DIFFERENTIATION IN MONOLAYER TISSUE CULTURE CELLS

Microenvironmental Regulation of the Initiated Cell

In the classical skin model of tumor initiation, keratinocytes treated once with carcinogen retain their normal appearance and growth behavior indefinitely unless promoted to growth into papillomas. Because many of the papillomas regress and may recur with further promotion, their cells can also be considered as initiated. The growth of initiated keratinocytes can be inhibited either in vitro or in vivo by close association with an excess of normal keratinocytes, but it is enhanced by dermal fibroblasts. Chick embryo fibroblasts (CEF) in culture produce transformed foci after infection with Rous sarcoma virus (RSV) on a background of normal CEF in a medium containing 10% or less calf serum (CS), but they retain normal appearance and growth regulation in 10% fetal bovine serum (FBS) or 20% CS. Transformation of a carcinogen-treated line of mouse embryo fibroblasts is prevented, and can be reversed, in high concentrations of FBS in the presence of an excess of normal cells. FBS has high, broad-spectrum antiprotease activity. Increased protease production occurs in a variety of transformed cells and is correlated with progression in tumors. Protease treatment stimulates DNA synthesis and mitosis in confluent, contact-inhibited normal cell cultures. Synthetic inhibitors of proteases suppress transformation in carcinogen-treated cultures and inhibit tumor formation in animals. Several different classes of protease may be overexpressed in the same transformed cells. It is proposed that excessive protease production accounts for major features of neoplastic transformation of initiated cells, but that transformation can be held in check by protease inhibitors present in serum and released from surrounding cells. It would be informative to determine whether high concentrations of FBS would inhibit the neoplastic development of initiated keratinocytes.

Cell aging in vivo and in vitro

It has become a staple assumption of biology that there is an intrinsic fixed limit to the number of divisions that normal vertebrate cells can undergo before they senesce, and this limit is in some way related to aging of the organism. The notion of such a limited replicative lifespan arose from the often repeated observation that diploid fibroblasts cannot proliferate indefinitely in monolayer culture, and that the number of divisions before senescence is directly related to the in vivo lifespan of different species. The in vitro evidence is countered by estimates that the number of cell divisions in some organs of rodents and man are one or more orders of magnitude higher than the in vitro limit, with no indication of the degenerative changes seen in culture. Serial transplantation experiments in animals also exhibit many more cell divisions than the in vitro studies, with some indicating an indefinite replicative lifespan. I present evidence that vertebrate cells are severely stressed by enzymatic dispersion and sustain cumulative damage during serial subcultivations. The evidence includes large increases in cell size and its heterogeneity, reductions in replicative efficiency at low seeding densities, appearance of abnormal structures in the cytoplasm, changes in metabolism to a common cell culture type, continuous loss of methyl groups and reiterated sequences from DNA, and a constant rate of decline of growth rate with passage. This evidence is complemented by the reduction induced in the replicative life span of diploid cells by a large array of treatments which have different primary targets in the cells. The most consistent and general observation of cell behavior in aging animals, with only a few exceptions, is a reduction in the rate of cell proliferation. This reduction is perpetuated when the cells are grown in culture, indicating it is an enduring and intrinsic property of the cells rather than a systemic effect of the aging organism. A similar heritable reduction in growth rate can be induced in established cell lines by prolonged incubation at quiescence. The reduction can be exaggerated by subculturing the quiescent cells under suboptimal conditions, just as the effects of age are exaggerated under stress. The constant decline of growth rate that occurs during serial passage of diploid cells may represent a similar decay of cell function. I propose that the limit on replicative lifespan is an artifact that reflects the failure of diploid cells to adapt to the trauma of dissociation and the radically foreign environment of cell culture. It is, however, a useful artifact that has given us much information about cell behavior under stressful conditions. The overall evidence indicates cell in vivo accumulate damage over a lifetime that results in gradual loss of differentiated function and growth rate accompanied by an increased probability for the development of cancer. Such changes are normally held to a minimum by the organized state of the tissues and homeostatic regulation of the organism. The rejection of an intrinsic limit on the number of cell divisions eliminates the need for a cellular clock, such as telomere length, that counts mitoses. I offer a heuristic explanation for the gradual reduction of cell function and growth capacity with age based on a cumulative discoordination of interacting pathways within and between cells and tissues. I also make a case for the use of established cell lines as model systems for studying heritable damage to cell populations that simulates the effects of aging in vivo, and represents a relatively unexplored area of cell biology.

