Insulin control of cyclic AMP phosphodiesterase

A critical role of Sp1 transcription factor in regulating gene expression in response to insulin and other hormones

Specificity protein 1 (Sp1) belongs to a family of ubiquitously expressed, C(2)H(2)-type zinc finger-containing DNA binding proteins that activate or repress transcription of many genes in response to physiological and pathological stimuli. There is emerging evidence to indicate that in addition to functioning as 'housekeeping' transcription factors, members of Sp family may be key mediators of gene expression induced by insulin and other hormones. The founding member of the family, Sp1, by virtue of its multi-domain organization, potential for posttranslational modifications and interactions with numerous transcription factors, represents an ideal mediator of nuclear signaling in response to hormones. Insulin regulates the sub-cellular localization, stability and trans-activation potential of Sp1 by dynamically modulating its post-translational modification by O-linked beta-N-acetylglucosamine (O-GlcNAc) or phosphate residues. We briefly review the recent literature demonstrating that an involvement of Sp-family of transcription factors in the regulation of differential gene expression in response to hormones is more common than previously appreciated and may represent a key regulatory mechanism.

Insulin deprivation leads to deficiency of Sp1 transcription factor in H-411E hepatoma cells and in streptozotocin-induced diabetic ketoacidosis in the rat

Members of the family of Sp transcription factors include Sp1, Sp3, and Sp4 and are important regulators of eukaryotic gene expression. We previously reported that Sp1 mediated stimulation of rat calmodulin I gene expression in response to insulin. To test whether other members of the Sp family are direct targets of insulin action, we compared the levels of Sp1 and Sp3 proteins from nuclear extracts obtained from both insulin-treated and untreated rat hepatoma (H-411E) cells. We demonstrated by Western blot analysis that levels of Sp1 and Sp3 proteins were increased more than 2-fold in the insulin-treated group. Additionally, the up-regulation of both Sp1 and Sp3 transcription factors by insulin was antagonized by tumor necrosis factor-alpha, a known inhibitor of insulin action. Immunohistochemical analysis demonstrated that H-411E cells treated with insulin (10,000 microU/ml) had a marked increase in demonstrable Sp1 in the nucleus compared with cells incubated in insulin-free medium. We extended these in vitro observations to in vivo studies in the streptozotocin-diabetic rat model. We demonstrated in rat liver tissue by both Western blot and immunohistochemical staining with anti-Sp1 antibody that 1) livers of fully diabetic streptozotocin rats have low levels of Sp1 transcription factor; and 2) insulin treatment of the diabetic rat rapidly reversed this process by markedly stimulating accumulation of Sp1 in rat liver. Studies of the signal transduction mechanisms involved in insulin's effect on Sp1 demonstrate a facilitating role for phosphoinositol 3-kinase and an inhibitory role for cyclic nucleotides. In summary, insulin stimulates Sp1 protein, a transcription factor that is shown to regulate calmodulin gene expression and most likely other, as yet untested, genes.

Members of the Sp transcription factor family regulate rat calmodulin gene expression

We have previously demonstrated that insulin positively regulates transcription of the rat calmodulin (CaM) I gene and that both basal and insulin stimulation of this gene are critically dependent on Sp1. Furthermore, a 392 bp CaM promoter was stimulated by insulin equal to the full promoter but lost activity with deletion of any of the three Sp1 sites (Solomon SS, Palazzolo MR, Takahashi T, Raghow R. Endocrinology 1997;138:5052-5054). Herein we document that Sp1 preferentially binds to the upstream sites Sp1(2) and Sp1(3) but not Sp1(1). Furthermore, gel-mobility super-shift assays demonstrate that both Sp1 and Sp3 protein are found in these complexes. When pPac-Spl, pPac-Sp3, pPac-USp3, and pPac-Sp4 were cotransfected with rCaM 1-392 promoter into Drosophila SL2 cells and challenged with 10,000 microU/mL insulin, we discovered that (1) Sp1 enhanced both basal and insulin-stimulated CaM I gene expression; (2) USp3, a "long" form of the Sp3 molecule, had a stimulatory effect on CaM I gene expression; (3) Sp1 or USp3 is involved in mediating insulin-stimulation of the CaM I gene in SL2 cells; and (4) Sp3, a "short" form of the Sp3 molecule, and Sp4 inhibited Spl-stimulated and insulin-stimulated Sp1-mediated CaM I gene expression. Together these data corroborate and extend our previous observations on Sp1 and elucidate that other members of the Sp family of transcription factors may also be involved in regulating the activity of the CaM promoter.

