Lipid phosphate phosphatase inhibitors locally amplify lysophosphatidic acid LPA(1) receptor signalling in rat brain cryosections without affecting global LPA degradation

BACKGROUND: Lysophosphatidic acid (LPA) is a signalling phospholipid with multiple biological functions, mainly mediated through specific G protein-coupled receptors. Aberrant LPA signalling is being increasingly implicated in the pathology of common human diseases, such as arteriosclerosis and cancer. The lifetime of the signalling pool of LPA is controlled by the equilibrium between synthesizing and degradative enzymatic activity. In the current study, we have characterized these enzymatic pathways in rat brain by pharmacologically manipulating the enzymatic machinery required for LPA degradation. RESULTS: In rat brain cryosections, the lifetime of bioactive LPA was found to be controlled by Mg(2+)-independent, N-ethylmaleimide-insensitive phosphatase activity, attributed to lipid phosphate phosphatases (LPPs). Pharmacological inhibition of this LPP activity amplified LPA(1) receptor signalling, as revealed using functional autoradiography. Although two LPP inhibitors, sodium orthovanadate and propranolol, locally amplified receptor responses, they did not affect global brain LPA phosphatase activity (also attributed to Mg(2+)-independent, N-ethylmaleimide-insensitive phosphatases), as confirmed by P(i) determination and by LC/MS/MS. Interestingly, the phosphate analog, aluminium fluoride (AlF(x)(-)) not only irreversibly inhibited LPP activity thereby potentiating LPA(1) receptor responses, but also totally prevented LPA degradation, however this latter effect was not essential in order to observe AlF(x)(-)-dependent potentiation of receptor signalling. CONCLUSIONS: We conclude that vanadate- and propranolol-sensitive LPP activity locally guards the signalling pool of LPA whereas the majority of brain LPA phosphatase activity is attributed to LPP-like enzymatic activity which, like LPP activity, is sensitive to AlF(x)(-) but resistant to the LPP inhibitors, vanadate and propranolol.