Roger White

The oxidation of low density lipoprotein (LDL) in the arterial wall is thought to contribute to human atherosclerotic lesion formation, in part by the high affinity uptake of oxidized LDL (OxLDL) by macrophages, resulting in foam cell formation. We have utilized cloning by expression to identify CD36 as a macrophage receptor for OxLDL. Transfection of a CD36 clone into 293 cells results in the specific and high affinity binding of OxLDL, followed by its internalization and degradation. An anti-CD36 antibody blocks 50% of the binding of OxLDL to platelets and to human macrophage-like THP cells. Furthermore, like mouse macrophages, 293 cells expressing CD36 recognize LDL which has been oxidized only 4 h, whereas more extensive oxidation of the LDL is required for recognition by the other known OxLDL receptors, the acetylated LDL (AcLDL) receptor and Fc gamma RII-B2. CD36 may play a role in scavenging LDL modified by oxidation and may mediate effects of OxLDL on monocytes and platelets in atherosclerotic lesions.

The mouse estrogen receptor was shown to be constantly shuttling between the nucleus and cytoplasm although under steady-state conditions it is detected predominantly in the cell nucleus in both the absence and presence of estradiol. Shuttling was demonstrated by monitoring the transfer of protein between nuclei in heterokaryons and by examining the subcellular distribution of mutant receptors. In the presence of the partial antiestrogen 4-hydroxytamoxifen the receptor was retained in the nucleus whereas it accumulated in the cytoplasm when cells were treated with the pure antiestrogen ICI 182780. The effect of the pure antiestrogen was to inhibit nucleocytoplasmic shuttling of the receptor by blocking its nuclear uptake. Thus although ligand binding is not required by the estrogen receptor to undergo nucleocytoplasmic shuttling, this process can be disrupted by the binding of a pure antiestrogen.

A cDNA for human adipsin was isolated and shown to encode a protein sharing 98% amino acid sequence similarity with the protein sequence previously determined for purified natural human complement factor D. Like mouse adipsin, recombinant human adipsin displays the enzymatic activity of human complement factor D, cleaving complement factor B only when B is complexed with activated complement component C3. We conclude that human adipsin is equivalent to complement factor D and that adipsin is the homologue of factor D in rodents. Adipose tissue is a major site of synthesis of human adipsin/complement factor D mRNA, but unlike the case in rodents, human adipsin mRNA is also expressed in monocytes/macrophages. The data presented here, demonstrating the equivalence of human adipsin to complement factor D and its high level of expression in fat, suggest a previously unsuspected role for adipose tissue in immune system biology.

Pulmonary surfactant is a lipid-rich material that promotes alveolar stability by lowering the surface tension at the air-fluid interface in the peripheral air spaces. The turnover of surfactant phospholipids in the alveolar space is fast, and several lines of evidence suggest there is rapid formation and replenishment of the phospholipid surface film during normal respiration. Specific proteins may regulate these dynamic surface properties. The predominant surfactant protein is a well-characterized, lipid-associated glycoprotein, SP 28-36 (28-36 kDa). A second group of very hydrophobic proteins has recently been shown to affect the surface activity of surfactant phospholipids. We have isolated this group of hydrophobic proteins, herein called SP 5-18 (5-18 kDa), from canine surfactant and have shown by NH2-terminal sequence analysis that at least two proteins, SP 5-8 and SP 18, are present in this group. We have derived the full amino acid sequence of SP 18 from the nucleotide sequence of the cDNAs identified with oligonucleotide probes that were based on the NH2-terminal amino acids of SP 18. The protein isolated from extracellular surfactant appears to be a fragment of a much larger precursor protein (40 kDa). The amino acid sequence of SP 18 is markedly hydrophobic and contains two possible bilayer-spanning domains. We have shown that SP 18 and the glycoprotein SP 28-36 have a cooperative, calcium-dependent action in promoting the formation of phospholipid surface films.