A model of cell cycle control: sequential events regulated by growth factors

A Short History of the B-Cell-Associated Surface Molecule CD40

This perspective traces developments using monoclonal antibody technology that led to the discovery of CD40, a receptor that on B cells mediates “T cell help” and on dendritic cells helps to program CD8 T cell responses. I discuss some things that we got right during the path of discovery and some things we missed. Immunotherapies that block or stimulate the CD40 pathway hold great promise for treatment of autoimmune diseases and cancers.

Ammonia-induced senescence in cultured rat astrocytes and in human cerebral cortex in hepatic encephalopathy

Hepatic encephalopathy (HE) is a frequent complication of liver cirrhosis and is due to a low-grade cerebral edema associated with oxidative/nitrosative stress. Recent reports suggest that cognitive impairment in cirrhotic patients may not resolve completely after an attack of manifest HE. As astrocyte dysfunction is central to the pathogenesis of HE and astrocytes are critically involved in synaptic plasticity, we tested for sustained impairment of astrocyte function by analyzing expression levels of senescence biomarkers in ammonia-treated cultured rat astrocytes and in postmortem brain samples from cirrhotic patients with or without HE. NH4 Cl time- and dose-dependently inhibited proliferation of cultured astrocytes by up to 45% (5 mmol/L, 72 h) and strongly increased senescence-associated β-galactosidase activity. Inhibition of astrocyte proliferation by ammonia was mediated by a l-methionine sulfoximine-, oxidative stress-, and p38(MAPK) -dependent activation of p53 associated with enhanced transcription of cell cycle inhibitory genes GADD45α and p21. Mitochondria and the nucleus were identified as sources of oxygen radical formation after prolonged NH4 Cl exposure. Concurrently, NH4 Cl (5 mmol/L) treatment inhibited both epidermal growth factor- and brain-derived neurotrophic factor (BDNF)-induced proliferation as well as BDNF-mediated astrocyte morphology changes through downregulation of the respective growth factor receptors epidermal growth factor receptor and truncated tyrosine receptor kinase B. Increased mRNA expression levels of senescence-associated genes were also found in post mortem brain samples from patients with liver cirrhosis with HE, but not in those without HE. The data suggest that ammonia toxicity and HE are associated with premature astrocyte senescence, which may impair neurotransmission and contribute to persistence of cognitive disturbances after resolution of episodes of overt HE.

Resolvins E1 and D1 inhibit interstitial fibrosis in the obstructed kidney via inhibition of local fibroblast proliferation

Resolvin E1 (RvE1) is a naturally occurring lipid-derived mediator generated during the resolution of inflammation. The anti-inflammatory effects of RvE1 have been demonstrated in a variety of disease settings; however, it is not known whether RvE1 may also exert direct anti-fibrotic effects. We examined the potential anti-fibrotic actions of RvE1 in the mouse obstructed kidney-a model in which tissue fibrosis is driven by unilateral ureteric obstruction (UUO), an irreversible, non-immune insult. Administration of RvE1 (300 ng/day) to mice significantly reduced accumulation of α-smooth muscle actin (SMA)(+) myofibroblasts and the deposition of collagen IV on day 6 after UUO. This protective effect was associated with a marked reduction of myofibroblast proliferation on days 2, 4 and 6 after UUO. RvE1 treatment also inhibited production of the major fibroblast mitogen, platelet-derived growth factor-BB (PDGF-BB), in the obstructed kidney. Acute resolvin treatment over days 2-4 after UUO also had a profound inhibitory effect upon myofibroblast proliferation without affecting the PDGF expression, suggesting a direct effect upon fibroblast proliferation. In vitro studies established that RvE1 can directly inhibit PDGF-BB-induced proliferation in primary mouse fibroblasts. RvE1 induced transient, but not sustained, activation of the pro-proliferative ERK and AKT signalling pathways. Of note, RvE1 inhibited the sustained activation of ERK and AKT pathways seen in response to PDGF stimulation, thereby preventing up-regulation of molecules required for progression through the cell cycle (c-Myc, cyclin D) and down-regulation of inhibitors of cell cycle progression (p21, cip1). Finally, siRNA-based knock-down studies showed that the RvE1 receptor, ChemR23, is required for the anti-proliferative actions of RvE1 in cultured fibroblasts. In conclusion, this study demonstrates that RvE1 can inhibit fibroblast proliferation in vivo and in vitro, identifying RvE1 as a novel anti-fibrotic therapy.

Platelet-derived growth factor (PDGF) receptor-alpha-activated c-Jun NH2-terminal kinase-1 is critical for PDGF-induced p21WAF1/CIP1 promoter activity independent of p53

Platelet-derived growth factor (PDGF) is a potent mitogen for mesenchymal cells. PDGF AA functions as a "competent factor" that stimulates cell cycle entry but requires additional (progression) factors in serum to transit the cell cycle beyond the G1/S checkpoint. Unlike PDGF AA, PDGF B-chain (c-sis) homodimer (PDGF BB) and its viral counterpart v-sis can serve as both competent and progression factors. PDGF BB activates alpha- and beta-receptor subunits (alpha-PDGFR and beta-PDGFR) and induces phenotypic transformation in NIH 3T3 cells, whereas PDGF AA activates alpha-PDGFR only and fails to induce transformation. We showed previously that alpha-PDGFR antagonizes beta-PDGFR-mediated transformation through activation of stress-activated protein kinase-1/c-Jun NH2-terminal kinase-1, whereas both alpha-PDGFR and beta-PDGFR induce mitogenic signals. These studies revealed a striking feature of PDGF signaling; the specificity and the strength of the PDGF growth signal is modulated by alpha-PDGFR-mediated simultaneous activation of growth stimulatory and inhibitory signals, whereas beta-PDGFR mainly induces a growth-promoting signal. Here we demonstrate that PDGF BB activation of beta-PDGFR alone results in more efficient cell cycle transition from G1 to S phase than PDGF BB activation of both alpha-PDGFR and beta-PDGFR. PDGF AA activation of alpha-PDGFR or PDGF BB activation of both alpha- and beta-PDGFRs up-regulates expression of p21WAF1/CIP1, an inhibitor of cell cycle-dependent kinases and a downstream mediator of the tumor suppressor gene product p53. However, beta-PDGFR activation alone fails to induce p21WAF1/CIP1 expression. We also demonstrate that alpha-PDGFR-activated JNK-1 is a critical signaling component for PDGF induction of p21WAF1/CIP1 promoter activity. The ability of PDGF/JNK-1 to induce p21WAF1/CIP1 promoter activity is independent of p53, although the overall p21WAF1/CIP1 promoter activities are greatly reduced in the absence of p53. These results provide a molecular basis for differential regulation of the cell cycle and transformation by alpha- and beta-PDGFRs.

