Ultrastructural and cytochemical changes in cultured human lung cells

Lysosome biogenesis: Regulation and functions

Lysosomes are degradation centers and signaling hubs in cells and play important roles in cellular homeostasis, development, and aging. Changes in lysosome function are essential to support cellular adaptation to multiple signals and stimuli. Therefore, lysosome biogenesis and activity are regulated by a wide variety of intra- and extracellular cues. Here, we summarize current knowledge of the regulatory mechanisms of lysosome biogenesis, including synthesis of lysosomal proteins and their delivery via the endosome-lysosome pathway, reformation of lysosomes from degradative vesicles, and transcriptional regulation of lysosomal genes. We survey the regulation of lysosome biogenesis in response to nutrient and nonnutrient signals, the cell cycle, stem cell quiescence, and cell fate determination. Finally, we discuss lysosome biogenesis and functions in the context of organismal development and aging.

Lysosome activity is modulated by multiple longevity pathways and is important for lifespan extension in C. elegans

Lysosomes play important roles in cellular degradation to maintain cell homeostasis. In order to understand whether and how lysosomes alter with age and contribute to lifespan regulation, we characterized multiple properties of lysosomes during the aging process in C. elegans. We uncovered age-dependent alterations in lysosomal morphology, motility, acidity and degradation activity, all of which indicate a decline in lysosome function with age. The age-associated lysosomal changes are suppressed in the long-lived mutants daf-2, eat-2 and isp-1, which extend lifespan by inhibiting insulin/IGF-1 signaling, reducing food intake and impairing mitochondrial function, respectively. We found that 43 lysosome genes exhibit reduced expression with age, including genes encoding subunits of the proton pump V-ATPase and cathepsin proteases. The expression of lysosome genes is upregulated in the long-lived mutants, and this upregulation requires the functions of DAF-16/FOXO and SKN-1/NRF2 transcription factors. Impairing lysosome function affects clearance of aggregate-prone proteins and disrupts lifespan extension in daf-2, eat-2 and isp-1 worms. Our data indicate that lysosome function is modulated by multiple longevity pathways and is important for lifespan extension.

The concept of telomeric non-reciprocal recombination (TENOR) applied to human fibroblasts grown in serial cultures: concordance with genealogical data

Since the discovery of the limited life span of human fibroblasts some 50 years ago, many genealogical studies have been undertaken to describe growth kinetics of fibroblasts in serial cultures by their individual division behavior. It is now accepted that proliferation capacities of human fibroblasts strongly depend on their telomere lengths and integrity. Telomeres shorten with each replication round, and there is a direct correlation between cell division capacity and telomere lengths; that is, the consumption of disposable telomeric DNA repeats during cell divisions progresses until critically short telomeres determining the replicative senescence of the cells are present. Recently, we have suggested that telomeres in fibroblasts can also become elongated during DNA replication by telomeric non-reciprocal recombination (TENOR). Here we discuss genealogical data collected over the last decades as well as more recent findings on the telomere-driven replicative senescence process, and we summarize both to give an integrated picture.

Replicative senescence: a critical review

Human cells in culture have a limited proliferative capacity. After a period of vigorous proliferation, the rate of cell division declines and a number of changes occur in the cells including increases in size, in secondary lysosomes and residual bodies, nuclear changes and a number of changes in gene expression which provide biomarkers for senescence. Although human cells in culture have been used for over 40 years as models for understanding the cellular basis of aging, the relationship of replicative senescence to aging of the organism is still not clear. In this review, we discuss replicative senescence in the light of current information on signal transduction and mitogenesis, cell stress, apoptosis, telomere changes and finally we discuss replicative senescence as a model of aging in vivo.

Induction of apoptosis in human replicative senescent fibroblasts

Cellular senescence is an irreversible growth phase characteristic of normal cells. We have found that human senescent fibroblasts can be induced to undergo programmed cell death (apoptosis) by ceramide, TNF-alpha, or okadaic acid. The most profound effects were induced by TNF-alpha and okadaic acid treatment. In the present study, we also evaluated the contribution of lysosomal activation as a possible mechanism underlying the induction of apoptosis. Four lysosomal enzyme activities were measured: beta-galactosidase, alpha-galactosidase A, beta-glucoronidase, and acid phosphatase. Using an in situ assay, we have found that the activity of beta-galactosidase, which is also a biochemical marker of senescence, is induced in young proliferating fibroblasts following exposure to all three apoptotic inducing agents. The other enzymes were not significantly induced in young fibroblasts following exposure to agents that induce apoptosis. During replicative senescence, three of the four lysosomal enzymes tested (beta-galactosidase, alpha-galactosidase A, and beta-glucoronidase) are constitutively expressed at high levels. TNF-alpha was the only agent that induced lysosomal activity in senescent fibroblasts, of which only alpha-galactosidase A activity was induced. Our studies show that senescent fibroblasts can be induced to undergo apoptosis in a signal-dependent manner. However, the lysosomal enzymes examined do not appear to be correlated with apoptotic induction.

