Studies on cultured rat Schwann cells. II. Comparison with a rat Schwann cell line.

Cultured rat Schwann cells do not exhibit the ring-like changes in cell shape previously reported to be induced in the Schwann cell line RN22 by elevation of intracellular cyclic AMP. They do, however, undergo different shape changes on treatment with cholera toxin or low serum concentration. Furthermore, DNA synthesis in the cell line is inhibited by treatment with cholera toxin and unaffected by bovine pituitary extract, though both of these agents stimulate DNA synthesis in normal Schwann cells. Our results, therefore, do not support the hypothesis that elevation of intracellular cyclic AMP is a positive signal for myelination by the Schwann cell. Moreover, they illustrate the need for caution in drawing conclusions about normal cells of the nervous system from studies on neural cell lines.     

Studies on cultured rat Schwann cells

Summary Cultured rat Schwann cells do not exhibit the ring-like changes in cell shape previously reported to be induced in the Schwann cell line RN22 by elevation of intracellular cyclic AMP. They do, however, undergo different shape changes on treatment with cholera toxin or low serum concentration. Furthermore, DNA synthesis in the cell line is inhibited by treatment with cholera toxin and unaffected by bovine pituitary extract, though both of these agents stimulate DNA synthesis in normal Schwann cells. Our results, therefore, do not support the hypothesis that elevation of intracellular cyclic AMP is a positive signal for myelination by the Schwann cell. Moreover, they illustrate the need for caution in drawing conclusions about normal cells of the nervous system from studies on neural cell lines. Paper I in this series is reference 2.     

Increases in cyclic AMP potentiate competence formation in BALB/c-3T3 cells

The ability of platelet-derived growth factor and fibroblast growth factor to stimulate the initiation of DNA synthesis in quiescent BALB/c-3T3 cells was enhanced by cholera toxin. However, the addition of cholera toxin to unsupplemented medium was not mitogenic, nor did cholera toxin increase the mitogenic potential of mediuum supplemented with platelet-poor plasma. The enhancement of serum-induced DNA synthesis by cholera toxin was due to a specific effect on competence formation and not plasma-controlled progression. Cholera toxin increased the rate of competence formation during a transient exposure of quiescent cells to platelet-derived growth factor; this rate was further increased by the addition of isobutylmethylxanthine, a cyclic nucleotide phosphodiesterase inhibitor. Intracellular cyclic AMP concentrations in quiescent BALB/c-3T3 cells were increased 2- to 3-fold after the addition of cholera toxin. The addition of cholera toxin plus 30 m?M isobutylmethylxanthine caused an even greater (7- to 8-fold) increase in the cellular levels of cyclic AMP. That these increases in cyclic AMP concentrations mediated at least part of the increased sensitivity of quiescent cells to competence factors was substantiated by the observation that 0.01 to 1 mM monobutrylcyclic AMP or 8-bromocyclic AMP also caused a concentration-dependent potentiation of competence formation in quiescent cells during a transient exposure to platelet-derived growth factor.     

Differential sensitivity of fibroblasts to epidermal growth factor is related to cyclic AMP concentration

The differential sensitivity of various cell lines to the mitogenic effects of epidermal growth factor (EGF) was investigated. Two lines of evidence suggest that cellular capacity to respond proliferatively to EGF is related to intracellular cyclic AMP concentration. First, the ability of three density-arrested cell lines to synthesize DNA in response to EGF was directly proportional to the basal cyclic AMP level of the cells at quiescence. Second, treatment of cultures with various agents known to promote intracellular cyclic AMP accumulation increased the sensitivity of all three cell lines to EGF. The mechanism whereby cyclic AMP modulates EGF responsiveness is not known; cholera toxin did not affect the cellular capacity to bind or internalize and process EGF. Although platelet-derived growth factor (PDGF) had no effect on cyclic AMP levels, transient treatment of quiescent cultures with this polypeptide also enhanced EGF sensitivity. In agreement with previous data and in contrast to cholera toxin, PDGF induced the down-regulation of EGF receptors in the three cell lines. These data suggest that the capacity of various cell types to respond to EGF is subject to both intracellular regulation by cyclic AMP and extracellular modulation by factors such as PDGF which can affect EGF receptor activity.     