Atrial natriuretic factor inhibits mitogen-induced growth in aortic smooth muscle cells

Atrial natriuretic factor (ANF) is a polypeptide able to affect cardiovascular homeostasis exhibiting diuretic, natriuretic, and vasorelaxant activities. ANF shows antimitogenic effects in different cell types acting through R(2) receptor. Excessive proliferation of smooth muscle cells is a common phenomenon in diseases such as atherosclerosis, but the role of growth factors in the mechanism which modulate this process has yet to be clarified. The potential antimitogenic role of ANF on the cell growth induced by growth factors appears very intriguing. Aim of the present study was to investigate the possible involvement of ANF on rat aortic smooth muscle (RASM) cells proliferation induced by known mitogens and the mechanism involved. Our data show that ANF, at physiological concentration range, inhibits RASM cell proliferation induced by known mitogens such as PDGF and insulin, and the effect seems to be elicited through the modulation of phosphatidic acid (PA) production and MAP kinases involvement.

Cell synchronization for the purposes of nuclear transfer in the bovine

We have examined the reprogramming ability of donor fibroblast nuclei in various phases of the cell cycle, upon transfer to cytoplasts, using a bovine nuclear transfer (NT) model. Bovine fetal fibroblasts were cultured in reduced serum and conditioned medium to induce quiescence (G0) and treated with nocodazole to induce M phase arrest. Unsynchronized actively dividing cells (control) were mainly in G1. Cells synchronized in G0, M, and G1 phase were transferred to enucleated bovine MII oocytes by direct injection using the Piezo-Drill microinjector. NT oocytes were artificially activated following injection. Cells at the M phase were also transferred to enucleated oocytes after artificial activation. Cells induced into quiescence by serum starvation and unsynchronized donor cells produced the highest rates of development to the morula/blastocyst stage (20% and 18%, respectively). Development to blastocyst was significantly higher in parthenogenetic controls compared to NT embryos. The transfer of M phase nuclei to MII cytoplasts was not associated with high development to the blastocyst stage. Nevertheless, determining the viability of these embryos requires transfer to recipient animals and assessment of in vivo development.

Cellular effects of purvalanol A: a specific inhibitor of cyclin-dependent kinase activities

We have studied the effects of purvalanol A on the cell cycle progression, proliferation and viability. In synchronized cells, purvalanol A induced a reversible arrest the progression in G1 and G2 phase of the cell cycle, but did not prevent the completion of DNA synthesis in S-phase cells. The specificity of action of the drug was supported by the selective inhibition of the phosphorylation of cyclin-dependent kinase (cdk) substrates such as Rb and cyclin E. The cell contents of cyclins D1 and E were lower in cells incubated with purvalanol A compared to controls, but the level of the cdk inhibitory protein p21(WAF1/CIP1) was increased, indicating that the drug did not cause a general inhibition of gene expression. Purvalanol A did not inhibit transcription under cell-free conditions. This compound, however, caused an inhibition of the estradiol-induced expression of an integrated luciferase gene, suggesting that cdk or related enzymes may participate in the regulation of the activity of certain promoters. When exponentially growing cells, both mouse fibroblasts and human cancer cell lines, were incubated with purvalanol A for prolonged periods of time (24 hr), a lasting inhibition of cell proliferation as well as cell death were observed. In contrast, a 24 hr incubation of quiescent (non-transformed) cells with purvalanol A did not prevent their resumption of cell cycle after removal of the drug.

Regulation of thyroid cell proliferation by TSH and other factors: a critical evaluation of in vitro models

TSH via cAMP, and various growth factors, in cooperation with insulin or IGF-I stimulate cell cycle progression and proliferation in various thyrocyte culture systems, including rat thyroid cell lines (FRTL-5, WRT, PC Cl3) and primary cultures of rat, dog, sheep and human thyroid. The available data on cell signaling cascades, cell cycle kinetics, and cell cycle-regulatory proteins are thoroughly and critically reviewed in these experimental systems. In most FRTL-5 cells, TSH (cAMP) merely acts as a priming/competence factor amplifying PI3K and MAPK pathway activation and DNA synthesis elicited by insulin/IGF-I. In WRT cells, TSH and insulin/IGF-I can independently activate Ras and PI3K pathways and DNA synthesis. In dog thyroid primary cultures, TSH (cAMP) does not activate Ras and PI3K, and cAMP must be continuously elevated by TSH to directly control the progression through G(1) phase. This effect is exerted, at least in part, via the cAMP-dependent activation of the required cyclin D3, itself synthesized in response to insulin/IGF-I. This and other discrepancies show that the mechanistic logics of cell cycle stimulation by cAMP profoundly diverge in these different in vitro models of the same cell. Therefore, although these different thyrocyte systems constitute interesting models of the wide diversity of possible mechanisms of cAMP-dependent proliferation in various cell types, extrapolation of in vitro mechanistic data to TSH-dependent goitrogenesis in man can only be accepted in the cases where independent validation is provided.

Cellular ras and cyclin D1 are required during different cell cycle periods in cycling NIH 3T3 cells

Novel techniques were used to determine when in the cell cycle of proliferating NIH 3T3 cells cellular Ras and cyclin D1 are required. For comparison, in quiescent cells, all four of the inhibitors of cell cycle progression tested (anti-Ras, anti-cyclin D1, serum removal, and cycloheximide) became ineffective at essentially the same point in G1 phase, approximately 4 h prior to the beginning of DNA synthesis. To extend these studies to cycling cells, a time-lapse approach was used to determine the approximate cell cycle position of individual cells in an asynchronous culture at the time of inhibitor treatment and then to determine the effects of the inhibitor upon recipient cells. With this approach, anti-Ras antibody efficiently inhibited entry into S phase only when introduced into cells prior to the preceding mitosis, several hours before the beginning of S phase. Anti-cyclin D1, on the other hand, was an efficient inhibitor when introduced up until just before the initiation of DNA synthesis. Cycloheximide treatment, like anti-cyclin D1 microinjection, was inhibitory throughout G1 phase (which lasts a total of 4 to 5 h in these cells). Finally, serum removal blocked entry into S phase only during the first hour following mitosis. Kinetic analysis and a novel dual-labeling technique were used to confirm the differences in cell cycle requirements for Ras, cyclin D1, and cycloheximide. These studies demonstrate a fundamental difference in mitogenic signal transduction between quiescent and cycling NIH 3T3 cells and reveal a sequence of signaling events required for cell cycle progression in proliferating NIH 3T3 cells.