Ceroid/lipofuscin-loaded human fibroblasts show decreased survival time and diminished autophagocytosis during amino acid starvation

To test whether heavy accumulation of ceroid/lipofuscin can disturb important functions of the lysosomal system, AG-1518 human fibroblasts, ceroid/lipofuscin-loaded (following prolonged culture at normobaric hyperoxia) or not, were exposed to amino acid starvation. Ceroid/lipofuscin-loading resulted in decreased cellular survival. Also, there was an inverse relationship between amounts of ceroid/lipofuscin and the survival time of individual cells within the same cultures. Ceroid/lipofuscin-loaded fibroblasts displayed diminished autophagocytotic capacity, as demonstrated by electron microscopy and by treatment of cell cultures with NH4Cl (which inhibits autophagocytotic degradation by increasing intralysosomal pH) for 1 week before ensuing starvation. The latter treatment increased survival of control cells (due to deposition of nondegraded autophagocytosed material before start of starvation), but not that of ceroid/lipofuscin-loaded cells. Moreover, when NH4Cl treatment was combined with starvation, both groups of cells showed approximately the same shortened survival times, testifying to the causal relationship between diminished autophagocytosis and decreased survival of starving ceroid/lipofuscin-loaded cells. We hypothesize that large amounts of undegradable ceroid/lipofuscin within the acidic vacuolar compartment may interfere with lysosomal function, resulting in poor renewal of long-lived proteins and worn-out/damaged organelles, decreased adaptability, and cell death.

Ceroid/lipofuscin-loaded human fibroblasts show increased susceptibility to oxidative stress

To test whether the possibly enhanced sensitivity of aged cells to oxidative stress may depend on their content of ceroid/lipofuscin, AG-1518 human fibroblasts with various amounts of the pigment accumulated due to prolonged cultivation under normobaric hyperoxia were exposed to acute oxidative stress (2.5 microM naphthazarin, 15 min) and then returned to standard culture conditions. Twenty-four hours after the naphthazarin treatment, 37% of the cells were still vital, whereas others had undergone oxidative stress-induced apoptosis with ensuing postapoptotic necrosis. The average amount of ceroid/lipofuscin within the surviving cells was only about half of that of the initial population of cells, as measured before the naphthazarin exposure. This finding suggests that ceroid/lipofuscin-rich cells have an increased sensitivity to oxidative stress. The ceroid/lipofuscin quantity strongly positively correlated with the size of the acidic compartment (as evaluated by uptake of the weakly basic lysosomotropic fluorochrome acridine orange) and with its content of the lysosomal protease cathepsin D, as assayed by immunocytochemistry. We hypothesize that the enhanced sensitivity of ceroid/lipofuscin-loaded cells to oxidative stress may be caused by the increased amounts of lysosomal enzymes, known as mediators of oxidative damage, and/or by catalysis of intralysosomal oxidative reactions by lipofuscin-associated iron.

Ceroid/lipofuscin formation in cultured human fibroblasts: the role of oxidative stress and lysosomal proteolysis

The mechanisms involved in the accumulation of ceroid/lipofuscin within non-dividing cells are not totally understood. Oxidative stress, as well as diminished activity of lysosomal proteolytic enzymes, are known to induce ceroid/lipofuscin accumulation in a variety of cell types. In order to clarify the roles of oxidative stress and lysosomal proteolysis in ceroidogenesis/lipofuscinogenesis, and to study the fate of already formed ceroid/lipofuscin, confluent cultures of AG-1518 human fibroblasts were exposed to oxidative stress (40% ambient oxygen) and/or treated with the thiol protease inhibitor leupeptin for 2 weeks. Both oxidative stress and protease inhibition caused accumulation of ceroid/lipofuscin per se (estimated by fluorescent, confocal and electron microscopy). The combined effect of these factors was, however, almost three times as large as the sum of their isolated effects. The pigment accumulated progressively as long as the oxidative stress and/or protease inhibition acted; was not eliminated after re-establishment of normal conditions; and decreased in amount after subsequent passage. The results suggest that (i) ceroid/lipofuscin forms within secondary lysosomes due to peroxidative damage of autophagocytosed material, and (ii) it is not substantially eliminated from non-dividing cells by degradation or exocytosis.

