Role of cyclic nucleotides in growth control

In vitro conversion of porcine thyroid cells growing in monolayer into functional follicular cells

Porcine thyroid cells in primary cultures form either monolayers or, when they are stimulated by thyrotropin (TSH), follicles. This system, monolayer-follicle associated cell culture, determines two morphofunctional states of the thyroid cell in vitro. These cells divide, when grown in monolayer. In this article we describe the precise conditions which allow the conversion of monolayer cells into follicles. The rate of cell growth was lowered using a serum-free medium. Cells were concentrated, stimulated by TSH and cultured on poly-L-lysine pretreated plastic flasks. Light and electron microscope studies show that cells reorganize into follicles containing thyroglobulin. Active iodide transport by the cells, as well as detection of thyroid hormones in the cell culture media, demonstrate that these follicles are functional. Formation of monolayers from follicles is in vitro a spontaneous phenomenon. It is linked to the loss of cell polarity, iodide transport, synthesis of hormones and to the decrease of the number of TSH receptor-sites. These main characteristics of differentiation may be regained in vitro after conversion of monolayer thyroid cells into active follicles up to at least generation five.

Patterns of cyclic nucleotides in normal and leukaemic human leucocytes

Because recent observations indicate that metabolism of cyclic nucleotides may be altered in neoplastic cells, the intracellular levels of cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) were measured in mononuclear leukaemic and normal human leucocytes. The activities of adenylate cyclase, guanylate cyclase and cyclic nucleotide phosphodiesterases were also determined. Under basal conditions, cAMP levels were always higher in the normal leucocytes, whilst cGMP levels were of the same order of magnitude in both normal and leukaemic cells, causing the cAMP/cGMP ratios to be significantly lower in leukaemic leucocytes. Leukaemic cells significantly increased cyclic nucleotide levels in response to theophylline, but did not respond to serotonin, carbamylcholine or D,L-isoproterenol. Preincubation of these leucocytes with theophylline produced a detectable cAMP response to D,L-isoproterenol but no cGMP response to serotonin or carbamylcholine was found. Adenylate cyclase and guanylate cyclase were significantly lower in leukaemic than in normal cells, which could largely explain the abnormal cyclic nucleotide pattern found in human leukaemic leucocytes. In our experiments, cAMP phosphodiesterase activity was comparable in normal and leukaemic cells, whereas cGMP phosphodiesterase activity was undetectable inall mononuclear-leucocyte preparations with the methods used.

A comparison between adenylate cyclase solubilized from normal and Rous sarcoma virus-transformed chicken embryo fibroblasts

The specific activity of adenylate cyclase in membrane preparations obtained from Rous Sarcoma virus-transformed chicken embryo fibroblasts is two to four times lower than that found in untransformed membranes. Adenylate cyclase was solubilized from normal and transformed membranes in order to evaluate the influence of the membrane phase on the properties of the enzyme. Adenylate cyclase in normal and transformed membranes differed in specific activity, V for ATP, activation entropies, sensitivity to Ca2+, and stability at 37 degrees C. Solubilization with Brij 96 abolished or greatly reduced these differences. These data suggest that the differences between adenylate cyclase activities in normal and transformed chicken embryo fibroblasts are due either to differential modulation of enzyme activity by an effector which requires intact membranes for its effects, or indirect effects due to altered membrane properties.

Mitogenic hormone-induced intracellular message: assay and partial characterization of an activator of DNA replication induced by epidermal growth factor

