Diffusible factor(s) controlling density inhibition of 3T3 cell growth: a new approach

Characteristics of skeletal muscle in mdx mutant mice

We review the extensive research conducted on the mdx mouse since 1987, when demonstration of the absence of dystrophin in mdx muscle led to X-chromosome-linked muscular dystrophy (mdx) being considered as a homolog of Duchenne muscular dystrophy. Certain results are contradictory. We consider most aspects of mdx skeletal muscle: (i) the distribution and roles of dystrophin, utrophin, and associated proteins; (ii) morphological characteristics of the skeletal muscle and hypotheses put forward to explain the regeneration characteristic of the mdx mouse; (iii) special features of the diaphragm; (iv) changes in basic fibroblast growth factor, ion flux, innervation, cytoskeleton, adhesive proteins, mastocytes, and metabolism; and (v) different lines of therapeutic research.

Inhibition of proliferation in 8-week-old mdx mouse muscle fibroblasts in vitro

Our purpose is to understand why mdx muscle does not show the progressive degeneration observed in human Duchenne muscular dystrophy (DMD) muscle. In the mouse, the regenerative process compensates for the necrosis of the muscle fibers, particularly during the acute phase of the disease (5-9 weeks). In DMD muscle, there is a gradual failure of the regenerative process and the muscle fibers are replaced by connective and fatty tissue. We propose that distinct properties of mdx and DMD muscle fibroblasts could be one of the reasons for the differences between the mdx and DMD phenotypes. We found that fibroblasts taken from human DMD and control muscle had similar in vitro proliferative capacities. The proliferation rate of mouse muscle fibroblasts decreased during the acute phase of the disease, and inhibition was complete in fibroblasts from 8-week-old mdx mice. Moreover, the medium conditioned by these cells inhibited fibroblast proliferation. The effect was specific for fibroblasts, since this conditioned medium stimulated myoblast proliferation, as did control fibroblast-conditioned medium. These results suggest that 8-week-old mdx mouse muscle fibroblasts produce an inhibitor of their own proliferation and a growth factor specific for myoblasts in vitro. If these factors are secreted in vivo, the growth inhibitory factory may stop fibroblast proliferation whereas the mitogenic activity could stimulate satellite cell proliferation, thus favouring muscle regeneration.

Modulation by the src oncogene of the effect of inhibitory diffusible factor IDF45

Density-dependent inhibition of growth has been assumed to be under the control of inhibitory molecules diffusing from dense cell cultures. Growth inhibitory factors have been fractionated or purified from medium conditioned by different cell types. In the present work, it was shown that IDF45 (inhibitory factor diffusing from 3T3 cells) decreased DNA synthesis in chick embryo fibroblasts (CEF) and was an inhibitor of CEF growth; this inhibition was reversible. Since similitudes between oncogene products and growth factors have been observed, it was of interest to compare the inhibitory effect of IDF45 upon the stimulation of DNA synthesis induced either by serum or by pp60-src. CEF infected by Ny68 virus (a mutant of Rous sarcoma virus ts for the expression of transformation) were density-inhibited at 41 degrees C, but were stimulated at this temperature by addition of 1% serum. This stimulation was 94% inhibited by IDF45. The same Ny68-infected cells could also be stimulated by transfer to 37 degrees C, the permissive temperature (in the absence of serum). The stimulation of DNA synthesis by src expression was poorly inhibited by IDF45. From our results, it appears that oncogene expression in CEF induces a loss in their sensitivity to IDF45. This would explain why transformed cells escape DDI of growth.