Lipofuscin: mechanisms of formation and increase with age

Lipofuscin (age pigment) is a brown-yellow, electron-dense, autofluorescent material that accumulates progressively over time in lysosomes of postmitotic cells, such as neurons and cardiac myocytes. The exact mechanisms behind this accumulation are still unclear. This review outlines the present knowledge of age pigment formation, and considers possible mechanisms responsible for the increase of lipofuscin with age. Numerous studies indicate that the formation of lipofuscin is due to the oxidative alteration of macromolecules by oxygen-derived free radicals generated in reactions catalyzed by redox-active iron of low molecular weight. Two principal explanations for the increase of lipofuscin with age have been suggested. The first one is based on the notion that lipofuscin is not totally eliminated (either by degradation or exocytosis) even at young age, and, thus, accumulates in postmitotic cells as a function of time. Since oxidative reactions are obligatory for life, they would act as age-independent enhancers of lipofuscin accumulation, as well as of many other manifestations of senescence. The second explanation is that the increase of lipofuscin is an effect of aging, caused by an age-related enhancement of autophagocytosis, a decline in intralysosomal degradation, and/or a decrease in exocytosis.

Ultrastructure of dermal fibroblasts during development and aging: relationship to in vitro senescence of dermal fibroblasts

One approach to understanding the relationship between in vitro cellular senescence and in vivo aging is to define the development and aging of cells in vivo and then to compare these in vivo properties with the in vitro behavior of the same cells. The Syrian hamster is being used as an experimental aging model to investigate the intrinsic developmental program of dermal fibroblasts in vivo (proliferation, extracellular matrix (ECM) production, quiescence, and reactivation during wound repair) in order to determine whether the in vivo differentiation program and mature function of these cells is related to their in vitro proliferation and senescence pattern. The ultrastructure of dermal fibroblasts from midfetal development through old age is described, and a working hypothesis of the development and aging of dermal fibroblasts is proposed as a framework for further evaluation of the relationship between in vitro proliferative capacity of dermal fibroblasts and in vivo developmental and age-related changes in the dermis.

Cellular senescence revisited: a review

The field of cellular senescence (cytogerontology) is reviewed. The historical precedence for investigation in this field is summarized, and placed in the context of more recent studies of the regulation of cellular proliferation and differentiation. The now-classical embryonic lung fibroblast model is compared to models utilizing other cell types as well as cells from donors of different ages and phenotypes. Modulation of cellular senescence by growth factors, hormones, and genetic manipulation is contrasted, but newer studies in oncogene involvement are omitted. A current consensus would include the view that the life span of normal diploid cells in culture is limited, is under genetic control, and is capable of being modified. Finally, embryonic cells aging in vitro share certain characteristics with early passage cells derived from donors of increasing age.

Quantitative measures of aging in the nematode Caenorhabditis elegans: II. Lysosomal hydrolases as markers of senescence

In an attempt to provide additional quantitative markers of senescence in the nematode Caenorhabditis elegans, we have identified age-dependent increases in four lysosomal enzymes: acid phosphatase, beta-N-acetyl-D-glucosaminidase, beta-D-glucosidase, and alpha-D-mannosidase. These enzymes were judged to be lysosomal on the basis of their resemblance to analogous mammalian lysosomal enzymes with regard to subcellular fractionation, lectin binding, Km, molecular weights, inhibitor sensitivities, and pH optima. In nematode populations which had a median lifespan of 8.9 +/- 0.7 days and a maximum lifespan of 14-16 days, we observed the following increases in acid hydrolase activities per animal from day 3 (early adulthood) to day 10: (1) up to 2.5-fold for acid phosphatase; (2) 8-fold for beta-N-acetyl-D-glucosaminidase; (3) 9-fold for beta-D-glucosidase; and (4) 4-fold for alpha-D-mannosidase. Three forms of acid phosphatase and two forms of beta-D-glucosidase were separated by ion-exchange chromatography, but in each case only one form of the enzyme was primarily responsible for the age-dependent increase in total activity: acid phosphatase I increased 18-fold, while beta-D-glucosidase I increased 100-fold. By contrast, there were only slight age-dependent changes in choline acetyltransferase, acetylcholinesterase, or alpha-D-glucosidase activities after early adulthood. The age-dependent increases in acid phosphatase, beta-N-acetyl-D-glucosaminidase, beta-D-glucosidase, and alpha-D-mannosidase activities are sufficiently large and reproducible to be useful quantitative markers of senescence in C. elegans.

