An electron microscopy study of the effects on dibutyryl cyclic AMP on Chinese hamster ovary cells

Membrane and cytoplasmic changes in 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU)-sensitive and resistant human malignant glioma-derived cell lines

Human glioma-derived cell lines previously determined by a microtiter chemotherapy assay to be either 'sensitive' or 'resistant' to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) were treated with BCNU (1-80 micrograms/ml) and observed using microcinematography, scanning electron microscopy, and transmission electron microscopy. Striking bleb formation and cell retraction were observed to occur in a dose-dependent relationship within minutes in the cells known to be BCNU-sensitive. At 15 micrograms/ml, 69% of cells showed blebs by 30 min, 87% by 90 min, and 100% by 4 hr. This activity was not seen in BCNU-resistant cells. These morphological changes occur at a time too early to be accounted for by the known BCNU mechanism of DNA alkylation and cross-link formation and suggest that cytoplasmic and/or membrane events may be significant initial events in the cytotoxic actions of BCNU.

Free and polymerized tubulin in cultured bone cells and Chinese hamster ovary cells: the influence of cold and hormones

Free and polymerized tubulin were measured in bone cells and Chinese hamster ovary (CHO) cells cultured on plastic substrata. Polymerized tubulin was stabilized in a microtubule- stabilizing medium (MSM) containing 50 percent glycerol and separated from free tubulin by centrifugation. Tubulin content was assayed in both fractions by the colchicines- binding assay. The measured degree of polymerization in both bone cells and CHO cells varied with stabilixation conditions. The degree of polymerization in both bone cells and CHO cells varied with stabilization conditions. The degree of polymerization in both bone cells and CHO cells varied with stabilization conditions. The degree of polymerization in attached cells was found to increase up to 73 percent during the first 20 min after addition of the MSM at 24 degrees C, and remained constant thereafter. Stabilization of 0 degrees C resulted in a decrease down to 62 percent in the degree of constant thereafter. Stabilization at 0 degrees C resulted in a decrease down to 62 percent in the degree of polymerization during the first 20 min after addition of the MSM at 24 degrees C, and remained constant thereafter. Confluent bone cells maintained at 0 degrees C for 1 h before stabilization contained significantly less polymerized tubulin than control cells kept at 37 degrees C using stabilization both at 0 degrees C and at 24 degrees C. Changes in bone cell morphology induced by incubation of cells with prostaglandin E(1) or E(2), parthyroid hormone, and dibutyryl cyclic AMP were not associated with a change in the degree of tubulin polymerization. This was confirmed morphologically by immunofluorescence using affinity-purified tubulin antibodies: microtubules in hormone- treated cells were not noticeably reorganized when compared to microtubule organization in control cells. They were, however, squeezed closer together in cellular pseudopods due to the altered cell shape. This altered cell shape appears to be correlated with disorganization of the microfilament system, since microfilaments, detected using affinity-purified actin antibodies, did alter drastically their appearance and distribution after hormone addition.

Temperature-dependent alteration of cellular morphology by cholera toxin in rat liver epithelial cells which are ts for maintenance of transformed properties

Cholera toxin via its ability to increase intracellular cyclic AMP levels can induce drastic changes in cell morphology. This report describes a temperature sensitive mutant of chemically transformed rat liver epithelial cells which only display cell shape alterations in response to cholera toxin at the permissive temperature. Shift up-shift down experiments indicate that the change in the response occurs fairly rapidly, i.e., within 2 hours at the new temperature. The behavior of the temperature sensitive cells at the nonpermissive temperature mimics that of the untransformed rat liver epithelial cells (i.e., no morphological change in response to cholera toxin) while at the permissive temperature the positive cell shape change is identical to that exhibited by chemically transformed rat liver epithelial cells. The temperature sensitive response to cholera toxin is not a function of cyclic AMP production, since the amount of cyclic AMP found as a function of either time or concentration of cholera toxin is quite similar in cells treated at either temperature.