Increase in Gs and cyclic AMP generation in HIT cells. Evidence that the 45-kDa alpha-subunit of Gs has greater functional activity than the 52-kDa alpha-subunit

Cyclic AMP accumulation in response to forskolin, cholera toxin, or isoproterenol is dramatically increased in HIT T-15 cells, a clonal cell line of Syrian hamster pancreatic islet beta cells, as a function of passage number. Forskolin and cholera toxin elevate cyclic AMP levels 5- to 10-fold higher in later passages (87-100) than in earlier passages (70-80). A similar phenomenon is observed with isoproterenol (10 microM) which increases cyclic AMP levels 56-fold in older HIT cells (passage 94), whereas only marginally stimulating cyclic AMP production in younger cells (passage 70-82). To determine whether a change in the stimulatory or inhibitory guanine nucleotide regulatory proteins, Gs or Gi, was responsible for these observations, ADP-ribosylation of HIT cell membranes with cholera toxin and pertussis toxin was examined. All passages contained two cholera toxin substrates at 52 and 45 kDa. The amount of 52 kDa did not appear to change with passage number, but the amount of 45 kDa increased in the later passages (89 and 94). The ratio of 45 to 52 kDa cholera toxin substrate, as determined by densitometric analysis, increased from 0.1 in passages 70, 75, and 82 to 0.45 at passage 89. No passage related changes in a 40-kDa pertussis toxin substrate were observed. An increase in the amount of the 45-kDa alpha-subunit of Gs was confirmed on immunoblots using antisera specific for the alpha-subunits of Gs. The amount of functional Gs present in various HIT cell passages was examined by determining the extent to which extracts from HIT cell membranes reconstituted guanine nucleotide-sensitive adenylyl cyclase in S49 cyc- membranes. Extracts derived from passage 94 reconstituted three to four times more adenylyl cyclase activity in cyc- membranes than extracts from passages 70, 75, and 82. These data indicate that an increase in functional Gs in later passages may be the underlying cause for the increased responsiveness to isoproterenol and forskolin in later passages. These data also suggest that functional differences exist between the Gs alpha-subunits, with the smaller 45-kDa subunit being more efficacious in coupling to cyclic AMP synthesis than the larger 52-kDa subunit. This is a departure from the commonly held view that the two subunits have similar efficacies in stimulating adenylyl cyclase.     

Platelet-derived growth factors and fibroblast growth factors are mitogens for rat Schwann cells

Rat sciatic nerve Schwann cells in culture respond to a limited range of mitogens, including glial growth factor, transforming growth factors beta-1 and beta-2 (TGF-beta 1, TGF-beta 2), some cell membrane-associated factors, and to agents such as cholera toxin and forskolin which raise intracellular levels of cAMP. These responses require the presence of FCS, which exhibits little or no mitogenic activity in the absence of other factors. However, we recently found that forskolin greatly potentiates the mitogenic signal from TGFs-beta 1 and beta 2, raising the possibility that cAMP might couple other factors to mitogenesis. We have therefore screened a range of candidate mitogens using DNA synthesis assays. Other than TGFs-beta and glial growth factor, none of the factors tested were mitogenic in the presence of 10% serum alone. With the addition of forskolin, however, porcine PDGF, human PDGF, acidic and basic FGF were potent mitogens for rat Schwann cells, stimulating DNA synthesis and increasing cell number. Cholera toxin and dibutyrylcyclicAMP, but not 1,9-dideoxyforskolin, can substitute for forskolin indicating that the mitogenic effect is mediated via adenylyl cyclase activation. Porcine PDGF gave half-maximal stimulation at 15 pM, and human PGDF an equivalent response at 1 nM. Basic FGF was half maximal at 5 pM, acidic FGF at 1 nM. The recognition of PDGFs and FGFs as mitogens for Schwann cells has many implications for the study of Schwann cell proliferation in the development and regeneration of nerves, and in Schwann cell tumorigenesis.     

Action of cholera toxin on dispersed acini from guinea pig pancreas.