Quantitative morphological analysis of proliferating and nonproliferating subpopulations of IMR-90 fibroblasts during aging in vitro

Early-, mid- and late-passage cultures (population doubling levels 12, 35, and 51, respectively) of IMR-90 fibroblasts were exposed to 3H-thymidine for 48 h prior to fixation in situ for morphometric analysis in order to determine quantitatively what ultrastructural changes accompany the loss of proliferative capacity during aging in vitro. Analysis of autoradiographs, both at the light and electron microscopic levels, with an image analyzer followed by ANOVA statistical scrutiny demonstrated that a significant increase in relative cell area, an indicator of cell size, was characteristic of cells unable to incorporate 3H-TdR at both mid- and late-passage, but not at early-passage levels. Nuclear size also increased significantly with progressive passage level but was not related to proliferative capacity. No significant difference in the area fraction of nucleoli per unit area of nucleus or of mitochondria, Golgi, or lysosomes was seen in either subpopulation at any passage level. Dilated cisternae of rough endoplasmic reticulum in early-passage cells were seen if cells were harvested with trypsin and fixed either before or after centrifugation, but were not seen in labeled or unlabeled cells from any passage level when cultures were fixed in situ. We conclude that a significant increase in cell size is the only significant morphological change associated with the loss of proliferative capacity of IRM-90 fibroblasts. Furthermore, our data indicate that there is no accumulation of secondary lysosomes in human diploid fibroblasts during aging in vitro; we therefore cannot support any hypothesis of aging or proliferative decline that is based mechanistically upon this phenomenon.

The role of limited cell replicative capacity in pathological age change. A review

Physiological functions are carried out by differentiated cells, with finite lifespans, which age and need to be replaced. In young individuals, tissue functions are sustained at optimal levels because cellular dysfunction and cell loss are balanced by the emergence of newly differentiated cells as stem cells and their partially differentiated descendants replicate. However, with the passage of time the mitotic rates of these cells diminish. Eventually, replications occur too infrequently to offset the loss. It is at this point that the tissue begins to show structural changes and declining function which, as they become pervasive, are identified as "ageing". In this paper the theory is set forth that: (1) Diminishing mitotic activity in older tissues results from limited stem cell replicative capacity. (2) All stem cells, regardless of tissue, exhibit similar replicative patterns over time, progressing from the actively proliferating to the nonproliferating state. However, stem cells in different kinds of tissue have different rates of replicative decline, with the result that some tissues show age earlier than others. (3) The combination of two cellular properties--differentiated cell ageing and limited stem cell replicative capacity--is sufficient to establish the framework in which other pathological changes characteristic of aged people and animals take place.

Changes in nuclear shape and mitochondrial structure do not accompany the loss of division potential in human fibroblasts in vitro

Previous studies on ultrastructural changes that occur in cultured human fibroblasts during their in vitro life-span indicate that "senescent" cells characteristically possess structurally altered mitochondria, highly lobed nuclei, and an abundance of secondary lysosomes when compared to early passage cells. In the present study, we demonstrate that improper preparative methods can induce altered mitochondrial morphology in preparations of both IMR-90 and HF730A fibroblasts, regardless of passage level. We also show that nuclei of both living and fixed IMR-90 fibroblasts are ovoid in shape, not lobulate, in well-spread cells, regardless of either the passage level or the proliferative capacity of the cell. Fibroblasts contain lobulated nuclei only when they have not spread completely on the culture substrate. Lobulations can be induced at any passage level by collagenase/trypsin or trypsin/EDTA treatment prior to fixation, but not by cytochalasin B treatment or by cold temperatures. We conclude that any treatment that affects cytoskeleton-membrane-culture substrate interactions will induce this aberrant nuclear morphology, but that this is not indicative of "senescence" and does not relate to proliferative decline.