Growth inhibitors in serum, platelets, and normal and malignant tissues

The non-malignant epithelial cell line from Buffalo rat liver (BRL) and its malignant transformant obtained by Rous sarcoma virus infection (RSV-BRL) were used as the indicators. A survey was made in animals for growth inhibitors which could inhibit the growth of BRL, but not the growth of RSV-BRL in culture, under the consideration that the loss of the sensitivity to such inhibitors might be the most dominant cause for acquisition of tumorigenicity. From human platelets, a growth inhibitor was extracted with acid-ethanol and prepared to a homogeneous purity, separated from the major TGF-beta activity, which stimulated the anchorage-independent growth of NRK cells in the presence of EGF. The purified inhibitor (Mr 25-27k; pI 8.7-9.1) showed 50% inhibition on the growth of BRL at 20 pg/ml (0.7 pM), but hardly influenced the growth of RSV-BRL. Its inhibition was specific to epithelial cell lines, thus called epithelial growth inhibitor (EGI). It consisted of two subunits with practically identical Mr, which were cross-linked with disulfide bonds. Its sequence of N-terminal 10 amino acids was consistent with that of TGF-beta. In rats, serum contained a growth inhibitor with Mm 220k. The inhibitor, when treated with high concentrations of urea or guanidine-HCl, liberated an active component with Mr 30k and pI 5.2. When the growth inhibitor with Mm 220k was treated with ethanol-acetone or 1 M acetic acid, its activity increased more than 10-fold. Growth inhibitors were extracted with acid-ethanol from various tissues of rats. The activities of the extracts to inhibit the growth of BRL cells were high in spleen, lung and tumors with relatively high mitotic activities, but low in muscle, heart and brain. Acid-ethanol extract from BRL cells showed growth inhibitory activity toward BRL cells themselves significantly higher than that from RSV-BRL cells. The serum-free media conditioned with BRL and RSV-BRL cells showed a little growth inhibitory activity toward BRL cells. On treatment with acetic acid, their activities increased significantly, comparable for both types of cells. The media thus activated inhibited the growth of BRL cells, but not that of RSV-BRL cells. All the growth inhibitors described above including human platelet inhibitor, whether present in extracts or purified, resembled one another in stability against acid, heat and reducing reagents.

Purified epidermal pentapeptide inhibits proliferation and enhances terminal differentiation in cultured mouse epidermal cells

Skin extracts contain an epidermal mitosis inhibitor that recently has been purified and identified as a pentapeptide. To develop an in vitro assay system for further biologic characterization, primary mouse epidermal cells and an established mouse epidermal cell line (line 308) were used for testing of the purified pentapeptide. In primary cell cultures the mitotic activity, as estimated by means of vinblastine, was reversibly inhibited by 44% at a peptide concentration of 10(-8) M in high-calcium (1.2 mM Ca++), and by 27-38% at peptide concentrations of 10(-10) and 10(-8) M in low-calcium (0.02 mM Ca++) medium. The 308 cells were inhibited by 46% at a peptide concentration of 10(-6) M but only after the cells had reached near-confluence and had a moderate rate of proliferation. A low concentration of adrenaline (0.18 micrograms/ml) in the medium rendered the primary cultures more sensitive to the peptide. After repeated peptide treatments over 24 h, the number of cornified envelopes (a marker of terminal differentiation) was increased both in primary cultures and in the 308 cells. The epidermal pentapeptide thus seems to influence both proliferation and terminal differentiation in cultured mouse epidermal cells.

Inhibitory diffusible factor IDF45, a G1 phase inhibitor

An inhibitory diffusible factor of 45 kDa (IDF45) was isolated from medium conditioned by dense cultures of 3T3 cells. The procedure involved Bio-Gel P150 chromatography and 2 reverse-phase FPLC. After the final step of purification, 60 ng/ml of IDF45 inhibited 50% of alpha-globulin-stimulated DNA synthesis. It was shown that IDF45 acted in the G1 phase of the cell cycle. When added for 8 h in the G1 phase of the cell cycle, it was able to inhibit DNA synthesis in the S phase which followed this G1 phase. Furthermore, IDF45 inhibited the early stimulation of RNA synthesis induced by alpha-globulin.

Cell cycle controls in higher eukaryotic cells: resting state or a prolonged G1 period?

We express the viewpoint that control over cell growth in higher eukaryotes is achieved predominantly by regular transition of cells from proliferation to rest and vice versa as a result of coordinated interrelationship between intracellular growth inhibitors and extracellular growth factors. The resting state is considered as a special physiological state of a cell where the prereplicative reactions necessary for the onset of DNA synthesis are inhibited. Cells pass into a resting state at each successive cell cycle, with regard to the next cycle, once the threshold level of growth inhibitors has been attained. Cellular rest may thus initiate and proceed in parallel with conventional periods of the cell cycle but in a hidden way. Its termination strictly depends on the appropriate concentration of extracellular growth factors. In the absence of growth factors cells, after completing mitosis, pass into an overt state of rest metabolically different from any period of the cell cycle including G1.