Morphological evidence for cyclic AMP-induced reverse transformation in vole cells infected with avian sarcoma virus

Normal fibroblasts of the vole displayed moderately spread or flattened, spindle-shaped, or polygonal morphologies and attached firmly to a substrate. Topographic features of these cells included sparse microvilli, ruffles, and filopodia. Microfilament bundles, intermediate filaments, and long microtubules generally parallel to each other, and the long axis of the cell or its extensions were present in the cytoplasm. Fibronectin was abundant, and fibronectin fibrils often formed junctions at the cell membrane with microfilament bundles. Transformation with avian sarcoma virus converted 90% of the cells to spheres 5 to 10 microns in diameter. In contrast to the normal vole cells, microfilament bundles were absent, microtubules were short and randomly arranged, and fibronectin was no longer visible. Exposure to dibutyryl cyclic AMP and testololactone caused a majority of the spherical cells to stretch and flatten, a process referred to as reverse transformation. Microtubules radiated out to the cell periphery and became parallel in cell extensions, while long microfilament bundles appeared in the cytoplasm. Parallel intermediate filaments were arranged throughout the cell. This ultrastructural analysis of reverse transformation in avian sarcoma virus-transformed vole cells detailed the status of the cytoskeletal system and showed agreement with earlier findings (Puck et al., J. Cell. Physiol. 107:399-412, 1981) using indirect immunofluorescence.

Decreases in tubulin and actin gene expression prior to morphological differentiation of 3T3 adipocytes

The differentiation of 3T3-F442A preadipocytes is characterized by numerous enzymatic events and by a programmed change in cell morphology from a fibroblastic form to a nearly spherical shape. Accompanying this morphological change are large and specific decreases in biosynthetic rates for beta and gamma actin, vimentin and alpha and beta tubulin, as detected by one- and two-dimensional gel electrophoresis. In cells undergoing adipose differentiation, actin synthesis decreases by 90%, while the decrease in tubulin synthesis is more than 95%. Translation in vitro of mRNA isolated from differentiating cultures indicates that the decreases in biosynthetic rate for cytoskeletal proteins result from altered levels of active mRNA. Using cloned cDNA probes for beta actin and alpha tubulin, we show that changes in mRNA activity correspond to a specific loss of these sequences during cellular differentiation. Quantitatively, this loss of tubulin and actin mRNA sequences accounts virtually completely for the changes in protein biosynthetic rates. Examination of the synthesis and accumulation of cytoskeletal proteins and of their temporal relation to morphological conversion indicates that the biosynthetic changes are very early events in the differentiation, and suggests strongly that they participate in the development of the adipocyte morphology. The early occurrence of decreased cytoskeletal-protein synthesis also suggests that subsequent biosynthetic events specific to adipocyte differentiation may be influenced by alterations in the cytoskeleton.

Evaluation of the drug-induced morphological differentiation of rat glioma cells (C-6) from the aspects of S-100 protein level and con A binding pattern

The intracellular content of the nervous system specific protein S-100 began to increase with 4 days latency following the morphological differentiation of cultured rat glioma cells (C-6) with 1 mM dibutyryl cyclic AMP (dbcAMP), rising to approximately 10-fold over the control level at 15 days after the treatment. The concanavalin A (Con A) binding pattern on the external cell surface of C-6 cells exposed to dbcAMP appeared as a smooth layer of 40-60 nm thickness whereas that of control cells was irregularly thick and patchy. The correlation between morphological and biochemical changes of C-6 cells after dbcAMP treatment is discussed in relation to the mechanism controlling the differentiation of glioma cells.

Reverse transformation of vole cells transformed by avian sarcoma virus containing the src gene

Vole cells transformed by avian sarcoma virus carrying the src gene lose their fibroblastic morphology, the organized cytoskeletal system of the normal fibroblastic cell, the typical fibronectin deposit around the cell membrane, and the ability to shut off multiplication when suspended in liquid medium. All of these transformation characteristics are reversed by treatment with cAMP derivatives. Moreover, the cAMP treatment does not cause loss of activity of the src gene product. These data imply that cAMP exerts its effect at or after the point in the metabolic pathway affected by the src gene product, pp60src. Presumably, the decision to adopt the transformed or the normal state is determined by the degree to which the src gene or cAMP-mediated kinase activities respectively predominante in the cell. The development of all four transformation characteristics as a result of introduction of the src gene, and their coordinate reversal by cAMP derivatives, supports the previous thesis that in the normal vole or CHO fibroblast all four properties are part of a common regulatory system.

