Bovine lens epithelium: a suitable model for studying growth control mechanisms. C6-substituted purines inhibit cell flattening and growth stimulation of G0 cells

Fatty acid cytotoxicity to human lens epithelial cells

Data obtained with the neutral red cytotoxicity assay reveal that human lens epithelial cells in culture are highly sensitive to low micromolar concentrations of unsaturated, cis-configured fatty acids in the following order: arachidonic acid>linolenic acid=linoleic acid=oleic acid, whereas the saturated fatty acids are much less effective. Though the cytotoxic effects of the unsaturated fatty acids could not be discerned from effects of their oxidation products, the fact that oleic acid is equally cytotoxic as linoleic acid or linolenic acid as well as previously reported findings with bovine lens epithelial cells support the idea that the unsaturated fatty acid molecules directly account for the cytotoxicity and not their products of lipid peroxidation. Bleb formation and cell retraction are early morphological signs of fatty acid-induced lens cell damage. These cellular alterations are accompanied by an aggregation of intermediate filaments in a first step, whereas the disorganization of microfilaments occurs at a later time and only at higher fatty acid concentrations. Measurements of protein-, RNA- and DNA-synthesis turned out to be much less sensitive parameters for the fatty acid-induced damage of lens cells. The uptake rate of linoleic acid by human lens cells is relatively high (4.35 fmol sec(-1) per 1000 cells), 30 and 50% higher as compared with diploid human embryonal lung fibroblasts and chemically transformed mouse fibroblasts, respectively. Saturation kinetics in combination with competition between linoleic acid, oleic acid and palmitic acid on one hand and ineffectiveness of trypsin and DIDS treatment on the other hand hint at cytoplasmic fatty acid binding proteins as receptors with high binding affinity (5.55 micromol l(-1), calculated for the linoleic acid-albumin complex) to be involved in the fatty acid uptake in human lens cells. Cellular fatty acid uptake is mainly influenced by the albumin concentrations present in physiological solutions. Albumin determinations in aqueous humor from 177 cataract patients reveal an age-dependent, statistically significant albumin rise with average values below 2 micromol l(-1) up to the age of 40 years to about 4 micromol l(-1) at the age between 80 and 90 years with single values up to 10 micromol l(-1). Using physiological fatty acid mixtures it is demonstrated that fatty acid-induced lens cell damage is strongly increased by elevated albumin concentrations found in aqueous humor of the elderly, who already have cataracts. Free fatty acid induced lens cell damage as a possible cause for age-dependent cataracts as well as a molecular link between systemic diseases such as diabetes and cataract formation is discussed.

Mechanical responses of single non-confluent epithelial cells to low extracellular calcium

Single non-confluent MDCK cells respond immediately to a sharp decrease in extracellular Ca2+ (< or = 5 microM) with an intense reversible retraction, along with an increase in cell height, correlating in overall rate and extent with initial cell size. Optical sectioning of individual cells by confocal microscopy showed that this structural response, observed in about 50% of the population, involves narrowing and even furrowing near the base of the cell by a thickened peripheral belt of actin filaments, which remains associated with the cortex instead of being internalized in the cytoplasm. Single cells retracted significantly in response to low Ca2+ under conditions that have been found largely inhibitory for retraction of confluent cells, such as Ca2+ replacement with Ba2+ and the substitution of Na+ with choline, a non-permeant cation. Conversely, the Na(+)-ionophore monensin applied in the normal-Ca2+ medium elicited by itself an earlier and much greater retraction in single cells than in confluent cultures. These observations indicate that single cells can retract more readily than those forming confluent cultures, suggesting that retraction in typical monolayers is resisted in part by the cell junctions. According to this view, inward actin-myosin mediated tension around the periphery of individual cells precedes and probably helps dissociation of E-cadherins in confluent cultures exposed to low Ca2+.