Direct effect of parathyroid hormone on the proliferation of osteoblast-like cells; a possible involvement of cyclic AMP

Serum-starved chick osteoblast-like cells (OB cells) and periosteal fibroblasts (PF cells) were used to study the proliferative effects of parathyroid hormone (PTH) and prostaglandin E2 (PGE2). Both PTH (10(-11) to 10(-8) M) and PGE2 (10(-9) to 10(-5) M) had a direct, dose-related effect on the de novo synthesis of DNA in OB cells. The PF cells only showed a dose-dependent effect in the presence of PGE2 (10(-9) to 10(-5) M). The hormonally induced proliferation of these cells was shown to be dependent on cell density and stimulation time. An optimal response for both cell types was observed in the cell density range 1.5 to 3.5 micrograms DNA/2 cm2, when stimulated for 18 hours. As cAMP-enhancing substances (N6-dBcAMP, forskolin and IBMX) could mimic the PTH- and PGE2-induced proliferation in OB cells, the increased DNA synthesis was concluded to be mainly caused by enhanced cAMP concentrations.

Regulation of cell proliferation inhibitory and stimulatory factors diffused by 3T3 cultured cells

The growth rate of normal cells multiplied in vitro decreases as the cell density of the culture increases. Previous results suggested that this density-dependent inhibition of growth in nontransformed cells was due to the diffusion of growth inhibitory substances in the medium of dense cultures. In this paper, we demonstrate that dense cultures of 3T3 cells secrete inhibitory and stimulatory factors. Macromolecules of conditioned medium were fractionated on Biogel P150 and the different fractions were tested on quiescent cultures of 3T3 cells stimulated or not to proliferate by addition of alpha globulin. When target cells were not stimulated to proliferate by addition of exocrine growth factors, we observed the inhibitory activity of a large molecular weight inhibitor (IDF45) and the stimulatory activity of autocrine growth factors (fraction about 35 and 10 K molecular weight), on the incorporation of 14C inosine into nucleotide pool and RNA. However, DNA synthesis was significantly stimulated with fraction 10 K only. This discrepancy between the stimulation of RNA and DNA synthesis may be explained by the presence, simultaneously, of inhibitory and stimulatory factors in fraction 35 and 10 K molecular weight. The presence of inhibitory factor was demonstrated when the fractions were tested on target cells stimulated to proliferate by alpha globulin addition and labeled with 14C thymidine. In these conditions, the stimulatory activity of autocrine growth factors was not observable, and only the inhibitory activity on DNA synthesis of fractions 35 and 10 K appeared. It is tempting to assume that the regulation of in vitro cell proliferation is determined by the balance between these antagonist stimulatory and inhibitory autocrine growth factors.

Relationship of finite proliferative lifespan, senescence, and quiescence in human cells

Cell hybrids were formed between human diploid fibroblasts (HDF) and carcinogen-transformed HDF to determine the relationship among: (1) finite proliferative lifespan, which we define as an age-related failure of a population to achieve one population doubling in 4 weeks; (2) arrest in a senescent state, which we define as cessation of DNA synthesis in a viable culture that is at the end of its lifespan by the above definition; and (3) arrest in a quiescent state, which we define as cessation of DNA synthesis in a young culture that is crowded or mitogen-deprived. HDF express all three of these phenotypes, which we have abbreviated FPL+, S+, and Q+, respectively. Carcinogen-transformed HDF are transformed to immortality (FPL-) and inability to achieve quiescence (Q-). They have no S phenotype because, by definition, this phenotype only exists in FPL+ cells. Fusion of FPL+, Q+, S+ HDF X FPL-, Q- carcinogen-transformed HDF produced hybrid clones that were FPL+, Q-, and S-, where the S- phenotype means that individual cells continued to synthesize DNA in cultures that had reached the end of their lifespan by our definition. These results are consistent with our hypothesis that senescent HDF and quiescent HDF may share a common mechanism for arrest in G1 phase. We have suggested that this could occur if the aging mechanism that is responsible for the FPL+ phenotype is a progressive decrease in the ability of cells to recognize or respond to mitogenic growth factors. If so, then cells would become physiologically mitogen-deprived at the end of their lifespan, which would cause them to arrest in the senescent state by the same mechanism that causes young cells to arrest in the quiescent state when they are mitogen-deprived. This hypothesis predicts that the FPL+ phenotype can be separated from the S+ phenotype--i.e., FPL+ cells can be S+ or S- --and that the Q and S phenotypes are linked--i.e., FPL+ cells are either Q+ and S+ or Q- and S-. Both these predictions are supported by the present data.

