Overexpression of G(s)alpha in NG108-15, neuroblastomaXglioma cells: effects on receptor regulation of the stimulatory adenylyl cyclase cascade

Role of G proteins and ERK activation in hemin-induced erythroid differentiation of K562 cells

Heterotrimeric G proteins which couple extracellular signals to intracellular effectors play a central role in cell growth and differentiation. The pluripotent erythroleukemic cell line K562 that acquires the capability to synthesize hemoglobin in response to a variety of agents can be used as a model system for erythroid differentiation. Using Western blot analysis and RT-PCR, we studied alterations in G protein expression accompanying hemin-induced differentiation of K562 cells. We demonstrated the presence of G(alpha s), G(alpha i2) and G(alpha q) and the absence of G(alpha i1), G(alpha o) and G(alpha 16) in K562 cells. We observed the short form of G(alpha s) to be expressed predominantly in these cells. Treatment of K562 cells with hemin resulted in an increase in the levels of G(alpha s) and G(alpha q). On the other hand, the level of G(alpha i2) was found to increase on the third day after induction with hemin, followed by a decrease to levels lower of those of uninduced cells. The mitogen-activated protein kinase ERK1/2 pathway is crucial in the control of cell proliferation and differentiation. Both Gi- and Gq-coupled receptors stimulate MAPK activation. We therefore examined the phosphorylation of ERK1/2 during hemin-induced differentiation of K562 cells. Using anti-ERK1/2 antibodies, we observed that ERK2 was primarily phosphorylated in K562 cells. ERK2 phosphorylation increased gradually until 48 h and returned to basal values by 96 h following hemin treatment. Our results suggest that changes in G protein expression and ERK2 activity are involved in hemin-induced differentiation of K562 cells.

Biochemical and pharmacological control of the multiplicity of coupling at G-protein-coupled receptors

For decades, it has been generally proposed that a given receptor always interacts with a particular GTP-binding protein (G-protein) or with multiple G-proteins within one family. However, for several G-protein-coupled receptors (GPCR), it now becomes generally accepted that simultaneous functional coupling with distinct unrelated G-proteins can be observed, leading to the activation of multiple intracellular effectors with distinct efficacies and/or potencies. Multiplicity in G-protein coupling is frequently observed in artificial expression systems where high densities of receptors are obtained, raising the question of whether such complex signalling reveals artefactual promiscuous coupling or is a genuine property of GPCRs. Multiple biochemical and pharmacological evidence in favour of an intrinsic property of GPCRs were obtained in recent studies. Thus, there are now many examples showing that the coupling to multiple signalling pathways is dependent on the agonist used (agonist trafficking of receptor signals). In addition, the different couplings were demonstrated to involve distinct molecular determinants of the receptor and to show distinct desensitisation kinetics. Such multiplicity of signalling at the level of G-protein coupling leads to a further complexity in the functional response to agonist stimulation of one of the most elaborate cellular transmission systems. Indeed, the physiological relevance of such versatility in signalling associated with a single receptor requires the existence of critical mechanisms of dynamic regulation of the expression, the compartmentalisation, and the activity of the signalling partners. This review aims at summarising the different studies that support the concept of multiplicity of G-protein coupling. The physiological and pharmacological relevance of this coupling promiscuity will be discussed.

Different roles for Gi and Go proteins in modulation of adenylyl cyclase type-2 activity

The effect of Gi/o protein-coupled receptors on adenylyl cyclase type 2 (AC2) has been studied in Sf9 insect cells. Stimulation of cells expressing AC2 with the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) led to a twofold stimulation of cAMP synthesis that could be blocked with the protein kinase C inhibitor GF109203X. Activation of a coexpressed alpha2A-adrenoceptor or muscarinic M4 receptor inhibited the stimulation by TPA almost completely in a pertussis toxin-sensitive manner. Activation of Gs proteins switched the response of the alpha2A-adrenoceptor to potentiation of prestimulated AC2 activity. The potentiation, but not the inhibition, could be blocked by a Gbetagamma scavenger. A novel methodological approach, whereby signalling through endogenous G proteins was ablated, was used to assess specific G protein species in the signal pathway. Expression of Go proteins (alphao1 + beta1gamma2) restored both the inhibition and the potentiation, whereas expression of Gi proteins (alphai1 + beta1gamma2) resulted in a potentiation of both the TPA- and the Gs-stimulated AC2 activity. The data presented supports the view of AC2 as a molecular switch and implicates this isoform as a target for Go protein-linked signalling.

