Showing papers on "Cell growth published in 1978"

Role of cell shape in growth control

Abstract: Tissue culture plastic adhesivity was precisely varied by applying different concentrations of poly(2-hydroxyethyl methacrylate). The extent of cell spreading was thus accurately controlled so that cells cultured on these substrata could be held at any one of a graded series of quantitated cell shapes. Cell shape was found to be tightly coupled to DNA synthesis and growth in nontransformed cells. These findings suggest a mechanism that is important in growth control of mammalian cells, and provide a more fundamental interpretation of such phenomena as density dependent inhibition of cell growth and anchorage dependence.

Nuclear volume control by nucleoskeletal DNA, selection for cell volume and cell growth rate, and the solution of the DNA C-value paradox.

Abstract: The 40,000-fold variation in eukaryote haploid DNA content is unrelated to organismic complexity or to the numbers of protein-coding genes. In eukaryote microorganisms, as well as in animals and plants, DNA content is strongly correlated with cell volume and nuclear volume, and with cell cycle length and minimum generation time. These correlations are simply explained by postulating that DNA has 2 major functions unrelated to its protein-coding capacity: (1) the control of cell volume by the number of replicon origins, and (2) the determination of nuclear volume by the overall bulk of the DNA: cell growth rates are determined by the cell volume and by the area of the nuclear envelope available for nucleocytoplasmic transport of RNA, which in turn depends on the nuclear volume and therefore on the DNA content. During evolution nuclear volume, and therefore DNA content, has to be adjusted to the cell volume to allow reasonable growth rates. The great diversity of cell volumes and growth rates, and therefore of DNA contents, among eukaryotes results from a varying balance in different species between r-selection, which favours small cells and rapid growth rates and therefore low DNA C-values, and K-selection which favours large cells and slow growth rates and therefore high DNA C-values. In multicellular organisms cell size needs to vary in different tissues: size differences between somatic cells result from polyteny, endopolyploidy, or the synthesis of nucleoskeletal RNA. Conflict between the need for large ova and small somatic cells explains why lampbrush chromosomes, nurse cells, chromatin diminution and chromosome elimination evolved. Similar evolutionary considerations clarify the nature of polygenes, the significance of the distribution of haploidy, diploidy and dikaryosis in life cycles and of double fertilization in angiosperms, and of heteroploidy despite DNA constancy in cultured cells, and other puzzles in eukaryote chromosome biology. Eukaryote DNA can be divided into genic DNA (G-DNA), which codes for proteins (or serves as recognition sites for proteins involved in transcription, replication and recombination), and nucleoskeletal DNA (S-DNA) which exists only because of its nucleoskeletal role in determining the nuclear volume (which it shares with G-DNA, and performs not only directly, but also indirectly by coding for nucleoskeletal RNA). Mechanistic and evolutionary implications of this are discussed.

Control of normal cell differentiation and the phenotypic reversion of malignancy in myeloid leukaemia

Abstract: A cell system has been developed to determine and dissect the controls that regulate normal myeloid cell growth, differentiation and malignancy, and to suggest a novel approach to the therapy of myeloid leukaemia.

Epidermal growth factor stimulates phosphorylation in membrane preparations in vitro.

Abstract: EPIDERMAL GROWTH FACTOR (EGF) forms a complex with plasma membrane receptors in intact cells that initiates a series of biochemical events resulting in increased cell growth in vivo and in vitro1. The interaction of EGF with membrane receptors has been demonstrated in crude membrane preparations2, but no biochemical alteration of the membrane resulting from hormone binding has been detected. To clarify the molecular mechanisms regulating cell proliferation, specific biochemical reactions initiated by mitogens such as EGF need to be investigated in cell-free systems. As the human epidermoid carcinoma cell line A-431 has an extraordinarily high concentration of EGF receptors3,4 (2–3 x 106 receptors per cell), we have used a crude membrane preparation from these cells to look for an EGF-dependent alteration of membrane structure and/or function. We report here that (1) membranes may be prepared from A-431 cells that retain the ability to bind 125I-labelled EGF in a specific manner, and (2) the binding of EGF to these membranes in vitro results in a marked stimulation of the phosphorylation of endogenous proteins in the presence of [γ-32P]ATP.

