Alkaline phosphatase relieves desensitization of adenylate cyclase-coupled beta-adrenergic receptors in avian erythrocyte membranes

Receptor phosphorylation mediates estradiol reduction of alpha2-adrenoceptor coupling to G protein in the hypothalamus of female rats

Estrogen increases evoked norepinephrine release in the hypothalamus of female rodents, in part by reducing the ability of alpha2-adrenoceptors to act as negative feed-back inhibitors of norepinephrine release. Estrogen enhancement of norepinephrine release in the hypothalamus correlates with decreased coupling of the alpha2-adrenoceptor to G protein. To determine the mechanism by which estrogen uncouples alpha2-adrenoceptors from G protein, we tested the hypothesis that estrogen increases alpha2-adrenoceptor phosphorylation. Short-term activation of endogenous serine/threonine phosphatases with protamine or treatment with exogenous phosphatase restored alpha2-adrenoceptor coupling to G protein to control levels in hypothalami from estrogen-exposed female rats. Additional experiments examined whether estrogen alters G protein-coupled receptor kinase expression or activity or serine/threonine phosphatase activity. These proteins are involved in G protein-coupled receptor phosphorylation, internalization, and recycling. Estrogen exposure reduced G protein-coupled receptor kinase mRNA, protein, and activity in the hypothalamus. Furthermore, estrogen treatment reduced serine/threonine phosphatase activity in the hypothalamus. Analysis of ligand binding in subcellular fractions demonstrated that estrogen decreases the fraction of internalized alpha2-adrenoceptors in the hypothalamus.Therefore, estrogen promotes norepinephrine release in the hypothalamus by stabilizing alpha2-adrenoceptor phosphorylation, uncoupling the receptor from G protein. Estrogen may stabilize alpha2-adrenoceptor phosphorylation by inhibiting receptor internalization and dephosphorylation.

Evidence that oestradiol attenuates beta-adrenoceptor function in the hypothalamus of female rats by altering receptor phosphorylation and sequestration

Activation of beta-adrenoceptors in the hypothalamus (HYP) and preoptic area (POA) inhibits both gonadotropin release and reproductive behaviour in female rats. Exposure of female rats for 48 h to physiologically relevant doses of oestrogen attenuates beta-adrenoceptor function in the HYP and POA as indicated by reduced isoproterenol (beta-adrenoceptor agonist) stimulation of adenylyl cyclase activity. Reduced beta-adrenoceptor coupling to G protein in the HYP-POA from oestrogen-exposed female rats correlates with attenuation of beta-adrenoceptor function. To examine potential mechanisms underlying receptor-G protein uncoupling, initial experiments tested the hypothesis that oestrogen attenuation of beta-adrenoceptor function in the HYP and POA involves receptor phosphorylation. Activation of endogenous serine/threonine phosphatases with protamine restores agonist-stimulated cAMP accumulation in HYP slices from oestrogen-exposed female rats to control levels. Additional experiments examined whether oestrogen-induced changes in beta-adrenoceptor binding density and/or subcellular localization correlate with the attenuation of beta-adrenoceptor function in the HYP and POA. Oestrogen treatment does not alter total beta-adrenoceptor binding density in the HYP or POA. However, oestrogen significantly reduces cell surface binding of the hydrophilic beta-adrenoceptor antagonist [3H] CGP 12177 to intact HYP and POA slices. At the same time, oestrogen decreases the fraction of beta-adrenoceptors localized in a light vesicle fraction following sucrose density gradient centrifugation. Therefore, oestrogen attenuates beta-adrenoceptor signalling in the HYP-POA by uncoupling the beta-adrenoceptor from G protein, perhaps by promoting receptor phosphorylation. Furthermore, a significant fraction of beta-adrenoceptors in the HYP and POA are no longer accessible to hydrophilic ligands, but are not internalized. Thus, physiological doses of oestrogen may facilitate reproductive behaviour and gonadotropin release, in part, by stabilizing beta-adrenoceptor phosphorylation in the HYP and POA, thereby uncoupling the receptors from G protein.

Age-related reductions in the activities of antioxidant enzymes in the rat inferior colliculus

The inferior colliculus (IC) is a major relay and processing center of auditory signals in the midbrain and receives inputs from most other auditory nuclei. A number of studies have indicated age-related declines in the GABAergic and excitatory amino acid systems in the IC, including losses in both GABA immunoreactive (+) and GABA immunonegative (-) synapses. The goal of this project was to identify potential biochemical and morphological changes in the IC that may contribute to deficits in the functions of these neurotransmitters, using three age groups of Fischer-344 rats. Homogenates obtained from the IC showed age-dependent reductions in activities of the antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT), with a concomitant increase in lipid peroxidation. Dephosphorylation of IC homogenates with alkaline phosphatase reduced the activities of SOD and CAT in all age groups, which could be restored by protein kinase C (PKC)-dependent phosphorylation. Restoration of enzyme activity was specific to the PKC-alpha isozyme, but not to the beta1, beta2, delta or gamma forms. No age-dependent change in the levels of PKC isoforms (alpha, beta1, beta2 and gamma) was detectable in IC homogenates. Morphological analyses indicate decreases in mitochondrial density in the somata of both GABA+ and GABA- IC neurons in 19- and 28-month-old rats when compared to 3-month-olds, along with significantly higher matricial abnormalities. These data indicate age-related increases in oxidative stress in the IC, which could be partially restored by PKC. The progressive increase in oxidative stress with age may underlie changes in neuronal morphology and function of the IC.

