Binding of α2ML1 to the Low Density Lipoprotein Receptor-Related Protein 1 (LRP1) Reveals a New Role for LRP1 in the Human Epidermis

BACKGROUND: The multifunctional receptor LRP1 has been shown to bind and internalize a large number of protein ligands with biological importance such as the pan-protease inhibitor α2-macroglobulin (α2M). We recently identified Α2ML1, a new member of the α2M gene family, expressed in epidermis. α2ML1 might contribute to the regulation of desquamation through its inhibitory activity towards proteases of the chymotrypsin family, notably KLK7. The expression of LRP1 in epidermis as well as its ability to internalize α2ML1 was investigated. METHODS AND PRINCIPAL FINDINGS: In human epidermis, LRP1 is mainly expressed within the granular layer of the epidermis, which gathers the most differentiated keratinocytes, as shown by immunohistochemistry and immunofluorescence using two different antibodies. By using various experimental approaches, we show that the receptor binding domain of α2ML1 (RBDl) is specifically internalized into the macrophage-like cell line RAW and colocalizes with LRP1 upon internalization. Coimmunoprecipitation assays demonstrate that RBDl binds LRP1 at the cell surface. Addition of RAP, a universal inhibitor of ligand binding to LRP1, prevents RBDl binding at the cell surface as well as internalization into RAW cells. Silencing Lrp1 expression with specific siRNA strongly reduces RBDl internalization. CONCLUSIONS AND SIGNIFICANCE: Keratinocytes of the upper differentiated layers of epidermis express LRP1 as well as α2ML1. Our study also reveals that α2ML1 is a new ligand for LRP1. Our findings are consistent with endocytosis by LRP1 of complexes formed between α2ML1 and proteases. LRP1 may thus control desquamation by regulating the biodisponibility of extracellular proteases.