Cross-talk between m1 muscarinic acetylcholine and beta 2-adrenergic receptors. cAMP and the third intracellular loop of m1 muscarinic receptors confer heterologous regulation.

Genes encoding the m1 muscarinic (m1 mAChR) and beta 2-adrenergic receptors (beta 2AR) were stably co-expressed into Chinese hamster ovary (CHO) cells to study receptor regulation and cross-talk. Persistent activation of the beta 2AR/adenylate cyclase pathway by isoproterenol leads to heterologous desensitization, internalization, and down-regulation of the m1 mAChR which is comparable, but smaller in magnitude, with that seen with persistent activation of the m1 mAChR by carbachol. This heterologous effect was mimicked by dibutyryl cAMP and forskolin and antagonized by the protein kinase A (PKA) inhibitor H-8. A potential consensus sequence for phosphorylation by PKA (Lys351-Arg-Lys-Thr354) exists on the third intracellular loop of the m1 mAChR, suggesting that receptor phosphorylation by PKA may be involved in heterologous regulation. The loss of m1 mAChRs induced by carbachol was not reversed by H-8, indicating that homologous regulation is not dependent on PKA. Recent evidence suggests that muscarinic agonist-mediated internalization of the m1 mAChR involves the third intracellular loop (i3) (Maeda, S., Lameh, J., Mallet, W. G., Philip, M., Ramachandran, J., and Sadee, W. (1990) FEBS Lett. 269, 386-388). Three deletion mutant receptors were constructed in which the majority, or small regions, of i3 were eliminated but the membrane proximal portions of the loop were left intact. Each of the mutants was co-expressed with the beta 2AR in CHO cells. A small region in i3 was identified which is crucial for carbachol- and isoproterenol-promoted internalization and down-regulation. This region contains a series of 6 serine residues within an 8-amino acid stretch. A similar domain has been identified in the carboxyl tail of the beta 2AR and has been proposed to participate in receptor internalization (Hausdorff, W. P., Campbell, P. T., Ostrowski, J., Yu, S. S., Caron, M. G., and Lefkowitz, R. J. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 2979-2983).