The Tumor Suppressor Par-4 Activates an Extrinsic Pathway for Apoptosis

Abstract

Summary Prostate apoptosis response-4 (Par-4) is a pro-apoptotic protein with intracellular functions in the cytoplasm and nucleus. Unexpectedly, we noted Par-4 protein is spontaneously secreted by normal and cancer cells in culture, and by Par-4 transgenic mice that are resistant to spontaneous tumors. Short exposure to endoplasmic reticulum (ER) stress-inducing agents further increased cellular secretion of Par-4 by a brefeldin A-sensitive pathway. Secretion occurred independently of caspase activation and apoptosis. Interestingly, extracellular Par-4 induced apoptosis by binding to the stress response protein, glucose-regulated protein-78 (GRP78), expressed at the surface of cancer cells. The interaction of extracellular Par-4 and cell surface GRP78 led to apoptosis via ER stress and activation of the FADD/caspase-8/caspase-3 pathway. Moreover, apoptosis inducible by TRAIL, which also exerts cancer cell-specific effects, is dependent on extracellular Par-4 signaling via cell surface GRP78. Thus, Par-4 activates a novel extrinsic pathway involving cell surface GRP78 receptor for induction of apoptosis.

Cell Surface Relocalization of the Endoplasmic Reticulum Chaperone and Unfolded Protein Response Regulator GRP78/BiP

“…In this model, the C-terminal region of GRP78 is expected to be exposed extracellularly. In support of this, studies using antibodies against the C terminus of GRP78 in FACS analysis detected surface GRP78 expression (18,33). Here we observed that the His epitope tagged at the C terminus of GRP78 is also exposed.…”

“…However, evidence is accumulating that the N terminus of GRP78 is also exposed on the cell surface. Thus, using antibody that targets the N-terminal region of GRP78, it was demonstrated that the interaction between surface GRP78 and extracellular Par-4 and GPI-anchored Cripto and T-cadherin could be negatively affected (29,30,33). Here using FACS analysis of ectopically expressed GRP78 bearing the FLAG epitope at the N terminus, we demonstrated directly that the N terminus of GRP78 is exposed.…”

“…Studies on cell surface GRP78 expression performed in tissue culture show much variability, ranging from expression of GRP78 in various cancer cell lines to no expression in normal fibroblast cell lines (18) to positive reactivity with GRP78 binding peptide motifs whether or not they are tumor cells (17). There are also conflicting reports of whether GRP78 is expressed on specific cell lines, such as the PC-3 prostate cancer cells (33,39). The discrepancies are probably due to different techniques used to detect cell surface GRP78, which is present at very low amounts and in only a subpopulation of cells, and the divergent results could also represent intrinsic variabilities among cultured cell lines.…”

“…GRP78 interacts with the major histocompatibility complex class I molecules and is implicated as a co-receptor for viral entry (31,32). Surface GRP78 is also required for the activation of an extrinsic apoptotic pathway mediated by extracellular Par-4 and TRAIL (33).…”

PAWR-mediated suppression of BCL2 promotes switching of 3-azido withaferin A (3-AWA)-induced autophagy to apoptosis in prostate cancer cells

“…From that standpoint, activation of apoptotic pathways by inducing the expression of the tumor suppressor PAWR would be more purposeful as a therapeutic strategy. Immunocytochemical studies revealed the localization of PAWR in the ER and plasma membrane, 58 and in response to severe ER stress, it might be possible that 3-AWA-induced PAWR in the ER vicinity negatively regulates both the ER pool of BCL2 and BECN1 to suppress autophagy and simultaneously activates the mitochondrial machinery (through BAX (BCL2-associated x protein) or BAK (BCL2-antagonist/killer 1) to suppress the mitochondrial pool of BCL2, 5 and promotes CASP3-mediated apoptosis. 40 Following, we assume that 3-AWA-induced activation of PAWR would be more advantageous to control disease in the ER-stressed condition rather than blocking autophagy by inhibitors.…”