In the present study an attempt was made to further elucidate the molecular mechanisms whereby protein kinase C (PKC) modulates the beta-cell stimulus-secretion coupling. Regulation of Ca2+ channel activity, [Ca2+]i, and insulin release were investigated in both normal pancreatic mouse beta-cells and in similar beta-cells deprived of PKC activity. [Ca2+]i was measured with the intracellular fluorescent Ca2+ indicator fura-2 and the Ca2+ channel activity was estimated by the whole cell configuration of the patch-clamp technique. To reveal the various isoenzymes of PKC present in the mouse beta-cell, proteins were separated by one-dimensional gel electrophoresis and Western blotting was performed. The production of inositol phosphates was measured by ion-exchange chromatography and insulin release was measured radioimmunologically. Acute stimulation with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate resulted in suppression of both the carbamylcholine-induced increase in [Ca2+]i and production of inositol 1,4,5-trisphosphate. Under these conditions the increase in [Ca2+]i in response to glucose was similar to that found in control cells. When beta-cells were deprived of PKC, by exposure to 200 nM 12-O-tetradecanoylphorbol-13-acetate for 24-48 h, there was an enhanced response to carbamylcholine. This response constituted increases in both the [Ca2+]i signal and production of inositol 1,4,5-trisphosphate. Interestingly, cells with down-regulated PKC activity responded more slowly to glucose stimulation, when comparing the initial increase in [Ca2+]i, than control cells. On the other hand, the maximal increase in [Ca2+]i was similar whether or not PKC was present. Moreover, PKC down-regulated cells exhibited a significant reduction of maximal whole cell Ca2+ currents, a finding that may explain the altered kinetics with regard to the [Ca2+]i increase in response to the sugar. Both the alpha and beta 1 forms of the PKC isoenzymes were present in the mouse beta-cell and were also subjected to PKC down-regulation. Hence, either of these isoenzymes or both may be involved in the modulation of phospholipase C and Ca2+ channel activity. Since insulin release under physiological conditions is critically dependent on Ca(2+)-influx through the voltage-gated L-type Ca2+ channels, the kinetics of hormone release was expected to demonstrate a similar delay as that of the [Ca2+]i increase. Although not as pronounced, such a delay was indeed also observed in the onset of insulin release. There was, however, no effect on the total amounts of hormone released.(ABSTRACT TRUNCATED AT 400 WORDS)
To examine the involvement of Na+ ions in adrenergic responses in brown adipose tissue, a method is described for measuring Na+ influx into isolated brown adipocytes, using short (30 s) incubations with 22Na+, followed by a two-step centrifugation recovery procedure. Using this method, a clear norepinephrine-stimulated accumulation of intracellular 22Na+ was observed, which was enhanced by the addition of ouabain, was insensitive to amiloride (a Na+/H+ exchange blocker), and could not be mimicked by the total removal of oxygen from the incubation medium. The norepinephrine-stimulated Na+ influx was dose-dependent for the hormone with an EC50 of 250 nM, was blocked by the beta-antagonist propranolol but not by the alpha 1-antagonist prazosin, and could be induced by adrenergic agonists with the order of potency: isoproterenol greater than norepinephrine greater than phenylephrine, indicating a beta-receptor-mediated process. The Na+ influx was found to be cAMP-dependent since it could be induced by both theophylline (a phosphodiesterase inhibitor) and forskolin (an adenylate cyclase activator), but it was independent of other known cellular cAMP-dependent responses since neither addition of fatty acid substrates (octanoate or palmitate), nor of the mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxyphenyl-hydrazone could induce the phenomenon, despite having significant stimulatory effects on cellular respiration. Furthermore, total respiratory inhibition with rotenone, or total oxygen depletion of the medium with dithionite, did not prevent the normal norepinephrine-induced Na+ influx. The possibility that this beta-mediated norepinephrine-stimulated Na+ influx plays an important physiological role in brown adipose tissue activity is discussed, perhaps as one of the, as yet undefined, signals initiating tissue growth in the chronically beta-stimulated tissue of animals facing long-term increases in thermogenic demands.