Differential and synergistic actions of nerve growth factor and cyclic AMP in PC12 cells

When a clonal line of rat pheochromocytoma (PC12) was exposed to beta-nerve growth factor (beta NGF), N6, O2-dibutyryl adenosine 3':5' cyclic monophosphate (Bt2cAMP), or a combination of the two, 10, 26, or 70% of the cell clumps, respectively, displayed neurites after 1.d. Increases in the cellular RNA concentration were also found to be additive or greater when both agents were present. Neurites induced by Bt2cAMP alone were not maintained after replacement with beta NGF. The degree of potentiated neurite outgrowth was a function of the time of simultaneous exposure to both agents. The initiation of neurite outgrowth in the presence of Bt2cAMP was independent of RNA synthesis, in contrast to that induced by beta NGF alone. We conclude that beta NGF-induced initiation of morphological differentiation of these cells is not mediated by a cAMP-dependent mechanism. Consideration of Bt2cAMP effects upon other cell lines suggest that Bt2cAMP causes a rapid, but unstable, reorganization of the PC12 cytoskeleton, resulting in the initiation of neurite outgrowth from these cells. In contrast, beta NGF alone achieves a more stable cytoskeleton reorganization by an RNA synthesis-dependent mechanism.

Effects of Ca+2 and Mg+2 deprivation of cell shape in cultured ovarian granulosa cells

After 3 days in culture, rat ovarian granulosa cells assume a flattened epitheloid organization. Ca+2 and Mg+2 deprivation results in cellular rounding which is reversible and was monitored by phase-contrast time-lapse cinematography. Concomitant with the shape change is a dispersion of the structural proteins actin and alpha-actinin. The arrays of large actin-containing bundles (stress fibers) are converted to a diffuse network as observed by electron microscopy. Alpha-actinin, which was observed by immunocytochemistry to be in a periodic array along the actin bundles, is disrupted also and redistributed in the periphery of the cell upon rounding. Measurements made of the culture medium during the rounding process indicate that there is a loss of Ca+2 and Mg+2 from the cell interior. These data led us to speculate that Ca+2 and/or Mg+/ are necessary in order to maintain the integrity of stress fibers and/or restrict the movement of alpha-actinin anchoring sites within the membrane.

Temperature- and cyclic nucleotide-induced phase transitions of Histoplasma capsulatum

The transition from yeast to mycelia of Histoplasma capsulatum could be accomplished by shifting the temperature of incubation from 37 to 25 degrees C. It was accompanied by many changes in cellular metabolism, including changes in respiration, intracellular cyclic adenosine 3',5'-monophosphate (cAMP) levels, and activities of two enzymes specific for the yeast phase, cystine reductase (EC 1.6.4.1) and cysteine oxidase (EC 1.13.11.20). Even at 37 degrees C, the yeast to mycelial transition could be induced by cAMP and agents which raise the intracellular levels of cAMP (theophylline, acetylsalicylic acid, prostaglandin E1, and nerve growth factor). During this morphogenesis the same pattern of changes occurred as in the temperature-induced transition. Therefore, these changes were not simply dependent on a shift in temperature, but rather were part of the process of the phase transition.

Growth regulation, reverse transformation, and adaptability of 3T3 cells in decreased Mg2+ concentration

A nontransformed and a spontaneously transformed clone of BALB/c 3T3 cells were compared for their capacity to multiply in decreased concentrations of Mg(2+). Cells of the nontransformed clone were flat, formed regularly patterned, nonoverlapping arrays, required high serum concentration for multiplication, had a low saturation density, and did not make colonies in agar. Cells of the transformed clone were slender and spiky, formed random, overlapping arrays, multiplied in low serum concentrations, and had no fixed saturation density, and 20-30% of them formed colonies in agar. The saturation density of the nontransformed clone was decreased in a growth-limiting supply of Mg(2+) in proportion to the reduction in initial rate of multiplication. At very low Mg(2+) concentrations, saturation occurred when less than half of the surface of the dish was covered with cells. The transformed cells did not reach a stable saturation density in low Mg(2+) concentrations, but their growth rate did slow down when they became crowded, and a transient saturation density was reached at the lowest Mg(2+) concentrations that allowed multiplication. Limiting the supply of Mg(2+) caused the transformed cells to flatten and to assume a regularly patterned, non-overlapping relationship to one another, resembling that of the nontransformed cells. This also occurred in BALB/c 3T3 cells transformed by infection with Moloney mouse sarcoma virus. After 1 week in low concentrations of Mg(2+), the nontransformed cells began to multiply and to incorporate [(3)H]thymidine at a rapid rate. The transformed cells did so also and, in addition, reverted to their transformed appearance. The intracellular content of Mg(2+) was not significantly decreased when the extracellular concentration was decreased to 1/50th. The results suggest that: (a) limited contact among cells already multiplying at a reduced rate is sufficient to halt further multiplication; (b) a very small decrease in intracellular Mg(2+) content or in membrane-associated Mg(2+) causes transformed cells to assume aspects of the appearance and behavior of nontransformed cells (i.e., Mg(2+)-regulated reactions may be involved in determining the transformed phenotype); and (c) cells multiplying at a slow rate in low concentrations of Mg(2+) begin to multiply faster after about 1 week, due either to an adaptation of the cells or to a change in the cellular microenvironment.

