Mechanisms of bombesin-induced arachidonate mobilization in Swiss 3T3 fibroblasts.
A peptide mitogen bombesin, which activates the phospholipase C-protein kinase C signaling pathway, induces a mepacrine-sensitive, dose-dependent increase in the release of [3H]arachidonic acid and its metabolites ([3H]AA) from prelabeled Swiss 3T3 fibroblasts. The effect is temporally composed of two phases, i.e. an initial transient burst that is essentially independent of extracellular Ca2+, and a following sustained phase that is absolutely dependent on the extracellular Ca2+. The initial transient [3H]AA liberation occurs concomitantly with bombesin-induced 45Ca efflux from prelabeled cells: both responses being substantially attenuated by loading cells with a Ca2+ chelator quin2. However, bombesin-induced intracellular Ca2+ mobilization by itself is not sufficient as a signal for the initial transient [3H]AA liberation, since A23187 potently stimulates 45Ca efflux to an extent comparable to bombesin but fails to induce [3H]AA release in the absence of extracellular Ca2+. The second sustained phase of the bombesin-induced [3H]AA release is abolished by reducing extracellular Ca2+ to 0.03 mM, although bombesin effects on phospholipase C and protein kinase C activation are barely affected by the same procedure. A protein kinase C activator phorbol 12,13-dibutyrate induces an extracellular Ca(2+)-dependent, slowly developing sustained increase in [3H]AA release, and markedly potentiates both phases of bombesin-induced [3H]AA release. Down-regulation of cellular protein kinase C completely abolishes all of the effects of phorbol dibutyrate, and partially inhibits the second but not the first phase of bombesin-induced [3H]AA release. These results indicate that bombesin-induced receptor-mediated activation of phospholipase A2 involves multiple mechanisms, including intracellular Ca2+ mobilization for the first phase, protein kinase C activation plus Ca2+ influx for the second phase, and as yet unknown mechanism(s) independent of intracellular Ca2+ mobilization or protein kinase C for both of the phases.
Takuwa
N
N
Department of Physiology, Faculty of Medicine, University of Tokyo, Japan.
Kumada
M
M
Yamashita
K
K
Takuwa
Y
Y
D000818:Animals; D001095:Arachidonic Acids / Q000378:metabolism*; D001839:Bombesin / Q000494:pharmacology*; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002413:Cations, Divalent; D002478:Cells, Cultured; D003035:Cobalt / Q000378:metabolism; D015536:Down-Regulation; D004789:Enzyme Activation; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism; D051379:Mice; D015240:Phorbol 12,13-Dibutyrate / Q000494:pharmacology; D010741:Phospholipases A / Q000378:metabolism; D054467:Phospholipases A2; D011493:Protein Kinase C / Q000378:metabolism; D011796:Quinacrine / Q000494:pharmacology; D010738:Type C Phospholipases / Q000378:metabolism
D001095:Arachidonic Acids; D002413:Cations, Divalent; D015240:Phorbol 12,13-Dibutyrate; D000001:Calcimycin; D003035:Cobalt; D011493:Protein Kinase C; D010741:Phospholipases A; D054467:Phospholipases A2; D010738:Type C Phospholipases; D011796:Quinacrine; D001839:Bombesin; D002118:Calcium