Abstract

β₂-Adrenoceptors (β₂ARs) are concentrated in caveolar lipid raft domains of the plasma membrane in airway smooth muscle (ASM) cells, along with adenylyl cyclase type 6 (AC6). This is believed to contribute to how these receptors can selectively regulate certain types of cAMP-dependent responses in these cells. The goal of the present study was to test the hypothesis that β₂AR production of cAMP is localized to specific subcellular compartments using fluorescence resonance energy transfer (FRET)-based cAMP biosensors targeted to different microdomains in human ASM cells. Epac2-MyrPalm and Epac2-CAAX biosensors were used to measure responses associated with lipid raft and non-raft regions of the plasma membrane, respectively. Activation of β₂ARs with isoproterenol produced cAMP responses most readily detected in lipid raft domains. Furthermore, overexpression of AC6 somewhat paradoxically inhibited β₂AR production of cAMP in lipid raft domains, without affecting β₂AR responses detected in other subcellular locations or cAMP responses to EP₂ prostaglandin receptor activation, which were confined primarily to non-raft domains of the plasma membrane. The inhibitory effect of overexpressing AC6 was blocked by inhibition of phosphodiesterase type 4 (PDE4) activity with rolipram, inhibition of protein kinase A (PKA) activity with H89, and inhibition of A kinase anchoring protein (AKAP) interactions with the peptide inhibitor Ht31. These results support the idea that overexpression of AC6 leads to enhanced feedback activation of PDE4 via phosphorylation by PKA that is part of an AKAP-dependent signaling complex. This provides insight into the molecular basis for localized regulation of cAMP signaling in human ASM cells.