This paper explores the pathway from nuclear quiescence to mitogenesis. It describes an in vitro assay for an activator of DNA replication induced by epidermal growth factor (EGF) in responsive cells. Cytoplasmic extracts from EGF-treated 3T3 cells were found to contain substances that can stimulate DNA synthesis in isolated nuclei from spleen cells of adult frogs. Extracts from untreated resting 3T3 cells lack this activity, and EGF itself is incapable of stimulating DNA synthesis in these cell-free systems. The extract-induced stimulation of incorporation of [3H]dTTP into nuclear DNA is ATP dependent and requires the presence of the four deoxyribonucleoside triphosphates, suggesting the occurrence of replication rather than repair synthesis. This cell-free assay has been used to obtain some initial insights into the mechanism of induction and biochemical characterization of the intermediate in EGF action. Half-maximal induction of the active intracellular substance is achieved at about 0.08 nM EGF, a concentration that correlates well with the concentration required for half-maximal mitogenesis. Studies on the biochemical characteristics of this active substance strongly suggest that the activity is associated with a protein. The activity is nondialyzable and sensitive to trypsin and heat. Sucrose gradient centrifugation of the extract revealed three peaks of activity with molecular weights of 46,000, 110,000, and 270,000 (sedimentation coefficients: 3.7 S, 6.6 S, and 12 S, respectively). These results indicate that receptor-EGF interaction at the cell surface leads to the intracellular generation of protein that are capable of stimulating quiescent nuclei into activity.

Some enzyme and hormonal attributes of hepatoma cell membranes

Adenylate cyclase activity was measured in plasma membranes isolated form Morris Hepatomas (44 and 47C) and from their host livers. We found that the enzyme activity in the tumours was very low, approx. 5% of the level in control and host livers. The amount of cAMP and cGMP in the tumours was also lower than in the host livers but the ratio of cGMP to cAMP in the tumours was increased by a factor of 4-5. The membrane binding capacities for the pancreatic hormones insulin and glucagon were measured. Hepatoma membranes bound less glucagon than those of livers. A decrease in the number of the glucagon receptors was found but there were no changes in the affinity constant. For insulin, we found the same binding capacity as the host and control livers; thus there was an increase in the ratio of insulin bound/glucagon bound in tumours as compared to controls. The plasma levels of insulin in the tumour bearing animals were approximately half of those in control, whereas the glucagon levels in plasma were 60-62% higher in tumour bearing animals. These results are discussed in terms of the characterization of normal, foetal and regenerating liver, in comparison with slow growing hepatomas. The levels of cAMP and cGMP are discussed with respect to control mechanisms of cell proliferation.

On the role of cyclic AMP in the cytotoxic effect of fluoride

The possible role of adenosine 3',5'-monophosphate (cAMP) in the cytotoxic effect of fluoride was investigated in fluoride sensitive mouse fibroblasts (LS) and a subline of LS resistant to 6 mM fluoride (FR6). In both cell lines, growth was inhibited by dibutyryl-cAMP, prostaglandin (PG)E2 and theophylline, FR6 being somewhat more sensitive to these agents than LS. FR6 had lower basal cAMP levels in the intact cells and lower basal adenylate cyclase activity in the homogenate preparation than LS, but the percentual response of intact cells or adenylate cyclase preparations to PGE1 or PGE2 was about the same in the two cell lines, and the sensitivity of the adenylate cyclase to fluoride was similar. No measurable increase in cAMP content was found in either LS or FR6 after exposure of the intact cells to various concentrations of fluoride for various times. The present results indicate that the development of fluoride resistance in these cells is not due to decreased sensitivity to cAMP, and probably not due to altered cAMP-formation in response to fluoride. The growth inhibitory and cytotoxic effects of fluoride in LS cells is probably not mediated through cAMP.

Increase in levels of cyclic AMP during avian limb chondrogenesis in vitro

In the present study the level of cAMP was measured during in vitro chondrogenesis of wing mesenchyme of stage 24 chick embryos and was found to increase significantly from 6.3 pmol/mg protein at the end of the first day of culture to 9.7 pmol/mg protein on the second day, when chondrogenic expression is first detected by the appearance of an Alcian blue staining extracellular matrix. Nonchondrogenic cultures derived from wings of stage 19 embryos had a lower level of cAMP (4.4 +/- 0.07 pmol/mg protein). The level of cAMP in intact wings was 4.5 +/- 0.4 pmol/mg protein and did not change between stages 19 through 25. The correlatin between increased levels of cAMP and the onset of chondrogenesis is consistent with a role of cAMP in the expression of differentiated functions in chondrocytes, as well as in some other cell types.

