The stimulation of the initiation of DNA synthesis by fibroblast growth factor in Swiss 3T3 cells: interactions with hormones during the pre-replicative phase

Gap junctions and growth control in liver regeneration and in isolated rat hepatocytes

The hepatocytes in the mature normal liver are tightly coupled through gap junctions, except during compensatory hyperplasia (regeneration) after partial hepatectomy when the gap junctions become down-regulated. The significance of this down-regulation has been a long-standing enigma. The present study of hepatocytes in primary culture and in the regenerating liver aimed at defining the relationship, if any, between hepatocyte gap junctional communication and proliferation. Gap junctional down-regulation in the regenerating liver appeared to be a specific phenomenon because desmosomes and the surface contact area between neighboring hepatocytes remained constant. All agents and conditions (dexamethasone in vivo; dexamethasone, cyclic adenosine monophosphate, serum, and high cell density in vitro) delaying gap junctional down-regulation also increased the lag before the cells reached competence to enter S phase. This raised the possibility that hepatocyte DNA replication was inhibited through preservation of gap junctions. However, we disproved this assumption by showing that the DNA replication (more specifically the G1/S transition rate constant) was inhibited even in hepatocytes completely devoid of gap junctional communication. The teleological advantage of linking gap junctional down-regulation to hepatocyte G1 progression therefore may not be to trigger DNA replication but to ensure that proliferating hepatocytes and hepatocytes responsible for liver-specific metabolic functions maintain separate pools of metabolites and signaling molecules.

Late signals are required for the stimulation of DNA synthesis in rat mammary fibroblasts by growth factors

Maximal stimulation of DNA synthesis in quiescent rat mammary (Rama) 27 fibroblasts is elicited by epidermal growth factor (EGF) or basic fibroblast growth factor (bFGF) 18 h after the initial addition of the growth factors-the 'lag' period. At maximally-stimulating concentrations, EGF and bFGF are interchangeable 9 h after their initial addition. When the initial concentration of growth factor is below that required to elicit a maximal response, it is possible to increase the level of DNA synthesis by increasing the concentration of growth factor 9 h after its initial addition. When the initial concentration of growth factor is high, substitution by a lower concentration of growth factor after 9 h allows a greater proportion of cells to synthesize DNA than would be expected from a continuous low dose of growth factor. Similar results are obtained when both the growth factor and its concentration are changed 9 h after the initial addition of growth factor. However, when EGF at a low concentration is substituted for a high concentration of EGF or bFGF the resulting increase in the levels of DNA synthesis is greater when EGF rather than bFGF is added for a second time. The half-life of the growth-stimulatory signals delivered by EGF and by bFGF 9 h after their initial addition is 1-2 h. These results suggest that to stimulate DNA synthesis: (i) EGF or bFGF must deliver a signal(s) continuously; (ii) the initial signals produced by EGF and bFGF are equivalent; (iii) the signals produced between 9-18 h by EGF may be different to those produced by bFGF.

Okadaic acid, cAMP, and selected nutrients inhibit hepatocyte proliferation at different stages in G1: modulation of the cAMP effect by phosphatase inhibitors and nutrients

The protein phosphatase inhibitor okadaic acid (> 100 nM) caused an abrupt and complete cessation of primary rat hepatocyte cell cycle progression at the restriction point in late G1. A decline in the G1/S transition rate was observed in response to elevated cAMP, excess selected nutrients, and okadaic acid (< 100 nM). Excess nutrients (40 mM glucose +/- 5 mM dihydroxyacetone) acted by imposing an incomplete block in early G1. The cAMP action was potentiated by the phosphatase inhibitor microcystin, which in itself did not affect DNA replication. This suggests that cAMP acted by phosphorylating substrate(s) that is dephosphorylated by a microcystin-sensitive phosphatase. The additive effects of submaximal concentrations of okadaic acid and cAMP analogs indicated that okadaic acid and cAMP acted via different pathways. In conclusion, okadaic acid, cAMP, and excess nutrients, acting through distinct pathways, inhibited hepatocytes in different parts of the G1 phase.

