Hepatocyte beta-adrenergic responsiveness and guanine nucleotide-binding regulatory proteins

Canonical and non-canonical Wnt signaling are simultaneously activated by Wnts in colon cancer cells

The Wnt signaling pathway is a crucial regulator of the intestinal epithelium homeostasis and is altered in most colon cancers. While the role of aberrant canonical, β-catenin-dependent Wnt signaling has been well established in colon cancer promotion, much less is known about the role played by noncanonical, β-catenin-independent Wnt signaling in this type of cancer. This work aimed to characterize the noncanonical signal transduction pathway in colon cancer cells. To this end, we used the prototype noncanonical ligand, Wnt5a, in comparison with Wnt3a, the prototype of a canonical β-catenin activating ligand. The analysis of the expression profile of Wnt receptors in colon cancer cell lines showed a clear increase in both level expression and variety of Frizzled receptor types expressed in colon cancer cells compared with non-malignant cells. We found that Wnt5a activates a typical Wnt/Ca++ - noncanonical signaling pathway in colon malignant cells, inducing the hyperphosphorylation of Dvl1, Dvl2 and Dvl3, promoting Ca++ mobilization as a result of phospholipase C (PLC) activation via pertussis toxin-sensitive G-protein, and inducing PLC-dependent cell migration. We also found that while the co-receptor Ror2 tyrosine kinase activity is not required for Ca++ mobilization-induced by Wnt5a, it is required for the inhibitory effects of Wnt5a on the β-catenin-dependent transcriptional activity. Unexpectedly, we found that although the prototype canonical Wnt3a ligand was unique in stimulating the β-catenin-dependent transcriptional activity, it also simultaneously activated PLC, promoted Ca++ mobilization, and induced Rho kinase and PLC-dependent cell migration. Our data indicate, therefore, that a Wnt ligand can activate at the same time the so-called Wnt canonical and noncanonical pathways inducing the formation of complex signaling networks to integrate both pathways in colon cancer cells.

Pivotal role of cAMP in the activation of liver glycogen breakdown in high-fat diet fed mice

AIMS

Liver glycogen catabolism was evaluated in male Swiss mice fed a high-fat diet rich in saturated fatty acids (HFD) or normal fat diet (NFD) during one week.

MAIN METHODS

Liver glycogenolysis (LG) and liver glucose production (LGP) were measured either under basal or stimulated conditions (infusion of glycogenolytic agents). Thus, isolated perfused livers from HFD and NFD mice were infused with glycogenolytic agents, i.e., glucagon, epinephrine, phenylephrine, isoproterenol, adenosine-3'-5'-cyclic monophosphate (cAMP), N(6),2'-O-dibutyryl-cAMP (DB-cAMP), 8-bromoadenosine-cAMP (8-Br-cAMP) or N(6)-monobutyryl-cAMP (N6-MB-cAMP). Moreover, glycemia and liver glycogen content were measured.

KEY FINDINGS

Glycemia, liver glycogen content and basal rate of LGP and LG were not influenced by the HFD. However, LGP and LG were lower (p<0.05) in HFD mice during the infusions of glucagon (1 nM), epinephrine (20 μM) or phenylephrine (20 μM). In contrast, the activation of LGP and LG during the infusion of isoproterenol (20 μM) was not different (HFD vs. NFD). Because glucagon showed the most prominent response, the effect of cAMP, its intracellular mediator, on LGP and LG was investigated. cAMP (150 μM) showed lower activation of LGP and LG in the HFD group. However, the activation of LGP and LG was not influenced by HFD whether DB-cAMP (3 μM), 8-Br-cAMP (3 μM) or N6-MB-cAMP (3 μM) were used.

SIGNIFICANCE

The activation of LGP and LG depends on the intracellular availability of cAMP. It can be concluded that cAMP played a pivotal role on the activation of LG in high-fat diet fed mice.