Ultrastructural and autoradiographic observations of hamster cheek pouch epithelium grown in vitro

Epithelial outgrowths from hamster cheek pouch explants were cultured for varying periods of time up to 22 days. Growth of the epithelial sheets was monitored, employing colcemid for demonstrating mitotic activity and tritiated thymidine for DNA synthesis. Mitoses and thymidine uptake were observed among epithelial outgrowths at a considerable distance form the original explant. The epithelial nature of the growing cell sheets was confirmed, employing electron microscopic techniques. The cells exhibited the presence of tonofilaments, desmosomes, ribosomes, Golgi, mitochondria, and rough endoplasmic reticulum. The cultured explants were treated with cyclic nucleotides in order to investigate their modulatory effects on epithelial cell differentiation. Dibutyryl cAMP induced marked mitotic inhibition (46.3%) in our assay, which was increased to 57% with the addition of theophylline. Dibutyryl cGMP showed only a mild (5%) stimulatory effect on mitotic activity. Dibutyryl cAMP enhanced keratinization in the epithelial cell outgrowths with the biogenesis of keratohyalin granules, whereas dibutyryl cGMP did not produce any observable alterations.

Regulation of protein synthesis

Synthesis of globin in reticulocyte lysates depends on the presence of heme, the prosthetic group of hemoglobin. In the absence of heme, an inhibitor of polypeptide chain initiation is activated. The inhibitor is a cyclic AMP-independent protein kinase that catalyzes the phosphorylation by ATP of the small subunit of the initiation factor eIF-2. This blocks the interaction of eIF-2 with eIF-2-stimulating protein (ESP) that is essential for initiation. Our observations are consistent with the view that the inhibitor is activated by phosphorylation catalyzed by a cyclic AMP-dependent protein kinase. Heme inhibits this enzyme and, in this way, prevents activation of the inhibitor of chain initiation.

The influence of insulin, cAMP and the calcium ionophore X 537 A on the growth of the cartilage analagen of limb buds in vitro

Limb buds of 11-day-old mouse embryos were cultured for 6 days with insulin, dibutyryl cAMP and X 537 A. The cartilage anlage was reduced by insulin and enlarged by dibutyryl cAMP and X 537 A. The effects are due to changes in the amount of intercellular substance.

The cyclic AMP-dependent initiation of DNA synthesis by T51B rat liver epithelioid cells

The brief rise in the cellular cyclic AMP content which occurs late in the prereplicative phases of rat hepatocytes in vivo and T51B rat liver epitheloid cells in vitro seems to be necessary for the initiation of DNA synthesis. Thus, the extracellular calcium-deprivation in T51B rat liver cells in culture which induces a late G-1 block is rapidly reversible (cells surge into S phase within one hour) either by creating a cyclic AMP surge by the addition of calcium or 3-isobutyl-1-methyl xanthine (a cyclic 3',5'-nucleotide phosphodiesterase inhibitor) or by the exogenous addition of low concentrations of cyclic AMP itself (i.e., 10(-8)-10(-5) M). On the other hand, prevention of the calcium-induced cyclic AMP surge by imidazole (a cyclic 3',5'-nucleotide phosphodiesterase activator) blocked the initiation of DNA synthesis by the calcium-deprived T51B cells.

Effects of dipyridamole in vivo on ATP and cAMP content in platelets and arterial walls and on atherosclerotic plaque formation

In rabbits receiving an atherogenic diet for 2 months, the ATP content of platelet rich plasma (PRP) and arterial tissue was significantly elevated as compared to normal rabbits. This increase in ATP levels of platelets from atherosclerotic rabbits was paralleled by higher basal as well as PGI2-induced cAMP levels. In arterial tissues, an increase was only obtained in PGI2-stimulated cAMP content. Treatment with dipyridamole (DPD) for 4 weeks resulted in a reduction of the ATP content in platelets and arterial tissue from atherosclerotic rabbits to values seen in normal animals. Again, the reduction of ATP content was reflected in a decrease of basal as well as PGI2-induced cAMP levels in platelets, whereas in arterial tissue a decrease was only obtained in PGI2-induced cAMP content. At the same time, DPD treatment enhanced atherosclerotic plaque formation in the aortic wall. The enhanced atherosclerotic plaque formation seen in DPD treated atherosclerotic rabbits may be linked to the inhibition of adenosine uptake, resulting in a decrease of the adenine nucleotide pools of arterial wa-l cells. The decrease also caused a reduction in PGI2-induced cAMP content. This effect may be linked to altered proliferative activity, since in many cell types, stimulation of cAMP levels results in reduced proliferation rates.