Late signals from the PDGF receptors leading to the activation of the p70S6-kinase are necessary for the transition from G1 to S phase in AKR-2B cells

Platelet-derived growth factor AB (PDGF-AB) has to be permanently present in the culture medium to achieve full proliferation (>90%) of AKR-2B fibroblasts. Upon removal after 1 h incubation time, only a small number of cells (<20%) entered the cell cycle. Concomitantly there was no increase in RNA- and protein-synthesis. The PDGF-receptor autophosphorylation reached a maximum after 30 min incubation with PDGF-AB. Tyrosine phosphorylation was no longer detectable after 2-4 h. The clustering of receptors into coated pits, analyzed by indirect immunofluorescence using a specific antibody against PDGF-beta-receptor, showed in contrast to autophosphorylation a biphasic kinetic. A first maximum was reached after 30 min, followed by a complete disappearance of coated pits, which regenerated in a second phase after 3 h and were long lasting. If PDGF-AB was removed after 1 h, the second phase was obliterated. The involvement of two different signalling pathways in these two phases was investigated in detail: (1) The ras-raf-MAP-kinase pathway and (2) the PI-3-kinase/p70(S6)-kinase pathway. PDGF-AB addition caused a fast (10 min) activation of MAP-kinase, which returned to background level after 1 h without any further activation later on. In contrast PDGF-AB led to a rapid (15-30 min) activation of the p70(S6)-kinase that persisted for 8-12 h just prior to the entry of the cells into S-phase. If PDGF-AB was removed after 1 h, the activation of this kinase ceased 3 h later. PDGF-AA, which is unable to promote division of AKR-2B cells, induced only a shortlasting p70(S6)-kinase activation. These observations add further evidence for the involvement of the p70(S6)-kinase pathway in the proliferation control of AKR-2B fibroblasts in the late G1 phase (4-8 h after growth factor addition). On the other hand, if the p70(S6)-kinase activation was prevented by the addition of 10 nM rapamycin, the cell division was not inhibited but only delayed by 4 h. Similar kinetics were observed when the PI-3-kinase was inhibited by 400 nM wortmannin. It is suggested that a regulatory element exists upstream of the p70(S6)-kinase and the PI-3-kinase. This regulatory element should be responsible for the transmission of late signals required for the progression through the cell cycle. This element is not involved in the immediate responses after PDGF-AB addition but must be stimulated within a second later phase of PDGF activation.

Role of insulin-like growth factor-I in esophageal mucosal healing processes

The current study examines the stimulation of healing processes and signal transduction that is mediated by insulin-like growth factor-I (IGF-I) in an ex vivo esophageal explant model when using tyrphostin inhibition of receptor tyrosine kinase. The explant model provides a 3-dimensional cellular environment of multiple interacting cells isolated from the neural and vascular supply. Tyrphostins previously characterized for their interactions with epithelial growth factor (EGF) receptor-associated protein tyrosine kinases were tested for their potential effects on IGF-I growth-promoting activity. Explants of rabbit esophagus were incubated in media with or without IGF-I. Tyrphostins 1, 23, 25, 46, 47, 51, and 63 were added. We assessed DNA synthesis by tritiated thymidine incorporation. Outgrowth from the edge of the primary mucosa of the explant was evaluated on histologic sections, and cell proliferation was confirmed with immunohistology. IGF-I increased the incorporation of tritiated thymidine by 50% to 100%. Tyrphostins 23 and 47 eliminated IGF-I-induced proliferation in a dose-dependent manner. Tyrphostins 25, 46, and 51--along with negative controls tyrphostin 1 and tyrphostin 63--were ineffective, inasmuch as IGF-I-stimulated growth remained unchanged in their presence. Proliferative activity demonstrated by PCNA staining was confined to new mucosa. Two of 5 tyrphostins originally developed as EGF receptor protein tyrosine kinase inhibitors were effective in inhibiting the actions of exogenous IGF-I. We conclude that IGF-I stimulation may play an important role in repair processes in the esophagus and that this stimulation can be inhibited by using specific tyrphostins.

Comparative studies between freshly isolated and spontaneously immortalized bovine granulosa cells: protein secretion, steroid metabolism, and responsiveness to growth factors

A bovine granulosa cell line (BGC-1) has been obtained by spontaneous immortalization of primary cultures. BGC-1 cells have retained some characteristics of primary cultures, such as the hormonal regulation of fibronectin biosynthesis. In the present study we have compared BGC-1 cells and primary cultures of bovine granulosa cells in terms of protein secretion, steroid metabolism, and mitogenic responses to growth factors. The pattern of protein secretion in BGC-1 cells was qualitatively similar to that of primary cultures. The main differences were a higher proportion of fibronectin and the relative amounts of several other unidentified proteins. Progesterone levels in BGC-1 cultures were undetectable. When BGC-1 cells and primary cultures were incubated with [3H]-pregnenolone, the former showed a lower conversion rate to progesterone. In contrast, the conversion rate of [3H]-progesterone to 5 alpha-reduced metabolites was markedly increased in BGC-1 cells. We also examined the effects of epidermal growth factor (EGF), insulin like growth factor-I (IGF-I), and transforming growth factor-beta (TGF-beta) on DNA synthesis under serum-free conditions. Both primary cultures and BGC-1 cells exhibited a stimulatory response to EGF and IGF-I on [3H]-thymidine incorporation. Neither BGC-1 cells nor primary cultures showed a significant response to TGF-beta when added alone. However, in the presence of a combination of EGF and IGF-I, TGF-beta displayed an inhibitory effect on primary cultures while it stimulated DNA synthesis in BGC-1 cells even further. The addition of conditioned medium from BGC-1 cells (BGC-1-CM) stimulated DNA synthesis on primary cultures to a greater extent than the addition of conditioned medium from primary cultures. These results suggest that BGC-1 cells may be a useful model to study the regulation of granulosa cell function during the period previous to the preovulatory stage of follicular development. The differential responses of the immortalized cells to growth regulators may offer some clues on the mechanisms that control cell proliferation in normal tissues.