Studies on the mechanism of desensitization of the parathyroid hormone-stimulated calcium signal in UMR-106 cells: reversal of desensitization by alkaline phosphatase but not by protein kinase C downregulation

The involvement of protein kinase C (PKC), cAMP-dependent protein kinase (PKA), and other phosphorylation mechanisms in the rapid desensitization of the [Ca2+]i response to parathyroid hormone (PTH) stimulation was investigated in osteoblast-like UMR-106 cells. A 5 minute preincubation of the cell suspension with phorbol 12,13-dibutyrate (PDB) decreased the response to PTH in a concentration-dependent manner. 1-Oleoyl-2-acetyl-r-glycerol (OAG) pretreatment likewise decreased the PTH response. Staurosporine, a potent protein kinase inhibitor, completely prevented the desensitization caused by PDB. These PDB and staurosporine effects were also observed in 3 mM EGTA-containing medium ([Ca2+]free < 10(-8) M). A 5 minute pretreatment of cells with 1 microM forskolin had no effect on the calcium response to PTH. Homologous and PDB-induced desensitizations differed in several respects. Staurosporine pretreatment resulted in only a slight restoration of the PTH response under conditions of homologous desensitization. Chronic treatment with phorbol ester prevented the desensitization of the PTH response by acute phorbol treatment but not the homologous desensitization. Both homologous and PDB-induced desensitization were relieved by alkaline phosphatase treatment, consistent with the involvement of phosphorylation in the desensitization. This alkaline phosphatase effect on desensitization was inhibited by L-phenylalanine. These results suggest that PTH receptor homologous desensitization involves phosphorylation process(es) other than or in addition to those of PKC.

Identification of two distinct isoforms of the guanine nucleotide binding protein G0 in neuroblastoma X glioma hybrid cells: independent regulation during cyclic AMP-induced differentiation

Three distinct antipeptide antisera generated against synthetic peptides that represent parts of the primary sequence of the alpha-subunit of the (pertussis toxin-sensitive) guanine nucleotide binding protein G0 were used in two-dimensional immunoblots of membranes of neuroblastoma X glioma (NG108-15) cells. Each antiserum identified two distinct polypeptides of some 39 kDa. These had apparent isoelectric points of 5.5 and 5.8. Differentiation of NG108-15 cells in response separately to dibutyryl cyclic AMP (cAMP), 8-bromo cAMP, forskolin, and prostaglandin E1 produced elevated levels of G0 alpha, as has previously been noted in one-dimensional immunoblots. Two-dimensional analysis demonstrated that the cAMP-induced increases in levels of G0 alpha were only of the more acidic isoform. The two isoforms were both substrates for pertussis toxin-catalysed ADP-ribosylation and did not appear to represent differentially phosphorylated forms of the same polypeptide. Separation of the two forms of G0 alpha could be achieved in one-dimensional sodium dodecyl sulphate-polyacrylamide gel electrophoresis when 4 M deionized urea was included in the resolving gel. The more slowly migrating band was the acidic form and corresponded exactly in mobility with the major form of G0 from both rat and mouse brain. There was no equivalent in brain of the more rapidly migrating form of G0 from the cells. In agreement with the data from two-dimensional gels, only the more slowly migrating form was expressed in considerably higher amounts following cAMP-induced differentiation of NG108-15 cells. Of these two forms of "G0," the acidic species is equivalent to G0 from brain, but the basic form is not identical with G0*, which has been purified from bovine brain.

Inhibition of adenylate cyclase activity by polyamines in human erythrocyte plasma membranes

Polyamines (spermidine, spermine and putrescine) inhibited the adenylate cyclase activity in a concentration dependent manner in human erythrocyte plasma membranes. Spermidine (Spd) exhibited more inhibitory effect than spermine (Spm) and putrescine (Put). On the contrary, the addition of amino acids (arginine, glutamine and lysine) did not influence the basal enzyme activity. Other cations (polylysine, polyarginine and polyglutamine) also did not affect the enzyme activity. Addition of all the three polyamines (Spd, Spm and Put) in the reaction mixture exhibited moderate inhibitory effect on the adenylate cyclase activity whether it was basal or activated with sodium fluoride or with forskolin. Since the three polyamines exhibited maximum inhibitory effect at 10 microM concentration which is within physiological limit for mammalian tissues, we suggest that there may be a regulatory function of these molecules on adenylate cyclase activity in human erythrocytes.