The role of increased proteolysis in the atrophy and arrest of proliferation in serum-deprived fibroblasts

When cultured fibroblasts are deprived of serum, the degradation of long-lived proteins and RNA increases, the cells stop proliferating, and they decrease in size. To determine the role of the increased protein catabolism in these responses, we studied the effects of inhibitors of intralysosomal proteolysis in Balb/c 3T3 cells. When these cells were placed in serum-deficient medium (0.5% serum), the rate of degradation of long-lived proteins increased about twofold within 30 min. This increase was reduced by 50-70% with inhibitors of lysosomal thiol proteases (Ep475 and leupeptin) or agents that raise intralysosomal pH (chloroquine and NH4Cl). By contrast, these compounds had little or no effect on protein degradation in cells growing in 10% serum. Thus, in accord with prior studies, lysosomes appear to be the site of the increased proteolysis after serum deprivation. When 3T3 cells were deprived of serum for 24-48 hours, the rate of protein synthesis and the content of protein and RNA and cell volume decreased two- to fourfold. The protease inhibitor, Ep475, reduced this decrease in the rate of protein synthesis and the loss of cell protein and RNA. Cells deprived of serum and treated with Ep475 for 24-48 hours had about twice the rate of protein synthesis and two- to fourfold higher levels of protein and RNA than control cells deprived of serum. The Ep475-treated cells were also about 30% larger than the untreated cells. Thus, the protease-inhibitor prevented much of the atrophy induced by serum deprivation. The serum-deprived fibroblasts also stopped proliferating and accumulated in the G1 phase of the cell cycle. The cells treated with Ep475 accumulated in G1 in a manner identical to untreated serum-deprived cells. Other agents which inhibited protein breakdown in serum-deprived cells also did not prevent the arrest of cell proliferation. Thus the enhancement of proteolysis during serum deprivation appears necessary for the decrease in size and protein synthesis, but probably not for the cessation of cell proliferation. When cells deprived of serum in the presence or absence of Ep475 were stimulated to proliferate by the readdition of serum, the larger Ep475-treated cells began DNA synthesis 1-2 hours later than the smaller untreated cells. Thus, after treatment with Ep475, the rate of cell cycle transit following serum stimulation was not proportional to the cell's size, protein, or RNA content, or rate of protein synthesis.     

Serum-dependent control of entry into S phase of next generation in rat 3Y1 fibroblasts. Effect of large T antigen of simian virus 40

When rat 3Y1 fibroblasts are deprived of serum in S phase and/or G2 phase in the first generation, the cells delay entry into S phase in the second generation for the duration of the serum deprivation. We can now show that when resting 3Y1 cells are infected with Simian virus 40 (SV40), the removal of serum through S and G2 phases in the first generation does not markedly delay entry into S phase in the second generation. These observations suggest that the serum-dependent control of entry into S phase of the second generation continues from the first generation, and that the abolition of this control by infection with SV40 in the first generation involves the mechanism operative when the resting cells are stimulated to enter S phase (of the first generation) by infection with SV40.     

Elevation of intracellular glutathione content associated with mitogenic stimulation of quiescent fibroblasts

The relationship between total glutathione (GSH) content and cell growth was examined in 3T3 fibroblasts. The intracellular GSH level of actively growing cultures gradually decreases as these cells become quiescent by either serum deprivation or high cell density. Upon mitogenic stimulation of sparse, quiescent (G0/G1) cultures with serum, there is a rapid 2.3-fold elevation in intracellular GSH levels which is maximal by 1 h and returns to baseline by 2 h. This is followed by a more gradual increase in GSH content as cells enter the S phase. In addition, the elevation in GSH content is required for maximum induction of DNA synthesis. Treatments that prevent the early increase in intracellular GSH levels do not affect protein synthesis but result in a reversible dose-dependent decrease in the percent of cells capable of entering S phase. These results indicate that GSH may be important in the regulation of cellular proliferation.     

Effects of serum deprivation on the initiation of DNA synthesis in the second generation in rat 3Y1 cells

Rat 3Y1 cells arrested at early S by hydroxyurea traversed the remainder of S and G2 and completed mitosis after removal of the drug, irrespective of the absence of serum from the culture medium. When cells were deprived of serum for a period between early S and mitosis after removal of hydroxyurea, the cells delayed entry into S in the presence of serium in the second generation for the time length approximately equal to that of serum deprivation. When mitotic cells, which had been continously exposed to serum after removal of hydroxyurea, were deprived of serum for the next 24 hours and then were reexposed to serum, the cells delayed entry into S for more than 24 hours (more than the time length of serum deprivation). On the other hand, the cells already deprived of serum between early S and G2 in the first generation were less delayed in entry into S after postmitotic 24-hour serum deprivation than were the cells exposed to serum between early S and G2 in the first generation. These results suggest that serum-dependent events continue to occur in the first generation for on-time entry into S in the next generation, and that these premitotic events (the potential for entry into S) decay if serum is absent for a long period of time after mitosis.     