Growth factors and growth modulators in human uterine endometrium: their potential relevance to reproductive medicine

OBJECTIVE

To provide an up-to-date, comprehensive review on the presence and regulation of growth factors (GFs), GF receptors, and GF regulatory proteins in human endometrium in an effort to understand the potential roles of these proteins in endometrial cell mitosis and differentiation and in endometrial-trophoblast interactions.

DESIGN

Relevant studies were identified through a computerized bibliographic search (MEDLINE; BRS Information Technologies, a division of Maxwell Online, Inc., McLean, VA) and through manual scanning of recent relevant journals.

RESULTS

Several GFs, their receptors, and regulatory proteins have been identified in endometrium, and cellular localization and steroid-dependence of these proteins as well as action of several growth modulators on endometrial cell function have been studied. Epidermal growth factor, transforming growth factor (TGF)-alpha, platelet-derived growth factor, insulin-like growth factors (IGFs) and their binding proteins, fibroblast growth factor (FGF), TGF-beta, colony-stimulating factor (CSF)-1, and interferon-gamma regulate mitosis of endometrial cellular components in vitro. Endothelin-1 may participate in vasoconstriction and FGF may participate in angiogenesis in this tissue in vivo. Interleukins-1 and -6 are believed to be involved in endometrial T-cell activation, and TGF-beta, CSF-1, the interleukins, and the IGFs likely mediate endometrial-trophoblast interactions. The role of tumor necrosis factor in endometrium remains uncertain.

CONCLUSIONS

Current evidence supports the thesis that GFs play a central role in cyclic mitosis and differentiation of endometrial cellular components, recruitment of macrophages in decidualizing endometrium, endometrial-trophoblast interactions, early pregnancy maintenance, tissue shedding in the absence of implantation, and endometrial functionalis regeneration.

Modulation of growth and function of human gingival fibroblasts by fibroblast-activating factor derived from Porphyromonas gingivalis W50

The effect of a 24-kDa fibroblast-activating factor (FAF) isolated from outer membrane vesicles of Porphyromonas gingivalis W50 on the modulation of [3H]thymidine uptake and cell proliferation was examined in selected fibroblast and transformed cell lines. FAF enhanced the proliferation of human gingival fibroblasts, human skin fibroblasts, and human umbilical vein endothelial cells in subconfluent and confluent cells, suggesting that FAF might be functioning as a competence factor. The transformed cell lines, U-937 and HEp-2, were not responsive. FAF and several human-derived growth factors showed a synergistic effect on proliferation. [3H]proline and [3H]leucine were rapidly incorporated into fibroblasts in the presence of FAF; however, there was no selective induction of collagen synthesis. FAF was not active in the induction of interleukin-1 and interleukin-6. It is hypothesized that FAF from P. gingivalis functions as a growth factor for human fibroblasts but is without activity for transformed cells.

Cell cycle-dependent gene expression in V point-arrested BALB/c-3T3 cells

Density-arrested BALB/c-3T3 cells stimulated to proliferate in an amino acid-deficient medium arrest in mid-G1 at a point termed the V point. Cells released from V point arrest require 6 hr to traverse late G1 and enter S phase. As data presented here show that mRNA synthesis is needed for 2-3 hr after release of cells from the V point, after which inhibition of mRNA synthesis does not prevent entry into S phase, we used this mid-G1 arrest protocol to analyze gene expression in late G1. We found that although stimulation of cells in amino acid-deficient medium did not inhibit the induction of genes expressed in early G1, genes normally expressed in late G1 were expressed only after release from the V point. The expression of late G1 genes in cells released from the V point was temporally similar, in respect to G1 location, as was seen in stimulation of quiescent G0 cells. As this protocol effectively divides gene expression into early (pre-V point) and late (post-V point) categories, it should be useful in studies of growth factor-modulated events that regulate traverse of late G1 and commitment to DNA synthesis. In addition, we used c-myb antisense oligonucleotides to show that c-myb expression, which occurs in late G1, is required for BALB/c-3T3 fibroblasts to traverse late G1 and initiate DNA synthesis.

Altered steroidogenic activity of human granulosa-lutein cells at different cell densities in culture

In the present study, the relationship between human granulosa-lutein cell (hGLC)-plating density and steroidogenic activity was evaluated. Increasing hGLC-plating density 32-fold, from 0.25 x 10(4) to 8 x 10(4) cells/well, was associated with a concomitant increase in the total amount of progesterone (P4), testosterone (T), and estradiol-17 beta (E2) secretion. The daily amount of each steroid (P4, T, and E2) secreted by hGLC at different cell-plating densities was further normalized per 10(3) cells. Thus, an increase in hGLC-plating density from 0.25 x 10(4) to 1 x 10(4) cells/well was associated with approximate increases of 1.3-fold in P4 and 3-fold in T and a 50% decrease in E2 secretion, per 10(3) cells. A further increase in hGLC-plating density, from 1 x 10(4) to 8 x 10(4) cells/well, was associated with a significant decrease of approximately 3.7-fold in P4 and 6-fold in T per 10(3) cells. A similar increase in hGLC-plating density was associated with no change or a 2-fold decrease, per 10(3) cells, in E2 secretion during days 0-3 or days 3-5 of culture, respectively. The P4/E2 ratio was increased and the E2/T ratio decreased with extension of the culture period. These two ratios had a tendency to be altered inversely, concurrent with the increase in cell-plating densities. At 1-2 x 10(4) cells/well, P4/E2 was maximal, whereas E2/T was minimal.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of antibodies against IGF-I, insulin or their receptors on proliferation of human acute lymphoblastic leukemia cell lines

