Uncoupling of ATP-induced inositol phosphate formation and Ca²⁺ mobilization by phorbol ester in canine cultured tracheal epithelial cells

The regulation of the increase in inositol phosphates (IPs) production and intracellular Ca²⁺ concentration ([Ca²⁺]_i) by protein kinase C (PKC) was investigated in canine cultured tracheal epithelial cells (TECs). Pretreatment of TECs with phorbol 12-myristate 13-acetate (PMA, 1 μM) for 30 min attenuated the ATP- and UTP-induced IPs formation and Ca²⁺ mobilization. The concentrations of PMA that gave half-maximal (EC₅₀) inhibition of ATP- and UTP-induced IPs accumulation and an increase in [Ca²⁺]_i were 5-10 and 4-12 nM, respectively. Prior treatment of TECs with staurosporine (1 μM), a PKC inhibitor, partially inhibited the ability of PMA to attenuate ATP- and UTP-induced responses, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. Furthermore, analysis of cell extracts by Western blotting with antibodies against different PKC isozymes revealed that TECs expressed PKC-α, -βI, -βII, -γ, -δ, -ε, -θ, and -ζ. With PMA treatment of the cells for various times, translocation of PKC-α, -βI, -βII, -γ, -δ, -ε, and -θ from the cytosol to the membrane was seen after 5- and 30-min and 2- and 4-h treatment. However, 6-h treatment caused a partial down-regulation of these PKC isozymes. PKC-ζ was not significantly translocated and down-regulated at any of the times tested. In conclusion, these results suggest that activation of PKC may inhibit the phosphoinositide (PI) hydrolysis and consequently attenuate the [Ca²⁺]_i increase or inhibit independently both responses to ATP and UTP. The translocation of PKC-α, -βI, -βII, -δ, -ε, -γ, and -θ induced by PMA caused an attenuation of ATP- and UTP-induced IPs accumulation and Ca²⁺ mobilization in TECs.