Increased cyclic AMP content directly correlated with morphological transformation of cells infected with a temperature-sensitive mutant of mouse sarcoma virus

Normal rat kidney cells infected with a cold-sensitive mutant of mouse sarcoma virus [NRK(MSV-lb)] morphologically transform when exposed to adenosine 3':5' cyclic monophosphate (cAMP) at the restrictive temperature. The cAMP-induced morphological changes occur rapidly and are reversible. Agents capable of elevating endogenous levels of cAMP [prostaglandin E1 (PGE1) and cholera toxin (CT)] induced morphological transformation of NRK(MSV-lb) cells at the restrictive temperature that was concentration dependent, potentiated by cAMP phosphodiesterase inhibitors, and not prevented by inhibitors of DNA, RNA, and protein synthesis. Prostaglandin E1 stimulated a transient increase in the intracellular level of cAMP with a concomitant morphological transformation and reversion of cells as cAMP levels decline. The maximum increase is reached by 10 min, followed by a decline to near basal level by 80 min. In contrast, incubation of cells with CT resulted in irreversible morphological transformation and increased levels of cAMP first detectable by 1 hr with maximum levels reached by 24 hr. Heated CT (100 degrees C, 20 min) was without effect. Addition of CT to reverted PGE1-treated cells resulted in morphological transformation suggesting the existence of discrete receptors in NRK(MSV-lb) cells.

Induction of surface glycoprotein expression by cyclic AMP in Chinese hamster ovary cells

Surface expression of a membrane glycoprotein of 135,000 molecular weight (P135) was inducible by adenosine 3',5' -cyclic monophosphate in Chinese hamster ovary cells, CHO-K1 clone. Cells were cultured in the presence or absence of cyclic AMP derivatives, chemicals influencing cytoplasmic cyclic AMP levels, or inhibitors of protein or RNA synthesis. Surface proteins wre radiolabeled by a lactoperoxidase-catalyzed iodination reaction and analyzed by two-dimensional polyacrylamide gel electrophoresis. Surface expression of P135 increased 3---5-fold inthe presence of N6,O2' -dibutyryl cyclic AMP or 8-parachlorophenylthio cyclic AMP. Induction was also observed after treatment with prostaglandins E1 and F2 alpha, but not with sodium butyrate. Phosphodiesterase inhibitor, Roche compound Ro20-1724, enhanced the effect of N6,O2' -dibutyryl cyclic AMP. Metabolic incorporation of [35S]methionine into P135 was increased by N6,O2' -dibutyryl cyclic AMP. The induction was sensitive to inhibitors of protein and RNA biosynthesis. These results are consistent with a proposal that cyclic AMP controls the synthesis of this protein. Metabolic incorporation of a radioactive precursor suggested that P135 was a glucos-amine-containing glycoprotein. P135 appeared to be strongly associated with cell membrane because it was resistant to extraction of plasma membrane by cole 0.1 N NaOH.