Targeted expression of activated Q227L G(alpha)(s) in vivo

We report the creation of transgenic mice with an inducible, tissue-targeted expression of a constitutively active mutant form (Q227L) of G(alpha)(s). Mice expressing activated G(alpha)(s) in fat tissue, liver, and skeletal muscle displayed normal body mass and blunted glucose metabolism. cAMP accumulation in adipose tissue was increased in the basal state, but far less than would be expected. Marked adaptation to elevated cAMP levels occurred, leading to an increase in total cAMP-specific phosphodiesterase activity, a 50% decline in cAMP-dependent protein kinase (protein kinase A) activity, and an increased expression of G(alpha)(i2). The reduction in kinase activity coincided with >50% increase in the expression of RIalpha and RIIalpha regulatory subunits of protein kinase A, with no change in the amount of catalytic subunit. These data demonstrate the existence of adaptive responses of protein kinase A, phosphodiesterase, and G(alpha)(i2) in tissues expressing constitutively active G(alpha)(s) that may act to rectify the impact of increased cAMP accumulation.

The human 5-HT7 serotonin receptor splice variants: constitutive activity and inverse agonist effects

1. Using membranes from stably or transiently transfected HEK293 cells cultured in 5-HT-free medium and expressing the recombinant human 5-HT(7) receptor splice variants (h5-HT(7(a)), h5-HT(7(b)) and h5-HT(7(d))), we compared their abilities to constitutively activate adenylyl cyclase (AC). 2. All h5-HT(7) splice variants elevated basal and forskolin-stimulated AC. The basal AC activity was reduced by the 5-HT(7) antagonist methiothepin and this effect was blocked by mesulergine (neutral 5-HT(7) antagonist) indicating that the inhibitory effect of methiothepin is inverse agonism at the 5-HT(7) receptor. 3. Receptor density correlated poorly with constitutive AC activity in stable clonal cell lines and transiently transfected cells. Mean constitutive AC activity as a percentage of forskolin-stimulated AC was significantly higher for the h5-HT(7(b)) splice variant compared to the h5-HT(7(a)) and h5-HT(7(d)) splice variants but only in stable cell lines. 4. All eight 5-HT antagonists tested inhibited constitutive AC activity of all splice variants in a concentration-dependent manner. No differences in inverse agonist potencies (pIC(50)) were observed between the splice variants. The rank order of potencies was in agreement and highly correlated with antagonist potencies (pK(b)) determined by antagonism of 5-HT-stimulated AC activity (methiothepin >metergoline> mesulergine > or = clozapine > or = spiperone > or = ritanserin > methysergide > ketanserin). 5. The efficacy of inverse agonism was not receptor level dependent and varied for several 5-HT antagonists between membrane preparations of transiently and stably transfected cells. 6. It is concluded that the h5-HT(7) splice variants display similar constitutive activity and inverse agonist properties.

Treatment with inverse agonists enhances baseline atrial contractility in transgenic mice with chronic beta2-adrenoceptor activation

In this study, we investigate whether chronic treatment with beta-adrenoceptor (betaAR) ligands with inverse agonist activity enhances myocardial beta2AR-mediated atrial tension more than neutral antagonists in transgenic mice (TG35). These mice exhibit chronic adrenoceptor activation because they possess a greater number of constitutively active receptors than wild type mice due to cardiac-specific overexpression of human betaARs. TG35 and wild type mice were chronically treated for 90 h with three inverse agonists, ICI-118,551, propranolol, and carvedilol, and one neutral antagonist, alprenolol. After 96 h, we compared the basal and isoprenaline-stimulated (10 microM) increase in atrial tension in treated or untreated TG35 mice and wild type mice. In parallel, to determine the effect of chronic betaAR ligand treatment on the amounts of G protein receptor kinase-2 (GRK-2) and G proteins, we performed Western blotting on myocardial cytosolic and membrane proteins. Atria from the TG35 mice treated with inverse agonists showed increases in the baseline tension compared to those from alprenolol/vehicle-treated mice. ICI-118,551 and propranolol treatment restored the elevated myocardial G-inhibitory protein (Gialpha) levels to that of wild type. Also, treatment with inverse agonists upregulated G-stimulatory protein (Gsalpha) levels and GRK2 above those levels in vehicle-treated TG35 or wild type mice. The increased baseline atrial tension was reversed by the addition of ICI-118,551. Overall, our data suggests that inverse agonists enhance baseline atrial tension more than neutral antagonists. Based on this, we propose that upregulation of the active conformation of the beta2ARs, Gsalpha protein and restoration of Gialpha as three possible mechanisms to explain this enhanced receptor activity. Therefore, the favourable effects of some ligands used in pathological conditions involving chronic adrenoceptor activation may be due to the inverse agonist activity of the ligand.

Modulation of forskolin-mediated adenylyl cyclase activation by constitutively active G(S)-coupled receptors

In transfected CHO cells constitutively active histamine H2 receptors not only increase the basal cAMP level, but also enhance forskolin-induced cAMP production. The increased forskolin response was inhibited by inverse H2 agonists with potencies similar to those determined at basal levels. The modulation of the forskolin response was also observed after H2 receptor expression in HEK-293 and Sf9 cells or TSH receptor expression in COS-7 cells. The enhancement of forskolin-induced cAMP production seems to be a general characteristic of constitutively active G(S)-coupled receptors and can be very useful to study inverse agonism at wild-type receptors.