A-kinase anchor protein 75 increases the rate and magnitude of cAMP signaling to the nucleus

Abstract

A-kinase anchor protein 75 (AKAP75) binds regulatory subunits (RIIα and RIIβ) of type II protein kinase A (PKAII) isoforms and targets the resulting complexes to sites in the cytoskeleton that abut the plasma membrane [1–7]. Co-localization of AKAP75–PKAII with adenylate cyclase and PKA substrate/effector proteins in cytoskeleton and plasma membrane effects a physical and functional integration of up-stream and downstream signaling proteins, thereby ensuring efficient propagation of signals carried by locally generated cyclic AMP (cAMP) [4–9]. An important, but previously untested, prediction of the AKAP model is that efficient, cyclic nucleotide-dependent liberation of diffusible PKA catalytic subunits from cytoskeleton-bound AKAP75–PKAII complexes will also enhance signaling to distal organelles, such as the nucleus. We tested this idea by suing HEK-A75 cells, in which PKAII isoforms are immobilized in cortical cytoskeleton by AKAP75. Abilities of HEK-A75 and control cells (with cytoplasmically dispersed PKAII isoforms) to respond to increases in cAMP content were compared. Cells with anchored PKAII exhibited a threefold higher level of nuclear catalytic subunit content and 4–10-fold greater increments in phosphorylation of a regulatory serine residue in cAMP response element binding protein (CREB) and in phosphoCREB-stimulated transcription of the c-fos gene. Each effect occurred more rapidly in cells containing targeted AKAP75–PKAII complexes. Thus, anchoring of PKAII in actin cortical cytoskeleton increases the rate, magnitude and sensitivity of cAMP signaling to the nucleus.