Coupling of proadipocyte growth arrest and differentiation. II. A cell cycle model for the physiological control of cell proliferation

Loss of proliferative potential during terminal differentiation coincides with the decreased abundance of a subset of heterogeneous ribonuclear proteins

The decrease in abundance of a subset of highly conserved basic nuclear proteins is established to correlate with the loss of proliferative potential in association with the process of terminal differentiation in murine mesenchymal stem cells and human keratinocytes. These proteins, designated P2Ps for proliferation potential proteins, have apparent molecular masses of 30-40 kD, are associated with the 30-40S substructures of nuclear hnRNP complexes, and are recognized by antibodies made against core proteins of hnRNP particles. They also share an epitope in common with heat shock protein-90 (hsp90) and are recognized by two mAbs against hsp90. Two-dimensional electrophoretic Western blots furthermore show that P2Ps make up a subset of hnRNP proteins. Cells that possess these proteins express the potential to proliferate whether or not they are traversing the cell cycle. These include rapidly growing cells, reversibly growth-arrested cells, and nonterminally differentiated cells. In contrast, cells that have irreversibly lost their proliferative potential, such as terminally differentiated cells, show a marked reduction in the abundance of P2Ps as determined by immunodetection on Western blots. A correlation, therefore, exists between the presence of this subset of nuclear proteins and the proliferative potential in two cell types. These results raise the possibility that as a subset of hnRNP proteins, P2Ps may mediate posttranscriptional control of the processing of specific RNAs required for cell proliferation.

Induction of preadipocyte differentiation by mature fat cells in the rat

In this study we investigated the influence of mature adipocytes, derived from rat adipose tissue, on the replication and differentiation of preadipocytes in primary culture. Mature fat did not inhibit preadipocyte replication within the 6-d period studied. Preadipocyte differentiation, as assessed by both cytoplasmic lipid accretion and an increase in glycerophosphate dehydrogenase (GPDH) activity, was significantly stimulated by the presence of mature fat tissue or isolated adipocytes. The proportion of cells containing visible lipid droplets by oil red O staining was 47 +/- 10 to 58 +/- 10% (depending on the site of origin of the preadipocytes) when cocultured with mature fat compared with less than 1 to 2 +/- 1% when cultured in medium alone, while GPDH activity was 344 +/- 9 compared with 43 +/- 3 nM NADH/min per mg protein, respectively. This effect was not due to release of triacylglycerols from damaged adipocytes. Fatty acids added to the medium promoted lipid accumulation but did not stimulate a rise in GPDH activity. We concluded that mature adipocytes may release factor(s) that promote preadipocyte differentiation (and maturation).

Inhibition of simian virus 40 T-antigen expression by cellular differentiation

Murine 3T3T stem cells transfected with pSV3neo DNA were employed to study the effects of somatic cell differentiation on simian virus 40 (SV40) T-antigen expression. This experimental approach was used because the 3T3T cell line is a well-characterized in vitro adipocyte differentiation system and the pSV3neo plasmid contains the early region of the SV40 genome and a selective marker, G418 resistance. Cell clones containing stably integrated pSV3neo which expressed T antigen were isolated in G418-containing medium. Most of these cell clones differentiated poorly. However, several clones retained the ability to efficiently differentiate into adipocytes, and with these cell clones, it was established that adipocyte differentiation markedly repressed T-antigen expression. The differentiation-specific repression of T-antigen expression did not result from a loss of proliferative potential associated with terminal differentiation, because it was observed in adipocytes that could be restimulated to proliferate. In such cells, restimulation of cell growth induced reactivation of T-antigen expression. Repression of T-antigen expression was also demonstrated during differentiation of SV40 T-antigen-immortalized human keratinocytes. These results establish that the process of cellular differentiation can repress T-antigen expression in at least two distinct biological systems.

Nonterminally differentiated cells express decreased growth factor responsiveness

In 3T3 T mesenchymal stem cells, at least four types of biological states exist that can mediate the control of cell differentiation and/or proliferation. These include the predifferentiation growth arrest state, the nonterminal differentiation state, the terminal differentiation state, and a growth arrest state induced by growth factor/serum deficiency. The current studies were performed to investigate the relative mitogenic responsiveness of cells at these four states and specifically to determine if nonterminally differentiated cells show decreased responsiveness to specific mitogens. Twenty-five different serum, plasma, and growth factor combinations were evaluated. The results show that undifferentiated, growth-arrested cells are highly responsive to numerous mitogens and that by definition terminally differentiated cells are not responsive to any mitogens. In contrast, nonterminally differentiated cells demonstrate a unique pattern of mitogenic responsiveness. Whereas nonterminally differentiated cells can be stimulated to proliferate by high concentrations of serum or plasma supplemented with growth factors, they cannot be stimulated to proliferate by combinations of multiple purified growth factors. These results suggest that the process of nonterminal differentiation is associated with a significant change in factors/cofactors required to stimulate cell proliferation and that these factors/cofactors are present in plasma.

