Role of cyclic AMP and protein kinase on the steroidogenic action of ACTH, prostaglandin E1 and dibutyryl cyclic AMP in normal adrenal cells and adrenal tumor cells from humans.

The role of the cyclic AMP-protein kinase system in mediating the steroidogenic effect of ACTH, prostaglandin E1 and dibutyryl cyclic AMP, induced similar stimulations of protein kinase activity, cyclic AMP was studied using human adrenal cells isolated from normal and adrenocortical secreting tumors. At high concentrations of ACTH, complete activation of protein kinase of normal adrenal cells was observed within 3 min, at the time when cyclic AMP production was slightly increased and there was still no stimulation of steroidogenesis. At supramaximal concentrations, ACTH, PGE1 and dibutyryl cyclic AMP and cortisol productions in adrenal cells isolated from normal and from one adrenocortical tumor. In one tumor in which the adenylate cyclase activity was insensitive to ACTH, the hormone was unable to stimulate protein kinase or steroidogenesis, but the cells responded to both PGE1 and dibutyryl cyclic AMP. In another tumor in which the adenylate cyclase was insensitive to PGE1, this compound also did not increase protein kinase activity or steroidogenesis, but both parameters were stimulated by ACTH and dibutyryl cyclic AMP. After incubation of normal adrenal cells with increasing concentrations of ACTH (0.01-100 nM) marked differences were found between cyclic AMP formation and cortisol production. However at the lowest concentrations of ACTH exerting an effect on steroid production a close linked correlation was found between protein kinase activation and cortisol production, but half-maximal and maximal cortisol production occurs at lower concentration of ACTH than was necessary to induce the same stimulation of protein kinase. Similar findings were found after incubating the adrenal cells with dibutyryl cyclic AMP (0.01-10 mM). The results implicate an important role of the cyclic AMP-protein kinase system during activation of adrenal cell steroidogenesis by low concentrations of steroidogenic compounds.     

The correlation of cyclic AMP and protein kinase activity in adipocytes with lipolysis stimulated by ACTH: the effect of adenosine deaminase and actinomycin D.

ACTH at levels as low as 0.05 mU/ml stimulated lipolysis, protein kinase and cyclic AMP accumulation in isolated fat cells from fed and fasted rats. Changes in cyclic AMP levels and in the protein kinase activity ratio were well correlated temporally. The protein kinase activity ratio was potentiated by adenosine deaminase. A sudden increase or decrease in either ACTH or dibutyryl cyclic AMP concentration was associated with a rapid and corresponding change in the rate of glycerol production. With ACTH, the changes in glycerol production were accompanied by appropriate changes in cyclic AMP levels. Actinomycin-D (10 UM) did not affect lipolysis or cyclic AMP accumulation activated by ACTH in fat cells.     

Mechanisms of action of corticotropin-releasing factor and other regulators of corticotropin release in rat pituitary cells

The role of cyclic AMP in the stimulation of corticotropin (ACTH) release by corticotropin-releasing factor (CRF), angiotensin II (AII), vasopressin (VP), and norepinephrine (NE) was examined in cultured rat anterior pituitary cells. Synthetic CRF rapidly stimulated cyclic AMP production, from 4- to 6-fold in 3 min to a maximum of 10- to 15-fold at 30 min. Stimulation of ACTH release by increasing concentrations of CRF was accompanied by a parallel increase in cyclic AMP formation, with ED50 values of 0.5 and 1.3 nM CRF for ACTH and cyclic AMP, respectively. A good correlation between cyclic AMP formation and ACTH release was also found when pituitary cells were incubated with the synthetic CRF(15-41) fragment, which displayed full agonist activity on both cyclic AMP and ACTH release with about 0.1% of the potency of the intact peptide. In contrast, the CRF(21-41) and CRF(36-41) fragments were completely inactive. The other regulators were less effective stimuli of ACTH release and caused either no change in cyclic AMP (AII and VP) or a 50% decrease in cyclic AMP (NE). Addition of the phosphodiesterase inhibitor, methylisobutylxanthine, increased the sensitivity of the ACTH response to CRF but did not change the responses to AII, VP, and NE. In pituitary membranes, adenylate cyclase activity was stimulated by CRF in a dose-dependent manner with ED50 of 0.28 nM, indicating that the CRF-induced elevation of cyclic AMP production in intact pituitary cells is due to increased cyclic AMP biosynthesis. The intermediate role of cyclic AMP in the stimulation of ACTH release by CRF was further indicated by the dose-related increase in cyclic AMP-dependent protein kinase activity in pituitary cells stimulated by CRF with ED50 of 1.1 nM. These data demonstrate that the action of CRF on ACTH release is mediated by the adenylate cyclase-protein kinase pathway and that the sequence requirement for bioactivity includes the COOH-terminal 27 amino acid residues of the molecule. The other recognized regulators of ACTH release are less effective stimuli than CRF and do not exert their actions on the corticotroph through cyclic AMP-dependent mechanisms.     

