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Prepublished online as a Blood First Edition Paper on March 20, 2003; DOI 10.1182/blood-2002-10-3055.
Blood, 15 July 2003, Vol. 102, No. 2, pp. 682-688 Sorting of soluble TNF-receptor for granule storage in hematopoietic cells as a principle for targeting of selected proteins to inflamed sitesFrom the Department of Hematology and the Department of Cell and Molecular Biology, Lund, Sweden; and The Netherlands Cancer Institute, Amsterdam, the Netherlands
Hematopoietic cells have secretory lysosomes that degranulate at the inflammatory site upon stimulation. We asked whether one could target exogenous proteins with a therapeutic potential to secretory lysosomes in hematopoietic cells. For this purpose, we expressed a soluble tumor necrosis factor (TNF) receptor form (sTNFR1) in hematopoietic cell lines. In order to accomplish targeting to secretory lysosomes, both endoplasmic reticulum (ER) retention and constitutive secretion have to be prevented. ER export was facilitated by addition of a transmembrane (tm) sequence, and constitutive secretion was overcome by incorporating a cytosolic sorting signal (Y) from CD63. This signal directed the resulting sTNFR1-tm-Y to secretory lysosomes. Confirmation of these results was provided by biosynthetic radiolabeling, subcellular fractionation, immunofluorescence microscopy, and immunoelectron microscopy. The tm-Y fragment was cleaved by proteolysis, resulting in generation of the membrane-free sTNFR1 in secretory lysosomes. Our results suggest a potential for using the storage organelles of hematopoietic cells as vehicles for targeting sites of inflammation with therapeutically active agents.
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