Elevated cAMP gives short-term inhibition and long-term stimulation of hepatocyte DNA replication: roles of the cAMP-dependent protein kinase subunits

The study reports the role of the isozyme forms (cA-PKI and cA-PKII) and subunits (R and C) of cAMP-dependent protein kinase in mediating the acute depression of hepatocyte DNA replication by elevated cAMP. Combinations of cAMP analogs preferentially activating cA-PKI or II showed that either isozyme could inhibit DNA replication. The effects of glucagon and cAMP analogs were counteracted by the cAMP antagonist RpcAMPS, implicating the necessity for cA-PK dissociation in cAMP action. The effect of elevated cAMP was mimicked by microinjected C subunit, but not by the RI subunit of cA-PK. Hepatocytes under continuous cAMP challenge more than regained their replicative activity. This tardive stimulatory effect of cAMP was enhanced by insulin and blocked by dexamethasone, and was preceded by downregulation of cA-PK. In conclusion, a burst of cAMP acutely inhibits hepatocyte G1/S transition in late G1 regardless of hormonal state. In the presence of high glucocorticoid/low insulin the inhibition persists. At high insulin/low glucocorticoid the inhibitory phase is followed by a prolonged stimulation of DNA replication. Downregulation of endogenous cA-PK is a mechanism for escape from the inhibitory action of highly elevated cAMP.

The N-terminal hexapeptide fragment of IGF II stimulates thymidine incorporation into fibroblasts

We synthesized the N-terminal hexapeptide fragment of IGF II to study potential binding to NMDA receptors in analogy to the N-terminal tripeptide of IGF I. The amino acid sequence of the hexapeptide is furthermore identical with the C-terminal sequence of the casiragua insulin B chain. The hexapeptide did not bind to the NMDA receptors, but was found to promote [3H]-thymidine incorporation into fibroblasts at concentrations of 10(-8) - 10(-5) M in a dose-dependent manner. Since [125I]-hexapeptide did not bind to IGF receptors, indirect competition studies using either labelled IGFs or insulin had to be used. The competition of hexapeptide at a concentration of 10(-5) M with labelled IGF I or II was about equal to that of 10(-9) M IGF I or II. IGF receptors were apparently up-regulated by the hexapeptide, as has also been described for insulin. When using casiragua insulin as labelled ligand, IGF II and casiragua insulin competed with equal potency, whereas the hexapeptide at 10(-7) M caused an apparent up-regulation of the casiragua insulin binding sites. Our results that the hexapeptide stimulates [3H]-thymidine incorporation and up-regulates IGF II and casiragua insulin binding sites may be connected to one or several of the following findings: the hystricomorph insulins--of which the casiragua insulin is a member--stimulate DNA synthesis to a greater extent than other insulins; the insulin and type 1 IGF receptor binding regions are localized predominantly in the C-terminal region of the insulin B chain; and the "cooperative" site regulating the affinity of the insulin receptor is also located in the C-terminal region of the insulin B chain. Further experiments will be needed to clarify the exact mechanism.

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.

Cyclic adenosine monophosphate acts synergistically with dexamethasone to inhibit the entrance of cultured adult rat hepatocytes into S-phase: with a note on the use of nucleolar and extranucleolar [3H]-thymidine labelling patterns to determine rapid changes in the rate of onset of DNA replication