In dispersed acini from guinea pig pancreas cholera toxin bound reversibly to specific membrane binding sites to increase cellular cyclic AMP and amylase secretion. Cholera toxin did not alter outflux of 45Ca or cellular cyclic AMP. Binding of 125I-labeled cholera toxin could be detected within 5 min; however, cholera toxin did not increase cyclic AMP or amylase release until after 40 min of incubation. There was a close correlation between the dose vs. response curve for inhibition of binding of 125I-labeled cholera toxin by native toxin and the action of native toxin on cellular cyclic AMP. With different concentrations of cholera toxin, maximal stimulation of amylase release occurred when the increase in cellular cyclic AMP was approximately 35% of maximal. Cholera toxin did not alter the increase in 45Ca outflux or cellular cyclic GMP caused by cholecystokinin or carbachol but significantly augmented the increase in cellular cyclic AMP caused by secretin or vasoactive intestinal peptide. The increase in amylase secretion caused by cholera toxin plus secretin or vasoactive intestinal peptide was the same as that with cholera toxin alone. On the other hand, the increase in amylase secretion caused by cholera toxin plus cholecystokinin or carbachol was significantly greater than the sum of the increases caused by each agent alone.     

Action of cholera toxin on dispersed acini from guinea pig pancreas

In dispersed acini from guinea pig pancreas cholera toxin bound reversibly to specific membrane binding sites to increase cellular cyclic AMP and amylase secretion. Cholera toxin did not alter outflux of ⁴⁵Ca or cellular cyclic AMP. Binding of ¹²⁵I-labeled cholera toxin could be detected within 5 min; however, cholera toxin did not increase cyclic AMP or amylase release until after 40 min of incubation. There was a close correlation between the dose vs. response curve for inhibition of bindind of ¹²⁵I-labeled cholera toxin by native toxin and the action of native toxin on cellular cyclic AMP. With different concentrations of cholera toxin, maximal stimulation of amylase release occurred when the increase in cellular cyclic AMP was approximately 35% of maximal. Cholera toxin did not alter the increase in ⁴⁵Ca outflux or cellular cyclic GMP caused by cholecystokinin or carbachol but significantly augmented the increase in cellular cyclic AMP caused by secretion or vasoactive intestinal peptide. The increase in amylase secretion caused by cholera toxin plus secretin or vasoactive intestinal peptide was the same as that with cholera toxin alone. On the other hand, the increase in amylase secretion caused by cholera toxin plus cholecystokinin or carbachol was significantly greater than the sum of the increases caused by each agent alone.     

Effects of cholera toxin on vasoconstriction and cyclic AMP content of the isolated rabbit ear artery

We have studied the effect of cholera toxin on the constrictor responses of the isolated, perfused rabbit ear artery to nerve stimulation and to norepinephrine infusion. We found that when we perfussed arteries with cholera toxin (1–9 μg/ml) for five minutes or longer, the toxin gradually inhibited the responses to intermittent stimulation of the adrenergic nerves and to brief infusion of norepinephrine. The constrictor responses began to decrease between one and two hours after we added cholera toxin, and the responses were still depressed after 24 hours. Cholera toxin inhibited both the rapid, initial phase and the slower, sustained phase of the biphasic response of the ear artery to nerve stimulation. Propranolol and indomethacin did not block the effect of cholera toxin on vasoconstriction. However, when we mixed the toxin with antitoxin or G_M1 ganglioside, we prevented the inhibitory effect on vasoconstriction. Levels of adenosine 3′:5′-cyclic monophosphate (cyclic AMP) in arteries treated with cholera toxin were greater than levels of cyclic AMP in untreated arteries. The cyclic AMP content increased and the constrictor responses decreased with a similar time course after the arteries were exposed to the toxin. Thus an increase in cyclic AMP may be involved in the relaxation of vascular smooth muscle induced by cholera toxin.     

Elevated intracellular concentrations of cyclic AMP inhibited serum-stimulated, density-arrested BALB/c-3T3 cells in mid G1

The stimulation of DNA synthesis in quiescent, density-arrested BALB/c-3T3 cells by platelet-derived growth factor in plasma-supplemented medium was inhibited by the presence of isobutylmethylxanthine (IBMX) and cholera toxin, although neither IBMX or cholera toxin when used alone inhibited the stimulation of DNA synthesis. The cells were reversibly inhibited in mid G1 at a point 6 hr prior to the initiation of DNA synthesis. The inhibition of cell cycle traverse was associated with a 10-15 fold increase in cellular cyclic AMP concentration over basal levels. The reversal of this inhibition by removal of IBMX was correlated with a dramatic decrease in cyclic AMP levels. The traverse of G1 and the initiation of DNA synthesis after release from the cholera toxin and IBMX inhibition was dependent on the presence of plasma in the medium. Either somatomedin C (10-20 ng/ml) or insulin (10(-6)-10(-5) M) completely replaced the plasma requirement for late G1 progression and entry into S phase. Once the inhibited cells were released from the IBMX and cholera toxin block a subsequent increase in cyclic AMP did not prevent entry into S phase. The presence of cholera toxin alone inhibited the stimulation of human dermal fibroblasts. The elevation of intracellular cyclic AMP levels in the human dermal fibroblasts by cholera toxin was two to three fold greater than that found in the BALB/c-3T3 cells in the presence of cholera toxin and the IBMX.     

