PKC-dependent inhibition of CA2+-dependent exocytosis from astrocytes

Astrocytes release various bioactive substances via Ca(2+) - and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-dependent exocytosis; however the regulatory mechanisms of glial exocytosis are still poorly understood. In the present study, we investigated the effect of protein kinase C (PKC) on exocytosis in glial cells using primary cultured astrocytes and clonal rat glioma C6 cells. Mass spectrometry and Western blot analysis using phospho-specific antibodies revealed that phorbol 12-myristate 13-acetate (PMA) treatment induced the phosphorylation of synaptosomal-associated protein of 23 kDa (SNAP-23) on Ser(95), Ser(120), and Ser(160) in cultured astrocytes and C6 cells. Phosphorylation at these sites was suppressed by treatment with the PKC inhibitor, bisindolylmaleimide I (BIS). In contrast, Ser(110) of SNAP-23 was constitutively phosphorylated in these cells and was dephosphorylated in a PKC-dependent manner. Exogenously expressed human growth hormone (hGH) accumulated in cytoplasmic granular structures in cultured astrocytes, and its release after ATP-treatment was Ca(2+) - and SNARE-dependent. PMA treatment suppressed the ATP-induced hGH release from astrocytes and this inhibition was reversed by BIS. We also observed PMA-dependent suppression and an attenuation of that suppression by BIS in ionomycin-induced hGH release from C6 cells. These results suggest that intracellular activation of PKC suppresses Ca(2+) - and SNARE-dependent exocytosis in astroglial cells.