Inhibition of the proliferation of Nb2 cells by femtomolar concentrations of cholera toxin and partial reversal of the effect by 12-O-tetradecanoyl-phorbol-13-acetate

Signalling in response to sub-picomolar concentrations of active compounds: Pushing the boundaries of GPCR sensitivity

There is evidence for ultra-sensitive responses to active compounds at concentrations below picomolar levels by proteins and receptors found in species ranging from bacteria to mammals. We have recently shown that such ultra-sensitivity is also demonstrated by a wide range of prototypical GPCRs, and we have determined the molecular mechanisms behind these responses for three family A GPCRs: the relaxin receptor, RXFP1; the β₂ -adrenoceptor; and the M₃ muscarinic ACh receptor. Interestingly, there are reports of similar ultra-sensitivity by more than 15 human GPCR families, in addition to other human receptors and channels. These occur through a diverse range of signalling pathways and produce modulation of important physiological processes, including neuronal transmission, chemotaxis, gene transcription, protein/ion uptake and secretion, muscle contraction and relaxation, and phagocytosis. Here, we summarise the accumulating evidence of ultra-sensitive receptor signalling to show that this is a common, though currently underappreciated, property of GPCRs. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Adenosine acts as an inhibitor of lymphoma cell growth: a major role for the A3 adenosine receptor

In this study, we demonstrated several mechanisms exploring the inhibitory effect of low-dose adenosine on lymphoma cell growth. Adenosine, a purine nucleoside present in plasma and other extracellular fluids, acts as a regulatory molecule, by binding to G-protein associated cell-surface receptors, A1, A2 and A3. Recently we showed that low-dose adenosine released by muscle cells, inhibits tumour cell growth and thus attributes to the rarity of muscle metastases. In the present work, a cytostatic effect of adenosine on the proliferation of the Nb2-11C rat lymphoma cell line was demonstrated. This effect was mediated through the induction of cell cycle arrest in the G0/G1 phase and by decreasing the telomeric signal in these cells. Adenosine was found to exert its antiproliferative effect mainly through binding to its A3 receptor. The cytostatic anticancer activity, mediated through the A3 adenosine receptor, turns it into a potential target for the development of anticancer therapies.

Control of cell proliferation by embryonal-origin factors

Embryogenesis can be paralleled and contrasted with cancerous cell proliferation; both embryogenesis and cancer are associated with extremely rapid cell proliferation. However, unlike cancer, embryogenesis is characterized by a delicate balance of proliferative and anti-proliferative processes. We have found two chromatographically separated fractions derived from human embryonal neural tissue extracts that significantly suppress the proliferation of human breast cancer cells. The reduction in cell number was time dependent, with maximal inhibition (70%) observed after 4 days of incubation while maintaining cell viability. The anti-proliferative effect was also evidenced by decreased [3H]-thymidine incorporation. Significant inhibition of proliferation of osteosarcoma, fibrosarcoma, and Balb/c 3T3 cell lines was also obtained with a low concentration of the active fractions. Embryonal factors inhibited mouse and rat cell lines, indicating cross-species effectiveness. The SDS-PAGE of the biologically active approximately 10.7 kDa region revealed several protein bands, while the biologically active approximately 4.5 kDa fraction contained only weakly stainable bands. Thus, the embryo contains factors that control the proliferation of malignant cells. These potent and possibly novel compounds should be investigated for their potential therapeutic role in cancer and other proliferative disorders.