Status of mitochondria in living human fibroblasts during growth and senescence in vitro: use of the laser dye rhodamine 123

Rhodamine 123, a fluorescent laser dye that is selectively taken up into mitochondria of living cells, was used to examine mitochondrial morphology in early-passage (young), late-passage (old), and progeric human fibroblasts. Mitochondria were readily visualized in all cell types during growth (mid-log) and confluent stages. In all cell strains at confluence, mitochondria became shorter, more randomly aligned, and developed a higher proportion of bead-like forms. Treatment of cells for six days with Tevenel, a chloramphenicol analog that inhibits mitochondrial protein synthesis, brought about a marked depletion of mitochondria and a diffuse background fluorescence. Cyanide produced a rapid release of preloaded mitochondrial fluorescence followed by detachment and killing of cells. Colcemid caused a random coiling and fragmentation of mitochondria particularly in the confluent stage. No gross differences were discernible in mitochondria of the three cell strains in mid-log and confluent states or after these treatments. Butanol-extractable fluorescence after loading with rhodamine 123 was lower in all cell strains in confluent compared to mid-log stages. At confluence all three cell strains had similar rhodamine contents at zero-time and after washout up to 24 h. At the mid-log stage, young cells contained more rhodamine initially and lost it more rapidly than old or progeria cells, in that order. The data indicate no gross derangement in the morphology or number of mitochondria in old and progeria fibroblasts but there is a reduction of protonmotive force evident in these cells at the mid-log stage that may be growth limiting.

Lysosomal enzyme activities during ageing of adult human liver cell lines

Ten lysosomal enzyme activities have been compared during the growth and ageing of adult human liver cell lines. Arylsulfatase A, beta-D-galactosidase and beta-D-glucuronidase activities were significantly lower and arylsulfatase B activity was significantly higher in senescent cells than in actively growing cells. Furthermore, hexosaminidase activity was lower and acid phosphatase activity higher in old cells in every cell line tested but the differences were not significant. On the other hand, no change occurred in alpha-L-fucosidase, alpha-D-mannosidase, alpha-D-galactosidase and alpha-D-glucosidase activities. These results demonstrate that the increase in size and number of secondary lysosomes during ageing is accompanied for a few lysosomal enzymes by an increase or a decrease in activity depending on the enzyme.

Effect of cell density and senescence of WI-38 cells on cytochemically demonstrable phosphatases

Exponentially growing and stationary phase young and old cultures of the human cell line WI-38 were studied using cytochemistry at the ultrastructural level. 5'-Nucleotidase activity was present in the plasma membrane of all cells examined; in exponentially growing cultures the reaction was more intense in mitotic cells than in interphase cells. An increase in the amount of the reaction product was observed at confluencey, especially in older cells. The reaction of Mg2+-activated adenosine 5'-triphosphatase was absent or very weak in exponentially growing cells and increased at confluency, especially in older cells. Alkaline phosphatase was detectable only in the cell membranes and in intercellular spaces of young cells at confluency. Acid phosphatase activity was increased in old cells, especially at confluency. In these old cells, positive reactions appeared in numerous small lysosomes, autophagic vacuoles and in some flattened sacs of the Golgi apparatus. The obtained results confirm and extend previous biochemical observations and indicate that changes in phosphatase activity are associated with proliferative activity and senescence of cells growing in vitro.

Aging model for unexposed human dermis

Young human dermis is charterized by the presence of many fibroblasts with distended endoplasmic reticulum, extensive Golgi bodies, and orderly and "clean"-appearing groundwork. Older dermis contains inactive fibrocytes with lipofuscin-like granules, macrophages with dense granules, and extracellular spaces with evidence of degeneration. We have found the sex skin of cycling pigtail macaques (Macaca nemestrina) a predictable model for studies on aging changes in human dermis. Swollen sex skin has many biochemical and ultrastructural properties that are similar to those of young dermis, and deflated sex skin resembles older human skin. Sex skin, however, is unique in that it becomes "rejuvenated" with each succeeding ovarian cycle. This animal model may prove useful to researchers attempting to increase their understanding of aging in connective tissue.

Comparison by autoradiography of macromolecular biosynthesis in "young" and "old" human diploid fibroblast cultures. A brief note

The biosynthetic abilities of WI-38 fibroblasts from early and late population-doubling-level cultures were compared by autoradiography of cells grown with labeled precursors of DNA, RNA, protein and lipids. Incorporation of radioactive thymidine, uridine, protein-hydrolysate, acetate, oleic acid and cholesterol, as measured by the number of grains per cell surface, decreased with the progressive aging of the culture. However, the decrease in the incorporation of acetate, oleic acid and cholesterol was much smaller than that of the other precursors, indicating that lipid synthesis is affected to a lesser degree than protein and nucleic acid synthesis on aging. This result is in accord with the higher lipid content and proliferation of intracellular membranes in cells of "old" WI-38 cultures reported by others.

