Cathepsin D is partly endocytosed by the LRP1 receptor and inhibits LRP1-regulated intramembrane proteolysis

Abstract

The aspartic protease cathepsin-D (cath-D) is a marker of poor prognosis in breast cancer that is overexpressed and hypersecreted by human breast cancer cells. Secreted pro-cath-D binds to the extracellular domain of the Β-chain of the LDL receptor-related protein-1 (LRP1) in fibroblasts. The LRP1 receptor has an 85-kDa transmembrane Β-chain and a noncovalently attached 515-kDa extracellular α-chain. LRP1 acts by (1) internalizing many ligands via its α-chain, (2) activating signaling pathways by phosphorylating the LRP1Β-chain tyrosine and (3) modulating gene transcription by regulated intramembrane proteolysis (RIP) of its Β-chain. LRP1 RIP involves two cleavages: the first liberates the LRP1 ectodomain to give a membrane-associated form, LRP1Β-CTF, and the second generates the LRP1Β-intracellular domain, LRP1Β-ICD, that modulates gene transcription. Here, we investigated the endocytosis of pro-cath-D by LRP1 and the effect of pro-cath-D/LRP1Β interaction on LRP1Β tyrosine phosphorylation and/or LRP1Β RIP. Our results indicate that pro-cath-D was partially endocytosed by LRP1 in fibroblasts. However, pro-cath-D and ectopic cath-D did not stimulate phosphorylation of the LRP1Β-chain tyrosine. Interestingly, ectopic cath-D and its catalytically inactive D231N cath-D, and pro-D231N cath-D all significantly inhibited LRP1 RIP by preventing LRP1Β-CTF production. Thus, cath-D inhibits LRP1 RIP independently of its catalytic activity by blocking the first cleavage. As cath-D triggers fibroblast outgrowth by LRP1, we propose that cath-D modulates the growth of fibroblasts by inhibiting LRP1 RIP in the breast tumor microenvironment. © 2012 Macmillan Publishers Limited All rights reserved.