We have shown previously that MDA-MB-231 breast cancer cells express only one membrane-associated form of the CA….i.e., CAIX. Thus, cell surface activity measurements reflect the activity of only this isoform. This form is induced by hypoxia, and we show here using the 18O-exchange GM6001 chemical structure technique that membranes isolated from hypoxic cells have a substantial increase in CA activity. We then utilized this technique in whole cells. These data demonstrated that the activity of CAIX can be distinguished from that of CAII and infers a role for the bicarbonate transporter in their individual catalytic activities. Application of an impermeant sulfonamide,

which selectively blocks CAIX activity, confirmed its specific contribution to cell-surface CA activity. Ferrostatin-1 Further, inhibition of bicarbonate transport demonstrated the requirement of this component

in the cross-talk between the two CAs. A BAY 11-7082 model predicted by these studies will be presented. Poster No. 42 Cathepsin D Binds to the Extracellular Domain of the Beta Chain of LRP1 and Inhibits LRP1 Regulated Intramembrane Proteolysis, Stimulating LRP1-dependent Fibroblast Invasive Growth Mélanie Beaujouin1, Christine Prébois1, Danielle Derocq1, Valérie Laurent-Matha1, Olivier Masson1, Peter Coopman2, Nadir Bettache2, Hongyu Zhang3, Bradley Hyman4, Peter van Der Geer5, Gary Smith6, Emmanuelle Liaudet-Coopman 1 1 Inserm U896, IRCM, Montpellier, France, 2 CNRS UMR5237, CRBM, montellier, France, 3

University of Ottawa, Ottawa, ON, Canada, 4 Alzheimer Disease Research Laboratory, Harvard Medical School, Charlestown, MA, USA, 5 San Diego University, San Diego, CA, USA, 6 Glaxosmithkline, NC, USA The protease cathepsin-D (cath-D) is secreted at high levels by breast cancer cells and triggers fibroblast outgrowth via a paracrine loop (Laurent-Matha et al., 2005). Here, we evidence that cath-D interacts with the extracellular domain of the beta chain of the LDL receptor-related protein-1, LRP1, in fibroblasts. LRP1 is composed of a 515 kDa extracellular alpha chain and an 85 kDa Sclareol beta chain. The beta chain contains an extracellular domain, a trans-membrane region and a cytoplasmic tail. LRP1 originally identified as an endocytosis receptor, is also involved in signal transduction by tyrosine phosphorylation of its cytoplasmic NPXY motifs. LRP1 was then shown to participate in cell signalling by regulated intramembrane proteolysis (RIP). In the RIP process, LRP1betae chain undergoes ectodomain shedding, generating the membrane-associated LRP1 fragment, that becomes a substrate for constitutive intramembrane cleavage by gamma-secretases, producing the LRP1 cytoplasmic intracellular domain that acts as a transcriptional modulator. In this study, we show that cath-D binds to residues 349–394 of LRP1beta and this binding is not competed by the chaperone protein RAP.