Effects of chemical denervation with 6-hydroxydopamine on myocardial responsiveness to isoproterenol in rabbits

This study examined the hypothesis that chemical denervation with 6-hydroxydopamine (6-OHDA) would increase myocardial responsiveness to isoproterenol. Five days previously, 15 New Zealand white rabbits were given 60 mg/kg 6-OHDA intravenously. Fifteen control rabbits received vehicle. Hemodynamic, coronary blood flow (CBF), and cardiac output measurements were obtained before and during isoproterenol infusion (0.5 microgram/kg/min for 15 min). Norepinephrine tissue content, beta-adrenoceptor number (Bmax) and affinity (Kd), cyclic AMP content and cyclic AMP-phosphodiesterase (PDE) activity were measured in the subepicardium (EPI) and subendocardium (ENDO). Myocardial norepinephrine content was significantly decreased from 1263 +/- 292 (EPI) and 874 +/- 221 ng/g tissue (ENDO) in the control to 148 +/- 33 (EPI) and 90 +/- 45 ng/g tissue (ENDO) in the denervated group. There were no significant changes in cyclic AMP-PDE activity or Bmax and Kd of beta-adrenoceptors. Cyclic AMP content was similar at baseline, but controls had a significantly larger increase (123-155%) during isoproterenol infusion when compared to the denervated group (27-37%). The denervated animals showed a smaller increase in cardiac output during isoproterenol infusion (from 203 +/- 30 to 235 +/- 26 ml/min), when compared to the control animals (from 135 +/- 18 to 216 +/- 42 ml/min). Baseline CBF was significantly higher in the EPI but not ENDO of the denervated group (185 +/- 20 ml/100 g/min in EPI and 150 +/- 8 in ENDO) compared to the control group (108 +/- 13 in EPI and 133 +/- 17 in ENDO). The relative increase in CBF during isoproterenol infusion was smaller in the denervated group (44-45%) than the control group (107-109%). Isoproterenol infusions of 0.1 and 2.5 micrograms/kg/min showed similarly depressed coronary blood flow responses in denervated animals. Thus, the chemically denervated animals did not have beta-adrenoceptor upregulation, exhibited a lesser increase in cyclic AMP with isoproterenol, and had a reduced functional and coronary blood flow response to isoproterenol. This occurred without any significant change in beta-adrenoceptor number or affinity, or in cyclic AMP-phosphodiesterase activity, indicating there may be receptor uncoupling or other changes in the signal transduction pathway.

Insulin-mediated sensitization of adenylyl cyclase activation

1. Insulin may be an important regulator of vascular function. We have previously studied lymphocyte beta-adrenoceptors as a model for the human vascular beta-adrenoceptor. To examine the effects of insulin on human beta-adrenoceptor responsiveness, adenylyl cyclase activity, cyclic AMP-dependent protein kinase activity and beta-adrenoceptor radioligand binding assays were performed on permeabilized mononuclear leukocytes. 2. With acute exposure to insulin in vitro, followed by washing and permeabilization there was a dose-dependent increase in both lymphocyte NaF-stimulated activity and beta-adrenoceptor-stimulated adenylyl cyclase activity paralleling an increase in beta-adrenoceptor-stimulated protein kinase A activity. Manganese-, forskolin- and forskolin plus guanylimidodiphosphate-stimulated adenylyl cyclase activities were not altered by insulin pretreatment. Additionally, mononuclear leukocyte beta-adrenoceptor density, proportion of externalized receptors and receptor affinity for agonist were not altered. 3. The data indicate that acute exposure to insulin sensitizes G-protein-stimulated adenylyl cyclase activity. These findings suggest a potential role for insulin in the regulation of beta-adrenoceptor responsiveness in man.