Lactogenic hormones rapidly activate p21( ras )/mitogen-activated protein kinase in Nb2-11C rat lymphoma cells

Lactogenic hormone-dependent Nb2-11C cells proliferate in response to prolactin (PRL) or human growth hormone (hGH). We have investigated the activation of p21( ras ) and mitogen-activated protein kinase (MAP-kinase) by hGH in lactogen-dependent Nb2-11C and in autonomous hormone-independent Nb2-SP rat lymphoma cells. Exposure of Nb2-11C cells to hGH resulted in a dose-dependent activation of p21( ras ) and of MAP-kinase. Activation occurs at physiological hGH concentration and with a rapid onset (∼1 min) reaching maximal level at 10-20 min. In contrast, in Nb2-SP autonomous lactogen-independent cells, p21( ras ) and MAP-kinase are constitutively activated and a challenge with lactogenic hormone had a modest additional activating effect. TPA, an activator of protein kinase C, enhanced p21( ras ) and MAP-kinase activity in Nb2-11C cells but failed to induce proliferation. The mechanism of activation of p21( ras ) in Nb2-11C cells by lactogenic hormones involves both an increased binding of guanine nucleotides to p21( ras ) as well as an increase in GTP/GDP+GTP ratio. In summary, we have demonstrated here that activation of the p21( ras )/MAP-kinase pathway follows PRL receptor activation but is not sufficient for the lactogenic hormone-dependent mitogenesis.

Regulation of heat-shock protein (hsp70) gene expression by hGH and IL2 in rat Nb2 lymphoma cells

A comparative study of hGH and IL2 post-signaling effects, as examined by RNA expression (Nb29) and protein levels of the heat-shock protein hsp70, was performed in a hormone-dependent rat lymphoma cell line, Nb2-11C. Optimal doses of hGH or IL2 increased Nb29 expression in a dose-dependent manner. Addition of both mitogens to cell cultures affected Nb29 expression and mitogenesis synergystically, indicating a possible interaction between the post-receptoral mechanisms of these mitogens. Pretreatment of the cells with cholera toxin (CT) inhibited Nb29 expression, protein levels and mitogenesis of hGH- or IL2-induced cells up to 50%, indicating the involvement of Gs-proteins in the post-signaling processes of both hGH and IL2. Incubation of cell cultures with low concentrations of pertussis toxin (IAP) (0.01 ng/ml) markedly increased Nb29 expression in hGH but not in IL2-induced cells, suggesting specific involvement of the Gi-protein in post-signaling processes of hGH-induced cells. Addition of the PKC activator 12-O-tetra-decanoyl phorbol ester (TPA) to control cell cultures markedly increased the expression of Nb29 RNA levels but not mitogenesis, indicating that induction of these proteins in the cells is not sufficient for cell proliferation. Furthermore, incubation of hGH- or IL2-induced cells with the potent PKC inhibitor staurosporin (ST) decreased the levels of Nb29 in both hGH- and IL2-induced cells, although the effect of the mitogens differed significantly in their inhibition slopes. These results indicate that activation of PKC is one of the signaling pathways differentially involved in hGH and IL2 stimulation of cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced responsiveness of adenylate cyclase to forskolin in human lymphoma cells

The beta 2-adrenergic transmembrane signal transduction was investigated in malignant B-cells from 15 patients with low grade non-Hodgkin's lymphoma as compared with normal lymphocytes of seven healthy adults. The number of beta 2-adrenoceptors and the response of adenylate cyclase (AC) to isoproterenol were slightly decreased in lymphoma cells. The responsiveness of AC to forskolin was 8-fold lower in lymphoma cells, whereas the response to cholera toxin showed no difference. These findings demonstrate an impairment of the beta 2-adrenergic signal transduction in low grade lymphoma cells that particularly affects the function of AC. The comparison with forskolin resistant mutants of an adrenocortical tumor cell line, Y1 (Schimmer et al., J Biol Chem 262: 15521-15526, 1987), suggests that the availability of functional active alpha subunits of stimulatory G proteins (Gs) might be reduced in human B-cell lymphoma, although other mechanisms known to inhibit the AC activity might be involved.

