The leukotriene E4 puzzle: finding the missing pieces and revealing the pathobiologic implications

Abstract

The intracellular parent of the cysteinyl leukotrienes (cysLTs), leukotriene (LT) C(4), is formed by conjugation of LTA(4) and reduced glutathione by LTC(4) synthase in mast cells, eosinophils, basophils, and macrophages. After extracellular export, LTC(4) is converted to LTD(4) and LTE(4) through sequential enzymatic removal of glutamic acid and then glycine. Only LTE(4) is sufficiently stable to be prominent in biologic fluids, such as urine or bronchoalveolar lavage fluid, of asthmatic individuals and at sites of inflammation in animal models. LTE(4) has received little attention because it binds poorly to the classical type 1 and 2 cysLT receptors and is much less active on normal airways than LTC(4) or LTD(4). However, early studies indicated that LTE(4) caused skin swelling in human subjects as potently as LTC(4) and LTD(4), that airways of asthmatic subjects (particularly those that were aspirin sensitive) were selectively hyperresponsive to LTE(4), and that a potential distinct LTE(4) receptor was present in guinea pig trachea. Recent studies have begun to uncover receptors selective for LTE(4): P2Y(12), an adenosine diphosphate receptor, and CysLT(E)R, which was observed functionally in the skin of mice lacking the type 1 and 2 cysLT receptors. These findings prompt a renewed focus on LTE(4) receptors as therapeutic targets that are not currently addressed by available receptor antagonists.

Figure 1

Biosynthesis and molecular structures of cys-LTs. Cytosolic phospholipase A₂α (cPLA₂α) catalyzes the liberation of arachidonic acid from nuclear membranes. 5-lipoxygenase (5-LO) translocates to the nuclear envelope, requiring the integral membrane protein 5-LO-activating protein (FLAP) to convert arachidonic acid to the precursor LTA₄. LTA₄ is further conjugated to reduced glutathione (GSH) by LTC₄S, forming LTC₄, the first committed molecule of the cys-LTs. Following energy-dependent export, LTC₄ is converted by the extracellular enzymes γ-glutamyl transpeptidase (γ-GT) or γ-glutamyl leukotrienase (γ-GL) to LTD₄, and to LTE₄ by dipeptidases (DiPs).

Figure 2

Resolution by RP-HPLC of three major peaks of SRS-A produced in the peritoneal cavity of rats. (A) Retention times of synthetic cysteinyl leukotrienes, (B) retention time of the resolved of natural components of SRS-A, and (C) arbitrary units of biologic activity of these natural components for contraction of the guinea pig ileum. Published from reference #6 with permission from Elsevier Inc.

Figure 3

LTD₄-elicted contraction of guinea pig Ileum and associated metabolism to LTE₄. A) Time course of contractile response to 3.6 ng of [³H]LTD₄ from guinea ileum expressed as percentage of maximal response (closed circles) and of metabolism of [³H]LTD₄ (open squares) to [³H]LTE₄ (open triangles) expressed as percent of total labeled leukotrienes recovered. B) Time course of contractile response to 3.6 ng of [³H]LTD₄ from longitudinal muscle strips of guinea pig ileum expressed as percent of maximal response (closed circles) and of metabolism of [³H]LTD₄ (open squares) to [³H]LTE₄ (open triangles) expressed as percent of total leukotrienes recovered. Published from reference #15 with permission from the American Society for Clinical Investigation.

Figure 4

Wheal formation occurring with intracutaneous injections of various eicosanoids into 3 human subjects. The agonists were LTC₄ (closed square, 1.0 nmole/site), LTD₄ (closed circle, 1.0 nmole/site), LTE₄ (closed triangle, 1.0 nmole/site), LTB₄ (open triangle, 1.6 nmole/site), PGD₂ (open circle, 3.0 nmole/site) and saline (open square). The greatest diameter of the wheal in mm is depicted vs. time from 10 minutes to 6 hrs. Published from reference #26 with permission from Nature Publishing Group.

Figure 5

Dose-dependence of LTC₄-, LTD₄-, and LTE₄-induced ear edema in WT and Cysltr1/Cysltr2^−/− mice. WT (A, C, and E) and Cysltr1/Cysltr2^−/− (B, D, and F) mice received intradermal injections of LTC₄ (A and B), LTD₄ (C and D), or LTE₄ (E and F) in the right ear and vehicle in the left ear (2 mice per group). Ear thickness was measured with calipers at the indicated times after the injection. Error bars indicate S.D. Published from reference #32 from the Proceedings of the National Academy of Sciences.

Figure 6

Schematic presentation of the diversity of the cys-LT receptor system.