Growth-inhibitory activity of lymphoid cell plasma membranes. II. Partial characterization of the inhibitor

We have shown that plasma membranes from lymphoid cells have inhibitory activity for the growth of normal lymphocytes and lymphoid tumor cells (Stallcup, K. C., A. Dawson, and M. F. Mescher, J. Cell Biol. 99:1221-1226). This growth-inhibitory activity has been found to co-purify with major histocompatibility complex class I antigens (H-2K and D) when these cell surface glycoproteins are isolated from detergent lysates of cells by affinity chromatography on monoclonal antibody columns. When incorporated into liposomes, the affinity-purified H-2 antigens inhibited the growth of both normal lymphocytes and tumor cells at concentrations of 1-3 micrograms/ml. Inhibition was readily reversed upon removal of the liposomes from the cell cultures, even after several days of exposure of cells to the inhibitor. Inhibitory activity was insensitive to protease digestion or heat treatment, indicating that it was not due to the H-2 glycoproteins. This was confirmed by the demonstration that inhibitory activity could be separated from the H-2 protein by gel filtration in the presence of deoxycholate and could be extracted from membranes or H-2 antigen preparations with organic solvents. The results demonstrate that the growth-inhibitory component(s) of the plasma membrane is a minor lipid or lipid-like molecule which retains activity in the absence of other membrane components. The findings reported here and in the preceding article suggest that this novel membrane component may have a role in control of lymphoid cell growth, possibly mediated by cell contacts.

Density-dependent inhibition of growth: inhibitory diffusible factors from 3T3- and Rous sarcoma virus (RSV)-transformed 3T3 cells

We recently fractionated, from the culture medium of 3T3 cells, a thermolabile inhibitory diffusible factor (IDFN) with a molecular weight of about 40,000 daltons, which decreased nucleic acids synthesis of stimulated target 3T3 cells. In the present publication the inhibitory activities of IDFN (produced by 3T3 cells) and IDFT (produced by RSV-transformed 3T3 [3T3 SRA/H] cells) on 3T3 and 3T3 SRA/H cells have been compared. The inhibitory activity of IDFN decreased (by a mean of 57%) when it was tested on transformed instead of 3T3 cells. On the other hand, IDFT was able to decrease 14C-inosine incorporation in target 3T3 cells. However, the inhibitory activity of IDFT decreased (by mean 50%) when tested on 3T3 SRA/H instead of 3T3 cells. Therefore, transformed cells produced an inhibitory factor but were less sensitive than 3T3 cells to its inhibitory activity. The inhibitory activity of IDFT on 3T3 SRA/H cells was only 20% of the inhibitory activity of IDFN on 3T3 cells. This appreciable difference is of particular interest, since it could explain the release of density-dependent inhibition of growth (DDI) in transformed 3T3 SRA/H cells. Furthermore, it provides more evidence for the hypothesis that, in 3T3 cells, DDI of growth is due to the release of an inhibitory molecule into the medium, and that IDFN is in fact, the inhibitory molecule involved in this phenomenon.

Properties of a cell growth inhibitor produced by mouse embryo fibroblasts

Secondary mouse embryo fibroblasts produce a growth inhibitor with the character of a thermolabile, nondialysable protein. The inhibitor was harvested from conditioned medium, and following G-75 Sephadex fractionation it was isolated in one peak which consisted of two fractions eluting at approximately two thirds of the bed volume of the column where approximately 80 percent of the original activity was recovered with an increase in specific activity of about tenfold. Polyacrylamide gradient gel electrophoresis of fractions from L-[35S] methionine-labelled conditioned medium showed that the two fractions with growth inhibitory activity contained some 4-5 bands and shared the two major components. Cell cycle studies showed that the growth inhibitory effect was exerted after addition during early and late G1 and during S phase, and morphological studies showed that where growth was inhibited the morphological expression of the cells was altered.