Role of cyclic-AMP responsive element binding (CREB) proteins in cell proliferation in a rat model of hepatocellular carcinoma

The role of cyclic adenosine monophosphate (cAMP) is poorly understood in the regulation of normal and abnormal hepatic cell growth. In this study, we examined the regulation of intracellular cAMP levels and its effect on nuclear cAMP responsive elements (CREs) in a rat model of hepatocellular carcinoma (HCC). Tumorigenic liver cells were cultured from an in vivo model of HCC and the role of cAMP in cell mitogenesis determined. These data demonstrated agents that elevate intracellular cAMP ([cAMP]i) levels caused significant dose-dependent inhibition of serum-stimulated mitogenesis in HCC cells. Cells were next analyzed for transcription factor expression and activity following increased [cAMP]i. These data demonstrated time- and dose-dependent increases in CRE binding protein (pCREB) activity, a maximal response occurring after 10-20 min before returning to basal levels within 60 min. In contrast, increased [cAMP]i levels led to sustained inducible cAMP early repressor (ICER) II/IIgamma mRNA and protein induction. To understand these data in relation to the in vivo setting, HCC tumors were analyzed and compared to pair-matched normal liver (NL) samples. These studies demonstrated significantly elevated Gsalpha-protein expression in HCC versus NL in the absence of significant changes in basal cAMP levels. Analysis of total and active CREB demonstrated significantly increased total CREB/pCREB in HCC versus NL. Further analysis of CRE expression demonstrated significantly increased expression of ICER mRNA and protein in HCC versus sham operated (Sh). These data demonstrate cAMP, while capable of stimulating promitogenic CREB activation inhibits cell mitogenesis in HCC possibly via ICER induction.

Mechanisms of regulation of G(11)alpha protein by dexamethasone in osteoblastic UMR 106-01 cells

We have previously demonstrated that glucocorticoids increased G(q/11)alpha protein expression and phospholipase C activity in the rat osteosarcoma cell line UMR 106-01. In this study, we demonstrated that G(11)alpha is the primary G(q)-subtype family member expressed in UMR cells. Dexamethasone treatment increased the expression of G(11)alpha protein in both a time- and a dose-dependent manner. Glucocorticoid treatment significantly increased the half-life of G(11)alpha protein from 20.3 to 63 h. Steady-state G(11)alpha mRNA level was also increased by glucocorticoid treatment by approximately 70%. This change was not the result of changes in RNA stability but rather the result of increased transcription, because the glucocorticoid-mediated upregulation of G(11)alpha mRNA was blocked by the transcription inhibitor actinomycin D. The dexamethasone induction of G(11)alpha mRNA occurred after a time lag of 12-24 h and was blocked by the protein synthesis inhibitor cycloheximide. These results suggest that the dexamethasone-induced rise in G(11)alpha protein results primarily from changes in the degradation rate of the protein, whereas changes in G(11)alpha mRNA play a smaller role and require de novo synthesis of regulatory protein(s).

Time sequence of changes in the responsiveness of glycogen breakdown to adrenergic agonists in perfused liver of rats with insulin-induced hypoglycemia

The time-course changes of the responsiveness of glycogen breakdown to alpha- and ss-adrenergic agonists during insulin-induced hypoglycemia (IIH) were investigated. Blood glucose levels were decreased prior to the alteration in the hepatic responsiveness to adrenergic agonists. The activation of hepatic glucose production and glycogenolysis by phenylephrine (2 microM) and isoproterenol (20 microM) was decreased in IIH. The changes in the responsiveness of glycogen catabolism were first observed for isoproterenol and later for phenylephrine. Hepatic ss-adrenergic receptors showed a higher degree of adrenergic desensitization than did alpha-receptors. Liver glycogen synthase activity, glycogen content and the catabolic effect of dibutyryl cyclic AMP (the ss-receptor second messenger) were not affected by IIH.

Angiotensin AT1 receptors in Clone 9 rat liver cells: Ca2+ signaling and c-fos expression

In C9 (Clone 9) liver cells, angiotensin 11 increased the intracellular Ca2+ content, inositol phosphate production and c-fos mRNA expression. Other angiotensins were also active with the order of potency being angiotensin II = angiotensin III >> angiotensin I > angiotensin IV. Losartan, but not PD 123177 (1-(4-amino-3-methyl)-5-diphenylacetyl-4,5,6,7-tetrahydro-1H-imida zo [4,5c]pyridine-6-carboxylic acid), blocked the effects of angiotensin II. Pertussis toxin did not alter these actions of angiotensin II. These data indicate that the effects were mediated through angiotensin AT1 receptors involving pertussis toxin-insensitive G-proteins. Phorbol myristate acetate was also able to increase c-fos mRNA expression. The action of angiotensin II was consistently greater than that of the active phorbol ester. Staurosporine but not genistein inhibited this effect of angiotensin II. Angiotensin II- and phorbol myristate acetate-induced proto-oncogene mRNA expression was attenuated in cells incubated overnight with the active phorbol ester, which suggests a major role of protein kinase C.

