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Members of a subfamily of G protein-coupled receptors (GPCRs), encoded by five different endothelial differentiation genes (edgs), specifically mediate effects of lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) on cellular proliferation and differentiation. Mechanisms of suppression of apoptosis by LPA and S1P were studied in the Tsup-1 cultured line of human T lymphoblastoma cells, which express Edg-2 and Edg-4 GPCRs for LPA and Edg-3 and Edg-5 GPCRs for S1P. At 10-10 M to 10-7 M, both LPA and S1P protected Tsup-1 cells from apoptosis induced by Abs to Fas, CD2, and CD3 plus CD28 in combination. Apoptosis elicited by C6 ceramide was inhibited by S1P, but not by LPA, in part because ceramide suppressed expression of Edg-2 and Edg-4 surface receptors for LPA without affecting Edg-3 surface receptors for S1P. At 10-9 M to 10-7 M, LPA and S1P significantly suppressed cellular levels of the apoptosis-promoting protein Bax, without altering the levels of Bcl-xL or Bcl-2 assessed by Western blots and immunoassays. Transfections of pairs of antisense plasmids for Edg-2 plus Edg-4 and Edg-3 plus Edg-5, and hygromycin selection of transfectants with reduced expression of the respective Edg R proteins in Western blots, inhibited both protection from apoptosis and reduction in cellular levels of Bax by LPA and S1P. Thus, LPA and S1P protection from apoptosis is mediated by distinct Edg GPCRs and may involve novel effects on Bax regulatory protein.
Goetzl et al. (Mon,) studied this question.