LEUKOTRIENE RECEPTOR ON U-937 CELLS - DISCRIMINATORY RESPONSES TO LEUKOTRIENES C4 AND D4

Dimethyl sulfoxide (DMSO)-differentiated U-937 cells develop cell surface receptors for leukotrienes that, when stimulated, initiate a transient increase in intracellular calcium concentration ([Ca2+]i). We investigated the calcium transient that occurs after addition of leukotriene C4 (LTC4) to determine whether it occurs due to 1) the bioconversion of LTC4 to leukotriene D4 (LTD4), which then acts at the LTD4 receptor; 2) the direct action of LTC4 at the LTD4 receptor; or 3) the action of LTC4 at a receptor selective for LTC4. Bioconversion of [H-3]LTC4 to [H-3]LTD4 was inhibited by 98% when DMSO-differentiated U-937 cells were incubated with 10 mM AT-125 compared with control cells. The dose-response curve for LTC4, with [Ca2+]i as the index of response, was parallel to that for LTD4 but was significantly (P < 0.0001) shifted 1.6 +/- 0.11 log units to the right. AT-125 did not change the response to LTD4 but the LTC4 dose-response curve was shifted on additional 1.7 logo units to the right. The antagonists SKF 104353 (1-mu-M) and LY 171883 (10-mu-M) shifted the dose-response curve for LTD4 3.0 +/- 0.23 and 2.5 +/- 0.23 log units, respectively, to the right and completely inhibited the change in [Ca2+]i due to LTC4 in the presence of 10 mM AT-125. Molecular modeling studies demonstrated a striking difference in the spatial configuration of LTC4 and LTD4, likely accounting for the ability of cell surface receptors to discriminate between the effects of these two molecules. We conclude that the predominant mechanism for the LTC4-induced calcium transient in U-937 cells is through gamma-glutamyltranspeptidase-mediated bioconversion of LTC4 to LTD4 and subsequent action at the LTD4 receptor.