Elevated cAMP gives short-term inhibition and long-term stimulation of hepatocyte DNA replication: roles of the cAMP-dependent protein kinase subunits

The study reports the role of the isozyme forms (cA-PKI and cA-PKII) and subunits (R and C) of cAMP-dependent protein kinase in mediating the acute depression of hepatocyte DNA replication by elevated cAMP. Combinations of cAMP analogs preferentially activating cA-PKI or II showed that either isozyme could inhibit DNA replication. The effects of glucagon and cAMP analogs were counteracted by the cAMP antagonist RpcAMPS, implicating the necessity for cA-PK dissociation in cAMP action. The effect of elevated cAMP was mimicked by microinjected C subunit, but not by the RI subunit of cA-PK. Hepatocytes under continuous cAMP challenge more than regained their replicative activity. This tardive stimulatory effect of cAMP was enhanced by insulin and blocked by dexamethasone, and was preceded by downregulation of cA-PK. In conclusion, a burst of cAMP acutely inhibits hepatocyte G1/S transition in late G1 regardless of hormonal state. In the presence of high glucocorticoid/low insulin the inhibition persists. At high insulin/low glucocorticoid the inhibitory phase is followed by a prolonged stimulation of DNA replication. Downregulation of endogenous cA-PK is a mechanism for escape from the inhibitory action of highly elevated cAMP.

Further studies of the mechanism(s) of polyunsaturated-fatty-acid-mediated increases in intracellular cAMP formation in N1E-115 neuroblastoma cells

Following earlier observations that increasing the polyunsaturated fatty-acid (PUFA) content of N1E-115 neuroblastoma cells elevated basal and adenosine (Ado)-stimulated intracellular cyclic AMP (cAMP) formation, we carried out studies to determine the mechanism(s) by which PUFA exerted their modulatory effects. Basal increases in cAMP in the PUFA-enriched (PUFA+) cells were evident with short (60 sec) exposure to a phosphodiesterase inhibitor (Ro 20-1724), and increased to a maximum at 20 min; they were not observed in the absence of Ro 20-1724. Forskolin-stimulated cAMP formation in the presence of the Ro compound was 2- to 3-fold higher in the PUFA+ cells. Basal elevations in cAMP were reduced by approximately 70% by exposing the PUFA+ cells to Ado deaminase (ADA) or to an Ado antagonist, and were further increased by inhibiting ADA, which suggested that they could be producing endogenous Ado that activated stimulatory Ado receptors. However, this did not appear to involve PUFA-mediated stimulation of 5'-nucleotidase activity or inhibition of [3H]Ado uptake. Overall, the results of this study indicated that multiple mechanisms are involved in PUFA modulation of cAMP formation.

Regulation of cyclic AMP synthesis and degradation is modified in rat liver at late gestation

Cyclic AMP (cAMP) is known to play a key role in regulating insulin action, and it is well documented that in several cases of physiological insulin resistance its concentration is increased. Since late pregnancy in the rat is associated with liver insulin resistance, we have studied possible alterations of some cellular mechanisms regulating the cAMP metabolism. (1) Liver cAMP concentration was shown to be increased by some 30% and 50% at 18 and 22 days of pregnancy respectively, compared with virgins. (2) Basal adenylate cyclase activity was higher only in the 18-days-pregnant rat, and the forskolin-stimulated maximal activity was similar in the three groups of animals. (3) alpha s protein is decreased in term-pregnant rats; however, coupling between Gs and adenylate cyclase is only impaired in the 18-days-pregnant animals, and stimulation by glucagon is impaired in both groups of pregnant animals. (4) Gi-2 protein was shown to be unable to elicit the tonic inhibition of adenylate cyclase in pregnant rats, although it was only decreased at 22 days of gestation. The increased alpha i-2 level detected by immunoblotting at 18 days of gestation did not correlate with its decreased ADP-ribosylation, suggesting that the protein is somehow modified at this stage. (5) Pregnancy is associated with a decrease in membrane phosphodiesterase activity. Our results show that late pregnancy is associated with increases in liver cAMP levels that might be involved in eliciting the characteristic insulin-resistant state, and suggest that mechanisms leading to these increments are changing during this phase of gestation.

