β-endorphin endopeptidase: Expression and induction during murine macrophage differentiation

Mature, differentiated macrophages derived in vitro from murine bone marrow progenitors stimulated with GM-CSF display significant surveillance and host defense against infection compared to immature progenitor macrophages. Opioid peptides such as β-endorphin are known to affect the immune functions of haematopoietic cells and the biological activity of opioid peptides is terminated or regulated by enzymatic degradation. We recently reported that a β-endorphin metabolizing endopeptidase isolated from EL4 cells was identified as insulin degrading enzyme and was present in antiCD3 activated murine CD4⁺ and CD8⁺ T cells. Here we report this same T cell endopeptidase is involved in the hydrolysis of β-endorphin by intact and lysed cell extracts of GM-CSF derived murine mature bone marrow macrophages. The expression of β-endorphin endopeptidase catalytic activity, mRNA accumulation (RT-PCR amplification), and protein level (Western blot analysis) is much higher in mature macrophages compared to progenitor macrophages. Metalloprotease inhibitors inhibit the β-endorphin degradation by the intact mature macrophages and approximately 84% of the β-endorphin degrading activity is immunoprecipitated from the cell extracts by a polyclonal antibody raised against the EL4 endopeptidase. These results suggest that the metabolism of β-endorphin by the mature bone marrow macrophages is due to a novel metallo endopeptidase (insulin degrading enzyme), whose activity is required during the morphological and biochemical differentiation of macrophages.