Date Published: October 1, 2012
Publisher: Public Library of Science
Author(s): Danilo Maddalo, Antje Neeb, Katja Jehle, Katja Schmitz, Claudia Muhle-Goll, Liubov Shatkina, Tamara Vanessa Walther, Anja Bruchmann, Srinivasa M. Gopal, Wolfgang Wenzel, Anne S. Ulrich, Andrew C. B. Cato, Salvatore V. Pizzo. http://doi.org/10.1371/journal.pone.0045690
The molecular chaperone GRP78/BiP is a key regulator of protein folding in the endoplasmic reticulum, and it plays a pivotal role in cancer cell survival and chemoresistance. Inhibition of its function has therefore been an important strategy for inhibiting tumor cell growth in cancer therapy. Previous efforts to achieve this goal have used peptides that bind to GRP78/BiP conjugated to pro-drugs or cell-death-inducing sequences. Here, we describe a peptide that induces prostate tumor cell death without the need of any conjugating sequences. This peptide is a sequence derived from the cochaperone Bag-1. We have shown that this sequence interacts with and inhibits the refolding activity of GRP78/BiP. Furthermore, we have demonstrated that it modulates the unfolded protein response in ER stress resulting in PARP and caspase-4 cleavage. Prostate cancer cells stably expressing this peptide showed reduced growth and increased apoptosis in in vivo xenograft tumor models. Amino acid substitutions that destroyed binding of the Bag-1 peptide to GRP78/BiP or downregulation of the expression of GRP78 compromised the inhibitory effect of this peptide. This sequence therefore represents a candidate lead peptide for anti-tumor therapy.
The glucose regulated protein GRP78 (also known as BiP, immunoglobuling heavy chain binding protein) is a member of the heat shock protein family and plays an important role in maintaining cellular homeostasis . It is the key regulator of the unfolded protein response (UPR), a pathway activated upon accumulation of unfolded peptides during stressful conditions such as heat shock, acidosis, nutrient starvation and hypoxia .
GRP78/BiP is expressed in many human cancers where it mediates tumor growth by enhancing proliferation, protecting against apoptosis and promoting tumor angiogenesis. . GRP78/BiP also favors cell survival and contributes to tumor progression and drug resistance during ER stress that arises in the tumor microenvironment as a result of hypoxia and nutrient deprivation , . The maintenance of cellular homeostasis by GRP78/BiP occurs in different tumors including prostate cancer , ,  and an increased expression of GRP78 has been associated with castration resistance and androgen deprivation in prostate cancer . As a result, several attempts have been made to target GRP78/BiP to trigger apoptosis in prostate cancers and other forms of cancers. For example, a peptidic ligand of GRP78/BiP fused to a programmed cell death-inducing sequence was shown to suppress tumor growth in xenograft and isogenic models of prostate and breast cancer . Furthermore a peptidic ligand of GRP78/BiP conjugated to taxol has been shown to exhibit selective cytotoxicity against highly metastatic melanoma cells . In addition to peptide-drug conjugates, a number of peptides and antibodies binding to the ATPase and substrate binding domains of GRP78/BiP have been reported , , . Some of these affect the growth promoting and angiogenic action of GRP78/BiP positively or negatively but their modes of action have not been extensively investigated.