Cyclic nucleotide metabolism in tumours

Many hormones act by combining with cell surface receptors and stimulating adenylate cyclase activity. The cyclic AMP generated is the mediator of a number of cellular metabolic processes. Other processes may be influenced by changes in cyclic GMP levels. Although much evidence from cultured cells suggested that low cellular levels of cyclic AMP and high levels of cyclic GMP are a feature of rapid cell growth and of malignant transformation, review of the data reveals many inconsistencies. Thus in established tumours growing in vivo, for example, cyclic AMP levels appear to be unrelated to tumour growth rates. It seems that tumour cell cyclic AMP is more likely concerned with the regulation of tumour cell function than of growth. This would have implications for therapy, in that drugs which influence cyclic nucleotide metabolism could influence tumour cell function. The control of cyclic nucleotide production in normal and tumour cells is discussed, together with the possible ways in which abnormalities of this may occur.

Regulation of cyclic nucleotide phosphodiesterase forms by serum and insulin in cultured fibroblasts

A rapid reduction of cyclic nucleotide phosphodiesterase activity occurs after the replating of confluent cultures of BHK 21 c/13 fibroblasts into fresh medium. This reduction in activity depends on the density to which the cultures are reseeded and the concentration of serum in the medium. Enzyme activity in BHK cells is restored after 24 to 48 hours if cells are diluted into medium containing 10% fetal calf serum or 0.5% fetal calf serum supplemented with insulin (10(-6)M), but not into 0.5% serum alone. The restoration in enzyme activity is blocked by cycloheximide or Actinomycin D. When BHK cells become quiescent by maintanance in 0.5% serum conditions for 48 hours, a rapid (15--60 minutes) increase in cyclic AMP phosphodiesterase activity occurs when 10% serum is added to the cultures. Enzyme activity is increased even further after 24 to 48 hours in the 10% serum. Cycloheximide or Actinomycin D do not affect the rapid increase in enzyme activity in response to serum, but completely inhibit the long term increase. In contrast to serum, insulin (10(-8) to 10(-6)M) has no short term effect, but does increase enzyme activity after 24 to 48 hours to levels comparable to those seen with addition of 10% serum. As is the case with serum, this long term effect of insulin on enzyme activity is prevented by inhibitors of protein and RNA synthesis. Kinetic analyses of cyclic AMP phosphodiesterase activity in homogenates of quiescent BHK cells indicate the presence of only high Km (congruent to 20 muM) enzyme activity. Addition of serum or insulin to quiescent cells results in the appearance of apparent low Km enzyme activity in homogenates. Sucrose gradient analysis of BHK cells displays two forms of cyclic AMP phosphodiesterase enzyme activity: a 3--4 S form and 5--6 S form. In quiescent cells, the 5--6 S form greatly predominates relative to the 3--4 S form. Addition of serum to quiescent cells results in a rapid appearance of increased 3--4 S form enzyme activity. Insulin also increases the activity of this higher affinity 3--4 S enzyme form after 24 to 48 hours in culture. The functional significance of short and long term regulation of cyclic nucleotide phosphodiesterase(s) in cells is discussed.

In vitro studies on the self-differentiating capacities of quail adenohypophysis epithelium

Isolated epithelial rudiments of 3--4 days quail embryo adenohypophysis were cultivated in vitro. Differentiation of glandular cells occurred when culture conditions allowed the formation of explants characterized by the presence of a fibroblastic sheet and of an epithelial roof surrounding a central cavity. Differentiation did not occur when culture conditions did not allow the fibroblastic sheet to be established or resulted in the absence of a cavity. The importance of the explant structure in differentiation is also indicated by the location of glandular cells. In vitro they differentiate near the cavity derived from Rathke's pouch lumen while in situ they first appear at the periphery of the rudiment, near the basement membrane. The results of this study show that the adenohypophyseal primordium can differentiate without any mesenchymal influence. The observed differentiation seems to be correlated with fibroblast-secreted material, and this hypothesis is discussed.