Responsiveness of glycogen catabolism to adrenergic agonists during insulin-induced hypoglycemia in rat livers

1. Insulin-induced hypoglycemia (IIH) promoted decreased responsiveness of hepatic glycogen catabolism to phenylephrine and isoproterenol, but not to glucagon and cyanide. 2. In addition, glycogen phosphorylase activity and glycogen levels were not affected by IIH. 3. It was concluded that hypoglycemia promoted changes in hepatic responsiveness to adrenergic agonists. 4. However, the ability of the liver to mobilize glycogen was not influenced by hypoglycemia.

Activation of endothelin ETA receptors induces phosphorylation of alpha1b-adrenoreceptors in Rat-1 fibroblasts

The effect of endothelin-1 on the phosphorylation of alpha1b-adrenoreceptors, transfected into rat-1 fibroblasts, was studied. Basal alpha1b-adrenoreceptor phosphorylation was markedly increased by endothelin-1, norepinephrine, and phorbol esters. The effect of endothelin-1 was dose dependent (EC50 approximately 1 nM), reached its maximum 5 min after stimulation, and was inhibited by BQ-123, an antagonist selective for ETA receptors. Endothelin-1-induced alpha1b-adrenoreceptor phosphorylation was attenuated by staurosporine or genistein and essentially abolished when both inhibitors were used together. The effect of norepinephrine was not modified by either staurosporine or genistein alone, and it was only partially inhibited when both were used together. These data suggest the participation of protein kinase C and tyrosine kinase(s) in endothelin-1-induced receptor phosphorylation. However, phosphoaminoacid analysis revealed the presence of phosphoserine and traces of phosphothreonine, but not of phosphotyrosine, suggesting that the putative tyrosine kinase(s), activated by endothelin, could act in a step previous to receptor phosphorylation. The effect of endothelin-1 on alpha1b-adrenoreceptor phosphorylation was not mediated through pertussis toxin-sensitive G proteins. Calcium mobilization induced by norepinephrine was diminished by endothelin-1. Norepinephrine and endothelin-1 increased [35S]GTPgammaS binding to control membranes. The effect of norepinephrine was abolished in membranes obtained from cells pretreated with endothelin-1. Interestingly, genistein plus staurosporine inhibited this effect of the endothelial peptide. Endothelin-1 did not induce alpha1b-adrenoreceptor internalization. Our data indicate that activation of ETA receptors by endothelin-1 induces alpha1b-adrenoreceptor phosphorylation and alters G protein coupling.

Cyclic AMP signalling and cellular proliferation: regulation of CREB and CREM

In eukaryotes, transcriptional regulation upon stimulation of the adenylyl cyclase signalling pathway is mediated by a family of cAMP-responsive nuclear factors. This family consists of a large number of members which may act as activators or repressors. These factors contain the basic domain/leucine zipper motifs and bind as dimers to cAMP-response elements (CRE). The function of CRE-binding proteins (CREB) is modulated by phosphorylation by the cAMP-dependent protein kinase. The ICER (inducible cAMP early repressor) protein is the only inducible member of this family and is a product of the CREM gene. The induction of this powerful repressor is likely to be important for the transient nature of cAMP-induced gene expression. CREB proteins have been found to play an important role in the physiology of neuroendocrine functions. In addition, recent results indicate that CREB and CREM could be involved in the proliferation of hepatocytes which follows partial hepatectomy.

The involvement of the stimulatory G protein in sexual dimorphism of beta-adrenergic receptor-mediated functions in rat liver

In rat hepatocytes, beta-adrenergic receptor (beta-AR)-mediated cAMP generation was found to be higher in the female than in the male. As compared to the male, the number of beta-AR, detected by [125I]iodocyanopindolol, was elevated in the female. In agonist competition experiments, the proportion of beta-AR in the high-affinity state was promoted in the female than in the male. The alpha subunit of the stimulatory G protein (Gs alpha) was quantified using ADP-ribosylation catalyzed by cholera toxin. The amount of Gs alpha, both small, 42 kDa (Gs alpha S), and large, 47 kDa (Gs alpha L), forms increased in parallel with enhancement of catecholamine-sensitive adenylate cyclase activity in the female. The female showed a disproportionate increase in Gs alpha L, which is preferentially coupled to beta-AR, compared with Gs alpha S. In addition, 17 beta-estradiol facilitated isoproterenol-induced cAMP generation in both male and female rats, whereas castration or testosterone had no effect on this response. It is proposed that the cellular sites for sexual dimorphism in hepatic beta-adrenergic functions are the coupling state of beta-AR to Gs and the amount of Gs alpha as well as the level of beta-AR.

