Signal transduction pathways associated with ATP‐induced proliferation of cell progenitors in the intact embryonic retina

ATP and ADP induce retinal cell proliferation through activation of PKC and extracellular signal‐regulated kinases (ERKs). Here, we characterized the effect of purinergic agonists on the turnover of phosphoinositides and activation of ERKs during development of the chick embryo retina. When intact retinas were incubated with ATP, ADP or UTP, a dose‐dependent accumulation of [3H]‐phosphoinositides was observed (% of control, EC50: 548 ± 20.5%, 0.18 mM; 314 ± 53.8%, 0.51 mM; 704 ± 139.9%, 0.018 mM, respectively). Only the response promoted by ADP was completely inhibited by the P2 receptor antagonists, PPADS and suramin. All the responses decreased with the progression of retinal development. Western blot assays revealed that ATP, ADP and UTP stimulated the phosphorylation of ERKs in the chick embryo retina very early during development (% of control: 174 ± 16; 199 ± 16.4 and 206 ± 37, respectively). The responses to ADP and UTP were transient and dose‐dependent, showing EC50 values of 0.12 mM and 0.009 mM. The response to ADP was inhibited by the antagonists PPADS and suramin and by U73122 and chelerythrine chloride, which block PLC and PKC, respectively. Conversely, chelerythrine chloride did not block the response induced by UTP. Immunohistochemical analysis revealed that ATP and ADP induced the phosphorylation of ERKs in cells of the neuroblastic layer of retinas from embryos at E8. Our data showed that ATP, ADP and UTP stimulate the turnover of InsPs and promoted the activation of ERKs in the chick embryo retina. ADP, through activation of P2Y1 receptors, activated ERK pathway through PLC and PKC and UTP, via P2Y4‐like receptors, induced the phosphorylation of ERKs through a pathway that did not involve PKC.