Calcium and microtubule dependence for increased ornithine decarboxylase activity stimulated by beta-adrenergic agonists, dibutyryl cyclic AMP, or serum in a rat astrocytoma cell line

Ornithine decarboxylase (ODCase; L-ornithine carboxy-lyase; EC 4.1.1.17) activity was increased 5-20 fold above basal activity by N(6),O(2')-dibutyryl cyclic AMP, isoproterenol, epinephrine, or fetal calf serum in confluent C6-2B rat astrocytoma cells. Serum increased ODCase activity by a mechanism apparently independent of cyclic AMP because it decreased intracellular cyclic AMP. Calcium ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) buffers (pCa = 6.4) reduced basal ODCase activity and blocked the increase in activity by beta-adrenergic agonists, dibutyryl cyclic AMP, or serum. Readdition of calcium to pCa = 4.0 restored the ability of the cells to respond to these agents. The rise in ODCase activity stimulated by isoproterenol, dibutyryl cyclic AMP, or serum was also blocked by the microtubule disrupting agents, vinblastine and colchicine, and by the microfilament disrupting agent, cytochalasin B. Lumicolchicine, an analog of colchicine that does not disrupt microtubules, was ineffective. The microtubule and microfilament disrupting agents and EGTA had no effect on the intracellular accumulation of cyclic AMP. Cycloheximide inhibited protein synthesis >95% within 30 min, and caused an immediate decline of stimulated ODCase activity with a half-time of 20-30 min. The inhibition of ODCase activity by colchicine or EGTA was distinct from that seen with cycloheximide and could not be correlated to their effects on general protein synthesis. Colchicine or EGTA each caused identical rates of decline in ODCase activity with a half-time of 20-30 min after an initial lag period of about 60 min.

Isolation and characterization of a morphologic variant of Chinese hamster (CHO) cells

An epithelioid clone of Chinese hamster (CHO) cells which is spontaneously transformed was exposed to the mutagen N-methyl-N-Nitro-N-Nitrosoguanidine (MNNG), and a fibroblastic variant, clone CHO-F2, was isolated. This clone is partially reverted in several of the in vitro properties characteristic of transformed cells. When compared to wild type CHO, CHO-F2 has a longer doubling time, a lower saturation density and less piling up at high cell density, and a higher serum requirement. CHO-F2 also elaborates less plasminogen activator and has more abundant microtubules and actin cables. On the other hand, both CHO and CHO-F2 grow in agar suspension (although CHO-F2 grows with a lower efficiency), both lack detectable LETS protein, and both are tumorigenic in nude mice. Thus, expression of the individual properties frequently associated with transformation and tumorigenicity can be dissociated. The most critical biochemical change in CHO-F2 appears to be an increase in the intracellular level of cyclic AMP, when compared to CHO, and several growth and morphological properties of CHO-F2 resemble those induced in wild type CHO exposed to exogenous dibutyryl cyclic AMP. The role of cyclic AMP in expression of the transformed phenotype and the significance of individual in vitro parameters of transformation with respect to tumorigenicity are discussed.

Deposition of fibronectin in the course of reverse transformation of Chinese hamster ovary cells by cyclic AMP

The Chinese hamster ovary (CHO) cell, like other transformed cells, has lost the fibronectin deposit around its membrane. Treatment with cyclic AMP derivatives restores the typical fibroblastic deposit of fibronectin. Thus, the reverse transformation process induced by cyclic AMP (cAMP) in the CHO cell restores this important property as well as other morphological, biochemical, and growth behavioral characteristics of the normal fibroblastic state. The fibronectin deposit occurs significantly later in time than do other characteristics of the reverse transformation reaction and may therefore reflect a metabolic action that requires other cAMP effect to precede it. The restoration of fibronectin deposition in response to cAMP derivatives is also exhibited by vole cells transformed by avian sarcoma virus, but it is not by HeLa cell. Addition of Colcemid, which disrupts microtubules, to CHO cells containing a fibronectin deposit induced by cAMP derivatives causes little or no erosion of the deposit, but cytochalasin B, which disrupts 5-nm microfilaments, eliminates it completely. Thus, various features of the action of cAMP derivatives on CHO and related cells require integrity of the cellular microfibrils--in some cases microtubules only, in some cases 5-nm microfilaments only, and in some cases both classes of fibrils.

