Permissive stimulation of Ca(2+)-induced phospholipase A2 by an adenosine receptor agonist in a pertussis toxin-sensitive manner in FRTL-5 thyroid cells: a new 'cross-talk' mechanism in Ca2+ signalling.
We have described the pertussis toxin (PTX)-sensitive potentiation of P2-purinergic agonist-induced phospholipase C activation, Ca2+ mobilization and arachidonic acid release by an adenosine receptor agonist, N6-(L-2-phenylisopropyl)adenosine (PIA), which alone cannot influence any of these cellular activities [Okajima, Sato, Nazarea, Sho and Kondo (1989) J. Biol. Chem. 264, 13029-13037]. In the present study we have found that arachidonic acid release was associated with lysophosphatidylcholine production, and conclude that arachidonic acid is produced by phospholipase A2 in FRTL-5 thyroid cells. This led us to assume that PIA augments P2-purinergic arachidonic acid release by increasing [Ca2+]i which, in turn, activates Ca(2+)-sensitive phospholipase A2. The arachidonic acid-releasing response to PIA was, however, always considerably higher (3.1-fold increase) than the Ca2+ response (1.3-fold increase) to the adenosine derivative. In addition, arachidonic acid release induced by the [Ca2+]i increase caused by thapsigargin, an endoplasmic-reticulum Ca(2+)-ATPase inhibitor, or calcium ionophores was also potentiated by PIA without any effect on [Ca2+]i and phospholipase C activity. This action of PIA was also PTX-sensitive, but not affected by the forskolin- or cholera toxin-induced increase in the cellular cyclic AMP (cAMP), suggesting that a PTX-sensitive G-protein(s) and not cAMP mediates the PIA-induced potentiation of Ca(2+)-generated phospholipase A2 activation. Although acute phorbol ester activation of protein kinase C induced arachidonic acid release, P2-purinergic and alpha 1-adrenergic stimulation of arachidonic acid release was markedly increased by the protein kinase C down-regulation caused by the phorbol ester. This suggests a suppressive role for protein kinase C in the agonist-induced activation of arachidonic acid release. We conclude that PIA (and perhaps any of the G1-activating agonists) augments an agonist (maybe any of the Ca(2+)-mobilizing agents)-induced arachidonic acid release by activation of Ca(2+)-dependent phospholipase A2 in addition to enhancement of agonist-induced phospholipase C followed by an increase in [Ca2+]i.
Shimegi
S
S
Department of Physical Biochemistry, Gunma University, Maebashi, Japan.
Okajima
F
F
Kondo
Y
Y
D000818:Animals; D016718:Arachidonic Acid / Q000378:metabolism; D002118:Calcium / Q000494:pharmacology; D002147:Calmodulin / Q000378:metabolism; D002478:Cells, Cultured; D004789:Enzyme Activation; D004790:Enzyme Induction; D006160:Guanosine Triphosphate / Q000494:pharmacology; D015759:Ionomycin / Q000494:pharmacology; D037342:Pertussis Toxin; D010660:Phenylisopropyladenosine / Q000494:pharmacology*; D010741:Phospholipases A / Q000096:biosynthesis*; D054467:Phospholipases A2; D011493:Protein Kinase C / Q000378:metabolism; D051381:Rats; D018047:Receptors, Purinergic P1 / Q000187:drug effects*; D015398:Signal Transduction; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin; D013961:Thyroid Gland / Q000166:cytology / Q000187:drug effects* / Q000201:enzymology; D010566:Virulence Factors, Bordetella / Q000494:pharmacology*
D002147:Calmodulin; D018047:Receptors, Purinergic P1; D013729:Terpenes; D010566:Virulence Factors, Bordetella; D016718:Arachidonic Acid; D010660:Phenylisopropyladenosine; D015759:Ionomycin; D019284:Thapsigargin; D006160:Guanosine Triphosphate; D037342:Pertussis Toxin; D011493:Protein Kinase C; D010741:Phospholipases A; D054467:Phospholipases A2; D002118:Calcium