Growth control in cultured 3T3 fibroblasts II. Molecular properties of a fraction enriched in growth inhibitory activity

Treatment of sparse, proliferating cultures of 3T3 cells with medium conditioned by exposure to density-inhibited 3T3 cultures resulted in an inhibition of growth and division in the target cells when compared to similar treatment with unconditioned medium. This growth inhibitory activity was fractionated by ammonium sulfate precipitation and gel filtration, yielding one fraction that was 35-fold enriched in specific activity. Analysis of the chemical and biological properties of this highly active fraction indicated that: (a) it is an endogenous cell product, synthesized by the 3T3 cells and shed into the medium; (b) it is a protein and its activity is sensitive to treatment with pronase; (c) the constituent polypeptide chains have molecular weights of 10,000 and 13,000; and (d) it is not cytotoxic and its effect on target cells are reversible. These results suggest that we have partially purified from conditioned medium an endogenous growth regulatory factor that may play a role in density-dependent inhibition of growth in cultured fibroblasts. We propose the term Fibroblast Growth Regulator to describe this class of molecules.

Density-dependent regulation of cell growth: an example of a cell-cell recognition phenomenon

Cell-to-cell contact can result in a variety of changes in the cell's physiology. For different cell types, this may include both the initiation as well as the cessation of cell growth and changes in the state of differentiation. This review examines in detail one such phenomenon, density-dependent inhibition of growth, which is observed with many fibroblasts in culture. Data are summarized which demonstrate that the cessation of growth at high cell density is in part a consequence of cell-to-cell contact. An approach to the study of the molecular basis of this phenomenon is presented based on the demonstration that plasma membranes, when bound to sparse growing cells, mimic contact inhibition of growth. The present status of attempts to purify plasma membrane proteins responsible for this effect are summarized, and the properties of these membrane proteins are compared to those of previously described "soluble" proteins that inhibit cellular growth.

Further evidence for an inhibitor of proliferation elaborated by normal human fibroblasts in culture: partial characterization of the inhibitor

We present evidence that extends our earlier preliminary report on the stimulatory effect of saline washes on confluent cells in culture (Lacey et al., '77). That work suggested that an inhibitory substance was being removed by the washes. The present work suggests that the inhibitor is in the 10,000-30,000 MW range, is reversibly bound, is cationic and is also a protease inhibitor. It is heat stable, but is apparently degraded with time in our experimental systems.

Age-dependent decrease of growth responsiveness in density inhibited 3T3 cell populations and their interactions with SV 40-3T3 cells

An aging process has been detected in stationary 3T3 cell cultures, especially in the presence of plasma-derived serum (PDS) from adult bulls. It leads to irreversible conversion of an increasing percentage of initially responsive cells of a stationary population into cells unresponsive to growth stimulation by newborn calf serum (NBCS) or reseeding at low cell density in the presence of NBCS. These unresponsive cells are viable in the sense that, following trypsinization, they reattach and spread on a new culture plate and can be maintained for many days. The conversion process is accelerated by increasing PDS concentration. It is antagonized by NBCS. It is accompanied by enhancement of growth-inhibiting interactions exerted by stationary 3T3 cell populations on SV 40-3T3 cells.

Serum-dependent regulation of alpha-aminoisobutyric acid uptake in bovine granulosa cells

The removal of serum from the medium of ovarian granulosa cells in exponential or confluent stages of growth results in a rapid and pronounced decrease in the rate of transport of the non-metabolizable amino acid, alpha-aminoisobutyric acid. This decrease is rapidly and completely reversed by the addition of serum. The decrease and its reversal are insensitive to inhibitors of RNA and protein syntheisis and are unaffected by a number of other metabolic inhibitors. The serum requirement cannot be replaced by peptide hormones known to stimulate cell division and secretion by these cells. These data are consistent with a model of post-translational control of AIB transport by a high-molecular-weight component of serum.