Initiation of chemical carcinogenesis requires cell proliferation

Abstract: CELL proliferation has often been implicated in the development of cancer with chemicals1–3. Supportive evidence for this is the observation that several carcinogens that normally do not induce liver cancer in intact adult animals, especially with a single dose, become carcinogenic if administered as a single injection after partial hepatectomy (PH)1. In these conditions, it is thought that PH may act during initiation, presumably by fixation of some carcinogen-induced DNA damage through replication of the altered DNA. However, because carcinogenesis is a multi-step process, often involving the appearance of several new cell populations between the initial target cells and the ultimate cancer4,5, it is possible that the regenerative response of liver following PH could have a major effect on one or more steps subsequent to initiation. Thus, the use of a very late end-point (cancer) makes it difficult, if not impossible, to relate with any accuracy some very early event to any specific step in the carcinogenic process. Solt and Farber6 have recently developed a new approach to the sequential analysis of carcinogenesis in vivo that delineates the first few steps in the process and is a quantitative assay for initiation of liver cancer. We have used this model to investigate whether cell replication exerts its first effect on initiation or on some later step in the carcinogenic process. And if initiation is at least one site of action, when in the regenerative cell cycle is a carcinogen most effective and what biochemical events might be involved at this phase of the cell cycle?

Factors involved in the modulation of cell proliferation in vivo and in vitro: the role of fibroblast and epidermal growth factors in the proliferative response of mammalian cells.

Abstract: The control of proliferation of mesoderm-derived cells by EGF and FGF has been examined taking, as an example, the vascular endothelium. The mechanisms by which cell proliferation can be brought to a stop in vivo and in vitro have been reviewed.

Inhibitory Effects on Lipid Metabolismn in Cultured Cells of ML‐236B, a Potent Inhibitor of 3‐Hydroxt‐3‐methylglutaryl‐Cornzyme‐A Reductase

Abstract: Effects of ML-236B, a potent competiove inhibitor 3-hydrocy-3-methyglutaryl-CoA reductase, on the lipiod metabolism were studied using mouse L cells and human smin fibrobalsts. ML-236B completely inhibited sterol sythesis from both [14C]acetate and [14C]octanoate in these cells at a tconcentration of 0.5 μg/ml (11.117 μM). withour affecting thqat form [14C]mevalonate. Syntheses of other lipids adn macromolecules like DNA, RNA and protein were not affected at concentrations up to 5 μg/ml. Concentrations of ML-236B giving 50% inhibitionof sterol synthesis for [14C]acetate were as low homozygote with familial hypercholesteroloemia. The ML-236B-mediated inhibition of sterol syunthesis was readitly reversivle. The complkete inhibition of endogenous sterol synthesis at a higher concentration (5μg/ml) of ML-236-B caused a marked inhibition of cell growth even in the presence of exogenous cholesterol contained in while gfatel calf serum as lipoproteins. This inhivition of growth was prevented by teh presence in the culture medium of mevalonate, but not by acetate, thus indficating that ML-236B inhibit cell growth by specifically interferring with mevalonate wynthesis. It is further concluded thjat slight activity of endogenous sterol synthesis, which provides enfogenous copounds generated from meavalonate, may be essential to the growth of cells even when sufficeient amounts of cholesterol are availavel to the cells.

The effect of oxygen tension on haemopoietic and fibroblast cell proliferation in vitro.

Abstract: The effects of providing low oxygen tension in the gas phase of two different types of cell culture systems were investigated. The clonal growth of granulocyte-macrophage progenitor cells in an agar culture system was improved markedly by incubation within a low oxygen tension gas phase (48 mmHg--6.8%) instead of the conventional air (135 mmHg--19%), the effects being measured by increases in numbers of colony forming cells detected and in the colony sizes. The increased efficiency of colony formation was observed both with mouse and human marrow cells. A similar effect was observed in a liquid adherence culture system with primary cultures of foetal mouse fibroblasts both at clonal and higher cell densities.

Coordinate control of 3T3 cell proliferation by platelet-derived growth factor and plasma components.