Alterations in the stimulatory G protein of the rat liver after partial hepatectomy

In adult male rat livers, cAMP generation in response to beta-adrenergic agonists was dramatically stimulated after partial hepatectomy. Quantitation of the alpha subunits of the stimulatory G protein (Gs alpha) using ADP-ribosylation catalyzed by cholera toxin revealed the increment in the amounts of two forms of Gs alpha, Gs alpha-S and Gs alpha-L, during liver regeneration. These increases in the amounts of both Gs alpha proteins were associated with the stimulation in their mRNA levels. In addition, partial hepatectomy gave rise to a shift in the proportion of beta-adrenergic receptor (beta-AR) in the high affinity state produced by beta-AR-Gs complex. The susceptibility of Gs alpha to trypsin was used as a probe for beta-AR-Gs coupling. The GTP-bound forms of both Gs alpha-S and Gs alpha-L were more trypsin-sensitive than their GDP-bound forms. Preincubation of liver plasma membranes prepared from partially hepatectomized rats with the agonist isoproterenol resulted in an enhancement of trypsin-sensitivity of Gs alpha-L, but not Gs alpha-S. This effect was retarded by the addition of the antagonist propranolol. We conclude that the increase in the amount of Gs alpha can be contributed to the rise in beta-response after partial hepatectomy, and suggest that beta-AR is preferentially coupled with Gs alpha-L rather than Gs alpha-S.

Characterization of the alpha 1B-adrenergic receptors of chicken hepatocytes. Signal transduction and actions

1. In chicken hepatocytes, alpha 1-adrenoceptor activation increased: (a) phosphatidylinositol labeling; (b) production of inositol trisphosphate; (c) cytosol calcium; and (d) phosphorylase activity. 2. Prazosin (Ki approximately 0.2-0.4 nM) was more potent in inhibiting these actions than 5-methyl-urapidil (Ki approximately 30-60 nM); these actions were sensitive to chlorethylclonidine suggesting the involvement of alpha 1B-adrenoceptors. 3. The stimulation of phosphoinositide turnover was insensitive to pertussis toxin. 4. In chicken liver membranes, [3H]prazosin binding sites (Bmax 872 fmol/mg protein) with high affinity for prazosin (KD 0.3 nM; Ki 0.4 nM) and lower affinity for 5-methyl-urapidil (Ki 46 nM) were detected, consistent with the presence of alpha 1B-adrenoceptors.

Intracellular signalling of epinephrine in rat hepatocytes during fetal development and hepatic regeneration

The changes in intracellular calcium concentration and IP3 production after the addition of epinephrine were analysed in adult, fetal (20th-22nd day of intrauterine life), and regenerating rat hepatocytes (4 h-24 h after partial hepatectomy) to determine whether the signal transduction is the same in quiescent proliferating and differentiating cells. The epinephrine treatment causes a significative cytosolic calcium transient in hepatocytes isolated in the last day of fetal life (22-day old) and in the early stage of regeneration (4 h). This effect is not significant in the previous stage of fetal life (20-day old) and at the onset of M phase of cell cycle after partial hepatectomy (24 h). [3H]myo inositol incorporation into IP3 and IP4 is higher in 20 day fetal and regenerating hepatocytes with respect to the control. In these cells the epinephrine does not affect basal level of IP3 and IP4, while it causes a substantial increase of these inositol phosphates in adult hepatocytes. [3H]myo inositol incorporation into PIP2 is very low at the 20th day of fetal life. Epinephrine has no effect on this parameter in fetal and regenerating hepatocytes. Our results show that the epinephrine signal is mediated differently in proliferating and in quiescent hepatocytes.

Hepatocyte homologous beta 2-adrenergic desensitization is associated with a decrease in number of plasma membrane beta 2-adrenoceptors

Preincubation of rat hepatocytes with isoproterenol induces homologous beta-adrenergic desensitization evidenced both in whole cells (cyclic AMP accumulation) and membranes (adenylyl cyclase activity). This desensitization is associated with and quantitatively similar to a loss of beta 2-adrenoceptors from the plasma membrane. Desensitization did not alter the affinities of isoproterenol for the [125I]iodocyanopindolol binding sites nor reduce the ability of guanine nucleotides to modulate agonist affinity, i.e., the receptors that remain in the surface of plasma membrane after desensitization (approximately 50%) retain their functional integrity. When membranes from isoproterenol-desensitized hepatocytes were treated with alkaline phosphatase, no attenuation of the desensitization was observed. Cholera toxin-catalyzed ADP-ribosylation was not decreased but rather slightly increased in membranes from desensitized cells as compared to the controls. Our data indicate that in hepatocytes, a loss of beta 2-adrenoceptors from the plasma membrane is closely associated to the homologous desensitization induced by isoproterenol.