To evaluate the potential role of IGF-I and insulin as growth-promoting factors in malignant lymphocyte development, we examined established T-acute lymphoblastic leukemic (ALL) cell lines with increasing stage of differentiation, HSB2, HUT78, CEM, MOLT3, Jurkat, JM-P, JM-886, and four established preB- and B-ALL cell lines REH, SKW6 CESS, BJAB for production of IGF-I and growth in the presence of antibodies, directed against IGF-I or insulin or their receptors. Basal DNA synthesis of the early differentiated T-cell lines HSB2 and HUT78, as well as the B-cell line REH, could be inhibited in a dose-dependent manner by both monoclonal antibodies against IGF-I (ASC41) and antibodies against the IGF-I receptor (alpha-IR3), suggesting that IGF-I acts as an auto- or paracrine growth factor for these cells via the IGF-I receptor. From these cells HUT78 and REH secreted IGF-I into cell culture medium. DNA synthesis of the further differentiated T-cell lines CEM and MOLT3 was inhibited by alpha-IR3 and antibodies directed against the insulin receptor (RPN.538) and against insulin (RPN.1661). These results suggest that insulin via the IGF-I receptor or insulin receptor can function as an autocrine or paracrine growth factor in these T-ALLs. Proliferation of the most differentiated T-ALL Jurkat and JMP was inhibited only by alpha-IR3 and 2C2, an antibody directed against the IGF-II receptor, suggesting that IGF-I or IGF-II acting via the IGF-I receptor or IGF-II receptor may be involved in proliferation of these cell lines. Inhibition of the DNA synthesis by RPN.538 and RPN.1661 indicate a more important role for insulin in growth of leukemias of the B-ALL cell lines SKW6 and CESS.

PDGF-AA effectively stimulates early events but has no mitogenic activity in AKR-2B mouse fibroblasts

The response of AKR-2B mouse fibroblasts, which express approximately equal numbers of platelet-derived growth factor (PDGF)-alpha and -beta receptors on their surface (V. Hoppe et al. Eur. J. Biochem. 187, 207-214, 1990) to all three isoforms of PDGF, was studied. All isoforms stimulated early events, i.e., receptor autophosphorylation on tyrosine, total cellular phosphorylation, increase in 32P-labeled phospholipid content, but there was no correlation between the extents measured for the different effects. Although rPDGF-AA effectively stimulated these early events, it was unable to induce [3H]thymidine incorporation and cell growth whereas rPDGF-BB and -AB stimulated the division of more than 90% of the cells. This activity was restored by addition of insulin-like growth factor I (IGF-I), which itself exhibited only a low mitogenic activity. rPDGF-AB or -BB did not require the presence of IGF-I to fully stimulate cells for [3H]thymidine incorporation and cell division. Apparently, rPDGF-AA induced only a "competence" state of the cells whereas rPDGF-AB or -BB was also able to initiate "progression". It is speculated that some early events occurring during the competence phase might be part of a "maintenance" program elicited by growth factors.

Growth-regulatory effects of glucagon, insulin, and epidermal growth factor in cultured hepatocytes. Temporal aspects and evidence for bidirectional control by cyclic AMP

Data presented indicate that in hepatocytes insulin and glucagon promote growth by acting in a relatively early part of the prereplicative period (G0 or early G1) whereas cells (if pretreated with insulin) become more sensitive to EGF at the later stages, ie, nearer the S phase entry. The data indicate that at least two effects of glucagon (cAMP) on hepatocyte proliferation exist; in addition to a growth-promoting modulation early in the prereplicative period, there is also an inhibitory effect of glucagon (as well as other cAMP-elevating agents) that is exerted at a point shortly before the G1-to-S transition. Because both effects occur dose-dependently in the normal range of glucagon concentrations in portal blood, it is conceivable that glucagon/cAMP is involved both when liver growth is initiated and terminated.

Possible involvement of a lipocortin in the initiation of DNA synthesis by human endothelial cells

This work focused on three themes. First, evidence was obtained for the presence of proteins of 34, 35, 32, and 69 kDa immunologically related to lipocortins I, II, V, and VI, respectively, in human umbilical vein endothelial (HUVE) cells. The 69-kDa protein (p69), but not proteins related to lipocortins I, II, and V, exhibited an increased phosphorylation after exposure of cells to basic fibroblast growth factor (bFGF) and phorbol ester PMA. Second, treatment of HUVE cell particulate fractions with EGTA and hydrophobic affinity chromatography in combination with conventional techniques provided extracts rich in p69 and purified p69. p69 from control cells and extracts from control, bFGF-treated, and PMA-treated cells were found to possess anti-phospholipase A2 (PLA2) activity of lipocortin. In contrast, a striking reverse effect occurred when extracts were obtained from cells exposed to bFGF plus PMA. Third, the combination of bFGF and PMA induced a stimulated PLA2-catalyzed release of arachidonic acid in HUVE cells. This arachidonate production was shown to be involved in the decision of cells to enter into DNA synthesis. Taken together, the present results suggest that phosphorylation of p69 is causally involved in the control of commitment to growth in HUVE cells by acting as a coupling mechanism between surface stimuli and arachidonate pathways.

Activation of signal transduction pathways protects quiescent Balb/c-3T3 fibroblasts against death due to serum deprivation

Platelet-derived growth factor (PDGF), epidermal growth factor (EGF), insulin-like growth factor-1 (IGF-1), and insulin protect density-inhibited murine Balb/c-3T3 fibroblasts against death by distinctive mechanisms. Determination of the cell survival-enhancing activity of growth factors by cell enumeration and neutral red uptake measurement gives equivalent results. PDGF displays a steep dose-response relationship in the 1-5 ng/ml range. The other factors display shallow log-linear relationships in the following ranges: EGF: 0.2-5 ng/ml; IGF-1: 2-80 ng/ml; and insulin: 57-4,500 ng/ml. Agonists that lead to the activation of protein kinase A, including forskolin, 8-bromoadenosine 3':5'-cyclic monophosphate (Br-cAMP) and N6,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate (db-cAMP), markedly increase both short-term (5-h) and long-term (20-h) survival of cells. 2-Isobutyl-1-methylxanthine (IBMX) markedly enhances short-term survival, but its effect decays with time. The protein kinase C agonist 12-O-tetradecanoyl phorbol-13-acetate (TPA) has a moderate protective effect at concentrations of 16-32 nM, and 64 nM TPA is highly effective. The synthetic diaclglycerols 1,2-dioctanoylglycerol (DiC8) and 1-oleoyl-2-acetylglycerol (OAG) and the calcium ionophore ionomycin show low activity. Supplementation of EGF with a protein kinase A or C agonist results in a varying additive increase in short-term (5-h) cell survival and supplementation of EGF + insulin or PDGF + EGF + insulin increases further the already high level of protection given by the growth factor combinations. Combining a protein kinase A and a protein kinase C agonist in the absence of growth factors gives an approximately additive increase in cell survival. Results obtained with kinase, RNA, and protein synthesis inhibitors suggest that: 1) activated protein kinase C catalyzes one or more phosphorylation events in quiescent Balb/c-3T3 cells that lead to gene expression with the protein product(s) mediating protection of quiescent cells against death, and 2) phosphorylation events catalyzed by protein kinase A largely serve to protect cells by a mechanism not requiring de novo RNA and protein biosynthesis.