Abstract: DNA synthesis and cell division were measured in Swiss mouse 3T3 cells cultured in different concentrations of cell-free plasma-derived serum and increasing amounts of a platelet-derived growth factor In plasma-derived serum alone, the cells were quiescent and they were arrested in the Go/G1 phase of the cell cycle Addition of a platelet-derived growth factor to quiescent cells maintained in plasma-derived serum stimulated both DNA synthesis and cell division When plasma components were present at high concentration (5%, vol/vol), the amount of platelet factor added to the cultures determined the number of cell doublings Plasma-derived molecules were required for the platelet factor to stimulate DNA synthesis and cell division in the maximal number of cells In addition, plasma components had to be present for recently divided cells to respond to the platelet factor When 3T3 cells were cultured in excess platelet factor and limiting amounts of plasma-derived serum (05%, vol/vol), the cells underwent one doubling and then ceased to proliferate Addition of fresh plasma-derived serum to these cells induced a second round of cell division Plasma components and the platelet-derived growth factor acted in a coordinate fashion to regulate the proliferation of Swiss 3T3 cells

Cell division from a genetic perspective

Abstract: A novel view of the eukaryotic cell cycle is taking form as genetic strategies borrowed from investigations of microbial gene regulation and bacteriophage morphogenesis are being applied to the process of cell division. It is a genetic construct in which mutational lesions identify the primary events, thermolabile gene products reveal temporal order, mutant phenotypes yield pathways of causality, and regulatory events are localized within sequences of gene controlled steps.

Dexamethasone modulates binding and action of epidermal growth factor in serum-free cell culture

Abstract: Experiments probing the mechanism by which glucocorticoids modulate cell proliferation were carried out on serum-free cell cultures of quiescent human diploid foreskin (HF) cells. Added alone, the synthetic glucocorticoid dexamethasone had no effect on cell number. However, dexamethasone enhanced the mitogenic response of HF cells to epidermal growth factor (EGF) by 50% at all EGF concentrations. The mitogenic action of EGF was maximally promoted by a dexamethasone concentration of 100 ng/ml (0.25 μM). Binding studies with 125 I-labeled EGF ( 125 I-EGF) suggested that dexamethasone caused this “permissive” effect by modulating cell surface receptors for EGF. Paralleling their increased responsiveness to EGF growth stimulation, dexamethasone-treated cells exhibited a 50-100% increased ability to bind physiological concentrations of 125 I-EGF. A binding increase was apparent after a 4-hr dexamethasone treatment. The dexamethasone-treated cells maintained an increased ability to bind 125 I-EGF during the prolonged exposure to EGF that was required to stimulate cell division. Moreover, the increase in 125 I-EGF binding exhibited a dexamethasone dose-dependence similar to that for the enhancement of EGF mitogenesis, suggesting a relationship between the dexamethasone effects on binding and growth. An investigation of the binding increase showed that it was specific for glucocorticoids, and required protein synthesis. The enhancement of 125 I-EGF binding diminished with increasing concentrations of 125 I-EGF, indicating that dexamethasone caused a qualitative change in the EGF receptors (possibly a change in receptor affinity or cooperativity). The alteration in 125 I-EGF binding may occur as part of a far-reaching dexamethasone-mediated change in the cell surface, because dexamethasone treatment slightly increased the ability of HF cells to bind 125 I-insulin, and decreased by half their ability to bind 125 I-thrombin.

Mutants altered in the control co-ordinating cell division with cell growth in the fission yeast Schizosaccharomyces pombe.

Abstract: The control co-ordinating cell division with cell growth has been investigated in the fission yeast Schizosaccharomyces pombe Twenty-five mutants altered in this control have been isolated which have the same growth rate as wild type but divide at a smaller cell size The mutants define two genes wee 1 and wee 2, both of which are involved in a control initiating mitosis when the cell attains a critical size

Growth of a human breast cancer cell line in serum-free hormone-supplemented medium.

Abstract: ZR-75-1, a human breast cancer cell line, has been grown in hormone-supplemented medium without serum. The factors required for optimal growth include 17β-estradiol, insulin, transferrin, dexamethasone, and l-triiodothyronine. If estradiol, insulin, or l-triiodothyronine is omitted, cells cease division within 7 days, but viability is retained for at least 14 days. Omission of transferrin leads to cell death within 7 days. The cells have been continuously maintained in this environment without morphological alteration or cessation of growth for more than 5 months. Addition of the anti-estrogen, Tamoxifen (10-6m), inhibited cells below the growth rate seen when estradiol was omitted from the medium, even when Tamoxifen was added 4 days and two medium changes after the removal of estradiol from the medium, thus suggesting an action of Tamoxifen which may be independent of competition with estradiol. The availability of a human breast cancer cell line that can be propagated in hormone-supplemented medium without serum should aid in the study of the mechanisms by which hormones effect cell growth.