Regulation of Myelin P0 Glycoprotein Synthesis in Cultured Rat Schwann Cells and Continuous Rat PNS Cell Lines

We studied the effects of agents that raise intracellular cyclic AMP on synthesis of myelin components by cultured neonatal rat sciatic nerve Schwann cells and by continuous PNS cell lines derived from the fusion of neonatal rat sciatic nerve Schwann cells with rat RN22 Schwannoma. Treatment with N6,2'-O-dibutyryl cyclic AMP (dibutyryl cyclic AMP) caused a fourfold increase in Schwann cell incorporation of 35SO4 into sulfogalactosylceramide (sulfatide), and elicited a 10- to 20-fold increase in such incorporation by the continuous PNS cell lines; a similar effect on PNS cell line sulfatide radiolabelling was obtained with forskolin. Cultured Schwann cells expressed barely detectable levels of myelin P0 glycoprotein (P0) mRNA and myelin basic protein (MBP) mRNA. Treatment of the Schwann cells with axolemmal fragments or with dibutyryl cyclic AMP did not elicit a detectable increase in the levels of these mRNAs. The PNS cell lines constitutively expressed much higher levels of P0 mRNA than did the Schwann cells, and synthesized immunochemically demonstrable P0 glycoprotein, but did not express MBP. Treatment of the PNS cell lines with dibutyryl cyclic AMP markedly reduced expression of P0 mRNA and also diminished immunoreactive P0 glycoprotein. These PNS cell lines should prove useful for further studies of the control of Schwann cell differentiation.     

Action of cholera toxin on dispersed acini from rat pancreas. Post-receptor modulation involving cyclic AMP and calcium

In dispersed acini from rat pancreas, cholera toxin caused a significant increase in cellular cyclic AMP but little or no change in amylase secretion. The presence of a secretagogue that causes mobilization of cellular calcium (e.g., cholecystokinin, carbamylcholine, bombesin or ionophore A23187) caused a substantial increase in the effect of cholera toxin on enzyme secretion. Cholera toxin did not alter calcium transport or the changes in calcium transport caused by other secretagogues, and secretagogues that mobilize cellular calcium did not alter cellular cyclic AMP or the increase in cyclic AMP caused by cholera toxin. These results indicate that in dispersed acini from rat pancreas there is post-receptor modulation of the action of cholera toxin by secretagogues that mobilize cellular calcium and that this modulation is a major determinant of the effect of the toxin on enzyme secretion.     

Action of cholera toxin on dispersed acini from rat pancreas: Post-receptor modulation involving cyclic AMP and calcium

In dispersed acini from rat pancreas, cholera toxin caused a significant increase in cellular cyclic AMP but little or no change in amylase secretion. The presence of a secretagogue that causes mobilization of cellular calcium (e.g., cholecystokinin, carbamylcholine, bombesin or ionophore A23187) caused a substantial increase in the effect of cholera toxin on enzyme secretion. Cholera toxin did not alter calcium transport or the changes in calcium transport caused by other secretagogues, and secretagogues that mobilize cellular calcium did not alter cellular cyclic AMP or the increase in cyclic AMP caused by cholera toxin. These results indicate that in dispersed acini from rat pancreas there is post-receptor modulation of the action of cholera toxin by secretagogues that mobilize cellular calcium and that this modulation is a major determinant of the effect of the toxin on enzyme secretion.     

Induction of refractoriness to isoproterenol by prior treatment of C6-2B rat astrocytoma cells with cholera toxin.