Plasma membrane phosphoproteins in normal and Rous sarcoma virus transformed chick embryo fibroblasts: characterization by in vitro phosphorylation

Plasma membranes isolated from normal and RSV transformed chick embryo fibroblasts were phosphorylated in vitro using endogenous protein kinase and ATP (gamma32P) and the labeled phosphoproteins were analyzed by SDS-PAGE. A number of protein phosphorylation changes were observed following transformation, however in most cases they were relatively small quantitative differences. The four major changes were in proteins of 47,000, 58,000, 75,000 and 135,000 daltons. Decreased phosphorylation of the 47,000 dalton polypeptide was found in transformed cell membranes but this alteration was shown to be due to differences in cell growth rather than transformation. Increase phosphorylation of the 75,000 dalton protein was at least partially related to virus infection. However, increased phosphorylation of the 58,000 and 135,000 dalton polypeptides were entirely transformation specific.

Cyclic nucleotides levels in the liver of rats treated with CCl4

Treatment of rats with two different doses of CCl4 (respectively 2.5 and 0.5 ml/kg body wt. intragastrically) is followed by a rapid increase in the cAMP content of the liver. With 0.5 ml of CCl4, the increase occurs as early as 30 min after poisoning, namely about 4-5 h before the onset of triglyceride accumulation in the liver. The maximum increase has been at 6 h after administration of the hepatotoxin. In both experimental conditions, normal values are recovered only after 36-48 h. cGMP level appears unmodified during the whole observation period. Therefore the ratio cGMP/CAMP decreases consistently. The ATP level decreases significantly between 2 and 12 h. The increase in liver triglycerides level after CCl4 can be also a consequence of an impairment of microtubule function, leading to a decreased release of lipoprotein micelles from hepatocytes into the blood stream.

High alkaline phosphatase activity in isoproterenol stimulated fibroblast cultures from patients with numerically unbalanced chromosomal aberrations

Treatment of cultured fibroblasts from patients with unbalanced chromosomal aberrations with a mixture of isoproterenol, theophylline and ascorbic acid resulted after 48 hours in an at least three-fold increase of alkaline phosphatase activity on a per cell basis, whereas cells from normal healthy individuals did not show this dramatic response. Cells were studied from patients with trisomy 21 (14 cases), trisomy 18 (3 cases), trisomy 13 (1 case), pentasomy X (1 case), Turner syndrome (2 cases), and Klinefelter syndrome (1 case), and no exception was noted. The mechanism of this phenomenon is not clear, but it is speculated that increased cyclic-AMP levels caused by the action of isoproterenol on adenylcyclase may account for excessive reactions of unbalanced cells as compared to normal cells. This simple biochemical diagnostic procedure might become useful in screening programs for unbalanced chromosomal abberations.

Isolation of functionally active acini from bovine mammary gland

Conditions to obtain high yields of intact acini from lactating bovine mammary glands and certain structural and functional characteristics of isolated acini were investigated. A two-factor experiment with three collagenase concentrations (100, 150, and 200 mg/100 ml) and incubation periods (40, 60, and 90 min) demonstrated that increases in both factors significantly increased net acini yield. Largest amounts of acini obtained, based on content of deoxyribonucleic acid, were 10.3% of the original tissue. Morphologically, fractions consisted primarily of acini or large cell clumps, and nearly all cells excluded trypan blue. Acini cultured in complete nutrient medium incorporated radioactive leucine into proteins. When acini were incubated in medium without supplemental amino acids, specific activity of synthesized proteins was correlated negatively with incubation time. During pulse labeling with radioactive L-leucine over 16 min, true labeling of acinar proteins occurred after 4 min. Sequential kinetics of pulse-chase labeling demonstrated a response pattern unique to the in vitro acinar system. Acinar protein synthesis was inhibited by cycloheximide and strongly stimulated by by 3',5'-cyclic adenosine monophosphate.