c-myc antisense transcripts accelerate differentiation and inhibit G1 progression in murine erythroleukemia cells

Friend murine erythroleukemia (F-MEL) cells were transfected with a plasmid bearing tandemly arranged mouse c-myc antisense and dihydrofolate reductase transcription units. Sixteen clones were isolated, each containing unrearranged c-myc sequences and expressing high levels of antisense transcripts. All antisense clones examined contained reduced amounts of cytoplasmic endogenous c-myc transcripts. The kinetics of reaccumulation of endogenous c-myc mRNA during a 24-h exposure to dimethyl sulfoxide (DMSO) were also retarded and the ultimate transcript levels attained were less than in control cells. Antisense clones grew as well as control F-MEL cells in medium containing 10% fetal calf serum but at only a half and a quarter of the control rates in media containing 5 and 2% serum, respectively. Antisense clones differentiated faster and to a greater degree than control cells following DMSO exposure. myc antisense transcript expression was increased by growing cells in methotrexate, which resulted in an enhanced response to DMSO. Fluorescence-activated cell sorter (FACS) analysis of cellular DNA content indicated that a greater fraction of antisense nuclei contained a G0/G1 2n DNA content following a 24-h exposure to DMSO. When density-arrested antisense clones were diluted into fresh medium to allow reentry into the cell cycle, they incorporated less [3H]thymidine than control cells. FACS analysis showed that this was because only a portion of the cell population was entering S phase. Whereas control cells did not increase in size following release from density arrested antisense cells contained a subpopulation which were initially smaller and which eventually attained the same size as control cells. Quiescent antisense cells thus comprise two populations, each arrested at a different point in G1. Dilutional replating allowed both populations to reenter the cell cycle. We propose a model which postulates that certain minimal myc levels are necessary for cells to traverse G1. Those with insufficient levels, due, for example, to antisense inhibition, are unable to completely traverse G1 during density arrest and synchronize at an earlier point than do control cells. This earlier point may be along the differentiation pathway and may account for the greater responsiveness of antisense cells to DMSO induction. This model postulates that F-MEL cells overexpressing myc fail to differentiate because myc levels are never sufficiently low enough to allow cells to enter the differentiation pathway.

Nonterminal differentiation represses the neoplastic phenotype in spontaneously and simian virus 40-transformed cells

The potential of nonterminal cellular differentiation to stably repress the expression of the neoplastic phenotype of transformed cells is established. Nonterminal differentiation induces spontaneously transformed 3T3 T cells to revert to a nontransformed state and induces the revertant cell clones to become resistant to retransformation by UV irradiation or 4-nitroquinoline oxide treatment. Nonterminal differentiation also induces simian virus 40-transformed 3T3 T cells to repress expression of the large tumor antigen and to revert to a nontransformed state. Although the molecular mechanisms that mediate these and other forms of anticancer activity have not been definitively established, data are presented which suggest that differentiation-induced repression of large tumor antigen expression can be regulated at the level of transcription and/or RNA processing.

Tumor necrosis factor inhibits the terminal event in mesenchymal stem cell differentiation

Control of the terminal event in cellular differentiation is an important normal regulatory process, and the expression of defects in the control of this process has been implicated in the pathogenesis of cancer. To determine if tumor necrosis factor (TNF), which is an important biological response modifier, can inhibit terminal differentiation, we have studied 3T3 T mesenchymal stem cells. This experimental cell system was employed because a well-defined series of steps in differentiation has been defined and cells at each stage of differentiation can be isolated. For example, nonterminal differentiated cells can be isolated, and their transition to a terminal differentiation state can be evaluated. The most interesting results in the current studies show that TNF blocks the terminal event in mesenchymal stem cell differentiation. Inhibition of the terminal event of differentiation by TNF is reversible and is not associated with inhibition of selective or general protein synthesis. Evidence is also presented that cell clones that are defective in their ability to undergo the terminal event in differentiation secrete factor(s) that inhibit the terminal event in differentiation. These observations suggest that the inhibition of the terminal event in differentiation may be mediated via autocrine or paracrine regulatory molecules such as tumor necrosis factor.

c-myc antisense transcripts accelerate differentiation and inhibit G1 progression in murine erythroleukemia cells

