Upon platelet activation, numerous proteins are known to be tyrosine phosphorylated. To investigate the mechanisms of the regulation of tyrosine phosphorylation and its physiological significance, the effects on tyrosine phosphorylation of agents that elevate the platelet level of the cyclic nucleotides cAMP and cGMP were examined in aspirin-treated gel-filtered platelets by Western blotting with a specific antiphosphotyrosine antibody. The effects of these agents on other aspects of platelet activation, i.e., aggregation, secretion, and elevation of the concentration of cytosolic ionized calcium ([Ca2+]i), were also examined in parallel experiments. Tyrosine phosphorylation in platelets activated by alpha-thrombin (1 nM) was inhibited by prostaglandin (PG) E1 (2 microM) or by sodium nitroprusside (100 microM). Elevation of [Ca2+]i, aggregation, and serotonin secretion was also strongly inhibited. On the other hand, a higher concentration of alpha-thrombin (10 nM) induced tyrosine phosphorylation of the same proteins, elevation of [Ca2+]i, platelet aggregation, and serotonin secretion, irrespective of pretreatment of platelets by either PGE1 or sodium nitroprusside. Inhibition by sodium nitroprusside of tyrosine phosphorylation induced by alpha-thrombin (1 nM) was accompanied by an increased concentration of cGMP. 8-BrcGMP (2 mM) also inhibited tyrosine phosphorylation and aggregation, although less than sodium nitroprusside. ADP (20 microM) induced platelet shape change and tyrosine phosphorylation of only a few proteins; these effects were also inhibited by either PGE1 or sodium nitroprusside. Thus tyrosine phosphorylation in platelets can be inhibited by elevation of either cAMP or cGMP, an effect that is overcome by a high concentration of thrombin, resulting in granule secretion and aggregation. Some of the proteins that are tyrosine phosphorylated may be important in the regulation of platelet functions.
With the use of proteins derived from Escherichia coli cells expressing the v-H-ras gene product as immunogens and an enzyme-linked immunosorbent assay with whole cells for a screening method, 4 BALB/c mouse hybridoma cell lines (rp-12, rp-28, rp-35, and rp-38) were isolated that produced monoclonal antibodies (MoAbs) showing higher reactivity with murine ras gene-activated cell lines than with normal cell lines. All the MoAbs complexed p21ras from the ras gene-activated cell lines in Western immunoblot analysis and demonstrated a binding property of p21ras to guanine nucleotides. The indirect immunofluorescence assay revealed that MoAbs rp-12 and rp-28 stained the murine and human H- or K-ras-activated cell lines, and MoAbs rp-35 and rp-38 not only stained these cell lines but also weakly stained a human N-ras-activated cell line. All these MoAbs stained the murine fibroblast lines with lower intensity, but they did not stain a human fibroblast line. Further, positive reactions with MoAb rp-12 were seen against human melanomas, but there was no reaction against nevi. The rp-12, rp-28, rp-35, and rp-38 antibodies are useful additions to the MoAbs reacting with p21ras reported previously.
Calpain is distributed ubiquitously in virtually every tissue (Croall, D. E., and DeMartino, G. N. (1991) Physiol. Rev. 71, 813-846), but its physiological role remains to be determined. The identification of its natural endogenous substrates would be of great interest. Since pp60src, a major tyrosine kinase in platelets, is known to be easily cleaved during purification from cells (Feder, D., and Bishop, J. M. (1990) J. Biol. Chem. 265, 8205-8211), we examined the possibility that it is an endogenous substrate of calpain. In the whole cell lysate from resting platelets, which was analyzed by Western blotting with monoclonal antibody 327, we found pp60src almost exclusively in a 60-kDa form, with a trace of 52-kDa form. Addition of A23187 (a calcium ionophore) or dibucaine, which are known to be activators of platelet calpain (Croall and DeMartino, 1991; Fox, J. E., Reynolds, C., Morrow, J. S., and Phillips, D. R. (1987) Blood 76, 2510-2519; Fox, J. E., Austin, C. D., Boyles, J. K., and Steffen, P. K. (1990b) J. Cell Biol. 111, 483-493), caused dose- and time-dependent cleavage of actin-binding protein and p235 protein (talin). At the same time, loss of the 60-kDa species of pp60src and generation of the 52-kDa (occasionally seen as doublets) and 47-kDa species were detected by the Western blotting. In platelets aggregated by 1 unit/ml thrombin, apparently identical cleavage products were found. The cleavage of pp60src was inhibited by calpeptin (20 microM), an inhibitor of calpain (Tsujinaka, T., Kajiwara, Y., Kambayashi, J., Sakon, M., Higuchi, N., Tanaka, T., and Mori, T. (1988) Biochem. Biophys. Res. Commun. 153, 1201-1208; Tsujinaka, T., Ariyoshi, H., Uemura, Y., Sakon, M., Kambayashi, J., and Mori, T. (1990) Life Sci. 46, 1059-1066; Fox, J. E., Clifford, C. C., and Austin, C. D. (1990) Blood 76, 2510-2519; Fox, J. E., Austin, C. D., Boyles, J. K., and Steffen, P. K. (1990) J. Cell. Biol. 111, 483-493; Fox, J. E., Austin, C. D., Clifford, C. C., and Steffen, P. K. (1991) J. Biol. Chem. 266, 13289-13295). Addition of EGTA (3 mM) to the extracellular media completely inhibited the cleavage of actin-binding protein, talin, and pp60src in response to A23187 (1 microM). Intact pp60src was distributed in both cytosolic and particulate (membrane) fractions. Cleaved species were found exclusively in the cytosolic fraction. pp60src-associated enolase kinase activity was reduced. Thus, pp60src is an endogenous substrate for calpain, the cleavage of which may have regulatory effects on the kinase.
SummaryA night-break (i.e., a short exposure to light near the middle of the dark period) inhibits flowering in chrysanthemum (Chrysanthemum morifolium Ramat.), a short-day plant. We studied the effect of night-break light quality (wavelength) on flowering and on expression of the FLOWERING LOCUS T (FT)-like gene, CmFTL3. Night-break treatments with the yellow-to-red wavelength range showed strong inhibitory effects on flowering. Further studies using monochromatic light from panels of light emitting diodes (LEDs) showed that the maximum effect on the inhibition of flowering was around 596 nm, and that levels of CmFTL3 mRNA were reduced in a pattern consistent with the observed inhibition of flowering. Wavelengths from ultraviolet-A to blue, and far-red, had no inhibitory effects on flowering. Our results also showed that the inhibitory effect of red light on flowering responses could be reduced by far-red light. These results suggest that phytochromes are involved in night-break responses, and the absorption spectrum of leaf extracts indicated that the effect of wavelength might be distorted by the screening effect of the other pigments found in green leaves. We also discuss further developments of lighting techniques for commercial production of chrysanthemum.