Genetic toxicology of bleomycin.

Abstract: Bleomycin (BLM), an antibiotic obtained from Streptomyces verticillus, is of significance as an antineoplastic agent. The compound is actually the mixture of some 200 related forms which differ from each other in the amine moiety. The drug, at low concentrations, can cause elimination of bases, particularly thymine. This causes strand breakage of DNA and inhibition of cell growth. The influence of BLM on cell growth may be unrelated to the effects on DNA. In general, mitotically dividing cells show more DNA damage than non-dividing cells. G2 seems to be the most sensitive phase indicating that cell death may not be related to a direct effect of BLM on DNA replication. The antibiotic shows specific effects on chromatin and causes chromosomal damage in all sub-phases of interphase. It can affect early prophase chromosomes also. Suggestion has been made that BLM-induced breakage and cell death are similar to those induced by densely ionizing radiations. Whereas the antibiotic affects the frequency of somatic crossing over and produces micronuclei, the data on mutation induction and production of sister-chromatid exchanges do not permit classifying BLM as a potent inducer of these phenomena. The genetic effects of BLM can be modified quantitatively by thiol compounds, caffeine, hyperthermia and H2O2. It is concluded that the available data do not permit assessment of genetic damage in the offsprings of BLM-treated patients. Such studies are urgently needed, as are the studies to find out the effects of BLM on meiotic phenomena.

Transforming viruses directly reduce the cellular growth requirement for a platelet derived growth factor.

Abstract: Early passage mouse embryo fibroblasts, mouse 3T3 cell lines, and early passage diploid human fibroblasts grew to higher cell densities in tissue culture medium supplemented with serum than in medium supplemented with defibrinogenated platelet-poor plasma (PPP). Unlike the mouse cells, the human fibroblasts displayed this differential growth response only in the presence of hypophysiologic concentrations of calcium. The addition of heat-treated extracts of human platelets to PPP-supplemented medium stimulated the replication of both the normal mouse cells and early passage human embryo fibroblasts. Human or mouse fibroblasts transformed by either retroviruses or by SV40, including SV40 infected “serum revertants” and “flat transformants,” grew to equal cell densities in medium supplemented with either serum or PPP. Infection of Balb/c-3T3 cells with SV40 rapidly induced them to grow in PPP-supplemented medium demonstrating that the ability of SV40-transformed cell lines to proliferate in PPP-supplemented medium does not arise from the cell culture selection procedures usually employed to obtain stable virus-transformed cell lines. 3T3 cells infected but not transformed by retroviruses do not replicate in PPP-supplemented medium demonstrating that reduction of the growth requirement for the platelet growth factor(s) by retroviruses is a transformation-specific response. Cell cultures that did not proliferate well in PPP-supplemented medium did not form tumors when inoculated into athymic nude mice. Many, although not all, of the lines which grew well in PPP medium were tumorigenic in nude mice. Together, these findings indicate that: (1) normal fibroblast-like cells display a growth requirement for factor(s) present in serum but not found in PPP; (2) this serum specific growth factor is derived from platelets; (3) a primary response to viral transforming genes is a reduction in the growth requirement for these platelet-derived factors; and (4) cells that have a reduced requirement for the platelet-derived growth factor are often tumorigenic.

Proliferation Kinetics of a Human Breast Cancer Line in Vitro following Treatment with 17β-Estradiol and 1-β-d-Arabinofuranosylcytosine

Abstract: The effect of 17β-estradiol on an estrogen receptor-positive human breast cancer cell line (MCF-7) was studied. Low concentrations (10 -9 m) of 17β-estradiol enhanced the rate of cell proliferation; the overall cell cycle time was shortened; and the proportion of cells in the S phase increased. Higher concentrations (10 -7 m) suppressed proliferation and slightly decreased the proportion of cells in DNA synthesis. When combined with 1-β-d-arabinofuranosylcytosine, an S-phase-specific chemotherapeutic agent, 10 -9 m 17β-estradiol enhanced cell killing. This enhancement was not observed with 10 -7 m 17β-estradiol. Kinetic changes caused by hormones have profound implications in clinical therapy, since the efficacy of cycle active agents may be altered.