Specific protein phosphorylation during cyclic AMP-mediated morphological reversion of transformed cells

Treatment of transformed Chinese hamster ovary cells with dibutyryl cAMP or other agents that elevate cAMP results in the acquisition of growth and morphology characteristic of normal fibroblasts. The role of specific protein phosphorylation in this process of morphological reversion has been examine using metabolic labeling of Chinese hamster ovary (CHO) cells with 32P-orthophosphate in the presence or absence of N6O2'-dibutyryladenosine 3':5'-cyclic monophosphoric acid (Bt2cAMP). Analysis of labelled cultures by SDS gel electrophoresis and radioautography demonstrate dramatic changes in the phosphorylation of only 2 cellular proteins during reverse transformation. A 55,000 dalton protein (pp55) was phosphorylated and a 20,000 dalton protein (pp20) was dephosphorylated. The time course of these events was consistent with the kinetics of morphological reversion. The lower molecular weight species, pp20, was dephosphorylated within 15-30 minutes, prior to all morphological changes except membrane tranquilization. The higher molecular weight protein, pp55, was maximally phosphorylated over 1-2 hours following addition of Bt2cAMP, paralleling early stages in the establishment of fibroblastic form. The phosphorylated forms of pp20 and pp55 were both extracted from cellular cytoskeletons by 0.5% Triton X-100, but analysis of 35S-methionine-labelled cultures suggested that unphosphorylated pp 20 may be bound to the cytoskeleton. Since pp20 was found to comigrate with the 20,000 dalton myosin light chain, it is possible tht dephosphorylation of CHO cell myosin induced by cAMP may alter its interaction with actin microfilaments and modulate the assembly of stress fibers during morphological reversion.

Studies on cell transformation

Seven different transformation stigmata of the transformed CHO cell line, including morphological characteristics, growth behavior, cell membrane biochemical properties, and failure of fibronectin deposition, are reversed by addition of cAMP derivatives to the medium. Simultaneously the microtubular pattern changes from a sparse, relatively random set to an orderly arrangement of tubules largely parallel to each other and to the long axis of the resulting fibroblastic cell. Agents like colcemid and cytochalasin B, respectively disorganizing microtubular and particular microfilamentous structures, prevent at least certain aspects of the reverse transformation reaction induced by cAMP in interphase cells. It is proposed that malignant transformation can be effected by damage to the microtubular and microfilamentous structures which changes cell constitution and behavior in two ways: (1) chromosomal instability is introduced which promotes continuous selection for variants better able to resist environmental signals to limit reproduction and (2) a variety of metabolic defects in biochemical processes such as specific membrane functions are introduced which may alter the growth responses of the cell. This picture offers a reasonable explanation for a number of aspects of normal and malignant cell behavior.

An analysis of concanavalin A-mediated agglutination in two Chinese hamster ovary subclones whose surface phenotypes respond to maintenance in medium supplemented with dibutyryl cyclic AMP. V. Biochemical composition of the plasma membrane

We have used two Chinese hamster ovary subclones whose surface phenotype has been extensively investigated with regard concanavalin A-mediated cell-cell agglutination and concanavalin A-induced receptor site clustering to investigate what changes in membrane composition, if any, can be correlated with the concanavalin A-detected changes in surface phenotype. These cell clones are uniquely disposed for this purpose since maintenance of the cells under different growth conditions produces changes in agglutinability and receptor site mobility in one cell clone (H-7W) but not the other (K-1). After extensive characterization of the surface membranes of these two subclones we have been unable to identify any change in the membrane peptides, glycopeptide, cholesterol, or fatty acid composition which can be directly correlated with the concanavalin A-detected surface phenotypes. It is of particular interest to note that we have been unable to correlate the presence or absence of the large external transformation-sensitive glycoprotein with the relative mobility of the lectin receptors or with the degree of concanavalin A-mediated cell agglutination. Furthermore we have been unable, in this system, to corroborate earlier data suggesting a role for cholesterol in determining the relative mobility of the lectin receptors. Thus using a cell system consisting of genetically matched cell clones, we have been unable to identify any changes in the biochemical composition of the plasma membrane which might be associated with the surface phenotypes detected by concanavalin A.

