Density-induced down regulation of epidermal growth factor receptors

Summary Previous studies have shown that cell density can regulate the binding of several growth factors. To determine whether cell density exerts a uniform effect on the expression of epidermal growth factor (EGF) receptors, seven cell lines were examined in detail. For each cell line, EGF binding was found to decrease as cell density increases. Scatchard analysis of the binding data reveals that decreases in EGF binding are due to reductions in the number of cell surface EGF receptors. The only apparent exception is the effect of cell density on the binding of EGF to A-431 cells. For these cells, increases in cell density lead to two effects: decreases in the number of high affinity EGF receptors and increases in the total number of EGF receptors. In addition to the effects of cell density on EGF receptors, it was determined that increases in cell density can coordinately down-regulate receptors for as many as four different growth factors. Overall, the findings described in this report for EGF and those previously described for transforming growth factor type-β (TGF-β) and fibroblast growth factor (FGF) demonstrate the existence of a common mechanism for down-regulating growth factor receptors. This work was supported by grants from the Nebraska Department of Health (89-51), the National Cancer Institute (Laboratory Research Center Support Grant, CA36727), and the American Cancer Society (Core Grant ACS SIG-16). EDITOR'S STATEMENT The paper by Rizzino et al. demonstrates that receptor number decreases as a function of cell density. This may represent a mechanism by which cell proliferation is reduced as cell density increases.     

Protein profiles of Capnocytophaga species

Ninety-seven strains of Capnocytophaga isolated from the oral cavity and the type strains of C. ochracea, C. sputigena and C. gingivalis were compared by one-dimensional SDS-PAGE of whole cell proteins. The protein patterns were highly reproducible and were used as the basis for numerical taxonomic analysis. The clusters containing the type strains of C. ochracea and C. sputigena segregated at the 78% similarity level. Some of the eight clusters obtained at this level showed good correlation with grouping based on the results of biochemical testing for lactose and galactose fermentation and nitrate reduction. No consistent association was found between protein profiles and colony type, size or colour or cell length but all agar-adherent colony types segregated into a single cluster. and accepted 25 July 1989     

The Population Dynamics of Maternal-Effect Selfish Genes

We use population genetic methods to describe the expected population dynamics of the selfish-gene chromosomal factor, Medea (maternal-effect dominant embryonic arrest), recently discovered in flour beetles, genus Tribolium. In the absence of deleterious effects on gross fecundity, Medea factors spread to fixation for all degrees of maternal-effect lethality greater than zero and the rate of spread is proportional to the strength of the maternal-effect. The rate of spread when rare is very slow, on the order of the frequency squared p(2), but this can be accelerated to order p when there is density regulation at the level of families as is known to occur for some genetic strains of flour beetles. When there are general deleterious effects of Medea on fecundity, affecting all offspring genotypes in addition to the genotype-specific maternal effect, then a stable interior polymorphism is possible. The location of the interior equilibrium and the probability of loss or fixation are sensitive to the degree of dominance of these fecundity effects.     

Cdk inhibitors: on the threshold of checkpoints and development

The regulation of cyclin-dependent kinases is at the heart of cell cycle control and, by inference, the control of cell proliferation. Recent advances in regulation of these kinases have uncovered a group of small proteins that bind to and inhibit them, thus preventing cell cycle progression. Linking these proteins to tumor suppressor functions has provided a much sought after connection between cancer and cell cycle control.     

Gating of the active site of triose phosphate isomerase: Brownian dynamics simulations of flexible peptide loops in the enzyme.

The enzyme triose phosphate isomerase has flexible peptide loops at its active sites. The loops close over these sites upon substrate binding, suggesting that the dynamics of the loops could be of mechanistic and kinetic importance. To investigate these issues, the loop motions in the dimeric enzyme were simulated by Brownian dynamics. The two loops, one on each monomer, were represented by linear chains of appropriately parameterized spheres, each sphere corresponding to an amino acid residue. The loops moved in the electrostatic field of the rest of the enzyme, which was held rigid in its crystallographically observed conformation. In the absence of substrate, the loops exhibited gating of the active site with a period of about 1 ns and occupied "closed" conformations for about half of the time. As the period of gating is much shorter than the enzyme-substrate relaxation time, the motion of the loops does not reduce the rate constant for the approach of substrate from its simple diffusion-controlled value. This suggests that the flexible loops may have evolved to create the appropriate environment for catalysis while, at the same time, minimizing the kinetic penalty for gating the active site.     

A dual fluorescence technique for visualization ofStaphylococcus epidermidis biofilm using scanning confocal laser microscopy

A new dual fluorescence technique is described which, when combined with scanning confocal laser microscopy (SCLM), can be used to visualize the components of biofilm produced byStaphylococcus epidermidis. Chemostat cultures of RP62A (a well-characterized slime-producing strain ofS. epidermidis) were used to produce mature biofilm on polyvinylcholoride (PVC) disks immobilized in a modified Robbins device using a seed and feed model system. Serial horizontal and vertical optical thin sections, as well as three-dimensional computer reconstructions, were obtained onin situ biofilm using the dual fluorescence procedure. Bacteria were visualized by green autofluorescence excited at 488 nm with an Argon laser. Cell-associated and exocellular matrix material (slime) was visualized by red fluorescence excited at 568 nm with a Krypton laser after interaction of the biofilm with Texas Red-labeled wheat germ agglutinin which is a slime-specific lectin marker. Structural analysis revealed that the cocci grew in slime-embedded cell clusters forming distinct conical-shaped microcolonies. Interspersed open channels served to connect the bulk liquid with the deepest layers of the mature, hydrated biofilm which increased overall surface area and likely facilitated the exchange of nutrients and waste products throughout the biofilm. The combined dual fluorescence technique and SCLM is potentially useful as a specific noninvasive tool for studying the effect of antimicrobial agents on the process of biofilm formation and for the characterization of the architecture ofS. epidermidis biofilm formedin vivo andin vitro on medical grade virgin or modified inert polymer surfaces.     

Evolution of human immunodeficiency virus type 1 in perinatally infected infants with rapid and slow progression to disease.

We addressed the relationship between the origin and evolution of human immunodeficiency virus type 1 (HIV-1) variants and disease outcome in perinatally infected infants by studying the V3 regions of viral variants in samples obtained from five transmitting mothers at delivery and obtained sequentially over the first year of life from their infected infants, two of whom (rapid progressors) rapidly progressed to having AIDS. Phylogenetic analyses disclosed that the V3 sequences from each mother-infant pair clustered together and were clearly distinct from those of the other pairs. Within each pair, the child's sequences formed a monophyletic group, indicating that a single variant initiated the infection in both rapid and slow progressors. Plasma HIV-1 RNA levels increased in all five infants during their first months of life and then declined within the first semester of life only in the three slow progressors. V3 variability increased over time in all infants, but no differences in the pattern of V3 evolution in terms of potential viral phenotype were observed. The numbers of synonymous and nonsynonymous substitutions varied during the first semester of life regardless of viral load, CD4+-cell count, and disease progression. Conversely, during the second semester of life the rate of nonsynonymous substitutions was higher than that of synonymous substitutions in the slow progressors but not in the rapid progressors, thus suggesting a stronger host selective pressure in the former. In view of the proposal that V3 genetic evolution is driven mainly by host immune constraints, these findings suggest that while the immune response to V3 might contribute to regulating viral levels after the first semester of life, it is unlikely to play a determinant role in the initial viral decline soon after birth.