Effects of streptozotocin diabetes on amylase release and cAMP accumulation in rat parotid acinar cells

Rat parotid responses to sympathetic nerve stimulation in vivo are impaired 2-4 weeks after the induction of streptozotocin diabetes. In this study, the effects of experimental diabetes of similar duration and severity on noradrenaline-stimulated amylase release and cAMP accumulation were examined in vitro. Amylase levels were significantly lower in acinar cells isolated from diabetic animals than in controls, and cellular amylase increased after treatment of the diabetic animals with either thyroxine (T4) or insulin. Diabetes and T4 had no apparent affect on amylase release measured as a percentage of the total. In contrast, giving insulin resulted in a significant reduction in maximal secretion (20.4 +/- 2.4% compared with 43.6 +/- 7.6%). Similar results were observed when amylase release was stimulated with forskolin. Basal cAMP levels were unaffected by diabetes or T4 (7.8 +/- 2.3 pmol/mg protein), but stimulated cAMP levels were significantly greater in diabetic acinar cells than in controls. Insulin reversed the effects of diabetes on cAMP accumulation, whereas T4 had no effect. Thus, diabetes (2-4 weeks) and insulin in vivo appear to have paradoxical effects on parotid amylase release and cAMP accumulation in vitro. Further, the effects of diabetes appear to be unrelated to thyroid status.

Effects of sequence and timing of hormonal additions on adipose tissue: activation of low-Km cyclic adenosine monophosphate phosphodiesterase

Epinephrine (EPI) is lipolytic and insulin (INS) antilipolytic in the isolated fat cell (IFC). We have previously demonstrated that in a perifusion system the antilipolytic action of INS is more powerful when IFC are exposed to INS before EPI. In contrast to their opposite effects on lipolysis, both INS and EPI stimulate low-Km cyclic adenosine monophosphate (cAMP) phosphodiesterase (PDE) in adipose tissue. In view of these observations, we decided to determine the effects of sequential addition of EPI and INS on stimulation of PDE from rat adipose tissue. Using previously published methods, the effects of INS and EPI on PDE were assessed alone, together with INS followed by EPI, and then with EPI followed by INS. The resulting data demonstrate that EPI and INS individually both stimulate PDE (P less than .001); EPI plus INS together stimulate PDE minimally compared with EPI or INS alone (P less than .001); when adipose tissue is included with INS first, then followed by EPI, activation of PDE is much less than INS or EPI alone (P less than .001); and when adipose tissue is stimulated by EPI then INS, there is no activation of PDE, different from EPI or INS alone (P less than .001). In conclusion, in perifused IFC, INS and EPI always oppose each other. In studies using activation of PDE, EPI and INS each stimulate PDE, but INS opposes EPI when incubated simultaneously. When adipose tissue is incubated first with INS followed by EPI, PDE is activated. In contrast, when the reverse order is applied, no activation of PDE is observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclic AMP phosphodiesterases and Ca2+ current regulation in cardiac cells

At least four different isoforms of phosphodiesterases (PDEs) are responsible for the hydrolysis of cAMP in cardiac cells. However, their distribution, localization and functional coupling to physiological effectors (such as ion channels, contractile proteins, etc.) vary significantly among various animal species and cardiac tissues. Because the activity of cardiac Ca2+ channels is strongly regulated by cAMP-dependent phosphorylation, Ca(2+)-channel current (ICa) measured in isolated cardiac myocytes may be used as a probe for studying cAMP metabolism. When the activity of adenylyl cyclase is bypassed by intracellular perfusion with submaximal concentrations of cAMP, effects of specific PDE inhibitors on ICa amplitude are mainly determined by their effects on PDE activity. This approach can be used to evaluate in vivo the functional coupling of various PDE isozymes to Ca2+ channels and their differential participation in the hormonal regulation of ICa and cardiac function. Combined with in vitro biochemical studies, such an experimental approach has permitted the discovery of hormonal inhibition of PDE activity in cardiac myocytes.

Cyclic nucleotide phosphodiesterases: pharmacology, biochemistry and function

This article is a review of cyclic nucleotide phosphodiesterase(s) (CN PDE) from the point of view of the relationships between the newer aspects of the complex enzymology of CN PDE and recent major advances in CN PDE pharmacology. A consolidation of isozyme nomenclature to the proposed family designations is recommended. Emphasis is placed on the importance of defining the subcellular localization of isozymes expressed in a given tissue and cyclic GMP substrate and regulatory roles in CN PDE isozyme functions. CN PDE inhibitors that may be useful for experimental and clinical purposes are discussed. Examples of these inhibitors include CGS 9343B, TCV-3B, KW-6, MIMAX, Dihydroisoquinolines, Trequinsin, bipyridine and dihydropyridazinone cardiotonics, Rolipram, SQ 65442, Zaprinast and Dipyridamole.