Immunology of ovarian failure

Ovarian failure is the result of depletion of ovarian follicles. Naturally occurring ovarian failure usually takes place around 50 years of age in the human. Premature ovarian failure occurs in 1% of women and is the result of acceleration of rate of ovarian follicular depletion in the majority of cases. Cytokines are involved in the mechanisms of ovarian follicular atresia, whether it occurs at a normal or accelerated rate. It is the balance between the actions of TGF alpha and TGF beta upon the granulosa cell that determines the fate of a nonluteinized follicle and between LH and INF gamma that determines destiny of a luteinized follicle. When granulosa cells express MHC antigens in response to IFN gamma or genetic stimulus, an autoimmune reaction ensures resulting in follicular atresia. If the immune processes proceed continuously rather than cyclically, premature ovarian failure occurs. Thus, not only do the immunologic and endocrinologic systems need to communicate to allow normal ovarian function, evidence exists to support the concept that they interact in the pathophysiology of ovarian failure.

Steady-state mRNA expression for growth factors in DMBA-induced rat mammary tumors

Steady-state mRNA levels were examined for insulin-like growth factors (IGF) I and II, transforming growth factor alpha (TGF alpha) and its receptor and the epidermal growth factor (EGF) receptor in mammary tumors induced by DMBA in rats. An abundant 4.8 kb TGF alpha transcript was identified in all tumors, along with a 7.5-8.0 kb IGF-I transcript. A presumptive 2.9 kb IGF-II transcript was also identified in all tumors. Northern analyses and receptor autophosphorylation studies failed to detect EGF receptors in any mammary tumors. These findings suggest the potential for autocrine or paracrine influences of IGF-I and IGF-II in this tumor model and a possible paracrine influence of TGF alpha in tumor-induced neovascularization.

Mitogenic effects of transforming growth factor type e on epithelial and fibroblastic cells--comparison with other growth factors

Transforming growth factor type-e (TGFe) is a novel TGF which was first described as a growth factor possibly involved in autocrine stimulation of anchorage-independent growth of carcinoma cells. Its later identification in normal tissues, plasma, and platelets suggested a role for TGFe in normal cell growth. This study shows that TGFe stimulates both anchorage-dependent and -independent growth of epithelial and fibroblastic cells of nonneoplastic origin. The mitogenic activity of TGFe in monolayer is slightly less than that of basic fibroblast growth factor, equipotent to that of epidermal growth factor, and greater than that of IGF-1. TGFe acts as a progression factor for both AKR-2B and Balb-3T3 cells. TGFe is also a potent mitogen for normal human epidermal keratinocytes and may therefore play a role in epidermal growth and regeneration.

Suppression of protein kinase C and the stimulation of glucocorticoid receptor synthesis by dexamethasone in human fibroblasts derived from tumor tissue

Exposure of fibroblasts derived from keloid tissues, desmoid and dermal tissue from individuals with Gardner's syndrome (GS) to dexamethasone resulted in the suppression of protein kinase C (PKC) activity and [3H]thymidine incorporation into DNA, and a 20-fold induction of glutamine synthetase activity. Treatment of GS and keloid fibroblasts with 0.1 microM dexamethasone for 36 h increased glucocorticoid receptor (GR) synthesis, as determined by [35S]methionine labeling and immunoprecipitation with a monoclonal antibody to the human GR. The suppression of PKC activity by dexamethasone was shown to result from a loss of protein mass as determined by immunoblotting using an antibody to PKC type III. In contrast to these results, exposure of fibroblasts isolated from normal tissues to dexamethasone did not result in the suppression PKC and [3H]thymidine incorporation, there was only a sixfold induction of glutamine synthetase, and a decrease of GR synthesis. As no primary receptor binding defect could be detected, the altered response of tumor cells to steroid-occupied receptor indicates a partial post-receptor binding defect in GS and keloid cells.

The growth-regulated gene 1B6 is identified as the heavy chain of calpactin I

The expression of 1B6, a growth-regulated sequence isolated from a Syrian hamster fibroblast cDNA library, was studied in BALB/c 3T3 cells. The level of cytoplasmic 1B6 mRNA (1600 bases) was low in quiescent cells and plateaued in mid/late G1 after the cells were stimulated with 15% fetal calf serum (FCS). Protein synthesis was not required for the induction of 1B6 mRNA; therefore, the expression of 1B6 is a primary response to serum stimulation. The induction of 1B6 mRNA was also observed after stimulation with insulin, epidermal growth factor, and fibroblast growth factor but not with platelet-derived growth factor. When quiescent cells were serum-stimulated, the percentage of cells that became committed to enter DNA synthesis was proportional to the length of their incubation with serum. To determine if 1B6 expression was also correlated with the time of exposure to serum, quiescent cells were stimulated with a pulse of 15% FCS and the abundance level of 1B6 induced by that pulse was determined. The amount of 1B6 mRNA increased with increasing time of exposure to serum and paralleled the increase in the percentage of nuclei that were induced into DNA synthesis by the serum pulse. Comparison of the nucleotide sequence of the p1B6 cDNA to the GenBank database revealed a striking identity of 1B6 to the 3' end of p36, the heavy chain of calpactin I. The previous characterization of p36 as a substrate for tyrosine kinases suggests a possible role for 1B6/p36 in cell proliferation.