Friend murine erythroleukemia (F-MEL) cells were transfected with a plasmid bearing tandemly arranged mouse c-myc antisense and dihydrofolate reductase transcription units. Sixteen clones were isolated, each containing unrearranged c-myc sequences and expressing high levels of antisense transcripts. All antisense clones examined contained reduced amounts of cytoplasmic endogenous c-myc transcripts. The kinetics of reaccumulation of endogenous c-myc mRNA during a 24-h exposure to dimethyl sulfoxide (DMSO) were also retarded and the ultimate transcript levels attained were less than in control cells. Antisense clones grew as well as control F-MEL cells in medium containing 10% fetal calf serum but at only a half and a quarter of the control rates in media containing 5 and 2% serum, respectively. Antisense clones differentiated faster and to a greater degree than control cells following DMSO exposure. myc antisense transcript expression was increased by growing cells in methotrexate, which resulted in an enhanced response to DMSO. Fluorescence-activated cell sorter (FACS) analysis of cellular DNA content indicated that a greater fraction of antisense nuclei contained a G0/G1 2n DNA content following a 24-h exposure to DMSO. When density-arrested antisense clones were diluted into fresh medium to allow reentry into the cell cycle, they incorporated less [3H]thymidine than control cells. FACS analysis showed that this was because only a portion of the cell population was entering S phase. Whereas control cells did not increase in size following release from density arrested antisense cells contained a subpopulation which were initially smaller and which eventually attained the same size as control cells. Quiescent antisense cells thus comprise two populations, each arrested at a different point in G1. Dilutional replating allowed both populations to reenter the cell cycle. We propose a model which postulates that certain minimal myc levels are necessary for cells to traverse G1. Those with insufficient levels, due, for example, to antisense inhibition, are unable to completely traverse G1 during density arrest and synchronize at an earlier point than do control cells. This earlier point may be along the differentiation pathway and may account for the greater responsiveness of antisense cells to DMSO induction. This model postulates that F-MEL cells overexpressing myc fail to differentiate because myc levels are never sufficiently low enough to allow cells to enter the differentiation pathway.

Flow cytometric evaluation of cell cycle characteristics during in vitro differentiation of chick embryo chondrocytes

The cell cycle kinetic characteristics of chick endochondral chondrocytes differentiating in vitro were studied by flow cytometry. In addition, the synthesis of type I and type X collagens of the same cells was evaluated by immunoprecipitation. Dedifferentiated cells, derived from chick embryo tibiae and grown attached to a substratum, were characterized by type I collagen synthesis, a high growth fraction (GF = 0.94), minimal cell loss factor (phi = 0.02), and a total cell cycle time of the proliferating cells of about 17 h (tG1 = 8 h, tS = 5 h, and tG2 + M = 4 h). Transfer of dedifferentiated cells to suspension culture on agarose-coated dishes induced differentiation to hypertrophic chondrocytes. These were characterized by type X collagen synthesis, a low growth fraction (GF = 0.52), maximal cell loss factor (phi = 1.0), and a total cell cycle time of the proliferating cells of about 73 h (tG1 = 53 h, tS = 12 h, and tG2 + M = 8 h). The transition from dedifferentiated chondrocytes to hypertrophic chondrocytes was accompanied by large increases of the duration of all the cell cycle phases and of the number of quiescent and degenerating cells. Associated with these alterations in cell cycle kinetics was a switch from type I to type X collagen synthesis. Further preliminary data suggest that the population of differentiating chondrocytes (a state between dedifferentiated and hypertrophic chondrocytes) comprises a heterogeneous population of fast and slow growing cells.

Biologic mechanisms for the regulation of normal human keratinocyte proliferation and differentiation

Normal human keratinocytes can be grown in serum-free medium, and the integrated control of their proliferation and differentiation can be modulated experimentally. The growth of cultured human keratinocytes can also be specifically arrested at either reversible or irreversible growth arrest states. Reversible growth arrest is induced by culture in medium containing TGF-beta or ethionine or in medium deficient of isoleucine. Irreversible growth arrest is induced by culture in razoxane-containing medium or by routine passage of keratinocytes until senescence results. The current studies were performed to determine from which growth arrest states keratinocyte differentiation occurs. Cells were therefore growth-arrested at each state, and they were then incubated in several different differentiation-promoting culture conditions. The results show that differentiation, as determined by morphologic, cytochemical, and immunofluorescent assays, can be induced from multiple reversible and irreversible growth arrest states by a series of complex biologic mechanisms. More specifically, at least three distinct stages appear to be involved in the process of keratinocyte differentiation. First, cells arrest their growth at a reversible predifferentiation state. Second, cells irreversibly lose their proliferative potential. Finally, cells express the terminally differentiated keratinocyte phenotype.

Cell cycles and in vitro transdifferentiation and regeneration of isolated, striated muscle of jellyfish

Isolated, mononucleated, cross-striated muscle cells of a medusa can transdifferentiate in vitro to various new cell types and even form a complex regenerate. The transdifferentiation events follow a strict pattern. The first new cell type resembles smooth muscle and is formed without a preceding DNA replication. This cell type behaves like a stem cell and by quantal cell cycles produces all other new cell types. Some preparations develop an inner and an outer layer separated by a basal lamella. Formation of these layers does not depend on DNA replication. When layers do not form, each division results in nerve cells and smooth muscle cells. If separation into layers occurs, then a regenerate will be formed, and in the course of only two cell cycles all necessary cell types to form a functional regenerate will differentiate.