Analogs of cyclic adenosine monophosphate (cAMP) (N6benzoyl cAMP and N6monobutyryl cAMP) as well as agents that increased the intracellular level of cAMP (glucagon and isobutylmethylxanthine) inhibited the EGF-stimulated DNA replication of adult rat hepatocytes in primary culture independently of cell density. This inhibition was strongly potentiated by the glucocorticoid dexamethasone. The effect of cAMP (and dexamethasone) was not due to toxicity, because the inhibition was reversible and the cell ultrastructure preserved. cAMP acted by decreasing the rate of transition from G1- to S-phase, the duration of G2- and S-phase of the hepatocyte cell cycle being unaffected. DNA replication started in the extranucleolar compartment of the nucleus and ended in the nucleolar compartment as described earlier for cells grown in the absence of cAMP (O.K. Vintermyr and S.O. Døskeland, J. Cell. Physiol., 1987, 132:12-21). The action of cAMP was very rapid: significant inhibition of the transition was noted 2 hr after the addition of glucagon/IBMX and half-maximal inhibition after 4 hours. The determination of extranucleolarly labelled nuclei in cells pulse-labelled with [3H]thymidine allowed precise analysis of rapid changes in the probability of transition from G1- to S-phase. The extranucleolar labelling index could also be determined in cells continuously exposed to [3H]thymidine.

Platelet-derived growth factor, epidermal growth factor, and insulin-like growth factor I regulate specific cell-cycle parameters of human diploid fibroblasts in serum-free culture

The growth regulation of human diploid fibroblasts by platelet-derived growth factor (PDGF), epidermal growth factor (EGF), insulin-like growth factor I (IGF-I) (somatomedin C), dexamethasone, and transferrin was investigated in a serum-free, chemically defined culture system. Cell-cycle kinetic parameters were determined using 5'-bromodeoxyuridine (BrdU) incorporation and flow cytometric analysis with the DNA-specific dye Hoechst 33258. We found that PDGF and EGF regulate the proportion of cells capable of entering the cell cycle from the quiescent state, with smaller effects upon the rate of cell transition from G1 into S phase. IGF-1, on the other hand, regulates the rate of cell exit from G1 without affecting the cycling fraction. Transferrin and dexamethasone showed less effect upon the cell-cycle kinetics under these culture conditions. The data provide functional evidence that PDGF and EGF regulate similar cell-kinetic parameters in human fibroblast cultures. IGF-I is functionally distinct from both PDGF and EGF in its role of regulating G1 exit rate without affecting the cycling fraction. These observations made by BrdU-Hoechst flow cytometric techniques provide a novel perspective on the regulatory effects exerted by different classes of growth factors, and suggest a mode of interdependence of these mitogens in regulating the net growth rate which could be a feature of growth regulation in vivo. These data also provide a different perspective on the regulation of the growth of fibroblast-like cells than that of the "competence/progression" cell-cycle model.

Characterization of the inhibitory effect of glucocorticoids on the DNA replication of adult rat hepatocytes growing at various cell densities

Dexamethasone inhibited the basal and EGF-stimulated DNA synthesis of adult rat hepatocytes in primary culture. The inhibition was glucocorticoid-specific: It was shown by dexamethasone and hydrocortisone, but not by progesterone, testosterone, or estradiol; and was counteracted by the glucocorticoid antagonist RU-38486 in a concentration-dependent manner. Dexamethasone acted by decreasing the rate of entry into S-phase (kG1/S), while cell cycle parameters were unaffected. The steroid was able to decrease the kG1/S severalfold even when added more than 20 hr after EGF, half-maximal effect occurring 11 hr after the addition of dexamethasone. Densely populated areas were much more sensitive to the inhibition by dexamethasone than sparsely populated areas within the same culture dish: A moderate (10 nM) concentration of dexamethasone nearly abolished the DNA synthesis in densely populated areas of hepatocyte cultures with only marginal effect on sparsely populated cells.

Involvement of three intracellular messenger systems, protein kinase C, calcium ion and cyclic AMP, in the regulation of c-fos gene expression in Swiss 3T3 cells

In quiescent cultures of Swiss 3T3 cells, platelet-derived growth factor or fibroblast growth factor known to induce both protein kinase C activation and Ca2+ mobilization raised c-fos mRNA. This action of the growth factors was mimicked by the specific activators for protein kinase C, such as phorbol esters and a membrane-permeable synthetic diacylglycerol, and also by the Ca2+ ionophores, such as A23187 and ionomycin. Prostaglandin E1 known to elevate cyclic AMP also raised c-fos mRNA, and this action was mimicked by 8-bromo-cyclic AMP, dibutyryl cyclic AMP and forskolin. These results suggest that expression of the c-fos gene is regulated by three different intracellular messenger systems, protein kinase C, Ca2+ and cyclic AMP, in Swiss 3T3 cells.