Modulation of G proteins and second messenger responsiveness by steroid hormones in GH3rat pituitary tumour cells

We have investigated the modulation of different G protein alpha- and beta-subunit levels in prolactin (PRL) and growth hormone producing rat pituitary adenoma cells (GH3 cells) in culture after prolonged exposure (6-48 h) to the steroid hormones 17 beta-oestradiol and dexamethasone. Gi-3 alpha- and G beta-subunits were the only G protein subunits which increased in response to 10(-6) M oestradiol (to approximately 150 and 200% of controls, respectively), while the other alpha-subunits investigated (Gs alpha, Gi-2 alpha and G(o) alpha) remained relatively unchanged. Thyroliberin (TRH)--and guanosine 5'-[beta gamma-imido]trisphosphate (Gpp(NH)p)-elicited adenylyl cyclase (AC) activities were reduced during 6-12 h of oestradiol treatment (by 60 and 20%, respectively), while the inhibitory effect of somatostatin (SRIF) increased by approximately 100%. Dexamethasone (10(-6) M) increased levels of the stimulatory G protein Gs alpha (to approximately 340%) and decreased levels of Gi-3 alpha (to 25%). After 48 h, the AC response to TRH was reduced by approximately 70%, whereas the effect of the other modulators remained close to controls. We conclude that G protein subunits in GH3 cells are subject to specific regulation by steroid hormones and that this may be important in the tuning of the responsiveness of PRL secretion to hormones in the in vivo situation.

Histamine activates phosphorylase and inositol phosphate production in guinea pig hepatocytes

In guinea pig hepatocytes, histamine increased phosphorylase activity and inositol phosphate production. Similar effects were obtained with 2-(2-aminoethyl)-thiazole, a histamine H1 receptor agonist, but not with dimaprit or impromidine, H2 receptor agonists. These effects of histamine were dose-dependently inhibited by the H1 antihistamines, (+)-chlorpheniramine and mepyramine (pyrilamine) but not by cimetidine or ranitidine, H2 antagonists. (+)-Chlorpheniramine and mepyramine had similar potencies (apparent Ki values approximately 3 nM) when incubated with the cells for 1 min (phosphorylase a assays) but the former was 15-20-fold more potent than the latter at longer incubation times (apparent Ki values approximately 3-4 nM and 45-90 nM, respectively) indicating that mepyramine is actively metabolized by guinea pig hepatocytes. Histamine increased cytosol calcium approximately 2-fold, an effect also mediated through H1 receptors. The actions of histamine were not affected by in vivo ADP-ribosylation by pertussis toxin. Our data clearly indicate that histamine modulates the metabolism of guinea pig hepatocytes via activation of H1 receptors. These receptors are coupled to the phosphoinositide turnover-calcium mobilization signalling pathway through a pertussis toxin-insensitive process.

Guinea pig hepatocyte α1A-adrenoceptors: characterization, signal transduction and regulation

Activation of guinea pig hepatocyte alpha 1-adrenoceptors increases phosphatidylinositol (PI) labeling, [3H]inositol phosphate production and phosphorylase activity. These adrenergic actions were not altered by pretreatment with chlorethylclonidine but were blocked by 5-methyl urapidil and prazosin (the former being 3- to 10-fold more potent than the latter), indicating that alpha 1A-adrenoceptors were involved. When the cells were incubated in buffer without calcium and containing EGTA, the alpha 1A-adrenergic stimulation of PI labeling was diminished but not abolished and that of phosphorylase was not affected. The alpha 1A-adrenergic effects were insensitive to pertussis toxin treatment. Phorbol myristate acetate inhibited the alpha 1A-adrenergic actions, although at relatively large concentrations, and also those of other agents such as angiotensin II and NaF. Our data clearly indicate that guinea pig hepatocytes express alpha 1A-adrenoceptors whose activation stimulates phosphoinositide turnover, via a pertussis toxin-insensitive process; the alpha 1A-adrenergic effects were at least partially independent of extracellular calcium.

