Cell cycle progression is regulated by a wide variety of external factors, amongst them are growth factors and extracellular matrix factors. During the last decades evidence has been obtained that reactive oxygen species (ROS) may also play an important role in cell cycle progression. ROS may be generated by external and internal factors. In this overview we describe briefly the generation of ROS and their effects on processes that have been demonstrated to play an essential role in cell cycle progression, including such systems as signal transduction cascades, protein ubiquitination and degradation, and the cytoskeleton. These different effects of ROS influence cell cycle progression dependent upon the amount and duration of ROS exposure. Activation of growth factor stimulated signaling cascades by low levels of ROS result in increased cell cycle progression, or, in case of prolonged exposure, to a differentiation like growth arrest. From many studies it seems clear that the cyclin kinase inhibitor protein p21 plays a prominent role, leading to cell cycle arrest at higher but not directly lethal levels of ROS. Dependent upon the nature of p21 induction, the cell cycle arrest may be transient, coupled to repair processes, or permanent. At high concentrations of ROS all of the above processes are activated, in combination with enhanced damage to the building blocks of the cell, leading to apoptosis or even necrosis.

Research in microgravity is indispensable to disclose the impact of gravity on biological processes and organisms. However, research in the near-Earth orbit is severely constrained by the limited number of flight opportunities. Ground-based simulators of microgravity are valuable tools for preparing spaceflight experiments, but they also facilitate stand-alone studies and thus provide additional and cost-efficient platforms for gravitational research. The various microgravity simulators that are frequently used by gravitational biologists are based on different physical principles. This comparative study gives an overview of the most frequently used microgravity simulators and demonstrates their individual capacities and limitations. The range of applicability of the various ground-based microgravity simulators for biological specimens was carefully evaluated by using organisms that have been studied extensively under the conditions of real microgravity in space. In addition, current heterogeneous terminology is discussed critically, and recommendations are given for appropriate selection of adequate simulators and consistent use of nomenclature.

This study focuses on factors increasing the effectiveness of collaborative learning. Results show that challenging, open, and complex group tasks that required the students to create something new and original evoked effective collaboration.

Abstract. In a number of recent studies it has been shown that in vivo part of the EGF receptor (EGFR) population is associated to the actin filament system. In this paper we demonstrate that the purified EGFR can be cosedimented with purified filamentous actin (F-actin) indicating a direct association between EGFR and actin. A truncated EGFR, previously shown not to be associated to the cytoskeleton, was used as a control and this receptor did not cosediment with actin filaments. Determination of the actin-binding domain of the EGFR was done by measuring competition of either a polyclonal antibody or synthetic peptides on EGFR cosedimentation with F-actin. A synthetic peptide was made homologous to amino acid residues 984-996 (HL-33) of the EGFR which shows high homology with the actin-binding domain of Acanthamoeba profilin. A polyclonal antibody raised against HL-33 was found to prevent cosedimentation of EGFR with F-actin. This peptide HL-33 was shown to bind directly to actin in contrast with a synthetic peptide homologous to residues 1001-1013 (HL-34). During cosedimentation, HL-33 competed for actin binding of the EGFR and HL-34 did not, indicating that the EGFR contains one actin-binding site. These results demonstrate that the EGFR is an actin-binding protein which binds to actin via a domain containing amino acids residues 984-996.

Abstract. Many cell types display two classes of epidermal growth factor receptor (EGFR) as judged from EGF binding studies; i.e., a major class of low affinity EGFR and a minor class of high affinity EGFR. We have studied their respective contribution to the cascade of events elicited by EGF in human A431 carcinoma cells, using anti-EGFR mAb 2E9. This antibody specifically blocks EGF binding to low affinity EGFR, without activating receptors in intact cells, and thus enables us to study the effects of exclusive EGF binding to high affinity EGFR. We show that blocking of low affinity EGFR by mAb 2E9 has almost no effect on the activation of the receptor protein-tyrosine kinase by EGF, suggesting that EGFR kinase activation occurs exclusively through the subclass of high affinity EGFR (5-10%). In addition, we provide evidence that high affinity EGFR exists both in monomeric and dimeric forms, and that cross-phosphorylation of low affinity EGFR by high affinity EGFR may take place in dimers of both receptor types.We demonstrate that the following early cellular responses to EGF are also unimpaired in the presence of mAb 2E9: (a) inositol phosphate production, (b) release of Ca ~+ from intracellular stores, (c) rise in intracellular pH, (d) phosphorylation of EGF on threonine residue 654, (e) induction of c-fos gene expression, and (f) alteration in cell morphology. As possible nonspecific side effects, we observed that the EGF induced Ca 2+ influx and fluid-phase pinocytosis were inhibited in A431 cells in the presence of mAb 2E9. We conclude, therefore, that the activation of the EGFR signal transduction cascade can occur completely through exclusive binding of EGF to the subclass of high affinity EGFR.

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Abstract. In this paper we demonstrate that cytoskeletons isolated from A431 cells have associated with them high activities of several kinases involved in inositol lipid metabolism, such as phosphatidylinositol kinase, phosphatidylinositol phosphate kinase, and diacylglycerol kinase . In addition also phospholipase C activity was detected on isolated cytoskeletons . Controlled extraction of the cytoskeletons followed by in vitro polymerization of actin demonstrated an association of the kinases to the actin filament system con-

During the cell cycle, a cell may encounter one of five different fates: it can proliferate, differentiate, become quiescent or senescent, or go into apoptosis. The initiation of such fates is often seen in the G1 phase. The aim of this review is to describe an integrative model of G1 phase progression and cell fate determination. Along the G1 phase, the cell will encounter an early checkpoint after which apoptosis can result. For a quiescent state and for differentiation, the cell will exit G1 before the restriction point and a subsequent differentiation checkpoint will decide the fate of the cell, quiescence or differentiation. After the restriction point, the cell can be arrested in response to stress stimuli, such as telomere depletion, and a decision between senescence and apoptosis occurs.