Increase of epidermal growth factor-stimulated cell-cycle progression and induction of thermotolerance by heat shock: temperature and time relationship

When quiescent confluent cultures were incubated at increased temperature and then incubated at 37 degrees C prior to a second increase of temperature (46 degrees C) it appeared that heat-induced morphological alteration and ability to proliferate could be influenced by the previous thermal history of the cells. Incubations for 20 min in a temperature range of 41-46 degrees C caused cells to develop thermo-tolerance within 3 h of incubation at 37 degrees C. Confluent quiescent Swiss mouse 3T3 cells were incubated at 41.8, 43.7 or 45.6 degrees C and then reincubated at 37 degrees C to determine the effects of heat shock on the mitogenic effects of epidermal growth factor (EGF). Preincubation at 43.7 degrees C or 45.6 degrees C enhanced stimulation of G1-S progression by EGF. Preincubation at 43.7 degrees C markedly increased the rate at which cells enter the S phase without changing the length of the lag phase. A comparison of the duration of incubation at 43.7 degrees C for potentiation of EGF-induced DNA synthesis and that for induction of thermotolerance showed that a similar time interval for induction of effect could be implied.

Apparent desensitization of Swiss 3T3 cells to the mitogens FGF and vasopressin

The growth factors FGF and vasopressin were found to have only a transient effect on confluent quiescent monolayers of Swiss 3T3 cells. Whether measured as cumulative entry into S-phase by autoradiography, or as cell division by time-lapse filming, the elevated rate of cell proliferation was maintained only over 10-15 hr. Several trivial or artifactual explanations for this transience were ruled out, including toxicity of 3H-thymidine; exhaustion or degradation of medium components, nutrients or growth factors (although some medium depletion was observed); and the generation during quiescence of cells incapable of division. We have also eliminated heritable variation in the capacity to respond to individual growth factors. However, unstable phenotypic heterogeneity in growth factor requirements between cells may play some part, as found elsewhere for the response to low concentrations of serum (Brooks et al, 1984). Cell populations that had ceased to respond to vasopressin recovered their sensitivity after 2-3 days' incubation in conditioned medium lacking vasopressin. The phenomenon thus resembles the mitogen-induced desensitization described by Collins and Rosengurt (1982, 1983). However, in our case, the loss of sensitivity was not selective for vasopressin but applied also to epidermal growth factor (EGF) and to prostaglandin F2 alpha. Furthermore, changes in responsiveness to vasopressin with time were associated with changes in cell density. Although some element of selective desensitization has not been ruled out, the transient response in our experiments can be accounted for in terms of unstable heterogeneity in growth factor requirements and/or in terms of density-dependent regulation of growth.

Regulation of the rate of cell cycle progression in quiescent cytolytic T cells by T cell growth factor: analysis by flow microfluorometry

We have previously shown that greater than 90% of B6.1 cells, a murine cytolytic T lymphocyte (CTL) cloned line which is solely dependent on T cell growth factor (TCGF) for continuous growth in vitro, accumulates in the G1 phase of the cell cycle after transfer into culture medium containing no TCGF. Moreover, when such quiescent cells are exposed again to TCGF, greater than 85% reenter the S phase and subsequently divide in a relatively synchronous fashion. In this study, the regulation of the rate of cell cycle progression of quiescent B6.1 cells after exposure to TCGF was analyzed using two complementary DNA staining techniques, namely, the propodium iodide method (to enumerate cells entering the S phase) and the Hoechst 33342-bromodeoxyuridine substitution technique (to enumerate cells which have gone through mitosis). After TCGF addition, quiescent B6.1 cells resumed DNA synthesis and divided after a lag phase of 10 and 20 h, respectively. The duration of the lag phase was found to be dependent on the length of time during which quiescent B6.1 cells had been deprived of TCGF, but was independent of the concentration of TCGF used for restimulation. In contrast, the proportion of cells responding to TCGF as well as the rate of their first passage through mitosis was dependent on TCGF concentration. The presence of TCGF for at least 6 h was required for a maximal response. Moreover, direct evidence was obtained that TCGF by itself was able to stimulate proliferation of quiescent B6.1 cells in the absence of other growth factors and serum constituents other than bovine serum albumin, transferrin, and lipids.