Control of the adipogenic differentiation of 3T3-F442A cells by retinoic acid, dexamethasone, and insulin: a topographic analysis

Differentiation of 3T3-F442A adipocytes, monitored by accumulation of neutral lipid and by using the sensitive marker glycerophosphate dehydrogenase, is inhibited by incubation of confluent 3T3-F442A fibroblasts in medium containing retinoic acid or dexamethasone. When added together, dexamethasone (0.25 microM) potentiates about 50-fold the inhibitory effect of retinoic acid (10 microM). Insulin cannot counteract the retinoic acid blockade; however, it can overcome the inhibition of differentiation elicited by dexamethasone. These differential effects of insulin are used for characterizing the adipose conversion cycle. We describe cell culture conditions where terminal differentiation of 3T3-F442A preadipocytes is achieved by low, physiological levels of insulin. They include the switch from a high-serum medium containing isobutyl methyl xanthine and dexamethasone to a serum-free, hormone-supplemented medium. The data reported establish the existence of two successive states for commitment to adipogenic differentiation: a first commitment point (CA) to differentiation which requires serum adipogenic factors, and a second commitment point (CH) controlled by lipogenic hormones, namely insulin, after which terminal maturation can resume. We demonstrate that retinoic acid can prevent and interrupt differentiation by blocking the cells within the early differentiation phase.

Polypeptide changes associated with loss of proliferative potential during the terminal event in differentiation

The differentiation of murine mesenchymal stem cells occurs in nonterminal and terminal phases. In previous reports we established the characteristics of nonterminally differentiated cells and showed that transition from the nonterminal to the terminal state of differentiation can be induced by human plasma. We also showed that this transition is blocked by protein synthesis inhibitors and other pharmacological agents. In this paper, we have employed two-dimensional gel electrophoresis to evaluate changes in specific polypeptides that are induced when cells lose proliferative capacity associated with the terminal event in differentiation. Using silver staining procedures for analysis of electrophoretograms, we detected only seven major polypeptide differences between nonterminally differentiated and terminally differentiated cells. Six polypeptides were expressed only in preparations of terminally differentiated cells; these included two polypeptides identified in cytosolic fractions and four polypeptides identified in nuclear fractions. One polypeptide was also found to be selectively expressed only in nuclear fractions of nonterminally differentiated cells. Based on these observations we conclude that the loss of proliferative potential that occurs during the terminal event in mesenchymal stem cell differentiation is associated with changes in the composition of a limited number of specific polypeptides. We suggest that one or more of these polypeptides may be important in the regulation of cellular proliferation.

Aproliferin--a human plasma protein that induces the irreversible loss of proliferative potential associated with terminal differentiation

Cellular proliferation is regulated not only by the action of growth factors and growth inhibitors whose effects are reversible but also by factors that induce the irreversible loss of proliferative potential associated with the terminal event in cellular differentiation. The authors have employed 3T3 T mesenchymal stem cells as a model system to study the terminal event in cellular differentiation because in these cells' distinct nonterminal and terminal states of differentiation can be identified and because transition from the nonterminal to the terminal states of differentiation can be induced by human plasma. In this paper is reported the 20,000-fold purification of a component of human plasma that induces the terminal event in differentiation. This factor is shown to have an apparent molecular weight of approximately 45,000 and an isoelectric point of approximately 7.6. It is trypsin-sensitive, acid and heat-labile, and is resistant to treatment with dithiothreitol and alkali. The ability of this human plasma protein to induce the irreversible loss of proliferative potential associated with the terminal event in differentiation serves as the basis for its designation "aproliferin." The data in this paper in addition show that no other pharmacologic or physiologic agents have been identified that can mimic the biologic effect of aproliferin. Therefore, aproliferin appears to be a functionally distinct protein in human plasma.

Distribution of interdivisional times in proliferating and differentiating Friend murine erythroleukaemia cells

The interdivisional times of Friend murine erythroleukaemia cells which are growing continuously, or during terminal erythroid differentiation after exposure to dimethyl sulphoxide (DMSO), were determined by time lapse video photography. The median interdivisional times were found to increase from 11.75 hr before exposure to DMSO, to 24.0 hr at 72 hr after exposure. This increase in median interdivisional time was accompanied by an increase in heterogeneity of interdivisional times (%CV = 8.5----40.8), by an increase in the similarity of sister interdivisional times (ryy = 0.622----0.925), and by a decrease in the fraction of cells observed to divide (F = 1.0----0.807). Cells exposed to DMSO for 72 hr can be induced to divide at least once with nearly normal interdivisional times, if they are resuspended at a tenfold higher cell concentration. Computer simulations of cell cycle regulation, based on the opposing reactions model of Murphy, generate interdivisional time distributions which resemble the experimental data better than the single transition probability model of Smith and Martin.

