Nitric oxide (NO) is a signaling molecule that is produced by many different kinds of cells, and it is known to mediate actions such as blood vessel dilation, communication between nerve cells, and killing of bacteria in infections. The cytoskeleton is involved in many important cellular functions, among them intracellular transport of organelles, migration, and cell division. The aim of the present studies was to examine the effects of NO on some of the indicated functions. Homotypic adhesion of human neutrophils, which is mediated by ß2 integrins, is an early step in the inflammatory process. Addition of L-argiriine (the substrate of NO production) to fMLP-stimulated neutrophils increased and prolonged aggregation of the cells. Stimulation of L-arginine-pretreated neutrophils by cross-linking of ß2 integrins attenuated the increase in F-actin, as compared to control cells. These results suggest that the aggregation is prolonged by activation of ß2 integrins and endogenous NO production, two events that together seem to inhibit actin polymerization, possibly via ADP ribosylation.
The effect of NO on intracellular translocation of organelles along the cytoskeleton was studied in Xenopus laevis pigment cells. Inhibition of NO production induced by the drug L-NAME was found to inhibit aggregation of the pigment organelles (melanosomes) and to induce dispersion. Activation of PKC, MEK, and ERKl, but not PKA, was associated with the dispersion, thus NO may negatively regulate these kinases, which, when activated, would induce movement of melanosomes. During melanosome aggregation, the cell center increases in height by approximately 30%. Experiments were performed to determine whether the cell membrane is pushed upwards by actin polymerization and water influx through HgCl2-sensitive aquaporins. The results gave no evidence that either two of these mechanisms affects the upward movement. However, L-NAME caused dispersion and a decrease in cell height, thus NO may play a role in maintaining an aggregated, elevated state. In conclusion, many factors regulate both homotypic aggregation and intracellular organelle transport, and NO seems to prolong homotypic aggregation of neutrophils and regulate melanosome transport by inhibiting PKC, MEK and ERKl.
Linköping: Linköpings universitet , 2001. , 48 p.
2001-03-02, Elsa Brändströmssalen, Universitetssjukhuset, Linköping, 09:00 (Swedish)