Actin cytoskeleton role in the structural response of epithelial (MDCK) cells to low extracellular Ca2+

Kinetic and stereometric assessment of the mechanical responses of epithelial cells to variations in the concentration of extracellular Ca2+ was carried out in vivo at the single cell level. Continuous monitoring of individual MDCK cells in subconfluent cultures attested to an intense, immediately relaxable, and cytochalasin D-sensitive contraction, equivalent to that seen in confluent monolayers following depletion of external Ca2+ (<0.1 mM). Increasingly greater and less readily reversible contractions were performed upon repeated stimulation with short-term cycles of alternating normal (30 min) and low Ca2+ (30 min) media. Constriction of a narrow horizontal girdle corresponding in position to the major ring-like bundle of actin filaments eventually develops into a deep lateral furrow in intensely contracted cells. Substantial membrane infolding in the contracted state is indicated also by stereometric estimates of apparent bounding surface area. Irrespective of the contracted or relaxed cell condition, rhodamine-phalloidin labeling showed a marginal position of the ring-like bundle of microfilaments and other components of the actin cytoskeleton. These results suggest, contrary to prevalent views, that the actin-myosin system stays associated to the cortex and retains contractile capability in epithelial cells deprived of external Ca2+. Hence, the mechanical responses to variations of Ca2+ may be an overstrained expression of a physiological mechanism.

Fatty acid cytotoxicity to bovine lens epithelial cells: investigations on cell viability, ecto-ATPase, Na(+), K(+)-ATPase and intracellular sodium concentrations

Unsaturated non-esterified fatty acids have been shown to be cytotoxic in micromolar concentrations to bovine lens epithelial cells, in the following order: arachidonic acid > linoleic acid > oleic acid = linolenic acid. As unsaturated free fatty acids are known to be Na(+), K(+)-ATPase inhibitors, the aim of the study was to investigate whether or not the fatty acid cytotoxicity is correlated with effects on Na(+), K(+)-ATPase activity and function in bovine lens epithelial cells. Furthermore, we also examined the effects of linoleic acid on an ecto-ATPase activity which could be demonstrated on the outside of primarily cultured bovine lens epithelial cells. It has already been shown that 10 micro mol l(-1)linoleic acid was cytotoxic but did not impair the ecto-ATPase activity of intact cells nor the Na(+), K(+)-ATPase in enriched membrane fractions. Na(+), K(+)-ATPase activity was slightly activated with 10 micro mol l(-1)linoleic acid and inhibited by about 50% with 100 micro mol l(-1). Using the sodium-binding benzofuran isophthalate, measurements of intracellular sodium concentrations were carried out. In serum-starved bovine lens epithelial cells the basal [Na(+)](in)was clearly lower than 5 mmol l(-1). When the function of the Na(+), K(+)-ATPase was interrupted by omitting K(+)-ions from the medium, [Na(+)](in)increased at a rate of 0.318 mmol l(-1)min(-1). Linoleic acid intensified that increase strongly in a concentration dependent manner. However, in K(+)-containing medium the linoleic acid-induced increase of [Na(+)](in)was completely prevented. Therefore, the high linoleioc acid cytotoxicity cannot be mediated by linoleic acid effects on Na(+), K(+)-ATPase activity and function in bovine lens epithelial cells.

Lens epithelial cell response to isoforms of platelet-derived growth factor

It has been reported previously that platelet-derived growth factor (PDGF) may play an important role in the regulation of lens growth and differentiation. To evaluate PDGF-induced effects at the cellular level, we investigated the response of cultured bovine lens epithelial cells (BLEC) to PDGF-AB, -AA, and -BB isoforms at the cellular level. Stimulation of BLEC with PDGF isoforms showed no increase in cell proliferation under the culture conditions of this study. In contrast, measurement of cytosolic free calcium concentration ([Ca2+]i), which has been shown to be an important second messenger for controlling multiple cellular processes in the lens, revealed a dose-dependent rise in [Ca2+]i upon stimulation with PDGF-AB and -BB isoforms. PDGF-AA used in similar concentrations was not effective. Our data suggest that PDGF-AB and -BB may play a role in the regulation of cellular functions in BLEC via modulation of intracellular calcium homeostasis.

Alkalinization stimulates Ca2+-dependent spreading during the activation of resting lens epithelial cells in primary culture

Lens epithelium, when attached to its natural substratum, the lens capsule, can be maintained in culture for more than 2 weeks in a simple HEPES- and EDTA-buffered salt solution (HBS). In HBS, the epithelium shows the same characteristic phenomena of locomotion, initial retraction and respreading which in MEM plus serum precedes the inception of DNA synthesis. These phenomena have been shown to be dependent on extracellular Ca2+. 0.05 mM Ca2+ is necessary for maintaining cell-to-cell contacts of the in vivo epithelium. Higher concentrations of Ca2+ cause the epithelium to retract initially. In contrast, Mg2+ greatly favours cell-substratum interactions leading to the formation of lamellopodia and an initial spreading of the epithelium. After some hours in culture the epithelium changes markedly in response to extracellular Ca2+ and Mg2+; it respreads and flattens in the presence of Ca2+, while Mg2+ becomes less effective in maintaining cell-to-substratum contacts. Mg2+-dependent initial spreading is promoted at pH values near 7.0 but the Ca2+-dependent respreading requires an alkalinization of the salt solution.