Transforming growth factor type beta is a specific inhibitor of 3T3 T mesenchymal stem cell differentiation

In the current studies we examined the effects of transforming growth factor type beta (TGF-beta) on the control of differentiation of BALB/c 3T3 T stem cells. We report that TGF-beta is a potent, reversible inhibitor of adipocyte differentiation (50% inhibition at approximately 0.06-0.08 ng/ml), while other biologically active polypeptides, such as epidermal growth factor (EGF), human growth hormone (hGH), and somatomedin C, have no specific effect on differentiation at even higher concentrations (200 ng/ml). We also report that TGF-beta inhibits differentiation in a cell cycle-dependent manner by its effect on a specific phase in the differentiation process. We therefore suggest that if TGF-beta is an important regulatory factor, one of its critical mechanisms of action may be its ability to inhibit the process of cell differentiation.

Suppression of tumorigenicity by the cell-cycle-dependent control of cellular differentiation and proliferation

The experiments described in this report were designed to determine if suppression of tumorigenicity can be mediated by cell-cycle-dependent mechanisms that control cellular differentiation and/or proliferation in mesenchymal stem cells of the 3T3 T type. These cells were employed because they possess distinct, well-characterized cell-cycle-dependent mechanisms to control both cellular differentiation and proliferation. To achieve our goal we developed by non-mutagenic procedures 23 clonal variants of 3T3 T stem cells that expressed one of 4 distinct phenotypes for the regulation of cellular differentiation and proliferation. Six clones expressed combined defects in the control of differentiation and proliferation; 6 expressed intact mechanisms to control proliferation but defects in the control of differentiation; and 3 expressed intact mechanisms to control differentiation but defects in the control of proliferation. Finally, 8 clones expressed no detectable phenotypic defects in the control of either differentiation or proliferation. Once isolated and characterized, each of these clones was assayed for its tumorigenic potential. The results establish that clones which express combined defects in the control of differentiation and proliferation are highly tumorigenic. By contrast, tumorigenicity is markedly suppressed in clones that maintain the ability to control their proliferation or their differentiation. Furthermore, clones that maintained the ability to control both proliferation and differentiation showed no evidence of tumorigenicity. These data are interpreted to suggest that stringently regulated control of cellular differentiation and/or proliferation can act as a cancer suppressor mechanism.

Regulation of the terminal event in cellular differentiation: biological mechanisms of the loss of proliferative potential

Human plasma has been demonstrated to contain factors that induce the sequential expression of nonterminal and terminal adipocyte differentiation in 3T3 T mesenchymal stem cells. We now report the development of methods for the isolation of purified populations of nonterminally differentiated cells and terminally differentiated cells, and we show that it is possible to experimentally induce transition from the nonterminal to the terminal state of differentiation. With this model system it is therefore now possible to examine the biological and molecular processes associated with the terminal event in differentiation, i.e., the irreversible loss of proliferative potential. In this regard, we demonstrate that transition from the nonterminal to terminal state of differentiation is a complex metabolic process that consists of at least two steps and that this process can be triggered by pulse exposure to an inducer for approximately 12 h but that approximately 24-48 h is required for the process to be completed. The data also establish that induction of the terminal event in differentiation requires protein synthesis but not RNA and DNA synthesis. These and additional results suggest that loss of proliferative potential associated with the terminal event in cellular differentiation is a distinct regulatory process, and we suggest that defects in this regulatory process may be of etiological significance in the pathogenesis of specific human diseases, especially cancer.

Two functionally distinct classes of growth arrest states in human prokeratinocytes that regulate clonogenic potential

Rapidly growing normal human neonatal prokeratinocytes (HPK) cultured in serum-free medium can be induced to undergo either reversible or irreversible growth arrest at distinct cell cycle states. Reversible G1 arrest was induced by culture of low-density cells in human lymphocyte conditioned medium, by culture in high-density stationary phase conditioned medium, and by culture in isoleucine-deficient medium. Irreversible arrest of HPK growth predominantly in G1 was induced by culture in growth factor-deficient medium. Irreversible arrest of HPK growth in G1 and G2 was also induced by culture in suspension in methylcellulose prepared in complete MCDB 153 medium or by culture in serum-containing medium. Finally, the drug razoxane was employed to induce irreversible arrest of HPK in G2. These data establish that there are 2 distinct classes of growth arrest states for HPK and suggest that each arrest mechanism may serve a unique role in the control of keratinocyte differentiation in normal cells. It is also possible that the development of selective defects in either of these processes could be of etiologic significance in certain epidermal disease states.

