Abstract
GRP78 conducts protein folding and quality control in the ER and shows elevated expression and cell surface translocation in advanced tumors. However, the underlying mechanisms enabling GRP78 to exert novel signaling functions at cell surface are just emerging. CD44 is a transmembrane protein and an important regulator of cancer metastasis, and isoform switch of CD44 through incorporating additional variable exons to the extracellular juxtamembrane region is frequently observed during cancer progression. Using super-resolution dual-color single-particle tracking, we report that GRP78 interacts with CD44v in plasma membrane nanodomains of breast cancer cells. We further show that targeting cell surface GRP78 by the antibodies can effectively reduce cell surface expression of CD44v and cell spreading of tamoxifen-resistant breast cancer cells. Our results uncover new functions of GRP78 as an interacting partner of CD44v and as a regulator of CD44v membrane homeostasis and cell spreading. This study also provides new insights into anti-CD44 therapy in tamoxifen-resistant breast cancer.
Highlights
GRP78 (78 kD glucose-regulated protein, referred to as BiP or HSPA5) belongs to the heat shock protein 70 (HSP70) family and is a major ER chaperone protein that facilitates protein folding, quality control, and regulates the unfolded protein response (Ni & Lee, 2007; Luo & Lee, 2013; Lee, 2014; Pobre et al, 2019)
A positive correlation of Cell surface GRP78 (csGRP78) and CD44v expression levels was observed in MCF7-LR cells as demonstrated by flow cytometry, with the specificity for the signals confirmed by the absence of primary antibody (M.O.M. control) and isotype control staining (Fig 1E)
Because acquired tamoxifen resistance of breast cancer cells is accompanied with an elevated level of csGRP78, the tamoxifen-resistant MCF7-LR breast cancer cells represent a clinically relevant model to study the interaction of csGRP78 with its partner proteins
Summary
GRP78 (78 kD glucose-regulated protein, referred to as BiP or HSPA5) belongs to the heat shock protein 70 (HSP70) family and is a major ER chaperone protein that facilitates protein folding, quality control, and regulates the unfolded protein response (Ni & Lee, 2007; Luo & Lee, 2013; Lee, 2014; Pobre et al, 2019). Overexpression of GRP78 is associated with cancer cell growth, invasion, and drug resistance (Lee, 2014; Cook & Clarke, 2015; Gifford et al, 2016). Cell surface GRP78 (csGRP78) has emerged as a novel regulator of cell surface signaling, beyond the traditional protein foldase activity as a chaperone protein in the ER (Ni et al, 2011; Zhang et al, 2013; Tsai et al, 2018). Because GRP78 exists on the cell surface primarily as a peripheral protein (Tsai et al, 2015), the identification of plasma membrane proteins that interact with csGRP78 is critical toward understanding how GRP78 is anchored on the cell surface and exerts its signaling functions
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