Augmentation of the in vitro humoral immune response by pharmacologic agents. II: comparison of the effects of antiproliferative agents with DBcAMP

We have compared the stimulatory activity of DBcAMP with various antiproliferative agents on the induction of the humoral immune response. When they are present only during an early stage of immune induction, DBcAMP, colcemid, cytosine-arabinoside, hydroxy urea, and high specific activity 3H-thymidine can all enhance the primary 19s antibody response to SRBC. In contrast, each of these agents inhibits the PFC response, when they are incubated with the cells during late stages of induction of humoral immunity. Because all of these agents can inhibit proliferation of cultured cells, the results suggested that DBcAMP and other agents that elevate cAMP could augment humoral immunity via their effects on cellular proliferation. However, we also found that although each agent could modulate induction of the immune response to SRBC, only DBcAMP produced a dose- and time-dependent augmentation of the response to DLF. We conclude that although antiproliferative effects of drugs may contribute to augmentation of some humoral antibody responses, this effect alone is insufficient to account for the mechanism by which agents that elevate intracellular levels of cAMP produce enhancement of humoral immunity.

Evidence suggesting that a cyclic AMP-dependent protein kinase is a positive regulator of proliferation in Cloudman S91 melanoma cells

Wild-type Cloudman S91 melanoma cells have a retarded rate of division when agents which raise cyclic AMP levels such as melanotropin, protaglandin E1, or cholera toxin are supplemented to the culture medium. A mutant cell line was isolated which had the opposite response, i.e., the mutant grew very slowly unless agents which raised cyclic AMP levels were present (Pawelek et al., '75a). In this report evidence is presented indicating that the molecular basis for the mutant phenotype resides in the major cyclic AMP-dependent protein kinase found in the cells. The mutant kinase had increased thermolability and an elevated activation constant for cyclic AMP over the corresponding wild-type kinase. It is proposed that the elevated requirement for cyclic AMP for the proliferation of cAdep cells is related to the elevated activation constant of the kinase, suggesting that the kinase is a positive regulator of proliferation in Cloudman S91 cells.

Growth factors and gangliosides: a possible new perspective in neuronal growth control

For many permanent cell lines the transition from a growing (P) to a resting (R) state is reversibly controlled by growth factors present in serum. This P-to-R transition was studied in a neuronal cell line (B104) with respect to the action of serum, dibutyryl cyclic AMP (DBcAMP), gangliosides, and a glioma cell-produced growth factor GGF. In this cell system gangliosides seem to act as differentiation and survival factors. The kinetics of uptake of radioactively labeled gangliosides and survival experiments both support the idea of the stable incorporation of exogenously added gangliosides into the cells. Based on the experimental evidence a new model of cell development is proposed. Thus in addition to the R or Go state, which in this cell system is rather unstable and probably regulated by cyclic nucleotides, we postulate a differentiated D state, which is controlled by gangliosides and which is characterized by its stability (survival time). This D compartment seems to be closer to the in vivo differentiated neuron than does the R or P state. tthe possible mechanisms for the action of gangliosides are discussed.

Isolation of cholera toxin receptors from a mouse fibroblast and lymphoid cell line by immune precipitation

Cholera toxin receptors have been isolated from both a mouse fibroblast (Balbc/3T3) and mouse lymphoid cell line labeled by the galactose oxidase borotritiide technique. Tritiated receptor-toxin complexes solubilized in NP40 were isolated by addition of toxin antibody followed by a protein A-containing strain of Staphylococcus aureus. In both cell types by far the major species of toxin receptor isolated was ganglioside in nature, although galactoproteins were also present in the immune complexes. Whether the galactoproteins form part of a toxin-receptor complex or are artifacts of the isolation procedure is presently unclear. The relative specificity of cholera toxin for a carbohydrate sequence in a glycolipid suggests that the toxin might prove a useful tool in establishing the function and organization of glycolipids in membranes. For example, interaction of cholera toxin with the mouse lymphoid cell line was shown to result in patching and capping of bound toxin, raising the possibility that the glycolipid receptor interacts indirectly with cytoskeletal elements. Cholera toxin might also be used to select for mutant fibroblasts lacking the toxin receptor and therefore having an altered glycolipid profile. Such mutants might prove useful in establishing the relationship (if any) between modified glycolipid pattern and other aspects of the transformed phenotype. Attempts to isolate mutants, based on the expectation that growth of cells containing the toxin receptor would be inhibited by the increase in cAMP levels normally induced by cholera toxin, proved unsuccessful. Cholera toxin failed to inhibit significantly the growth of either Balbc or Swiss 3T3 mouse fibroblasts although it markedly elevated cAMP levels.

