Actin-directed processes such as membrane ruffling and cell migration are regulated by specific signal transduction pathways that become activated by growth factor receptors. The same signaling pathways that lead to modifications in actin dynamics also activate cPLA 2 a. Moreover, arachidonic acid, the product of cPLA 2 a activity, is involved in regulation of actin dynamics. Therefore, it was investigated whether cPLA 2 a plays a role in actin dynamics, more specifically during growth factor-induced membrane ruffling and cell migration. Upon stimulation of ruffling and cell migration by growth factors, endogenous cPLA 2 a and its active phosphorylated form were shown to relocate at protrusions of the cell membrane involved in actin and membrane dynamics. Inhibition of cPLA 2 a activity with specific inhibitors blocked growth factorinduced membrane and actin dynamics, suggesting an important role for cPLA 2 a in these processes.

Simulation of weightlessness is a desired replenishment for research in microgravity since access to space flights is limited. In real microgravity conditions, the human epidermoid cell line A431 exhibits specific changes in the actin cytoskeleton resulting ultimately in the rounding-up of cells. This rounding of A431 cells was studied in detail during exposure to Random Positioning Machine (RPM) rotation and magnetic levitation. Random rotation and magnetic levitation induced similar changes in the actin morphology of A431 cells that were also described in real microgravity. A transient process of cell rounding and renewed spreading was observed in time, illustrated by a changing actin cytoskeleton and variation in the presence of focal adhesions. However, side effects of both methods easily can lead to false linking of cellular responses to simulated microgravity. Therefore further characterization of both methods is required.

No abstract

Both growth factor directed and integrin dependent signal transduction were shown to take place directly after completion of mitosis. The local activation of these signal transduction cascades was investigated in early G1 cells. Interestingly, various key signal transduction proteins were found in blebs at the cell membrane within 30 min after mitosis. These membrane blebs appeared in round, mitotic-like cells and disappeared rapidly during spreading of the cells in G1 phase. In addition to tyrosine-phosphorylated proteins, the blebs contained also phosphorylated FAK and phosphorylated MAP kinase. The formation of membrane blebs in round, mitotic cells before cell spreading is not specific for mitotic cells, because similar features were observed in trypsinized cells. Just before cell spreading also these cells exhibited membrane blebs containing active signal transduction proteins. Inhibition of signal transduction did not affect membrane bleb formation, suggesting that the membrane blebs were formed independent of signal transduction.

Regulation of cell proliferation is dependent on the integration of signal transduction systems that are activated by external signal molecules, such as growth factors and extracellular matrix components. Dependent on these signal transduction networks, the cells decide in the G1 phase to continue proliferation or, alternatively, to stop cell-cycle progression and undergo apoptosis, differentiation, or quiescence. The MAP kinase and PI-3 kinase pathways have been demonstrated to play an essential role in these G1-phase decisions. Interestingly, actin has been demonstrated to mutually interfere with signal transduction. In addition, it has been indicated that the FOXO transcription factors are involved in these decisions, as well. Actin has been demonstrated to play an important role in the regulation of G1-phase progression. Because of its properties as a structural protein, actin is essential in cytokinesis and in cell spreading and, thus, is involved in G1-phase progression. As an intermediate factor in signal transduction, actin is likely to be involved in cell-cycle regulation induced by external signal molecules. And, finally, actin has been demonstrated to play a direct role in transcription. These observations indicate a prominent role of actin in the regulation of G1-phase progression.

The subcellular localization of the PDGFβ-receptor was investigated in relation with PDGF-induced actin and membrane dynamics in mouse C3H10T1/2 fibroblasts. Serum-starved cells exhibit a nonhomogenous distribution of PDGFβ-receptors. However, the observed pattern does not resemble the localization of PDGF-induced actin structures. Interestingly, the PDGFβ-receptor showed a changed subcellular distribution in relation to the formation of PDGF-BB-induced actin structures. Upon PDGF exposure, PDGFβ-receptors were found to accumulate in dorsal circular ruffles. The presence of both macropinosomes and clathrin in the induced circular ruffles suggests that the accumulation of PDGFβ-receptors in circular ruffles results in the efficient internalization of PDGFβ-receptors.