The effect of cAMP on tumour promoter responses mediated by C-kinase

Neurochemical and morphological studies on differentiation of NG108-15 cells by phorbol ester and forskolin

12-O-tetradecanoylphorbol 13-acetate (TPA), forskolin or dibutyryl cAMP induced neurite outgrowth and inhibition of cell growth in NG108-15 cells. TPA, forskolin and dibutyryl cAMP significantly increased specific activity of choline acetyltransferase. Forskolin markedly stimulated cAMP accumulation, but not TPA, suggesting that forskolin could induce differentiation by increasing the cAMP content via adenylate cyclase activation, but TPA-induced differentiation seems not to be due to the raise of the cAMP level. Incubation of the cells with TPA, forskolin or dibutyryl cAMP for 24 h resulted in enhancement of 50 mM K(+)-evoked Ca(2+) influx and neurite elongation, although incubation with these agents for 1 h didn't affect these events. From these results, it is suggested that TPA and forskolin induce differentiation of NG108-15 cells to acetylcholine neurons via different mechanisms: protein kinase C activation by TPA and cAMP-dependent protein kinase activation by forskolin. In addition, it is likely that Ca(2+) channels in cells differentiated by TPA, forskolin or dibutyryl cAMP become sensitive to depolarization.

Protein kinase C and meiotic regulation in isolated mouse oocytes

In this study, the possible role of protein kinase C (PKC) in mediating both positive and negative actions on meiotic maturation in isolated mouse oocytes has been examined. When cumulus cell-enclosed oocytes (CEO) were cultured for 17-18 hr in a medium containing 4 mM hypoxanthine (HX) to maintain meiotic arrest, each of the five different activators and five different antagonists of PKC stimulated germinal vesicle breakdown (GVB) in a dose-dependent fashion. One of the activators, phorbol-12-myristate 13-acetate (PMA), also triggered GVB in CEO arrested with isobutylmethylxanthine or guanosine, but not in those arrested with dibutyryl cyclic AMP. When denuded oocytes (DO) were cultured for 3hr in inhibitor-free medium, all PKC activators suppressed maturation (<10% GVB compared to 94% in controls), while the effect of PKC antagonists was negligible. Four of the five antagonists reversed the meiosis-arresting action of HX in DO. PMA transiently arrested the spontaneous maturation of both CEO and DO, with greater potency in DO. The stimulatory action of PMA in HX-arrested oocytes was dependent on cumulus cells, because meiotic induction occurred in CEO but not DO. PKC activators also preferentially stimulated cumulus expansion when compared to antagonists. A cell-cell coupling assay determined that the action of PMA on oocyte maturation was not due to a loss of metabolic coupling between the oocyte and cumulus oophorus. Finally, Western analysis demonstrated the presence of PKCs alpha, beta1, delta, and eta in both cumulus cells and oocytes, but only PKC epsilon was detected in the cumulus cells. It is concluded that direct activation of PKC in the oocyte suppresses maturation, while stimulation within cumulus cells generates a positive trigger that leads to meiotic resumption.

In situ regulation of cell-cell communication by the cAMP-dependent protein kinase and protein kinase C

The effects of cAMP-dependent protein kinase A and protein kinase C on cell-cell communication have been examined in primary ovarian granulosa cells microinjected with purified components of these two regulatory cascades. These cells possess connexin43 (alpha 1)-type gap junctions, and are well-coupled electrotonically and as judged by the cell-to-cell transfer of fluorescent dye. Within 2-3 min after injection of the protein kinase A inhibitor (PKI) communication was sharply reduced or ceased, but resumed in about 3 min with the injection of the protein kinase A catalytic subunit. A similar resumption also occurred in PKI-injected cells after exposure to follicle stimulating hormone. Microinjection of the protein kinase C inhibitor protein caused a transient cessation of communication that spontaneously returned within 15-20 min. Treatment of cells with activators of protein kinase C, TPA or OAG for 60 min caused a significant reduction in communication that could be restored within 2-5 min by the subsequent injection of either the protein kinase C inhibitor or the protein kinase A catalytic subunit. With a longer exposure to either protein kinase C activator communication could not be restored and this appeared to be related to the absence of aggregates of connexin43 in membrane as detected immunologically. In cells injected with alkaline phosphatase communication stopped but returned either spontaneously within 20 min or within 2-3 min of injecting the cell with either the protein kinase A catalytic subunit or with protein kinase C. When untreated cells were injected with protein kinase C communication diminished or ceased within 5 min. Collectively these results demonstrate that cell-cell communication is regulated by both protein kinase A and C, but in a complex interrelated manner, quite likely by multiple phosphorylation of proteins within or regulating connexin-43 containing gap junctions.