Neurite formation by neuroblastoma-glioma hybrid cells (NG108-15) in defined medium: stochastic initiation with persistence of differentiated functions

Neurite formation and proliferation by NG108-15 cells were studied in short-term serum-free N2 medium. Neuritogenesis by individual cells was observed at widely differing times, suggesting a stochastic component to initiation of differentiation. Cells with and without neurites could also enter one or more rounds of proliferation at varying times. The initial choice between these divergent behaviors influenced subsequent growth. Cells initially extending neurites had a high probability of continuing neuritic elongation. Cells initially dividing had a high probability of yielding further progeny. Addition of dibutyryl cyclic AMP to cells cultured in N2 medium rapidly increased the probability of differentiating and decreased the probability of proliferating. To test whether or not cells with highly differentiated morphologies had irreversibly lost the capacity for proliferation, induced cultures were washed and challenged by the addition of serum-containing medium. The length of time required for individual cells to divide increased with increasing time of preincubation in the induction medium. However, few cells appeared to be permanently removed from the proliferative pool. These observations suggest that differentiating cells exhibit persistence, a tendency to continue on the differentiation pathway. Persistence is extinguished following one round of proliferation in serum-containing medium.

Tumorigenicity associated with loss of differentiation and of response to insulin in the adipogenic cell line 1246

The adipogenic cell line 1246, which grows and differentiates in defined medium, stringently requires insulin for both processes. From this cell line, insulin-independent variants were isolated and characterized. Unlike 1246 cells, the variant cell lines proliferate without insulin, have lost their differentiation ability, produce factor(s) able to replace insulin to stimulate 1246 cell growth but not differentiation and are tumorigenic. Because of these properties, this system is appropriate to examine the correlation (if any) between the loss of response to an extra-cellular factor and of ability to differentiate, and between the production of endogenous growth factor and the acquisition of tumorigenic properties.

An initiator of carcinogenesis selectively and stably inhibits stem cell differentiation: a concept that initiation of carcinogenesis involves multiple phases

A concept of carcinogenesis was recently devised in our laboratory that suggests the development of defects in the control of cell differentiation is associated with an early phase of carcinogenesis. To test this proposal directly, the effects of an initiator of carcinogenesis (i.e., UV irradiation) on proadipocyte stem cell differentiation and proliferation was assayed. In this regard, 3T3 T proadipocytes represent a nontransformed mesenchymal stem cell line that possesses the ability to regulate its differentiation at a distinct state in the G1 phase of the cell cycle as well as the ability to regulate its proliferation at two additional G1 states that are induced by culture in growth factor-deficient or nutrient-deficient microenvironments. The results establish that a low dosage of 254 nm UV irradiation selectively and stably inhibits the differentiation of a high percentage of proadipocyte stem cells without significantly altering their ability to regulate cellular proliferation in growth factor-deficient or nutrient-deficient culture conditions. Differentiation-defective proadipocyte stem cells are demonstrated not to be completely transformed but to show an increased spontaneous transformation rate, as evidenced by the formation of type III foci in high density cell cultures. These data support the role of defects in the control of differentiation in the initiation of carcinogenesis. Other data, however, also suggest that additional cellular defects must be expressed for a cell to be completely initiated. These observations support a concept that the initiation of carcinogenesis involves multiple phases.

Changes in self-renewal potential of human leukemic cells (K562): a bidirectional stochastic process

Daughter cells arising from a single cell division in the leukemic cell line K562 have equivalent self-renewal potential with respect to their ability to form clones in semisolid medium. However, individual cells isolated from these clones in sequence have vastly different abilities in their self-renewal potentials. Thus, cells originating from a clone with any particular self-renewal potential exhibit the full range of self-renewal potentials--from highly renewing to none renewing, cells. These results show that self-renewal potential in the K562 cell line is a random, reversible and partially noninherited characteristic. It is suggested that the stochastic variability of the intraclonal self-renewal potential of K562 progeny cells either reflects the initial expression of a differentiation program or the expression of the predeterministic portion of the normal myelopoietic differentiation pathway.

Inhibition of ornithine decarboxylase by retinoic acid and difluoromethylornithine in relation to their effects on differentiation and proliferation

Murine embryonal carcinoma F9 cells can be induced to differentiate by 2-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase (ODC). The differentiated phenotype is similar to that of retinoic acid (RA)-treated F9 cells. In contrast to F9 cells the differentiated cells secrete plasminogen activator and express keratin intermediate filaments. Both DFMO and RA reduce ornithine decarboxylase activity, polyamine levels and inhibit cell proliferation of F9 cells. These compounds also reduce ODC, polyamine levels and proliferation of mouse BALB/c 3T6 fibroblasts. RA inhibits the induction of ODC by insulin, serum and to a lesser extent that of epidermal growth factor (EGF) and 12-O-tetradecanoylphorbol-13-acetate (TPA). The action of DFMO and RA can be distinguished by their response to putrescine. The induction of differentiation and the inhibition of cell proliferation by DFMO can be totally abolished upon the addition of putrescine, whereas the actions of RA are not affected at all. These results suggest that the inhibition of ODC and reduction of polyamines are not causal in the induction of differentiation and the inhibition of proliferation by RA.