G1 specific increases in cyclic AMP levels and protein kinase activity in Chinese hamster ovary cells.

Chinese hamster ovary cells were synchronized by selective detachment of cells in mitosis. The adenosine 3':5'-cyclic monophosphate (cyclic AMP) intracellular concentrations and cyclic AMP-dependent protein kinase activities were measured as these cells traversed G1 phase and entered S phase. Protein kinase activity, assayed in the presence or absence of saturating exogenous cyclic AMP in the reaction mixture, was lowest in early G1 phase (2 h after mitosis), increased 2-fold (plus exogenous cyclic AMP in reaction mixture) or 3.5-fold (minus cyclic AMP in reaction mixture) to maximum values in mid to late G1 phase (4-5 h after mitosis), and then decreased as cells entered S phase. Intracellular cyclic AMP concentrations were minimal 1 h after mitosis, increased 5-fold to maximum levels at 4-6 after mitosis, and decreased as cells entered S phase. Similar to the fluctuations in intracellular cyclic AMP, the cyclic AMP-dependent protein kinase activity ratio increased more than 40% in late G1 or early S phase. Puromycin (either 10 mug/ml or 50 mug/ml) administered 1 h after mitosis inhibited cyclic AMP-dependent protein kinase activity up to 50% by 5 h after mitosis, while similar treatment (10 mug/ml) had no effect on the increase in cyclic AMP formation. These data demonstrate that: (1) total protein kinase activity changed during G1 phase and this increase was dependent on new protein synthesis; (2) the increased intracellular concentrations of cyclic AMP were not dependent on new protein synthesis; and (3) the activation of cyclic AMP-dependent protein kinase was temporally coordinated with increased intracellular concentration of cycli AMP as Chinese hamster ovary cells traversed G1 phase and entered S phase. These results suggest that cyclic AMP acts during G1 phase to regulate the activation of cyclic AMP-dependent protein kinase.     

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 by 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 twofold 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.     

Effects of ACTH (corticotropin) analogues on steroidogenesis and cyclic AMP in rat adrenocortical cells. Evidence for two different steroidogenically responsive receptors.

Comparative studies on the mechanism of action of ACTH1-39 and ACTH5-24 [corticotropin-(1-39)- and corticotropin-(5-24)-peptides] on isolated rat adrenal cells were performed. The relationship between stimulated steroidogenesis and cyclic AMP was very different, suggesting that cyclic AMP does not play the same role in mediating the action of the two agonists. Data from three separate experiments showed that the competitive antagonist ACTH6-24 [corticotropin-(6-24)-peptide] had mean inhibitor constants of 13.4 +/- 3.1 nM against ACTH1-39 and 3.4 +/- 1.0 nM against ACTH5-24, indicating that the steroidogenic actions of these two agonists are mediated by two different receptor types. We conclude that there are two possible mechanisms for corticotropin action, only one of which involves the necessary production of cyclic AMP.     

The viral Ki-ras gene must be expressed in the G2 phase if ts Kirsten sarcoma virus-infected NRK cells are to proliferate in serum-free medium.