Modulation by protein kinase C of the hormonal responsiveness of hepatocytes from lean (Fa/fa?) and obese (fa/fa) Zucker rats

The effect of phorbol myristate acetate (PMA) on the hormonal responsiveness of hepatocytes from lean and obese Zucker rats was studied. Phenylephrine-stimulated phosphatydylinositol labeling and phosphorylase activation were antagonized by PMA in cells from obese and lean animals; bigger residual effects were observed in cells from obese animals even at high PMA concentrations. Cyclic AMP accumulation induced by isoproterenol, glucagon, forskolin and cholera toxin was higher in cells from lean animals than in those from obese rats. PMA diminished glucagon- and cholera toxin-induced cyclic AMP accumulation; cells from lean animals were more sensitive to PMA. Two groups of isoforms of protein kinase C (PKC) were observed in hepatocytes from Zucker rats using DEAE-cellulose column chromatography: PKC 1 and PKC 2. The PKC 1 isozymes were separated into four peaks using hydroxylapatite: aa, 1a (PKC-beta), 1b (PKC-alpha) and 1c. Short treatment with PMA decreased the activity of PKC 1 (peaks 1b (PKC-alpha) and 1c) and to a lesser extent of PKC 2; cells from lean animals were more sensitive to PMA than those obtained from obese rats. Our results indicate that cells from genetically obese Zucker rats are in general less sensitive to this activator of protein kinase C than those from their lean littermates. The possibility that alterations in the phosphorylation/dephosphorylation cycles, that control metabolism and hormonal responsiveness, may contribute to this obese state is suggested.

Differential expression of guanine nucleotide-binding proteins enhances cAMP synthesis in regenerating rat liver

Events leading to cAMP accumulation after partial hepatectomy (PH) and effects of cAMP on hormonal induction of DNA synthesis in hepatocytes were characterized. Hepatic cAMP peaked biphasically post-PH and paralleled changes in adenylyl cyclase activity. Fluctuations in cyclase activity were not explained by variations in glucagon receptor kinetics, but reflected altered G-protein expression. Membrane levels of the stimulatory G-protein, Gs alpha, increased early after PH and were sustained. Levels of the inhibitory G-protein, Gi2 alpha, increased more slowly, peaked later, and quickly fell. Levels of both G-proteins correlated poorly with levels of their mRNAs, suggesting posttranscriptional factors modify their membrane concentrations. When growth factor-induced DNA synthesis was compared in hepatocyte cultures grown with or without agents that increase intracellular cAMP, DNA synthesis was inhibited by sustained high levels of cAMP but was enhanced when high cAMP levels fell. In both regenerating liver and hepatocyte cultures, the expression of a "differentiated" hepatocyte gene, phosphoenolpyruvate carboxykinase, correlated with elevated cAMP levels. These data suggest that the differential expression of G-proteins integrates signals initiated by several growth factors so that the accumulation of cAMP is tightly regulated post-PH. The ensuing variations in cAMP levels modulate both growth and differentiated functions during liver regeneration.

Levels of G-proteins in liver and brain of lean and obese (ob/ob) mice

G-protein levels were assessed in liver and brain membranes of lean and obese mice. ADP-ribosylation and immunodetection studies revealed a decrease in the abundance of Gs and Gi alpha-subunits in the liver membranes of obese mice compared with lean mice. In contrast, in brain membranes, the abundance of these proteins was not significantly different between lean and obese mice. Studies at the mRNA level in both liver and brain revealed no difference in gene expression between lean and obese mice. Protein and mRNA studies both showed that Gs, Gi alpha 1, Gi alpha 2, Go alpha and G beta subunits are present in brain membranes, and Gi alpha 3 is barely detectable. In liver, Ga alpha, Gi alpha 2 and G beta subunits are the major constituents, whereas Gi alpha 1, Gi alpha 3 and Go alpha are barely detectable. It is possible that the differences observed at the protein level are due to different rates of translation of the mRNA. Different rates of release of the alpha-subunits from the membrane and/or different rates of degradation would also explain these results.