Regulation of the transmodulated epidermal growth factor receptor by cholera toxin and the protein phosphatase inhibitor okadaic acid

Addition of tumor promoting phorbol esters, such as phorbol 12-myristate 13-acetate (PMA), to many cell lines results in a decrease of 125I-epidermal growth factor (EGF) binding and increased serine/threonine phosphorylation of the EGF receptor in a process termed transmodulation. It is, however, unclear whether or not receptor phosphorylation is causally related to the inhibition of high affinity EGF binding. We have investigated the significance of phosphorylation/dephosphorylation events in the mechanism of PMA-induced transmodulation using the adenylate cyclase activator cholera toxin and the serine/threonine protein phosphatase inhibitor okadaic acid. In Rat-1 fibroblasts treated at 37 degrees C, PMA induced a rapid decrease in EGF binding which persisted for 3 hours. In contrast, cells exposed to PMA in the presence of cholera toxin exhibited a marked recovery of binding within 60 minutes. The PMA-stimulated decrease in binding correlated with a rapid increase in the phosphorylation state of the EGF receptor. While phosphorylation of the receptor was sustained at an elevated level for at least three hours in cells receiving PMA alone, EGF receptor phosphorylation decreased between 1 and 3 hours in cells treated with PMA and cholera toxin. Furthermore, the cholera toxin-stimulated return of EGF binding was inhibited by treatment with the phosphatase inhibitor okadaic acid. These results suggest that a cholera toxin-activated phosphatase can increase binding capacity of the transmodulated EGF receptor in Rat-1 cells. Cholera toxin treatment elicited a qualitatively similar response in cells transmodulated by platelet-derived growth factor (PDGF). Okadaic acid antagonized the natural return of binding observed in cells stimulated with PDGF alone, indicating that a dephosphorylation event may be required for the recovery of normal EGF binding after receptor transmodulation.

Response of malignant glioma cell lines to activation and inhibition of protein kinase C-mediated pathways

To evaluate the role of protein kinase C-mediated pathways in the proliferation of malignant gliomas, this study examined the effect of a protein kinase C (PKC)-activating phorbol ester (12-O-tetradecanoyl-13-phorbol acetate or TPA) and a protein kinase C inhibitor (polymyxin B) on deoxyribonucleic acid (DNA) synthesis of malignant glioma cells in vitro. A serum-free chemically defined medium, MCDB 105, was employed for all studies. Two established human malignant glioma cell lines (T98G and U138), two rat glioma lines (9L and C6), and two low-passage human glioma lines (obtained from surgical specimens) were studied. With the exception of the C6 line, all tumors responded in a dose-dependent fashion to nanomolar concentrations of TPA with a median effective dose that varied from 0.5 ng/ml for the U138 glioma to 1 ng/ml for the T98G glioma. At optimal concentrations (5 to 10 ng/ml), TPA produced a two- to five-fold increase in the rate of DNA synthesis (p less than 0.05) as assessed by incorporation of 3H-thymidine. However, TPA had no additive effect on the mitogenic response produced by epidermal growth factor (EGF) or platelet-derived growth factor (PDGF). Inhibition of PKC using the antibiotic polymyxin B (20 micrograms/ml) abolished the TPA-induced mitogenic response in the five responsive lines tested. In two tumors (U138 and 9L), polymyxin B also eliminated EGF-, PDGF-, and serum-induced DNA synthesis as well as abolishing baseline DNA synthesis. These cells remained viable, however, as assessed by trypan blue exclusion; after removal of polymyxin B from the medium, they were able to resume DNA synthesis in response to TPA and serum. In the three other tumors (T98G and the two low-passage human glioma lines), growth factor-induced and serum-induced DNA synthesis were inhibited by approximately 25% to 85%. It is concluded that PKC-mediated pathways affect DNA synthesis in the human malignant glial tumors studied. The response of the glioma cells to TPA is similar to the responses seen in fetal astrocytes, but differs significantly from those reported for normal adult glial cultures. Because the response of the 9L glioma to TPA is similar to the responses seen in the human tumors, the 9L rat glioma model may prove useful for examining the role of PKC-mediated pathways in controlling glioma growth in vivo.

