We recently reported the positional cloning of a candidate gene for hereditary hemochromatosis called HFE. The gene product, a member of the major histocompatibility complex class I-like family, was found to have a mutation, Cys-282 3 Tyr (C282Y), in 85% of patient chromosomes. This mutation eliminates the ability of HFE to associate with ␤ 2 -microglobulin (␤ 2 m) and prevents cellsurface expression. A second mutation that has no effect on ␤ 2 m association, H63D, was found in eight out of nine patients heterozygous for the C282Y mutant. In this report, we demonstrate in cultured 293 cells overexpressing wild-type or mutant HFE proteins that both the wild-type and H63D HFE proteins form stable complexes with the transferrin receptor (TfR). The C282Y mutation nearly completely prevents the association of the mutant HFE protein with the TfR. Studies on cell-associated transferrin at 37°C suggest that the overexpressed wild-type HFE protein decreases the affinity of the TfR for transferrin. The overexpressed H63D protein does not have this effect, providing the first direct evidence for a functional consequence of the H63D mutation. Addition of soluble wild-type HFE͞␤ 2 m heterodimers to cultured cells also decreased the apparent affinity of the TfR for its ligand under steady-state conditions, both in 293 cells and in HeLa cells. Furthermore, at 4°C, the added soluble complex of HFE͞␤ 2 m inhibited binding of transferrin to HeLa cell TfR in a concentration-dependent manner. Scatchard plots of these data indicate that the added heterodimer substantially reduced the affinity of TfR for transferrin. These results establish a molecular link between HFE and a key protein involved in iron transport, the TfR, and raise the possibility that alterations in this regulatory mechanism may play a role in the pathogenesis of hereditary hemochromatosis.

Here we report the first structure of a mammalian 15-lipoxygenase. The protein is composed of two domains; a catalytic domain and a previously unrecognized beta-barrel domain. The N-terminal beta-barrel domain has topological and sequence identify to a domain in the mammalian lipases, suggesting that these domains may have similar functions in vivo. Within the C-terminal domain, the lipoxygenase substrate binding site is a hydrophobic pocket defined by a bound inhibitor. Arachidonic acid can be docked into this deep hydrophobic pocket with the methyl end extending down into the bottom of the pocket and the acid end tethered by a conserved basic residue on the surface of the enzyme. This structure provides a unifying hypothesis for the positional specificity of mammalian lipoxygenases.

Arachidonate 15-lipoxygenase (arachidonate:oxygen 15-oxidoreductase, EC 1.13.11.33) is a lipidperoxidating enzyme that is implicated in oxidizing low density lipoprotein to its atherogenic form. Monocyte/macrophage 15-lipoxygenase is present in human atherosclerotic lesions. To pursue a basis for induction of the enzyme, which is not present in blood monocytes, the ability of relevant cytokines to regulate its expression was investigated. Interleukin 4 (IL4), among 16 factors tested, specifically induced 15-lipoxygenase mRNA and protein in cultured human monocytes. Interferon y and hydrocortisone inhibited this induction. High-performance liquid chromatography analysis of lipid extracts from IL-4-treated monocytes detected 15-lipoxygenase products esterified to the cellular membrane lipids, indicating enzymatic action on endogenous substrates. Stimulation of IL-4-treated monocytes with calcium ionophore or opsonized zymosan A enhanced the formation of 15-lipoxygenase products. These data identify 1L14 and interferon y as physiological regulators of lipoxygenase expression and suggest an important link between 15-lipoxygenase function and the immune/inflammatory response in atherosclerosis as well as other diseases.

Oxidation of low density lipoprotein (LDL) enhances its atherogenicity, and inhibition of such oxidation decreases the rate of progression of atherosclerotic lesions. The mechanism of LDL oxidation in vivo remains uncertain, but in vitro studies have suggested that cellular lipoxygenases may play a role by initiating lipid peroxidation in LDL. In situ hybridization studies using a 15-lipoxygenase riboprobe and immunostaining using antibodies against 15-lipoxygenase showed strongly positive reactivity largely confined to macrophage-rich areas of atherosclerotic lesions. The mechanism of LDL oxidation in vivo has not been elucidated. In principle, it might result from the release of superoxide anion from cells, a direct action of membrane enzymes on LDL making contact with the cells, or the transfer of lipid peroxides generated within the cell membrane to LDL in the extracellular space. In any case, the initial "seeding" of LDL with a lipoperoxide would be followed by propagation, a process that may require metal ion catalysis. In vitro studies suggest that cellular lipoxygenases may play an important role (5,25,26). Thus, LDL treated with soybean lipoxygenase (LO) closely resembles LDL oxidized by incubation with cells (25). Moreover, several lipoxygenase inhibitors block cell-induced oxidation of LDL (26). Atherosclerotic lesions of rabbit aorta have been reported to show higher 15-LO activity than normal aorta (27,28). However, those studies did not establish in which cell types the 15-LO resides or whether that expression relates to oxidation of LDL. The purpose of the present study was to determine whether LOs are present in atherosclerotic lesions, which cell types express LO mRNAs, and whether LOs colocalize with oxidized LDL in atherosclerotic lesions. METHODSStudies were done utilizing the WHHL rabbit, a strain deficient in functional LDL receptors and therefore showing marked elevation of LDL and spontaneous atherosclerosis (29). Grossly visible lesions were dissected from the aortic arch and the thoracic aorta of six WHHL rabbits (1-2 years old). Animals were anesthetized with intramuscular ketamine (35 mg/kg) and xylazine (5 mg/kg), exsanguinated, and Abbreviations: PCR, polymerase chain reaction; LO, lipoxygenase; 4-HNE, 4-hydroxynonenal; MDA, malondialdehyde; nt, nucleotide; LDL, low density lipoprotein; WHHL rabbit, Watanabe heritable hyperlipidemic rabbit. fTo whom reprint requests should be addressed