NRK cells infected with a temperature-sensitive Kirsten sarcoma virus (ts371 KSV) are transformed at 36 degrees C, but are untransformed at 41 degrees C which inactivates the abnormally thermolabile oncogenic p21Ki product of the viral Ki-ras gene. At 41 degrees C, tsKSV-infected NRK cells were arrested in G0/G1 when incubated in serum-free medium, but could then be stimulated to transit G1, replicate DNA, and divide by adding serum at 41 degrees C or dropping the temperature to a p21-activating 36 degrees C without adding serum. When quiescent cells at 41 degrees C were stimulated to transit G1 in serum-free medium by activating p21 at 36 degrees C and then shifted back to the p21-inactivating 41 degrees C in the mid-S phase, they continued replicating DNA but could not transit G2. Reactivating p21 in the G2-arrested cells by once again lowering the temperature to 36 degrees C stimulated a rapid entry into mitosis. By contrast, while serum-stimulated quiescent G0 cells at 41 degrees C replicate DNA and divide, serum did not induce G2-arrested cells to enter mitosis, indicating that serum growth factors may trigger events in the G1 phase that ultimately determine G2 transit. These observations made with the viral ras product suggest that cellular ras proto-oncogene products have a role in G2 transit of normal cells.     

Effect of cyclic AMP on chromatin-bound protein kinases in WI-38 fibroblasts stimulated to proliferate.

When resting confluent monolayers of WI-38 fibroblasts are stimulated to proliferate by serum, DNA synthesis begins to increase between 15-18 h after stimulation. Chromatin-bound protein kinase activity increases in stimulated cells within 1 h after the nutritional change, concomitant with an increase in the template activity of nuclear chromatin. Addition of dibutyryl 3' : 5'-cyclic adenosine monophosphate (dibutyryl cyclic) AMP to the stimulating medium inhibits the entrance of cells into S phase, but only if dibutyryl cyclic AMP (5-10(-4) M) is added before the onset of DNA synthesis. The increases in chromatin template activity and in the chromatin-bound kinase activity are not inhibited by dibutyryl cyclic AMP in the early hours after stimulation, but are completely inhibited after the 5th hour from the nutritional change. This seems to indicate that in stimulated WI-38 cells, dibutyryl cyclic AMP exerts its inhibitory action somewhere between 5 and 12 h after stimulation. A number of protein kinase activities were extracted from chromatin with 0.3 M NaCl and partially resolved on a phosphocellulose column. Two distinct peaks of protein kinase activity appeared to be markedly increased in WI-38 cells 6 h after serum stimulation. Both peaks of increased activity were inhibited by dibutyryl cyclic AMP in vivo. Adenosine, sodium butyrate and adenosine 5'-monophosphate (AMP) do not inhibit the increase in DNA synthesis nor the increase in protein kinase activity. The results suggest that stimulation of cell proliferation in confluent monolayers of WI-38 cells causes an increase (or the new appearance) of certain chromatin-bound protein kinases, and that this increase is inhibited by cyclic AMP in vivo.     

Indirect evidence that protein kinase C plays a critical role in signal transduction of both vasopressin and corticotropin-releasing factor on pituitary cells in culture

The possible role of protein kinase C (PKC) in the cyclic AMP-dependent mechanism of action of corticotropin-releasing factor (CRF) on proopiomelanocortin cells of anterior and intermediate pituitary glands was examined after pretreatment of cells in culture with the PKC inhibitor retinal or the phorbol ester PMA, which depletes cell stores of the kinase. We found that these drugs not only abolished ACTH response to PMA and vasopressin, which both activate PKC, but unexpectably also dampened by 80-90% the stimulatory effect of CRF. Cell treatment with retinal failed to prevent CRF-induced accumulation of cyclic AMP. Retinal and PMA pretreatments of intermediate pituitary cells likewise inhibited alpha-MSH secretion stimulated by CRF. These data provide evidence to suggest that the mechanism of action of CRF on pituitary cells involves both cyclic AMP and PKC messenger systems.     

Evidence against direct involvement of cyclic AMP-dependent protein phosphorylation in the exocytosis of amylase.

To examine whether or not the activation of cyclic AMP-dependent protein kinase is coupled to the exocytosis of amylase from rat parotid cells, the effect of protein kinase inhibitors on amylase release and protein phosphorylation was studied. A membrane-permeable inhibitor of cyclic AMP-dependent protein kinase, N-[2-(methylamino)ethyl]-5-isoquinolinesulphonamide (H-8), and peptide fragments of the heat-stable protein kinase inhibitor [PKI-(5-24)-peptide and PKI-(14-24)-amide] strongly inhibited cyclic AMP-dependent protein kinase activity in the cell homogenate. However, H-8 had no inhibitory effect on amylase release from either intact or saponin-permeabilized parotid cells stimulated by isoproterenol or cyclic AMP. Moreover, PKI-(5-24)-peptide and PKI-(14-24)-amide did not inhibit cyclic AMP-evoked amylase release from saponin-permeabilized cells, whereas cyclic AMP-dependent phosphorylations of 21 and 26 kDa proteins in intact or permeabilized cells were markedly inhibited by these inhibitors. These results suggest that cyclic AMP-dependent protein phosphorylation is not directly involved in the exocytosis of amylase regulated by cyclic AMP.     