Endothelin-1 stimulates DNA synthesis and anchorage-independent growth of Rat-1 fibroblasts through a protein kinase C-dependent mechanism

Stimulation of quiescent cultured fibroblasts with a variety of growth-promoting factors induces release of diacylglycerol (DG) and subsequent activation of protein kinase C (pkC), but the role of pkC in the induction of DNA synthesis and cell proliferation remains unclear. We have investigated the involvement of pkC in the response of Rat-1 fibroblasts to the newly described peptide endothelin-1 (Et-1), an agonist that is secreted by the vascular endothelium and that may play a role in the proliferative response of cells in the vessel wall. Addition of Et-1 to serum-deprived Rat-1 cells promoted DNA synthesis in the absence of additional factors and stimulated anchorage-independent growth in the presence of epidermal growth factor (EGF), indicating that Et-1 has many of the characteristics of a mitogen. The ability of Et-1 to stimulate both DNA synthesis and anchorage-independent growth was markedly reduced by the depletion of cellular pkC activity induced by prolonged exposure to 12-O-tetradecanoylphorbol-13-acetate (TPA). In contrast, the ability of Et-1 to induce both second messenger production and transcription of c-fos and c-jun was largely independent of cellular pkC activity. Production of DG in response to Et-1 persisted for greater than 12 h and may account for the ability of Et-1 to augment the G1-S phase transition. Although these observations indicate that functional pkC is not an essential component of the proximal pathway leading to rapid changes in gene transcription and second messenger production in response to Et-1 treatment, the data suggest that activation of pkC is an essential component of the downstream events responsible for the stimulation of cell proliferation and anchorage-independent growth in Rat-1 cells exposed to Et-1.

Transforming signals generated by the polyoma virus tumor antigens

The secretion of a threshold amount of SAGF can elicit one of the major features of the transformed phenotype, anchorage-independent growth. It appears that, like other cells, NIH 3T3 cells make and secrete SAGF, but not enough to enable them to proliferate in soft agar. The stream of signals from polyoma virus MTAg:pp60c-src:PI-kinase complexes attached to the inner surface of the cell membrane is not enough to raise SAGF secretion to the threshold level for agar growth, but these signals can powerfully enhance the responsiveness to added SAGF. Only when NIH 3T3 cells express all three polyoma T antigens do they secrete enough SAGF and become responsive enough to the factor to form colonies in soft agar. This is in contrast to F111 cells which produce adequate amounts of, and respond to, SAGF after MTAg expression alone. The reasons for this difference remain to be investigated. Obviously, factors expressed during the course of the spontaneous establishment are different for these two lines and they affect the cells' response to MTAg. Clearly, knowing how polyoma virus transforms cells will require a deeper understanding of the deregulation of membrane signaling enzymes by the viral MTAg and the identification and characterization of the autocrine SAGF's that mediate at least part of the neoplastic response to the viral signals.

Fibroblast growth factor inhibits epidermal growth factor-induced responses in rat astrocytes

The signals which regulate the proliferation of astrocytes have relevance to normal developmental processes, transformational loss of growth control, and reactive gliosis present in many brain disease states. We have studied, in primary cultures of rat astrocytes, a sequential interaction of two growth factors, epidermal growth factor (EGF) and fibroblast growth factor (FGF), which may be relevant to the brain in these conditions. EGF is a strong mitogen and stimulator of 2 deoxyglucose (2 DG) transport with no effect on plating of cells, and FGF is a lesser mitogen and 2 DG uptake stimulator. However, when FGF is given to the cells as a pretreatment, FGF strongly inhibits the ability of EGF to stimulate both DNA synthesis and 2 DG uptake. The inhibition of EGF stimulation by FGF is across the EGF dose-response curve, present at high and low culture densities, and stable for at least 3 days. Specificity is indicated by lack of inhibition by PDGF pretreatment and much less inhibition of fetal calf serum-induced stimulations than EGF-induced stimulation. Cell counts confirmed that the FGF pretreatment also inhibits EGF stimulation of cell division. Because of FGF brain derivation and angiogenic and neurotropic functions, it may serve as a regulator of EGF-astrocyte interactions in processes such as development, gliomatous transformation, and neural regeneration.

Combined DC and ELF magnetic fields can alter cell proliferation

Effects of extremely-low-frequency (ELF) electromagnetic fields on proliferation of rabbit-ligament fibroblasts were examined. Markedly different effects, ranging from inhibition to stimulation of proliferation, were obtained, depending on the signal parameters of amplitude, frequency, and DC magnetic field. These results demonstrate that simple, single-frequency signals can have dramatic effects on ligament cells, which may be useful for bioelectrical stimulation of growth and repair. Also, the range of frequencies examined in this study covered those commonly used in transmission of electrical energy (50 or 60 Hz), which indicates that further study of possible effects of exposure to fields generated by power transmission equipment is warranted.

Proliferation-related responses in rat astrocytes to epidermal growth factor

The signals which regulate the proliferation of astrocytes have relevance to both normal developmental processes and abnormal states of gliosis or glial tumor formation. We have extended studies of astrocyte proliferation and related responses in primary cultures of rat telencephalic cortical astrocytes as a result of treatment with epidermal growth factor. Epidermal growth factor stimulates the rate of DNA synthesis five fold and maintains the rate of protein synthesis. The stimulation occurs at a dose of 2 ng/ml and is greater in higher density cultures than in lower density cultures, perhaps representing a relative starvation for the growth factor. The astrocyte response is still present even after being cultured 3 1/2 weeks in serum-free and non-growth factor or hormone-supplemented media. Combined immunofluorescence and thymidine autoradiography disclose that glial fibrillary acidic protein containing cells are the cells synthesizing DNA in response to the growth factor, and combined rhodamine and fluorescein-linked stains disclose that epidermal growth factor is in the glial fibrillary acidic protein containing cells. Proliferation-related 2-deoxyglucose uptake is stimulated at approximately the same dose as DNA synthesis is stimulated, but the time course is relatively slow, maximizing at 48 hr. Ornithine decarboxylase is stimulated in 6 hr indicating more rapid nuclear stimulation by the signal. In conclusion, epidermal growth factor has a clear direct interaction with glial fibrillary acidic protein-containing cells which is greater in higher density cultures, is still present in long-quiescent cells, and includes DNA synthesis, cell cycle progression, hexose uptake, and polyamine synthesis.

Influence of selection for increased body weight on the incidence of leiomyomas and leiomyosarcomas in Japanese quail

Leiomyomas and leiomyosarcomas were observed in adult laying hens from lines of Japanese quail selected solely (HW) or partly (HW-HP; HW-LP) for increased 4-wk BW and the corresponding randombred control (R1). No neoplasms were observed in a line (LW) selected for decreased 4-wk BW based on observations in one generation. Line R1 served as the base population for Lines HW and LW and was maintained without conscious selection for any trait, Lines HW-HP and HW-LP were sublines of Line HW in which the males were selected for increased 4-wk BW and the females were selected for high or low levels of plasma total phosphorus, respectively. Hens of all lines were necropsied after completing a 120-day production period. A high incidence of neoplasms were found in the dorsal and ventral ligaments of the oviduct in the immediate vicinity of the magnum, and the incidence was much greater in the lines selected for increased growth than in Line R1. These results suggest that selection for increased BW in Lines HW, HW-HP, and HW-LP has resulted in genetic changes that are conducive to neoplastic growth. Based on the results of one generation, it appears that selection for decreased 4-wk BW reduced incidence of neoplasms.