Biological mechanisms for the loss of the differentiated phenotype by non-terminally differentiated adipocytes

The differentiation of 3T3 T proadipocyte stem cells is controlled at two related yet distinct states in the G1 phase of the cell cycle. They are designated GD and GD'. GD is the G1 state at which cells must growth arrest prior to differentiation, and GD' is the G1 state at which non-terminal differentiation occurs. Cells arrested at the GD and GD' states have distinct characteristics; yet cells at both states can mediate the integrated control of cellular proliferation and differentiation. In this paper we report on studies designed to further characterize the relationship of these two states, specifically to determine whether non-terminally differentiated GD'-arrested cells can be induced to lose the adipocyte phenotype and revert to the GD state. We report that retinoic acid (RA) and methyl isobutyl xanthine (MIX) can induce non-terminally differentiated GD'-arrested cells to lose the adipocyte phenotype without undergoing DNA synthesis. Such cells that have lost the adipocyte phenotype are also shown to remain in the G1 phase of the cell cycle and to reacquire most of the characteristics of GD-arrested cells. Most importantly, they demonstrate the capacity to redifferentiate without DNA synthesis. We therefore conclude that when non-terminally differentiated GD'-arrested cells are induced to lose the adipocyte phenotype they do indeed revert to the GD state and they thereby become more responsive to environmental influences which can further regulate the integrated control of cellular proliferation and differentiation.

Hormonal regulation of the transformation phenotype in simian virus 40-transformed rat embryonic preadipose cell lines

The regulation of transformed phenotypes was studied in newly isolated preadipose cell lines which were established after infection with simian virus 40 tsA58 dl2009. The clonal cell lines isolated exhibited most of the characteristics typical of transformed cells. The transformants, however, were able to differentiate into adipocytes in the presence of low calf serum (0.5%) and a combination of several hormones, including hydrocortisone and insulin. Treatment with insulin alone stimulated the growth of these cells but did not induce lipid accumulation without added hydrocortisone. The effect of hydrocortisone was accompanied by a restoration of growth control in the transformants after they reached high cell density. The blot hybridization analysis of cellular DNAs digested by restriction enzymes revealed that simian virus 40 genomes were integrated at multiple separate sites at which a head-to-tail oligomeric insertion took place. Large T antigen was synthesized in growing cells but was regulated at high cell density when cells were committed to differentiate by glucocorticoids. These results suggest that the glucocorticoid hydrocortisone is capable of restoring growth regulation at high cell densities to simian virus 40-transformed preadipose cell lines.

Hormonal regulation of the transformation phenotype in simian virus 40-transformed rat embryonic preadipose cell lines

The regulation of transformed phenotypes was studied in newly isolated preadipose cell lines which were established after infection with simian virus 40 tsA58 dl2009. The clonal cell lines isolated exhibited most of the characteristics typical of transformed cells. The transformants, however, were able to differentiate into adipocytes in the presence of low calf serum (0.5%) and a combination of several hormones, including hydrocortisone and insulin. Treatment with insulin alone stimulated the growth of these cells but did not induce lipid accumulation without added hydrocortisone. The effect of hydrocortisone was accompanied by a restoration of growth control in the transformants after they reached high cell density. The blot hybridization analysis of cellular DNAs digested by restriction enzymes revealed that simian virus 40 genomes were integrated at multiple separate sites at which a head-to-tail oligomeric insertion took place. Large T antigen was synthesized in growing cells but was regulated at high cell density when cells were committed to differentiate by glucocorticoids. These results suggest that the glucocorticoid hydrocortisone is capable of restoring growth regulation at high cell densities to simian virus 40-transformed preadipose cell lines.

Mechanisms for the initiation and promotion of carcinogenesis: a review and a new concept

Carcinogenesis in humans is a multistage process, and the two major stages have been designated initiation and promotion. Although the biochemical basis for initiation and promotion remains to be established, recent research has provided important insights into potentially significant biologic mechanisms. These data are reviewed, and a new concept of carcinogenesis is presented. This concept suggests that the initiation of carcinogenesis may result from cellular immortalization and the development of defects in the integrated control of stem cell proliferation and differentiation and that the promotion of carcinogenesis may result when such initiated stem cells develop aberrant autoregulatory growth-control properties.

Induction of differentiation in HL60 leukaemic cells: a cell cycle dependent all-or-none event

The human leukaemia cell line (HL60) shows a limited capacity to differentiate spontaneously, but this property can be greatly enhanced by chemical inducers. Sodium butyrate induced differentiation in virtually 100% of HL60 cells over a four-day interval to cells with multiple phenotypic markers of monocytes. Clonogenic analysis in agar demonstrated that differentiated cells (either spontaneous or induced) irreversibly lost clonogenic potential. This appeared to be an all-or-none process with unaffected cells exhibiting unaltered clonogeneity. A study of the kinetics of colony formation showed that most, if not all, cells completed one division in the presence of butyrate and sometimes several divisions before loss of proliferative potential. Despite the uniform spectrum of cell cycle states present in HL60 cultures when butyrate was added, all differentiated cells were shown to be arrested in G1. Evidence was obtained suggesting that the 'switch' into the differentiation pathway occurred during a restricted stage of the cell cycle, either late in the cycle (G2-M) or early in G1.