Regulation of p27Kip1 by intracellular iron levels

Enhanced intracellular iron levels are essential for proliferation of mammalian cells. If cells have entered S phase when iron is limiting, an adequate supply of deoxynucleotides cannot be maintained and the cells arrest with incompletely replicated DNA. In contrast, proliferating cells that are not in S phase, but have low iron pools, arrest in late G1. In this report the mechanism of iron-dependent G1 arrest in normal fibroblasts was investigated. Cells were synchronized in G0 by contact inhibition and serum deprivation. Addition of serum caused the cells to re-enter the cell cycle and enter S phase. However, if the cells were also treated with the iron chelator deferoxamine, S phase entry was blocked. This corresponded to elevated levels of the cyclin dependent kinase inhibitor p27^Kip1 and inhibition of CDK2 activity. Expression of other cell cycle regulatory proteins was not affected, including the induction of cyclins D1 and E. When the quiescent serum starved cells were supplemented with a readily usable form of iron in the absence of serum or any other growth factors, a significant population of the cells entered S phase. This was associated with downregulation of p27^Kip1 and increased CDK2 activity. Using an IPTG-responsive construct to artificially raise p27^Kip1 levels blocked the ability of iron supplementation to promote S phase entry. Thus it appears that p27^Kip1 is a mediator of G1 arrest in iron depleted Swiss 3T3 fibroblasts. We propose that this is part of an iron-sensitive checkpoint that functions to ensure that cells have sufficient iron pools to support DNA synthesis prior to entry into S phase.     

Resumption of cell cycle in BALB/c-3T3 fibroblasts arrested by polyamine depletion: relation with "competence" gene expression

Serum deprivation arrests BALB/c-3T3 fibroblasts (clone A31) in G0 phase, where resumption of the cell division cycle can be induced by addition of serum or of specific growth factors in a defined sequence: PDGF (inducer of a state of "competence," characterized by the expression of a family of genes including c-myc), epidermal growth factor EGF and IGF1 (Leof et al., 1982, 1983). When exponentially growing A31 cells are placed for greater than or equal to 2 days in a medium containing the alpha-difluoromethylornithine (alpha DFMO), an irreversible inhibitor of ornithine decarboxylase, they become arrested in G1 phase as a consequence of polyamine depletion (Medrano et al., 1983). In the alpha DFMO-arrested cells, addition of putrescine (60 microM) in a culture medium containing 6% fetal calf serum (FCS), but not in serum-free medium, is sufficient to induce G1 progression and entry into S phase (as determined by 3H-thymidine incorporation). The level of "competence" mRNAs is high in alpha DFMO-arrested cells. After addition of putrescine in FCS-containing medium, these mRNAs continue to be present for at least 3 h. A large proportion of alpha DFMO-arrested cells can be induced to progress to S phase by insulin (1 microM, acting via IGF1 receptor) plus putrescine in a serum-free medium (greater than or equal to 50% of FCS effect). In this case, the levels of "competence" mRNAs become low or undetectable within 3 h, EGF (10 nM) plus insulin had only slightly greater effect than insulin alone on the progression of alpha DFMO-arrested cells. When the alpha DFMO-arrested cells are subsequently incubated during 3 days in a low-serum-containing medium (0.25% FCS), they do not replenish their supply of polyamines, and then continue to express the c-myc gene. The recruitment of the polyamine-depleted, serum-deprived cells into the cell division cycle does not require PDGF and can be induced by addition of EGF and insulin plus putrescine. These data indicate that alpha DFMO arrests majority of the cells at a point situated beyond the PDGF- and EGF-dependent portion of G1 phase. During the subsequent serum deprivation, the alpha DFMO-arrested cells remain "competent" (PDGF-independent), probably as a consequence of their continued expression of c-myc (and possibly other PDGF-inducible genes).     