Observation on the morphological heterogeneity of WI-38 cells

WI-38 cells of intermediate and late-passage cultures were examined by light microscopy, scanning, conventional transmission and high voltage electron microscopy for evidence of heterogeneity among the cells within a single culture. Two morphologically distinct cell sizes and shapes were noted in all passages, (1) a typical fibroblastic type, and (2) a much larger, non-fusiform type. The larger cells generally had a nucleus that was positioned to one side of the bulk of the cytoplasm. The smaller was consistently fusiform with a centrally placed nucleus. The surfaces of intermediate-passage cells were uniform in showing small microvilli and corticall pits but were otherwise smooth and with out distinctive featues. The late-passage cells, on the other band, were consistent in showing numerous blebs and marginal ruffing. The internal structure of these cells in all passages studied was complicated by many age-related changes. The observations indicate that there is in WI-38 cells a high degree of intraculture heterogeneity. An awareness of this is important in studies which characterize the biochemical properties of this cell strain.

Direct association of messenger RNA with microsomal membranes in human diploid fibroblasts

Messenger RNA (mRNA) of membrane-bound polysomes in a membrane fraction of WI-38 cells remains associated with the microsomal membranes even after ribosomes and their nascent polypeptide chains are removed by using puromycin in a high salt buffer or by disassembling the ribosomes in a medium of high ionic strength lacking magnesium. mRNA either was specifically labeled in the presence of actinomycin D, or it was recognized by virtue of its affinity for oligo-dT. Poly A segments in bound mRNAs have an electrophoretic mobility in acrylamide gels which is characteristic of cytoplasmic mRNAs and corresponds to 150-200 adenyl residues. Extensive RNase treatment did not lead to release of the poly A segments of membrane-associated mRNA molecules either from an intact membrane fraction or from a membrane fraction previously stripped of ribosomes. On the other hand, RNase treatment led to the release and digestion of the nonpoly A segments of the mRNA molecules, indicating that the site of attachment of mRNA to the ER membranes is located near or at the 3' end of the molecule which contains the poly A. A direct association of mRNAs and endoplasmic reticulum membranes is considered in a modelto explain the assembly of bound polysomes and protein synthesis in a membrane-associated apparatus.

Characteristics of proliferative cells from young, old, and transformed WI 38 cultures

Mitotic cells were obtained by a shake off procedure from cultures of normal WI 38 cells at various passage levels, and from SV-40 virus transformed cells. The size of all mitotic WI 38 cells was similar regardless of in vitro age, whereas cells from the monolayer displayed an age-related increase in size. Mitotic transformed cells were similar to normal in size, but no size changes were observed in transformed monolayer cells during serial passage. Ultrastructural studies of mitotic WI 38 cells revealed no consistent change in the numbers of mitochondria or lysosome-like bodies during aging in culture. Mitotic transformed cells displayed numbers of mitochondria comparable to normal cells, but lysosome-like bodies occurred less frequently. Size distribution and structural characteristic are presented in relation to the ability of cells to synthesize DNA and to divide. These results support the contention that aging in WI 38 cultures is characterized by a declining fraction of homogeneous, actively dividing cells, and an increasing fraction of heterogenous nondividers that display senescent changes.

The interaction of cationic liposomes containing entrapped horseradish peroxidase with cells in culture

Cationic liposomes composed of sphingomyelin, cholesterol, and stearylamine were prepared with horseradish peroxidase trapped inside. Stable particles were formed in which 10-12% of the enzymic activity appeared to be located at, or near, the outer surface of the liposome. Adsorption and uptake of liposomes by HeLa cells were followed cytochemically by electron microscopy and quantitated by enzyme assay and by the distribution and fate of particles labeled with [(14)C]cholesterol and [(125)I]horseradish peroxidase. The particles were adsorbed by HeLa cells at least 300 times as efficiently as was free horseradish peroxidase. Many of the particles remained at the cell surface, but numerous membrane-bound cytoplasmic inclusions were observed to contain peroxidase-staining material. In addition, many areas of the cell membrane gave a positive staining reaction. It was concluded that many particles (presumably the larger ones) did not gain access to the interior of the cells, many were phagocytized, and some enzyme was transferred to the cell membrane, perhaps as a result of fusion of the liposomal membrane with the cell membrane.