Retinol induces density-dependent growth inhibition and changes in glycolipids and LETS.

Abstract: CHEMICAL carcinogenesis and viral transformation are prevented or inhibited by vitamin A or its analogues1,2. Vitamin A may also play a part in membrane glycosylation as a lipid intermediate3,4. Recently, effects of retinoic acid on proliferation and density-dependent growth of mouse L cells5 and many other untransformed and transformed cells in vitro have been described6. In view of these observations, we have studied the effect of retinol on cell growth behaviour, glycolipid synthesis, and surface-exposed profile of glycoproteins in hamster fibroblasts NIL, NILpy and mouse (BALB/c) 3T3 cells. We report here that culturing NIL or 3T3 cells in vitamin A-containing medium markedly enhances, whereas a medium with ultraviolet-irradiated serum markedly reduces, contact orientation and cell-density dependent inhibition of cell growth. Associated changes of cell surface membrane GM3 level, ganglioside contact response, and in LETS (‘Gap a’) were observed.

A runaway-replication mutant of plasmid R1drd-19: temperature-dependent loss of copy number control.

Abstract: A two-step mutant plasmid, originating from the resistance plasmid R1drd-19, was shown to replicate without control of the plasmid copy number when the host bacteria were grown at temperatures above 35°C The uncontrolled plasmid replication, so called runaway-replication, eventually inhibits the host cell growth and results in a conditionally lethal phenotype As much as 75% of the total DNA in the cells was found to be covalently closed circular plasmid DNA at the stage when the cell growth stopped At 30°C the mutant plasmid had a strictly controlled copy number and the onset and kinetics of runaway-replication was studied by temperatureshift experiments The synthesis of plasmid DNA after a temperature shift from 30°C to 40°C (or 37°C) was found to be exponential with a doubling time of about half the doubling time of the host bacteria, in broth medium as well as in minimal salt-glucose medium In broth medium the plasmid population had a doubling time of about 12 min which is longer than expected if it is assumed that the rate of elongation during the DNA polymerization is the only limitation on the uncontrolled replication The results indicate that the minimum time required between successive replications of a plasmid copy is determined by some rate-limiting step(s) presumably after the duplication process The uncontrolled plasmid replication was shown to be inhibited by the protein synthesis inhibitor chloramphenicol The plasmid synthesis stopped within 20 min after the addition of the drug and the plasmid DNA increment was less than 50%, suggesting that the uncontrolled plasmid replication requires de novo protein synthesis Similarly, the RNA polymerase inhibitor rifampicin did inhibit the plasmid replication The mutational alteration could be reversed, and the copy number control restored, by lowering the temperature before the host cell growth was inhibited This type of plasmid mutant offers new possibilities in studies of host cell-plasmid interactions

Physiological quiescence in plasma-derived serum: influence of platelet-derived growth factor on cell growth in culture.

Abstract: A platelet-derived growth factor can be shown to be the principal stimulant of DNA synthesis in whole blood serum for those cells that require serum for maintenance and growth in culture. Cell free plasma-derived serum lacks such platelet-derived material. 3T3 cells and primate arterial smooth muscle cells can be maintained in a quiescent state in culture for as long as six weeks in plasma-derived serum. Such cells can grow logarithmically after exposure to 5% whole blood serum or as little as 100 ng/ml of partially purified platelet factor. The cell cycle of smooth muscle cells has been studied in the quiescent (5% plasma-derived serum) and growing state (5% whole blood serum or 5% plasma-derived serum plus platelet factor). The generation time of smooth muscle cells is 16 to 18 hours as shown by autoradiographic frequency of labelled mitoses. The generation time is the same for cells in the growth fraction in either 5% whole blood serum or 5% plasma-derived serum. Thus, platelet factor acts by recruiting cells into the growth fraction rather than effecting a change in the duration of the cell cycle. Flow microfluorimetry studies on cells growing logarithmically in 5% whole blood serum give the following phase durations: G1 = 5.6 hours; S = 7.6 hours; and G2 + M = 3.8 hours. Based on these studies the argument is presented that cells cultured in 5% plasma-derived serum provide a more physiological base for the study of quiescence than do cells in low concentrations of whole blood serum or confluent, density inhibited cells at high (5% or greater) concentrations of whole blood serum. Furthermore, 5% plasma-derived serum represents an appropriate state to examine the perturbation of quiescent cells.

