Human Scavenger Receptor B1 Is Involved in Recognition of Apoptotic Thymocytes by Thymic Nurse Cells
There are two isoforms of the macrophage scavenger receptor (MSR I and II). Both are expressed on macrophages and mediate internalization of oxidized lipoproteins and several other ligands. MSR expression is regulated by cytokines but the individual regulation of each isoform is not well documented. We have therefore developed a PCR method to quantify mRNA levels of MSR isoforms. The analysis is based on relating the amount of reverse transcribed and amplified human macrophage MSR transcripts to a synthetic internal standard, using a 32P-labeled 5'-primer to allow quantitation of the products. Each MSR isoform and its corresponding standard amplified with equal efficiency and the amount of MSR mRNA could be determined from 1 to 100 ng of total RNA. Using this method, we estimated that each monocyte-derived macrophage contains 10-130 molecules of MSR I and 30-640 copies of MSR II mRNA. Both isoforms were down-regulated by bacterial endotoxin (LPS), but the effect was more pronounced for MSR II transcripts. However, cycloheximide induced a selective degradation of MSR I transcripts, leaving MSR II levels unaltered. This suggests that both transcriptional and posttranscriptional control mechanisms are important in the regulation of MSR expression.