Prolactin-provoked alterations of cytosolic, membrane, and nuclear protein kinase C following partial hepatectomy

The adenohypophyseal polypeptide hormone prolactin is a potent liver mitogen, stimulating cell cycle progression, an effect that appears coupled to increasing protein kinase C activity in membrane and nuclear fractions. Here, we examine whether hepatocyte proliferation, stimulated by partial hepatectomy, is associated with altered serum prolactin or protein kinase C activation. Within 5-15 min of liver resection, serum prolactin concentrations elevate significantly. Protein kinase C activity in hepatic cytosol decreases significantly, and membrane and nuclear PKC activity increase by 30 min. Hypophysectomy prior to partial hepatectomy abrogates any effect of liver resection on protein kinase C activation in the hepatic remnant. Based upon these data, it is suggested that the rapid increase in serum prolactin seen after partial hepatectomy may be linked to protein kinase C activation, which in turn stimulates the hepatic proliferative response that is essential for hepatic regeneration.

Epidermal growth factor receptor gene expression in avian epiphyseal growth-plate cartilage cells: effect of serum, parathyroid hormone and atrial natriuretic peptide

Avian chondrocytes and fibroblasts, derived from epiphyseal growth-plate and skin, respectively, were cultured in vitro. In chondrocytes, epidermal growth factor (EGF) caused a dose-dependent stimulation of proliferation. EGF receptor mRNA was not detected with the v-erb B probe in chondrocytes cultured in the presence of 5% fetal calf serum (FCS). In the absence of FCS in the medium, a time-dependent increase in the level of EGF receptor mRNA was observed. Parallel changes were also observed in the level of EGF receptor, as demonstrated by immunofluorescence using antibodies directed against avian EGF receptor. In avian fibroblasts, EGF receptor mRNA and EGF receptor levels were not affected by FCS. Furthermore, FCS did not affect the level of thyroid hormone receptor mRNA (using v-erb A as a probe) in either chondrocytes or fibroblasts. Parathyroid hormone (PTH), which acts as a mitogen in avian chondrocytes attenuated--whereas atrial natriuretic peptide (ANP), a suppressor of chondrocyte proliferation, enhanced--EGF receptor mRNA. The present results show that avian growth-plate chondrocytes respond to EGF and bear EGF receptors. The levels of EGF mRNA and EGF receptor are inversely related to cell proliferation. The results also support previous suggestions that PTH and ANP play important roles in chondrocyte proliferation, possibly through their effect on the synthesis of the EGF receptor.

Growth and protein phosphorylation in the Nb2 lymphoma: effect of prolactin, cAMP, and agents that activate adenylate cyclase

The Nb2 T lymphoma is unique in that these lymphocytes proliferate in response to prolactin as well as in response to interleukin-2. In this study, we have examined the responsiveness of the adenylate cyclase system in Nb2 cells and the role of this signaling system in regulating proliferation and protein phosphorylation. An analog of cAMP inhibited prolactin-stimulated proliferation and blocked a prolactin-induced decrease in protein phosphorylation. Forskolin, a potent activator of adenylate cyclase in T lymphocytes, did not elevate cAMP levels in Nb2 cells and was not an effective inhibitor of prolactin-induced proliferation. In fact, one preparation of forskolin stimulated proliferation of quiescent Nb2 cells. Like forskolin, prostaglandin E2 did not stimulate cAMP production in Nb2 cells even though it increased cAMP in a preparation of rat peripheral blood lymphocytes. Cholera toxin appeared to ADP-ribosylate a stimulatory guanine nucleotide-binding protein in Nb2 cells, but the toxin did not increase intracellular levels of cAMP nor was it a potent anti-mitogenic agent. Pertussis toxin, an agent that can increase cAMP production through suppression of the inhibitory guanine nucleotide-binding protein, exerted only minor anti-proliferative actions on prolactin-stimulated Nb2 cells. These data suggest that cAMP inhibits Nb2 cell proliferation and prolactin-induced changes in protein phosphorylation but that the adenylate cyclase system in our clone of Nb2 cells responds poorly to agents that normally increase cAMP.

