Apoptosis inhibition in cancer cells: A novel molecular pathway that involves BAG3 protein

Abstract

Stress-induced apoptosis regulates neoplasia pathogenesis and response to therapy. Indeed, cell transformation induces a stress response, that is overcome, in neoplastic cells, by alterations in apoptosis modulators; on the other hand, antineoplastic therapies largely trigger the apoptosis stress pathway, whose impairment results in resistance. Therefore, the study of the roles of apoptosis-modulating molecules in neoplasia development and response to therapy is of key relevance for our understanding of these processes. Among molecules that regulate apoptosis, a role is emerging for BAG3, a member of the BAG co-chaperone protein family. Proteins that share the BAG domain are characterized by their interaction with a variety of partners (heat shock proteins, steroid hormone receptors, Raf-1 and others), involved in regulating a number of cellular processes, including proliferation and apoptosis. BAG3, also known as CAIR-1 or Bis, forms a complex with the heat shock protein (Hsp) 70. This assists polypeptide folding, can mediate protein delivery to proteasome and is able to modulate apoptosis by interfering with cytochrome c release, apoptosome assembly and other events in the death process. It has been recently shown that, in human primary lymphoid and myeloblastic leukemias and other neoplastic cell types, BAG3 expression sustains cell survival and underlies resistance to therapy, through downmodulation of apoptosis.

This review summarizes findings that assign an apoptotic role to BAG3 in some neoplastic cell types and identify the protein as a candidate target of therapy.

Introduction

A relevant trait that sustains tumor growth is neoplastic cell ability of evading apoptosis induced by stressful stimuli, such as cell cycle alteration, increase in the intracellular levels of reactive oxygen species (ROS) and others (Evan et al., 2005; Spurgers, Chari, Bohnenstiehl, & McDonnell, 2006). Furthermore, resistance to therapy also largely relies on a lowered sensitivity to chemotherapy-induced stress (Gatti & Zunino, 2005). Therefore, the understanding of pathways that modulate stress response in neoplastic cells can lead to the identification of novel targets and strategies for therapy.

Cell stress-inducing conditions trigger mechanisms (activation of JNK, Bax and/or Bak, alterations in mitochondrial membrane, etc.) able to mediate cell death, and others (increase in Akt levels, stimulation of NF-κB activity, etc.) involved in maintaining cell survival. The balance between these two kinds of responses determines cell fate (Beere, 2005; Pirkkala, Nykanen, & Sistonen, 2001). The anti-apoptotic events triggered by stress-inducing agents involve increase in levels of heat shock proteins (Hsps). In addition to function as molecular chaperones that assist damaged protein re-folding or delivery to proteasome, Hsps can interfere in more than one step in the apoptosis process, including mitochondrial membrane depolarization (through their effects on members of the Bcl-2 family), proteasome formation and other events (Beere, 2005; Young, Barral, & Ulrich-Hartl, 2003). Recent evidence implicates Hsp70 in regulating the stability of Bim mRNA (Matsui, Asou, & Inaba, 2007). Inducible Hsp expression is regulated by the heat shock transcription factors (HSFs). In response to various inducers such as elevated temperature, oxidants, heavy metals, bacterial and viral infections, most HSFs acquire DNA binding activity to the heat shock elements (HSE), thereby mediating transcription of the heat shock genes. Several members of the HSF family have been found in vertebrates (HSF1–4) and plants; this suggests that different HSFs mediate the responses to various forms of physiological and environmental stimuli. HSF1, an HSF prototype, and HSF3 are responsible for heat-induced Hsp expression, whereas HSF2 is refractory to classical stressors. HSF4 is expressed in a tissue-specific manner; similar to HSF1 and HSF2, HSF4 have alternatively spliced isoforms, adding further complexity to its regulation (Pirkkala et al., 2001).

Co-chaperone proteins that share the BAG (Bcl-2 associated athanogene) domain are characterized by their interaction with heat shock proteins of the Hsp70 family. Furthermore they can bind other partners (steroid hormone receptors, Raf-1 and others), implicated in a number of cellular processes, including proliferation and apoptosis (Doong et al., 2000, Doong et al., 2002; Takayama, Xie, & Reed, 1999; Takayama & Reed, 2001). Among BAG family members there is BAG3, also known as CAIR-1 or Bis (Antoku, Maser, Scully, Delach, & Johnson, 2002; Bonelli et al., 2004, Briknarova et al., 2002, Chen et al., 2004; Chroboczek, Gout, Favier, & Galinier, 2002; Dong et al., 2005, Doong et al., 2000, Doong et al., 2002, Doong et al., 2003, Homma et al., 2006; Kassis, Guancial, Doong, Virador, & Kohn, 2006; Lee et al., 1999, Lee et al., 2002a; Lee, Kim, Shin et al., 2002; Liao et al., 2001; Pagliuca, Lerose, Cigliano, & Leone, 2003; Romano, Festa, Pagliuca et al., 2003; Romano, Festa, Petrella et al., 2003; Zhang et al., 2006, Seo et al., 2005, Tabuchi et al., 2006; Takayama & Reed, 2001). BAG3 forms a complex with Hsc70/Hsp70 (Takayama et al., 1999). In addition, BAG3 polypeptide binds to phospholipase C (PLC)-γ (Doong et al., 2000) and possibly other partners (Doong et al., 2002; Takayama & Reed, 2001). Notably, BAG3 expression is induced by heavy metals (zinc, cadmium) or high temperature in cells of the human neoplastic epithelial cell line HeLa, with time-dependent kinetics of accumulation of mRNAs similar to two other stress-regulated genes, metallothioneins (MTs) and Hsp70 (Pagliuca et al., 2003). BAG3 induction by stressors has been observed also in other systems: pancreatic cancer cells exposed to heat stress (Liao et al., 2001), light damaged murine retinal cells (Chen et al., 2004), human leukemia Molt-4 cells treated with non-thermal low intensity pulsed ultrasound (Tabuchi et al., 2006), human primary microglia subjected to HIV-induced oxidative stress (Rosati et al., 2007).

These findings indicate that BAG3 expression is part of stress-induced programs. This is in accord with the presence of HSF-responsive elements (HSE) in bag3 gene (Ensembl Gene ID ENSG00000151929). Among proteins of the BAG family, BAG3 is the only one reportedly inducible by stressors.

This review illustrates experimental evidence that assign to BAG3 a role in modulating stress response and, specifically, in sustaining neoplastic cell survival and impairing response to therapy.