Fibroblast growth factor induces a transient net K+ influx carried by the bumetanide-sensitive transporter in quiescent BALB/c 3T3 fibroblasts

The bumetanide-sensitive transport system performed a net efflux of K+ in serum deprived quiescent cells. The addition of partially purified fibroblast growth factor (FGF) to G0/G1 phase 3T3 fibroblasts induced a transient net influx of K+, carried out by the bumetanide-sensitive transport system for 2-6 minutes. The stimulation of the bumetanide-sensitive K+ influx by FGF was followed by stimulation of the ouabain-sensitive K+ influx. In addition, both the bumetanide-sensitive and the ouabain-sensitive K+ influxes were found to be similarly stimulated when the G0/G1 3T3 cells were treated with insulin. These results suggest that growth factors such as FGF and insulin induce a change in the action of the bumetanide-sensitive transporter from performing net K+ efflux along its concentration gradient to an uphill transport pumping of K+ into the cell. We propose, therefore, that the bumetanide-sensitive transporter contributes to the increase in the intracellular K+ (and probable Na+) stimulated by growth factors such as FGF and insulin in early G1 phase of the cell cycle.     

The suppression of cellular proliferation in SV40-transformed 3T3 cells by glucocorticoids

Glucocorticosteroids, when added two hours after cell plating to SV40-transformed, 3T3 mouse fibroblasts in low serum (0.3% v/v), biotin-supplemented medium, suppress cellular proliferation by 24 hours. While some cell death probably occurs, the growth inhibition is not primarily due to cytotoxicity and cytolysis. This conclusion is supported by the following: 1) both dead and viable cell numbers are suppressed, 2) little cell debris is evident in the medium, and 3) very high concentrations of glucocorticoids do not cause an increase in the dead cell count. Furthermore, this growth suppression, which is specific for glucocorticoids since several non-glucocorticoid steroids have no inhibitory effect, is not permanent nor irreversible. Removal of the glucocorticoid and replacement with 10% serum restore rapid proliferation. Although higher concentrations (1% and 10%) of serum afford some protection against glucocorticoid inhibition, this protection is not simply a consequence of faster growth rates. SV3T3 cells can be grown in serum-free medium supplemented with biotin, transferrin, insulin, and epidermal growth factor (EGF). Under these conditions growth rates are comparable to high serum media, yet glucocorticoids are still powerful inhibitors. However, the omission of insulin from serum-free, glucocorticoid cultures does result in observable cell death and lysis. Flow microfluorometry and autoradiographic studies have determined that glucocorticoid-inhibited cells are partially blocked in G1. The proportions of S phase and G2 + M cells are greatly reduced with an accompanying accumulation of G1 cells. These results suggest that glucocorticoids regulate a biochemical step(s) in G1 which is critical for DNA initiation.     

Epidermal growth factor induces rapid centrosomal separation in HeLa and 3T3 cells

Using indirect immunofluorescence, we have found that epidermal growth factor (EGF), at 100 ng/ml, induces centrosomal separation within 20 min in HeLa and 3T3 cells. The effect was evident both in unsynchronized cultures and in HeLa cells blocked in early S phase by hydroxyurea. EGF also induced centrosomal separation in quiescent 3T3 cells blocked in G0/G1 by serum deprivation, indicating that DNA replication is not necessary for this effect. The mechanism of this rapid centrosomal separation and its role in the mitogenic effects of EGF remains to be determined.     

Mitogenic response to epidermal growth factor (EGF) modulated by platelet-derived growth factor in cultured fibroblasts

The mitotic effects of epidermal growth factor (EGF) were investigated in two cultured fibroblast lines, BALB/c-3T3 and C3H 10T1/2 cells. EGF (30 ng/ml) added to quiescent 3T3 cells in medium containing either platelet-poor plasma or 10(-5) M insulin caused only minimal increases in the percentage of cells stimulated to initiate DNA synthesis. In contrast, EGF acted synergistically with either insulin or plasma to stimulate DNA synthesis in quiescent cultures of 10T1/2 cells, although the maximum effects of EGF were measured at concentrations several-fold greater than those found in either serum or plasma. In either 3T3 or 10T1/2 cells a transient preexposure to platelet-derived growth factor (PDGF) caused over a 10-fold increase in the sensitivity to the mitogenic effects of EGF. It is therefore possible that a primary action of PDGF is to increase the sensitivity of fibroblasts to EGF, independent of whether EGF alone is found to be mitogenic.     

Hormonal regulation of discrete portions of the cell cycle: commitment to DNA synthesis is commitment to cellular division

Density-arrested human fibroblasts were stimulated to traverse G0/G1 and initiate DNA synthesis by the addition of medium containing either serum or a combination of platelet-derived growth factor and platelet-poor plasma. Medium containing a combination of epidermal growth factor and high concentrations of insulin also stimulated DNA synthesis in platelet factor-treated quiescent cells. Platelet factor was required only to initiate proliferation. Epidermal growth factor and insulin then allowed G1 traverse and commitment to DNA synthesis. Cells could complete S, G2, and M in unsupplemented medium lacking peptide growth factors.     

