PACAP acts through a cyclic AMP‐initiated ERK activation pathway independent of PKA and requiring calcium co‐signaling for transcription linked to differentiation in PC12‐G cells

Abstract

PACAP via the Gs coupled PAC1 receptor, and forskolin by direct stimulation of adenylate cyclase, elevates cAMP leading to ERK-dependent differentiation of PC12 cells (Ravni et al., J. Neurochem. 98: , 2006). However, the relative roles of PKA and PKA-independent cAMP signaling in ERK activation are unclear. We employed the PACAP-, cAMP-, and ERK-dependent gene ier-3 as a read-out for the role of PKA in cAMP-dependent signaling required for differentiation in PC12 cells. PACAP activation of ERK and induction of ier-3 were independent of PKA (not inhibited by H89 and persisting in PKA-deficient A126-1B2 PC12 cell variants). Forskolin effects on ERK and ier-3 were PKA-dependent (inhibited by H89 and absent in A126-1B2 cells), but were switched to an H89-resistant stimulation when PC12 cells were concomitantly depolarized with elevated potassium. The relative contributions of PKA-dependent and –independent pathways for activation of ERK and immediate early response genes may also be serum-dependent. Since PACAP stimulation of ier-3 is reduced by the plasma membrane voltage-dependent cation channel blocker D600, we suggest that concerted elevation of cAMP and calcium, a unique property of PAC1 receptor activation, plays a key role in recruitment of cAMP signaling to a PKA-independent mode for ERK activation, and ERK-dependent transcription of the ier-3 gene.