Angiotensin II potentiates prostaglandin stimulation of cyclic AMP levels in intact bovine adrenal medulla cells but not adenylate cyclase in permeabilized cells

The level of cyclic AMP in primary cultures of bovine adrenal medulla cells is elevated by prostaglandin E1. Angiotensin II is commonly reported to act on receptors linked to phosphoinositide metabolism or to inhibition of adenylate cyclase. We have investigated the effect of angiotensin II on prostaglandin E1-stimulated cyclic AMP levels in these primary cultures. Rather than reducing cyclic AMP levels, we have found that angiotensin II powerfully potentiates prostaglandin E1-stimulated cyclic AMP accumulation in intact cells, both in the presence and absence of phosphodiesterase inhibitors. The 50% maximal response was similar to that for stimulation of phosphoinositide breakdown by angiotensin II in these cultures. The potentiation of stimulated cyclic AMP levels was seen, although to a smaller maximum, with the protein kinase C (Ca2+/phospholipid-dependent enzyme) activating phorbol ester tetradecanoyl phorbolacetate and with the synthetic diacylglycerol 1-oleoyl-2-acetylglycerol; pretreatment (24 h) with active phorbol ester, which would be expected to diminish protein kinase C levels, attenuated the angiotensin II potentiation of cyclic AMP. Using digitonin-permeabilized cells we showed that adenylate cyclase activity was stimulated by prostaglandin E1 with the same dose-response relationship as was cyclic AMP accumulation in intact cells, but the permeabilized cells showed no response to angiotensin II. The results are discussed with respect to the hypothesis that the angiotensin II influence on cyclic AMP levels is mediated, in part, by diacylglycerol stimulation of protein kinase C.     

Corticotropin-Peptide Regulation of Intracellular Cyclic AMP Production in Cortical Neurons in Primary Culture

Previous studies have provided evidence for adrenocorticotropic hormone (ACTH) effects on a wide variety of behaviors. However, the precise sites of action and the mechanisms by which these effects may be mediated have yet to be clearly elucidated. Although ACTH was shown to augment cyclic AMP levels in glial cells isolated from whole brain, other studies found little or no effect of ACTH peptides on cyclic nucleotide metabolism in slices of cerebral cortex or homogenates of whole brain. In the present study, our objective was to determine whether ACTH peptides regulate intracellular cyclic AMP levels in neurons of the cerebral cortex in primary culture. ACTH peptides stimulated cyclic AMP synthesis up to threefold in a dose-dependent manner; stimulation was complete within 5-10 min of exposure to agonists. Neurohormone efficacy was augmented by 0.1 microM forskolin (which was virtually ineffective alone); potency was unaffected. The order of potency (EC50) for increasing intracellular cyclic AMP levels was as follows: ACTH (1-24), ACTH (1-17) (10 nM) greater than alpha-melanocyte stimulating hormone, beta-melanocyte stimulating hormone (alpha-MSH, beta-MSH) (100 nM) greater than ACTH (1-10) (1 microM) greater than ACTH (4-10) (5 microM). The hexapeptide ACTH (4-9) as well as ACTH (11-24) were inactive at concentrations as high as 10 microM. Other neuropeptides derived from proopiocortin, such as beta-endorphin and Met- and Leu-enkephalin were without effect on basal or hormonally stimulated cyclic AMP synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Altered cyclic AMP-dependent protein kinase activity in a mutant adrenocortical tumor cell line

