The Secreted Protein Isthmin Selectively Triggers Apoptosis in Cancer Cells and Cancer Endothelial Cells through Cell-Surface GRP78 Mediated Mitochondria Targeting

Isthmin (ISM) is a secreted protein of 60 kDa that inhibits angiogenesis and induces endothelial cell apoptosis. Cell-surface glucose-regulated protein of 78 kDa (GRP78) is a high-affinity receptor for ISM, mediating its pro-apoptotic function. Through screening a large number of human cancer cell lines, we found that the cell-surface GRP78 level correlates with the invasiveness and metastatic potential of the cancer cell line. Correspondingly, ISM selectively induces apoptosis only in cancer cells that harbor high level cell-surface GRP78. Hence, ISM is a dual targeting agent, targeting both cancer cells and the cancer stromal endothelial cells.

Mechanistic investigation revealed that ISM-GRP78 interaction initiates clathrin-dependent endocytosis and ISM internalization. The internalized ISM-GRP78 complex are co-targeted to mitochondria via endosomes. Inside mitochondria, ISM blocks ATP release to cytosol through interacting with ADP/ATP carriers on the mitochondria inner membrane, causing mitochondrial dysfunction without affecting its membrane integrity. We discovered a novel protein transport pathway from extracellular environment to mitochondria. We demonstrate that this pathway is not unique to ISM, and other secreted antiangiogenic proteins may also use similar pathways to reach mitochondria and induce apoptosis.