Ascorbate induction of collagen synthesis as a means for elucidating a mechanism of quantitative control of tissue-specific function

Ascorbic acid displays the characteristics of an ideal inducer of tissue-specific function in primary avian tendon cells in culture. It is a highly specific, potent stimulator of collagen synthesis, it demonstrates slow reversible kinetics, and it has no effect on growth rate of the cultured cells. Kinetic analysis of ascorbate induction of collagen synthesis was used to determine the critical steps in this complex biosynthetic pathway. Full hydroxylation of the proline residues in collagen, although probably a necessary step for collagen induction, was in itself not sufficient for achieving either increased secretion or increased synthesis. On the other hand, an increase in secretion rate, which required both the presence of ascorbate and a high cell density, did correlate with the later stimulation in procollagen production. The process of procollagen secretion, therefore, meets the minimal requirements for the rate-limiting step. The fact that the cells maintained a large pool of intracellular procollagen despite changes in the rates of translation or secretion led us to postulate a possible feedback between the level of the internal procollagen pool and the rate of procollagen synthesis.

'Spontaneous' transformation as aberrant epigenesis

The NIH 3T3 line of cells has particular advantages for studying the dynamics of change in cellular phenotype in response to environmental conditions. Similar to stem cell growth during development, the cell line changes its phenotype under growth constraints that elicit differentiation or, alternatively, it maintains its original state over many replication cycles when grown without constraint. Unlike many cell types which respond by undergoing terminal differentiation, the NIH 3T3 cells continue to multiply indefinitely following an induced alteration in phenotype; the heritability of this change may thus be analyzed under stringent conditions of cell culture. During the course of over two years of frequent passage at low density in high calf serum (CS) concentration, a new subline developed which exhibited a consistent capacity to respond quickly and pervasively to growth constraints with an increase in saturation density, development of transformed foci in confluent cultures and altered appearance of isolated colonies. A retrospective study was undertaken, with cells from cryopreserved samples, of the course of changes in responsiveness of the cells to growth constraint leading up to the highly responsive state. Three stages were discerned, the first with an initially high capacity of a small fraction of cells to produce diffuse foci, but with a rapid decline in this capacity with frequent low density passages; the second stage, extending over more than 200 passages, of refractoriness to transformation; and the third stage (which probably arose by mutation) in which there is a consistent transformation-related response by the entire population to growth constraint, a response which has remained relatively constant over some 100 passages. A striking and novel feature of the third stage is seen on cloning the cells. Almost all the colonies obtained by cloning cells from post-confluent, growth-inhibited cultures are distinctly different in morphology from those obtained by cloning cells from the frequent low density passages. The pervasiveness of this morphological change among the clones is unmistakable evidence for a heritable adaptive response to growth constraint by most if not all of the cells in the population. The population-wide response of the cells of the third stage offers the opportunity for a rigorous, quantitative analysis of the nature of this type of persistent cellular change. Although cells of the third stage may be of mutational origin, their pervasive heritable response once the variant population is established supports the concept of progressive state selection which postulates that transformation can arise by the continuous fluctuation of growth states within cells, accompanied by the progressive selection of those states best suited to function under the selecting constraint. Relevance of the concept to the process of differentiation under growth constraint is considered.

Ascorbate induction of collagen synthesis as a means for elucidating a mechanism of quantitative control of tissue-specific function

Ascorbic acid displays the characteristics of an ideal inducer of tissue-specific function in primary avian tendon cells in culture. It is a highly specific, potent stimulator of collagen synthesis, it demonstrates slow reversible kinetics, and it has no effect on growth rate of the cultured cells. Kinetic analysis of ascorbate induction of collagen synthesis was used to determine the critical steps in this complex biosynthetic pathway. Full hydroxylation of the proline residues in collagen, although probably a necessary step for collagen induction, was in itself not sufficient for achieving either increased secretion or increased synthesis. On the other hand, an increase in secretion rate, which required both the presence of ascorbate and a high cell density, did correlate with the later stimulation in procollagen production. The process of procollagen secretion, therefore, meets the minimal requirements for the rate-limiting step. The fact that the cells maintained a large pool of intracellular procollagen despite changes in the rates of translation or secretion led us to postulate a possible feedback between the level of the internal procollagen pool and the rate of procollagen synthesis.

Properties of Na-K pump in primary cultures of kidney cells

Activities related to Na-K transport were measured in cell cultures of ground squirrel kidney cortex in order to compare these cells with those of intact kidney and of continuous cell lines. A microsomal preparation containing plasma membrane Na,K-ATPase from fresh kidney showed twice the activity of a similar preparation from 72-hour cultured cells. Na,K-ATPase of homogenates of 72-hour cells showed one-third to one-fourth the specific activity of that from 6-hour cultured cells. The associated K-dependent phosphatase activity also declined as a function of time in culture. The ouabain-sensitive influx of K into 6-hour cultured cells was twice as great as the K influx into 72-hour cells. The number of sites binding 3H-ouabain in intact cultured cells declined 81% on a cell protein basis between 6 and 72 hours in culture. This decline in ouabain binding sites was relatively greater than that of K influx, so that the K turnover number increased over this same time period. The decline in ouabain-sensitive K influx during culture was complementary to an increase in furosemide-sensitive K influx. Measurements of unidirectional and net K fluxes showed that there were three components of K influx into 3-day cultured cells: ouabain-sensitive Na:K exchange, furosemide-sensitive K:K exchange, and K diffusion. In the 6-hour cultures, however, there was no furosemide-sensitive K:K exchange. Thus, after three days in culture ground squirrel kidney cells lose a feature characteristic of the original parent cells (high Na,K-ATPase activity), and gain a feature common to many undifferentiated cultured cells (furosemide-sensitive K:K exchange).

