P2Y6 nucleotide receptor activates PKC to protect 1321N1 astrocytoma cells against tumor necrosis factor-induced apoptosis

1. We recently reported that the activation by UDP of rat P2Y₆ nucleotide receptors expressed in 1321N1 astrocytoma cells protected them from TNF-induced apoptosis by suppressing activation of caspase 3 and 8. This study aims to characterize the involvement of intracellular signaling pathways, including kinases, involved in the antiapoptotic effect of UDP.2. Cell death was induced in 1321N1 astrocytoma cells permanently expressing the rat P2Y₆ receptor by exposure to TNF in the presence of cycloheximide. The apoptotic fraction was analyzed using flow cytometry.3. The activation of P2Y₆ receptors by UDP both protected the astrocytes from TNF- induced apoptosis and activated protein kinase C (PKC) isotypes. The phorbol ester PMA also activated PKC and protected the cells from TNF-induced cell death. The - and -isotypes of PKC were both activated in a persistent fashion upon 5-min exposure to either UDP (10 M) or the phorbol ester PMA (100 nM). The PKC isotype was markedly activated upon UDP treatment.4. The addition of PKC inhibitors, GF109203X or Gö6976, partially antagonized the protective effect of UDP and reduced the UDP-induced phosphorylation of extracellular signal-regulated protein kinases (Erk). The inhibitors of Erk, PD98,059 or U0126, antagonized UDP-induced protection.5. The antiapoptotic protein, Akt, was not affected by P2Y₆ receptor activation. Incubation of the astrocytes with calcium modifiers, BAPTA-AM or dantrolene, did not affect the UDP-induced protection from apoptosis.6. The addition of phospholipase C (PLC) inhibitors, D609 or U73122, partially antagonized both UDP-induced protection and PKC activation.7. Therefore, it is suggested that P2Y₆ receptors in 1321N1 cells, through coupling to PC-PLC and PI-PLC, activate PKC to protect against TNF -induced apoptosis, in which the activation of Erk is involved in part.