Rat C6-2B astrocytoma cells responded to cholera toxin treatment with an 8-fold increase in intracellular cyclic AMP concentrations. Cyclic AMP levels began to rise 60--90 minutes after addition of the toxin and reached maximal concentrations in 3 hours. Cells exposed to cholera toxin and the phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine (MIX), displayed an increase in cyclic AMP of 15-fold. The peak isoproterenol response was reduced 80--90% in cells previously treated with cholera toxin. Cholera toxin-induced refractoriness was time dependent and was not altered by concurrent treatment with propranolol. Prolonged exposure of the cells to isoproterenol reduced the cyclic AMP response to cholera toxin by 80%. MIX augmented both cholera toxin-induced refractoriness and isoproterenol-induced refractoriness. Cycloheximide inhibited the full development of refractoriness to both cholera toxin and isoproterenol. These results indicate that C6-2B cell refractoriness to cholera toxin is mediated by cyclic AMP and requires new protein synthesis. Refractoriness in C6-2B cells does not appear to be agonist-specific and probably involves a common locus of action on adenylate cyclase beyond that of the membrane receptors for cholera toxin and isoproterenol.     

Potentiation of cholera toxin-stimulated cyclic AMP production in cultured cells by inhibitors of RNA and protein synthesis

Cyclic AMP increased 8- to 10-fold after a 3-h treatment with 6 nM cholera toxin in rat C6-2B astrocytoma cells. In the presence of cycloheximide, cholera toxin increased intracellular cyclic AMP about 50-fold. Qualitatively similar potentiation of cholera toxin action by cycloheximide was observed in isolated swine aortic vascular smooth muscle cells. Cycloheximide, by itself, had no effect upon cyclic AMP levels and did not alter the apparent Ka for cyclic AMP generation by cholera toxin in the cells. Also, cycloheximide did not appear to augment cholera toxin action via inhibition of cyclic nucleotide phosphodiesterase. Puromycin and actinomycin D also augmented cholera toxin action in C6-2B cells. Potentiation of cholera toxin-increased cyclic AMP formation by cycloheximide was correlated with the inhibition of [14C]leucine incorporation into protein. These results indicate that the ability of cholera toxin to stimulate cyclic AMP production in C6-2B astrocytoma and swine vascular smooth muscle cells is enhanced by inhibition of de novo protein synthesis.     

Dissociation by cooling of hormone and cholera toxin activation of adenylate cyclase in intact cells.

Cholera toxin, through adenylate cyclase activation reproduced cyclic AMP-mediated effects of thyroid-stimulating hormone (TSH) in dog thyroid slices, i.e. protein iodination, [1-14C]glucose-oxidation and hormone secretion. Iodide and carbamylcholine decreased the cyclic AMP accumulation induced by cholera toxin as well as by TSH, which supports the hypothesis of an action of these agents beyond the steps of hormone-receptor and receptor-adenylate cyclase interaction. Cooling to 20 degrees C did not impair the TSH induced cyclic AMP accumulation in thyroid slices, but completely suppressed the cholera toxin effect. This observation has been extended to other hormones and target tissues, such as the parathyroid hormone (PTH) (kidney cortex), adrenocorticotropic hormone (ACTH) (adrenal cortex) and luteinizing hormone (LH) (ovary systems). As in thyroid, cooling dissociated the cholera toxin and hormonal effects on cyclic AMP accumulation. In homogenate, cooling decreased cyclic AMP generation in the presence of cholera toxin but at 20 degrees C and 16 degrees C a cholera toxin stimulation was still observed. These results bear strongly against the hypothesis that the glycoprotein hormones TSH and LH acetivate adenylate cyclase by a mechanism identical to cholera toxin.     

Dissociation by cooling of hormone and cholera toxin activation of adenylate cyclase in intact cells

Cholera toxin, through adenylate cyclase activation reproduced cyclic AMP-mediated effects of thyroid-stimulating hormone (TSH) in dog thyroid slices, i.e protein iodination, [1-¹⁴C]glucose-oxidation and hormone secretion. Iodide and carbamylcholine decreased the cyclic AMP accumulation induced by cholera toxin as well as by TSH, which supports the hypothesis of an action of these agents beyond the steps of hormone-receptor and receptor-adenylate cyclase interaction. Cooling to 20°C did not impair the TSH induced cyclic AMP accumulation in thyroid slices, but completely suppressed the cholera toxin effect.This observation has been extended to other hormones and target tissues, such as the parathyroid hormone (PTH) (kidney cortex), adrenocorticotropic hormone (ACTH) (adrenal cortex)_and luteinizing hormone (LH) (ovary systems). As in thyroid, cooling dissociated the cholera toxin and hormonal effects on cyclic AMP accumulation. In homogenate, cooling decreased cyclic AMP generation in the presence of cholera toxin but at 20°C and 16°C a cholera toxin stimulation was still observed. These results bear strongly against the hypothesis that the glycoprotein hormones TSH and LH activate adenylate cyclase by a mechanism identical to cholera toxin.     