Sulfated glycosaminoglycans of cells grown in culture: dermatan sulfate disappearance in successive fibroblast subcultures

Sulfated glycosaminoglycan content of primary cultures of guinea pig embryo fibroblast in reported. It is shown that changes of sulfated glycosaminoglycans occur in successive subcultures: 1) disappearance of dermatan sulfate and 2) a slight increase in the proportion of heparitin sulfate. The sulfated glycosaminoglycan composition of the mammalian cell established lines: VERO, SIRK, Hep-2, MK-2, BGM, IB-RS-11 (S6) and 3T6 is also reported. It is shown that chondroitin sulfate AC and/or heparitin sulfate are the main glycosaminoglycans of the established cell lines (except 3T6). A possible role of these glycosaminoglycans in cell biology is discussed.

Phospholipid biosynthesis and secretion by a cell line (A549) which resembles type II aleveolar epithelial cells

The A549 cell line is a continuous cell line derived from a human adenocarcinoma of the lung. At low cell population density the cells contain relatively few lamellar bodies, but in mature cells in very confluent cultures lamellar bodies are abundant. The lamellar bodies from these cells are enriched for phosphatidylcholine and disaturated phosphatidylcholine. In mature cells, 45% of newly synthesized phosphatidylcholine is disaturated. Stimulation with the calcium ionophore A23187 produces exocytosis of phosphatidylcholine (46% disaturated). The A549 cell synthesizes, stores in lamellar bodies, and secretes phosphatidylcholine, and thus has many important biological properties of the alveolar epithelial type II cell.

Effects of hormones and serum on glycogen metabolism in adult rat liver parenchymal cell primary cultures

Parenchymal cells from adult rat liver, isolated by a collagenase perfusion technique, have been maintained in primary culture and a detailed study on carbohydrate metabolism carried out over the initial 48-hour culture period. The glucose concentration of the medium exerts a major influence on glycogen accumulation by the cells. Insulin, particularly at high glucose concentrations, stimulates glycogen biosynthesis, whereas glucagon prevents glycogen accumulation. Dexamethasone was without effect on glycogen metabolism. Glucose appears to stimulate glycogen accumulation by activation of glycogen synthetase enzyme. However, there is a gradual loss of synthetase activity throughout the culture period. Similar decreases in activity were noted for pyruvate kinase, aldolase and hexokinase. Glucose, insulin and dexamethasone were unable to prevent these decreases in enzyme activity. Foetal bovine serum contains fructose and this hexose appears to be the factor in serum which is responsible for the activation of glycogen accumulation in the presence of physiological glucose concentrations. The lactic acid content of the serum may also stimulate glycogen accumulation. In general, there is a gradual loss of the pattern of carbohydrate metabolism typical of differentiated hepatocytes during the culture period.

Parenchymal cells from adult rat liver in nonproliferating monolayer culture. I. Functional studies

Parenchymal cells from adult rat liver have been established in primary monolayer culture. Donor animals are subjected to a partial hepatectomy and, 4 days later, cells are prepared by collagenase perfusion of the regenerated liver. The hepatic parenchymal cells, separated from nonparenchymal material and suspended in serum-free medium, are placed in plastic tissue culture dishes, where they form a monolayer within 24 h. The monolayer cells exhibit minimal mitotic activity and demonstrate several major metabolic functions characteristic of liver in vivo; these include albumin synthesis and secretion, gluconeogenesis from 3-carbon precursors, responsiveness to insulin and glucagon, glycogen synthesis, and activity of two microsomal enzymes. These functions are present in the monolayer cells for several days at activities similar to those observed in the liver in vivo. The findings indicate that hepatic parenchymal cells in this monolayer system are viable and behave in many respects like normal adult rat liver.

Retention of differentiation potentialities during prolonged cultivation of myogenic cells

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Cystathionine synthase in tissue culture derived from human skin: enzyme defect in homocystinuria

Fibroblasts derived from normal human skin and from cells in amniotic fluid and grown in tissue culture have cystathionine synthase activity. Skin from homocystinuric patients gives rise to fibroblast lines with normal activities of methionine-activating enzyme, but with very low or undetectable cystathionine synthase activity. Thus, the enzyme lesion in homocystinuria is demonstrable in readily available human cells. Neither cystathionine synthase nor methionine-activating enzyme could be detected in intact normal skin.