Cultured liver epithelial cells can incorporate monoclonal antibodies directed against intracellular cytokeratin structures without microinjection

T51B rat liver cells in the exponential phase of growth were arrested in late G1 by medium calcium deprivation, exposed to mouse monoclonal anti-cytokeratin (Mr = 55,000) by addition of the antibody to the medium, induced to enter the S phase by readdition of 1.5 mM calcium, and incubated for a further 18 h. After fixing and staining with FITC-conjugated anti-mouse IgG, the cells exhibited an intact intracellular cytokeratin-staining pattern. No effect of antibody was observed on entry of cells into the S phase. These results show that rat liver-derived epithelial cells can actively take up macromolecules like cytokeratin antibodies without microinjection.     

Cell cycle-dependent morphological changes in the actin cytoskeleton induced by agents which elevate cyclic AMP

Agents which increase the intracellular concentration of cyclic adenosine-5′-monophosphate, induce a highly arborised morphology in a proportion of sub-confluent Swiss 3T3 fibroblasts. During this process the organisation of actin filaments progessively changes from a characteristic stress fibre pattern to leave a network of actin filaments within each and every arborisation. Despite this massive reorganisation of the actin cytoskeleton no changes are observed in the extent of polymerisation of actin during arborisation. The proportion of cells in asynchronous cultures undergoing arborisation at maximal concentrations of agents reaches a maximum of 30%; suggesting that the effect might be mediated only in cells during a restricted period of the cell cycle. More than 80% of serum-starved cells responded to these agents between 1 and 8 hours after readdition of serum, but not at other times, suggesting that the arborisation response can occur only in the G1 phase of the cell cycle.     

Effects of polyamine depletion on serum stimulation of quiescent 3T3 murine fibroblast cells

Numerous reports have shown that polyamines are required for cell proliferation. A current model for regulating commitment to DNA replication in cultured fibroblasts stimulated from quiescence by serum addition postulates sequential action by specific growth factors. To temporally localize polyamine-dependent steps within this defined sequence, mouse Balb/c-3T3 fibroblasts were partially depleted of polyamines by treatment with DL-alpha-difluoromethylornithine (DFMO), next rendered quiescent by serum deprivation, then stimulated by 10% serum with or without exogenous putrescine (Pu). Depletion of polyamines was verified by HPLC, and entry of cells into S phase was monitored by autoradiography. After 24 h of incubation with [3H]-thymidine, polyamine-depleted cells had labeling indices similar to quiescent cells if they were serum-stimulated without Pu, but progressed to S phase to the same degree as control cultures if polyamines were restored by adding Pu at the time of serum stimulation. These observations suggested that commitment of quiescent cells to DNA replication may require polyamines. To determine if polyamine-dependent steps occur during the pre-commitment period (up to 12 h after serum stimulation) or only in traverse of G1 (12 h to 24 h, post-commitment), polyamine-depleted quiescent cells were serum-stimulated for 12 h without Pu, then returned to low serum with Pu. Labeling indices of these cultures remained nearly as low as those of unstimulated cells. Reducing serum concentration from 10% to 0.5% at 12 h after stimulation did not effect labeling indices of control cells not depleted of polyamines by DFMO. These results supported the postulated requirement for polyamines during pre-commitment events. However, polyamine-deficient quiescent cells serum-stimulated without Pu for periods longer than 24 h had labeling indices at 36 and 48 h significantly greater than at 24 h. This suggested that polyamine depletion may decrease the rate at which quiescent cells commit to DNA replication, rather than producing an absolute blockade during the pre-commitment period.     

Arrest of 3T3 cells in G1 phase in suspension culture.

3T3 cells do not grow in Methocel suspension culture, while other permanent cell lines do. The viability of 3T3 cells in suspension remains unchanged for at least three days with respect to plating efficiency, vital staining and resumption of normal growth when transferred into monolayer culture. When monolayer 3T3 cells in G1 phase are suspended they remain in G1 phase. Cells already in S phase which are suspended complete ongoing DNA synthesis and mitosis and then are arrested in the G1 phase. Progress through the cell cycle is reinitiated after suspended cells attach to a surface. When monolayer cells in late G1 phase (just before entering S phase) are put in suspension cultures they do not initiate DNA synthesis.     