Calmodulin plus cyclic AMP-dependent phosphorylation of a Mr 22,000 pituitary protein

Protein phosphorylation was examined in cytosolic extracts of adult rat anterior pituitary. In the presence of both cyclic AMP and calmodulin, the phosphorylation of a Mr 22,000 protein was markedly stimulated. Cyclic AMP and calmodulin must both be present in order for this effect to be observed; cyclic GMP does not substitute for cyclic AMP, and the effect is abolished by either trifluoperazine or the heat-stable inhibitor of cyclic AMP-dependent protein kinase. Two-dimensional isoelectric focusing sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicates that there are three molecular species of the Mr 22,000 phosphoprotein, with pI values ranging from 6.8 to 8.1. Phosphorylation of this protein is maximally stimulated by 5 microM cyclic AMP and 5.7 microM calmodulin. The effect of cyclic AMP plus calmodulin is enhanced by preincubation and requires a divalent cation; maximal phosphorylation takes place at 100 microM Mn2+, although higher concentrations of Mg2+ and Co2+ support an equivalent degree of phosphorylation. Cyclic AMP plus calmodulin-dependent protein phosphorylation was not detected in other rat tissues surveyed, including brain, testes, adrenal, kidney, liver, spleen, skeletal muscle, pineal, or posterior pituitary. These results help to explain the previous findings of Brattin and Portanova (Brattin, W.J., Jr., and Portanova, R. (1981) Mol. Cell. Endocr. 23, 77-90) of in vivo but not in vitro phosphorylation of three Mr 20,000 anterior pituitary proteins and indicate a possible point of convergence for calcium and cyclic AMP actions in the anterior pituitary.     

Enhanced GTP-dependent activities of the adenylate cyclase system: basis for increased hormonal responsiveness.

Normal rat kidney (NRK) cells growth arrested by picolinic acid and isoleucine deprivation exhibit an increased response to certain agents (i.e., prostaglandin E₁, (?)-isoproterenol, and cholera toxin) which elevate intracellular cyclic AMP levels. The enhanced hormonal response is apparently due, at least in part, to increased adenylate cyclase activity. Adenylate cyclase activities measured in the presence of GTP, GTP plus prostaglandin E₁, and GTP plus (?)-isoproterenol are increased two- to threefold in membranes prepared from treated cells. In contrast, basal activity is potentiated only 20 to 50% and activity determined in the presence of fluoride is only marginally altered. Also of interest is the increase in cholera toxin activation of cyclase activity in the treated cells. Lower concentrations of cholera toxin (5 ng/ml) are required to achieve maximal stimulation of cyclase activity from picolinic acid-treated and isoleucine-deprived cells; maximal stimulation of control cell adenylate cyclase is attained with 25 to 50 ng/ml cholera toxin. Picolinic acid treatment and isoleucine deficiency both have been shown to arrest NRK cell growth in the G₁ phase of the cell cycle. However, results with cells arrested in G₁ by serum starvation and by growth to high cell population density indicate that G₁ specific growth arrest does not appear to account for the increase in hormonal responsiveness. Chelation of inhibitory metals and proteolytic activation also do not appear to be involved in the mechanism by which picolinic acid enhances cyclic AMP formation. Rather, the results suggest that the treated cells have an increased amount of an active GTP-dependent function required for hormone and cholera toxin stimulation of adenylate cyclase. Thus, picolinic acid treatment and isoleucine deprivation may provide a useful means of modulating the GTP-dependent step required to potentiate hormonal responsiveness.     

Bacterial adenylate cyclase increases cyclic AMP and hormone release in pituitary tumor cells

Calmodulin-activated, adenylate cyclase toxin, a virulence factor produced by the human respiratory pathogen Bordetella pertussis, elicits marked accumulation of cyclic AMP in cell lines from rat pituitary tumors. This effect is associated with and apparently responsible for an enhanced release of prolactin and/or growth hormone from GH3, GH4C1 and 235-1 cells. The utility of this novel toxin in probing cyclic AMP-mediated responses is supported by these observations and studies with pertussis and cholera toxins.