A somatic cell genetic approach has been used to evaluate the role of cyclic AMP-dependent protein kinase in ACTH action on adrenal steroidogenesis. A mutant clone, 8BrcAMP^r-1, previously was isolated from an ACTH-sensitive adrenocortical tumor cell line (clone Y1) following mutagenesis and selective growth in 8-bromoadenosine 3′, 5′-monophosphate. This study demonstrates that the 8BrcAMP⁴-1 cells have an altered cyclic AMP-dependent protein kinase. The protein kinase in the cytosol of the mutant characteristically requires, for half-maximal activity, concentrations of cyclic AMP 7-fold higher than those required by the enzyme in preparations from the parent. The cytosolic cyclic AMP-dependent protein kinases of Y1 and 8BrcAMP^r-1 cells chromatograph similarly on columns of DEAE-cellulose. From each cell line, a major peak of activity (≥ 70% of recovered activity), designated as Peak I, elutes with 0.04–0.06 M NaCl; a second peak of activity, designated as Peak II, elutes with 0.12–0.14 M NaCl. Protein kinase activity in the Peak I fraction of mutant cells has a decreased apparent affinity (4-fold) for cyclic AMP relative to the corresponding fraction of parental Y1 cells. The protein kinase activities present in Peak II fractions from Y1 and mutant cells are indistinguishable. The protein kinase mutant exhibits poor steroidogenic responses to added ACTH and cyclic AMP; and as shown previously does not display the growth arrest and morphological changes produced in Y1 by these agents. These results suggest that cyclic AMP-dependent protein kinase is important in the regulation of adrenal steroidogenesis, morphology and growth by ACTH.     

Differential response to adrenocorticotropic hormone analogs of bovine adrenal plasma membranes and cells.

The ability of three analogs of ACTH1-24 ([Gln5, Phe9] ACTH1-24, [Gln5, Ala9[Acth1-24, and [Gln5, Lys8, Phe9[ ACTH1-24) embodying tryptophan substitutions to activate the adenylate cyclase system of a bovine adrenal plasma membrane preparation was compared to the effect of the analogs on adenosine 3':5'-monophosphate (cyclic AMP) accumulation and steroidogenesis in viable bovine adrenocortical cells. The results were not comparable. Whereas the analogs antagonized the ACTH1-24-activated membrane cyclase they stimulated cyclic AMP accumulation as well as steroid production of the cells. None of the analogs inhibited steroidogenesis of ACTH1-24-stimulated cells, but two of them, at very high dose levels, inhibited cyclic AMP production. The ability of the analogs to stimulate steroidogenesis of the adrenal cells half-maximally decreased in the order tryptophan greater than phenylalanine greater than alanine, indicating that the aromaticity of the indole ring of tryptophan is necessary for maximal interaction between hormone and receptor. Both the absolute and relative steroidogenic potencies were the same for several analogs when assayed with rat adrenal cells. Although only a small fraction of the cell's potential to produce cyclic AMP was necessary to induce maximum steroid production, the relative activities of a series of analogs were the same for steroidogenesis as for cyclic AMP accumulation. Furthermore, the concentration of cyclic AMP necessary for full steroidogenesis was practically identical for a series of peptides that differed widely in potency. These findings support the postulate that cyclic AMP accumulation and steroidogenesis in adrenocortical cells are coupled processes. The differential behavior of bovine adrenal plasma membranes and bovine adrenocortical cells toward ACTH analogs indicates that structure-function studies using cyclase assays may not reflect events that take place in the intact adrenal or in cell preparations derived therefrom.     

Inhibition of ACTH secretion in mouse pituitary tumor cells by activation of muscarinic cholinergic receptors

Hormonally stimulated secretion of ACTH from AtT-20 mouse pituitary tumor cells is a cyclic AMP-mediated process. The presence of inhibitory cholinergic muscarinic receptors on these cells was recently reported, and in this study, the relationship between the activation of these receptors and the consequent inhibition of cyclic AMP formation and ACTH secretion was investigated. The muscarinic agent, oxotremorine, antagonized both cyclic AMP synthesis and ACTH secretion in response to corticotropin-releasing factor (CRF), vasoactive intestinal peptide, a 27-amino acid peptide with an N-terminal histidine and a C-terminal isoleucine amide, and forskolin. Other muscarinic agents, carbachol and bethanechol, had similar inhibitory effects. The cholinomimetics reduced basal (unstimulated) ACTH secretion without decreasing basal cyclic AMP levels, and also antagonized hormone release in response to cyclic AMP-independent agonists such as K+, A-23187, and phorbol ester. Scopolamine reversed the inhibitory effects of the muscarinic agents on basal and stimulated ACTH secretion and cyclic AMP formation. Increasing the extracellular calcium concentration reversed the muscarinic antagonism of basal and CRF-stimulated hormone release without affecting the cyclic AMP response. Pertussis toxin pretreatment attenuated the inhibitory effects of the muscarinic agents on forskolin-stimulated cyclic AMP synthesis and ACTH secretion as well as the inhibitory effect of carbachol on basal ACTH release. The data suggest that cyclic AMP is an essential mediator in the ACTH secretory pathway, but that an alternate cyclic AMP-independent ACTH pathway also exists in the clonal cells, and that both pathways may be modulated by a common postcholinergic receptor mechanism.     

