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

Induction of Epstein-Barr virus lytic cycle by tumor-promoting and non-tumor-promoting phorbol esters requires active protein kinase C

Exposure to the tiglian 12-O-tetradecanoylphorbol-13-acetate (TPA) represents one of the most efficient and widely used protocols for inducing Epstein-Barr virus (EBV)-infected cells from latent into lytic cycle. Since TPA is both a potent tumor promoter and a potent activator of the cellular protein kinase C (PKC), we sought to determine whether either of these activities was closely linked to EBV lytic cycle induction. A panel of TPA structural analogs, encompassing tiglians with different spectra of biological activities, was assayed on a number of EBV-positive B-lymphoid cell lines. Lytic cycle induction correlated with the capacity to activate PKC, not with tumor promoter status; some nonpromoting tiglians were as efficient as TPA in inducing lytic cycle antigen expression. We then sought more direct evidence for an involvement of PKC in the induction process. In initial experiments, 1-(5-isoquinolinyl sulphonyl)-2-methylpiperazine (H-7), the best available pharmacological inhibitor of PKC, completely blocked the induction of the lytic cycle by TPA and its active analogs. This is consistent with, but does not prove, a requirement for active PKC in the induction process, since H-7 targets PKC preferentially but also has some effects on other kinases. We therefore turned to the synthetic pseudosubstrate peptide PKC(19-36) as a means of specific PKC inhibition and to the closely related but inactive peptide PKC(19-Ser-25-36) as a control. Using the technique of scrape loading to deliver the peptides into cells of an adherent EBV-positive target line, we found that the pseudosubstrate peptide PKC(19-36) completely and specifically blocked tiglian-induced entry of the cells into the lytic cycle. The evidence both from TPA analogs and from enzyme inhibition studies therefore indicates that the pathway linking TPA treatment to lytic cycle induction involves active PKC. Interestingly, inhibition of PKC had no effect upon the spontaneous entry into lytic cycle which occurs in naturally productive cell lines, suggesting that spontaneous entry is signalled by another route.

GTP gamma S inhibits early c-myc protein accumulation but not DNA synthesis in Swiss 3T3 fibroblasts

Quiescent Swiss 3T3 fibroblasts stimulated with epidermal growth factor and insulin showed large transient increases in c-myc mRNA and c-myc protein accumulation which were maximal at about 2 h after addition of the co-mitogens. When the cells were loaded with 0.1 mM of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) by transient permeabilisation immediately before mitogenic stimulation, the increase in c-myc mRNA was similar to that observed in unloaded cells but the corresponding c-myc protein peak was reduced by at least 95%. The GTP gamma S completely blocked incorporation of [35S]methionine into cell proteins for 3-4 h after addition of the mitogens, but not thereafter, and caused a delay in the subsequent onset of DNA synthesis by the same period. The data show that less than 5% of the early increase in c-myc protein normally observed after mitogenic stimulation is required for its obligatory role in the progression of cells to S phase implied by other evidence.

Dissection of a single round of vesicular transport: sequential intermediates for intercisternal movement in the Golgi stack

We take advantage of a cell-free system that reconstitutes essentially a single round of transport of the VSV-encoded G protein between Golgi cisternae to identify discrete stages in the maturation of carrier vesicles. Using GTP gamma S and N-ethylmaleimide (NEM) as selective inhibitors to accumulate coated and uncoated vesicles, respectively, we find these to be successive and obligatory transport intermediates. We find that the coated and uncoated vesicles that accumulate when transport is blocked have already transferred from donor to acceptor stacks but not yet fused. Similar coated and uncoated vesicles accumulate in appropriately treated whole cells. Our studies imply that a coated bud (pit)-coated vesicle-uncoated vesicle system analogous to that responsible for receptor-mediated endocytosis carries biosynthetic protein transport across the Golgi stack. However, "Golgi"-coated buds do not contain clathrin and seem to act as bulk carriers, whereas endocytic clathrin-coated pits carry a highly selective cargo.