Prolactin and known modulators of rat splenocytes activate nuclear protein kinase C

Prolactin (PRL) and other trophic factors rapidly activate a nuclear pool(s) of protein kinase C (nPKC) in purified splenocyte nuclei. The PRL also enhanced [2-3H]glycerol incorporation into nuclear mono- and triacylglycerol. An assay was devised which not only probed the ability of the hormone to activate protein kinase C (PKC) but also demonstrated the presence of nuclear substrates. Using this methodology, a biphasic concentration-response curve to PRL was observed. Heterologous species of PRL and various growth factors also activated nPKC. The PRL-induced nPKC stimulation was antagonized by various immunomodulators, G protein-coupling inhibitors, PKC inhibitors, a calmodulin inhibitor, and a peripheral benzodiazepine agonist and antagonist. A monoclonal antibody to PKC, anti-rat PRL antiserum and a monoclonal anti-rat PRL receptor antibody antagonized PRL-induced PKC-dependent nuclear phosphorylation, further implicating nPKC and a PRL receptor-mediated activation process. Nuclear PKC may be a major target for trophic regulation in response to both positive and negative growth signals.

Proliferation of bovine undifferentiated mammary epithelial cells in vitro is modulated by G-proteins

Several cAMP-elevating agents such as cholera toxin (CT), forskolin and 3-isobutyl-1-methylxanthine (IBMX) exhibited weak mitogenic activity on bovine undifferentiated mammary epithelial cells in three-dimensional collagen culture. CT and IBMX strongly synergized with epidermal growth factor (EGF), insulin-like growth factor I (IGF-I) or both, but not with 10% fetal calf serum (FCS). Permeable cAMP analogs also synergized with IGF-I. Other hormones such as ovine prolactin, bovine growth hormone, estrogen or progesterone were not mitogenic and not synergistic with EGF, IGF-I, CT and FCS. Pertussis toxin (PT) reduced the DNA synthesis in cells cultured in the basal medium and attenuated 40-90% of the mitogenic activity stimulated by 10% FCS. PT inhibition of DNA synthesis was accompanied by ADP-ribosylation of 40 kDa and 41 kDa membrane proteins. The 41 kDa protein cross-reacted with antibodies that recognize the Gi-protein of the adenylate cyclase system, indicating the involvement of the latter in the mitogenic process. The nature of the second protein remains unknown. The present results suggest that the mitogenesis of normal mammary epithelial cells which is stimulated by IGF-I, EGF and other factors found in FCS is mediated through both cAMP-dependent and independent pathways. These pathways include PT-sensitive GTP-binding proteins.

Nb2 cell mitogenesis: effect of lactogens on cAMP and protein phosphorylation

Rat lymphoma cells (Nb2) are exquisitely sensitive to lactogenic hormones and are an ideal system to study receptor-mediated signal transduction. The effect of human growth hormone (hGH) on macromolecular synthesis, intracellular cAMP concentrations and protein phosphorylation was investigated in Nb2 cells maintained in serum-free medium. hGH stimulated the incorporation of radiolabeled precursors into protein, RNA and DNA in a time-dependent manner. The concentration of hGH inducing half-maximal DNA synthesis was 11 pM, indicating that Nb2 cells cultured in serum-free medium maintain the same sensitivity to lactogen as cells in horse serum-containing medium. hGH over a period of 4 h had no effect on intracellular cAMP regardless of the presence or absence of isobutylmethylxanthine (IBMX). IBMX (250 microM), increased intracellular cAMP levels 2-fold indicating that the cAMP assay was sufficiently sensitive to detect relatively small changes in intracellular cAMP. Cyclic AMP had no effect on protein phosphorylation. However, hGH, prolactin and placental lactogen enhanced phosphorylation of many protein targets, as well as that of a specific protein (Mr = 29,000). Rat growth hormone, which is not mitogenic, had no effect on protein phosphorylation. These results suggest that lactogen-mediated Nb2 mitogenesis does not involve modulation of intracellular cAMP concentration and that cAMP-independent protein phosphorylation may play a role.