Lysophosphatidylcholine and Endothelial Cell Signalling

Access to files

•   FULL-TEXT.PDF (pdf)

Abstract

Lysophosphatidylcholine (LPC) is a by product of phospholipid metabolism, that under physiological conditions is maintained at a low level. However, through an enhanced degradation of phospholipids and/or a reduced catabolism, LPC accumulates in the plasma and fluids of patients with disorders underscored by inflammation – such as atherosclerosis, diabetes, ischaemia and epilepsy. Previous studies have demonstrated LPC to possess vasoactive properties, able to both induce and inhibit vasodilation. Furthermore, a variety of proteins are sensitive to LPC, including non-selective cation (NSC) channels and Ca2+-activated K+ (KCa) channels. These channels are intimately associated with the maintenance and regulation of vascular tone. The aim of this study was to elucidate the mechanisms underlying the vascular effect of LPC.Aortic segments were constricted with phenylephrine and exposed to cumulative concentrations of LPC, with an ensuing endothelium-dependent, concentration-dependent vasodilation. Inhibitors of nitric oxide synthase (NOS) and soluble guanylyl cyclase (sGC) abolished LPC-induced responses, implicating nitric oxide (NO) as the mediator. Two cation fluxes were implicated in the dilator activity of LPC - Ca2+ and K+. NSC channel antagonists and reduced extracellular Ca2+ concentration attenuated dilation and reduced the Ca2+ signal activated in isolated rat aortic endothelial cells (RAEC) by LPC, implicating endothelial Ca2+ influx in the response. In addition, LPC also evoked a robust hyperpolarisation of isolated RAEC membrane potential. The K+ channel antagonists TEA+, TRAM-34 and apamin, inhibitors of KCa channels, attenuated both the LPC-induced dilation and RAEC membrane hyperpolarisation, highlighting their potential role in mediating both these processes.HEK293 cells, which lack many of the channels and signalling pathways possessed by other cells, mimicked RAEC in their sensitivity to LPC, generating robust elevations of intracellular Ca2+ when exposed to this lysolipid. Likewise, membrane hyperpolarisations were also observed in HEK293 cells, however, these only occurred when cells expressed recombinant KCa channels. This suggests that KCa channel activation is dependent upon Ca2+ influx, not vice versa.Phospholipase C (PLC) inhibitor U73122, attenuated LPC-induced hyperpolarisation, raising the question as to the possible involvement of G-protein coupled receptors in the bioactivity of LPC. Alternately, LPC might initiate PLC activity, and subsequent NSC channel opening and Ca2+ influx via a perturbation of membrane integrity, like certain local anaesthetics. It is proposed that endothelial NSC-channel activation by LPC initiates endothelial cell signalling, with concomitant activation of Ca2+-sensitive proteins such as NOS, to bring about vasodilation, and KCa channels, which modulate membrane potential and in turn the driving force for Ca2+ entry.

Keyword(s)

Calcium; Calcium-activated potassium channel; Endothelial cell; Hyperpolarisation; Lysophosphatidylcholine

Bibliographic metadata

Institutional metadata

Record metadata