Date Published: November 25, 2013
Publisher: Public Library of Science
Author(s): Marie Cohen, Pascale Ribaux, Manuella Epiney, Olivier Irion, Hong Wanjin.
Glucose-regulated protein 78 (GRP78) is an endoplasmic reticulum (ER) molecular chaperone that belongs to the heat shock protein 70 family. GRP78 is also present on the cell surface membrane of trophoblastic cells, where it is associated with invasive or fusion properties of these cells. Impaired mechanism of GRP78 relocation from ER to the cell surface was observed in preeclamptic cytotrophoblastic cells (CTB) and could take part in the pathogenesis of preeclampsia. In this study, we have investigated whether prostate apoptosis response 4 (Par-4), a protein identified as a partner of GRP78 relocation to the cell surface in prostate cancer cells, is present in trophoblastic cells and is involved in the translocation of GRP78 to the cell surface of CTB. Par-4 is indeed present in trophoblastic cells and its expression correlates with expression of membrane GRP78. Moreover, overexpression of Par-4 led to an increase of cell surface expression of GRP78 and decreased Par-4 gene expression reduced cell surface localization of GRP78 confirming a role of Par-4 in relocation of GRP78 from ER to the cell surface. Accordingly, invasive property was modified in these cells. In conclusion, we show that Par-4 is expressed in trophoblastic cells and is involved in transport of GRP78 to the cell surface and thus regulates invasive property of extravillous CTB.
GRP78 is an ER molecular chaperone that belongs to the heat shock protein 70 family (for a review ). The primary functions of GRP78 are related to its capacity to bind hydrophobic regions on nascent polypeptides in the ER and to its pivotal role in the signalling cascade producing the unfolded protein response (UPR) . GRP78 expression can be stimulated by a variety of environmental and physiological stress conditions such as glucose starvation or hypoxia , . GRP78 is well-known to reside inside the ER lumen. However, this chaperone is also located at the cell surface of cancer cells and cells undergoing ER stress . The mechanisms responsible for the translocation of this protein from the ER to the cell surface remain poorly understood . The KDEL sequence of GRP78 present in its C-terminal part is involved in maintaining proteins within the ER lumen. It was thus hypothesized that overexpression of GRP78 observed under stress conditions may exceed the retention capacity of the KDEL retrieval system, resulting in relocation of GRP78 from the ER to the cell surface . It was also hypothesized that the masking of the KDEL may be implicated in GRP78 transport to the cell surface. Additionally, particular GRP78-interacting protein partners are involved in the transport of GRP78 from the ER to the cell surface, and this can be cell-type-specific . For example, MTJ-1 binds GRP78 and silencing MTJ-1 expression decreases cell-surface GRP78 expression in macrophages . In prostate cancer cells, Par-4 seems to be required for the translocation of GRP78 from the ER to the plasma membrane . On the outer plasma membrane, GRP78 functions as a receptor for a wide variety of ligands  and several small proteins can bind to surface GRP78 and modulate properties of cells .
Par-4 is a multi-domain protein of 340 amino acids. The key domains are highly conserved among different species and are leucine zipper domain at the C-terminal part, two nuclear localization sequences at the N-terminal part, a nuclear export sequence and the SAC (Selective for Apoptosis of Cancer cells) domain. In addition, Par-4 contains a number of conserved consensus sites for phosphorylation by kinases suggesting that Par-4 may be tightly regulated by post-translational modification, localization and interactions with proteins of biological consequence. The main biological role of Par-4 described in literature is pro-apoptotic and depends on interactions with other proteins such as PKCζ, WT-1, ZIP kinase, THAP-1 , , . More generally, functions of Par-4 are dependent on its partners. Secreted Par-4 could also induce apoptosis and this would be rendered possible by its interaction with cell surface GRP78 in cancer cells . In this last study, Par-4 knock-down does not alter total GRP78 levels in whole-cell lysates, but decreases the cell surface GRP78 expression in prostate cancer cells, suggesting that endogenous Par-4 is involved in the cell surface expression of GRP78.