Ajuba receptor mediates the internalization of tumor-secreted GRP78 into macrophages through different endocytosis pathways
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Xiaoqin La1,*, Lichao Zhang2,*, Hanqing Li3, Zhuoyu Li1,2,3, Guisheng Song4, Peng Yang1 and Yufei Yang1
1Institute of Biotechnology, Key Laboratory of Chemical Biology and Molecular Engineering of National Ministry of Education, Shanxi University, Taiyuan 030006, China
2Institute of Biomedical Sciences, Shanxi University, Taiyuan 030006, China
3School of Life Science, Shanxi University, Taiyuan 030006, China
4Department of Bioengineering and Therapeutic Sciences, University of California, San Francisco, CA 94143, USA
*These authors contributed equally to this work
Zhuoyu Li, email: firstname.lastname@example.org
Keywords: tumor-secreted GRP78; macrophages; endocytosis; Ajuba; tumor microenvironment
Received: October 20, 2017 Accepted: January 02, 2018 Epub: January 09, 2018 Published: March 20, 2018
Glucose-regulated protein 78 (GRP78), an ER chaperone, is overexpressed in cancer cells. Solid tumor cells can secrete GRP78 that can promote tumor angiogenesis, differentiation of bone marrow-derived mesenchymal stem cells, tumor cell proliferation and polarization of tumor-associated macrophages. However, the mechanism by which GRP78 functions as a tumor promoter either by staying on the membrane to stimulate intracellular signals or directly entering into cytosolic remains unknown. Here, we reported that an endotoxin-free His-GRP78 protein was purified in vitro that simulates original secreted GRP78. Through analyzing GRP78 concentration in serum samples from 32 colon cancer patients, 40 nM His-GRP78 was selected as an optimized dose to treat cells. Biochemical analysis revealed that secreted GRP78 was able to enter into RAW264.7 and THP-1 cells directly rather than stay on the plasma membrane to transfer signals. Further studies showed that GRP78 internalization was endocytosis-dependent, and both phagocytosis and clathrin, caveolin-1 and micropinocytosis-mediated endocytosis pathways contributed to internalization of secreted GRP78 into cells. Mechanistically, Ajuba is able to interact with GRP78. Ablation of Ajuba suppressed the internalization of secreted GRP78 into cells, indicating that Ajuba was responsible for internalization of secreted GRP78 into RAW264.7. Furthermore, we observed that internalized GRP78 could entered into the mitochondrion and endoplasmic reticulum, which provided a suitable place and enough time for GRP78 to function in molecular and cellular processes. Together, these results reveal a novel mechanism by which secreted GRP78 internalizes into macrophages in the tumor microenvironment, which provides a potential target for drug development.
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