Growth-promoting effects of acidic and basic fibroblast growth factor on rabbit articular chondrocytes aging in culture

Rabbit articular chondrocytes have a limited growth potential in vitro. After four passages in culture, chondrocytes have accomplished more than 50% of their life span. At this stage of culture, they are considered to be senescent-like, since a dramatic decrease in proliferative capacity and enhanced cell size and protein content are observed. These aged cells are, however, still able to respond to fibroblast growth factor (FGF). The addition of either acidic or basic FGF (10 ng/ml) to culture medium permitted an enhanced proliferation. The attenuation of FGF mitogenic activity during aging was not observed for both fractions. Moreover, when treated with acidic or basic FGF, aged chondrocytes had a smaller size and a lower protein content. The acidic FGF was less potent than the basic FGF in delaying the evolution of aged chondrocytes to senescence.

Centriole ciliation and cell cycle variability during G1 phase of BALB/c 3T3 cells

Although variability in the duration of the cell cycle is thought to reflect growth-regulatory processes that control cell cycle progression, the precise timing of the variable period within the G1 phase of the cell cycle has not been defined. In particular, the timing of cell cycle variability in relation to the cell's commitment (R point) to the initiation of DNA synthesis remains controversial. In order to investigate cell cycle variability, indirect immunofluorescence was used to measure the formation of the primary cilium as a possible marker of G1 events in both stimulated quiescent and exponentially growing cells. The primary cilium, an internal "9 + 0" nonmotile structure formed by one of the interphase centrioles, was first detected in postmitotic BALB/c 3T3 cells 5 hr before the initiation of DNA synthesis, an interval similar to that for the reassembly of the primary cilium in serum-stimulated quiescent fibroblasts. This similarity in the timing of ciliation suggests that serum-stimulated quiescent cells reenter the cell cycle in early G1 and recapitulate much of G1. Moreover, the rate of cilia formation in both postmitotic and serum-stimulated quiescent cells was identical to the rate of DNA synthesis initiation. Thus, cell cycle variability occurs before ciliation in both stimulated quiescent and exponentially growing cells. Furthermore, since ciliation also precedes the R point, variability in the centriole cycle occurs before the R point and thus may reflect processes controlling the cell's commitment to the initiation of DNA synthesis.

Oncogene-related growth factors and growth factor receptors in human malignant glioma-derived cell lines

Oncogenes induce malignant transformation of cells. Two oncogenes are closely related to genes coding for a mitogenic growth factor (v-sis to the PDGF gene) and a receptor for a mitogenic growth factor (v-erb B to the EGF receptor gene). We studied the possibility that cells derived from malignant gliomas produce mitogenic factors that bind to cell surface receptors, the activation of which could lead to excessive stimulation of cell proliferation. All six cell lines tested secrete into their medium factors that stimulate DNA synthesis. The factor secreted by one cell line was characterized and found to resemble PDGF. Six of 11 cell lines had receptors for PDGF demonstrable by binding and receptor autophosphorylation assays. Six of six cell lines tested had EGF receptors demonstrable by binding and receptor autophosphorylation experiments. The extremely high levels of EGF receptor in one cell line may reflect excessive expression of the erb B oncogene associated with abnormalities of chromosome 7 that occur in this cell line.

Placental keratinocyte growth factor: partial purification and comparison with epidermal growth factor

A water-soluble extract of term human placenta, which was previously shown to promote proliferative growth of human keratinocytes in defined medium, enhanced both cellular attachment and proliferative growth. We have partially purified the activity which enhanced cell growth and examined its action in keratinocytes. Activity was precipitated from the crude extract by (NH4)2SO4 between 33 and 60% saturation and chromatographed by gel filtration. The activity did not bind to heparin-Sepharose at low ionic strength but was adsorbed to DEAE-cellulose from which it was eluted with NaCl and then passed over phenyl-HPLC to remove bovine serum albumin previously added to protect the activity. The active fraction was applied to gel exclusion HPLC in the presence of 0.02% octyl-beta-D-glucopyranoside, which yielded an apparent Mr 35,000 for the factor. Purification was approximately 200-fold with approximately 4% recovery. The factor appears to be a protein, since activity is destroyed by trypsin. Autoradiography of cultures treated with the placental factor or epidermal growth factor (EGF) revealed that approximately 50% of cells were labeled after treatment with either growth factor compared to 9% in control cultures after a [3H]thymidine pulse. Protein synthesis was increased by about 50% 42 h after treatment with either agent, consistent with a 50% increase in nuclear labeling. Cell number was increased fivefold after 6 days in the presence of the partially purified factor, whereas EGF increased cell number eightfold. Stimulation of [3H]thymidine incorporation by the partially purified factor, in contrast, was about twice that produced by EGF, indicating that thymidine incorporation is preferentially stimulated by the placental factor and does not correlate well with other parameters of proliferative growth. The placental keratinocyte growth factor is a unique factor with a novel effect on incorporation of thymidine into DNA.

The cyclic AMP-mediated stimulation of cell proliferation

The role of cyclic AMP (cAMP) in the regulation of mammalian cell proliferation has been the subject of controversy. Negative control was demonstrated in the 1970s, but evidence of positive control in other cell types has been neglected. Recent evidence which demonstrates such a control in the yeast Saccharomyces cerevisiae has now made this concept acceptable.

Fibroblast growth factor pretreatment reduces epidermal growth factor-induced proliferation in rat astrocytes

Epidermal Growth Factor (EGF) is mitogenic for purified rat astrocytes in primary tissue culture. A combined concomitant treatment by EGF and Fibroblast Growth Factor (FGF) does not reduce the proliferation effect of EGF, however when the astrocytes are pretreated with FGF, their response to an EGF stimulation is reduced by 70%. This inhibition of EGF stimulation by FGF pretreatment is consistent across the EGF dose response curve and perhaps represents a mechanism for local modulation of astrocyte mitogenic effects.

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).