Correlation between phosphatidylinositol degradation and cell division in embryonic chicken lens epithelia

Differentiation of embryonic chicken lens epithelial cells to form lens fibers is associated with a marked decrease in both the rate of phosphatidylinositol degradation and the rate of cell division. In cells of the central region of the lens epithelium, the rate of cell division also declines with developmental age. The present study measures phosphatidylinositol degradation in cultured explants of the central lens epithelium of chicken embryos of different ages to determine the extent of the correlation between phosphatidylinositol degradation and cell division in this tissue. The results show that the rate of phosphatidylinositol degradation also decreases during development and is proportional to the rate of cell division throughout the period from 6 to 19 days of development. Furthermore, stimulating cell division in central explants of lens epithelia of 19-day-old chicken embryos by culturing them in the presence of fetal calf serum produces a proportional increase in the rate of phosphatidylinositol degradation. These findings indicate that cell division and phosphatidylinositol degradation are tightly coupled in this tissue, and raise the possibility that phosphatidylinositol metabolism may regulate some aspect of the cell cycle.

Effects of extracellular calcium depletion on membrane topography and occluding junctions of mammary epithelial cells in culture

Ca2+ dependence of occluding junction structure and permeability, well documented in explanted or cultured epithelial sheets, presumably reflects inherent control mechanisms. As an approach to identification of these mechanisms, we induced disassembly of zonulae occludentes in confluent monolayers of mouse mammary epithelial cells by exposure to low concentrations of the chelators, EGTA or sodium citrate. Stages in disassembly were monitored during treatment by phase-contrast microscopy and prepared for transmission and scanning electron microscopy. Cellular response included several events affecting occluding junctions: (a) Centripetal cytoplasmic contraction created tension on junction membranes and displaced intramembrane strands along lines determined by the axis of tension. (b) Destabilization of junction position, probably through increased membrane fluidity, augmented tension-induced movement of strands, resulting in fragmentation of the junction belt. (c) Active ruffling and retraction of freed peripheral membranes remodeled cell borders to produce many filopodia, distally attached by occluding-junction fragments to neighboring cell membranes. Filopodia generally persisted until mechanically ruptured, when endocytosis of the junction and adhering cytoplasmic bleb ensued. Junction disassembly thus resulted from mechanical tensions generated by initial centripetal contraction and subsequent peripheral cytoskeletal activity, combined with destabilization of the junction's intramembrane strand pattern.

Nutrient transport in a bovine lens epithelial cell line

A bovine calf lens epithelial cell line (CLE-1) that synthesizes crystallin has been established in culture and some of its transport properties have been characterized using both cells and membrane vesicles derived from them. The membrane vesicles fractionate with high recovery of plasma membrane markers, showing a 40-fold purification of 5'-AMPase and a 20-fold decrease in the specific activity of the mitochondrial marker enzyme succinic dehydrogenase relative to a cell homogenate. Transport sites demonstrated higher specific activity than has been seen in vesicles from cell lines studied previously. The uptake of alpha-amino isobutyric acid (AIB) (an alanine analog) by CLE-1 cells is stimulated four- to fivefold by Na+ and exhibits a Km of 5.4 mM with a Vmax of 50 pmoles/min.microgram of cell protein. The uptake of leucine was not Na+ stimulatable. The uptake of AIB by the cells was reduced by 43% at confluence. Thus, the cell density dependent behavior of the uptake of the alanine amino acid family in CLE-1 is similar to that of various fibroblast cells. The Na+ caused a threefold stimulation of AIB uptake in the membrane vesicles, while vesicular uptake of leucine was unaffected by Na+. The uptake of adenine, guanine, uridine, and guanosine was also tested in these vesicles. The substrates were rapidly accumulated, came to a steady state distribution within 1-2 minutes, and were recovered as the unaltered compounds after uptake.