Cytochalasin inhibition of hexose transport by platelets

GLUT-3 (brain-type) glucose transporter polypeptides in human blood platelets

Antiserum directed against a C-terminal peptide of the human GLUT-3 (brain type) equilibrative glucose transporter isoform reacted with polypeptides M(r) 46,000-48,000 on immunoblots of human platelets. Photoirradiation of human platelets in the presence of 3H-cytochalasin B led to radiolabeling of polypeptides of identical mobility. This labeling was substantially reduced by preincubation the cells with 440 mM D-glucose, but not 440 mM L-glucose, consistent with glucose transporter function. Only traces of GLUT-1 (erythrocyte type) glucose transporter polypeptides were detected, and there was no evidence of GLUT-2 (liver type) transporter on immunoblots of platelet proteins.

Structure-activity relationships among mycotoxins

Relationships between structural features and biological effects of mycotoxins are reviewed. Structure-activity relationships are characterized at the molecular, subcellular, cellular, or supracellular level. Major chemical and physicochemical factors responsible for bioactivity of mycotoxins are stressed. A variety of chemical families of mycotoxins are then discussed from the point of view of structure-activity relationships. The structurally related families comprise small lactones, macrocyclic lactones, isocoumarin derivatives, aflatoxins and related compounds trichothecenes, anthraquinones, indole-derived tremorgens and selected amino acid-derived mycotoxins such as sporidesmins and cyclosporines. Biological effects of mycotoxins include acute and chronic toxicity, antimicrobial activity, mutagenicity and genotoxicity, carcinogenicity and biochemical modes of action.

Binding of cytochalasin B to platelets

The binding of cytochalasin B (CB) to human platelets and to isolated platelet cytosol and membranes has been analyzed with [3H]CB. High- and low affinity classes of saturable binding sites were associated with intact platelets. Binding at very low concentrations of CB (i.e., high-affinity binding) was partially prevented by 100 mM D-galactose or D-glucose and to a much lesser extent by L-glucose. Binding to platelet cytosol also involved two classes of sites with affinities and capacities similar to those observed with the whole cells. None of this binding, however, was affected by 100 mM D-galactose. Saturable binding to platelet membranes occurred at sites with a uniform binding affinity. Approximately 52% of this binding was prevented by 1 M D-galactose and another 15% by cytochalasin E (CE). We hypothesize that binding in the cytosol is to monomeric (low-affinity) and polymerized (high-affinity) actin, whereas membrane binding (high-affinity only) occurs primarily at sites involved with galactose transport.

Cytochalasin E enhances the protein kinase C-dependent process of secretion

Platelet dense body secretion, monitored as released serotonin, when induced by 20 nM tetradecanoylphorbol acetate was enhanced by 10 microM cytochalasin E even in the absence of external calcium ions and aggregation. Secretion induced by 500 nM A23187 in the presence of 15 microM indomethacin was not changed by cytochalasin but the synergy between A23187 and phorbol ester was enhanced. Therefore, protein kinase C-mediated secretion is not dependent on filipodal development, granule centralization, external calcium ion concentration or aggregation but is enhanced selectively by cytochalasin. Because the synergy between kinase C and A23187 is very rapid, the rate determining step in the slow secretion induced by phorbol ester alone appears to be the step accelerated by a rise in cytosolic Ca2+ concentration.