Modulation of lysophosphatidic acid-induced Cl^- currents by protein kinases A and C in the Xenopus oocyte

The roles of protein kinase C (PKC) and protein kinase A (PKA) in the regulation of lysophosphatidic acid (LPA)-induced Cl^- currents in Xenopus oocytes were examined. PKC activation by phorbol 12-myristate 13-acetate (PMA) treatment completely blocked LPA-induced Cl^- currents by inhibiting inositol 1,4,5-trisphosphate (IP₃) elevation. This inhibitory effect of PMA on the LPA response was blocked by pretreatment of oocytes with staurosporine and 3-[N-(dimethylamino)propyl-3-indolyl]-4-[3-indolyl]maleimide (GF109203X), PKC inhibitors. In addition, treatment of oocytes with GF109203X enhanced the LPA response by increasing IP₃ production. Elevation of the intracellular adenosine 3',5'-cyclic monophosphate (cAMP) concentration by treating oocytes with either forskolin (FK) plus isobutylmethylxanthine (IBMX) or 2'-O- dibutyryl-cAMP (dB-cAMP) reduced LPA-induced Cl^- currents. The effect of activation of the cAMP pathway appears to be mediated by PKA, since treatment of oocytes with FK plus IBMX or dB-cAMP enhanced PKA activity. Furthermore, the inhibitory effect of dB-cAMP on the LPA response was blocked by treatment of oocytes with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulframide- 2HCl (H-89), a selective inhibitor of PKA. Both FK plus IBMX and dB-cAMP treatment reduced IP₃ generation in response to LPA stimulation. Inhibition of PKA activity with H-89 or Rp-cyclic 3',5'-hydrogen phosphorothioate adenosine triethylammonium had no effect on LPA-induced Cl^- currents. Finally, inhibition of the LPA response by activation of PKA was independent of extracellular Ca²⁺. These results demonstrate that both PKC and PKA play active roles in modulating the LPA-induced signaling pathway.