Characterization and detoxification of an easily prepared acellular pertussis vaccine. Antigenic role of the A protomer of pertussis toxin

An acellular pertussis vaccine was prepared from pertussis vaccine concentrates by eliminating the cells by centrifugation and subsequent acid precipitation. SDS-PAGE analysis indicated the presence of 17 clearly defined bands including the subunits of pertussis toxin. The acellular preparation was detoxified with glutaraldehyde and lost its undesirable effects (leukocytosis-promoting activity, histamine-sensitizing activity and decrease in weight gain) but retained a very good protective activity (PD50 approximately 2 micrograms protein/mouse). Sera from mice immunized with this preparation indicated an antibody response against the A protomer of pertussis toxin.

Regulation of G protein mRNA levels by thyroliberin, vasoactive intestinal peptide and somatostatin in prolactin-producing rat pituitary adenoma cells

We have investigated the regulation of mRNA levels of alpha- and beta-subunits of guanine nucleotide-binding regulatory proteins (G proteins) by peptide hormones in prolactin producing rat pituitary adenoma cells (GH3 cells) in culture. The cells were treated with thyroliberin (1 microM), vasoactive intestinal peptide (1 microM) or somatostatin (10 microM) for 6 to 48 hours. Thyroliberin and vasoactive intestinal peptide increased the levels of Gs alpha Go alpha, Gi-2 alpha, Gi-3 alpha, Gx alpha, G beta 36 and mRNAs. The effect of vasoactive intestinal peptide was however earlier and more pronounced. Gi-2 alpha mRNA levels showed the quantitatively largest alterations. Somatostatin upregulated Gs alpha and downregulated Go alpha and Gi-2 mRNAs. G protein mRNAs for Gi-2 alpha and Go alpha were increased by exposure of the cells to a medium devoid of serum. We conclude that G protein mRNA levels are subjected to alterations by hormones that act through the corresponding G proteins in the regulation of prolactin synthesis and secretion.

Regulation of beta 2-adrenergic receptors in keratinocytes: glucocorticoids increase steady-state levels of receptor mRNA in foetal rat keratinizing epidermal cells (FRSK cells)

Glucocorticoids increase the beta-adrenergic adenylate cyclase response of epidermal keratinocytes. Using FRSK cells, a cultured cell line of foetal rat keratinocytes, the regulatory mechanism of the beta-adrenergic augmentation effect was investigated. Treatment with dexamethasone (1 x 10(-6) M increase by 1.5-fold the beta-adrenergic adenylate cyclase response of FRSK cells. The effect was observed at 6 h incubation and remained for at least 48 h. The prostaglandin E-adenylate cyclase response was also increased 1.5-fold by glucocorticoid treatment. Neither the adenosine-adenylate cyclase response nor cholera toxin- or forskolin-induced cyclic AMP accumulations were altered. Northern blot hybridization showed that levels of the beta 2-adrenergic receptor mRNA increased within 3 h, while actin-, Gs-alpha, Gi-2 alpha, Gi-3 alpha mRNA levels were unchanged. Testosterone, 17 beta-oestradiol, and progesterone had no effect on either the beta 2-adrenergic adenylate cyclase response or the expression of beta 2-adrenergic receptor mRNA. The increase in the numbers of the beta-adrenergic receptors was visualized by immunofluorescence with an antibody specific for the beta 2-adrenergic receptor. Our results indicate that glucocorticoids regulate the beta 2-adrenergic adenylate cyclase response of FRSK cells through the enhanced expression of the receptor.

Interactive effects of ethanol and caffeine on rat fetal hepatocyte replication and EGF receptor expression

This study reports on the interactive effects of ethanol and caffeine on growth of rat fetal hepatocytes. Exposure of cultured rat fetal hepatocytes (RFH) to ethanol in concentrations above 1 mg/ml, causes a blockade of EGF-dependent cell replication along with an overexpression of surface EGF receptors (EGF-R). However, RFHs exposed for 24 hours to ethanol at a concentration of 1 mg/ml alone had little effect on cell replication. Caffeine, when combined with this concentration of alcohol, progressively impaired RFH growth by up to 100%. Caffeine alone up to 10 micrograms/ml, on the other hand, caused a progressive increase in RFH replication associated with a 69% enhancement of DNA synthesis. Caffeine concentrations in excess of 50 micrograms/ml had no effect on replicative capacity. Concomitant caffeine exposure had no effect on the ethanol-related increase in cell DNA content, yet it caused a further enhancement of the cell protein accural induced by ethanol alone. Caffeine (10 micrograms/ml) alone had no effect on EGF-R expression, while ethanol (2 mg/ml) increased it by almost 200%. Addition of caffeine to ethanol reduced this enhanced EGF binding by 45%. Scatchard analysis indicated that no treatment altered ligand affinity for the receptor, but that the alterations in binding caused by ethanol and the caffeine/ethanol combination reflected changes in binding capacity, in both low and high affinity components. It is concluded that (1) ethanol blocks EGF-mediated replication accompanied by a reduction in DNA synthesis, (2) caffeine alone at low concentrations has the opposite effect and can actually potentiate the EGF-mediated mitogenic response, (3) caffeine in combination with ethanol acts synergistically to reduce RFH replication.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of protein kinase C inhibits hormonal stimulation of the GTPase activity of Gi in human platelets

