The novel cannabinoid receptor GPR55 is expressed by rodent islets and it has been implicated in β-cell function in response to a range of ligands. This study evaluated the effects of GPR55 ligands on intracellular calcium ([Ca2+ ]i ) levels and insulin secretion from islets isolated from GPR55 knockout (GPR55-/- ) mice, age-matched wildtype (WT) mice, and human pancreas.
MATERIALS AND METHODS:
GPR55 expression was determined by western blotting and fluorescent immunohistochemistry. Changes in [Ca2+ ]i were measured by Fura-2 microfluorimetry. Dynamic insulin secretion was quantified by radioimmunoassay following perifusion of isolated islets. RhoA activity was monitored using a Rho binding domain pull down assay.
Western blotting indicated that MIN6 β-cells, mouse and human islets express GPR55 and its localisation on human β-cells was demonstrated by fluorescent immunohistochemistry. The pharmacological GPR55 agonist O-1602 (10μM) significantly stimulated [Ca2+ ]i and insulin secretion from WT mouse islets and these stimulatory effects were abolished in islets isolated from GPR55-/- mice. In contrast, while the putative endogenous GPR55 agonist lysophosphatidylinositol (LPI, 5μM) and the GPR55 antagonist cannabidiol (CBD, 1μM) also elevated [Ca2+ ]i and insulin secretion, these effects were sustained in islets from GPR55-/- mice. Stimulatory effects of O-1602 on [Ca2+ ]i and insulin secretion were also observed in experiments using human islets, but O-1602 did not activate RhoA in MIN6 β-cells.
Our results therefore suggest that GPR55 plays an important role in the regulation of mouse and human islet physiology, but LPI and CBD exert stimulatory effects on islet function by a GPR55-independent pathway(s).
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