Facilitation of actin polymerization by interfilamentous factors

Cytoskeletal components enhance the autophosphorylation of retinal insulin receptor

Insulin receptor (IR) signaling provides a trophic signal for transformed retinal neurons in culture, and we recently reported that deletion of IR from rod photoreceptors resulted in stress-induced photoreceptor degeneration. Retinal insulin receptor has a high basal level autophosphorylation compared to liver and the reasons for higher autophosphorylation are not known. In the current study we report a novel finding that cytoplasmic actin associates with and activates the retinal IR in vivo. Similar to insulin, actin also induced autophosphorylation at tyrosines 1158, 1162 and 1163 in the catalytic loop of IR. Our studies also suggest that globular actin activates the retinal IR more effectively than does filamentous actin. Retinal IR kinase activity has been shown to decrease in hyperglycemia and we found a decreased binding of actin to the IR under hyperglycemia. This is the first study which demonstrates that cytoplasmic actin regulates autophosphorylation of the retinal IR.

Regulation of endothelial nitric oxide synthase by the actin cytoskeleton

In the present study, the association of endothelial nitric oxide synthase (eNOS) with the actin cytoskeleton in pulmonary artery endothelial cells (PAEC) was examined. We found that the protein contents of eNOS, actin, and caveolin-1 were significantly higher in the caveolar fraction of plasma membranes than in the noncaveolar fraction of plasma membranes in PAEC. Immunoprecipitation of eNOS from lysates of caveolar fractions of plasma membranes in PAEC resulted in the coprecipitation of actin, and immunoprecipitation of actin from lysates of caveolar fractions resulted in the coprecipitation of eNOS. Confocal microscopy of PAEC, in which eNOS was labeled with fluorescein, F-actin was labeled with Texas red-phalloidin, and G-actin was labeled with deoxyribonuclease I conjugated with Texas red, also demonstrated an association between eNOS and F-actin or G-actin. Incubation of purified eNOS with purified F-actin and G-actin resulted in an increase in eNOS activity. The increase in eNOS activity caused by G-actin was much higher than that caused by F-actin. Incubation of PAEC with swinholide A, an actin filament disruptor, resulted in an increase in eNOS activity, eNOS protein content, and association of eNOS with G-actin and in a decrease in the association of eNOS with F-actin. The increase in eNOS activity was higher than that in eNOS protein content in swinholide A-treated cells. In contrast, exposure of PAEC to phalloidin, an actin filament stabilizer, caused decreases in eNOS activity and association of eNOS with G-actin and increases in association of eNOS with F-actin. These results suggest that eNOS is associated with actin in PAEC and that actin and its polymerization state play an important role in the regulation of eNOS activity.