Phosphorylated tyrosine residues in receptor tyrosine kinases serve as binding sites for signal transduction molecules. We have identified two autophosphorylation sites, Tyr-988 and Tyr-1018, in the platelet-derived growth factor (PDGF) alpha-receptor carboxyl-terminal tail, which are involved in binding of phospholipase C-gamma (PLC-gamma). The capacities of the Y988F and Y1018F mutant PDGF alpha-receptors, expressed in porcine aortic endothelial cells, to bind PLC-gamma are 60 and 5% of that of the wild-type receptor, respectively. Phosphorylated but not unphosphorylated peptides containing Tyr-1018 are able to compete with the intact receptor for binding to immobilized PLC-gamma SH2 domains; a phosphorylated Tyr-988 peptide competes 10 times less efficiently. The complex between PLC-gamma and the PDGF alpha-receptor is more stable than that of PLC-gamma and the PDGF beta-receptor. However, PDGF stimulation results in a smaller fraction of tyrosine-phosphorylated PLC-gamma and a smaller accumulation of inositol trisphosphate in cells expressing the alpha-receptor as compared with cells expressing the beta-receptor. We conclude that phosphorylated Tyr-988 and Tyr-1018 in the PDGF alpha-receptor carboxyl-terminal tail bind PLC-gamma, but this association leads to only a relatively low level of tyrosine phosphorylation and activation of PLC-gamma.
The cell signaling docking protein p130cas became tyrosine-phosphorylated in SH-SY5Y human neuroblastoma cells during induced differentiation with 12-O-tetradecanoylphorbol-13-acetate (TPA) and serum or a combination of basic fibroblast growth factor (bFGF) and insulin-like growth factor-I (IGF-I). The differentiating cells develop a neuronal phenotype with neurites and growth cones and sustained activation of protein kinase C (PKC) and pp60c-src. The TPA-induced p130cas phosphorylation increased within 5 min of stimulation and persisted for at least 4 days, whereas bFGF/IGF-I-induced p130cas phosphorylation was biphasic. However, the increase in tyrosine phosphorylation of p130cas was not restricted to differentiation inducing stimuli. The phosphorylation was blocked by the specific PKC inhibitor GF 109203X, and transient transfection with active PKC-epsilon induced p130cas tyrosine phosphorylation. pp60c-src, known to directly phosphorylate p130cas in other cell systems, was not activated after stimulation with TPA or bFGF/IGF-I for up to 30 min, and the initial p130cas phosphorylation was resistant to the Src family kinase inhibitor herbimycin A. However, in long term stimulated cells, herbimycin A blocked the induced phosphorylation of p130cas. Also, overexpression of src induced phosphorylation of p130cas. p130cas protein and phosphorylated p130cas were present in growth cones isolated from differentiated SH-SY5Y cells. Inhibition of PKC activity in differentiating cells with GF 109203X leads to a rapid retraction of growth cone filopodia, and p130cas phosphorylation decreased transiently (within minutes). Growth cones isolated from these cells were virtually devoid of phosphorylated p130cas. These data suggest a function for p130cas as a PKC downstream target in SH-SY5Y cells and possibly also in their growth cones.
The potent mitogen platelet-derived growth factor (PDGF) induced a rapid increase in Ras.GTP in permeabilized human and murine fibroblasts. The effect was initiated by both PDGF-AA acting exclusively through PDGF alpha-receptors, and by PDGF-BB interacting with both alpha- and beta-type receptors. The dose-response curves suggest that both receptor types mediate the response. PDGF-dependent Ras activation, measured as increased formation of Ras.GTP, was rapid and reversible. At 37 degrees C the effect had a duration of around 10 min. The PDGF-dependent increase in Ras.GTP was followed by a simultaneous increase in Ras.GDP. Under no experimental condition could a relative increase in Ras.GTP be detected. 0.5 microM GDP and 0.5 microM GTP were equally potent competing for the formation of Ras.[alpha-32P]GTP upon PDGF stimulation. Furthermore, when the basal nucleotide exchange rate on Ras was elevated by omission of Mg2+ from the medium, PDGF had no further effect on the formation of Ras.GTP. We therefore conclude that PDGF activates Ras through a mechanism leading to an increased nucleotide exchange on Ras.