Role of cyclic AMP in the inhibition of mouse hepatocyte intercellular communication by liver tumor promoters

The liver tumor promoters phenobarbital (PB) (20-500 micrograms/ml) and 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT) (1-10 micrograms/ml) inhibited intercellular communication between primary cultured B6C3F1 mouse hepatocytes after 8 hr of treatment. Intercellular communication was detected autoradiographically as the passage and incorporation of [5-3H]uridine nucleotides from prelabeled donor hepatocytes to recipient hepatocytes. The addition of either dibutyryl cyclic AMP (N6,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate) (0.001-0.1 mM) or caffeine (0.01-1 mM) decreased or completely abolished the inhibitory effects of PB and DDT on intercellular communication. Cyclic AMP (adenosine 3':5'-cyclic monophosphate; cAMP) in primary cultured mouse hepatocytes was measured by radioimmunoassay. Cyclic AMP in nontreated, freshly plated cultures declined from 4.2 +/- 0.7 pmol/mg protein after 1 hr in culture to 2.4 +/- 0.5 pmol/mg protein after 8 hr in culture. Phenobarbital at 250 and 500 micrograms/ml significantly decreased cyclic AMP below control values after 1 hr of treatment. However, no difference in the amount of cyclic AMP was detected between control and PB-treated cultures after 2, 4, and 8 hr in culture or with lower PB concentrations. DDT at 10 micrograms/ml decreased cAMP levels in the hepatocytes after 1, 2, 4, and 8 hr of treatment. No effects were seen after 8 hr of treatment or with lower DDT concentrations. DDT (10 micrograms/ml) also decreased cAMP levels in 24-hr-old cultures while PB (500 micrograms/ml) had no effect. Addition of dibutyryl cAMP (0.1 mM) or caffeine (1.0 mM) to freshly plated cultures elevated cAMP levels 50-fold and twofold, respectively. These data suggest that the inhibition of mouse hepatocyte intercellular communication by PB and DDT at the highest concentrations tested may be mediated by transient decreases in intercellular cAMP levels.

Epidermal growth factor receptor expression during morphological differentiation of pheochromocytoma cells, induced by nerve growth factor or dibutyryl cyclic AMP

Rat pheochromocytoma cells (clone PC12) possess functional surface receptors for both nerve growth factor (NGF) and epidermal growth factor (EGF). PC12 cells respond to NGF as well as to dibutyryl cyclic AMP (dbcAMP) by arrest of cell proliferation and initiation of morphological differentiation, while EGF acts as a mitogen. Exposure of PC12 cells to NGF for several days resulted in a complete loss of rapid EGF responses, such as membrane ruffling and activation of active K+ transport. EGF binding studies revealed that this loss of EGF responses was due to an almost complete reduction of the number of EGF binding sites. In contrast, exposure of PC12 cells to dbcAMP for 2 days did not affect the rapid EGF responses, despite the morphological differentiation. Moreover, EGF binding studies demonstrated a twofold increase in the number of high-affinity binding sites and a small increase in the number of low-affinity sites. In addition, exposure of the cells to dbcAMP caused a twofold increase of EGF-receptor phosphotyrosine kinase activity. These results indicate that neither EGF-binding or the presence of EGF receptors nor the rapid EGF responses are sufficient for persistent proliferation, on one hand, or sufficient to avoid morphological differentiation, on the other.