Mitogenic effect of prostaglandin E1 in Swiss 3T3 cells: role of cyclic AMP

Addition of prostaglandin E1 (PGE1) to quiescent cultures of Swiss 3T3 cells rapidly elevates the intracellular levels of cAMP and increases the activity of adenylate cyclase in particulate fractions of these cells. In the presence of insulin, PGE1 stimulates the reinitiation of DNA synthesis. Both effects (increase in cellular cAMP and stimulation of DNA synthesis) are markedly potentiated by 1-methyl-3-isobutyl xanthine (IBMX) or by 4-(3-butoxy-4 methoxy benzyl)-2-imidazolidine (Ro 20-1724), both of which are potent inhibitors of cyclic nucleotide phosphodiesterase activity. In the presence of 50 microM IBMX, PGE1 caused a dose-dependent increase in cAMP levels and in [3H]thymidine incorporation into acid-insoluble material at concentrations (5-50 ng/ml) that are orders of magnitude lower than those used in previous studies (50 micrograms/ml) to demonstrate growth-inhibitory effects. Thus, the inhibitory effects produced by adding high concentrations of PGE1 on the initiation of DNA synthesis in Swiss 3T3 cells are not mediated by cAMP and should be regarded as nonspecific. In contrast, the mitogenic activity of PGE1 parallels its ability to increase the intracellular levels of cAMP. The findings support the proposition that a sustained increase in the level of this cyclic nucleotide acts as a mitogenic signal for confluent and quiescent Swiss 3T3 cells.

Control of entry of Swiss 3T3 cells into S phase by fibroblast growth factor under serum-free conditions

Swiss 3T3 cells can be made quiescent at low density by plating in medium MCDB 402 supplemented with dexamethasone (DEX), insulin (INS) and bovine plasma fibronectin (BPFn) for 3 days. One hour after stimulation of these cells by fibroblast growth factor (FGF), an increase in the rate of protein synthesis can be measured. Nine hours after stimulation by FGF, the rate at which the cells enter S phase increases abruptly. This increased rate of entry into S phase is delayed when methylamine is added to the medium before FGF treatment and later removed. The delay is only for the amount of time that the cells are exposed to methylamine, with no subsequent effect on the rate at which the cells enter S. The early increase in rate of protein synthesis caused by FGF is not blocked by concentrations of methylamine that stop the progression of FGF-treated cells toward S phase. The assay system that has been developed provides a means for detailed analysis of the prereplicative phase of Swiss 3T3 cells in a serum-free medium and in the absence of density-dependent inhibition.

Multiplication of Swiss 3T3 cells in a serum-free medium

Gently trypsinized Swiss 3T3 cells inoculated into medium MCDB 402 attach readily to polylysine-coated surfaces and remain viable for several days in the absence of exogenously added protein. Short-term multiplication under defined conditions can be obtained by supplementing the MCDB 402 with fibroblast growth factor (FGF), insulin (INS), and dexamethasone (DEX). Addition of bovine plasma fibronectin further improves attachment and viability. This system does not require initial plating in serum or the addition of poorly defined extracts for cellular attachment or for multiplication. In the complete system minus FGF, cells plated at a low density attach to the culture surface and become quiescent. The addition of FGF or PDGF 48-72 h after plating stimulates a high level of DNA synthesis during the following 24 h. EGF also stimulates DNA synthesis in these cells, but to a lesser extent. Insulin and dexamethasone are not needed for the initial DNA synthesis response to FGF, but are needed for continuing multiplication over a period of several days. This system provides a means for studying the effects of specific mitogens on Swiss 3T3 cells in the absence of undefined supplements, and without complications due to density-dependent inhibition.