The effect of 12-tetradecanoyl phorbol 13-acetate on the GTPase activity of Gi was investigated. Treatment with TPA did not alter basal GTPase activity of membranes or the stimulatory effect of prostaglandin E1 (putatively via Gs). In contrast, the phorbol ester markedly diminished stimulation of GTPase by agents whose receptors are coupled to Gi such as epinephrine (alpha-adrenergic action), platelet activating factor or thrombin. Pertussis toxin catalyzed ADP-ribosylation was also decreased in membranes from TPA-treated platelets as compared to the controls. It is suggested that the alteration in the hormonal activation of the GTPase activity of Gi is secondary to a perturbation in the receptor-Gi interaction.

Quantification of the alpha and beta subunits of the transducing elements (Gs and Gi) of adenylate cyclase in adipocyte membranes from lean and obese (ob/ob) mice

The abundance of the alpha and beta subunits of the GTP-binding proteins (G-proteins) that transduce hormonal messages to adenylate cyclase was assessed in adipocyte membranes from lean (+/+) and obese (ob/ob) mice, using ADP-ribosylation with bacterial toxin and immunodetection. Both methods revealed two Gs alpha species (48 and 42 kDa) in the membranes. Compared with those of lean mice, the membranes from obese mice contained substantially less of the 48 kDa species of Gs alpha, as assessed by both methods. ADP-ribosylation by pertussis toxin showed that only half as much ADP-ribose was incorporated into Gi alpha in the membranes from obese as compared with lean mice. Immunodetection revealed two separate Gi alpha peptides (39 and 40 kDa) and showed that the 40 kDa species was less abundant in the membranes from obese mice, whereas the amount of the 39 kDa species was similar in membranes from both lean and obese animals. Based on ADP-ribosylation assays, in membranes from lean mice the ratio Gs alpha/Gi alpha was 1:16, whereas in the membranes from obese mice it was 1:10. Similar amounts of immunodetectable beta peptide were found in both types of membranes. On the basis of the currently accepted dissociation model of adenylate cyclase activation, the decrease in the abundance of the Gi alpha subunit in adipocyte membranes from obese mice could account for the abnormal kinetics of the enzyme in these membranes.

Epinephrine regulation of amino acid transport in rat hepatocytes isolated during development

The effect of epinephrine on the amino acid transport mediated by system A was investigated by determining the uptake of 2-amino [1-14C]isobutyric acid (AIB) in rat hepatocytes, freshly isolated at different stages of pre- and postnatal development. The data obtained show that the hormone increased AIB uptake, enhancing the Vmax, while Km was unchanged. This effect was evident in cells from adult, 18- to 20-day-old fetus, and neonate rat. Actinomycin D or cycloheximide abolished the hormone dependent increase. Experiments carried out with alpha- and beta-antagonists showed that the effect of epinephrine was beta-mediated in fetal life and alpha-mediated in adult life. Membrane binding experiments showed a higher value for epinephrine and beta-agonist dihydroalprenolol in the fetus versus the adult. The calcium depletion obtained after cell incubation with EGTA or calcium ionophore A23187 reduced the hormonal stimulation in the adult, and was ineffective in the prenatal period. An involvement of cAMP was present in the epinephrine modulation of AIB transport, both in adult and in fetal life.

Glucocorticoids modulate mRNA levels for G-protein beta-subunits

Adrenalectomy decreases, whereas glucocorticoid treatment increases, the steady-state levels of G-protein beta-subunits (G beta) in rat fat-cells. A DNA-excess solution-hybridization assay was established to define the steady-state mRNA levels for G beta [5.8 +/- 0.4 amol/micrograms of RNA (n = 5) in control fat-cells]. G beta mRNA levels decrease by 20% after adrenalectomy; dexamethasone treatment reverses the decline. Dexamethasone treatment itself increases G beta mRNA levels by 50%.