Efficient differentiation of proadipocyte stem cells on nonadherent surfaces: evidence for differentiation without DNA synthesis

The differentiation of low density BALB/3T3 T proadipocytes that are cultured in standard tissue culture flasks can be induced by heparinized medium containing human plasma. It has been shown that under these conditions, cells first growth-arrest at a distinct state in the G1 phase of the cell cycle, designated GD, and thereafter differentiate within 8 to 12 days. In this paper, we report that the kinetics of proadipocyte differentiation can be significantly accelerated by culture of cells in differentiation-promoting medium on non-adherent surfaces, such as agarose-coated plates or bacteriological Petri dishes. Data also show that in a nonadherent microenvironment extensive differentiation can occur in the absence of DNA synthesis. This was established most convincingly by the demonstration that placement of mitotic cells in heparinized medium containing human plasma and hydroxyurea on agarose-coated Petri dishes induced 70-80% of the cells to GD arrest and differentiate without traversing the S phase of the cell cycle. It is concluded that under appropriate microenvironmental conditions metabolic events that occur solely in the late M or early G1 phase of the cell cycle can mediate the integrated control of proadipocyte proliferation and differentiation.

Differentiation in mouse melanoma cells: initial reversibility and an on-off stochastic model

Various proposals that a stochastic event, "commitment," is the first and rate-limiting step in mammalian cell differentiation were tested in one cell type, B16C3 mouse melanoma cells. Differentiation (pigment production) was observed in time-lapse films and in cloned single cells. As predicted by all the theories, onset of differentiation was at widely variable times in different cells after stimulation; and selection experiments showed that little of the variability was genetic. Contrary to some theories, differentiation appeared unrelated to cell division. Two properties of the melanoma cells did not fit any of the theories: times of differentiation were highly correlated in sister cells; and differentiation could be reversed in a proportion of cells, which was highest at the lowest levels of pigmentation. Dedifferentiation was associated with cell proliferation, so that most pigmented clones were small and most unpigmented clones large. These findings are accommodated by a model in which functions associated with differentiation can switch on and off, but an inhibition of the off transition builds up in the on state.

Spontaneous and agonist-induced 86Rb efflux from rabbit aortic smooth muscle cells in culture: a comparison with fresh tissue

Potassium efflux was measured in rabbit aortic strips and smooth muscle cells cultured from them by monitoring the release of isotope from preparations preloaded with 86Rb. The basal rate of 86Rb efflux from rabbit subcultured aortic smooth muscle cells was eightfold higher than from freshly isolated strips, but calculations of reuptake of isotope in the tissue indicated that the measured rate constant for efflux from aortic strips underestimated the true rate by about fourfold. The rate constant for efflux from freshly dispersed cells was less than half that of subcultured cells and remained unchanged for 5 days in culture. It then rose and by around day 10 had reached the value for subcultured cells. The increase in efflux coincided with the onset of cell division. The increased rate of efflux was accompanied by an increased rate of uptake so that the internal potassium content of the cells remained constant. Heparin decreased the efflux of 86Rb from subcultured cells to that of freshly isolated cells concomitant with a reduction in the rate of proliferation. The onset of cell division and increased basal efflux of potassium was associated with a loss of responsiveness to noradrenaline and histamine as assessed by monitoring 86Rb efflux, although depolarising solutions of potassium chloride were still able to elicit a response. Responsiveness to noradrenaline and histamine could be restored by the addition of heparin. These results suggest that the lack of responsiveness of subcultured cells is not due to irreversible dedifferentiation but to a reversible loss in proliferating cells of receptors for vasoactive agents or of a coupling mechanism between receptor occupation and ion gating.

Differential mitogenic effects of methyl isobutyl xanthine and a tumor growth factor on G1-arrested 3T3 T proadipocytes at the predifferentiation GD state and the growth-factor deficiency GS state

Two growth-states exist in the G1 phase of the 3T3 T proadipocyte cell cycle. GD is the arrest state at which proadipocytes must growth-arrest prior to differentiation. GS is the arrest state at which proadipocytes growth-arrest following deprivation of serum or growth factors. In an attempt to further distinguish these arrest states, we have compared the relative ability of a variety of mitogens to induce GD- and GS-arrested cells to initiate DNA synthesis. The data show that GD-arrested cells at both high and low densities can be induced to proliferate by methyl-isobutyl-xanthine (MIX), whereas high and low density GS-arrested cells are not. The data also show that a tumor growth factor can stimulate the proliferation of both high and low density GD- and GS-arrested cells, whereas other agents are poor mitogens for high density GD-arrested cells. We conclude that MIX and a tumor growth factor (TUGF) can serve as probes to study the characteristics of the GD arrest state.