metabolism is similar in both the normal cells and their transformed counterparts. In addition, they suggest that the activated Con A receptors generate very similar signals in the two cell types, and that any perturbations of primary signal transduction to the secondary phosphoinositide and [Ca2+]i responses in the S49 phenotype are quantitative rather than qualitative.

GTP gamma S activation of proto-oncogene expression in transiently permeabilised Swiss 3T3 fibroblasts

A technique of transient permeabilisation has been used to show that the introduction of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), a non-hydrolysable analogue of GTP, into intact Swiss 3T3 fibroblasts stimulates phosphoinositide hydrolysis, cyclic AMP accumulation and the activation of c-fos and c-myc proto-oncogenes. Of a number of nucleotide triphosphates introduced into the cells, only GTP and its non-hydrolysable analogues activated inositol phosphate release, suggesting that this response is mediated by guanine nucleotide regulatory (G) protein(s). The data demonstrate that transient permeabilisation provides a method of examining the involvement of G-proteins in nuclear activation.

C-terminal bombesin sequence requirements for binding and effects on protein synthesis in Swiss 3T3 cells

1. Synthetic peptides corresponding to the five, seven, nine and eleven C-terminal amino acids of the tetradecapeptide bombesin as well as bombesin itself and gastrin-releasing peptide have been evaluated in Swiss 3T3 cells in order to define the minimal peptide length needed for biological responsiveness. 2. Gastrin-releasing peptide, bombesin, the undecapeptide and nonapeptide had nearly equipotent abilities to compete for binding of labelled gastrin-releasing peptide to the cell receptors and showed half-maximal competition at 5-10 nM. The heptapeptide and pentapeptide were ineffective. 3. Cross-linking experiments demonstrated specific binding of gastrin-releasing peptide to a 100 kDa receptor subunit. 4. Total cell protein synthesis was stimulated equally by the nonapeptide and longer peptides with a half-maximal effect at 0.5 nM, while a more than 1000-fold higher concentration of the heptapeptide was required to produce a similar response. Comparable results were found when insulin was also present. 5. Neither an inhibition of protein breakdown nor a stimulation of DNA labelling could be demonstrated by bombesin or gastrin-releasing peptide. 6. We conclude that a C-terminal peptide ligand comprising more than seven but no more than nine amino acids is required to achieve high-affinity binding and receptor-mediated responses via the bombesin receptor.

Anti-phospholipase C antibodies inhibit the lectin-induced proliferation of human lymphocytes

A novel approach was used to assess the role of phosphoinositide hydrolysis in the mitogenic action of phytohemagglutinin (PHA) or concanavalin A (ConA). The treatment of human peripheral blood leukocytes (PBL) with monospecific antibodies against phospholipase C (PLC) produced a dose-dependent inhibition (up to 100%) of PHA (10 micrograms/ml) or ConA (25 micrograms/ml) proliferative effects. Thus, the activation of membrane-bound PLC is a sine-qua-non condition for lectin-induced proliferation of T lymphocytes. The key-role of PLC versus protein kinase C (PKC) is stressed by the fact that the inhibition of PKC with Hidaka's compound H-7 (40 microM) produced only a partial blockade (about 25%) of lectin mitogenic effect.

Mitogenic stimulation transiently increases the exchangeable mitochondrial calcium pool in rat thymocytes

Exchangeable calcium pools were measured in rat thymocytes by 45Ca labelling and selective depletion of intracellular pools with oligomycin in the presence or absence of rotenone. The mitochondrial pool increased by 150% after 3 min of treatment with the mitogen concanavalin A, and decreased to zero 10 min after mitogen addition. No significant change in the ATP-dependent pool could be detected.

Differential control of proto-oncogene c-myc and c-fos expression in lymphocytes and fibroblasts

In lymphocytes stimulated with the mitogen phytohaemagglutinin, an inhibitor of the enzyme ADP-ribosyltransferase (ADPRT) completely blocks the proliferative response and the increase in expression of the proto-oncogene c-myc without affecting c-fos significantly. Conversely, in fibroblasts the serum-induced growth is not affected by the ADPRT inhibitor, and both oncogenes are dramatically super-induced. Hence there are differences between lymphocyte and fibroblast early responses to mitogenic stimulation and also between regulation of c-fos and c-myc gene expression.

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

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