Epidermal growth factor initiates DNA synthesis after a time-dependent sequence of regulatory events in Swiss 3T3 cells--interactions with hormones and growth factors

Epidermal growth factor (EGF) stimulates the initiation of DNA synthesis in Swiss 3T3 cells after a constant prereplicative period of 14-15 hours. The final rate of initiation follows apparent first-order kinetics and can thus be quantified by a rate constant k. The value of k can be changed by later additions during the prereplicative period: When cells stimulated by a very low concentration of EGF, alone or with insulin, which results in a relatively low value of k, receive a saturating amount of EGF at 15 hours, then k is markedly increased after 4-6 hours. Insulin alone (up to 200 ng/ml) is unable to set the lag phase, but does have a synergistic effect on the value of k given by EGF. When added at 15 hours, insulin also increases K, but after a delay of 4-6 hours. In contrast, both hydrocortisone and prostaglandin E1 (PGE1) inhibit the stimulation of DNA synthesis by EGF only during the first 8 hours of the prereplicative period of decreasing the value of k. Prostaglandin F2 alpha), which stimulates DNA synthesis in a similar mode as EGF, when added with EGF has a synergistic effect on DNA synthesis. This suggests that EGF and PGF2 alpha, nevertheless, act through different regulatory events.

Microtubule-disrupting agents affect two different events regulating the initiation of DNA synthesis in Swiss 3T3 cells

The stimulatory effect of epidermal growth factor, alone or with insulin, on the rate of initiation of DNA synthesis in Swiss 3T3 cells can be synergistically enhanced by the addition of either Colcemid or colchicine at 1 microM. However, both Colcemid and colchicine can exert the synergistic effect only when added earlier than 8 hr of the prereplicative period (lag phase). Removal of Colcemid (which allows for rapid reassembly of microtubules) earlier than 10 hr of the lag phase results in a loss of the synergistic effect. This suggests that microtubules must remain disrupted for longer times to accomplish some putative event(s) necessary for increasing the rate of initiation of DNA synthesis. Preincubation of quiescent cells with either Colcemid or colchicine for 8 hr prior to adding epidermal growth factor, alone or with insulin, shortens the lag phase by about 4 hr, irrespective of the resulting rate of initiation of DNA synthesis. These results suggest that the state of microtubules is affecting independently at least two different events involved in regulating time initiation of DNA synthesis.

Glucocorticoids inhibit the stimulatory effect of epidermal growth factor on the initiation of DNA synthesis

Confluent, quiescent Swiss 3T3 cells in culture can be stimulated to initiate DNA synthesis and divide by addition of growth factors to the culture medium. Here we show that hydrocortisone and other steroids which have glucocorticoid activity inhibit the stimulation of these cells by epidermal growth factor (EGF) in contrast to their reported enhancement of stimulation by fibroblast growth factor (FGF). Binding studies using [3H]-triamcinolone acetonide show that Swiss 3T3 cells contain a single class of glucocorticoid receptor of uniform affinity (KD = 2.0 nM), and about 34,000 receptor sites per cell. Those steroids which displace bound [3H]-triamcinolone acetonide are also effective in inhibiting the stimulation of DNA synthesis by EGF in the presence or absence of insulin, and the concentration of triamcinolone acetonide required for one-half maximal biological effect is in the same range as the KD. A similar concentration is required for one-half maximal enhancement of the effect of FGF. These results suggest that both the inhibitory and stimulatory effects of glucocorticoids may be mediated via these receptors, the different effects thus being due to differences in the intracellular events triggered by each growth factor.