ß1-adrenergic receptor N-terminal cleavage by ADAM17; the mechanism for redox-dependent downregulation of cardiomyocyte ß1-adrenergic receptors.

ß1-adrenergic receptors (ß1ARs) are the principle mediators of catecholamine action in cardiomyocytes. We previously showed that the ß1AR extracellular N-terminus is a target for post-translational modifications that impact on signaling responses. Specifically, we showed that the ß1AR N-terminus carries O-glycan modifications at Ser37/Ser41, that O-glycosylation prevents ß1AR N-terminal cleavage, and that N-terminal truncation influences ß1AR signaling to downstream effectors. However, the site(s) and mechanism for ß1AR N-terminal cleavage in cells was not identified. This study shows that ß1ARs are expressed in cardiomyocytes and other cells types as both full-length and N-terminally truncated species and that the truncated ß1AR species is formed as a result of an O-glycan regulated N-terminal cleavage by ADAM17 at R31↓L32. We identify Ser41 as the major O-glycosylation site on the ß1AR N-terminus and show that an O-glycan modification at Ser41 prevents ADAM17-dependent cleavage of the ß1-AR N-terminus at S41↓L42, a second N-terminal cleavage site adjacent to this O-glycan modification (and it attenuates ß1-AR N-terminal cleavage at R31↓L32). We previously reported that oxidative stress leads to a decrease in ß1AR expression and catecholamine responsiveness in cardiomyocytes. This study shows that redox-inactivation of cardiomyocyte ß1ARs is via a mechanism involving N-terminal truncation at R31↓L32 by ADAM17. In keeping with the previous observation that N-terminally truncated ß1ARs constitutively activate an AKT pathway that affords protection against doxorubicin-dependent apoptosis, overexpression of a cleavage resistant ß1AR mutant exacerbates doxorubicin-dependent apoptosis. These studies identify the ß1AR N-terminus as a structural determinant of ß1AR responses that can be targeted for therapeutic advantage.