Opposite effects of β2-adrenoceptor gene deletion on insulin signaling in liver and skeletal muscle

BACKGROUND AND AIM

β₂-Adrenoceptors (β₂-ARs) are G protein-coupled receptors (GPCRs) expressed in the major insulin target tissues. The interplay between β₂-AR and insulin pathways is involved in the maintenance of glucose homeostasis. The aim of this study was to explore the consequences of β₂-ARs deletion on insulin sensitivity and insulin signaling cascade in metabolically active tissues.

METHODS AND RESULTS

We evaluated glucose homeostasis in skeletal muscle and liver of β₂-AR-null mice (β₂-AR^-/-) by performing in vivo (glucose tolerance test and insulin tolerance test) and ex vivo (glucose uptake and glycogen determination) experiments. β₂-AR gene deletion is associated with hepatic insulin resistance and preserved skeletal muscle insulin sensitivity. Importantly, we demonstrate that hepatic β₂-AR regulates insulin-induced AKT activation via Grb2-mediated SRC recruitment through a G_i-independent mechanism.

CONCLUSIONS

β-AR stimulation contributes to the development of early stages of insulin resistance progression in the liver. Our findings indicate that the cross-talk between β₂-AR and insulin signaling represents a fundamental target towards the development of novel therapeutic approaches to treat type 2 diabetes and metabolic syndrome.

Multiple endothelial cells constitute the tip of developing blood vessels and polarize to promote lumen formation

Blood vessel polarization in the apical-basal axis is important for directed secretion of proteins and lumen formation; yet, when and how polarization occurs in the context of angiogenic sprouting is not well understood. Here, we describe a novel topology for endothelial cells at the tip of angiogenic sprouts in several mammalian vascular beds. Two cells that extend filopodia and have significant overlap in space and time were present at vessel tips, both in vitro and in vivo. The cell overlap is more extensive than predicted for tip cell switching, and it sets up a longitudinal cell-cell border that is a site of apical polarization and lumen formation, presumably via a cord-hollowing mechanism. The extent of cell overlap at the tip is reduced in mice lacking aPKCζ, and this is accompanied by reduced distal extension of both the apical border and patent lumens. Thus, at least two polarized cells occupy the distal tip of blood vessel sprouts, and topology, polarization and lumenization along the longitudinal border of these cells are influenced by aPKCζ.