The phorbol ester PMA and cyclic AMP activate different Cl(-) and HCO3(-) fluxes in C127 cells expressing CFTR

In mouse mammary epithelial C127 cells expressing wild-type cystic fibrosis transmembrane conductance regulator (CFTR), chloride efflux, measured with the Cl(-)-sensitive dye 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ), was stimulated by activation of protein kinase A with cyclic AMP elevating agents forskolin plus 3-isobutyl-1-methyl-xanthine (IBMX) and, to a less extent, by activation of protein kinase C with the phorbol 12-myristate 13-acetate (PMA). Conversely, bicarbonate influx, determined by intracellular alkalinization of cells incubated with the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluoresceintetraacetoxymethyl ester (BCECF-AM), was stimulated by cyclic AMP elevation, but not by PMA. Patch clamp analysis revealed that PMA activated a Cl(-) current with the typical biophysical characteristics of swelling-activated current and not of CFTR.     

The role of protein kinase activation in the control of steroidogenesis by adrenocorticotrophic hormone in the adrenal cortex

A method has been developed for investigation of the effect of adrenocorticotrophic hormone (ACTH) on the state of activation of a cyclic AMP-dependent protein kinase within cells of the adrenal cortex. Enzyme activity was measured in terms of the quantity of (32)P transferred from [gamma-(32)P]ATP to histone under conditions in which bound cyclic AMP did not dissociate from the regulatory subunit of the protein kinase ACTH (1x10(-2)i.u./ml) caused a rapid and complete activation of the cyclic AMP-dependent protein kinase activity within 2min of hormone addition to the isolated cells. In response to a range of ACTH concentrations a sigmoid log dose-response curve for protein kinase activation was obtained, with half-maximal stimulation attained at about 1x10(-3)i.u./ml. However, some low doses of ACTH that elicited a marked (but submaximal) steroidogenic response failed to cause a clear stimulation of protein kinase activity in isolated adrenal cells. Theophylline (2mm) potentiated the effect of ACTH on protein kinase activity. The results implicate an important role for protein kinase in ACTH action on the adrenocortical cell.     

Induction of 3T3 cell division at the monolayer stage : Early changes in macromolecular processes

We have previously demonstrated that 3T3 cells at the monolayer stage can be induced to divide by brief treatment with a variety of proteases. We have now used this system to investigate the macromolecular changes occurring in 3T3 cells induced to divide in this manner. Immediately after pronase treatment, 3T3 cells become agglutinable with concanavalin A. This is rapidly followed in order by a decrease in cyclic AMP concentration within the cell, and an increase in RNA synthesis and uridine transport. The increases in RNA synthesis and uridine transport are dependent on concomitant protein synthesis. Specific protein synthesis follows 1 h after protease treatment, with DNA synthesis occurring 24 h after treatment and mitosis 30 h after treatment. This work suggests that following alteration of the surface membrane a variety of intercellular macromolecular processes occur which eventually culminate in DNA synthesis and cell division. Such a series of events may occur during each cell cycle in non-confluent 3T3 cells.     

Atrial natriuretic factor regulation of cyclic GMP levels and steroidogenesis in isolated fasciculata cells of rat adrenal cortex

Isolated fasciculata cells of rat adrenal cortex, when incubated with atrial natriuretic factor (ANF), stimulated the levels of cyclic GMP and corticosterone production in a concentration-dependent manner without a rise in the levels of cyclic AMP. The ANF-dependent elevation of cyclic GMP was rapid, with a detectable increment in 30 s. ANF also stimulated the particulate guanylate cyclase. These results not only indicate the coupling of cyclic GMP and corticosterone production with ANF signal, but also demonstrate that, like the ACTH signal, cyclic AMP is not the mediator of ANF-induced adrenocortical steroidogenesis.