Role of the microfibrillar system in knob action of transformed cells

Transformed cells often display knobs (or blebs) distributed over their surface throughout most of interphase. Scanning electron microscopy (SEM) and time-lapse cinematography on CHO-K1 cells reveal roughly spherical knobs of 0.5-4 micron in diameter distributed densely around the cell periphery but sparsely over the central, nuclear hillock and oscillating in and out of the membrane with a period of 15-60 sec. Cyclic AMP derivatives cause the phenomenon of reverse transformation, in which the cell is converted to a fibroblastic morphology with disappearance of the knobs. A model was proposed attributing knob formation to the disorganization of the jointly operating microtubular and microfilamentous structure of the normal fibroblast. Evidence for this model includes the following: 1) Either colcemid or cytochalasin B (CB) prevents the knob disappearance normally produced by cAMP, and can elicit similar knobs from smooth-surfaced cells; 2) knob removal by cAMP is specific, with little effect on microvilli and lamellipodia; 3) immunofluorescence with antiactin sera reveals condensed, amorphous masses directly beneath the membrane of CB-treated cells instead of smooth, parallel fibrous patterns of reverse-transformed cells or normal fibroblasts; 4) transmission electron microscopy (TEM) of sections show dense, elongated microfilament bundles and microtubules parallel to the long axis of the reverse-transformed CHO cell, but sparse, random microtubules throughout the transformed cell and an apparent disordered network of 6-nm microfilaments beneath the knobs; 5) cell membranes at the end of telophase, when the spindle disappears and cleavage is complete, display typical knob activity as expected by this picture.

Hormonally induced cell shape changes in cultured rat ovarian granulosa cells

Cultured rat ovarian granulosa cells undergo a dramatic morphological change when exposed to follicle-stimulating hormone (FSH). Exposure to FSH causes the flattened epithelioid granulosa cells to assume a nearly spherical shape while retaining cytoplasmic processes which contact the substrate as well as adjacent cells. This effect of FSH is preceded by a dose-dependent increase in intracellular cAMP, is potentiated by cyclic nucleotide phosphodiesterase inhibitors, and is mimicked by dibutyryl cAMP. Prostaglandins E1 or E2 and cholera enterotoxin also cause the cells to change shape. A subpopulation of the cells responds to luteinizing hormone. These morphological changes, which are blocked by 2,4-dinitrophenol, resemble those produced by treating cultures with cytochalasin B. Electron microscopy shows that the unstimulated, flattened cells contain bundles of microfilaments particularly in the cortical and basal regions. After FSH stimulation, microfilament bundles are not found in the rounded granulosa cell bodies but they are present in the thin cytoplasmic processes. These data suggest that the morphological change results from a cAMP-mediated, energy-dependent mechanism that may involve the alteration of microfilaments in these cells.

Characterization of two adenosine 3':5'-phosphate-dependent protein kinase species from Chinese hamster ovary cells

Chinese hamster ovary cells exhibit several characteristic morphological and physiological responses upon treatment with agents which increase the intracellular level of adenosine 3':5'-phosphate (cyclic AMP). To better understand the mechanism of these cyclic AMP-mediated responses, we separated two cyclic AMP-dependent protein kinases (ATP:protein phosphotransferase, EC 2.7.1.37) (protein kinase I and protein kinase II) from the cytosol of Chinese hamster ovary cells by DEAE-cellulose chromatography and studied their properties. Protein kinase I is eluted at a lower salt concentration than protein kinase II and is stimulable to 10 times its basal catalytic activity, while protein kinase II is stimulable only 2-fold. Both kinases are completely dissociated by cyclic AMP and inhibited by specific cyclic AMP-dependent protein kinase inhibitor. They have similar Km values for magnesium (approximately 1 mM), cyclic AMP (approximately 60 nM), and ATP (approximately 0.1 mM), and the dissociation constant (Kdis) for cyclic AMP (approximately 13 nM) is the same for both enzymes. However, they appear to have different substrate preferences and cyclic AMP-binding properties in that cyclic AMP bound to protein kinase II exchanges readily with free cyclic AMP, while that bound to protein kinase I is not exchangeable. The native enzymes have different sedimentation coefficients (6.4 S for protein kinase I and 4.8 S for protein kinase II), whereas those of the activated enzymes are the same (2.9--3.0 S). It appears that the two cyclic AMP-dependent protein kinases which differ from each other in their regulatory subunits may play different roles in the mediation of cyclic AMP action in Chinese hamster ovary cells.

