Transforming growth factor-beta 1 regulates Kir2.3 inward rectifier K+ channels via phospholipase C and protein kinase C-delta in reactive astrocytes from adult rat brain.

The multifunctional cytokine, transforming growth factor beta(1) (TGF-beta(1)), exerts complex effects on astrocytes with early signaling events being less well characterized than transcriptional mechanisms. We examined the effect of TGF-beta(1) on the 14-pS Kir2.3 inward rectifier K(+) channel in rat primary cultured reactive astrocytes. Immunofluorescence study showed that cells co-expressed TGF-beta(1) receptors 1 and 2, Kir2.3, and glial fibrillary acidic protein (GFAP). Patch clamp study showed that TGF-beta(1) (0.1-100 ng/ml) caused a rapid (<5 min) depolarization because of dose-dependent down-regulation of Kir2.3 channels, which was mimicked by the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (10-500 nm) and which was inhibited by the PKC inhibitor calphostin C (100 nm), by PKC desensitization produced by 3 h of exposure to phorbol 12-myristate 13-acetate (100 nm), and by the PKC-delta isoform-specific inhibitor rottlerin (50 microm). Immunoblot analysis and confocal imaging showed that TGF-beta(1) caused PKC-delta translocation to membrane, and co-immunoprecipitation experiments showed that TGF-beta(1) enhanced association between Kir2.3 and PKC-delta. Additional electrophysiological experiments showed that Kir2.3 channel down-regulation was blocked by the phospholipase C inhibitors, neomycin (100 microm) and D609 (200 microm). Given the commonality of signaling involving PLC-PKC-delta, we speculate that TGF-beta(1)-evoked depolarization may be an early signaling event related to gene transcription in astrocytes.