β1-adrenergic receptor N-terminal cleavage by ADAM17; the mechanism for redox-dependent downregulation of cardiomyocyte β1-adrenergic receptors

beta(1)-adrenergic receptors (beta(1)ARs) are the principle mediators of catecholamine action in cardiomyocytes. We previously showed that the beta(1)AR extracellular N-terminus is a target for post-translational modifications that impact on signaling responses. Specifically, we showed that the beta(1)AR N-terminus carries O-glycan modifications at Ser(37)/Ser(41), that O-glycosylation prevents beta(1)AR N-terminal cleavage, and that N-terminal truncation influences beta(1)AR signaling to downstream effectors. However, the site(s) and mechanism for beta(1)AR N-terminal cleavage in cells was not identified. This study shows that beta(1)ARs are expressed in cardiomyocytes and other cells types as both full-length and N-terminally truncated species and that the truncated beta(1)AR species is formed as a result of an O-glycan regulated N-terminal cleavage by ADAM17 at R-31 down arrow L-32. We identify Ser(41) as the major O-glycosylation site on the beta(1)AR N-terminus and show that an O-glycan modification at Ser41 prevents ADAM17-dependent cleavage of the beta(1)-AR N-terminus at S-41 down arrow L-42, a second N-terminal cleavage site adjacent to this O-glycan modification (and it attenuates beta(1)-AR N-terminal cleavage at R-31 down arrow L-32). We previously reported that oxidative stress leads to a decrease in beta(1)AR expression and catecholamine responsiveness in cardiomyocytes. This study shows that redox-inactivation of cardiomyocyte beta(1)ARs is via a mechanism involving N-terminal truncation at R-31 down arrow L-32 by ADAM17. In keeping with the previous observation that N-terminally truncated beta(1)ARs constitutively activate an AKT pathway that affords protection against doxorubicin-dependent apoptosis, overexpression of a cleavage resistant beta(1)AR mutant exacerbates doxorubicin-dependent apoptosis. These studies identify the beta(1)AR N-terminus as a structural determinant of beta(1)AR responses that can be targeted for therapeutic advantage.

Automated summary

The study revealed that beta(1)-adrenergic receptors are expressed in cardiomyocytes and other cell types as both full-length and N-terminally truncated species, with the truncated species being formed as a result of O-glycan regulated N-terminal cleavage by ADAM17. It was also found that oxidative stress leads to redox-inactivation of cardiomyocyte beta(1)ARs through N-terminal truncation by ADAM17, and overexpression of a cleavage resistant beta(1)AR mutant exacerbates cell apoptosis. These findings suggest that the beta(1)AR N-terminus plays a crucial role in beta(1)AR responses and may be targeted for therapeutic advantage.