Permissive stimulation of Ca2+-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

Abstract

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. In the present study we have found that arachidonic acid release was associated with lysophosphatidyl-choline 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 Ca2+-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 Ca2+-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 Ca2+-generated phospholipase A2 activation. Although acute phorbol ester activation of protein kinase C induced arachidonic acid release, P2-purinergic and α1-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 Ca2+-mobilizing agents)-induced arachidonic acid release by activation of Ca2+-dependent phospholipase A2 in addition to enhancement of agonist-induced phospholipase C followed by an increase in [Ca2+](i).