Stereoselective actions of hepoxilins A₃ and B₃ and their cyclopropane analogs (HxΔA₃ and HxΔB₃) on bradykinin and PAF-evoked potentiation of vascular leakage in rat skin

Abstract

Native hepoxilins (Hx) A₃ and B₃ as well as their synthetic cyclopropane analogs, HxΔA₃ and HxΔB₃ are inactive on their own in causing changes in vascular permeability in rat skin measured by leakage of plasma-bound Evans Blue dye. Several of these compounds, however, were observed to potentiate the leakage of dye evoked by bradykinin (BK) and platelet-activating factor (PAF). The syn epimer of HxA₃ was effective in potentiating dye leakage evoked by BK but not by PAF. The syn epimer of HxB₃, on the other hand, was capable of potentiating both BK- and PAF-evoked plasma protein leakage. The anti epimer of both hepoxilins was inactive. In contrast, the anti epimer of the cyclopropane analog HxΔA₃ potentiated only the BK-evoked changes, whereas the anti epimer of HxΔB₃ potentiated only the PAF-evoked changes in dye leakage. The corresponding other epimer of each compound was inactive. Our findings indicate that the hepoxilin cyclopropane analogs appear to mimic the actions of the native compounds.

Introduction

Hepoxilins are hydroxyepoxy metabolites formed by the 12(S)-lipoxygenase pathway Pace-Asciak and Martin 1984, Pace-Asciak et al. 1983. They were shown to be formed by the brain Pace-Asciak 1988, Reynaud et al. 1994a, Reynaud et al. 1994b, pineal gland (Pace-Asciak et al., 1993), blood vessels (Laneuville et al., 1991b), human neutrophils (Reynaud et al., 1995b), and pancreatic islets (Pace-Asciak et al., 1985). They were recently reported to be greatly enhanced in psoriatic skin (Antón et al., 1998). Hepoxilins are formed when 12(S)-lipoxygenase metabolizes arachidonic acid to 12(S)-hydroxyeicosa-5,8,10,14-tetraenoic acid [12(S)-HPETE] Pace-Asciak 1984, Pace-Asciak et al. 1983 and hepoxilin synthase subsequently rearranges this compound into mostly hepoxilin A₃ (HxA₃), although some hepoxilin B₃ (HxB₃) also is formed Reynaud et al. 1994a, Reynaud et al. 1994b. The last reaction is stereospecific in that it utilizes only 12(S)-HPETE; the corresponding 12(R) epimer is inactive as substrate for hepoxilin synthase. We previously showed that HxA₃ potentiates the vascular leakage of plasma proteins evoked by bradykinin (BK) in a stereospecific manner—that is, only the 8R (syn) epimer being active Laneuville et al. 1991a, Laneuville et al. 1991c. Because native hepoxilins are relatively unstable in a biological system owing to epoxide ring opening by epoxide hydrolase (Pace-Asciak and Lee, 1989), the chemical synthesis of hepoxilin analogs in which the epoxide ring is replaced by a biologically stable cyclopropyl ring was carried out (Demin and Pace-Asciak, 1993). The present study was intended to investigate the actions of these analogs on plasma protein leakage evoked by two agonists, BK and platelet-activating factor (PAF), to determine whether the analogs may have potential in the modulation of inflammation in the skin.