Arrest of 3T3 cells in G1 phase in suspension culture

3T3 cells do not grow in Methocel suspension culture, while other permanent cell lines do. The viability of 3T3 cells in suspension remains unchanged for at least three days with respect to plating efficiency, vital staining and resumption of normal growth when transferred into monolayer culture. When monolayer 3T3 cells in G1 phase are suspended they remain in G1 phase. Cells already in S phase which are suspended complete ongoing DNA synthesis and mitosis and then are arrested in the G1 phase. Progress through the cell cycle is reinitiated after suspended cells attach to a surface. When monolayer cells in late G1 phase (just before entering S phase) are put in suspension cultures they do not initiate DNA synthesis.     

Growth effects of lithium chloride in BALB/c 3T3 fibroblasts and madin-darby canine kidney epithelial cells

The ability of LiCl to initiate DNA synthesis was studied in Madin-Darby canine kidney (MDCK) cells, and mouse BALB/c 3T3 fibroblasts. In a defined culture medium lacking serum, LiCl increased DNA synthesis in BALB/c 3T3 cells 100–200% over control values. Maximum DNA synthesis was observed with concentrations of LiCl between 10 and 25 mM and increases from 40–50% over control were observed with concentrations as low as 1 mM. Exposure of BALB/c 3T3 cultures to LiCl resulted in an increase in the percentage of cells initiating DNA synthesis, total DNA content and cell number. Lithium chloride, in combination with insulin or epidermal growth factor (EGF), had either an additive or synergistic effect upon the growth of BALB/c 3T3 fibroblasts. MDCK cells proved refractory to the growth actions of LiCl, although they responded to EGF and insulin with increased DNA synthesis. Lithium chloride appears to have a direct effect on cell proliferation in some but not all cell types.     

Conditional responsiveness of a chemically transformed cell line to growth factor stimulated DNA synthesis

BP3T3, a clonal benzo(a)pyrene-transformed BALB/c-3T3 cell line, is conditionally responsive to growth factor stimulation. Density arrested cell populations deprived of growth factors by pretreatment with 0.5% platelet-poor plasma synthesized DNA both in response to ng/ml concentrations of PDGF, EGF, and somatomedin C, and in response to insulin, plasma, and serum. The above agents acted singly to induce DNA synthesis, but synergism is suggested because a higher percentage of cells were stimulated to enter the S phase when the growth factors were added in combination. Desensitization to growth factors occurred when cultures were pretreated with the high concentration of growth factors present in 10% serum (or plasma). In desensitized cultures none of the above agents, added singly or in combination, stimulated DNA synthesis. This effect appears to be global because pretreatment with one growth factor (e.g., insulin) inhibited the action of another (e.g., PDGF). Cell density appears to play a critical role in regulating DNA synthesis. Unlike nontransformed BALB/c-3T3 cells whose density is regulated by the serum concentration, the density of BP3T3 cells reached a plateau when cultures were grown in a serum (or plasma) concentration of 3% or greater. Such density arrested cultures were growth factor unresponsive; however, the cells rapidly responded to growth factors by synthesizing DNA and replicating when reseeded at a lower cell density. Thus the growth of BP3T3 cells is regulated by both growth factors and cell density.     

Mitogen response and cell cycle kinetics of Swiss 3T3 cells in defined medium: differences from human fibroblasts and effects of cell density

The mitogen requirement and proliferative response of Swiss 3T3 cells in serum-free, chemically defined culture medium were compared with those of early-passage human diploid fibroblasts. The effects of platelet-derived growth factor (PDGF), epidermal growth factor (EGF), insulin, transferrin, and dexamethasone on cell-cycle parameters were measured using 5'-bromo-deoxyuridine-Hoechst flow cytometry. Swiss 3T3 cells differ from human fibroblasts in several ways: (1) Swiss 3T3 cells showed a much higher dependence on PDGF than human fibroblasts; the growth of the latter, but not of the former, could be stimulated by the combination of EGF, insulin, and dexamethasone to the full extent of that when PDGF was present; (2) in the absence of PDGF, insulin was an absolute requirement for Swiss 3T3 cells to initiate DNA synthesis, while a substantial proportion of human fibroblasts could enter DNA synthesis without exogenous insulin or IGF-I; and (3) in the absence of PDGF, increasing insulin concentration increased the cycling fraction of Swiss 3T3 cells without an appreciable effect on the rate of cell exit from G0/G1, while under similar culture conditions, insulin showed its major effect on regulation of the G1 exit rate of human fibroblasts, without much effect on the cycling fraction. In addition, the proliferative response of high-density versus low-density, arrested Swiss 3T3 cells showed that the interaction of mitogens varied with cell density. At high cell density, the PDGF requirement was consistent with the "competence/progression" cell-cycle model. This growth response was not seen, however, when cells were plated at low density.