BAG1

Abstract: Burkitt lymphoma (BL) is a highly aggressive B-cell non-Hodgkin lymphoma (B-NHL), which originates from germinal center (GC) B cells and harbors translocations deregulating v-myc avian myelocytomatosis viral oncogene homolog (MYC). A comparative analysis of microRNAs expressed in normal and malignant GC B cells identified microRNA 28 (miR-28) as significantly down-regulated in BL, as well as in other GC-derived B-NHL. We show that reexpression of miR-28 impairs cell proliferation and clonogenic properties of BL cells by modulating several targets including MAD2 mitotic arrest deficient-like 1, MAD2L1, a component of the spindle checkpoint whose down-regulation is essential in mediating miR-28–induced proliferation arrest, and BCL2-associated athanogene, BAG1, an activator of the ERK pathway. We identify the oncogene MYC as a negative regulator of miR-28 expression, suggesting that its deregulation by chromosomal translocation in BL leads to miR-28 suppression. In addition, we show that miR-28 can inhibit MYC-induced transformation by directly targeting genes up-regulated by MYC. Overall, our data suggest that miR-28 acts as a tumor suppressor in BL and that its repression by MYC contributes to B-cell lymphomagenesis.

Abstract: Bag 1 acts as a co-chaperone for Hsp70/Hsc70. We report here that stable over-expression of Bag1 in immortalized neuronal CSM14.1 cells prevents death following serum deprivation. Bag1 over-expression slowed the proliferative rate of CSM14.1 cells, resulted in increased levels of phospo-MAP kinases and accelerated neuronal differentiation. Immunocytochemistry revealed mostly nuclear localization of Bag1 protein in these cells. However, during differentiation in vitro, Bag1 protein shifted from predominantly nuclear to mostly cytosolic in CSM14.1 cells. To explore in vivo parallels of these findings, we investigated Bag1 expression in the developing mouse nervous system using immunohistochemical methods. Early in brain development, Bag1 was found in nuclei of neuronal precursor cells, whereas cytosolic Bag1 staining was observed mainly after completion of neuronal precursor migration and differentiation. Taken together, these findings raise the possibility that the Bag1 protein is expressed early in neurogenesis in vivo and is capable of modulating neuronal cell survival and differentiation at least in part from a nuclear location.

Abstract: Bcl-2-associated athanogene 1 (BAG1) is the first identified member of the BAG protein family, which consists of at least six different members identified in mammals to date. With the exception of BAG5, all BAG family members share the highly conserved BAG domain near the C terminus (37). The BAG family proteins differ in various other domains, allowing interactions with cellular binding partners and differential subcellular targeting. BAG1 binds to the ATPase domain of the 70-kDa heat shock protein (Hsp70) and its cognate protein Hsc70 (hereafter referred to as Hsp70), which are chaperones, through the BAG domain, thereby serving as a cochaperone and modulating chaperone activity (37-39). Moreover, multiple functions of BAG1 have been proposed; these include interaction and activation of the serine/threonine-specific protein kinase Raf-1 via its N-terminal domain (34, 45) as well as binding to steroid hormone and other receptors (2, 45, 50). Many studies dealing with BAG1 function revealed that it increases resistance to death stimuli when overexpressed in vitro, including in neuronal cells (19, 31, 35, 38, 43, 44). The neuroprotective effects were confirmed in vivo with transgenic mice overexpressing BAG1. Explanted primary neurons displayed increased resistance against glutamate toxicity. Moreover, BAG1 proved to be neuroprotective against stroke induced by middle cerebral artery occlusion in these mice (18). Apart from its antiapoptotic effects, BAG1 also accelerates differentiation when overexpressed in neurons (19). In fact, BAG1−/− mice die during embryogenesis due to failed neurogenesis (46; unpublished observations), indicating an important role for the BAG1 gene in neuronal differentiation, neuronal survival, or both in vivo. It has been proposed that these multifunctional properties of BAG1 depend on its interactions with cellular binding partners. For example, there is evidence for a balance between BAG1 binding to Raf-1 or Hsp70 (34). This scenario raises the possibility that BAG1 promotes cell growth by binding to and stimulating the activity of Raf-1, whereas its antiapoptotic effects are modulated by its interaction with Hsp70. Thus, BAG1 may serve as a molecular switch, encouraging cells to proliferate and differentiate under permissive conditions and allowing survival under nonpermissive conditions (34, 37, 45). Moreover, conflicting data have been reported regarding the modulation of chaperone activity by BAG1. While BAG1 was originally characterized as an inhibitor of Hsp70 activity in vitro, recent evidence suggests that BAG1 can act as a stimulatory or an inhibitory cochaperone, depending on cell type, cofactor expression, and concentration (10, 41). To resolve the question of how the interaction of BAG1 and Hsp70 influences chaperone activity in single neuronal cells, we generated a chaperone-dependent yellow fluorescent protein (YFP) folding mutant (cdYFP) and measured its refolding in rat nigral CSM14.1 and human SHSY-5Y neuroblastoma cells stably overexpressing full-length BAG1 or a truncated BAG1 construct (BAGΔC) lacking the ability to bind Hsp70. Cells overexpressing BAGΔC were also characterized and compared to those containing full-length BAG1 (19) with regard to neuronal differentiation and susceptibility to apoptosis. Here we show for the first time, using intact cells of different species, that BAG1 is a potent inducer of Hsp70 chaperone activity in neurons and that BAG1 neuroprotectivity is dependent on this cochaperone effect. In contrast, the ability of BAG1 to promote neuronal differentiation is independent of Hsp70 binding and cochaperone activity.

Abstract: BAG3, a multifunctional HSP70 co-chaperone and anti-apoptotic protein that interacts with the ATPase domain of HSP70 through its C-terminal BAG domain plays a key physiological role in cellular proteostasis. The HSP70/BAG3 complex determines the levels of a large number of selective client proteins by regulating their turnover via the two major protein degradation pathways, i.e. proteasomal degradation and macroautophagy. On the one hand, BAG3 competes with BAG1 for binding to HSP70, thereby preventing the proteasomal degradation of its client proteins. By functionally interacting with HSP70 and LC3, BAG3 also delivers polyubiquitinated proteins to the autophagy pathway. BAG3 exerts a number of key physiological functions, including an involvement in cellular stress responses, proteostasis, cell death regulation, development, and cytoskeletal dynamics. Conversely, aberrant BAG3 function/expression has pathophysiological relevance correlated to cardiomyopathies, neurodegeneration, and cancer. Evidence obtained in recent years underscores the fact that BAG3 drives several key hallmarks of cancer, including cell adhesion, metastasis, angiogenesis, enhanced autophagic activity, and apoptosis inhibition. This review provides a state-of-the-art overview on the role of BAG3 in stress and therapy resistance of cancer, with a particular focus on BAG3-dependent modulation of apoptotic signaling and autophagic/lysosomal activity.