Effect of dibutyryl cyclic AMP and analogs on the rate of contractions of myocytes in culture

2O, 6N-butyryl, 3', 5'-cyclic monophosphate (dibu cAMP) when added to fetal rat heart cells in culture inhibits myocyte contraction. This inhibition is 100, 84 and 51% complete when the dibu cAMP concentration used is 2, 0.2 and 0.02 mM, respectively. The potency of dibu cAMP derivatives in myocyte contraction inhibition follows the order, dibu cAMP greater than 6N-bu cAMP greater than 2O-bu cAMP = AMP greater than butyrate. The inhibition caused by the first three chemicals is greater than 70%.

Induced glial differentiation of fetal rat brain cells in culture: an ultrastructural study

Primary and secondary cultures of fetal rat brain cells (FBC) from 18th day of gestation have been investigated by scanning and transmission electron microscopy. Primary cultures consisted of a monolayer of flat, undifferentiated epithelioid cells, with some oligodendrocytes, astrocytes and immature neuronal cells. In secondary cultures, cells with glia morphology disappeared. Following addition of extracts from adult rat brains to secondary cultures, a dramatic change of the epithelioid cells took place. They detached from the palstic surface, extruded long cytoplasmic processes with numerous microvilli and cytoplasmic blebs as well as parallel arrays of microtubules and filaments. The differentiated cells resembled astrocytes, and characteristic glia filaments were also observed. An increase of ribosomes and rough endoplasmatic reticulum suggested enhancement of protein synthesis. At the same time S-100 protein and glial fibrillary acidic protein accumulated within the cells. The morphological changes were mostly reversible within 48 h of removal of the brain extract.

Tubulin assembly protein: immunochemical and immunofluorescent studies on its function and distribution in microtubules and cultured cells

Cytoplasmic microtubule assembly from tubulin monomers requires an accessory protein or proteins present is isolated microtubules. These proteins have been designated "tau" factors. One such factor, tubulin assembly protein (TAP), has been purified to homogeneity from calf brain microtubules. A precipitating, monospecific antibody against the protein has been prepared. The antibody has been used to investigate the mechanism of TAP action in microtubule assembly and the distribution of TAP in cellular microtubules. Immunochemical, immunofluorescent and electron microscopic studies indicate that TAP functions stoichiometrically by binding physically to tubulin to form a complex active in microtubule assembly. TAP is an elongation protein which is required throughout the growth of a microtubule and which is actually present along the entire microtubule. Immunofluorescence microscopy has been used to demonstrate that TAP is distributed throughout the cytoplasmic microtubule network of cultured human, hamster and rat cells-both normal and virally transformed. Immunofluorescence of cells in mitosis shows that TAP is present in the mitotic spindle. These results demonstrate the biological importance of tubulin assembly protein and suggest that it or immunologically related "tau" proteins represent ubiquitous cofactors in cytoplasmic microtubule assembly.

Cell adhesion and cell surface topography in aggregates of 3T3 and SV40-virus-transformed 3T3 cells. Visualization of interior cells by scanning electron microscopy

A technique for exposing the interior of aggregates of cultured cells has been developed and is described in this report. Using this technique, we have examined for the first time, by scanning electron microscopy, cell morphology and cell contact ultrastructure in the interior of aggregates of BALB/c 3T3 and SV40-transformed 3T3 cells. The 3T3 cells make initial intercellular contact by means of microvillar processes. Over a period of 3-8 h, some of these microvillar contacts are replaced by broader projections. In contrast, the SV40-transformed cells make initial intercellular contact by means of blebs or blunt projections which are also broadened and extended over a period of 3-8 h. For both 3T3 and SV40-3T3 cells, the surfaces of the cells which form the outer layer of the aggregate resemble the surfaces of single cells fixed in suspension, regardless of how long the aggregates have been cultured. Thse cells are covered with many cellular processes and are roughly hemispherical in profile. The surfaces of the internal cells of the aggregates, however, lose many of their cellular processes, develop smooth patches, and many become irregular in shape. This smooth morphology was also observed on the interior surfaces of the peripheral cell layer. From these observations we conclude that: (a) the stabilization of adhesive contacts is a slow process which takes at least 3-8 h; (b) the outer surfaces of peripheral cells differ significantly from the surfaces of interior cells; and (c) clear differences in surface topography exist between nonmalignant 3T3 cells and their malignant SV40 transformants.