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.
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