GPR55-dependent stimulation of insulin secretion from isolated mouse and human islets of Langerhans

Aims: 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. Results: Western blotting indicated that MIN6 β-cells, mouse and human islets express GPR55 and its localization 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. Conclusions: 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).