Involvement of Protein Kinase C in Ca²⁺‐Signaling Pathways to Activation of AP‐1 DNA‐Binding Activity Evoked via NMDA‐ and Voltage‐Gated Ca²⁺ Channels

Abstract: Stimulation of cultured cerebellar granule cells with N‐methyl‐d‐aspartate (NMDA) or kainic acid (KA) leads to activation of activator protein‐1 (AP‐1) DNA‐binding activity, which can be monitored by an increase in 12‐O‐tetradecanoylphorbol 13‐acetate (TPA)‐responsive element (TRE)‐binding activity, in concert with c‐fos induction. For this increase in TRE‐binding activity, Ca²⁺ influx across the plasma membrane is essential. Treatment of cells with an intracellular Ca²⁺ chelator, BAPTA‐AM, abolished this increase. Close correspondence between the dose‐response curves of ⁴⁵Ca²⁺ uptake and TRE‐binding activity by NMDA or KA suggested that Ca²⁺ influx not only triggered sequential activation of Ca²⁺‐signaling processes leading to the increase in TRE‐binding activity, but also controlled its increased level. Stimulation of non‐NMDA receptors by KA mainly caused Ca²⁺ influx through voltage‐gated Ca²⁺ channels, whereas stimulation of NMDA receptors caused Ca²⁺ influx through NMDA‐gated ion channels. The protein kinase C (PKC) inhibitors staurosporine and calphostin C inhibited the increase in TRE‐binding activity caused by NMDA and KA at the same concentration at which they inhibited that caused by TPA. Furthermore, down‐regulation of PKC inhibited the increase in TRE‐binding activity by NMDA and KA. Thus, a common pathway that includes PKC could, at least in part, be involved in the Ca²⁺‐signaling pathways for the increase in TRE‐binding activity coupled with the activation of NMDA‐ and non‐NMDA receptors.