Effects of castration, dietary fat and adipose tissue sites on adipocyte plasma-membranes cyclic AMP phosphodiesterase activity in the pig

1. cAMP Phosphodiesterase activity and kinetic parameters (Km and Vmax) were measured in subcutaneous and perirenal adipocyte plasma membranes from Large White male and castrated pigs. The animals were fed a control low fat diet or a sunflower diet enriched with linoleic acid (C18:2 n-6). 2. Phosphodiesterase activity, low Km and Vmax were lowered by castration. 3. In animals fed the sunflower diet, phosphodiesterase activity decreased without affecting either Km or Vmax. 4. Phosphodiesterase activity was higher in perirenal sites than in subcutaneous ones, particularly in male pigs. This may be explained by a lower Km or a higher cAMP phosphodiesterase affinity to cAMP in perirenal sites. 5. Theophylline was a potent inhibitor of phosphodiesterase activity principally in perirenal sites. 6. The intermediate role of cAMP phosphodiesterase in adenylate cyclase activity and lipolytic processes is discussed.

Effects of dietary fat and adipose tissue location on insulin action in young boar adipocytes

1. Regulation of lipogenesis and lipolysis by insulin was studied on adipocytes isolated from 100 kg Large white male pigs. Two adipose tissues were studied: subcutaneous and perirenal. Animals were fed either a control low fat diet or a diet containing 14.7% sunflower seed oil. 2. The cell diameter was higher in the group fed the sunflower diet. 3. De novo lipogenesis was decreased for each adipose tissue in the group fed the sunflower diet. The perirenal site had a higher lipogenic activity than subcutaneous site whatever the diet. 4. Insulin did not significantly stimulate lipogenesis but had an important antilipolytic effect on stimulated lipolysis by isoproterenol. 5. The antilipolytic action of insulin was higher in perirenal adipocytes with the control diet. With the sunflower diet, the decrease was about 54.4% for subcutaneous adipocytes, whereas the inhibition was decreased in perirenal adipocytes. Addition of theophylline reversed the antilipolytic action of insulin. 6. Insulin binding was not affected neither by the dietary fat nor by the adipose tissue location. 7. Absence of de novo lipogenesis stimulation by insulin was not due to an impairment in insulin binding. 8. The different effects of dietary fat and adipose tissue location on the antilipolytic action of insulin could not be explained by a modification of insulin binding but rather by a latter event, probably at a post-insulin binding stage.

Expression of calmodulin gene is down-regulated in diabetic BB rats

In past studies, we have demonstrated that in streptozotocin-induced diabetic or spontaneously diabetic (BB) animal models, low Km cAMP phosphodiesterase and calmodulin are decreased while a low MW inhibitor of calmodulin is increased. To extend these studies, we have determined the rate of [35S]-methionine incorporation into calmodulin in isolated fat cells from these diabetic animals, i.e. streptozotocin-induced diabetic and the BB rats, spontaneous diabetic rat, non-diabetic rat, and control. We found markedly decreased rates of synthesis of calmodulin in the fully diabetic BB rat. In order to investigate the mechanism of the reduced calmodulin biosynthesis, we probed poly A+ mRNA from control and diabetic rat livers with a calmodulin specific anti-sense oligonucleotide probe and found that the fully diabetic animals, streptozotocin-induced diabetic and genetically diabetic BB, contained markedly reduced levels of calmodulin transcripts. Thus, both calmodulin protein and its putative mRNA are decreased in diabetic rat liver. We believe that in uncontrolled diabetes, the observed elevation in the levels of cyclic AMP in plasma and tissue results in part from decreased activity of phosphodiesterase. The insulin-sensitive phosphodiesterase appears to be regulated by calmodulin. We hypothesize that cyclic AMP phosphodiesterase inactivation in diabetes results in part from insulin insufficiency and to a less well-defined genetic lesion leading to calmodulin down-regulation.