Sphingosine 1-phosphate lyase inhibition by 2-acetyl-4-(tetrahydroxybutyl)imidazole (THI) under conditions of vitamin B6 deficiency

Abstract

Caramel food colorant 2-acetyl-4-(tetrahydroxybutyl)imidazole (THI) causes lymphopenia in animals through sphingosine 1-phosphate lyase (SPL) inhibition. However, this mechanism of action is partly still controversial because THI did not inhibit SPL in vitro either in cell-free or in cell-based systems. It is thought that the in vitro experimental conditions which have been used so far were not suitable for the evaluation of SPL inhibition, especially in case of cell-based experiments. We speculated that the key factor might be the coenzyme pyridoxal 5'-phosphate (PLP), an active form of vitamin B6 (VB6), because media used in cell-based assays usually contain an excess amount of VB6 which leads to the activation of SPL. By the use of VB6-deficient culture medium, we could regulate apo- (without PLP) and holo- (with PLP) SPL enzyme in cultured cells, resulting in the successful detection of SPL inhibition by THI. Although the observed inhibitory effect was not as strong as that of 4-deoxypyridoxine (a VB6 analog SPL inhibitor), these findings may be useful for further understanding the mechanism of action of THI.

Sphingosine-1-Phosphate Signaling in Immune Cells and Inflammation: Roles and Therapeutic Potential

“…CD68 + cells on the parenchymal side of marginal reticular cells express SPL in human LN [ 62 ]. Inhibition of SPL by caramel food colorant, 2-acetyl-4-tetrahydroxybutylimidazole (THI), also prevents T cell egress from the thymus and secondary lymphoid organs under conditions of vitamin B6 deficiency [ 63 ].…”

Sphingolipids in Ventilator Induced Lung Injury: Role of Sphingosine-1-Phosphate Lyase

“…CS was also found to modulate the unfolding of inflammatory cascade in the cells [ 62 ]. Inhibiting S1PL with 4-DP, an analog of pyridoxal phosphate that is required for S1PL activity [ 32 , 63 ] attenuated CS induced gap formation and apoptosis in the alveolar epithelium. S1P plays an important role as a barrier stabilizing lipid mediator [ 64 ].…”

Inhibitors of sphingosine-1-phosphate metabolism (sphingosine kinases and sphingosine-1-phosphate lyase)

“…(Schwab et al, 2005) Very recently, Ohtoyo et al investigated the mode of action of THI by focusing in particular on the vitamin B6 concentration in the assay system. Using a vitamin B6-deficient culture medium in a cell-based assay, they reported the first successful detection of SPL inhibition by THI in in vitro experiments (Ohtoyo et al, 2015). In addition, derivatives of THI, such as LX2931 and LX2932, have also been described.…”