Isolation and characterization of a variant of B16-mouse melanoma resistant to MSH growth inhibition

A variant of B-16 F1 mouse melanoma was selected for its ability to survive and replicate in the presence of melanocyte-stimulating hormone (MSH). Although the variant (MR-4) was completely resistant to growth inhibition of MSH, cyclic AMP was still able to block cell replication. Tyrosinase activity in MR-4 cells was considerably lower than in B-16 F1 cells. MSH induced a two fold to three-fold increase in tyrosinase activity in both cell types, but the absolute activity in MR-4 remained significantly less than in the parental cells. MR-4 cells were also found to have a markedly depressed cyclic AMP-dependent protein kinase activity relative to B-16 F1 cells. The protein kinase from both cell types was stimulated by cyclic AMP, but the level of MR-4 kinase activity at maximal cyclic AMP concentrations remained considerably lower than B-16 F1 kinase activity under the same conditions. In both cell types adenylate cyclase activity was markedly stimulated by MSH. When equal numbers of viable F1 and MR-4 cells were injected subcutaneously into C57/B1 mice, the MR-4 cells formed tumors earlier and killed the host sooner than the parental F1 cells. We conclude that the biochemical alteration which allows MR-4 cells to replicate in the presence of MSH is a low level of tyrosinase activity, which in turn may be the result of low cyclic AMP-dependent protein kinase activity.

Growth of kidney epithelial cells in hormone-supplemented, serum-free medium

Madin Darby canine kidney cells can grow in synthetic medium supplemented with 5 factors--insulin, transferrin, prostaglandin E1, hydrocortisone and triiodothyronine--as a serum substitute. These 5 factors permit growth for one month in the absence of serum, and a growth rate equivalent to that observed in serum-supplemented medium. Dibutyryl cAMP substitutes for prostaglandin E1 in the medium, suggesting that increased growth of Maden Darby canine kidney cells results from increased intracellular cAMP. Potential applications of the serum-free medium are discussed. The medium permits the selective growth of primary epithelial cell cultures in teh absence of fibroblast overgrowth, and a defined analysis of the mechanisms by which hormones regulate hemicyst formation.

Progesterone-induced meiosis in Xenopus laevis oocytes: a role for cAMP at the "maturation-promoting factor" level

Cholera toxin inhibition of progesterone-induced meiosis of Xenopus laevis oocytes in vitro has been correlated with increased cAMP levels. Inhibition of germinal vesicle breakdown (Gvbd) and cAMP increase occurred after a lag period of 2 hr, when cholera toxin was injected, or 4--5 hr, when applied externally. The ability of the maturation-promoting factor (Mpf) to provoke Gvbd when injected into recipient oocytes was found to be dependent upon whether the oocytes had been exposed to cholera toxin alone or to toxin and progesterone. With the former, cAMP levels were elevated and Mpf activity was abolished, whereas with the latter, the increase in cAMP was less pronounced and Mpf activity was observed. Injection of cAMP or its 8-thio derivatives shortly before the appearance of progesterone-induced Mpf abolished Gvbd. If injected earlier or later, no inhibition was observed. In contrast, cholera toxin inhibited maturation even when added several hours before progesterone, suggesting a sustained accumulation of cAMP. No Gvbd occurred when 8-thio-methyl-cAMP was injected together with Mpf. These data suggest that cAMP is involved in the control of the formation/amplification and/or activity of Mpf-a result which may be of general significance in cell division mechanisms.