Collagen required for proliferation of cultured connective tissue cells but not their transformed counterparts.

Abstract: THE proline analogue, cis-hydroxyproline, when incorporated into collagen, prevents collagen secretion1 and also inhibits the growth of fibroblasts in vitro2. We describe here studies on the mechanism of action of cis-hydroxyproline on cell proliferation of primary cultures of murine connective tissue cells and of their spontaneously transformed, tumorigenic counterparts. We find that the analogue inhibits the attachment, spreading and proliferation of the connective tissue cells and that these effects are completely reversed when the cells are grown on collagen-coated dishes. The tumorigenic cells are less affected by cis-hydroxyproline even though their collagen synthesis is inhibited to a similar extent to the connective tissue cells. These results suggest that the presence of a collagen matrix is required for the growth of some connective tissue cells in vitro but not for the growth of their tumorigenic counterpart.

A human fibrosarcoma cell line producing multiplication stimulating activity (MSA)-related peptides

Abstract: WE suggested previously that transformed cells may produce and release cellular growth factors capable of interacting with a specific class of cellular receptors, making them unavailable as receptors for external ligands1,2. A prediction of this model was that a human fibrosarcoma line, which had been shown not to bind 125I-labelled multiplication stimulating activity (MSA)2, might be secreting molecules functionally related to MSA. We describe here some properties of growth factors produced by this human tumour cell line, derived from a 25-yr-old female with fibrosarcoma of the leg. The cells produce a family of MSA-related compounds with different molecular weights that can be separated on Sephadex G-75. The major activities correspond to proteins of approximate molecular weight (MW) of 11,000 and 7,000. Both of these fractions stimulate cell division, and will compete with 125I-labelled MSA for its receptors on mouse, rat or human cell membranes. Fibrosarcoma cells in culture may constitute an important alternate source to human serum and plasma for the isolation and characterisation of growth stimulating factors.

Mechanism of sensitivity of cultured pancreatic carcinoma to asparaginase

Abstract: The effects of E. coli L-asparaginase on cultured human pancreatic carcinoma (MIA PaCa-2) have been studied. The enzyme (1 U/ml) inhibited growth and protein synthesis in both MIA PaCa-2 and PANC-1, another pancreatic carcinoma cell line, but had little or no effect on human breast carcinoma or melanoma cells. The inhibition of protein synthesis by E. coli L-asparaginase was largely reversed by L-glutamine but not by L-asparagine. The growth of both MIA PaCa-2 and PANC-1 showed absolute dependence on L-glutamine. These results indicate that the effect of E. coli L-asparaginase on cultured pancreatic carcinoma cells is exerted at least in part through its L-glutaminase activity. Although the addition of L-glutamine to the culture appeared to prevent cell death caused by L-asparaginase, it did not restore the ability of the cells to proliferate. Asparaginase derived from vibrio succinogenes, which is virtually free of L-glutaminase activity, was equally inhibitory to MIA PaCa-2 cell growth but did not affect protein synthesis. It is concluded that the inhibition of growth of cultured pancreatic carcinoma cells by E. coli asparaginase is a combined function of both its L-asparaginase and L-glutaminase activity.

Glucocorticoid Potentiation of the Adenosine 3′,5′- Monophosphate Response to Parathyroid Hormone in Cultured Rat Bone Cells

Abstract: We have previously demonstrated the presence of glucocorticoid receptors in bone cells and have suggested that direct inhibition of cell proliferation may contribute to glucocorticoid-induced osteoporosis. The present studies examine the issue of whether glucocorticoids might also alter bone status by interacting with parathyroid hormone (PTH) a major regulator of bone homeostasis. Since cyclic AMP (cAMP) is the putative mediator of PTH-induced inhibition of collagen synthesis as well as resorption, we assessed the ability of dexamethasone to modulate the cAMP response to PTH. Bone cells from calvaria of 20-21 day fetal rats were cultured for 6–7 days and the ability of PTH to stimulate cAMP was measured. The cells responded to both the synthetic (1–34) fragment and native PTH with a dose-dependent increase in cAMP. Dexamethasone treatment did not alter base-line levels of cAMP but did potentiate the cAMP response to PTH: 1- to 2- fold when assayed in the presence of theophylline and 2- to 4-fold in its a...